Behaviour of uranyl and neptunyl cations during ion exchange between silicate gels pSiO2−(A,B)O−nH2O and aqueous solutions (A2+, B2+) (A2+ = UO22+, NpO22+; B2+ = Mg2+, Ca2+, Ni2+): an experimental study

Ion exchange equilibria between silicate gels p SiO 2 −( A,B )O− n H 2 O and aqueous solutions ( A 2+ , B 2+ ), where A 2+ is UO 2 2+ or NpO 2 2+ and B 2+ is Mg 2+ , Ni 2+ or Ca 2+ , were studied. For the uranyl cation, all the pairs ( A 2+ , B 2+ ) investigated exhibit normal simple ion-exchange behaviour, except the pair (UO 2 2+ −Ca 2+ ). Although normal, this behaviour is not totally in conformity with the thermodynamic cation-hydroxide model, previously proposed for Mg 2+ and the divalent cations of the transition elements of the first series. Several explanations are put forward: structural, chemical and, possibly, thermodynamic. For the pair UO 2 2+ −Ca 2+ , the anomalous behaviour recorded can be explained by the formation of a stoichiometric analogue of uranophane, Ca(UO 2 ) 2 (SiO 3 OH) 2 , which precludes any quantitative comparison with the model. Supplementary distribution data obtained with the pairs Mg 2+ −Ca 2+ and Ni 2+ −Ca 2+ give indirect evidence of an order of affinity of UO 2 2+ for silicate gels: Ni 2+ 2+ 2+ , also in concordance with the one for hydroxides as in the model. The behaviour of the neptunyl cation, which was only investigated in the high dilution range, appears to be similar to that of the uranyl cation. The overall results suggest a selective fixation of both actinide cations by silicate gels, which appear to be materials of interest in the problems of radioactive waste repository contamination.