Potassium-calcium exchange in clays of selected Moroccan vertisols

The clay fractions of five moroccan vertisols (chaouia,gharb,doukalla,zair,and sais) as well as a reference smectite (montmorillonite swy-1) were ca-saturated and dispersed in k+ca2+ binary exchange solutions with constant ionic forces of 10-2 at 25°c. exchange isotherms established showed that k is preferentially adsorbed compared to ca2+ in all soil clays and the reference smectite. the clay fraction of the ch-chaouia soil,which is dominantly beidellitic,represents the highest affinity for k+. for all other soil clays,the values of the vanselow (kv) exchange selectivity coefficient,gaines and thomas (kgt),and gapon (kg) were higher than 1. this corroborates the fact that clays fractions of vertisols dominated by smectites are highly selective to k+ compared to ca2+. the regression coefficient show that a two-site adsorption model predicts well the adsorption parameters α,kv1,kv2 and n0. the site 1 (α) portion is more important in the chaouia soil clay (α = 0.248) and the least in the doukkala soil clay (α = 0.078). the selectivity coefficient of sites 1 (kv1) is less important for the reference montmorillonite swy-1 compared to all soil clays. for sites 2,the selectivity coefficients (kv2) are of less importance and vary from 1.77 (mole/l)-1 for swy-1 to 12.54 (mole/l)-1 for the chaouia soil clay. the equilibrium constants (kéq) of the k-ca exchange at 25°c are greater than 1. they are highest for the chaouia clay and the lowest for the swy-1 reference montmorillonite. for all clays,the standard free enthalpy (δg°) has negative values varying from -5 to 0 kj/éq.