Mengqiang Zhu

Arsenite oxidation by a poorly crystalline manganese-oxide. 2. Results from X-ray absorption spectroscopy and X-ray diffraction.

Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (δ-MnO₂) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the δ-MnO₂ structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the δ-MnO₂ structure appears to increase as the reaction proceeds. Binding of As(V) to δ-MnO₂ also changes as Mn(III) becomes more prominent in the δ-MnO ₂ structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline δ-MnO₂ is greatly affected by Mn oxidation state in the δ-MnO₂ structure.

Cation effects on the layer structure of biogenic Mn-oxides.

Biologically catalyzed Mn(II) oxidation produces biogenic Mn-oxides (BioMnO(x)) and may serve as one of the major formation pathways for layered Mn-oxides in soils and sediments. The structure of Mn octahedral layers in layered Mn-oxides controls its metal sequestration properties, photochemistry, oxidizing ability, and topotactic transformation to tunneled structures. This study investigates the impacts of cations (H(+), Ni(II), Na(+), and Ca(2+)) during biotic Mn(II) oxidation on the structure of Mn octahedral layers of BioMnO(x) using solution chemistry and synchrotron X-ray techniques. Results demonstrate that Mn octahedral layer symmetry and composition are sensitive to previous cations during BioMnO(x) formation. Specifically, H(+) and Ni(II) enhance vacant site formation, whereas Na(+) and Ca(2+) favor formation of Mn(III) and its ordered distribution in Mn octahedral layers. This study emphasizes the importance of the abiotic reaction between Mn(II) and BioMnO(x) and dependence of the crystal structure of BioMnO(x) on solution chemistry.

Ni(II) Sorption on Biogenic Mn-Oxides with Varying Mn Octahedral Layer Structure

Biogenic Mn-oxides (BioMnO(x)), produced by microorganisms, possess an extraordinary ability to sequester metals. BioMnO(x) are generally layered structures containing varying amounts of Mn(III) and vacant sites in the Mn layers. However the relationship between the varying structure of BioMnO(x) and metal sorption properties remains unclear. In this study, BioMnO(x) produced by Pseudomonas putida strain GB-1 was synthesized at either pH 6, 7, or 8 in CaCl(2) solution, and Ni(II) sorption mechanisms were determined at pH 7 and at different Ni(II) loadings, using isotherm and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses. Our data demonstrate that Ni(II) sorbs at vacant sites in the interlayer of the BioMnO(x) and the maximum Ni(II) sorption capacity increases as the formation pH of BioMnO(x) decreases. This relation indicates that the quantity of BioMnO(x) vacant sites increases as formation conditions become more acidic, which is in good agreement with our companion study. Contents of the vacant sites were quantitatively estimated based on maximum Ni(II) sorption capacity. Additionally, this study reveals that imidazole groups are involved in Ni(II) binding to biomaterials, and have a higher Ni(II) sorption affinity, but a lower site density compared to carboxyl groups.

Early stage formation of iron oxyhydroxides during neutralization of simulated acid mine drainage solutions.

The phases and stability of ferric iron products formed early during neutralization of acid mine drainage waters remain largely unknown. In this work, we used in situ and time-resolved quick-scanning X-ray absorption spectroscopy and X-ray diffraction to study products formed between 4 min and 1 h after ferric iron sulfate solutions were partially neutralized by addition of NaHCO(3) ([HCO(3)(-)]/[Fe(3+)] < 3). When [HCO(3)(-)]/[Fe(3+)] = 0.5 and 0.6 (initial pH ∼ 2.1 and 2.2, respectively), the only large species formed were sulfate-complexed ferrihydrite-like molecular clusters that were stable throughout the duration of the experiment. When [HCO(3)(-)]/[Fe(3+)] = 1 (initial pH ∼ 2.5), ferrihydrite-like molecular clusters formed initially, but most later converted to schwertmannite. In contrast, when [HCO(3)(-)]/[Fe(3+)] = 2 (initial pH ∼ 2.7), schwertmannite and larger ferrihydrite particles formed immediately upon neutralization. However, the ferrihydrite particles subsequently converted to schwertmannite. The schwertmannite particles formed under both conditions aggregated extensively with increasing time. This work provides new insight into the formation, stability and reactivity of some early products that may form during the neutralization of natural acid mine drainage.

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.

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.

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.

In Situ Structural Characterization of Ferric Iron Dimers in Aqueous Solutions: Identification of μ-Oxo Species

The structure of ferric iron (Fe(3+)) dimers in aqueous solutions has long been debated. In this work, we have determined the dimer structure in situ in aqueous solutions using extended X-ray absorption fine structure (EXAFS) spectroscopy. An Fe K-edge EXAFS analysis of 0.2 M ferric nitrate solutions at pH 1.28-1.81 identified a Fe-Fe distance at ∼3.6 Å, strongly indicating that the dimers take the μ-oxo form. The EXAFS analysis also indicates two short Fe-O bonds at ∼1.80 Å and ten long Fe-O bonds at ∼2.08 Å, consistent with the μ-oxo dimer structure. The scattering from the Fe-Fe paths interferes destructively with that from paths belonging to Fe(OH2)6(3+) monomers that coexist with the dimers, leading to a less apparent Fe shell in the EXAFS Fourier transform. This might be a reason why the characteristic Fe-Fe distance was not detected in previous EXAFS studies. The existence of μ-oxo dimers is further confirmed by Mössbauer analyses of analogous quick frozen solutions. This work also explores the electronic structure and the relative stability of the μ-oxo dimer in a comparison to the dihydroxo dimer using density function theory (DFT) calculations. The identification of such dimers in aqueous solutions has important implications for iron (bio)inorganic chemistry and geochemistry, such as understanding the formation mechanisms of Fe oxyhydroxides at molecular scale.

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), 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. At both 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 = 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.

Fe-doped cryptomelane synthesized by refluxing at atmosphere: Structure, properties and photocatalytic degradation of phenol

Fe-doped cryptomelanes were synthesized by refluxing at ambient pressure, followed by characterization with multiple techniques and test in photocatalytic degradation of phenol. The introduction of Fe(III) into the structure of cryptomelane results in a decrease in particle size and the contents of Mn and K(+), and an increase in the Mn average oxidation state (AOS), specific surface area and UV-vis light absorption ability. Mn and Fe K-edge extended X-ray absorption fine structure spectroscopy analysis indicates that some Fe(III) is incorporated into the framework of cryptomelane by replacing Mn(III) while the remaining Fe(3+) is adsorbed in the tunnel cavity. These Fe-doped cryptomelanes have significantly improved the photocatalytic degradation rate of phenol, with the sample of ∼3.04 wt.% Fe doping being the most reactive and achieving a degradation rate of 36% higher than that of the un-doped one. The enhanced reactivity can be ascribed to the increase in the coherent scattering domain size of the crystals, Mn AOS and light absorption, as well as the presence of sufficient K(+) in the tunnel. The results imply that metal doping is an effective way to improve the performance of cryptomelane in pollutants removal and has the potential for modification of Mn oxide materials.