B18-Core EXAFS
|
Diamond Proposal Number(s):
[7593, 10163, 12767]
Abstract: Technetium is a problematic contaminant at nuclear sites and little is known about how repeated microbiologically-mediated redox cycling impacts its fate in the environment. We explore this question in sediments representative of the Sellafield Ltd. site, UK, over multiple reduction and oxidation cycles spanning ~ 1.5 years. We found the amount of Tc remobilised from the sediment into solution significantly decreased after repeated redox cycles. X-ray Absorption Spectroscopy (XAS) confirmed that sediment bound Tc was present as hydrous TcO2-like chains throughout experimentation and that Tc’s increased resistance to remobilisation (via reoxidation to soluble TcO4-) resulted from both shortening of TcO2 chains during redox cycling and association of Tc(IV) with Fe phases in the sediment. We also observed that Tc(IV) remaining in solution during bioreduction was likely associated with colloidal magnetite nanoparticles. These findings highlight crucial links between Tc and Fe biogeochemical cycles that have significant implications for Tc’s long-term environmental mobility, especially under ephemeral redox conditions.
|
Nov 2017
|
|
I18-Microfocus Spectroscopy
|
Alexander
Morrell
,
J. Frederick W.
Mosselmans
,
Kalotina
Geraki
,
Konstantin
Ignatyev
,
Hiram
Castillo-michel
,
Peter
Monksfield
,
Adrian T.
Warfield
,
Maria
Febbraio
,
Helen M.
Roberts
,
Owen
Addison
,
Richard A.
Martin
Diamond Proposal Number(s):
[16458]
Abstract: Synchrotron radiation X-ray fluorescence microscopy is frequently used to investigate the spatial distribution of elements within a wide range of samples. Interrogation of heterogeneous samples that contain large concentration ranges has the potential to produce image artefacts due to the profile of the X-ray beam. The presence of these artefacts and the distribution of flux within the beam profile can significantly affect qualitative and quantitative analyses. Two distinct correction methods have been generated by referencing the beam profile itself or by employing an adaptive-thresholding procedure. Both methods significantly improve qualitative imaging by removing the artefacts without compromising the low-intensity features. The beam-profile correction method improves quantitative results but requires accurate two-dimensional characterization of the X-ray beam profile.
|
Nov 2018
|
|
I14-Hard X-ray Nanoprobe
I18-Microfocus Spectroscopy
|
Open Access
Abstract: Biological exposures to micro- and nano-scale exogenous metal particles generated as a consequence of in-service degradation of orthopaedic prosthetics can result in severe adverse tissues reactions. However, individual reactions are highly variable and are not easily predicted, due to in part a lack of understanding of the speciation of the metal-stimuli which dictates cellular interactions and toxicity. Investigating the chemistry of implant derived metallic particles in biological tissue samples is complicated by small feature sizes, low concentrations and often a heterogeneous speciation and distribution. These challenges were addressed by developing a multi-scale two-dimensional X-ray absorption spectroscopic (XAS) mapping approach to discriminate sub-micron changes in particulate chemistry within ex-vivo tissues associated with failed CoCrMo total hip replacements (THRs). As a result, in the context of THRs, we demonstrate much greater variation in Cr chemistry within tissues compared with previous reports. Cr compounds including phosphate, hydroxide, oxide, metal and organic complexes were observed and correlated with Co and Mo distributions. This variability may help explain the lack of agreement between biological responses observed in experimental exposure models and clinical outcomes. The multi-scale 2D XAS mapping approach presents an essential tool in discriminating the chemistry in dilute biological systems where speciation heterogeneity is expected.
|
Jun 2019
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[11043]
Open Access
Abstract: Heterogeneous catalysis performed in the liquid phase is an important type of catalytic process which is rarely studied in situ. Using microfocus X-ray fluorescence and X-ray diffraction computed tomography (μ-XRF-CT, μ-XRD-CT) in combination with X-ray absorption near-edge spectroscopy (XANES), we have determined the active state of a Mo-promoted Pt/C catalyst (NanoSelect) for the liquid-phase hydrogenation of nitrobenzene under standard operating conditions. First, μ-XRF-CT and μ-XRD-CT reveal the active state of Pt catalyst to be reduced, noncrystalline, and evenly dispersed across the support surface. Second, imaging of the Pt and Mo distribution reveals they are highly stable on the support and not prone to leaching during the reaction. This study demonstrates the ability of chemical computed tomography to image the nature and spatial distribution of catalysts under reaction conditions.
|
Jul 2015
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[11043]
Abstract: Heterogeneous catalysis performed in the liquidphase is an important type of catalytic process whichisrarelystudied in situ. Using microfocus X-ray fluorescence and X-raydiffraction computed tomography (m-XRF-CT, m-XRD-CT) incombination with X-ray absorption near-edge spectroscopy(XANES), we have determined the active state of aMo-promoted Pt/C catalyst (NanoSelect) for the liquid-phasehydrogenation of nitrobenzene under standardoperatingconditions.First, m-XRF-CT and m-XRD-CT reveal the activestate of Pt catalyst to be reduced, noncrystalline,and evenlydispersed across the support surface.Second, imaging of the Ptand Mo distribution reveals they are highly stable on thesupport and not prone to leaching during the reaction. Thisstudy demonstrates the ability of chemical computed tomog-raphy to image the nature and spatial distribution of catalystsunder reaction conditions.
|
Aug 2015
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[7405]
Abstract: A windowless electrochemical cell for the spectroscopic investigation of the liquid–liquid interface, using a dual droplet configuration, has been designed. The setup permits in situ probing of the bulk solutions and the interfacial region by fiber-optic UV–vis spectroscopy, microfocus X-ray fluorescence (XRF) elemental mapping, and microfocus X-ray absorption near-edge structure (?XANES) spectroscopy. The electrodeposition of Au, induced by ion transfer of the tetrachloroaurate complex from a halogenated solvent (containing a weak reducing agent) to the aqueous phase, has been monitored by a combination of the three techniques. The reaction can be followed in situ by UV–vis spectroscopy by detecting the oxidized form of the reducing agent. Voltammetric evidence suggests the formation of interfacial Au(I) species, whereas ?XANES detect the presence of metallic Au(0).
|
Feb 2013
|
|
I18-Microfocus Spectroscopy
|
Peter
Kille
,
John
Morgan
,
John Michael
Charnock
,
Mike
O'reilly
,
Jane
Andre
,
Michael
Turner
,
Paul
Gunning
,
Peter C.
Fisher
,
Andrew
Bennett
,
Carol
Winters
,
Fred
Mosselmans
Diamond Proposal Number(s):
[4693]
Abstract: Understanding the relationships between accumulated metal speciation in cells and tissues of ecologically significant taxa such as earthworms will improve risk assessments. Synchrotron-based ?-focus X-ray spectroscopy was used to detect, localize, and determine ligand-speciation of Zn and Pb in thin sections of two epigeic earthworm species collected from a Pb/Zn-mine soil. The findings indicated that Zn and Pb partition predominantly as typical hard acids (i.e., strong affinities for O-donors) within liverlike chloragocytes. Moreover, Zn speciation was very similar in the chloragog and intestinal epithelia but differed subtly in the kidneylike nephridial tubules; neither Zn nor Pb was detectable in the ventral nerve cord. High resolution X-ray mapping of high pressure-frozen, ultrathin, freeze-substituted sections in a transmission electron microscope (TEM), combined with conventional TEM structural analysis, identified a new cell type packed with highly organized rough endoplasmic reticulum and containing deposits of Cd (codistributed with S); there was no evidence that these cells are major depositories of Zn or Pb. These data may be used in a systems biology approach to assist in the interpretation of metal-evoked perturbations in whole-worm transcriptome and metabolome profiles.
|
Dec 2012
|
|
I18-Microfocus Spectroscopy
|
Diamond Proposal Number(s):
[12907]
Open Access
Abstract: One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.
|
Oct 2018
|
|
I18-Microfocus Spectroscopy
|
Open Access
Abstract: The effect of nitrate on the iron dissolution in artificial corrosion pits in chloride-containing solutions has been studied with in-situ
synchrotron X-ray diffraction and ex-situ Raman spectroscopy. A black layer containing Fe3C and carbon was found in the pit in
pure 1 M HCl at 0.1 V(Ag/AgCl), but at higher voltages up to 1 V(Ag/AgCl), the black layer was not observed and Fe3C was not
detected. The dissolution of Fe3C at low potentials may be inhibited by a carbon layer. In the presence of nitrate, the black layer
containing Fe3C was present at all the voltages tested, and current fluctuations were observed and found to be potential-dependent,
with greater fluctuations at higher voltages. After dissolution in 1 M HCl, the metal surface appeared slightly roughened with some
small cavities, but the surface became crystallographically etched after dissolution in 1 M HCl and 50 mM NaNO3.
|
Feb 2015
|
|
I18-Microfocus Spectroscopy
|
Open Access
Abstract: The effect of nitrate on the salt layers in iron artificial corrosion pits in acidic chloride solutions has been studied using in-situ synchrotron X-ray diffraction. During dissolution in 1 M HCl, there is a salt layer of FeCl2.4H2O on the electrode surface, which is isotropic. With addition of trace nitrate, the salt layer remains FeCl2.4H2O and no nitrate phase is observed, but the diffraction pattern becomes anisotropic, consistent with the formation of platelets with (1 2 0) planes settling horizontally. In nitrate solution containing trace of chloride (0.1 M HNO3 + 10 mM HCl), a salt layer is formed that is isostructural with Co(NO3)2.6H2O, and therefore assumed to be Fe(NO3)2.6H2O. This is the first reported crystal structure of ferrous nitrate. The salt layer is also found to give an anisotropic diffraction pattern, consistent formation of platelets with (0 2 0) planes settling horizontally.
It is well known that salt layers can form at the bottom of growing corrosion pits due to supersaturation of metal salts,14 and that the presence of salt layers is important for continued pit growth.5 This information is significant both in the field of electrochemical machining (ECM), where nitrate solutions are often used,68 and in corrosion of steel in radioactive waste solutions.911
These salt layers are a slurry of crystallites that form on a dissolving metal surface12 when the rate of metal ion production (dissolution) is greater than the rate that they can diffuse from the interface, leading to supersaturation and thus crystallite nucleation. The equilibrium thickness of the layer is determined by a self-regulating process.13 The formation of the salt layer leads to a resistance to ion flow in the electrolyte since ions can only flow in channels between the crystallites. This resistance decreases the interfacial potential, decreasing the dissolution rate. The steady state thickness of the salt layer is such that the rate of metal ion production is equal to the rate of escape.
Investigations of salt layers in pits can only be carried out in situ, since they dissolve as soon as the interfacial potential driving dissolution is removed. Studies are often carried out in artificial corrosion pits, in which a wire or foil is embedded in resin and dissolved back to give a one-dimensional cavity,1315 which is simpler to study and model.
Electrochemical impedance measurements on the salt layer of iron in a chloride solution have led to the suggestion that the salt layer is duplex, comprising a compact semiconducting inner film and a porous outer film.1618 Rayment et al.12 carried out an in-situ synchrotron study on salt layers on Fe and stainless steel in HCl-containing artificial pits, and in both systems, the salt layers were found to be FeCl2.4H2O, but the size of the crystallites on Fe was much smaller than that on stainless steel.
However, despite their practical importance, salt layers in nitrate-containing solutions have received scant attention except in electrochemical machining (ECM) studies. Surface brightening was reported to involve salt precipitation on an iron surface in a nitrate-containing electrolyte.19 It was proposed that, during ECM processing of iron in NaNO3 (potential above 2 V, current density up to 80 A/cm2), a salt layer with a duplex structure was present on iron surfaces, comprising a solid oxide inner film and a supersaturated outer layer, which is a meta-stable viscous solution or molten salt of Fe(NO3)3.9H2O/Fe(NO3)2.6H2O due to Joule heating at high current densities.7,20,21 The lack of free water in the outer layer was presumed to suppress oxygen evolution.8
In this work, the salt layer composition and structure in nitrate-containing solutions have been characterized using in-situ synchrotron X-ray diffraction combined with electrochemical measurements on iron artificial pits
|
Feb 2015
|
|