Giannantonio Cibin

REDUCTION AND SORPTION OF CHROMIUM BY Fe(II)-BEARING PHYLLOSILICATES: CHEMICAL TREATMENTS AND X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

Chromium-containing muscovite crystal chemistry and XANES spectroscopy

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C 2/ c to an agreement factor (R obs ) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2 M 1 . Electron density at the M2 site is greater than that required for the ideal muscovite-2 M 1 structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, α, decreases. XANES spectra at the Cr K-edge in these chromium-containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site.

Experimental and theoretical XANES and EXAFS study of tetra-ferriphlogopite

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance of trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 A, Fe-Si = 3.24 A, Fe-O-Si = 138°. Moreover, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 A. This latter contribution (20 mol % of the total Fe ca. ) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data.

Structural origin of light emission in germanium quantum dots

We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.

Local and average Fe distribution in trioctahedral micas Analysis of Fe K-edge XANES spectra in the phlogopite-annite and phlogopite-tetra-ferriphlogopite joins on the basis of single-crystal XRD refinements

In Fe-bearing trioctahedral 1M micas Fe2+ occurs essentially in the octahedral O sheet whereas Fe3+ either coexists with Fe2+ in the octahedral M sites or, more rarely, enters the tetrahedral T sites. The Fe K -edge absorption spectra of twelve micas were recorded in an effort to relate the observed multiple-scattering absorption features to the results of single-crystal X-ray structure refinements obtained on crystals from the same rock sample. This study first addresses the three end-members phlogopite, annite and tetra-ferriphlogopite, used as reference materials, and enters later in the interpretation of nine trioctahedral micas intermediate in composition along the joins phlogopite-tetra-ferriphlogopite and phlogopite-annite. A detailed analysis of the observed XANES features singles out the independent effects of coordination and valence, and shows distinctly different position vs. intensity trends that depend upon the local Fe2+ and Fe3+ environments. Bond distances evaluated from XANES spectra in the full multiple-scattering region of the K -edge spectrum agree well with those obtained by XRD refinement. Distinct variations in the local to intermediate degrees of order of certain micas can be determined by the analysis of their intermediate multiple-scattering regions; they supplement the information on their long-range order that is best obtained through the XRD refinement.

Crystal-chemical study by XANES of trioctahedral micas: the most characteristic layer silicates

A crystal-chemical study of trioctahedral micas previously characterized by single-crystal XRD has been performed by XANES spectroscopy at the Si and Al K edges. XANES, being a local structural probe, can investigate distortion and modification of the tetrahedral sheet with increasing Fe for Mg substitution in the octahedral sheet. Comparison of XANES spectra allows determining the size of the tetrahedral site occupied by either Si or Al. The Si-O distance remains essentially unchanged whereas the Al-O distance appears to increase. The behavior may be interpreted as a tilt of the tetrahedra, initially rotated to match the ideal mica geometry, with increasing Fe substitution in the octahedral sheet.

Performance of B18, the Core EXAFS Bending Magnet beamline at Diamond

B18 has been operational since April 2010 and has hosted a total of 63 EXAFS user experiments up to Jan 2012. B18 is contributing to research programs across a wide range of scientific disciplines, e.g. solid state physics and materials, catalysis, chemistry, soft matter, surfaces and biomaterials. We present a review of its present performance and capabilities.

Iodine sequestration by thiol-modified MIL-53(Al)

A thiol-modified version of the porous metal organic framework MIL-53 is synthesised in a single step using the functionalised linker precursor 2,5-dithiol-1,4-benzenedicarboxylic acid and aluminium as the framework metal. Careful washing is needed to remove unreacted and dimerised linker from the material after synthesis, but once performed profile fitting of powder X-ray diffraction shows that thiol-modified MIL-53(Al) presents a closed, narrow-pore, structure with unit cell volume ∼1103 A3 (space group C2/c). The presence of intact thiol groups is confirmed using sulfur K-edge XANES spectroscopy and IR spectroscopy, while nitrogen BET surface area analysis and krypton and xenon adsorption isotherms reveal the porosity of the material. The thiol-modified solid is capable of iodine adsorption from the vapour phase and from solution and an equilibrium uptake of ∼325 mg per g is reached, which is higher than other reported modified forms of MIL-53. Infrared spectroscopy shows the disappearance of the S–H stretch after iodine adsorption, while sulfur K-edge XANES shows a complex spectrum, consistent with the formation of sulfenyl iodide but also oxidation of some sulfur to disulfide having occurred. We therefore propose that formation of covalent S–I bonds allows the sequestration of iodine by the porous solid, but that a proportion of the thiol groups are also in close enough proximity for the formation of disulfide links.

XRF-XANES characterization of deep ice core insoluble dust

An analytical protocol based on X-ray spectroscopic measurements (XRF and Fe and Ti K-edges XAS) has been applied to evaluate the mineralogical phase composition of dust trapped in deep ice cores. Information on the insoluble dust composition and local and species-selective information as oxidation and coordination states has been obtained to recognize the main mineralogical families of aerosol particles contained in deep ice cores. In this framework we point out the possibility to obtain the mineralogical characterization of dust from deep ice core samples from Antarctica by means of XANES spectroscopy.

Angular dependence of potassium k-edge xanes spectra of trioctahedral micas: Significance for the determination of the local structure and electronic behavior of the interlayer site

Abstract The X-ray absorption angle-dependent behavior at the potassium K-edge has been determined for two end-members and two intermediate trioctahedral micas, which are representative of the Fe2+Mg-1, Fe3+Al-1, SiAl-1FeLi-1, and F-1OH exchange vectors, using horizontally polarized synchrotron radiation. Experimental spectra are interpreted by the multiple scattering theory following decomposition according to the angular dependence approach of Brouder (1990). The experimental spectra sum up anisotropic effects deriving from in-plane and out-of-plane photoelectron interactions with the potassium near- and next-near neighbors up to the fifth/sixth coordination sphere. For the first time, the absorption edge of a low Z atom is decomposed so as to produce two partial patterns giving, respectively, the full in-plane absorption spectrum (σ||) and the full out-of-plane spectrum (σ⊥). They fully describe the complete X-ray absorption dichroic behavior of layered compounds, of which mica is a prototype. σ|| essentially reflects the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets. Differences among the four micas depend on structural changes induced by the adaptation of their tetrahedral sheets to the increasing sizes of their octahedral sheet due to chemical substitutions (i.e., the Fe2+Mg-1 vector, mainly) and to chemical changes at the octahedral anion site (F-1OH vector). By contrast, σ⊥ reflects multiple-scattering interactions entering deep into the mica structure, beyond the tetrahedral sheet and well into the octahedral sheet. Such contributions reflect both changes in the anions coordinating the octahedral cations and increasing total amount of heavy atoms (essentially Fe), and differ based on electronic properties such as oxidation states (e.g., Fe2+ and Fe3+). Therefore, angle-dependent XANES spectroscopy is one of the rare spectroscopic techniques able not only to detect, but also to quantify structural effects in the atoms at the medium to long-range order.