ATR-FTIR spectroscopic characterization of coexisting carbonate surface complexes on hematite

The speciation of carbonate adsorbed to hematite in air-equilibrated aqueous solutions has been studied using ATR-FTIR spectroscopy. Samples were measured over a range of pH conditions, at 0.1 M NaCl and at low ionic strength, and in H₂O and D₂O solutions to permit a multispecies analysis of the data. Second-derivative analyses and fits to the spectra indicate the presence of two major and two minor surface-bound carbonate species. The two major complexes coexist at near-neutral pH and low ionic strength. One of these two complexes is relatively sensitive to ionic strength, being displaced at 0.1 M NaCl, whereas the other is not. Comparison of experimental to DFT/MO-calculated frequencies suggest these two major species to be (a) a monodentate binuclear inner-sphere carbonate surface complex, and (b) a fully or partially solvated carbonate (CO₃²⁻) species that is symmetry broken and appears to reside in the structured vicinal water layers at the hematite-water interface, retained by hydrogen bonding and/or other forces. Minor carbonate complexes include diffuse layer CO₃²⁻ and an unidentified inner-sphere species. Both of the dominant species observed here are likely to be significant controls of the surface charge and sorptive properties of Fe-oxides.