John Bargar, SSRL
Redox transitions between U(IV) and U(VI) are fundamental steps in geochemical processes with local and global impact, including clean-up of uranium-contaminated aquifers, formation of uranium deposits, extraction of uranium from ore bodies, and use of uranium isotope fractionation as a paleoredox indicator for Earth’s oceans through geologic time. The reactions that control uranium redox transitions occur at the molecular scale in chemically and physically complex mesoscale assemblages comprised of reactive nanoscale minerals, microbial cells, and biofilms, all residing within pores between sediment grains and in grain coatings. Unlocking the reaction pathways that govern uranium behavior requires understanding the behavior of key system components under representative conditions, developing robust model systems, and ultimately, observing the systems as a whole in their native environments, i.e., underground, embedded in living aquifer systems. Our research collaboration has systematically investigated the chemical and physical forms and reactivity of uranium reduced by axenic microbial cultures, in whole sediments, and in an uranium contaminated aquifer at the DOE-BER funded Rifle IFRC field site in Rifle, CO. Synchrotron x-ray absorption spectroscopy and x-ray microprobe studies have played major roles in these investigations because of their ability to observe and interrogate metal and sulfur speciation in complex natural systems. These studies have helped to demonstrate that a previously unrecognized form of U(IV), bound to biological polymers as mononuclear complexes and coordination polymers, plays a major role in low temperature sediments and is likely of global importance to uranium redox biogeochemistry.
