Electrochemical Conversion of Micropollutants in Gray WaterAndrii Butkovskyi,†,‡,* Adriaan W. Jeremiasse,§ Lucia Hernandez Leal,† Ton van der Zande,†

Huub Rijnaarts,‡ and Grietje Zeeman‡

†Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands‡Sub-department Environmental Technology, Wageningen University, P.O. Box 17, 6700 AA Wageningen, The Netherlands§MAGNETO Special Anodes B.V., Calandstraat 109, 3125 BA Schiedam, The Netherlands

*S Supporting Information

ABSTRACT: Electrochemical conversion of micropollutants in real graywater effluent was studied for the first time. Six compounds that arefrequently found in personal care and household products, namelymethylparaben, propylparaben, bisphenol A, triclosan, galaxolide, and 4-methylbenzilidene camphor (4-MBC), were analyzed in the effluent of theaerobic gray water treatment system in full operation. The effluent wasused for lab-scale experiments with an electrochemical cell operated inbatch mode. Three different anodes and five different cathodes have beentested. Among the anodes, Ru/Ir mixed metal oxide showed the bestperformance. Ag and Pt cathodes worked slightly better than Ti and mixedmetal oxide cathodes. The compounds that contain a phenolic ring(parabens, bisphenol A, and triclosan) were completely transformed onthis anode at a specific electric charge Q = 0.03 Ah/L. The compounds,which contain a benzene ring and multiple side methyl methyl groups (galaxolide, 4-MBC) required high energy input (Q ≤ 0.6Ah/L) for transformation. Concentrations of adsorbable organohalogens (AOX) in the gray water effluent increased significantlyupon treatment for all electrode combinations tested. Oxidation of gray water on mixed metal oxide anodes could not berecommended as a post-treatment step for gray water treatment according to the results of this study. Possible solutions toovercome disadvantages revealed within this study are proposed.

1. INTRODUCTION

The increasing scarcity of fresh water makes reuse of(waste)water more and more attractive.1 Consequently, theinterest in so-called “new sanitation” concepts is growing. Newsanitation concepts are based on the separate collection ofwastewater from toilets (black water), and wastewater fromshowers, dishwashers, laundry etc (gray water).1 Gray water isconsiderably less polluted than black water, and therefore,potentially more attractive for reuse. Various authorsemphasized the need of post-treatment to remove pathogensand micropollutants prior to gray water reuse.2,3 Eriksson et al.found that hundreds of micropollutants might be present ingray water, from which many are toxic and persistent.4

Hernandez Leal et al. showed that a number of toxiccompounds are still present in gray water after aerobictreatment using a sequencing batch reactor.3 Gray water evenafter extended aerobic treatment still retains a slightly yellowishcolor, which also reduce possibilities for its reuse.5

Ozonation and adsorption to activated carbon do effectivelyreduce micropollutants concentrations in the effluents of thewastewater treatment plants.6 However, ozonation of persistentorganics leads to the formation of byproducts. Moreover,ozonation requires equipment for generation and/or trans-portation of oxidative reagent, a full-time operator, andnumerous safety restrictions to be followed.7,8 Sorption to

activated carbon does not lead to byproducts formation, but thesorbent has to be frequently changed and an additionaldisinfection step is usually required for water reuse.6

Alternatively, micropollutants may be removed throughelectrochemical process. In this process, the anode is used tooxidize organic and inorganic compounds (e.g., NH4

+) presentin wastewater. Oxidation of organic compounds proceeds viadirect and mediated anodic oxidation.7 Direct oxidation occurson and near the anode surface by physically adsorbed hydroxylradicals (•OH) or chemisorbed active oxygen.9,10 Mediatedoxidation takes place in the bulk solution due to the action ofelectrogenerated oxidizing agents, such as active chlorine,ozone, peroxide, peroxide sulfate, and so forth.8

Electrochemical oxidation has a number of advantages incomparison with other advanced oxidation processes appliedfor micropollutants removal. A number of authors reportubiquitous nonselective oxidation of organic compounds by theprocess.10,11 Besides, electrochemical oxidation providessimultaneous disinfection of the effluent, which is not thecase for activated carbon adsorption.12,13 The formation of

Received: October 3, 2013Revised: December 17, 2013Accepted: December 23, 2013Published: December 23, 2013

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halogenated byproducts is the principal disadvantage of theprocess.10 The choice of proper electrode material is the maindirection for overcoming this problem. To decrease theformation of halogenated disinfection byproducts, such aschloromethanes, a limited number of studies investigated thecoupling of anodic oxidation to cathodic reduction.14,15 Azzamet al. demonstrated a considerable influence of cathode materialon the rate of destruction of chlorophenol.16 Thus, the term“conversion” is more appropriate for description of thetransformation of organic molecules in undivided electro-chemical cell, because the final products are evolving as a resultof both oxidation and reduction processes.Electrochemical oxidation has been successfully used for

leachate,17 textile,18 and tannery19 wastewaters. Besides,successful applications of the process for oxidation of phenoliccompounds,20 pharmaceuticals,11,21,22 and pesticides23 havebeen reported. Polishing of the effluent of the sewage treatmentplants (STPs) is another area of the process application. Forexample, Menapace et al. treating an effluent of STP spikedwith pharmaceuticals and complexing agents on a boron-dopeddiamond (BDD) anode achieved higher removal rates withlower energy input than ozonation.24 Frontistis et al.demonstrated complete removal of synthetic hormoneethynilestradiol, spiked to the effluent of STP at 100 μg/L byoxidation on BDD electrode at the specific electric charge of0.02 Ah/L.13

Aerobically treated gray water is a wastewater stream withlow concentration of pollutants and low ionic strength, similarto the STP effluents. The aim of this study is to establish theperformance of different combinations of anodes and cathodesfor the electrochemical conversion of micropollutants typicallypresent in gray water. The choice of the electrodes is based onthe enhanced lifetime and stability of the electrode material andtheir different oxidizing power. Parabens, bisphenol A, triclosan,galaxolide, and 4-methylbenzilidene camphor (4-MBC) areamong the micropollutants selected for this study. To the bestof our knowledge, no studies on electrochemical conversion ofmicropollutants in real gray water have yet been reported.Additionally, not much is known of the electrochemicalconversion of micropollutants originating from personal careand household products, and the electrochemical conversion ofgalaxolide and 4-MBC is reported here for the first time.Detection of chlorinated byproducts evolving as a result ofelectrochemical treatment is being reported here. Theprospects of the process application for gray water treatmentare briefly being discussed.

2. MATERIALS AND METHODS

2.1. Chemicals. The micropollutants methylparaben, bi-sphenol A (Sigma-Aldrich, Germany), 4-methylbenzilidenecamphor (Chemos, Germany), triclosan, propylparaben(Fluka, Germany), and galaxolide (SAFC, Germany) werespiked into gray water and used as standard compounds forcalibration and recovery experiments. The chemical structuresof these compounds are given in Supporting Information, SI,Figure S1. Benzophenone-d5, bisphenol A-d16 (Chemos,Germany), and tonalide-d3 (Sigma-Aldrich, Germany) wereused as internal standards. The standard solutions wereprepared in methanol of analytical grade (99.9%, VWR,Belgium). L-Ascorbic acid, Na2CO3, and NaCl, required formicropollutants analyses were purchased from Sigma-Aldrich(Germany) and acetic anhydride from Fluka (Germany).

2.2. Treated Gray Water: Source and Composition.The lab-scale electrochemical cell was fed with effluent from thegray water treatment system of the DeSaR (DecentralisedSanitation and Reuse) full scale plant at Sneek (TheNetherlands).25 This plant is designed to treat the separatedwastewater streams of 250 households. Gray water is treated inan aerobic adsorption/bio-oxidation (AB) system, whichdetailed description is provided by Bohnke.26 The effluent ofthe AB-system is a relatively clean stream, with a COD below30 mg/L and a slightly yellowish color (SI Table S1). Theeffluent was transported in 10-L plastic jerry cans and stored at4 °C for not more than 14 days prior to the experiment.Directly before the experiment, it was spiked with micro-pollutants to obtain concentrations in the range of 150−200μg/L for each compound. Spiking was necessary to obtainsignificantly high GC-MS response (>100 times higher thanLOQ). Concentrations of micropollutants in the effluent ofadsorption/bio-oxidation (AB) system (average, minimal, andmaximal values of 15 measurements) are present in SI TableS2.

2.3. Experimental Setup. An undivided electrochemicalcell was used that consisted of a plexiglas plate with a flowchannel, placed in between an anode and a cathode (activeelectrode surface area 22 cm2). The flow channel was 11 cmlong, 2 cm high, and 1.5 cm wide. A membrane-less cell waschosen for the study because such a configuration will be moreeconomically feasible than a configuration with membrane.Anodes were made of a Ti sheet coated with Ru/Ir mixed metaloxide (MMO), Pt/Ir MMO, or Pt (MAGNETO SpecialAnodes BV, Schiedam, The Netherlands). For anode experi-ments, the cathode was an uncoated Ti sheet. For cathodeexperiments, the best performing Ti-based anode wascombined with a cathode of a Ti sheet coated with Ru/IrMMO, Pt/Ir MMO, Pt, or Ag sheet. The measurements ofanode and cathode potentials were not performed becauseaddition of background electrolyte, required for such measure-ments, would change the gray water matrix.The electrochemical cell was operated in recycling batch

mode and the temperature was maintained at 25 °C using aheating jacket. Gray water effluent spiked with micropollutants(V = 2 L) was constantly recycled (200 mL/min) over theelectrochemical cell and a glass vessel, which was continuouslymixed on a magnetic stirrer plate with glass-coated magnets.Stainless steel tubes were used to minimize adsorption ofmicropollutants. The glass vessel was wrapped in aluminum foilto prevent photodegradation of micropollutants. A constantcurrent of 120 mA (10.9 mA/cm2) was applied for 5 h. Sampleswere taken from the glass vessel at t = 0, 0.25, 0.5, 1, 2, 3, and 5h. Sample volume at t = 0 and 5 h was 100 mL, at t = 0.25, 0.5,1, 2, and 3 h − 15 mL. Directly after sampling 0.5 mL of the0.003 M L-ascorbic acid was added to the samples to avoidoxidation of micropollutants by free chlorine. All experimentswere performed in duplicate. Control experiments wereperformed under the same conditions, but without appliedcurrent to confirm the absence of micropollutant degradationdue to the factors other than electrochemical processes.Micropollutant concentrations were expressed as a functionof specific electrical charge applied (Q = I × t × V−1, where I isa current applied; t, treatment time; and V, total volume ofelectrolyte).

2.4. Analytical Methods. WTW Multi 340i pH/con-ductivity meter was used for pH measurements. Chloride wasmeasured using Ion Chromatography (761 Compact IC

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Metrohm) according to Standard Methods.27 Color of the graywater was spectrophotometrically determined at λ = 455 nmbased on the color unit standards.27 Adsorbable Organo-halogens (AOX), free chlorine, and total chlorine were analyzedwith Dr. Lange test kits. The difference between total chlorineand free chlorine, i.e., combined chlorine, corresponds to theorganically bound chlorine and chlorine of inorganic chlor-amines.Micropollutants were extracted from aqueous phase using

Stir Bar Sorptive Extraction (SBSE) with 2 cm PolyDiMe-thylSiloxane (PDMS) twisters, as described by Hernandez Lealet al.3 The twisters were analyzed using a thermal desorptionGC-MS system consisting of a Gerstel Thermal DesorptionUnit (TDU), a Gerstel Cooled Injection System (CIS4), aGerstel MPS2XL automatic sampler, an Agilent 6890N gaschromatograph (GC), and an Agilent 5975XL mass spec-trometer (MS) modified with a Chromtech Evolution triplequadrupole. Micropollutants were desorbed from the twister inthe TDU at 280 °C, then trapped in the CIS at −50 °C, usingliquid nitrogen and separated in the GC unit on a HP5MScolumn (length, 30 m; internal diameter, 25 mm; and filmthickness, 0.25 μm) using temperature programming (15 °C/min) from 60° to 280 °C. Quantification was done bycompound specific Selective Reaction Monitoring (SRM)detection in the triple quadrupole MS. For detection ofchlorinated parabens MS scanning from 50 to 550 mass unitswas performed. Since calibration standards of chlorinated

parabens were not available, their amount in the samples wasdetermined semiquantitatively.Internal standards were added to all of the samples for

quality assurance, namely, benzophenone-d5 for parabens andtriclosan, bisphenol A-d16 for bisphenol A, tonalide-d3 forgalaxolide and 4-MBC. Recoveries and limits of quantificationare presented in SI Table S3. Standard deviations of theindividual sampling points were not determined, since all themeasurements were done in duplicate. Instead, each individualvalue was divided by the sample mean, and a common standarddeviation for these values was calculated.

3. RESULTS AND DISCUSSION3.1. Influence of Type of Anode on Conversion of

Micropollutants. The results of the batch experimentsdemonstrate that the composition of the anodes significantlyinfluenced the degradation of the spiked micropollutants(Figure 1).The highest conversion was found with the Ru/Ir MMO

anode. Using this anode, parabens, triclosan, and bisphenol Awere completely converted at Q = 0.06 Ah/L. Rapid conversionwas observed despite the relatively low chloride content (60 ±9 mg/L) of the gray water. Most reported studies on theelectrochemical conversion in presence of Cl− were done atmuch higher chloride concentrations, which was usually addedto the electrolyte artificially.7 Conversion of parabens,bisphenol A, and triclosan was also achieved with the Pt/Ir

Figure 1. Effect of application of different anodes on the electrochemical removal of selected micropollutants from spiked gray water effluent at 10.9mA/cm2 (cathode, Ti).

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MMO anode, although six times higher specific electricalcharge had to be applied for 99% conversion (Q = 0.36 Ah/L).In contrast, none of the spiked micropollutants were

converted on the Pt anode. Pt anodes have a poor efficiencytoward oxidation of organic compounds due to the lowoverpotential for oxygen evolution.10 As reported previously byChen, platinum anodes are effective only at low currentdensities and high chloride concentrations in the electrolyte.10

The efficiency of chlorine evolution on the anodes alsodecreases in order Ru/Ir > Pt/Ir > Pt, which is confirmed bythe measured production of chlorine-containing oxidativespecies and explains the observed differences in the conversionof micropollutants (SI Table S4).An increase of pH from 8.0 ± 0.1 to 9.2 ± 0.2 was observed

on all of the anodes, including Pt (SI Table S4). Since the pHof gray water was slightly alkaline, ClO− is suspected to be thepredominant oxidative compound among the active chlorinespecies.28 As ClO− has a lower oxidative potential than that ofHClO− (predominant form of active chlorine at pH 3−8), theoxidation efficiency can be higher at lower pH values. Colorremoval was achieved with Pt/Ir and Ru/Ir anodes (SI TableS4). In case of Pt anode, the gray water had a slightly yellowishcolor (29.8 ± 0.5 CU in the effluent).

It was also shown that the spiked micropollutants haddifferent degradation rates (Figure 1). Parabens, triclosan, andbisphenol A, in general, were converted with both MMOanodes, but not with Pt. These compounds contain a phenolicring in their molecule, which is converted by mediatedelectrochemical oxidation in presence of active chlorine speciesin the solution.20 Rapid oxidation of bisphenol A, triclosan, andparabens on different anodes (PbO2, Sb−SnO2, SnO2, RuO2,and IrO2) was also observed by other authors.

29−34 Contrary tothis study, Tanaka et al. could oxidize bisphenol A on Pt anodein a 0.1 M H2SO4 electrolyte solution.29 High sulfateconcentration favors formation of persulfate with consequentgeneration of hydrogen peroxide.35 However, low concen-trations of sulfate in gray water (0.23 mM SO4−/L) andabsence of bisphenol A transformation on Pt anode indicatesthat persulfate pathway was unlikely to be involved in theoxidation of bisphenol A in this study.Galaxolide and 4-MBC were more stable toward electro-

chemical conversion than phenolic compounds. An 80%conversion of galaxolide and 40% conversion of 4-MBC wasachieved after 5 h of treatment on Ru/Ir anode (Q = 0.6 Ah/L). No data on electrochemical conversion of galaxolide and 4-MBC have been previously reported in the literature. Both

Figure 2. Effect of application of different cathodes on the electrochemical removal of selected micropollutants from spiked gray water effluent at10.9 mA/cm2 (anode, Ru/Ir).

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compounds contain a benzene ring, which, according to theHuckel’s rule of aromaticity, is stable toward electrochemicaloxidation.36 The side methyl groups, which are present in bothgalaxolide and 4-MBC, are also likely to enhance persistence ofthese compounds toward electrochemical oxidation.36

3.2. Influence of Type of Cathode on Conversion ofMicropollutants. To check whether the cathode material hasan influence on the micropollutant degradation, severalcathodes (Pt/Ir MMO, Ru/Ir MMO, Pt, and Ag) were testedagainst the Ru/Ir MMO anode. The choice of anode was basedon the previous set of experiments, where Ru/Ir MMO showedthe highest removal efficiency (see Section 3.1). The use of Pt,Ru/Ir, and Pt/Ir cathodes was inspired by the fact that polarityreversal can be used to remove scaling from their surface.Uncovered Ti metal sheet was used because Ti is commonlyused as a cathode material in commercial cells. Ag was chosenas a cathode material because it can provide a dehalogenationeffect.37

All cathodes showed similar removal patterns toward thetested micropollutants (Figure 2). However, complete trans-formation of parabens and bisphenol A was already achieved atQ = 0.03 Ah/L applied with Pt and Ag cathodes. With Ti, Pt/Ir,and Ru/Ir MMO cathodes complete transformation of thesemicropollutants was achieved at Q = 0.06 Ah/L. Additionally,97 and 92% removal of galaxolide was achieved with Pt and Agcathodes respectively. Removal of 4-MBC was more efficientwith the Ag cathode (61% degradation), than with the otherfour cathodes (40−54% removal).The increased conversion of parent compounds on Pt and

Ag cathodes is presumably attributed to their catalyticproperties, such as enhanced formation of oxidative speciesthrough oxygen reduction, and direct or mediated reductionprocesses. The reduction rate of chlorine on the cathodesurface has an impact on the amount of free chlorine releasedinto the gray water.38 Depending on the amount of free

chlorine, oxidation of micropollutants can be enhanced ordecreased. This is likely to cause the increased transformationof micropollutants on Ag cathode, because it has provendehalogenation activity, as stated above. Cathodic H2O2formation mainly occurs on carbon-based electrodes, such asa carbon cloth gas diffusion electrode. Bergmann did measureH2O2 formation up to 0.3 ppm with Ru/Ir MMO cathodes, inchloride-free waters.39 It is thus possible that a small amount ofH2O2 is formed cathodically. Consequently, hydrogen peroxideformed on cathodes can play a role in the transformation ofmicropollutants in this study.Direct reduction of micropollutants on cathodes could also

occur. According to Knust et al. triclosan can be electrochemi-cally reduced to 2-phenoxyphenol in dimethylformamidesolution.40 Mediated electrocatalytic hydrogenation of a widerange of organic compounds was shown by Cleghorn andPletcher.41 The impact of direct and mediated cathodicreduction on micropollutants transformation cannot beindividually quantified within this study, because all of theexperiments were run in undivided cells. However, the similartransformation rates achieved with five different cathodes implythat the contribution of the cathode on micropollutantstransformation in gray water was less than that of the anode.

3.3. Byproduct Formation. Chlorinated organic sub-stances are the main byproducts of electrochemical oxidationprocesses in presence of chloride ions. The spectrophotometricanalysis, which was used in the current study for free and totalchlorine determination, is based on the oxidation of diethyl-p-phenylenediamine (DPD). This substance does not react withfree chlorine alone, but rather with all kinds of oxidizing agents,present in the sample, e.g., chlorine dioxide, hydrogen peroxide,and ozone. To assess to what extent free chlorine itselfcontributed to the oxidation process, the decrease in theconcentration of chloride ions in the gray water was monitoredwith ion chromatography (IC). Application of the Ru/Ir MMO

Figure 3. TIC chromatogram of the samples, taken at 0 (red line) and 60 (green line) minutes of electrochemical treatment. The peaks observed forthe sample taken at 60 min are colored green. Anode, Ru/Ir MMO; cathode, Pt.

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anode caused high (35.2 ± 1.7 mg/L) concentrations ofoxidizing species in the effluent. Apart from free chlorine,additional oxidative species were formed, because the decreasein the concentration of chloride ions was 2 times lower than theincrease in concentrations of the free chlorine (SI Table S4).Concentrations of AOX in the spiked gray water before the

experiment were 0.1 ± 0.05 mg/L (SI Table S4). An increase inAOX concentration up to 1.8 ± 0.2 mg/L was found in theexperiments with Ru/Ir MMO anodes. In the experiments withPt/Ir anode, an increase of AOX was also observed to a finalconcentration of 0.7 ± 0.3 mg/L. While no conversion ofmicropollutants was found on the Pt anode, AOX formation onthis anode was not observed either.The cathode material had a substantial impact on the

presence of chlorinated organics in the effluent after electro-chemical oxidation. The AOX concentrations in the effluent ofthe cells with Pt/Ir MMO (1.4 mg/L), Ru/Ir MMO (1.8 ± 0.1mg/L), and Ti (1.8 ± 0.2 mg/L) cathodes were lower thanthose in the cells with Pt (2.0 ± 0.1 mg/L) and Ag (2.4 ± 0.3mg/L) cathodes (SI Table S4). Higher AOX formation on Ptand Ag was observed, possibly due to the lower chlorinereduction on these cathodes, i.e., more free chlorine remainsactive for oxidation in the bulk solution. Free chlorine led to theenhanced formation of chlorinated compounds and oxidationof chloramines. This has been indicated by the amount ofcombined chlorine, which includes chlorine of organicchlorinated compounds and inorganic chloramines. Contraryto the AOX concentrations, combined chlorine was lower inthe effluent of the cells with Pt (1.7 ± 0.7 mg/L) and Ag (3.0 ±0.5 mg/L) cathodes, while it exceeded 5 mg/L in the effluent ofthe cells with Pt/Ir and Ru/Ir MMO cathodes (SI Table S4).Therefore, higher chloramine conversion was achieved on Ptand Ag cathodes, whereas on Pt/Ir, Ru/Ir, and Ti cathodes, thechloramine conversion was lower.

Apart from the detection of AOX, which is a general measureof halogenated byproducts, specific byproducts of the micro-pollutants, spiked to gray water, were identified. Chromato-graphic peaks of unknown compounds were found in the GC-MS full scan chromatograms from samples taken at Q = 0.12Ah/L. The compounds were evolving as a result of theelectrochemical oxidation, since they were not present in thespiked gray water before treatment.The compounds, evolving at retention times 11.08 and 12.35,

were identified as dichlorinated byproducts of the oxidation ofparabens (Figure 3). Their chromatographic peaks appear toshow strong isotopic mass fragments at m/z 189 and 191 (SIFigure S2). These mass fragments, which are typical forcompounds containing two chlorine atoms, also appear in thelibrary spectrum of 3,5-dichloro-4-hydroxybenzoic acid.42 Theycorrespond to the main mass fragment of this compound(C7H3O2Cl2: 3,5-dichloro-4-hydroxybenzoic acid without −OHof the acid group). Chlorinated parabens, being alkyl esters of4-hydroxybenzoic acid, have the same main mass fragment (m/z = 189 and 191). The chromatographic peaks at t = 11.08 and12.35 also reveal, in a lower abundance, the presence of othermolecular ions, characteristic for the dichlorinated parabens.The specific isotopic distribution of these ions (m/z = 220, 222,224 for dichloromethylparaben at t = 11.08, and m/z = 248,250, and 252 for dichloropropylparaben at t = 12.35) wastypical for dichlorinated compounds (ratio 9:6:1).The dependence of the formation of dichlorinated parabens

on the type of anode and cathode materials is shown in Figures4 and 5. Formation of dichlorinated parabens was found on alltypes of tested anodes, except for Pt, where no conversion wasobserved. The evolution of detected species (dichlorinatedmethyl- and propylparaben) had similar trends in formationwith increasing specific electrical charge. For Ru/Ir MMOanode, a rapid increase in the abundance of dichlorinated

Figure 4. Effect of different anodes on the formation of dichlorinated parabens (cathode, Ti).

Figure 5. Effect of different cathodes on the formation of dichlorinated parabens (anode, Ru/Ir).

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parabens was observed already between Q = 0.03 Ah/L and Q= 0.06 Ah/L. It reached a maximum at Q < 0.12 Ah/L and thengradually decreased. Another pattern was observed in theexperiments with Pt/Ir MMO anode, where the amount ofdichlorinated parabens in the gray water increased progressivelyafter Q = 0.12 Ah/L with the highest levels reached at the endof the oxidation process (Q = 0.6 Ah/L).It was concluded that decomposition of parabens starts with

initial chlorination of the phenolic ring. This is confirmed bythe increase of the amount of dichlorinated parabens in the graywater with application of electric charge. These compounds arenot the final products since their amount decreases withincrease of the electrical charge. The results, however, did notallow conclusions on the further fate of these intermediates. Forthe other compounds studied, byproducts and intermediateswere not identified. However, Gallard et al. found thatchlorinated byproducts of bisphenol A are further decomposedby oxidation in solution at chlorine concentrations higher than2 mg/L.43 Thus, complete oxidation of bisphenol A is expectedat the chlorine concentrations observed in our study.Among the cathodes, Ag led to the fastest transformation of

dichlorinated parabens in the gray water (Figure 5). In contrast,the final AOX concentration was the highest (2.4 ± 0.3 mg/L)with this type of cathode. The higher conversion of compounds(both parent and chlorinated) on the Ag and Pt cathodes mayhave been caused by a lower chlorine reduction, higher H2O2production or direct dehalogenation on these cathodes, asstated previously.Obtained results show that a number of micropollutants

present in gray water can be converted via electrochemicalprocess, with highest yields obtained on Ru/Ir MMO anode.Although ubiquitous degradation of organic matter isconsidered to be an advantage of the process by some authors,not all of the personal care products used in this study weredegraded, in spite of the high current densities used.10,11 Thecompounds, which are easily oxidized electrochemically,namely parabens and triclosan, are also biodegradable inaerobic conditions, as reported by Hernandez Leal et al.3

Meanwhile, the compounds, which are recalcitrant in aerobicbiological systems, namely galaxolide and 4-MBC, are difficultto electrochemically oxidize on MMO anodes as well. At thesame time, galaxolide and 4-MBC are efficiently removed fromgray water both by ozonation and adsorption to granularactivated carbon.5 Thus, utilization of MMO anodes forelectrochemical conversion of personal care products in graywater is at this stage not a promising solution when comparedto the other competing post-treatment methods mentioned.Another important drawback is the formation of halogenated

byproducts, such as halogenated parabens, which was observedwith all the combinations of electrodes tested. Although thehalogenated parabens are eventually converted further on, thisrequires considerable extra charge input. Additionally, the studygives no evidence of the cathodic dechlorination of halogenatedmicropollutants, which is a reported mechanism for conversionof chlorinated micropollutants.44 Moreover, the final AOXconcentrations are still higher than 1.5 mg/L. Theseconcentrations exceed the values permitted for discharge. Forexample, in German guidelines for wastewater discharge,permitted AOX concentrations vary between 0.1 and 1 mg/L,depending on the type of wastewater.45 However, controversialdata exist on the relationship between AOX and toxicity,because AOX, as a complex parameter, does not correlateexactly with the toxicity of individual chlorinated compounds.

Gellert found a strong correlation of Microtox, algae, anddaphnia toxicity tests with the AOX concentrations in thewastewater, whereas O’Connor concluded that the AOXconcentration is not suitable for predicting toxicity of theeffluents.46,47 In some cases, chlorinated byproducts can be lesstoxic than the parent ones, as was shown by Wang and Farrell.These authors studied the toxicity of triclosan and itschlorinated byproducts, formed after electrochemical oxidationof the compound.33

The presence of halogenated compounds and free chlorine inthe effluent makes the electrochemical oxidation inappropriateas a sole post treatment of gray water effluent. Formation ofbyproducts requires addition of the postfiltration step, such asbiological sand filter or granular activated carbon filter. Thesame post-treatment step is usually included after ozonation,because that process also leads to the formation of undesirablebyproducts.6 Compared with granular activated carbonfiltration, electrochemical oxidation provides an extra contri-bution to gray water effluent quality improvement, namely thedisinfection.48 As shown by Frontistis et al., complete removalof E. coli is accomplished by electrochemical oxidation of STPeffluent on BDD anode already at Q = 0.01 Ah/L.13 That ismuch lower than the specific electric charge applied in thisstudy for micropollutants destruction. Moreover, chlorinatedderivatives, which are produced in the electrochemicaloxidation step, are usually more hydrophobic, than the parentcompounds. Thus, their adsorption to the activated carbonapplied after electrochemical oxidation, will be increased.49

When the electrochemical oxidation process is followed by apostadsorption step, operation of the electrochemical processfor just disinfection and conversion of micropollutants to theirchlorinated forms is recommended. The latter is observedalready at 0.03 Ah/L with a combination of Ru/Ir MMO anodeand Ti or Ag cathode. This specific electric charge is equivalentto the energy requirements of 0.66 kWh/m3. This energyconsumption is on the same order of magnitude as the energyconsumption of a full-scale ozonation plant, as reported by Josset al. (0.1−0.3 kWh/m3).6 Although the value, reported by Joss,includes the energy consumption of the filtration step, the latteris equal to 0.02 kWh/m3 only.50 Meanwhile, the calculatedenergy consumption of electrochemical oxidation is based onthe results of a lab study performed in a nonoptimized cell.Optimization of the cell configuration and the use of strongeroxidizing electrodes, as proposed above, will considerablydecrease the energy consumption. To show the competitive-ness of the technology with ozonation, further studies shouldfocus on the utilization of a BDD anode and optimization ofthe electrochemical cell.

■ ASSOCIATED CONTENT*S Supporting InformationMolecular structure of the analyzed micropollutants (FigureS1); mass spectra of the compounds, identified in the treatedgray water on the total ion chromatogram (TIC) at theretention times 11.08 and 12.35 (Figure S2); physico-chemicalcharacteristics of gray water (Table S1); concentrations ofselected micropollutants in the effluent of adsorption/bio-oxidation system (Table S2); retention times, QQQ transitions,recoveries, and limits of quantification (LOQs) of themicropollutants selected for the study (Table S3); and changesin pH, conductivity, color, and concentrations of chlorine-containing substances in the beginning at Q = 0 Ah/L and Q =0.6 Ah/L with different combinations of anodes and cathodes

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used (Table S4). This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: Andrii.Butkovskyi@wetsus.nl.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was performed in the cooperation framework ofWetsus, Centre of Excellence for Sustainable Water Technol-ogy (www.wetsus.nl). Wetsus is cofunded by the DutchMinistry of Economic Affairs and Ministry of Infrastructureand Environment, the European Union Regional DevelopmentFund, the Province of Fryslan, and the Northern NetherlandsProvinces. The authors would like to thank the participants ofthe research theme “Separation at Source” for the fruitfuldiscussions and their financial support. The authors would alsolike to thank Lina Taparaviciute for assistance in theexperimental part of the study.

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■ NOTE ADDED AFTER ASAP PUBLICATIONThis paper was originally published ASAP on January 15, 2014,with a mistake in the Abstract graphic. The corrected versionwas reposted on January 22, 2014.

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