EFFECT OF HyA-INTERLAYERING ON NH₄/Ca SELECTIVITY OF EXPANDABLE LAYER SILICATES

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Hydroxy-interlayering in vermiculites (Vt) and montmorillonites (Mt) is very common in acid soils and sediments which causes significant changes in many physical, chemical, electrochemical and mineralogical properties of the host clays. Cation exchange capacity and selectivity are essential parameters that influence cation distribution between adsorbed and solution phases, and their mobility in the soil profile. Adsorption and exchange isotherms for NH₄-Ca system were derived using Vt and Mt clays interlayered with hydroxyaluminum (HyA) ions of initial basicities of 1.0 and 2.5. The principal objective of this study was to clarify the effect of hydroxy-interlayering on the NH₄/Ca selectivity of the host clays. Vermiculite in contrast to Mt, showed more preferential adsorption and exchange of NH₄ over Ca. The HyA-interlayering greatly increased NH₄/Ca selectivity of Vt, whereas that of Mt was little affected. The greater NH₄/Ca selectivity of Vt in comparison to Mt might be a result of higher layer charge density in the former predominantly of tetrahedral origin. The HyA-interlayering might have increased the NH₄/Ca selectivity through making the clay interlayers propped open, occupying the Ca adsorbing sites and/or exerting steric hindrance to the entry of more hydrated, larger Ca ions. Larger amount of Hy A adsorption at higher initial basicity of the HyA ionic solution appeared to increase NH₄/Ca selectivity, especially of Vt. The greater NH₄/Ca selectivity of Vt, Mt, and HyA-Vt/Mt as well as the effect of Hy Ainterlayering on this parameter were limited to low NH₄-saturation.