Hematite reactivity with supercritical CO2 and aqueous sulfide

Abstract Hematite nanoparticles were reacted in a solution containing sulfide in contact with supercritical CO 2 , to simulate conditions relevant to carbon dioxide sequestration in the Earth's subsurface. X-ray diffraction was used to identify reaction products. Rietveld analysis of the X-ray diffractograms and thermogravimetric analysis were used to estimate the amounts of reaction product formed. Under the experimental conditions used (70 °C), a near equimolar amount of siderite (FeCO 3 ) and pyrite (FeS 2 ) formed. The equimolar amounts of pyrite and siderite products were consistent with the stoichiometry associated with the following overall reaction, 2Fe 3+  + 2S 2−  + CO 3 2–  → FeS 2  + FeCO 3 . In situ infrared spectroscopy suggested that the conversion of the hematite was rapid and largely occurred within 4 h at 70 °C. Analysis of the reaction product with electron microscopy showed the presence of framboidal pyrite and rhombohedral siderite particles that were micron-sized. The observed morphologies suggest that the transformation from hematite to siderite occurred via a dissolution–reprecipitation reaction.