Edge Structure Spectroscopy Research Articles

Mineralogy-dependent sulfide oxidation via polysulfide and thiosulfate pathways during weathering of mixed-sulfide bearing mine waste rock

Tracking the S oxidation pathway in sulfide-bearing mine waste-rock piles is complicated by variations in water content, O2 and Fe3+ concentrations, microbial diversity, mineralogy, the occurrence of a variety of S species associated with incomplete oxidation, and nonlinear coupling between physicochemical processes. Synchrotron-based S K-edge X-ray absorption near edge structure (XANES) spectroscopy facilitates the identification and quantification of S species (i.e., S2−, S22−, S8, S2O32−, and SO42−) in a weathered mine waste-rock pile containing pyrite, sphalerite, galena, pyrrhotite, and chalcopyrite. Mineralogy-dependent polysulfide and thiosulfate pathways both affect the S oxidation within the waste-rock pile. Currently, the polysulfide pathway, with S8 as the intermediate S species, generates high concentrations of dissolved metals (from the rapid oxidation of monosulfides including sphalerite, galena, pyrrhotite, and chalcopyrite). As these monosulfides are depleted, the thiosulfate pathway of pyrite oxidation, including intermediate oxidation products (i.e., S8 and S2O32−), will become the dominant oxidation pathway, resulting in the potential for generation of additional acidic drainage. This information highlights the importance of identifying the sulfide-mineral oxidation reaction pathways when attempting to estimate acid generation potential, and when developing remediation strategies for the storage and management of sulfide-bearing waste rock.

Improved Capacity Retention of Lithium Ion Batteries Under Fast Charge Via Metal-Coated Graphite Electrodes

The need to develop Li-ion batteries which can be charged in approximately 10 minutes is critical for the widespread implementation of electric vehicles. Fast charging capability of current state of the art Li-ion batteries is currently limited by the occurrence of Li plating at the graphite anode, which limits cycle life and negatively impacts safety. To overcome this challenge, we deliberately modify the interface of graphite electrodes with nanoscale layers of Cu and Ni to increase the overpotential for Li deposition and suppress Li plating under high rate charge conditions.1, 2 Due to their nanoscale, the deposited surface films have minimal impact on cell level theoretical energy density. Interfacial properties of the anodes are thoroughly characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and spatially resolved mapping X-ray absorption near edge structure (XANES) spectroscopy. Li plating is quantified by X-ray diffraction and associated electrochemistry measurements revealing that the surface treatment effectively reduces the quantity of plated Li metal by ~50% compared to untreated electrodes. Extended galvanostatic cycling of control and metal-coated electrodes in graphite/NMC622 pouch cells show that the metal films improve capacity retention under fast charge rates. The effect of metal film loading on capacity retention improvement has also been quantified. The findings establish that with rational design of an electrode interface, the overpotential for Li deposition can be modulated, providing a new conceptual approach for reducing Li plating on graphite anodes under fast charge conditions.

Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and non-paddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-microaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

Investigation of the Ag species and synergy of Ag-TiO2 and g-C3N4 for the enhancement of photocatalytic activity under UV–Visible light irradiation

Herein, UV–visible light-responsive g-C3N4/Ag-TiO2 photocatalysts were successfully synthesized. The Ag species in the composites before and after photocatalysis were investigated by X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS). The Ag species in the Ag-TiO2 were all metallic Ag, while a combination of metallic Ag and Ag+ attributed to Ag2O was found in the composites. In addition, the change of Ag0/Ag+ oxidation after photocatalysis was conspicuously explored. The composite with optimal ratios of g-C3N4 to Ag-TiO2 at 2 to 1 (2CN:1ST) exhibited photocatalytic efficiencies for the degradation of rhodamine B (RhB, cationic dye) and methyl orange (MO, anionic dye) of 95.01 % and 98.13 % and 72.92 % and 92.82 % under UV and visible light irradiation, respectively. The results suggested that Ag-TiO2 is a predominantly UV-responsive photocatalyst, whereas g-C3N4 can effectively promote photocatalytic degradation under visible light irradiation. Therefore, the synergy of the synthesized g-C3N4 and Ag-TiO2 photocatalysts results in high-performance cationic and anionic dye degradation under both UV and visible light irradiation. In addition, the ability of Ag0/Ag+ to switch oxidation states under irradiation is advantageous to the suppression of recombination.

Phase evolution of the surface iron (hydr)oxides to the iron sulfide through anion exchange during sulfidation of zero valent iron

The naturally-formed iron (hydr)oxides on the surface of zero valent iron (ZVI) have long been considered as passivation layer and inert phases which significantly reduce the reaction activities when they are employed in environmental remediation. Although it seems there are no direct benefits to keep these passivation layers, here, we show that such phases are necessary intermediates for the transformation to iron sulfides through an anion exchange pathway during sulfidation of ZVI. The pre-formed (hydr)oxides undergo a phase evolution upon aging and specific phases can be effectively trapped, which can be confirmed by a combination of different characterization techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), and X-ray absorption near edge structure (XANES) spectroscopy. Interestingly, after sulfidation, the resultant samples originated from different (hydr)oxides demonstrate different activities in the Cr(VI) sequestration. The XANES investigation of Fe K edge and Fe L2,3 edge indicates Fe remains the same after sulfidation, suggesting a non-redox, anion exchange reaction pathway for the production of iron sulfides, where O2- anions are directly replaced with S2-. Consequently, the structural characteristics of the parent (hydr)oxides are inherited by the as-formed iron sulfides, which make them behave differently because of their different structural natures.

Bismuth telluride anode boosting highly reversible electrochemical activity for potassium storage

To ensure high capacity with reasonable cycling stability using the conversion or K-alloy reaction, the anode should minimize or embrace the volume expansion during the discharge (potassiation) and charge (depotassiation) processes. Herein, we report on a Bi2Te3 composite modified by nanosized carbon that boosts the highly reversible capacity with long cycling stability. The as-synthesized Bi2Te3 powders and acetylene black carbon were compositized via high-energy ball-milling, resulting in improvement of the electrical conductivity to ∼10−4 S cm−1 from ∼10−7 S cm−1. Electrochemical investigation revealed that the proposed Bi2Te3@C retained over 79.8% of its initial capacity (311 mAh g− 1) for 500 cycles at a current of 1000 mA g− 1. We unveil the reaction chemistry behind the acceptable performance using operando X-ray diffraction, X-ray absorption near edge structure spectroscopy, and time-of-flight secondary-ion mass spectrometry. The Bi2Te3 undergoes a conversion reaction to form Bi0 metal and K2Te as a conversion byproduct, after which Bi is further potassiated to K3Bi. The highly reversible behavior is attributed to the enlargement of the active area along with the filling of voids among Bi2Te3 particles by nanosized carbons in the composite electrode, enabling not only facile electron transport but also preservation of the electrode shape even after long-term cycling. Our approach highlights the feasibility of applying Bi2Te3@C as a sustainable anode material for potassium-ion batteries.

X-ray Spectroscopic Quantification of Phosphorus Transformation in Saharan Dust during Trans-Atlantic Dust Transport.

Saharan dust is an important phosphorus (P) supply to remote and oligotrophic parts of the oceans and American lowland tropical rainforests. Phosphorus speciation in aeolian dust ultimately controls the release and bioavailability of P after dust deposition, but the speciation in Saharan dust and its change during the trans-Atlantic transport remains unclear. Using P K-edge X-ray absorption near edge structure (XANES) spectroscopy, we showed that with increasing dust traveling distance from the Sahara Desert to Cape Verde and to Puerto Rico, about 570 and 4000 km, respectively, the proportion of Ca-bound P (Ca-P), including both apatite and non-apatite forms, decreased from 68-73% to 50-71% and to 21-37%. The changes were accompanied by increased iron/aluminum-bound P proportion from 14-25% to 23-46% and to 44-73%, correspondingly. Laboratory simulation experiments suggest that the changes in P speciation can be ascribed to increasing degrees of particle sorting and atmospheric acidification during dust transport. The presence of relatively soluble non-apatite Ca-P in the Cape Verde dust but not in the Puerto Rico dust is consistent with the higher P water solubility of the former than the latter. Our findings provide insights into the controls of atmospheric processes on P speciation, solubility, and stability in Saharan dust.

Photoactivated Osmium Arene Anticancer Complexes

Half-sandwich Os-arene complexes exhibit promising anticancer activity, but their photochemistry has hardly been explored. To exploit the photocytotoxicity and photochemistry of Os-arenes, O,O-chelated complexes [Os(η6-p-cymene)(Curc)Cl] (OsCUR-1, Curc = curcumin) and [Os(η6-biphenyl)(Curc)Cl] (OsCUR-2), and N,N-chelated complexes [Os(η6-biphenyl)(dpq)I]PF6 (OsDPQ-2, dpq = pyrazino[2,3-f][1,10]phenanthroline) and [Os(η6-biphenyl)(bpy)I]PF6 (OsBPY-2, bpy = 2,2'-bipyridine), have been investigated. The Os-arene curcumin complexes showed remarkable photocytotoxicity toward a range of cancer cell lines (blue light IC50: 2.6-5.8 μM, photocytotoxicity index PI = 23-34), especially toward cisplatin-resistant cancer cells, but were nontoxic to normal cells. They localized mainly in mitochondria in the dark but translocated to the nucleus upon photoirradiation, generating DNA and mitochondrial damage, which might contribute toward overcoming cisplatin resistance. Mitochondrial damage, apoptosis, ROS generation, DNA damage, angiogenesis inhibition, and colony formation were observed when A549 lung cancer cells were treated with OsCUR-2. The photochemistry of these Os-arene complexes was investigated by a combination of NMR, HPLC-MS, high energy resolution fluorescence detected (HERFD), X-ray adsorption near edge structure (XANES) spectroscopy, total fluorescence yield (TFY) XANES spectra, and theoretical computation. Selective photodissociation of the arene ligand and oxidation of Os(II) to Os(III) occurred under blue light or UVA excitation. This new approach to the design of novel Os-arene complexes as phototherapeutic agents suggests that the novel curcumin complex OsCUR-2, in particular, is a potential candidate for further development as a photosensitizer for anticancer photoactivated chemotherapy (PACT).

Electronic properties, bonding structure and mechanical behaviours of a-CNx: Si (: B) thin films

The electronic properties, bonding structure and mechanical behaviours of amorphous carbon nitride (a-CNx), silicon doped a-CNx (a-CNx:Si) and boron doped a-CNx (a-CNx:B) thin films alloys were studied by the C K-edge, N K-edge x-ray absorption near edge structure (XANES) and valence band photoelectron spectroscopy (VB-PES). The a-CNx:Si and a-CNx:B shows higher Young’s modulus compare to a-CNx thin film that are due to increase of carbon sp3-hybridized cluster in C and N K-edge XANES spectra. We have observed from the C and N K-edge XANES spectra that the formation of different carbon, nitrogen, silicon and boron bonding with carbon in the a-CNx:Si(:B) film structure. VB-PES spectra showed that the p-σ band became more intense than the p-π band upon the addition of silicon and boron in a-CNx film structure, which confirmed the role played by the σ bonds in controlling the electronic structure of a-CNx films and hence their mechanical behaviour. These films could be very useful in the surface coating technology.

Water electrolysis system based on a lithium cation-exchange membrane

The electronic properties, bonding structure and mechanical behaviours of amorphous carbon nitride (a-CNx), silicon doped a-CNx (a-CNx:Si) and boron doped a-CNx (a-CNx:B) thin films alloys were studied by the C K-edge, N K-edge x-ray absorption near edge structure (XANES) and valence band photoelectron spectroscopy (VB-PES). The a-CNx:Si and a-CNx:B shows higher Young’s modulus compare to a-CNx thin film that are due to increase of carbon sp3-hybridized cluster in C and N K-edge XANES spectra. We have observed from the C and N K-edge XANES spectra that the formation of different carbon, nitrogen, silicon and boron bonding with carbon in the a-CNx:Si(:B) film structure. VB-PES spectra showed that the p-σ band became more intense than the p-π band upon the addition of silicon and boron in a-CNx film structure, which confirmed the role played by the σ bonds in controlling the electronic structure of a-CNx films and hence their mechanical behaviour. These films could be very useful in the surface coating technology.

Correction to: Hepatocyte growth factor administration increases bone soluble phosphate and alters bone chemical structure in diabetic hypertensive rats

Hepatocyte growth factor (HGF) is a novel potential therapy for improving bone health in patients with type II diabetes and hypertension, but its effect on the bone molecular structure is not revealed yet. Here, X-ray absorption near edge structure (XANES) spectroscopy was used to explore the effects elicited by HGF on the bone chemical structure. This study assessed local calcium (Ca) and phosphorus (P) coordination of diabetic hypertensive rat bones, each with and without HGF treatment. Results revealed that HGF has significant effects on Ca and P coordination chemistry as confirmed by presence of more soluble phosphates in the HGT-treated groups. Data indicated that treated bones have a poorly developed phosphate structure as evidenced by drastic drop in post-edge shoulder in P L2, 3-edge compared to diabetic hypertensive and diabetic control bone. Presence of soluble Ca and P, products of bone resorption, with HGF treatment suggests unbalanced bone resorption and formation.

Controllable nitrogen-doped carbon layers coated on NiCoO2 as electrodes for high-performance hybrid supercapacitors

Controllable nitrogen-doped carbon layers coated on NiCoO2 (NiCoO2@N-C) as anode materials are successfully synthesised by facile hydrothermal method and post-annealing treatment. Benefiting from the synergistic merits of carbon and nitrogen, the as-synthesised NiCoO2@N-C delivered an excellent capacitance of 1000 F g−1 at 1 A g−1 and a high specific capacitance of 491 F g−1 at 50 A g−1. After 5000 cycles at 10 A g−1, the cyclic stability remains 91% of the initial capacitance. Moreover, the assembled asymmetric supercapacitor exhibits a high energy density of 45.9 Wh kg−1 at a power density of 375 W kg−1. The NiCoO2@N-C hybrid nanostructure was investigated by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) techniques. In situ X-ray absorption near edge structure (XANES) spectroscopy was used to track the changes of NiCoO2@N-C electrode under electrochemical working conditions and confirmed that Ni had more significant electrochemical activity than Co. These findings provide new insights into the role of heteroatom doping for high-performance supercapacitors in future applications.

Role of Fe3+ in altering the degrees of freedom in ZnAl2O4 spinel

Distribution of Zn2+, Al3+, and Fe3+ ions on tetrahedral and octahedral sites in Fe3+-doped ZnAl2O4 nanocrystals is studied through detailed x-ray diffraction (XRD) analysis. Significant changes in the intensity ratio of (220), (311), (400), and (422) diffraction peaks upon Fe addition were perceived, whose correlation with lattice inversion, Fe3+ ions distribution, and ionic vacancies is probed through simulations and Rietveld refinement. Fe3+ ions were found to be occupying tetrahedral sites in increasing proportion with the doping concentration, leading to decreased spinel inversion. However, different orders of defects, viz., cation and anion vacancies were also found to be playing their part in the statistical distribution of cations on octahedral/tetrahedral interstices. Fe L2,3-edge x-ray absorption near edge structure spectroscopy indicated Fe3+ ions being present on tetrahedral as well octahedral interstices and distortion in cation polyhedra, while O K-edge inferred formation of different orders of ionic vacancies upon Fe addition, thereby corroborating the results from XRD. The photoluminescence study illustrated the quenched emission intensity upon Fe doping, with almost no change in emission color, and signatures of tetrahedral/octahedral Fe3+ occupancy in the lattice. Thus, this study highlights the variation in degrees of freedom associated with atomic arrangements in spinel ZnAl2O4 lattice upon Fe3+ addition.

Characterization of Dimeric Vanadium Uptake and Species in Nafion™ and Novel Membranes from Vanadium Redox Flow Batteries Electrolytes

A core component of energy storage systems like vanadium redox flow batteries (VRFB) is the polymer electrolyte membrane (PEM). In this work, the frequently used perfluorosulfonic-acid (PFSA) membrane Nafion™ 117 and a novel poly (vinylidene difluoride) (PVDF)-based membrane are investigated. A well-known problem in VRFBs is the vanadium permeation through the membrane. The consequence of this so-called vanadium crossover is a severe loss of capacity. For a better understanding of vanadium transport in membranes, the uptake of vanadium ions from electrolytes containing Vdimer(IV–V) and for comparison also V(II), V(III), V(IV), and V(V) by both membranes was studied. UV/VIS spectroscopy, X-ray absorption near edge structure spectroscopy (XANES), total reflection X-ray fluorescence spectroscopy (TXRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and micro X-ray fluorescence spectroscopy (microXRF) were used to determine the vanadium concentrations and the species inside the membrane. The results strongly support that Vdimer(IV–V), a dimer formed from V(IV) and V(V), enters the nanoscopic water-body of Nafion™ 117 as such. This is interesting, because as of now, only the individual ions V(IV) and V(V) were considered to be transported through the membrane. Additionally, it was found that the Vdimer(IV–V) dimer partly dissociates to the individual ions in the novel PVDF-based membrane. The Vdimer(IV–V) dimer concentration in Nafion™ was determined and compared to those of the other species. After three days of equilibration time, the concentration of the dimer is the lowest compared to the monomeric vanadium species. The concentration of vanadium in terms of the relative uptake λ = n(V)/n(SO3) are as follows: V(II) [λ = 0.155] > V(III) [λ = 0.137] > V(IV) [λ = 0.124] > V(V) [λ = 0.053] > Vdimer(IV–V) [λ = 0.039]. The results show that the Vdimer(IV–V) dimer needs to be considered in addition to the other monomeric species to properly describe the transport of vanadium through Nafion™ in VRFBs.

Hepatocyte growth factor administration increases bone soluble phosphate and alters bone chemical structure in diabetic hypertensive rats.

Hepatocyte growth factor (HGF) is a novel potential therapy for improving bone health in patients with type II diabetes and hypertension, but its effect on the bone molecular structure is not revealed yet. Here, X-ray absorption near edge structure (XANES) spectroscopy was used to explore the effects elicited by HGF on the bone chemical structure. This study assessed local calcium (Ca) and phosphorus (P) coordination of diabetic hypertensive rat bones, each with and without HGF treatment. Results revealed that HGF has significant effects on Ca and P coordination chemistry as confirmed by presence of more soluble phosphates in the HGT-treated groups. Data indicated that treated bones have a poorly developed phosphate structure as evidenced by drastic drop in post-edge shoulder in P L2,3-edge compared to diabetic hypertensive and diabetic control bone. Presence of soluble Ca and P, products of bone resorption, with HGF treatment suggests unbalanced bone resorption and formation.

Lead speciation and availability affected by plants in a contaminated soil

Changes in lead (Pb) speciation in the rhizosphere may be plant species-dependent and dictate Pb fate and behavior in the soil-plant system. X-ray absorption near edge structure (XANES) spectroscopy can explain how these changes affect Pb availability in soils and its uptake by plants. We investigated the changes in Pb speciation and availability in the rhizosphere of eucalypt (Eucalyptus urophylla x Eucalyptus grandis), palisade grass (Urochloa brizantha cv. Marandu), and Indian mustard (Brassica juncea L.) using XANES spectroscopy. A greenhouse experiment was performed in a complete randomized design, with three plant species and a no plant control treatment. After three months, rhizosphere and bulk soil samples were collected, Pb speciation was assessed by Pb L3-edge XANES spectroscopy, and Pb concentration was determined in plant tissue. In bulk soil, we found Pb primarily as Pb-Il (Pb sorbed to illite; 48%) and lead monoxide (PbO; 36%). In the rhizosphere, Pb-Ka (Pb sorbed to kaolinite; 33–56%) and (CH₃COO)₂Pb (25–41%) were the main Pb forms, the latter suggesting Pb complexation by low molecular weight organic acids (LMWOAs). Palisade grass rhizospheric soil had a lower abundance of Pb-Ka and a higher abundance of (CH₃COO)₂Pb than eucalypt and mustard, which led to low Pb concentration in plant tissue. LMWOAs exudation followed by Pb2+ complexation is the apparent mechanism used by palisade grass to detoxify the rhizosphere and control Pb uptake. Given its low Pb uptake and potential to complex Pb in organic forms, palisade grass may be a promising species for Pb phytostabilization in contaminated soils.

X-ray absorption near edge spectroscopy of the electronic structure of potassium adsorbed single walled carbon nanotubes

Carbon nanotubes (CNTs) show exceptional properties which make them extremely attractive for many applications. The electronic structure of potassium adsorbed single-walled carbon nanotubes (SWCNTs) is investigated by x-ray near edge structure (XANES) spectroscopy. Gas molecules (H2O, O2, CO2, CO) and potassium (K) adsorbates showed non negligible sensitivity to the geometry, thermal annealing treatment, and hybridization. The SWCNT type is shown by transmission electron microscopy (TEM) whereas an aleatory arrangement of SWCNTs and the spherical iron particles along carbon nanotubes are observed by scanning electron microscopy (SEM). XANES shows that the charge transfer effect in K-doping SWCNTs increases the conductivity to 62.7%. The adsorption study demonstrates that the adsorption mechanism to remove potassium from aqueous solutions using SWCNTs is assimilated to the ion exchange process. The functional groups OH−, C=O and C≡C are involved in the adsorption process. They are correlated to the assignment of the adsorbates given by XANES.

Optimizing pyrolysis conditions for recycling pig bones into phosphate fertilizer.

Selecting pyrolysis parameters for recycling P-rich and hazardous biowastes, such as bones, into fertilizers is still a challenge. Our objective was to improve pyrolysis procedures of pig bones for the production of P fertilizers. Bone chars were produced by pyrolysis at 400, 550, or 800°C with no gas addition; 550 and 800°C under N2; 800°C under steam flux, using calcination at 800°C as control treatment. Synchrotron-based X-ray diffraction and X-ray absorption near edge structure spectroscopy at the P and Ca K- and L-edges showed that these bone chars were largely composed of hydroxyapatite. Hydroxyapatite crystallization was inhibited by pyrolysis conducted in the absence of oxygen at 400, 550, or 800°C, either under no gas or under N2 flux. The clogging of pores by lack of organic compounds removal was hypothesized to cause low surface area of 400°C bone char, resulting in a fertilizer with citric-acid soluble P as low as calcination, while 550 and 800°C bone chars obtained in absence of oxygen showed greater porosity, surface area, and citric acid-soluble P than steamed or calcined samples at 800°C. Although extractable phosphate in water and neutral-ammonium-citrate showed trends comparable to those from citric acid, it was negligible for all heated materials. Since it is possible to produce bone chars with different chemical, physical and crystallographic properties by managing pyrolysis conditions, bone chars can be designed to increase their suitability as P fertilizers for different purposes, such as high solubility or slow P release.

Local structure and cation distribution analysis of Mn1-xZnxFe2O4 powders by X-ray Absorption Near Edge Structure spectroscopy

In this work, Mn1-xZnxFe2O4 powders (x = 0.0, 0.2, 0.4, 0.6 and 0.8) were prepared by a conventional solid-state reaction method. The phase formation, local structure and cation distribution of Mn1-xZnxFe2O4 ferrite samples were investigated by X-ray Diffraction (XRD) and X-ray Absorption Near Edge Structure (XANES) Spectroscopy techniques. XRD analysis reveals that the single cubic spinel phase was formed in all prepared samples. The average crystallite size decreases with increasing zinc concentration. XANES measurements showed that the oxidation states of Fe and Zn ions in doped samples are +3 and + 2, respectively. The feature of the measured Fe, Mn and Zn K-edges XANES spectra are compared with the simulated XANES spectra obtained by using linear combination of the proportion of the distribution of Fe, Mn and Zn ions within the spinel structure. It is shown that a satisfactory agreement for the XANES Fe, Mn and Zn K-edges of Mn1-xZnxFe2O4 powders can be obtained introducing a mixed spinel structure with the majority of Fe3+ ions occupying tetrahedral sites and the majority of Zn2+ ions occupying octahedral sites. While, Mn2+ and Mn3+ ions were found to occupy both tetrahedral and octahedral sites with a higher stronger preference to be at the tetrahedral sites.

Contribution of biological soil crusts to soil organic matter composition and stability in temperate forests

Biological soil crusts (biocrusts) play an important role in the biogeochemical C-, N- and P-cycles but there is limited knowledge about their effect on the composition and stability of organic matter (OM) in soil. In pine and beech dominated temperate forests, the different compartments of biocrust, crust-adhering soil and crust-free soil were analysed by pyrolysis field ionization mass spectrometry (Py-FIMS) and C K-edge X-ray absorption near edge structure (XANES) spectroscopy. Phototrophic biocrust communities had been determined morphologically. Predominant tree species affected OM composition and stability but an overriding effect of biocrust organisms was observed. In biocrusts, the OM mineralization potential was high due to a high carbohydrate/ketone ratio and a high proportion of the fatty acid C22:0. A high proportion of free fatty acids in combination with a high carbohydrate proportion was identified as a relative marker for biocrusts and additional high proportions of lignin dimers were specific for samples under pine. In biocrust under pine, OM in soil compartments was characterized by large proportions of long-chain aliphatics from plant cuticular material. In A-horizon substrate under beech, OM compound classes were not affected by phototrophic species richness, suggesting functional redundancy of these biocrust organisms with regard to OM composition. In contrast to A-horizon substrate, OM in C-horizon substrate was characterized by a higher proportion of easily decomposable material. Under pine, forest management intensity (SMI) affected OM by stand age and density due to effects on phototrophic biocrust species. We conclude that compartment, vegetation, horizon substrate and SMI affected OM composition and stability, and biocrusts hence contributed to soil OM diversity in forest ecosystems.