Probing the structural role of Cr in stabilized tannery wastes with X-ray absorption fine structure spectroscopy.

The effective stabilization of tannery sludge wastes is explored using X-Ray Absorption Fine Structure (XAFS) spectroscopies. Solidification of the Cr-rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr(VI) and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr(VI) and Cr(III) species are identified, corresponding to Cr(VI)O4 glass formers and Cr(III)O6 network modifiers. The toxic Cr(VI) prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr(VI) increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass-ceramics transforms almost all Cr(VI) to Cr(III), while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr(III) from the silica matrix and not from the reduced Cr(VI); yet, Cr-removal from the glass matrix does not impair the chemical stability of the devitrified products.

Crystalline and amorphous calcium carbonate as structural components of the Calappa granulata exoskeleton.

The exoskeleton of crustaceans consists of chitin biopolymers where the embedded inorganic biominerals, mainly CaCO3, affect strongly its mechanical properties. Raman and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopies and Transmission Electron Microscopy (TEM) are applied to investigate the CaCO3 structure in various parts of the Calappa granulata crab exoskeleton. The shape of the main Raman peak of CaCO3 reveals the presence of two phases which are identified as calcite and amorphous calcium carbonate (ACC). The relative concentration of the two phases in various parts of the exoskeleton is determined from the area ratio under the corresponding peaks. The results of the Ca L3,2-edge NEXAFS analysis are in line with the Raman findings, since the energy separation of peaks that appear in the lower frequency region of the main L2 and L3 peaks due to crystal field splitting, is directly related to the percentage of the ACC phase in the total CaCO3 mineral content. The C K-edge spectra are used for the determination of the extent of calcification of the exoskeleton. Furthermore, dark and bright field TEM images reveal the presence of nanocrystallites with an average size of 20 nm. The structure of the nanocrystallites, as derived from the Selected Area Electron Diffraction patterns, is calcite. The results suggest that ACC plays a structural role in the exoskeleton of Calappa granulata.

Optimization of tetravalent manganese feroxyhyte's negative charge density: A high-performing mercury adsorbent from drinking water.

This study demonstrates an optimization procedure for the development of an Hg-specified adsorbent able to comply with the regulation limit for drinking water of 1µg/L. On this purpose, the synthesis of Mn(IV)-feroxyhyte was modified to achieve high negative charge density by combining alkaline and extreme oxidizing conditions. In particular, precipitation of FeSO4 at pH9 and excess of KMnO4 follows a very fast nucleation step providing a product with very small nanocrystal size (1-2nm), high specific surface area (300m2/g) and maximum negative charge density (1.8mmol H+/g). The adsorbent was validated for Hg removal in batch experiments and column tests using natural-like water indicating an adsorption capacity as high as 2.5µg/mg at equilibrium concentration 1µg/L under reliable conditions of application. Importantly, the adsorption is an exothermic spontaneous process, resulting in the formation of inner sphere complexes by sharing both A-type and B-type oxygen atoms with the metal surface octahedral as revealed by the X-ray absorption fine structure results.

Ca L2,3-edge XANES and Sr K-edge EXAFS study of hydroxyapatite and fossil bone apatite.

Upon burial, the organic and inorganic components of hard tissues such as bone, teeth, and tusks are subjected to various alterations as a result of interactions with the chemical milieu of soil, groundwater, and presence of microorganisms. In this study, simulation of the Ca L 2,3-edge X-ray absorption near edge structure (XANES) spectrum of hydroxyapatite, using the CTM4XAS code, reveals that the different symmetry of the two nonequivalent Ca(1) and Ca(2) sites in the unit cell gives rise to specific spectral features. Moreover, Ca L 2,3-edge XANES spectroscopy is applied in order to assess variations in fossil bone apatite crystallinity due to heavy bacterial alteration and catastrophic mineral dissolution, compared to well-preserved fossil apatite, fresh bone, and geologic apatite reference samples. Fossilization-induced chemical alterations are investigated by means of Ca L 2,3-edge XANES and scanning electron microscopy (SEM) and are related to histological evaluation using optical microscopy images. Finally, the variations in the bonding environment of Sr and its preference for substitution in the Ca(1) or Ca(2) sites upon increasing the Sr/Ca ratio is assessed by Sr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.

Monitoring the role of Mn and Fe in the As-removal efficiency of tetravalent manganese feroxyhyte nanoparticles from drinking water: An X-ray absorption spectroscopy study.

The implementation of amorphous tetravalent manganese feroxyhyte (TMFx) nanoparticles, prepared via co-precipitation synthesis, as an efficient As(V)-removal material is investigated using X-ray absorption fine structure (XAFS) spectroscopy at the Fe-, Mn- and As-K-edges. The optimum synthesis conditions and chemical composition of the TMFx adsorbent were determined by the degree of polymerization in the adsorbents' microstructure. Under synthesis into mildly acidic conditions, the change in the polymerization of the metal-oxyhydroxyl chains (metal=Fe, Mn) provides more adsorption sites at edges and corner sites in the bonding environment of Fe and Mn, respectively, thereby enhancing As uptake. After exposure to As-polluted water, similar microstructural changes related to As-bidentate and monodentate geometries are generated: As(V) preferentially occupies the high energy adsorption sites ((2)C complexes) available in the Mn-oxyhydroxyl groups and the low energy edge sites offered by Fe ((2)E complexes). It is revealed that optimum arsenic-removal by TMFx occurs into mildly acidic synthesis pH and for iron to manganese molar ratio equal to 3.

Detailed spectroscopic study of the role of Br and Sr in coloured parts of the Callinectes sapidus crab claw.

The exoskeleton of crustaceans consists mainly of calcium carbonate (CaCO3) minerals and in many cases exhibits vivid colouration due to the presence of proteins rich in carotenoid chromophores. The exposure of aquatic animals in sea water results often in the incorporation of trace elements in their exoskeleton. The bonding configuration of Br and Sr trace elements in regions with different staining (white, orange and blue) of the exoskeleton of the Callinectes sapidus in crab claw are systematically investigated by a number of complementary spectroscopic techniques, including X-ray absorption fine structure spectroscopy (EXAFS), X-ray fluorescence, Raman and visible light reflectivity spectroscopies. It is found that Sr substitutes for Ca and the Sr/Ca ratio is constant along the claw. In the orange region that includes the claw fingers, CaCO3 adopts a calcite-like structure, whereas in the blue and white regions, located in the palm of the claw, an aragonite-like structure dominates. On the other hand, Br, present only in the blue and orange stained parts of the claw, is bound to phenyl and/or phenol rings of amino acid residues, most probably to phenylalanine and/or tyrosine, of the chromophore protein.

An X-ray absorption study of synthesis- and As adsorption-induced microstructural modifications in Fe oxy-hydroxides.

Synthetic adsorbents based on Fe oxy-hydroxides (FeOOH) prepared under a wide range of pH-values via intense oxidation conditions of FeSO4 as well as the As(III) and As(V) adsorption mechanism are investigated using X-ray absorption fine structure (XAFS) spectroscopies at the Fe- and As-K-edges. Synthesis in an alkaline environment promotes the face-connectivity of the Fe(O,OH)6 chains at the expense of edge- and corner-sharing linkage, which is consistent with the lower surface charge density and in turn with the lower arsenic adsorption capacity. Microstructural changes are also detected after As(V) adsorption onto FeOOH synthesized at pH 5.5: the ratio of face-/edge-sharing sites increases from approximately 0.4-0.7 as a function of the As(V)-loading. This modification of the polymeric Fe(O,OH)6 structure at higher As/Fe ratios is attributed to strong As(V) bidentate mononuclear ((2)E) and binuclear ((2)C) adsorption. In contrast, no alterations in the FeOOH microstructure were observed, possibly due to the weaker (2)E linkage of As(III).

A XAFS study of plain and composite iron(III) and chromium(III) hydroxides.

Reduction of hexavalent Cr(VI) to the trivalent state is the common strategy for remediation of Cr(VI) contaminated waters and soils. In the presence of Fe the resulting compounds are usually mixed Fe(III)-Cr(III) phases, while, under iron-free conditions, reduction leads to formation of plain Cr(III) hydroxides. Environmental stability of these compounds depends on their structure and is important to understand how different precipitation conditions affect the local atomic order of resulting compounds and thus their long term stability. In current study, typical Cr(VI) environmental remediation products, i.e. plain and mixed Fe(III)-Cr(III) hydroxides, were synthesized by hydrolysis and redox reactions and their structure was studied by X ray diffraction and X ray absorption fine structure techniques. Plain Cr(III) hydroxide was found to correspond to the molecular formula Cr(OH)3·3H2O and was identified as crystalline in XRD. However, the same compound when examined by EXAFS did not exhibit any clear local order in the range of EXAFS detectable distances, i.e. between 0 and 5Å. Namely, EXAFS spectroscopy detected only contribution from the first nearest neighboring (Cr-O) shell, suggesting that CrO6 octahedra interconnection is loose, in accordance with the suggested anti-bayerite structure of this compound. Mixed Fe(III)-Cr(III) systems resembled 2-line ferrihydrite irrespective of the synthesis route. Analysis of Fe-K-EXAFS and Cr-K-EXAFS spectra indicated that FeO6 octahedra are bonded by sharing both edges and corners, while CrO6 octahedra seem to prefer edge sharing linkage. EXAFS data also suggest that Fe-Cr hydroxide produced by hydrolysis presents a better arrangement of CrO6 octahedra compared to the redox product.

Reducing the inversion degree of MnFe2O4 nanoparticles through synthesis to enhance magnetization: evaluation of their (1)H NMR relaxation and heating efficiency.

Manganese ferrite (MnFe2O4) nanoparticles of identical size (9 nm) and with different inversion degrees were synthesized under solvothermal conditions as a candidate theranostic system. In this facile approach, a long-chain amine, oleylamine, was utilized as a reducing and surface-functionalizing agent. The synthesized nanoparticles were shown to have a cubic-spinel structure as characterized by TEM and XRD patterns. Control over their inversion degree was achieved by a simple change of manganese precursor from Mn(acac)2 to Mn(acac)3. The variation in the inversion degree is ascribed to the partial oxidation of Mn(2+) to Mn(3+), as was evidenced by X-ray absorption near edge structure spectroscopy and extended X-ray absorption fine structure spectroscopy at both the Fe and Mn K-edges. The reduction of the inversion degree from 0.42 to 0.22 is close to the corresponding bulk value of 0.20 and led to elevated magnetization (65.7 emu g(-1)), in contrast to the Néel temperature, which was decreased owing to the weaker superexchange interactions between the tetrahedral and octahedral sites within the spinel structure. In order to evaluate the performance of these nanoprobes as a possible bifunctional targeting system, the (1)H NMR relaxation of the samples was tested together with their specific loss power under an alternating magnetic field as a function of concentration. The hydrophobic as prepared MnFe2O4 nanoparticles converted to hydrophilic nanoparticles with cetyltrimethylammonium bromide (CTAB). The MnFe2O4 nanoparticles, well-dispersed in aqueous media, were shown to have r2 relaxivity of up to 345.5 mM(-1) s(-1) and heat release of up to 286 W g(-1), demonstrating their potential use for bioapplications.

Study of fossil bones by synchrotron radiation micro-spectroscopic techniques and scanning electron microscopy.

Earlymost Villafranchian fossil bones of an artiodactyl and a perissodactyl from the Milia excavation site in Grevena, Greece, were studied in order to evaluate diagenetic effects. Optical microscopy revealed the different bone types (fibro-lamellar and Haversian, respectively) of the two fragments and their good preservation state. The spatial distribution of bone apatite and soil-originating elements was studied using micro-X-ray fluorescence (µ-XRF) mapping and scanning electron microscopy. The approximate value of the Ca/P ratio was 2.2, as determined from scanning electron microscopy measurements. Bacterial boring was detected close to the periosteal region and Fe bearing oxides were found to fill bone cavities, e.g. Haversian canals and osteocyte lacunae. In the perissodactyl bone considerable amounts of Mn were detected close to cracks (the Mn/Fe weight ratio takes values up to 3.5). Goethite and pyrite were detected in both samples by means of metallographic microscopy. The local Ca/P ratio determined with µ-XRF varied significantly in metal-poor spots indicating spatial inhomogeneities in the ionic substitutions. XRF line scans that span the bone cross sections revealed that Fe and Mn contaminate the bones from both the periosteum and medullar cavity and aggregate around local maxima. The formation of goethite, irrespective of the local Fe concentration, was verified by the Fe K-edge X-ray absorption fine structure (XAFS) spectra. Finally, Sr K-edge extended XAFS (EXAFS) revealed that Sr substitutes for Ca in bone apatite without obvious preference to the Ca1 or Ca2 unit-cell site occupation.

Local bonding geometry of oxygen implanted in GaN: a depth-dependent study.

The bonding environment of oxygen implanted in GaN is studied using near edge X-ray absorption fine structure spectroscopy at the O-K-edge. The 70 keV oxygen ions form a 200 nm-thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 x 10(15) cm-2 to 1 x 10(17) cm(-2). The information depth of the fluorescence photons varies from 50 to 63 nm, depending on the angle of incidence. The spectra are simulated using the FEFF8 code and assuming various models, e.g., O interstitial, O substitutional in N sites, Ga and N vacancies, and various polymorphs of Ga2O3. The lattice disorder is modelled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulation results reveal that at the low fluence limit, the O atoms occupy interstitial sites preferentially in the empty channels aligned to the c-axis and in the atomic planes containing the Ga atoms. When the fluence is equal to 1 x 10(16) cm(-2) the O atoms substitute N while at 1 x 10(17) cm(-2) they form mixed GaO(x)N(y) phases with the N/O ratio decreasing with increasing depth, i.e., as we approach the peak of the implanted O profile.

Application of conventional and microbeam synchrotron radiation x-ray fluorescence and absorption for the characterization of human nails.

The synchrotron radiation based spectroscopies X-ray fluorescence and X-ray absorption fine structure are used to detect illness-related changes in the elemental distribution and bonding environment of metals in human nails. The effective atomic number of a collection of nails is determined using two methods, the X-ray transmittance and the scattering method, and is found equal to 7.5 +/- 0.5. X-ray fluorescence maps of the elemental distributions, recorded with a lateral resolution of 5 microm, reveal that S, Ca and Zn are distributed homogeneously while Fe tends to cluster. In the Fe-rich clusters, which have a diameter in the range 15-30 microm, the Fe concentration is 10 times higher than in the matrix. The Zn K edge X-ray Absorption Fine Structure spectra reveal that Zn, in the nails from healthy donors and patients suffering from lung diseases, is four-fold coordinated with N and S and the Zn-N and Zn-S distances are equal to 2.03 A and 2.25 A, respectively. Finally the signature of various bonds in the C-, O- and N- K near edge X-ray absorption fine structure spectra is discussed.

On the distribution and bonding environment of Zn and Fe in glasses containing electric arc furnace dust: a mu-XAFS and mu-XRF study.

We apply synchrotron radiation assisted X-ray fluorescence (SR-XRF), SR-XRF mapping as well as micro- and conventional X-ray absorption fine structure (mu-XAFS and XAFS) spectroscopies in order to study the bonding environment of Fe and Zn in vitrified samples that contain electric arc furnace dust from metal processing industries. The samples are studied in the as-cast state as well as after annealing at 900 degrees C. The SR-XRF results demonstrate that annealing does not induce any significant changes in the distribution of either Fe or Zn, in both the as-cast and annealed glasses. The mu-XAFS spectra recorded at the Fe-K and Zn-K edges reveal that the structural role of both Fe and Zn remains unaffected by the annealing procedure. More specifically, Fe forms both FeO(6) and FeO(4) polyhedra, i.e. acts as an intermediate oxide while Zn occupies tetrahedral sites.

Verification of a distortion in the microstructure of GaN detected by EXAFS using ab initio density functional theory calculations.

X-ray absorption fine structure (XAFS) measurements on a series of epitaxially grown GaN samples have shown a distortion in the microstructure of GaN. More specifically the central N atom is 4-fold coordinated but the four Ga atoms are not equidistant. It has been shown that 2.9 to 3.5 of them (depending on the growth conditions) are found in the expected from XRD distance of 1.94 A and the remaining are at a distance longer by approximately 15%. Second derivative calculation of the conformation energy using the Density Functional Theory (DFT) is used to investigate if the symmetric GaN cluster as given by XRD is the most energetically favorable configuration and if not which distorted structure corresponds to the most energetically favorable one. A very good agreement between DFT results and experimental XAFS spectra has been found. Generalization this technique to other dislocated clusters is also discussed.