Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species
Photocatalytic Oxidation Mechanism of As(III) on TiO2: Unique Role of As(III) as a Charge Recombinant Species
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DOI：
10.1021/es102507u
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发表时间：
2010-12-01
影响因子：
11.4
通讯作者：
Majima, Tetsuro
中科院分区：
文献类型：
--
作者：
Choi, Wonyong;Yeo, Jiman;Majima, Tetsuro
使用TiO 2光催化剂，砷化物，As（III），可以快速氧化为砷酸盐，As（V），其在水环境中毒性较小且移动的较少。因此，TiO 2/UV工艺可作为砷污染水的有效预处理方法。自2002年首次报道超氧阴离子（或过氧化氢自由基）在TiO 2/UV过程中起As（III）的主要氧化剂的作用以来，关于超氧阴离子、空穴和OH自由基中主要光氧化剂的真实身份一直存在很大争议。的关键问题是集中在为什么更强的OH racicals不能氧化As（III），它已被提出，作为外部电荷复合中心的紫外线激发的TiO 2颗粒上的As（III）的独特作用是负责这个电子中心的机制。尽管该机理已得到了大量实验证据的支持，但人们对它的怀疑仍未完全消除。在这项研究中，我们提供了直接和无可争议的证据来支持的作用，作为（III）在c大复合动力学使用时间分辨瞬态吸收光谱。As（III）的存在确实介导了TiO 2中的电荷重组。在此条件下，OH自由基的作用由于零循环而被抑制，并且较弱的氧化剂，超氧化物，应该占上风。超氧化物的作用以前一直怀疑的基础上，观察到，过量的甲酸d（空穴清除剂，应提高生产的超氧化物）抑制光催化氧化的As（III）。然而，这项研究证明，这是由于还原自由基（HCO 2中心点）的光生作用，使As（V）/As（IV）再循环回As（III）。研究还表明，4-氯苯酚/TiO 2体系在可见光下既不能产生OH自由基，也不能产生价带空穴，通过超氧化物途径将As（III）转化为As（V）。
Using TiO2 photocatalyst, arsenile, As(III), can be rapidly oxidized to arsenate, As(V), which is less toxic and less mobile in the aquatic environment. Therefore, the TiO2/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 TiO2/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 racicals 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 TiO2 particle is responsible for this e;:centric 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 c large recombination dynamics using time-resolved transient absorption spectroscopy. The presence of As(III) indeed mediated the charge recombination in TiO2. 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 ac d (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 (HCO2 center dot) that recycle As(V)/As(IV) back to As(III). It was also demonstrated that the 4-chlorophenol/TiO2 system under visible light that cannot generate neither OH radicals nor valence band holes converted As(III) to As(V) through the superoxide pathway.