RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The surface charge of dolomite (CaMg(CO₃)₂) was measured as a function of pH (6.5–11.5), pCO₂ (10^−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 (ζ) 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 Ca²⁺, Mg²⁺, and CO₃²⁻ concentrations. The point of zero charge (PZC) and isoelectric point (IEP) of dolomite are the same (pH ∼8 at pCO₂ ∼10^−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: >CO₃H°, >CaOH°, and >MgOH°. 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 ζ-potential values for a wide range of solution compositions. In most natural aquatic environments, dolomite surface speciation can be modeled using the following species: >CO₃⁻, >CO₃Me⁺, >MeOH₂⁺, >MeHCO₃^o, and >MeCO₃⁻, 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 >CO₃H° surface complexes at pH < 6 and by hydrolysis of >MeOH₂⁺ groups at higher pH.