Mineral carbonation for subsurface carbon storage [electronic resource] : an experimental investigation of olivine (Mg, Fe)2SiO4 dissolution and carbonation
Keywords: Chemical Engineering.

Concern about effects of climate change motivates the development of carbon capture and storage (CCS) technologies that facilitate both the capture of carbon dioxide (CO2) from point sources, such as power plants, and injection of CO2¬ into the subsurface for long-term storage. Compared with other approaches to reduce CO2 emissions, the advantages of CCS technologies include the possibility of near-term, large-scale implementation and reduction of CO2 emissions without major grid infrastructure changes. However, ensuring the stability of CO2 in the subsurface for hundreds to thousands of years is a major technological challenge. One approach for improving the stability is to convert CO2 into a mineral via chemical reactions, a process known as "mineral carbonation." Carbon dioxide reacts with divalent cations (e.g., Mg) to form carbonate minerals (e.g., magnesite, MgCO3) that are stable and negatively buoyant in the subsurface. Field studies have established that mineral carbonation occurs in nature, but the kinetics of the reaction are not fully understood. Of particular interest is the formation of a Si-rich surface layer that may passivate the surface of the reacting minerals. In order to investigate the reaction kinetics of mineral carbonation reactions at conditions representative of geologic storage, we performed a series of batch reactions at 60 °C and 100 bar pCO2 using Twin Sisters olivine ((Mg1.84Fe0.16)SiO4) as the reactive silicate mineral. Results demonstrate that mineral carbonation is feasible at subsurface storage conditions, with reaction extent reaching 7 mol% carbonated over 94 days. The reaction rate is strongly affected by SiO2(aq), decreasing two orders of magnitude as SiO2(aq) approaches amorphous silica (SiO2(am)) saturation, but remains constant for up to 94 days after that and does not depend strongly on pH. We consistently observed that a Si-rich layer forms on reacted olivine grains in less than 2 days, persists for at least 94 days, and increases in thickness with reaction extent. A high-resolution transmission electron microscopy (HR-TEM) study of mineral cross-sections showed two distinct types of amorphous surface layers, both rich in Si (or depleted in Mg). Analysis of olivine reacted with a 29Si isotopic tracer using a Sensitive High Resolution Ion Microprobe Reverse Geometry (SHRIMP-RG) resulted in strong evidence for the presence of a precipitated layer on olivine reacted for 19+ days but not on olivine reacted for 2 days. Observations from the HR-TEM and SHRIMP-RG studies allowed us to generate a new model for the formation of Si-rich layers on multi-oxide silicate minerals. Previous studies have suggested either a leached surface layer or a precipitated surface layer. In contrast, we have evidence that both types of surface layers occur simultaneously. The leached or "active" layer forms in the first hours to days of reaction and persists for 19+ days; the removal of Mg and Si from this layer controls the bulk dissolution rate. The precipitated layer forms after the bulk solution reaches saturation with respect to SiO2(am) and grows in thickness as the reaction continues at a constant rate, suggesting that it is does not passivate the surface. New knowledge linking the measured dissolution rate with surface dynamics is useful for the design of an engineered system for subsurface carbon storage. The model presented here for Si-rich surface layer formation on olivine may be applicable to other multi-oxide silicate minerals, providing a new framework for geochemical models that include mineral dissolution.