Collaborative Research: Chromium isotopes as redox indicators- Oxidation and isotopic equilibration experiments

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Chromium in its hexavalent form, Cr(VI), is mobile and toxic in the environment. Reduction of Cr(VI) to the trivalent form, Cr(III), is a very important process as it renders Cr immobile and less toxic. Reduction induces stable isotope fractionation, and the PIs have led an effort to develop 53Cr/52Cr isotope ratio measurements as a new and much needed reduction indicator. This approach is quicker and less complicated than the traditional mass balance approach, and it appears to work as expected. Application of Cr stable isotopes is expanding beyond contaminant geochemistry to studies exploring biogeochemical cycling of Cr and attempting to constrain past and present marine redox reactions and conditions. However, whereas Cr isotope fractionation caused by Cr(VI) reduction has been studied in some detail, fractionation caused by oxidation of Cr(III) to Cr(VI) and by Cr(III)-Cr(VI) exchange reactions is poorly understood and must be explored to enable accurate interpretation of Cr isotope data. Preliminary experiments with MnO2-induced Cr(III) oxidation have revealed that the reaction product is variably enriched (up to 1.1o/oo) in the heavier isotope. This cannot be a simple kinetic isotope effect, which would produce an opposite shift. The observed isotopic fractionation must be the composite effect of a multi-step redox reaction. Considerable variation as a function of pH, solution chemistry, and Mn oxide chemistry is expected, as these variables can affect the relative rates of the reaction steps. This project will explore a range of these variables to develop a systematic understanding of Cr isotopic fractionation during Cr(III) oxidation, and to determine fractionation factors relevant to natural conditions. Previous studies of isotopic exchange between Cr(III) and Cr(VI) indicate that the process is slow, but the two species can coexist in aquifers for many years. It is not clear if the rate is slow enough that the extent of Cr(VI) reduction can be determined from Cr isotope data using Rayleigh distillation models, which assume no exchange. This project includes highly sensitive experiments needed to detect and quantify slow exchange over workable reaction durations (less than several months). This project will measure exchange rates, and also determine the isotopic fractionation factor for Cr(III)-Cr(VI) equilibrium. Development of the Cr isotope approach to detecting reduction will have impacts beyond academia; this new technique is already in demand for Cr(VI) contaminant studies by environmental consultants. Use of Cr isotopes in a wider array of geoscience studies has just begun, but we expect important applications in oceanography, earth history (e.g., the history of earth redox changes), and earth surface process studies (e.g., the role of shale weathering in carbon cycle studies) as the understanding of Cr isotope systematics becomes more complete. The project will synergistically benefit both institutions, as it will bring students from an underrepresented group to the Illinois campus to increase its diversity, while providing those students access to a smoothly functioning mass spectrometry facility with a cadre of students working on the same techniques.