Photochemical Oxidation of Arsenic(III) to Arsenic(V) using

Jul 2, 2008 - The photochemical oxidation of arsenic, As(III), to the less toxic As(V) using peroxydisulfate ions (S2O82−) as the oxidizing agent un...
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Environ. Sci. Technol. 2008, 42, 6179–6184

Photochemical Oxidation of Arsenic(III) to Arsenic(V) using Peroxydisulfate Ions as an Oxidizing Agent BERNAURDSHAW NEPPOLIAN, EVRIM CELIK, AND HEECHUL CHOI* Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-Gu, Gwangju, South Korea 500-712

Received January 17, 2008. Revised manuscript received May 27, 2008. Accepted May 28, 2008.

The photochemical oxidation of arsenic, As(III), to the less toxic As(V) using peroxydisulfate ions (S2O82-) as the oxidizing agent under UV light irradiation was investigated. The photochemical oxidation of As(III) to As(V) assisted using peroxydisulfate ions (KPS) proved to be a simple and efficient method, and the rate of oxidation for As(III) was exceptionally high in accordance with the concentration of KPS. In this study, the UV light intensity was of primary importance for the dissociation of the KPS in generating sulfate anion radicals (SO4-•). Upon intense UV light irradiation, very efficient oxidation was achieved due to the complete decomposition of KPS into SO4-• radicals which favor a higher reaction rate. Subsequent pH variation from 3 to 9 was seen to have no influence on the photolytic cleavage of KPS, and hence, the reaction was unaltered. There was also no significant effect from the continuous purging of oxygen or dissolved oxygen before the reaction as the air-equilibrated condition was found to be sufficient for efficient oxidation. However, the continuous purging of nitrogen substantially reduced the reaction rate (20%), confirming that the dissolved oxygen plays a role in this reaction, although at high concentrations of KPS, this situation was overcome. Humic acid was also found to have no detrimental effect on the reaction rate, even at 20 ppm concentration. The resultant SO42- obtained in this study was, thus, not considered a pollutant. Moreover, there was no need for a sensitizer or other metals with highly alkaline conditions that are normally used in conjunction with KPS. Natural solar light could also effectively oxidize As(III) at room temperature. This simple technique was, thus, considered a cost-effective and safe method for the oxidation of As(III) to As(V).

Introduction Arsenic is one of the most toxic metalloids present on the earth’s crust and can be found in groundwater as a persistent metalloid or in combination with the ores of other metals such as gold, lead, and copper (1, 2). Southeast Asian countries such as India, Bangladesh, and Nepal have been adversely affected by high arsenic levels in the environment (3–6), which has led to the Environmental Protection Agency reporting that the maximum allowable arsenic level in safe drinking * Corresponding author phone: +82-62-970-2441; fax: +82-62970-2434; e-mail: [email protected]. 10.1021/es800180f CCC: $40.75

Published on Web 07/02/2008

 2008 American Chemical Society

water is 10 µg/L (7, 8). Arsenic normally exists in its common oxidation states of As(III) and As(V) (9); among these two states, As(III) is significantly more toxic and mobile than As(V) (10, 11). There have been many studies on the oxidation of As(III) to As(V) by various methods, as reported in previous literature, with recent reports showing the oxidation and removal of As(III) by photocatalysis and the use of zerovalent iron (8, 12–18). Once As(III) is oxidized to As(V), it can be easily removed by common physicochemical treatments. KPS (S2O82-) has been used as a potential oxidant for organic pollutants under various advanced oxidation technologies and also in combination with photocatalysts to enhance reaction rates (19–25). However, the rate of chemical oxidation with KPS is very slow at room temperature (r.t.) (26), whereas KPS can easily be decomposed into SO4-• by photolysis or thermal decomposition (21, 25). SO4-• (2.6 V) has a high oxidizing power to decompose various pollutants, followed by OH• at 2.8 V (19, 20). Nishida and Kimura (26) have reported on the chemical oxidation of arsenic using KPS at high pH conditions of around 0.5 M NaOH. However, chemical oxidation is not considered an economically feasible process due to the highly alkaline conditions that are required. Few researchers have worked on the oxidation of As(III) to As(V) using coupled Fe-KPS in the presence of a tungsten lamp (27, 28), though Woods et al. (27, 28) studied the oxidation of As(III) to As(V) using coupled peroxydisulfate-iron(II) with and without the influence of oxygen. They have explained that the KPS oxidation of As(III) was slow using a tungsten lamp, and thus used KPS coupled with iron, where the reaction rate was much faster. They also found that the reaction rate was dependent on the concentration of perchloric ions. Similarly, Nishida and Kimura (29) studied the oxidation of As(III) to As(V) using KPS under visible light in the presence of the tris (2,2′-bipyridine) ruthenium(II) ion as a sensitizer to enhance the decomposition of KPS. Both Woods et al. (27, 28) and Yamazaki and Kimura (29) used a tungsten lamp as the irradiation source; however, it was considered a rather weak source with insufficient light intensity for inducing an efficient photolysis reaction. The production of SO4-• is, therefore, very slow in the absence of a sensitizer or metals. To the best of our knowledge, there have not been any reports on the photochemical oxidation of As(III) to As(V) with KPS using a high intensity UV light source and without the use of any sensitizers or metals to enhance the oxidation process. Moreover, KPS is cost-efficient, readily available, and its end product, the SO42- anion, is normally inert and nonpolluting, as has been reported by Lau et al. (21). Here, we study and discuss the KPS ion-assisted oxidation of As(III) using high energy UV light radiation with different UV light sources in the absence of sensitizers or metals.

Experimental Section Chemicals and Reagents. Potassium peroxydisulfate (K2S2O8) was obtained from Yakuri Pure Chemicals Co., Japan, humic acid was purchased from Fluka, and ammonium peroxydisulfate [(NH4)2S2O8], sodium peroxydisulfate (Na2S2O8), NaAsO2, benzoic acid, and other chemicals were obtained from the Aldrich Chemical Co.. The chemicals were used without further purification, and the solutions were prepared with 18 MΩ deionized water obtained from a water purification system (Millipore, Synergy). For pH adjustment, 0.1 M HCl was used and all of the experiments were carried out under identical conditions. An As(III) stock solution of 1.35 mM was prepared and diluted to one tenth of the initial concentration. Similarly, the required amount of KPS solution VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effect of the initial KPS concentration on the oxidation of As(III) to As(V). Experimental conditions: [As(III)] ) 0.135 mM, pH 3. (from 1 M solution) was added just before the irradiation experiments. Reactor Set-Up and Analysis Methods. Four different UV lamps were used in this study: Lamp A emitting radiation in the range of 315-400 nm (Philips, TL4W/05, Holland), Lamp B emitting light in the range of 270-300 nm (Sankyo Denki, G4T5E, Japan), Lamp C emitting radiation at a wavelength of 254 nm (Philips, TUV 4W/G4 T5, Holland), and a 300 W Xe Arc lamp (Oriel Lamp, model 66984, U.S.) emitting light in the range of 200-800 nm. A 410 cutoff filter was used to cut the UV light and allow only visible light to go through. The reactor consisted of a double-walled glass reactor and an inside wall made from quartz, with the lamp being mounted inside. Between the reactor walls, 200 mL of the reaction solution was used for the reaction. The distance between the lamp and solution was around 3 cm (Figure S1, Supporting Information). For the Oriel lamp, the reactor setup was made up of quartz having a 200 mL capacity, and was mounted 10 cm away from the reactor (Figure S2, Supporting Information). Prior to UV irradiation, the As(III) solution was stirred for 30 min in an oxygen atmosphere under dark conditions. A magnetic stirrer was used to ensure that the solution was continuously stirred at a constant speed. At regular intervals, around 3 mL of the sample was taken out of the reactor, filtered using 0.20 µm membrane filter paper (Millipore) with a disposable syringe filter unit (Dismac-13, JP) and analyzed using an ICP-MS instrument. Next, the As(III) ion concentration was measured using an anionexchange cartridge (Supelclean LCSAX SPE 3 mL) method to separate the As(III) and As(V); the anion exchange cartridge retained the As(V) while leaving As(III) in the filtrate. The As(III) concentration was then measured using an ICP-MS instrument (Agilent 7500ce, TCP-MS, U.S.). The stability of the SPE cartridge was subsequently tested at different pH values. It was found to retain 100% of the initial As(V) until the solution reached pH 2. When the pH dropped to 1.5, 80% As (V) was retained in the SPE cartridge, but at pH 1 all the arsenic eluted from the cartridge. However, the pH values did not affect As (III) since, irrespective of the initial pH values, 100% As (III) eluted from the cartridge. In this study, even for higher KPS concentrations, the final pH of the solution did not go below 2.43.

Results and Discussion Reaction Mechanism of KPS. The mechanisms for the KPS ion-assisted oxidation of As(III) to As(V) are illustrated in eq 6180

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1–6. The initial step for this photochemical reaction is the photolytic cleavage of KPS into SO4-• via homolysis (19, 21, 25). Once the SO4-• radicals are generated due to photolysis, the As (III) ions immediately start to oxidize in a fast reaction into As (IV) by the donation of an electron to the SO4-• radical (27) as an electron acceptor (25), which then becomes a sulfate anion (SO42-) (21) (eq 2). Further oxidation of As(IV) to As (V) occurs either with another SO4-• radical or with dissolved oxygen, as established by Lee and Choi (18). As(IV) is highly unstable and immediately oxidizes into As(V) through the two reaction mechanisms in eqs 3 and 4. Moreover, SO4-• reacts with water to produce OH radicals (eq 5) which assist the oxidation of As(III) into As(IV), as shown in eq 6. Reaction mechanisms k1

S2O82- + hυor heat 98 2SO4-• k2

As(III) + SO4-• 98 As(IV) + SO42k3

As(IV) + SO4-• 98 As(V) + SO42k4

(1)

(2)

(3)

As(IV) + O2 98 As(V) + O2-•

(4)

SO4-• + H2O f HO• + H+ + SO42-

(5)

k5

As(III) + HO• 98 As(IV) + HO-

(6)

The overall reaction is as follows: As(III) + S2O82- f As(V) + 2SO42-

(7)

Nishida and Kimura (29) have found the ratio of As (III)/ (S2O82-) to be almost 1:1. Thus, it is clear that the S2O82- ion dissociates to give two SO4-• radicals which are sufficient to oxidize one As(III) to As(IV) and, subsequently, to As(V). In order to confirm whether or not the reaction follows a photochemical or chemical oxidation at r.t., As(III) oxidation was performed in the presence of 0.1 mM KPS and without UV light irradiation, where no conversion of As (III) was observed, as shown in Figure 1. The figure clearly shows that the photochemical oxidation of KPS is the main factor in the

FIGURE 2. Effect of different initial concentrations of As(III) on the oxidation of As(III) to As(V). Experimental conditions: [S2O82-] ) 0.1 mM, pH 3. conversion of As(III) to As(V). Photolytic reactions in the absence of KPS were also carried out and, similarly, no conversion of As(III) was observed. In other words, for the photochemical oxidation reaction to occur, both light and KPS are essential. Effect of the Peroxydisulfate Ion Concentration on the Oxidation of As(III) to As(V). An enhancement in the oxidation of As(III) to As(V) in the presence of KPS was investigated. The effects of different amounts of KPS on the conversion efficiency at 0.135 mM As(III) to As(V) was examined by varying the KPS concentration from 0.005 to 1 mM, as shown in Figure 1. It can be seen that when the concentration of KPS increases, As(III) conversion also increases. Within 5 min reaction time, more than 90% conversion of As(III) could be achieved with 1 mM KPS. However, the optimum amount of KPS for this preliminary study was found to be 0.1 mM, which was sufficient in oxidizing 99% As(III) to As(V) in 1 h reaction time. The rate of these reactions was observed at pseudofirst order with respect to the concentration of the peroxydisulfate. When the amount of KPS increased, the formation of SO4-• radicals also increased, leading to the enhanced conversion of As(III) to As(V). Using the optimized value, i.e., 0.1 mM peroxydisulfate, further experiments were carried out to study the influence of other parameters on the oxidation kinetics of As(III) to As(V). Lau et al. (21) reported that (NH4)2S2O8 contributed to a higher rate of degradation of butylated hydroxyanisole, an organic compound used as a comparison to K2S2O8 as a different source for SO4-• radicals. In this study, the influence of different precursors for peroxydisulfate ions, such as (NH4)2S2O8, K2S2O8, and Na2S2O8, on the oxidation of As(III) was investigated; no noticeable difference was observed on the percentage of oxidation using three different peroxydisulfate ions. Therefore, among the three, K2S2O8 was chosen for further research in this work. The oxidation of As(III) was then carried out with natural lake water (obtained from the Yeongsan Reservoir in South Korea) and spiked with 0.135 mM As(III) using 0.1 mM of KPS under identical experimental conditions. As a result, about 82% oxidation of As(III) was obtained within 1 h, whereas 100% oxidation of As(III) was achieved within 10 min using 1 mM of KPS, irrespective of the different contaminants present in the water (see the complex matrices

in Table S1, Supporting Information). The results of this study offer a promising technique for the treatment of wastewater. Effect of pH. The effect of pH on the rate of oxidation from As(III) to As(V) was examined over a pH range of 3-9. It was clearly observed that the oxidation rate was unaltered by the initial pH of the reaction mixture. KPS can, thus, be considered a potential oxidant in such photochemical oxidation reactions. Normally, in an As(III) oxidation reaction with zerovalent iron or TiO2, the initial pH is a key factor in the speedy conversion of As(III) to As(V). To this end, Nishida and Kimura (26) have reported that As(III) oxidation mainly depends on the pH of the reaction using KPS as the chemical oxidant. They reported that the reaction is not followed by a free radical mechanism but rather an electron transfer reaction in which two electrons participate. The reaction was observed to be dependent on the pH and the rate of the reaction was faster above pH 9. Similarly, in photocatalytic reactions using TiO2, the initial rate of As(III) oxidation mainly depends on the pH of the reaction medium. Zhang and Itoh (15) have reported that the oxidation of As(III) is very low at a neutral pH of 7.5 and fast at low and high pH values due to variations in the production of OH• and H+ ions. Here, the reaction rate was mainly influenced by SO4-• and not the pH value. An advantage in KPS ion-mediated oxidation is that the photo-oxidation of As(III) can be carried out irrespective of the initial pH of the mixture. Thus, an initial pH of 3 was fixed for the rest of the experiments, and the final pH was around 2.6 in 1 h. This result is due to the formation of SO42ions by the reduction of SO4-•, further supporting our results that SO4-• is mainly involved in this reaction as an oxidant and, hence, the pH of the final solution dropped to 2.6 (eqs 2 and 3), as reported by Lau et al. (21). Effect of Arsenic Concentration. The effect of the initial concentration of arsenic on the oxidation rate was investigated by varying the initial concentrations of As(III) from 0.07 mM to 0.675 mM using 0.1 mM KPS. Figure 2 shows that the oxidation rate of arsenic decreased with an increase in its initial concentration. The oxidation of As(III) is mainly associated with the number of SO4-• radicals as the main oxidant in this process with some additional contribution from dissolved oxygen and OH radicals. It has been reported that the stoichiometric ratio of arsenic and KPS is around 1:1. The extent of As(III) oxidation is 100% within 30 min for 0.07 mM of the initial VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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As(III) concentration, whereas it takes 1 h for the 0.135 mM As(III) concentration. The reaction mainly depends on the sulfate ion concentration, hence, when the concentration of As(III) increases, the rate of oxidation decreases due to less availability of SO4-•, indicating the reaction occurred at pseudofirst order with respect to the concentration of As(III). When the amount of KPS was increased, the reaction became faster than using 0.1 mM peroxydisulfate. The oxidation rate of As(III) to As(V) can be expressed as follows (based on eqs 1–4 and 6, respectively): -d[As(III)] ⁄ dt ) [k1 + k2 + k3 + k4 + k5] ) K[As(III)] (8) where K denotes the overall rate constant of the oxidation reaction and [As(III)] signifies the oxidation of As(III) to As(V). Effect of Dissolved Oxygen. The oxidation of As(III) to As(V) is greatly enhanced in the presence of oxygen in photocatalytic reactions using TiO2 photocatalysts (18). The intermediate state, As(IV), can be oxidized by dissolved oxygen, forming As(V). Dissolved oxygen is, thus, an important oxidant in the conversion of As(IV) to As(V) (18). In order to understand the reaction mechanism, i.e., whether or not As(IV) is oxidized to As(V) by SO4-• or dissolved oxygen, the reaction was carried out with dissolved oxygen, the continuous purging of oxygen, and the continuous purging of nitrogen. No changes were observed in the conversion of As(III) to As(V) with or without the purging of oxygen before the reaction (Figure S3, Supporting Information). However, when oxygen was continually purged during the reaction, the oxidation rate showed little enhancement (5% higher) than with only dissolved oxygen. These results substantiate the suggestion that there is no need for purging before the reaction or a continuous flow of oxygen as an oxidant for the conversion of As(IV) to As(V), in contrast to photocatalytic reactions in which the purging of oxygen is of greater importance. These investigations revealed that existing (airequilibrated) dissolved oxygen is sufficient for the oxidation of As(IV) to As(V). This is similar to results reported by Dutta et al. (14) for the oxidation of As(III) to As(V) using TiO2. When N2 was purged during oxidation, the reaction rate was reduced to around 20% less than in the presence of dissolved oxygen. These results clearly imply that the conversion was assisted by both SO4-• and the dissolved oxygen, whereas SO4-• was seen to play a key role in the oxidation process. On the other hand, when N2 was purged in a photocatalytic system, the reaction rate abruptly decreased since dissolved oxygen is essential in the initiation of photocatalysis (14). Around 64% As(III) was oxidized into As(V) when N2 was purged in this system (Figure S3, Supporting Information). It was also evident that the dissolved oxygen was required for enhancing the reaction speed. In order to confirm that SO4-• was sufficient to oxidize As (IV) to As(V), the oxidation reaction was performed with a continuous flow of N2 using KPS concentrations of 0.5 mM instead of 0.1 mM. The conversion rate was not affected by N2, indicating that a 20% reduction of As(III) oxidation was prevented in the presence of excess oxidants. The proposed reaction mechanism shows that the SO4-• radicals themselves were sufficient for the complete oxidation of As(III) to As(V). With 0.1 mM peroxydisulfate, the amount of dissolved oxygen was sufficient for oxidation. Effect of Initial Temperature. Generally, KPS is dissociated into SO4-• either through photolysis or thermolytic cleavage at high temperatures (21). Temperature has been a greater influence on the decomposition rate of KPS in the production of more SO4-•. The experiments were carried out at three different temperatures ranging from 20 to 35 °C, and oxidation is shown in Figure 3. The oxidation percentage of As(III) was enhanced with a gradual rise in the temperatures. At r.t., 86% conversion was observed within 30 min, whereas 100% oxidation of As(III) 6182

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FIGURE 3. Effect of initial temperature on the oxidation of As(III) to As(V). Experimental conditions: [S2O82-] ) 0.1 mM, [As(III)] ) 0.135 mM, pH 3.

FIGURE 4. Effect of the different UV/visible light sources on the oxidation of As(III) to As(V). Experimental conditions: [S2O82-] ) 0.1 mM, [As(III)] ) 0.135 mM, pH 3. was achieved within the same time period with an initial solution temperature of 35 °C. This is due to the fast decomposition of KPS at high temperatures, which in turn generates more SO4-• at a much faster rate than at low temperatures. KPS could, thus, be useful as an oxidant at high temperatures, quite beneficial in Southeast Asia where an abundance of As(III) can be found. Effect of Different Light Sources. The influence of different light sources of varying intensities was examined using four different lamps. Lamp C, which emits light at around 254 nm (high energy) showed a profound effect on the oxidation of As(III) to As(V), as shown in Figure 4. This is due to the high rate of homolytic cleavage of KPS into SO4-• upon high energy UV irradiation. Less oxidation of As (III) was observed with Lamp A (λ range ) 315-400 nm) and Lamp B (λ range ) 270-300 nm) due to the low energy of light radiation. Similarly, the Xe Arc lamp (300 W) emits light of all ranges but not in a specific wavelength. The reactor setup for the Xe Arc lamp, placed around 10 cm away, led to As(III) oxidation of almost similar results as that of Lamp B. It was also noted that visible light alone is not useful as an irradiation source since no oxidation occurred with visible light using the Xe Arc lamp (300 W) with a 420 nm cutoff filter. In addition to these lamps, natural solar light was used as an irradiation source and studies were carried out in open atmosphere under identical reaction conditions. As a result, about 50% oxidation of As(III) to As(V) was achieved within 30 min (Figure 4), similar to Lamp B and with much better results than with the Xe Arc lamp; similar results have also been observed by Ferguson and Hering (30) with TiO2. UV light of 5%, which can be found in the solar spectrum, was

FIGURE 5. Effect of benzoic acid (BA) on the oxidation of As(III) to As(V). Experimental conditions: [S2O82-] ) 0.1 mM, [As(III)] ) 0.135 mM, [BA] ) 0.3 mM and 0.9 mM, pH 3. strong enough to decompose the KPS to SO4-•, as observed during the degradation of a textile dye with TiO2 in the presence of KPS using natural solar light (19). Furthermore, Nishida and Kimura (29) reported that the decomposition of KPS to SO4-• depends mainly on the intensity of the light source. They have also reported that, with an increase in the number of lamps, the decomposition of KPS increased, generating more SO4-• and resulting in a more efficient degradation of pollutants. This is due to the high rate of homolytic cleavage of KPS into SO4-• upon high energy UV irradiation, as previously observed by Maurino et al. (25). This method can be employed under natural solar light irradiation at around temperatures of 25-30 °C with 0.135 mM initial arsenic concentration and 0.1 mM KPS concentration, irrespective of the initial pH of the solution, whereas with natural water contaminated with arsenic, a somewhat high concentration of KPS (around 0.5 mM KPS) could effectively oxidize the As(III) to As(V). From this, a useful and simple method in utilizing natural and abundant sunlight for the oxidation of As(III) from drinking water or for groundwater remediation processes can be proposed. Effect of Humic Acid. Humic acids (HA) are macromolecular organic substances and one of most abundant materials on the earth’s crust (18, 31, 32). They are normally composed of alkyl and aromatic compounds with different functional groups such as phenolic hydroxyl or carboxylic and quinine moieties attached, and are a major contaminant that needs to be removed from natural water as they may cause health-related and environmental problems (31). In this study, we investigated their effect on the photochemical oxidation of As(III) to As(V) in the presence of varying concentrations of HA from 5 to 20 ppm (Figure S4, Supporting Information). Initially, some detrimental effects of HA on the oxidation reaction of As(III) to As(V) were observed, however, after 30 min reaction, oxidation reached the same level as without HA. To this end, Giasuddin et al. (31) and Lin et al. (32) have also reported the negative impact of HA on the removal of arsenic and different organic compounds with zerovalent iron and TiO2, respectively. Conversely, Lee and Choi (18) have reported the positive effects of HA on the oxidation of As(III) using TiO2 photocatalysts. HA, which is present in natural water along with naturally occurring arsenic, did not affect the oxidation rate of As(III) using KPS, another advantage of using this method. Effect of OH Radical Scavenger. Benzoic acid (BA) is commonly used as an OH radical scavenger for photocatalytic reactions in which BA is employed as a means of determining whether or not a reaction has occurred with OH radicals. In this study, OH radicals were produced during the photolytic

decomposition of KPS into SO4-• which react with water to produce OH radicals (eq 6). In order to determine whether OH radicals are involved in the As(III) oxidation in addition to SO4-•, as reported in eq 6, we carried out As(III) oxidation with 0.3 mM BA and As(III) in which a 15% reduction of As(III) oxidation was observed, as shown in Figure 5. When the concentration of BA was increased to 0.9 mM, the same 15% reduction of As(III) was obtained, as similarly reported by Dutta et al. (14) with TiO2. They reported that the rate of As(III) oxidation to As(V) is reduced in the presence of BA, however, the same trend is noted even at high concentrations of BA. These results show direct evidence that the reduction percentage of As(III) oxidation in the presence of BA is mainly due to the contribution of OH radicals. When the concentration of KPS increased to 0.5 mM instead of 0.1 mM, As(III) oxidation was not affected by BA and complete oxidation of As(III) occurred due to the higher amounts of SO4-•. Meanwhile, the BA oxidation rate also increased from 15 to 80% with an increase in the KPS ion concentration. This increase is in direct proportion to the amount of KPS, which led to the generation of more OH radicals. This study revealed that, to a certain extent, OH radicals also contribute to the oxidation of As (III).

Acknowledgments This research was supported by a grant (M1050000012805J000012810) from the Korea Ministry of Science and Technology, through the National Research Laboratory Program and partially supported by the Korea Science and Engineering Foundation (KOSEF).

Supporting Information Available Additional details are shown in four Figures and one Table. This material is available free of charge via the Internet at http://pubs.acs.org.

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