Solubility, Speciation and Thermodynamics of PuCO₃OH(cr) in Carbonate Containing NaCl Solutions

The solubility, aqueous speciation, and solid phase formation of trivalent plutonium, Pu(III) was investigated for the first time in pH near-neutral 0.10 M NaCl solutions under the presence of carbonate at T = (23 ± 2) °C in the frame of a comprehensive experimental study. Crystalline PuCO₃OH(cr) was synthesized by long-term equilibration of the binary hydroxide Pu(OH)₃(am) in aqueous Na₂CO₃ solutions and comprehensively characterized by VIS/NIR absorption spectroscopy, SEM/EDX, powder XRD and Rietveld analysis, and Pu L3-edge EXAFS. Batch solubility experiments with PuCO₃OH(cr) equilibrated in 0.10 M NaCl-Na₂CO₃ solutions were performed from undersaturation as a function of pH_m and followed over time for up to 90 days. A chemical model was derived and the corresponding thermodynamic constants evaluated. These novel data demonstrate that under specific conditions Pu(III) equilibrium concentrations are up to three orders of magnitude lower when controlled by the PuCO₃OH(cr) compared to the concentration level defined by Pu(OH)₃(am), and reveal a remarkable stabilization of the entire Pu(III) oxidation state in carbonate containing aqueous solutions. The relevance of the new data for geochemical modeling of environmental systems and nuclear waste repositories is discussed.