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, equivalent toS(S)OEu(2+), equivalent toS(S)OEuOH(+) and equivalent toS(S)OEu(OH)((3) over bar), 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, (equivalent toS(S)O)(2)Eu+ and (equivalent toS(S)O)(2)Eu(OH)((2) over bar), and cation exchange. To model the sorption isotherm measurements up to pH = 7.2, only one additional weak site surface complex was required, equivalent toS(W1)OEu(2+) for the monodentate case and (equivalent toS(W1)O)(2)Eu+ 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. Copyright (C) 2002 Elsevier Science Ltd.