title: Oxidation-reduction transformations of chromium in aerobic soils and the role of electron-shuttling quinones.
abstract: Oxidation of Cr(III) and reduction of Cr(VI) can occur simultaneously in aerobic soils, but the mechanisms involved are not well-understood, especially how electron shuttling by redox-active organic acids is involved. A and B soil horizons from three topohydrosequences from the Coastal Plain and Piedmont physiographic provinces of Maryland were chosen to investigate oxidation-reduction transformations of Cr under field moist conditions. Reduction of added Cr(VI) to Cr(III) was observed in all 18 samples, and 11 demonstrated enhanced reduction with added anthraquinone-2,6-disulfonate (AQDS) acting as an electron shuttle in 24 h quick tests under aerobic conditions. Oxidation of Cr(III) to Cr(VI) was observed in 12 samples, with 7 demonstrating diminished oxidation with AQDS added. Cr(VI) was undetectable after 11 d of incubation when lactic acid was added as a reductant for Cr(VI) to the Watchung soil A horizon. This reduction occurred in the presence of AQDS and a high salt background to suppress microbial growth, suggesting abiotic reduction was the dominant pathway. The results of this study demonstrate that in field-moist, aerobic soils, the electron shuttle, AQDS, enhanced Cr(VI) reduction and inhibited Cr(III) oxidation. This suggests redox-active organic C amendments and electron shuttles can be important in enhancing rates and extent of Cr(VI) reduction, while inhibiting Cr(III) oxidation in the in situ remediation of Cr(VI)-contaminated soils.
authors.affiliation: Department of Environmental Science and Technology, University of Maryland, College Park, Maryland 20742, United States. dbrose@umd.edu
authors.forename: Dominic A
authors.initials: DA
authors.lastname: Brose
authors.forename: Bruce R
authors.initials: BR
authors.lastname: James
mesh_terms: D002857:Chromium / Q000032:analysis / Q000737:chemistry*; D004583:Electrons; D007700:Kinetics; D010084:Oxidation-Reduction; D011809:Quinones / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis / Q000737:chemistry*
chemical_list: D011809:Quinones; D012989:Soil Pollutants; D002857:Chromium