Ca-montmorillonite, 2SPNE/CE model; also sorption isotherm (1E-3 - 1E-9 M Eu3+, pH6 and 1E-3 - 1E-10 M, pH6.9, resp.); additional modeling: same data are well described by considering bidentate surface complexes (=XO)2Eu+ and (=XO)2Eu(OH)2-, and cation exchange; bidentate complex; selectivity coefficient for cation exchange Ca-Eu (I=0)=13