X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 5. Halide ion (F-, Cl-, Br-, I-) adsorption.

Halide anion (F(-), Cl(-), Br(-), and I(-)) adsorption and its impact on sodium adsorption at the hematite/water interface were studied by cryogenic X-ray photoelectron spectroscopy (XPS). Measurements were carried out on frozen, centrifuged wet hematite pastes that were previously equilibrated in 50 mM electrolytic solutions in the pH 2-11 range. XPS-derived halide ion surface loadings decreased in the order F(-) > I(-) ≈ Cl(-) > Br(-), whereas sodium loadings were in the order Na(F) > Na(I) > Na(Br) > Na(Cl). The greater sodium loadings in NaF and in NaI resulted from larger anion loadings in these systems. Bromide ion had the lowest loading among all halide ions despite having a charge-to-size ratio that is intermediate between those of Cl(-) and I(-). This unexpected result may have arisen from specific properties of the hematite/water interface, such as water structure and electric double layer thickness. Fluoride ion adsorption proceeded via the formation of hydrogen bonds with the surface hydroxo groups (e.g., ≡Fe-OH(2)···F(-) or ≡Fe-OH···F(-)). Surface-bound fluoride ions exert a greater charge-screening effect than the other halide anions, as demonstrated by considerably small zeta potential values. Fe-F bond formation was excluded as a possible interfacial process as the F 1s peak binding energy (684.2 eV) was more comparable to that of NaF (684.6 eV) than FeF(3) (685.4 eV). Overall, these findings motivate further refinements of existing thermodynamic adsorption models for predicting the ionic composition of hematite particle surfaces contacted with sodium halide aqueous solutions.