Ryu Jungho, Choi Wonyong
School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea.
Environ Sci Technol. 2006 Nov 15;40(22):7034-9. doi: 10.1021/es0612403.
Previously we have reported that superoxide plays the primary role as oxidant of As(III) in the UV/TiO2 system, however, since then there has been a controversy over the true identity of the major As(III) oxidant. This study aims to establish a comprehensive understanding of the oxidative mechanism which satisfactorily explains all of the observed results during the photocatalytic oxidation (PCO) of As(III). The key step that has masked the true oxidative mechanism is related to the fact that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. This was confirmed by the observation that the photoanodic current obtained with a TiO2 electrode immediately decreased upon spiking with As(III), portraying the superoxide-mediated PCO as the dominant pathway. The degradation of competitive substrates (benzoic acid and formic acid) was delayed until As(III) was fully converted into As(V) since the normal PCO mechanism that is based on the action of adsorbed OH radicals (or holes) is not working as long as As(III) is present on the TiO2 surface. However, the As(III) PCO mechanism is entirely altered when alternative electron acceptors (Ag+, Cu2+, polyoxometalate) are present. When these alternative electron acceptors are more efficient than 02 they are able to intercept the CB electron, impeding the recombination pathway and enabling an anoxic oxidation mechanism in which OH radicals and holes play the role of main As(III) oxidant. In the presence of polyoxometalate or Cu2+, the above-mentioned photoanodic current immediately increases upon spiking As(III), indicating that the PCO mechanism has changed in the presence of more efficient electron acceptors. Comprehensive mechanisms of As(III) PCO and experimental factors that alter the mechanism are discussed.
此前我们曾报道，在紫外光/二氧化钛体系中，超氧化物作为砷（III）的氧化剂起主要作用，然而，自那时起，关于主要砷（III）氧化剂的真实身份就一直存在争议。本研究旨在全面了解氧化机制，该机制能令人满意地解释砷（III）光催化氧化（PCO）过程中所有观察到的结果。掩盖真实氧化机制的关键步骤与以下事实有关：吸附在二氧化钛上的砷（III）充当外部电荷复合中心，在该中心，砷（III）与羟基自由基（或空穴）反应后紧接着发生电子转移，形成无效循环。这一点通过以下观察得到证实：用二氧化钛电极获得的光阳极电流在加入砷（III）后立即下降，表明超氧化物介导的PCO是主要途径。竞争性底物（苯甲酸和甲酸）的降解被延迟，直到砷（III）完全转化为砷（V），因为只要二氧化钛表面存在砷（III），基于吸附的羟基自由基（或空穴）作用的正常PCO机制就不起作用。然而，当存在替代电子受体（银离子、铜离子、多金属氧酸盐）时，砷（III）PCO机制会完全改变。当这些替代电子受体比氧气更有效时，它们能够拦截导带电子，阻碍复合途径，并实现一种缺氧氧化机制，其中羟基自由基和空穴充当主要的砷（III）氧化剂。在存在多金属氧酸盐或铜离子的情况下，加入砷（III）后上述光阳极电流立即增加，表明在存在更有效电子受体的情况下PCO机制发生了变化。本文讨论了砷（III）PCO的综合机制以及改变该机制的实验因素。
