Specific ion effects: The role of anions in the aggregation of permanently charged clay mineral particles

Purpose Understanding how anions affect the colloidal behavior of clay mineral is greatly important in determining soil particle dispersibility. The surface of soil particles, especially for silicate clay mineral components, is often net negatively charged. However, how anions influence soil particle interactions and affect soil structure has not been fully studied. This study investigated how specific anions (SO 4 2− , Cl − , and phosphate) affect the aggregation of permanently charged montmorillonite particles. Methods The effects of anions on colloidal montmorillonite particle aggregation were investigated at different ionic strengths using dynamic light scattering. Anion adsorption on montmorillonite particles in equilibrated solutions was measured by ion chromatography. Results Anions were adsorbed onto a permanently charged clay mineral surface, which induced strong specific effects in inhibiting montmorillonite particle aggregation. Phosphate had the greatest inhibiting effect on coagulation, followed by Cl − and SO 4 2− . The critical coagulation ionic strength (CCIS) for K 2 SO 4 , KCl, and KH 2 PO 4 was 9.915, 11.38, and 180.7 mmol/L, respectively. The activation energy between particles in the different anion solutions quantitatively characterized this specific anion effect. The difference in the various anion solutions increased with decreasing ionic strength (i.e., increasing electric field). Second, the amount of phosphate adsorbed by montmorillonite was the greatest among the three anions, followed by Cl − and SO 4 2− , under the given solution conditions, consistent with the observed specific anion effects. Conclusions Phosphate, Cl − , and SO 4 2− adsorbed onto the surface of permanently charged mineral particles and increased the negative surface charge through non-classic polarization. The strength of the electric field strongly influenced this polarization. The combined roles of cations and anions regulate aggregation, whereas CCIS is determined by specific ion adsorption. Our findings emphasize the importance of specific anion adsorption and double-layer interfacial effects on the aggregation of permanently charged clay mineral colloids. Our findings have important implications for interpreting the behavior of clay minerals in the environment.