63 Characterization and Application of Nanomaterials (2021) Volume 4 Issue 1 doi:10.24294/can.v4i1.1291 Original Research Article Preparation and characterization of magnetic graphene oxide nano- composite (GO-Fe3O4) for removal of strontium and cesium from aqueous solutions Sule Aytas1, Sabriye Yusan1*, Senol Sert1, Cem Gok2 1 Institute of Nuclear Sciences, Ege University, 35100 Bornova, Izmir, Turkey. E-mail: sabriye.doyurum@ege.edu.tr 2 Faculty of Technology, Department of Metallurgical and Materials Engineering, Pamukkale University, 20160 Kinikli Denizli, Turkey ABSTRACT Magnetic graphene oxide nanocomposites (M-GO) were successfully synthesized by partial reduction co-precipi- tation method and used for removal of Sr(II) and Cs(I) ions from aqueous solutions. The structures and properties of the M-GO was investigated by X-ray diffraction, Fourier transformed infrared spectroscopy, X-ray photoelectron spectros- copy, transmission electron microscopy, scanning electron microscopy, vibrating sample magnetometer (VSM) and N2- BET measurements. It is found that M-GO has 2.103 mg/g and 142.070 mg/g adsorption capacities for Sr(II) and Cs(I) ions, respectively. The adsorption isotherm matches well with the Freundlich for Sr(II) and Dubinin–Radushkevich model for Cs(I) and kinetic analysis suggests that the adsorption process is pseudo-second-ordered. Keywords: Graphene Oxide; Magnetite; Nanocomposite; Strontium; Cesium, Sorption ARTICLE INFO Received: 26 January 2021 Accepted: 12 March 2021 Available online: 18 March 2021 COPYRIGHT Copyright © 2021 Sule Aytas, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ 1. Introduction Nuclear and Industrial Safety Agency (Japan) reported that the Fukushima Daiichi reactor meltdowns have thus far released 15,000 tera becquerels of radioactive cesium-137 into the environment[1]. Also, strontium was measured in plant samples in four others villages, with values ranging from 12 to 61 Bq/kg for Sr-89 and 1.8 to 5.9 Bq/kg for Sr-90[2]. This accident caused a great environmental disaster for living metabolisms and plants. Furthermore, diverse anthropogenic activities like nuclear research reactors, the production and use of radioisotopes and radiopharmaceuticals bring about the spread of radioactive wastes in the environment[3]. Some radionuclides as cesium and strontium are biologically toxic and of great importance due to their long-lasting nature and high solu- bility in aqueous systems[4]. For this reason, it is a significant subject to find out efficient, economic method that can be used in the removal and recovery of cesium and strontium from contaminated environments. Different types of physicochemical methods as ion exchange, chemical precipitation, membrane separation and adsorption, etc. are used for removal and recovery of radionuclides. Considering many parameters, one of these techniques comes to the forefront. Adsorption is wide- ly-used technique that is fast and effective approach in eliminating pol- lutants from aqueous solutions[5]. 64 The most critical point in the development of new adsorption methods is developed new adsorbent materials. Among the previously developed adsor- bents, nanomaterial and especially nano-composites have been received great attention owing to high adsorption capacity, selectivity, high surface area, fast kinetic performances, and reusability for several cycles use[6,7]. Furthermore, nano-engineered mag- netic adsorbents can be widely applied in contami- nant removal due to the magnetism and high surface area. The magnetic particles can be quickly separat- ed from the water after adsorption and this provides easily controlled process[8]. In recent years, graphene oxide nanomaterials, which has a large theoretical surface area and high sorption capacity for the metallic cations, has with wide range of surface oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl[9,10]. According to literature, graphene oxide has a notable affinity toward hard and semi-hard cations like urani- um, thorium, lanthanides, and also strontium. These superior properties make graphene oxide proper for efficient adsorbent of cesium and strontium. Un- fortunately, disadvantage of the graphene oxide is colloidal behavior of its dispersion, which makes separation of its reaction products with metallic cations quite unfairable[9]. Preparation of composite is one of the possible solutions to solve this prob- lem. In this study, magnetic-nano composites were synthesized by using graphene and also magnetite, which is economic and readily available material; on the other side it is not selective. Integration of these two materials solved agglomeration problem and chemical instability in acidic media[11,12]. Preparation of nano-composite material with graphene and mag- netite, not only brings about the chemical resistance, but also increase adsorption capacity. In recent years, combining of magnetite and graphene into nanocom- posites has become an important subject of research due to their new and/or enhanced functionalities that cannot be obtained by either component alone. So, this topic holds a great promise for a wide variety of applications in removal of contaminants from waste- water, surface enhanced Raman scattering, biomedi- cal fields, catalysis, etc.[13,14] In despite of various studies, there is lack of efficient, low cost, secure, high capacity, modifiable, dopeable and reusable technique about cesium and strontium removal from aqueous solution. Based on that, magnetic-nano graphene composites were synthesized and characterized to investigate the re- movability of radiotoxic strontium and cesium ions from aqueous solutions. Adsorption performance of prepared composite by batch experiment was stud- ied by physicochemical parameters. The adsorption isotherm parameters were estimated by linear regres- sion analysis, thermodynamic and kinetic parameters have been also calculated to clarify the adsorption mechanism. The interaction mechanism of cesium and strontium on the magnetic nano-composites was discussed from the experimental results. 2. Experimental procedure 2.1. Reagents and materials The graphene oxide powder was purchased from the Sigma Aldrich. The stock standard solutions of strontium and cesium were prepared by dissolving an appropriate amount of Sr(NO3)2 (Merck) and CsNO3 (Merck) in distilled deionized water. Considering the radioactivity of the 90Sr, non-radioactive 88Sr was used. All reagents used were of analytical reagent grade. 2.2. Instrumental and analytical conditions The strontium and cesium concentration mea- surements were done using a Perkin-Elmer Optima 2000 DV ICP–OES. The shaking was carried out in a thermostated electronic shaker bath (GFL-1083 model). To analyze the characteristics of the M-GO, scanning electron microscope (SEM, COXEM EM- 30), transmission electron microscopy (TEM, FEI 120kV CTEM), Fourier transform infrared spectros- copy (FTIR, PERKIN ELMER SPECTRUM TWO), X-ray diffraction (XRD, Thermo Scientific ARL K-Alpha), X-ray photoelectron spectroscopy (XPS, Thermo Scientific Al K-Alpha), vibrating sample magnetometer (VSM, VSM550-100, Dexing Magnet Tech. Co) and N2-BET adsorption–desorption were 65 determined at 77 K using Micromeritics ASAP 2020. 2.3. Synthesis of M-GO The M-GO nanocomposite was prepared by co-precipitation method, as reported in the liter- ature[15]. 0.05 g graphene oxide powder was used during the synthesis of the nanocomposite. 2.4. Batch adsorption experiments All sorption experiments were performed by the batch technique using 0.01 g of the sorbent sus- pended in 10 mL of strontium/cesium solution in a polyethylene (PE) flask at selected pH (2–9) and pH (2–12) for Sr(II) and Cs(I), respectively. The effects of sorption parameters such as contact time (15–300 min), Sr(II) concentration (10–50 mg/L), Cs(I) con- centration (200–500 mg/L), adsorbent dosage (m/V) ratio (1–10) and temperature (25–45 °C) on the sorp- tion of Sr(II)/Cs(I) were determined by changing a parameter and keeping others constant. The pH was adjusted by adding 0.1 mol/L HCl and NaOH to the solutions at the each experiment. After reaction, the solid and liquid was separated by the magnetic sep- aration method. The concentrations of total Sr and Cs were determined by using ICP-OES. Each exper- iment was repeated three times and average values were used for calculation. The percentage sorption of metal ions from aqueous solution was computed as follows: where Ci and Ce are the initial and final metal ions concentration, respectively. 2.5 Kinetic studies Kinetic studies were carried out in a thermo- stated shaker with polyethylene tubes at room tem- perature. In each run, 0.01 g of M-GO was added to 10 mL of Sr and Cs solution (50, 100, 150 mg/L and 200, 250, 300 mg/L), respectively and adjusted to the desired pH level. The contact time varied from 5 to 180 min. Samples were filtered from each tube at specified time intervals and analyzed for the remain- ing Sr/Cs ion concentrations by ICP-OES. 3. Results and discussion 3.1. Characterization The micro-structure of the Fe3O4-GO(M-GO) was characterized by XRD (Figure 1). The peaks at 2θ values of 30.42°, 35.06°, 43.48°, 53.22°, 57.78° and 63.06° are the characteristic peaks of the Fe3O4 crystal with the cubic spinal structure for magnetic graphene nanocomposites matching well with those from the JCPDS card (19-0629) [16,17]. The small peak at 2θ = 26.5° corresponds to well-ordered graphene layers of GO skeleton and indicates that this way the formation of the magnetic composite as reported by other groups[18]. Generally we can conclude from the XRD pattern, the nanocomposite contains mostly Fe3O4 and it was synthesized with GO. Figure 1. XRD pattern of M-GO. SEM and TEM images of the obtained M-GO nanocomposite are showed in Figure 2 and Figure 3. SEM images confirm the Fe3O4 nanoparticles are attached to the surface of the GO sheet in homoge- neously. Nevertheless, TEM images of the M-GO shows that Fe3O4 nanoparticles are well decorated and clearly observed on the surface of graphene sheet. XPS technique was used to verify the chemical state of M-GO and the results were shown in Figure 4. The wide scan XPS spectra of the M-GO shows the binding energy peaks about 285, 530 and 710 eV, which are attributed to C1s, O1s and Fe 2p, respec- tively[19,20]. In the figure spectrum, the peaks of Fe 2p 3/2 and Fe 2p 1/2 were located at about 711.12 and 724.79 eV, confirming that Fe3O4 was fairly synthe- sized on the GO. 66 The FT-IR spectrum of GO and M-GO is de- picted in Figure 5. The GO sheet showed apparent adsorption bands for the carboxyl groups (stretching vibrations from C=O; 1,716 cm−1), aromatic (C=C; 1,580 cm−1), and alkoxy (stretching vibration from C– O (1,041 cm−1). The intense peak at 1,415 cm–1 can Figure 2. SEM image of M-GO. Figure 3. TEM image of M-GO. 67 be attributed to epoxy groups (C=C vibration) also showed at 1,122, 897 and 799 cm–1 in M-GO spec- trum that related to symmetric stretching, asymmet- ric stretching, and deformation vibrations, respec- tively[18]. For M-GO, band at 1,635 cm−1 is assigned to H– O bending vibration. The peak at 553 cm−1 showed Fe–O bond from Fe3O4 The peak around 1,400 cm–1 can be explained by symmetric vibration of COO- groups which indicates the carboxylate groups of GO coordination with the iron cations[21]. The magnetic properties of the Fe3O4 nanopar- ticles, GO and the M-GO nanocomposite were de- termined at room temperature. The hysteresis loop of magnetite, GO and M-GO composite are shown in Figure 6, where the magnetization hysteresis loops appear S-like, and saturation magnetization is 16.16 and 10.74 emu/g for magnetite and M-GO, respectively. Nevertheless, as shown in the figure, GO sample has no magnetic property. The reduction in the saturation magnetization could be related to existence of GO and impurities on the surface of the magnetite nanoparticles[22]. However, M-GO still could be separated rapidly under the external mag- net. BET analysis was performed to investigate the specific surface area and pore size of the syn- thesized material. The BET surface area and pore size of M-GO was found as 124.37 m2/g and 0.386 nm, respectively. In the literature, Cheng et al. and Hur et al. found the BET surface area for magnetic graphene oxide composites are 111. 8 m2/g and 49.9 m2/g, respectively[23,24]. The results obtained in this study are consistent with the literature. Figure 6. Magnetization versus magnetic field for magnetite, GO and M-GO. 3.2 Adsorption studies 3.2.1 pH effect The pH value of solutions is a determining factor of the removal efficiency because it affects surface charge of the sorbent, and also the degree of ionization and speciation of the metal in solution. The effect of pH on Sr2+ and Cs+ adsorption is shown in Figure 7 (a-b). As can be seen, the strontium and cesium removal on M-GO adsorbent are affected by the pH change of the solution. The maximum stron- tium and cesium uptake were found 40% at pH 4 and 17.92 mg Sr(II)/g and 59.5% at pH 10 and 148.77 mg Cs(I)/g as adsorbent, respectively. For this rea- son, pH 4 and pH 10 were used in subsequent exper- iments for Sr(II) and Cs(I), respectively. In addition, M-GO depicted higher adsorption capacity for Cs+ than for Sr2+ under the same experimental conditions. This can be attributed to the smaller hydrated ionic Figure 4. XPS survey scan spectrum of M-GO. Figure 5. FT-IR spectra of M-GO. 68 radius of Cs+[25]. 3.2.2 Concentration effect The Sr(II) and Cs(I) ions adsorption capacities of the M-GO were given as a function of the initial concentrations of metal ions in Figure 8(a-b). The solution concentration of Sr(II) and Cs(I) was varied in the range 25–125 mg/L and 200–500 mg/L, re- spectively. As shown in Figure 8a, when the strontium con- centration increased, % adsorption value increased but the concentration value decreased from the 50 mg/L. The highest uptake for M-GO adsorbent was calculated as 29.98% at 50 mg/L strontium concen- tration. This concentration was used in subsequent parameter assays. As shown in Figure 8b, when the cesium con- centration increased, % adsorption value increased but the concentration decreased from 250 mg/L concentration to the equilibrium. The highest uptake for M-GO adsorbent was calculated as 57.2% at a cesium concentration of 250 mg/L. In subsequent parameter assays, this concentration has been studied at maximum adsorption. 3.2.3 Effect of dosage (m/V) The adsorption of Sr(II) and Cs(I) ions decreas- es by increasing the ratio of the mass of the M-GO to volume of aqueous phase (m/V) (Figure 9). The highest values for the adsorption was obtained using 0.01 mL Sr/Cs solution and 0.01 g adsorbent (m/ V) = 1 and it was taken as the optimum amount for other experiments for Sr and Cs adsorption. It can be concluded that low amount of nanocomposite can gives higher metals adsorption. Increasing the adsor- bent dose above 1 g/L have a little or no change on Cs(I) removal while it can lead to significant remov- al for Sr(II). Figure 7. a) The effect of pH on the adsorption of Sr(II) ions with M-GO (c: 50 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C); b) The effect of pH on the adsorption of Cs(I) ions with M-GO (c: 250 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C). Figure 8. a) The effect of initial concentration on the adsorption of Sr(II) ions with M-GO (pH: 4, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C); b) The effect of initial concentration on the adsorption of Cs(I) ions with M-GO (pH: 10, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C). 69 Figure 9. The effect of adsorbent dosage on the adsorption of Sr(II) and Cs(I) ions with M-GO (Sr; pH: 4, c: 50 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C: Cs; pH: 10, c: 250 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C). 3.3 Adsorption equilibrium and isotherm models The adsorption process is a mass transfer op- eration that can be described mathematically by equilibrium and a rate process. The equilibrium is established between the concentration of the metal ions dissolved in aqueous phase and that bound to the adsorbent. The data obtained from experimental results is fundamental requirements for the design of adsorption systems. The data are used to develop equations and also to calculate isotherm parameters. By this way, the data, provide some insight into both the sorption mechanism and the surface properties and affinity of the sorbent can be used to compare different adsorbents under different operational con- ditions and to design and optimize an operating pro- cedure[26–28]. In order to analyze the equilibrium data of the adsorption system, experimental data were fitted to Langmuir, Freundlich, Dubinin–Radushkev- ich, Temkin, Flory-Huggins and Brunauer, Emmer & Teller isotherms among the varied models. The constants of isotherm models along with correlation coefficients (R2) have been calculated from the plots for adsorption of cesium and strontium on the com- posite material and the results are given in Table 1. Adsorption equilibrium in the concentration range of 25–125 mg/L and 200–500 mg/L was stud- ied with 10 mg of magnetic nanocomposite at 25 °C, 120 min contact time and pH 4 and 10.0 strontium and cesium, respectively. The Langmuir isotherm, probably the most widely used model, assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface[29]. The linear forms of the this model are expressed by the following equations: where qe is the amount of cesium and strontium ions adsorbed onto adsorbent; Ce is the equilibrium con- centration of these metals in solution, and Q0 and bL are Langmuir constants related to adsorption capac- ity and adsorption energy, respectively. Q0 and bL were calculated from the slope and intercept of linear plots of Ce/qe versus Ce. The Freundlich model has been used to describe adsorption of strontium and cesium from solution onto composite material. This model is not restrict- ed to the formation of the monolayer coverage. It is assumes an empirical expression encompassing the surface heterogeneity and the exponential distribu- tion of the energy of active sites as well as multilayer adsorption. Linear form of Freundlich model can be represented as follows[30]: where KF represents the adsorption capacity (mg/g), nF is a constant related to adsorption intensity (di- mensionless). The data obtained are well described by the Freundlich isotherm equation when plotted as logqe versus logCe (Figure 10). Dubinin–Radushkevich is the other model that used extensively to determine the type of adsorption for the removal of strontium and cesium[31]. This model was used to calculate the apparent free energy of adsorption, proposed an equation to find out the adsorption mechanism on the basis of the potential theory assuming a porous structure of the sorbent and heterogeneous surface. The linearized equation form of the D-R iso- therm is given as: where Cads (mmol/g ) is the amount of solute ad- 70 sorbed per unit weight of solid, Xm (mmol/g or mg/ g) is the adsorption capacity, β (mol/K)2 is a constant related to energy and ε is the Polanyi potential. Po- lanyi potential can be computed by the following equation: where R is a gas constant in kJ/mol and T is the tem- perature in Kelvin. If lnCads is plotted against ε2, β and Xm can be obtained from the slope and intercept, respectively (Figure 11). The adsorption mean ener- gy (E), the free energy change when one mol of ion is transferred to the surface of the solid from infinity in the solution, is assumed by the following equation using the constant β: The Temkin isotherm model contains a factor that obviously assuming adsorbent–adsorbate inter- actions. This model takes into account that heat of adsorption of all molecules in the layer would de- crease linearly rather than logarithmic with coverage by neglecting the extremely low and large value of concentrations[32]. The Temkin isotherm has general- ly been applied as follow: Where, bTe is the constant of Temkin related to adsorption heat (J/mol); aTe is the Temkin isotherm constant (L/mg); R is the gas constant and T is the absolute temperature (K). bTe and aTe constants were calculated from the intercept and slope of straight line of the plot of the qe versus lnCe.. The Flory–Huggins isotherm model was exam- ined to account for the degree of surface coverage characteristics of the sorbate on the sorbent[33,34]. The equation of the isotherm is as follows: Where, Θ is the degree of surface coverage, KFH is the Flory–Huggins model equilibrium constant and nFH is the Flory–Huggins model exponent. Θ is calculated using the following equation: The linearized equation of the model is given as: The constants of isotherm were extrapolated from plots of plot of log(Θ/Ci) versus log(1-Θ), and values of KFH and nFH calculated from the slope and intercept of the plot and are shown in Table 1. Equi- librium constant (KFH) was used for the calculation of spontaneity of the Gibbs free energy (ΔG0) on fol- lowing equation: The negative values of ΔG0 confirmed the fea- sibility of the process and the spontaneous nature of adsorption ontoadsorbent. Brunauer–Emmett–Teller (BET) isotherm mod- el, related to the liquid–solid interface, is a theoret- ical equation, most widely applied in the gas–solid equilibrium systems[35]. This equation is presented as: where CBET, Cs, qs and qe are the BET adsorption iso- therm constants relating to the energy of interaction with the surface (L/mg), adsorbate monolayer satu- ration concentration (mg/L), theoretical isotherm sat- uration capacity (mg/g) and equilibrium adsorption capacity (mg/g), respectively[36]. Linearized equation of the model is as follows: The curve was plotted between Ce/qe(Cs–Ce) and Ce/Cs, and values of both constants qs and CBET were calculated from the intercept and slope. BET isotherm parameter for linear regression analyses and error functions are given in Table 1. The high determination coefficients for linear 71 models show applicability of the model for metals adsorption using the present adsorbent. According to the correlation coefficients, the adsorption of strontium could be well described by Freundlich equation. Freundlich’s model theory is regarded as the heterogeneous adsorption and the exponential distribution of the energy of active sites as well as multilayer adsorption. Dubinin and Radushkevich isotherm provide a particularly good model for the adsorption of cesium. This model has reported that the characteristics of sorption curve is related to the porous structure of the sorbent. According to the results, the maximum adsorp- tion capacities of strontium and cesium were cal- culated as 2.103 mg/g from Freundlich model and 142.07 mg/g from Dubinin–Radushkevich model, respectively (Table 1). The 1/nF value between 0 and 1 indicates that the adsorption is favorable un- der the experimental conditions. As seen in Table 1, cesium adsorption on magnetic graphene composite was found high enough for separation. Moreover, the value of 1/nF is known as heterogeneity factor and ranges between 0 and 1; the more heterogeneous the surface, the closer 1/nF value is to 0. The numerical value of 1/nF (< 1) indicates that adsorption capac- ity is only slightly suppressed at lower equilibrium concentration and the isotherm does not present any saturation of the solid surface of the sorbent by the sorbate[36]. One of the unique features of the Dubinin–Ra- dushkevich isotherm model lies on the fact that it is temperature-dependent, which when adsorption data at different temperatures are plotted as a func- tion of logarithm of amount adsorbed vs the square of potential energy, all suitable data will lie on the same curve, named as the characteristic curve[36]. The calculated E value is used to estimate the reac- tion mechanism of adsorption process. If value of E is smaller than 8 kJ/mol, it indicates a physical adsorption. If value of E is higher than 8 kJ/mol, the adsorption process is of a chemical nature. The E values obtained were 0.016 kJ/mol and 0.129 kJ/mol for strontium and cesium, respectively. Therefore, the magnitudes of E values are in the energy range of physical adsorption for strontium and cesium[7,8]. Table 1. Isotherm constants of models for strontium and cesium adsorption onto M-GO Isotherm models Parameters Strontium Cesium Langmuir Qo (mg/g) 256.410 434.78 bL (L/mg) 0.004 0.035 R2 0.9249 0.2483 Freundlich KF (mg/g) 2.103 29.058 nF 0.772 1.973 R2 0.9987 0.4260 Dubinin–Radushkevich Xm (mmol/g) 0.638 1.069 β (mol/kJ)2 2.10–4 3.10–5 E (kJ/mol) 0.016 0.129 R2 0.8843 0.9611 Temkin aTE (L/g) 5.820 6.991 bTE (kJ/mol) 0.245 0.025 R2 0,9235 0.4867 Flory-Huggins KFH 2.38.10–3 3.93.10–5 nFH 1.190 1.860 ∆G0 (kj/mol) 14.972 25.142 R2 0.1920 0.5004 Brunauer, Emmer & Teller qs (mg/g) 3.753 39.745 CBET (L/mg) 0.421 2.020 R2 0.3481 0.5785 72 Figure 10. Linear isotherm models of Freundlich for strontium adsorption on magnetic graphene oxide composite. Figure 11. Linear isotherm models of Dubinin and Radushkev- ich for cesium adsorption on magnetic graphene oxide compos- ite. 3.4 Kinetic parameters of adsorption A study on the kinetics of adsorption is carrying out to obtain information about the adsorption mech- anism, which is important for the efficiency of the process[37]. Therefore, two well-known kinetic equa- tions were adopted to model the experimental data and identify the adsorption mechanism. In order to analyze the sorption of Sr(II)/Cs(I) onto M-GO, the pseudo first equation and pseudo second order equation was employed[38–40]: (15) 1 2 2 eet q t qkq t += Where, qe is the amount of metal ion adsorbed onto adsorbent at equilibrium (mg/g); qt is the amount of metal ion adsorbed at various times; t (min) is the time of adsorption duration and k1 is the first order rate constant (min–1); k2 (g/mol∙min) is the second-order rate constant. The experiments were conducted at different concentrations 50, 100, 150 ppm for Sr(II) and 200, 250, 300 ppm for Cs(I). From the slope ofeach linear trace, the rate constants were calculated and the re- sults are presented in the Table 2 and Table 3 (pseu- do-first-order model was not shown as figure because the R2 values of the adsorption of Sr(II) and Cs(I) are low at the studied concentrations). The data ob- tained separately for each of the kinetic models from the slopes of plots show a good compliance with the pseudo second order equation. High R2 values for the linear plots showed that kinetic data fitted the pseudo second order adsorption kinetic equation for Sr(II) and Cs(I) removal (Figuer 12 and Figure 13). The theoretical values of qe for Sr(II) and Cs(I) removal also agree very well with the experimental ones. Both facts suggest that the adsorption of Sr(II) and Cs(I) onto M-GO follows the pseudo-second-or- der kinetic model. Therefore, the rate-limiting step may be chemical sorption or chemisorption through sharing or exchange of electrons between sorbent and adsorbate[5]. Figure 12. Pseudo-second-order plot for the adsorption of Sr(II) by M-GO. Table 2. Rate parameters for the adsorption of Sr(II) onto M-GO at various initial concentrations Concentration 50 ppm 100 ppm 150 ppm Pseudo-first-order model qe (mg/g) 1.8539 1.2600 4.5790 k1 (1/min) 0.0999 0.0096 0.0145 R2 0.0913 0.3406 0.0321 Pseudo-second-order model qe (mg/g) 7.4404 16.7504 33.55 k2 (g/mol∙min) 1.1579 0.0627 1.1100 R2 0.9646 0.9823 0.7632 Experimental qe (mg/g) 7.89 18.22 32.30 73 Figure 13. Pseudo-second-order plot for the adsorption of Cs(I) by M-GO. Table 3. Rate parameters for the adsorption of Cs(I) onto M-GO at various initial concentrations 3.5 Thermodynamic studies In this study, the adsorption of Sr(II) and Cs(I) onto M-GO was examined in the temperature range of 25–40 °C under optimized conditions (Sr: pH = 4, C: 50 mg/L, m/V: 1, t: 120 min; Cs: pH = 10, C: 250 mg/L, m/V: 1, t: 120 min and adsorbent amount of 0.01 g). Figure 14 (a-b) show the effect of tem- perature on the adsorption of Sr(II) and Cs(I) on the nanocomposite, respectively. Thermodynamic pa- rameters like enthalpy change (ΔH0), entropy change (ΔS0) and free energy change (ΔG0) were estimated using the following equations. The enthalpy ΔH0 (kJ/mol) and the entropy ΔS0 (J/molK) of adsorption can be determined from the slope and the intercept of the linear fits which are gained by drawing lnKd against 1/T respectively. The negative amounts ΔG0 show that the adsorption process is spontaneous for both of the ions. The val- ues are well under those related to chemical bond constitution, showing the physical property of the adsorption process[41]. Besides, the enthalpy varia- tion ΔH0 following adsorption is negative in all cases representing the exothermic nature of the adsorption. The results indicated that the reaction efficiency decreased as the temperature increased for Sr(II) removal but the reaction efficiency increased as the temperature increased for Cs(I) removal. The negative value of ΔS0 for Sr(II) shows the change in the randomness at the M-GO-solution in- terface during the adsorption. The entropy variations ΔS0 of the system along with the adsorption of Cs(I) ions on the M-GO is positive in all cases, showing that more discover is generated following adsorption. The results were calculated in Table 4. Concentration 200 ppm 250 ppm 300 ppm Pseudo-first-order model qe (mg/g) 69.47 100.74 120.46 k1 (1/min) 0.0203 0.0205 0.0257 R2 0.9586 0.7875 0.7567 Pseudo-second-order model qe (mg/g) 104.17 96.15 86.96 k2 (g /mol min) 0.0051 0.0003 0.0059 R2 0.9965 0.8965 0.999 Experimental qe (mg/g) 102.8 95.0 85.5 Figure 14. Plots of ln Kd versus 1/T for Sr (a) and Cs (b) adsorption on M-GO. Table 4. Thermodynamic parameters for Sr(II) and Cs(I) sorption on M-GO as a function of temperature M-GO ΔHo (kJ/mol) ΔSo (J/molK) ΔGo (kJ/mol) 298 K 303 K 308 K 318 K Sr(II) –22.97 –18.37 –17.49 –17.42 –17.33 –17.24 Cs(I) –5.17 33.35 –15.11 –15.28 –15.44 –15.61 74 4. Conclusion M-GO nanocomposite was synthesized using partial reduction co-precipitation method, which is simple, effective, economical and environmentally friendly technique for the removal of Sr(II) and Cs(I) from aqueous solutions. Prepared nanocomposite was characterized by SEM, TEM, XRD, FTIR, XPS and VSM. According to all characterization meth- ods and literature data, we can conclude that M-GO was successfully prepared and possessed with the desired properties. The adsorption capacity of M-GO for Sr(II) and Cs(I) were found as 2.103 mg/g and 142.070 mg/g, respectively. Kinetic results indicated that the adsorption process could be defined by the pseudo-second-order kinetic model under the select- ed strontium and cesium concentration range which provides the best correlation of the data in all cases and the experimental qe values agree with the cal- culated ones and the adsorption isotherm was fitted well to Freundlich model and D-R model for Sr(II) and Cs(I), respectively. The thermodynamic analysis of the sorption process for both of the radionuclides indicates that the system is spontaneous and exother- mic. The values of ΔG0 for Sr(II) and Cs(I) are well under those related to chemical bond constitution, showing the physical property of the adsorption pro- cess. It could be therefore concluded that the sorp- tion mechanism was dominated by physisorption, but the overall observations suggest that the sorption process was administrated by combination of several mechanisms, such as physical sorption, ion exchange and complexation. Based on the results, M-GO can effectively remove the strontium and cesium ions from aqueous solutions. Author contributions Sule Aytas, Sabriye Yusan and Senol Sert per- formed the experiments and analyzed the data. Sule Aytas supervised and designed data. Sabriye Yusan and Cem Gok designed and analyzed data, and pre- pared the manuscript. Senol Sert realized ICP-OES measurements and also designed the manuscript. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements This research project was supported by Ege Uni- versity Scientific Research Project Unit Project No. 2014 NBE 005. References 1. Japan’s Challenges Towards Recovery. 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