Investigation of Chromate Coordination on Ferrihydrite by in Situ ATR-FTIR Spectroscopy and Theoretical Frequency Calculations

Abstract

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.

Surface complexation modeling of Cr(VI) adsorption at the goethite–water interface

“…Since the net charge of bidentate complex is neutral, the ionic strength mainly affects the contribution of the monodentate complex: higher adsorption of monodentate complex with lower ionic strength is observed. Similar observation was reported for Cr(VI) adsorption on ferrihydrite analyzed by in situ ATR-FTIR spectroscopy, monodentate complex was particularly susceptible to ionic strength [20]. It is mainly because that most of surface charges of "FeOCrO 3 À3/2 is located at the outer plane, then the adsorption of "FeOCrO 3 À3/2 somehow behaves like an outer-sphere complex.…”

“…However, most of the surface reactions were derived directly from macroscopic adsorption data without considering spectroscopic evidence. Previous spectroscopic analysis showed direct evidence that Cr(VI) forms inner-sphere complexes on ferric oxide surfaces [18][19][20]. For example, Fendorf et al [18] found both monodentate and bidentate complexes surface complex of Cr(VI) on goethite surfaces using extended X-ray absorption fine structure (EXAFS) spectroscopy.…”

“…They also found at low surface coverage, the monodentate complex is favored while at high surface coverage, the bidentate is more prevalent. Johnston and Chrysochoou [20] also found the formation of monodentate and bidentate surface complexes in the adsorption process of Cr(VI) on ferrihydrite using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Weerasooriya and Tobschall [13] proposed a Triple Plane Model (TPM) to predict the chromate adsorption onto goethite based on the spectroscopic observation.…”

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…outer-sphere) bound to the surface via hydrogen bonding. In a more recent study, chromate sorption to ferrihydrite was probed using this combined experimental and theoretical approach (Johnston and Chrysochoou, 2012). The authors conclude that at pH greater than pH 6.5 monodentate complexes are prevalent, whereas bidentate surface species are observed at pH < 6.…”

Chromate adsorption on selected soil minerals: Surface complexation modeling coupled with spectroscopic investigation

“…Different techniques have been previously tested for the identification of Cr(VI) complexes on Fe or Al oxyhydroxide surfaces [22][23][24][25]. In addition, the redox transformation of Cr(VI) during adsorption onto environmentally relevant surfaces, including oxides and clay minerals have been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS) [22,[26][27][28]. Therefore, studies using coupled modeling and spectroscopic approaches to soil mineral surfaces would provide valuable information.…”