A thermodynamic model for the solubility of NpO2(am) in the aqueous K+-HCO3--CO32--OH--H2O system

Solubility of NpO2(am) was determined in the aqueous K+HCO3-CO32 -OH -H2O s ystem extending to high concentrations of carbonate, bicarbonate, and mixed carbonate-hydroxide. Several reducing agents (Fe powder, Na2S2O4, NH . NH2 , and NH2OH . HCl) were tested for their effectiveness to maintain neptuniu m in the tetravalent state. Of these reducing agents, Na2S2O4 was found to be the most effective. Even in the presence of Na2S2O4. significant oxidati on of Np(IV) to Np(V) occurred in samples containing relatively low concent rations of carbonate/bicarbonate, relatively high concentrations of hydroxi de, and samples equilibrated for relatively long periods. X-ray absorption spectroscopy (XAS) and solvent extraction were used to identify aqueous spe cies and oxidation states and to help select appropriate data sets for ther modynamic interpretations. The dominant aqueous species in CO: and relative ly concentrated HCO, solutions was found by XAS to be Np(CO3)(5)(6-). Solub ility of NpO2(am) in carbonate and bicarbonate solutions increased dramatic ally with increasing molal concentrations (carbonate >0.1 moles per kg H2O (m) and bicarbonate >0.01m), indicating that carbonate makes strong complex es with Np(IV). The dominant Np(IV)-carbonate species that reasonably descr ibed all of the experimental data were Np(CO,): in low to high concentratio ns of carbonate and hy droxide and in high concentrations of bicarbonate, a nd Np(OH)(2)(CO3)(2)(2-) in low concentrations of bicarbonate. The logarith m of the thermodynamic equilibrium constants for the NpO2(am) dissolution r eactions involving these species [(NpO2(am) + 5 CO32- + 4 H+ reversible arr ow Np(CO3)(5)(6-) + 2 H2O) and (NpO2(am) + 2 HCO3- reversible arrow Np(OH)( 2)(CO3)(2)(2-))] were found to be 34.85 and -4.4, respectively. These value s, when combined with the solubility product of NpO2(am) [log K-Sp = -54.9 [1, and recent unpublished data from Rai et al.]], provided logarithm of th e equilibrium constants of 33.79 and 43.17, respec tively for (Np4+ + 5 CO3 2- reversible arrow Np(CO3)(5)(6-)) and (Np4+ + 2 CO32- 2 OH reversible arr ow Np(OH)(2)(CO3)(2)(2-)).