EXCHANGE SELECT IV ITY OF LANTHANIDE IONS IN MONTMORILLONITE

ABSTRACT: The exchange ofCa 2+ and Na + by tanthanide ions (Ln 3+ = Pr 3+, Gd 3+, Er 3+) in montmorillonite was investigated at two different ionic strengths (0-01 and 0.1 mol/kg). Preferential sorption of Ln 3+ was observed and variable selectivity coefficients were found depending upon the lanthanide concentration n the solid, and ionic strength. The highest exchange extent of Ln 3+ always occurred for the system Na+/Ln 3+, but the exchange s lectivities of Ln 3+ were generally higher in the exchange system Ca2+/Ln 3+. Although the relative affinity of montmorillonite for the three lanthanide ions was similar, distinctive b haviour between Pr3+ and the heavier lanthanides, Gd3+ and Er ~+, was noted. The study of Ln 3+ adsorption i trace amounts showed specific adsorption f lanthanides at high concentrations of Na + in the external solution and that the exchange stoichiometries in the interlayer regions were 3: 1 at equilibrium pH = 4. The interaction of lanthanide ions (Ln 3+) with clay minerals has been the subject of considerable study for purposes as varied as application to industrial catalytic processes (Shabtai, 1980), radioactive waste storage (Komarneni & White, 1983), and determination of origin of parent rocks (Steinberg & Courtois, 1976). All these processes involve ion-exchange reactions. Different aspects dealing with the ion-exchange of lanthanide ions i