Interactions between anionic complex species of actinides and negatively charged mineral surfaces

Sorption of actinides onto negatively charged mineral surfaces was investigated under conditions that actinides were predominantly present as anionic complex species. U(VI), Np(V) and Sn(IV) were prepared as 1 mol kg-1 Na-ClO4/OH solutions and equilibrated with γ-Al2O3 at pcH 11-13.6 where these elements form UO2(OH)42-, NpO2(OH)2- and Sn(OH)5- species, respectively. Sorption of Np(IV), Th(IV) and Am(III) on amorphous SiO2 and on γ-Al2O3 was investigated in 0.03-0.3 mol kg-1 NaHCO3/CO3 solutions where these actinides are expected to be stable as anionic carbonate complexes. Distribution coefficients of U(VI), Np(V), Sn(IV), Th(IV) and Am(III) decreased with the increasing pH or with the increasing carbonate concentrations. The monotonous decrease in the distribution coefficients in the investigated pcH range suggests that anionic complex species of actinides were not sorbed on negatively charged mineral surfaces. A proportional relation was found between the experimentally obtained Kd values and calculated mole fraction of electrically neutral species in equilibrated solutions. Sorption occurs under the condition that both actinides and mineral surfaces are negatively charged because minor neutral species are sorbed on mineral surfaces. (orig.)