Effect of K+ on Na-Ca exchange and the SAR-ESP relationship

In Australia, application of winery wastewater to land is increasingly being viewed as the most environmentally sound and cost-effective means of disposal. This wastewater contains high concentrations of both sodium (Na+) and potassium (K+), which have the potential to accumulate in the profile of irrigated soils and adversely alter physical properties such as aggregate stability and hydraulic conductivity. Cation exchange equilibria in soil of mixed illite and kaolinite mineralogy have been investigated in binary Ca-Na and Ca-K systems and in a ternary Ca-Na-K system. In the respective binary systems, resulting exchangeable potassium percentage was nearly twice the corresponding exchangeable sodium percentage (ESP), indicating a high binding affinity of K+ in this soil. In a ternary system, soils were equilibrated with solutions of differing sodium adsorption ratio (SAR) and potassium adsorption ratio (PAR) within ranges typical of winery wastewater. The presence of K+ had a significant effect on the relationship between SAR and ESP, whereby ESP decreased with increasing PAR. Resulting ESP in the ternary system was consistently lower than in the binary system. Cation selectivity between solid and solution phases in the ternary system was calculated from the Vanselow and K-selectivity coefficients and showed a decreasing selectivity for Na+ with increasing K+ in solution. It is expected that, due to the high K+ content of winery wastewater (i.e. > 400 mg/L), adsorption of Na+, and subsequent ESP, will be less than in wastewaters of comparable Na+ concentration yet absent K+.