Ryu Jungho, Choi Wonyong
School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea.
Environ Sci Technol. 2004 May 15;38(10):2928-33. doi: 10.1021/es034725p.
Using TiO2 photocatalyst, arsenite [As(III)] can be rapidly oxidized to arsenate [As(V)], which is less toxic and less mobile in the aquatic environment. Superoxides have been recently proposed as a main photocatalytic oxidant of As(III) whereas OH radicals are dominant oxidants in most TiO2 photocatalytic oxidation (PCO) reactions. This study confirms that superoxides are mainly responsible for the As(III) PCO by investigating PCO kinetics in pure and modified TiO2 systems. The rate of As(III) oxidation drastically increased on Pt-TiO2, which could be ascribed to the enhanced superoxide generation through an efficient interfacial electron transfer from the conduction band (CB) to O2. Since the addition of tert-butyl alcohol (OH radical scavenger) had little effect on the PCO rate in both naked and Pt-TiO2 suspensions, OH radicals do not seem to be involved. The addition of polyoxometalates (POMs) as an electron shuttle between TiO2 CB and 02 highly promoted the PCO rate whereas the POM alone was not effective at all in oxidizing As(III). Fluorinated TiO2 that had a markedly reduced adsorptive capacity for As(III) did not show a reduced PCO rate, which indicates that the direct hole transfer path is not important. The arsenite oxidation proceeded under visible light with a similar rate to the case of Pt-TiO2/UV when dye-sensitized Pt-TiO2 was used. Since only superoxides can be generated as a photooxidant in this visible light system, their role as a main oxidant of As(III) is confirmed. In addition, the PCO rate was significantly reduced in the presence of superoxide dismutase.
使用二氧化钛光催化剂，亚砷酸盐[As(III)]可迅速氧化为毒性较低且在水生环境中迁移性较小的砷酸盐[As(V)]。最近有人提出超氧化物是As(III)的主要光催化氧化剂，而在大多数二氧化钛光催化氧化（PCO）反应中，羟基自由基是主要氧化剂。本研究通过研究纯二氧化钛体系和改性二氧化钛体系中的PCO动力学，证实超氧化物是As(III)光催化氧化的主要原因。As(III)在Pt-TiO₂上的氧化速率急剧增加，这可归因于通过从导带（CB）到O₂的有效界面电子转移增强了超氧化物的生成。由于添加叔丁醇（羟基自由基清除剂）对裸TiO₂和Pt-TiO₂悬浮液中的PCO速率影响很小，因此羟基自由基似乎未参与其中。添加多金属氧酸盐（POMs）作为TiO₂导带和O₂之间的电子穿梭体极大地促进了PCO速率，而单独的POMs在氧化As(III)方面根本无效。对As(III)吸附能力明显降低的氟化TiO₂并未显示出PCO速率降低，这表明直接的空穴转移路径并不重要。当使用染料敏化的Pt-TiO₂时，亚砷酸盐在可见光下的氧化速率与Pt-TiO₂/紫外光的情况相似。由于在该可见光体系中只能产生超氧化物作为光氧化剂，因此证实了它们作为As(III)主要氧化剂的作用。此外，在超氧化物歧化酶存在的情况下，PCO速率显著降低。
