MODELING OF H+ AND CU2+ ADSORPTION ON CALCIUM-MONTMORILLONITE

The interaction of H+- and Cu2+-ions with Ca-montmorillonite was investigated in 0.1 mol/dm3 solutions of Ca(CIO4)2 at 298.2 K by potentiometric titrations using both glass electrodes (for H+) and ion specific electrodes (for Cu2+). The experimental data were interpreted on the basis of the surface complexation model. The calculations were performed with the least-squares program FITEQL (Westall, 1982) using the constant capacitance approximation. The best fit was obtained with a set of equilibria of the general form p H+ + q Cu2+ + = SOH double line arrow pointing left and right (H+)p(Cu2+)q(= SOH)(p+21)+ beta(p,q(int)s = [H(p)Cu(q)(= SOH)(p+2q)]/[H+]p[Cu2+]q[= SOH] and the constants log beta1,0(int)s = 8.16 (+/- 0.04), log beta-1,0(int)s = -8.71 (+/- 0.08), log beta0,1(int)s = 5.87 (+/- 0.06), log beta-1,1(int)s = -0.57 (+/- 0.12), log beta-2,1(int)s = -6-76 (+/- 0.02). An appropriate modeling of the H+ adsorption data requires the introduction of a second surface group = TOH with the acidity constant = TOH - H+ double line arrow pointing left and right = TO- log beta-1,0(int)s = -5.77 (+/- 0.07). In addition, the ion exchange equilibria Ca2+ - Cu2+ and Ca2+ - H+ had to be taken into account. Arguments are presented to identify the groups = SOH and = TOH as surface aluminol groups-Al(OH)(H2O) and surface silanol groups = Si-OH, respectively.