Adsorbed ion activity coefficients in K - Ca exchange on soil fractions derived from volcanic materials

Abstract

The effects of temperature and iron oxide coatings on the adsorbed ion activity coefficients obtained by the Gaines–Thomas treatment for the K–Ca exchange equilibria on clay fractions of 3 Chilean Andisols were studied. The comparison with the values calculated through the Vanselow approach, using conversion equations, was also considered. Adsorbed Ca activity coefficient curves present a similar shape for the Gaines–Thomas and Vanselow approaches; for K, maxima were observed only when the Gaines–Thomas treatment was used. Iron oxide coated samples showed higher adsorbed ion activity coefficients than allophanic samples. The adsorbed K activity coefficients increased when temperature increased from 4°C to 26·5°C, but only a slight effect on Ca ions was observed. In both types of samples, the presence of 3 possible selective sites was detected: K-selective sites, Ca-selective sites, and sites where K–Ca competition occurs.