RhB Adsorption Performance of Magnetic Adsorbent Fe3O4/RGO Composite and Its Regeneration through A Fenton-like Reaction

Adsorption is one of the most effective technologies in the treatment of colored matter containing wastewater. Graphene related composites display potential to be an effective adsorbent. However, the adsorption mechanism and their regeneration approach are still demanding more efforts. An effective magnetically separable absorbent, Fe3O4 and reduced graphene oxide (RGO) composite has been prepared by an in situ coprecipitation and reduction method. According to the characterizations of TEM, XRD, XPS, Raman spectra and BET analyses, Fe3O4 nanoparticles in sizes of 10-20 nm are well dispersed over the RGO nanosheets, resulting in a highest specific area of 296.2 m/g. The rhodamine B adsorption mechanism on the composites was investigated by the adsorption kinetics and isotherms. The isotherms are fitting better by Langmuir model, and the adsorption kinetic rates depend much on the chemical components of RGO. Compared to active carbon, the composite shows 3.7 times higher adsorption capacity and thirty times faster adsorption rates. Furthermore, with Fe3O4 nanoparticles as the in situ catalysts, the adsorption performance of composites can be restored by carrying out a Fenton-like reaction, which could be a promising regeneration way for the adsorbents in the organic pollutant removal of wastewater.


Introduction
Colored matter containing wastewater discharged from dyeing, textile, leather, cosmetics or food industries has brought serious environmental problems in our daily life. Even at a low concentration, dyes in water would bring aesthetically displeasing, and most of them are toxic to both aquatic life and humans [1]. The complex conjugated molecular structures make them stable and recalcitrant to oxidant and bacteria, also resistant to be bleached by sunlight. Accordingly, traditional biological or chemical oxidation methods are difficult to remove them from wastewater. Simultaneously, dyes are always highly soluble in water, rendering them hard to be dealt with by precipitation or flocculation. Therefore, cost-effective method is still urgently demanded to remove colored substance from wastewater. Adsorption is an effective and classic depth processing in the field of wastewater treatment, by which a substance at the interfacial layer of the adsorbent would accumulate owing to the operation of surface forces [2]. Compared to other advanced water-treatment technology, adsorption has such merits as a low invest-cost, simplicity of design and operation, and no discharge of harmful substances. Although the history of adsorption in wastewater treatment is more than one hundred years, there are still two challenges in the practical applications. One is to realize selective adsorption, which is the prerequisite for recovering valuable substances from wastewater without being disturbed by coexisting matters [3,4]. Another is to obtain adsorbents possessing excellent adsorption performance and the ability to be facilely separated and regenerated, which is significant to lower the operating cost [5,6].
In previous years, carbonaceous materials, including active carbon and carbon nanotubes, have been widely investigated and applied in the removal of diverse pollutants from water due to their high adsorption capacity [1,7]. Compared with them, graphene and graphene oxide (GO) nanosheets, a type of two-dimensional nanomaterial with a single layer framework of six-member carbon rings, also show strong adsorption ability for both organic or inorganic pollutants because of their large theoretical specific surface area of 2630 m 2 /g and special π-π conjugate planar geometric structure [8][9][10][11][12]. Moreover, the oxygen-containing groups and surface amphiphilicity can be tuned by synthesizing modified GO or reduced GO (RGO) for specific adsorption objectives [13,14]. On the other hand, multifunctional graphene-related nanocomposites would bring additional properties to graphene and become a focused field of graphene materials [11,15,16]. Among them, incorporation of magnetic nanoparticles with graphene has attracted tremendous attention. The inherent superparamagnetic behavior of magnetic nanoparticles enables them to be easily separated and collected by an external weak magnetic field [17,18]. Moreover, the presence of Fe 3 O 4 nanoparticles can prevent the aggregation and restacking of graphene sheets, and consequently retain a high surface area [19]. In addition, Fe 3 O 4 nanoparticles can serve as both a powerful adsorbent [17,20] and a catalyst for heterogeneous Fentonlike reaction [21,22]. The superparamagnetic Fe 3 O 4 nanoparticles possess excellent adsorption capacity for arsenic removal [17], and the performance was much improved when they are dispersed over RGO to form a hybrid adsorbent [23]. In addition, considering the accumulated organic molecules at the interface layer by adsorption is similar to a concentrated process, combining Fenton-like oxidation after adsorption would significantly enhance the reaction kinetics and possibly develop into a new regeneration approach [5].
In general, the Fe 3 O 4 /GO (or RGO) nanocomposites can be synthesized by solvothermal, self-assembly with covalent bond, and in-situ chemical coprecipitation methods, and display efficient adsorption performance in the removals of organic dyes and inorganic metal ions in wastewater [19,[23][24][25][26][27]. The abundant oxygen-containing groups on GO enable a hydrogen bond and electrostatic interactions with organic adsorbates, while the hydrophobic polyaromatic islands of unoxidized benzene rings of RGO offer hydrophobic interaction and π-π stacks toward the organic molecules, especially conjugation molecules. However, much more efforts are required to understand the optimized structural factors for adsorption. Furthermore, it was found that under a photo-Fenton reaction GO would transform into graphene quantum dots [28]. Although GO is unstable in the Fenton conditions, RGO is recalcitrant in the heterogeneous Fenton-like reaction, because the less oxygen containing groups in RGO makes the initial decomposition unfeasible [28], and the oxidation species in the Fenton-like system are localized over the surface of catalysts and less accessible to the graphene partial. Due to the large specific surface area, tunable structures and components of RGO, its application in Fenton(-like) reactions has attracted increasing attentions in the latest years [29][30][31].
Herein, a Fe 3 O 4 /RGO composite with efficient adsorption performance for rhodamine B (RhB) removal has been synthesized by optimizing the iron source, and the structural controllable adsorption mechanism was discussed by the analyses of isotherms and kinetics. Moreover, the regeneration of the adsorbent has been preliminarily studied via a way of heterogeneous Fenton-like reaction.

Materials
Ferrite chloride (FeCl 3 ·6H 2 O), graphite (Spec. pure) and active carbon (AC) powder were procured from Sinopharm Chemical Reagent Co., Ltd. RhB were obtained from Shanghai Hyperheal Biotech Co., Ltd. Ferrous chloride (FeCl 2 ·4H 2 O) was purchased from Aladdin Chemistry Co. Ltd. All chemicals used in this study without special states were of analytical grade and used as received. All solutions were prepared with deionized water.

Synthesis of Fe 3 O 4 /RGO adsorbents
Graphene oxide was prepared by a modified Hummers' method [32,33]. Briefly, graphite oxide was obtained by oxidizing graphite powder with the presence of H 2 SO 4 , NaNO 3 and KMnO 4 . The graphite oxide was washed with diluted HCl solution and exfoliated by ultrasonic treatment at 400 W for 30 min. The Fe 3 O 4 /RGO adsorbents were synthesized through an in situ method. In a typical synthesis, FeCl 2 ·4H 2 O and FeCl 3 ·6H 2 O (weight ratio 3.17) were firstly dissolved in 60 mL GO aqueous suspension (1 mg/mL). The mixture was mechanical agitated in a nitrogen atmosphere at 90 • C for 20 min, to ensure that ferric and ferrous salts were completely dissolved and anchored at the surface of GO nanosheets by chemical interactions. Then 20 mL aqueous ammonia was dropwise added, following with 4 h reaction at 90 • C to complete the precipitation and formation of Fe 3 O 4 and the reduction of GO. Finally the precipitation was magnetically separated, washed with copious water, and vacuum dried overnight at 60 • C. The obtained dark powder is a composite of Fe 3 O 4 and RGO. By changing the amount of iron source, various composites with different theoretic weight ratios of Fe 3 O 4 (according to the amount of total iron dosages) to RGO (according to the GO dosages) were synthesized, that is 1:1, 2:1, 3:1 and 4:1 and assigned as M-1 to M-4, respectively. The control sample of Fe 3 O 4 nanoparticles were also prepared according to the same procedure but without the addition of GO in the reaction solution.

Adsorption kinetics and isotherms
The kinetics of the adsorption process was determined in batch experiments with 50 mg/L RhB solutions. A 100 mL volume of the test solution and 0.2 g weight of adsorbent were shaken in a flask at a speed of 150 r/min at 30 • C in a thermostatic oscillatory water bath. At several points in time, 5 mL sample was taken out from the water bath, and the supernatant solution was collected by preliminarily magnetic separation and subsequently centrifugation to ensure completely removal of solid adsorbents. The residual concentration was analyzed by a UNICO UV-1202 spectrometer. The control (no sorbent) test displays there is negligible change of the dye concentration in the whole process.
RhB adsorption isotherms were determined by batch adsorption experiments at 30 • C in the dark. Briefly, various amounts of solid adsorbent were introduced into 50 mL flasks, receiving 20 mL RhB aqueous solution with an initial 50 mg/L concentration. The samples were shaken for 2 h at the same conditions in the kinetic experiment. The time was sufficient to reach apparent adsorption equilibrium based on the results of adsorption kinetics. Then the equilibrium concentra-tion in the aqueous phase was measured. The adsorbed capacity was calculated according to equation (1): where Q e (mg/g) is the equilibrium adsorption capacity of adsorbent, C i (mg/L) is the initial aqueous phase concentration, C e (mg/L) is the equilibrium concentration, V (mL) is the volume of solution, M s (g) is the mass of adsorbent. A Fenton-like reaction was carried out to regenerate the saturated adsorbents. Briefly, 0.04 g adsorbents saturated by RhB were transferred into a 30 mL H 2 O 2 (30 mmol/L, pH=3.0) aqueous solution and shaken for 30 min at 50 • C to completely remove the adsorbed RhB molecules. The regenerated adsorbents were magnetically separated from the solution and dried for recycled adsorption tests.

Characterization
The nanometric structure and the morphology of the composites were observed by a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan). The crystal structure of Fe 3 O 4 and Fe 3 O 4 /RGO composites were measured using a powder X-ray diffraction (XRD, D/max-2200/PC, Rigaku Corporation, Japan) with Cu K α radiation at 40 kV and 30 mA. Raman spectra were obtained on a Senterra R200-L dispersive Raman microscope (Bruker Optik Gmbh, Germany) with a 532 nm laser source. The chemical states of elements in the nanocomposites were determined by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Kratos Ltd., Japan). The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were analyzed by a Micromeritics ASAP 2010 M+C nitrogen adsorption apparatus (USA). All of the samples were degassed at 180 • C before nitrogen adsorption measurements were taken. Fourier transform infrared spectroscopy (FTIR) was performed at a scanning range of 4000-400 cm −1 on a Nicolet 6700 Infrared-Raman Spectroscope (Thermo Fisher Co., USA) with DTGS detector.

Characterizations of adsorbents
The morphology and microstructure of GO and Fe 3 O 4 /RGO composite (M3) were characterized by TEM. As shown in Fig. 1(a), GO displays a typical flake-like and crumpled shape of graphene morphology. Corrugation and scrolling are the intrinsic nature of graphene, because its ultra-thin and large twodimensional structure would become thermodynamically stable via bending [34][35][36]. The GO nanosheets are transparent and very stable under the electron beam. The ordered graphitic lattices are visible in the HRTEM image of the insert in Fig. 1(a). The folding and scrolling edges of nanosheets allow for a cross-section view of several GO layers. The TEM in Fig. 1 increases of iron oxide, the intensity of characteristic peaks in the composites is enhanced. Figure 3 shows the Raman spectra of GO and the composite M3. In the range below 800 cm −1 , some bands including 215.9, 273.3, 390.7, 482.3 and 642.1 cm −1 appear in the spectrum of M3. These bands are also observed in the composite of Fe 3 O 4 /MWCNTs and attributed to the vibration modes of Fe-O and Fe-C bonds [5]. There is no observable Raman mode at around 300 or 410 cm −1 , indicating the absence of hematite phase in the composite [38,39]. On the other hand, Raman spectroscopy can provide information on the disorder and defect structures of carbonaceous materials. Both spectra of GO and M3 show the fundamental D and G bands of graphene at around 1340 and 1580 cm −1 , respectively. The former one is corresponding to A1g symmetry mode of the disordered sp 3 carbon with the structural defects, amorphous carbon, or edges that break the symmetry and selection rule, while the latter one is attributed to the in-plane vibration of sp 2 -bonded carbon domains [33,40]. Moreover, the intensity ratio of the two bands (I D /I G ) is used to measure the degree of ordering in the carbon materials. The ratio I D /I G of GO is calculated as 2.70, indicating the presence of a large amount of sp 3 carbons in the GO nanosheets. D and G bands of GO are centered at 1332 and 1581 cm −1 , respectively. In comparison to GO, M3 has a much lower I D /I G value (1.86), and the center of the two bands red shift to 1341 and 1589 cm −1 , respectively. It represents that GO has been partially reduced, and there are chemical interactions, but not only physical adsorption between Fe 3 O 4 nanoparticles and RGO carbon [41].
The chemical states of elements in M3 were further determined by XPS analyses, and the results are shown in Fig. 4. The binding energies of peaks at about 284, 530 and 711 eV are corresponding to the C 1s, O 1s and Fe 2p, respectively. The carbon spectrum could be deconvoluted into two bands at 284.6 and 287.3 eV, corresponding to C=C/C-C, and C=O respectively. The absence of C-O group in the nanocomposite indicates the reduction of GO. In Fig. 4(c), the two peaks at 710.9 and 724.8 eV of Fe 2p are corresponding to Fe 2p 1/2 and Fe 2p 3/2 of Fe 3 O 4 , respectively [42]. This can also be supported by the deconvoluted peak at 530.2 eV in the O 1s spectrum (Fig. 4(d)), which is the binding energy of crystal oxygen in Fe 3 O 4 . Another peak located at 531.8 eV can be attributed to the residual C=O group over the RGO. N 2 adsorption-desorption isotherms for Fe 3 O 4 and the composites are shown in Fig. 5. According to the IUPAC classification, Fe 3 O 4 displays a type II isotherm, which is the normal form for non-porous materials. It is obviously that the composites exhibit higher N 2 adsorption quantity than pure Fe 3 O 4 , because the highly dispersion of Fe 3 O 4 nanoparticles over the RGO support results in enhanced surface area and improved porosity and adsorb ability. The isotherm curves of the composites are quite different with Fe 3 O 4 and ascribed to type IV. The sharp steps at the relative pressure around 0.4 P /P 0 is associated with capillary condensation of N 2 molecules inside the pores. However, the Type H 2 characteristic of their hysteresis loops above 0.4 P/P 0 presents that the distribution of pore size and shape is not well-defined, which appears in many typical porous adsorbents (e.g. inorganic oxide gels and porous glasses), and is attributed to the complicated mechanism related with pores with narrow necks and wide bodies and effects of the whole network [43]. M2 has the highest N 2 adsorption, corresponding to its largest specific surface area, which is found to be 296.2 m 2 /g by using a BET equation, higher than 264.6 and 208. Nitrogen adsorption isotherms of Fe3O4 and Fe3O4/RGO composites.

Adsorption performance of the Fe 3 O 4 /RGO composites
In order to evaluate the adsorption performance of the Fe 3 O 4 /RGO composites toward organic dyes in wastewater, the basic dye RhB was employed as the model pollutant. Figure 6 is the adsorption kinetics of RhB on various adsorbents. Fe 3 O 4 nanoparticles display low adsorption capacity due to its small surface area and lack of chemical interaction between the nanoparticles and RhB molecules. The active carbon (AC) shows a high dye adsorption capacity, which can be attributed to its large surface area (>500 m 2 /g), hydrophobia surface and porous structure. RhB adsorption reaches equilibrium at 300 min, achieving more than 98% RhB decolorization. As our expectation, all composites show high adsorption capacity when reach-ing equilibrium. However, according to Fig. 6, it is interesting to observe that the composite M1, which has the highest amount of RGO and even a larger surface area than M3, shows relatively poorer adsorption performance, both in the capacity and kinetic rate. The RhB adsorption on M1 approaches balance after more than 10 h, while the time on M2 and M3 is only 20 and 10 min, respectively. It is known that RGO has the ability to adsorb RhB efficiently at very low concentration, indicating specific interactions toward RhB molecules [24,[44][45][46]. However, the adsorption kinetic depends much on the chemical components of RGO. Composites M2 and M3 display even better adsorption capacities and faster kinetic rates than AC, although AC has more than double surface area than the composites. It also suggests that the RhB adsorption mechanisms on AC and composites are different. Figure 6(b) shows the RhB adsorption kinetics on M3 at different pH conditions. In both acidic and basic solution, the adsorption rate and capacity show negligible decrease. The binding mechanisms of RhB on other molecular or materials have been discussed and most of them are pH dependent [46,47], including hydrogen bond, electrostatic interaction and hydrophobic interaction. Here the RhB adsorption on the composite is efficient in a wide pH range from 3.6 to 9.6. The reason is that the influences of pH on these mechanisms are different. The function of hydrogen bond is strong at acidic solution, whereas the role of electrostatic interaction would be important at basic conditions due to the more negative charged materials. In additional, in a higher pH solution, the inhibited aggregation of RhB molecules also favors a better adsorption [48]. Moreover, the planar structure of RGO and the aromatic conjugate RhB molecules renders a strong π-π stack between them, which enables the efficient adsorption work at a wide pH range.
The isotherms have been measured to explore the adsorption process. Figure 7 shows the adsorption isotherms of RhB on various adsorbents. The most frequently-used isotherm models are Langmuir and Freundlich models. The former one is represented for the monolayer adsorption by the equation (2), whereas the latter one is described by equation (3) for the multilayer adsorption. ln where Q max (mg/g) is the maximum adsorption capacity of adsorbent; K L (L/mg) is the Langmuir constant related to energy of adsorption; K f (L/g) and n are Freundlich constants corresponding to the factors affecting the adsorption capacity and intensity of adsorption, respectively. The linear fitting results and parameters by both models are listed in Table 1. According to the linear correlation coefficient in the two models, RhB adsorption on Fe 3 O 4 /RGO composites can be well described by Langmuir model, but poorly by Freundlich model. The fitting results suggest that RhB adsorption on composites is ascribed to monolayer adsorption, an interaction between the groups on RGO and RhB molecules playing principal role for the effective adsorption. However, the better linear fitting for active carbon and GO is Freundlich model, indicating RhB adsorptions on them are more complicated than monolayer format, which could be ascribed to the porous structure of AC and the different mechanism on RhB reduction by GO. The adsorption capacity of GO was measured by centrifuging the suspension after adsorption equilibrium. Q max for GO is only 14.2 mg/g, which is comparable to the reference result [49], but much lower than those of composites. This indicates in the composites, the micro components of those carbon rings have been much altered. In the composites, M2 possesses the largest Q max and smallest K L , corresponding to its highest adsorption capacity and lowest adsorption energy. Considering the presence of Fe 3 O 4 nanoparticles, we normalize the maximum adsorption capacity of adsorbent according to the carbon amount in the composites. Then Q max of M2 is 432.91 mg/g, more than 3.7 times higher than AC, and at the same time the adsorption rate is about thirty times faster. However, compared to M2, M3 shows even higher adsorption rate but a little lower adsorption capacity, indicating the influent factors to adsorption rate and adsorption capacity are different. The excellent adsorption performance of composites can be attributed to the combination effects of electrostatic interactions, hydrophobic interactions and π-π stacks toward the organic conjugation molecules. However, there is only van der Waal's hydrophobic interaction for the adsorption on AC, which is relatively weak and results in a much slower adsorption rate and a smaller adsorption capacity [46,48]. It was reported that GO can act as a surfactant because of its hydrophilic edges and hydrophobic basal plane [13]. The dosage of iron source contributes to the extent of GO reduction. With the reduction, the increase of hydrophobic characteristic and yet partially retained hydrophilic groups must be one reason for the improved adsorption. A strong hydrophobic interaction toward RhB molecules induces the fast adsorption. Moreover, the enhanced π electrons in the conjugated structure of RGO lead to a facile π-π stack with the molecules with aromatic groups. Considering the fastest adsorption kinetic and relatively higher adsorption capacity of M3, this composite was chosen for the following studies.

Recycle and regeneration
The recyclable property of the composites was investigated by the magnetically separable performance and the regeneration after heterogeneous Fenton-like reaction or methanol washing. Magnetic hysteresis of M3 was measured as shown in Fig. 8. The composite has a typical ferromagnetic hysteresis, with a saturated magnetization of 51.76 emu/g. It was known that Fe 3 O 4 nanoparticles can be separated at a very low magnetic field due to its superparamagnetic behavior [17]. Incorporation of Fe 3 O 4 nanoparticles with RGO would makes the composites be easily separated from aqueous solution. If the adsorbent can be regenerated, the adsorbent could be repeatedly used for dyestuff wastewater purification with a low running cost. Traditional method for adsorbent regeneration is to eliminate the adsorbates by washing with a certain solution or calcining at a high temperature. However, these methods would be requiring a high cost, causing secondary pollution or time-consuming. A control test shows that the adsorbed RhB on M3 could not release into the water at a wide range of pH solution. However, the adsorbent can be regenerated by rinsing with methanol. The regenerated M3 by rinsing with methanol was used for RhB adsorption, and the adsorption performance is almost restored, as shown in Fig. 9. As we known, Fe 3 O 4 in nano sizes can act as a heterogeneous Fenton-like catalyst [21,22]. According to the Haber-Weiss mechanism, the primary processes can be described by equation 4 to 6, in which the catalytic H 2 O 2 decomposition includes the adsorption of H 2 O 2 , the reduction of Fe III and the formation of radicals by Fe II and Fe III species [50]. Fe 3 O 4 /RGO composites could be a more suitable Fenton-like catalyst due to the enhanced dispersion of Fe 3 O 4 nanoparticles over GO nanosheets and the concentrated RhB over the surface of adsorbents. To regenerate the adsorbent by Fenton-like reaction, the RhB saturated adsorbents were treated in H 2 O 2 solution to remove adsorbed dye molecules. The final solution is colorless, indicating that adsorbed RhB was degraded and escaped from the adsorbents, which was further confirmed by the FTIR spectra, as shown in Fig. 10. Compared to the as-prepared M3, there are several obvious new bands in the spectrum of RhB saturated M3 (M3 A), such as the bands at around 821, 1331 and 1460 cm −1 , corresponding to the rock vibration of -CH 2 -, the stretching vibration of C-N, and the bending vibration of -CH 2 -, respectively [46,51]. After Fenton-like reaction, these bands originated from RhB molecules have disappeared in the sample of M3 F, indicating that RhB molecules on the adsorbent have been degraded and removed. Furthermore, the regenerated M3 F was employed for RhB adsorption to evaluate the adsorption capacity of the treated composites, as shown in Fig. 9. After twice treatments, although the RhB adsorption on M3 F becomes slower, the removal efficiency of RhB is still more than 90% after 60 min adsorption by M3 F, indicating that the adsorption capacity of M3 can be retained after regenerated by the Fenton-like treatment method. Further optimizing the conditions in the Fenton-like reaction could obtain a promising regeneration technology for Fe 3 O 4 /RGO adsorbents.

Conclusions
Effective magnetic absorbents, Fe 3 O 4 /RGO composites have been synthesized by an in situ coprecipitation and reduction method. Fe 3 O 4 nanoparticles in sizes of 10-20 nm are well dispersed over the RGO nanosheets. According to the adsorption kinetics and isotherms toward RhB, the composite with a Fe 3 O 4 to RGO ratio of 2:1 shows the highest adsorption capacity and relatively faster adsorption rate, more than 3.7 times higher and thirty times faster than commercial active carbon. Considering the monolayer RhB adsorption behavior by the Langmuir model isotherms, strong interactions between RhB molecules and RGO nanosheets, which depend much on the chemical components of RGO, contribute mainly to the surprising fast adsorption rates. Furthermore, with Fe 3 O 4 nanoparticles as the heterogeneous Fenton-like catalysts, the adsorbed RhB can be degraded and removed from the adsorbents after a Fenton-like reaction, which could be developed into a promising regeneration way of adsorbents in the removal of organic molecules.