by Oldenbourg Wissenschaftsverlag, München Solubility and hydrolysis of tetravalent actinides

Np(IV), and Pu(IV) are critically reviewed and a comprehen-presented. The hydrolysis constants are selected preferentially from experimental studies at actinide trace concentrations, where the interference of colloid formation can be excluded. Unknown formation constants of mononuclear complexes An(OH)n42n are estimated by applying a semi-empirical elec-trostatic model and an empirical correlation with the known constants of other actinide ions. Based on the known and estimated hydrolysis constants, the solubility products of An(OH)4(am) or AnO2 2 imental solubility data available in the literature. The SIT is used for ionic strength corrections. The solubilities of U(IV), Np(IV), and Pu(IV) hydroxides or hydrous oxides can be calculated by accounting only for mononuclear hydrolysis species. The considerably higher solu-bilities of amorphous Th(IV) precipitates at pH,5 include major contributions of polynuclear species. The solubility data in acidic solutions depend strongly on the preparation and crystallinity of the solid phase. In neutral and alkaline solu-tions, where An(OH)4(aq) are the predominant aqueous spe-cies, the solubilities of AnO2(cr) become equal to those of the amorphous solids. This indicates that the crystalline dioxides are covered by amorphous hydroxide layers. 1