Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, Delaware 19716, USA.
Environ Sci Technol. 2010 Nov 15;44(22):8460-6. doi: 10.1021/es102013p.
Manganese-oxides (Mn-oxides) are quite reactive, with respect to arsenite (As(III)) oxidation. However, studies regarding the pathways of As(III) oxidation, over a range of time scales, by poorly crystalline Mn-oxides, are lacking. In stirred-flow experiments, As(III) oxidation by δ-MnO₂ (a poorly crystalline form of hexagonal birnessite) is initially rapid but slows appreciably after several hours of reaction. Mn(II) is the only reduced product of δ-MnO₂ formed by As(III) oxidation during the initial, most rapid phase of the reaction. There seems to be evidence that the formation of Mn(III) observed in previous studies is a result of conproportionation of Mn(II) sorbed onto Mn(IV) reaction sites rather than from direct reduction of Mn(IV) by As(III).The only evidence of arsenic (As) sorption during As(III) oxidation by δ-MnO₂ is during the first 10 h of reaction, and As sorption is greater when As(V) and Mn(II) occur simultaneously in solution. Our findings indicate that As(III) oxidation by poorly crystalline δ-MnO₂ involves several simultaneous reactions and reinforces the importance of studying reaction mechanisms over time.
锰氧化物（Mn-oxides）对亚砷酸盐（As(III)）的氧化具有相当高的反应活性。然而，对于非晶态 Mn-oxides 在不同时间尺度范围内氧化 As(III)的途径的研究还很缺乏。在搅拌流实验中，δ-MnO₂（六方纤锌矿型的一种非晶态形式）氧化 As(III)的初始速率很快，但在反应数小时后会明显减缓。在反应的初始、最快阶段，As(III)氧化形成的 δ-MnO₂中只有 Mn(II)是唯一的还原产物。似乎有证据表明，先前研究中观察到的 Mn(III)的形成是由于 Mn(II)在 Mn(IV)反应位点上的共沉淀，而不是 As(III)对 Mn(IV)的直接还原。在 δ-MnO₂氧化 As(III)的过程中，只有在反应的最初 10 小时内观察到砷（As）的吸附，并且当 As(V)和 Mn(II)同时存在于溶液中时，As 的吸附更大。我们的发现表明，非晶态 δ-MnO₂氧化 As(III)涉及几个同时发生的反应，并强调了在不同时间尺度上研究反应机制的重要性。
