Dissolution of hectorite in inorganic acids

The effect of acid type and concentration on the reaction rate and products of dissolution of hectorite in inorganic acids was investigated. The dissolution of hectorite in hydrochloric (HCl), nitric (HNO 3 ) and sulphuric (H 2 SO 4 ) acids was characterized using quantitative chemical analysis, infrared (IR) and multinuclear MAS NMR spectroscopies. The rate of dissolution increased with acid concentration and decreased in the order HCl ≥ HNO 3 > H 2 SO 4 at the same molar concentration. No differences were found in the reaction products of hectorite treated with the three acids. The rate of Li dissolution was slightly greater than that of Mg at lesser acid concentrations (0.25 M), indicating that protons preferentially attack Li octahedra. The gradual changes in the Si-O IR bands reflects the extent of hectorite dissolution. The analysis of 29 Si MAS NMR spectra relative peak intensities with dissolution time and acid concentration provided direct dissolution rates for tetrahedral (Q 3 ) Si. After acid dissolution, most Si was bound in a three dimensional framework site (Q 4 ), but a substantial part also occurred in the Si(OSi) 3 OH (Q 3 1OH) and Si(OSi) 2 (OH) 2 (Q 2 2OH) environments. These three sites probably occur in a hydrous amorphous silica phase. Both Al IV and Al VI rapidly disappeared from 27 Al MAS NMR spectra of the dissolution products with acid treatment. The changes in IR and MAS NMR spectra of hectorite due to acid dissolution are similar to those of montmorillonite.