Preferential adsorption of selenium oxyanions onto {1 1 0} and {0 1 2} nano-hematite facets

As the commercial use of nano metal oxides, including iron oxides, becomes more prevalent, there is a need to understand functionality as it relates to the inherent properties of the nanomaterial. Many applications of nanomaterials rely on adsorption, ranging from catalysis to aqueous remediation. In this paper, adsorption of selenium (Se), an aqueous contaminant, is used as a model sorbate to elucidate the relationships of structure, property, and (adsorptive) function of nano-hematite (nα-Fe₂O₃). As such, six nα-Fe₂O₃ particles were synthesized controlling for size, shape and surface area without capping agents. Sorbent characteristics of the six particles were then assessed for their impact on selenite (HSeO₃^-) and selenate (SeO₄^2-) adsorption capacity and mechanism. Mechanism was assessed using in-situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine edge spectroscopy (EXAFS). Regression analyses were then performed to determine which characteristics best describe adsorption capacity and binding mechanisms of Se on nα-Fe₂O₃. The results demonstrate that crystal surface structure, specifically presence of the {0 1 2} facet promotes adsorption of Se and the presence of {0 1 2} facets promotes SeO₄^2- sorption to a greater extent than HSeO₃^-. The data further indicates that {1 1 0} facets bind HSeO₃^- with binuclear complexes while {0 1 2} facets bind HSeO₃^- via mononuclear inner-sphere complexes. Specific nα-Fe₂O₃ facets also likely direct the ratio of inner to outer-sphere complexes in SeO₄^2- adsorption.