Determination of isotopic fractionation of hexavalent Cr induced by reduction with sulfide

Abstract:

Reduction of Cr(VI) to Cr(III) results in decreased Cr solubility as well as lower toxicity and mobility. The light isotope (⁵²Cr) reacts at a greater rate compared to the heavy isotope, leading to enrichment of the heavy isotope (⁵³Cr) in the remaining Cr(VI) pool. Thus, Cr isotope variations can be used as an indicator of the extent of Cr(VI) reduction, to monitor the natural attenuation of Cr contamination in groundwater or to explore the environmental redox conditions of ancient earth systems. Cr(VI) reduction can be driven by various electron donors. However, even though sulfide is a common reductant in the natural environment, the Cr(VI) isotopic fractionation induced by reaction with sulfide is poorly understood. Isotopic fractionation associated with this reaction can help us better understand the geochemical redox processes in the oceans and expand the prospects of using Cr as a paleo redox proxy. Moreover, it can enhance our quantification of the Cr and sulfide reactions happening in both vadose zones (in-situ sulfide gas injection) and saturated zones (in the presence of sulfate-reducing bacteria and/or decaying organic matter).

Here, we report Cr isotope fractionation factors for Cr(VI) reduction by sulfide under circumneutral pH conditions in homogeneous batch experiments. The experiments are carried out using methods similar to those of Kim et al¹with excess hydrogen sulfide. Anaerobic conditions were maintained throughout the experiments. Samples were taken out periodically and the reaction was stopped by sparging the sampled solutions with nitrogen gas. Preliminary results indicate that α is 0.9979 for pH 7.45. Moreover, it appears that fractionation factors for Cr reduction by sulfide within circumneutral pH range are not sensitive to changes of pH.

1. Kim C, Zhou Q, Deng B, Thornton EC, Xu H. Chromium(VI) reduction by hydrogen sulfide in aqueous media: Stoichiometry and kinetics. Environ Sci Technol. 2001;35(11):2219-2225. doi:10.1021/es0017007