Photocatalytic Oxidation of Arsenite in TiO 2 Suspension: Kinetics and Mechanisms HYUNJOO LEE AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea Arsenite [As(III)] and arsenate [As(V)] are highly toxic aquatic contaminants. Since arsenite is more mobile in natural waters and less efficiently removed in adsorption/ coagulation processes than arsenate, the oxidation of arsenite to arsenate is desirable in water treatment. We performed the photocatalytic oxidation of arsenite in aqueous TiO 2 suspension and investigated the effects of pH, dissolved oxygen, humic acid (HA), and ferric ions on the kinetics and mechanisms of arsenite oxidation. Arsenite oxidation in UV-illuminated TiO 2 suspension was highly efficient in the presence of dissolved oxygen. Homogeneous photooxidation of arsenite in the absence of TiO 2 was negligibly slow. Since the addition of excess tert -butyl alcohol (OH radical scavenger) did not reduce the rate of arsenite oxidation, the OH radicals should not be responsible for As(III) oxidation. The addition of HA increased both arsenite oxidation and H 2 O 2 production at pH 3 under illumination, which could be ascribed to the enhanced superoxide generation through sensitization. We propose that the superoxide is the main oxidant of arsenite in the TiO 2 /UV process. The addition of ferric ions also significantly enhanced the arsenite photooxidation. In this case, the addition of tert -butyl alcohol reduced the arsenite oxidation rate, which implied that the OH radical- mediated oxidation path was operative in the presence of ferric ions. Since both Fe 3+ and HA that were often found with the arsenic in groundwater were beneficial to the photocatalytic oxidation of arsenite, the TiO 2 /UV process could be a viable pretreatment method. This can be as simple as exposing the arsenic-polluted water in a TiO 2 -coated trough to sunlight. Introduction Recent health threats by arsenic contamination of ground- water in Bangladesh and northern Vietnam have raised much concern. It is estimated that 85 million people are at risk of developing cancer due to chronic arsenic poisoning in Bangladesh (1). Studies on long-term human exposure reported that arsenic in drinking water was associated with liver, lung, kidney, bladder, and skin cancers (2, 3), which suggested that arsenic could be more dangerous than previously thought. Accordingly, the U.S. Environmental Protection Agency (EPA) has recently proposed to decrease the maximum contaminant level(MCL)forarsenicin drinking water from 50 μg/L to around 2-20 μg/ L (4). Most ofarsenicpollution in naturalwatersoriginatesfrom the oxidative weathering or reductive dissolution of As- containingmineralswith itscommon oxidation state ofAs(III) [arsenite] or As(V) [arsenate] (5). While As(III) species could be prevalent in anoxic groundwater, it is more toxic and mobile than As(V) with low affinity for adsorbents. The oxidation of As(III) species is highly desirable for enhancing the immobilization of arsenic. Therefore, common arsenic removal technologies involve the pretreatment of As(III) oxidation followed by coprecipitation/adsorption of As(V) onto metal oxyhydroxides (6). The tested arsenite oxidants include O2 and/or ozone (7), hydrogen peroxide (8, 9), manganese dioxide (10, 11), UV/ iron (12, 13), and TiO2/UV (14, 15). Since each process has advantages and disadvan- tages, developments of more efficient arsenite oxidation processesand studiesofthe oxidation mechanism are needed to meet more stringent arsenic MCL. TiO2 photocatalysis has been extensively studied as an efficient method ofremediating polluted water and air (16- 20). Its excellent performance in pollutant destruction is mainly ascribed to the strong oxidation potential of the photogenerated valence band (VB) holes in TiO2 (EVB )+2.7 V vs NHE at pH 7). Although most of TiO2 photocatalytic oxidations have been applied to organic substrates, photo- catalytic reactions with inorganic species have been also demonstrated (17).In particular,the photocatalyticoxidation of As(III) to As(V) in UV-irradiated TiO2 suspension has been reported by Rajeshwar and co-workers (14). Arsenite could be effectively oxidized to arsenate at pH 9 in the presence of dissolved oxygen. However, the detailed mechanism of arsenite oxidation and the factors affecting its oxidation remain to be investigated. In this work, we carried out the photocatalytic oxidation of arsenite to arsenate in UV-irradiated TiO2 suspension in order to provide detailed mechanistic understanding.Unlike manyotherTiO2 photocatalytic reactions,where VBholes or hydroxyl radicals are playing the role of major oxidants, superoxides are proposed to be largely responsible for arsenite oxidation from this study. The presence of humic acids or ferric ions that often correlated with the arsenic level in groundwater greatly increased the arsenite photo- oxidation rate.Theireffectson the photocatalyticmechanism and implications for arsenic remediation are discussed. Experimental Section Materials and Chemicals. NaAsO2 (Aldrich) and Na 2HAsO4‚ 7H2O (Kanto) were used as the source of arsenite [As(III)] and arsenate [As(V)], respectively. FeCl3 (Kanto), 2,9-dim- ethyl-1,10-phenanthroline (DMP) (Aldrich), CuSO4‚5H2O (Shinyo), KH2PO4 (Kanto), N ,N -diethyl- p -phenylenediamine (DPD) (Aldrich), bipyridine (Kanto), ethylenediaminetetra- acetic acid (EDTA) (Aldrich), and tert -butylalcohol(Shinyo) were ofreagent grade and used asreceived.Peroxidase (POD) from horseradish (Roche, grade II, lyophilized) was stored at 2-8 °C. The humic acid (HA) was obtained from Aldrich as a sodium salt. HA was dissolved in alkaline solution (pH 11-12)at 200mg/L,then acidified to pH 3with HClsolution, and finally filtered through a 0.45- μm pore-sized Millipore membrane to remove the undissolved fraction of HA. The finalHAstocksolution wasestimated to have a concentration of 100 mg/Land was stored in the dark. The water used was ultrapure (18 MΩ‚cm) and prepared by a Barnstead puri- fication system. Titanium dioxide (Degussa P25), a mixture of 80% anatase and 20% rutile with an average surface area of 50 m 2 /g, was used as photocatalyst without further activation. Photolyses and Analyses. All TiO2 suspensions were prepared at a concentration of1.5g/L,and the initialarsenite *Corresponding author e-mail: wchoi@postech.ac.kr; phone: +82-54-279-2283; fax: +82-54-279-8299. Environ. Sci. Technol. 2002, 36, 3872-3878 3872 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 17, 2002 10.1021/es0158197 CCC: $22.00  2002 American Chemical Society Published on Web 08/03/2002