RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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⁺, >CaHCO₃^o, >MgHCO₃^o, >CaCO₃⁻, >MgCO₃^-, >CO₃Ca⁺, >CO₃Mg⁺, and >CO₃^- surface species from two primary hydration sites, >CaOH^o (>MgOH^o) and >CO₃H^o. 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⁺}/{>MeHCO₃^o + >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.