Evaluation of polymer inclusion membranes containing crown ethers for selective cesium separation from nuclear waste solution

Transport behaviour of (137)Cs from nitric acid feed was investigated using cellulose triacetate plasticized polymer inclusion membrane (PIM) containing several crown ether carriers viz. di-benzo-18-crown-6 (DB18C6), di-benzo-21-crown-7 (DB21C7) and di-tert-butylbenzo-18-crown-6 (DTBB18C6). The PIM was prepared from cellulose triacetate (CTA) with various crown ethers and plasticizers. DTBB18C6 and tri-n-butyl phosphate (TBP) were found to give higher transport rate for (137)Cs as compared to other carriers and plasticizers. Effect of crown ether concentration, nitric acid concentration, plasticizer and CTA concentration on the transport rate of Cs was also studied. The Cs selectivity with respect to various fission products obtained from an irradiated natural uranium target was found to be heavily dependent on the nature of the plasticizer. The present work shows that by choosing a proper plasticizer, one can get either good transport efficiency or selectivity. Though TBP plasticized membranes showed good transport efficiency, it displayed poor selectivities. On the other hand, an entirely opposite separation behaviour was observed with 2-nitrophenyloctylether (NPOE) plasticized membranes suggesting the possible application of the later membranes for the removal of bulk (137)Cs from the nuclear waste. The stability of the membrane was tested by carrying out transport runs for nearly 25 days.

Extraction of the uranyl ion with 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) and neutral donors from dilute nitric acid medium

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 UO2(PBI)2·nH2O 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 UO2(PBI)2·D2EHAA, UO2(PBI)2·D2EHPrA, UO2(PBI)2·D2EHiBA and UO2(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 pK 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 UO2(PBI)2·D2EHAA.

N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide impregnated magnetic particles for the uptake of lanthanides and actinides from nuclear waste streams

A novel, simple technique based on magnetically assisted chemical separation (MACS) has been developed for the uptake of lanthanides and actinides from pure nitric acid solutions as well as from Simulated Pressurized Heavy Water Reactor-High Level Waste (PHWR-SHLW). Uptake profiles of various metal ions, such as Pu(IV), U(VI), Th(IV), Am(III), Eu(III), Sr(II), Cs(I) and Fe(III) were investigated as a function of time and nitric acid concentrations using N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) impregnated magnetic particles and compared with N,N′-dimethyl-N,N′-dibutyl tetradecyl malonamide (DMDBTDMA) and trialkyl phosphine oxide coated magnetic particles and also with TODGA coated extraction chromatographic resins. The uptake of various metal ions follows the order Eu(III) > Am(III) > Th(IV) > Pu(IV) > U(VI) > Sr(II) > Fe(III) > Cs(I) in pure nitric acid solutions. On the other hand, distinctly high distribution coefficient (K d) value has been observed for Pu(IV) as compared to Eu(III) and Am(III) in SHLW. Further, no significant extraction of Cs(I) or Sr(II) was observed in SHLW. Also, at any acidity the K d value of a given metal ion is less in the SHLW as compared to that in pure HNO3, apparently due to the co-extraction of the other metal ions present in SHLW. The loading capacity of the TODGA impregnated magnetic particles with respect to Th(IV), U(VI) and Eu(III) was determined, along with the distribution isotherms to simulate multiple contacts. The adsorption models of Langmuir and Freundlich were fitted to the experimental data and best correlations was obtained for Langmuir model suggesting monolayer adsorption is the prevalent mechanism. The stability and reusability of the TODGA coated magnetic particles was also assessed.

N, N′-dimethyl-N, N′-dibutyl tetradecyl malonamide impregnated magnetic particles for the extraction and separation of radionuclides from nuclear waste streams

N, N′-dimethyl-N, N′-dibutyl tetradecyl malonamide (DMDBTDMA) coated magnetic particles are being evaluated for the possible application in the partitioning of actinides, lanthanides and fission products from pure nitric acid solutions as well as from Simulated Pressurized Heavy Water Reactor-High Level Waste (PHWR-SHLW). Uptake profiles of various metal ions, such as Pu(IV), U(VI), Am(III), Eu(III), Sr(II) and Cs(I) were obtained as a function of time and nitric acid concentration by batch studies using DMDBTDMA coated magnetic particles. The order of uptake follows the order Pu(IV)>U(VI)>Am(III)>Eu(III)>Sr(II)∼Cs(I) in both nitric acid and SHLW. The uptake of various trivalent lanthanides was also investigated as a function of nitric acid concentration and found the uptake order as Pr(III)>La(III)>Eu(III)>Tb(III)>Ho(III)>Er(III)>Yb(III)>Lu(III). The sorption capacity of the DMDBTDMA coated magnetic particles with respect to U(VI) and Eu(III) was determined, along with the sorption isotherms to simulate multiple contacts. The maximum sorption capacity of DMDBTDMA coated magnetic particles was found to be 1.58 mmol/g and 0.36 mmol/g for U(VI) and Eu(III), respectively. The adsorption models of Langmuir and Freundlich were fitted to the experimental data and best correlations were obtained for both the models. The Langmuir model predicts a loading capacity of 1.61 mmol/g and 0.37 mmol/g for U(VI) and Eu(III), respectively, which is close to the experimental values. The stability and recycling capacity of the DMDBTDMA coated magnetic particles was also assessed.

Distribution studies on Th(IV), U(VI) and Pu(IV) using tri-n-butylphosphate andN,N-dialkyl amides

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 chainN,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 thoriumviz.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.

Solvent extraction behaviour of some fission product elements withN,N-dihexyl octanamide and tri-n-butyl phosphate under simulated PUREX process conditions

Extraction behaviour of some fission product contaminants like Zr(IV), RuNO(III), La(III), Ce(III) and Nd(III) with 1.1 M solutions ofN,N-dihexyl octanamide (DHOA) and tri-n-butyl phosphate (TBP) inn-dodecane from nitric acid medium has been investigated under conditions prevailing in the PUREX process. Results of the study reveal that the extraction of Zr(IV), La(III), Ce(III) and Nd(III) with 1.1 M DHOA is significantly lower than that of TBP. However, the extraction of RuNO(III) with DHOA is of the same order as that with TBP. The influence of uranium(VI) saturation of the organic phase on the extraction of Zr(IV) and RuNO(III) is evaluated. The effect of degradation products of DHOA (caprylic acid/n-dihexylamine) and TBP (di-n-butyl phosphoric acid) on the extraction of Zr(IV) is also examined. The study indicates the innocuous nature of the degradation products of DHOA towards the extraction of Zr(IV) under the PUREX process conditions. This is unlike the degradation products of TBP, the presence of which produce noticeable increase in the extraction of Zr(IV).

Effect of humic acid on cesium sorption on silica colloids

Effect of humic acid (HA) on the sorption behaviour of137Cs on silica colloids has been studied by batch procedure over a pH range of 3-10 and at ionic strength of 0.05 M (NaClO4). The sorption of137Cs on bare silica colloids was found to be negligible up to pH 7 above which it was found to increase sharply with pH. In the presence of HA significant enhancement in the sorption of137Cs on silica colloids was observed at lower pH values with no effect at pH > 7. Zeta potential of the colloidal suspension of HA, silica and their binary mixture were also measured. Binary interaction between137Cs and HA, and that between HA and silica were investigated in order to explain the observations in terms of linear additive model. The data obtained in the present work confirmed the role of non-additive factors in the ternary sorption system.

Extraction of tetravalent neptunium isoxazolonates as their TOPO adducts

The extraction behaviour of Np(IV) by 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) in the presence of tri-n-octyl phosphine oxide (TOPO) was investigated from nitric acid medium. The speciation studies, carried out by the slope analysis method, yielded binary species of the type Np(PBI)4 when only HPBI was used as the extractant and ternary species of the type Np(PBI)4·TOPO when both HPBI and TOPO were used as the extractants. The two-phase extraction constant for the binary species (log k ex) was calculated to be 10.11±0.03, while that for the ternary species was found to be 17.24±0.05. Thermodynamic parameters for the binary and ternary extraction systems were also determined. Both the binary and ternary extraction systems were found to be highly spontaneous processes with predominant enthalpy stabilization. The very high negative entropy factor suggested contribution of unfavourable steric factors.

Role of ligand basicity and stereochemistry in the extraction of plutonium(IV) isoxazolonates

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(ClO4)·(TOPO) and Pu(PBI)4·TOPO were co-extracted from perchloric acid medium, a mixture of Pu(PBI)2(NO3)2·2TOPO, Pu(PBI)3(NO3)·(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.

The role of oxidants in Pu-Th separation studies employing di(2-ethylhexyl)isobutyramide (D2EHIBA) as extractant

Distribution studies of Pu and Th have been carried out under varying acidity/ligand concentrations to optimise the conditions for their separation employing di(2-ethylhexyl)isobutyramide (D2EHIBA) in n-dodecane as extractant. In view of the selective complexation of D2EHIBA towards hexavalent metal ions over tetravalent metal ions, the focus of the present work is to identify a suitable oxidant of Pu. Amongst various oxidising agents investigated for the oxidation of Pu to Pu(VI), AgO appears to be a promising candidate as (i) it is thermodynamically as well as kinetically a favourable oxidant and (ii) unlike Cr(VI) and Ce(IV), it is not co-extracted with Pu towards the organic phase.

Extraction of actinides using N, N, N′, N′-tetraoctyl diglycolamide (TODGA): a thermodynamic study

The effect of temperature on the extraction behaviour of Am(III), Pu(IV) and U(VI) from nitric acid medium was studied employing N,N,N′,N′-tetraoctyl diglycolamide (TODGA) in n-dodecane. The two-phase equilibrium constants (log K′ex) were calculated and compared with those of other extractants proposed for actinide partitioning, viz. octyl-(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and N,N,N′,N′-dimethyl dibutyl tetradecyl malonamide (DMDBTDMA). Thermodynamic parameters, viz. ΔG, ΔH and ΔS for the extraction of actinides by TODGA were also compared with those of CMPO and DMDBTDMA. These studies indicate that the extraction processes of Am(III) and U(VI) are enthalpy driven whereas entropy factor counteracts the extraction. However, in the case of Pu(IV), the extraction process is enthalpy as well as entropy favoured. Role of diluent on the loading of Nd(III) in 0.1 M TODGA has also been investigated.

Interaction of Eu3+ with N,N,N',N'-tetraoctyl diglycolamide: a time resolved luminescence spectroscopy study

N,N,N',N'-tetraoctyl diglycolamide (TODGA) has been identified as one of the promising extractants for the partitioning of minor actinides from high-level nuclear waste solutions. Solvent extraction studies have shown that stoichiometry of the extracted species of Eu(3+) with TODGA depend on the nature of diluent. Time resolved luminescence spectroscopy (TRLS) has been employed to investigate the complexation of Eu(3+) with TODGA under different experimental conditions. The effects of different experimental parameters such as aqueous phase acidity, nature of diluent, and TODGA concentration on the luminescence lifetime of Eu(3+) ions have been investigated. The lifetime measurements of the complexed fraction of Eu(3+) with TODGA suggested the absence of water molecules in the inner coordination sphere of the metal ion in different solvents. In ethanol-water (60/40%) mixture, the complexation of Eu(3+) with TODGA under varying ligand-to-metal ratios suggested the formation of 1:1, 1:2, and 1:3 species, viz., Eu(TODGA)(3+), Eu(TODGA)(2)(3+), and Eu(TODGA)(3)(3+), respectively. The conditional stability constants logbeta(1), logbeta(2), and logbeta(3) were calculated as 6.1+/-0.5, 10.8+/-0.7, and 14.3+/-0.6, respectively. The nature of diluent did not influence the luminescence spectra of Eu(3+) in the presence of TODGA.

Role of magnetite and humic acid in radionuclide migration in the environment

Sorption of (137)Cs, (90)Sr, (154)Eu and (141)Ce by magnetite has been studied at varying pH (4 to 11) in the presence and absence of humic acid. The sorption studies have also been carried out at varying ionic strength (0.01 to 0.2 M NaClO(4)) and humic acid concentration (2 to 20 mg/L). Percentage sorption of (137)Cs and (90)Sr was found to be pH dependent, with the sorption increasing with increasing pH of the suspension. At any pH, the percentage sorption of (90)Sr was higher than that of (137)Cs. The results have been explained in terms of the electrostatic interaction between the positively charged metal ions and the surface charge of the magnetite which becomes increasingly negative with increasing pH. On the other hand, (154)Eu and (141)Ce were found to be strongly sorbed by the magnetite at all pH values, with the sorption being independent of pH. The strong sorption of trivalent and tetravalent metal ions suggests the role of complexation reactions during sorption, apart from the electrostatic interactions. However, in the case of (141)Ce surface precipitation of Ce(III) formed by reduction of Ce(IV) in the presence of magnetite cannot be ruled out. Presence of humic acid (2 mg/L) was found to have negligible effect on sorption of all metal ions.