The Solubility of Th(IV) and U(IV) Hydrous Oxides in Concentrated Nahco3 and Na2Co3 Solutions

Abstract

AbstractThe solubility of Th(IV) and U(IV) hydrous oxide was studied in the aqueous HCO3−-CO32−-OH−-H2O system extending to high concentration. The solubility of the Th(IV) and U(IV) hydrous oxides increases dramatically in both high bicarbonate and carbonate solutions and decreases with the increase in hydroxide at a fixed carbonate concentration. In general, the observed solubility of Th(IV) hydrous oxide at a given total carbonate concentration was three to four orders of magnitude higher than the solubility of U(IV) hydrous oxide. In the studies of the U(IV) system, extreme caution was used to ensure that the dissolved U was present as U(IV). Oxidation state analyses and systematic trends in the U(IV) solubility data similar to those for Th(IV), which is not redox sensitive, indicated that the dissolved U was present as U(IV).