Volume 95, Issue 3

Excitation functions for 201 Pb, 202m Pb, 203 Pb and 204m Pb radionuclides which are formed via proton induced reactions with natural thallium target have been measured from their respective threshold ( E thr ) to 27.5 MeV using activation technique. Natural copper foils were used to monitor the cyclotron beam. The integral yields (MBq/μA h) of the produced radionuclides were calculated from the measured excitation functions. The optimum proton energy range for the production of 203 Pb with low amount of impurities is (16–10 MeV) after 5 h of EOB. The experimental cross-sections for nat Tl( p , xn ) reactions were compared with the cross-sections recommended by the IAEA and with earlier published data when it was possible.

Activation of natural Sb 2 O 3 with 48 MeV 7 Li 3+ beam results in the formation of no-carrier-added 123 I in the matrix along with the radionuclides 123,125 Xe and 122 Sb. The 123 I yield amounts to about 400 kBq/μAh and the radionuclidic impurity of 124 I to ∼1.2% of 123 I. Attempts to separate no-carrier-added iodine from bulk antimony target involved liquid-liquid extraction with TOA and HDEHP as well as precipitation of Sb 2 S 3 with thioacetamide. The precipitation technique was found to be the most effective for quantitative separation of 123 I from the bulk antimony oxide target.

Sorption of tracer curium, Cm 3+ , on SiO 2 · H 2 O (silica gel) was investigated under different experimental conditions such as time of equilibration, concentration of Sm 3+ , pcH, and temperature. Sm 3+ was used as a carrier for Cm 3+ ions in this study. From the sorption kinetics data, the diffusion coefficient of Cm 3+ ion was calculated to be 1.20(±0.05)×10 -11 m 2 s -1 under particle diffusion-controlled conditions. The sorption rate was determined to be 3.47(±0.19)×10 -3 s -1 at 298 K, pcH 5.25(±0.03) in 0.20 M (NaClO 4 ). The sorption data were fitted in Freundlich, Langmuir and Dubinin–Radushkevich (D–R) isotherms. A synergistic enhancement in the sorption of Cm 3+ (Sm 3+ )-ligand ( e.g. , CO 3 2- , F - , H 2 PO 4 - and C 2 O 4 2- ) complexes on silica gel was observed. The presence of these anions increase the sorption of Cm 3+ (Sm 3+ ) in the following order: CO 3 2- < F - < C 2 O 4 2- < H 2 PO 4 - . Temperature variation experiments gave enhanced sorption with increased temperature for Cm 3+ indicating an endothermic enthalpy.

Colloid formation and solubility of U(IV) in acidic HClO 4 /NaClO 4 solutions are investigated by coulometric titration. Quantification of traces of U(VI) by laser fluorescence spectroscopy proves that the tetravalent state of uranium had been maintained. Laser-induced breakdown detection (LIBD) is applied for the detection of traces of uranium colloids as the pH is increased. The pH values at the onset of colloid formation are used for thermodynamic calculations aimed at determining the solubility products K ° sp of crystalline and amorphous uranium dioxides. The particle size of the colloids determined by means of LIBD is taken into account since it influences the solubility product. The results indicate the formation of crystalline UO 2 (cr) at low pH∼1 (log K ° sp = -59.6±1.0) whereas the amorphous hydrous oxide UO 2 · x H 2 O(am) is formed at pH ∼ 3 (log K ° sp = -54.1 ± 1.0). Measurements by EXAFS, XRD and REM confirm the occurrence of these different phases. The obtained solubility products fit well in the known series of solubility products of the other tetravalent actinides.

The complex formation of uranium(VI) with salicylhydroxamic,benzohydroxamic, and benzoic acid in 0.1 M NaClO 4 was studied by UV-vis spectroscopy at pH 3 and 4. Uranium(VI) species of the type M p L q H r were identified from the UV-vis spectra in all three systems. An increase in the absorption combined with a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 22.5 ± 2 nm was observed in the uranium (VI)-salicylhydroxamic acid-system. Besides indications for a 1:2 complex, the formation of a 1:1 complex with a stability constant of log β 111 = 17.12 ± 0.10 could be demonstrated by its individual absorption spectrum and molar absorption coefficient. Also in the uranium(VI)-benzohydroxamic acid-system a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of 27 ± 1.4 nm indicate the complex formation. The stability constants are log β 110 = 7.96 ± 0.05 for UO 2 [C 6 H 4 CONHO] + and log β 120 = 15.25 ± 0.11 for UO 2 [C 6 H 4 CONHO] 2 . In contrast to the hydroxamic acids, benzoic acid shows a red shift of the absorption maxima of 2.5 ± 2 nm. Only the 1:1 complex UO 2 [C 6 H 4 COO] + with a stability constant of log β 110 = 3.37 ± 0.14 is existent. An estimate is made in order to discuss the dependencies observed in the absorption spectra in relation to possible coordination modes of uranium(VI). The strength of the complex formation between uranyl and the three aromatic acids is discussed.

A novel, simple technique based on magnetically assisted chemical separation (MACS) has been developed for the uptake of lanthanides and actinides from pure nitric acid solutions as well as from Simulated Pressurized Heavy Water Reactor-High Level Waste (PHWR-SHLW). Uptake profiles of various metal ions, such as Pu(IV), U(VI), Th(IV), Am(III), Eu(III), Sr(II), Cs(I) and Fe(III) were investigated as a function of time and nitric acid concentrations using N , N , N ′, N ′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) impregnated magnetic particles and compared with N , N ′-dimethyl- N , N ′-dibutyl tetradecyl malonamide (DMDBTDMA) and trialkyl phosphine oxide coated magnetic particles and also with TODGA coated extraction chromatographic resins. The uptake of various metal ions follows the order Eu(III) > Am(III) > Th(IV) > Pu(IV) > U(VI) > Sr(II) > Fe(III) > Cs(I) in pure nitric acid solutions. On the other hand, distinctly high distribution coefficient ( K d ) value has been observed for Pu(IV) as compared to Eu(III) and Am(III) in SHLW. Further, no significant extraction of Cs(I) or Sr(II) was observed in SHLW. Also, at any acidity the K d value of a given metal ion is less in the SHLW as compared to that in pure HNO 3 , apparently due to the co-extraction of the other metal ions present in SHLW. The loading capacity of the TODGA impregnated magnetic particles with respect to Th(IV), U(VI) and Eu(III) was determined, along with the distribution isotherms to simulate multiple contacts. The adsorption models of Langmuir and Freundlich were fitted to the experimental data and best correlations was obtained for Langmuir model suggesting monolayer adsorption is the prevalent mechanism. The stability and reusability of the TODGA coated magnetic particles was also assessed.

TRLFS has been used to study the U VI O 2 2+ -PO 4 3- system, in the 1 to 5 pH range, for [PO 4 3- ] = 2 × 10 -3 to 1.02 × 10 -1 M range and [UO 2 2+ ] = 1.2 × 10 -5 M. The data acquisition procedure was based on four different sets of acquisition parameters, which were chosen in order to maximize the fluorescence signal for four different time domains. The fitting procedure used to simulate the corresponding fluorescence decays curves was based on the crosscheck of the fitting results and on the inter-comparison between results for solutions of different concentrations. Five different lifetimes τ 1 to τ 5 were evidenced and associated with five uranyl species (UO 2 (H 2 PO 4 ) 2 , UO 2 H 2 PO 4 + , UO 2 HPO 4 , UO 2 PO 4 - and UO 2 2+ ). The fitting results, moreover, allowed the distinction between mono-, bi- and tri-exponential fluorescence decay behaviours. The isolated fluorescence spectra showed an expected wavelength shift between the fluorescence spectra of all the complexes, except in the case of the (UO 2 (H 2 PO 4 ) 2 , UO 2 H 2 PO 4 + ) couple, which spectra were not distinguishable. Finally, the obtained results have shown that the data acquisition and fitting procedure enabled a complete and reliable characterization of the studied solutions.

The radioactive isotopes 131 I and 82 Br were used to trace the ion-isotopic exchange reactions using industrial grade ion exchange resins Amberlite IRA-400. The experiments were performed to understand the effect of temperature and concentration of ionic solution on kinetics of exchange reactions. Both the exchange reactions were greatly influenced by rise in temperature, which result in higher percentage of ions exchanged. For bromide ion-isotopic exchange reactions, the calculated values of specific reaction rate/min -1 , and amount of ions exchanged/mmol were obtained higher than that for iodide ion-isotopic exchange reactions under identical experimental conditions. The observed variation in the results for two ion-isotopic exchange reactions was due to the difference in the ionic size of bromide and iodide ions.

The heterogeneous isotopic anion exchange kinetics and equilibria between calcium molybdate and sodium molybdate solutions have been studied by using 99 Mo as tracer in batch experiments. The values of exchange ratio lower than unity suggest that rate-limiting step is particle diffusion process and the effect of re-crystallization can be neglected. The self-diffusion coefficients calculated using both Paterson´s and Nernst–Plank approximations are increased by the temperature. The observed values for isotope exchange characteristics such as exchange fractions, exchanging amounts and fractional attainment of equilibrium are consistent with those of their calculated values. Activation energy and thermodynamic parameters calculated based on transition state theory indicate the existence of both energy and entropy barrier in the system.

A chelating resin was synthesized by incorporating amide group into Amberlite IRC-50, a weakly acidic polymer. The new resin was employed in the separation of 137m Ba from its parent 137 Cs from an equilibrium mixture. The adsorbed daughter was recovered using 0.01 M EDTA at pH 10.