K-edge Extended X-ray Absorption Research Articles

Enrichment mechanism of heavy rare earth elements in magmatic-hydrothermal titanite: Insights from SXAS/XPS experiments and first-principles calculations and implications for regolith-hosted HREE deposits

Abstract Heavy rare earth elements (HREE), as critical materials for the global transition to a low-carbon economy, are mostly supplied by regolith-hosted ion adsorption deposits formed from the weathering of granites. Magmatic-hydrothermal titanite of pronounced HREE enrichment in granites has been suggested to play an important role in forming HREE-dominated regolith-hosted deposits. However, the crystal-chemical mechanism and geological process responsible for HREE enrichment in titanite remain unknown. This study investigated two texturally distinct types of HREE-enriched titanite (titanite I and II) in granites from the Gucheng regolith-hosted HREE deposit in South China. Secondary ion mass spectrometry (SIMS) U-Pb dating of rutile associated with titanite I and II yielded similar ages of 101.0 ± 1.7 Ma and 97.9 ± 6.2 Ma, respectively, supporting HREE remobilization during auto-metasomatism. Microbeam synchrotron X-ray absorption spectroscopic analyses show that titanite I and II have distinct Y K-edge X-ray absorption near-edge structure spectra. The best-fit results of Y K-edge extended X-ray absorption fine structure data suggest that titanite I and II have a Y-O first shell with coordination numbers and distances of 7.8 and 7.2 and 2.31–2.51 Å and 2.30–2.47 Å, respectively. Measured Y 3d and Dy 3d X-ray photoelectron spectroscopic data also support different Y,HREE-O first shells between titanite I and II. These results suggest that Y3+ and REE3+ occupy the 7-coordinated Ca site via three substitutions (Y,REE)3+ + (Al,Fe)3+ ↔ Ca2+ + Ti4+, 2(Y,REE)3+ + □Ca ↔ 3Ca2+, and 2(Y,REE)3+ + O2− ↔ 2Ca2+. First-principle calculations and the lattice strain model predict preferential uptakes of Y3+ and HREE3+ as the (Y,HREE)O8 polyhedra arising from the latter substitution mechanism at the Ca site in titanite. Also, we link the 2Y3+ + O2− ↔ 2Ca2+ substitution in titanite to highly oxidized and HREE-enriched melts/fluids originating from the subducted slab. These findings further support our previous suggestion that the crystallization of HREE-enriched titanite in granites plays an important role in forming HREE-dominated regolith-hosted deposits.

The fate of cadmium during ferrihydrite phase transformation affected by dissolved organic matter: Insights from organic-mineral interaction

Ferrihydrite (Fh) is an important scavenger for sequestering cadmium (Cd). However, Fh is poorly crystallized and tends to undergo phase transformation under the impact of dissolved organic matter (DOM). The roles of DOM in Fh transformation pathway and the consequent Cd2+ remobilization behavior remain debate due to its structural heterogeneity and diversity. Herein, formic acid (FA), oxalic acid (Ox), and citric acid (CA) were selected as model DOM, which have one, two, and three carboxyl ligands, respectively. Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra revealed an organic-specific correlation between Fh transformation and Cd2+ transport. FA with monodentate carboxyl ligand, lacking binding with Fh particles, had negligible influence on transformation dynamics and Cd2+ transport. The complexation of bidentate Ox with Fh particles impaired the stability of Fe(O,OH)6 octahedra, accelerating Fh phase transformation, thereby increasing Cd2+ release. During the subsequent recrystallization, Ox guided crystal growth via oriented attachment, which possibly occluded the remaining Cd2+ within structural defects of the secondary minerals. Conversely, tridentate CA exhibited a strong affinity with Fh, resulting in the formation of ferric citrate surface complexes that stabilized Fh against transformation and thus enhanced the long-term immobilization of Cd2+. These findings highlight the significant role of organic-ion/mineral interactions in Fh transformation, which are fundamentally linked to predicting Cd2+ transport behavior in environmental systems.

Effect of aliovalent substitution on the local structure of CaKFe4As4 superconductor

We have investigated the local structure of the iron-based CaKFe4As4superconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFe4As4, CaK0.9Sr0.1Fe4As4, CaK0.9Ba0.1Fe4As4and Ca0.9Na0.1K0.9Ba0.1Fe4As4. Temperature-dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements are used to determine the near-neighbors bondlengths and their stiffness. The EXAFS analysis reveals that the Fe-As bondlength undergoes negligible changes by substitution, however, the Fe-Fe bondlength and the As height are affected by the Sr substitution. The superconducting transition temperatures of CaK0.9Sr0.1Fe4As4and CaK0.9Ba0.1Fe4As4are very similar even if the mean As heights are significantly different suggesting that the anion height may not be a unique parameter to describe the superconductivity in CaKFe4As4. The mean As heights show a peculiar temperature dependence characteristic of CaKFe4As4system. Furthermore, the temperature-dependent mean square relative displacements reveal similar Fe-Fe bond stiffness in all samples, instead the Fe-As bond is substantially stiffer in case of CaK0.9Sr0.1Fe4As4. The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.

Antimony sorption to schwertmannite in acid sulfate environments

Schwertmannite is a poorly-crystalline Fe(III) oxyhydroxysulfate mineral that may control Sb(V) mobility in acid sulfate environments, including acid mine drainage and acid sulfate soils. However, the mechanisms that govern uptake of aqueous Sb(V) by schwertmannite in such environments are poorly understood. To address this issue, we examined Sb(V) sorption to schwertmannite across a range of environmentally-relevant Sb(V) loadings at pH 3 in sulfate-rich solutions. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Sb(V) sorption (at all loadings) involved edge and double-corner sharing linkages between SbVO6 and FeIIIO6 octahedra. The coordination numbers for these linkages indicate that sorption occurred by Sb(V) incorporation into the schwertmannite structure via heterovalent Sb(V)-for-Fe(III) substitution. As such, Sb(V) sorption to schwertmannite was not limited by the abundance of surface complexation sites and was strongly resistant to desorption when exposed to 0.1 M PO43-. Sorption of Sb(V) also conferred increased stability to schwertmannite, based on changes in the schwertmannite dissolution rate during extraction with an acidic ammonium oxalate solution. This study provides new insights into Sb(V) sorption to schwertmannite in acid sulfate environments, and highlights the role that schwertmannite can play in immobilizing Sb(V) within its crystal structure.

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography.

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te Kα X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

Local Structure and Crystallization Transformation of Hydrous Ferric Arsenate in Acidic H2O-Fe(III)-As(V)-SO42- Systems: Implications for Acid Mine Drainage and Arsenic Geochemical Cycling.

Hydrous ferric arsenate (HFA) is a common thermodynamically metastable phase in acid mine drainage (AMD). However, little is known regarding the structural forms and transformation mechanism of HFA. We investigated the local atomic structures and the crystallization transformation of HFA at various Fe(III)/As(V) ratios (2, 1, 0.5, 0.33, and 0.25) in acidic solutions (pH 1.2 and 1.8). The results show that the Fe(III)/As(V) in HFA decreases with decreasing initial Fe(III)/As(V) at acidic pHs. The degree of protonation of As(V) in HFA increases with increasing As(V) concentrations. The Fe K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure results reveal that each FeO6 is linked to more than two AsO4 in HFA precipitated at Fe(III)/As(V) < 1. Furthermore, the formation of scorodite (FeAsO4·2H2O) is greatly accelerated by decreasing the initial Fe(III)/As(V). The release of As(V) from HFA is observed during its crystallization transformation process to scorodite at Fe(III)/As(V) < 1, which is different from that at Fe(III)/As(V) ≥ 1. Scanning electron microscopy results show that Oswald ripening is responsible for the coarsening of scorodite regardless of the initial Fe(III)/As(V) or pH. Moreover, the formation of crystalline ferric dihydrogen arsenate as an intermediate phase at Fe(III)/As(V) < 1 is responsible for the enhanced transformation rate from HFA to scorodite. This work provides new insights into the local atomic structure of HFA and its crystallization transformation that may occur in AMD and has important implications for arsenic geochemical cycling.

Distribution of soil organic carbon between particulate and mineral-associated fractions as affected by biochar and its co-application with other amendments

The application of biochar to soils, either alone or combined with other amendments, represents a management practice aimed at storing carbon (C) while enhancing soil fertility. However, the long-term effects of biochar application on soil organic C protection against microbial decomposition are uncertain. This study investigated, in a 9-year-long-term field experiment, the protection of organic C by minerals and iron (Fe) in soils amended with biochar alone or combined with either municipal solid waste compost or sewage sludge. Particulate (not protected by minerals) and mineral-associated organic matter fractions were separated, quantified, and finally the Fe-mediated protection mechanisms were examined by Fe K-edge extended X-ray absorption fine structure spectroscopy. With respect to the unamended control soil, soils amended with biochar, especially when combined with municipal solid waste compost and sewage sludge, had 3 times greater content of particulate organic C. Biochar combined with municipal solid waste compost and sewage sludge also increased mineral-associated organic C content (1.5×), although the magnitude of the effect was smaller than for the particulate organic C fraction. The contents of Fe(III)-organic matter complexes in particulate and mineral-associated organic matter fractions of the amended soils were similar to those of the unamended soils. Hematite represented the main Fe oxide phase in the particulate organic matter fraction of all the soils as well as in the mineral-associated organic matter fraction of the unamended soils, whereas ferrihydrite was more abundant in the mineral-associated organic matter fraction of the amended soils. As a whole, the obtained results in general, and the positive effect on the mineral-associated organic matter possibly mediated by ferrihydrite occurrence in particular, highlight the potential of biochar, alone or in combination with other amendments, as a strategy to store and preserve C in soils.

Extended X-ray Absorption Fine Structure Revealed the Mechanism of Arsenate Removal by the Fe/Mn Oxide-Based Composite under Conditions of Fully Saturated Sorption Sites.

Molecular mechanism of arsenate removal by a promising inorganic composite based on Fe/Mn oxides and MnCO3 was studied under the rarely investigated conditions of fully saturated sorption sites (characteristic of dynamic sorption, such as water treatment plants) at the pH of 4/6/7/8 using As K-edge extended X-ray absorption fine structure (EXAFS)/X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). Comparison of arsenic speciation in the initial adsorbate solution (calculated by Visual MINTEQ) and after sorption (determined by As 3d XPS) allowed the interpretation of the initializing forces of the interfacial processes. Contribution of various solid phases of this composite anion exchanger to the removal of arsenate was disclosed by examining the Fe 2p3/2 and Mn 2p3/2 XPS spectra supported by FTIR. As K-edge EXAFS simulation not only proved the chemisorptive binding of aqueous As(V) anions to the Fe/Mn oxide-based adsorbent but also demonstrated the presence of a variety of sorption sites in this complex structured porous material, which became available step-wise upon an increasing pressure on the interface with high arsenate loading during the long-term sorption process. The type of inner-sphere complexation of As(V) on the saturated surface discovered by As K-edge EXAFS modeling was a function of pH. Analysis of EXAFS fitting data resulted in suggestion of a methodological idea on how the EXAFS-derived coordination numbers can be used to distinguish the localization of adsorbed ions (surface versus structure emptiness). This work also provides more insights into the superiority of composite adsorbents (compared to the materials based on individual compounds) in terms of their capability to adapt/change the molecular sorption mechanism in order to inactivate (remove) more toxic aqueous anions.

Examining the Effect of Cu and Mn Dopants on the Structure of Zinc Blende ZnS Nanopowders.

It is known that doping zinc sulfide (ZnS) nanoparticles with Mn or Cu ions significantly affects their luminescent properties. Herein, we investigated how dopant atoms are incorporated into the structure of ZnS using X-ray diffraction and multi-edge X-ray absorption spectroscopy. The observed broadening of the X-ray diffraction patterns indicates an average crystallite size of about 6 nm. By analyzing the Zn, Mn, and Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra using the reverse Monte Carlo method, we were able to determine the relaxations of the local environments around the dopants. Our findings suggested that upon the substitution of Zn by Mn or Cu ions, there is a shortening of the Cu-S bonds by 0.08 Å, whereas the Mn-S bonds exhibited lengthening by 0.07 Å. These experimental results were further confirmed by first-principles density functional theory calculations, which explained the increase in the Mn-S bond lengths due to the high-spin state of Mn2+ ions.

Atomic-Level Structural Differences between Fe(III) Coprecipitates Generated by the Addition of Fe(III) Coagulants and by the Oxidation of Fe(II) Coagulants Determine Their Coagulation Behavior in Phosphate and DOM Removal.

In situ Fe(III) coprecipitation from Fe2+ oxidation is a widespread phenomenon in natural environments and water treatment processes. Studies have shown the superiority of in situ Fe(III) (formed by in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (using a Fe(III) coagulant directly) in coagulation, but the reasons remain unclear due to the uncertain nature of amorphous structures. Here, we utilized an in situ Fe(III) coagulation process, oxidizing the Fe(II) coagulant by potassium permanganate (KMnO4), to treat phosphate-containing surface water and analyzed differences between in situ and ex situ Fe(III) coagulation in phosphate removal, dissolved organic matter (DOM) removal, and floc growth. Compared to ex situ Fe(III), flocs formed by the natural oxidizing Fe2+ coagulant exhibited more effective phosphate removal. Furthermore, in situ Fe(III) formed through accelerated oxidation by KMnO4 demonstrated improved flocculation behavior and enhanced removal of specific types of DOM by forming a more stable structure while still maintaining effective phosphate removal. Fe K-edge extended X-ray absorption fine structure spectra (EXAFS) of the flocs explained their differences. A short-range ordered strengite-like structure (corner-linked PO4 tetrahedra to FeO6 octahedra) was the key to more effective phosphorus removal of in situ Fe(III) than ex situ Fe(III) and was well preserved when KMnO4 accelerated in situ Fe(III) formation. Conversely, KMnO4 significantly inhibited the edge and corner coordination between FeO6 octahedra and altered the floc-chain-forming behavior by accelerating hydrolysis, resulting in a more dispersed monomeric structure than ex situ Fe(III). This research provides an explanation for the superiority of in situ Fe(III) in phosphorus removal and highlights the importance of atomic-level structural differences between ex situ and in situ Fe(III) coprecipitates in water treatment.

Unraveling the interlayer and intralayer coupling in two-dimensional layered MoS[formula omitted] by X-ray absorption spectroscopy and ab initio molecular dynamics simulations

Understanding interlayer and intralayer coupling in two-dimensional layered materials (2DLMs) has fundamental and technological importance for their large-scale production, engineering heterostructures, and development of flexible and transparent electronics. At the same time, the quantification of weak interlayer interactions in 2DMLs is a challenging task, especially, from the experimental point of view. Herein, we demonstrate that the use of X-ray absorption spectroscopy in combination with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations can provide useful information on both interlayer and intralayer coupling in 2DLM 2Hc-MoS2. The analysis of the low-temperature (10–300 K) Mo K-edge extended X-ray absorption fine structure (EXAFS) using RMC simulations allows for obtaining information on the means-squared relative displacements σ2 for nearest and distant Mo–S and Mo–Mo atom pairs. This information allowed us further to determine the strength of the interlayer and intralayer interactions in terms of the characteristic Einstein frequencies ωE and the effective force constants κ for the nearest ten coordination shells around molybdenum. The studied temperature range was extended up to 1200 K employing AIMD simulations which were validated at 300 K using the EXAFS data. Both RMC and AIMD results provide evidence of the reduction of correlation in thermal motion between distant atoms and suggest strong anisotropy of atom thermal vibrations within the plane of the layers and in the orthogonal direction.

Silica-magnesium-titanium Ziegler-Natta catalysts. Part 1: Structure of the pre-catalyst at a molecular level

In this paper, which is the first part of a more extended work, we elucidate the molecular level structure of a highly active SiO2-supported Ziegler-Natta precatalyst obtained by reacting a dehydroxylated silica and a solution of an organomagnesium compound with TiCl4. The synergetic combination of Ti K-edge and Ti L3-edge X-ray Absorption spectroscopy (XAS) and diffuse reflectance UV–Vis spectroscopies, complemented by Density Functional Theory (DFT) simulations, indicate that small TiCl3 clusters similar to β-TiCl3 coexist with isolated monomeric Ti(IV) species. Ti K-edge Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy allows the quantification of these two phases and demonstrates that the Ti(IV) sites are 6-fold coordinated (either by six chlorine ligands or by five chlorine and one oxygen ligands), but highly distorted, similar to what is modelled for TiCl4-capped MgCl2 nanoplatelets. Finally, IR spectroscopy suggests that the MgCl2 phase has a molecular character (Far-IR) and that the only accessible Mg2+ sites are uncoordinated cations acting as Lewis acid sites (IR of CO adsorbed at 100 K). Based on these experimental findings, we propose the co-existence in the precatalyst of small TiCl3 clusters and of mixed oxo-chloride magnesium-titanium structures deposited at the silica surface. The evolution of the precatalyst in the presence of the activator and of the monomer is discussed in the second part of this work.

Spectroscopic studies for identifying the chemical states of the periostracum of the Corbicula species in Lake Biwa

Corbicula clam shells consist of thin periostracum and calcareous layers made of calcium carbonate (CaCO3). Depending on habitat conditions, the shell exhibits various colorations, such as yellow, brown, and black. The chemical state of the periostracum of the Corbicula species in Lake Biwa was studied by X-ray absorption fine structure (XAFS) and Raman scattering spectroscopies. Fe K-edge X-ray absorption near edge structure (XANES) revealed that the Fe3+ intensity increases as the color of the shell changes from yellow to black. Raman spectra suggested that quinone-based polymers cover the yellow shell, and the black shell is further covered by dihydroxyphenylalanine (DOPA) rings of amino acid derivatives. From Fe K-edge extended X-ray absorption fine structure (EXAFS), it was found that Fe3+ in the periostracum was surrounded by five to six oxygen atoms with an average Fe-O ligand distance of 2.0 Å. Accordingly, a tris-DOPA-Fe3+ complex is formed, which is responsible for the periostracum’s black color.

Possible local order in the high entropy TrZr[formula omitted] superconductors

We have investigated the local structure of TrZr2 (Tr = transition metal) superconductor with variable configurational entropy using x-ray absorption spectroscopy. Temperature dependent Zr K-edge extended x-ray absorption fine structure (EXAFS) measurements are carried out on three different samples including the pristine CoZr2, medium entropy Co0.33Rh0.34Ir0.33Zr2 and high entropy Co0.2Fe0.2Ni0.2Rh0.2Ir0.2Zr2. We have found that the local bondlengths remain largely temperature independent or tend to show negative thermal expansion, instead the Zr–Zr bondlengths get stiffer with the configurational entropy. The x-ray absorption near edge structure (XANES) spectra at Zr K-edge and Ir L3-edge indicate Zr 4d - Tr nd (n = 2, 3, 4) hybridization to be related with the superconducting transition temperature. The results provide important information on the local structure that can be useful to understand the physical properties of these materials and design new high entropy alloys (HEAs) superconductors.

Short range order and topology of binary Ge-S glasses

Short range order and topology of GexS100-x glasses over a broad composition range (20 ≤ x ≤ 42 in at%) was investigated by neutron diffraction, X-ray diffraction, and Ge K-edge extended X-ray absorption fine structure (EXAFS) measurements. The experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo simulation method. It was found that both constituents (Ge and S) satisfy the Mott-rule in all investigated glasses: Ge and S atoms have 4 and 2 neighbours, respectively. The structure of these glasses can be described with the chemically ordered network model: Ge-S bonds are preferred; S-S bonds are present only in S-rich glasses. Dedicated simulations showed that Ge-Ge bonds are necessary in Ge-rich glasses. Connections between Ge atoms (such as edge-sharing GeS4/2 tetrahedra) in stoichiometric and S-rich glasses were analysed. The frequency of primitive rings was also calculated.

Efficiently photocatalytic H2O overall splitting within the strengthened polarized field by reassembling surface single atoms

Surface single atom Ni-coordinated carbon nitride fibers (Ni/C3N4Fs) were reassembled into tiny NiP nanoparticle-integrated C3N4Fs (NiP/C3N4Fs) with the strengthened polarized electric fields accompanied by a weakened interface barrier. Photogenerated e- and h+ dynamically transfer to the convex-integrated NiP and concave of C3N4Fs under the synergetic electric fields via inner single atoms of Ni as charge transfer bridges within C3N4Fs interlayers, which is confirmed by high-angle annular dark field scanning transmission electron microscopy, K-edge extended X-ray absorption fine structure, X-ray photoelectron spectroscopy and theoretical prediction. H2- and O2-evolution rates of 1.89 and 0.92 mmol g−1 h−1 with solar-to-hydrogen efficiency of 2.33% are obtained over NiP/C3N4Fs system under visible light irradiation. This research provides a rational strategy for constructing the connection of H2 fuel production with its practical application.

Structural analysis of high-energy implanted Ni atoms into Si(100) by X-ray absorption fine structure spectroscopy

Ni silicide synthesis by Ni ion beam irradiation into Si attracts attention due to its advantages including the ability of formation of local structures, the controllability of ion beams, the formability of silicide without heat treatment and the high reproducibility of the resulting specimen. In this work, we investigate the local atomic structure of Si implanted with 3.0 MeV Ni+ ions. Analysis of Ni K-edge fluorescent-yield extended X-ray absorption fine structure reveals that Ni atoms have mixed structure of metallic-like face-centered cubic Ni and NiSi2 phases at the initial stage of the irradiation and the formation of NiSi2 promotes significantly with the ion fluence above 1015 ions⋅cm−2. With consideration of the agreement between the ion fluence threshold for the structural transition and the critical Si-amorphization fluence (7.1 × 1014 ions⋅cm−2), it is concluded that the amorphization of Si plays an important role in the synthesis of the NiSi2 phase in Ni+-irradiated Si.

The Local Structure and Metal-Insulator Transition in a Ba3Nb5-xTixO15 System.

The local structure of the filled tetragonal tungsten bronze (TTB) niobate BaNbTiO (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.

Insight into the effect of SO42− on the precipitation and solubility of ferric arsenate in acidic solutions: Implication for arsenic mobility and fate

The effect of sulfate (SO42−) on the precipitation of As(V) and Fe(III) and the stability of the generated solid phase as well as its underlying mechanisms, which strongly influences the transportation and fate of As in acidic waters (such as acid mine drainage), is still not well known. This work systematically investigated the precipitation, decomposition, and settling properties of the ferric arsenate phase in the presence of different amount of SO42− at pH 1.2 and 1.8. The results showed that on the one side, the presence of SO42− delayed the removal of As and Fe from the aqueous phase and enhanced the solubility of solid ferric arsenate. The geochemical modelling results showed that the coexisting SO42− induced the formation of aqueous SO42−-Fe(III) complexes, inducing the decomposition of produced ferric arsenate solid phase. On the other side, the addition of SO42− promoted the growth rate of generated ferric arsenate and accelerated the settling of solids. The particle size distribution and transmission electron micorscopy analyses elucidated that the presence of SO42− significantly enlarged the ferric arsenate aggregate size. The infrared spectroscopy and S K-edge X-ray absorption near edge structure results proved that SO42− is incorporated into the structure of ferric arsenate by forming ferric arsenate sulfate solid solution, with each SO42− tetrahedron coordinating with approximately two FeO6 octahedra. The S K-edge extended X-ray absorption fine structrure result gave an averaged S-Fe interatomic distance of 3.21 Å. These results demonstrated that the presence of SO42− enlarged the aggregate size of ferric arsenate dominantly via the S-O-Fe bridging, and then accelerated the settling of SO4-ferric arsenate. The contrasting role of SO42− in influencing As obtained in this study is significant for understanding the mobilization and fate of As and the formation mechanisms of ferric arsenate minerals in acidic systems.

Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2.

The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS2. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS2 remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS2 under reaction conditions (H2 atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H2 atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS2 is herein elucidated for applications in optoelectronics and catalysis.