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 C_2v 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.