Sulfate adsorption on hematite via surface complexation affects sulfate mobility and fate in tropical soils where hematite is abundant, but the sulfate complexation mechanism remains ambiguous. Here, sulfate adsorption behavior on hematite of two different sizes (8 nm versus 35 nm) were investigated using S K-edge XANES, EXAFS and ATR-FTIR spectra as well as the ionic strength effect test. Results indicate that sulfate forms both inner-sphere and outer-sphere complexes on hematite surfaces, but the inner-sphere complexation is dominant. The inner-sphere complex fraction increases with increasing ionic strength, sulfate loading, and drying, white decreasing with increasing pH, based on the relative pre-edge peak intensity and white line peak positions. Particle sizes do not show obvious effects. EXAFS fitting determined the S−Fe interatomic distance to be 3.23 ± 0.03 Å, indicative of sulfate bidentate-binuclear inner-sphere complexation, which is similar to sulfate complexation on ferrihydrite surfaces and in the structure of schwertmannite in previous studies. In addition, OH<sup>-</sup> released kinetics during sulfate adsorption were also measured. This study clarified that outer-sphere complexes co-exist with inner-sphere complexes, which have not been unambiguously revealed in previous studies. Thus, the finding in this study have important implications for understanding sulfate behavior in hematite-rich environment, such as tropical soils, and provide a basis for building physically-meaningful surface complexation modeling for predicting sulfate fate and mobility in the environment.

Sulfate complexation on hematite surfaces

251st National Meeting (2016)

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