Chemistry of soil minerals. Part IX. Ion exchange and ion fixation in synthetic fluorhectorites

An investigation and comparison has been made of exchange reactions in synthetic fluorhectorites of exchange capacities 150 and 90 mequiv. per 100 g. The exchanges studied were of Na+ by K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Cu2+, and Cu(NH3)42+. Isotherms for exchanges of Na+ by the alkaline earth metal ions and by Co2+, Ni2+, and Cu2+ were reversible, and were analysed to give thermodynamic equilibrium constants, standard free energies, and activity coefficients of the intracrystalline ions. The exchanges involving K+, Rb+, Cs+, and Cu(NH3)42+ were irreversible and resulted in ion fixation. The fluorhectorites were notable for the strong selectivities shown towards K+, Rb+, and especially Cs+. The latter ion was quantitatively removed up to the exchange capacities of the fluorhectorites. The fixation reactions resulted from preferential solvation of K+, Rb+, and Cs+ by lattice oxygens, so that the exchanged forms were anhydrous and the cations, partially recessed into sheets of oxygens, were immobilised.