Thermodynamic characteristics of ion-exchange sorption of Co2+ cations on Na-vermiculite
| Autor: | D. A. Krysenko, V. E. Polyakov, Yu. I. Tarasevich |
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| Rok vydání: | 2014 |
| Předmět: | |
| Zdroj: | Colloid Journal. 76:202-206 |
| ISSN: | 1608-3067 1061-933X |
| Popis: | 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 ΔGm of the mixed form of the mineral is determined by the entropy factor. It is shown that, as θ increases, the ΔGm(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. |
| Databáze: | OpenAIRE |
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