Removal and Recovery of Dissolved Oil from High-Salinity Wastewater Using Graphene–Iron Oxide Nanocomposites

: We report the synthesis of reduced graphene oxide (rGO)- α -Fe 2 O 3 nanocomposite and its application to remove and recover dissolved oil from a high-salinity oil–water emulsion in batch and column/breakthrough setups. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and nitrogen adsorption characterized the synthesized nanocomposite’s structure, morphology, and surface properties. Both batch and continuous breakthrough adsorption studies were investigated. The effect of the adsorption parameters on the adsorption capacity and removal efﬁciency was analyzed. The rGO-Fe 2 O 3 nanocomposite (rGO-Fe 2 O 3 -NC) demonstrated a superior adsorption capacity, both when measured experimentally (1213 mg/g) and predicted using the Freundlich isotherm (1301 mg/g). The adsorption process followed pseudo-second-order kinetic, and the rGO-Fe 2 O 3 -NC exhibited a very rapid removal, with more than 60% of oil being removed within 10 min. Breakthrough conﬁrmed the exceptional removal capacities with good regeneration and cycling ability under a short contact time. Moreover, the adsorption capacity was enhanced with an emulsion salinity of up to 100,000 ppm, conﬁrming the suitability for high-salinity wastewater.


Introduction
Produced water (PW) is the largest waste stream associated with the production and processing of oil and gas. PW from oil production is a mixture of reservoir-trapped water, formation water, and injection water injected into the oil and gas reservoir during waterflooding or enhanced oil recovery (EOR) operations [1]. The physicochemical properties of PW are influenced by the reservoir hydrocarbon type, geographic location of the reservoir field, and hydrocarbon properties. PW from an oil reservoir is a complex mixture of highsalinity water containing organic/inorganic components, heavy metals, oil droplets, and dissolved oil [2]. Properly treated PW can be used for water flooding or EOR instead of disposing of it in the environment [3].
The main stages for PW treatment are deoiling, desalination, suspended particle removal, radioactive and soluble organic materials, softening, and disinfection [4]. To achieve these objectives and fulfill the stringent environmental regulations, especially for oilcontaminated water, researchers have studied combined and standalone physical, chemical, and biological techniques for PW treatment [5]. Among these methods, adsorption is efficient for removing dissolved oil from water in terms of time, cost, and industrial applicability. Adsorption is a surface phenomenon by which mass transfer from a fluid to a solid surface proceeds due to the adhesion of a substance at the interface of two phases [6].
Developing new functional adsorbents for deoiling oily wastewater depends on factors such as surface charge density, surface area, porosity, pore size distribution, and thermal and chemical stability [7]. There is remarkable potential for the remediation of environmental issues with the advent of nanomaterials [8][9][10]. Unlike conventional materials, nanomaterials

Materials
Graphene oxide was supplied by HK E-Sun International Co. Ltd., Guangdong, China. It was prepared using the Hummer method. Ammonium hydroxide 28-30% solution (Merck, Burlington, MA, USA) and iron (II) acetate, 95% (Sigma Aldrich, St. Louis, MO, USA), were used as delivered. Diesel was obtained from Wooqod Gas Station (Doha, Qatar) and was utilized as a model oil.

Oil-Water Emulsion: Preparation and Characterization
The oil in the water emulsion was prepared by mixing diesel and water using probe sonication (Cole-Parmer Ultrasonic Processors CV334) for 30 min and 3 s on and 1 s off pulses, plus 30 min of bath sonication. The emulsion salinity was regulated using pure sea salt (NaCl). The emulsion zeta potential and droplet size distribution were analyzed using (Zeta Analyzer Nano ZS instrument, Malvern, UK) laser-scattering particle size and zeta potential analyzer. The droplet size distribution was calculated by analyzing the scattered light using the Mie principle. The zeta potential measurement assessed the electrophoretic mobility of the colloidal particles. The diesel concentration of oil-water was measured by a combustion-type TOC analyzer (Shimadzu, Kyoto, Japan, model TOC-L). For the column experiments, emulsified oil concentration was measured by UV-visible spectrophotometer (Shimadzu) using a calibration curve and the Beer-Lambert Law: where A is the absorbance, I 0 and I are the incident and transmitted light intensity, E is constant, l is the path length, and c is oil concentration [42].

Characterization of rGO-Fe 2 O 3
rGO-Fe 2 O 3 with 47 wt.% iron oxide was prepared hydrothermally using iron II acetate as iron precursors. In a typical experiment, GO (500 mg) was dispersed in water (100 mL) by probe sonication (CV334, Cole-Parmer, Court Vernon Hills, IL, USA) for 5 min. Iron acetate was dissolved in 100 mL of deionized water and added to the GO suspension under stirring with dropwise addition of NH 4 OH solution to adjust the pH to 10 ± 0.1. The mixture was then transferred to 200 mL, Teflon-lined stainless-steel cell, sealed, heated to 80 • C, and maintained for four hours. After cooling the hydrothermal cell to 25 • C, the solid product was retrieved via filtration, washed with ethanol, dispersed in water, and freeze-dried (Freeze-Dryer 6 Plus, LABCONCO, Kansas City, MO, USA). The dry sample was calcined at 400 • C in a tube furnace for 4 h under 40 mL/min Argon flow.

Batch Adsorption
The impacts of initial oil concentration (C 0 ), adsorbent dosage, adsorption time, and salinity on the adsorption capacity and removal efficiency were investigated via batch adsorption experiments. C 0 varied from 25 to 200 ppm, and the rGO-Fe 2 O 3 dose varied between 1 and 20 mg, yielding a dissolved oil to the adsorbent ratio of 50 to 8000 mg/g. The emulsion salinity varied between 0 and 200,000 mg/L using NaCl.
The adsorption capacity (q t , mg/g) and removal efficiency (RE, %) were calculated using Equations (2) and (3): where C t is the oil concentration (ppm) at time t, and V and m g are the emulsion volume (L) and the adsorbent mass (g).

Breakthrough/Column Experiments
Column adsorption measurements were carried out using a fixed-bed adsorber at 22 ± 1 • C using a glass column with a 1" diameter and a bed (2 cm height) containing 0.2 g of adsorbent mixed with 4 mm glass spheres. A syringe pump fed the oil emulsion (100 ppm) to the bottom of the column at 200 mL/h. Effluent samples from the exit of the adsorption column were collected every 10 min, and the oil concentration was measured using a UV-VIS spectrometer. The continuous experiment was continued until a breakthrough was observed. A control experiment was carried out in the same manner, except no adsorbent was added to the glass spheres bed to account for any adsorption by the column of the Teflon-fritted support. At the end of each adsorption cycle, the adsorbent regeneration was carried out by adding 50 mL of ethanol to dissolve the adsorbed oil.

Adsorption Isotherms
The batch experiment results were analyzed to reveal the adsorption mechanism using Langmuir, Freundlich, Temkin, and Dubinin-Radushevich (D-R) isotherms, and the results are consistent with the Freundlich isotherm. Analysis of the results against these models indicated the Freundlich isotherm provides the best fit for the results. The Freundlich isotherm is an empirical equation that describes heterogeneous adsorption. The Freundlich isotherm model is given by: where n and k f are the constants related to the sorption intensity and capacity of the adsorbent [43]. The linear form of the Freundlich model is:

Adsorption Kinetics
To study the kinetics of the adsorption process, 5 mg of the adsorbent was added to 400 mL of a 100-ppm emulsion, and the mixture was agitated at 350 rpm and 25 • C using a mechanical shaker. The residual oil concentration was measured at 10 min intervals. The pseudo-first-order and pseudo-second-order models were tested. The pseudo-firstorder (Lagergren model) assumes the adsorption rate is proportional to the number of unoccupied sites [44]. It is linearized as: where q t is the sorption capacity (mg/g) at time t and k 1 is the rate constant (min −1 ). To calculate the adsorption rate constant, ln(q e − q t ) is plotted versus t [45], and the slope and intercept of the linear fit are calculated. The linear form of the pseudo-second-order model is: where k 2 is the pseudo-second-order rate constant (g/mg.min), and t is the constant time (min). A linear fit of t q t versus t indicates the model applicability with slope = 1 q e and the intercept = 1 k 2 q e 2 [46], allowing for calculating k 2 and q e [44].

Adsorbent Characterization
Pristine graphene has a very high surface area but no functional groups to facilitate the attachment of metals or metal oxides on its surface [3]. On the other hand, GO readily disperses in water and has strong interaction with other substances, making it promising support for depositing different inorganic compounds on its surface. GO can be reduced hydrothermally [3].
The surface topography of the rGO-Fe 2 O 3 nanocomposite was analyzed using SEM. The SEM image ( Figure 1a) shows a thin platelet-like structure with lateral dimensions of a few to several µm. Moreover, the HR SEM image in Figure 1b shows the 10-20-nm Fe 2 O 3 nanoparticles that are highly distributed on the surface of rGO. Figure 1c shows the XRD patterns of GO and rGO-Fe 2 O 3 . The polar oxygen functionalities on the surface of GO and with the adsorbed water molecules expanded the interlayer spacing of GO compared to graphite, shifting the characteristic (002) peak to 2θ of 10.56 • , corresponding to the d-spacing of 7.22 Å. Upon reducing GO to rGO, the sharp (002) peak of GO disappeared, confirming the reduction/exfoliation of GO to rGO. The diffraction pattern of the nanocomposite had diffraction peaks of the (012), (104), (110), (113), (024), and (116) planes of the α-Fe 2 O 3 crystal consistent with the JCPDS profile no. 33-0664.
The N 2 adsorption-desorption isotherms (80 K) for the rGO-Fe 2 O 3 nanocomposite ( Figure 1d) were consistent with the type IV isotherm characterized by a hysteresis loop at high P/P 0 due to the presence of mesopores. The pore volume was calculated at lower pressure using the Barrett-Joyner-Halenda (BJH) analysis. The Kelvin equation was used to determine the pore size distribution of rGO-Fe 2 O 3 . Moreover, micropore volume was calculated via density functional theory (DFT). The nanocomposite had a good surface area of 150 m 2 /s and a reasonable pore volume of 0.36 cm 3 /g. Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 13  The N2 adsorption-desorption isotherms (80 K) for the rGO-Fe2O3 nanocomposite ( Figure 1d) were consistent with the type IV isotherm characterized by a hysteresis loop at high / due to the presence of mesopores. The pore volume was calculated at lower pressure using the Barrett-Joyner-Halenda (BJH) analysis. The Kelvin equation was used to determine the pore size distribution of rGO-Fe2O3. Moreover, micropore volume was calculated via density functional theory (DFT). The nanocomposite had a good surface area of 150 m 2 /s and a reasonable pore volume of 0.36 cm 3 /g.

Emulsion Characterization
The droplet size distribution of different concentrations of the diesel emulsion, as shown in Table 2, reveals a bimodal droplet size distribution with a droplet size range between 50 and 106 nm for the small population and 140 and 4800 nm for the large population. The dissolved oil's zeta potential at various concentrations indicates the emulsion had a sizeable negative potential of −41 to −27 mV. Therefore, droplet coalescence would be unlikely due to electrostatic repulsion force [47].

Emulsion Characterization
The droplet size distribution of different concentrations of the diesel emulsion, as shown in Table 2, reveals a bimodal droplet size distribution with a droplet size range between 50 and 106 nm for the small population and 140 and 4800 nm for the large population. The dissolved oil's zeta potential at various concentrations indicates the emulsion had a sizeable negative potential of −41 to −27 mV. Therefore, droplet coalescence would be unlikely due to electrostatic repulsion force [47].

Effect of Adsorption Parameters
To evaluate the contact time effect on the adsorption capacity and removal efficiency, measurements were performed with contact times between 0 and 60 min using adsorbent dosage, initial oil concentration, temperature, and a sample trembling speed of 22 • C, 3 mg adsorbent, 100 ppm, and 350 rpm. As shown in Figure 2, the removal efficiency of the emulsified oil by rGO-Fe 2 O 3 was very rapid in the first several minutes, with 95% of the equilibrium capacity achieved in 10 min. This initial rapid adsorption was due to the abundance of empty binding sites on the rGO-Fe 2 O 3 surface. As the rGO-Fe 2 O 3 surface became saturated with the adsorbed oil molecules, the adsorption rate slowed, approaching an equilibrium. In addition, the repulsion between the rGO-Fe 2 O 3 and the emulsified oil molecules could occur due to the increased electrostatic (repulsive) force as the active sites on the rGO-Fe 2 O 3 surface became saturated with dissolved oil molecules [48].
To evaluate the contact time effect on the adsorption capacity and removal effic measurements were performed with contact times between 0 and 60 min using adso dosage, initial oil concentration, temperature, and a sample trembling speed of 22 mg adsorbent, 100 ppm, and 350 rpm. As shown in Figure 2, the removal efficiency emulsified oil by rGO-Fe2O3 was very rapid in the first several minutes, with 95% equilibrium capacity achieved in 10 min. This initial rapid adsorption was due abundance of empty binding sites on the rGO-Fe2O3 surface. As the rGO-Fe2O3 su became saturated with the adsorbed oil molecules, the adsorption rate slowed, appr ing an equilibrium. In addition, the repulsion between the rGO-Fe2O3 and the emul oil molecules could occur due to the increased electrostatic (repulsive) force as the sites on the rGO-Fe2O3 surface became saturated with dissolved oil molecules [48]. Batch measurements were performed to study the influence of the rGO-Fe2O and the initial oil concentration on the adsorption capacity and removal efficiency dosage varied between 1 and 20 mg, and the initial oil concentrations were betwe and 200 ppm. All other parameters, including temperature, agitation speed, and co Batch measurements were performed to study the influence of the rGO-Fe 2 O 3 dose and the initial oil concentration on the adsorption capacity and removal efficiency. The dosage varied between 1 and 20 mg, and the initial oil concentrations were between 25 and 200 ppm. All other parameters, including temperature, agitation speed, and contact time, were constant at 30 min, 350 rpm, and 22 ± 1 • C. Figure 2c,d show the adsorption capacity and removal efficiency dependence on the rGO-Fe 2 O 3 nanocomposite dose and the initial oil concentration. The adsorption capacity rose with the initial oil concentration due to the increased mass transfer driving force caused by the higher concentration gradient between the bulk solution and the rGO-Fe 2 O 3 surface at a higher initial oil concentration [49].
Furthermore, the adsorption capacity declined with rGO-Fe 2 O 3 doses of up to 5 mg but remained constant for dosages of more than 5 mg, possibly because of the adsorbent saturation with oil. Therefore, 1 mg of rGO-Fe 2 O 3 was chosen as the standard adsorbent dosage for the remaining experiments. Meanwhile, the removal efficiency of emulsified oil dropped with increasing C 0 , while the capacity increased with C 0 . Thus, the maximum removal efficiency occurred at lower initial concentrations due to the limited vacant adsorption sites.

Effect of Salinity
The influence of the emulsion salinity on the oil adsorption is shown in Figure 3. The rGO-Fe 2 O 3 adsorption capacity and removal efficiency increased with a salinity of up to 15,000 ppm, reaching a plateau at higher salinity. At higher NaCl concentrations, the dissolved oil becomes less stable in the aqueous phase, leading to droplet size expansion as confirmed by zeta potential. Therefore, the dissolved oil adhered more easily to the rGO-Fe 2 O 3 surface, modifying the emulsified oil-water interactions and boosting the oil adsorption onto the surface of rGO-Fe 2 O 3 . sorption sites.

Effect of Salinity
The influence of the emulsion salinity on the oil adsorption is shown in Figure 3. The rGO-Fe2O3 adsorption capacity and removal efficiency increased with a salinity of up to 15,000 ppm, reaching a plateau at higher salinity. At higher NaCl concentrations, the dissolved oil becomes less stable in the aqueous phase, leading to droplet size expansion as confirmed by zeta potential. Therefore, the dissolved oil adhered more easily to the rGO-Fe2O3 surface, modifying the emulsified oil-water interactions and boosting the oil adsorption onto the surface of rGO-Fe2O3.

Adsorption Isotherm and Kinetics
Modeling the adsorption results is essential to reveal the adsorption mechanism and thermodynamics. The equilibrium analysis determines the maximum dissolved oil adsorbed per unit mass of adsorbent. Our results were examined against Langmuir, Freundlich, Temkin, and D-R isotherms. The regression coefficient R 2 was found, and an error analysis was carried out. Here, we provide the analysis of the results using the Freundlich model, as it provides the best fit for the experimental results. The details of the analysis of the results using these isotherm models are provided in the Supplementary Information.
The Freundlich isotherm (Figure 4a) provided an excellent fit for the experimental adsorption data, indicating that the adsorption mechanism is not limited to the monolayer structure. Therefore, the amount of oil adsorbed on rGO-Fe2O3 adsorbents was the total of

Adsorption Isotherm and Kinetics
Modeling the adsorption results is essential to reveal the adsorption mechanism and thermodynamics. The equilibrium analysis determines the maximum dissolved oil adsorbed per unit mass of adsorbent. Our results were examined against Langmuir, Freundlich, Temkin, and D-R isotherms. The regression coefficient R 2 was found, and an error analysis was carried out. Here, we provide the analysis of the results using the Freundlich model, as it provides the best fit for the experimental results. The details of the analysis of the results using these isotherm models are provided in the Supplementary Information.
The Freundlich isotherm (Figure 4a) provided an excellent fit for the experimental adsorption data, indicating that the adsorption mechanism is not limited to the monolayer structure. Therefore, the amount of oil adsorbed on rGO-Fe 2 O 3 adsorbents was the total of adsorption on all sites, with more substantial energy binding sites at the beginning and exponentially decaying as the adsorption process moved towards completion.
The parameters of the Freundlich isotherm model (Table 3) validate the excellent agreement between the model and the experimental results, indicating that oil adsorption on heterogeneous sites is the adsorption mechanism [6]. The heterogeneous nature of the adsorption process can be attributed to the different sizes of hydrocarbon molecules comprising the diesel oil and the presence of rGO and Fe 2 O 3 sites. The Freundlich parameter, n, was greater than 1, suggesting that the adsorption of emulsified oil onto rGO-Fe 2 O 3 -NC is favorable, and the mechanism is physical adsorption. The Freundlich model is the optimal isotherm to describe the physical adsorption mechanisms [50]. Moreover, the measured adsorption capacity was consistent with the model prediction, confirming that Freundlich is the correct isotherm for this process, indicating the multi-layer adsorption characteristic. Appl. Sci. 2022, 12, x FOR PEER REVIEW 9 of 13 adsorption on all sites, with more substantial energy binding sites at the beginning and exponentially decaying as the adsorption process moved towards completion. The parameters of the Freundlich isotherm model (Table 3) validate the excellent agreement between the model and the experimental results, indicating that oil adsorption on heterogeneous sites is the adsorption mechanism [6]. The heterogeneous nature of the adsorption process can be attributed to the different sizes of hydrocarbon molecules comprising the diesel oil and the presence of rGO and Fe2O3 sites. The Freundlich parameter, n, was greater than 1, suggesting that the adsorption of emulsified oil onto rGO-Fe2O3-NC is favorable, and the mechanism is physical adsorption. The Freundlich model is the optimal isotherm to describe the physical adsorption mechanisms [50]. Moreover, the measured adsorption capacity was consistent with the model prediction, confirming that Freundlich is the correct isotherm for this process, indicating the multi-layer adsorption characteristic. Practical applications require understanding the kinetics of the adsorption mechanism. The adsorption kinetic study is essential to evaluate the performance of a batch and continuous flow adsorber. Additionally, it can verify the solute uptake rate to assess the residence time needed to complete adsorption. Numerous mathematical models describe the adsorption kinetics, classified as adsorption reaction and diffusion models. This study employed the most widely used sorption reaction models, pseudo-first-order (Lagergren model) and pseudo-second-order kinetic models, using the accomplished experimental results.
The primary assumption of the pseudo-second-order kinetic model is that the ratelimiting step is potentially chemical adsorption, involving valent forces between the  Practical applications require understanding the kinetics of the adsorption mechanism. The adsorption kinetic study is essential to evaluate the performance of a batch and continuous flow adsorber. Additionally, it can verify the solute uptake rate to assess the residence time needed to complete adsorption. Numerous mathematical models describe the adsorption kinetics, classified as adsorption reaction and diffusion models. This study employed the most widely used sorption reaction models, pseudo-first-order (Lagergren model) and pseudo-second-order kinetic models, using the accomplished experimental results.
The primary assumption of the pseudo-second-order kinetic model is that the ratelimiting step is potentially chemical adsorption, involving valent forces between the adsorbate and adsorbent [51]. In Figure 4, the R 2 > 0.999 indicates that this kinetic model can explain nearly 100% of the difference in the response variable [51,52]. Therefore, the adsorption process of emulsified oil on rGO-Fe 2 O 3 follows pseudo-second-order kinetics. Thus, the adsorption process is hindered by intraparticle diffusion, boundary-layer, or external diffusion. The calculated adsorption rate was 1.5 × 10 −3 g adsorbent min·mg oil , and the predicted adsorption capacity was 661.6 mg/g.

Column (Breakthrough) Study
The study aimed to verify the recyclability of rGO-Fe 2 O 3 and establish the boundaries necessary to design manufacturing-scale adsorption columns. Several adsorptionregeneration breakthrough cycles were performed at 22 ± 1 • C using a column operating in the adsorption-desorption rhythm. Upon completing the breakthrough experiment, the adsorbed oil was recovered from rGO-Fe 2 O 3 using ethanol. All experiments were conducted at 22 ± 1 • C and a 3.3 mL/min emulsion flow rate. The mass of rGO-Fe 2 O 3 varied between 0.04 and 1 g, and the initial oil concentration was 100 ppm. When the rGO-Fe 2 O 3 adsorption saturated, 30 mL of ethanol was added into the column at 3.3 mL/min, followed by washing using water to eliminate residual ethanol. The emulsion was collected from the exit of the fixed-bed outlet every 10 min, the oil concentration was measured, and the breakthrough curves were generated ( Figure 5). The upper area above the breakthrough curve reflects the fixed-bed capacity and is given by [53]: where G is the flow rate (L/min), C 0 and C are the inlet and outlet concentration (mg/L), and t s is the time to complete saturation of the bed.
The study aimed to verify the recyclability of rGO-Fe2O3 and establish the boundaries necessary to design manufacturing-scale adsorption columns. Several adsorption-regeneration breakthrough cycles were performed at 22 ± 1 °C using a column operating in the adsorption-desorption rhythm. Upon completing the breakthrough experiment, the adsorbed oil was recovered from rGO-Fe2O3 using ethanol. All experiments were conducted at 22 ± 1 °C and a 3.3 mL/min emulsion flow rate. The mass of rGO-Fe2O3 varied between 0.04 and 1 g, and the initial oil concentration was 100 ppm. When the rGO-Fe2O3 adsorption saturated, 30 mL of ethanol was added into the column at 3.3 mL/min, followed by washing using water to eliminate residual ethanol. The emulsion was collected from the exit of the fixed-bed outlet every 10 min, the oil concentration was measured, and the breakthrough curves were generated ( Figure 5). The upper area above the breakthrough curve reflects the fixed-bed capacity and is given by [53]: where G is the flow rate (L/min), and are the inlet and outlet concentration (mg/L), and is the time to complete saturation of the bed. It is noted that removal efficiency (90%) did not reach 100 at the beginning of each cycle due to the short residence time (<1 min), which was due to the high flow rate. The effect of regeneration on the dynamic adsorption capacity of rGO-Fe2O3 is shown in Table  4. The capacity declined by 5.7% after the second cycle and 17% after the third cycle. Moreover, as shown in Table 4, the adsorption capacity of rGO-Fe2O3-NC was higher than that of GO. Regardless of the cycle number, the final capacity retention after three cycles of rGO-Fe2O3-NC was higher than GO [9]. Although there was a 17% reduction in adsorption capacity, the performance could be optimized by extruding the nanocomposite into porous pellets, a process routinely carried out for adsorbents and catalysts to preserve the mechanical integrity of the adsorbent. It is noted that removal efficiency (90%) did not reach 100 at the beginning of each cycle due to the short residence time (<1 min), which was due to the high flow rate. The effect of regeneration on the dynamic adsorption capacity of rGO-Fe 2 O 3 is shown in Table 4. The capacity declined by 5.7% after the second cycle and 17% after the third cycle. Moreover, as shown in Table 4, the adsorption capacity of rGO-Fe 2 O 3 -NC was higher than that of GO. Regardless of the cycle number, the final capacity retention after three cycles of rGO-Fe 2 O 3 -NC was higher than GO [9]. Although there was a 17% reduction in adsorption capacity, the performance could be optimized by extruding the nanocomposite into porous pellets, a process routinely carried out for adsorbents and catalysts to preserve the mechanical integrity of the adsorbent.

Conclusions
The sustainable, eco-friendly, cost-effective, and magnetic responsive nanocomposite was synthesized to remove dissolved oil in wastewater. Hematite (α-Fe 2 O 3 ) nanoparticles were incorporated into rGO to prepare the magnetic nanohybrid composite. The magnetic nanocomposite (rGO-Fe 2 O 3 ) was developed using a one-step hydrothermal method with 47 wt.% iron oxides. The batch and column adsorption tests were carried out under different industrial conditions to evaluate the adsorption capacity of the prepared nanocomposite for dissolved oil removal. rGO-Fe 2 O 3 exhibited extremely fast (within 30 min) and 90% removal of dissolved oil from its solution, having 100 mg/L concentrations with 3 mg of rGO-Fe 2 O 3 . The nanocomposite showed a maximum Langmuir adsorption capacity of 1301 mg/g using 200 mg/L of dissolved oil solution and 1 mg of rGO-Fe 2 O 3 . The adsorption capacity improved substantially when the wastewater salinity was increased compared to the 0 mg of NaCl/L solution. The adsorption kinetics followed the pseudosecond-order model and provided the best mathematical fit to the experimental data, and a good correlation was achieved with the intraparticle diffusion model.
The fixed-bed prototype with 0.2 g of rGO-Fe 2 O 3 was used in a breakthrough experiment that confirmed its ability to treat 12 L of a 100 mg/L emulsion. The removal efficiency was~90%, which is very promising because the residence time of the 3.3 mL/min emulsion stream was very short (<1 min). The nanocomposite was regenerated with ethanol and went through three cycles of adsorption-regeneration, with more than 80% retention of the adsorption capacity and near 100% retention of the removal efficiency. Overall, this nanocomposite had superior adsorption performance to other graphene nano-adsorbents, making rGO-Fe 2 O 3 an optimum choice for dissolved oil removal from high-salinity wastewater as the cost of large-scale GO production continues to decrease.
Author Contributions: A.D. was responsible for data curation, visualization, analysis, and writing the first draft; H.R.M. was responsible for analysis, writing, review, and editing; G.M. was responsible for analysis, writing-review, and editing; A.A. was responsible for conceptualization, methodology, analysis, supervision resources, writing-review and editing. All authors have read and agreed to the published version of the manuscript.