Infrared spectra from binuclear bridging complexes of sulphate adsorbed on goethite (α-FeOOH)

Abstract

Spectra of sulphate adsorbed on synthetic goethite (α-FeOOH) show replacement of A-type (one coordinated to Fe3+) surface hydroxyls and the formation of adsorbed sulphate groups with C2v symmetry. High affinity adsorption isotherms at low pH and replacement of all A-type hydroxyls (∼400 µmol g–1) by the adsorbed sulphate (200 µmol g–1) suggests that a binuclear bridging complex (i.e., Fe—O—SO2—O—Fe) is formed, similar to that found for phosphate adsorption on goethite.

Absorption near 1300 cm–1 in the infrared spectra of the binuclear complex suggests that infrared criteria, recently suggested in the literature of metal sulphato complexes, for distinguishing between bridging (i.e., all frequencies < 1200 cm–1) and bidentate chelating (i.e., band > 1200 cm–1) sulphato complexes may need to be re-examined.