School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea.
Environ Sci Technol. 2010 Dec 1;44(23):9099-104. doi: 10.1021/es102507u. Epub 2010 Nov 9.
Using TiO(2) photocatalyst, arsenite, As(III), can be rapidly oxidized to arsenate, As(V), which is less toxic and less mobile in the aquatic environment. Therefore, the TiO(2)/UV process can be employed as an efficient pretreatment method for arsenic contaminated water. Since we first reported in 2002 that the superoxide (or hydroperoxyl radical) plays the role of main oxidant of As(III) in the TiO(2)/UV process, there has been much debate over the true identity of the major photooxidant among superoxides, holes, and OH radicals. The key issue is centered on why the much stronger OH radicals cannot oxidize As(III), and it has been proposed that the unique role of As(III) as an external charge recombination center on the UV-excited TiO(2) particle is responsible for this eccentric mechanism. Although the proposed mechanism has been supported by many experimental evidences, doubts on it were not clearly removed. In this study, we provided direct and undisputed evidence to support the role of As(III) in the charge recombination dynamics using time-resolved transient absorption spectroscopy. The presence of As(III) indeed mediated the charge recombination in TiO(2). Under this condition, the role of the OH radical is suppressed because of the null cycle, and the weaker oxidant, superoxide, should prevail. The role of the superoxide has been previously doubted on the basis of the observation that the addition of excess formic acid (hole scavenger that should enhance the production of superoxides) inhibited the photocatalytic oxidation of As(III). However, this study proved that this was due to the photogeneration of reducing radicals (HCO(2)·) that recycle As(V)/As(IV) back to As(III). It was also demonstrated that the 4-chlorophenol/TiO(2) system under visible light that cannot generate neither OH radicals nor valence band holes converted As(III) to As(V) through the superoxide pathway.
利用 TiO(2) 光催化剂，亚砷酸盐（As(III)）可以迅速被氧化为砷酸盐（As(V)），后者在水生态环境中的毒性和迁移性较低。因此，TiO(2)/UV 工艺可以作为一种有效的含砷水预处理方法。自 2002 年我们首次报道超氧自由基（或过氧自由基）在 TiO(2)/UV 工艺中作为 As(III)的主要氧化剂以来，关于超氧自由基、空穴和 OH 自由基中哪种物质是主要光氧化剂，一直存在很多争议。关键问题集中在为什么强得多的 OH 自由基不能氧化 As(III)，有人提出，As(III)作为 UV 激发 TiO(2)颗粒上的外部电荷复合中心的独特作用是造成这种偏心机制的原因。尽管所提出的机制得到了许多实验证据的支持，但对其仍存在疑问。在这项研究中，我们使用时间分辨瞬态吸收光谱法直接提供了无可争议的证据，证明了 As(III)在电荷复合动力学中的作用。As(III)的存在确实介导了 TiO(2)中的电荷复合。在这种情况下，由于空循环的存在，OH 自由基的作用被抑制，较弱的氧化剂超氧自由基应占主导地位。超氧自由基的作用先前曾受到质疑，其依据是观察到添加过量甲酸（应该会增加超氧自由基生成）会抑制 As(III)的光催化氧化。然而，本研究证明，这是由于还原自由基（HCO(2)·）的光生成，该自由基将 As(V)/As(IV)循环回 As(III)。还证明，在可见光下不能产生 OH 自由基和价带空穴的 4-氯苯酚/TiO(2)体系通过超氧自由基途径将 As(III)转化为 As(V)。
