The effects of electrolyte concentration, ion species and pH on the zeta potential and electrokinetic charge density of montmorillonite

In this study, the influence of pH, electrolyte concentration and type of ionic species (such as LiCl, NaCl, KC), RbCl, CsCl, CaCl2, AlCl3) on the electrokinetic properties (zeta potential and electrokinetic charge density) of montmorillonite has been quantified. The zeta potential of montmorillonite particles did not change significantly with change in pH. The valencies of the ions have proven to have a great influence on the electrokinetic behaviour of the suspension. There is a gradual decrease in the zeta potential (from -24 mV to -12 mV) with increase in monovalent electrolyte concentration (from 10(-4) m to 10(-1) m). At any monovalent electrolyte concentration, the magnitude of the zeta potential increased with the electrolytes in the order Li+ > Na+ > K+ > Rb+ > Cs+. The zeta potential of the montmorillonite minerals in CaCl2 solutions illustrated the same behaviour as the monovalent cations. Less negative values were obtained for the CaCl2 electrolyte (similar to-10 mV) due to the greater valence of the ions. A sign reversal was observed at an AlCl3 concentration of 5 x 10-4 m, and, at greater concentrations, zeta potential values had a positive sign (similar to 20 mV). The electrokinetic charge density of montmorillonite showed similar trends of variation in mono- and divalent electrolyte solutions. Up to concentrations of similar to 10(-3) M, it remained practically constant at similar to 0.5 x 10(-3) C m(-2), while for greater electrolyte concentrations the negative charge produced more negative values (- 16 x 10(-3) C m(-2)). The electrokinetic charge density of montmorillonite particles was constant at low AlCl3 concentrations, but at certain concentrations it increased rapidly and changed sign to positive.