Facile synthesis of cysteine functionalized magnetic graphene oxide nanosheets: Application in solid phase extraction of cadmium from environmental sample

Accepted Manuscript Title: Facile synthesis of cysteine functionalized magnetic graphene oxide nanosheets: application in solid phase extraction of cadmium from environmental sample Author: Alireza Banazadeh Shahla Mozaffari Bita Osoli PII: DOI: Reference:

S2213-3437(15)30003-8 http://dx.doi.org/doi:10.1016/j.jece.2015.10.003 JECE 796

To appear in: Received date: Revised date: Accepted date:

9-6-2015 10-9-2015 1-10-2015

Please cite this article as: Alireza Banazadeh, Shahla Mozaffari, Bita Osoli, Facile synthesis of cysteine functionalized magnetic graphene oxide nanosheets: application in solid phase extraction of cadmium from environmental sample, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2015.10.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of cysteine functionalized magnetic graphene oxide nanosheets: application in solid phase extraction of cadmium from environmental sample Alireza Banazadeha* [email protected], Shahla Mozaffarib, Bita Osolib a

Faculty of Chemistry and Petrochemical Engineering, Department of Petrochemical

Engineering, Standard Research Institute (SRI), Karaj, P.O. Box 31745-139, Iran. b

Faculty of Payame Noor University, Department of Chemistry, Tehran, P.O. Box 19395-3697, Iran.

*

Corresponding author. Tel.: +98 2632861125, Fax: +98 2632803881.

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Abstract A facile method for synthesis of cysteine functionalized magnetic graphene oxide nanosheets is introduced. In comparison with other metal ions, cysteine has more affinity to coordinate with cadmium, therefore, the modified graphene oxide nanosheets were used as a selective sorbent for solid-phase extraction and determination of trace cadmium in different food samples (rice, wheat, milk and shrimp). Satisfactory recoveries were obtained by using 0.8 mg ml-1 of sorbent in a pH range of 5-9. The results showed good adsorption capacities (24.39-30.30 mg g−1 at 298-328 K) of the adsorbent with the high selectivity toward cadmium ions. The process was relatively fast and the equilibrium was established within 5 min and its kinetics followed the pseudo-second order mechanism. The best interpretation for the equilibrium data was given by Langmuir isotherm and the thermodynamic parameters showed that the adsorption process was spontaneous, endothermic and chemical in nature. Keywords: Solid phase extraction; Cadmium; Cysteine; Magnetic graphene oxide

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1. Introduction Heavy metals contamination is known to be a significant problem, which threatens the environment and human life. The main threats to human health from heavy metals are associated with exposure to lead (Pb), cadmium (Cd), mercury (Hg) and arsenic (As) [1-3]. Cd and its compounds are highly toxic and exposure to this metal is known to cause cancer and targets the body’s cardiovascular, renal, gastrointestinal, neurological, reproductive and respiratory systems [4-6]. It is considered more toxic than either lead or mercury. The most common sources of Cd toxicity are foods such as rice and wheat which are grown in soil contaminated by sewage sludge, super phosphate fertilizers and irrigation water. Ocean fish such as tuna, codfish and haddock concentrate within their tissues relatively large amounts of Cd. Therefore, the effective removal of Cd from the environmental, biological and food samples has been a crucial issue related to the quality of human life. Several analytical methods including inductively coupled plasma optical emission spectrometry, inductively coupled plasma-mass spectrometry, flame atomic absorption spectrometry, electrothermal atomic absorption spectrometry and graphite furnace atomic absorption spectrometry, have been proposed for the determination of Cd in various matrices [7-17]. However, the direct determination of this metal in real samples with above techniques was very difficult in most cases, because of matrix effect and low existing level. Under these circumstances, in order to determine trace levels of Cd a separation and enrichment step prior to the determinations may be beneficial. For this purpose, several methods have been applied, including, liquid–liquid extraction, cloud point extraction, liquid-phase microextraction and solid-phase extraction [18-23]. Among these techniques, solid-phase extraction (SPE) as a popular technique for achieving separation and preconcentration of metal ions in 3 

 

environmental samples has been developed and widely used because of its high enrichment factor, simple operation, minimal cost, reusability of the adsorbent and the ability to combine with different detection techniques whether in on-line or off-line mode [23-26] . In this sense, graphene, a single layer of sp2 bonded carbon atoms in a two-dimensional hexagonal lattice, has been attracted more attentions due to its physico-chemical properties, such as a large surface area, high dispersibility and hydrophilicity [27-29]. However, it is easy to aggregate, which will lead to great reduction in the surface area and the adsorption. Therefore, chemical modification of graphene is imperative. In comparison with graphene, graphene oxide (GO) has oxygen containing groups such as epoxides, hydroxyl, carboxyl, and carbonyl on its surface. These functional groups may serve as binding sites for metal ions.

However,

unfunctionalized GO doesn't have enough selectivity for efficient removal of heavy metals and also, the oxygen-containing functional groups cannot provide strong coordination with metals ions.

In order to further improve selectivity and adsorption properties, various

graphene-based nanocomposites have been fabricated by chemical modification of GO [3034]. Moreover, it is difficult to separate graphene or GO from aqueous solution via traditional centrifugation and filtration method because of its small particle size. Therefore, magnetic graphene based adsorbents that facilitate separation by magnetic field have begun to be used in the field of environmental remediation [35-40]. Owing to the exceptional characteristics of amino acids (such as: relatively high affinity to metal ions, biocompatibility, structural flexibility and durability and also, significantly lower material and manufacturing cost) these compounds can be used as a suitable ligand in extraction of heavy metals from different media [41-43]. Cysteine, a nonessential, watersoluble, sulfur-containing amino acid with three functional groups (-SH, -NH2, -COOH), has 4 

 

a strong tendency to coordinate with Cd2+ ions [44,45]. Herein, we report the synthesis, characterization and application of cysteine functionalized magnetic GO (Cys-MGO) nanosheets as a selective sorbent for solid-phase extraction and determination of Cd in rice, wheat, milk and shrimp samples. 2. Experimental 2.1. Apparatus and Reagents Graphite powder (diameter <150 μm and purity 99.99% trace metals basis, Sigma-Aldrich Co.) and cysteine (purity 97%, Sigma-Aldrich Co.) were used to prepare the cysteine functionalized graphene oxide. Other chemicals used in this study were purchased from Merck Chemical Co. with analytical grade and were used without further purification. Ultrapure water (Millipore) was used throughout the whole experiments. The structure and morphology of cysteine functionalized magnetic GO nanosheets were characterized by X-ray diffraction (XRD-D8, BRUKER) and transmission electron microscopy (TEM-CM10, Philips). Magnetic properties were investigated by using a vibrating sample magnetometer (VSM) with an applied field between−10000 and 10000 Oe at room temperature (MDKF, Iran). Thermogravimetric analyses (TGA) were carried out with thermogravimetric analyzer (STA 503, Bahr, Germany) at a heating rate of 10 °C/min under N2 flow (10 ml/min). ICP-OES spectrometer (Thermo Scientific, IRIS Intrepid II, USA) were used for measuring the concentrations of elements. Microwave digester (Milestone, Japan) was used to digest the samples and temperature was controlled by use of an ATC-400CE inner temperature sensor probe.

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2.2 Synthesis of cysteine functionalized magnetic GO nanosheets Graphite oxide was synthesized using the harsh oxidation of graphite according to the modified Hummers method [46]. Briefly, graphite powders (1 g) were sonicated in concentrated H2SO4 (22 mL) for 30 min. while keeping the temperature below 20 °C by using an ice bath, KMnO4 (3 g) was gradually added with stirring. The mixture was then stirred at 35 °C for 2 h. The resulting solution was diluted by adding 50 mL of water under vigorous stirring and was kept for 15 min. The reaction was terminated by adding 150 mL of distilled water followed by 10 mL of 30% H2O2 solution. The graphite oxide precipitate was separated by centrifugation, washed repeatedly with 5% HCl solution and then with distilled water until the pH of the solution became neutral and was kept in an oven at 80 °C for 48 h to complete drying. Magnetic GO (MGO) nanosheets were fabricated according to Ding et. al [47]. First, Graphite oxide (100 mg) was dispersed in deionized water under sonication for 1 h for the exfoliation of the graphitic oxide into a GO monolayer. At the same time, 100 mg of Fe3O4 MNPs which were prepared via the co-precipitation method [48], were suspended in 0.1M HNO3 solution (100 mL). Next, the obtained positive surface charge Fe3O4 MNPs was added to the GO dispersion under sonication. After 1 h, the resulting nanocomposites were collected by using an external magnet and dried at 60 °C in vacuum oven. Cys-MGO nanosheets were synthesized by facile and green procedure [49]. Briefly, MGO (100 mg) was dispersed in distilled water (100 mL) and 10 ml of cysteine solution (20 mg mL-1) then was added. After addition an equimolar amount of NaOH, the mixture was sonicated for 1h at room temperature. The resulting nanocomposite was centrifuged, washed 6 

 

well with H2O/EtOH mixture and finally dried at 60 °C in vacuum oven. The overall synthesis procedure was shown in scheme 1. 2.3 Sample preparation Rice, wheat, milk and shrimp samples were purchased from local markets in Tehran (Iran). Where possible, each product was chosen from a different manufacturer. The procedure used for preparing samples for determination of metal ions was as follows. 0.4 g of each sample was weighed accurately and transferred into a Teflon vessel and 5 mL of concentrated nitric acid was then added. The vessels were sealed and placed into the microwave oven. Acid digestion was performed with the heating programme, which consist of three successive steps at maximum power. First, the temperature was increased to 120 °C and kept for 5 min. Next, the temperature was increased to 180 °C and kept for 15 min for complete digestion. Finally the digests were cooled to reach room temperature, transferred to a 50 mL volumetric flask and the volume was completed to the mark with phosphate buffer solution (pH 6.5). 2.4 Procedure of solid phase extraction The adsorption of Cd2+ ions by using Cys-MGO nanosheets was investigated in aqueous solution. For this purpose, an optimum amount of functionalized GO (8.0 mg) were put into 10.0 mL of aqueous solution containing Cd2+ ions (2.0 µg mL−1) and mixed by sonication for several minutes until the equilibrium was established. The adsorbent was then magnetically separated by using an external magnet. Next, 5 mL of 0.5 mol L−1 HCL was added as eluent and the mixture was sonicated again for 5 min. Finally, the adsorbent was removed and the supernatant was collected for the determination of Cd by ICP-OES.

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3. Results and discussion 3.1. Characterization of cysteine functionalized magnetic graphene oxide nanosheets Fig .1 shows the FT-IR spectra of the synthesized MGO and Cys-MGO nanosheets. The peak at 588 cm-1 is assigned to the stretching vibration of Fe–O bond [50]. The absorption peaks at 1724, 1620, 1401, 1257 and 1074 cm-1 in the FTIR spectrum of MGO are attributed to C=O,

C-C, C-OH, C-O-C and C-O vibrations, respectively [46]. The peaks at 870, 960,

1205 and 1577 cm-1 in Cys-MGO represent the N-H wag, S–H bend, C-N stretch and N-H bending vibrations, respectively [35,42,45]. In addition, for better comparisons, the spectrum of Cys-MGO was compared with the spectrum of pristine cysteine (Fig. 1c). Appearance of cysteine absorption peaks in the spectrum of functionalized graphene, confirmed successful functionalization process. The crystal structure of the as prepared samples was investigated by X-ray diffraction technique. Fig. 2 shows typical XRD patterns of Fe3O4 MNPs, GO and Cys-MGO nanosheets. As can be seen, six characteristic peaks (2θ = 30.1◦, 35.8◦, 43.3◦, 53.7◦, 57.7◦ and 63.1◦) of Fe3O4 MNPs, related to their corresponding indices ((2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0)) are observed. Deduced from Debye–Scherrerr’s formula, the average size of the Fe3O4 MNPs is about 12.6 nm. Also, a wide diffraction peak at 2θ = 11.78 with 0.83 nm d-spacing is appeared, which is attributed to the (001) crystalline plane of GO. As reported previously, due to the introduction of oxygenated functional groups on the carbon sheets, the interlayer spacing increases from 0.34 nm for pristine graphite to 0.83 nm for the GO [33,46].

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Thermal stability of graphite, GO, MGO and Cys-MGO nanosheets was examined by TGA analysis (Fig. 3). It is evident that, graphite was extremely stable up to 400 °C, with only a weight loss of 4.7% at 400 0C. Comparing with the graphite, GO shows much lower thermal stability which is because of introduction of oxygen-containing groups destroy the original multilayer stack structure of graphite. As can be seen, GO decomposes in three steps. The first weight loss at 50-120 °C relates to the loss of water molecules. The major mass reduction at approximately 200 °C corresponds to the removal of oxygen-containing functional groups and the third step above 300 °C relates to an unstable carbon remaining in the structure and the pyrolysis of oxygen functional groups in the main structure to yield CO and CO2. The total weight loss of GO at 400 0C was 25.7%. The results show that MGO has better thermal stability (total weight loss at 400 0C is 19.9%) than pure GO as a result of less oxygen-containing groups on MGO nanosheets. Moreover, as reported by Caruntu et al. [51], oxidation of Fe3O4 to Fe2O3 at temperature between 120 to 200 °C causes small weight gain. The mass loss of Cys-MGO nanosheets was 31.2%, which indicating the decrease of thermal stability. Assuming bonded cysteine was completely pyrolyzed, the approximately weight percentage of cysteine was 11.3%. The magnetization curves of Fe3O4 MNPs and Cys-MGO nanosheets are shown in Fig. 4, which indicates superparamagnetic behavior of functionalized GO nanosheets. The magnetic saturation values are 76.32 and 20.71 emu g-1, for Fe3O4 MNPs and Cys-MGO, respectively. The measured saturation magnetization of Cys-MGO nanosheets was strong enough to separate from aqueous solution because saturation magnetization of 16.3 emu/g was sufficient for magnetic separation with a conventional magnet [52].

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To further characterize the exact structures of functionalized GO nanosheets, TEM analysis was conducted. The TEM image of Cys-MGO nanosheets (Fig. 5) shows that the GO possesses a transparent sheet structure with uniform dispersion of spherical Fe3O4 MNPs (with an average particle size of 11.6 nm) on the GO nanosheets. 3.2 Evaluation the proficiency of cysteine functionalized magnetic graphene oxide nanosheets in cadmium removal 3.2.1 Optimization of the parameters As reported previously, cysteine has more affinity to coordinate with Cd2+ ions in comparison with other metal ions [44,45]. In this project, cysteine functionalized magnetic graphene oxide nanosheets were used as a sorbent for extraction and determination of Cd in various food samples. For this purpose, the effect of different parameters such as pH, extraction time, adsorbent content and extracting solvent were investigated in order to establish the optimum conditions for the extraction and determination of Cd. It is known that pH value has a critical role on solid phase extraction of metal ions. In order to determine the optimal pH, the effect of pH on adsorption of Cd was studied over the range of 2–9. The pH was adjusted by using phosphate buffer solution (PBS 0.1M). As can be seen in Fig. 6a, The adsorption efficiency increased with increasing the solution pH and then remained almost unchanged at pH 5–9. At lower pH values, more active sites (N-H and SH) are protonated and the numbers of free sites on the outer surface of Cys-MGO nanosheets are decreased, which decreases the adsorption capacity.

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In order to determine the minimum dosage of adsorbent required for bringing down the Cd level to the tolerance limit, the amount of Cys-MGO nanosheets was varied from 0.1 to 1 mg ml-1. In general, increasing the amount of adsorbent would increase the number of available adsorption sites. Satisfactory recoveries were obtained by using 0.8 mg ml-1 of sorbent for aqueous solution containing 2 µg ml-1 Cd (Fig. 6b). Other important factors which affect the preconcentration procedure are the type, volume, and concentration of the eluent used for the removal of metal ions from the sorbent. For this purpose, desorption studies were carried out by using HNO3, HCl and CH3COOH as eluents with different concentrations and volumes and all the regeneration experiments were carried out at room temperature. The results showed that the quantitative recoveries of the analytes (≥95%) could be obtained by using 5 mL of 0.5 mol L−1 HCl solution. To obtain an appropriate experimental time, the effect of ultrasonic time on the extraction process was examined. For this purpose, the ultrasonic time was varied in the range of 1–10 min for both adsorption and desorption experiments. Satisfactory results were obtained after 2 and 5 min for adsorption and desorption, respectively (Fig. 6c,d). The fast extraction rate indicates high affinity of cysteine’s functional groups to the Cd2+ ions with the adequately high binding constant. 3.2.2 Adsorption isotherms To understand the adsorption mechanism of Cd2+ ions on the Cys-MGO nanosheets, adsorption experiments were carried out by adding 5.0 mg of adsorbent to 10.0 mL solution containing 5.0, 10.0, 15.0 and 20 mg L−1 Cd at different temperatures ( t= 25, 35, 45 and 55 11 

 

°C). The equilibrium data were analyzed by Freundlich and Langmuir isotherm models [53,54]. The dimensionless equilibrium parameter RL also was determined according to the Langmuir model to predict whether the adsorption process is favorable or unfavorable. The adsorption equations and corresponding parameters are presented in Table 1. The Langmuir model assumes that adsorption takes place at specific homogeneous sites within the adsorbent, while the Freundlich model is used to describe heterogeneous adsorption systems. The fitting parameters for Cd adsorption isotherms were summarized in Table 2. In comparison to Freundlich equations, the Langmuir equations yielded R2 values higher than 0.98 in all testing temperatures, demonstrating that the adequacy of this model and favorable monolayer adsorption of the Cd onto the surface of Cys-MGO nanosheets at the concentrations of adsorbent and adsorbate applied. 3.2.3 Adsorption kinetics The adsorption rate is an important parameter used to image the adsorption process. Kinetic experiments were carried out by adding 20.0 mg of Cys-MGO nanosheets to 50.0 mL solution containing 5.0, 10.0, 15.0 mg L-1 of Cd at 25 °C. The result shown in Fig. 7 indicated that adsorption equilibriums were reached within 5 min. This rapid adsorption implied that the adsorption occurred mainly on the surface of the adsorbent. To further elucidate adsorption kinetics, the pseudo-first order and pseudo-second order kinetic models were used to examine experimental data. The kinetic equations and corresponding parameters are presented in Table 3. The higher correlation coefficients (R2) for pseudo-second order kinetic model indicated that the sorption followed this mechanism and the process was controlled by chemisorption [53].

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3.2.4 Thermodynamic parameters There are three thermodynamic parameters that must be considered to characterize the adsorption process which are the changes in standard free energy (ΔG◦), standard enthalpy (ΔH◦) and standard entropy (ΔS◦). The equations and related parameters with the corresponding calculated values for ΔG◦, ΔH◦ and ΔS◦ are summarized in Table 4. The negative values of ΔG◦ confirmed that the adsorption was spontaneous and the decreasing of ΔG◦ as temperature rises indicated that the adsorption was more favorable at high temperatures. The positive value of ΔH◦ confirmed the endothermic nature of adsorption process and also the positive value of ΔS◦ suggested the increasing randomness at the solid/liquid interface during the adsorption of Cd2+ ions on Cys-MGO nanosheets [53,55]. 3.2.5 Regeneration study In order to evaluate the possibility of regeneration and reusability of Cys-MGO nanosheets, repetitive adsorption-desorption cycles were performed. The results showed that, no significant difference in adsorption capacity was observed after 10 cycles and it remains almost constant. The acceptable reusability and stability indicate that the cysteine functionalized magnetic graphene oxide nanosheets can be potential adsorbents in practical environmental remediation. 3.2.6 Selectivity of the Cys-MGO nanosheets for extraction of Cd2+ ions To demonstrate the selectivity of the Cys-MGO nanosheets for extraction of Cd, interferences from other metal ions (K+, Na+, Mg2+, Ca2+, Fe3+, Cr3+, Ni2+, Cu2+, Pb2+ and Zn2+) on preconcentration and determination of 50 µg L-1 Cd2+ were examined. The effect is expressed

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as the recovery of Cd2+ in the presence of interfering ions relative to the interference-free response. The results showed that 1000-fold of (Ca2+, Mg2+, K+ and Na+), 500-fold of (Zn2+, Fe3+, Cr3+ and Ni2+) and 100-fold of (Cu2+ and Pb2+ ) had no significant interferences, when the tolerance limit was set as the amount of ions causing recoveries of the examined elements to be less than 90.0%. As reported previously, due to the more affinity of cysteine to coordinate with Cd2+ ions in comparison with other metal ions, the selectivity for extraction of Cd2+ improves clearly [45]. 3.2.7 Applications to real samples In order to investigate the accuracy and applicability of the optimized method in food samples, the concentration of Cd was determined in rice, wheat, milk and shrimp samples. The analytical results are shown in Table 5. Satisfactory results with the recoveries in the range of 94.1–98.4, were obtained. The world health organization recommends a maximum of 3 to 100 µg mL-1 for cadmium, in their guidelines for drinking water and daily foods. The results demonstrate the applicability of Cys-MGO nanosheets for solid-phase extraction of Cd in a variety of environmental and biological samples. 4. Conclusion In this study, cysteine functionalized magnetic graphene oxide nanosheets were prepared for the effective removal of cadmium from aqueous solutions. The characteristic methods (FTIR, XRD, TGA, VSM and TEM) confirmed succefull functionalization process. The kinetics of the adsorption process was evaluated utilizing the pseudo-first and pseudo-second order models. The equilibrium data were also analyzed using Langmuir and Freundlich isotherms. The results showed that, its kinetics followed the pseudo-second order mechanism, 14 

 

evidencing chemical sorption as the rate-limiting step of sorption mechanism. The best interpretation for the equilibrium data was given by Langmuir isotherm with the maximum adsorption capacities of 24.39-30.30 mg g−1 at 298-328 K. Thermodynamic parameters indicated that the adsorption process was spontaneous, endothermic and chemical in nature. The regeneration studies also showed that cysteine functionalized magnetic graphene oxide nanosheets can be used several times for the adsorption of cadmium without loss of their adsorption properties. Acknowledgement The authors wish to express their gratitude to Research Council of Standard Research Institute and Payame Noor University for the support of this work.

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Figure Captions

Fig.1 FT-IR spectra of (a) MGO (b) Cys-MGO and (c) Cysteine.

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Fig.2 XRD patterns of (a) Fe3O4 MNPs (b) GO and (c) Cys-MGO.

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Fig. 3 TGA curves of (a) graphite (b) GO (c) MGO and (d) Cys-MGO.

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Fig. 4 Magnetization curves of Cys-MGO and Fe3O4 MNPs (inset).

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Fig. 5 TEM image of Cys-MGO nanosheets.

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Fig. 6 Effect of (a) pH (b) catalyst loading (c) adsorption time (d) desorption time , on cadmium removal. Condition: [Cd2+]: 2 µg ml-1 , solution volume: 10 mL.

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Fig.7 Kinetic data for Cd removal at different initial Cd2+ concentration (a) 5 mg L-1 (b) 10 mg L-1 and (c) 15 mg L-1.

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Hummer method Fe3 O4 NPs dispersion

HS

O OH

Cysteine H2N

Scheme 1. Synthesis of cysteine functionalized magnetic graphene oxide nanosheets.

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Tables Table 1. Adsorption isotherm modes and related parameters Adsorption Linear equations Parameters models Langmuir

Ce 1 C   e qe K L qm qm RL  1 / (1  k LC0 )

Freundlich

Ln qe  Ln K F 

1 Ln Ce n

Plot

Ce: Equilibrium concentration (mg L−1) C0: Initial concentration (mg L−1) qe: Equilibrium adsorption capacity (mg g-1) qm: Maximum adsorption capacity (mg g-1) KL: Langmuir adsorption equilibrium constant (L mg−1) RL: Dimensionless constant separation factor*

Ce vs Ce qe

KF: Freundlich constant n: Empirical parameter related to the intensity of adsorption**

Ln qe vs Ln Ce

*The adsorption process could be irreversible, favorable, linear or unfavorable for (RL = 0), (0 < RL < 1), (RL = 1) or (RL > 1) respectively. **For favorable adsorption process the value of n should lie in the range of 1–10.

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Table 2. Isotherm constants for the adsorption of Cd2+ at various temperatures. T (K) Langmuir Freundlich qm (mg g−1) KL(L mg−1) R2 RL range KF (L g−1) 298 24.39 0.14 0.997 0.26-0.58 0.108 308 22.72 0.18 0.994 0.21-0.51 0.098 318 38.46 0.12 0.984 0.29-0.62 0.131 328 30.30 0.22 0.987 0.18-0.47 0.037

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n 0.589 0.597 0.731 0.562

R2 0.987 0.939 0.947 0.965

Table 3. Kinetic parameters of different models fitted to the experimental data. Linear equations Parameters Calculated parameters Cadmium concentration (mg L-1) 5 10 15 qe:2.817 qe:3.669 Ln ( qe  qt )  Ln qe  k1t qe: Equilibrium adsorption capacity qe:1.311 Pseudo-first order (mg g-1) k1: 0.113 k1: 0.122 k1: 0.124 qt: Adsorption capacity at time t R2:0.954 R2:0.985 R2:0.931 k1: Pseudo-first order rate constant (min-1) Kinetic models

Pseudo-second order

t 1 t   qt k2 qe2 qe h  k2 qe2

qe: Equilibrium adsorption capacity (mg g-1) qt: Adsorption capacity at time t k2: Pseudo-second order rate constant (g mg−1 min−1) h:Initial sorption rate (mg g−1min−1)

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qe: 6.944

qe: 13.15

qe: 18.51

K2: 0.377

K2: 0.186

K2: 0.138

h:18.18

h: 32.25

h: 47.62

2

R :0.999

2

R :0.999

R2:0.998

Table 4. Thermodynamic parameters for the adsorption of Cd2+ at different temperatures Linear equations Parameters ΔG◦= -RT Ln K R: Gas constant (8.314 J (mol K)−1) ΔG◦ (kJ mol-1) T: Absolute temperature (K) 298K 308 K 318 K K: Equilibrium constant at various temperatures* -2.68 -3.26 -3.76 ln K  

H 0 S 0  RT R

R: Gas constant (8.314 J(mol K)−1) T: Absolute temperature (K) K: Equilibrium constant at various temperatures*

 

-4.80

ΔH◦ (kJ mol-1)

ΔS◦ (J mol-1k-1)

16.79

67.89

*The values of K can be calculated according to the method of Lyubchik et al. [56].

34 

328 K

Table 5. Determination of cadmium in food samples (mean ± S.D., n = 3) Added (ng ml-1) Found (ng ml-1) Recovery (%) 0 11.27 ± 0.17 20 30.86 ± 0.14 98.0 40 49.32 ± 0.27 96.2 Wheat 0 18.70 ± 0.34 20 37.81 ± 0.26 97.7 40 56.53 ± 0.27 96.3 Milk 0 B.D.L. 20 19.68 ± 0.38 98.4 40 37.64 ± 0.21 94.1 Shrimp 0 21.07 ± 0.43 Sample Rice

20 40

39.92 ± 0.51 58.26 ± 0.57

97.2 95.4

*B.D.L.: Below Detection Limit

35