Volume 91, Issue 12 -

The centennials of the discovery of radioactivity by Antoine Henri Becquerel in 1896 and of the discovery of polonium and radium by Pierre and Marie Curie in 1898 have been amply celebrated. In 1903 the Nobel Prize for Physics was awarded to the three scientists. The vicissitudes of the awarding and sharing of the prize are reviewed.

Irradiation experiments were performed at the HIMAC (Heavy Ion Medical Accelerator in Chiba) facility, National Institute of Radiological Sciences, Japan. The radioactivity distributions of spallation products in a thick Cu target were obtained by irradiating 230 and 100MeV/nucleon Ne, C, He, p and 230MeV/nucleon Ar ions. The gamma-ray spectra from thin irradiated samples of C, Al and Cu inserted into a Cu target were measured with a HPGe detector. From the gamma-ray spectra, we obtained the spatial distribution of radioactive yields of spallation products of about 40 nuclides in Cu sample in the Cu target. Our results agree with other experimental data. We also calculated the spatial distribution of residual radioactivities in the Cu target by the PHITS (Particle and Heavy-Ion Transport code System) code and compared with measured results. The PHITS code provides good results on residual activity calculations.

Metal-Affinity metalloprotein chromatography has been suggested to have potential for selectively removing actinides from solution. To evaluate this potential, characterization of Pu(IV) and Fe(III) binding to an ovotransferrin-Sepharose™ 4B matrix, with oxalate and carbonate synergistic anions, was performed. Metal binding capacity was determined for metal concentrations ranging from 2×10 -6 to 1×10 -3 M Fe(III) and 2.9×10 -5 to 2.6×10 -4 M Pu(IV), where both metals were added as a solution of 10:1 nitrilotriacetic acid:metal. Metal binding capacity was also determined over the pH range 3.0 to 9.2. The metal binding properties of the immobilized ovotransferrin were found to differ from those reported for free-protein studies. Distribution coefficients were low: for metal solution concentrations of 0.02, 0.04, 0.10, and 0.20mM, coefficients were 540, 180, 23, and 12mL/g for Fe(III) and 20, 12, 7.2, and 6.0mL/g for Pu(IV), respectively. The capacity of the material was less dependent on pH than expected, varying by only two μmoles metal/g matrix for both Fe(III) and Pu(IV) over the pH range of 4.2 to 9.5. This study illustrates limitations of metalloprotein chromatography for Pu(IV) separation.

The extraction behaviour of plutonium (IV) was investigated from nitric acid and perchloric acid using 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) as well as a mixture of HPBI and tri- n -octyl phosphine oxide (TOPO) in xylene. Studies on the binary extraction system Pu(IV)-HPBI indicated extraction of species of the type Pu(PBI) 4 . In the presence of TOPO, while species of the type Pu(PBI) 3 (ClO 4 )·(TOPO) and Pu(PBI) 4 ·TOPO were co-extracted from perchloric acid medium, a mixture of Pu(PBI) 2 (NO 3 ) 2 ·2TOPO, Pu(PBI) 3 (NO 3 )·(TOPO) and Pu(PBI) 4 ·TOPO were co-extracted from nitric acid medium. In addition to the electronic factors, steric factors were found to play a significant role during the formation of binary/ternary/quaternary complexes. The steric hindrance due to the unusual conformation of the isoxazolone led to lowering of the equilibrium constants for both the binary and ternary extraction systems. Possible steric factor influences are explained with the help of molecular modelling.

Plutonium (Pu) is one of the primary actinides of concern for long-term disposal and storage of nuclear waste. Strong sorption of Pu onto colloids of iron oxide, clay, and silica could result in colloid-facilitated transport of this actinide in groundwater systems. However, fundamental data on Pu sorption to colloids is sparse, resulting in large uncertainties in long-term predictions of colloid-facilitated Pu transport. This sparseness of data and the potential to significantly reduce uncertainties in predictive models served as a motivation for this study. The authors investigated the sorption and desorption behaviors of 239 Pu(V) on three types of inorganic colloids (hematite, montmorillonite and silica), with adsorption being measured as a function of temperature, ionic strength and colloid concentration for each colloid. Natural ground water collected from Well-13 near Yucca Mountain, Nevada, and synthetic ground water (SYN.J-13) were used in the experiments. The results indicated that the adsorption of 239 Pu(V) onto hematite colloids was faster and had a higher partition coefficient ( K p , ml g -1 ) than onto montmorillonite or silica colloids in both J-13 and SYN.J-13 waters. Temperature did not significantly influence the adsorption of 239 Pu(V) onto hematite and silica colloids, but the adsorption of 239 Pu(V) onto montmorillonite colloids increased significantly with increasing temperature. While ionic strength did not significantly influence the adsorption of 239 Pu(V) onto hematite colloids, an increase in ionic strength decreased the adsorption of 239 Pu onto montmorillonite and silica colloids. Adsorption of 239 Pu(V) per unit mass of colloid was much higher at low colloid concentrations than at high colloid concentrations. Desorption of 239 Pu from 239 Pu-loaded colloids was considerably slower than the adsorption process.

The sorption of Np(V) to synthetic hydroxyapatite was determined in batch experiments in a 0.1M NaClO 4 solution. The hydroxyapatite used was of high purity as determined by SEM, EDS, XRD, FT-IR and ICP-MS analysis. Results from kinetic experiments with an initial Np(V) concentration of 1×10 -7 to 1×10 -6 M indicate the sorption process is relatively fast with more than 90 of the Np(V) being sorbed in approximately 3 hours. Equilibrium experiments performed over the pH range of 6 to 11 indicated sorption is strongly pH dependent with distribution coefficients, K d values (mL/g), increasing from 123L/mole at pH 6 to 69200L/mole at pH 8.5. K d values are observed to decrease as pH further increases. Data points over a range of Np(V) concentrations were collected at pH 8 and fitted to the Langmuir isotherm model for simple adsorption. The Langmuir equation gave an excellent representation of the data. Langmuir parameters were determined to be C a =0.032mole/mole and K =1.22×10 6 L/mole, indicating the high affinity of hydroxyapatite for Np(V) adsorption.

The extraction behaviour of Uranium (VI) from aqueous nitric acid medium employing 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) has been studied in the presence of various amides, e.g , di-2-ethylhexylacetamide (D2EHAA), di-2-ethylhexylpropanamide (D2EHPrA), di-2-ethylhexylisobutylamide (D2EHiBA) and di-2-ethylhexylpivylamide (D2EHPvA) in toluene medium. The extraction constant (log k ex ) for the binary species UO 2 (PBI) 2 · n H 2 O was found to be 1.42±0.14 which is by far the largest amongst the corresponding values known for the other β -diketones. The overall equilibrium constant (log K ) for the ternary species UO 2 (PBI) 2 ·D2EHAA, UO 2 (PBI) 2 ·D2EHPrA, UO 2 (PBI) 2 ·D2EHiBA and UO 2 (PBI) 2 ·D2EHPvA were estimated to be 6.51, 6.21, 6.11 and 5.77 respectively. This trend in equilibrium constants was in accordance with the substitution of the amides as sterically crowded amides show relatively lower equilibrium constant values. The extraction constants follow an inverse correlation with the p K a values of the β -diketonates while the equilibrium constants vary linearly with the size of the amides. The steric factors have been found to play a significant role in the co-ordination of the amides to the uranyl β -diketones when the carbonyl group is a part of the heterocyclic ring. The distribution ratios of the metal were measured in the temperature range 15 to 52 °C using each of the amide donors in combination with HPBI. Calculation of enthalpy and entropy therefrom indicated that the extraction was thermodynamically favoured process and maximum extraction resulted in the case of UO 2 (PBI) 2 ·D 2 EHAA.

Cellulose has been functionalized by coupling with 5-chloro-quinoline-8-ol in the presence of alkali. The resulting polymer material has been characterized by IR spectra and has been used for preconcentrating thorium prior to its determination by spectrophotometry. The optimum pH value for quantitative extraction is 5.0–7.0 and can be eluted with 5cm 3 of 1mol dm -3 of HCl. The retention capacity of the quinoline-8-ol modified cellulose is 20.0mg g -1 . The effect of various cations, anions and neutral electrolytes on the preconcentration of thorium in conjunction with the determination procedure has been studied and we have found that none of interferents including uranium (up to 10 fold amounts) interfere. The enrichment factor for preconcentration of thorium was found to be 200. Ten replicate determinations of 10μg of thorium (IV) present in 1dm 3 of sample gave a mean absorbance of 0.084 with a relative standard deviation 1.52. The detection limit (corresponding to three times the standard deviation of the blank) was found to be 2μg dm -3 . The thorium content of mixed rare earth chloride solution obtained from Indian Rare Earths Limited (IRE), India was estimated by using quinoline-8-ol immobilized cellulose for preconcentration and subsequent determination by spectrophotometry.

The humic colloid mediated transport of tetravalent actinide and technetium ions is discussed. Numerous published and new results are used from comparable experimental systems and conditions. The results originate from investigations on two humic rich Gorleben groundwater/sediment systems under near-natural conditions. The conditioning time between groundwater and radionuclides vary from few minutes to four years. The migration time (residence time in the column) is varied from four hours to three days. The concentration of radionuclides added is varied between about 10 -12 and 10 -4 mol/L. Humic colloids in natural groundwater contain trace metal ions in varying concentrations. In addition to studying the behavior of radionuclides added to the groundwater in the laboratory, the dissociation behavior of these natural tetravalent trace metal ions is studied by scavenging dissociating ions with a cation exchanger. The results show a kinetic behavior of the trace metal ion interaction as frequently reported in the literature, i.e. with increasing contact time, slow dissociation modes are progressively populated resulting in an enhanced radionuclide transport. The strong dependency of the radionuclide transport on the radionuclide concentration shows that humic colloid mediated transport may be overestimated if radionuclide concentrations used in laboratory studies are too high. Dissociation of the natural inventory of humic associated tetravalent trace metal ions proceeds with a kinetics comparable to the radionuclides added. This, however, is only observed for a small portion of the inventory and extrapolation beyond the observation range is not permissible. Therefore, the key question for application to predictive modeling of the mobility of tetravalent radionuclides in natural groundwater is if the behavior of the natural trace element inventory is applicable or the behavior of radionuclides added in laboratory studies.

The solubility of ThO 2 (am) in the aqueous Na + -H + -OH - -NO 3 - -H 2 O-EDTA system as a function of pC H⁺ (= -log[H + ]) and variable NaNO 3 (0.5M to 6.0M) has been determined. The experimental observations show that between pC H⁺ values 4.2 to 8.2, a stoichiometric 1:1 Th-EDTA complex forms that completely saturates the added chelate concentration. Th concentrations then decrease linearly with increasing pC H⁺ at pC H⁺ >9.0. These changes in solubility are not predicted by currently available thermodynamic models. The ion-interaction model of Pitzer was used to interpret these solubility data. Thermodynamic analysis indicates that the speciation under basic conditions is dominated by monomeric mixed Th-OH-EDTA complexes. X-ray absorption near-edge spectroscopy confirmed the absence of the higher order ( e.g. dimeric) species. The equilibrium constants for the following reactions were determined from analysis of the solubility data: ThO 2 (am) + EDTA 4- + 2H 2 O ↔ Th(OH) 2 EDTA 2- + 2OH - , log K = -6.0; and ThO 2 (am) + EDTA 4- + 2H 2 O ↔ Th(OH) 3 EDTA 3- + OH - , log K∼-7.5. Pitzer ion-interaction parameters for the Th(OH) 2 EDTA 2- and the Th(OH) 3 EDTA 3- species were calculated. It was also determined that the solubility method for examining the complexation of tetravalent actinides with EDTA is limited to relatively high aqueous EDTA concentrations, relative to the amount of ThO 2 (am) precipitate, owing to the adsorption of the EDTA chelator by the solid phase.

The kinetics of pure Mo, Re and Ru metals, as well as their alloys 52.5at. Mo – 47.5at. Re and 90at. Mo – 10at. Ru electrochemical dissolution during potential controlled electrolysis at a temperature T=25 °C in 0.5–6.0M HNO 3 solutions were studied. The dissolution kinetics for all metal and alloy electrodes under study were found to be of zero order vs. metal concentration in the electrolyte. Rate constants for Mo electrochemical dissolution increased from 1.68×10 -3 to 5.99×10 -3 mol cm -2 h -1 with the growth of HNO 3 concentration in the electrolyte from 0.5 to 4.0M. Further increase of the acid concentration in the solution to 6.0M did not result in the acceleration of Mo electrochemical dissolution. Rate constants of Re and Ru metals electrochemical dissolution increased monotonously from 4.27×10 -4 to 1.7×10 -3 mol cm -2 h -1 for Re and from 1.28×10 -4 to 1.61×10 -3 mol cm -2 h -1 for Ru with the rise of HNO 3 concentration from 0.5 to 6.0M. Dissolution of Mo-Re and Mo-Ru alloys was congruent. With the increase of HNO 3 concentration in the electrolyte from 0.5 to 6.0M, the dissolution rate constant of Mo-Re alloy was increasing from 1.33 to 7.60×10 -3 mol(Mo+Re)cm -2 h -1 . Maximum rates of Mo-Ru alloy electrochemical dissolution from 2.21×10 -3 to 3.28×10 -3 mol(Mo+Ru)cm -2 h -1 were observed in 2.0–4.0M HNO 3 at the electrolysis potential E =1.6V/SCE. The main electrochemical and chemical processes occurring at the metal and alloy electrodes and resulting in the metals dissolution are pointed out.

Complexation of hafnium with two ligands viz. diethylene triamine pentaacetic acid (DTPA) and its dimethyl ester derivative (DMDTPA) has been studied by using time differential perturbed angular correlation (TDPAC) technique. Nuclear quadrupole coupling constant ( ν Q ), asymmetry of the EFG ( η ) and the frequency distribution ( δ ) for these complexes have been obtained. Square antiprism structures have been suggested for these complexes using these parameters. A systematic trend of the ν Q has been observed in these and other hafnium complexes with similar ligands and has been explained in terms of the electronegativity of the donor atoms in the ligands.

Actinide cation–cation complexes in aqueous solution have long been a subject of interest. Since the former work of Sullivan et al. , several complexes of this type have been discovered, all of them involving an actinide(V) oxocation and either another actinide oxocation or a transition metal cation. Although many studies have been devoted to the structure analysis of such complexes, they have been lacking evidences for the definition of a structural model. In this report, EXAFS spectroscopy has been tentatively used, considering two possible interaction modes: bridging or linear Np–Np interaction.