Dolomite surface speciation and reactivity in aquatic systems

The surface charge of dolomite (CaMg(CO3)(2)) was measured as a function of pH (6.5-11.5), pCO(2) (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 resi dence time reactor. Dolomite zeta potential (xi) was determined using strea ming potential and electrophoresis techniques at pH 2 to 12 in solutions ha ving ionic strengths from 0.001 to 0.1 M NaCl as st function of aqueous Ca2 +, Mg2+, and CO32- concentrations. The point of zero charge (PZC) and isoel ectric point (IEP) of dolomite are the same (pH similar to 8 at pCO(2) simi lar to 10(-3.5) atm) and very close to those of calcite and magnesite. On t he basis of these results, a surface complexation model (SCM) is proposed t hat postulates the presence of three distinct primary hydration sites: >CO3 H degrees, >CaOH degrees, and >MgOH degrees. The intrinsic stability consta nts of dolomite surface reactions were determined by fitting the pH depende nce of the surface charge: and taking into account the isoelectric points a nd xi-potential values for a wide range of solution compositions. In most n atural aquatic environments, dolomite surface speciation can be modeled usi ng the following species: >CO3- >CO3Me+, >MeOH2+ >MeHCO(3)degrees, and >MeC O3- where Me = Ca, Mg. The speciation model presented in this study allows description of metal and ligand adsorption onto dolomite surface and provid es new insights on the mechanisms that control dolomite dissolution/crystal lization in aqueous solutions. In particular, it is shown that dolomite dis solution is controlled by the protonation of >CO3H degrees surface complexe s at pH < 6 and by hydrolysis of >MeOH2+ groups at higher pH. Copyright (C) 1999 Elsevier Science Ltd.