Coupled redox transformation of chromate and arsenite on ferrihydrite.

The redox chemistry of chromate (Cr(VI)) and arsenite (As(III)) on the iron oxyhydroxide, ferrihydrite (Fh), was investigated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the adsorbed layer on Fh during and after exposure to solution-phase Cr(VI) and As(III). The individual exposure of Cr(VI) or As(III) on Fh resulted in the adsorption of the respective species, and there was no change in the oxidation state of either species. In contrast, exposure of Fh simultaneously to Cr(VI) and As(III) led to an adsorbed layer that was primarily Cr(III) and As(V). This redox transformation occurred over various experimental conditions at pH 3, 5, and 7 and in the presence or absence of O2, as demonstrated by in situ ATR-FTIR results. A similar redox transformation was not observed at a solution of pH 9, due to minimal Cr(VI) adsorption. Postreaction XPS showed that the majority of adsorbed arsenic existed as As(V) at pH 3, 5, and 7, while As(III) was the main species detected at pH 9. At pH 3 the redox chemistry between Cr(VI) and As(III) led to a As(V) product surface loading of ∼600 mmol/kg. Experiments performed in the absence of dissolved O2 resulted in less As(V) on the surface compared to experiments in which O2 was present for equivalent reaction times.

Photoinduced oxidation of arsenite to arsenate in the presence of goethite.

The photochemistry of an aqueous suspension of goethite in the presence of arsenite (As(III)) was investigated with X-ray absorption near edge structure (XANES) spectroscopy and solution-phase analysis. Irradiation of the arsenite/goethite under conditions where dissolved oxygen was present in solution led to the presence of arsenate (As(V)) product adsorbed on goethite and in solution. Under anoxic conditions (absence of dissolved oxygen), As(III) oxidation occurred, but the As(V) product was largely restricted to the goethite surface. In this circumstance, however, there was a significant amount of ferrous iron release, in stark contrast to the As(III) oxidation reaction in the presence of dissolved oxygen. Results suggested that in the oxic environment ferrous iron, which formed via the photoinduced oxidation of As(III) in the presence of goethite, was heterogeneously oxidized to ferric iron by dissolved oxygen. It is likely that aqueous reactive oxygen species formed during this process led to the further oxidation of As(III) in solution. Results from the current study for As(III)/goethite also were compared to results from a prior study of the photochemistry of As(III) in the presence of another iron oxyhydroxide, ferrihydrite. The comparison showed that at pH 5 and 2 h of light exposure the instantaneous rate of aqueous-phase As(V) formation in the presence of goethite (12.4 × 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 × 10(-6) M s(-1) m(-2)). It was proposed that this increased rate of ferrous iron oxidation in the presence of goethite and dissolved oxygen was the primary reason for the higher As(III) oxidation rate when compared to the As(III)/ferrihydrite system. The surface area-normalized pseudo-first-order rate constant, for example, associated with the heterogeneous oxidation of Fe(II) by dissolved oxygen in the presence of goethite (1.9 × 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 × 10(-7) L s(-1) m(-2)) at a solution pH of 5.

Photoinduced oxidation of arsenite to arsenate on ferrihydrite.

The photochemistry of an aqueous suspension of the iron oxyhydroxide, ferrihydrite, in the presence of arsenite has been investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption near edge structure (XANES), and solution phase analysis. Both ATR-FTIR and XANES show that the exposure of ferrihydrite to arsenite in the dark leads to no change in the As oxidation state, but the exposure of this arsenite-bearing surface, which is in contact with pH 5 water, to light leads to the conversion of the majority of the adsorbed arsenite to the As(V) bearing species, arsenate. Analysis of the solution phase shows that ferrous iron is released into solution during the oxidation of arsenite. The photochemical reaction, however, shows the characteristics of a self-terminating reaction in that there is a significant suppression of this redox chemistry before 10% of the total iron making up the ferrihydrite partitions into solution as ferrous iron. The self-terminating behavior exhibited by this photochemical arsenite/ferrihydrite system is likely due to the passivation of the ferrihydrite surface by the strongly bound arsenate product.

Neutron pair distribution function study of two-line ferrihydrite.

Pair distribution function (PDF) analysis of neutron total scattering data from deuterated two-line ferrihydrite is consistent with the Keggin-related structural model for ferrihydrite published by Michel et al. (2007). Other models proposed in the literature, such as that of Drits et al. (1993), lead to inferior fits. Bond valence sums indicate that O(1) is bonded to a hydrogen atom, but the quality of the data is such that the exact position of the hydrogen could not be elucidated with confidence.

Photodissolution of ferrihydrite in the presence of oxalic acid: an in situ ATR-FTIR/DFT study.

The photodissolution of the iron oxyhydroxide, ferrihydrite, in the presence of oxalic acid was investigated with vibrational spectroscopy, density functional theory (DFT) calculations, and batch geochemical techniques that determined the composition of the solution phase during the dissolution process. Specifically, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR- FTIR) was used to determine the structure of the adsorbed layer during the dissolution process at a solution pH of 4.5. DFT based computations were used to interpret the vibrational data associated with the surface monolayer in order to help determine the structure of the adsorbed complexes. Results showed that at pH 4.5, oxalate adsorbed on ferrihydrite adopted a mononuclear bidentate (MNBD) binding geometry. Photodissolution at pH 4.5 exhibited an induction period where the rate of Fe(II) release was limited by a low concentration of adsorbed oxalate due to the site-blocking of carbonate that was intrinsic to the surface of the ferrihydrite starting material. Oxalate displaced this initial carbonate over time, and the dissolution rate showed a corresponding increase. Irradiation of oxalate/ferrihydrite at pH 4.5 also ultimately led to the appearance of carbonate reaction product (distinct from carbonate intrinsic to the starting material) on the surface.

Investigation of surface structures by powder diffraction: a differential pair distribution function study on arsenate sorption on ferrihydrite.

Differential pair distribution function (d-PDF) analysis of high energy powder X-ray diffraction data was carried out on 2-line ferrihydrite nanoparticles with arsenate oxyanions adsorbed on the surface to investigate the binding mechanism. In this analysis, a PDF of ferrihydrite is subtracted from a PDF of ferrihydrite with arsenate sorbed on the surface, leaving only correlations from within the surface layer and between the surface and the particle. As-O and As-Fe correlations were observed at 1.68 and 3.29 A, respectively, in good agreement with previously published EXAFS data, confirming a bidentate binuclear binding mechanism. Further peaks are observed in the d-PDF which are not present in EXAFS, corresponding to correlations between As and O in the particle and As-2nd Fe.

Adsorption of carbon dioxide on Al/Fe oxyhydroxide.

The structure and reactivity of 0-70mol% Al/Fe iron oxyhydroxides (ferrihydrite in the absence and presence of Al) toward gaseous CO2 were investigated with X-ray photoelectron spectroscopy (XPS), atomic absorption (AA), scanning transmission electron microscopy with electron dispersive X-ray spectroscopy (STEM/EDS), X-ray diffraction (XRD), and attenuated total reflectance Fourier transform Infrared spectroscopy (ATR-FTIR) combined with density functional theory (DFT) calculations. Results showed that Al/Fe oxyhydroxide particles containing more than 20 mol% Al consisted at least in part of Fe-oxyhydroxide with incorporated Al and a discrete AlOOH phase. Results from ATR-FTIR experiments and DFT calculations suggested that the bicarbonate complex formed by passing CO2 over the particles was accommodated on at least three distinct binding sites. At the lowest Al concentrations bicarbonate was bound to individual sites with primarily Fe or Al character. At the highest concentrations of Al (>20 mol%) bicarbonate bound to discrete AlOOH phases became apparent. Results also suggested that the amount of CO2 adsorption for a given particle mass increased as the Al concentration was increased from 0 to 30%. This increase was likely due in large part to differences in the morphology of the particle aggregates that formed in the dry state, which would be expected to affect the amount of surface that was available to adsorb CO2.

Ferrihydrite reactivity toward carbon dioxide.

The reaction of ferrihydrite with gaseous CO(2) was investigated with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculations. ATR-FTIR results show that CO(2) reacts with ferrihydrite resulting in surface adsorbed carbonate species. The carbonate species experimentally observed in view of theoretical calculations are shown to be in large part monodentate binuclear complexes. These carbonate complexes exist as both inner-sphere and outer-sphere hydrogen-bonded complexes. Under "dry" conditions CO(2) reacts with free OH sites on the ferrihydrite surface resulting in a metastable bent CO(2) (bicarbonate-like) complex. Removal of the gaseous reactant leads to the loss of this metastable surface complex. The reaction of CO(2) with hydrated ferrihydrite results in only carbonate formation (no bicarbonate). In this circumstance, experiments and theoretical calculations suggest that hydrogen bound water on surface OH sites prevents the formation of the metastable bicarbonate species. Ferrihydrite that was allowed to react with atmospheric levels of CO(2) and water vapor resulted in the formation of surface carbonate coordinated as both inner and outer-sphere complexes.