Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide

Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide

Author’s Accepted Manuscript Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide Cuicui Wang, Heyi Ge, Yueying Zhao...

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Author’s Accepted Manuscript Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide Cuicui Wang, Heyi Ge, Yueying Zhao, Shanshan Liu, Yu Zou, Wenbo Zhang www.elsevier.com/locate/jmmm

PII: DOI: Reference:

S0304-8853(16)31189-1 http://dx.doi.org/10.1016/j.jmmm.2016.09.128 MAGMA61909

To appear in: Journal of Magnetism and Magnetic Materials Received date: 23 June 2016 Revised date: 29 September 2016 Accepted date: 30 September 2016 Cite this article as: Cuicui Wang, Heyi Ge, Yueying Zhao, Shanshan Liu, Yu Zou and Wenbo Zhang, Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.09.128 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 galley proof before it is published in its final citable 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.

Study on the adsorption of Cu(II) by folic acid functionalized magnetic graphene oxide Cuicui Wang1,2, Heyi Ge1,2*, Yueying Zhao3, Shanshan Liu1,2, Yu Zou1,2, Wenbo Zhang1,2 1

Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials,

University of Jinan, Jinan 250022, P.R. China 2

School of Material Science and Engineering, University of Jinan, Jinan 250022, P. R. China Shandong Xiaguang Industrial Co., LTD, Jining 272000, P. R. China * Corresponding author: Tel.: +86 531 82769579. [email protected] 3

Abstract The folic acid functionalized magnetic graphene oxide (FA-mGO) as a new adsorbent has been synthesized in this work for the elimination of Cu(II) from waste water. The as-prepared FA-mGO was tested by SEM, TEM, particle size analyzer, FTIR, XRD, Roman spectrum, TGA and magnetic properties analyzer. Some factors, such as adsorbent dose, pH, contact time, initial concentration of adsorbate and temperature were explored. The results showed that the FA-mGO had the better adsorption performance than mGO. After 40 min, the adsorption equilibrium could be reached. Furthermore, the adsorption property obeyed the pseudo-second order kinetic model and the Temkin isotherms well. The maximum adsorption capacity was 283.29 mg g-1 for Cu(II) from Pseudo-second-order model at pH = 5 and 318 K. The chelation action between FA and Cu(II) along with electrostatic incorporation between GO and Cu(II) determined the favourable adsorption property. Besides, thermodynamic studies results ∆G0<0, ∆H0>0, ∆S0>0 suggested that the adsorption mechanism was an endothermic and spontaneous process essentially. Finally, desorption and reusability studies imply FA-mGO has an excellent reproducibility and is benefit to environmental protection and resource conservation. 1

Graphical abstract: Schematic illustration of preparing FA-mGO Keywords: Folic acid; Magnetic GO; Cu(II) ions; Adsorption kinetics; Adsorption isotherm

1. Introduction With the rapid development of the modern industry, water pollution of heavy metals has been a burgeoning worldwide environmental issue. Thereinto, copper (Cu(II)) is taken for one of the most pernicious metals. 1 Therefore, an efficient method should be explored urgently to eliminate Cu(II) from waste water. 2-5 So far, more and more scientists proposed many ways to remove heavy metal ions from waste water.

6

The common methods are chemical precipitation,

7

membrane filtration,

8

electrolysis,

9

coagulation, 10 ion exchange, 11 biological treatment 12 and adsorption strategies. 13-20 Among them, adsorption process is considered the most feasible techniques due to its reproducibility, economy, easy for operation, effective and sensitive to toxic substance.

21-22

So adsorbent is a very important

step in the adsorption procedure. Therefore, preparation of adsorbent with excellent adsorption behavior is a key step for adsorption research. If the adsorbent is simple synthesis, inexpensive, environmental and efficient for removal heavy metal ions, it will be very employable. 23,24 Activated carbon,

25

resin,

26

zeolite

27

and clay

28

are the traditional materials for the

adsorption of heavy metal ions from waste water. However, the above materials have some disadvantages such as wide distribution of pore size, small specific area, uneven pore structure and poor selectivity to metal ions and there is energy intensive and hard to regenerate.

17

What’s more,

the majority mesoporous materials with the adsorptive action to heavy metal ions are modified by

2

organic thiol/ether

20

and amine groups

18

which hold the inevitable toxicity. It is easy to cause

secondary pollution after adsorption process. Folic acid (FA) is a derivative of amino acid and equivalent to pteroyglutamic acid. So far, FA has been widely applied in sensor, health, especially in drug delivery

29-32

due to the role of

prevention of cancer. FA has the abundant functional groups, including carboxyl, hydroxyl and amidogen functional groups which can form coordination complexes with most metal ions. It is easily used to synthesize multiple adsorbents with other materials for the removal of heavy metal ions. Furthermore, FA is nontoxic. The water will not be contaminated after adsorption. Therefore, FA has the better environmental protection than EDTA which has been widely reported.

16,21,22

Based on this principle, FA will be an excellent candidate of adsorbent. There are mainly two aspects to evaluate the adsorption property of compound functionalized materials. The one is the performance of base materials, the other is intermediates used for fixing compound on base materials. 33,34 Graphene oxide (GO) is considered as a good selection for the base material. 35-38 GO has vast functional groups introduced by the oxidation process of graphite and high surface area. The groups are the key chemical skeletons which can be used as anchoring sites for metal ions connection, making it become a potential material as a super and ideal adsorbent.

39,40

In recent

years, nano-Fe3O4 as a popular magnetic material has been applied in wastewater treatment field due to the achievement of the easy solid-liquid separation under an external magnetic field, without complex centrifugation or filtration.

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Good magnetic recycle can save cost and avoid unknown

damage to environment. 42,43 There are many reports on absorption performance of magnetic graphene oxide (mGO), GO/polyaniline,

15,44

GO/polypyrrole,

13,14

mGO/polythiophene,

45

42

mGO supported by

3

cyclodextrin46 and EDTA-mGO.

21

However, very few reports about FA and FA functionalized

magnetic GO (FA-mGO) as adsorbents were reported. In this work, mGO was synthesized using a solvothermal system and then 3-Triethoxysilylpropylamine (APTES) was decorated on mGO (f-mGO). Finally, FA-mGO with eco-friendly and high surface area property was facilely synthesized by grafting FA on f-mGO. The relevant factors that influenced the adsorption performance including adsorbent dose and pH, etc were discussed. The adsorption kinetics, isothermals and thermodynamic were carried out to investigate the interaction between FA-mGO and Cu(II)) ions. The recyclability and reproducibility of FA-mGO were measured to explore its potential environment and economic benefits.

2. Experimental section 2.1. Materials Graphite powder (8000 meshes, 99.95%) and all chemicals agents supplied by Aladdin Industrial Corporation (Shanghai, China) were analytical grade. APTES was kindly provided by Taishan Glass Fiber INC. 2.2. Synthesis of mGO The GO aqueous suspension was prepared using the modified Hummers method and had related in our previous reports. 47-50 Then mGO was synthesized using a solvothermal system.

21

3.24 g FeCl3·6H2O was added

into ethylene glycol under magnetic stirring for dissolution. Then, 16.22 g GO aqueous suspension (10 mg/g) was dispersed into the above solution. With continuous magnetic stirring, 3 ml ammonium hydroxide was added to the solution by continuous dipping method. After forming the uniform mixture solution, polyvinylpyrrolidone was slowly added. Finally, the solution was transferred into a teflon-lined stainless steel autoclave for 20 h at 180 ℃. When the autoclave was cooled to room temperature naturally, the obtained product (mGO) was separated by magnet then

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washed by ultrapure water and ethanol repeatedly. Finally, it was dried at 50 ℃ in the vacuum drying oven and grinded into powder for the following experiments. 2.3. Synthesis of f-mGO 2.5 g of mGO and 8 g of APTES were added into 200 g of ethanol. Then, 16 g of ultrapure water was slowly added into the above solution. The mixture was stirred using a mechanical stirrer at 25 ℃ for 4 h. The obtained material was washed with ethanol and ultrapure water for several times and dried in a vacuum oven at 50 ℃ then grinded into powder for the following experiments. 2.4. Synthesis of FA-mGO Firstly, triethylamine (0.50 mL), N-Hydroxysulfosuccinimide (0.52 g, 5.00 mmol), dicyclohexycarbodiimide (0.50 g, 2.50 mmol) and FA (1.0 g, 2.27 mmol) were added into the 40 mL dry dimethyl sulfoxide with stirring for 20min for activating carboxyl function groups. Then, the prepared f-mGO was pour into the mixture and stirred at room temperature in the dark for 24 h. The obtained material was washed with ethanol and ultrapure water for several times and dried in a vacuum oven at 50 ℃ then grinded into powder for the measurements. The prepared process chart and reaction principle were demonstrated in Fig. 1. 2.5. Characterizations The GO sheets were characterized by Dimension Icon type atomic force microscope (AFM) system (Bruker, USA). The external features of FA-mGO were characterized by FEI QUANTA FEG 250 field emission scanning electron microscopy (SEM) (FEI, USA) system and a JEM 2010 transmission electron microscopy (TEM) (Electronics Co., Ltd, Japan). A LS-13320 dynamic laser particle analyzer (Beckman Coulter, United States) was used to test particle size and distribution. 48 The relevant properties of the adsorbent were measured by fourier transform infrared (FTIR) on a Nicolet 380 infrared spectrometer (Thermo electron corporation, USA), X-ray diffraction (XRD) (D8 ADVANCE, Bruker, Germany), Raman spectra (an inVia Reflex confocal Laser MicroRaman spectrometer, Renishaw, Britain), the thermogravimetric analysis (TGA) (TGA/DSC1/1600HTA)

5

made by Mettler-Toledo with a heating rate of 10 ℃/min under argon atmosphere, vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series). The UV-vis spectra were obtained using a Shimadzu UV2550 spectrometer. The zeta potentials of the FA-mGO aqueous suspension with the different pH values were tested by JS9H type electrophoresis apparatus made by Shanghai Zhongchen Digital Technic Apparatus Co., Ltd, China. 2.6. Cu(II) ions adsorption The homogeneous solution was produced by dissolving an appropriate mass of CuSO4·5H2O in ultrapure water. The final concentration of Cu(II) varied from 0.2 g L-1 to 2.0 g L-1. 0.1 M HCl was used to adjust pH of Cu(II) solution. 0.06 g FA-mGO was introduced into 15 mL Cu(II) solution and then the compound was oscillated. After the adsorption process, the FA-mGO-Cu was removed using a portable magnet. The supernatant was tested by UV-vis spectrophotometer. Unless special provisions, the experiments were carried out in aqueous solution at pH=5 and 25 ℃ circumstance. The adsorbed amount of Cu(II) and the removal efficiency onto FA-mGO were calculated by the following equations: 21

qt 

(C0  Ce )V m

Removal efficiency (%) 

(1)

C0  Ce 100% C0

(2)

Where V (L) is the volume of aqueous solution. C0 and Ce (mg L-1) are the initial and equilibrium concentrations of adsorbate, respectively. qt (mg g-1) is the adsorption amount of adsorbate per unit mass of the adsorbent at time t. m (g) is the mass of adsorbent. 21

3. Results and discussion 6

3.1. Characterization of FA-mGO From Fig. 2(a-c), the AFM images exhibit the GO sheets characteristic of single layer about 754.837 pm which is in good agreement with the previous report. 51 Fig. 2(d-g) shows the representative SEM and TEM images of different kinds of magnetic particles. Fe3O4 presents a uniform spherical morphology with the average size of 123 nm (size distribution as shown in Fig. 3(a)) and tends to self-assemble into chainlike structure by high surface energy and strong magnetic dipole directed interactions (Fig. 2(d)). Fig. 2(e) is the SEM image of FA-mGO composite and shows the regular spheres are immobilized uniformly onto GO substrates. Different from Fe3O4, the mean diameter of FA-mGO is 793 nm (size distribution as shown in Fig. 3(d)), which is much larger than that of Fe3O4 itself. This result indicates that FA composite has been successfully loaded onto the surface of mGO. The folded sheet structure of GO can be recognized obviously, which provided sufficient loading sites for FA and was beneficial for improving the adsorption capacity of the composites. In addition, the aggregation of Fe3O4 was reduced in FA-mGO, which indicated the presence of GO was beneficial for the homogeneous distribution of Fe3O4 particles. Low aggregation rate and homogeneous distribution endowed the FA-mGO with large surface area for removal of heavy metal ions. The representative TEM images of the FA-mGO are shown in Fig. 2(f and g). The piece of translucent GO sheet is homogeneously decorated by Fe3O4 nanoparticles, which is consistent with the SEM analysis. Particle size distributions of Fe3O4, mGO, f-mGO and FA-mGO are shown in Fig. 3. Fig. 3 reveals that the different kinds of magnetic nanoparticles have the various diameter sizes. The Fe3O4 nanoparticle mean diameter is 0.123 μm while mGO, f-mGO and FA-mGO mean diameters are 0.443 μm, 0.672 μm, and 0.793 μm, respectively, which demonstrates that the grafting process

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significantly affected the result of the diameter size. This statement has been confirmed in SEM images. The chemical structures of materials were measured by FT-IR, as shown in Fig. 4(a). For Fe3O4, the peaks at 604 cm-1, 1380 cm-1 and 1620 cm-1 are attributed to Fe-O stretching vibration, C-H bending vibration and C=O stretching vibration, respectively. The Fe3O4 was synthesized using a solvothermal system and polyvinylpyrrolidone was used as a catalyzer. Therefore, it is an inevitable phenomenon of the presence of C=O and C-H. For mGO, there is no obvious change in peaks compared with Fe3O4, which suggests that GO had been reduced to graphene. It can be inferred that it is hard to graft FA directly. As for f-mGO, the characteristic peaks at 1090 cm-1, 1200 cm-1, 1560 cm-1 and 3440 cm-1 appear for Si-O, C-N, N-H and O-H, respectively.

48

This

demonstrates that the mGO surface had been connected with amino groups. For FA-mGO, the peak at 1250 cm-1 belongs to C-N and the characteristic peaks at 2930 cm-1 and 2850 cm-1 reveal C-H. Furthermore, it can be found that the peak at 1620 cm-1 is enhanced and the peak at 1560 cm-1 is weakened obviously. This demonstrates that FA had connected with f-mGO through the reaction of the carboxyl group and amino group. The XRD patterns of Fe3O4, mGO, f-mGO and FA-mGO are presented in Fig. 4(b). The prominent diffraction peaks at 2θ values of about 30.30°, 35.55°, 43.25°, 55.55°, 57.21° and 62.80° of the Fe3O4 nanoparticles are corresponding to the (220), (311), (400), (422), (511) and (440) crystallographic planes, respectively [(JPPDS No. 19-0629)]. 52 As for mGO, f-mGO and FA-mGO, two new characteristic peaks at 2θ values of about 33.15° and 48.48° are corresponding to the (104) and (024) crystallographic planes growth respectively, due to the hybridization of mGO. Moreover, from the XRD patterns results of mGO, f-mGO and FA-mGO, eight characteristic peaks position

8

and intensity almost have no change. This demonstrates that the introduction of APTES and FA did not influence crystal structure of mGO. The Raman spectra of five samples were shown in Fig. 4(c). As for all the samples except for Fe3O4, the Raman spectra show two characteristic peaks between 1300 and 1600 cm-1, which are corresponding to the D (arose from the disordered carbon atoms) and G (originated from the crystalline graphitic carbon atoms) bands, respectively.

42,53

As shown in Fig. 4(c), the D and G

bands values for FA-mGO are 1343 and 1591 cm-1 while FA-mGO-Cu are 1343 and 1581 cm-1, respectively. It indicated that the adsorption reaction between Cu(II) and FA-mGO resulted of the G band blue-shift and a bandwidth narrowing. The ratio of the intensity of the D band to G band (ID/IG) reflects the disordered structure of carbonaceous materials.

42

As shown in Fig. 4(c), the intensity

ratios of ID/IG of mGO, f-mGO, FA-mGO and FA-mGO-Cu are 1.08, 1.05, 1.02 and 0.94, respectively, which attributes to the introduction of APTES, FA and Cu(II) increasing the order of carbonaceous materials structure. The TGA curves of Fe3O4 and its derivative are presented in Fig. 4(d). As shown in the Fig. 4(d), from the temperature of 30 ℃ to 900 ℃, Fe3O4 only has 2.5% mass loss. The reason is that Fe3O4 has no phase transition in the argon atmosphere, and its thermal decomposition temperature is 1400 ℃. Therefore, the decrease of mass can be attributed to the evaporation of surface water of the magnetic particles. As for the derivative of Fe3O4, the weight loss process can be divided into three stages (mGO and FA-mGO) or four stages (f-mGO). The small proportion of weight loss below 200 ℃ is attributed to the evaporation of absorbed solvent. Then, a slow weight loss appearing from 400 to 450 ℃ is assigned to the removal of the residual oxygen-containing functional groups on GO sheets. With the increasing temperature, a rapid mass loss appears at

9

around 700 ℃, which is attributed to pyrolysis of carbon skeleton. 54 The percentages of residues are 64.7%, 60.5% and 55.6% corresponding to the mGO, f-mGO and FA-mGO, respectively. The decomposition temperature decreasing with the mGO, f-mGO and FA-mGO indicates that the grafted functional groups of nanoparticles impacted the destruction of carbon skeleton. The distinct mass loss appears at the temperature higher than 200 ℃, implies that as-prepared FA-mGO has a good thermal stability and can be used as a suitable absorbent in wastewater treatment. The Fig. 4(e) shows the magnetization spectrum of magnetic materials which was measured at room temperature to explore its specific saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc). The hysteresis loops of all samples are S type showing the typical ferromagnetic of materials. The values of Ms, Mr and Hc of materials are given in Table 1. It can be observed in Table 1 that the Fe3O4 has the maximum of Ms about 81.57 emu g-1. After the modified by GO and FA, the Ms decreases to 37.15 emu g-1 and 25.97 emu g-1, respectively. Moreover, the Mr and Hc also decline. It might be two reasons: firstly, with the introduction of GO, the content of Fe3O4 particles decreased, resulting in the decrease of Ms. Secondly, FA-mGO particles had the fewer agglomerations which decreased Ms. In addition, Fig. 4(f) shows a simple experiment to evaluate magnetic property of FA-mGO by a portable magnet. A uniform FA-mGO aqueous suspension is obtained when the magnetic field does not exist. However, after the adsorption, in the presence of external magnet FA-mGO is thoroughly separated from aqueous solution within 5 s, which can make the adsorption post-processing easier and save more time or economic costs. 21 3.2 Factors affecting adsorption 3.2.1. Effect of adsorbent dosage on the removal efficiency

10

The influence of adsorbent dosage was explored by adding a series masses of FA-mGO into 15 mL of Cu(II) solution (0.4 g L-1, pH=6) and shaking for 3 h at room temperature (Fig. 5(a)). The adsorption efficiency of Cu(II) ions increases with the increase of FA-mGO dosage. The dosage of FA-mGO controlled the contact sites between the adsorbent and adsorbate. The higher dosage of adsorbent was added, the more available contact sites between the adsorbent and adsorbate for increasing the removal efficiency. However, the adsorbent dosage gradually reached the saturation state with the continue adding adsorbent. In this state, if the adsorbent was further added, the large number of spare contact sites appeared, which would bring a waste of material. Therefore, an appropriate dosage of adsorbent was needed. As shown in the Fig. 5(b), the adsorption efficiency reaches to equilibrium of 94% for Cu(II), corresponding to the FA-mGO dosage of 0.06 g. Consequently, the adsorbent dosage was maintained at 0.06 g for Cu(II) in all the following experiments. 3.2.2. Effect of pH on the removal efficiency The pH value affects the charge properties of the FA-mGO surface and then influences the adsorption efficiency in waste water. The 0.06 g FA-mGO was added into 15 mL of Cu(II) solution (0.4 g L-1) and shaking for 3 h at room temperature. The UV-vis spectra lines, removal efficiency and zeta potential results with varied pH value from 1.0 to 7.0 are shown in Fig. 6. Fig. 6 shows that the minimum absorption curve is obtained at pH=5 (Fig. 6(a)), with removal efficiency of 96.8% (Fig. 6(b)). In the first place, it was very hard for FA to release of H+ at low pH (high H+ concentration) of aqueous solution and the coordination interaction between Cu(II) and FA was difficult, too. Therefore, the lower adsorption efficiency appeared. On the contrary, too high pH (high OH- concentration) of the aqueous solution was also a harmful situation for adsorption of

11

Cu(II), which was due to that CuOH+ and Cu(OH)2 would be emerged.

55,56

This obviously

influenced the adsorption performance and adsorption efficiency. The possible relevant reaction equilibrium is shown in Equ. (3). Where HY- on behalf of FA molecules.

2HY   Cu 2

CuY2  2H 

(3)

In addition, the adsorption performance might be related to the electrostatic attraction between Cu(II) and FA-mGO. Therefore, the protonation and deprotonation of FA-mGO carboxyl and hydroxyl groups might be another non-ignorable reason influencing the adsorption performance.

57

The possible relevant adsorption process is shown in Fig. 7. The above results are consistent with the presentation of zeta potential. Therefore, all the following experiments were conducted at pH=5 to guarantee the best adsorption efficiency. 3.2.3. Effect of contact time on the removal efficiency The result of contact time on the adsorption efficiency of different adsorbent concentration is shown in Fig. 8(a). The increasing contact time is advantageous to the sufficient loading of Cu(II) on the adsorption sites of FA-mGO. Take 0.2 g L-1 as example, the adsorption efficiency increases quickly within 10 min and reaches the maximum equilibrium in 20 min. For FA-mGO, the adsorption sites for Cu(II) were adequate in the initial. With the increase of time, more and more adsorption sites were taken up and eventually reached its saturating adsorption capacity. In summary, the optimum saturating contact time was 40 min. 3.3. Adsorption kinetics The kinetics as a key aspect was used for characterization the adsorption rate of adsorbent. Five models including Pseudo-first-order, Pseudo-second-order, Elovich, Intra-particle diffusion and Bangham models were investigated for the adsorption kinetics.

58-61

The adsorption kinetics

12

model formulas were listed in supporting information. The linear fitting results are displayed in Fig. 8. The relevant parameter values are listed in Table 2. Fig. 8(a) shows that Cu(II) adsorption is quite rapid firstly and then retards as time grows, finally, reaches the equilibrium value within 40 min, which indicates that the FA-mGO is quite effective at a short time. The relevant mechanisms have been explained in section of 3.2.3. From the Fig. 8(b), the pseudo-first order kinetic model correlation coefficient (R2) varies from 0.88732 to 0.98214. However, the pseudo-second order kinetic model (Fig. 8(c)) can well describe the adsorption pcocess (0.2 g L-1: R2=0.99609, 0.4 g L-1: R2=0.99749, 0.8 g L-1: R2=0.99658, 1.2 g L-1: R2=0.99552, 1.6 g L-1: R2=0.99682, and 2.0 g L-1: R2=0.99869), which indicates that the rate-limiting step in adsorption was worked at fast rate and mainly determined by chemical process. The Fig. 8(d) describes the Elovich models which is used to explain a series of reaction mechanism, such as the diffusion of solute in the solution phase or at the interface, surface activation and deactivation etc.. It well fits for the activation energy changes in the large range during the reaction process, for example, at soil and sediment interface and highly suitable of heterogeneous diffusion process.

22

The Fig. 8(d) reveals that the Elovich models can fit the first

half of adsorption process and not suit the adsorption equilibrium. The intra-particle diffusion model explains the movement of ions from liquid to the solid phase.22 From the Fig. 8(e), all the lines do not go through the origin. In other words, all the rate constants C are not zero, indicating that the intra-particle diffusion model is not the only way to control the adsorption process. 22 The adsorption process of the material is divided into two types, the surface adsorption and the diffusion of the channel in the intra-particle. The Bangham model (Fig. 8(f)) is used for describing

13

the diffusion mechanism of porous channel. The R2 varies from 0.63848 to 0.8733, which indicates the adsorption process is not only depending on the Bangham model. The non-linear results of pseudo-first order and pseudo-second order kinetic models are shown in Fig. 9 and Table 3. The result shows that the pseudo-second order model can fit the adsorption kinetics behavior well, which is consistent with the linear method. From all the R2 and above analysis, it can be concluded that the pseudo-second order kinetic model was the most compatible for description adsorption process of Cu(II) onto FA-mGO, which also demonstrated that the electron transfer or sharing was generated and chemical bond was formed in the adsorption process. 21 3.4. Adsorption isotherms The adsorption process continues until adsorbent and adsorbate reach a dynamic equilibrium. The adsorption isotherms were used for describing this equilibrium relationship and providing the useful information for the reaction pattern between adsorbent surface and metal ions. The adsorption isotherm data were tested by four different isotherms including Herry, Langmuir, Freundlich and Temkin isotherms. 39,62,63 These isotherm model formulas are listed in the supporting information. Fig. 10 shows the four important adsorption isotherms by linear regression at thermodynamic temperature of 298 K, 308 K and 318 K. The relevant data are illustrated in Table 4. The linear correlation coefficient R2 for Freundlich, and Temkin models are all higher than 0.93. Especially, Temkin models are all higher than 0.97, while as for Henry model, all the R2 are only lower than 0.48. This result indicates that the two models of Freundlich and Temkin match the adsorption system sufficiently well. It is demonstrated that the adsorption process refers to

14

multimolecular layers of coverage, meanwhile heterogeneous nature of adsorptive sites on FA-mGO surface. Furthermore, 1/n value is smaller than 0.1 indicating the heterogeneous surface for FA-mGO. The results of non-linear method for the isotherm models are shown in Fig. 11 and Table 5. The fitted results show that Freundlih and Temkin match the adsorption system sufficiently well, which is consistent with the linear method. 3.5. Thermodynamic parameters In order to investigate the spontaneous adsorption process and thermal properties, the thermodynamic parameters were obtained by the formulations listed in the supporting information.21 The values of ∆S0, ∆H0 and ∆G0 are illustrated in Table 4. The small value of ∆G0 suggests the action forces of adsorption were strong enough to conquer the potential barrier. The negative of ∆G0 implies that the adsorption was a spontaneous process. Furthermore, it can be seen that a better adsorption performance was obtained at a higher temperature. ∆H0>0 and ∆S0>0 suggests the endothermic nature of the adsorption and the randomness was increased at the solid-solution interface. 21 3.6. Desorption and reusability The reusability efficiencies results after desorption by the eluant (HCl) from FA-mGO are shown in Fig. 12. 0.5 mol L-1 HCl was chosen as the desorption eluants to conduct the recycle experiment. The surface of FA-mGO would be protonated and the connection between Cu(II) and FA would be slow down and even forbidden. These results are consistent with the discussion of the factor of pH influence on the adsorption capacity. As shown in Fig. 12, after six recycles, FA-mGO still holds good reusable adsorption property (86.5% removal efficiency). The good regeneration

15

indicates that FA-mGO is an environmental, economic and efficient adsorbent in the practical wastewater treatment. 3.7. Performance evaluation between mGO and FA-mGO The SEM images of the mGO-Cu, FA-mGO-Cu powder composites (Fig. 13(a) and (a’)) and the corresponding elemental mapping (Fig. 13(b) to (d’)) show the homogeneous distribution of copper, nitrogen and oxygen in the entire range. The results imply that FA-mGO composite has better binding sites than mGO for Cu ions. Finally, Table 6 lists the maximum capacity of comparative tests between FA-mGO and the reported adsorbents. The qm of FA-mGO for Cu(II) is clearly more than that of other reported adsorbents.

4. Conclusion The excellent adsorbent FA-mGO with the favourable adsorption performance for Cu(II) was synthesized in this work. The adsorbent reached the adsorption equilibrium at adsorbent dose 0.06 g and contact time 40 min. Moreover, FA-mGO had the best adsorption performance at pH=5 and temperature 298 K. The adsorption mechanism of FA-mGO mainly depended on chelation action and electrostatic incorporation between Cu(II) and oxygenic functional groups of FA-mGO surface. The pseudo-second-order kinetic model could describe adsorption process well. The Temkin models could well describe the adsorption isotherm process. It demonstrated that the electron transfer, exchange and sharing were generated and chemical bonding was formed in the adsorption process. The adsorption process referred to multimolecular layers of coverage and heterogeneous nature of adsorptive sites on FA-mGO surface. Furthermore, desorption and reusability studies implied FA-mGO had an excellent reproducibility in water treatment and was benefit to environmental protection and resource conservation.

16

Acknowledgments The authors sincerely acknowledge the financial support from the major special project of science a nd technology in Shandong Province (Grant No. 2015JMRH0110).

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Fig. 1 The process of preparing FA-mGO: (a) Schematic illustration of preparing FA-mGO; (b) Chemical reaction equations of preparing FA-mGO 23

Fig. 2 Characterization of as-prepared adsorbent: (a) AFM ichnography for GO sheets; (b) Three-dimensional diagram for GO sheets; (c) Section analysis of ichnography; (d) SEM images of Fe3O4; (e) SEM images of FA-mGO; (f) TEM images of FA-mGO; (g) The larger magnification of FA-mGO

Fig. 3 Particle size distribution analysis: (a) Fe3O4; (b) mGO; (c) f-mGO; (d) FA-mGO

Fig. 4 Characterization of as-prepared adsorbent: (a) The FTIR spectra of Fe3O4, mGO, f-mGO and FA-mGO; (b) XRD pattern of Fe3O4, mGO, f-mGO and FA-mGO; (c) Raman spectra of Fe3O4, mGO, f-mGO, and FA-mGO; (d) TGA analysis of Fe3O4, mGO, f-mGO and FA-mGO; (e) Magnetization curve of Fe3O4, mGO and FA-mGO at room temperature inset is the expanded low field hysteresis curves; (f) Demonstration of magnetic separation

Fig. 5 Effect of adsorbent dosage on the removal efficiency: (a) UV-vis absorption spectra of Cu(II)-DDTC complex with different adsorbent dosage; (b) Removal efficiency of Cu(II)

Fig. 6 Effect of pH on the removal efficiency: (a) UV-vis absorption spectra of Cu(II)-DDTC complex with different pH; (b) Removal efficiency of Cu(II) and zeta potential of FA-mGO aqueous suspension

Fig. 7 Schematic formation process of pH effect on adsorption

24

Fig. 8 The linear fitting of adsorption kinetic models: (a) The kinetic data for Cu(II) uptake by FA-mGO at different initial Cu(II) concentration; (b) The pseudo-first order; (c) The pseudo-second order; (d) Elovich; (e) Intra-particle diffusion; (f) Bangham kinetic models for Cu(II) adsorption on FA-mGO under different initial concentrations (0.2 g L-1, 0.4 g L-1, 0.8 g L-1, 1.2 g L-1, 1.6 g L-1, 2.0 g L-1) at room temperature

Fig. 9 The non-linear fitting of adsorption kinetic models: (a) The pseudo-first order; (b) The pseudo-second order kinetic models for Cu(II) adsorption on FA-mGO under different initial concentrations (0.2 g L-1, 0.4 g L-1, 0.8 g L-1, 1.2 g L-1, 1.6 g L-1, 2.0 g L-1) at room temperature

Fig. 10 The linear adsorption isotherm models: (a) Henry; (b)Langmuir; (c) Freundlich; (d) Temkin adsorption isotherm fit of Cu(II) (m=0.06 g, C0=2.0 g·L-1, V=10 mL, pH at 5, temperature at 298 K-318 K)

Fig. 11 The non-linear adsorption isotherm models: (a) Henry; (b)Langmuir; (c) Freundlich; (d) Temkin adsorption isotherm fit of Cu(II) (m=0.06 g, C0=2.0 g·L-1, V=10 mL, pH at 5, temperature at 298 K-318 K)

Fig. 12 Adsorption-desorption recycles

Fig. 13 Performance evaluation between mGO and FA-mGO: (a) SEM image of the mGO-Cu; (a’)

25

SEM image of FA-mGO-Cu; (b, b’) The elemental mapping images: copper mapping; (c, c’) nitrogen mapping; (d, d’) oxygen mapping

Table 1 Magnetic properties of magnetic materials Sample

Ms (emu/g)

Hc (Oe)

Mr (emu/g)

Fe3O4 mGO FA-mGO

81.57 37.15 25.97

104.55 68.12 42.21

9.28 2.80 1.32

Table 2 Kinetic parameters of various models linear fitted to experimental data Kinetic model

Cu(II) c0 (mg L-1)

Parameter

0.2

0.4

0.8

1.2

1.6

2.0

0.05359

0.05412

0.06878

0.0655

0.7307

0.05646

29.40

62.01

113.93

194.14

234.12

194.45

0.88732 0.00294

0.90056 0.00142

0.92423 0.00088

0.94702 0.00045

0.9861 0.00049

0.98214 0.00048

63.45

121.95

178.25

243.90

263.16

283.29

0.99609 37.59

0.99749 63.05

0.99658 75.04

0.99552 65.05

0.99682 91.37

0.99869 127.02

0.08880

0.04533

0.02988

0.02064

0.02002

0.01985

0.7499 5.04

0.81386 9.78

0.84264 14.53

0.89897 20.13

0.88669 21.46

0.9162 22.57

19.89 0.63765 18.99

36.30 0.67724 36.67

49.30 0.70859 51.35

53.50 0.78357 58.58

65.49 0.75605 71.79

75.84 0.75542 86.10

3.5066 0.63848

3.5690 0.72842

3.4836 0.79055

3.1527 0.8679

3.4290 0.85105

3.7727 0.8733

-1

Pseudo-first-order

Pseudo-second-order

Elovich

k1 (mg min g ) qe (mg g-1) R2

k2 (mg min g-1) qe (mg g-1) R2

α (mmol g-1 min-1) β (g mmol-1) R2

Intra-particle diffusion

kdif (mg g-1 min-1/2) C R2

Bangham

kb (mg g-1) m R2

Table 3 Kinetic parameters of various models non-linear fitted to experimental data 26

Kinetic model

Parameter

Pseudo-first-order

Cu(II) c0 (mg L-1)

k1 (mg min g-1) qe (mg g-1) R2

0.2

0.4

0.8

1.2

1.6

2.0

0.10261

0.09493

0.08492

0.0690

0.07706

0.08528

59.36

113.80

166.68

221.90

241.35

256.70

0.97895 0.00102

0.97553 0.00062

0.95061 0.00035

0.96728 0.00038

0.96782 0.00041

128.65

188.94

254.89

274.83

290.28

0.98495

0.98456

0.98396

0.98583

0.99363

0.97366 0.00211 Pseudo-second-order

k2 (mg min g-1) qe (mg g-1) R2

67.01 0.98197

Table 4 The linear fitting of isotherm parameters for adsorption of Cu(II) onto FA-mGO at 298 K, 308 K, 318 K Temperature Adsorption ∆G0 (kJ ∆H0 (kJ ∆S0 (J mol-1 2 (k)

isotherm Henry Langmuir

298

Freundlich Temkin Henry Langmuir

308

Freundlich Temkin Henry Langmuir

318

Freundlich Temkin

Parameter

Value

KH b (L mg-1) qm(mg g-1) KF 1/n bT AT KH b (L mg-1) qm(mg g-1) KF 1/n bT AT KH b (L mg-1) qm(mg g-1) KF 1/n bT AT

2.04331 0.0041 243.90 207.67 0.0958 130.90 96091.29 2.10917 0.00389 257.07 215.33 0.09583 128.82 80917.75 2.22872 0.00377 265.25 223.15 0.09632 127.53 75097.71

R

mol-1)

mol-1)

K-1)

0.4703 0.8329 0.97959

-7.29

-

-

-7.66

3.58

36.49

-8.02

-

-

0.98608 0.47047 0.88221 0.93221 0.97182 0.46685 0.87548 0.94597 0.97275

Table 5 The non-linear fitting of isotherm parameters for adsorption of Cu(II) onto FA-mGO at 298 K, 308 K, 318 K 27

Temperature (k)

Adsorption isotherm Henry Langmuir

298

Freundlich Temkin Henry Langmuir

308

Freundlich Temkin Henry Langmuir

318

Freundlich Temkin

Parameter

Value

KH b (L mg-1) qm(mg g-1) KF 1/n bT AT KH b (L mg-1) qm(mg g-1) KF 1/n bT AT KH b (L mg-1) qm(mg g-1) KF 1/n bT AT

7.2166 0.7116 247.76 213.40 0.0826 146.41 481500.5 7.4878 0.0498 267.16 218.55 0.0868 128.82 80917.99 7.8163 0.0465 276.93 226.28 0.0876 127.53 75097.65

R2 0.5233 0.55378 0.99429 0.98810 0.5234 0.75949 0.95533 0.97464 0.5202 0.76303 0.95463 0.97547

Table 6 Comparison of adsorption capacities with various adsorbents for Cu(II) Adsorbents EDTA-mGO Sulfonated magnetic graphene oxide Chitosan-coated bentonite beads Cation-exchange resin-supported polyethyleneimine Graphene oxide-ethylenediamine triacetic acid Activated carbon modified by poly(N,N-dimethylaminoethyl methacrylate) EDTA functionalized Fe3O4 magnetic nano-particles Diethylenetriamine functionalized magnetic nano-particles Treating Fe3O4 nano-particles with gum arabic

Kinetic PSO PSO PSO PSO

Isotherm Freundlich Temkin Langmuir Freundlich Langmuir Langmuir

Capacity (mg g-1)

Equilibrium time (min)

Ref.

301.2

90

21

62.7

120

64

12.2

-

65

99

200

66

-

Langmuir

108.7

90

67

-

Langmuir

31.46

30

68

PSO

Langmuir

46.27

5

22

-

Langmuir

12.43

-

69

-

Langmuir

38.5

2

70 28

Monodisperse chitosan-bound Fe3O4 nano-particles Humic acid (HA) coated Fe3O4 nano-particles FA-mGO

-

Langmuir

21.5

1

71

-

Langmuir Temkin

46.3

15

283.29

40

72 This work

PSO

Highlights

    

The FA-mGO as a novel magnetic nano-adsorbent was synthesized for the removal of Cu(II) ions from aqueous solution. The chelation action between FA and Cu(II) along with electrostatic incorporation between GO and Cu(II) determined the favourable adsorption property. The adsorption property obeyed the pseudo-second order kinetic model and the Temkin isotherms well. The thermodynamic studies results suggested that the adsorption mechanism was an endothermic and spontaneous process essentially. Desorption and reusability studies implied FA-mGO has an excellent reproducibility.

29

Graphical abstract: Schematic illustration of preparing FA-mGO

Fig. 13 Performance evaluation between mGO and FA-mGO: (a) SEM image of the mGO-Cu; (a’) SEM image of FA-mGO-Cu; (b, b’) The elemental mapping images: copper mapping; (c, c’) Nitrogen mapping; (d, d’) Oxygen mapping

Fig. 12 Adsorption-desorption recycles

Fig. 11 The non-linear adsorption isotherm models: (a) Henry; (b)Langmuir; (c) Freundlich; (d) Temkin adsorption isotherm fit of Cu(II) (m=0.06 g, C0=2.0 g·L-1, V=10 mL, pH at 5, temperature at 298 K-318 K)

Fig. 10 The linear adsorption isotherm models: (a) Henry; (b)Langmuir; (c) Freundlich; (d) Temkin adsorption isotherm fit of Cu(II) (m=0.06 g, C0=2.0 g•L-1, V=10 mL, pH at 5, temperature at 298 K-318 K)

Fig. 9 The non-linear fitting of adsorption kinetic models: (a) The pseudo-first order; (b) The pseudo-second order kinetic models for Cu(II) adsorption on FA-mGO under different initial concentrations (0.2 g L-1, 0.4 g L-1, 0.8 g L-1, 1.2 g L-1, 1.6 g L-1, 2.0 g L-1) at room temperature

Fig. 8 The linear fitting of adsorption kinetic models: (a) The kinetic data for Cu(II) uptake by FA-mGO at different initial Cu(II) concentration; (b) The pseudo-first order; (c) The pseudo-second order; (d) Elovich; (e) Intra-particle diffusion; (f) Bangham kinetic models for Cu(II) adsorption on FA-mGO under different initial concentrations (0.2 g L-1, 0.4 g L-1, 0.8 g L-1, 1.2 g L-1, 1.6 g L-1, 2.0 g L-1) at room temperature

Fig. 7 Schematic formation process of pH effect on adsorption

Fig. 6 Effect of pH on the removal efficiency: (a) UV-vis absorption spectra of Cu(II)-DDTC complex with different pH; (b) Removal efficiency of Cu(II) and zeta potential of FA-mGO aqueous suspension

Fig. 5 Effect of adsorbent dosage on the removal efficiency: (a) UV-vis absorption spectra of Cu(II)-DDTC complex with different adsorbent dosage; (b) Removal efficiency of Cu(II)

Fig. 4 Characterization of as-prepared adsorbent: (a) The FTIR spectra of Fe3O4, mGO, f-mGO and FA-mGO; (b) XRD pattern of Fe3O4, mGO, f-mGO and FA-mGO; (c) Raman spectra of Fe3O4, mGO, f-mGO, and FA-mGO; (d) TGA analysis of Fe3O4, mGO, f-mGO and FA-mGO; (e) Magnetization curve of Fe3O4, mGO and FA-mGO at room temperature inset is the expanded low field hysteresis curves; (f) Demonstration of magnetic separation

Fig. 3 Particle size distribution analysis: (a) Fe3O4; (b) mGO; (c) f-mGO; (d) FA-mGO

Fig. 2 Characterization of as-prepared adsorbent: (a) AFM ichnography for GO sheets; (b) Three-dimensional diagram for GO sheets; (c) Section analysis of ichnography; (d) SEM images of Fe3O4; (e) SEM images of FA-mGO; (f) TEM images of FA-mGO; (g) The larger magnification of FA-mGO

Fig. 1 The process of preparing FA-mGO: (a) Schematic illustration of preparing FA-mGO; (b) Chemical reaction equations of preparing FA-mGO