Molecular interactions of SO2 with carbonate minerals under co-sequestration conditions: A combined experimental and theoretical study

Abstract

We present a combined experimental and theoretical study investigating the reactivity between select and morphologically important surfaces of carbonate minerals with supercritical CO2 and co-existing H2O and SO2. Trace amounts of SO2 cause formation of CaSO3 in the form of hannebachite in the initial stages of SO2 adsorption and transformation. Atomistic simulations based on density functional theory of these initial steps indicate accumulation of water over the magnesium sites, and suggest depletion of Mg over the Ca from the mineral surface. Under co-sequestration conditions with wet scCO2, water is not likely to cause carbonate dissolution of a perfect surface, however, it stabilizes pre-existing low coordination oxygen atoms by creating surface hydroxyl groups on the CO2-defect sites. Formation of bisulfites (surface-SO2OH) occurs with a low barrier of ca 0.5eV, estimated by the climbing image nudged elastic band method (CI-NEB). Estimates of the effective transformation rates are in the range of 4.0×101 to 4.0×104 s−1. The sulfur-containing species bind preferentially on surface calcium atoms creating the first nucleation sites. Molecular dynamics simulations also show dynamic tautomerization of the adsorbed bifulfites (s-SO2OH ⇌ s-S(H)O3), which is likely to slow down further oxidation to sulfates in less oxidative environments. From the same simulations, we extract local geometries of the resulting CaSO3H···OH species, similar to the crystallographic structure of hannebachite. Collectively, the experimental results and ab initio molecular dynamics simulations suggest potential of carbonate reservoirs for in situ chemical scrubbing of CO2 captured from fossil fuel sources, which could be stored permanently for sequestration purposes or extracted and utilized for enhanced oil recovery (EOR).