Specific ion effects of Cu2+, Ca2+ and Mg2+ on montmorillonite aggregation

Abstract Differences in ion-surface reactions among various divalent cations may result in large differences in clay particle interaction behaviors. Specific ion effects of Cu 2+ , Ca 2+ and Mg 2+ in montmorillonite (Mt) aggregation were studied by laser light scattering. The aggregation rate in the presence of Cu 2+ was much higher than that in the presence of Ca 2+ and Mg 2+ . The critical coagulation concentrations (CCCs) were Cu 2+ (0.650 mM) 2+ (2.38 mM) 2+ (7.99 mM). The differences in aggregation rates and CCC values were attributed to non-electrostatic interaction forces between the cations and the clay mineral surface. These non-electrostatic forces generated from (i) non-classical polarization of cations in the strong electric field near the clay mineral surface; and (ii) the quantum-mechanical interaction between the strongly polarized cations, especially for the d -type orbital electronic configuration of Cu 2+ , and the strongly polarized surface oxygen atoms. It was assumed that a new, polarization-induced covalent bond formed between Cu 2+ and the surface O.