Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration

Abstract

Injecting supercritical CO2 (scCO2) into basalt formations for long-term storage is a promising strategy for mitigating CO2 emissions. Mineral carbonation can result in permanent entrapment of CO2; however, carbonation kinetics in thin H2O films in humidified scCO2 is not well understood. We investigated forsterite (Mg2SiO4) carbonation to magnesite (MgCO3) via amorphous magnesium carbonate (AMC; MgCO3·xH2O, 0.5 < x < 1), with the goal to establish the fundamental controls on magnesite growth rates at low H2O activity and temperature. Experiments were conducted at 25, 40, and 50 °C in 90 bar CO2 with a H2O film thickness on forsterite that averaged 1.78 ± 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO2 as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by 2 to 3 orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low H2O activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO2 fate and transport.

Nanoscale Mg-Depleted Layers Slow Carbonation of Forsterite (Mg2SiO4) When Water Is Limited

“…Nonetheless, the question of how this nanometer-thin, Mgdepleted layer slows carbonation of divalent metal silicates at <100% RH where conditions remain far from equilibrium persists, that is, highly supersaturated with respect to Mg carbonates. 12 We hypothesize that low-water conditions impact the dissolution of Mg from forsterite and the precipitation of Mg carbonates, as both mechanisms rely on hydration of Mg for transport. 3,58,59 Silica under the same conditions likely requires more water than Mg to solubilize and transport, leading to the relatively rapid formation of a largesurface area Mg-depleted layer, as hypothesized by previous forsterite dissolution studies.…”

“…A fundamental understanding of silicate mineral carbonation in low-water, humidified fluids is needed to optimize sequestration of CO 2 in select applications: basaltic geologic reservoirs, − carbonation of low-grade mafic ores for efficient recovery of critical elements, − and decarbonization of agriculture by enhanced rock weathering. , Under these conditions, water adsorbs on silicate minerals as films that are only angstroms to nanometers thick. ,− Reactivity in thin water films is markedly different from that in bulk water due to high surface area/water volume ratios (SA/ V ), strong water–mineral interactions, low diffusivity, and low dielectric constants. ,,− …”

Formation and Dissolution of Surface Metal Carbonate Complexes: Implications for Interfacial Carbon Mineralization in Metal Silicates

“…Unlike reactions mediated by water-rich fluids, the carbonation reactions with humidified CO 2 -rich fluids might appear hindered owing to uncertainties arising in mass transport processes and ion solvation extents . However, well-controlled CO 2 -rich laboratory and field-scale experiments on olivine, − brucite, and feldspar show that the carbonation in such environments can be potentially facile due in parts to the formation of nanometer-thick adsorbed water films.…”

Nanoconfinement matters in humidified CO2 interaction with metal silicates

“…These gaps are difficult to address experimentally and are key for parameterizing realistic MD simulations of these interfaces. 49,[52][53][54] Although continual progress is being made in determining water film thicknesses at mineral-H 2 O-CO 2 interfaces, [54][55][56][57][58][59][60][61][62][63][64] including for forsterite surfaces, the initial hydration state of the mineral surface to measurements is often unknown, and the influence of pressure-temperature-composition on water film thicknesses has not been systematically explored. Second, we investigate whether carbonic acid and bicarbonate can potentially form at the water-solid interface.…”