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Feasibility and applicability of the scaling-up of bio-electro-Fenton system for textilewastewater treatment

Zou, Rusen; Angelidaki, Irini; Jin, Biao; Zhang, Yifeng

Published in:Environment International

Link to article, DOI:10.1016/j.envint.2019.105352

Publication date:2020

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Zou, R., Angelidaki, I., Jin, B., & Zhang, Y. (2020). Feasibility and applicability of the scaling-up of bio-electro-Fenton system for textile wastewater treatment. Environment International, 134, [105352].https://doi.org/10.1016/j.envint.2019.105352

Contents lists available at ScienceDirect

Environment International

journal homepage: www.elsevier.com/locate/envint

Feasibility and applicability of the scaling-up of bio-electro-Fenton systemfor textile wastewater treatment

Rusen Zoua, Irini Angelidakia, Biao Jinb, Yifeng Zhanga,b,⁎

a Department of Environmental Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmarkb State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

A R T I C L E I N F O

Handling Editor: Frederic Coulon

Keywords:Textile wastewaterBio-electro-FentonScaling-upContinuous operationH2O2

A B S T R A C T

Textile wastewater entering natural water bodies could cause serious environment and health issues. Bio-electro-Fenton (BEF) as an efficient and energy saving wastewater treatment technology has recently attracted wide-spread attention. So far, there is no research available on the scaling-up of BEF process. In this work, an in-novative 20 L up-scaled BEF system was constructed for the treatment of methylene blue (MB) containingwastewater. The system was first tested in batch mode. The results showed that the system performance wasmajorly related to the operating parameters including initial MB concentration, catholyte pH and concentration,cathodic aeration rate, Fe2+ dosage, and applied voltage. At the optimal condition, 20 mg L−1 of MB wasefficiently removed following the apparent first order kinetics. The corresponding rate constants for the deco-lorization and mineralization were 0.68 and 0.20 h−1, respectively. Furthermore, MB decolorization efficiencyof 99% and mineralization efficiency of 74% were observed when the hydraulic retention time was 28 h incontinuous mode. This work demonstrates the scaling-up potential of BEF for recalcitrant wastewater treatment.

1. Introduction

With the widespread use of synthetic dyes, dye-containing waste-waters are extensively discharged by various industries including col-oring textiles, leather, foods, drinks, pharmaceuticals, cosmetics, papermanufacturing and so on. It has been reported that the concentration ofazo dyes generated by textile industries in actual wastewater wasranged from 5 to 1500 mg L−1 (Gottlieb et al. 2003). These wastewatersare highly persistent in the environment and thus bring negative im-pacts on the natural environment and society (Suteu and Bilba 2005).Therefore, various of technologies including biological, chemical,physicochemical, electrochemical and photochemical approaches havebeen invented for removing dyes from wastewaters (Brillas andMartínez-Huitle 2015). Among which, advanced oxidation processes(AOPs) based technologies, especially the electrochemical advancedoxidation processes (EAOPs), have been demonstrated as broadly pro-mising and high efficient technologies for dyes wastewater treatment(DWWT) (Barrera-Díaz et al. 2014; Brillas et al. 2009; Oturan andAaron 2014). Advantages of EAOPs include high degradation rates,feasibility of total mineralization, mild operation conditions and beingeco-friendly. However, there are still some shortcomings such as ex-tensive energy and chemical (acid and iron) consumption. In contrast tohigh cost of EF, biodegradation is a cost-effective way for DWWT, but

the slow degradation rate and toxicity of high concentration of dyes tomicroorganisms restrict its widespread application.

In recent years, a new process termed Bio-electro-Fenton (BEF)process, which combines the strong oxidation ability of traditional EFand low cost of biodegradation, was proposed as promising alternativetreatment technology (Feng et al. 2010a; Olvera-Vargas et al. 2016; Zhuand Ni 2009). The BEF process has several common properties withtraditional EF in terms of reactor configuration and cathodic reaction.The key factor that makes BEF process different and more cost-effectivethan the common EF process, is that the electric power in the BEFprocess is provided from oxidation of organic matter, instead of tradi-tional electricity input. For example, Zhang et al. reported the use ofelectrical energy generated by a microbial fuel cell (MFC) to drive an-other BEF system for MB removal (Zhang et al., 2015). In the anode ofBEF reactor, electrons and protons are first produced due to organicmatter (e.g. from domestic wastewater) degraded by electroactivebacteria, and then transfer through external circuit and proton ex-change membrane respectively into cathode chamber. In cathode ofBEF reactor, in-situ H2O2 production is achieved following two-electronoxygen reduction reaction according to Eq. (1), which subsequentlyreacts with Fe2+ and generates hydroxyl radicals (%OH) through Eq. (2).In addition, in-situ regeneration of Fe2+ can be achieved by reductionof Fe3+ (Eq. (3)).

https://doi.org/10.1016/j.envint.2019.105352Received 5 November 2019; Received in revised form 18 November 2019; Accepted 19 November 2019

⁎ Corresponding author.E-mail addresses: yifz@env.dtu.dk, yifzmfc@gmail.com (Y. Zhang).

Environment International 134 (2020) 105352

0160-4120/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

2H+ + O2 + 2e− → H2O2 (1)

Fe2+ + H2O2 + H+ → Fe3+ + %OH + H2O (2)

Fe3+ + e− → Fe2+ (3)

There has been a lot of research devoted to the application of BEFprocess to remove organic pollutants from water bodies such as organicdyes (de Dios et al. 2014; Li et al. 2017b; Zhang et al., 2015), phar-maceuticals and personal care products (Nadais et al. 2018; Wang et al.2018; Wang et al. 2017), municipal, agriculture and industrial chemi-cals (Hassan et al. 2017; Li et al. 2017a; Yu et al. 2018). Nevertheless,the current research on BEF process is still in lab-scale, where most ofthe reported systems are less than 1 L. The scaling-up feasibility of BEFsystem has never been reported.

Therefore, the present work aims to demonstrate the feasibility ofscaling up BEF process from milliliters to 20 L for DWWT.Subsequently, the various including catholyte pH, Fe2+ dosage, aera-tion rate, applied voltage, cathode electrolyte concentration and initialdyes concentration on system performance were evaluated due to theseparameters can affect treatment efficiency. Finally, the applicability ofthe scaled-up BEF reactor was further validated with six different dyeswastewaters in continuous flow mode.

2. Materials and methods

2.1. Scaled-up reactor and operation

A rectangular two-chamber BEF reactor (20 cm × 20 cm × 25 cmfor each chamber) was developed. A cation exchange membrane (CMI7001, Membrane international, NJ) was placed between the twochambers. The total volume of each chamber was 10 L, out of which 9 Lwas working volume. There are 20 electrodes in each chamber, 10 ofwhich are located on the front side of each chamber, and the remaining

10 are located on the back of the each chamber and more details can beseen in Fig. 1. A commercial carbon fiber brush (diameter 5.9 cm,length 6.9 cm, Mill-Rose, USA) was used as anode electrode. Eachcarbon brush needs to be pretreated as described before (Zhang andAngelidaki 2015b). A commercial graphite plate in the size of4.5 cm × 4.5 cm and a Ag/AgCl electrode (+0.197 V vs SHE, PineInstrument Company, USA) were used as cathode and reference elec-trode in the cathode chamber, respectively.

This BEF reactor was firstly running in MFC mode using sodiumacetate (1.6 g L−1) and domestic wastewater (Primary clarify, LyngbyWastewater Treatment Plant, Copenhagen, Denmark) as substrates andinoculum, respectively, to get mature anodic biofilm (Zhang andAngelidaki 2015a). Thereafter, the reactor was converted into micro-bial electrolysis cell (MEC) mode with applied voltage at 0.2 V. Inanode chamber, the synthetic nutrient medium (detail see Text.S1,Supplementary data)) was continuously fed to avoid substrate limita-tion on anode performance (Zhang et al., 2015). The hydraulic reten-tion time (HRT) of anode chamber was 58 h. Six dyes named as Me-thylene blue (MB), Orange G (OG), Toluidine blue (TB), Meldola’s blue(MeB), Rhodamine B (RhB) and Rhodamine 6G (Rh6G), which arecommon dyes widely used for dyeing, printing and biological indicator,were used as pollutants in the abiotic cathode. Preparation of syntheticazo wastewater (17.62 ms cm−1) was done by adding 20 mg L−1 of MBand 50 mM of Na2SO4 into deionized water, unless stated otherwise.The operation parameters including pH, initial concentration of Fe2+,aeration rate, applied voltage, cathode electrolyte concentration andinitial concentration of MB and their impact on the system were in-vestigated. Firstly, the effect of initial catholyte pH of 2, 3, 4, 5, 7 and 8on system performance were explored. Secondly, different catholyteFe2+ dosage of 0.05, 0.1, 0.2, 0.5 and 1.0 mM were tested. Thirdly, theaeration rate (between 87.5 and 350 mL min−1) was continuouslyaerated through a peristaltic pump in order to investigate the effect onsystem performance. Fourthly, different external voltage of 0.3, 0.4,

Fig. 1. Front view of the 20 L scaled-up BEF system. 1, Anode chamber; 2, Cathode chamber.

R. Zou, et al. Environment International 134 (2020) 105352

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and 0.5 V were applied to see the effect on system performance. Fifthly,the initial cathode electrolyte concentration was 50 mM Na2SO4, unlessthe test of different cathode electrolyte concentration, where Na2SO4

concentration at 0, 25 and 100 mM were adjusted. Finally, the impactof initial dye concentration (5, 10, 20, 50 and 100 mg L−1) on treat-ment performance was explored. When the cathode chamber was alsooperated in continuous flow mode, the HRT was 28 h. 0.1 Ω resistorwas connected between the two chambers, unless stated otherwise. Apower supply (CT-4008 W, Neware Battery Testing System, China) wasused to provide the constant voltage (0.3, 0.4 and 0.5 V). Severalcontrol experiments including the one in open circuit (Control 1), onewithout cathodic aeration (Control 2), one without Fe2+ addition in thecathode (Control 3) and one without external power supply (Control 4)were also conducted. The stirring rate was 200 rpm in both chambers.All the duplicated experiments were conducted at room temperature(25 ± 5 ℃).

2.2. Chemical, electrochemical analysis and calculations

Current and cathode potential were recorded by using a potentiostatwith 30 min intervals (CT-4008W, Neware Battery Testing System,China). A ultraviolet-visible spectrophotometry detector (Spectronic20D+, Thermo Scientific) was used to determine the concentration ofMB, OG, TB, MeB, RhB and Rh6G with the wavelength at 665, 478, 623,565, 555 and 526 nm, respectively (Abdelsalam and Birkin 2002; Aiet al. 2008; FaridaYunus et al. 2009; Li et al. 2017b; Rauf et al. 2009;Zhang et al., 2015). The pH in solution was measured by a pH meter(PHM 210 pH meter, Radiometer). The mineralization rate of selectedsix dyes in the wastewater was estimated through total organic carbon(TOC) analysis (Shimadzu TOC 5000 A). The electric energy consumedby the pumping system was estimated as previously stated (Zhang andAngelidaki 2015b). The apparent decolorization rate constant (Kapp)and mineralization rate constant (KTOC) were calculated as described inText.S2. Furthermore, the TOC removal and corresponding electricalenergy consumption were estimated in order to evaluate the economicfeasibility of the BEF process and its values were calculated as describedin Text.S3.

3. Results and discussion

3.1. The performance of the scaled-up BEF reactor

Fig. 2A shows the removal of 20 mg L−1 of MB during this scaled-upBEF process. The decolorization efficiency of MB achieved after 8 hoperation of the BEF was approximately 98% (Fig. 2A). In comparison,the decolorization efficiency of MB was less than 30% after 8 h inControl 1. The reason could be attributed to the sorption of dye on thereactor inner wall, cathode electrodes and the membrane surfacing tothe cathode side. The decolorization efficiency of MB without aeration(Control 2), without Fe2+ (Control 3) in the cathode chamber wasabout 75% and 94%, respectively, which was much higher than thatreported for lab-scale reactor (Li et al. 2017b; Zhang et al., 2015). Thisresult could be due to the following two aspects. On the one hand, MBas an electron acceptor can be directly degraded into colorless inter-mediate with shorter molecules under the behavior of current andhigher current could accelerate this process. The current obtained incontrol 2 (21 mA) and control 3 (35 mA) was higher that reported inlab-scale reactors (Fig. 2C), which can explain this phenomenon (Liet al. 2017b; Zhang et al., 2015). On the other hand, as the reactorworking volume and current increased, more H2O2 was produced,which could still accelerate the decolorization of MB even withoutformation of %OH (Brillas et al. 2009). It is well known that some ofintermediates generated during oxidation of dyes could be persistentand even more toxic compared to the original dyes themselves (Rizzo2011). Thus, it is particularly important to assess the degree of mi-neralization throughout the treatment process. The TOC removal

showed that the mineralization of MB had similar trend as decoloriza-tion. According to Fig. 2B, 72% of TOC removal was obtained after 8 h,while only 28%, 45% and 45% were observed in Control 1, 2 and 3,respectively. For Control 1, the TOC removal efficiency can only beexplained that the TOC in solution was removed, but it was actuallycaused by the adsorption of MB on the inner wall of the reactor, whileMB was entirely removed from whole part of the reactor in normaloperating condition. Furthermore, the MB decolorization and miner-alization efficiency were only 66% and 37% in control 4, respectively.The reason can be attributed to the much lower H2O2 productioncompared with scaled-up BEF system with external power supply (Liet al., 2018). Hence, less %OH produced for MB degradation and thus

Fig. 2. The decolorization (A), mineralization (B) and current (C) during MBwastewater treatment in the BEF system. Control 1, open circuit; Control 2,without cathode aeration; Control 3, without Fe2+ dose in cathode, Control 4,without external power supply. BEF operational conditions: MB of 20 mg L−1,initial pH of 2, Fe2+ of 0.2 mM, Na2SO4 of 50 mM, applied voltage of 0.4 V, andcathode aeration rate of 350 mL min−1.

R. Zou, et al. Environment International 134 (2020) 105352

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Table1

Prev

ious

stud

ieson

BEFproc

essde

grad

ationof

dyes.

Dye

sReactor

design

Volum

eof

reactor

Ope

rating

parameters

Efficien

cyOpe

ration

mod

eReferen

ce

Methy

lene

blue

Twoch

ambe

rs30

0mLof

each

cham

ber

50mgL−

1,2

mM

Fe2+,p

H3an

daeration

rate

of10

mLmin

−1.

97%

deco

lorization

in8han

d99

.6%

TOC

remov

alin

16h.

Batch

(Zha

nget

al.,20

15)

Orang

eG

Twoch

ambe

rs50

mLof

each

cham

ber.

400mgL−

1,1

0mM

Fe2+,p

H2,

50mM

Na 2SO

4an

daeration

rate

of10

mLmin

−1.

100%

deco

lorization

and99

.6%

TOCremov

alin

6h.

Batch

(Liet

al.2

017b

)

Reactiveblack5,

Poly

R-478

and

Indigo

carm

ine

Twoch

ambe

rs15

0mLof

cathod

ech

ambe

r.50

,20an

d80

mgL−

1,[Fe

3+@algina

tege

lbeads]of

150mgL−

1,

pH7.8an

daeration

rate

of1Lmin

−1.

89%,9

7%an

d19

%de

colorization

in15

mins.

Batch

(deDioset

al.2

014)

Orang

eII

Twoch

ambe

rs75

.6mLof

each

cham

ber.

0.1mM,6

6gL−

1[γ-FeO

OH@CNT]

,pH

7an

daeration

rate

of10

0mLmin

−1.

100%

deco

lorization

in14

han

d10

0%TO

Cremov

alin

43h.

Batch

(Fen

get

al.2

010a

)

Orang

eII

Twoch

ambe

rs75

mLof

each

cham

ber.

0.2mM,[

γ-Fe

OOH]of

1gL−

1,p

H7an

daeration

rate

of10

0mLmin

−1.

100%

deco

lorization

in30

h.Ba

tch

(Fen

get

al.2

010b

)

Orang

eII

Twoch

ambe

rs40

0mLof

each

cham

ber

2mM,ironplatewithasize

of2×

10cm

2,p

H3an

d2mM

ofH2O2ad

dition

.85

%de

colorization

in30

mins.

Batch

(Liu

etal.2

012)

Amaran

thTw

och

ambe

rs80

mLof

each

cham

ber.

75mgL−

1,F

e2+

of1mM

orFe

3+

of0.5mM,p

Hof

2an

dO2flow

rate

of15

0mLmin

−1.

83%

deco

lorization

withFe

2+

and76

%de

colorization

withFe

3+

in1h.

Batch

(Fuet

al.2

010)

Dye

sReactor

design

Volum

eof

reactor

Ope

rating

parameters

Efficien

cyOpe

ration

mod

eReferen

ce

Rho

damineB

Twoch

ambe

rs75

mLof

each

cham

ber.

15mgL−

1,[

Fe@Fe

2O3]of

0.2gL−

1,p

H3an

daeration

rate

of30

0mLmin

−1.

95%

deco

lorization

and90

%TO

Cremov

alin

12h.

Batch

(Zhu

anget

al.2

010)

Acidblue

113

Twoch

ambe

rs25

0mLof

each

cham

ber.

100mgL−

1,F

e2+

of10

mgL−

1,p

H3an

dO2flow

rate

of10

mLmin

−1.

71%

deco

lorization

and29

%TO

Cremov

alin

90mins.

Batch

(Asgha

ret

al.2

017)

Methy

loran

geTw

och

ambe

rs10

00mLof

each

cham

ber

300mgL−

1,F

e2+

of5mM,p

H3an

daeration

rate

of15

0mLmin

−1.

86%

deco

lorization

and84

%COD

remov

alin

72h.

Batch

(Hua

nget

al.2

018)

Methy

loran

geTw

och

ambe

rs55

0mLof

each

cham

ber.

5mgL−

1,[

Fe@Fe

2O3]of

0.5gL−

1,p

H2an

daeration

rate

of75

0mLmin

−1.

87%

deco

lorization

in2h

Batch

(Linget

al.2

016)

Crystal

violet

andLissam

inegreen

BTw

och

ambe

rs25

0mLof

each

cham

ber.

10mgL−

1,[

Fe3+@algina

tege

lbe

ads]

of15

0mgL−

1,p

H2an

daeration

rate

of2Lmin

−1.

83%

deco

lorization

and82

%TO

Cremov

alin

9h.

Batch

(deDioset

al.2

013)

Methy

lene

blue

Twoch

ambe

rs10

Lof

each

cham

ber.

20mgL−

1,F

e2+

of0.2mM,p

H2,

appliedvo

ltag

e0.4Van

daeration

rate

of35

0mLmin

−1.

95%

deco

lorization

in6h

and89

%TO

Cremov

alin

28h.

Batchan

dco

ntinue

This

stud

y

R. Zou, et al. Environment International 134 (2020) 105352

4

poor system performance was observed. According to the previous re-sults, the degradation kinetics of MB followed with apparent first-orderkinetics, which was also consistent with traditional Fenton processesand previously BEF process (Feng et al. 2010a; Li et al. 2017b; Melgozaet al. 2009; Zhang et al., 2015). The Kapp and KTOC were 0.68 h−1 and0.20 h−1 corresponded to scaled-up BEF process, respectively. It shouldbe noted that the value of mineralization rate constant was far belowthe decolorization rate constant probably because MB was firstly easierto oxidize to colorless intermediates which were further oxidized at arelatively slower rate to CO2 (Feng et al. 2010a). Compared to the rapidand easier decolorization, azo dyes mineralization was more difficultdue to its high molecular weight and complex structure. The Kapp of0.43 h−1 and KTOC of 0.22 h−1 has been reported for MB removal,while Kapp of 0.212 h−1 and KTOC of 0.0827 h−1 have been obtainedwith Orange II in previous BEF system, which were lower than the onesobserved in this study (Feng et al. 2010a; Zhang et al., 2015). Theseresults indicated that the scaled-up BEF process could be more efficientthan lab-scale BEF process (≤1L, see Table 1) for DWWT and providedknowledge for large-scale applications in the future.

3.2. System performance with varied catholyte pH

According to previous studies on application of EF and BEF forDWWT, we found that degradation performance was easily affected byinitial pH of dye-containing wastewater and was varied greatly in dif-ferent systems (Li et al. 2017b; Ling et al. 2016; Moreira et al. 2017).Therefore, the effect of catholyte pH on system performance was con-ducted at 6 pH levels of 2, 3, 4, 5, 7 and 8. The results showed thecatholyte pH can significantly affect the removal of MB (Fig. 3). A dropof the decolorization efficiency, TOC removal efficiency, Kapp and KTOC

of MB was observed when pH increased from 2 to 8 and achievedmaximum removal efficiency at pH 2 (99%, 89%, 0.68 h−1 and0.22 h−1). The pH of 2 found here as optimum was consistent with thatreported by milliliters-scale BEF systems (de Dios et al. 2013; Li et al.2017b; Ling et al. 2016). The pH in the cathode increased with theincreasing of operational time, regardless of initial cathodic pH (detailsee Fig.S1). For initial pH of 2 and 3, the pH increased to 2.6 and 10.07after 28 h, respectively. Based on previous studies on EF process, thehighest Fenton reaction rate was obtained at pH 2.8, where %OH can begenerated and disseminated with the Fe3+/Fe2+ couple as the catalyst(Brillas et al. 2009; Sirés and Brillas 2012). For initial pH above 4, thepH increased quickly and reached above 11 at 28 h. Hence, the poortreatment performance was observed (pH 4, 5, 7 and 8), compared tothe results when pH below 3 (pH 2 and 3). The increasing of pH maybring several impacts to the system. Firstly, the generated H2O2 coulddecompose into O2 and H2O when pH exceed 5 (Moreira et al. 2017).Secondly, higher pH caused the formation of Fe(OH)3 precipitate byFe3+ species, which may reduce the Fe3+/Fe2+ couple content(Nadaiset al. 2018). Thirdly, the MB could be removed by electro-coagulationdue to formation of Fe(OH)n (Nadais et al. 2018). The current andcathode potential showed the opposite trend as that of MB degradation(Fig.S1). Overall, these results demonstrated that scaled-up BEF processwas sensitive to pH values as same as the traditional Fenton and EFprocesses, and the pH of 2 was chosen for the following experimentswith current system.

3.3. MB Removal at different initial catholyte Fe2+ concentration

Fe2+ ions, as a widely used iron source, was usually added directlyinto BEF process. For a defined condition, the concentration of H2O2

produced from MEC can be considered to be relatively constant. Hence,Fe2+ concentration is an important parameter determining the forma-tion of %OH, and thus, can significantly affect the treatment perfor-mance (de Dios et al. 2013; Fu et al. 2010; Nadais et al. 2018; Yuanet al. 2017; Zhang et al., 2015). The effect of initial Fe2+ concentration(0.05, 0.1, 0.2, 0.5 and 1 mM) on MB degradation with a set of defined

parameters (20 mg L−1 of MB, pH of 2, Na2SO4 of 50 mM, appliedvoltage of 0.4 V, and aeration rate of 350 mL min−1) was investigated.According to Fig. 4, the different concentration of Fe2+ was shown tohave obvious influence on decolorization and mineralization of MB. Arapid decolorization of above 95% at first 6 h (then above 99% at 28 h),TOC removal efficiency of above 89% at 28 h and Kapp of 0.68 h−1 wereobtained with initial Fe2+ concentration of 0.2 mM. The relatively slowrate of decolorization and lower TOC removal efficiency of MB at ahigher concentration of Fe2+ (0.5 and 1 mM) may be explained asfollowing. On one hand, the formation of Fe3+ may be caused by theexcessive Fe2+ reacting with%OH according to Eq. (4).

Fe2+ + OH → Fe3+ + OH− (4)

On the other hand, excessive Fe2+supply could lead to surplus %OHproduction (Eq. (2)) which could further react with H2O2 (Eq. (5)).

%OH + H2O2 → HO2 + H2O (5)

Those reactions lead to simultaneous loss of %OH and H2O2, andthereby resulting in the poor performance of MB degradation. In ad-dition, for lower TOC removal efficiency of MB at higher Fe2+ con-centration could also be due to the generation of Fe3+-carboxylic acidcomplexes (Brillas et al. 2000; Sirés et al. 2007). Such complexes in-cluding Fe(C2O4)33−, Fe(C2O4)− and Fe(C2O4)+ were hard to be oxi-dized by %OH. Moreover, with the lower concentration of Fe2+ (0.05and 0.1 mM) addition, the MB degradation showed a similar trend asthat observed at relatively higher concentration of Fe2+(0.5 and 1 mM).The reason can be attributed to insufficient Fe2+, which led to in-sufficient %OH production (Eq. (2)). Thus, based on the above results,

Fig. 3. The effect of initial pH on the degradation of MB (A), TOC removalefficiency (B) rate in the BEF process. Operational conditions: MB of 20 mg L−1,0.2 mM Fe2+ dose, 50 mM Na2SO4, applied voltage of 0.4 V, and aeration rateof 350 mL min−1.

R. Zou, et al. Environment International 134 (2020) 105352

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0.2 mM of Fe2+ dosage was chosen for the subsequent experiments,which was far below than that reported previously (Fu et al. 2010; Liet al. 2017b; Zhang et al., 2015). The reason can be attributed to thescaled-up BEF system could enhance the regeneration of Fe2+ (Eq. (3))compared with lab-scale BEF system and thus less initial Fe2+ dosagewas needed. This could reduce the sludge production and save costsimultaneously.

3.4. System performance with varied catholyte airflow rate

The electrochemical production of H2O2 lies on the reduction ofoxygen through the two electrons reaction according to Eq. (1). Hence,the cathodic aeration rate which determines the dissolve oxygen (DO)level could directly affect the generation of H2O2 and subsequentlyaffect the generation of%OH (Eq. (2)). Thus, the effect of aeration rates(i.e., 87.5, 175, 262.5 and 350 mL min−1) on the BEF process was in-vestigated under a certain operational condition (20 mg L−1 of MB, pHof 2, Na2SO4 of 50 mM, 0.4 V, and Fe2+ of 0.2 mM). The results in Fig. 5showed that decolorization efficiency, Kapp and TOC removal efficiencyof MB increased when the aeration rate was increased, and the corre-sponding maximum value (99%, 89%, 0.68 h−1) was achieved at350 mL min−1. Table.S1 showed the DO concentration in the cathodewith different aeration rate. Furthermore, the cathodic DO was ap-proaching a saturated concentration around 8.45 mg L−1 at 24 ℃ whenaeration rate reached 350 mL min−1. It was always found in previousstudies demonstrated that over a certain range increasing the airflow

rate cannot continue improve the pollutants degradation (Liu et al.2007). It could be due to that the DO level was saturated and the largebubbles might prevent the adsorption of O2 to the cathode electrode.The air flow rate of 350 mL min−1 (corresponding to 0.0082 mL O2

min−1 mL−1 based on cathode volume) was chosen as cathodic aera-tion rate for the following experiments, which was much lower thanprevious studies (de Dios et al. 2013; Li et al. 2017b; Nadais et al. 2018;Zhuang et al. 2010). Since the aeration rate was related to the elec-tricity consumption on pumping, thus operating the scaled-up BEFprocess at an optimum value could enhance the wastewater treatmentefficiency and significantly decline the overall operating expenditure.

3.5. System performance at different applied voltage

Previous studies have shown that applied voltage, as an importantparameter, can affect the lab-scale BEF process by regulating the pro-duction of %OH (Li et al. 2017a; Li et al. 2017b; Nadais et al. 2018;Zhang et al., 2015). Hence, different applied voltages (0.3, 0.4 and0.5 V) were applied to explore its impact on the decolorization andmineralization in the scaled-up system. Fig. 6A exhibited the decolor-ization efficiency of MB firstly enhanced when the applied voltage wasincreased from 0.3 to 0.4 V, but it started to drop with the appliedvoltage further increased to 0.5 V. Generally, the higher applied vol-tage, the higher current can be reached (Fig.S4). Nevertheless, currentover a certain range may accelerate the reduction of H2O2 to H2O, andthereby reducing the production of %OH for further degradation of MB(Liu et al. 2007). The corresponding Kapp and TOC removal efficienciesof MB were presented in Fig. 6B, which showed the similar trends asdecolorization. Similar results were observed in previous lab-scalestudies (Li et al. 2017a; Nadais et al. 2018). Therefore, applied voltage

Fig. 4. The effect of initial Fe2+ concentration on the degradation of MB (A),TOC removal efficiency (B) in the BEF. Operational conditions: MB of20 mg L−1, initial pH of 2, 50 mM Na2SO4, applied voltage of 0.4 V andaeration rate of 350 mL min−1.

Fig. 5. The effect of aeration rate on the degradation of MB (A), TOC removalefficiency and rate constant (B) in the BEF process. Operational conditions:initial pH of 2, Fe2+ of 0.2 mM, MB of 20 mg L−1, Na2SO4 of 50 mM, andapplied voltage of 0.4 V.

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of 0.4 V was chosen for the following experiments, based on the con-sideration of TOC removal efficiency and energy consumption.

3.6. System performance at different cathode electrolyte concentration

Ionic strength in the cathode is crucial to the treatment performanceof EF and BEF process, since it can support the conductivity and allowefficient flow of electrical current (Moreira et al. 2017). Supportingcathode electrolytes including Na2SO4, NaCl, KCl, NaClO4, NaNO3 andNa2CO3 have used in EAOPs (Moreira et al., 2017). It has been foundthat system using Na2SO4 showed higher mineralization ability thanusing NaCl (Thiam et al., 2015b, 2014). Moreover, Fan et al. found thatdecolorization ability of BEF system were in the order of catholyteNa2SO4 ＞ Na2NO3 ＞ Na2CO3 (Fan et al. 2010). As for NaClO4, it wasworth noting that ClO4

− was toxic for aquatic organisms. Therefore,Na2SO4 was chosen as supporting cathode electrolyte and differentinitial concentrations (0, 25, 50, 75 and 100 mM Na2SO4) were used toinvestigate its effect on the MB removal. As exhibited in Fig. 7, it can befound that the final decolorization of MB were independent of its initialconcentration, which was consistent with previous lab scale study (Liet al. 2017b). However, the Kapp and TOC removal efficiencies of MBfirstly increased as Na2SO4 concentration increased from 0 to 50 mM(0.68 h−1 and 89%), and then decreased with further increasing ofNa2SO4 concentration. Notably, the decolorization and mineralizationof MB were still efficient (100% and 83%) even without addition ofNa2SO4. The reason could be due to the conductivity of dye solutionwas 7.63 ms cm−1, which was probably sufficient to support thetreatment process. The conductivity contributed by MB alone wasslightly lower than that of 25 and 50 mM Na2SO4 (Table S2), and thus, aslight lower current output (30 mA) was obtained (Fig.S5). The cathode

electrolyte concentration (50 mM Na2SO4) was selected for the fol-lowing experiments, which was consistent with the majority of studiesreported. The current slightly increased as the Na2SO4 concentrationincreased (Fig.S5). The reason may be that the relatively higher Na2SO4

concentration contributed to higher cathode conductivity (Table.S2)and thereby lowering the internal resistance (D’Angelo et al. 2015).Furthermore, a slight decrease of the Kapp and TOC removal efficiencieswere observed with higher electrolyte concentration (75 and 100 mMNa2SO4), which can be explained as following: firstly, the excess SO4

2−

could be absorbed on the surface of electrode and thus caused the de-crease of active site of electrode for H2O2 generation and subsequentlyreduced the %OH production (Chen et al., 2015). Secondly, SO4

2− couldreact with %OH to generate SO4%

− (Eq. (6)), then the produced SO4%−

could further consume the H2O2 (Eq. (7) and Eq. (8)) (Moreira et al.,2017). The same trends also observed in previous study (Luo et al.,2015; Thiam et al., 2015a).

SO4%2− + %OH → SO4·− + OH− (6)

SO4%− + H2O2 → SO4

2− + H+ + HO2% (7)

SO4%− + HO2 → SO4

2− + H+ + O2 (8)

3.7. System performance at different MB concentrations

Different MB concentrations were chosen to investigate its influenceon the system performance. Fig. 8 depicted that the decolorization ef-ficiency, Kapp and TOC removal efficiency declined as initial MB con-centration increased. Approximately 100% color removal of 5, 10, 20and 50 mg L−1 MB solution has reached by this scaled-up BEF process

Fig. 6. The effect of applied voltage on the removal of MB (A), TOC removalefficiency and rate constant (B) in the BEF process. Operational conditions:initial pH of 2, Fe2+ of 0.2 mM, MB of 20 mg L−1, Na2SO4 of 50 mM, andaeration rate of 350 mL min−1.

Fig. 7. The effect of cathode electrolyte on the degradation of MB (A), TOCremoval efficiency and rate constant (B) in the BEF process. Operational con-ditions: initial pH of 2, Fe2+ of 0.2 mM, MB of 20 mg L−1, applied voltage of0.4 V and aeration rate of 350 mL min−1.

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after 28 h. However, only 89% color removal of 100 mg L−1 MB wasobserved after 28 h (Fig. 8A). Furthermore, variation of concentrationof MB (C) with time t of the BEF process for degradation of differentinitial MB concentration was conformed to first-order kinetics reaction(Fig. 8B). Hence, the Kapp was calculated based on Eq. (4). For instance,the Kapp and TOC removal efficiency were 1.41 h−1 and approximately98%, respectively, when initial MB concentration was 5 mg L−1.However, Kapp of 0.06 h−1 and TOC removal efficiency of 39% wereobserved with 100 mg L−1 of MB. The same trends were observed in thelab-scale BEF and EF process, which indicated that longer treatmenttime might be required for the wastewater containing relatively higherdye concentration (Hammami et al. 2007; Labiadh et al. 2015; Li et al.2017b; Zhang et al., 2015). Moreover, a drop of Kapp with the increasingof MB concentration was observed, which was consisted with previousstudies (El-Desoky et al. 2010; Garcia-Segura and Brillas 2014; Moreiraet al. 2015). The results could be explained as following. Firstly, theelectrode charge transfer process could probably replace the diffusionprocess as the rate-determining step, which may result in a reduction ofKapp and a transition of the kinetics from pseudo-first-order to zero-order (Moreira et al., 2017). Secondly, slower diffusion and/or masstransport between H2O2 and Fe2+ species may lead to the less pro-duction of %OH when the MB concentration was higher. Meanwhile,numerous Fe3+ complexes can be formed due to the reaction betweenFe3+ and MB and its degradation products (Moreira et al., 2017).Thirdly, this kinetic model might have limitations to give accurate in-terpretation of the decay profiles of MB, thus a more comprehensivemodel that may contain all reactions associated with MB degradation,especially involving its oxidation products, should be pursued in future(Moreira et al., 2017).

3.8. Applicability to different dye wastewaters

Five more different dyes wastewaters were selected to evaluate theperformance of this scaled-up BEF reactor under the identified optimalconditions (initial pH of 2, Fe2+ of 0.2 mM, Na2SO4 of 50 mM, appliedvoltage of 0.4 V, aeration rate of 350 mL min−1 and each dye con-centration of 20 mg L−1). Fig. 9 showed that the decolorization effi-ciency after 28 h reached about 98%, 99%, 100%, 100% and 100% forTB, MeB, OG, RhB and Rh6G, respectively. In contrast to decoloriza-tion, mineralization of the organic dyes were more diffcult due to theirhigh molecular weight and the complicated structure. Especially for thestudied five dyes, the benzene rings in the aromatic molecules are quitestable and diffcult to break apartly by %OH. As a result, the TOC removalefficiency reached 78%, 78%, 87%, 71% and 73% for TB, MeB, OG, RhBand Rh6G, respectively. The results demonstrated that the selected dyescan be almost completely decolorized with high mineralization per-centages by this scaled-up BEF reactor. Therefore, when the reactiontime was further increased, the complete decolorization and miner-alization of the studied dyes can be expected.

3.9. System performance under continuous operation

There were no reports available so far on the application of BEFtechnology in the treatment of dye-containing wastewater under con-tinuous mode. Therefore, it is of importance to test this 20 L scaled-upreactor in continuous mode, which will help to evaluate the industrialapplicability of the system. The optimal parameters obtained frombatch experiments (initial pH of 2, Fe2+ of 0.2 mM, MB of 20 mg L−1,Na2SO4 of 50 mM, applied voltage of 0.4 V, and cathode areation rate of350 mL min−1) were adopted for the continuous flow test. The HRT

Fig. 8. The effect of initial MB concentration on the degradation of MB (A), rateconstant and TOC removal efficiency (B) in the BEF process. Operational con-ditions: initial pH of 2, Fe2+ of 0.2 mM, Na2SO4 of 50 mM, applied voltage of0.4 V, and aeration rate of 350 mL min−1.

Fig. 9. Application on the degradation of 6 different dyes (A), TOC removalefficiency and rate constant (B) in the BEF process. Operational conditions:initial pH of 2, Fe2+ of 0.2 mM, 6 different dyes of 20 mg L−1, Na2SO4 of50 mM, applied voltage of 0.4 V and aeration rate of 350 mL min−1.

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was 28 h and continuous operation for 10 days. For feeding anodechamber, the flow rate of pump was 5.84 mL min−1. For cathodechamber, the flow rate of pump was 5.36 mL min−1. According toFig. 10, the decolorization efficiency reached about 90% in first 12 hand continuously increased to above 99% after 72 h operation,whereafter it kept stable. The TOC removal efficience was stable ataround 74%. The current and cathode potential were also keep stable ataround 0.4 V and 35 mA, respectively. The decolorization and TOCremoval efficiency were slightly lower than that observed in batchmode. It could be due to the different mass transfer characteristics incontinuous operation mode, compared to batch mode. The same trendwas also observed in EF process (Nidheesh and Gandhimathi 2014). Theresults demonstrated that the scaled-up BEF reactor can be operatedunder continuous flow conditions with a stable treatment performance,and the system need to be further optimized when it was switched frombatch to continous mode.

3.10. Energy consumption and costs

Previous studies have demonstrated that the BEF process is aneconomical alternative to conventional EF processes for dye wastewatertreatment (Li et al. 2017a; Li et al. 2017b; Nadais et al. 2018). Thepurpose of calculating energy consumption in this paper is to evaluatethe economic feasibility of the scaled-up BEF reactor. The electricalenergy consumption was mainly composed of two parts, one is from theapplied voltage and the other is from the pump and magnetic stirrer.For the first part, the calculation was based on Eq. (9):

= × ×E I V t/1000 (9)

Herein, the E (kWh) is the electrical energy consumption, V (V) is

the voltage applied to the system, I (A) is the calculated current in-tensity, and t (h) is the time for one batch cycle. The electrical energyconsumption in first part during one batch cycle with the optimumoperational parameters was 0.000392 kWh, while the electrical energyconsumption in second part was 0.48 kWh for one batch cycle.Therefore, the total electrical energy consumption was 0.480392 kWh.According to calculation, of the normalized electrical energy with re-spect to TOC removal was 7469.9 kWh (kg TOC)−1. It was worth notingthat 99.92% of the energy consumption from the scaled-up BEF reactorwas mainly due to the aeration consumption by using pump. The en-ergy consumption from the electricity directly applied to the reactorwas only 5.96 kWh (kg TOC)−1 in this study, which was far below theenergy consumption from traditional EF process (Barros et al. 2014;Thiam et al. 2015b). Moreover, the electrons derived from biologicaloxidation of organic matter in BEF process significantly decreased theenergy need for H2O2 generation, compared to the traditional EF pro-cesses. In addition, normalized electrical energy with respect to TOCremoval was 11526.6 kWh (kg TOC)−1 when this scaled-up BEF reactorrunning without external power supply (see detailed TOC removal inFig.S8). The much higher energy consumption of MFC-based scaled-upBEF process was mainly due to the low treatment efficient which pro-longed the reaction time and thereby increasing the aeration costs. Theresults indicated that MEC-based scaled-up BEF process was more sui-table for wastewater treatment in future large scale application. Fur-thermore, our previous study also demonstrated that wastewatertreatment capacity of MEC based BEF system was much larger than thatof MEC based BEF system in smaller lab scale (0.3 L) (Zhang et al.,2015). Further optimization of the aeration in the cathode (e.g., em-ploying the advanced aeration strategy such as air diffusion electrode)and using renewable energy such as solar energy and additional MFCsto replace the direct current power will make the BEF process evenmore attractive from economic point of view. The aeration in thecathode could also be greatly reduced or even avoided during large-scale application if the wastewater is collected in open to air storagetanks. Moreover, the cost of the addition of chemicals for pH adjust-ment may also need to be considered. The cost of addition of chemicalsincluding iron and acids were not calculated here due to followingconsideration. On the one hand, as shown in Fig.S1B, we only adjustedthe initial pH and didn’t control the pH during treatment. When theinitial pH was 2, there was only a slight increase of pH (below 3) in thegiven HRT of 28 h. Thus, the pH adjustment only needed in the be-ginning. On the other hand, pH adjustment/control is the commonchallenge to traditional Fenton or EF processes. Several strategies havebeen reported to address this issue. For instance, using carbon nanotube(CNT)/γ-FeOOH composite as cathode could complete the treatment atneutral pH (Feng et al., 2010). In addition, the waste acid from in-dustries could be used for pH adjustment if it is needed. All thesestrategies could be considered for the future practical applications andcould be the goals of our future studies.

4. Conclusions

The textile wastewater was efficiently degraded by the scaled-upBEF process in a 20 L reactor. Batch experiments indicated that theoptimum operating conditions for MB removal are pH of 2, 0.2 mM ofFe2+, 50 mM of Na2SO4, cathode aeration rate of 350 mL min−1 andpower supply of 0.4 V. Dye removal was consistent with first-orderkinetic reaction under the above optimum operating conditions.Furthermore, the degradation of dye wastewaters by this scaled-up BEFreactor was operated successfully and performed stable in a continuousflow mode. Notably, the energy consumption from the electricity di-rectly input in the reactor was only 5.96 kWh (kg TOC)−1. Thoughpromising, further optimization on the reduction of aeration cost is stillneeded before large-scale application. This study offers insight intodevelopment of sustainable and eco-friendly BEF process for the textilewastewater treatment at industrial scale.

Fig. 10. Operation with continue flow on the degradation and TOC removalefficiency of MB (A), cathode potential and current (B) in the BEF. Operationalconditions: initial pH of 2, Fe2+ of 0.2 mM, MB of 20 mg L−1, Na2SO4 of50 mM, and power supply 0.4 V and aeration rate of 350 mL min−1.

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CRediT authorship contribution statement

Rusen Zou: Conceptualization, Methodology, Investigation,Validation, Data curation, Writing - original draft. Irini Angelidaki:Funding acquisition, Methodology, Validation, Writing - review &editing, Supervision. Biao Jin: Validation, Writing - review & editing,Funding acquisition. Yifeng Zhang: Funding acquisition, Methodology,Validation, Writing - review & editing, Funding acquisition,Supervision.

Declaration of Competing Interest

The authors declare that there is no conflict of interest.

Acknowledgement

The authors would like to acknowledge China Scholarship Councilfor the finial support. The authors thank the Xiaohu Li for his con-tribution on reactor setup. This research was supported by NovoNordisk Foundation (NNF16OC0021568) and SKLOG grant (Grant No.SKLOG-201930) from State Key Laboratory of Organic Geochemistry(SKLOG), Guangzhou Institute of Geochemistry, Chinese Academy ofSciences (GIG-CAS). Yifeng Zhang specially thanks the CarlsbergFoundation (CF18-0084) for the grant and support.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envint.2019.105352.

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