K/Ca and NH_4/Ca Selectivity of Hydroxyaluminum-Interlayered Vermiculite and Montmorillonite : Contribution from Regular and Frayed Edge Exchange Sites

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Cation distribution in adsorbed and solution phases and their mobility in the 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 hydroxyaluminum (HyA). Such interlayering causes significant changes in various properties of Vt and Mt. We evaluated the contribution of HyA-induced frayed edge sites (FES) in Vt and Mt to the overall selectivity of K/Ca and NH_4/Ca. A more preferential adsorption and exchange of K to Ca was observed in the case of Vt in contrast to Mt. The overall K/Ca selectivity of Vt increased markedly 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 HyA ionic solutiout. Selective adsorption of K and NH_4 predominantly occurred on the FES. Interlayering substantially enhanced such FES contribution. The overall K/Ca and NH_4/Ca selectivity coefficients did not indicate any striking competitiveness of NH_4 with K, unlike that for the natural micaceous clay minerals. However, the parameter of FES contribution suggested that NH_4 could be slightly more competitive with K for the FES. The relative abundance of FES on different clay specimens determining K/Ca and NH_4/Ca selectivity and a slightly higher preference of NH_4 to K on FES played a major role at low K or NH_4-saturation.
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K/Ca and NH_4/Ca Selectivity of Hydroxyaluminum-Interlayered Vermiculite and Montmorillonite : Contribution from Regular and Frayed Edge Exchange Sites

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