Linear Combination Fitting Research Articles

Rare Earth Element Speciation in Coal and Coal Combustion Byproducts: A XANES and EXAFS Study.

Transitioning to a low-carbon economy, necessary to mitigate the impacts of anthropogenic climate change, will lead to a significant increase in demand for critical minerals such as rare earth elements (REE). Meeting these raw materials requirements will be challenging, so there is increasing interest in new sources of REE including coal combustion byproducts (CCBs). Extraction of REE from CCBs can be advantageous as it involves reusing a waste product, thereby contributing to the circular economy. While a growing body of literature reports on the abundance of REE in CCBs globally, studies examining the key factors which control their recovery, including speciation and mode of occurrence, are lacking. This study employed synchrotron-based X-ray absorption spectroscopy to probe the speciation and local bonding environment of yttrium in coals and their associated CCBs. Linear Combination Fitting identified silicate and phosphate minerals as the dominant REE-bearing phases. Taken together with the results of extended X-ray absorption fine structure (EXAFS) curve fitting, we find there is minimal transformation in the REE host phase during combustion, indicating it is transferred in bulk from the coals to the CCBs. Accordingly, these findings can be incorporated into the development of an efficient, environmentally conscious recovery process.

Effect of Cu incorporation in silicate and modified borate bioglass materials for health applications: insights from synchrotron-based x-ray absorption spectroscopy

This contribution investigates the effect of variable copper incorporation (x = 0.2, 1.0, 2.0, and 4.0) in silicate (45 SiO2, 24.5 CaO, 24.5 Na2O, 6P2O5wt%) and modified borate (45 B2O3, 24.5 CaO, 24.5 Na2O, 6P2O5wt%) bioglass materials to be used for bone bonding applications. X-ray absorption fine structure spectroscopy (XAFS) has been used to determine the oxidation states and local coordination structure of Cu atoms in silicate-based and borate-based glasses at the Cu K-edge (~ 8979 eV). The oxidation states of Cu atoms have been determined by near-edge XAFS (XANES) fingerprinting employing reference standard compounds of Cu. Cu (I) and Cu (II) XANES spectra of the standard reference compounds were linearly combined to fit the normalized μ(E) data of the collected XANES spectra using linear combination fitting (LCF approach). The obtained results prove that most of the silicate glass samples contain Cu2O almost exclusively, while modified borate glass samples contain a significant mixture of Cu2O and CuO phases. According to the literature, the remarkable coexistence of Cu2O and CuO phases within the borate sample, particularly when x = 4, promotes the conversion process to allow the more facile formation of hydroxy carbonate apatite (HCA). The best fit structural parameters derived from extended-XAFS (EXAFS) fitting show that the ratio between Cu (I) and Cu (II) in borate glass agreed well with that extracted from XANES analysis. XANES and EXAFS conclude that borate glass with x = 4 is the most suitable composition for bone bonding applications.

Microscale heterogeneity of phosphorus species associated with secondary mineral phases in the B horizons of two boreal Podzols

Poorly crystalline Al and Fe minerals have high sorption capacities for phosphate anions. However, there is still uncertainty about the molecular-level distribution of phosphorus (P) species between Al and Fe mineral phases, which can be relevant for bioavailability. The present study employed synchrotron X-ray microscopic techniques to distinguish between Al- and Fe-associated P phases in two Podzol B horizons (Flakaliden and Skogaby). First, P species were clustered into major groups depending on the main element association (Al-P, Fe-P, Ca-P + organic P) according to the color of P-containing spots in the tricolor μ-XRF quantitative maps that combined P, Al, and Fe fluorescence intensities. Second, the results were analyzed using quantitative linear combination fitting (LCF) of the P K-edge μ-XANES spectra. Third, in cases when the results diverged, a careful visual comparison between the diagnostic features of the XANES spectra of the samples and the reference standards was made. In 45 of 54 investigated spots, the color method and XANES-LCF produced consistent results. In 2 spots where the results were inconsistent, the diagnostic features of the XANES spectra agreed with the color method but not with XANES-LCF. In 39 of the 47 spots where the major P phase could be identified, Al-P was found to predominate. Ca-P was the major phase in 5 spots, Fe-P in 2, and organic P in 1. Most of the Al-P probably consisted of PO4 adsorbed to imogolite-type materials (ITM), as evidenced by oxalate extraction and Fourier Transform Infrared (FTIR) spectroscopy, although at Skogaby, adsorbed organic P may have played an important additional role. Overall, the P speciation showed substantial spatial heterogeneity, particularly in the upper B horizon of the Skogaby soil and in the lower B horizon of the Flakaliden soil. In conclusion, Al-P, particularly ITM-adsorbed PO4, is the main P phase in the B horizons of the studied Podzols, which may have implications for bioavailability as ITM is more easily dissolved than Fe oxides. Also, the combination of XANES-LCF, colors of the μ-XRF maps, and diagnostic features of the XANES spectra led to robust estimates of the major P phases in soil microsites.

In situ arsenic immobilization by natural iron (oxyhydr)oxide precipitates in As–contaminated groundwater irrigation canals

Arsenic-contaminated groundwater is widely used in agriculture. To meet the increasing demand for safe water in agriculture, an efficient and cost-effective method for As removal from groundwater is urgently needed. We hypothesized that Fe (oxyhydr)oxide (FeOOH) minerals precipitated in situ from indigenous Fe in groundwater may immobilize As, providing a solution for safely using As-contaminated groundwater in irrigation. To confirm this hypothesis and identify the controlling mechanisms, we comprehensively evaluated the transport, speciation changes, and immobilization of As and Fe in agricultural canals irrigated using As-contaminated groundwater. The efficiently removed As and Fe in the canals accumulated in shallow sediment rather than subsurface sediment. Linear combination fitting (LCF) analysis of X–ray absorption near edge spectroscopy (XANES) indicated that As(V) was the dominant As species, followed by As(III), and there was no FeAsO4 precipitate. Sequential extraction revealed higher contents of amorphous FeOOH and associated As in shallower sediment than in the subsurface layer. Stoichiometric molar ratio calculations, SEM‒EDS, FTIR, and fluorescence spectroscopy collectively demonstrated that the microbial reductive dissolution of amorphous FeOOH proceeded via reactive dissolved organic matter (DOM) consumption in subsurface anoxic porewater environment facilitating high labile As, whereas in surface sediment, the in situ-generated amorphous FeOOH was stable and strongly inhibited As release via adsorption. In summary, groundwater Fe2+ can efficiently precipitate in benthic surface sediment as abundant amorphous FeOOH, which immobilizes most of the dissolved As, protecting agricultural soil from contamination. This field research supports the critical roles of the phase and reactivity of in situ-generated FeOOH in As immobilization and provides new insight into the sustainable use of contaminated water.

A method for correcting self-absorption shown in fluorescence XAFS spectra

X-ray-absorption fine-structure (XAFS) measurement by the fluorescence-yield (FY) method can be applied to samples of various shapes and concentrations. However, it is known that XAFS spectra obtained by the FY method do not precisely reflect the original X-ray absorption of the sample due to the self-absorption effect. We therefore propose a method for correcting the self-absorption by using FY XAFS spectra of original and diluted samples. The proposed self-absorption correction method can be applied to samples with unknown compositions. The effectiveness of the correction method was evaluated by applying it to the FY XAFS spectra of several iron oxides and their mixtures. As a result, the corrected FY XAFS of Fe K-edge spectra were almost identical to the XAFS spectra obtained by the transmission method. The suppressed EXAFS oscillations of the FY XAFS spectra were also recovered. These results validate the effectiveness of the proposed correction method. They also demonstrate that the corrected XANES spectra can be applicable to linear combination fitting analysis.

Phosphorus speciation in manure and fertilizer impacted Mid-Atlantic coastal plain soils.

Historical applications of manures and fertilizers at rates exceeding crop P removal in the Mid-Atlantic region (United States) have resulted in decades of increased water quality degradation from P losses in agricultural runoff. As such, many growers in this region face restrictions on future P applications. An improved understanding of the fate, transformations, and availability of P is needed to manage P-enriched soils. We paired chemical extractions (i.e., Mehlich-3, water extractable P, and chemical fractionation) with nondestructive methods (i.e., x-ray absorption near edge structure [XANES] spectroscopy and x-ray fluorescence [XRF]) to investigate P dynamics in eight P-enriched Mid-Atlantic soils with various management histories. Chemical fractionation and XRF data were used to support XANES linear combination fits, allowing for identification of various Al, Ca, and Fe phosphates and P sorbed phases in soils amended with fertilizer, poultry litter, or dairy manure. Management history and P speciation were used to make qualitative comparisons between the eight legacy P soils; we also speculate about how P speciation may affect future management of these soils with and without additional P applications. With continued P applications, we expect an increase in semicrystalline Al and Fe-P, P sorbed to Al (hydro)oxides, and insoluble Ca-P species in these soils for all P sources. Under drawdown scenarios, we expect plant P uptake first from semicrystalline Al and Fe phosphates followed by P sorbed phases. Our results can help guide management decisions on coastal plain soils with a history of P application.

The Effect of Water Co-Feeding on the Catalytic Performance of Zn/HZSM-5 in Ethylene Aromatization Reactions.

During the methanol-to-aromatics (MTA) process, a large amount of water is generated, while the influence and mechanism of water on the activity and selectivity of the light olefin aromatization reaction are still unclear. Therefore, a study was conducted to systematically investigate the effects of water on the reactivity and the product distribution in ethylene aromatization using infrared spectroscopy (IR), intelligent gravitation analyzer (IGA), and X-ray absorption fine structure (XAFS) characterizations. The results demonstrated that the presence of water reduced ethylene conversion and aromatic selectivity while increasing hydrogen selectivity at the same contact time. This indicated that water had an effect on the reaction pathway by promoting the dehydrogenation reaction and suppressing the hydrogen transfer reaction. A detailed analysis using linear combination fitting (LCF) of Zn K-edge X-ray absorption near-edge spectroscopy (XANES) on Zn/HZSM-5 catalysts showed significant variations in the state of existence and the distribution of Zn species on the deactivated catalysts, depending on different reaction atmospheres and water contents. The presence of water strongly hindered the conversion of ZnOH+ species, which served as the active centers for the dehydrogenation reaction, to ZnO on the catalyst. As a result, the dehydrogenation activity remained high in the presence of water. This study using IR and IGA techniques revealed that water on the Zn/HZSM-5 catalyst inhibited the adsorption of ethylene on the zeolite, resulting in a noticeable decrease in ethylene conversion and a decrease in aromatic selectivity. These findings contribute to a deeper understanding of the aromatization reaction process and provide data support for the design of efficient aromatization catalysts.

A Fine Analysis of Zn Species Structure and Distribution in Zn/ZSM-5 Catalysts by Linear Combination Fitting Analysis of XANES Spectra

Investigating the distribution of different Zn species on Zn-containing zeolite catalysts is crucial for identifying the active sites and establishing the relationship between the catalyst’s structure and its activity in the process of ethylene aromatization. By utilizing X-ray absorption near edge spectra (XANES) of various reference samples, this study employed linear combination fitting (LCF) analysis on XANES spectra of real samples to accurately measure the changes in the distribution of Zn species in Zn-containing HZSM-5 zeolites under different Zn sources and loadings. The results showed that ZnOH+, ZnO clusters, and ZnO crystalline structures coexist in Zn/HZSM-5 catalysts prepared through physical mixing and incipient wet impregnation methods. A similar trend was observed for catalysts prepared using different methods, with an increase in Zn content resulting in a decrease in the proportion of ZnOH+ and a significant increase in the amount of larger ZnO crystals. Furthermore, ZnO clusters were confined within the zeolite pores. The findings of this study established a direct correlation between the amount of ZnOH+ determined through LCF analysis and both the rate of hydrogen production and the rate of aromatics formation, providing strong evidence for the catalytic role of ZnOH+ as an active center for dehydrogenation, which plays a key role in promoting the formation of aromatics. The method of LCF analysis on XANES spectra allows for the determination of the local structure of Zn species, facilitating a more precise analysis based on the distribution of these species. This method not only provides detailed information about the Zn species but also enhances the accuracy of the overall analysis.

X-ray absorption spectroscopic studies of Fe environments in borosilicate waste glasses synthesized under a variety of redox conditions

Fe K-edge X-ray absorption spectroscopy (XAS) was performed on two borosilicate waste glass series to determine Fe valence and coordination changes over a wide range of redox synthesis conditions. Data were also collected on eight Fe-silicate standards, including the rare phase, pellyite, that was used as a tetrahedral Fe2+O4 standard. X-ray absorption near-edge structure (XANES) linear combination fits of the Fe2+O4, Fe2+O5, and Fe3+O4 standards data to the glass spectra determined Fe2+/Fetotal ratios from 0.07 to 0.90 that are within 10 % of Mössbauer findings. As the glasses become reduced, XANES indicates Fe3+O4 tetrahedra are being replaced by Fe2+O4 tetrahedra, and possibly, Fe2+O5 sites, while extended X-ray absorption fine structure analyses show average Fe-O distances increase from 1.88 to 1.96 Å. The XANES-determined three Fe-site population distributions in the glasses are similar to findings reported in a recent molecular dynamics study of Fe redox effects in waste glass.

Multivariate regression analysis of factors regulating the formation of synthetic aluminosilicate nanoparticles.

Interest is growing in nanoparticles made of earth abundant materials, like alumino(silicate) minerals. Their applications are expanding to include catalysis, carbon sequestration reactions, and medical applications. It remains unclear, however, what factors control their formation and abundance during laboratory synthesis or on a larger industrial scale. This work investigates the complex system of physicochemical conditions that influence the formation of nanosized alumino(silicate) minerals. Samples were synthesized and analyzed by powder X-ray diffraction, in situ and ex situ small angle X-ray scattering, and transmission electron microscopy. Regression analyses combined with linear combination fitting of powder diffraction patterns was used to model the influence of different synthesis conditions including concentration, hydrolysis ratio and rate, and Al : Si elemental ratio on the particle size of the initial precipitate and on the phase abundances of the final products. These models show that hydrolysis ratio has the strongest control on the overall phase composition, while the starting reagent concentration also plays a vital role. For imogolite nanotubes, we determine that increasing concentration, and relatively high or low hydrolysis limit nanotube production. A strong relationship is also observed between the distribution of nanostructured phases and the size of precursor particles. The confidences were >99% for all linear regression models and explained up to 85% of the data variance in the case of imogolite. Additionally, the models consistently predict resulting data from other experimental studies. These results demonstrate the use of an approach to understand complex chemical systems with competing influences and provide insight into the formation of several nanosized alumino(silicate) phases.

Operando and Dynamic XAS Studies of Metal-Support Interactions of Iridium-Based Catalysts for Oxygen Evolution Reaction (OER) in Proton Exchange Membrane Water Electrolysis (PEMWE)

Iridium (Ir) is an important electrocatalyst material for the oxygen evolution reaction (OER) in acidic media. However, high loadings of very costly and scarce Ir are needed to achieve sufficient performance in proton exchange membrane water electrolysis (PEMWE). [1] To establish a large-scale green hydrogen economy based on PEMWE in the near future, the Ir loading needs to be further reduced. In the last decades, promising Ir-based catalyst strategies has been developed to improve the OER activity by controlling the size of Ir nanoparticles (NPs) [2] and tuning the electronic interactions of the Ir NPs with the catalyst support material, referred to as strong metal support interaction (SMSI). The latter has frequently been observed for heterogeneous catalysts (e.g. Au NPs supported on titanium oxide [3]), while the SMSI for Ir-based OER electrocatalysts is poorly understood to date.This study focuses on operando and dynamic X-ray absorption spectroscopy (XAS) analysis of Ir NPs deposited on three support materials: graphitized carbon (C), titanium oxide (TiO2) and antimony-doped tin oxide (ATO). The surfactant-free Ir NPs with controlled size of around 1 nm were obtained by a colloidal approach in low-boiling point alcohols. [2, 4] Afterwards, the Ir NPs were homogeneously distributed on different support materials with similar loadings and particle sizes confirmed by TEM and XRD. Solute components from the aged catalyst materials (Ir and/or support material) in the electrolyte solution were determined by ICP-OES, while the solid catalyst powders were analysed by TGA and µ-XRF. Our first/initial electrochemical results by recording cyclic voltammetry (CV) profiles show that after the immobilization the Ir NPs obtained from the same batch are strongly oxidized on TiO2, while a metallic character of the Ir is still observed on the C and ATO. The OER mass activity of 120±8 A/gIr for Ir/C is 2-times higher than for Ir/TiO2 (60±6 A/gIr) and 1.2-times higher than for Ir/ATO (95±4 A/gIr) at 1.5 VRHE,iR-free and 25 °C obtained from rotating disc electrode (RDE) setup.Operando and dynamic Ir LIII edge XANES measurements during the cycling from 0.06 to different upper potentials (0.8, 1.0, 1.2, 1.4 and 1.6 VRHE) at 2 mV/s were performed in acidic media at the SuperXAS (X10DA) beamline, Switzerland. All measurements were carried out using a home-made spectro-electrochemical flow-cell. Principal component analysis (PCA) of Quick-XANES data confirmed the tendencies of the formation of irreversible Irz+ species on TiO2 even at low upper potentials, while ATO and C tend to retain the metallic character of the Ir NPs. From the PCA, two independent components were identified to describe the changes in the series of XANES data. The compositions of these components were then analyzed using linear combination fitting (LCF).For 60 wt.% Ir/C, the cycling up to 1.4 VRHE shows two components with 50:25:25 wt.% of Ir0:Ir3+:Ir4+ and 42:0:58 wt.% of Ir0:Ir3+:Ir4+ obtained from the PCA and LCF analysis. During the cycling, the ratios of these components change gradually. In spite of the strong hysteresis behavior, we observed that the ratio between both components reverses to the initial state. In other words, only the component with 42:0:58 wt.% of Ir0:Ir3+:Ir4+ is present at 1.4 VRHE, while the other component is dominant at lower anodic potential.In the same experiment, the analysis of our data for 60 wt% Ir/ATO using PCA indicates two independent components with 74:11:15 wt.% of Ir0:Ir3+:Ir4+ and 70:0:30 wt.% of Ir0:Ir3+:Ir4+. The cycling up to 1.4 VRHE shows similar behavior like 60 wt.% Ir/C.Although the same batch of colloidal Ir NPs were used for the immobilization, the PCA and LCF analysis for 60 wt.% Ir/TiO2 reveal two components with high content of Ir4+ species, more precisely 36:64 wt.% of Ir0:Ir4+ and 2:98 wt.% of Ir0:Ir4+. The cycling up to 1.4 VRHE shows mainly a reversible behavior due to high content of Ir4+ species on TiO2. With increasing anodic potential, the content of metallic Ir further decreases and the main component with almost only Ir4+ species is stable at 1.4 VRHE. In the backward direction, the reduction to metallic Ir takes place to small extent. Therefore, the component with 36:64 wt.% of Ir0:Ir4+ is mainly present at 0.06 VRHE.In summary, our PCA and LCF analysis of the operando and dynamic Quick-XANES data provide further insights into the electronic structure of Ir species and the mechanism of the complex metal-support interaction for Ir NPs immobilized on C, ATO and TiO2.

Local structure and ordering of Al atoms in AlxGa1−xN epilayers

In this study, we investigate the local structure of aluminum (Al) in a comprehensive series of AlxGa1−xN epilayers, where the Al concentration spans from the dilute limit to 100%. We analyze grazing incidence Al K-edge tender x-ray absorption spectroscopy data using both linear combination fitting based on reference limit spectra and full quantitative analysis. The results indicate deviations from random cation distribution with varying signs within the explored concentration range. Additionally, we observe a reduction in cationic interatomic distances over a wide concentration range, which contrasts with some previous studies conducted at hard x-ray absorption edges.

The oxidation state of titanium in silicate melts

Titanium occurs as Ti3+, in addition to the more usual Ti4+, in extraterrestrial materials such as Lunar basalts and chondritic meteorites. The proportion of Ti as Ti3+ was investigated by Ti K-edge X-ray absorption near edge structure (XANES) spectroscopy for five silicate glass compositions quenched from melts equilibrated at 1400 °C, atmospheric pressure, and oxygen fugacities (fO2) in log units relative to the fayalite-magnetite-quartz (FMQ) buffer from FMQ+3.3 to FMQ-10.2 (+6.6 to −6.9 log units relative to the iron-wüstite, IW, buffer). All spectra could be well fit using a linear combination of the spectra recorded from the most oxidised and reduced samples of the same composition, indicating that the samples only contain two Ti species. Ti3+/ΣTi (where ΣTi = Ti3+ + Ti4+) = 0 for the most oxidised samples but is unknown for the most reduced. Thus, the linear combination fit results were used in a regression model in which Ti3+/ΣTi of the reduced end-member was varied to give Ti3+/ΣTi values of the other samples that best fit the thermodynamically expected dependence of Ti3+/ΣTi on fO2. The most reduced samples were found to have Ti3+/ΣTi ∼ 0.6. The resulting modified equilibrium constants of the Ti oxidation reaction, logK', are linearly correlated with the optical basicity (Λ) parameterisation of melt composition, such that as Λ increases, Ti3+/ΣTi decreases, at constant fO2. This correlation allows Ti3+/ΣTi to be predicted for other compositions and, assuming that the temperature dependence of Ti3+/Ti4+ is parallel to FMQ, a general equation relating Ti3+/Ti4+ to fO2 was obtained: log(Ti3+/Ti4+) = −0.25ΔFMQ − 0.32(19) − 3.44(32)Λ. This equation was used to predict Ti3+/ΣTi as a function of fO2 for high-Ti Mare basalt, chondrule (CV and CM), and calcium aluminium inclusion (CAI; Type A and B) compositions. For melts of these compositions Ti3+ = Ti4+ at ∼ FMQ-10.8, −9.5, −9.3, −10.6, and −10.2 (∼IW-7.5, −6.2, −6.0, −7.3, and −6.9), respectively, independent of temperature.

X-ray Absorption Spectroscopy as a Process Analytical Technology: Reaction Studies for the Manufacture of Sulfonate-Stabilized Calcium Carbonate Particles.

Process analytical technologies are widely used to inform process control by identifying relationships between reagents and products. Here, we present a novel process analytical technology system for operando XAS on multiphase multicomponent synthesis processes based on the combination of a conventional lab-scale agitated reactor with a liquid-jet cell. The preparation of sulfonate-stabilized CaCO3 particles from polyphasic Ca(OH)2 dispersions was monitored in real time by Ca K-edge XAS to identify changes in Ca speciation in the bulk solution/dispersion as a function of time and process conditions. Linear combination fitting of the spectra quantitatively resolved composition changes from the initial conversion of Ca(OH)2 to the Ca(R-SO3)2 surfactant to the ultimate formation of nCaCO3·mCa(R- SO3)2 particles. The system provides a novel tool with strong chemical specificity for probing multiphase synthesis processes at a molecular level, providing an avenue to establishing the relationships between critical quality attributes of a process and the quality and performance of the product.

Importance of inner-sphere P-O-Fe bonds in natural and synthetic mineral-organic associations

Sorption of organic molecules on mineral surfaces can occur through several binding mechanisms of varying strength. Here, we investigated the importance of inner-sphere P-O-Fe bonds in synthetic and natural mineral-organic associations. Natural organic matter such as water extracted soil organic matter (WESOM) and extracellular polymeric substances (EPS) from liquid bacterial cultures were adsorbed to goethite and examined by FTIR spectroscopy and P K-edge NEXAFS spectroscopy. Natural particles from a Bg soil horizon (Gleysol) were subjected to X-ray fluorescence (XRF) mapping, NanoSIMS imaging, and NEXAFS spectro-microscopy at the P K-edge. Inner-sphere P-O-Fe bonds were identified for both, adsorbed EPS extracts and adsorbed WESOMs. Characteristic infrared peaks for P-O-Fe stretching vibrations are present but cannot unambiguously be interpreted due to possible interferences with mono- and polysaccharides. For the Bg horizon, P was only found on Fe oxides, covering the entire surface at different concentrations, but not on clay minerals. Linear combination fitting of NEXAFS spectra indicates that this adsorbed P is mainly a mixture of orthophosphate and organic P compounds. By combining atomic force microscopy (AFM) images with STXM-generated C and Fe distribution maps, we show that the Fe oxide surfaces were fully coated with organic matter. In contrast, clay minerals revealed a much lower C signal. The C NEXAFS spectra taken on the Fe oxides had a substantial contribution of carboxylic C, aliphatic C, and O-alkyl C, which is a composition clearly different from pure adsorbed EPS or aromatic-rich lignin-derived compounds. Our data show that inner-sphere P-O-Fe bonds are important for the association of Fe oxides with soil organic matter. In the Bg horizon, carboxyl groups and orthophosphate compete with the organic P compounds for adsorption sites.

Uranium Speciation in a Chalky Soil.

In this article, the speciation and behavior of anthropogenic metallic uranium deposited on natural soil are approached by combining EXAFS (extended X-ray absorption fine structure) and TRLFS (time-resolved laser-induced fluorescence spectroscopy). First, uranium (uranyl) speciation was determined along the vertical profile of the soil and bedrock by linear combination fitting of the EXAFS spectra. It shows that uranium migration is strongly limited by the sorption reaction onto soil and rock constituents, mainly mineral carbonates and organic matter. Second, uranium sorption isotherms were established for calcite, chalk, and chalky soil materials along with EXAFS and TRLFS analysis. The presence of at least two adsorption complexes of uranyl onto carbonate materials (calcite) could be inferred from TRLFS. The first uranyl tricarbonate complex has a liebigite-type structure and is dominant for low loads on the carbonate surface (<10 mgU/kg(rock)). The second uranyl complex is incorporated into the calcite for intermediate (∼10 to 100 mgU/kg(rock)) to high (high: >100 mgU/kg(rock)) loads. Finally, the presence of a uranium-humic substance complex in subsurface soil materials was underlined in the EXAFS analysis by the occurrence of both monodentate and bidentate carboxylate (or/and carbonate) functions and confirmed by sorption isotherms in the presence of humic acid. This observation is of particular interest since humic substances may be mobilized from soil, potentially enhancing uranium migration under colloidal form.

X-ray Absorption Near Edge Structure (XANES) Spectrum Interpretation for LiFePO4-Silicon Doped Cathode Material

LiFePO4-Silicon doped has been widely developed as a battery cathode material. Due to the doping of silicon atoms, this study aims to determine the peak energy (E0), oxidation state and percentage of component composition for Fe K-edge. The experimental sample used are LFP Si-0%, LFP Si-1%, LFP Si-3% and LFP Si-6%. These samples were characterized using XAS (X-ray Absorption Spectroscopy). XANES spectrum showed the peak energy of Pre-edge area is 7113.08 – 7114.49 eV. The oxidation state of Fe atoms were increased and tend to be Fe3+ than Fe2+. Moreover, Linear Combination Fitting (LCF) used to determine the ratio composition FeO/Fe2O3 in experimental samples. The ratio composition indicated Fe atom is multivalent, when the percentage of silicon-doped increases, the ratio of Fe3+ increased.It has been found that the presence of silicon atoms in the LiFePO4 structure has an effect in electronic structure.

Temporal Ni K-Edge X-ray Absorption Spectroscopy Study Reveals the Kinetics of the Ni Redox Behavior of the Iron-Nickel Oxide Bimetallic OER Catalyst

Operando X-ray absorption spectroscopy (XAS) can be utilized to probe the phase and structural changes of FeNiOx and similar transition metal oxide electrocatalysts during electrocatalytic reactions. However, capturing the temporal changes occurring in the operando chemistry of electrocatalysts has been little studied. In this work, we successfully capture the time-resolved changes at the Ni K-edge for both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions of an aqueous phase synthesized FeNiOx bimetallic nanoparticle electrocatalyst. During a stepped voltage experiment, the temporal change in the Ni K-edge was observed as the applied voltage was stepped from 0.7 to 0.8 V versus the silver/silver chloride reference electrode, and the change observed is associated with the Ni redox transition from the 2+ to 3+/4+ oxidation state. Individual XAS spectra were obtained in 90 s, and a total of 9 scans post voltage step showed a unique transition from a hydroxide to an oxyhydroxide phase. The shift in absorption edge energy position and the changes in spectral shape for individual scans explain the typically observed broadening of the Ni K-edge XANES spectrum during time-averaged operando XAS, and a kinetic analysis revealed a first-order observed rate constant, |kobs|, of 0.00426 s–1 and a half-life (t1/2) of 163 s. Multivariate curve resolution-alternating least squares analysis followed by linear combination fitting analysis for time-averaged versus time-resolved Ni K-edge changes upon voltage step show distinct differences in estimated contributions from Ni2+ oxide/hydroxide phases. Detailed EXAFS modeling shows the phase transition from hydroxide to oxyhydroxide on top of the unchanged metallic core.

Magnesium-enriched poultry manure enhances phosphorus bioavailability in biochars

Pyrolysis of calcium-rich feedstock (e.g., poultry manure) generates semi-crystalline and crystalline phosphorus (P) species, compromising its short-term availability to plants. However, enriching poultry manure with magnesium (Mg) before pyrolysis may improve the ability of biochar to supply P. This study investigated how increasing the Mg/Ca ratio and pyrolysis temperature of poultry manure affected its P availability and speciation. Mg enrichment by ∼2.1% increased P availability (extracted using 2% citric and formic acid) by 20% in Mg-biochar at pyrolysis temperatures up to 600 °C. Linear combination fitting of P K-edge XANES of biochar, and Mg/Ca stoichiometry, indicate that P species, mainly Ca–P and Mg–P, are altered after pyrolysis. At 300 °C, adding Mg as magnesium hydroxide [Mg(OH)2] created MgNH4PO4 (18%) and Mg3(PO4)2.8H2O (23%) in the biochar, while without addition of Mg Ca3(PO4)2 (11%) predominated, both differing only for pyrophosphate, 33 and 16%, respectively. Similarly, the P L2,3 edge XANES data of biochar made with Mg were indicative of either MgHPO4.3H2O or Mg3(PO4)2.8H2O, in comparison to CaHPO4.2H2O or Ca3(PO4)2 without Mg. More importantly, hydroxyapatite [Ca5(PO4)3(OH)] was not identified with Mg additions, while it was abundant in biochars produced without Mg both at 600 (12%) and 700 °C (32%). The presence of Mg formed Mg–P minerals that could enhance P mobility in soil more than Ca–P, and may have resulted in greater P availability in Mg-enriched biochars. Thus, a relatively low Mg enrichment can be an approach for designing and optimize biochar as a P fertilizer from P-rich excreta, with the potential to improve P availability and contribute to the sustainable use of organic residues.

Tissue-variation of iron stable isotopes in marine fish coupled with speciation analysis using X-ray absorption fine structure

The Fe stable isotope ratio (δ56Fe) in tissues is a potential parameter for examining the Fe metabolism in marine fish. Although the effect of ferritin storage has been proposed as a possible cause of heavy isotope (56Fe) enrichment in the liver, no speciation and stable isotope ratio coupling data are available. Here, we report the δ56Fe values measured by multicollector ICP-MS and the result of Fe K-edge X-ray absorption near-edge structure (XANES) analysis of multiple tissues obtained from a skipjack tuna (Katsuwonus pelamis) and a chub mackerel (Scomber japonicus). Apparent isotopic fractionation between the liver and the muscle samples (Δ56FeL–M = δ56Feliver − δ56Femuscle) from these species was observed (0.85 ‰ and 0.57 ‰, respectively). The dominant Fe species in the muscle was heme Fe (the sum of methemoglobin, oxyhemoglobin, and deoxyhemoglobin), while ferritin was not detected according to the linear combination fitting of the XANES spectra. In the liver, ferritin contribution was ca. 28 %–54 % of the total Fe content. The apparent difference in δ56Fe between heme Fe and ferritin was estimated to range from 1.41 ‰ to 1.52 ‰ based on the tissue-specific δ56Fe values and the XANES results. These results indicate that the Fe storage as ferritin does not induce the lowering of δ56Fe in the muscle, considering the low contribution of the liver Fe to the total Fe content in the body.