Chromium Mobilization by Microbially-Driven Iron and Manganese Redox Cycling

Abstract:

Chromium, a naturally occurring contaminant, poses a significant threat to California groundwater quality when ultramafic rocks weather leaving Cr-enriched serpentine soils. Benign and of limited solubility, Cr(III) can oxidize into soluble and carcinogenic Cr(VI). Under most environmental conditions, Mn-oxides are the principal oxidant of Cr(III). Here we investigate Cr(III) oxidation by both abiotically synthesized birnessite and biogenically produced Mn-oxides. Further, we explore chromium dynamics within artificial soil aggregates composed of Cr(OH)₃- and Cr_0.25Fe_0.75(OH)₃-coated quartz grains surrounded by aerated solute flow. Abiotic aggregates contained synthetic birnessite, while biotic aggregates were inoculated with Leptothrix cholodnii, a manganese-oxidizing bacterium, and Shewanella putrefaciens, an iron-reducing bacterium. Results show aqueous Cr(VI) concentrations scaling with Cr-mineral solubility. When Leptothrix sp.-inoculated Cr(III),Fe(III)-aggregates are supplied with aqueous Mn(II), Mn-oxides precipitate in the aerobic aggregate. Cr(VI) production occurs similar to that via synthetic birnessite. With the addition of Shewanella sp., coupled biotic and abiotic processes occur causing the reduction, and subsequent immobilization, of chromium by microbial metabolites (e.g., Fe(II)). This study shows the importance of microbial community composition on chromium dynamics within diffusion-limited zones, and suggests the potential for biological immobilization of Cr even in the presence of Mn-oxidizing bacteria.