Volume 95, Issue 12

In 30% TBP/OK Np(V) is unstable and disproportionates to Np(IV) and Np(VI). Np(V) readily coordinates to Np(IV) in solution to form a “cation–cation” complex by bonding through an axial oxo group on Np(V). The rate of disproportionation in 30% TBP/OK is >500 times that in aqueous solution.

Current reactors spent nuclear UO 2 fuel, that will probably enter geologic formations for disposal, is assumed to get into contact with groundwater of a repository earliest after several thousands of years. After this time, its alpha activity will be still high enough to induce a process of water radiolysis that leads to production of oxidants and, probably, to the oxidative dissolution of the material, that could start a mobilisation of radiotoxic nuclides. The main oxidant produced during this process would be hydrogen peroxide (H 2 O 2 ), which is thermodynamically unstable and can decompose during reactions with certain groundwater ions. Information on the stability of H 2 O 2 is therefore important for assessing the behaviour of the material in the repository. We investigated the stability of H 2 O 2 and the correlated dissolution of U from UO 2 as a function of the concentration of H 2 O 2 and of the groundwater ions carbonate, sulphate, and silicate. Decomposition of H 2 O 2 was measured in the presence and absence of UO 2 (cr). We monitored the concentrations of H 2 O 2 , U diss , groundwater ions, O 2diss , and the pH in aqueous solution under Ar atmosphere. The lowest stabilities (in average) for H 2 O 2 were found in carbonate solutions. When comparing the effect of carbonate systems containing UO 2 (cr) to carbonate solutions without a solid phase, we found that in some of the homogeneous tests H 2 O 2 was consumed faster. This effect was not observed with the other groundwater ions. Also, U dissolution rates were significantly lower in the carbonate experiments than in those with sulphate or silicate. The results suggest that a radical-controlled mechanism (similar to the Halpern–Smith-mechanism of dissolved U(IV) oxidation) takes place at the UO 2 surface during dissolution. The hypothesis is supported by ESR measurements on hydroxyl radical scavenging by the carbonate ion.

The cyclic voltammetric behavior of uranyl ion was investigated at hanging mercury drop electrode in perchlorate supporting electrolyte. The standard heterogeneous electron transfer rate constant “ k s ” for the reduction of uranyl ion in perchlorate medium was (2.74±0.03)×10 −3 cm s −1 while in acetate and formate medium the value of k s was (2.83±0.02)×10 −3 and (2.66±0.02)×10 −3 cm s −1 , respectively. The effect of temperature on the values of k s was investigated and thermodynamic constants Δ H *, Δ S * and Δ G 298 * were evaluated. The values of Δ H *, Δ S *; and Δ G 298 * were 8.06±0.03 kJ mol −1 , −23.42±0.05 J mol −1 K −1 and 15.02±0.07 kJ mol −1 in simple perchlorate medium and 3.67±0.02 kJ mol −1 , −38.04±0.05 J mol −1 K −1 , and 15.01±0.03 kJ mol −1 in acetate medium and 0.79±0.02 kJ mol −1 , −48.24±0.04 J mol −1 K −1 and 15.17±0.02 kJ mol −1 in formate medium, respectively. The thermodynamic data showed that the process of reduction of uranyl ion at hanging mercury drop electrode was endothermic and non-spontaneous. The stability constants of uranyl acetate and uranyl formate were calculated from an analysis of the shift in peak potential data. The logβ 1 and logβ 2 values for 1:1 and 1:2 uranyl acetate complexes were 2.67±0.04 and 4.71±0.01, respectively. The values of logβ 1 and logβ 2 of uranyl formate complex were 1.61±0.02 and 2.57±0.01, respectively, which are reported for the first time.

Feasibility of supercritical CO 2 extraction of thorium from tissue paper matrix was studied and various organophosphorus reagents (TBP, TOPO, TPP, TPPO, TBPO) were evaluated as co-solvent. Effects of pressure (100–300 atm), temperature (313–353 K), flow rate (0.5–3 mL min −1 ), static time (0–40 min) and extraction time (5–40 min) on thorium extraction efficiency were investigated. Pressure of 200 atm, temperature of 333 K, CO 2 flow rate of 2 mL min −1 , 20 min static period followed by 20 min of extraction were found to be optimum. Under optimum conditions employing various organophosphorus reagents as co-solvent, extraction efficiency indicated the trends: (i) Among phosphates, if aliphatic group attached to P was replaced by aromatic group, then extraction efficiency decreased e.g. TBP (53%±8%) and TPP (39%±5%). (ii) Among the phosphine oxides, higher the aliphatic chain length attached to P, higher was the extraction efficiency e.g. TOPO (68%±4%) and TBPO (50%±9%). (iii) In case of phosphine oxides, if phenyl group was attached to P i.e. TPPO (59%±15%) then extraction efficiency was intermediate to TOPO and TBPO. (iv) Between phosphates and phosphine oxides, for aliphatic hydrocarbons, the extraction efficiencies were comparable e.g. TBP and TBPO. For aromatic ring, extraction efficiency was higher in phosphine oxides e.g. TPP and TPPO. Both TBPO and TPP have shown higher extraction efficiency in combination with TBP. The solvents dissolving oraganophosphorus reagents were found to influence extraction efficiency. The extraction efficiency was found to decrease slightly with increasing aliphatic chain length of the alcohol. The maximum extraction efficiency was obtained with 0.2 M TOPO in methanol where thorium was extractable with 68%±4 efficiency.

Technetium-99 is a β -emitting fission product of 238 U. It is important when considering a safety case for nuclear waste disposal because of its high yield in radioactive waste and long half-life. The aqueous chemistry of Tc in a cementitious repository is likely to be dominated by Tc(VII), as TcO 4 − , in aerobic areas and by Tc(IV), as TcO 2 (am), in anaerobic. Some previous studies have shown an increase in Tc(IV) solubility with increasing pH above pH 9.5 whilst others have not observed this increase. This study found that the solubility of Tc(IV) remains independent of pH until around 13.5, when a small increase can be seen which continues to increase linearly with pH. Modelling suggests that this increase occurs as the species TcO(OH) 3 − is formed. The formation constant was estimated with data from this study and was found to be log K 2 =−21.6±0.3.

The basic chemistry for the immobilization of pertechnetate (TcO 4 − ) by hydrogen sulfide was investigated in aqueous solution under both aerobic and anaerobic environments. Pertechnetate immobilization was acid dependent, with accelerated rates and increased immobilization yields as the acid concentration increased. Oxygen had no effect under acidic conditions. Under anaerobic alkaline conditions, the pH, and therefore the speciation of sulfide, was the determining factor on the immobilization of pertechnetate. Only 53% of the TcO 4 − was immobilized at pH 8, while the yield increased to 83% at pH 9 as HS − became the dominant sulfide species. The immobilization yield then decreased to 73% at pH 13. No reaction was observed between TcO 4 − and sulfide under aerobic alkaline conditions, indicating that oxygen suppressed this reaction. Pertechnetate immobilization was found to be first order with respect to both sulfide and pertechnetate in acidic solutions, and in alkaline solution under anaerobic conditions. The results of stoichiometry studies and product analysis under alkaline anaerobic environments indicated that Tc 2 S 7 was obtained at pH 9. EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) studies suggested that the samples obtained from acidic, aerobic solution and alkaline anaerobic solution were both Tc 2 S 7 . The stability of Tc 2 S 7 is affected by O 2 with accelerated dissolution at high pH.

A molybdenum-doped sodium tungstate phase (Mo-HTB) and a porous, granular polyacrylonitrile composite variant (PAN-Mo-HTB) have been shown to effectively remove polonium-210 and lead-210 from concentrated sulphuric acid solutions (1–8 M) and a copper electrowinning electrolyte. Both forms of the exchanger were characterised by high polonium-210 and lead-210 distribution coefficients and very good selectivity for polonium-210 and lead-210 over divalent copper and iron.

The synergistic solvent extraction of trivalent lanthanides such as Ce(III), Tb(III) and Lu(III) has been studied with a mixture of hexafluoroacetylacetone (HHFA) acting as an acidic chelating agent and triphenylphosphine oxide (TPPO) as a neutral ligand in benzene from aqueous solution of pH 2.5 having ionic strength 0.01 mol dm −3 (K + /H + , Cl − ). The composition of the synergistic adduct has been determined by slope analysis method and found to be M(HFA) 3 ·2TPPO [M=Ce(III), Tb(III) Lu(III)]. The effect of various anions and cations on the extraction of these metal ions has also been studied and only oxalate among the anion and Cr(III) among the cations has some deleterious effect. The formation constant and separation factor from various metal ions have also been determined and discussed.