A new bisglycolamide substituted calix[4]arene-benzo-crown-6 for the selective removal of cesium ion: combined experimental and density functional theoretical investigation

Abstract

A new bisglycolamide substituted calix-benzo-crown-6 (CBCBGA) ionophore has been synthesized and characterized by using ¹H, ¹³C NMR, ESI-MS and elemental analysis. Detailed investigations on the effect of various parameters such as the aqueous phase acidity, ionophore concentration and nitrate ion concentration on extraction of cesium have been carried out. The new ionophore is found to be highly selective for Cs over other metal ions present in the simulated high level liquid waste solution. The complexing ability of this novel ionophore towards Cs⁺ and Na⁺ metal ions was further complimented by predicting the structure, extraction free energy and fraction of charge transfer using the B3LYP density functional employing a split-valence plus polarization (SVP) basis set in conjunction with the conductor like screening model. The unusually high selectivity of CBCBGA for Cs⁺ ion over Na⁺ ion was established by the calculated value of difference in the free energy, ΔΔG (ΔG_ext,Cs⁺ − ΔG_ext,Na⁺), −9.46 kcal mol⁻¹, which is in good agreement with the experimentally determined value of −5.59 kcal mol⁻¹. The calculated values of ρ and ∇2ρ at the bond critical point for the hydrated Na⁺ ion are found to be reduced during complexation with the CBCBGA, whereas for the hydrated Cs⁺ ion, though ρ remains same, the value of ∇2ρ is increased which seems to play a decisive role in the higher selectivity to Cs⁺ ion over Na⁺ ion of CBCBGA. The new CBCBGA ionophore shows promise for the selective removal of Cs⁺ ion over other metal ions present in the simulated high level liquid waste solution.

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Introduction

The major challenge for the nuclear community is safe management of nuclear waste generated during the reprocessing of the spent nuclear fuel. The dissolver solution of the spent fuel contains mainly U and Pu, long-lived minor actinides such as Np, Am, Cm and other radioactive fission products, mainly rare earths and ⁹⁰Sr, ¹³⁷Cs, ⁹⁹Tc etc. Safe management of the radioactive waste by efficient separation processes for elimination of harmful radiotoxic elements is the key to success and public acceptance of nuclear energy programs.^1,2 The PUREX process used worldwide selectively (and nearly quantitatively) removes the U and Pu from rest of fission products and activation products.^3,4 The extractants such as carbamoyl-phosphonate (CMP), carbamoylmethylphosphine oxide (CMPO), diglycolamides (DGA) etc. demonstrated their utility for the actinide partitioning from acidic high level liquid waste solutions.^4–7

Cs-137 is one of the major heat generating radionuclide and its removal leads to reduction in heat load hence the less volume of repository required for final vitrification. The radionuclide Cs-135 is long lived (t_1/2 ∼ 106 years) and one of the most mobile therefore can cause adverse effect on the environment after leaching out from repository along with other radionuclide. Also Cs-137 is a β,γ-emitter and can be used as alternate source for cobalt-60 for medical and industrial applications like, preservation of foodstuffs by killing bacterial growth, blood irradiation and medical accessories sterilization, cancer treatment by brachytherapy and scintillation cameras.⁸ This makes the cesium separation and recovery valuable. In fact, the effective separation of these radionuclides has been one of most difficult task.

For the separation of cesium form high level liquid waste, many of the specific ionophore that has been developed is based on the molecular platform of crown ethers. Among them, calix[4]arene-crown-6 ethers seems to have relatively well optimized structural properties especially when fixed in 1,3-alternate conformation for cesium ions extraction over the large amount of sodium ions.^9–11 On the other hand the 1,3-alternate calix-bis-crown (BOBCalix6 and BEHBCalix6) has been found to serve as the potential selective ionophore for cesium from the alkaline waste solutions.^12–16 In the various reports the effective removal of cesium has been demonstrated from the real high level liquid waste solutions at pilot plant scale along with their mixer-settler runs.^17–19

In this paper, we report for the first time, a new diglycolamide substituted calix[4]arene benzocrown-6 (CBCBGA) in 1,3-alternate conformation. The synthesis, characterization and preliminary evaluation of their feasibility for cesium and other metal ion extraction from nitric acid medium has been carried out. The high level liquid waste (HLW) generated during the reprocessing of spent fuel is having radioactive elements in nitric acid medium and cesium being a potential radioactive element presents in this waste. Hence, the extraction studies are carried in nitric acid medium.

Quantum chemical calculations plays an important role in elucidating the complexing ability of a ionophores towards a particular ion by predicting the structure, free energy of complexation and various bonding analysis.^20,21 Hence, in addition to the experimental studies, DFT based calculation was also performed to obtain a molecular level insights which includes the structure of the free calix-crown ether and its complex with Cs⁺ and Na⁺ ions and their free energy of extraction in aqueous-organic biphasic system.

Experimental section

Chemicals and solutions

Calix[4]arene was purchased from M/s Numex Chemical Corporation, Mumbai, India and purified by recrystallization. o-NPHE and 2-chloro-N,N-diisobutylacetamide was procured from M/s Orion Chem. Pvt. Ltd, Mumbai, India and used after distillation. 9-BBN and allyl bromide were purchased from Sigma Aldrich. All the other reagents used were of Analytical-Reagent grade and purchased from Spectrochem Pvt. Ltd., Maharashtra, India. Solvents were further purified and dried by standard methods prior to use. The simulated waste solution was prepared by dissolving the nitrates of Cs and Sr, radiotracer of ²⁴¹Am, chlorides of Na, K and Ba, oxide of Ce, Nd, Zr and ammonium salts of Mo in nitric acid solutions.

Physicochemical measurements

NMR spectra were obtained with a Bruker 500 MHz FT-NMR spectrometer in CDCl₃ with TMS as internal standard. Melting points were determined with a Mel-Temp melting point apparatus. Elemental analysis was performed at CHNS-Thermo apparatus and ESI-MS spectra obtained by Q-TOF micro mass (YA-105) Bruker in positive ion mode. The nitric acid concentrations were measured by potentiometric titration, using Metrohm 905 Titrando device and 0.1 M NaOH solution.

Distribution ratio studies

For the determination of distribution ratios (D_M) the organic phase was constituted with CBCBGA/o-NPHE. It was seen that o-NPHE, the diluent, was inert to cesium extraction. Distribution ratio measurements were performed by equilibrating 2 ml each of organic and aqueous phase for 15 minutes in 10 ml stopper glass vials. Distribution ratios were calculated by taking the ratio of metal ion concentration (radioactivity) in the organic phase to the metal ion concentration in the aqueous phase.

Distribution ratio (D_M) = [M]_org./[M]_aq. (1)

where [M]_org. is the concentration of metal ion in the organic phase and [M]_aq.. is the concentration of metal ion in aqueous phase.

Usually, 0.1 ml aliquots were removed from organic and aqueous phase and were assayed radiometrically. Radiometry assay of ¹³⁴Cs, ^85,89Sr and ²⁴¹Am, was carried out by gamma counting employing an intrinsic Ge detector connected to 4 K MCA at 604.7 keV, 511 keV and 60 keV respectively. Cesium in SHLW was analyzed by tracer technique by using ¹³⁴Cs tracer and inactive metal ions of SHLW were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) of Horiba Jobin-Yvon make. Estimation of rubidium was carried out by ion chromatography, using Dionex ICS 1600 device. All the experiments were carried out in a thermostatic bath 25 ± 10 °C. After equilibrium, phases were separated by centrifugation. Equilibrium was reached in 5 minutes, as it was seen that there was no considerable change in distribution ratio observed for the subsequent intervals of time. The reproducibility of results was within ±5%.

Synthesis of CBCBGA

According to above Scheme 1, the compound (2) was synthesized by the reaction of calix[4]arene with allyl bromide in presence of base potassium carbonate as the procedure reported.²² This 1,3-dipropenyloxy calix[4]arene (2) was reacted with the compound (3) in presence of cesium carbonate by refluxing in acetonitrile for 2 days to obtain the 1,3-dipropenyloxy calix-benzo-crown (4). The compound (3) ditosylate was synthesized by refluxing catechol with 2-(2-chloroethoxy) ethanol and K₂CO₃ in acetonitrile for 48 hours followed by conversion to ditosylate by reacting with tosylchloride reported elsewhere.¹⁵ 1,3-Dipropenyloxy calix[4]arene-benzocrown diol (5) was prepared by the reaction of 1,3-dipropenyloxy calix[4]arene-benzocrown (4) with 9-BBN followed by hydrogen peroxide oxidation. Calix[4]arene-benzocrown-bisglycolamide 6 was synthesized by the condensation of the 2-chloro-N,N-diisobutylacetamide and 1,3-dipropenyloxy calix[4]arene-benzocrown-6 diol (5) in presence of sodium hydride.

Scheme 1 Synthesis of 1,3-alternate calix[4]arene-benzocrown-bisglycolamide (CBCBGA) reagents and conditions: (a) allylbromide, MeCN, K₂CO₃, reflux; (b) Cs₂CO₃, MeCN, reflux; (c) 9-BBN, hydrogen peroxide, NaOH; (d) NaH, THF, rt.

Synthesis of 1,3-dipropenyloxy calix-benzo-crown (4)

To the stirred solution of 2 (20 gram, 39.6 mmol) in 4 l, of dry acetonitrile the 3 (12 gram 20.2 mmol) and excess of cesium carbonate (39 gram, 120 mmol) were added. The reaction mixture was refluxed for 24 hours then the left over amount of 3 (12 gram, 20.2 mmol) was added in the reaction mixture and reflux was continued further for 24 hours. The progress of reaction was monitored on TLC with 1 : 3, ethylacetate/Hexane as the mobile phase. The solvent was evaporated by vacuum distillation on rotavapour. The product was taken in dichlomethane (250 ml) and washed with 0.1 N HCl (3 × 250 ml) and distill water (3 × 250 ml). During the washing the third phase formed in between the organic phase and aqueous phase was discarded. The organic phase was dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure on the rotary evaporator. The crude product was chromatographed on silica gel (100–200 mesh size) with 1 : 4, ethylacetate : hexane as eluent to obtain 21.5 gram (72%) of 4 with mp 151–153 °C as the white shiny crystals. ¹H NMR: ¹H NMR (600 MHz, CDCl₃, δ in ppm): 3.64–3.69 (m, 8H, 4-OCH₂), 3.70 (s, 8H, 4ArCH₂Ar), 3.71–3.76 (m, 4H, 2-OCH₂), 4.0–4.11 (m, 4H, –2OCH₂), 4.11–4.17 (m, 4H, –2OCH₂), 4.8–4.9 (m, 2H, –CH₂=CH–CH₂), 5.0–5.6 (m, 2H, –CH₂=CH–CH₂), 5.69–5.81 (m, 2H, –2CH₂=CH–CH₂), 6.66–6.69 (t, 4H, J = 7.3 Hz, Ar–H), 6.96–7.08 (m, 8H, Ar–H), 7.08–7.26 (d, 4H, J = 7.4 Hz, Ar–H). ¹³C NMR (150 MHz, CDCl₃, δ in ppm): 37.51, 69.42, 70.16, 70.44, 70.77, 70.86, 76.78, 76.96, 77.18, 114.98, 115.83, 121.78, 122.21, 122.36, 130.12, 131.01, 133.43, 133.52, 134.29, 148.91, 155.78, 156.05. Anal. calcd for C₄₈H₅₀O₈: C, 76.30; H, 6.62; O, 16.95. Found: C, 76.38; H, 6.62; O, 16.99. ES-MS (ES)⁺, 755.3: [M + H]⁺, 756.3: [M + 2H]⁺.

Synthesis of 1,3-dipropenyloxy calix[4]arene-benzocrown diol (5)

Compound 4 was dissolved (15 gram, 19.0 mmol) in 100 ml anhydrous THF and kept at 0–50 °C in ice water bath. To this 9-borabicyclo[3.3.1]nonane (17 gram, 0.15 mol) in 100 ml anhydrous THF was added dropwise for 30 min, maintaining the temperature ∼0 °C. Reaction mixture was allowed reaching at room temperature. The reaction mixture was refluxed for 24 h then allowed to cool at room temperature. The resulting organoborane was oxidized by adding slowly 30 ml, 10 N NaOH and 100 ml, 50% v/v H₂O₂. The solution was stirred for 30 min then refluxed for 6 h to ensure the complete oxidation. The mixture was concentrated to remove the solvents by vacuum evaporation and extracted with dichloromethane (3 × 100 ml). The organic layer was washed with 0.1 N HCl (2 × 250 ml) and de-ionized water (2 × 250 ml). The organic was dried over anhydrous sodium sulphate, filtered and concentrated by vacuum evaporation. Crude product was chromatographed on silica gel with 10 : 1, CH₂Cl₂–acetone as the mobile phase to yield 11 gram, 70%. mp 137–140 °C: ¹H NMR: ¹H NMR (600 MHz, CDCl₃, δ in ppm) 1.52–1.60 (s, 2H, 2-OH), 1.68–1.72 (t, 4H, J = 5.9 Hz, –2OCH₂CH₂), 3.31–3.35 (t, 4H, J = 6.4 Hz, 2-OCH₂), 3.42–3.44 (m, 4H, –2OCH₂), 3.56–3.58 (t, 4H, J = 6.4 Hz, –2OCH₂), 3.69–3.71 (m, 4H, 2-OCH₂), 3.74–3.76 (m, 4H, 2-OCH₂), 3.82 (s, 8H, –4ArCH₂Ar), 4.13–4.16 (t, 4H, J = 4.5 Hz, 2-OCH₂), 6.70–6.72 (t, 3H, J = 7.3 Hz, Ar–H), 6.88–6.91 (t, 3H, J = 7.3 Hz, Ar–H), 6.93–7.01 (m, 3H, Ar–H), 7.08–7.11 (m, 7H, Ar–H). ¹³C NMR (150 MHz, CDCl₃, δ in ppm): 32.73, 38.00, 60.53, 69.24, 69.42, 69.70, 69.91, 70.22, 76.78, 76.96, 77.18, 94.38, 115.56, 122.00, 122.67, 122.79, 129.94, 133.68, 133.80, 149.22, 156.21, 156.94. Anal. calcd for C₄₈H₅₄O₁₀: C, 72.86; H, 6.82; O, 20.22. Found: C, 72.93; H, 6.82; O, 20.24. ES-MS (ES)⁺, 793.4: [M + 3H]⁺, 813.4: [M + Na]⁺, 829.4 [M + K]⁺.

Synthesis of calix[4]arene-benzocrown-bisglycolamide (6)

Suspended NaH (7.6 gram, 60 mmol), 60 percent dispersion in mineral oil in the 100 ml of dry THF and cooled to 0–5 °C in ice/water bath. Dissolved the 5 (9 gram, 11.3 mmol) in 100 ml dry THF and added slowly in the above cooled reaction mixture using pressure equalizer dropping funnel while maintaining the reaction temperature 0–5 °C. Ice bath was removed and the solution was warmed at 50 °C for 1 h and after the solution gets clarify, the 2-chloro-N,N-diisobutylacetamide (2.47 gram, 12.0 mmol) dissolved in 50 ml dry THF, added dropwise for 30 min. Refluxed the reaction mixture for 16 hours than allowed to reach at room temperature. The reaction was stopped by adding slowly 75 ml of distilled water. The reaction mixture was concentrated under reduced pressure and product was extracted with CH₂Cl₂ (2 × 100 ml). Both the organic fractions combined and washed with 0.1 N HCl (3 × 150 ml) and distill water (3 × 150 ml). The organic was dried over anhydrous MgSO₄, filtered and concentrated by vacuum evaporation. After the chromatography on silica gel with 1 : 4 ethylacetate/hexane, final product was obtained 8.7 gram, yield 68% viscous liquid. ¹H NMR: ¹H NMR (600 MHz, CDCl₃, δ in ppm), 0.82–0.98 (m, 24H, –8CH₃), 1.62–1.72 (m, 4H, –4NCH₂CH), 1.81–2.12 (bs, 8H, –4NCH₂), 3.11–3.15 (d, 4H, J = 7.9 Hz, –2ArOCH₂CH₂), 3.16–3.25 (d, 4H, J = 7.4 Hz, –2OCH₂), 3.41–3.50 (m, 4H, 2-OCH₂), 3.51–3.68 (m, 12H, 6-OCH₂), 3.74 (s, 8H, –4ArCH₂Ar), 4.12–4.18 (m, 8H, 2-ArOCH₂ + 2-OCH₂CO), 6.66–6.71 (t, 3H, J = 7.3 Hz, Ar–H), 6.72–6.81 (t, 3H, J = 7.3 Hz, Ar–H), 6.95–7.04 (m, 10H, Ar–H). ¹³C NMR (150 MHz, CDCl₃, δ in ppm): 19.9, 22.86, 26.10, 27.25, 28.8, 29.58, 29.70, 37.6, 38.62, 52.26, 54.06, 68.01, 68.05, 68.6, 70.11, 70.21, 76.71, 76.92, 77.13, 115.32, 121.83, 122.067, 122.06, 122.14, 129.79, 129.92, 133.83, 133.92, 149.05, 156.23, 156.50, 169.53. Anal. calcd for C₆₈H₉₂O₁₂N₂: C, 72.24; H, 8.14; O, 16.99; N, 2.47. Found: C, 72.31; H, 8.13; O, 17.05; N, 2.50. ES-MS (ES)⁺, 1152.6: [M + H + Na]⁺, 1153.9: [M + 2H + Na]⁺, 1167.8 [M + K]⁺.

Computational methods

The structure of free CBCBGA and its complexes with Cs⁺ and Na⁺ ions with nitrate ion has been optimized using hybrid Becke-Lee-Young-Parr (B3LYP) density functional²³ employing split-valence plus polarization (SVP) basis set²⁴ as supplied in the TURBOMOLE suite of program.²⁵ The 46 core electron based effective core potentials (ECP) was used for Cs⁺ ion.²⁶ The free energy was computed at 298.15 K using B3LYP functional.²³ The hybrid B3LYP functional was shown to be quite successful in predicting the thermodynamic properties of Cs.²¹ The solvent phase was accounted for using popular conductor like screening model (COSMO).²⁷ The dielectric constant of water, nitrobenzene, NPHE, octanol and chloroform was taken to be 80, 34.81, 25.7, 10.4, and 4.80 respectively. The model complexation reaction was used as follows:

M⁺(H₂O)_n(aq.) + NO₃⁻_(aq.) + CBCBGA_(org.) → MNO₃CBCBGA_(org.) + nH₂O_(aq.) (M/n = Cs/8 and Na/6) (2)

The extraction energy, ΔE_ext for the above complexation reaction can be evaluated as:

ΔE_ext = E_{MNO3CBCBGA(org.)} + nE_H2O(aq.) − E_{M⁺(H2O)(aq.)} − E_NO3⁻(aq.) − E_CBCBGA(org.) (3)

where E is the total electronic energy of the respective chemical species in aqueous and organic phase. The free energy of extraction has been calculated using standard thermodynamic procedure.²¹

Results and discussion

Effect of ionophore concentration on extraction of cesium

The CBCBGA has shown almost zero solubility in dodecane hence, in order to study the extraction behavior a polar diluent named o-NPHE (ortho nitrophenyl hexyl ether) was used. o-NPHE has been preferred as it can dissolve the calix-crown in high concentration, also being a high dielectric constant it can improve the extraction by solvating the counter anion.²⁸ Extraction of cesium from nitric acid medium can be represented as eqn (4)

Cs⁺_aq. + NO₃⁻_aq. + nCBCBGA_org. + mHNO_3aq. ⇄ Cs⁺·nCBCBGANO₃⁻·mHNO_3org. (4)

The conditional extraction equilibrium constant, K_ex, in reaction (4) is described as:

K_ex = [Cs⁺·nCBCBGA·NO₃⁻·mHNO₃]_org./[Cs⁺]_aq.[NO₃⁻]_aq.[CBCBGA]ⁿ_org.[HNO₃]^m_aq. (5)

D_Cs is the distribution coefficient and defined as:

D_Cs = [(Cs⁺·nCBCBGA)·NO₃⁻·mHNO₃]_org./[Cs⁺]_aq. (6)

By substituting eqn (6) into (5) and converting into logarithmic form the following equations are obtained

K_ex = D_Cs/[CBCBGA]ⁿ_org.[HNO₃]^m_aq. (7)

log K_ex = log D_Cs − n log[CBCBGA]_org. − m log[HNO₃]_aq. (8)

The species with the subscripts ‘aq.’ and ‘org.’ refer to those in the aqueous phase and in the organic phase. The aqueous phases were consisted of ∼100 ppm of Cs(I) as cesium nitrate and CBCBGA concentration was varied while keeping other parameters fixed. It was observed that the D_Cs value increases with the increase in ionophore concentration. The graph between the log D_Cs, V s⁻¹ log[CBCBGA] was plotted. From the Fig. 1, slope(n) of 1.017 suggests that one molecule of CBCBGA is participating in the extracted complex. The same extracted complex was also analyzed with ESI-MS and the cesium binding at 1261.6, 1262.7 m/z were observed† which further proves the 1 : 1 complex of Cs : CBCBGA in the extraction condition. Furthermore the 1 : 1 stoichiometry can be confirmed by binding isotherm analyses and Job plot analysis based on either NMR or UV-Vis spectral titration studies, which here was not performed due to the technical reason.

Fig. 1 Extraction dependency of cesium as a function of initial ionophore concentration (aqueous phase ∼100 ppm of cesium at 3.2 M HNO₃ and organic phase as CBCBGA in NPHE).

Effect of nitric acid concentration

The variation of the distribution ratio value with the feed acidity was studied using 0.01 M CBCBGA/o-NPHE as organic phase with the phase ratio 1 : 1 as shown in Fig. 2. It was observed from Fig. 2 that at lower nitric acid concentration, increase in nitric acid favors extraction of cesium and this increase in D_Cs was up to ∼3.2 M nitric acid and beyond this acidity the distribution ratio falls, due to competitive extraction of H⁺ ions. Dozol et al. have reported D_Cs for different calix-crowns dissolved in o-NPHE, their values are comparable with our results.¹⁵ It is important to note that in most of calix-crowns studied with o-NPHE diluent system the distribution ratios are maximum at 2 M nitric acid but in our case this maxima is around 3 M nitric acid concentration with a D_Cs of 12.5, this effect is due to the presence of amidic groups in the molecule. These amidic groups facilitate in neutralizing the acid effect resulting in low competitive extraction of H⁺ ions vis-â-vis intramolecular buffering effect.²⁹

Fig. 2 Distribution coefficients of cesium as a function of initial nitric acid. (Organic phase 0.01 M CBCBGA in NPHE and aqueous phase ∼100 ppm cesium with varying aqueous acidity).

For further interpretation on stoichiometry of extracted complex in solvent extraction process the D_Cs was determined from nitric acid solutions at fixed ionic strength 3.5 M (H, Na) NO₃⁻. log D_Cs vs. log[HNO₃] plot (Fig. 3) shows a slope (m) of 1.065 suggest, that the CBCBGA·Cs to HNO₃ ratio is 1 : 1.

Fig. 3 Extraction dependency of cesium as a function of initial nitric acid under fixed ionic strength. Organic phase 0.01 M CBCBGA/o-NPHE; aqueous phase: 3.5 M (H + Na) NO₃⁻.

Effect of nitrate ion and extraction equation

In order to investigate the effect of counter ion during cesium extraction, D_Cs was determined with increase in [NO₃⁻] of aqueous medium. The initial concentration of feed solution was fixed to 1 M nitric acid to ensure a considerable distribution ratio (3.6) and then calculated the D_Cs with the increase of sodium nitrate concentration. It was observed that there is no significant effect of nitrate ion on cesium extraction which is in contrary to other neutral extractants where a considerable effect of nitrate ion was present. This suggests that the nitrate ion is not participating in the complex formation. But for maintaining the charge neutrality the complex must contains the nitrate ion in the organic phase. The effect may be nullified due to the ion-pair dissociation in organic phase to give free Cs⁺·CBCBGA·HNO_3org. and NO₃⁻_org. Reports are there for polar diluents solvent systems like nitrobenzene for which such type of ion-pair dissociation in organic phase is favorable which in turns depends upon the bulk dielectric constant and inter nuclear separation between the ion centers.³⁰ In our present case the solvent is highly polar (o-NPHE) to bring an sufficient ion-pair dissociation in organic phase resulting in an dissociated solvated Cs⁺·CBCBGA·HNO_3org. cation. From the slope analysis of above discussed graphs it was obtained that the one cesium ion is bounded with one CBCBGA and nitric acid molecule and the polar nature of diluents keeps the nitrate ion dissociated in organic phase. Thus the overall extraction equation can be written as eqn (9).

Cs⁺_aq. + NO₃⁻_aq. + CBCBGA_org. + HNO_3aq.Cs·CBCBGA⁺·HNO_3org. + NO₃⁻_org. (9)

Effect of diluents

The diluent plays an important role on extraction of metal ions. When an extractant molecule is dissolved in a particular diluent, the diluent molecule rearranges themselves around the extractant molecules by forming a solvation sphere. The CBCBGA being a polar molecule requires polar diluents for its dissolution prior to extraction process. These polar diluents like nitrobenzene, 1-octanol and o-NPHE etc. affect the extraction process by solvating the extracted complex in organic phase which depends on the their dielectric constant, dipole moment and H-bonding ability. The CBCBGA has limited solubility in aliphatic diluents like n-dodecane. Therefore, the role of diluents on extraction of cesium was investigated using different diluents. The distribution ratios of cesium were carried out with different diluents system at 3 M nitric acid concentration. The results are presented in Table 1.

Table 1 Effect of organic diluents on the extraction of cesium. [CBCBGA]: 0.01 M

Diluent	Dielectric constant	DCs (3 M HNO3)
Nitrobenzene	34.8	15.7
o-NPHE	25.7	11.4
1-Octanol	10.4	5.2
Chloroform	4.8	2.1

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The distribution constant was found to be highest with nitrobenzene and lowest with chloroform and follows the deceasing trend with decreasing dielectric constant of the diluents: nitrobenzene > o-NPHE > 1-octanol > chloroform.

Extraction of alkali metal ions

The amount of cesium present in HLW depends upon the burn-up of a nuclear reactor. In a typical high level waste from pressurized heavy water reactor (PHWR) with a burn-up of 6700 MWD/ton consists of 0.32 g L⁻¹ of cesium. Generally, in HLW cesium is present along with very high concentration sodium with a lesser concentration of potassium and rubidium. Due to similarities in their chemical properties they compete with the extraction of cesium and thereby reduce the effectiveness of cesium extraction process. Therefore, the extraction of individual alkali metal ion dissolved in 3.0 M nitric acid was tested with 0.01 M CBCBGA/o-NPHE. The values of distribution ratio of metal ions (D_M = M_org./M_aq., where M_org./aq. refers to the metal ion concentration in the organic and aqueous phase respectively) are presented in Table 2. The results indicate very high selectivity of cesium extraction over sodium and potassium and relatively smaller for rubidium (S.F_(Cs/Rb) = D_Cs/D_Rb = 158).

Table 2 Extraction of alkali metal ions, organic phase: 0.01 M CBCBGA/o-NPHE: aqueous phase: alkali metal ions (∼100 mg L⁻¹ each) dissolved in 3.5 M HNO₃, O/A = 1)

Metal ion	DM	Separation factor (S.F)
Cs	12.5	—
Na	<0.001	>15 000
K	0.01	1250
Rb	0.08	158

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Selectivity

In order to study the extraction of cesium with respect to other metal ions, the extraction of cesium was carried out from simulated high level liquid waste solution (Table 3) using 0.01 M CBCBGA/o-NPHE. The feed solution concentration of metal ions in Simulated High Level Waste (SHLW) is according to spent fuel of long-cooled pressurized heavy water reactor with a burn up of ∼6500 MWD per ton.³¹ Although CBCBGA contains the two monoglycolamide units but its exhibits almost nil An/Ln extraction at these particular conditions, this may be explained due to the large spacing between two monoglcolamide (MGA) units and non-fulfillment of the required An/Ln coordination numbers by single bulky molecule. In general, 2–3 diglycolamides ionophore has been required to meet An/Ln coordination sphere. Here, it is difficult for three bulky ionophores to come closer to participate in complex formation with An/Ln's.

Table 3 Distribution ratios of metal ions from simulated high level liquid waste solution

M	Cs	K	Sr	Am	Mo	Ce	Nd	Ba	Na	Zr
Conc ppm	100	220	150	Tracer	250	300	60	120	5000	80
DM	12.5	≤10^−2	≤10^−3	≤10^−3	≤10^−2	≤10^−3	≤10^−3	≤10^−3	≤10^−2	≤10^−2

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The results indicates that the cesium is having much higher distribution coefficient as compared to other metal ion present in simulated high level liquid waste, hence can be used a promising ionophore for selective removal of cesium from the waste. The addition of glycolamide units impart extra rigidity to the calix-crown ionophore which in turn enhances the selectivity. In addition to this, these two bulky moieties also provide the selectivity towards cesium ion over other metal ions by blocking the other metal ion approach toward the crown-cavity.

Computational results

Structural parameters

The minimum energy structure of free CBCBGA is displayed in Fig. 4. From the figure it is seen that the CBCBGA offers a nice cavity to trap the metal ions inside it.

Fig. 4 Optimized structure of the free CBCBGA at the B3LYP/SVP level of theory.

The optimized structure of complex of Cs⁺ with CBCBGA in the presence of nitrate ion is presented in Fig. 5.

Fig. 5 Optimized structure of the complex of Cs⁺ with CBCBGA at the B3LYP/SVP level of theory.

From the figure it is seen that the Cs⁺ ion is coordinated to the 6 donor O atoms from the crown moiety and two donor atoms from one nitrate anion leading to 8-coordinated complex. The average Cs–O (O of crown moiety) bond distance is found to be 3.30 Å which is longer than that of Cs–O (3.19 Å) bond with nitrate O. Three donor O atoms of crown moiety were found to be coordinated from the top and three from the down as clearly shown in Fig. 6. The optimized structure of complex of Na⁺ with CBC in the presence of nitrate ion is presented in Fig. 7. From the figure it is seen that Na⁺ ion is coordinated to the 4 donor O atoms from the crown moiety and one donor atoms from one nitrate anion leading to 5-coordinated complex. The average Na–O (O of crown moiety) bond distance is found to be 2.67 Å which is longer than that of Na–O (2.19 Å) bond with the O atom of nitrate ion.

Fig. 6 Optimized structure of the complex of CBCBGA with Cs⁺ at the B3LYP/SVP level of theory.

Fig. 7 Optimized structure of the complex of CBCBGA with Na⁺ at the B3LYP/SVP level of theory.

Free energy of extraction during complexation

The free energy in the gas phase and solution phase was evaluated as per eqn (2) and are presented in Table 4. The free energy was found to be negative indicating highly exergonic nature of the extraction complexation for both the Cs⁺ and Na⁺ ions. The solution phase free energy of extraction was seen to be reduced considerably from their respective gas phase free energy due to the dielectric screening of the organic solvent. It is interesting to note that the gas phase free energy is seen to be higher with the Na⁺ ion over Cs⁺ ion as expected, because, the size of the Na⁺ ion smaller than that of Cs⁺ ion. Whereas the solution phase free energy for Cs⁺ ion is found to be higher than that of Na⁺ ion. This is due to the higher free energy of solvation for Na⁺ ion than that of Cs⁺ ion in aqueous solution. The calculated value of difference in free energy, ΔΔG (ΔG_ext,Cs⁺ − ΔG_ext,Na⁺) is found to be −9.46 kcal mol⁻¹, which is in good agreement with the experimentally determined value of −5.59 kcal mol⁻¹. The very high value of ΔΔG is responsible for the unusually high selectivity of Cs⁺ ion over Na⁺ towards CBCBGA. The extraction free energy was found to be highest with nitrobenzene and lowest with chloroform for both the Cs⁺ and Na⁺ ions as observed in the solvent extraction experiments. Further, the selectivity is found to be reduced with decreasing dielectric constant of the solvent.

Table 4 Calculated value of free energy in gas and solution phase at the B3LYP level of theory using SVP basis set

Complexation reaction	Free energy of complexation kcal mol^−1
	Gas	Solvent
		NB	NPHE	OCT	CF
Cs^+(H2O)8 + NO3^− + CBC = CBC–CsNO3 + 8H2O	−81.52	−26.18	−25.98	−24.92	−23.08
Na^+(H2O)6 + NO3^− + CBC = CBC–CsNO3 + 6H2O	−87.07	−16.68	−16.52	−15.64	−14.11

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In order to understand the type of various molecular interaction, the LUMO–HOMO energy gap for free ionophore and metal ion was determined³² and the calculated values are given in Table 5.

Table 5 Calculated quantum chemical descriptors in gas phase at the B3LYP/SVP level of theory

System	ΔELUMO–HOMO	η	χ	ΔN
Cs^+	14.54	7.27	12.54	0.484
Na^+	32.26	16.13	23.02	0.536
CBC	5.62	2.81	2.76

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The pictorial representation of HOMO and LUMO of CBCBGA ionophore is displayed in Fig. 8.

Fig. 8 Calculated HOMO–LUMO of the CBCBGA at the B3LYP/DZP level of theory.

The ionophore studied here represents the class of hard base because of hard O donor which is evident from the high value of E_LUMO–HOMO, χ and η. Further, the amount of charge transfer, ΔN (ΔN = (χ_M − χ_L)/{2(η_M + η_L)}, here, M stands for metal ion, which acts as Lewis acid i.e. acceptor and L stands for the ionophore i.e. CBCBGA, which acts as Lewis base i.e. donor.)³³ was also calculated for the donor acceptor complexation and the values are listed in Table 5. The high metal-ionophore interaction can be correlated with the higher value of charge transfer, ΔN. Here, the value of ΔN was seen to be quite high indicating strong interaction. Furthermore, the value of ΔN is seen to be higher with the Na⁺ ion over Cs⁺ ion which is well correlated with the gas phase free energy values. The calculated molecular electrostatic potential (Fig. 9) clearly demonstrate the site for binding of the metal ions within the crown cavity.

Fig. 9 Calculated molecular electrostatic potential of the CBCBGA ionophore at the B3LYP/DZP level of theory.

The charge transfer was calculated using natural population analysis.³⁴ The residual charge on the Na⁺ ion was found to be 0.900 whereas it was 0.906 for Cs⁺ ion. The transfer of charge is found to be marginally higher for Na⁺ compared to Cs⁺ as evident from the value from Natural population analysis suggesting the higher binding energy for Na⁺ ion compared to Cs⁺ ion in gas phase.

Second order bonding analysis

Natural bond order (NBO) analysis program NBO6.0 (ref. 35 and 36) was used to evaluate the second order stabilization energy to investigate the unusually high selectivity of Cs⁺ over Na⁺ ion towards CBCBGA at the B3LYP/DZP level of theory. The second-order interaction energies E⁽²⁾_ij indicate the strength of the metal ion-ionophore coordinated interactions. The second order stabilization energy E⁽²⁾_ij is defined as

E⁽²⁾_ij = q_ixF⁽²⁾_ij/(ε_i − ε_j) (10)

where, q_i, ε_i, ε_j and F⁽²⁾_ij represent the donor orbital occupancy, diagonal elements (orbital energies), and off-diagonal NBO Fock matrix element respectively. The strong donor–acceptor interaction is reflected in the large value of stabilization energy. The calculated value of E⁽²⁾_ij for Cs⁺ ion was found to be 1.42 kcal mol⁻¹ whereas for Na⁺ ion it was 1.15 kcal mol⁻¹ (details are given in Table S2, ESI†). The stabilization energy for Cs⁺ ion with CBCBGA was found to be higher compared to its corresponding Na⁺ ion complex whereas, the gas phase free energy follows the reverse trend. For this reason, second order stabilization energy might not be helpful for the forecast of selectivity in the solution phase.

Bond critical points

The bond critical point (BCP) analysis are commonly used to measure the strength of the bond between the metal ion and the donor atom of the ionophore. Hence, the electron density at the BCP, ρ and the Laplacian of electron density, ∇²ρ for metal ion-ionophore systems are evaluated using the atom in molecule methods.^36,37 The higher the value of ρ stronger is the bond, whereas, the negative value of ∇²ρ points to covalent bond and positive value points to ionic, coordinated, hydrogen bond or van der Waals type of interaction. The ellipticity (ε) parameter measures the cylindrical symmetry of the bond.

The calculated values of ρ, ∇²ρ and ε are presented in Table 6. The details are presented in Table S1, ESI.†

Table 6 Calculated values of the average electron density and Laplacian of electron density and ellipticity at B3LYP/DZP level of theory using Bader's AIM calculation

Complex	BCP	ρ	∇^2ρ	ε = (λ1/λ2) − 1
Cs^+–CBCBGA	Cs–O	0.008	0.039	0.069
Na^+–CBCBGA	Na–O	0.012	0.079	0.062
Cs^+–(H2O)8	Cs–O	0.008	0.031	0.078
Na^+–(H2O)6	Na–O	0.015	0.096	0.055

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The higher value of charge density for Na⁺ ion over Cs⁺ ion reflects the stronger interaction with the former than the later ion and is well correlated to the higher complexation free energy of Na⁺ ion than Cs⁺ ion in gas phase. The calculated positive values of ∇²ρ demonstrate that the interaction of Na⁺ and Cs⁺ ions with O donor atoms of the ionophore are of closed-shell type. The small values of ρ, ∇²ρ and ε indicate little covalency in the bonding and the covalency is slightly higher for Cs⁺ ion. The very low value of ε suggests that the deviation from cylindrical symmetry is not significant. Furthermore, the values of ρ, ∇²ρ and ε for the hydrated metal ion cluster are also evaluated to study the nature of bonding in hydrated cluster. The calculated electron density, ρ at the BCP for 8 Cs–O bonds and 6 Na–O bond are found to be almost equal, thought it is much higher for Na–O bond over Cs–O bond. Whereas, ρ at the BCP for 8 Cs–O bonds and 5 Na–O bond are found to be dissimilar (Table S2, ESI†). It is worthwhile to mention that the values of ρ and ∇²ρ for hydrated Na⁺ ion are reduced during complexation with CBCBGA, whereas for hydrated Cs⁺ ion though ρ remains same, the value of ∇²ρ is increased. Therefore, the increased bond strength might be responsible for the higher selectivity of Cs⁺ ion over Na⁺ ion towards novel CBCBGA ionophore.

Conclusions

Combined experimental and density functional theoretical studies were conducted to understand the unusually high selectivity of the Cs⁺ ion over Na⁺ ion towards Calix-benzo-crown-6 bisglycolamide (CBCBGA) ionophore. The CBCBGA ionophore in 1,3-alternate conformation has been synthesized and characterized by using ¹H, ¹³C NMR, ESI-MS and elemental analysis.† The effect of various parameters like feed acidity, CBCBGA concentration and nitrate ion were studied to determine the stoichiometry of extracted complex. The slope analysis method reveals that 1 : 1 : 1 molar ratio for CsNO₃ : CBCBGA : HNO₃ in the extracted complex. The CBCBGA molecule is found to serve a highly selective ionophore for cesium over other metal ions present in simulated high level liquid waste solution. The present DFT predicts the exergonic nature of complexation in the gas phase as well as solution phase for the complexation of Cs⁺ and Na⁺ ions with CBCBGA as observed in the experiments. The calculated value of difference in free energy, ΔΔG (ΔG_ext,Cs⁺ − ΔG_ext,Na⁺) is found to be −9.46 kcal mol⁻¹, which is in good agreement with the experimentally determined value of −5.59 kcal mol⁻¹. The extraction free energy was found to be highest with nitrobenzene and lowest with chloroform for both the Cs⁺ and Na⁺ ions as observed in the solvent extraction experiments. The free energy of extraction was seen to be increased with increase dielectric constant of the organic solvents as observed in the solvent extraction experiments. The calculated electron density, ρ at the BCP for 8 Cs–O bonds and 6 Na–O bonds are found to be almost equal, thought it is much higher for Na–O bond over Cs–O bond. Whereas, ρ at the BCP for 8 Cs–O bonds and 5 Na–O bonds are found to be dissimilar. It is worthwhile to mention that the values of ρ and ∇²ρ for hydrated Na⁺ ion are reduced during complexation with CBCBGA, whereas for hydrated Cs⁺ ion though ρ remains same, the value of ∇²ρ is increased. Therefore, the increased bond strength seems to play decisive role for the higher selectivity of Cs⁺ ion over Na⁺ ion towards CBCBGA ionophore. A corroborated experimental and theoretical studies were very helpful to understand the unusually high selectivity of Cs⁺ ion over Na⁺ ion towards the new CBCBGA ionophore which shows promise for the selective extraction of Cs⁺ ion from the radioactive waste.

Acknowledgements

We are thankful to Dr S. B. Roy, Associate Director, Chemical Engineering Group and Mr K. T. Shenoy, Head, ChED for continuous support and encouragement. Computer Division BARC is acknowledged for providing ANUPAM Supercomputing facility.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1: ¹H NMR of 1,3-dipropenyloxy calix-benzo-crown (4), Fig. S2: ¹³C NMR of 1,3-dipropenyloxy calix-benzo-crown (4), Fig. S3: ESI-MS spectra of 1,3-dipropenyloxy calix-benzo-crown (4), Fig. S4: ¹H NMR of 1,3-dipropyloxy calix[4]arene-benzocrown diol(5), Fig. S5: ¹³C NMR of 1,3-dipropyloxy calix[4]arene-benzocrown diol (5), Fig. S6: ESI-MS spectra of 1,3-dipropyloxy calix[4]arene-benzocrown diol (5), Fig. S7: ¹H NMR of 1,3-alternate calix[4]arene-benzocrown-bisglycolamide (6), Fig. S8: ¹³C NMR of 1,3- alternate calix[4]arene-benzocrown-bisglycolamide (6), Fig. S9: ESI-MS spectra of 1,3-alternate calix[4]arene-benzocrown-bisglycolamide (6), Fig. S10: ESI-MS of CBCBGA complexes with cesium, Table S1: computed values of electron density and Laplacian of electron density and ellipticity of MNO₃–CBCBGA and hydrated metal ion complexes at the B3LYP/DZP level using Bader's AIM calculation, Table S2: calculated values of average second order stabilization energies E⁽²⁾_ij using NBO analysis at the B3LYP/DZP level of theory. See DOI: 10.1039/c6ra05814k

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