Polymeric Nanocomposite Membranes for Water Purification and Scalability

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This entry is adapted from the peer-reviewed paper 10.3390/nano12203637

Water shortage is a major worldwide issue. Filtration using genuine polymeric membranes demonstrates excellent pollutant separation capabilities; however, polymeric membranes have restricted uses. Nanocomposite membranes, which are produced by integrating nanofillers into polymeric membrane matrices, may increase filtration. Carbon-based nanoparticles and metal/metal oxide nanoparticles have received the greatest attention.

polymeric nanocomposite membranes nanoparticles located polymerisation physical combining sol-gel

1. Membranes Made of Polymeric Nanocomposite (PNC)

PNCs may be classified into three categories based on the functional properties of the membranes. Type I is conventional PNCs, which are PNCs created from polymeric nanocomposites to increase their characteristics but without adding a function to the membrane. The membrane is enhanced in this scenario, but it remains a passive element in separation systems, with the primary and unique role of separating components of the feed phase through two different mechanisms, ion exchange and ionic exchange, in addition to size exclusion or dissolution–diffusion ^[1]^[2]^[3]^[4]^[5]^[6]^[7]. Type II is the active-bulk phase PNCs, which are PNCs made from polymeric nanocomposites that give the membrane the ability to perform dual functions, one major function linked to mass transfer between two separated phases and a second function linked to specific properties of the material from which it is made; an example of this is a PNC made from inorganic nanoparticles distributed in a polymeric phase based on a conductive polymer such as poly(aniline) ^[8]^[9]^[10]^[11]^[12]. Type III is the active-surface PNC, which exhibits a second functionality on the surface in contact with the feed phase or the inner of the pores, but not in the bulk of the material ^[13]^[14]^[15]^[16]^[17]^[18]. The categorisation previously given is implicitly dependent on the nature of the active layer; hence, the same membrane might be classed as two distinct categories due to the effect of polymer bulk characteristics on polymeric surface properties.

Several research have proposed a second category for the different kinds of PNCs based on the membrane structure and placement of nanomaterials: (i) mix-matrix PNCs, which are membranes made of some nanocomposite material, (ii) nanomaterial thin-film PNCs, which are membranes surface coated with some sort of nanomaterial, (iii) thin-film PNCs with nanocomposite substrate, which are membranes made of a thin-film on a support made of nanocomposite material, (iv) nanocomposite thin-film PNCs, which are membranes coated with some type of polymer nanomaterial ^[19].

Despite the fact that this classification aids in visualising the configuration of membrane layers, it is deemed unsuitable for describing the structure–function relationship of PNCs because all separation properties of porous membranes are dependent on the “active layer”, which would be defined as the layer with the littlest pore size distribution constituting the membrane structure, and as a result, the porous substrate must be understood to explain the separation properties. An asymmetric polymeric membrane, for example, is defined as porous polymeric support covered on its surface by a thin film with a pore-size distribution less than the support’s pore-size distribution; as a result, all retention characterisations in this type of membrane are largely decided by the thin film comprising the active layer, as the porous polymeric support’s function is to enhance the mechanical characteristics of the multi-layered system ^[20]. In the cases of “nanomaterial thin-film PNC” (symmetric membrane|nanomaterial), “thin-film PNC with nanocomposite substrate” (nanocomposite support|active layer), and “surface-coated PNC” (support|activelayer|nanomaterial), the previous classification describes multilayer membranes rather than PNCs. Note that the vertical bar was utilised to denote the contact between the membrane’s distinct layers. Note that “mix-matrix PNC” corresponds to symmetric PNC whereas “thin-film PNC” relates to asymmetric PNC ^[21].

2. Methods of Membrane Preparation

2.1. Traditional Methods for the Preparation of Nanocomposite Membranes

Currently, the conventional techniques for fabricating hybrid nanocomposite membranes fall into three categories. The methods include (1) in situ polymerisation, (2) sol-gel, and (3) physical mixing. These three methods may be utilised alone or in combination to create membrane architectures of choice ^[22]^[23]^[24].

2.1.1. Located Polymerisation

Infiltration, often known as in situ polymerisation, is a typical approach for producing PNC membranes. Before polymerisation, the nanofillers are mixed in bulk or solution with the monomer. When exposed to heating, high-energy radiation, or plasma, certain functional groups on the surface of nanofillers, such as hydroxyl and carboxyl groups, may create activated species (radicals, cations, or anions). These activated species may cause the monomer’s surface to polymerise. The polymer chain grows after the initiation phase, and inorganic fillers may be physically or chemically connected to or integrated into the polymer matrix. This method is particularly useful for nanocomposite membranes with inorganic exfoliated structures because the monomer penetrates the exfoliated structure’s inorganic galleries and, after polymerisation, achieves more uniform filler dispersion within the polymer matrix. Ring-opening, atom transfer radical, live anionic/cationic, and nitroxide-mediated polymerisations, in addition to standard polymerisation procedures ^[25]^[26]^[27], are among the polymerisations that may be employed in this methodology. The aggregation of inorganic nanofillers in the produced membranes is difficult to prevent or minimise using this approach ^[24]^[28]^[29]^[30]^[31].

As previously stated, many polymerisation techniques may be utilised during this preparation procedure, such as polystyrene (PS), which typically polymerises by generating radicals. This kind of polymerisation may occur in bulk, solution, or suspension. For this reason, the selection of the proper surfactant must take into consideration several criteria. The first is because the surfactant alters the reactivity of the inorganic filler so that it may react with the monomer. The second theory depends on the notion that a surfactant’s structure must have long alkyl chains or tetrahedral structures for interlayer space to be increased ^[32]. A team from the Toyota Research Institute was the first to use in situ polymerisation to create nylon-6/clay nanocomposite membranes. Between the layers, polymerisation occurs, and macromolecular chains are created. As these chains expand, they remove the disoriented clay layers, resulting in an exfoliated structure ^[33].

Doucouré et al. ^[34] created certain gas permeation membranes by using in situ plasma polymerisation on mesoporous silica containing various fluorinated monomers. The monomers used were octafluorocyclobutane (C₄F₈), trifluoromethane (CHF₃), and 1,1,1,2-tetrafluoroethane (CF₃-CH₂F), with argon serving as the carrier gas. The Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) findings revealed that CF₃-CH₂F produces the most crosslinked polymers and C₄F₈ produces the least crosslinked polymers, indicating that C₄F₈ is the most flexible polymer. Using permeability and selectivity procedures, it was determined that N₂ molecules could pass through this polymer with the least amount of crosslinking. Using CF₃-CH₂F, however, it proved difficult to penetrate tiny atoms such as He.

Patel et al. ^[35]^[36] produced various hybrid membranes by combining dispersed silica nanofillers with diacrylate-terminated poly(ethylene glycol) (PEG) or poly(propylene glycol) (PPG) and initiating the radical polymerisation using 2,2-azobisisobutyronitrile (AIBN). After silica incorporation, the polymer’s elastic modulus increased and its gas permeability decreased, but the CO₂/H₂ selectivity was practically unaffected. Moreover, Nunes et al. ^[37] produced a PNC for gas separation by dispersing silica in poly (ether imide) (PEI). In situ hydrolysis and condensation of tetraethoxysilane (TEOS) were used to produce polymer nanocomposite membranes to scatter silica, and aminosilanes, which interact strongly with the imide groups of PEI, were added into this inorganic polymer network. By measuring the glass transition temperature (Tg), it was discovered that aminosilanes raised the Tg, which is directly connected to the increase in rigidity; this rigidity explains why a PNC is more stable under pressure. In addition, the inclusion of silica transformed the membrane’s structure from finger-like to sponge-like ^[38].

2.1.2. Sol-Gel

The sol-gel method is a low-temperature synthesis technique that has been widely used to produce PNCs since the 1980s. This approach involves the mixing of dissolved inorganic nanofillers with monomers, oligomers, or polymers. Then, the inorganic precursors hydrolyse and condense into nanofillers that are uniformly scattered in the polymer matrix. The mid-reaction circumstances in this approach, such as room temperature and pressure, are advantageous. Additionally, the concentration of the various species in the solution is simple to regulate. Due to the dispersion of chemicals at the molecular or nanometre level ^[38]^[39]^[40]^[41]^[42]^[43], the membranes formed are homogenous.

Clearly, the actual process is more complicated, and intermediate compounds, such as metal oxo alkoxides, may be produced. To produce monodisperse TiO₂ powder, for instance, hydrolysis, condensation, nucleation, and particle growth are undertaken in sequence. Oxo oligomers, polymers, and crosslinked macromolecules are also produced and coexist with the sol ^[44]. Typically, monomers and oligomers in gels condense and reprecipitate, resulting in a phase change. Some inorganic fillers are not overly sensitive to hydrolysis, and if water is introduced within a few days, gelation may occur. Therefore, hydrolysis and condensation happen without catalysts for non-silicate metal alkoxides ^[45], whereas acid or base catalysts are needed for silicon alkoxides.

Iwata et al. ^[46] produced gas-permeable PNC membranes to separate N₂ and O₂ by combining tetraethoxysilane (TEOS) with 1.3 to 20% fumed silica in polyacrylonitrile (PAN) membranes. Si–O–Si networks are created using TEOS. The scientists exhibited an excellent separation of the O₂/N₂ mixture by increasing the silica content of this PNC, thus producing a thick Si–O–Si network. In another instance, Gomes et al. ^[47] used sol-gel copolymerisation of TEOS with various organoalkoxysilanes to create a poly(1-trimethylsilyl-1-propyl) (PTMSP)/silica PNC. The kind and content of organoalkoxysilanes, as well as the temperature and duration of the synthesis process and gas permeability of the PNC, were examined. Butane permeability and butane/methane selectivity are enhanced by using 20–40 nm particle size and increasing silica concentration in the PNC.

In addition, this approach eliminates the need for elevated temperatures in the MOF’s development, since a uniform growth of ZIF-8 may be obtained at room temperature, as seen in the following example. For instance, Guo et al. ^[48] employed this approach to produce a PNC of MOFs at room temperature to remove Rhodamine B dye molecules from wastewater. Using this method, a coating of ZIF-8 was applied to the hollow PVDF fibre’s macroporous surface. All the components were submerged in a methanol/water solution for 12 h at room temperature. The preparation was then submerged in Zn(NO₃)26H₂O in an ethanol/water solution at 30 degrees Celsius for six hours. Thus, the creation of ZIF-8 membranes on the inner or outer side of hollow PVDF fibres may be regulated so long as the pre-coating of one or the other surface with the Zn precursor or ZIF-8 seeds is regulated.

2.1.3. Physical Combining

Physical blending or casting is regarded as the easiest technique for producing hybrid polymer membranes. In this procedure, nanofillers and polymer matrix are combined directly. There are two primary ways to obtain the mixture: solution blending and melt blending. These two blending techniques are further examined independently.

Solution Blending

Solution blending or solution casting is a simple and efficient process for producing PNC membranes from any inorganic material. Moreover, the concentrations of chemicals in the combination may be readily regulated ^[49]. However, membranes may accumulate inorganic components ^[50]. In order to dissolve the inorganic material, the polymer must be soluble in the solvent. Stirring in three consecutive phases allows the two compounds to be mixed and distributed: (a) dissolving the polymer and then adding the nanofillers, (b) dissolving the nanofillers and then adding the polymer, and (c) dissolving both species to combine them. The polymer chains intercalate into inorganic structures by exchanging with solvent molecules. The membrane is then formed by evaporating the solvent by precipitation or under vacuum ^[51]. However, owing to health, financial, and environmental issues, this technique’s commercial viability is diminished by its high solvent use. Nevertheless, water-soluble polymers, such as poly(ethylene glycol) and polyvinyl alcohol, may be used with this approach in a water solution. Moreover, low or non-polar polymers may be employed to produce intercalated nanocomposites using this technique ^[29]^[31].

Panwar et al. ^[32] noted that by immersing the clay in the proper solvent, the solvent molecules permeate and extend the clay channels. However, they also noted that one of the main advantages of this procedure over melt mixing is the reduction in viscosity. This facilitates the movement of polymer molecules to the platelet surface. However, the solvent is also adsorbed on the clay’s surface, thus the polymer must better adsorb on the clay’s surface in order to displace the solvent. This last argument demonstrates that this procedure is superior for producing hybrid membranes from weak polar polymers.

Ragab et al. ^[52] removed micropollutants from water using a polytetrafluoroethylene (PTFE) double-layer membrane with ZIF-8. The PTFE membrane was dipped in various concentrations of ZIF-8 (20 wt.%) for the manufacture of hybrid membranes. In a comparison of modified and unmodified membranes, it was discovered that the changed membranes enhance the adsorption capacity of micropollutants by 40% while increasing their water permeability. This permeability is also highly intriguing since it would minimise the process’s energy usage. In addition, following three regeneration cycles using poly(ethylene glycol)-400, it was discovered that the membrane retained 95% of its original efficiency.

Low et al. ^[53] created a novel two-dimensional ZIF with a leaf-like architecture (ZIF-L). The influence of incorporating this ZIF-L onto polyethersulfone (PES) membranes on its ultrafiltration characteristics was investigated. The membrane modified with 0.5% ZIF-L yielded the greatest results, as its water flow was enhanced to 75% and its resistance to fouling with bovine serum albumin was almost doubled (BSA). These findings achieved with the modified membrane may be explained by the decrease in zeta potential, the rise in hydrophilicity, and the decrease in surface roughness, which made it more difficult for BSA to bind to the membrane surface.

Melt Blending

When it comes to synthesising PNCs, the melt blending method (also known as melt compounding, melt casting, or melt intercalation method) is by far the most popular, flexible, and favoured methodology. As an added bonus, the method is widely employed in industry since it is solvent-free and safe for the surrounding ecosystem. It is the most exciting method for PNC synthesis with industrial applications ^[24], and one of the best methods overall since it is compatible with cutting-edge industrial processing equipment such as injection moulding and extrusion. Since it does not need the selection of a solvent, it is also simpler than in situ polymerisation, sol-gel, or solution mixing ^[24]^[29]^[30]^[54] and is accessible for commercial polymers that are incompatible with these techniques. This process includes combining thermoplastic polymers and nanofillers using force. Various processing methods are available, including single- or double-screw extrusion, internal mixers, and manual mixing. The external pressures used during processing enhance the dispersion of nanofillers into the polymer matrix to be simpler and more uniform. Paul et al. ^[26] investigated the dispersion and intercalation of nanofillers in polymers and determined that shear forces due to extrusion melt processing and mixing conditions owing to screw speed, mixing duration, and temperature are both crucial characteristics. In addition, the type of the polymer, its molecular weight, and its polarity impact the interaction between phases, which is crucial for obtaining a homogenous filler dispersion ^[55]. The blended hybrid material undergoes annealing at a temperature greater than the Tg of the polymer, enabling the appropriate mixing of components for membrane fabrication. However, this feverish temperature might be a drawback since hot temperatures can occasionally breakdown polymers. Using solid-state NMR, Van der Hart et al. ^[30] studied different exfoliated nylon-6/clay membrane combinations and compared them to pure nylon-6. Using mixing and in situ polymerisation, the membranes were produced. The clays were first treated with a cationic modifier, which attaches ionically to the surface of the clay layers and causes them to expand. In terms of the spacing, crystallinity, and mobility of the non-crystalline nylon-6 segments, the findings obtained for modified hybrid membranes produced by both preparation procedures were comparable. Bhiwankar and Weiss ^[54] used tetra-octyl and tetra-decyl ammonium salts of sulfonated polystyrene (SPS) as ionic compatibilizers of polymer PS and pure Na-montmorillonite clay using this approach. After combining all the components, the compatibilizers exhibited excellent exfoliation and dispersion of the clay, most likely as a result of the separation created by the quaternary ammonium ion, which exchanges with the Na in the channels. In addition, it was noticed that exfoliation increased when the alkyl chain length of the counterion of the compatibilizer increased. In addition, the membranes containing the compatibilizer exhibited a larger storage modulus. Motamedi and Bagheri ^[56] examined the structure and characteristics of polymer composite membranes made with polypropylene (PP), nanoclay, and polyamide-6 (PA6) utilising this preparation process, creating PP/organoclay, PA6/organoclay, and PP/PA6 and PP/PA6/layered silicate combinations. The PA6/organoclay PNC had an exfoliated structure because the clay was contained inside the PA6 particles, resulting in an increase in viscosity. In contrast, the silicate layers in the PP/PA6 PNC were oriented near the PP/PA6 interface. In this sample, it was also noticed that the organic clay altered the form and size of the PA6 particles. Finally, the mechanical characteristics of the produced samples with two components were enhanced. In a separate work, Yoon et al. ^[55] examined the influence of polar comonomers in PS chains on melt intercalation in organosilicate galleries. The comonomer composition and two production techniques of styrene/organosilicate membranes were investigated. PS and three distinct styrenic copolymers containing methylvinyl oxazoline and acrylonitrile units were employed as polymers. At the same temperature, the two synthesis procedures were used with and without shearing. In every instance and very rapidly, the polymers intercalate flawlessly inside the organosilicate framework. The structural stability of polymer/organosilicate hybrids seems to rely on the interactions between both components. Because polymer chains do not diffuse effectively into organosilicate layers, the weak interactions between components resulted in an unstable hybrid. However, when polar comonomers were used, the resulting PNCs were stable because of the strong interaction between their constituents.

2.2. Electrospinning

Electrospinning is regarded as one of the most significant techniques for obtaining polymeric (or composite) nanofibers with diameters between the nanometric and micrometric scales ^[57]. One of its greatest benefits is its adaptability, which enables simple, inexpensive, and customizable membrane production with variable pore size and distribution, aspect ratio, elasticity, and stiffness, among others. Thin-film nanocomposites, mix-matrix hybrids, two- or three-layered composites, and metal–organic membranes with a broad range of components have all been produced using this approach ^[58]. Electrospun membranes have been used in a wide range of environmental applications, including MF ^[59], UF ^[60], NF ^[61], RO ^[62], oil/water separation ^[63], MD ^[64], and bio-separation ^[65].

The management of the structural integrity, stability, and functioning of electrospun nanofibrous membranes (ENMs) across a broad range of operating conditions, such as temperature, fluid pressure, and pH ^[58], is critical for their usage in a variety of applications. These features are integrally connected to the diverse polymeric or composite systems as well as the morphology/topology of the electrospun fibres. There are approximately 200 polymers that have been utilised effectively for the creation of fibres by electrospinning ^[66], providing a broad variety of compositions for membrane manufacturing. Moreover, electrospinning is compatible with a variety of inorganic fillers, such as nanoparticles (NPs) of various natures (SiO₂, TiO₂, ZnO, carbon nanotubes, etc.) ^[67]^[68]. The plethora of available combinations enables the enhancement and/or modification of membrane mechanical characteristics to meet a variety of operating situations and, in certain instances, to offer secondary capabilities such as photocatalytic activity ^[68]. Compared to other conventional fabrication techniques such as non-solvent-induced phase inversion (NIPS) and thermally induced phase inversion (TIPS), electrospun membranes have an interconnected open pore structure and an easily tenable thickness, giving them superior porosity and permeability ^[69].

2.2.1. Instrumentation and Conceptual Framework

A notable advantage of the electrospinning method is that the required equipment is quite simple, which makes the method easily accessible to almost any research centre. The following are the major constituents: (i) a static DC high-voltage power supply source between 1–30 kV, (ii) a syringe pump that allows the control of the fluid flux between 0.01–2 mL·h⁻¹, (iii) syringes and needles, in most cases, hypodermic blunt tip type needles are used, with different lengths and inner diameters between 0.3–1 mm, and (iv) a metallic collector, in most instances, an aluminium one. It is possible to manufacture materials with nanofibers that are randomly oriented by setting the grounded collector so that it is perpendicular to the needle, or it is possible to produce materials with nanofibers that are oriented in the desired deposition direction by using a rotatory system.

Electrospinning is based on an electrohydrodynamic process in which a drop of polymer solution or melt is subjected to the electric potential difference caused by the electrical field in which it is immersed. This difference in potential is what makes electrospinning possible. Taylor’s cone is the name given to the shape that the drop takes on, which resembles a jet or a cone and is analogous to the point of a fountain pen. This cone goes through elongation and flapping processes because electrostatic tensile forces exerted on it are greater than the surface tension of the fluid it encounters. These pressures cause the polymer to be driven into a connected metal manifold, and it is in this manifold that the polymeric fibres that will eventually make up the final membrane are deposited and overlaid.

2.2.2. Parameters of Control in an Electrospinning Process

Every step of the electrospinning process is determined by several factors, each of which has one of three unique qualities: (i) electrospinning characteristics, such as the needle inner diameter, the fluid flow velocity, the applied voltage, and the distance between the needle tip and the collector; (ii) the parameters of the fluid, such as its viscosity and conductivity, as well as, for solution electrospinning, the kind of solvent and the concentration of the polymer; (iii) environmental variables, including temperature and moisture ^[58]. The true difficulty of this technology is in the adjustment of these factors to produce fibres with a certain diameter and shape. Each of these factors has a distinct critical value for each distinct electrospinning system. Setting a parameter outside of this range will always result in fibres with undesirable morphologies, pearl-beaded formations, discontinuous fibres, or insufficient fluid solidification prior to deposition on the collector. The versatility of the electrospinning process is enabled by the simplicity of standard electrospinning equipment. Multi-needle electrospinning and coaxial electrospinning are examples ^[58]. Multi-needle electrospinning increases production by as much as 18 m²·h⁻¹ ^[70] but adds additional control factors such as the number of needles, the distance between them, and their arrangement ^[71]. A coaxial spinneret, formed by two concentric needles, permits the extrusion of two distinct systems that, upon meeting at the needle’s tip, create a core-sheath structure ^[72]. Coaxial electrospinning adds the parameter of miscibility between the two extruded fluids and makes it desirable for both fluids to have comparable dielectric characteristics ^[73] to provide comparable electrical pilings.

2.2.3. Membrane Electrospinning of Composites

When it comes to generating membranes by electrospinning, these two methods are by far the most common ones to use. They both concentrate on composite nanofiber membranes. When creating ceramic-based composite fibres, it is common practise to add sol-gel precursors ^[74]. On the other hand, this is not always the case, and the approach that is utilised the most often for composite fibres with polymeric matrices is the integration of nanoscale components into polymeric solutions ^[75]. Over the course of the last several years, it has come to widespread public attention that viable composite nanofiber materials for use in water purification applications have been produced employing these two methods.

It is essential to recognise that the incorporation of the presented nanocomponents or any other nanocomponents will always have the ultimate effect of modifying the aspect ratio, the mechanical properties, the water flux ^[70], and/or the hydrophobicity of the material. Because of this, it is essential to optimise the proportion to be used to adjust these properties to their optimal values for each application. Chemical compatibility, contact angle, pore size, porosity, and surface roughness are some of the most important considerations to make when choosing the best appropriate composite system for a particular application ^[76].

2.3. The 3D Printing Innovation

Additive manufacturing or 3D printing technologies have emerged as extremely adaptable and promising approaches for a variety of applications. From aerospace ^[77]^[78]^[79], automotive ^[80]^[81], and construction ^[82] sectors to biomedicine ^[83]^[84] and the food sector ^[85], the range of fields in which this technology might be utilised has grown significantly. Even though it is currently relatively understudied, the number of membranes produced by 3D printing technology for desalination and water remediation rises annually.

In addition to the reduction in membrane fouling and the chemical stability of the membrane, one of the driving forces behind the search for more efficient and lucrative membranes is the necessity to remove new pollutants such as emerging contaminants. This requirement is one of the driving forces behind the search. A manufacturing process that can manage membrane structure has been adopted as an economical and solvent-free approach for the manufacture of new membranes thanks to the development and expansion of 3D printing technology. Therefore, three-dimensional printing, which is capable of producing complex structures despite their reduced size, has been identified as one of the most promising methods for the production of membranes in the years to come.

According to the material feed or deposition process, 3D printing technologies are often categorised into four categories: (1) powder, (2) material extrusion, (3) lamination, and (4) photo polymerisation. The most used 3D printing technology is fused deposition modelling (FDM), a filament extrusion process that typically prints thermoplastics (PC, ABS, PLA, etc.) with a resolution between 50 and 200 microns. In three-dimensional printing (3DP), selective laser sintering (SLS), selective laser melting (SLM), and liquid binding jetting are among the powder-processing-based techniques employed. In these techniques, the powder is applied to the construction platform and is used to strategically combine material layers. SLS resolution is close to 80 m, but 3DP resolution ranges from 100 to 250 m. Laminated object manufacturing (LOM) and selective deposition lamination (SDL) utilise cut and laminated polymer sheets as feedstock in lamination-based 3D printing. Lastly, photo-polymerisation-based 3D printing, also known as stereolithography (SLA), employs UV light to commence the polymerisation process, solidifying a polymer/resin layer that binds the following layers together. In this instance, the resolution is around 10 m. In addition to the popular 3D printing methods, additional printing technologies have been improved during the last several years. Among the revolutionary techniques, two-photon polymerisation (TPP) and continuous liquid interface production (CLIP) stand out because of their superior resolution and enhanced mechanical characteristics when compared to conventional techniques ^[86]. Moreover, these procedures are extensively used in several applications, but when it comes to membrane manufacturing, photo polymerisation has emerged as the most prevalent process ^[87].

Up to now, when contemplating 3D-printed materials for water clean-up, the prevailing technical limits, resolution beyond 10 m, have limited their usage to module spacer production rather than membrane synthesis ^[88]. Nonetheless, recent advancements in advanced manufacturing (AM) technologies have increased the viability of 3D printing for the production of membranes, and various instances of membranes based on this method have been published ^[89]. Koh and co-workers, for instance, produced an antifouling oil/water cellulose membrane by direct printing. The average pore size of the cellulose mesh produced was less than 240 m, and its separation efficiency was almost 95% ^[90].

This progress has also enhanced the creation of hybrid materials that can be printed using the many current techniques. Sangiorgi et al. ^[91] created hybrid polylactic acid (PLA) and titanium dioxide (TiO₂) membranes using FDM technology.

3. Mechanism of Separation of PNC

The separation mechanism of conventional membranes and PNC is, in general, the same; however, the presence of nanocomponents on the surface and in the internal structure of the pores, or, in the case of dense membranes, the occlusion of the nanophase within the membrane can lead to minor changes in their descriptions. The impact of nanocomponents on separation based on the usage of PNC may be examined by examining the separation mechanisms or membrane performance. In general, a large number of research ^[19]^[92]^[93]^[94]^[95] have concentrated on antifouling qualities and operational performance (i.e., the improvement of mechanical and thermal properties for their use in operations at high temperatures and high pressures). Therefore, the rejection coefficient and permeability of a porous polymeric membrane are the characteristics that characterise its use in the separation of analytes dissolved in water. Because of this, the description of the process contains the initial values of the rejection coefficient and permeability at a starting temperature that has already been established. Alterations in the temperature at which the process is carried out contribute to an increase in permeability because temperature has a direct correlation to permeability. Despite this correlation, an increase in temperature causes the polymer phase to expand, which in turn results in an increase in the effective pore size. If size exclusion is the mechanism of separation, then an increase in effective pore size will lead to a large fall in the rejection coefficient. This will be the case if the rejection coefficient decreases. In contrast, if a porous polymeric membrane is nanostructured, its thermal characteristics may be enhanced, and as a result, it may be conceivable for PNCs to increase their permeability in response to an increase in temperature without compromising their rejection coefficient values. Globally, the membrane performance is enhanced in PNCs compared to non-nanostructured membranes because of this example. However, the enhancement of attributes is contingent on the type and concentration of the nanomaterial, the properties of the polymeric phase, and the interaction between the nanostructured and polymeric phases.

The examination of separation processes is essential for a complete comprehension of PNCs. In general, two mechanisms can be identified for membrane separation processes: (i) separation by size-based exclusion and (ii) separation by dissolution–diffusion ^[20]. In the first case, particles that are larger than the pore in the membrane are retained, while those that are smaller are allowed to pass through. This process takes place in membranes, referred to as nano-, ultra-, and microfiltration (NF, UF, and MF, respectively), with water being the majority of the fluid phase. The input stream’s liquid phase might be homogeneous (e.g., aqueous dissolution for macromolecule removal) or heterogeneous (e.g., aqueous dispersion for the removal of suspended solids). In some settings (e.g., air filtration), it may be seen as a gaseous fluid containing suspended particulates ^[20]^[96]. During the second step, known as separation by dissolution–diffusion, the substance that penetrates the membrane is first dissolved inside the membrane (during the stage referred to as the dissolution stage), and then it diffuses across the membrane (i.e., diffusion stage). This mechanism is used to describe the separation process in dense membranes (for example, bulk, emulsion, and supported liquid membranes), but it can also be used to describe the separation by porous membranes of reverse osmosis (RO). This is due to the fact that the nanoconfinement of matter in the inner of very small pores increases the interaction between the permeant substance and membrane phase ^[20]^[96].

3.1. Impact on the Size Exclusion Mechanism

Understanding the effect of nanometric components on porous PNC separation requires looking at the process, rather than focusing just on the membrane itself. Because of this, the composition of the membrane is of no consequence to the procedure of separation if a system is envisioned in which the fluid and the species contained within it do not interact with the membrane, which may be an NF, MF, or UF membrane. In this scenario, the nature of the membrane is irrelevant. Regardless of whether the membrane is ceramic, polymeric, or nanocomposite, the separation will be controlled by the cut-off of the membrane, which is determined by the pore size and particle size distribution in the fluid. This means that ceramic, polymeric, and nanocomposite membranes all have the same effect on the separation. It is important to note that this hypothetical “zero interaction membrane” or “non-interacting membrane” should not be confused with the concept of “zero retention membrane”, which is used for the modelling of hybrid methods of membrane separation such as polymer-enhanced ultrafiltration, micellar ultrafiltration, and liquid membranes enhanced with liquid-phase polymer-based retention ^[97]^[98]. This is because the separation mechanism being analysed is size exclusion, as the process involves the addition of liquid membranes that are enhanced

On the other hand, when the membrane interacts with one or more fluid components, the nature of the membrane becomes a primary component if the interaction is significant. This is due to the fact that the nature of the membrane is associated not only with the type of interaction that occurs between the membrane and solutes, but also with the phenomena that are triggered as a result of these interactions. These may be both good and negative occurrences, with the major negative consequence being an increase in fouling and, as a result, a loss in operating capacity due to decreased permeability and selectivity. Positive benefits might include less fouling, increased permeability and disinfection, longer working durations, fewer secondary cleaning stages, a longer half-life time, and improved microbiological quality of retentate and permeate ^[13]^[14]^[15]^[16]^[17]^[18].

When a PNC that is not interacting with anything else is investigated, however, various cases may be found, including the following: (i) If the nanomaterial that makes up the PNC only improves its mechanical and thermal characteristics, it is predicted that the membrane’s permeability will improve, but there will be no effect on the mechanism that controls separation, and as a result, the PNC will not be an active component when it comes to the separation process. (ii) If the nanomaterial that makes up the PNC improves the performance of the membrane and the separation process, then there is some kind of beneficial interaction with one or more solutes. For instance, PNCs that are based on silver nanoparticles (AgNPs) are an active element during the separation process because they reduce biofouling. One or more additional interactions at the level surface may take place in comparison to the equivalent non-nanostructured membrane. This is due to the fact that the surface of the nanocomposite that makes up the active layer of the membrane in these systems has AgNPs that are partly exposed and come into contact with the feed stream. In the case of AgNPs, the nanoparticle surface chemically links molecules that have thiol groups on their structures. As a result, even though small molecules such as cysteine, which have dimensions of 0.64 nm (length) and 0.37 nm (width) ^[99], cannot be removed by the size exclusion mechanism using NF, UF, or MF membranes, the formation of RSH-Ag0 bonds can produce the retention of cysteine molecules without causing changes to them. In addition, washing the membrane under certain pH conditions might facilitate the recovery of cysteine that is linked to the membrane ^[100]^[101]. In addition, the anti-biofouling characteristics of PNCs based on AgNPs have been comprehensively established in several studies throughout the scientific community. AgNPs, in general, have the potential to function as a reservoir for Ag⁺ ions, diffuse off the surface of the membrane, and enter the bacterium, where they interact with a variety of biomolecules, ultimately leading to the death of the microorganism ^[98].

In these membranes, AgNPs are reduced and stabilised using an appropriate stabilising agent (e.g., citrate, glucamine, PVA, etc.) before being added to the polymer phase to generate the nanocomposite, which is then employed to make the membrane. In addition to the preferential retention of thiolate compounds and anti-biofouling capabilities, it is clear that the addition of AgNPs impacted other material properties, including dielectric properties. There have not been many studies that have focused on describing the dielectric response of PNC in terms of the chemical composition of the polymer phase and nanomaterial, despite the fact that this type of membrane has the potential to be used as a sensing surface for both monitoring a particular effluent and performing process self-control operations ^[102]^[103]. To produce anti-biofouling membranes or anchoring surfaces of thiolate compounds, AgNPs can only be added at the surface level. However, this type of membrane could be referred to as “nanostructured” and “active”, but not a PNC, because the active polymer phase is not a nanocomposite. It is important to note that AgNPs can only be added at the surface level.

3.2. Influence on the Dissolution–Diffusion Mechanism

Liquid membranes are not included here since the research presented below is restricted to polymeric membranes alone. The dissolution–diffusion mechanism may be broken down into two main stages: the first step is the dissolution stage, and the second stage is the diffusion stage. It is essential to keep in mind that the dissolving stage is determined by the phases that are in touch with one another; hence, it is inextricably linked to the constituent parts of the solute and the membrane. Interfacial contact is favourable and the transfer of analytes to the membrane phase is promoted if the feed phase has a high affinity for the membrane phase. This step is comparable to size exclusion because of the fact that size is used when it is specified as a criterion for affinity. Particles that are larger than the membrane cut-off are unable to enter the membrane phase, therefore their affinity for the membrane phase is low. On the other hand, particles that are smaller than the membrane cut-off have a high affinity for the membrane phase since they can enter it. In a similar manner, the dissolution of analytes in the membrane phase serves as an exclusion criterion. As a result, solutes that have a weak affinity for the membrane phase are not allowed to pass through it, while analytes that have a strong affinity are permitted to do so. This affinity is connected to the structural similarity of the membrane-forming polymer and the solute, or, to put it in molecular terms, to the collection of intermolecular interactions that take place between the membrane material, the analyte, and the transporting phase ^[20]^[104]^[105]. Because of this, it is challenging to understand how the nanomaterial will affect the very first step. However, further investigation reveals several fascinating components that contribute to a better understanding of the mechanism behind the process.

To elaborate on the idea presented previously, take into account the simplest system that could possibly exist, which would be equivalent to a feed that did not contain any solutes. In this scenario, for the dissolution stage, a single interaction should be analysed for both non-nanostructured membranes and PNCs, and that interaction is the solvent–membrane interaction. The fluid phase in the feed is referred to as the “solvent.” In the case of gases, the analysis is limited to the examination of a single gas that is allowed to pass through the membrane. If the molecular diameter of the gas molecules was smaller than the cut-off for the membrane, then the gas molecules would have a strong affinity for the membrane in the case of porous membranes such as RO membranes. In very certain situations, such as when the gas molecules are enormous in relation to the cut-off point of the membrane, the affinity may be significantly diminished. However, the affinity in dense membranes, which are crosslinked polymers that are embedded in a liquid, is governed by the solubility of the gas in the liquid; hence, the structure of the polymeric material is analogous to that of a hydrogel when water is the solvent. However, the solubility of the gas will be determined by operational factors such as pressure and concentration (chemical potential), which are the fundamental characteristics that have an impact on the gas’s Henry constant. This will be the case because pressure and concentration are the fundamental characteristics that impact the gas’s Henry constant. ^[20]^[104]^[105].

In the case of thick membranes, however, if the feeding stream is a liquid, three scenarios are possible: (i) the feeding phase has a high affinity for the liquid contained in the membrane, (ii) the feeding phase and the liquid in the inner of the membrane are the same, and (iii) the feeding phase has no affinity for the liquid in the membrane phase. The solvent will be able to infiltrate the membrane in the first situation, and the membrane’s stability will be governed by the mechanical strength of the polymer network. The nano-structuring of the membrane becomes critical at this time. In the second situation, nothing changes. As a result, in order to deepen the analysis, at least one solute must be included in the simplification performed. As a result, if the solute is physically like the medium and the medium is structurally similar to the membrane, the solute will have a high affinity in the membrane phase. However, in the scenario when the solute carries a net charge, the membrane surface features may be exploited to offer better selectivity to this initial step. In this way, if the membrane has charged, even if two solutes have a high affinity with the medium and the medium contained in the inner of the membrane has a high affinity with the membrane, it is possible to increase the rejection of one of the solutes with respect to the other by selecting the appropriate polymer structure via the use of an anionic barrier to prevent negatively charged particles from passing through. In the third instance, when the liquids are not structurally connected, the solute affinity is determined by the partition or distribution coefficient between the phases ^[20]^[104]^[105].

In the case of porous membranes, more specifically RO membranes, the solvent will penetrate the structure of the membrane, which will lead to two outcomes: the polymer phase has a high affinity with the membrane, and as a result, it is possible that strong solvation interactions will occur, which will promote the decrease in the permeability, which will result in a decrease of the effective pore radius; however, it is also very likely that the membrane will exhibit poor mechanical performance under these conditions. In circumstances such as these, the nanocomponent of the PNC contributes to the attenuation of the effect ^[1]^[2]^[3]^[4]^[5]^[6]. Because there is no affinity between the solvent and the membrane, solvation does not take place, the membrane material’s mechanical properties remain unchanged, and solute dissolution in the membrane phase can be easily understood by using the solute–membrane affinity in terms of relative size in relation to the membrane cut-off.

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Nouf F. Al Harby

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