Volume 103, Issue 4 - Special Issue: Separation Science in Nuclear Technology in India / Prasanta Kumar Mohapatra, Vijay Kumar Manchanda

This paper reports the extraction behavior of uranium and thorium by diamylbutyl phosphonate (DABP). This extractant was synthesized, characterized by various techniques like carbon and hydrogen analysis, IR, NMR ( 1 H, 13 C and 31 P) and mass spectra. This paper reports the physicochemical properties like density and viscosity of this ligand. The tracer and macro extraction of uranium and thorium and stoichiometry of the extracted species of actinides by DABP is presented in this paper. This compound is an unsymmetrical phosphonate and is compared with symmetrical member of the same family, diamylamyl phosphonate (DAAP) and a member of trialkyl phosphate family, viz. tributyl phosphate (TBP).

In recent years, Supercritical Fluid Extraction (SFE) technique has been widely used for the extraction of metal ions. In the present study, extraction of uranium from nitric acid medium was investigated using supercritical carbon dioxide (Sc-CO 2 ) containing various organophosphorous compounds such as trialkyl phosphates e.g. tri- iso -amyl phosphate (TiAP), tri- sec -butyl phosphate (TsBP) and tri- n -butyl phosphate (TBP), dialkylalkyl phosphonates, e.g. diamylamyl phosphonate (DAAP) and dibutyl butyl phosphonate (DBBP), dialkyl hydrogen phosphonates, e.g. dioctyl hydrogen phosphonate (DOHP), dioctylphosphineoxide (DOPO), trioctyl phosphine oxide (TOPO), n-octylphenyl N,N-diisobutyl carbamoylmethylphosphine oxide (CMPO) and di-2-ethyl-hexyl phosphoric acid (HDEHP). Some of these ligands have been investigated for the first time in the supercritical phase for the extraction of uranium. The extraction efficiency of uranium was studied with TiAP, DAAP and DBBP as a function of nitric acid concentration; the kinetics of the equilibration period (static extraction) and transportation of the metal complex (dynamic extraction) was investigated. The influence of pressure and temperature on the extraction behaviour of uranium with DAAP was studied from 4 N HNO 3 . The extraction efficiency of uranium from 4 N nitric acid medium was found to increase in the order of phosphates < phosphonates < HDEHP < TOPO < CMPO. In the case of phosphates and phosphonates, the maximum extraction of uranium was found to be from 4 N HNO 3 medium. The acidic extractants, HDEHP and DOHP showed relatively higher extraction at lower acidities. The relative extraction of uranium and thorium from their mixture was also examined using Sc-CO 2 containing phosphates, phosphonates and TOPO. The ligand, TsBP provided better fractionation between uranium and thorium compared to trialkyl phosphates, dialkyl alkyl phosphonates and TOPO.

Polymer inclusion membranes (PIM) containing N , N , N ′, N ′-tetra(2-ethylhexyl) diglycolamide (T2EHDGA) were evaluated for the separation of actinide ions such as Am 3+ , Pu 4+ , UO 2 2+ and Th 4+ from acidic feeds. The PIMs were prepared using cellulose triacetate (CTA) as the polymer matrix, 2-nitrophenyloctyl ether (NPOE) as the plasticizer and T2EHDGA as the carrier extractant and the optimized membrane composition was found to be 68.4% T2EHDGA, 17.9% NPOE and 13.7% CTA which resulted in 74% Am 3+ uptake at 1 M HNO 3 in 2 h. The uptake studies were carried out using feed solutions containing varying concentrations of nitric acid (0.5–3.0 M) and showed the trend: Pu 4+ > Am 3+ > Th 4+ > UO 2 2+ . Quantitative stripping (> 99%) of the sorbed Am 3+ was possible using a solution containing 0.01 M EDTA at pH 3.0. Reusability studies indicated deterioration of the PIM on continuous use.

The extraction and stripping behavior of various metal ions present in the fast reactor simulated high-level liquid waste (FR-SHLLW) was studied using a solvent phase composed of a neutral extractant, N , N ,-didodecyl- N ′, N ′-dioctyl-3-oxapentane-1,5-diamide (D 3 DODGA) and an acidic extractant, di-2-ethylhexyl diglycolamic acid (HDEHDGA) in n -dodecane ( n -DD). The third phase formation behavior of the solvent formulation D 3 DODGA + HDEHDGA/ n -DD, was studied when it was contacted with FR-SHLLW, and the concentration of neutral and acidic extractant needed to avoid the third phase formation was optimized. The distribution ratio of various metal ions present in FR-SHLLW was measured in a solution of 0.1 M D 3 DODGA + 0.2 M HDEHDGA/ n -DD. The extraction of Am(III) was accompanied by the co-extraction of lanthanides and unwanted metal ions such as Zr(IV), Y(III), and Pd(II). A procedure was developed to minimize the extraction of unwanted metal ions by using aqueous soluble complexing agents in FR-SHLLW. Based on those results, the counter-current mixer-settler run was performed in a 20-stage mixer-settler. Quantitative extraction of Am(III), Ln(III), Y(III), and Sr(II) in 0.1 M D 3 DODGA + 0.2 M HDEHDGA/ n -DD was observed. The recovery of Am(III) from the loaded organic phase was carried out by the optimized aqueous formulation composed of 0.01 M diethylenetriaminepentaacetic acid (DTPA) + 0.5 M citric acid (CA) at pH 1.5. The stripping of Am(III) was accompanied by co-stripping of some early lanthanides. However the later lanthanides (Eu(III) and beyond) were not back extracted to Am(III) product. Therefore, the studies foresee the possibility of intra-lanthanides as well as lanthanide-actinide separation in a single-processing cycle.

The present work investigates the role of increase in the basicity of organophosphorus extractant (dialkylalkyl phosphonates) on the uptake of actinides and fission products vis-à-vis tributyl phosphate (TBP), currently employed as a universal extractant. Two dialkylalkyl phosphonates viz. dibutylpropyl phosphonate (DBPrP) and dibutylpentyl phosphonate (DBPeP) were synthesized, characterized and evaluated for their solvent extraction behavior towards U(VI), Th(IV), Eu(III) and Tc(VII) in nitric acid medium ranging from 0.01–6 M. It was observed that increasing the basicity of the phosphoryl oxygen enhanced the uptake of the actinides and the distribution coefficient values were significantly larger as compared to TBP. The limiting organic concentration (LOC) value was estimated for Th(IV) for these extractants and compared with the TBP system. The separation factors of actinides with phosphonates over Tc(VII) are distinctly better than that with TBP.

An experimental system has been setup inside an argon atmosphere glove box to carry out studies on the actinide draw down process, an important step of the pyroprocess flow sheet. Studies have been carried out at 723 K on the extraction of uranium from LiCl-KCl eutectic salt containing 0.5% UCl 3 using lithium-0.03% cadmium alloy. In these experiments, 5 L of the molten salt and 5 L of the molten alloy are fed to a single stage extractor where 250 mL of the salt and 250 mL of the alloy come into contact. A three stage stirrer having straight as well as pitched blade turbine impellers was used to mix the molten alloy and salt phases having high interfacial tension of 450 dyne/cm. The results of the studies indicate that a U extraction efficiency of 99% could be achieved.

Batch sorption experiments were carried out under aerobic conditions to understand the sorption behavior of U(VI) onto bentonite clay under varying pH (2–8) and ionic strength ( I  = 0.01 – 1 M (NaClO 4 )) conditions. The influences of different complexing anions (1 · 10 – 4  M) such as oxalic acid (ox), carbonate (CO 3 2 – ), citric acid (cit), and humic acid (HA, 10 mg/L) on the sorption behavior were also investigated. The sorption of U(VI) increased with increasing pH up to pH 6 beyond which a decrease was attributed to the formation of anionic carbonate species. Marginal influence of the change in the ionic strength of the medium on the sorption profile of uranium suggested inner-sphere complexation onto the bentonite surface. The presence of humic acid showed interesting sorption profile with varying pH. Initially, there was an enhancement in the sorption with increased pH followed by a plateau and finally a decrease thereafter due to the formation of aqueous U(VI)-humate complexes. Spectroscopic studies such as UV spectrophotometry, luminescence and extended X-ray absorption fine structure (EXAFS) measurements were also performed to understand the changes in aqueous speciation of U(VI) ion. The luminescence yields of different aqueous U(VI) species followed the order: U(VI) Hydroxy > U(VI) HumicAcid > U(VI) carbonate > U(VI) citrate . The lower luminescence yield of U(VI)carbonate complex can be attributed to the strong dynamic quenching by carbonate at room temperature. The U(VI) samples shows two distinct life-time suggesting the presence of the different luminescent U(VI) species. Similar trend was observed for U(VI)-bentonite suspension in presence/absence of the complexing ligands. There was luminescence quenching for the sorbed U(VI) due to surface complexation. These observations were further supported by spectrophotometric measurements. EXAFS spectra of U(VI) samples were recorded in luminescence mode at the U  L 3 edge. There was shift in the absorbance edge which was attributed to decrease in electron density at U(VI) due to surface or ligand complexation. The R space spectra are mainly dominated by the back-scattering from the axial oxygens in the first shell. The inner-sphere multinuclear complex formation takes place during the U(VI) sorption onto bentonite.

Citric acid is one of the low molecular weight organic acids which are ubiquitous in nature. In this study the sorption of citric acid at titania (anatase)/water interface was investigated using batch adsorption experiment and attenuated total reflectance – Fourier transform infrared spectroscopy (ATR-FTIR) over wide experimental conditions of pH, surface loading and ionic strengths. The batch adsorption experiment showed typical anionic characteristics; higher sorption was observed at lower pH values and sorption decreases with increasing pH. Strong dependence of sorption on ionic strength shows formation of citrate surface species in outer-sphere binding mode. At lower pH, ATR-FTIR spectra of aqueous citrate differs from that of sorbed citrate ions indicating lowering of p K a values of surface sorbed citrate compared to that present in solution. Detailed analysis of FTIR spectra of sorbed citrate in light of that of aqueous and Ca(II)/Eu(III) complexed citrate revealed formation of an inner sphere complex of citrate on anatase surface along with the outer sphere complex. Wavenumber difference between the asymmetric and symmetric COO – peaks ( Δυ  = 159 cm – 1 ) implies inner-sphere complex in bidentate bridging mode. These results help in explaining the sorption behaviour of Eu(III) on anatase in presence of citric acid.

While neutron irradiation of natural Xe gas followed by wet chemical dissolution of activation products constitutes a successful paradigm for the small scale production 125 I, the concomitant production of 134 Cs and 137 Cs emerged as the primary impediment which necessitates purification of 125 I solution. This paper describes an ion-exchange chromatographic technique using Resorcinol Formaldehyde (RF) resin to purify 125 I solution from 134 Cs and 137 Cs impurities. A thorough investigation of the adsorption parameters of RF resin was carried out to arrive at the experimental conditions resulting optimum retention of 134 Cs and 137 Cs impurities. Based on the experimental findings, an optimized separation procedure was developed in which the neutron irradiated dissolved products at pH ∼ 13 was passed through a chromatography column containing RF resin where in 134 Cs and 137 Cs impurities gets adsorbed leaving behind 125 I to appear in the effluent. The overall recovery of 125 I was >90% with acceptable purity amenable for clinical applications.

Measurement of 210 Po in environmental matrices is important due to its very high specific activity, present in every compartment of the environment due to a daughter product of uranium ( 238 U), accumulative and highly toxic in nature. Conventional method for 210 Po estimation is by auto-deposition onto both sides of a silver disc followed by alpha spectrometry of both the sides. A new deposition unit having the facility to hold the silver disc and magnetic stirring bar has designed and fabricated for 210 Po estimation in which only one side is counted. In the conventional method, the total activity is distributed to the both sides of the silver disc and more counting time is required whereas in the improved deposition unit, only one side contain all the activity so that one time counting is required with better statistical significance. The same has been observed in spike recovery and water sample assessment. The tracer recovery in the conventional method was 72% – 88% and 70% – 85% whereas for the new deposition the recovery is 87% – 99% and 78% – 94% for spike recovery study and environmental samples, respectively. Certified tracers were analysed for the assurance of the reliability of the method and the results were in good agreement with the recommended value with a relative error <20%. The MDA of the method is 1.5 mBq for the estimation of 210 Po at 3 σ confidence level, 86400 s. counting time and 100 ml of water sample, taking the detector efficiency and chemical yield into consideration. The results obtained from both the methods were compared statistically. χ 2 test, repeatability parameters, relative bias measurement and linearity test was performed for both the methods. The % difference between the two methods in terms of linearity is 0.2%. From the χ 2 test it can be concluded that the measured data by two methods falls within 99% confidence interval. The modified deposition unit enhance the statistical significance, reduce the counting time, improved detection level, accuracy and precision and more suitable for determination of 210 Po in environmental samples as the levels in various environmental matrices is very low and close to the method detection level.