Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy
Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy
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DOI：
10.1016/j.gca.2011.02.009
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发表时间：
2011-05
影响因子：
5
通讯作者：
A. Neumann;S. Petit;T. Hofstetter
中科院分区：
文献类型：
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作者：
A. Neumann;S. Petit;T. Hofstetter
粘土矿物中结构铁的氧化还原过程在生物地球化学循环和污染物在土壤和含水层中的转化动力学中起着重要作用。粘土矿物中Fe(II)/Fe(III)的反应依赖于多种矿物学和环境因素，这使得评价Fe的氧化还原活性具有挑战性。在这里，我们使用中红外(IR)光谱来确定四个蒙脱石中的反应性结构Fe(II)排列，这些蒙脱石的总铁含量、八面体阳离子组成、负过剩电荷的位置以及八面体羟基的构型都不同。此外，我们还研究了导致铁还原和再氧化过程中结构变化可逆性的矿物性质。对于结构铁含量较低(2.8wt%)的蒙脱石-怀俄明蒙脱石(SWY-2)，我们确定了八面体AlFe(II)-OH是唯一的活性Fe(II)物种，而结构铁含量较高(>；12wt%)是在富铁蒙脱石和含铁蒙脱石(SWA-1)以及合成非硬质蒙脱石中形成多个Fe(II)实体(双八面体AlFe(II)-OH、MgFe(II)-OH、Fe(II)Fe(II)-OH和三八面体Fe(II)-OH)的必要条件。根据总体阳离子组成和过量电荷的位置，富铁蒙脱石在还原过程中形成了不同的活性Fe(II)物种，包括合成非硬质石中的四面体Fe(II)基团。在四面体带电的含铁蒙脱石和合成非钙石中发现了还原状态的三八面体Fe(II)结构域，而在Ölberg蒙脱石中没有这些Fe(II)实体，呈现八面体层状电荷。富铁蒙脱石中的Fe(III)还原伴随着强烈的脱羟基和结构重排，但通过二次氧化只有部分可逆。相反，怀俄明州蒙脱石的再次氧化恢复了原来的矿物结构。以硝基芳香族化合物为活性探针的Fe(II)氧化实验被用来将我们的光谱证据与结构Fe(II)在广义动力学模型中的表观反应性联系起来，该模型考虑了具有明显不同反应活性的Fe(II)实体的存在以及Fe(II)重排的动力学。
Redox processes of structural Fe in clay minerals play an important role in biogeochemical cycles and for the dynamics of contaminant transformation in soils and aquifers. Reactions of Fe(II)/Fe(III) in clay minerals depend on a variety of mineralogical and environmental factors, which make the assessment of Fe redox reactivity challenging. Here, we use middle and near infrared (IR) spectroscopy to identify reactive structural Fe(II) arrangements in four smectites that differ in total Fe content, octahedral cationic composition, location of the negative excess charge, and configuration of octahedral hydroxyl groups. Additionally, we investigated the mineral properties responsible for the reversibility of structural alterations during Fe reduction and re-oxidation. For Wyoming montmorillonite (SWy-2), a smectite of low structural Fe content (2.8wt%), we identified octahedral AlFe(II)–OH as the only reactive Fe(II) species, while high structural Fe content (>12wt%) was prerequisite for the formation of multiple Fe(II)-entities (dioctahedral AlFe(II)–OH, MgFe(II)–OH, Fe(II)Fe(II)–OH, and trioctahedral Fe(II)Fe(II)Fe(II)–OH) in iron-rich smectites Ölberg montmorillonite, and ferruginous smectite (SWa-1), as well as in synthetic nontronite. Depending on the overall cationic composition and the location of excess charge, different reactive Fe(II) species formed during Fe reduction in iron-rich smectites, including tetrahedral Fe(II) groups in synthetic nontronite. Trioctahedral Fe(II) domains were found in tetrahedrally charged ferruginous smectite and synthetic nontronite in their reduced state while these Fe(II) entities were absent in Ölberg montmorillonite, which exhibits an octahedral layer charge. Fe(III) reduction in iron-rich smectites was accompanied by intense dehydroxylation and structural rearrangements, which were only partially reversible through re-oxidation. Re-oxidation of Wyoming montmorillonite, in contrast, restored the original mineral structure. Fe(II) oxidation experiments with nitroaromatic compounds as reactive probes were used to link our spectroscopic evidence to the apparent reactivity of structural Fe(II) in a generalized kinetic model, which takes into account the presence of Fe(II) entities of distinctly different reactivity as well as the dynamics of Fe(II) rearrangements.