Surface speciation models of calcite and dolomite/aqueous solution interfaces and their spectroscopic evaluation

The composition and density of surface hydroxyl and carbonate groups on cal cite and dolomite after contact at 25 degrees C with solutions of different pH (3 to 12) and carbonate concentration (10(-4) less than or equal to Sig ma CO2 less than or equal to 0.1 M) were monitored by means of diffuse refl ectance infrared (DRIFT) spectroscopy. Both for calcite and dolomite, broad high-intensity absorbance bands at about 3400 and 1600 cm(-1) were observe d at pH below 6 and carbonate concentration below 10(-3) M. These bands are assigned to hydroxyl groups present at the mineral surfaces. At higher pH and Sigma CO2, the intensity of these bands significantly decreases. On the contrary the intensity of the broad double band at about 1400 cm(-1) due t o carbonate species (surface and bulk) for both minerals was found to incre ase significantly with increasing solution pH and carbonate concentration, being the lowest at pH less than or equal to 5 and Sigma CO2 less than or e qual to 10(-3) M. These observations correlate well with the surface specia tion for calcite or dolomite/aqueous solution interface predicted based on surface complexation models (SCM). These models were proposed based on the electrokinetics and surface titration experimental results and they postula te the formation of >CaOH2+, >MgOH2+, >CaHCO3o, >MgHCO3o, >CaCO3-, >MgCO3-, >CO3Ca+, >Co3Mg+, and >CO3- surface species from two primary hydration sit es, >CaOHo (>MgOHo) and >CO3Ho. Very good relationships were found between the predicted concentration of the surface OH groups (>MeOH2+) and the meas ured density of the surface hydroxyl groups corresponding to a band at arou nd 3400 cm(-1). Moreover, the experimental ratio of band intensities I-3400 /I-1420 (OH/CO3) was found to correlate well with the predicted concentrati on ratio of the adsorbed surface hydroxyl and carbonate groups, { >MeOH2+}/ {>MeHCO3o + >MeCO3-}. External addition of Mg2+ or Ca2+ ions to alkaline do lomite suspensions leads to an increase of the surface density of the OH gr oups. This increase is explained, in accordance with the SCM, by the format ion of >CO3Me+ x nH(2)O outer sphere species that yield an increase of surf ace adsorbed water.