Potential for Reoxidation of Iron-Chromium Precipitates by Manganese Oxide
Reductive immobilization of hexavalent chromium (Cr(VI)), often forming iron-chromium (Fe-Cr) precipitates, is a frequent remediation alternative, yet the relationship between the conditions of precipitate formation, the structural and chemical properties of the precipitates, and the rate and extent of precipitate oxidation by Mn oxides is needed. This study provided a systematic investigation of the rates of Cr(VI) reduction by both abiotic minerals and a chromium-reducing bacterium, the properties of the resulting Fe-Cr precipitates, and the susceptibility for reoxidation and remobilization of Cr(VI) upon precipitate exposure to the manganese oxide birnessite.
The properties of the resulting Fe-Cr solids and their behavior upon exposure to birnessite differed significantly. In microcosms where Cr(VI) was reduced by Desulfovibrio vulgaris strain RCH1, and where hematite or Al-goethite were present as iron sources, there was significant initial loss of Cr(VI) in a pattern consistent with adsorption, and significant Cr(VI) was found in the resulting solids. The solid formed when Cr(VI) was reduced by FeS contained a high proportion of Cr(III) and was poorly crystalline. Reaction between birnessite and the abiotically formed Cr(III) solids led to production of significant dissolved Cr(VI) compared to the no-birnessite controls. This pattern was not observed in the solids generated by microbial Cr(VI) reduction, and could be due to re-reduction of any Cr(VI) generated upon oxidation by birnessite via active bacteria or microbial enzymes.
The results of this study suggest that Fe-Cr precipitates formed in groundwater remediation may remain stable only in the presence of active anaerobic microbial reduction. If exposed to environmentally common Mn oxides such as birnessite in the absence of microbial activity, there is the potential for rapid (re)formation of dissolved Cr(VI) above regulatory levels.
University of Oklahoma
Funding for this work was provided by the Subsurface Biogeochemical Research Program (grant DE-SC0006902) of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science. We thank Dr Romy Chakraborty at Lawrence Berkeley National Lab for providing the culture of D. vulgaris strain RCH1, Jingling Hu and Preston Larson at the University of Oklahoma for help with laboratory and SEM measurements, respectively, and Yuanzhi Tang at Harvard University for preparing the Al-goethite. Part of this research was conducted at the Stanford Synchrotron Radiation Lightsource. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank the anonymous reviewers for careful review and insightful comments.
Butler, E. C., Chen, L., Hansel, C. M., Krumholz, L. R., Elwood Madden, A. S., and Lan, Y. “Biological versus mineralogical chromium reduction: Potential for reoxidation by manganese oxide.” Environmental Science: Processes & Impacts 17, 1930–40 (2015). [DOI:10.1039/C5EM00286A].