Cation Dependent Carbonate Speciation and the Effect of Water

Carbon storage in geological formations relies on knowledge of the effect of carbonate ligand formation for both dissolution and mineralization to permit realistic reservoir modeling and to enhance confidence in storage security both spatially and temporally. Reaction rates are intrinsically slow and thus are not amenable to in situ study, often determined from high pressure batch reactions providing largely bulk structural information through product analysis. Here we present a methodology to prepare silicate mineral analogues by placing reactive cations (Na and Ca) on the surface of amorphous silica, greatly enhancing the rate of carbonate formation. Traditional characterization techniques provide thorough structural and morphological information related to the effects of doping. In situ Fourier transform infrared spectroscopy (FTIR) is used to follow carbonate formation upon exposure to CO₂. Carbonate speciation is found to be cation dependent and sensitive to the presence of water. Speciation shows further dependence on whether water is present during CO₂ exposure or subsequently. Low pressure CO₂ adsorption isotherms (T = 35 °C) were measured to quantify carbonate formation, fitted to an empirical adsorption isotherm model, and related to cation coverage as determined by energy-dispersive X-ray spectroscopy (EDX). CO₂ uptake at low pressure was related to carbonate formation on the surface and at higher pressures was shown to depend on surface area as modified by cation adsorption. Although envisaged as mineralization analogues, this system provides information on the impact of water on the formations of carbonates relevant to wider carbon capture and storage processes at the fundamental molecular level.