Methodology to obtain exchange properties of the calcite surface--application to major and trace elements: Ca(II), HCO3-, and Zn(II).

Sorption of inorganic elements onto carbonate minerals has been intensively described in the literature by two reaction steps: (1) a first one rapid and completed within a few hours and (2) a second one slower, eventually irreversible, and occurring at a constant rate. The first step is often attributed to an ion-exchange process, but its reversibility is rarely investigated. Consequently, discrimination of the global sorption phenomenon into two different mechanisms is not always justified. In this study, we investigated, by batch experiments, both sorption and desorption of Ca(II), HCO(3)(-), and Zn(II), radiolabeled with isotopes (45)Ca(II), H(14)CO(3)(-), and (65)Zn(II), respectively, onto synthetic pure calcite. Solutions were preequilibrated with atmospheric p(CO2) and saturated with respect to calcite. Therefore, our purpose was to: (1) obtain experimental distribution coefficients of major elements (Ca(II) and HCO(3)(-)) and a trace element (Zn(II)) onto calcite from sorption and desorption experiments, (2) test the validity of a first-occurring ion-exchange process generally noted in the literature, by calculating distribution coefficients for the "sole" exchange process, and (3) quantify the amounts of Ca(II), HCO(3)(-), and Zn(II) sorbed on the calcite surface by the sole "exchange process" and compare them with surface crystallochemical data. Ca(II) or HCO(3)(-) sorption experimental data suggest that a significant fraction of these two elements was sorbed irreversibly onto or in the calcite. By using a method based on isotopic ratios, the Ca(II) or HCO(3)(-) concentrations, which are reversibly adsorbed on the calcite, have been quantified. These concentrations are respectively estimated at 4.0+/-2.0 x 10(-4) and 7.0+/-1.5 x 10(-4) mol/kg. The obtained Ca(II) surface concentration value is one order of magnitude lower than the one obtained from isotopic measurement by former authors [Geochim. Cosmochim. Acta 55 (1991) 1549; Geochim. Cosmochim. Acta 51 (1987) 1477; Geochim. Cosmochim. Acta 52 (1988) 2281] at the same pH. On the other hand, the kinetics of Zn(II) sorption onto calcite was followed over more than 1000 h. Sorption/desorption experimental results suggest that the sorption is totally reversible at least when total aqueous Zn concentration is less than 10(-6) mol/L and when experiments are performed in equilibrium with both calcite and p(CO2)=10(-3.5) atm. Under these conditions and at pH 8.3, the occupancy rate of Zn(II) onto the calcite surface is estimated to represent approximately 1% of the total surface-site density.