Thermodynamic characteristics of ion-exchange sorption of Co2+ cations on Na-vermiculite

The exchange of Na⁺ ions of vermiculite for Co²⁺ cations is measured by sorption-analytical and microcalorimetric methods. The previously revealed growth of the equilibrium constant with a rise in the degree of filling θ of vermiculite exchange sites with Co²⁺ cations is confirmed. This result is explained by the segregation of Co²⁺ and Na⁺ cations being exchanged in separate interlayer regions of vermiculite. The negative heats of exchange of Na⁺ cations for Co²⁺ cations are associated with the energy consumption for the rearrangement of the segregated interlayer regions in vermiculite. The positive entropy changes resulting from the exchange indicate the formation of a more disordered Co-Na-layered “cake” in comparison with the initial Na-form vermiculite. A decrease in the integral molal free-energy change ΔG _m of the mixed form of the mineral is determined by the entropy factor. It is shown that, as θ increases, the ΔG _m(θ) dependence passes from a positive to a negative range of values. This pattern of the curve may be interpreted as a gradual phase transition from the Na-form to the mixed Co-Na-segregated form. The first derivatives of the enthalpy and entropy changes with respect to the degree of filling θ exhibit distinct maxima at θ ～ 0.20 as a result of the transition from the ideal mixing of Co²⁺ and Na⁺ cations being exchanged to their segregation.