Surface Speciation Models of Calcite and Dolomite/Aqueous Solution Interfaces and Their Spectroscopic Evaluation

Abstract

The composition and density of surface hydroxyl and carbonate groups on calcite and dolomite after contact at 25 °C with solutions of different pH (3 to 12) and carbonate concentration (10-4 ≤ ∑CO2 ≤ 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 ∑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 ≤ 5 and ∑CO2 ≤ 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 I3400/I1420 (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+ × nH2O outer sphere species that yield an increase of surface adsorbed water.