Abstract:

Comparison study between Mg2+ and Ca2+ adsorption kinetics, as influenced by different particle surface potentials, was investigated by the miscible displacement technique. In this study, important new features of ion adsorption were discovered. (1) The initial stage of experiment the adsorption process had zero-order kinetics because of strong adsorption. First-order kinetics then resulted because of weak adsorption and the transition point from zero-order to first-order kinetics was very sharp for each system. (2) At equivalent supporting electrolyte concentrations, the adsorption rate of Ca2+ is obviously faster than that of Mg2+，and the equilibrium adsorption capacity of Ca2+ is more than that of Mg2+. The coverage of Ca2+ on the solid soil particle's surface is also higher than that of Mg2+. (3) The differences between the relative effective charge coefficient and the surface electrochemical properties are the reason why the Ca2+, Mg2+ adsorption kinetics are different. (4) We also determined some important properties of the adsorbed ions such as the rate coefficients, the adsorption quantities, the surface coverage of the adsorbed ions and the distributed space in the fixed liquid film of the adsorbed ions. These parameters allow a future quantitative evaluation of the effects of different colloid surface potentials on the ion diffusion/adsorption kinetics.

Key words: Solid-liquid interface, Surface electric field, Relative effective charge coefficient, Ion adsorption, Kinetics