Combined LIBD and XAFS investigation of the formation and structure of Zr(IV) colloids

Zirconium Coordination Chemistry and Its Role in Optimizing Hydroxymate Chelation: Insights from Molecular Dynamics

In the past decade, there has been a growth in using Zirconium-89 (89Zr) as a radionuclide in nuclear medicine for cancer diagnostic imaging and drug discovery processes. Although one of the most popular chelators for 89Zr, desferrioxamine (DFO) is typically presented as a hexadentate ligand, our work suggests a different scenario. The coordination structure of the Zr4+-DFO complex has primarily been informed by DFT-based calculations, which typically ignore temperature and therefore entropic and dynamic solvent effects. In this work, free energy calculations using molecular dynamics simulations, where the conformational fluctuations of both the ligand and the solvent are explicitly included, are used to compare the binding of Zr4+ cations with two different chelators, DFO and 4HMS, the latter of which is an octadentate ligand that has been recently proposed as a better chelator due to the presence of four hydroxymate groups. We find that thermally induced disorder leads to an open hexadentate chelate structure of the Zr4+-DFO complex, leaving the Zr4+ metal exposed to the solvent. A stable coordination of Zr4+ with 4HMS, however, is formed by involving both hydroxamate groups and water molecules in a more closely packed structure.

Structural Approach to Understanding the Formation of Amorphous Metal Hydroxides

This study investigates the formation of amorphous tetravalent metal hydroxides, M(OH)₄, based on the structural analysis by small- and wide-angle X-ray scattering (SWAXS) and on the electrical potential charge near the surface of M(OH)₄ particles. The amorphous zirconium hydroxide solid phases that aged in NaCl and CaCl₂ solutions at 25 °C exhibited a hierarchical structure consisting of primary particles of a few nanometers in size and their aggregates more than 100 nm in size. The SWAXS profiles suggested that the size of the primary particles depends on the ionic strength and electrolytes in the sample solutions. The smaller size of the primary particles observed in solutions with higher ionic strength can be explained by the thinner electrical double layer. Additionally, we focused on the ζ potentials of M(OH)₄ suspensions in NaCl, NaNO₃, and CaCl₂ solutions. With the aid of reference systems of metal oxides, MO₂, it was found that the ζ potentials were well interpreted by a traditional surface ionization and complexation model, and the size distributions of large aggregates were explained by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with the ζ potential values. The present study suggests the formation mechanism of amorphous metal hydroxides through a combination of structural analysis and investigation of electrical potentials.

Giant zirconium-bisphosphonate nano-ribbons and their liquid crystalline phase behaviour in water

In decimolar aqueous solutions, zirconium oxychloride octahydrate forms several micrometer long and approximately 15 nm wide thin ribbons through the reaction with excess amounts of the sodium salt of 1-hydroxyethane-1,1-diphosphonic acid (HEDP, known as etidronic acid). Primarily deduced from SAXS, TEM, EXAFS and solid-state NMR analyses, a consistent structural model enables congruous explanations for the colloidal behaviour of the purified ribbons as well as of their reaction products with ammonia and amines, respectively. Properties of the lyotropic, liquid crystalline phases are discussed in the light of potential applications in aqueous coatings.

Structural Approach to Understanding the Solubility of Metal Hydroxides

We report the hierarchical structure of zirconium hydroxide after aging at different temperatures to elucidate the factors governing zirconium solubility in aqueous solutions. Zirconium hydroxide solid phases after aging at 25, 40, 60, and 90 °C under acidic to alkaline conditions were investigated using extended X-ray absorption fine structure (EXAFS), wide- and small-angle X-ray scattering (WAXS and SAXS), and transmission electron microscopy (TEM) techniques to reveal the bulk and surface structures of the solid phases from the nanoscale to sub-microscale. After aging at 25 °C, the fundamental building unit of the solid phase was considered to be tetrameric and dimeric hydroxide species. These polynuclear species formed amorphous primary particles that are approximately 3 nm in size, which in turn formed aggregates that are hundreds of nanometers in size. This hierarchical structure was found to be stable up to 60 °C under acidic and neutral conditions and up to 40 °C under alkaline conditions. After aging at 90 °C under acidic conditions and at 60 and 90 °C under alkaline conditions, the WAXS and EXAFS measurements suggested the crystallization of the solid phase. The SAXS profiles and TEM observations supported the existence of crystallized large particles about 60 nm in size, and the appearance of the Guinier region in the SAXS profiles indicated that the crystallization of the amorphous primary particles leads to the reduction of the size of the large aggregates. The transformation of the solid-phase structure by temperature was discussed in relation to the solubility product to understand the solubility-limiting solid phase. The solubility of zirconium hydroxide after aging at different temperatures was governed not only by the size of the amorphous primary particles or crystallized large particles but also by their surface configuration.

Hydrolysis of trivalent plutonium and solubility of Pu(OH)3 (am) under electrolytic reducing conditions

The aim of this work is to determine the solubility product of plutonium hydroxide under reducing conditions and to ascertain the stability of Pu(OH)₃ (am) in water. Hydrolysis of Pu(iii) and solubility of Pu(OH)₃ (am) were investigated at a constant ionic strength of 0.1 M NaClO₄. Coulometric titration was adopted to adjust the pH of plutonium solutions, during which the electrolytic reducing conditions maintained the oxidation state of Pu(iii). Chemical speciation for dissolved plutonium was investigated using sensitive spectrophotometry coupled with a liquid waveguide capillary cell. The spectroscopic investigations indicated that dissolved Pu(iv), Pu(v), and Pu(vi) species were ignorable under these experimental conditions. The absorbance of Pu³⁺ ions decreased due to hydrolysis of Pu(iii) but the absorbance of Pu(iii) hydrolysis species was not distinguishable. The formation constant for the first hydrolysis species (log ^*β'₁) determined in the present study is -6.62 ± 0.25. The non-crystalline structure of the plutonium precipitate was observed through X-ray diffraction. The solubility product of Pu(OH)₃ (am), log ^*K'_s,0 is determined to be 15.23 ± 0.50. These results indicate a stronger tendency for the hydrolysis of Pu(iii) and higher stability (lower solubility) of Pu(OH)₃ (am) compared to Am(iii).

Structural analysis of cellulose acetate and zirconium alkoxide hybrid fibres

We investigated the detailed structures of organic–inorganic hybrid fibres composed of cellulose acetate (CA) and zirconium alkoxides (Zr(OR)₄) using various measurements.

A monomeric [Zr(CO3)4]4− complex in an ammonium zirconium carbonate aqueous solution studied by extended X-ray absorption fine structure, Raman and nuclear magnetic resonance spectroscopy

A monomeric complex, [Zr(CO₃)₄]⁴⁻, is able to exist under certain conditions.

Understanding the role of aqueous solution speciation and its application to the directed syntheses of complex oxidic Zr chlorides and sulfates

The lack of an in-depth understanding of solution-phase speciation and its relationship to solid-state phase formation is a grand challenge in synthesis science. It has severely limited the ability of inorganic chemists to predict or rationalize the formation of compounds from solutions. The need to investigate mechanisms that underlie self-assembly has motivated this study of aqueous Zr-sulfate chemistry as a model system, with the goal of understanding the structures of oligomeric clusters present in solution. We used high-energy X-ray scattering (HEXS) data to quantify Zr correlations in a series of solutions as a function of sulfate concentration. The pair distribution function (PDF) from the sulfate-free sample reveals that the average oligomeric Zr moiety is larger than the tetrameric building unit, [Zr4(OH)8(H2O)16](8+), generally understood to dominate its solution speciation. At sulfate concentrations greater than 1 m (molal), bidentate sulfate is observed, a coordination not seen in Zr(SO4)2·4H2O (2), which forms upon evaporation. Also seen in solution are correlations consistent with sulfate-bridged Zr dimers and the higher-order oligomers seen in 2. At intermediate sulfate concentrations there are correlations consistent with large Zr hydroxo-/oxo-bridged clusters. Crystals of [Zr18(OH)26O20(H2O)23.2(SO4)12.7]Cl0.6·nH2O (3) precipitate from these solutions. The Raman spectrum of 3 has a peak at 1017 cm(-1) that can be used as a signature for its presence in solution. Raman studies on deuterated solutions point to the important role of sulfate in the crystallization process. These solution results emphasize the presence of well-defined prenucleation correlations on length scales of <1 nm, often considered to be within the structurally amorphous regime.

Synthesis and characterization of thorium, uranium and cerium oxide nanoparticles

We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2 samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.

Minimization of absorption contrast for accurate amorphous phase quantification: application to ZrO2nanoparticles

Monoclinic and tetragonal zirconia samples were characterized by X-ray diffraction, pycnometry, thermogravimetric analysis (TGA), Fourier transform (FT) IR and mass (MS) spectroscopies, and scanning and transmission electron (TEM) microscopies. The results show, for the particular case of a tetragonal zirconia sample, an X-ray-undetected subproduct identified as an amorphous organic phase by FTIR–ATR (attenuated total reflection) and TGA–MS. The observations by TEM allowed this amorphous phase to be localized on the surface as a shell coating the nanoparticles. Moreover, this amorphous phase was quantified by Rietveld refinementviathe addition of an internal silicon standard. Because zirconia and silicon have different linear absorption coefficients, the microabsorption effect was minimized by using small particle sizes. The amorphous phase was calculated to constitute 11.4 (30)% of the initial mass before Brindley correction and 10.6 (30)% of the initial mass after Brindley correction. The closeness of these values shows that the contribution of the Brindley correction can be neglected if precautions are taken on the microabsorption effect. This work has also highlighted the importance of thoroughly characterizing commercial products, which are not necessarily pure. Indeed, the presence of impurities could become a non-negligible parameter for physical and chemical properties studies related to commercial materials.

Humic acid metal cation interaction studied by spectromicroscopy techniques in combination with quantum chemical calculations

Humic acids (HA) have a high binding capacity towards traces of toxic metal cations, thus affecting their transport in aquatic systems. Eu(III)-HA aggregates are studied by synchrotron-based scanning transmission X-ray microscopy (STXM) at the carbon K-edge and laser scanning luminescence microscopy (LSLM) at the (5)D(0) --> (7)F(1,2) fluorescence emission lines. Both methods provide the necessary spatial resolution in the sub-micrometre range to resolve characteristic aggregate morphologies: optically dense zones embedded in a matrix of less dense material in STXM images correspond to areas with increased Eu(III) luminescence yield in the LSLM micrographs. In the C 1s-NEXAFS of metal-loaded polyacrylic acid (PAA), used as a HA model compound, a distinct complexation effect is identified. This effect is similar to trends observed in the dense fraction of HA/metal cation aggregates. The strongest complexation effect is observed for the Zr(IV)-HA/PAA system. This effect is confirmed by quantum chemical calculations performed at the ab initio level for model complexes with different metal centres and complex geometries. Without the high spatial resolution of STXM and LSLM and without the combination of molecular modelling with experimental results, the different zones indicating a ;pseudo'-phase separation into strong complexing domains and weaker complexing domains of HA would never have been identified. This type of strategy can be used to study metal interaction with other organic material.

Solubility measurement of zirconium(IV) hydrous oxide

The solubility of zirconium(IV) hydrous oxide was measured in the hydrogen ion concentration (pHc) range from 1 to 13 at 25 °C. The experiment was carried out with I=0.1, 0.5 and 1.0 mol/dm3 (M) of NaClO4 solution. The concentration of Zr was measured by ICP-MS after separating the aqueous phase from the solid phase by ultrafiltration methods. Some differences between the obtained values and literature data were observed due to the size distribution of chemical species. In the lower pHc region, the size distribution of filtrate species was investigated by different pore-size filtration in the range of 3 k to 100 kDa NMWL (nominal molecular weight limit) membranes. The solubility values of mononuclear species were obtained by the Flory model, and by using the specific ion interaction theory (SIT) for ionic strength corrections and the known literature value of the hydrolysis constant β1°, the tentative value of the solubility product (K sp°) was determined to be −56.94±0.32.

The INE-Beamline for actinide research at ANKA

The Institut für Nukleare Entsorgung (INE) at the Forschungszentrum Karlsruhe (FZK), Germany, has constructed and is operating a beamline at the synchrotron source ANKA dedicated to actinide speciation investigations related to nuclear waste disposal as well as applied and basic actinide research. Experiments on nuclides not suited as nuclear fuel with activities up to 106 times the limit of exemption inside a safe and flexible containment concept are now possible. The design is for a multi-purpose beamline, i.e., a number of methods (XAFS, surface sensitive and spatially resolved techniques) are envisaged on one and the same sample, with X-ray energies from 2472 eV (SK edge) to 23220 eV (RhK edge). The commissioning of the INE-Beamline was officially completed in September 2005 and meanwhile regular user operation has been started. Here we present a description of this new facility and examples for X-ray absorption studies on actinides and homologues performed during the beamline commissioning phase.

Solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides in CaCl2solutions and the formation of ternary Ca-M(IV)-OH complexes

The solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides is investigated at 22±2 °C in alkaline 0.1−4.5 M CaCl2solutions. Further studies are performed with Zr(IV) over the entire pH range in NaCl and CaCl2media, and with Zr(IV) and Th(IV) in alkaline Ca(ClO4)2solutions. The comparison of Zr(IV) data in different ionic media (NaCl, NaClO4, CaCl2and Ca(ClO4)2) of similar ionic strength shows that the solubility in the acidic and neutral pH range is not affected by strong interactions between the aqueous M(IV) species and the medium ions. However, in alkaline CaCl2and Ca(ClO4)2solutions the formation of ternary Ca-M(IV)-OH complexes causes unexpectedly high solubilities of Zr(IV) at pHc=10−12 and [Ca2+]>0.05 M and of Th(IV) at pHc=11−12 and [Ca2+]>0.5 M. The dependence of the Zr(IV) and Th(IV) solubilities on the H+and CaCl2concentrations shows that the complexes Zr(OH)62−and Th(OH)84−with an unusual large number of OH−ligands are stabilized by the formation of associates or ion pairs with Ca2+ions. The SIT is used to derive equilibrium constants at zero ionic strength for the complexes Zr(OH)62−(in calcium-free solutions), Ca2[Zr(OH)6]2+, Ca3[Zr(OH)6]4+and Ca4[Th(OH)8]4+. In analogous studies with Pu(IV) hydrous oxide, the solubility increasing effect of ternary complex formation with Ca2+ions is only observed at CaCl2concentrations above 2 M.

Investigation of polynuclear Zr(IV) hydroxide complexes by nanoelectrospray mass-spectrometry combined with XAFS

Polynuclear species of zirconium in acidic aqueous solution are investigated by combining X-ray absorption spectroscopy (XAFS) and nanoelectrospray mass spectrometry (ESI-MS). Species distributions are measured between pHC 0 and pHC 3 for [Zr]=1.5-10 mM. While the monomer remains a minor species, with increasing pH the degree of polymerization increases and the formation of tetramers, pentamers, octamers, and larger polymers is observed. The high resolution of the mass spectrometer permits the unambiguous determination of polynuclear zirconium hydroxide complexes by means of their isotopic patterns. The relative abundances of mononuclear and polynuclear species present simultaneously in solution are measured, even if one of the species contributes only 0.1% of the Zr concentration. For the first time it has been directly observed that the hydrolysis of polynuclear Zr species is a continuous process which leads to charge compensation through the sequential substitution of water molecules by hydroxide ligands until doubly charged polymers dominate at conditions (H+ and Zr concentrations) close to the solubility of Zr(OH)4(am). The invasiveness of the electrospray process was minimized by using very mild declustering conditions, leaving the polynuclear species within a solvent shell of approximately 20 water molecules.

Probing particle size distributions in natural surface waters from 15 nm to 2 μm by a combination of LIBD and single-particle counting

We present a technique for measuring colloid size distributions between 15 nm and 2 microm at concentrations relevant to natural surface waters. Two particle-measuring methods are combined: laser-induced breakdown detection (LIBD), which allows the quantification of colloid size distributions below 400 nm, and a commercial single-particle counter that extends the accessible size range up to two mum. Centrifugation was used in order to separate micrometer sized particles for the LIBD measurement. The feasibility is demonstrated on water of Lake Brienz (Switzerland) and the River Pfinz (Germany) and the particle size distributions follow Pareto's law even down to 15 nm in both cases.