Sulfate Local Coordination Environment in Schwertmannite

Schwertmannite, a nanocrystalline ferric oxyhydroxy-sulfate mineral, plays an important role in many environmental geochemical processes in acidic sulfate-rich environments. The sulfate coordination environment in schwertmannite, however, remains unclear, hindering our understanding of the structure, formation, and environmental behavior of the mineral. In this study, sulfur K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses in combination with infrared spectroscopy were used to determine the sulfate local atomic environment in wet and air-dried schwertmannite samples after incubation at various pHs and ionic strengths. Results indicate that sulfate exists as both inner- and outer-sphere complexes in schwertmannite. Regardless of the sample preparation conditions, the EXAFS-determined S–Fe interatomic distances are 3.22–3.26 Å, indicative of bidentate-binuclear sulfate inner-sphere complexes. XANES spectroscopy shows that the proportion of the inner-sphere complexes decreases with increasing pH for both wet and dried samples and that the dried samples contain much more inner-sphere complexes than the wet ones at any given pH. Assuming that schwertmannite is a distorted akaganéite-like structure, the sulfate inner-sphere complexation suggests that, the double chains of the edge-sharing Fe octahedra, enclosing the tunnel, must contain defects, on which reactive singly-Fe coordinated hydroxyl functional groups form for ligand exchange with sulfate. The drying effect suggests that the tunnel contains readily exchangeable H₂O molecules in addition to sulfate ions

Solid-State NMR Spectroscopic Study of Phosphate Sorption Mechanisms on Aluminum (Hydr)oxides

Sorption reactions occurring at mineral/water interfaces are of fundamental importance in controlling the sequestration and bioavailability of nutrients and pollutants in aqueous environments. To advance the understanding of sorption reactions, development of new methodology is required. In this study, we applied novel ³¹P solid-state nuclear magnetic resonance (NMR) spectroscopy to investigate the mechanism of phosphate sorption on aluminum hydroxides under different environmental conditions, including pH (4–10), concentration (0.1–10 mM), ionic strength (0.001–0.5 M), and reaction time (15 min–22 days). Under these conditions, the NMR results suggest formation of bidentate binuclear inner-sphere surface complexes was the dominant mechanism. However, it was found that surface wetting caused a small difference. A small amount (<3%) of monodentate mononuclear inner-sphere surface complexes was observed in addition to the majority of bidentate binuclear surface complexes on a wet paste sample prepared at pH 5, which was analyzed in situ by a double-resonance NMR technique, namely, ³¹P­{²⁷Al} rotational echo adiabatic passage double resonance (REAPDOR). Additionally, we found that adsorbents can substantially impact phosphate sorption not only on the macroscopic sorption capacity but also on their ³¹P NMR spectra. Very similar NMR peaks were observed for phosphate sorbed to gibbsite and bayerite, whereas the spectra for phosphate adsorbed to boehmite, corundum, and γ-alumina were significantly different. All of these measurements reveal that NMR spectroscopy is a useful analytical tool for studying phosphorus chemistry at environmental interfaces

Quantification of Coexisting Inner- and Outer-Sphere Complexation of Sulfate on Hematite Surfaces

Sulfate adsorption on hematite surfaces controls sulfate mobility and environmental behavior but whether sulfate forms both inner- and outer-sphere complexes and the type of the inner-sphere complexes remain contentious. With ionic strength tests and S K-edge X-ray absorption near-edge structure spectroscopy, we show that sulfate forms both outer- and inner-sphere complexes on hematite surfaces. Both S K-edge extended X-ray absorption fine structure spectroscopy and the differential pair distribution function analyses determine the S–Fe interatomic distance (∼3.24 Å) of the inner-sphere complex, suggesting bidentate-binuclear complexation. A multivariate curve resolution (MCR) analysis of the attenuated total reflection–Fourier-transform infrared spectra of adsorption envelope samples shows that increasing ionic strength does not affect the inner-sphere but decreases the outer-sphere complex adsorption loading, consistent with the ionic strength effect. The extended triple layer model directly and successfully models the MCR-derived inner- and outer-sphere surface loadings at various ionic strengths, indicating weaker sulfate inner-sphere complexation on hematite than on ferrihydrite surfaces. Results also show that sample drying, lower pH, and higher ionic strength all favor sulfate inner-sphere complexation, but the hematite particle size does not affect the relative proportions of the two types of complexes. Sulfate adsorption kinetics show increasing ratio of exchanged OH^– to adsorbed sulfate with time, attributed to inner- and outer-sphere complexation dominating at different adsorption stages and to the changes of the relative abundance of surface OH^– and H₂O groups with time. This work clarifies sulfate adsorption mechanisms on hematite and has implications for understanding sulfate availability, behavior and fate in the environment. Our work suggests that the simple macroscopic ionic strength test correlates well with directly measured outer-sphere complexes

Effect of Ferrihydrite Crystallite Size on Phosphate Adsorption Reactivity

The influence of crystallite size on the adsorption reactivity of phosphate on 2-line to 6-line ferrihydrites was investigated by combining adsorption experiments, structure and surface analysis, and spectroscopic analysis. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the ferrihydrite samples possessed a similar fundamental structure with a crystallite size varying from 1.6 to 4.4 nm. N₂ adsorption on freeze-dried samples revealed that the specific surface area (SSA_BET) decreased from 427 to 234 m² g^–1 with increasing crystallite size and micropore volume (<i>V</i>_micro) from 0.137 to 0.079 cm³ g^–1. Proton adsorption (<i>Q</i>_H) at pH 4.5 and 0.01 M KCl ranged from 0.73 to 0.55 mmol g^–1. Phosphate adsorption capacity at pH 4.5 and 0.01 M KCl for the ferrihydrites decreased from 1690 to 980 μmol g^–1 as crystallite size increased, while the adsorption density normalized to SSA_BET was similar. Phosphate adsorption on the ferrihydrites exhibited similar behavior with respect to both kinetics and the adsorption mechanism. The kinetics could be divided into three successive first-order stages: relatively fast adsorption, slow adsorption, and a very slow stage. With decreasing crystallite size, ferrihydrites exhibited increasing rate constants per mass for all stages. Analysis of OH^– release and attenuated total reflectance infrared spectroscopy (ATR-IR) and differential pair distribution function (d-PDF) results indicated that initially phosphate preferentially bound to two Fe–OH₂^1/2+ groups to form a binuclear bidentate surface complex without OH^– release, with smaller size ferrihydrites exchanging more Fe–OH₂^1/2+ per mass. Subsequently, phosphate exchanged with both Fe–OH₂^1/2+ and Fe–OH^1/2– with a constant amount of OH^– released per phosphate adsorbed. Also in this stage binuclear bidentate surface complexes were formed with a P–Fe atomic pair distance of ∼3.25 Å

Mechanism of Myo-inositol Hexakisphosphate Sorption on Amorphous Aluminum Hydroxide: Spectroscopic Evidence for Rapid Surface Precipitation

Inositol hexakisphosphates are the most abundant organic phosphates (OPs) in most soils and sediments. Adsorption, desorption, and precipitation reactions at environmental interfaces govern the reactivity, speciation, mobility, and bioavailability of inositol hexakisphosphates in terrestrial and aquatic environments. However, surface complexation and precipitation reactions of inositol hexakisphosphates on soil minerals have not been well understood. Here we investigate the surface complexation–precipitation process and mechanism of myo-inositol hexakisphosphate (IHP, phytate) on amorphous aluminum hydroxide (AAH) using macroscopic sorption experiments and multiple spectroscopic tools. The AAH (16.01 μmol m^–2) exhibits much higher sorption density than boehmite (0.73 μmol m^–2) and α-Al₂O₃ (1.13 μmol m^–2). Kinetics of IHP sorption and accompanying OH^– release, as well as zeta potential measurements, indicate that IHP is initially adsorbed on AAH through inner-sphere complexation via ligand exchange, followed by AAH dissolution and ternary complex formation; last, the ternary complexes rapidly transform to surface precipitates and bulk phase analogous to aluminum phytate (Al-IHP). The pH level, reaction time, and initial IHP loading evidently affect the interaction of IHP on AAH. In situ ATR-FTIR and solid-state NMR spectra further demonstrate that IHP sorbs on AAH and transforms to surface precipitates analogous to Al-IHP, consistent with the results of XRD analysis. This study indicates that active metal oxides such as AAH strongly mediate the speciation and behavior of IHP via rapid surface complexation–precipitation reactions, thus controlling the mobility and bioavailability of inositol phosphates in the environment

Binding Geometries of Silicate Species on Ferrihydrite Surfaces

Silicate sorption on ferrihydrite surfaces, as monomers, oligomers, and polymers, strongly affects ferrihydrite crystallinity, thermodynamic stability, and surface reactivity. How these silicate species bind on ferrihydrite surfaces is, however, not well understood. We have determined silicate binding geometries using a combination of X-ray absorption spectroscopy (XAS), differential atomic pair distribution function (d-PDF) analysis, and density functional theory (DFT) calculations. Silicon K-edge absorption pre-edges and DFT-predicted energies indicate that silicate forms monomeric monodentate–mononuclear (MM) complexes at low silicate sorption loadings. With increasing silicate loading, the pre-edge peak shifts to higher energies, suggesting changes in the silicate binding geometry toward multidentate complexation. The d-PDF analysis determines the Si–Fe interatomic distance to be ∼3.25 Å for the high-loading samples. The DFT calculations indicate that such distance corresponds to an oligomer in the bidentate–binuclear (BB) binding geometry. The transition of the silicate sorption geometry accompanied by polymerization can affect stability of ferrihydrite and its adsorption and redox reactivity and increase the degree of Si isotopic fractionation upon silicate sorption on Fe oxides. MM monomeric complexes and BB oligomeric complexes should be used for surface complexation models predicting silicate sorption on Fe oxide surfaces

Enhanced Dissolution and Transformation of ZnO Nanoparticles: The Role of Inositol Hexakisphosphate

The toxicity, reactivity, and behavior of zinc oxide (ZnO) nanoparticles (NPs) released in the environment are highly dependent on environmental conditions. <i>Myo</i>-inositol hexakisphosphate (IHP), a common organic phosphate, may interact with NPs and generate new transformation products. In this study, the role of IHP in mediating the dissolution and transformation of ZnO NPs was investigated in the laboratory kinetic experiments using powder X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, ³¹P nuclear magnetic resonance spectroscopy, high-resolution transmission electronic microscopy, and synchrotron-based extended X-ray absorption fine structure spectroscopy. The results indicate that IHP shows a dissolution–precipitation effect, which is different from citrate and EDTA that only enhances Zn dissolution. The enhanced dissolution and transformation of ZnO NPs by IHP (<0.5 h) is found to be strikingly faster than that induced by inorganic phosphate (Pi, > 3.0 h) at pH 7.0, and the reaction rate increases with decreasing pH and increasing IHP concentration. Multitechnique analyses reveal that interaction of ZnO NPs with IHP induces rapid transformation of ZnO NPs into zinc phytate complexes initially and poorly crystalline zinc phytate-like (Zn–IHP) phase finally. Additionally, ZnO NPs preferentially react with IHP and transform to Zn–IHP when Pi and IHP concurrently coexist in a system. Overall, results from this study contribute to an improved understanding of the role of organic phosphates (e.g., IHP) in the speciation and structural transformation of ZnO NPs, which can be leveraged for remediation of ZnO-polluted water and soils

Effects of <i>Myo</i>-inositol Hexakisphosphate on Zn(II) Sorption on γ‑Alumina: A Mechanistic Study

<i>Myo</i>-inositol hexakisphosphate (IHP), a most common organic phosphorus in many soils, can strongly interact with aluminum (Al) oxides and influence the fate of metal ions. In this study, the effects of presorbed IHP on γ-Al₂O₃ (γ-alumina) surfaces on Zn­(II) sorption were investigated in batch experiments using a combination of powder X-ray diffraction (XRD), <i>in situ</i> attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), ³¹P and ²⁷Al solid-state nuclear magnetic resonance spectroscopies (NMR), and Zn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). The results of the batch experiments show that the presorption of IHP increases the sorption density of Zn­(II) on γ-Al₂O₃ surfaces. The XRD data indicate that the presorption of IHP hinders the formation of Zn–Al layered double hydroxide (LDH) precipitates by raising the critical concentration of Zn­(II) required to precipitate the complex. Solid-state NMR spectra further suggest that the chemical environment and speciation of IHP presorbed change, i.e., from inner-sphere surface complexes to ternary surface complexes and to zinc phytate precipitates (Zn-IHP) with the increase in Zn­(II) concentration or pH. Linear combination fittings (LCFs) of the EXAFS spectra indicate that the proportion of Zn­(II) in binary or ternary surface complexes decreases and that in Zn–Al LDH increases with increasing concentration of Zn­(II) at pH 7. Furthermore, the order at which IHP and Zn are added in the reaction can influence the cosorption mechanism. At pH 7, more binary or ternary Zn surface complexes and Zn-IHP form, and less Zn–Al LDH precipitates form if Zn is added first. These results demonstrate that both the timing and concentration of IHP and divalent metals have sweeping influences on their solubility and speciation and these intricacies need to be taken into consideration toward predicting their fates in the environment

Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry

Birnessite, a phyllomanganate and the most common type of Mn oxide, affects the fate and transport of numerous contaminants and nutrients in nature. Birnessite exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two types of birnessite contain contrasting content of layer vacancies and Mn­(III), and accordingly have different sorption and oxidation abilities. OrthLayBir can transform to HexLayBir, but it is still vaguely understood if and how the reverse transformation occurs. Here, we show that HexLayBir (e.g., δ-MnO₂ and acid birnessite) transforms to OrthLayBir after reaction with aqueous Mn­(II) at low Mn­(II)/Mn (in HexLayBir) molar ratios (5–24%) and pH ≥ 8. The transformation is promoted by higher pH values, as well as smaller particle size, and/or greater stacking disorder of HexLayBir. The transformation is ascribed to Mn­(III) formation via the comproportionation reaction between Mn­(II) adsorbed on vacant sites and the surrounding layer Mn­(IV), and the subsequent migration of the Mn­(III) into the vacancies with an ordered distribution in the birnessite layers. This study indicates that aqueous Mn­(II) and pH are critical environmental factors controlling birnessite layer structure and reactivity in the environment