[en] The hydrolysis of palladium was investigated in 0.6 mol kg^-1 NaCl at 298.2 K. Potentiometric titrations of solutions at various total concentrations of palladium(II) revealed that dilute (millimolar) conditions can be used to monitor the proton release due to hydrolysis reactions up to 2 protons per palladium(II) as long as the equilibration time is kept small. Spectrophotometric titrations were used to corroborate the homogeneous changes in speciation for the PdCl₃OH^2- species and to extract its correlative molar absorption coefficients in the 210-320 nm range. The molar absorption coefficients are similar to those of PdCl42- but exhibit a broader distribution of excitation energies resulting from the blue shift of the dominant charge transfer bands due to the presence of OH-. The longer-term potentiometric titrations systematically yielded, on the other hand, precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments at six different total palladium(II) concentrations in the 3-11 pH range showed the dominant precipitating phase as Pd(OH)1.72Cl0.28. The coordination environment of Pd in this solid was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and yielded an average 1.75 O and 0.25 Cl per Pd atoms with a Pd-O distance of 2.0 (angstrom) and Pd-Cl of 2.1 (angstrom). Finally, the precipitation experiments showed the final products to be of larger solubility than a literature Pd(OH)2 solubility study in which the KCl media induced a solid phase transformation to Pd(OH)1.72Cl0.28. Polynuclear complexes Pdq(OH)r2q-r with q=r=[3,9] explain the combined precipitation and hydrolysis data and may represent subsets of [Pd(OH)2]n and/or [Pd(OH)1.72Cl0.28]n chains coiled into nanometer-sized spheroids previously described in the literature

[en] Ru speciation is being investigated systematically from models of high level waste solutions right through to the calcination process and the vitrified glass product. The characterisation of these species is complicated due to the fact that a wide range of ruthenium nitrosyl/nitrite/nitrate complexes can be present in nitric acid waste solutions. The general formula for these complexes is RuNO(NO₃)_x(NO₂)_y(OH)_z(H₂O)_5-x-y-z^+3-x-y-z. A range of different techniques has been used for the characterisation of these species in solution, including electron absorption spectroscopy, vibrational spectroscopy, multi-nuclear magnetic resonance spectroscopy and X-ray absorption spectroscopy. (authors)

[en] An integrated approach involving the use of ion chromatography-inductively coupled plasma-mass spectrometry (IC-ICP-MS), X-ray absorption spectroscopy (XAS) and sequential extraction procedures has been employed to elucidate the solution and solid phase speciation and partitioning of As in a polluted urban watercourse. Dissolved As concentrations exceeding 130 μg l^-1 and comprising entirely inorganic species were determined in the waters of Tinker Brook, a contaminated stream. Upon mixing with a relatively As-free stream, White Ash Brook, both the total concentration of dissolved As and the proportion of As(V) were observed to decrease dramatically below values expected for conservative mixing. This was ascribed to adsorption onto the Fe (oxyhydr)oxides that characterise White Ash Brook on the basis of sequential extraction and direct analysis of the solids via XAS . The shift in oxidation state is speculated to be due to the faster rate of adsorption of As(V) on Fe (oxyhydr)oxides than As(III) in this fast flowing stream system. During periods of reduced supply of anthropogenic As, a small, secondary input of As(III) to White Ash Brook is detectable, delivered by a small ochreous seepage. The Fe (oxyhydr)oxide As-rich deposits surrounding this discharge may also act as a significant source of As upon dissolution during stormflow conditions

[en] There is currently a great deal of research interest in the development of molten salt technology, both classical high temperature melts and low temperature ionic liquids, for the electrochemical separation of the actinides from spent nuclear fuel. We are interested in gaining a better understanding of actinide and key fission product speciation and electrochemical properties in a range of melts. Our studies in high temperature alkali metal melts (including LiCl and LiCl-KCl and CsCl-NaCl eutectics) have focussed on in-situ species of U, Th, Tc and Ru using X-ray absorption spectroscopy (XAS, both EXAFS and XANES) and electronic absorption spectroscopy (EAS). We report unusual actinide speciation in high temperature melts and an evaluation of the likelihood of Ru or Tc volatilization during plant operation. Our studies in lower temperature melts (ionic liquids) have focussed on salts containing tertiary alkyl group 15 cations and the bis(tri-fluor-methyl)sulfonyl)imide anion, melts which we have shown to have exceptionally wide electrochemical windows. We report Ln, Th, U and Np speciation (XAS, EAS and vibrational spectroscopy) and electrochemistry in these melts and relate the solution studies to crystallographic characterised benchmark species. (authors)

[en] Technetium, uranium and neptunium may all occur in the environment in more than one oxidation state (IV or VII, IV or VI and IV or V respectively). The surface of mackinawite, the first-formed iron sulfide phase in anoxic conditions, can promote redox changes so a series of laboratory experiments were carried out to explore the interactions of Tc, U and Np with this mineral. The products of reaction were characterised using X-ray absorption spectroscopy. Technetium, added as TcO₄^-, is reduced to oxidation state IV and forms a TcS₂-like species. On oxidation of the mackinawite in air to form goethite, Tc remains in oxidation state IV but in an oxide, rather than a sulfide environment. At low concentrations, uranium forms uranyl surface complexes on oxidised regions of the mackinawite surface but at higher concentrations, the uranium promotes surface oxidation and forms a mixed oxidation state oxide phase. Neptunium is reduced to oxidation IV and forms a surface complex with surface sulfide ions. The remainder of the Np coordination sphere is filled with water molecules or hydroxide ions

[en] The ability of metal-reducing bacteria to produce nanoparticles, and their precursors, can be harnessed for the biological manufacture of fluorescent, semiconducting nanomaterials. The anaerobic bacterium Veillonella atypica can reduce selenium oxyanions to form nanospheres of elemental selenium. These selenium nanospheres are then further reduced by the bacterium to form reactive selenide which could be precipitated with a suitable metal cation to produce nanoscale chalcogenide precipitates, such as zinc selenide, with optical and semiconducting properties. The whole cells used hydrogen as the electron donor for selenite reduction and an enhancement of the reduction rate was observed with the addition of a redox mediator (anthraquinone disulfonic acid). A novel synchrotron-based in situ time-resolved x-ray absorption spectroscopy technique was used, in conjunction with ion chromatography and inductively coupled plasma-atomic emission spectroscopy, to study the mechanisms and kinetics of the microbial reduction of selenite to selenide. The products of this biotransformation were also assessed using electron microscopy, energy-dispersive spectroscopy, x-ray diffraction and fluorescence spectroscopy. This process offers the potential to prepare chalcogenide-based nanocrystals, for application in optoelectronic devices and biological labelling, from more environmentally benign precursors than those used in conventional organometallic synthesis

[en] Four thallium(I) uranates(VI), Tl₄UO₅, Tl₂UO₄, Tl₂U₂O₇ and Tl₂U₃O₁₀, were prepared and their IR, and, for the first time, their Raman, electronic and X-ray absorption (EXAFS) spectra have been measured. These uranates are thermally stable under nitrogen up to 660 deg. C but above this temperature they decompose and their uranium content increases, due to loss of Tl₂O. The thermal stability of these uranates increases with decreasing thallium content. Comparison with the reported values for the standard enthalpies of formation of the corresponding alkali metal uranates, which are essentially cation independent, allows suggested values for the above uranates, respectively, of -2437, -1900, -3182, and -4437 kJ mol^-1. Structural information and atom arrangements were obtained from analysis of powder XRD and EXAFS spectroscopy measurements. U-O distances, both primary and secondary, were obtained from EXAFS measurements and these were essentially identical with those calculated from vibrational IR spectra

[en] Uranium (and plutonium) can be separated from spent fuel by treatment in molten carbonates and air sparging. UO₂ is converted into insoluble uranate species that can be filtered off. The rare earth and other fission product elements remaining in solution can be later precipitated using phosphates as precipitants. A sequence is described that can (in principle) be used to recover and recycle the carbonate melt used. Molten chloride eutectics are more convenient solvents than carbonates for obtaining the detailed chemistry of fission products in molten salts and their selective precipitation as phosphates. We have investigated the behavior of: Cs, Mg, Sr, Ba, lanthanides (La to Dy), Zr, Cr, Mo, Mn, Re (to simulate Tc), Fe, Ru, Ni, Cd, Bi and Te, but here report results for the rare earths. The distribution coefficients of these elements between chloride melts and precipitates were determined. Lithium-free melts favored formation of double phosphates. Rare earth elements and zirconium can be removed from chloride melts but Sr, Ba and Mg are melt cation specific. EXAFS and XANES measurements have established the speciation of both transition and non-transition elements in molten chlorides. EXAFS is largely model dependent, and to ensure the correct coordination number requires absorption spectroscopy data. EXAFS provides the element-Cl distance in the melt, not previously available. At high concentrations, the presence of bridging chlorine atoms can be established by EXAFS. The effect of diluting such melts and preliminary data on the solubilities of rare earth fission product elements in carbonate melts are reported

[en] Highlights: → Barium and radium react with carbonate minerals. → Radium reacts with all the phases studied. → Barium reacts only with dolomite, magnesite and siderite. → We study the development of secondary phases formed in these reactions. → Trace components of the minerals can dictate the outcome of reaction. - Abstract: Radium-226 is a naturally-occurring radioisotope with potentially significant radiological impact and whose environmental behaviour is of concern. The reactions of tracer (0.1-1 nM) dissolved Ra and its chemical analogue Ba with the surfaces of a range of carbonate minerals have been studied. All of the minerals react with Ra but, whereas calcite, dolomite, strontianite, rhodocrosite, ankerite and witherite all show increased uptake with increasing Ra concentration, suggesting a coprecipitation reaction (hence with phase formation limiting uptake), siderite, magnesite and ankerite show behaviour suggesting simple sorption (with decreasing uptake as Ra concentration increases, or with no dependence on [Ra]). Magnesite, in particular, has a low sorption capacity. Barium has been used at higher (0.1-1 mM) concentrations to enable the use of surface analytical and imaging techniques in addition to bulk uptake measurements. Although the same eight carbonates were studied, measurable uptake occurs only on dolomite, magnesite and siderite. For siderite and magnesite, there is an approximately linear relationship between the increasing solid and solution phase Ba concentrations, suggesting a simple sorption process. Dolomite shows more complex behaviour suggesting simple sorption at the lowest concentrations and phase formation at higher concentrations (>0.4 mmol L^-1). The latter observation is consistent with spectroscopic evidence for the formation of witherite. Surface analysis and imaging of the three carbonate substrates that react with Ba show a diversity of behaviour, partly as a result of using natural minerals in these experiments. Witherite is commonly formed as a surface precipitate although the presence of even trace SO₄^2- leads to barite formation. The surface phases display a range of characteristic morphologies, and the surface structure has the effect of templating growth. The presence of even minor amounts of Fe (hydr)oxide phases as alteration products or precipitates on the carbonates is also important, since Ba has a strong affinity for these phases.