Department of Earth Sciences, Dartmouth College , Hanover, New Hampshire 03755, United States.
Environ Sci Technol. 2017 Apr 4;51(7):3733-3741. doi: 10.1021/acs.est.6b06018. Epub 2017 Mar 21.
The specific Eh-pH conditions and mechanism(s) for the reduction of arsenite, As(III), by pyrite are incompletely understood. A fundamental question is what role the pyrite surface plays in the reduction process. We used electrochemical methods to evaluate the reduction of As(III) under controlled redox conditions. As(III) reduction to elemental As(0) occurs on the pyrite surface under suboxic-reducing conditions and is promoted at low pH. Remarkably, As(III) reduction on pyrite occurs at similar potentials to those for reduction on platinum metal, suggesting a similar mechanism and kinetics for these surfaces. The onset for As(III) reduction at pH ≤ 3.5 coincides with the potential for hydrogen electroadsorption on pyrite, E ≈ +0.1 V (versus RHE). Batch reactions show that As(III) is reduced on pyrite at the Eh-pH predicted by the electrochemical study. X-ray photoelectron spectroscopy reveals that, at pH ≤ 3.5, a significant fraction of the surface arsenic (30-60%) has an oxidation state consistent with As(0). Here, we propose a mechanism whereby atomic hydrogen that forms on ferric (hydr)oxide surface layers promotes As(III) reduction at low Eh and pH. Insights provided by this study will have implications for understanding the controls on dissolved As(III) concentrations in suboxic-anoxic environments.
黄铁矿还原亚砷酸盐（As(III)）的具体 Eh-pH 条件和机制尚不完全清楚。一个基本问题是黄铁矿表面在还原过程中扮演什么角色。我们使用电化学方法在受控的氧化还原条件下评估 As(III)的还原。在亚缺氧还原条件下，As(III)在黄铁矿表面还原为元素砷（As(0)），并且在低 pH 值下得到促进。值得注意的是，As(III)在黄铁矿上的还原发生在与铂金属还原相似的电位，表明这些表面具有相似的机制和动力学。在 pH ≤ 3.5 时，As(III)的还原起始电位与黄铁矿上氢电吸附的电位 E ≈ +0.1 V（相对于 RHE）重合。批处理反应表明，在电化学研究预测的 Eh-pH 条件下，As(III)在黄铁矿上被还原。X 射线光电子能谱揭示，在 pH ≤ 3.5 时，表面砷的很大一部分（30-60%）的氧化态与 As(0)一致。在这里，我们提出了一种机制，即形成于铁（氢）氧化物表面层的原子氢在低 Eh 和 pH 值下促进 As(III)的还原。这项研究提供的见解将对理解亚缺氧-缺氧环境中溶解态 As(III)浓度的控制具有重要意义。
