Volume 108, Issue 3

The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (An V,VI O 2 +/2+ (aq) ) form strong complexes with available ligands, like carbonate (CO 3 2− ), which may inhibit reduction to relatively insoluble An IV O 2(s) . Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO 2 (CO 3 ) 3 ] 5−/4− ) with two different reductants, [Fe(OH) 2 (H 2 O) 4 ] 0 and [H 2 S(H 2 O) 6 ] 0 . Energetically-favorable outer-sphere complexes (OSC) are found to form rapidly, on the order of milliseconds to seconds over a wide actinyl concentration range (pM to mM). The systems then encounter energy barriers ( E a ), some of which are prohibitively high (>100 kJ/mol for some neptunyl and plutonyl reactions with Fe 2+ and H 2 S), that define the transition from outer- to inner-sphere complex (ISC; for example, calculated E a of ISC formation between UO 2 + and UO 2 2+ with Fe 2+ are 35 and 74 kJ/mol, respectively). In some reactions, multiple OSCs are observed that represent different hydrogen bonding networks between solvent molecules and carbonate. Even when forming ISCs, electron transfer to reduce An 6+ and An 5+ is not observed (no change in atomic spin values or lengthening of An–O ax bond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe 2+ to U 6+ and Pu 6+ . Not all proton transfer reactions yielded reduction of An 6+ to An 5+ and only a few pathways were energetically-favorable (e. g. H + transfer from H 2 O to drive Pu 6+ reduction to Pu 5+ with Δ E  = −5 kJ/mol). The results suggest that the tricarbonate complex serves as an effective shield against actinide reduction in the tested reactions and will maintain actinyl solubility at elevated pH conditions. The results highlight reaction steps, such as inner-sphere complex formation and electron transfer, which may be rate-limiting. Thus, this study may serve as the basis for future research on how they can be catalyzed by a mineral surface in a heterogeneous process.

An improved apparatus is used for nucleation measurements according to Nielsen’s method. A new method is proposed to calculate the dilution ratio N of the reaction solution during nucleation rate determination. With the rule, when the initial apparent supersaturation ratio S′  =  f ( N ) in the dilution tank is controlled from 1.2 to 2.7, crystal nucleus dissolving and secondary nucleation can be avoided satisfactorily. Experiments are realized by varying the supersaturation ratio from 26.0 to 297.5 and temperature from 30 °C to 50 °C. Uranium(IV) oxalate is precipitated by mixing equal volumes of tetravalent uranium nitrate and oxalic acid solution. The experimental results show that the nucleation rate of uranium(IV) oxalate in the supersaturation range as show above is characterized by the primary homogeneous mechanism and can be expressed by the equation RN=ANexp(−Ea/RT)exp[−B/(ln S)2],${R_N} = {A_N}{\rm{exp}}( - {E_a}/RT){\rm{exp}}[ - B/{({\rm{ln }}S)^2}],$ where A N  = 1.9 × 10 27 m −3 s −1 , E a  = 71.2 kJ mol −1 , and B  = 34.3. The crystal growth rate can be expressed by the equation G(t)=kgexp(−E′a/RT)(c−ceq)g,$G(t) = {k_g}{\rm{exp(}} - {E^{\prime}_a}/RT{\rm{)(}}c - {c_{{\rm{eq}}}}{{\rm{)}}^g},$ where k g  = 7.1 × 10 5 (mol/L) −0.98 (m/s), E′a=33.1 kJ mol−1,${E^{\prime}_a} = 33.1 \ {\rm{ kJ \ mo}}{{\rm{l}}^{ - 1}},$ and g  = 0.98. The results indicate that the crystal growth of tetravalent uranium(IV) oxalate is controlled by the BCF model.

A two-dimensional lanthanum(III) porous coordination polymer was prepared, characterized and applied as an efficient adsorbent for the removal of uranium from aqueous solution. Lanthanum(III) was the metal center of MOFs, and the deprotonated anions of pyridine-2,6-dicarboxylic acid (H 2 PDA), PDA 2− was the organic ligand, this MOF was name as La-PDA, which was synthesized by hydrothermal reaction method. Scanning electron microscope (SEM), Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD) and thermal gravimetric (TG) analysis were used for characterization, and the results indicated that the La-PDA composites were successfully prepared. Compared with traditional adsorbents of uranium, La-PDA showed excellent adsorption properties. The adsorption capacity was 247.6 mg g −1 at 298 K and pH 4.0. The adsorption equilibrium achieved within 120 min, and the adsorption process was exothermic and spontaneous. The absorption mechanism of La-PDA was also explored, from the XPS spectra, the pyridine-like nitrogen atoms (C=N–C) and carboxyl oxygen atoms (–COO–) contributed to the adsorption of uranium. The results suggested that PDA 2− was a potential ligand of uranium adsorption, La-PDA composites were effective adsorbents for the removal of uranium from aqueous solution.

Scandium-47 is one of the most useful radioisotopes which is gaining great importance in cancer theranostics applications due to its favorable nuclear and chemical properties. MCNPX2.7.0 code was used to simulate the neutron activation of natural calcium target positioned at a thermal neutron flux of 1.8 × 10 14 n cm −2 s −1 in the Egypt Second Research Reactor (ETRR-2). The burn card was used to calculate 47 Ca and 47 Sc radioactivities during 3 days irradiation and 20 days post-irradiation. The undesirable impurities generated during this period were also calculated. The obtained calculations were found to be in agreement with the experimental measurements. The distribution coefficient value (K d ) of 47 Sc(III) as well as 47 Ca(II) ions was determined using the commercially available ion-exchanger Chelex 100 in HNO 3 and/or HCl media. Radiochemical separation of 47 Sc(III) from 47 Ca(II) was studied using HNO 3 and HCl solutions and the results showed that HNO 3 is a better medium than HCl for complete retention and recovery of 47 Sc(III), where the recovery yields were 85 ± 1.2 and 95 ± 0.87 % using 1 M HCl and 1 M HNO 3 solutions, respectively. The recovery yield obtained in our work was higher than in the reported procedures. Radionuclidic, radiochemical and chemical purities were investigated to ensure the suitability of 47 Sc(III) for nuclear medicine applications.

Selenium (Se) has been a focus of attention as an important micronutrient with its impact on human health, with there being consequences either due to excess or deficiency in intake. Radiochemical neutron activation analysis (RNAA) is well established method for determination of a number of elements at trace level concentrations with high sensitivities. In this work, we used the RNAA method for determination of selenium content in Mentha spicata L. The result of this study was compared with those samples from India and Serbia. The result obtained show that, Se concentration obtained in M. spicata L., is close to the minimal FAO recommendation.

The current study covers the effect of gamma rays on a natural biopolymer Gum Arabic (GA). Gum acacia has a very wide range applications in various fields, that a comparison between two dosimetry systems; the first system depends on the radiolysis of GA (mixing poly-vinyl alcohol) as a thin-film dosimeter. The second system depends on the same material GA as solutions dosimeter, which is more sensitive to γ-radiation due to free mobility of free radicals released by the action of gamma rays. The prepared GA/film/solutions have a considerable peak at 289 nm, which increases (cross-linking of GA polymer) upon irradiation until 28 and 6 kGy for two systems, respectively in case of the first step. However, upon increased the absorbed doses and the intensity of the signal decrease with increasing radiation dose (degradation of GA polymer) up to 80 and 15 kGy for film and solutions. Radiation chemical yield G-(value); dose-response function; gamma radiation sensitivity; also pre- and post-irradiation stability under various conditions were illustrated.

In the current study, we employ the polyvinyl alcohol doped with nickel sulphate (PVA-Ni +2 ) as a new film for monitoring the absorbed dose in radiation processing facility. The effect of irradiation doses on the structure of PVA-Ni +2 composite films was analyzed through Fourier transform infrared and X-ray diffraction techniques. The PVA-Ni +2 composite films have a response curve that extends from 5 to 60 kGy and labeling character figured through color change from pale green to intense purple color on exposing to gamma photons. Although the films are dose rate dependent, the films have excellent post stability.