In the natural environments, mineral-microbe interactions are pervasive. Not only can active microbial metabolism induce mineral formation and transformation, microorganisms also produce a wide range of metabolic byproducts (e.g. organic acids, organic ligands, and redox active metabolic byproducts) that can alter mineral structure and properties. This talk will use the dissolution of chromium (Cr) containing minerals as an example to demonstrate the importance of considering such processes in the natural environment. Cr is a significant contaminant with environmental and human health hazards. Common oxidation states of Cr are Cr(VI) (highly mobile, toxic, and bioavailable) and Cr(III) (typically insoluble in circumneutral conditions, and a micronutrient). The remediation of Cr employees the reduction of Cr(VI) to Cr(III) and the subsequent precipitation of the insoluble Cr(III)-Fe(III)-(oxy)hydroxides. Yet, the role of microbial activities on the transformation of these remediation products has generally been overlooked. Our research investigated the stability of Cr(III)-Fe(III)-(oxy)hydroxide solid solution series in the presence of common microbial exudates, siderophores, and small organic acids. The presence of microbial exudates can promote the release of Cr(III) from remediation products via both ligand complexation and increased solid solubility, producing dissolved ligand-complexed or free Cr(III) ions that are susceptible to subsequent oxidation to Cr(VI), leading to the occurrence of recontamination. On the other hand, the only environmentally significant oxidant of Cr(III) are Mn oxides, the formation of which also require microbial activities. Our study demonstrated that Cr(III) oxidation by bacterial Mn oxides are affected by the presence of light and organics, and the Mn(II) produced during the redox reaction can be cycled in the system, leading to continuous Cr(III) oxidation even in the presence of small amounts of biogenic Mn oxides. Taken together, our study demonstrated the importance of taking into account microbial activities on the weathering and transformation of Cr-containing phases, and have important implication for both environmental protection and paleoenvironment applications using the Cr isotope system as a proxy.

Microbial metabolite promoted mineral transformation and implications

256th National Meeting (2018)

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