Removal of hexavalent chromium from water by functionalized magnetic nano porous graphene (NPG / Fe3O4 @ COOH).

1. Introduction
Hexavalent chromium Cr(VI) is one of the top-priority toxic heavy metals in wastewater that can be found in many industrial effluents such as mining, battery and etc. There are many adsorbents like carbon materials purolite, chitosan zero-valent iron, and metal oxides have been used for Cr contaminated-water treatment. We used chemical vapor deposition (CVD) technique to synthesize nano-porous graphene (NPG) on transition metals that has recently been used for synthesizing large graphene domains. , magnetic nano-particles (MNPs) prepared for synthesis of NPG/Fe3O4 as an adsorbent for the economic and efficient removal of Cr+6 ions from aqueous. Modification of graphene with different materials can produce various adsorbents for improving their adsorption capacity.The purpose of this work was the synthesis of nano-porous graphene (NPG) by CVD method and magnetite with MNPs (NPG/Fe3O4) for economic separation from water then functionalized with the carboxyl group for provide different nano-composite (COOH@NPG/Fe3O4) as an adsorbent for the removal of Cr+6‏ ions from aqueous solutions. The characterization of COOH@NPG/Fe3O4 surface was analyzed by several techniques such as FTIR, SEM and TEM. The impacts of optimal parameters such as pH of solution, contact time, temperature, initial ion concentrations and adsorbent dosage were studied. In addition, the adsorption experiments were conducted under varying conditions to investigate the equilibrium isotherms, kinetic models and thermodynamics.
2. Materials and methods
2.1. Materials
Anhydrous iron (III/II) chloride, (99.9 %) ammonia solution, potassium dichromate and (28 %) hydrazine hydrate were purchased from Merck, Co, Germany. A hand magnet was also prepared to separate adsorbents from solution. All the solution were prepared by ultrapure water and kept at 4 ˚C prior to use.
2.2. Preparation of nano-porous graphene (NPG)
Nano-porous graphene was synthesized by chemical vapor deposition (CVD) technique which is a highly effective and low-cost method.
2.3. Synthesis of magnetite nano-porous graphene (NPG/Fe3O4)
NPG/Fe3O4 nano-composite was synthesized according to method reported in the previous studies (Juang et al. 2010) with some modifications.
2.4. Functionalization of magnetite nano-porous graphene with Carboxyl (COOH@NPG/Fe3O4)
Nano-porous graphene was functionalized by carboxyl. About 1 g of graphene was treated in 25 ml of nitric acid and 75 ml of sulfuric acid for 3 h at 60 ˚C. The mixture was kept in an ultrasonic bath and then washed by distilled water until reaching to the natural pH.
2.5. Characterization of the synthesis adsorbents
A scanning electron microscope (SEM, model MIRA3, Tescan, Czech Republic) was used to measure surface morphology, size and distribution of synthesized adsorbents. The XRD pattern of the adsorbent was analyzed (Quantachrome, NOVA 2000) using graphite monochromatic copper radiation (Cu Ka, λ =1.54 Å) in the 2θ range of 10-70˚ at 25 ˚C. The morphological and shape of the adsorbent were recorded by a transmission electron microscope (TEM, model PHILIPS, EM 208 S) with 100 keV. In addition, the surface functional groups were characterized by fourier transforms infrared spectroscopy (FTIR) (Tensor 27, Bruker, Germany). VSM (7400, Lakeshare) was also applied to determine the magnetic properties of adsorbent at 10 kOe at 25 ˚C.
. The effect of operating parameters such as pH of solution (2-10), contact time (5-120 min), adsorbent dose (20, 35, 50, 100, 150 and 200 mg/L), temperature (283, 298, 303 and 323 K) and different concentrations of Cr+6 (25, 50, 100,150 and 200 mg/L) on the adsorption efficiency was investigated.
2.7. Isotherm, Kinetic and thermodynamic of adsorption
The Langmuir and Freundlich isotherm models were used to evaluate Cr+6 adsorption onto the adsorbent . The adsorption kinetic models of Cr+6 on COOH@NPG/Fe3O4 adsorbents along with their corresponding regression coefficients are calculated. The thermodynamic diagram of Cr+6 adsorption is demonstrated.
3. Results and discussion
3.1. Characterization of the synthesized adsorbent
The FTIR spectra of synthesized composite of COOH@NPG/Fe3O4+Cr+6 and NPG/Fe3O4+Cr+6 that characterize the functional groups on the adsorbent surfaces that can play an important role in the adsorption mechanism, is demonstrated in Fig 1d. View of magnetic nanoparticles at a wavelength of 582 cm-1 confirms the Fe-O bonds between groups are in the form of tetrahedron. The other groups that have emerged in the wavelength of 1421.86 cm-1 and 3312 cm−1 can be assigned to represents aromatic C=C bonds and OH stretching vibrations of the carboxylic acid group, respectively. Finally, alkoxy CO bond is defined at 1029 cm-1 which indicates the graphene structure of NPG. The peak around 1700 cm−1 appears in the spectra of COOH@NPG/Fe3O4+Cr+6 .
In addition, Fig 1e represents the morphology, size and surface area of the NPG that were analyzed by SEM in high magnification; it can be see good porosity and high adsorption capacity. By TEM technique could be pointed out that a high density of Fe3O4 nanoparticles is noticed on the NPG layers (Fig 1f) low-magnification and Fig 1g high-magnification).
3.3. The effect of pH
The amount of Cr+6 removal in various ranges of pH between two adsorbents is shown in Fig 2a. According to a similar study, the adsorption of Cr+6 on COOH@NPG/Fe3O4 was significant at acidic conditions.
3.3. The effect of time on Cr (VI) adsorption onto COOH@NPG/Fe3O4
The contact time is one of the most essential parameters in designing a batch system that affects the adsorption of contaminants. As shown in Fig 2b.
3.4. The effect of adsorbent dosage on Cr (VI) adsorption onto COOH@NPG/Fe3O4
The effect of the optimal absorbent concentration (20, 35, 50, 100, 150 and 200 mg/L) on the 100 mg/L Cr (VI) removal under optimal condition (pH=3, t=60 min, 200 rpm, 25 ˚C) is shown in Fig 3a
3.5. Effects of different Cr+6 concentrations
The effect of various chromium concentrations (25, 50, 100, 150 and 200mg/L) under optimum conditions (pH=3, time=60, 200 rpm, m0=0.2 g/L) is shown in Fig 3b.
3.6. Adsorption isotherm models
Equilibrium adsorption isotherm models are used for better explanation of adsorption capacity between adsorbent and adsorbate which is an important factor in optimizing the application of adsorbents. The obtained values based on both Langmuir and Freundlich models for Cr+6 sorption on COOH@NPG/Fe3O4 at ambient temperature and optimum conditions are shown

3.7. Kinetic study
The adsorption kinetic models of Cr+6 on adsorbent along with their corresponding regression coefficients are given in Table 3. These are further verified by the diagrams presented in Fig 7. According to the regression coefficient (R2) in Table 3, the adsorption kinetic data was well-fitted by the pseudo second-order model.
3.8. Thermodynamic of adsorption
The thermodynamic diagram of Cr+6 adsorption is demonstrated. The results were obtained by the curve where the values of ΔH◦ and ΔS◦ can be achieved from the slope and intercept of the plot of lnK◦ against 1/T.
4. Conclusion
In this study, nano-porous graphene (NPG) was synthesized by chemical vapor deposition (CVD) method then magnetized by Fe3O4 and Fe2O3 powder for both rapid and economic separation by external magnetic field, due to its magnetism contributed from Fe3O4 instead of older method. The NPG/Fe3O4 was also functionalized with carboxyl (COOH@NPG/Fe3O4) for using as an adsorbent for removal of Cr+6 from aqueous solution. The structural, functional and morphological properties of synthesized adsorbent were characterized using SEM, TEM, XRD, FTIR, BET and VSM techniques. The optimum experimental conditions of Cr+6 removals using NPG/Fe3O4 was investigated in batch adsorption experiments. The adsorption efficiency of Cr+6 was increased with decreasing the pH of solution and initial Cr+6 concentrations. But, an increasing trend was happened in Cr+6 adsorption efficiency with increasing the adsorbent dosage and contact time until 60 min. In addition, the adsorption data was fitted well with Langmuir isotherm model. The Langmuir model indicated that it is monolayer adsorption of Cr+6 on the adsorbent surface. Kinetic data of adsorption can be best described by a pseudo second-order model. The sorption reaction onto adsorbent was an endothermic and spontaneous process. It should be noted that the synthesized adsorbent has promising potential in wastewater treatment which can easily be separated via an external magnet