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Adsorption of 60Co and 154+152Eu Using Graft Copolymer of Starch-Polyacrylic Acid-Polyvinylsulfonic Acid

Starch-polyacrylic acid-polyvinylsulfonic acid (St-g-PAA-PVSA) graft copoymer was synthesized by gamma radiation as an initiator. The chemical structure, morphology, and thermal stability of the graft copolymer were investigated using FTIR, SEM, and TGA. The St-g-PAA-PVSA graft copolymer was employed as an adsorbent for the removal of Co(II) and Eu(III) radionuclides from their aqueous solutions by batch adsorption method. Several experimental factors such as pH, contact time, initial concentration of adsorbate, and temperature were used to find the best conditions for the removal of Co(II) and Eu(III) radionuclides. The pseudo-second order kinetic model better fits the adsorption kinetic data of radionuclides. Langmuir models had the ability to well describe the equilibrium data of adsorption of radionuclides. Thermodynamic parameters were calculated and suggested the adsorption process of Co(II) was endothermic while exothermic in the case of Eu(III) adsorption and both adsorption systems were spontaneous in nature. Among the examined desorbing agents, both AlCl3 and HCl were succeeded to desorb most of the radionuclides.


Introduction
Recently, many papers have shown an increasing interest in modifying the renewable source-based biodegradable polymers as a substitution of conventional synthetic materials because they are non-toxic, biodegradable, cheap, and easy to develop [1,2]. Starch (St) is a polysaccharides polymer and has advantages above other biodegradable polymers such as its low cost, wide using and total composability without toxic remains. Because of the drawbacks of St such as lowly process capacity and poor long-standing stability, high sensitivity to water, and weak mechanical properties [3][4][5][6]. Many study attempts had been utilized both in industrial and academic organizations to solve these problems through chemical modification or cross-linking St. Also, modification of St was tested with various synthetic monomers, e.g., acrylamide, methacrylamide, acrylic acid, vinyl imidazole, acrylonitrile, vinyl alcohol, styrene [6][7][8]. El-hoshoudy and Desouky synthesized the acryloylated St then it's grafted by poly(acrylamide-vinylmethacrylate/1-vinyl-2-pyrrolidone) terpolymer in presence of diallylamine and dimethylphenylvinylsilane as a crosslinker through emulsified polymerization process [9]. Xiong et al., prepared St-based wood adhesive by grafting vinyl acetate and butyl acrylate with different ratios [10]. Abdelmonem et al., created and investigated a low-cost St-AA-VSA/f-MWCNTs nanocomposite [11]. Worzakowska prepared the St grafted-terpene acrylate by free radical polymerization [12]. Wang et al. synthesized a ternary flocculants based on St, acrylic acid, and chitosan by radical reaction [13]. Işıklan and Geyik synthesized novel temperature and pH-sensitive graft copolymer of κ-carrageenan with N,N-dimethylaminoethyl methacrylate as well as acrylic acid using 4,4-Azobis (4-cyanovaleric acid) under microwave irradiation [14]. Mittal et al. prepared polyvinyl alcohol/St and cellulosic material barley husk based composite films [15]. Superabsorbent polymers consisting of St, acrylic acid, acrylamide, poly(vinyl alcohol), 2-hydroxyethyl methacrylate, and 2-acrylamido-2-methylpropane sulfonic acid were produced and characterized by Czarnecka and Nowaczyk [5,7].
Polyacrylic acid (PAA) is a high-absorbency polymer that can absorb and retain water while swelling to many times its original volume. It has been employed in water treatment as a result of this unique property [5,7,11,16,17]. Polyvinylsulfonic acid (PVSA) (as sodium salt) is a polyelectrolyte with negatively charged sulfonate groups. Vinylsulfonic acid (VSA) is a monomer used to make extremely acidic and anionic homopolymers and copolymers. These polymers are employed as photoresists and ion-conductive polymer electrolyte membranes for fuel cells in the industry. Polyvinylsulfonic acid, for example, can be used to make translucent membranes with high ion exchange capacity and proton conductivity [5,7,11,[16][17][18][19].
Radioactive materials are being used in a growing number of industrial investigations, including industrial radiography, nuclear medicine, agriculture, and oil production, as well as academic research. Therefore, radioactive waste management becomes a worldwide problem. 60 Co(II) and 152+154 Eu(III) radionuclides are among the most dangerous radionuclides. These isotopes emit γ-rays of high energies and have long half-lives. They may cause dangerous human disorders. As a result, removing such nuclides from radioactive wastewater is critical. Various technologies are existing to remove the radionuclides such as ion exchange, chemical precipitation, flocculation, coagulation, and reverse osmosis. Unfortunately, these technologies' generality is expensive and/ or environmentally unfriendly. Adsorption has been popular as a method for eliminating radionuclides because of its many benefits, including high efficiency, environmental friendliness, and low cost [11,[20][21][22][23]. There is little research that has been interested in St and its derivatives as adsorbents for the removal of radionuclides and radioactive waste treatment [11,24,25]. The goal and novelty of this study are to develop and test a low-cost and new starchgrafted poly(acrylic acid-vinylsulfonic acid) (St-g-PAA-PVSA) graft copolymer for removing Co(II) and Eu(III) radionuclides from the radioactive waste under various experimental settings. The physical and chemical characterization of the St-g-PAA-PVSA can aid in determining the sorption mechanism as well as provide information on heat stability.

Materials
St was obtained from Nice Chemical Pvt. Ltd. (India). Co(II) chloride, Eu(III) oxide. N, N′-Methylenebisacrylamide (NMBA), and acrylic acid (AA), were purchased from Merck (Germany). Vinyl sulfonic acid sodium salt solution monomer (VSA) was purchased from Aldrich. Methanol was obtained from ADWIC, Egypt. Bi-distilled water was used in all experiments, for preparation, dilution, and analytical purposes. all chemicals and reagents were of the highest purity grade.

Synthesis of St-g-PAA-PVSA Graft Copolymer
St-g-PAA-PVSA graft copolymer was synthesized by dispersing 2.50 g of St in 40 mL bi-distilled water and heated at 80 °C in a water bath until a homogeneous solution was obtained. Then, 8.75 g of AA, 3.75 g of VSA monomers, and 0.25 g of NMBA were added to the suspension via stirring and the mixture was completed to 60 mL and sonicated for 30 min. The mixture was subjected to gamma irradiation at dose of 20 KGy and room temperature using a 60 Co -γ -ray field (Co -60 gamma cell of type MC -20, Cyclotron Project, Inshas, Egypt). The sample was then cut into small pieces and washed in a water-methanol solution. The grafted material was separated, filtered, and dried at 75 °C overnight to a constant weight.

Characterization
The structural features of the St-g-PAA-PVSA graft copolymer were released by Fourier transform infrared analysis (FT-IR). The FT-IR spectrum was recorded in the midinfrared range (4000-400 cm −1 ) with 4 cm −1 resolution using a Shimadzu infrared spectrometer (BOMEM, FT-IR, Japan) by KBr disc method with 98:2% of KBr: polymer concentration and the number of averaged scans equal 16. The morphological structure was obtained by scanning electron microscope (SEM) (JEOL -JSM 6510 LA, Japan) at high magnification and resolution. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed at a heating rate of 20 °C/min using a Shimadzu DTG -60 thermal analyzer, Japan.

Adsorption Studies of Radioactive Isotopes
The adsorption behavior of Co(II) and Eu(III) radionuclides toward the St-g-PAA-PVSA graft copolymer was investigated using the batch technique. 0.01 g of the graft copolymer was equilibrated with 5 mL of the solution containing a definite amount of 60 Co(II) or 152+154 Eu(III) radionuclides stock solution, individually, by mechanical shaking at 25 ± 1 °C for 24 h. Aliquots of the solution before and after the equilibration were taken, and the radioactivity was assayed radiometrically by a single-channel analyzer (Spetech ST 360 to crystal, USA). Removal percentage of the radionuclides was calculated using the following equation [22]: The removal capacity (q) (mg/g) was obtained from: where C o and C e (mg/L) are the beginning and the final radionuclides concentration, V is the volume of the radionuclide solution (L), and m is the adsorbent mass (g).

Desorption Experiments
Desorption investigations were carried out in the order to recover and reuse adsorbents in a variety of adsorption processes. 0.05 g of the St-g-PAA-PVSA graft copolymer was contacted with 5 mL of 250 mg/L of Co(II) and Eu(III) ion, individually, spiked with 60 Co(II) and 152+154 Eu(III) radionuclides at pH ~ 5 and ~ 4, respectively, for 24 h. The Co(II)-loaded St-g-PAA-PVSA graft copolymer and Eu(III) loaded St-g-PAA-PVSA graft copolymer were separated by centrifugation and dispersed into 5 mL of the desorbing agent (0.001-0.1 M of HCl, NaCl, MgCl 2 , and AlCl 3 ) and leave under shaking at 120 rpm for 24 h. the Co(II) and Eu(III) radionuclides were measured radiometrically in the supernatant. The desorption percentage of the concerned radionuclide was calculated using the following equation.

Synthesis of St-g-PAA-PVSA Adsorbent
The starch grafted-vinyl monomers mechanism was discussed in [26][27][28][29][30][31]. In this study, the St-PAA-PVSA graft copolymer was synthesized by free radical polymerization using gamma irradiation in the presence of NMBA. When water is irradiated, three primary reactive species emerge: hydrated electrons, hydroxyl radicals, and hydrogen radicals. When the aqueous solution of the St, AA and VSA is irradiated by gamma radiation, the hydroxyl radicals are created from water. The hydroxyl radicals abstract hydrogen atoms from St backbone, resulting in the formation of macroradicals. The scheme shown in Fig. 1 is based on a fact that the C1-C2 (end groups) and C2-C3 are predominant sites for the graft copolymerization initiation [26].

FT-IR
FT-IR spectrum of the St and the St-g-PAA-PVSA graft copolymer is shown in Fig. 2. In the FT-IR spectrum of the St, the absorption band at 1386 cm −1 is the characteristic PVSA graft copolymer contain irregular morphology with different size granules and appeared as a porous surface with interconnected pores. This irregular morphology may be caused by exposure to heat during the production of the graft copolymer. Heat and moisture can generate a minor gelatinization of the granule's surface, causing the granules to stick together and form aggregates [34]. In addition, irregular shape promotes the production of sufficient holes due to the binding of hydroxyl groups, which allows hydrogen and covalent connections to form between St chains [35]. Rough surfaces of the graft copolymer lead to enhancement of its surface area and increase the interaction between the graft copolymer and the radionuclides and decrease the diffusion limitations in radionuclides adsorption [11]. Figure 4 shows the TGA and the TDA of the St and the St-g-PAA-PVSA graft copolymer. The St shows three-step thermal degradation. The first step occurs in the temperature range 81-172ºC with 11.82% weight loss due to dehydration and pyrolytic volatilization processes. The second and the third stages are consecutive overlapping steps that occurred in the temperature range 275-490ºC with 72.53% weight loss. This is attributed to loss by depolymerization and oxidation of the organic matter. St-g-PAA-PVSA graft copolymer shows the fifth decomposition stage, the first stage continued up to the temperature of 200 °C and count for about 7.15% weight loss. This corresponds to a loss of moisture.

Batch Adsorption Optimization
Effect of pH At low pH, the surface charge of the graft copolymer will become positive and competition between H + and Co(II) or Eu(III) radionuclides for occupancy of the active sites was increased. Whereas at higher pH values the surface charge of the graft copolymer becomes negative, increasing radionuclides adsorption. The adsorption capacity of Eu (III) is higher than that of Co(II), this may be attributed to the adsorption capacity values are proportional to the ionic potential, i.e., Z/r of ions; along with other factors, especially in the case of Eu(III) (as an f-block element) whose coordination bonds are predominantly electrostatic. Consequently, it can be expected that the adsorption capacity of Eu 3+ ions should be higher than that of Co 2+ ions. [22,23,36]. Figure 6 shows the influence of the contact time on the adsorption of Co(II) and Eu(III) radionuclides. The results showed that adsorption has occurred rapidly. Then the adsorption nearly remains at a constant with increasing time, the adsorption was appeared to proceed rapidly when the numbers of available sites are larger than the number of adsorbed radionuclides. Lagergren's pseudo-first-order (Eq. 4), pseudo-secondorder (Eq. 5), and Intra-particle diffusion models (Eq. 6) were applied to describe the adsorption process of Co(II) and Eu(III) radionuclides onto St-g-PAA-PVSA graft copolymer [37].

Adsorption Kinetics
where qe (mg/g) is the amount of absorbed radionuclides at equilibrium, qt (mg/g) is the amount of absorbed radionuclides at t, t is the time. k 1 (min −1 ), K 2 (g/mg min) and K diff (mg/ g min 0.5 ) are the constants of pseudo-first order, pseudo-second order and intra-particle diffusion models, respectively. m is denoting the adsorption mechanism for the intra-particle diffusion to be the rat-determine step, the value of m should be equal to 0.5. Figure 7 depicted the nonlinear fitting of the experimental kinetics data. The calculated kinetic data of the adsorption of Co(II) and Eu(III) radionuclides were given in Table 1. According to the high correlation coefficient (R 2 ), it can be observed that the experimental data for the adsorption of 60 Co(II) was closed to pseudo-second-order model (R 2 = 0.907) more than intra-particle diffusion model (R 2 = 0.727) and pseudo-first-order (R 2 = 0.701).
(4) q t = q e 1 − exp −k 1 t (5) q t = q 2 e k 2 t 1 + q e k 2 t (6) q t = k diff t m In the case of the Eu(III) adsorption, the pseudo-firstorder model and the pseudo-second-order model both have high R 2 values of 0.922 and 0.925, respectively, indicating the simultaneous occurrence of physical diffusion and chemical adsorption. A higher R 2 (0.925) and lower stander error (SE) (0.146) confirmed that the adsorption process predominantly followed the pseudo-second-order kinetic model. It's worth noting that on the surface of an adsorbent, both physisorption and chemisorption can occur at the same time since a layer of molecules can be physically adsorbed on top of an underlying chemisorbed layer [38]. The intraparticle diffusion model for both radionuclides demonstrated the least fitting to experimental kinetic data. However, the m value, not equal to 0.5 for both systems implies that the intra-particle diffusion mechanism does not solely limit the overall adsorption process.

Adsorption Isotherms
The adsorption isotherm is helping in understanding the interaction between the graft copolymer and the radionuclides in the adsorption process. Figure 8 shows that the adsorption capacity of 60 Co(II) and 152+154 Eu(III) radionuclides increase by increasing the initial concentration of radionuclides. It is maybe a result of an increase of driving force for mass transfer at high initial concentration.
where Ce is the concentration of radionuclides at equilibrium in solution (mg/L), qm is the maximum capacity of monolayer coverage (mg/g), and K L (L/mg), K F (mg 1−n L n /g), K T (L/g) and K DR (mol/J) 2 are constants Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models. n represents a constant of the adsorption intensity. B(J/mol) = RT/b, b is related to the heat of adsorption, R is the universal gas constant (8.314 J /mol K), and T is the absolute temperature (K), ε (J mol −1 ) is the Polanyi potential which is related to equilibrium.   Figure 9 shows non-linear fits of the experimental isotherm data. The parameters of the studied isotherm models were summarized and are listed in Table 2. The high correlation coefficient (R 2 ) value indicated that the experimental data for the adsorption of 60 Co(II) and 152+154 Eu (III) radionuclides by the St-g-PAA-PVSA graft copolymer follow the Langmuir model better than other models. According to the Langmuir model, adsorption happens by monolayer adsorption onto a homogenous surface.

Thermodynamic Studies
The effect of temperature on the adsorption of Co(II) and Eu(III) radionuclides onto the St-g-PAA-PVSA graft copolymer was determined by varying the temperature between 20 and 70 °C. Figure 10 shows that the removal capacities of Co(II) radionuclides slightly increase with the increasing temperature this may be attributed to decreases the viscosity of the solution and correspondingly increases the diffusion rate of the radionuclides within the pores of the graft copolymer by increasing temperature. While the removal capacities of Eu(III) radionuclides slightly decrease, showing that the sorption reaction is exothermic.
Thermodynamic parameters such as the adsorption standard free energy changes (ΔG o ), the standard enthalpy change (ΔH o ) and the standard entropy change (ΔS o ) are obtained from experiments at various temperatures using the following equations: [39,40,40,41] where K L (L/mg) is the Langmuir constant. The values of ΔH o (kJ/mol) and ΔS o (kJ/mol K) can be calculated from the slopes and the intercepts of the linear straight lines by plotting lnK L against 1/T of Arrhenius reaction. The values of ΔG o (kJ/mol) can be calculated from Eq. (11). the values of ΔH°, ΔS°, and ΔG° are given in Table 3.
In case of the adsorption of Co(II) radionuclides: -The positive value of ΔH° indicated that the adsorption was endothermic. -The positive value of ΔS o for Co(II) radionuclides indicated that the randomness is increased at the solid-solute interface, In case of the adsorption of Eu(III) radionuclides: -The negative value of ΔH° indicated that the adsorption was exothermic. -The negative value of ΔS o for Eu(III) radionuclides assumed is the arrangement of Eu(III) radionuclides is shaped more ordered onto the surface of the graft copolymer after adsorption The negative value of ΔG o for the adsorption of both radionuclides indicated the feasibility of the reaction and spontaneous nature of the adsorption at a given temperature.  Figure 11 illustrates the influence of desorbing agent concentrations (0.001-0.1 M of HCl, NaCl, MgCl 2 and AlCl 3 ) on the desorption percentage of Co(II) and Eu(III). the data in the figure show that the desorbing agent concentration played a significant role in the desorption of Co(II) and Eu(III) radionuclides. Increasing the desorbing agent concentration, resulting in an increase in the desorption percentage of Co(II) and Eu(III) radionuclides. The maximum desorption percentage of Co(II) radionuclides was ~ 59.60     Fig. 11 Desorption of Co(II) and Eu(III) radionuclides loaded onto St-g-PAA-PVSA graft copolymer using various desorbing agents and ~ 54.72% which was achieved by ≥ 0.1 M of AlCl 3 and HCl, respectively. While The maximum desorption percentage of Eu(III) radionuclides was ~ 63.72 and ~ 63.05% which were achieved by ≥ 0.1 M of AlCl 3 and HCl, respectively. These desorption results proposed that the radionuclides are adsorbed onto the active sites of the graft copolymer. While the fraction remained onto the adsorbent, 45-40% of radionuclides could not be desorbed, which clarified that an insignificant amount of radionuclides are adsorbed onto the internal adsorption sites [42,43].

Conclusion
The St-g-PAA-PVSA graft copolymer was successfully synthesized and its chemical structure, morphology and thermal stability were investigated by FTIR, SEM, and TGA. The St-g-PAA-PVSA graft copolymer was applied as an adsorbent for Co(II) and Eu(III) radionuclides from their aqueous solutions. Modeling of the kinetic data showed that the pseudo-second-order model was the best one for describing the Co(II) radionuclides adsorption process. In the case of the adsorption of Eu(III) radionuclides, both pseudo-first order and pseudo-second order kinetic models better fit the adsorption kinetic data. But a higher R 2 (0.925) and lower SE (0.146) confirmed that the adsorption process predominantly followed the pseudo-second-order kinetic model. Langmuir isotherm models had the ability to represent the equilibrium isotherms data. Thermodynamic parameters deduced that Co(II) radionuclides was endothermic while and Eu(III) radionuclides was exothermic and both adsorption systems were spontaneous in nature. At concentrations of 0.1 M, AlCl 3 and HCl obtain a maximum desorption percentage of Co(II) radionuclides of about 59.60 and 54.72%, respectively, whereas AlCl 3 and HCl achieve a maximum desorption percentage of Eu(III) ions of about 63.72 and 63.05%, respectively.

Conflict of interest
The authors have not disclosed any competing interests.
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