Quantitative Determination of Chromium Valence in Environmental Samples Using Xafs Spectroscopy

1996 ◽  
Vol 432 ◽  
Author(s):  
M. L. Peterson ◽  
G. E. Brown ◽  
G. A. Parks
Keyword(s):  
Ab Initio ◽  
Sample Pretreatment ◽  
Complex Oxide ◽  
Quantitative Information ◽  
Model Compounds ◽  
Xafs Spectroscopy ◽  
Industrial Sites ◽  
X Ray Absorption ◽  
Exafs Spectra

AbstractSynchrotron x-ray absorption fine structure (XAFS) spectroscopy provides quantitative information on the speciation of Cr in complex oxide samples, including the Cr(VI):Cr(III) ratio, without any sample pretreatment. We discuss several XAFS methods that can be used for this purpose, including: (1) measurement of the height of the pre-edge peak of Cr K-edge spectra which is caused by ls→3d electronic transitions occurring predominately in the Cr(VI)O4 tetrahedron; (2) fitting observed XANES spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; (3) fitting observed EXAFS spectra using combinations of experimental spectra from Cr(III)- and Cr(VI)-containing model compounds; and (4) ab-initio modeling of observed Cr K-EXAFS spectra using theoretical phase-shift and amplitude parameters for Cr(III)O6 and Cr(VI)O4 models. The first method is shown to be the simplest and most accurate. The samples examined using these methods include highly pure model compounds, Cr3+2O3 and Na2Cr6+O4·4H42O, as well as more complex soil samples taken from Cr-contaminated industrial sites and Cr waste disposal pits. We present XAFS evidence for the reduction of Cr(VI) to Cr(III) after reaction with the Fe(Il)-bearing oxide magnetite, Fe3O4, and show that Cr associated with magnetite in soils is reduced, whereas that associated with non-Fe(II)-bearing silicate minerals is not. Also presented are ab-initio multiplescattering calculations of XANES spectra of an Cr(VI)O4 tetrahedron, which help explain the XANES features of this Cr species in solids and aqueous solutions.

EELS and Xanes Analysis of Plutonium and Cerium Edges From Titanate Ceramics for Fissile Materials Disposal

1999 ◽  
Vol 5 (S2) ◽  
pp. 768-769
Author(s):  
J.A. Fortner ◽  
E.C. Buck ◽  
A.J. Kropf ◽  
A.J. Bakel ◽  
M.C. Hash ◽  
...  
Keyword(s):  
Oxidation State ◽  
Electron Loss ◽  
Edge Structure ◽  
X Ray ◽  
Titanate Ceramics ◽  
X Ray Absorption ◽  
Photon Source ◽  
Titanate Ceramic ◽  
Exafs Spectra

We report x-ray absorption near edge structure (XANES) and extended x-ray fine structure analysis (EXAFS) spectra from the plutonium Llll and cerium Lm edges in prototype titanate ceramic hosts for disposal of surplus fissile materials. These spectra were obtained using the MRCAT beamline at the Advanced Photon Source (APS). The XANES and EXAFS results are compared with electron loss spectra (EELS) determination of oxidation state from the plutonium MlV,V and cerium MlV,V edges [1,2]. The titanate ceramics studied are based upon the hafniumpyrochlore and zirconolite mineral structures and will serve as an immobilization host, containing as much as 10 weight % fissile plutonium, and 20 weight % (natural or depleted) uranium. Similar formulations were composed using cerium as a “surrogate” element, replacing both plutonium and uranium in the ceramic matrix. We find the plutonium to be present almost entirely as Pu (IV), while the cerium is clearly in a mixed III-IV oxidation state in the surrogate ceramic.

Photochemical reactions of charge-transfer complexes. III. Molecular orbital studies of the charge-transfer complexes between 1,2-dimethoxyethylenes and 1,2-dicyanoethylenes

1981 ◽  
Vol 59 (6) ◽  
pp. 974-981 ◽  
Author(s):  
Po Cheong Wong ◽  
Russell J. Boyd
Keyword(s):  
Charge Transfer ◽  
Ab Initio ◽  
Molecular Orbital ◽  
Photochemical Reactions ◽  
Charge Transfer Complexes ◽  
Double Bonds ◽  
Mo Calculations ◽  
Model Compounds ◽  
Projected Distance

Ab initio and PCILO molecular orbital (MO) calculations for the four π–π* charge-transfer complexes formed between maleonitrile (MN) and fumaronitrile (FN), and cis (CS) and trans-1,2-dimethoxyethyIene (TR) are reported. The donor and the acceptor molecules are stacked in parallel planes. Trans and cis-l,2-dihydroxyethylene are used as the model compounds for TR and CS in the determination of the optimum geometries of the complexes. Both types of MO calculations indicate that the eclipsed geometry (with the two central double bonds parallel and the centres of the double bonds centred over one another) is relatively unstable. With limited geometry searching, we find that the slipped geometry (with the two central double bonds parallel but separated at a projected distance of 2.2 Å and the oxygen atoms above the cyanocarbon atoms) is the preferred structure. According to the ab initio calculations the relative order of the calculated heats of formation (ΔHθ's) for these complexes is: MN−CS > FN−CS > FN−TR > MN−TR.

scholarly journals Determination of Chromium(VI) Reduction by Acinetobacter haemolyticus using X-ray Absorption Fine Structure Spectroscopy

2014 ◽  
Vol 4 (2) ◽  
Author(s):  
Quek Hsiao Pei ◽  
Shafinaz Shahir ◽  
Liu Tao ◽  
Wan Azlina Ahmad
Keyword(s):  
Fine Structure ◽  
Enzymatic Reactions ◽  
Edge Structure ◽  
X Ray ◽  
Xafs Spectroscopy ◽  
Absorption Fine ◽  
Chromium Vi ◽  
X Ray Absorption ◽  
Acinetobacter Haemolyticus ◽  
Exafs Spectra

Several types of microorganisms have been reported to reduce Cr(VI) to the less toxic Cr(III) via enzymatic reactions. The main purpose of this study is to determine the reduction of Cr(VI) by a locally isolated bacterium, Acinetobacter haemolyticus (A. haemolyticus) using the x-ray absorption fine structure (XAFS) spectroscopy. XAFS analysis consisted of both the x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectra. XANES spectra of the rinsed Cr(VI)-laden A. haemolyticus biomass indicated that chromium was only present in the form of Cr (III). The nearest atoms coordinated to Cr(III) were oxygens in an octahedral geometry. The longer Cr-O bond lengths (~1.97 Å) obtained from the EXAFS spectra corroborated the presence of chromium as Cr(III) bonded to oxygen. These findings demonstrate the ability of A. haemolyticus to reduce Cr(VI) to Cr(III) that was bound to oxygen atoms of the ligands in A. haemolyticus.

Accurate determination of the As(v) coordination environment at the surface of ferrihydrite using synchrotron extended X-ray absorption fine structure spectroscopy and ab initio Debye–Waller factors

2019 ◽  
Vol 6 (8) ◽  
pp. 2441-2451
Author(s):  
Shaofeng Wang ◽  
Guoqing Zhang ◽  
Jinru Lin ◽  
Xin Wang ◽  
Yongfeng Jia
Keyword(s):  
Fine Structure ◽  
Ab Initio ◽  
Accurate Determination ◽  
Coordination Environment ◽  
X Ray ◽  
Absorption Fine ◽  
Mononuclear Complexes ◽  
X Ray Absorption

The synchrotron EXAFS fitting using ab initio Debye–Waller factors suggests that ferrihydrite adsorbs arsenate dominantly via monodentate mononuclear complexes.

A comparative assessment of the zinc–protein coordination in 2Zn–insulin as determined by X-ray absorption fine structure (EXAFS) and X-ray crystallography

1983 ◽  
Vol 219 (1214) ◽  
pp. 21-39 ◽  
Keyword(s):  
Fine Structure ◽  
Metal Ion ◽  
Radial Distance ◽  
Exafs Spectrum ◽  
Ion Concentration ◽  
Model Compounds ◽  
X Ray ◽  
Absorption Fine ◽  
X Ray Absorption ◽  
Exafs Spectra

Accurate ( ca .0.1Å) knowledge of the metal environment in metalloproteins is essential to understanding their function. Single crystal X-ray analysis has provided detailed descriptions of metal environments in a number of crystallizable proteins but their accuracy has often been limited by their restricted diffraction patterns. The technique of X-ray absorption fine structure (EXAFS) is not limited to crystals and can provide very accurate radial distances between metal ions and their ligands. It has therefore great potential for the study of biochemical metal-containing systems in solution. The method depends on the analysis of the oscillations in the absorption or fluorescence spectrum extending over several hundreds of electron volts above the metal absorption edge. The very intense synchrotron X-ray sources make EXAFS applicable to biological systems where the metal ion concentration is low, typically in the millimolar range. We have determined EXAFS spectra for 2Zn–insulin and a variety of Zn–ligand model compounds of known crystal structures in both the absorption and the fluorescence modes. The X-ray crystallographic refinement of 2Zn-insulin with 1.5 Å data provides estimated standard deviations for well defined atoms ranging from 0.03 to 0.06 Å which thus determine the zinc-ligand distances sufficiently accurately for comparison with those derived from EXAFS. The experimental procedures for obtaining the spectra with the use of the storage ring doris at the Deutsche Elektrononen Synchrotron (DESY) are described. The shapes of the exafs spectra of 2Zn-insulin and the zinc complexes are remarkably similar. These results emphasize one of the major weaknesses of the technique: the difficulty in distinguishing between atoms of similar atomic mass such as oxygen and nitrogen in the present instance. The small but real effect of solvent on the spectrum of one complex has important implications; it reveals the ability of EXAFS to provide evidence for structural changes in the metal coordination and that other structures possibly more relevant to function may exist. For the detailed analysis we used ab initio calculations on the model compounds. The method used is outlined and the programs are cited; the theoretical basis for these calculations is to be found in Lee & Pendry ( Phys . Rev . B 11, 2795-2811 (1975)). Its application to the 2Zn-insulin EXAFS spectrum showed that there is good agreement in both the exafs and the X-ray crystallographic methods for the CE1 and NE2 to Zn radial distance; significant discrepancies, however, exist for the other atoms in the coordinating structure. This failure stems essentially from the smaller contribution of outer atoms to the EXAFS spectrum. To resolve these correctly the amplitudes in the EXAFS spectrum need to be more accurate, and there needs to be a more adequate theory to deal with multiple scattering effects. Until these have been achieved it is probably more profitable to make use of our exact knowledge of the bonding behaviour and geometry of such coordinating groups as imidazole rings.

New developments in the ab initio determination of transition metal alloys phase diagram

1993 ◽  
Vol 90 ◽  
pp. 249-254 ◽  
Author(s):  
C Wolverton ◽  
M Asta ◽  
S Ouannasser ◽  
H Dreyssé ◽  
D de Fontaine
Keyword(s):  
Phase Diagram ◽  
Transition Metal ◽  
Ab Initio ◽  
Metal Alloys ◽  
New Developments ◽  
Transition Metal Alloys

$\mathit{Ab\!-\!initio}$ Determination of Magnetic Interface Coupling Constants for Magnetic Multilayers

1997 ◽  
Vol 7 (11) ◽  
pp. 1299-1304 ◽  
Author(s):  
P. Weinberger ◽  
C. Sommers ◽  
U. Pustogowa ◽  
L. Szunyogh ◽  
B. Újfalussy
Keyword(s):  
Ab Initio ◽  
Magnetic Multilayers ◽  
Coupling Constants ◽  
Interface Coupling
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