Uranium immobilization by Cellulomonas sp. ES6

Vaideeswaran Sivaswamy

Removal of uranium (U) from aqueous solution was studied using a Gram-positive facultative anaerobe, Cellulomonas sp. strain ES6, under anaerobic, non-growth conditions in bicarbonate and PIPES buffer. During aerobic growth on tryptic soy broth, cells accumulate excess phosphate, which can be hydrolyzed and released as inorganic phosphate (Pi) under anaerobic starvation conditions. Inorganic phosphate released by the cells precipitated U from the medium as uranyl phosphate. The saturation concentration of phosphate required to initiate U precipitation from solution was dependent on the buffer and the amount of U present in solution. A Monod-based kinetic model was used to describe the Pi release process. Examination of the cultures by high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) showed both extracellular and intracellular U accumulation. The uranyl phosphate precipitates were nanometer sized needle-like fibrils and EDS analysis suggested a 1:1 molar ratio of U and phosphorus in these precipitates. Studies of U immobilization with strain ES6 and anthraquinone-2,6-disulfonate (AQDS), a model humic substance, showed that U reduction is the predominant mechanism and not precipitation by phosphate ligands. X-ray absorption near-edge spectroscopy (XANES) analysis showed that the predominant oxidation state of U precipitates was +4 in bicarbonate buffer, +6 in PIPES buffer and +4 in AQDS treatments. Uranium immobilization by Cellulomonas sp. was previously reported as reduction, however present work suggests that strain ES6 can precipitate U via both precipitation with phosphate ligands and enzymatic reduction, depending on geochemical conditions. In the presence of AQDS, complete reduction of U(VI) to U(IV) by Cellulomonas sp. ES6 was observed. Cellulomonadaceae are environmentally relevant subsurface bacteria and for the first time we report U immobilization by multiple mechanisms using the Gram positive subsurface organism Cellulomonas sp. ES6.

Uranium immobilization by Cellulomonas sp. ES6

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