Thermodynamics of K-Ni and Ca-Ni exchange reactions on kaolinite clay

Exchange equilibria involving nickel ions on potassium and calcium kaolinites were studied using exchange isotherms and derived thermodynamic parameters. The equilibrium was attained within 3 h and maximum adsorption occurred at pH 5.5. The exchange at pH 5.5 and an equilibrium time of 3 h yielded isotherms, equilibrium constants and standard fne energy values indicating a higher preference for nickel ions on the kaolinite surface. A stronger binding of nickel ions on the kaolinite surface was suggested by the enthalpy changes and the entropy changes indicate an increase in order of the system which may be due to changes in hydration states of nickel, potassium and calcium ions. The surface-phase activity coefficients of the cations showed that with increasing nickel saturation, in the Ca-Ni system f_Ni continuously increased while f_Ca decreased; in the K-Ni system f_K first increased and then decreased while f_Ni first decreased and then increasad, which may be attributed to differences in the distribution of ions in the Gouy-Stem layer. The excess thermodynamic functions showed that the exchange reactions were non-ideal: that is the mixture of cations oh the kaolinite surface was more stable and more tightly bound than the homo-ionic forms.