Competitive exchange of K+ , Na+ , and Ca2+ during transport through soil

Abstract

Competitive exchange affects the transport of cations through soils with negatively charged surfaces, and when more than 2 cations are present in the soil solution the effects are complex. There has been little previous experimental work to examine these effects in ternary (or higher order) cation systems. Cation transport experiments involving both binary and ternary cation exchange were therefore carried out. Calcium, which initially saturated both the solution and exchange phases of the soil, was displaced by infiltrating solutions of Na⁺ and Ca²⁺, K⁺ and Ca²⁺, or K⁺ and Na⁺. In all cases, the equivalent fraction of Na⁺ or K⁺ in the inflowing solution was 0·5. The spatial distributions of cation concentrations in both the solution and exchange phases were measured by destructive sampling. The Na⁺ front advanced into the soil much more rapidly than K⁺ in all experiments because Ca²⁺–K⁺ exchange was much stronger than Ca²⁺–Na⁺ exchange. In the ternary experiments (Na⁺ and K⁺ displacing Ca²⁺), this led to the K⁺ and Na⁺ fronts being distinctly separate from each other, with the result that Ca²⁺–K⁺ exchange occurred in a background of almost constant Na⁺ concentration, and Ca²⁺–Na⁺ exchange occurred in a background of constant (near-zero) K⁺ concentration. The presence of the highly competitive K⁺ had little influence on the transport of Na⁺, but the presence of Na⁺ did affect the distribution of K⁺. For experiments involving both binary and ternary exchange, the positions of the cation fronts and the shapes of the cation distributions in both the solution and exchange phase could be predicted using the appropriate binary exchange isotherms and approximate analytical solutions published previously.