title: Ion exchange model for reversible sorption of divalent metals on calcite: implications for natural environments.
abstract: Most of the thermodynamic models available in the literature describing the speciation of the calcite surface do not predict a significant concentration of sorbed Ca(II), whereas previous electrokinetics studies clearly show that Ca(2+) is the main cation determining the potential of the calcite surface. This study proposes a new thermodynamic model based on ion-exchange theory that is able to describe the reversible sorption of Ca(2+) on calcite. To constrain the model, concentrations of Ca(II) sorbed reversibly on the mineral surface were obtained as a function of pH. Such experimental data were obtained using solutions in equilibrium with both calcite and fixed p(CO2(g)) values (from 10(-5) to 10(-2) atm). The concentration of (de)sorbed Ca(II) is almost constant in the [7-9.5] pH range, having a value of approximately 1.2 × 10(-6) ± 0.4 × 10(-7) eq·g(-1). Such a value agrees with total sorption site densities that were previously calculated by crystallography and is used to obtain a selectivity coefficient between H(+) and Ca(2+) species by fitting the experimental data. Then, selectivity coefficients between H(+) and different metallic cations (Zn(2+), Cd(2+), Pb(2+)) that are able to accurately describe previously published data are proposed. Finally, the model is used to predict the contribution of calcite in the overall sorption of Cd(II) on a natural and complex solid (calcareous aquifer sand).
authors: Université de Poitiers/CNRS, UMR 7285 IC2MP, Equipe HydrASA, Poitiers, France. emmanuel.tertre@univ-poitiers.fr
authors: Emmanuel
authors: E
authors: Tertre
authors: Jacques
authors: J
authors: Page
authors: Catherine
authors: C
authors: Beaucaire
mesh_terms: D000327:Adsorption; D002104:Cadmium / Q000302:isolation & purification; D002118:Calcium / Q000302:isolation & purification; D002119:Calcium Carbonate / Q000737:chemistry*; D002254:Carbonates / Q000737:chemistry; D002413:Cations, Divalent / Q000302:isolation & purification*; D007474:Ion Exchange; D007854:Lead / Q000302:isolation & purification; D008670:Metals / Q000302:isolation & purification*; D008956:Models, Chemical; D013499:Surface Properties; D013816:Thermodynamics; D015032:Zinc / Q000302:isolation & purification
chemical_list: D002254:Carbonates; D002413:Cations, Divalent; D008670:Metals; D002104:Cadmium; D007854:Lead; D002119:Calcium Carbonate; D015032:Zinc; D002118:Calcium