Effects of Salts and pH on the Critical Coagulatior Concentration of Montmorillonite

Abstract

The critical coagulation concentration (CCC) of montmorillonite was measured at various salts and pH levels, and compared with that predicted by the DLVO theory. The CCC of monovalent cations was in accordance with the hydrated cation radius, and ratios of divalent to monovalent cations agreed with the Schulze-Hardy rule. The CCC in Na₂SO₄ solution was found to be high compared to that of NaCl or NaNO₃ below pH 6.5. Since SO₄^2- cancels more the positive charge at the edge than Cl^- or NO₃^-, pH 6.5 at which the effects of anion valence disappeared was considered the point of zero charge (PZC) of the edge. Effective PZC as a net charge of the edge including the face negative charge in the vicinity of the edge is introduced. Predicted CCC agreed with that measured under alkaline conditions assuming the effective PZC of the edge at pH 4. However it did not agree under acidic conditions where multiple interactions disturb edge to edge (EE) or edge to face (EF) interactions with small contact areas. Coagulation with EE and EF linkages may not occur easily at low pH unless face to face repulsion is depressed by salts.