Fabrication of rGO/Fe 3 O 4 Magnetic Composite for the Adsorption of Anthraquinone-2-Sulfonate in Water Phase

: In the last few decades, anthraquinone and its derivatives (AQs) have been intensively applied to electrochemical, textile and dye, and photovoltaic industries. This has increased the levels of AQs in the natural environment and threatens human health. To remove AQs from the aqueous phase and recover these multi-functional molecules, a binary magnetic adsorbent, reduced graphene/Fe 3 O 4 (rGO/Fe 3 O 4 ), was synthesized via a hydrothermal method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectra, and thermogravimetric analysis (TGA) were then used to characterize the samples. The adsorption capacities of rGO/Fe 3 O 4 to AQs were investigated by selecting anthraquinone-2-sulfonate (AQ2S) as a model molecule. The adsorption process followed the Langmuir adsorption isotherm and the second-order kinetics. The regeneration of adsorbents and the recycling of AQ2S and solvent were simultaneously achieved by Soxhlet extraction and rotary evaporation. These results conﬁrm the high adsorption efﬁciency of rGO/Fe 3 O 4 for removing AQs from water and provide a promising approach to recover the valuable molecules from the aqueous phase.


Introduction
Recently, anthraquinone and its derivatives (AQs) have attracted increasing attention because of their excellent electrochemical properties and good visible-light responses [1]. As non-precious metal electrode materials, AQs have been intensively applied to develop environmentally friendly batteries that have high voltage and better energy-storage capacities [2]. They are also used to degrade refractory organic pollutants in advanced electrochemical oxidation processes (EAOPs) [3]. Their conjugated double bonds increase their capacity for light absorption [4]. Furthermore, AQs are effective photosensitizers, and are widely used for solar energy storage, in the textile dyeing processes [5,6], and for the development of biosensors [7]. However, the high solubility of AQs in water limits their applications as they enter the natural environment in dissolution and threaten human health.
To overcome this, continuous efforts have been made to eliminate AQs from aqueous environments. These methods include bacterial degradation, advanced oxidation processes, physical techniques, and electrochemical oxidation [8][9][10]. Due to their numerous applications and complex synthesis, advanced recycling methods are required to remove AQs without damaging their inherent activity during the cycling process. Adsorption is recognized as an easy, low-cost, and efficient pollutant removal method, which has been widely used to remove pollutants and capture valuable molecules from water [11,12]. Adsorbents play critical roles in improving the extraction of target pollutants from water.
Graphene, a two-dimensional carbon material, is widely used in material science and technology, due to its distinctive structural, mechanical, and electronic properties [13,14]. Graphene's ultrahigh-specific surface area and high delocalized p-electron system make it a viable option for the removal of organic contaminants, with a strong affinity and high adsorption capacity [15]. Theoretically, AQs with abundant conjugated double bonds can firmly attach to the surface of graphene, which has a specifically developed, π-π stacked, two-dimensional conjugated structure, without a covalent bond [16,17]. Therefore, using graphene as an absorbent is a suitable solution for the absorption of AQs from water. However, graphene must be centrifuged or filtered after water treatment, to separate and recycle the absorbent, which is inconvenient and tedious [18]. To overcome these limitations, magnetic materials such as iron oxide nanomaterials have been widely used for the simple removal of dispersive tiny targets using an external magnet [19]. Yao et al. [20] applied a coprecipitating method to generate Fe 3 O 4 particles, and then added hydrazine hydrate as a reduction into a graphene oxide and Fe 3 O 4 mixed solution to form an rGO/Fe 3 O 4 composite [20]. Although this as-prepared binary material showed a good performance of Congo red (CR) adsorption methylene blue (MB), the use of hydrazine hydrate may be a safety hazard because it is highly corrosive. Liu et al. [18] constructed a GO/Fe 3 O 4 magnetic adsorbent for Cr(VI) removal, via an intricate electrostatic assembling method [18]. Wei et al. [21] provided a solvothermal method to generate Fe 3 O 4 particles, and then obtained the GO/Fe 3 O 4 composite by electrostatic interaction [21]. Due to the defect of the π-π conjugate on the GO surface, the as-obtained material is not ideal as an adsorbent for the removal of AQ2S. Therefore, the development of an effective method to produce rGO/Fe 3 O 4 is required.
To fulfill ecological and economic demands, the regeneration and reuse of adsorbents are essential in practical applications, which also reduces the overall cost of the adsorption process. The Soxhlet extraction process is an ideal approach for the regeneration of adsorbents due to very small mass loss, reduced solvent cost, and little damage to the structures of adsorbates and adsorbents [22,23].
In this study, a binary magnetic adsorbent, reduced graphene oxide-Fe 3 O 4 (rGO-Fe 3 O 4 ), was constructed by a one-pot hydrothermal method and used to remove organic pollutants from the aqueous phase. A typical AQ, anthraquinone-2-sulfonate (AQ2S), was used as a model pollutant. The as-prepared composite had an excellent adsorption capacity for AQ2S and showed good separation performance from the suspension. Soxhlet extraction was used as the regeneration method, and the extract was concentrated by a rotary evaporator to recover the AQ2S. This combination of Soxhlet extraction and rotary evaporation not only recovered the AQ2S, but also significantly reduced the organic solvent consumption. In addition, a five-cycle experiment was performed to investigate the stability of the adsorbent. The flow chat of the AQ2S adsorption/recovery process is shown in Figure 1.
Water 2021, 13, x FOR PEER REVIEW 2 of 13 widely used to remove pollutants and capture valuable molecules from water [11,12]. Adsorbents play critical roles in improving the extraction of target pollutants from water. Graphene, a two-dimensional carbon material, is widely used in material science and technology, due to its distinctive structural, mechanical, and electronic properties [13,14]. Graphene's ultrahigh-specific surface area and high delocalized p-electron system make it a viable option for the removal of organic contaminants, with a strong affinity and high adsorption capacity [15]. Theoretically, AQs with abundant conjugated double bonds can firmly attach to the surface of graphene, which has a specifically developed, π-π stacked, two-dimensional conjugated structure, without a covalent bond [16,17]. Therefore, using graphene as an absorbent is a suitable solution for the absorption of AQs from water. However, graphene must be centrifuged or filtered after water treatment, to separate and recycle the absorbent, which is inconvenient and tedious [18]. To overcome these limitations, magnetic materials such as iron oxide nanomaterials have been widely used for the simple removal of dispersive tiny targets using an external magnet [19]. Yao et al. [20] applied a coprecipitating method to generate Fe3O4 particles, and then added hydrazine hydrate as a reduction into a graphene oxide and Fe3O4 mixed solution to form an rGO/Fe3O4 composite [20]. Although this as-prepared binary material showed a good performance of Congo red (CR) adsorption methylene blue (MB), the use of hydrazine hydrate may be a safety hazard because it is highly corrosive. Liu et al. [18] constructed a GO/Fe3O4 magnetic adsorbent for Cr(VI) removal, via an intricate electrostatic assembling method [18]. Wei et al. [21] provided a solvothermal method to generate Fe3O4 particles, and then obtained the GO/Fe3O4 composite by electrostatic interaction [21]. Due to the defect of the π-π conjugate on the GO surface, the as-obtained material is not ideal as an adsorbent for the removal of AQ2S. Therefore, the development of an effective method to produce rGO/Fe3O4 is required.
To fulfill ecological and economic demands, the regeneration and reuse of adsorbents are essential in practical applications, which also reduces the overall cost of the adsorption process. The Soxhlet extraction process is an ideal approach for the regeneration of adsorbents due to very small mass loss, reduced solvent cost, and little damage to the structures of adsorbates and adsorbents [22,23].
In this study, a binary magnetic adsorbent, reduced graphene oxide-Fe3O4 (rGO-Fe3O4), was constructed by a one-pot hydrothermal method and used to remove organic pollutants from the aqueous phase. A typical AQ, anthraquinone-2-sulfonate (AQ2S), was used as a model pollutant. The as-prepared composite had an excellent adsorption capacity for AQ2S and showed good separation performance from the suspension. Soxhlet extraction was used as the regeneration method, and the extract was concentrated by a rotary evaporator to recover the AQ2S. This combination of Soxhlet extraction and rotary evaporation not only recovered the AQ2S, but also significantly reduced the organic solvent consumption. In addition, a five-cycle experiment was performed to investigate the stability of the adsorbent. The flow chat of the AQ2S adsorption/recovery process is shown in Figure 1.

Chemicals and Reagents
Sodium anthraquinone-2-sulfonate was purchased from Shanghai Aladdin Corporation. NaHCO 3 , HCl, NaCl, methanol, and NaOH were given by Tianjin Continental Chemical Reagent Factory (Tianjin, China). Sodium dodecylbenzene sulfonate (SDBS) was supplied by Tianjin Tianli Chemical Reagent Co., Ltd. Graphene oxide (GO) was obtained from Tangshan Jianhua Science Technology Co., Ltd. Ferrous chloride tetrahydrate (FeCl 2 ·4H 2 O) and Iron chloride hexahydrate (FeCl 3 ·6H 2 O) were obtained from Tianjin Shuangchuan Chemical Reagent Factory. All the reagents were analytical grade and did not require any further purification. All the experiment was done using deionized (DI) water.

Synthesis of Fe 3 O 4 Nanoparticles
In an alkaline solution, Fe 2+ and Fe 3+ were chemically coprecipitated to produce magnetic Fe 3 O 4 [24]. FeCl 2 ·4H 2 O (0.06 g) and FeCl 3 ·6H 2 O (0.12 g) were solubilized in deionized water, and 1 M NaOH solution was used to maintain the pH at 10. After 120 min of stirring at 50 • C, the solution was cooled and black precipitates from the solution were then separated with external magnet. Before being dried at 60 • C under decreased pressure, the precipitate was washed several times with ethanol and DI water.

Synthesis of rGO/Fe 3 O 4 Composite
A one-pot hydrothermal approach was employed to make the rGO/Fe 3 O 4 composites, which used sodium dodecylbenzene sulfonate (SDBS) as an anionic surfactant for micelles formation in water [25]. Typically, 25 mg of SDBS was dissolved in 10 mL DI water. The, GO (8.1 mg) and Fe 3 O 4 (0.323 g) were introduced to the SDBS solution and ultrasonicated for 40 min. For the hydrothermal process, the suspension was heated to 180 • C in a 100 mL Teflon-lined autoclave for 16 h. The solution was permitted to cool down to ambient temperature. The solid was rinsed with deionized water (DI) and ethanol multiple times before being dried in a vacuum oven for 12 h at 60 • C.

Characterization
The structures and morphologies of prepared composites were analyzed through transmission electron microscopy (TEM FEI Tecnai F20, Waltham, MA, USA). A diffractometer of X-ray (X'Pert, Panalytical, Almelo, The Netherlands) was used to acquire sample X-ray diffraction (XRD) patterns using the radiation of Cu Kα in the 2θ range from 10 • to 90 • . A spectrometer of Raman (inVia-Reflex, Renishaw, Wotton-under-Edge, UK) equipped with a 532 nm laser was used to record the products' Raman spectra. Over the range of 4000-400 cm −1 , Fourier transform infrared (FT-IR) spectra were collected on an infrared spectrometer (Nicolet iS50, Thermo Fisher, Waltham, MA, USA) with KBr as a background. Quantachrome Autosorb IQ (Waltham, MA, USA) examined the pore size distribution and specific surface area of photosensitive particles. The chemical states and surface elemental compositions of each specimen were assessed using X-ray photoelectron spectroscopy (XPS, PHI 5300, Perkin Elmer, Waltham, MA, USA) with a source of Al Kα X-ray. The C1s peak at 284.8 eV was implemented to calibrate all binding energies.

Adsorption Experiments
Batch experiments were used to evaluate SDZ adsorption characteristics and the AQ2S adsorption mechanism on rGO/Fe 3 O 4 . Kinetic experiments were analyzed by adding 10 mg adsorbent to a series of 250 mL AQ2S solutions with 50 mg·L −1 . Samples were taken and analyzed after 0.5, 1, 2, 3, and 4 h adsorption, at 288, 298, and 308 K temperature, which represent the average temperatures of wastewater-treatment plants in winter, spring/autumn, and summer, respectively. The vials were transferred to an incubator shaker and shaken at 150 rpm. The adsorbent was separated magnetically from the suspension and HPLC was used to measure residual AQ2S concentrations. After magnetic separation, the adsorption amounts qt (mg/g) were calculated by Equation (1): where νdemonstrates the volume of AQ2S solution (mL), m demonstrates the mass of the adsorbent (mg), and C 0 and Ct imply the concentration (mg·L −1 ) at zero time (initial) and concentration (mg·L −1 ) at time t, respectively. Adsorption isotherms were obtained by adding 250 mL AQ2S solutions at different concentrations (5-100 mg·L −1 ) in 500 mL vials with 10 mg rGO/Fe 3 O 4 at three temperatures (288, 298, and 308 K) for 8 h. The samples were taken at 0 and 8 h to measure the AQ2S concentrations. All experiments were performed in duplicate.

Regeneration
The saturated adsorbent was regenerated via Soxhlet extraction using methanol, ethanol, and acetone as solvents. rGO/Fe 3 O 4 (10 mg) was placed in the Soxhlet extractor cartridge and repeatedly refluxed with 100 mL methanol at 70, 80, 90, and 100 • C. After extraction, the adsorbent in the cartridge was removed and rinsed several times with DI water followed by its drying at 60 • C. The extract containing AQ2S was poured into the Rotary Evaporator to recover the concentrated AQ2S and methanol.

Characterizations of rGO/Fe 3 O 4 Composite
The morphology and structure of the as-prepared rGO/Fe 3 O 4 are shown in the TEM image ( Figure 2). Figure 2a shows that the GO sheets are single-layered and twodimensional planar structures, with slight wrinkles on their surface. Figure 2b FT-IR spectroscopy further characterized carbon species and the functional groups of GO, Fe 3 O 4 , and rGO/Fe 3 O 4 ( Figure 3a). The characteristic absorption peak of GO and rGO/Fe 3 O 4 was present at 1618 cm −1 , ascribed to the C-C stretching vibration on the sp2 carbon skeleton [13]. The peaks detected at 1368 and 3433 cm −1 associated with the stretching vibration of C-O and -OH, respectively. The peak at 580 cm −1 for Fe 3 O 4 particles and rGO/Fe 3 O 4 is due to Fe-O vibrations [21].
The phase compositions and crystal structures were investigated using XRD techniques ( Figure 3b). The presence of distinct characteristic peaks in all samples suggests that the composite has high crystallinity. The primary diffraction peaks for Fe 3 O 4 crystals were found at 30.  . The diffraction angle of GO was found to be 10.7 • , which corresponded to the GO plane (001). Compared with GO, no obvious peak was present at 10.7 • in the composite XRD pattern, which might be due to the reduction of GO during synthesis.
The spectra of Raman revealed the lattice structure of GO and rGO/Fe 3 O 4 . As seen in Figure 3c, the GO peaks at 1350 and1590 cm −1 belong to the D and G bands, accordingly, which are slightly blue-shifted when compared to pure GO. The average size of the sp2 domains [14] is inversely related to the intensity ratio ID/IG, which fell from 1.12 to 1.06 following Fe 3 O 4 addition, indicating that the hydrothermal process reduced the rGO and regained the conjugate structure. FT-IR spectroscopy further characterized carbon species and the functional groups of GO, Fe3O4, and rGO/Fe3O4 (Figure 3a). The characteristic absorption peak of GO and rGO/Fe3O4 was present at 1618 cm −1 , ascribed to the C-C stretching vibration on the sp2 carbon skeleton [13]. The peaks detected at 1368 and 3433 cm −1 associated with the stretching vibration of C-O and -OH, respectively. The peak at 580 cm −1 for Fe3O4 particles and rGO/Fe3O4 is due to Fe-O vibrations [21].
The phase compositions and crystal structures were investigated using XRD techniques (Figure 3b). The presence of distinct characteristic peaks in all samples suggests that the composite has high crystallinity. The primary diffraction peaks for Fe3O4 crystals were found at 30.2°, 35.5°, 43.2°, and 62.7°, which correspond to the crystal faces (220), (311), (400), (422), (511), and (440). (JPPDS No. 19-0629). The diffraction angle of GO was found to be 10.7°, which corresponded to the GO plane (001). Compared with GO, no obvious peak was present at 10.7° in the composite XRD pattern, which might be due to the reduction of GO during synthesis. The spectra of Raman revealed the lattice structure of GO and rGO/Fe3O4. As seen in Figure 3c, the GO peaks at 1350 and1590 cm −1 belong to the D and G bands, accordingly, which are slightly blue-shifted when compared to pure GO. The average size of the sp2 domains [14] is inversely related to the intensity ratio ID/IG, which fell from 1.12 to 1.06 following Fe3O4 addition, indicating that the hydrothermal process reduced the rGO and regained the conjugate structure.
The chemical states and surface composition of the elements in the rGO/Fe3O4 com- The chemical states and surface composition of the elements in the rGO/Fe 3 O 4 composite are detected using XPS (Figure 4). The as-prepared composite material is made up of O, Fe, and C, as seen in the XPS entire survey spectrum in Figure 4a. C1s and O1s are responsible for the peaks at 285 and 530 eV, respectively [19]. The characteristic peaks located at 710 eV correspond to Fe 2p. In Figure 4b, peaks at 725.0 and 711.5 eV are allocated to Fe 2p1/2 and Fe 2p3/2, accordingly [20]. property of rGO/Fe3O4 is sufficient for rapid magnetic separation, which is critical for practical application in water treatment.  Table 1 shows the BET surface areas of the as-obtained materials. According to the BET analysis, the addition of Fe3O4 decreased the BET surface areas to 522.134 m 2 /g, compared with GO (635.21 m 2 /g). Although the reduction of rGO will increase the surface areas, the decrease caused by inclusion of FeO4 which increases the adsorption site occupancy.

Adsorption Kinetics
The adsorption of AQ2S on the surface of rGO/Fe3O4 is greatly influenced by temperature. The adsorption process of AQ2S is investigated at different temperatures from 288 to 308 K with a 50 mg·L −1 initial concentration of AQ2S, and 10 mg rGO/Fe3O4. The adsorption of AQ2S increased as the temperature was raised, showing that the procedure is endothermic (Figure 5a). Because of the fast rate of diffusion of AQ2S on the composite surface at temperatures of 288 and 308 K, all three adsorption processes reached adsorption/desorption equilibrium in 4 h. Furthermore, the AQ2S equilibrium adsorption capacity increases from 288 to 308 K, making it as sensitive as the adsorption rate. The kinetics of the AQ2S adsorption process at different temperatures were studied using the second-order models and pseudo-first. These models are shown in Equations (2) and (3), respectively. The percentage of each component in rGO/Fe 3 O 4 composite was determined by thermogravimetric analysis (TGA), which was carried out at a heating rate of 10 • C min −1 in an air atmosphere. Figure 4c shows the weight loss stages of the composite in air: the first stage occurs at below 150 • C, attributable to water adsorbed on the material's surface evaporating. The elimination of labile oxygen-containing functional groups, including CO and CO 2 , and H 2 O vapors from the specimen produced through the breakdown of the oxygenated functional groups, causes the second stage of weight loss, which occurs between 150 and 300 • C, to be considerably more evident than the first. Then significant weight loss occurs from 300 to 800 • C, indicating the decomposition of graphene in air. The amount of Fe 3 O 4 microspheres in the rGO/Fe 3 O 4 composites was estimated to be 47.11% from the residual weight.
The magnetic hysteresis loop of the as-prepared rGO/Fe 3 O 4 samples at room temperature is shown in Figure 4d. The composite's saturation magnetization (Ms) value was 22.7 emu/g. Under the influences of external magnetic fields, the dispersed composite in aqueous solution was drawn to the sides of the bottle, demonstrating that the magnetic property of rGO/Fe 3 O 4 is sufficient for rapid magnetic separation, which is critical for practical application in water treatment. Table 1 shows the BET surface areas of the as-obtained materials. According to the BET analysis, the addition of Fe 3 O 4 decreased the BET surface areas to 522.134 m 2 /g, compared with GO (635.21 m 2 /g). Although the reduction of rGO will increase the surface areas, the decrease caused by inclusion of FeO 4 which increases the adsorption site occupancy. a The Burnauer-Emmett-Teller (BET) method was used to measure N2 adsorption. b Determination of total pore volume was carried out at P/P 0 = 0.97. c The Barrett-Joyner-Halenda (BJH) method was used to calculate the diameter of the pores based on the desorption data.

Adsorption Kinetics
The adsorption of AQ2S on the surface of rGO/Fe 3 O 4 is greatly influenced by temperature. The adsorption process of AQ2S is investigated at different temperatures from 288 to 308 K with a 50 mg·L −1 initial concentration of AQ2S, and 10 mg rGO/Fe 3 O 4 . The adsorption of AQ2S increased as the temperature was raised, showing that the procedure is endothermic (Figure 5a). Because of the fast rate of diffusion of AQ2S on the composite surface at temperatures of 288 and 308 K, all three adsorption processes reached adsorption/desorption equilibrium in 4 h. Furthermore, the AQ2S equilibrium adsorption capacity increases from 288 to 308 K, making it as sensitive as the adsorption rate. The kinetics of the AQ2S adsorption process at different temperatures were studied using the second-order models and pseudo-first. These models are shown in Equations (2) and (3), respectively.
where q t (mg·g −1 ) and q e represent the capacities of adsorption at time t (h) and equilibrium, accordingly.; v represents the total volume of the suspension (mL); m demonstrates the mass of the adsorbent (mg); k 1 and k 2 represent the pseudo first-order rate constants (h −1 ) and the rate constant for pseudo second-order adsorption (g·mg −1 ·h −1 ), respectively [26]. The calculated results from both pseudo first-order and pseudo second-order models are given in Table 2. The model of pseudo first-order is not fit to represent the adsorption of AQ2S by rGO/Fe 3 O 4 based on the correlation coefficient (r 2 ). Better fit lines at each temperature ( Figure 5b) are obtained by plotting t/q t versus t based on the model of pseudo second-order with a high correlation coefficient > 0.99, which indicates that the adsorption process followed pseudo second-order kinetics. According to a better fit to the model of pseudo second-order, the rate of adsorption is responsible for the distribution of adsorption sites rather than the concentration of adsorbate in the solution.

Adsorption Isotherms
Adsorption isotherms are frequently employed to reveal the adsorbate distribution in the solid and liquid phases at equilibrium. The adsorption of AQ2S by rGO/Fe3O4 is explained by employing the Freundlich and Langmuir isotherm models. The linear forms of these two isotherms are listed below: Freundlich: lnq = + lnk (6) where kl and kf imply the Langmuir constant and Freundlich constant, respectively; qm illustrates the maximum capacity of adsorption (mg·g −1 ); n demonstrates the intensity of adsorption; and Ce is the equilibrium concentration (mg·L −1 ) [26]. The linear fit of Langmuir and Freundlich isotherms is shown in Figure 5c, and the calculated parameters are given in Table 3. The Langmuir model better describes the adsorption isotherm of AQ2S due to greater correlation coefficient values (r2 > 0.99). As a result, the adsorption is monolayer adsorption, with a finite number of adsorption sites on the rGO/Fe3O4 surface. Once all the sites are occupied by AQ2S, the surface of the adsorbent reaches saturation point [27]. Due to the better fit of this model, Arrhenius equation (Equation (4)) parameters are reasonable for the determination of the AQ2S adsorption type.

Adsorption Isotherms
Adsorption isotherms are frequently employed to reveal the adsorbate distribution in the solid and liquid phases at equilibrium. The adsorption of AQ2S by rGO/Fe 3 O 4 is explained by employing the Freundlich and Langmuir isotherm models. The linear forms of these two isotherms are listed below: Langmuir : c e q = c e q m + 1 q m k l (5) Freundlich : lnq e = lnc e n + lnk f (6) where k l and k f imply the Langmuir constant and Freundlich constant, respectively; qm illustrates the maximum capacity of adsorption (mg·g −1 ); n demonstrates the intensity of adsorption; and Ce is the equilibrium concentration (mg·L −1 ) [26].
The linear fit of Langmuir and Freundlich isotherms is shown in Figure 5c, and the calculated parameters are given in Table 3. The Langmuir model better describes the adsorption isotherm of AQ2S due to greater correlation coefficient values (r 2 > 0.99). As a result, the adsorption is monolayer adsorption, with a finite number of adsorption sites on the rGO/Fe 3 O 4 surface. Once all the sites are occupied by AQ2S, the surface of the adsorbent reaches saturation point [27]. Thermodynamics analyses are widely used to determine energy changes during the adsorption process, in a wide range of temperatures. Thermodynamic parameters including enthalpy (H • ), Gibbs free energy (G • ), and entropy (S • ) were estimated by utilizing Equations (7) and (8) to examine the effects of temperature on the adsorption of AQ2S by rGO/Fe 3 O 4 : The calculated results are shown in Table 4. The adsorption of AQ2S on the surface of rGO/Fe 3 O 4 is irreversible and endothermic, as indicated by the positive ∆S • and ∆H • values. Furthermore, the negative standard Gibbs free energy ∆G • values at different temperatures indicate that the degree of spontaneity of the reaction and the spontaneous adsorption increase as temperature rise [29].

Regeneration of Adsorbent and AQ2S Recovery
To improve the regeneration performance, different parameters such as the effect of solvent, temperature, and regeneration time were optimized. Ethanol showed the best desorption of AQ2S in a shorter time (90 min) and restored approximately 98% adsorption capacity of rGO/Fe 3 O 4 . The regeneration performance remains constant after 90 min extraction, demonstrating 90 min as the optimum time for the regeneration of rGO/Fe 3 O 4 . Although the regeneration rate can reach nearly 99% at 90 and 100 • C considering the energy cost, 80 • C is a suitable choice, with a 98% regeneration rate. Therefore, the regeneration of composite with methanol for 90 min at 80 • C results in optimum performance.
Under the optimum conditions, five consecutive adsorption/regeneration cycles were conducted to measure the adsorbent's reproducibility and stability. The result is shown in Figure 6a which reveals that the rGO/Fe 3 O 4 composite maintains its adsorption capacity at 85.7% after five cycles, indicating good stability of this composite. The recovery of AQ2S and methanol was achieved by the rotary evaporation process under negative pressure. Figure 6b shows that, with the increasing index of the adsorption/regeneration cycles, the recovery rate of AQ2S decreased from 92 to 81%, which may be attributed to the mass loss of adsorbent during transferring.
Water 2021, 13, x FOR PEER REVIEW 11 of 13 Figure 6. Adsorption capacity (a) and AQ2S recovery rate (b) variation over the adsorption/regeneration cycle.

Conclusions
In this study, a binary magnetic rGO/Fe3O4 adsorbent was synthesized by the hydrothermal method for AQ2S removal from the water. The as-synthesized composite was characterized by TEM, FTIR, XRD, XPS, Raman spectra, and TGA. The composite displayed good magnetism and high reusability. The adsorption capacities of the magnetic adsorbent were demonstrated by kinetics and equilibrium models. The adsorption of AQ2S by rGO/Fe3O4 was better fitted to the Langmuir isotherm model and described more precisely by a pseudo second-order kinetic model. According to thermodynamic studies, the adsorption reaction was found to be spontaneous and endothermic. The spontaneity of the adsorption procedure increased with the rising temperatures. Therefore, the adsorption of AQ2S on rGO/Fe3O4 could be regarded as an endothermic physical adsorption process dominated by π-π stacking. This corresponds to previous studies; Yu et al. [30] calculated the binding energy of AQs on monolayer graphene via DFT study and confirmed that van der Waals dispersion contributed to >80% of the total attractive interaction for this adsorption process. Through the combination of Soxhlet extraction and rotary evaporation, saturated adsorbents maintained over 90% of their original adsorption capacity for AQ2S after five cycles. The used solvent and AQ2S were also recycled for future use.

Conclusions
In this study, a binary magnetic rGO/Fe 3 O 4 adsorbent was synthesized by the hydrothermal method for AQ2S removal from the water. The as-synthesized composite was characterized by TEM, FTIR, XRD, XPS, Raman spectra, and TGA. The composite displayed good magnetism and high reusability. The adsorption capacities of the magnetic adsorbent were demonstrated by kinetics and equilibrium models. The adsorption of AQ2S by rGO/Fe 3 O 4 was better fitted to the Langmuir isotherm model and described more precisely by a pseudo second-order kinetic model. According to thermodynamic studies, the adsorption reaction was found to be spontaneous and endothermic. The spontaneity of the adsorption procedure increased with the rising temperatures. Therefore, the adsorption of AQ2S on rGO/Fe 3 O 4 could be regarded as an endothermic physical adsorption process dominated by π-π stacking. This corresponds to previous studies; Yu et al. [30] calculated the binding energy of AQs on monolayer graphene via DFT study and confirmed that van der Waals dispersion contributed to >80% of the total attractive interaction for this adsorption process. Through the combination of Soxhlet extraction and rotary evaporation, saturated adsorbents maintained over 90% of their original adsorption capacity for AQ2S after five cycles. The used solvent and AQ2S were also recycled for future use.

Conflicts of Interest:
The authors declare no conflict of interest.