Mechanism unravelling for highly efficient and selective 99TcO4− sequestration utilising crown ether based solvent system from nuclear liquid waste: experimental and computational investigations

Selective and efficient separation of pertechnetate (TcO4−) from nuclear waste is desirable for the safe and secure management of radioactive waste. Here, we have projected dibenzo-18-crown-6 ether (DB18C6) in a highly polar nitrobenzene medium for enhancing the removal efficiency of 99Tc from reprocessing plant low level waste (LLW). An effort was made to determine the stoichiometry of metal–ligand complex by slope ratio method, revealing that one ligand (DB18C6) binds with one TcO4− moiety. Optimum ligand concentration for 99Tc extraction was evaluated. Relevant interference of the anions was studied systematically. The effect of solution pH was analysed on the extraction efficiency of 99Tc. A kinetic study was carried out for maximum extraction of metal ions. A quantitative stripping study was also achieved for metal ions with a suitable stripping solution. After evaluation of all essential parameters, selectivity and feasibility studies were finally carried out with actual low level reprocessing plant waste to demonstrate a laboratory scale process for effective separation of TcO4− ions. Density functional theory (DFT) calculations were carried out to understand the nature of the complexation of TcO4− ion with DB18C6 in different solvents systems and to elucidate the key aspect behind ionic selectivity and enhanced the 99Tc extraction efficiency of DB18C6 in the studied diluent systems. The ΔE and ΔG values for different modeled complexation reactions were evaluated systematically. From the calculated free energy of complexation of TcO4− with DB18C6, it was observed that the consideration of explicit solvent plays a vital role in predicting the experimental selectivity.


Introduction
Presently, nuclear power is a growing technology that can fulll the high demand for low-carbon energy globally. [1][2][3][4][5][6] The progress of nuclear industries has been associated with some issues such as accidental and intentional release of harmful radionuclides in the environment. 7,8 The success and public acceptance of nuclear industries depend on the safe and secure management of hazardous radioactive waste. Out of several radionuclides, one potentially troublesome radionuclide is 99 Tc, a beta emitting radionuclide (E b-max ¼ 295.5 keV) having long half-life (t 1/2 ¼ 2.13 Â 10 5 y), 9 99 Tc is generated during the ssion of 235 U with a 6.13% yield. 10 99 Tc is mostly found as TcO 4 À species in the aqueous system, where it exists in a +7 oxidation state. Unlike MnO 4 À ions, 99 TcO 4 À does not have a strong oxidizing capacity, making it extremely stable under different environmental conditions. Additionally, 99 TcO 4 À moiety possesses high symmetric geometrical conguration (T d ) and low charge density, making it quite non-complexing in nature, along with high solubility (11.3 M at 20 C). [11][12][13][14] Consequently, 99 TcO 4 À can easily migrate into the environment through a natural water system, and at the same time, it is difficult to capture by using a majority of conventional materials, thereby posing severe environmental risks. Due to the volatile nature of 99 Tc compounds (e.g., Tc 2 O 7 ), vitrication of nuclear waste at high temperatures gets affected, and it creates challenges in the off-gas system design for waste management facilities. 15 99 Tc vapor inhalation and dust contamination can create a major cancer risk, 16 while aer digestion, 99 Tc can build up in the thyroid and mammary tissue. 17 Considering all these factors, separation of 99 Tc is an urgent need to ensure safe and secured management of highly toxic radioactive waste as well as to reduce 99 Tc release through off-site migration to contaminate groundwater. 15 Therefore, efficient and selective separation of radioactive 99 Tc has drawn signicant attention in the nuclear industry.
Due to reprocessing of spent nuclear fuel, high level liquid waste (HLLW) and intermediate level liquid waste (ILLW) are generated in the reprocessing plant. ILLW is made alkaline with the addition of sodium hydroxide and sodium carbonate to store in carbon steel waste tank farms. Treatment of alkaline ILW is carried out via resorcinol formaldehyde (RF) resin and imino diacetic acid resin using ion exchange method to separate 137 Cs and 90 Sr, respectively. Aer separation of 137 Cs and 90 Sr from ILLW, it reduces its activity and gets converted to low level waste (LLW), which contains mainly 99 Tc and 106 Ru. It is very much essential to separate 99 Tc and 106 Ru before discharge, mainly to bring down the activity level below the discharge limit. 18,19 As LLW contains high concentrations of monoanions like OH À and NO 3 À , it is very challenging to separate TcO 4 À from alkaline waste due to the common ion effect. Preferably, TcO 4 À should be separated at the beginning stage while the spent nuclear fuel is chopped, followed by dissolution in concentrated nitric acid even before the PUREX (plutonium uranium redox extraction) process in the reprocessing plant. However, this early stage TcO 4 À separation is associated with a huge challenge given the extreme conditions prevailing like high ionic strength, super acidity as well as strong ionizing radiation. Remarkable efforts have been given for designing efficient anion-exchange materials for the separation of TcO 4 À ions. 15 Conventional polymeric anion-exchange resins revealed efficient separation of TcO 4 À under acidic conditions, 20-22 but it is not radiation resistant. The kinetics of anion-exchange is usually quite slow, resulting in prolonged exposure of resins under radiation dose causing damage to the material due to the radiation effect. In this regard, some promising materials have been reported, like layered rare-earth hydroxides (LRHs), 23 layered double hydroxides (LDHs), 24,25 and thorium borates (NDTB-1). 26,27 These materials can work efficiently under radiation, but nally, these materials are not applicable in real time due to their poor selectivity as well as low sorption efficiency. In recent times, other potential candidates, known as cationic metal-organic frameworks (MOFs), have been reported for the separation of TcO 4 À from aqueous solution. 28 ions (À251 kJ mol À1 ) as compared to OH À (À410 kJ mol À1 ) and NO 3 À (À289 kJ mol À1 ) ions, respectively. 40 The extraction efficiency is strongly controlled by complex formation and its stability. 39 Through computational studies, it has been conrmed that DB18C6 shows a strong binding affinity towards sodium ions in the solution phase. 41 Based on these understandings, we have selected a crown-ether, namely dibenzo-18crown-6 (DB18C6), which is easily available in large scale, commercially cheap, and good in the extraction of sodium ions, and we have studied its extracting properties for TcO 4 À ions.
Note that dibenzo-18-crown-6 is reasonably simple to synthesize with reproducible product quality.
In the extraction process, solvent polarities oen play an important role. Thus, the dielectric constants of the used diluents have a direct inuence over the extraction efficiency of metal ions. Accordingly, in addition to the selection of an appropriate ligand, the choice of a proper diluent is also an essential parameter to achieve enhanced efficiency of the extraction process. Keeping this in mind, here we have attempted to achieve an enhanced distribution ratio for TcO 4 À ions using well known ligand DB18C6 in a highly polar nitrobenzene medium, considering that metal extraction efficiency increases with the increased polarity of the organic solvent system. 42

Materials: chemicals, isotopes, and solutions
Nitrobenzene (NB) (99.9%) and DB18C6 ( Fig. 1) were used as received (S.D. Fine). Each experiment was conducted in triplicate to conrm the reproducibility of the observations. 99 Tc radioisotope was separated from plant LLW by Sonar et al., 43 which was used in the present investigation as a radiotracer.
Compositions of LLW samples represented in Table 1 were generated from our reprocessing plant. Before reprocessing, the spent fuel from the pressurized heavy water reactor (burn up of 6500 MWD tone À1 ) was cooled for 3 years and used subsequently for extraction studies in batch mode. During the experiment, proper personal protective equipment (PPE) and personal radiation monitoring instruments were used. We kept the solution activity less than 1 mCi L À1 , and also, the gamma radiation eld was less than 1 mR h À1 . Supra-pure HNO 3 and quartz double distilled water were used throughout the present investigation.

Percent extraction, distribution studies, and radiometry
The extent of 99 Tc removal from the aqueous phase was determined by calculating the distribution ratio (D Tc ) or percent extraction (PE). All the studies were performed at (25 AE 1) C, maintained using a thermostated water bath. The distribution ratio D Tc for 99 Tc or D M for any other metal can be calculated by estimating the ratio of the radioactivity/concentration in the organic phase to the radioactivity/concentration in the aqueous phase at equilibrium. Following the method adopted by Sonar et al., the concentration of pertechnetate ion in an aqueous medium was calculated using the Tctetraphenyl arsonium chloride (TPAC) complex method. 43 Analysis of 99 Tc radionuclide was carried out by precipitating with TPAC complex followed by beta activity counting by GM counter to estimate the 99 Tc concentration in waste streams. During beta activity counting, self absorption of low energy beta particles through the precipitate introduces error. The concentration of 99 Tc was conrmed using its cgross beta cut-off (95%) by evaporation onto a planchet and Al-absorber having thickness 100 mg cm À2 . 43 Solution pH was measured using Benchtop pH meters. ZnS/Ag scintillator, GM counter, and NaI/Tl scintillator were used for the estimation of gross a, b and g activities. To determine PE, the organic phase (DB18C6 in 100% nitrobenzene) and aqueous phase (pH 13, containing 99 Tc) were equilibrated in a 1 : 1 ratio for 20 min in a glass vial and centrifuged subsequently to separate the two phases. The error/uncertainty of the experimental result was within AE5%. The percentage of 99 Tc extraction was calculated using the following equation: Extraction% ¼ Tc activity in organic Tc activity in initial Â 100 (1) where Tc activity in organic ¼ 99 Tc extracted from the organic phase, Tc activity in initial ¼ total 99 Tc taken for extraction in the aqueous phase.

Computational protocol
The structure of DB18C6 and its complexes with TcO 4 À and Na + ions were optimized using B3LYP 44,45 functional with triple zeta valence plus polarization (TZVP) basis set as implemented in the Turbomole package. 46 The frequency calculations were carried at the same level, which is used for geometry optimization. The gas phase free energy, DG g was computed at T ¼ 298.15 K and P ¼ 1 atm. The solvent effects were considered using the Conductor like Screening Model (COSMO) 47,48 solvation model. The default COSMO radii were used for all the elements. In the present study, we have considered three solvents, nitrobenzene, chlorobenzene, and trichloroethylene, to understand the experimental observations. Despite a lower dielectric constant, trichloroethylene has strong complexation compared to chlorobenzene, and nitrobenzene has a stronger complexation ability compared to the other two studied solvents because of its high dielectric constant. The dielectric constant, 3 of water, nitrobenzene, chlorobenzene, and trichloroethylene, was taken as 80, 34.82, 5.62, and 3.42, respectively. The gas phase minimum energy structures were used for the calculation of single point energy in the COSMO phase. 49-54

Kinetics study of 99 Tc extraction
Before starting the extraction studies, it is very important to evaluate the equilibrium time for maximum extraction of TcO 4 À ions. Keeping this in mind, the extraction prole for TcO 4 À was Fig. 1 Optimized structure of DB18C6.  (11)(12)(13) established by varying the equilibration time. It was observed that at the time of about 15 min (Fig. 2), the reaction reached equilibrium, and subsequently, the D Tc value was found to be almost unchanged. Hence, for all the extraction studies throughout the experiments, the equilibrium time was kept as 20 min. As discussed earlier, LLW contained 1.3-1.8 M Na + in the form of nitrate. Therefore, the extraction kinetics shown above pertained to the mass transfer of Tc aer the formation of the complex associated with Na + as well as BD18C6. It meant, while discussing the kinetics, it was not only the attachment of TcO 4 À to BD18C6, but it was the overall mass transfer of Tc aer suitable complexation with ligand as well as Na + forming [Na + $BD18C6$TcO 4 À ] n like species.

Determination of metal-ligand binding stoichiometry
To determine the optimum ligand concentration for extraction of 99 Tc as well as to understand the complexation nature between the metal and ligand, an extraction study was carried out by varying the concentration of the DB18C6 ligand in the nitrobenzene solvent system (Fig. 3). The calculated D Tc value increases with an increase in DB18C6 concentration. For 0.1 M DB18C6 concentration in nitrobenzene, the removal of 99 Tc is about 89% (D Tc ¼ 7.6) in a single contact, which was found optimum. The extraction equilibrium for the TcO 4 À ions by the ligands can be expressed as Following eqn (2), we can write, Eqn (3) can be simplied further as, While the Gibb's free energy change for the extraction equilibrium can be expressed as, A plot of log D Tc against log[BD18C6] NB (shown in Fig. 3) gave a straight line with a slope of 0.93, revealing the formation of a 1 : 1 metal-ligand complex. From the intercept, the conditional extraction constant and the change in Gibb's free energy have been evaluated as 79.43 mol À1 and À1.0708 kJ mol À1 , respectively, which are very close to the values reported from theoretical calculations. 41 The negative DG value for the extraction equilibrium suggests that the process is thermodynamically favourable and spontaneous.

Interference of nitrate and hydroxide ions on the extraction efficiency of 99 Tc
As the LLW contains a high concentration of NaNO 3 and NaOH, it is essential to know the interfering effects of NO 3 À and OH À  À (triplicate measurements were done, and the error limit is AE5%). À (triplicate measurements were done, and the error limit is AE5%).
increases steadily, and beyond this concentration, the extraction efficiency gets leveled off effectively. Beyond 3 M concentration, it is observed that the competitive effect of NO 3 À ions is somewhat stronger than OH À ions, as the chemical behavior of NO 3 À ions is relatively closer to TcO 4 À ions as compared to the OH À ions. Fig. 4b depicts a similar investigation using noncomplexing counter ion like NH 4 + , and the results were found to be similar to that obtained by NaNO 3 and NaOH.

Determination of thermodynamic parameters
The temperature of the medium plays an important role in the extraction of metal ions using a ligand. Thus, temperature dependent measurements were also carried out to determine the extraction efficiency of TcO 4 À ions. So, experiments using organic phase 0.1 M DCH18C6 in nitrobenzene and aqueous phase spiked with 99 TcO 4 À ion radiotracer with varying temperatures were carried out. Experimental results reveal (Fig. 5) that with increasing temperature, the extraction of TcO 4 À ions by DB18C6 decreases substantially, indicating the overall extraction process to be largely exothermic in nature. These results may be explained in terms of the aqueous solubility of the metal-ligand complex. With the rise in temperature, the solubility of the complex increases substantially, and hence the organic phase contains a lesser amount of metal ions.
The enthalpy change (DH) of the extraction equilibrium (6) can be evaluated using the Van't Hoff equation as, From eqn (6), the plot of log D vs. 1/T would give a straight line with a slope as DH/2.303R. Further, the Gibb's free energy change (DG) for the process can be evaluated from eqn (7).
Subsequently, the change in entropy (DS) can be determined at a particular temperature using eqn (8).
From the slope of the log D vs. I/T plot, the DH value for the extraction equilibrium (6) is found to be about À18.2 kJ mol À1 . Accordingly, following eqn (6) and (7), the DG and DS values for the overall extraction process are estimated to be about À1.07 kJ mol À1 and À57.48 J mol À1 , respectively. The largely negative DS value can be attributed to the reduction in the number of free ions in the reaction medium during complex formation.   to predict the redox nature of Tc in dilute solutions to concentrated MgCl 2 and NaCl solutions. The E h value of the system and also an aqueous concentration of Tc(IV) with TcO 2 $1.6H 2 O(s) in equilibrium are signicantly affected by high ionic strength, particularly in the case of 4.5 M MgCl 2 solutions. In the presence of concentrated brines and alkaline conditions (pH ¼ pH max 9) Tc(VII) is reduced to Tc(IV) using magnetite, mackinawite 56 at pH ¼ 8-9 in the presence of concentrated MgCl 2 and NaCl solutions. Here, we have obtained the extraction prole of TcO 4 À ions at different pH values of the medium (pH 1-14) (Fig. 6), maintaining the ionic strength in solution in the aqueous phase. The extraction prole of 99 Tc increases from pH 1 to 3 and subsequently decreases till up to neutral pH. Subsequently, the extraction efficiency starts to increase again in the alkaline region and reaches a kind of saturation from pH 12 onward. These results can be explained in terms of the binding selectivity of DB18C6 towards alkali metal (Na + ) ions that determines the stability of the Na + -DB18C6$TcO 4 À complex. In an aqueous solution, the solvation energy of Na + -DB18C6 plays a vital role in the binding selectivity of DB18C6 for Na + . 41 The interaction via non-electrostatic dispersion between the solvent and solute, which also depends on the complex structure as well as accountable for the solvation energies of Na + -DB18C6, which in turn contributes to the stability of the Na + -DB18C6$TcO 4 À complex. Solvation energy of Na + -DB18C6 has a signicant role in relative bond dissociation free energies (BDFE) of Na + -DB18C6 followed by the binding selectivity of DB18C6 towards Na + ions in aqueous solution. So, the non-electrostatic and electrostatic dispersion interactions between the solvent and solute play a vital role, which also inuences the stability of the Na + -DB18C6 complex along with a large contribution for solvation energies of the Na + -DB18C6 complex. Therefore, these interactions, which strongly control the solvation energy, are followed by the stability and binding selectivity of complex formation. At neutral pH, where charge species in the solution are less, the electrostatic dispersion interaction does not operate that strongly, resulting in lowering the binding selectivity of DB18C6 for Na + ions and thereby reducing the Na + -DB18C6$TcO 4 À complex formation effectively. On the other hand, at the acidic and alkaline pH conditions, where the charge species are abundant, the electrostatic dispersion interaction operates very strongly, leading to the strong binding selectivity of DB18C6 towards Na + ions and consequently enhancing the stability of the Na + -DB18C6$TcO 4 À complex, giving higher extraction efficiency in effect.

Stripping study
For waste management, for the reuse of the DB18C6 ligand, a stripping study is very much required. Hence, in the present work stripping studies were carried out using water and nitric acid solution as the stripping agents. Stripping of 99 Tc was increased with the increase in acidity of the stripping solution, as shown in Fig. 7. Using water as a stripping agent, only 5% Tc was found to be back extracted from the loaded organic phase, while using 0.1 M HNO 3 and 0.01 M HNO 3 , the % stripping was found to be within 20-25%. However, a sharp enhancement with maximum stripping of $77% was observed when 1 M HNO 3 was used as a stripping agent.

Selectivity and feasibility study of TcO 4 À extraction process
Aer evaluating all necessary parameters for the extraction of TcO 4 À ions by DB18C6, experiments were carried out to evaluate the feasibility as well as selectivity of the present process for extraction of TcO 4 À ions from reprocessing alkaline (pH ¼ 13) plant LLW sample using 0.1 M DB18C6 in nitrobenzene as the extractant. Experimental result reveals that about 90% of 99 Tc activity is trapped in the DB18C6/organic phase. As the results in Table 2 indicate, the separation of 99 Tc is much higher as compared to 137 Cs and 90 Sr. This is due to the binding selectivity of DB18C6 in the nitrobenzene solvent system towards alkali metal ions, which is strongly dependent on the size of the metal ions as well as their hydration energy.

Proposed TcO 4 À extraction and stripping mechanism
The extraction and stripping mechanism of TcO 4 À ions by DB18C6 can be explained by the size selective reversible complex formation between alkali metal Na + ions and DB18C6, as the cavity size of DB18C6 matches with Na + radius. 41 This Fig. 6 Effect of solution pH on the extraction efficiency of TcO 4 À by DB18C6. Aqueous phase: solution at different pH (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) values spiked with 99 TcO 4 À radiotracer; organic phase: 0.1 M DB18C6 in nitrobenzene (triplicate measurements were done and the error limit is AE5%). [DB18C6$Na + ] complexes subsequently bind the TcO 4 À ions to form [DB18C6$Na + $TcO 4 À ] complexes, as shown schematically in Fig. 8 and eqn (9). The LLW samples from the reprocessing spent fuel contain a very high concentration of Na + ions. Thus, these Na + ions derive the overall reaction favorably towards the formation of [DB18C6$Na + $TcO 4 À ] complexes participating in the extraction process. In the extraction process, the formation of size selective stable complexes between Na + and DB18C6 enhances the extraction process over the conventional salting out effect at high salt concentrations. In the present process, since DB18C6 is a neutral ligand and Na + is a unipositive cation, the resultant [DB18C6$Na + ] complex possesses a unit positive charge. Accordingly, for the enhancement of the stability, the so formed DB18C6$Na + complex requires to be associated with an anion, in this the TcO 4 À ion, to achieve the charge neutralization for the overall [DB18C6$Na + $TcO 4 À ] complex, which is effectively extracted in the organic phase. From Fig. 5 it is clear that DB18C6 is directly participating in the mass transfer of Tc. Tc exists as TcO 4 À ion in the acidic feed under consideration. It was also evidenced from this slope ratio method that one DB18C6 ligand was attached with one Tc species. Therefore, the expected species would be TcO 4 À [DB18C6]. Since the overall charge of the complex was unit negative, so to maintain electrical neutrality, the unit positive charge cation must be associated with it during the effective mass transfer of Tc. As reported earlier, Na + had a tendency to sit in the cavity of DB18C6 due to size selectivity. Therefore, in the presence of Na + , most likely, Na[DB18C6]TcO 4 could be the species involved in mass transfer. The argument is quite logical and can be accepted. Stripping is accomplished by simply reversing this complexation equilibrium. The organic phase containing the [DB18C6$Na + $TcO 4 À ] complex is contacted with deionized water and acid, promoting the dissociation of the complex whereby the free Na + and TcO 4 À ions are partitioned into the aqueous phase, and the free crown ether remains in the organic phase. Eqn (9) represents the extraction reaction.

S. No. Species
Before extraction (activity mCi L À1 )   bond distance in the CB solvent is found to be longer compared to the NB solvent and may be the reason for more stability of the DB18C6-Na-TcO 4 complex in the NB solvent compared to CB solvent. Whereas in the case of TCE solvent, correlation cannot be drawn as the Na-O(DB18C6) distance was found to increase compared to its gas phase DB18C6-Na-TcO 4 complex.

Solvent phase
TcO 4 À (aq) + Na + (aq) + DB18C6(org) / DB18C6-Na- The selectivity for a particular metal ion towards a complexion ligand can be explained using binding energy or free energy. Hence, the gas phase binding energy is computed for the complexation reaction of eqn (10) as follows.
The thermal correction to the electronic energy (E el ), enthalpy (H), and free energy (G) of the optimized complexes has been performed following the earlier reported prescription. 1,57,58 The calculated values of B.E., DE, and free energy of complexation DG in the gas and solvent phases using implicit solvation model are presented in Table 4 (equation (A)). Equation (A) represents treating "Na" as a bare metal ion without solvation. From the calculated values of DE and DG in the gas phase, it is observed that DG value is found to be lower than DE, which is due to the complex formation process. Further, from the calculated values of DE in the solvent phase, it is observed that the complexation of TcO 4 À follows the order NB > CB > TCE, which is not in line with experimental results (see Section 3.11, Table 6) and the calculated DG values were found to be positive.
To retrieve the experimental selectivity, the above complexation reaction is modelled by including the Na + -(H 2 O) 6 cluster instead of Na + ion, which is strongly hydrated compared to TcO 4 À in the calculations (equation (B)). In equation (B), the Na + ion is replaced by Na + -(H 2 O) 6 cluster in eqn (10) and (11). Now the complexation reaction can be modelled as TcO 4 À (g) + Na + -(H 2 O) 6 (g) + DB18C6(g) / DB18C6-Na- TcO 4 À (aq) + Na + -(H 2 O) 6 (aq) + DB18C6(org) / DB18C6-Na- The calculated values of B.E., DE, and free energy of complexation DG in the gas and solvent phases using Na + -(H 2 O) 6 (equation (B)) cluster are presented in Table 2. From the calculated values of DE and DG in the gas phase, it is observed that DG value is found to be higher than DE, which is due to the release of water molecules in the complexation process. The calculated DE values were found to be positive and DG values were found to be negative, which is the reverse of earlier observation with equation (A). Further, from the calculated is not in line with experimental results (see Section 3.11, Table  6). So, consideration of the Na + -(H 2 O) 6 cluster was also not able to predict the experimental selectivity.
In the next step, the DB18C6-Na-TcO 4 complex is explicitly solvated with NB, CB, and TCE solvent molecules to explain the experimental selectivity of TcO 4 À with DB18C6. The complexation reactions with explicit solvent molecules in the complex using equation (A) and (B) can be modelled as.

Gas phase
TcO 4 À (g) + Na + (g) + DB18C6(g) + (S) m / DB18C6-Na-TcO 4 - (where, S ¼ NB, CB or TCE, m ¼ 3-4). Solvent phase TcO 4 À (g) + Na + (aq) + DB18C6(org) + (S) m org / DB18C6-Na- Gas phase The calculated values of DE and DG of TcO 4 À in the gas and solvent phases using Na + (equation (A)) and Na + -(H 2 O) 6 (equation (B)) cluster with explicit solvents in the complex are presented in Table 5 Table 6). Hence, we found the importance of explicit solvent molecules in complexation in the present study. Further, the studies can be extended with the incorporation TcO 4 À hydration and variation of solvent molecules in the complex, which is beyond the scope of the present study.
3.11 Comparative evaluation of TcO 4 À extraction Table 6 summarizes the experimental results for TcO 4 À extraction by the two most promising crown ether based ligands in different solvent systems that are reported in the literature to compare the present experimental results from the earlier studies reported. It is established that the formation of a 1 : 1 metal-ligand complex as well as the size selectivity are the key driving forces for the mass transfer of the target metal ions (TcO 4 À ) to the organic phase from an aqueous medium. The maximum D Tc value achieved in the present case is found to be considerably higher with respect to all the solvent systems  reported, as indicated clearly in Table 6. Among the other literature reports, the result reported by Sharma et al. using di-t-BuDB18C6 in isodecyl alcohol/n-dodecane solvent shows a signicantly higher D Tc value, 39 but this value is still much lower than that estimated in the present study. Comparing the effects of the diluents on 99 Tc extraction using DB18C6 and di-t-BuDB18C6 ligands reveal that the extraction efficiency follows the order: Nitrobenzene > isodecyl alcohol/n-dodecane > 1,2-dichloromethane > o-dichlorobenzene > trichloroethylene > chloroform > chlorobenzene > 1,2,4-trichlorobenzene > benzene. This comparative evaluation denitely indicates that DB18C6 in nitrobenzene (present case) is the most efficient system for the extraction of 99 TcO 4 À from LLW samples.

Conclusion
In summary, we herein present the study on 99 TcO 4 À separation using DB18C6 ligand in a highly polar nitrobenzene solvent system, showing promising results for the extraction of TcO 4 À ions (D TC ¼ 7.6, around 88% removal) from aqueous alkaline waste. From the experimental results, it is inferred that the extraction of 99 TcO 4 À can be considerably enhanced using DB18C6 as an extractant in the presence of a highly polar solvent, nitrobenzene. The high selectivity of 99 TcO 4 À separation is mainly due to the strong binding selectivity of DB18C6 towards Na + ions resulting in high stability of the resultant DB18C6$Na + $TcO 4 À complex, where TcO 4 À plays the role of a charge neutralizing agent for the initially formed cationic DB18C6$Na + complex. The effect of relevant anions present in the waste, like the nitrate and hydroxide ions, on the extraction behaviour of 99 Tc has also been evaluated. The optimum contact time for maximum extraction is around 20 min. The effect of pH on the extraction process has also been carried out systematically. Thermodynamic parameters of the extraction equilibrium have been estimated from temperature dependent extraction studies. It is found that with the rise in temperature, the extraction efficiency of TcO 4 À ions by DB18C6 decreases substantially. DFT calculations were carried out to understand the complexation of TcO 4 À ion with DB18C6 in different solvents. The DE and DG values for different modelled complexation reactions were evaluated. From the calculated free energy of complexation of TcO 4 À with DB18C6, it is observed that the consideration of an explicit solvent play a very important role for predicting the experimental selectivity. The analytical implication of this research work lies with the much higher estimate of the distribution ratio (D Tc ) as compared to the literature reports, and this methodology can be used for the selective extraction of 99 Tc from LLW, which is of major importance to minimize the hazardous effect of TcO 4 À in LLW, to avoid the long term radiological surveillance.

Conflicts of interest
There are no conicts to declare.