Chromate mobility, reactivity, and bioavailability in soil environments are affected by adsorption reactions on iron oxide minerals, but the adsorption mechanisms remain controversial. In this study, we employed in situ attenuated total reflectance Fourier transform infrared spectroscopy and theoretical frequency calculations to characterize chromate adsorption on 2-line ferrihydrite. The effects of pH, aqueous chromate concentration, ionic strength, and deuterium exchange were investigated. Results suggest the formation of monodentate and bidentate surface complexes. It was determined that monodentate complexes are dominant at low surface coverage and pH ≥ 6.5 and that bidentate complexes form at high surface coverage and pH < 6. Deuterium exchange experiments indicated that the inner-sphere complexes are not protonated. Difference spectra revealed that monodentate complexes are particularly susceptible to ionic strength effects under acidic conditions.

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.

No abstract

No abstract

No abstract