Surface Reactivity of Talc: Acid-Base Behaviour and Cd Adsorption

Using surface titration and electrokinetic measurements in aqueous solutions at 25 °C, the surface of natural talc and cadmium adsorption were studied. Using the Mular – Roberts titration method, the point of zero salt effect, which in the case of talc is close to the point of zero charge was determined to be pH_pzse = 8.1 ± 0.1, while the isoelectric point pH_iep is approximately 2, which corresponds to the predominance of SiO₂ in the surface layer due to the preferential removal of Mg in the acidic region. When titrating from pH 3 to the alkaline region, ζ-potential of the talc surface exhibits strong hysteresis, becoming positive, and pH_iep shifts toward the alkaline region due to the retardation of the reverse exchange reaction Mg²⁺ → 2 H⁺. This circumstance suggests that proton adsorption (desorption) occurs only at the outer adsorption sites of the edges, making it possible to apply the theory of surface complexation to adsorption modeling. Cadmium adsorption on talc is described by the Freundlich isotherm, and the adsorption kinetics obey a pseudo-second-order equation. Almost complete (90–98%, depending on concentration) adsorption of cadmium is observed in neutral and slightly alkaline solutions (pH 7.5–8). Coupled modeling of proton and cadmium adsorption using the Extended Constant-|Capacitance Model showed that the best agreement with experimental data on both cadmium and proton adsorption is provided by a model with two inner-sphere surface cadmium complexes, > MgOCdOH and > SiOHCd²⁺. Cadmium adsorption (at pH > 6) is primarily determined by the surface complex > MgOCdOH, while minor absorption in the range 4 < pH < 6 is associated with the formation of the silanol complex > SiOHCd²⁺.