Fe3O4@SiO2@CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II)

Accepted Manuscript Title: Fe3 O4 @SiO2 @CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II) Authors: Fan Wang, Lijuan Zhang, Yeying Wang, Xijian Liu, Sohrab Rohani, Jie Lu PII: DOI: Reference:

S0169-4332(17)31535-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.179 APSUSC 36114

To appear in:

APSUSC

Received date: Revised date: Accepted date:

23-3-2017 12-5-2017 21-5-2017

Please cite this article as: Fan Wang, Lijuan Zhang, Yeying Wang, Xijian Liu, Sohrab Rohani, Jie Lu, Fe3O4@SiO2@CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II), Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.179 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.

Fe3O4@SiO2@CS-TETA functionalized graphene oxide for the adsorption of methylene blue (MB) and Cu(II)

Fan Wang a,1, Lijuan Zhang a,1,*, Yeying Wang a, Xijian Liu a, Sohrab Rohani b, Jie Lu a,*

a

School of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China

b

Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario N6A 5B9, Canada

*

Corresponding authors: Tel./fax: +86 21 67791216. E-mail addresses: [email protected] (J. Lu); [email protected] (L. Zhang).

1

These authors contributed equally to this work.

-1-

Graphical abstract

HIGHLIGHTS:  The magnetic composites Fe3O4@SiO2@CS-TETA-GO were synthesized.  The composites exhibited a high capacity for the fast adsorption of MB and Cu(II).  The composites can be easily recycled and reused as a novel absorbent.

ABSTRACT The graphene oxide (GO) functionalized by Fe3O4@SiO2@CS-TETA nanoparticles, Fe3O4@SiO2@CS-TETA-GO, was firstly fabricated in a mild way as a novel adsorbent for the removal of Cu(II) ions and methylene blue (MB) from aqueous solutions. The magnetic composites showed a good dispersity in water and can be conveniently collected for reuse through magnetic separation due to its excellent magnetism. When the Fe3O4@SiO2@CSTETA-GO was used as an absorbent for the absorption of MB and Cu(II), the adsorption kinetics and isotherms data well fitted the pseudo-second-order model and the Langmuir model, respectively. Under the optimized pH and initial concentration, the maximum adsorption capacity was about 529.1 mg g-1 for MB in 20 min and 324.7 mg g-1 for Cu(II) in 16 min, respectively, exhibiting a better adsorption performance than other GO-based -2-

adsorbents reported recently. More importantly, the synthesized adsorbent could be effectively regenerated and repeatedly utilized without significant capacity loss after six times cycles. All the results demonstrated that Fe3O4@SiO2@CS-TETA-GO could be used as an excellent adsorbent for the adsorption of Cu(II) and MB in many fields. Keywords: Graphene oxide; Functionalization; Fe3O4@SiO2@CS-TETA-GO; Adsorption; Cu(II) ions; Methylene blue (MB)

1. Introduction Dyes and heavy metal ions in water are deemed to be the most basic contaminants which are threatening to the ecological balance even the survival of human due to their non-biodegradability, toxicity, carcinogenicity and bioaccumulation [1,2]. The sewages containing methylene blue (MB) or Cu(II), which affect the health of all creatures, are released from various industries, including the mining, electroplating, dyes manufacturing, textile, leather tanning, paper coloring as well as the preparation of cosmetics [3-6]. Nowadays, multifarious techniques, including ion exchange [7], adsorption [8], membrane filtration [9,10], electrochemical treatment [11], and photocatalytic degradation [12,13], have been developed to remove the overmuch MB and Cu(II) from wastewaters. Among these methods, adsorption is frequently regarded as a preferred approach in wastewater treatment for the advantages such as workability, high efficiency, simple operation and low-cost over other methods. Over the past decade, a variety of adsorbent materials have been widely applied to remove Cu(II) and MB from wastewater, including natural polymers such as cellulose [14] and chitosan (CS) [15], carbonaceous material represented by activated carbons [16], zeolite [17], -3-

graphene oxide (GO) and its derivatives [8], clay [18] and so forth. Among these various materials, GO and CS have attracted considerable attention for their unique properties. Graphene oxide nanosheets, equipped with a one-atom-thick 2D layer of sp2-hybridized carbon, have drawn widespread interest in adsorption of all kinds of organic and heavy metals contaminants due to fantastic physicochemical properties such as excellent mechanical strength, extraordinary large specific surface area, π-π interactions and relatively facile modification [19-22]. However, poor solubility in water and less functional groups limits their application in the wastewater treatment and thus a number of work have been carried out to modify graphene oxide with multifunctional materials to break these restrictions [3,23,24]. For example, Liu et al. [4] reported that the maximum adsorption capacity of β-cyclodextrin/poly(acrylic acid) grafted graphene oxide (β-CD/PAA/GO) was 247.9 mg g-1 in the removal of MB from aqueous solutions. Though it can be simply prepared, the adsorption capacity of β-CD/PAA/GO for MB sounded still undesirable for industrial applications. Hu et al. [8] functionalized graphene oxide by chitosan (CS-GO) to adsorb MB, and reported its high adsorption capacity as 598.2 mg g-1. But its preparation through self-assembly method as well as separation from wastewater seems complicated and impractical. CS and its derivatives, a promising and extensive bio-adsorbent, have always been applied into the treatment of heavy metals and dye pollution due to their abundant functional groups [25]. For instance, Labidi et al. [26] prepared the ethylenediaminetetra acetic acid (EDTA) modified chitosan to adsorb Cu(II) and the maximum adsorption capacity was found to be 110 mg g-1. Nevertheless, it is far from a desirable adsorbent because of its weak mechanical -4-

property. Recently an efficient way to improve its practicability is to form a rigid and magnetic core [27]. Sun et al. [28] synthesized a kind of magnetic chitosan microspheres with quaternary ammonium groups as adsorbents to remove Cr(VI)，and the equilibrium time was determined to be 40-120 min which could be attributed to the large size (223.2 μm) of the adsorbent. Ren et al. [29] compounded magnetic Fe3O4@SiO2@CS particles modified with EDTA (EDCMS), which were estimated to reach 10 μm as adsorbent to treat wastewaters containing Cu(II), the maximum adsorption capacity and equilibrium time was 44.4 mg g-1 and 360 min respectively due to large size and low specific surface area. When graphene oxide grafted by xanthated Fe3O4-chitosan was used to remove Cu(II) from aqueous solutions, the maximum adsorption capacity was 426.8 mg g-1 which was the highest reported [5]. The high adsorption capacity can be attributed to abundant functional groups. However, the time to reach adsorption equilibrium of the material was up to 45 min, and the reusability did not meet the requirement due to its bad acid resistance. Recent studies suggest that the materials with nanoscale structure and modified with amino groups could markedly increase not only the specific surface area but also the adsorption capacity and rate [4,24]. In this paper, a new kind of adsorbent based on Fe3O4@SiO2@CS-TETA functionalized graphene oxide sheets, Fe3O4@SiO2@CS-TETA-GO, was successfully synthesized. As shown in Scheme 1, Fe3O4 magnetic cores were firstly synthesized by a convenient solvothermal method. Next, a thin layer of SiO2 was coated on the Fe3O4 cores using revised Stöber method to improve the acid resistance of cores. And next, CS thin-film was coated on Fe3O4@SiO2 by crosslinking with glutaraldehyde. After that, triethylenetetramine (TETA), possessing a good deal of amino groups, was grafted on the surface of Fe3O4@SiO2@CS. Finally, the -5-

Fe3O4@SiO2@CS-TETA nanocomposites were attached to the surfaces of GO by an amidation reaction between amino groups on TETA and carboxyl groups on GO. Thanks to the material and preparation process, the Fe3O4@SiO2@CS-TETA-GO exhibited excellent hydrophily and magnetism which afforded it to easily separate from the wastewater for reuse. The performance of the adsorbent was evaluated through the adsorption isotherms and kinetics of MB and Cu(II). Strikingly, the maximum adsorption capacity was 529.1 mg g-1 for MB and 324.7 mg g-1 for Cu(II), respectively, and in particular, had no obvious loss after six cycles.

2. Experimental 2.1. Materials Ferric chloride hexahydrate (FeCl3·6H2O), trisodium citrate dihydrate (C6H5Na3O7·2H2O), sodium acetate anhydrous (NaAC), tetraethylorthosilicate (TEOS), (3-aminopropyl) triethoxysilane (APTES), N-hydroxysuccinimide (NHS), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), monochloroacetic acid (MCA), N,N-dimethylformamide (DMF) and triethylenetetramine (TETA) were purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Ethylene glycol (C2H6O2), ammonium hydroxide (NH3·H2O, 28％-30％), powdery chitosan (CS, Mw 1.3×105 Da, 90% deacetylation), anhydrous ethanol, isopropanol alcohol, glutaraldehyde (GA, 25% w/w solution in water), acetate (99.5％), Cu(NO3)2·3H2O and methylene blue trihydrate (MB) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Epichlorohydrin was purchased from Shanghai Macklin Biochemical Co., Ltd. Graphene Oxide was purchased from XFNANO. Deionized water (DI water, resistivity 18.2 MΩ cm-1) used in all steps was produced by a Milli-Q purification system -6-

from Millipore (Bedford, MA). 2.2. Preparation of magnetic Fe3O4 nanoparticles A convenient solvothermal method previously reported was used to prepare the magnetic nanoparticles [30]. In a representative step, 3.13 g of FeCl3·6H2O and 0.81 g of Na3C6H5O7·2H2O were dissolved in 70 mL of ethylene glycol under steady stirring to form a clear solution, followed by a speedy addition of 4.81 g of NaAC with the help of magnetic stirring. Then the mixture was transferred into a Teflon reactor to react for 8 h at 473 K. After the reaction mixture was cooled down to room temperature, the black precipitate was collected with a magnet, washed several times with DI water and ethanol, respectively, and dried under vacuum at 333 K for further use. 2.3. Preparation of core-shell Fe3O4@SiO2 nanoparticles The Fe3O4@SiO2 core-shell structured nanoparticles were fabricated according to a revised Stöber method reported by previous studies [31]. Briefly, 0.12 g of Fe3O4 particles were dispersed into a mixture containing 85 mL of ethanol and 6 mL of DI water under the help of ultrasound. Then 2 mL of NH3·H2O was added into the above mixed solution. Next, a mixture containing 300 µL of TEOS and 5 mL of ethanol was added drop by drop into the above mixed solution and the reaction was carried out at room temperature for 15 h under vigorous stirring. The resulting Fe3O4@SiO2 nanoparticles were collected by a magnet, washed with DI water and ethanol thoroughly, and dried in a vacuum oven at 333 K. 2.4. Preparation of Fe3O4@SiO2-COOH nanoparticles The Fe3O4@SiO2-COOH nanoparticles were synthesized according to literatures [32,33]. In detail, 0.22 g of Fe3O4@SiO2 nanoparticles were firstly dispersed in a mixture containing -7-

90 mL of ethanol and 2 mL of DI water with the help of ultrasound. Next 1.8 mL of APTES was added and the mixture was stirred for 10 h at 323 K. And then, the product was collected and redispersed in 30 mL of alcohol. And 10 mL of monochloroacetic acid was added slowly under stirring. Next, the mixture was refluxed for 10 h at 353 K after the pH value was adjusted to 8.0. The final product was collected by a magnet, washed with DI water and ethanol several times, and dried in a vacuum oven at 333 K. 2.5. Preparation of Fe3O4@SiO2@CS nanoparticles The Fe3O4@SiO2@CS nanoparticles were produced by virtue of glutaraldehyde crosslinking method reported by previous studies [28,29]. At length, 0.11 g of CS was dissolved into 15 mL of 5％ w/w acetic acid aqueous solution under magnetic stirring till complete dissolution. Next, 0.01 g of Fe3O4@SiO2-COOH composites were dispersed into the CS solution with the assistance of ultrasonic vibration and the mixture was stirred for 12 h at room temperature. After that, the mixture was centrifuged and the product was redispersed in 15 mL of 5％ w/w acetic acid aqueous solution to remove the redundant CS which was not adsorbed on the surface of the magnetic composites. 0.5 mL of 25% w/w glutaraldehyde aqueous solution was dropwise added into the above dispersion and reacted for 3 h at 333 K. Finally, the resulting product was collected by a magnet, washed with DI water thoroughly, and dried in a vacuum oven at 333 K. 2.6. Preparation of Fe3O4@SiO2@CS-TETA nanoparticles The Fe3O4@SiO2@CS-TETA nanoparticles were synthesized in the light of literature [34]. Briefly, 0.52 g of Fe3O4@SiO2@CS composites were dispersed into 50 mL isopropanol alcohol at 313 K with stirring for 1 h, after that 10 mL of epichlorohydrin was added -8-

gradually under stirring. And 10 h later, the composites were collected by a magnet and washed with DI water to remove the excess epichlorodydrin. Next, the composites were redispersed in 30 mL of N,N-dimethylformamide at 323 K with stirring for 1 h, after that 20 mL of triethylenetetramine was dropped into the mixture slowly under stirring for 4 h. At the end, the final product was collected by a magnet, washed with DI water several times, and dried at 333 K under vacuum for further use. 2.7. Preparation of Fe3O4@SiO2@CS-TETA-GO composites The synthesis of Fe3O4@SiO2@CS-TETA-GO through a linkage reaction can be described as follows. Firstly, 0.08 g of GO was dissolved in 200 mL DI water under ultrasonic for 30 min, after that 3.80 g of EDC and 2.32 g of NHS were added and meanwhile the mixture’s pH was adjusted to 7 to activate the carboxyl groups of the GO. When the above mixture was mechanically stirred for 2 h at 313 K, 0.20 g of Fe3O4@SiO2@CS-TETA was added and stirred constantly at 338 K for 2 h. Afterwards the resulting product was collected by a magnet, washed with DI water, and dried in a vacuum oven at 333 K. 2.8. General characterization The morphology and size of a series of particles were observed by field emission scanning electron microscopy (FE-SEM; Sirion, 200, FEI, Netherlands) and transmission electron microscopy (TEM; JEM-2100F, 200 kV, JEOL, Japan). FTIR spectra were collected by PerkinElmer Fourier transform infrared spectrometer with KBR sheets as platform. X-ray diffraction (XRD) patterns for internal structure description were collected using an X’Pert PRO X-ray diffractometer (PANalytical, Netherlands). Hysteresis loops were acquired with a JDAW-2000D vibrating sample magnetometer (VSM, Lake Shore, CA) at room temperature. -9-

The concentration of Cu(II) in solution was determined by an Agilent 4100 microwave plasma-atomic emission spectrometer (MP-AES, Agilent Technologies, Australia), whereas the concentration of MB was determined by Cary 5000 UV-vis-NIR spectrophotometer (Agilent, CA). 2.9. Adsorption experiments The adsorption performance of Fe3O4@SiO2@CS-TETA-GO was evaluated through batch adsorption experiments using MB and Cu(II) as two types of model pollutants. The initial concentrations of MB and Cu(II) in stock solutions were 1.0 g L-1 and 0.5 g L-1. All adsorption experiments were performed in 50 mL of plastic tubes which were shaken by a water-bath oscillator at 200 rpm. All adsorption data had been repeated three times and all relative standard deviation (RSD) of adsorption isotherms and adsorption kinetics were less than 5%. 2.9.1. Effect of initial pH The initial pH of solutions was adjusted by 1.0 mol L-1 HCl solution and 1.0 mol L-1 NaOH solution in the ranges of 2-10 for MB and 2-6 for Cu(II), respectively. In fact, Cu(II) would precipitate when the pH surpassed 6. 0.01 g of Fe3O4@SiO2@CS-TETA-GO particles were added into 20 mL of stock solutions and the mixture was shaken for 24 h at 303 K. After the adsorption balance achieved, the equilibrium concentrations of MB solution and Cu(II) solution were determined using Cary 5000 UV-vis-NIR spectrometer and Agilent 4100 microwave plasma-atomic emission spectrometer respectively. Adsorption capacity was calculated from the following equation: Qe 

C0  Ce V m

(1) - 10 -

where C0 (mg L-1) and Ce (mg L-1) are the initial and the equilibrium concentrations of MB and Cu(II), respectively, Qe (mg g-1) is the adsorption capacity of adsorbent for MB and Cu(II), V (mL) is the solution volume, m (g) is the mass of the adsorbent. 2.9.2. Adsorption isotherms The initial concentrations of MB and Cu(II) were set from 50 mg L-1 to 1000 mg L-1 and 50 mg L-1 to 500 mg L-1, respectively. In a typical adsorption experiment, 10 mg of Fe3O4@SiO2@CS-TETA-GO particles were dispersed in 20 mL of a certain pollutant solution at the most appropriate pH and different temperature (293 K, 303 K 313 K). 2.9.3. Adsorption kinetics In the adsorption kinetics experiments, 0.10 g of adsorbent was added into 200 mL of each pollutant solution under stirred. The amount of adsorbed pollutant ( Qt ) at the time t can be calculated by: Qt 

C0  Ct V m

(2)

Where C0 (mg L-1) and Ct (mg L-1) are the initial and the time t concentrations of MB and Cu(II), respectively. 2.9.4. Desorption and reusability experiments. In adsorption-desorption cycles, 0.01 mol L-1 HCl and 0.01 mol L-1 NaOH aqueous solutions were employed as the desorbing agents to desorb Cu(II) and MB from the adsorbent at room temperature respectively. 0.2 g of MB- or Cu(II)- loaded adsorbent was added into a conical flask containing 20 mL of the desorbing agent and the mixture was stirred for 2 h at 298 K. Then the adsorbent was collected from the solution by a magnet and washed with DI water three times. The residual concentration of MB or Cu(II) was also determined. The - 11 -

adsorption-desorption cycle was executed 6 times totally.

3. Results and discussion 3.1. General characterization The morphology and structure of the particles Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA-GO were observed by SEM and TEM, respectively, as shown in Fig. 1. Fe3O4 nanoparticles presented a spherical shape with an average diameter of ~460 nm (Fig. 1(a)). After capsulation with SiO2, the surface of Fe3O4 nanoparticles became smooth (Fig. 1(c)). Meanwhile, Fig. 1(d) demonstrated that the Fe3O4@SiO2 nanoparticles possessed core-shell structure and the thickness of SiO2 shell was about ~64 nm, proving that silica shell was successfully coated on the surface of Fe3O4 nanoparticles by the aforementioned Stöber method. As shown in Fig. 1(e) and Fig. 1(f), CS was coated on the silica shell and the surface of Fe3O4@SiO2@CS was much rougher than that of Fe3O4@SiO2. Fig. 1(g) and Fig. 1(h) showed that Fe3O4@SiO2@CS-TETA was steadily attached onto the surface of GO. The FTIR spectra of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-COOH, Fe3O4@SiO2@CS, Fe3O4@SiO2@CS-TETA and Fe3O4@SiO2@CS-TETA-GO particles were compared in Fig. 2. As shown in Fig. 2(a), the peak at 577 cm-1 was attributed to the Fe−O vibration. Meanwhile, the peaks at 1630 and 1388 cm-1 were allocated to carboxyl stretching vibrations which resulted from NaAc adsorbed on the surface of Fe3O4 nanoparticles [35]. In the spectrum of Fe3O4@SiO2, the new peaks at 799 and 949 cm-1 corresponded to the symmetric stretching vibration of Si−O−Si and the stretching vibration of Si−OH, respectively, and the peaks at 1090 and 1209 cm-1 were attributed to stretching vibrations of Si−O−Si, indicating that Fe3O4 particles were successfully coated with SiO2 through chemical method [30,36]. In - 12 -

the Fe3O4@SiO2-COOH spectrum, the stronger peak at 1630 cm-1 and a new peak at 1403 cm-1 was assigned to the vibration of carboxymethyl bands, implying that the carboxymethyl substituent was linked to Fe3O4@SiO2 nanoparticles [33]. Compared with Fe3O4@SiO2-COOH nanoparticles, the well-resolved peaks at 1641 and 1578 cm-1 were assigned to the C＝O stretching vibration and N−H bending vibration of −CONH− formed through the reaction between the carboxyl group of Fe3O4@SiO2-COOH and the amide group of chitosan, which indicated that the Fe3O4@SiO2 nanoparticles were successfully coated with chitosan [36]. By comparison of the IR spectrum of Fe3O4@SiO2@CS, the new peak appeared around 1449 cm-1 in the spectrum of Fe3O4@SiO2@CS-TETA was assigned to C−N−C bond, which was introduced by TETA molecules. In addition, the stretching vibration intensity of the peak at 2929 cm-1 was greatly improved due to methylene in the TETA molecules, which verified that the TETA molecules were conjugated to the Fe3O4@SiO2@CS nanoparticles [34,37]. In the spectrum of Fe3O4@SiO2@CS-TETA-GO, several characteristic FTIR peaks were observed, for example, C−O−C (970 cm-1), C=O (1729 cm-1), and the peak at 3432 cm-1 was attributed to the stretching vibration of O−H, confirming that Fe3O4@SiO2@CS-TETA was successfully grafted onto the surface of GO [35]. Fig. 3 showed the powder X-ray diffraction patterns of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA-GO. The six characteristic peaks at 30.09°, 35.42°, 43.05°, 53.39°, 56.94° and 62.51° (Fig. 3a) were assigned to the (220), (311), (400), (422), (511), and (440) planes of pure Fe3O4 (JCPDS card No. 19-0629) respectively. Compared with Fe3O4, the intensity of the peaks of Fe3O4@SiO2 was decreased as a result of the coating by amorphous SiO2 layer (Fig. 3b). As shown in Fig. 3c, a new diffraction peak at - 13 -

21.32° presented in the pattern of Fe3O4@SiO2@CS, which was corresponded to a mixture of (001) and (100) planes of chitosan in the sample, that is, chitosan was successfully cross-linked on the surface of Fe3O4@SiO2 [38]. In addition, the strong characteristic peaks of Fe3O4 remained in Fig. 3d, indicating that the crystal structure of Fe3O4 was unchanged in the preparation process of Fe3O4@SiO2@CS-TETA-GO.

3.2. Magnetic separation performance The magnetic properties of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA-GO particles were tested using VSM, and the magnetic hysteresis loops were shown in Fig. 4. The saturation magnetization values at room temperature were discovered to be 40.49, 28.20, 13.27 and 8.22 emu g-1 for Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA-GO, respectively. Not surprisingly, the saturation magnetization values decreased in turn due to the coating of silica and chitosan which weakened the magnetic intensity of particles. For all this, the final product Fe3O4@SiO2@CS-TETA-GO still remained adequate magnetism to meet the need of magnetic separation by an external magnetic field from wastewaters, which could be illustrated by the result from the inset in Fig. 4. 3.3. Adsorption of MB and Cu(II) 3.3.1. Effect of pH on adsorption Because the pH of aqueous solutions will strongly affect the existence forms of Cu(II) and MB in the solutions [39], in this work the effect of pH was investigated in wide pH ranges from 2-11 for the adsorption of MB and from 2-6 for the adsorption of Cu(II), respectively. As - 14 -

shown in Fig. 5, the adsorbed amounts of both Cu(II) and MB increased with the increasing of pH value, and were 301.3 mg g-1 of Cu(II) at pH = 6 and 404.2 mg g-1 of MB at pH = 10.70, respectively. The results demonstrated that the adsorption capacity of synthesized absorbent for MB and Cu(II) was distinctly restrained by hydrogen ion which can compete active sites with MB and Cu(II) at low pH [40]. In other words, with the increasing of the solution pH, the active sites were released by the hydrogen ion and accordingly the adsorption capacity of synthesized absorbent for both MB and Cu(II) increased. 3.3.2. Adsorption isotherms Adsorption isotherms were studied to assess the adsorption performance of synthesized adsorbent as well as the interaction between the adsorbate and adsorbent surface [3]. As demonstrated in Fig. 6, with the increasing of the initial concentrations of MB and Cu(II), the adsorption capacity of Fe3O4@SiO2@CS-TETA-GO composite microspheres increased until a plateau at C0 = 400 mg L-1 of Cu(II) and C0 = 900 mg L-1 of MB, at which the adsorptions of MB and Cu(II) reached saturation. The saturated adsorption capacity were calculated as 301.6 (mg g-1) for Cu(II) and 495.1 (mg g-1) for MB, respectively. The adsorption isotherms of MB and Cu (II) on the Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA are shown in Fig. S1 (Supplementary Materials). As shown in Fig. S1, with the increase of the initial concentration, the adsorption capacity of MB and Cu(II) on Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA composite microspheres increased. Besides, the saturated adsorption capacity of MB and Cu(II) on Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA increased in turn due to the increase of the content ratio of functional groups such as −NH2, −NH−, −COOH, −OH and benzene rings. - 15 -

In order to reveal the adsorption isotherm mechanism, conventional Langmuir (Eq. (3)) and Freundlich (Eq. (4)) adsorption isotherm models were selected to describe adsorption equilibria [41], which can be expresses as: Ce C 1  e  Qe Qmax K LQmax log Qe  log K F 

(3)

1 log Ce n

(4)

where Qe (mg g-1) and Qmax (mg g-1) are the equilibrium adsorption capacity and maximum adsorption capacity respectively; and Ce (mg L-1) is the concentration of the adsorbate at equilibrium; K L is the Langmuir isothermal constant; K F is the Freundlich adsorption constant and n is the heterogeneity factor. The fitting lines of Langmuir isotherm and Freundlich isotherm are shown in Fig. 7(A) and Fig. 7(B) respectively, and the calculated isotherm parameters are listed in Table 1. The Langmuir isotherm model’s R2 = 0.999 of Cu(II) and R2 = 0.990 of MB are higher than the Freundlich isotherm model’s R2 = 0.933 of Cu(II) and R2 = 0.956 of MB, suggesting that adsorption isotherm data of both Cu(II) and MB fit the Langmuir model better than the Freundlich isotherm model. The adsorption process of both Cu(II) and MB on the surface of Fe3O4@SiO2@CS-TETA-GO composites was the monolayer adsorption [23]. 3.3.3. Thermodynamic parameters In order to investigate the type of adsorption and the influence of temperature on the adsorption of MB and Cu(II), the thermodynamic parameters (△H, △S, △G) can be evaluated by the following equations [23]:

G   RT ln Kd

(5)

- 16 -

ln K d 

S H  R RT

(6)

Where △G (kJ mol-1) is the Gibbs free energy change, R (8.314 J mol-1 K-1) is the gas constant, T (K) is Kelvin temperature, Kd is the thermodynamic equilibrium constant, △S (J mol-1 K-1) is the entropy change and △H (kJ mol-1) is the enthalpy change. As shown in Table 2, the thermodynamic parameters (△H, △S, △G) in the process of adsorption of MB were as follows: △H = -43.84 kJ mol-1, △S = -127.40 J mol-1 K-1, and △G = -6.51 kJ mol-1, -5.24 kJ mol-1, and -3.96 kJ mol-1, at 293 K, 303 K, 313 K, respectively. And the thermodynamic parameters (△H, △S, △G) in the process of adsorption of Cu(II) were as follows: △H = 25.41 kJ mol-1, △S = 103.07 J mol-1 K-1, and △G = -5.31 kJ mol-1, -5.82 kJ mol-1, and -6.85 kJ mol-1, at 293 K, 303 K, 313 K, respectively. The negative values of △G for both MB and Cu(II) indicated the process of adsorption is spontaneous. Moreover, the △G absolute values of MB decreased with the increase of temperature, suggested a lower temperature strengthened the process of adsorption of MB. In addition the △G absolute values of Cu(II) increased with the increasing of temperature, suggested a higher temperature enhanced the process of adsorption of Cu(II). The negative values △H and △S of MB implied the exothermic nature and decreasing randomness at the solid-solution interface during the process of adsorption of MB, and The positive values △H and △S of Cu(II) implied the endothermic nature and the increasing randomness at the solid-solution interface during the process of adsorption of Cu(II). In addition, the absolute value of both MB and Cu(II) approximated the general reaction enthalpy (40 kJ mol-1), which signified chemical adsorption was dominant in the process of adsorption [34]. 3.3.4. Adsorption kinetics - 17 -

Adsorption kinetics was used to evaluate the adsorption performance and application of in-house synthesized adsorbent. Fig. 8 shows the adsorption kinetics curves of MB and Cu(II) onto Fe3O4@SiO2@CS-TETA-GO. It can be observed that the adsorption equilibrium time of MB and Cu(II) was 20 min and 16 min, respectively. The fast adsorption rate may be attributed to the multiple functional groups on the surface of Fe3O4@SiO2@CS-TETA-GO which can interact with MB and Cu(II) [5], and the nanostructure of the adsorbent which is beneficial to reducing the mass transfer resistance [4]. The adsorption kinetics of MB and Cu (II) on the Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA are showed in Fig. S2 (Supplementary Materials), indicating that the adsorption equilibrium time of MB and Cu(II) on the Fe3O4@SiO2@CS and Fe3O4@SiO2@CS-TETA had no obvious change because the size of adsorbent had not changed after the introduction of TETA and GO. To further reveal the adsorption mechanism predominating the adsorption process of MB and Cu(II) on the Fe3O4@SiO2@CS-TETA-GO. The frequently-used pseudo-first-order and pseudo-second-order kinetics models were employed and appraised as follows [42]: 1 K 1  1  Qt Qet Qe

(7)

t 1 t   2 Qt K 2Qe Qe

(8)

where Qe (mg g-1) and Qt (mg g-1) are the adsorption capacities at equilibrium and at time t (min), respectively. K1 and K2 (g mg-1 min-1) are the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. Fig. 9 shows the linear plots correlating the experimental data by pseudo-first-order and pseudo-second-order kinetics models. All the kinetics model parameters from the linear fitting - 18 -

are listed in Table 3. The correlation coefficients R2 (MB: 0.954; Cu(II): 0.973) of pseudo-first-order model were inferior to those (MB: 0.999; Cu(II): 0.998) of pseudo-second-order model. In addition, the calculated Qe by the pseudo-second-order model was 309.8 (mg g-1) for Cu(II) and 476.2 (mg g-1) for MB, which were more closer to the experimental Qe of 301.6 (mg g-1) for Cu(II) and 495.1 (mg g-1) for MB than the calculated Qe by the pseudo-first-order model, suggesting that the adsorption of Cu(II) and MB on the in-house constructed Fe3O4@SiO2@CS-TETA-GO was predominated by chemical adsorption [6]. Furthermore, the adsorption capacity and adsorptive equilibrium time of various materials for the adsorption of MB and Cu(II) are compared in Table 4 and Table 5, respectively. The constructed Fe3O4@SiO2@CS-TETA-GO composites exhibited a best adsorption performance. 3.3.4. Adsorption mechanism XPS, which is a useful technique to identify the chemical elements on the surface of adsorbents, was introduced for further exploring the complex adsorption mechanism between adsorbents and adsorbates. Typical XPS spectra of Fe3O4@SiO2@CS-TETA-GO before (a: black line) and after the adsorption of MB (b: red line) and Cu(II) (c: blue line) were shown in Fig. 10A. It can be obviously found that Fe3O4@SiO2@CS-TETA-GO showed peaks of C1s, O1s and N1s before and after adsorption. New peaks of Cu2p appeared in the spectrum of Fe3O4@SiO2@CS-TETA-GO after the adsorption of Cu(II) (Fig. 10B) and of S2p after the adsorption of MB (Fig. 10C), indicating that Cu(II) and MB were respectively adsorbed on the Fe3O4@SiO2@CS-TETA-GO. As shown in Fig. 10B, the satellite peaks around 943.7 eV and 963.0 eV were attributed to the existence of the oxidation state of Cu(II), and the peaks at - 19 -

935.4 eV and 955.3 eV should be assigned to Cu2p3/2 and Cu2p1/2, respectively [49]. As shown in Fig. 11A, the N1s XPS spectrum of Fe3O4@SiO2@CS-TETA-GO before adsorption, the peak at 399.5 eV can be assigned to −NH− groups in TETA, the peak at 399.3 eV can be assigned to −NH2 groups in TETA and CS, and the peak at 401.5 eV can be assigned to protonated amine functions (−NH3+) [49,50]. The O1s XPS spectrum (Fig. 11D) has three peaks at 530.9 eV (O−Fe), 531.9 eV (O=C) and 532.8 eV (C−O−C, O−H) [51]. The peak assignment and group content of N1s and O1s are summarized in Table.6. After the adsorption of Cu(II), large changes can be found in N1s and O1s high resolution spectra (Fig. 11B, E). At length, the content of −NH3+ decreased from 16.3% to 0%, and those of −NH− and −NH2 decreased from 50.2% to 18.4% and from 33.5% to 10.3%, respectively. In Fig. 11E, the content of O=C and O−H decreased from 49.1% to 43.7% and from 18.6% to 15.6%, respectively, which suggested that the hydroxyl and the carboxyl were involved in the adsorption of Cu(II) due to oxygen atoms possess two pairs of lone electrons. Based on the above results, the chemical adsorption of Cu(II) on the surface of Fe3O4@SiO2@CS-TETA-GO can be expressed by the following processes [50,51]: 

R  NH 3  Cu2  R  NH 2Cu2  H 

(7)

R  NH 2  Cu 2  R  NH 2Cu 2

(8)

2R  NH 2   Cu2  R  NH 2 2 Cu2

(9)

R  COOH  Cu 2  ( R  COOH )Cu 2

(10)

R  OH  Cu2  ( R  OH ) Cu2

(11)

When MB was adsorbed on Fe3O4@SiO2@CS-TETA-GO, the peak of −NH3+ disappeared due to the high pH, as shown in Fig. 11C, and the peaks of O1s in Fig. 11F had - 20 -

no drastic change, revealing that the chemical adsorption was driven by such weak interactions as electrostatic interactions, hydrogen bonding, aromatic-aromatic interactions, etc. As illustrated as Fig. 12, the benzene rings, −OH, −NH2 and −COO− on the surface of the Fe3O4@SiO2@CS-TETA-GO interacted with the benzene rings, amines, quaternary ammonium through π-π stacking, hydrogen bonding and electrostatic interactions [52,53]. 3.4. Desorption and reusability Reusability of adsorbent is an important index in practical application. The adsorption capacity of desorption-adsorption cycles were shown in Fig. 13. It can be found that the adsorption capacity dropped to 81.1% and 83.2% of MB and Cu(II) after six recycles respectively, which could be explained by the inadequacy desorption. Even so, the results indicated that the Fe3O4@SiO2@CS-TETA-GO nanocomposites had desirable reusability for the practical adsorption of Cu(II) and MB.

4. Conclusions In this study, magnetic composites Fe3O4@SiO2@CS-TETA-GO were successfully constructed as a novel adsorbent for the adsorption of Cu(II) and MB. Characterizations and batch adsorption experiments demonstrated that the adsorbent had a satisfactory adsorption capacity up to 324.7 mg g-1 for Cu(II) and 529.1 mg g-1 for MB, respectively. In particular, the adsorption equilibria were reached in 20 min for MB and 16 min for Cu(II), respectively. The excellent adsorption performance can be attributed to the function groups on the absorbent surface contributed by CS, TETA and GO simultaneously. Furthermore, good reusability and magnetic responsiveness also endow them a high potential application in practice for the removal of Cu(II) and MB from waters. - 21 -

Acknowledgements This research work was financially supported by the National Natural Science Foundation of China (Nos. 21176102, 21176215 & 21476136), the Natural Science Foundation of Jiangsu Province (No. BK20131100), the Connotation Construction Project of SUES (No. Nhky-2015-05), Science and Technology Commission of Shanghai Municipality (No. 15430501200) and the Sino-German Center for Research Promotion (No. GZ935).

- 22 -

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757-766.

- 30 -

Figure captions: Fig. 1. Photos of SEM (a) and TEM (b) of Fe3O4, SEM (c) and TEM (d) of Fe3O4@SiO2, SEM (e) and TEM (f) of Fe3O4@SiO2@CS, SEM (g) and TEM (h) of Fe3O4@SiO2@CS-TETA-GO. Fig. 2. The FIIR spectra of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2-COOH (c), Fe3O4@SiO2@CS (d), Fe3O4@SiO2@CS-TETA (e), and Fe3O4@SiO2@CS-TETA-GO (f). Fig. 3. Powder X-ray diffraction patterns of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2@CS (c), Fe3O4@SiO2@CS-TETA-GO (d). Fig. 4. (A) The magnetic hysteresis loops of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2@CS (c) and Fe3O4@SiO2@CS-TETA-GO (d). (B) The inset illustrates the magnetic separation of Fe3O4@SiO2@CSTETA-GO from MB aqueous solution using a magnet was achieved within 30s. Fig. 5. Effect of initial solution pH on the adsorption of Cu(II) and MB by Fe3O4@SiO2@CS-TETA-GO at 303 K. Initial concentrations of both Cu(II) and MB were 500 mg L-1. Fig. 6. Adsorption isotherms and error bars of Fe3O4@SiO2@CS-TETA-GO for Cu(II) and MB at 303K (MB: pH=10, Cu(II): pH=6). Fig. 7. (A) Langmuir isotherm and (B) Freundlich isotherm of Cu(II) and MB on the surface of Fe3O4@SiO2@CS-TETA-GO. Fig. 8. Adsorption kinetics and error bars of Cu(II) and MB on Fe3O4@SiO2@CS-TETA-GO at 303 K (MB: pH=10, Cu(II): pH=6). Fig. 9. (A) Pseudo-first-order and (B) pseudo-second-order models for the adsorption of Cu(II) and MB on the Fe3O4@SiO2@CS-TETA-GO composites. Fig. 10. The XPS spectra of Fe3O4@SiO2@CS-TETA-GO (a: black line), after the adsorption of MB (b: red line) and after the adsorption of Cu(II) (c: blue line): survey (A), Cu2p (B), S2p (C).

- 31 -

Fig. 11. The XPS spectra of N1s and O1s of adsorbent (A and D), loading Cu(II) (B and E) and loading MB (C and F). Fig. 12. The proposed adsorption mechanism of MB on Fe3O4@SiO2@CS-TETA-GO. Fig. 13. Adsorption-desorption cycles performance of Fe3O4@SiO2@CS-TETA-GO for removal of MB and Cu(II). Scheme 1. Schematic illustration of the synthesis of Fe3O4@SiO2@CS-TETA-GO.

- 32 -

Table heads: Table 1 Isotherm parameters of MB and Cu(II) adsorbed onto Fe3O4@SiO2@CS-TETA-GO. Table 2 Thermodynamic parameters for adsorption of MB and Cu(II) on Fe3O4@SiO2@CS-TETA-GO. Table 3 Model parameters and correlation coefficients of the kinetics models for the adsorption of MB and Cu(II) on the Fe3O4@SiO2@CS-TETA-GO. Table 4 Adsorption capacity and adsorptive equilibrium time of various materials for the adsorption of MB. Table 5 Adsorption capacity and adsorptive equilibrium time of various materials for the adsorption of Cu(II). Table 6 High resolution XPS analysis of Fe3O4@SiO2@CS-TETA-GO after adsorption.

- 33 -

Table 1 Isotherm parameters of MB and Cu(II) adsorbed onto Fe3O4@SiO2@CS-TETA-GO. Adsorbate

Langmuir model

Freundlich model

Qmax (mg g-1)

KL (g mg-1)

R2

KF

n

R2

Cu(II)

324.8

0.028

0.999

42.942

2.948

0.933

MB

529.1

0.013

0.990

46.871

2.703

0.956

- 34 -

Table 2 Thermodynamic parameters for adsorption of MB and Cu(II) on Fe3O4@SiO2@CS-TETA-GO. Parameters InKd △G (KJ mol-1) △H (KJ mol-1) △S (J mol-1 K-1)

MB

Cu(II)

293 K

303 K

313 K

293 K

303 K

313 K

2.67 -6.51

2.08 -5.24 -43.84 -127.40

1.52 -3.96

2.14 -5.31

2.31 -5.82 25.41 103.07

2.63 -6.85

- 35 -

Table 3 Model parameters and correlation coefficients of the kinetics models for the adsorption of MB and Cu(II) on the Fe3O4@SiO2@CS-TETA-GO. Pollutants

Pseudo-first order

Pseudo-second order

Qe (mg g-1)

K1 (min-1)

R2

Qe (mg g-1)

K2 (g mg-1 min-1)

R2

MB

456.6

1.630

0.954

476.191

0.001

0.999

Cu(II)

318.5

3.153

0.973

309.579

0.002

0.998

- 36 -

Table 4 Adsorption capacity and adsorptive equilibrium time of various materials for the adsorption of MB. Adsorbents

Qm (mg g-1)

Equilibrium time (min)

Refs

MG@m-SiO2

178.5

120

[6]

β-CD/PAA/GO

248.0

40

[4]

MCGO

180.8

-

[24]

CA-mGO/CS

315.5

-

[43]

Iron-oxide/polymer

298.0

-

[44]

PAA/MnFe2O4

53.3

-

[45]

Fe3O4@CPTES@AG

246.3

10

[46]

Fe3O4@SiO2@CS-TETA-GO

529.1

20

This work

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Table 5 Adsorption capacity and adsorptive equilibrium time of various materials for the adsorption of Cu(II). Adsorbents

Qm (mg g-1)

Equilibrium time (min)

Refs

EDTA-mGO

301.2

90

[23]

PET-AA-CS

69.0

60

[25]

CS

67.0

-

[26]

CS-EDTA

110.0

-

[26]

CMS

31.4

180

[29]

EDCMS

44.4

360

[29]

RL-LDH

116.1

-

[47]

HMS-NHs

138.6

-

[48]

Fe3O4@SiO2@CS-TETA-GO

324.7

16

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Table 6 High resolution XPS analysis of Fe3O4@SiO2@CS-TETA-GO after adsorption. Region

O1s

N1s

Assignment

Before adsorption

After adsorption Cu(II)

After adsorption MB

Be (eV)

%

Be (eV)

%

Be (eV)

%

O−Fe

530.9

32.3

530.9

40.7

530.9

30.8

O=C

531.9

49.1

531.9

43.7

531.9

51.3

C−O−C, O−H

532.8

18.6

532.8

15.6

532.8

17.9

−NH−

399.5

50.2

399.5

18.4

399.5

41.3

−NH2

399.3

33.5

399.3

10.3

399.3

58.7

−NH3+

401.5

16.3

401.5

0

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