XANES Spectroscopy Research Articles

Long-term land use and tillage influence on phosphorus species in Brazilian Oxisols: A multi-technique assessment by chemical P fractionation, 31P NMR and P K-edge XANES spectroscopies

Phosphorus (P) interacts with soil constituents in many ways, forming compounds of distinct stabilities in a number of chemical inorganic and organic forms. The identification of these P compounds in tropical agro-ecosystems is especially relevant to understanding potential P availability in these common P-fixing soils, providing information to improve P use efficiency. In this study, we investigated P partitioning and speciation in Brazilian Cerrado Oxisols under different management systems using a combination of sequential chemical P fractionation (SPF), P K-edge XANES and 31P NMR spectroscopy. Topsoil (0–10 cm) samples were obtained from soils under three management regimes, including native vegetation (Cerrado), no-tillage (NT), and conventional tillage (CT) agriculture. Four representative agricultural sites in the Brazilian Cerrado were selected: Ox-1 (Costa Rica-MS), Ox-2 (Sapezal-MT), Ox-3 (Cristalina-GO) and Ox-4 (Tasso Fragoso-MA). In all sites, the soils are classified as Typic Hapludox, varying in cultivation time, soil clay content and oxide types. Long-term cultivation in NT and CT led to large P accumulation in the soil. Chemical P fractionation indicated that labile P was the smallest pool at all sites. P K-edge XANES showed that P is mostly sorbed to the Fe and Al hydr(oxides), goethite, hematite and gibbsite. It also showed that phytate was the predominant organic P species, depleting in sequence from native vegetation to NT and CT management systems. 31P NMR showed that monoester P was present in all soils and management systems, whereas diester-P was found only in soils under native vegetation. The combined use of SPF, XANES and 31P NMR data was necessary to fully understand inorganic and organic P chemistry and reactivity in these tropical agricultural croplands.

Phosphorus hotspots in pedogenic carbonate coatings determined by zoned microscale arrangement and organo-mineral interactions

Subsoils of calcareous soils are often characterized by pedogenic carbonate precipitates coating soil aggregate and rock surfaces. Numerous studies identified the key mechanisms of pedogenic carbonate formation in calcareous soils. Still, little is known about the ecosystem function of carbonate precipitates for nutrient supply, their microscale structure and the interaction of microscale structure with nutrient distribution such as P. We investigated the mineral composition and P speciation of lithogenic carbonate coated by pedogenic carbonate precipitates. Subsoil horizons of a Rendzic Leptosol (Tuttlingen, C horizon) and a Eutric Cambisol (Benediktenwand, BC horizon) in Central Europe were sampled. We combined wet-chemical methods with X-ray diffraction, synchrotron-based P XANES spectroscopy, and specific surface area analysis (N2-BET). To investigate the spatial arrangement and elemental distribution at the microscale, a complementary approach using light microscopy, scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX) as well as XANES spectroscopy at the microscale (µXANES) was conducted. In both soils, the pedogenic carbonate coatings showed an intricate porous microstructure and large specific surface area. The δ13C values of the coatings were halfway between those of the primary bedrock and soil organic matter (SOM), proving their formation by re-precipitation of dissolved carbonate through organic and inorganic (H2CO3) acids. Compared to the primary rocks, the coatings were enriched in Al, Fe, Mn, and K and in organic C, N, and P; additionally, they contained silicate minerals (muscovite, illite) and quartz. Whereas the P in the rock interior was mainly present as apatite, the P in the coatings was mainly organic P bound to Ca and Al minerals. NanoSIMS and µXANES analyses at Tuttlingen revealed a zoned microstructure of the pedogenic coating: An inner zone with most P co-localized and bound to Al minerals and an outer zone enriched again in Ca and SOM. The formation of these zones indicates the infiltration and residual accumulation of Al minerals which may contribute to mitigate P losses by leaching. Intensive penetration by fine plant roots and observations of fungal hyphae within the coatings indicate the potential of pedogenically altered carbonates as microbial habitat and ecosystem nutrient source, thus contributing to the scarce ecosystem P nutrition of initial calcareous soils.

Magnetic transitions and different valence states of manganese ions in solid solutions (Ni,Cu,Mn)-=SUB=-3-=/SUB=-BO-=SUB=-5-=/SUB=-

The content of metal ions and their valence state for three NixCuyMn3-x-yBO5 compositions were estimated by EXAFS and XANES spectroscopy methods. In all three compositions, the copper y content did not exceed 0.25; in the third composition, some manganese ions entered the compound not only in the trivalent, but also in the divalent state. A detailed study of the magnetic properties showed that two magnetic transitions are observed in all compositions: one in the region of 60-75 K, and the second in the region of 10 K. Based on the exchange interactions calculated within the framework of the empirical model, the magnetic ordering temperatures for two- and three-lattice magnets were determined and it was assumed that the magnetic transition in the region of 60-75 K is associated with the ordering of magnetic moments in positions 2 and 3. Keywords: ludwigites, magnetic phase transition, indirect exchange interactions.

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model.

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Mixed Valanced V3+,V2+ Phosphate Na7V4(PO4)6: A Structural Analogue of Mineral Yurmarinite

Two sodium vanadium phosphates, synthetic analogues of the minerals kosnarite, Na3V2(PO4)3, and yurmarinite, Na7V4(PO4)6, were obtained by hydrothermal synthesis simulating a natural hydrothermal solution. While the Na3V2(PO4)3 phase belongs to the NASICON family and is well-known for its high-ionic conductivity, the new Na7V4(PO4)6 compound is a rare case of V2+-containing oxosalts, which are hard to prepare due to their instability in air. Here we report the crystal structure of heterovalent vanadium phosphate studied by single crystal X-ray diffraction, XANES spectroscopy, and topological ion migration modelling. A discussion of divalent vanadium compounds of both natural and synthetic origin is also given, with a review of the methods for their synthesis and a comparative analysis of V–O bond lengths.

Distribution of Mn Oxidation States in Grassland Soils and Their Relationships with Soil Pores.

Manganese (Mn) is known to be an active contributor to processing and cycling of soil organic carbon (C), yet the exact mechanisms behind its interactions with C are poorly understood. Plant diversity in terrestrial ecosystems drives feedback links between plant C inputs and soil pores, where the latter, in turn, impact the redox environment and Mn. This study examined associations between soil pores (>36 μm Ø) and Mn within intact soils from two grassland ecosystems, after their >6-year implementation in a replicated field experiment. We used μ-XRF imaging and XANES spectroscopy to explore spatial distribution patterns of Mn oxidation states, combined with X-ray computed microtomography and 2D zymography. A high plant diversity system (restored prairie) increased soil C and modified spatial distribution patterns of soil pores as compared to a single species system (monoculture switchgrass). In switchgrass, the abundance of oxidized and reduced Mn oxidation states varied with distance from pores consistently with anticipated O2 diffusion, while in the soil from restored prairie, the spatial patterns suggested that biological activity played a greater role in influencing Mn distributions. Based on the findings, we propose a hypothesis that Mn transformations promote C gains in soils of high plant diversity grasslands.

(Invited) Why Is Pacman so Hungry? Femtosecond M-Edge XANES Explains the Low Quantum Yield of µ-Oxo Bis(Fe Porphyin) Photocatalysts

Bridged μ-oxo iron bisporphyrins serve as photocatalysts for oxidative organic transformations, but suffer from low quantum efficiency. This low performance has been primarily attributed to the short lifetime of Fe(II)/Fe(IV)=O state formed after LMCT excitation, but a competing proposal claims that the majority photoproduct is a catalytically inactive ligand-centered ion pair. We use femtosecond optical and M2,3-edge XANES spectroscopy to investigate the early photodynamics of the μ-oxo iron bisporphyrin, (TPPFe)2O, providing evidence for the preferential formation of an TPPFe(III)+/TPPFe(III)-O- ion pair state instead of the desired TPPFe(II)/TPPFe(IV)=O. This work provides new guidance for catalyst design, suggesting that the excited-state potential surfaces must be redesigned to promote formation of the catalytically active state.

XANES spectroscopy for the clinician

XANES spectroscopy, which uses synchrotron radiation as a probe, offers substantial information about the local structure of biological samples, encompassing those without long range order such as Pt anticancer molecules, and nanometre scale or amorphous particles of calcium phosphate. Its subcellular spatial resolution, as well as its capacity to operate at room temperatures and pressures represent major advantages for medical research. Moreover, paraffin embedded biopsy samples can be analysed without any further preparation, Key publications which illustrate these capacities are presented.

Iron (II) fluoride cathode material derived from MIL-88A

We report an eco-friendly, simple, and scalable method of FeF 2 cathode production. MIL-88A was synthesized in a water medium without any additives. It was used as a source of iron (3 +) ions during pyrolysis. The porous structure of such a sacrificial agent allowed us to incorporate poly-vinylidene fluoride molecules as a guest component into a host MIL-88A framework. Pyrolysis in Ar-flow results in two simultaneous processes: reducing Fe 3+ into Fe 2+ and forming porous carbon shells for FeF 2 nanoparticles. Applying complex analysis of high-resolution TEM images, porosity measurements, and XANES spectroscopy, we have revealed that obtained iron fluoride is composed of nanoparticles with elongated and hexagonal shapes. Both iron fluorides were attributed to tetragonal FeF 2 structure type, contained only Fe 2+ ions, and were covered with porous carbon shells. The obtained material was used as a cathode for a lithium-ion battery and showed good stability and a high capacity of 425–330 mA h/g. The proposed water-based synthesis of MIL-88A as a precursor in combination with mild pyrolysis conditions and good electrochemical performance make this material promising for cathode application. • We report the green and reproducible synthesis of MIL-88a. • For the first time, MIL-88a was applied as an iron and carbon source to produce FeF 2 . • MIL-88a was used as a host for polyvinylidene fluoride. • We obtained stable FeF 2 nanoparticles in a porous conductive carbon matrix. • This material was tested in a lithium-ion battery as a conversion cathode.

Soil chemical extractions can alter potassium coordination in agricultural soils: A combined wet chemical and X-ray absorption spectroscopic approach

• Potassium XANES spectroscopy from soils is paired with six wet chemical extractions. • Dampening of post-edge XANES oscillations was caused by the Mehlich III extraction. • XANES indicates Mehlich III can dislodge K from inner-sphere siloxane bonds. • Dampening was more significant in the loamy sand soil versus the sandy clay loam soil. • Dampening increased the LCF component of K associated with montmorillonite by 25% Soil wet chemical extractions are used to determine plant available potassium (K); yet problems with K bioavailability still persist in soils. Bioavailability is directly related to how elements are bonded and coordinated to the solid phases of soil. However, there is a critical knowledge gap with respect to identifying the in situ, solid-phase K coordination environments in soils treated with wet chemical extractions. Synchrotron-based X-ray absorption spectroscopy (XAS) measures the molecular coordination environment of elements and has been used here to study K chemistry in two agricultural field soils. The objective of this work is to pair XAS with wet chemical K extractions and analyze changes in the coordination environment of K throughout the soil profile. Soil profiles collected down to 1.2 m of a loamy sand and a sandy clay loam were treated with six different extraction methods: (1) Mehlich III, (2) ammonium acetate, (3 & 4) Haney, Haney, Hossner, and Arnold 1 and 2, (5) citrate dithionite, and (6) water extractable potassium. The residual solid was analyzed using XAS and X-ray diffraction (XRD). Citrate dithionite extracted the largest amount of K (1860 mg kg −1 ), and the water extraction removed the least (30 mg kg −1 ). XAS and XRD indicate K sources in these soils are illite, illite–smectite, smectite, and potassium feldspar. A Linear Combination Fit (LCF) approach indicated changes of interest in the post-edge region of the K, K-edge XANES spectra due to the Mehlich extraction. XANES data indicate that the Mehlich extraction can remove loosely bound K and dislodge a portion of K from interlayer space of illite and illite–smectite. This is significant because the Mehlich extraction is effectively decreasing the number of coordinating O atoms surrounding K and causing a dampening in the oscillations of the XANES spectra in the post-edge region (3,620–3,650 eV). The dislodgement of K due to the Mehlich III extraction indicates that the inner-sphere bonds that K forms to the siloxane surfaces in the interlayer space can be broken, and K can become more hydrated with water molecules also present in the interlayer space. This dampening was dependent on soil texture, was more significant in the coarser textured soil, and would indicate an increased accommodation of K by montmorillonite or an increase in K hydration in illite–smectite. The dampening resulted in an average increase in K intercalated in montmorillonite interlayer spaces by 25% in Mehlich treated samples according to the LCF results. Future studies involving K sorption to silicon and aluminum oxides will further clarify behavior of K on mineral surfaces, particularly during surface precipitation.

Phosphorus speciation in the organic layer of two Swedish forest soils 13–24 years after wood ash and nitrogen application

Application of wood ash to forests can restore pools of phosphorus (P) and other nutrients, which are removed following whole tree harvesting. Yet, the mechanisms that affect the fate of ash-P in the organic layer are less well known. Previous research into the extent to which ash application leads to increased P solubility in the soil is contradictory. We combined synchrotron PK-edge XANES spectroscopy, µ-XRF microscopy, and chemical extractions to examine the speciation and solubility of P. We studied organic horizons of two long-term field experiments, Riddarhyttan (central Sweden), which had received 3, 6, and 9 Mg ash ha−1, and Rödålund (northern Sweden), where 3 Mg ash ha−1 had been applied alone or combined with N every-three years since 2003. At the latter site, we also determined P in aboveground tree biomass. Overall, the ash application increased P in the organic layer by between 6 and 28 kg P ha−1, equivalent to 17–39 % of the initial P content in the applied ash. At Rödålund, there was 4.6 kg Ca-bound P ha−1 (9.5 %) in the ash treatment compared to 1.6 kg ha−1 in the ash + N treatment and < 0.4 kg ha−1 in the N treatment and the control. At Riddarhyttan, only the treatment with the highest ash dose had residual Ca-bound P (3.8 kg ha−1). In contrast, the ash application increased Al-bound P (p < 0.001) with up to 15.6 kg P ha−1. Moreover, the ash increased Olsen-P by up to two times. There was a strong relationship between the concentrations of Olsen-P and Al-bound P (R2 = 0.83, p < 0.001) as well as Fe-bound P (R2 = 0.74, p = 0.003), suggesting that the ash application resulted in an increased amount of relatively soluble P associated with hydroxy-Al and hydroxy-Fe compounds. Further, there was an 18 % increase in P uptake by trees in the ash treatment. By contrast, repeated N fertilization, with or without ash, reduced Olsen-P. The lower P extractability was concomitant with a 39 % increase in plant P uptake in the N treatment, which indicates elevated P uptake in response to higher N availability. Hence, the application of wood ash increased Al-bound P, easily available P, and P uptake. N fertilization, while also increasing tree P uptake, instead decreased easily available P and did not cause a shift in soil P speciation.

Kaznakhtite, Ni6Co3+2(CO3)(OH)16⋅4H2O, a new natural layered double hydroxide, the member of the hydrotalcite supergroup

AbstractKaznakhtite, ideally Ni6Co3+2(CO3)(OH)16⋅4H2O, is a new member of the hydrotalcite group within the hydrotalcite supergroup. The mineral was discovered at the Kaznakhtinskiy ultrabasic massif, Altai Republic, SW Siberia, Russia. It occurs as powdery aggregates forming flattened lenses up to 1.5 × 0.5 cm and veinlets up to 1 cm long and up to 1 mm thick in aggregates of chrysotile, lizardite, stichtite and dolomite. Other associated minerals include brucite, chromite, heazlewoodite, manganochromite, magnetite and magnesioferrite. Kaznakhtite aggregates are composed of tiny platy grains up to 0.01 mm across. Kaznakhtite is light green and translucent in aggregates. It has an earthy lustre and white streak. Cleavage is micaceous on {001}. Dcalc = 2.864 g cm–3. The mineral is optically uniaxial (–) with ɛ = 1.657(3) and ω = 1.676(3), and weakly pleochroic in greenish hues, ω &gt; ɛ. Chemical composition (wt.%, electron microprobe, Co valence state determined by XANES spectroscopy, CO2 and H2O calculated by stoichiometry) is: MgO 2.15, NiO 47.40, ZnO 0.22, Al2O3 0.16, Cr2O3 0.98, Co2O3 17.42, Cl 0.63, CO2 5.05, H2O 24.60, –O=Cl –0.14, total 98.47. The empirical formula calculated based on the sum of all metal cations = 8 apfu is (Ni5.54Mg0.47Zn0.02)Σ6.03(Co3+1.83Cr0.11Al0.03)Σ1.97C1.00O2.99(OH)15.84Cl0.16⋅4H2O. Infrared spectroscopy confirmed the presence of CO32– anions, OH– groups and H2O molecules. The crystal structure was refined by the Rietveld method with RB = 0.19%. Kaznakhtite is trigonal, space group R͞3m, a = 3.0515 (3), c = 23.180 (3) Å, V = 186.93 (4) Å3 and Z = 3/8. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.72 (100) (003); 3.863 (24) (006); 2.630 (4) (101); 2.576 (10) (012); 2.294 (6) (015); 1.950 (4) (018); 1.526 (4) (110); and 1.497 (4) (113). Kaznakhtite is a Co3+ analogue of reevesite, Ni6Fe3+2(CO3)(OH)16⋅4H2O. The mineral is named after its type locality.

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

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

Exploring spectroscopic X-ray nano-imaging with Zernike phase contrast enhancement

Spectroscopic full-field transmission X-ray microscopy (TXM-XANES), which offers electrochemical imaging with a spatial resolution of tens of nanometers, is an extensively used unique technique in battery research. However, absorption-based bright-field imaging has poor detection sensitivity for nanoscale applications. Here, to improve the sensitivity, we explored spectroscopic X-ray nano imaging with Zernike phase contrast (ZPC-XANES). A pinhole-type Zernike phase plate, which was optimized for high-contrast images with minimal artifacts, was used in this study. When the absorption is weak, the Zernike phase contrast improves the signal-to-noise ratio and the contrast of images at all energies, which induces the enhancement of the absorption edge step. We estimated that the absorption of the samples should be higher than 2.2% for reliable spectroscopic nano-imaging based on XANES spectroscopy analysis of a custom-made copper wedge sample. We also determined that there is a slight absorption peak shift and sharpening in a small absorption sample due to the inflection point of the refractive index at the absorption edge. Nevertheless, in the case of sub-micron sized cathode materials, we believe that better contrast and higher resolution spectroscopic images can be obtained using ZPC-XANES.

Structural, redox and isotopic behaviors of iron in geological silicate glasses: A NRIXS study of Lamb-Mössbauer factors and force constants

We report a combined conventional Mössbauer and synchrotron Nuclear Resonant Inelastic X-ray Scattering (NRIXS) study of a series of basalt, andesite, dacite, and rhyolite glasses at temperatures ranging from 5 to 1223 K. These glasses were synthesized under controlled oxygen fugacities and span a wide range of Fe3+/Fetot ratios. As expected from theory, we find that in these glasses, the inverse of the Lamb-Mössbauer factor (the recoil-free fraction) correlates linearly with the fraction of the conventional Mössbauer spectrum that can be ascribed to Fe3+. Extrapolating the linear relationships to pure Fe3+ and Fe2+ endmembers yields a ratio for the Lamb-Mössbauer factors C of Fe3+ and Fe2+ of 1.203 ± 0.017 (1σ), with no clear dependence on the chemical composition of the glass. We show that the materials studied follow the harmonic approximation up to 1223 K for olivine and 773 K for basaltic glass, allowing us to extrapolate C in temperature. The temperature-dependence of C is well approximated by the formula C=1.203±0.033T/300 over the temperature range 50–750 K. This calibration is used to correct previous redox ratio determinations. We investigate how chemical composition and iron redox state influence the mean force constant of iron in glasses, a key driver of Fe isotopic fractionation during magma generation and differentiation. As previously documented by Dauphas et al. (2014), we find clear correlation between bond strength and iron redox ratio that is secondarily modulated by coordination effects in more alkali-rich felsic magmas. New data on simplified glass compositions reveal that network modifiers, notably K and Na, seem to exert an important control on Fe2+ bond strengths. The refined ratio of Mössbauer factors for Fe2+ and Fe3+ is used to improve on calibrations of iron redox state in geological glasses using XANES spectroscopy. We reevaluate the Fe3+/Fetot ratio of MORBs based on previously published XANES and Mössbauer data. 0.087±0.005

Pinus nigra bark from a mercury mining district studied with high resolution XANES spectroscopy.

Tree bark near former mercury (Hg) mines and roasting plants is known to have exceptionally high (up to several mg kg-1) Hg concentrations. This study explores the change of Hg speciation with depth (down to 25-30 mm from the outermost surface) in black pine (Pinus nigra) bark by means of high-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy at the Hg LIII-edge. Principal component analysis and linear combination fitting applied to the HR-XANES spectra suggested that in the outermost layer (∼0-2 mm from the surface), roughly 50% of Hg is in the form of nanoparticulate metacinnabar (nano-β-HgS). A progressive increase in Hg-organic species (Hg bound to thiol groups) is found in deeper bark layers, while nano-β-HgS may decrease below the detection limit in the deepest layers. Notably, bark layers did not contain cinnabar (α-HgS), which was found in the nearby soils along with β-HgS (bulk), nor Hg0, which is the main Hg species in the atmosphere surrounding the sampled trees. These observations suggested that nano-β-HgS, at least in part, does not originate from mechanically trapped wind-blown particulates from the surrounding soil, but may be the product of biochemical reactions between gaseous elemental Hg and the bark tissue.

Chromium in lead metasilicate glass: Solubility, valence, and local environment via multiple spectroscopy

In this article the doping of chromium in a lead metasilicate glass is explored using Raman, UV–Vis, and Cr K -edge XANES spectroscopy. Our results indicate that the Cr ions are predominantly present as CrO 4 2− complexes, with a minor amounts of Cr(III). The solubility of Cr is limited to ∼1 mol % Cr 2 O 3 . This boundary was inferred from the intensity changes to the CrO 4 2− stretching vibrations and confirmed by the inability to produce a homogeneous glass containing 2 mol % Cr 2 O 3 . The establishment of the solubility limit of Cr in lead glasses is important to design more effective glass compositions depending on the desired application. Finally, this low solubility indicates that Cr 2 O 3 could be used as a potential nucleating agent for lead-silica based glass-ceramics.

Influence of Spartina alterniflora invasion on mercury storage and methylation in the sediments of Yangtze River estuarine wetlands

The influence of Spartina alterniflora invasion on mercury (Hg) cycling in the coastal wetlands of China and in other parts of the world is not well understood. By combining field sampling with laboratory-scale microcosm incubation experiments, we examined the effect of S. alterniflora invasion on mercury (Hg) storage and net methylmercury (MeHg) production in the sediments of the wetlands in the Yangtze River estuary. The results showed that (1) the estimated mean annual total Hg (THg) input to the wetlands from litterfall was 8.0–26.5 μg/m2 from S. alterniflora, Phragmites australis and Scirpus mariqueter, with the highest levels (26.5 ± 9.0 μg/m2) coming from S. alterniflora due to its high primary productivity. (2) The depth-integrated (<40 cm) mean concentrations of THg in vegetated sediments ranged from 47.5 to 109.1 ng/g, which showed significant positive correlations with the fraction of <16 μm particles and the loss-on-ignition content (P < 0.01). (3) The MeHg concentrations and MeHg/THg (%MeHg) spanned 0.14–1.30 ng/g and 0.2–1.7%, respectively, and showed similar decreasing patterns with depth across all the sites, thereby reflecting the higher rates of Hg methylation in the upper surface sediments. In addition, no significant differences in MeHg concentrations were observed among the sites vegetated by the three plants (P > 0.05). (4) During plant decomposition, net MeHg production was enhanced significantly, but it was inhibited in the presence of the sulfate reduction inhibitor molybdate; these results suggest that sulfate-reducing bacteria could be important methylation mediators in sediments. Moreover, a large fraction of reduced inorganic sulfur (S) species such as iron monosulfide formed during plant decomposition, as revealed by S K-edge XANES spectroscopy. All of these results suggest that S. alterniflora invasion facilitates Hg storage in wetland sediments and affects sulfur cycling but that its effects on MeHg production are similar to those of native plants in the Yangtze River estuarine wetlands.

X-ray spectral diagnostics of the local environment of zinc in the arachidic acid layers

The article describes the behaviour of the Zn ions in aqueous subphase in presence of arachidic acid films at the air/liquid interface first studied by XANES spectroscopy in the fluorescent mode under total external reflection conditions. Fingerprint analysis of experimental spectra was carried out to determine the local structure of zinc ions in arachidic acid. It showed that zinc is surrounded by oxygen atoms, which may correspond to the interaction of zinc ions with polar groups of arachidic acid.

Immobilization of lead by amendments in a mine-waste impacted soil: Assessing Pb retention with desorption kinetic, sequential extraction and XANES spectroscopy

Chemical stabilization is an in-situ remediation that uses amendments to reduce contaminant availability in polluted soils. Rates of phosphate, lime, biochar, and biosolids were evaluated as affecting Pb speciation and mobility in soil samples of a mining area located in Vazante, state of Minas Gerais, Brazil. Chemical and mineralogical characterization, desorption kinetics, sequential extraction, leaching evaluation in columns and speciation using X-ray absorption near edge structure were performed. Pb adsorbed on bentonite and on anglesite were the predominant species in the unamended soil. The treatments with phosphate and lime transformed part of the Pb species to pyromorphite. Conversely, part of Pb species was transformed to Pb adsorbed on citrate in the soil amended with biochar, while PbCl2 was formed in soil samples amended with biosolids. Phosphate and lime increased the Pb extracted in the residual fraction, thus showing that more recalcitrant species, such as pyromorphite, were formed. Biosolids and biochar treatments decreased the Pb in the residual fraction, and the fraction associated to organic matter increased after the addition of biosolids. Phosphate and lime were effective to immobilize Pb and to decrease Pb desorption kinetics, but the organic amendments increased the desorption kinetics of Pb in all rates applied. The soil amended with phosphate decreased the Pb leached in the experiment with leaching columns.