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Journal of Photochemistry and Photobiology C: Photochemistry Reviews 31 (2017) 1–35

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology C:Photochemistry Reviews

jo ur nal home p ag e: www.elsev ier .com/ locate / jphotochemrev

pplied photoelectrocatalysis on the degradation of organicollutants in wastewaters

ergi Garcia-Segura ∗, Enric Brillas ∗

aboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí iranquès 1-11, 08028 Barcelona, Spain

r t i c l e i n f o

rticle history:eceived 20 October 2016eceived in revised form 16 January 2017ccepted 30 January 2017vailable online 9 February 2017

eywords:ydroxyl radicalrganics degradationhotocatalysishotoelectrocatalysisiO2 photoanodeastewater treatment

a b s t r a c t

A large variety of electrochemical advanced oxidation processes (EAOPs) have been recently developedto remove organic pollutants from wastewaters to avoid their serious health-risk factors from their envi-ronmental accumulation and to reuse the treated water for human activities. The effectiveness of EAOPsis based on the in situ production of strong reactive oxygen species (ROS) like hydroxyl radical (•OH).Photoelectrocatalysis (PEC) has emerged as a promising powerful EAOP by combining photocatalytic andelectrolytic processes. It consists in the promotion of electrons from the valence band to the conductionband of a semiconductor photocatalyst upon light irradiation, with production of positive holes. The fastrecombination of the electron/hole pairs formed is avoided in PEC by applying an external bias potentialto the photocatalyst that extracts the photogenerated electrons up to the cathode of the electrolytic cell.Organics can be oxidized directly by the holes, •OH formed from water oxidation with holes and otherROS produced between the electrons and dissolved O2. This paper presents a general and critical reviewon the application of PEC to the remediation of wastewaters with organic pollutants. Special attentionis made over the different kinds of photocatalysts utilized and preparation methods of the most ubiqui-
tous TiO2 materials. Typical PEC systems and main operation variables that affect the effectiveness of thedegradation process are also examined. An exhaustive analysis of the advances obtained on the treat-ment of dyes, chemicals and pharmaceuticals from synthetic solutions, as well as of real wastewaters, isperformed. Finally, research prospects are proposed for the future development of PEC with perspectivesto industrial application.
© 2017 Elsevier B.V. All rights reserved.

Abbreviations: A, absorbance of the most intense UV/Vis peak; A0, initial absorbancelectrode of Ag/AgCl (KCl saturated); ALD, atomic layer deposition; AOP, advanced oxidaiochemical oxygen demand at 5 days; c, organic concentration (mM or mg L−1); c0, initiaeposition; COD, chemical oxygen demand (mg O2 L−1); COD0, initial chemical oxygeneflectance spectroscopy; DSA® , dimensionally stable anode; EAOP, electrochemical advanpectrometry; EF, electro-Fenton; EIS, electrochemical impedance spectroscopy; EO, elecathodic potential (V); Ecell , potential difference of the cell (V); Efb, flat band energy (V); Fand; HPLC, high-performance liquid chromatography; I, current (A or mA); ITO, indium-timax, maximum wavelength of UV/Vis spectrum; LC–MS, liquid chromatography–mass spehotocatalysis; PEC, photoelectrocatalysis; PEF, photoelectro-Fenton; PTFE, polytetrafluoalomel electrode; SCL, space charge layer; SEM, scanning electron microscopy; SPEC, solarOC, total organic carbon (mg C L−1); TOC0, initial total organic carbon (mg C L−1); VB, val; Vis, visible; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy.∗ Corresponding authors.

E-mail addresses: [email protected] (S. Garcia-Segura), [email protected] (E. Brillas).

ttp://dx.doi.org/10.1016/j.jphotochemrev.2017.01.005389-5567/© 2017 Elsevier B.V. All rights reserved.

of the most intense UV/Vis peak; ACF, activated carbon fiber; Ag/AgCl, referencetion process; APS, atmospheric plasma spray; BDD, boron-doped diamond; BOD5,l organic concentration (mM or mg L−1); CB, conductive band; CVD, chemical vapor

demand (mg O2 L−1); DO, direct ozonation; DP, direct photolysis; DRS, diffuseced oxidation process; e−

CB, electron in the conduction band; EDS, energy dispersivetrochemical oxidation; Eanod, anodic potential (V); Ebg, band gap energy (eV); Ecat,ESEM, field-emission SEM; FTO, fluor-doped tin dioxide; h+

VB, hole in the valencen oxide; janod, anodic current density (mA cm−2); �, wavelength of UV/Vis spectrum;ctrometry; NB, nanobelt; NT, nanotube; NTA, nanotube array; PANI, polyaniline; PC,roethylene; PZC, point of zero charge; ROS, reactive oxygen species; SCE, saturated

photoelectrocatalysis; t, electrolysis time; TEM, transmission electron microscopy;ence band; UV, ultraviolet; UVA, ultraviolet A; UVB, ultraviolet B; UVC, ultraviolet

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S. Garcia-Segura, E. Brillas / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 31 (2017) 1–35

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Fundamentals of photoelectrocatalysis (PEC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43. Experimental conditions for PEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1. Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.1. TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.2. WO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.3. ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.4. Other semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.1.5. Modified TiO2 materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2. Preparation of TiO2 photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.1. Thin-film electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.2. Nanostructured materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.3. Characterization of synthesized photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3. PEC systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.4. Operation parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4. Destruction of organic pollutants by PEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.1. Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.1.1. TiO2 photoanodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.1.2. Doped TiO2 and composites with TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.1.3. Other photocatalytic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2. Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.2.1. TiO2 and composites with TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.2.2. Other photoanodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3. Pharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.4. Real wastewaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5. Conclusions and prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30Acknowlegments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Dr. Sergi Garcia-Segura is a researcher dedicated tothe development of Electrochemical Advanced OxidationProcesses to remove organic pollutants from environ-ment such photoelectrocatalysis. He has conducted hisresearch at the Universitat de Barcelona (Spain), Univer-sity of Queensland (Australia), Universidade Federal doRio Grande do Norte (Brazil), Chia Nan University of Phar-macy and Science (Taiwan), Bonn Universität (Germany)and Arizona State University (USA). He received theInternational Society of Electrochemistry Prize for Envi-ronmental Electrochemistry 2014, the Green Talentsaward 2015 and the Antonio Aldaz prize 2016. He haspublished 40 peer-reviewed papers (h-index = 19).

Dr. Enric Brillas is Full Professor of Physical Chem-istry at the Universitat de Barcelona since 1987. He waspresident of the Electrochemistry Group of the RealSociedad Espanola de Química (2004–2008). His researchis pre-eminently devoted on organic electrochemistry,photocatalysis, photoelectrocatalysis, electrocatalysis andelectrochemical treatments of organic pollutants. Hereceived the Chemviron Carbon 1995 and the CIDETEC2014 Awards. Associate Editor of Chemosphere and mem-ber of the Editorial Board of Journal of Hazardous Materialsand Applied Catalysis B: Environmental. He has published330 peer-reviewed papers (h-index = 61) and 22 booksand book chapters, and presented 280 communications

o scientific congresses.

. Introduction

One of the main current worldwide concerns is the growth ofater pollution by organic compounds arising from many indus-

rial, agricultural and urban human activities. The vast majority ofhese compounds are persistent organic pollutants, owing to theiresistance to conventional chemical, biological and photolytic pro-esses. As a result, they have been detected in rivers, lakes, oceansnd even drinking water all over the world. This constitutes a seri-

Dyes, chemicals and pharmaceuticals are some of the most com-mon recalcitrant organic pollutants. For instance, many industriesincluding textile, cosmetic, paper, leather, light-harvesting, arrays,agricultural research, photoelectrochemical cells, pharmaceuticaland food produce large volumes of wastewater polluted with highconcentration of dyes and other components. As a result, about280,000 tons of textile dyes are currently discharged in effluentsevery year and introduced in the aquatic environment [5]. Thishas induced many governments to apply legislation that prescribesand limits the emission of pollutants. To face this environmen-tal problem, three different approaches have been considered: (i)the development of Green Chemical and Technological Processes,(ii) the use of the 3R (reduce, reuse and recycle) sustainabilityconsciousness and (iii) the application of wastewater remedia-tion technologies. The latter approach has received great attentionbecause it is easily usable and can solve the contamination prob-lems.

Current methods for wastewater treatment have been based onoxidation processes including physicochemical, biological, chem-ical and electrochemical treatments [5]. Note that no universalstrategy on wastewater remediation is feasible because of theextremely diverse composition of industrial waste that usually con-tains a complex mixture of organic and inorganic compounds andmainly depends on the nature and concentration of pollutants.

Physicochemical techniques require high cost of equipment andusually present low effectiveness, particularly over dyes and phar-maceuticals. Biological treatments are environmentally friendly,produce less sludge than physicochemical systems and are rela-tively inexpensive. Nevertheless, their application is rather limitedsince treatment needs a large land area, has sensitivity toward tox-icity of certain chemicals and operation time is very long. Overthe past three decades, many advanced oxidation processes (AOPs)

us environmental health problem mainly due to their toxicity andotential hazardous health effects (carcinogenicity, mutagenicitynd bactericidality) on living organisms, including human beings1–4].

have been developed as more effective technologies to remove per-sistent organic pollutants from wastewaters [6]. AOPs are based onthe in situ production of highly reactive hydroxyl radicals (•OH)that non-selectively react with most organics and are able to

nd Photobiology C: Photochemistry Reviews 31 (2017) 1–35 3

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S. Garcia-Segura, E. Brillas / Journal of Photochemistry a

egrade even highly recalcitrant compounds [5–7]. This radicals the second strongest oxidant known after fluorine, displaying

high standard reduction potential of E◦(•OH/H2O) = 2.80 V (SHE)nd rate constants for reaction with several contaminants in therder of 106 to 1010 M−1 s−1 [6,8]. •OH has so short lifetime, aew nanoseconds in water [9], that can be rapidly self-eliminatedrom the treatment system. The most ubiquiotous AOPs are chem-cal, photochemical and photocatalytic systems such as H2O2 withVC radiation (H2O2/UVC), ozone and ozone based processes (O3,3/UVC, O3/H2O2 and O3/H2O2/UVC), titanium dioxide based pro-esses (TiO2/UV and TiO2/H2O2/UV) and Fenton’s reaction basedethods (Fenton (Fe2+/H2O2) and photo-Fenton (Fe2+/H2O2/UV))

10]. Photocatalysis (PC) uses a semiconductor, usually TiO2 knowns the photocatalyst, under light illumination (UV or solar) toenerate electron/hole pairs with ability to degrade most organicollutants by producing the strong oxidant •OH at its surface.his technique has prominent advantages including non-toxicity,ow cost, no secondary pollution and thorough mineralization.owever, PC is restricted by its low photonic efficiency. The fast

ecombination of photogenerated electron/hole pairs at the photo-atalyst surface represents the major drawback for PC applications.

Over the last two decades, electrochemical advanced oxidationrocesses (EAOPs) have gained increasing attention as a promisinglass of AOPs [5,6,11,12]. They have emerged as novel treatmentechnologies for the elimination of a broad-range of organic con-aminants from wastewaters. Several advantages of EAOPs includeigh energy efficiency, amenability to automation, simple equip-ent, safety because they operate under mild conditions (ambient

emperature and pressure) and versatility. From the beginning ofhe XXI Century, the combination of electrochemistry and PC in theo-called photoelectrocatalysis (PEC) method has deserved increas-ng attention. It is based on a semiconductor photoanode that isrradiated by light with energy equal or greater than its band gapnd simultaneously biased by a gradient of potential. PEC offers thepportunity to separate the charges of the photogenerated elec-ron/hole pairs, strongly enhancing the mineralization of organicollutants in wastewaters. Note that urban and industrial wastew-ters usually present conductivity enough to effectively performhe PEC treatment of their organic pollutants because these water

atrices already contain electrolytes like salts of sulfate, chloridend carbonate. Since PEC is still in development, most researchas been made using synthetic wastewater prepared with ultra-ure water and addition of electrolytes. If electrolytes are addedor real PEC application, they should be removed from the finalreated solution before disposal. PEC has also been successfullypplied to inorganic ion reduction, microorganism inactivation,O2 reduction, and production of electricity and hydrogen fromater splitting [13–15]. When sunlight is used as energy source, it

s known as solar PEC (SPEC).Fig. 1a shows the increasing number of papers over PEC classi-

ed for wastewater remediation since 2001, showing the growingnterest over its development. Fig. 1b highlights that TiO2 is thereferred material as photoanode in PEC, although modified TiO2nd to smaller extent WO3, ZnO and other materials have been uti-ized as photoanodes as well. Special attention has been made onhe applications with TiO2 and its modified materials because theyffer important advantages as a result of their non-toxic, optical,ow-cost and biocompatibility properties. Thus, PEC has been usedo remove a large variety of organic pollutants, most of them dyesnd to less extent chemicals, drugs and real wastewaters, as cane seen in Fig. 1c. The large development of PEC over the last 15ears has open novel ways for the application of this EAOP to water
emediation.
The aim of this paper is to present a general and critical reviewn the use of the PEC technology for the remediation of organicollutants in wastewaters. The fundamentals of the technique are

organic pollutants in wastewaters as a function of: (a) published year, (b) photoan-ode used and (c) degraded pollutant.

firstly exposed, followed by an overview about its experimen-tal conditions including the photocatalysts used, the preparationmethods for the most common TiO2 materials, the kinds of PECsystems utilized and the main operation parameters that affect thedegradation performance. The application of PEC to the treatmentof dyes, chemicals, pharmaceuticals and real wastewaters, remark-ing the use of different photoanodes, is discussed. Examples aregiven for the different kinds of organics to evidence the advancesachieved in this technique.

Several reviews on the environmental application of PEC havebeen published based on the advances of this technology upto 2011–2012 [16–18]. The present paper shows a much moredetailed review from the research mainly devoted on the latter fiveyears, where PEC has achieved more mature developments in thefollowing points: (i) the preparation of new photoanodes not onlybased on TiO2, but also on WO3, ZnO and other photocatalytic mate-
rials involving �-PbO2, BiVO4, BiPO4 and �-Fe2O3, among others,(ii) the design of new PEC systems for using direct illumination withsunlight and filter-press flow cells at lab-scale, (iii) the couplingof PEC with O3 and other EAOPs to enhance its oxidation power,

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nd (iv) the use of PEC to treat pharmaceuticals and real wastew-ters, pre-eminently reported from 2012. Moreover, this reviewescribes comprehensively the main characteristics or organicsegradation by PEC such as their kinetic decay, mineralization ratend degree, identification of aromatic and cyclic intermediates, asell as final carboxylic acid generated, and reaction sequences pro-osed. The significant effect of operation parameters over organicegradation is extensively documented and exemplified in selectedgures for better understanding.

. Fundamentals of photoelectrocatalysis (PEC)

The PEC technique combines both electrolytic and photocat-lytic processes and has received considerable attention because ofts ability to retard the recombination of electron-hole (e−

CB/h+VB)

airs, increasing the lifetime of the holes. The basic process of PEConsists in the ejection of an electron (e−

CB) from the valence bandVB) of a semiconductor, which is fully filled, to the conductive bandCB), which is completely empty, generating a positively chargedacancy or hole (h+

VB). The band gap is related to the light irradia-ion utilized. The semiconductor has to be exposed to an irradiationith greater energy than that of its band gap (Ebg), giving rise to thehotoexcitation of the e−

CB from VB to CB. The light then allows theeneration of e−

CB/h+VB pairs via Reaction (1) [5,18–24]:

emiconductor + hv → e−CB + h+

VB (1)

Photogenerated h+VB is a strong oxidizing species, whereas the

romoted e−CB is a potential reductor. Organic pollutants are then

xidized by the photogenerated h+VB up to their complete min-

ralization. It is also proposed the reaction of h+VB with adsorbed

ater to form the strong oxidant •OH from Reaction (2) that min-ralizes the organic pollutants, although there is no clear evidenceor the formation of free hydroxyl radical from h+

VB. The e−CB can

eact with adsorbed O2 to form the superoxide radical O2•− from

eaction (3).

+VB + H2O → •OH + H+ (2)

−CB + O2 → O2

•− (3)

Other weaker reactive oxygen species (ROS) such as H2O2 andydroperoxyl radical HO2

• can be produced via Reactions (4) and5):

2•− + H+ → HO2

• (4)

HO2• → H2O2 + O2 (5)

Nevertheless, the promoted e−CB is an unstable species of an

xcited state and tends to return to the ground state either withdsorbed •OH by Reaction (6) or pre-eminently by recombinationith the unreacted h+

VB from Reaction (7)) [18–24].

−CB + •OH → OH− (6)

−CB + h+

VB → Catalyst + heat (7)

The last reaction represents the main drawback for the efficientse of absorbed photons in the classical PC. To improve the abate-ent of organic pollutants from wastewaters by this technique,

he separation of charges formed from Reaction (1) has been per-ormed using nanoparticulated photocatalysts with high specificrea in suspension in the effluent, but, unfortunately, the recoveryf these materials after treatment is complicated [25–27]. Researchfforts to solve this problem have then been devoted to the synthe-is of nanoparticulated photocatalysts immobilized onto different
ubstrates [28–31]. The fixation of photocatalysts on supports pro-uces an inevitable and significant reduction of their active specificrea with the consequent drop in pollutant removal. However,he efficiency of the immobilized photocatalyst can be strongly
otobiology C: Photochemistry Reviews 31 (2017) 1–35

improved if it is deposited onto a conductive substrate to act asa photoanode in a photoelectrolytic system leading to the PEC pro-cess [18,20–24].

The PEC process on water splitting was firstly described byFujishima and Honda on 1972 and analyzed from the electrochem-ical point of view by Brockis et al. [33,34] on the early 1980s, butit was not up to the beginning of XXI Century when it was appliedto wastewater treatment, as stated above. It uses an electrolyticsystem containing a thin-film active photoanode subjected to lightillumination with application of a constant bias potential to theanode (Eanod), a constant cell potential (Ecell) or a constant anodiccurrent density (janod). This promotes the extraction of photoin-duced e−

CB by the external electrical circuit, thereby yielding anefficient separation of the e−

CB/h+VB pairs since Reactions (3)–(7)

are inhibited [18,20–24]. The prevention of charge recombinationupgrades the photocatalytic efficiency of the anode with genera-tion of higher quantities of holes by reaction (1) and acceleration oforganics oxidation compared to classical PC. The lifetime of holesis increased and they have more opportunities either to directlyoxidize the organic pollutants adsorbed on the photoanode surfaceor indirectly destroy them with more amounts of •OH formed fromReaction (2). In PEC, the photocatalyst can be easily recovered afterusage and recycled for consecutive treatments.

The material more extensively used as semiconductor photocat-alyst in PEC is the anatase crystalline form of TiO2, since it can act asan active photoanode with low cost, low toxicity, high stability andwide band gap of 3.2 eV. Fig. 2 illustrates the mechanism of the pro-cesses taking place in an n-type semiconductor such as TiO2 [18].The e−

CB/h+VB pairs are produced by irradiating photons of UV light

with h� > Ebg of TiO2. The photogenerated electrons flow throughthe external circuit due to a potential gradient from the photoan-ode to the cathode. The holes can thus attack directly to organics Rand/or generate high amounts of •OH via photo-oxidation of waterin much larger extent than ROS formed from e−

CB via O2 photore-duction because most electrons are lost from the photoanode to thecathode [18].

As shown in Fig. 2, a typical cathode involves the reduction ofwater to H2. However, it is feasible to enhance the oxidation powerof PEC by using carbonaceous cathodes that allow the in situ elec-trogeneration of H2O2 from two-electron reduction of injected O2by Reaction (8) [5,11]. The presence of H2O2 in the electrolyticmedium upgrades the production of oxidant •OH from its reductionwith the promoted e−

CB by Reaction (9) [34–38]. Although Reaction(9) is accepted for authors working in PC and PEC, a recent researchhas reported controversial results over its validity [39].

O2 + 2H+ + 2 e− → H2O2 (8)

e−CB + H2O2 → •OH + OH− (9)

Apart from the above photolytic processes, a semiconductor Msubmitted to an anodic potential can remove the organics by elec-trochemical oxidation (EO). This is feasible because it oxidizes thewater to adsorbed hydroxyl radical M(•OH) as follows [40–42]:

M + H2O → M(•OH) + H+ + e− (10)

Unfortunately, the low conductivity of semiconductors used inPEC only allows the pass of small janod values, usually < 10 mA cm−2,leading to low M(•OH) production with very poor ability for themineralization of organic pollutants. In the PEC treatment of awastewater, it is very difficult to know the contribution of EO to theoverall process, since the photogeneration of electron/hole pairs
from Reaction (1) competes with M(•OH) generation from Reaction(10) at the photocatalyst surface, although the large predominanceof the former process explains the much greater oxidation ability ofPEC than EO [31]. The use of high janod values in PEC causes normally

S. Garcia-Segura, E. Brillas / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 31 (2017) 1–35 5

hotoca

ap

3

iFtctdacc

3

rioait

3

ranmboTiatdtcTf

Fig. 2. Mechanism for PEC process using TiO2 p

loss of its performance as a result of a change of the photocatalystroperties, as will be discussed below.

. Experimental conditions for PEC

Most papers devoted to the PEC treatment of organic pollutantsn wastewaters present an experimental study in consecutive steps.irstly, the pollutant and photocatalyst are selected. Since the pho-oactive material is not commercially available, its synthesis andharacterization are subsequently described. Finally, the PEC sys-em used to explore the process performance is detailed and theegradation results are reported as function of the operation vari-bles chosen. This section gives an overview all these experimentalonditions, which are essential to understand the potential appli-ation of PEC.

.1. Photocatalysts

The PEC efficiency on organic pollutant remediation is directlyelated to the semiconductors selected as photoanodes by theirntrinsic photocatalytical properties. In general, they are metalxides with an appropriate Ebg value between the fulfilled VBnd the empty CB to photogenerate electron/hole pairs upon lightrradiation. The different materials used in PEC for wastewaterreatment are detailed below.

.1.1. TiO2TiO2 is the most utilized metal oxide photocatalyst for envi-

onmental remediation applications, as shown in Fig. 1b. It isn n-type semiconductor with three main crystalline structures,amely rutile, anatase and brookite. The former is the most ther-odynamically stable phase, whereas the metastable anatase and

rookite phases can be transformed into the most stable rutilene under annealing in the range 600−800◦ C [43,44]. The Ebg foriO2 is slightly superior to 3.0 eV, with little differences betweents crystalline phases, with values of 3.02 eV for rutile, 3.23 eV fornatase and 3.14 eV for brookite [45,46]. However, anatase withhe higher Ebg value is the most active phase in PC upon UV irra-iation [45,47]. This has been justified by the prolonged lifetime of
he photogenerated charge carriers (e−
CB and h+VB) and the spatial

harge separation promoted by anatase crystalline structure [45].his behavior is very different under PEC conditions where the per-ormance between the different phases is quite similar since the

talyst. Adapted with permission from ref. [18].

photogenerated electrons are dragged out by an external electricalcircuit (see Fig. 2) [18].

3.1.2. WO3WO3 is other n-type semiconductor metal oxide as TiO2. The

crystalline structure of WO3 is temperature dependent, althoughthe most common is the monoclinic phase between 17 and 330 ◦C[48]. The Ebg values of tungsten oxides are around 2.5–2.7 eV, whichare appreciably lower than those of TiO2, and for this reason, WO3presents better performance under visible light irradiation due tothe easier formation of photogenerated electron/hole pairs [49,50].The main drawback of this material is that it is not as innocuous asTiO2, since it is a hazardous, toxic and irritant metal oxide [51]. Thefirst usage of WO3 photoanode for wastewater remediation wasreported by Hepel and Luo in 2001 [52,53], who well-proven thepotential applicability of this material to the abatement of azo dyes.This possibility has been recently explored over different pollutantsby a moderate number of articles, as can be seen in Fig. 1b.

3.1.3. ZnOZnO is extensively found in the nature and is considerably

cheaper than the above photocatalysts. It is also considered as anenvironment-friendly material because of its innocuous characterover the health of living beings [54]. ZnO presents two crys-talline structures, the hexagonal wurtzite and the cubic zinc blende.Wurtzite is the most thermodynamically stable structure at ambi-ent conditions [55]. Its Ebg value is 3.4 eV, with high electrochemicalstability and possible use under natural sunlight irradiation [56,57].ZnO thin films are transparent and this improves the light pen-etration into the material and hence, the photogeneration ofelectron/hole pairs from Reaction (1). Although the number of pub-lications related to ZnO as photoanode in PEC is rather limited (seeFig. 1b), Li et al. [58] have reported that it can present better perfor-mance even than TiO2, thus appearing as an alternative functionalmaterial for PEC application to wastewater treatment.

3.1.4. Other semiconductorsOther many pure metal oxide semiconductors have been

checked as photoanode materials in PEC aiming to search inex-
pensive, efficient and highly stable photocatalysts to the electricalcurrent with ability to absorb greater energy from solar spectrumto photoinduce electron/hole pairs. Nevertheless, the performanceof such materials cannot be easily compared with that found with

6 nd Photobiology C: Photochemistry Reviews 31 (2017) 1–35

ca

nhtwwodirhwcSrecutcbwEeo

3

tracptnmaomCbnp

ioTagrtoisetlaesecce

Fig. 3. SEM micrographs for thin films of: (a) TiO2 thin film deposited onto stainlesssteel by atmospheric plasma spray. (Reproduced with permission from Ref. [31])and (b) TiO2 nanotube arrays (NTAs) prepared by anodization of a Ti foil in 10 wt%water + 0.5 wt% NH4F in glycerol at 30 V for 50 h (Reproduced with permission from


lassical TiO2 because different PEC systems and operation vari-bles are used for environmental remediation.

The hematite �-Fe2O3 has been considered one of these alter-ative candidates because of its low cost, chemical stability andigh light absorbance. Its low Ebg = 2.2 eV allows its direct applica-ion to visible light [59,60]. Other promising photocatalyst is MnOith a very low Ebg = 1.3 eV, being an interesting functional materialith low cost, large surface, electrochemical stability and innocu-

us character [61,62]. The combination of MnO with polyanilineemonstrated an enhancement of its PEC properties owing to the

nteraction with the polyaniline bandgap of 2.8 eV that reduces theecombination reaction of electrons and holes [62]. On the otherand, SnO2 is a chemically and thermally stable semiconductorith so wide Ebg of 3.5–3.8 eV that is difficultly used as photo-

atalyst [63]. Pure non-conductive SnO2 can be easily doped withb improving considerably its electrical conductivity, although theesulting Sb-SnO2 material employed as photocatalyst presents lowlectrochemical stability with short lifetime upon current appli-ation [64]. Other semiconductor such as �-PbO2 has also beentilized as photocatalyst due to its very small Ebg = 1.4 eV [65], buthis material is only useful in alkaline medium since under acidiconditions, it leaches toxic lead ions to the medium. In contrast, sta-le bismuth materials like BiVO4 with Ebg = 2.5 eV [66] and Bi2WO6ith Ebg = 2.8 eV [67] under visible light irradiation and BiPO4 with

bg = 3.8 eV [68] under UV illumination have shown an excellentffectiveness and good performance for the PEC treatment of somerganic pollutants.

.1.5. Modified TiO2 materialsThe PEC performance of a photocatalyst mainly depends on

he following factors: (i) the light absorption properties, (ii) theeduction and oxidation rates on the surface by the photogener-ted electrons and holes, and (iii) the recombination rate of suchharges. To enhance the PEC activity of the most ubiquiotous TiO2hotocatalyst, various strategies have been developed including: (i)he construction of TiO2 nanotubes (NTs), nanotube arrays (NTAs),anobelts (NBs) or nanorods structures [69,70], (ii) the doping withetals like Cr [71], Cu [72] and Fe [73] or non-metals like B [74]

nd N [75] (second generation of photocalysts), (iii) the synthesisf composites with metals like Pd [76], Ag [77] and Au [78], otheretal oxides like SiO2 [79], WO3 [80], Fe3O4 [81], SnO2 [82] and

u2O [83], metal sulfides like CdS [84] and Sb2S3 [85], and car-onaceous materials like carbon cloth and graphene [86], and (iv)ew titanium compounds like TiNbO5 [87] and the cubic double-erovskite CaCu3Ti4O12 [88] (third generation of photocalysts).

TiO2 thin films prepared on substrates like Ti, stainless steel,ndium-tin oxide (ITO) and fluor-doped tin dioxide (FTO), amongthers, by several procedures have been extensively used in PEC.hese films have granulate, compact and low porosity structure,s exemplified in the scanning electron microscopy (SEM) micro-raph of Fig. 3 [32,89], and this restricts the light absorption to UVadiation with high recombination rate of the photogenerated elec-ron/hole pairs, as stated above. Since 2007, highly crystalline andrdered TiO2 nanostructures with large surface area and porosityn the form of nanotubes, nanobelts or nanorods have been synthe-ized to improve the PEC activity of thin-film photoanodes. A higherfficiency for organics removal is described using thin nanostruc-ures as a result of a synergistic effect of absorbing visible light,arger semiconductor/electrolyte interface due to higher surfacerea, minority carriers generated within a distance from the surfacequal to the sum of the width of the depletion layer and the diffu-ion length escape recombination, and smaller recombination of
lectron/hole pairs [89]. Fig. 3b shows a SEM micrograph of a typi-al nanotubular TiO2 photoanode synthesized by Ti anodization andomposed of one-dimensional crystalline NTAs with uniform diam-ters of about 100 nm and wall thicknesses near 10 nm [89]. Using
Ref. [89]).

the same preparation technique, Liao et al. [69] found an Ebg = 2.7 eVfor crystalline anatase Ti/TiO2 NTAs, a value much lower than 3.2 eVof a granulate anatase thin film, and that allowed an enhanced PECactivity using visible light provided by a 60 W incandescent lamp.

The doping of TiO2 with metals or non-metals introduces newVB and/or CB related to the impurity energy levels that enhance thedegradation of organic pollutants by PEC compared with undopedphotocatalyst. Kerkez et al. [72] synthesized TiO2 nanorod arraysfilms deposited onto FTO doped with CuO up to 0.26% molar of Cu2+

(Cu2+-TiO2). The Ebg value of these materials decreased graduallywith raising dopant proportion, from 3.1 eV for the bare TiO2/FTOto 2.6 eV for the higher doped material, which allowed operatingunder visible light improving PEC treatment of organics. This wasexplained by the transference of the photogenerated e−

CB in TiO2to the less energetic CB of CuO, producing Cu+ from Reaction (11).
Cu2+ sites then act as traps of photogenerated e−
CB increasing theelectron/hole lifetime, and the resulting Cu+ can be oxidized to Cu2+


F issioR

et

C

C

mw[aeTvuossd

ig. 4. Proposed PEC mechanism for: (a) doped B-TiO2 NTs (Reproduced with permef. [85]).

ither at the photoanode or with O2 via Reaction (12) to form O2•−

hat can originate the sequence of ROS by Reactions (4)–(5).

u2+ + e−CB → Cu+ (11)

u+ + O2 → Cu2+ + O2•− (12)

A different behavior was found when doping TiO2 with a non-etal compound. An interesting case is the doping of TiO2 NTsith B in the form of NaBF4. For this photocatalyst, Bessegato et al.

74] determined two Ebg values, one of 3.3 eV related to TiO2 andnother of 2.2 eV due to the formation of intermediary energy lev-ls between the VB and CB of TiO2 created by substituted B atoms.he proposed PEC mechanism with absorption of UV (for TiO2) andisible light (from B dopant level) is illustrated in Fig. 4a. This fig-re also shows the bending of all energetic bands upon application
f a bias potential due to the change of the Fermi potential of theemiconductor [74,85]. This bending leads to the formation of apace charge layer (SCL) or depletion layer, characterized by theepletion of photogenerated electrons and corresponding to the
n from ref. [74]) and (b) Sb2S3/TiO2 composite (Reproduced with permission from

distance between the photocatalyst surface and the beginning ofthe flat bands. No SCL exists at the flat band potential (Efb), whereall bands are flats, and its value grews with increasing the appliedEanod respect to Efb.

Other strategy for improving the TiO2 photoactivity is the prepa-ration of composites with noble metals and other semiconductors.An enhancement of the PEC process has been found by decorat-ing TiO2 NTs with Pd, Ag and Au nanoparticles [76–78]. This hasbeen ascribed to the lower work function of the noble metal thanthe electron affinity of TiO2 NTs that originates a Schottky bar-rier potential in the interfaces making energetically favorable theelectron transfer from the CB of the semiconductor to the metaland significantly reducing the recombination of electron/hole pairs[76]. However, the authors do not consider the feasible simultane-ous EO of organics by physisorbed hydroxyl radicals formed at the
metal surface from water oxidation by Reaction (10). The integraltreatment by PEC and EO has been reported for a SnO2/TiO2 com-posite by assembling sieve-like macroporous Sb-doped SnO2 filmon vertically aligned TiO2 NTs [82]. In contrast, the strong hetero-

8 S. Garcia-Segura, E. Brillas / Journal of Photochemistry and Ph

Fe

S

jpPc1UmtTttesirTbtr•

woR

vwo

3

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3

Tois2safi

ig. 5. Schematic diagram illustrating the transfer of the photogenerated holes andlectrons in a TiO2/graphene/Cu2O interface.

ource: Reproduced with permission from ref. [86].

unction between TiO2 and other n-type mixed oxides causes largerhotoactivity of their bicomposites. Fig. 4b shows a diagram of theEC mechanism for Ti/TiO2 NTAs decorated with Sb2S3 nanoparti-les in orthorhombic phase (stibinite), with Ebg values of 3.2 and.7 eV, respectively [85]. The photogenerated electrons in Sb2S3 byV and visible light are injected into the CB of TiO2 because of theore cathodic potential of the former (-0.77 V) respect to the lat-

er (−0.50 V). Furthermore, the photogenerated holes in the VB ofiO2 are transferred to that of Sb2S3, creating a high concentra-ion of holes in the sensitizer/electrolyte interfaces. This decreaseshe recombination of the electron/hole pair and increases the PECfficiency process, along with the advantage that Sb2S3 acts as aensitizer practically in the entire visible spectrum. The PEC behav-or of an interesting TiO2/graphene/Cu2O tricomposite has beenecently explored by Yang et al. [86]. As can be seen in Fig. 5, bothiO2 NTAs and Cu2O (a p-type semiconductor with Ebg = 2.2 eV) cane excited by UV/Vis light to produce electron/holes pairs. The elec-rons of the CB of TiO2 then across the inserted graphene layer andecombine with the holes of the VB of Cu2O. In this way, oxidantsOH can be formed from the holes of VB of TiO2 from Reaction (2)ithout possibility of recombination with the electrons of the CB

f Cu2O, which can originate the chain of ROS from the startingeaction (3) with O2.

The use of new titanium-metal oxides like CaCu3Ti4O12 withery low Ebg = 1.5 eV [88] and high stability represents a promisingay to be explored in the next future for improving the PEC process

f organic pollutants in wastewaters.

.2. Preparation of TiO2 photocatalysts

One of the most crucial points of PEC for the remediation ofolluted waters is the preparation of stable and re-utilizable pho-ocatalysts onto conducting materials as support. Great researchfforts have been carried out to develop novel methodologieso synthesize a large variety of photoanodes. This subsection isevoted to detail the most important methodologies for the prepa-ation of TiO2 thin-film and nanostructured photocatalysts, sincehey are the most used materials, as stated above. Unfortunately,he possible effect of the supporting material on the photocatalyticesponse is barely addressed in the literature and will not be con-idered here.

.2.1. Thin-film electrodesThe sol-gel method is the most common technique to synthesize

iO2 thin-film photocatalysts [90–92]. It consists in the preparationf a titanium dioxide colloidal suspension by mixing titanium(IV)sopropoxide (Ti(i-OPr)4) with glacial acetic acid under constanttirring and keeping a molar ratio H+/Ti = 4, followed by dilution in
-propanol with 1:1 Ti/alcohol ratio. A stable colloidal suspensionolution is then obtained by adding water and HNO3 under stirringnd keeping molar ratios of H2O/Ti = 25 and H+/Ti = 0.5. The thinlm over a typical Ti foil is prepared following a sequence involv-

ing the dipping of the foil into the colloidal suspension, drying andannealing at 100–300 ◦C for ca. 3 h. Although the resulting Ti/TiO2photocatalysts are the easiest applicable ones at lab scale, the coat-ings obtained by this method present low stability because theyare easily cracked by the gas evolution at the photoanode surfaceduring PEC treatment. Low janod values are needed to be appliedto upgrade their operational life. The alternative use of the sol-gel coating laser calcination method allows an improvement of theadherence of TiO2 coatings onto Ti [93].

Other interesting method is the spray painting and thermaldecomposition [94,95], which consists in spraying a solution oftitanyl acetylacetonate (C10H14O5Ti) onto a conductive substrate.The solution concentration, solution flow rate, nozzle-to-substratedistance, and substrate temperature are crucial parameters thataffect the coating properties. Clear and colorless thin films are thenconsistently formed through thermal decomposition of the pre-cursor under oxygen. The painted coating could also be furtherannealed at 400- 600 ◦C to favor the thermal decomposition andto oxidize the precursors giving a best oxide layer coating. Thismethodology is faster than the sol-gel method, allowing coatinglarger substrate surfaces in appreciably lower times. However, theobtained Ti/TiO2 thin-film photoanodes still present low mechan-ical stability like in the case of sol-gel methodology.

TiO2 thin films can also be prepared by magnetron sputtering,a physical vapor deposition technology based on a plasma coatingprocess [96,97]. The sputtering material or precursor is ejected dueto bombardment of ions to the target surface in a vacuum cham-ber. The ionized plasma beam is accurately directed and focusedin front of the target of the substrate by magnetic fields, givingexcellent layer uniformity and smooth coatings. One strong pointof this technology is that any conducting substrate can be sputteredwithout decomposition or coating mechanical failure, because nosubstrate heading is required. He et al. [97] prepared TiO2 coatingover ITO and Ti substrates. The main disadvantages of magnetronsputtering are: (i) the slow deposition speed which makes the pro-cess more expensive, (ii) only small surfaces can be coated, and (iii)the low adhesion of the coatings that could diminish the opera-tion life of photoanodes, although it is considerably superior to theconventional sol-gel coatings.

Chemical vapor deposition (CVD) is an extensively used coatingtechnology for a wide range of applications [98]. A precursor gasis flowed into a chamber containing one or more heated objectsto be coated, occurring the chemical reactions to form the coat-ing on/near the hot surface to be coated. The chemical by-productsrequired are exhausted out of the chamber along with the unre-acted precursor gases usually at sub-torr total pressures and attemperatures typically ranging from 200 to 1600 ◦C [98]. CVD yieldsbetter adherence of the TiO2 coating onto the conductive sub-strate than sol-gel, and it allows controlling the film thickness fromnanometers up to micrometers as well as the crystallinity degree ofthe coating, although it is much more expensive. The main limita-tions of CVD are related to the higher cost of the precursors and thekinds of substrates with smaller surface areas that can be coated.The different thermal expansion coefficients between the substrateand the coating can stress the deposited films causing mechani-cal failure or coating breaking. The first application of CVD coatedphotoanodes in PEC remediation of wastewaters was reported byHitchman and Tian [99], who prepared TiO2 thin films by low pres-sure between 257 and 400 ◦C using titanium tetra-tertbutoxide aschemical precursor. The coatings obtained presented great photo-catalytic activity and stability, allowing their reuse.

Atomic layer deposition (ALD) is other chemical thin-film tech-
nology in which the substrate surface is exposed to gaseousprecursors [100,101]. In ALD, the precursors are injected sequen-tially in non-overlapping pulses after clearing the reactor ofprevious ones. The coating reaction ends when all the reactive

nd Photobiology C: Photochemistry Reviews 31 (2017) 1–35 9

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ites on the surface are consumed and thus, the maximum mate-ial depositable in a single exposure cycle is determined by theature of the surface-precursor interaction. This technology allowsrowing the coatings uniformly in small surfaces with high preci-ion by increasing the number of ALD cycles. Only the synthesis ofiO2 thin films by ALD and its application to wastewater treatmentas been reported by Heikkila et al. [100], who used Ti(OMe)4 andater as precursors injected into the reactor at 10 mbar, 325 ◦C andith pulses of 0.5 s. The coating obtained were highly stable andresented good adherence to the substrate.

Finally, thermal spray coating technologies allow the synthe-is of high quality and reliable TiO2 coatings with great control ofhysical properties like thickness, porosity, roughness and hard-ess [102]. Atmospheric plasma spray (APS) is the most commonnd versatile thermal spray process, where the coating is producedsing a nano or micropowder precursor that is partially or totallyelted during the deposition process by the plasma temperature

etween 6000 and 15,000 ◦C. Typically, Ar is used to generate thelasma superheated in a DC arc and the powder feedstock intro-uced in it is accelerated at high temperature toward the substratey the plasma jet. Thus, coating of large surfaces at low operationimes is feasible by APS. Garcia-Segura et al. [31] reported for therst time the use of TiO2 coatings onto stainless steel obtained byPS (see Fig. 3a) to treat polluted wastewaters by SPEC. The hightability and reusability of the coating during consecutive treat-ents was well-proven, showing good efficiencies for Acid Orange

decolorization.

.2.2. Nanostructured materialsSince 2007 TiO2 NTs have been synthesized for PEC purposes.

he main advantage of these tridimensional and highly orderedanostructures compared to thin-films electrodes is due to theirigher effective area that allows higher light absorption.

The TiO2 NTs are usually prepared from the anodization ofi plates or foils. Before the anodization process, the titaniumheets were: (i) ultrasonically degreased with water/acetone,ethanol/acetone or ethanol/acetone mixtures for above 30 min

ollowed by (ii) mechanical polished with abrasive papers [103]r chemical polished with HF/HNO3/H2O mixtures [104,105]. Theubstrates were subsequently rinsed with water or subjected tonother ultrasonic degrease. Once the anodic surface was pre-reated, it was anodized in a two-electrode cell with an inertathode of Ni or Pt under vigorous magnetic stirring, employinglycerol/water mixtures with H3PO4, HF or NH4F as electrolyte89,104–107]. Different anodization conditions have been reportedn the literature involving Ecell values from 10 to 60 V for elec-rolysis times from 30 min to 17 h in order to obtain TiO2 NTs orne-dimensionally TiO2 NTAs of various diameters and lengths. Thelectrochemically synthesized deposits were finally subjected tonnealing between 400 and 600 ◦C for 1–2 h to induce TiO2 crys-allization and improve the PEC performance [105,108,109]. Fig. 3bxemplifies the TiO2 NTAs obtained by this procedure by anodizingt +30 V for 50 h [89].

A similar electrochemical procedure is utilized for the synthesisf TiO2 NBs [70]. In contrast, FTO/TiO2 nanorod arrays are preparedy a hydrothermal method involving the deposition of the photo-atalyst onto the cleaned FTO-coated glass from a TiCl4 and HClolution heated in a muffle at 180 ◦C for 2 h [72].

Several authors [110] proposed an alternative sonoelectro-hemical method for the preparation of TiO2 NTs. The procedures a modification of the electrochemical procedure described above
ased on the use of ultrasonic waves instead of a magnetic stirrer.he irradiation with ultrasonic waves during anodization enhanceshe mobility of ions inside the solution for leading to more efficientroduction of the nanotubes.
of the TiO2 thin-film photoanode prepared by atmospheric plasma spray and com-posed of 29% rutile, 9% anatase and 62% Ti7O13 non-stoichiometric phase.

Source: Reproduced with permission from ref. [31].

It has also been reported the formation of coral-like struc-tures by changing the electrolyte of the cell during Ti anodization.Liu et al. [111] described the oxidation of Ti plates in an organicelectrolyte with 1-butyl-3-methylimidazolium tetrafluoroborate([BMIM][BF4]) at +60 V for 20 min leading to a hierarchical TiO2structure containing nanoholes resembling a coral.

3.2.3. Characterization of synthesized photocatalystsThe physical properties of all the synthesized photocatalysts

are characterized by means of many techniques. The morphologyof deposits can be analyzed by SEM, field-emission SEM (FESEM)and transmission electron microscopy (TEM). From the imagesobtained by these methods, the distribution and size of TiO2 NTscan be determined, as described above for the SEM micrograph ofFig. 3b. For thin-film photocatalysts, the porosity, roughness, adhe-sion and hardness of deposits can be measured [31]. The chemicalcomposition of coatings can be assessed by energy dispersive spec-trometry (EDS) and X-ray photoelectron spectroscopy (XPS) andtheir crystalline structure ascertained by X-ray diffraction (XRD).As an example, Fig. 6 presents the XRD pattern of a TiO2 thin filmcoating a stainless steel sheet and prepared by APS [31]. Fromthe crystallographic planes detected, a composition of 29% rutile,9% anatase and 62% Ti7O13 (non-stoichiometric phase) was deter-mined. Furthermore, the crystallites size can be estimated by thewell known Scherrer’s equation:

� = K�

� cos �(13)

where � refers to the mean size of ordered crystalline domains,K is the Scherrer constant, a dimensionless shape factor usuallywith values close to unity, � is the X-ray wavelength, � is the linebroadening at the half value of the maximum intensity in radiansand � is the Bragg angle in degrees.

The optical properties of coatings are usually evaluated bydiffuse reflectance spectroscopy (DRS) using, for instance, anUV/Vis/NIR spectrometer. Fig. 7a exemplifies the DRS spectraobtained for bare TiO2 NTs and TiO2-NTs doped with differentproportions of B [74]. All doped materials presented a secondabsorption shoulder in the visible region, at � > 400 nm, due tothe introduction of B atoms into the TiO2 lattice. The inset ofFig. 7a shows the Tauc’s graph constructed from the DRS data ofthe undoped and a B-doped material, which was utilized to esti-
mate the two Ebg values of 3.2 and 2.2 eV, obtained as the interceptwith the X axis corresponding to the energy h� irradiated, accord-ing to ref. [74]. These data allowed the proposal of the previouslydiscussed PEC mechanism of B-TiO2 NTs shown in Fig. 4a.

10 S. Garcia-Segura, E. Brillas / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 31 (2017) 1–35

Fig. 7. (a) DRS spectra of: (1) bare TiO2 NTs and doped B-TiO2 NTs with (2) 70, (3)560, (4) 140 and (5) 280 ppm of NaBF4. The inset shows the Tauc’s plot and Ebg valuefor (1) B280ppm–TiO2 NTs and (2) undoped TiO2 NTs. (b) Photocurrent density vs.potential curves for (1) dark current, (2) TiO2 NTs before annealing, (3) bare TiO2

NEa

mEBctoUfpr4aacuoc

vtpc5iPTp

Fig. 8. (a) Nyquist plots for TiO2 NTs and Pd/TiO2 NTs photoanodes in the dark and

Ts and B-doped TiO2 NTs with (4) 70, (5) 140, (6) 560 and (7) 280 ppm of NaBF4.lectrolyte 0.1 M Na2SO4, scan potential rate 10 mV s−1 and UV/Vis irradiation from

125 W mercury lamp. Adapted with permission from ref. [74].

The electronic properties of photocatalysts can be assessed byeasuring the spontaneous photocurrent as function of the applied

anod in an electrolytic cell. An example is depicted in Fig. 7b for the-doped materials reported in Fig. 7a [74]. The experiments wereonducted using a 0.1 M Na2SO4 solution of pH 6 and by scanninghe potential at a rate of 10 mV s−1. Scarce photocurrent can bebserved in the dark (curve 1) and before annealing (curve 2) underV/Vis illumination, whereas the photoactivity upgraded strongly

or the bare TiO2 NTs (curve 3). The B-doped samples were morehotoactive than the bare one and showed an increasing photocur-ent at higher amount of dopant agent from 70 to 280 ppm (curves, 5 and 7) due to the enhancing photoactivity of the B, whereast 560 ppm (curve 6) the photocurrent decayed since the dopantlso trapped the electrons photogenerated in the material. In allases, the photocurrent rose with increasing Eanod because a grad-al value higher than Efb was applied and this caused a bendingf the CB and VB bands leading to greater SCL (see Fig. 4a) andonsequently, photocurrent.

An interesting electrochemical method to determine the Efbalue of the photocatalyst and its electron carrier density (ND) ishe electrochemical impedance spectroscopy (EIS). The interfacialroperties between the solution and the photocatalyst are typi-ally studied from frequencies of 105 to 0.1 Hz using amplitudes of–10 mV at open circuit. Fig. 8a shows the Nyquist plots (imag-

nary impedance vs real impedance) obtained for TiO2 NTs andd/TiO2 NTs composite in the dark and under 300 W Xe light [76].he semicircles observed are characteristics of a charge transferrocess and its diameter is equal to the charge transfer resistance.

under 300 W Xe illumination. (b) Mott−Schottky plot at a fixed frequency of 5 kHzon TiO2 NTs and Pd/TiO2 NTs under illumination.

Source: Reproduced with permission from ref. [76].

The much smaller arch of the Pd/TiO2 NTs than that of the TiO2NTs in the dark and also under illumination indicates a reductionof the recombination of electron/holes pairs by Pd upgrading theelectron mobility. From the corresponding equivalent circuit, thecharge capacitance (C) of the photocatalyst can be determined andsubsequently, related to Efb and ND by means of the following equa-tion:

1C2

= 2ND e ε0 ε

[(Eanod − Efb) − kT

e] (14)

where e is the elemental charge, �0 is the permittivity of thevacuum; � is the relative permittivity of the semiconductor, k isthe Boltzmann constant and T is the absolute temperature. Thecorresponding Mott-Schottky plots for the TiO2 NTs and Pd/TiO2NTs under illumination are presented in Fig. 8b, where C valueswere acquired beginning at negative Eanod. Thus, an Efb = −0.294 Vwas found for the former material, a value more negative than−0.137 V determined for the latter one that facilitates the electrontransfer at any positive Eanod. This was also reflected in the lowerND = 2.05×1021 cm−3 for TiO2 NTs compared with 8.55 × 1021 cm−3

for Pd/TiO2 NTs that is obtained, indicating a faster carrier (electron)transfer for the latter, which presupposes a best PEC performance.

The structural characteristics of the photoanodes can affect thephotoelectrocatalytic efficiency. Parameters such coating thicknessor roughness should not be miss considered. However, the studyof the influence of these structural parameters has been barely

nd Ph

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pospdt

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d


ttended in the literature to the difficulties to modify and con-rol these parameters during the anode synthesis. For instance,he effect of the film thickness on the PEC performance could bescribed to two main effects: (i) the light penetration into thelm to photogenerate electron/hole pairs by Reaction (1) and (ii)he coating conductivity. Obviously both effects are related to thehotocatalyst material. According to Hitchman and Tian [99], theaximum penetration depth of incident irradiation into TiO2 coat-

ngs is defined by 1/�, where � is the absorption coefficient of TiO2t the incident light wavelength. It should be noted that the holesenerated in the depletion layer width should diffuse to the pho-oanode surface in order to react with water to release •OH byeaction (2) or directly mineralize organics. These holes generated

n the minority carrier length, which is the distance that the holesove into a field-free region before recombination from Reaction

7), may diffuse to the depletion layer boundary and transported tohe photoanode surface. On the other hand, the material resistivityises when thickness increases, diminishing the efficient removal ofhotopromoted electrons by the external electric circuit [35,112].hereby, the increase of coating thickness enhances the photogen-ration up to a limit, where a further increase diminishes the PECerformance.

Since photocatalytical processes are directly related to surfacerocesses, the surface roughness of thin films plays a key rolen the PEC efficiency [112]. Photoanode materials with higheruperficial roughness quadratic average will then result in highererformance. Consequently, the photocatalyst roughness should beetermined in order to be able to compare in equality conditionshe different catalytic materials response.

.3. PEC systems

Lab-scale photoelectrochemical devices have been utilized upo present to check the PEC treatment of organic wastewater. Theystems are basically composed of an electrolytic cell with the pho-ocatalyst, a potentiostat or power source to provide the electricalnergy and a light source to illuminate the photoanode. Never-heless, a large variety of PEC systems have been described, withifferent cells and kinds and positions (outside or inside the cell)f the light source, which can be UVC (� < 300 nm), UVA (� from20 to about 400 nm) or UV/Vis lamps, as well sunlight that cane simulated with a Xe lamp. In most works, any criterion is givenbout the selection of the PEC system applied and as much, the usef a visible radiation is argued to justify larger absorption in thisegion from photoanodes different from classical TiO2. This varietyf conditions makes impossible a realistic comparison of the PECerformance reported by different authors over a given organic pol-

utant, thereby being needed a vast development of optimized PECystems for their possible scale-up to be applicable to industrialevel.

The electrolytic cells can be differentiated by their form andhe number of compartments and electrodes. The most ubiquitousell is a tank reactor, although flow cells are also utilized. One canhen operate with: (i) a one-compartment or undivided cell andii) a two-compartment or divided one, with a separator betweenhe anolyte (anodic solution) and catholyte (cathodic solution). Theormer arrangement is usually preferred for avoiding the potentialenalty of the separator of the latter one. In both cases, it is feasiblehe use of two or three electrodes. The systems with two electrodesontain the photoanode and cathode (inert for the degradation pro-ess or not), providing a constant Ecell or janod by a power source. Inontrast, the three-electrode cells are equipped with an additional
eference electrode that makes possible the supply of a constantanod to the photoanode by a potentiostat.
Examples of undivided three-electrode PEC systems areepicted in Fig. 9. The arrangement of Fig. 9a was composed of

otobiology C: Photochemistry Reviews 31 (2017) 1–35 11

a quartz glass tank reactor, an UVC light source and a potentiostat[113]. Note that this kind of high energetic light can also photolyzedirectly the organic pollutants. The tank reactor contained a Ti/TiO2photoanode and an inert Cu cathode, both placed in parallel, and thereference electrode to control the potential of the photoanode wasa saturated calomel electrode (SCE). The lamp was positioned verti-cally in a double-walled U-tube outside the reactor, surrounded bycirculating water to decrease the heating effect of the lamp. The UVCirradiation supplied by the lamp was perpendicular to the photo-catalyst surface and crossed the quartz glass wall without intensityloss. A porous titanium plate was used to provide dissolved oxygenfor enhancing the degradation process by O2

− production via Reac-tion (3) and pre-eminently, the mass transport of organics towardthe photocatalyst by the stirring of the solution. On the other hand,Fig. 9b shows a similar three-electrode configuration, but with aninside thermostated Xe light source [114]. The photocatalyst was aTi/B-TiO2 NTs, the cathode was an inert Ni sheet and the referenceelectrode was a SCE.

A recent paper of Martin de Vidales et al. [115] reported a flowPEC system with a three-electrode cell to degrade methyl orangeazo dye from wastewater. Fig. 10a depicts an expanded view of theundivided electrochemical cell utilized, in which the photoanodewas a Ti plate coated with TiO2 NTs and the inert cathode was aTi mesh coated with RuOx attached over a glass end for the pas-sage of the UV light [115]. The reference electrode was an Ag/AgCl(3 M KCl), connected to the electrolyte exit of the cell, near thephotoanode. The arrangement used is illustrated in Fig. 10b. Theexperiments were made with 1 L of solution, which was introducedin the reservoir, thermostated at 15 ◦C with water bath and recir-culated through the circuit with a centrifugal pump (batch mode).After passing through the cell, its flow rate was regulated between20 and 100 L h−1 with a flowmeter before come back to the reser-voir.

Fig. 11a and b presents two examples of undivided two-electrode PEC systems for wastewater remediation [31,36]. Daghriret al. [37] studied the oxidation of chlortetracycline hydrochlo-ride in aqueous solution using the arrangement of Fig. 11a witha Ti/TiO2 thin film as the photoanode. The tank reactor was madeof acrylic material and a quartz window was disposed on one face ofit to illuminate the photocatalyst with UVC light of �max = 254 nm.Apart from the Ti/TiO2, one cathode of the same surface areawas placed in parallel. Different cathode materials such as stain-less steel, vitreous carbon, graphite and amorphous carbon wereutilized. The solution was stirred with a magnetic bar. Fig. 11billustrates a sketch of the set-up used for Garcia-Segura et al.[31] to degrade Acid Orange 7 azo dye solutions submitted tosolar irradiation by SPEC. A 5 cm2 TiO2 photoanode and a 3 cm2

carbon-polytetrafluoroethylene (PTFE) air-diffusion cathode wereused as electrodes. The TiO2 thin-film material was synthesizedover stainless steel by APS technology and the cathode producedcontinuously H2O2 from the two-electron reduction of oxygen frominjected air by Reaction (8), upon vigorous magnetic stirring. Thiscathode acted in the degradation process since the generated H2O2can directly attack organic pollutants or its oxidation productsand/or produce more potent ROS like •OH by Reaction (9).

A limited number of papers have described the use of dividedcells for PEC treatment. Fig. 12a exemplifies the three-electrodearrangement utilized by Selcuk et al. [116] to study the removalof humic acid. The anolyte of the tank reactor of 100 mL capac-ity was equipped with a TiO2 photoanode and a SCE, whereasthe catholyte with the same solution volume contained a Pt foilcathode. Both compartments were separated by a Nafion 117 mem-
brane and the photocatalyst was illuminated with an outside Xelamp through a quartz glass window. An interesting study of Dinget al. [117] compared the performance of divided and undividedtwo-electrode semi-circular quartz glass cylinder cells to degrade


Fig. 9. Typical set-up of undivided three-electrode cells for wastewater treatment by PEC. (a) TiO2 thin-film photocatalyst for the degradation of a 20 mg L−1 Acid Blue 7 azod opeds

2woodtawctc(

H

sos

ye solution with external light (Reproduced with permission from Ref. [113]). (b) Dolution with inner light (Reproduced with permission from Ref. [114]).

00 mL of rhodamine B dye, as shown Fig. 12b. The photoanodeas formed by Bi2WO6 nanoplates deposited on FTO and the cath-

de was composed of Fe@Fe2O3 core-shell nanoparticles supportedn activated carbon fiber (ACF). The anolyte and catholyte of theivided cell were connected with a saturated KCl salt bridge andhe photocatalyst was illuminated with a visible light by means ofn outside 300 W tungsten halogen lamp with � > 420 nm. Fresh airas injected near the cathode to generate H2O2 by Reaction (8) for

oupling PEC process with electro-Fenton (EF) one. This occurredhanks to the iron ions released by the cathode during current cir-ulation, which originated •OH in the bulk from Fenton’s Reaction15) [5,11,118]:

2O2 + Fe2+ → Fe3+ + •OH + OH− (15)

The coupling of PEC with EF mineralized more largely the dyeolution in the catholyte of the divided cell than in the undividedne as a result of a greater accumulation of H2O2 in the formerystem because it was partly anodically oxidized in the latter one.

B-TiO2 NTs photocatalyst for the degradation of a 20 mg L−1 methyl orange azo dye

3.4. Operation parameters

The degradation of organics from wastewaters during PECprocess is monitored from the variation of experimental parame-ters such as the absorbance at the maximum wavelength (�max)of the solution, usually for dyes, determined by UV/Vis spec-trometry, the concentration of the parent molecule measured byhigh-performance liquid chromatography (HPLC) and total organiccarbon (TOC) or chemical oxygen demand (COD) of the solution.The two former parameters assess the destruction of the startingpollutant/s, the third one informs about its mineralization (conver-sion into CO2) and the latter one evaluates its transformation intosmall by-products like short-linear carboxylic acids and oxidizableinorganic species. The change of these parameters can be expressedin terms of:

Decolorization = A(16)

A0

Percentage of color removal = A0 − A

A0× 100 (17)


F otocat1 L h−1.

R

P

P

P

ig. 10. (a) Expanded view of the undivided electrochemical cell with TiO2 NTs ph L of a 0.25 mM methyl orange azo dye solution at pH 3, 15 ◦C and flow rate of 100

eproduced with permission from ref. [115].

ercentage of organic removal = c0 − c

c0× 100 (18)

TOC0 − TOC
ercentage of TOC removal =
TOC0× 100 (19)

ercentage of COD removal = COD0 − CODCOD0

× 100 (20)

alyst and (b) scheme of the set-up of the flow PEC system used for the removal of

where A0. c0, TOC0 and COD0 denote the absorbance, concentra-tion, total organic carbon and chemical oxygen demand at initialtime t, and A, c, TOC and COD are the corresponding values at elec-trolysis time t. It is necessary to remark the typical confusion in
many papers over the indiscriminate use of Eq. (18) instead of Eq.(17) for dyes solutions. It is well-known that dyes oxidation origi-nates colored by-products that can absorb at the same �max as the


Fig. 11. Schematic diagram of coupled systems of PEC with generated H2O2 at the cathode using TiO2 thin-film photocatalysts. Treatment of: (a) 1 L of chlortetracyclineh f a 15w

psobli

toaAEnwtmidpt

ydrochloride solutions (Reproduced with permission from Ref. [36]). (b) 100 mL oith permission from Ref. [31]).

arent molecule and hence, the percentage of color removal of theolution determined from absorbance decay from Eq. (17) increasesver time much more slowly than the decay of dye content foundy HPLC from Eq. (18) [119]. Decolorization results reported in the

iterature then need to be carefully analyzed for avoiding such badnterpretation.

Several operation variables can modify the above experimen-al parameters and affect the effectiveness of the PEC treatment ofrganic pollutants in wastewaters. The most important variablesre the external bias potential and light intensity [13–15,18,20–24].s stated above, the application of an increasing Eanod respect tofb accelerates the pass of photogenerated electrons to the exter-al circuit causing a bending of the CB and VB of the photocatalystith the consequent formation of a SCL. Nevertheless, it is found

hat the degradation process of organics is enhanced up to a maxi-um Eanod when the majority of photoexcited electrons are drawn

nside the semiconductor and extracted at the back contact, con-itions under which the reaction kinetics is only limited by thehoton flux. At superior Eanod values, the width of SCL can exceedhe thickness of the photocatalyst and the charge is redistributed

mg L−1 Acid Orange 7 azo dye solution under direct solar irradiation (Reproduced

in the SCL with the consequent reduction of PEC activity and grow-ing of the energy consumption [86]. The light intensity has then toinfluence largely the degradation by PEC since it regulates the quan-tity of photogenerated electrons that can be extracted at maximumEanod. Nevertheless, most authors do not consider this variable anduse a high UV or visible intensity to ensure the highest effectivenessof PEC.

Other important variables are the solution pH, its conductivityrelated to the electrolyte concentration and the organic concen-tration that determines the oxidation ability of ROS and dissolvedO2 [18,21,22]. The effect of pH in PC is explained from the pointof zero charge (PZC) corresponding to the pH where the surfacecharge of the photocatalyst is zero, a value approximately 6 forTiO2, and the adsorption of organics below and above it [18]. Incontrast, the PZC is not valid in PEC because the surface chargebecomes strongly positive upon a given Eanod and the degradation
process is limited by the generated oxidizing ROS due to the lowrecombination of electron/hole pairs. The best pH then has to beexperimentally determined in each case since it depends on themolecule nature and its interaction with the charged photocata-


Fig. 12. (a) Set-up of a divided PEC system with a TiO2 photocatalyst for treating 100 mL in the photoanode (anolyte) compartment of a 25 mg L−1 humic acid solution( eft) ana e solutp

lsjRonatrttd[

r

Reproduced with permission from Ref. [116]). (b) Schematic diagrams of divided (l Fe@Fe2O3/ACF cathode for H2O2 generation to degrade 10.44 M rhodamine B dyermission from Ref. [117]).

yst. On the other hand, Na2SO4 is usually added to the treatedolution for upgrading its conductivity in order to obtain a higheranod at a given Eanod, improving the PEC effectiveness by a largerOS production. A sufficient amount of dissolved O2 also promotesxidizing conditions in PEC, because it helps to avoid the recombi-ation of photogenerated electron/hole pairs by acting as electroncceptor from Reaction (3) to form more quantity of ROS as illus-rated in Figs. 2 and 5. The effect of the temperature and the stirringate or flow rate, which determines the reaction rate and the massransfer of reactants/products toward/from the electrodes, respec-ively, has been reported by a limited number of authors in PEC,espite their basic role in the EAOPs treatments of wastewaters
11,118].
It should be noteworthy that the elucidation of the oxidationeaction of organics, the changes in toxicity and biodegradability

d undivided (right) PEC semi-cylindrical cells with a Bi2WO6/FTO photoanode andions. In both systems, PEC was coupled with electro-Fenton (EF) (Reproduced with

and energy costs have been scarcely determined for the PEC treat-ment of organic pollutants [82,86,107,120,121]. More research overthese points should be made in order to corroborate the viability ofPEC for future scale-up at industrial level.

4. Destruction of organic pollutants by PEC

The application of PEC to wastewater remediation has beenpreferentially centered on the treatment of dyes, chemicals andpharmaceuticals spiked in synthetic aqueous solutions, usuallycontaining Na2SO4 as electrolyte. Unfortunately, less attention has
been pay on the remediation of real wastewaters due to the com-plexity of their nature and composition. This section is devotedto show and discuss the main results and advances found for thetreatments of all these kinds of organic pollutants, which will be


sa

4

mtbaAo[ctunis

4

ocpc6e(u<Ettor

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idNpPIsPp•

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 30 60 90 120 150 180 210

noitaziroloceD

Time (min)

Fig. 13. Change of decolorization with electrolysis time for 36 mL of 50 mg L−1 ofmethyl orange azo dye in 0.01 M Na2SO4 using an undivided cell with Ti/TiO2 NTAsphotocatalyst at Eanod = +0.75 V (Ag/AgCl) upon 15 W UVC light. ( ) Direct photolysis

6 S. Garcia-Segura, E. Brillas / Journal of Photochemistry a

eparately analyzed for a better remark of their degradation char-cteristics.

.1. Dyes

As highlighted in Fig. 1c, synthetic dyes solutions represent theajor number of wastewaters treated by PEC. Their fast decoloriza-

ion upon the action of generated ROS can be easily monitoredy UV/Vis spectrometry, even for very low dye concentrations,nd this allows checking the performance of this technology.part from the large variety of dyes tested, synthesized photoan-des such as TiO2 thin films [31,90,93,95,113,122–136], TiO2 NTs69,106,108,115,137–143], doped TiO2 [72,74,114,129,144–146],omposites with TiO2 [38,76,79,80,84,85,147–158] and other pho-ocatalytic materials [52,53,59,60,65,66,117,159–166] have beentilized in these studies. The main reason for preparing such highumber of photocatalysts is the search of materials with larger abil-

ty to photogenerate electron/hole pairs than TiO2 thin films uponunlight irradiation.

.1.1. TiO2 photoanodesTable 1 collects the percentages of color and TOC removals

btained for different dye solutions treated by PEC under selectedonditions using undivided cells with either TiO2 thin-film or NTshotocatalyst (see Figs. 9–11). A look of these data allows con-luding that artificial UVC (�max = 254 nm), UV (from 180 to ca.00 nm) and sometimes Xe lamps with high intensity were pref-rentially used to irradiate the photocatalyst. Exposure to UVA�max = 360 nm) lamps [90] and direct sunlight [31,95] has beentilized, as well. Dye concentrations were typically low (normally

25 mg L−1) due to the small oxidation ability of PEC, because lowanod values, associated with small photocurrents, were utilizedo preserve the fragile photoactivity of the TiO2 photoanode. Forhis reason, operation in batch mode with long electrolysis timesf 2–3 h were commonly applied to achieve more than 70% of coloremoval (see Table 1).

Comparative tests for given dyes using both, TiO2 thin-filmnd TiO2 NTs photocatalyst under the same experimental PEConditions have been scarcely reported. The superior mechani-al integrative structure and photoactivity of the nanoestructuredaterial is assumed by most authors without verification. The

anoni’s group [138,140] described that after 240 min of PEF treat-ent at Eanod = +1.0 V (Ag/AgCl) under UV irradiation, 0.05 mMisperse Red 1, Disperse Orange 1 and Disperse Red 13 in chloride

olutions undergone 87–97% mineralization using a TiO2 NTs pho-oanode prepared by anodization, values much higher than 54–60%btained for a TiO2 thin-film one prepared by sol-gel. These authorslso found a strong reduction of the toxicity of the treated dyeolutions, thereby showing that PEC is a very interesting methodor wastewater remediation. This is due to the conversion of theoxic aromatic dye and its aromatic intermediates into biodegrad-ble products like short-linear carboxylic acids. The formation ofartaric, succinic, acetic and oxamic acids and their removal overlectrolysis time has been reported for the SPEC process of Acidrange 7 [31].

Several papers have described the best performance of PEC overndividual processes for dye decolorization. Fig. 13 exemplifies theecolorization abatement for 50 mg L−1 methyl orange in 0.01 Ma2SO4 by different methods [106]. Individual processes like directhotolysis (DP) under 15 W UVC, EO at Eanod = +0.75 V (Ag/AgCl) andC under 15 W UVC led to a poor decolorization lower than 20%.n contrast, the PEC treatment yielded 97% color removal, demon-
trating that it was much more potent and synergistic to EO andC because the larger separation of the photoexcited electron/holeairs under the applied Eanod generated much greater amount ofOH from photogenerated holes via Reaction (2) that oxidized more
(DP) without photoanode, ( ) photocatalysis (PC) without applied current, ( )electrochemical oxidation (EO) without UVC illumination and ( ) PEC. Adaptedwith permission from Ref. [106].

rapidly the dye. The same synergistic behavior for PEC has beenshown for Acid Orange 7 [31], Remazol Brilliant Orange 3R [90],rhodamine B [93] and orange G [136].

The operation variables most studied in dye degradation arethe solution pH, the applied Eanod, the dye concentration, the elec-trolyte concentration and the kind of electrolyte used. Fig. 14illustrates the effect of the above operation variables over thedegradation of 350 mL of methyl orange solutions in sulfatemedium by PEC with a TiO2 thin film upon 350 W UV illumination[129]. Fig. 14a evidences that the best pH for the process was 2.0,with percentages of color removal slightly higher than that of pH 3.5and much more superior to those of pH 5.2. This behavior is diffi-cult to explain and differs from the optimum pH values reportedby other authors using Ti/TiO2 thin films. Fu et al. [113] alsodescribed an optimum acidic pH = 3.4 with 90% color removal for20 mg L−1 of Acid Blue 7 in sulfate medium using the cell of Fig. 9a atEanod = +0.68 V (SCE) upon 11 W UVC for 1 h (see Table 1), whereasGarcia-Segura et al. [31] found a pH = 7.0 as optimal for 100% decol-orization of 100 mL of 15 mg L−1 of Acid Orange 7 solutions in0.05 M Na2SO4 using the cell of Fig. 11b at janod = 1.0 mA cm−2 and35 ◦C after 120 min of electrolysis under direct sunlight. Conversely,Li et al. [93] reported an increasing decolorization from pH 2.0–10.0when 50 mL of a 0.015 mM rhodamine B solution in 0.10 M Na2SO4was treated at Eanod = +0.50 V (SCE) exposed to 30 W UVA, attain-ing the best color removal of 95% after 70 min of electrolysis atthe higher pH = 10.0. This discrepant behavior for pH can be mainlyrelated to: (i) the different systems, photocatalysts and operationconditions utilized that change the amounts of •OH produced fromphotogenerated holes and the interaction of organics with the pho-tocatalyst and (ii) the different colored products formed from eachdye that interfere the absorbance measurements for decoloriza-tion [31]. When 0.50 M NaCl was used as electrolyte, Zanoni et al.[90] reported a maximum TOC removal of 28% for 30 mL of 0.05 mMRemazol Brilliant Orange 3R at pH 6.0 as the anolyte of a divided celloperating at Eanod = +1.0 V (SCE) upon 450 W Xe–Hg arc illumina-tion for 30 min. The superior degradation at this pH explored withinthe range 2–11 could be explained by the synergistic action of thestrong oxidant •OH formed from Reaction (2) and active chlorinespecies like HClO and Cl• produced from Cl− oxidation by Reactions
(21)–(23), respectively [129].
2Cl− → Cl2 + 2 e− (21)

Cl2 + H2O → HClO + Cl− + H+ (22)


Table 1Per cent of color and TOC removals for selected PEC treatments of dyes using undivided cells with TiO2 photoanodes.

Dye co (mg L−1) Experimental conditions % color removal % TOCdecay

Ref.

Acid Blue 7 20 Solution of pH 3.4, Eanod = +0.68 V (SCE),11 W UVC for 1 ha

90 −c [113]

Methyl orange 1020

165 mL of solution, Eanodl = +0.6 V,125 W UV for 2 ha

100 −c

78[122]

Triazo leather dye 10 70 mL of solution with 0.1 M KCl, 35 ◦C,Eanod = +0.8 V (Ag/AgCl), 125 W UV for8 ha

98(pH 6.14)72(pH 3.35)

−c [127]

Methyl orange 20 350 mL of solution with 0.1 M Na2SO4,pH 2.0, Eanod = +0.75 V (Ag/AgCl), 350 WUV for 150 mina

97 −c [129]

Reactive Orange 16 100 70 mL of solution with 0.1 M KCl,pH 3.35, 30 ◦C, Ecell = 5 V, 125 W UVfor 5 ha

100 −c [134]Reactive Black 5 100 92 −c

Reactive Red RB 133 100 70 −c

Rhodamine B 5 20 mL of solution with 0.1 Mphosphate, pH 7, Eanod = +0.8 V(Ag/AgCl), 500 W Xe for 2 ha

53 −c [135]

Rhodamine B 0.01d 45 mL of solution with 0.1 M Na2SO4,Eanod = +0.6 V (SCE), 60 W visible lightfor 3 hb

72 −c [69]

Methyl orange 50 36 mL of solution with 0.01 M Na2SO4,Eanod = +0.75 V, 15 W UVC for 3 hb

98 −c [106]

Methyl orange 0.25d 1 L of solution with 0.1 M Na2SO4, pH 3,15 ◦C, Eanod = +1.50 V (Ag/AgCl), flowrate 100 L h−1, 20.5 A of UV for150 minb

51 −c [115]

Drimaren Red 243 X −6BN 25 2 L of solution with 4.2 mM KCl,Eanod = +1.50 V (Ag/AgCl), 30 W UVC for1 hb

93 −c [142]

a TiO2 thin film.

H

(badotooi[b

idawo[

0ormetsipl

b TiO2 NTs.c Not determined.d Initial concentration in mM.

ClO + hv → Cl• + •OH (23)

Fig. 14b highlights that increasing Eanod from +0.2 to +1.0 VAg/AgCl) accelerated the degradation process of methyl orangeecause more current was gradually generated enhancing the sep-ration of electrons and holes. However, the degradation rateecreased for Eanod = +2.0 V (Ag/AgCl) suggesting a redistributionf charges in SCL with greater electron/hole recombination despitehe circulation of a higher current through the electrodes. Lowptimum Eanod values have also been shown for the degradationf several dyes using TiO2 thin-films or TiO2 NTs photoanodesn different systems, varying between +0.6 and +1.2 V (SCE)90,93,106,113,122,127,129], although up to +1.50 V (Ag/AgCl) haseen reported [115].

The expected decay of the percentage of color removal withncreasing methyl orange concentration from 10 to 30 mg L−1 isepicted in Fig. 14c. This is a typical behavior of EAOPs and can beccounted for by the destruction of lower percentage of organicsith increasing their concentration since a similar amount of •OH

xidants is produced from photogenerated holes via Reaction (2)5].

Fig. 14d shows that increasing Na2SO4 concentration up to.10 M as bakcground electrolyte caused greater degradation ratef the dye as a result of the rise of the solution conductivity. givingise to a higher current that produced more holes and consequently,ore quantity of •OH to oxidize methyl orange. Finally, Fig. 14e

vidences that the decolorization rate for different 0.10 M elec-rolytes slightly increased in the order: NaNO3 < KCl < Na2SO4. This
equence could be tentatively ascribed to the growing conductiv-ty of the solutions at pH 2.0, with little influence of the additionalroduction of: (i) •OH from NO3
− photolysis by Reaction (24), (ii)esser oxidant active chlorine species from Cl− oxidation by Reac-

tions (21)–(23), and (iii) H2O2 from SO42− oxidation via Reactions

(25)–(26), which could generate extra •OH by Reaction (9).

NO3− + H2O + h� → NO2

• + •OH + OH− (24)

2SO42− + h� → S2O8

2− + 2e− (25)

S2O82− + 2H2O → 2HSO4

− + H2O2 (26)

The action of these additional oxidants differed for other dyes.For instance, Zanoni et al. [90] described a large rise in TOC removalin the order: Na2SO4 < NaNO3 < KCl when 0.05 mM Remazol Bril-liant Orange 3R solutions in 0.50 M of such electrolytes at pH 6.0as the anolyte of a divided cell were treated at Eanod = +1.0 V (SCE).In this case, the dye molecule was more effectively attacked bygenerated active chlorine species than by •OH.

The positive effect of temperature from 20 to 35 ◦C and oflight intensity from 44 to 132 mW m−2 on the decolorization of25 mg L−1 Acid Black 1 dye using an undivided cell are shown inFig. 15a and b, respectively [108]. Macedo et al. [127] described asimilar behavior respect to temperature for the color abatement ofa 10 mg L−1 triazo leather dye solution in 0.10 M KCl of pH 3.35 and6.14 using a TiO2 thin film at Eanod of +0.8 and +1.2 V (SCE) under125 W UV operating between 21 and 35 ◦C. This trend is expectedby the concomitant enhancement of the mass transport of reac-tants toward/from the photoanode and the increase in rate of allchemical and electrochemical reactions involved in the decoloriza-tion process. For much higher intensities between 5 and 20 W m−2

of UVC illumination, Fu et al. [113] found that 16 W m−2 was opti-mal for the decolorization of a 20 mg L−1 Acid Blue 7 solution usingthe cell of Fig. 9a at Eanod = +0.68 V (SCE), indicating a saturation of
photoexcited electrons in the VB of a TiO2 photocatalyst at highenough light intensity.
Martin de Vidales et al. [115] used the flow cell of Fig. 10 todegrade 1 L of 0.25 mM methyl orange in 1 M Na2SO4 at pH 3, 15 ◦C


0

20

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100

0 30 60 90 12 0 150 180

pH 2.0pH 3.5pH 5.2

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0.2 V0.6 V1.0 V1.5 V

0

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10 mg L-120 mg L-130 mg L-1

10 mg L-1

20 mg L-1

30 mg L-1

0

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0 M0.05 M0.10 M

c

ba

d

Time (min)

%

lavomer

roloC

0

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40

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0 30 60 90 12 0 150 180

NaNO3KClNa2SO4

NaNO3

Na2SO

4

KCl

e

Fig. 14. Percentage of color removal vs. electrolysis time for the degradation of 350 mL of methyl orange solutions in sulfate medium by PEC using an undivided cell withT 1 met2 t, 0.10a d Ean

ao1calowt

a[tpbt0UHoe(at

i/TiO2 thin-film photocatalyst upon 350 W UV illumination. (a) Effect of pH, 20 mg L−

0 mg L−1 methyl orange, 0.10 M Na2SO4, pH 2.0, (c) effect of methyl orange contennd (e) effect of different 0.10 M electrolytes for 20 mg L−1 methyl orange, pH 2.0 an

nd Eanod = +1.50 V (Ag/AgCl) by varying the flow rate. After 150 minf electrolysis, the color removal grew from 20% at 20 L h−1 to 51% at00 L h−1. At the higher flow rate the process achieved the best effi-iency because it was controlled by the mass-transport of reactantsnd the oxidation products were rapidly removed to the solutioneaving the surface photoactive sites available for further organicxidation. This condition can also be easily attained in tank reactorsith vigorous stirring (mechanic, magnetic or by bubbling O2) of

he solution, as shown in the systems of Figs. 9–12.The coupling of PEC with cathodically generated H2O2 has

lso been explored aiming to enhance its degradation power31,123,126,136]. Xie and Li [126] proposed the use of an undividedank reactor similar to that of Fig. 9a equipped with a Ti/TiO2 meshhotocatalyst upon outside 8 W UVA and a reticulated vitreous car-on cathode fed with 120 mL min−1 of air. The anode mesh allowedhe illumination of 30 mL of a 30 mg L−1 Orange G dye solution in.01 M Na2SO4 with 0.15 mM Fe2+ of pH 3.0 and room temperature.nder the application of a cathodic potential (Ecat) of −0.71 V (SCE),2O2 was continuously produced at the cathode by Reaction (8) andxidizing •OH radicals were pre-eminently formed from photogen-rated holes by Reaction (2) and in the bulk from Fenton’s Reaction
15). Furthermore, the UVA illumination of the solution allowed thepplication of the photoelectro-Fenton (PEF) process involving: (i)he additional generation of •OH with Fe2+ regeneration by photol-
hyl orange, 0.10 M Na2SO4, Eanod = +1.0 V (Ag/AgCl), (b) influence of the applied Eanod, M Na2SO4, pH 2.0, Eanod = +1.0 V (Ag/AgCl). In (d) influence of sulfate concentration

od = +1.0 V (Ag/AgCl). Adapted with permission from ref. [129].

ysis of Fe(OH)2+ species (the most stable Fe(III) species at pH 3.0)from Reaction (27) and (ii) the photolysis of Fe(III) complexes withfinal carboxylic acids by the general Reaction (28) [5,11]:

Fe(OH)2+ + h� → Fe2+ + •OH (27)

Fe(OOCR)2+ + h� → Fe2+ + CO2 + R• (28)

The coupled PEC/PEF process led to total decolorization in300 min with 74% mineralization. This method was much morepowerful than the PEC treatment alone, without Fe2+ addition,which yielded only 50% color removal at the same time.

Another proposed coupled system involves the combination ofPEC and EF in a divided tank reactor similar to that of Fig. 12b [123].The anolyte contained 100 mL of 30 mg L−1 of Acid Scarlet 3R dyein 0.02 M Na2SO4 and a Ti/TiO2 thin-film electrode illuminated by a125 W UV light. The catholyte was separated of the anolyte by a saltbridge and contained 50 mL of the same solution adjusted to pH 3.0with 0.036 mM Fe2+ and a graphite electrode fed with an air flow of2.5 L min−1. Operating at Ecat = −0.66 V (SCE) for 70 min, 60% colorremoval was achieved in the anolyte through PEC process, whereasa much higher decolorization of 92% was found in the catholyte by
EF. This work evidences that a coupled PEC/EF process with a “dualcell” can upgrade the dye degradation compared with single PEC.
Recently, Bessegato et al. [143] showed the combination of PECand ozonation for improving the destruction of Acid Yellow 1 dye


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70

20 ºC25 ºC30 ºC35 ºC

noitaziroloceD

Time (min)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70

44 mW m--288132no itaz iroloce

D

Time (min)

44 mW m-2

132 mW m-288 mW m-2

a

b

Fig. 15. Decolorization vs. electrolysis time for the PEC treatment of 100 mL of a25 mg L−1 Acid Black 1 dye solution using an undivided cell with Ti/TiO2 NTs photo-catalyst irradiated by UVC light (E was not specified). (a) Effect of temperatureaw

spwoawc

4

soammttsamoadtpdtse

0

20

40

60

80

100

0 30 60 90 120 150

lavomer

roloC

%

Time (m in)

0

20

40

60

80

100

0 30 60 90 120 150

lavomer

CO

T%

Time (m in)

a

b

Fig. 16. Percentage of (a) color removal at � = 393 nm and (b) TOC removal for thedegradation of 500 mL of a 100 mg L−1 Acid Yellow 1 with 0.1 M Na2SO4 solution atpH 2.0 and 25 ◦C in an undivided cell. Method: ( ) EO with Ti/B-TiO2 NTs, ( ) DP,

anod

t light intensity of 44 mW m−2 and (b) influence of light intensity at 25 ◦C. Adaptedith permission from Ref. [105].

olutions up to 100 mg L−1 in 0.01 M Na2SO4 at pH 3.0. A TiO2 NTshotoanode in an annular bubble reactor operating at Ecell = 2.0 Vith 36 W UVB irradiation in the presence of 1.25 × 10−4 mol min−1

f O3 was employed. Total decolorization and mineralization werettained in 20 and 60 min for the coupled PEC/O3, respectively,hich presented much higher oxidation ability than individual pro-

esses.

.1.2. Doped TiO2 and composites with TiO2A high number of modified TiO2 photocatalysts has been synthe-

ized with the aim of upgrade the performance of the PEC processf dyes using visible light as radiation source. Most of publishedrticles have only detailed the comparative decolorization andineralization of these pollutants in sulfate medium between theodified TiO2 and the pristine material. Fig. 16a and b exemplify

he change of the percentage of color and TOC removals with elec-rolysis time, respectively, for different treatments of 100 mg L−1 ofolutions of a nitronaphthol dye like Acid Yellow 1 in 0.1 M Na2SO4t pH 2 and 25 ◦C [74]. The cell was a tank reactor with an arrange-ent similar to that of Fig. 9b equipped with either a TiO2 NTs

r a doped B-TiO2 photoanode under 125 W UV/Vis irradiation bypplying an Eanod = +1.2 V (Ag/AgCl) in EO and PEC [74]. EO in theark practically did not yield any removal of the dye, indicatinghat it was not oxidized at the anode. A large decolorization withoor TOC reduction can be observed under DP, suggesting that the
ye and some of its by-products were directly and slowly pho-olyzed by the incident light. The decay of both parameters wastrongly enhanced by PC pre-eminently by the larger oxidationffectiveness of •OH radicals formed from Reaction (2). As expected,
( ) PC with Ti/TiO2 NTs, ( ) PC with Ti/B-TiO2 NTs, ( ) PEC with Ti/TiO2 NTs and( ) PEC with Ti/B-TiO2 NTs. Irradiation with 125 W UV/Vis light and Eanod = +1.2 V(Ag/AgCl) in EO and PEC. Adapted with permission from ref. [74].

the much smaller recombination of the electron/hole pairs underEanod = +1.2 V (Ag/AgCl) in PEC yielded quicker destruction of organ-ics. Under these conditions, as well as in PC, the doped B-TiO2 NTsphotocatalyst gave better degradation than TiO2 NTs one. This isdue to the reduction of the Ebg value of the former material from3.2 to 2.2 eV due to the B-doping level (see Fig. 4a). The better resultsof 100% color removal and 93% TOC decay were obtained with anamount of 280 ppm of B as doping agent.

A similar enhancement of PEC with dopant B has been reportedby Su et al. [114], who used the arrangement of Fig. 9b to remark thebeneficial of B-TiO2 NTs photocatalysts under Xe illumination overmethyl orange decolorization. While improved PEC processes fordyes were also found for dopant Cu2+ [72] and codopants W andN [145], any effect was observed using either Fe3+ [144] or V, Ceand F [129] as doping agents of TiO2 thin films. On the other hand,positive PEC performance of different dyes has been described byusing composites of TiO2 with metals [76,147] and other materials[79,80,84,85,148–157].

The percentage of color and TOC removals for several dyesdegraded by PEC using different doped TiO2 and composites withTiO2 photoanodes under selected conditions are summarized inTable 2. The smaller Ebg values of these photocatalysts compared tothat of TiO2 allowed their best irradiation with visible light alterna-tively to UVC and UVA. The data of Table 2 show the use of potentXe lamps in many cases for photo-excitation, which simulate inthe laboratory the role of sunlight. Higher Eanod values than thoseused for TiO2 in Table 1 can be observed in Table 2 because ofthe larger stability of the modified materials. Good color removals,usually >80%, were obtained, although electrolysis times as long
as 2–3 h were needed operating in batch mode. Similarly, TOCremovals >70% were found under these conditions when dye con-tents <50 mg L−1 were degraded. Greater contents required muchlonger electrolysis times. For instance, a 105 mg L−1 rhodamine 6G


Table 2Percentage of color and TOC removals for selected degradations of dyes by PEC using undivided cells with doped TiO2 and composites with TiO2 photoanodes.

Dye co (mg L−1) Experimental conditions % color removal % TOC decay Ref.

Acid Yellow 1 50 500 mL of solution with 0.01 M Na2SO4, pH 2,25 ◦C, B-TiO2 NTs photocatalyst, Eanod = +1.2 V(Ag/AgCl), 125 W UV for 2 h

100 93 [74]

Methyl orange 20 500 mL of solution with 0.1 M Na2SO4, 25 ◦C,B-TiO2 NTs photocatalyst, Eanod = +2.0 V (SCE),300 W Xe for 2 h

92 −a [114]

Methyl orange 10 50 mL of solution with 0.2 M Na2S, CdS/TiO2

NTAs photocatalyst, Eanod = +0.5 V (SCE), 300 WXe for 3 h

89 −a [84]

Methyl orange 10 200 mL of solution with 0.1 M NaCl, pH 2.7,SiO2/TiO2 photocatalyst, Eanod = +0.8 V (SCE),9 W UVC for 30 min

97 −a [79]

Methylene blue 5 3 mL of solution with 0.5 M Na2SO4, pH 6, 25 ◦C,Cu-TiO2 NTAs photocatalyst, Eanod = +1.0 V(Ag/AgCl), 105 W visible light for 4 h

63 −a [72]

Methylene blue 4 10 mL of solution with 0.1 M KCl,graphene/TiO2/carbon cloth, Eanod = +0.9 V(SCE), 150 W Xe for 160 min

89 −a [154]

Methylene blue 4 Each solution with 0.05 M Na2SO4, Pd/TiO2 NTsphotocatalyst, Eanod = +0.5 V (Ag/AgCl), 300 WXe for 3 h

100 −a [76]

Rhodamine B 2.5 84 −a

Rhodamine B 10 Solution with 0.01 M Na2SO4,TiO2/carbon/Al2O3 membrane, Eanod = +1.2 V(SCE), 500 W Xe for 2 h

−a 82 [148]

Rhodamine 6G 105 250 mL of solution with 0.1 M Na2SO4, 25 ◦C,= +0.3

100 69 [152]

sTo

aTaoR0uci(9ofPb

bivwacPnspawTtrlfm

W-Ti NTAs photocatalyst, Eanod

80 W UV for 8 h

a Not determined.

olution needed up to 8 h to achieve 69% TOC reduction [152] (seeable 2). All these results evidence again the low oxidation abilityf PEC, useful for low organics loads.

Few studies have described the influence of pH [74,79] andpplied Eanod [74,79,114] over the degradation of dyes by PEC.he analysis of their decolorization process revealed that theylways obeyed a pseudo-first-order kinetics owing to the attackf a constant •OH content produced from photogenerated holes viaeaction (2) [72,76,84,114,156]. Fraga et al. [80] treated 30 mL of a.033 mM Basic Red 51 solution in 0.1 M Na2SO4 of pH 2.0 using anndivided cell with a WO3/TiO2 bicomposite photoanode and a Ptathode. After 120 min of electrolysis at janod = 1.25 mA cm−2 uponrradiation of the photocatalyst with UV (280–400 nm) and visible420–630 nm) light, total decolorization was always attained with4% and 83% TOC abatement, respectively. Using comparable PC,nly 25% and 30% color removal and 11% and 7% TOC decay wereound. These results corroborate again the superiority of PEC overC and the feasible sustainable solar-assisted remediation of waterodies contaminated with dyes.

The effectiveness of the coupled PEC/PEF process was assessedy Almeida et al. [38] for 500 mL of 85 mg L−1 orange G solutions

n 0.05 M Na2SO4 at optimum pH 3.0 and 25 ◦C using an undi-ided cell like of Fig. 9a by applying 16.67 mA cm−2. This methodas conducted with a Pt/TiO2 NTs photoanode irradiated with

80 W UV/Vis lamp and a carbon-PTFE air-diffusion cathode forontinuous H2O2 electrogeneration. It was compared with singleEC using an alternative Pt cathode and with EF using an alter-ative Pt anode. A 0.5 mM Fe2+ concentration was added to theolution for PEC/PEF and EF treatments. The dye disappeared com-letely in 16, 12 and 6 min for PEC, EF and PEC/PEF, respectively,lways obeying a pseudo-first-order kinetics. After 190 min, TOCas reduced by 80%, 87% and 97% for the above degradations.

he coupled PEC/PEF process then showed higher oxidation abilityhan PEC alone. LC–MS/MS and HPLC analysis of treated solutions
evealed the formation of 12 aromatic intermediates and 4 short-inear carboxylic acids. From these products, the reaction sequenceor orange G mineralization of Fig. 17 was proposed [35], where the
ain oxidizing agents are •OH produced from Reaction (2) and (15).

V (SCE),

The degradation of the dye (1) is initiated with its desulfonationand hydroxylation to give the compound 2. This intermediate canthen undergo: (i) successive hydroxylation to compounds 3 and 4or (ii) C N cleavage to form the naphthalene products 5 and 6 withdesulfonation. Compound 5 evolves to the benzoic acid derivative7, whereas the oxidation of 6 yields the benzene compounds 8 and9, along with the naphthalene compound 10, which can also be pro-duced from 4. Degradation of 7 leads to the sulfobenzene product 11that is oxidized to the p-benzoquinone derivative 12, also formedfrom compounds 8-10. The ring opening of benzenic intermediatesgives the aliphatic carboxylic acids 13 and 14, which are then trans-formed into the ultimate oxalic (15) and formic (16) acids. Theseacids form Fe(III)-oxalate and Fe(III)-formate complexes that arephotolyzed to CO2 with Fe2+ regeneration via Reaction (28).

The coupling of membrane filtration with PEC has been designedby Wang et al. [148]. The membrane was constructed deposit-ing graphitic carbon layer and TiO2 nanoparticles on an Al2O3membrane support. When this TiO2/carbon/Al2O3 membrane wasirradiated with a 500 W Xe lamp and an Eanod = +1.2 V (SCE) wasprovided for 2 h, 82% TOC reduction was obtained for a 10 mg L−1

rhodamine B solution with 0.01 M Na2SO4 flowing through themembrane (see Table 2). This removal was 1.3 or 3 times higherthan that found for filtration with light irradiation (using PC pro-cess) or filtration alone. The coupled PEC/membrane became moreefficient thanks to the greater mineralization of the dye with thelarger amounts of •OH formed from Reaction (2) upon the action ofthe applied Eanod.

4.1.3. Other photocatalytic materialsSemiconductor materials like WO3 and composites

[52,53,160,161,166], Bi2WO6 [117], ZnO [162], BDD-ZnWO4[159], �-PbO2 [65], BiVO4 [66], BiPO4 [164,165] and �-Fe2O3[59,60,163], with an appropriate Ebg to photo-excite electron/holepairs similarly to Reaction (1) for TiO2, have been checked for
the remediation of dye solutions by PEC. Table 3 collects thedegradation parameters obtained for several dye treatments withthe above photocatalysts using undivided cells under selectedconditions. As can be seen, UVC, UV, visible light and Xe lamps


F EC/PEFi ref. [3

w(le

ig. 17. Proposed reaction sequence for orange G mineralization by the coupled Prradiation and a carbon-PTFE air-diffusion cathode. Adapted with permission from

ere successfully applied to decolorize low dye concentrations−1
<25 mg L ) for long times, usually between 2 and 3 h, due to the
ow generation of •OH from photogenerated holes. Note that thexperimental conditions used for the photocatalysts of Table 3 are,

process using an undivided cell with Pt/TiO2 NTs photoanode upon 80 W UV/Vis8].

in general, quite similar to those applied with undoped TiO2 (see
Table 1) and doped TiO2 and composites with TiO2 (see Table 2).
Most of the above papers corroborated the superior oxida-tion power of PEC compared to individual processes like DP, EOand PC [60,65,159,161,163,164]. For example, Chatchai et al. [161]


Table 3Per cent of color and TOC removals for selected PEC treatments of dyes in undivided cells with other photocatalytic materials.

Dye co (mg L−1) Experimental conditions % colorremoval

% TOCdecay

Ref.

Direct Red 80 525

200 mL of solution with 5 g L−1 Na2SO4, 25 ◦C,�-PbO2 photocatalyst, I = 5 mA, 7 W UVC for 6 h

8872

1217

[65]

X-3B 20 60 mL of solution with 0.1 M Na2SO4,ZnWO4/BDD photocatalyst, Eanod = +2.0 V (SCE),20 W UV for 3 h

97 −a [159]

Basic Red 51 0.01b 30 mL of solution with 0.1 M Na2SO4, pH 2,W/WO3 photocatalyst, janod = 1.25 mA cm−2,150 W Xe for 2 h

100 63 [160]

Rhodamine B 0.1 b 200 mL of solution, WO3 photocatalyst,Eanod = +1.5 V (SCE), direct sunlight for 160 min

98 81 [166]

Methylene blue 5 Solution with 0.1 M Na2SO4, pH 6.5,WO3/BiVO4 photocatalyst, Eanod = +0.2 V(Ag/AgCl), 500 W Xe (> 420 nm) for 2 h

83 −a [161]

Methylene blue 0.01b 5 mL of solution with 0.1 M Na2SO4, BiVO4

photocatalyst, Eanod = +1.4 V (Ag/AgCl), 50 Wvisible light for 40 min

51 −a [66]

Methylene blue 5 25 mL of solution with 0.01 M Na2SO4, pH 7,�-Fe2O3 photocatalyst, Eanod = +0.6 V (SCE),350 W Xe for 105 min

78 −a [163]

Methyl orange 5 50 mL of solution with 0.1 M Na2SO4, �-Fe2O3

photocatalyst, Eanod = +1.0 V (SCE), 300 Wvisible light for 2 h

79 −a [59]

Methyl blue 10 Solution with 0.1 M Na2SO4, pH 7, BiPO4

photocatalyst, Eanod = +3.0 V (SCE), 11 W UVC80 −a [164]

rmovaTyaru

e[awtcit(sir•

adrrdassFdtrtf

0

1

2

3

4

5

6

0.0 0.2 0.4 0.6 0.8 1.0 1.2

e tarcificepslaitinI

(10-9

M c

m-2

s-1)

[NaCl] (M)

0

1

2

3

4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

etarcificep slaitinI

(10-9

M c

m-2

s-1)

[dye] (10-4 M)

a

b

Fig. 18. (a) Initial specific rate vs. NaCl concentration for the decolorization of8.8 × 10−5 M naphthol blue black diazo dye solutions by PEC using an undivided tank

for 5 h

a Not determined.b Initial concentration in mM.

eported the progressive decay of the more intense visible band ofethyl orange at �max = 662 nm up to 83% after 2 h of PEC treatment

f a 5 mg L−1 dye solution in 0.1 M Na2SO4 at pH 6.5 using an undi-ided cell equipped with a WO3/BiVO4 composite photocatalystt Eanod = +0.2 V (Ag/AgCl) under a 500 W Xe (>420 nm) irradiation.his process was much more powerful than PC alone, which onlyielded about 26% color removal, whereas the decolorization by DPnd EO at Eanod = +0.2 V (Ag/AgCl) was rather insignificant. Theseesults confirm an excellent separation of the electron/hole pairspon a bias potential for a WO3/BiVO4 composite.

Nevertheless, little information has been given over the influ-nce of operation variables like pH [52,53,161], applied Eanod52,53,65,159,164], light [52,160,161], dye concentration [52,65]nd background electrolytes [52,66]. In their pioneering worksith a WO3, Luo and Hepel [52,53] studied the degradation of

he naphthol blue black diazo dye in different electrolytes with aonventional undivided three-electrode cell like of Fig. 9a underrradiation with an outside 500 W UV/Vis light. For a dye concen-ration of 8.8 × 10−5 M with 0.5 M electrolytes and Eanod = +1.08 VSCE), the decolorization of the PEC process was enhanced in theequence: Na2SO4 < NaNO3 < NaClO4 < NaCl. The best performancen 0.5 M NaCl can be attributed to the very fast attack of active chlo-ine species formed from Reactions (21)–(23) compared to that ofOH produced from photogenerated holes by Reaction (2), whichre the only oxidants expected in the stable 0.5 M NaClO4. The fasterecolorization in 0.5 M NaNO3 than in the latter one can then beelated to the generation of more amounts of •OH via the photolyticeaction (24), whereas the lower oxidation ability of S2O8

2− pro-uced from Reaction (25) could justify the slowest loss in colorchieved in 0.5 M Na2SO4. The oxidation action of active chlorinepecies was confirmed by the linear dependence between the initialpecific rate for decolorization and NaCl concentration depicted inig. 18a [52], assessed under optimum conditions of 8.8 × 10−5 Mye concentration and Eanod = +1.08 V (SCE). It was also found that
he initial specific rate in 0.5 M NaCl was quite similar in the pHange 1.0-4.5, whereas it dropped progressively up to pH 12.0 byhe loss of oxidation power of active chlorine species due to theormation of the weaker oxidant ClO−. Moreover, the illumination
reactor with WO3 photocatalyst upon 500 W UV/Vis light at Eanod = +1.08 V (SCE). (b)Initial specific rate vs. naphthol blue black content for the PEC decolorization of dyesolutions in 0.50 M NaCl by illuminating the WO3 photocatalyst with 500 W of ( )UV/Vis and ( ) visible light at Eanod = +1.08 V (SCE). Adapted with permission fromref. [52].


0

10

20

30

40lavo

merC

OT

%

5 mg L-1 5 mA 25 mg L-1 50 mA25 mg L-1 5 mA

EOPEC

a

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3 4 5 6

noitaziroloceD

Time (h)

b

Fig. 19. (a) Percentage of TOC removal for EO and PEC treatments of 200 mL of 5 and25 mg L−1 Direct Blue 80 dye solutions in 5 g L−1 Na2SO4 at 25 ◦C using an undividedcell equipped with �-PbO2 photocatalyst upon 7 W UVC illumination for 6 h at 5 mAand 4 h at 50 mA (adapted with permission from Ref. [65]). (b) Decolorization vs.electrolysis time of a 10 mg L−1 methyl blue dye solution with 0.1 M Na2SO4 at pH 7by ( ) EO at Eanod = +3.0 V (SCE), ( ) PC and PEC at Eanod of ( ) +1.0 V, ( ) +2.0 V,(u

wloo

o8wFPcopb[sFt(tot

P6ov

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

0.4

0.6

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1.0

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0 20 40 60 80 100 120 140

Time (min)

noit az ir oloc eD

CO T

/C

OT

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

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0.6

0.8

1.0

1.2

0 20 40 60 80 100 120 140

Time (min)

noitaziroloceD

CO T

/C

OT0

a

b

Fig. 20. Time-course of decolorization decay (in red) and normalized TOC removal(in blue) in the (a) undivided and (b) divided semi-cylindrical tank reactors of Fig. 12bfor the treatments of 200 mL of a 1.044 × 10−5 M rhodamine B solution in 0.05 MNa2SO4 of pH 6.2 upon illumination of the Bi2WO6/FTO photoanode with 300 Wvisible light and at janod = 0.1 mA cm−2. (a) Systems: ( ) Bi2WO6/FTO|Pt, ( )Pt|Fe@Fe O /ACF and ( ) Bi WO /FTO|Fe@Fe O /ACF. (b) Catholyte degrada-

treated by PEC aiming to show the interest of this technology for

) +3.0 V and ( ) +4.0 V (SCE) in an undivided cell with a BiPO4 photocatalystnder 11 W UVC irradiation (adapted with permission from Ref. [164]).

ith UV/Vis light gave much better decolorization rate than visibleight, as can be seen in Fig. 18b, indicating that the photo-excitationf electrons in WO3 was mainly due to UV light, despite its low Ebgf 2.5–2.7 eV (see subsection 3.1.2).

The combined effect of dye concentration and applied currentn TOC decay for the EO and PEC processes of 200 mL of Direct Blue0 dye solutions in 5 g L−1 Na2SO4 at 25 ◦C using an undivided cellith a �-PbO2 photocatalyst upon 7 W UVC light was analyzed by

lorencio et al. [65]. Fig. 19a evidences a strong improvement ofEC over EO at the greater dye content of 25 mg L−1 and higherurrent of 25 mA, which can be ascribed to a large enhancementf the separation electron/hole pairs upon the action of externalotential. On the other hand, the high optimum Eanod value that cane applied to a BiPO4 photocatalyst was evidenced by Zeng et al.164] when exploring the decolorization of a 10 mg L−1 methyl blueolution with 0.1 M Na2SO4 at pH 7 using 11 W UVC illumination.ig. 19b shows that the rise of Eanod from 0 to +3 V (SCE) enhancedhe removal of the dye up to a maximum 80% color removal at 5 hsee Table 3). Further increase to +4 V (SCE) caused the redistribu-ion of charges in SCL with greater electron/hole recombination andnly 67% color removal was obtained at the same time. For all theserials, a pseudo-first-order kinetics was always found.

Ding et al. [117] examined the effectiveness of the coupledEC/EF treatment of rhodamine B in 0.05 M Na SO at neutral pH
2 4.2 in the undivided and divided semi-cylindrical tank reactorsf Fig. 12b using a Bi2WO6/FTO photocatalyst exposed to 300 Wisible illumination. The cathode was a Fe@Fe2O3/ACF electrode
2 3 2 6 2 3

tion in ( ) Pt||Fe@Fe2O3/ACF and ( ) Bi2WO6/FTO||Fe@Fe2O3/ACF. Adaptedwith permission from ref. [117].

with ability to release iron ions and electrogenerate H2O2 to pro-duce •OH from Fenton’s Reaction (15), as stated above. A verylow janod of 0.1 mA cm−2 was used, thus avoiding the formation of•OH from water oxidation by Reaction (10). In the undivided cell,Fig. 20a illustrates that the decolorization and TOC decay for thecoupled PEC/EF process (Bi2WO6/FTO|Fe@Fe2O3/ACF system) weregreater than the sum of those of PEC (Bi2WO6/FTO|Pt system) andEF (Pt|Fe@Fe2O3/ACF system) [117]. The synergistic process of thecoupled treatment can then be accounted for by the larger gener-ation of •OH from both Reactions (2) and (15). The production of•OH was enhanced when the dye degradation was performed inthe catholyte of the divided cell, with better performance using aBi2WO6/FTO photocatalyst than a Pt anode, as shown in Fig. 20b.These results are rather doubtful, because if a janod = 0.1 mA cm−2

was applied in both cases, the same degradation parameters shouldbe expected for the EF process taking place in the catholyte. In fact,the degradation of the catholyte in the divided cell can only beassociated with EF but not with a coupled PEC/EF since the photo-generated •OH cannot attack the organic load of the solution.

4.2. Chemicals

A high number of wastewaters containing chemicals have been

their remediation (see Fig. 1c). Most of these compounds are widelyused toxic and biorefractory industrial products such as anilines,phenols, bisphenol A and carboxylic acids. Photocatalysts including


T[mmd

4

iwcowtotdiTsafcmatT(atcisPl

fi[opw[lTtmptta

rihnorttbmas•

t

0

20

40

60

80

100

2.0 4.0 6.0 8.0 10.0

]enir olhce vit c

A [(m

g L-1

)

pH

0

20

40

60

80

100

0 30 60 90 12 0 15 0 18 0 210

lavomer

CO

T%

Time (min)

a

b

Fig. 21. (a) Percentage of TOC removal with time for 250 mL of a 5.0×10−6 M 4,4′-oxydianiline solution in 0.1 M Na2SO4 of pH 2.0 in an undivided cell under 150 WUVA light. ( ) DP, ( ) PC with Ti/TiO2 thin film, ( ) PC with Ti/TiO2 NTAs, ( )PEC with Ti/TiO2 thin film at Eanod = +1.5 V (Ag/AgCl) and ( ) PEC with Ti/TiO2

NTAs at Eanod = +1.5 V (Ag/AgCl) (Reproduced with permission from Ref. [89]). (b)


iO2 thin films and NTs [30,44,89,99,109,116,167–173], doped TiO2174–176], composites with TiO2 [70,75,82,86,177–187] and other

aterials [60,64,67,188–190] under different Eanod values and illu-ination sources have been utilized in these studies, as will be

escribed below.

.2.1. TiO2 and composites with TiO2Table 4 collects the degradation characteristics of several chem-

cals destroyed by PEC under the action of TiO2 and compositesith TiO2 photoanodes under selected conditions. High chemi-

al removals (>92%) along with excellent TOC reductions can bebserved in most cases using any kind of photoanode illuminatedith UV, visible and/or simulated solar light. Long times from 1

o 5 h were again required in view of the relatively low amount ofxidizing agents originated during the PEC treatment of low con-ents of chemicals. In some cases, the decay of solution COD wasetermined. This parameter reflects the amounts of organic and

norganic species that can be oxidized by dichromate ion, whereasOC accounts for by the amount of organic carbon present in theolution. The COD/TOC ratio varies with the compound checkednd, for example, attains a value of 2.38 for phenol and only 0.18or oxalic acid. Thus, the COD value for wastewaters with aromaticompounds decreases more rapidly than TOC because of the for-ation of final short-linear aliphatic carboxylic acids like formic

nd oxalic [5]. A COD removal up to 95% has been reported forhe destruction of a 10 mM formic acid solution by PEC using Ag-iO2 [177] photocatalysts under 500 W visible light at Eanod = +0.8 VSCE) for 60 min. On the other hand, it was found that 50 mL with

817 mg L−1 solution of this acid underwent a lower COD reduc-ion of 63.8% after 60 min of PEC treatment in a packed-bed reactorontaining a granular Ti/TiO2 thin film exposed to 500 W visiblellumination at Ecell = 30.0 V [168]. These findings suggest the fea-ibility of a large mineralization of organic pollutants because theEC process is potent enough to remove final carboxylic acids toarge extent.

Similarly to dyes, most works demonstrate the best per-ormance of PEC to destroy chemicals from wastewaters thanndividual processes. The superiority of TiO2 NTs [89], doped TiO2174,175] and composites with TiO2 [177,178,185] over thin filmsf this material has also been well-proven. Fig. 21a shows the com-arative TOC removal for a 5.0 × 10−6 M 4,4′-oxydianiline solutionith 0.1 M Na2SO4 at pH 2.0 by several methods and photocatalysts

89]. DP and PC with Ti/TiO2 thin film and NTAs under 150 W UVAight only led to 40–50% mineralization in 180 min. At that time,OC was more largely reduced by 78% using PEC with a Ti/TiO2hin-film photocatalyst at Eanod = +1.5 V (Ag/AgCl), whereas total

ineralization was attained in a shorter time of 120 min when thehotocatalyst was a Ti/TiO2 NTAs material. This work shows clearlyhe enhancement of the separation of the photogenerated elec-ron/holes by highly ordered NTAs with much greater photoactiverea than conventional thin films.

Contradictory relative oxidation abilities have been describedeferring to the electrolytes used in synthetic solutions. While anncreasing PEC performance in the order NaCl < NaNO3 < Na2SO4as been found for 4,4′-oxydianiline [89], humic acid [116] and p-itrophenol [167], the sequence NaNO3 < Na2SO4 < NaCl has beenbtained for microcystin-LR [183]. Moreover, Jorge et al. [167]eported a superior performance of HClO4 instead of the abovehree electrolytes, whereas Daskalaki et al. [172] obtained a bet-er removal of bisphenol A using NaCl instead HClO4. This differentehavior can be associated with the different reactivity of parentolecules and its oxidation products with generated ROS and/or

ctive chlorine species, as pointed out above. Several studies inulfate medium have confirmed the oxidation preponderance ofOH formed from reaction (2) over O2

•− produced from reac-ion (3) during the PEC treatment of tetrabromobisphenol A [70]

pH-dependence of active chlorine concentration produced in 250 mL of a 0.1 MNaCl in an undivided cell with a Ti/TiO2 thin-film photocatalyst upon 125 W UV(315–400 nm) irradiation for 30 min (Reproduced with permission from Ref. [170]).

and 2,4-dichlorophenol [185] from addition of radical scavengerslike tert-butanol (•OH scavenger), oxalate (hole scavenger) and p-benzoquinone (O2

•− scavenger).The effect of pH on the PEC performance was also dependent on

the molecule nature. In most cases, the best removal of chemicalswas obtained in an acidic pH range 1–4 [89,116,169,172,175,183],although for aniline [44] and dodecyl-benzenesulfonate [174] theoptimum pH values were close to 10 and 6.2, respectively. For 0.05-0.10 M NaCl, Fraga et al. [170] confirmed the larger production ofactive chlorine species at pH 4.0 using an undivided reactor with aTi/TiO2 thin-film photocatalyst upon 125 W UV light, as can be seenin Fig. 21b. They obtained 95% TOC reduction of a 50 g L−1 micro-cystin solution at this pH using janod = 30 mA cm−2 for 180 min.

Chai et al. [82] presented an interesting study on the degrada-tion of a 200 mg L−1 p-nitrophenol solution in 0.1 M Na2SO4 in anundivided cell similar to that of Fig. 9a to check the performance ofa SnO2/TiO2 NTs composite under 300 W UV illumination by sev-eral treatments. A high content of this pollutant was utilized tobetter determine the changes in biodegradability and relative tox-icity index of the treated solution, and the evolution of detectedintermediates. The biodegradability was determined as the ratioof biochemical oxygen demand at 5 days (BOD5) and COD, need-
ing a threshold value of 0.4 for biological post-treatment. Therelative toxicity index with respect to the starting sample was cal-culated from the change in luminescence of the marine bacteriumVibrio fisheri. Fig. 22a reveals a very small p-nitrophenol concentra-


Table 4Percentage of chemical and TOC removals for selected PEC processes of several organics in undivided cells with TiO2 and composites with TiO2 photocatalyts.

Chemical co (mg L−1) Experimental conditions % chemical removal % TOC decay Ref.

4,4′-oxydianiline 0.1a 250 mL of solution with 0.1 M Na2SO4, pH 6.0,TiO2 NTs photocatalyst, Eanod = +1.5 V(Ag/AgCl), 150 W UVA for 180 min

92 31 [89]

Phenol 9.4 40 mL of solution with 0.1 M Na2SO4, pH 6,bifunctional TiO2 NTs and Ni-Sb-SnO2

photocatalyst, Eanod = +2.0 V (SCE), 150 W Xe for100 min

95 18 [186]

p-Nitrophenol 200 100 mL of solution with 0.1 M Na2SO4, 25 ◦C,SnO2/TiO2 NTs photocatalyst, Eanod = +1.5 V(SCE), 300 W UV for 4 h

98 91 [81]

p-Nitrophenol 5 60 mL of solution, pH 2, PANI/Cr-TiO2 NTsphotocatalyst, Eanod = +0.8 V (SCE), 15 W UVCfor 2 h

99 76c [175]

Bisphenol A 0.2a 60 mL of solution with 0.1 M NaCl, pH 5,ITO/TiO2 photocatalyst, janod = 0.32 mA cm−2,150 W Xe for 75 min

98 −b [172]

Bisphenol A 15 160 mL of solution, pH 4.5,TiO2/graphene/Cu2O, Eanod = +0.5 V (SCE),500 W Xe for 150 min

97 −b [86]

Tetrabromobisphenol A 5 35 mL of solution with 0.5 M Na2SO4, Au/TiO2

NBs photocatalyst, janod = 20 mA cm−2, 35 Wvisible light for 100 min

97 37 [70]

Humic acid 25 100 mL of solution with 0.0125 M NaCl, pH 3,Ti/TiO2 photocatalyst, Eanod = +1.0 V (SCE),450 W Xe for 150 min

100 83 [169]

Microcystin-LR 1 40 mL of solution with 0.01 M NaNO3,TiO2/AgCl/Ag photocatalyst, Eanod = +0.6 V(SCE), 60 W visible light for 5 h

92 78 [183]

a Initial concentration in mM.b Not determined.c COD decay.

0255075

100125150175200225

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

[p]lonehporti

N-(m

g L-1

)

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

lavomer

COT

%

a b

0.0

0.2

0.4

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0.8

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BO

D5/C

OD

0

1

2

3

4

5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

xedniyticixot

evitaleR

Time (h)

c d

Fig. 22. Change of (a) p-nitrophenol concentration, (b) percentage of TOC removal, (c) biodegradability and (d) relative toxicity index with time for the treatment of 100 mLo −1 thod:

w inatio

S

tNEdtt

f 200 mg L pollutant solution with 0.1 M Na2SO4 in an undivided cell for 4 h. Meith SnO2/TiO2 NTs and ( ) PEC with SnO2/TiO2 NTs. Eanod = +1.5 V (SCE) and illum


ion abatement by PC processes with both Ti/SnO2 and SnO2/TiO2Ts photocatalysts [81], which was strongly enhanced by EO at
anod = +1.5 V (SCE), more pronounced for the composite material,ue to the larger generation of •OH via Reaction (10). The addi-ional photogeneration of ROS by Reactions (2)–(5) allowed theotal disappearance of the parent molecule by PEC with SnO2/TiO2
( ) PC with Ti/SnO2, ( ) PC with SnO2/TiO2 NTs, ( ) EO with TiO2 NTs, ( ) EOn with 300 W UV light.

NTs at the same bias anodic potential in 4 h. The p-nitrophenol con-centration decay always obeyed a pseudo-first-order decay. The
same trend can be observed in Fig. 22b for TOC removal, where themost powerful PEC process attained a larger mineralization of 91%.Under EO and PEC conditions, the biodegradability of the solutionupgraded to values >0.4, whereas the relative toxicity index was


rNqrobt

+(b4naIocdsduhufEat

idntawATauemriu9wibb

wbt6bjtc

4

[tgvae

0.0

0.2

0.4

0.6

0.8

1.0

0 30 60 90 120 150 180 210 240 270

A / A

0

Time (m in)

0.0

0.2

0.4

0.6

0.8

1.0

0 30 60 90 12 0 150 180Time (m in)

A / A

0

a

b

Fig. 23. Variation of the relative absorbance decay at � max = 276 nm with elec-trolysis time for 160 mL of 15 mg L−1 bisphenol A at pH 4.5 under (a) 85 and (b)100 mW cm−2 Xe illumination. Method: ( ) DP, ( ) PC with TiO2/Cu2O mesh,


educed to practically zero using these methods with the SnO2/TiO2Ts composite. However, low contents of by-products like hydro-uinone, p-benzoquinone and maleic, fumaric and oxalic acids stillemained in the final solution treated by PEC. These results corrob-rate the potential applicability of this procedure at industrial level,eing feasible its coupling with a cheaper biological post-treatmento produce reusable water.

For the PEC process in an undivided cell, optimum Eanod values of0.8 V (SCE) for p-nitrophenol with PANI/Cr-TiO2 NTs [175], +1.0 VSCE) for p-nitrophenol with TiO2 thin film [167] and dodecyl-enzenesulfonate with doped W-TiO2 [174], +1.5 V (Ag/AgCl) for,4′-oxydianiline with Ti/TiO2 NTAs [89] and +2.0 V (SCE) for phe-ol with bifunctional TiO2 NTs and Ni-Sb-SnO2 photocatalyst [186],long with optimum janod = 0.32 mA cm−2 for bisphenol A withTO/TiO2 [172], have been found. Moreover, the expected decreasef the pseudo-first-order rate constant for organic decay and per-entage of TOC removal with increasing chemical concentrationue to the presence of more organic matter has been confirmed byeveral authors [29,44,89,167,174,175]. Similar results have beenescribed for divided cells [116,169,178]. Selcuk et al. [116,169]tilized the divided cell of Fig. 12a to degrade 100 mL of 25 mg L−1

umic acid in sulfate or chloride media, contained in the anolyte,sing a Ti/TiO2 photocatalyst exposed to 450 W Xe irradiation. Theyound for 0.0125 M NaCl, optimum operation conditions of pH 3 andanod = +1.0 V (SCE) for total pollutant removal with 83% TOC decayfter 150 min of electrolysis (see Table 4), a value quite similar tohat obtained in 0.01 M Na2SO4 at the same conditions.

The best performance of the PEC process with increasing lightntensity has also been confirmed by Yang et al. [86] from theecay of the absorbance of the UV band at �max = 276 nm of bisphe-ol A. Fig. 23a and b evidence the large stability of 15 mg L−1 ofhis chemical at pH 4.5 upon DP when it was illuminated with

Xe lamp at 85 and 100 mW cm−2, respectively. The use of PCith a TiO2/Cu2O mesh caused a notable abatement in bisphenol

concentration, which was significantly improved by applying aiO2/graphene/Cu2O mesh due to the effective electron/hole sep-ration, as shown in Fig. 5. The bisphenol A removal was stronglypgraded by PEC at Eanod = +0.5 V (SCE), as expected by a greaterxtraction of electrons from the CB of Cu2O enhancing the for-ation of holes at the VB of TiO2 with higher formation of •OH

adicals from Reaction (2). The removal of this chemical was largelymproved by the production of more quantities of these radicalspon addition of 50 mM H2O2 via Reaction (9). Fig. 23b reveals a7% reduction of bisphenol A after 150 min of PEC (see Table 4),hich decreased to 90 min upon 50 mM H2O2 addition. Compar-

son of Fig. 23a and b clearly shows a more rapid destruction ofisphenol at higher light intensity for both PC and PEC processesecause of the greater photogneration of electron/hole pairs.

It has also been reported the positive action of H2O2 on PEChen it was electrogenerated on site at an ACF/PTFE [106] or

oron-doped diamond (BDD) cathode [172] via Reaction (8). Withhe latter cathode, for example, the bisphenol A concentration in0 mL of a 0.2 mM solution in 0.1 M NaCl at pH 5 was reducedy 98% using an ITO/TiO2 photocatalyst under 150 W Xe light at

anod = 0.32 mA cm−2 for 75 min (see Table 4), whereas the alterna-ive use of a Zr cathode (no H2O2 production) only led to 42% ofoncentration removal.

.2.2. Other photoanodesThe effective application of �-Fe2O3 [60], Sb-SnO2 [64], Bi2WO6

67,188], WO3 [189] and ZnO [190] semiconductors to the destruc-ion of phenolic compounds by PEC has also been reported. The
ood degradation results obtained for such treatments in undi-ided cells after long irradiation time with UVA, UVC or Xe lampsre summarized in Table 5. All these studies described again thenhancement of PEC performance over individual processes, but
( ) PC with TiO2/graphene/Cu2O mesh, ( ) PEC with TiO2/graphite/Cu2O mesh atEanod = +0.5 V (SCE) and ( ) PEC with TiO2/graphite/Cu2O mesh and 50 mM H2O2

addition at Eanod = +0.5 V (SCE). Adapted with permission from ref. [86].

little information was given over the optimization of operation vari-ables. On the other hand, Nissen et al. [189] checked the removal of60 mL of 40 mg L−1 of 2,4-dichlorophenol in water (without elec-trolyte) of pH 5.5 in the anolyte of a divided H-cell by supplying35 W visible irradiation to the immersed WO3 photocatalyst. Sincea very low photocurrent of 7.5 A was applied owing to the lowconductivity of the medium, only 53% of the content of this pollu-tant was removed after 48 h of treatment.

Phenolic compounds can be directly oxidized at the Bi2WO6photocatalyst surface, e.g., at +0.8 V (SCE) for 4-chlorophenol [188],and hence, a higher Eanod upgrades their direct destruction. The firststep in EO of phenol or chlorinated phenols involves the formationof a phenoxy radical, which can undergo either radical–radical orradical–substrate coupling to give polymeric products that remainadsorbed on the electrode surface and inhibit the process, espe-cially in alkaline medium where their deprotonated forms areinvolved. In contrast, the photogenerated ROS species in PEC canactivate the electrode surface and promote pollutant oxidation.Fig. 24a shows that the quicker removal of 4-chlorophenol con-centration at pH 6.5 with Bi2WO6 exposed to 500 W Xe lamp wasachieved at Eanod = +2.0 V (SCE) [188], where the separation of theelectron/hole pairs was maximal. At higher Eanod values up to +4.0 V(SCE) the SCL became gradually larger and the photogeneratedelectrons were redistributed, facilitating its recombination withholes and making the process more inefficient. Fig. 24b illustratesthe gradual drop in 2,4-dichlorophenol content abatement for theabove PEC treatment at optimum Eanod = +2.0 V (SCE) when pH var-ied from an alkaline medium of pH 8.5 to an acidic one of 4.3[188]. This behavior can be related to a progressive larger adsorp-
tion of the molecule that favors its reaction with oxidizing agentsformed at the photoanode surface, although the smaller forma-tion of inhibitory polymeric films with decreasing pH could alsocontribute to the better efficiency of the process in acidic medium.


Table 5Per cent of chemical and TOC removals for selected PEC treatments of phenols using undivided cells with other photocatalytic materials.

Chemical co (mg L−1) Experimental conditions % chemical removal % TOC decay Ref.

Phenol 20 100 mL (without electrolyte), SnO2-Sb2O4

photocatalyst, Ecell = 2.0 V, 250 W UVA for 2 h100 85 [64]

Phenol 10 15 mL of solution with 0.5 M Na2SO4, Bi2WO6

photocatalyst, Eanod = +0.6 V (SCE), 500 W Xe for3 h

78 65 [67]

p-Nitrophenol 0.1a 50 mL of solution with 0.1 M Na2SO4, pH 3.0,ZnO photocatalyst, Eanod = +1.4 V (SCE), 15 WUVC for 3 h

91 74b [190]

p-Nitrophenol 0.05a 50 mL of solution with 0.1 M Na2SO4, �-Fe2O3

photocatalyst, Eanod = +1.0 V (SCE), 300 Wvisible light for 4 h

47 −c [59]

4-Chlorophenol 10 100 mL of solution with 0.5 M Na2SO4, pH 4.3,Bi2WO6 photocatalyst, Eanod = +2.0 V (SCE),500 W Xe for 12 h

68 65 [188]

a Initial concentration in mM.b COD decay.c Not determined.

0

10

20

30

40

50

60

0 1 2 3 4 5

lavomerlonehporolhc-4

%

Eanod

(V v s. S CE)

0

2

4

6

8

10

12

0 3 6 9 12 15

]lonehporolhC- 4[

(mg

L-1)

Time (h)

a

b

Fig. 24. (a) Percentage of 4-chlorophenol removal vs. anodic bias potential for thedegradation by PEC of 100 mL of 10 mg L−1 organic solution with 0.5 M Na2SO4 at pH6.5 using an undivided cell like of Fig. 9a with a Bi2WO6 photoanode under 500 W Xelight for 8 h. (b) 4-Chlorophenol content decay with electrolysis time for the aboves

S

Cnupuata

from microtox and microalga tests. Moreover, the primary oxi-

olution at pH: ( ) 8.5, ( ) 6.5 and ( ) 4.3 and Eanod = +2.0 V (SCE).


Fao et al. [190] found a 91% pollutant reduction with 74%OD removal after 3 h of PEC treatment of 50 mL of a 0.1 mM p-itrophenol solution in 0.1 M Na2SO4 with a ZnO photocatalystpon 15 W UVC illumination at optimum Eanod = +1.4 V (SCE) andH 3.0 (see Table 5). When the p-nitrophenol concentration grewp to 0.5 mM, a gradual loss in its abatement up to 33% was obtained
s a result of the larger organic matter treated with similar quan-ities of oxidants photogenerated. A pseudo-first-order kinetics forll concentration decays was always determined.
4.3. Pharmaceuticals

Despite the social alarm over the presence of pharmaceu-ticals in the aquatic environment, their feasible degradationby PEC has only been assessed for few compounds in thelast years [37,81,88,107,191–200]. The trials were pre-eminentlymade with TiO2 photoanodes, although composites with TiO2[81,191,198,199] and new Ti compounds [88] have been utilized aswell. Table 6 summarizes the percentages of drug and TOC decaysachieved by this technique for some compounds under selectedconditions using undivided cells with TiO2 thin-film and NTs photo-catalyst. As seen previously, UVA, UVC and Xe lamps were applied totreat the solutions. Low drug concentrations (usually ≤ 20 mg L−1)were used to check the oxidation ability of the technique and goodper cent of drug removals (regularly >80%) were obtained for timeslonger than 1 h.

Most authors confirmed the expected larger oxidation abilityof PEC to remove drugs over individual processes. The effect ofsome operation variables such as pH [37,192,196–198], appliedEanod or I [192,196–198] and drug [37,196–198] and electrolyte[197] concentrations over the process performance was assessed.The kinetics for drug decay [37,88,191,192,196,197], the detectionof oxidation products [37,107,191,198], the drop in toxicity duringtreatment [37,107,196], economical assessment [37,191] and eventhe use of scavengers [197,199] has been reported as well.

An interesting work of Daghrir et al. [37] showed that theoptimum PEC treatment of 1 L of 0.025–0.230 mg L−1 chlortetra-cycline solutions in 0.050 M Na2SO4 took place at pH close to 6,at I = 0.39 A under 6.9 mW cm−2 UVC irradiation using the arrange-ment of Fig. 11a with a vitreous carbon cathode as compared tostainless steel, graphite and amorphous carbon ones. Under theseconditions, 98% drug decay and 67% TOC removal after 120 min oftreatment of a 0.025 mg L−1 solution were obtained (see Table 6),with similar values in the concentration range tested and an esti-mated cost of 4.52 US$ m−3 for chemicals plus energy consumption[37,193]. The superiority of the vitreous carbon cathode was relatedto its ability to produce H2O2 from Reaction (8), which can bephotolyzed to OH by UVC from Reaction (29), thus enhancing theorganics removal from •OH generated by Reaction (2).

H2O2 + hv → 2 •OH (29)

The authors reported a strong reduction of the toxicity of the0.025 mg L−1 solution after optimum treatment during 120 min

dation products were detected by LC–MS proposing the reactionsequence of Fig. 25 [37]. The degradation of chlortetracycline(1) under the main action of OH is initiated by the formation


Table 6Per cent of drug and TOC removals for selected PEC treatments of pharmaceuticals using undivided cells with TiO2 photoanodes.

Drug co (mg L−1) Experimental conditions % drug removal % TOC decay Ref.

Chlortetracycline 0.025 1 L of solution with 0.05 M Na2SO4, 25 ◦C, pHnear 6, I = 0.39 A, 6.9 mW cm−2 UVC for120 mina

98 67 [37]

Chlortetracycline 25 1 L of solution with 0.05 M Na2SO4, pH 4.8,25 ◦C, I = 0.39 A, 6.9 mW cm−2 UVC for 120 mina

74 −c [193]

Tetracycline 10 500 mL of mixed solution with 0.1 M Na2SO4,pH 7.4, Eanod = +1.0 V (SCE), 15 W UVA for180 minb

81 −c [192]Oxytetracycline 10 83 −c

Chlortetracycline 10 85 −c

Triclosan 20 50 mL of solution with 0.02 M Na2SO4, pH 5.8,Eanod = 0 V (SCE), 125 W UVC for 30 minb

79 60 (1 h) [107]

Acyclovir 20 0.1 mL of solution with 0.2 M NaNO3 in athin-layer reactor, pH 5.8, Eanod = +1.0 V(Ag/AgCl), 10 mWcm−2 UVA for 370 sb

97 −c [194]

Ofloxacin 40 70 mL of solution with 0.1 M Na2SO4, pH 3.0,Eanod = +0.8 V (SCE), 15 W UVC for 35 minb

100 70 [196]

Sulfamethoxazole 1.3 50 mL of solution with 10 mM NaCl, pH 2.7,Eanod = +0.5 V (Ag/AgCl), 4 W UVA for 150 mina

100 −c [197]

Diclofenac 5 80 mL of solution with 0.1 M Na2SO4,Eanod = +0.4 V (SCE), 35 W Xe for 10 hb

100 −c [200]

oddlthc

oboapi[

[Hmswrtc

tToramptdtmwc

ttw

a TiO2 thin film.b TiO2 NTs.c Not determined.

f six derivatives: isochlortetracycline (2) by dechlorination, epi-emeclocycline (3) and 4-epi-N-demethylchlortetracycline (4) byemethylation, 4-epi-N-dedismethylchlortetracycline (5) by the

oss of two methyl groups, isochlortetracycline (6) with forma-ion of a 5-ring heterocycle and its hydroxylated product 7. Furtherydroxylation, demethylation or dedismethylation of 7 yields theompounds 8, 9 and 10, respectively.

Optimum degradation for this antibiotic and its mixture withther tetracyclines in the pH range 6–9 has also been reportedy Liu et al. [192], whereas acidic media of pH 1–3 were foundptimal for the oxidation of ofloxacin [196,198] and sulfamethox-zole [197]. The best performance of chlortetracycline at neutralH was explained from the higher electrostatic attraction between

ts zwitterionic form and the positively charged surface of Ti/TiO237].

The decay of chlortetracycline [37,192] and sulfamethoxazole197] was analyzed from a kinetic equation involving a Langmuir-inshelwood model that presupposes a rapid adsorption of theolecule at the photocatalyst surface and the limiting step is the

ubsequent transformation of the adsorbed species. This modelas found valid for relatively high drug concentrations, but it was

educed to a pseudo-first-order equation at low contents. The lat-er equation has been directly applied to explain the abatement ofiprofloxacin [88], paracetamol [191] and ofloxacin [196].

The reduction of toxicity of the treated solution by PEC is due tohe destruction of the parent molecule and its oxidation products.his was clearly shown by Liu et al. [107] for the oxidation of 50 mLf 20 mg L−1 of the bactericide triclosan in 0.02 M Na2SO4 at natu-al pH 5.0 using an undivided cell with a Ti/TiO2 NTs photocatalystt Eanod = 0 V (SCE) under 125 W UVC light for 60 min. They deter-ined the solution toxicity from the luminescent Photobacterium

hosphoreum T3 spp. bacterium, expressed as equivalent HgCl2 con-ent. The toxicity was largely reduced during the first 10 min of PECue to the loss of the toxic triclosan. Further, it was increased upo 30 min where it reached a quasi-steady value owing to the for-

ation of the more toxic intermediate 2,7-dichlorodibenzodioxin,hich was degraded more slowly than the parent molecule to be

ompletely mineralized.The addition of scavengers to the solution has demonstrated

he generation of oxidizing species in PEC. Fig. 26a exemplifieshe abatement of the normalized sulfamethoxazole concentrationhen 50 mL of 2.4 mg L−1 of this antibiotic in 10 mM NaCl of pH 2.7

was degraded in an undivided cell with a Ti/TiO2 thin-film photo-catalyst by applying Eanod = +0.5 V (Ag/AgCl) exposed to 4 W UVAlight [197]. After 80 min of PEC treatment, the sulfamethoxazolecontent dropped to 72%, which was reduced to 60% or 12% by adding2.4 mg L−1 humic acid or 10% methanol, respectively. The two lat-ter compounds are well-known scavengers of generated ROS andactive chlorine species and then, it react more rapidly with theseoxidants diminishing the destruction rate of the antibiotic. Thedecrease of the corresponding pseudo-first-order rate constant isdepicted in Fig. 26b. This figure also reveals a photocurrent densitynear 0.14 mA cm−2 for the systems with sulfamethoxazole aloneor humic acid added, but it underwent an unexpected spectulargrowing to 0.46 mA cm−2 by adding 10% methanol. The excep-tional enhancement of the photocurrent density in the presence ofmethanol was justified by current doubling that refers to the releaseof additional electrons from reactive solute-hole intermediates. Itwas hypothesized the oxidation of a methanol molecule by a holeto produce a methoxy radical (CH3O•) by Reaction (30), followedby the oxidation of this radical to formaldehyde with release ofone electron by Reaction (31). Nevertheless, more information isneeded to confirm this assumption.

CH3OH + h+VB → CH3O• + H+ (30)

CH3O• → CH2O + H+ + e−CB (31)

4.4. Real wastewaters

The remediation of a very limited number of real wastewaters byPEC has been investigated at lab-scale using several photocatalystsupon exposure to UV and visible lights. Good color removal and/orCOD abatements at relatively short times have been found for riverwater contaminated with humic acid using Ti/TiO2 thin film [201],sugarcane factory wastewater using ZnO and doped N-ZnO [202],pharmaceutical wastewater using Ni/TiO2 thin fim [203], landfillleachate using codoped Cu/N-TiO2 [205] and textile wastewatersusing Ti/TiO2 thin film decorated or not with Pt [204] and Ti/TiO2NTs [206]. Relevant results of some of these works are listed inTable 7. These data encourage for a further intense research to show
the benefits of PEC for treating many real wastewaters in order todevelop a powerful technology at industrial level.
Operation variables were optimized in some cases. For example,Fang et al. [203] degraded 500 mL of a pharmaceutical wastewa-


Cl

OH

OHCH3

O OHOH

O

OH

N

O

NH2

H3C CH3 Cl

OH

OHCH3

O OHOH

O

OH

NH

O

NH2

H3CCl

OH

OHCH3

O OHOH

O

OH

NH2

O

NH2

Cl

OH

OH

O OHOH

O

OH

N

O

NH2

H3C CH3

OH

OHCH3

O OHOH

O

OH

N

O

NH2

H3C CH3

Cl

OH

H3C

O OHOH

O

OH

N

O

NH2

H3C CH3

OCl

OH

H3C

O OOH

O

OH

N

O

NH2

H3C CH3

O

HO

Cl

OH

H3C

O OOH

O

OH

NH2

O

NH2

O

HO

Cl

OH

H3C

O OOH

O

O

N

O

NH2

O

HOCl

OH

H3C

O OOH

O

OH

NH

O

NH2

H3C

O

HO

OH

1

2 3

45

67

8 9

10

CH3H3C

Fig. 25. Proposed reaction sequence for chlortetracycline degradation by PEC process using the arrangement of Fig. 11a with TiO2 thin-film photocatalyst upon 6.9 mW cm−2

UVC irradiation at 0.39 A. Adapted with permission from ref. [37].

Table 7Per cent of color and COD removals for selected PEC treatments of real wastewaters using undivided reactors with different photoanodes.

Wastewater COD0 (mg L−1) Experimental conditions % color removal % COD decay Ref.

Pharmaceutical 3150 500 mL, 0.5 M NaCl, pH 3.0, Ni/TiO2

photocatalyst, Ecell = 10 V, 250 W UVA for 2 h78 93 [203]

Landfill leachate 4378 500 mL, pH 2, 30 ◦C, Cu/N-TiO2 photocatalyst,Ecell = 20 V, 50 W visible light for 210 min

−a 69 [205]

Textile effluent 153 8.5 L, pH 3, 2.9 g L−1 O3, cylindrical Ti/TiO2 NTs0 W U

98 33 [206]

ttNMec

photocatalyst, Ecell = 2.0 V, 1060 min

a Not determined.

er with COD0 = 3150 mg L−1 of pH 5.8 in an undivided cell similaro that of Fig. 9a equipped with a photocatalyst composed of ai foam coated with TiO2 thin film and a cathode formed by
nO2 and multi-walled carbon nanotube composite. Air injection
nsured additional •OH generation from MnO2 reduction at theathode during the EO and PEC processes and 250 W UVA was

VB for

irradiated on the photocatalyst in PC and PEC. The best PEC condi-tions were obtained by applying an Ecell = 10 V and operating with0.50 M NaCl and pH 3.0, showing the positive influence of the active
chlorine species produced. Under these conditions, a loss of CODof 93%, much greater than 79% of color, was found for PEC (seeTable 7), values much higher than 66% and 53% for EO and 43% and


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70 80 90

c / c

0

Time (min)

0.000

0.005

0.010

0.015

0.020

0.025

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1 2 3

k dapp

nim(

-1)

yt isn

e dtn

erru

coto

hP(m

A cm

-2)

Treatment

a

b

Fig. 26. (a) Relative sulfamethoxazole concentration decay with time for the PECtreatment of 50 mL of several antibiotic solutions in 10 mM NaCl of pH 2.7 usingan undivided cell with Ti/TiO2 thin-film photocatalyst at Eanod = +0.5 V (Ag/AgCl)under 4 W UVA light. (b) Apparent rate constant and photocurrent density for thesame trials. Treatment: ( ,1) 2.4 mg L−1 sulfamethoxazole, ( ,2) 2.4 mg L−1 sul-f −1 −1

m

5vTeiaiaiTewe

daAFTp1fawb3Wiw

Fig. 27. (a) Scheme of the annular bubble reactor developed for the degradationof a real textile wastewater by direct ozonation (DO), DP, PC and PEC processes.1: reactor glass wall; 2: gas inlet; 3: sintered glass diffusers for injecting O2/O3;4: rubber tip; 5: glass tube to accommodate the UVB 100 W lamp; F: cylindricalTiO2 NTs; G: DSA used as cathode. (b) Electrical energy per order after 60 min of

amethoxazole + 10 mg L humic acid and ( ,3) 2.4 mg L sulfamethoxazole + 10%ethanol. Adapted with permission from ref. [197].

9% for PC, demonstrating again the superiority of PEC over indi-idual processes. For a landfill leachate with COD0 = 4378 mg L−1,OC0 = 2583 mg L−1 and a very low BOD5/COD = 0.01 of pH 7.8, Zhout al. [205] studied the degradation by PEC of 500 mL wastewatersn a cell like of Fig. 9a with a codoped Cu/N-TiO2 photoanode and

graphite cathode, but without air injection. An outside 50 W vis-ble light was utilized to illuminate the catalyst and pH 2, 30 ◦Cnd Ecell = 20 V were found as optimum operation variables, lead-ng to the best COD reduction of 69% in 210 min (see Table 7).hese authors also proposed a doubtful dynamic kinetic model toxplain the COD decay rate as a function of COD, Ecell and pH, fromhich an activation energy of 63.5 kJ mol−1 for the process was

stimated.A more recent paper of Cardoso et al. [206] compared the degra-

ation of a real textile wastewater with color0 = 1080 mg Pt/Co L−1

nd COD0 = 153 mg L−1 by direct ozonation (DO), DP, PC and PEC.dditionally, the latter three processes were combined with ozone.ig. 27a presents the 4 annular reactors used by bubbling O2 or O3.he solution volume was 12 L for DO and 8.5 L for the other threehotoassisted treatments due to the volume occupied by the inside00 W UVB lamp with �max = 315 nm. The PEC process was per-ormed with a Ti/TiO2 NTs photoanode and a DSA

®cathode with

n optimum Ecell = 2.0 V. Poor color removal of 32%, 42% and 55%,ith scarce COD removal, was obtained by DP, PC and PEC by bub-

ling O2 at acidic pH 3 for 60 min, whereas 98% color decay with3% COD abatement was found for DO at 2.9 g h−1 O3 flow rate.
hen ozonation was coupled to the photoassisted processes, sim-
lar results were always obtained. A slightly slower performanceas obtained at pH 8 due to the presence of scavengers bicarbon-

the different treatments at pH 3 and 8 by injecting O3 at 2.9 g h−1. Adapted withpermission from ref. [206].

ate ions, despite the production of •OH and other ROS from O3decomposition in alkaline medium:

O3 + OH− → •OH + (O2•− ↔ HO2

•) (32)

For DO and coupled DP/O3, PC/O3 and PEC/O3 trials, a compara-tive energetic study was made from the electrical energy per orderof magnitude consumed during 60 min, defined as follows:

Electrical energy per order (kWh m−3order−1)

= 1000P t

Vs log (color0/color f)(33)

where P stands for the rated power (in kW) of the oxidationsystem, t is the treatment time (in h), Vs denotes the treated vol-ume (in L) and color0 and colorf mean the initial and final colormeasurements in Pt/Co units. As expected, Fig. 27b makes in evi-dence an enhancement of this energetic parameter in the order:DO < DP/O3 < PC/O3 < PEC/O3, with greater values at pH 8 than atpH 3. These differences may be cancelled by using free sunlightand solar photovoltaic panels, for example, to power the electricaldevices, which need to be taken into account in the next future todesign competitive PEC systems for wastewater remediation.

5. Conclusions and prospects

Over the last 15 years, the potential applicability of PEC asan emerging EAOP to the remediation of wastewaters containingtoxic and/or biorefractory organic pollutants has been well-proven.

nd Ph

ItlbtwtuoeutncgiotrwlpfoulSptmPCP

rpncilrrcfsitvbrsaettaptmbeotbs


t appears as one of the most highly promising water treatmentechnologies to deal with recalcitrant organic contaminants pol-ution in water streams. The development of this technology haseen feasible thanks to the preparation of many novel semiconduc-ors such as nanostructured, doped and composite TiO2 materialsith ability enough to absorb more light intensity and/or expand

heir photoactivity to the visible region compared with UV lampssed to illuminate classical TiO2 thin-film, WO3 and ZnO photoan-des, initially checked for an effective separation of photogeneratedlectron/hole pairs. PEC systems have been designed at lab-scale,sually being composed of tank reactors or flow cells with two- orhree-electrodes, upon illumination of the photoanode with inter-al or external UVA, UVC, visible and/or Xe lamps. Good removal ofolor or initial substrate with smaller COD or TOC abatement wereenerally obtained for synthetic wastewaters containing dyes, typ-cal industrial chemicals and pharmaceuticals. Nevertheless, lowrganic loads with long treatment times were tested because ofhe low generation of oxidizing ROS due to the small photocur-ent produced. The superiority of PEC over individual processes wasell-demonstrated in many cases. The effect of operation variables

ike pH, applied Eanod or janod, kind and concentration of electrolyte,ollutant concentration and temperature on the degradation per-ormance of synthetic effluents was also analyzed to ascertain theptimum PEC conditions. The decay of low pollutant concentrationssually obeyed a pseudo-first-order kinetics, but high contents fol-

owed an adsorption Langmuir-Hinshelwood model in some cases.carce studies have been devoted to the detection of oxidationroducts formed, the changes of toxicity and biodegradability, andhe estimation of the energy costs of the PEC process. An enhance-

ent of the oxidation ability of PEC was found for coupled PEC/O3,EC/EF and PEF/SPEF systems. Promising good loses of color andOD have been described for few real wastewaters by PEC andEC/O3 processes.

On the basis of the present available data, more fundamentalesearch is still needed to develop a mature PEC technology witherspectives to treating real wastewaters at industrial scale in theext future. It seems necessary to continue searching novel photo-atalysts with enhanced photoactivity under sunlight irradiation tomprove the actual photoefficiencies and PEC performance on pol-utants abatement. These materials should be coupled to suitableeactors in pilot plants to show the viability of PEC degradation overeal wastewaters for its further scale-up to industrial level. The effi-ient design of novel photoelectrochemical reactors will become auture challenge to the scientist and engineers, since these reactorshould embody the photocatalytical requirements in electrochem-cal reactors. In these systems, the use of free sunlight to illuminatehe photoanodes, along with renewable energies such as photo-oltaic or wind devices to power the electrical instruments, shoulde designed to have a competitive process with minimum costequirements for wastewater remediation. This point is highly fea-ible due to the lower energy requirement of PEC systems as fars we know. Since no overall removal of the organic pollutants isxpected, viability trials with pilot plants should include at least: (i)he optimization of color reduction and COD and/or TOC removal ofhe treated effluent and (ii) the assessment of the decay in toxicitynd the enhancement of biodregradability during the degradationrocess. Coupling or combination with other powerful oxidationechniques could also be investigated in order to develop treat-

ents with higher oxidation power of the organic load. It can thene envisaged that if the biodegradability of a given industrial efflu-nt can be enhanced up to values >0.4 with an adequate singler coupled PEC process, the resulting wastewater could be post-
reated by a biological process, suitable for high volumes of water,efore disposal to the public sewage or reuse in human activitiesuch as agriculture or industrial manufacture of goods.

Acknowlegments

This research did not receive any specific grant from fundingagencies in the public, commercial, or not-for-profit sectors.

References

[1] H. Zollinger, Color Chemistry: Syntheses, Properties, and Applications ofOrganic Dyes and Pigments, 3rd ed., VHCA and Wiley-VCH, Zurich, 2003.

[2] K.P. Sharma, S. Sharma, S. Sharma, P.K. Singh, S. Kumar, R. Grover, P.K.Sharma, A comparative study on characterization of textile wastewaters(untreated and treated) toxicity by chemical and biological tests,Chemosphere 69 (2007) 48–54.

[3] K. Kümmerer, The presence of pharmaceuticals in the environment due tohuman use −Present knowledge and future challenges, J. Environ. Manage.90 (2009) 2354–2366.

[4] C.A. Damalas, I.G. Eleftherohorinos, Pesticide exposure safety issues, and riskassessment indicators, Int. J. Environ. Res. Public Health 8 (2011) 1402–1419.

[5] E. Brillas, C.A. Martinez-Huitle, Decontamination of wastewaters containingsynthetic organic dyes by electrochemical methods. An updated review,Appl. Catal. B: Environ. 166–167 (2015) 603–643.

[6] M.A. Oturan, J.J. Aaron, Advanced oxidation processes in water/wastewatertreatment: principles and applications. A review, Crit. Rev. Environ. Sci.Technol. 44 (2014) 2577–2641.

[7] W.H. Glaze, J.-W. Kang, D.H. Chapin, The chemistry of water treatmentprocesses involving ozone, hydrogen peroxide and ultraviolet radiation,Ozone Sci. Eng. 9 (1987) 335–352.

[8] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, Critical review of rateconstants for reactions of hydrated electrons, hydrogen atoms and hydroxylradicals (OH/O−) in aqueous solution, J. Phys. Chem. Ref. Data 17 (1988)513–886.

[9] E.G. Janzen, Y. Kotake, R.D. Hinton, Stabilities of hydroxyl radical spinadducts of PBN-type spin traps, Free Radical Biol. Med. 12 (1992) 169–173.

[10] M.N. Chong, A.K. Sharma, S. Burn, C.P. Saint, Feasibility study on theapplication of advanced oxidation technologies for decentralisedwastewater treatment, J. Clean. Prod. 35 (2012) 230–238.

[11] I. Sirés, E. Brillas, M.A. Oturan, M.A. Rodrigo, M. Panizza, Electrochemicaladvanced oxidation processes: today and tomorrow. A review, Environ. Sci.Pollut. Res. 21 (2014) 8336–8367.

[12] B.P. Chaplin, Critical review of electrochemical advanced oxidationprocesses for water treatment applications, Environ. Sci. Processes Impacts16 (2014) 1182–1203.

[13] H.J. Lewerenz, C. Heine, K. Skorupska, N. Szabo, T. Hannappel, T. Vo-Dinh,S.A. Campbell, H.W. Klemm, A.G. Munoz, Photoelectrocatalysis: principles,nanoemitter applications and routes to bio-inspired system, Energy Environ.Sci. 3 (2010) 748–760.

[14] A.G. Munoz, H.J. Lewerenz, Advances in photoelectrocatalysis withnanotopographical photoelectrodes, ChemPhysChem 11 (2010) 1603–1615.

[15] V. Augugliaro, G. Camera-Roda, V. Loddo, G. Palmisano, L. Palmisano, J. Soria,S. Yurdakal, Heterogeneous photocatalysis and photoelectrocatalysis: fromunselective abatement of noxious species to selective production ofhigh-value chemicals, J. Phys. Chem. Lett. 6 (2015) 1968–1981.

[16] T. Ochiai, A. Fujishima, Photoelectrochemical properties of TiO2

photocatalyst and its applications for environmental purification, J. Photoch.Photobio. C: Photochem. Rev. 13 (2012) 247–262.

[17] Y. Zhang, X. Xiong, Y. Han, X. Zhang, F. Shen, S. Deng, H. Xiao, X. Yang, G.Yang, H. Peng, Photoelectrocatalytic degradation of recalcitrant organicpollutants using TiO2 film electrodes: an overview, Chemosphere 88 (2012)145–154.

[18] R. Daghrir, P. Drogui, D. Robert, Photoelectrocatalytic technologies forenvironmental applications, J. Photoch. Photobio. A: Chem. 238 (2012)41–52.

[19] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surfacephenomena, Surf. Sci. Rep. 63 (2008) 515–582.

[20] D. Li, J. Qu, The progress of catalytic technologies in water purification: areview, J. Environ. Sci. 21 (2009) 713–719.

[21] I. Sirés, E. Brillas, Remediation of water pollution caused by pharmaceuticalresidues based on electrochemical separation and degradation technologies− A review, Environ. Int. 40 (2012) 212–229.

[22] J. Georgieva, E. Valova, S. Armyanov, N. Philippidis, I. Poulios, S. Sotiropoulos,Bi-component semiconductor oxide photoanodes for thephotoelectrocatalytic oxidation of organic solutes and vapors: a shortreview with emphasis to TiO2-WO3 photoanodes, J. Hazard. Mater. 211–212(2012) 30–46.

[23] G.G. Bessegato, T.T. Guaraldo, J. Ferreira de Brito, M.F. Brugnera, M.V.B.Zanoni, Achievements and trends in photoelectrocatalysis: fromenvironmental to energy applications, Electrocatalysis 6 (2015) 415–441.

[24] X. Meng, Z. Zhang, X. Li, Synergetic photoelectrocatalytic reactors forenvironmental remediation: a review, J. Photochem. Photobiol. C:
Photochem. Rev. 24 (2015) 83–101.
[25] M.A. Sousa, C. Gonc alves, V.J.P. Vilar, R.A.R. Boaventura, M.F. Alpendurada,Suspended TiO2-assisted photocatalytic degradation of emergingcontaminants in a municipal WWTP effluent using a solar pilot plant withCPCS, Chem. Eng. J. 198–199 (2012) 301–309.

http://refhub.elsevier.com/S1389-5567(16)30074-0/sbref0005




















































































































































































































































































































































































































































































































































































































































































































































3 nd Ph
[26] L. Chen, C. Zhao, D. Dionysiou, K.E. O’shea, TiO2 photocatalytic degradationand detoxification of cylindrospermopsin, J. Photochem. Photobiol. A: Chem.307–308 (2015) 115–122.

[27] P. Fernández-Ibánez, M.I. Polo-López, S. Malato, S. Wadhwa, J.W.J. Hamilton,P.S.M. Dunlop, R. D’sa, E. Magee, K. O’shea, D.D. Dionysiou, J.A. Byrne, Solarphotocatalytic disinfection of water using titanium dioxide graphenecomposites, Chem. Eng. J. 261 (2015) 36–44.

[28] T. Yazawa, F. Machida, N. Kubo, T. Jin, Photocatalytic activity of transparentporous glass supported TiO2, Ceram. Int. 35 (2008) 3321–3325.

[29] G. Palmisano, V. Loddo, H.H. El Nazer, S. Yurdakal, V. Augugliaro, R. Criminna,M. Pagliaro, Graphite-supported TiO2 for 4-nitrophenol degradation in aphotoelectrocatalytic reactor, Chem. Eng. J. 155 (2009) 339–346.

[30] F. Mazille, T. Schoettl, N. Klamerth, S. Malato, C. Pulgarin, Field solardegradation of pesticides and emerging water contaminants mediated bypolymer films containing titanium and iron oxide with synergisticheterogeneous photocatalytic activity at neutral pH, Water Res. 44 (2010)3029–3038.

[31] S. Garcia-Segura, S. Dosta, J.M. Guilemany, E. Brillas, Solarphotoelectrocatalytic degradation of Acid Orange 7 azo dye using a highlystable TiO2 photoanode synthesized by atmospheric plasma spray, Appl.Catal. B: Environ. 132–133 (2013) 142–150.

[32] A. Fujishima, K. Honda, Electrochemical photolysis of water at asemiconductor electrode, Nature 238 (1972) 37–38.

[33] J. O’M. Bockris, S.U.M. Khan, O.J. Murphy, M. Szklarczyk,;1 Technical brief onmodels of photoelectrocatalysis, Int. J. Hydrogen Energy 9 (1984) 243–244.

[34] A.Q. Contractor, J. O’M. Bockris, Photoelectrocatalysis of oxygen evolution onn-TiO2, Electrochim. Acta 32 (1987) 121–123.

[35] K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J.Photochem. Photobiol. C: Photochem. Rev. 13 (2012) 169–189.

[36] T. Hirakawa, C. Koga, N. Negishi, K. Takeuchi, S. Matsuzawa, An approach toelucidating photocatalytic reaction mechanisms by monitoring dissolvedoxygen: effect of H2O2 on photocatalysis, Appl. Catal. B: Environ. 87 (2009)46–55.

[37] R. Daghrir, P. Drogui, M.A. El Khakani, Photoelectrocatalytic oxidation ofchlortetracycline using Ti/TiO2 photo-anode with simultaneous H2O2

production, Electrochim. Acta 87 (2013) 18–31.[38] L.C. Almeida, B.F. Silva, M.V.B. Zanoni,

Photoelectrocatalytic/photoelectro-Fenton coupling system using ananostructured photoanode for the oxidation of a textile dye: kinetics studyand oxidation pathway, Chemosphere 136 (2015) 63–71.

[39] Y. Nosaka, A. Nosaka, Understanding hydroxyl radical (OH) GenerationProcesses in Photocatalysis, ACS Energy Lett. 1 (2016) 356–359.

[40] M. Panizza, G. Cerisola, Direct and mediated anodic oxidation of organicpollutants, Chem. Rev. 109 (2009) 6541–6569.

[41] E.B. Cavalcanti, S. García-Segura, F. Centellas, E. Brillas, Electrochemicalincineration of omeprazole in neutral aqueous medium using a platinum orboron-doped diamond. Degradation kinetics and oxidation products, WaterRes. 47 (2013) 1803–1815.

[42] X. Florenza, A.M.S. Solano, F. Centellas, C.A. Martínez-Huitle, E. Brillas, S.Garcia-Segura, Degradation of the azo dye Acid Red 1 by anodic oxidationand indirect electrochemical processes based on Fenton’s reactionchemistry. Relationship between decolorization, mineralization andproducts, Electrochim. Acta 142 (2014) 276–288.

[43] P.J. Huang, H. Chang, C.T. Yeh, C.W. Tsai, Phase transformation of TiO2

monitored by Thermo-Raman spectroscopy with TGA/DTA, Termochim.Acta 297 (1997) 85–92.

[44] W.H. Leng, Z. Zhang, J.Q. Zhang, Photoelectrocatalytic degradation of anilineover rutile TiO2/Ti electrode thermally formed at 600 ◦C, J. Mol. Catal. A:Chem. 206 (2003) 239–252.

[45] A. Di Paola, M. Bellardita, L. Palmisano, Brookite, the least known TiO2

photocatalyst, Catalysts 3 (2013) 36–73.[46] T. Luttrell, S. Halpegamage, J. Tao, A. Kramer, E. Sutter, M. Batzill, Why is

anatase a better photocatalyst than rutile? −Model studies on epitaxial TiO2

films, Sci. Rep. 4 (2015) 4043–4050.[47] R. Kaplan, B. Erjavec, G. Drazic, J. Grdadolnik, A. Pintar, Simple synthesis of

anatase/rutile/brookite TiO2 nanocomposite with superior mineralizationpotential for photocatalytic degradation of water pollutants, Appl. Catal. B:Environ. 18 (2016) 465–474.

[48] E. Lassner, W.D. Schubert, Tungsten: Properties, Chemistry, Technology ofthe Element, Alloys and Chemical Compounds, first ed., Springer, New York,1999.

[49] M. Yagi, S. Maruyama, K. Sone, K. Nagai, T. Norimatsu, Preparation andphotoelectrocatalytic activity of a nano-structured WO3 platelet film, J. SolidState Chem. 181 (2008) 175–182.

[50] Z. Xu, X. Li, J. Li, L. Wu, Q. Zeng, Z. Zhou, Effect of CoOOH loading on thephotoelectrocatalytic performance of WO3 nanorod array film, Appl. Surf.Sci. 284 (2013) 285–290.

[51] J.M. Stellman, Encyclopaedia of Occupational Health and Safety: Chemical,Industries and Occupations, fourth ed., International Labour Office, Geneva,1998.

[52] M. Hepel, J. Luo, Photoelectrochemical mineralization of textile diazo dye
pollutants using nanocrystalline WO3 electrodes, Electrochim. Acta 47(2001) 729–740.
[53] J. Luo, M. Hepel, Photoelectrochemical degradation of naphthol blue blackdiazo dye on WO3 film electrode, Electrochim. Acta 46 (2001) 2913–2922.


[54] L. Schmidt-Mende, J.L. MacManus-Driscoll, ZnO − nanostructures defects,and devices, Mater. Today 10 (2007) 40–48.

[55] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J.Cho, H. Morkoc , A comprehensive review of ZnO materials and devices, J.Appl. Phys. 98 (2005) 1–103.

[56] C. Klingshirn, ZnO: Material, physics and applications, ChemPhysChem 8(2007) 782–803.

[57] A. Janotti, C.G. Van de Walle, Fundamentals of zinc oxide as asemiconductor, Rep. Prog. Phys. 72 (2009) 126501 (29pp).

[58] Z. Li, F. Gao, W. Kang, Z. Chen, M. Wu, L. Wang, D. Pan, Layer-by-layergrowth of ultralong ZnO vertical wire arrays for enhancedphotoelectrocatalytic activity, Mater. Lett. 97 (2013) 52–55.

[59] M. Mahadik, S. Shinde, V. Mohite, S. Kumbhar, K. Rajpure, A. Moholkar, J.Kim, C. Bhosale, Photoelectrocatalytic oxidation of rhodamine B withsprayed �-Fe2O3 photocatalyst, Mater. Express 3 (2013) 247–255.

[60] M. Zhang, W. Pu, S. Pan, O.K. Okoth, C. Yang, J. Zhang, Photoelectrocatalyticactivity of liquid phase deposited �-Fe2O3 films under visible lightillumination, J. Alloy. Compd. 648 (2015) 719–725.

[61] S.E. LeBlanc, H.S. Fogler, The role of conduction/valence bands and redoxpotential in accelerated mineral dissolution, AIChE J. 32 (1986) 1702–1709.

[62] S. Yu, M. Xi, K. Han, Z. Wang, W. Yang, H. Zhu, Preparation andphotoelectrocatalytic properties of polyaniline/layered manganese oxideself-assembled film, Thin Solid Films 519 (2010) 357–361.

[63] C.M. Fan, B. Hua, Y. Wang, Z.H. Liang, X.G. Hao, S.B. Liu, Y.P. Sun, Preparationof Ti/SnO2-Sb2O4 photoanode by electrodeposition and dip coating for PECoxidations, Desalination 249 (2009) 736–741.

[64] Y. Wang, C. Fan, B. Hua, Z. Liang, Y. Sun, Photoelectrocatalytic activity of twoantimony doped SnO2 films for oxidation of phenol pollutants, Trans.Nonferreous Met. Soc. China 19 (2009) 778–783.

[65] J. Florencio, M.J. Pacheco, A. Lopes, L. Ciriaco, Application of Ti/Pt/�-PbO2

anodes in the degradation of DR80 azo dye, Port. Electrochim. Acta 31(2013) 257–264.

[66] M. Rodrigues da Silva, A.C. Lucilha, R. Afonso, L.H. Dall’Antonia, L.V.A. Scalvi,Photoelectrochemical properties of FTO/m-BiVO4 electrode in differentelectrolytes solutions under visible light irradiation, Ionics 20 (2014)105–113.

[67] S. Sun, W. Wang, L. Zhang, Efficient contaminant removal by Bi2WO6 filmswith nanoleaflike structures through a photoelectrocatalytic process, J.Phys. Chem. C 116 (2012) 19413–19418.

[68] C.S. Pan, Y.F. Zhu, New type of BiPO4 oxy-acid salt photocatalyst with highphotocatalytic activity on degradation of dye, Environ. Sci. Technol. 44(2010) 5570–5574.

[69] W. Liao, J. Yang, H. Zhou, M. Murugananthan, Y. Zhang, Electrochemicallyself-doped TiO2 nanotube arrays for efficient visible lightphotoelectrocatalytic degradation of contaminants, Electrochim. Acta 136(2014) 310–317.

[70] Q. Chen, H. Liu, Y. Xin, X. Cheng, Coupling immobilized TiO2 nanobelts andAu nanoparticles for enhanced photocatalytic and photoelectrocatalyticactivity and mechanism insights, Chem. Eng. J. 241 (2014) 145–154.

[71] J. Gong, W. Pu, C. Yang, J. Zhang, A simple electrochemical oxidation methodto prepare highly ordered Cr-doped titania nanotube arrays with promotedphotoelectrochemical property, Electrochim. Acta 68 (2012) 178–183.

[72] O. Kerkez, I. Boz, Photo(electro)catalytic activity of Cu2+-modified TiO2

nanorod array thin films under visible light irradiation, J. Phys. Chem. Solids75 (2014) 611–618.

[73] W. Tang, X. Chen, J. Xia, J. Gong, X. Zeng, Preparation of an Fe-dopedvisible-light-response TiO2 film electrode and its photoelectrocatalyticactivity, Mater. Sci. Eng. B: Adv. 187 (2014) 39–45.

[74] G.C. Bessegato, J.C. Cardoso, M.V.B. Zanoni, Enhanced photoelectrocatalyticdegradation of an acid dye with boron-doped TiO2 nanotube anodes, Catal.Today 240 (2015) 100–106.

[75] Z.-L. Cheng, S. Han, Preparation and photoelectrocatalytic performance ofN-doped TiO2/NaY zeolite membrane composite electrode material, WaterSci. Technol. 73 (2016) 486–492.

[76] Z. Zhang, Y. Yu, P. Wang, Hierarchical top-porous/bottom-tubular TiO2

nanostructures decorated with Pd nanoparticles for efficientphotoelectrocatalytic decomposition of synergistic pollutants, ACS Appl.Mater. Interfaces 4 (2012) 990–996.

[77] K. Xie, L. Sun, C. Wang, Y. Lai, M. Wang, H. Chen, C. Lin, Photoelectrocatalyticproperties of Ag nanoparticles loaded TiO2 nanotube arrays prepared bypulse current deposition, Electrochim. Acta 55 (2010) 7211–7218.

[78] A. Pandikumar, S. Murugesan, R. Ramaraj, Functionalized silicatesol-gel-supported TiO2-Au core-shell nanomaterials and theirphotoelectrocatalytic activity, ACS Appl. Mater. Interface 2 (2010)1912–1917.

[79] Z. Zhou, L. Zhu, J. Li, H. Tang, Electrochemical preparation of TiO2/SiO2

composite film and its high activity toward the photoelectrocatalyticdegradation of methyl orange, J. Appl. Electrochem. 39 (2009) 1745–1753.

[80] L.E. Fraga, J.H. Franco, M.O. Orlandi, M.V.B. Zanoni, Photoelectrocatalyticoxidation of hair dye basic red 51 at W/WO3/TiO2 bicomposite photoanodeactivated by ultraviolet and visible radiation, J. Env. Chem. Eng. 1 (2013)
194–199.
[81] X. Hu, J. Yang, J. Zhang, Magnetic loading of TiO2/SiO2/Fe3O4 nanoparticleson electrode surface for photoelectrocatalytic degradation of diclofenac, J.Hazard. Mater. 196 (2011) 220–227.






























































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































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[82] S. Chai, G. Zhao, P. Li, Y. Lei, Y. Zhang, D. Li, Novel sieve-like SnO2/TiO2

nanotubes with integrated photoelectrocatalysis: fabrication andapplication for efficient toxicity elimination of nitrophenol wastewater, J.Phys. Chem. C 115 (2011) 18261–18269.

[83] W. Siripala, A. Ivanovskaya, T.F. Jaramillo, S. Baeck, E.W. McFarland, ACu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis, Sol.Energ. Mat. Sol C 77 (2003) 229–237.

[84] G. Li, L. Wu, F. Li, P. Xu, D. Zhang, H. Li, Photoelectrocatalytic degradation oforganic pollutants via a CdS quantum dots enhanced TiO2 nanotube arrayelectrode under visible light irradiation, Nanoscale 5 (2013) 2118–2125.

[85] G.G. Bessegato, J.C. Cardoso, B. Ferreira da Silva, M.V.B. Zanoni, Enhancedphotoabsorption properties of composites of Ti/TiO2 nanotubes decoratedby Sb2S3 and improvement of degradation of hair dye, J. Photochem.Photobiol. A: Chem. 276 (2013) 96–103.

[86] L. Yang, Z. Li, H. Jiang, W. Jiang, R. Su, S. Luo, Y. Luo, Photoelectrocatalyticoxidation of bisphenol A over mesh of TiO2/graphene/Cu2O, Appl. Catal. B:Environ. 183 (2016) 75–85.

[87] O. Zhang, B. Lin, Y. Chen, B. Gao, L. Fu, B. Li, Electrochemical andphotoelectrochemical characteristics of TiNbO5 nanosheet electrode,Electrochim. Acta 81 (2012) 74–82.

[88] H.S. Kushwaha, N.A. Madhar, B. Ilahi, P. Thomas, A. Halder, R. Vaish, Efficientsolar energy conversion using CaCu3Ti4O12 photoanode for photocatalysisand photoelectrocatalysis, Sci. Reports 6 (2016) 18557.

[89] J.C. Cardoso, T.M. Lizier, M.V.B. Zanoni, Highly ordered TiO2 nanotube arraysand photoelectrocatalytic oxidation of aromatic amine, Appl. Catal. B:Environ. 99 (2010) 96–102.

[90] M.V.B. Zanoni, J.J. Sene, M.A. Anderson, Photoelectrocatalytic degradation ofRemazol Brilliant Orange 3R on titanium dioxide thin-film electrodes, J.Photoch. Photobio. A: Chem. 157 (2003) 55–63.

[91] W.Y. Gan, M.W. Lee, R. Amal, H. Zao, K. Chiang, Photoelectrocatalytic activityof mesoporous TiO2 films prepared using the sol-gel method with tri-blockcopolymer as structure directing agent, J. Appl. Electrochem. 38 (2008)703–712.

[92] J. Marugán, P. Christensen, T. Egerton, H. Purnama, Synthesis,characterization and activity of photocatalytic sol-gel TiO2 powders andelectrodes, Appl. Catal. B: Environ. 89 (2009) 273–283.

[93] J. Li, L. Li, L. Zheng, Y. Xian, L. Jin, Photoelectrocatalytic degradation ofrhodamine B using Ti/TiO2 electrode prepared by laser calcination method,Electrochim. Acta 51 (2006) 4942–4949.

[94] P.S. Shinde, S.B. Sadale, P.S. Patil, P.N. Bhosale, A. Brüger, M.Neumann-Spallart, C.H. Bhosale, Properties of spray deposited titaniumdioxide thin films and their application in photoelectrocatalysis, Sol. Energ.Mat. Sol. C 92 (2008) 283–290.

[95] P.S. Shinde, P.S. Patil, P.N. Bhosale, A. Brüger, G. Nauer, M.Neumann-Spallart, C.H. Bhosale, UVA and solar light assistedphotoelectrocatalytic degradation of AO7 dye in water using spraydeposited TiO2 thin films, Appl. Catal. B: Environ. 89 (2009) 288–294.

[96] L. Sirghi, T. Aoki, Y. Hatanaka, Friction force microscopy study of thehydrophilicity of TiO2 thin films deposited by radio frequency magnetronsputtering, Surf. Rev. Lett. 10 (2003) 345–349.

[97] C. He, X.Z. Li, N. Graham, Y. Wang, Preparation of TiO2/ITO and TiO2/Tiphotoelectrodes by magnetron sputtering for photocatalytic application,Appl. Catal. A: Gen. 305 (2006) 54–63.

[98] G. Li Puma, A. Bono, J.G. Collin, Preparation of titanium dioxidephotocatalyst loaded onto activated carbon support using chemical vapourdeposition: a review paper, J. Hazard. Mater. 157 (2008) 209–219.

[99] M.L. Hitchman, F. Tian, Studies of TiO2 thin films prepared by chemicalvapour deposition for photocatalytic and photoelectrocatalytic degradationof 4-chlorophenol, J. Electroanal. Chem. 538-539 (2002) 165–172.

100] M. Heikkilä, E. Puukilainen, M. Ritala, M. Leskelä, Effect of thickness of ALDgrown TiO2 films on photoelectrocatalysis, J. Photochem. Photobiol. A:Chem. 204 (2009) 200–208.

101] T. Wang, Z. Luo, C. Li, J. Gong, Controllable fabrication of nanostructuredmaterials for photoelectrochemical water splitting via atomic layerdeposition, Chem. Soc. Rev. 43 (2014) 7469–7484.

102] M. Gardon, J.M. Guilemany, Milestones in functional titanium dioxidethermal spray coatings: a review, J. Therm. Spray Technol. 23 (2014)577–595.

103] Y. Su, X. Zhang, M. Zhou, S. Han, L. Lei, Preparation of high efficientphotoelectrode of N-F-codoped TiO2 nanotubes, J. Photochem. Photobiol. A:Chem. 194 (2008) 152–160.

104] Y. Xie, D. Fu, Photoelectrocatalysis reactivity of independent titaniananotubes, J. Appl. Electrochem. 40 (2010) 1281–1291.

105] Q. Wang, J. Shang, T. Zhu, F. Zhao, Efficient photoelectrocatalytic reductionof Cr(VI) using TiO2 nanotube arrays as the photoanode and a large-areatitanium mesh as the photocathode, J. Mol. Catal. A: Chem. 335 (2011)242–247.

106] A. Zhang, M. Zhou, L. Liu, W. Wang, Y. Jiao, Q. Zhou, A novelphotoelectrocatalytic system for organic contaminant degradation on a TiO2

nanotube (TNT)/Ti electrode, Electrochim. Acta 55 (2010) 5091–5099.107] H. Liu, X. Cao, G. Liu, Y. Wang, N. Zhang, T. Li, R. Tough, Photoelectrocatalytic

degradation of triclosan on TiO2 nanotube arrays and toxicity change,Chemosphere 93 (2013) 160–165.

108] D. Tekin, B. Saygi, Photoelectrocatalytic decomposition of acid black 1 dyeusing TiO2 nanotubes, J. Environ. Chem. Eng. 1 (2013) 1057–1061.


[109] X. Cheng, G. Pan, X. Yu, Construction of high-efficient photoelectrocatalyticsystem by coupling with TiO2 nano-tubes photoanode and activecarbono/polytetrafluoroethylene cathode and its enhancedphotoelectrocatalytic degradation of 2,4-dichlorophene and mechanism,Chem. Eng. J. 279 (2015) 264–272.

[110] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, A novel method for thesynthesis of titania nanotubes using sonoelectrochemical method and itsapplication for photoelectrochemical splitting water, J. Catal. 246 (2007)362–369.

[111] Y. Liu, K. Mu, G. Yang, H. Peng, F. Shen, L. Wang, S. Deng, X. Zhang, Y. Zhang,Fabrication of a coral/double-wall TiO2 nanotube array film electrode withhigher photoelectrocatalytic activity under sunlight, New. J. Chem. 39(2015) 3923–3928.

[112] S. Dosta, M. Robotti, S. Garcia-Segura, E. Brillas, I.G. Cano, J.M. Guilemany,Influence of atmospheric plasma spraying on the solarphotoelectron-catalytic properties of TiO2 coatings, Appl. Catal. B: Environ.189 (2016) 151–159.

[113] J.F. Fu, Y.Q. Zhao, X.D. Xue, W.C. Li, A.O. Babatunde, Multivariate-parameteroptimization of acid blue-7 wastewater treatment by Ti/TiO2

photoelectrocatalysis via the Box-Behnken design, Desalination 243 (2009)42–51.

[114] Y. Su, S. Han, X. Zhang, X. Chen, L. Lei, Preparation and visible-light-drivenphotoelectrocatalytic properties of boron-doped TiO2 nanotubes, Mater.Chem. Phys. 110 (2008) 239–246.

[115] M.J. Martin de Vidales, L. Mais, C. Saez, P. Canizares, F.C. Walsh, M.A.Rodrigo, C.D.A. Rodrigues, C. Ponce de Leon, Photoelectrocatalytic oxidationof methyl orange on a TiO2 nanotubular anode using a flow cell, Chem. Eng.Technol. 39 (2016) 135–141.

[116] H. Selcuk, J.J. Sene, M.A. Anderson, Photoelectrocatalytic humic aciddegradation kinetics and effect of pH, applied potential and inorganic ions, J.Chem. Technol. Biotechnol. 78 (2003) 979–984.

[117] X. Ding, Z. Ai, L.A. Zhang, Dual-cell wastewater treatment system withcombining anodic visible light driven photoelectro-catalytic oxidation andcathodic electro-Fenton oxidation, Sep. Purif. Technol. 125 (2014) 103–110.

[118] F.C. Moreira, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Electrochemicaladvanced oxidation processes: a review on their application to syntheticand real wastewaters, Appl. Catal. B: Environ. 202 (2017) 217–261.

[119] E.J. Ruiz, C. Arias, E. Brillas, A. Hernández-Ramírez, J.M. Peralta-Hernández,Mineralization of Acid Yellow 36 azo dye by electro-Fenton and solarphotoelectro-Fenton processes with a boron-doped diamond anode,Chemosphere 82 (2011) 495–501.

[120] M.E. Olya, A. Pirkarami, M. Soleimani, M. Bahmaei, Photoelectrocatalyticdegradation of acid dye using Ni-TiO2 with the energy supplied by solar cell:mechanism and economical studies, J. Environ. Manage. 121 (2013)210–219.

[121] A. Pirkarami, M.E. Olya, S.R. Farshid, UV/Ni-TiO2 nanocatalyst forelectrochemical removal of dyes considering operating costs, Water Res.Ind. 5 (2014) 9–20.

[122] F.-B. Li, X.-Z. Li, Y.-H. Kang, X.-J. Li, An innovative Ti/TiO2 meshphotoelectrode for methyl orange photoelectrocatalytic degradation, J.Environ. Sci. Health A: Toxic/Hazard. Subst. Environ. Eng. A37 (2002)623–640.

[123] M. Li, L. Xiong, Y. Chen, N. Zhang, Y. Zhang, H. Yin, Studies onphoto-electro-chemical catalytic degradation of Acid Scarlet 3R dye, Sci.China B: Chem. 48 (2005) 297–304.

[124] Y.-F. Su, T.-C. Chou, Comparison of the photocatalytic andphotoelectrocatalytic decolorization of methyl orange on sputtered TiO2

thin films, Z. Naturfor. B: Chem. Sci. 60 (2005) 1158–1167.[125] T.A. Egerton, H. Purnama, S. Purwajanti, M. Zafar, Decolourization of dye

solutions using photoelectrocatalysis and photocatalysis, J. Adv.Oxid.Technol. 9 (2006) 79–85.

[126] Y.B. Xie, X.Z. Li, Interactive oxidation of photoelectrocatalysis andelectro-Fenton for azo dye degradation using TiO2-Ti mesh and reticulatedvitreous carbon electrodes, Mater. Chem. Phys. 95 (2006) 39–50.

[127] L.C. Macedo, D.A.M. Zaia, G.J. Moore, H. de Santana, Degradation of leatherdye on TiO2: a study of applied experimental parameters onphotoelectrocatalysis, J. Photochem. Photobiol. A: Chem. 185 (2007) 86–93.

[128] B. Liu, L. Wen, X. Zhao, Efficient degradation of aqueous methyl orange overTiO2 and CdS electrodes using photoelectrocatlysis under UV and visiblelight irradiation, Progress Org. Coat. 64 (2009) 120–123.

[129] J. Li, J. Wang, L. Huang, G. Lu, Photoelectrocatalytic degradation of methylorange over mesoporous film electrodes, Photochem. Photobiol. Sci. 9(2010) 39–46.

[130] M. Neumann-Spallart, Photoelectrochemistry on a planar, interdigitatedelectrochemical cell, Electrochim. Acta 56 (2011) 8752–8757.

[131] H. Su, H. Du, Study on photoelectrocatalysis of three-dimensional electrodeusing TiO2 coatings particle electrode, Adv. Mater. Res. 156–157 (2011)344–349.

[132] W. Zhanga, J. Bai, J. Fu, Photoelectrocatalytic degradation of methyl orangeon porous TiO2 film electrode in NaCl solution, Adv. Mater. Res. 213 (2011)15–19.

[133] W. Zhang, Q. Li, H. He, Photoelectrocatalytic properties of porous TiO2 filmsprepared using ODA as template, Adv. Mater. Res. 457–458 (2012) 521–524.

[134] T.N.M. Cervantes, D.A.M. Zaia, G.J. Moore, H. Santana, Photoelectrocatalysisstudy of the decolorization of synthetic azo dye mixtures on Ti/TiO2,Electrocatalysis 4 (2013) 85–91.








































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































3 nd Ph
[135] B. Tan, Y. Zhang, M. Long, Large-scale preparation of nanoporous TiO2 filmon titanium substrate with improved photoelectrochemical performance,Nanoscale Res. Lett. 9 (2014) 190 (6 p.).

[136] C.-F. Liu, C.P. Huang, C.-C. Hu, Y. Juang, C. Huang, Photoelectrochemicaldegradation of dye wastewater on TiO2-coated titanium electrode preparedby electrophoretic deposition, Sep. Purif. Technol. 165 (2016) 145–153.

[137] L.G. Devi, N. Kottam, Enhanced activity of nano-TiO2 in photoelectrocatalysisover photocatalysis process for the degradation of Indanthrene BR Violetdye in presence of UV-light, Polish J. Chem. 81 (2007) 1819–1827.

[138] M.E. Osugi, M.V.B. Zanoni, C.R. Chenthamarakshan, N.R. de Tacconi, G.A.Woldemariam, S.S. Mandal, K. Rajeshwar, Toxicity assessment anddegradation of disperse azo dyes by photoelectrocatalytic oxidation onTi/TiO2 nanotubular array electrodes, J. Adv. Oxid. Technol. 11 (2008)425–434.

[139] J. Bai, B. Zhou, J. Li, W. Cai, Enhanced photoelectrocatalytic degradation ofazo-dye pollutants using transparent titania nanotube arrays glasselectrode, Adv. Mater. Res. 311–313 (2011) 2089–2092.

[140] E.R.A. Ferraz, G.A.R. Oliveira, M.D. Grando, T.M. Lizier, M.V.B. Zanoni, D.P.Oliveira, Photoelectrocatalysis based on Ti/TiO2 nanotubes removes toxicproperties of the azo dyes Disperse Red 1, Disperse Red 13 and DisperseOrange 1 from aqueous chloride samples, J. Environ. Manage. 124 (2013)108–114.

[141] M.V.B. Zanoni, T.T. Guaraldo, Photoelectrochemical hydrogen generationand concomitant organic dye oxidation under TiO2 nanotube, ECS Trans. 50(2013) 63–70.

[142] S. Franz, D. Perego, O. Marchese, M. Bestetti, Photoelectrochemical advancedoxidation processes on nanostructured TiO2 catalysts: decolorization of atextile azo-dye, Khim. Tekhnol. Vody 37 (2015) 207–219.

[143] G.G. Bessegato, J.C. Cardoso, B. Ferreira da Silva, M.V.B. Zanoni, Combinationof photoelectrocatalysis and ozonation: a novel and powerful approachapplied in Acid Yellow 1 mineralization, Appl. Catal. B: Environ. 180 (2016)161–168.

[144] E.C. Rodrigues, L.A. Soares, J.A. Modenes Jr., J.J. Sene, G. Bannach, C. Carvalho,M. Ionashiro, Synthesis and characterization of Fe(III)-doped ceramicmembranes of titanium dioxide and its application in photoelectrocatalysisof a textile dye, Eclet. Quim. 36 (2011) 0002, 19 pp.

[145] J. Gong, W. Pu, C. Yang, J. Zhang, Tungsten and nitrogen co-doped TiO2

electrode sensitized with Fe-chlorophyllin for visible lightphotoelectrocatalysis, Chem. Eng. J. 209 (2012) 94–101.

[146] H. Liao, W. Zhang, X. Sun, L. Shi, M. Qin, Synthesis and photoelectrocatalyticproperty of two-nonmetal-codoped TiO2 nanotube arrays with high aspectratio, Adv. Mater. Res. 412 (2012) 219–222.

[147] F.-J. Zhang, M.-L. Chen, W.-C. Oh, Visible light photoelectrocatalyticdegradation of methylene blue by platinum-treated carbonnanotube/titania composites, Asian J. Chem. 22 (2010) 5636–5648.

[148] G. Wang, S. Chen, H. Yu, X. Quan, Integration of membrane filtration andphotoelectrocatalysis using a TiO2/carbon/Al2O3 membrane for enhancedwater treatment, J. Hazard. Mater. 299 (2015) 27–34.

[149] Y.-N. Zhang, G. Zhao, Y. Lei, P. Li, M. Li, Y. Jin, B. Lv, CdS-encapsulated TiO2

nanotube arrays lidded with ZnO nanorod layers and theirphotoelectrocatalytic applications, ChemPhysChem 11 (2010) 3491–3498.

[150] Y. Shen, F. Li, D. Liu, S. Li, L. Fan, Fabrication of TiO2 nanotube films modifiedwith Ag2S and photoelectrocatalytic decolorization of methyl orange undersolar light, Sci. Adv. Mater. 4 (2012) 1214–1219.

[151] D. Wang, X. Li, J. Chen, X. Tao, Enhanced photoelectrocatalytic activity ofreduced graphene oxide/TiO2 composite films for dye degradation, Chem.Eng. J. 198–199 (2012) 547–554.

[152] M. Li, G. Zhao, P. Li, Y. Zhang, M. Wu, Photoelectrocatalytic properties of avertically aligned Ti-W alloy oxide nanotubes array and its applications indye wastewater degradation, Environ. Technol. 33 (2012) 191–199.

[153] Q. Wang, J. Shang, H. Song, T. Zhu, J. Ye, F. Zhao, J. Li, S. He, Visible-lightphotoelectrocatalytic degradation of rhodamine B over planar devices usinga multi-walled carbon nanotube-TiO2 composite, Mater. Sci. Semicond.Process. 16 (2013) 480–484.

[154] C. Zhai, M. Zhu, F. Ren, Z. Yao, Y. Du, P. Yang, Enhanced photoelectrocatalyticperformance of titanium dioxide/carbon cloth based photoelectrodes bygraphene modification under visible-light irradiation, J. Hazard. Mater. 263(2013) 291–298.

[155] M. Zhang, C. Yang, W. Pu, Y. Tan, K. Yang, J. Zhang, Liquid phase deposition ofWO3/TiO2 heterojunction films with high photoelectrocatalytic activityunder visible light irradiation, Electrochim. Acta 148 (2014) 180–186.

[156] T.T. Guaraldo, V.R. Goncales, B.F. Silva, S.I.C. de Torresi, M.V.B. Zanoni,Hydrogen production and simultaneous photoelectrocatalytic pollutantoxidation using a TiO2/WO3 nanostructured photoanode under visible lightirradiation, J. Electroanal. Chem. 765 (2016) 188–196.

[157] W. Wang, F. Li, D. Zhang, D.Y.C. Leung, G. Li, Photoelectrocatalytic hydrogengeneration and simultaneous degradation of organic pollutant via CdSe/TiO2

nanotube arrays, Appl. Surf. Sci. 362 (2016) 490–497.[158] J. Liu, L. Ruan, S.B. Adeloju, Y. Wu, BiOI/TiO2 nanotube arrays, a unique

flake-tube structured p-n junction with remarkable visible-lightphotoelectrocatalytic performance and stability, Dalton Trans. 43 (2014)
1706–1715.
[159] X. Zhao, H. Liu, J. Qu, Photoelectrocatalytic degradation of organiccontaminant at hybrid BDD-ZnWO4 electrode, Catal. Commun. 12 (2010)76–79.


[160] L.E. Fraga, M.V.B. Zanoni, Nanoporous of W/WO3 thin film electrode grownby electrochemical anodization applied in the photoelectrocatalyticoxidation of the Basic Red 51 used in hair dye, J. Braz. Chem. Soc. 22 (2011)718–725.

[161] P. Chatchai, A.Y. Nosaka, Y. Nosaka, Photoelectrocatalytic performance ofWO3/BiVO4 toward the dye degradation, Electrochim. Acta 94 (2013)314–319.

[162] S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Solar light assisted photocatalysis ofwater using a zinc oxide semiconductor, J. Semicond. 34 (2013),043002/1-043002/4.

[163] Q. Zeng, J. Bai, J. Li, L. Xia, K. Huang, X. Li, B. Zhou, A novel in situ preparationmethod for nanostructured �-Fe2O3 films from electrodeposited Fe films forefficient photoelectrocatalytic water splitting and the degradation of organicpollutants, J. Mater. Chem. A: Mater. Energy Sustain. 3 (2015) 4345–4353.

[164] T. Zeng, X. Yu, K.-H. Ye, Z. Qiu, Y. Zhu, Y. Zhang, BiPO4 film on ITO substratesfor photoelectrocatalytic degradation, Inorg. Chem. Commun. 58 (2015)39–42.

[165] Y. Cong, J. Wang, H. Jin, X. Feng, Q. Wang, Y. Ji, Y. Zhang, Enhancedphotoelectrocatalytic activity of a novel Bi2O3-BiPO4 composite electrodefor the degradation of refractory pollutants under visible light irradiation,Ind. Eng. Chem. Res. 55 (2016) 1221–1228.

[166] S.V. Mohite, V.V. Ganbavle, K.Y. Rajpure, Solar photoelectrocatalyticactivities of rhodamine-B using sprayed WO3 photoelectrode, J. AlloysComp. 655 (2016) 106–113.

[167] S.M.A. Jorge, J.J. de Sene, A.O. Florentino, Photoelectrocatalytic treatment ofp-nitrophenol using Ti/TiO2 thin-film electrode, J. Photochem. Photobiol. A:Chem. 174 (2005) 71–75.

[168] T. An, Y. Xiong, G. Li, X. Zhu, G. Sheng, J. Fu, Improving ultraviolet lighttransmission in a packed-bed photoelectrocatalytic reactor for removal ofoxalic acid from wastewater, J. Photochem. Photobiol. A: Chem. 181 (2006)158–165.

[169] H. Selcuk, M. Bekbolet, Photocatalytic and photoelectrocatalytic humic acidremoval and selectivity of TiO2 coated photoanode, Chemosphere 73 (2008)854–858.

[170] L.E. Fraga, M.A. Anderson, M.L.P.M.A. Beatriz, F.M.M. Paschoal, L.P. Romao,M.V.B. Zanoni, Evaluation of the photoelectrocatalytic method for oxidizingchloride and simultaneous removal of microcystin toxins in surface waters,Electrochim. Acta 54 (2009) 2069–2076.

[171] H. Chen, D. Li, X. Li, J. Li, Q. Chen, B. Zhou, Adsorption andphotoelectrocatalytic characteristics of organics on TiO2 nanotube arrays, J.Solid State Electrochem. 16 (2012) 3907–3914.

[172] V.M. Daskalaki, I. Fulgione, Z. Frontistis, L. Rizzo, D. Mantzavinos, Solarlight-induced photoelectrocatalytic degradation of bisphenol-A on TiO2/ITOfilm anode and BDD cathode, Catal. Today 209 (2013) 74–78.

[173] L. Suhadolnik, A. Pohar, B. Likozar, M. Ceh, Mechanism and kinetics of phenolphotocatalytic, electrocatalytic and photoelectrocatalytic degradation in aTiO2-nanotube fixed-bed microreactor, Chem. Eng. J. 303 (2016) 292–301.

[174] J. Gong, C. Yang, W. Pu, J. Zhang, Liquid phase deposition of tungsten dopedTiO2 films for visible light photoelectrocatalytic degradation ofdodecyl-benzenesulfonate, Chem. Eng. J. 167 (2011) 190–197.

[175] K. Yang, W. Pu, Y. Tan, M. Zhang, C. Yang, J. Zhang, Enhancedphotoelectrocatalytic activity of Cr-doped TiO2 nanotubes modified withpolyaniline, Mater. Sci. Semicond. Process. 27 (2014) 777–784.

[176] C. He, Y. Xiong, J. Chen, C. Zha, X. Zhu, Photoelectrochemical performance ofAg-TiO2/ITO film and photoelectrocatalytic activity towards the oxidation oforganic pollutants, J. Photochem. Photobiol. A: Chem. 157 (2003) 71–79.

[177] X.Z. Li, C. He, N. Graham, Y. Xiong, Photoelectrocatalytic degradation ofbisphenol A in aqueous solution using Au-TiO2/ITO film, J. Appl.Electrochem. 35 (2005) 741–750.

[178] M. Antoniadou, V.M. Daskalaki, N. Balis, D.I. Kondarides, C. Kordulis, P.Lianos, Photocatalysis and photoelectrocatalysis using (CdS-ZnS)-TiO2

combined photocatalysts, Appl. Catal. B: Environ. 107 (2011) 188–196.[179] H. Su, H. Du, Study on photoelectrocatalytic of three-dimensional electrode

using TiO2 coated -Al2O3 and scrap iron particle electrode, Appl. Mech.Mater. 71–78 (2011) 972–975.

[180] H. Su, H. Du, Photoelectrocatalytic performance of TiO2 film treatment ofLa(NO3)3, Energy Procedia 11 (2011) 2333–2338.

[181] X. Zhao, H. Liu, J. Qu, Photoelectrocatalytic degradation of organiccontaminants at Bi2O3/TiO2 nanotube array electrode, Appl. Surf. Sci. 257(2011) 4621–4624.

[182] W. Liao, Y. Zhang, M. Zhang, M. Murugananthan, S. Yoshihara,Photoelectrocatalytic degradation of microcystin-LR using Ag/AgCl/TiO2

nanotube arrays electrode under visible light irradiation, Chem. Eng. J. 231(2013) 455–463.

[183] N. Mojumder, S. Sarker, S.A. Abbas, Z. Tian, V. Subramanian, Photoassistedenhancement of the electrocatalytic oxidation of formic acid on platinizedTiO2 nanotubes, ACS Appl. Mater. Interface 6 (2014) 5585–5594.

[184] S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Photodegradation of organicpollutants using N-titanium oxide catalyst, J. Photochem. Photobiol. B: Biol.141 (2014) 186–191.

[185] J. Pan, X. Li, Q. Zhao, T. Li, M. Tade, S. Liu, Construction of Mn0.5Zn0.5Fe2O4

modified TiO2 nanotube array nanocomposite electrodes and theirphotoelectrocatalytic performance in the degradation of 2,4-DCP, J. Mater.Chem. C 3 (2015) 6025–6034.

[186] S.Y. Yang, W. Choi, H. Park, TiO2 nanotube array photoelectrocatalyst andNi-Sb-SnO2 electrocatalyst bifacial electrodes: a new type of bifunctional










































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































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photoelectrocatalysis on landfill leachate using codoped TiO2/Tiphotoelectrodes, J. Nanomater. (2015), ID 810579, 11p.

[206] J.C. Cardoso, G.C. Bessegato, M.V.B. Zanoni, Efficiency comparison ofozonation photolysis, photocatalysis and photoelectrocatalysis methods inreal textile wastewater decolorization, Water Res. 98 (2016) 39–46.


hybrid platform for water treatment, ACS Appl. Mater. Interface 7 (2015)1907–1914.

187] J. Su, L. Zhu, P. Geng, G. Chen, Self-assembly graphitic carbon nitridequantum dots anchored on TiO2 nanotube arrays: an efficientheterojunction for pollutants degradation under solar light, J. Hazard. Mater.316 (2016) 159–168.

188] X. Zhao, T. Xu, W. Yao, C. Zhang, Y. Zhu, Photoelectrocatalytic degradation of4-chlorophenol at Bi2WO6 nanoflake film electrode under visible lightirradiation, Appl. Catal. B: Environ. 72 (2007) 92–97.

189] S. Nissen, B.D. Alexander, I. Dawood, M. Tillotson, R.P.K. Wells, D.E. Macphee,K. Killham, Remediation of a chlorinated aromatic hydrocarbon in water byphotoelectrocatalysis, Environ. Pollut. 157 (2008) 72–76.

190] M. Fan, C. Yang, W. Pu, J. Zhang, Liquid phase deposition of ZnO film forphotoelectrocatalytic degradation of p-nitrophenol, Mater. Sci. Semicond.Process. 17 (2014) 104–109.

191] H.C.A. Valdez, G.G. Jiménez, S.G. Granados, C. Ponce de León, Degradation ofparacetamol by advanced oxidation processes using modified reticulatedvitreous carbon electrodes with TiO2 and CuO/TiO2/Al2O3, Chemosphere 89(2012) 1195–1201.

192] C. Liu, D. Fu, H. Li, Behaviour of multi-component mixtures of tetracyclineswhen degraded by photoelectrocatalytic and electrocatalytic technologies,Environ. Technol. 33 (2012) 791–799.

193] R. Daghrir, P. Drogui, I. Ka, M.A. El Khakani, Photoelectrocatalyticdegradation of chlortetracycline using Ti/TiO2 nanostructured electrodesdeposited by means of a Pulsed Laser Deposition process, J. Hazard. Mater.199–200 (2012) 15–24.

194] X. Nie, J. Chen, G. Li, H. Shi, H. Zhao, P.-K. Wong, T. An, Synthesis andcharacterization of TiO2 nanotube photoanode and its application inphotoelectrocatalytic degradation of model environmental pharmaceuticals,J. Chem. Technol. Biotechnol. 88 (2013) 1488–1497.

195] X. Cheng, H. Liu, Q. Chen, J. Li, P. Wang, Enhanced photoelectrocatalyticperformance for degradation of diclofenac and mechanism with TiO2

nano-particles decorated TiO2 nano-tubes arrays photoelectrode,Electrochim. Acta 108 (2013) 203–210.

196] R. Li, S.E. Williams, Q. Li, J. Zhang, C. Yang, A. Zhou, Photoelectrocatalyticdegradation of ofloxacin using highly ordered TiO2 nanotube arrays,Electrocatalysis 5 (2014) 379–386.


[197] Y.-F. Su, G.-B. Wang, D.T.F. Kuo, M.-L. Chang, Y.-H. Shih,Photoelectrocatalytic degradation of the antibiotic sulfamethoxazole usingTiO2/Ti photoanode, Appl. Catal. B: Environ. 186 (2016) 184–192.

[198] L. Liu, R. Li, Y. Liu, J. Zhang, Simultaneous degradation of ofloxacin andrecovery of Cu(II) by photoelectrocatalysis with highly ordered TiO2

nanotubes, J. Hazard. Mater. 308 (2016) 264–275.[199] Z. Hua, Z. Dai, X. Bai, Z. Ye, P. Wang, H. Gu, X. Huang, Copper nanoparticles

sensitized TiO2 nanotube arrays electrode with enhancedphotoelectrocatalytic activity for diclofenac degradation, Chem. Eng. J. 283(2016) 514–523.

[200] X. Cheng, Q. Cheng, X. Deng, P. Wang, H. Liu, A facile and novel strategy tosynthesize reduced TiO2 nanotubes photoelectrode for photoelectrocatalyticdegradation of diclofenac, Chemosphere 144 (2016) 888–894.

[201] H. Selcuk, J.J. Sene, H.Z. Sarikaya, M. Bekbolet, M.A. Anderson, An innovativephotocatalytic technology in the treatment of river water containing humicsubstances, Water Sci. Technol. 49 (2004) 153–158.

[202] S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Hydroxyl radical’s role in theremediation of wastewater, J. Photochem. Photobio. B: Biol. 116 (2012)66–74.

[203] T. Fang, L. Liao, X. Xu, J. Peng, Y. Jing, Removal of COD and colour in realpharmaceutical wastewater by photoelectrocatalytic oxidation method,Environ. Technol. 34 (2013) 779–786.

[204] K. Li, H. Zhang, T. Tang, Y. Wang, Y. Xu, D. Ying, J. Jia, Optimization andapplication of TiO2/Ti-Pt photo fuel cell (PFC) to effectively generateelectricity and degrade organic pollutants simultaneously, Water Res. 62(2014) 1–10.

[205] X. Zhou, Y. Zheng, J. Zhou, S. Zhou, Degradation kinetics of



























































































































































































































































































































































































































































































































































































































































































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