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.