Selectivity coefficients of K, Na, Ca, and Mg in binary exchange systems in some calcareous soils

One of the major issues in surveying the sorption and mobility of elements is awareness of the behavior and reactions of elements in soils and their distribution coefficient (K_d) values. This study was conducted to investigate the adsorption of potassium (K), sodium (Na), calcium (Ca), and magnesium (Mg) in six calcareous soils saturated with mentioned cations. The initial equivalent fraction of K, Na, Ca, and Mg in solutions increased from 0.1 to 1 at a total electrolyte of 30 meq L^-1. The shape of isotherm curves and the amount of cations adsorbed varied in different binary exchange systems. The adsorption of cations increased gradually with increasing initial concentrations. The average values of K_d (L kg^-1) in different binary systems were arranged in the following order: Ca-Mg (23.4) > Mg-Na (21.5) > Mg-K (20.7) > Mg-Ca (20.1) > Ca-Na (16.0) > Ca-K (15.4) > K-Mg (5.6) > K-Na (5.0) > K-Ca (4.6) > Na-Ca (2.7) > Na-Mg (1.9) > Na-K (1.7). The average values of Gaines-Thomas selectivity coefficients of cations in different binary exchange systems followed this order: K-Ca (16.5) > K-Mg (7.8) > Ca-Mg (4.1) > Mg-Ca (3.1) > Mg-Na (2.1) > K-Na (1.7) > Mg-K (1.0) > Ca-Na (0.8) > Ca-Na (0.6) > Na-Ca (0.5) > Na-Mg (0.1) = Na-K (0.1). So, the affinity of cations for adsorption by soils followed this order: K > Ca > Mg > Na which differed from the classical lyotropic series. The high affinity of K for adsorption by soils was attributed to the presence of illite. The free cations were the most abundant species (above 90%) in all soil solutions after reaching equilibrium with initial concentrations of 3 and 30 meq L^-1. The saturation index (SI) values of minerals in all exchange systems were negative.