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Fractionation of pure solvent components from degraded PUREX solvent using room temperature ionic liquids
Separation and Purification Technology 122 (2014) 67–72
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Fractionation of pure solvent components from degraded PUREX solvent using room temperature ionic liquids S. Panja ⇑, S.K. Misra, S.C. Tripathi ⇑, M. Bindu, P.M. Gandhi Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 18 August 2013 Received in revised form 26 October 2013 Accepted 29 October 2013 Available online 4 November 2013
With the view point of recovery of pure solvent components from degraded PUREX (Plutonium URanium Extraction) solvent, comprising of mixture of 30% TBP–HDBP-n-dodecane, three homologues of room temperature ionic liquids (CnmimCl, n = 4, 6, 8) have been investigated in batch equilibration mode (liquid–liquid extraction). While lower homologues of RTIL (C4 and C6) could effectively separate out HDBP from pure TBP–HDBP-n-dodecane mixture, the higher homologue C8mimCl could quantitatively remove TBP and HDBP together leaving behind pure n-dodecane as organic phase. Efficacy of RTIL solutions have also been studied as a function of RTIL concentration, composition of various components in organic mixture, kinetics of extraction, etc. These ionic liquids were found to retain their efficacy even after five cycles of operation after their regeneration. Studies from actual degraded solvent showed that C8mimCl could remove HDBP quantitatively and TBP (45% of the initial concentration) whereas C6mimCl and C4mimCl showed very little extraction even for HDBP. The mechanism of extraction was found to be formation of micelles and ion-pair formation. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: RTIL PUREX HDBP TBP Reusability
1. Introduction Tri-butyl Phosphate has been recognized as the work horse of nuclear industry [1]. In spent nuclear fuel reprocessing 30% TBP in n-dodecane has been used as a solvent for the past 3–4 decades to reprocess spent nuclear fuel [2]. The process known as PUREX (Plutonium Uranium EXtraction) aims at complete recovery of uranium and plutonium from fission products and also from each other in nitric acid medium. The solvent used in this process is 30% TBP in n-dodecane. In this process, prevailing harsh chemical and radiolytic conditions cause hydrolysis and de-alkylation of TBP during repeated use. As a result a host of degradation products of TBP viz, dibutyl phosphate (HDBP), monobutyl phosphate (MBP) and phosphoric acid [3–5] are formed. The formation of HDBP is only the first step towards complete de-alkylation of TBP that may eventually lead to the formation of orthophosphoric acid. MBP and orthophosphoric acid being water soluble do not hinder much in the process performance. But HDBP forms organic soluble complexes with several cations like (UO2)2+, Pu4+ and ZrðOHÞ2þ 2 , normally encountered in PUREX process. The net result of the presence of HDBP in PUREX process is a progressive reduction in the efficiency of removal of fission products, loss of uranium and plutonium into waste effluent streams and emulsification or precipita⇑ Corresponding authors. Tel.: +91 22 25591201. E-mail addresses: (S.C. Tripathi).
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tion within the contactors. Hence, separation and removal of HDBP from the TBP is of importance for efficient operation of PUREX process. There are various methods for the regeneration and management of this degraded organic solvent such as fractional distillation, alkaline hydrolysis and incineration. These methods are very poor with respect to high energy consumption, secondary waste generation, formation of highly corrosive phosphorous compounds like P2O5. In view of the problems associated with the presently adopted methods, a procedure for the separation of TBP and HDBP by a new class of organic compounds known as Room Temperature Ionic Liquids (RTIL) has been tried and reported here. Room temperature ionic liquids have gained considerable importance worldwide due to various properties like non-volatility, non-flammability, moisture stability, chemical tunability to task specific actions [6–10], etc. They are being used in diverse applications like (i) Synthesis of novel materials [11]. (ii) As solvents in catalytic reactions [12,13]. (iii) Separation of CO2 [14,15]. (iv) Separation of metal ions [16–19]. (v) As sensors of volatile organic vapors [20], etc. Application of RTILs for the separation of organic compounds from various streams has also been explored extensively. McFarlanea et al. [21] explored the possibility of RTILs for separating organics from water. They investigated a host of imidazolium, pyrrolidinium, phosphonium based hydrophobic ionic liquids for separation of different polar organic compounds typical of water contaminants. It was observed that some of the ionic liquids studied showed large uptake for certain organics. Hu et al. [22] reported the possibility of separation of ethyl acetate and
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ethanol using ionic liquids with tetrafluoroborate anion. The experimental results show that 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate {[C2OHmim]BF4} is a potential candidate for separation of ethyl acetate and ethanol by liquid–liquid extraction. Galan Sanchez et al. [23] investigated the potential of room temperature ionic liquids for separation of ethylene from ethane. The olefin/paraffin selectivity at a pressure of 1 bar was 30–50 higher in the silver based RTILs than in standard ionic liquids as reported by them. Ortiz et al. [24] reported room temperature ionic liquids containing silver salt as reactive media for separation of propylene/propane gas mixtures. Propylene absorption was found to be chemically enhanced in the silver-based RTILs and was considerably higher than that in the standard RTILs. Absorption of propane in the silver-based RTILs was based on the physical interactions only. Branco et al. [25] reported highly selective transport of organic compounds across supported liquid membranes using ionic liquids. They studied three organic isomeric amines hexylamine, diisopropylamine and triethylamine using 1n-butyl-3-methylimidazolium hexafluorophosphate as carrier. The transport of diisopropylamine was found to be much higher compared to two other amines. But so far no report in the literature is available on the separation of HDBP and TBP from degraded solvent using room temperature ionic liquids. In the current study, different RTILs were investigated for their behavior towards fractionation of TBP and HDBP from degraded PUREX solvent. Extraction of TBP and HDBP from organic mixture using liquid–liquid extraction by RTIL was studied to optimize the following parameters: (i) RTIL concentration. (ii) Extraction kinetics. (iii) Effect of HDBP/TBP concentration in organic phase. Studies were also carried out to investigate the efficacy of the RTILs towards removal of HDBP and TBP from degraded solvent generated during repeated recycling of the PUREX extraction system. 2. Experimental 2.1. Reagents RTILs (1-alkyl-3-methyl-imidazolium chloride, alkyl = butyl, hexyl and octyl) were procured from Aldrich and were used as such. Commercial grade TBP from Heavy Water Plant, Talchar, India, was purified by washing it with 2% (w/v) solution of sodium carbonate followed by distilled water. The purity of TBP was found better than 99% (v/v) by gas–liquid chromatographic (GLC) assay. The n-dodecane from Aldrich was used for dilution of TBP to 30% (v/v). HDBP was procured from Fluka AG (Switzerland). Stock solutions of different concentrations of HDBP in 30% (v/v) TBP–n-dodecane were prepared by suitable dilution from stock solution and standardization by gas chromatography. The diazomethane was prepared by treating p-tolylsulphonyl methylnitrosamide with alcoholic potassium hydroxide. All the other reagents used in this study were of analytical grade. 2.2. Liquid–liquid extraction 5 mL of 30% TBP in n-dodecane in presence or in absence of desired concentration of HDBP (1–5 g/L) were equilibrated with 5 mL of aqueous solutions of RTIL in a separating funnel for a fixed time interval. After equilibration the phases were kept for settlement, after which organic phase was taken for analysis. The concentration of organic phase was determined by gas chromatography as described below. The regeneration of aqueous RTIL salt solution was done by acidifying the aqueous solution followed by removal of organophosphates by scrub with benzene–benzyl alcohol mixture. In this study (1:1) (w/v) aqueous solution of RTILs was equilibrated with organic phase containing 3 g/L HDBP in 30%
TBP–n-dodecane. After equilibration, DBP was removed from equilibrated aqueous solution using 10% benzene–benzyl alcohol solution. The aqueous solution containing RTILs, depleted of HDBP, was re-contacted with fresh lot of organic solution having similar composition of DBP/TBP as to that of first contact. Such procedure was followed for 5 consecutive cycles. These regenerated RTILs were used as such for further extraction studies. 2.3. Gas–liquid chromatographic analysis of TBP, HDBP and dodecane TBP and HDBP was analyzed by GLC (Shimadzu, Japan) using thermal conductivity detector. For HDBP, GLC was carried out after esterifying the HDBP into volatile methyldibutyl phosphate with ether solution of diazomethane prior to its gas chromatographic assay. The quantification of HDBP was made by using a calibration plot. A 10% XE-60 column (1.5 m 0.32 cm) was used under temperature programming along with other operating parameters. Initial column temperature of 170 °C was maintained for 1 min. The heating rate of the column was 10 °C/min till the final temperature of column reached to 230 °C, which was maintained for 10 min. Throughout the study, the injection port temperature and carrier gas (He) flow were maintained at 240 °C and 40 mL/min, respectively. 3. Results and discussion 3.1. Extraction of HDBP 3.1.1. Kinetics of extraction Kinetics of the extraction of HDBP was investigated for the three RTILs mentioned earlier. It was observed that all three RTILs showed relatively fast kinetics for the uptake for HDBP. The kinetics of HDBP extraction for the three different RTILs is shown in Fig. 1. Concentration of DBP in 30% TBP was maintained constant at 3 g/L in each of batch equilibration experiments using three different RTILs. It was observed that 15 min of equilibration time resulted in quantitative uptake of DBP by each of the three RTIL solutions from the organic phase. From Fig. 1 it is also clear that the kinetics of HDBP uptake by these three RTILs was independent of carbon chain length attached to the imidazolium group. 3.1.2. HDBP concentration Amount of DBP in the degraded PUREX solvent is dependent on the extent of radiation exposure encountered during extraction process. Efficacy of the three RTILs under optimized batch equilibration process was therefore studied as a function of varying
Conc of HDBP in Org Phase (g/L)
68
3.0
Cl Cl C 8 mim Cl
C4 mim 2.5
C 6 mim
2.0 1.5 1.0 0.5 0.0 0
5
10
15
20
25
30
Time (min) Fig. 1. Kinetics of HDBP extraction from n-dodecane using (1:1) solution of CnmimCl (n = 4, 6, 8) in H2O, feed – 3 g/L DBP in 30% TBP–n-dodecane.
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% TBP in n-dodecane phase
HDBP concentration in 30% TBP–n-dodecane system. The concentration of HDBP was varied from 1 g/L to 5 g/L in the organic phase whereas (1:1) (w/v) aqueous solution of each of three RTILs was chosen as the aqueous solution. The maximum concentration of HDBP observed in degraded solutions from PUREX process is found in the range of 4–5 g/L and so 5 g/L was chosen as the maximum concentration of HDBP in our studies. From the studies it was evident that HDBP (at concentration of 5 g/L) was quantitatively separated by three RTILs indicating that DBP extraction by the three studied RTILs (at chosen concentration level) is independent of carbon chain length attached to the imidazolium group.
40 C8 mimCl 35 30 25 20 15 10 0
3.1.3. Reusability of RTILs In order to prove the economic viability of RTILs for bulk scale application, it is imperative to assess the extent of its recyclability/reuse along with its efficacy of the used ionic liquids. Experimental details for the study have been described earlier. It was observed that even after 5 cycles of operation there was no reduction in the extraction efficiency of all the three RTILs. Analysis of organic phase after each equilibration showed quantitative extraction of HDBP for all the three RTILs studied. Hence, the extraction potential of the RTILs towards HDBP removal from DBP–TBP–ndodecane system (organic phase) remains unaffected for 5 successive cycles of extraction and depletion. The observed reusability potential of the aqueous solutions of RTILs thus offers hope for their economically viability for large scale purification of degraded PUREX solvent with respect to removal of major harmful species DBP (an acidic metal complexant) from a mixture of TBP and DBP in n-dodecane. 3.2. Removal of TBP for recovery of pure diluent Regeneration of degraded PUREX solvent follows two different pathways. The first one is removal HDBP from degraded TBP–ndodecane mixture for reuse of purified TBP–n-dodecane. The second one is removal of TBP and HDBP simultaneously from mixed DBP–TBP–n-dodecane phase to recover pure dodecane for recycling. Hence the current study was also extended to understand the extraction behavior of TBP if any from the organic mixture using RTILs. Initial experiment with distilled water only as aqueous phase showed no extraction of TBP from the 30% TBP–n-dodecane phase. Even for lower homologues of RTILs (viz, hexyl and butyl), (1:1) (w/v) aqueous solution showed no extraction efficiency for TBP. But when we used higher homologue (octyl) of RTIL in the aqueous phase, reasonably good extraction efficiency for TBP was observed. In contrary to extraction of HDBP where quantitative extraction was observed, TBP extraction was found to be only 45% of initial feed concentration. So in contrary to HDBP, the carbon chain length attached to the imidazolium group can be said to have profound impact on the extractability of TBP. Hence forth, further studies on removal of TBP from TBP–n-dodecane mixture were carried out using C8mimCl only. 3.2.1. Kinetics of extraction Kinetics of extraction of TBP from an organic solution containing 30% TBP in n-dodecane was studied using (1:1) (w/v) aqueous solution of C8mimCl. The results (given in Fig. 2) show that it requires 30 min of equilibration time to attain equilibrium. Hence it can be inferred that the kinetics of extraction of TBP is slower compared to that of HDBP. 3.2.2. RTIL concentration Cost intensive nature of RTIL makes it imperative to find out the lowest possible concentration of RTIL in the aqueous solution that can be used for maximum removal of TBP from TBP–n-dodecane phase. With this purpose we varied the ratio of RTIL to water
15
30
45
60
75
90
Time (min) Fig. 2. Kinetics of TBP extraction for C8mimCl; feed – 30% TBP in n-dodecane, extractant – (1:1) solution of C8mimCl in water.
(w/v) from 0.5 to 5 for C8mimCl and followed its influence on TBP removal by batch equilibration method. The results from the experiment which are given in Table 1 indicate that increasing the concentration of RTIL in aqueous solution lead to increase in percentage extraction of TBP and it reaches maximum for (1:1) (w/v) ratio. Further increase in RTIL concentration does not lead to improvement in percentage extraction of TBP. Hence during further studies for TBP extraction optimized concentration of (1:1) (w/v) aqueous solution of C8mimCl was used. 3.2.3. TBP concentration in organic phase As mentioned earlier for 30% TBP–n-dodecane phase the percentage extraction by (1:1) (w/v) aqueous solution of C8mimCl was 45%. In order to understand whether increase or decrease in TBP concentration in n-dodecane phase have any effect in% extraction of TBP, batch equilibration studies were carried out employing different concentrations of TBP in n-dodecane. The concentration of TBP in n-dodecane was varied from 10% to 40% in this experiment. Table 2 shows the results on extraction efficiency of (1:1) (w/v) aqueous solution of C8mimCl against varying TBP concentration in the organic phase. The result as evident from Table 2 was found to be surprising. Extraction efficiency of C8mimCl was found to be 45% (distribution ratio of 1), irrespective of the initial concentration of TBP. The reason behind this extraction behavior (distribution ratio being independent of the initial TBP concentration in the feed phase) is yet to be understood. 3.2.4. Successive extraction stages For the purpose of recycling the pure diluent, n-dodecane, from TBP–HDBP-n-dodecane phase, it is necessary to remove HDBP and TBP quantitatively from the TBP–n-dodecane phase. As reported earlier, quantitative extraction of HDBP is achievable in a single extraction cycle by (1:1) (w/v) aqueous solution of C8mimCl. But as per the discussion above, the extraction efficiency of C8mimCl solution in a single contact for TBP was found to be 45% of initial TBP concentration in the organic phase. Hence successive contact
Table 1 Effect of C8mimCl concentration on the extraction of TBP from n-dodecane phase; feed – 30% TBP in n-dodecane, contact time: 30 min. Water: (RTIL)
Conc. of TBP (%) in organic phase after extraction
5:1 4:1 3:1 2:1 1:1 1:2
27.4 24.5 22.3 19.8 16.4 16.6
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Table 2 Effect of TBP concentration on the extraction performance of C8mimCl; feed – different percentage of TBP in n-dodecane, extractant – (1:1) aqueous solution of C8mimCl, contact time – 60 min. Initial concentration of TBP (%)
Concentration of TBP left in the organic phase after extraction
10 20 30 40
5.3 11.5 16.5 22.5
of the TBP–n-dodecane phase with aqueous solution of RTIL was found necessary to extract TBP quantitatively. In this study 30% TBP–n-dodecane solution was given successive contact by fresh (1:1) (w/v) aqueous solution of C8mimCl after each batch of equilibration. After each extraction the organic phase was analyzed and then was subjected to further extraction using fresh C8mimCl aqueous solution. The results from the experiment are given in Table 3 which shows that the cumulative extraction of more than 95% of TBP after 5 successive contacts from an initial concentration of 30% TBP–n-dodecane. 3.2.5. Reusability of RTIL As only 45% of initial TBP concentration gets extracted by a single contact of (1:1) aqueous solution of C8mimCl and due to the exotic nature of the RTIL, reusability of the RTIL is a must for possible process application. The experimental details to study the reusability of C8mimCl, are given in the experimental section. The result from the experiment showed that even after 5 cycles of extraction and stripping, there was no reduction in the extraction performance of (1:1) aqueous solution of C8mimCl for TBP. The percentage extraction of TBP was found to be 45% of the initial TBP concentration even after 5th contact which is identical to that observed after 1st contact. 3.3. Studies from actual degraded solvent – a case study Effects of actual conditions present in degraded solvent such as effect of extracted metal ion, extracted nitric acid, residual activity and diluent degradation products on the extraction performance of TBP and HDBP for RTILs were studied using actual degraded solvent from PUREX process. The degraded solvent consisted of 4.9 g/L HDBP and 29% TBP in n-dodecane along with some residual activity and Uranium complexed with HDBP. The result as indicated in Table 4 showed that only aqueous solution of C8mimCl was able to remove HDBP completely from the degraded solvent, but for C4mimCl and C6mimCl salt solutions the efficiency of HDBP removal was very less. To understand the role of extracted uranium metal towards HDBP extraction for the studied RTILs, the degraded solvent was washed with H2SO4 to remove the complexed uranium. But to our surprise, acid wash was not able to improve the extraction efficiency of HDBP for C4mimCl and C6mimCl solution. Hence we can conclude that metal extraction was not responsible for the non-extraction of HDBP by C4mimCl and C6mimCl salt solutions. Along with uranium, the degraded solvent also extract Table 3 Cumulative extraction of TBP for C8mimCl; feed – 30% TBP in n-dodecane, extractant – (1:1) aqueous solution of C8mimCl, contact time – 1 h. No. of contact
% TBP in feed after extraction
1 2 3 4 5
16.1 8.9 4.8 2.5 1.3
significant amount of nitric acid during process operation. To understand the role of extracted nitric acid in the unexpected extraction behavior of C6mimCl and C4mimCl salt solutions towards HDBP extraction, the acid washed solvent was given further wash with water. As evident from Table 4, significant improvement in the extraction of HDBP was observed for C6mimCl and C4mimCl salt solutions and this improvement was found to be higher for C6mimCl solution compared to C4mimCl solution. Hence we can infer that the poor extraction behavior of HDBP from actual process solvent for C6mimCl and C4mimCl salt solutions was due to co-extraction of nitric acid which prevented HDBP from being extracted by the aqueous solutions of the mentioned RTILs. As for TBP extraction, C4mimCl and C6mimCl did not show any extraction of TBP from the degraded solvent which was expected from their behavior towards pure TBP–n-dodecane solution. Similarly C8mimCl salt solution was found to extract 45% of the initial TBP concentration from the degraded solvent. Fig. 3 shows the GC profile of real process solvent before and after contacting with (1:1) aqueous solution of C8mimCl. Successive five contacts were given to quantitatively extract TBP and after the contacts the concentration of TBP in the organic phase was found to be 1.4%. The organic phase was found to be pure dodecane (with 1.4% TBP) free from any residual activity and complexed metal ion. Hence it was proved very clearly that (1:1) aqueous solution of C8mimCl could remove HDBP from the degraded solvent in a single contact as well as can separate pure diluent, n-dodecane in successive contacts. 3.4. Mechanism of extraction It was reported earlier by Misra et al. [26] that dissolved HDBP and TBP can be removed by Micellar Enhanced Ultrafiltration Technique using SDS as surfactant. It was reported by them that the dissolved organics (i.e., HDBP and TBP) get solubilized in the tail of the micelles formed by the surfactant. Micelle formation behavior of ionic liquids in aqueous solution was reported recently by Blesic et al. [27]. They used a variety of methods like interfacial tension, fluorescence and 1H NMR measurements to monitor the adsorption at the aqueous solution–air interface and self-aggregation behavior (critical micelle concentration, CMC) of room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium family of cations, [Cnmim]+, with different linear alkyl chain lengths, CnH2n+1, and different counter-ions, namely [Cnmim]Cl (n = 2–14), [Cnmim][PF6] (n = 4 or 10) and [C10mim][NTf2]. They observed that only [Cnmim]Cl with n P 8 unambiguously form aggregates in solution In contrast, the shorter chains behave as simple salts. In order to explain the extraction mechanism of TBP and HDBP by CnmimCl, initially we tried to observe the micelle formation of the three RTILs studied in the present work by conductivity measurement. As reported by Blesic et al. [27], we observed the formation of micelles for aqueous solution of C8mimCl (CMC = 195 mM) and no micelle formation for C6mimCl and C4mimCl aqueous solutions. After that we tried to understand whether the extraction of TBP by C8mimCl aqueous solution takes place via micelle formation only. For that purpose we studied the extraction behavior of TBP from n-dodecane medium using non-micellar C8mimCl aqueous solution. We added a little amount (10 mM) of 1-propanol as micelle breaking reagent to break the micelles formed by C8mimCl in aqueous solution. We observed that no extraction of TBP takes place by (1:1) solution of C8mimCl in presence of 1-propanol. Further the same solution of C8mimCl in presence of 1-propanol was also used to study the extraction of HDBP from n-dodecane phase. Quantitative extraction of HDBP was observed by C8mimCl aqueous solution even in presence of 1-propanol. This signifies that the extraction of TBP takes place via micelle formation whereas extraction of HDBP takes place by a different mechanism, i.e., ion-pair extraction (Cnmim+-HDBP). This is also proved by the fact
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Table 4 Extraction of HDBP and TBP from degraded solvent of PUREX process; feed – 29% TBP along with 4.9 g/L HDBP in n-dodecane, activity of the organic phase – 5 mCi/L, [U] = 4 g/L, extractant – (1:1) solution of CnmimCl, contact time – 1 h. RTIL
C8mimCl C6mimCl C4mimCl
[TBP] after extraction (%)
15.3 29.1 28.9
[HDBP] after extraction (g/L) in organic phase Without treatment
After H2SO4 wash
After water wash
BDL 4.5 4.6
– 3.8 4.0
– 1.5 1.8
Fig. 3. GC profile of 30% TBP–HDBP-n-dodecane before and after extraction using (1:1) aqueous solution of C8mimCl.
that aqueous solutions of C6mimCl and C4mimCl extract HDBP though they do not form micelles in aqueous solution. To further prove the mechanism we carried out the extraction studies from actual degraded solvent. As discussed above, the extraction of HDBP is insignificant for aqueous solutions of C6mimCl and C4mimCl though they extract HDBP quantitatively from pure HDBP–TBP–n-dodecane solution. This can be explained by the fact that TBP extracts acid (HNO3) from aqueous feed solution during PUREX process. This co-extracted nitric acid competes with RTILs to form ion-pair ðCþ nmim NO3 Þ. The strength of the ion-pair comþ plex formed by nitrate ðCn mim NO 3 Þ is expected to be stronger þ than ðC nmim DBP Þ. Thus even after giving H2SO4 acid wash no improvement in the extraction efficiency of HDBP for C4mimCl and C6mimCl aqueous solution was found. Because H2SO4 removes the co-extracted metal ions from the organic phase not the co-extracted acid. To remove the co-extracted acid from the organic phase, degraded solvent was washed with water. Under this condition the aqueous solutions of C6mimCl and C4mimCl are able to form ion-pair complex with DBP as now there is no competition for the available RTIL sites. But for C8mimCl quantitative HDBP extraction was observed from the degraded solvent without any acid or water wash. This is due to the reason that though the ionic interaction between HDBP and RTIL is weaker in degraded solvent due to competition from co-extracted HNO3, but due to the formation of micelles by C8mimCl in aqueous solution, HDBP gets solubilized in the tail of the micelles as reported earlier [26]. 4. Conclusions In conclusion it can be said that room temperature ionic liquids containing Chloride as anion work as excellent extractant for cleaning up of degraded solvent from PUREX process. Quantitative
extraction of HDBP was observed in the concentration range expected in PUREX process by all the three RTILs studied (CnmimCl, n = butyl, hexyl and octyl). But only C8mimCl was found to remove HDBP from actual degraded solvent. Mechanism of extraction was ion-pair and/or via micelle formation. TBP extraction was found to take place only via micelle formation and hence C8mimCl only was able to extract TBP from n-dodecane and degraded solvent alike. The RTIL can be reused for more than five cycles of operation and so can be economically feasible. Further studies to test the feasibility of the RTILs for solvent clean-up in large scale are under progress.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Fractionation of pure solvent components from degraded PUREX solvent using room temperature ionic liquids S. Panja ⇑, S.K. Misra, S.C. Tripathi ⇑, M. Bindu, P.M. Gandhi Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 18 August 2013 Received in revised form 26 October 2013 Accepted 29 October 2013 Available online 4 November 2013
With the view point of recovery of pure solvent components from degraded PUREX (Plutonium URanium Extraction) solvent, comprising of mixture of 30% TBP–HDBP-n-dodecane, three homologues of room temperature ionic liquids (CnmimCl, n = 4, 6, 8) have been investigated in batch equilibration mode (liquid–liquid extraction). While lower homologues of RTIL (C4 and C6) could effectively separate out HDBP from pure TBP–HDBP-n-dodecane mixture, the higher homologue C8mimCl could quantitatively remove TBP and HDBP together leaving behind pure n-dodecane as organic phase. Efficacy of RTIL solutions have also been studied as a function of RTIL concentration, composition of various components in organic mixture, kinetics of extraction, etc. These ionic liquids were found to retain their efficacy even after five cycles of operation after their regeneration. Studies from actual degraded solvent showed that C8mimCl could remove HDBP quantitatively and TBP (45% of the initial concentration) whereas C6mimCl and C4mimCl showed very little extraction even for HDBP. The mechanism of extraction was found to be formation of micelles and ion-pair formation. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: RTIL PUREX HDBP TBP Reusability
1. Introduction Tri-butyl Phosphate has been recognized as the work horse of nuclear industry [1]. In spent nuclear fuel reprocessing 30% TBP in n-dodecane has been used as a solvent for the past 3–4 decades to reprocess spent nuclear fuel [2]. The process known as PUREX (Plutonium Uranium EXtraction) aims at complete recovery of uranium and plutonium from fission products and also from each other in nitric acid medium. The solvent used in this process is 30% TBP in n-dodecane. In this process, prevailing harsh chemical and radiolytic conditions cause hydrolysis and de-alkylation of TBP during repeated use. As a result a host of degradation products of TBP viz, dibutyl phosphate (HDBP), monobutyl phosphate (MBP) and phosphoric acid [3–5] are formed. The formation of HDBP is only the first step towards complete de-alkylation of TBP that may eventually lead to the formation of orthophosphoric acid. MBP and orthophosphoric acid being water soluble do not hinder much in the process performance. But HDBP forms organic soluble complexes with several cations like (UO2)2+, Pu4+ and ZrðOHÞ2þ 2 , normally encountered in PUREX process. The net result of the presence of HDBP in PUREX process is a progressive reduction in the efficiency of removal of fission products, loss of uranium and plutonium into waste effluent streams and emulsification or precipita⇑ Corresponding authors. Tel.: +91 22 25591201. E-mail addresses: (S.C. Tripathi).
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Panja),
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1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.10.046
tion within the contactors. Hence, separation and removal of HDBP from the TBP is of importance for efficient operation of PUREX process. There are various methods for the regeneration and management of this degraded organic solvent such as fractional distillation, alkaline hydrolysis and incineration. These methods are very poor with respect to high energy consumption, secondary waste generation, formation of highly corrosive phosphorous compounds like P2O5. In view of the problems associated with the presently adopted methods, a procedure for the separation of TBP and HDBP by a new class of organic compounds known as Room Temperature Ionic Liquids (RTIL) has been tried and reported here. Room temperature ionic liquids have gained considerable importance worldwide due to various properties like non-volatility, non-flammability, moisture stability, chemical tunability to task specific actions [6–10], etc. They are being used in diverse applications like (i) Synthesis of novel materials [11]. (ii) As solvents in catalytic reactions [12,13]. (iii) Separation of CO2 [14,15]. (iv) Separation of metal ions [16–19]. (v) As sensors of volatile organic vapors [20], etc. Application of RTILs for the separation of organic compounds from various streams has also been explored extensively. McFarlanea et al. [21] explored the possibility of RTILs for separating organics from water. They investigated a host of imidazolium, pyrrolidinium, phosphonium based hydrophobic ionic liquids for separation of different polar organic compounds typical of water contaminants. It was observed that some of the ionic liquids studied showed large uptake for certain organics. Hu et al. [22] reported the possibility of separation of ethyl acetate and
S. Panja et al. / Separation and Purification Technology 122 (2014) 67–72
ethanol using ionic liquids with tetrafluoroborate anion. The experimental results show that 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate {[C2OHmim]BF4} is a potential candidate for separation of ethyl acetate and ethanol by liquid–liquid extraction. Galan Sanchez et al. [23] investigated the potential of room temperature ionic liquids for separation of ethylene from ethane. The olefin/paraffin selectivity at a pressure of 1 bar was 30–50 higher in the silver based RTILs than in standard ionic liquids as reported by them. Ortiz et al. [24] reported room temperature ionic liquids containing silver salt as reactive media for separation of propylene/propane gas mixtures. Propylene absorption was found to be chemically enhanced in the silver-based RTILs and was considerably higher than that in the standard RTILs. Absorption of propane in the silver-based RTILs was based on the physical interactions only. Branco et al. [25] reported highly selective transport of organic compounds across supported liquid membranes using ionic liquids. They studied three organic isomeric amines hexylamine, diisopropylamine and triethylamine using 1n-butyl-3-methylimidazolium hexafluorophosphate as carrier. The transport of diisopropylamine was found to be much higher compared to two other amines. But so far no report in the literature is available on the separation of HDBP and TBP from degraded solvent using room temperature ionic liquids. In the current study, different RTILs were investigated for their behavior towards fractionation of TBP and HDBP from degraded PUREX solvent. Extraction of TBP and HDBP from organic mixture using liquid–liquid extraction by RTIL was studied to optimize the following parameters: (i) RTIL concentration. (ii) Extraction kinetics. (iii) Effect of HDBP/TBP concentration in organic phase. Studies were also carried out to investigate the efficacy of the RTILs towards removal of HDBP and TBP from degraded solvent generated during repeated recycling of the PUREX extraction system. 2. Experimental 2.1. Reagents RTILs (1-alkyl-3-methyl-imidazolium chloride, alkyl = butyl, hexyl and octyl) were procured from Aldrich and were used as such. Commercial grade TBP from Heavy Water Plant, Talchar, India, was purified by washing it with 2% (w/v) solution of sodium carbonate followed by distilled water. The purity of TBP was found better than 99% (v/v) by gas–liquid chromatographic (GLC) assay. The n-dodecane from Aldrich was used for dilution of TBP to 30% (v/v). HDBP was procured from Fluka AG (Switzerland). Stock solutions of different concentrations of HDBP in 30% (v/v) TBP–n-dodecane were prepared by suitable dilution from stock solution and standardization by gas chromatography. The diazomethane was prepared by treating p-tolylsulphonyl methylnitrosamide with alcoholic potassium hydroxide. All the other reagents used in this study were of analytical grade. 2.2. Liquid–liquid extraction 5 mL of 30% TBP in n-dodecane in presence or in absence of desired concentration of HDBP (1–5 g/L) were equilibrated with 5 mL of aqueous solutions of RTIL in a separating funnel for a fixed time interval. After equilibration the phases were kept for settlement, after which organic phase was taken for analysis. The concentration of organic phase was determined by gas chromatography as described below. The regeneration of aqueous RTIL salt solution was done by acidifying the aqueous solution followed by removal of organophosphates by scrub with benzene–benzyl alcohol mixture. In this study (1:1) (w/v) aqueous solution of RTILs was equilibrated with organic phase containing 3 g/L HDBP in 30%
TBP–n-dodecane. After equilibration, DBP was removed from equilibrated aqueous solution using 10% benzene–benzyl alcohol solution. The aqueous solution containing RTILs, depleted of HDBP, was re-contacted with fresh lot of organic solution having similar composition of DBP/TBP as to that of first contact. Such procedure was followed for 5 consecutive cycles. These regenerated RTILs were used as such for further extraction studies. 2.3. Gas–liquid chromatographic analysis of TBP, HDBP and dodecane TBP and HDBP was analyzed by GLC (Shimadzu, Japan) using thermal conductivity detector. For HDBP, GLC was carried out after esterifying the HDBP into volatile methyldibutyl phosphate with ether solution of diazomethane prior to its gas chromatographic assay. The quantification of HDBP was made by using a calibration plot. A 10% XE-60 column (1.5 m 0.32 cm) was used under temperature programming along with other operating parameters. Initial column temperature of 170 °C was maintained for 1 min. The heating rate of the column was 10 °C/min till the final temperature of column reached to 230 °C, which was maintained for 10 min. Throughout the study, the injection port temperature and carrier gas (He) flow were maintained at 240 °C and 40 mL/min, respectively. 3. Results and discussion 3.1. Extraction of HDBP 3.1.1. Kinetics of extraction Kinetics of the extraction of HDBP was investigated for the three RTILs mentioned earlier. It was observed that all three RTILs showed relatively fast kinetics for the uptake for HDBP. The kinetics of HDBP extraction for the three different RTILs is shown in Fig. 1. Concentration of DBP in 30% TBP was maintained constant at 3 g/L in each of batch equilibration experiments using three different RTILs. It was observed that 15 min of equilibration time resulted in quantitative uptake of DBP by each of the three RTIL solutions from the organic phase. From Fig. 1 it is also clear that the kinetics of HDBP uptake by these three RTILs was independent of carbon chain length attached to the imidazolium group. 3.1.2. HDBP concentration Amount of DBP in the degraded PUREX solvent is dependent on the extent of radiation exposure encountered during extraction process. Efficacy of the three RTILs under optimized batch equilibration process was therefore studied as a function of varying
Conc of HDBP in Org Phase (g/L)
68
3.0
Cl Cl C 8 mim Cl
C4 mim 2.5
C 6 mim
2.0 1.5 1.0 0.5 0.0 0
5
10
15
20
25
30
Time (min) Fig. 1. Kinetics of HDBP extraction from n-dodecane using (1:1) solution of CnmimCl (n = 4, 6, 8) in H2O, feed – 3 g/L DBP in 30% TBP–n-dodecane.
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% TBP in n-dodecane phase
HDBP concentration in 30% TBP–n-dodecane system. The concentration of HDBP was varied from 1 g/L to 5 g/L in the organic phase whereas (1:1) (w/v) aqueous solution of each of three RTILs was chosen as the aqueous solution. The maximum concentration of HDBP observed in degraded solutions from PUREX process is found in the range of 4–5 g/L and so 5 g/L was chosen as the maximum concentration of HDBP in our studies. From the studies it was evident that HDBP (at concentration of 5 g/L) was quantitatively separated by three RTILs indicating that DBP extraction by the three studied RTILs (at chosen concentration level) is independent of carbon chain length attached to the imidazolium group.
40 C8 mimCl 35 30 25 20 15 10 0
3.1.3. Reusability of RTILs In order to prove the economic viability of RTILs for bulk scale application, it is imperative to assess the extent of its recyclability/reuse along with its efficacy of the used ionic liquids. Experimental details for the study have been described earlier. It was observed that even after 5 cycles of operation there was no reduction in the extraction efficiency of all the three RTILs. Analysis of organic phase after each equilibration showed quantitative extraction of HDBP for all the three RTILs studied. Hence, the extraction potential of the RTILs towards HDBP removal from DBP–TBP–ndodecane system (organic phase) remains unaffected for 5 successive cycles of extraction and depletion. The observed reusability potential of the aqueous solutions of RTILs thus offers hope for their economically viability for large scale purification of degraded PUREX solvent with respect to removal of major harmful species DBP (an acidic metal complexant) from a mixture of TBP and DBP in n-dodecane. 3.2. Removal of TBP for recovery of pure diluent Regeneration of degraded PUREX solvent follows two different pathways. The first one is removal HDBP from degraded TBP–ndodecane mixture for reuse of purified TBP–n-dodecane. The second one is removal of TBP and HDBP simultaneously from mixed DBP–TBP–n-dodecane phase to recover pure dodecane for recycling. Hence the current study was also extended to understand the extraction behavior of TBP if any from the organic mixture using RTILs. Initial experiment with distilled water only as aqueous phase showed no extraction of TBP from the 30% TBP–n-dodecane phase. Even for lower homologues of RTILs (viz, hexyl and butyl), (1:1) (w/v) aqueous solution showed no extraction efficiency for TBP. But when we used higher homologue (octyl) of RTIL in the aqueous phase, reasonably good extraction efficiency for TBP was observed. In contrary to extraction of HDBP where quantitative extraction was observed, TBP extraction was found to be only 45% of initial feed concentration. So in contrary to HDBP, the carbon chain length attached to the imidazolium group can be said to have profound impact on the extractability of TBP. Hence forth, further studies on removal of TBP from TBP–n-dodecane mixture were carried out using C8mimCl only. 3.2.1. Kinetics of extraction Kinetics of extraction of TBP from an organic solution containing 30% TBP in n-dodecane was studied using (1:1) (w/v) aqueous solution of C8mimCl. The results (given in Fig. 2) show that it requires 30 min of equilibration time to attain equilibrium. Hence it can be inferred that the kinetics of extraction of TBP is slower compared to that of HDBP. 3.2.2. RTIL concentration Cost intensive nature of RTIL makes it imperative to find out the lowest possible concentration of RTIL in the aqueous solution that can be used for maximum removal of TBP from TBP–n-dodecane phase. With this purpose we varied the ratio of RTIL to water
15
30
45
60
75
90
Time (min) Fig. 2. Kinetics of TBP extraction for C8mimCl; feed – 30% TBP in n-dodecane, extractant – (1:1) solution of C8mimCl in water.
(w/v) from 0.5 to 5 for C8mimCl and followed its influence on TBP removal by batch equilibration method. The results from the experiment which are given in Table 1 indicate that increasing the concentration of RTIL in aqueous solution lead to increase in percentage extraction of TBP and it reaches maximum for (1:1) (w/v) ratio. Further increase in RTIL concentration does not lead to improvement in percentage extraction of TBP. Hence during further studies for TBP extraction optimized concentration of (1:1) (w/v) aqueous solution of C8mimCl was used. 3.2.3. TBP concentration in organic phase As mentioned earlier for 30% TBP–n-dodecane phase the percentage extraction by (1:1) (w/v) aqueous solution of C8mimCl was 45%. In order to understand whether increase or decrease in TBP concentration in n-dodecane phase have any effect in% extraction of TBP, batch equilibration studies were carried out employing different concentrations of TBP in n-dodecane. The concentration of TBP in n-dodecane was varied from 10% to 40% in this experiment. Table 2 shows the results on extraction efficiency of (1:1) (w/v) aqueous solution of C8mimCl against varying TBP concentration in the organic phase. The result as evident from Table 2 was found to be surprising. Extraction efficiency of C8mimCl was found to be 45% (distribution ratio of 1), irrespective of the initial concentration of TBP. The reason behind this extraction behavior (distribution ratio being independent of the initial TBP concentration in the feed phase) is yet to be understood. 3.2.4. Successive extraction stages For the purpose of recycling the pure diluent, n-dodecane, from TBP–HDBP-n-dodecane phase, it is necessary to remove HDBP and TBP quantitatively from the TBP–n-dodecane phase. As reported earlier, quantitative extraction of HDBP is achievable in a single extraction cycle by (1:1) (w/v) aqueous solution of C8mimCl. But as per the discussion above, the extraction efficiency of C8mimCl solution in a single contact for TBP was found to be 45% of initial TBP concentration in the organic phase. Hence successive contact
Table 1 Effect of C8mimCl concentration on the extraction of TBP from n-dodecane phase; feed – 30% TBP in n-dodecane, contact time: 30 min. Water: (RTIL)
Conc. of TBP (%) in organic phase after extraction
5:1 4:1 3:1 2:1 1:1 1:2
27.4 24.5 22.3 19.8 16.4 16.6
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Table 2 Effect of TBP concentration on the extraction performance of C8mimCl; feed – different percentage of TBP in n-dodecane, extractant – (1:1) aqueous solution of C8mimCl, contact time – 60 min. Initial concentration of TBP (%)
Concentration of TBP left in the organic phase after extraction
10 20 30 40
5.3 11.5 16.5 22.5
of the TBP–n-dodecane phase with aqueous solution of RTIL was found necessary to extract TBP quantitatively. In this study 30% TBP–n-dodecane solution was given successive contact by fresh (1:1) (w/v) aqueous solution of C8mimCl after each batch of equilibration. After each extraction the organic phase was analyzed and then was subjected to further extraction using fresh C8mimCl aqueous solution. The results from the experiment are given in Table 3 which shows that the cumulative extraction of more than 95% of TBP after 5 successive contacts from an initial concentration of 30% TBP–n-dodecane. 3.2.5. Reusability of RTIL As only 45% of initial TBP concentration gets extracted by a single contact of (1:1) aqueous solution of C8mimCl and due to the exotic nature of the RTIL, reusability of the RTIL is a must for possible process application. The experimental details to study the reusability of C8mimCl, are given in the experimental section. The result from the experiment showed that even after 5 cycles of extraction and stripping, there was no reduction in the extraction performance of (1:1) aqueous solution of C8mimCl for TBP. The percentage extraction of TBP was found to be 45% of the initial TBP concentration even after 5th contact which is identical to that observed after 1st contact. 3.3. Studies from actual degraded solvent – a case study Effects of actual conditions present in degraded solvent such as effect of extracted metal ion, extracted nitric acid, residual activity and diluent degradation products on the extraction performance of TBP and HDBP for RTILs were studied using actual degraded solvent from PUREX process. The degraded solvent consisted of 4.9 g/L HDBP and 29% TBP in n-dodecane along with some residual activity and Uranium complexed with HDBP. The result as indicated in Table 4 showed that only aqueous solution of C8mimCl was able to remove HDBP completely from the degraded solvent, but for C4mimCl and C6mimCl salt solutions the efficiency of HDBP removal was very less. To understand the role of extracted uranium metal towards HDBP extraction for the studied RTILs, the degraded solvent was washed with H2SO4 to remove the complexed uranium. But to our surprise, acid wash was not able to improve the extraction efficiency of HDBP for C4mimCl and C6mimCl solution. Hence we can conclude that metal extraction was not responsible for the non-extraction of HDBP by C4mimCl and C6mimCl salt solutions. Along with uranium, the degraded solvent also extract Table 3 Cumulative extraction of TBP for C8mimCl; feed – 30% TBP in n-dodecane, extractant – (1:1) aqueous solution of C8mimCl, contact time – 1 h. No. of contact
% TBP in feed after extraction
1 2 3 4 5
16.1 8.9 4.8 2.5 1.3
significant amount of nitric acid during process operation. To understand the role of extracted nitric acid in the unexpected extraction behavior of C6mimCl and C4mimCl salt solutions towards HDBP extraction, the acid washed solvent was given further wash with water. As evident from Table 4, significant improvement in the extraction of HDBP was observed for C6mimCl and C4mimCl salt solutions and this improvement was found to be higher for C6mimCl solution compared to C4mimCl solution. Hence we can infer that the poor extraction behavior of HDBP from actual process solvent for C6mimCl and C4mimCl salt solutions was due to co-extraction of nitric acid which prevented HDBP from being extracted by the aqueous solutions of the mentioned RTILs. As for TBP extraction, C4mimCl and C6mimCl did not show any extraction of TBP from the degraded solvent which was expected from their behavior towards pure TBP–n-dodecane solution. Similarly C8mimCl salt solution was found to extract 45% of the initial TBP concentration from the degraded solvent. Fig. 3 shows the GC profile of real process solvent before and after contacting with (1:1) aqueous solution of C8mimCl. Successive five contacts were given to quantitatively extract TBP and after the contacts the concentration of TBP in the organic phase was found to be 1.4%. The organic phase was found to be pure dodecane (with 1.4% TBP) free from any residual activity and complexed metal ion. Hence it was proved very clearly that (1:1) aqueous solution of C8mimCl could remove HDBP from the degraded solvent in a single contact as well as can separate pure diluent, n-dodecane in successive contacts. 3.4. Mechanism of extraction It was reported earlier by Misra et al. [26] that dissolved HDBP and TBP can be removed by Micellar Enhanced Ultrafiltration Technique using SDS as surfactant. It was reported by them that the dissolved organics (i.e., HDBP and TBP) get solubilized in the tail of the micelles formed by the surfactant. Micelle formation behavior of ionic liquids in aqueous solution was reported recently by Blesic et al. [27]. They used a variety of methods like interfacial tension, fluorescence and 1H NMR measurements to monitor the adsorption at the aqueous solution–air interface and self-aggregation behavior (critical micelle concentration, CMC) of room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium family of cations, [Cnmim]+, with different linear alkyl chain lengths, CnH2n+1, and different counter-ions, namely [Cnmim]Cl (n = 2–14), [Cnmim][PF6] (n = 4 or 10) and [C10mim][NTf2]. They observed that only [Cnmim]Cl with n P 8 unambiguously form aggregates in solution In contrast, the shorter chains behave as simple salts. In order to explain the extraction mechanism of TBP and HDBP by CnmimCl, initially we tried to observe the micelle formation of the three RTILs studied in the present work by conductivity measurement. As reported by Blesic et al. [27], we observed the formation of micelles for aqueous solution of C8mimCl (CMC = 195 mM) and no micelle formation for C6mimCl and C4mimCl aqueous solutions. After that we tried to understand whether the extraction of TBP by C8mimCl aqueous solution takes place via micelle formation only. For that purpose we studied the extraction behavior of TBP from n-dodecane medium using non-micellar C8mimCl aqueous solution. We added a little amount (10 mM) of 1-propanol as micelle breaking reagent to break the micelles formed by C8mimCl in aqueous solution. We observed that no extraction of TBP takes place by (1:1) solution of C8mimCl in presence of 1-propanol. Further the same solution of C8mimCl in presence of 1-propanol was also used to study the extraction of HDBP from n-dodecane phase. Quantitative extraction of HDBP was observed by C8mimCl aqueous solution even in presence of 1-propanol. This signifies that the extraction of TBP takes place via micelle formation whereas extraction of HDBP takes place by a different mechanism, i.e., ion-pair extraction (Cnmim+-HDBP). This is also proved by the fact
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Table 4 Extraction of HDBP and TBP from degraded solvent of PUREX process; feed – 29% TBP along with 4.9 g/L HDBP in n-dodecane, activity of the organic phase – 5 mCi/L, [U] = 4 g/L, extractant – (1:1) solution of CnmimCl, contact time – 1 h. RTIL
C8mimCl C6mimCl C4mimCl
[TBP] after extraction (%)
15.3 29.1 28.9
[HDBP] after extraction (g/L) in organic phase Without treatment
After H2SO4 wash
After water wash
BDL 4.5 4.6
– 3.8 4.0
– 1.5 1.8
Fig. 3. GC profile of 30% TBP–HDBP-n-dodecane before and after extraction using (1:1) aqueous solution of C8mimCl.
that aqueous solutions of C6mimCl and C4mimCl extract HDBP though they do not form micelles in aqueous solution. To further prove the mechanism we carried out the extraction studies from actual degraded solvent. As discussed above, the extraction of HDBP is insignificant for aqueous solutions of C6mimCl and C4mimCl though they extract HDBP quantitatively from pure HDBP–TBP–n-dodecane solution. This can be explained by the fact that TBP extracts acid (HNO3) from aqueous feed solution during PUREX process. This co-extracted nitric acid competes with RTILs to form ion-pair ðCþ nmim NO3 Þ. The strength of the ion-pair comþ plex formed by nitrate ðCn mim NO 3 Þ is expected to be stronger þ than ðC nmim DBP Þ. Thus even after giving H2SO4 acid wash no improvement in the extraction efficiency of HDBP for C4mimCl and C6mimCl aqueous solution was found. Because H2SO4 removes the co-extracted metal ions from the organic phase not the co-extracted acid. To remove the co-extracted acid from the organic phase, degraded solvent was washed with water. Under this condition the aqueous solutions of C6mimCl and C4mimCl are able to form ion-pair complex with DBP as now there is no competition for the available RTIL sites. But for C8mimCl quantitative HDBP extraction was observed from the degraded solvent without any acid or water wash. This is due to the reason that though the ionic interaction between HDBP and RTIL is weaker in degraded solvent due to competition from co-extracted HNO3, but due to the formation of micelles by C8mimCl in aqueous solution, HDBP gets solubilized in the tail of the micelles as reported earlier [26]. 4. Conclusions In conclusion it can be said that room temperature ionic liquids containing Chloride as anion work as excellent extractant for cleaning up of degraded solvent from PUREX process. Quantitative
extraction of HDBP was observed in the concentration range expected in PUREX process by all the three RTILs studied (CnmimCl, n = butyl, hexyl and octyl). But only C8mimCl was found to remove HDBP from actual degraded solvent. Mechanism of extraction was ion-pair and/or via micelle formation. TBP extraction was found to take place only via micelle formation and hence C8mimCl only was able to extract TBP from n-dodecane and degraded solvent alike. The RTIL can be reused for more than five cycles of operation and so can be economically feasible. Further studies to test the feasibility of the RTILs for solvent clean-up in large scale are under progress.
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