Magnetite nanoparticles decorated reduced graphene oxide

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Magnetite nanoparticles decorated reduced graphene oxide composite as an efficient and recoverable adsorbent for the removal of cesium and strontium ions Cherukutty Ramakrishnan Minitha, Rahul Suresh, Ujjwal Kumar Maity, Yuvaraj Haldorai, Vijayakumar Subramaniam, Periasamy Manoravi, Mathew Joseph, and Ramasamy Thangavelu Rajendra Kumar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05340 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Industrial & Engineering Chemistry Research

Magnetite nanoparticles decorated reduced graphene oxide composite as an efficient and recoverable adsorbent for the removal of cesium and strontium ions Cherukutty Ramakrishnan Minitha,1 Rahul Suresh,2 Ujjwal Kumar Maity,3 Yuvaraj Haldorai4, Vijayakumar Subramaniam,5 Periasamy Manoravi,3 Mathew Joseph,3 Ramasamy Thangavelu Rajendra Kumar 1,4* 1

Advanced Materials and Devices Laboratory (AMDL), Department of Physics, Bharathiar

University, Coimbatore – 641 046. Tamil Nadu, India. 2

3

Department of Physics, Bharathiar University, Coimbatore – 641 046. Tamil Nadu, India.

Radioactive Chemical Laboratory (RCL), Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam-603102, Tamil Nadu, India.

4

Department of Nanoscience and Technology, Bharathiar University, Coimbatore – 641 046. Tamil Nadu, India

5

Department of Medical Physics, Bharathiar University, Coimbatore – 641 046. Tamil Nadu, India.

Contact. No: +91-9789757888 *Email: [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Magnetite nanoparticles (Fe3O4) decorated reduced graphene oxide (rGO) composite was synthesized by the solvothermal method and utilized as a potential adsorbent for the removal of cesium (Cs+) and strontium (Sr2+) ions from aqueous solution. The effects of adsorbate concentration and reaction time on the removal efficiencies of Cs+ and Sr2+ were investigated. The adsorption capacity increases as the initial concentration of Cs+/Sr2+ increased from 1 to 170 mg/L, which might be due to the more available adsorption sites, and the adsorbent reached equilibrium at 360 min. The adsorption isotherm was fitted to the Freundlich model with a maximum adsorption capacity of Cs+ and Sr2+ were 128.2 and 384.6 mg g-1, respectively. The kinetic study showed that the adsorption behavior followed pseudo-secondorder kinetics. The rGO/Fe3O4 nanocomposite showed excellent selectivity towards Cs+ and Sr2+ even in the presence of competitive cations (Na+, K+, and Mg2+) having a higher concentration.

KEYWORDS: Magnetic particle, reduced graphene oxide, composite, cesium/strontium adsorption, selectivity.


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INTRODUCTION Rapid use of nuclear energy has become an ineluctable for improvement of living conditions and economy, as the per capita energy consumption is considered as an index of overall growth of a country. In order to have an efficient nuclear fuel economy, the closing of fuel cycle is essential. Reprocessing of irradiated nuclear fuel will result in the generation of radioactive high-level liquid waste (HLLW), which is inevitable. The presence of long-lived and high fission yield radioisotopes such as cesium (137Cs, t1/2 = 30.1 years) and strontium (90Sr t1/2 = 28.5 years) are the main source of radiotoxicity in such HLLW, which makes the waste management process critical.1,

2

The

137

Cs and

90

Sr are gamma (661 keV) and beta

emitters (2.2 MeV) respectively, adding a lot of heat to the HLLW.3,

4

Removal of these

radionuclides will reduce the radioactivity burden considerably. It is very difficult to remove these radionuclides from the highly acidic medium, that too in the presence of a host of other fission products and actinides.5 Hence, the most challenging task is to develop an efficient method for selective removal of these radionuclides (Cs & Sr) from HLLW having a large quantity of interfering ions in the highly aggressive medium6. Several attempts have been reported for removal of these radionuclides from simulated high-level nuclear waste (SHLW) using various techniques such as inorganic ion exchangers,4 photochemical methods,7 chemical precipitation,8 and organic membrane filtration.9 But the thermal, chemical and radiation degradation of membranes in organic membrane filtration and low selectivity in chemical precipitation limit their use in the nuclear waste management process. On the contrary, the widely used inorganic ion exchangers are stable under high radiation, chemical, and thermal environment1. But, ion exchange processes have auxiliary drawbacks - sensitive to the pH of the waste solution and the presence of dissolved species in high concentration often is a limiting factor.6 Adsorption is a cost-effective, environmentally friendly process for the separation of toxic metal ions. Hence, in the present scenario, adsorption process can



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be an alternative technique to the inorganic ion exchangers.10 The added advantages of adsorption process are the comparable selectivity over a wide range of pH, cost-effective, and more significantly, environmentally friendly compared to inorganic ion exchangers. The adsorption process can also be an economical method for nuclear liquid waste management. Many researchers have used carbon/montmorillonite11, iron terephthalate metal organic frame work12 to remove toxic from the contaminated water. Particularly, materials like natural clay,13 hydroxyapatite,14 magnetite incorporated metal oxides,15 charcoal16 and zeolites17 for the removal radionuclides. Recently, other materials like metal organic framework18 and Prussian blue (PB)/alginate composite3 were also reported for selective removal of Cs+ and Sr2+ from nuclear waste with coexisting metal ions such as sodium and potassium. Fe3O4 combined with GO based materials were also investigated by macroscopic, spectroscopic and modeling techniques to study the mechanism for U(VI) removal19,

20

. The rGO/WO3

composite effectively remove an individual radioactive strontium. The effect of temperature and Na ionic strength shown the exothermic and lower the adsorption sites respectively. rGO/Fe3O4 was also investigated for the removal of U(VI)

21, 22

. Adsorption of radionuclide

in the mixture of co-existing metal ions (closer to HLLW) is less explored. Graphene oxide-based materials have high surface area and they are a cationic scavenger in aqueous solution10. Major quantum of work reported so far in the literature, are for the removal of individual ions from aqueous solution.2, 3, 10, 23 As mentioned above, in HLLW, the radionuclides and actinides present in the highly acidic medium. The HLLW also contains a considerable amount of sodium ions, which is a reactor fuel for efficient heat transport. Therefore, it is very crucial to remove Cs+ and Sr2+ ions from such medium. Very few researchers have addressed this issue.4, 6 Over the last decade, few reports have been reported that the graphene-based materials.2, 3, 24 could be used for the removal of Cs+ and Sr2+ ions. However, the results indicated that the removal efficiency of metal ions in highly acidic


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condition was poor.24, 25 The investigation of removal of Cs+ and Sr2+ in the highly acidic medium is essential since the wastewater coming out from the nuclear reactor after fission process is extremely acidity. Furthermore, most of the adsorbents have the drawback of either low adsorption capacity and/or too hard to be separated from the treated metal ion solution. Improvement of the adsorbent with high uptake capacity and easy removal of contaminants is a prerequisite. GO reduced to rGO to form rGO/Fe3O4 nanocomposites. Though the density of oxygen-containing functional groups is low for rGO (compared with GO), it reestablishes its sp2 domains and expose negatively charged π electron clouds could be favourable adsorption sites for metal ions26, 27. In the present study, reduced graphene oxide/magnetite (rGO/Fe3O4) nanocomposite synthesized by the solvothermal method and it was used as an efficient adsorbent for the removal of Cs+ and Sr2+ from the aqueous solution. The as-synthesized adsorbent was confirmed by X-ray diffraction (XRD) and Raman spectroscopy. The effect of adsorbate concentration and reaction time on the removal efficiencies of Cs+ and Sr2+ was investigated. In addition, the adsorbent can be easily recovered from aqueous solution using an external magnetic field, thereby offering facile and economic separation of the adsorbent. Furthermore, we investigated the selectivity of the composite towards selective adsorption of Cs+ and Sr2+ removal under acidic condition with a mixture of coexisting metal ions (Mg2+, Na+, and K+). EXPERIMENTAL Materials High purity 150 µm sized graphite flakes (>99.99 %), CsNO3 and Sr(NO3)2 were purchased from Sigma-Aldrich, India. KMnO4, H2SO4, and diethylene glycol (DEG) were purchased from Merck, India. All other reagents were of analytical grade and were used as received. Double distilled water was used throughout the study.



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Synthesis of Fe3O4/rGO nanocomposite GO was prepared by modified Hummers method.28,

29

The Fe3O4 nanoparticles were

synthesized according to the previous report.30 In a typical synthesis of rGO/Fe3O4 nanocomposite, 0.1 g GO was dispersed in 60 ml of DEG by ultrasonic dispersion. After dispersion, 0.12 g FeCl3.6H2O and 0.37 g sodium acetate were added to the GO/DEG solution under stirring (30 min) and allowed to dissolve completely. To maintain the solution pH to 10, few drops of aqueous ammonia was slowly added to the solution. The resulting homogeneous solution was then transferred to Teflon – lined stainless – steel autoclave and sealed and then heated at 180°C for 12 h. The obtained rGO/Fe3O4 nanocomposite was washed several times with double distilled water and ethanol followed by centrifugation, dried at 60°C for 12 h in a vacuum oven. Adsorption Experiment The adsorption experiments were conducted (in a 2 mL solution) in batches at room temperature by varying the initial Cs+/Sr2+ concentrations (1–170 mg/L) at pH 7 for 24 h with a constant adsorbent dosage (0.3 mg/L). After the adsorption process, the composite was recovered by an external magnet and analyzed by an Inductive Coupled Plasma-Mass Spectrometer (ICP-MS) to quantify the amount of Cs+/Sr2+ adsorbed. The percentage removal of Cs+/Sr2+ and the adsorption capacity(qt) of the rGO/Fe3O4 at a given time were calculated using the following equations31.

(%) Removal =

q t (mg/g) =

(C0 − Ct ) X 100 C0

(1)

(C 0 − Ct ) ×V M

(2)


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where C0 (mg/L), is the initial concentration of adsorbate, Ct (mg/L), is the concentration of adsorbate at a time t, V and M are the initial volume of the solution (L) and weight of the adsorbent (g) respectively. Selectivity Selectivity experiments were carried out using 2 mL of the Cs+/Sr2+solution (100 mg/L) containing 0.3 mg of the adsorbent at different acidic pH values. The pH of the solution was adjusted by using 4 M HNO3. After 24 h of agitation at 120 rpm, the adsorbent was recovered with a magnet and the solution was analyzed by ICP-MS to quantify the initial and residual concentrations of Cs+ and Sr2+. Characterization The crystallinity and phases of the as-prepared Fe3O4/rGO nanocomposite were investigated by XRD (Bruker Advanced D8). The morphology was examined by field emission scanning electron microscopy (FEI – QUANTA – FEG 250). Raman spectroscopy was performed on a Raman microscope (Horiba Jobin Yvon, HR800) with an excitation wavelength of 514 nm. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, K–Alpha) was measured using an Al Kα X-ray source. Transmission electron microscopy (TEM, JEOL 2100F) was performed with an accelerating voltage of 200 kV and equipped with an energy-dispersive Xray spectrometer (EDS). The residual Cs+ and Sr2+ concentrations were measured using ICPMS (SPECTRO MS, MSS001). The magnetic moment was analyzed by vibrating sample magnetometer (EZ9-Microsense Inc. USA) at 300 K. The Brunauer-Emmett-Teller (BET) surface area and pore size distributions were recorded by measuring N2 gas adsorptiondesorption (NOVA 2000e). Zeta potential was measured using a zeta potential and particle size analyzer (Type Nano-Z, Malvern). RESULTS AND DISCUSSION



In this study, we synthesized a rGO/Fe3O4 composite by the solvothermal method. A schematic representation of the preparation of composite for efficient removal of Cs+ and Sr2+ is shown in Fig. 1. Fig. 2a shows the powder XRD pattern of the rGO/Fe3O4 composite. The diffraction peaks at 30.5° (220), 36.0° (311), 43.5° (400), 53.9° (422), 57.5° (511), and 63.2° (440) observed were consistent with the standard XRD pattern of cubic Fe3O4 (JCPDS, no. 19-0629).32 The peak at 25.2o corresponds to the (002) reflection of graphitic carbon.31 The XRD patterns of as-synthesized rGO and Fe3O4 are shown in Fig. 2a for comparison. Fig. 2b shows the Raman spectrum of the composite, which exhibited bands at 210, 268, 375, and 582 cm-1 corresponding to the T2g(1), Eg, T2g(2), and A1g modes of the crystalline Fe3O4.33 In addition, the D and G bands at 1341 cm-1 and 1596 cm-1, respectively, confirmed the presence of rGO. The D band originates from the disordered carbon structure, and the G band is related to the sp2 hybridized carbon atoms.31 The BET surface area and pore size of the composite were determined from the N2 sorption isotherm at 77 K up to 1 bar (Fig. 2c). The isotherm displayed a steep uptake in the low-pressure region which means it follows type IV isotherm.34 The BET surface area of the composite was calculated as 89 m2 g−1. The BarrettJoyner-Halenda (BJH) pore size distribution (Fig. 2d) suggested a mesoporous nature of the material,14 with an average pore size and pore volume of 5.4 nm and 0.23 cm3 g−1, respectively. Magnetic characterization of rGO/Fe3O4 composite was performed using a vibrating sample magnetometer at room temperature (Fig. 2e). Pristine Fe3O4 and rGO/Fe3O4 composite exhibit superparamagnetic nature and saturation of magnetization (Ms) were found to be 35.1 and 18.6 emu/g, respectively. The difference in Ms between the pristine and composite suggests that the presence of rGO layers could be the reason for the lowering of Ms value. Inset in Fig. 2e shows that the composite exhibited low coercivity and remnant magnetization.35 Zeta potential values were measured using a zeta analyzer (Fig. 2f). The rGO, Fe3O4 nanoparticles, and composite have different charges, with zeta potential values of


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-22.5 mV, 16.6 mV, and -18.4 mV, respectively. The as-synthesized rGO/Fe3O4 composite has negative surface charge (-18.4 mV) at pH 7, which indicates the presence of functional group (-OH, -COOH) present in the rGO perhaps responsible for the surface negative charge which is the main factor for the adsorption of Sr2+ or Cs+. The FESEM image of rGO is shown in Fig. 3a indicated that the sheets were not perfectly flat and it was observed to be crumpled. It is well known that Fe3O4 nanoparticles prepared by the solvothermal method were highly agglomerated due to magnetic dipole interactions between the nanoparticles.36 The FESEM image of Fe3O4 revealed that the particles were quasispherical in shape with an average particle size of less than 10 nm. In the case of the composite (Fig. 3c), the Fe3O4 nanoparticles were densely dispersed on to the rGO surface. However, aggregated nanoparticles were observed in certain areas. The TEM image (Fig. 3d) shows that the Fe3O4 nanoparticles were well dispersed in the rGO matrix; however, some nanoparticle aggregation was evident in a few places. The close inspection of TEM image (Fig. 3e) clearly showed that the average size of Fe3O4 nanoparticles was less than 10 nm. Fig. 3f shows the corresponding selected area diffraction pattern (SAED) patterns, revealing a crystalline nature of Fe3O4 that was consistent with that indicated by the XRD analysis. The removal efficiency of the composite as a function of the Cs+/Sr2+ concentration was examined by changing the initial concentration of Cs+/Sr2+ from 1 to 170 mg/L and keeping the adsorbent dosage constant for different time intervals (10 min to 24 h). Samples were collected at different times to evaluate the adsorption capacity. Fig. 4 shows the adsorption capacity of Cs+ and Sr2+ versus the contact time. The amount of Cs+/Sr2+ removal increased with increasing Cs+/Sr2+ concentrations from 1 mg/L to 170 mg/L. This might be because at higher concentrations, the driving forces for ion migration between the aqueous phase and solid phase increases. The adsorption reached equilibrium at 360 min at all Cs+/Sr2+ concentrations tested. After this time, no appreciable increase in the Cs+/Sr2+ removal was



observed because there were no active sites available for Cs+/Sr2+adsorption. The adsorption capacities of Cs+ and Sr2+ after 360 min for the initial (1 mg/L) and maximum (170 mg/L) concentrations were calculated as 0.59 mg/g and 256 mg/g for Cs+ and 1.98 mg/g and 168 mg/g for Sr2+, respectively. The comparative adsorption experiments for the rGO and pure Fe3O4 showed that the adsorption capacities of Cs+ and Sr2+ for the rGO and pure Fe3O4 were calculated as 216 mg/g and 162.3 mg/g for Cs+ and 92 mg/g and 89.7 mg/g for Sr2+, respectively. The data are shown in Fig. S1. The effective adsorption of rGO/Fe3O4 composite is derived from the following factors: (i) the large surface area and surface functional groups of rGO; the electrostatic non-covalent interaction between the surface functional groups of rGO and Sr2+/Cs+, and (ii) the non-covalent interaction between the πelectron cloud of rGO and Sr2+/Cs+. After adsorption process, we analyzed the Fe ions leaching from the rGO/Fe3O4 composite by ICP-MS. The detection limit of ICP-MS instrument was ~1-5 ppb for Fe ions. After adsorption, the supernatant was subjected to analysis and the amount of Fe ions leaching was below its detection limit. Therefore, it is clear that the amount of Fe ions leached was very minimal or negligible. To further investigate the interaction of adsorbate with rGO/Fe3O4 composite XPS analysis was recorded. Fig. 5 shows the survey XPS analysis of the composite before and after adsorption of Cs+ and Sr2+. The spectra revealed peaks of C 1s (285 eV), O 1s (531 eV), Fe 2p (725 eV), Cs 3d (738 eV), and Sr 3d (134 eV) which endorsed the existence of these elements in the composite after adsorption. The core-level spectra of C 1s and O 1s are shown in Fig. S2. Batch experiments were carried out by adding 0.3 mg of the adsorbent to a test tube containing different initial Cs+/Sr2+ concentration (1 to 170 mg/L). The tube was shaken at 120 rpm for 24 h. Once the adsorption process reached equilibrium, the adsorbent was removed by centrifugation/magnetic separation and the supernatant was analyzed by ICP-MS to check the residual Cs+ and Sr2+ concentrations. The adsorption process on the surface of


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the adsorbent is described by Langmuir and Freundlich adsorption isotherm models. The Langmuir isotherm3, 23 refers to homogeneous monolayer adsorption, in which adsorption can occur at the active surface sites that are identical and equivalent. The linear and nonlinear forms of the equations are written as,

Ce C 1 = + e qe q m K L q m K L Ce qe = q m 1 + K L Ce

(3)

(4)

where qe and qm are the equilibrium adsorption capacity (mg/g) and maximum adsorption capacity (mg/g) of the adsorbate, respectively; Ce (mg/L) is the concentration of adsorbate at equilibrium, and KL (L/g) is a constant related to the affinity between the adsorbent and adsorbate. The Freundlich isotherm model14 is an empirical equation commonly used to describe the adsorption characteristics of a heterogeneous surface and can be applied to multilayer adsorption of an adsorbate on the surface of an adsorbent. The isotherm equations can be expressed as,

1 ln qe = ln K F +  n 

qe = K F Ce

1

n

  ln Ce  

(5)

(6)

Where qe (mg/g), Ce (mg/L) are the same as mentioned in Langmuir model.

n is the

heterogeneity coefficient, and KF (L/g) is characteristic constants related to the multilayer adsorption capacity. The experimental results were fitted to the adsorption isotherms (Fig. 6a and 5b) and the resulting constants and correlation coefficient (R2) were calculated. The experimental data of composite was a better fitted to the Freundlich model (Fig. 6b) for the adsorption of both Cs+ and Sr2+ with the correlation coefficient (R2) of 0.9974 and 0.9992 11 ACS Paragon Plus Environment


than the Langmuir model (Fig. 5a) (R2 = 0.5844 and 0.9275). The composite possessed a qm of 128.2 and 384.6 mg/g for Cs+ and Sr2+ from aqueous solution, respectively. The n (1.13) and KF (2.21) values confirmed the easy recovery of Sr2+ from the aqueous solution and indicate favorable adsorption. The isotherm results indicated that the as-prepared composite could be an excellent material for Cs+ and Sr2+ adsorption. We have compared the adsorption capacity of rGO/Fe3O4 composite with previous reported carbonaceous adsorbents and tabulated in Table S1. The kinetic experiments were carried out by adding 0.3 mg of the adsorbent to a test tube containing 2 mL of 170 mg/L Cs+/Sr2+ solution. The tube was shaken at 120 rpm for different time intervals (10 min to 24 h). The adsorbent was then removed by an external magnet and the supernatant was analyzed by ICP-MS to check the residual Cs+ and Sr2+ concentrations. The kinetics of present adsorption studies at different time intervals was investigated by pseudo-first-order and pseudo-second-order kinetic models. Processes such as rate of reaction and the mass transfer were determined by applying the pseudo-first-order model.3, 31 The pseudo-first-order rate equation can be expressed as: dq t = k1 (q e − q t ) dt

(7 )

where qe is the equilibrium Cs+/Sr2+ removal capacity and qt is the Cs+/Sr2+ removal capacity (mg/g) at time t, respectively, and k1 is the pseudo first-order rate constant (min-1). By applying the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, the linearized form of the rate equation can be expressed as:

 k  log(qe − qt ) = log qe −  1 t  2.303 

(8)

Fig. 6c shows a plot of ln (qe - qt) vs. t gave a linear line with R2 values of 0.9857 and 0.9314 for Cs+ and Sr2+, respectively. From the slope and intercept of the straight line, the qe and k1 12 ACS Paragon Plus Environment

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values were calculated as 48.67 mg/g and 0.0105 min-1 for Cs+ and 20.92 mg/g and 0.0186 min-1 for Sr2+. The pseudo-second-order kinetic model is represented by the following equation: dq t 2 = k 2 (q e − q t ) dt

(9 )

t 1 t = + 2 qt k 2 qe qe

(10)

From the linear plot of t/qt vs t (Fig. 6d), the adsorption capacity (qe) and second-order rate constant (k2) values were calculated as 285.71 mg/g and 0.0002 g.mg-1min-1, respectively, with an R2 value of 0.9992 for Cs+ and 175.43 mg/g and 0.0009 g.mg-1min-1 for Sr2+ (R2 = 0.9999). Compared to the first-order model, the second-order model had a higher R2 value (Table. 1), implying that the adsorption may be the rate-limiting step involving electron transfer between the adsorbent and the adsorbate. Effect of pH and selectivity with competing cations The selectivity of composite for Cs+ and Sr2+ (Fig. 7a) was evaluated by measuring the efficient removal of Cs+ and Sr2+ in the presence of competing cations such as Na+ (1000 mg/L), and K+ (1000 mg/L), and Mg2+ (500 mg/L) in aqueous solution. Since HLLW contains large quantities of interfering ions in the highly acidic medium, we have carried out the selectivity experiments in acidic pH. The required amount of adsorbent (0.3 mg) was added to a 2 mL tube containing 100 mg/L of the Cs+ and Sr2+ solution. The experiment was conducted for 24 h.

The composite showed excellent selectivity towards Cs+ and Sr2+

because the concentration of Cs+ and Sr2+ in any environment is significantly lower than the concentrations of coexisting cations, such as Na+, K+, and Mg2+. The removal efficiencies of Cs+ and Sr2+ from aqueous solution at different pH values were 16.9 and 16.0 (pH 1), 18.2 and 17.2 (pH 4), and 16.1 and 8.4 (pH 7), respectively. The results demonstrated that the Cs+



and Sr2+ adsorption was not significantly affected by the presence of coexisting cations. The metal ions (Cs+ and Sr2+) bind preferentially and more strongly to the COO- groups of the rGO than Na+ and K+.31,32 This is due to the relative binding affinity of COO- groups to the metal ions in the order of Sr2+ >> Cs+ >> Na+. In addition, in the acidic medium, the H+ ions compete with Na+ and K+ for adsorption site and therefore the adsorption of Na+ and K+ was less as compared to the Cs+ and Sr2+. Pašalić et al.,27 theoretically investigated the Cation–π interactions. They claimed the barrier less transitions and high mobilities of Cs cation on the aromatic surface than Na and K ions.

rGO reestablishes its sp2 domains and expose

negatively charged π electron clouds could be favourable adsorption sites for metal ions. Fig. 7b displays a FESEM image of the adsorbent after adsorption and the corresponding EDS analysis is shown in Fig. 7c-7g. It was observed that adsorbed Cs and Sr ions are higher than the Na and K ions on adsorbent. CONCLUSION In the present study, a magnetic composite adsorbent was synthesized for the removal of Cs+ and Sr2+ from aqueous solution. The experimental results indicated that the removal efficiency initially increased slowly, and the maximum adsorption reached within 360 min. A further increase in reaction time did not show significant changes in the equilibrium concentration. The adsorbent was shown to be an effective material for Cs+ and Sr2+ removal with maximum adsorption capacities of 128.2 and 384.6 mg g-1, respectively. The adsorption and kinetic studies showed that the process was in good agreement with the Freundlich model and followed second-order kinetics. The adsorbent exhibited high selectivity for Cs+ and Sr2+ even in the presence of higher concentrations of co-existing cations in acidic condition. In addition, the magnetic property made it possible to recover the adsorbent effectively from the contaminated water. Nevertheless, more directions such as the interaction analysis at the vacancy sites with rGO/Fe3O4 surfaces are open to research. These results can help in


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engineering efficient adsorbents capable of removing radionuclides and selectively from competitive ions solutions for radioactive wastewater management.

ASSOCIATED CONTENT SUPPORTING INFORMATION (1) Adsorption studies of Cs+ and Sr2+ using pure rGO, pure Fe3O4, and rGO/Fe3O4 composite; (2) Core-level C1s and O1s spectra of the rGO/Fe3O4 composite; (3) Table S1: Comparison adsorption studies of Cs+ and Sr2+ using an rGO/Fe3O4 composite with other carbonaceous adsorbents.

NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors C.R.M and R.T.R. would like to thank the Department of Science and Technology, Government of India, for financial support under the SERB project (SR/FTP/PS- 099/2011). The authors are thankful for DST- PURSE, New Delhi, India for providing FESEM facility.

C.R.M would also like to thank Dr. K. Rajavel (Zhejiang

University, Hangzhou, China) and Dr. D. Prabhu (ARCI, Chennai) for BET and VSM measurements. The authors would like to thank Prof. Yun Suk Huh, (Inha University, Incheon, Republic of Korea) for analysis of TEM and XPS measurements.



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Figure captions Fig. 1. Schematic representation of the synthesis of rGO/Fe3O4 nanocomposite for efficient removal of Cs+ and Sr2+. Fig. 2. (a) XRD, (b) Raman, (c) BET, (d) pore-size distribution, (e) magnetic property, and (f) zeta potential measurement of the rGO/Fe3O4 nanocomposite. Inset in Fig. 2e shows the lower coercivity and photographic image of rGO/Fe3O4 nanocomposite in aqueous solution with response to an external magnet. Fig. 3. SEM images of (a) rGO, (b) Fe3O4, (c) rGO/Fe3O4 nanocomposite, (d,e) TEM images of composite, and (f) SAED pattern of composite. Fig. 4. Removal efficiency of (a) Cs+ and (b) Sr2+ using the rGO/Fe3O4 nanocomposite. The legends for (a &b) are similar. Fig. 5. XPS survey spectra of rGO/Fe3O4 nanocomposite before and after adsorption of Cs+ and Sr2+. Fig. 6. Freundlich isotherm (a) Cs+, (b) Sr2+, (c) pseudo-first-order kinetics and (d) secondorder kinetics of the nanocomposite. Fig. 7. (a) Selectivity of Cs+ and Sr2+ in the presence of competitor cations, such as Na+, K+, and Mg2+, (b) SEM image of the adsorbent after selective removal of Cs+ and Sr2+, and (c-g) the corresponding EDS mapping of the adsorbent.



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Table 1. Parameters of pseudo-first-order and second-order kinetic models. Adsorption system

qe, exp (mg/g)

(170 mg/L) Cs(I) Sr(II)

Pseudo-first order k1 (min-1)

R

qe,cal

Pseudo-second order 2

(mg/g)

256 168

0.011 0.018

48.67 20.92

0.9857 0.9314

qe,cal

k2

(mg/g)

(g.mg-1min-1)

285.71 175.43

0.0002 0.0009


R2

0.9992 0.9999

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Fig. 1



Fig. 2


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Fig. 3

Fe3O4

rGO



Fig. 4


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Fig. 5

Fig. 6



Fig. 7


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Schematic representation of the synthesis of rGO/Fe3O4 nanocomposite for efficient removal of Cs+ and Sr2+. 447x190mm (150 x 150 DPI)

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Magnetite nanoparticles decorated reduced graphene oxide

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