Comparative reactivity study of forsterite and antigorite in wet supercritical CO2 by in situ infrared spectroscopy

The carbonation reactions of forsterite (Mg2SiO4) and antigorite [Mg3Si2O5(OH)(4)], representatives of olivine and serpentine minerals, in dry and wet supercritical carbon dioxide (scCO(2)) at conditions relevant to geologic carbon sequestration (35 degrees C and 100 bar) were studied by in situ Fourier transform infrared (FT-IR) spectroscopy. Our results confirm that water plays a critical role in the reactions between metal silicate minerals and scCO(2). For neat scCO(2), no reaction was observed in 24 hr for either mineral. When water was added to the scCO(2), a thin water film formed on the minerals' surfaces, and the reaction rates and extents increased as the water saturation level was raised from 54% to 116% (excess water). For the first time, the presence of bicarbonate, a key reaction intermediate for metal silicate reactions with scCO(2), was observed in a heterogeneous system where mineral solids, an adsorbed water film, and bulk scCO(2) co-exist. In excess-water experiments, approximately 4% of forsterite and less than 2% of antigorite transformed into hydrated Mg-carbonates. A precipitate similar to nesquehonite (MgCO3 center dot 3H(2)O) was observed for forsterite within 6 hr of reaction time, but no such precipitate was formed from antigorite until after water was removed from the scCO(2) following a 24-hr reaction period. The reduced reactivity and carbonate-precipitation behavior of antigorite was attributed to slower, incongruent dissolution of the mineral and lower concentrations of Mg2+ and HCO3- in the water film. The in situ measurements employed in this work make it possible to quantify metal carbonate precipitates and key reaction intermediates such as bicarbonate for the investigation of carbonation reaction mechanisms relevant to geologic carbon sequestration. (c) 2013 Elsevier Ltd. All rights reserved.