In recent decades, rapid industrialization and economic growth have significantly sped up urbanization, industrial output, and overall quality of life. However, this expansion has also increased the production of industrial waste, especially oily wastewater, which poses a serious environmental threat if released untreated into natural ecosystems [1,2]. The United States Environmental Protection Agency (USEPA) classifies oil-contaminated wastewater as one of the most toxic and hazardous effluents because of its persistent nature and damaging ecological effects [3]. Oily wastewater comes from various industrial activities, including oil and gas exploration, petroleum refining, chemical and pharmaceutical manufacturing, metal processing, food and beverage production, and even household waste. Of these, the oil and gas industry and the petrochemical sector are the biggest sources [4]. Although technology exists to remove oil from water, the complex makeup and stable emulsions in these wastewaters make treatment very difficult. Food processing industries also release large amounts of oily wastewater, which contain different petroleum fractions like diesel, gasoline, kerosene, and lubricants, either dispersed or emulsified in water [5]. If not properly treated, oily wastewater released into water bodies can severely harm ecosystems. Oil films on the surface block oxygen transfer, raising biochemical oxygen demand (BOD) and chemical oxygen demand (COD). This causes oxygen depletion, stops photosynthesis by blocking sunlight, and harms aquatic life [6,7].

To address these concerns, a range of physical, chemical, and biological treatment technologies has been developed to reduce pollutant loads [8]. However, conventional treatment methods often fall short in fully eliminating complex contaminants; they may only transfer pollutants from one phase to another (e.g., liquid to sludge), rather than remove them entirely [9,10]. Many of these methods are inefficient at handling highly emulsified or stable oil-water mixtures and often require extensive post-treatment processes, which increase operational costs and complexity [11].

Within the oil and gas sector, traditional approaches such as gravity separation, centrifugation, coagulation, and flotation are commonly employed to remove dispersed and free-floating oil. However, these techniques are limited in effectiveness, especially for emulsified oils with small droplet sizes [12]. Membrane filtration has emerged as a more promising and efficient alternative, offering superior separation performance and scalability for various water treatment applications worldwide [13].

Polymeric membranes, in particular, are widely used due to their excellent mechanical properties, ease of processing, chemical resistance, tunable pore structure, and cost-effectiveness [14]. These membranes function as semi-permeable barriers that selectively allow water molecules to pass while rejecting oil droplets, suspended solids, and organic matter. Despite their advantages, polymeric membranes suffer from limitations such as fouling, thermal and chemical instability, reduced permeability over time, and degradation under harsh operating conditions [15]. These drawbacks limit long-term operation and necessitate frequent maintenance or replacement.

To overcome these limitations, researchers have investigated the modification of membranes by blending different polymers or incorporating nanoparticles (NPs) to enhance membrane structure and performance. Polymer blending is an effective and economical strategy that combines the beneficial properties of individual polymers, improving mechanical strength, elasticity, thermal resistance, and fouling resistance [16]. Blended polymer membranes can be either homogeneous or heterogeneous, depending on the miscibility and compatibility of the constituent polymers. Homogeneous blends are particularly valuable for achieving thermodynamically stable membranes with enhanced separation performance [17].

The polymer blending technique is advantageous because it simplifies membrane fabrication compared to synthesizing new copolymers. It offers greater design flexibility, allowing optimization of water permeability, surface hydrophilicity, structural integrity, and antifouling behavior [18]. These features are critical for ensuring membrane efficiency and durability during oily wastewater treatment. In parallel with polymer blending, the incorporation of nanomaterials into polymer matrices has gained significant attention. Nanoparticles can modify membrane surface characteristics, improve pore structure, enhance hydrophilicity, and impart additional functionalities such as antibacterial or photocatalytic activity [19]. Among the most studied nanomaterials are titanium dioxide (TiO₂), graphene oxide (GO), and iron-based nanoparticles. These materials possess high surface area, unique physicochemical properties, and excellent compatibility with polymer matrices. Their hydrophilic functional groups enable strong interactions with the membrane matrix and water, leading to reduced fouling and improved oil-water separation [19,20].

Recent research has also shifted toward green nanotechnology that uses plant-based materials to synthesize nanoparticles. This eco-friendly approach offers advantages such as cost-effectiveness, non-toxicity, rapid synthesis, and compatibility with biodegradable materials [21]. Plant-extracted nanoparticles often contain hydroxyl (−OH), carboxyl (−COOH), and amine (−NH₂) functional groups, which assist with surface modifications and interactions with both polymers and contaminants [22,23]. The use of such green nanomaterials promotes sustainable membrane development and supports global environmental goals. These studies confirm that polymeric membranes enhanced with green NPs not only deliver high separation performance but also exhibit improved fouling resistance and mechanical strength, making them ideal for sustainable oily wastewater treatment. In addition, their low-cost raw materials and simple preparation methods increase their usefulness at both laboratory and industrial levels.

The present review aims to thoroughly explore recent advances in the development of eco-friendly polymeric membranes made from polymer blends, especially those enhanced with green or traditional nanomaterials, for treating oily wastewater. Specifically, this work assesses the performance of polymer blend membranes, compares them with conventional polymer membranes, and examines the role of both synthetic and plant-based NPs in boosting membrane effectiveness. It also highlights key challenges and suggests future directions for improving membrane materials and fabrication methods. Furthermore, this review considers the real-world applicability of these advanced membranes, evaluating factors such as environmental impact, economic viability, and safety regulation compliance. The goal of this review is to provide a valuable resource for researchers, engineers, and policymakers pursuing sustainable and effective solutions for managing industrial oily wastewater.

Oily wastewater is one of the most challenging and hazardous forms of industrial effluent due to its complex composition, persistence, and toxicity. The primary sources of oily wastewater are the petrochemical industry, petroleum exploration and extraction activities, food processing, metal and machinery industries, textile and leather manufacturing, paint production, marine transportation, and the automotive sector. These industries generate wastewater streams that are often contaminated with oil and grease, contributing significantly to environmental degradation [24]. The harmful effects of oily wastewater are multifaceted, including toxicity to aquatic life, poisoning of animals and humans, and the inhibition of plant growth due to oil-coated soils and oxygen depletion in water bodies [25]. In addition to industrial origins, domestic activities such as kitchen waste disposal and urban surface runoff also contribute to oily wastewater pollution. The composition of oily wastewater varies depending on its source, but it typically contains a wide spectrum of hydrocarbons ranging from light oils and greases to heavy oils, bitumen, waxes, cutting fluids, and lubricants [26]. For instance, domestic sewage usually contains oil and grease concentrations ranging from 50 to 100 ppm, while industrial effluents especially those from petroleum operations can have oil concentrations as high as 4000 to 6000 ppm [27,28]. Fig. S1 in the Supplementary Materials depicts the typical sources, characteristics, environmental effects, and treatment advantages of oily wastewater [29].

Based on droplet size and stability, oily wastewater is commonly classified into four types: free-floating oil, dispersed oil, emulsified oil, and dissolved oil. Free-floating oil consists of large droplets (>150 μm in diameter) that quickly rise to the water surface and can be easily removed through gravity separation. Dispersed oil contains droplet sizes of 20–150 μm, which are moderately stable and require processes like sedimentation or flotation for removal [30]. Emulsified oil droplets are smaller than 20 μm and are stabilized by surfactants or natural emulsifiers, making them significantly harder to separate. Dissolved or "melted" oil, with droplet sizes below 5 μm, presents the most difficult challenge and typically requires advanced treatment methods to achieve effective separation [31].

To address these challenges, various treatment techniques have been employed, including physical, chemical, mechanical, and biological methods. Traditional physical methods include gravity separation, skimming, filtration, and flotation. Flotation and gravity separation are typically applied to remove oil droplets larger than 150 μm. In contrast, emulsified oil droplets below 20 μm require advanced coagulation or membrane-based separation methods [32,33]. Dissolved oils with droplet sizes under 5 μm necessitate sophisticated chemical or membrane filtration technologies to achieve effective treatment [34]. Chemical approaches often rely on coagulants, flocculants, or surfactants to destabilize emulsions and aggregate oil particles. Mechanical techniques such as centrifugation are used to enhance separation efficiency, while biological treatment can degrade certain hydrocarbon compounds using microorganisms [35]. The most prominent advantages of these traditional methods include: Ease of operation and maintenance, making them suitable for facilities with limited resources; and low capital costs compared to advanced solutions. It is widely used in various applications within the industrial sector. However, these methods face several challenges, including poor efficiency in removing fine emulsified oils, which limits the quality of the resulting water. Heavy reliance on chemicals, which results in potentially polluting by-products [36]. Furthermore, these processes require large surface areas and long treatment times to achieve satisfactory purification levels. Their performance can also be significantly affected by variations in inlet water quality, resulting in reduced operational stability. The effectiveness of biological treatment is limited in cases of high oil concentration or the presence of contaminants toxic to microorganisms. As a result of these challenges, membrane separation technologies have emerged as promising alternatives, proving their effectiveness in selectively and effectively removing oils and contaminants, supported by ongoing advances in membrane design and improving their surface properties to resist fouling and enhance their long-term performance. However, these conventional techniques are often more effective for treating free and dispersed oils rather than emulsified or dissolved oils. In addition, their efficiency declines at higher oil concentrations and they frequently require multiple treatment stages [37,38]. Fig. S2 in the Supplementary Materials displays examples of oily industrial wastewater collected from oil production facilities in Iraq.

Membrane separation technology has emerged as one of the most promising and effective methods for treating oily wastewater, owing to its ability to selectively separate dissolved substances, emulsified oils, and fine particulates without the need for extensive chemical additives. Membranes act as physical barriers composed of polymeric, metallic, or ceramic materials with finely controlled pore sizes, allowing certain components (e.g., water) to pass through while retaining others (e.g., oil droplets, suspended solids, and contaminants) [39]. Their role is especially critical in the context of sustainable water treatment, where minimizing chemical usage, energy consumption, and waste generation is essential.

By definition, a membrane is a selective barrier that enables the separation of two phases typically oily compounds and water based on size, charge, or molecular affinity. The advancement of membrane materials and configurations has led to broad applicability across industries including oil and gas, petrochemicals, food processing, pharmaceuticals, and municipal water treatment [40]. One of the key advantages of membrane-based processes is their ability to operate without the need for additional chemical reagents, making them more environmentally benign. Moreover, they often require less energy compared to conventional thermal or chemical separation methods, contributing to operational sustainability. Despite their advantages, membrane technologies face several challenges. High capital and maintenance costs, membrane fouling, pore clogging, and degradation under long-term operation can hinder efficiency and economic feasibility [41,42]. Polymeric membranes, in particular, are susceptible to fouling by oil, organic matter, or biofilms, which can reduce permeability and membrane lifespan. Nonetheless, membrane processes remain among the most efficient treatment options for oily wastewater due to their high selectivity and ability to achieve excellent separation performance under optimized conditions [43,44].

Membrane separation systems are primarily classified based on the mechanism driving the filtration process most notably, pressure-driven membrane filtration. This category includes microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Each process is distinguished by its characteristic pore size, operating pressure, and ability to retain particles of specific sizes and types [45].

Microfiltration (MF) is one of the coarser membrane separation techniques, using membranes with pore sizes ranging from approximately 100 to 10,000 nanometers. It is mainly used to remove large suspended solids, emulsified particles, and some bacteria. The operating pressure for MF membranes typically ranges between 0.2 and 5 bar. MF is widely applied in the food and dairy industries, industrial water clarification, pre-treatment for finer membrane processes, and oil-water separation in oily wastewater treatment [46,47]. While effective at removing free-floating oil and coarse particles, MF membranes cannot retain emulsified oil or dissolved contaminants.

Ultrafiltration (UF) offers finer separation capabilities, with membrane pore sizes between 10 and 100 nanometers. These membranes are capable of rejecting macromolecules, emulsified oils, and colloidal particles. UF membranes operate at pressures between 1 and 10 bar and can separate substances with molecular weights ranging from 1,000 to 100,000 Daltons [48,49]. In oily wastewater treatment, UF membranes are widely used due to their high flux, fouling resistance, and ability to retain emulsified oils and proteins. Applications span a range of industries, including pharmaceuticals, dairy, food processing, petrochemical wastewater recycling, and seawater pre-treatment prior to reverse osmosis. Many studies confirm UF to be a cost-effective and environmentally sustainable alternative to traditional treatment methods, particularly for separating oil-soluble organic matter and fine particulates from water streams [50,51].

Nanofiltration (NF) bridges the gap between ultrafiltration and reverse osmosis. NF membranes typically possess pore sizes between 1 and 10 nanometers and can separate molecules in the 200 to 1,000 Dalton range. Operating at moderate pressures of 5–10 bar, NF membranes are capable of removing divalent and trivalent ions, small organic molecules (such as carboxylic acids, glucose, and amino acids), and residual oils [50]. Their selective permeability makes them suitable for various applications, including dye and salt removal, water softening, wastewater reuse, and concentration of valuable organics in industrial effluents. In the context of oily wastewater treatment, NF can serve as a polishing step following MF or UF, ensuring that emulsified oils and dissolved contaminants are efficiently removed [51].

Reverse osmosis (RO) represents the most refined level of pressure-driven membrane filtration. RO membranes have dense structures with pore sizes less than 1 nanometer and a molecular weight cutoff of around 100 Daltons. Operating at high pressures ranging from 10 to 150 bar, RO membranes are capable of rejecting virtually all dissolved salts, organics, bacteria, and viruses, making them ideal for producing high-purity water from heavily contaminated sources [52]. RO is widely used in desalination, ultrapure water production, and the final treatment stage for industrial wastewater, including that containing emulsified or dissolved oils. The selective nature of RO membranes, however, comes with high energy requirements and increased susceptibility to fouling, necessitating robust pre-treatment processes and regular membrane cleaning [51,53]. Fig. 1 demonstrates the relative pore size ranges and operating pressures associated with MF, UF, NF, and RO processes, demonstrating how membrane processes span the continuum of separation from macro- to micro-scale pollutants [51].

In summary, pressure-driven membrane technologies offer flexible and efficient treatment options for oily wastewater across varying contamination levels and industrial settings. While MF and UF are effective for removing large particulates and emulsified oils, NF and RO provide deeper purification by targeting dissolved and fine contaminants. The choice of membrane process depends on the nature of the oily wastewater, the desired quality of treated water, and operational constraints such as cost, pressure requirements, and membrane fouling propensity.

Continued research into membrane materials especially those incorporating nanomaterials or derived from sustainable sources is critical to overcoming current limitations. For example, Ghadhban et al. [54] demonstrated the successful fabrication of a membrane composed of a PLA/PBAT polymer blend modified with banana peel nanoparticles (BP-NPs). The incorporation of just 0.05% BP-NPs significantly improved the membrane’s wettability, decreasing the water contact angle from 73.7° to 38.99°, while increasing water flux and mechanical stability. The modified membrane achieved a high oil removal efficiency of 95.2% at a flux of 105.3 L/m².h substantially higher than the 88% efficiency of the unmodified membrane. Similarly, Khader et al. [55] evaluated polyacrylonitrile (PAN)-based membranes modified with eggplant waste (EGW) nanoparticles. The modified membranes exhibited improved hydrophilicity, with a contact angle reduction from 68.56° to 39.66°, and enhanced surface porosity from 69% to 91%. The tensile strength increased from 5.1 to 8.2 MPa with 0.1 wt.% EGW nanoparticles. These changes translated into higher water permeability (204.71 L/m².h) and greater oil rejection efficiency achieving 99.95% at 1000 ppm oil concentration, 95.5% at 100 ppm, and 90% at 10 ppm while maintaining stable performance over multiple filtration cycles and meeting the World Health Organization (WHO) discharge standards (< 5 ppm).

Innovations in membrane design, surface modification, and hybrid system integration are key to enhancing oil rejection efficiency, reducing fouling, and extending membrane lifespan. Ultimately, membrane separation technologies are poised to play an increasingly central role in sustainable oily wastewater management, contributing to both environmental protection and resource recovery.

Polymer blending refers to the process of combining two or more different polymers to form a composite material with tailored properties [81]. The primary objective of polymer blending is to produce a membrane material that incorporates the favorable characteristics of each constituent polymer, while also reducing fabrication costs and enhancing performance for specific applications [82]. Blends can be formulated to improve mechanical strength, thermal stability, hydrophilicity, chemical resistance, and overall separation efficiency. In many cases, the motivation behind blending is not only to enhance functional attributes but also to achieve economic and scalable production [83].

The properties of the resulting polymer blend are highly dependent on the characteristics of the individual polymers, their ratios in the mixture, and, most importantly, their compatibility or miscibility. Miscibility determines whether a homogeneous or heterogeneous structure will form and influence the final morphology and performance of the membrane. Developing entirely new polymers for membrane fabrication is often costly and complex, whereas blending provides a more practical approach by leveraging existing materials to generate membranes with customized and reproducible properties [84].

Polymer blending has become a preferred strategy over other membrane modification techniques due to its simplicity, repeatability, and flexibility. Membranes produced through polymer blending are now widely adopted in water and wastewater treatment, offering effective separation performance and long-term operational stability. Based on miscibility, polymer blends are generally classified as homogeneous (miscible at the molecular level) or heterogeneous (phase-separated domains) [85].

Blending technologies are often integrated with phase inversion techniques during membrane fabrication. This combination allows the development of membranes with well-defined porous structures, enhanced surface characteristics, and improved mechanical performance. The phase inversion process facilitates the transformation of a polymer solution into a solid membrane through controlled precipitation, typically induced by solvent–non-solvent exchange. This method has proven effective for optimizing membrane properties for oily wastewater treatment applications [86].

The performance of membranes produced through polymer blending can be significantly enhanced by selecting suitable hydrophilic additives. Common hydrophilic polymers used in blends include PVDF, PAN, polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP). These additives improve surface wettability, increase water permeability, and reduce membrane fouling, which are critical factors in oily wastewater separation [87,88].

Several studies have demonstrated the effectiveness of polymer blending in producing membranes with superior oil rejection performance. For example, Masuelli et al. [89] investigated PVDF/SPC (sulfated polycarbonate) blend membranes, where polycarbonate was treated with acetyl sulfate and subsequently blended with PVDF and PVP. The resulting membranes achieved an oil removal efficiency exceeding 96.63%. However, fouling resistance decreased slightly with the incorporation of 2–4 wt.% SPC. Scanning electron microscopy (SEM) revealed that while the overall structure of the membrane remained stable, the pore density increased, indicating improved permeability. These structural changes are presented Fig. S4 in the Supplementary Materials.

In another study, Johari et al. [90] developed hollow fiber membranes using a blend of polyamide-imide (PAI) and sulfonated polyether ether ketone (SPEEK). At a PAI/SPEEK ratio of 85/15, the membrane exhibited a porosity of 79%, a water contact angle of 58°, and bimodal average pore sizes of 12 nm and 81 nm. Field emission scanning electron microscopy (FESEM) images showed fiber diameters of 0.4 mm (inner) and 0.65 mm (outer), with finger-like pore structures increasing as the SPEEK content rose. As shown in Fig. S5 in the Supplementary Materials, the membrane structure was optimized for oil separation, achieving a removal efficiency of over 95%.

The integration of polymer blending into membrane fabrication not only enhances structural and surface characteristics but also contributes to improved oil-water separation efficiency, durability, and fouling resistance. Table S2 in the Supplementary Materials summarizes various studies involving polymer blends for the treatment of industrial oily wastewater, highlighting their compositions and separation performance.

Nanoparticles can enhance structural integrity, provide anti-bacterial properties, and increase the hydrophilicity of the membrane surface, all of which contribute to higher rejection rates and improved permeate quality [91]. Nanoparticles used in membrane development vary widely and are classified based on their chemical composition, size, morphology, and functional behavior [92]. These include polymer nanostructures, carbon-based nanoparticles, ceramic-based nanoparticles, semiconductor materials, lipid nanoparticles, and metallic nanomaterials [93]. Such nanoparticles have a high surface area and unique physicochemical properties, enabling greater interaction with oil droplets and contaminants. Incorporating them into membranes results in better antifouling characteristics, increased mechanical stability, and superior separation efficiency.

Common nanomaterials used in polymeric membranes include iron, TiO₂, and GO. These nanoparticles are especially popular due to their natural hydrophilic nature, which helps form a water-attracting (wettable) layer on the membrane surface. This enhances the membrane’s resistance to fouling and improves its separation efficiency in oily wastewater treatment [93].

Hydrophilic inorganic nanofillers, such as nanoparticles, nanosheets, and nanotubes, have been widely incorporated into both polymeric and ceramic membranes in recent years. Their inclusion addresses several performance issues, including low permeability, decreased rejection efficiency, and a high tendency to foul. Nanoparticles can be added into membranes mainly via two methods: (i) blending into the polymer dope solution during membrane casting, and (ii) post-fabrication surface coating. Among these, bulk blending is becoming more popular because it provides uniform nanoparticle dispersion and reduces surface delamination or uneven coating problems [94,94].

Adding nanoparticles creates a favorable interface between the polymer matrix and filler, improving surface roughness, pore distribution, and interfacial bonding. This facilitates better water flux, solute rejection, and anti-fouling properties. Various nanoparticles including metal oxides (e.g., Al₂O₃, TiO₂, SiO₂, ZnO, MgO, Fe₂O₃, and zeolite), metals (e.g., Cu and Ag), and carbon-based nanomaterials (e.g., graphene, carbon nanotubes (CNTs), and carbon nanofibers (CNFs)) have been successfully used as fillers. These materials provide multifunctional benefits, such as enhanced hydrophilicity, mechanical and thermal stability, improved solute rejection, antibacterial activity, modified surface charge, and adjustable pore size and distribution [96,97]. However, one challenge lies in the limited compatibility between hydrophilic nanomaterials and hydrophobic polymer matrices, which may lead to nanoparticle aggregation or phase separation.

Recent advances in membrane technology have focused on integrating nanoparticles into polymeric materials to improve membrane functionality, structure, and overall separation performance. The main goal of adding nanoparticles is to address common limitations of traditional polymeric membranes, like fouling, low flux, and limited durability. By modifying the bulk, surface, or both, nanoparticles can significantly change the membrane’s morphology and physicochemical properties, leading to enhanced water purification and oil-water separation capabilities [98].

Several studies have investigated the incorporation of conventional nanoparticles into polymeric membranes. For instance, Al-Jadir et al. [99] produced nanocomposite membranes using phase transition technology from PPSU polymer reinforced with TiO₂ particles. The membranes demonstrated effective oil removal due to the presence of microchannels of varying sizes, thermal stability up to 240°C, and good mechanical properties. The membrane containing 2% TiO₂ (P2) outperformed in oil separation performance and fouling resistance, achieving a removal rate of 92.95% and a flux recovery rate of 82.56%. Another study by Senol-Arslan et al. [100] showed that incorporating SiO₂ nanoparticles into PSF and PVDF membranes enhanced oil removal efficiencies to 92.2% and 94.1%, respectively, with a lower contact angle indicating improved hydrophilicity. Water permeability was also improved, and the membranes demonstrated high stability during reuse experiments, with the ability to recycle more than 90% of the oily water. This study confirms the effectiveness of these nanomembranes in the sustainable treatment of oily wastewater. The study, conducted by Akaood et al. [101], addressed the preparation of PAN:HPMC and PAN:HPMC:Gr nanocomposite membranes using electrospinning techniques. The results demonstrated the successful incorporation of graphene into the fibers, resulting in a significant effect on the diameter, porosity, and pore size. The PAN:HPMC:Gr (2%) membranes recorded the best oil rejection performance of 72.47% and a permeability flux of 750 LMH. However, the rejection efficiency decreased with increasing graphene content, attributed to its aggregation and the expansion of the pore size within the fiber structure. Junaidi et al. [102] produced flat and hollow fiber composite membranes of graphene oxide and polyethersulfone (GO-PES) using 0.5 and 1.0 wt.% GO for oil/water separation. The prepared membranes showed significant improvements in hydrophilicity and increased permeability by up to 150%, especially when 1.0 wt.% GO was added. The hollow fiber membranes recorded high oil rejection rates of up to 99%, while the flat membranes' oil rejection performance decreased to about 50%. A research study by Anvari et al. [103] investigated the effects of TiO₂ nanoparticles incorporated into PVDF/PAN membranes. These ultrafiltration membranes were prepared using the phase inversion technique, where TiO₂ was dispersed in the casting solution before membrane formation. The impact of varying TiO₂ concentrations on membrane morphology, water absorption, filtration performance, and fouling resistance was systematically examined. SEM analysis revealed that the presence of TiO₂ reduced the number of finger-like pores and large gaps in the membrane matrix, resulting in a denser and more uniform structure. The addition of TiO₂ also significantly improved water affinity, as indicated by a reduced contact angle, and enhanced pure water flux. The optimized membrane, incorporating 1 wt.% TiO₂ achieved a water flux of 398.5 L/m²·h and a high flux recovery ratio of 93.64%, indicating excellent anti-fouling performance. In another study, Jalal Sadiq et al. [104] developed polyvinyl chloride (PVC)-based membranes incorporated with multi-walled carbon nanotube-grafted graphene oxide (MWCNT-g-GO) using the phase inversion method. These membranes were designed for treating oily wastewater from the Daura refinery in Baghdad, Iraq. Different weight percentages (0.0599, 0.119, and 0.219 wt.%) of the MWCNT-g-GO nanofillers were used to evaluate their effects on membrane characteristics. Comprehensive analyses including SEM, FTIR, AFM, XRD, DSC, porosity, contact angle, and mechanical strength measurements were conducted. The findings showed that the inclusion of 0.119 wt.% MWCNT-g-GO significantly enhanced membrane performance. The contact angle decreased drastically from 74.5° to 13.9°, indicating increased hydrophilicity. Porosity increased from 69.3% to 81.4%, which contributed to an impressive rise in water flux from 153 L/m².h to 254 L/m².h, which is a 66% increase. In addition, chemical oxygen demand (COD) rejection improved from 60% to 88.9%. At the highest concentration tested (0.219 wt.%), the membrane's mechanical strength improved from 2.004 to 2.37 MPa, indicating enhanced structural integrity (see Figs. S6 and S7 in the Supplementary Materials).

These findings clearly demonstrate that incorporating nanoparticles into polymeric blends can significantly improve membrane characteristics relevant to oily wastewater treatment. Enhanced surface hydrophilicity, better pore architecture, improved mechanical strength, and higher rejection efficiency are among the key advantages gained from nanoparticle modification.

Table 1 presents a summary of various polymer blends and nanoparticle additives used in membrane fabrication for oil removal from wastewater. It includes the types of polymers, types of nanoparticles, and performance outcomes. According to Table 1, the incorporation of nanoparticles into polymer blends significantly improved the functional properties of oil/water separation membranes. Among the tested formulations, the PAN membrane reinforced with GO/SiO₂ particles 4% demonstrated outstanding performance, achieving the highest flux recovery ratio (FRR) of 94%, an oil removal efficiency of 96%, and a low contact angle (29.30°), indicating a hydrophilic surface, enhancing fouling resistance and ease of cleaning. On the other hand, the PES/PVP composite membrane with Fe₃O₄ (4%) achieved the highest flux of 3227.7 L/m²·h, along with an extremely low contact angle (21.78°), reflecting excellent wetting properties, making it suitable for applications requiring high permeability. Although the FRR for this membrane was relatively lower (79.5%), its removal efficiency reached 94.01%, reflecting a good balance between wetting and operational efficiency. The PEI/PF127 membrane reinforced with GO (0.6%) particles offered a balanced performance, recording a flux of 325.00 L/m²·h with a removal efficiency of 95.00% and an FRR of 93.00%, indicating remarkable performance retention over multiple operating cycles. In contrast, membranes with high contact angles, such as membranes PET/PEG/CRHA (4%) contact angle of 70.42° and PPSU/PVPBiOCl-AC (2%) with an angle of 67.4°, exhibited poorer anti-fouling performance, as evidenced by a lower FRR of 55.97%, reflecting the importance of modifying surface properties with nanoparticles to improve separation efficiency.

Based on available studies in the literature, it is evident that the choice of nanoparticle type and concentration, along with their mixing with polymer blends, directly affects the physicochemical properties of the membrane, particularly in terms of surface wettability, flow rate, bioresistance, and contaminant removal efficiency. This is attributed to the structural and surface interactions that arise between the polymer and the nanomaterial, which contribute to the improvement of the membrane microstructure, pore distribution, and structural stability. Therefore, strategies directed at designing and effectively integrating nanomaterials within the polymer matrix are essential for developing high-performance membranes that are operationally efficient and sustainable in complex oily and industrial water treatment applications.

Green nanoparticles have emerged as sustainable and advanced materials widely applied in fields such as membrane technology and water purification, primarily due to their eco-friendly synthesis routes utilizing natural resources. However, a comprehensive evaluation of the environmental footprint of these synthesis processes is crucial to ensure their alignment with overall environmental sustainability principles [115]. Green synthesis methods predominantly employ biological materials such as plant extracts, microorganisms, and natural polymers, significantly reducing the reliance on toxic chemicals and harsh reaction conditions common in conventional nanoparticle production [116,117]. Nevertheless, some biogenic synthesis approaches require controlled parameters, including elevated temperatures, precise pH adjustments, and extended reaction times that may lead to considerable energy consumption. Optimizing energy efficiency through alternative techniques such as ambient-temperature reactions, microwave-assisted synthesis, or ultrasound irradiation is an active research focus aimed at minimizing the energy demand of these processes [118]. Biological synthesis processes generate biomass residues, including spent plant materials or microbial cultures, and may produce aqueous wastes containing organic compounds. Improper disposal of such waste streams can result in secondary environmental pollution. Thus, integrating advanced waste treatment strategies such as biodegradation, recycling, or valorization of by-products as raw materials for other industrial applications is essential to enhance process sustainability [119]. Green synthesis significantly lowers the emissions of volatile organic compounds (VOCs) and toxic chemicals compared to traditional chemical routes. It also minimizes the consumption of organic solvents and hazardous reagents, directly mitigating environmental hazards and occupational health risks associated with nanoparticle manufacturing. Incorporating renewable energy sources like solar or wind power into the nanoparticle synthesis workflow can drastically reduce the carbon footprint of production. Moreover, adopting circular economy strategies such as material reuse, process intensification, and waste minimization further improves resource efficiency and environmental compatibility [120].

Despite the evident environmental benefits of green nanoparticle synthesis, ongoing efforts must focus on enhancing energy efficiency, implementing sustainable waste management practices, and leveraging renewable energy integration to realize genuinely eco-friendly production processes [121,122]. Future research should emphasize comprehensive life cycle assessments (LCA) to identify critical environmental impacts and guide the development of greener, scalable, and economically viable synthesis protocols.

Green nanoparticles, synthesized using biological materials such as plant extracts and microorganisms, generally show lower toxicity than traditional nanoparticles made through chemical methods. Their natural biodegradability and minimized use of harmful chemicals align well with strict environmental regulations aimed at reducing ecological and human health risks. Studies have shown that many green-synthesized nanoparticles break down naturally in environmental conditions, lowering concerns about bioaccumulation and persistence [123,124]. Regulatory frameworks such as REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) and Environmental Protection Agency (EPA) guidelines increasingly stress the importance of safer nanomaterials. The eco-friendly production and better toxicological profiles of green nanoparticles make them strong candidates for regulatory approval, which can speed up their adoption in industries like water treatment and biomedical fields. However, comprehensive toxicological testing and long-term environmental impact studies are still vital to fully confirm their safety. Continued research and standardized testing procedures will further enhance regulatory acceptance and public trust in green nanoparticle technology [125,126].

In recent years, the integration of green, plant-based nanoparticles into polymeric membranes has emerged as a highly promising and environmentally sustainable approach for enhancing membrane performance. Green nanoparticles are synthesized from natural extracts, especially plant materials, and offer several advantages over conventional physical or chemical synthesis techniques. These include cost-effectiveness, non-toxicity, simplicity, fast reaction rates, and minimal energy consumption, all of which contribute to their environmental friendliness and scalability for industrial applications [127].

Compared to traditional wet chemical and physical methods, the synthesis of nanoparticles using plant extracts is notably more sustainable. It facilitates compatibility with biological systems and generates nanoparticles that are often more stable and easier to process [20]. One of the key benefits of using plant-derived nanoparticles is the presence of multiple functional groups such as −OH, −COOH, and −NH₂ on their surfaces. These functional groups provide active sites for interactions with polymers and contaminants, and allow for the formation of nanoparticles with various shapes, sizes, and surface charges [21]. Due to their broad availability and the low cost of plant waste, green nanoparticles represent a sustainable and practical route to improving membrane-based separation technologies [22].

Green membranes incorporate these environmentally friendly nanoparticles to modify and enhance key membrane characteristics. These include mechanical strength, surface wettability, hydrophilicity, thermal and chemical stability, antifouling performance, and pollutant rejection efficiency [93]. Recent research has increasingly focused on utilizing agricultural and food waste such as banana and citrus peels, green tea leaves, rice husks, ginger rhizomes, and fruit skins as raw material for synthesizing such nanoparticles [128]. These waste materials are rich in surface functional groups that promote interactions with pollutants via electrostatic forces, hydrogen bonding, and hydrophobic interactions. Thus, they are capable of adsorbing heavy metals, dyes, phenolic compounds, and oils from wastewater streams [129].

The process of producing these nanoparticles involves simple mechanical and chemical techniques such as drying, grinding, chopping, decolorizing, and size reduction to achieve particle sizes in the millimeter to micrometer range. These processes enhance the surface area and pore characteristics, resulting in highly porous and functional nanomaterials for membrane enhancement [130]. Given the increasing environmental impact of oily wastewater generated by various industries, it is essential to develop effective, scalable, and eco-friendly treatment solutions. Researchers have therefore focused on engineering green membranes that not only provide high separation efficiency but are also biodegradable, low-cost, and capable of being produced using simple methods [131].

Among the various green additives, hesperidin-based nanoparticles (HSP NPs) and nanoparticles derived from banana peels and eggplant waste have gained considerable interest. These materials have been shown to improve the membrane’s structural stability, oil rejection efficiency, permeability, and long-term performance [132,133]. This was confirmed in a study by Ghadhban et al. [54], which investigated the modification of poly lactic acid and poly butylene adipate-co-terephthalate (PLA/PBAT) membranes using nanoparticles synthesized from banana peel extracts (BP-NPs). The study evaluated how different concentrations of BP-NPs influenced membrane morphology, pore structure, contact angle, porosity, and separation efficiency for diesel oil-contaminated water. The incorporation of banana peel nanoparticles enhanced several key membrane properties.

The membrane labeled MB4, which contained the highest concentration of BP-NPs, showed a marked increase in permeability (63%) compared to the unmodified MB1 membrane (50%). This improvement was attributed to the enlargement of pore size, which reached an average diameter of 29 nm in MB4. Also, SEM images revealed that the modified membranes exhibited more open and uniform pore structures, supporting better water transport and oil rejection. However, it was noted that excessive addition of nanoparticles could result in aggregation on the membrane surface, which might reduce pore uniformity and slightly hinder performance. Fig. 4 (a) displays the changes in membrane thickness and permeability with different BP-NP concentrations, while Fig. 4 (b) shows the corresponding variations in average pore diameter. Another important aspect investigated was the hydrophilicity of the membranes. The water contact angle as a measure of surface wettability dropped significantly from 73.71° in the unmodified MB1 membrane to 41.3° in the MB3 membrane containing BP-NPs, indicating a substantial increase in hydrophilicity. This change in wettability is crucial, as hydrophilic surfaces resist oil fouling more effectively and support higher water flux.

Fig. 4 (c) shows the visual contrast between water contact angles on unmodified and modified membranes. With regard to oil rejection, the membranes modified with BP-NPs exhibited a remarkable increase in separation efficiency. The oil rejection ratio improved from 88% in the unmodified PLA/PBAT membrane to 95.2% in the BP-NP-added membrane. This enhancement is mainly due to the improved hydrophilicity and the ability of the functionalized membrane surface to repel oil droplets, thereby preventing oil accumulation and fouling during operation (Fig. 4 (d)).

These findings confirm that green nanoparticle additives derived from biodegradable waste can significantly enhance membrane properties and contribute to sustainable oily wastewater treatment solutions. Moreover, this approach aligns with global efforts toward circular economy practices, where agricultural waste is valorized into high-value functional materials for environmental remediation.

Also, study conducted by Khadar et al. [55] explored the integration of nanoparticles derived from eggplant waste (EGW) into PAN membranes. The incorporation of these bio-based nanoparticles significantly enhanced the structural and functional properties of the membranes, particularly in terms of oil–water separation performance, hydrophilicity, porosity, and mechanical stability. This investigation exemplifies the growing interest in using agro-waste as a sustainable and efficient additive for improving membrane technology.

The EGW nanoparticles, when embedded within the PAN matrix, notably reduced the water contact angle of the membrane surface from 68.56° to 39.66° at a nanoparticle loading of 0.1 wt.% indicating a significant enhancement in surface hydrophilicity (Fig. 5 (a)). This increase in wettability reduces the likelihood of oil fouling and promotes water permeation by minimizing interfacial tension between the membrane surface and the water phase.

In terms of porosity, the modified membranes showed a significant improvement. Porosity increased from 69% in the unmodified PAN membranes to 91% with the addition of 0.1 wt.% EGW nanoparticles. This enhancement allows a greater volume of water to pass through the membrane without compromising separation performance. Concurrently, the membrane thickness decreased from 136 to 102 μm, which likely contributed to reduced transport resistance and improved water flux (Fig. 5 (b)). Further characterization using field emission scanning electron microscopy (FE-SEM) revealed that both pristine and EGW- modified membranes exhibited a highly porous, heterogeneous structure. The modified membranes displayed a typical asymmetric morphology, consisting of a thin selective skin layer, a porous sub-layer with finger-like pores for mechanical support, and a sponge-like layer at the bottom. This composite structure plays a critical role in balancing mechanical strength and mass transport efficiency (Fig. S8 in the Supplementary Materials). Performance testing demonstrated a significant improvement in separation capabilities. At a nanoparticle loading of 0.1 wt.%, the modified membrane achieved an oil-contaminated water flux of 151.32 L/m².h and an oil separation efficiency of 95.5%. In contrast, the unmodified PAN membrane only achieved a water flux of 68.26 L/m²·h and an oil rejection rate of 83% under identical conditions (oil concentration: 100 ppm, pressure: 2 bar, temperature: 25°C) (Fig. 5 (c)).

These findings confirm that EGW nanoparticles significantly improve the physicochemical and operational performance of PAN membranes. The combination of increased porosity, enhanced hydrophilicity, and reduced membrane thickness contributes to superior flux and rejection rates, making PAN-EGW membranes a promising and eco-friendly candidate for oily wastewater treatment.

In summary, green nanoparticle additives offer a highly attractive and sustainable approach for improving polymeric membranes used in oil–water separation. They provide a combination of performance, environmental compatibility, and cost-effectiveness. As interest in green technologies continues to grow, further research is needed to optimize synthesis methods, improve dispersion techniques within polymer matrices, and scale up fabrication processes for industrial applications. The integration of green nanomaterials into membrane technologies represents a transformative step toward eco-friendly, high-efficiency, and low-cost wastewater treatment systems.

A review of the comparative performance of nanoparticle-enhanced separation membranes reveals that the use of green or bio-based nanoparticles represents a promising approach for developing more sustainable technologies to treat industrial wastewater, particularly oily wastewater. These particles, often derived from plant sources or natural materials such as fruit peels or bio-treated clay minerals, possess surface and interaction properties that enable improved membrane performance without negative environmental impact.

As summarized in Table 2, several studies have used polymeric membranes embedded with eco-friendly additives derived from natural and agricultural waste sources, showing promising results in treating synthetic oily wastewater. These environmentally friendly modifications enhanced permeability, oil rejection, and antifouling properties under various operating conditions. The results in Table 2 indicate that adding these particles into polymer blends helped improve several key membrane features, such as lowering contact angle (indicating a hydrophilic surface), increasing flow rate, boosting rejection efficiency, and strengthening resistance to biofouling through better recovery rates (FRR). The proper mixing of suitable polymers (such as PET/PEG, PVC, PPSU/PVP, PLA/PBAT, and PAN) with green nanoparticles also showed a clear effect on the nanostructure of the material, positively influencing the membrane's physical and chemical characteristics. The comparative analysis table is a valuable tool for assessing the performance of each formulation, highlighting how the type of nanoparticles relates to their performance in parameters like flow rate, contact angle, oil removal efficiency, and operational stability. This emphasizes the need to develop green nanomaterials tailored to improve membrane performance in oily wastewater treatment, considering environmental sustainability and cost-effectiveness.

Conventional nanoparticles, when blended with polymeric matrices, have generally demonstrated lower effectiveness in membrane applications compared to green-synthesized nanoparticles. This reduced performance is particularly evident in key parameters such as water contact angle and oil removal efficiency. Thus, conventional nanoparticles offer lower efficiency in performance, and in addition to their high production cost, reliance on toxic chemicals, and non-biodegradable residues pose environmental and safety concerns [138]. In contrast, membranes embedded with plant-based nanomaterials tend to exhibit improved hydrophilicity, leading to better surface wettability and, consequently, enhanced oil separation capabilities in water treatment processes. Therefore, the performance of green nanoparticles synthesized from plant extracts demonstrates notable efficiency in removing oil from water, primarily due to their high surface area, functional groups, and environmental compatibility. These particles can modify membrane surface properties, enhance hydrophobicity or selectivity, and offer antimicrobial properties that support long-term use [128].

Polymeric membranes play a vital role in separation processes and have emerged as the dominant membrane technology for wastewater treatment due to their tunable properties, cost-effectiveness, and ease of fabrication. However, their overall efficiency is determined by a complex interplay of several key factors, including material selection, fabrication methods, surface characteristics, and operating conditions. Understanding these variables is essential to optimize performance in terms of permeability, selectivity, fouling resistance, and long-term stability [19].

Despite notable progress in the development and application of environmentally friendly polymeric membranes for oily wastewater treatment, several critical challenges persist. One of the primary issues is the limited long-term durability of polymeric membranes. These membranes are often susceptible to chemical degradation, mechanical failure, and fouling over prolonged operational periods, which significantly reduces their performance and lifespan. Membrane fouling, in particular, remains a major obstacle, leading to decreased permeability, increased energy consumption, and higher maintenance requirements.

Another significant challenge lies in the economic feasibility and scalability of these eco-friendly membrane technologies. Although promising at the laboratory scale, many green polymeric blends and nanoparticle-enhanced membranes face difficulties when translated to large-scale industrial applications. The costs associated with raw materials, fabrication techniques, and operational requirements can pose barriers to commercial implementation. To overcome these limitations, future research should focus on enhancing the mechanical strength, thermal stability, and anti-fouling performance of polymeric membranes by optimizing the composition of polymer blends and incorporating advanced nanomaterials. Green nanoparticles, synthesized from low-cost, biodegradable sources such as banana peels, eggplant waste, rice husks, and citrus residues, show great potential for improving membrane wettability, porosity, and rejection efficiency while maintaining environmental sustainability. Moreover, further efforts are needed to develop scalable and energy-efficient membrane fabrication techniques, such as 3D printing, electrospinning, and low-temperature phase inversion. Investigating membrane cleaning and regeneration strategies is also essential to extend membrane life and reduce operational costs.

Improving membrane durability remains a key challenge. Crosslinking techniques can be employed to reinforce polymer networks, improving both mechanical stability and chemical resistance. For instance, Alshahrani et al. [177] demonstrated that crosslinking polysulfone membranes with glutaraldehyde significantly increased their structural strength and tolerance to harsh chemical conditions. Moreover, incorporating hybrid materials such as green-synthesized silver nanoparticles within polymer matrices has shown promise in enhancing anti-fouling properties and prolonging membrane lifespan [178].

Cost reduction remains a key challenge. Waste-to-resource strategies offer a viable solution by using agricultural or industrial biomass as raw materials for membrane fabrication or nanoparticle production. Fionah et al. [179] utilized agricultural waste to produce biochar-based additives, which lowered material costs and improved membrane performance. Similarly, Abdelbasir et al. [180] reported the successful recycling of industrial sludge as a precursor for nanoparticles, promoting both economic and environmental sustainability.

Optimizing manufacturing methods is equally important. Techniques such as conducting phase inversion at room temperature can significantly reduce energy consumption and overall production costs. Green synthesis of nanoparticles is generally more environmentally friendly than conventional methods, as it utilizes natural resources such as plant extracts and microorganisms, minimizing the reliance on toxic chemicals.

However, some green synthesis processes may still require substantial energy inputs due to the need for controlled conditions such as precise pH and temperature. They may also generate biomass and liquid waste that must be properly managed to avoid secondary pollution. Therefore, improving the energy efficiency of green synthesis and incorporating waste recycling or biodegradation techniques are essential steps toward achieving genuinely sustainable and scalable membrane technologies.

Despite the increasing interest in sustainable nanomaterials, several challenges should be addressed to fully unlock their potential. Hybrid green nanoparticles, created through eco-friendly methods using plant extracts or biopolymers, have become a promising class of materials due to their improved functionality, stability, and environmental compatibility. By combining several components such as metal oxides with carbon-based materials or biogenic supports, these hybrids demonstrate synergistic effects that enhance performance in applications like wastewater treatment, antimicrobial coatings, and catalysis. Importantly, their green synthesis minimizes hazardous by-products and energy use, supporting sustainable development goals. For industrial scalability, recent progress focuses on optimizing reaction conditions, using low-cost biomass feedstocks, and adopting continuous flow or microwave-assisted processes. These approaches not only lower production costs but also enable large-scale, reproducible manufacturing, making hybrid green nanoparticles more viable for commercial and environmental uses. Ongoing efforts are necessary to overcome scalability challenges and standardize green synthesis techniques for broader industrial adoption.

Lastly, interdisciplinary collaboration between material scientists, environmental engineers, and industry stakeholders will be vital in designing next-generation green membranes that meet performance, economic, and regulatory requirements. With continued innovation, these advanced polymeric systems can play a transformative role in achieving sustainable oily wastewater treatment at both local and global scales.

Polymeric membranes made from blended polymers have shown significant potential in tackling the challenges of oily wastewater treatment. Their adjustable structural features, combined with chemical and mechanical durability, make them highly effective for oil–water separation. By blending polymers such as polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and polyvinyl alcohol (PVA), researchers have improved membrane porosity, surface hydrophilicity, and structural strength, leading to better water flux and oil rejection. Adding nanoparticles like TiO₂), graphene oxide (GO), and carbon nanotubes has further improved membrane performance. These nanoparticles increase porosity, enhance surface wettability, and lower fouling, thereby extending membrane lifespan and increasing separation efficiency. They also help improve thermal and mechanical stability, which is important for long-term operation in harsh conditions. At the same time, the use of eco-friendly nanoparticles from plant-based sources like banana peels, eggplant waste, and rice husks has gained popularity. These environmentally friendly nanomaterials are low-cost, biodegradable, and sustainable alternatives to synthetic nanoparticles. Their rich surface chemistry featuring hydroxyl, carboxyl, and amine groups—allows strong interactions with membrane materials and pollutants, boosting antifouling properties, permeability, and oil rejection. Despite these advances, several challenges remain, including high production costs, limited scalability, and membrane fouling that can reduce performance over time. Ensuring long-term reliability and optimizing cleaning methods are also necessary. Future research should focus on: a) developing advanced green nanomaterials with controlled size, shape, and functionality; b) improving membrane fabrication techniques such as electrospinning, 3D printing, and phase inversion, for large-scale, cost-effective manufacturing; c) increasing membrane durability and ease of regeneration, especially under real industrial conditions with variable feedstreams; and d) encouraging cross-disciplinary research to align material innovations with industrial and environmental needs. In addition, comprehensive pilot-scale and long-term studies are essential to verify membrane performance in real-world settings. Broad industrial adoption will depend on ensuring that polymeric membrane systems comply with environmental, safety, and regulatory standards for wastewater discharge.