Fluorinated-Polyether-Grafted Graphene-Oxide Magnetic Composite Material for Oil–Water Separation

Abstract

In this study, a new type of highly efficient and recyclable magnetic-fluorine-containing polyether composite demulsifier (Fe₃O₄@G-F) was synthesized by the solvothermal method to solve the demulsification problem of oil–water emulsion. Fe₃O₄@G-F was successfully prepared by grafting fluorinated polyether onto Fe₃O₄ and graphene-oxide composites. Fe₃O₄@G-F was characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Taking the self-made crude-oil emulsion as the experimental object, the demulsification mechanism of the demulsifier and the influence of external factors, such as the temperature and pH value, on the demulsification performance of the demulsifier are discussed. The results show that the demulsification efficiency of the Fe₃O₄@G-F emulsion can reach 91.38% within 30 min at a demulsifier dosage of 750 mg/L, pH of 6, and a demulsification temperature of 60 °C. In neutral and acidic environments, the demulsification rate of the demulsifier is more than 90%. In addition, Fe₃O₄@G-F has been proven to have good magnetic effects. Under the action of an external magnetic field, Fe₃O₄@G-F can be recycled and reused in a two-phase system four times, and the demulsification efficiency is higher than 70%. This magnetic nanoparticle demulsifier has broad application prospects for various industrial and environmental processes in an energy-saving manner.

1. Introduction

As oil fields approach their later stages of exploitation, ensuring the continuous and stable production of crude oil is paramount. The implementation of tertiary oil recovery technologies, such as polymer flooding, has significantly enhanced the stability of crude-oil-produced fluid [1,2,3]. The improper treatment of oil–water emulsions may lead to elevated production costs, dimished quality of petroleum products, and severe ecological consequences that are difficult to predict [4,5]. Achieving efficient demulsification is an urgent problem for each oilfield due to the uncertainty and complexity of the composition of crude oil emulsions. The use of demulsifiers, which are amphiphilic and structurally adjustable chemical reagents, has become the most widely used demulsification method because of their convenient and efficient characteristics [6,7,8]. The distinctive nanoscale dimensions of nanoparticles and the magnetic characteristics of materials have garnered significant research attention, focusing on their application in demulsification. Researchers aim to diversify demulsifiers and enhance their performance by incorporating these unique features [9,10,11,12].

The properties of a compound change after it is combined with the fluorine element. The interaction between the fluorine atom and the carbon chain skeleton ensures that the compound has good friction resistance, weather resistance, thermodynamic stability, high surface activity, etc., so the fluorine-containing material has a unique role in coating, sterilization, cleaning, and medicine [13,14,15]. A fluorinated polyether demulsifier is used to introduce the fluorine element with high surface activity into the original polyether demulsifier. It was found that [16,17,18] a fluorinated polyether demulsifier can quickly reduce the oil–water interfacial tension and accelerate the aggregation of water droplets. Compared with traditional demulsifiers, it has better demulsification effects. Zhang et al. [19] prepared a series of novel fluorosilicone copolymers through atom-transfer radical polymerization (ATRP). The demulsification performance of the coal tar/water emulsion at low temperatures was evaluated using a bottle test. Studies have shown that the copolymer has little effect on the interfacial tension of the emulsion, but that it significantly reduces the elasticity of the interfacial film. The removal rate of low-temperature coal tar (LCT) in the emulsion is more than 90%, and the copolymer has a good demulsification performance. Wei et al. [20] synthesized an FB-series fluorinated polyether demulsifier via block polymerization. The results showed that at a demulsification temperature of 60 °C and an ffb4 fluorinated polyether demulsifier dosage of 100 mg/L, the overall dehydration rate (%) of 50 mL of crude oil emulsion in the Liaohe Oilfield reached 90.33% within 2 h, which was better than the demulsifier currently used in Liaohe crude oil. Therefore, the use of fluorine-containing surfactants and demulsifier-composite demulsification has become a research focus.

Fluorinated polyether demulsifiers have good demulsification performance, but they are expensive and toxic to the environment. The aim of this study was to synthesize an environmentally friendly magnetic fluorinated polyether demulsifier by introducing magnetic nanoparticles. Magnetic nano-demulsifiers can be separated from the emulsion under the action of an external magnetic field without causing secondary pollution and can be reused after washing [21,22]. In addition, the π–π bond formed on the surface of the demulsifier and the oil droplet accelerates oil droplet polymerization via the magnetic effect. Among various magnetic nanomaterials, Fe₃O₄ has become a research focus in recent years due to its low toxicity, easy availability in raw materials, relatively simple synthesis process, and excellent performance [23,24,25,26]. Azizi et al. [12] synthesized Fe₃O₄ magnetic nanoparticles (MNPs) through an electrochemical method and mixed them with commercially available demulsifiers to improve the demulsification of high asphaltene content (13 wt%). The experimental results showed that Fe₃O₄ magnetic nanoparticles can be circulated at least three times, the demulsification efficiency (%) can be increased by 10%, and the settling time is shortened by 6 h. The efficient demulsification ability and performance of Fe₃O₄ magnetic nanoparticles in W/O demulsification processes were revealed. Feng et al. [27] used shellfish-excited polydopamine as a binder to immobilize commercial polyether demulsifiers (AE1910, SP169, and AR321) on the surfaces of Fe₃O₄ nanoparticles to synthesize a simple new magnetic recoverable demulsifier. The demulsification efficiencies of the three nanoparticles in the crude oil emulsion were 98.00%, 91.63%, and 94.33%, respectively. Furthermore, Fe₃O₄@PDA@AE1910 can be used for eight cycles of water/n-decane emulsion, and the residual demulsification rate of crude oil emulsions can reach 77.81% after three cycles. After three cycles of Fe₃O₄@PDA@AE1910, the residual demulsification rate of crude oil emulsions can reach 77.81%. Evidently, these experimental demulsification results are gratifying. We speculate that this is due to the good synergistic demulsification effect of Fe₃O₄ with the three types of commercial polyether demulsifiers. The presence of polydopamine coating provides more sites for the grafting of polyether demulsifiers so that the polyether molecules are better attached to Fe₃O₄. Therefore, it is necessary to introduce a material that can provide more grafting sites. GO has a large specific surface area, excellent stability, and a large number of oxygen-containing active groups such as carboxyl, hydroxyl, and epoxy groups on the surface. It is easy to modify the surface using biologically active factors and is one of the most popular materials [28,29]. In the field of oil–water emulsion treatment, graphene oxide sheet is highly regarded as an amphiphilic surfactant due to its hydrophobic carbon skeleton and hydrophilic carboxyl and hydroxyl groups. By combining these properties, graphene oxide sheets can exhibit excellent oil–water separation performance. This is achieved via a synergistic demulsification effect when they are combined with other demulsification materials. Therefore, the utilization of graphene oxide sheets in conjunction with other demulsification materials has the potential to significantly enhance the efficiency of emulsion treatment processes [30,31,32,33].

This paper focuses on optimizing the performance of a new fluorine-containing block polyether demulsifier synthesized by the team [34] by incorporating graphene oxide and iron oxide. In order to achieve this, a magnetic demulsifier with properties, such as being environmentally friendly, having a high demulsification rate, and recyclability, was synthesized by grafting fluorinated polyether onto graphene oxide and combining it with ferric oxide. The structure and physicochemical properties of the demulsifier were characterized. Furthermore, the effects of factors such as demulsifier dosage, emulsion pH value, and temperature on demulsification were investigated using a bottle test. The optimal demulsification conditions were determined and the demulsification mechanism of Fe₃O₄@G-F was explored. Due to the reusability of the demulsifier, it can be used for crude oil demulsification multiple times while maintaining the performance of the demulsifier.

2. Experimental Section

2.1. Materials

Fluorinated polyether was prepared according to the report [35]. Fe(acac)₃ was purchased from Shanghai Aladdin Industrial Co., Ltd., Shanghai, China. Oleic acid, oleylamine, and graphene were purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). Benzyl Ether was purchased from Merck Chemical (Shanghai, China). All chemicals used in the production process were of analytical grade. The dehydrated crude oil was supplied by a heavy oil block in Liaohe Oilfield.

2.2. One-Pot Preparation of Magnetic Demulsifiers

The preparation process of the fluorinated polyether and Fe₃O₄@F can be found in the attachment. Fe₃O₄@G-F was prepared using a one-pot method (Figure 1). Firstly, 0.001 mol iron acetylacetonate, 0.003 mol oleic acid, 0.003 mol oleylamine, 5 g fluorinated polyether, 30 mL dibenzyl ether solvent, and 0.05 g/0.10 g/0.15 g/0.20 g GO were added to the beaker. Subsequently, the material in the beaker was stirred evenly and ultrasonically dispersed for 30 min. The reagents in the beaker were poured into the reactor, and the air in the reactor was removed using nitrogen. The reactor was then placed in a muffle furnace at 230 °C for 5 h. After the heat-preservation experiment, the reactor was taken out and placed in the air for natural cooling. After the reaction, the product in the kettle was taken out, the magnetic demulsifier product was separated by a magnet, and a 1:1 volume mixture of ethanol and n-hexane was added to the product to clean the magnetic demulsifier product 3–5 times. Finally, the product was dried at 60 °C for 12 h to obtain a magnetic demulsifier. The samples with GO content of 0.05 g, 0.1 g, 0.15 g, and 0.2 g were recorded as Fe₃O₄@G1-F, Fe₃O₄@G2-F, Fe₃O₄@G-F, and Fe₃O₄@G4-F, respectively.

2.3. Preparation of Crude Oil Emulsion

The oil–water emulsion with a mass fraction of 50% was prepared. NaCl was dissolved in deionized water (0.5 mol/L, NaCl) to prepare the brine, and then 100 mL of crude oil and 100 mL of brine were heated to 60 °C, respectively. The brine was added to the preheated distilled water three times and stirred at 11,000 rpm for 30 min to obtain a stable oil–water emulsion.

2.4. Demulsification Performance Test

According to SY-T 5281-2000, the demulsification performance of Fe₃O₄@G-F was evaluated using a bottle test. First, the pre-prepared crude oil emulsion was packaged separately, and each test tube was filled with 20 mL of the emulsion and then placed in a water bath at 60 °C for 5 min to prevent the solidification of droplets in the emulsion. Then, according to the experimental requirements, the pre-configured demulsifier was added without adding the control group. The test tube was shaken back and forth 100 times within 1 min to completely mix the demulsifier with the crude oil emulsion. Finally, the test tube was placed in a 60 °C water bath for static sedimentation, and the demulsification effect was observed after 5 min, 10 min, 15 min, 20 min, 25 min, and 30 min, respectively. The demulsification performance is expressed by the following equation:

$$R=\frac{C_0-C}{C_0}\times 100\%$$

where R (%) is the oil removal ratio, $C_0$ (mgL⁻¹) represents the initial oil content of the emulsions, and C (mgL⁻¹) is the oil content during the process of demulsification.

2.5. Demulsification Recovery Test

The recyclability of the Fe₃O₄@G-F magnetic demulsifier was evaluated by repeated use times. After the demulsification experiment, the obtained reagent was placed in an external magnetic field to recover the Fe₃O₄@G-F magnetic demulsifier. The recovered product underwent a series of washing steps, which included washing with n-hexane, followed by an ultrasonic cleaner for 15 min. After the demulsifier was fully dispersed, it was recovered by an external magnetic field and washed 3-4 times until the solution was colorless and transparent. Finally, the product was dried in a vacuum oven at 60 °C for 6 h, and the next experiment was repeated.

3. Results and Discussion

3.1. Characterization of Modified Fluoropolyether

3.1.1. Infrared Spectra of Different Materials

Figure 2 shows the infrared spectra of graphene oxide, Fe₃O₄, and their composites. The infrared spectrum was collected using the KBr tabletting method. The scanning number was set to 32, the range of spectral acquisition was 400~4000 cm⁻¹, and the optical frequency was 1 cm⁻¹. It can be seen from the spectrum comparison that the Fe₃O₄@GO composite was similar to GO. There was a stretching vibration absorption peak of -OH at 3375 cm⁻¹, a stretching vibration peak of C=O at 1725 cm⁻¹, and a bending vibration peak of C=C at 1631 cm⁻¹. There were stretching vibration absorption peaks of C-OH and bending vibration peaks of C-O-C were observed at 381 cm⁻¹ and 1052 cm⁻¹, respectively. However, the C-O-C stretching vibration absorption peak of GO at 1052 cm⁻¹ disappeared in Fe₃O₄@GO, and the characteristic peak intensity of the other functional groups was significantly weakened. The characteristic stretching vibration peak of Fe-O-Fe was observed at 556 cm⁻¹ in the Fe₃O₄ spectrum. The characteristic stretching vibration peak of Fe-O-Fe can be clearly seen in the Fe₃O₄/GO spectrum at 523 cm⁻¹, which has a red shift, indicating that Fe₃O₄ may be compounded with GO by a coordination reaction [35,36]. In the FTIR spectrum of Fe₃O₄@G-F, the tensile vibration peaks of CH and CH₂ appeared at 2912 cm⁻¹ and 2840 cm⁻¹, respectively, and the tensile vibration absorption peak of the C-F bond appeared at 1186 cm⁻¹. The spectral results showed that the surface of Fe₃O₄@G was grafted with fluorinated polyether, and Fe₃O₄@G-F was successfully prepared.

3.1.2. XPS of Modified Fluorinated Polyether

The valence state and chemical composition of Fe₃O₄@G-F were further studied using XPS. Figure 3 shows the full XPS full spectrum of Fe₃O₄@G-F and the Fe 2p, O 1s, and C 1s peak diagrams. In the full spectrum of Fe₃O₄@G-F, the signals of Fe, F, O, and C elements appeared at about 711.0 eV, 683.8 eV, 532.6 eV, and 285.1 eV, respectively. It can be seen from the Fe2p spectrum in Figure 3b that the two dominant peaks at 710.4 and 726.3 eV belong to Fe 2p_3/2 and Fe 2p_1/2 spin-orbit peaks, respectively. At the same time, the peaks at 712.4 eV and 726.0 eV are consistent with the Fe³⁺ ions of the Fe-O bond. The other two peaks at 710.2 eV and 723.6 eV were assigned to Fe²⁺ ions [37]. In the C1s peak spectrum (Figure 3c), C1s can be divided into three strong peaks and one weak peak, which are C-C/C-H, C-OH, epoxy group, and O=C=O, respectively. The two peaks in Figure 3d correspond to C=O and C-O, respectively. This result is in excellent correspondence with the elemental composition of the sample obtained from the infrared spectrum. Spectroscopic and crystallographic evidence further confirmed the successful combination of the fluorinated polyether, Fe₃O₄, and GO.

3.1.3. TEM and SEM of Composite Materials

The SEM and TEM images of the composites are shown in Figure 4. Figure 4a demonstrates that the morphology of the Fe₃O₄@F particles is irregular and the agglomeration between particles is significant. It was further observed by TEM that the Fe₃O₄ nanoparticles were tightly wrapped by outer granular fluorinated polyether molecules. Consequently, it can be inferred that the encapsulation of fluorinated polyether molecules led to different aggregation sizes and adhesion of the Fe₃O₄@F particles. Comparing Fe₃O₄@G-F with different GO (Figure 4e–f), it can be seen that Fe₃O₄@G-F is a stacked nanosheet with a smooth surface and obvious wrinkles at the edge of the sheet. Fe₃O₄@G2-F consists of stacked nanoflakes, and the crystal is slightly rough without obvious edge folds. The surfaces of the Fe₃O₄@G1-F and the Fe₃O₄@G4-F are not smooth and are attached to Fe₃O₄@F particles, without obvious edge wrinkles. There are obvious differences in the morphology of Fe₃O₄@G-F with different GO content. This is due to the lack of polymerization sites with fluorinated polyether and Fe₃O₄ when the amount of GO added is small, resulting in the three materials not being fully compounded. The excessive addition of GO causes graphene to stack, leading to a lack of polymerization sites. Further observation of Fe₃O₄@G-F (Figure 4b) by TEM shows that fluorinated polyether molecules are uniformly dispersed in GO, and most of the orthorhombic Fe₃O₄ is attached to the lamellar folds and corners of graphene oxide. This is because the surface area of the GO wrinkle is large, which is beneficial for the attachment of the magnetic Fe₃O₄ nanoparticles. Therefore, we can conclude that the optimal addition amount of GO in Fe₃O₄@G-F is 0.15 g.

3.1.4. TGA of Modified Fluoropolyether

The thermal properties of various magnetic composites were studied using TGA analysis. From Figure 5, it can be seen that at 50 °C–100 °C, each composite material has a weight loss rate of 2%, which is due to the water in the air adsorbed by the nanoscale ferroferric oxide. The weight loss behavior of Fe₃O₄@F at 328 °C–406 °C is related to the pyrolysis of the polymer chain of the fluorinated polyether. The relatively slight weight loss at 400 °C to 500 °C is due to the partial self-condensation of iron hydroxide. Compared to Fe₃O₄@F, the thermal stability of the modified fluorinated polyether decreased. The modified fluorinated polyether has two obvious weight loss stages. The first stage of weight loss occurs at 170 °C–250 °C. This is mainly due to the cracking of oxygen-containing functional groups in graphene oxide to produce gas, and the large amount of heat released during the cracking process further aggravates the cracking reaction. The second stage occurs between 328 °C and 406 °C. The weight loss in this stage is not only related to the pyrolysis of the polymer chain of the fluoropolyether ether but also includes the oxidation and evaporation of carbon in GO. According to the thermogravimetric curves of magnetic fluoropolyethers with different GO contents, thermal weight loss is the most obvious when the GO content is 0.15 g. Insufficient amounts of added GO do not significantly decrease thermal weight loss. Excessive amounts of added GO cause graphene sheets to stack due to strong van der Waals attraction, increasing stability. The discussion that the optimal addition amount of GO in the modified fluoropolyether is 0.15 g was further verified. The subsequent performance tests of the demulsifiers were all based on Fe₃O₄@G-F.

3.1.5. VSM of Modified Fluorinated

The magnetic properties of Fe₃O₄ and its composites were tested using a VSM magnetization loop test. It can be seen from Figure 6 that Fe₃O₄, Fe₃O₄@G, Fe₃O₄-F, and Fe₃O₄@G-F all exhibit superparamagnetic behavior, and their magnetic saturation values are 65.9, 41.05, 45.8, and 35.6 emu/g, respectively. The magnetic saturation values of the composites decreased due to the magnetic field-shielding effect of the fluorinated polyether and GO. Among all types of Fe₃O₄ composites, Fe₃O₄-F has the highest magnetic saturation value since the smaller fluorinated polyether molecules have little effect on the magnetic response of Fe₃O₄. Despite a decrease in magnetic saturation, the Fe₃O₄@G-F composite demulsifier retains its superparamagnetic properties at room temperature. This allows for the effortless separation of the demulsifier from the emulsion using an external magnetic field.

3.2. The Demulsification Performance of Modified Fluorinated Polyether

3.2.1. Effect of Demulsifier Dosage on Demulsification Performance

Figure 7 shows the effect of different doses of the Fe₃O₄@G-F magnetic demulsifier (150, 300, 450, 600, 750, and 900 mg/L) on the demulsification performance of the crude oil emulsion at an experimental temperature of 60 °C. It can be seen from Figure 7a that the demulsification performance is the best when the dosage of Fe₃O₄@G-F magnetic demulsifier is 750 mg/L, and the demulsification rate can reach 90.54%. After the addition of the Fe₃O₄@G-F magnetic demulsifier, the demulsification efficiency was more than 50% within 5 min. After demulsification, the separated water has high transparency, and there is a small amount of crude oil sticking to the wall, with an obvious phase separation boundary. This indicates that the Fe₃O₄@G-F magnetic demulsifier is a very effective demulsifier for oily wastewater. When the amount of demulsifier is 0–750 mg/L, the demulsification rate increases with an increase in the demulsifier dose. When the amount of demulsifier is more than 750 mg/L, the demulsification rate decreases with an increase in the amount of demulsifier. This is due to the amphiphilicity of Fe₃O₄@G-F. At a certain mass fraction, the demulsifier molecules become saturated and reach their maximum adsorption capacity at the oil–water interface. The excessive demulsifier molecules continue to migrate to the oil–water interface, resulting in emulsification. Thickening the oil–water interface film enhances emulsion stability but reduces demulsification efficiency. Therefore, the optimum dosage of Fe₃O₄@G-F magnetic demulsifier is 750 mg/L.

3.2.2. Effects of Time and Temperature on Demulsification Performance

Four groups of control experiments at different temperatures (35 °C, 45 °C, 60 °C, and 75 °C) were set up to observe the demulsification effect of Fe₃O₄@G-F within 30 min when the addition amount was 750 mg/L. The demulsification effect of the Fe₃O₄@G-F magnetic demulsifier at different times and different temperatures is shown in Figure 8. It can be observed from the figure that the demulsification efficiency increases with an increase in the demulsification time and demulsification temperature. When the demulsification temperature is 75 °C and the action time is 30 min, the demulsification efficiency reaches the highest value of 91.02%. When the demulsification temperature is 60 °C and the action time is 25 min, the demulsification efficiency is 90.03%. When the action time reaches 25 min, the solution becomes clear and the oil droplets hang less. With an increase in the action time, the change in solution clarity is not obvious. The horizontal comparison curve shows that although extending the action time to 30 min improves the demulsification efficiency of the demulsifier, the increase is not significant. Evidently, increasing the temperature and prolonging the action time increases the production cost. Therefore, from the perspective of engineering applications, the action time of the Fe₃O₄@G-F magnetic demulsifier is set to 25 min, and the temperature is set to 60 °C.

3.2.3. Effect of pH on Demulsification Performance

Figure 9 shows the effect of different pH values on the demulsification performance of Fe₃O₄@G-F at an experimental temperature of 60 °C and an additional amount of Fe₃O₄@G-F at 750 mg/L. When pH = 6, Fe₃O₄@G-F has the best demulsification performance of up to 91.38%. The demulsification rate was 89.43% at pH = 5, and the demulsification rate exceeded 85% under neutral conditions, indicating that the demulsifier had better performance in neutral and acidic environments. This is because the Fe₃O₄@G-F surface carries a positive charge under acidic and neutral conditions. The negative charge on the oil droplets attracts Fe₃O₄@G-F, promoting the adsorption of demulsifier molecules. This aids in the coalescence and separation of the oil and water. However, the demulsification rate was low at pH 8 and 9, and the demulsification rate did not exceed 85%. This is because, under alkaline conditions, the negative charge on the surface of Fe₃O₄@G-F and the oil droplets undergo electrostatic repulsion, which reduces the adsorption capacity of the demulsifier and leads to a decrease in the demulsification rate.

3.2.4. Recovery Performance Test

The experimental temperature of the recycling test was 60 °C, the amount of demulsifier was 750 mg/L, and the action time was 25 min. The results are shown in Figure 10. Although the demulsification efficiency of the Fe₃O₄@G-F decreased with an increase in the number of cycles, it showed excellent demulsification performance in the first four cycles, and the demulsification rate exceeded 70%. The demulsification efficiency of the demulsifier sharply decreased after six cycles, reaching only 16.91% and 9.8% in the 7th and 8th cycles, respectively. This may be due to the fact that the asphalt and resin in the crude oil are adsorbed on the surface of the demulsifier molecule during the demulsification process. The n-hexane solvent cannot dissolve all the components of the crude oil, and the decrease in the magnetic response of Fe₃O₄@G-F leads to a decrease in the recovery rate. In summary, the Fe₃O₄@G-F magnetic demulsifier not only has excellent demulsification performance but can also can be recycled from oil–water two-phase system due to its unique magnetic response performance. It solves a series of environmental problems caused by the difficulty in separating traditional chemical demulsifiers after the reaction.

3.3. The Demulsification Mechanism of Fe₃O₄@G-F

Here, we propose a demulsification mechanism of the Fe₃O₄@G-F magnetic demulsifier, as shown in Figure 11. First, the Fe₃O₄@G-F magnetic demulsifier has ultra-high surface activity due to the presence of C-F bonds. After Fe₃O₄@G-F was added to the crude oil emulsion, it could quickly reach the oil–water interfacial film, replacing the natural emulsifiers such as asphaltenes and resins on the oil–water interfacial film to form a new easily broken interfacial film. The surface of the demulsifier contains a dendritic fluorine-containing polyether, which acts as a bridge for promoting the coalescence of oil droplets via a pulling effect. When oil droplets accumulate to a certain extent, they float to the upper part of the test tube under the action of gravity to achieve the separation of the oil phase and water phase. Second, GO can produce strong π-π interactions with asphaltenes, resins, and other organic compounds by virtue of its conjugated aromatic ring structure, so the demulsifier can be adsorbed to the oil droplets in the emulsion. Then, the hydrophobic Fe₃O₄ carries positive charges under acidic and neutral conditions. The adsorption capacity of Fe₃O₄@G-F was enhanced by the electrostatic interactions between Fe₃O₄ and negatively charged oil droplets. In addition, the nanoscale GO and Fe₃O₄ particles have the characteristics of a large specific surface area and many adsorption sites, and the two can have a better synergistic effect. Finally, Fe₃O₄@G-F has magnetic responsiveness. Under the action of an external magnetic field, it can be recovered from the two-phase system and recycled after cleaning.

4. Conclusions

An efficient and recyclable magnetic fluorinated polyether demulsifier (Fe₃O₄@G-F) was prepared using a solvothermal method. The demulsification performance of the magnetic composite materials in different external environments was compared using the bottle test method, and the optimal demulsification conditions were optimized. The higher demulsification rate of Fe₃O₄@G-F was due to the synergistic demulsification effect of GO and Fe₃O₄. On the other hand, due to the existence of the C-F bond in Fe₃O₄@G-F, the demulsifier has an ultra-high surface activity. Fe₃O₄@G-F can quickly reach the oil–water interface film and replace natural emulsifiers, such as asphaltenes and resins on the oil–water interface film to form a new fragile interface film. The dendritic fluorine-containing polyether on the surface of the demulsifier can act as a bridge to promote the coalescence of oil droplets via the pulling effect. The demulsification performance of Fe₃O₄@G-F was greatly affected by the pH, which is suitable for neutral and acidic environments. The results showed that the demulsification efficiency of the Fe₃O₄@G-F emulsion can reach 91.38% within 30 min at a demulsifier dosage of 750 mg/L, pH of 6, and demulsification temperature of 60 °C. In addition, Fe₃O₄@G-F was recovered and recycled from the two-phase system four times under the action of an external magnetic field. Fe₃O₄@G-F provides a new approach for the preparation of fluorinated high-efficiency demulsifiers and the treatment of environmental problems caused by crude oil pollution.