Separation and Purification Technology 188 (2017) 206–218

Contents lists available at ScienceDirect

Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Development of adsorption and electrosorption techniques for removalof organic and inorganic pollutants from wastewater using novelmagnetite/porous graphene-based nanocomposites

http://dx.doi.org/10.1016/j.seppur.2017.07.0241383-5866/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: Department of Chemistry, Alfaisal University, P.O.Box 50927, Riyadh 11533, Saudi Arabia.

E-mail address: sribharath7@gmail.com (G. Bharath).

G. Bharath a,b,⇑, Emad Alhseinat c, N. Ponpandian b, Moonis Ali Khan d, Masoom Raza Siddiqui d,Faheem Ahmed a, Edreese H. Alsharaeh a

aDepartment of Chemistry, Alfaisal University, P.O. Box 50927, Riyadh 11533, Saudi ArabiabDepartment of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, IndiacDepartment of Chemical Engineering, Khalifa University for Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab EmiratesdChemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 April 2017Received in revised form 17 June 2017Accepted 11 July 2017Available online 16 July 2017

Keywords:GrapheneMagnetiteDye adsorptionElectrochemical adsorptionEnvironmental remediation

Herein, we report a new synthesis route for generating porous on graphene with a scalable and control-lable size of micron to submicron through the successive insertion of potassium atoms into interlayers ofgraphite at low temperature. Comprehensive studies such as physico-chemical analysis confirm that theas-obtained porous graphene has few layers with fewer defects. Further, the magnetically separable mag-netite (Fe3O4)/porous graphene nanocomposites were synthesized through a facile, cost-effectivehydrothermal process. The as-prepared nanocomposites were characterized by different analytical tech-niques. In the nanocomposites, superparamagnetic Fe3O4 nanoparticles with an average size of 30 nmnanoparticles uniformly dispersed on the porous graphene sheets, and they acted as mutual spacers inthe nanocomposites to avoid aggregation of the magnetic nanoparticles and restacking of the porous gra-phene layers. In addition, Fe3O4/porous graphene nanocomposites possessed high adsorption capacitiesof dyes and heavy weight metal ions from wastewater. An organic dye methyl violet was used as anadsorbate for investigating the adsorption characteristics of the Fe3O4/porous graphene nanocomposites.Fe3O4/porous graphene exhibited rapid adsorption (5 min), high adsorption capacity (Qo-460 mg/g), easyseparation and reuse owing to the high specific surface area with porous nature of graphene and highmagnetic property of Fe3O4 nanoparticles. Also, the nanocomposite used as an ultrahigh performanceof novel capacitive deionization electrodes (CDI) for removal of Pb2+ and Cu2+ ions at constant appliedpotential 1.2 V and constant flow rate 4 ml/min. The results indicate that the Fe3O4/graphene nanocom-posites exhibit an ultrahigh electrosorption for Pb2+, Cu2+ and Cd2+ ions. The progress made so far willguide further development of graphene based nanostructures with porous and exploration of such porousnanomaterials in environmental remediation toward removal of organic and inorganic contaminants.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Graphene, a one-atom-thick sheet of sp2 bonded carbon atomswith a two-dimensional hexagonal lattice structure, has attractedintense attention because of its distinctive properties such as elec-trical, optical, mechanical, high thermal conductivity and largespecific surface area with potential applications in various applica-tions [1]. In numerous applications, such as energy, catalysis,biosensing, drug delivery, batteries, surface coatings, composites,

fuel cells and solar cells, they need to be synthesized in large quan-tities and most preferably at low cost [2,3]. Highly demanded todevelop time consumes, simple and efficient methods for the syn-thesis of graphene sheets with large quantities and specified sizefor their several of technological applications. Commonly, gra-phene can be produced by various methods including chemicalvapor deposition (CVD), micromechanical cleavage of graphite, ballmilling, sonochemical, epitaxial growth on silicon carbide, andimproved chemical method [2,4–6]. A chemical method to synthe-size graphene and its derivatives were prepared with large scale,which involved the oxidation and reduction of graphene dispersionin organic solvents. However, the multistep oxidization processexposes a large number of structural defects on the surfaces of

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 207

graphene sheets and the harsh chemicals required to reduce gra-phene derivatives increase the safety, economic and environmentalcosts involved in large scale production.

Alternatively, very few researchers were prepared high qualitywith large scale graphene via hydroxylation of potassium-graphite intercalation compound (KC8) [7–9]. A prototypical gra-phite intercalation compound (GIC) KC8 (potassium-graphite) istypically formed by heating a mixture graphite and potassium ina solid-state reaction at higher temperatures of 180–260 �C for4 h in an inert atmosphere [7,10]. When a small amount of waterwas dropped into KC8 with an oxygen-free atmosphere, the stagestructure of the intercalation compounds was broken and therebyagglomerated graphene was obtained [10]. Also, sonication wasused to disperse the agglomerated graphene in buffered water con-taining sodium dodecyl sulfate followed by rinsing with hot wateryield gray color samples, resulting in few layers of a graphene.However, the as-prepared graphene from KC8 exhibits low specificsurface area (SSA) because it has contains multilayers of graphene.Therefore, the lower SSA of graphene may affect the potentialapplications including, energy, sensors, biomedical and environ-mental remediation. On the other hand, graphene based adsorbentwidely used for environmental applications including the removalof heavy metals and various organic dyes from polluted water [11].Specifically, the toxic metal ions such as lead (Pb), copper (Cu),cadmium (Cd) and nickel (Ni) ions and various dyes are toxic andnot biodegradable, which causes various health hazards and eco-logical disequilibrium [12,13]. Therefore, efficient removal of theseheavy metal ions and dyes from contaminated industrial wasteeffluents is important in water treatment. Activate carbon (AC), sil-ica gel, polymeric porous materials/frameworks and graphenederivatives have been broadly studied in adsorption of heavy metalions and dyes for their large specific surface area (SSA) with micro-porous structure [11,14–16]. Also, graphene oxide was used asadsorbent to removal of Zn(II) ions and aluminum metal–organicframework/reduced graphene oxide composite provide largeadsorption sites toward removal of p-nitrophenol from aqueoussolutions [17,18]. However, these adsorbents suffer from lowadsorption capacity and separation inconveniences; moreover,almost all of them become problematic on the concern of recyclingand reuse.

In order to solve these problems, Fe3O4/graphene-basednanocomposites are established for the efficient removal of dyesand heavy metal ions due to the high loading capacity and easymanipulation by external permanent magnets [19,20]. Recentstudies were established that the Cu(II) ions and fulvic acidsremoved from an aqueous solutions by graphene oxide nanosheetsdecorated with Fe3O4 nanoparticles [21]. However, when introducepore in the graphene may enhance the adsorption ability towardremoval of organic and inorganic pollutants due to admirableSSA. Therefore, structural features of porous graphene with Fe3O4

nanocomposites including its large SSA, which enhances theopportunity to contact organic/inorganic pollutants, and its well-defined porous structure, which facilitates the diffusion of pollu-tant into the 2D structure, enable magnetic Fe3O4 to be an idealmaterial for pollutant separation management due to its excellentmagnetic properties and easy recyclability. Moreover, Fe3O4/graphene-based nanocomposites can offer high electroactiveregions and short diffusion path for the efficient access of the elec-trolyte to the electrode because of the high SSA, signifying they arecapable nanomaterials for capacitive deionization (CDI) [22]. TheCDI is an emerging water treatment technique that uses pairs ofelectrodes to remove charged (positive and negative ions) speciesfrom water. Subsequently CDI technique trusts on the electrosorp-tion of positive and negative ions at the surfaces of the preparedelectrodes. A design and the nature of the prepared electrodematerial is a major key constituent of the high performance of

CDI. The supreme electrodes for CDI technique might have a largeSSA with a porous nature offering conductivity, high ion mobilityand electrochemically stable [22]. Recent advances in CDI elec-trode nanomaterials have been an effort on development ofFe3O4/graphene-based nanocomposites with novel porous nanos-tructures toward heavy metal ions removal from wastewater[23,24]. Based on that, we have decided to synthesis the nanocom-posites of Fe3O4/porous graphene to remove dyes as well as heavymetal ions from wastewater, and especially we can focus theadsorption method for dyes removal and CDI technique for heavymetal ions.

Taking the above important factors into account in our presentstudy, we report a novel strategy to synthesize scalable porous gra-phene obtained from potassium-graphite intercalation compound(KC8). Meanwhile, the Fe3O4 nanoparticles dispersed on surfacesof porous graphene sheets by using hydrothermal process at180 �C for 12 h. Comprehensive synthesis strategies of porous gra-phene and formation mechanism of Fe3O4/porous graphenenanocomposites were discussed based on experimental results.The dispersion of Fe3O4 nanoparticles on porous graphene sheetsprovides a decrease in the possibility of severe agglomeration tomaintain their high SSA which was used as a good nanomaterialbased adsorbent for dye removal and heavy metal ions adsorptionfrom wastewater. The as-obtained nanocomposite exhibits severaladvantages including effective adsorption of methyl violet (MV)and the adsorbent can be removed completely from aqueous solu-tions by an external permanent magnetic field. Also, the initial CDIperformance of Fe3O4/porous graphene-based electrodes was fur-ther investigated and compared with graphene and pure Fe3O4

based electrodes. The results indicate that the Fe3O4/graphenenanocomposites exhibit an ultrahigh electrosorption capacity forPb2+, Cu2+ and Cd2+ ions were found to be 47, 40 and 49 mg g�1,respectively in NaCl solution with an initial concentration of500 mg L�1 and a cell potential of 1.6 V. Therefore, the Fe3O4/-porous graphene nanocomposites should be a promising adsorbentfor removal of dyes as well as good electrode material for high-performance CDI.

2. Experimental

2.1. Materials

All the chemical were analytical grade and received withoutfurther purification. Graphite flakes (�105 lm flakes), potassiumpermanganate (KMnO4), aspartic acid, ferrous chloride tetrahy-drate (FeCl2�4H2O) and ferric trichloride hexahydrate (FeCl3�6H2O)were supplied by Sigma Aldrich. Acetone, Sodium hydroxide(NaOH), ethanol and hydrochloric acid 5% (HCl) and were pur-chased from Himedia Laboratory Pvt. Ltd.

2.2. Synthesis of graphene form KC8

In the other process, stoichiometric amount of potassium (K)(0.10 g (25.6 mmol)) and graphite (2.458 g (204.8 mmol) werepre-heated and degassed to remove adsorbed oxygen and water.Further, the potassium (K) and graphite (G) were mixed and dis-solved in 30 ml of toluene by probe sonic dispersion for 30 min.The intercalation process has been completed only after 60 minof sonication. Then, this was placed in a tubular furnace at 400 �Cfor 4 h under Argon atmosphere. After the intercalation process,the tubular furnace was cooled down naturally to room tempera-ture, and we obtained the KC8. Then, 200 mg of KC8 was added to50 ml of the selected solvents and kept at elevated temperaturesin the range of 0–80 �C for 24 h to allow the formation of graphenenanosheets (GNS) with regular interlayers. Then, the expanded

208 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

graphene sheets were exfoliated to graphene flakes with sonicationin a water bath for 60 min and treated with acid (2 M HNO3) atroom temperature to obtain the high-quality porous graphenewith large scale.

2.3. Synthesis of Fe3O4/porous graphene nanocomposite byhydrothermal technique

In a typical synthesis process, 50 mg of porous graphene wasdispersed in 50 ml double distilled water by ultra-sonication for2 h to form a stable black color solution and the supernatant wascollected for the further synthesis process. Afterwards, 50 mM ofFeCl2�4H2O and 100 mM of FeCl3�6H2O solutions added to theabove solution and pH was maintained in the range of 10–11 byadding of ammonium hydroxide solution (30%, NH4OH). Then,the solution was stirred continuously for 30 min and the final mix-ture was transferred to the Teflon-lined stainless steel autoclave at180 �C for 12 h. The precipitate was washed with double distilledwater and ethanol several times and dried in vacuum oven at70 �C for 12 h before further physico-chemical characterizations.

2.4. Characterization

Morphologies of the graphene and Fe3O4/porous graphene wereanalysed via field emission scanning electron microscopy (FESEM)(FEI Quanta-250 FEG) coupled with EDX spectroscopy and high-resolution transmission electron microscopy (HRTEM) carried outusing a FEI Tecnai G2 S-twin instrument with a UHR pole piece.The structural analysis of the samples was performed through X-ray diffraction analysis (XRD) at room temperature using a PANa-lytical X’Pert-Pro X-ray diffractometer with Cu Ka1 radiation(k = 1.5406 Å). The average crystal structure and crystallite sizesof the samples were calculated using the Scherrer formula by usingthe X-ray line broadening. Raman measurement of the as-preparedsamples was performed by using LabRam HR Raman spectrometerusing a 520 nm laser source. The infrared spectrum of the as-prepared samples was obtained by using a Fourier transform infra-red (FTIR) spectrometer (Bruker Tensor 27, Germany). The sampleswere prepared by a KBr pellet by investigating the peaks within therange of 4000–450 cm�1. Dye adsorption performance was carriedout using UV–Visible spectral analysis was done by using JoscoV-650 spectrophotometer. The X-ray photoelectron spectroscopy(XPS) was carried out using Kratos Axis Ultra-DLD X-ray photoelec-tron spectroscopy (Manchester, U.K). Magnetic properties of thesamples were measured using a Quantum Design Versa Lab 3TVibrating Sample Magnetometer (VSM). The specific surface area(SSA) and pore diameter of the samples were analysed with a sur-face area analyser (ASAP 2020, Micromeritics) using physicaladsorption–desorption of N2 at liquid-N2 temperature.

2.5. Batch mode adsorption

In this adsorption experiments, 50 ml of the crystal violet (CV)dye solution with different concentrations (10–40 mg/L) and10 mg of Fe3O4/porous graphene nanocomposites was introducedinto a 200 ml glass-stoppered flask at 28� ± 0.5 �C, and the mixturesolution was agitated in an orbital shaker at 300 rpm. The super-natant liquid samples were taken withdrawn during agitation atpreset time intervals. The adsorbent was separated from the solu-tion using external applied magnetic field. The absorbance spectraof the collected supernatant solutions were estimated for quantifi-cation of the remaining dye concentration in the solution throughJasco V-650 UV–Vis spectrophotometer. All the experiments werecarried out three times, and calculate the average dyes adsorptionvalues. The initial dye concentration in the test solution and thedosage of adsorbent were varied to explore their effect on the

adsorption kinetics and isotherms. The pH effect on adsorptionquantity was performed by studying the adsorption of dye overthe pH range from 2 to 10. The NaOH or HCl solutions were usedto adjust the pH of the dye solutions.

The amount of sorption at time t, qt (mg/g), was calculatedusing the following the formula:

The dye removal percentage can be calculated as follows:

ð%Þ of dye removal ¼ ðC0 � CeÞðC0Þ � 100 --- ð4:11Þ

where C0 and Ce (mg L�1) are the initial and equilibrium con-centrations of the dye in solution.

2.6. Fabrication of CDI electrode

Fe3O4/porous graphene nanocomposites and PVDF (polyvinyli-dene fluoride) as a binder ratio (9:1) are used to prepare the CDIelectrode. The graphite sheet electrode dimension was 80length � 30 mm wide � 0.3 mm thick. The carbon nanocompositeslurry was prepared by mixing of the raw material Fe3O4/porousgraphene (90 wt%) PVDF (10 wt%) dissolved in N-methyl-2-pyrrolidone (NMP). The mixture was sonicated and mixed by stir-red for about 12 h. The homogenous carbon slurry was then castonto a graphite sheet used as a current collector by applying adoctor-blade technique. The coated electrodes were dried in theoven at 60 �C for 12 h to remove entirely humidity and residualorganic materials.

3. Results and discussion

3.1. Formation mechanism of porous graphene from potassiumintercalated compounds

The formation mechanism was schematically illustrated inFig. 1. Graphite intercalation compounds (GICs) are formed whenalkali metal atoms inserted into the spaces between the grapheneplanes. The mixed potassium and graphite powders were placed ina tubular furnace and heated at 400 �C for 4 h under Ar atmo-sphere. During this process, the molten potassium enters into themicrospores of graphite through capillary action and sponta-neously intercalated to graphite interlayers. Due to the intercala-tion process, the interlayer distance of graphite increases from0.34 to 0.57 nm. Then, the gold-colored potassium (K) graphiteintercalation compounds (KC8) in Fig. 1 were gradually sliced inpyridine solution at 0 �C for 24 h to form dark-colored graphenenanosheets and further it was washed with 5% HCl. Afterwards,the suspension was treated with 2 M of HNO3 resulting nano-scaled pores could be created on the surfaces of graphene sheets.In this process, HNO3 can react with carbon atoms (C@C) at theexisting defect sites and edge sites of the graphene sheets, leadingto partial removal and detachment of several carbon atoms fromthe graphene sheets [25]. Furthermore, the porous graphenenanosheets were exfoliated to form negatively charged grapheneunder probe sonication for 30 min. The ultra-probe sonic wave atsufficient acoustic energy produced frictional forces and highstrain rates may broke the carbon framework, resulting in the for-mation of nano-scaled pores in the graphene sheets [26,27]. Thenegatively charged porous graphene is uniformly dispersed in n-methyl 2-pyrolidine (NMP) solution without any coagulation orflocculation. Fig. S1 shows the surface charge of the synthesizedporous graphene in NMP solution was measured to be �17.0 mV.This result confirms that the synthesized porous graphene exhibitsnegative charges in NMP solvents. Further, the obtained porousgraphene sheets were used to composite with Fe3O4 nanoparticlesusing hydrothermal process. The broadly accepted growth mecha-

Fig. 1. Schematic illustrations for the scalable preparation of porous graphene sheets from potassium-graphite intercalation compound (KC8) and formation mechanism ofFe3O4/porous graphene nanocomposites.

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 209

nism for the synthesis of metal oxides nanoparticles decorated gra-phene is the electrostatic attraction of the positively-charged metaloxides ions by the polarized bonds of the functional groups on thesurfaces of graphene. As demonstrated in Fig. 1, during the synthe-sis, the porous graphene sheets were uniformly dispersed in aque-ous solutions, in which the Fe2+ and Fe3+ (1:2) precursor salts fullydissolved. A hydrothermal temperature of 180 �C for 12 h is a suf-ficient temperature for the formation of the thermodynamicallystable magnetite phase on the porous graphene sheets, whilst itis considered that the growth mechanism of magnetite nanoparti-cles may have involved the growth and dissolution of intermediateiron (oxy)hydroxide (Fe(OH)3) phase. Further, the Fe(OH)3 wasreduced by hydrothermal temperature to formed homogeneousFe3O4 phase with a relatively high crystallinity could be obtainedon surfaces of porous graphene sheets.

3.2. Morphological analysis

Field-emission scanning electron microscopy (FESEM) with EDSand high-resolution transmission electron microscopy (HRTEM)were performed to characterize the morphology, compositionand crystalline nature of the graphene and Fe3O4/porous graphenenanocomposites. Fig. 2(a)–(d) shows different magnificationsFESEM and TEM micrographs of the graphene nanosheets pro-duced after intercalation and exfoliation. The as-obtained gra-phene sheets indicate a flexible morphology with porous nature.The FESEM images with different magnification images of Fe3O4/porous graphene nanocomposites are shown in Fig. 2(e)–(g). Thelow magnification FESEM images clearly indicate that the Fe3O4

nanoparticles are homogenously embedded on the surface of theporous graphene sheets and that no significant changes in the mor-phology of the porous graphene. High-magnification FESEMimages (inset Fig. 2(g)) shows that the Fe3O4 nanoparticles havespheres like morphology and the average diameter of primary par-ticles measured to be 30 nm. A, significance of this study demon-strates that the Fe3O4 nanoparticles and porous graphene were invery close contact with each other, and they acted as mutual spac-ers in the as-prepared nanocomposites to maintain the restackingof the graphene layers and prevent the aggregation of the Fe3O4

nanoparticles. To further characterize the elemental compositionof the nanocomposite using energy dispersive X-ray spectroscopy(EDS). The EDS spectrum in Fig. 2(h) shows that the Fe3O4/porousgraphene nanocomposite sample reveals the presence of elementsiron (Fe), oxygen (O), and carbon (C) and no other impurity wasdetected. HRTEM micrographs and Fourier-transform (FT) patternwere used to evaluate the crystalline phase and the lattice fringeinformation of the Fe3O4/porous graphene nanocomposites. Thetypical HRTEM image of Fe3O4/porous graphene nanocomposite isshown in Fig. 2(i), the surfaces and edges of porous graphene aredensely covered by dispersed Fe3O4 nanoparticles, whose nanopar-ticle sizes agree well with the FESEM micrographs. The lattice-resolved HRTEM image (inset Fig. 2(k)) of the Fe3O4/porous gra-phene nanocomposites reveals the (3 1 1) orientation of the wellcrystalline single-phase Fe3O4 core, and the estimated lattice-fringe or d spacing value of the magnetic Fe3O4 nanoparticles is0.38nm, which agrees well with that of standard magnetite.Fourier-transform (FT) pattern (Fig. 2(l)) of the Fe3O4 could beindexed as 220, 311 and 400 diffractions of the face-centered cubic(fcc) magnetite structure and also the bright diffraction rings showthat the Fe3O4 nanoparticles were well crystallized.

3.3. X-ray diffraction analysis

Fig. 3 displays the XRD patterns of graphite (Gr), intercalationgraphite (IGr), graphene (G) and Fe3O4/porous graphene nanocom-posite. The strong diffraction peak of natural graphite flakesappears at 26.7� in the XRD pattern as shown in inset Fig. 3, corre-sponding to the interlayer d-spacing of 0.32 nm calculated via theBragg equation. In contrast, the IGr diffraction pattern shows abroadening characteristic peak (higher full width at half maxima,FWHM (b)) at 26.5�, corresponding to an interlayer spacing of0.72 nm, indicating the potassium (K atoms) metal atoms interca-late into graphene layers. These K atoms enlarge the interlayerspacing between graphene sheets to 0.72 nm from 0.32 nm[9,10]. This increase in the interlayer distance weakens the vander Waals force between the graphite interlayers, consequentlyenabling graphite exfoliation. The XRD pattern of the obtainedporous graphene showed a downshifted in the direction angle of

Fig. 2. (a–c) FESEM and (d) TEM images of graphene synthesized from KC8, (e–g) FESEM images of Fe3O4/porous graphene nanocomposites, (h) EDS analysis, (i–k) HRTEMimages and lattices fringes of Fe3O4/porous graphene nanocomposites and (l) Fourier-transform (FT) pattern of single Fe3O4 nanoparticles.

Fig. 3. X-ray diffraction pattern of graphene (G) and Fe3O4/porous graphenenanocomposites. Inset Figure: graphite (Gr) and intercalation Graphite (IGr).

210 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

the present sample and also peaks intensity decreased. After inter-calation process, the IGr was suspended and exploited in 5% HCland n-methyl 2-pyrolidine (NMP) with maintaining at 75 �C for2 h. This process could prompt the functionalization of the hydro-xyl and carboxylic groups on the surfaces of porous grapheneresulting increase in the interlayer d-spacing and decreases thepeak intensity at 26.2�. Meanwhile, the temperature induces the

epoxy functional groups on the surfaces of graphite sheets. Theseall negatively charged functional groups can produce binding sitesfor positively charged metal ions, resulting in metal nanoparticlesuniformly grown on the surface of porous graphene sheets. Mag-netite (Fe3O4) nanoparticles uniformly grown on the surfaces ofporous graphene through hydrothermal process at 180 �C for12 h. Fig. 3 shows the XRD pattern of Fe3O4/porous graphenenanocomposite displays a series of diffraction peaks at2h = 30.25, 35.6, 43.31, 53.7, 57.34, and 62.8�, corresponding tothe reflections from the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and(4 4 0) crystal planes of the cubic inverse spinel structure of Fe3-O4 (PDF card No. 88-0866) with the average grain size is calculatedto be 28 nm. Such a cubic inverse spinel structure is characterizedby O2� anions at the cube corners and face centres of the FCC lat-tice with all the tetrahedral sites being occupied by Fe3+ cationsand the octahedral sites equally by both Fe2+ and Fe3+ ions [4,20].

3.4. FTIR analysis

Fig. 4 shows the FTIR spectra of natural graphite powder, porousgraphene, and the Fe3O4/porous graphene nanocomposite. TheFTIR spectrum of the pure natural graphite exhibits a sharp, highintense peak at 3435 cm�1, which associated to bending modesof water-related with KBr used for the preparation of the FTIR pel-let. The intense peak at 1630 cm�1 is ascribed to the existence of aC@C bond in pure graphite powder and graphene [6]. A broad peakat 1227 cm�1 in the graphene spectrum is attributed to the pres-ence of phenolic, ketone and epoxy groups in graphene, whichmight have been caused by the oxidation of graphene during

Fig. 4. FT-IR spectra of graphite, porous graphene, and Fe3O4/porous graphenenanocomposites.

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 211

exploration and washing process. The intense peak observed at560 cm�1 corresponds to the intrinsic stretching vibration(Fetetra–O) of metal–oxygen at a tetrahedral site [4,19]. The peaksat 2920 and 2850 cm�1 show the asymmetric and symmetricvibrations of CH2 groups.

3.5. Raman analysis

Raman spectroscopy is a powerful tool to examine the struc-tural properties in graphene-based materials. Fig. 5 shows theRaman spectra of natural graphite powder (G), intercalation gra-phite (IGr), graphene (G), and Fe3O4/porous graphene nanocom-posite and the spectra entail of four prominent peaks, namelythe D, G, 2D and D + G band. Analysis of the four main signalsin the Raman spectra, the so-called D-band around 1350 cm�1,G-band around 1582 cm�1 and the dispersive double-resonancepeak in the range between 2650 and 2900 cm�1 (which carriesboth the name 2D band and D + G band), offers the completeevidence; e.g., it allows determination of the number of porous

Fig. 5. Raman spectra of graphite, intercalation graphite, porous graphene andFe3O4/porous graphene nanocomposites.

graphene layers, ordered and disordered structure of graphene,induced strain in the structure, and charging. The intensity (I)ratio of the D and G band (ID/IG) indicates the density of struc-tural defects on the porous graphene surface. In Fig. 6, the ID/IGratio of IGr is 0.78, 0.89 (G), and 1.01 (Fe3O4/G), which is muchhigher than the calculated value of pure graphite (0.17). Basedon this observation the intensity ratio of the IGr shows highervalue associated to pure graphite demonstrating the presence oflocalized sp3 defects in the sp2 (C@C) network upon intercalationof the graphite and also the 2D band was disappeared. The ID/IGratio of the porous graphene shows higher than the intensity ratioof IGr. This result also clearly indicates that the phenolic, ketoneand epoxy functional groups were presented on surfaces of theporous graphene. Meanwhile, the 2D band at 2750 cm�1 appearedin the Raman spectrum of porous graphene, which indicated thatthe porous graphene consists of only a few layers. This study con-firms that the few layers of porous graphene successfully pre-pared from KC8. The ID/IG ratio of Fe3O4/porous graphenenanocomposites is 1.01 which is much higher than the calculatedvalues of Gr, IGr, and porous graphene. These ID/IG ratio clearlyindicate that the Fe3O4 modification introduced additional defectsinto the graphene structure.

3.6. XPS analysis

The chemical composition of the Fe3O4/porous graphenenanocomposites was studied using XPS analysis. In the XPS spectra(Fig. 6a), the three most important peaks such as, C1s, O1s, andFe2p were observed in the survey scans of the Fe3O4/porous gra-phene nanocomposites. The inset Fig. 6b shows that the C1s XPSspectrum of porous graphene deconvoluted into two differentpeaks associated with sp2 hybridized carbon atoms (C@C at284.35 eV), and sp3 carbon atoms bonded with oxygen (CAO at286.29 eV), respectively [4,19,20]. The XPS spectrum of O1s in aninset Fig. 6(c) shows two intense peaks at 530 and 531.82 eV forthe anionic oxygen in magnetite and residual oxygen functionalgroups present in graphene sheets. Moreover, in the high-resolution Fe 2p XPS spectrum of the Fe3O4/porous graphenenanocomposites, the binding energy peaks at 710.73 and724.57 eV agreed well with that of Fe 2p3/2 and Fe 2p1/2 (Fig. 6(d)), respectively.

3.7. VSM analysis

The field dependence of magnetization for the Fe3O4

nanoparticles and Fe3O4/porous graphene nanocomposites wasinvestigated using a vibrating sample magnetometer. VSManalysis was carried out at room temperature with an appliedmagnetic field of �2 to 2 KOe for both the Fe3O4 nanoparti-cles and the Fe3O4/porous graphene nanocomposites, and theresults shown in Fig. 7(a) and (b). The Fe3O4 nanoparticlesand the Fe3O4/porous graphene nanocomposites displayed atypical S-shaped magnetization hysteresis loops with negligiblecoercivity (Hc) and remnant (Mr); inferring that the as-prepared samples show super-paramagnetism, while theirmagnetization behaviors removed in the absence of theapplied magnetic field. The saturation magnetization (Ms)obtained from the magnetic hysteresis loops are 63 and38 emu g�1 for Fe3O4 nanoparticles and the Fe3O4/porous gra-phene nanocomposites, respectively. Magnetization resultsclearly show that the as-prepared Fe3O4/porous graphenenanocomposites have excellent magnetic property, suggestingtheir possible applications such as the organic and inorganiccompounds removal from wastewater with efficient separationability.

Fig. 6. XPS spectrum of Fe3O4/porous graphene nanocomposites (a) wide spectrum, (b) C1s, (c) O1s and (d) Fe2p.

Fig. 7. Magnetic hysteresis loops of (a) pure Fe3O4 nanoparticles and (b) Fe3O4/porous graphene nanocomposites.

212 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

3.8. Specific surface area analysis

To determine the adsorption capacity toward organic and inor-ganic pollutants, we performed the Brunauer–Emmett–Teller (BET)specific surface area (SSA) of porous graphene and Fe3O4/porousgraphene nanocomposites using liquid nitrogen adsorption–des-orption isotherms. Fig. 8(a) and (b) show the isotherm curves ofgraphene and Fe3O4/porous graphene nanocomposites and theinset Fig. 8(a) and (b) exhibits the corresponding pore-size distri-bution calculated using the BJH method. Conferring to the Interna-tional Union of Pure and Applied Chemistry classification, thenitrogen adsorption-desorption isotherms of the graphene andFe3O4/porous graphene nanocomposites were type IV, with

hysteresis loops type H3. Generally, a type IV physisorption(adsorption-desorption) isotherm indicates the presence ofmesopores in the synthesized both samples, although a type H3

hysteresis loop may associated with slit-shaped pores, probablybetween parallel layers of graphene. The specific surface area andpore size of the graphene was 410 m2 g�1 and 4 nm respectively.A higher BET specific surface area and pore size may have beendue to the activation effect of potassium (K) atoms between thegraphene layers. After successfully removal of potassium atoms,it may create the mesopores on surfaces of graphene resultingenhance the SSA. In contrast to the Fe3O4/porous graphenenanocomposites, Fe3O4 nanoparticles show a much broader poresize distribution up to 7 nm and BET specific surface area

Fig. 8. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curve of (a) porous graphene and (b) Fe3O4/porous graphene nanocomposites.

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 213

determined to be 380 m2 g�1. The density of the Fe3O4 nanoparti-cles on porous graphene may slightly decreases the specific surfacearea; however obtained specific surface area shows much higherthan already available graphene based nanostructures [25,27]. Sig-nificance of this study well supported with the conformation ofporous on nanocomposites with higher specific surface area couldbe utilized to adsorb organic and inorganic pollutants fromwastewater.

3.9. Adsorption of crystal violet

The Fe3O4/porous graphene nanocomposites act as a usefuladsorbent for adsorption of organic dyes from aqueous solutions.Commonly used adsorbents such as activated carbon (AC), gra-phene and graphene oxide could provide more adsorption sitesfor the adsorption of dye molecules; however after adsorption pro-cess it’s very hard to separation. The introduction of magneticproperties into graphene may combine the high adsorption capac-ity of graphene and the separation convenience of the magneticmaterials. The Fe3O4/graphene nanocomposites can be easily sepa-rated from the aqueous solution using a permanent magnet. There-fore, it could be used as a magnetic adsorbent to remove organicand inorganic contaminants from wastewater. Crystal violet (CV)is a model organic dye for determining of adsorption capacity of

Fig. 9. (a) Effects of agitation time and (b) effect of pH with the initial concentration of d

Fe3O4/porous graphene nanocomposites. The effects of contacttime, pH effect, and initial dye concentrations on the adsorptionprocess were discussed and analysis was done using validatedspectrophotometric analysis.

3.9.1. Effects of agitation time and pH effectEffects of agitation time and crystal violet concentrations (10,

20, 30 and 40 mg/L) on the removal of crystal violet with constantadsorbent dosage of 10 mg of Fe3O4/porous-graphene nanocom-posites are presented in Fig. 9(a). The% of adsorption for crystalviolet increased with increase in agitation time 0–300 min andreached equilibrium at 210 mm. The% of adsorption crystal violetremoval at equilibrium decreased from 93 to 51 as the dye concen-tration was increased from 10 to 40 mg/L. It is clear that theadsorption of crystal violet depends on the initial concentrationof the crystal violet. The adsorption curves are smooth, singleand continuous leading to saturation. At higher dye concentrations,the ratio of the initial number of dye molecules to the availablesurface area is low, therefore decreases the dye adsorption per-centages. Moreover, the control dye adsorption experiment wasconducted on 10 mg/L crystal violet concentrations without addi-tion of adsorbent. This results shows that 5% of crystal violet losesdue to the environmental factors of light and room temperature.

ye (10, 20, 30, 40 mg/L and control) on the Fe3O4/porous graphene nanocomposites.

214 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

Subsequently, the fractional adsorption becomes independentof initial concentration. However, at high concentration of dyes,the available sites of adsorption become fewer and hence theadsorption% of dye is dependent upon initial concentration. ThepH is most significant parameters for the adsorption of crystal vio-let on the Fe3O4/porous graphene nanocomposites. In this study,the adsorption of crystal violet on Fe3O4/porous graphenenanocomposites was explored with the pH in the range of 2–10by fixing the concentration of Fe3O4/porous graphene nanocom-posites dosage to 10 mg/L for the different initial concentrationsof crystal violet as 10, 20, 30 and 40 mg/L. The various adsorp-tions% of dye were attributed by changing the pH value of the solu-tions cause to an increase or decrease in the adsorption capacity.The adsorption capacity can be allocated to the chemical form ofcrystal violet in a solution at the specific pH or due to differentfunctional groups on the adsorbent surface. Fig. 9 (b) shows theadsorption percentages were decreased with the pH of 2–6 andthe maximum adsorption were attributed at pH 7. In the pH 7,the negatively charged Fe3O4/porous graphene-based adsorbentfavors to capturing cationic dyes molecules through electrostaticattraction. Therefore, pH 7 is the optimum pH level for all dyeadsorption experiments. Especially, crystal violet and other catio-nic dyes produce an intense molecular cation (C+) and reduced ions(CH+). The adsorption mechanisms of crystal violet on Fe3O4/porous graphene nanocomposites are shown in Fig. 10. Conse-quently, the normal pH of solution contains OH– ions and it waspresent on the surface of adsorbent, which was favoring theadsorption of cationic dye molecules.

Fig. 10. Schematic illustration for adsorption of crystal

Therefore, the adsorption of crystal violet usually increases athigher pH value. At lower dye initial concentrations the adsorptionpercentage of crystal violet was higher due to the larger specificsurface area (SSA) of the Fe3O4/porous graphene nanocompositesand it was provides more adsorption sites. The higher initial con-centrations (40 mg/L) of crystal violet fully occupied on the adsorp-tions site of Fe3O4/porous graphene nanocomposites causesdecreases the SSA and therefore the removal percentage of thecrystal violet decreases. Meanwhile, the optimum pH for crystalviolet adsorption by Fe3O4/porous graphene was determined tobe pH 7.

3.9.2. Adsorption kineticsThe study of dyes adsorption kinetics could describe the solute

uptake rate and obviously this rate controls the residence time ofadsorbate uptake at the interface of solid–solution. The kineticsof crystal violet adsorption on the Fe3O4/porous graphenenanocomposite was analysed using Lagergren first-order andpseudo second order models as shown in Fig. 11(a) and (b). Theconventionality between experimental data and the kinetics mod-els was expressed by the correlation coefficients (R2) value. Lager-gren (1898) presented a first-order rate equation to describe thekinetic process of liquid-solid phase adsorption of oxalic acid andmalonic acid onto charcoal, which is believed to be the earliestmodel relating to the adsorption rate based on the adsorptioncapacity. It can be described as follows:

The rate constant of adsorption is determined from the first-order rate expression given by.

violet on Fe3O4/porous graphene nanocomposites.

Fig. 11. Linear fit of experimental data obtained using pseudo first order kinetics (a), second order kinetics (b), Langmuir isotherm (c) and Frendlich isotherm (d) for theadsorption of crystal violet on Fe3O4/porous graphene nanocomposites.

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 215

log ðqe� qÞ ¼ log qe� k1t2:303

where qe and q are the amounts of crystal violet adsorbed (mg g�1)at equilibrium and at time t (min), respectively and k1 is the rateconstant of adsorption (min�1). Linear plots of log (qe-q) versus tfor different concentrations of crystal violet were obtained asshown in Fig. 11(a) and (b).

Pseudo second-order kinetic model can be represented as

tq¼ 1

k2q2eþ 1qe

where k2 is the equilibrium rate constant of pseudo second-ordercrystal violet adsorption (g mg�1 min�1). Values of k2 and qe werecalculated from the plots of t/q versus t (Fig. 11(c) and (d)). The cal-culated results obtained from the first and second-order kineticmodels along with the experimental qe values are presented inTable 1. The calculated R2 values of the pseudo second order kinet-ics (R2 = 0.9991) are generally closer to the calculated R2 valuesfrom Lagergren first order kinetics (R2 = 0.9735) in the initial

Table 1Langmuir and Freundlich constants and correlation coefficients for adsorption of crystal v

Dye Crystal Violet Langmuir

Q0 (mg g�1) b R2

Fe3O4/porous graphene 460 0.361 0.9

concentration of 10 mg/L. Therefore, the adsorption process followspseudo second order kinetic model.

3.9.3. Adsorption isothermAdsorption isotherms are mathematical models that define the

distribution of the adsorbate species (dyes, heavy metal ions, etc.)between liquid and adsorbent (Fe3O4/porous graphene nanocom-posite) based on a set of assumptions that are mainly associatedwith the heterogeneity/homogeneity of adsorbents and type ofcoverage and probability of interaction between the adsorbate spe-cies. The adsorption data of crystal violet on the Fe3O4/porous gra-phene nanocomposite were used to control the typical adsorptionconstants by using Langmuir and Freundlich isotherms models.The adsorption isotherms of Langmuir and Freundlich models ofcrystal violet are shown in Fig. 11(c) and (d). The experimental datawere fitted based on Langmuir isotherm and Freundlich models toexplain the adsorption performances of organic dyes on theadsorbents.

The Langmuir and Freundlich equations can be expressed by thelinearized form as:

iolet on Fe3O4/porous graphene nanocomposites.

Freundlich

Kf (mg1�1/n L1/n g�1) n R2

412 56.96 1.7438 0.9112

216 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

Ce

qe¼ 1

Q0bþ Ce

Q0ð1Þ

log qe ¼ logKf þ logCe

nð2Þ

where Ce and qe are the equilibrium concentration (mg/L) andamount of adsorption at equilibrium (mg/g). Qo and b is Langmuirconstants related to the theoretical monolayer adsorption efficiency(mg/g) and energy of adsorption (L/mg), respectively. The linearplots of Ce/qe vs Ce suggest the applicability of the Langmuir iso-therms. Values of Qo and b were determined from the slope andintercepts of the plots. The values of Qo and b indicate the maximumadsorption corresponds to a saturated monolayer of adsorbatemolecules on adsorbent surface. The fitted parameters from theexperimental data with both Langmuir and Freundlich isothermswere presented in Table 1. The maximum adsorption efficiency(Qo) observed based on Langmuir model were 460 mg/g for thecrystal violet of Fe3O4/porous graphene nanocomposite.

The constants and correlation coefficients of crystal violet onFe3O4/porous graphene were compared and reported in Table 1for comparison. It shows that the Fe3O4/porous graphene exhibitsmaximum adsorption capacity due to the high specific surface area(SSA) and surface charges of graphene. The Freundlich isothermundertakes heterogeneous adsorption of dyes due to the varietyof adsorption sites. The Freundlich constants Kf and n were calcu-lated from the linear plot of log qe vs log Ce and were presentedin Table 1. The values of the adsorption isotherm constants n andR2 indicate the experimental values are well fitted with the Fre-undlich model. Therefore, it confirms that the adsorption of crystalviolet from aqueous solution by Fe3O4/porous graphene nanocom-posites shows good heterogeneous adsorption capacity than theother reported nanomaterials [20,21]. Earlier studies on variousadsorbents for removal of crystal violet have been presented inTable 2. Especially, cellulose-based adsorbent (CGS), NaOH-treated almond shell, yeast-treated peat, bottom ash, Artocarpusodoratissimus leaf, and almond shell has very low adsorptioncapacity and very hard to separate after dye adsorption [28–40].The magnetic properties of the Fe3O4/porous graphene adsorbentallow convenient separation from the wastewater by a magneticfield, leading to the development of a safe and clean process forwaste water pollution remediation. Therefore, the Fe3O4/porous

Table 2Comparison of crystal violet adsorption capacity (Qo) of Fe3O4/porous graphene withother reported adsorbents.

S.No

Adsorbents Maximum AdsorptionCapacity (Qo) mg/g

Reference

1 Chitosan–graphite oxidemodified polyurethane

64.935 [28]

2 Cellulose-based adsorbent(CGS)

218.82 [29]

3 NaOH-treated almond shell 123 [30]4 Surfactant modified alumina 254 [31]5 Diatomite earth & carbon 87.05 [32]6 Magnetic Zeolite 117.647 [33]7 Yeast-treated peat 17.95 [34]8 Bottom Ash 28.74 [35]9 Artocarpus odoratissimus leaf 50.5 [36]10 Magnetically modified

activated carbon67.1 [37]

11 CaFe2O4 nanoparticles 10.67 [38]13 Grafted polyacrylamide on

SiO2 nanocomposites378.8 [39]

14 Chitosan–graphite oxidemodified polyurethane

64.935 [40]

12 Fe3O4/porous graphene 460 Presentstudy

graphene nanocomposites excellent adsorption capacity andthereby it can be used in the environmental remediation process.

3.10. CDI unit cell performance

Electrodes used in CDI technique have a high SSA, better electri-cal conductivity and suitable pore arrangement for removal ofheavy metal ions in aqueous solutions. The synthesized Fe3O4/porous graphene based modified electrodes exhibits more highSSA with porous nature and electrical conductivity may utilize todevelop high efficient electrodes for adsorption of Pb2+, Cu2+ andCd2+ ions from aqueous solutions. The electrosorption capacity(mg/g) is defined as the adsorbed heavy metal ion amounts pergram of the electrode and can be observed by the conductivity orconcentration variation of ions in aqueous solutions during thecharging process [41]. The CDI is conducted to evaluate the Pb2+,Cu2+ and Cd2+ ions removal ability of the electrode.

3.10.1. Assemble of CDI unit cellThe electrodes were assembled into a CDI device. Two elec-

trodes were placed in face to face at both sides of plexiglass sheetpacking and kept parallel with a 0.2 cm apart. The total volume ofsolution is 150 ml with a feed solution flow rate 4 ml/min. Thesolution was continuously pumped from a peristaltic pump (RavelRH-P120L India) into the cell. During the measurements, thepotential difference between the two electrodes was kept at a con-stant voltage of V cell = 1.2 V using a (CHI6038D, CH Instruments,USA) at room temperature respectively. The electrosorption ofmetal ions (Pb2+, Cu2+ and Cd2+ ions) and two different concentra-tions, 0.03 mM and 0.3 mM of each ions. The pH of all the feedsolutions was in the range of 6.1–7.1, which is close to groundwa-ter pH. The metal removal efficiency was analysed by atomicabsorption spectrophotometer (240 AA, Agilent Technologies).The electrosorption removal experiments were carried out usingbare Fe3O4, porous graphene and Fe3O4/porous graphenenanocomposites based electrodes.

3.10.2. The electrochemical removal of Pb2+, Cu2+ and Cd2+ ionsFig. 12(a) and (b) shows the effect of flow rate (0.3 and

0.03 mM) on the complete electrochemical removal of Pb2+, Cu2+

and Cd2+ ions using bare Fe3O4, porous graphene, and Fe3O4/porousgraphene based electrodes through CDI technique. Fig. 12(a) showsthe 0.03 mM flow rate on the removal of metal ions at 1.2 V byusing various as-prepared electrodes. The Fe3O4/porous graphenebased electrode exhibits higher removal% of metal ions than por-ous graphene and bare Fe3O4 nanoparticles at applied potentialof 1.2 V with 0.03 mM of metal ions concentration. This is becausein a high electrochemical activity and large SSA can facilitate thediffusion of several metal ions within porous graphene networksand it’s provide more adsorption sites for adsorption of metal ions.Increasing the flow rate to 0.3 mM concentrations of metal ionssignificantly preserve the efficiency and removal rate as shown inFig. 12 (b). Therefore, the removal rate does not much affect byvarying the flow rate and concentrations of metal ions. Recently,Fe3O4/rGO nanocomposite was used as electrodes for CDI tech-nique with attractive structures, such as high performance and fas-ter ion electrosorption at 1.5 V which is three times higher thanrGO [41]. However, Fe3O4/porous graphene based electrode exhi-bits higher metals removal% at 1.2 V than Fe3O4/rGO nanocompos-ite with working potential of 1.5 V. This result described in thisstudy demonstrates that in the CDI process Fe3O4/porous graphenenanocomposite electrodes to enhance ions removal efficiency at1.2 V are 90% and this composite shows high performance. The bestperformance of bare Fe3O4, porous graphene and, Fe3O4/porousgraphene nanocomposite based electrodes to remove metal ionswas polarized at +1.2 V. Also, the high specific surface area (SSA)

Fig. 12. Electrochemical removal of Pb2+, Cu2+ and Cd2+ ions using bare Fe3O4, porous graphene, Fe3O4/porous graphene based electrodes at constant applied potential 1.2 Vand constant flow rate 4 ml/min with different concentrations of 0.03 (a) and 0.3 mM (b).

G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218 217

of graphene-based nanocomposite-modified electrode providesmore sorption sites for metal ions, resulting in enhanced removalcapacity. In summary, newly fabricated nanocomposite electrodematerial exhibits an excellent potential as a promising electrodefor CDI application in waste water treatment. Consequently, Fe3-O4/porous graphene nanocomposite shows excellent adsorptionbehavior of organic dyes from industrial and municipality wastew-ater. Therefore, Fe3O4/porous graphene nanocomposite is animportant nanomaterial toward practical use for water purificationand desalination.

4. Conclusion

In summary, high-quality and scalable porous graphene havebeen successfully prepared in large quantities from potassium-graphite intercalation compound (KC8). The structural, chemicaland morphological analysis demonstrates that the as-preparedporous graphene exhibits low structural defects and wrinkled likemorphology with submicron sized sheets. The Fe3O4 nanoparticleshave been successfully dispersed on the surfaces of porous gra-phene sheets via hydrothermal process at 180 �C for 12 h. The as-prepared nanocomposites were characterized by various analyticaltechniques. X-ray diffraction analysis confirmed the formation ofthe Fe3O4/porous graphene nanocomposites, while HRTEM imagesshowed that the Fe3O4 nanoparticles with an average size of 30 nmwere uniformly dispersed on the surface of porous graphenesheets. The Fe3O4/porous graphene nanocomposites showed typi-cal S-shaped curve and the saturation magnetization (Ms) obtainedto be 38 emu g�1. Further, the as-prepared nanocomposites used asan effective adsorbent to remove the organic dye methyl violetfrom aqueous solutions with good adsorption ability (460 mg g�1)for methyl violet dye and the Langmuir isotherm model coulddescribe the adsorption of crystal violet dye. Furthermore, thecomplete adsorption process followed pseudo second-order kinet-ics. In addition, Fe3O4/porous graphene nanocomposites have beenprepared for high-performance capacitive deionization (CDI)toward the removal of heavy metal ions from aqueous solutions.The Fe3O4/porous graphene nanocomposites, high electrochemicalconductivity, and high specific surface area enable ultra-high elec-trochemical removal of Pb2+, Cu2+ and Cd2+ ions. Therefore, thesuperparamagnetic property of the Fe3O4/porous graphene

adsorbent allows convenient separation of dyes from the wastew-ater by an external magnetic field and in application to CDI forheavy metals removal, prominent to the development of a safeand clean process for water pollution remediation.

Acknowledgments

The authors would like to extend their sincere appreciation tothe Deanship of Scientific Research at King Saud University forfunding this work through the Research group No. RG-1437- 031and the authors would like to acknowledge the University GrantCommission (UGC), New Delhi, and Government of India for thefinancial support through the major research project.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.seppur.2017.07.024.

References

[1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191.[2] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang,

Graphene-based materials: synthesis, characterization, properties, andapplications, Small 7 (2011) 1876–1902.

[3] X. Chen, R. Meng, J. Jiang, Q. Liang, Q. Yang, C. Tan, X. Sun, S. Zhang, T. Ren,Electronic structure and optical properties of graphene/stanene heterobilayer,Phys. Chem. Chem. Phys. 18 (2016) 16302–16309.

[4] G. Bharath, M. Rajesh, C. Shen-Ming, V. Veeramani, D. Mangalaraj, N.Ponpandian, Solvent-free mechanochemical synthesis of graphene oxide andFe3O4–reduced graphene oxide nanocomposites for sensitive detection ofnitrite, J. Mater. Chem. A 3 (2015) 15529–15539.

[5] G. Bharath, V. Veeramani, C. Shen-Ming, M. Rajesh, M. Manivel Raja, A.Balamurugan, D. Mangalaraj, C. Viswanathan, N. Ponpandian, Edge-carboxylated graphene anchoring magnetite-hydroxyapatite nanocompositefor an efficient 4-nitrophenol sensor, RSC Adv. 5 (2015) 13392–13401.

[6] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B.Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4(2010) 4806–4814.

[7] K.H. Park, D. Lee, J. Kim, J. Song, Y.M. Lee, H.T. Kim, J.K. Park, Defect-free, size-tunable graphene for high-performance lithium ion battery, Nano Lett. 14(2014) 4306–4313.

[8] M. Inagaki, Y.A. Kim, M. Endo, Graphene: preparation and structural perfection,J. Mater. Chem. 21 (2011) 3280–3294.

218 G. Bharath et al. / Separation and Purification Technology 188 (2017) 206–218

[9] J. Kwon, S.H. Lee, K.H. Park, D.H. Seo, J. Lee, B.S. Kong, K. Kang, S. Jeon, Simplepreparation of high-quality graphene flakes without oxidation usingpotassium salts, Small 7 (2011) 864–868.

[10] A. Yamashita, Y. Mori, T. Oshima, Y. Baba, Preparation of few-layer graphene bythe hydroxylation of a potassium–graphite intercalation compound, Carbon 76(2014) 469–470.

[11] S. Wang, H. Sun, H.M. Ang, M.O. Tadé, Adsorptive remediation ofenvironmental pollutants using novel graphene-based nanomaterials, Chem.Eng. J. 226 (2013) 336–347.

[12] K.K. Christian, H. Seema, S. Muhammad, H.L. Nhien, M. Kandula, C. Vimlesh, S.K. Kwang, Environmental applications using graphene composites: waterremediation and gas adsorption, Nanoscale 5 (2013) 3149.

[13] R. Sitko, E. Turek, B. Zawisza, E. Malicka, E. Talik, J. Heimann, A. Gagor, B. Feist,R. Wrzalik, Adsorption of divalent metal ions from aqueous solutions usinggraphene oxide, Dalton Trans. 42 (2013) 5682.

[14] K.A. Krishnan, T.S. Anirudhan, Uptake of heavy metals in batch systems bysulfurized steam activated carbon prepared from sugarcane bagasse pith, Ind.Eng. Chem. Res. 41 (2002) 5085–5093.

[15] S. Radi, S. Tighadouini, M.E. Massaoudi, M. Bacquet, S. Degoutin, B. Revel, Y.N.Mabkhot, Thermodynamics and kinetics of heavy metals adsorption on silicaparticles chemically modified by conjugated b-ketoenol furan, J. Chem. Eng.Data 60 (2015) 2915–2925.

[16] D. Wang, W. Yang, S. Feng, H. Liu, Amine post-functionalized POSS-basedporous polymers exhibiting simultaneously enhanced porosity and carbondioxide adsorption properties, RSC Adv. 6 (2016) 13749.

[17] H. Wanga, X. Yuana, Y. Wuc, H. Huanga, G. Zenga, Y. Liua, X. Wanga, N. Lina, Y.Qi, Adsorption characteristics and behaviors of graphene oxide for Zn(II)removal from aqueous solution, Appl. Surf. Sci. 279 (2013) 432–440.

[18] Z. Wu, X. Yuan, H. Zhong, H. Wang, G. Zeng, Enhanced adsorptive removal of p-nitrophenol from water by aluminum metal–organic framework/reducedgraphene oxide composite, Sci. Rep. 6 (2016) 25638.

[19] J. Ding, B. Li, Y. Liu, X. Yan, S. Zeng, X. Zhang, L. Hou, Q. Cai, J. Zhang, Fabricationof Fe3O4@reduced graphene oxide composite via novel colloid electrostaticselfassembly process for removal of contaminants from water, J. Mater. Chem.A 3 (2015) 832–839.

[20] N.A. Zubir, C. Yacou, J. Motuzas, X. Zhang, J.C. Diniz da Costa, Structural andfunctional investigation of graphene oxide–Fe3O4 nanocomposites for theheterogeneous Fenton-like reaction, Sci. Rep. 4 (2014) 4594.

[21] J. Li, S. Zhang, C. Chen, G. Zhao, X. Yang, J. Li, X. Wang, Removal of Cu(II) andfulvic acid by graphene oxide nanosheets decorated with Fe3O4 nanoparticles,ACS Appl. Mater. Interfaces 4 (2012) 4991–5000.

[22] F. Perreault, A. Fonseca de Faria, M. Elimelech, Environmental applications ofgraphene-based nanomaterials, Chem. Soc. Rev. 44 (2015) 5861–5896.

[23] W. Li, S. Gao, L. Wu, S. Qiu, Y. Guo, X. Geng, M. Chen, S. Liao, C. Zhu, Y. Gong, L.Mingsheng, X. Jianbao, W. Xiangfei, S. Mengtao, L. Liwei, High-density three-dimension graphene macroscopic objects for high-capacity removal of heavymetal ions, Sci. Rep. 3 (2013) 2125.

[24] W. Shi, H. Li, X. Cao, Z.Y. Leong, J. Zhang, T. Chen, H. Zhang, H. Ying, YangUltrahigh performance of novel capacitive deionization electrodes based on athree-dimensional graphene architecture with nanopores, Sci. Rep. 6 (2016)18966.

[25] L. Jiang, Z. Fan, Design of advanced porous graphene materials: from graphenenanomesh to 3D architectures, Nanoscale 6 (2014) 1922–1945.

[26] X. Zhao, C.M. Hayner, M.C. Kung, H.H. Kung, Flexible holey graphene paperelectrodes with enhanced rate capability for energy storage applications, ACSNano 5 (2011) 8739–8749.

[27] X. Wang, L. Jiao, K. Sheng, C. Li, L. Dai, G. Shi, Solution-processable graphenenanomeshes with controlled pore structures, Sci. Rep. 3 (2013) 1996.

[28] Q. Jiao, Q. Fengxian, R. Xinshan, Y. Jie, Z. Hao, Y. Dongya, Adsorption behavior ofcrystal violet from aqueous solutions with chitosan–graphite oxide modifiedpolyurethane as an adsorbent, J. Appl. Polym. Sci. 41828 (2015) 1–10.

[29] Z. Yanmei, Z. Min, W. Xinhai, H. Qi, M. Yinghao, M. Tongsen, N. Jingyang,Removal of crystal violet by a novel cellulose-based adsorbent: comparisonwith native cellulose, Ind. Eng. Chem. Res. 53 (2014) 5498–5506.

[30] M. Ishaq, F. Javed, I. Amad, H. Ullah, Adsorption of crystal violet dye fromaqueous solutions onto low-cost untreated and NaOH treated almond shell,Iran. J. Chem. Chem. Eng. 35 (2016) 97–106.

[31] Z. Javad, B. Maryam, S. Tahere, Simultaneous removal of binary mixture ofBrilliant Green and Crystal Violet using derivative spectrophotometricdetermination, multivariate optimization and adsorption characterization ofdyes on surfactant modified nano-c-alumina, Spectrochim. Acta Mol. Biomol.Spectrosc. 137 (2015) 1016–1028.

[32] Z. Yanzhuo, L. Jun, C. Xiaojie, B. Wei, C. Guanghui, L. Yun, L. Wenjing, Z.Zhaoming, Efficient removal of crystal violet by diatomite and carbon in thefixed bed column: influence of different glucose/diatomite, RSC Adv. 6 (2016)51337–51346.

[33] S.A. Olusola, V.O. Tunde, K.N. Seteno, S.A. Olushola, Rapid adsorption of crystalviolet onto magnetic zeolite synthesized from fly ash and magnetitenanoparticles, J. Encapsul. Adsorpt. Sci. 5 (2015) 191–203.

[34] Z. Tasneem, P. Namal, B. Linda, L. Lim, Removal of crystal violet dye fromaqueous solution using yeast-treated peat as adsorbent: thermodynamics,kinetics, and equilibrium studies, Environ. Earth Sci. 75 (2016) 357.

[35] P.V. Nidheesh, R. Gandhimathi, S.T. Ramesh, T.S. Singh Anantha, Investigationof equilibrium and thermodynamic parameters of crystal violet adsorptiononto bottom Ash, J. Int. Environ. Appl. Sci. 6 (2011) 461–470.

[36] B.L.L. Linda, P. Namal, H.C. Hui, A.H.M.Z. Nur, Adsorption characteristics ofArtocarpus odoratissimus leaf toward removal of toxic crystal violet dye:isotherm, thermodynamics and regeneration studies, J. Environ. Biotechnol.Res. 4 (2016) 32–40.

[37] H. Soheila, T. Marzieh, J.S. Seyed, Removal of crystal violet from water bymagnetically modified activated carbon and nanomagnetic iron oxide, J.Environ. Health Sci. Eng. 13 (2015) 8.

[38] A. Shuai, L. Xueyan, Y. Lijun, Z. Lei, Enhancement removal of crystal violet dyeusing magnetic calcium ferrite nanoparticle: study in single- and binary-solutesystems, Chem. Eng. Res. Des. 94 (2015) 726–735.

[39] S. Ghorai, A. Sarkar, M. Raoufi, A.B. Panda, H. Schönherr, S. Pal, Enhancedremoval of methylene blue and methyl violet dyes from aqueous solutionusing a nanocomposite of hydrolyzed polyacrylamide grafted xanthan gumand incorporated nanosilica, ACS Appl. Mater. Interfaces 6 (2014) 4766–4777.

[40] J. Qin, F. Qiu, X. Rong, J. Yan, H. Zhao, D. Yang, Adsorption behavior of crystalviolet from aqueous solutions with chitosan–graphite oxide modifiedpolyurethane as an adsorbent, J. Appl. Polym. Sci. 132 (2015) 41828.

[41] N. Tuan Trinh, S. Chung, J. Kwang Lee, J. Lee, Development of high qualityFe3O4/rGO composited electrode for low energy water treatment, J. EnergyChem. 25 (2016) 354–360.