to the situation where sulfate replaces one or several surface An in situ attenuated total reflection (ATR)-Fourier transform hydroxyl groups, resulting in the coordination of at least one infrared (FTIR) method, in which the ATR element is coated with sulfate oxygen atom with a surface metal site. hematite particle layers in direct contact with the aqueous phase, Quantitative studies of sulfate adsorption from aqueous was used to measure sulfate adsorption as a function of aqueous solution and interpretations based on the surface complexsulfate concentration and pH. Between pH 3 and 6, monitoring ation models reviewed by Stumm et al. (5) and Schindler the spectral region 900-1300 cm 01 , an IR spectrum with three and Stumm (6) provide indirect information about possible bands between 950 and 1150 cm 01 , indicative of C 3Β£ symmetry surface structures. Experimental data on sulfate adsorption and monodentate sulfate coordination, was observed. The spectral on ferric hydrous oxides ( ) were fitted with a generalized amplitudes varied with sulfate concentration and solution pH in agreement with independent measurements and as described by two-layer model (diffuse double layer model) by formulation quantitative surface complexation models. Previous studies on of an inner-sphere mononuclear (mono-or bidentate is not dried samples found a spectrum with a fourth maximum (consisdistinguished) and an outer-sphere complex (8). Ali and tent with C 2Β£ symmetry) at ΓΊ1200 cm 01 , which has been interpre-Dzombak (1) recently investigated competitive adsorption of ted as evidence of bidentate coordination. In the present study, a sulfate and small organic acids on goethite and used a doublefourth band at 1200 cm 01 appeared only on drying of the sulfated layer model with an additional inner-sphere bisulfate surface hematite layer or with an aqueous phase pH Γ΅2. Based on these complex; however, surface complexation models with more observations and on a comparison with various reported IR meaadjustable parameters (constant capacitance and triple-layer surements, it is suggested that in the presence of an aqueous phase model), and possibly with different proposed surface combetween pH 3 and 5, predominantly monodentate sulfate surface plexes, lead to equally good fits, so that such models can complexes are formed on hematite. Spectral changes on removal usually not prove or disprove certain structures. More direct of the solvent indicate formation of monodentate bisulfate or of bidentate sulfate on dry hematite.