Molecular interactions of SO ₂ with carbonate minerals under co-sequestration conditions: A combined experimental and theoretical study

We present a combined experimental and theoretical study investigating the reactivity between select and morphologically important surfaces of carbonate minerals with supercritical CO ₂ and co-existing H ₂O and SO ₂. Trace amounts of SO ₂ cause formation of CaSO ₃ in the form of hannebachite in the initial stages of SO ₂ 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 scCO ₂, 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 CO ₂-defect sites. Formation of bisulfites (surface-SO ₂OH) 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 4.0×10 ¹ to 4.0×10 ⁴ 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-SO ₂OH s-S(H)O ₃), 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 CaSO ₃H···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 CO ₂ captured from fossil fuel sources, which could be stored permanently for sequestration purposes or extracted and utilized for enhanced oil recovery (EOR).