Specific ion effects on ion exchange kinetics in charged clay

Colloids and Surfaces, A.

Du, WeiLi, RuiLiu, Xin-MinTian, RuiLi, Hang

Ion exchange adsorption is a physical-chemical process ubiquitously occurring in a wide range of systems, and it is possible that specific ion effects exist universally in cation adsorption processes. In this study, the adsorption kinetics of Li+, Na+ and Cs+ in the exchange systems (Li+/K+, Na+/K+ and Cs+/K+) occurring at a negatively charged montmorillonite surface were explored under different dilute electrolyte concentrations. The results show that (1) the adsorption kinetics exhibited strong specific ion effects; and (2) the strength of the specific ion effects could be quantitatively characterized by the defined Hofmeister energy, w(H)(0), additionally, the w(H)(0) values for Cs+, Na+ and Li+ were respectively 1.40, 0.942, 0.180 and 0.0630 time larger than the corresponding Coulomb energies. Based on those results, new kinetic theories of cationic adsorption taking specific ion effects into account were established. We theoretically conclude that: large w(H)(0) value will result in fast adsorption rate, and the adsorption will obey a zero order kinetics during the first stage of adsorption, and subsequently changed to a first-order kinetics; however, for a cation with low w(H)(0) value, only the first-order kinetics could be observed. All those theoretical predictions were verified by experimental results. (C) 2016 Elsevier B.V. All rights reserved.