Ion Exchange Model for Reversible Sorption of Divalent Metals on Calcite: Implications for Natural Environments
Résumé
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 Ca2+ 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 Ca2+ 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 pCO2(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 Ca2+ species by fitting the experimental data. Then, selectivity coefficients between H+ and different metallic cations (Zn2+, Cd2+, Pb2+) 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).