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Microbial Effects in Promoting the Smectite to Illite Reaction: Role of Organic Matter Intercalated in the Interlayer

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Cysteine and toluene as model organic molecules were intercalated into Fe-rich smectite nontronile, NAu-2. The illitization of these intercalated smectites as induced by microbial reduction of structural Fe3 was investigated. Iron-reducing bacterium Shewanella putrefaciens CN32 was incubated with lactate as the sole electron donor and structural Fe3 in cysteine- and toluene-intercalated NAu-2 referred to as cysteine-NAu-2 and toluene-NAu-2 hereafter as the sole electron acceptor. Anthraquinone- 2, 6-disulfonate AQDS was used as an electron shuttle in bicarbonate buffer. The extent of Fe3 reduction in cysteine-NAu-2 and toluene-NAu-2 was 15.7 and 5.4, respectively, compared to 20.5 in NAu-2 without organic matter intercalation. In the bioreduced NAu-2, X-ray diffraction, and scanning and transmission electron microscopy did not detect any discrete illite, although illite smectite mixed layer or high charge smectite phases were observed. In bioreduced cysteine-NAu-2, discrete illite and siderite formed. In contrast, bioreduction of toluene-NAu-2 did not result in any mineralogical changes. The contrasting bioreduction results between cysteine- and toluene-intercalated nontronite may be ascribed to the nature of organic matter-bacteria interactions. Whereas cysteine is an essential amino acid for bacteria and can also serve as an electron shuttle, thus enhancing the extent of Fe3 bioreduction and illitization, toluene is toxic and inhibits Fe3 reducing activity. This study, therefore, highlights the significant role of organic matter in promoting the smectite to illite reaction under conditions typical of natural environments i.e., non-growth condition for bacteria.

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  • Microbiology

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