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Sodium-Calcium Exchange on Brazilian Soils: Modeling the Variation of Selectivity Coefficients

ORSTOM, 70 Route d'Aulnay, 93143 Bondy Cedex, France

J. Touma

ORSTOM, B.P. 1386, Dakar, Sénégal

H. R. Gheyi

Univ. Fed. da Paraiba, Dep. de Eng. Agric., Cx. Postal 518, 58100 Campina Grande, PB., Brazil

*Corresponding author.

ABSTRACT

Because routine experimental determination of exchangeable cations is time consuming, models allowing for the prediction of the composition of the adsorbed phase based on that of the solution are of great interest. Thermodynamically based models allow the determination of exchange equilibrium constants from the variation of selectivity coefficients, as defined by Vanselow, Gapon, or Gaines-Thomas, and usually modeled by a direct fit of experimental data. Empirical models, based on the relationship between the sodium adsorption ratio (SAR) and either the exchangeable sodium ratio (ESR) or the exchangeable fraction ratio (EFR) provide an estimate of adsorbed cation ratios. Both thermodynamically based and empirical models were tested on 74 Na-Ca exchange equilibria measurements, carried out on calcic Cambisols representative of the Paraiba region of northeast Brazil. Because of their sensitivity to errors in measurement, questionable selectivity-coefficient values were obtained by a direct fit of experimental data, so models involving these values appeared roughly approximate. Selectivity-coefficient values calculated from empirical relations between SAR and either ESR or EFR were more accurate. Therefore, models of the variation of selectivity coefficients were derived from these relationships. Linear relations enabled consideration of the constancy of the selectivity coefficients across a range of SAR 60, as implicitly assumed by the well-known relationship between SAR and ESR. Parabolic relations exhibited a restricted range of application, whereas power-law relationships appeared very simple and accurate for producing both a direct estimate of the composition of the adsorbed phase and the calculation of the equilibrium exchange constant.

Received for publication October 11, 1989.