X-ray Absorption Near-edge Structure Techniques Research Articles

Mechanistic understanding of speciated oxide growth in high entropy alloys

Complex multi-element alloys are gaining prominence for structural applications, supplementing steels, and superalloys. Understanding the impact of each element on alloy surfaces due to oxidation is vital in maintaining material integrity. This study investigates oxidation mechanisms in these alloys using a model five-element equiatomic CoCrFeNiMn alloy, in a controlled oxygen environment. The oxidation-induced surface changes correlate with each element’s interactive tendencies with the environment, guided by thermodynamics. Initial oxidation stages follow atomic size and redox potential, with the latter becoming dominant over time, causing composition inversion. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to elucidate the oxidation process and surface oxide structure evolution. Our findings deconvolute the mechanism for compositional and structural changes in the oxide film and will pave the way for a predictive design of complex alloys with improved resistance to oxidation under extreme conditions.

Migration of phosphorus in pig manure during pyrolysis process and slow-release mechanism of biochar in hydroponic application

Pyrolysis is an effective method for treating of livestock and poultry manure developed in recent years. It can completely decompose pathogens and antibiotics, stabilize heavy metals, and enrich phosphorus (P) in biochar. To elucidate the P migration mechanism under different pig manure pyrolysis temperatures, sequential fractionation, solution 31P nuclear magnetic resonance, X-ray photoelectron spectroscopy, X-ray diffraction, and K-edge X-ray absorption near-edge structure techniques were used to analyze the P species in pig manure biochar (PMB). The results indicated that most of the organic P in the pig manure was converted to inorganic P during pyrolysis. Moreover, the transformation to different P groups pathways was clarified. The phase transition from amorphous to crystalline calcium phosphate was promoted when the temperature was above 600 °C. The content of P extracted by hydrochloric acid, which was the long-term available P for plant uptake, increased significantly. PMB pyrolyzed at 600 °C can be used as a highly effective substitute for P source. It provides the necessary P species (e.g. water-soluble P.) and metal elements for the growth of water spinach plants, and which are slow-release comparing with the Hogland nutrient solution.

Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO3 Nickelate

Rare-earth perovskite nickelates RNiO3 (R stands for Y, rare earths, Tl or Bi) have attracted wide attention in the past few decades because of their unique electronic properties that emerge from the insulator–metal transition (IMT). This transition can be tuned by chemical substitutions or by external variables, like pressure and temperature, but the mechanism of this transition still remains debated. While most previous studies focused on nickelates with partially filled 4f cations on the R site, here we studied the mechanism of the pressure-induced phase transition on a rare Tl3+ nickelate, which comprises fully filled 4f orbitals. We probed the pressure-induced structural and electronic changes taking place at the bulk, medium, and local atomic level at room temperature by using synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) as probing techniques, respectively. High-resolution XRD data under ambient conditions show evidence for the monoclinic phase of TlNiO3, as characterized by the charge ordering between Ni3−δ and Ni3+δ in the insulator regime. High-pressure XRD data elucidated an anomaly in the evolution of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P21/n → Pbnm. High-pressure EXAFS data at the Ni K-edge provide a solid proof for a short-order charge disproportionation under near ambient conditions. The Ni–O pair-bond compression curves unveiled an isotropic overlapping Ni t2g6eg1 → O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distance variance (Debye–Waller exponents) lodged a hardening at this pressure point, suggesting constrained atomic displacements in the average range of ±0.04(5) Å. We propose that this reduced motional freedom could be linked to an electron–phonon coupling, which is the driving force behind the mechanisms of the IMT. We demonstrated that the XANES technique can be applied to ascertain the pressure-induced transition, by tracking the anomalous behavior of the position and width (full width at half-maximum) of the XANES features.

Electron Paramagnetic Resonance and Synchrotron X-ray Absorption Spectroscopy for Highly Sensitive Characterization of Calcium Arsenates.

Calcium arsenates such as pharmacolite (CaHAsO4·2H2O), haidingerite (CaHAsO4·H2O), and weilite (CaHAsO4) are important sinks for arsenic in mine tailings as well as other natural and contaminated sites and are useful for reducing the mobility and bioavailability of this toxic metalloid in the environment. However, calcium arsenates usually occur in trace amounts dominated by other phases, making their detection, identification, and quantification challenging. In this contribution, pharmacolite, haidingerite, and weilite are shown to exhibit subtle but distinct postedge differences in As K-edge X-ray absorption near-edge structure (XANES) spectra and feature characteristic [AsO3]2-, [AsO4]2-, and [AsO4]4- radicals, all derived from the diamagnetic [HAsO4]2- precursor during γ-ray irradiation, in electron paramagnetic resonance (EPR) spectra. In particular, the 75As (nuclear spin I = 3/2 and natural isotope abundance = 100%) hyperfine coupling constants of the [AsO3]2- radicals in pharmacolite and haidingerite as well as other minerals (e.g., calcite and gypsum) are clearly distinct, allowing the unambiguous identification of calcium arsenates by the EPR technique readily at ∼0.1 wt %. Similarly, linear combination fittings of As K-edge XANES spectra demonstrate that pharmacolite and haidingerite at ∼0.1 wt % each in gypsum-rich mixtures can be detected and quantified as well. Therefore, a combination of the EPR and XANES techniques is a powerful approach for the highly sensitive characterization of calcium arsenates in the quest for the safe management and remediation of arsenic contamination. This work demonstrates the highly sensitive characterization of calcium arsenates by integrated electron paramagnetic resonance and synchrotron X-ray absorption spectroscopy.

Influence of in-situ and ex-situ Cu-Fe doping in K-OMS-2 catalysts on dye degradation via Fenton-like reaction with focus on catalytic properties and performances

The aims of this work were to synthesize Cu and Fe doping on manganese-based octahedral molecular sieves (K-OMS-2) via the in-situ hydrothermal method (Cu-Fe-OMS-2) and impregnation on K-OMS-2 for ex-situ techniques (Cu-Fe/OMS-2). The catalytic activities of prepared samples were evaluated through the degradation of reactive red 120 (RR-120) applying a Fenton-like reaction with peroxymonosulfate (PMS). The physicochemical properties of the catalysts were investigated using X-ray diffraction (XRD), N2 adsorption-desorption, Field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) techniques. The effects of catalyst loading, initial pH of the solution and PMS concentration were examined by applying the design of the experiment (DOE) based on the Box-Behnken design (BBD) on RR-120 degradation. Results demonstrated that cryptomelane crystalline phases of the Cu-Fe-OMS-2 and Cu-Fe/OMS-2 catalysts were successfully synthesized, confirmed by XRD measurement. The morphology of the in-situ catalyst represented in rod-like shapes and their size apparently decreased when doped with Cu and Fe metals. On the other hand, the surface morphology of the ex-situ catalyst displayed the aggregation of copper and iron oxides particles on the rod-like of K-OMS-2. The specific surface area of in-situ catalyst showed higher than ex-situ catalyst about 6 times. The oxidation states of Mn, Cu and Fe atoms were analyzed using XPS and XANES techniques. The valence states of the Mn atom in all prepared samples presented Mn3+ and Mn4+, while the Fe atom demonstrated Fe3+ in both modified samples. Simultaneously, the Cu atom in both modified samples illustrated different valence states, ex-situ catalyst (Cu+ and Cu2+) and in-situ catalyst (Cu2+ and Cu3+). The RR-120 removal by Cu-Fe-OMS-2 was more effective than Cu-Fe/OMS-2 in every run order. The significant effects on dye degradation for Cu-Fe-OMS-2 were the initial pH of the solution and catalyst loading. For Cu-Fe/OMS-2, only the catalyst loading effect was significant. The significant interaction term was found only Cu-Fe-OMS-2 catalyst between the initial pH of the solution and PMS concentration. The degradation percentage of the Cu-Fe-OMS-2 catalyst demonstrated higher than the Cu-Fe/OMS-2 catalyst about 5% at pH 4, catalyst loading of 0.85 g/L and PMS of 4.70 mM conditions.

Elusive Coordination of the Ag+ Ion in Aqueous Solution: Evidence for a Linear Structure.

X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag+ ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag-O bond distance of 2.34(2) Å, different from the first generally accepted tetrahedral model with a longer mean Ag-O bond distance. Ab initio molecular dynamics simulations with the Car-Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag+ ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag+ ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag+ in water. Altogether these data show how the description of the hydration of the Ag+ ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag-O bond distance in the linear [Ag(H2O)2]+ ion to be ca. 0.2 Å longer than in isolated ions in solid state.

An X-ray Absorption Near-Edge Structure (XANES) Study on the Oxidation State of Chromophores in Natural Kunzite Samples from Nuristan, Afghanistan

Kunzite, the pink variety of spodumene is famous and desirable among gemstone lovers. Due to its tenebrescent properties, kunzite always remains a hot research candidate among physicists and mineralogists. The present work is continuing the effort towards value addition to kunzite by enhancing its color using different treatments. Before color enhancement, it is essential to identify the chromophores and their oxidation states. In this paper, the authors investigated the main impurities in natural kunzite from the Nuristan area in Afghanistan and their valence states. Some impurities in the LiAlSi2O6 spodumene structure were identified and quantified by using sensitive techniques, including Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), UV−VIS and X-ray absorption near-edge structure (XANES). LA-ICP-MS indicated many trace elements as impurities in kunzite, among which Fe and Mn are the main elements responsible for coloration. The oxidation states of these two transition elements were determined by the XANES technique. The study reveals that Mn is present in both Mn2+ and Mn3+ oxidation states, while Fe is present only in Fe3+ oxidation state.

The insights from X-ray absorption spectroscopy into the local atomic structure and chemical bonding of Metal–organic frameworks

After a short introduction highlighting the potentialities of metal–organic frameworks (MOFs) and the power of X-ray absorption spectroscopies (XAS), a list of selected examples follows. The first part of this review provides the basic concepts that are behind the most used XAS techniques: Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. Moreover, it reports also the basis of an innovative approach to the EXAFS data analysis based on the wavelet transform approach, that allows a more solid attribution to the different paths contributing to the overall signal. The second part is devoted to provide a section of examples where EXAFS and XANES techniques have been determinant in understanding the structural and electronic properties of metal centers in MOFs. The selected examples have been ordered in subsections related to the application foreseen for the investigated MOFs materials: MOFs functionalization for catalytic applications; MOFs as single site catalyst; MOFs as photocatalysts; MOFs for gas sorption and storage; MOFs for energy application. Conclusions and perspectives are provided at the end.

Synchrotron-based XAS on structure investigation of La0.99−xSrx(Na, K, Ba)0.01MnO3 nanoparticles: Evidence of magnetic properties

This work presents the structural study of La0.99−xSrx(Na, K, Ba)0.01MnO3 or LSAM nanoparticles synthesized using thermal-hydro decomposition method where A denotes Na, K, Sr and Ba, respectively. The effect of ionic radii size of A dopants or rA from the substitution of A for La and Sr on the MnO6 octrahedral structure, where the average size of the cations occupying in A-site or 〈rA〉 is fixed at ∼ 1.24 Å, is focused. The LSAM nanoparticles are carefully studied using X-ray diffraction (XRD) including Rietveld refinement and X-ray Absorption Spectroscopy (XAS) including X-ray Absorption Near edge Structure (XANES) and X-ray Absorption Fine Structure (EXAFS). The Rietveld refinement shows all nano-powder samples have rhombohedral structure. By XANES technique we found that the effect of A substitutions at A-site causes a slight change of mean oxidation state of Mn between 3.54 and 3.60. Furthermore, the structural distortion of MnO6 octrahedral in samples is analysed and obtained from EXAFS. The observed trend of ferromagnetism for all LSAM samples can be clearly explained by evidences of A-site doping, structural distortion around Mn atoms and mixing Mn3+/Mn4+ valence states.

Extraction of copper species from the nanoporous sorbent with an ionic liquid

Abstract The extraction of copper species from the nanoporous sorbent into an ionic liquid ([C 4 mim][PF 6 ], 1-butyl-3-methylimidazolium hexafluorophosphate) was studied herein by means of X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS) spectroscopies. The nanoporous sorbent is composed of adsorbed humic acid (HA) on nanoporous SiO 2 to simulate copper-contaminated nanoporous soils. During extraction with [C 4 mim][PF 6 ] at 298, 323 and 393 K, approximately 20, 40 and 54%, respectively, of the copper species on the nanoporous sorbent were extracted into [C 4 mim][PF 6 ]. Using the least-square fitted XANES spectra, the major copper species on the nanoporous sorbent was determined to be Cu(II)-HA (Cu(II) adsorbed on HA) (86%). At 298, 323 and 393 K, the copper dissociated from the extracted Cu(II)-HA and immediately formed Cu(mim) 4 2 + (mim = 1-methylimidazole) in [C 4 mim][PF 6 ]. At 323 and 393 K, Cu(mim) 4 2 + was oxidized with oxygen in [C 4 mim][PF 6 ] to form nanosized CuO. In this study, it was found that at least three reaction paths for the extracted copper to take from the nanoporous sorbent to [C 4 mim][PF 6 ] at 298–393 K: (1) extraction of Cu(II)-HA from the nanoporous sorbent to [C 4 mim][PF 6 ]; (2) dissociation of copper from extracted Cu(II)-HA to form Cu(mim) 4 2 + in [C 4 mim][PF 6 ]; and (3) oxidation of Cu(mim) 4 2 + to form nanosized CuO in [C 4 mim][PF 6 ]. Combining 1 H and 31 P NMR spectra, nanosized CuO interacted with [C 4 mim][PF 6 ] at 393 K. The fitted EXAFS spectra show the Cu(mim) 4 2 + in [C 4 mim][PF 6 ] possessed a Cu N bond distance of 1.82 A at 298 and 323 K. It is also found that the Cu O bond distance was 1.93 A at 393 K. This work demonstrates that the XANES technique can reveal structural changes of the copper species from the nanoporous sorbent into [C 4 mim][PF 6 ] during extraction.

Synchrotron X-ray microfluorescence measurement of metal distributions in Phragmites australis root system in the Yangtze River intertidal zone.

This study investigates the distributions of Br, Ca, Cl, Cr, Cu, K, Fe, Mn, Pb, Ti, V and Zn in Phragmites australis root system and the function of Fe nanoparticles in scavenging metals in the root epidermis using synchrotron X-ray microfluorescence, synchrotron transmission X-ray microscope measurement and synchrotron X-ray absorption near-edge structure techniques. The purpose of this study is to understand the mobility of metals in wetland plant root systems after their uptake from rhizosphere soils. Phragmites australis samples were collected in the Yangtze River intertidal zone in July 2013. The results indicate that Fe nanoparticles are present in the root epidermis and that other metals correlate significantly with Fe, suggesting that Fe nanoparticles play an important role in metal scavenging in the epidermis.

The effect of Gd doping in La1−x−yGdxSryMnO3 compound on nanocrystalline structure by X-ray Absorption Spectroscopy (XAS) technique

This work presents the structural study of La1−x−yGdxSryMnO3 or LGSM compositions where 0⩽x⩽0.15 and 0⩽y⩽0.45, respectively. In particular, the effect of doping Gd concentration and the size mismatch of A-site cations from the substitution of Gd for La on the MnO6 octahedral structure is focused as the average size of the cations occupying in A-site is fixed. The LGSM nanoparticles are carefully studied and analyzed using X-ray diffraction (XRD) and X-ray Absorption Spectroscopy (XAS) including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). The XRD results obtained from Rietveld refinement show that all nano-powder samples have rhombohedral structure. From the XANES technique we found that the effect of Gd substitutions causes the slight increase of mean oxidation state of Mn from 3.50 to 3.65. Furthermore, the results of MnO6 octahedral analysis obtained from EXAFS technique show that the local structure around Mn atoms for LG05 and LG10 samples can be explained by 4(s)+2(l) model while the LG15 sample can be explained by 6 model.

Assessment of Hedley’s Sequential Extraction Method for Phosphorus Forms in Biosolids Using P K-edge X-ray Absorption Near-edge Structure Spectroscopy

Abstract We determined P species in biosolids and their residues after applying Hedley’s sequential extractions using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. HCl extraction successfully removed P associated with Ca minerals from the biosolids. The majority of P associated with Fe minerals (FeP) was extracted by the NaOH fluid. However, recalcitrant FeP phases remained persistent even after the final extraction process, suggesting that NaOH-extracted P fractions may be underestimated in biosolids with abundant Fe. The XANES technique is essential to identify secondary P phases in biosolids, which could not be detected by an XRD technique.

Soil phosphorus bioavailability assessed by XANES and Hedley sequential fractionation technique in a glacier foreland chronosequence in Gongga Mountain, Southwestern China

The primary productivity of terrestrial ecosystems is influenced by soil phosphorus bioavailability, which depends largely on chemical fractions of phosphorus. The sequential fractionation technique developed by Hedley et al. or its subsequent modification is a well-known method to determine soil phosphorus forms. Hedley sequential fractionation technique separates the phosphorus into fractions based on their different chemical solubilities in extractants with certain chemical properties. Recently, synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy has been employed to measure soil phosphorus species directly and non-invasively. The XANES method provides information concerning local structure and chemical information of target elements at a molecular level. Thus, it can distinguish phosphorus fractions bound by metal oxides or hydroxides (such as Fe, Al, and Ca). In this present work, the phosphorus speciation of topsoil along a glacial foreland chronosequence in Gongga Mountain is determined using these two methods. The changes in soil phosphorus bioavailability along the 120-year-old chronosequence are assessed based on comparisons of the results obtained by these two methods. The results indicate that Hedley sequential fractionation technique shows a greater ability to determine soil bioavailable phosphorus (Resin-P and NaCHO3-P), while XANES is effective in distinguishing phosphorus bound by metal compounds. In the chronosequence, Ca- and Al-bound phosphorus were derived mainly from primary minerals, whose phosphorus contents decreased within 120 years of moraine weathering and soil development. The content of soil bioavailable phosphorus increased rapidly after 30 years since deglaciation. The increasing phosphorus bioavailability promoted the colonizing and primary succession vegetation.

X-ray absorption near-edge structure anomalous behaviour in structures with buried layers containing silicon nanocrystals

Substructure and phase composition of silicon suboxide films containing silicon nanocrystals and implanted with carbon have been investigated by means of the X-ray absorption near-edge structure technique with the use of synchrotron radiation. It is shown that formation of silicon nanocrystals in the films' depth (more than 60 nm) and their following transformation into silicon carbide nanocrystals leads to abnormal behaviour of the X-ray absorption spectra in the elementary silicon absorption-edge energy region (100-104 eV) or in the silicon oxide absorption-edge energy region (104-110 eV). This abnormal behaviour is connected to X-ray elastic backscattering on silicon or silicon carbide nanocrystals located in the silicon oxide films depth.

On the possibility of using XANES to investigate bromide-based ionic liquids

X-ray absorption near edge structure (XANES) spectra have been collected for 1-alkyl-3-methylimidazolium bromide [Cnmim]Br ILs with different alkyl chains (n=2, 4). The XANES technique has been found to be sensitive to the subtle differences in the Br− coordination environment present in [C2mim]Br and [C4mim]Br ILs, that are solid at room temperature. With this Letter we show that XANES is a new effective tool for studying the local structural arrangements around selected atoms in ILs both in the solid and liquid phase.

Beneficial influence of EDTA on the structure and catalytic properties of sulfided NiMo/SBA-15 catalysts for hydrotreating of light gas oil

SBA-15 supported NiMo hydrotreating catalysts with different EDTA/Ni molar ratio were prepared by incipient wetness impregnation method. Hydrotreating activities of these catalysts were studied with Athabasca bitumen derived light gas oil and comparison was done with NiMo/SBA-15 and NiMo/γ-Al2O3 catalysts. A beneficial effect of chelating ligand was seen in hydrotreating activity and 28% increase in HDS of LGO was found as compared to NiMo/γ-Al2O3 in the case of EDTA/Ni molar ratio 4. Detailed mechanistic aspect of interaction between support – EDTA (ethylene diamine tetraacetic acid), EDTA-metallic species, support – metal, metallic species – metallic species at different reaction conditions is studied at different process parameters by using different characterization techniques such as X-ray absorption near-edge structure (XANES), Fourier transform infrared spectroscopy (FTIR), HRTEM, XRD, Raman, temperature programmed reduction (TPR), ICP-MS, CO chemisorption and N2 adsorption. A detailed scheme has been developed to explain the behavior of chelating agent in mesoporous SBA-15 supported catalysts. HRTEM results explain the high dispersion of Ni and MoS2 active phases in sulfided state even though big crystallite of MoO3 was seen in oxide state characterized by XRD and Raman spectra which explains role of EDTA in redistribution of active phases during sulfidation. XANES technique is used as predominant method for atomic level study of various structural changes in sulfided and oxide catalysts to find out the appropriate mechanism of the reaction. Characterization by XANES reveals that the presence of chelating agent delayed Ni2+ sulfidation which is main cause of improvement in HDS and HDN activity of the catalysts prepared in presence of organic chelates. Nickel sulfidation starts only when the EDTA–Ni complex decomposes and released nickel atoms move to the reactive edges of the MoS2 to form a finely dispersed sulfide NiMoS type II active phase.

Evidence for a new mechanism of Fe2O3 decomposition in lightweight aggregate formation

Hematite (Fe2O3) chemical reduction into FeO and Fe3O4 by releasing O2 at high temperatures is considered one of the generally accepted mechanisms for processing waste minerals and clay into lightweight aggregate construction materials. In many case studies, this mechanism has not been strictly confirmed. To verify whether hematite can effectively release O2 at 1,000–1,260°C, a material containing hematite, simulating waste sediments from a Taiwanese reservoir, was shaped into pellets and fired into lightweight aggregates at high temperatures for 20 min and studied with various techniques. As revealed by the X-ray absorption near-edge structure technique, almost all the hematite remained as Fe(III) in the pellets when fired at 1,000–1,260°C, implying a negligible release of O2 leading to the creation of pores. This finding shows that the generally accepted mechanism for lightweight aggregate formation associated with hematite decomposition into FeO, Fe3O4, and O2 is invalid. Furthermore, Fe(III)-containing composites were formed in the fired pellets. Although firing at 1,000°C can trigger the decomposition of the components K2CO3, Na2CO3, and CaCO3 with a release of CO2, the sintering reaction was seemingly too weak to encapsulate the gases effectively. For pellets fired at 1,050–1,150°C, pores grew in size because the sintering reaction sufficed to generate a glassy phase that could better encapsulate gases.

Speciation and fractionation of nickel in airborne particulate matter: comparison between selective leaching and XASspectroscopy

Nickel speciation and fractionation using a multidisciplinary approach are discussed for different particulate matter samples collected in industrial and rural atmospheres. The technologies utilized in this research span from X-ray Absorption Near Edge Structure (XANES) and X-Ray Diffraction (XRD) to a wet chemistry sequential leaching assay (including determination by inductively coupled plasma atomic emission spectroscopy, ICP-AES). The Zatka sequential leaching method provides an inexpensive assay to differentiate among ‘soluble’, ‘sulfidic’, ‘metallic’, and ‘oxidic’ chemical forms of Ni. The XANES technique is especially well suited for Ni speciation between and to a lesser extent within the 4 defined Ni species groups of the Zatka sequential leaching procedure. Limitations for interpretation in the present study with respect to XANES are the availability of pure phase Ni species for uptake as reference spectra and the collinearity between the spectra of Ni compounds within a Ni species group (e.g. NiSO4·6H2O and Ni(NO3)2·6H2O). The Ni speciation and fractionation results on the particulate matter samples reflect in general a good agreement between the modified Zatka sequential leaching procedure and the XANES data. For the particulate matter collected in and close to a stainless steel factory, Ni included in a spinel structure (NiFe2O4) was identified as the principal Ni species. The particulate matter collected in rural atmosphere showed a 50/50 distribution between soluble and oxidic Ni species.

XRD and XANES study of reactions in particulate Al–Mg alloy composites

Reactivity in stir cast Al Uju Mg alloy composites reinforced with fly ash was investigated by means of X‐ray diffraction (XRD) and X‐ray absorption near edge structure (XANES) spectroscopy at Al and Si K‐edges. High purity Al, α‐Al2O3, MgAl2O4 (spinel), MgO and SiO2 powders were used as reference materials in the XANES investigation. The XRD and XANES data acquired from the raw fly ash used in making the composites were compared with fly ash particles extracted from the as cast composites. The major reaction product formed during fabrication of the composites was spinel. The XANES technique was useful in verifying the presence of various oxides in raw and extracted fly ash particles.On a investigué la réactivité de composites de l’alliage Al–Mg renforcé avec des cendres volantes et moulé par agitation, au moyen de la diffraction des rayons X (XRD) et de la spectroscopie de structure près du front d’absorption des rayons X (XANES) aux seuils K de l’Al et du Si. On a utilisé des poudres d’Al, d’Al2O3‐α, de MgAl2O4 (spinelle), de MgO et de SiO2 de haute pureté comme matériaux de référence dans l’investigation par XANES. On a comparé les données de XRD et de XANES acquises à partir des cendres volantes brutes utilisées dans la fabrication des composites, avec les particules de cendres volantes extraites des composites de brut de coulée. Le produit de réaction majeur formé lors de la fabrication des composites était le spinelle. La technique de XANES était utile dans la vérification de la présence d’oxydes variés dans les particules de cendres volantes brutes et extraites.