K/Ca and NH4/Ca selectivity of hydroxyaluminum-interlayered vermiculite and montmorillonite: Contribution from regular and frayed edge exchange sites

Cation distribution in adsorbed and solution phases and their mobility in t he soil profile are largely influenced by the cation exchange capacity and selectivity coefficient of the soil. Vermiculites (Vt) and montmorillonites (Mt) in acid soils and sediments are frequently interlayered by hydroxyalu minum (HyA). Such interlayering causes significant changes in various prope rties of Vt and Mt. We evaluated the contribution of HyA-induced frayed edg e sites (FES) in Vt and Mt to the overall selectivity of K/Ca and NH4/Ca. A more preferential adsorption and exchange of K to Ca was observed in the c ase of Vt in contrast to Mt. The overall K/Ca selectivity of Vt increased m arkedly by HyA-interlayering, whereas that of Mt was slightly affected. An increased overall K/Ca selectivity, especially of Vt was also observed due to the larger amount of HyA adsorption at higher initial basicity of the Hy A ionic solution. Selective adsorption of K and NH4 predominantly occurred on the FES. Interlayering substantially enhanced such FES contribution. The overall K/Ca and NH4/Ca selectivity coefficients did not indicate any stri king competitiveness of NH4 with K, unlike that for the natural micaceous c lay minerals. However, the parameter of FES contribution suggested that NH4 could be slightly more competitive with K for the FES. The relative abunda nce of FES on different clay specimens determining K/Ca and NH4/Ca selectiv ity and a slightly higher preference of NH4 to K on FES played a major role at low K or NH4-saturation.