KINETICS OF ION-EXCHANGE ON MONTMORILLONITE CLAYS

The kinetics for the exchange of Li+, K+, Rb+, and Cs+ for Na+ as the exchangeable cation on bentonite and montmorillonite K10 and KSF have been studied using conductimetric stopped-flow. Dilute aqueous suspensions of the clays, of particle sizes of a few micrometers, were used, so that diffusion was fast and the rate-determining step was the substitution of one cation by another on the lattice surface. The kinetics were treated in terms of relaxation from equilibrium. Relaxation times ranged from 100 to 250 ms, and forward rate constants from 30 to 500 M-1 s-1. The reactions had very low activation enthalpies (7-25 kJ mol-1) and were only slow enough to be studied by the stopped-flow technique because of the large negative entropies of activation (-120 to -170 J K-1 mol-1).