Extraction of pertechnetates from HNO3 solutions into ionic liquids

The extraction of pertechnetate ions from aquous solutions containing various concentrations of nitric acid into hydrophobic ionic liquids (ILs) has been examined at 25, 50 and 70 °C. The results show that the distribution ratio of Tc (DTc) between both phases weakly depends on the temperature and HNO3 concentration when IL’s with relatively short aliphatic chains are used. The DTc obtained for all examined ILs, except methyltrioctylammonium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidasolium hexafluorophosphate, are lower than 1.5. In the case of methyltrioctylammonium bis(trifluoromethylsulfonyl)imide a decrease of Tc concentration in aqueous solutions facilitates pertechnetate extraction into the organic phase.


Introduction
Technetium, whose 99 Tc isotope is one of the major longlived products of 235 U fission, is of particular interest in context of nuclear waste immobilization and reprocessing [1]. A high mobility of pertechnetate ions in aqueous media may lead to a long term contamination of environment with 99 Tc. Although numerous works have been devoted to the extraction of technetium in liquid-liquid systems many aspects of 99 Tc separation from spent nuclear fuel during reprocessing of the latter are nor fully understood and efficient removal of this isotope from other products of nuclear fission continues to pose challenging problems for nuclear industry. The distribution ratio of TcO 4 -, D Tc , has been determined for aqueous solutions in contact with organic systems, e.g. derivatives of ammonium chlorides in toluene [2], TBP in kerosine/dodecane [3][4][5][6], crown ether in m-xylene [7], tetraphenylphosphonium in chloroform/ nitrobenzene [8], acetohydroxamic acid(AHA)/TBP [9] or tricaptylmethyl ammonium salt (Aliquat-336) [10]. Summary of the extraction properties of various extracting agents has been presented by Schwochau [11] and Spitsyn et al. [12]. Recent studies on this topic report an influence of presence of actinides on liquid-liquid Tc extraction [13][14][15].
Trace amounts of technetium can be extracted from aqueous basic solutions into an organic phase with the distribution ratio of up to 10 3 [7]. Cocalia et al. [16] reported that D Tc exceeds 700 in a system containing conc. K 3 PO 4 and an ionic liquid (IL) phase and increases with increasing length of the aliphatic chain constituting the IL cation. These observations are in line with the results published by Schanker et al. [17]. They also reported an influence of the length of the aliphatic chain of the quaternary ammonium salts used as extracting agents on the extraction of pertechnetates in aqueous/chloroform systems. In the latter system the distribution ratio of Tc increases from 3.40 for tetrabutylammonium iodine to 9.89 for tetraheptylammonium iodine. This effect corresponds to direction of changes of other physical properties of organic derivatives of ammonium pertechnetates which are caused by increasing length of the aliphatic chain of the cation, i.e. a decrease in solubility in water and an increase of hydrophobic properties of the organic cation shown for tetra(C n H 2n?1 )ammonium pertechnetates series [18].
The solubility of ionic liquids in water is determined mainly by properties of their anions. As a general trend, ILs with [Tf 2 N -] ions are less soluble in water than ILs with [PF 6 -] anions. The respective solubility, which is represented as a molar fraction of the IL in water, increases from 10 -3.0-5.0 for [A ? ][Tf 2 N -] to 10 -2.5 for [A ? ][PF 6 -], [19]. Owing to their low solubility in water, the ionic liquids containing bis(trifluoromethylsulfonyl)imide or hexafluoro-phosphate anions represent an interesting alternative to classical extractants used in novel strategies for spent nuclear fuel reprocessing [20]. The main disadvantage of these extractans is their relatively low radiation stability [21] which limits their applications to reprocessing of the low level radioactive.
According to the literature [7], the extraction of pertechnetates in TcO 4 -(aq) /IL (A ? B -) systems follows the anion-exchange mechanism presented by Eq. (1): This mechanism may change when crown-ethers (CE) are added to the IL [7]. Under such conditions the predominant mode of Tc transfer is described as the ion-pair production, according to Eq. (2): The structure of the technetium complex extracted into the organic phase from aqueous HNO 3 solutions was discussed also by El-Kot [4] who investigated tri-octylamine (TOA) and tri-n-butylphosphate (TBP) as extracting agents. It was proposed that pertechnetate anions must be neutralized by H ? in order to form neutral complexes with TOA or TBP:  6 ] were obtained from IoLiTec and were used as received. The aqueous solutions were prepared using high purity distilled water (Millipore Ò , 18.2 Mohm cm) and high purity chemicals: potassium pertechnetate, K 99 TcO 4 (Forschungszentrum Dresden-Rossendorf-Institute of Radiopharmacy) and HNO 3 (POCh, Poland).
The extraction experiments were performed by contacting 1 ml of the aqueous solution containing pertechnetates with 1 ml of the ionic liquid. Temperature of the solutions studied was set at 25, 50 and 70°C and was controlled using a Huber MPC E thermostat. After 30 min. of intense shaking the samples were centrifuged to facilitate phase separation, and the aliquots of both phases were assayed by a liquid scintillation counting method (Perkin Elmer Tri-Carb 2910TR) using Ultima Gold liquid scintillator cocktail. For each of the phases three samples of 20 microliters were collected and measured and the obtained results were averaged for each phase. The distribution ratio of Tc between organic and aqueous phase has been defined as the ratio of 99 Tc activity in organic to the activity of 99 Tc present in aqueous phase, D Tc = A Tc(org) /A Tc(aq) .
The water content in the organic phases was determined by means of using Karl Fisher titration method (Metrohm, 716 DMS Titrino).

Results and discussion
The The results of the studies on pertechnetates extraction from aqueous HNO 3 solutions into the ILs studied are summarized in Table 1 6 ] for which the distribution ratio is greater than 1 but still lower than 20.
The concentration of nitric acid in the aqueous phase does not play a significant role in the extraction of pertechnetates from aqueous into the organic phase for the systems containing ionic liquids with cations with relatively short aliphatic chains, i.e. [N 4,111 This process manifests itself by disappearance of the boundary between concentrated nitric acid (C4 M HNO 3 ) and the organic phase. Without a doubt, the reaction (5) takes place also in the systems with lower HNO 3 concentration but is too slow as to be completed in the timescale of the experiment. This leads to higher uncertainties of D  4 extraction shows that the highest D Tc value obtained for the ILs studied in this work (18.85) is higher than D Tc obtained for 30% TBP (D Tc equal to unity) but is significantly lower than the value of 250 measured for TOA (0.1 M) by El Kot [4]. Billard et al. [22] reported that the solubility of HNO 3 in ILs increases with increasing the acid concentration which indicates that the interaction between TcO 4 and HNO 3 dissolved in the organic phase should be considered. This point may be discussed on the basis of the measured influence of HNO 3 concentration on the extraction of TcO 4 -anions. An increase in HNO 3 concentration impedes transfer of the pertechnetates into the organic phase. This observation suggests that the formation of a neutral complex, e.g. [HTcO 4 ][A ? B -], can but probably does not play a significant role in transferring of pertechnetates into the hydrophobic phase. Moreover, at high concentrations of HNO 3 the NO 3 anions may compete with pertechnetates in the extraction process.
The protonation of pertechnetate ions in acidic solutions results in existence of undissociated pertechnetic acid molecules. Various values of the HTcO 4 acidity constant, log 10 K c , are reported in the literature with the range of -0.4-0.6 [23]. These undissociated and neutrally charged HTcO 4 molecules can be directly transferred into the organic phase according to the following equations (Eqs. 6-7): As it was mentioned earlier, the solubility of bis(trifluoromethylsulfonyl)imide salts with organic cations in the aqueous phase is low [19] but replacement of organic cations with potassium increases the solubility of such IL up to 10 g/dm -3 = 31 mmol dm -3 [24], which is the value sufficiently high as to transfer the organic anion into the aqueous phase. Thus, taking into account Eq. 1 one may suggest that the technetium complex in the organic phase could be also described as: When the aqueous and organic phases are brought into contact, a small amount of the water dissolves in the organic phase. The amount of such dissolved water was determined for all examined ILs after completing the extraction experiments with the aqueous phase not containing HNO 3 (   (Fig. 2). Repeated application of the same IL in the extraction process requires its purification by means of the backextraction of Tc species into an aqueous phase. Usually, the removal of pertechnetates from the IL phase cannot be completed in a single cycle of the back-extraction process. Therefore, usually several consecutive back-extraction cycles are performed in order to obtain satisfying level of removal of TcO 4 from the ionic liquid. Figure 3 presents changes in concentration of pertechnetates remaining in the organic phase after specified number of the back-extraction cycles. -] but also undissociated HTcO 4 molecules, the first contact of organic phase (saturated in technetium species) with water, results in effective transfer of HTcO 4 into aqueous phase and there This observation is in line with the results presented by Peretrukhin et al. [18] which show an extremely low solubility of tetra(C n H 2n?1 )ammonium pertechnetates salts with long aliphatic chain in water. For the other investigated ILs a slow decrease in the Tc concentration in the organic phase with the number of purification cycles is observed.

Conclusions
The extraction properties of ionic liquids in respect to pertechnetates depend on the length of the aliphatic chain of the IL cation. The longer the aliphatic chain the higher the distribution ratio of pertechnetates between the aqueous and the organic phase. D Tc higher than ten is observed only for [N 1,888 ][Tf 2 N], the other ILs investigated in this work exhibit lower extent of the pertechnetates extraction. This factor increases with the decreases in Tc concentration in the aqueous phase. The presence of nitric acid in the aqueous phase leads to a decrease in the Tc transfer between the aqueous and the organic phases. Back-extraction of pertechnetates from the organic into the aqueous phase is efficient only for weakly extracting ILs.