An Explanation for the Variability in Sr-Ca Exchange Selectivity of Soils, Clays and Humic Acid

Abstract

The reason for the variability among soils in their effect on the Sr-Ca exchange selectivity was investigated in systems where Sr and Ca were the only competing ions and Sr was present in small amounts. The exchange selectivity theory of Eisenman was used to explain the results. The dominant factor governing the relative strengths of adsorption by the clay minerals studied was the relative energy of hydration of the two cations, so that Sr, the more weakly hydrated, was adsorbed more strongly than Ca. However on humic acid, where the carboxyl group was mainly responsible for the exchange adsorption, the field strength of the exchange site was important so that a reversed sequence occurred and Ca was adsorbed more strongly than Sr. Investigations with soils confirmed the results using soil components. As the fraction of Sr on the exchanger phase was reduced, the relative Sr adsorption increased indicating that variable sites were present on all of the materials studied.