FTIR Spectral Components of Schwertmannite

Fourier transform infrared (FTIR) spectral components of three dominant groups of sulfate species in synthetic schwertmannite (Fe₈O₈(OH)_6−x(SO₄)_x·nH₂O) are presented. These components were extracted by multivariate curve resolution analysis of spectra obtained from N₂(g)-dry samples initially reacted in aqueous solutions (pH 3−9) at room temperature. Each component contains complex sets of bands that correspond to mixtures of similar species. We tentatively assign these components to sulfate ions that are hydrogen- (components I and III) and iron-bonded (component I) to schwertmannite. Another component (II) is assigned to protonated sulfate species. Heating experiments to 130 °C moreover confirmed this possibility for component II. The spectral components extracted from this study can be used to identify dominant sulfate species in FTIR spectra of naturally occurring schwertmannite samples.