Potassium Selectivity of Clays as Affected by the State of Oxidation of Their Crystal Structure Iron

Fawzy M. Kishk* and H. M. El-Sheemy
Department of Soil and Water Science, Faculty of Agriculture, University of Alexandria, Egypt
* Present address: c/o Chemistry Department, Acheson Hall, SUNYAB, Buffalo, New York 14214, U.S.A.

Abstract: The Ca-K exchange isotherms were determined at two temperatures for two highly montmorillonitic iron-rich soil clays in their oxidized and reduced states. The thermodynamic parameters K, ΔG0, ΔH0 and δS0 were calculated for the exchange reactions.

It was found that the formation of K-clay from C-clay in both the oxidized and reduced state was accompanied by negative free energy, enthalpy and entropy changes. The results indicate that K is more strongly bound than Ca by the clay and the Ca-preference shown by the isotherms may be due to entropy changes in solution.

The oxidation of crystal structure iron resulted in an increase in K selectivity of the clay and a decrease in the free energy, enthalpy and entropy changes of the Ca-K exchange reaction. It was concluded that K is more strongly held by the oxidized clay than the reduced one, which is possibly due to a more nearly dioctahedraI character in the oxidized than in the reduced state.

Clays and Clay Minerals; February 1974 v. 22; no. 1; p. 41-47; DOI: 10.1346/CCMN.1974.0220107
© 1974, The Clay Minerals Society
Clay Minerals Society (www.clays.org)