EXAFS Spectroscopy Research Articles

Antimony(V) sorption and coprecipitation with ferrihydrite: An examination of retention mechanisms and the selectivity of commonly-applied extraction procedures

We investigated the mechanisms that control Sb(V) sorption and coprecipitation with ferrihydrite across a range of Sb(V) loadings, and examined the associated effects on Sb(V) extractability during the commonly-applied 1M HCl extraction scheme and the BCR and Wenzel sequential extraction schemes. EXAFS spectroscopy reveals that Sb(V) sorption and coprecipitation mainly involved Sb(V) incorporation into the ferrihydrite structure via edge sharing and double-corner sharing between SbO6 and FeO6 octahedra. Large amounts of these linkages partially stabilized ferrihydrite against extraction with 1M HCl. Negligible (<0.5%) ferrihydrite-bound Sb(V) was recovered in the “acid extractable” and “reducible” fractions of the BCR scheme, while 1-16% was recovered in the “oxidizable” fraction. As such, the BCR scheme risks ferrihydrite-bound Sb(V) being misidentified as Sb residing mainly in “residual” phases. In contrast, in the Wenzel scheme, almost all sorbed- and coprecipitated-Sb(V) was recovered in the “amorphous hydrous oxide-bound” fraction, with only 0.6-3.3% in the “specifically-bound” fraction (consistent with our finding of Sb(V) retention via incorporation into ferrihydrite, as opposed to adsorption by the ferrihydrite surface). Collectively, the results provide new insights into the retention mechanisms and extraction behaviour of ferrihydrite-bound Sb(V), enhancing our ability to assess Sb contamination in soils, sediments and geogenic wastes.

Physicochemical Characterization and Antimicrobial Properties of Lanthanide Nitrates in Dilute Aqueous Solutions.

This work presents the results of studying dilute aqueous solutions of commercial Ln(NO3)3 · xH2O salts with Ln = Ce-Lu using X-ray diffraction (XRD), IR spectroscopy, X-ray absorption spectroscopy (XAS: EXAFS/XANES), and pH measurements. As a reference point, XRD and XAS measurements for characterized Ln(NO3)3 · xH2O microcrystalline powder samples were performed. The local structure of Ln-nitrate complexes in 20 mM Ln(NO3)3 · xH2O aqueous solution was studied under total external reflection conditions and EXAFS geometry was applied to obtain high-quality EXAFS data for solutions with low concentrations of Ln3+ ions. Results obtained by EXAFS spectroscopy showed significant contraction of the first coordination sphere during the dissolution process for metal ions located in the middle of the lanthanide series. It was established that in Ln(NO3)3 · xH2O solutions with Ln = Ce, Sm, Gd, Yb (c = 134, 100, 50 and 20 mM) there are coordinated and, to a greater extent, non-coordinated nitrate groups with bidentate and predominantly monodentate bonds with Ln ions, the number of which increases upon transition from cerium to ytterbium. For the first time, the antibacterial and antifungal activity of Ln(NO3)3 · xH2O Ln = Ce, Sm, Gd, Tb, Yb solutions with different concentrations and pH was presented. Cross-relationships between the concentration of solutions and antimicrobial activity with the type of Ln = Ce, Sm, Gd, Tb, Yb were established, as well as the absence of biocidal properties of solutions with a concentration of 20 mM, except for Ln = Yb. The important role of experimental conditions in obtaining and interpreting the results was noted.

Superhydrophobic Co/Fe sulfide nanoparticles and single-atoms doped carbon nanosheets composite air cathode for high-performance zinc-air batteries

Development of a high-performance bifunctional catalyst is essential for the actual implementation of zinc-air batteries in practical applications. Herein, a bifunctional cathode of Co3S4/FeS heterogeneous nanoparticles embedded in Co/Fe single-atom-loaded nitrogen-doped carbon nanosheets is designed. Cobalt-iron sulfides and single atomic sites with Co-N4/Fe-N4 configurations are confirmed to coexist on the carbon matrix by EXAFS spectroscopy. 3D self-supported super-hydrophobic multiphase composite cathode provides abundant active sites and facilitates gas–liquid-solid three-phase interface reactions, resulting in excellent electrocatalytic activity and batteries performance, i.e., an OER overpotential (η10) of 260 mV, a half-wave potential (E1/2) of 0.872 V for ORR, a ΔE of 0.618 V, and a discharge power density of 170 mW cm−2, a specific capacity of 816.3 mAh g−1. DFT analysis shows multiphase coupling of sulfide heterojunction through single-atomic metal doped carbon nanosheets reduces offset on center of electronic density of states before and after oxygen adsorption, and spin density of adsorbed oxygen with same spin orientation, leading to weakened charge/spin interactions between adsorbed oxygen and substrate, and a lowered oxygen adsorption energy to accelerate OER/ORR.

Codoping Eu3+ with Sc3+ and La3+ in CaSnO3 for possible site selective doping: Is size selection in site selective doping really effective?

Local site of dopant in host lattice with multiple cationic sites is very important from light emission perspective in dictating its various photophysical features. The same is very important particularly in perovskites which is considered a very important invention for optoelectronic applications. Here in this work we have investigated the local site of dopant and time resolved photoluminescence in CaSnO3:Eu red emitting phosphor on co-doping La3+ and Sc3+ based on their size proximity to A and B-site respectively. EXAFS spectroscopy suggested increase in coordination number of Eu3+ from 6 to 8 and formation of Sn and Ca vacancies as charge compensating defects (CCDs) in La3+ and Sc3+ co-doped samples respectively. Co-doping with either La3+or Sc3+ is directing more fraction of Eu3+ to asymmetric 8-coordinated Ca2+ site compared to relatively symmetric 6-coordinated Sn4+ site. Sc3+ codoping direct more Eu3+ to Ca2+ site but with less distortions in the lattice owing to smaller size of Eu3+ than Ca2+ and higher size of Sc3+ than Sn4+. In case of La3+ co-doping, directing Eu3+ to Sn4+ site is not so effective owing to larger size of Eu3+ than Sn4+ as well as La3+ than Ca2+ and both cause significant distortions in the lattice. PL intensity also improved in the co-doped samples with Sc3+ co-doping emerging victorious over La3+ due to better charge compensation encountered in the former. Excitation dependent PL suggested Eu3+ at Ca2+ sites is preferentially excited under 250 nm and Eu3+ at Sn4+ symmetric site is preferentially excited under 300 nm.

Entrapping aliphatic and heterocyclic gaseous amines on copper-exchanged montmorillonite: A straight and green process to reduce air pollution

The capture of a number of aliphatic (APAs) and heterocyclic (HCAs) amines, known as widespread pollutants, by Cu2+-exchanged montmorillonite (Mt-Cu) was studied through elemental analysis diffuse-reflectance ultraviolet-visible and infra-red spectroscopies, X-rays diffraction, thermal analysis coupled with mass spectrometry of the evolved gases, XANES and EXAFS spectroscopy, and solid-state NMR. The investigated APAs and HCAs were: ethylamine (EtNH2), triethylamine (Et3N), ethylenediamine (En) and morpholine (Morph), piperidine (Piper), and pyridine (Py), respectively. Mt-Cu saturation occurred within 25 h at worst, the immobilized amount was relevant and involved the binding of the amino nitrogen to the interlayer Cu2+ ions. While for APAs the entrapped amount changed with the structure of the molecule, for HCAs it did not. The EXAFS and XANES studies showed that the interlayer Cu2+ ions are strongly coordinated to the amino groups. This resulted in the coordination number changing from four to five coordination, with the exception of En for which the CuII ions remained tetra-coordinated as in pristine Mt-Cu. Residual water molecules were found to be bound to the copper also after exposure to the amines. For EtNH2, En, Morph, and Piper the coordination to CuII occurred without the deprotonation of the amino group. NMR measurements showed the presence of physisorbed amines whose amount depended on the structure: while En was mostly bound to CuII (physisorption was negligible), for EtNH2, Et3N, Morph, and Piper the physisorbed amount ranged from 55 to 30% and for Py it was even the 70%. In addition, En was found to be the amine entrapped with the highest strength and covalent character. The obtained results supported the effectiveness of Cu2+-modified layered silicates as an air/gas purification system. The exploitation of devices based on Cu2+-exchanged montmorillonite could therefore become a very appealing future challenge as the interlayer modification was very simple and fast and the entire process turned out to be green and economic.

Mechanisms of Arsenic and Antimony Co-sorption onto Jarosite: An X-ray Absorption Spectroscopic Study.

Jarosite, a common mineral in acidic sulfur-rich environments, can strongly sorb both As(V) and Sb(V). However, little is known regarding the mechanisms that control simultaneous co-sorption of As(V) and Sb(V) to jarosite. We investigated the mechanisms controlling As(V) and Sb(V) sorption to jarosite at pH 3 (in dual and single metalloid treatments). Jarosite was found to sorb Sb(V) to a greater extent than As(V) in both single and dual metalloid treatments. Relative to single metalloid treatments, the dual presence of both As(V) and Sb(V) decreased the sorption of both metalloids by almost 50%. Antimony K-edge EXAFS spectroscopy revealed that surface precipitation of an Sb(V) oxide species was the predominant sorption mechanism for Sb(V). In contrast, As K-edge EXAFS spectroscopy showed that As(V) sorption occurred via bidentate corner-sharing complexes on the jarosite surface when Sb(V) was absent or present at low loadings or by formation of similar complexes on the Sb(V) oxide surface precipitate when Sb(V) was present at high loadings. These results point to a novel mechanism by which Sb(V) impacts the co-sorption of As(V). Overall, these findings highlight a strong contrast in the sorption mechanisms of Sb(V) versus As(V) to jarosite under acidic environmental conditions.

Uranium sorption to organic matter and long-term accumulation in a pristine alpine wetland

Understanding the controls on uranium (U) mobility in the environment is key to improve the management of sites contaminated by U mining activities. Previous research has shown that natural or engineered wetlands are particularly able to scavenge high amounts of U(VI) and U(IV) under noncrystalline forms. However, questions remain on the respective roles of sorption and reduction processes in the removal of U from running waters in wetlands, as well as on the long-term stability of U storage. Here, we performed a series of geochemical, isotopic (δ238U, (234U/238U)), microscopic (SEM-EDXS, EPMA) and spectroscopic (µ-XRF, µ-XAS, XANES and EXAFS at the U L3 and M4-edges and Fe K-edge) investigations to determine the modes of U accumulation and assess U mobility in a natural exceptionally U-enriched (up to 5000 µg/g) wetland on the shore of Lake Nègre (Mediterranean Alps, France). Uranium (VI) was largely dominant in the two studied soil cores, except a few samples containing as much as ∼50 % U(IV). At the particle scale, U is associated to a variety of organic constituents of the soil matrix with a homogenous oxidation state. Bulk EXAFS spectroscopy at the U L3-edge shows that U is mostly mononuclear, with dominant monodentate binding to organic moieties (C neighbors at ∼3.45 Å). An additional minor fraction of U under polymeric forms is inferred from wavelet (CCWT) analysis of the EXAFS data. These observations are reinforced by 1 M bicarbonate extractions that result in the dissolution of 82–96 % of total U, including putative polymeric species. At the wetland scale, similar or slightly fractionated isotopic ratios (δ238U) between the wetland-feeding creek waters and the wetland soils are observed, supporting the idea that U(VI) sorption on organic matter is the primary U scavenging mechanism. Furthermore, it indicates that partial U(VI) reduction to U(IV) occurs as a second step, after sorption. Analysis of U decay chain disequilibria in the cores as a function of depth suggests that U accumulation in this wetland has lasted for several thousand years. We propose that the wetland acts as an active reactor where U has been massively accumulating for ∼14500 years, especially as U(VI) forms associated to organic matter, and is further partly exported to the lake through soil erosion.

Cover Feature: Shedding Light on the Enigmatic TcO2 ⋅ xH2O Structure with Density Functional Theory and EXAFS Spectroscopy (Chem. Eur. J. 59/2022)

Radioactive Tc in deep nuclear waste repositories can be immobilized by reduction of the highly soluble TcO4− anion by agents like magnetite (Fe3O4) and H2, which create an amorphous TcO2⋅x H2O precipitate. EXAFS measurements combined with DFT calculations reveal that the precipitate is an inorganic polymer formed of zigzag TcO2⋅2 H2O chains and give evidence of an increasing polymerization over time. More information can be found in the Research Article by A. F. Oliveira, A. C. Scheinost, and co-workers (DOI: 10.1002/chem.202202235).

Contribution of Molecular Dynamics in pNMR for the Structural Determination of AnV and AnVI Complexes in Solution.

In this study, we propose to use classical molecular dynamics (MD) coupled with 1H NMR spectroscopy to study the conformations of different actinyl AnVI (An = U, Np, and Pu) and AnV (An = Np) complexes with tetra-ethyl dyglicolamide (TEDGA) ligands in order to have a better representation of such complexes in solution. Molecular dynamics simulations showed its effectiveness in interpreting the experiments by the calculation of geometric factors needed for the determination of magnetic properties of these complexes. We demonstrated that different conformations of the AnV and AnVI complexes with TEDGA exist in solution with different coordination modes, which is experimentally confirmed by 1H NMR and EXAFS spectroscopies. Furthermore, MD simulations provide additional insights into the structures of complexes in solution since conformations with fast exchanges, which are not accessible from NMR experiments, have been observed by MD simulations.

Chemical status of zinc in plant phytoliths: Impact of burning and (paleo)environmental implications

Phytoliths are microscopic structures made of amorphous opal (opal-A), an amorphous hydrated silica, dispersed within plant tissues and persisting after the decay of the plant. Silicon is known to alleviate metal toxicity in plants, but the role of phytoliths in metal sequestration and detoxification is unclear. Dry ashing, the most common protocol for phytolith extraction, was previously shown to lead to sequestration of metals by the phytoliths; however, the mechanisms of this process remained elusive. The purpose of this study was to evaluate whether the association between metals and phytoliths results from dry ashing or pre-exists in plant tissues. Thus, we compared phytoliths extracted by dry ashing at 700 °C and plant leaves before and after dry ashing. A combination of ICP-MS, XRD, SEM-EDX and Zn-K-edge EXAFS spectroscopy was used to assess elemental concentrations, morphology and crystallography of silica, and chemical status of Zn. Results demonstrated a phase transition from amorphous opal (opal-A) to opal-CT and α-cristobalite, and the sequestration of metal in phytoliths during dry ashing. For Zn, Mn and Pb, a linear relationship was found between the concentration in phytoliths and in leaves. In the phytoliths, Zn was sequestered in silica in tetrahedral configuration. We hypothesize that this association results form a solid-state reaction during ashing, involving a redistribution of Zn from the organic material to the silica, possibly promoted by the release of structural water from amorphous opal throughout the heating procedure. This study improves our understanding of the impact of high temperature treatments on plant biomass and phytoliths. It suggests that Zn toxicity alleviation in plants by silicon does not rely on its sequestration by phytoliths. In natural settings, wild fire events and biomass burning may lead to metal sequestration in low-soluble form, which should be considered in modeling of biogeochemical cycles and in paleoenvironmental studies.

Study of Spin-Crossover in Iron(II) Complexes with 2,6-Bis(4,5-Dimethyl-1H-Imidazol-2-yl)Pyridine and closo-Borate Anions

New iron(II) coordination compounds with 2,6-bis(4,5-dimethyl-1H-imidazol-2-yl)pyridine (L) and closo-borate(2–) anions [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O have been synthesized. The compounds have been identified and studied by CHN analysis, electron spectroscopy (diffuse reflection spectroscopy), IR, Mössbauer, and EXAFS spectroscopies, X-ray powder diffraction, and static magnetic susceptibility. The structures of the coordination knots of complexes [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O has been obtained by modeling the EXAFS spectra. The ligand is coordinated by the iron(II) ion in a tridentate-cyclic manner by two nitrogen atoms of imidazole cycles and a nitrogen atom of pyridine to form the FeN6 coordination knot. The study of the temperature dependence of the magnetic susceptibility in the range of 80–500 K has showed that the high-temperature spin-crossover 1А1 ↔ 5Т2 manifests itself in the obtained compounds.

Features of the Phase Preferences, Long- and Short-Range Order in Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) with Their Relation to Hydration Behavior

The effect of synthesis conditions on the features of the long- and short-range order of Ln2(WO4)3 (Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was found that crystallization of amorphous precursors begins at 600 °C/3 h and leads to the formation of the monoclinic structure with sp. gr. C12/c1(15) for Ln2(WO4)3 (Ln = Gd, Dy) and with sp. gr. P121/a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature. After annealing at 1000 °C, the P121/a1(14) phase becomes the dominant phase component for all heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates with the P121/a1(14) monoclinic structure correspond to trihydrates Ln2(WO4)3·3H2O formed due to a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to hydration is due to a specific structure of the P121/a1(14) phase with large voids available to water molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure type change and hydration are monitored using EXAFS spectroscopy.

Interaction Mechanism of U(VI) with Redox Active Covalent Organic Framework: EXAFS Spectroscopy and XPS analysis

Interaction Mechanism of U(VI) with Redox Active Covalent Organic Framework: EXAFS Spectroscopy and XPS analysis

Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst.

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Understanding the role of guest ions in the control of thermal expansion of FeFe(CN)6

Understanding the role of guest species in framework materials is important to successfully achieve the control of thermal expansion. In this work, the thermal expansion of the Prussian blue analogue FeFe(CN)6 has been effectively tailored from negative, to zero to positive through the insertion of Na+ ions. To shed light on the role of Na+ ions on thermal expansion behavior, a joint study was conducted by means of high-resolution synchrotron X-ray diffraction, EXAFS spectroscopy and lattice dynamics calculations. It has been found out that the insertion of Na+ ions reduces the structural flexibility of the CN linkages and affects the low-energy phonons, thus allowing the tuning of the overall thermal expansion. This work demonstrates the possibility of precisely controlling the thermal expansion of open-framework materials by inserting guest ions and clarifies the physical mechanism underlying this control.

An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, CrVIO42- can substitute for SO42- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4−8) and the degree of Cr(VI) substitution (0.16–13 mol% CrVIO42--for-SO42- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.

Local atomic environment of Yb3+ in alkaline-earth fluorohalide nanocrystals

The local atomic environment of Yb3+ ions doped into alkaline-earth fluorohalide nanocrystals was probed using Yb L2 edge EXAFS spectroscopy. Two structural models are proposed for the first coordination shell of Yb3+.

Influence of preparation regimes and composition on structure, phase state and electric properties of nanogranular (FeCoZr)x(PZT)100-x (30 ≤ x ≤ 85 at.%) films

Granular films containing Fe50Co50Zr10 alloy nanoparticles inside the ferroelectric matrix Pb0.81Sr0.04(Na0.5Bi0.5)0.15(Zr0.575Ti0.425)O3 (PZT) are characterized by a complex of functional magnetic and electrical characteristics that can be effectively controlled by an external electric or magnetic field. The formation of the necessary granular structure in the case of a PZT matrix is possible only during synthesis in an oxygen-containing atmosphere, leading to significant oxidation of metal nanoparticles. In this regard, an urgent task is to study the degree of oxidation of metal nanoparticles depending on the synthesis conditions, as well as the influence of the forming phases on the electrical properties of films.The relationship of the phase composition and electrical characteristics of granular films (FeCoZr)x(PZT)100-x (30 ≤ x ≤ 85, at.%) obtained in an oxygen-containing atmosphere at a pressure of PO in the range (2.4—5.0) ⋅ 10-3 Pa was studied by X-ray diffraction analysis, EXAFS spectroscopy (Extended X-ray Absorption Fine Structure) and four-probe electrical resistivity measurements.A comparative complex analysis of the structural-phase composition and local atomic order in films (FeCoZr)x(PZT)100-x for the first time showed the fundamental influence of oxygen pressure during synthesis on the oxidation of nanoparticles and their phase composition. It is shown that in the case of oxygen pressure up to the values of PO = 3.2 ⋅ 10-3 Pa, a transition from nanoparticles of complex Fe(Co,Zr) oxides occurs with increasing x to the superposition of complex oxides and ferromagnetic nanoparticles α-FeCo(Zr,O) (or their agglomerations). At a higher oxygen pressure PO = 5.0 ⋅ 10-3 Pa, complete oxidation of nanoparticles is observed with the formation of a complex oxide (FexCo1-x)1-δO with a wustite structure.The observed structural-phase composition allows us to explain the measured temperature dependences of the electrical resistance of granular films, characterized by a negative temperature coefficient of electrical resistance (TKR) in the entire range of film compositions at high oxygen pressure (PO = 5.0 ⋅ 10-3 Pa), and the transition to positive TKR at lower oxygen pressure (PO = 3.2 ⋅ 10-3 Pa) in the synthesis atmosphere and the value x ≤ 69 at.% in films. The transition from negative to positive TKR, indicating the presence of a metallic contribution to conductivity, is fully correlated with the detection by XRD and EXAFS methods of non-oxidized ferromagnetic nanoparticles α-FeCo(Zr,O) or their agglomerations.

Uranium speciation control by uranyl sulfate and phosphate in tailings subject to a Sahelian climate, Cominak, Niger

Long-term uranium mobility in tailings is an environmental management issue. The present study focuses on two U-enriched layers, surficial and buried 14.5 m, of the tailings pile of Cominak, Niger. The acidic and oxidizing conditions of the tailings pile combined with evapotranspiration cycles related to the Sahelian climate control U speciation. Uraninite, brannerite, and moluranite as well as uranophane are relict U phases. EXAFS spectroscopy, HR-XRD, and SEM/WDS highlight the major role of uranyl sulfate groups in uranium speciation. Uranyl phosphate neoformation in the buried layer (paleolayer) acts as an efficient trap for uranium.

Mineralogical characterization and speciation of sulfur, zinc and lead in pyrite cinder from Bergvik, Sweden

Roasting of sulfide ores to produce sulfuric acid began on an industrial scale in the middle 1800′s and is still used extensively worldwide. The residual, pyrite cinders, have commonly been disposed in the environment where they pose a potential and serious threat to aquatic life. In this project, two profiles in a pyrite cinder deposit in Bergvik, Sweden, have been sampled and a comprehensive mineralogical characterization have been carried out. The objectives were to investigate the composition and morphology of pyrite cinder grains and the speciation of sulfur, Pb and Zn in the solid phase. Scanning electron microscopy showed pyrite cinder grains with a core of the ore minerals pyrite and sphalerite enclosed in layers of iron oxides/hydroxides (mainly hematite). XANES analysis, supported by X-ray diffraction analysis, SEM-EDS and bulk element analysis, showed that content of the residual sulfur in the cinder is mainly sulfides, 55–80%, predominately sphalerite. The remaining mass of sulfur is assumed to be adsorbed or precipitated as secondary sulfate minerals, predominantly associated with the grain surfaces. Calculated saturation indexes indicated barite, anglesite and perhaps aluminite. EXAFS spectroscopy indicated that about half of the Zn was bound to O and half was bound to S. LCF analysis of EXAFS spectra indicated that the main Zn species were sphalerite (40–50%) and franklinite (10–20%). The remaining Zn was associated with iron oxides/hydroxides and Zn minerals like hydrozincite or possibly zinc oxides. SEM-EDS analysis showed Pb precipitate located on both the surface of the grains and in the interior as inclusions. The analytical interpretation indicated anglesite.