Ferrihydrite phase transformation in the presence of aqueous sulfide and supercritical CO2

The transformation of ferrihydrite to siderite (FeCO3) in the presence of supercritical CO2 (scCO2) and aqueous sulfide has been investigated by in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and ex situ X-ray diffraction (XRD). The chemical transformation of iron-bearing minerals to form iron carbonates may play a role in the chemistry related to the geological sequestration of carbon dioxide in the Earth's (sub)surface. The iron oxyhydroxide ferrihydrite in an aqueous solution containing sodium sulfide in contact with scCO2 at 100 °C reacts within 24 h to form siderite and elemental sulfur. In situ studies of this reaction with ATR-FTIR show that ferrihydrite exists with an adsorbed layer of carbonate. However, the presence of the carbonate adsorbate does not impede the reduction of ferric to ferrous iron and, ultimately, siderite is observed. While ferrihydrite has been used in this study as a model material in the investigation of mineral storage of CO2 at carbon sequestration conditions it may have potential use above ground in surface mineral carbonation.