A New Mechanism in Electrochemical Process for Arsenic Oxidation

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A New Mechanism in Electrochemical Process for Arsenic Oxidation: Production of H2O2 from Anodic O2 Reduction on the Cathode under Automatically Developed Alkaline Conditions Ao Qian,†,‡ Songhu Yuan,*,†,‡ Peng Zhang,† and Man Tong†,‡ †

State Key Lab of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, P. R. China ‡ School of Environmental Studies, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, P. R. China S Supporting Information *

ABSTRACT: Electrochemical cathodes are often used to reduce contaminants or produce oxidizing substances (i.e., H2O2). Alkaline conditions develop automatically around the cathode in electrochemical processes, and O2 diffuses onto the cathode easily. However, limited attention is paid to contaminant transformation by the reactive species produced on the cathode under oxic and alkaline conditions due to the inapplicability of pH for Fenton reaction. In this study, a new oxidation mechanism on the cathode is presented for contaminant transformation under automatically developed alkaline conditions. In an electrochemical sand column, 6.67 μM As(III) was oxidized by 36% when it passed through the cathode under the conditions of 30 mA current, an initial pH of 7.5 and a flow rate of 2 mL/min. Under the alkaline conditions (pH 10.0−11.0) that developed automatically around the cathode, the reduction potential of As(III) decreased greatly, allowing a pronounced oxidation by the small quantities of H2O2 produced from O2 reduction on the cathode. As(III) oxidation was further increased by the presence of soil pore water and groundwater solutes of HCO3−, Ca2+, Mg2+ and humic acid. The new oxidation mechanism found for the cathode under localized alkaline conditions supplements the fundamentals of contaminant transformation in electrochemical processes.

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INTRODUCTION Electrochemical processes are increasingly used for treating contaminated soils and groundwater.1−4 The main mechanisms of electrochemical transformation of contaminants include anodic oxidation,5−7 cathodic reduction,8−10 and indirect oxidation by the oxidizing substances (i.e., H2O2, •OH) produced.11−13 H2O2 can be produced through the twoelectron reduction of O2 on the cathode under acidic conditions, and is then utilized for contaminant degradation in the presence of ferrous iron (termed “electro-Fenton”).3,14 Although previous investigations on the electro-Fenton process were carried out under acidic or near neutral conditions,11−13 localized alkaline conditions appear on the vicinity of the cathode when the OH− produced from the cathode (eq 1) is not neutralized quickly by the H+ produced from the anode (eq 2). For example, alkaline conditions, that is, pH 9.0−12.0, are automatically attained near the cathode in the electrokinetic soil/groundwater remediation process,15−17 in the electrocoagulation process18−20 and in the divided electrolytic system.21−23 Meanwhile, O2 entering from the air or produced from the anode diffuses onto the cathode. Nevertheless, contaminant degradation under the automatically developed alkaline conditions around the cathode has not been reported. 2H 2O → 4H+ + O2 + 4e−

(1)

2H 2O + 2e− → 2OH− + H 2

(2)

© 2015 American Chemical Society

The reduction of O2 on the cathode generally proceeds through a series of steps. O2 is first reduced to superoxide anion (O2−) followed by further reduction to H2O2 and H2O in alkaline media (eqs 3−5). H2O2 is a versatile oxidant over the whole pH range (E0 = 1.763 V at pH 0, E0 = 0.878 V at pH 14). Contaminant degradation by H2O2 is well recognized under acidic conditions, particularly in the presence of Fe(II), but is rarely tested under alkaline conditions. The oxidation/ reduction potential of superoxide anion has also been reported.24 With the solution pH ascending, it could oxidize reduced Cr(III) to Cr(VI).25 Therefore, it is possible that these reactive oxygen species (ROSs) have the potential to transform a certain range of contaminants. This transformation represents a new mechanism for contaminant degradation under alkaline conditions in electrochemical processes. O2 + e− → O2−

(3)

O2− + 2H+ + e− → H 2O2

(4)

H 2O2 + 2H+ + 2e− → 2H 2O

(5)

Received: Revised: Accepted: Published: 5689

February 13, 2015 March 28, 2015 April 8, 2015 April 8, 2015 DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

Article

Environmental Science & Technology Table 1. Parameters and Results Associated with Column Experiments steady-state pH

oxidation efficiency (%)

no.

solution composition (10 mM Na2SO4)a

current (mA)

cathode

effluent

anodeb

cathodec

totald

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14

10 mM Na2SO4 10 mM Na2SO4 10 mM Na2SO4 0.5 mM NaHCO3 2 mM NaHCO3 5 mM NaHCO3 2 mM NaCl 4 mM NaCl 2 mM CaSO4 2 mM MgSO4 1 mM CaSO4+1 mM MgSO4 5 mg/L HAf 10 mg/L HA 20 mg/L HA

10 30 50 30 30 30 30 30 30 30 30 30 30 30

9.3 10.8 11.1 10.8 10.9 10.1 10.9 10.8 10.1 9.6 9.3 11.0 10.9 10.6

6.7 8.6 8.8 8.0 7.5 8.7 8.1 8.0 7.2 6.2 6.4 8.3 7.8 7.8

23 28 28 25 25 26 57 95 30 15 20 44 57 60

7 36 42 65 72 45 43 5 70 85 80 48 40 18

30 64 70 90 97 71 100 100 100 100 100 92 97 78

a

10 mM Na2SO4 is the background electrolyte. The other electrolytes were added in C4−C14, respectively. bThe oxidation efficiency of As(III) from Ports 2 to 4. cThe oxidation efficiency of As(III) from Ports 5 to effluent. dThe total oxidation efficiency of As(III) in the whole column. fThe concentration of HA is represented by humic acid sodium salt.

components in soil pore water and groundwater such as inorganic ions (HCO3−, Cl−, Ca2+, Mg2+) and humic acid (HA) on the oxidation are also investigated.

Arsenic (As) contamination in soils and groundwater is a worldwide environmental issue. As is mostly found in the inorganic forms as oxyanions of arsenite (As(III)) and arsenate (As(V)) in natural waters. Electrochemical processes are proven to be effective and promising for As remediation. For example, electrokinetic process has been used to remove As from soil wastes and low-permeability contaminated subsurface in laboratory or field tests,17,26 and iron-electrode electrocoagulation is widely used for treating As in wastewaters and groundwater.27−29 The mechanism of As removal in electrokinetic process is ascribed to the mobilization by electromigration and electroosmosis, and the oxidation of As(III) in electrocoagulation process is mainly attributed to the oxidizing substance produced from the reaction of Fe(II) and O2.30 Although alkaline conditions develop automatically near the cathode and the system is exposed to O2, the transformation of As(III) by the products of O2 reduction on the cathode under alkaline conditions has not been addressed. Oxidation of As(III) to As(V) is beneficial to the remediation because of the dramatic decrease in toxicity and mobility. pH and redox potential (Eh) are the most important factors controlling As transformation. As suggested in SI Figure S1 in the Supporting Information (SI), oxidation of As(III) becomes easier with raising pH. The oxidation potential of H2O2 is much higher than that of As(III) in the whole pH range. The oxidation potential of O2− (E0= −0.33 V) is also higher than that of As(III) when solution pH reaches above 10.0. From a thermodynamic view, both H2O2 and O2− can oxidize As(III) under alkaline conditions. This study aims to reveal the contribution of O2 reduction on the cathode under alkaline conditions to the oxidation of contaminants, thus provides extra fundamentals to elucidate contaminant transformation in electrochemical processes. As(III) is employed as the target contaminant because its contamination is widespread31 and its reduction potential is pH-dependent.32,33 An electrochemical sand column is employed to produce an automatic pH increase in the vicinity of cathode as well as to transfer O2 produced from the anode to the cathode (SI Figure S2). The contributors to As(III) oxidation near the cathode are identified and evaluated, and the associated mechanisms are elucidated. The effects of common

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EXPERIMENTAL METHODS Chemicals. As2O3 (99.8%) was purchased from Shanghai General Reagent Factory, China. Na2HAsO4·7H2O (99.99%) was obtained from Sigma-Aldrich. One gram of 2,9-dimethyl1,10-phenanthroline (DMP) reagent (Sigma-Aldrich), which was used as received, was dissolved in 100 mL of ethanol (>99.7%, Sinopharm Chemical Reagent Co. Ltd.) for detection of produced H2O2 in the cathodic compartment. Nitro blue tetrazolium (NBT) were of analytical grade and were used without further purification. Humic acid sodium salt (HA, 99.5%) was purchased from Sigma-Aldrich, which contains 0.75% N, 34.2% C, 2.9% H, 0.3% S (determined by elemental analysis), 2.55 mmol/g carboxyl and 0.72 mmol/g phenolic hydroxyl (determined by potentiometric titration). Mixed metal oxides (MMO, IrO2 mixed Ta2O5 coating on titanium diamond mesh, Shanxi Kaida Chemical Ltd.) with dimensions of 5.0 cm diameter and 1.7 mm thickness were used as both the anode and cathode due to its superior stability over carbonaceous electrodes. Deionized (DI) water (18.2 MΩ·cm) obtained from a Heal Force NW ultrapure water system was used in all the experiments. All the other chemicals were above analytical grade. Column Experiments. A vertical electrolytic column (5.0 cm inner diameter × 30 cm length) (SI Figure S2) was employed to justify the proposed new mechanism. Two pieces of MMO mesh were installed in an upward sequence as the anode and the cathode with a distance of 10 cm so that a local alkalinity was produced around the cathode. The distance of the anode from the bottom is 10 cm. The remaining space in the column was packed with 3 mm glass beads with a porosity of 0.4. The total and pore volume (PV) of the column were 580 and 230 mL, respectively. Solutions were pumped into the bottom using a peristaltic pump (Luxi, model HL-2, China). The flow rate was maintained at 2 mL/min, resulting in Darcy’s velocity of 0.26 cm/min. A total current of 30 mA, corresponding to the voltage varying from 23 to 30 V, was maintained by a direct current (DC) power supply (GPC5690

DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

Article

Environmental Science & Technology

Analysis. A high performance liquid chromatography (HPLC) system consisting of a ternary pump, an injector and a 100-μL sampling loop coupled to an atomic fluorescence spectrometer (AFS 9600, Beijing Kechuang Haiguang Instrument Co., Ltd.) was used for the analysis of As(III) and As(V) in the solutions. Separation of As(III) and As(V) was performed on a Hamilton PRP X-100 anion-exchange column (10 μm, 250 mm × 4.1 mm i.d; Hamilton, Reno, NV). Hydride generation of volatile arsines was conducted by adding online solutions of 5% HCl and 2% KBH4 using a peristaltic pump. The quantification of arsenic was performed on an atomic fluorescence spectrometer equipped with a boosted-discharge hollow cathode lamp. The analog signal output was connected to a computer equipped with chromatographic software (Beijing Kechuang Haiguang Instrument Co., Ltd.). H2O2 concentrations were measured at 540 nm after coloration with Cu2+ and DMP by a spectrometer (UV-1800 PC, Shanghai Mapada Spectrum Instrument Co., Ltd.).34

3060D, Taiwan Goodwill Instrument). Simulated deoxygenated-electrolyte containing 6.67 μM (500 μg/L) As(III) and 10 mM Na2SO4 was stored in a glass bottle under N2. Different solutes were added to the electrolyte to test their influence. Prior to electrolysis, the column was flushed by 2 PVs of electrolyte. Control experiments show that the adsorption of As(III) and As(V) on glass beads were negligible. At regular time intervals, about 1.5 mL of groundwater was sampled from the six ports (see SI Figure S2 for locations) and filtered through a 0.45 μm micropore filter membrane for analysis of As(III), As(V) and pH. A total of 14 column experiments were conducted with parameters listed in Table 1. The first set (Columns C1−C3) evaluated the oxidation at different currents. The second (Columns C4−C8) and third (Columns C9−C11) sets investigated the effects of anions such as HCO3− and Cl− and cations such as Ca2+ and Mg2+ on the oxidation. And the last set (Columns C12−C14) assessed the influence of HA. Batch Experiments for Mechanistic Study. The mechanisms of As(III) oxidation in the column were investigated through a series of batch experiments in a divided electrolytic system. Two 750 mL cells were used as the anodic and cathodic compartments. A salt bridge consisting of 2% agar in saturated K2SO4 was used to connect the two compartments. Two pieces of MMO mesh were used as the anode and the cathode. For each test, 6.67 μM As(III) in 500 mL 10 mM Na2SO4 solution was added into both compartments. Solution pH was adjusted to 7.5 by addition of dilute H2SO4 and NaOH before electrolysis and was not adjusted during the process unless otherwise specified. The reactor was stirred at 300 rpm using a Teflon-coated magnetic stirring bar. The reaction was initiated by switching on the DC power supply. The effects of solution pH, dissolved oxygen (DO) and current on As(III) oxidation in the cathodic compartment were investigated to identify the required components and conditions for the oxidation. Then, the concentration of H2O2, a possible intermediate of O2 reduction in the cathodic compartment, was determined under the same conditions and its contribution was explored by the addition of 6.67 μM As(III)/As(V) into the cathodic compartment. The contribution of O2− to As(III) oxidation was also evaluated by the addition of 6.67 μM As(III) and 0.1 mM NBT, a O2− scavenger, into the cathodic compartment which was powered off immediately after 1 h electrolysis. The addition of NBT prior to electrolysis was not feasible due to the formation of a layer of black purple coating on the cathode. The divided electrolytic system was also used to elucidate the effect of HA on As(III) oxidation on the cathode. At predetermined time intervals, 1.5 mL of aqueous solution was sampled and filtered through a 0.45 μm micropore filter membrane for analysis of As(III) and As(V). Each experiment was performed at least in duplicate. Voltammograms of O2 Reduction on the Cathode. The reduction of O2 by the MMO (1 × 1 cm) mesh was analyzed by voltammograms on a CS150 electrochemical workstation (Wuhan CorrTest Instrument, China). A Pt plate (2 × 2 cm) was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. For the measurement of O2 reduction, the voltammograms were scanned at 5 mV/s under a stirring speed of 300 rpm. Na2SO4 electrolytes at 0.1 M with oxygen-saturated and free (N2 atmosphere) at pHs 10.0−12.0 were tested. The potential versus SCE are presented in this study.

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RESULTS AND DISCUSSION As(III) Oxidation Around the Cathode in the Columns. In the electrolytic sand column, the solution pH near the anode dropped to about 2.5 and around the cathode rose to about 11.0 when the operation reached a steady state at the current of 30 mA (Figure 1a). H+ produced from the anode (eq 1) decreased the pH near the anode and OH− generated from the cathode (eq 2) increased the pH around the cathode. The distance (10 cm) between the anode and cathode in the column led to the appearance of localized acidity and alkalinity around the anode and cathode, respectively. The electromigration rate of H+ is 1.76 times of OH−. In the top of the column, OH− produced at the cathode was neutralized by the H+ moved from the anode, allowing for a neutral effluent. The variations of localized pH around the anode and cathode are similar to those reported in electrochemical groundwater treatments20,22 and electrokinetic soil remediation.15−17 Due to the variations of solution pH at the different locations, As(III) transforms from nonionic H3AsO3 (pKa1 = 9.22 at 25 °C) near the anode to the charged H2AsO3− around the cathode. As(III) concentration was decreased by 28% when passing through the anode, stabilized afterward and was further decreased by 23% when the solution passed through the cathode (Figure 1b). As(V) concentration increased accordingly. The slight decrease of total As along the column could be ascribed to the adsorption of As(V) at a higher pH condition. Direct oxidation of As(III) by the anodes including MMO has been extensively reported.35,36 However, it is the first time reporting that As(III) can be oxidized by the cathode under alkaline conditions. Although previous investigations on As(III) oxidation were reported in Fenton and electro-Fenton systems, the processes were carried out under acidic or near neutral conditions.37,38 The oxidation of As(III) and variation of pH are further tested at different currents. With increasing the current from 10 to 30 and 50 mA, the oxidation around the anode increased slightly, but around the cathode increased considerably from 7 to 36 and 42% (Figure 1c, Table 1); the solution pH around the anode dropped slightly, but around the cathode rose dramatically from 9.3 to 10.8 and 11.1 (Figure 1c, Table 1). Increase in O2 production and pH with increasing current could be attributable to the increased oxidation around the cathode. Production of O2 increased with the increase in 5691

DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

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Environmental Science & Technology

cathode through its reduction, which might be associated with As(III) oxidation. It is notable that the oxidation increased from 36 to 42% with increasing current from 30 to 50 mA when dissolved O2 concentrations have exceeded the saturated concentration. Then, we can infer that the rise of solution pH around the cathode was also responsible for the dramatic increase in As(III) oxidation. It is reported that As(III) can be oxidized more easily at a higher alkaline pH.33 It is likely that O2 reduction on the cathode increased with the rise of current, and the increase in O2 reduction increased As(III) oxidation under alkaline conditions. Mechanism of As(III) Oxidation around the Cathode. In order to explore the mechanism of As(III) oxidation around the cathode, a divided electrolytic reactor was employed to simulate the reactions. In the cathodic compartment exposed to air simulating the oxidation around the cathode in the column, 94% of 6.67 μM As(III) was oxidized within 60 min at the initial pH of 7.5 and current of 30 mA (Figure 2a). It is noted that the solution pH in the cathodic compartment rose gradually to 11.0 (SI Figure S5). The oxidation was minimal under N2 or without electrolysis at the same conditions, and was also marginal when the solution pH was buffered at pH 7.5 (Figure 2a). As a result, O2, electrolysis and alkaline condition are all necessary for the oxidation. It is therefore reasonable to deduce that reduction of O2 on the cathode under alkaline conditions led to the significant oxidation of As(III) around the cathode. Then, which species produced from O2 reduction is responsible for the oxidation? On the surface of the cathode, O2 is first reduced to O2− followed by further reduction to H2O2 and H2O in basic media.25 Hence, H2O2 (pKa = 11.75 at 25 °C) and O2− can be assumed as the ROSs for As(III) oxidation in the cathodic compartment under alkaline conditions. (a). Contribution of ROSs. The cathodic compartment was electrolyzed for 1 h without As(III) and was then powered off, thus produces and retains the ROSs in the solution. Immediately, 6.67 μM As(III) was added into the solution. As(III) was transformed remarkably (red curve in Figure 2c), approaching the transformation in the cathodic compartment with electrolysis at the same time (red curve in Figure 2a). This validates the predominant contribution of ROSs to As(III) oxidation in the cathodic compartment. (b) Identif ication of H2O2. H2O2 was measured in the cathodic compartment, demonstrating an increase in concentration with time (Figure 2b). It is found that the presence of As(III) considerably decreased the cumulative concentration of H2O2, while the presence of As(V) had minimal influence. However, H2O2 was undetectable in the anodic compartment, and As(III) cannot be oxidized when it was added into the anodic compartment which was powered off after 1 h electrolysis and was adjusted to pH 11.0. Thus, the oxidation caused by H2O2 produced on the anode and transported to the cathode can be ruled out. Therefore, H2O2 produced from O2 reduction on the cathode can be suggested as an ROS for As(III) transformation. (c) Preclusion of O2− Contribution. With addition of 0.1 mM NBT (O2− scavenger) and 6.67 μM As(III) into the cathodic compartment which was power off after 1 h electrolysis, As(III) transformation was marginally affected (Figure 2c), indicating a negligible contribution of O2− to As(III) transformation in the cathodic compartment. This is also consistent with the fact that O2− is only stable in concentrated alkaline solutions or aprotic media.43,44 Then, production of H2O2 from O2 reduction on the cathode was characterized by voltammograms from 0.5 to −1.0 V (vs

Figure 1. Profiles of (a) variations of pH along column under steadystate conditions at different currents, (b) As(III) oxidation in column at the current of 30 mA and (c) As(III) oxidation at different currents. As(III) oxidation reached a steady state after 180 min of operation (1.57 PVs) in the column (SI Figure S3). The operation conditions were based on 6.67 μM initial As(III) concentration, initial pH of 7.5 and 2 mL/min flow rate.

current according to Faradaic Law. The solubility of O2 in water is about 1.3 mM at room temperature.39 Assuming that all the O2 produced at the anode quickly dissolved into the solution, variations of dissolved O2 concentration at different currents are plotted (SI Figure S4). At 10 mA, the dissolved O2 concentration was less than the saturated concentration; whereas at 30 and 50 mA, the dissolved O2 concentrations theoretically exceeded the saturated concentration. Within the operation time, O2 cannot cause a significant oxidation of As(III) in acidic, neutral or even alkaline media.40−42 As a consequence, the increase in O2 production with increasing the current was not the direct reason for the dramatic increase in As(III) oxidation around the cathode. However, the increase in O2 production could generate more reactive species on the 5692

DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

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Environmental Science & Technology

Figure 2. (a) As(III) oxidation in anodic and cathodic compartments of a divided electrolytic system under different conditions; (b) production of H2O2 in the absence and presence of As in the cathodic compartment; (c) effect of NBT on As(III) oxidation by the reactive intermediates produced after 1 h electrolysis of the background solution without As; (d) voltammogram curves of O2 reduction on MMO cathode at different pHs. The scanning conditions were based on 0.1 M Na2SO4 electrolyte saturated with O2, 5 mV/s and 300 rpm. Unless otherwise specified, the oxidation conditions were based on 6.67 μM initial As(III) concentration, initial pH of 7.5 and 30 mA current exposed to air.

complex therein due to the predominant existence of H2CO3. So, minimal influence of HCO3− on As(III) oxidation on the anode was obtained. Nevertheless, As(III) oxidation around the cathode increased from 65 to 72% with the increase in HCO3− concentration from 0 to 2 mM, but decreased to 45% with the further increase to 5 mM (Figure 3a, Table 1). This variation can be attributed to the contrary influence of HCO3− on the variation of solution pH and the formation of CO32−−As(III) complex around the cathode. As a buffer, HCO3− could suppress the rise of solution pH around the cathode. Presence of low concentrations (0.5 and 2 mM) of HCO3− had negligible influence on the variation of solution pH around the cathode (SI Figure S8a), so formation of complex enhanced As(III) oxidation. However, presence of a high concentration (5 mM) inhibited the rise of solution pH around the cathode (SI Figure S8a), decreasing the solution pH from 10.8 (without HCO3−) to 10.1. Because As(III) oxidation is highly dependent on solution pH (>9),33,42 the decrease in solution pH around the cathode suppressed As(III) oxidation. The suppression offset the enhancement by forming the CO32−-As(III) complex, so the oxidation decreased with further increasing HCO3− concentration from 2 to 5 mM. Overall, the enhancement by forming the CO32−-As(III) complex is superior to the inhibition by buffering pH for As(III) oxidation in the tested concentration range of HCO3−. The presence of Cl− considerably promoted the anodic oxidation of As(III) in the column (SI Figure S9). As(III) oxidation reached up to 57 and 95% in the presence of 2 and 4

SCE) at different pHs (Figure 2d). Using MMO as the cathode, the reactions were carried out in oxygen-saturated and free solutions (SI Figure S6). In oxygen-saturated solution, two peaks appeared at −0.36 V and −0.80 V when the potential decreased from 0.5 to −1.0 V. The peak at −0.36 V is assigned to the reduction of O2 to H2O2 via the overall reactions of (3) and (4) mentioned above, and the peak at −0.8 V is ascribed to the reduction of H2O2 to H2O. The peaks became more obvious with the increase in solution pH from 10.0 to 12.0, suggesting that O2 reduction on the cathode increased with the rise of pH. It is also reported that reduction of O2 through oneelectron transfer produced intermediate, O2−, only in strongly alkaline media.25 Therefore, O2 was reduced on the cathode through 2 steps of two-electron transfer process with H2O2 as an intermediate. As a summary, H2O2 is the predominant ROS produced from O2 reduction on the cathode under alkaline conditions contributing to As(III) oxidation. Effects of Solution Compositions on As(III) Oxidation in the Columns. (a). HCO3− and Cl−. HCO3− and Cl− are ubiquitous in soil pore water and groundwater. As shown in Figure 3a, HCO3− at a concentration less than 5 mM showed a minimal influence on the anodic oxidation of As(III) in the columns. It is reported that HCO3− (pKa = 10.25 at 25 °C) accelerated As(III) oxidation by forming CO32−-As(III) complex.45 In this complex, CO32− served as a bridge to transfer electrons from As(III) to H2O2, thereby enhancing the oxidation. As the local pH around the anode decreased to about 2.5 (SI Figure S8a), it is difficult to form CO32−-As(III) 5693

DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

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Environmental Science & Technology

was tested at pH 11.0 in a batch mode. Mg(OH)2 precipitated in the presence of 2 mM Mg2+ at pH 11.0. As(III) oxidation was obviously promoted in the presence of Mg(OH) 2 compared to the absence (Figure 4a). In presence of 2 mM

Figure 4. (a) Influence of different cation precipitates on As(III) oxidation by exogenous H2O2; (b) adsorption of oxidized As(V) on the precipitates. Operation conditions were based on 2 mM CaSO4, MgSO4, 0.5 mg/L H2O2 and constant solution pH at 11.0.

Ca2+ at pH 11.0, there was no visible precipitation. However, As(III) oxidation was also accelerated. Meanwhile, Figure 4b shows that As(V) concentration in the solution phase in the presence of Ca2+ was less than that in the absence, suggesting the removal by precipitation. As a consequence, the enhanced removal of As(III) around the cathode in the columns by the presence of Ca2+ and Mg2+ can be attributed to the enhanced oxidation of As(III) by the formation of Ca and Mg precipitates or clusters followed by the adsorption of As(V). This conclusion is supported by presence of As(V) in the solid phase in the columns (SI Table S1). It is noteworthy that the solution pH around the cathode in the columns decreased to less than 10.0 (i.e., pH 9.3) by the presence of Ca and Mg precipitates (SI Figure S8b). The decrease in solution pH could have suppressed As(III) oxidation around the cathode according to the aforementioned results. However, the significant enhancement of As(III) oxidation by the presence of Ca and Mg precipitates indicates that the enhancement by the precipitates completely outcompeted the suppression by the pH decrease. Therefore, the presence of Ca2+ and Mg2+ in the solution may extend the pH to weakly alkaline conditions, that is, pH 9.0−10.0, for As(III) oxidation by O2 reduction on the cathode. This pH range can be generally encountered in electrocoagulation process.18−20 (c). Humic Acid. HA is ubiquitous and plays an important role in contaminants transformation. It is interesting that As(III) oxidation was also greatly promoted by the addition of HA (SI Figure S7c). Figure 3c shows that the presence of 5, 10,

Figure 3. Effects of (a) bicarbonate, (b) calcium and magnesium cations and (c) HA on As(III) oxidation in the column. As(III) oxidation achieved stability after about 180 min (SI Figure S7). Operation conditions were based on 6.67 μM initial As(III) concentration, initial pH of 7.5 and 2 mL/min flow rate.

mM Cl−, respectively. This promotion can be ascribed to the formation of HClO by the oxidation of Cl− on the anode.36 It is noted that As(III) oxidation was also enhanced around the cathode in the presence of Cl−. With addition of 2 mM Cl−, As(III) oxidation increased to 43% compared to 36% in absence of Cl−. The reduction potential of As(III) decreased at the high pH conditions, in which the residual HClO promoted As(III) oxidation around the cathode. (b). Ca2+ and Mg2+. Ca2+ and Mg2+ are widespread cations in the subsurface environment. Under alkaline conditions, both cations are detrimental for electrochemical remediation by forming precipitates (i.e., Ca(OH)2, CaSO4 and Mg(OH)2) on the cathode. Figure 3b shows that the presence of 2 mM Ca2+, 2 mM Mg2+ or the mixture slightly affected As(III) oxidation in the anodic region, but dramatically promoted the removal of As(III) to 100% around the cathode. The removal could be caused by the adsorption of As(III) on Ca and/or Mg precipitates. However, this possibility was precluded by the negligible adsorption obtained in batch experiments (SI Figure S10). Then, the promotion can be exclusively ascribed to be due to the formation of Ca and/or Mg precipitates. To justify this, the effect of Ca2+ and Mg2+ on As(III) oxidation by H2O2 5694

DOI: 10.1021/acs.est.5b00808 Environ. Sci. Technol. 2015, 49, 5689−5696

Environmental Science & Technology

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and 20 mg/L HA increased the anodic oxidation of As(III) to 44, 57, and 60%, respectively. It appears that HA at a concentration higher than 10 mg/L led to a similar enhancement. Quinone groups in NOM serve as electron shuttles accelerating the electrons transfer from As(III) to the anode.46 Notably, As(III) was further oxidized around the cathode by 48, 40, and 18% in the presence of 5, 10, and 20 mg/L HA. Compared with the absence of HA, As(III) oxidation was promoted with addition of 5 and 10 mg/L HA. Solution pHs around the cathode were almost the same in the presence of 5 and 10 mg/L HA (SI Figure S8c), which ruled out the influence of pH variation on As(III) oxidation. Batch experiments in the divided electrolytic system reveal that As(III) oxidation was promoted with the addition of 10 mg/L HA preoxidized on the anode (SI Figure S11). It is thus reasonable to infer that the oxidized HA has an enhancement effect on As(III) oxidation around the cathode. This enhancement can be translated to the columns, wherein HA was first oxidized on the anode and then passed through the cathode. However, a high concentration of HA (20 mg/L) restrained As(III) oxidation around the cathode. The solution pH decreased from 11.0 to 10.6. At the same time, a high concentration of HA affected the physical properties of the solution. As observed in the experiments, more anodic O2 was retained in the vicinity of the anode with increasing HA concentrations which was adverse to production of H2O2. Moreover, high concentrations of HA may compete with O2 for the electrons on the cathode. As a consequence, less enhancement on As(III) oxidation was observed around the cathode when HA concentration increased from 10 to 20 mg/ L. Implications. This study presents a new mechanism in electrochemical processes for As(III) oxidation. O2 produced from the anode dissolves into the solution and moves to the surface of cathode, wherein it undergoes two-electron reduction to H2O2. The solution pH around the cathode automatically rises to 10.0−11.0. Under the localized alkaline conditions, As(III) dissociates to H2AsO3−, accompanied by a significant decrease in reduction potential. Then, As(III) is oxidized to As(V) by H2O2. The presence of HCO3− significantly enhances the oxidation of As(III) by forming a CO32−-As(III) complex. The presence of Ca2+ and Mg2+ dramatically promotes the oxidation through forming Ca and Mg precipitates. More importantly, As(III) can be oxidized at a lower pH of 9.0−10.0 in the presence of Ca2+ and Mg2+ than in the absence. The presence of HA accelerates the oxidation, but the acceleration is more pronounced at a low concentration (