Dolomite surface speciation and reactivity in aquatic systems
The surface charge of dolomite (CaMg(CO{sub 3}){sub 2}) was measured as a function of pH (6.5--11.5), pCO{sub 2} (10{sup {minus}3.5}, 0.01, and 0.96 atm) and ionic strength (0.01, 0.1, and 0.5 M NaCl) using potentiometric titrations in a limited residence time reactor. Dolomite zeta potential ({zeta}) was determined using streaming potential and electrophoresis techniques at pH 2 to 12 in solutions having ionic strengths from 0.001 to 0.1 M NaCl as a function of aqueous Ca{sup 1+}, Mg{sup 2+}, and CO{sub 3}{sup 2{minus}} concentrations. The point of zero charge (PZC) and isoelectric point (IEP) of dolomite are the same (pH {approximately}8 at pCO{sub 2} {approximately}10{sup {minus}3.5 atm}) and very close to those of calcite and magnesite. On the basis of these results, a surface complexation model (SCM) is proposed that postulates the presence of three distinct primary hydration sites: {gt}CO{sub 3}H{degree}, {gt}CaOH{degree}, and {gt}MgOH{degree}. The intrinsic stability constants of dolomite surface reactions were determined by fitting the pH dependence of the surface charge and taking into account the isoelectric points and {zeta}-potential values for a wide range of solution compositions. In most natural aquatic environments, dolomite surface speciation can be modeled using the following species: {gt}CO{sub 3}{sup {minus}}, {gt}CO{sub 3}Me{sup +}, {gt}MeOH{sub 2}{sup +}, {gt}MeHCO{degree}{sub 3}, and {gt}MeCO{sub 3}{sup {minus}}, where Me = Ca, Mg. The speciation model presented in this study allows description of metal and ligand adsorption onto dolomite surface and provides new insights on the mechanisms that control dolomite dissolution/crystallization in aqueous solutions. In particular, it is shown that dolomite dissolution is controlled by the protonation of {gt}CO{sub 3}H{degree} surface complexes at pH {gt} 6 and by hydrolysis of {gt}MeOH{sub 2}{sup +} groups at higher pH.
- Research Organization:
- UPS-CNRS, Toulouse (FR)
- OSTI ID:
- 20003970
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 19-20; Other Information: PBD: Oct 1999; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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