Reactivity of organic complexes at mineral-CO2-H2O interfaces

Abstract:

Understanding the interactions between minerals and organics in H₂O-CO₂ fluids is important, as they are the two most abundant volatiles in the crust. CO₂-rich fluids in natural and anthropogenic environments, such as metamorphic aureoles and carbon storage reservoirs, respectively, produce a complex geochemical setting in which CO₂-rich fluids containing dissolved water and organic compounds interact with rocks and minerals. We have undertaken experimental and theoretical studies to evaluate how organics impact carbonate mineralization and to determine the partitioning behavior of organic complexes between CO₂, H₂O, and mineral interfaces. The first groups of experiments have clarified how the type and concentration of simple organic ligands impact the degree and type of carbonation in interfacial water films. In these experiments, salts of simple organic ligands were equilibrated with wet supercritical CO₂, which was reacted with the model mineral forsterite (Mg₂SiO₄). The forsterite dissolution and coupled carbonate precipitation reactions were followed with time-resolved pressurized X-ray diffraction (XRD) at 50 °C and 90 bar. The extent of carbonation and the relative abundance of anhydrous magnesite (MgCO₃) precipitated relative to hydrated nesquehonite (MgCO₃·3H₂O) was impacted by the type of organic ligand. Magnesite enhancement was observed with the trend of citrate>oxalate≈malonate>acetate>organic-free control. This indicates that the organic ligands complexed Mg²⁺ in the interfacial water film environment and helped alleviate kinetic barriers to magnesite formation. Additional XRD experiments with varying concentrations of citrate verified the dependence of magnesite enhancement and the degree of overall carbonation on the amount of organic present in the water film. Lastly, our ongoing work concerning the partitioning of organic and metal-organic complexes between CO₂, H₂O, and interfacial water films will be presented. This experimental work, which includes in situ spectroscopic measurements, has been coupled with atomistic simulations to clarify the energy barriers and reaction mechanism. The implications for cycling of C-O-H-metal compounds (e.g. H₂O, carbonates, metal-organics, and CO₂) in a broad range of subsurface environments will be discussed.