- Email: [email protected]
Diluents induced tuning of the extraction characteristics of radioactive Cs from acidic nuclear waste solution using calix crown ether
Journal of Environmental Chemical Engineering 7 (2019) 103216
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Diluents induced tuning of the extraction characteristics of radioactive Cs from acidic nuclear waste solution using calix crown ether
T
⁎
Arijit Sengupta , Rajeswari B., R.M. Kadam Radiochemistry Division, Bhabha Atomic Research Centre Mumbai, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Cesium Calixcrown ether Extraction Speciation Radiolytic stability
The present investigation deals with understanding the effect of different diluents on the extraction properties of Cs+ using novel calix crown ether. The extraction efficiency was found follow the same order as the dielectric constant of the diluents. The extraction mechanism and the species involved during extraction were evaluated. The extraction process was found to be fast. Suitable stripping procedure was also optimized using different aqueous complexing agents. The effect of radiation exposure on the performance of these solvent systems in extraction of Cs+ ion was investigated up to 1000 kGy. The performance of octanol and isodecyl alcohol was found to be optimized with respect to radioactive plant scale operations.
1. Introduction Nuclear energy is considered to be a potential source to fulfill the ever increasing worldwide energy demands as most of the other sources are limited in nature [1]. But there are several challenges in the nuclear energy programme mainly associated with the safety issues. In a country like India, where we have very scarcity of sufficient amount of high quality uranium, closed nuclear fuel cycle was proposed for the recovery of useful fuel elements [2–5]. During processing of the spent fuel from nuclear waste, 137Cs is one of the isotopes responsible for the high radiotoxicity, mainly due to its long half-life (30.17 years).137Cs decays by beta emission to a metastable isomer of barium while the rest populates the ground state of barium and is responsible for all of the emissions of gamma rays in samples of 137Cs. This isotope of Cs finds application to calibrate radiation-detection equipment, in radiation therapy, in flow meters, moisture-density gauges, thickness gauges, and gamma ray well logging devices [6–8].The accidental intake of 137Cs was reported to be uniformly distributed throughout the body, with a preference to the soft tissue while fortunately, the biological half-life of 137 Cs is short (70 days). In view of this, there is a requirement of efficient and selective separation of 137Cs from the nuclear waste solution. The separation of 134, 137 Cs from nuclear power plant low-level waste solutions by ion exchange with potassium cobalt hexacyano-ferrate(II), potassium copper cobalt hexacyanoferrate(II) and ammonium phosphomolybdate was studied [9]. Electrically switched ion exchange method (a combination of ion exchange and electrochemical deposition) was also used for the
⁎
separation of Cs+ using high surface area electrodes and a flow system [10]. The sorption of Cs+ was also carried out using newly synthesized sorbent, (Calix [4] + Dodecanol)/SiO2–P [11]. The lower chemical potential of Cs+ ion makes the complexation chemistry less efficient hence several efforts were seen in literature for the separation of Cs+ based on the size inclusion [12–15]. Ligand molecules having proper cavity for fitting of Cs+ ion was used to induce the selective and efficient separation of Cs+. Crown ether of suitable cavity size was used for the effective extraction of Cs+ ion from nuclear waste. Such crown ether supported liquid membrane was also explored for clean supported membrane based clean separation [16,17]. Different substituent on calix crown ether was suitably used for enhancing the selectivity of the extraction. The 5-aminomethylcalix [4]arene-[bis4-(2-ethylhexyl)benzo-crown-6] was used for the extraction of Cs+ from acidic and basic mixtures of sodium nitrate and other concentrated salts [18]. Distribution studies on Cs+ were carried out from pressurized heavy water reactor (PHWR) simulated high level waste (SHLW) solution using calix[4]-bis-2,3-naphtho-crown-6 as the ligand [19]. A mixture of 1:1 nitrobenzene and toluene was evaluated as a suitable diluent. The importance of the diluents in the metal ion extraction is acceptable to fine tune the extraction efficiency, selectivity and other associated parameters [20,21]. In view of these, a systematic study was carried out to understand the extraction properties of Cs+ using the novel calix crown ether based ligand, i.e.,1, 3 di hexyloxy calix[4] arene crown 6 (DHCC) in different diluents. Efforts were also put to understand how the diluents properties play significant role in formation of metal-ligand complex and a
Corresponding author. E-mail address: [email protected] (A. Sengupta).
https://doi.org/10.1016/j.jece.2019.103216 Received 27 March 2019; Received in revised form 10 June 2019; Accepted 16 June 2019 Available online 17 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
separation, suitable aliquots of organic as well as aqueous phase were collected for radiometric determination of Cs. Finally, from the experimental results, LogDCs values were plotted as a function of log [NO3−], M. For kinetic studies, the extraction of 0.01 M DHCC dissolved in different solvents was done for different time intervals from 5 min to 120 min.. In this case 5 min centrifugation was also done for complete phase separation followed by determination of Cs concentration. For D values of stripping studies, the organic phase 0.01 M DHCC in different organic solvents was equilibrated with different 137Cs spiked stripping solutions. The solvent systems were nitric acid equilibrated for 4 h prior to its exposure to different radiation doses from 100 to 1000 kGy. After chemical and radiation exposure the solvents were checked for DCs extraction studies using 3.5 M HNO3137Cs spiked aqueous system. Fig. 1. Structure of DHCC ligand.
3. Results and discussion complete evaluation was performed to choose better diluents for the efficient Cs+ separation.
3.1. Effect of diluents on the extraction of Cs+ The extraction of Cs+ using DHCC can be broadly divided into three steps. In the first step, the ligand molecules; i.e. DHCC gets partitioned between aqueous and the organic phase. The nature of diluents and its interaction with DHCC molecules highly influence the step. In the second step, the Cs+ ions get coordinated by the ligand molecules in water. Here the coordinating power of the ligand molecules to the Cs+ ion plays the important role. The third step is the dissolution of the metal-ligand complex into the organic phase. This step is influenced by the interaction of metal-ligand complex with the diluent molecules. In general, the metal-ligand complex exhibits an overall charge separation, leading to the dipole moment. Therefore, the diluents with more polar character is expected to have large partitioning on the organic solvent. The DCs value for the DHCC in the different solvents were found follow the trend nitrobenzene > 1 octanol > iso-decanol > iso-decanol-dodecane mixture > chloroform. This trend was found to be similar to their dielectric constant. Fig. 2 depicts the DCs value for DHCC using different diluents.
2. Experimental 2.1. Reagents All the diluents and reagents used were of AR grade, procured from local market and were used directly. DHCC was purchased from Orion company and used without further treatment (Fig. 1). 137Cs tracer was procured from BRIT, Mumbai and its radiochemical purity was checked using gamma spectrometer having HPGe detector. 2.2. Distribution ratio studies For determining the DCs extraction studies, equal volumes of organic and aqueous phase were taken. The organic phase used was 0.01 M DHCC dissolved in different solvents and the aqueous phase used was 137 Cs tracer in 3.5 M nitric acid medium. The batch size was kept 2 mL each. The equilibriation time was kept 10 min. Both the phases were counted by gamma spectrometry using NaI(Tl) detector. The DCs values were calculated as the ratio of concentration of 137Cs in organic phase to the concentration of 137Cs in aqueous phase after extraction. The extraction studies of DHCC (0.01 M DHCC in different diluents) was studied in varying aqueous nitric acid concentrations from 0.01 M to 6 M to study the effect of nitric acid on the solvent systems. The distribution ratio of Cs (DCs) has been expressed by the following equation
DCs =
3.2. Effect of aqueous feed acidity To achieve the maximum extraction of Cs+ and to have an idea of extraction mechanism, the aqueous phase of the feed solution was varied in the range of 0.01 M–6 M HNO3 for the ligand in all the diluents under investigation. For octanol, iso-decanol and chloroform, the D values for Cs+ was found to increase with aqueous feed acidity attend maxima in the range of 3–4 M HNO3 with DCs values 8.36, 7.80 and 4.14, respectively followed by a decrease, while in case of the diluent comprises of a mixture of 50% iso decanol in dodecane, DCs values
[Cs+]org [Cs+]aq
(1) +
where, [Cs+]or and [Cs+]aq are the analytical concentration of Cs in organic and aqueous phase, respectively. The DCs value was monitored as a function of various DHCC concentration in the corresponding diluents with a feed of Cs+ tracer in 3.5 M HNO3. 2 mL of organic phase was allowed to equilibrate with 2 mL of aqueous phase containing Cs+ tracer for 10 min. After 10 min of equilibration, the system was centrifuged for 5 min. Then suitable aliquots from aqueous phase as well as organic phase were collected for the radiometric determination of 137Cs. Hence the DCs values were evaluated and logDCs was plotted as a function of log[DHCC], M. The variation of DCs value as a function of nitrate ion concentration have been carried out to investigate the number of nitrate ion associated with each Cs+ ion. The DHCC concentration was kept constant, 0.01 M in different diluents as specified. The aqueous phase feed acidity was kept constant, 0.01 M HNO3. The final nitrate ion concentration was achieved by dissolving different extent of NaNO3 in the aqueous phase. Like previous case, 2 mL of each phase was mixed and equilibrated for 10 min. After 5 min centrifugation for proper phase
Fig. 2. The variation of DCs value as a function of dielectric constant of the diluents. 2
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 4. The variation of H+ ion concentration after extraction of Cs+ ion from feed solutions having different Cs+ concentrations: evidence of cation exchange mechanism in nitrobenzene.
either positive or negative ion from the organic phase should come to the aqueous phase depending upon the value of ‘m’. If ‘m’ is zero, then the overall extracted species is positively charged, hence some positive charge from organic phase should come to aqueous phase. If ‘m’ is one, then the overall species will be neutral and no need of charge balance. If ‘m’ is more than 1, then the extracted species will be negatively charged and anions from the organic phase is expected to come to the organic phase. Since nitro benzene is a neutral diluent, therefore any charged species from nitrobenzene is very difficult to be expected. But there is probability of H+ and NO3− coming from diluent phase, as it was preequilibrate with the nitric acid. During pre-equilibration, depending upon the basicity of the ligand in the diluent H+ got extracted into organic phase. Now when this preequilibrated organic phase was allowed to have contact with the aqueous phase containing Cs for Cs extraction, the cationic extracted species would go from aqueous phase to organic phase, while to maintain charge neutrality equivalent H+ ion from organic phase would come down to aqueous phase. This argument was further confirmed by varying Cs concentration in aqueous phase and monitoring the change in aqueous phase acidity. Fig. 4 is showing that how the aqueous phase feed acidity has been changed after extraction of different concentration of Cs from 3 M HNO3 feed solution (before extraction acidity). When the Cs concentration in feed was 1 mM, the feed acidity became 3.23 M after extraction from original value of 3.0 M HNO3. In case of 10 mM, 100 mM and 1 M Cs initial concentrations; the feed acidity after extraction became 3.62 M, 4.08 M and 4.31 M, respectively. This enhancement in H+ ion concentration after extraction revealed that Cs+ gets extracted as cationic species and hence H+ ion gets exchanged confirming the predominance of 'cation exchange' mechanism in case of nitrobenzene. To prove the predominance of cation exchange mechanism, it is essential to prove the extracted species as [Cs(DHCC)]+. Based on the argued species, no anion is associated during the extraction of Cs+, which indicates that the extraction efficiency should be independent of the nature of acid, provided the concentration of the acid is same. To understand the effect of anions from different inorganic acids, we monitored the extraction efficiency of Cs in the form of DCs as a function of 3 M of HNO3, HCl, H2SO4 and HClO4. No significant change in DCs is observed (Fig. 5). This fact proves the argument of no anion association of extracted species and hence predominance of cation extraction mechanism in nitrobenzene.
Fig. 3. The variation of DCs values as a function of aqueous feed acidity for diluents (a) octanol, iso-decanol, 50% iso decanol in dodecane, chloroform and (b) nitrobenzene.
increased up to 3.5 M HNO3 followed by a plateau. This trend in D value profile suggested that the Cs+ extraction predominantly proceed via ‘solvation mechanism’ though neutral species [Fig. 3] [22,23]. The initial increase DCs value can be attributed to the law of mass action. At higher feed acidity, H+ ion competes strongly with Cs+ ion due to its large availability. + Csaq + nLorg + NO3−aq = CsNO3 . nLorg
(1)
+ Haq + n'Lorg + NO3−aq = HNO3 . n'Lorg
(2)
Where, n and n' are the number of ligand molecules associated with each Cs+ ion and H+ ion, respectively. On the contrary, in case of nitrobenzene as diluent, the maximum DCs value was obtained at 0.01 M HNO3 (89) and it gradually decreased with increase in aqueous feed acidity. This indicated that the extraction mechanism for nitrobenzene is entirely different than that of the other diluents. Similar trend was reported for the extraction of metal ion using selective ligand in ionic liquid diluents and can be ascribed to ‘ion exchange’ mechanism [24–26]. However, the variation in DCs as a function of nitrate ion concentration can reveal the number of nitrate ion associated with each Cs+ ion (subsequent study) and this also indicated the overall charge of the extracted species is positive for this particular case and hence confirmed the existence of cation exchange mechanism. + Csaq + nLorg + m NO3−aq → [Cs (NO3) m . nLorg ] m − 1
(3)
The Eq. (3) does not show the charge balance in both the phases, i.e. aqueous and organic phase. To maintain the overall electro neutrality, 3
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
constant for the extraction Kex can be expressed as
K ex =
[CsNO3 . nL] or + [Csaq ][CC 6or ] n [NO3−] m
(4)
On simplification the equation reduced to
K ex =
DCs [CC 6or ] n [NO3−] m
logDCs = logK ex' + nlog [CC 6or ] + mlog [NO3−]
(5) (6)
Now a logarithmic plot of DCs as a function of logarithm of ligand concentration at a particular concentration of nitrate ion should give straight line with a slope equal to the number of ligand molecule associated with each Cs+ ion. On the other hand, a variation of logarithm of DCs as a function of logarithm of nitrate ion concentration at constant ligand concentration should also give the straight line with a slope equal to the number of nitrate ion associated with each Cs+ ion. Fig. 6 showed the logDCs plots as a function of log[ligand] at constant [NO3−] while Fig. 7 showed the logDCs as a function of log[NO3−] at constant [ligand]. For all the diluents Cs+ ion was found to form 1:1 metalligand complex as evaluated from the slope values while Cs+ ion was found to associated with 1 nitrate ion for all diluents except nitrobenzene. Therefore, the overall speciation of the extracted complex was evaluated as Cs(NO3), DHCC for all the diluents except nitrobenzene, whereas in case of nitrobenzene it is Cs+ (DHCC). The mono positive extracted complex of Cs+ in nitrobenzene also reflects the proposed cation exchange mechanism.
Fig. 5. The DCs values for different acid with 3 M HNO3 using DHCC in nitrobenzene.
3.3. Determination of metal-ligand stoichiometry The metal – ligand stoichiometry decides the size and the nature (hydrophilicity/hydrophobicity) of the complex. To evaluate metal ligand stoichiometry, the DCs value was varied as a function of ligand concentration. The increase in DCs value with increase in the concentration of the ligand primarily suggested the participation of the ligand in the formation of the extracted species. The equilibrium
Fig. 6. The variation of DCs value as a function of [DHCC] at a constant nitrate ion concentration for different diluents. 4
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 7. The variation of DCs value as a function of nitrate ion concentration at a constant DHCC concentration for different diluents.
determining the mass transfer to the organic phase and hence the kinetics. Since the minimum time of equilibration was 5 min and 5 min for centrifugation, approximately, total 10 min, both the phases were in contact with each other. The viscosity coefficients for the organic diluents were not very high. As a consequence, within 10 min of equilibration, maximum DCs values were achieved. More data points were collected in the zone of 5–10 min of equilibration time to find out the relative viscosity effect of these diluent systems. Some minor changes were observed regarding the time of equilibration. However, it is very difficult to point out whether they are due to difference in viscosity coefficients of the diluents or they are within the experimental error limits. This investigation can only revealed that the extraction kinetics are very fast for all these diluent systems under investigation. Within an equilibration of 10 min, the complete extraction is being taken place.
3.4. Extraction kinetics The extraction kinetics is one of the important factors which decides the time of contact to reach the maximum distribution ratio values for the solvent systems. In plant scale operation also it very important because slower extraction kinetics require more time to reach equilibrium distribution ratio values. Therefore, more time, more energy is required in terms of keeping the mixing of both the phases and hence the associated cost of operation is expected to be more. Generally, the viscosity of the diluents and the conformational change of the supramolecular ligand might lead to slow down the extraction kinetics. For a highly viscous diluent, the mass transfer is expected to be slow leading to slower kinetics. If the achievement of preferred conformation for complexation from the most stable conformation of ligands require appreciable time, that also can slow down the overall extraction kinetics. Fortunately, all the diluents under the present investigation were found to demonstrate fast kinetics. It was observed that the maximum DCs value was achieved for all the diluents within 5 min and followed by plateau. Therefore, the time of equilibration is 5 min for all the diluents, which is an artifact of less viscous solvents. Fig. 8 is depicting the variation of DCs as a function of time of contact. The viscosity of the diluents plays the predominant role in
3.5. Effect of Na+, Sr2+, UO22+ Fig. 9 is depicting the effect of different concentration of interfering elements like Na+, Sr2+, UO22+ on the DCs values using DHCC in the diluents of interest. The concentrations of interfering elements were kept as 10 ppm, 100 ppm, and 1000 ppm, respectively. In case of UO22+ there is almost no change in the DCs values, while some minor reduction 5
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 8. The variation of DCs value as a function of time of contact between aqueous and organic phase.
Fig. 9. The effect of different concentrations of interfering ions on the DCs values for DHCC in different diluents.
in DCs values were observed in case of Na+. However, the presence of Sr2+ was found to decrease the DCs values drastically. This fact revealed the selectivity of DHCC for Cs+ over UO22+ and Na+. But the Sr2+ was found to get co-extracted in the organic phase resulting reduction in DCs values. In this kind of calix crown ether, mainly the selectivity depends on the size of the cavity of DHCC vis-avis the ionic radii of the competing ions.
back extraction. In iso-decanol, mixture of iso-decanol and dodecane and octanol; except aluminum nitrate and oxalic acid; the other reagents were found to be effective for stripping. It was also revealed that oxalic acid and sodium carbonate are the best strippant for all the diluents except nitrobenzene. Fig. 10 shows the DCs values using different aqueous phase as strippant and from the extracted complex in different diluents. The ionic radius of Cs+ is 300 pm, while that of Na+ is 227 pm. However that of H+ is 120 pm. During extraction, the Cs+ ion can suitably sit in the cavity of calix crown ether efficiently. During stripping with NaNO3, the Na+ is getting replaced and sits inside the cavity of DHCC. The H+ ion being too small compared to these metals cannot be exchanged as efficiently as Na+. Hence, NaNO3 was found to be more effective than HNO3. Moreover, NaNO3, being salt completely dissociated as ions. Therefore, the effective concentration of nitrate ion is more compared to that in HNO3 and hence, the complexing ability of nitrate ion. As a consequence, the NaNO3 has shown better stripping characteristics compared to HNO3 of same concentration
3.6. Back extraction of Cs+ from the loaded organic phase The stripping of the metal ion from the extracted phase is required to be investigated in order to investigate reusability of the solvent systems. The back extraction can be investigated either in terms of % stripping or the D values. Dilute nitric acid, nitrate ion in the form of sodium nitrate, aluminium nitrate; aqueous complexing agents: like oxalic acid and sodium carbonate were used for back extraction studies. In case of nitrobenzene, 0.01 M NaNO3 was found to be the best solution for stripping. In case of chloroform, sodium carbonate, oxalic acid, 0.01 M NaNO3 as well as 0.01 M HNO3 were found to be effective for 6
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 10. The DCs value by using different stripping agents in aqueous phase from the extracted complex at different diluents.
of gamma dose ranging from 100 kGy to 1000 kGy. For octanol, the DCs values were found to decrease only marginally with increase in the gamma exposure. There is drastic decrease in the DCs values was observed for irradiated nitrobenzene containing the ligand. The performance of chloroform was also poor. For IDA and octanol solvent system, the decrease in D value was observed as the radiation exposure increases. However, the difference in the D value for acid equilibration prior to irradiation is better indicating its chemical and radiation stability (%Extraction decreases from 90% to 75%). Fig. 11 showed the DCs values as a function of gamma dose for different solvents. The data shows the significant role of nitric acid in affecting the DCs values for assessing the role of solvent systems prior its application in plant scale purposes. 4. Conclusion Fig. 11. The performance degradation of the solvent systems at various gamma dose.
A systematic investigation was carried out in understanding the effect of diluents on the extraction and complexation of Cs+ with novel calix crown ether. Nitro benzene choloroform, isodecanol, octanol and a mixture of isodecanol-dodecane have been investigated as diluent in the extraction. In nitrobebzene ion exchange mechanism was found to be predominating while for other diluents conventional solvation mechanism was found to play major role. The metal ion was found to form 1:1 complex with DHCC, while the complex was found to be associated with a single nitrate ion for all the diluents except nitrobenzene. Suitable stripping procedure was optimized for extracted Cs ion. Stripping of nitrobenzene was poor in 0.01 M HNO3 among others. The extraction kinetics study revealed that the maximum D values were achieved within 20 min for all the solvent systems. The trend in DCs followed: nitrobenzene > octanol > iso-decanol > isodecanol-dodecane > chloroform. The chemical and radiolytic stability was also investigated for these solvent systems at various radiation exposure upto 1000 kGy. The IDA and octanol system was found to be better among others. As the solubility of octanol is marginally more than IDA in aqueous system, so IDA was taken for further studies. Now the solubility of DHCC is better in IDA-dodecane system than in 100%IDA system, so the IDA-dodecane solvent system was checked using laboratory scale mixer settlers. It indicated that more than 99.5% of the 137Cs could be separated from the HLLW. Stripping of the loaded organic phase was carried with 0.01 M HNO3 where more than 99% of the extracted 137Cs could be stripped from organic phase. The IDA-dodecane solvent system is then deployed in plant scale application purpose for separation of cesium from high level waste solutions and more than 2 lakh curie of 137 Cs was successfully separated from HLLW in plant scale and used for
3.7. Radiolytic stability Radiolytic stability of the solvent system plays a very crucial role for choosing appropriate separation system on plant scale. An interesting report was also found in the literature concerning chemically durable and radiation-resistant material, 3D uranyl organic framework having umbellate distortions in the uranyl equatorial planes for the sorption of actinides and long lived fission products like 137Cs due to their unique structural arrangement [27]. The solvent system, which essentially comprises of DHCC as extractant and various diluents were studied by (a) exposing them directly to irradiation and (b) equilibrating with 4 M HNO3 prior to irradiation for different doses such as 100 to 1000 KGy. Acidity of HNO3 was chosen 4 M as the high level waste streams are 3–4 M acidic in nature. The study was planned to check the conjugate effect of nitric acid as well as radiation exposure together. The solvent systems will be in the direct contact of the radio toxic metal ions and nitric acid for long time. Depending on the nature and amount of isotope and the contact time, their energy depositions into the solvent system varies. This deposited energy into the system may lead to the breaking of the weakest bond in the solvent system. Consequently, the selectivity and the efficiency of extraction for the solvent system will change. In the present investigation our main aim was to investigate separation procedure for 137Cs, which is a known gamma emitter. Therefore, this radiolytic stability is relevant in the present investigation. In this case the solvent systems were exposed to different amount 7
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
societal applications. [14]
Acknowledgment The authors wish to acknowledge Dr. P.K.Pujari, Associate Director, RC & I group and Head, Radiochemistry Division
[15]
References
[16]
[1] R. Duffey, Nuclear power as a basis for future electricity generation, J. Nucl. Eng. Rad. Sci. 1 (1) (2015), https://doi.org/10.1115/1.4029420. [2] J. Magill, V. Berthou, D. Haas, J. Galy, R. Schenkel, H.-W. Wiese, G. Heusener, J. Tommasi, G. Youinou, Impact limits of partitioning and transmutation scenarios on the radiotoxicity of actinides in radioactive waste, Biol. Sci. 42 (5) (2003) 263–277. [3] M. Salvatores, G. Palmiotti, Radioactive waste partitioning and transmutation within advanced fuel cycles: achievements and challenges, Prog. Part. Nucl. Phys. 66 (1) (2011) 144–166. [4] M. Salvatores, Nuclear fuel cycle strategies including partitioning and transmutation, Nucl. Eng. Des. 235 (7) (2005) 805–816. [5] V.K. Manchanda, P.N. Pathak, Amides and diamides as promising extractants in the back end of the nuclear fuel cycle: an overview, Sep. Purif. Technol. 35 (2) (2004) 85–103. [6] L. Boni, Rapid ion exchange analysis of radiocesium in milk, urine, sea water, and environmental samples, Anal. Chem. 38 (1) (1966) 89–92. [7] T.P. Valsala, S.C. Roy, J.G. Shah, J. Gabriel, K. Raj, V. Venugopal, Removal of radioactive caesium from low level radioactive waste (LLW) streams using cobalt ferrocyanide impregnated organic anion exchanger, J. Hazard. Mater. 166 (2–3) (2009) 1148–1153. [8] D. Bnarjee, M.A. Rao, J. Gabriel, S.K. Samanta, Recovery of purified radiocesium from acidic solution using ammonium molybdophosphate and resorcinol formaldehyde polycondensate resin, Desalination 232 (1–3) (2008) 172–180. [9] J. Lento, R. Harzula, Separation of cesium from nuclear waste solutions with hexacyanoferrate(ii)s and ammonium phosphomolybdate, Solv. Extr. Ion Exc. 5 (2) (1987) 343–352. [10] B. Sun, X.-G. Hao, Z.-D. Wang, G.-Q. Guan, Z.-L. Zhang, Y.-B. Li, S.-B. Liu, Separation of low concentration of cesium ion from wastewater by electrochemically switched ion exchange method: experimental adsorption kinetics analysis, J. Hazard. Mater. 233–234 (2012) 177–183. [11] Z. Chen, Y. Wu, Y. Wei, Cesium removal from high level liquid waste utilizing a macroporous silica-based calix[4]arene-R14 adsorbent modified with surfactants, Energy Procedia 39 (2013) 319–327. [12] J. Kříž, J. Dybal, E. Makrlík, P. Vaňura, B.A. Moyer, Interaction of cesium ions with calix[4]arene-bis(t-octylbenzo-18-crown-6): NMR and theoretical study, J. Phys. Chem. B 115 (23) (2011) 7578–7587. [13] H. Luo, S. Dai, P.V. Bonnesen, A.C. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Extraction of cesium ions from aqueous solutions using calix[4]arene-
[17]
[18]
[19]
[20]
[21] [22]
[23] [24]
[25]
[26] [27]
8
bis(tert-octylbenzo-crown-6) in ionic liquids, Anal. Chem. 76 (11) (2004) 3078–3083. B. Grüner, J. Plešek, J. Báča, J.F. Dozol, V. Lamare, I. Císařová, M. Bělohradský, J. Čáslavský, Crown ether substituted cobalta bis(dicarbollide) ions as selective extraction agents for removal of Cs+ and Sr2+ from nuclear waste, New J. Chem. 26 (2002) 867–875. P.K. Mohapatra, S.A. Ansari, A. Sarkar, A. Bhattacharya, V.K. Manchanda, Evaluation of calix-crown ionophores for selective separation of radio-cesium from acidic nuclear waste solution, Anal. Chim. Acta 571 (2) (2006) 308–314. Z. Asfari, C. Bressot, J. Vicens, C. Hill, J.-F. Dozol, H. Rouquette, S. Eymard, V. Lamare, B. Tournois, Doubly crowned calix[4]arenes in the 1,3-alternate conformation as cesium-selective carriers in supported liquid membranes, Anal. Chem. 67 (18) (1995) 3133–3139. P.K. Mohapatra, D.S. Lakshmi, D. Mohan, V.K. Manchanda, Selective transport of cesium using a supported liquid membrane containing di-t-butyl benzo 18 crown 6 as the carrier, J. Membr. Sci. 232 (1–2) (2004) 133–139. B.W. Harmon, D.D. Ensor, L.H. Delmau, B.A. Moyer, Extraction of cesium by a Calix [4]arene crown6 ether bearing a pendant amine group, Solv. Extr. Ion Exc. 25 (3) (2007) 373–388.. D.R. Raut, P.K. Mohapatra, S.A. Ansari, V.K. Manchanda, Use of Calix[4]-bis-2,3naphthocrown-6 for separation of cesium from pressurized heavy water reactor simulated high level waste solutions (PHWR-SHLW), Sep. Sci. Technol. 44 (15) (2009) 3664–3678. A. Sengupta, P.K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, Role of organic diluent on actinide ion extraction using a both-side diglycolamide-functionalized calix[4]arene, Supramol. Chem. 25 (9-11) (2013) 688–695. Y. Marcus, Diluent effects in solvent extraction, Solv. Extr. Ion Exch. 7 (4) (1989) 567–575. M. Singh, A. Sengupta, M.S. Murali, R.M. Kadam, Selective separation of uranium from nuclear waste solution by bis(2,4,4-trimethyl) pentylphosphinic acid in ionic liquid and molecular diluents: a comparative study, J. Radioanal. Nucl. Chem. 309 (3) (2016) 1199–1208. X. Sun, H. Luo, S. Dai, Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle, Chem. Rev. 112 (4) (2012) 2100–2128. N.K. Gupta, A. Sengupta, Substituted sulphoxide ligands in piperidinium based ionic liquid: novel solvent systems for the extraction of Pu4+ and PuO22+, J. Radioanal. Nucl. Chem. 311 (3) (2017) 1729–1739. A. Sengupta, M. Singh, M. Sundarajan, L. Yuan, Y. Fang, X. Yuan, W. Feng, Understanding the extraction and complexation of thorium using structurally modified CMPO functionalized pillar[5]arenes in ionic liquid: experimental and theoretical investigations, Inorg. Chem. Commun. 75 (2017) 33–36. Z. Lei, B. Chen, D.R. MacFarlane, Introduction: ionic liquids, Chem. Rev. 117 (10) (2017) 6633–6635. Y. Wang, Z. Liu, Y. Li, Z. Bai, W. Liu, Y. Wang, X. Xu, C. Xiao, D. Sheng, J. Diwu, J. Su, Z. Chai, T.E. Albrecht-Schmitt, S. Wang, Umbellate distortions of the uranyl coordination environment result in a stable and porous polycatenated framework that can effectively remove cesium from aqueous solutions, J. Am. Chem. Soc. 137 (19) (2015) 6144–6147.
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Diluents induced tuning of the extraction characteristics of radioactive Cs from acidic nuclear waste solution using calix crown ether
T
⁎
Arijit Sengupta , Rajeswari B., R.M. Kadam Radiochemistry Division, Bhabha Atomic Research Centre Mumbai, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Cesium Calixcrown ether Extraction Speciation Radiolytic stability
The present investigation deals with understanding the effect of different diluents on the extraction properties of Cs+ using novel calix crown ether. The extraction efficiency was found follow the same order as the dielectric constant of the diluents. The extraction mechanism and the species involved during extraction were evaluated. The extraction process was found to be fast. Suitable stripping procedure was also optimized using different aqueous complexing agents. The effect of radiation exposure on the performance of these solvent systems in extraction of Cs+ ion was investigated up to 1000 kGy. The performance of octanol and isodecyl alcohol was found to be optimized with respect to radioactive plant scale operations.
1. Introduction Nuclear energy is considered to be a potential source to fulfill the ever increasing worldwide energy demands as most of the other sources are limited in nature [1]. But there are several challenges in the nuclear energy programme mainly associated with the safety issues. In a country like India, where we have very scarcity of sufficient amount of high quality uranium, closed nuclear fuel cycle was proposed for the recovery of useful fuel elements [2–5]. During processing of the spent fuel from nuclear waste, 137Cs is one of the isotopes responsible for the high radiotoxicity, mainly due to its long half-life (30.17 years).137Cs decays by beta emission to a metastable isomer of barium while the rest populates the ground state of barium and is responsible for all of the emissions of gamma rays in samples of 137Cs. This isotope of Cs finds application to calibrate radiation-detection equipment, in radiation therapy, in flow meters, moisture-density gauges, thickness gauges, and gamma ray well logging devices [6–8].The accidental intake of 137Cs was reported to be uniformly distributed throughout the body, with a preference to the soft tissue while fortunately, the biological half-life of 137 Cs is short (70 days). In view of this, there is a requirement of efficient and selective separation of 137Cs from the nuclear waste solution. The separation of 134, 137 Cs from nuclear power plant low-level waste solutions by ion exchange with potassium cobalt hexacyano-ferrate(II), potassium copper cobalt hexacyanoferrate(II) and ammonium phosphomolybdate was studied [9]. Electrically switched ion exchange method (a combination of ion exchange and electrochemical deposition) was also used for the
⁎
separation of Cs+ using high surface area electrodes and a flow system [10]. The sorption of Cs+ was also carried out using newly synthesized sorbent, (Calix [4] + Dodecanol)/SiO2–P [11]. The lower chemical potential of Cs+ ion makes the complexation chemistry less efficient hence several efforts were seen in literature for the separation of Cs+ based on the size inclusion [12–15]. Ligand molecules having proper cavity for fitting of Cs+ ion was used to induce the selective and efficient separation of Cs+. Crown ether of suitable cavity size was used for the effective extraction of Cs+ ion from nuclear waste. Such crown ether supported liquid membrane was also explored for clean supported membrane based clean separation [16,17]. Different substituent on calix crown ether was suitably used for enhancing the selectivity of the extraction. The 5-aminomethylcalix [4]arene-[bis4-(2-ethylhexyl)benzo-crown-6] was used for the extraction of Cs+ from acidic and basic mixtures of sodium nitrate and other concentrated salts [18]. Distribution studies on Cs+ were carried out from pressurized heavy water reactor (PHWR) simulated high level waste (SHLW) solution using calix[4]-bis-2,3-naphtho-crown-6 as the ligand [19]. A mixture of 1:1 nitrobenzene and toluene was evaluated as a suitable diluent. The importance of the diluents in the metal ion extraction is acceptable to fine tune the extraction efficiency, selectivity and other associated parameters [20,21]. In view of these, a systematic study was carried out to understand the extraction properties of Cs+ using the novel calix crown ether based ligand, i.e.,1, 3 di hexyloxy calix[4] arene crown 6 (DHCC) in different diluents. Efforts were also put to understand how the diluents properties play significant role in formation of metal-ligand complex and a
Corresponding author. E-mail address: [email protected] (A. Sengupta).
https://doi.org/10.1016/j.jece.2019.103216 Received 27 March 2019; Received in revised form 10 June 2019; Accepted 16 June 2019 Available online 17 June 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
separation, suitable aliquots of organic as well as aqueous phase were collected for radiometric determination of Cs. Finally, from the experimental results, LogDCs values were plotted as a function of log [NO3−], M. For kinetic studies, the extraction of 0.01 M DHCC dissolved in different solvents was done for different time intervals from 5 min to 120 min.. In this case 5 min centrifugation was also done for complete phase separation followed by determination of Cs concentration. For D values of stripping studies, the organic phase 0.01 M DHCC in different organic solvents was equilibrated with different 137Cs spiked stripping solutions. The solvent systems were nitric acid equilibrated for 4 h prior to its exposure to different radiation doses from 100 to 1000 kGy. After chemical and radiation exposure the solvents were checked for DCs extraction studies using 3.5 M HNO3137Cs spiked aqueous system. Fig. 1. Structure of DHCC ligand.
3. Results and discussion complete evaluation was performed to choose better diluents for the efficient Cs+ separation.
3.1. Effect of diluents on the extraction of Cs+ The extraction of Cs+ using DHCC can be broadly divided into three steps. In the first step, the ligand molecules; i.e. DHCC gets partitioned between aqueous and the organic phase. The nature of diluents and its interaction with DHCC molecules highly influence the step. In the second step, the Cs+ ions get coordinated by the ligand molecules in water. Here the coordinating power of the ligand molecules to the Cs+ ion plays the important role. The third step is the dissolution of the metal-ligand complex into the organic phase. This step is influenced by the interaction of metal-ligand complex with the diluent molecules. In general, the metal-ligand complex exhibits an overall charge separation, leading to the dipole moment. Therefore, the diluents with more polar character is expected to have large partitioning on the organic solvent. The DCs value for the DHCC in the different solvents were found follow the trend nitrobenzene > 1 octanol > iso-decanol > iso-decanol-dodecane mixture > chloroform. This trend was found to be similar to their dielectric constant. Fig. 2 depicts the DCs value for DHCC using different diluents.
2. Experimental 2.1. Reagents All the diluents and reagents used were of AR grade, procured from local market and were used directly. DHCC was purchased from Orion company and used without further treatment (Fig. 1). 137Cs tracer was procured from BRIT, Mumbai and its radiochemical purity was checked using gamma spectrometer having HPGe detector. 2.2. Distribution ratio studies For determining the DCs extraction studies, equal volumes of organic and aqueous phase were taken. The organic phase used was 0.01 M DHCC dissolved in different solvents and the aqueous phase used was 137 Cs tracer in 3.5 M nitric acid medium. The batch size was kept 2 mL each. The equilibriation time was kept 10 min. Both the phases were counted by gamma spectrometry using NaI(Tl) detector. The DCs values were calculated as the ratio of concentration of 137Cs in organic phase to the concentration of 137Cs in aqueous phase after extraction. The extraction studies of DHCC (0.01 M DHCC in different diluents) was studied in varying aqueous nitric acid concentrations from 0.01 M to 6 M to study the effect of nitric acid on the solvent systems. The distribution ratio of Cs (DCs) has been expressed by the following equation
DCs =
3.2. Effect of aqueous feed acidity To achieve the maximum extraction of Cs+ and to have an idea of extraction mechanism, the aqueous phase of the feed solution was varied in the range of 0.01 M–6 M HNO3 for the ligand in all the diluents under investigation. For octanol, iso-decanol and chloroform, the D values for Cs+ was found to increase with aqueous feed acidity attend maxima in the range of 3–4 M HNO3 with DCs values 8.36, 7.80 and 4.14, respectively followed by a decrease, while in case of the diluent comprises of a mixture of 50% iso decanol in dodecane, DCs values
[Cs+]org [Cs+]aq
(1) +
where, [Cs+]or and [Cs+]aq are the analytical concentration of Cs in organic and aqueous phase, respectively. The DCs value was monitored as a function of various DHCC concentration in the corresponding diluents with a feed of Cs+ tracer in 3.5 M HNO3. 2 mL of organic phase was allowed to equilibrate with 2 mL of aqueous phase containing Cs+ tracer for 10 min. After 10 min of equilibration, the system was centrifuged for 5 min. Then suitable aliquots from aqueous phase as well as organic phase were collected for the radiometric determination of 137Cs. Hence the DCs values were evaluated and logDCs was plotted as a function of log[DHCC], M. The variation of DCs value as a function of nitrate ion concentration have been carried out to investigate the number of nitrate ion associated with each Cs+ ion. The DHCC concentration was kept constant, 0.01 M in different diluents as specified. The aqueous phase feed acidity was kept constant, 0.01 M HNO3. The final nitrate ion concentration was achieved by dissolving different extent of NaNO3 in the aqueous phase. Like previous case, 2 mL of each phase was mixed and equilibrated for 10 min. After 5 min centrifugation for proper phase
Fig. 2. The variation of DCs value as a function of dielectric constant of the diluents. 2
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 4. The variation of H+ ion concentration after extraction of Cs+ ion from feed solutions having different Cs+ concentrations: evidence of cation exchange mechanism in nitrobenzene.
either positive or negative ion from the organic phase should come to the aqueous phase depending upon the value of ‘m’. If ‘m’ is zero, then the overall extracted species is positively charged, hence some positive charge from organic phase should come to aqueous phase. If ‘m’ is one, then the overall species will be neutral and no need of charge balance. If ‘m’ is more than 1, then the extracted species will be negatively charged and anions from the organic phase is expected to come to the organic phase. Since nitro benzene is a neutral diluent, therefore any charged species from nitrobenzene is very difficult to be expected. But there is probability of H+ and NO3− coming from diluent phase, as it was preequilibrate with the nitric acid. During pre-equilibration, depending upon the basicity of the ligand in the diluent H+ got extracted into organic phase. Now when this preequilibrated organic phase was allowed to have contact with the aqueous phase containing Cs for Cs extraction, the cationic extracted species would go from aqueous phase to organic phase, while to maintain charge neutrality equivalent H+ ion from organic phase would come down to aqueous phase. This argument was further confirmed by varying Cs concentration in aqueous phase and monitoring the change in aqueous phase acidity. Fig. 4 is showing that how the aqueous phase feed acidity has been changed after extraction of different concentration of Cs from 3 M HNO3 feed solution (before extraction acidity). When the Cs concentration in feed was 1 mM, the feed acidity became 3.23 M after extraction from original value of 3.0 M HNO3. In case of 10 mM, 100 mM and 1 M Cs initial concentrations; the feed acidity after extraction became 3.62 M, 4.08 M and 4.31 M, respectively. This enhancement in H+ ion concentration after extraction revealed that Cs+ gets extracted as cationic species and hence H+ ion gets exchanged confirming the predominance of 'cation exchange' mechanism in case of nitrobenzene. To prove the predominance of cation exchange mechanism, it is essential to prove the extracted species as [Cs(DHCC)]+. Based on the argued species, no anion is associated during the extraction of Cs+, which indicates that the extraction efficiency should be independent of the nature of acid, provided the concentration of the acid is same. To understand the effect of anions from different inorganic acids, we monitored the extraction efficiency of Cs in the form of DCs as a function of 3 M of HNO3, HCl, H2SO4 and HClO4. No significant change in DCs is observed (Fig. 5). This fact proves the argument of no anion association of extracted species and hence predominance of cation extraction mechanism in nitrobenzene.
Fig. 3. The variation of DCs values as a function of aqueous feed acidity for diluents (a) octanol, iso-decanol, 50% iso decanol in dodecane, chloroform and (b) nitrobenzene.
increased up to 3.5 M HNO3 followed by a plateau. This trend in D value profile suggested that the Cs+ extraction predominantly proceed via ‘solvation mechanism’ though neutral species [Fig. 3] [22,23]. The initial increase DCs value can be attributed to the law of mass action. At higher feed acidity, H+ ion competes strongly with Cs+ ion due to its large availability. + Csaq + nLorg + NO3−aq = CsNO3 . nLorg
(1)
+ Haq + n'Lorg + NO3−aq = HNO3 . n'Lorg
(2)
Where, n and n' are the number of ligand molecules associated with each Cs+ ion and H+ ion, respectively. On the contrary, in case of nitrobenzene as diluent, the maximum DCs value was obtained at 0.01 M HNO3 (89) and it gradually decreased with increase in aqueous feed acidity. This indicated that the extraction mechanism for nitrobenzene is entirely different than that of the other diluents. Similar trend was reported for the extraction of metal ion using selective ligand in ionic liquid diluents and can be ascribed to ‘ion exchange’ mechanism [24–26]. However, the variation in DCs as a function of nitrate ion concentration can reveal the number of nitrate ion associated with each Cs+ ion (subsequent study) and this also indicated the overall charge of the extracted species is positive for this particular case and hence confirmed the existence of cation exchange mechanism. + Csaq + nLorg + m NO3−aq → [Cs (NO3) m . nLorg ] m − 1
(3)
The Eq. (3) does not show the charge balance in both the phases, i.e. aqueous and organic phase. To maintain the overall electro neutrality, 3
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
constant for the extraction Kex can be expressed as
K ex =
[CsNO3 . nL] or + [Csaq ][CC 6or ] n [NO3−] m
(4)
On simplification the equation reduced to
K ex =
DCs [CC 6or ] n [NO3−] m
logDCs = logK ex' + nlog [CC 6or ] + mlog [NO3−]
(5) (6)
Now a logarithmic plot of DCs as a function of logarithm of ligand concentration at a particular concentration of nitrate ion should give straight line with a slope equal to the number of ligand molecule associated with each Cs+ ion. On the other hand, a variation of logarithm of DCs as a function of logarithm of nitrate ion concentration at constant ligand concentration should also give the straight line with a slope equal to the number of nitrate ion associated with each Cs+ ion. Fig. 6 showed the logDCs plots as a function of log[ligand] at constant [NO3−] while Fig. 7 showed the logDCs as a function of log[NO3−] at constant [ligand]. For all the diluents Cs+ ion was found to form 1:1 metalligand complex as evaluated from the slope values while Cs+ ion was found to associated with 1 nitrate ion for all diluents except nitrobenzene. Therefore, the overall speciation of the extracted complex was evaluated as Cs(NO3), DHCC for all the diluents except nitrobenzene, whereas in case of nitrobenzene it is Cs+ (DHCC). The mono positive extracted complex of Cs+ in nitrobenzene also reflects the proposed cation exchange mechanism.
Fig. 5. The DCs values for different acid with 3 M HNO3 using DHCC in nitrobenzene.
3.3. Determination of metal-ligand stoichiometry The metal – ligand stoichiometry decides the size and the nature (hydrophilicity/hydrophobicity) of the complex. To evaluate metal ligand stoichiometry, the DCs value was varied as a function of ligand concentration. The increase in DCs value with increase in the concentration of the ligand primarily suggested the participation of the ligand in the formation of the extracted species. The equilibrium
Fig. 6. The variation of DCs value as a function of [DHCC] at a constant nitrate ion concentration for different diluents. 4
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 7. The variation of DCs value as a function of nitrate ion concentration at a constant DHCC concentration for different diluents.
determining the mass transfer to the organic phase and hence the kinetics. Since the minimum time of equilibration was 5 min and 5 min for centrifugation, approximately, total 10 min, both the phases were in contact with each other. The viscosity coefficients for the organic diluents were not very high. As a consequence, within 10 min of equilibration, maximum DCs values were achieved. More data points were collected in the zone of 5–10 min of equilibration time to find out the relative viscosity effect of these diluent systems. Some minor changes were observed regarding the time of equilibration. However, it is very difficult to point out whether they are due to difference in viscosity coefficients of the diluents or they are within the experimental error limits. This investigation can only revealed that the extraction kinetics are very fast for all these diluent systems under investigation. Within an equilibration of 10 min, the complete extraction is being taken place.
3.4. Extraction kinetics The extraction kinetics is one of the important factors which decides the time of contact to reach the maximum distribution ratio values for the solvent systems. In plant scale operation also it very important because slower extraction kinetics require more time to reach equilibrium distribution ratio values. Therefore, more time, more energy is required in terms of keeping the mixing of both the phases and hence the associated cost of operation is expected to be more. Generally, the viscosity of the diluents and the conformational change of the supramolecular ligand might lead to slow down the extraction kinetics. For a highly viscous diluent, the mass transfer is expected to be slow leading to slower kinetics. If the achievement of preferred conformation for complexation from the most stable conformation of ligands require appreciable time, that also can slow down the overall extraction kinetics. Fortunately, all the diluents under the present investigation were found to demonstrate fast kinetics. It was observed that the maximum DCs value was achieved for all the diluents within 5 min and followed by plateau. Therefore, the time of equilibration is 5 min for all the diluents, which is an artifact of less viscous solvents. Fig. 8 is depicting the variation of DCs as a function of time of contact. The viscosity of the diluents plays the predominant role in
3.5. Effect of Na+, Sr2+, UO22+ Fig. 9 is depicting the effect of different concentration of interfering elements like Na+, Sr2+, UO22+ on the DCs values using DHCC in the diluents of interest. The concentrations of interfering elements were kept as 10 ppm, 100 ppm, and 1000 ppm, respectively. In case of UO22+ there is almost no change in the DCs values, while some minor reduction 5
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 8. The variation of DCs value as a function of time of contact between aqueous and organic phase.
Fig. 9. The effect of different concentrations of interfering ions on the DCs values for DHCC in different diluents.
in DCs values were observed in case of Na+. However, the presence of Sr2+ was found to decrease the DCs values drastically. This fact revealed the selectivity of DHCC for Cs+ over UO22+ and Na+. But the Sr2+ was found to get co-extracted in the organic phase resulting reduction in DCs values. In this kind of calix crown ether, mainly the selectivity depends on the size of the cavity of DHCC vis-avis the ionic radii of the competing ions.
back extraction. In iso-decanol, mixture of iso-decanol and dodecane and octanol; except aluminum nitrate and oxalic acid; the other reagents were found to be effective for stripping. It was also revealed that oxalic acid and sodium carbonate are the best strippant for all the diluents except nitrobenzene. Fig. 10 shows the DCs values using different aqueous phase as strippant and from the extracted complex in different diluents. The ionic radius of Cs+ is 300 pm, while that of Na+ is 227 pm. However that of H+ is 120 pm. During extraction, the Cs+ ion can suitably sit in the cavity of calix crown ether efficiently. During stripping with NaNO3, the Na+ is getting replaced and sits inside the cavity of DHCC. The H+ ion being too small compared to these metals cannot be exchanged as efficiently as Na+. Hence, NaNO3 was found to be more effective than HNO3. Moreover, NaNO3, being salt completely dissociated as ions. Therefore, the effective concentration of nitrate ion is more compared to that in HNO3 and hence, the complexing ability of nitrate ion. As a consequence, the NaNO3 has shown better stripping characteristics compared to HNO3 of same concentration
3.6. Back extraction of Cs+ from the loaded organic phase The stripping of the metal ion from the extracted phase is required to be investigated in order to investigate reusability of the solvent systems. The back extraction can be investigated either in terms of % stripping or the D values. Dilute nitric acid, nitrate ion in the form of sodium nitrate, aluminium nitrate; aqueous complexing agents: like oxalic acid and sodium carbonate were used for back extraction studies. In case of nitrobenzene, 0.01 M NaNO3 was found to be the best solution for stripping. In case of chloroform, sodium carbonate, oxalic acid, 0.01 M NaNO3 as well as 0.01 M HNO3 were found to be effective for 6
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
Fig. 10. The DCs value by using different stripping agents in aqueous phase from the extracted complex at different diluents.
of gamma dose ranging from 100 kGy to 1000 kGy. For octanol, the DCs values were found to decrease only marginally with increase in the gamma exposure. There is drastic decrease in the DCs values was observed for irradiated nitrobenzene containing the ligand. The performance of chloroform was also poor. For IDA and octanol solvent system, the decrease in D value was observed as the radiation exposure increases. However, the difference in the D value for acid equilibration prior to irradiation is better indicating its chemical and radiation stability (%Extraction decreases from 90% to 75%). Fig. 11 showed the DCs values as a function of gamma dose for different solvents. The data shows the significant role of nitric acid in affecting the DCs values for assessing the role of solvent systems prior its application in plant scale purposes. 4. Conclusion Fig. 11. The performance degradation of the solvent systems at various gamma dose.
A systematic investigation was carried out in understanding the effect of diluents on the extraction and complexation of Cs+ with novel calix crown ether. Nitro benzene choloroform, isodecanol, octanol and a mixture of isodecanol-dodecane have been investigated as diluent in the extraction. In nitrobebzene ion exchange mechanism was found to be predominating while for other diluents conventional solvation mechanism was found to play major role. The metal ion was found to form 1:1 complex with DHCC, while the complex was found to be associated with a single nitrate ion for all the diluents except nitrobenzene. Suitable stripping procedure was optimized for extracted Cs ion. Stripping of nitrobenzene was poor in 0.01 M HNO3 among others. The extraction kinetics study revealed that the maximum D values were achieved within 20 min for all the solvent systems. The trend in DCs followed: nitrobenzene > octanol > iso-decanol > isodecanol-dodecane > chloroform. The chemical and radiolytic stability was also investigated for these solvent systems at various radiation exposure upto 1000 kGy. The IDA and octanol system was found to be better among others. As the solubility of octanol is marginally more than IDA in aqueous system, so IDA was taken for further studies. Now the solubility of DHCC is better in IDA-dodecane system than in 100%IDA system, so the IDA-dodecane solvent system was checked using laboratory scale mixer settlers. It indicated that more than 99.5% of the 137Cs could be separated from the HLLW. Stripping of the loaded organic phase was carried with 0.01 M HNO3 where more than 99% of the extracted 137Cs could be stripped from organic phase. The IDA-dodecane solvent system is then deployed in plant scale application purpose for separation of cesium from high level waste solutions and more than 2 lakh curie of 137 Cs was successfully separated from HLLW in plant scale and used for
3.7. Radiolytic stability Radiolytic stability of the solvent system plays a very crucial role for choosing appropriate separation system on plant scale. An interesting report was also found in the literature concerning chemically durable and radiation-resistant material, 3D uranyl organic framework having umbellate distortions in the uranyl equatorial planes for the sorption of actinides and long lived fission products like 137Cs due to their unique structural arrangement [27]. The solvent system, which essentially comprises of DHCC as extractant and various diluents were studied by (a) exposing them directly to irradiation and (b) equilibrating with 4 M HNO3 prior to irradiation for different doses such as 100 to 1000 KGy. Acidity of HNO3 was chosen 4 M as the high level waste streams are 3–4 M acidic in nature. The study was planned to check the conjugate effect of nitric acid as well as radiation exposure together. The solvent systems will be in the direct contact of the radio toxic metal ions and nitric acid for long time. Depending on the nature and amount of isotope and the contact time, their energy depositions into the solvent system varies. This deposited energy into the system may lead to the breaking of the weakest bond in the solvent system. Consequently, the selectivity and the efficiency of extraction for the solvent system will change. In the present investigation our main aim was to investigate separation procedure for 137Cs, which is a known gamma emitter. Therefore, this radiolytic stability is relevant in the present investigation. In this case the solvent systems were exposed to different amount 7
Journal of Environmental Chemical Engineering 7 (2019) 103216
A. Sengupta, et al.
societal applications. [14]
Acknowledgment The authors wish to acknowledge Dr. P.K.Pujari, Associate Director, RC & I group and Head, Radiochemistry Division
[15]
References
[16]
[1] R. Duffey, Nuclear power as a basis for future electricity generation, J. Nucl. Eng. Rad. Sci. 1 (1) (2015), https://doi.org/10.1115/1.4029420. [2] J. Magill, V. Berthou, D. Haas, J. Galy, R. Schenkel, H.-W. Wiese, G. Heusener, J. Tommasi, G. Youinou, Impact limits of partitioning and transmutation scenarios on the radiotoxicity of actinides in radioactive waste, Biol. Sci. 42 (5) (2003) 263–277. [3] M. Salvatores, G. Palmiotti, Radioactive waste partitioning and transmutation within advanced fuel cycles: achievements and challenges, Prog. Part. Nucl. Phys. 66 (1) (2011) 144–166. [4] M. Salvatores, Nuclear fuel cycle strategies including partitioning and transmutation, Nucl. Eng. Des. 235 (7) (2005) 805–816. [5] V.K. Manchanda, P.N. Pathak, Amides and diamides as promising extractants in the back end of the nuclear fuel cycle: an overview, Sep. Purif. Technol. 35 (2) (2004) 85–103. [6] L. Boni, Rapid ion exchange analysis of radiocesium in milk, urine, sea water, and environmental samples, Anal. Chem. 38 (1) (1966) 89–92. [7] T.P. Valsala, S.C. Roy, J.G. Shah, J. Gabriel, K. Raj, V. Venugopal, Removal of radioactive caesium from low level radioactive waste (LLW) streams using cobalt ferrocyanide impregnated organic anion exchanger, J. Hazard. Mater. 166 (2–3) (2009) 1148–1153. [8] D. Bnarjee, M.A. Rao, J. Gabriel, S.K. Samanta, Recovery of purified radiocesium from acidic solution using ammonium molybdophosphate and resorcinol formaldehyde polycondensate resin, Desalination 232 (1–3) (2008) 172–180. [9] J. Lento, R. Harzula, Separation of cesium from nuclear waste solutions with hexacyanoferrate(ii)s and ammonium phosphomolybdate, Solv. Extr. Ion Exc. 5 (2) (1987) 343–352. [10] B. Sun, X.-G. Hao, Z.-D. Wang, G.-Q. Guan, Z.-L. Zhang, Y.-B. Li, S.-B. Liu, Separation of low concentration of cesium ion from wastewater by electrochemically switched ion exchange method: experimental adsorption kinetics analysis, J. Hazard. Mater. 233–234 (2012) 177–183. [11] Z. Chen, Y. Wu, Y. Wei, Cesium removal from high level liquid waste utilizing a macroporous silica-based calix[4]arene-R14 adsorbent modified with surfactants, Energy Procedia 39 (2013) 319–327. [12] J. Kříž, J. Dybal, E. Makrlík, P. Vaňura, B.A. Moyer, Interaction of cesium ions with calix[4]arene-bis(t-octylbenzo-18-crown-6): NMR and theoretical study, J. Phys. Chem. B 115 (23) (2011) 7578–7587. [13] H. Luo, S. Dai, P.V. Bonnesen, A.C. Buchanan, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Extraction of cesium ions from aqueous solutions using calix[4]arene-
[17]
[18]
[19]
[20]
[21] [22]
[23] [24]
[25]
[26] [27]
8
bis(tert-octylbenzo-crown-6) in ionic liquids, Anal. Chem. 76 (11) (2004) 3078–3083. B. Grüner, J. Plešek, J. Báča, J.F. Dozol, V. Lamare, I. Císařová, M. Bělohradský, J. Čáslavský, Crown ether substituted cobalta bis(dicarbollide) ions as selective extraction agents for removal of Cs+ and Sr2+ from nuclear waste, New J. Chem. 26 (2002) 867–875. P.K. Mohapatra, S.A. Ansari, A. Sarkar, A. Bhattacharya, V.K. Manchanda, Evaluation of calix-crown ionophores for selective separation of radio-cesium from acidic nuclear waste solution, Anal. Chim. Acta 571 (2) (2006) 308–314. Z. Asfari, C. Bressot, J. Vicens, C. Hill, J.-F. Dozol, H. Rouquette, S. Eymard, V. Lamare, B. Tournois, Doubly crowned calix[4]arenes in the 1,3-alternate conformation as cesium-selective carriers in supported liquid membranes, Anal. Chem. 67 (18) (1995) 3133–3139. P.K. Mohapatra, D.S. Lakshmi, D. Mohan, V.K. Manchanda, Selective transport of cesium using a supported liquid membrane containing di-t-butyl benzo 18 crown 6 as the carrier, J. Membr. Sci. 232 (1–2) (2004) 133–139. B.W. Harmon, D.D. Ensor, L.H. Delmau, B.A. Moyer, Extraction of cesium by a Calix [4]arene crown6 ether bearing a pendant amine group, Solv. Extr. Ion Exc. 25 (3) (2007) 373–388.. D.R. Raut, P.K. Mohapatra, S.A. Ansari, V.K. Manchanda, Use of Calix[4]-bis-2,3naphthocrown-6 for separation of cesium from pressurized heavy water reactor simulated high level waste solutions (PHWR-SHLW), Sep. Sci. Technol. 44 (15) (2009) 3664–3678. A. Sengupta, P.K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, Role of organic diluent on actinide ion extraction using a both-side diglycolamide-functionalized calix[4]arene, Supramol. Chem. 25 (9-11) (2013) 688–695. Y. Marcus, Diluent effects in solvent extraction, Solv. Extr. Ion Exch. 7 (4) (1989) 567–575. M. Singh, A. Sengupta, M.S. Murali, R.M. Kadam, Selective separation of uranium from nuclear waste solution by bis(2,4,4-trimethyl) pentylphosphinic acid in ionic liquid and molecular diluents: a comparative study, J. Radioanal. Nucl. Chem. 309 (3) (2016) 1199–1208. X. Sun, H. Luo, S. Dai, Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle, Chem. Rev. 112 (4) (2012) 2100–2128. N.K. Gupta, A. Sengupta, Substituted sulphoxide ligands in piperidinium based ionic liquid: novel solvent systems for the extraction of Pu4+ and PuO22+, J. Radioanal. Nucl. Chem. 311 (3) (2017) 1729–1739. A. Sengupta, M. Singh, M. Sundarajan, L. Yuan, Y. Fang, X. Yuan, W. Feng, Understanding the extraction and complexation of thorium using structurally modified CMPO functionalized pillar[5]arenes in ionic liquid: experimental and theoretical investigations, Inorg. Chem. Commun. 75 (2017) 33–36. Z. Lei, B. Chen, D.R. MacFarlane, Introduction: ionic liquids, Chem. Rev. 117 (10) (2017) 6633–6635. Y. Wang, Z. Liu, Y. Li, Z. Bai, W. Liu, Y. Wang, X. Xu, C. Xiao, D. Sheng, J. Diwu, J. Su, Z. Chai, T.E. Albrecht-Schmitt, S. Wang, Umbellate distortions of the uranyl coordination environment result in a stable and porous polycatenated framework that can effectively remove cesium from aqueous solutions, J. Am. Chem. Soc. 137 (19) (2015) 6144–6147.