Modeling of H⁺ and Cu²⁺ Adsorption on Calcium-Montmorillonite

Abstract

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 Potentiometrie 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

$$\begin{array}{c}pH^{+}+qCu^{2+}+\equiv{SOH}\Leftrightarrow(H^{+})_p(Cu^{2+})_q(\equiv{SOH})^{(p+2q)+}\\ \beta_{p,q(int)}^s=\frac{\text[H_pCu_q(\equiv{SOH})^{(p+2q)}]}{\text[H^+]^{p}[Cu^{2+}]^q[\equiv{SOH}]}\end{array}$$

and the constants logβ ^S_1,0(int) = 8.16 (± 0.04), logβ-1,0(int)^S = −8.71 (± 0.08), logβ_0,1(int)^S = 5.87 (± 0.06), logβ−1,1(int)^S = −0.57 (± 0.12), logβ_−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

$$\equiv{TOH}-H^+\Leftrightarrow{\equiv}{TO}^-log\beta_{-1,0(int)}^S=-5.77(\pm0.07).$$

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.