Manganese (Mn) oxides are strong oxidants ubiquitous in soils and can oxidize trace metals such as chromium (Cr). In the soil and sediment environments, trivalent chromium [Cr(III)] species are relatively benign, immobile micronutrients, whereas the hexavalent [Cr(IV)] form is present as a highly mobile, toxic chromate oxyanion that presents a contamination risk for drinking and groundwater wells. Although many studies have characterized the capacity of Mn(IV) oxides to oxidize Cr(III) to toxic Cr(VI), the oxidative capacity of the Mn(IV) oxides in the presence of other potentially passivating soil constituents, including reduced soluble iron [Fe(II)], remains unresolved. While the importance of understanding Cr(VI) dynamics is clear, a simultaneous adverse consequence of chromium(III) oxidation by Mn oxides is the solubilization of Mn through reductive dissolution of Mn oxides. Although Mn is an unregulated secondary contaminant in the U.S., a growing research body continues to demonstrate that consumption of Mn contaminated water leads to neurotoxic effects particularly in children. Since the fate of Mn in drinking water is greatly associated with the local biogeochemistry of the soil environment, we have investigated a potential source of Mn in drinking water through oxidation of geogenic metals, such as chromium, at soil redox interfaces and how disproportionate exposure to high Mn concentrations might impact human health in California. In this work we used a multi-chamber reactor to stimulate the diffusion-limited transport at soil redox interfaces to first investigate the capacity of Mn(IV) oxides to oxidize solid Cr(III) minerals to soluble Cr(VI) in the presence of aqueous Fe(II) and the extent of Mn(II) release. We then used this understanding of potential Mn release mechanisms through the oxidation of geogenic metals at soil redox interfaces to provide preliminary insights into the extent of Mn mobilization from California's Central Valley.

Biogeochemical controls of geogenic manganese and chromium release at soil redox interface and potential for groundwater exposure in California’s Central Valley

ACS Spring 2022

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