Band 94, Heft 4 -

Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48 Ca with 238 U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283 112 for the 238 U( 48 Ca,3n) 283 112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of element 112 with half-lives ( t 1/2 ) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of 283 112. Therefore, these events could not be unambiguously attributed to the decay of 283 112. In contradiction with earlier publications newer reports from FLNR Dubna claim that 283 112 decays by α-particle emission ( E α = 9.5 MeV) with t 1/2 = 4 s followed by a SF-decay of 279 Ds ( t 1/2 = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of 283 112. However, during this experiment neither short α-SF correlations nor SF coincidences were detected. The conclusion is that 283 112 was not unambiguously detected, neither in Experiment I nor in Experiment II.

Distribution studies on U, Pu and Th have been carried out employing tri- n -butyl phosphate (TBP) as extractant to optimise the conditions for the reprocessing of spent fuel of proposed Th based Advanced Heavy Water Reactor (AHWR). Due to their high basicity and organophilicity straight chain N , N -dialkyl amides have been evaluated for their third phase formation behaviour (usually quantified by limiting organic concentration) during the extraction of thorium vis a vis TBP. Dihexyl derivatives of octanamide (DHOA) and decanamide (DHDA) were found promising in view of higher limiting organic concentration of thorium viz. 33 g and 50 g/L, respectively as compared to 26 g/L for TBP. Extraction profiles of U, Pu and Th have been obtained using these amides and compared with those of TBP. Attempts have also been made to optimise the stripping conditions using aceto-hydroxamic acid (AHA) and acetaldoxime as reductants for the partitioning of Pu from the loaded organic phase.

Extraction of Th(IV) and U(VI) using PEG-2000/(NH 4 ) 2 SO 4 (40% w/w of each) aqueous biphasic system (ABS) with 18-crown-6 (18-C-6) as the extractant was studied at four fixed temperatures in the range 288 to 318 K. The species extracted were identified to be [Th·(18-C-6)](SO 4 ) 2 and [UO 2 ·(18-C-6)]SO 4 by the slope ratio method. The equilibrium constant ( K ) was evaluated for the extraction reactions. The thermodynamic parameters evaluated by the temperature coefficient method at 298 K showed that the reaction is favoured by decrease in enthalpy and counteracted by decrease in entropy. The large decrease in the enthalpy values was attributed to direct bonding of crown ether to the central metal atom ( i.e. the formation of inner sphere complex).

The aqueous complexation of thorium with citrate was investigated under neutral to basic conditions and over a broad range of ionic strengths. The solubility data for ThO 2 (am) as a function of citrate concentration indicate the presence of stable species with citrate-to-metal ratios of between two to three. The dependence of the ThO 2 (am) solubilities on hydrogen ion concentration can also be readily explained by the classical assumption of hydrolysis of the central Th(IV) ion to form mixed thorium-hydroxide-citrate complexes. 13 C NMR spectra of the species in solution confirm that the citrate-to-metal ratio of the species in solution is between two and three and show that the citrate attaches to the metal in a bidentate fashion through oxygens on the α-carboxylate and α-alkoxyl groups, rather than through the carboxylate groups. The 13 C NMR spectra, as well as a density functional theory (DFT) electronic structure study of the presumptive complexes, suggests that the associated α-hydroxyl proton can be displaced during complex formation. These findings indicate an alternative explanation for the observed changes in solubility as a function of hydrogen ion concentration, the displacement of protons from the citrate alkoxyl groups via metal binding. Removal of protons from the alkoxyl groups or hydrolysis of the central Th(IV) cannot be distinguished by thermodynamic measurements, however the species with the α-hydroxyl proton removed ( i.e. ThOH(Cit) 2 5- and Th(Cit) 3 8- ) would appear to better represent the microscopic binding. Apparent equilibrium constants for the solution phase reactions of these species and the hydrous thorium oxide have been calculated as a function of ionic strength.

The radiohalogen, 124 I, was produced by the 124 Te( p , n ) reaction from a glassy 124 TeO 2 /6%Al 2 O 3 melt on an 11 MeV proton cyclotron. A 20-degree inclined target withstands beam currents up to 20 μA with no physical damage to the substrate surface. Radioiodine was removed from the glassy melt by dry distillation techniques and trapped on cooled platinum wire. The radiochemical purity, determined by ion chromatography, shows a stable product in the iodide form over a lifetime of weeks.

Electrodeposition of rhodium metal on a copper backing was performed in acidic sulfate media using RhCl 3 · 3 H 2 O, Rh 2 (SO 4 ) 3 (recovered from hydrochloric acid solution) and also the commercially available Rhodex plating baths. This work describes the development of a high current density (2.4 A·cm -2 ) electrodissolution system that allows solubilisation of rhodium fragments, powder and pieces of foils and wires in the presence of hydrochloric and chlorine gas. Solvent-solvent extraction of no-carrier-added 103 Pd from the irradiated rhodium target with a-furyldioxime into chloroform under the influence of hydrochloric acid concentration was investigated. The extraction yield was 85.3% for a single extraction with 0.37 M HCl, and 103 Pd was more than 99% pure.

The sorption edges and the sorption isotherms of UO 2 2+ onto alumina were determined by a batch technique in the presence and absence of phosphate at pH range 3–6, ionic strength range 0.05–0.5 mol/L and UO 2 2+ concentrations 10 -5 –10 -4 mol/L. The dependence of sorption percentage of UO 2 2+ onto Al 2 O 3 as a function of initial concentration of phosphate was also investigated. The effects of phosphate and ionic strength on the sorption of UO 2 2+ onto Al 2 O 3 as a function of pH were emphatically investigated. It was found that the pH value and the presence of phosphate play a very important role in the UO 2 2+ sorption onto Al 2 O 3 , even if the concentration of phosphate is only a half of the concentration of UO 2 2+ . The sorption steeply increases with increasing pH from 3 to 6, and the sorption is enhanced with increasing concentration of phosphate at sorption percentage lower than 97%. The ionic strength is insensitive to UO 2 2+ sorption in the ionic strength range from 0.05 to 0.1 mol/L KNO 3 , while the sorption is slightly decreased with increasing ionic strength from 0.1 to 0.5 mol/L. The enhanced sorption of UO 2 2+ is attributed to the formation of ternary surface complexes involving UO 2 2+ and phosphate.

We have examined the effect of the gluconate anion, an analogue for cellulose degradation products, on the adsorption of trivalent lanthanum (La 3+ ) # to goethite. Lanthanum is investigated as an analogue for the trivalent actinides. Batch pH adsorption edge experiments were used to quantify the adsorption of La 3+ in the absence of gluconate and in solutions where gluconate was present at a 1:1 mole ratio to lanthanum. Using available thermodynamic data, it is calculated that lanthanum is primarily present in solution as the free La 3+ ion at pH values up to 8.5 in the absence of gluconate. Above pH 8.5, solid La(OH) 3 precipitates from solution. In the presence of gluconate, complexation decreases the free La 3+ concentration in solution. The fraction of La 3+ complexed increases, from 3% to 50%, as the concentrations of La 3+ and gluconate were increased. Very little effect on the adsorption of La 3+ to goethite was observed in the presence of gluconate below pH 7. At pH values above 7, however, gluconate doubled the maximum amount of La 3+ adsorbed when present at concentrations that saturated the goethite adsorption sites. The presence of gluconate did not appear to inhibit the formation of La(OH) 3 (s) at pH 8.5 and milli molar lanthanum concentrations. Adsorption to the goethite surface was represented with a surface complexation approach using the diffuse double-layer model. Intrinsic binding constants for the surface complexes were estimated from the pH adsorption edge data using the computer code FITEQL 4.0 and visual curve fitting. Two surface reactions were used to fit the adsorption data in the absence of gluconate: 1) a strong binding site with no proton release and 2) a much higher concentration of weak binding sites with release of two protons per La 3+ adsorbed. In the presence of gluconate, a third surface complex was needed that involved a ternary complex of two lanthanum atoms with one gluconate molecule.