Using TODGA in Room Temperature Ionic Liquids

Transcription

1 J Solution Chem (018) 47: Extraction of Np 4þ and NpO þ from Nitric Acid Medium Using TODGA in Room Temperature Ionic Liquids S. A. Ansari 1 P. K. Mohapatra 1 D. R. Raut 1 Received: 5 September 017 / Accepted: 1 January 018 / Published online: 3 April 018 Ó Springer Science+Business Media, LLC, part of Springer Nature 018 Abstract Extraction of Np 4? and NpO þ was carried out from nitric acid feeds using solutions of N,N,N 0,N 0 -tetra-n-octyldiglycolamide (TODGA) in two imidazolium-based room temperature ionic liquids, viz., 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl) imide ([C 4 mim[ntf ) and 1-octyl-3-methylimidazolium bis(trifluoromethanesulphonyl) imide ([C 8 mim[ntf ). The extraction equilibrium was attained within h for both the metal ions in both the ionic liquids. While a cation exchange mechanism is proposed for the extraction of NpO þ ; an ion-pair mechanism of extraction is proposed for the Np 4? ion. The nature of the extracted species was determined by carrying out experiments at varying concentrations of TODGA, and species of the type Np(L) (NO 3 ) 4 and NpO (L)? were found to be extracted in 3 moldm -3 HNO 3. The identification of these extracted species was also supported from the variable nitrate and C 4 mim? ion concentration experiments. Keywords Neptunium TODGA Extraction Ionic liquid 1 Introduction Neptunium, the major component of the minor actinides, with significant concentrations both in the dissolver solution (arising out of the dissolution of the spent nuclear fuel) as well as the high level radioactive waste (made after concentrating the raffinate emanating from the PUREX process [1), has not attracted as much attention from the researchers as other actinides such as uranium and plutonium. There are good reasons to study the chemistry of neptunium which include: (a) development of efficient and effective separation methods from acidic feeds which have relevance for radioactive waste remediation, & P. K. Mohapatra mpatra@barc.gov.in 1 Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai , India 13

2 J Solution Chem (018) 47: and (b) understanding the migration and speciation of neptunium under environmental conditions [. Though a major fraction of the 37 Np produced in a nuclear reactor, by neutron capture by 35 U as well as 38 U, end up in the high level radioactive waste, it is not advisable to vitrify this long lived radionuclide along with the fission product nuclides in glass blocks [3. In view of the long half-life of 37 Np (t 1/ = years), the vitrified blocks would need long surveillance periods for possible neptunium escape (due to leaching), potentially leading to long term environmental contaminations. Furthermore, the recovered 37 Np can be utilized for the production of 38 Pu that is used in pacemakers and also as a power source [4. From the basic chemistry point of view, understanding the aqueous chemistry of neptunium is fascinating due to the possibility of the coexistence of several different oxidation states, viz.,? 3,? 4,? 5 and? 6, and it is truly challenging to understand the complex chemistry of neptunium. Both Np 4? and NpO þ are known to behave in a manner similar to Pu 4? and UO þ and, hence, their chemistry can be assumed to be comparable. TBP (trin-butyl phosphate) can extract both Np 4? and NpO þ from nitric acid medium [5 and there are reports on the extraction of the metal ions by tertiary amines as well [6. Recent studies have indicated the extraction of Np 4? and NpO þ by CMPO [7, malonamides [8 and diglycolamides [9, which are well known extractants for the separation of the minor actinides using a proposed strategy known as Actinide Partitioning [10. Out of these extractants, diglycolamides are reported to be the most efficient, yielding very high extraction of the actinide ions at relatively low concentrations of the ligand and at moderate concentrations of HNO 3. These solvent extraction methods, however, have the drawback of using solvents with finite volatility which can lead to a high volatile organic carbon burden on the environment. On the other hand, room temperature ionic liquids (RTILs), a class of benign neoteric diluents of recent origin, which are non-volatile and non-inflammable with good solvation properties and large electrochemical windows, have been proposed as suitable alternatives to the commonly employed volatile molecular diluents [ Furthermore, RTILs are reported to have good radiolytic stability vis-àvis the molecular diluents, making a case for their use as a better option for extraction of radionuclides [15. During the past decade and half, a large number of publications have appeared on the application of RTILs as alternative diluents for metal ion extraction including actinide ions [16. However, there are very few publications available on the use of RTILs for neptunium separation from acidic feeds [3, 4. It is of interest to understand the mechanism of neptunium extraction in ionic liquid medium, especially with an extractant such as TODGA (Fig. 1a) that has been investigated by many researchers for actinide ion extraction into ionic liquids. As the cation-exchange extraction mechanism is prevalent in ionic liquids (Fig. 1b) such as [C 4 mim[ntf, while a solvation mechanism has been suggested for ionic liquids with longer alkyl chain such as [C 8 mim[ntf, it was of interest to investigate the extraction of neptunium in the? 4 and the? 6 oxidation states using solutions of TODGA in [C 4 mim[ntf as well as in [C 8 mim[ntf. To our knowledge, this is the first systematic study on the extraction and speciation of Np 4? and NpO þ ions using TODGA solutions in ionic liquids. 13

3 138 J Solution Chem (018) 47: Experimental.1 Materials TODGA ([ 98%) was purchased from Thermax Ltd. (Pune, India) and was used after ascertaining its purity by comparing its distribution ratios with those reported earlier [5. The ionic liquids, [C 4 mim[ntf and [C 8 mim[ntf, were procured from IoliTech, Germany (purity, 99%) and were used as received. Suprapur nitric acid (Merck, Germany) was used for preparing the dilute HNO 3 solutions with MilliQ water (Millipore, USA), and its concentrations were standardized by volumetric titration using standard NaOH ([ 99%, BDH) with phenolphthalein indicator (98%, Fluka). All the other reagents were of AR grade.. Radiotracer The 37 Np tracer was from a laboratory stock solution after ensuring its radiochemical purity by alpha spectrometry. Neptunium was converted to the? 4 state by following a literature method [6, using reducing agents such as ferrous sulfamate and hydroxylamine hydrochloride in 1 moldm -3 HNO 3.Np 4?, thus produced, was selectively extracted using a 0.05 moldm -3 HTTA (-thenoyltrifluoro acetone) solution in xylene. The extracted Np(IV) was stripped back using 7.5 moldm -3 HNO 3, which was subsequently used as the Np(IV) stock solution. On the other hand, NpO þ was prepared by oxidation to the? 6 state using K Cr O 7. The oxidation states of neptunium in the? 4 and? 6 states were checked by varying the HTTA concentration (in the range of moldm -3 )in experiments at fixed aqueous phase HNO 3 concentrations of 1 and 0.01 moldm -3, respectively, to yield slopes (of log 10 D vs. log 10 [TTA plots) of ca. 4 and. Confirmation from UV vis spectral studies was not possible due to the very low concentration of the metal ion used in the present study (vide infra). 37 Np was assayed radiometrically by alpha liquid scintillation counting (Hidex, Finland) using an Ultima Gold (Perkin Elmer) liquid scintillation cocktail. In the case of low count samples, long-time counting was done to avoid counting statistics errors. Fig. 1 Structural formula of a TODGA and b the ionic liquids used in the present study; R = n- butyl: [C 4 mim[ntf orr=noctyl: [C 8 mim[ntf 13

4 J Solution Chem (018) 47: Extraction Procedure Liquid liquid extraction studies (involving either Np 4? or NpO þ ) were carried out by equilibrating equal volumes (usually 1 ml) of the ionic liquid (containing the desired concentration of TODGA) and nitric acid solution (spiked with the Np tracer in the? 4or? 6 oxidation states) in leak-tight Pyrex glass tubes (volume 10 ml) in a thermostated water bath at 5 ± 0.1 C for 4 h. A separate study was carried out to optimize the equilibration time (vide infra), which was ca. h. Subsequently, the phases were separated after centrifugation (3000 rpm) for 5 min. The ionic liquid phases were heavier than the aqueous phases and adequate precaution was taken to avoid cross contaminations during removal of aliquots (usually 100 ll) for subsequent radiometric assay as mentioned above. The distribution ratio (D Np ) values were calculated as per the following expression: counts per unit time per unit volume in the ionic liquid phase D Np ¼ counts per unit time per unit volume in the aqueous phase : ð1þ The solvent extraction studies were carried out in duplicate and the accepted data points had mass balance within ± 5%. The reproducibility of the D Np values was within 3% of the average value. The concentration of the metal ion used in the solvent extraction studies was * 10-6 moldm Results and Discussion 3.1 Solvent Extraction Studies It is well known that neptunium exists in multiple oxidation states, viz.,? 4,? 5 and? 6 in nitric acid feeds. As the extraction of Np(V) is considered insignificant and those of Np(IV) as well as Np(VI) are relevant in the nuclear fuel cycle, the present study is focused on the extraction of neptunium in the? 4 and? 6 oxidation states. As ionic liquids are constituted of cations and anions of large molar volumes, metal ions can be extracted even in the absence of any extractant [. A previous paper reported quite significant extraction of tetravalent actinides such as Pu 4? from 6 moldm -3 HNO 3, the D Pu values being.70,.93 and 3.00 in [C 4 mim[ntf, [C 6 mim[ntf and [C 8 mim[ntf, respectively [7. In a separate study, Rout et al. [8 reported D Pu values as high as ca. 40 and 15 for [C 6 mim[ntf and [C 8 mim[ntf, respectively, for 6 moldm -3 HNO 3 as the feed. The very high D Pu values reported in the latter report may be due to quenching errors. However, there is no doubt that significant amounts of the tetravalent ion were extracted into the ionic liquid phase, even in the absence of any organic extractant. In view of this, it is quite natural to expect extraction of Np 4? into the RTIL phase, which should increase with increasing HNO 3 concentrations. Table 1 gives the D values of Np 4? obtained with [C 4 mim[ntf and [C 8 mim[ntf at different HNO 3 concentrations. Extraction of NpO þ was not studied in view of reported poor extraction of UO þ [7. The results indicated low D Np values, and hence may be of very less significance than that of Np 4? extraction which was reported to take place based on an anion exchange mechanism given by the following equation: MX ði 4Þ i aq þði 4ÞTf N IL MX ði 4Þ i IL þði 4ÞTf N aq ðþ 13

5 1330 J Solution Chem (018) 47: Table 1 Extraction of Np(IV) from various concentrations of HNO 3 by pure ionic liquids: equilibration time h, temperature 5 C Feed, HNO 3 (moldm -3 ) D Np(IV) [C 4 mim[ntf [C 8 mim[ntf 1 \ 10 3 \ (0.18) 0.05 (0.1) (0.40) 0.09 (1.38) (.70) 0.43 (3.01) The values inside parentheses refer to the values obtained with Pu(IV) where, M 4? denotes the tetravalent metal ion (for example, Pu 4? or Np 4? ), X denotes the nitrate ion and i (i [ 4) is the number of nitrate anions bound to the metal ion in forming the anionic complex. The species with the subscripts IL and aq refer to those present in the ionic liquid and aqueous phases, respectively. The results indicate that the extraction of Np 4? is significantly lower than that reported for Pu 4? (which may be aptly explained on the basis of their ionic potentials and complexation constants [9), and may not be of any practical consequence as the D Np values in the presence of TODGA are extremely high (vide infra) Extraction Kinetics The kinetics of metal ion extraction has been one of the most important parameters to be optimized in ionic liquid based solvent systems. In view of the moderate to high viscosity of the ionic liquids, there are reports where the time taken to attain equilibrium D values is enormously large [30, 31. On the other hand, there are reports where rather fast attainment of equilibrium has been achieved [3. A recent publication has clearly indicated that the aqueous phase acidity affects the extraction kinetics [33. In view of these conflicting reports, it was of paramount importance to understand the extraction kinetics in the present study. As shown in Fig., the results of the D Np versus time studies indicate that about h was sufficient to attain equilibrium in the cases of both Np 4? and NpO þ extraction using 0.01 moldm -3 TODGA in the two RTILs. This is in sharp contrast to the very slow kinetics reported by us for the extraction of Pu 4? using TODGA in RTILs [31. The exact reason for this discrepancy in the extraction kinetics is not understood and needs a separate thorough investigation. In view of this, all subsequent experiments were carried out by keeping the equilibration time as h Effect of Nitric Acid Concentration Figure 3 shows the effect of nitric acid concentration on the D values of Np 4? and NpO þ (both termed as D Np ) obtained with TODGA in both [C 4 mim[ntf and [C 8 mim[ntf. Though the HNO 3 concentration was varied from 0.1 to 6.0 moldm -3 for NpO þ ion extraction, the acid concentration range used for Np 4? ion extraction was moldm -3. This was due to the concern for hydrolysis of Np 4? ion below 0.5 moldm -3 HNO 3 [34. It can be seen from the figure that the D Np values decrease with 13

6 J Solution Chem (018) 47: Fig. Kinetics of extraction of Np 4? and NpO þ from 3 moldm -3 HNO 3 by 0.01 moldm -3 TODGA in [C 4 mim[ntf and [C 8 mim[ntf 10 3 Np(IV) - [C 4 mim[ntf Np(IV) - [C 8 mim[ntf Distribution ratio Np(VI) - [C 4 mim[ntf Np(VI) - [C 8 mim[ntf Time (h) Fig. 3 Effect of the HNO 3 concentration on the extraction of Np 4? and NpO þ using 0.01 moldm -3 solutions of TODGA in [C 4 mim[ntf and [C 8 mim[ntf ; equilibration time h, temperature 5 C Distribution ratio Np(IV) - [C 4 mim[ntf Np(IV) - C 8 mim[ntf Np(VI) - [C 4 mim[tf Np(VI) - [C 8 mim[ntf [HNO 3, M increasing nitric acid concentration for NpO þ ion extraction in the acid concentration range of moldm -3, beyond which a marginal increase in the D Np values was noticed. This behavior is typically seen with the proposed cation exchange extraction mechanism prevailing with neutral donor extractants such as TBP, for the extraction of 13

7 133 J Solution Chem (018) 47: hexavalent actinide ions such as UO þ [ The cation exchange mechanism was further confirmed by the lower extraction of the hexavalent actinide ion with [C 8 mim[tf N vis-à-vis [C 4 mim[ntf in the acid concentration range of moldm -3. Similar to the trend reported by Dietz and Stepinski for the extraction system: UO þ TBP RTIL, the extraction minimum occurs at a higher HNO 3 concentration (4.0 moldm -3 ) for [C 4 mim[ntf as compared that (3.0 moldm -3 ) seen with [C 8 mim[tf N. However, the present extraction system behaved in a different manner beyond the minimum. While for the UO þ TBP RTIL system, all extraction profiles were superimposable beyond the minima, the present study indicated that the extraction profile obtained with [C 4 mim[ntf lie below that obtained with [C 8 mim[ntf (Fig. 3). Although this behavior is at variance with the results obtained with the UO þ TBP RTIL system, it agrees with our recent report on the UO þ DHOA (dihexyloctanamide) RTIL system [38. The following cation exchange mechanism may then be proposed by the twophase extraction equilibrium: NpO þ aq þ xl IL þ C n mim þ IL NpO ðlþ þ x IL þ C n mim þ aq ð3þ where L represents TODGA, x represents the number of TODGA molecules participating in the extracted species and n is either 4 or 8. In the case where the species of the type NpO (NO 3 )? are invoked (similar to that reported for the UO þ ion [39), the extraction equilibrium can take the form: NpO ðno 3 Þ þ aq þ xl IL þ C n mim þ IL NpO ðno 3 ÞðLÞ þ x IL þ C n mim þ aq ð4þ Such nitrate bearing cationic species have been invoked in the case of the extraction of UO þ ion from HNO 3 medium using TBP as the extractant and [C n mim[ntf as the ionic liquid, the evidence comes from ESI MS data [36, 39. However, for convenience, we have not invoked the presence of NO 3 ion in the extracted species. As shown below, there was indeed no participation of the nitrate anion in NpO þ ion extraction. Figure 3 also shows the extraction of Np 4? with varying concentrations of nitric acid. The trend of increasing Np 4? ion extraction with increasing HNO 3 concentration conforms to a solvation, or more appropriately an ion-pair extraction mechanism. An analogous extraction trend has been reported in the case of extraction of Pu 4? using a phosphine oxide functionalized RTIL [40. In such a case, the extraction equilibrium may be given as: Np 4þ aq þ yl IL þ 4NO 3 aq Np(L) y ðno 3 Þ 4 IL ð5þ where y is the number of TODGA molecules associated with the extracted species. The extracted species presented by Eq. 5 is similar to that reported in molecular diluents. Huang et al. [41 reported an entirely different mechanism for Pu 4? ion extraction using TODGA in [C 6 mim[ntf and proposed the extraction of a PuL(NO 3 )? species involving a cation exchange mechanism. A cation exchange mechanism was also proposed for Pu 4? ion extraction using TODGA in RTILs [31, 4. Analogous cation exchange mechanisms were also proposed for the extraction of Np 4? with malonamide extractants in [C n mim[ntf (n =4,6,or8)[3. However, the ion-pair mechanism proposed in the present study for the Np 4? ion extraction was proven beyond doubt based on several other solvent extraction studies as described below. 13

8 J Solution Chem (018) 47: Effect of Varying the TODGA Concentrations As shown in Eqs. 3 and 5, the extracted species contains x and y units of TODGA for the NpO þ and Np 4? ions, respectively. The two-phase equilibrium constant (K ex ) for the above reactions for NpO þ and Np 4? ions can be written as: K ex;npðviþ ¼ ½NpO ðlþ þ x Š IL½C n mim þ Š aq ½NpO þ Š aq½lš x IL ½C nmim þ Š IL K ex;npðivþ ¼ ½NpðNO 3Þ 4 ðlþ y Š IL ½Np 4þ Š aq ½NO 3 Š4 aq ½LŠy IL ð6þ ð7þ These are conditional extraction constants that are valid for specific aqueous phase conditions such as nitric acid concentration, ionic strength, etc. By substituting ð½npo ðlþ þ y Š IL=NpO þ Š aq for D Np(VI) and ð½npðno 3 Þ 4 ðlþ x Š IL =½Np 4þ Š aq Þ for D Np(IV), Eqs. 6 and 7 are converted to K ex;npðviþ ¼ D NpðVIÞ½C n mim þ Š aq ½LŠ IL ½C n mim þ Š IL ð8þ and D NpðIVÞ K ex;npðivþ ¼ ½NO 3 Š4 aq ½LŠ IL ð9þ The ratio of ½C n mim þ Š aq =½C n mim þ Š IL is considered as a constant C, and taking logarithms at both sides, Eq. 8 becomes log 10 K ex;npðviþ ¼ log 10 D NpðVIÞ x log 10 ½LŠ IL þ log 10 C ð10þ which can be rearranged to yield log 10 D NpðVIÞ ¼ log 10 K ex;npðviþ þ x log 10 ½LŠ IL log 10 C ð11þ Similar manipulations can be done for Np 4? extraction, Eq. 9, to yield the following equation: log 10 D NpðIVÞ ¼ log 10 K ex;npðivþ þ y log 10 ½LŠ IL þ 4 log 10 NO 3 ð1þ aq This can be verified by slope analysis and the extraction of NpO þ and Np 4? was studied at a fixed HNO 3 concentration (3 moldm -3 ) and with various concentrations of TODGA in [C 4 mim[ntf as well as [C 8 mim[ntf. The results are presented in several log log plots (Fig. 4) which show linear dependences. As shown in Table, the slope values of the log log plots were close to for Np 4? extraction while those were close to 1 for the extraction of NpO þ : This indicates extraction of ML species in the case of Np 4? and ML species in the case of NpO þ : Normally, one would expect eight coordination for the Np4? ion while only five to six coordination for the NpO þ ion (in the equatorial plane). This is in sharp contrast to the ligand concentration dependence reported for Pu 4? extraction by TODGA by RTILs which suggested ML type species in [C 4 mim[ntf as well as in [C 8 mim[ntf [31. A 1:1 (M:L) species has also been reported by Huang et al. [41 and Panja et al. [4 for Pu 4? extraction. 13

9 1334 J Solution Chem (018) 47: Fig. 4 Effect of ligand concentration on the extraction of Np 4? and NpO þ using solutions of varying concentrations of TODGA in [C 4 mim[ntf and [C 8 mim[ntf aqueous phase; 3.0 moldm -3 HNO 3, equilibration time h, temperature 5 C Distribution ratio Np(IV) - [C 4 mim[ntf Np(IV) - C 8 mim[ntf 10-1 Np(VI) - [C 4 mim[tf Np(VI) - [C 8 mim[ntf [TODGA, mm 10 Table Slope values from the log 10 D versus log 10 [TODGA straight-line plots and the corresponding proposed extracted species Extraction systems [C 4 mim[ntf [C 8 mim[ntf References Slope Extracted species Slope Extracted species Np 4? TODGA IL.14 ± 0.31 Np(NO 3 ) 4 (L).13 ± 0.8 Np(NO 3 ) 4 (L) This work NpO þ TODGA IL 1.34 ± 0.15 NpO (L)? 1.7 ± 0.0 NpO (L)? This work Pu 4? TODGA IL 1.13 ± 0.03 a PuL(NO 3 Þ 4 x 1. ± 0.01 a x PuL(NO 3 Þ 4 x x [30 a Values obtained at 1.0 moldm -3 HNO Effect of Nitrate and C 4 mim? Ion Concentrations Equation 3 suggests that no nitrate ion participates in the extraction of NpO þ while Eq. 5 suggests that four nitrates are associated with the extracted species involving Np 4? following an ion-pair extraction. Furthermore, for a cation exchange mechanism (as proposed for NpO þ ion extraction as above), a dependence on the concentration of the C 4 mim? ion should be observed, i.e., in the presence of aqueous soluble salts such as [C 4 mim[cl the extraction of metal ion will not be favorable. The variable nitrate ion concentration experiments were carried with a mixture of HNO 3 (fixed concentration of 1.6 moldm -3 ) and NaNO 3 (varied) as the aqueous phase. The log 10 D Np versus log 10 [NO 3 plots are found to be straight lines for both Np4? and NpO þ : Figure 5 shows that the D Np values for Np 4? at 5.6 moldm -3 nitrate ion concentration are nearly comparable to those obtained at 3.6 moldm -3 NO 3 : This can be attributed to the errors in the aqueous phase counts which were very low and there was only marginal 13

10 J Solution Chem (018) 47: Fig. 5 Effect of nitrate concentration on extraction of neptunium by TODGA in RTIL: feed 1.6 moldm -3 HNO 3? varying amounts of NaNO 3, 10 mmoldm -3 ligand in RTIL Distribution ratio Np(IV) - [C 4 mim[ntf Np(IV) - C 8 mim[ntf Np(VI) - [C 4 mim[tf Np(VI) - [C 8 mim[ntf [1.6M H + + NO - 3, M 10 improvement even after long-time counting. In view of this, these data points were ignored for the linear fit of the variable nitrate ion concentration data. Slope values close to 4 (3.95 ± 0.60 for [C 4 mim[ntf and 3.77 ± 0.56 for [C 8 mim[ntf ) were obtained for Np 4? extraction while those of 0.4 ± 0.06 and 0.6 ± 0.11 were obtained for NpO þ extraction, and the results conform to our proposed extraction mechanisms as given by Eqs. 4 and 5, respectively. Variable [C 4 mim[cl concentration studies at a fixed HNO 3 concentration (1.6 moldm -3 ) were carried out using moldm -3 TODGA in [C 4 mim[ntf and [C 8 mim[ntf and the results (log 10 D Np versus log 10 [C 4 mim[cl) are plotted in Fig. 6. The straight line plots suggest no dependence for Np(IV) and a slope of for Np(VI), implying validation of the extraction mechanisms proposed above Calculation of Two Phase Extraction Constants The extracted species are given as NpðLÞ ðno 3 Þ 4;IL and NpO ðlþ þ IL for Np(IV) and Np(VI), respectively. As mentioned above, the conditional extraction constants (log 10 K ex ) can be obtained from the log 10 D Np versus log 10 [TODGA plots at a fixed HNO 3 concentration. The log 10 K ex,np(iv) and log 10 K ex,np(vi) values are listed in Table 3. While the log 10 K ex,np(iv) values can be calculated directly from Eq. 1, the same is not possible from Eq. 11. In view of the absence of the partition coefficient data of the cationic part of the ionic liquid (defined as the constant C above), Eq. 11 can be rearranged to: log 10 D NpðVIÞ ¼ log 10 K 0 ex;npðviþ þ x log 10½LŠ IL ð13þ is termed as the conditional extraction constant given by the fol- where log 10 Kex;NpðVIÞ 0 lowing equation 13

11 1336 J Solution Chem (018) 47: Fig. 6 Effect of C 4 mim? concentration on the extraction of neptunium by TODGA in RTIL: feed: 1.6 moldm -3 HNO 3? varying amounts of [C 4 mim[cl, 10 mmoldm -3 ligand in RTIL Distribution ratio Np(IV) - [C 4 mim[ntf Np(IV) - C 8 mim[ntf [1.6M HNO 3 + C 4 mim +, M 1.5 Table 3 Two-phase conditional extraction constant values for the extraction of Np 4? and NpO þ using TODGA in ionic liquids Extraction systems Extraction constants (log 10 K ex ) [C 4 mim[ntf [C 8 mim[ntf Np 4? TODGA IL - (1.90 ± 0.13) - (.05 ± 0.10) NpO þ TODGA IL - (1.06 ± 0.08)a - (1.04 ± 0.11) a a These values also include the partition coefficient term log 10 K 0 ex;npðviþ ¼ log 10 K ex;npðviþ log 10 C ð14þ The lesser extraction of NpO þ as compared to Np 4? is reflected in their respective extraction constants. 4 Conclusions The solvent extraction of Np 4? and NpO þ ions into solutions of TODGA in the ionic liquids ([C 4 mim[ntf and [C 8 mim[ntf ) was studied in detail from nitric acid feeds. The extraction kinetics were not slow (as reported in the case of analogous extraction systems) and was found to take only h. The extraction of neptunium was relatively higher in [C 4 mim[ntf as compared to that in [C 8 mim[ntf. Furthermore, the extraction of Np 4? was much higher as compared to that of NpO þ under comparable experimental conditions. The nature of the extracted species was investigated by slope analysis, by 13

12 J Solution Chem (018) 47: carrying out our extraction studies at various concentrations of HNO 3, TODGA, nitrate ion and C 4 mim? ion while keeping the other parameters constant. Although a cation-exchange mechanism was observed for the extraction of NpO þ with extraction of the NpO L? species into the ionic liquid phase for both the ionic liquids, ion-pair extraction was found to occur in the mechanism for Np 4? with a species of the type Np(L) (NO 3 ) 4 being extracted. Acknowledgements The authors thank Dr. P.K. Pujari, Head, Radiochemistry Division, Bhabha Atomic Research Centre for his keen interest in this work. References 1. Swanson, J.L.: PUREX process flowsheets. In: Schulz, W.W., Burger, L.L., Navratil, J.D., Bender, K.P. (eds.) Science and Technology of Tributyl Phosphate. CRC Press, Inc., Boca Raton (1984). Yoshida, Z., Johnson, S.G., Kimura, T., Krsul, J.R.: Chapter 6. In: Morss, L.R., Edelstein, N.M., Fuger, J., Katz, J.J. (eds.) Actinide and Transactinide Elements, vol.. Springer, Dordrecht (006) 3. Vienna, J.D., Ryan, J.V., Gin, S., Inagaki, Y.: Current understanding and remaining challenges in modeling long-term degradation of borosilicate nuclear waste glasses. Int. J. Appl. Glass Sci. 4, (013) 4. Clark, D.L., Hecker, S.S., Jarvinen, G.D., Neu, M.P.: In: Morss, L.R., Edelstein, N.M., Fuger, J., Katz, J.J. (eds.) Actinide and Transactinide Elements, vol., p Springer, Dordrecht (006) 5. Alcock, K., Best, G.F., Hesford, E., McKay, H.A.C.: Tri-n-butyl phosphate as an extracting solvent for inorganic nitrates V: further results for the tetra- and hexavalent actinide nitrates. J. Inorg. Nucl. Chem. 6, (1958) 6. Arhland, S.: Chapter 1. In: Katz, J.J., Seaborg, G.T., Morss, L.R. (eds.) The Chemistry of the Actinide Elements, nd edn, pp Chapman and Hall, New York (1986) 7. Wisnubroto, D.S., Nagasaki, S., Enokida, Y., Suzuki, A.: Effect of TBP on solvent extraction of Np(V) with n-octyl(phenyl)-n,n-diisobutylcarbamoylmethylphosphine oxide. J. Nucl. Sci. Technol. 9, (199) 8. Carrot, M.J., Gregson, C.R., Taylor, R.J.: Neptunium extraction and stability in the GANEX solvent: 0. M TODGA/0.5 M DMDOHEMA/kerosene. Solvent Extr. Ion Exch. 31, (013) 9. Ansari, S.A., Gujar, R.B., Prabhu, D.R., Pathak, P.N., Mohapatra, P.K.: Counter-current extraction of neptunium from simulated pressurized heavy water reactor high level waste using N,N,N 0,N 0 -tetraoctyl diglycolamide. Solvent Extr. Ion Exch. 30, (01) 10. Ansari, S.A., Pathak, P.N., Mohapatra, P.K., Manchanda, V.K.: Actinide partitioning of minor actinides by different processes. Sep. Purif. Rev. 40, (011) 11. Welton, T.: Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 99, (1999) 1. Binnemans, K.: Lanthanides and actinides in ionic liquids. Chem. Rev. 107, (007) 13. Baker, G.A., Baker, S.N., Pandey, S., Bright, F.V.: An analytical view of ionic liquids. Analyst 130, (005) 14. Rogers, R.D., Sheddon, K.R.: Ionic liquids: solvents for the future? Science 30, (003) 15. Allen, D., Baston, G., Bradley, A.E., Gorman, T., Haile, A., Hamblett, I., Hatter, J.E., Healey, M.J.F., Hodgson, B., Lewin, R., Lovell, K.V., Newton, B., Pitner, W.R., Rooney, D.W., Sanders, D., Seddon, K.R., Sims, H.E., Thied, R.C.: An investigation of the radiochemical stability of ionic liquids. Green Chem. 4, (00) 16. Huddleston, J.G., Willauer, H.D., Swatloski, R.P., Visser, A.E., Rogers, R.D.: Room temperature ionic liquids as novel media for clean liquid liquid extraction. Chem. Commun. (1998) /A803999B 17. Billard, I.: Ionic liquids: new hopes for efficient lanthanide/actinide extraction and separation. In: Bunzli, J.C.G., Pecharsky, V. (eds.) Handbook on the Physics and Chemistry of Rare Earths, pp Elsevier Science Publishers B.V., Amsterdam (013) 18. Visser, A.E., Swatloski, R.P., Reichert, W.M., Griffin, S.T., Rogers, R.D.: Traditional extractants in nontraditional solvents: groups 1 and extraction by crown ethers in room-temperature ionic liquids. Ind. Eng. Chem. Res. 39, (000) 13

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