A thermodynamic model for the solubility of PuO2(am) in the aqueous K+-HCO3--CO32--OH--H2O system

The solubility of PuO2(am) was determined in the aqueous K+-HCO3--CO32--OH- -H2O system extending to high concentrations of carbonate and bicarbonate. X-ray absorption spectroscopy (XAS) and solvent extraction were used to ide ntify species and oxidation states in the aqueous phase. The dominant aqueo us species in relatively concentrated CO32- and HCO3- solutions were determ ined by XAS to be Pu(CO3)(5)(6-). The solubility of PuO2(am) increased dram atically with increasing total carbonate concentrations, indicating that ca rbonate makes strong complexes with Pu(IV). The dominant Pu(IV)-carbonate s pecies that reasonably described all of the experimental data were Pu(CO3)( 5)(6-) in high concentrations of carbonate and bicarbonate and Pu(OH)(2)(CO 3)(2)(2-) in low concentrations of bicarbonate. Data suggest the possible p resence of another species in the low carbonate and high pH region. However , because of uncertainty in Pu(IV) concentrations in this region, no attemp t was made to fit the data in this region. The logarithm of the thermodynam ic equilibrium constants for the PuO2(am) dissolution reactions involving P u(CO3)(5)(6)- and Pu(OH)(2)(CO3)(2)(2-) [(PuO2(am) + 5 CO32- + 4 H+ reversi ble arrow Pu(CO3)(5)(6-) + 2 H2O) and (PuO2(am) + 2 HCO3- reversible arrow Pu(OH)(2)(CO3)(2)(2-))] were found to be 33.32 and -4.78, respectively. The se values, when combined with the solubility product of PuO2(am) (log K-sp = -56.85 [1]), provided logarithm of the equilibrium constants of 34.18 and 44.76, respectively, for (Pu4+ + 5 CO32- reversible arrow Pu(CO3)(5)(6-)) and (Pu4+ + 2 CO32- + 2 OH- reversible arrow Pu(OH)(2)(CO3)(2)(2-).