The mechanism of Zn 2+ adsorption on calcite

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The adsorption of Zn 2+ on calcite (CaCO 3(s) ) was investigated from aqueous solutions in equilibrium with CaCO 3(s) and undersaturated with respect to Zn 5 (OH) 6 (CO 3 ) 2(s) ). Zinc adsorption occurred via exchange with Ca 2+ in a surface-adsorbed layer on calcite. The validity of this exchange reaction was supported by adsorption isotherm and constant concentration experiments, where Ca 2+ (aq) was varied by systematically changing the pH and CO 2(g) . Greater adsorption of Zn 2+ occurred at higher pH and CO 2(g) levels, where Ca 2+ activities were lowest. Sites available for Zn 2+ sorption were less than 10% of Ca 2+ sites on the calcite surface. Surface exchange of Zn 2+ did not affect the solubility of calcite. Zinc sorption was apparently independent of surface charge, which suggested that the surface complex had covalent character. Desorption and isotopic exchange experiments indicated that the surface complex remained hydrated and labile as Zn 2+ was rapidly exchangeable with Ca 2+ . Careful analysis of the adsorption data showed that Zn 2+ and ZnOH + were the sorbing species. The exchange reaction was generalized as a power exchange function: Zinc adsorption on calcite was compared to and was consistent with that of Co 2+ , but Zn 2+ was more strongly sorbed.

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