Study on Complexation of Ternary $M-AnO_2-CO_3$ Species at Elevated Temperatures (M = Mg, Ca; An = U, Pu)

Abstract

The proper management of radioactive wastes and spent nuclear fuels produced by nuclear power generation is highly required not to pose any environmental impact and any threat to human health beyond the safety standards and regulations. Uranium constitutes a major portion of the spent nuclear fuels and plutonium is the most guardedly handled material regarding national security and non-proliferation of nuclear weapon. Here, the formation constants of $CaUO_2(CO_3)_3 ^{2-}$, $Ca_2U_O2(CO_3)_3(aq)$, and $MgUO_2(CO_3)_3 ^{2-}$, which are considered the most predominant U(VI) species in natural groundwater and seawater, at various temperatures ($10-85$ $^\circ C$) were obtained using time-resolved laser fluorescence spectroscopy (TRLFS), potentiometric titration with $Ca^{2+}$-selective electrode, and UV–Vis absorption spectroscopy. The reaction enthalpy of the ternary complexes was measured by isothermal titration calorimetry (ITC). Additionally, the evidence of the formation of plutonyl(VI) carbonate species, $CaPuO_2(CO_3)_3 ^{2-}$ and $MgPuO_2(CO_3)_3 ^{2-}$, was found for the first time by Vis–NIR absorption spectrometry. Similar to the ternary U(VI) complexes, $CaPuO_2(CO_3)_3 ^{2-}$ may be the most prevailing plutonyl(VI) species in the oxidizing natural waters at neutral and weakly alkaline pH. Based on the chemical thermodynamic data obtained in this work, the approximation models to predict the formation constant at desired temperatures were examined and successfully developed. This work can provide a more realistic understanding and prediction of uranium(VI) and plutonium(VI) migration in the environment relevant to the deep geological repository for radioactive wastes and the contaminant sites by uranium and plutonium.