Volume 93, Issue 3 -

Thermodynamics of adsorption of gaseous species on the surface of a gas chromatography column is considered using the knowledge of the electronic structure of the adsorbate. Relevant equations based on a model of mobile adsorption are offered to predict the adsorption temperature, T ads , of a heavy-element (or its compound) with respect to T ads of its lighter homolog (or the same type of compound). A case of adsorption of OsO 4 and HsO 4 on an inert (quartz or silicon nitride) surface of a chromatography column is taken, as an example. The influence of various properties of the adsorbate, such as molecular weight and size, on T ads is analyzed to show that those factors should not be ignored in intentionally accurate predictions of T ads . A comparison of the desorption constants of OsO 4 and HsO 4 obtained with the use of the calculated spectroscopic properties shows that HsO 4 should be significantly more volatile than OsO 4 , mainly due to the entropy factor.

The complexation of 248 Cm(III) with aqueous silicic acid is investigated in the pH range of 1.5–9.0 in 0.03 M NaCl with varying the silicate concentration under and over saturation in reference to the solubility of amorphous silica in order to characterize the reaction with monosilicic and polysilicic acid. Speciation is made by time-resolved laser fluorescence spectroscopy (TRLFS) in combination with radiometric quantification. Three different complexation products are observed: Cm-silicate(I), Cm-silicate(II) and Cm-silicate(III). The first and second species are formed in under-saturation of silicic acid, whereas all three species are produced in over-saturation of silicic acid. The formation of Cm-silicate(I) following the reaction of Cm 3+ with H 3 SiO 4 − appears in both under and over-saturation of silicic acid only as a minor fraction at pH=4–7. Cm-silicate(II) and Cm-silicate(III) are found to be colloidal. Cm-silicate(II) shows its spectroscopic characteristics varying with the experimental condition, whereas Cm-silicate(III) formed exclusively with polysilicic acid remains consistent and stable. The experimental results indicate that a species conversion takes place as Cm-silicate(I) → Cm-silicate(II) → Cm-silicate(III) with increasing pH and silicon concentration. Only the reaction leading to the formation of Cm-silicate(I) can be quantified for its stability constant as logβ°=7.74±0.08.

The influence of phenolic OH groups on the Np(V) complexation by humic acids was studied at pH 7 and pH 8 under nitrogen atmosphere applying chemically modified humic acids with blocked phenolic OH groups in comparison to their corresponding unmodified humic acids. The studies were performed by near-infrared (NIR) absorption spectroscopy using the metal ion charge neutralization model for evaluation of the experimental data. For all humic acids under investigation comparable complexation constants were determined. However, the studied modified humic acids Aldrich and M42 show significantly lower loading capacities for NpO 2 + ions ( e.g. , pH 7: 7.2%±1.1% and 5.3%±1.0%, respectively) than the corresponding unmodified humic acids ( e.g. , pH 7: 10.0%±1.5 and 11.2%±1.1, respectively). This result points to a lower amount of maximal available humic acid ligand sites after blocking of phenolic OH groups. From that, it can be concluded that humic acid phenolic OH groups contribute to the interaction between humic acid and Np(V) under the studied conditions. The comparison of the loading capacities obtained for unmodified and modified Aldrich humic acid at pH 7 and pH 8 shows that the impact of phenolic OH groups on the Np(V) complexation by humic acids is increased with increasing pH value.

Advanced nuclear fuel cycles are being developed in order to reduce the long-term radiotoxicity of highly radioactive waste. Pyrochemical partitioning techniques appear particularly attractive for advanced fuel cycles in which the minor actinides are recycled. The electrochemical processes of practical importance are the electrorefining process and the liquid–liquid extraction of transuranic (TRU) elements from fission products using either non-miscible molten metal or molten salt–metal phases. Analytical methods for the accurate assay of actinide elements in these matrices needed to be developed. A quantitative assay is required in order to establish a material balance for process development and – at a later stage – for accountancy and control purposes. To this end radiometric techniques such as energy-dispersive X-ray fluorescence analysis (XRF), neutron coincidence counting (NCC) and high-resolution gamma spectrometry (HRGS) were extensively employed for the quantitative determination of actinides (U, Np, Pu, Am, Cm) in process samples. Comparative analyses were performed using inductively coupled plasma mass spectrometry (ICP-MS). The respective samples were available in small quantities (≅100 mg) either in the form of eutectic salt or in metallic form with Cd, Zr or Bi as major metallic matrix constituents.

The release of mercury and thallium from liquid eutectic lead-bismuth alloy (LBE) under a flowing Ar/7%-H 2 atmosphere has been studied in the temperature range from 408 to 1292 K using γ-ray spectroscopy. For technical applications such as liquid metal spallation targets or accelerator driven systems, where liquid LBE is planned to be used as target material, the release of radioactive mercury isotopes produced by spallation is expected to be one of the major safety issues. During short-term experiments significant amounts of mercury begin to evaporate from liquid LBE at temperatures starting from about 475 K. 80% of the mercury present in the sample is released from samples of approximately 1.5–3 g within one hour at temperatures higher than 625 K. Thallium release in the temperature range investigated is below experimental error. Long-term experiments reveal that even at temperatures as low as 476 K about 25% of the mercury present in the samples is released per day under a flowing Ar/7%-H 2 atmosphere.

The contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO 2 )8O 2 (OH) 12 ](H 2 O) 10 were observed on the hydrated CSNF particles. Over the 2-year contact period, all evidence of metaschoepite disappeared, and the fuel particles were coated by a new alteration phase. Additionally, films of the reacted fuel were observed at the sample air-water interface of each sample. The corrosion phases on fuel powders and on the suspended films were examined by scanning electron microscopy, energy-dispersive X-ray fluorescence, and X-ray diffraction and were identified as studtite [(UO 2 )(O 2 )(H 2 O) 2 ](H 2 O) 2 and metastudtite (UO 4 ·2H 2 O), respectively. The reason for the partitioning of the latter phase to the sample air-water interface is unclear at this time but may be due to structural differences between the two phases. Scanning electron micrographs of the CSNF powders indicated surface corrosion along grain boundaries and fragmentation of the primary solid. The occurrence of studtite and metastudtite on CSNF could have implications for the potential attenuation of released radionuclides during oxidative corrosion of CSNF in a geologic repository.

Immersing commercial spent nuclear fuel (CSNF) in deionized water produced two corrosion products after a 2-year contact period. Suspensions of aggregates were observed to form at the air–water interface for each of five samples. These suspended aggregates were characterized by X-ray diffraction (XRD) to be metastudtite (UO 4 ·2H 2 O), while the corrosion present on the surface of the fuel itself was determined to be studtite [(UO 2 )(O 2 )(H 2 O) 2 ](H 2 O) 2 ]. The presence of unreacted UO 2 matrix was below the limits of detection by XRD for the three samples examined. The result prompted a radiochemical analysis of the solids collected from the sample air–water interface. The analysis indicated that high concentrations of 90 Sr, 137 Cs, and 99 Tc, relative to the fuel inventory, had concentrated at the air–water interface along with the aggregates of metastudtite. Concentrations of 241 Am were at least two orders of magnitude lower than expected in these solids, and retention of 237 Np and 239 Pu into the corrosion product was observed. The combined radiochemical analyses of the air–water interface aggregates and leachate samples are a rare example of radionuclide partitioning to an alteration phase and may provide preliminary evidence for mechanisms that give rise to such noticeable departures from fuel-inventory values. The leachate radiochemical data are compared to existing data from hydration of the same CSNF.

Mercury is a toxic environmental pollutant. A method has been developed to remove these metal ions by preconcentrating onto 1-(2-thiazolylazo)-2-naphthol (TAN) loaded polyurethane (PUR) foam that proves an excellent sorbent for these metal ions. The accumulation conditions are optimized with respect to pH, shaking time, loading capacity and adsorbent weight. The optimum pH for sorption was found to be 7, which is close to the natural water. Optimum shaking time for Hg(II) is 30 min. The kinetic studies indicate that sorption occurs through intraparticle diffusion process. Classical adsorption isotherms are applied to determine the nature of sorption. The sorption free energy value indicates that adsorption process is chemisorption. Effect of temperature has been used to compute the values of Δ H , Δ S and Δ G for Hg(II). These are 19.0±0.9 kJ mol −1 , 80.73±0.1 J K −1 mol −1 , −5.1±0.9 kJ mol −1 respectively. Adsorption of metal ions is endothermic, spontaneous and entropy driven. Effect of different anions and cations are observed. Adsorption via Hg(II)-TAN complex formation on PUR foam was supported by spectrophotometric studies.