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.

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Nano metal oxides are of interest for aqueous selenium (Se) remediation, and as such, nano-hematite (nα-Fe2O3) was examined for use as a Se adsorbent. The effect of surface area on adsorption was also studied. nα-Fe2O3 particles were synthesized from Fe(NO3)3 and FeCl3 via forced hydrolysis. The resulting particles have similar sizes, morphologies, aggregate size, pore size, and PZC. The nα-Fe2O3 from FeCl3 (nα-Fe2O3-C) differs from the nα-Fe2O3 from Fe(NO3)3 (nα-Fe2O3-N) with a ∼25±2m(2)/g greater surface area. Selenite Se(IV) adsorption capacity on nα-Fe2O3 has a qmax ∼17mg/g for the freeze-dried and re-suspended nα-Fe2O3. The Δqmax for nα-Fe2O3 from Fe(NO3)3 and FeCl3 that remained in suspension was 4.6mg/g. For selenate Se(VI), the freeze-dried and re-suspended particles realize a Δqmax= 1.5mg/g for nα-Fe2O3 from Fe(NO3)3 and FeCl3. The nα-Fe2O3 from Fe(NO3)3 and FeCl3 that remained in suspension demonstrated Se(VI) Δqmax=5.4mg/g. In situ ATR-FTIR isotherm measurements completed for Se(VI) at a pH 6 suggest that Se(VI) forms primarily outer-sphere complexes with nα-Fe2O3 synthesized from both salts.

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Calcium polysulfide (CPS) has been observed to maintain a reducing capacity for prolonged time periods when used to treat Cr(VI)-contaminated soils. This study utilized bulk and micro-X-ray absorption near edge structure (XANES) spectroscopy to investigate sulfur speciation in soil samples treated with CPS in batch and column studies and to determine the source of the reducing potential. Bulk XANES spectra indicated the presence of two dominant sulfur species: elemental sulfur, which is the product of the sulfide-chromate redox reaction, and thiosulfate (S2O3(2-)). Micro-XANES analyses confirmed these findings and showed that elemental sulfur precipitated as large particles, while thiosulfate was diffused within the soil grains and thus available to react with chromate that leached from slowly dissolving PbCrO4. Micro-X-ray fluorescence (μXRF) analyses indicated a close association of Pb and thiosulfate, so that PbS2O3 is a likely sink for thiosulfate, accounting for up to 20% of the total S added. Sorption of thiosulfate on iron oxides below pH 8 is a second retention mechanism for thiosulfate in the solid. Given that thiosulfate cannot reduce chromate but can reduce solid-bound Fe(III) under neutral pH conditions, it is hypothesized that ferrous iron production is an additional mechanism to maintain reductive conditions in CPS-treated soils.