Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions

The structure and Mn­(III) concentration of birnessite dictate its reactivity and can be changed by birnessite partial reduction, but effects of pH and reductant/birnessite ratios on the changes by reduction remain unclear. We found that the two factors strongly affect the structure of birnessite (δ-MnO₂) and its Mn­(III) content during its reduction by fulvic acid (FA) at pH 4–8 and FA/solid mass ratios of 0.01–10 under anoxic conditions over 600 h. During the reduction, the structure of δ-MnO₂ is increasingly accumulated with both Mn­(III) and Mn­(II) but much more with Mn­(III) at pH 8, whereas the accumulated Mn is mainly Mn­(II) with little Mn­(III) at pH 4 and 6. Mn­(III) accumulation, either in layers or over vacancies, is stronger at higher FA/solid ratios. At FA/solid ratios ≥1 and pH 6 and 8, additional hausmannite and MnOOH phases form. The altered birnessite favorably adsorbs FA because of the structural accumulation of Mn­(II, III). Like during microbially mediated oxidative precipitation of birnessite, the dynamic changes during its reduction are ascribed to the birnessite-Mn­(II) redox reactions. Our work suggests low reactivity of birnessite coexisting with organic matter and severe decline of its reactivity by partial reduction in alkaline environment

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

X‑ray Absorption Spectroscopic Quantification and Speciation Modeling of Sulfate Adsorption on Ferrihydrite Surfaces

Sulfate adsorption on mineral surfaces is an important environmental chemical process, but the structures and respective contribution of different adsorption complexes under various environmental conditions are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy, quantum chemical calculations, and surface complexation modeling (SCM), we have shown that sulfate forms both outer-sphere complexes and bidentate–binuclear inner-sphere complexes on ferrihydrite surfaces. The relative fractions of the complexes vary with pH, ionic strength (<i>I</i>), and sample hydration degree (wet versus air-dried), but their structures remained the same. The inner-sphere complex adsorption loading decreases with increasing pH while remaining unchanged with <i>I</i>. At both <i>I</i> = 0.02 and 0.1 M, the outer-sphere complex loading reaches maximum at pH ∼5 and then decreases with pH, whereas it monotonically decreases with pH at <i>I</i> = 0.5 M. These observations result from a combination of the ionic-strength effect, the pH dependence of anion adsorption, and the competition between inner- and outer-sphere complexation. Air-drying drastically converts the outer-sphere complexes to the inner-sphere complexes. The respective contributions to the overall adsorption loading of the two complexes were directly modeled with the extended triple layer SCM by implementing the bidentate–binuclear inner-sphere complexation identified in the present study. These findings improve our understanding of sulfate adsorption and its effects on other environmental chemical processes and have important implications for generalizing the adsorption behavior of anions forming both inner- and outer-sphere complexes on mineral surfaces

Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of Zn–Al LDH Formation on Al Oxide

While silicate has been known to affect metal sorption on mineral surfaces, the mechanisms remain poorly understood. We investigated the effects of silicate on Zn sorption onto Al oxide at pH 7.5 and elucidated the mechanisms using a combination of X-ray absorption fine structure (XAFS) spectroscopy, Zn stable isotope analysis, and scanning transmission electron microscopy (STEM). XAFS analysis revealed that Zn–Al layered double hydroxide (LDH) precipitates were formed in the absence of silicate or at low Si concentrations (≤0.4 mM), whereas the formation of Zn–Al LDH was inhibited at high silicate concentrations (≥0.64 mM) due to surface-induced Si oligomerization. Significant Zn isotope fractionation (Δ66Znsorbed‑aqueous = 0.63 ± 0.03‰) was determined at silicate concentrations ≥0.64 mM, larger than that induced by sorption of Zn on Al oxide (0.47 ± 0.03‰) but closer to that caused by Zn bonding to the surface of Si oxides (0.60–0.94‰), suggesting a presence of Zn–Si bonding environment. STEM showed that the sorbed silicates had a close spatial coupling with γ-Al2O3, indicating that >Si–Zn inner-sphere complexes (“>” denotes surface) likely bond to the γ-Al2O3 surface to form >Al–Si–Zn ternary inner-sphere complexes. This study not only demonstrates that dissolved silicate in the natural environment plays an important role in the fate and bioavailability of Zn but also highlights the potential of coupled spectroscopic and isotopic methods in probing complex environmental processes

Synthesis of Birnessite in the Presence of Phosphate, Silicate, or Sulfate

Layered manganese (Mn) oxides, such as birnessite, are versatile materials in industrial applications and common minerals mediating elemental cycling in natural environment. Many of birnessite properties are controlled by Mn­(III) concentration and particle sizes. Thus, it is important to synthesize birnessite nanoparticles with controlled Mn­(III) concentrations and sizes so that one can tune its properties for many applications. Birnessite was synthesized in the presence of oxyanions (phosphate, silicate, or sulfate) during reductive precipitation of KMnO₄ by HCl and characterized using multiple synchrotron X-ray techniques, electron microscopy, and diffuse reflectance UV–vis spectroscopy. Results indicate that all three anions decrease MnO₆ sheet sizes, attributed to oxyanion adsorption on edges of the sheets, inhibiting their lateral growth. As a result of decreased sizes, sheets undergo significant structural contraction. Meanwhile, Mn­(III) concentration significantly increases with increasing oxyanion/Mn ratio. The increased Mn­(III) concentration, along with the decreased size, enlarges both direct and indirect band gaps of birnessite. Phosphate imposes the strongest influence, followed by silicate and then by sulfate, consistent with their decreasing adsorption affinity. Reacting with 1 M KOH solution effectively removed the adsorbed oxyanions while leading to increased sheet sizes, probably due to oriented attachment-driven particle growth mechanisms. The results have important implications for developing highly performed birnessite materials, for example, small size Mn­(III)-rich birnessite for photochemical and catalytic applications, as well as for understanding chemical compositional variations of naturally occurring birnessite

Impacts of Ionic Strength on Three-Dimensional Nanoparticle Aggregate Structure and Consequences for Environmental Transport and Deposition

The transport of nanoparticles through aqueous systems is a complex process with important environmental policy ramifications. Ferrihydrite nanoparticles commonly form aggregates, with structures that depend upon solution chemistry. The impact of aggregation state on transport and deposition is not fully understood. In this study, small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) were used to directly observe the aggregate structure of ferrihydrite nanoparticles and show how the aggregate structure responds to changing ionic strength. These results were correlated with complementary studies on ferrihydrite transport through saturated quartz sand columns. Within deionized water, nanoparticles form stable suspensions of low-density fractal aggregates that are resistant to collapse. The particles subsequently show limited deposition on sand grain surfaces. Within sodium nitrate solutions the aggregates collapse into denser clusters, and nanoparticle deposition increases dramatically by forming thick, localized, and mechanically unstable deposits. Such deposits limit nanoparticle transport and make transport less predictable. The action of ionic strength is distinct from simpler models of colloidal stability and transport, in that salt not only drives aggregation or attachment but also alters the behavior of preexisting aggregates by triggering their collapse

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

Determination of the Three-Dimensional Structure of Ferrihydrite Nanoparticle Aggregates

Aggregation impacts the reactivity, colloidal stability, and transport behavior of nanomaterials, yet methods to characterize basic structural features of aggregates are limited. Here, cryo-transmission electron microscope (cryo-TEM) based tomography is utilized as a method for directly imaging fragile aggregates of nanoparticles in aqueous suspension and an approach for extracting quantitative fractal dimensions from the resulting three-dimensional structural models is introduced. The structural quantification approach is based upon the mass autocorrelation function, and is directly comparable with small-angle X-ray scattering (SAXS) models. This enables accurate characterization of aggregate structure, even in suspensions where the aggregate cluster size is highly polydisperse and traditional SAXS modeling is not reliable. This technique is applied to study real suspensions of ferrihydrite nanoparticles. By comparing tomographic measurements with SAXS-based measurements, we infer that certain suspensions contain polydisperse aggregate size distributions. In other suspensions, fractal-type structures are identified with low intrinsic fractal dimensions. The fractal dimensions are lower than would be predicted by simple models of particle aggregation, and this low dimensionality enables large, low-density aggregates to exist in stable colloidal suspension

Phosphorus Speciation and Solubility in Aeolian Dust Deposited in the Interior American West

Aeolian dust is a significant source of phosphorus (P) to alpine oligotrophic lakes, but P speciation in dust and source sediments and its release kinetics to lake water remain unknown. Phosphorus K-edge XANES spectroscopy shows that calcium-bound P (Ca–P) is dominant in 10 of 12 dust samples (41–74%) deposited on snow in the central Rocky Mountains and all 42 source sediment samples (the fine fraction) (68–80%), with a lower proportion in dust probably because acidic snowmelt dissolves some Ca–P in dust before collection. Iron-bound P (Fe–P, ∼54%) dominates in the remaining two dust samples. Chemical extractions (SEDEX) on these samples provide inaccurate results because of unselective extraction of targeted species and artifacts introduced by the extractions. Dust releases increasingly more P in synthetic lake water within 6–72 h thanks to dissolution of Ca–P, but dust release of P declines afterward due to back adsorption of P onto Fe oxides present in the dust. The back sorption is stronger for the dust with a lower degree of P saturation determined by oxalate extraction. This work suggests that P speciation, poorly crystalline minerals in the dust, and lake acidification all affect the availability and fate of dust-borne P in lakes

A Model for Nucleation When Nuclei Are Nonstoichiometric: Understanding the Precipitation of Iron Oxyhydroxide Nanoparticles

Despite years of study, quantitative models for the nucleation and growth of metal oxyhydroxide nanoparticles from aqueous solution have remained elusive. The problem is complicated by surface adsorption, which causes the stoichiometry of the nucleus to differ from that of the bulk precipitate and causes the surface tension of the precipitate-water interface to depend upon solution chemistry. Here we present a variation of classical nucleation theory that can accommodate surface adsorption, and apply it to understand the nucleation of β-FeOOH (akaganeite) nanoparticles from aqueous FeCl₃ solutions. We use small-angle X-ray scattering (SAXS) to quantify nucleation rates over a range of concentrations (5–200 mM FeCl₃) and temperatures (47–80 °C), then apply our model to estimate the critical nucleus size and surface tension at each condition. The surface tension varies from 0.07 J/m² in 200 mM solutions to 0.16 J/m² in 5 mM solutions. This behavior indicates that the nuclei contain an excess of Cl^– and H⁺ relative to the ideal FeOOH stoichiometry, and the coadsorption of Cl^– and H⁺ is critical for reducing surface tension into a range where classical nucleation pathways can operate. Furthermore, we find that the surface tension can be roughly estimated from aqueous solubility data alone, which may help to understand systems where surface tension data is unavailable