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 Cu2+, Ca2+ and Mg2+ in montmorillonite (Mt) aggregation were studied by laser light scattering. The aggregation rate in the presence of Cu2+ was much higher than that in the presence of Ca2+ and Mg2+. The critical coagulation concentrations (CCCs) were Cu2+ (0.650 mM)<<Ca2+ (2.38 mM)<Mg2+ (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 Cu2+, and the strongly polarized surface oxygen atoms. It was assumed that a new, polarization-induced covalent bond formed between Cu2+ and the surface O.