Stanford University

Reactions controlling the migration and toxicity of elements are critical in determining surface and groundwater quality. Coupled with the physical structure, solute transport, and mineral-shaped chemistry, microorganisms have a prominent control on the toxicity and mobility of chromium and other trace contaminants. Here, we study Cr and Mn cycling within artificial soil aggregates in order to understand how specific microbial communities, along with chemical reactions, shape the release of Cr(VI) in groundwater. To assess physical constraints of Cr(VI) genesis by Mn-oxides, the primary oxidant of Cr(III) under environmental conditions, artificial soil aggregates of mixed Fe(III),Cr(III)-hydroxide coated quartz grains were placed in flow-through reactor cells. Synthetic birnessite was mixed into abiotic controls, while the Mn-oxides in biotic aggregates were formed in situ by bacterial Mn(II)-oxidation of Leptothrix cholodnii. Cr(VI) generation in the presence of Mn-oxidizing bacteria occurrs similar to that of synthetic birnessite; however, the addition of Shewanella putrefaciens, a dissimilatory iron- and manganese-reducing bacterium, significantly suppresses Cr(VI) production in the presence of both synthetic birnessite and biogenic Mn-oxides. This study provides insight into Cr cycling within highly structured environments and illustrates the potential for microbially-driven reductive immobilization of chromium even in shallow, aerated sediments.