polymers Article Facile Fabrication of Superhydrophobic Graphene/Polystyrene Foams for Efficient and Continuous Separation of Immiscible and Emulsified Oil/Water Mixtures Chunxia Zhao 1,2, * , Haoran Huang 1 , Jiaxin Li 1 , Yuntao Li 1,3, *, Dong Xiang 1,2 , Yuanpeng Wu 1,2,3 , Ge Wang 1 and Mingwang Qin 4 1 2 3 4 * Citation: Zhao, C.; Huang, H.; Li, J.; Li, Y.; Xiang, D.; Wu, Y.; Wang, G.; Qin, M. Facile Fabrication of Superhydrophobic Graphene/Polystyrene Foams for Efficient and Continuous Separation of Immiscible and Emulsified Oil/Water Mixtures. Polymers 2022, 14, 2289. https://doi.org/10.3390/ polym14112289 Academic Editor: Jin-Hae Chang Received: 25 April 2022 School of New Energy and Materials, Southwest Petroleum University, Chengdu 610500, China; HuangHaoran0805@163.com (H.H.); ljx991022@sina.com (J.L.); dxiang01@hotmail.com (D.X.); ypwu@swpu.edu.cn (Y.W.); wangge12335@126.com (G.W.) The Center of Functional Materials for Working Fluids of Oil and Gas Field, Sichuan Engineering Technology Research Center of Basalt Fiber Composites Development and Application, Southwest Petroleum University, Chengdu 610500, China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China School of Engineering, Southwest Petroleum University, Nanchong 637001, China; QinMingwang123@126.com Correspondence: polychem2011@hotmail.com (C.Z.); yuntaoli@swpu.edu.cn (Y.L.) Abstract: Three-dimensional superhydrophobic/superlipophilic porous materials have attracted widespread attention for use in the separation of oil/water mixtures. However, a simple strategy to prepare superhydrophobic porous materials capable of efficient and continuous separation of immiscible and emulsified oil/water mixtures has not yet been realized. Herein, a superhydrophobic graphene/polystyrene composite material with a micro-nanopore structure was prepared by a singlestep reaction through high internal phase emulsion polymerization. Graphene was introduced into the polystyrene-based porous materials to not only enhance the flexibility of the matrix, but also increase the overall hydrophobicity of the composite materials. The resulting as-prepared monoliths had excellent mechanical properties, were superhydrophobic/superoleophilic (water/oil contact angles were 151◦ and 0◦ , respectively), and could be used to continuously separate immiscible oil/water mixtures with a separation efficiency that exceeded 99.6%. Due to the size-dependent filtration and the tortuous and lengthy micro-nano permeation paths, our foams were also able to separate surfactant-stabilized water-in-oil microemulsions. This work demonstrates a facile strategy for preparing superhydrophobic foams for the efficient and continuous separation of immiscible and emulsified oil/water mixtures, and the resulting materials have highly promising application potentials in large-scale oily wastewater treatment. Accepted: 2 June 2022 Published: 5 June 2022 Keywords: graphene; polystyrene; superhydrophobic; porous materials; oily wastewater treatment Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1. Introduction Treatment of oily wastewater has attracted growing attention from the academic and industrial sectors in recent years due to the growing global environmental hazards associated with the discharge of industrial and domestic sewage as well as oil spills [1,2]. Traditional disposal techniques used to treat oily wastewater, such as gravity-driven separation, oil skimmers, in situ burning, methods that rely on centrifugal separation, dispersion with chemical reagents and microbial treatments, have low efficiencies, high costs and can even cause secondary pollution to the environment [3–5]. Moreover, treatment of oily wastewater composed of surfactant-stabilized emulsions is even more challenging [6–10]. Therefore, novel techniques and materials that efficiently separate immiscible and emulsified oil/water mixtures are needed for practical use in wastewater treatment. Polymers 2022, 14, 2289. https://doi.org/10.3390/polym14112289 https://www.mdpi.com/journal/polymers Polymers 2022, 14, 2289 2 of 14 The high porosity, interconnected pore structure and unique wettability of porous materials make them highly valued in oily wastewater treatment [11,12]. Moreover, the surface wettability of porous materials can be adjusted to tune their selectivity and separation ability for oil/water mixtures [13–20]. For example, Li et al. [21] modified Fe3 O4 nanoparticles with siloxanes and then used the modified nanoparticles to adjust the hydrophobicity of polyurethane foams by a simple drop-coating method. The resulting magnetic materials were able to remotely magnetically absorb oil from water and achieve gravity-driven oil/water separation. Kang et al. [22] prepared a superhydrophobic/superoleophilic material based on the surface modification of wood fibers that showed excellent performance in oil/water separations. Moreover, the prepared material could be used in multiple environments and achieved highly efficient absorption- and filtration-based separation of oil/water mixtures. While these materials were able to absorb oil from wastewater, treating large-scale oil spills with such types of oil sorbents is a time-consuming and labor-intensive task. A large amount of absorbent material is often required because the absorbents have a limited capacity for the oils [23]. Moreover, absorbent-based separations are not scalable for largescale oily wastewater treatment because the oil/water mixtures must be collected before it can be filtered [24–27]. Therefore, in order to simplify the separation process, it is necessary to design and manufacture devices based on oil absorbing materials that can continuously and efficiently handle oily wastewater. Towards this goal, Ge et al. developed an in situ oil collection device that was aided by the application of external force [28]. Such continuous oil collection processes require oil absorbing materials with high mechanical performances as well as excellent hydrophobic and lipophilic properties [29]. Many strategies have been developed for the modification of commercial melamine foams, polyurethane foams or metal foams (pore sizes ranging from 100 to 500 µm) to create materials with the desired characteristics for the continuous clean-up of large-scale oil spills [22,30–32]. Although the design of these superwetting foams is exquisite, the pore structure of the matrix material is not sufficient to act as particle sieves and separate emulsions, especially emulsions with particle sizes less than 20 µm [33–35]. On the other hand, the surface modification of the matrix material requires either complicated procedures or expensive equipment, and even fluorochemicals are utilized to reduce the surface energy, causing secondary environmental pollution. Therefore, the exploration of new strategies for the preparation of superwettable materials with controlled pore structures is needed to realize the continuous and efficient separation of immiscible and emulsified oil/water mixtures. In recent years, the high internal phase emulsion (HIPE) templating method has been studied extensively to prepare polymer-based porous materials with adjustable pore structures and porosities [36–43]. However, the HIPE templating method is typically performed using rigid monomers, and the resulting porous materials have poor mechanical properties in that they are highly brittle and easily pulverized [44,45]. In addition, the hydrophobic properties of such materials need to be further improved for practical applications in handling oily wastewater [46]. To address these limitations, here we prepare graphene/polystyrene (GN/PSt) porous materials via the HIPE templating method. The introduction of graphene simultaneously improves the flexibility and enhances the hydrophobic properties of the polymer-based matrix. The as-prepared GN4 /PSt superhydrophobic porous materials achieved in situ continuous absorption of heavy and light oils from water with separation efficiencies that exceeded 99.6%. Moreover, the micro-nanopore structure and wettability of the prepared materials allowed for the efficient and continuous separation of surfactant-stabilized water-in-oil microemulsions. These advantages demonstrate the potential value of our prepared foams for the large-scale and continuous separation of immiscible and emulsified oil/water mixtures. 2. Materials and Methods 2.1. Materials Styrene (St) and Span 80 were obtained from Innochem Technology Co., Ltd., Beijing, China. Divinylbenzene (DVB), sodium persulfate (Na2 S2 O8 ) and sodium dodecyl 2. Materials and Methods 2.1. Materials Polymers 2022, 14, 2289 Styrene (St) and Span 80 were obtained from Innochem Technology Co., Ltd., Beijing, China. Divinylbenzene (DVB), sodium persulfate (Na 2S2O8) and sodium dodecyl sul3 of 14 fonate (SDS) were obtained from Aldrich, Shanghai, China. Graphene (GN, lamellae size 5–15 µ m) was supplied by Deyang Carbonene Technology Co., Ltd., and the lamellae were composed of five to six layers of graphene. Absolute ethanol, petroleum ether, chlosulfonate (SDS) were obtained from Aldrich, Shanghai, China. Graphene (GN, lamellae size roform, toluene and acetone were all purchased from Chengdu Kelong Chemical Reagent 5–15 µm) was supplied by Deyang Carbonene Technology Co., Ltd. (Deyang, China), and Factory. Ultrapure deionized water from an ultrapure water machine was used to prepare the lamellae were composed of five to six layers of graphene. Absolute ethanol, petroleum all ether, solutions. chloroform, toluene and acetone were all purchased from Chengdu Kelong Chemical Reagent Factory. Ultrapure deionized water from an ultrapure water machine was used to 2.2.prepare Fabrication of GN/PSt Foams all solutions. In a typical preparation process, 0.02 g of Na2S2O8, 0.004 g of SDS and 0.02 g of GN were ultrasonically dispersed in 20 mL deionized water to obtain a homogeneous soluIn a typical preparation process, 0.02 g of Na S O8 , 0.004 g of SDS and 0.02 g of GN tion. This solution was then gradually added to2 a2 beaker containing 0.3 g of St, 0.2 g of were ultrasonically dispersed in 20 mL deionized water to obtain a homogeneous solution. DVB and 0.15 g of Span 80, and the mixture was stirred at 200 rpm until a homogeneous This solution was then gradually added to a beaker containing 0.3 g of St, 0.2 g of DVB and viscous was which the at HIPE pre-polymerization mixture. and 0.15 g solution of Span 80, andformed, the mixture waswas stirred 200 rpm until a homogeneous and The beaker was sealedwas andformed, kept atwhich 65°C for h to allow the polymerization reaction to proceed. viscous solution was8the HIPE pre-polymerization mixture. The beaker ◦ After polymerization, the product was washed with absolute ethanol to remove the Span was sealed and kept at 65 C for 8 h to allow the polymerization reaction to proceed. After product was washed with absolutegray ethanol to remove thedried Span 80 80 polymerization, and unreactedthe organic monomers. The obtained monolith was at and 50 °C for organic The obtained gray monolith was dried at 50 ◦ C for 10 h in a 10 unreacted h in a blast oven.monomers. The prepared superhydrophobic polystyrene-based porous material blast oven. The prepared superhydrophobic polystyrene-based porous material containing containing 4 wt% GN was labeled as GN4/PSt. Samples containing 0 wt%, 2 wt%, 8 wt% 4 wt% GN was labeled as GN4 /PSt. Samples containing 0 wt%, 2 wt%, 8 wt% and 10 wt% and 10 wt% GN were prepared following the same protocol and were labelled as GN0/PSt, GN were prepared following the same protocol and were labelled as GN0 /PSt, GN2 /PSt, GNGN 2/PSt, GN8/PSt and GN10/PSt, respectively. The fabrication process is schematically il8 /PSt and GN10 /PSt, respectively. The fabrication process is schematically illustrated lustrated in1.Figure 1. in Figure 2.2. Fabrication of GN/PSt Foams Figure 1. 1. Schematic of the thefabrication fabricationofof superhydrophobic GN/PSt foams. Figure Schematicillustration illustration of superhydrophobic GN/PSt foams. Characterization 2.3.2.3. Characterization The microscopic pore structures in the prepared materials were characterized with The microscopic pore structures in the prepared materials were characterized with scanning electron microscopy (SEM, JEOL JSM-5009LV), and the pore size distribution scanning electron microscopy (SEM, JEOL JSM-5009LV), and the pore size distribution and porosity of the GN4 /PSt composites were determined using an automatic mercury and porosity of the 9500). GN4/PSt were determined usingangles an automatic porosimeter (Mike Thecomposites water contact angles (WCA), rolling (RA) andmercury oil porosimeter (Mike 9500). The water contact angles (WCA), rolling angles and oil contact angles (OCA) were characterized using a contact angle goniometer (OCA (RA) 25, Data contact angles (OCA) were characterized using a contact angle goniometer (OCA 25, Data physics Instruments GmbH, Filderstadt, Germany). Contact angle measurements were made by dropping 3 µL of deionized water or petroleum ethermeasurements on three different locations physics Instruments GmbH, Germany). Contact angle were made by of each sample surface, and the average value is presented. The compressive properties of the porous materials were evaluated using a universal material testing machine (CMT4104, Polymers 2022, 14, 2289 4 of 14 MTS, USA) with a compression rate of 2 mm/min. Samples used in the compressive tests were cylindrical with a height of 20 ± 2 mm and a diameter of 24 mm. 2.3.1. Oil Absorption Capacity The saturated oil absorption capacity (k) of the GN4 /PSt superhydrophobic porous material was calculated according to Equation (1) [12]: k= m1 − m0 m0 (1) where m0 is the original mass of the GN4 /PSt porous material, and m1 is the mass of the GN4 /PSt porous material saturated with oil. 2.3.2. Oil/Water Mixture Separation Efficiency The model oil/water mixtures were continuously separated using the prepared porous materials with the aid of an external pump, and the separation efficiency (η) was calculated following Equation (2): ma η = × 100% (2) mb where mb and ma denoted the water mass before and after separation, respectively. 2.3.3. Emulsified Oil/Water Mixture Separation Experiments Three surfactant-stabilized water-in-oil (W/O) emulsions (water/petroleum ether; water/toluene; water/chloroform) were prepared to simulate stable oil/water mixtures found in real world applications. The emulsions were prepared by mixing water and oil (Vwater :Voil = 1:100) with 1 g/L Span 80 under vigorous stirring for 1 h. The resulting emulsions were stable for at least 24 h. Continuous oil/water emulsion separation experiments were performed with the aid of an external power source. Microscopic images of the emulsions before and after separation with the GN/PSt composites were taken with an eyepiece inverted fluorescent digital microscope (AMG EVOSFL, USA). The separation efficiency (E) of the oil/water emulsion was calculated according to Equation (3) [47]:   Cs E = 1− × 100% (3) C0 where C0 and Cs represent the moisture content in the emulsion and the filtrate, respectively, and the moisture contents in the oil were determined using a Karl Fischer moisture meter (Mettler V10S). 3. Results and Discussion 3.1. Characterizations of the GN/PSt Composites During HIPE, water was dispersed in an organic continuous phase consisting of St and DVB, forming a water-in-oil emulsion stabilized by the lipophilic surfactant, Span 80, co-Pickering-surfactant and GN nanosheets. The organic phase, St and DVB, was then polymerized to form the skeleton of the porous material [48], and the water phase was removed from the pore cavities to give the GN/PSt porous materials with high porosities and open pore structures (Figure 2). As seen in the SEM images in Figure 2a, the GN/PSt composites contained interconnected polymer walls that formed large spherical pores (Figure 2a), and many smaller pores were present within the polymer skeleton (Figure 2b) [49]. GN randomly penetrated into the matrix of the porous materials, resulting in a rougher microstructure and lower surface energy (Figure 2c–f) [50]. Similar interconnected porous structures were seen in the GN0 /PSt, GN2 /PSt and GN4 /PSt composites, which should be advantageous for the better transfer of substances through the porous materials when the monoliths are used to separate oil/water mixtures. Meanwhile, a more closed-cell structure Polymers 2022, 14, 2289 interconnected porous structures were seen in the GN0/PSt, GN2/PSt and GN4/PSt compo sites, which should be advantageous for the better transfer of substances through the po 5 of 14 rous materials when the monoliths are used to separate oil/water mixtures. Meanwhile, a more closed-cell structure was seen in the GN/PSt composites prepared with 8% and 10% GN (Figure 2g–j) because the GN agglomerated and was harder to disperse at these high was seen in the GN/PSt composites prepared with 8% and 10% GN (Figure 2g–j) because concentrations and such a closed cell structure is not conducive to the transfer of matte the GN agglomerated and was harder to disperse at these high concentrations and such a through materials. closed cellthe structure is not conducive to the transfer of matter through the materials. Figure2.2. SEM SEM images imagesof ofthe theGN GN00/PSt /PSt (a,b), (a,b), GN GN22/PSt /PSt (c,d), GN44/PSt /PSt (e,f), Figure (c,d), GN (e,f), GN GN88/PSt /PSt(g,h) (g,h) and and GN10/PS (i,j)10composites. GN /PSt (i,j) composites. Moreover, composite are shown in 4 /PSt Moreover,the theRaman Ramanspectra spectraofofGN GNand andthe theGN GN 4/PSt composite are shown in Figure Figure S1. Two obvious peaks corresponding to D and G bands appeared at approxiS1. Two obvious peaks corresponding to D and G bands appeared at approximately 1358 mately 1358 cm−1 and 1586 cm−1 , respectively. The G band was generally associated with cm−1 and 1586 cm−1, respectively. The G band was generally associated with the E2g phonon the E2g phonon of the C sp2 atom, while the D band arose from the activation of the first2 of the C sp atom, while the D band arose from the activation of the first-order scattering order scattering process of the sp3 carbon atom in graphene sheets [51]. In the GN4 /PSt process ofthe thesame sp3 carbon atom in graphene sheets appeared. [51]. In the GNresult 4/PSt composite, the same composite, characteristic peaks as GN clearly This demonstrated characteristic peaks GN clearly appeared. This result the composite that the composite was as successfully prepared. Furthermore, the demonstrated intensity ratio ofthat D bands to was successfully prepared. Furthermore, the intensity ratio of D bands to G bands G bands (ID/IG) is usually adopted to evaluate the defects of graphene sheets. The ID/IG (ID/IG of used adopted in this experiment was the approximately and sheets. it increased to used in is GN usually to evaluate defects of 0.1071, graphene Thesubstantially ID/IG of GN 0.1897 for the GN /PSt composite. The higher ID/IG of GN /PSt resulted from a decrease 4 4 this experiment was approximately 0.1071, and it increased substantially to 0.1897 for the in the crystalline sp2 domains of GN, which may be caused by the intercalation of monomer GN 4/PSt composite. The higher ID/IG of GN 4/PSt resulted from a decrease in the polymerization in the GN layers [52]. crystalline sp2 domains of GN, which may be caused by the intercalation of m polymerization in the GN layers [52]. Polymers 2022, 14, 2289 6 of 14 3.2. Hydrophobicity of the GN/PSt Composites The wettability of the Composites GN/PSt porous materials was quantified by their wate 3.2. Hydrophobicity of the GN/PSt angles. As shown the WCA of was the quantified GN/PSt composite foams The wettability of in theFigure GN/PSt3, porous materials by their water contactwas hig that ofAsthe pureinfoam GN or GN0/PSt (WCA—140.5 ± 0.7°), su angles. shown Figureprepared 3, the WCAwithout of the GN/PSt composite foams was higher than ◦ that the addition pure foam of prepared without GN or GN (WCA—140.5 0.7 ), suggesting thatofthe the GN nanosheets made PSt-based±foams more hydropho 0 /PStthe that the addition of the GN nanosheets made the PSt-based foams more hydrophobic. The highest WCA was measured for the GN4/PSt composite monolith (WCA—150.9 highest WCA was measured for the GN4 /PSt composite monolith (WCA—150.9 ± 0.6◦ ), indicating that it was the most hydrophobic of the prepared materials. indicating that it was the most hydrophobic of the prepared materials. Figure WCAs ofGN/PSt the GN/PSt composite porous materials. Figure 3. 3. WCAs of the composite porous materials. 3.3. Mechanical Properties of GN/PSt Composites 3.3. Mechanical Properties of GN/PSt Composites To be useful in practical applications, the porous material used for oil/water separation To must be useful in practical applications, used for oil/water processes have excellent mechanical propertiesthe [53].porous Figure 4material shows that the overall trends in the compressive stress–strain curves measured forproperties the GN/PSt [53]. composites with tion processes must have excellent mechanical Figure 4 shows different GN contents were similar, and the curves could be divided into three stages. overall trends in the compressive stress–strain curves measured for the GN/PSt At strains less than 10%, the stress increased linearly with the applied strain. Between sites with different GN contents were similar, and the curves could be divided in 10% and 60% strain, the measured stress plateaued, indicating that the porous material stages. At strains lessofthan 10%, the stress increased with the st absorbed a large amount compressive energy and suggestinglinearly that the skeleton of applied the tween 10% and 60% strain,Atthe measured stress plateaued, that the por structure was highly deformed. strains greater than 60%, the materialindicating began to fracture under the applied pressure and underwent a densification process. At this time, the the ske terial absorbed a large amount of compressive energy and suggesting that stress increased sharply with the increase in the strain [54]. Materials prepared by HIPE the structure was highly deformed. At strains greater than 60%, the material b polymerization have poor mechanical properties, for example, they are highly brittle and fracture under due the to applied and underwent a densification process. At th easily pulverized, the use pressure of rigid monomers and the high crosslinking densities the increased with increase strainduring [54]. the Materials prep of thestress final materials [44]. sharply Here, we see thatthe as the amountin of the GN added HIPE polymerization increased, the compressive strength ofproperties, the resultingfor composite materials HIPE polymerization have poor mechanical example, they are hig first decreased and then increased. Small amounts of added GN increased the flexibility of tle and easily pulverized, due to the use of rigid monomers and the high cros the material, but the addition of excessive amounts of GN resulted in uneven dispersion and densities of the final materials [44]. Here, we see that as the amount of GN added agglomeration of the nanosheets, which concentrated the stress and reduced the flexibility the polymerization the compressive strength the resulting co of theHIPE final composite material. increased, From the present studies, the addition of 4%of GN was the optimal amount effectively improve theincreased. flexibility of the porous material while also materials firsttodecreased and then Small amounts of added GN incre increasing itsof hydrophobicity. GN4/PSt was selected foramounts the subsequent tests. flexibility the material,Therefore, but the addition of excessive of GN resulted in dispersion and agglomeration of the nanosheets, which concentrated the stress duced the flexibility of the final composite material. From the present studies, the of 4% GN was the optimal amount to effectively improve the flexibility of the por terial while also increasing its hydrophobicity. Therefore, GN4/PSt was selected subsequent tests. Polymers 2022, 14,14,x 2289 FOR PEER REVIEW Polymers 2022, 7 of 14 Figure 4. Compressive stress–strain curve measured of the composite porous materials. Figure 4. Compressive stress–strain curve measured of the composite porous materials. 3.4. Wettability of GN4 /PSt Composites 3.4.The Wettability of GN4/PSt Composites wettability of porous materials towards water and oil is a vital indicator of their performance in oil/water separations As seen in Figure 5a,b, the WCA andisOCA of the The wettability of porous[55]. materials towards water and oil a vital indicato ◦ and 0◦ , respectively. To more clearly demonstrate GN /PSt porous composite were 151 4 performance in oil/water separations [55]. As seen in Figure 5a,b, the WCA and the superhydrophobicity and oleophilicity of the prepared composite, water and oil were the GN4/PSt porous composite were 151° and 0°, respectively. To more clearly dropped onto the surface of the GN4 /PSt monolith, and the water droplets remained the shapes superhydrophobicity andwere oleophilicity of the(Figure prepared composite, wate asstrate spherical while the oil droplets rapidly absorbed 5c). Under the were dropped onto the surface the GN 4/PSt monolith, and the water r application of an external force, a waterofdroplet was forced to make contact and move droplets on the surface of the material, and the water droplet moved easily and did not remain on as spherical shapes while the oil droplets were rapidly absorbed (Figure 5c). U the surface of the suggesting thatathe water adhesion on forced the GN4to /PSt composite application of foam, an external force, water droplet was make contact and was extremely low (Figure 5d). Moreover, the rolling angle (RA) of the material was small the=surface of5e). the As material, and the 5f, water droplet moved easily andmaterial did not rema (RA 8◦ , Figure shown in Figure the lightweight GN4 /PSt porous surface of the suggesting that the placed wateron adhesion therolled GN 4off /PSt could be placed on afoam, thin blade, and a water droplet the surfaceon easily at compo only a slight incline due to the5d). lowMoreover, density and low of the angle prepared composite extremely low (Figure theRA rolling (RA) of the materials. material was s An automatic mercury porosimeter was used to characterize the pore structure of GN 4 /PSt = 8°, Figure 5e). As shown in Figure 5f, the lightweight GN4/PSt porous material composites, and the density and porosity were 0.0276 g/cm3 and 97.2%, respectively. The placed on a thin blade, and a water droplet placed on the surface easily rolled of results indicated that the GN4 /PSt foam had a hierarchical pore structure containing both a slight incline low density and (ESI; low Figure RA ofS2), thesuggesting preparedthat composite m nanopores (40–1000due nm) to andthe micropores (1–20 µm) the An automatic used to characterize the pore structure of prepared materialmercury should be porosimeter able to separatewas oil/water emulsions through a size sieving 3 effect [56]. Moreover, the properties of the GN4 /PStwere porous sampleg/cm were highly uniform, composites, and the density and porosity 0.0276 and 97.2%, respectiv and the monolith could be randomly cut into various segments without affecting the results indicated that the GN4/PSt foam had a hierarchical pore structure contain shapes of the water droplets on any of the surfaces (Figure 5g). Figure 5h shows that the nanopores (40–1000 nm) and micropores (1–20 μm) (ESI; Figure S2), suggesting GN 4 /PSt porous material also had a strong water-impact resistance [57]. In summary, prepared material ableresulted to separate emulsions through the addition of GN intoshould an HIPEbe system in GNoil/water materials with a siz 4 /PSt composite superhydrophobic/superoleophilic properties as well as a low density and high porosity, effect [56]. Moreover, the properties of the GN4/PSt porous sample were highly making this monolith material especially for thecut purification of oily sewage. and the could promising be randomly into various segments without affe shapes of the water droplets on any of the surfaces (Figure 5g). Figure 5h shows GN4/PSt porous material also had a strong water-impact resistance [57]. In summ addition of GN into an HIPE system resulted in GN4/PSt composite materials wi hydrophobic/superoleophilic properties as well as a low density and high poros ing this material especially promising for the purification of oily sewage. Polymers 2022, 14, 2289 Polymers 2022, 14, x FOR PEER REVIEW 8 of 14 8 of Figure WCA and OCA of GN 4/PSt, (c) photographs of water with brilliant Figure 5. 5. (a)(a) WCA and (b) (b) OCA of GN (c) photographs of water (dyed(dyed with brilliant green green an 4 /PSt, methyl orange) oilred redO) O)droplets droplets surface of 4GN 4/PSt, (d) dynamic adh and methyl orange)and andoil oil(dyed (dyed with with oil onon thethe surface of GN /PSt, (d) dynamic sion behavior ofofwater (e)the therolling rollingangle, angle, water droplets fallin adhesion behavior waterdroplets droplets on on GN GN44/PSt /PSt surface, surface, (e) (f)(f) water droplets on the surface of GN 4 /PSt, (g) water droplets on a random surface of GN 4 /PSt, (h) water impa falling on the surface of GN4 /PSt, (g) water droplets on a random surface of GN4 /PSt, (h) water resistance of GN 4 /PSt. impact resistance of GN4 /PSt. 3.5. Absorption Capacity and Continuous Oil/Water Separation Using theUsing the GN4/PSt 3.5.OilOil Absorption Capacity and Continuous Oil/Water Separation GN4 /PSt Composites Composites As shown in Figure 6a,b, both light (petroleum ether) and heavy (chloroform) oils As shown in Figure 6a,b, light (petroleum ether) and heavy (chloroform) oi were rapidly absorbed into the GNboth 4 /PSt composite by capillary forces, suggesting the were rapidly absorbed into thegood GN4performance /PSt composite by capillary forces,experiments. suggesting the pr prepared composites should show in water/oil separation The saturated oil absorption of theperformance GN4 /PSt foams various separation types of organic pared composites shouldcapacity show good in for water/oil experiment solvents (Figure 6c) and ranged from 27.44 to 56.9 g/g. These results suggested that theof organ The saturated oil absorption capacity of the GN4/PSt foams for various types prepared composites had a high absorption capacity for various organic solvents, and the that th solvents (Figure 6c) and ranged from 27.44 to 56.9 g/g. These results suggested variations in the different oil absorption capacities were due to differences in the densities prepared composites had a high absorption capacity for various organic solvents, and th and viscosities of the absorbed organic solvents [58]. The reusability of materials is also a variations in the different oil absorption capacities were due to differences in the densitie key factor in the actual treatment of oily wastewater. The reusability of GN4 /PSt foam was and viscosities of the absorbed organic [58]. Thereached reusability of materials examined by absorption-centrifugation. Aftersolvents the oil absorption saturation, the oil is also key factor in the actual treatment of oily wastewater. The reusability of GN in the GN4 /PSt foam was removed by centrifugation at 6000 rpm for 2 min, while the4/PSt foamfoam wa examined by absorption-centrifugation. After the cycle oil absorption reached saturation, th was regenerated for the next absorption/centrifugation without further treatment (Figure The4/PSt absorption/centrifugation test was repeated times andfor the2results oil in S3a). the GN foam was removed cycle by centrifugation at 10 6000 rpm min, while th are shown in Figure S3b. The porous material exhibited stable reuse performance foam was regenerated for the next absorption/centrifugation cycle withoutwith further trea almost change in oil absorption capacity, and approximately petroleum ment no (Figure S3a). The absorption/centrifugation cycle test84% wasofrepeated 10ether times and th was removed by each centrifugation. It is speculated that the residual petroleum ether was results are shown in Figure S3b. The porous material exhibited stable reuse performanc with almost no change in oil absorption capacity, and approximately 84% of petroleu ether was removed by each centrifugation. It is speculated that the residual petroleu ether was stably adsorbed and retained in the micropores by van der Waals and capillar forces. In addition, the GN4/PSt sample continuously absorb oil pumped through th Polymers 2022, 14, 2289 9 of 14 Polymers 2022, 14, x FOR PEER REVIEW 9 of 14 stably adsorbed and retained in the micropores by van der Waals and capillary forces. In addition, the GN4 /PSt sample continuously absorb oil pumped through the foam, which is a necessary feature of materials used in large-scale oily wastewater treatment. As shown in treatment. As Video shownS1, in Figure 6d and Video S1, both light oilheavy (petroleum ether) and heavy Figure 6d and both light oil (petroleum ether) and oil (chloroform) were oil (chloroform) were quickly and continuously separated from water without leaving any quickly and continuously separated from water without leaving any residual red oil in the residual red oil the water phase.6e, Asthe shown Figure 6e, maintained the GN4/PStafoams maintained water phase. Asinshown in Figure GN4in /PSt foams high separation a high separation efficiency for various organic solvents in water 99.6%),that further efficiency for various organic solvents in water (above 99.6%), further(above highlighting the highlighting that the GN 4 /PSt composite material has great application potential for effiGN4 /PSt composite material has great application potential for efficient, large-scale and cient, large-scale and of continuous treatment of oily wastewater. continuous treatment oily wastewater. Figure6.6. Photographs Photographs showing composite materials forfor selective oil/water sepFigure showing the the use useof ofGN GN4/PSt composite materials selective oil/water 4 /PSt aration. (a) Petroleum ether/water and (b) chloroform/water, (c) the saturated oil absorption capacseparation. (a) Petroleum ether/water and (b) chloroform/water, (c) the saturated oil absorption ity of GN a variety of organic solvents, (d) images of the continuous separation of light capacity of4/PSt GN4foam /PSt for foam for a variety of organic solvents, (d) images of the continuous separation oil/water/heavy oil mixtures using the GN4/PSt foams and corresponding (e) separation efficiencies of light oil/water/heavy oil mixtures using the GN4 /PSt foams and corresponding (e) separation for different oil/water mixtures. efficiencies for different oil/water mixtures. 3.6.Continuous ContinuousSeparation SeparationofofSurfactant-Stabilized Surfactant-StabilizedEmulsions EmulsionsUsing Usingthe theGN GN44/PSt /PStComposites Composites 3.6. Theseparation separationof ofemulsions emulsionsisismuch muchmore moredifficult difficultthan thanthe theseparation separationof ofimmiscible immiscible The water/oil mixtures, and the separation of surfactant-stabilized microemulsions withsmall small water/oil mixtures, and the separation of surfactant-stabilized microemulsions with droplet sizes and lower content of dispersive phase is especially challenging [59]. To droplet sizes lower content of dispersive phase is especially challenging [59]. test To the the feasibility of separating oil/water emulsions usingusing the GN three surfactest feasibility of separating oil/water emulsions the4/PSt GNfoams, /PSt foams, three 4 tant-stabilized water-in-oil emulsions with with micro-nano particle sizessizes werewere prepared, and surfactant-stabilized water-in-oil emulsions micro-nano particle prepared, the the separation of Span 80-stabilized water-in-petroleum ether emulsions areare discussed as and separation of Span 80-stabilized water-in-petroleum ether emulsions discussed an example. As shown in Figure 7a and Video S2, after the milky white emulsion was as an example. As shown in Figure 7a and Video S2, after the milky white emulsion pumped through the the monolith, thethe filtrate was clear and transparent indicating that in was pumped through monolith, filtrate was clear and transparent indicating that situ demulsification andand continuous oil/water separation were realized. Moreover, optical in situ demulsification continuous oil/water separation were realized. Moreover, optical micrographs and digital photographs the as-prepared water-in-oil emulsions micrographs and digital photographs of the of as-prepared water-in-oil emulsions before before andseparation after separation using GN /PSt foams are shown in Figure 7b–d. While and after using the GNthe 4/PSt foams are shown in Figure 7b–d. While numerous 4 numerous droplets seen in the feed solutions, no droplets were observed in disperseddispersed droplets were seenwere in the feed solutions, no droplets were observed in the filthe filtrate using an optical microscope, further highlighting the high-efficiency separation trate using an optical microscope, further highlighting the high-efficiency separation perperformance foams. Furthermore, droplet of the emulsions formance of of thethe GNGN 4/PSt foams. Furthermore, the the droplet sizesize of the feedfeed emulsions and 4 /PSt and filtrates measured using dynamic scattering. initial emulsions filtrates werewere measured using the the dynamic lightlight scattering. The The initial emulsions contained a wide size distribution of droplets ranging from 30 nm to 7 µ m (Figure 7e). In comparison, the droplets in the filtrate were smaller than 100 nm (Figure 7f), and the particle sizes were similar to those seen in a solution of the Span 80 surfactant in oil (Figure Polymers 2022, 14, 2289 10 of 14 contained a wide size distribution of droplets ranging from 30 nm to 7 µm (Figure 7e). In comparison, the droplets in the filtrate were smaller than 100 nm (Figure 7f), the particle 10 ofand 14 sizes were similar to those seen in a solution of the Span 80 surfactant in oil (Figure 7g). Based on these results, we speculate that the filtrate was composed of a small number of 7g). Based on these results, we speculate that the filtrate was composed of a small number nano-sized emulsified droplets and and surfactant micelles [56,60]. of nano-sizedwater emulsified water droplets surfactant micelles [56,60]. Polymers 2022, 14, x FOR PEER REVIEW Figure 7. (a) Photographs of the setup using GN4/PSt composites for the continuous separation of Figure 7. (a) Photographs ofether theemulsions setup using GN4 /PSt composites for the separation water-in-petroleum and corresponding optical microscopy images and continuous digital photos of the emulsions and after separation, (b) water-in-petroleum ethermicroscopy emulsions, (c) waterof water-in-petroleum ether before emulsions and corresponding optical images and digiin-toluene emulsions, (d) water-in-chloroform emulsions, (e) droplet size distributions in the watertal photos of the emulsions before (f) and after (b)filtrate, water-in-petroleum ether emulsions, in-petroleum ether emulsion, droplet sizeseparation, distribution in the and (g) droplet size distribution in a solution of 1 g/L Span 80 surfactant in petroleum ether. (c) water-in-toluene emulsions, (d) water-in-chloroform emulsions, (e) droplet size distributions in the water-in-petroleum ether emulsion, (f) droplet size distribution filtrate, and (g) droplet size 4/PSt foams, To more quantitatively assess the demulsification efficiencyin of the the GN the moisture contents of the emulsions before and after the separation were measured distribution in a solution of 1 g/L Span 80 surfactant in petroleum ether. using a Karl Fischer moisture meter, and the results are shown in Figure 8. The separation efficiencies of the GN4/PSt composites for the three emulsions were 98.2%, 98.5% and To more98.2%, quantitatively assess the demulsification ofsize-sieving the GN4 /PSt foams, respectively. This high separation efficiency is due to aefficiency combination of 4/PStemulsions in theof GN foams as well as the completely opposite towardswere oil the moisturefiltration contents the before and after thewettability separation measured and water. The long and tortuous micro-nano permeation channels in the foams were also using a Karl Fischer moisture meter, and the results are shown in Figure 8. The separation crucial in the emulsion separation process [35,61,62]. In summary, the as-prepared efficiencies of the GN4 /PSt composites for the three emulsions were 98.2%, 98.5% and 98.2%, respectively. This high separation efficiency is due to a combination of size-sieving filtration in the GN4 /PSt foams as well as the completely opposite wettability towards oil and water. The long and tortuous micro-nano permeation channels in the foams were also crucial in the emulsion separation process [35,61,62]. In summary, the as-prepared superhydrophobic GN4 /PSt porous materials could be used for the efficient, large-scale and continuous separation of emulsions to achieve rapid treatment of emulsified oily sewage. Polymers Polymers2022, 2022,14, 14,2289 x FOR PEER REVIEW 11 of of14 14 Figure8.8.Separation Separationefficiency efficiencyofofwater-in-oil water-in-oilemulsions. emulsions. Figure 4.4.Conclusions Conclusions In thethe superhydrophobic GN4GN /PSt4/PSt composite material with micro-nanopore Insummary, summary, superhydrophobic composite material with micro-nastructures was successfully fabricated fabricated using a facile, low-cost HIPE polymerization method. nopore structures was successfully using a facile, low-cost HIPE polymerizaCompared with a neat porous prepared with only PSt, porous monolith tion method. Compared with a material neat porous material prepared withthe only PSt, the porous ◦ to prepared with 4 wt% GN was more hydrophobic (WCA increased from 140.5 ± 0.7 monolith prepared with 4 wt% GN was more hydrophobic (WCA increased from 140.5 ± ◦ 150.9 ± 0.6 ) and more flexible. to the ideal excellent mechanical properties 0.7° to 150.9 ± 0.6°) and more Due flexible. Due to wettability, the ideal wettability, excellent mechanical and high porosity (97.2%), the(97.2%), GN4 /PSt monolith could be used properties and high porosity thecomposite GN4/PSt composite monolith couldtobecontinuused to ously separate immiscible oil/water mixtures with a separation efficiency above 99.6%. continuously separate immiscible oil/water mixtures with a separation efficiency above Arising from thefrom lengthy micro-nanopore permeation path, the GN4the /PSt efficient 99.6%. Arising the lengthy micro-nanopore permeation path, GNenable 4/PSt enable efand continuous separation of surfactant-stabilized water-in-oil microemulsions. The speficient and continuous separation of surfactant-stabilized water-in-oil microemulsions. cial wettability, hierarchical pore structure with micro-nanopore and tortuous permeation The special wettability, hierarchical pore structure with micro-nanopore and tortuous perchannel the significant roles in emulsion The superhydrophobic GN4 /PSt meationplay channel play the significant roles in separation. emulsion separation. The superhydrophobic composite material prepared here showed great performance for the efficient and conGN4/PSt composite material prepared here showed great performance for the efficient and tinuous separation of immiscible and emulsified oil/water mixtures and has promising continuous separation of immiscible and emulsified oil/water mixtures and has promising application prospects in the practical large-scale treatment of oily wastewater. application prospects in the practical large-scale treatment of oily wastewater. Supplementary Materials: The following supporting information can be downloaded at: https:// Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/polym14112289/s1, Figure S1. Raman spectra of GN and GN4/PSt.; www.mdpi.com/xxx/s1, Figure S1. Raman spectra of GN and GN4/PSt.; Figure S2. The pore size Figure S2. The size porous distribution of GN4/PSt porous material.; S3. (a) The process of distribution of pore GN4/PSt material.; Figure S3. (a) The process Figure of removal of petroleum ether removal of petroleum ether from oil/water mixture and the regeneration of the foam. (b) Regeneration from oil/water mixture and the regeneration of the foam. (b) Regeneration of GN4/PSt sponge durofing GN4/PSt spongeofduring the absorption petroleum ether for 10 cycles.; Video 1: of Continuous the absorption petroleum ether for 10ofcycles.; Video 1: Continuous separation immiscible separation of immiscible light oil/water/heavy oil by GN4/PSt porous material with the assistance light oil/water/heavy oil by GN4/PSt porous material with the assistance of a pump.; Video 2: Conoftinuous a pump.; Video 2: Continuous separation of surfactant-stabilized water-in-oil microemulsions separation of surfactant-stabilized water-in-oil microemulsions with the assistancewith of a the assistance of a pump. pump. Author AuthorContributions: Contributions:Conceptualization, Conceptualization,C.Z. C.Z.and andY.L.; Y.L.;methodology, methodology,H.H.; H.H.;software, software,D.X. D.X.and and Y.W.; Y.W.;validation, validation,H.H. H.H.and andJ.L.; J.L.;formal formalanalysis, analysis,H.H., H.H.,D.X. D.X.and andY.W.; Y.W.;investigation, investigation,H.H.; H.H.;resources, resources, C.Z. C.Z.and andM.Q.; M.Q.;data datacuration, curation,H.H.; H.H.;writing—original writing—originaldraft draftpreparation, preparation,H.H.; H.H.;writing—review writing—reviewand and editing, visualization, H.H. and G.W.; supervision, C.Z.C.Z. andand Y.L.;Y.L.; project administration, editing,C.Z. C.Z.and andY.L; Y.L; visualization, H.H. and G.W.; supervision, project administraC.Z., Y.L.; acquisition, C.Z and All authors have have read read and agreed to theto tion, H.H. C.Z., and H.H., andfunding Y.L.; funding acquisition, C.ZM.Q. and M.Q. All authors and agreed published version of theofmanuscript. the published version the manuscript. Funding: This work was supported by the Science and Technology Strategic Cooperation Special Project of Nanchong City and SWPU (SXHZ046), and the Innovation and Entrepreneurship Training Program for College Students of Sichuan Province (S202110615095). Polymers 2022, 14, 2289 12 of 14 Funding: This work was supported by the Science and Technology Strategic Cooperation Special Project of Nanchong City and SWPU (SXHZ046), and the Innovation and Entrepreneurship Training Program for College Students of Sichuan Province (S202110615095). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 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