Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria

The rapid kinetics of microbial U （Ⅵ） reduction and low solubility of uraninite make this process a promising strategy for removing uranium from groundwater and preventing its further migration. Nevertheless, some environmental factors that can influence U（Ⅵ） bioreduction, such as pH, the concentration of coexistent anions（sulfate, nitrate） and toxicity of heavy metal cationsCu（ Ⅱ ）, Zn（ Ⅱ ）, etc], are not well defined. In this paper, anaerobic batch experiments were performed to evaluate their effects on the enzymatic reduction of U （Ⅵ） by mixed cultures of sulfate-reducing bacteria （SRB）. Kinetics investigations under variable pH conditions demonstrated that U （Ⅵ） was mainly reduced during the first 48h. The yield of this bacterial reduction depended strongly on the pH and increased from 4.3% to 99.4% when the pH was raised from 2.0 to 6.0. No inhibition of U （Ⅵ） bioreduction occurred and the formation Of uraninite was concurrent with the precipitation of metal sulfide （ZnS/CuS） at an initial concentration of 20 mg/L Zn （ Ⅱ ） or 10 mg/L Cu （ Ⅱ ）. But addition of 25 mg/L of Zn （ Ⅱ ） or 15 mg/L of Cu （ Ⅱ ） to the bacterial medium stopped U （Ⅵ） reduction due to their toxicities to SRB. Assessment tests for heavy metal toxicity implied that Cu （ Ⅱ ） toxicity probably proceeds by a mechanism different from that of Zn （ Ⅱ ） toxicity. The Zn （ Ⅱ ）-induced inhibition of microbial activity can be eliminated, but Cu （ Ⅱ ） toxicity can generate permanent and irreversible damage to SRB. The sulfate concentration as high as 4000 mg/L did not appreciably interfere with U （Ⅵ） reduction, however, the anion level greater than 5000 mg/L significantly slowed the rate of U （Ⅵ） reduction. Moreover, it was found that U （Ⅵ） was not reduced by H2S produced during dissimilatory sulfate reduction.

Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria