Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242, United States of America.
Environ Sci Technol. 2011 Jan 15;45(2):540-5. doi: 10.1021/es102560m. Epub 2010 Dec 7.
Interfacial electron transfer has been shown to occur between sorbed Fe(II) and structural Fe(III) in Fe oxides, but it is unknown whether a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we used the isotopic specificity of (57)Fe Mössbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mössbauer spectra of NAu-2 reacted with aqueous (56)Fe(II) (which is invisible to (57)Fe Mössbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mössbauer spectra using aqueous (57)Fe(II) showed that sorbed Fe(II) is oxidized upon sorption to the clay and pXRD patterns indicate that the oxidation product is lepidocrocite. Spectra collected at different temperatures indicate that reduction of structural Fe(III) by sorbed Fe(II) induces electron delocalization in the clay structure. Our results also imply that interpretation of room temperature and 77 K Mössbauer spectra may significantly underestimate the amount of Fe(II) in Fe-bearing clays. These findings provide compelling evidence for abiotic reduction of Fe-bearing clay minerals by sorbed Fe(II), and require us to reframe our conceptual model for interpreting biological reduction of clay minerals, as well as contaminant reduction by reduced clays.
界面电子转移已被证明发生在被吸附的 Fe(II) 和结构 Fe(III) 之间的铁氧化物中，但尚不清楚类似的反应是否发生在被吸附的 Fe(II) 和含 Fe(III)的粘土矿物之间。在这里，我们使用(57)Fe Mössbauer 光谱的同位素特异性来证明含 Fe 蒙脱石粘土矿物 (NAu-2，非绿脱石) 中被吸附的 Fe(II)和结构 Fe(III)之间的电子转移。与水相 (56)Fe(II)（对 (57)Fe Mössbauer 光谱不可见）反应的 NAu-2 的 Mössbauer 光谱显示了被吸附的 Fe(II)还原 NAu-2 的直接证据。使用水相 (57)Fe(II)的 Mössbauer 光谱表明，被吸附的 Fe(II)在吸附到粘土上时被氧化，pXRD 图谱表明氧化产物是纤铁矿。在不同温度下收集的光谱表明，被吸附的 Fe(II)还原结构 Fe(III)导致粘土结构中的电子离域。我们的结果还表明，对室温和 77 K Mössbauer 光谱的解释可能大大低估了含 Fe 粘土中 Fe(II)的含量。这些发现为非生物还原含 Fe 粘土矿物提供了有力证据，需要我们重新构建我们的概念模型，以解释粘土矿物的生物还原以及还原粘土对污染物的还原。
