Sorption of divalent metals on calcite

Abstract

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 (lo-' to 10e4 mol/L) in constant ionic strength (I = 0. l), equilibrium CaC03(s)-CaCOs(aq) suspensions that varied in pH. At higher initial Me concentrations (10m5 to 10m4 mol/L) geochemical calculations indicated that the equilibrium solutions were saturated with discrete solid phases of the sorbates: CdCOx(s), MnCO,(s), Zn5(OH),(C0,)2(s), Co(OH),(s), and Ni(OH),(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 2 Mn > Co > Ni 4 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*+-Ca*+ 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.

Enhanced Cd(II) Uptake by the Bassanite Phase Contained in Waste Calcite Produced via the Carbonation of Flue Gas Desulfurization (FGD) Gypsum

“…It then diminishes reaching a limiting value after about 18 h. The fast initial uptake is often observed and interpreted as adsorption while the slow second uptake is explained as precipitation, recrystallization, and/or diffusion into solid for the sorption. 1,6,17) After 18 h, further uptake of Cd(II) does not occur at least in a macroscopic scale. This may be due to the unavailability of exchangeable adsorption sites for Cd(II) ions, which prevents further adsorption of Cd(II).…”

“…[1][2][3][4][5] Among the divalent metals, cadmium (Cd) has attracted the most scientific and practical interest because it is known to have a similar ionic radius to that of Ca (about 98% of that of Ca) and a high sorption affinity to calcite. 4,5) Generally accepted mechanisms involved in calcium dissolution and cadmium adsorption on calcite are explained in terms of the following equations.…”

“…4,5) Generally accepted mechanisms involved in calcium dissolution and cadmium adsorption on calcite are explained in terms of the following equations. 1,5,6) The dissolution of calcium from calcium carbonate with atmospheric CO 2 gas is established as follows:…”

“…For example, solutions pre-equilibrated with calcite and of low Cd(II) concentration were used to minimize the calcium dissolution and to prevent the formation of otavite (CdCO 3 ) during sorption processes. 1,6) Calcite single crystal cleavage surface was often utilized in microscopic studies 5,7) to better define the surface chemistry and structure of the sorption products. In addition, the effects of reaction parameters (i.g., temperature, ionic strength, pH, particle size, dissolved species, etc.)…”

Waste dolomite powder as an adsorbent of Cd, Pb(II), and Zn from aqueous solutions

“…More and more often, low-cost materials are investigated as adsorbents (Salam et al 2011;Hegazi 2013;Mikoda et al 2017). Experimental usage of calcite for heavy metal sorption, carried out by Zachara et al (1991), showed a good sorption capacity of that mineral. Research concerning the sorption of metals from wastewater (Brady et al 1999;Ayoub and Mehaweh 2007;Merrikhpour and Jalali 2012) showed that carbonate minerals had very good sorption properties.…”