Experimental study of organic ligand transport in supercritical CO₂ fluids and impacts to silicate reactivity

The interactions between water-saturated supercritical carbon dioxide, organics, and minerals are relatively unknown despite being important to carbon sequestration and enhanced hydrocarbon recovery activities. The goals of this study include verification of organic transport through scCO₂ and exploration of whether organic ligands can impact carbonate formation. An in situ near-infrared spectroscopic technique was used to probe supercritical CO₂ (scCO₂)-organic mixtures at 35 °C and 100 bar. Observation of C-H bands in the spectra collected from scCO₂ equilibrated with a Ca-acetate solution (1.7 m) provided direct evidence of organic partitioning into the scCO₂ phase. A series of high pressure X-ray diffraction experiments at 50 °C and 90 bar were performed to investigate how citrate (0.01-0.5 m) affected the coupled dissolution of forsterite and precipitation of magnesium carbonates in scCO₂. In control experiments where no citrate was present, nesquehonite (MgCO₃₃H₂O) was initially produced as a metastable intermediate that was then converted to magnesite (MgCO₃). However, experiments with citrate promoted magnesite, rather than nesquehonite, precipitation, and at the highest concentrations reduced the extent of the carbonation reaction. This paper discusses these unique findings on organic and scCO₂ interactions that are not currently being considered in the fate and transport of CO₂ in the subsurface.