title: Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al-Substituted Ferrihydrite: Implications for Ferrihydrite Surface Structure and Reactivity.
abstract: Ferrihydrite is a nanocrystalline Fe (hydr)oxide and important sink for environmental contaminants. Although Fe (hydr)oxides are rarely pure in natural systems, little is known about the effects of structural impurities such as Al on the surface properties and reactivity of ferrihydrite. In this study, we characterized the adsorption mechanisms of chromate, selenate, and sulfate on Al-substituted ferrihydrite (0, 6, 12, 18, and 24 mol % Al) using in situ attenuated total reflection Fourier transform infrared spectroscopy. Spectral data sets recorded as a function of pH were processed using a multivariate curve resolution technique to identify which types of surface species form and to generate their concentration profiles as a function of pH and Al content. Results show a significant increase in relative fraction of outer-sphere complexes for all three oxyanions with increasing Al substitution. In addition, the effect of Al substitution is found to be mechanism-specific in the case of chromate, with bidentate complexes disproportionately suppressed over monodentate complexes at higher Al contents. Overall, our findings have important implications for the fate of chromate, selenate, and sulfate in subsurface environments and offer new insight into the surface reactivity of Al-ferrihydrite.
authors.affiliation: Engineering Science Program, Loyola University Chicago , Chicago, Illinois 60660, United States.
authors.forename: Chad P
authors.initials: CP
authors.lastname: Johnston
authors.affiliation: Department of Civil and Environmental Engineering, University of Connecticut , Storrs, Connecticut 06269, United States.
authors.forename: Maria
authors.initials: M
authors.lastname: Chrysochoou
mesh_terms: D000327:Adsorption; D000535:Aluminum / Q000737:chemistry*; D002840:Chromates / Q000302:isolation & purification*; D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D015999:Multivariate Analysis; D064586:Selenic Acid / Q000302:isolation & purification*; D017550:Spectroscopy, Fourier Transform Infrared; D013431:Sulfates / Q000302:isolation & purification*; D013499:Surface Properties
chemical_list: D002840:Chromates; D005290:Ferric Compounds; D013431:Sulfates; C092844:ferric oxyhydroxide; D000535:Aluminum; D064586:Selenic Acid