Fe3O4

Synthesis of acetone reduced graphene oxide/Fe3O4 composite through simple and efficient chemical reduction of exfoliated graphene oxide for removal of dye from aqueous solution Kaushal R. Parmar, Isha Patel, Shaik Basha & Z. V. P. Murthy

Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 Volume 49 Number 19 J Mater Sci (2014) 49:6772-6783 DOI 10.1007/s10853-014-8378-x

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Author's personal copy J Mater Sci (2014) 49:6772–6783 DOI 10.1007/s10853-014-8378-x

Synthesis of acetone reduced graphene oxide/Fe3O4 composite through simple and efficient chemical reduction of exfoliated graphene oxide for removal of dye from aqueous solution Kaushal R. Parmar • Isha Patel • Shaik Basha Z. V. P. Murthy

•

Received: 13 March 2014 / Accepted: 2 June 2014 / Published online: 24 June 2014 Ó Springer Science+Business Media New York 2014

Abstract A simple and effective technique for reduction of graphene oxide at low temperature (70 °C) using acetone was reported for the first time. Magnetically recoverable acetone reduced graphene oxide (ARGO)/Fe3O4 composite was synthesized by uniformly decorating Fe3O4 on ARGO. The synthesized ARGO/Fe3O4 composite was characterized by the powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform-infrared spectroscopy and thermogravimetric analysis. An organic dye rhodamine 6G was used as an adsorbate for investigating the adsorption characteristics of the composite. The adsorption kinetic data were best described by the pseudo-second-order model, and equilibrium was achieved within 2 h. Dye adsorption was favored in basic conditions (pH 9–11) and governed by intraparticle diffusion process. The maximum dye adsorption on the composite was 93.37 mg/g at 293 K, and it followed the Langmuir–Freundlich model. The calculated thermodynamic

Electronic supplementary material The online version of this article (doi:10.1007/s10853-014-8378-x) contains supplementary material, which is available to authorized users. K. R. Parmar I. Patel S. Basha (&) Marine Biotechnology and Ecology Discipline, CSIR - Central Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, Gujarat, India e-mail: [email protected] K. R. Parmar Z. V. P. Murthy (&) Department of Chemical Engineering, S.V. National Institute of Technology Surat, Surat 395007, Gujarat, India e-mail: [email protected] Present Address: K. R. Parmar Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016, India

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parameters (DG°, DH° and DS°) showed that the dye adsorption onto composite was feasible, spontaneous and exothermic. The ARGO/Fe3O4 composite was easily controlled in magnetic field for desired separation, leading to an easy removal of the dye from wastewater, which holds great potential for dye decontamination.

Introduction Environmental contamination and human exposure to dyes have dramatically increased over the past decades because of their increasing use in industries such as textiles, paper, plastics, tannery and paints. It is projected that the amount of dyes (more than 100,000 commercial compounds) produced exceeds 700,000 tons annually worldwide, with 10–15 % being discharged in water bodies [1]. These dyes can cause deterioration in water quality by imparting color to the water and inducing the photosynthetic activity of aquatic organisms by hindering the light penetration. Moreover, some of the dyes are considered carcinogenic and mutagenic for human health. Therefore, efficient treatment and removal of dyes from wastewater have attracted considerable attention in recent years [2]. It is well known that adsorption is a promising process among the various remediation technologies due to its simplicity, high efficiency and ease of operation as well as the availability of a wide range of adsorbents. Development of lowcost adsorbents with high adsorption capacities and fast kinetics for dye removal is still a monumental task. Graphene, a novel one-atom-thick and two-dimensional graphitic carbon nanomaterial with a honeycomb-like assembly, has attracted tremendous attention in recent years from scientific community. This importance is greatly attributed to its unique physicochemical and excellent

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mechanical properties [3]. Lately, graphene oxide (GO), as a precursor for graphene preparation, proved to be a promising material to adsorb various pollutants including dyes, and catalyst supports due to its extraordinary mechanical strength, high surface area and its perfect match with sp2 hybridized structure, compared to carbon nanotubes and their carbon additives [4–6]. GO is a intensely oxygenated, highly hydrophilic-layered carbon material which can be readily exfoliated into single-layer sheets on many substrates, making it a promising material in functional nanomaterials. However, high-speed centrifugation is required to separate the graphene and GO after adsorption from the aqueous solution. In addition, the p–p interactions between adjacent sheets might lead to serious agglomeration and restacking, which reduces the effective surface area for adsorption [7]. Fe3O4 nanoparticles have long been used in biosensors and magnetic separation owing to their strong magnetic properties, low toxicity, good biocompatibility and easy preparation process in physiological environments [8–10]. Incorporation of Fe3O4 nanoparticles on graphene or GO will impart magnetic properties, making the graphenebased magnetic composites promising for a variety of applications including environmental remediation [5, 11, 12]. The combination of graphene or reduced graphene oxide (RGO) with Fe3O4 nanoparticles produces a magnetic graphene/Fe3O4 composite with certain advantages over conventionally used adsorbents, in terms of convenient magnetic separation, fast adsorption rate and elevated adsorption capacity. The preparation of graphene-based magnetic composites and their application in removal of organic pollutants have been reported recently [5, 12–14]. In this work, a simple and effective method for reduction of GO at low temperature using acetone was reported and magnetically recoverable acetone reduced graphene oxide (ARGO)/ Fe3O4 composite was synthesized by co-precipitation. The adsorption capacity of composite was compared with those of other adsorbents. The effects of the pH, dosage, contact time, initial dye concentration and temperature on adsorption have also been studied to determine the optimal conditions for removal of rhodamine 6G. Adsorption mechanism was studied through kinetic, isotherm and thermodynamic analysis.

Experimental section Materials Natural graphite powder (200 mesh) was purchased from Qualikems, India, and was used without further purification. Analytical grade reagents such as anhydrous iron(III) chloride (FeCl3, 98 %), iron(II) sulfate and rhodamine 6G were purchased from Sigma-Aldrich, India, while H2SO4 (98 %), H2O2 (30 %), KMnO4, NaNO3 and NaOH were obtained from

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Merck, India. Ultrapure Milli-Q water (resistivity of 18.5 MX/ cm) was used throughout all the experiments. Analytical grade rhodamine 6G was used to prepare stock solutions of 500 mg/ L, which were further diluted to the required concentrations. Synthesis of GO and ARGO/Fe3O4 composite GO was synthesized from natural flake graphite (99.95 % purity) using a modified Hummers method [15]. Briefly, 3.0 g of graphite, 1.5 g of NaNO3 and 69 mL of concentrated H2SO4 were mixed and stirred in a three-neck flask in an ice bath, and then 9.0 g of KMnO4 was slowly added to keep the reaction temperature \20 °C. After addition of KMnO4, the reaction temperature was taken to 35 ± 1 °C, stirred for 30 min, and then 138 mL Milli-Q water was added. The solution was stirred for 30 min at 98 ± 1 °C. Later, the reaction was cooled using water bath for 10 min. Additional 420 mL Milli-Q water and 30 mL of H2O2 (30 %) were added slowly, turning the color of the solution from dark brown to yellow. After cooling, the mixture was washed by Milli-Q water several times until pH 7 was obtained. These multiple washes and centrifugation give GO suspended in water. Finally, it was freeze-dried to obtain a dark brown GO powder. A simple and effective new technique was used for the reduction of GO. In this method, GO was dispersed in acetone and sonicated 1 h for complete exfoliation and then it was kept in oven at 70 °C. After 4–6 h, the acetone was evaporated and highly exfoliated fluffy material was obtained. This was designated as acetone reduced GO (ARGO). To make a composite of Fe3O4 with ARGO, co-precipitation was used. Here, solution A was prepared by adding measured amount of FeCl3 and FeSO47H2O such that Fe3?: Fe2? = 1.5:1, while solution B was obtained by adding ARGO (1 g/L) in aqueous NaOH solution. Now, solution B was sonicated for 1 h for further exfoliation and uniform dispersion of material, and solution A was slowly added (1 drop/s) to it with continuous sonication. As solution A comes in contact with solution B, immediately Fe3O4 as a black precipitate was formed. Due to sonication, they do not get chance for agglomeration and get directly attached with ARGO sheets and the sonication was continued for 1 h at 65 °C. Then the suspension was washed with Milli-Q water several times until pH becomes neutral, and the supernatant was dried in oven at 70 °C. By using above procedure, five different composites by varying Fe3O4 percentage (10, 25, 50, 75 and 90) were prepared. Thus, 10, 25, 50, 75 and 90 % Fe3O4 (w/w)/ ARGO composite materials were obtained. Adsorption experiments All adsorption experiments were performed on a model INFORS HT thermostat shaker (Ecotron, Switzerland)

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using a batch equilibrium technique in a serial of 20-mL glass vials. The effect of solution pH on the equilibrium uptake of rhodamine 6G dye from aqueous solution by composite was investigated between pH 3 and 11 by adjusting with HNO3 and NaOH solutions. The experiments were performed by adding a known weight of composite into 20-mL glass vials containing 45 mg/L of dye. The glass vials were shaken at 140 rpm and 298 K for 3 h to attain equilibrium, and the amount of dye remaining in solution was measured spectrophotometrically (UV–vis spectrophotometer, Shimadzu, UV-1800) at the wavelength of 526.5 nm (kmax) after the separation of composite from the solution by magnetic separation using permanent magnet. The kinetics of sorption studies were conducted with 100 mL of dye solution with initial concentrations of 25, 45 and 65 mg/L and known amount of composite. The samples were withdrawn at regular intervals, and the residual concentration of dye in the aqueous phase was analyzed after magnetic separation. Effect of initial concentration experiments at different temperatures (20–40 °C) was carried out in 20-mL glass vials containing dye solution (10 mL) of known concentrations (10–50 mg/L). Weighed amounts of composite (5 mg) were added to each vial, and the mixtures were agitated on a shaker at 140 rpm. After 3 h of agitation, the solution was separated from the composite by magnetic separation and analyzed for dye. Various doses of the composite (5–25 mg) were added to 10 mL of dye solution (45 mg/L) to investigate the effect of the adsorbent dosage on the adsorption. After shaking for 3 h, the composite was removed from the solution by magnetic separation and the resultant solution was analyzed. The amount of dye adsorbed at equilibrium, qe (mg/g) which represented the dye uptake, was calculated from the difference in dye concentration in the aqueous phase before and after adsorption, as per following equation: qe ¼

VðCi Ce Þ W

ð1Þ

where V is the volume of dye solution (L), Ci and Ce are the initial and equilibrium concentration of dye in solution (mg/L), respectively, and W is the mass of dry composite (g). All the adsorption experiments were repeated twice to substantiate the results with controls. Controls were employed to ensure that sorption was by composite only, and to rule out any effect of adsorption of dye onto the wall of the reaction bottles. Both isotherm and kinetic model parameters were evaluated by nonlinear regression using DATAFITÒ software (Oakdale Engineering, USA). Apart from the regression coefficient (R2), the residual or sum of square error (SSE) and the standard error (SE) of the estimate

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were also used to gauge the goodness of fit. The smaller SE and SSE values indicate the better curve fitting. Characterization The morphologies of prepared GO and ARGO/Fe3O4 composites were characterized by scanning electron microscopy (SEM) images (LEO series VP1430) equipped with energy dispersive X-ray (EDX) facility (Oxford Instruments) and transmission electron microscopy (TEM) images (JEOL JEM-2100) with acceleration voltage of 200 keV. Powered X-ray diffraction (XRD) patterns were measured on a Philips X’Pert MPD system using Cu-Ka ˚ ). Fourier transform-infrared (FTradiation (k = 1.5406 A IR) absorption spectra were recorded using a PerkinElmer FT-IR spectrometer (Model-FT-1730) equipped with a KBr beam splitter (KBr, FTIR grade) at room temperature. Thermogravimetric analysis (TGA) was conducted in Mettler TGA/SDTA 851e, and the data were processed using Star e software, in flowing air at a flow rate of 60 ml/ min and heating rate of 5 °C/min.

Results and discussion XRD XRD characterization was conducted to obtain the structural information about GO and ARGO/Fe3O4 composites. The diffraction lines a, b, c, d, e and f correspond to 10 % Fe3O4, 25 % Fe3O4, 50 % Fe3O4, 75 % Fe3O4, 90 % Fe3O4 and pure Fe3O4, respectively (Fig. 1b). The intense diffraction peaks at 2h equal to 30.48°, 35.69°, 43.37°, 53.83°, 57.27° and 63.07° represent the corresponding indices (220), (311), (400), (422), (511) and (440), respectively. The peak positions and relative intensities of the nanoparticles match well with those from the JCPDS data card (19-0629) for Fe3O4, which indicated the well-known inverse spinel crystal structure of Fe3O4 [13]. As shown in Fig. 1a, the diffraction peak at 2h = 11.2° with d-spacing ˚ can be confidently indexed as the (002) reflection of 7.46 A the GO [16]. After reduction, d-spacing of GO was reduced ˚ , which confirmed that oxygen groups from 7.46 to 3.74 A were removed [17]. The characteristic peak of GO located at 11.2° disappeared, confirming the formation of ARGO or graphene [5]. In addition, no obvious diffraction peak ascribed to graphite was evident, which suggested that the graphene sheets remained disordered and assigned to the reflection of crystal planes of cubic Fe3O4 with a facecentered cubic structure. The strongest reflection at the (311) plane is the characteristic phase of Fe3O4. The observations of XRD also confirmed the successful synthesis of ARGO/Fe3O4 composites.


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Fig. 1 XRD of a GO and ARGO, and b ARGO/Fe3O4 composites by varying % of Fe3O4; a, b, c, d, e and f correspond to 10 % Fe3O4, 25 % Fe3O4, 50 % Fe3O4, 75 % Fe3O4, 90 % Fe3O4 and pure Fe3O4, respectively

FT-IR Fourier transform-infrared spectroscopy (FT-IR) spectra were measured to characterize the GO, ARGO and ARGO/ Fe3O4 composites. Figure 2 A displays the FT-IR curve for GO and ARGO. For GO, the spectrum displayed the characteristic peaks of O–H (mO–H at 3410 cm-1), C–O (mC–O at 1734, 1411, 1224, 1097 and 1023 cm-1) and C–C (mC–C at 1612 cm-1). The FTIR spectrum of ARGO showed significant changes from that of GO. There was a strong decrease in the intensities of the characteristic absorption bands of oxygen functional groups (carboxy, epoxy, alkoxy and hydroxyl) and was removed effectively by acetone treatment at low temperature. This further indicates that GO had been reduced to graphene sheets, which is consistent with the XRD results. The spectra a, b, c, d, e and f in Fig. 2b correspond to RGO, 10 % Fe3O4, 25 % Fe3O4, 75 % Fe3O4, pure Fe3O4 and rhodamine 6G loaded 25 % Fe3O4 composites and show the variation of functional groups with change in weight percent of Fe3O4 in composites. In case of all composites, the characteristic peak at 562 cm-1 is related to the vibration of Fe–O functional groups, confirming the presence of Fe3O4 [18]. Figure 2d shows the FTIR for 25 % Fe3O4-ARGO before and after adsorption. Extra intense peak at 1384 cm-1 indicated the presence of rhodamine 6G on the surface of ARGO/Fe3O4 composite. SEM Figure 3a–c displays the morphological view of RGO, high resolution image of ARGO and 25 % Fe3O4–ARGO composites. Highly wrinkled morphology was observed for

RGO, while the morphology of 25 % Fe3O4–ARGO composite shows Fe3O4 nanoparticles at the edge of the sheet. Some nanosized particles are visible on the basal plane of ARGO sheet. The appearance of spherical beadlike Fe3O4 particles on the smooth layer of graphene could be seen from Fig. 3c, which is attributed to increase in the surface area. TEM The morphology of ARGO and ARGO/Fe3O4 composite was further investigated by transmission electron microscopy (TEM). Figure 4a shows TEM image of ARGO, whereas Fig. 4b–d displays TEM image of ARGO/Fe3O4 composites. As evident (Fig. 4b), the sheetlike, corrugated morphology of RGO was well preserved despite the high and homogeneous coverage of Fe3O4 nanoparticles. The vast majority of the Fe3O4 nanoparticles were found well dispersed on the RGO skeleton, and it should be emphasized that, without the presence of the RGO, Fe3O4 nanoparticles tend to aggregate into large clusters due to the coalescence. Moreover, even after ultrasonication of ARGO/Fe3O4 composite for a long time for TEM characterization, the Fe3O4 nanoparticles were firmly attached to the RGO sheets, suggesting a strong interaction between the two substances. The lattice spacing of Fe3O4 was 0.257 nm (Fig. 4d), which agreed well with the basal spacing of (311) lattice planes. The selected area electron diffraction (SAED) pattern in Fig. 4e also confirmed the highly crystalline nature of the ARGO/Fe3O4 composite, in which the bright dots corroborated the crystalline and inverse cubic spinel structure of Fe3O4, while the rings arise from the planes of ARGO [19].

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Fig. 2 FT-IR spectra of a GO and ARGO, b all composite as well as pure components, c ARGO, Fe3O4 pure and 25 % Fe3O4–ARGO composite and d 25 % Fe3O4–ARGO composite before (b) and after (a) adsorption

TGA The thermal behaviors of GO and RGO/Fe3O4 composite were investigated by thermogravimetric analysis (TGA) in a nitrogen atmosphere (Fig. S1). Both GO and 25 % Fe3O4ARGO samples showed a slight mass loss on heating even below 100 °C due to the loss of residual water adsorbed physically in the sample. With increase in temperature from 200 to 250 °C, a weight loss was observed in the TGA curve for GO, which can be attributed to the decomposition of oxygen functional groups [20]. The other step of mass loss from 400 to 600 °C was due to the burning of carbon in GO [21]. The TGA curve for the 25 % Fe3O4–ARGO composite (Fig. S1) exhibited a gradual mass loss spanning the range 500–600 °C, which could be ascribed to the bulk pyrolysis of the carbon skeleton [21]. Overall, the composite displayed

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good thermal stability, and no major weight loss was found. The above results confirm that the ARGO/Fe3O4 composites have been successfully fabricated. Possible mechanisms for the reduction of GO by acetone Since the analysis of XRD and FT-IR indicated reduction of the exfoliated GO results in considerable removal of oxygen, it is axiomatic that acetone played a key role by supplying the electrons necessary for the reduction of GO to reduced graphene oxide (RGO) through an unknown mechanism. However, establishing how the electrons were responsible for the reduction in the studied system is a great challenge since acetone is not known as an effective reducing agent. It may be possible that the OH- ion shred a


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Fig. 3 SEM images of a ARGO, b high-resolution image of ARGO and c 25 % Fe3O4/ARGO composite (inset of C is magnified image of top left portion)

proton from one of the methyl groups of acetone forming a very reactive species, carbanion/enolate anion in alkaline conditions [22]. These anions react quickly either with an acetone molecule (aldol condensation) or another carbanion/enolate anion releasing the electrons required for the reduction of GO. The detailed study to ascertain the reduction mechanism is in progress. The described method is an attractive route for the mass-scale production of RGO and its application in electronics, optoelectronics, composite materials and energy-storage devices [23]. Moreover, this technique eliminates the use of hazardous, explosive and costly materials in the reduction process. Adsorption isotherms It is paramount to establish the most suitable correlation for the adsorption isotherm data at equilibrium to optimize the design of an adsorption system. In this context, various isotherm equations were given by the Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Redlich–Peterson (R–P) to fit the experimental data of rhodamine 6G sorption on to ARGO/Fe3O4 composite at various temperatures.

The parameters of isotherm models along with their respective equations as well as the regression coefficients (R2), SE and SSE values are summarized in supporting information (Table S1). The regression coefficients demonstrate that the adsorption of dye had a better fit with the Langmuir isotherms (R2 [ 0.972) and the values of maximum adsorption capacity (qm), analogous to complete monolayer coverage, decreased from 108.97 to 94.27 mg/g with increase in temperature from 293 to 313 K, indicating that the sorption is exothermic process. The separation factor, RL (0 \ RL \ 1), indicated that the ARGO/Fe3O4 composite was a suitable adsorbent for removal of rhodamine 6G from aqueous solutions. The Freundlich isotherm is empirical and has been used widely to fit experimental data of liquid-phase sorption to heterogeneous surface. Statistically, the Freundlich model commensurates with the experimental data with high R2 (0.962–0.955), low SE (0.201–0.281) and SSE (0.203–0.394) values. The isotherm parameter (nF), which measures the adsorption intensity of dye on composite, showed values greater than unity (1.30–1.95) demonstrating that the isotherms can be characterized by a convex

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Fig. 4 TEM images of a ARGO, b SAED image of ARGO/Fe3O4 composite, c, d high-resolution images of ARGO/Fe3O4 composite, left and right portions of b, and e ARGO/Fe3O4 composite in which ARGO, Fe3O4 and grid are indicated

Freundlich isotherm. This implied that a significant adsorption may occur even at a high dye concentration. The Freundlich isotherm model was also used to describe adsorption of rhodamine 6G on carbon nanotubes [24]. As the Freundlich isotherm can be applied to multilayer as well

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as non-ideal sorption on heterogeneous surfaces, our isotherm data indicate that the multilayer adsorption would be involved in the process of dye removal by the ARGO/Fe3O4 composite. The D–R isotherm model is more general and used to estimate the characteristic porosity as well as

Author's personal copy J Mater Sci (2014) 49:6772–6783 Table 1 Adsorption characteristics of various graphene-based materials for dye removal

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Adsorbent

Dye

Sorption capacity/ efficiency (mg/g)/(%)

References

Graphene–Fe3O4 nanocomposite

Pararosaniline

198.2/–

[31]

Magnetic graphene oxide

Reactive black 5

164–188/–

[32]

Magnetic Fe3O4 @ graphene composite

methylene blue

45.27/–

[33]

Congo red

33.66/–

Magnetic graphene oxide

Methylene blue

64.23/–

[34]

Graphene oxide-based hydrogel

Ponceau S

–/98

[35] [36]

Magnetic-sulfonic graphene nanocomposite

GO–Fe3O4 hybrid composite

Safranine T

199.3/–

Neutral red

216.8/–

Victoria blue

200.6/–

Methylene blue

167.3/–

Neutral red

171.3/–

[37]

Reduced graphene oxide–Fe3O4 nanoparticles

Rhodamine B

40.01/–

[38]

Magnetic graphene–Fe3O4 @ carbon hybrid

Methylene blue

73.26/–

[39]

Ferromagnetic hematite @ graphene nanocomposites

Rhodamine B

–/90.8

[40]

Magnetite/reduced graphene oxide nanocomposites

Rhodamine B

–/91

[41]

Malachite green

–/94

Modified nanographite/Fe3O4 composite

Methylviolet

144.7–151.5/–

[42]

Acetone reduced graphene oxide (ARGO)/Fe3O4 composite

Rhodamine 6G

93.37/–

Present study

apparent free energy of adsorption. Evidently, the D–R model was more appropriate for dye adsorption isotherms with moderate regression coefficients ([0.881) and low SSE and SE values. The values of porosity factors (B) ranged from 0.0012 to 0.0023 mol2/kJ2 at various temperatures. These values are less than unity and so imply that the composite has fine micropores and indicated that a surface heterogeneity may result from the pore structure as well as adsorbate–adsorbent interactions [25]. The apparent free energy, E, values (14.74–20.41 kJ/mol) were comparable to that of chemisorption that amounted to 40 kJ/mol [26]. The three-parameter isotherm model, Langmuir–Freundlich, was applied to represent dye adsorption data under surface heterogeneity conditions that revealed from both the Freundlich and D–R isotherm models. Based on the regression coefficients (R2), SE and SSE values, the Langmuir–Freundlich adsorption isotherm model provides a better fit of our experimental data (Fig. S2). The ultimate sorption capacity, KLF, of composite (97.69–88.04 mg/g) calculated from the modeled isotherm data at various temperatures closely related to experimental values (93.38–88.04 mg/g). The exponent c value was in the range of 0.42–0.53, denoting that the rhodamine 6G sorption is more of a Freundlich form rather than that of Langmuir. The analysis of various isotherm models indicates that different models are appropriate in their merits in describing the potential of the ARGO/Fe3O4 for the removal of dyes from wastewater.

The qm values of rhodamine 6G adsorption on ARGO/ Fe3O4 composite, ARGO and Fe3O4 were 99.62, 66.37 and 48.29 mg/g, respectively, at 303 K. Compared to qm values of rhodamine 6G adsorption on other adsorbents—activated carbon (44.7 mg/g at pH 7 and T = 333 K) [27], acrylic acid-based superabsorbent polymers (60–80 mg/g) [28], Na?-montmorillonite clay (57.8 mg/g at 298 K) [29] and almond shell (32.6 mg/g at 298 K) [30]—the ARGO/ Fe3O4 composite showed highest sorption capacity. The reported work on the removal of dyes by various graphenebased materials is presented in Table 1. Effect of pH on dye adsorption The effect of initial pH on rhodamine 6G adsorption by ARGO/Fe3O4 composite was studied with the initial pH ranging from 3.0 to 11.0 (Fig. S3). It is evident that in the acidic pH range from 3.0 to 7.0, the adsorption capacity of composite increased sharply from 38.99 to 56.48 mg/g while for pH [7.0, though the adsorption capacity did increase; however, it was marginal (for pH 9 and 11, the adsorption capacity was 59.62 and 61.19 mg/g, respectively). Therefore, alkaline condition was favorable for adsorption of rhodamine 6G. This experimental behaviour was not unexpected, as surface charge density decrease with an increase in pH, the electrostatic repulsion between the positively charged rhodamine 6G and the surface of the

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qt (mg/g)

60 50 40 30 20

50 mg/L 75 mg/L 100 mg/L

10 0 0

30

60

90

120

Time (min) Fig. 5 Adsorption kinetics of rhodamine 6G onto ARGO/Fe3O4 composite at three different initial concentrations, pH 5.0, solid to liquid (s/l) ratio = 1 g/L. Symbols represent experimental data, and solid lines represent the fitting of the pseudo-second-order rate model

composite was lowered; this may result in an increase in the adsorption capacity. As the pH decreases, the extent of negative charges on the surface decreases, and therefore, the adsorption decreases. Similar results were obtained for the adsorption of rhodamine laser dyes on TiO2 hydroxylated surfaces [43]. Adsorption kinetics Adsorption of rhodamine 6G dye on the surface of ARGO/ Fe3O4 composite consisted of both fast and slow reaction process (Fig. 5). The fast process was completed in approximately 40 min, in which adsorption increased rapidly with contact time, while the slow process could extend over 2 h, in which the increase in dye adsorption reduced. Initially, the rate of adsorption was rapid due to the sorption of dye onto the exterior surface and may also be attributed to the presence of large number of binding sites for adsorption. Subsequently, the dye entered into pores, a relatively slow process, and the slower adsorption rates were due to the saturation of the binding sites and attainment of equilibrium [44]. An increase of the rhodamine 6G concentration accelerated the diffusion of dye from the bulk solution onto sorbent due to the increase in the driving force of the concentration gradient [45]. Therefore, the amount adsorbed at equilibrium increased from 53.43 to 76.33 mg/g with an increase in initial dye concentration from 50 to 100 mg/L. The initial concentration did not have a considerable effect on the time to reach equilibrium. Kinetic models such as pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion models were used to fit the experimental data to predict the mechanism involved during the adsorption process and potential ratecontrolling step. The best-fit model was selected based on

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nonlinear regression correlation coefficient (R2), SE and SSE values as well as the calculated qe values. The results of befitting experimental data with kinetic models for the removal of rhodamine 6G by ARGO/Fe3O4 composite were summarized (Table S2). For the pseudofirst-order model, the regression coefficients (R2) were relatively low and varied from 0.716 to 0.742. Moreover, the difference between experimental and theoretical adsorption capacities at equilibrium was high, and the experimental values were 10–25 % higher than the theoretical ones. As a corollary, adsorption was not an ideal pseudo-first-order reaction. Similar results were obtained for rhodamine 6G sorption onto Trichoderma harzianum mycelial waste [46]. In many cases, the pseudo-first-order model could not fit well to the whole range of contact times and is commonly applicable over the initial stage of the adsorption [47]. Kinetic parameters of the pseudo-second-order equation were characterized by higher regression coefficients (0.991–0.997) and lower values of SE (0.027–0.069) and SSE (0.025–0.038) which supports improved adjustment of that model to experimental data of ARGO/Fe3O4 composite. Likewise, the predicted equilibrium sorption capacities at various initial dye concentrations from the pseudo-second-order model were more consistent with the experimental values. These results suggested that the adsorption of rhodamine 6G onto ARGO/Fe3O4 composite follows the pseudo-second-order model, and hence, it can be presumed that more than one step including chemisorption may be involved in the sorption process [48]. Increase of sorption capacity (from 56.72 to 78.12 mg/g) and initial adsorption rate, h (from 7.299 to 18.518 mg/g/ min), of ARGO/Fe3O4 composite with initial concentration of the dye was observed. This phenomena can be attributed to the increase in driving force at higher concentration [49]. Further, the kinetic data were fitted to the Elovich rare equation, which described the chemical adsorption (chemical reaction) mechanism assuming that the actual solid surfaces were energetically heterogeneous [50]. The R2 (0.963–0.985) values revealed a moderate fit to the experimental kinetic data; however, both SE and SSE values were \1. With the increase in initial dye concentration, the Elovich constants aE and bE increased showing that both the rate of chemisorption and the available sorption surface would increase. The moderate applicability of the Elovich equation for the present kinetic data is generally in agreement with other researchers’ results that the Elovich model was able to describe the adsorption kinetics of acid blue 9 and food yellow 3 dyes adsorption on chitosan [51]. To summarize, among the kinetic models studied, the pseudo-second-order model was more appropriate for kinetic modeling of time-course rhodamine 6G adsorption data.


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The Weber and Morris intraparticle diffusion model has been used to predict the influence of the mass transfer resistance on the sorption of dye onto the ARGO/Fe3O4 composite (Table S2). Piecewise nonlinear regression of data showed that at all the tested concentrations of dye, q versus t0.5 plots (Figure not shown) had two distinct regions. The first curved portion comprised the sorption period of 0–40 min, representing external diffusion and binding of dye by active sites which were distributed onto the outer surface of the ARGO/Fe3O4 composite. The second linear portion included the sorption period of 40–120 min, representing intraparticle diffusion and binding of dye by active sites distributed to macropore, mesopore and micropores of composite and finally establishment of the equilibrium. A high regression coefficients (R2) (0.901–0.911), low SE (0.363–0.703) and SSE (0.756– 1.237) values suggested a significant affinity between q and t0.5 for dye at all the three tested concentrations of dye. The increase in intraparticle rate constant (Kid) values (from 1.684 to 2.271 mg/g/min0.5) with increasing initial dye concentration (from 50 to 100 mg/L) can be explained by the growing effect of driving force resulted in reducing the diffusion of dye in the boundary layer and enhancing the diffusion in the solid (Table S2). The value of intercept, C, increased with concentration of dye in the solution. This occurred because a high dye concentration appears to have provided better driving force to external mass transfer process. On the other hand, C was = 0 at all the test conditions, thereby suggesting that intraparticle diffusion was not the rate-limiting step and external mass transfer has also played an crucial role in dye sorption by the ARGO/Fe3O4 composite [52]. The contribution of external mass transfer and pore diffusion in the sorption was evaluated by Eqs. (4) and (5), respectively [53, 54]: Ct ¼ ekf ðS=VÞt Co 2 ! 2 qt 4p D log 1 t ¼ 2:3u2 qe

ð4Þ ð5Þ

where Ci and Ct, respectively, represent the initial concentration and at time t (mg/L), kf (cm/s), the external mass transfer coefficient, D (cm2/s) is the sum of pore and surface diffusion and S/V (cm-1) represents the ratio of the total interfacial area of the particles against the total solution volume. To assess the significance of film mass transfer versus intraparticle mass transfer resistances, the Biot number (BN) was calculated for pore diffusion, according to Eq. (6): BN ¼ k f

/ D

ð6Þ

where / is the mean particle diameter (cm). The values of kf, D and BN calculated from the above Eqs. (4) to (6) (Table S2). If the value of D lies in the range 10-13– 10-5 cm2/s, as it is the case here (0.22 9 10-12 to 0.32 9 10-12 cm2/s), it indicates that chemisorption phenomena occur during the adsorption process [53]. This result corroborated with the earlier result on very good fit of the pseudo-second-order model, which is governed by chemisorption, to the kinetic data. The increase in D values with initial concentration was attributed to the increase in the internal surface affinity [55]. The Biot number should be [100 for adsorption processes controlled by internal diffusion mechanisms [56]. Hence, the adsorption of rhodamine 6G onto the ARGO/Fe3O4 composite is best described by the internal diffusion mechanism rather than the external diffusion since the Biot number obtained (331.82–369.37) was higher than 100. Comparatively, the high regression coefficients (0.802–0.874) obtained with pore and surface mass diffusion model are in agreement with this conclusion. Adsorption thermodynamics The adsorption isotherms of rhodamine 6G on ARGO/ Fe3O4 composite at three different temperatures are shown in Fig. S2. Adsorption capacities were the highest at T = 313 K and lowest at T = 293 K, demonstrating that the adsorption of dye on composite is promoted at lower temperatures. The thermodynamic parameters [standard free-energy change (DG°), the standard enthalpy change (DH°) and the standard entropy change (DS°)] were calculated from the temperature-dependent adsorption isotherms [57]. The values of DH° and DS° were calculated from the slope and intercept of the plot of ln Kd versus 1/T (Figure not shown) by using the equation ln Kd = (-DH°/RT) ? DS°/R, while the Gibbs free energy (DG°) of specific adsorption was calculated from the equation DG° = DH° - TDS°, where R is the universal gas constant (8.314 J/mol/K), T is the absolute temperature (K) and Kd is distribution adsorption coefficient [58]. The thermodynamic parameters calculated at three different temperatures were listed in Table S3. The negative standard free-energy change (DG°) indicated that the adsorption was a spontaneous under the conditions applied, and the value of DG° became more negative with the decrease in temperature, indicating that lower temperature favors the adsorption of rhodamine 6G on ARGO/Fe3O4 composite. The positive DH° value suggested the exothermic nature of adsorption, which was also supported by the decrease in the adsorption capacity of composite with the increase in temperature. The positive value of DS0 (13.08 J/ K/mol) corresponds to an increase in the randomness at the

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solid–solute interface, which indicate that the affinity of dye toward the composite was high. It also suggested that there were some structural changes in the composite during adsorption process [58]. It is axiomatic that the adsorption of rhodamine 6G on the ARGO/Fe3O4 composite is an exothermic and spontaneous process. Desorption and regeneration Desorption experiments were performed to investigate the possibility of the regeneration of the ARGO/Fe3O4 composite. The desorption efficiency (%) was defined as the ratio of the amount of the dye desorbed to the amount of the dye adsorbed. The experimental procedure for the adsorption–desorption study includes adsorption of rhodamine 6G on to ARGO/Fe3O4 composite, separation of adsorbed composite by used in by a magnetic field, transfer of dye adsorbed composite into ethanol to release the dye and complete desorption of dye in ethanol to regenerate the composite, again separated by a magnetic field. The results showed that the desorption ratio for the dye was around 89 % when ethanol was used, and the ARGO/Fe3O4 adsorbent could be regenerated and reused at least three times without a significant loss (around 3–5 %) of the sorption capacity. It was observed that after three cycles of adsorption–desorption process, the ARGO/Fe3O4 composite exhibited high magnetic sensitivity under an external magnetic field, indicating a high stability of the composite. Both stability and regeneration ability of the adsorbent are very important for the industrial applications to avoid the secondary pollutions in the wastewater treatment.

Conclusions A simple and effective chemical method has been used to reduce the GO using acetone, which can be used for mass production of graphene and also prevents use of hazardous, explosive and costly materials. Also, Fe3O4 uniformly decorated on ARGO sheets was successfully synthesized and was used as an effective adsorbent to remove the organic dye rhodamine 6G from aqueous solutions. The kinetics and isotherm experimental data can be well described with the pseudo-second-order model and the Langmuir–Freundlich isotherm model, respectively. The rate-controlling mechanism study revealed that intraparticle diffusion was predominant during the adsorption process. The thermodynamic parameters indicated that the adsorption of rhodamine 6G on the ARGO/Fe3O4 composite was a spontaneous and exothermic process. Furthermore, the sorbent could be regenerated and used repeatedly. The magnetic properties of the ARGO/Fe3O4

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composite allow its easy separation from the medium after adsorption by applying a magnetic field, leading to the development of a clean and green route for environmental pollution treatment. Acknowledgements The authors wish to express their sincere thanks to Professor Bhavanath Jha, Discipline coordinator of Marine Biotechnology and Ecology and Dr. (Mrs) K.H. Mody, Group Leader, Marine Environment for their valuable suggestions, Mr. Vinod Agrawal for FT-IR spectra, Ms. Pragnya Bhatt for XRD, Mr. Jayesh Chaudhary for SEM, Mr. Gopal Ram Bhadu for TEM and Mr. Satyajit for TG analysis.

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