ENVIRONMENTAL FATE OF URANIUM
The potential for uranium transport in the subsurface is decreased in anaerobic conditions through the formation of sparingly soluble UO2(s), a process often facilitated by dissimilatory metal and sulfate reducing bacteria. Additionally, uranium may adsorb on a variety of mineral surfaces decreasing its solution concentration. Within my dissertation research, I explored the profound impact of uranyl speciation, and specifically the presence of dissolved calcium on uranium redox cycling. I demonstrated a novel remediation strategy of sequestering uranium via incorporation into iron oxide minerals (Stewart et. al, 2009) and demonstrated this in a natural setting at a redox interface in Rifle, CO (Stewart et. al, 2015).
SELENIUM IN MINE WASTE
Selenium can be extremely toxic, causing gastrointestinal and endocrine damage to both humans and animals at higher concentrations. During mining operations, Se is often oxidized, increasing its mobility in the environment, as a byproduct of mining for other metals; however, native microbial populations have the metabolic capacity to re-reduce Se back to a more stable form when oxygen concentrations decrease to micro-aerophilic levels. I measured soil respiration rates in mine waste rock samples to quantify how rapidly microbial populations in oxidized waste rock materials consume oxygen. We combined these observations with X-ray spectroscopy to identify different solid phase products from the selenium-bearing mine waste materials to understand native microbial populations’ influence on selenium biogeochemistry in these settings.
CHROMIUM REMOVAL FROM INDUSTRIAL STORMWATER
Treatment processes for metals in industrial stormwater can be cost prohibitive. Therefore, passive remediation strategies using natural materials and microbial processes have the potential to provide affordable solutions for removing metals from aqueous waste streams, which is important from regulatory and environmental quality perspectives. My current research explores key chemical, biological, and physical processes promoting removal of hexavalent chromium from an industrial stormwater treatment system by association with granular organic peat media.
A Multi-Modal Approach to Unpacking Iron Biogeochemical Processes in Buoyant Hydrothermal Plumes. Stewart, B.D., J.V. Sorensen, K. Wendt, J.B. Sylvan, C. R. German, K. Anantharaman, G.J. Dick, J.A Breier, and B. M. Toner. Chemical Geology. Submitted.
Large Nickel Isotope Fractionation Caused by Surface Complexation Reactions with Hexagonal Birnessite. Sorensen, J.V., B. Gueguen, B.D. Stewart, J. Peña, O. Rouxel, and B. M. Toner. Chemical Geology 2020, 537, 119481.
Reactivity of Uranium and Ferrous Iron with Natural Iron Oxyhydroxides. Stewart, B.D., A.C. Cismasu, K.H. Williams, B.M. Peyton, and P.S. Nico. Environmental Science and Technology 2015, 49(17), 10357-10365.