Hard and soft acid-base model applied to bivalent cation selectivity on a 2 : 1 clay mineral

We applied the hard and soft acid-base (HSAB) model (Xu and Harsh 1990a, 19 90b) to bivalent cation exchange on a purified Ca-montmorillonite. As a res ult, a satisfactory model is proposed to describe the gradual selectivity o f exchange with Ca for 4 of the 6 metals studied (Cd, Cu, Ph, Zn). The sele ctivity is predicted as a function of the differences of electronegativity and softness of the metals. The deviation of Ni and Co data from the predic ting model is interpreted in terms of hydration (Ni and Co being the most s trongly hydrated ions). The fitting parameters of the model, alpha and beta , are related to the electronegativity and softness characteristics of the surface, respectively. Their ratio gives information on the nature of bandi ng. Results suggest that covalent bonding modifies electrostatic interactio ns, which in turn affect selectivity with an increasing influence of covale nt bondings in the order: Pb < Cd < Zn < Cu. To balance the lack of representativity of the model for the small molar fr action (N-M), we propose to associate to the HSAB model an equation describ ing the variation of the Vanselow selectivity coefficient as a function of the molar fraction of metal on clay.