Final Report - Phase II - Biogeochemistry of Uranium Under Reducing and Re-oxidizing Conditions: An Integrated Laboratory and Field Study

Our understanding of subsurface microbiology is hindered by the inaccessibility of this environment, particularly when the hydrogeologic medium is contaminated with toxic substances. Past research in our labs indicated that the composition of the growth medium (e.g., bicarbonate complexation of U(VI)) and the underlying mineral phase (e.g., hematite) significantly affects the rate and extent of U(VI) reduction and immobilization through a variety of effects. Our research was aimed at elucidating those effects to a much greater extent, while exploring the potential for U(IV) reoxidation and subsequent re-mobilization, which also appears to depend on the mineral phases present in the system. The project reported on here was an extension ($20,575) of the prior (much larger) project. This report is focused only on the work completed during the extension period. Further information on the larger impacts of our research, including 28 publications, can be found in the final report for the following projects: 1) Biogeochemistry of Uranium Under Reducing and Re-oxidizing Conditions: An Integrated Laboratory and Field Study Grant # DE-FG03-01ER63270, and 2) Acceptable Endpoints for Metals and Radionuclides: Quantifying the Stability of Uranium and Lead Immobilized Under Sulfate Reducing Conditions Grant # DE-FG03-98ER62630/A001 In this Phase II project, the toxic effects of uranium(VI) were studied using Desulfovibrio desulfuricans G20 in a medium containing bicarbonate or 1, 4-piperazinediethane sulfonic acid disodium salt monohydrate (PIPES) buffer (each at 30 mM, pH 7). The toxicity of uranium(VI) was dependent on the medium buffer and was observed in terms of longer lag times and in some cases, no measurable growth. The minimum inhibiting concentration (MIC) was 140 M U(VI) in PIPES buffered medium. This is 36 times lower than previously reported for D. desulfuricans. These results suggest that U(VI) toxicity and the detoxification mechanisms of G20 depend greatly on the chemical forms of U(VI) present and the buffer present in a system. Phase II of this project was supported at a cost of $20,575 with most funds expended to support Rajesh Sani salary and benefits. Results have been published in a peer reviewed journal article