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ReviewReceived: 16 February 2014 Revised: 22 April 2014 Accepted article published: 13 June 2014 Published online in Wiley Online Library: 9 July 2014

(wileyonlinelibrary.com) DOI 10.1002/jctb.4460

Recent advances, challenges and prospectsof in situ production of hydrogen peroxidefor textile wastewater treatment in microbialfuel cellsAnam Asghar, Abdul Aziz Abdul Raman* and Wan Mohd Ashri Wan Daud

Abstract

Application of the Fenton process for textile wastewater treatment is limited due to high treatment cost, substantially con-tributed by the un-availability of cheap hydrogen peroxide. Therefore, alternative methods for hydrogen peroxide productionare in demand. One such option is in situ hydrogen peroxide production using a wastewater based microbial fuel cell (WBMFC).However, not much have been published regarding in situ production of hydrogen peroxide for textile wastewater treatment ina WBMFC. Therefore, in this work the concept, advantages, challenges and prospects of using WBMFC to treat textile wastew-ater by simultaneously producing hydrogen peroxide (hence in situ hydrogen peroxide) and power are reviewed. The conceptof WBMFC is the reduction of oxygen in the presence of electrons and protons from the anode chamber to produce hydrogenperoxide with simultaneous power production. This review confirms that use of dual chambers, proton exchange membrane,domestic or municipal wastewater/Geobacter Sulfurreducens or Shewanella species, pure graphite cathode, ammonia and heattreated carbon-based anode can treat most textile wastewaters. However, single chamber WBMFCs can be used as a low powersource for an electro-Fenton reactor. Power produced can be used to provide energy for aeration required in the WBMFC, thusproviding an integrated and sustainable solution for textile wastewater treatment.© 2014 Society of Chemical Industry

Keywords: microbial fuel cells; in situ hydrogen peroxide; Fenton oxidation; power density; anode modification

INTRODUCTIONTextile industries consume immense amounts of water andchemicals.1 Wastewater resulting from textile processes hasimpacts in terms of high COD, BOD, TOC, color, turbidity, tempera-ture, wide-ranging pH (5–12), suspended solids and toxic organiccompounds.2 – 8 Discharge of even a small quantity (ca.1 mg L−1)of dye is not acceptable and may produce toxic compounds atthe end of the treatment process.9 It has been observed throughmarket survey that the annual production of dyestuff is 7× 105

tons5,10,11 and approximately 10–15% of the dyestuff is lost duringthe dyeing operation.12 Globally, almost 280,000 tons of textiledyes are discharged into water sinks through textile effluents.13 Forinstance, in the textile cotton industry 0.6–0.8 kg NaCl, 30–60 gdyestuff, and 70–150 L water are required for dyeing 1 kg of cottonwith reactive dyes. However, the wastewater produced contains20–30% of the applied unfixed reactive dyes with an averageconcentration of 2000 ppm and high salt content.14 This incurseconomic glitches aside the environmental problems associatedwith waste discharge. Therefore, textile dyeing effluents requireintricate treatment.15 Thus, a highly oxidative and non-selectiveoxidizing agent is needed for the treatment of textile wastewater.

In general, Advance Oxidation Processes (AOPs) such asozonation, photocatalysis, sonolysis, electrochemical oxidation,Fenton and Fenton-like processes have the potential to pro-duce highly oxidative and non-selective hydroxyl radical (HO•).16

Compared with conventional methods, AOPs offer several par-ticular advantages such as easy operation, high efficiency andless sludge formation.17 Among AOPs, the process that is gainingthe attention of researchers and industry is Fenton process. Thisis because it can rapidly form HO• radicals in acidic mediumthrough interaction between iron salt and hydrogen peroxide(H2O2) (Equation (1)). The sequence of processes involved in theFenton oxidation of textile wastewater is shown in Fig. 1.

Fe+2 + H2O2 → Fe+3 + HO• + HO− (1)

However, reaction parameters,18 type and excessive quantity ofthe iron salt used,19 high cost of H2O2 and high chemicals require-ment for pH modulation of real textile wastewater, are factorsthat hamper application of the Fenton process in textile wastew-ater treatment.20,21 Moreover, safety issues associated with thetransport and handling of bulk quantities of commercially avail-able H2O2 and the energy intensive nature of the anthraquinoneprocess20 make the process hazardous and uneconomical.22 This

∗ Correspondence to: A.A. Abdul Raman, Chemical Engineering Department,Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia.E-mail: [email protected]

Chemical Engineering Department, Faculty of Engineering, University Malaya,50603 Kuala Lumpur, Malaysia

J Chem Technol Biotechnol 2014; 89: 1466–1480 www.soci.org © 2014 Society of Chemical Industry

1467In-situ Production of hydrogen peroxide in WBMFCs www.soci.org

(1) Addition of acid (H2SO4/HCl)for pH adjustment (pH=3)

(2) Addition of Iron salt and H2O2for performing Fenton oxidation

(3) Addition of base (NaOH) forneutralization (pH~ 8-9)

Influent(wastewater)

Filteredinfluent

Membrane/Screens

pHAdjustment(1)

Fentonoxidation(2)

Neutralizationstage(3)

SettlingTank

Treatedeffluent

FlocculationTank

Figure 1. Sequene of processes for Fenton oxidation of textile wastewater treatment.

highlights the need to develop an in situ Fenton process, which ispossible by in situ production of H2O2/HO• using AOPs.

Among all AOPs, iron catalyzed processes23 – 27 require acidicconditions and end up with sludge formation. Ozonation28 – 32 andphotocatalysis33 – 37 are energy intensive. Photocatalysis is limitedfor small streams of wastewater and low dye concentrations whileozonation relies on alkaline pH for HO• radical formation. However,the efficacy of these AOPs for H2O2/HO• radical production canbe improved through sonolysis as an auxiliary tool, which is alsohighly energy intensive.35,38 – 40 It is also evident from Table 1 thatfor almost all AOPs, operating costs make a significant contributionto the total cost of the process and thus make these processeshighly expensive for wastewater treatment applications.

Alternatively, smooth production of H2O2 via two electronsoxygen reduction reaction and continuous regeneration ofFe+2 from Fe+3 is possible with indirect electrochemical oxi-dation. Nonetheless, this process utilizes higher energy whichhinders its application in textile wastewater treatment.41 – 44

Also, the cost of electrodes contributes significantly to the totalcapital cost which can be estimated by the investigation inwhich the reported Boron doped diamond electrode costs ofUS$2.07E+ 04.45 Thus, the foregoing limitations associated withAOPs show the need to find and develop an alternative for in situH2O2/ HO• production that should be economical and energyefficient.

Scope of microbial fuel cells for hydrogen peroxideproductionWastewater based microbial fuel cells (WBMFCs) is sustainabletechnology46 which is emerging as a substitute for electrochem-ical oxidation for in situ production of H2O2. It consists of anode

and cathode compartments separated by a separator membrane.Substrate/organic matter in wastewater is oxidized by microbes inthe anode compartment resulting in the formation of electronsand protons. Protons and electrons enter the cathode compart-ment through the separator membrane and the external circuitas shown in Fig. 2. Here, these are reduced by oxygen or someother electron acceptor to produce water (Equation (2)) or H2O2(Equation (3)) through a four or two electron oxygen reductionreaction,47 respectively.

O2 + 4e− + 4H+ → 2H2O(

E∘ = 0.816)

(2)

O2 + 2e− + 2H+ → H2O2

(E∘ = 0.295

)(3)

Water synthesis following Equation (2) is commonly a recog-nized reaction with simultaneous power production. Over the lastdecade, the interest in generating power from wastewater treat-ment in WBMFCs has become a global quest with more and moreresearches emerging. The result obtained for a search with key-word ‘Microbial fuel cell’ from ‘Web of Science’ is presented inFig. 3.

Interestingly, there has been almost 90% increase in paperspublished over the last decade while 3 and 16 published articleswere found with keyword ‘in situ production of hydrogen peroxidein microbial fuel cells’ and ‘production of hydrogen peroxidein microbial fuel cells’, respectively (Web of knowledge, 2013).Two conclusions can be drawn from these results: (1) researchershave more interest in power production; and (2) the desired levelfor practical applications of power production in conventionalWBMFCs is still not achieved.48

J Chem Technol Biotechnol 2014; 89: 1466–1480 © 2014 Society of Chemical Industry wileyonlinelibrary.com/jctb

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www.soci.org A Asghar, AA Abdul Raman, WMAW Daud

Table 1. Estimated costs of AOPs1

AOPsCapital cost

($/1000gallons)Operating and maintenance

cost ($/1000 gallons)Total cost

($/1000 gallons) Status

Fenton/US 1.59E+ 08 (Roughly) 1.99E+ 06 (without US)6.35 E+ 06 (with US)

1.61E+ 08 (without US)1.65E+ 08 (with US)

Emerging

Photocatalysis 2.67E +07 8.14E+ 07 8.36 E+ 08 EmergingOzonation 4.53 E+ 05 1.15E+ 05 5.68E+ 05 EmergingUS 8.23E+ 09 5.02E+ 08 8.73E+ 09 EmergingUS+O3 1.92E+ 09 5.28E+ 07 1.97E+ 09 EmergingUS+UV+O3 1.50e+ 09 4.43E+ 07 1.54E+ 09 Emerging

*Capacity= 1000 L min.−1

Microorganism

PEM

Power produced isused for aeration

Current output

Wastewater(Cathode

Chamber)

Wastewater(Anode

Chamber) Protons

Through external circuit Electrons

H2O2

O2

Figure 2. Hydrogen peroxide production mechanism in dual chamber WBMFC.

0

100

200

300

400

500

600

Figure 3. Total published items on microbial fuel cells (Web of knowledge,2013).

However, WBMFCs have the potential for in situ production ofH2O2 not only for textile wastewater treatment17 but also forother recalcitrant contaminants49,50 as reported by Fu et al.51 Theauthors obtained 79 mg L−1 of H2O2 at the pure graphite cathodeusing anaerobic sludge as an inoculum in dual chamber WBMFC.However, Modin and Fukushi52 obtained 2.3 g L−1 of H2O2 at an

energy cost of 8.3kWh/kgH2O2 using municipal wastewater as aninoculum. It suggests that this technology has potential for H2O2

production at lower cost but it needs to be explored more forefficient wastewater treatment.

WBMFC has an edge over other methods as it offers severaladvantages for textile wastewater treatment. These include: (1) itis environment-friendly; (2) energy input is not required providedcathode chamber is aerated passively; (3) it does not evolve any offgas except CO2 which is combustion free in nature;53 and (4) in situproduction of H2O2 is cost saving.17

Existing data on in situ production of H2O2 in WBMFCs especiallyfor textile wastewater treatment is limited. Several factors includ-ing bacterial culture, type of anode and cathode used, electrodespacing, coulombic efficiency, power density, internal resistance,and configuration of WBMFCs must be studied in detail in orderto solve the problems and challenges with in situ H2O2 produc-tion. Besides, bioelectrical current generated in WBMFC is directlylinked with H2O2 production.51 Hence, the aim of this review is tohighlight the potential of WBMFCs for in situ production of H2O2

for textile wastewater treatment with simultaneous power produc-tion. Moreover, it will also discusses in detail the recent advancesin power production from wastewater treatment together with theweaknesses and strengths of WBMFCs.

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Table 2. Bacterial species used in microbial fuel cells

Bacterial species/Source Culture Reference

Shewanella oneidensisDSP10

Pure culture Ringeisen et al.72,Biffinger et al.73

S. Oneidensis MR-1 Pure culture Manohar et al.74,Biffinger et al.73

Geobacter sulfurreducens Pure culture R. Bond et al.66

Geobactermetallireducens

Pure culture Min et al.75

Geopsychrobacterelectrodiphilus gen.nov., sp. nov.,

Mixed culture Holmes et al.76

Geothrix fermentans Pure culture R. Bond et al.65

RhodopseudomonasPalustris DX-1

Pure culture Xing et al.77

E.coli Potter et al.78

Domestic wastewater Mixed culture Liu et al.79, Ahn andLogan80,81

Sediment wastewater Mixed culture Lowy et al82, Liu et al83,Holmes76

Brewery wastewater Mixed culture Feng et al.84, Scott et al.48

Municipal wastewater Mixed culture Modin and Fukushi69

MICROBIAL COMMUNITIESMicrobial communities have prime importance in WBMFCs.Microbes oxidize organic material present in wastewater andtransfer electrons to the anode.54 Most of the bacteria are elec-trochemically inactive in nature which employ mediators knownas electron shuttles for efficient electron transfer.55 Thionine,56

methyle viologen, methylene blue,57 humic acid,58 and neutralred59 are used as electron shuttles in mediator WBMFCs. Thesemediators are toxic in nature and easily washed away which resultsin operational losses.60

In mediator-less WBMFC, electricigens facilitate extra-cellularelectron transfer to the anode.61 Several attempts to evaluate theperformance of WBMFCs with mixed as well as pure culture havebeen made to date as summarized in Table 2. Generally, mixed cul-tures produce high power densities62 while it takes a longer timeto obtain stable power output in comparison with the pure culture.This can be because microbial communities are unable to degradethe complex material or competing processes, for example,methanogenesis consumes substrate and forms methane.46

However, the combination of high electrode surface area alongwith controlled substrate loading suppresses the growth ofmethanogenesis and increase the performance of WBMFCs.63

However, Geobacter species have the potential to producepower densities comparable with mixed culture because of theability to oxidize the organic substrate completely. This can beascribed to a difference in electron transfer mechanism in whichthey transfer electrons through direct established link with theanode.64,65 Moreover, formation of H2O2 is also possible in thepresence of G. sulfurreducens strain using graphite carbon ascathode.66 However, G. sulfurreducens species are highly sensitiveto oxygen therefore, Fernantez de Dios et al.67 suggested the useof Shewanella species which is a facultative exoelectrogen andobtained 82% mineralization of dye mixture through H2O2 pro-duction in the cathode chamber of WBMFCs. In lieu of pure cul-ture, Rozendal et al.49 investigated the synthesis of H2O2 by inoc-ulating anode chamber by microbial consortium obtained from

MFCs for carbon and nitrogen removal.68 In this study, an effi-ciency of ∼83% was achieved with a low energy requirement of∼0.93 kWh/kg H2O2. Fu et al.51 obtained 79 mg L−1 H2O2 usingmixed culture that is anaerobic sludge strengthened by adding700 mgCOD L−1 glucose with current output efficiency of 69.47%.Then this idea was extended in their next investigation by usingthe same inoculum for in situ Fenton process in which they suc-cessfully achieved 76% amaranth dye degradation with simulta-neous power production.17 Other cultures that have been inves-tigated for H2O2 production are municipal wastewater,69 brewerywastewater70 and pre-acclimated inoculums.71

There are few microbial consortium-based examples which showthe possibility of in situ production of H2O2 for textile wastewatertreatment with simultaneous power production. However, com-munity composition of mixed culture is poorly defined thus thereis a need to investigate the mechanistic and physiological behaviorof microbes64 that can serve as model for developing WBMFC forspecific applications such as in situ production of H2O2 for textilewaste water treatment.

MICROBIAL FUEL CELLSCONFIGURATION/ARCHITECTUREDesign of WBMFCs for in situ H2O2 production should be econom-ical in order to be practical at large scale. Several designs for WBM-FCs have been proposed for several applications.61,72,75,81,85 – 89

According to the literature, WBMFCs are broadly classified into twoconfigurations: (1) single chamber; and (2) dual chamber.

Dual chamber WBMFCs comprise anode and cathode chambersconnected by either a proton/cation exchange membrane51,90 ora salt bridge75 which physically separates the two compartmentswhile allowing protons/cations to travel to the cathode in orderto sustain electrical current as shown in Fig. 4(a).91 Single cham-ber WBMFCs mainly comprise a single compartment with cathodeforming one wall of the cell such that one of its side faces water andanother faces air49 as presented in Fig. 4(b). There are other config-urations for single and dual chamber as well as their modificationsas presented in Table 3.

Textile wastewater treatment through an in situ H2O2 pathway(hence in situ Fenton oxidation) normally employs a dual cham-ber configuration.17,51 Only one investigation of a single chamberWBMFC for H2O2 production can be found in the literature. There-fore, it is imperative to understand the operational aspects of thearchitectural design for developing WBMFCs in the textile sector.High internal resistance, large working volume, electrode spacing,oxygen diffusion and use of expensive separator membrane93,100

are important factors that must be taken into consideration beforeselecting a suitable design.

It is a general perception that single chamber WBMFCs have asimple configuration and offer high power output at lower costcompared with dual chamber.97 However, reduction in electrodespacing in dual chamber WBMFCs improves the power efficiencyas a consequence of decrease in ohmic losses. For example, Liuand Logan86 proposed a cost effective approach and achieved 60%increase in power output as a result of reduced electrode spacing.

Liu et al.85 achieved a reduction in internal resistance from 35to 16 Ω and an increase in power output from 720 to 1210mW m-2 by decreasing the distance between electrodes from 4to 2 cm. However, Du et al.94 observed contrasting results andreported a decline in voltage output from 670 to 430 mV whenelectrode spacing was reduced from 30 to 10 cm in membrane-lessWBMFCs. This suggests that decreasing electrode spacing may


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Wastewater +Substrate

Nutrientsolution H+

H+

Organic

H+ & CO2

CO2

e–

e–

R

Dye & Ironsolution

H+

PEM

Microbes

Anode

Cathode

Wastewater + Substrate

Nutrient

R

Air Organicsubstrate

Microbes

Anode

PEM

Cathode

O2 in

O2 out

O2

O2

H2O2

H2O

Figure 4. Schematic of (a) dual chamber WBMFC (b) Single Chamber WBMFC.

not be effective for single chamber WBMFCs. A possible reasoncould be the diffusion of oxygen from cathode to anode. It notonly hampers the metabolism of bacterial consortium but it alsocompetes with the anode to accept the electrons and lowers theoutput of WBMFCs.85,94

Other electron acceptors may also be used and include air,101

ferricyanide, permanganate,102 ferrous iron,103,104 bio-mineralizedmanganese105 nitrates and chloro-organics.47 However, oxygen isthe most suitable electron acceptor because of its high oxidationpotential, availability, sustainability, and lack of waste product.106

On the other hand, in situ production of H2O2 is only possible whenoxygen is used as an electron acceptor.49,50 This implies that a sin-gle chamber configuration cannot be used for in situ H2O2 produc-tion in WBMFCs. Therefore, the effect of oxygen diffusion shouldbe reduced by using separator electrode assembly in WBMFCs97,98

which significantly improves the coulombic efficiency even in sin-gle chamber configuration.86

To date, Nafion51 and cation exchange membrane49,90 have beenused for both direct reduction of dyes as well as in situ productionof H2O2. Inevitably, the membrane affected the maximum powerdensity. Moreover, high cost and high internal resistance rendersthe use of membranes in WBMFCs in large-scale applications.For instance, Nafion 117 is the most frequently used electrode

separator material and is generally regarded as having excellentproton conductivity.91 However, it is not economical to use as itcosts $1400 per m2 which is more expensive than a simple cationexchange membrane which costs around $80 per m2.107

To reduce the cost of the membrane, dual chamber WBMFCswith cellulose acetate microfiltration membrane and Nafionwere compared by Tang et al.96 The cellulose acetate mem-brane is cheaper and it costs $40 per m2 which is, 35 times lessexpensive than the frequently used Nafion membrane.108 Bothtypes of membrane produce almost similar results except forcoulombic efficiency, for which the cellulose acetate membrane ismarkedly lower than Nafion. This indicates that the microfiltrationmembrane is less efficient in blocking oxygen, as the diffusionco-efficient for the microfiltration membrane is 5.9× 104 cm s−1

which is much larger than that of Nafion 117, which is 1.3× 104

cm s−1.95

To encourage the use of membranes other than Nafion, a com-parison among cation, anion and three types of ultra-filtrationmembranes has been conducted while using acetate as carbonsource.95 It has been observed that cube shaped WBMFCs per-form well in comparison with bottle shaped WBMFC. In this study,Nafion was most permeable to the oxygen while ultra-filtrationwith a molecular cut of 0.5 K was the least permeable with oxygen



Table 3. Configurations

WBMFC Anode Cathode Membrane

Total

vol.(mL)

Power density

(mW m-2) Rint (Ω)

Electrode

spacing (cm)

Parameters

studied Ref.

SCMFC Plain carbon cloth Carbon Cloth/Pt/C Membrane less 21 1000 1 Flow pattern; Electrodespacing

92

ML-MFC ACC* Carbon cloth Membrane less 540 68.537.4

154.3282.5

10.513.5

Mixed/Pure culture 93

ML-MFC Granular Graphite Granular Graphite Membrane less – – 4172

1030

Electrode spacing 94

Mini-MFC RVC Graphite felt RVC Graphite felt Nafion 117 1.2 2410

– 0.0175 MFC type; Anodematerial

72

Air cathodeMFC

Carbon paper Carbon cloth/Pt Polymeric PEMMembrane less

28 262494

– – Separator membrane 86

DCMFC Carbon paper Carbon paper Nafion 117Salt bridge

500250

382.2

128619920

– Separator membrane 75

B-MFC Carbon paper Carbon paper /Pt UF-1KNafion

28 3638

1239 – Separator membrane 95

C-MFC Carbon paper Carbon paper /Pt UF-1K Nafion 28 462514

9884

– Separator membrane 95

DCMFC Graphite rod Graphite rod/Pt MF membranea 500 831 263 10 MF membrane 96

DCMFC Graphite rod Graphite rod/Pt PEM 500 872 267 10 MF membrane 96

BAFMFC Compound anodec Carbon cloth/Pt/PTFE

Membrane less 460 10.7 (mW m-3) 10.8 0.2 Architecture of MFC 87

Sensor typeMFC

Graphite felt Pt coated graphitefelt

Cathode specificmembrane

20 560 – – Type of MFC 61

SCMFC Graphite fiber brush Carbon cloth/Pt/PTFE

Textile separatorb 130 – 2580c

1800d– Multiple anodes in series 81

SCMFC Toray Carbon Paper Carbon paper/Pt Membrane less 2828

7201210

161 77 4 2 Electrode spacing;Internal resistance

85

SCMFC Carbon cloth Carbon Cloth/Pt/PTFE

J-clothe 2.5 1800 92 0.6 Membrane type;Continuous flow

97

SCMFC Carbon cloth Carbon Cloth/Pt/PTFE

J-clothe 2.5 1120 92 0.6 Membrane type; BatchFlow

97

C-SCMFC Ammonia treatedcarbon cloth

Carbon Cloth/Pt/PTFE

Glass Fiber 1 Glass Fiber 1 9 1.8 939895

5.92.2

20.3

Electrode Spacing 98

DCMFC Porous carbonpaper

Aerobic biocathode UFM-1K 800 122 Rct: 105.1 – Congo red color removal 99

DCMFC Graphite granules SPG rods PEM 80 25.13 150 – H2O2 synthesis 51

*ACC: Water proof activated carbon cloth; RVC: Reticulated vitreous carbon; DSEA2: Double separator electrode assembly with 2 cm electrode spacing; DSEA0.3: Doubleseparator electrode assembly with 0.3 cm electrode spacing; SPG: Spectrographically pure graphite.a Cellulose acetate microfiltration membrane.b 46% cellulose and 54% polyester.c Power density based on polarization data.d Power density based on EIS analysis.e CEA: Cloth electrode assembly.

diffusion co-efficient of 0.19 cm s−1. However, it exhibited the high-est internal resistance of 6009 Ω in the case of B-MFC. UF-1K andUF-3K showed comparable internal resistance and power densitywith oxygen diffusion co-efficient of 0.41 and 0.42, respectively.Thus, UF membrane is the most convenient option among all typesof membranes discussed as it is cheap and prohibitive for oxy-gen. Other membranes that have been utilized are bipolar mem-brane, J-cloth and glass wool109 with J-cloth demonstrating 100%increase in coulombic efficiency.97,103

Considering the disadvantages of membranes, Zhu and Logan71

proposed an energy efficient and cost effective approach usingmembrane-less single chamber WBMFCs as a low power sourcefor operating electro-Fenton reaction in an electro-Fenton reactor.In this study, anode and cathode of WBMFCs were connectedwith the cathode and anode of an electro-Fenton reactor respec-tively. In situ H2O2 production and high mineralization efficienciesof 75% were the advantages of this technology. However, theconcentration of pollutant used was very low, which predicts poorperformance for high pollutant concentration possibly because

of large distances between anode and cathode of both reac-tors. Another modification in WBMFCs design was successfullydemonstrated by Fernandez de Dios et al.67 In this study, dualchamber WBMFC was investigated for mineralization of low dyeconcentrations through in situ H2O2 production (in situ Fentonoxidation). Maximum 82% dye (Lissamine green dye and CrystalViolet) mineralization efficiency was obtained with simultaneouspower production of 12.3 W m-3. The power produced was usedto provide energy for operating the electro-Fenton reactor for insitu H2O2 production and 85% mineralization efficiency of 15 mgL−1 dye was obtained.

From the above discussion it is concluded that WBMFC is in itsdevelopment stage and is an efficient method for low concen-trations of recalcitrant wastes. Considering all the disadvantagesassociated with dual chamber WBMFCs, use of expensive mem-branes, and limitations of single chamber WBMFCs, it is stronglyrecommended to consider all aspects before selecting WBMFCsconfiguration for future research and development. These includeunwanted oxygen diffusion from cathode to anode, maximum


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availability of electrons for oxygen to be reduced in the cathodechamber, cost of membrane, substrate leakage, accumulation ofprotons and cations in the anode chamber.

ANODE AND CATHODE MODIFICATIONAnodeStudies focusing on in situ H2O2 production (hence in situ Fentonoxidation) in WBMFCs have not considered the anode materialand its modification as an objective. However, the performance ofWBMFCs for in situ H2O2 production depends on efficient electrontransfer from oxidized organic matter to the anode in the presenceof biocatalyst.82,100,110,111 Higher power densities can be achievedby combining factors including electrical conductivity, increasedactive surface area and biocompatible nature of anode surface.48

Moreover, the amount of H2O2 produced in a WBMFC is in directrelation with the current density.51 Keeping this in view, thissection attempts to collate investigations on anode modificationto improve current densities in WBMFCs.

Understanding a co-relation between the performance and sur-face chemistry of the anode is important to improve the effi-ciency of WBMFCs.112 Among the parameters that determine theanode performance, the nature of anode material and its con-figuration are important. They not only influence the growth ofmicrobial community but also affect the interfacial electron trans-fer resistance.113 They must be electrically conductive, biocompat-ible, non-porous and cost effective. Carbon based materials in theform of rods, mesh, paper or graphite rods are suitable for use asthey are chemically stable and non-toxic to the microbes.106,113,114

Therefore, these factors should be contemplated for in situ H2O2production for textile wastewater treatment.

To-date, graphite fiber brush,115,116 carbon paper,85 carboncloth,85,92 granular graphite,114,117 graphite felt electrode,107 stain-less steel,106,118 and copper119 have been tested to improve theperformance of WBMFCs. Among all these materials, the maxi-mum power density achieved was 1540 mW m-2 which resultedfrom advective flow from anode to cathode instead of the specificmaterial use. Power density of 720 mW m-2 was achieved withgraphite fiber brush which was improved to 1210 mW m-2 bydecreasing the electrode spacing from 4 to 2 cm.116 However,for in situ H2O2 production applications, Fu et al.51 and Rozendalet al.49 used graphite granules in the anode chamber and achieved79 mg L−1 in 12 h and 2 kg m-3 day−1 of H2O2, respectively. Thismarked difference was due to the applied voltage of 0.5 V whichresulted in an increase in the yield of H2O2.49 With the same anodematerial, Fu et al.17 achieved 76% removal efficiency of amaranthdye (75 mg L−1) with power density production of 28.3 W m-3.

Attempts to improve the performance of WBMFCs were madeby binding mediators82,120 to the anode. The use of mediatorsis highly discouraged as these are expensive and toxic. Theycan also be easily washed away which increases the operationalcost.116,121

Anode modification has been reported to be an effective alterna-tive to chemical binding of mediators. It has been investigated andsuggested by several authors based on economical effectivenessand expected successful application at large scale.87,110,116,121 – 126

Based on reviewed literature, ammonia and heat treatment arethe most economical and practical for large-scale implementation.Cheng et al.122 achieved 1970 mW m-2 power density and reported2.5 days start up time with ammonia treated plain carbon cloth assummarized in Table 4.

Zhu and Logan71 used heat treated carbon fiber brush as ananode in a single chamber WBMFC which was used as a low volt-age power source for in situ H2O2 production in electro-Fentonreactors. They used phenol as an organic pollutant and obtained75% mineralization efficiency. This implies that anode modifi-cation should be explored further for in situ H2O2 productionin WBMFCs as it is directly linked with higher/improved powerproduction.

CathodeResearch on material selection for the cathode and its modificationare rather limited. Cathodic activation, ohmic and mass transportare losses that limit the WBMFCs yield.47

Platinum (Pt) is the most commonly used catalyst in elec-trochemical systems due to its low overpotential for oxygenreduction reaction.130 However, its high cost always limits itsapplication.131 Like the anode, carbon based materials modifiedwith different metal catalysts have been found to improve oxygenreduction rate. Platinum,130,132 gold,119 lead oxide,133 cobalt andiron based materials134,135 and biocatalyst103, 136-138 have been usedas modifying materials for cathode. Metal based catalysts are sen-sitive to high pH values and as explained earlier, continuous oper-ation of WBMFCs results in alkalinization of catholyte.47 Besides,H2O2 production in the cathode compartment is favored at neu-tral pH values17. Thus, it may limit the use of metal catalyst for insitu production of H2O2 for dye wastewater treatment.139

GAC, 109,129 carbon felt140 graphite granules,140 rutile coatedgraphite cathode90 have been used for direct dye degradationwhereas gas diffusion cathode,49 carbon felt50 and graphite (SPG)rods51 have been frequently used for in situ H2O2 production.In fact, it has been explicitly reported by Fu el al.51 that H2O2is produced at the surface of graphite without Pt loading bytwo-electron reduction of oxygen. It suggests that WBMFCs canproduce H2O2 with simultaneous production of electricity ina very cost effective way without using expensive catalyst forcathode material. In another study, modification of carbon feltelectrode was performed by preparing Fe@Fe2O3/NCF compos-ite and 90% mineralization efficiency of 15 mg L−1 RhodamineB (RhB) dye was obtained with simultaneous power output of307mW m-2. Here, the purpose of modification was the contin-uous supply of iron for Fenton reaction with simultaneous pro-duction of H2O2.70 In this study, dye concentration was verylow but it shows that selection of cathode material should alsobe critically explored and modified for improvement in currenttechnologies.

From experimental studies it is clear that increasing the effec-tive surface area of anode and cathode enhances the perfor-mance of WBMFCs while there are other factors which also con-tribute to its overall performance. For instance, cost of materialsis important in determining the possibility of successful applica-tion of WBMFCs at large scale. Based on previous studies on powerproduction through WBMFCs, economic aspects and efficiencymust be considered while selecting and modifying electrodematerial.

EFFECT OF OPERATING PARAMETERSEffect of dye concentrationLiterature available on in situ production of H2O2 in WBMFCsis rather limited. However, dye concentration in the cathodechamber is an important parameter because it directly affects



Table 4. Anode modification

Microbial fuel cells Anode Anode modification Cathode Efficiency Ref

SCMFC (cube shape) Carbon cloth Nitrogen addition bydiazonium salt

Carbon cloth/ Pt/PTFE Start up time: 60 h; P: 938 mWm-2

112

DCMFC Carbon felt PPy-ADQS* Carbon felt Start up time:23 h; P: 1303 mWm-2

123

SCMFC Carbon fiber brush Acid soaking Carbon cloth/Pt Start up time:140 h; P: 1100 mWm-2; Surface area: 9.42 m2

g−1; O/C: 0.28; N/C: 0.0164

87

Carbon fiber brush Heating Carbon cloth/Pt Start up times:240 h; P: 1280mW m-2; Surface area: 49.3m2 g−1; O/C: 0.13; N/C: 0.0218

Carbon fiber brush Combined acid and heattreatment

Carbon cloth/Pt Start up times: 200 h; P: 1370mW m-2; Surface area: 43.9m2 g−1; O/C: 0.18; N/C: 0.0261

SCMFC (cube shape) Carbon mesh Heat treatment at 450 ∘C(30 mins)Acidtreatment

Carbon Cloth/Pt/ PTFE Startup times: 200 h: P: 922 mWm-2: Surface area: 58± 1 cm2

Startup times: 200 h; P: 1015mW m-2; Surface area: 58± 1cm2

116

SCMFC MWNT/Toray Carbonpaper

MWNT* and PEI* Carbon paper/Pt/PTFE P: 290 mW m-2 113

SCMFC Plain carbon cloth High Temp. AmmoniaTreatment

Carbon cloth/Pt/PTFE Startup times:60 h; P: 1970 mWm-2

122

SCMFC Graphite pallets(diameter between 0.2to 0.6 cm)

Irregular packed bed ofgraphite granules

Toray carbon paper/Pt P: 1.3W m-3; Startup times:1247cm-2 CE: 68%; COD removal:89%

114

B-SCMFC Recycled tire Graphite paint coating(4 layers)

Carbon cloth/Pt/PTFE P: 421 mW m-2; Surface area: 4.5m2 g−1; Ohmic resistance: 2.5Ωmm1; CE: 25.1%

121

DCMFC Plain porous carbonpaper

Iron oxide Plain porous carbonpaper/Pt

Startup times 20 h; P: 30 mWm-2; CE: 80%

127

DCMFC (rectangular shape) Plain carbon graphite +200mV Poisedpotential

Plain carbon graphite Startup times: 840 h; P:1.66 Wm-3; I: 0.42 mA

128

ML-SCMFC Stainless steel mesh Goethite (Nanosemiconductingmaterial) in AC*powder

Carbon mesh/Pt/PTFE Startup times: 20 days;P:693± 20 mW m-2

110

GACB MFC GACB / four graphiterods

– GAC bed P: 8 W m-3; Rint: 47 Ω 129

*CF: carbon fiber brush; PPy-ADQS: Polypyrrol/ Anthraquinone-2, 6-disulphonic disodium salt; MWNT: Multiwall Carbon Nano-tube; PEI: PolyelectrolytePolyethyleneimine; DCMFC: Dual chamber microbial fuel cell; AC: Activated carbon; CP: Carbon Paper; GACB: Granular activated carbon bed.

the mineralization efficiency of treated wastewater. It is obviousfrom the literature that in situ production of H2O2 in the cathodecompartment of WBMFCs forms an in situ Fenton process inthe presence of iron catalyst. In a conventional Fenton process,increase in initial dye concentration leads to lower degradationefficiency and prolongs the time required for degradation.27,40,141

For instance, 96% degradation efficiency was achieved after 35min with 29 mg L−1 of MG while higher concentrations of dyeslowed down the degradation rate. In the same study, 10% degra-dation efficiency was achieved with 100 mg L−1 of dye as moreH2O2 was required.141 The same concept can be applied to in situH2O2 production in WBMFCs.

Fu el al.17 confirmed a decrease in the efficiencies of degradationof amaranth azo dye while studying in situ production of H2O2 inWBMFCs. According to the reported data, degradation efficiencydeclined from 100% to 75.65% as concentration of amaranth dyewas increased from 25 to 75 mg L−1. The reason for decrease indye degradation efficiency may be two-fold: (1) at high concen-tration dye may compete with oxygen for electron acceptance in

the cathode chamber; and (2) changing dye concentration andkeeping other factors constant may result in a decrease in removalefficiency in WBMFCs.

Literature available on dye degradation through an in situ Fen-ton oxidation pathway reports high mineralization efficiency atlow concentrations. For instance, Zhuang et al.70 investigated 15mg L−1 of Rhodamine B (RhB) dye and achieved 90% miner-alization efficiency. Fernandez et al.67 also studied 10 mg L−1

of Lissamine Green B and 10 mg L−1 of Crystal Violet dye inboth ex situ and in situ Fenton oxidation systems and obtaineda maximum of 82% mineralization efficiency. This implies thatWBMFC in its initial stages of in situ H2O2 production is capa-ble of producing low concentrations of H2O2 which are insuffi-cient to treat high concentrations of recalcitrant contaminants.Moreover, yield of H2O2 in the cathode compartment of WBM-FCs depends on both anodic and cathodic conditions, whichresults in a decrease in dye degradation efficiency in responseto increased concentration if all other factors applied remain thesame.


1474


Effect of pHThe pH in WBMFCs strongly affects the current density.142 Accord-ing to the reaction mechanism, the anodic reaction is protonproducing while the cathodic reaction is proton consuming.46

Continuous operation in a WBMFC causes acidification in theanode compartment due to accumulation of protons and alka-linization in the cathode compartment. This leads to electro-chemical/thermodynamical limitations on WBMFCs performance.Hence, decreasing the operating pH of catholyte will enhanceoxygen reduction which increases current output from WBMFCs.Bacteria show optimal growth close to neutral pH and respondto changes in pH which consequently affect the current.73,143 – 148

Therefore, it is important to determine the effect of pH on in situproduction of H2O2 in the cathode compartment.

In order to determine the effect of pH, Fu et al.51 synthesizedH2O2 production in the cathode compartment and achieved acurrent efficiency of 69.47% along with 79 mg L−1 of H2O2 at anoptimized pH value of 7 in the cathode chamber. These resultswere supported by the fact that charge transfer resistance (Rct)for two-electron oxygen reduction reaction to produce H2O2 is2512 Ω at pH 7, lower than that at pH 3 (5708 Ω). Thus, itsuggests that neutral pH is favored for the production of H2O2for dye degradation in the cathode chamber. However, for in situFenton oxidation to occur, which is one of the focuses of thecurrent study, a pH value of 3 is necessary. While investigatingin situ Fenton oxidation in WBMFCs, Fu et al.17 achieved 76%degradation of amaranth dye at pH 3. This indicates that pH is asimportant as other parameters for in situ Fenton oxidation becausein situ H2O2 production is favored at neutral pH and Fentonoxidation takes place at pH 3 even in WBMFCs.70,149 Moreover, pHincrease in the cathode compartment as a result of continuousoperation is an issue that is required to be solved. Its effect onH2O2 production can be estimated by the studies conducted byModin and Fukushi,69 in which cathodic pH was increased to 12.9at the end of experiments and 9.7 g L−1 of H2O2 was produced atan energy requirement of 3 kWh kg−1 H2O2. Thus pH adjustment inboth chambers is an essential component of operating WBMFCs,that is why it is recommended to use buffer to maintain pH atdesired levels.95,122,150 – 152

Effect of temperatureLike pH, temperature is a crucial parameter that affects the per-formance of both anode and cathode chambers. In the anodecompartment, temperature increase improves the microbialactivity exponentially which results in an increase in poweroutput.154,155 Larrosa-Guerrero et al.156 confirmed that powerdensity was improved from 8.1 mW m-2 to 92.8 mW m-2 whentemperature was changed from 4∘C to 35∘C.155,157 The conceptbehind this increase is that microbes show maximum metabolicactivities at temperatures between 30∘C and 45∘C while lowerand higher temperature leads to decomposition of biofilm andinactivation of bacterial metabolic activities.151,155 – 157 Moreover,elevated temperature also accelerates the bacterial growth whichin turn reduces the start up time. Patil et al.155 reported that timerequired for biofilm formation in a primary wastewater treatmentplant reduced from 40 days to 3.5 days when temperature wasincreased from 15∘C to 35∘C.

In the cathode chamber, in situ H2O2 production forms an in situFenton process in the presence of iron salt. In one study, Mericet al.158 demonstrated the effects of temperature on COD and colorremoval while investigating the Fenton process for Reactive Black

5. They obtained 84% COD removal at 40∘C which decreased whentemperature rose to 60∘C, indicating the importance of tempera-ture optimization for the Fenton process. In contrast, temperaturesin the range 20–25∘C (room temperature) have been used in WBM-FCs applications for in situ H2O2 production. For instance, Fu el al.17

applied a temperature of 25∘C in all experiments for azo dye degra-dation through in situ produced H2O2. It may be because at hightemperature solubility of O2 in water decreases with increase intemperature which may result in reduction of H2O2 productionin the cathode compartment.159 However, this parameter has notbeen investigated yet which suggests that it should be furtherinvestigated for future development.

Effect of external loadNumerical models of WBMFCs predict that high external resis-tance causes higher biomass growth and decline in current gen-erated at the anode.160,161 Studies addressing the effect of exter-nal resistance on WBMFCs performance mainly focus on therelationship between external resistance, current and coulombicefficiency.162 – 166 It has been found that different external resis-tances characterize distinct microbial communities. Katuri et al.166

reported that low external resistance causes the enrichment ofelectrogenic bacteria and improves output current density. More-over, the production rate of H2O2 in the cathode compartment isin direct relation with the current density, which is linked to theapplied external resistance. In order to confirm it, a study con-ducted by Zhuang et al.70 is of great importance. In this study, anincrease in H2O2 production from 0.01 mmol L−1 to 0.02 mmolL−1 was observed when the external resistance was changed fromclose to short circuit (0 Ω). The reason for this increase in H2O2concentration was the increase in current flow from the anodeand cathode chamber. In order to confirm an increase in poweroutput in response to external resistance, Zhu and Logan71 main-tained the external resistance at zero in between the WBMFC andelectro-Fenton reactor and achieved a maximum power densityof 1964 mW m-2, which was much higher than that reported inprevious studies (300–500 mW m-2). Moreover, in a dual cham-ber WBMFC external resistance was varied from 1000 to 200 Ω andfaster degradation of Amaranth dye through the in situ H2O2 path-way was obtained at low resistance.17 This is because there is adecrease in cathode potential at lower external resistance. It sug-gests that a decrease in external resistance results in an increase incurrent flow which in turn increases cathodic current density andhence increases H2O2 production rate.

Effect of co-substratesSubstrate is one of the most important biological factors affectingWBMFC current yield.168 According to a simple model proposed byJadhav and Ghangrekar,143 current flow from anode to cathode isdirectly proportional to substrate removal in the anode chamber.This implies that substrate concentration plays an important partin current output and availability of electrons and protons in thecathode chamber for H2O2 production.

Since current production in WBMFCs is directly linked with oxy-gen reduction in the cathode chamber, suitable selection and opti-mum concentrations of substrate are important for H2O2 produc-tion. Also, H2O2 production in WBMFCs is favored at high cathodiccurrent density which is affected by anode conditions.71 That iswhy it is important to study the effect of different substrates onH2O2 production.



Various substrates and wastewater (as substrate) have beenused in WBMFCs for power as well as H2O2 production. Theseinclude glucose,51,164,169,170 cysteine,130 acetate,71,169 ethanol,169

sodium fumarate, brewery,70,84 domestic,79 synthetic and munic-ipal wastewater.69 In order to emphasize the importance ofco-substrate, Modin and Fukushi69 compared the performanceof WBMFCs in the presence of real municipal wastewater andsynthetic feed enriched with 500 mg L−1 of acetate. The resultsshow that by using synthetic feed, an increase in H2O2 produc-tion from 2.7 g L−1 to 9.7 g L−1 was observed with simultaneousdecrease in energy requirement from 8.3 kWh kg−1H2O2 to 3kWh kg−1H2O2. Fu et al.171 used anaerobic sludge from municipalwastewater and glucose equivalent to 700 mg L−1 was used tostrengthen the anolyte. In contrast to prior investigation, 79 mgL−1 of H2O2 was obtained without energy requirement. Further-more, Zhu and Logan71 used 1 g L−1 of sodium acetate as a carbonsource for a single chamber WBMFC to provide power for in situH2O2 production in an electro-Fenton reactor. The H2O2 concen-tration produced was sufficient to achieve 90% mineralizationefficiency.

Comparison of the above mentioned studies highlights theimportance of suitable selection of substrate and its concentrationrange. Besides H2O2 production yield, large variations in perfor-mance parameters such as power density, coulombic efficiencies,current density, voltage, were observed for different substrates assummarized in Table 5. For instance, in a dual chamber configura-tion, acetate produced the highest power density of 123 mW m-2 at1000 Ω due to simpler metabolism compared with glucose (P = 28mW m-2) and xylose (P = 32 mW m-2) under the same experimentalconditions.58

Besides H2O2 production, the start-up time of WBMFCs is alsoinfluenced by the type of substrate used. To date, this aspecthas not been taken into account for in situ H2O2 productionapplications. Nevertheless, a comparative study demonstratingthe effect of substrate on start up times has been undertaken.In this study, domestic wastewater was amended with differentsubstrates. The results obtained show that domestic wastewateramended with acetate took 110 h for start up, and the WBMFCfailed to start up when 100 mmol L−1 of phosphate buffer wasadded to domestic wastewater.79 This highlights the importance

Table 5. Substrates and inoculum used

MFC Culture Source of inoculum Substrate Output Parameters studied Ref

SCMFC Mixed Aerobic and anaerobicsludge

Glucose (500 mg L−1) P: 103 mW m-2; Rint: 97Ω Congo red degradation 170


Acetate (500 mg L−1) P: 85 mW m-2; Rint: 163Ω Congo red degradation 127


Ethanol (500 mg L−1) P: 63.2 mW m-2; Rint: 253Ω Congo red degradation 86

DCMFC Mixed Anaerobic sludge Glucose (2 g L−1) P: 3.6 mW m-2; Startup: 18 h Substrate loading 79

DCMFC Mixed Anaerobic sewagesludge

Acetate Startup:50 h Acclimation strategy 79

SCMFC Mixed Primary sediment tank Glucose (170–1200 mgL−1)

Startup:140 h; P: 28 mW m-2

(PEM)/146 mW m-2

(without PEM)

Proton exchangemembrane

79

SCMFC Mixed Domestic wastewater no Startup: 118 h; P: 91 mW m-2 Startup; Power density 79

SCMFC Mixed Domestic wastewater Acetate Startup: 110 h; Startup; Power density 79

SCMFC Mixed Domestic wastewater Glucose Startup: 181 h; Startup; Power density 79

SCMFC Mixed Domestic wastewater Fumarate Startup: 115 h; P: ∼50 mWm-2

Startup; Power density 93

SCMFC Mixed Domestic wastewater Fe(III) Startup: 353 h; P: ∼103 mWm-2


SCMFC Mixed Domestic wastewater PBS (25, 50 mmol L−1) Startup:149–251 h; P: 88–96mW m-2


ML-MFC Mixed Activated sludge fromwine bearing waste

Glucose (300 mg L−1) P: 68.5; V: 124.4 mV; Rin:154.3Ω

Mixed/pure culture 127

MFC (Air cathode) Mixed Starch processingwastewater treatmentplant

Starch processingwastewater (4852 mgL−1)

P: 239.4 mW m-2; V: 490.8mV;COD removal: 98%

Wastewater treatment 86

DCMFC Mixed Anolyte of MFCinoculated withdomestic wastewater

Poultry wastewater(566 mg L−1)

P: 746 mW m-2; COD: 81.6%;C.E: 35.3± 3.2%

Microfiltrationmembrane

96

DCMFC (Cube shaped) Mixed Aerobic and anaerobicsludge

Glucose (500 mg L−1) Startup; 60 days; P: 122 mWm-2; Decolorization: 96.4%(29 h)

Congo reddecolorization;Cathode types

99

SCMFC (Cube shaped) Mixed Aerobic and anaerobicsludge

Glucose (500 mg L−1) Startup: 30 days; P: 324 mWm-2; Decolorization: 96.4%(107 h)

Congo reddecolorization;Cathode types

99

DCMFC Mixed Real textile wastewater(COD: 2080mg/L)

Substrate less COD: 76%(cathode); P: 1.7 Wm-3; Non toxic (24 h)

Real textile waste watertreatment

109

DCMFC Mixed Anaerobic sludge frommunicipal wastewatertreatment plant

Glucose (700 mg L−1) H2O2: 78.85 mg L−1; Time:12 h; COD conversionefficiency: 8.51%;

Synthesis of H2O251


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of a selective and detailed literature survey to select a substratefor WBMFCs applications. This was further confirmed by anotherstudy in which dual chamber microbial fuel cells using anaerobicsludge from a potato processing plant enriched with glucose assubstrate started up within 18 h.170

Limitations and merits of in situ H2O2production in WBMFCsThe forgoing discussion on the possibility of in situ H2O2 produc-tion in WBMFCs concludes that its production in WBMFCs is practi-cally challenging. The motivation that has provoked the attentionof researchers towards WBMFCs is its low operating cost becauseof no or reduced energy investment and the high cost of commer-cially available H2O2. It has already been estimated that for 1000kg of industrial wastewater 0.63 kg of H2O2 is required and it costs$350–500 per ton.173 Thus, in this scenario, H2O2 production inWBMFCs is an appealing alternative. Being in its initial stages ofdevelopment, production yields of H2O2 and current output effi-ciencies, and percentage mineralization of recalcitrant waste havenot reached the level of pilot plant studies. To date a maximum of9.7 g L−1 of H2O2 at an energy requirement of 3 kWh kg−1 H2O2and 79 mg L−1 of H2O2 with a current efficiency of 70% has beenobtained in dual chamber WBMFCs.69 However, high internal resis-tance and high operational cost of dual chamber WBMFCs hamperits wide application. In dual chamber WBMFCs the cost of protonexchange membrane is $100 per square meter which adds sub-stantial amount to the total cost of the process.71

A possible way to cut the cost of the membrane is to usesingle chamber WBMFCs. One way is to connect the cathode andanodes of single chamber WBMFCs to the anode and cathode ofan electro-Fenton reactor, respectively. In this way, WBMFCs will beused as a power source for the reactor for in situ H2O2 production.However, large distances between these two assemblies mayreduce the efficiency of the process.171 WBMFC systems are alsolimited to low concentrations ranging from 10–75 mg L−1 andsmall volumes of 150–250 L, as also summarized in the tables.However, the opportunity to use the power output as a sourceto provide power to other systems such as an electro-chemicalfuel cell for in situ Fenton process and to the aeration system forproviding oxygen to the cathode compartment for in situ H2O2production are a few advantages that can possibly cut the externalutility requirement.

CONCLUSIONWastewater based microbial fuel cells (WBMFC), are devices inwhich microbes (biocatalysts) are used to convert chemical energyinto electrical energy. This electrical energy can possibly be usedfor the in situ production of H2O2 for textile wastewater treatment.The application of in situ H2O2 for textile wastewater treatmenthas been demonstrated successfully. Up to ∼79 mg L−1 of H2O2can be produced in a dual chamber air cathode WBMFC usinganaerobic sludge as an inoculum. Moreover, 83.6% degradation ofamaranth dye has been obtained with simultaneous production of28.3 W m-3 power using 700 mg L−1 glucose as substrate fuel. Thefollowing conclusions can be drawn:

(1) Electricity produced in water based WBMFCs can be used forthe production of H2O2 in the cathode chamber.

(2) Anaerobic sludge can be used as a mixed culture for H2O2production. Nevertheless, information about electricigens is

not conclusive. To study this, Geobacter sulfurreducens or She-wanella species can be used as these are found to have poten-tial for production of H2O2 with power density comparablewith mixed culture.

(3) A single chamber configuration offers cost effective and sim-ple structure but it cannot be used for hydrogen peroxideproduction. A dual chamber is a favorable option but it facesthe challenges of high internal resistance, expensive separatormembrane, and also oxygen diffusion from cathode to anodechamber, which may reduce the efficiency of the process bothin terms of H2O2 as well as power production. Single chamberWBMFCs can be used as a low power source for operating anelectro-Fenton reactor for in situ H2O2 production. Large dis-tances between the anode and cathode may reduce the poweroutput.

(4) Modification of anode material is an option for improvingpower density which in turn increases H2O2 yield. Most mod-ification methods are not cost effective. However, ammonia,heat and acid treatment are less expensive and can possiblybe used for increasing the yield of hydrogen peroxide. Puregraphite cathode is favored for in situ production of H2O2.

(5) Optimizing operating parameters such as pH, temperature,external loads and co-substrate concentration can furthermaximize both H2O2 concentration and power production.

Wastewater based microbial fuel cells offer integrated solutionfor in situ H2O2 production as power produced can be used foraeration in the cathode chamber. Numerous applications andmodifications of WBMFCs have been proposed but few of them arebased on dye degradation through in situ production of H2O2. TheWBMFC is facing many issues for H2O2 production but still it offersa green and sustainable solution for textile wastewater treatmentby simultaneous power production.

ACKNOWLEDGEMENTThe authors are grateful to the University of Malaya High ImpactResearch Grant (HIR-MOHE- D000037-16001) from the Ministry ofHigher Education Malaysia and University of Malaya Bright SparksUnit which financially supported this work.

ABBREVIATIONSWBMFCs, Wastewater based microbial fuel cells; COD, Chem-ical Oxygen Demand; BOD, Biological Oxygen Demand; TOC,Total Organic Carbon; AOPs, Advance Oxidation Processes; •OH,Hydroxyl radical; H2O2: Hydrogen peroxide; UF-1K, Ultra-filtrationmembranes with molecular weight cut off 1K; UF-3K,Ultra-filtration membranes with molecular weight cut off 3K;MG, Malachite green; MO, Methyl Orange. MF: Microfilteration,SCMFC: Single chamber wastewater based microbial fuel cell,DCMFC: Dual chamber wastewater based microbial fuel cells

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