Nanomaterials-modified reverse osmosis membranes: a comprehensive review

Because of its great efficiency and widespread application, reverse osmosis (RO) is a popular tool for water desalination and purification. However, traditional RO membranes have a short lifespan due to membrane fouling, deterioration, decreased salt rejection rate, and the low water flux with aging. As a result, membrane modification has received a lot of attention recently, with nanomaterials being extensively researched to improve membrane efficacy and lifespan. Herein, we present an in-depth analysis of recent advances of RO membranes modification utilizing nanomaterials. An overview of the various nanomaterials used for membrane modification, including metal oxides, zeolites, and carbon nanomaterials, is provided. The synthesis techniques and methods of integrating these nanomaterials into RO membranes are also discussed. The impacts of nanomaterial change on the performance of RO membranes are addressed. The underlying mechanisms responsible for RO membrane enhancements by nanomaterials, such as improved surface hydrophilicity, reduced membrane fouling via surface repulsion and anti-adhesion properties, and enhanced structural stability, are discussed. Furthermore, the review provides a critical analysis of the challenges and limitations associated with the use of nanomaterials to modify RO membranes. Overall, this review provides valuable insights into the modification of RO membranes with nanomaterials, providing a full grasp of the benefits, challenges, and future prospects of this challenging topic.


Introduction
The global water crisis threatens human beings, economies, and ecosystems worldwide.Access to clean and safe water is a fundamental human need, yet a signicant portion of the global population still lacks this vital resource.Population expansion, urbanization, climate change, water pollution, unsustainable agricultural practices, inadequate water infrastructure, and water conicts all contribute to the global water crisis.Understanding the complexities of this crisis and Mahmoud A: Ahmed Mahmoud Adel Ahmed earned his PhD degree in 2024.He has been actively engaged in research for the past eight years and his research focuses on the synthesis, characterization, and environmental applications of nanomaterials and their composites in water treatment and remediation.He has authored several reviews and book chapters on these topics.He also serves as a senior service engineer at Veolia Environmental Services, managing various sectors like reverse osmosis, boilers, cooling towers, and wastewater plants.developing innovative mitigation strategies is crucial for ensuring water security and sustainable development.Scientists and researchers are constantly looking for sustainable and costeffective tools to obtain clean and potable water. 1,2The majority of the Earth's water, approximately 97%, is found in the form of saline water in oceans and seas, making it a vast and untapped resource that could help alleviate the water crisis. 3Consequently, desalination has emerged as a highly efficient and costeffective solution to the global water scarcity problem.However, in areas where access to brackish or seawater is limited, wastewater reclamation provides a viable solution to the water crisis. 4RO desalination has become a widely adopted process for producing clean water, with numerous sources contributing to its feed, with about 60% coming from seawater, 20% from brackish water, and the remaining from other sources including rivers, and wastewaters. 5esalination methods can be categorized into two main groups based on their separation mechanisms: phase-change and membrane-based processes.7][8][9] On the other hand, membrane-based desalination relies on specialized membranes, which act as lters to allow water to pass through while retaining salts and minerals, e.g., reverse osmosis, forward osmosis, nanoltration, electrodialysis, membrane distillation, and capacitive deionization methods.
Currently, RO processes are the most widely used, accounting for over 65% of desalination plants globally.This is attributed to its simplicity, low-energy consumption, separation efficiency, stable product quality, and relatively low operational costs.The rst thin-lm composite (TFC) membrane was develop using an interfacial polymerization reaction, 10 such as the combination of trimesoyl chloride (TMC) and m-phenylene diamine (MPD) or piperazine (PIP), as shown in Fig. 1a. 11A conventional polyamide (PA)-based RO membrane typically comprises three layers: a very thin active layer made of aromatic PA, a support layer with small pores, and a layer of nonwoven fabric, as shown in Fig. 1b.These layers have approximate thicknesses of 200 nm, 20-50 mm, and 120-150 mm, respectively. 12,13The polyester support layer alone cannot offer a suitable surface for the PA active layer due to its uneven and porous structure.To overcome this issue, a microporous polysulfone interlayer is put between the selective layer and the support layer. 14This extra layer protects the ultra-thin selective layer from high-pressure compression. 15The PA layer is responsible for providing selectivity, with typical salt rejection of about 99%. 16TFC membranes have a high selectivity and water ux; however, its use in RO processes faces several challenges that signicantly impact their overall performance. 17ne major compromise that RO membranes face is the trade-off between membrane permeability and salt rejection/ selectivity.Finding a balance between high permeability and excellent salt rejection/selectivity is crucial in designing efficient RO membranes.Furthermore, fouling is a persistent issue that degrades RO membranes' performance.Membrane fouling occurs when organic substances, mineral scales, colloids, and biolms accumulate on the membrane's surface or within its pores. 18This process is complex and is controlled by factors such as the membrane's surface hydrophilicity, roughness, and charge, as well as feed water quality and operational parameters. 19,20A more hydrophilic and smoother membrane is less susceptible to fouling as it discourages foulants from attaching. 21Hydrophilic materials are more resistant to fouling because they can undergo hydration, mainly via hydrogen bonding.Conversely, surface roughness provides more area and surface defects for foulants to adhere to membrane surfaces, increasing the likelihood of fouling. 22Managing these variables is critical for minimizing membrane fouling.One of the most convenient approaches for preventing biofouling in RO units is the chlorination of feed water. 23However, the use of chlorine can have detrimental effects on the polyamide barrier layer, which is a critical component of the TFC-RO membrane. 23hlorine attacks the polyamide layer through several mechanisms Fig. 2. 24 One mechanism involves the substitution of the hydrogen in the amide group with a chlorine atom. 25Further, chlorine can cause rapid N-chlorination followed by intramolecular rearrangement, leading to the migration of chlorine to the aromatic ring. 26Another mechanism is the direct replacement of chlorine atoms for the aromatic ring of m-phenylene diamine. 24,27Moreover, chlorine promotes the hydrolysis of the amide group, resulting in the formation of amido and carboxylic groups.Oxidation of the membrane by chlorine degrades performance, specically water permeability and salt rejection. 28,29herefore, to address the challenges that RO membranes face, scientists have extensively researched the use of numerous materials and strategies to enhance membrane properties.Among these, the incorporation of nanomaterials derived from metal oxides, zeolites, and carbon-based nanoparticles into RO membranes have emerged as intriguing enhancement option.Nanomaterials have been proved to improve multiple aspects of RO membranes, including water permeability, chlorine   resistance, antifouling potential, and antimicrobial characteristics. 30Furthermore, nanomaterials have been found to improve the mechanical strength and thermal stability of RO membranes.2][33] For instance, carbon-based nanomaterials, including graphene, carbon nanotubes, and metal organic frameworks (MOF) have showed potential in improving RO membrane performance.These nanomaterials possess high surface area, and unique porosity and adsorption capabilities.They can act as molecular sieves, allowing water transport while effectively excluding salts and pollutants.Carbon-based nanomaterials, for instance, can reduce fouling potential and extend membrane lifespan.Additionally, carbon-based nanomaterials have excellent mechanical strength and chemical stability, which improve the durability and lifespan of the modied membranes.They can withstand harsh operating conditions, such as high pressures and varying pH levels, making them suitable for industrial-scale RO applications.Further, the microporous structure of zeolite nanoparticles selectively allows water molecules to ow through while hindering the transport of larger molecules, assuring effective rejection of pollutants and reducing the likelihood of fouling. 34Furthermore, metal oxide nanoparticles can exhibit photocatalytic properties, enabling the degradation of organic pollutants that contribute to fouling.This photocatalytic activity effectively reduces fouling and maintains the membrane's permeability and salt rejection capabilities over prolonged operation.Moreover, Silica's nanoparticles remarkable affinity for water makes it an appealing material for improving the hydrophilicity, water permeability, and efficiency of RO membranes. 31he characteristics of nanocomposite membranes undergo signicant alterations based on the nature, concentration, chemical properties, and size of the incorporated nanomaterials. 33Consequently, it is possible to customize the properties of nanocomposite membranes according to the specic nanomaterial employed. 33Nanomaterials can be integrated into thin lm composite (TFC) membranes through various primary approaches, as shown in Fig. 3. 33 Firstly, nanomaterials can be dispersed within the organic/water phase of polyamide (PA) monomers during their interfacial polymerization (IP) reaction whereby the nanomaterials become randomly captured and enveloped by the active PA matrix, resulting in TFNa membranes (the suffix "a" means: within the active PA layer). 35,36Alternatively, nanomaterials can be dispersed within the substrate matrix through the process of phase inversion, resulting in the formation of TFNs membranes with nanocomposite substrates (the suffix "s" means: within the substrate). 37Lastly, nanomaterials can be uniformly coated or deposited onto the substrate surface before the IP reaction, yielding TFNi membranes with nanomaterial interlayer (the suffix "i" means: at the interface of substrate). 38These various incorporation methods play a signicant role in the development of TFC membranes and are important aspects to consider when examining their performance.The escalating interest in the advancement of nanomaterial-modied RO membranes is clearly apparent from the growing number of research studies published, as demonstrated in Fig. 4.This Figure shows the major contribution of TFNa over both TFNs and TFNi membranes, as well as the major contributing journals to this interesting topic.This observation underscores the necessity to analyze and review the existing literature on this topic.
This review offers a thorough and in-depth analysis of recent advancements in the eld of nanomaterials-modied RO membranes.The article systematically explores the synthesis methods employed to incorporate carbonaceous and inorganicbased nanomaterials into membranes, along with the characterization techniques used to assess their properties.Moreover, in-depth characterization methods for evaluating the structural, morphological, and physicochemical properties of modied membranes will also be addressed.The impact of these modi-cations on critical performance parameters, such as water permeability, salt rejection, fouling resistance, and membrane stability, will be discussed in detail.This review will also address the challenges associated with scaling up these modi-cations from the laboratory to the industrial scale and will identify future research directions to promote further advancements in the eld.By critically analyzing the existing literature and providing valuable insights, this review aims to contribute to the development of innovative and effective RO membrane technologies for sustainable water treatment.

Nanomaterials and nanocomposites-enhanced polymeric membranes
Various nanomaterials can be used as llers to enhance water permeability and salt rejection rates of polymeric TFC membrane for NF and RO applications.These nanollers can be tailored based on various factors that operate collectively, including composition, size, shape, porosity, hydrophilicity, the interfacial polymerization method employed, and the membrane layer where these nanollers have been incorporated, as well as its preparation method.Various nanoller shapes have been identied including nanospheres, nanotubes, nanosheets, nanocrystals, nano-bowl, nano-cubes, hollow particles, and core shell shaped nanoparticles. 39,40Moreover, the researchers interestingly compared the performance of non- porous and porous nanollers having various compositions, sizes, inclusion membrane layer, interfacial polymerization process, and operated under different conditions. 39,40urthermore, the effect of nanoller positions on TFC membrane performance was interestingly established. 41embranes can be classed as TFNa, TFNs, or TFNi based on where nanomaterials are incorporated, i.e., in the active layer, the support layer, or at the interface between the two.The researchers found that TFNa membranes for RO and FO applications and TFNi membranes for NF were more effective in overcoming the trade-off between water permeability and selectivity.Furthermore, compared to conventional nanomaterials, the use of porous 1D or 2D nanollers, such as nanosheets, nanotubes, hollow and mesoporous NPs, in the fabrication of TFNi and TFNs membranes gave superior water permeability through their pores and channels, as well as enhanced selectivity due to their hydrophilicity. 41Nevertheless, nano-bowl-shaped nanollers with concave cavities reduced mass transfer resistance of water molecules and shortened their ow-through pathways.It is worth noting that these factors were not studied using monovariate optimizations due to the practical limitations involved in researching each factor separately while keeping all other factors constant.The current review discusses some representative examples of specially tailored nanollers with fruitful characteristics for their potential performance in modifying TFC membranes.

Carbon nanomaterial
Carbon based nanomaterials are attractive for membrane modication owing to their porous structures, biocidal activities and hydrophilic properties.A variety of carbon nanomaterials, such as carbon nanotubes (CNTs), graphene oxides (GO) and metal organic framework (MOF) have been used to modify various membranes.Table 1 Shows the performance evaluation of several TFN reverse osmosis membranes with incorporation various carbon-based nanoparticle.
2.1.1Graphene and graphene oxide.Graphene (G), a thin sheet and smooth two-dimensional nanomaterial composed of carbon atoms arranged in a hexagonal lattice, has garnered signicant attention in the eld of materials science.3][44][45][46][47] This has led to the emergence of graphene-based nanomaterials, including graphene, reduced graphene oxide (rGO), and graphene oxide (GO) which have found widespread applications in various applications, such as electrochemical processes, photocatalysis, and sensors. 43,48,49Water desalination has been a particular focus for graphenes in membrane separations due to its atomically smooth and thin structure.Graphene's hydrophobic characteristics and atomic-level smoothness allow water to ow with minimal friction between the graphenic layers.][52] However, GO and rGO nanomaterials are more hydrophilic than graphene itself and exhibit antifouling and antimicrobial properties that are advantageous for thin-lm composite membranes used in desalination.Long-term contact between microbes and GO or graphene sheets leads to disruptions in cell membrane integrity and oxidation of cellular components, ultimately leading to a loss of cell viability. 53,54Additionally, the hydrophilic nature of GO and rGO nanoparticles contributes to anti-adhesive features on the membrane surface, which reduces foulant deposition.Oxidative stress, penetration via lipid bilayers, and lipid extraction via graphene sheets are further potential interactions (Fig. 5). 55unctional groups, such as carboxyl, hydroxyl, and epoxide groups, present on GO nanosheets enhance the negative charge of the membrane surface.This strengthens the electrostatic repulsion between extracellular polymeric substances (EPS) and microorganisms, decreasing microbial adhesion to the membrane surface. 56The presence of hydrophilic groups increases the membrane's affinity towards water molecules, reducing interfacial tension and enhancing the membrane's wetting properties.As a result, water molecules can penetrate and ow more easily through the membrane, leading to increased water ux and improved permeate ow.The highly interconnected pores and channels formed by the GO nanosheets provide more and more direct pathways for water molecules to travel through the membrane. 57,58Furthermore, the addition of GO nanoparticles to the membrane matrix improves its mechanical strength and stability, particularly under high trans-membrane pressures, enhancing the longterm performance of the membrane. 59Furthermore, incorporating GO into the membrane structure can enhance chlorine resistance due to its unique properties.GO acts as a barrier that effectively hinders the penetration of chlorine molecules into the membrane matrix and reduces its reactivity with -NH 2 groups of MPD moieties of the membrane's PA layer. 60,61In another mechanism, the oxygen-containing functional groups of GO, such as hydroxyl and epoxide groups, provide preferential sites for chlorine molecules to react, where chlorine atoms can form covalent bonds with oxygen atoms on GO, resulting in chlorine-oxygen bonds that are more stable than chlorinecarbon bonds typically found in polymeric membrane material upon chlorination. 62,63hus, the distinctive properties of GO nanosheets, including their hydrophilicity, chemical durability, and rapid water permeation, were used to develop a dual-function barrier coating for PA-TFC membranes. 29This conformal coating of GO effectively enhanced the membrane's surface hydrophilicity and reduced its roughness, leading to improved proteins' fouling resistance.Moreover, due to the chemically inert nature of GO nanosheets, the coating layer also served as a chlorine barrier, preventing membrane degradation when exposed to chlorine, while maintaining high salt rejection capability. 29On the other hand, reducing the polymer solution concentration used in the PA support layer preparation enhanced the TFC membrane water ux but resulted in a porous sub-surface structure that degraded the support layer's mechanical strength. 64Therefore, the researchers incorporated GO nanosheets into the sublayer, which enhanced the mechanical strength, porosity, and the water ux.These enhancements were affected by the GO nanosheets wt% and thickness, where higher content and The results demonstrated that ZIF-8 can signicantly improve the polyamide fouling resistance and, in this study, the thicker nanosheets had negative effects on the mechanical strength.The dispersion and surface area of the nanollers also played important roles in their incorporation into the polymer matrix.
According to the ndings, incorporating GO nanosheets with a thickness of 1.5 nm and 0.9 wt% dose yielded the maximum mechanical strength.The incorporation of GO did not affect the polyamide layer, as the active layer roughness and thickness were retained, and the network structure of the surface layer remained unchanged.Furthermore, the impact of incorporating GO during interfacial polymerization on membrane performance was examined, Fig. 6a. 65Namely, the membrane's water ux, salt rejection, and resistance to chlorine were assessed under various operating conditions, as shown in Fig. 6b and c, where TFC/GO membrane exhibited consistent salt rejection, whereas the pristine TFC membrane experienced a sharp decrease in salt rejection at high pressures due to deformation.Additionally, the TFC/GO showed enhanced chlorine resistance, Fig. 6c, as evidenced by higher normalized salt rejection and stable normalized ux, which can be attributed to the hydrogen bonding between the oxygen-bearing functional groups of GO and the secondary amide groups in PA, offering protection against chlorine.Furthermore, prior to interfacial polymerization, GO was added to the aqueous MPD solution to be embedded into the PA layer, and the resulting GO-TFN membranes showed 98% improvement in water antibiofouling based on biovolume changes, as well as 80% increase in permeability features. 66These enhancements can be attributed to changes in surface charge, roughness, hydrophilicity, and PA layer thickness resulting from GO incorporation.In addition, the GO-TFN also exhibited a high salt rejection even when exposed to challenging conditions of 48 000 ppm h chlorination with a NaOCl solution, which can be attributed to the protection of PA-amide groups from chlorine attack.The study also emphasized the importance of GO concentration and size in inuencing the performance of GO-TFN membranes. 66urthermore, modication of RO membranes with GO improved surface properties, making them smoother and more hydrophilic, as well as reduced CaSO 4 scaling and biofouling, demonstrating its potential for enhanced RO membrane performance. 67In another study, incorporating GO into the PA layer of TFC membrane had no effect on the membrane's permeability and salt rejection; however, it resulted in a noteworthy decline of viable E. coli cells, reducing them by 64.5% within one hour. 68GO induced membrane damage, mediated by physical disruption, charge transfer, formation of reactive  oxygen species and lipid extraction from the cell membrane, as evidenced by SEM imaging, where most bacterial cells attached to the GO-TFC membrane appeared to be shrunk or attened with compromised integrity compared to the control-TFC membrane. 68.1.2Carbon nanotubes.Carbon nanotubes (CNTs) are made up of rolled-up cylindrical graphite sheets that resemble tubes and have the appearance of a mesh fence.69 CNTs exhibit superior thermal, mechanical, antibacterial and electrical properties.Because they can reduce the amount of energy required to remediate water, CNTs have intrigued researchers in the eld of advanced membrane technologies for water desalination.70,71 CNT-modied membranes enable seamless water ow while effectively capturing a wide range of water pollutants.The hollow inner cavity of CNTs has substantial desalination potential. Noably, CNTs' unique properties, such as smooth hydrophobic walls, high aspect ratios, and small pore diameters, allow for remarkably appropriate water-molecules mobility; therefore, CNTs have received signicant recognition in the eld of RO membrane modication.72 The hydrophilicity and porous structure of CNTs, when introduced into TFC, can augment the water pathways within membranes and improve the affinity between membrane surface and water.73 Furthermore, the integration of negatively charged CNTs has the potential to raise the charge density of the membrane surface, lower the adhesion forces between the membrane surface and fouling agents, and thereby enhance its antifouling capabilities of the membrane.Studies have suggested that the stability of membranes against chlorine can be enhanced through the interactions between the AP-amide groups and the -COOH of CNTs.74 Additionally, CNTs possess inherent antimicrobial properties, which can help mitigate biofouling in RO membranes.The nanotube structure and surface chemistry of CNTs provide a hostile environment for the growth and adhesion of microorganisms, reducing biolm formation and microbial fouling.75 Furthermore, the introduction of CNTs into the membrane composite may yield a synergistic effect, leading to an enhancement in the photocatalytic features of the membrane.76 CNTs can provide mechanical reinforcement to the membrane structure, enhancing its stability and resistance to physical stress.75 The robust nature of CNTs helps to prevent the membrane from deformation or damage during operation, leading to improved membrane performance and lifespan.
For instance, incorporating MWCNTs into the PA layer of a TFC membrane resulted in enhanced chlorine resistance that was attributed to the electron-rich MWCNTs protection of the amide linkage. 77Furthermore, TMC solutions in n-hexane were mixed with MPD solutions containing modied CNTs to prepare PA thin lms via controlled interfacial polymerization (IP) process, resulting in the formation of well-dispersed states. 78In that work, CNTs were previously treated for several hours (x) at predetermined settings with various mixtures of sulphuric and nitric acids to impart oxygen bearing functionalities to the modied CNTx.Among the prepared membranes, PA-CNT4 demonstrated enhanced durability, water ux, and chemical resistance compared to the pristine PA membranes.These improved performance features were attributed to the presence of hydrophobic nanochannels provided by the CNTs.Additionally, incorporating CNTs into PA lms of TFC membranes improved water permeability, leading to reduced energy consumption, high anti-fouling potential, and improved recovery efficiency during the cleaning stage due to the smoother surfaces. 79Furthermore, the effect of varying amounts of CNTs on the performance of PA membranes was examined. 75he experimental ndings demonstrated promising results, with notable improvements observed in salt rejection, high water ux, and remarkable anti-biofouling properties, as well as enhanced durability.However, when higher amounts of CNTs were incorporated, the anti-biofouling characteristics of the PA-CNT membrane coated with polyvinyl alcohol (PVA) were reduced.This decline in anti-biofouling efficacy was attributed to the formation of aggregated CNT clusters within the membrane.The superior anti-biofouling properties observed in the PA-CNT-PVA membrane were primarily attributed to the inherent antimicrobial properties of the CNTs.To validate this, both confocal laser scanning microscopy (CLSM) imaging and cell viability tests were conducted, providing evidence of the membrane's anti-biofouling efficacy. 75ayer to enhance its resistance to fouling and chorine as well. 73Experimental tests conducted using inorganic (Ca(HCO 3 ) 2 ) and BSA-protein foulants demonstrated that the TFN membrane experienced a lower decline in water ux and exhibited reduced irreversible fouling, as shown in Fig. 7a and  b, respectively.These improvements were primarily attributed to the enhanced electrostatic repulsion between the foulants and the membrane surface, as well as the improved hydrophilicity of the TFN membrane, which minimized foulant adhesion.Furthermore, the TFN membrane also exhibited high resistance to chlorine, which can be attributed to the MWCNTs electron-rich nature that provide protection to MPD active sites, and hence hindering the chlorine attack (Fig. 7c and d). 73urthermore, incorporation of raw and oxidized MWCNTs reduced inorganic-, organic-and bio-fouling owing to the increased membrane's negative charge and smooth surfaces. 80dditionally, the fabricated membrane exhibited twice the lifespan in comparison to existing commercially available membranes. 80Results reected that the hydrophilicity, as well as water ux, improved with increasing concentration of oxidized MWCNTs compared to bare PA or raw MWCNTs/PA.Despite their lower hydrophilicity, the membranes with raw MWCNTs exhibited signicant improvements in water ux and salt rejection due to their proper dispersion within the polyamide matrix.However, at higher concentrations of both MWCNT types, the water ux decreased.Assessment of fouling behavior in a 24 hours test using BSA/salt solution demonstrated that membranes with all concentrations of MWCNTs, whether raw or oxidized, exhibited better antifouling performance compared to unmodied membranes.Finally, optimized concentrations played a crucial role in achieving superior membrane characteristics. 80n another investigation, 81 the researchers follow the efficiently reduced protein adhesion and antifouling capabilities of MWCNT-PA by employing a multifaceted approach combining molecular dynamics (MD) simulations and experimental studies.MD simulations, suggested that the incorporation of MWCNTs within the PA framework gives rise to weaker interactions between BSA proteins and membrane surfaces.This weakened interaction can be attributed to three key factors: a more rigid PA structure resulting in reduced molecular conformity with BSA, enhanced hydrophilicity, and smoother morphology leading to the formation of an interfacial water layer. 81Similarly, a fabricated MWCNTs-PA exhibited resistance to chlorine attacks that as ascribed to the electronic-rich nature of the highly sp 2 -hybridized MWCNTs and the existence abundance of sacricial organic functionalities on nanotubes.Moreover, the physical crosslinking effect of MWCNT also contributes to the membrane's resistance by preventing the formation of pinholes.It should be noted that PA with a signicant degree of crosslinking is generally more resistant to chlorine attack. 82.1.3Metal-organic frameworks (MOF).MOFs are porous materials consisting of metal ions or clusters connected by organic ligands.Their unique structure and tunable properties make them versatile materials for addressing environmental challenges.MOFs typically exhibit exceptionally signicant surface areas, oen surpassing 1000 to 7000 m 2 g −1 . 83The structure of MOFs can be specically tailored to have different pore sizes, shapes, and functionalities. 84Moreover, many MOFs possess inherent catalytic activity, either due to the metal centers or the organic linkers in their structure. 85,86They possess the exibility of organic matter and the stiffness of porous inorganic material. 87,88he enhanced performance of reverse osmosis (RO) membranes upon incorporating metal-organic framework (MOF) materials can be attributed to several key mechanisms.Firstly, MOFs can provide a high density of active sites for selective interaction with water molecules, leading to improved water permeability.The porous structure of MOFs allows for the formation of nanoscale channels, increasing the overall water ux through the membrane. 89,90The permeation characteristics of a MOF-TFN are signicantly altered by the size of the pores of a MOF. 89Furthermore, MOFs can enhance the salt rejection capabilities of RO membranes.Additionally, by functionalizing the MOF materials with specic functional groups or modifying the ligands, the selectivity towards certain ions can be increased.
For instance, this selectivity can be especially valuable in applications like seawater desalination, where high salt rejection rates are crucial for obtaining fresh water.Thus, the addition of myristic acid (MA) to zirconium-based porphyrinic MFOs (PCN-222) reduces pore size and results in improved rejection compared to pristine MOFs. 89Additionally, MOFs can enhance the stability and durability of RO membranes.The incorporation of MOFs into the polymeric matrix of the membrane can improve its mechanical strength and resistance to fouling. 78,79Moreover, the high thermal stability of MOFs allows for their application in harsh operating conditions, such as high temperatures or corrosive environments.This expands the range of potential applications for RO membranes, making them suitable for a broader range of industries and processes.
Furthermore, the researchers examined the potential of a microporous and hydrophobic hybrid zeolitic imidazole frameworks substance (ZIF-8) as ller in TFC-RO membranes.They observed a remarkable enhancement in water permeance of up to 162% while keeping an impressive NaCl rejection rate of 98%.Additionally, the selective layer's surface, when integrated with ZIF-8, exhibited more hydrophilicity and reduced crosslinking compared to the pristine polyamide (PA). 79,91dditionally, to enhance the water permeability, and the support layer's porosity and hydrophilicity, researchers developed a polysulfone (PSf) layer incorporating a sulfuric acidtreated HKUST-1 MOF, [Cu 3 (1,3,5-benzenetricarboxylate) 2 $(H 2 -O) 3 ] n . 91,92The overall surface roughness of the TFC was affected by the support layer's porosity and hydrophilicity.By improving these features in the support layer, they successfully produced a TFC-RO membrane with reduced roughness which resulted in enhanced resistance to fouling and improved water ux compared to a TFC-RO with a bare PSf, whilst maintaining its excellent salt rejection capability. 93The rodlike structure of PCN-222 MOFs has been successfully fabricated and incorporated into TFC RO. 89 Furthermore, the PCN-222 was modied with MA aer synthesis, allowing for the adjustment of the MOF pore size (Fig. 8a).Membranes fabricated using both bare and modied PCN-222 MOF demonstrated slight reductions in selectivity and signicant increase in water ux compared to bare PA membranes.Specically, the modied PCN-222 -TNF membranes (MA-10) exhibited an almost 100% enhancement in ux while maintaining salt rejection above 95% (Fig. 8b). 89The authors proposed that the existence of hydrophobic MA chains hinders the inltration of MPD into the pores of the PCN-222 nanoparticles in suspension, leading to a reduction in pore penetration by the polyamide (Fig. 8a).This causes an increase in pore volume available for transport, resulting in higher ux and lower salt rejection compared to the control membrane (Fig. 8b).Additional mechanisms that could contribute to the enhanced ux in the PA/PCN-222 include interfacial movement occurring at the surface of the MOF aggregates, as well as transport taking place within the interstitial channels of the MOF aggregates. 89n another investigation, a microporous and hydrophilic hybrid MOFs, MIL-101 (Cr), was incorporated into a TCF RO membrane, resulting in improved overall membrane performance. 90This enhanced performance can be attributed to changes in membrane characteristics such as roughness, crosslinking extent, morphology, and wettability.Moreover, doped MIL-101 (Cr) also creates direct channels within the PA layer of the membrane.At a concentration of 0.05 w/v% of MIL-101 (Cr), water permeability increased by up to 44% while keeping NaCl salt rejection over 99. 91Overall, the integration of metal-organic framework materials into RO membranes can signicantly enhance their performance.MOFs offer improved water permeability, higher salt rejection rates, enhanced stability, and increased durability.

Incorporation of inorganic nanomaterials
The inclusion of inorganic nanomaterials into the polymer matrix has gained attention due to their hydrophilic characteristics, which can enhance a range of physical and biomedical benets, such as optical, mechanical, catalytic, and antimicrobial membrane properties. 94,95Through the addition of inorganic NPs, the permeability of pure polyamide membranes can be enhanced by facilitating water transport through a direct pathway or modifying the membrane structure. 217][98] Table 2. Shows the performance evaluation of several TFN reverse osmosis membranes with incorporation various inorganic-nanoparticle.
2.2.1 Zeolite nanoparticles.0][101] These minerals have a threedimensional porous structure with a regular arrangement of channels and cavities.With a porous structure composed of interconnected silicon and aluminum tetrahedra bridged by oxygen atoms, 101 zeolites offer a high surface area and tunable pore size and shape, leading to efficient molecule and ion interactions. 102,103Their ion exchange capabilities enable the selective removal of specic ions and pollutants.Moreover, zeolites exhibit exceptional thermal and chemical stability, making them suitable for harsh conditions. 102These unique characteristics have generated interest in incorporating zeolite nanoparticles into systems such as membranes separation processes.The presence of zeolite creates additional pathways for water molecules to pass through, promoting faster and more efficient water transport.This increased permeability results in higher water ux rates and improved overall membrane performance. 104hen RO is modied with zeolite, the enhanced performance can be attributed to several mechanisms.The negatively charged molecular sieve and super-hydrophilic zeolite will facilitate the selective movement of water by establishing specic pathways while giving effective solute rejection by utilizing a combination of Donnan and steric exclusion mechanisms. 96Zeolites contain pores with sizes ranging from approximately 0.3-0.8nm.These pore sizes are positioned between the diameters of hydrated salt ions, such as Na + (0.72 nm), and water molecules (0.27 nm). 104Taking advantage of the principle of size exclusion, zeolites are expected to facilitate the preferential permeation of water while impeding the passage of salt ions.
For instance a 12-16% improvement in water ow with zeolite-TFN sea water reverse osmosis (SWRO) membranes was observed compared to unmodied SWRO membranes with similar salt removal rates. 105The researchers attributed this enhancement to both molecular sieving and the formation of defects in the TFN-SWRO membranes, as well as the low degree of crosslinking due to the presence of zeolite despite of its low concentration (0.2 wt%). 105Further, incorporating sodiumaluminum-silicate zeolite (NaA-NPs) into the active PA layer of TFC-RO membranes resulted in more hydrophilic, smoother, and more negatively charged membrane surfaces owing to the characteristics of NaA zeolite which results in improved salt rejection and enhanced antifouling features. 96The highest zeolite loading content (0.5%) led to a reduction in contact angle from about 70°to 40°and increased permeability while maintaining solute rejection.This improvement can be attributed to the preferential ow paths for water permeation provided by NaA zeolite, along with steric and Donnan exclusion mechanisms. 96dditionally, two types of zeolite, hydrophilic (FAU, Al/Si = 1.0) and hydrophobic (MFI, Al/Si = 0), were incorporated into RO membranes. 106Both FAU and MFI membranes demonstrated a remarkable salt rejection rate of 100%, with improved membrane permeability reaching 720 L m −2 h −1 bar −1 for a membrane thickness below 3.5 nm, that is approximately 100 times higher than the permeability of commercial RO membranes. 106Additionally, the FAU membrane showed a lower pressure drop due to its hydrophilic nature, while the hydrophobic MFI zeolite membrane displayed a higher pressure drop attributed to capillary resistance. 106n addition, nonthermal glow discharge plasma was used to incorporate clinoptilolite zeolite into the polyamide (PA) layer of the TFC-RO membrane and the resulting composite membrane demonstrated signicant enhancement in water ux and fouling recovery ratio when compared to the unmodied membrane. 20Moreover, the researchers investigated the impacts of incorporating nano-NaX zeolite into PA membranes, and observed several improvements in the surface features of the nanocomposite. 107These enhancements included a reduction in root mean square (RMS) surface roughness, an increase in free energy of liquid-solid interface, and a decrease in contact angle.Furthermore, the addition of nano-NaX zeolite resulted in larger pore size, a thinner lm thickness, higher thermal stability, and improved water ux compared to pure PA membranes. 107Additionally, TFN membranes integrated with NaY zeolite nanoparticles were successfully fabricated on polysulfone supports through interfacial polymerization of MPD and TMC. 108The experimental ndings revealed that increasing the reaction time during IP process resulted in higher salt rejection, indicating a denser zeolite-polyamide layer, along with enhancing the water ux from 39.63 to 74.32 L m −2 h −1 , while maintaining a salt rejection of 98.8% under optimized conditions.Through post-treatment of the TFN membranes using a solution composed of camphor sulfonic acid-triethylamine salt, sodium lauryl sulfate, and glycerol, the water ux was further enhanced to 86.05 L m −2 h −1 , with a salt rejection of 98.4%.Comparatively, the water ux of the posttreated TFN membranes containing NaY zeolite nanoparticles was twice as high as that of a pristine TFC membrane without zeolite nanoparticles. 108 2.2 Silica nanoparticles.Silica, also known as silicon dioxide (SiO 2 ), is a naturally occurring compound that is one of the most abundant minerals on Earth.Silica possesses unique physical-chemical properties that make it a versatile and valuable material.It is chemically inert, which means that it does not react with most substances, making it an excellent choice for applications where chemical stability is crucial.0][111] Silica or mesoporous silica nanoparticles are known as hollow-based nanoparticles that have an appropriate size with a large pore volume and surface area.Silica nanoparticles have pores that range from 2-20 nm in size, which give them a large surface area (>1000 m 2 g −1 ), excellent pore volume, and superior chemical stability. 112,113Pristine silica membranes can form microporous structures, but they tend to become unstable during the process of desalination.This is because the water that passes through the micropores interacts with silane groups, causing the silica lm's pore sizes to increase. 114To enhance both the mechanical and chemical stability of these membranes, it may be necessary to incorporate silica nanoparticles into polymeric nanocomposites. 114Silica's incorporation into RO membranes provides several benets, including improved permeability efficiency, fouling resistance, and overall system durability.Silica promotes the formation of a more hydrophilic membrane surface which enhances the water ux through the membrane by reducing the membrane's resistance to water ow. 115Furthermore, silica incorporation enhances the durability of the RO membrane by reducing membrane degradation.][118] For instance, the researchers examined the utilization of SiO 2 NPs as nanollers in PA membranes to improve their performance.They created TFN membranes using SiO 2 NPs of various sizes (10-20 nm) and ratios.The inclusion of SiO 2 NPs led to structural modications in the membranes, with higher concentrations of SiO 2 resulting in a more extensive porous network.The incorporation of SiO 2 NPs led to a considerable improvement in permeability, increasing it by 58%.When compared to the pristine membrane, the modied membranes with SiO 2 demonstrated higher permeability and lower ux decline ratio, while ltering organic matter solutions.This investigation offers valuable insights into the potential of SiO 2 NPs as a viable strategy for enhancing the performance of PA membranes. 119For instance, the incorporation of SiO 2 NPs into RO membranes led to enhanced thermal stability, as indicated by a minimal decrease in salt rejection of less than 2.5% aer heat treatment at 95 °C for 180 minutes. 120Furthermore, the membranes demonstrated excellent resistance to chlorine exposure, with a salt rejection decrease of less than 2% even aer being exposed to 10 000 ppm h of NaOCl. 120Further, TFN membranes were fabricated by IP process of MPD and TMC organic solution on silicon nitride/polyethersulfone (PES) composite substrate, as shown in Fig. 9. 118 Researchers have found that by introducing amino functionalization to silica (SNPs) using p-aminophenol, the compatibility of silica NPs with the PA layer can be enhanced.This modication also reduces the occurrence of agglomeration during the interfacial polymerization process, thereby facilitating the successful production of high-quality TFN membranes.The TFN shows considerably improved ux, compared to the pristine TFC membrane, along with higher thermal stability.2.2.3 TiO 2 nanoparticles.The modication of RO membranes with titanium dioxide (TiO 2 ) has gained signicant attention due to its potential to enhance membrane performance. 121Several mechanisms contribute to the improved performance of TiO 2 -modied RO membranes.One key mechanism is the photocatalytic activity of TiO 2 nanoparticles.When irradiated with UV light, TiO 2 nanoparticles generate reactive oxygen species (ROS) that can degrade organic compounds and microbial pollutants present in the water. 49,1224][125] Another mechanism is the hydrophilicity enhancement of TiO 2 -modied membranes.TiO 2 nanoparticles possess a high surface area and can alter the surface properties of the membrane, making it more hydrophilic.This hydrophilic surface reduces fouling by inhibiting the adsorption of organic matter and preventing the attachment of foulants, thereby enhancing the water permeability and ux of the membrane. 126Additionally, TiO 2 can provide structural stability to the RO membrane.
For instance, PA/TiO 2 TFN membranes were synthesized using an in situ interfacial polymerization, 126 where two different approaches were utilized to incorporate TiO 2 nanoparticles into the membranes: dispersing them in the dodecane organic phase of TMC or the aqueous phase of MPD.The synthesized PA-TiO 2 TFN membranes exhibited a signicant enhancement in the permeate ux that increased from 33.6 to 40 L m −2 h −1 , indicating improved water productivity.Additionally, there was a slight increase in salt rejection, rising from 99.75% to 99.82%.Moreover, the TFN membrane with a lower TiO 2 concentration demonstrated improved resistance to organic fouling. 126urthermore, the impact of incorporating TiO 2 NPs, at different NP concentrations (0.01, 0.05, 0.2, and 0.5% w/v) into PA on the rejection of organic matter (OM) and salts as well as permeate ux, was assessed. 119The inclusion of TiO 2 nanollers in the PA membranes enhances their porosity, hydrophilicity, and consequently, their permeability that increased by 24% compared to membranes without NPs, where the rejection performance and fouling behavior of the membranes were assessed using salts (MgSO 4 and NaCl) and OM (humic acid [HA] and tannic acid [TA]).In addition, sub-10 nm TiO 2 NPs were incorporated, at different doses, into the PA layer to create TFN. 127The researchers found that these TFN membranes exhibited enhanced thermal stability and antifouling properties compared to pristine membranes.The number of E. coli colonies that formed over the UV-illuminated membranes was counted, Fig. 11.The results revealed that the exposure of TiO 2incorporated TFN membranes to UV light signicantly reduced the viability of E. coli. 127However, despite its effectiveness, the use of TiO 2 -based antifouling membranes is hindered by the requirement of costly UV irradiation. 123This has limited their practical application. 123However, there has been considerable research directed toward the development of visible-light responsive-TiO 2 that utilizes the localized surface plasmon resonance (LSPR) of metal nanoparticles, specically gold (Au) and silver (Ag) nanoparticles. 123,128verall, the enhanced performance of RO membranes modied with TiO 2 is driven by its photocatalytic activity, improved hydrophilicity, antimicrobial properties, and structural reinforcement.These mechanisms collectively enable the membrane to resist fouling, prevent microbial growth, increase water permeability, and extend its lifespan.
2.2.4 Silver NPs-modied TFC membranes.Silver NPs have proven to be effective in enhancing the performance of RO membranes, preventing biofouling, improving thermal response, facilitating photocatalytic degradation, and enhancing electro-conductivity. 129 Ag-NPs have widely been recognized for their antibacterial properties, which occur through various mechanisms, including the generation of reactive oxygen species (ROS), contact killing, and the release of silver ions (Ag + ) (Fig. 12). 130To further elaborate, Ag-NPs have a strong affinity for the cell membrane surface, allowing them to penetrate into the cell and disrupt its functions.Furthermore, the production of ROS by Ag-NPs leads to damage to the cell membrane and the alteration of its DNA structure.The release of Ag + ions can disrupt the production of adenosine triphosphate (ATP) and hinder DNA replication within the cell membrane. 131,132Additionally, the application of the Schottky effect 133 and surface plasmon resonance (SPR) 134 can enhance the absorption of visible light by photocatalytic materials when combined with Ag-NPs.These improvements in photocatalytic performance can enhance the antibacterial and organic degradation capabilities of modied membranes.[137] Fig. 11  Fig. 12 The proposed antimicrobial mechanisms of Ag NPs. 130or instance, the antibacterial characteristics of a covalently bonded cysteamine-modied TFC membrane incorporating Ag-NPs was investigated. 138Compared to the pristine TFC membrane, the Ag-NPs-graed TFC membrane exhibited slightly lower salt rejection but a higher water ux and potent antibacterial properties against E. coli, evidenced by the absence of bacterial growth on the membrane surface.The antimicrobial mechanism of Ag-NPs can be attributed to their ability to disrupt bacterial cell metabolism.The antimicrobial effect of Ag + ions involves the inactivation of membrane proteins followed by binding to bacterial DNA, which interferes with DNA replication. 138The facile loading of Ag-NPs onto TFC-RO membranes was achieved through the reaction of a reducing agent with a silver salt on the surface of the membrane to yield a uniform Ag-NPs coverage with rm bonding to the membrane. 97These Ag-NPs demonstrated strong antibacterial activity, resulting in a decrease of over 75% in the number of viable bacteria attached to the membrane for different bacterial strains.Additionally, using confocal microscopy, the researchers observed that Ag-NPs effectively inhibited biolm formation, leading to a signicant decrease in total biomass, as well as reduced levels of EPS and both live and dead bacteria on the membrane. 97Furthermore, a study of Ag-NPs TFN membrane unveils the creation of nanochannels with an average size of approximately 2.5 nm around the Ag-NPs. 139The formation of these nanochannels can be attributed to the hydrolysis of TMC monomers and the subsequent termination of IP by the water layer surrounding each hydrophilic Ag-NPs, as shown in Fig. 13a.Importantly, these nanochannels signicantly enhanced the water permeability of TFN membranes, nearly tripling it compared to the pristine membrane without nanochannels, Fig. 13b.Furthermore, the incorporation of Ag-NPs leads to improved rejection capabilities against various substances such as NaCl, boron, and small organic compounds like noroxacin, propylparaben, and ooxacin, Fig. 13.This enhanced rejection performance can be attributed to a combination of factors, including enhanced Donnan exclusion, improved size exclusion, and suppressed hydrophobic interaction. 139

Characterization of polyamide layers
Characterization techniques play a vital role in studying the physicochemical properties of nanoparticles incorporated into/ onto thin lm composite (TFC) membranes. 140These techniques enable researchers to acquire valuable information about the morphology, composition, and interactions between nanoparticles and membrane surfaces.This overview section presents a classication of characterization techniques, highlighting their types and importance in providing essential information for assessing the performance of nanoparticles-modied membranes.

Surface imaging techniques
Surface imaging techniques, such as atomic force microscopy (AFM), and scanning and tunneling electron microscopy (SEM, and TEM) are of signicant importance in characterizing nanoparticles-modied RO membranes. 141,142These techniques provide valuable insights into the surface morphology, roughness, thickness, and distribution of nanoparticles on the membrane surface.This information is critical for understanding the interaction between nanoparticles and the membrane surface, assessing the uniformity and coverage of the modication layer, and evaluating the potential impact on membrane performance.
AFM enables high-resolution imaging of the membrane surface, which allows for the assessment of nanoparticle adhesion, distribution, and stability.It provides information about the surface topography, roughness, and porosity, aiding in understanding the surface modications caused by the incorporated nanoparticles.AFM offers detailed insights into the surface features at the nanoscale level, which are crucial for evaluating the impact of nanoparticle modications on membrane performance.Additionally, AFM can measure the thickness of the nanoparticle layer, further assisting in optimizing the coating process.For instance, AFM analysis revealed that the PA membrane coated with 10% w/w graphene oxide (GO10-PA), displayed an average roughness (R a ) of approximately 21.5 nm.This value was signicantly lower compared to the pristine PA membrane, which exhibited an R a of approximately 46.5 nm (Fig. 14a and b). 29In another investigation, AFM images provided evidence showcasing that the surface roughness of membrane increased proportionally with the concentration of SiO 2 . 31Further, AFM analysis of the (PA/GO) composite revealed non-uniform coverage of GO NPs, with some areas showing folded GO NPs.Additionally, the surface roughness exhibited slight variations across different regions of the sample.These observations can be attributed to the uneven distribution of GO NPs within the PA matrix.Furthermore, due to the thickness of GO NPs ranging from 1.1 to 2.6 nm, the incorporation of GO NPs resulted in a slightly thicker membrane formation. 60In another study, 62 it was observed that the surface modication of PA resulted in a smoother surface, evidenced by a decrease in roughness (R a ) from 66.4 nm to 50.3 nm.Similarly, AFM analysis showed that as the content of the reduced graphene oxide (rGO)-TiO 2 nanocomposite increased, the TFN-RO membrane roughness gradually decreased. 30EM offers high-resolution imaging of the modied membrane surface, enabling researchers to scrutinize nanoscale features and assess the distribution of nanoparticles.It provides a larger eld of view compared to AFM, allowing for the examination of a larger sample area.With SEM, the morphology, cross-sectional views, shape, and spatial arrangement of nanoparticles on the membrane surface can be observed, aiding in understanding the efficacy of the modication process. 143,144Thus, SEM was employed to examine the surface structure of SiO 2 -modied TFC membrane. 145The researchers observed a nodular pattern on the membrane surface, consisting of densely packed granular bumps, where the addition of SiO 2 nanoparticles resulted in the formation of prominent ridge-like nanoribbons, clearly visible on the surface of the modied membrane.As the concentration of the added SiO 2 NPs increased, the substrate surface exhibited an enhanced presence of these distinct structural characteristics, resembling a nanoribbon polyamide (PA) layer. 145In another study, SEM images revealed the existence of spherical 10-20 nm sized Ag-NPs on the membrane surface. 138Additionally, SEM images of the original PA membrane surface displayed a rugged topography characterized by ridges and valleys; however, when planar aminated graphene oxide (AGO) and GO nanosheets are introduced, they effectively smoothened the surface by lling in the valleys (Fig. 14c and d). 29Further, by utilizing SEM analysis, researchers could obtain high-resolution images of the layers cross-section, revealing important information about the layerby-layer structure, thickness, and spatial arrangement of the nanomaterials in the modied RO membranes.Therefore, to examine the PA layer's cross-section, membrane samples are prepared by either removing the polyester backing layer and fracturing the polyamide support layers, or immersing the entire membrane in liquid nitrogen and then fracturing the polyamide support layer. 146][149] Additionally, SEM can be combined with energy-dispersive X-ray spectroscopy (EDS) to obtain elemental composition information, helping to identify the presence and distribution of nanoparticles on the membrane surface. 127,141

Chemical composition analysis
Chemical composition analysis techniques, such as X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR), provide in-depth insights into the elemental composition, chemical state, and bonding conguration of nanoparticles incorporated onto RO membranes.These techniques offer valuable information for understanding the surface modications, compatibility, and reactivity of the modied membranes.
XPS provides valuable information about the surface composition of modied RO membranes.By measuring the kinetic energy of photoelectrons emitted from the sample surface, XPS allows for the identication of elements present on the surface, their chemical states, and their binding energies. 150This information aids in determining the type and concentration of nanoparticles incorporated onto the membrane surface.Additionally, XPS can reveal changes in oxidation states, chemical functionalities, and the extent of nanoparticle dispersion.These insights are crucial for evaluating the distribution and stability of nanoparticle modications, which impact the performance and longevity of the modied membranes.Moreover, XPS can detect subtle changes in the chemical environment surrounding the nanoparticles, providing valuable information about potential interactions with the membrane materials and the formation of chemical bonds.For instance, the XPS analysis of GO-modied PA demonstrated the formation of bonds between the TMC moieties of AP and GO NPs; 60 however there was no clear evidence of covalent bonding between the MPD moieties of AP and GO NPs; nevertheless, it was assumed that H-bonding might be involved. 60TIR spectroscopy provides information about the functional groups and molecular vibrations present in modied RO membranes.By measuring the absorption of infrared light, FTIR can identify the types of chemical bonds within the modied layer, including both surface-bound nanoparticles and the membrane matrix.Through analysis of FTIR spectra, changes in peak positions, intensities, or shis can be correlated to chemical interactions occurring between nanoparticles and the membrane surface.These changes in chemical bonds and functional groups offer insights into modications in surface properties, hydrophilicity, and compatibility with the aqueous environment.Furthermore, FTIR enables the identication of specic functional groups present in the modied membranes, facilitating the understanding of potential changes in surface chemistry and enabling the optimization of membrane performance.For instance, The FTIR analysis of GO-modied PA indicated that covalent bonds are indeed formed between the acyl chloride groups of TMC moieties and the functional groups of GO. 60 This signies a strong chemical interaction between these two components. 60Likewise, the presence of silica in the composite was evident from a pronounced peak in the infrared spectrum at approximately 1040-1060 cm −1 , whose intensity increased with increasing the silica content of samples, indicating a clear correlation between the silica content and the strength of the interaction observed in the FTIR spectrum. 151Further, in GO-modied membranes, the existence of GO leads to peak broadening at 1585 cm −1 owing to combined contributions from graphene (G) band and vibrations of the polysulfone phenyl ring. 68he combined use of XPS and FTIR allows for a comprehensive analysis of the chemical composition and reactivity of modications applied to RO membranes.These techniques provide detailed information about the incorporation, distribution, and stability of nanoparticles, as well as the potential modications occurring at the membrane surface.Understanding these surface modications and interfacial interactions is crucial for tailoring the functional properties of RO membranes, such as improved fouling resistance, enhanced ux rates, and selective ion rejection.

Contact angle measurement
Contact angle measurement assesses the membrane's surface energy, the wetting behavior and surface hydrophilicity or hydrophobicity of modied membranes.By placing a water droplet on the membrane surface, contact angle analysis determines the angle between the liquid-vapor interface and the solid surface.Modications that enhance the hydrophilicity of RO membranes can improve water ux rates, reduce fouling propensity, and enhance overall membrane performance.For instance, the inclusion of GO NPs enhanced the membrane hydrophilicity, as demonstrated by a reduction in the contact angle. 60Namely, when 15 PA layers with 3 GO layers on top were assembled, the GO-modied PA sample (GO3/PA15) exhibited the highest GO NPs coverage on the PA surfaces and displayed the highest reduction of contact angle from 60 ± 2 to 20 ± 5 with an accompanying marginal increase in hydrophilicity. 60imilarly, the immobilization of Ag-NPs on the membrane led to a noticeable decrease in the contact angle, reducing it from 56.7 ± 2.2°to 32.2 ± 2.4°. 138In another investigation, the TFC membrane devoid of zeolite nanoparticles exhibited a relatively elevated contact angle of 68.4°. 108However, upon the introduction of zeolite, the contact angle of the modied membranes was reduced from 63.4°to 46.2°as the zeolite loading increased from 0.02 wt% to 0.2 wt%. 108

Zeta potential analysis
The characterization of modied RO membranes using zeta potential analysis is a valuable technique in assessing the surface charge and its impact on membrane performance.Zeta potential analysis also allows the assessment of surface charge changes due to membrane modications, such as the incorporation of nanoparticles, functional groups, or surface coatings, all of which can alter the surface charge, and result in improved anti-fouling properties or enhanced selectivity.A higher magnitude of zeta potential indicates a greater electrostatic repulsion between the membrane surface and charged solutes, reducing fouling potential and promoting higher rejection of dissolved species.Furthermore, monitoring zeta potential changes over time can help assess the stability and durability of modied RO membranes.Variations in zeta potential can indicate changes in membrane properties, such as fouling accumulation or chemical degradation, providing valuable information for membrane maintenance and cleaning protocols.For instance, incorporation of 0.1 weight percent of multi-walled carbon nanotubes (MWCNTs) reduced the modi-ed membrane's zeta potential and surface charge. 73

Thermal stability
Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) are two key techniques used in the characterization of nanoparticles-modied TFC membranes for reverse osmosis (RO) applications.These techniques provide valuable information on the thermal stability, decomposition behavior, and phase transitions of nanocomposite lms as a function of temperature.During a typical TGA experiment, the sample is subjected to a controlled temperature ramp, and as the temperature increases, the weight loss due to volatilization, degradation, or combustion of the constituents is recorded.TGA enables the determination of the temperature at which weight loss begins, which indicates the onset of decomposition or degradation.Moreover, DSC is a technique used to analyze the thermal properties, phase transitions, and heat ow characteristics of materials as a function of temperature. 152It measures the difference in heat ow to or from a sample and a reference during controlled heating or cooling.This technique gives valuable data about melting temperature (T m ) glass transition temperature (T g ), and crystallization temperature (T c ) of various polymer types. 152,153For instance, using TGA to examine the thermal stability of TFC and TFN membranes revealed that the distribution of TiO 2 NPs within the TFN membrane, produced a slightly higher onset temperature for intense degradation compared to the unmodied TFC membrane. 127amely, the onset degradation temperature increased from 530 to 550 °C upon the incorporation of TiO 2 NPs. 127Similarly, the TGA curves provided evidence supporting the correlation between the addition of SiO 2 and a reduction in thermal decomposition.The results indicated that as the SiO 2 content increased, the thermal decomposition of modied membranes decreased. 151verall, thermal stability data allows the determination of the degradation temperature, which is the temperature at which a signicant weight loss occurs.This information helps in assessing the temperature limits that the nanocomposite lm can tolerate during RO operations, ensuring its stability and preventing any detrimental effects on the lm's properties.

Challenges in TFC membrane
Most research papers of TFN membranes focus on membrane modication with a given nanomaterial to prepare a novel membrane and study its improved performance characteristics.However, there is a need for a systematic monovariate exploration of incorporating one nanomaterial with various membrane preparation methods and in various locations to produce TFNa, TFNs and TFNi containing the same controlled size, porosity, and hydrophilicity NPs.0][41] The primary challenge in incorporating nanomaterials in polyamide membrane fabrication is achieving a uniform dispersion of the nanollers within the polymer matrix. 154,155Poor dispersion can lead to accumulation, uneven properties, and reduced membrane performance. 33Factors such as the surface chemistry of nanomaterials, shear forces during processing, and compatibility with the polymer matrix all inuence dispersion quality.For instance, the dispersion of hydrophilic nanoparticles in non-polar chemical solvents is unstable. 156It is challenging for large hydrophilic nanoparticles in water to phase-transfer to a nonpolar organic solvent. 1567][158][159] Modifying the surface properties of nanoparticles can help minimize agglomeration in nonpolar solvents or polymer matrices. 33In addition to modifying the surface of nanollers, novel nanomaterials such as MOFs and COFs can be created with specic pore structure and surface charge to promote even dispersion of nanollers within the polymer matrix.Moreover, the intrinsic orientation of 1D and 2D nanollers during the IP processes can be another challenge, e.g., most nanotubes because of gravitational forces prefer horizontal alignment, creating longer paths and higher resistance for horizontal nanochannels water ow.Otherwise, nano-bowls that are randomly dispersed create extra resistance for water ow. 40,159,160Additionally, horizontal nanochannels of nanosheets can be disrupted with long pressurization, and lead to TFN membrane performance deterioration.It is common knowledge that the longer the path, the greater the hydraulic resistance.2D nanomaterials like COF have internal channels that allow water molecules to ow through with reduced distance, making them advantageous compared to traditional materials. 40Hence, the concept of the preferential pathway for water molecules requires further verication since it is only possible if the nanochannels are aligned toward the water ux direction and not blocked by the polymer matrix.So far, most studies on liquid separation reported the membrane performance results with only randomly arranged nanotubes.Furthermore, the trade-off between water permeability, salt rejection rate, fouling and Cl 2 -resistance should be carefully considered.The nanollers toxicity and appreciable leaching rates from TNF membranes should also be avoided.Moreover, green synthesis of nanollers and TFN membranes are preferred.Another signicant challenge in incorporating nanomaterials in polyamide membrane fabrication is ensuring the stability and durability of the composite membrane under harsh operating conditions.Nanomaterials, such as carbon nanotubes, graphene oxide, or metal nanoparticles, can be susceptible to degradation, oxidation, or leaching, which can compromise the long-term performance of the membrane.Finally, the scalability and cost-effectiveness of incorporating nanomaterials in polyamide membrane fabrication are critical factors that need to be considered to meet industrial production requirements.Advanced manufacturing processes, such as electrospinning, 3D printing, and roll-to-roll coating, can be employed to scale up the production of nanocomposite membranes while maintaining cost-effectiveness. 40,159,160

Economic analysis
The economic aspects of modied RO membranes incorporating nanomaterials (graphene, CNTs, MOF, zeolites, TiO 2 , and SiO 2 , Ag NPs, etc.) for water treatment applications are crucial considerations in determining their feasibility and commercial viability.Nanomaterial-modied RO membranes offer improved performance characteristics, such as increased permeability, salt rejection, reduced cleaning requirements, antimicrobial properties, and enhanced mechanical, chemical, and physical stability.These advancements translate into several economic benets, including higher water production rates, reduced energy consumption, improved water quality, and extended membrane lifespan.The increased productivity and efficiency of nanomaterial-modied membranes can result in cost savings over time, making them economically viable alternatives to traditional membranes.A research 161 highlighted the important role of membrane modication in lowering specic energy consumption (SEC) during desalination processes across different water sources (Fig. 15a and b).The study demonstrated considerable energy savings of up to 80% in scenarios involving feed waters with low osmotic pressure, such as water reuse.However, the cost of nanomaterials is a signicant consideration when assessing their economic feasibility.Additionally, nanomaterials may have limited availability and higher market costs due to their specialized nature and manufacturing constraints.These higher initial costs can pose a barrier to the widespread adoption of nanomaterial-modied membranes, particularly in large-scale applications.
For instance, metal oxides, silica, and zeolites are generally more economically viable due to their wide availability and competitive pricing.The use of metal oxides in RO membrane modication provides several economic advantages.They are readily available in bulk quantities, ensuring a steady supply and competitive pricing.Furthermore, the relatively low material costs associated with metal oxides make them economically viable for large-scale applications.The cost-effectiveness of these materials is particularly advantageous for water treatment facilities and industrial processes where long-term cost savings are essential.Similarly, zeolite nanoparticles are relatively costeffective and readily available in the market, making them a cost-effective option for membrane modication.
On the other hand, carbon-based nanomaterials, namely graphene, single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWCNTs), hold signicant potential for enhancing RO membrane performance.Their unique properties, including high permeability, hydrophilicity, and salt rejection rates, make them ideal candidates for improving desalination efficiency.However, the economic feasibility of carbon nanomaterials remains a challenge.The high cost of production and limited availability hinder their widespread adoption.In addition, the complex manufacturing processes required for these nanomaterials make large-scale production and local availability difficult.Further research and development are necessary to overcome these economic barriers and make carbon nanomaterials more cost-effective.Further, the use of silver coatings in RO membrane modication offers antimicrobial properties, effectively preventing biofouling.This reduces the need for frequent membrane cleaning and maintenance, leading to long-term cost savings.While silver is relatively more expensive compared to other materials, the potential economic benets resulting from reduced cleaning requirements should be considered.Moreover, the integration of these nanomaterials into the RO membranes involves additional processing steps, which might increase manufacturing costs.Techniques such as physical blending, electrospinning, and chemical deposition methods require specic instrumentation, energy consumption, and skilled labor.Hence, an economic analysis could provide insights into the long-term monetary benets of employing these modied membranes in terms of reduced operational expenses.

Conclusion and prospective
In conclusion, the incorporation of nanomaterials into RO membranes holds great promise for improving membrane performance and addressing challenges in water purication.Utilizing inorganic and porous organic material, TFN membranes can be fabricated through interfacial polymerization by dispersing NPs in the organic phase or the aqueous solution.The inclusion of nanomaterials in the PA layer not only creates nanochannels for enhanced water transport and consistent solute rejection but also changes membrane features such as hydrophilicity, surface roughness, cross-linking degree, and surface charge, which signicantly impact membrane performance.Extensive research studies have demonstrated the positive impacts of these nanomaterials on various membrane features, including rejection rates, water ux, selectivity, chlorine resistance, and fouling resistance.The optimal dose of nanomaterials and suitable fabrication methods play vital roles in achieving the overall desired performance.Incorporating materials like SiO 2 further enhances the mechanical strength and durability of the membrane, thereby extending its lifespan.Likewise, membranes containing CNTs, GO, SiO 2 and TiO 2 NPs exhibit versatility in RO applications due to their exceptional thermal, electrical, fouling resistance and mechanical stability, as well as their antibacterial and photocatalytic features.Additionally, it is worth noting that MOFs are particularly attractive porous nanomaterials that warrant intensive research attention in the fabrication of TFN membranes.However, there are still areas of concern and knowledge gaps in the existing literature that will require further investigation in the future: 1. Tailored material design: future research can focus on developing and tailoring carbon nanomaterials, and other NPs with specic properties for membrane modication.This could involve exploring new synthesis methods, optimizing material composition, and engineering advanced structures at the nanoscale.By ne-tuning material properties, such as surface chemistry, pore size, porosity, and hydrophilicity, researchers can enhance membrane selectivity, permeability, and fouling resistance.
2. Mechanistic insights: gaining a deeper understanding of the mechanisms underlying the interaction between modied membranes and various pollutants is crucial for optimizing membrane performance.
3. Scaling up and commercial viability: to enable the practical implementation of NPs-modied membranes, researchers need to address scalability and cost-effectiveness.Further research should focus on developing scalable and cost-efficient synthesis methods for incorporating nanomaterials into large-area membranes.The durability and stability of modied membranes under realistic operating conditions must also be thoroughly evaluated to ensure long-term performance.Collaborations between academia and industry can facilitate the translation of research ndings into commercially viable membrane products.
4. Environmental sustainability: as the eld progresses, it is crucial to consider the broader environmental implications of modied membranes.Research should explore the life cycle analysis of these membranes, including the environmental impact of raw material extraction, synthesis processes, and endof-life disposal.Additionally, efforts should be made to develop recycling and regeneration methods for used membranes to minimize waste generation.
Overall, the use of nanomaterials to modify reverse osmosis membranes holds promise for advancing membrane technology and addressing challenges in water treatment.Continued research and development in this eld will contribute to the improvement of membrane science and play a critical role in providing sustainable and efficient solutions for water purication and desalination processes on a global scale.

Safwat
Ashraf A: Mohamed Ashraf A. Mohamed is a professor of environmental analytical chemistry, at the Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt.He earned his MSc degree in 1991 and his PhD degree in 1995.He has been actively engaged in research for the past 35 years and his current research interests include analytical chemistry, nanomaterials, layered double hydroxides, molecularly imprinted polymers, water treatment and analysis, optical sensors, and paper micro-uidics.He has authored several reviews and book chapters on these topics.

Fig. 2
Fig. 2 Chlorine attack on PA membrane and the performance decline due to the attack.Reprinted with the permission of ref. 23, copyright 2024, 2024 Elsevier.

Fig. 1
Fig. 1 (a) The interfacial polymerization process between TMC and MPD on a microporous support layer.(b) Structure of the fabricated TFC membranes with the top and cross-sectional morphologies.148

Fig. 3
Fig. 3 Schematic illustrations demonstrating typical structures of nanomaterial-modified PA TFC membranes: (a) TFC membrane with nanomaterial-coated PA surface layer, (b) TFC membrane with nanocomposite substrate and (c) TFC membrane with nanomaterial interlayer.Reprinted with the permission of ref. 32, copyright 2024, 2024 Elsevier.

Fig. 4
Fig. 4 Number of publications related to nanomaterial-modified FN RO membranes (retrieved from Scopus keywords "(reverse osmosis OR RO) AND (nanoparticles OR nanomaterial OR nanocomposite)" on 27th May 2024).Insets show top contributing journals to the topic and the position of the nanofiller in the thin film nanocomposite.

Fig. 5
Fig. 5 The proposed antimicrobial mechanisms of GO.Reprinted with the permission of ref. 55, copyright 2024, RSC.

Fig. 6
Fig. 6 (a) Synthesis of PA with and without the addition of GO during interfacial polymerization; salt rejection and water flux of TFC and TFC/GO membranes at different (b) feed pressures, and (c) chlorine exposure time at a 500 ppm Cl 2 -dose.Reprinted with the permission of ref. 65, copyright 2024, Elsevier.

Fig. 8
Fig. 8 (a) Schematics of water transport through the pores of zirconium-based porphyrinic MOFs (PCN-222 nanorods) with increasing modifier loading (myristic acid, MA, at 0, 0.1, 1 and 10%), and (b) salt rejection and water flux results for MA-modified and pristine membrane reprinted with the permission of ref. 89, copyright 2024, American Chemical Society.
Fig. 11 (a) Images of the E. coli colonies formed in the plate of UV-treated (i) TFC, (ii) TFN2 (0.012 wt% TiO 2 ) and (iii) TFN4 (0.024 wt% TiO 2 ) membranes; (b) mechanism for photocatalytic activity of TiO 2 NPs under UV irradiation; (c) number of E. coli colonies counted on the plate of TFC, TFN2 and TFN4 membranes after 30 minutes of UV irradiation.127

Fig. 13 (
Fig. 13 (a) Schematic diagram of Ag-NPs induced nanochannels in the polyamide layer for efficient water transport, (b) separation performance results of the control and TFC-Ag20 membranes, and (c) membrane rejection results of the control TFC and TFC-Ag20 membranes (the suffix Ag20 means prepared using 20 mM solution of AgNO 3 ).Reprinted with the permission of ref. 147, copyright 2024, American Chemical Society.

Fig. 14
Fig. 14 AFM height images of (a) the pristine, uncoated polyamide (PA) and (b) the GO 10 -coated PA membranes; and the SEM images of (c) the pristine PA and (d) the GO 10 -coated PA membranes, (scale bar = 1 mm).Reprinted with the permission of ref. 29, copyright 2024, American Chemical Society.

Fig. 15
Fig. 15 Relationship between specific energy consumption (SEC, kW h m −3 ) and product flow in the context of (a) seawater desalination and (b) water reuse.Reprinted with the permission of ref. 29, copyright 2024, American Chemical Society.
A: Mahmoud nanotechnology, solid state physics, thin lm technology, material science, water treatment and optical sensors.

Table 1
Performance evaluation of several TFN reverse osmosis membranes incorporating various carbon-based nanomaterials

Table 1 (
Contd. ) modied membrane reduced the fouling by more than 75% MIL-101 (Cr) 2000, NA, 16 bar 24.3 to 35.2 99.3 to 99.1 MIL-101 (Cr)'s porous structures can provide direct water channels for water molecules to go through fast in the thick selective PA layer, enhancing the membranes' water permeance 101