Ferrihydrite reactivity toward carbon dioxide

Abstract

The reaction of ferrihydrite with gaseous CO 2 was investigated with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations. ATR-FTIR results show that CO 2 reacts with ferrihydrite resulting in surface adsorbed carbonate species. The carbonate species experimentally observed in view of theoretical calculations are shown to be in large part monodentate binuclear complexes. These carbonate complexes exist as both inner-sphere and outer-sphere hydrogen-bonded complexes. Under “dry” conditions CO 2 reacts with free OH sites on the ferrihydrite surface resulting in a metastable bent CO 2 (bicarbonate-like) complex. Removal of the gaseous reactant leads to the loss of this metastable surface complex. The reaction of CO 2 with hydrated ferrihydrite results in only carbonate formation (no bicarbonate). In this circumstance, experiments and theoretical calculations suggest that hydrogen bound water on surface OH sites prevents the formation of the metastable bicarbonate species. Ferrihydrite that was allowed to react with atmospheric levels of CO 2 and water vapor resulted in the formation of surface carbonate coordinated as both inner and outer-sphere complexes.