(330b) Identification of Iron Sulfides During Reoxidation of Uraninite Under Sulfate Reducing Conditions
Subsurface bacteria including sulfate reducing bacteria (SRB) can reduce soluble U(VI) to insoluble U(IV) with subsequent precipitation of uraninite (UO2). However, it has been shown that SRB-mediated uraninite nanocrystals can be reoxidized (and thus remobilized) by Fe(III)-(hydr)oxides, common constituents of soils and sediments, 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 µM U(VI). After U(VI) reduction, to stimulate the U reoxidation, hematite-goethite (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 (~20-25%) of the hematite was transformed. All treatments irrespective of the Fe(III) sources (hematite or FeCl3) or with or without U(VI) gave a similar ?iron sulfide? products. The iron sulfide was very unstable in air (when samples were exposed to air for 1 h - Fe(II) phase was disappeared). XRD data showed no iron sulfide compounds probably due to amorphous phases in nature of the samples or low concentrations of crystalline forms. Mössbauer data also showed that these iron sulfide products were not mackinawite, siderite, or pyrite. In addition to Mössbauer and XRD, HR-TEM, EDS, SEAD showed various forms of iron sulfides (most of the amorphous), 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.