X-ray Absorption Near-edge Fine Structure Research Articles

Consistent mineral-associated organic carbon chemistry with variable erosion rates in a mountainous landscape

Interactions between organic carbon (OC) and minerals represent a critical mechanism for stabilizing organic matter in soils. Because both mineral weathering and plant productivity are negatively affected by soil erosion, mineral-associated organic carbon (MOC) chemistry is also expected to vary with erosion intensity. Here we show that MOC chemistry, determined by carbon X-ray absorption near-edge fine structure spectroscopy (XANES), exhibits little difference across a large (10-fold) gradient in erosion-derived soil turnover times. Mineral-associated OC chemistry further fails to explain the variation in radiocarbon-based MOC turnover times. Our results suggest that soil OC longevity is largely independent of organic matter chemistry in steep mountainous landscapes where soil development is constrained by erosion.

Organic amendments stimulate co-precipitation of ferrihydrite and dissolved organic matter in soils

Ferrihydrite-organic matter (OM) complexes in soil play a vital role in preserving soil organic carbon (C), either through adsorption (reaction of OM with existing ferrihydrite) or co-precipitation (neo-formation of ferrihydrite in the presence of OM). However, there is still limited understanding of the effects of long-term fertilization on the pathways for the formation of ferrihydrite-OM in soil. In this study, the dissolved OM (DOM) extracted from a Ferralic Cambisol under 25-year inorganic nitrogen (N), phosphorus (P) and potassium (K) (NPK) or NPK plus organic pig manure (NPKM) was used to synthesize adsorption complexes and co-precipitates. Compared with the adsorption, the co-precipitation of ferrihydrite and DOM resulted in higher C/Fe ratios in the solid products, particularly under NPKM. Analysis of the iron K-edge X-ray absorption near-edge fine structure (XANES) showed that there was a preference for DOM from soil under NPKM treatment to co-precipitate with ferrihydrite, whereas adsorption of ferrihydrite on DOM was more likely with the long-term NPK fertilized soil. Fourier-transform infrared (FTIR) spectra suggested that a higher intensity of Si-O functional groups, clay minerals, and aromatic groups in the DOM from the NPKM treatment played important roles in the co-precipitation process. Desorption of co-precipitated OM was lower than that of adsorbed OM in soils, indicating a higher chemical stability of OM in the co-precipitates. In summary, our results demonstrate that 25-year long-term NPK and NPKM fertilizations have contrasting effects on the formation pathway of ferrihydrite-OM complexes. Organic amendments can stimulate the co-precipitation of DOM and ferrihydrite in soil, leading to greater biological C preservation than the adsorption process, and a further enhancement of long-term SOC protection.

Exploring the N2 Adsorption and Activation Mechanisms over the 2H/1T Mixed-Phase Ultrathin Mo1-xWxS2 Nanosheets for Boosting N2 Photosynthesis.

Solar-driven conversion of nitrogen (N2) to ammonia (NH3) is highly appealing, yet in its infancy, the low photocatalytic efficiency and unclear adsorption and activation mechanisms of N2 are still issues to be addressed. In this study, ultrathin alloyed Mo1-xWxS2 nanosheets with tunable hexagonal (2H)/trigonal (1T) phase ratios were proposed to boost photoreduction N2 efficiency, while the mechanisms of N2 adsorption and activation were explored simultaneously. The alloyed Mo1-xWxS2 nanosheets for the 1T phase concentration of 33.6% and Mo/W = 0.68:0.32 were proven to reach about 111 μmol gcat-1 h-1 under visible light, which is 3.7 (or 3)-fold higher than that of pristine MoS2 (or WS2). With the aid of density functional theory calculations and in situ N2 adsorption X-ray absorption near-edge fine structure techniques, the adsorption and activation behaviors of N2 over the interface of Mo1-xWxS2 nanosheets were investigated during the N2 reduction process. The results show that the W doping causes a higher electron density state in W 5d orbitals, which can further polarize the adsorbed N2 molecules for adsorption and activation. This work provides a new insight into the adsorption and activation mechanisms for the NH3 synthesis.

0D/2D MXene Quantum Dot/Ni-MOF Ultrathin Nanosheets for Enhanced N2 Photoreduction

To improve the photocatalytic efficiency of the N₂ reduction reaction, MXene quantum dots (Ti₃C₂-QDs) and two-dimensional (2D) nickel metal–organic framework (Ni-MOF) with different ratios were fabricated by a simple self-assembly strategy to form type II heterojunctions. The resultant composites showed enhanced absorption and excellent interfacial charge-transfer capabilities according to the optical and photoelectron properties. The optimal Ti₃C₂-QD/Ni-MOF heterostructure exhibited a considerable ammonia yield rate (88.79 μmol gcₐₜ–¹ h–¹). By means of X-ray absorption near-edge fine structure (XANES) technique, it was found that the interaction of Ti₃C₂-QD and Ni-MOF could accelerate the electron transfer, and Ni site would be the main area to enrich the electron density as well as adsorb and activate N₂. Finally, with the aid of density functional theory (DFT) calculations and in situ Fourier transform infrared spectrometry (FTIR), the reaction pathway and intermediate products were studied.

Surface‐interface investigations of an ultrathin pulsed laser deposited NiO/ZnO bilayer structure

We hereby report detailed structural and morphological studies for an ultrathin NiO/ZnO bilayer structure grown on sapphire (001) substrate using pulsed laser deposition technique. The combined X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) studies revealed formation of a low‐density defective ZnO interfacial layer of thickness ~32 Å at the ZnO/sapphire interface prior to growth of main ZnO layer. Our results further indicate that the variation of electron density across the NiO/ZnO bilayer structure is smooth and we do not observe presence of any interface layer between them. X‐ray diffraction measurements show that deposited ZnO layer is epitaxial in nature whereas NiO is highly oriented along (100) direction. The angle dependent X‐ray absorption near edge fine structure (XANES) measurements at Ni–K edge has been utilized to determine depth‐resolved oxidation state of Ni and the results have been correlated with the depth‐resolved electron density of NiO layer. The method described here offers nondestructive determination of the microstructural parameters as well as depth‐resolved mapping of oxidation state of a thin film‐based heterojunction device. It extends several advantages over destructive methods which are abundantly reported in literature.

Mineral vs. organic matter supply as a limiting factor for the formation of mineral-associated organic matter in forest and agricultural soils

Physical and chemical interactions between soil organic matter (OM) and minerals is one of the primary mechanisms for stabilizing OM in terrestrial ecosystems. Focusing on OM association with mineral surfaces, this study sought to examine mineral-associated OM from the perspectives of both mineral surface characteristics and organic matter chemistry. The research was conducted at paired-sites under North American Mid-Atlantic Coastal forest and crop production with shared environmental factors. Using carbon (C) and nitrogen (N) 1s micro- X-ray absorption near-edge fine structure (XANES) spectroscopy, we investigated the amounts and types of mineral-associated OM. Mineral specific surface area (SSA) of bulk soil was determined for three conditions: untreated, post OM removal and post iron (Fe) (oxyhydr)oxides removal. Amounts of mineral-associated OM were smaller in the agricultural soil, where greater SSA sourced from clay-sized phyllosilicates and Fe (oxyhydr)oxide minerals did not result in greater OM coverage of the mineral surface area. Although agricultural surface soil showed less abundance of phenolic C, speciation of mineral-associated OM did not differ between comparable horizons. Our results suggest that despite the plow-derived mixing of soil, which increased SSA and secondary minerals available to interact physically and chemically with OM in the plowed layer, the formation of mineral-associated OM in agricultural soil is ultimately limited by available OM.

Evolution of intermetallic GaPd2/SiO2 catalyst and optimization for methanol synthesis at ambient pressure

ABSTRACT The CO2 hydrogenation to methanol is efficiently catalyzed at ambient pressure by nanodispersed intermetallic GaPd2/SiO2 catalysts prepared by incipient wetness impregnation. Here we optimize the catalyst in terms of metal content and reduction temperature in relation to its catalytic activity. We find that the intrinsic activity is higher for the GaPd2/SiO2 catalyst with a metal loading of 13 wt.% compared to catalysts with 23 wt.% and 7 wt.%, indicating that there is an optimum particle size for the reaction of around 8 nm. The highest catalytic activity is measured on catalysts reduced at 550°C. To unravel the formation of the active phase, we studied calcined GaPd2/SiO2 catalysts with 23 wt.% and 13 wt.% using a combination of in situ techniques: X-ray diffraction (XRD), X-ray absorption near edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS). We find that the catalyst with higher metal content reduces to metallic Pd in a mixture of H2/Ar at room temperature, while the catalyst with lower metal content retains a mixture of PdO and Pd up to 140°C. Both catalysts form the GaPd2 phase above 300°C, albeit the fraction of crystalline intermediate Pd nanoparticles of the catalyst with higher metal loading reduces at higher temperature. In the final state, the catalyst with higher metal loading contains a fraction of unalloyed metallic Pd, while the catalyst with lower metal loading is phase pure. We discuss the alloying mechanism leading to the catalyst active phase formation selecting three temperatures: 25°C, 320°C and 550°C.

Impact of agricultural fertilization practices on organo-mineral associations in four long-term field experiments: Implications for soil C sequestration

Soil organic carbon (SOC) associated with minerals is considered to be one of the most fundamental long-term SOC storage strategies, but little research has integrated the organo-mineral complexes regulated by long-term fertilization. Here, soil samples under three fertilization treatments (Control, no fertilization; NPK, chemical nitrogen, phosphorus and potassium fertilization; NPKM, NPK plus manure) from four 23–34 years long-term field experiment sites across China were examined. Chemical analyses indicated that vigorous iron (Fe) mobilization could be regulated by long-term fertilization regimes. Meanwhile, Fe K-edge X-ray absorption near-edge fine structure (XANES) demonstrated that compared to NPK treated soils, NPKM treated soils contained significantly higher concentration of poorly crystalline ferrihydrite. Results from both the Fourier transform infrared combined with two-dimensional correlation spectroscopy analyses (FTIR-2DCOS) and C 1 s X-ray photoelectron spectroscopy (XPS) revealed that aliphatic carbohydrate might play an important role in binding exogenous Fe(III) in all tested four soils. In addition, greater amounts of aromatic C (the most resistant soil C fraction) were under long-term treated NPKM than NPK soils. Furthermore, multiple regression analyses showed a significantly positive relationship between poorly crystalline Fe minerals and SOC or aromatic C. Such relationships indicated that aromatic functional groups had been attached to the poorly crystalline Fe minerals, which could also be protected from being transformed to the crystalline counterpart. In conclusion, results from our integrated spectroscopic analyses have evidenced greater improvement of both poorly crystalline Fe minerals and aromatic C in organically fertilized than in chemically fertilized soils.

Long-term agricultural management and erosion change soil organic matter chemistry and association with minerals

The interaction of soil organic matter (SOM) and minerals is a critical mechanism for retaining SOM in soil and protecting soil fertility and long-term agricultural sustainability. The chemical speciation of carbon (C) and nitrogen (N) in mineral-associated SOM can be sensitive to both anthropogenic management practices and landscape positions, but these two aspects are rarely examined in tandem. Here we examined the effects of long-term (>100 years) agricultural management and erosion on mineral-associated SOM along grassland and agricultural hillslope transect. The mineral-associated SOM was obtained using particle size and density fractionation approaches. Chemical speciation of C and N in mineral-associated SOM was characterized using micro X-ray absorption near-edge fine structure (XANES) spectroscopy. The extent of SOM coverage and contribution of iron oxyhydroxides (Fe oxides) to the total specific mineral surface area (SSA) were determined using the BET–N2 adsorption method of soil samples under three conditions: untreated, SOM removal, and Fe oxides removal. The amount of SSA covered by SOM (SSASOM-covered) was lower by 61% and 37% in cultivated eroding and depositional topsoils, respectively, compared with the corresponding grassland. Depositional soils had higher SSASOM-covered than eroding positions. In the cultivated hillslopes, aromatic and phenolic C species were more abundant in depositional soils than in the eroding topsoils, indicating that deposition and burial of eroded or in-situ plant-derived phenolic C protected them from further transformation. Our results, therefore, highlight the importance of anthropogenic activities in the interaction of SOM and minerals, including C speciation changes, which may exert a considerable influence on SOM retention in soils.

Ab initio calculation and synchrotron X-ray spectroscopy investigations of tin oxides near the Sn L 3 absorption edges

The results of theoretical and experimental investigations of the electron-energy structure of the conduction band of tin oxides have been presented. The Sn L3 X-ray absorption near-edge fine structure (XANES) has been calculated for the first time for single crystals of metallic tin and tin monoxide, as well as for the orthorhombic and tetragonal phases of tin dioxide, using the linearized augmented plane wave (LAPW) method. The fine structure of the XANES spectra has been compared with the specific features of the energy distribution of the local partial densities of states of the tin compounds under investigation. A joint analysis of the results of the simulation and the experimental X-ray synchrotron Sn L3 XANES spectra of commercial bulk samples of metallic tin and tin oxides SnO and SnO2 has been carried out.

Sulfur dynamics during long-term ecosystem development

Long-term soil and ecosystem development involves predictable changes in nitrogen (N) and phosphorus (P) availability and limitation, but far less is known about comparable changes in sulfur (S) despite its importance as an essential plant macronutrient and component of soil organic matter. We used a combination of elemental analysis, X-ray absorption spectroscopy, hydrolytic enzyme assays, and stable S isotope ratios to examine S in soil and leaf tissue along the 120,000-year Franz Josef chronosequence, New Zealand. Total soil S concentrations increased during the early stages of pedogenesis and then declined as soils aged. There was little variation in soil N:S ratios along the chronosequence other than in the youngest (5 year old) soil, although the carbon (C):S ratio increased markedly in the oldest soils and the P:S ratio decreased continuously along the chronosequence. Foliar S concentrations and N:S ratios varied widely among common plant species but did not change consistently with increasing soil age, although foliar P:S declined for several species in the older stages of the chronosequence. The chemical nature of soil organic S extracted from mineral and organic horizons and determined by S K-edge X-ray absorption near-edge fine-structure (XANES) spectroscopy was dominated by C-bonded S distributed approximately equally in highly-reduced and intermediate oxidation states, although ester-bonded S was also abundant throughout the chronosequence. Soil sulfatase activity expressed on a soil C basis was highest in young soils, indicating low S availability in the early stage of pedogenesis. Enzymatic C:S and N:S ratios varied little during ecosystem development, although the enzymatic P:S ratio increased continuously along the chronosequence. Stable S isotope ratios (δ34S) increased along the chronosequence, particularly in the early stages of pedogenesis, reflecting a shift in S inputs from primary mineral S to oceanic sulfate in atmospheric deposition. Overall, this first comprehensive assessment of S along a long-term soil chronosequence suggests that S availability is low in the earliest stage of pedogenesis, but then remains stable throughout the progressive and retrogressive phases of ecosystem development, despite pronounced shifts in the chemistry and dynamics of other nutrients.

Spectroscopic Evidence of the Improvement of Reactive Iron Mineral Content in Red Soil by Long-Term Application of Swine Manure.

Mineral elements in soil solutions are thought to be the precursor of the formation of reactive minerals, which play an important role in global carbon (C) cycling. However, information regarding the regulation of mineral elements release in soil is scarce. Here, we examined the long-term (i.e., 23 yrs) effects of fertilisation practices on Fe minerals in a red soil in Southern China. The results from chemical analysis and Fourier-transform infrared spectroscopy showed that long-term swine manure (M) treatment released greater amounts of minerals into soil solutions than chemical fertilisers (NPK) treatment, and Fe played a dominant role in the preservation of dissolved organic C. Furthermore, Fe K-edge X-ray absorption near-edge fine structure spectroscopy demonstrated that reactive Fe minerals were mainly composed of less crystalline ferrihydrite in the M-treated soil and more crystalline goethite in the NPK-treated soil. In conclusion, this study reported spectroscopic evidence of the improvement of reactive Femineral content in the M-treated soil colloids when compared to NPK-treated soil colloids.

DNA adsorption by nanocrystalline allophane spherules and nanoaggregates, and implications for carbon sequestration in Andisols

This study provides fundamental knowledge about the interaction of allophane, deoxyribonucleic acid (DNA), and organic matter in soils, and how allophane sequesters DNA. The adsorption capacities of salmon-sperm DNA on pure synthetic allophane (characterised morphologically and chemically) and on humic-acid-rich synthetic allophane were determined, and the resultant DNA–allophane complexes were characterised using synchrotron-radiation-derived P X-ray absorption near-edge fine structure (XANES) spectroscopy and infrared (IR) spectroscopy. The synthetic allophane adsorbed up to 34μgmg−1 of salmon-sperm DNA. However, the presence of humic acid significantly lowered the DNA uptake on the synthetic allophane to 3.5μgmg−1 by occupying the active sites on allophane so that DNA was repulsed. Both allophane and humic acid adsorbed DNA chemically through its phosphate groups. IR spectra for the allophane–DNA complex showed a chemical change of the Si–O–Al stretching of allophane after DNA adsorption, possibly because of the alteration of the steric distance of the allophane outer wall, or because of the precipitation of aluminium phosphate on allophane after DNA adsorption on it, or both. The aluminol groups of synthetic allophane almost completely reacted with additions of small amounts of DNA (~2–6μgmg−1), but the chemical adsorption of DNA on allophane simultaneously led to the formation of very porous allophane aggregates up to ~500μm in diameter. The formation of the allophane nano- and microaggregates enabled up to 28μgmg−1 of DNA to be adsorbed (~80% of total) within spaces (pores) between allophane spherules and allophane nanoaggregates (as “physical adsorption”), giving a total of 34μgmg−1 of DNA adsorbed by the allophane. The stability of the allophane–DNA nano- and microaggregates likely prevents encapsulated DNA from exposure to oxidants, and DNA within small pores between allophane spherules and nanoaggregates may not be accessible to enzymes or microbes, hence enabling DNA protection and preservation in such materials. By implication, substantial organic carbon is therefore likely to be sequestered and protected in allophanic soils (Andisols) in the same way as demonstrated here for DNA, that is, predominantly by encapsulation within a tortuous network of nanopores and submicropores amidst stable nanoaggregates and microaggregates, rather than by chemisorption alone.

Comprehensive study of Pb (II) speciation in soil by X-ray absorption spectroscopy (XANES and EXAFS) and sequential fractionation

The study is aimed at the analysis of the spatial–structural organization of Pb(II) in Chernozem soils and the relationship between the metal ion and the soil components using X-ray absorption spectroscopy and chemical extractive fractionation. In a model experiment, soil samples were artificially contaminated with elevated rates of Pb(NO3)2 and PbO (2000 and 10,000 mg kg−1). The samples of mineral phases (bentonite, gibbsite, kaolinite, calcite, and hydromuscovite) were saturated with Pb2+ ions. The sequential fractionation of Pb in the soil was conducted by the Tessier method. X-ray absorption near-edge fine structure (XANES) spectra at the Pb LIII-edge (13.040 keV) were obtained on a Rigaku R-XAS Looper spectrometer. Extended X-ray absorption fine structure (EXAFS) LIII-edge Pb was measured at the Structural Materials Science beamline of the Kurchatov Center for Synchrotron Radiation. The results of successive extraction showed that Pb is associated with strongly bound organic substances, Fe and Mn (hydr)oxides, and carbonates. An increase in the portion of exchangeable fraction is observed under extreme loads. At the addition of Pb in the form of oxide and nitrate to the soil, the fractional compositions were similar, which indicates the good transformation of PbO in Chernozem. The features of XANES spectra indicate different orbital transitions in the electron shells of Pb2+ ions for monoxide (PbO) and soluble salt (Pb(NO3)2), which affect the ion properties and determine the individual structure of the coordination sphere. The analysis of XANES revealed that sorption of Pb in the soil samples and in the samples of mineral phases does not change its bond valence. The increased degree of soil contamination with Pb is accompanied by decreasing the stable connection between metal and soil components. Lead ions in bentonite, kaolinite, hydromuscovite, gibbsite, and calcite are incorporated in the positions of the inner-sphere complex replacing some aluminum ions in the octahedral sites. This results in changes the Pb–O distances in Pb-bearing octahedrons. We may suggest that Pb2+ is also sorbed by dimer (Pb–Pb) silicate and/or aluminum groups. The structure of adsorbent surface plays the key role in the sorption of Pb2+ by mineral phases.

Synergistic Catalysis between Pd and Fe in Gas Phase Hydrodeoxygenation of m-Cresol

In this work, a series of Pd/Fe2O3 catalysts were synthesized, characterized, and evaluated for the hydrodeoxygenation (HDO) of m-cresol. It was found that the addition of Pd remarkably promotes the catalytic activity of Fe while the product distributions resemble that of monometallic Fe catalyst, showing high selectivity toward the production of toluene (C–O cleavage without saturation of aromatic ring and C–C cleavage). Reduced catalysts featured with Pd patches on the top of reduced Fe nanoparticle surface, and the interaction between Pd and Fe, was further confirmed using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray absorption near edge fine structure (XANES). A possible mechanism, including Pd assisted H2 dissociation and Pd facilitated stabilization of the metallic Fe surface as well as Pd enhanced product desorption, is proposed to be responsible for the high activity and HDO selectivity in Pd–Fe catalysts. The synergic catalysis derived from Pd–Fe interaction found in this work was proved to be applicable to other precious metal promoted Fe catalysts, providing a promising strategy for future design of highly active and selective HDO catalysts.

Characteristics and chemical compositions of particulate matter collected at the selected metro stations of Shanghai, China

A campaign was conducted to assess and compare the air quality at the different metro platforms at Shanghai City, focusing on particulate matter (PM) levels, chemical compositions, morphology and mineralogy, as well as species of iron. Our results indicated that the average PM2.5 concentrations for the three metro lines were 177.7μg/m3, 105.7μg/m3 and 82.5μg/m3, respectively, and the average PM1 concentrations for the three lines were 122.3μg/m3, 84.1μg/m3 and 59.6μg/m3, respectively. Fe, Mn, Cr, Cu, Sr, Ba and Pb concentrations in all of the sampling sites were significantly higher than that in the urban ambient air, implicating that these trace metals may be associated with the metro systems working. Individual airborne dusts were studied for morphology and mineralogy characteristics. The results revealed that the presence of most individual particles were with no definite shape and most of them were with a large metal content. Furthermore, Fe-rich particles had significantly higher abundance in the metro systems, which were more frequently encountered in the underground lines than the aboveground line. The 2D distribution map of an interested Fe-rich particle showed an uneven Fe distribution, implying that a hollow or core of other substance exists in the particle center during the formation process. Cluster analysis revealed that Fe-rich particles were possibly a mixture of Fe species. Fitting of X-ray absorption near-edge fine structure spectra (XANES) showed the main iron species within the particles collected from the three contrasting metro lines of Shanghai to be hematite, magnetite, iron-metal and mineral Fe. Hematite and mineral Fe were all found in three lines, while magnetite only existed in aboveground metro line. Iron-metal was determined in both the older and younger underground lines, based on the X-ray diffraction (XRD) analysis. As diverse Fe species have different physical–chemical characteristics and toxicity, the speciation of Fe-containing metro particles is important in the context of public health and control measures.

Oxidation of the porous silicon surface under the action of a pulsed ionic beam: XPS and XANES studies

The changes in the electronic structure and phase composition of porous silicon under action of pulsed ionic beams have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) using synchrotron radiation. The Si 2p and O 1s core photoemission spectra for different photoelectron collection angles, valence band photoemission spectra, and X-ray absorption near-edge fine structure spectrain the region of Si L2,3 edges of the initial and irradiated samples have been analyzed. It has been found that, as a result of the irradiation, a thin oxide film consisting predominantly of higher oxide SiO2 is formed on the porous silicon surface, which increases the energy gap of the silicon oxide. Such film exhibits passivation properties preventing the degradation of the composition and properties of porous silicon in contact with the environment.

Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

Environmental context Uranium and arsenic, two elements of human health concern, are commonly found at sites of uranium mining, but little is known about processes influencing their environmental behaviour. Here we focus on understanding the chemical and physical processes controlling uranium and arsenic transport at an abandoned uranium mine. We find that the use of sedimentation ponds limits the mobility of uranium; however, pond conditions at our site resulted in arsenic mobilisation. Our findings will help optimise restoration strategies for mine tailings. Abstract Although As can occur in U ore at concentrations up to 10wt-%, the fate and transport of both U and As at U mine tailings have not been previously investigated at a watershed scale. The major objective of this study was to determine primary chemical and physical processes contributing to transport of both U and As to a down gradient watershed at an abandoned U mine site in South Dakota. Uranium is primarily transported by erosion at the site, based on decreasing concentrations in sediment with distance from the tailings. Sequential extractions and U X-ray absorption near-edge fine structure (XANES) fitting indicate that U is immobilised in a near-source sedimentation pond both by prevention of sediment transport and by reduction of UVI to UIV. In contrast to U, subsequent release of As to the watershed takes place from the pond partially due to reductive dissolution of Fe oxy(hydr)oxides. However, As is immobilised by adsorption to clays and Fe oxy(hydr)oxides in oxic zones and by formation of As–sulfide mineral phases in anoxic zones down gradient, indicated by sequential extractions and As XANES fitting. This study indicates that As should be considered during restoration of uranium mine sites in order to prevent transport.

Electrocatalysis of heat-treated cobalt-porphyrin/carbon for hydrogen peroxide formation

The electrocatalysis of heat-treated Co-TPP/carbon for O2 reduction and H2O2 formation using a fuel cell reactor with an H2SO4 aqueous electrolyte was studied (TPP: 5,10,15,20-tetrakis(phenyl)-21H,23H-porphyrin). Carbon supports, including vapor grown carbon fiber (VGCF), activated carbon (AC), carbon black XC-72 (XC72), and carbon black Black-Pearl-2000/3500 (BP2000/3500), strongly affected the electrocatalysis of the Co-TPP/carbon. The Co-TPP/VGCF heat-treated at 1023K, abbreviated as Co-TPP/VGCF(1073), showed a good electrocatalytic activity for H2O2 formation (1.0moldm−3 with 40% current efficiency (CE)); however, other carbon supports were unfavorable. Electrocatalytic activities were Co-TPP/VGCF(1073)>>Co-TPP/XC72(1073)>Co-TPP/AC(1073)>Co-TPP/BP2000(1073). Their electrocatalysis were studied by rotating ring-disk electrode voltammetry and kinetic studies of the decomposition of H2O2 and the electroreduction of H2O2 to water. These studies indicated that the electrocatalytic activities for the successive reduction of H2O2 to water determined the final H2O2 concentrations and CEs. An active site of heat-treated Co-TPP/VGCF was characterized by X-ray photoelectron spectroscopy, and X-ray absorption near-edge fine structure spectroscopy. Partial pyrolysis of Co-TPP on the carbon surface was essential to generate the active site for the H2O2 formation. A model of the Co active site, CoN2–carbon, was proposed.

Local hydrated structure of an Fe2+/Fe3+ aqueous solution: an investigation using a combination of molecular dynamics and X-ray absorption fine structure methods

The hydrated shell of both Fe2+ and Fe3+ aqueous solutions are investigated by using the molecular dynamics (MD) and X-ray absorption structure (XAS) methods. The MD simulations show that the first hydrated shells of both Fe2+ and Fe3+ are characterized by a regular octahedron with an Fe-O distance of 2.08Å for Fe2+ and 1.96Å for Fe3+, and rule out the occurrence of a Jahn-Teller distortion in the hydrated shell of an Fe2+ aqueous solution. The corresponding X-ray absorption near edge fine structure (XANES) calculation successfully reproduces all features in the XANES spectra in Fe2+ and Fe3+ aqueous solution. A feature that is located at energy 1 eV higher than the white line (WL) in an Fe3+ aqueous solution may be assigned to the contribution of the charge transfer.