In SituFourier Transform Infrared Measurements of Sulfate Adsorption on Hematite in Aqueous Solutions

Anin situattenuated total reflection (ATR)–Fourier transform infrared (FTIR) method, in which the ATR element is coated with hematite particle layers in direct contact with the aqueous phase, was used to measure sulfate adsorption as a function of aqueous sulfate concentration and pH. Between pH 3 and 6, monitoring the spectral region 900–1300 cm−1, an IR spectrum with three bands between 950 and 1150 cm−1, indicative ofC3vsymmetry and monodentate sulfate coordination, was observed. The spectral amplitudes varied with sulfate concentration and solution pH in agreement with independent measurements and as described by quantitative surface complexation models. Previous studies on dried samples found a spectrum with a fourth maximum (consistent withC2vsymmetry) at >1200 cm−1, which has been interpreted as evidence of bidentate coordination. In the present study, a fourth band at 1200 cm−1appeared only on drying of the sulfated hematite layer or with an aqueous phase pH <2. Based on these observations and on a comparison with various reported IR measurements, it is suggested that in the presence of an aqueous phase between pH 3 and 5, predominantly monodentate sulfate surface complexes are formed on hematite. Spectral changes on removal of the solvent indicate formation of monodentate bisulfate or of bidentate sulfate on dry hematite.