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Research paper

Fe-clay-plate as a heterogeneous catalyst in photo-Fenton oxidation of phenol as probe molecule for water treatment

Haithem Bel Hadjltaief a, Patrick Da Costa b,⁎, Patricia Beaunier c,d, María Elena Gálvez e, Mourad Ben Zina a

a Laboratoire Eau, Energie et Environnement (LR3E), Code: AD-10-02, Ecole Nationale d'Ingénieurs de Sfax, Université de Sfax, B.P1173.W.3038 Sfax, Tunisiab Sorbonne Universités, UPMC Paris 6, Institut Jean Le Rond d'Alembert, UMR CNRS 7190, 2 Place de la Gare de Ceinture, 78210 Saint Cyr L'école, Francec UPMC, Univ Paris 06, UMR 7197, Laboratoire Réactivité de Surface, Le Raphaël, 3 rue Galilée, 94200 Ivry, Franced CNRS, UMR 7197, Laboratoire Réactivité de Surface, Le Raphaël, 3 rue Galilée, 94200 Ivry, Francee ETH Zurich, Institute of Energy Technology, ML J 13, Sonneggstr. 3, CH-8092 Zurich, Switzerland

a b s t r a c ta r t i c l e i n f o

Article history:

Received 15 May 2013Received in revised form 16 January 2014Accepted 31 January 2014Available online 3 March 2014

Keywords:

Natural clayIron catalystHeterogeneous catalystPhenolWater treatmentPhoto-Fenton

A novel heterogeneous photo-Fenton plate catalyst was prepared by immobilizing iron species on the surface of natural Tunisian clay. The activity of this structured catalyst was assayed in the degradation of phenol under UV irradiation at two different wavelengths (245 nm, UVC, and 365 nm, UVA, radiation). Phenol removal rate fromthe aqueous solution always increased in the presence of the Fe-plate catalyst, even under dark-Fenton condi-tions and for both 254 and 365 nm UV radiation, conrming the ef ciency of the prepared catalytic system intheFenton process.HPLC analysisconrmeda phenol degradationmechanismtowards an almost completemin-eralization of the organic compound. An apparent activation energy of 48.7 kJ/mol was calculated from the re-moval experiments performed at different reaction temperatures in the presence of the Fe-plate catalyst.Catalytic activity remains almost unaltered after ve consecutive reaction cycles re-usingthe same Fe-plate. Cat-alyst stability was conrmed by means of TEM–EDX analysis, pointing to this novel plate catalyst as a promisingoption as a Fenton heterogeneous catalyst for the mineralization of organic compounds in wastewaters.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Industrial, agricultural, and domestic wastes have contributed to thecontamination of water sources with several organic compounds, fre-quently toxic and non-biodegradable ( Jin et al., 2010; Savage andDiallo, 2005). Such wastewaters have become a major social and eco-nomicproblem, as modern health-quality standards and environmentalregulations are becoming gradually more restrictive. Among the com-pounds contained in such wastewaters, phenol is considered as one of the most toxic pollutants, harmful to human health and to water life(ATSDR, 2008; Busca et al., 2008). It is moreover classied as a terato-genic and carcinogenic agent. Thus, phenol is listed in water hazardclass 2 in several countries. Biodegradability is only 90% in surfacewaters after seven days, and the aquatic toxicity of phenol (LC50) is12mgL −1 (Daphnia magna, 48 h). In EU countries, the maximum con-centration of phenol allowed in drinking water is 0.5 mg L −1 (Weberet al., 2008).

The group of technologies globally known as advanced oxidationprocesses (AOPs), is characterized by their high removal ef ciencies of refractory organic pollutants, i.e. the ones are dif cult to mineralize,that is, totally oxidize. They are based on the generation of reactive rad-icals, such as hydroxyls, which are able to oxidize the organic pollutants

up to either their mineralization, or the generation of easily biodegrad-able small molecules, at near-ambient temperature and atmosphericpressure (Kavitha and Palanivelu, 2004; Martins and Quinta-Ferreira,2011).

Among the different AOPs, Fenton and photo-Fenton oxidation pro-cesses areenvironmentallyfriendly,since theydo not involve the useof harmful chemical reagents. Besides such methods are easy to handleand can be operated using quite uncomplicated reactor designs. Homo-geneous-Fenton reaction has been considered in the last decades as oneof the most ef cient routes for the treatment of water polluted with re-calcitrant chemicals (Bobu et al., 2006; Chen et al., 2010; Garrido-Ramírez et al., 2010; Herney-Ramírez et al., 2011; Jin et al., 2010;Liotta et al., 2009; Pera-Titus et al., 2004; Saracco et al., 2001; Wang,2008). The Fenton process is based on an electron transfer between hy-drogen peroxide and a homogeneous metal in solution, generally iron(II) (Fe2+), resulting in the formation of hydroxyl radicals (Pera-Tituset al., 2004; Wang, 2008):

Fe2þ þ H2O2→Fe3þ

þ OH– þ •OH Fenton reactionð Þ: ð1Þ

Fe3+ ions are subsequently regenerated to active Fe2+, with H2O2:


þ HO2• þ Hþ Fe2þ regeneration

: ð2Þ

Applied Clay Science 91–92 (2014) 46–54

⁎ Corresponding author. Tel.: +33 1 30 85 48 65; fax: +33 1 30 85 48 99.E-mail address: [email protected] (P. Da Costa).

http://dx.doi.org/10.1016/j.clay.2014.01.020

0169-1317/© 2014 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Clay Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c l a y

http://dx.doi.org/10.1016/j.clay.2014.01.020http://dx.doi.org/10.1016/j.clay.2014.01.020http://dx.doi.org/10.1016/j.clay.2014.01.020http://dx.doi.org/10.1016/j.clay.2014.01.020http://dx.doi.org/10.1016/j.clay.2014.01.020mailto:[email protected]://dx.doi.org/10.1016/j.clay.2014.01.020http://www.sciencedirect.com/science/journal/01691317http://www.sciencedirect.com/science/journal/01691317http://dx.doi.org/10.1016/j.clay.2014.01.020mailto:[email protected]://dx.doi.org/10.1016/j.clay.2014.01.020http://crossmark.crossref.org/dialog/?doi=10.1016/j.clay.2014.01.020&domain=pdf

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Unfortunately, this last reaction, Eq. (2), proceeds considerablymore slowly than Eq. (1), i.e. k2 = 0.02 mol

−1∙ dm3 ∙ s−1 vs. k1 =

58 mol−1 ∙ dm3 ∙ s−1 (Bai et al., 2013). Therefore, Fe2+ ions are quicklyconsumed but slowly regenerated and the resulting low concentrationof Fe2+ makes the overall Fenton reaction slow down.

A combination of UV irradiation and Fenton process, namely photo-Fenton method, has been developed in order to improve the ef ciencyin the oxidation of the most refractory organic compounds. In this pro-

cess, oxidationrate increases, resultingin a higherdegree of mineraliza-tion, due to the enhanced production of •OH radicals. Hydroxyl radicalsaregenerateddue to both the photo-decompositionof hydrogen perox-ide and its iron catalyzed decomposition:

H2O2 þ UV → •OH þ •OH Photolysis of H2O2ð Þ: ð3Þ


þ OH– þ •OH Photo−Fenton reactionð Þ: ð4Þ

However, the homogeneous Fenton process has a signicant disad-vantage, for it needs up to 50–80 ppm Fe in solution. This is wellabove the limits set by EU directives, which allow a maximum of 2 ppm Fe in treated water to be discharged directly into the environ-ment (Walling, 1975). Due to this requirement, the application of thehomogeneous photo-Fenton treatment of large water ef uents mayproduce considerable amounts of sludge in the nal neutralizationstep (Sabhi and Kiwi, 2001). Thus, replacement of the homogeneouscatalystswith heterogeneous catalystswherethe activemetalcanbe in-corporated into a support stands out as a promising alternative.Although theuse of a heterogeneous catalyst mayresult in lower oxida-tion rates than in homogeneous conditions, due to diffusion resistancesof the reactants into theporeand products out of the pore, this problemcanbe minimizedor completely solvedby means of theproper choiceof a support of adequate surface area and pore size distribution. Differentcatalyst supports such as syntheticand natural zeolites, clay andpillaredclays,polymers,silica, carbonor resinshave been considered as possibleheterogeneous supports (Garrido-Ramírez et al., 2010; Liotta et al.,2009; Liu et al., 2009; Navalon et al., 2010; Polaert et al., 2002; Santoset al., 2006; Zazo et al., 2006).

Pillared interlayered clays (PILCs) containing iron oxide pillars (Fe-PILCs) are known as promising heterogeneous Fenton catalysts(Catrinescu et al., 2011; Herney-Ramírez et al., 2011; Tabet et al.,2006) and photo-Fenton catalysts (Chen and Zhu, 2007; Iurascu et al.,2009) forthe degradation of organic pollutants in wastewaters, combin-ing a good catalytic activity with high stability against iron leaching.Moreover, clays have been used for the immobilization of iron specieslike aqua-complexes or oxides (Du et al., 2009; Garrido-Ramírez et al.,2010; Luo et al., 2009). So far, the latter materials are used in slurryphoto-catalytic systems that face reactor design problems associatedwith thelight penetration into the bulk of treated water and thesepara-tion and recovering of heterogeneous photo-catalysts at the end of thetreatment. Guo and Al-Dahhan (2005) have previously reported a

ow packed-bed reactor containing a pellet-conformed pillared claycatalyst, but for phenol wet air oxidation. Three-phase trickle beds andcolumn reactors are quite common designs for this particular applica-tion (Habtu et al., 2011). However, photo-Fenton processes infer addi-tional and singular requirements from the reactor design point of viewthat make the application of such reactor concepts unfeasible. In fact,to thebest of our knowledge, only onerecent work focuses on the prep-aration of a non-dispersed heterogeneous catalyst,presented as an iron-

containing natural bentonite (clay) plate for photo-Fenton catalysis forresorcinol degradation (González-Bahamón et al., 2011).The present study focuses on the preparation of a stable plate-

structured heterogeneous photo-Fenton catalyst based on naturalTunisian clay. The activity of this Fe-clay plate catalyst was assayed inthe photo-Fenton degradation of phenol, under different experimentalconditions, considering as well its stability on a prolonged operationtime.

2. Materials and methods

2.1. Catalyst preparation

2.1.1. Puri cation of the natural clay

Natural clay from the Medenine region (Tunisia) was used as rawmaterial. The clay was puried by means of careful aqueous dispersionand decantation. The fraction with a particle size smaller than 2 μ mwas selected, dispersed in 1 M NaCl solution and stirred at room tem-perature for 12 h. The supernatant was removed after settling. This pro-cedurewasrepeated 3 times.After complete exchange, theNa-clay wasseparated by centrifugation, washed with distilled water, andnally di-alyzedto eliminatetheexcessof chloride ions (conrmedby theAgNO3test (Darder et al., 2005)). The solid was then dried at 60 °C, ground to100 mesh, and kept in a sealed vessel.

2.1.2. Synthesis of Fe-immobilized clay plate catalyst

Na-clay was combined with water and sand (Na-clay/sand weightratio of 1:1). The mix was molded forming circular plates (Ø =7.8 cm, 0.4 cm thickness) which were dried at room temperature

(25–30 °C), in order to avoid the formation of ssures. The dried solidwas then calcined at 250 °C for 4 h. For the deposition of an iron oxideactive phase, the plate was immersed in an aqueous solution of Fe(NO3)3 ( 5 g L

−1), heated at 60 °C and kept for 2 h under magnetic stir-ring. This procedure was repeated 3 times. The Fe-Clay plate catalystwas then dried was and subsequently calcined at 350 °C for 4 h. Animage of the catalyst is presented in Fig. 1.

2.2. Physico-chemical characterization

Thechemical composition and structural features of the natural claywas analyzed by means of X-ray uorescence (XRF, ARL® 9800 XPspectrometer), powder X-ray diffraction (XRD Philips® PW 1710

Fig. 1. The Fe-clay plate catalyst.

47H. Bel Hadjltaief et al. / Applied Clay Science 91–92 (2014) 46–54

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diffractometer, Kα, 40 kV/40 mA, with a scanning rate of 2θ per min)and infrared spectroscopy (IR, Digilab Excalibur FTS 3000 spectrome-ter;). Loss on ignition of the clay was determined after calcination at1000 °C, until mass change was no longer observed.

Themorphology of plateswas studied using scanning electronic mi-croscopy (SEM, Hitachi SU-70). This equipment has an Oxford X-Max50 mm2 X-ray spectroscopy system through dispersive energy (EDX),which enabled qualitative evaluation of chemical composition. High

resolution transmission electron microscopy(HRTEM) images were ac-quired on a JEOL JEM 2011 equipped with LaB6 lament and operatingat 200 kV. The images were collected with a 4008 × 2672 pixel CCDcamera (Gatan Orius SC1000) coupled with the DIGITAL MICROGRAPHsoftware. Chemical analyses were obtained by a EDX microanalyzer(PGT IMIX PC) mounted on the microscope. The plates were grinded,dispersed in ethanol and sonicated. A drop of the dispersion wasdepos-ited on a carbon-coated copper grid for the TEM observations.

2.3. Activity tests

The photo-catalytic oxidation experiments were carried outin a 250 mL Pyrex open vessel, placed on a magnetic stirrer andunder 2 parallel UV-lamps lamps (2 × 15 at 254 and 365 nm with930/1350 μ W cm−2). The distance between the solution and theUV source was kept constantat 15 cm, in allexperiments.A schemat-ic of this experimental installation is presented in Fig. 2.

After stabilization of the stirring speed (350 rpm) and pH (3.5), theFe-clay plate catalyst was introduced into the vessel containing100 mL of the aqueous solution of phenol (prepared from analyticalgrade phenol 99%, Merck). Then 8 mL of a 1000 mg L −1 H2O2 solution,prepared from H2O2 Merck reagent, was added to the reaction vessel.The addition of H2O2 was considered as the initial time for reaction.Thesolution wassubsequently stirred for 3 h. During reaction, liquid al-iquots were retrieved from the vesselat selected time intervals. Residu-al H2O2 in these samples was immediately quenched with MnO2(Merck), in order to avoid the occurrence of dark Fenton reactionthrough the possible presence of leached iron. Before analysis, liquidwas ltered using PTFE lters (0.45 μ m). Nevertheless, let us remark

here that the liquid solutions after reactions were analyzed by meansof ICP-OES. No Fe in these solutions was detected. Thus, we can assumethat the concentration of Fe ions being under the detection limit of theapparatus, that is, well belongs to the range of ppm.

The phenol concentration in the solution was analyzed by means of gas chromatography (GC), in an Agilent 2025 GC equipped with aZebron capillary column ZB-5MSi (30 m × 0.32 mm × 0.25 μ m) and

a

ame ionization detector (FID). Phenol removal ef

ciency was calcu-lated as follows:

η % ð Þ ¼ 100 C0–Ctð Þ=C0: ð5Þ

Where C0 and Ct (mg L −1) are the liquid-phase concentration of the

phenol at the initial and any time t respectively, measured by meansof GC.

In order to complete the information on the phenol oxidationprocess, additional analysis of the liquid samples extracted from thereaction vessel was performed by means of high-pressure liquid chro-matography (HPLC, VARIAN (pump and detector) equipped with elec-tronic injector JASCO and a pursuit 5 C18 150 × 4.6 mm column, witha detection wavelength of 254 nm under water and acetonitrile and atotal ow rate of 0.5 mL min−1).

3. Results and discussion

3.1. Natural clay and catalyst characterization

Table 1 shows the chemical composition of the natural clay, as ana-lyzed by means of XRF. The main constituents are silica, alumina, iron,calcium. Moreover, the absence of a correlation between SiO2 andAl2O3 contents indicates that the excess of SiO2 is due to the presenceof quartz, as conrmed by means of XRD. In fact, the powder X-raydiffraction pattern for this clay, presented in Fig. 3, evidences thatmain crystalline phases are quartz (26.7°), kaolinite (22.8°) and illite(12.6°). On the Fe-impregnated plate after calcination, the iron oxidephasesobservedare related to phaseα-Fe2O3, hematite,withdiffraction

peaks appearing at 2θ = 32.2, 35.7 and 36.4° (Ayodele and Hameed,2013; Carriazo, 2012; Nogueira et al., 2011).

The FT-IR absorption spectra, Fig. 4, show the presence of OHstretching bands at the vicinity of the 3500 cm−1 domain, as well asthe Si\O stretching bands near 1000 cm−1 (Balan et al., 2001). The

Fig. 2. Experimental set-up used in for the phenol degradation experiments (1: aquarium

mirror, 2: magnetic stirrer, 3: plate catalyst, 4: open Pyrex vessel, and 5: UV lamps).

Table 1

Chemical composition of natural clay.

Oxides (%)

SiO2 Al2O3 CaO Fe2O3 MgO K2O TiO2 SiO2/Al2O3

48.2 22.3 6.7 17.5 1.7 1.5 1.1 2.2

G

GDK

H

D

QHH

H

HH

C a S O

4

I

I

Q

Q

An

K

I

I

Calcined clay

Q Raw clay

Tetha (degr)

Q: quartz

An: anhydrid compouned(CaSO4)

H: hematite

D: dickite

G: gluconite

I: illite

K: kaolinite

0 5 10 15 20 25 30 35 40 45 50 55 60

Fig. 3. Powder X-ray diffraction pattern for the raw natural clay and the Fe impregnated

clay after calcination.

48 H. Bel Hadjltaief et al. / Applied Clay Science 91–92 (2014) 46–54

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band appearing at 1428 cm−1 corresponds to or is indicative of thepres-ence of carbonates, i.e. calcite (CaCO3) or dolomite (Ca, Mg(CO3)2), con-rmed by the presence of CaO and MgO revealed by the chemicalanalysis by means of XRF. Additionally, bands at 472 and 533 cm−1

can be assigned to Si\O\Mg and Si\O\Al, respectively.SEM images acquired for the already conformed plates, before and

after iron loading, are shown in Fig. 5a and c, and b and d, respectively.Imagesevidencea relatively noticeable changein thematerialmorphol-ogy upon iron addition.The clay plate surface, see Fig. 5a, appears moreuniform and at than after iron ion-exchange, Fig. 5b. A closer look,Fig. 5d, evidences some particle fragmentation and the alignment of the material in the form of cross-linked layers, as a consequence of iron loading. SEM–EDX analysis results are shown in Table 2. Initialiron content in the clay was as well evidenced by means of XRF analysis,

Table 1, a feature being relatively typical of Tunisian clays. However,iron content increase from 2.8 to 6.1 at.% conrms the ef ciency of theion-exchange treatment, which leads to the deposition of an approx.3.5 wt.%. TEM observation of the clay plate. Fig. 6a conrms the exis-tence of a large variety of constituents with different morphologiesand chemical compositions. Fig. 6b shows an illite particle near an ag-gregate of magnetite crystals. The EDX spectrum on the illite site,Fig.6e, evidences the presence of a small quantity of iron inside thepar-

ticle. Moreover, the study of the Fe-plate catalyst, i.e. Fig. 6f, proved thatFe species are effectively deposited on the surface of the clay.

3.2. Catalytic performance of Fe-plates and kinetic study

Fig. 7 shows the measured phenol removal ef ciency (η) as a func-tion of reaction time, forthe several experiments performedin thepres-enceofH2O2, Fe-plate catalysts, H2O2 andFe-platecatalyst,H2O2 andUV irradiation at 254 nm, H2O2 and UV irradiation at 365 nm, H2O2 + UV 254nm andFe-plate catalysts, andH2O2 + UV 365 nm and Fe-plate cat-alyst. In spite of its expected low oxidation potential (Zhou et al., 2011),it can be observed that phenol can be already oxidized in theonly pres-ence of hydrogen peroxide, reaching a maximal conversion of 8% after150 min of time-on-stream, and remaining almost constant in thenext 30 min of further reaction. The experimental run performed inthe presence of the Fe-plate catalyst, in the absence of H2O2 or UV irra-diation, evidences a maximal phenol conversion of around 22–25%,which becomes almost steady after 50 minof reaction time. This phenolremoval ef ciency can be ascribed to the adsorption of phenol on thecatalyst surface. For the sake of comparison, a non-impregnated platehas been tested and only a 15% removal ef ciency had been obtainedafter 120 min of time on stream. Phenol degradation extent increasessubstantially if H2O2 is added to the reaction mixture. In this case, a re-moval ef ciency of 68% is achieved after 180 min of reaction, which in-dicates that Fenton reaction takes place resulting in the enhancedformation of radicals that are involved in the oxidation of the organiccompound. Reaction may be hindered to some point by the presenceof originally inactive Fe species (Aleksić et al., 2010), or by the

-OH stretching Si-O

stretching

carbonates

Si-O-Mg

Si-O-Al

Fig. 4. FTIR spectra of the raw natural clay.

a) b)

d)c)

Fig. 5. SEM images for the clay plates a) and c) before; and b) and d) after Fe impregnation.


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essentially lower rate of Fe2+ regeneration reaction, Eq. (2), in compar-ison to Fentonradical generation, Eq. (1). UV irradiation is an importantkey for achieving higher phenol removal ef ciencies. When reaction isperformed in the presence of the Fe-plate catalyst, upon H2O2 additionand UV irradiation, 100% phenol removal can be nally attained. Thisfact proves thewell know highereffectiveness of thephoto-Fentonpro-cess in comparison to dark Fenton reaction, which is due to the en-hanced formation of radicals. Yield, however, is strongly dependent onthe wavelength of the UV radiation employed. The results presented

in Fig. 7 evidence that phenoloxidation proceeds fasterunder UV irradi-ation at a wavelength of 254 nm than when that of 365 nm wasemployed. The phenol removal ef ciency of 100% was attained afterlessthan60mininthe rstcase;whereas it takes 120min to completelydegrade phenol in the second case. This is rst of all due to fact thatUV 254 radiation is absorbed more effectively by the different iron spe-cies present in the system, as well as by phenol (Legrini et al., 1993).Moreover, it is well known that the photo-hydrolysis of H2O2, Eq. (3),

proceeds fasterand more effectively under UV radiation at wavelengthslower than 320 nm (Chen and Zhu, 2007; Legrini et al., 1993; Walling,1975). Therefore, a higherextent of radical formationthrough this path-way can be expected, resulting in faster oxidation of the organic com-pound. It is worth noting as well the differences in phenol removalef ciency as a function of reaction time measured under UV irradiationand upon H2O2 addition, in the absence of the Fe-plate catalyst. En-hanced photolysis of H2O2 under UV 254 irradiation results in phenol re-moval ef ciencies which are all the time higher than those measured inthe presence of the Fe-plate catalysts irradiated with UV 365. Infact 100%

Table 2

EDX of the samples.

C O Na Mg Al Si K Ca Fe

Atomic %

Plate 3.2 59.1 0.8 2.1 8.6 20.8 0.3 2.0 2.8Fe-plate 2.7 61.5 1 1.5 6.6 18.9 0.1 1.5 6.1

a)

c) d)

e)

f)

b)

Fig. 6. TEM images for the a) and b) clay plates; c) Fe-clay plate and d) Fe-clay plate after 5 consecutive cycles of phenol oxidation. EDX spectra of e) clay-plate and f) Fe-clay plate.


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removal ef ciency can be reached in this case, after 120 min of reaction,even in the absence of a catalyst.

Color change in the reaction solution during the degradationof phenol has been reported by Mijangos et al. (2006), who indicatedthat some highly colored intermediate compounds may includep-benzoquinone (yellow), o-benzoquinone (red), and hydroquinone(colorless), and the mixed solution of all intermediate compounds re-vealed a brown color. Moreover, these colored intermediates are morerefractory and dif cult to oxidize. Their increased stability is due tothe conjugated carbonyl groups contained in their chemical structure(Mijangos et al., 2006). Generally, they possess higher toxicity thanphenol itself (Yalfani et al., 2009). Fig. 8 reports a picture of each of the corresponding nal solution after 120 min of reaction. Incompletephenol oxidation in the case of the reaction performed upon H2O2 addi-tion either in the presence of the Fe-plate (non-irradiated), Fig. 8 solu-

tion (a), or under UV 365 irradiation (non-catalytic), Fig. 8 solution (b),results in fact in a certain brownish color, in agreement with the obser-vations of Mijangos et al. (2006), pointing to thepresence of such inter-mediates, i.e. p-benzoquinone and/or o-benzoquinone. On the otherhand, the solutions for those experimental runs resulting in 100% phe-nol removal ef ciency, UV 365 + H2O2 + Fe-plate, UV 254 + H2O2 andUV 254 + H2O2 + Fe-plate, Fig. 8 solutions (c), (d) and (e) respectively,appear colorless.

The mechanism of phenol degradation in water solutions has beenquite extensively studied (Davlin and Harris, 1984; Duprez et al.,1996; Santos et al., 2006; Soria-Sánchez et al., 2011). There is a generalagreement that phenol oxidation starts with the hydroxylation of themolecule leading either to hydroquinone or catechol, which are subse-quently oxidized to p-benzoquinone and o-benzoquinone, respectively.Therefore, the reaction is proceedingin two parallel pathways with ma-leic acid as common intermediate. Maleic acid can be then directly oxi-dized to CO2 or via oxalic and formic acids which are partially oxidizedto acetic acid, considered as the most refractory product of phenol deg-radation. Normally, 90% of phenol is oxidized into CO2 and 10% remainsas acetic acid in the media (Duprez et al., 1996).

Fig. 9 shows the HPLC chromatograms acquired for aliquots takenout of thereaction vesselat differentreaction times,duringphenol deg-radation experiment upon H2O2 addition in thepresence of the Fe-plate

catalyst and UV irradiation of 254 nm wavelength. Phenol peakappearsat a retention time of 5.53 min. After some minutes of reaction, the in-tensity of this phenol peak diminishes, whereas the intensity of thepeak appearing at retention times immediately near to that of phenolincreases. This peak can be assigned to the presence of hydrobenzoicacid, which has been considered as well as an intermediate in the phe-nol oxidation process (Eftaxias et al., 2006; Quintanilla et al., 2006). TheHPLC chromatogram obtained for the solution after 30 min of reaction

00

20

40

60

80

100H

2O

2 + Fe-plate + UV

254

H2O

2 + Fe-plate + UV

365

H2O

2 + UV

254

H2O

2 + UV

365

H2O

2 + Fe-plate

Fe-plate

η (

% )

Reaction time (min)

H2O

2

20 40 60 80 100 120 140 160 180

Fig. 7. Phenol removalef ciencymeasured duringexperimentalruns underdifferent reac-tion conditions.

Fig. 8. Aliquotsof thereaction solutionsafter 120minof reactiontime(a) H2O2 + Fe-plate, (b)H2O2 + UV 365,c)H2O2 + UV 365+ Fe-plate,d) H2O2 + UV 254ande)H2O2 + UV 254+ Fe-plate.

2 3 4 5 6 7

I n t e n s i t y ( a . u . )

Retention time (min)

initial

15 min

30 min

45 min

120 min

Fig. 9. HPLC chromatograms for the solutions extracted from thereaction vessel at differ-ent reaction times, during the photo-Fenton phenol removal experiment in the presenceof the Fe-plate upon H2O2 addition and UV 254 irradiation.


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time evidencesa higher phenol oxidation extent, reected in themark-edly lower intensity of the peak at 5.5 min, whereas a new peak appearsbetween those corresponding to fumaric andmaleic acids,which canbedueto theformationof aceticacid.Already after 45 minof reaction time,the phenol peak completely disappears from the HPLC chromatogram.After 120min of reaction time, only small amounts of thelow molecularweight acids are present.

3.3. In uence of phenol concentration and reaction temperature

It is of practical interest to investigate the effect of the initial pollut-ant – phenol – concentration on the removal ef ciency that can beattained, in order to evaluate the behavior of this particular water treat-ment system. Fig. 10 shows the inuence of various initial phenol con-centrations on the removal ef ciency measured in the photo-Fentonreaction, in the presence of the Fe-plate catalysts and under UV 254 irra-diation. Therate of phenoldegradation decreases for increasing concen-tration of this organic compound. In this sense, 100% removal ef ciencywas achieved after 17 min of reaction for an initial concentration of 25 mg L −1, 32 min for an initial concentration of 75 mg L −1, 60 minfor 150 mg L −1 and 143 min for 200 mg L −1. In the heterogeneousphoto-Fentonprocess, thereaction occurs at thesurface of Fe-plates be-tween the •OH radicals generated at the active sites and phenol mole-cules adsorbed on the surface. Thus, when phenol concentration ishigh enough, thenumber of active sites available decreasesdue to com-petitive adsorption of the phenol molecules on the catalytic surface(Chen et al., 2008). In addition, the intermediate products of phenol ox-idation might also compete for adsorption sites with phenol molecules,which may block their interaction with the Fe(II)/Fe(III) active phase(Chen et al., 2008).

Theinuence of reaction temperature on the photo-Fenton degrada-tion of phenol in the presence of the prepared Fe-plates catalyst was aswell studied and the results obtained are reported in Fig. 11. Tempera-turesubstantially inuencesthe rate of thephenol degradation reaction.For example, after 20 min of reaction, degradation ef ciency increasesfrom about 50% at 20 °C to almost 100%at 50 °C. The increase in temper-

ature enhances the rate of hydroxylation at the catalyst active sitesresulting in an increased production of •OH radicals. Moreover, a higherreaction temperature can provide more energy for the reactants to over-come the activation energy barrier (Xu et al., 2008). This excess energyleads to higher collision frequency between the •OH radicals and phenol

(or intermediate) molecules which eventually results in faster degrada-

tion (Sun et al., 2007).The apparent activation energy Ea, for the degradation of phenol byphoto-Fenton oxidation can be calculated from the correspondingArrhenius equation:

k ¼ A·exp −Ea=RTð Þ: ð6Þ

An activation energy value of 48.7 kJ ∙ mol−1 was obtained. General-ly, the reaction activation energy of ordinary thermal reactions is usual-ly between 60 and 250 kJ ∙mol−1 (Chenand Zhu, 2007). With respect tosimilar catalytic processes, Shukla et al. reported values of around 61.7–75.5 kJ/mol for Co/SiO2 catalysts (Shukla et al., 2011), and around67.4 kJ/mol for Co/SBA-15 (P. Shukla et al., 2010; P.R. Shukla et al.,

2010), whereas activation energy was found to be slightly lower butstill around 60 kJ/mol for Co supported on activated carbon (P. Shuklaet al., 2010; P.R. Shukla et al., 2010). This result implies that the degra-dation of phenol in an aqueous solution by the photo-Fenton oxidationprocess in the presence of the Fe-plate catalyst prepared requires loweractivation energy than the average of the degradation systems, andpoints to this material and derived structured catalyst as a perspectiveand effective option for the decontamination of wastewater containingsuch organic pollutant.

3.4. Stability of the catalytic system

In order to evaluate catalyst stability and the possible change in its

activity or in its physic-chemical features, successive removal experi-ments were performed. The Fe-plate catalyst was therefore used inve consecutivecycles, i.e. 1 h of operationusing fresh phenolsolutions,under UV 254 irradiation. After each run, the Fe-plate catalyst was re-moved from the reaction vessel, carefully washed with distilled waterand dried at 60 °C for 12 h. Regarding the changes in activity observedwith subsequent reaction cycles, a slight decay of about 10% in the re-moval ef ciency attained after 1 h of reaction time was measured butonly after the4thconsecutivereuseof thecatalyst.Moreover,no weightloss of the Fe-plate catalyst was measured after the ve reaction cycles.TEM observations conrmed this stability. After successive phenol deg-radation experiments, the Fe-plate surface was not modied. The cata-lyst surface presented still the same aspect than the one observed inFig. 6d. Moreover, EDX analysis conrmed the same quantitative pres-

ence of the Fe species.

0 180

0

20

40

60

80

100

25 mg/L

75 mg/L

150 mg/L

200 mg/L

Reaction time (min)

20 40 60 80 100 120 140 160

η (

% )

Fig. 10.Phenol removalef ciency asa functionof reactiontimemeasuredduringthe H2O2 +UV 254 + Fe-plate experiment for different initial phenol concentrations (25, 75, 150 and

200 mg L −1

).

0

0

20

40

60

80

100

20ºC

40ºC

50ºC

Reaction time (min)

10 20 30 40 50 60

η (

% )

Fig. 11. Phenol removal ef ciency as a function of reaction time measured duringthe H2O2 + UV 254 + Fe-plate experiment at different temperatures (20, 40 and 50 °C).


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4. Conclusions

A Fe-clay plate catalyst was prepared using Tunisian clay as astarting material. Physico-chemical characterization of the catalyst evi-denced successful immobilization of the Fe-active phase on the claymatrix.

The activity of this catalyst was assayed in the heterogeneousphoto-Fenton oxidation of a probe molecule: phenol in aqueous so-

lution which is representative of water contaminant. Phenol remov-alef ciency of 100% wasattained after less than 60 min of reaction inthe presence of the prepared Fe-plate catalyst and under UV irradia-tion of 254 nm wavelength. For equivalent reaction conditions, i.e.H2O2 addition, UV irradiation and wavelength, andaftera certain re-action time, phenol removal ef ciency measured was all the timehigher in the presence of the Fe-plate catalyst. In other words, therate of phenol degradation was higher in the presence of such cata-lytic system, proving its ef ciency as a Fenton catalyst even underthe less favorable reaction conditions. Moreover, HPLC analysis of samples periodically extracted from the reaction vessel conrmedphenol degradation following the reaction pathways described inliterature, and almost reaching complete mineralization of the or-ganic compound, with a small amount of the more refractoryshort-chain acids remaining in the solution after 120 min of reactiontime.

Negligible loss of activity was observed after ve consecutivereaction cycles performed re-using the same Fe-plate catalyst.TEM observation of the catalyst, as well as its EDX analysis,evidenced no visible modication of surface morphologyand chemical composition upon consecutive reaction cycles,conrming the good stability of the Fe-plate heterogeneouscatalytic system.

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