The purposes of this research were to: (1) establish new analytical kinetic equation for describing the effect of strong electrostatic force on adsorption; (2) experimentally determine if it is a strong or weak electrostatic force adsorption process; and (3) evaluate the adsorption energies of the strong and weak force adsorptions based on the proposed new theory of cation adsorption kinetics.
The constantly charged material-montmorillonite was used in the experiment. The montmorillonite was saturated with two Cation species: K+ and Ca2+, respectively, using KNO3 or Ca(NO3)2 before it was used for the experiment. The miscible displacement technique under a steady flow condition was adopted to the kinetic studies of Mg2+ (Mg(NO3)2) adsorption. In the experiment, 0.5000 g of K+- or Ca2+-saturated montmorillonite was layered on the exchange column, the thickness of sample layer was approximately 0.2–0.3 mm, the cross-sectional area of the column (sample area) was 15 cm2. The concentration of Mg2+ in the flowing liquid was 10−4 mol L−1. The flow velocity of the flowing liquid was 1.0 mL·min−1. Effluent was collected at 10-min intervals.
Firstly, new and exact rate models for describing ion adsorption have been advanced. Secondly, based on the experiments of Mg2+/K+ and Mg2+/Ca2+ exchange in K+ and Ca2+-saturated montmorillonites, Mg2+ adsorption in K+-saturated sample appeared zero-order kinetic process in the initial stage of the strong force adsorption for t = 0–405 min, and then the adsorption process converted to the first-order kinetics of the weak force adsorption, which agrees with the theoretical prediction. However, for the Ca2+-saturated sample, merely first-order kinetic process appeared for Mg2+ adsorption. Either for Mg2+/K+ exchange or Mg2+/Ca2+ exchange, the quantities of Mg2+ by weak force adsorption at equilibrium were almost the same. Thirdly, several important dynamic and thermodynamic parameters can be theoretically calculated based on the new theory in describing cation adsorption.
For Mg2+/K+ exchange, both strong and weak electrostatic force adsorptions exist, but for Mg2+/Ca2+ exchange only the weak electrostatic force adsorption occurs. The strong and weak force adsorption processes can be quantitatively described by the new analytical kinetic equations of the zero- and the first-order kinetics, respectively. Because each parameter in the analytical kinetic equations has its definitive physical meaning, several important dynamic and thermodynamic parameters in cation exchange can be theoretically estimated.