Sunday, August 1, 2010
Pacific Concourse (Hyatt Regency San Francisco)
Sulfate reducing bacteria (SRB) can reduce soluble U(VI) to insoluble U(IV) with subsequent precipitation of uraninite. It has been shown that SRB-mediated uraninite nanocrystals can be reoxidized by Fe(III)-(hydr)oxides, common constituents of soils, once the electron donor is exhausted. This reoxidation has also been shown to occur under sulfate reducing conditions; however the mechanism of this process remains unresolved. To understand these mechanisms, Desulfovibrio desulfuricans G20 (SRB) was grown in lactate-limited medium containing 2 mM or 180 mM U(VI). After U(VI) reduction, to stimulate the U reoxidation, hematite (15 mmol of Fe(III)/L) or FeCl3 (15 mM) was spiked under anaerobic conditions. Fe or S secondary minerals formed during U reoxidation were analyzed using Mössbauer, XRD, HR-TEM, EDS, and SEAD. Results showed that no goethite peaks were present in the samples, and a fraction (~20%) of the hematite was transformed. All treatments irrespective of the Fe(III) sources or with or without U(VI) gave a similar iron sulfide products. XRD data showed no iron sulfide compounds probably due to amorphous phases of the samples. Mössbauer data also showed that these iron sulfide products were not mackinawite, siderite, or pyrite. HR-TEM, EDS, SEAD showed various forms of iron sulfides, and intracellular and extracellular coexistence of uraninite and iron sulfides. However, Mössbauer data of the sample and its temperature behavior suggested that the precipitate could be greigite. Roles of various iron sulfides in reoxidation are currently being studied. These results would be applicable in the assessment of long term sequestration of U by SRB.