Development of Polymeric Membranes for Oil/Water Separation

Article Development of Polymeric Membranes for Oil/Water Separation

Arshad Hussain and Mohammed Al-Yaari *

Chemical Engineering Department, King Faisal University, P.O. Box 380, Al-Ahsa 31982, Saudi Arabia; [email protected] * Correspondence: [email protected]; Tel.: +966-13-589-8583

Abstract: In this work, the treatment of oily wastewater was investigated using developed cellulose acetate (CA) membranes blended with Nylon 66. Membrane characterization and permeation results in terms of oil rejection and flux were compared with a commercial CA membrane. The solution casting method was used to fabricate membranes composed of CA and Nylon 66. Scanning Electron Microscopy (SEM) analysis was done to examine the surface morphology of the membrane as well as the influence of solvent on the overall structure of the developed membranes. Mechanical and thermal properties of developed blended membranes and a commercial membrane were examined by thermogravimetric analysis (TGA) and universal (tensile) testing machine (UTM). Membrane characterizations revealed that the thermal and mechanical properties of the fabricated blended membranes better than those of the commercial membrane. Membrane fluxes and rejection of oil as a function of Nylon 66 compositions and transmembrane pressure were measured. Experimental results revealed that the synthetic membrane (composed of 2% Nylon 66 and Dimethyl Sulfoxide (DMSO) as a solvent) gave a permeate flux of 33 L/m2h and an oil rejection of around 90%, whereas the commercial membrane showed a permeate flux of 22 L/m2h and an oil rejection of 70%.

Keywords: oil–water separation; polymeric membrane; cellulose acetate; Nylon 66; permeability

Citation: Hussain, A.; Al-Yaari, M. Development of Polymeric Membranes for Oil/Water 1. Introduction Separation. Membranes 2021, 11, 42. Industrialization has given birth to many problems like increased greenhouse gas https://doi.org/10.3390/membranes emissions, environmental pollution, and energy crises [1–3]. Especially, water is indispens- 11010042 able for the existence and survival of life on earth. In the past few years, the world has faced severe scarcity in water resources. Extensive use of water, extreme drought periods, Received: 4 December 2020 upsurge in population, and water pollution [4] have resulted in a global concern about Accepted: 29 December 2020 secure drinking water supply. Polluted water is a threat to the life of humans, animals, Published: 8 January 2021 and plants both ashore and in the oceans. The situation will get worse if timely actions are not taken to combat the issue of water pollution, which would be a hindrance for the Publisher’s Note: MDPI stays neu- sustainable development of society [5]. Therefore, clean drinking water and sanitation are tral with regard to jurisdictional clai- among the seventeen Sustainable Development Goals of the United Nations Environment ms in published maps and institutio- Programme (UNEP) [6]. According to the United Nations World Water Development nal affiliations. (UNWWD) report, about 748 million people do not have access to clean drinking water. In addition, by 2050, the demand of water in industry will increase to 400 percent [7], thus aggravating the water crisis even further. In developing countries, 3.2 million children die Copyright: © 2021 by the authors. Li- annually due to poor sanitation and unsafe drinking water [8]. This daunting situation has censee MDPI, Basel, Switzerland. urged society and especially scientists to explore efficient and sustainable technologies and This article is an open access article materials (i.e., such which can be justified from an ecological and economical viewpoint to distributed under the terms and con- be applied by and for society now and in the future) for the mitigation of water pollution. ditions of the Creative Commons At- In a variety of industrial processes, water comes into contact with several pollutants tribution (CC BY) license (https:// such as hydrocarbons, hazardous chemicals, sewage sludge from boilers, cooling towers, creativecommons.org/licenses/by/ and heat exchangers [9]. Industrial wastewater is usually not reusable and may have drastic 4.0/).

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impacts on living species [10,11]. The wastewater coming from oil processing facilities poses a serious threat to aquatic life and pollutes groundwater [9]. Recycling of polluted water or reduction of pollutants concentration to an acceptable level is considered to be the only way out in these grim circumstances. Conventional techniques including decant- ing, biological treatment, chemical treatment, gravity separation, skimming, air flotation, coagulation, and flocculation are being used for the treatment of oily wastewater [12,13]. Such processes require some chemicals and biological solvents to treat wastewater which could cause devastating effects on mankind [12]. High cost, use of toxic compounds, and large space requirements are some other drawbacks that limit their usage as suitable techniques [12]. Hence, there is a need to develop suitable treatment processes that produce less hazardous pollutants. Membrane technology offers great potential to treat oily wastewater. In this context, a range of polymer membranes have been prepared using various techniques [14,15] and the feasibility of a membrane-based oil–water separation process has been reported by many researchers [16–19]. The cellulose-acetate-based membranes often show high hydrophilicity, high water permeability, and low membrane fouling tendency [20,21]. Li et al. [13] developed a hydrophilic cellulose acetate (CA) membrane for oil–water separation. The membrane was comprised of CA/monohydrate/N-methyl morpholine- N-oxide (NMMO.H2O)/polyethylene glycol (PEG 400). Membrane was tested for pure water flux first and then tested for the separation of oil and water. Fouling resistance of the membrane was also measured and analyzed using osmotic-pressure-adsorption model. Over 99% of oil retention was reported and total oil content in the permeate was found to be 10 mg/L. Flux reduction was noted and dominated by concentration polarization. In order to get a high permeation flux with high antifouling property, a new membrane was developed which was composed of CA grafted with polyacrylonitrile [22]. The new membrane was tested for flux and antifouling property, and results were very promising. Yan et al. [23] modified polyvinylidene fluoride-based membranes with nanosized inorganic alumina particles in order to purify the oily wastewater stream. Results obtained after the processing of the membrane revealed that the oil content in the permeate stream was reduced to 1 mg/L. Suspended particles in the permeate stream were also reduced to less than 1 mg/L, which is desirable by oilfield drainage. In 2012, Kota et al. [24] reported the membranes having hygro-responsive properties for oil–water separations. In their study, poly (ethylene glycol) diacrylate (PEGDA) and fluoro-decyl polyhedral oligomeric silsesquioxane (FPOSS) were used as substrate materials. The results demonstrated that the membranes are both super hydrophilic and super oleophobic. A separation efficiency of greater than 99% was achieved. After 100 h of operation, the water flux was nearly 210 L/m2 h. Mansourizadeh et al. [25] developed a membrane composed of polyethersulfone (PES) and CA. Polyethylene glycol (PEG400) was also added into the membrane dope to increase the number of pores in the membrane. CA was added into PES to increase the overall hydrophilicity of the membrane and to gain the desired structure for oil–water separation. Hydrophilic PES/CA membrane showed an increase in water flux up to 27 L/m2 s and oil rejection of 88%. This study is aimed at further improving the performance of cellulosic membranes for oil–water separation. Recently, antibacterial nanofiber membranes for potential application for oil–water emulsions has been reported by Mousa et al. [26]. Polysulfone (PSF) and CA were used as base polymers. Zinc oxide (ZnO) nanoparticles were also incorporated to enhance the properties of the polymeric membranes. The membranes were modified with NaOH and showed better properties than unmodified ones. The highest reported water flux was 420 L/m2h. The objective of this research work is to synthesize and characterize polymeric membrane comprising of CA and nylon 66 blends in different solvents. The effect of solvent on the overall morphology and structure of the membrane is studied. Mechanical, thermal, and morphological properties of the membrane were characterized using tensile testing Membranes 2021, 11, 42 3 of 15

machine, thermogravimetric analysis (TGA), and Scanning Electron Microscopy (SEM), respectively. Overall permeate ﬂux and rejection of oil were determined as functions of nylon 66 concentration. In addition, the effect of the transmembrane pressure on the overall ﬂux and oil rejection was examined using the permeation experiment setup.

2. Materials and Methods 2.1. Materials CA with a molecular weight of 30,000 g/mol was acquired from Sigma-Aldrich (St. Louis, MO, USA). CA was available in a powder form and no pretreatment was required for its further usage. Formic acid was also procured from Sigma-Aldrich (St. Louis, MO, USA) having a purity of 98%. Nylon 66 was purchased from Zytel Dupont (Pulau Sakra Island, Singapore) and was available in a pellet form. Dimethyl Sulfoxide (DMSO) was procured from Sigma-Aldrich with a purity of 99% and a transparent color. Commercial CA membrane with a pore diameter of 0.22 µm was purchased from Merck Millipore (Boston, MA, USA).

2.2. Preparation of Polymeric Membranes Polymer solutions of different wt% of CA and Nylon 66 in DMSO and Formic Acid were cast on a glass plate at room temperature. Membrane thickness of 0.3 mm was maintained with the help of a doctor blade. Solvents were ﬁrst evaporated from the casted membrane and then immersed into nonsolvent (water). After immersion, the ﬁlm color changed from clear to white and also detached from the glass plate. Membranes were collected from the coagulation bath and preserved in plastic bags for the characterization and permeation tests. Tables1 and2 show the composition of membrane solution using DMSO and Formic Acid, respectively.

Table 1. Composition (wt%) of the developed membrane using Dimethyl Sulfoxide (DMSO) as a solvent.

Solution Cellulose Acetate (wt%) Nylon 66 (wt%) DMSO (wt%) D1 11 2 87 D2 11 3 86 D3 11 5 84

Table 2. Composition (wt%) of the developed membrane using Formic Acid as a solvent.

Solution Cellulose Acetate (wt%) Nylon 66 (wt%) Formic Acid (wt%) F1 11 2 87 F2 11 3 86 F3 11 5 84

2.3. Scanning Electron Microscopy (SEM) To investigate the morphology of the membrane, dry polymeric membrane was dipped in liquid nitrogen and then fractured in order to get the ﬁbrous cross-section of the membrane. Before the analysis, the surface of the samples was sputter coated with a thin layer of gold to avoid the accumulation of charges. An acceleration voltage of 20 kV was used for investigation. For the cross-section analysis, the formulations were soaked in liquid nitrogen to freeze crack. Afterwards, they were also sputter coated with gold. Surface morphology and cross-section of the membrane were then studied using JEOL JSM-6490A SEM (Tokyo, Japan).

2.4. Permeate Flux Determination A crossﬂow permeation testing unit, shown in Figure1, was used to evaluate the overall ﬂux across the membrane. The permeation testing unit consists of a feed tank, Membranes 2021, 11, x FOR PEER REVIEW 4 of 15

Membranes 2021, 11, 42 4 of 15 2.4. Permeate Flux Determination A crossflow permeation testing unit, shown in Figure 1, was used to evaluate the overall flux across the membrane. The permeation testing unit consists of a feed tank, per‐ permeatemeate collector, collector, and and centrifugal centrifugal pump. pump. The The effect effect of transmembrane of transmembrane pressure pressure was was rec‐ recordedorded with with the the help help of ofa ball a ball valve valve mounted mounted on on the the retentate retentate stream. stream. AnAn oil–wateroil–water emulsion emulsion was was prepared prepared by by mixing mixing lubricating lubricating oil oil with with water water in differentin differ‐ proportionsent proportions at 900 at rpm 900 forrpm 30 for min. 30 Themin. membrane The membrane was washed was washed with distilled with distilled water before water determiningbefore determining the permeate the permeate flux. The flux. membrane The membrane was placed was placed on a porous on a porous Teflon Teflon support sup to‐ preventport to prevent bending bending at various at feedvarious pressures. feed pressures. The total The membrane total membrane area exposed area toexposed the feed to was about 0.0094 m2. Flux was calculated by varying the transmembrane pressures (0.5, the feed was about 0.0094 m2. Flux was calculated by varying the transmembrane pres‐ 1.0, and 1.5 bar). Permeate flux was calculated using the following equation. sures (0.5, 1.0, and 1.5 bar). Permeate flux was calculated using the following equation. Q𝑄 Jw 𝐽= (1)(1) 𝑤 A 𝐴× 𝑡t wherewhere 2 JJww denotes the permeate fluxflux (mL/m(ml/m2 h),h), QQ denotesdenotes thethe permeatepermeate volumevolume (mL),(ml), AA denotesdenotes thethe membranemembrane areaarea exposedexposed to to the the feed feed (m (m22),), andand tt isis thethe samplingsampling timetime forfor permeatepermeate (h).(h).

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the membrane membrane testing testing unit. unit.

2.5. Oil Rejection 2.5. Oil Rejection Percentage of oil rejection (R%) was calculated using the same method as used for oil Percentage of oil rejection (R%) was calculated using the same method as used for oil and grease calculation. Concentrations of oil in feed and permeate were calculated using and grease calculation. Concentrations of oil in feed and permeate were calculated using n‐Hexane and HCL. The rejection (%) was calculated using the following equation: n-Hexane and HCL. The rejection (%) was calculated using the following equation: 𝐶 𝑅 % 1 !100 (2) Cp𝐶 R % = 1 − × 100 (2) Cf where wherecp denotes the permeate concentration (g/L), cf denotes the feed concentration (g/L). cp denotes the permeate concentration (g/L), cf denotes the feed concentration (g/L). 2.6. Thermal Studies 2.6. ThermalThermogravimetric Studies analysis (TGA) was carried out by using Perkin Elmer S11 dia‐ mondThermogravimetric TG/DTA (Boston, MA, analysis USA). (TGA) The sample was carried weighing out by3 mg using was Perkin dried at Elmer a heating S11 dia-rate mond(β) of TG/DTA15 °C/min (Boston, from 300 MA, °C USA).to 600 The°C in sample an inert weighing atmosphere. 3 mg was dried at a heating rate (β) of 15 ◦C/min from 300 ◦C to 600 ◦C in an inert atmosphere.

2.7. Mechanical Properties Mechanical properties of the membrane were tested using a Universal Testing Machine (ELE International, Loveland, CO, USA). The mechanical analysis of polymeric membranes was carried out according to the ASTM D638 standard. All the specimen were cut into lengths of 50 mm and widths of 15 mm. The crosshead speed was 50 mm/min. The Membranes 2021, 11, 42 5 of 15

thickness of each sample was 0.3 ± 0.05 mm. The tensile strength (MPa) and elongation at break (%) were measured at room temperature.

3. Results and Discussion 3.1. Morphological Analysis of Membranes Morphology of a commercial membrane and the developed membranes were analyzed by SEM. Cross-section, outer surface, and skin layer morphologies of the polymeric formic acid-based and DMSO-based membranes are shown in Figures2 and3, respectively. As shown in Figure2, the commercial membrane (C1) has a homogeneous spongy structure with no skin layer on the surface. Solubility between solvents and nonsolvents is a critical factor in membrane synthesis. Solvents dissolve a wide variety of polymers and, based on their solubility parameter, they give porous and anisotropic membranes. The solubility parameter is a numerical value that indicates the relative solvency behavior of a specific solvent [15]. Generally, polar solvents like DMSO and Formic Acid are considered to be the best solvents for casting CA membranes as they precipitate rapidly when immersed in water and thus produce anisotropic membranes with high pore density and high flux. As reported, DMSO has the lowest water contact angle but the highest hydrophilicity [27]. Therefore, in this work, DMSO and Formic Acid have been used as solvents. In most of the cases, a membrane having fingerlike voids along its cross-section (F1, F3, D1, and D3) is preferable than having a spongy cross-section (C1) in the case of oil–water treatment [22] (see Figures2 and3). Therefore, the developed membranes are preferable over the commercial membrane. In addition, SEM results showed that Nylon 66 was homogeneously dispersed on the membrane surface (Figure2). The presence of Nylon 66 on the membrane surface increased its surface roughness whereas the cross-sectional analysis of the membrane confirmed its asymmetrical structure. The compact structure and the decreasing porosity of the support layer across its cross-section can be attributed to the affinity of solvent with nonsolvent [28]. The affinity of Formic Acid was low, which resulted in the compact structure of the membrane (Figure2). Nylon is a natural hydrophilic polymer and has a wide range of compatibility and resistance to organic solvents. The effect of nylon concentration on the overall morphology of the membrane was also examined by SEM. Results showed that as the concentration of nylon increased from 2 wt% to 5 wt% in the polymer solution, the roughness of the surface increased as shown in Figures2 and3. An increase in the Nylon 66 concentrations also affects the cross-sectional morphology of the membrane. The thickness of the skin layer increases as the amount of Nylon 66 is increased. The thickness of the skin layer is inversely proportional to the permeate flux and hence, permeate flux decreases as the amount of Nylon 66 increases in the casting solution. Moreover, the cavities available along the cross-section of the membrane also ceased to exist. The same effect was noted by Young et al. [29]. An increase in the polymer amount resulted in a slow liquid–liquid demixing which causes a decrease in porosity and pore-to-pore distance was increased leading to less porosity of the membrane [30]. Membranes 2021, 11, 42 6 of 15 Membranes 2021, 11, x FOR PEER REVIEW 6 of 15

FigureFigure 2. 2.SEM SEM images images of of surface surface and and cross-section cross‐section of of the the Formic-Acid-based Formic‐Acid‐based membranes membranes (F1 (F1 and and F3) andF3) commercial and commercial membrane membrane (C1). (C1).

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Figure 3. SEMFigure images 3. SEM of surface images and of cross-section surface and cross of DMSO-based‐section of DMSO membranes‐based (D1 membranes and D3). (D1 and D3). SEM surface images revealed that nylon is finely dispersed in CA, which confirms SEM surface images revealed that nylon is finely dispersed in CA, which confirms their compatibility with each other. Moreover, the hydrophilicity of the membrane surface their compatibility with each other. Moreover, the hydrophilicity of the membrane surface has been enhanced by the addition of Nylon 66, as indicated by the increase in the oil has been enhanced by the addition of Nylon 66, as indicated by the increase in the oil rejection rate. rejection rate. Pores were also available at the surface, which confirmed the ultrafiltration struc- Pores were also available at the surface, which confirmed the ultrafiltration structure ture [27]. Cavities were available across the cross-section of the membrane separated by a [27]. Cavities were available across the cross‐section of the membrane separated by a hon‐ honeycomb structure (Figure3). In addition, Figure3 shows the asymmetrical structure of eycombthe membrane structure which (Figure is also 3). In confirmed addition, by Figure other 3 researchers shows the asymmetrical [29]. The porosity structure of the of skin the membranelayer, as well which as the is availability also confirmed of cavities by other across researchers the cross-section [29]. The of theporosity membrane, of the could skin layer,be justified as well by as the the fact availability that DMSO of hascavities a higher across affinity the cross with‐section nonsolvents of the (water membrane, in this could case). beDue justified to the high by the solubility fact that parameter DMSO has of DMSO, a higher the affinity polymer with membrane nonsolvents is relatively (water porousin this case).(see Figure Due to3) comparedthe high solubility to the membrane parameter formed of DMSO, by using the polymer Formic Acid membrane ( see Figure is relatively2) . Less porousnonsolvent (see Figure dissolved 3) compared in a high amountto the membrane of solvent formed resulted by in using the production Formic Acid of (see cavities Fig‐ ureacross 2). theLess cross-section nonsolvent dissolved and pores in on a the high surface. amount of solvent resulted in the production of cavitiesThis phenomenonacross the cross has‐section also confirmed and pores thaton the the surface. affinity of solvent with nonsolvent resultsThis in phenomenon the production has of also fingerlike confirmed cavities that the as affinity well as of thin solvent skin with layer nonsolvent [29]. Similar re‐ sultsbehavior in the of production Nylon 66 was of fingerlike observed cavities by Lin as et al.well [31 as]. thin Nylon skin has layer become [29]. Similar more prominent behavior ofon Nylon the surface 66 was of membranes. observed by The Lin surface et al. [31]. of the Nylon membrane has become had porosity more owingprominent to the on lesser the surfacewidth of of the membranes. skin layer. The The surface addition of of the Nylon membrane 66 results had in porosity an increase owing in the to solution’sthe lesser widthviscosity of the [28 ].skin In the layer. case The of a addition lower concentration of Nylon 66 ofresults Nylon in 66, an large increase cavities in the were solution’s seen as viscosityshown in [28]. Figure In 2the. As case the of amount a lower of concentration nylon increases of Nylon in polymer 66, large solution, cavities the were number seen ofas showncavities in was Figure replaced 2. As with the amount solid material of nylon having increases less porosityin polymer [30 ].solution, the number of cavities was replaced with solid material having less porosity [30].

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3.2. Thermogravimetric Analysis (TGA) TGA was performed3.2. Thermogravimetric to analyze Analysisthe thermal (TGA) stability of the developed membrane (Figure 4). The TGA TGAcurve was showed performed three zones to analyze of weight the thermalloss at the stability temperature of the developedranges membrane of below 100 °C,(Figure 320–4004). °C, The and TGA above curve 400 showed °C. In three the case zones of ofthe weight developed loss at membranes, the temperature ranges of the first drop inbelow the TGA 100 curve◦C, 320–400 below ◦100C, and°C is above due to 400 the◦ C.loss In of the moisture. case of the Major developed weight membranes, the loss of the membranefirst drop was in noticed the TGA between curve below320–400 100 °C.◦C A is drop due in to the curve loss of would moisture. be the Major weight loss result of the degradationof the membrane of CA was[32]. noticed The last between drop in 320–400 the TGA◦C. curve A drop starting in the curveat 400 would °C be the result indicated the overallof the decomposition degradation of of CA the [32 membrane.]. The last drop in the TGA curve starting at 400 ◦C indicated TGA of CAthe‐based overall commercial decomposition membrane of the (C1) membrane. was also performed to compare its thermal properties withTGA those of CA-based of developed commercial membranes membrane at the (C1) same was conditions. also performed TGA to compare its curve of the C1 thermalmembrane properties showed with that those a major of developed drop in the membranes curve was at noticed the same at conditions. about TGA curve 180 °C and it continuesof the C1 until membrane 250 °C, showedwhere complete that a major degradation drop in the of the curve membrane was noticed was at about 180 ◦C observed. and it continues until 250 ◦C, where complete degradation of the membrane was observed. TGA results revealedTGA results that the revealed developed that themembranes developed (D1 membranes and F1) are (D1 more and resistant F1) are more resistant to to high temperatureshigh temperatures and showed and less showed mass loss less even mass at loss high even temperatures at high temperatures when com when‐ compared to pared to the commercialthe commercial membrane membrane (C1). This (C1). may This be may attributed be attributed to the toformation the formation of extra of extra hydrogen hydrogen bondingbonding interactions interactions between between CA CAand and Nylon Nylon 66. 66.Consequently, Consequently, extra extra energy energy was required was required toto break break these these additional additional bonds bonds [33]. [33]. Moreover, Moreover, the the solvent has nono significantsignifi‐ effect on the cant effect on thethermal thermal properties properties of of the the membrane.membrane. This finding finding also also suggests suggests that that both both polymers are polymers are compatiblecompatible with with each each other other and and the the addition addition of of Nylon Nylon 66 66 enhances enhances the the ther thermal‐ properties mal properties ofof CA CA [34]. [34].

Figure 4.Figure Thermogravimetric 4. Thermogravimetric analysis (TGA) analysis curves (TGA) of curves different of differentused membranes. used membranes.

3.3. Tensile Testing3.3. of Tensile Membranes Testing of Membranes Polymer membranesPolymer must membranes have the necessary must have mechanical the necessary properties mechanical to withstand properties the to withstand the pressure gradientpressure which gradient acts as a which driving acts force as a for driving the separation force for the process separation [35]. processSymmet [35‐ ]. Symmetrical rical or dense membranesor dense membranes have better have mechanical better mechanical properties propertieswhen compared when comparedwith asym with‐ asymmetric metric membranes.membranes. Asymmetrical Asymmetrical membranes membranes generally generally have a skin have layer a skin and layer large and cavi large‐ cavities which ties which deterioratedeteriorate the mechanical the mechanical strength strength of the of membrane. the membrane. However, However, symmetrical symmetrical membranes membranes areare compact, compact, do do not not have have fingerlike ﬁngerlike voids, voids, and and provide provide good mechanical properties [35]. properties [35]. Both developed developed and and commercial commercial membranes membranes were were tested tested to compare to compare their their mechanical mechanical propertiesproperties and and the theaverage average results results are are presented presented in inTable Table 33 and and Figure Figure 5.5. It was observed was observed thatthat commercial commercial membrane membrane (C1) (C1) shows shows high high tensile tensile strength strength up up to to 9 9 MPa. MPa. SEM image of SEM image of thethe commercial membranemembrane (Figure (Figure2) 2) revealed revealed its compact,its compact, symmetrical symmetrical structure along the structure along thecross-section cross‐section which which results results in high in high tensile tensile strength. strength. On On the the other other hand, hand, elongation at the break of the commercial membrane was about 2.9% which may be because of its porous

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elongation at the break of the commercial membrane was about 2.9% which may be because of its porous structure. The porous structure of the membrane reduces both the overallstructure. flexibility The porous and elongation structure at of break. the membrane reduces both the overall ﬂexibility and elongationDeveloped at break. membrane D1 (CA with 2 wt% Nylon 66 blended membrane having DMSODeveloped as a solvent) membrane was also D1 tested (CA withfor its 2 wt%satisfac Nylontory 66 mechanical blended membrane properties. having D1 mem- DMSO braneas a showed solvent) a was tensile also strength tested for of its6.9 satisfactoryMpa and elongation mechanical at break properties. of about D1 2.5%. membrane It is alsoshowed reported a tensile in the strengthliterature of that 6.9 the Mpa tensile and strength elongation of CA at break membrane, of about having 2.5%. an It asym- is also metricalreported structure, in the literature has a maximum that the tensile tensile strength strength of of CA 6.9 membrane, MPa [35]. SEM having images an asymmetrical of the D1 membranestructure, (Figure has a maximum 3) show tensilethat the strength membrane of 6.9 had MPa an [35 asymmetrical]. SEM images structure of the D1 with membrane large (Figure3) show that the membrane had an asymmetrical structure with large cavities which cavities which results in a decrease in elongation. results in a decrease in elongation. Developed membrane F1 (CA with 2 wt% Nylon 66 blended membrane having For- Developed membrane F1 (CA with 2 wt% Nylon 66 blended membrane having Formic mic Acid as the solvent) was also tested for its mechanical properties and results were Acid as the solvent) was also tested for its mechanical properties and results were very very promising. These membranes showed a tensile strength of about 14.3 MPa and elon- promising. These membranes showed a tensile strength of about 14.3 MPa and elongation gation at break of approximately 6%. SEM images revealed that Nylon 66 was homogene- at break of approximately 6%. SEM images revealed that Nylon 66 was homogeneously ously dispersed on the surface of the membrane. Such a homogeneous dispersion of nylon dispersed on the surface of the membrane. Such a homogeneous dispersion of nylon in the in the CA matrix imparts the inherent mechanical properties of nylon to the overall prop- CA matrix imparts the inherent mechanical properties of nylon to the overall properties of erties of the membrane. Membrane has fingerlike cavities beneath the skin layer, but these the membrane. Membrane has ﬁngerlike cavities beneath the skin layer, but these cavities cavities are not too wide. Moreover, the cavities extend to a porous, spongelike structure are not too wide. Moreover, the cavities extend to a porous, spongelike structure which whichprovides provides more more elongation elongation without without any breakage any breakage in the in membrane. the membrane. It was It observed was observed that as thatthe as amount the amount of Nylon of Nylon 66 increases 66 increases in polymer in polymer solution, solution, tensile strength tensile strength as well as as elongation well as elongationat break increases. at break increases. This increase This in increase mechanical in mechanical properties properties may be due may to be more due interaction to more interactionbetween polymericbetween polymeric chains. Moreover, chains. Moreover, the membrane the membrane structure also structure changed also from changed porous fromto dense porous which to dense resulted which in anresulted increase in inan the increase values in of the mechanical values of properties mechanical [35 properties], as shown [35in], Figureas shown5. in Figure 5.

Table 3. Tensile testing of membranes. Table 3. Tensile testing of membranes. Membrane Tensile Strength (Mpa) Elongation at Break (%) Membrane Tensile Strength (Mpa) Elongation at Break (%) C1 9 3 C1 9 3 D1 D16.9 6.92.5 2.5 D2 D27.4 7.45.7 5.7 D3 D310 106 6 F1 F114.3 14.36 6 F2 20 6.3 F2 20 6.3 F3 36 7 F3 36 7

Figure 5. MechanicalFigure properties 5. Mechanical of the fabricated properties membranes. of the fabricated (a) Tensile membranes strength;. ( (ab)) T elongationensile strength at break; (b) (%).elongation at break (%).

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3.4. Flux 3.4. FluxPermeate flux of commercial and developed membranes was determined by varying transmembranePermeate flux pressure of commercial and Nylon and 66 developed composition. membranes was determined by varying transmembrane pressure and Nylon 66 composition. 3.4.1. Effect of Composition on the Permeate Flux 3.4.1.The Effect composition of Composition of Nylon on the66 in Permeate polymer Flux solution was varied to evaluate the effect of nylonThe on compositionmembrane flux. of Nylon Results 66 showed in polymer that solution as the composition was varied toof evaluateNylon 66 the increased, effect of thenylon membrane on membrane permeate flux. flux Results decreased. showed As that the as amount the composition of Nylon of 66 Nylon increased 66 increased, in the pol the‐ ymericmembrane solution, permeate cavities flux were decreased. filled Asup thealong amount the cross of Nylon‐section 66 increasedof the membrane. in the polymeric Moreo‐ ver,solution, a higher cavities amount were of filledNylon up 66 along increases the cross-sectionthe surface roughness of the membrane. which results Moreover, in more a adhesionhigher amount of oil ofdroplets Nylon over 66 increases the membrane the surface surface roughness and the which formation results of in a moreresistant adhesion layer whichof oil droplets may hinder over the the contact membrane of CA surface with andwater the at formation the surface of and a resistant thus results layer whichin an over may‐ allhinder reduction the contact of flux. of Both CA with membrane water at materials the surface (CA and and thus Nylon results 66) swell in an overallwhen exposed reduction to waterof flux. [36,37]. Both membrane The polymer materials swelling (CA reduces and Nylon the overall 66) swell porosity when exposed of the membrane. to water [36 As,37 a]. result,The polymer a compact swelling structure, reduces having the overall less porosity porosity and of ultimately the membrane. less flux, As a was result, formed. a compact Fig‐ uresstructure, 6a and having 7a show less the porosity behavior and of ultimatelyflux as a function less flux, of wastime formed. for various Figures concentrations6a and7a ofshow Nylon the 66 behavior in the case of flux of D as‐ and a function F‐type ofmembranes, time for various respectively. concentrations of Nylon 66 in the case of D- and F-type membranes, respectively. 3.4.2. Effect of Transmembrane Pressure on Flux 3.4.2. Effect of Transmembrane Pressure on Flux The effect of transmembrane pressure (TMP) on the overall flux of the membrane The effect of transmembrane pressure (TMP) on the overall flux of the membrane was also studied. Results for both D‐ and F‐type membranes showed that an increase in was also studied. Results for both D- and F-type membranes showed that an increase transmembrane pressure resulted in an increase in the overall flux across the membrane in transmembrane pressure resulted in an increase in the overall flux across the mem- which may be attributed to the low thickness of the skin layer. An increase in the trans‐ brane which may be attributed to the low thickness of the skin layer. An increase in the membranetransmembrane pressure pressure may cause may some cause deformation some deformation in polymeric in polymeric chains as chains well as as in well pores’ as dimensionsin pores’ dimensions [38]. More [ permeate38]. More would permeate pass would through pass the through voids as thea result voids of as chain a result move of‐ ment.chain movement.However, over However, time, membrane over time, membraneflux decreases flux because decreases oil because droplets oil that droplets managed that tomanaged pass through to pass the through surface the of the surface membrane of the membrane later block laterthe pores block available the pores on available the surface on andthe surfacethroughout and throughoutthe cross‐section. the cross-section. Moreover, the Moreover, overall theflux overall of the fluxmembrane of the membrane decreases asdecreases a result asof athe result formation of the formationof a resistant of a layer resistant on the layer surface on the of surfacethe membrane. of the membrane. However,However, owing to the greater thickness of the skin layer of F F-type‐type membranes (than thatthat of the the D D-type‐type membrane), the the change change in in flux flux was was not significant. significant. FiguresFigures 66bb andand7 7bb showshow the the effect effect of of transmembrane transmembrane pressure pressure on on the the overall overall flux flux of of the the D D-‐ and and F F-type‐type mem mem-‐ branes,branes, respectively.

Figure 6. FluxFlux of of D D-type‐type membranes as a function of ( a) time and ( b) transmembrane pressure.

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FigureFigure 7. 7. FluxFlux of of F F-type‐type membranes membranes as as a a function function of of ( (aa)) time time and and ( (bb)) transmembrane transmembrane pressure. pressure.

3.5. Rejection of Oil 3.5. RejectionPermeate of was Oil treated in order to calculate the amount of oil that manages to pass throughPermeate the membrane. was treated The inoil order and grease to calculate hexane the extraction amount method of oil that was manages used to examine to pass thethrough amount the of membrane. oil. The oil and grease hexane extraction method was used to examine the amount of oil. 3.5.1. Oil Rejection as a Function of Composition 3.5.1. Oil Rejection as a Function of Composition Oil rejection was calculated by varying the composition of the polymer solution. The Oil rejection was calculated by varying the composition of the polymer solution. The amount of Nylon 66 in polymeric solution was varied and its effect on oil rejection was amount of Nylon 66 in polymeric solution was varied and its effect on oil rejection was studied. Results showed that as the amount of nylon increases, oil rejection increases as studied. Results showed that as the amount of nylon increases, oil rejection increases well. This increase in oil rejection could be attributed to a change in the membrane struc‐ as well. This increase in oil rejection could be attributed to a change in the membrane ture due to the addition of Nylon 66. While the overall membrane porosity decreased, the structure due to the addition of Nylon 66. While the overall membrane porosity decreased, thickness of the skin layer increased and, thus, more oil is rejected. Moreover, Nylon 66 the thickness of the skin layer increased and, thus, more oil is rejected. Moreover, Nylon offers a barrier to oil and any increase in the amount of nylon in polymeric solution de‐ 66 offers a barrier to oil and any increase in the amount of nylon in polymeric solution creases the amount of oil in the permeate. Figures 8a and 9a show the oil rejection of the decreases the amount of oil in the permeate. Figures8a and9a show the oil rejection of the D‐type and the F‐type membranes, respectively, as a function of time for various Nylon D-type and the F-type membranes, respectively, as a function of time for various Nylon 66 66 concentrations at constant pressure of 0.5 bar. concentrations at constant pressure of 0.5 bar.

3.5.2.3.5.2. Oil Oil Rejection Rejection as as a a Function Function of of Transmembrane Transmembrane Pressure OilOil rejection rejection was was also also calculated calculated as as a a function function of of transmembrane transmembrane pressure pressure to to examine examine thethe effect effect of of pressure pressure on on membrane membrane performance. performance. Results Results showed showed that that as as transmembrane transmembrane pressurepressure across across the the membrane membrane increases, increases, the the overall overall oil oil rejection rejection decreases. This This behavior behavior ofof the the membrane membrane could could be be due due to to pore pore blockage blockage or or the the formation formation of of a a resistant resistant layer layer over over thethe surface surface of of the the membrane membrane [36]. [36 An]. increase An increase in transmembrane in transmembrane pressure pressure across across the mem the‐ branemembrane resulted resulted in a higher in a higherpassage passage of oil droplets of oil droplets across the across membrane. the membrane. However, However, stiffness impartedstiffness impartedby Nylon by66 Nylonto the membrane 66 to the membrane reduced the reduced effect of the high effect pressure of high on pressure the mem on‐ branethe membrane [34] and oil [34 rejection] and oil rejectionof membrane of membrane did not rapidly did not decrease. rapidly decrease.However, However, it is worth it mentioningis worth mentioning that oil rejection that oil rejectionwas higher was in higher the case in theof F case‐type of membranes F-type membranes than D‐type than membranes.D-type membranes. This increase This increasein oil rejection in oil rejectionmight be mightdue to bethe due high to miscibility the high miscibility of Nylon 66 of inNylon Formic 66 inAcid. Formic Figures Acid. 8b Figures and 9b8b show and9 bthe show effect the of effect transmembrane of transmembrane pressure pressure on the on oil rejectionthe oil rejection through through all tested all membranes. tested membranes.

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Figure 8. Oil rejection of D‐type membranes as a function of (a) time at 0.5 bar and (b) pressure. Figure 8. Oil rejection of D-typeD‐type membranes asas aa functionfunction ofof ((aa)) timetime atat 0.50.5 barbar andand ((bb)) pressure.pressure.

Figure 9. Oil ejection of F-type membranes as a function of (a) time at 0.5 bar and (b) pressure. Figure 9. Oil ejection of F‐type membranes as a function of (a) time at 0.5 bar and (b) pressure. Figure 9. Oil ejection of F‐type membranes as a function of (a) time at 0.5 bar and (b) pressure. Experimental results in terms of permeate flux and oil rejection have been compared Experimental results in terms of permeate flux and oil rejection have been compared with the results of other research groups (Table4). Developed membranes have the potential withExperimental the results of results other researchin terms ofgroups permeate (Table flux 4). and Developed oil rejection membranes have been have compared the po‐ towith be usedthe results for oil/water of other separation. research groups It is evident (Table from 4). Developed the comparison membranes that membrane have the haspo‐ atential higher to oil be rejection used for with oil/water modest separation. water flux. It is It evident is also worth from the mentioning comparison here that that mem the ‐ tentialbrane tohas be a usedhigher for oil oil/water rejection separation. with modest It wateris evident flux. from It is alsothe comparisonworth mentioning that mem here‐ polymerbrane has used a higher is CA, oil which rejection is among with themodest cheapest water polymers flux. It is and also has worth the potential mentioning to make here thethat process the polymer cost-effective used is forCA, large which scale is among applications. the cheapest polymers and has the potential thatto make the polymer the process used cost is CA,‐effective which for is amonglarge scale the cheapestapplications. polymers and has the potential Tableto make 4. Comparison the process of experimentalcost‐effective results for large with scale other applications. membranes. Table 4. Comparison of experimental results with other membranes. 2 Membrane MaterialsTable 4. Comparison of experimental Oil Rejection, results with % otherWater membranes Flux, L/m. h Reference Membrane Materials Oil Rejection, % Water Flux, L/m2h Reference Polystyrene 96 230 [39] Polystyrene Membrane Materials Oil Rejection,96 % Water Flux,230 L/m2h Reference[39] Polysulfone 94.21 100 [40] PolystyrenePolysulfone 94.2196 230100 [39][40] Polysulfone/Polyvinyl pyrrolidone 99.7 43.6 [41] PolysulfonePolysulfone/Polyvinyl pyrrolidone 94.2199.7 10043.6 [40][41] Polysulfone/Cellulose acetate/Polyethylene glycol 80 700 [25] Polysulfone/PolyvinylPolysulfone/Cellulose pyrrolidoneacetate/Polyethylene glycol 99.780 43.6700 [41][25] Polyamide 60 100 [42] Polysulfone/CellulosePolyamide acetate/Polyethylene glycol 8060 700100 [25][42] Polysulfone/Cellulose acetate 60 40 [43] PolyamidePolysulfone/Cellulose acetate 6060 10040 [42][43] Polysulfone 140 - Polysulfone/CellulosePolysulfone acetate 60140 ‐ 40 [43] Polysulfone/Iron acetate 170 - PolysulfonePolysulfone/Iron acetate 140170 ‐ ‐ [44] Polysulfone/Polyamide film 310 - [44] Polysulfone/IronPolysulfone/Polyamide acetate film 170310 ‐ ‐ [44] Polysulfone/PolyamidePolysulfone/IronPolysulfone/Iron acetate/Polyamide acetate/Polyamide film film film 380310380 ‐ ‐ - Polysulfone/IronCelluloseCellulose acetate/Nylon acetate/Nylon acetate/Polyamide 66/Dimethyl 66/Dimethyl film Sulfoxide Sulfoxide (D1) (D1) 8938089 ‐ 3333 ThisThis work work CelluloseCelluloseCellulose acetate/Nylon acetate/Nylon acetate/Nylon 66/Dimethyl 66/Formic 66/Formic Acid Sulfoxide (F3) Acid (D1) (F3) 958995 233323 ThisThisThis work work Cellulose acetate/Nylon 66/Formic Acid (F3) 95 23 This work

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4. Conclusions In this work, cellulose acetate membranes reinforced with Nylon 66 were successfully fabricated with the solution casting technique. The developed membranes were subjected to extensive characterization in terms of surface analysis, flux permeation, oil rejection, thermal analysis, and mechanical analysis. In comparison to commercial pristine CA membrane, a significant improvement in properties was found. Scanning electron micrographs reveled the uniform dispersion of Nylon 66 into the polymer matrix, this can be a reason for the enhancement of mechanical and permeation properties. The onset of backbone degradation started at 180 ◦C in case of pure CA membrane. However, it was shifted to 320 ◦C after the addition of 2% Nylon 66 in CA matrix. The results confirmed the improvement in thermal properties after the addition of Nylon 66 into CA matrix. The tensile strength of the composite membranes improved drastically after the addition of Nylon 66 when formic acid was used as solvent. A similar trend was found for elongation at break. The maximum tensile strength and elongation at break of 36 MPa and 7%, respectively, were depicted by F3 membrane. The results encourage the application of prepared membranes at relatively high pressures. Likewise, the flux permeation and oil rejection were significantly higher than pure CA membrane. As the transmembrane pressure increased, the flux increased accordingly. Permeate flux of 33 L/m2h and oil rejection of 95% were achieved by the membrane developed in this work. All these results confirm the effectiveness of CA membranes reinforced by Nylon 66 for oil–water separations.

Author Contributions: Both authors contributed significantly to the completion of this article, but they had different roles in all aspects. Conceptualization, A.H. and M.A.-Y.; data curation, A.H.; formal analysis, A.H. and M.A.-Y.; funding acquisition, A.H.; investigation, A.H.; methodology, A.H.; project administration, M.A.-Y.; validation, A.H. and M.A.-Y.; visualization, A.H. and M.A.-Y.; writing—original draft, A.H.; writing—review & editing, M.A.-Y. All authors have read and agreed to the published version of the manuscript. Funding: This research and the APC were funded by the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia through the project number IFT20156. Acknowledgments: The authors extend their appreciation to the Deputyship for Research & Inno- vation, Ministry of Education in Saudi Arabia for funding this research work through the project number IFT20156. Authors hereby also thank Tariq Mehmood for his great help to produce excellent experimental work. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Bhaduri, A.; Bogardi, J.; Siddiqi, A.; Voigt, H.; Vörösmarty, C.; Pahl-Wostl, C.; Bunn, S.E.; Shrivastava, P.; Lawford, R.; Foster, S. Achieving sustainable development goals from a water perspective. Front. Environ. Sci. 2016, 4, 64. [CrossRef] 2. Conard, B.R. Some challenges to sustainability. Sustainability 2013, 5, 3368–3381. [CrossRef] 3. Jänicke, M.; Jörgens, H.; Jörgensen, K.; Nordbeck, R. Governance for Sustainable Development in Germany: Institutions and Policy Making. Report for the OECD iLibrary. 2001. Available online: www.oecd.org/puma/strat/pubs/GermanySD.pdf (accessed on 15 October 2020). [CrossRef] 4. Singh, P.; Shandilya, P.; Raizada, P.; Sudhaik, A.; Rahmani-Sani, A.; Hosseini-Bandegharaei, A. Review on various strategies for enhancing photocatalytic activity of graphene based nanocomposites for water puriﬁcation. Arab. J. Chem. 2020, 13, 3498–3520. [CrossRef] 5. Chen, D.; Zhu, H.; Yang, S.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. Micro–nanocomposites in environmental management. Adv. Mater. 2016, 28, 10443–10458. [CrossRef][PubMed] 6. Bebbington, J.; Unerman, J. Achieving the United Nations sustainable development goals. Account. Audit. Account. J. 2018, 31, 2–24. [CrossRef] 7. Haberl, R. Constructed wetlands: A chance to solve wastewater problems in developing countries. Water Sci. Technol. 1999, 40, 11–17. [CrossRef] 8. Bouwer, H. Integrated water management: Emerging issues and challenges. Agric. Water Manag. 2000, 45, 217–228. [CrossRef] 9. Yu, L.; Han, M.; He, F. A review of treating oily wastewater. Arab. J. Chem. 2013, 10, S1913–S1922. [CrossRef] 10. Baker, R.W. Membrane Technology and Applications, 2nd ed.; John Wiley & Sons, Ltd.: Hoboken, NJ, USA, 2004; Available online: https://onlinelibrary.wiley.com/doi/book/10.1002/0470020393 (accessed on 15 October 2020). Membranes 2021, 11, 42 14 of 15

11. Princy, K.; Holaday, M.D.; Gopinath, A. Marine macroalgae biofabricated silver nanoparticles as naked-eye colorimetric and turn-on fluorescent sensor for cyanide ions in aqueous media. Environ. Nanotechnol. Monit. Manag. 2020, 100399. [CrossRef] 12. Cheryan, M.; Rajagopalan, N. Membrane processing of oily streams. Wastewater treatment and waste reduction. J. Membr. Sci. 1998, 151, 13–28. [CrossRef] 13. Li, H.J.; Cao, Y.M.; Qin, J.J.; Jie, X.M.; Wang, T.H.; Liu, J.H.; Yuan, Q. Development and characterization of anti-fouling cellulose hollow fiber UF membranes for oil–water separation. J. Membr. Sci. 2006, 279, 328–335. [CrossRef] 14. Zoubeik, M.; Ismail, M.; Salama, A.; Henni, A. New developments in membrane technologies used in the treatment of produced water: A review. Arab. J. Sci. Eng. 2018, 43, 2093–2118. [CrossRef] 15. Wang, J.; Hou, L.A.; Yan, K.; Zhang, L.; Yu, Q.J. Polydopamine nanocluster decorated electrospun nanofibrous membrane for separation of oil/water emulsions. J. Membr. Sci. 2018, 547, 156–162. [CrossRef] 16. De Coutinho, C.; Chiu, M.C.; Basso, R.C.; Ribeiro, A.P.B.; Gonçalves, L.A.G.; Viotto, L.A. State of art of the application of membrane technology to vegetable oils: A review. Food Res. Int. 2009, 42, 536–550. [CrossRef] 17. Mohammadi, T.; Esmaeelifar, A. Wastewater treatment using ultrafiltration at a vegetable oil factory. Desalination 2004, 166, 329–337. [CrossRef] 18. Salahi, A.; Noshadi, I.; Badrnezhad, R.; Kanjilal, B.; Mohammadi, T. Nano-porous membrane process for oily wastewater treatment: Optimization using response surface methodology. J. Environ. Chem. Eng. 2013, 1, 218–225. [CrossRef] 19. Salahi, A.; Gheshlaghi, A.; Mohammadi, T.; Madaeni, S.S. Experimental performance evaluation of polymeric membranes for treatment of an industrial oily wastewater. Desalination 2010, 262, 235–242. [CrossRef] 20. He, Y.; Jiang, Z.-W. Technology review: Treating oilfield wastewater. Filtr. Sep. 2008, 45, 14–16. [CrossRef] 21. Lv, J.; Zhang, G.; Zhang, H.; Zhao, C.; Yang, F. Improvement of antifouling performances for modified PVDF ultrafiltration membrane with hydrophilic cellulose nanocrystal. Appl. Surf. Sci. 2018, 440, 1091–1100. [CrossRef] 22. Chen, W.; Su, Y.; Zheng, L.; Wang, L.; Jiang, Z. The improved oil/water separation performance of cellulose acetate-graft- polyacrylonitrile membranes. J. Membr. Sci. 2009, 337, 98–105. [CrossRef] 23. Yan, L.; Shui, Y.; Xiang, C.B. Preparation of poly(vinylidene fluoride)(pvdf) ultrafiltration membrane modified by nano-sized alumina (Al2O3) and its antifouling research. Polymer 2005, 46, 7701–7706. [CrossRef] 24. Kota, A.K.; Kwon, G.; Choi, W.; Mabry, J.M.; Tuteja, A. Hygro-responsive membranes for effective oil–water separation. Nat. Commun. 2012, 3, 1025. [CrossRef][PubMed] 25. Mansourizadeh, A.; Azad, A.J. Preparation of blend polyethersulfone/cellulose acetate/polyethylene glycol asymmetric membranes for oil–water separation. J. Polym. Res. 2014, 21, 375. [CrossRef] 26. Mousa, H.M.; Alfadhel, H.; Abouel Nasr, E. Engineering and Characterization of Antibacterial Coaxial Nanofiber Membranes for Oil/Water Separation. Polymers 2020, 12, 2597. [CrossRef][PubMed] 27. De Oliveiraa, P.N.; Mendes, A.M.M. Preparation and Characterization of an Eco-Friendly Polymer Electrolyte Membrane (PEM) Based in a Blend of Sulphonated Poly(Vinyl Alcohol)/ Chitosan Mechanically Stabilised by Nylon 6,6. Mater. Res. 2016, 19, 954–962. [CrossRef] 28. Gao, W.; Liang, H.; Ma, J.; Han, M.; Chen, Z.L.; Han, Z.S.; Li, G. Membrane fouling control in ultrafiltration technology for drinking water production: A review. Desalination 2011, 272, 1–8. [CrossRef] 29. Young, T.H.; Chen, L.W. Pore formation mechanism of membrane from phase inversion process. Desalination 1995, 103, 233–247. [CrossRef] 30. Sivakumar, M.; Malaisamy, R.; Sajitha, C.J.; Mohan, D.; Mohan, D.; Rangarajan, R. Preparation and performance of cellulose acetate-polyurethane blend membranes and their applications-II. J. Membr. Sci. 2000, 169, 215–228. [CrossRef] 31. Lin, D.J.; Chang, C.L.; Lee, C.K.; Cheng, L.P. Fine structure and crystallinity of porous Nylon 66 membranes prepared by phase inversion in the water/formic acid/Nylon 66 system. Eur. Polym. J. 2006, 42, 356–367. [CrossRef] 32. Rodríguez, F.; Castillo-Ortega, M.M.; Encinas, J.C.; Grijalva, H.; Brown, F.; Sanchez-Corrales, V.M.; Castano, V.M. Preparation, characterization, and adsorption properties of cellulose acetate-polyaniline membranes. J. Appl. Polym. Sci. 2009, 111, 1216–1224. [CrossRef] 33. Joshi, M.K.; Tiwari, A.P.; Maharjan, B.; Won, K.S.; Kim, H.J.; Park, C.H.; Kim, C.S. Cellulose reinforced nylon-6 nanofibrous membrane: Fabrication strategies, physicochemical characterizations, wicking properties and biomimetic mineralization. Carbohydr. Polym. 2016, 147, 104–113. [CrossRef][PubMed] 34. Guruprasad, K.H.; Shashidhara, G.M. Grafting, Blending, and Biodegradability of cellulose acetate. J. Appl. Polym. Sci. 2003, 91, 1716–1723. [CrossRef] 35. Bai, H.; Zhou, Y.; Wang, X.; Zhang, L. The Permeability and Mechanical Properties of Cellulose Acetate Membranes Blended with Polyethylene glycol 600 for Treatment of Municipal Sewage. Procedia Environ. Sci. 2012, 16, 346–351. [CrossRef] 36. Chakrabarty, B.; Ghoshal, A.K.; Purkait, M.K. Ultrafiltration of stable oil-in-water emulsion by polysulfone membrane. J. Membr. Sci. 2008, 325, 427–437. [CrossRef] 37. Hoeck, E.M.V.; Bhattacharge, S.; Elimelech, M. Effect of Membrane Surface Roughness on Colloid- Membrane DLVO Interactions. Langmuir 2003, 19, 4836–4847. [CrossRef] 38. Hua, F.L.; Tsang, Y.F.; Wang, Y.J.; Chan, S.Y.; Chua, H.; Sin, S.N. Performance study of ceramic microfiltration membrane for oily wastewater treatment. Chem. Eng. J. 2007, 128, 169–175. [CrossRef] Membranes 2021, 11, 42 15 of 15

39. Moatmed, S.M.; Khedr, M.H.; El-dek, S.; Kim, H.-Y.; El-Deen, A.G. Highly efficient and reusable superhydropho- bic/superoleophilic polystyrene@ Fe3O4 nanofiber membrane for high-performance oil/water separation. J. Environ. Chem. Eng. 2019, 7, 103508. [CrossRef] 40. Pagidi, A.; Saranya, R.; Arthanareeswaran, G.; Ismail, A.; Matsuura, T. Enhanced oil–water separation using polysulfone membranes modified with polymeric additives. Desalination 2014, 344, 280–288. [CrossRef] 41. Sadeghi, I.; Aroujalian, A.; Raisi, A.; Dabir, B.; Fathizadeh, M. Surface modification of polyethersulfone ultrafiltration membranes by corona air plasma for separation of oil/water emulsions. J. Membr. Sci. 2013, 430, 24–36. [CrossRef] 42. Zhang, Z.; Yin, C.; Yang, G.; Xiao, A.; Shi, X.; Xing, W.; Wang, Y. Stitching nanosheets of covalent organic frameworks to build aligned nanopores in nanofiltration membranes for precise ion separations. J. Membr. Sci. 2021, 618, 118754. [CrossRef] 43. Idris, A.; Ahmad, I. Cellulose acetate-polyethersulfone (CA-PS) blend ultrafiltration membranes for palm oil mill effluent treatment. In Survival and Sustainability; Springer: Berlin/Heidelberg, Germany, 2010; pp. 1149–1159. 44. Mousa, H.M.; Alfadhel, H.; Ateia, M.; Abdel-Jaber, G.T.; Gomaa, A.A.; Gomaa, A.A. Polysulfone-iron acetate/polyamide nanocomposite membrane for oil-water separation. Environ. Nanotechnol. Monit. Manag. 2020, 14, 100314. [CrossRef]