Geomicrobiology, Center for Applied Geosciences, University of Tuebingen, Germany.
Environ Sci Technol. 2010 Jan 1;44(1):102-8. doi: 10.1021/es901274s.
The redox state and speciation of the metalloid arsenic (As) determine its environmental fate and toxicity. Knowledge about biogeochemical processes influencing arsenic redox state is therefore necessary to understand and predict its environmental behavior. Here we quantified arsenic redox changes by pH-neutral goethite [alpha-Fe(III)OOH] mineral suspensions amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis. Goethite itself did not oxidize As(III) and, in contrast to thermodynamic predictions, Fe(II)-goethite systems did not reduce As(V). However, we observed rapid oxidation of As(III) to As(V) in Fe(II)-goethite systems. Mössbauer spectroscopy showed initial formation of (57)Fe-goethite after (57)Fe(II) addition plus a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer, EXAFS, SEM, XRD, or HR-TEM. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite. In summary this study indicates that in the simultaneous presence of Fe(III) oxyhydroxides and Fe(II), as commonly observed in environments inhabited by iron-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.
砷（As）的氧化还原状态和形态决定了其环境归宿和毒性。因此，了解影响砷氧化还原状态的生物地球化学过程对于理解和预测其环境行为是必要的。在这里，我们使用湿化学和同步辐射 X 射线吸收（XANES）分析，定量研究了 pH 中性针铁矿[α-Fe（III）OOH]矿物悬浮液中添加 Fe（II）时砷的氧化还原变化。针铁矿本身不会氧化 As（III），与热力学预测相反，Fe（II）-针铁矿体系也不会还原 As（V）。然而，我们观察到 Fe（II）-针铁矿体系中 As（III）迅速氧化为 As（V）。穆斯堡尔光谱表明，（57）Fe（II）加入后最初形成了（57）Fe-针铁矿，同时还存在一种迄今为止尚未确定的额外 Fe（II）相。穆斯堡尔光谱、EXAFS、SEM、XRD 或 HR-TEM 均未检测到其他 Fe（III）相。这表明，在 Fe（II）加入并向针铁矿体内部转移电子后，会形成一种活性 Fe（III）中间相，但在新形成的 Fe（III）作为针铁矿结晶之前，这种相就已经存在。总之，这项研究表明，在铁还原微生物栖息的环境中通常存在的 Fe（III）水合氧化物和 Fe（II）同时存在的情况下，As（III）可能会被氧化。这可能解释了还原地下水含水层中存在 As（V）的原因。
