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

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⁺-CO₃^2- accounted for the decreased rate of bioreduction, e.g., [Formula: see text]  = -0.6797 V ≪ [Formula: see text]  = 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 E_h(HA_ox/HA_red) was lower than E_h(PuO₂(am)/Pu³⁺), e.g., E_h(HA_ox/HA_red) ≦ E_h(PuO₂(am)/Pu³⁺) if humic acids accepted ≧ 7.952 × 10^-7 mol of electrons. The standard redox potentials, E_h^o(PuO₂(am)/Pu³⁺) = 0.9295 V ≫ [Formula: see text]  = -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., E_h(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 E_h(PuO₂(am)/Pu³⁺) at trace-level Pu(IV)_am, was responsible for the relatively small reduction extent of Pu(IV)_am.