Perrhenate ion uptake by aluminium hydroxide gels

The adsorption of perrhenate ions onto aluminium hydroxide gels was investigated. These gels were prepared by the sol-gel method in non aqueous solutions using a tertiary amine as template. They were amorphous and presented high specific surface areas. The amount of perrhenate ions adsorbed from aqueous solutions depended on the specific surface area of the gels. Removal of perrhenate ions up to 84% could be achieved without reaching saturation.     

Uranium sorption from aqueous solutions on sodium-form of HEU-type zeolite crystals

The uranium sorption from aqueous solutions (concentration range 50–20.000 mg/l) by the sodium-form (Na-form) of HEU-type zeolite crystals (particle-size <20 m) has been investigated by means of a batch-type method. The INAA, RI-XRF, powder-XRF, SEM-EDS and FT-IR techniques were used for the study of the experimental products. The absolute uranium uptake by the material reached the value of 11.68 mg/g in the case of initial concentraton 20,000 mg/l. On the other hand theK _d -values indicated that the relative uranium uptake, and consequently the percentage of removal, is higher for concentrations below 100 mg/l. The uranium uptake by the zeolite is attributed to different sorption processes such as ion-exchange, adsorption and surface precipitation, taking place both to the interior and the surface of the crystals and strongly depending on the pH of the solutions. The investigated zeolitic material was sufficiently resistant at the low initial pH of the solutions with dealumination phaenomena only observed in the case of the most acidic solution used.     

Characterisation of thin sputtered silicon nitride films by NRA, ERDA, RBS and SEM

Summary Thin, amorphous silicon nitride (a-SIN_x) films were deposited on n-type (100) silicon substrates using an argon ion beam for sputtering a HPSN block under high vacuum conditions. The substrates were kept at room temperature. Nitrogen depth distributions were determined by NRA using the resonance reaction ¹⁵N(p,)¹²C at 429 keV. Hydrogen profiles were analysed by NRA (¹H(¹⁵N,)¹²C at E_o=6.385 MeV) and by ERDA (²⁰Ne²⁺, E_o=10 MeV). The NRA was used to determine the depth distributions (concentration vs. areal density) of nitrogen and hydrogen taking calibration standards into consideration. The silicon depth distributions and the N/Si ratios of the deposited a-SiN_x films were determined by RBS (⁴He⁺, E_o=2.0 MeV). Film thicknesses were obtained by SEM. The density of the deposited a-SiN_x films was found to be =2.7 (±0.1) g/cm³ by correlating RBS data and real film thicknesses as obtained by SEM.     

Investigation of HEU-type zeolite crystals after interaction with Sr2+ cations in aqueous solution using nuclear and surface analytical techniques

Pure HEU-type zeolite (heulandite) crystals were allowed to interact with Sr²⁺ cations in aqueous solution. The powdered solid experimental products obtained from batch-type sorption experiments, using solutions of Sr²⁺-concentrations between 10 and 1000 mg/l, were investigated using INAA, RI-XRF and SEM-EDS. The Sr uptake by the mineral which can adequately be described with a Freundlich-type isotherm, varies from 3.14 to 6.22 mg/g. The distribution coefficients increase progressively by decreasing the solutions concentration reaching a value of 1800 ml/g. The investigated zeolite interacts with Sr²⁺ cations through ion exchange reactions and initial exchangeable Ca²⁺ cations are replaced into the structural micropores. However, in the best case, only ca. 43% of the theoretical CEC can be covered because of the limited availability of the extra framework Ca²⁺ cations that can be removed from the lattice under ambient treatment conditions. The XPS investigation of Sr-loaded single crystals indicated that adsorption of Sr²⁺ cations on the outer surface also occurs while surface precipitation phenomena must be excluded. Similar surface analyses by means of¹²C-RBS showed that the Sr depth-distribution at near-surface layers is quite homogenous in contrast to a previous relevant study revealing an intense surface Sr-accumulation on a natural Ca-zeolite of different structural characteristics (scolecite).     

Sorption of caesium and strontium on clinoptilolite-and mordenite-containing sedimentary rocks

The sorption of caesium and strontium from its aqueous solutions by ten clinoptilolite-and mordenite-containing sedimentary materials from Slovakia, Bulgaria, Ukraine and Greece was investigated by batch-type procedure and radiotracer techniques. The concentration of the solutions, that were performed in and without the presence of competing cations (0.005 mol·dm⁻³ KCl) varied between 1·10⁻⁴ and 5·10⁻² mol·dm⁻³. The uptake and distribution coefficient (K _d ) values determined for the materials of the different origin, were correlated with their mineralogical composition and gross cation exchange capacity (CEC). The identification of the specific uptake sites was attempted on the basis of the sorption isotherms and the content of exchangeable cations. The experimental results provide information on the suitability of the individual materials for the treatment of radioactive wastes and their application as backfills in potential nuclear waste repositories.     

Uranium-bearing francolites present in organic-rich limestones of NW Greece: a preliminary study using synchrotron radiation and fission track techniques

Synchrotron radiation techniques (μ-XRF and μ-XANES) were applied to the study of organic-rich phosphatized limestones of NW Greece (Epirus). The results revealed uranium accumulation in areas of the material containing, among others, carbonate apatite (francolite) and organic matter. The UL ₃-edge of μ-XANES spectra showed that uranium was present in tetravalent form. U-bearing francolite crystals were separated from the rock and characterized by Raman spectroscopy and microprobe. The analysis of the crystals also indicated the presence of sodium and sulfur. The uranium presence in the crystals was also visualized, after neutron irradiation and etching, by the observation of the fission tracks.

     

Thorium(IV) and neptunium(V) uptake from carbonate containing aqueous solutions by HDTMA-modified natural zeolites

The ²³²Th-uptake ([Th(IV)]° = 9.7 × 10^?5 M) from carbonate solutions ([CO ₃ ² ]_tot = 0.25 M, 9.0 < pH_c < 10.8) by raw and HDTMA-modified HEU-type zeolitic-, chabazitic- and phillipsitic-tuffs was investigated. The strong uptake by the HDTMA-tuffs at pH_c≈9 was assigned to the Th(CO₃) ₅ ^6? and ThOH(CO₃) ₄ ^5? predominance. The sorption coefficients (R _d) decreased with increasing pH_c indicating carbonate competition. Enhanced R _d values for pH_c > 10.5 are likely due to ThO₂(am)-precipitation. The ²³⁷Np-uptake ([Np(V)]° = 2.6 × 10^?5 M) from carbonate solutions ([CO ₃ ² ]_tot = 0.25 and 3.0 × 10^?4 M) by raw and HDTMA-modified HEU-type zeolitic tuff and pulverized pure heulandite crystals was studied under Ar-atmosphere at 6 < pH_c < 11. The R _d values for both elements indicated the modified tuffs potential to remove tetravalent- and pentavalent actinides from environmental matrices.