N,N,N ,N -tetraoctyl-3-oxapentane-1,5-diamide ...

N,N,N,N-tetraoctyl-3-oxapentane-1,5-diamide impregnated magnetic particles for the uptake of lanthanides and actinides from nuclear waste streams By B. S. Shaibu1 , M. L. P. Reddy1 , ∗, M. S. Murali2 and V. K. Manchanda2 1 2

Chemical Sciences and Technology Division, Regional Research Laboratory (CSIR), Thiruvananthapuram 695019, India Radiochemistry Division, B.A.R.C, Trombay, Mumbai 400085, India

(Received May 23, 2006; accepted in revised form October 27, 2006)

Magnetic microparticles / TODGA / Lanthanides / Actinides / Uptake profiles / Simulated high level waste Summary. 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 (PHWRSHLW). 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-oxapentane1,5-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.

1. Introduction Nuclear reactors have now become an important power source of many countries and as a result the amount of radioactive waste has also increased considerably. Cumulative nuclear spent fuel arising from existing nuclear technology could be around 3.5 × 105 tonnes by 2010. The concern of nuclear industry today is the management and safe *Author for correspondence (E-mail: [email protected]).

disposal of this high level radioactive waste. Therefore, it is desirable to develop simple, effective and environmentfriendly methods for the extraction and separation of radionuclides from nuclear waste, which helps in safe disposal of the same. Ion-exchange and solvent extraction are the two primary technologies currently being employed for this purpose. However, significant chemical additives, solvent losses, complex equipments, large secondary waste, prefiltration problems and time consuming, limit the application of these techniques. Magnetically Assisted Chemical Separation (MACS) process is one among the methods that was recently proposed for this quest. MACS process, which uses nano or micro sized magnetic particles, combines the selectivity afforded by solvent extractants with magnetic separation to provide an effective and efficient separation. In the MACS process, magnetic particles modified with solvent extractant are used to selectively separate radionuclides. The metal ions binding magnetic microparticles are then easily separated from waste solutions by the aid of a magnetic field. The metal ion bound to the magnetic particles are then removed using a suitable stripping agent and the particles are then recycled. The MACS concept, for separation of metal ions is opening a new methodology in separation chemistry. Both equipment and materials are inexpensive and generation of secondary wastes is minimized. Availability of large active surface area for a given mass of particles, ability to process solution that contains suspended solids, avoidance of channeling effects that are common in packed beds are some distinct advantages of MACS over other traditional techniques. Further, the process can be tailored for all applications which solvent extraction or ion-exchange processes have been developed. The effectiveness of MACS process has been already demonstrated for the separation of transuranics [1–5], cobalt and nickel [6] cadmium, and zinc [7] and copper [8]. This method also can be applied in the field of biomedicine [9], bioinorganic chemistry and catalysis [10]. CMPO ligands incorporated into a calixarene scaffold attached to surface of magnetic particles have been synthesized and utilized for the effective removal of Eu(III), Am(III) and Ce(III) from nuclear waste streams [11]. Recently, in our laboratory Cyanex 923 [12] or DMDBTDMA [13] coated magnetic

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Radiochim. Acta 95, 159–164 (2007) / DOI 10.1524/ract.2007.95.3.159 © by Oldenbourg Wissenschaftsverlag, München

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2. Experimental 2.1 Reagents and radionuclides N,N,N ,N -tetraoctyl-3-oxapentane-1,5-diamide (TODGA) was synthesized according to the procedure reported elsewhere [18]. The magnetic particles, MagaCharc™-AA (cross-linked polyacrylamide and acrylic acid entrapping charcoal and iron oxide (Fe3 O4 ) in the ratio of 1 : 1 : 1; size 1–60 µm) was procured from Cortex-Biochem, USA. 233 U tracer was purified by anion exchange in HCl medium to eliminate the daughter products of 232 U (formed as a byproduct during the irradiation of 232 Th for 233 U production) and its purity was confirmed by α-spectrometry [19]. Pu (principally 239 Pu) was purified from 241 Am in HNO3 medium and its radiochemical purity was ascertained by gamma spectrometry for the absence of 241 Am [20]. Pu(IV) was extracted by 0.5 M HTTA (2-thenoyltrifluoroacetone) in xylene at 1 M HNO3 and stripped by 8 M HNO3 and was used

Fig. 1. N,N,N ,N -tetraoctyl-3-oxapentane-1,5-diamide (TODGA).

as stock for Pu(IV) [21]. Further, plutonium valency in the aqueous phase was adjusted and maintained in tetravalent state by the addition of 0.05 M NaNO2 + 0.005 M NH4 VO3 (holding oxidants). Other radionuclides viz. 241 Am, 85,89 Sr, 152,154 Eu and 137 Cs were procured from Board of Radiation and Isotope Technology (BRIT), Mumbai, India. All other reagents used were of analytical reagent grade.

2.2 Instrumentation A Nicolet FT-IR 560 Magna spectrometer using KBr (neat) was used to obtain the infrared spectra of the coated magnetic particles. A JEOL JSM 5600LV scanning electron microscope (Tokyo, Japan) was used for morphological studies. Assay of 241 Am, 154 Eu, 85,89 Sr and 137 Cs was carried out by gamma counting in a well type NaI(Tl) scintillation counter (ECIL, India). Alpha counting for 233 U and 239 Pu was carried out by liquid scintillation counter employing a toluene-based scintillator containing 10% (v/v) di(2-ethylhexyl) phosphoric acid (HDEHP), 0.7% (w/v) 2,5-diphenyloxazole (PPO), 0.03% (w/v) 1,4-di-[2(5-phenyloxazoyl)]-benzene (POPOP). Th(IV) in the aqueous phase was determined spectrophotometrically as its Arsenazo-III complex at 660 nm [22]. Fe(III) was analyzed spectrophotometrically using 1,10-phenanthroline [23]. In the present study, 0.1 mM of Th(IV) and 0.1 mM of Fe(III) were used as carrier solutions. The preparation and composition of SHLW has been described in our recent publications [12, 13].

2.3 Preparation of TODGA impregnated magnetic particles The magnetic particles were washed repeatedly with deionized water and 0.1 M NaOH, followed by washing with deionized water alone and drying prior to use. A known amount of TODGA was diluted in acetone (1 : 1) and was mixed with equal weight of magnetic particles. The slurry was equilibrated for 24 h in a mechanical shaker followed by solvent removal by flushing nitrogen gas and vacuum dried to a constant weight. The weight percentage of the TODGA loaded on the magnetic particles was calculated from the difference in the weight of the magnetic particles before and after equilibration and was found to be ∼ 50% (W/W) in all cases. The SEM micrograph of the bare magnetic particles (Fig. 2a) show that there is no uniform distribution and the particle size varies typically between 1 to 60 µm whereas, the 50% TODGA coated magnetic particles shows large surface area and agglomerated (Fig. 2b). 2.4 Measurements of Distribution coefficient (K d ) The sorption of metal ions from nitric acid medium as well as from SHLW by TODGA impregnated magnetic particles was investigated by equilibrating a known volume of aqueous phase (1.0 mL) with a known amount of magnetic particles (∼ 50 mg) in an equilibration tube. The mixing of the two phases was carried out in a thermostated water bath maintained at 25 ± 0.1 ◦ C for 1 h. Subsequently, the tubes were centrifuged and kept on a magnetic separator (Cortex


particles have been successfully employed for the uptake of lanthanides and actinides from nuclear waste streams. Diamides are becoming increasingly more relevant as extractants for minor actinide partitioning due to their special characteristics like: 1) complete incinerability which helps in reducing the secondary waste volume significantly, 2) relatively innocuous degradation products which can easily be washed out, and 3) reasonably high decontamination factors obtained for the trivalent actinide ions vis-avis fission products at moderate acidity (3–5 M HNO3 ). To increase the efficiency of diamides towards the forward extraction of trivalent actinides several modifications have been attempted. It has been observed that the introduction of one etheric oxygen in between the two amide groups (diglycolamide) causes significant enhancement in the extraction efficiency of minor actinides due to their tridentate nature [14]. Recently developed, tridentate ligand, N,N,N ,N -tetraoctyl-3-oxapentane-1,5-diamide (TODGA) has been identified as one of the most powerful extractants for the partitioning of trivalent actinides and lanthanides from HLW solutions [15–17]. Hence, in the present study, TODGA (Fig. 1) has been chosen as the reagent for impregnating in magnetic particles. TODGA was impregnated on to the magnetic particles based on simple physical adsorption and have been evaluated with respective to the uptake of various actinides, lanthanides and other fission products. Different experimental conditions such as effect of contact time, nitric acid concentration, loading capacity, reusability of the impregnated magnetic particles on the uptake of metal ions were systematically investigated. Finally, MACS process has been employed for the uptake of lanthanides and actinides from Simulated Pressurized Heavy Water Reactor-High Level Waste.

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Fig. 2. (a) SEM photograph of the bare magnetic particles. (b) SEM photograph of 50% (Weight %) TODGA coated magnetic particles.

Biochem USA). The aqueous phase was separated and centrifuged a second time. 100 µL of the aqueous phase was taken before and after equilibration and assayed radiometrically. The distribution coefficient (K d ) was calculated using the following equation: Ci − Cf V Kd = , (1) Cf m where Ci and Cf are the metal ion concentration (in counts per unit time per unit volume) in the aqueous phase before and after equilibration, V is the volume of the aqueous phase (in mL) and m is the weight of the coated magnetic particles (in g). All the experiments were performed in duplicate and the general agreement of K d values obtained was within ±5%.

3. Results and discussion 3.1 Uptake kinetics The kinetics of sorption of U(VI), Th(IV), Pu(IV), Am(III), Eu(III) and Fe(III) by TODGA coated magnetic particles (weight % of TODGA coated magnetic particles = 50% W/W) was investigated from 2 M HNO3 and the results are depicted in Fig. 3. It is clear from the results that the TODGA coated magnetic particles exhibits faster uptake kinetics for U(VI) (15 min), Pu(IV) (10 min), Am(III) (10 min) and Eu(III) (10 min) as compared to previously reported DMDBTDMA (Am(III) 20 min) [13] or Cyanex 923

(U(VI) 30 min; Am(III) 30 min) [12] coated magnetic particles, and extraction chromatographic resin material (U/TEVA-2) coated with an equimolar mixture of diamyl amyl phosphonate and Cyanex 923 (U(VI) 30 min) [24]. However, the present kinetic data is in general consistent with the prior data reported for TODGA coated many chromatographic resin materials [18, 25, 26]. Though the sorption equilibrium was attained within 10−20 min, for all the subsequent experiments an equilibration time of 1 h was used.

3.2 Uptake studies of metal ions as a function of HNO3 concentration The uptake behavior of various metal ions, such as U(VI), Th(IV), Pu(IV), Am(III), Eu(III), Fe(III), Sr(II) and Cs(I) were obtained from pure nitric acid as well as from SHLW using TODGA coated magnetic particles (weight % of TODGA in coated magnetic particles = 50% W/W). Fig. 4. shows the uptake of metal ions as a function of HNO3 concentration. It is clear from the results that the uptake of Am(III), Eu(III), Th(IV) and Pu(IV) increases sharply with increase in acidity up to 2 M HNO3 beyond which saturation was observed. The uptake follows the order Eu(III) > Am(III) > Th(IV) > Pu(IV) > U(VI), which is similar to that obtained earlier in TODGA coated extraction chromatographic resins [24, 26, 27]. It is interesting to note that the commonly used neutral diamides act as bidentate ligands whereas the presence of additional etheric oxygen in TODGA makes the molecule tridentate thereby increasing the complexation with metal ions. The results obtained clearly demonstrate that the extraction properties of tridentate diglycolamide for Am(III) are superior to those of the bidentate diamides (DMDBTDMA). The weaker complexation of U(VI) by TODGA can be attributed to steric hindrance for tridentate complexation of the diglycolamide in the equilateral positions around the central U atom of


Fig. 3. Uptake of U(VI), Th(IV), Pu(IV) Am(III), Eu(III) and Fe(III) as a function of time by TODGA impregnated magnetic particles at 2 M HNO3 ; temperature 25 ◦ C.

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the linear UO2 2+ cation [23]. Further, it is assumed that the diglycolamide is bidentate in the uranium complex. In the case of Sr(II), the K d value increased upto 3 M HNO3 and decreased thereafter, which is in good agreement with the earlier reported data for TODGA coated chromatographic resin materials [26, 27]. For the entire range of acidity investigated the uptake of Cs(I) and Fe(III) was very low and the K d value was found to be less than 0.5 suggesting the insignificant uptake of these metal ions. Table 1 presents the comparison of K d values at 4 M HNO3 for different actinide ions with various extractant coated magnetic particles (weight % of DMDBTDMA/ Cyanex 923/TODGA in coated magnetic particles = 50% W/W) and also TODGA coated Chromosorb W resin particles (weight % of TODGA in Chromosorb W = 50% W/W). Relatively high K d values for all actinide ions were observed in the present system as compared to TODGA/Chromosorb W. The K d values for Am(III) at 4 M HNO3 follows the order TODGA > DMDBTDMA > TRPO, suggesting TODGA as the most promising extractant for trivalent actinides as explained earlier. The FT-IR spectra of the TODGA coated magnetic particles with and without loading of U(VI) are shown in Fig. 5. It is clear from the IR spectrum of the U(VI) loaded TODGA that the stretching frequency of the C=O has shifted from 1653 to 1612 cm−1 . The nitrate stretching frequencies were characterized at 1280, 1037, 734 cm−1 for uranium nitrate complex [28]. A strong absorption occurring around 933 cm−1 may be assigned to ν (O=U=O) of uranyl ion [28]. It is evident from the above specTable 1. Uptake of metal ions by different extraction systems at 4 M HNO3 ; temperature 25 ◦ C.

Extraction system

TODGA/MACS TODGA/Chromosorb W DMDBTDMA/MACS Cyanex 923/MACS

Fig. 5. FT-IR spectra of a) 50% TODGA coated magnetic particles. b) U(VI) loaded 50% TODGA coated magnetic particles at 5 M HNO3 .

tral changes that uranyl ion is interacting with the C=O of TODGA. Fig. 6 shows the variation in K d values of U(VI), Pu(IV), Am(III), Eu(III), Sr(II) and Cs(I) as a function of HNO3 concentration from SHLW solution by TODGA coated magnetic particles. The K d value is distinctly higher for Pu(IV) when compared with Am(III) and Eu(III). Also, no significant extraction of Sr(II) and Cs(I) was observed from SHLW as maximum K d , Sr(II) and Cs(I) was ∼ 0.2 in the entire range of acidity investigated. It is clear from the results that the K d value of any metal ion is less in the SHLW as compared to that in pure HNO3 , apparently due to the coextraction of other metal ions present in SHLW. The major cations responsible for decrease in the Am(III) sorption by the TODGA coated magnetic particles may be lanthanides present in the SHLW solutions.

3.3 Distribution isotherms Determination of loading capacity of the magnetic particles is necessary for the scale up of MACS process. Many isotherms have been reported in the literature for calculating the loading capacity of magnetic particles. However, the most commonly used models are Langmuir, Freundlich and Brunauer, Emmett and Teller (BET) [29]. In the present study, Langmuir and Freundlich models have been fitted to

K d (mL/g) of different metal ions Am(III) Pu(IV) U(VI) 1.05 × 104 7.5 × 103 31 2.21

8.9 × 103 5.0 × 103 5.0 × 103 –

420 156 1.3 × 103 1.0 × 104

Reference

Present work [26] [13] [12]


Fig. 4. Uptake of metal ions as a function of HNO3 concentration by TODGA impregnated magnetic particles; temperature 25 ◦ C.

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Fig. 6. Uptake of metal ions as a function of HNO3 concentration from SHLW by TODGA impregnated magnetic particles; temperature 25 ◦ C.

the experimental data to find out the binding capacity as well as the binding parameters. The Freundlich model can be expressed as: ae = K F Ce 1/n ,

(2)

where ae is the amount of solute adsorbed per unit weight of adsorbent, Ce is the residual liquid phase concentration, and K F and 1/n are characteristic constants, which are determined. The quantity K F gives the adsorption capacity of the magnetic particles. 1/n gives the intensity of the reaction, with a value less than one indicating favorable adsorption. The Langmuir model is formulated as: ae = Q 0 b Ce /1 + b Ce ,

Fig. 8. Distribution isotherms for Th(IV) from 5 M HNO3 with TODGA impregnated magnetic particles; temperature 25 ◦ C.

(3)

where Q 0 is the limiting adsorption capacity and b is related to the enthalpy of adsorption. The distribution isotherms for U(VI), Th(IV) and Eu(III) from 5 M HNO3 using 50% TODGA coated magnetic particles are given in Figs. 7, 8 and 9, respectively. The loading capacity from the experimental data is found to be 0.63 mmol/g, 0.80 mmol/g and 0.93 mmol/g for U(VI) Th(IV) and Eu(III), respectively. These results clearly indicate that the loading capacity of TODGA impregnated magnetic particles for Eu(III) is higher as compared to the previously reported DMDBTDMA [13] and Cyanex 923 [12] coated magnetic particles, highlighting that TODGA coated magnetic particles are found to be promising for the uptake of trivalent lanthanides and actinides. The isotherm models were fitted and the model parameters were obtained by non-linear least square fitting of the experimental data using Microcal Origin 6.0 program (Microcal Software Inc., 2000). The isotherm that correlates better with the experimental data is the one that exhibits the coefficient (R2 ) value closer to 1. The fitting constants or binding parameters obtaining by fitting Langmuir and Freundlich models to the experimental adsorption isotherm of TODGA impregnated magnetic particles for U(VI), Th(IV) and Eu(III) are shown in Table 2. The best fit observed in all the three cases was for Langmuir model suggesting that

Fig. 9. Distribution isotherms for Eu(III) from 5 M HNO3 with TODGA impregnated magnetic particles; temperature 25 ◦ C.

monolayer adsorption is the prevalent mechanism. The Langmuir model predicts a loading capacity of 0.652 mmol/g, 0.848 mmol/g and 0.964 mmol/g for U(VI), Th(IV) and Eu(III), respectively, for TODGA impregnated magnetic particles which are closer to the experimental values.

3.4 Stripping and recycling capacity of TODGA coated magnetic particles 0.01 M EDTA and 0.01 M HNO3 were evaluated for the stripping of Am(III) from the loaded magnetic particles. 0.01 M EDTA was found to be the better eluent among the two. To study the recycling capacity of the coated mag-


Fig. 7. Distribution isotherms for U(VI) from 5 M HNO3 with TODGA impregnated magnetic particles; temperature 25 ◦ C.

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U(VI)

Th(IV)

Eu(III)

Langmuir

Freundlich

Langmuir

Freundlich

Langmuir

Freundlich

b = 28.326 Q 0 = 0.652 mmol/g R2 = 0.973

K F = 0.65 1/n = 0.23 R2 = 0.75

b = 7.618 Q 0 = 0.848 mmol/g R2 = 0.982

K F = 0.67 1/n = 0.26 R2 = 0.83

b = 10.75 Q 0 = 0.964 mmol/g R2 = 0.977

K F = 0.79 1/n = 0.24 R2 = 0.78

netic particles, 50% TODGA coated particles was subjected to several extraction and back extraction processes (extraction of Am(III) from 4 M HNO3 and 0.01 M EDTA as a stripping agent). Even after five cycles of extractionback extraction, the recovery of Am(III) from the solution was found to be quantitative (K d values: first cycle = 11 502, second cycle = 11 485, third cycle = 11 532, fourth cycle = 11 473, fifth cycle = 11 562). Further, no loss of magnetic particles was noticed. The results demonstrate the stability and recycling capacity of TODGA coated magnetic particles.

4. Conclusion The magnetic particles prepared by impregnating TODGA displayed high affinities for trivalent actinides and moderate affinity towards hexavalent actinides at HNO3 concentrations in the range 1.0–6.0 M. On the other hand, poor uptake of fission products and Fe(III) were noticed. The K d values of actinides are less in the SHLW as compared to that in pure HNO3 , apparently due to the co-extraction of other metal ions present in SHLW. The loading capacity of TODGA coated magnetic particles was found to be 0.652 mmol/g, 0.848 mmol/g and 0.964 mmol/g for U(VI), Th(IV) and Eu(III), respectively. The stability, reusability, faster uptake kinetics and high loading capacity and extraction efficiency of TODGA coated magnetic particles make the MACS process an interesting alternative for the partitioning of radionuclides from nuclear waste streams. Acknowledgment. The authors wish to thank Prof. T. K. Chandrasekhar, Director, RRL, Trivandrum, for his constant encouragement and the Board of Research in Nuclear Sciences (BRNS), Mumbai, for financial support.

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Table 2. Model constants for extraction data of U(VI), Th(IV) and Eu(III). Q 0 is the limiting adsorption capacity, K F and 1/n are characteristic constants, R2 is the coefficient of determination and b is related to the enthalpy of adsorption.