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 calcite 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 Sigma CO2 less than or equal to 0.1 M) were monitored by means of diffuse reflectance infrared (DRIFT) spectroscopy. Both for calcite and dolomite, broad high-intensity absorbance bands at about 3400 and 1600 cm(-1) were observed 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 to carbonate species (surface and bulk) for both minerals was found to increase 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 equal to 10(-3) M. These observations correlate well with the surface speciation 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 postulate the formation of >CaOH2+, >MgOH2+, >CaHCO3o, >MgHCO3o, >CaCO3-, >MgCO3-, >CO3Ca+, >Co3Mg+, and >CO3- surface species from two primary hydration sites, >CaOHo (>MgOHo) and >CO3Ho. Very good relationships were found between the predicted concentration of the surface OH groups (>MeOH2+) and the measured density of the surface hydroxyl groups corresponding to a band at around 3400 cm(-1). Moreover, the experimental ratio of band intensities I-3400/I-1420 (OH/CO3) was found to correlate well with the predicted concentration ratio of the adsorbed surface hydroxyl and carbonate groups, { >MeOH2+}/{>MeHCO3o + >MeCO3-}. External addition of Mg2+ or Ca2+ ions to alkaline dolomite suspensions leads to an increase of the surface density of the OH groups. This increase is explained, in accordance with the SCM, by the formation of >CO3Me+ x nH(2)O outer sphere species that yield an increase of surface adsorbed water.