COMPETITIVE EXCHANGE OF K+, NA+, AND CA2+ DURING TRANSPORT THROUGH SOIL

Competitive exchange affects the transport of cations through soils wi th negatively charged surfaces, and when more than 2 cations are prese nt in the soil solution the effects are complex. There has been little previous experimental work to examine these effects in ternary (or hi gher order) cation systems. Cation transport experiments involving bot h binary and ternary cation exchange were therefore carried out. Calci um, 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 m easured 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 d istinctly separate from each other, with the result that Ca2+-K+ excha nge 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 i nfluence on the transport of Na+, but the presence of Na+ did affect t he distribution of K+. For experiments involving both binary and terna ry 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 approxim ate analytical solutions published previously.