Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment.

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 µm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 µg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

X-SPEC: a 70†eV to 15†keV undulator beamline for X-ray and electron spectroscopies.

X-SPEC is a high-flux spectroscopy beamline at the KIT (Karlsruhe Institute of Technology) Synchrotron for electron and X-ray spectroscopy featuring a wide photon energy range. The beamline is equipped with a permanent magnet undulator with two magnetic structures of different period lengths, a focusing variable-line-space plane-grating monochromator, a double-crystal monochromator and three Kirkpatrick-Baez mirror pairs. By selectively moving these elements in or out of the beam, X-SPEC is capable of covering an energy range from 70 eV up to 15 keV. The flux of the beamline is maximized by optimizing the magnetic design of the undulator, minimizing the number of optical elements and optimizing their parameters. The beam can be focused into two experimental stations while maintaining the same spot position throughout the entire energy range. The first experimental station is optimized for measuring solid samples under ultra-high-vacuum conditions, while the second experimental station allows in situ and operando studies under ambient conditions. Measurement techniques include X-ray absorption spectroscopy (XAS), extended X-ray absorption fine structure (EXAFS), photoelectron spectroscopy (PES) and hard X-ray PES (HAXPES), as well as X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS).

Retention of Iodide and Chloride by Formation of a Green Rust Solid Solution GR-Cl1-xIx: A Multiscale Approach.

The uptake of iodide and chloride during the synthesis of green rust (GR), the Fe endmember of the layered double hydroxide (LDH) group, was investigated. GR compounds were prepared by aerial oxidation of Fe(OH)2 in suspension, considering various I/Cl ratios at constant ionic strength. Only GR compounds formed in all experiments, and the associated I/Cl ratio increased with that of the starting suspension. No preferential uptake of any halide could be detected, and all compounds had comparable morphology. Furthermore, the height of the interlayer gallery increased with the I/Cl ratio from ∼7.7 Šfor the chloride endmember to ∼8.3 Šfor the iodide endmember, and the observed linear increase was attributed to increasing interlayer iodide content. In all compounds, Fe K-edge X-ray absorption spectroscopy evidenced the presence of sixfold coordinated iron with a Fe2+/Fe3+ ratio of 3, homogeneously distributed within flattened octahedral sites, with six Fe as next-nearest neighbors. The Fe short-range environment was not affected by the interlayer composition, and no halide from the interlayer could be detected. Furthermore, iodide and chloride anions are located in a water-like environment, being loosely bound by weak electrostatic interactions to the octahedral sheet likely above ferric iron. Results consistently hint at the formation of a solid solution between chloride and iodide GR endmembers, certainly facilitated by the crystallization of both compounds in the same space group. This study provides further insights into the ability of LDH to simultaneously accommodate several anionic species of various sizes. The formation of such LDH compounds in a deep geological repository for nuclear waste thus represents a possible retention barrier to the migration to the far field of anionic species like 36Cl- and 129I- mobilized from the waste matrix. The extent of retention in disposal sites will depend, among others, on the availability of GR and on the concentration of competing anions.

In Operando Synchrotron Diffraction and in Operando X-ray Absorption Spectroscopy Investigations of Orthorhombic V2O5 Nanowires as Cathode Materials for Mg-Ion Batteries.

Orthorhombic V2O5 nanowires were successfully synthesized via a hydrothermal method. A cell-configuration system was built utilizing V2O5 as the cathode and 1 M Mg(ClO4)2 electrolyte within acetonitrile, together with Mg xMo6S8 ( x ≈ 2) as the anode to investigate the structural evolution and oxidation state and local structural changes of V2O5. The V2O5 nanowires deliver an initial discharge/charge capacity of 103 mAh g-1/110 mAh g-1 and the highest discharge capacity of 130 mAh g-1 in the sixth cycle at C/20 rate in the cell-configuration system. In operando synchrotron diffraction and in operando X-ray absorption spectroscopy together with ex situ Raman and X-ray photoelectron spectroscopy reveal the reversibility of magnesium insertion/extraction and provide information on the crystal structure evolution and changes of the oxidation states during cycling.

Fully automated beamline control system for XAS beamlines.

Owing to the growing demand for complex in situ experiments, the handling of synchrotron experiments has become increasingly complicated. The control system at the XAS beamline of the Karlsruhe Institute of Technology synchrotron was automated based on a user-driven development cycle. By combining intelligent remote-controlled hardware and highly integrated software layers it is now possible to record data with a high level of reproducibility, eliminating human malperformance. The additional advantage of automation is the possibility of real 24/7 operation without tiring. This paper focuses on the options of optimization and enhancement in the data acquisition.

Observation of electrochemically active Fe3+/Fe4+ in LiCo0.8Fe0.2MnO4 by in situ Mössbauer spectroscopy and X-ray absorption spectroscopy.

LiCo0.8Fe0.2MnO4 has been investigated as an active material for the positive electrode in lithium-ion batteries (LIBs) with a discharge potential of around 5 V (vs. Li+|Li). After synthesis by a Pechini based sol-gel route, the structural and morphological properties have been investigated by X-ray diffraction, scanning electron microscopy, 7Li MAS NMR spectroscopy, and 57Fe Mössbauer spectroscopy. With galvanostatic cycling, it was possible to obtain a specific discharge capacity of 117 mA h g-1, which is more than 80% of the theoretical capacity. The lithium extraction/insertion mechanism has been characterized by in situ synchrotron powder diffraction. The reversible oxidation process of Fe3+ to Fe4+ has been observed by in situ Mössbauer spectroscopy and in situ XAS measurements.

Identifying the redox activity of cation-disordered Li-Fe-V-Ti oxide cathodes for Li-ion batteries.

Cation-disordered oxides have recently shown promising properties on the way to explore high-performance intercalation cathode materials for rechargeable Li-ion batteries. Here, stoichiometric cation-disordered Li2FeVyTi1-yO4 (y = 0, 0.2, 0.5) nanoparticles are studied. The substitution of V for Ti in Li2FeVyTi1-yO4 increases the content of active transition metals (Fe and V) and accordingly the amount of Li(+) (about (1 + y)Li(+) capacity per formula unit) that can be reversibly intercalated. It is found that Fe(3+)/Fe(2+) and V(4+)/V(3+) redox couples contribute to the overall capacity performance, whereas Ti(4+) remains mainly inert. There is no evidence for the presence of Fe(4+) species after charging to 4.8 V, as confirmed from the ex situ(57)Fe Mössbauer spectroscopy and the Fe K-edge absorption spectra. The redox couple reactions for iron and vanadium are examined by performing in situ synchrotron X-ray absorption spectroscopy. During charging/discharging, the spectral evolution of the K-edges for Fe and V confirms the reversible Fe(3+)/Fe(2+) and V(4+)/V(3+) redox reactions during cycling between 1.5 and 4.8 V.

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis.

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Thallium speciation and extractability in a thallium- and arsenic-rich soil developed from mineralized carbonate rock.

We investigated the speciation and extractability of Tl in soil developed from mineralized carbonate rock. Total Tl concentrations in topsoil (0-20 cm) of 100-1000 mg/kg are observed in the most affected area, subsoil concentrations of up to 6000 mg/kg Tl in soil horizons containing weathered ore fragments. Using synchrotron-based microfocused X-ray fluorescence spectrometry (µ-XRF) and X-ray absorption spectroscopy (µ-XAS) at the Tl L3-edge, partly Tl(I)-substituted jarosite and avicennite (Tl2O3) were identified as Tl-bearing secondary minerals formed by the weathering of a Tl-As-Fe-sulfide mineralization hosted in the carbonate rock from which the soil developed. Further evidence was found for the sequestration of Tl(III) into Mn-oxides and the uptake of Tl(I) by illite. Quantification of the fractions of Tl(III), Tl(I)-jarosite and Tl(I)-illite in bulk samples based on XAS indicated that Tl(I) uptake by illite was the dominant retention mechanism in topsoil materials. Oxidative Tl(III)uptake into Mn-oxides was less relevant, probably because the Tl loadings of the soil exceeded the capacity of this uptake mechanism. The concentrations of Tl in 10 mM CaCl2-extracts increased with increasing soil Tl contents and decreasing soil pH, but did not exhibit drastic variations as a function of Tl speciation. With respect to Tl in contaminated soils, this study provides first direct spectroscopic evidence for Tl(I) uptake by illite and indicates the need for further studies on the sorption of Tl to clay minerals and Mn-oxides and its impact on Tl solubility in soils.

Arsenic-rich acid mine water with extreme arsenic concentration: mineralogy, geochemistry, microbiology, and environmental implications.

Extremely arsenic-rich acid mine waters have developed by weathering of native arsenic in a sulfide-poor environment on the 10th level of the Svornost mine in Jáchymov (Czech Republic). Arsenic rapidly oxidizes to arsenolite (As2O3), and there are droplets of liquid on the arsenolite crust with high As concentration (80,000-130,000 mg·L(-1)), pH close to 0, and density of 1.65 g·cm(-1). According to the X-ray absorption spectroscopy on the frozen droplets, most of the arsenic is As(III) and iron is fully oxidized to Fe(III). The EXAFS spectra on the As K edge can be interpreted in terms of arsenic polymerization in the aqueous solution. The secondary mineral that precipitates in the droplets is kaatialaite [Fe(3+)(H2AsO4)3·5H2O]. Other unusual minerals associated with the arsenic lens are behounekite [U(4+)(SO4)2·4H2O], stepite [U(4+)(AsO3OH)2·4H2O], vysokýite [U(4+)[AsO2(OH)2]4·4H2O], and an unnamed phase (H3O)(+)2(UO2)2(AsO4)2·nH2O. The extremely low cell densities and low microbial biomass have led to insufficient amounts of DNA for downstream polymerase chain reaction amplification and clone library construction. We were able to isolate microorganisms on oligotrophic media with pH ∼ 1.5 supplemented with up to 30 mM As(III). These microorganisms were adapted to highly oligotrophic conditions which disabled long-term culturing under laboratory conditions. The extreme conditions make this environment unfavorable for intensive microbial colonization, but our first results show that certain microorganisms can adapt even to these harsh conditions.

Phosphorus adsorption on iron-coated sand under reducing conditions.

Mitigation measures are needed to prevent large loads of phosphate originating in agriculture from reaching surface waters. Iron-coated sand (ICS) is a residual product from drinking water production. It has a high phosphate adsorption capacity and can be placed around tile drains, taking no extra space, which increases the farmers' acceptance. The main concern regarding the use of ICS filters below groundwater level is that limited oxygen supply and high organic matter concentrations may lead to the reduction and dissolution of iron (hydr)oxides present and the release of previously adsorbed phosphate. This study aimed to investigate phosphate adsorption on ICS at the onset of iron reduction. First, we investigated whether simultaneous metal reduction and phosphate adsorption were relevant at two field sites in the Netherlands that use ICS filters around tile drains. Second, the onset of microbially mediated reduction of ICS in drainage water was mimicked in complementary laboratory microcosm experiments by varying the intensity of reduction through controlling the oxygen availability and the concentration of degradable organic matter. After 3 yr, ICS filters in the field removed phosphorus under low redox conditions. Over 45 d, the microbial reduction of manganese and iron oxides did not lead to phosphate release, confirming field observations. Electron microscopy and X-ray absorption spectroscopy did not evince systematic structural or compositional changes; only under strongly reducing conditions did iron sulfides form in small percentages in the outer layer of the iron coating. Our results suggest that detrimental effects only become relevant after long periods of operation.

Thallium adsorption onto phyllosilicate minerals.

The adsorption of thallium (Tl) onto phyllosilicate minerals plays a critical role in the retention of Tl in soils and sediments and the potential transfer of Tl into plants and groundwater. Especially micaceous minerals are thought to strongly bind monovalent Tl(I), in analogy to their strong binding of Cs. To advance the understanding of Tl(I) adsorption onto phyllosilicate minerals, we studied the adsorption of Tl(I) onto Na- and K-saturated illite and Na-saturated smectite, two muscovites, two vermiculites and a naturally Tl-enriched soil clay mineral fraction. Macroscopic adsorption isotherms were combined with the characterization of the adsorbed Tl by X-ray absorption spectroscopy (XAS). In combination, the results suggest that the adsorption of Tl(I) onto phyllosilicate minerals can be interpreted in terms of three major uptake paths: (i) highest-affinity inner-sphere adsorption of dehydrated Tl+ on a very low number of adsorption sites at the wedge of frayed particle edges of illite and around collapsed zones in vermiculite interlayers through complexation between two siloxane cavities, (ii) intermediate-affinity inner-sphere adsorption of partially dehydrated Tl+ on the planar surfaces of illite and muscovite through complexation onto siloxane cavities, (iii) low-affinity adsorption of hydrated Tl+, especially in the hydrated interlayers of smectite and expanded vermiculite. At the frayed edges of illite particles and in the vermiculite interlayer, Tl uptake can lead to the formation of new wedge sites that enable further adsorption of dehydrated Tl+. On the soil clay fraction, a shift in Tl(I) uptake from frayed edge sites (on illite) to planar sites (on illite and muscovite) was observed with increasing Tl(I) loading. The results from this study show that the adsorption of Tl(I) onto phyllosilicate minerals follows the same trends as reported for Cs and Rb and thus suggests that concepts to describe the retention of (radio)cesium by different types of phyllosilicate minerals in soils, sediments and rocks are also applicable to Tl(I).

Assessment of chemical species of lead accumulated in tidemarks of human articular cartilage by X-ray absorption near-edge structure analysis.

A highly specific accumulation of the toxic element lead was recently measured in the transition zone between non-calcified and calcified normal human articular cartilage. This transition zone, the so-called `tidemark', is considered to be an active calcification front of great clinical importance. However, little is known about the mechanisms of accumulation and the chemical form of Pb in calcified cartilage and bone. Using spatially resolved X-ray absorption near-edge structure analysis (µ-XANES) at the Pb L(3)-edge, the chemical state of Pb in the osteochondral region was investigated. The feasibility of the µ-XANES set-up at the SUL-X beamline (ANKA synchrotron light source) was tested and confirmed by comparing XANES spectra of bulk Pb-reference compounds recorded at both the XAS and the SUL-X beamline at ANKA. The µ-XANES set-up was then used to investigate the tidemark region of human bone (two patella samples and one femoral head sample). The spectra recorded at the tidemark and at the trabecular bone were found to be highly correlated with the spectra of synthetic Pb-doped carbonated hydroxyapatite, suggesting that in both of these very different tissues Pb is incorporated into the hydroxyapatite structure.

Spectroscopic set-up for simultaneous UV-Vis/(Q)EXAFS in situ and in operando studies of homogeneous reactions under laboratory conditions.

A novel experimental set-up for in operando studies of homogeneous catalyzed reactions under laboratory conditions has been developed and tested. It combines time-resolved X-ray absorption spectroscopy with UV/Vis spectroscopy. The reaction solution is stirred in a vessel and pumped in a circle by a peristaltic free gear-wheel through a measurement cell. The X-ray and UV/Vis beams probe the same sample volume of the cell orthogonally. Reactants can be added to the reaction mixture in the course of the measurements and a defined gas atmosphere can be adjusted up to a pressure of 10 bar. The in situ reduction of cerium(IV) ammonium nitrate to cerium(III) by isopropanol is studied as a test reaction with quick-XANES and UV/Vis measurements with a time resolution of 60 s and 1 s, respectively.

Thallium and co-genetic trace elements in hydrothermal Fe-Mn deposits of Central Spain.

Thallium (Tl) is a hazardous trace metal that can harm human and environmental health. Tl pollution can result from the mining and smelting of Tl-bearing minerals, but also the natural weathering of Tl-bearing sulfide minerals may induce Tl release to the environment. In this study, hydrothermal deposits hosted in dolostone rocks sited along fossil thermal springs in the Lodares region (Soria province, central Spain) were studied. In this hydrothermal mineralization zone, Tl association with primary minerals, identified Tl-bearing secondary products resulting from natural weathering of primary minerals, as well as the dispersion from its natural source along a seasonal small streambed were explored. Samples were analyzed by chemical, microscopic and spectroscopic techniques and epithermal pyrite, sphalerite, galena and barite and secondary gypsum, jarosite, scorodite, anglesite, goethite, epsomite and elemental sulfur produced by both inorganic and bacterial processes were found. The highest Tl contents were found in hydrothermal pyrite (188 mg kg-1), jarosite (142 mg kg-1), Mn-oxides (27 mg kg-1) or kerogen (13 mg kg-1). Feldspar was identified by electron probe microanalysis as potential host phase of Tl. XANES results confirmed the association of Tl(I) with metal sulfides in pyrite-rich samples and highlighted the role of jarosite-like minerals for Tl(I) sequestration upon pyrite oxidation, even in carbonate-rich samples at near-neutral pH. In addition to micaceous minerals, jarosite-group minerals and K-feldspars may contribute to the natural attenuation of Tl in soils and sediments.

In Situ X-ray Diffraction and X-ray Absorption Spectroscopic Studies of a Lithium-Rich Layered Positive Electrode Material: Comparison of Composite and Core-Shell Structures.

Lithium- and manganese-rich transition-metal oxide (LMR-NMC) electrodes have been designed either as heterostructures of the primary components ("composite") or as core-shell structures with improved electrochemistry reported for both configurations when compared with their primary components. A detailed electrochemical and structural investigation of the 0.5Li2MnO3-0.5LiNi0.5Mn0.3Co0.2O2 composite and core-shell structured positive electrode materials is reported. The core-shell material shows better overall electrochemical performance compared to its corresponding composite material. While both configurations gave the same initial charge capacity of ∼300 mAh/g when cycled at a rate of 10 mA/g at 25 °C, the core-shell sample gives a discharge capacity of 232 mAh/g compared to 208 mAh/g delivered by the composite sample. Also, the core-shell sample gave better rate capability and a smaller first-cycle irreversible capacity loss than the composite sample. The improved performance of the core-shell material is attributed to its lower surface reactivity and limited structural change since the more stable Li2MnO3 shell screens the more reactive Ni-rich core material from interacting with either air or electrolyte at high potentials, thereby preventing electrode surface modification. In situ X-ray diffraction correlated with electrochemical data revealed that the composite sample shows stronger volumetric changes in the lattice parameters during charging to 4.8 V. In addition, X-ray absorption spectroscopy showed an incomplete Ni reduction process after the first discharge for the composite sample. From these results, it was shown that this leads to a more severe degradation in the composite material that affects Li+ intercalation in the subsequent discharge, thereby resulting in its poorer performance. Furthermore, to confirm these results, another LMR-NMC material with a different composition (having a Ni-poor core)-0.5Li2MnO3-0.5LiNi0.33Mn0.33Co0.33O2-was investigated. The core-shell structured positive electrode material also gave an improved electrochemical performance compared to the corresponding composite positive electrode material. These results show that the core-shell configuration could effectively be used to improve the performance of the LMR-NMC materials to enable future high-energy applications.

Unveiling the Reaction Mechanism during Li Uptake and Release of Nanosized "NiFeMnO4": Operando X-ray Absorption, X-ray Diffraction, and Pair Distribution Function Investigations.

Here, we report that the trimetallic nanosized oxide NiFeMnO4 consists of a mixture of NiO and a strained cubic spinel phase, which is clearly demonstrated by analysis of the pair distribution function (PDF) and synchrotron X-ray data. Such a finding can easily be overlooked by using only inhouse X-ray powder diffraction, leading to inaccurate assumption of the stoichiometry and oxidation states. Such advanced characterization is essential because a homogeneous distribution of the elements is observed in energy-dispersive X-ray spectroscopy maps, giving no hints for a phase separation. Cycling of the sample against Li delivers a high reversible capacity of ≈840 mAh/g in the 50th cycle. Operando X-ray absorption spectroscopy (XAS) experiments indicate that ≈0.8 Li/fu is consumed without detectable changes of the electronic structure. Increasing amounts of Li, Mn3+, and Fe3+ are simultaneously reduced. The disappearance of the pre-edge features in X-ray absorption near-edge spectroscopy indicates movement of these cations from tetrahedral sites to octahedral sites. PDF analysis of the pattern after an uptake of 2 Li/fu evidences that the principal structure can be sufficiently well modeled assuming coexisting NiO, a mixed monoxide, and a small amount of residual spinel phase. Thus, the majority of cations is located on octahedral sites. Furthermore, an improvement of the PDF model is achieved taking into account small amounts of LiOH. The 7Li MAS NMR spectrum of this sample clearly shows the signal of Li in a diamagnetic environment, excluding Li-O-TM bonds. A further increase of the Li content leads to a successive conversion of the cations to nanosized metal particles embedded in a LiOH/Li2O matrix. Ex situ XAS results indicate that Fe can be reversibly reoxidized to Fe3+ during charge whereas Mn does not reach the oxidation state observed in the pristine material. After excessive cycling, reoxidation of metallic Ni is suppressed and contributes to a capacity loss compared with the early discharge/charge cycles.

The role of soil components in synthetic mixtures during the adsorption and speciation changes of Cr(VI): Conjunction of the modeling approach with spectroscopic and isotopic investigations.

This study investigates redox transitions associated with the adsorption of Cr(VI) on commonly occurring soil components (silicates, oxides and humic acids) and their synthetic mixtures by coupling the mechanistic surface complexation modeling with spectroscopic and isotopic analyses. The mixtures of soil components were prepared to reflect the composition of the real anthroposol sample, determined by X-ray Powder Diffraction (XRD), total organic carbon (TOC) measurement and extraction methods. The effect of different initial Cr(VI) concentrations (2×10-2, 5×10-4, 10-4, 10-5, and 10-6M), background electrolyte (10-3, 10-2, and 10-1M KNO3), pH values (3-9), and sorbate/sorbent ratios (2g/L - 20g/L) were investigated. Maghemite and ferrihydrite were confirmed to be the main phases controlling Cr(VI) adsorption with increasing Cr(VI) concentration. Humic acids were primarily responsible for Cr(VI) reduction, especially at low pH values. The reduction of Cr(VI) was also proved in case of illite and kaolinite by XAS and isotopic analyses. Illite revealed higher reduction capacity in comparison with kaolinite based on XAS measurements. Chromium isotopic fractionation, resulting from Cr(VI) reduction, was the highest in the case of humic acids, followed by kaolinite and illite. However, a dissolution of intrinsic Cr originally present within kaolinite and illite might affect the final Cr isotopic composition of the supernatants due to its different Cr isotopic signature. In general, the combination of three different approaches was confirmed to offer more comprehensive information about Cr(VI) adsorption and/or reduction in soils. Detailed studies using soil mixtures can help to predict how the soil components affect Cr(VI) behavior in natural soils and possibly could improve the environmental remediation processes.

Nature and reactivity of layered double hydroxides formed by coprecipitating Mg, Al and As(V): Effect of arsenic concentration, pH, and aging.

Arsenic (As) co-precipitation is one of the major processes controlling As solubility in soils and waters. When As is co-precipitated with Al and Mg, the possible formation of layered double hydroxides (LDHs) and other nanocomposites can stabilize As in their structures thus making this toxic element less available. We investigated the nature and reactivity of Mg-Al-arsenate [As(V)] co-precipitated LDHs formed in solution affected by As concentration, pH, and aging. At the beginning of the co-precipitation process, poorly crystalline LDH and non-crystalline Al(Mg)-oxides form. Prolonged aging of the samples promotes crystallization of LDHs, evidenced by an increase in As K XANES intensities and XRD peak intensities. During aging Al- and/or Mg-oxides are likely transformed by dissolution/re-precipitation processes into more crystalline but still defective LDHs. Surface area, chemical composition, reactivity of the precipitates, and anion exchange properties of As(V) in the co-precipitates are influenced by pH, aging, and As concentration. This study demonstrates that (i) As(V) retards or inhibits the formation and transformation of LDHs and (ii) more As(V) is removed from solution if co-precipitated with Mg and Al than by sorption onto well crystallized LDHs.

Composition-dependent structure of polycrystalline magnetron-sputtered V-Al-C-N hard coatings studied by XRD, XPS, XANES and EXAFS.

V-Al-C-N hard coatings with high carbon content were deposited by reactive radio-frequency magnetron sputtering using an experimental combinatorial approach, deposition from a segmented sputter target. The composition-dependent coexisting phases within the coating were analysed using the complementary methods of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS). For the analysis of the X-ray absorption near-edge spectra, a new approach for evaluation of the pre-edge peak was developed, taking into account the self-absorption effects in thin films. Within the studied composition range, a mixed face-centred cubic (V,Al)(C,N) phase coexisting with a C-C-containing phase was observed. No indication of hexagonal (V,Al)(N,C) was found. The example of V-Al-C-N demonstrates how important a combination of complementary methods is for the detection of coexisting phases in complex multi-element coatings.