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        Influences on the Oxidation and Mobilization of Naturally Occurring Chromium: Chris Mills

Project Title: Microbial Influences on the Oxidation and Mobilization of Naturally Occurring Chromium
Mendenhall Fellow: Chris Mills, (303) 236-5529,
Duty Station: Denver, CO
Start Date: February 19, 2007
Education: Ph.D. (applied chemistry) Colorado School of Mines, 2007
Research Advisors: Martin Goldhaber, (303) 236-1521,; Andrea Foster, (650) 329-5437,; Larry Gough, (703) 645-4404,; Michael Pribil, (303) 236-3202,; Martin Schoonen (SUNY Stony Brook), (631) 632-8007,
  Chris Mills

Project Description: Hexavalent chromium, Cr(VI), is a human health hazard due to both its toxicity and mobility (Costa, 1997). Environmental Cr hazards can arise from its use in wood preservatives, anti-corrosive agents, and chrome plating processes. However, Cr can also be naturally elevated in soils derived from the weathering of Cr-bearing ultramafic rocks and serpentinites which cover approximately 1 percent of the global land surface (Oze and others, 2004, 2007). This is the case for soils of northern California where a pilot study of the USGS Geochemical Landscapes project has measured total soil Cr concentrations as high as 5000 mg kg-1. Elevated concentrations of Cr in Sacramento Valley soils appear to be associated with refractory Cr(III)-bearing mineral phases such as chromite (FeCr2O4), which have likely been transported from Cr-enriched metamorphic belts in both the Sierra Nevada and Coast Range mountains. Because the Cr(III) in these mineral phases is relatively immobile, mechanisms by which geologic Cr(III) is oxidized to Cr(VI) are of primary concern for the net release of Cr and for its potential as a human health hazard. USGS data sets show that elevated concentrations of Cr(VI) exceeding 50 μgl-1are recognized in Sacramento Valley groundwaters and Cr(VI)-containing aerosols have been detected in the western Sacramento Valley. These findings suggest that there are oxidative dissolution mechanisms within soils that mobilize refractory Cr and which may pose significant human health-related hazards in northern California and in other serpentine environments.

Because soil microbial communities play an extensive role in the redox cycling of numerous elements, this research will focus on the role of soil microbes in the oxidative mobilization of natural Cr. Although there is no evidence that soil microbes can directly oxidize Cr(III), several different types of heterotrophic soil bacteria and fungi produce manganese oxides (Tebo and others, 2004; Miyata and others, 2004), which can abiotically oxidize Cr(III) (Eary and Rai, 1987; Murray and others, 2005; Wu and others, 2005; Oze and others, 2007). Therefore, it is likely that Cr(III) oxidation is temporally coupled to the soil microbial activity and that soils with enhanced microbial-based Mn cycling may display greater rates of Cr(III) oxidation. Research under this project will target mountainous soils derived from both Sierra Nevada and Coast Range metamorphic rocks as well as valley soils derived from more weathered and transported Cr rich rocks. The results will provide a better understanding of the links between the geochemistry and mineralogy of natural Cr in soils, land use, and Cr related human health hazards.

Research Objectives:

References Cited:

Costa, M., 1997, Toxicity and carcinogenicity of Cr(VI) in animal models and humans: Critical Reviews in Toxicology, v. 27, no. 5, p. 431–442.

Eary, L.E., and Rai, D., 1987, Kinetics of chromium(III) oxidation to chromium(VI) by reaction with manganese dioxide: Environmental Science and Technology, v. 21, p. 1187–1193.

Miyata, N., Tani, Y., Iwahori, K., and Soma, M., 2004, Enzymatic formation of manganese oxides by an Acremonium-like hyphomycete fungus, strain KR21-1: FEMS Microbiology Ecology, v. 47, p. 101–109.

Murray, K.J., Mozafarzadeh, M.L., and Tebo, B.M., 2005, Cr(III) oxidation and Cr toxicity in cultures of the manganese(II)-oxidizing Pseudomonas putida strain GB-1: Geomicrobiology Journal, v. 22, p. 151–159.

Oze, C., Fendorf, S., Bird, D.K., and Coleman, R.G., 2004, Chromium geochemistry in serpentinized ultramafic rocks and serpentine soils from the Franciscan complex of California: American Journal of Science, v. 304, no. 304, p. 67–101.

Oze, C., Bird, D.K., and Fendorf, S., 2007, Genesis of hexavalent chromium from natural sources in soil and groundwater: Proceedings of the National Academy of Sciences, v. 104, p. 6544–6549.

Tebo B.M., Bargar, J.R., Clement, B.G., Dick, G.J., Murray, K.J., Parker, D., Verity, R., and Webb, S.M., 2004, Biogenic manganese oxides: Properties and mechanisms of formation: Annual Review of Earth and Planetary Sciences, v. 32, p. 287–328.

Wu, Y., Deng, B., Xu, H., and Kornishi, H., 2005, Chromium(III) oxidation coupled with microbially mediated Mn(II) oxidation: Geomicrobiology Journal, v. 22, p. 161–170.

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