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Naturally occurring semiconducting metal (oxyhydr)oxides (MeOx) are ubiquitous geosorbents and geocatalysts, regulating the geochemical behaviors of the chemicals in the environment. The interactions between these MeOx and redox-sensitive elements may involve both the photo-catalytic redox reactions and surface adsorption, and the interplay of the two processes, therefore, makes the interactions rather complex and may have unique environmental effects. The complex interactions between Cr (Cr(III) or Cr(VI)) and hematite under simulated sunlight irradiation were systematically studied under different geochemical conditions (pH, light irradiation, beginning Cr species, etc.). The batch experimentals, X-ray photoelectron spectroscopy (XPS), transmitted electron microscopy (TEM), and energy-dispersive spectroscopy (EDS) were used to determine the concentration and distribution of Cr species (Cr(III), Cr(VI), or the sum of the two) in the aqueous and solid phases. Here we found that both the oxidation of Cr(III) and the reduction of Cr(VI) were accelerated by hematite under sunlight irradiation, resulting in the enhanced immobilization of Cr on hematite. The reduction and oxidation efficiencies were pH-dependent and more evident under acidic condition. In the Cr (III) reaction systems, the reactions can be characterized by (1) the adsorption of aqueous Cr(III) (Cr(III)aq) to hematite surfaces, (2) the oxidation of adsorbed Cr(III) (Cr(III)ad) to Cr(VI)ad by the photogenerated holes on hematite, i.e., heterogeneous oxidation, (Bargar et al., 1997) the accumulation of Cr(III)ad on hematite enhanced by Cr(VI)ad, and even the formation of Cr-riched precipitation, which was observed by TEM, and (3) the homogeneous oxidation of Cr(III)aq to Cr(VI)aq by the hydroxyl radicals, generating through interaction between O2 and the photogenerated electrons (more evident under acidic environment, e.g., pH 3). In the Cr(VI) reaction systems, the reduction of Cr(VI)ad by photogenerated electrons from hematite was also noticeable, with the yielded Cr(III) being fixed on the surfaces. Although the reactions in reduction systems were less extensive than those in the oxidation systems, the reduction reactions would reduce the environmental toxicity of Cr species; while in the oxidation systems, inspite of the higher removal efficiency, the oxidation processes actually increased the environmental risk of Cr. Our results clearly show that hematite could either oxidize Cr(III) or reduce Cr(VI), influencing the speciation and immobilization of Cr under sunlight irradiation. In this term, the natural semiconducting MeOx (e.g., hematite) on Earth's surfaces should play a significant role in affecting the environmental behaviors and toxicities of the redox-sensitive elements (e.g., Cr) through adsorption and photo-catalytic redox processes.
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