Facet-Dependent Cr(VI) Adsorption of Hematite Nanocrystals

In this study, the adsorption process of Cr(VI) on the hematite facets was systematically investigated with synchrotron-based Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, density-functional theory calculation, and surface complexation models. Structural model fitting of EXAFS spectroscopy suggested that the interatomic distances of Cr Fe were, respectively, 3.61 angstrom for the chromate coordinated hematite nanoplates with exposed {001} facets, 3.60 and 3.30 angstrom for the chromate coordinated hematite nanorods with exposed {001} and {110} facets, which were characteristic of inner-sphere complexation. In situ ATR-FTIR spectroscopy analysis confirmed the presence of two inner-sphere surface complexes with C-3 and C2,, symmetry, while the C3 and C2,, species were assigned to monodentate and bidentate inner-sphere surface complexes with average Cr Fe interatomic distances of 3.60 and 3.30 A, respectively. On the basis of these experimental and theoretical results, we concluded that HCr04 as dominated Cr(VI) species was adsorbed on {001} and {11O} facets in inner-sphere monodentate mononuclear and bidentate binuclear configurations, respectively. Moreover, the Cr(VI) adsorption performance of hematite facets was strongly dependent on the chromate complexes formed on the hematite facets.