SnO2–Sb and boron-doped diamond electrodes

SnO2–Sb and boron-doped diamond electrodes

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Electrochemical oxidation of electrodialysed reverse osmosis concentrate on Ti/PteIrO2, Ti/SnO2eSb and boron-doped diamond electrodes Arseto Y. Bagastyo a, Damien J. Batstone a, Korneel Rabaey a,b, Jelena Radjenovic a,* a b

Advanced Water Management Centre, The University of Queensland, Level 4, Gehrmann Bld. (60), St Lucia, QLD 4072, Australia Laboratory for Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, B-9000 Ghent, Belgium

article info

abstract

Article history:

Reverse osmosis concentrate from wastewater reclamation contains biorefractory trace

Received 30 July 2012

organic contaminants that may pose environmental or health hazard. Due to its high

Received in revised form

conductivity, electrochemical oxidation of brine requires low voltage which is energetically

18 September 2012

favourable. However, the presence of chloride ions may lead to the formation of chlori-

Accepted 1 October 2012

nated by-products, which are likely to exert an increased toxicity and persistence to further

Available online 17 October 2012

oxidation than their non-chlorinated analogues. Here, the performance of Ti/PteIrO2, Ti/

Keywords:

presence of chloride, nitrate or sulfate ions (0.05 M sodium salts). In order to investigate the

Electrochemical oxidation

electrooxidation of ROC with nitrate and sulfate ions as dominant ion mediators, chloride

Electrodialysis

ion concentration was decreased 10 times by electrodialytic pretreatment. The highest

Reverse osmosis concentrate

Coulombic efficiency for chemical oxygen demand (COD) removal was observed in the

Boron-doped diamond

presence of high chloride ions concentration for all anodes tested (8.3e15.9%). Electro-

Mixed-metal oxide electrode

oxidation of the electrodialysed concentrate at Ti/SnO2eSb and Ti/PteIrO2 electrodes

SnO2eSb and Si/BDD anodes was evaluated for the electrochemical oxidation of ROC in the

exhibited low dissolved organic carbon (DOC) (i.e. 23 and 12%, respectively) and COD removal (i.e. 37e43 and 6e22%, respectively), indicating that for these electrodes chlorinemediated oxidation was the main oxidation mechanism, particularly in the latter case. In contrast, DOC removal for the electrodialysed concentrate stream was enhanced at Si/BDD anode in the presence of SO42 (i.e. 51%) compared to NO32 electrolyte (i.e. 41%), likely due

to the contribution of SO4$ and S2 O82 species to the oxidative degradation. Furthermore, decreased concentration of chloride ions lead to a lower formation of haloacetic acids and trihalomethanes at all three electrodes tested. ª 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Reverse osmosis membranes have been applied in municipal wastewater reclamation as they represent an excellent barrier for organic and inorganic contaminants, as well as biological constituents. While in reverse osmosis membrane filtration

high-quality water is produced, 15e25% of the feed water is converted into a waste stream, reverse osmosis concentrate (ROC), which contains most of the dissolved salts and recalcitrant organics (Bellona et al., 2004). There has been an increased focus on further treatment of ROC since its direct and indirect discharge (e.g. via municipal sewer) poses an

* Corresponding author. Tel.: þ61 7 3346 3234; fax: þ61 7 3365 4726. E-mail addresses: [email protected] (A.Y. Bagastyo), [email protected] (J. Radjenovic). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.10.001

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 2 4 2 e2 5 0

environmental risk associated with high concentrations of pollutants rejected at the membrane (Khan et al., 2009). Among the existing treatment options, advanced oxidation processes are considered to be the most efficient in degrading non-biodegradable organic matter (Perez-Gonzalez et al., 2011). Electrochemical oxidation has recently emerged as a novel advanced oxidation process due to its capability to generate hydroxyl radicals (OH ) by water electrolysis, without any addition of chemicals (Bejan et al., 2012). Moreover, electrochemical oxidation is a robust and versatile process, capable of dealing with different types of wastewater at ambient temperature and pressure (Panizza and Cerisola, 2009). One of the most commonly investigated electrodes for environmental applications of electrochemical oxidation are mixed metal oxide electrodes. Their electrocatalytic activity is determined by the coating material and its interaction with electrogenerated OH . Anodes coated with Pt-, IrO2- and RuO2 have a strong interaction with the chemically-sorbed OH and are characterised by a low overpotential for O2 evolution, which results in their low oxidising power. In contrast, nonactive anodes such as SnO2eSb and PbO2-coated electrodes have a higher oxidising power, and are more capable of degrading organic compounds by the physisorbed OH at the electrode surface (Panizza and Cerisola, 2009). Among the non-active anodes, boron-doped diamond (BDD) exhibits the highest overpotential for O2 evolution, allowing the generation of quasi free OH and a variety of other reactive oxygen species (ROS), e.g. HO$2 , H2O2 and O3 at high anodic potentials applied (Bejan et al., 2012). Besides ROS, active species generated in-situ from ionic mediators will also participate in the indirect oxidation of organics. For example, S2 O82 and SO4$ , and P2 O48 and PO4$2 are formed from the oxidation of sulfate and phosphate-based electrolytes at BDD anodes and will significantly contribute to the mineralisation of organic matter (Costa et al., 2009; Zhu et al., 2008). For mixed metal oxide electrodes, particularly active anodes with low capability for OH generation, addition of chloride ions has been reported to improve the removal of organic pollutants, including pesticides and dyes, via electrogenerated reactive chlorine species, e.g. Cl2, HClO/ClO, ClO$ , Cl2$ (Bonfatti et al., 2000; Malpass et al., 2006; Rajkumar and Kim, 2006). The evolution of active chlorine is not exclusive to mixed metal oxide electrodes. Although oxidative degradation of organic compounds by OH is enhanced at BDD anodes, electrochlorination occurs simultaneously with oxidation by OH or other ROS in the presence of high concentrations of Cl ions (Boudreau et al., 2010; Scialdone et al., 2009; Bagastyo et al., 2012). Concerns have been raised regarding the formation of trihalomethanes and haloacetic acids during the electrochemical oxidation of chloride-containing waste streams, such as landfill leachate and ROC, at mixed metal oxide and BDD electrodes (Anglada et al., 2011; Bagastyo et al., 2011; Perez et al., 2010). Furthermore, electrochlorination of higher molecular weight organics will significantly increase the toxicity of the treated stream (Radjenovic et al., 2011). In order to decrease the toxicity of the treated effluent, Rajkumar et al. (2005) proposed the removal of halogenated organics formed in electrooxidation of wastewater containing phenols by adsorption onto activated 

243

carbon. In addition, anaerobic treatment has been studied for the removal of halogenated organics from pulp and paper wastewater (Deshmukh et al., 2009). This study aimed to investigate the effect of electrogenerated ROS/active chlorine, ROS or S2 O82 /SO4$ /ROS on electrooxidation of a real ROC stream. Three electrode materials were studied, Ti/PteIrO2, Ti/SnO2eSb and Si/BDD anode. Electrodialysis was applied to decrease the concentration of chloride ions prior to the oxidation, and enhance the contribution of OH in the presence of inert nitrate electrolyte (Pignatello et al., 2007). To investigate the effect of S2 O82 and SO4$ generation at BDD anode, Na2SO4 electrolyte was added to the electrodialysed ROC. The process evaluation was based on the removal of chemical oxygen demand (COD) and dissolved organic carbon (DOC), whereas the formation of low molecular weight halogenated by-products was determined as individual haloacetic acids and trihalomethanes. 













2.

Materials and methods

2.1.

Experimental set-up for electrodialysis of ROC

The ROC was collected directly at an inland water recycling plant which reclaims secondary effluents from four wastewater treatment plants. This ROC was then subjected to electrodialysis in order to decrease the concentration of chloride ions. In electrodialysis experiments, titanium mesh coated with RuO2eIrO2 (12 g m2 of RuO2:IrO2 ¼ 70:30) was used as an anode, with a projected surface area of 24 cm2 (4.8  5  0.1 cm) (Magneto Special Anodes, Netherlands). The cathode was a stainless steel mesh of the same dimensions, while Ag/AgCl electrode (3 M KCl, 0.210 V vs. standard hydrogen electrode (SHE), Bio-analytical, U.S.A) was used as reference electrode, placed in the anodic compartment. As illustrated in Fig. 1a, each compartment (internal dimensions: 20  5  2 cm, active surface: 100 cm2, active volume, VACT: 190 mL) was sequentially separated by cation exchange membrane and anion exchange membrane (CMI-7000 and AMI-7001, Membranes International, U.S.A.). 10 L of ROC was pumped into the central compartment and recirculated for 8 days at a flow rate of 80 mL min1. A detailed description of the electrodialysis experiments is given in Text S1.

2.2. Experimental set-up for electrochemical oxidation experiments



The anodes used in electrochemical oxidation experiments were Ti-based PteIrO2 and SnO2eSb electrodes and Si-based BDD, while stainless steel electrodes of the same shape and dimensions were used as cathodes. The two mixed metal oxide titanium mesh anodes had a projected surface area of 24 cm2 (4.8  5  0.1 cm) coated with Pt-IrO2 (12.5 g m2 of Pt:IrO2 ¼ 70:30) and SnO2eSb (15 g m2 SnO2eSb2O5 doped), and were supplied by Magneto Special Anodes (Netherlands). The monopolar plate Si/BDD anode (4.8  8.5  0.2 cm; 2e3 mm coating thickness of 500 ppm boron) with an active area of 40.8 cm2 was purchased from Adamant Tech., Switzerland. The Ag/AgCl electrode (3 M KCl, 0.210 V vs. SHE, Bio-analytical, U.S.A) was used as reference electrode.

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Fig. 1 e Schematic diagram of the experimental set-up for: (a) electrodialysis pre-treatment of ROC and (b) electrochemical oxidation of ROCED. In electrodialysis cell, ROC is sent to the central compartment where chloride other negatively charged ions permeate through anion exchange membrane, while cations diffuse towards the other side through cation exchange membrane. Note: 1) Power supply, 2) Data acquisition, 3) Ti-based current feeder, 4) Gas ventilation, 5) Peristaltic pump, 6) Stirrer and (7) pH monitoring and controlling (dosing) point.

The electrochemical oxidation experiments were conducted in a divided, two compartment reactor of the same cell dimensions as in the case of electrodialysis experiments, whereas anode and cathode compartments were separated by a cation exchange membrane (CMI-7000) only as shown in Fig. 1b. The electrochemical oxidation experiments were conducted in batch mode at controlled constant current (I ) of 510 mA using a potentiostat/galvanostat (KP07, Bank Elektronik, Germany). Data was recorded every 60 s using a data acquisition unit (Agilent 34970A, U.S.A.). Both anolyte (i.e. total volume, VTOT ¼ 1 L of electrodialysed ROC (ROCED)) and catholyte (i.e. 1 L of 0.5 M H2SO4) were continuously recirculated during 10 h at a rate of 120 mL min1. The supporting ion mediators added to ROCED were 0.05 M NaCl, NaNO3 or Na2SO4. Re-addition of NaCl to ROCED was performed due to the observed changes in COD, DOC and colour during the electrodialysis pre-treatment of ROC in order to investigate the performance of chlorine-mediated electrooxidation of ROC having the same background matrix as in the case of NaNO3 and Na2SO4 electrolytes, and simulating the original concentration of Cl ions in ROC. Since ion exchange membranes used in electrodialysis were of limited selectivity for particular ions, some of the charged organic ions electrodialysed together with the Cl ions. In order to assess the role

of generated active chlorine, ROS and S2 O82 /SO4$ /ROS, the experiments were performed at a controlled neutral pH by dosing 2 M NaOH. The pH and temperature values were monitored using an Endress þ Hauser pH controller (Germany). In order to characterise the employed electrodes in the working solutions, linear sweep voltammetry experiments were performed for ROCED and ROCED with added electrolytes, i.e. NaCl, NaNO3 and Na2SO4 (all at 0.05 M) in the same electrochemical reactor set-up.

2.3.

Analytical methods

Samples taken after 0, 1, 2, 4, 6, 8 and 10 h of electrooxidation were filtered using a 0.22 mm Millipore syringe unit. COD, free available chlorine and total chlorine (i.e. sum of active and combined chlorine) concentrations were measured directly after sampling. COD was determined using the COD tube tests range 10e150 mg L1 (Merck) by a spectrophotometric method, whereas active and total chlorine were measured with the N,N-diethyl-p-phenylenediamine ferrous titrimetric method. It is important to emphasise that oxidants other than HClO/ClO present in the solution (e.g. H2O2, O3) may react with N,N-diethyl-p-phenylenediamine similarly to chlorine to

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 2 4 2 e2 5 0

form a red dye, thus interfering with the measurements. The remaining samples were then quenched by adding specific amounts of 0.24 M Na2SO3 solutions, i.e. 1.2 mol of sulphite per mol of HClO (assuming all active chlorine was in HClO form), in order to eliminate its further reaction before the chemical analysis. Duplicate sample analyses were performed, and the obtained data was reported as a mean value (error margin of maximum and minimum values was lower than 5%). Measurement of DOC, Cl, NO3  and SO42 ions, conductivity, colour, haloacetic acids (i.e. monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, bromochloroacetic acid, monobromoacetic acid and dibromoacetic acid), and trihalomethanes (i.e. trichloromethane, bromodichloromethane, dibromochloromethane and tribromomethane) are described in detail in Text S2. The calculation of Coloumbic efficiency for COD removal, Cl ions oxidation and generation of Cl2 are given in Text S3.

3.

Results and discussion

3.1. Electrodialysis pre-treatment for chloride separation from ROC

obtained for ROCED with added NaCl electrolyte (Fig. 2a, c and e) indicated that the active Ti/PteIrO2 electrode has the highest activity for oxidation of Cl ions. Previously, excellent electrocatalytic properties of Ti/PteIrO2 electrodes for the production of active chlorine species were described by Szpyrkowicz et al. (2005). From the voltammogram of ROCED ([Cl] ¼ 142 mg L1 (4.0 mM)), O2 evolution could be observed without the interference of Cl2 generation with lower current response (Figure b, d and f). The onset potentials for oxygen evolution (EO2 ) increased in the order: Ti/PteIrO2 < Ti/ SnO2eSb < Si/BDD (i.e. EO2 ¼ 1.2, 1.5 and 1.8 V vs. SHE, respectively, at pH 7.0). The obtained voltammograms are in accordance with the higher oxidising power expected for BDD and non-active Ti/SnO2eSb anodes than in the case of active PteIrO2-coated electrode (Panizza and Cerisola, 2009). When NaNO3 and Na2SO4 were added to ROCED, the same onset potentials of oxygen and chlorine evolution were observed (Figure S2). It should be noted that the electrogeneration of Cl2 (ECl2) occurred at 0.9, 1.4 and 1.7 V vs. SHE for Ti/PteIrO2, Ti/ SnO2eSb and Si/BDD, respectively, which were lower values than their respective onset potentials for O2 evolution (i.e. 1.2, 1.5 and 1.8 V vs. SHE, respectively).

3.3. The characteristics of ROC and ROCED are summarised in Table 1. Although the objective of electrodialysis was to separate chloride ions contained in the ROC, concentrations of COD and DOC were lowered for 17% and 12%, respectively (Figure S1). Furthermore, the initial colour concentration (absorbency at 475 nm) in ROCED was 52% lower than in ROC, i.e. 274 vs. 132 mg PteCo L1. The decrease in colour intensity, as well as COD and DOC during electrodialysis pre-treatment can be explained by the loss of colour-causing organic compounds through the membrane. These compounds could be negatively charged organics, likely of low molecular weight, since molar mass plays an important role in the transport of organic ions by the electrical field (Zhang et al., 2009).

245

Chlorine-mediated electrooxidation of ROC

Fig. 3 illustrates the removals of COD, DOC and colour, as well as the measured active chlorine concentration during the anodic oxidation of ROCED with the addition of 0.05 M NaCl. The lowest COD removal was observed for Ti/PteIrO2, with 87% removal obtained at the end of the experiment (Q ¼ 5.6 Ah L1) (Fig. 3a). The non-active anodes, Ti/SnO2eSb and Si/BDD, achieved 93% and 100% COD removal after 5.6 Ah L1 and 3.3 Ah L1, respectively. The Coulombic efficiency of COD removal (CECOD) in chlorine-mediated electrooxidation at Si/BDD was 15.9% calculated at 3.3 Ah L1 when

3.2. Linear sweep voltammetry at Ti/PteIrO2, SnO2eSb and BDD electrodes Prior to galvanostatic electrochemical oxidation experiments, linear sweep voltammetry was performed to obtain the onset potentials for O2 and Cl2 evolution at all three anodes, at a scan rate of 10 mV s1. The voltammogram of Cl2 evolution

Table 1 e Main characteristics of the ROC and ROCED used in this study. Measures pH Conductivity Colour COD DOC Cl NO3  SO42

Unit

ROC

ROCED

mS cm1 mg Pt-Co L1 mg O2 L1 mg C L1 mg L1 mg L1 mg L1

7.8 5.2 274 175 53 1526 14.1 253

6.8 1.4 132 145 46 142 2.1 90

Fig. 2 e Linear sweep voltammograms (vs. SHE) for the oxidation of: (a, c and e) ROCED D 0.05 M NaCl and (b, d, f) ROCED only at Ti/PteIrO2 (a and b), Ti/SnO2eSb (c and d) and Si/BDD (e and f) anodes. The current vs. potential was scanned from 0 to D2 V (for Ti/PteIrO2), to D2.5 V (for Ti/ SnO2eSb) and to D3.5 V (for Si/BDD). Note: pH [ 7; scan rate [ 10 mV sL1.

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Fig. 3 e Electrochemical oxidation of ROCED D 0.05 M NaCl at ((;P) Ti/PteIrO2, (CB) Ti/SnO2eSb and (-,) Si/BDD anodes: (a) removal of COD and DOC, and (b) decolourisation and formation of active chlorine.

COD was completely removed. On the other hand, CECOD achieved by Ti/PteIrO2 and Ti/SnO2eSb were only 8.3 and 8.9%, respectively, at the end of experiment, i.e. 5.6 Ah L1 (Table 2). Previous studies reported that BDD was more efficient than both active and non-active mixed metal oxide electrodes in chlorine-mediated oxidation of organic matter (Scialdone et al., 2009; Van Hege et al., 2004; Zhou et al., 2011). For the Si/BDD anode, besides the chloro-species, OH likely contributed to the oxidative degradation of organics. Nevertheless, limited DOC removal was obtained for all anodes investigated. In electrooxidation of ROCED þ NaCl at Si/ BDD, 40% of the initial DOC was removed after 5.6 Ah L1, while in the case of Ti/SnO2eSb and Ti/PteIrO2 the obtained DOC removals were 31% and 28%, respectively (Fig. 3a). Considering that no COD could be measured at this point, the remaining organic compounds measured as DOC were apparently not oxidisable by the dichromates in the COD test. Faster COD removal than DOC removal observed at all electrodes tested indicates the formation of oxidation byproducts, likely chlorinated organics (Bagastyo et al., 2012; Radjenovic et al., 2011). At anodes with high overpotential for O2 evolution such as Ti/SnO2eSb and Si/BDD (anodic potential, EAN ¼ 2.3 and 3.5 V vs. SHE, respectively e Table 2), electrogenerated OH can be expected to enhance oxidative degradation of the organic matter. A similar observation of superior performance of BDD over RuO2-coated titanium electrodes for ROC oxidation was 



reported by Van Hege et al. (2004). However, chloride ions and active chlorine (HClO/ClO) act as efficient scavengers of OH (Grebel et al., 2010), whereas the equilibrium of reactive halogen species and OH will be pH-dependant (De Laat et al., 2004). In the pH range of the experiment (i.e. pH 6e7), enhanced contribution of OH and other ROS to the indirect oxidation of organics is expected (De Laat et al., 2004). However, the scavenging of OH by chloride ions is likely to be substantial due to the high initial concentration of chloride present in ROCED þ NaCl, i.e. 1318 mg L1 (37.2 mM), which lowers the participation of OH in the degradation of organics. Chloride ions are oxidised by OH , forming reactive radical and non-radical halogen species, which are less capable of bond breaking than OH . During oxidation of ROCED þ NaCl, the removal of the remaining colour was slower for Ti/PteIrO2 than for the nonactive anodes at the beginning of electrooxidation (e.g. up to 1 Ah L1) (Fig. 3b). However, at the end of electrooxidation nearly 95% decolourisation was achieved by all anodes tested. This likely corresponds to oxidative degradation of higher molecular weight organic compounds, e.g. humic substances as observed in our previous study (Bagastyo et al., 2011). Similar decrease in absorbency at 455 nm during electrochemical oxidation of ROC at BDD and mixed metal oxide anodes was reported in Van Hege et al. (2004). Long-lived active chlorine was likely the main oxidant species, as up to 10.6e16.5 mM of residual active chlorine 













Table 2 e Calculated CECOD, CEClL , CECl2 (and its corresponding measured total chlorine) and anodic potential in the presence of NaCl ([ClL]0 [ 37.2 mM), NaNO3 ([ClL]0 [ 4.0 mM) and Na2SO4 ([ClL]0 [ 4.0 mM) during electrochemical oxidation of ROCED on Ti/PteIrO2, Ti/SnO2eSb and Si/BDD anodes after Q [ 5.6 Ah LL1. NaCl Ti/PteIrO2 CECOD, % CECl % Total chlorine, mM CECl2 , % EAN, V vs. SHE

8.3 7.4 17.4 18.3 1.6

Ti/SnO2eSb 8.9 8.1 14.1 14.8 2.3

Na2SO4

NaNO3 Si/BDD 15.9 11.5 12.9 13.5 3.5

Ti/PteIrO2

Ti/SnO2eSb

Si/BDD

Ti/PteIrO2

Ti/SnO2eSb

Si/BDD

2.2 0.6 1.1 1.1 1.7

3.7 1.9 1.1 1.1 2.4

5.7 1.9 0.1 0.1 3.5

0.6 0.2 0.2 0.2 1.8

4.3 1.7 0.6 0.6 2.4

6.8 1.9 0.3 0.3 3.6

a

a Calculated after 3.3 Ah L1 when COD was completely removed.

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(Fig. 3b) and 12.9e17.4 mM of total chlorine (Figure S3a) accumulated in the bulk at all three anodes after 5.6 Ah L1, both increasing in the order: Si/BDD < Ti/SnO2eSb < Ti/PteIrO2. The corresponding Coulombic efficiencies for Cl2 evolution (CECl2 ) were 13.5%, 14.8% and 18.3% for Si/BDD, Ti/SnO2eSb and Ti/ PteIrO2, respectively (Table 2). The Coulombic efficiencies for chloride ion oxidation (CECl ) were similar for all three anodes, i.e. in the range from 7.4 (Ti/PteIrO2) to 11.5% (Si/BDD) after 5.6 Ah L1 (Table 2). Similar values calculated for CECl2 and CECl at Si/BDD indicate that at this electrode Cl and ClO may have been oxidised further to toxic chlorate or perchlorate ions (Bagastyo et al., 2011; Perez et al., 2010), which were not measured in this study. Overall, in the presence of chloride the highest oxidation efficiency was observed for Si/BDD, followed by Ti/SnO2eSb and Ti/PteIrO2. Considering its higher oxidising power and the lowest concentration of accumulated chlorine, oxidant species other than chlorine (e.g. ROS, ClO$ and Cl2$ ) likely contributed to the oxidation of organic matter at Si/BDD electrode.

3.4. Electrooxidation of ROCED in the presence of NaNO3 and Na2SO4 As seen in Fig. 4a and b, the lowest COD and DOC removals were observed for Ti/PteIrO2 in the presence of either NO3  or SO42 , i.e. 6e22% and 12%, respectively. It is expected that Ti/ PteIrO2 has poor performance when the concentration of Cl is low because active mixed metal oxide electrodes mainly

rely on the chlorine-mediated electrooxidation (Panizza and Cerisola, 2009). In the case of the Ti/SnO2eSb anode, around 37e43% of COD and 20e23% of DOC was removed in the presence of both NO3  and SO42 . The enhanced performance of Ti/SnO2eSb can be explained by the non-active nature of the SnO2eSb coating and thus lower adsorption of OH , which enables their participation in the oxidative degradation of ROCED. The overall removal of colour was slower without Cl on all anodes tested (Fig. 4c). However, at the end of electrooxidation similar colour removal was achieved, i.e. 85e95%, although the decrease in colour intensity was faster with NaNO3 than with Na2SO4 electrolyte at the beginning of electrooxidation. Enhanced generation of active chlorine could be responsible for the more intense oxidative degradation of colour-causing organic compounds in the presence of NO3  . In the case of Ti/PteIrO2 and Ti/SnO2eSb, addition of NO3  or SO42 seemed to affect the electrogeneration of chlorine in spite of identical EO2 and ECl2 potentials determined (see Section 3.2), and 0.45 and 1.13 mM of residual active chlorine concentration was measured in the presence of nitrate for Ti/ PteIrO2 and Ti/SnO2eSb, which was higher than in the presence of sulfate (i.e. 0.16 and 0.52 mM, respectively) (Fig. 4d). Furthermore, a faster decrease of the Cl concentration was noted at both electrodes in the presence of NO3  (Figure S3b and S3c). Both SO42 and NO3  ions will compete with Cl ions for electroactive sites at the anode surface, whereas higher molar conductivity and larger hydrated ionic radius of SO42 (Zhang et al., 2011) may limit the oxidation of Cl ions at 

Fig. 4 e Electrochemical oxidation of ROCED D 0.05 M NaNO3 (open symbols) and ROCED D 0.05 M Na2SO4 (closed symbols) on: (a) COD removal, (b) DOC removal, (c) colour removal and (d) formation of active chlorine during electrochemical oxidation of ROCED at (;P) Ti/PteIrO2, (CB) Ti/SnO2eSb and (-,) Si/BDD anodes.

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the anode surface more than NO3  ions. Thus, a lower production of chlorine in the presence of Na2SO4 than in the presence of NaNO3 could be attributed to a faster adsorption of than monovalent NO3  at the surface of the divalent SO2 4 electrode. More intense generation of chlorine was likely responsible for the higher COD removal observed at Ti/PteIrO2 in the presence of NO3  than in the presence of SO42 ions (Fig. 4a). On the other hand, in the presence of NO3  superior performance of Si/BDD over Ti/PteIrO2 and Ti/SnO2eSb anodes was observed, with 60% COD and 41% DOC removal after 5.6 Ah L1. Low concentration of Cl together with an inert electrolyte NaNO3 allows a higher participation of OH and other ROS in electrochemical oxidation. While in the case of Ti/SnO2eSb electrode the generated OH will be partially adsorbed at the electrode surface, BDD is capable of producing quasi free OH (Zhu et al., 2008). Furthermore, a recent study demonstrated that the OH radicals electrogenerated at BDD have similar activity to free-aqueous OH (Bejan et al., 2012). The latter represent the main advantage of BDD over the other two anodes tested, as they enhance the degradation rates of recalcitrant organic pollutants, including by-products such as carboxylic acids (Scialdone et al., 2011). Nevertheless, the highest COD and DOC removal (i.e. 74% and 51%, respectively) was obtained with Si/BDD after adding Na2SO4. Previously, a higher mineralisation of a model compound was achieved during anodic oxidation on BDD in the chloride-free medium (Murugananthan et al., 2008; Scialdone et al., 2009). In electrooxidation at BDD in the presence of SO42 , long-lived persulfate ions (S2 O82 ) are generated via direct electron transfer and/or via reaction with the generated OH by forming sulfate radicals (SO4$ ) as intermediates (Khamis et al., 2010; Ozcan et al., 2008). Sulfate radical is an excellent one-electron oxidant, with a standard redox potential (E0) of 2.6 V vs. SHE (Liang et al., 2007). Once SO4$ is formed, it can rapidly attack oxidisable compounds including organic compounds and inorganics such as Cl and SO42 ions. In addition, sulfate radicals can also react with water to form OH (Norman et al., 1970). On the other hand, although persulfate has relatively high E0, i.e. 2.01 V vs. SHE, it 



typically has slow oxidative kinetics at ordinary temperatures (Liang et al., 2007). However, S2 O82 ions are capable of diffusing away from the electrode surface into the bulk liquid where they may be further activated, e.g. by ferrous ion, forming the reactive SO4$ (Romero et al., 2010; Wu et al., 2012). Therefore, both S2 O82 and SO4$ were likely responsible for enhanced oxidation of ROCED þ Na2SO4 at BDD electrode. In summary, the oxidation efficiency was inferior for both mixed metal oxide anodes in the presence of low concentration of chloride ions, particularly Ti/PteIrO2 exhibited low performance. The calculated CECOD for Ti/PteIrO2 was only 2.2% (5.6 Ah L1), nearly four times lower than in the presence of a high Cl ion concentration (i.e. 8.3%) (Table 2). This result is in accordance with the previous studies (Malpass et al., 2006; Panizza and Cerisola, 2009; Szpyrkowicz et al., 2005).











3.5. Formation of halogenated by-products in the presence of NaCl, NaNO3 and Na2SO4 The formation of haloacetic acids (HAAs) and trihalomethanes (THMs) was expected to increase with increasing Cl concentration, irrespective of the electrode material used. As shown in Table 3, significantly higher formation of total HAAs and total THMs was noted for ROCED þ NaCl at all three anodes. Si/BDD exhibited the highest formation of both total HAAs and total THMs among the anodes investigated (62.4 and 9.1 mM, respectively at the end of the experiment, Q ¼ 5.6 Ah L1). In the case of Ti/SnO2eSb, 34.2 mM of total HAAs and 8.9 mM of total THMs were formed after 5.6 Ah L1 was supplied, while 27.4 mM of total HAAs and 5.6 mM of total THMs were measured for the Ti/PteIrO2 anode. Formation of HAAs and THMs has been reported during the electrochemical oxidation of ROC using mixed metal oxide and BDD anodes (Bagastyo et al., 2011; Perez et al., 2010). Although Ti/PteIrO2 was the most efficient anode for the production of chlorine, the highest formation of THMs and HAAs was observed for Si/ BDD. This could be a consequence of the formed chlorine atoms (Cl ) at the anode surface which quickly react with chloride ions and form reactive chlorine radical species, e.g. 

Table 3 e Formation of THMs and HAAs during electrochemical oxidation of ROCED with the addition of NaCl, NaNO3 and Na2SO4 after Q [ 5.6 Ah LL1. DBP

HAAs (mM)

THMs (mM)

Compound

Monochloroacetic acid Dichloroacetic acid Trichloroacetic acid Bromochloroacetic acid Monobromoacetic acid Dibromoacetic acid Total HAAs Trichloromethane Bromodichloromethane Dibromochloromethane Tribromomethane Total THMs

NaCl

Na2SO4

NaNO3

Ti/ PteIrO2

Ti/ SnO2eSb

Si/ BDD

Ti/ PteIrO2

Ti/ SnO2eSb

Si/ BDD

Ti/ PteIrO2

Ti/ SnO2eSb

Si/ BDD

1.48 11.58 14.10 0.16 0.04 0.02 27.38 5.35 0.26 0.01 0.00 5.63

2.17 14.62 17.29 0.10 0.04 0.02 34.24 8.73 0.18 0.00 0.00 8.92

6.38 25.78 30.06 0.13 0.04 0.02 62.41 8.92 0.15 0.00 0.00 9.08

0.69 4.66 3.31 0.20 0.04 0.02 8.91 3.10 0.18 0.01 0.00 3.29

0.94 7.24 6.63 0.13 0.04 0.02 15.00 5.66 0.25 0.01 0.00 5.93

0.28 0.55 0.13 0.09 0.04 0.04 1.12 0.21 0.01 0.00 0.00 0.23

0.75 9.73 1.48 0.41 0.04 0.02 12.43 4.80 0.26 0.02 0.00 5.08

1.29 10.33 9.95 0.13 0.04 0.02 21.76 5.79 0.21 0.01 0.00 6.02

0.67 11.08 1.40 0.33 0.12 0.02 13.62 1.85 0.04 0.00 0.00 1.90

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 2 4 2 e2 5 0

ClO$ and Cl2$ (Ferro et al., 2000; Park et al., 2009), resulting in enhanced organic degradation with an increased concentration of HAAs and THMs as the by-products. The formation of HAAs and THMs was considerably decreased when the concentration of Cl ions was lowered from 37.2 to 4.0 mM in ROCED. The measured concentrations of total HAAs and total THMs were decreasing in the order Ti/ SnO2eSb > Ti/PteIrO2 > Si/BDD regardless of the electrolyte added (i.e. NaNO3 or Na2SO4). Although the initial concentration of Cl ions in ROCED was the same, the formation of total HAAs and total THMs at Si/BDD in the presence of SO42 was an order of magnitude higher than in NO3  . The measured total HAAs and total THMs concentrations were 13.6 and 1.9 mM in Na2SO4, and 1.1 and 0.2 mM, in NaNO3, respectively. This could be explained by the oxidation of Cl to reactive chloro-species in the bulk by S2 O82 ions, and/or in the vicinity of the electrode surface by SO4$ radicals (Khamis et al., 2010; Provent et al., 2004). Therefore, bulk electrochlorination and formation of THMs and HAAs is enhanced in the presence of sulfate through indirect oxidation. On the other hand, concentrations of total HAAs and total THMs measured with the addition of SO42 and NO3  for both Ti/SnO2eSb and Ti/ PteIrO2 anodes were in the same order of magnitude, since they are not capable of generating S2 O82 and SO4$ species (Table 3). In the presence of high Cl concentrations polychlorinated by-products, i.e. trichloroacetic acid, dichloroacetic acid and trichloromethane were the dominant HAA and THM species measured for all three anodes, while dichloroacetic acid and trichloromethane were the dominant species measured in the case of lowered concentration of Cl ions. Furthermore, brominated HAAs and THMs were detected at low concentrations of maximum 0.12 and 0.01 mM, respectively, as the Br ion concentration was also lowered during electrodialysis.

4.

Conclusions

This study demonstrated that chlorine-mediated oxidation represents an important mechanism of electrochemical oxidation of real ROC at Ti/PteIrO2, Ti/SnO2eSb and Si/BDD electrodes. The most efficient COD removal was achieved in the presence of a high chloride ions concentration. Removal of the chloride ions by electrodialysis and substitution with inert nitrate anions highlighted the superior performance of Si/BDD by generating quasi-free OH and other ROS, leading to not only enhanced mineralisation (up to 41% DOC removal), but also a minimised formation of HAAs and THMs (i.e. from 62.4 to 1.1 mM of total HAAs and from 9.1 to 0.2 mM of total THMs). Although COD removal at BDD was diminished at a lower Cl concentration, the electrode was capable of achieving the same oxidative degradation (i.e. DOC removal) of the organic matter as in the case of chlorine-mediated oxidation. In the presence of sulfate ions, the performance of BDD anodes in terms of COD and DOC removal was enhanced from 60% to 74%, and from 41% to 51%, respectively, compared to nitrate electrolyte, due to the participation of S2 O82 and SO4$ species in oxidation. In contrast, oxidation on Ti/PteIrO2 and Ti/ SnO2eSb appears to rely on HClO/ClO species. 

249

Considering the expected high energy demand of a combined electrodialysis and electrochemical oxidation process, anion exchange membranes selective for monovalent anions should be used. Since electrodialysed brine has a lower conductivity, improved reactor design (e.g. columntype reactors, bipolar BDD electrodes) is required in order to reduce the energy investment of electrooxidation at high BDD anodic potentials.

Acknowledgements This study was supported by the Australian Research Council (LP0989159), Veolia Water Australia, Seqwater, Magneto Special Anodes and the Urban Water Security Research Alliance. The authors would like to acknowledge Dr. Renu Patel from Queensland Health Forensic and Scientific Service for performing the analyses of trihalomethanes and haloacetic acids. Arseto Bagastyo is currently also a staff member on leave at Institut Teknologi Sepuluh Nopember, Indonesia.

Appendix A. Supporting information Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2012.10.001.

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