The bioreduction of U(VI) and Pu(IV): Experimental and thermodynamic studies

Abstract

The experimental and thermodynamic bioreduction of U(VI)aq and Pu(IV)am was studied in order to more accurately predict their transport velocities in groundwater and assess the contamination risks to the associated environments. The results obtained in this study emphasize the impact of carbonate-calcium and humic acids at 7.1 and anoxic solutions on the rate and extent of U(VI)aq and Pu(IV)am bioreduction by Shewanella putrefaciens. We found that the bioreduction rate of U(VI)aq became slow in the presence of NaHCO3/CaCl2. The more negative standard redox potentials of the ternary complexes of U(VI)–Ca2+-CO32− accounted for the decreased rate of bioreduction, e.g., Eho(Ca2UO2(CO3)3(aq)/UO2) = −0.6797 V ≪ Eho((UO2)4(OH)7+/UO2(am)) = 0.3862 V. The bioreduction of Pu(IV)am seemed feasible, while humic acids accepted the adequate extracellular electrons secreted by S. putrefaciens, and the redox potential of Eh(HAox/HAred) was lower than Eh(PuO2(am)/Pu3+), e.g., Eh(HAox/HAred) ≦ Eh(PuO2(am)/Pu3+) if humic acids accepted ≧ 7.952 × 10−7 mol of electrons. The standard redox potentials, Eho(PuO2(am)/Pu3+) = 0.9295 V ≫ Eho(Ca2UO2(CO3)3(aq)/UO2) = −0.6797 V, cannot explain the reduction extent of Pu(IV)am (8.9%), which is notably smaller than that of U(VI)aq (74.9%). In fact, the redox potential of Pu(IV)am was distinctly negative under the experimental conditions of trace-level Pu(IV)am (∼2.8 × 10−9 mol/L Pu(IV) if Pu(IV)am was completely dissolved), e.g., Eh(PuO2(am)/Pu3+) = −0.1590 V (α(Pu3+) = 10−10 mol/L, pH = 7.1). Therefore, the chemical factor of Pu3+ activity, leading to a rapid drop in Eh(PuO2(am)/Pu3+) at trace-level Pu(IV)am, was responsible for the relatively small reduction extent of Pu(IV)am.