Change in the site density and surface acidity of clay minerals by acid or alkali spills and its effect on pH buffering capacity

Abstract

Changes in the site density and surface acidity constants (i.e. pKa1 and pKa2) of kaolinite and montmorillonite were determined after acid or alkali spills, and pH buffering capacity was evaluated as a parameter of soil function change. Surface complexation modeling with potentiometric titrations and Fourier-transform infrared spectroscopy showed that acid or alkali spills did not significantly change the surface properties of kaolinite. In montmorillonite, however, acid spills decreased the basal site density from 832 to 737 mmol kg−1 by dissolving substituted octahedral cations and decreased pKa2 from 7.32 to 5.42 by dissolving SiOH. In response to alkali spills, the basal site density increased to 925 mmol kg−1, and the edge site density increased from 84.8 to 253 mmol kg−1 due to AlOH and SiOH formation; thus, pKa2 decreased to 6.78. The pH buffering capacity of acid- or alkali-spilled kaolinite at pH 6 did not significantly change, while that of acid- or alkali-spilled montmorillonite increased from 30.3 to 35.9 and 56.0 mmol kg−1, respectively. Our results indicate that these spills greatly altered the surface properties of montmorillonite, but unexpectedly, increased the pH buffering capacity of montmorillonite.