Previous studies of divalent orthosilicate dissolution in bulk water have evidenced tens of nm thick M2+-depleted/silica rich coatings on reacted parent minerals. These altered surface layers are believed to form by either M2+ leaching and repolymerization of relic SiO4 tetrahedra or congruent dissolution followed by precipitation of SiO2(am). They significantly slow orthosilicate dissolution by reducing the number of reactive surface sites and hindering the release of M2+ that must now diffuse through porous channels in the SiO2(am). We have observed a passivating effect during the carbonation of forsterite (Mg2SiO4) in confined H2O films that are Å-to-nm in thickness under wet supercritical CO2 conditions relevant to geologic carbon sequestration. For example, the carbonation of ~30 nm sized forsterite particles in humidified scCO2 proceeds rapidly for the first ~24 hours but then dramatically slows so that only ~ 20% of the forsterite is consumed after 3 days of reaction. This results in a SiO2(am) layer that is less than 1 nm thick. Interestingly, carbonation of the same nano-sized forsterite in bulk water with a 90 bar CO2 headspace leads to complete reaction in ~24 hours with no apparent passivation effect. In this presentation, we will share results from a combination of in situ infrared and electrical impedance spectroscopic results that reveal a passivation mechanism during forsterite carbonation in thin H2O films. Our findings suggest that the secondary SiO2(am) dramatically reduces dissolution/carbonation rates. The inferred mechanism is distinct from bulk-water studies and is dependent on H2O film thickness. This study furthers an understanding of divalent silicate carbonation under low-H2O CO2-dominated fluids as part of a comprehensive effort to predict the fate of CO2 stored in geologic reservoirs.

Passivation effect during forsterite carbonation in thin H2O films

ACS Fall 2021

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