Highly Efficient Pertraction of Tetravalent Neptunium Ions Across a Flat Sheet Supported Liquid Membrane Containing Two Different Aza-Crown Ether-Based Multiple Diglycolamide Ligands

ABSTRACT Pertraction of Np(IV) across a PTFE membrane containing two different aza-crown ether-based multiple diglycolamide (DGA) ligands termed as L I (3 DGA arms) or L II (4 DGA arms) was investigated using nitric acid as feed solution. A few of the studies were also carried out with Pu(IV) for comparison purposes. The kinetics of the extraction of Np(IV) and Pu(IV) was fast at 3 M HNO3 and within 10 min reaches the equilibrium distribution (D) ratio with both ligands. However, the stripping kinetics was slower especially with ligand L II . The extraction and transport studies were carried out using 1.0 × 10−3 M ligand solution in 95% n-dodecane + 5% isodecanol. The transport rates were slower with ligand L II than with L I at higher nitric acid concentrations (>1 M HNO3) and the transport rates decreased with nitric acid concentration with both ligands. The effective diffusion coefficients (D eff) of Np(IV) were estimated using the lag-time method as 6.2 × 10−8 cm2 s−1 (L I ) and 4.6 × 10−8 cm2 s−1 (L II ). The stability data suggest that the L I containing supported liquid membrane (SLM) is comparatively more stable than that of L II and the transport flux remains the same up to nine days of operation in case of ligand L I .


Introduction
The separation of the long-lived minor actinides ( 237 Np, 241 Am, 243 Cm), especially 237 Np, from the high level waste (HLW) is very important not only because of its very long half-life (t 1/2 : 2.14 × 10 6 yr), but also of its use as a target material for the production of 238 Pu which is employed as a power source in the thermoelectric generator in the space program due to its high heat output. [1] However, separation of neptunium from HLW possesses challenges to the nuclear chemist due to the presence of its different oxidation states (+4, +5, +6) in the acidic HLW (3-4 M HNO 3 ). [2] Generally, more than 40% of Np stays in the aqueous raffinate, while 60% Np goes to the TBP (tributyl phosphate) bearing phase during the first cycle of the PUREX (Plutonium Uranium Redox Extraction) process. [3] TBP forms degraded products on radiolysis, which can interfere while stripping of the metal ions from the organic phase and also forms insoluble phosphates limiting the waste loading capacity of the glass matrix. [4] Diglycolamide (DGA)-based extractants are known as benign to the Environment as they consist of C, H, O, N elements [5] which form innocuous degradation products . [6] These extractants show very high distribution ratios (D M ) for the trivalent lanthanides/actinides and tetravalent actinides at 3-4 HNO 3 . For example, 0.05 M N,N,N',N'-tetraoctyl diglycolamide (TODGA; Figure 1) in 95% n-dodecane + 5% isodecanol medium showed D Np(IV) and D Pu(IV) -values of 78.5 ± 5.9 and 108.0 ± 8.3, respectively, at 3 M HNO 3 . [7] However, with multiple diglycolamide-based extractants, where generally three or four DGA units are tethered to a carbon or nitrogen center, generally, much higher D-values are obtained under similar experimental conditions due to cooperative action of binding of the metal ions by the ligating sites. In case of 0.001 M of a tripodal DGA ligand with an isopropyl group substituted at the inner amidic 'N' atom (termed as i-Pr 3 -TREN-DGA; Figure 1) in 95% n-dodecane + 5% isodecanol, D Np(IV) and D Pu(IV) -values of 147.1 ± 7.3 and 315.0 ± 2.7, respectively, were obtained at 3 M HNO 3 . [8] In view of this, we have extensively studied the extraction behavior of tetravalent actinides and trivalent actinides/lanthanide ions from acidic feed solutions using various multiple DGA-based extractants. [9] Recently, Ansari et al. reported the very promising extraction behavior of tetravalent neptunium and plutonium using two aza-crown ether scaffolds functionalized with DGA units, namely, a 9-membered aza-crown ether containing three 'N' atoms (L I ) and a 12-membered aza-crown ether containing four 'N' atoms (L II ) ( Figure 1) in a n-dodecane + isodecanol mixture from nitric acid solution. [10] In view of the lower inventory of the carrier ligands and other advantages of the use of a supported liquid membrane (SLM) over liquid-liquid extraction, such as simultaneous extraction/stripping, no thirdphase formation, higher selectivity, it is of interest to investigate the pertraction behavior of tetravalent actinide ions using L I and L II as carrier ligands. [11] The present investigation deals with the pertraction behavior of Np(IV) and Pu(IV) (for comparison) ions across a SLM using these two ligands in a 95% n-dodecane + 5% isodecanol mixture from acidic solutions. The preliminary understanding of this carrier mediated extraction/pertraction behavior of these metal ions across the flat sheet SLM using these ligands is very important in order to design a hollow fiber membrane contactor system [12] for its real application for the separation of these metal ions from HLW solution.

Chemicals
The aza-crown ether-based multiple-DGA ligands L I and L II were synthesized and characterized as described previously. [13] The diluents n-dodecane and isodecanol (>99% purity) were procured from Lancaster, UK and SRL, Mumbai, respectively, and were used as obtained. Oxalic acid (SRL, India), 2-thenoyltrifluoroacetone (TTA) (Fluka, Switzerland) were procured and were used as such. The different concentrations of the feed nitric acid were prepared by appropriate dilution of suprapur nitric acid (Merck) using milliQ water (Milipore, USA). Subsequently, the acids were standardized using a known strength NaOH solution and the end point was detected by phenolphthalein (Fluka) as indicator. A ferrous sulfamate solution (0.3 M) was prepared by dissolving iron powder (BDH) in sulfamic acid (Aldrich) taken in the proper ratio and heating under an infrared (IR) lamp. All the other chemicals used here for the experiments were of AR grade.

Membranes
Porous PTFE (polytetrafluoroethylene) flat sheet membranes (Sartorius, Germany) of diameter 47 mm, thickness 80 µm, pore size 0.45 µm, and porosity 72% were used in the present study. The pore size of the membrane was confirmed by Hg porosiometry measurements, [14] while the thickness was measured by a Mututoya digital micrometer.

239
Np tracer used for the present study was prepared on irradiating uranyl nitrate hexahydrate in the Dhruva reactor (thermal neutron flux: 1 × 10 14 n cm −2 s −1 ) at BARC, Mumbai. 239 Np was formed by (n, γ) reaction with 238 U followed by β − emission. Subsequently, 239 Np tracer was separated from the host of the matrix by adding ca. 2 mL of 1 M HNO 3 to dissolve the irradiated product and a few drops of ferrous sulfamate (FS) (0.3 M) and 50-100 mg of solid hydroxylamine hydrochloride (HA) to convert Np into its +4 state. [15] Subsequently, the mixture was vigorously shaken with 2 mL of 0.5 M HTTA in xylene (Merck) for a few minutes. This two-phase mixture was centrifuged at 5000 rpm for 10 min and the organic phase was contacted with 2 mL of 8 M HNO 3 to strip neptunium. Subsequently, the aqueous phase (containing 239 Np tracer at 8 M HNO 3 ) was contacted twice with xylene to remove any traces of organic material present in the aqueous phase. This aqueous phase containing the 239 Np(IV) tracer in 8 M HNO 3 was used as the 239 Np stock for the subsequent studies. The radionuclide purity (>99%) was checked using gamma ray spectrometry and scintillation counting with an alphabeta discriminator. Pu (mainly 239 Pu) was purified from the laboratory stock using an anion exchange resin (DOWEX 1 × 4) to remove 241 Am traces before use and the purity was checked by alpha as well as gamma ray spectrometry. [16] The oxidation state of Pu and Np in the acidic solution was adjusted to +4 using NaNO 2 and a FS-HA mixture, respectively, and the oxidation state of +4 was confirmed from the slope of the log-log plot of D Pu or D Np with HTTA concentrations in xylene. [17] 239 Np was assayed by gamma ray counting using a NaI(Tl) scintillation counter (Para Electronics) coupled to a multi-channel analyzer (ECIL, India), while Pu was assayed by liquid scintillation counting (Hidex, Finland) using the Ultima gold scintillation cocktail (Perkin Elmer). The concentration of the metal ions, viz., Np 4+ and Pu 4+ in the studies was ~10 −12 M and ~10 −6 M, respectively.

Liquid-liquid extraction studies
Liquid-liquid extractions were carried out in a 10 mL capacity stoppered glass tube in which equal volumes (usually 1 mL) of the aqueous and the organic phases were mixed on constant shaking of the tube in a thermostated water bath at 25 ± 0.1 ºC for about 1 h. After that, the tubes were settled and centrifuged (5000 rpm for 1 min). Subsequently, equal aliquots (100 µL) were removed from the organic and the aqueous phases and assayed radiometrically. The organic phase contained 1.0 × 10 −3 M L I /L II in 95% n-dodecane + 5% isodecanol. Isodecanol is used to increase the solubility of the ligand in the diluent mixture. [18] The aqueous feed was prepared by spiking the radio tracer in the nitric acid solution (0.5 M to 6 M HNO 3 ). The D-value is defined as the ratio of the activity of the metal ion per unit volume in the organic phase to that in the aqueous phase (Equation (1)). Since we used equal volumes of the organic and aqueous phases, the percentage stripping (% S) can be defined as follows: Activity of the metal ion per unit volume in the organic phase Activity of the metal ion per unit volume in the aqueous phase (1) The liquid-liquid extraction studies were carried out in duplicate and the relative standard deviation was found to be 2%, whereas the mass balance was within ±5%. The mass balance is defined as the difference of the counts per minute per 100 µL of the initial aqueous phase and the total counts in 100 µL per minute of the organic and aqueous phases after equilibration.

Supported liquid membrane transport studies
The supported liquid membrane (SLM) studies were conducted in a twocompartment (20 mL each) transport cell ( Figure S1, ESI) in which the PTFE membrane, containing the L I /L II ligand solution was fixed in between the two compartments and was tightened using a Parafilm strip. Both compartments were clamped together using a metallic clamp. However, before fixing the PTFE membrane in between the two compartments, it was soaked in the ligand solution for 60 min to ensure uniform soaking of the solution onto the membrane. The extra liquid onto the soaked membrane was gently wiped out using a tissue paper before fixing it into the transport cell. One compartment of the transport cell was filled with the nitric acid solution containing the radiotracer, while the other compartment was filled with the stripping solution. Both the solutions were continuously stirred by a stirrer bar and a sturdy synchronous magnetic stirrer. The samples (usually 100 µL) were removed at a regular time interval from both the source and the strip phases simultaneously through their respective sampling port as shown in Figure S1 and assayed radiometrically. All the transport studies were carried out at room temperature (25 ± 1°C) under ambient conditions. Different transport parameters, viz., the effective diffusion coefficient (D eff ), the permeability coefficient (P), were determined from the transport data (vide infra). In addition, the cumulative percentage transport (%T) of the metal ion to the receiver phase was determined from Equation (3) to quantify the transport of a metal ion at a given time.
The "% Feed" in the feed phase at a given time t is determined by Equation (4): where C f,t and C r,t represent the concentrations of the metal ions in the source and the receiver phases, respectively, at time 't' and C f,0 represents that in the feed phase at the start of the experiment. The permeability coefficient (P) is determined using the following equation [19] : Where Q stands for the effective surface area (cm 2 ), which is obtained by multiplying the geometrical surface area (A) (4.94 cm 2 ) and the porosity (ε) (72%) of the PTFE membrane, and V f is the feed volume (20 mL) in the present study and time (t) is expressed in seconds. The transport studies were carried out in duplicate and the relative standard deviation was found to be 5%.

Extraction and stripping kinetics
In carrier mediated transport of metal ions through an SLM, the metal ion is first extracted in the membrane at the source-membrane interface, followed by diffusion of the metal ion-carrier complex through the membrane and finally stripping of the metal ion from the complex at the membrane-strip interface. Generally, diffusion of the metal ion-carrier complex across the SLM is the slowest step and therefore the overall mass transfer kinetics depends on the kinetics of the diffusion of the metal ion-carrier complex across the SLM. However, there are reports where the extraction/stripping kinetics also can be the rate-limiting step. As for example, Reichwein-Buitenhuis et al. [20] have shown that slow kinetics of alkali cation decomplexation is the rate-limiting step in the transport of alkali cations using macrocyclic carriers like calix[4]arenes and calixspherands in a SLM. Therefore, it is important to investigate the extraction and stripping kinetics of the Np(IV) and Pu(IV) metal ions with both carrier ligands. For an efficient transport of the metal ions across the SLM, a reasonably fast extraction and stripping kinetics is required. We investigated the extraction kinetics of Np(IV) and Pu(IV) by extracting the respective metal ion from 3 M HNO 3 with 1 × 10 −3 M L I /L II in 95% n-dodecane + 5% isodecanol. [10b] The stripping kinetics was followed by contacting the stripping phase (0.5 M HNO 3 + 0.5 M oxalic acid) with the metal ion extracted organic phase containing 1 × 10 −3 M L I /L II in 95% n-dodecane + 5% isodecanol. The use of 0.5 M HNO 3 + 0.5 M oxalic acid as the strippant was decided based on our previous studies with different DGA ligands. [8] The extraction and stripping data are plotted in Figure 2.  hand, the stripping kinetics data indicate a relatively slower stripping process for Np(IV)/Pu(IV) from the L II containing organic phase compared to that containing L I . This may be due to the higher stability of the Np(IV)/Pu(IV)-L II complex compared to that of Np(IV)/Pu(IV)-L I , which causes difficulty in the stripping of the metal ions from the metal -ion complex. Higher stability of the metal-ion complex with L II was also reported in solvent extraction and DFT studies. [10b] Nevertheless, the extraction and stripping kinetics data are quite encouraging to investigate the transport behavior of Np(IV)/Pu(IV) with these ligands.

Effect of nitric acid concentration
The effect of the nitric acid concentration on the transport of metal ions across a SLM is important to investigate as it significantly affects the extraction of metal ions. The nitrate ion affects the extraction of metal ion according to the following equations.
The nitrate ions in the metal ion complex are not only satisfying the charge of the central metal ion but also the coordination number required for the metal ion complex as shown by Ansari et al. They proposed that two nitrate ions are coordinating through bidentate mode and two nitrate ions are in monodentate fashion, whereas one ligand is coordinating through its single DGA arm satisfying all the coordination numbers of the metal ion. [10b] Therefore, it is highly expected that extraction of a metal ion complex at the source-membrane interface by the membrane will depend on the nitrate ion concentration in the feed phase. From the above equation, it is expected that with the increase of the nitrate ion concentration, the extraction of the metalion complex increases and therefore transport across the SLM should also increase.
The transport of Np(IV) across the SLM containing 1 × 10 −3 M L I /L II was studied at 1 M, 3 M and 6 M HNO 3 in the feed phase using 0.5 M HNO 3 + 0.5 M oxalic acid as the strip phase. As can be seen from Figure 3 (Table 1). Therefore, maximum transport after 4 h was observed when the nitric acid concentration was 3 M and 1 M with L I and L II , respectively, while studying the nitric acid concentration effect at 1 M, 3 M and 6 M HNO 3 in the feed phase. The different behavior of the Np(IV) transport with the nitric acid concentration by the two analogous ligands may be explained based on the acid uptake tendency of these ligands. The acid uptake tendency of L II is found to be more compared to that of L I at a given nitric acid concentration (Table S1, ESI). Therefore, more free ligand (ligand which is not forming an adduct with acid; Equation (7)) concentration is available with L I compared to L II at a given nitric acid concentration. Transport of HNO 3 into the strip phase decreases the stripping efficiency and therefore decreases the transport efficiency of the metal ion. However, in view of the very low concentration of the ligands used here (1 × 10 −3 M), the transport of HNO 3 into the strip phase is expected to be negligible. In addition, the presence of hydroxylamine hydrochloride plus ferrous sulfamate in the feed solution confirms that neptunium remains in + 4 oxidation state and the presence of + 5 or +6 state is negligible. Therefore, the ligand.HNO 3 adduct formation could be the deciding factor for decrease of transport efficiency with acid concentration. Since, the tendency of adduct formation is more with L II  (as evident from the more sharper decrease of D Np(IV) with HNO 3 at higher HNO 3 concentration [17] and Table S1, ESI), we observe a decreasing trend of transport/P-value starting from 1 M HNO 3 with L II , whereas the same was observed at higher nitric acid concentration with L I and we get a peak value of P at 3 M HNO 3 when transport studies were carried out at 1 M, 3 M and 6 M HNO 3 with L I . The higher transport of Np(IV) with L II compared to L I at 1 M HNO 3 (Table 1)

Transport of Pu(IV) and Np(IV)
It is of interest to investigate the transport behavior of another tetravalent radionuclide, viz., Pu(IV) which is also present along with Np(IV) in the spent fuels and in the HLW. Therefore, the transport behavior of Pu(IV) was studied at 3 M HNO 3 , i.e., at the acidity of HLW (vide supra) using 1 × 10 −3 M L I /L II in 95% n-dodecane + 5% isodecanol and 0.5 M HNO 3 + 0.5 M oxalic acid as the strippant. The transport profiles are included in Figure 4 showing that the transport of Pu(IV) with both the ligands is lower compared to that of Np(IV) at 3 M HNO 3 . As for example, the percentage transport of Pu(IV) and Np(IV) with L I after 4 h under similar experimental conditions are 59.5 ± 3.0 and 97.6 ± 4.9, respectively, while that with L II are 48.7 ± 2.4 and 89.9 ± 4.5, respectively (Table 1). This observation is in opposite trend in view of the higher D-ratios of Pu(IV) compared to that of Np(IV) with both ligands at 3 M HNO 3 .
[10b] The higher transport of Np(IV) than that of Pu(IV) across the membrane, even though the former metal ion has a lower D-ratio than the latter one, may be due to the difference in kinetics of the stripping of the metal ions from the membrane. A similar phenomenon in the transport of Np(IV) and Pu(IV) was also observed in our earlier studies with TRPN-DGA and i-Pr 3 -TREN-DGA, due to the availability of more free ligand for Np(IV) than for Pu(IV). [8] The lower transport of Pu(IV) with L II compared to that with L I is in the same line as with Np(IV) (vide supra) at 3 M HNO 3 .

Comparative transport behavior with different ligands
The transport behavior of the present ligand systems was compared with that of analogous ligand systems already published by us. As can be seen from Table 2, the highest P-value of Np(IV) transport was obtained with L I followed by L II among the other ligands. Not only this, the selectivity of the transport of Pu(IV) TRPN-DGA 0.67 ± 0.01 0.35 ± 0.01 a - [8] i-Pr 3 -TREN-DGA 0.80 ± 0.05 0.24 ± 0.01 a - [8] TREN-G1-DenDGA b,c 0.643 ± 0.015 0.316 ± 0.030 2.03 [21] T-DGA d 0.12 ± 0.04 0.86 ± 0.17 0.14 [22] TODGA e -1.52 ± 0.03 - [22] L I Np(IV) with respect to Pu(IV) (which is defined as the ratio of the permeability coefficient of Np(IV) to Pu(IV) under similar experimental conditions) was found to be highest with L I followed by L II . The other ligand systems, viz., T-DGA and TREN-G1-DenDGA, show a poorer selectivity of transport ( Table 2). The selectivity of transport for other ligand systems was not estimated because the permeability coefficients of Np(IV) and Pu(IV) were measured under different experimental conditions. This shows that the present ligands, especially L I , is very efficient for the transport of Np(IV) from 3 M HNO 3 as the feed phase. The high D-value (41.2 ± 4.1) for the extraction along with fast extraction and stripping kinetics and very efficient stripping (>99%) with L I makes this system very efficient for Np(IV) transport from acidic medium.

Diffusion coefficient measurement
The effective diffusion coefficient (D eff ) values of Np(IV) were estimated using the lag-time method using the following formula: Where d o, ε and t lag are the membrane thickness (here 0.007 cm), membrane porosity and the lag-time, respectively. The lag-time is defined as the time required (seconds) for the first appearance of the Np activity in the strip phase from the start of the experiments. It can be estimated by plotting the counts per minute per mL of the strip phase on the y-axis versus time in the x-axis from the start of the experiment and estimating the time corresponding to the intersection of the two straight lines as shown in Figure 5. It exhibits that the lag-time of the Np(IV)-complex with L II is higher than that with L I indicating slower diffusion of the former metal nitrate-ligand complex. The estimated D eff -values were also compared with those of the diffusion coefficients (D o ) calculated using the Wilke-Chang equation [23] which is defined as: Where M is the molecular weight of the complex, M(NO 3 ) 3 .L (M = Np and L = L I /L II ), χ and η are the solvent association parameter and the viscosity of the solvent, respectively, V m is the molar volume of the ML complex and T is the temperature in degree Kelvin. The molar volume of L I and L II was calculated as 2085.36 cm 3 mol −1 and 2996.8 cm 3 mol −1 , [24] whereas the molecular weight of Np(NO 3 ) 4 ·L I and that of Np(NO 3 ) 4 ·L II was 1633 g mol −1 and 2015 g mol −1 , respectively. The dynamic viscosity of n-dodecane was used as an approximation for the solvent system containing 5% isodecanol in n-dodecane which is 1.34 mPa.s and χ is taken as unity for the solvent system. The diffusion coefficients determined by the lag-time method and by the Wilke-Chang equation are given in Table 3. Those obtained from the Wilke-Chang equation are higher (more than two orders) than those obtained from the lag-time method. This is since the Wilke-Chang model has the assumption that the diluent is non-interacting and with a diluent mixture containing n-dodecane and isodecanol the assumption may not be quite true. The smaller value of diffusion coefficients with L II as compared to that with L I is due to the formation of relatively bulkier complex with the former ligand due to the presence of larger aza-crown ether ring and the presence of more number of DGA groups in its structure.

Stability study
The stability of the membrane is an important issue while scaling up the process for industrial use. The SLMs are generally not very stable and show decrease in the flux for metal ion transport after a few cycles of operation. Therefore, it is very important to investigate the stability of the present membrane system. It is monitored by estimating the % transport (or P-value) of neptunium through the membrane for both the ligands over nine days and the data are shown in Table 4 as well as in Figure 6. The experiments were conducted by monitoring the flux for different cycles (days) while keeping the membrane intact and changing the source and strip Table 3. Determination of diffusion coefficients of Np(IV) complexes in the transport across the flat sheet SLM containing 1 × 10 −3 M ligand solution in 95% n-dodecane + 5% isodecanol. Feed: 3 M HNO 3 . Strip: 0.5 M HNO 3 + 0.5 M oxalic acid. Volume: 20 mL.
Ligand t lag (s) D eff (cm 2 s −1 ) D o (cm 2 s −1 ) L I 84 ± 5 (6.2 ± 0.5) × 10 −8 6.8 × 10 −6 L II 114 ± 6 (4.6 ± 0.3) × 10 −8 6.1 × 10 −6 phase solution before the start of a new cycle. Figure 6 and Table 4 show that the transport of Np decreases with both membrane systems, the L I system showing a better stability than that of L II over a period of nine days. As for example, the percentage transport data of Np(IV) with L I after 4 h are 97.9 ± 4.9, 97.2 ± 4.9, 96.8 ± 4.8, 93.7 ± 4.7 on days 1, 2, 3 and 9, respectively, while that with L II are 89.9 ± 4.5, 83.6 ± 4.2, 76.0 ± 3.8 and 48.2 ± 2.4, respectively. The relatively poor stability with L II may be due to its higher tendency of acid uptake than that seen with L I , which decreases the free ligand concentration available for metal ion transport over nine days.

Conclusions
Two different aza-crown ether-based multiple diglycolamide (DGA) ligands (viz., L I and L II ) were studied for SLM transport studies of Np(IV) and Pu(IV) from acidic medium. The extraction and stripping kinetics data suggest that both processes are relatively fast (within 10 min). The cumulative percentage transport data indicate very efficient transport of Np(IV) with 1 × 10 −3 M L I and L II containing SLMs, especially that with L I . As for example, the percentage transport values of Np(IV) at 4 h using L I and L II are 97.6 ± 4.9 and 89.9 ± 4.5, respectively, whereas that for Pu(IV) are 59.5 ± 3.0 and 48.7 ± 2.4,  respectively, at 3 M HNO 3 . Such a high transport rate of Np(IV) is not seen with other DGA-based ligands studied by us. The stability study showed that the L I -based membrane is more stable than that of L II over a period of nine days. The higher D eff -value obtained with the Np(IV)-L I complex compared to that of the Np(IV)-L II complex is due to the comparatively less bulkier species transported through the membrane in the case of L I as the carrier ligand.
Overall, it can be concluded that although L II has a better extraction ability for Np(IV) than L I , the SLM transport study just shows the opposite order.