Environ. Sci. Technol. 2005, 39, 9695-9701
Oxidation Mechanism of As(III) in the UV/TiO2 System: Evidence for a Direct Hole Oxidation Mechanism SUNG-HWAN YOON AND JAI H. LEE* Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-Dong, Buk-Gu, Gwangju 500-712, South Korea
Although it is well-known that As(III) is oxidized to As(V) in the UV/TiO2 system, the main oxidant for that reaction is still not clear. Accordingly, the present study aims at reinvestigating the TiO2-photocatalyzed oxidation mechanism of As(III). We performed a series of As(III) oxidation experiments by using UV-C/H2O2 and UV-A/TiO2, focusing on the effects of competing compounds. The experiment with UV-C/H2O2 indicated that HO2°/O2-° is not an effective oxidant of As(III) in the homogeneous phase. The effects of oxalate, formate, and Cu(II) on the photocatalytic oxidation of As(III) contradicted the controversial hypothesis that HO2°/ O2-° is the main oxidant of As(III) in the UV/TiO2 system. The effect of As(III) on the TiO2-photocatalyzed oxidations of benzoate, terephthalate, and formate was also incompatible with the superoxide-based As(III) oxidation mechanism. Instead, the experimental observations implied that OH° and/or the positive hole are largely responsible for the oxidation of As(III) in the UV/TiO2 system. To determine which species plays a more significant role, the effects of methanol and iodide were tested. Since excess methanol did not retard the oxidation rate of As(III), OH° seems not to be the main oxidant. Therefore, the best rationale regarding the oxidation mechanism of As(III) in the UV/TiO2 system seems to be the direct electron transfer between As(III) and positive holes. Only with this mechanism, it was possible to explain the data of this study. Besides the mechanistic aspect, an application method for this technology was sought. The usage of UV/TiO2 for oxidizing As(III) requires a posttreatment in which both As(V) and TiO2 should be removed from water. For this objective, we applied FeCl3 and AlK(SO4)2 as coagulants, and the result implied that the combined usage of TiO2 and coagulation might be a feasible solution to treat arsenic contamination around the world.
Introduction UV/TiO2 has been broadly studied for water remediation. Regarding its mechanism, electrons (ecb-) and holes (hvb+) are produced under UV light with energy greater than 3.2 eV (reaction 1) (1). A portion of hvb+-ecb- pairs escape the recombination process and migrate to the TiO2 surface, where they are converted to OH° and HO2°/O2-° by reacting with OH- and O2, respectively (reactions 2 and 3). Alternatively, * Corresponding author phone: +82-62-970-2444; fax: +82-62970-2434; e-mail:
[email protected]. 10.1021/es051148r CCC: $30.25 Published on Web 11/11/2005
2005 American Chemical Society
holes and electrons can react directly with substrates near the TiO2 surface (reactions 4 and 5).
TiO2 + hv (λ < 387 nm) f TiO2 (hvb+ + ecb-)
(1)
hvb+ + OH- f OH°
(2)
ecb- + O2 f O2-°
(3)
hvb+ + R f R+ (oxidized)
(4)
ecb- + A f A- (reduced)
(5)
Generally, there exist two oxidation mechanisms in this system. While OH° is generally considered as the major oxidant (2, 3), there are several papers suggesting that the valence band (VB) hole also plays a significant role (2, 4-7). For example, the hole oxidation mechanism has been proposed to explain the oxidations of carboxyl-bearing compounds (2, 4) and iodide (5-7). Whether oxidations occur “indirectly” by OH° or “directly” by hvb+ seems to depend on target substrates. Arsenic contamination is a serious problem, because a tremendous number of people in Southeast Asia are exposed to a groundwater environment with arsenic concentrations above accepted limits (8-10). Due to its high toxicity in humans, the U. S. Environmental Protection Agency (EPA) has recently lowered the maximum contaminant level of arsenic in drinking water from 50 to 10 µg/L (11). Of the two common oxidation states (+III and +V), As(III) (arsenite) is more mobile, more difficult to remove by coagulation/ precipitation, and more acutely toxic than As(V) (arsenate) (12). Accordingly, the preoxidation of As(III) is necessary to increase the removal efficiency of total arsenic (As(III)+As(V)). Many techniques for oxidizing As(III) have been reported: UV/TiO2 (12-17), H2O2 (18), O3 (19), MnO2 (20), UV/H2O2 (13), UV/Fe(III) complexes (21-23), Fe(II)/H2O2 (24), and Fe(VI) (25). Among them, potassium ferrate (K2FeO4:Fe(VI)) has been reported as an excellent chemical for the remediation of As(III)-contaminated water; it not only oxidizes As(III) but also removes As(V) with Fe(III) oxyhydroxides in a subsequent coagulation/precipitation procedure (25). However, K2FeO4 is expensive, because it is commercially unavailable. Inexpensive As(III) oxidation techniques are still necessary, and UV/TiO2 could be a feasible solution. It can oxidize As(III) under sunlight without chemicals. After the preoxidation, however, coagulants (e.g., FeCl3 and AlK(SO4)2) are needed to remove both As(V) and TiO2 (26-29). Several studies have been already conducted to investigate the kinetics and mechanism of As(III) oxidation in the UV/ TiO2 system. The oxidation of As(III) occurs through a transient species (i.e., As(IV)). Because the As(IV) is readily oxidized to As(V) even by oxygen (k(As(IV) + O2) ) 1.1 × 109 M-1 s-1 (30)), it is important to know which species (hvb+, OH°, or HO2°/O2-°) is mainly responsible for the ratedetermining step (As(III) f As(IV)). So far, different opinions have been proposed on this issue. In the early papers regarding this issue, both Rajeshwar et al. (13) and Frimmel et al. (12) presumed that As(III) is oxidized by OH° and/or hvb+. However, they did not investigate which one is more significant for oxidizing As(III). Later, Choi and co-workers (14, 15) performed detailed studies on the same subject and proposed that HO2°/O2-° is the most important oxidant of As(III) in the UV/TiO2 system. However, HO2°/O2-° is a much VOL. 39, NO. 24, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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weaker oxidant than OH° (E(OH°) ) 2.8 VNHE, E(HO2°) ) 1.7 VNHE) (31), and it is established that OH° readily reacts with As(III) (k(As(III) + OH°) ) 9.0 × 109 M-1 s-1 (32)). Thus, it is difficult to understand why HO2°/O2-° preferentially reacts with As(III) rather than OH° and/or hvb+. Furthermore, HO2°/ O2-° has been proposed to be an insignificant oxidant of As(III) in the Fenton system by Hug and Leupin (24). Meanwhile, during the period that this paper was prepared, two more papers (16, 17) were published regarding the TiO2photocatalyzed oxidation of As(III). While Ferguson et al. (16) did not refute the controversial superoxide-based As(III) oxidation mechanism, Sharma et al. (17) have proposed that OH° is the principal oxidant of As(III) in TiO2 photocatalysis. However, it needs to be mentioned that the oxidation of As(III) by UV/TiO2 has different characteristics from those of homogeneous advanced oxidation processes (AOPs). For example, Lee and Choi (14) reported that 0.53 M tert-butyl alcohol (k(t-BuOH + OH°) ) 6.0 × 108 M-1 s-1 (32)) caused a negligible effect on the photooxidation of 500 µM As(III) with 1.5 g/L TiO2. Such a phenomenon would not occur in other free OH°-mediated AOPs. Hence, currently, the mechanism of photocatalytic oxidation of As(III) remains as a controversial issue. This study has been conducted to clarify which species (hvb+, OH°, or HO2°/O2-°) is mainly responsible for the oxidation of As(III) in the UV/TiO2 system. Additionally, FeCl3 and AlK(SO4)2 were tested as coagulants for removing total arsenic and TiO2.
Experimental Section Materials. All chemicals including NaAsO2, Na2HAsO4‚7H2O, and tetranitromethane (TNM) were reagent grade and used as received. Degussa P25 TiO2 (isoelectric point ≈ 6.25 (33)) and ultrapure water (18.2 MΩ cm) were used for all experiments. Experiments. All experiments were performed using a cylindrical reactor (9.5 cm i.d. × 20 cm height). It was open to ambient air through a sampling hole (i.d. ) 2 cm) to attain air-equilibrated conditions. A 4-W UV lamp (Sankyo Denki, Japan, G4T5 (λmax ) 254 nm) or F4T5BL (λmax ) 352 nm)) positioned at the center with a quartz sleeve was used as the light source. The distance between lamp sleeve and reactor wall was 2.95 cm. TiO2 suspensions were dispersed in an ultrasonic cleaning bath (Branson 8510) and magnetically stirred throughout the reaction. HCl and NaOH were used for pH adjustment. The solution volume was initially 1000 mL. By ferrioxalate actinometry (34), the light intensities of UV-C and UV-A were measured as 2.08 × 10-6 and 1.56 × 10-6 einstein L-1 s-1, respectively. Normally, experiments were performed at air-equilibrated conditions. When anoxic or oxygen-saturated conditions were necessary, gas purging (O2 or N2) was applied for at least 20 min before the experiment and continued until the end of the reaction. Samples (
