Graphene oxide enhanced adsorption capacity of chitosan/magnetite nanocomposite for Cr(VI) removal from aqueous solution

Graphene oxide enhanced adsorption capacity of chitosan/magnetite nanocomposite for Cr(VI) removal from aqueous solution

Graphene oxide enhanced adsorption capacity of chitosan/

magnetite nanocomposite for Cr(VI) removal from aqueous solution

Hoang V Tran1 , Tuong L Tran2, Truong D Le1 , 3, Thu D Le1, Hang M T Nguyen3 and Le T Dang3

1 School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi, Vietnam

2 Agro-Forestry-Aquaculture Department, Quang Binh University, 312 Ly Thuong Kiet Street, Dong Hoi City, Quang Binh Province, Vietnam

3 Faculty of Environment, Thuyloi University, 175 Tay Son Street, Hanoi, Vietnam E-mail: hoang.tranvinh@hust.edu.vn

Keywords: Fe3O4/graphene oxide/chitosan (FGCs) nanocomposite, Cr(VI) removal, recoverable and recyclable adsorbent, magnetite, heavy metal ions removal

Supplementary material for this article is available online

Abstract

In this work, we propose a simple method for preparing of Fe3O4/graphene oxide/chitosan (FGCs) nanocomposite and its application for removal of Cr(VI) from aqueous solution by adsorption process The advantages of this adsorbent that it can be recovered by an external magnet as well as it can be regenerated For that, FGCs nanocomposite has been synthesized co-precipitation method and synthesized FGCs has been characterized by x-ray diffraction (XRD), Fourier-Transform Infrared spectroscopy (FT-IR), vibrating sample magnetometer (VSM), Transmission Electron Microscopy (TEM), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive x-ray spectroscopy (EDX) The sorption efficiency of FGCs was evaluated by adsorption Cr(VI) ions from aqueous solution to optimize contents in FGCs adsorbent material and also for adsorption process The results shown that the adsorption of Cr(VI) on FGCs can be reached to maximal with the content

of FGCs is 68 wt% of Fe3O4nanoparticles, 2 wt% of graphene oxide (GO) and 30 wt% of chitosan (CS) (sample FGCs-68/2/30) The adsorption process was carried out at pH3 (pH=3) with the contact time ca 40 min and dosage of adsorbent around 0.04 mg.mL−1.The adsorption isotherm fits the Langmuir model with adsorption capacity of FGCs-68/2/30 for Cr(VI) is 200 mg.g−1 In addition, the reusability of FGCs nanocomposite was tested and about 75% of removal efficiency was obtained after

6 cycles Therefore, the FGCs nanocomposite has a good stability and may be a promising material for removal of heavy metal ions from aqueous solution to clean up the environment.

1 Introduction The pollution in groundwater and seawater is a serious problem affecting the human life With the development

of the industry, textile, dyeing, leather and plasticK factories are constantly developing and create the pollution

to the environment Water pollution with toxic heavy metal ions such as Hg2+; Pb2+; Cd2+; Cu2+; Ni2+K contaminant constitutes a serious environmental hazard in particular for aquatic bio-systems, where symbiotic processes may be affected According to the World Health Organization(WHO) , the limit of Cr(VI) in potable water is 0.05 mg L−1(EPA, 1990) [1]; Pb(II) is 0.015 mg L−1(EPA, 1992) [2]; Cd(II) is 50 μg/LK These heavy metal ions are easy to be absorbed to the human body, they can invade the human organs, lead to liver damage, skin diseases and may cause cancers In the aquatic environment, chromium exits mainly in two states, Cr(III) and Cr(VI) where Cr(VI) is the most toxic [3] Because of Cr(VI) ions are easy to dissolve and diffuse in the tissue therefore they are necessary to remove them from contaminated water There are various technologies for removal of Cr(VI) ions from aqueous solution such as: ion exchange, chemical precipitation, membrane filtration and biological treatment [4,5] However, these methods shown practical disadvantages such as poor stability, high cost, create a secondary pollution in extreme conditions[6]

RECEIVED

12 September 2018

ACCEPTED FOR PUBLICATION

1 October 2018

PUBLISHED

9 November 2018

© 2018 IOP Publishing Ltd

The adsorption processes are the most common method to remove Cr(VI) from aqueous solution because of its high efficiency and low cost, can adsorb effectively even in low concentration of heavy metal ions [7,8] A recent literature review focused on the use of the GO materials as promising adsorbent materials for the removal

of heavy metal ions from the water[9–11] Graphene oxide is made of single layer of carbon atoms which are closely packed into honeycomb two dimensional(2D) lattices [9,10,12–14] Having the large surface area (∼2630 m2

g−1) [15], oxygen containing surface functionalities such as hydroxyl, carboxylic, carbonyl, and epoxide groups, and high water solubility makes GO become a material of great interest in adsorption-based technologies as well as in otherfields Many research groups have reported the use of GO as adsorbent material for heavy metal ions removal: Zhang et al[6] have used reduced GO (rGO) to adsorb Cr(VI) with the maximum adsorption capacity can reach 198 mg g−1, Yang et al[16] showed the adsorption capacity of Cu(II) ions on GO is 46.6 mg/g, Wang et al [17] concluded that the suitable pH for Zn(II) removal was 7.0 with the adsorption capacity was up to 246 mg/g or rGO is even used for enhanced ultraviolet protection applications However, in experiment, the dispersion of single-layered GO are difficult to remove out from water after adsorption

processes On the other hand, the hybrids of graphene with magnetic nanomaterials have been extensively exploited for removal of pollutants from water Fe3O4is the most frequently used materials for water

purification due to its high biocompatibility which ensures safety and also the magnetic properties which makes

it easy to collect post- treatment After the adsorption, Fe3O4–graphene composite can be easily separated out from the solution via an external magnet bar Yao et al used Fe3O4@graphene to dye removal from aqueous media with the adsorption capacities on methylene blue were 45.27 mg g−1and Congo red were 33.66 mg g−1 [18] Uheida et al used Fe3O4andɤ-Fe2O3for the removal of Co2+ions[19] Zhou et al synthesized Fe3O4

composed polypyrrole and grapheme oxide to mercury adsorption[20]

In the previous work, we have presented the use of Fe3O4/chitosan for removal of Ni2 +and Pb2+by

complexation of Ni2+and Pb2+ions with amino(–NH2) groups of chitosan [21] After that, this material was used to removal of Cr(VI) ions by attraction of negative charge of Cr(VI) ions with positive charge of –NH3

groups of chitosan[22] In this work, we have used GO to improve the surface area of Fe3O4/chitosan Kinetic, adsorption isotherm and recycle ability were also studied to understand the mechanism of adsorption

2 Experimental

2.1 Materials

Graphite was extracted from pencils which were purchased from a local bookstore Other chemicals, such as sulfuric acid(H2SO4) 98 wt%, sodium nitrate (NaNO3), potassiumpermanganate (KMnO4), hydrogen peroxide (H2O2) solution 30 wt%, FeCl3.6H2O, FeSO4.4H2O, acetic acid(CH3COOH) solution 30 wt%, were purchased from Sigma Aldrich.Sodium hydroxide(NaOH) flakes and hydrochloric acid (HCl) solution were purchased from Duc Giang Chemical Company(Vietnam) Chitosan was extracted from shrimp shell following previous report[21]

2.2 Preparation of Fe3O4/graphene oxide/ chitosan nanocomposite (FGC)

Graphene oxide was synthesized from pencil’s graphite using Hummer’s method [23] Then after, the

Fe3O4/graphene oxide/chitosan nanocomposite (FGCs) was synthesized following previous report using co-precipitation method from three solutions of precursor materials:(i) a solution of Fe2 +and Fe3+ions was prepared by dissolving of FeSO4.4H2O and FeCl3.6H2O appropriate molar ratio of Fe2+: Fe3+of 2: 1 into distilled water;(ii) GO solution was prepared by dispersion on graphene oxide into distilled water under

sonication condition.(iii) Chitosan solution (0–60 wt%) was prepared by adding chitosan into an acid acetic

1 v/v.% solution These solutions were mixed and neutralize to pH8 by NaOH 1 M FGC has formed as black precipitation in solution and it will be collected by external magnet bar and then washed by distillated water and dried at 80°C for 8 h

2.3 Cr(VI) removal procedure

0.04 g FGC powder was added into a 10 ml of 200 mg.L−1Cr(VI) solution The mixture was incubated for various contact times at different temperatures To adjust pH in range of pH2 to pH10, the 0.1 M HCl and 0.1 M NaOH solutions were used The residue concentration of Cr(VI) in solution after adsorption process has been obtained by measure UV–vis spectra and using a calibration curve for determining of Cr(VI) concentration in solution(figure SI.2(b) is available online atstacks.iop.org/MRX/6/025018/mmedia)

2.4 Batch adsorption and kinetic experiment

The amount of Cr(VI) uptake by FGCs (qe, mg.g−1) was calculated following equation:

e

a

The Langmuir equation(2) and Freundlich equation (3) isotherms can be linearized into the following forms:

C q

1

1

e

e

Where: C0and Ce(mg.L−1) are the initial and equilibrium concentrations of Cr(VI) in solution, respectively;

mais the concentration of FGCs(g.L−1); qe, qmaxis the equilibrium Cr(VI) concentration on the adsorbent and the monolayer capacity of the adsorbent(mg.g−1), respectively KLis the Langmuir constant(L.mg−1) and related to the free energy of adsorption; KFis the Freundlich constant(L.g−1) and n (dimensionless) is the heterogeneity factor

2.5 Materials characterization

Absorbance measurements(UV–vis) spectra were measured using Agilent 8453 UV–vis spectrophotometer system with the wavelength in a range of 200–1200 nm X-ray Diffraction (XRD) patterns of CS, GO and FGCs samples were obtained at room temperature by D8 Advance, Bruker ASX, using CuKα radiation (λ=1.5406 Å)

in the range of 2θ=10°–60°, and a scanning rate of 0.02 s−1 Morphology of GO and FGCs nanocomposite were analyzed by Field Emission Hitachi S-4500 Scanning Electron Microscope(FE-SEM) and Transmission Electron Microscope(TEM, JEOL, Voltage: 100 kV, magnification: ×200,000), respectively The magnetic behaviors of the samples were measured at room temperature using a vibrating sample magnetometer(VSM 880 DMS/ADE Technologies, USA) at fields ranging from −10 to 10 kOe at 25 °C, with accuracy of 10−5emu Chemical composition of samples was determined by JEOL Scanning Electron Microscope and Energy

Dispersive x-ray(SEM/EDS) JSM-5410 Spectrometer The infrared (IR) spectra were recorded by Nicolet FT-IR Spectrometer model 205 with KBr pellets in transmission mode in the region from 500 cm−1to 4000 cm−1

3 Results and discussion

3.1 Characterizations of FGCs

Figure1(A) showed XRD patterns of pure CS (curve a); GO (curve b); pure Fe3O4(curve c); Fe3O4/GO/CS (FGC) (curve d) Six characteristic peaks for Fe3O4corresponding to(220), (311), (400), (422), (511) and (440) were observed in Fe3O4and FGC sample(JCPDS file, PDF No 65-3107) [21,23], which indicated the forming of FGC composite did not result in phase change of Fe3O4 XRD pattern of GO(figure1(A), curve b) shows (002) diffraction peak at 2θ=9.98° indicating the distance between graphene layers However, in FGCs nanomposite samples(figure1(A), curve d), this peak disappeared due to a very low content of GO in these samples From the XRD parameters of graphene oxide(figure1(A), curve b), the interlayer distances (d(002)) in the graphene oxide were estimated using Bragg’s Law [24] of ca d(002)=0.885 nm The mean dimension of the crystallite

perpendicular to the plane of graphene samples L002can be determined using Debye–Scherrer equation (4):

002

l

Where, k=0.94 is the shape factor, β is the full width at half maximum given in radians, λ is a wavelength of x-ray(λ=0.154 06 nm), and θ is the angle between the incident ray and the scattering planes Therefore, L002

was calculated of ca 3.23 nm and the thickness of obtained GO was calculated of around N=4÷5 layers (detailed calculating was described in section I of supporting information)

Figure1(B) shows FT-IR spectra of CS (curve a), the adsorption at 3578 cm−1is due to the stretching

vibration of O-H group, the band at approximately 2881 cm−1reflects the C–H stretching vibration of the –CH2

groups in chitosan and the characteristic adsorption bands at 1674 and 1589 cm−1ascribes to C=O stretching and N-H blending in amide groups of chitosan[25,26] In FT-IR spectrum of pure Fe3O4(figure1(B), curve b), the peak at 610 cm−1is attributed to Fe–O group, the peak around 3420–3422 cm−1can be related to the–OH group of adsorbed water[21] In the spectrum of GO (figure1(B), curve c), the peaks at 1739, 1610 and

1463 cm−1is due to the stretching vibration of C=O bond of carboxyl groups, the skeletal C–C vibrations of un-oxidized graphene domains and the vibrations of alkoxy C–O [7] The adsorption at 3568 cm−1is attributed to the stretching vibration of O–H group of adsorbed water and a sharp absorption peak at 3371 cm−1depicts the

stretching vibrations of the surface hydroxyl(–OH) groups on the GO [27] In the specific spectrum of FGC (curve d), compared with the spectra of CS and GO, we see that, a O–H group band at 3578 cm−1had a weak intensity, proves the physical water in chitosan was evaporated during synthesis and calcination It also shows that there are some shift bands of amide groups in spectrum of FGC(1674 and 1589 cm−1in chitosan shifted to

1597, 1516 and 1394 cm−1, respectively) Besides, there are some peaks at around 563 cm−1ascribe to Fe–O groups of Fe3O4nanoparticles in FGC nanocomposite, however, it can be seen that the peak at 610 cm−1of Fe–

O group in pure Fe3O4(figure1(B), curve b) was shifted to 563 cm−1in FGC(figure1(B), curve d), which indicates that Fe3O4are linked successfully to GO and CS These results indicate an enhanced hydrogen-bonding interaction between chitosan and thefillers by using both Fe3O4and GO[25]

The magnetization hysteresis loops of the pure Fe3O4nanoparticles and FGC nanocomposite are presented

infigure SI.1 Based on the plot of magnetization (M), magnetic field (H) and its enlargement near the origin, the saturation magnetization(Ms), remanence magnetization (Mr), coercivity (Hc) and squareness (Sr=Mr/Ms) could be calculated Because of no remanence and coercivity, it can be suggested that the pure Fe3O4and FGC sample are superparamagnetic[21,23] In addition, results in figure SI.1 shown that the saturation

magnetization values(Ms) for pure Fe3O4and FGCs nanocomposite was 40.4 emu.g−1and 20.2 emu.g−1, respectively These results indicated that FGC sample exhibited typical superparamagnetic behavior and high saturation magnetization value(Ms), therefore the FGC nanocomposite sample can be easily removed from solutions and recycled by applying an external magneticfield

Figures2(a) and (b) show FE-SEM images of the obtained GO flakes The GO material consisted of randomly aggregated, thin, crumpled sheets closely associated with each other to form a disordered solid The images of

Fe3O4/chitosan composite are showed in the figures2(c) and (d) It can be seen that the material has porous Figure 1 (A) XRD patent of (a) CS; (b) GO; (c) Fe 3 O4; and (d) FGC (with Fe 3 O4/GO/CS ratio was 68/2/30 as wt%); (B) FT-IR spectra

of (a) chitosan (CS); (b) Graphene Oxide (GO); (c) Fe 3 O4and (d) FGC nanocomposite.

surface and much holes Figures2(e) and (f) showed that FGCs have the surface more porous than Fe3O4/CS material Infigures2(f), it can be seen that Fe3O4nanoparticles, which particles size around of 30–40 nm, were deposited onto GO sheets It can be explained that the role of GO in FGC creating the new 3D structures, make increasing the surface area with high porosity It is very promissory in applications FGC for adsorbing

Cr(VI) ions

The morphologies of GO and FGCs were also investigated by TEM(figure3) The results revealed that GO (figure3(a)) has layered-like structure with some wrinkled edges; the layer is very thin and transparent TEM of

Fe3O4nanoparticles(figure3(b)) showed strong agglomeration of 20–40 nm of Fe3O4nanoparticles With

Fe3O4/chitosan nanocomposite (figure3(c)) the agglomeration degree is lower than in Fe3O4only In case FGCs sample, the spherical Fe3O4nanoparticles with diameter is about 25 nm were obtained and uniform on the surface of GO sheets(figure3(d))

3.2 Adsorption Cr(VI) ions

The UV–vis spectra of Cr(VI) solutions with different concentrations are shown in figure SI.2(a) and calibration curve for determining of Cr(VI) concentration in solution has been generated by drawing the optical density at Figure 2 FESEM of (a), (b) pristine GO; (b), (c) CS/Fe 3 O 4 nanocomposite; (e), (f) FGC nanocomposite FESEM images were taken at (a), (c), (e) low magnification and (b), (d), (f) high magnification.

wavelength of 360 nm(OD360nm) vs Cr(VI) concentration in solution (figure SI.2(b)) This curve has been used

to determine the residue concentration of Cr(VI) in solution after adsorption process

Contact time of Cr(VI) ions adsorption was studied from determination of absorbance after adsorbed Effect

of contact time was shown infigure SI.3 Results showed that the longer contact time, the higher adsorption capacity It can be seen infigure SI.3, in the first 40 min, the concentration of Cr(VI) ions is adsorbed rapidly From 40 to 60 min, the adsorption was on the slight increase and achieves balance at 60 min The adsorption rate

of Cr(VI) on FGCs adsorbent (v, mg.L−1.s−1) can be estimated using following equation:

dt

t

-( )

Here, Ctand C0(mg.L−1) are considering time and the initial concentrations of Cr(VI) in solution, respectively Following the Lambert Beer’s equation:

Where: A is absorption; C is concentration of chromium(VI) solution vs time so that the absorption (A) respects

to concentration(C) Therefore, equation (4) can be modified to equation (6) below:

t

The reaction order respect to chromium ions is calculated by the plot between velocity(inversely proportional to time) and the absorption (figure SI.4) The obtained plot is nearly linear with the correlation coefficients

(R2=0.9843) that means the reaction order respect to Cr(VI) is 1 (the first order law)

Beside of contact time, pH of solution also affects the adsorption process For study the effect of pH on Cr(VI) adsorption, experiments have been done with different pH values from 1 to 10 at an initial Cr(VI) concentration of 100 mg L−1and with adsorbent doses of 0.04 mg The results indicate that,figure4(a), in acid condition from pH1 to pH3, the equilibrium Cr(VI) concentration on the adsorbent (qe, mg.g−1) were of 42 to

95 mg g−1, however, when pH was changed from pH3 to pH10, the obtained qehas been decreased These results suggested that in acidic condition of solution, chromium ion removed was many To explain this trend, the form of chromium ion was considered At low pH, chromium ion exits mainly in Cr2O7 −; HCrO4 −, Cr3O102−

and Cr4O132−form whereas hydrogenchromate ion(HCrO4 −) is dominated between pH 1.0 and 4.0 [1] As the

pH increases, this form shifts to Cr2O7 − The high adsorption efficiency at low pH is attributed that there are so Figure 3 TEM of (a) GO; (b) Fe 3 O4nanoparticles; (c); Fe 3 O4/CS nanocomposite and (d) and FGC nanocomposite.

many hydronium ions(H+) surround the surface of Fe3O4/GO, which will promote the attraction of HCrO4 −

(or Cr2O7 −) anions on the surface by electrostatic attraction With increasing pH, the lesser hydronium ions, the electrostatic attraction between Fe3O4/GO and Cr(VI) anions is weaken, thus reducing the sorption efficiency

In this work, the maximum of Cr(VI) ions adsorption is at pH3 (figure4(a))

The influence of temperature on the adsorption capacity of chromium ions is shown in figure4(b) It can be seen that the adsorption capacity increases with the increasing of temperature of 30°C Over this temperature, the adsorption decreases It may be due to this material is less stable at high temperature At the high

temperature, the pore site of chitosan is miniatured and inactivation of adsorbent surface[6]

Figure 4 (a) Effect of pH on equilibrium adsorption (q e , mg.g−1) of Cr(VI) by FGC nanocomposite; (b) Influence of temperature on the Cr (VI) adsorption on FGC nanocomposite Experiment conditions: C 0 =200 mg l −1 ; T =10 °C; 20 °C; 30 °C; 40 °C; 50 °C;

pH =3; contact time: 1 h); (c) Effect of GO content in FGC nanocomposite to Cr(VI) removal Experiment conditions:

C0=200 mg l −1 ; T =25 °C; contact time: 1 h; (d) Effect of CS content in FGC nanocomposite to Cr(VI) removal Experiment conditions: C0=200 mg l −1 ; T =25 °C; contact time: 1 h; (e) Influence of adsorbent amount on equilibrium adsorption (q e ,

mg.g−1) Experiment conditions: C 0 =200 mg l −1 ; T =25 °C; m=0,01; 0,02; 0,04; 0,06; 0,08 (g); pH=3;1 h.

The thermodynamic parameters of the adsorption process such as enthalpy change(ΔH), entropy change (ΔS) and free enthalpy change (ΔG) of reaction are obtained from experiments using the following equations [28]:

q C

S R

H RT

e

Where, R is gas constant(R=8,314 J.mol−1.K−1), Ceis the equilibrium concentration of Cr(VI) ions

(mol.L−1), T is absolute temperature (oK) From the linear fit of ln(qe/Ce) versus 1/T, we will calculate ΔH0

and

ΔS0

values are 25.72 kJ.mol−1and 182 J.mol−1.K−1respectively The positive value of enthalpy change and entropy indicate the adsorption process is endothermic and the increasing of randomness during the sorption of Cr(VI) ions onto FGCs surface [28] The values of ΔG0

at 283, 293, 303, 313 and 323°K are −25.8; −27.6;

−29.4; −31.2 and −33.1 (kJ.mol−1) respectively The negative values indicate that the adsorption Cr(VI) ions process is spontaneous[29]

To investigate the effect of GO content on the adsorption capacity of Cr(VI) ions, we keep the amount of chitosan is constant(except one sample has only Fe3O4in its content) and change content of GO from 0 to 12.5 wt% The amount of adsorbent is still 0.04 g at pH3 and room temperature Figure4(c) shows that over

2 wt% of GO, the adsorption capacity decreases The reason maybe dues to with the increasing of GO amount, the adsorption sites on the surface increase and get equilibrium state with thefix amount (2 wt%) After that, the distribution of chitosan and GO on the surface decreases make adsorption site decreases The result is the removal decreases respectively[6] Especially, without CS and GO, the adsorbent cannot adsorb any heavy metal (chromium) (the dash line in figure4(c)) That means Fe3O4only has magnetic properties which makes the adsorbent easy to collect post- treatment In next works, wefixed the GO content of 2 wt% and change CS content in range from 0 to 60 wt% The results show that(figure4(d)) the adsorption capacity increases with the increasing of CS content Over 30 wt%, the adsorption capacity of FGC nearly gets balance The adsorbent amount greatly affects the adsorption capacity of chromium ions To optimize the adsorbent amount, we change the content of substance from 0.01 to 0.08 mg Figure4(e) shows that the optimized amount of adsorbent

is 0.02 g

3.3 Adsorption isotherm

In order to optimize the use of FGCs for Cr(VI) removal, it is important to establish the most appropriate adsorption isotherm The amounts of Cr(VI) in the solution were determined after equilibration and its

concentration in solution was be extracted from calibration curve The result is shown infigure5and table1 The data of the Cr(VI) adsorbed at equilibrium (qe, mg.g−1) and the equilibrium Cr(VI)concentration (Ce, mg.L−1) were fitted to the linear form of Langmuir adsorption model The obtained results are shown on figure5

with the obtained correlation coefficients (RLangmuir2 =0.9557 and RFreundlich2 =0.947 06) showed that dye adsorption equilibrium data werefitted well to the Langmuir isotherm (figure5(A)) rather than Freundlich isotherm(figure5(B)) The maximum monolayer capacity qmaxand KLthe Langmuir constant(L.mg−1) were calculated from the Langmuir model as 200 mg.g−1and 0.0245 L.mg−1, respectively Compare to the other adsorbent, such as magnetite/chitosan (55.8 mg.g−1), acid activated carbon (71 mg.g−1) [30], nano iron oxide impregnated in chitosan bead(NIOC) (69.8 mg.g−1) [31] or waste tire (174.55 mg.g−1) for adsorption of Cr(VI) (table2), FGCs has a higher adsorption efficiency

3.4 Regeneration studies

The adsorption mechanism of Cr(VI) onto FGCs adsorbent is proposed in figure6(A) Here, amino (–NH2) groups of chitosan coating on FGCs surface in acidic solution(pH3) have been protonated as (–NH3 +) as positive charge on FGCs’s surface By excess hydrogen ion, the FGCs adsorbent becomes cations and they are easy to attract chromium anions(as CrO4 −, HCrO4 −K as negative charge) on the surface by electrostatic attraction and thereby the results the adsorption efficiency increases

To confirm that Cr(VI) ions have been loaded on FGC after adsorption, we have analyzed FESEM and EDX

of FGCs before and after Cr(VI) adsorption FESEM micrographs of FGC nanocomposite before and after their exposure to Cr(VI) solution was shown on figures6(b) and (c), respectively It can be seen that the different surface morphology of FGC before and after Cr(VI) adsorption Energy dispersive x-ray spectroscopy (EDX) of original FGCs sample includes only C, O, Fe form their compositions(figure6(d)), however, after adsorption

Cr(VI), two new peaks have appeared at 0.5 keV and 5.5 keV which can be attributed to successfully adsorbed Cr(VI) on FGC surface

After Cr(VI) adsorption, FGCs can be recovered from working solution using an external magnet

(figure6(B)) Then, FGCs can be recycled using NaOH solution, because of in alkaline solution, –NH3 +groups will be de-protonated to be–NH2groups therefore Cr(VI) ions have been de-adsorbed and so that FGC will be

Figure 5 (a) Langmuir plot and (b) Freundlich plot.

Table 1 The parameters for the Langmuir and Freundlich models.

Table 2 Comparison of the Cr (VI) adsorption capacity of some adsorbents.

Reduced graphene oxide /NiO (RGO/NiO) nanocomposites 198 [ 6 ]

Fe3O4hollow microspheres /graphene oxide composite 32.33 [ 7 ]

Nano iron oxide impregnated in chitosan bead (NIOC) 69.8 [ 34 ]

Modi fied graphene oxide /chitosan composite (GEC) 86.17 [ 35 ]

Chitosan /montmorillonite magnetic microspheres 58.82 [ 36 ]

regenerated To evaluate the recyclable of the FGC nanocomposite, we performed the desorption experiments.

As can be seen infigure6(C), the sorption capacity of Cr(VI) ions decreases with the increasing cycle number After 6 cycles, the adsorbed efficient is about 75% of the first cycle (after 6 cycles, qmax∼150 mg.g−1)

Figure 6 (a) Illustration scheme for adsorption of Cr(VI) onto FGC surface; (b), (c) FESEM of FGC nanocomposite surface: (b), (c) before and (e), (f) after adsorption Cr(VI), respectively; EDX of FGC nanocomposite (d) before and (g) after adsorption of Cr(VI), respectively; (h) digital photos of (1) initial Cr(VI) solution; (2) a mixture solution of Cr(VI) solution and FGC nanocomposite as adsorbent; and (3) removal of FGC nanocomposite from solution after adsorption process by external magnet; (i) Effect of recycling adsorbents on FGC nanocomposites at experiment conditions: C0=200 mg l −1 ; m =0,04 g; T=25 °C; pH=3; and contact time

of 1 h.