Mechanisms of wollastonite weathering and applications to CO2 storage

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Wood, Cameron
Wollastonite , Enhanced weathering , Silicon isotopes , CO2 Storage , Silicate weathering , Incongruent dissolution
The mechanisms of wollastonite dissolution and its potential applications in Enhanced Rock Weathering (ERW) initiatives that sequester atmospheric CO2 were investigated in two sets of experiments. Mixed flow reactor experiments, carried out from pH 2.0 to 9.5, revealed close to stoichiometric dissolution at neutral to alkaline pH but non-stoichiometric dissolution at pH 2.0, where Ca was released to solution and most Si was retained within an amorphous silica surface layer. At pH 2.0, the Si isotope composition of the fluid become progressively isotopically lighter during wollastonite dissolution over time. The Δ30Sisolid-fluid values between the bulk reacted solid and the pH 2.0 solution are 2.49 to 2.61 ± 0.28 ‰. These values indicate a greater magnitude of enrichment of the solid in Si isotopes of heavier mass relative to the fluid than has previously been documented during precipitation of amorphous silica. Condensation reactions within a retained amorphous silica layer could lead the more polymerized layer to become increasingly refractory, favoring the release of Si isotopes of lighter mass. The pH dependence of wollastonite dissolution stoichiometry and the behaviour of Si isotopes at pH 2.0 are thus not consistent with a coupled dissolution-precipitation process as the mechanism governing incongruent dissolution of wollastonite under acidic conditions. Column experiments that contained wollastonite, soil, and soil-wollastonite mixtures were irrigated with H2O or K3PO4 solutions. Experiments showed substantially greater dissolution of wollastonite and greater carbon capture in soil-wollastonite mixtures compared to wollastonite-only columns, but P exerted little influence on wollastonite dissolution. Adsorption to soils resulted in rapid removal of P, limiting its ability to catalyze the dissolution of wollastonite. Adsorption also significantly retarded Ca, making Ca concentrations unreliable as a means of estimating wollastonite dissolution rates. Mixed column effluent concentrations of HCO3- increased by up to 2.48 mM over time, suggesting that the presence of wollastonite resulted in retention of microbially generated CO2. These results emphasize the importance of site-specific characterization of mineral-soil mixtures. In such complex systems, silicon and other isotopic systems may be a valuable tool for gaining greater insight into weathering processes.
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