Arsenic Redox Changes by Microbially and Chemically Formed Semiquinone Radicals and Hydroquinones in a Humic Substance Model Quinone
Arsenic Redox Changes by Microbially and Chemically Formed Semiquinone Radicals and Hydroquinones in a Humic Substance Model Quinone
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DOI：
10.1021/es803112a
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发表时间：
2009-05-15
影响因子：
11.4
通讯作者：
Kappler, Andreas
中科院分区：
文献类型：
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作者：
Jiang, Jie;Bauer, Iris;Kappler, Andreas
砷是一种具有氧化还原活性的类金属，其毒性和流动性很大程度上取决于其氧化状态，其中亚砷酸盐（As(III)）比砷酸盐（As(V)）更具毒性和流动性。腐殖质（HS）也具有氧化还原活性，可能与砷发生反应并改变其氧化还原状态。在这项研究中，我们发现在微生物或化学还原HS模型醌（AQDS, 9,10-蒽醌-2,6-二磺酸）过程中产生的半醌自由基是强氧化剂。它们将亚砷酸盐氧化为砷酸盐，从而降低砷的毒性和迁移率。该反应强烈依赖于pH值，与pH值7（12.6%氧化）和pH值3（0.5%氧化）相比，pH值11时更多的亚砷酸盐（高达67.3%）被氧化。除了As（III）被半醌自由基氧化外，醌还原过程中产生的对苯二酚也在中性和酸性pH值（小于12%）下将As(V)还原为As(III)，而在碱性pH值下则不会。为了了解亚砷酸盐/砷酸盐与还原/氧化HS之间的氧化还原反应，我们量化了还原醌溶液中的自由基含量，并构建了E-h-pH图来解释所观察到的氧化还原反应。本研究结果可用于更好地预测环境中砷的命运，并可能解释缺氧环境中氧化As(V)的发生。
Arsenic is a redox-active metalloid whose toxicity and mobility strongly depends on its oxidation state, with arsenite (As(III)) being more toxic and mobile than arsenate (As(V)). Humic substances (HS) are also redox-active and can potentially react with arsenic and change its redox state. In this study we show that semiquinone radicals produced during microbial or chemical reduction of a HS model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) are strong oxidants. They oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. This reaction depends strongly on pH with more arsenite (up to 67.3%) being oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In order to understand redox reactions between arsenite/arsenate and reduced/oxidized HS, we quantified the radical content in reduced quinone solutions and constructed E-h-pH diagrams that explain the observed redox reactions. The results from this study can be used to better predict the fate of arsenic in the environment and potentially explain the occurrence of oxidized As(V) in anoxic environments.