SORPTION OF EU ON NA- AND CA-MONTMORILLONITES: EXPERIMENTAL INVESTIGATIONS AND MODELLING WITH CATION EXCHANGE AND SURFACE COMPLEXATION

Abstract

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, =SSOEu2+, =SSOEuOH+ and =SSOEu(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, (=SSO)2Eu+ and (=SSO)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, =SW1OEu2+ for the monodentate case and (=SW1O)2Eu+ for the bidentate case. Selectivity coefficients are given for Eu3+- Ca2+ and Eu3+- Na+ exchange on the planar sites and surface complexation constants for monodentate and bidentate Eu surface species on the edge sites of montmorillonite.