科研之友
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摘要
Reactions with aqueous sulfide are important in determining uranium (U) geochemistry under sulfate reducing conditions. This paper reports on abiotic reduction of U(VI) by aqueous sulfide under a range of experimental conditions using batch reactors. Dissolved U concentration was measured as a function of time to study the effects of chemical variables including pH, U(VI), S(-II), total dissolved carbonate (CARB = H2CO3* + HCO3 + CO32 ), and Ca2+ concentration on the U(VI) reduction rate. Solid phase reaction products were characterized using X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. The chemical variables had impacts on the solid phase U(VI) reaction products as well as the reduction rates by aqueous sulfide. The solid U reaction product at circumneutral pH was identified as uraninite (UO2+x(s)). Under basic pH conditions, whether a precipitate occurred depended on Ca2+ and CARB concentrations. U(VI) reduction was faster under higher S(-II) concentrations but was slowed by increased dissolved Ca2+ or CARB concentration. In the absence of dissolved CARB and Ca2+, a rapid decrease in dissolved U concentration occurred at circumneutral pH, while virtually no decrease was observed at pH 10.7 within the experimental timeframe of two days. The U(VI) reduction rate was proportional to the total concentration of free uranyl plus its hydrolysis complexes even at minor to trace concentrations. Dissolved Ca2+ and CARB slow abiotic U(VI) reduction by forming stable Ca-U(VI)-carbonato soluble complexes that are resistant to reaction with aqueous sulfide. U(VI) reduction was slow in a synthetic solution representative of groundwater at a uranium mill tailings site. This study illustrates that abiotic U reduction by aqueous sulfide can significantly vary under typical ranges of chemical conditions in groundwater and newly demonstrates the importance of dissolved Ca2+ in the abiotic U(VI) reduction by aqueous sulfide. The results contribute to our understanding of the impact of sulfate reducing conditions on U speciation in groundwater systems undergoing bioreduction conversion of U(VI) to less mobile U(IV) solid phases.
- 出版日期2014-11
