Sorption of Eu on Na- and Ca-montmorillonites: experimental investigations and modelling with cation exchange and surface complexation
The 2-site protolysis no electrostatics surface complexation and cation exchange (2SPNE/CE) model used in previous work to model the sorption of Ni and Zn on Na- and Ca-montmorillonites was applied to sorption edges and isotherms measured for Eu on these two montmorillonite forms. The aim was to further test the applicability of the sorption model on a trivalent element with a more complex aqueous chemistry. An additional reason for choosing Eu was that it is considered to be a good chemical analogue for other lanthanides and trivalent actinides. With site types, site capacities, and protolysis constants fixed at the values in the Ni/Zn studies, all of the measured sorption edge data could be modelled using cation exchange and the monodentate surface species, ≡S SOEu 2+, ≡S SOEuOH + and ≡S SOEu(OH) 3-, on the strong site type. However, an additional modelling study showed that the same data were almost equally well described by considering bidentate surface complexes, (≡S SO) 2Eu + and (≡S SO) 2Eu(OH) 2-, and cation exchange. To model the sorption isotherm measurements up to pH = 7.2, only one additional weak site surface complex was required, ≡S W1OEu 2+ for the monodentate case and (≡S W1O) 2Eu + for the bidentate case. Selectivity coefficients are given for Eu 3+- Ca 2+ and Eu 3+- Na + exchange on the planar sites and surface complexation constants for monodentate and bidentate Eu surface species on the edge sites of montmorillonite.