10. Sorption Values for Niobium and Tin

Niobium and tin exist in the penta- and tetravalent state, respectively, over the entire redox span of aqueous solutions (>pH 3). Both Sn(IV) and Nb(V) hydrolyse extensively and exist under cementitious conditions as negatively charged hydroxo-complexes. Tin and niobium are contained in various wastes from nuclear power production. Both elements sorb strongly on cementitious materials under all conditions, in accordance with their hydrolysis behaviour and with the affinity of the hydrolytic species to calcium ions. While the significant extent of sorption is well established, the mechanisms of sorption on cement phases are not well understood. Spectroscopic information indicates that the structural environment of Sn sorbed to CSH and hydrated cement paste differs between the two solids, but corresponds to the formation of inner-sphere complexes in both cases. However, it has to be admitted that the available information is not conclusive with regard to (i) identifying the sorption-dominating mineral phase and (ii) the distinction between surface sorption or/and incorporation.