Determination of layer charge of soil smectites by alkylammonium method : effect of removal of hydroxy-interlayering

Abstract

Hydroxy-interlayering in smectites of shrink-swell soils of Peninsular India is more common than exception. These hydroxy-interlayers prevent the determination of actual layer charge by alkylammonium method by obstructing the normal intrusion of alkyalmmonium ions into the interlayers. This may have resulted in the formation of relatively more layers with bi-pseudotrilayer transition and of paraffin type which overestimates the layer charge of core lattice mineral. Several extractant were tried to remove the hydroxy-interlayered materials and to determine the actual layer charge of the cleaned clays. But they did not yield desired results. However, Stripping of clay samples with 0.25 N EDTA solution (pH 7.0) removed the hydroxy-interlayers. The clay samples (<0.2 m) from a Vertisol (Amravati district, Maharashtra) were treated with different alkylammonium chlorides varying in chain length before and after EDTA treatment. The layer charge of the pre-treated fine clay smectites varied from 0.40 to 0.46 mol(-)/(Si,Al)4O10(OH)2. The charge was highest at the surface and decreases with depth. It is interesting to note that the degree of hydroxy-interlayering also decreased with depth. After EDTA treatment the layer charge decreased from the original values and varied from 0.27 to 0.33 mol(-)/(Si,Al)4O10(OH)2. The distribution of layer charge with depth before and after EDTA treatment followed the same trend. The results thus suggest that (i) 0.25 N EDTA solution (pH 7.0) is an effective extractant to remove the interlayer materials of the soil smectites and (ii) this method allows to estimate the actual layer charge of the crystal lattice of soil smectites of Penisular India.