Sorption of divalent metals on calcite

The sorption of seven divalent metals (Ba, Sr, Cd, Mn, Zn, Co, and Ni) was measured on calcite over a large initial metal (Me) concentration range (10 −8 to 10 −4 mol/L) in constant ionic strength ( I = 0.1), equilibrium CaCO 3(s)-CaCO 3(aq) suspensions that varied in pH. At higher initial Me concentrations (10 −5 to 1 −4 mol/L) geochemical calculations indicated that the equilibrium solutions were saturated with discrete solid phases of the sorbates: CdCO 3(s), MnCO 3(s), Zn 5(OH) 6(CO 3) 2(s), Co(OH) 2(s), and Ni(OH) 2(s), implying that aqueous concentrations were governed by solubility. However, significant sorption of all the metals except for Ba and Sr was observed at aqueous concentrations below saturation with Me-solid phases. Divalent metal ion sorption was dependent on aqueous Ca concentration, and the following selectivity sequence was observed: Cd > Zn ≥ Mn > Co > Ni > Ba = Sr. The metals varied in their sorption reversibility, which was correlated with the single-ion hydration energies of the metal sorbates. The strongly hydrated metals (Zn, Co, and Ni) were most desorbable. A sorption model that included aqueous speciation and Me 2+-Ca 2+ exchange on cation-specific surface sites was developed that described most of the data well. The chemical nature of the surface complex used in this model was unspecified and could represent either a hydrated or dehydrated surface complex, or a surface precipitate. A single exchange constant for Cd, Mn, Co, and Ni could describe the sorption of that metal over a wide range in pH, Ca concentration, and surface concentration. Zinc, however, exhibited nonlinear sorption behavior and required exchange constants that varied with surface coverage. Our data suggested that (i) Cd and Mn dehydrate soon after their adsorption to calcite and form a phase that behaves like a surface precipitate, and (ii) Zn, Co, and Ni form surface complexes that remain hydrated until the ions are incorporated into the structure by recrystallization.