Carbonation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modeling

Carbonation reactions of wollastonite have been investigated in experiments conducted at conditions relevant to geologic CO2 sequestration in subsurface environments (T = 90 °C and pCO2 = 25 MPa). Experiments were performed in batch reactors on single-mineral powders of selected grain size, either in supercritical CO2 or in aqueous solutions with different alkalinities. The main reaction products were calcite and amorphous silica. Calcite occurred either as a compact, continuous coating of small crystals in the case of supercritical CO2 and in circum-neutral pH aqueous solutions, or as isolated larger crystals in acidic aqueous solutions. Measured extents of carbonation were faster in aqueous solutions than in supercritical CO2. Extents of carbonation in aqueous acidic solutions were modelled using a geochemical code incorporating kinetic parameters for wollastonite dissolution and calcite precipitation taken from the literature. Comparison with experimental results shows that, in aqueous solutions, wollastonite dissolution is the rate-limiting step of the carbonation reaction and that silica layers have only a minor passivation effect on the overall reaction rate. In acidic solutions, secondary calcite coating has only a minor effect on carbonation extent, whereas the experimental data in circum-neutral pH aqueous solutions are consistent with dissolution rates inhibited by the observed dense calcite coating. A geochemical code incorporating a modified reactive surface model successfully accounted for this passivation effect. The more efficient carbonation in acidic than in circum-neutral pH solutions is thus ascribed to differences in microstructures of the secondary calcite layers due to differences in supersaturation states of calcite when its precipitation begins. Inhibitory effects of dense calcite coating are also invoked for limiting carbonation extents of wollastonite in supercritical CO2. Secondary calcite precipitations are thus shown to play a major role both as coatings of reactive surfaces and by maintaining large undersaturation with respect to wollastonite dissolution.