Polymer Inclusion Membrane Containing a Tripodal Diglycolamide

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Polymer Inclusion Membrane Containing a Tripodal Diglycolamide Ligand: Actinide Ion Uptake and Transport Studies Bholanath Mahanty,† Prasanta K. Mohapatra,*,‡ Dhaval R. Raut,‡ Dillip K. Das,† Praveen G. Behere,† Mohammed Afzal,† and Willem Verboom§ †

Advanced Fuel Fabrication Facility, Bhabha Atomic Research Centre, Tarapur 401502, India Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India § MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ‡

S Supporting Information *

ABSTRACT: A cellulose triacetate (CTA)-based polymer inclusion membrane (PIM) containing a C-pivot tripodal diglycolamide (T-DGA) as the carrier extractant and 2-nitrophenyl octyl ether (NPOE) as the plasticizer shows potential for the uptake of actinides from acidic feed solutions. The uptake of actinides (Am3+, Pu4+, Th4+, UO22+) was studied at 1 M HNO3 with PIMs containing 25.6% T-DGA, 53.9% NPOE, and 20.5% CTA. The metal ion uptake followed the trend: Pu4+ > Am3+ > Th4+ > UO22+, which was in synch with the trend reported previously with PIMs containing N,N,N′,N′-tetrakis(2-ethylhexyl) diglycolamide (T2EHDGA) as the carrier. The sorbed actinide ion could be desorbed from the PIMs using a solution of α-hydroxy-iso-butyric acid (AHIBA). Transport studies carried out with 1 M HNO3 as the feed and 1 M AHIBA (at pH 3.0) as the receiver phase showed the trend: Am3+ > Pu4+ > Th4+ > UO22+, which was different than that observed from the uptake studies. Membrane stability was found to be reasonably good. actinide ion separations.14−16 Though in PIM based transport studies, the basic steps involved in mass transfer from the feed to the receiver side are the same as those in the SLMs, the mechanism of mass transfer is different due to suggested mechanisms such as possibility of a “percolation threshold” and “fixed-site jumping”, which are typical of the PIMs.6 In view of these, it is required to reassess the metal ion transport using PIMs which can give valuable information which are not otherwise available from the SLM studies. Diglycolamide (DGA) ligands are reported to be highly selective for actinide ion extraction from acidic feeds.17 Recently, we have demonstrated the promising extraction ability of PIMs containing two very efficient diglycolamide (DGA) extractants, viz., N,N,N′,N′-tetraoctyl diglycolamide (TODGA),18 N,N,N′,N′-tetra(2-ethylhexyl) diglycolamide (T2EHDGA)19 and very recently with several other tetraalkyldiglycolamides to understand the role of alkyl chain length on metal ion uptake and transport.20 In all these cases, the extracted species contains three to four DGA ligands around the metal center and, in addition, requires special conditions for favorable extraction such as using a nonpolar diluent such as ndodecane, which helps in the formation of molecular aggregate

1. INTRODUCTION Detection of trace concentrations of important actinide elements such as U, Pu, and Am in the environment arising out of accidents such as Chernobyl or Fukushima has great relevance. Furthermore, analysis of these actinides, in addition to another important actinide element, Th, in environmental or waste samples is a challenging task.1,2 Although solvent extraction (SX) is widely employed for the removal of actinides from various streams of effluents in the nuclear fuel cycle, the above-mentioned applications involving lean feeds necessitates to opt for eco-friendly separation methods such as membranes in view of the very low ligand inventory used in the latter. Out of the membrane-based separation methods, the overall effect of the use of supported liquid membranes (SLMs) is quite similar to that of SX, as it involves extraction and stripping (though simultaneously).3−5 However, it has the issue of liquid membrane stability, which is a major disadvantage. The advent of polymer inclusion membranes (PIMs), where a composite plasticized membrane containing the carrier extractant is cast using a polymer and a plasticizer, can alleviate the stability issues. This has resulted in a great deal of ongoing research in this interesting area.6−10 Over the last 35 years, PIMs have been employed in the fabrication of many ion selective electrodes for the potentiometric study of cations and anions.6,11 Moreover, many studies have shown the potential use of PIMs as solid sorbents for the removal of metal ions.12,13 There are only a few reports available on the use of PIMs for © XXXX American Chemical Society

Received: December 4, 2015 Revised: January 28, 2016 Accepted: February 1, 2016

A

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research formation21,22 which favors metal ion extraction in a unique manner, i.e., trivalent actinide ions are extracted to a much larger extent than the tetravalent actinide ions and a much lower extraction of uranyl ion. In view of efficient metal ion extraction in the DGA aggregates, it was thought that multiple DGA-functionalized ligands may be more efficient than TODGA or T2EHDGA. Such ligands can form complexes under any experimental conditions and do not have to depend on the diluent characteristics as in case of TODGA. In view of this, three DGA units were appended to form a C-pivot tripodal diglycolamide ligand (T-DGA, Figure 1), which showed a

were used from laboratory stocks after confirming their radiochemical purity by alpha spectrometry (233U and Pu) as well as by gamma spectrometry (for 241Am). 234Th was freshly obtained by separating the daughter product of 238U (natural U) following a reported method25 in which U was selectively extracted from a feed containing a mixture of the radionuclides (present in secular equilibrium26) in 6 M HCl using an Aliquat 336 solution in chloroform. After three successive extraction stages, the raffinate solution, which contained >99% 234Th, was used as the tracer stock solution (Supporting Information). The concentrations of the radiotracers of the elements, Am, Pu, U, and Th used in the present studies, were 10−7 M, 10−6 M, 10−5 M, and 10−9 M, respectively. 2.2. Radiometric Assay of the Actinides. The alpha emitting radionuclides (233U and Pu) were assayed by liquid scintillation counting using an automated counting system (Hidex, Finland), which used a toluene-based scintillator cocktail (SRL, Mumbai). On the other hand, the gamma ray emitting 241Am and 234Th were assayed by gamma ray spectrometry using a well type NaI(Tl) counter (Para Electronics) coupled to a multichannel analyzer (ECIL, India). The quenching effects in the liquid scintillation counting were found to be negligible. The counting statistics errors were limited to 99% purities and were used as received. Suprapur nitric acid (Merck) was used for preparing dilute nitric acid solutions, which were subsequently standardized volumetrically using a standard NaOH solution and phenolphthalein (Merck) as the indicator. Europium oxide (>99.99%, Sigma-Aldrich) was dissolved in nitric acid and a stock solution of Eu3+ was prepared in 1 M HNO3. All the other reagents were of AR grade. The radiotracers, 241Am, Pu (mainly 239Pu), and 233U

%uptake =

C0 − Ct × 100 C0

(1)

where C0 and Ct are the metal ion concentrations in the aqueous phase at the beginning of the experiment and at time t, respectively. The uptake studies were carried out in duplicate at ambient temperature (∼24 °C) and the error in the uptake data was within ±5%. 2.5. Transport Studies. The transport studies involving the actinide ions, Am3+, Pu4+, UO22+, and Th4+ were carried out using a conventional two compartment transport cell of 20 mL feed as well as receiver compartment volumes.12 The feed (containing the required amount of the actinide ion in 1 M B

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research HNO3) and the receiver (containing 1 M α-hydroxy-iso-butyric acid) compartments were continuously stirred at 200 rpm using a magnetic stirrer (Corning, U.K.) and samples were taken out at regular intervals for radiometric assay (Supporting Information). Percentage metal ion transport (%T) and permeability coefficient (P) were calculated from the metal ion transport data using the following equations: %T = 100 × (Cf,0 − Cr, t )/Cf,0

(2)

ln(Cf, t /Cf,0) = − (Q /V )Pt

(3)

where Cf,t and Cf,0 are the concentrations of the actinide ions in the feed at a given time and at the start of the transport experiment, respectively, while Cr,t is the concentration of the metal ion in the receiver compartment at any given time. In eq 3, Q is effective surface area of the PIM (4.94 cm2) and V is the volume of the feed solution. The transport studies were carried out for 24 h without any interruption. All transport experiments, carried out at ambient temperature (∼24 °C), were repeated at least twice.

Figure 2. Effect of T-DGA content (in %) on Am(III) uptake by the PIMs prepared with CTA (as the polymer) and NPOE (as the plasticizer). Feed acidity: 1 M HNO3 (20 mL) and 0.020 g of PIM.

on increasing the T-DGA content from 6.5% to 12.4%, the Am3+ uptake (after 24 h) improved from 26.6% to 55.4% and further increased to 87.6% with 21.6% T-DGA in the PIM. However, further increasing the T-DGA content to 25.6%, the uptake percentage of Am3+ after 24 h remained unchanged (Table S1, Supporting Information). Though the initial increase in Am3+ uptake is a consequence of eq 4, the saturation beyond 21.6% T-DGA may be due to steric hindrance of the DGA moieties in the rather bulky T-DGA molecule causing some of these molecules not to be accessible for metal ion uptake anymore. Salima et al.30 observed a similar increase followed by a plateau in the extraction of methylene blue as a function of the D2EHPA concentration (5−30 μmol/ cm2) of a CTA-based PIM. Gherasim et al.31 reported a slight decrease in the sorption capacity of a polyvinyl chloride (PVC)based PIM at higher Aliquat-336 content (>40%) due to increasing steric hindrance. Figure 3 lists the uptake data of Am3+ along with the other actinide ions studied, viz., Pu4+, Th4+, and UO22+, at two different T-DGA concentrations, viz., 6.5% and 25.6%. While

3. RESULTS AND DISCUSSION 3.1. Uptake Studies. 3.1.1. Optimization of the PIM Composition. The PIMs used in the present study have three components, viz., CTA, T-DGA, and NPOE. PIMs which did not contain NPOE appeared to be brittle and, as discussed below, those without any T-DGA did not show any uptake of the actinide ions. The metal ion uptake capacity of a PIM will be unique for a given composition. Furthermore, in view of the interaction between the components, optimization of the PIM composition is required to achieve an efficient metal ion uptake capacity. Though all three components are important in the PIM, the carrier (T-DGA) plays the most important role in the extraction of a metal ion from its solution by forming a metal-carrier complex at the PIM-solution interface. The metal ion uptake at the PIM-aqueous feed interface occurs in the same manner as reported in solvent extraction studies23 and is given by eq 4: Maq n + + 2T‐DGAM + n NO3−aq = M(NO3)n·2T‐DGAM (4)

where the species with the subscripts “aq” and “M” represent those in the aqueous and the membrane phases, respectively. The number of nitrate ions (n) taking part in eq 4 are different for different metal ions are 3, 4, 4, and 2, respectively, for Am3+, Pu4+, Th4+, and UO22+. It is clear from eq 4 that the nitrate ion concentration has a direct role in metal ion uptake. Once the metal ion is complexed by T-DGA molecules present at the PIM surface, it may diffuse into the bulk of the PIM by the “fixed-site jumping” mechanism28 or the “mobile carrier” mechanism,29 reported previously. In both cases, the metal ion uptake is expected to increase with increasing concentration of T-DGA in the PIM. The effect of the carrier concentration was studied from a series of experiments by varying the T-DGA content (20−100 mg) in the PIM, while keeping the NPOE (210 mg) and CTA (80 mg) content fixed and the results are presented in Figure 2. The Am3+ uptake was negligible ( T2EHDGA > T-DGA may be partly attributed to the fraction of the carrier extractant present. It may be pointed out here that the kinetic effects due to the presence of three DGA moieties attached to the central carbon atom in T-DGA is discounted while making a comparison with TODGA and T2EHDGA. The plasticizer plays the role of solvent for ion transport in PIMs and makes the membrane flexible.6 Our studies on Am3+ uptake (2 h) from 1 M HNO3 using PIMs, containing a varying NPOE content (24.5−56.8%) at a given CTA and T-DGA content, show (Table S2, Supporting Information) an increased Am3+ uptake from 77.7% (24.5% NPOE) to 87.1% (39.6% NPOE). Further increase of the NPOE content in the PIM to 56.8% resulted in a decrease in the Am3+ uptake efficiency to 66.9% after 2 h. The Am3+ uptake profiles are plotted in Figure 4. The increase in the metal ion uptake efficiency with increasing NPOE content can be explained considering the

Figure 5. Uptake of Am(III) from feeds containing varying fractions of HNO3 and NaNO3. PIM composition: 25.6% T-DGA, 53.9% NPOE, and 20.5% CTA.

hydrolysis of Am3+) and 1 M NaNO3 + 2 M HNO3 solutions. On the other hand, though the best uptake efficiency takes place with 2 M HNO3 (up to 1.5 h), a plateau is reached at 2 h and above and the uptake efficiency matches with those observed for 3 M NaNO3 and 1 M NaNO3 + 2 M HNO3. The solution containing 2 M NaNO3 + 1 M HNO3 was found to be the least effective feed composition for Am3+ uptake. It was surprising to note that the uptake efficiency found with 1 M HNO3 was inferior to that with 2 M NaNO3 + 1 M HNO3 up to 2 h, whereupon it shows a better uptake efficiency. The uptake data obtained in the present study are in variance with

Figure 4. Effect of NPOE content on Am(III) uptake in PIMs with fixed CTA (80 mg) and T-DGA (80 mg) content. Feed: 1 M HNO3 (20 mL) and 0.020 g of PIM. D

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research those reported with TODGA18 where 2 M NaNO3 + 1 M HNO 3 resulted in the most effective Am 3+ uptake. Furthermore, while TODGA-based PIMs have shown a near quantitative Am3+ uptake in ∼2 h, much longer time was needed with T-DGA-based PIMs. These data, though not very conclusive, broadly suggest that (i) nitrate is the key ion for the metal ion uptake, (ii) HNO3 is much more effective that NaNO3, (iii) in some cases, NaNO3 has a negative effect on the metal ion uptake. Though nitric acid assisted reverse micelle formation33 has been reported which perhaps facilitates metal ion extraction, a similar effect is not ruled out with the T-DGA containing PIMs. The uptake studies were not carried out at higher acidity (>2 M HNO3), as it can lead to hydrolysis of the CTA-based PIM. 3.1.3. Comparative Uptake Behavior of Actinides. The uptake of Am3+, Pu4+, Th4+, and UO22+ in 1 M HNO3 was studied by using PIMs of the composition: 25.6% T-DGA, 53.9% NPOE, and 20.5% CTA. The uptake data are presented in Figure 6 showing the trend: Pu4+ > Am3+ ≫ UO22+ > Th4+.

butyric acid (AHIBA) is an effective strippant for the back extraction of the sorbed metal ions from PIMs.19 In view of this, a sorption−desorption study was performed in which the sorption was studied by following the uptake of Am3+ from 1 M HNO3 using a 4 cm2 piece of the PIM (as mentioned above). Desorption of Am3+ from the loaded PIM piece was carried out using 1 M AHIBA with reasonably good success. The sorption−desorption cycle, consisting of 24 h of sorption and 3 h of desorption (as desorption was much faster), was continued for three successive cycles; the results are presented in Figure 7. Though quantitative uptake was not observed,

Figure 7. Sorption−desorption profiles for Am3+ uptake using a PIM of composition: 25.6% T-DGA, 53.9% NPOE, and 20.5% CTA. Feed: 1 M HNO3 (20 mL). Desorption was done using 1 M AHIBA solution (20 mL).

desorption data at time zero were normalized to 100% for convenience sake. From Figure 7 it is clear that the uptake efficiency of the PIM was best in the fresh PIM and it deteriorated with successive cycles inferring the limited use of the PIM as a sorbent. Furthermore, the desorption study showed the quantitative desorption of Am3+ in 90 min using 1 M AHIBA as the strippant solution. The desorption profiles, shown in Figure 7, also indicate the deterioration tendency of the PIM on successive use as a sorbent. These results have relevance in the stability of the PIMs when used to carry out metal ion transport studies (discussed below in section 3.2.3). 3.2. Transport Studies. Transport studies were carried out using a two-compartment cylindrical transport cell with 1 M HNO3 as the feed and 1 M AHIBA as the strippant solution. Similar to the uptake studies, the PIM with no carrier showed negligible transport ( Am3+ > Th4+ > UO22+.19 On the other hand, a contrasting trend of actinide ion uptake was observed with a TODGA-based PIM.18 Interestingly, in contrast to the T2EHDGA-based PIM, the uptake of Pu4+ was found to be significantly higher (∼90%) by the T-DGA-based PIM even at a 10 times lower concentration of the carrier present in the PIM.19 3.1.4. Sorption−Desorption Study of the PIM. The longterm usage of PIMs depends on two factors, viz., the effective stripping of the sorbed metal ions and the effective reuse of the membrane. We have previously reported that α-hydroxy-isoE

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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compartment. It appears that in view of the very low concentration of Th ( Pu4+ ≫ UO22+ ≥ Th4+, which is contrasting to that observed with the uptake studies (Pu4+ > Am3+ > Th4+ ∼ UO22+). The difference between uptake and transport is that the latter also involves the stripping of the metal ions. Since the extraction and the stripping of a metal ion is a faster process, the slow rate-determining step is the bulk diffusion of the metal ion across the membrane. Thus, the transport efficiency strongly depends on the bulk diffusion of the metal ion. It is clear from the mass balance in Figure 8 that a considerable amount of the actinide ions was held up within the PIM during the 2 h transport period, indicating slow diffusion of the metal ion through the PIM. This was confirmed by counting the PIM used for the Am3+ transport, which accounted for the missing counts (total counts at the beginning of the experiment minus the sum of counts in the feed and receiver compartment after 2 h). We have also observed retention of actinides by a CTA-based PIM when TODGA was the carrier.18 The permeability coefficient values were calculated and the transport as well as uptake data of the actinides after 2 h are summarized in Table 2. The P values as seen from the feed follow the trend: Am3+ > Pu4+ > Th4+ > UO22+. It was surprising not to detect any Th4+ ion in the receiver

metal ion

T-DGA content (mg)

0.00 0.03 0.04 0.04

a

With the composition, 6.5% T-GA, 67.7% NPOE, 25.8% CTA; feed, 20 mL of 1 M HNO3; receiver, 20 mL of 1 M AHIBA; using 4.94 cm2 PIMs with average weight of 62 ± 1 mg. bValues inside parentheses refer to the metal ion uptake and transport after 24 h. cN.D.: Not detected. F

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 4. Stability Studies of PIM with Compositiona day

% Am(III) remained in the feed phaseb

1 2 3 4 5

4.2 (nil) 10.0 (0.3) 6.9 (0.2) 31.0 (0.4) 34.4 (1.0)

% of Am(III) transported into the receiver phaseb 87.4 84.4 77.1 52.5 55.2

% Am(III) held in PIMb

(99.9) (95.8) (88.0) (82.8) (88.4)

8.4 (0.05) 5.7 (3.9) 16.1 (11.7) 16.5 (16.8) 10.4 (10.7)

P (10−5 cm/s)

% deterioration in P as compared to day 1

± ± ± ± ±

0 29.1 37.0 65.7 66.8

2.89 2.05 1.82 0.99 0.96

0.03 0.04 0.08 0.02 0.03

a 21.6% T-DGA, 56.8% NPOE, 21.6% CTA on Am3+ transport. Feed, 1 M HNO3; receiver, 1 M AHIBA at pH 3.0. bData inside parentheses refer to after 24 h.

large T-DGA molecule weakening the metal extraction by the PIM. Th4+ could not be detected after 2 h transport, despite a higher cumulative uptake by the PIM as compared to UO22+, pointing to trapping of Th4+ within the PIM matrix. The stability study showed minor structural weakening of the membrane after a 5 days run, even though the permeability coefficient deteriorated sharply from day 1 up to day 4, followed by a marginal decrease in the permeability coefficient.

To test the long-term stability of the membrane, a particular PIM was reused, while the feed and strippant solutions were replaced with fresh solutions in each successive cycle.34 In this case, the feed solution was 1 M HNO3 containing Am3+ tracer, whereas the stripping solution was 1 M AHIBA and the experiment was conducted for five successive days. The permeability coefficient of Am3+ transport in each replicate run was determined (Table 4), while the transport profiles are shown in Figure S3 (Supporting Information). As can be seen from Table 4, the transport efficiency of Am3+, as detected from the receiver phase samples, decreases from day 1. The permeability coefficient values of Am3+ transport decrease in each run, comparing the second with the first run there is a decrease of 29.1%. It decreases further to 65.7% after 4 days and stabilizes at 66.8% lower after 5 days, indicating the limited stability of the PIMs. These results are in conformity with the reusability of the PIMs during batch uptake studies by a series of sorption and desorption studies mentioned above (section 3.1.4). The deterioration of the PIMs was investigated by various techniques. The FT-IR bands characteristic of T-DGA in the pristine PIM were found to be present in the PIM used for stability studies (Figure S3, Supporting Information) suggesting insignificant leaching of the extractant. On the other hand, the transmission infrared mapping microscopy (TIMM) profiles, showing the chemical images of the PIMs (Figure S4, Supporting Information), suggested significant degradation of CTA. The AFM profiles of the PIMs exposed to 1 M HNO3 for 5 days showed considerable reduction in the roughness (Figure S5, Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04621. Am uptake tables, figures with transport and time-lag plot of Am(III), and stability of PIM (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-22-25505151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors (B.N.M., D.R.R., and P.K.M.) thank Head, Radiochemistry Division, BARC, for his constant encouragement during this work.

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REFERENCES

(1) Kim, G.; Burnett, W. C.; Horwitz, E. P. Efficient preconcentration and separation of actinide elements from large soil and sediment samples. Anal. Chem. 2000, 72, 4882−4887. (2) La Rosa, J. J.; Burnett, W.; Lee, S. H.; Levy, I.; Gastaud, J.; Povinec, P. P. Separation of actinides, cesium and strontium from marine samples using extraction chromatography and sorbents. J. Radioanal. Nucl. Chem. 2001, 248, 765−770. (3) Bartsch, R. A.; Way, J. D.; Chrisstoffels, L. A. J.; de Jong, F.; Reinhoudt, D. N. Chemical Separations with Liquid Membranes; ACS Symposium Series, Number 642; American Chemical Society: Washington, DC, 1996. (4) Mohapatra, P. K.; Manchanda, V. K. Liquid membrane based separations of actinides and fission products. Indian J. Chem. 2003, 42A, 2925−2938. (5) Kocherginsky, N. M.; Yang, Q.; Seelam, L. Recent advances in supported liquid membrane technology. Sep. Purif. Technol. 2007, 53, 171−177. (6) Nghiem, L. D.; Mornane, P.; Potter, I. D.; Perera, J. M.; Cattrall, R. W.; Kolev, S. D. Extraction and transport of metal ions and small organic compounds using polymer inclusion membranes (PIMs). J. Membr. Sci. 2006, 281, 7−41. (7) Sugiura, M.; Urita, S.; Kikkawa, M. Effect of plasticizer on carriermediated transport of zinc ion through cellulose triacetate membranes. Sep. Sci. Technol. 1987, 22, 2263−2268.

4. CONCLUSIONS It was found that T-DGA-containing PIMs were more efficient than TODGA-based PIMs in the batch uptake studies. However, as compared to solvent extraction systems, where a 100 times lower concentration of T-DGA (as compared to TODGA) was found to be enough, here a much larger concentration of T-DGA was required. The actinide uptake with the T-DGA-based PIMs follows the trend: Pu4+ > Am3+ ≫ UO22+ > Th4+, which is slightly different (Am3+ > Pu4+ > Th4+ ≫ UO22+) from that reported with TODGA-containing PIMs18 but similar to that of T2EHDGA-containing PIMs.19 The sorption−desorption study indicated a limited use of the PIM as a sorbent. T-DGA containing PIMs were found to be inferior to the TODGA containing PIMs reported earlier.18 Transport studies showed a better permeation of Am3+ into the strippant solution as compared to the Pu4+ions. This is in contrast to the uptake results, which is a unique observation of this work. The transport of UO22+ ion to the strippant solution is small, probably due to steric hindrance of the bulky uranyl ion and the G

DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b04621 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX