Spatially resolved micro-X-ray fluorescence and micro-X-ray absorption fine structure study of a fractured granite bore core following a radiotracer experiment

Spatially resolved X-ray absorption and fluorescence investigation with a micrometer-scale resolution on actinide-containing samples provide information necessary for safety assessment of nuclear waste disposal. In this paper one example of such an experiment is presented. This example entails neptunium speciation in a fractured granite bore core from the Swedish Äspö Hard Rock Laboratory following a radiotracer experiment using µ-XAFS and µ-XRF. In order to probe micro-volumes below the surface in the granite samples and thereby avoid potential changes in the Np speciation during cutting of the bore core, a confocal irradiation–detection geometry is employed. µ-XAFS results for a selected granite bore core cross section with ~ 3 nmol Np/g reveal that Np, originally introduced as Np(V) in the tracer cocktail, is present in the granite in its reduced Np(IV) form. The Np(IV) is often present as particles, tens of µm in size. Elemental distribution maps show the tracer Np to be located in fissures and permeable channels not larger than 100 µm. The Np distribution appears often correlated with Zn also present in some fissures. We observe small granite fissures containing Fe (presumably Fe(II)), where we do not detect any Np. It is feasible that inflowing Np(V) has a shorter residence time in large fractures, while in the smaller fissures migration is slower, leading to longer residence times, i.e., reaction times, where it is reduced to less soluble Np(IV) and becomes thereby immobilized.     

Development and applications of grazing exit micro X-ray fluorescence instrument using a polycapillary X-ray lens

A polycapillary X-ray lens is an effective optics to obtain a μm-size X-ray beam for micro-X-ray fluorescence spectrometry (μ-XRF). We developed a μ-XRF instrument using a polycapillary X-ray lens, which also enabled us to perform Grazing Exit μ-XRF (GE-μ-XRF). The evaluated diameter of the primary X-ray beam was 48 μm at the focal distance of the X-ray lens. Use of this instrument enabled two-dimensional mapping of the elemental distributions during growth of the plant “Quinoa”. The results of the mapping revealed elemental transition during growth. In addition, a small region of thin film was analyzed by GE-μ-XRF. We expect that GE-μ-XRF will become an effective method of estimating the film thickness of a small region.     

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.     

Electrochemical and complexation behavior of neptunium in aqueous perchlorate and nitrate solutions

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.     

In situ examination of uranium contaminated soil particles by micro-X-ray absorption and micro-fluorescence spectroscopies

Two complimentary spectroscopic techniques, X-ray absorption and fluorescence spectroscopy have been conducted at spatial scales of 1 to 25 μm on uranium contaminated soil sediments collected from two former nuclear materials processing facilities of the DOE: Fernald, OH and Savannah River Site, SC. A method of imbedding particles in a non-reactive Si polymer was developed such that individual particles could be examined before and after extraction with a wide range of chemicals typically used in sequential extraction techniques and others proposed forex situ chemical intervention technologies. Using both the micro-X-ray fluorescence (XRF) and micro-X-ray Absorption Near Edge Structure (XANES) techniques, both elemental and oxidation state distribution maps were generated on individual particles before and following chemical extraction. XANES can determine the relative proportion of U(VI) and U(IV) in phases comprising individual particles before and after extraction and showed that greater than 85% of the uranium existed as hexavalent U(VI). Fluorescence spectra of contaminated particles containing mainly U(VI) revealed populations of uranyl hydroxide phases and demonstrated the relative efficacy and specificity of each extraction method. Correlation of XAS and fluorescence data at micron scales provides information of U oxidation state as well as chemical form in heterogeneous samples.     

Sensitive redox speciation of neptunium by CE–ICP–MS

Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1?×?10^-9 and 5?×?10^-10 mol L^-1 for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10^-9 to 10^-6 mol L^-1. The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5?×?10^-7 mol L^-1 Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe²⁺ led to complete sorption of the Np onto the clay. After desorption with HClO₄, a mixture of Np(IV) and Np(V) was found in solution by CE–ICP–MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe²⁺.     

Advantages of combined μ-XRF and μ-XRD for phase characterization of Ti–B–C ceramics compared with conventional X-ray diffraction

Design and processing of new materials with improved high-temperature properties is one of the most challenging tasks of modern engineering. Among such materials, nonoxidic ceramics hold an important place. When optimizing the synthesis conditions of these new materials in an largely empirical manner, the use of analytical methods that can fully document the resulting phase compositions is of great importance. In this paper, we demonstrate the advantages of using combined microbeam X-ray diffraction and X-ray fluorescence over conventional X-ray diffraction as the characterization method in the specific case of Ti–B–C ceramics. Ceramic samples were synthesized by the pulse plasma method starting from high-purity powders of titanium, boron, graphite, and nickel. Different mixtures were heated in a pulsed fashion and sintered by combustion synthesis at various temperatures and time durations, as is the case during empirical optimization of a synthesis procedure. Conventional X-ray diffraction showed the presence of two phases at the end of the sintering process, TiB₂ and TiC, irrespective of the conditions employed. Scanning μ-XRF/μ-XRD on the other hand allowed one to detect and visualize the distribution of additional phases present in the sintering products, during which a dependence on sintering conditions was apparent. The μ-XRD results showed that three phases (TiB₂, TiC, and TiB) instead of two were present in samples sintered during a short time interval. The addition of metallic Ni to the initial mixture as a sintering facilitator resulted in the formation of a Ni₃B phase. All phases proved to have strongly heterogeneous distributions above the 15-μm level with the presence of TiB₂ anticorrelated to that of TiC and TiB, emphasizing the necessity of the use of laterally resolved methods of characterization.     

Surface arsenic speciation of a drinking-water treatment residual using X-ray absorption spectroscopy

Drinking-water treatment residuals (WTRs) present a low-cost geosorbent for As-contaminated waters and soils. Previous work has demonstrated the high affinity of WTRs for As, but data pertaining to the stability of sorbed As is missing. Sorption/desorption and X-ray absorption spectroscopy (XAS), both XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) studies, were combined to determine the stability of As sorbed by an Fe-based WTR. Arsenic(V) and As(III) sorption kinetics were biphasic in nature, sorbing >90% of the initial added As (15,000 mg kg(-1)) after 48 h of reaction. Subsequent desorption experiments with a high P load (7500 mg kg(-1)) showed negligible As desorption for both As species, approximately <3.5% of sorbed As; the small amount of desorbed As was attributed to the abundance of sorption sites. XANES data showed that sorption kinetics for either As(III) or As(V) initially added to solution had no effect on the sorbed As oxidation state. EXAFS spectroscopy suggested that As added either as As(III) or as As(V) formed inner-sphere mononuclear, bidentate complexes, suggesting the stability of the sorbed As, which was further corroborated by the minimum As desorption from the Fe-WTR.     

Review of development of a silica-based thermoluminescence dosimeter

Development of a silica-based material suitable for thermoluminescence dosimetry (TLD) is described. Doped silica samples were prepared in-house using the sol–gel technique. Results from a micro-X-ray fluorescence (μ-XRF) study of Zn-doped silica have confirmed the capability of the sol–gel processing steps in producing homogeneously doped samples. The ability of sol–gel processing in producing doped samples with different dopant charge states has been illustrated in the case of copper (I)- and copper (II)-doped silica samples. The charge states of the dopants have been verified using the technique of X-ray absorption near-edge structure (XANES). X-ray diffraction (XRD) investigations have shown the structure of samples doped with erbium, copper (I) and copper (II) (listed in order of decreasing effect) to be altered by the dopants, albeit with the samples remaining in an amorphous state. Local structure studies, carried out using the method of extended X-ray absorption fine structure (EXAFS), reveal that in most cases the local environment of the dopant is similar to the respective native structure of the respective metal oxides. Conversely, in a number of cases, the dopant atoms occupy the silicon sites in the silica tetragonal geometry. Thermoluminescence (TL) studies were carried out on aluminium, copper (I), germanium, manganese, tin, and zinc-doped silica samples. Weight for weight, the most sensitive thermoluminescent material was found to be 4.0 mol% aluminium-doped silica, providing 3.5 times the TL yield of TLD100 and 5.4 times that of germanium-doped silica. The photon dose response of aluminium-doped silica was observed to be linear over the range of investigated dose, 0.5–10.0 Gy.     

The mechanism of the reduction of [AnO2]2+ (An = U, Np, Pu) in aqueous solution, and by Fe(II) containing proteins and mineral surfaces, probed by DFT calculations

The fate of actinyl species in the environment is closely linked to oxidation state, since the reduction of An(VI) to An(IV) greatly decreases their mobility due to the precipitation of the relatively insoluble An(IV) species. Here we study the mechanism of the reduction of [AnO(2)](2+) (An = U, Np, Pu) both in aqueous solution and by Fe(II) containing proteins and mineral surfaces, using density functional theory calculations. We find a disproportionation mechanism involving a An(V)-An(V) cation-cation complex, and we have investigated how these complexes are formed in the different environments. We find that the behaviour of U and Pu complexes are similar, but the reduction of Np(V) to Np(IV) would seems to be more difficult, in line with the experimental finding that Np(V) is generally more stable than U(V) or Pu(V). Although the models we have used are somewhat idealised, our calculations suggest that there are strong similarities between the biotic and abiotic reduction pathways.     

Synchrotron radiation induced μ-X-ray fluorescence spectroscopy on municipal solid waste fly ashes

The high brightness of synchrotron radiation sources of the 3rd generation and the development of focusing optical elements for X-rays make it possible to create beams of micrometer size with high intensity. These beams can be used to reveal spatially resolved information about structural and chemical properties of particles. Here, the possibility of using synchrotron radiation micro-X-ray fluorescence (μ-SRXRF) for the determination of the major and minor components and their distribution within municipal solid waste (MSW) fly ashes has been investigated. By using an excitation energy of 27.0 keV, trace elements of environmental concern, like Cd, Pb and Zn can be detected to their low concentrations (ppm) routinely. The aim of the work was to gain a better understanding of the factors that determine the environmental mobility of each trace element and, in particular, the potential for their dissolution and leaching. Such detailed investigation of the content and distribution of toxic metals on/in individual particles is a valuable complement to the usual elemental analysis of bulk samples.     

Determination of boron by (p, a) reaction

Back-extraction of tri- and tetravalent actinides from diisodecylphosphoric acid (DIDPA) is studied using hydrazine carbonate as back-extractant. In experiments using 0.5M DIDPA–0.1M TBP n-dodecane solution, Am(III), Eu(III), Pu(IV) and Np(IV) are back-extracted, and the distribution ratios are decreased with an increase of hydrazine carbonate concentration. The back-extraction equilibria are confirmed by slope analysis in consideration of neutralization between DIDPA and hydrazine carbonate, which occurs quantitatively during back-extraction. In particular, oxidation of Np(IV) to Np(V) during back-extraction is observed by measuring absorption spectra. The hydrazinium ion acts as an oxidation reagent in the back-extraction of Np(IV). Separation factors of those metals are compared with the results of HDEHP. Hydrazine carbonate back-extracts Np(IV) more selectively from DIDPA than from HDEHP.     

Uptake of Pu(IV) on Bio-Rex 63 from nitric acid medium: A kinetic study

Back-extraction of tri- and tetravalent actinides from diisodecylphosphoric acid (DIDPA) is studied using hydrazine carbonate as back-extractant. In experiments using 0.5M DIDPA–0.1M TBP n-dodecane solution, Am(III), Eu(III), Pu(IV) and Np(IV) are back-extracted, and the distribution ratios are decreased with an increase of hydrazine carbonate concentration. The back-extraction equilibria are confirmed by slope analysis in consideration of neutralization between DIDPA and hydrazine carbonate, which occurs quantitatively during back-extraction. In particular, oxidation of Np(IV) to Np(V) during back-extraction is observed by measuring absorption spectra. The hydrazinium ion acts as an oxidation reagent in the back-extraction of Np(IV). Separation factors of those metals are compared with the results of HDEHP. Hydrazine carbonate back-extracts Np(IV) more selectively from DIDPA than from HDEHP.     

1,1—二氰乙烯基—2,2—二硫醇盐第一过渡系列金属配合物的研究 Ⅱ.钒(Ⅳ)和铁(Ⅲ)配合物的研究

文献中研究了1,1-二氰乙烯基-2,2-二硫醇盐(简称i-mnt)的Cu(Ⅱ)和 Ni(Ⅱ)配合物.现合成出i-mnt的钒(Ⅳ)和铁(Ⅲ)配合物,根据钒(Ⅳ)配合物的ESR谱和铁(Ⅲ)配合物的Mossbauer谱及其他光谱探讨它们的键合和结构。     

Confocal microscopic X-ray fluorescence at the HASYLAB microfocus beamline: characteristics and possibilities

The possibilities of performing non-destructive elemental analysis in three dimensions on a variety of heterogeneous materials by means of an innovative variation of the microscopic X-ray fluorescence analysis (μ-XRF) method are described. Next to employing focusing optics for concentration of the primary beam of X-rays, a second optical element between the sample and the energy-dispersive detector is used in confocal μ-XRF. Thus, only X-ray fluorescence signals from a cube-like volume (within certain limits imposed by the absorption of the radiation in the sample) can be arbitrarily positioned within the sample. The distribution of major, minor and trace elements (down to the sub-ppm concentration level in some matrices) along lines and planes within the sample can be visualized with a spatial resolution of the order of 15–40 μm. The lowest detectable amounts in confocal mode using pink-beam excitation are situated at the sub-femtogram level.     

Applications of synchrotron μ-XRF to study the distribution of biologically important elements in different environmental matrices: A review

Environmental matrices including soils, sediments, and living organisms are reservoirs of several essential as well as non-essential elements. Accurate qualitative and quantitative information on the distribution and interaction of biologically significant elements is vital to understand the role of these elements in environmental and biological samples. Synchrotron micro-X-ray fluorescence (μ-SXRF) allows in situ mapping of biologically important elements at nanometer to sub-micrometer scale with high sensitivity, negligible sample damage and enable tuning of the incident energy as desired. Beamlines in the synchrotron facilities are rapidly increasing their analytical versatility in terms of focusing optics, detector technologies, incident energy, and sample environment. Although extremely competitive, it is now feasible to find stations offering complimentary techniques like micro-X-ray diffraction (μ-XRD) and micro-X-ray absorption spectroscopy (μ-XAS) that will allow a more complete characterization of complex matrices. This review includes the most recent literature on the emerging applications and challenges of μ-SXRF in studying the distribution of biologically important elements and manufactured nanoparticles in soils, sediments, plants, and microbes. The advantages of using μ-SXRF and complimentary techniques in contrast to conventional techniques used for the respective studies are discussed.     

Dosage spectrophotometrique du neptunium

(Spectrophotometric determination of neptunium.) Use of the absorption peak of the NpO⁺₂ ion at 981 nm is discussed. Quantitative conversion to Np(V) requires oxidation of Np(IV) by Ce(IV), reduction of Np(VI) and excess of Ce(IV) with hydrazinium nitrate, and destruction of excess of hydrazine by nitrite. The measurable concentration range in the cuvette is 2–1000 mg l^-1 and the precision is± 1% in the higher range. Uranium and plutonium at ratios Me/Np ? 10^-3 do not interfere.     

Symmetry and optical spectra: a "silent" 1 : 2 Np(V)-oxydiacetate complex

The 1 : 2 Np(V)-oxydiacetate complex, NpO(2)(ODA)(2)(3-), identified by single-crystal X-ray diffractometry for the first time, is centrosymmetric around the Np atom so that the f-f transitions of Np(V) are forbidden, resulting in the "silence" of the absorption and diffuse reflectance spectra of this complex in solution and the solid state in the near-IR and visible regions.     

Macro- and micro-scale studies on U(VI) immobilization in hardened cement paste

Wet chemistry and synchrotron-based (micro-)spectroscopic investigations have been carried out to determine the uptake and speciation of U(VI) in hardened cement paste (HCP). The wet chemistry experiments included kinetic studies and the determination of the sorption isotherm. The latter measurements allowed conditions for linear sorption to be distinguished from those where precipitation occurred. Micro-X-ray fluorescence and X-ray absorption spectroscopy (μ-XRF/XAS) were used to determine the elemental distribution and the coordination environment of U(VI) in an intact HCP sample at the atomic level. The sample was prepared by in-diffusion of U(VI) into HCP over 9 months. Micro-XRF maps revealed a heterogeneous distribution of U(VI) in a ten micron thick layer on the surface of the HCP disk. Micro-XAS measurements on a U(VI) hot spot showed that the coordination environment of U(VI) is similar to that in U(VI) doped HCP and in C-S-H sorption samples. To the best of our knowledge this is the first synchrotron-based micro-spectroscopic study on the speciation of diffusing uranyl ions with micro-scale spatial resolution.