The solubility of plutonium hydroxide in dilute solution and in high-ionic-strength chloride brines

The solubility of Pu(OH)3 is determined in deionized water and in chloride brines that contained Fe powder to maintain Pu in the Pu(III) oxidation state. The presence of aqueous Pu(III) was verified experimentally. The solubility of Pu(OH)3 is several orders of magnitude greater in brines than in deionized water at the same hydrogen ion concentration. The logarithm of the equilibrium constant, extrapolated to zero ionic strength from experiments in deionized water, for the reaction Pu(OH)3↔Pu3++3 OH- is determined to be -26.2±0.8. The data for solubility in brines are interpreted in terms of the ion-interaction model of Pitzer and coworkers for the excess solution free energy. These solubility data can be explained to within experimental error by a relatively simple ion-interaction model that includes only binary interactions between Pu3+ and Cl-. Ion pairs or ternary interaction parameters for Pu3+ were not required in order to predict the solubility of Pu(OH)3 in natural chloride-dominated brines at -log[H+] values of <9, where brackets represent concentration. (orig.)