Selective separation of ¹⁵²Eu from a mixture of ¹⁵²Eu and ¹³⁷Cs using a chitosan based hydrogel

A rapid and novel technique was developed to separate long-lived fission products ¹⁵²Eu (T_1/2 = 13.33 years) and ¹³⁷Cs (T_1/2 = 30.17 years) using a solid liquid extraction (SLX) technique with a chitosan biopolymer based hydrogel (ChG). In this technique, ChG selectively adsorped ¹⁵²Eu (∼87%) from the mixture of ¹⁵²Eu and ¹³⁷Cs without any contamination of ¹³⁷Cs. Simple back extraction using 1 M aqueous HCl from ¹⁵²Eu-adsorped-ChG leads to pure ¹⁵²Eu (∼72%). The adsorption behavior of ¹⁵²Eu on ChG depends on pH and radiation dose. The structural, mechanical, thermal and morphological characterization of ChG was confirmed by FT-IR, solid state NMR, rheology measurement, SEM, TG-DTG, DSC and XRD studies. The rheological study shows a higher value of the storage modulus (G′) than the loss modulus (G′′), demonstrating that the rheological behavior in the ChG has a majority of visco elastic solid nature.

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Introduction

Serious environmental pollution is caused by radionuclides, which are traced in the surroundings from the effluents of nuclear reactors, nuclear weapon tests and even from undesirable nuclear reactor accidents. ¹⁵²Eu and ¹³⁷Cs are fission products and produced from nuclear reactors. ¹⁵²Eu also can be produced from nuclear reactor control rods by neutron activation reactions. Hence, separation of these two fission products is necessary to keep them in separate confinement.

Reported separation techniques for fission products ¹⁵²Eu and ¹³⁷Cs are many and a few are listed in this paper. A few such ones are by the use of an amide type open chain crown ether, N,N,N,N-etraphenyl-3,6-dioxaoctanediamide (TDD).¹ Separation of ¹⁵²Eu and ¹³⁴Cs was also reported using inorganic ion exchangers such as zirconium vanadate and ceric vanadate.² Roy et al. reported the greener analytical technique like PEG based aqueous biphasic technique to study the extraction and separation behaviour of ^152,154Eu and ¹³⁷Cs.³ But we have developed a new medium for separation of ¹⁵²Eu (T_1/2 = 13.3 years) and ¹³⁷Cs (T_1/2 = 30 years) which employs solid liquid extraction (SLX) technique using biopolymer chitosan based hydrogel (ChG) to achieve faster and high purity separation.

Hydrogels are cross linked viscoelastic solids with highly porous three dimensional hydrophilic network chains.⁴ Softness, good adsorption capacity and water storing capacity make hydrogels unique. Hydrogels are also called ‘Smart Materials’ due to the presence of solid as well as liquid-like properties.⁵ Sensitivity of hydrogel structure towards pH, temperature, and the concentration of metabolite were reported in literature.⁶ Besides, the hydrogels should possess the following properties: insolubility in water, biocompatibility, biodegradability, high swelling capacity, high mechanical strength and stability in an acidic condition.⁷ The important application of hydrogels are in the field for purification of waste water, as a recyclable organocatalyst, adsorption of various metal ions including actinides, in agriculture, biomedical as sustained release drug delivery systems, in tissue engineering.^8–11

In recent past, biopolymers have drawn special attention in both academic and industrial worlds.¹² Among numerous biopolymers, cellulose and chitin are abundant as well as nontoxic. Chitin and chitosan are co-polymers containing N-acetyl-glucosamine and N-glucosamine units in a random manner. When more than 50% of N-acetyl-glucosamine monomer is present, the biopolymer is termed chitin and conversely, if the number of N-glucosamine units is more than 50%, the biopolymer is termed as chitosan.¹³ Chitosan is important for research as dilute acidic solution of chitosan can be easily cast into films and fibers and can be coagulated into well-defined spherical particles by spraying into alkaline solution. Chitosan has greater crosslinking ability due to the presence of amino (–NH₂) group.¹⁴ Park et al., Monier et al. and L. Wang et al. reported the synthesis route of hydrogel using chitosan biopolymer and glyoxal. Park et al. synthesized the superporus hydrogels using aqueous acetic acid solution of chitosan and glycol chitosan, mixed with glyoxal solutions and then NaHCO₃ powder was added and the time taken for the preparation of hydrogel was 12 h.¹⁵ Monier et al. synthesized chitosan/glyoxal resin by crosslinking chitosan with glyoxal in presence of L-aspartic acid and the time taken for the hydrogel preparation was 3 h.¹⁶ L. Wang et al. prepared chitosan/collagen hydrogel using glyoxal as crosslinking agent.¹⁷ We have synthesized hydrogel in a modified route using simple Schiff base formation reaction taking 1% aqueous acetic acid solution of chitosan biopolymer and glyoxal (crosslinking agent) as a starting material at 50–60 °C temperature within 5 min.

To know the efficacy of this newly developed hydrogel, separation studies of two long lived fission products such as ¹⁵²Eu (T_1/2 = 13.3 years) and ¹³⁷Cs (T_1/2 = 30 years) was carried out by Solid Liquid Extraction (SLX) technique. In the present work, SLX technique using biopolymer, chitosan based hydrogel as solid phase was developed for the first time to separate the long lived fission products such as ¹⁵²Eu and ¹³⁷Cs. In the SLX technique, analyte of the liquid samples are selectively adsorbed on the active sites of the solid surface.

Experimental

Materials

Starting materials for the synthesis of the hydrogel (ChG) were low molecular weight chitosan (LMWC) (SRL), glacial acetic acid (RANKEM, purity 99%), glyoxal (40.0%) (Sigma-Aldrich) and absolute ethanol (Merck). All other reagents employed for this study are of analytical grade. The radioisotopes, ¹⁵²Eu and ¹³⁷Cs, were procured from Board of Radiation and Isotope Technology (BRIT), Trombay, India. A 2 mL stock solution was prepared by mixing 20 000 dps ¹⁵²Eu and ∼60 000 dps ¹³⁷Cs solution, both individually procured from BRIT. From this stock solution 100 μL were used for each batch of extraction.

Synthesis of chitosan based hydrogel

The chitosan based hydrogel was prepared by using simple Schiff base condensation reaction. The procedure for preparation of the hydrogel was as follows: chitosan (0.2 g) was dissolved in 1% aqueous acetic acid (14 mL). After removal of insoluble impurity this solution of chitosan was stirred and heated to 60 °C. To this mixture, 2.5 M ethanolic solution of glyoxal (6 mL) (crosslinking agent) was added drop-wise. The formation of a light yellow hydrogel (Schiff base) took place within 5 min. The so formed hydrogel was then washed thoroughly with distilled water, followed by ethanol and then dried in a freeze drier and used for further experimental application.

Characterizations of hydrogel

FTIR, solid-state ¹³C CP-MAS NMR were employed to investigate the chemical structure of a compound. FTIR of the sample were recorded on Fourier-Transform Infrared (FTIR) spectrophotometer, (JASCO, FT/IR-460 PLUS) using KBr pellet method in the range of 500–4000 cm⁻¹.

Solid-state ¹³C CP-MAS NMR spectra were performed on a Bruker Avance^III 500 spectrometer using tetramethylsilane (TMS) as an internal standard at 25 °C equipped with an HP amplifier 1H 500 MHz, 200 W CW and with a pulse amplifier M3205. Mass of the dry powdered of ChG hydrogel was taken 8 K. In this measurement, spin rate, ð/2 pulse, relaxation delay and contact time for the cross-polarization experiment were 8.0 kHz, 3.5 μs, 5 s and 1 ms respectively. Spectra were obtained with 1024 words in the time domain, zero-filled, and Fourier-transformed with a size of 2048 words. For each sample 8000 scans were performed and total time taken 22 h.

To know the mechanical properties of hydrogel, rheological study was performed. Strain and frequency-sweep experiments were carried out on Anton Paar Physical Rheometer (MCR 301, Austria) with cone plate geometry using 20 mm in diameter with a gap size of 50 μm and 1° angle and temperature was controlled at 25 °C. The strain and frequency-sweep experiments of the ChG hydrogel were measured as a function of frequency in the range of 0.01–100 rad s⁻¹ with a constant strain value 0.01%. To determine the exact strain for frequency sweep experiments, the linear viscoelastic (LVE) regime was performed and a constant frequency is maintained of 10 rad s⁻¹. 200 μL of hydrogel was used for rheological measurements.

Scanning electron microscopic studies, for observing the surface topography of powder of the dried hydrogel were carried out with a Hitachi S4800. Completely dry powder of hydrogel was coated with gold using a gold sputter module in a high-vacuum evaporator. The gold coated samples were randomly scanned and then imaged at an acceleration voltage of 15.0 kV.

To check the thermal behaviour of the ChG, thermogravimetric (TG) analysis and differential (DTG) thermogravimetric analysis were carried out using Perkin Elmer Diamond TG-DTA (TG/DTA 6300). 3.228 mg of dry powdered sample was taken in an aluminum pan under dynamic nitrogen at a flow rate of 20 mL min⁻¹ atmosphere⁻¹ and heated between 32.78 to 800 °C at a heating rate of 10 °C min⁻¹.

The thermal property of ChG was measured by Differential Scanning Calorimetry using DSC Q100 V9.9 Build 303 instrument. About 5 mg of powder form of hydrogel was placed inside an aluminium sample pan. An empty sample pan was used as a reference. The thermal analysis was performed from −50 °C to 150 °C and from 150 °C to −50 °C at the heating rate of 10 °C min⁻¹ under dry nitrogen atmosphere with a flow rate of 50 mL min⁻¹. But we are presented a small temperature range 0 to 150 °C of the DSC where the main change occurred. Thermal Advantage (TA) universal analysis software was used for data analysis. The inflection of the DSC curve was used to determine glass transition temperature (T_g) by TA universal analysis software, and in the DSC curve crystallization peak was referred as the crystallization transition temperature (T_c).

To check the effect of cross-linking on the hydrogel morphology, X-ray diffraction studies of the ChG have been carried out by the D8 Advance X-ray diffractometer of BRUKER with a parallel beam optics attachment. The X-ray diffraction study was carried out at a 35 kV voltage and 30 mA current. The X-ray diffraction was employed using Ni-filtered CuKα radiation. The instrument was calibrated with a standard silicon sample. Samples were scanned from 5° to 80° (2θ) in the step scan mode (step size 0.030, preset time 2 s) and the diffraction patterns were recorded using a scintillation scan detector. The crystallinity index (CrI) can be calculated using the following equation¹⁸

CrI = [(I_f − I_s)/I_f] × 100

where, I_f is the peak intensity of the fundamental band and I_s is the peak intensity of the secondary band. This is a time-save empirical method to measure of relative crystallinity of a sample.

pH dependent swelling study

The ChG is a crosslinked polymeric hydrogels which is swelled but not dissolve when water or any other solvent enters it. The swelling properties, which is usually used to measure the degree of swelling (S_w) to define hydrogels was measured using 0.1 g of hydrogel in 2 mL of a buffer solution of various pH (2.5–8.5) at room temperature. The final weight (W_t) of the hydrogel was calculated after 2 h. The swelling ratio was calculated using the following equation

S_W (%) = [(W_s − W_d)/W_s] × 100

where W_s weight of the swollen hydrogel at experimental temperature and W_d is the weight of the dry hydrogel.

Radiochemical separation

Solid liquid extraction was performed for the separation of long-lived radionuclides ¹⁵²Eu (T_1/2 = 13.33 a) and ¹³⁷Cs (T_1/2 = 30.1 a) at pH range 1–6 using 3 mL aqueous HCl solution against 0.1 g of hydrogel as solid phase. After that 0.1 mL stock solution containing ¹⁵²Eu and ¹³⁷Cs was added to this system and was mechanically shaken for 10 min. The system was then kept for 10 min and finally the system was centrifuged at 6000 rpm for 4 min to achieve complete phase separation before collecting 2 mL of each phase for the γ-spectroscopic studies.

The SLX was also carried out using the varying amount (0.05 to 0.5 g) of hydrogel as solid phase and 3 mL of HCl as liquid phase at pH 4.

Similarly, shaking time (5 to 20 min) was also varied keeping the amount of hydrogel (0.4 g) and acidity of the solution (pH 4) fixed. Finally the SLX was done by varying the settling time (5 to 20 min) against 0.4 g of hydrogel as solid phase and 3 mL of HCl as liquid phase at pH 4.

Desorption of ¹⁵²Eu from hydrogel phase was carried out with 1 M HCl to obtain ¹⁵²Eu in liquid phase.

Radiation stability and its effect on the adsorption of ¹⁵²Eu on hydrogel were measured by gamma irradiation of 0.4 g of hydrogel phase using GAMMA CHAMBER 1200 at IUC-DAEF, Kolkata. In this process, hydrogel phase was irradiated with various gamma doses ranging from 1 to 15 kGy. After irradiation, SLX technique was carried out using irradiated hydrogel phase. In each set of extraction 3 mL of HCl at pH 4 and 0.2 mL of ¹⁵²Eu and ¹³⁷Cs was added and the SLX was carried out as before.

All gamma-spectroscopic measurements were carried out using HPGe detector (make CANBERRA) of 2.7 keV resolutions at 1332 keV along with DSA-1000. The energy as well as efficiency calibration of the detector was carried out with the standard ¹⁵²Eu (T_1/2 = 13.33 years) source of known activity. The GENIE 2k software was used for gamma spectroscopic analysis.

Results and discussion

The synthesis of ChG is shown in Scheme 1. The IR spectrum of Chitosan and ChG was represented in Fig. S1(a) and (b)† respectively. Chitosan (Fig. S1(a)†) shows a broad band in 3400–3800 cm⁻¹ due to stretching vibration of O–H and N–H groups, amide (amide I) displays its stretching vibration at 1655.26 cm⁻¹, N–H (amide II) angular deformation at 1600.8 cm⁻¹, symmetrical angular deformation of CH₃ at 1383.16 cm⁻¹, C–N stretching vibration at 1422.0 cm⁻¹ and C–N amino groups stretching frequency at 1324.86 cm⁻¹ and stretching vibration of C–O–C at 1079.9 cm⁻¹.

Scheme 1 Synthesis of ChG.

The ChG (Fig. S1(b)†) exhibits a strong characteristic absorption band at 1636.3 due to C=N bonds of imine and stretching vibration of bridged C–O–C bond at 1073.3 cm⁻¹.^19,20 The IR spectroscopy confirms the Schiff base formation in the hydrogel by the absorption peak at 1636.3 cm⁻¹.

The ¹³C CP-MAS NMR spectrum is revealed in Fig. S2.† Due to the poor solubility of the hydrogel, solid-state ¹³C CP-MAS NMR spectra were performed. At 103.49 ppm a resonance of the C₁ carbon of the glucosamine unit is observed, while at 95 ppm a much less intense signal of C_1′ of the acetylglucosamine unit is observed, and at 171.68 ppm (ref. 21) the most significant peak for C₇ carbon²² is found which shows the Schiff base formation in the hydrogel.

Strain and frequency-sweep experiments were performed to know the dynamic mechanical characterization for understanding the formation mechanism of the hydrogels and consequently their possible applications. So, we have to know mainly the storage modulus (G′) and loss modulus (G′′), which measure, respectively, the elastically stored energy and the energy lost as heat within the hydrogel on application of a shearing force. Fig. 1 represents storage modulus (G′) and loss modulus (G′′) vs. strain% at frequency 1 Hz and the most important observation is that the G′ is substantially larger than G′′. So, storage modulus (G′) is higher than loss modulus (G′′) and it never intersects each other throughout the applied frequency range, indicating that the gel shows the predominant property like a visco elastic solid²³ that means the stronger gel intensity. These are the characteristic features of a “strong gel”.

Fig. 1 Rheological study of ChG.

The damping factor can be defined as G′′/G′, calculated from the rheology data, was between 2 × 10⁻² and 8 × 10⁻¹. The lower the damping factor of a hydrogel higher will be its elasticity,²⁴ which implies that these chitosan hydrogels are highly elastic.

Hydrogels are cross-linked 3-dimensional networks with various morphologies such as fibers, tape, twisted ribbons, coli flower, globules, etc. The SEM image makes it clear that there is sufficient difference in the surface morphology between chitosan (Fig. 2(a)) and the ChG (Fig. 2(b)). Chitosan has a smooth, dense and flat morphology while ChG hydrogel has a rough fiber like structure.^25,26 The rough surface morphology of ChG hydrogel which is evident from its SEM image may be due to the cross-linking of glyoxal with chitosan. This rough surface of ChG is conducive to adsorption.

Fig. 2 SEM image of (a) Chitosan (b) ChG.

Thermogravimetric and differential thermogravimetric (DTG) plots for ChG hydrogel are displayed in Fig. 3. For the ChG hydrogel, thermal degradation takes place in two stages. The first stages of degradation takes place from 41–156 °C with a weight loss of 12.7% could be due to the loss of both the loosely bound water and tightly bound water. The free water and the hydrogen bonded water are released in the temperature region 41–100 °C. The hydrogel contains many hydrophilic groups that hold water more tightly in the backbone^27–29 of hydrogel through polar interaction. So, it is more difficult to lose. So, this tightly bound water is released in the temperature region 100–156 °C. The second stage of degradation of the hydrogel started at 156 to 433 °C with weight loss of 59%. This phase of the weight loss mainly could be caused by a series of thermal and oxidative decomposition in the process including the sugar ring dehydration, degradation, molecular chain acetaminophen and N-deacetylation of the cracking unit of chitosan and vaporization and elimination of volatile products. It is further stated that pyrolysis of polysaccharides starts by a random split of the glycosidic bonds, followed by further decomposition forming acetic and butyric acids and a series of lower fatty acids, where C₂, C₃, and C₆ predominate. As the ChG is absolutely stable at room temperature it has been successfully employed for the separation of ¹⁵²Eu and ¹³⁷Cs using SLX technique under ambient conditions.

Fig. 3 Thermogravimetric analysis of ChG.

The DTG curve shows its maxima at about 219 °C. This is probably due to the formation of C=N and this proves that biopolymer based Schiff base is thermally less stable.²

Fig. 4 shows the DSC thermal pattern of ChG hydrogel. The thermal analysis was performed from −50 °C to 150 °C and from 150 °C to −50 °C and ChG shows a sharp melting endothermic peak at 60.08 °C. From this we can see that water evaporation occurred during the first DSC run and these results confirm the previous result, obtained by TGA study and it indicates that the samples were not completely anhydrous implying some bound water was not completely removed. The water holding capacity of cross-linked polymeric network in ChG is higher^28,30,31 as ChG contains lots of hydrophobic –OH group, resulting in a strong hydrogen bonding.

Fig. 4 Differential Scanning Calorimetry of ChG.

We can also obtain glass transition temperature (T_g) from DSC. The glass transition temperature is a controversial aspect because during the course of formation of ChG we used natural biopolymer chitosan and it was found that some properties of chitosan such as crystallinity, molecular weight and degree of deacetylation show wide variations according to the source and method of extraction and thus influence the T_g. For ChG T_g and crystallization transition temperature (T_c) were −18.57 °C and 26.27 °C respectively.

The powder XRD pattern of chitosan and ChG are represented in Fig. 5. Chitosan exhibit a sharp crystalline peak at 20.06° and one of low intensity peaks at 36.99°.³² The Crystallisation Index (CrI) value of chitosan is 69.58 being a semicrystalline polymer.³³ In case of ChG incorporation of the glyoxal cross-linker into chitosan moiety severely weakened and broadened the intensity of both the peaks and calculated CrI of ChG was 61.82 suggesting the amorphous nature of ChG as chitosan destroyed free NH₂ groups via crosslinking and as a consequence a large number of hydrogen bonds in the chitosan powder was also destroyed. Thus regularity of the packing of the original chitosan is efficiently destroyed and lead to the formation of amorphous hydrogels, ChG.³⁴

Fig. 5 Powder XRD patterns Chitosan and ChG.

pH dependent swelling studies

To check the sensitivity of ChG hydrogel, the swelling of the ChG hydrogel was studied by varying pH at room temperature. Fig. 6 shows the pH dependent swelling behaviors of the hydrogels. From the figure it is evident that hydrogels show a lower swelling at basic pH as compared with acidic pH. At low pH high concentration of charged ionic groups in the hydrogel increases swelling due to osmosis and charge repulsion and at higher pH swelling decreases as the degree of ionization of hydrogel bound groups' decreases. At low pH unreacted amino groups of ChG converted into the ammonium ion and thus the ammonium ion would be attached to the hydrogels by ionic bonds. Therefore, swelling of the hydrogels increased in acidic buffer.

Fig. 6 pH dependent swelling studies of ChG.

At high pH, the unreacted amino group of ChG remains intact, resulting in lower swelling ratio.^35–37 From this we can conclude that greater number of unreacted amino groups is an important criterion for higher swelling ratio.

Radiochemical separation

The biopolymer chitosan based hydrogel is stable at the pH range of 1–6. Therefore, extraction behaviour of long-lived fission products such as ¹⁵²Eu and ¹³⁷Cs was studied using SLX technique at the pH range of 1 to 6 using aqueous HCl solution as liquid phase against 0.1 g of hydrogel as solid phase and result is shown in Fig. 7. Also, there is possibility of formation of Eu hydroxide in high pH environment, which also justifies that the acidic condition is best suited for present study.

Fig. 7 SLX profile of ¹⁵²Eu and ¹³⁷Cs in solid phase (ChG) by varying pH of liquid phase against 0.1 g of ChG.

Fig. 8 SLX profile of ¹⁵²Eu and ¹³⁷Cs in solid phase (ChG) by varying weight of hydrogel at pH 4.

It was observed from the profile that extraction of ¹⁵²Eu increases with increase in pH of liquid phase. It was also observed that the ability of extraction of ¹³⁷Cs was very low and it was almost zero throughout the whole pH range of liquid phase. On the other hand, maximum extraction of ¹⁵²Eu at pH 4 was ∼61% into solid phase without any contamination by ¹³⁷Cs. This could be due to: (i) the formation of [Eu(H₂O)]⁺³ complexes at pH ∼ 4. (ii) At lower pH the H⁺ ions compete with the metal ions for occupancy of the active site of the hydrogel.⁸ Therefore, the extraction profile of ¹⁵²Eu and ¹³⁷Cs indicating the cation exchange behaviour of the hydrogel at pH ∼ 4. The extraction profile also nicely corroborates with our earlier studies.^38–40

In some studies, the weight of the adsorbent was increased to see the effect of ¹⁵²Eu extraction on the solid phase. It was observed that by increasing the amount of the hydrogel from 0.1–0.4 g at pH 4, the extraction of ¹⁵²Eu also increased. However, the extraction of ¹³⁷Cs in the solid phase was almost the same with the increasing amount of hydrogel and it was almost zero. The maximum extraction of ¹⁵²Eu was ~87% with 0.4 g of hydrogel at pH 4 (Fig. 8). This could be due to the presence of more cation exchange sites for ¹⁵²Eu extraction with the increased amount of hydrogel.

Studies were also carried out by varying the shaking and settling time and results are shown in Fig. S3(a) and (b).† It is seen from the plot that there is small effect on the shaking and settling. When the shaking and settling time was 10 min, the extraction of ¹⁵²Eu was ∼87% and ¹³⁷Cs was almost 0%. After increasing shaking and settling time more than 10 min, there was no definite change in the extraction.

After removing ¹³⁷Cs into liquid phase from the Eu and Cs mixture using SLX technique, back extraction studies of solid phase containing ¹⁵²Eu was carried out to obtain pure ¹⁵²Eu in liquid phase. In this process, solid phase was mixed with 3 mL of 1 M HCl as liquid phase. About 72% of ¹⁵²Eu was back extracted from solid phase into liquid phase in a single run. Back extraction of ¹⁵²Eu at high concentration of HCl region (1 M) could be due to the formation of [EuCl₂]⁺ complexes.

Distribution ratios (D) and separation factors (S) of ¹⁵²Eu and ¹³⁷Cs in various experimental conditions were calculated (Table 1).

Table 1 Separation factor of ¹⁵²Eu & ¹³⁷Cs

Experimental Condition	DEu	DCs	SEu/Cs
Solid phase: 0.1 g ChG hydrogel	1.61	1.45 × 10^−4	1.11 × 10^4
Liquid phase: 3 mL of 10^−4 M HCl
Solid phase: 0.4 g ChG hydrogel	6.48	1.38 × 10^−4	4.70 × 10^4
Liquid phase: 3 mL of 10^−4 M HCl

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The initial concentration study is very significant because the initial concentration of Eu in solute can strongly affect the adsorption kinetics and more precisely, the mechanism that controls the overall kinetic coefficient. Therefore, we spiked ¹⁵²Eu radiotracer with bulk quantity of Eu and studied the uptake of Eu as a function of Eu concentration. Fig. 9 shows the residual uptake of Eu by ChG at aqueous HCl solution of pH 4 as a function of initial concentration. The maximum adsorption was observed at lower concentration and reaching equilibrium when we increase the concentration of Eu. This could be due to the fact that, at low initial concentration the gradient between the Eu solution sample and the center of particle enhances the adsorption of Eu in the crosslinked porous network of the ChG.¹¹ Eventually, when the initial concentration is higher and after a certain time, the adsorbed Eu starts to block the pores near the outer surface so Eu can no longer be adsorbed to the active sites of the ChG and reaches equilibrium.

Fig. 9 SLX profile of Eu in solid phase (ChG) by varying the concentration of Eu at pH 4.

Adsorption isotherm

Adsorption isotherm is very significant in describing how the initial concentration of Eu strongly affects with the use of ChG as an adsorbent. So, it is necessary to correlate isotherm data by experimental or by theoretical equations which is crucial for the operating of adsorption systems and practical design. The well-known Freundlich isotherms were used to analyze the adsorption isotherm. Fig. 10 shows the adsorption equilibrium isotherms obtained for Eu by ChG. The Freundlich isotherm^41,42 models were used to study the adsorption isotherm. The linear from of Freundlich isotherm is expressed as:

log(X/m) = log K_f + (1/n) log C_e

Fig. 10 Freundlich isotherm for adsorption of Eu by ChG.

This isotherm is frequently used in special cases of heterogeneous surface energy which is characterized by the heterogeneity factor 1/n where X and m are the mass of adsorbate and adsorbent respectively. So, X/m is the adsorbate concentration in solid phase at equilibrium (mg g⁻¹), C_e is the equilibrium concentration of the adsorbate in solution at (mg L⁻¹), K_f is Freundlich constant. Therefore, K_f and 1/n can be determined from the plot of log(X/m) vs. log C_e. The calculated results are listed in Table 2.

Table 2 Freundlich isotherm model constants and correlation coefficient

Isotherm	Kf	1/n	R^2
Freundlich	2.5	1	1

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Attempt has been made to fit the data with other isotherm, but minimum correlation was obtained in these cases.

Effect of γ-irradiation on adsorption

Fig. 11 shows the extraction behaviour of ¹⁵²Eu and ¹³⁷Cs on gamma irradiated ChG. ChG has three-dimensional network structure. Figure shows that the extraction of ¹⁵²Eu is increased (∼83%) upto 1 kGy radiation and then decreased with the increase of radiation dose but ¹³⁷Cs remain below 4%. This trend is possibly due to the increase of macromolecular network upto 1 kGy radiation dose. Above 1 kGy, breakdown of the macromolecular network structure (Fig. S5(a)–(c)†) is possibly responsible for lower extraction of ¹⁵²Eu radionuclides in the ChG.⁴³ It is noteworthy to mention that absorbed dose in the polymer network due to the presence of radioisotopes (i.e., ¹⁵²Eu and ¹³⁷Cs) is negligible.

Fig. 11 SLX profile of ¹⁵²Eu and ¹³⁷Cs in solid phase (ChG) of γ-irradiate ChG at pH 4.

Conclusions

The synthesis of this hydrogel is easy and much faster compared to the other reported methods. Rheological study shows, the ChG demonstrate its property like a viscoelastic property with high elasticity and is also fairly radiation stable. We have developed a rapid, simple, environmentally benign and cost effective method for separation of two long-lived fission products, ¹⁵²Eu and ¹³⁷Cs using this biopolymer based ChG.

Acknowledgements

Part of this work has been carried out under the support of SINP-DAE 12^th five year plan project TULIP. Authors also want to thank Prof. Abhijit Saha of UGC-DAE consortium, Kolkata centre for his help in gamma irradiation studies. Arpita Datta and Santu Maity are also thankful to Council of Scientific and Industrial Research for providing CSIR-RA fellowship and University Grant Commission (UGC) for providing UGC-BSR fellowship respectively.

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Footnote

† Electronic supplementary information (ESI) available: IR spectra, NMR spectra, SLX profile of ¹⁵²Eu and ¹³⁷Cs in solid phase (ChG) by varying shaking time and settling of the medium. See DOI: 10.1039/c5ra14976b

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