Charge Density and Na-K-Ca Exchange on Smectites1

I. Shainberg, N. I. Alperovitch and R. Keren
Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel
1 Contribution No. 1707E, 1986 Series, Agriculture Research Organization, The Volcani Center, Bet Dagan, Israel.

Abstract: Ca-K-, Na-K-, and Ca-Na-exchange isotherms on four reference smectites were studied. The electric charge on the four smectites was due predominantly to cationic substitution in the octahedral layer and varied between 90 and 122 meq/g (cmole(+)/kg) clay. The affinity of the clays for K increased with a decrease in the fraction of K in the exchange phase. The selectivity coefficients of the smectites for K increased with an increase in charge density. The effect was similar in the Na-K- and Ca-K-exchange isotherms. Charge densities on the smectites had no effect on the Ca-Na-exchange isotherm. The electrostatic attraction forces between the platelets increased with increase in charge density. The low hydration energy of adsorbed K was apparently not enough to overcome these attraction forces as the charge density on the smectites increased. Equilibrium between the low hydration energy of adsorbed K and the electrostatic attraction energies between the platelets is postulated to explain the results.

Key Words: Cation exchange • Charge density • Selectivity coefficient • Smectite • Swelling

Clays and Clay Minerals; February 1987 v. 35; no. 1; p. 68-73; DOI: 10.1346/CCMN.1987.0350109
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)