Selenium and chromium stable isotopes and the fate of redox-active contaminants in the environment

Redox active contaminants like selenium and chromium have soluble and mobile oxidized species (selenate/selenite and chromate/bichromate respectively) and less mobile reduced species (elemental Se and Cr(III) respectively). Both Se and Cr are environmental toxins at high concentration and it is important that we are able to reliably monitor these toxins and the biogeochemical processes that control their distribution. Redox reactions cause isotope fractionation and variations in isotope values can be used to monitor redox reactions. I, along with co-authors, develop an innovative method to monitor redox active contaminants using stable isotopes.; In the first part of this study, I measured Se stable isotope fractionation resulting from reduction of Se(VI) and Se(IV) oxyanions by natural microbial consortia in sediment slurry experiments under near-natural conditions, with no substrate added. I conducted experiments with a wide range of initial Se concentrations and with sediment and water from three locations with contrasting environmental settings. The products of Se(VI) and Se(IV) reduction were enriched in the lighter isotopes relative to the reactants; shifts of −2.6 to −3.1‰ and −5.5 to −5.7‰, respectively, were observed in the 80Se/76Se ratio. The results provide estimates of Se isotope fractionation in organic-rich wetland environments and may be used in field studies under similar conditions to estimate extent of reduction.; In the second part of this study, I developed the chromium stable isotope system as a biogeochemical indicator and monitoring tool. After initial work designing the techniques, I measured chromium stable isotope fractionation in laboratory experiments and natural waters. I observed that the products of Cr(VI) reduction had δ53Cr values that were 3.4 ± 0.1‰ less than that of the reactant. The 53Cr/ 52Cr ratios in water samples from three chromium-contaminated sites ranged from 1.1‰ to 5.8‰, relative to the standard and indicate the extent of reduction, a critical process that renders toxic Cr(VI) immobile and less toxic. Finally, I show that Cr stable isotope fractionation during sorption on alumina and goethite in laboratory experiments is insignificant and variations in 53Cr/52Cr values can be used to determine extent of reduction in natural waters.