Electrochemical Conversion of Micropollutants in Gray Water

Article pubs.acs.org/est

Electrochemical Conversion of Micropollutants in Gray Water Andrii 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 gray water effluent was studied for the first time. Six compounds that are frequently found in personal care and household products, namely methylparaben, propylparaben, bisphenol A, triclosan, galaxolide, and 4methylbenzilidene camphor (4-MBC), were analyzed in the effluent of the aerobic gray water treatment system in full operation. The effluent was used for lab-scale experiments with an electrochemical cell operated in batch mode. Three different anodes and five different cathodes have been tested. Among the anodes, Ru/Ir mixed metal oxide showed the best performance. Ag and Pt cathodes worked slightly better than Ti and mixed metal oxide cathodes. The compounds that contain a phenolic ring (parabens, bisphenol A, and triclosan) were completely transformed on this 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 ≤ 0.6 Ah/L) for transformation. Concentrations of adsorbable organohalogens (AOX) in the gray water effluent increased significantly upon treatment for all electrode combinations tested. Oxidation of gray water on mixed metal oxide anodes could not be recommended as a post-treatment step for gray water treatment according to the results of this study. Possible solutions to overcome 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, the interest in so-called “new sanitation” concepts is growing. New sanitation concepts are based on the separate collection of wastewater from toilets (black water), and wastewater from showers, dishwashers, laundry etc (gray water).1 Gray water is considerably less polluted than black water, and therefore, potentially more attractive for reuse. Various authors emphasized the need of post-treatment to remove pathogens and micropollutants prior to gray water reuse.2,3 Eriksson et al. found that hundreds of micropollutants might be present in gray water, from which many are toxic and persistent.4 Hernandez Leal et al. showed that a number of toxic compounds are still present in gray water after aerobic treatment using a sequencing batch reactor.3 Gray water even after extended aerobic treatment still retains a slightly yellowish color, which also reduce possibilities for its reuse.5 Ozonation and adsorption to activated carbon do effectively reduce micropollutants concentrations in the effluents of the wastewater treatment plants.6 However, ozonation of persistent organics leads to the formation of byproducts. Moreover, ozonation requires equipment for generation and/or transportation of oxidative reagent, a full-time operator, and numerous safety restrictions to be followed.7,8 Sorption to © 2013 American Chemical Society

activated carbon does not lead to byproducts formation, but the sorbent has to be frequently changed and an additional disinfection step is usually required for water reuse.6 Alternatively, micropollutants may be removed through electrochemical process. In this process, the anode is used to oxidize organic and inorganic compounds (e.g., NH4+) present in wastewater. Oxidation of organic compounds proceeds via direct and mediated anodic oxidation.7 Direct oxidation occurs on and near the anode surface by physically adsorbed hydroxyl radicals (•OH) or chemisorbed active oxygen.9,10 Mediated oxidation takes place in the bulk solution due to the action of electrogenerated oxidizing agents, such as active chlorine, ozone, peroxide, peroxide sulfate, and so forth.8 Electrochemical oxidation has a number of advantages in comparison with other advanced oxidation processes applied for micropollutants removal. A number of authors report ubiquitous nonselective oxidation of organic compounds by the process.10,11 Besides, electrochemical oxidation provides simultaneous disinfection of the effluent, which is not the case for activated carbon adsorption.12,13 The formation of Received: Revised: Accepted: Published: 1893

October 3, 2013 December 17, 2013 December 23, 2013 December 23, 2013 dx.doi.org/10.1021/es404411p | Environ. Sci. Technol. 2014, 48, 1893−1901

Environmental Science & Technology

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2.2. Treated Gray Water: Source and Composition. The lab-scale electrochemical cell was fed with effluent from the gray water treatment system of the DeSaR (Decentralised Sanitation and Reuse) full scale plant at Sneek (The Netherlands).25 This plant is designed to treat the separated wastewater streams of 250 households. Gray water is treated in an aerobic adsorption/bio-oxidation (AB) system, which detailed description is provided by Böhnke.26 The effluent of the AB-system is a relatively clean stream, with a COD below 30 mg/L and a slightly yellowish color (SI Table S1). The effluent was transported in 10-L plastic jerry cans and stored at 4 °C for not more than 14 days prior to the experiment. Directly before the experiment, it was spiked with micropollutants to obtain concentrations in the range of 150−200 μg/L for each compound. Spiking was necessary to obtain significantly high GC-MS response (>100 times higher than LOQ). Concentrations of micropollutants in the effluent of adsorption/bio-oxidation (AB) system (average, minimal, and maximal values of 15 measurements) are present in SI Table S2. 2.3. Experimental Setup. An undivided electrochemical cell was used that consisted of a plexiglas plate with a flow channel, placed in between an anode and a cathode (active electrode surface area 22 cm2). The flow channel was 11 cm long, 2 cm high, and 1.5 cm wide. A membrane-less cell was chosen for the study because such a configuration will be more economically feasible than a configuration with membrane. Anodes were made of a Ti sheet coated with Ru/Ir mixed metal oxide (MMO), Pt/Ir MMO, or Pt (MAGNETO Special Anodes BV, Schiedam, The Netherlands). For anode experiments, the cathode was an uncoated Ti sheet. For cathode experiments, the best performing Ti-based anode was combined with a cathode of a Ti sheet coated with Ru/Ir MMO, Pt/Ir MMO, Pt, or Ag sheet. The measurements of anode and cathode potentials were not performed because addition of background electrolyte, required for such measurements, would change the gray water matrix. The electrochemical cell was operated in recycling batch mode and the temperature was maintained at 25 °C using a heating jacket. Gray water effluent spiked with micropollutants (V = 2 L) was constantly recycled (200 mL/min) over the electrochemical cell and a glass vessel, which was continuously mixed on a magnetic stirrer plate with glass-coated magnets. Stainless steel tubes were used to minimize adsorption of micropollutants. The glass vessel was wrapped in aluminum foil to prevent photodegradation of micropollutants. A constant current of 120 mA (10.9 mA/cm2) was applied for 5 h. Samples were taken from the glass vessel at t = 0, 0.25, 0.5, 1, 2, 3, and 5 h. 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 the 0.003 M L-ascorbic acid was added to the samples to avoid oxidation of micropollutants by free chlorine. All experiments were performed in duplicate. Control experiments were performed under the same conditions, but without applied current to confirm the absence of micropollutant degradation due to the factors other than electrochemical processes. Micropollutant concentrations were expressed as a function of specific electrical charge applied (Q = I × t × V−1, where I is a current applied; t, treatment time; and V, total volume of electrolyte). 2.4. Analytical Methods. WTW Multi 340i pH/conductivity meter was used for pH measurements. Chloride was measured using Ion Chromatography (761 Compact IC

halogenated byproducts is the principal disadvantage of the process.10 The choice of proper electrode material is the main direction for overcoming this problem. To decrease the formation of halogenated disinfection byproducts, such as chloromethanes, a limited number of studies investigated the coupling of anodic oxidation to cathodic reduction.14,15 Azzam et al. demonstrated a considerable influence of cathode material on the rate of destruction of chlorophenol.16 Thus, the term “conversion” is more appropriate for description of the transformation of organic molecules in undivided electrochemical cell, because the final products are evolving as a result of 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 phenolic compounds,20 pharmaceuticals,11,21,22 and pesticides23 have been reported. Polishing of the effluent of the sewage treatment plants (STPs) is another area of the process application. For example, Menapace et al. treating an effluent of STP spiked with pharmaceuticals and complexing agents on a boron-doped diamond (BDD) anode achieved higher removal rates with lower energy input than ozonation.24 Frontistis et al. demonstrated complete removal of synthetic hormone ethynilestradiol, spiked to the effluent of STP at 100 μg/L by oxidation on BDD electrode at the specific electric charge of 0.02 Ah/L.13 Aerobically treated gray water is a wastewater stream with low concentration of pollutants and low ionic strength, similar to the STP effluents. The aim of this study is to establish the performance of different combinations of anodes and cathodes for the electrochemical conversion of micropollutants typically present in gray water. The choice of the electrodes is based on the enhanced lifetime and stability of the electrode material and their different oxidizing power. Parabens, bisphenol A, triclosan, galaxolide, and 4-methylbenzilidene camphor (4-MBC) are among the micropollutants selected for this study. To the best of our knowledge, no studies on electrochemical conversion of micropollutants in real gray water have yet been reported. Additionally, not much is known of the electrochemical conversion of micropollutants originating from personal care and household products, and the electrochemical conversion of galaxolide and 4-MBC is reported here for the first time. Detection of chlorinated byproducts evolving as a result of electrochemical treatment is being reported here. The prospects of the process application for gray water treatment are briefly being discussed.

2. MATERIALS AND METHODS 2.1. Chemicals. The micropollutants methylparaben, bisphenol A (Sigma-Aldrich, Germany), 4-methylbenzilidene camphor (Chemos, Germany), triclosan, propylparaben (Fluka, Germany), and galaxolide (SAFC, Germany) were spiked into gray water and used as standard compounds for calibration and recovery experiments. The chemical structures of these compounds are given in Supporting Information, SI, Figure S1. Benzophenone-d5, bisphenol A-d16 (Chemos, Germany), and tonalide-d3 (Sigma-Aldrich, Germany) were used as internal standards. The standard solutions were prepared in methanol of analytical grade (99.9%, VWR, Belgium). L-Ascorbic acid, Na2CO3, and NaCl, required for micropollutants analyses were purchased from Sigma-Aldrich (Germany) and acetic anhydride from Fluka (Germany). 1894

dx.doi.org/10.1021/es404411p | Environ. Sci. Technol. 2014, 48, 1893−1901


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

Metrohm) according to Standard Methods.27 Color of the gray water was spectrophotometrically determined at λ = 455 nm based on the color unit standards.27 Adsorbable Organohalogens (AOX), free chlorine, and total chlorine were analyzed with Dr. Lange test kits. The difference between total chlorine and free chlorine, i.e., combined chlorine, corresponds to the organically bound chlorine and chlorine of inorganic chloramines. Micropollutants were extracted from aqueous phase using Stir Bar Sorptive Extraction (SBSE) with 2 cm PolyDiMethylSiloxane (PDMS) twisters, as described by Hernandez Leal et al.3 The twisters were analyzed using a thermal desorption GC-MS system consisting of a Gerstel Thermal Desorption Unit (TDU), a Gerstel Cooled Injection System (CIS4), a Gerstel MPS2XL automatic sampler, an Agilent 6890N gas chromatograph (GC), and an Agilent 5975XL mass spectrometer (MS) modified with a Chromtech Evolution triple quadrupole. Micropollutants were desorbed from the twister in the TDU at 280 °C, then trapped in the CIS at −50 °C, using liquid nitrogen and separated in the GC unit on a HP5MS column (length, 30 m; internal diameter, 25 mm; and film thickness, 0.25 μm) using temperature programming (15 °C/ min) from 60° to 280 °C. Quantification was done by compound specific Selective Reaction Monitoring (SRM) detection in the triple quadrupole MS. For detection of chlorinated parabens MS scanning from 50 to 550 mass units was performed. Since calibration standards of chlorinated

parabens were not available, their amount in the samples was determined semiquantitatively. Internal standards were added to all of the samples for quality assurance, namely, benzophenone-d5 for parabens and triclosan, bisphenol A-d16 for bisphenol A, tonalide-d3 for galaxolide and 4-MBC. Recoveries and limits of quantification are presented in SI Table S3. Standard deviations of the individual sampling points were not determined, since all the measurements were done in duplicate. Instead, each individual value was divided by the sample mean, and a common standard deviation for these values was calculated.

3. RESULTS AND DISCUSSION 3.1. Influence of Type of Anode on Conversion of Micropollutants. The results of the batch experiments demonstrate that the composition of the anodes significantly influenced 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 A were completely converted at Q = 0.06 Ah/L. Rapid conversion was observed despite the relatively low chloride content (60 ± 9 mg/L) of the gray water. Most reported studies on the electrochemical conversion in presence of Cl− were done at much higher chloride concentrations, which was usually added to the electrolyte artificially.7 Conversion of parabens, bisphenol A, and triclosan was also achieved with the Pt/Ir 1895



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Figure 2. Effect of application of different cathodes on the electrochemical removal of selected micropollutants from spiked gray water effluent at 10.9 mA/cm2 (anode, Ru/Ir).

It was also shown that the spiked micropollutants had different degradation rates (Figure 1). Parabens, triclosan, and bisphenol A, in general, were converted with both MMO anodes, but not with Pt. These compounds contain a phenolic ring in their molecule, which is converted by mediated electrochemical oxidation in presence of active chlorine species in the solution.20 Rapid oxidation of bisphenol A, triclosan, and parabens on different anodes (PbO2, Sb−SnO2, SnO2, RuO2, and IrO2) was also observed by other authors.29−34 Contrary to this study, Tanaka et al. could oxidize bisphenol A on Pt anode in a 0.1 M H2SO4 electrolyte solution.29 High sulfate concentration favors formation of persulfate with consequent generation of hydrogen peroxide.35 However, low concentrations of sulfate in gray water (0.23 mM SO4−/L) and absence of bisphenol A transformation on Pt anode indicates that persulfate pathway was unlikely to be involved in the oxidation of bisphenol A in this study. Galaxolide and 4-MBC were more stable toward electrochemical conversion than phenolic compounds. An 80% conversion of galaxolide and 40% conversion of 4-MBC was achieved after 5 h of treatment on Ru/Ir anode (Q = 0.6 Ah/ L). No data on electrochemical conversion of galaxolide and 4MBC have been previously reported in the literature. Both

MMO anode, although six times higher specific electrical charge 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 efficiency toward oxidation of organic compounds due to the low overpotential for oxygen evolution.10 As reported previously by Chen, platinum anodes are effective only at low current densities and high chloride concentrations in the electrolyte.10 The efficiency of chlorine evolution on the anodes also decreases in order Ru/Ir > Pt/Ir > Pt, which is confirmed by the measured production of chlorine-containing oxidative species and explains the observed differences in the conversion of 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 pH of gray water was slightly alkaline, ClO− is suspected to be the predominant oxidative compound among the active chlorine species.28 As ClO− has a lower oxidative potential than that of HClO− (predominant form of active chlorine at pH 3−8), the oxidation efficiency can be higher at lower pH values. Color removal was achieved with Pt/Ir and Ru/Ir anodes (SI Table S4). In case of Pt anode, the gray water had a slightly yellowish color (29.8 ± 0.5 CU in the effluent). 1896



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

chlorine, oxidation of micropollutants can be enhanced or decreased. This is likely to cause the increased transformation of micropollutants on Ag cathode, because it has proven dehalogenation activity, as stated above. Cathodic H2O2 formation mainly occurs on carbon-based electrodes, such as a carbon cloth gas diffusion electrode. Bergmann did measure H2O2 formation up to 0.3 ppm with Ru/Ir MMO cathodes, in chloride-free waters.39 It is thus possible that a small amount of H2O2 is formed cathodically. Consequently, hydrogen peroxide formed on cathodes can play a role in the transformation of micropollutants in this study. Direct reduction of micropollutants on cathodes could also occur. According to Knust et al. triclosan can be electrochemically reduced to 2-phenoxyphenol in dimethylformamide solution.40 Mediated electrocatalytic hydrogenation of a wide range of organic compounds was shown by Cleghorn and Pletcher.41 The impact of direct and mediated cathodic reduction on micropollutants transformation cannot be individually quantified within this study, because all of the experiments were run in undivided cells. However, the similar transformation rates achieved with five different cathodes imply that the contribution of the cathode on micropollutants transformation in gray water was less than that of the anode. 3.3. Byproduct Formation. Chlorinated organic substances are the main byproducts of electrochemical oxidation processes in presence of chloride ions. The spectrophotometric analysis, which was used in the current study for free and total chlorine determination, is based on the oxidation of diethyl-pphenylenediamine (DPD). This substance does not react with free 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 itself contributed to the oxidation process, the decrease in the concentration of chloride ions in the gray water was monitored with ion chromatography (IC). Application of the Ru/Ir MMO

compounds contain a benzene ring, which, according to the Hückel’s rule of aromaticity, is stable toward electrochemical oxidation.36 The side methyl groups, which are present in both galaxolide and 4-MBC, are also likely to enhance persistence of these compounds toward electrochemical oxidation.36 3.2. Influence of Type of Cathode on Conversion of Micropollutants. To check whether the cathode material has an influence on the micropollutant degradation, several cathodes (Pt/Ir MMO, Ru/Ir MMO, Pt, and Ag) were tested against the Ru/Ir MMO anode. The choice of anode was based on the previous set of experiments, where Ru/Ir MMO showed the highest removal efficiency (see Section 3.1). The use of Pt, Ru/Ir, and Pt/Ir cathodes was inspired by the fact that polarity reversal can be used to remove scaling from their surface. Uncovered Ti metal sheet was used because Ti is commonly used as a cathode material in commercial cells. Ag was chosen as a cathode material because it can provide a dehalogenation effect.37 All cathodes showed similar removal patterns toward the tested micropollutants (Figure 2). However, complete transformation of parabens and bisphenol A was already achieved at Q = 0.03 Ah/L applied with Pt and Ag cathodes. With Ti, Pt/Ir, and Ru/Ir MMO cathodes complete transformation of these micropollutants was achieved at Q = 0.06 Ah/L. Additionally, 97 and 92% removal of galaxolide was achieved with Pt and Ag cathodes respectively. Removal of 4-MBC was more efficient with the Ag cathode (61% degradation), than with the other four cathodes (40−54% removal). The increased conversion of parent compounds on Pt and Ag cathodes is presumably attributed to their catalytic properties, such as enhanced formation of oxidative species through oxygen reduction, and direct or mediated reduction processes. The reduction rate of chlorine on the cathode surface has an impact on the amount of free chlorine released into the gray water.38 Depending on the amount of free 1897



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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).

anode caused high (35.2 ± 1.7 mg/L) concentrations of oxidizing species in the effluent. Apart from free chlorine, additional oxidative species were formed, because the decrease in the concentration of chloride ions was 2 times lower than the increase 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 in AOX concentration up to 1.8 ± 0.2 mg/L was found in the experiments with Ru/Ir MMO anodes. In the experiments with Pt/Ir anode, an increase of AOX was also observed to a final concentration of 0.7 ± 0.3 mg/L. While no conversion of micropollutants was found on the Pt anode, AOX formation on this anode was not observed either. The cathode material had a substantial impact on the presence of chlorinated organics in the effluent after electrochemical oxidation. The AOX concentrations in the effluent of the cells with Pt/Ir MMO (1.4 mg/L), Ru/Ir MMO (1.8 ± 0.1 mg/L), and Ti (1.8 ± 0.2 mg/L) cathodes were lower than those in the cells with Pt (2.0 ± 0.1 mg/L) and Ag (2.4 ± 0.3 mg/L) cathodes (SI Table S4). Higher AOX formation on Pt and Ag was observed, possibly due to the lower chlorine reduction on these cathodes, i.e., more free chlorine remains active for oxidation in the bulk solution. Free chlorine led to the enhanced formation of chlorinated compounds and oxidation of chloramines. This has been indicated by the amount of combined chlorine, which includes chlorine of organic chlorinated compounds and inorganic chloramines. Contrary to the AOX concentrations, combined chlorine was lower in the 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 of the cells with Pt/Ir and Ru/Ir MMO cathodes (SI Table S4). Therefore, higher chloramine conversion was achieved on Pt and Ag cathodes, whereas on Pt/Ir, Ru/Ir, and Ti cathodes, the chloramine conversion was lower.

Apart from the detection of AOX, which is a general measure of halogenated byproducts, specific byproducts of the micropollutants, spiked to gray water, were identified. Chromatographic peaks of unknown compounds were found in the GCMS full scan chromatograms from samples taken at Q = 0.12 Ah/L. The compounds were evolving as a result of the electrochemical oxidation, since they were not present in the spiked gray water before treatment. The compounds, evolving at retention times 11.08 and 12.35, were identified as dichlorinated byproducts of the oxidation of parabens (Figure 3). Their chromatographic peaks appear to show strong isotopic mass fragments at m/z 189 and 191 (SI Figure S2). These mass fragments, which are typical for compounds containing two chlorine atoms, also appear in the library spectrum of 3,5-dichloro-4-hydroxybenzoic acid.42 They correspond to the main mass fragment of this compound (C7H3O2Cl2: 3,5-dichloro-4-hydroxybenzoic acid without −OH of the acid group). Chlorinated parabens, being alkyl esters of 4-hydroxybenzoic acid, have the same main mass fragment (m/ z = 189 and 191). The chromatographic peaks at t = 11.08 and 12.35 also reveal, in a lower abundance, the presence of other molecular 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) was typical 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 Figures 4 and 5. Formation of dichlorinated parabens was found on all types of tested anodes, except for Pt, where no conversion was observed. The evolution of detected species (dichlorinated methyl- and propylparaben) had similar trends in formation with increasing specific electrical charge. For Ru/Ir MMO anode, a rapid increase in the abundance of dichlorinated 1898



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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 then gradually decreased. Another pattern was observed in the experiments with Pt/Ir MMO anode, where the amount of dichlorinated parabens in the gray water increased progressively after Q = 0.12 Ah/L with the highest levels reached at the end of 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 by the increase of the amount of dichlorinated parabens in the gray water with application of electric charge. These compounds are not the final products since their amount decreases with increase of the electrical charge. The results, however, did not allow conclusions on the further fate of these intermediates. For the other compounds studied, byproducts and intermediates were not identified. However, Gallard et al. found that chlorinated byproducts of bisphenol A are further decomposed by oxidation in solution at chlorine concentrations higher than 2 mg/L.43 Thus, complete oxidation of bisphenol A is expected at 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 may have been caused by a lower chlorine reduction, higher H2O2 production or direct dehalogenation on these cathodes, as stated previously. Obtained results show that a number of micropollutants present in gray water can be converted via electrochemical process, with highest yields obtained on Ru/Ir MMO anode. Although ubiquitous degradation of organic matter is considered to be an advantage of the process by some authors, not all of the personal care products used in this study were degraded, in spite of the high current densities used.10,11 The compounds, which are easily oxidized electrochemically, namely parabens and triclosan, are also biodegradable in aerobic conditions, as reported by Hernandez Leal et al.3 Meanwhile, the compounds, which are recalcitrant in aerobic biological systems, namely galaxolide and 4-MBC, are difficult to electrochemically oxidize on MMO anodes as well. At the same time, galaxolide and 4-MBC are efficiently removed from gray water both by ozonation and adsorption to granular activated carbon.5 Thus, utilization of MMO anodes for electrochemical conversion of personal care products in gray water is at this stage not a promising solution when compared to the other competing post-treatment methods mentioned. Another important drawback is the formation of halogenated byproducts, such as halogenated parabens, which was observed with all the combinations of electrodes tested. Although the halogenated parabens are eventually converted further on, this requires considerable extra charge input. Additionally, the study gives no evidence of the cathodic dechlorination of halogenated micropollutants, which is a reported mechanism for conversion of chlorinated micropollutants.44 Moreover, the final AOX concentrations are still higher than 1.5 mg/L. These concentrations exceed the values permitted for discharge. For example, 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, controversial data exist on the relationship between AOX and toxicity, because AOX, as a complex parameter, does not correlate exactly with the toxicity of individual chlorinated compounds.

Gellert found a strong correlation of Microtox, algae, and daphnia toxicity tests with the AOX concentrations in the wastewater, whereas O’Connor concluded that the AOX concentration is not suitable for predicting toxicity of the effluents.46,47 In some cases, chlorinated byproducts can be less toxic than the parent ones, as was shown by Wang and Farrell. These authors studied the toxicity of triclosan and its chlorinated byproducts, formed after electrochemical oxidation of the compound.33 The presence of halogenated compounds and free chlorine in the effluent makes the electrochemical oxidation inappropriate as a sole post treatment of gray water effluent. Formation of byproducts requires addition of the postfiltration step, such as biological sand filter or granular activated carbon filter. The same post-treatment step is usually included after ozonation, because that process also leads to the formation of undesirable byproducts.6 Compared with granular activated carbon filtration, electrochemical oxidation provides an extra contribution to gray water effluent quality improvement, namely the disinfection.48 As shown by Frontistis et al., complete removal of E. coli is accomplished by electrochemical oxidation of STP effluent on BDD anode already at Q = 0.01 Ah/L.13 That is much lower than the specific electric charge applied in this study for micropollutants destruction. Moreover, chlorinated derivatives, which are produced in the electrochemical oxidation step, are usually more hydrophobic, than the parent compounds. Thus, their adsorption to the activated carbon applied after electrochemical oxidation, will be increased.49 When the electrochemical oxidation process is followed by a postadsorption step, operation of the electrochemical process for just disinfection and conversion of micropollutants to their chlorinated forms is recommended. The latter is observed already at 0.03 Ah/L with a combination of Ru/Ir MMO anode and Ti or Ag cathode. This specific electric charge is equivalent to the energy requirements of 0.66 kWh/m3. This energy consumption is on the same order of magnitude as the energy consumption of a full-scale ozonation plant, as reported by Joss et al. (0.1−0.3 kWh/m3).6 Although the value, reported by Joss, includes the energy consumption of the filtration step, the latter is equal to 0.02 kWh/m3 only.50 Meanwhile, the calculated energy consumption of electrochemical oxidation is based on the results of a lab study performed in a nonoptimized cell. Optimization of the cell configuration and the use of stronger oxidizing electrodes, as proposed above, will considerably decrease the energy consumption. To show the competitiveness of the technology with ozonation, further studies should focus on the utilization of a BDD anode and optimization of the electrochemical cell.

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ASSOCIATED CONTENT

* Supporting Information S

Molecular structure of the analyzed micropollutants (Figure S1); mass spectra of the compounds, identified in the treated gray water on the total ion chromatogram (TIC) at the retention times 11.08 and 12.35 (Figure S2); physico-chemical characteristics of gray water (Table S1); concentrations of selected micropollutants in the effluent of adsorption/biooxidation system (Table S2); retention times, QQQ transitions, recoveries, and limits of quantification (LOQs) of the micropollutants selected for the study (Table S3); and changes in pH, conductivity, color, and concentrations of chlorinecontaining substances in the beginning at Q = 0 Ah/L and Q = 0.6 Ah/L with different combinations of anodes and cathodes 1899



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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is cofunded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the European Union Regional Development Fund, the Province of Fryslân, and the Northern Netherlands Provinces. The authors would like to thank the participants of the research theme “Separation at Source” for the fruitful discussions and their financial support. The authors would also like to thank Lina Taparaviciute for assistance in the experimental part of the study.

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

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Electrochemical Conversion of Micropollutants in Gray Water

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