Effect of autotransformation on the layer charge of smectites determined by the alkylammonium method

Hydrogen-forms of <2 mu m fractions of six bentonites of various Fe contents were prepared by H(+)-->OH(-)-->H(+) ion exchange using resins. Potentiometric titration curves revealed that the number of strong acid sites varied and accounted for 60-95% of the total acidity in the freshly prepared H-forms. The number of strong acid sites decreased and that of the weak acid sites increased on ageing. The process of autotransformation in aqueous dispersion at 90 degrees C was completed within four days. Layer-charge distributions of all samples were inhomogeneous with layer charges from 0.25-0.39 Eq/unit O(10)(OH)(2). Oxalate pretreatment of the samples resulted in changes in the layer-charge distribution due to the removal of readily soluble phases which may have blocked exchange sites. After autotransformation, the alkylammonium exchange method revealed inhomogeneous charge density distributions; the fraction of layers of the highest charge decreased. Comparison of total CEC obtained from potentiometric curves and interlamellar CEC calculated from the mean layer charge confirmed attack of protons from particle edges. However, for several samples the structural attack may also occur from the interlayer space. Autotransformation of the H-smectites decreased the mean layer charge. Protons probably attack the Mg(O,OH)(6) octahedra preferentially during the autotransformation.