Volume 91, Issue 9 -

A method is given for the preparation of a Pu tracer solution in 0.10M NaHSO 4 containing only Pu(IV). This solution is stable for several years.

Neptunium(IV), being quite stable in 0.5–3M HNO 3 , undergoes quick oxidation in the presence of 1 Pt/SiO 2 catalyst. It was found that the catalytic oxidation of neptunium(IV) in nitric acid solutions proceeds even in the presence of anti-nitrous agent (such as NH 2 SO 3 H), though the rate of the reaction decreases to some extent. The kinetics of the heterogeneous catalytic oxidation of neptunium(IV) in nitric acid were studied for the following conditions: [HNO 3 ] = 0.5–4.0M; [NH 2 SO 3 H] = 0 and 0.1M; T = 40–70°C; S/L = 0.025–0.1g/mL 1 Pt/SiO 2 . It was proposed that the mechanism of the neptunium(IV) oxidation is a result of the heterogeneous interaction of the Np(IV) aqua-ion with the nitronium ion NO 2 + on the surface of the catalyst. The reaction deceleration and the increase of the apparent activation energy with the increase of the nitric acid concentration is explained by the complex formation of neptunium(IV) with nitrate ion.

The uranyl iodate system has been studied by using three speciation methods: laser-induced time-resolved fluorescence spectroscopy (TRLIFS), UV-visible spectrophotometry and capillary zone electrophoresis with direct UV detection (CZE). Two uranyl iodate complexes were found: UO 2 IO 3 - and UO 2 (IO 3 ) 2 in 0.05M NaClO 4 solutions of pH 2.6, depending on the iodate concentration. The three methods are convenient and efficient for determining the complex formation constants of the two complexes. The variation of the uranyl mobility as a function of the ligand concentration is very simple to analyse and its fitting leads to accurate thermodynamic data. The values obtained for log β 11 (1.6±0.1) and log β 12 (3.05±0.1) are in good agreement with the previously published values. Both complexes were also characterized by their fluorescence spectra and fluorescence lifetime. From the analysis of the fluorescence spectra, taking into account the dynamic quenching, the complex formation constants were calculated, in reasonable agreement with the ones obtained by capillary electrophoresis: log β 11 (1.5±0.2) and log β 12 (3.6±0.2).

The influence of humic acids (HA) on the retention of thorium (IV) onto the surface of silica colloids is investigated. Thorium is considered as an analogue of tetravalent actinides (U, Np, Pu), except for the fact that it has no f electrons. Silica (SiO 2 ) is chosen as a model surface because it is a component of many minerals and because of its weak HA sorption properties. Retention experiments are performed by batch procedure at constant ionic strength ( I = 0.1M NaClO 4 ), at various pH (2 to 9) and HA concentrations (1–100mg/L). The sorption of Th(IV) onto colloidal amorphous silica is characterised using a surface complexation model. The ternary system ( i.e. HA-Th-Silica) clearly shows the influence of HA on Th(IV) retention. This can lead to a strong reduction of the amount of Th(IV) sorbed onto silica in the presence of HA compared to silica colloids without HA, due to a predominant part of thorium present in solution as humic complexes. In a pH range where no organic coating onto silica occurs, there is a competitive reaction between Th(IV) sorption onto the silica surface and onto HA reactive functional groups. In this case, Schubert´s method is applied to obtain a global interaction constant for the Th(IV)-HA system. The large interaction constant values indicate a strong affinity of HA for Th(IV).

Citric acid, a naturally occurring hydroxycarboxylic acid, forms bidentate, tridentate, dinuclear, or polymeric species, depending on the metal. In the presence of iron and uranium a ternary Fe: U: citric acid complex is formed. We determined the molecular structure of the ternary complex in both the aqueous and solid phase. Fourier transform infrared spectroscopy (FTIR) showed the presence of uni- and bi-dentate bonding of the citric acid to the metals, and the involvement of the α -hydroxyl group. Analysis of molecular fragments generated from time-of-flight secondary ion mass spectroscopy (TOF-SIMS) confirmed the bonding of terminal carboxylate groups of citric acid to uranium and iron. Extended X-ray absorption fine structure (EXAFS) analysis revealed the dinuclear nature of the Fe bonding to citrate and mononuclear uranium species with citrate. Coordination of a sodium atom to the iron was also noted. The proposed empirical formula for the complex is [NaFe 2 (μ-O)(μ-citrato)(OH) 2 (H 2 O)(C 6 H 4 O 7 )bis(UO 2 )(C 6 H 5 O 7 )] 7- . The structure consists of a dinuclear ferric ion core coordinated through an oxy (μ-O) and a carboxylate (μ-citrato) group of citric acid. The mononuclear uranyl ions are coordinated in bidentate fashion to the central carboxylate groups of citric acid and tridentate coordinated to citric acid.

The sorption of Hg(II) ions onto coconut ( Cocos nucifera ) husk has been studied using radiotracer technique. Maximum sorption (96) of Hg(II) ions (7.39×10 -6 M) onto sorbent surface is achieved from 10 -3 M HNO 3 solution in 30min agitation time using 100mg of coconut husk. The sorption data follow the Freundlich and Dubinin–Radushkevich (D–R) isotherms. Sorption capacity (6.84 ± 0.45 mmol g -1 ) and sorption energy (10.6 ± 0.13 kJ mol -1 ) have been evaluated using these isotherms. Among the ions tested to monitor their influence on the sorption, Ba(II), fluoride and tartarate increased the sorption, while thiosulfate, bromide and thiocyanate reduced (< 63 > 26) the sorption. The cations K(I), Ce(III), Cr(III), Fe(III) and Zr(IV) partially suppressed the sorption. The variation of sorption with temperature yields thermodynamic parameters Δ H = -37.42 kJ mol -1 Δ S = -1057J mol -1 K -1 and Δ G = -2.58 ± 0.05 kJ mol -1 at 298 K. The negative values of enthalpy and free energy reflect the spontaneous and exothermic nature of sorption, respectively. The selectivity studies of sorbent show that the coconut husk column can be used to separate Hg(II) ions from Se(IV), Zn(II), I(I) and Tc (VII). The sorbent has a potential in radiochemistry to separate gamma energies of 203 Hg (279 keV) from 75 Se (265 and 280 keV).

Performance tests were conducted using novel getter materials that can immobilize or delay the transport of radioiodine that would be released during physical and chemical degradation of solidified low-level waste packages. The results showed that metal-capped novel getters such as Hg-thiol and Ag-thiol Self-Assembled Monolayers on Mesoporous Silica (SAMMS), designed specifically to adsorb soft anions such as I - , had very high affinities for adsorption of radioiodine ( K d ∼ 1×10 4 – 4×10 5 ml/g). The iodide adsorption performance of these novel getters was from one to two orders of magnitude better than many natural mineral and modified mineral getters. These data indicate that the novel getter materials are capable of significantly retarding the mobility of radioiodine leaching from physically and chemically weathered low-level waste glass packages during various physical and chemical weathering reactions expected during long-term disposal in the vadose zone.

The principal objective pursued in this study is to establish the base-line data on the status of elemental composition in human milk from Pakistani subjects of Rawalpindi/Islamabad area. Radiochemical neutron activation analysis (RNAA) methodology was developed and successfully employed to determine the concentration of 18 minor and trace elements (essential, toxic and nonessential) in human milk. This methodology has significantly improved the detection limits of most of these elements due to suppression of Compton background. The data provide the base-line values of these elements in human milk of low- and medium-income group subjects of the region. The results obtained show good compatibility with the data reported by the WHO on elemental composition of human milk from different geological regions.

The distribution coefficients of Ra 2+ and Ac 3+ on cryptomelane MnO 2 from acidic aqueous solutions were determined in order to find the best conditions for separation of both cations. Very high affinity of cryptomelane MnO 2 for Ra 2+ ions make possible to separate 225 Ac from 225 Ra, and 228 Ac from 228 Ra in single-step and rapid procedure. The obtained results enable to design simple and effective generators of 225 Ac and 228 Ac from their mother radium isotopes.