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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 Å for the chromate coordinated hematite nanoplates with exposed 001 facets, 3. 60 and 3. 30 Å 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 C3ν and C2ν symmetry, while the C3ν and C2ν species were assigned to monodentate and bidentate inner-sphere surface complexes with average Cr-Fe interatomic distances of 3. 60 and 3. 30 Å, respectively. On the basis of these experimental and theoretical results, we concluded that HCrO4 (-) as dominated Cr (VI) species was adsorbed on 001 and 110 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.
Huang et al. (Wed,) studied this question.