Chitosan-Based Membranes: A Comprehensive Review of Nanofiltration, Pervaporation, and Ion Exchange Applications

Abstract

Innovations for separation via membranes are extremely energy-efficient, and through the previous decade, attention to this technology has spiked tremendously. Biopolymers are becoming widely recognized as membrane materials since they are sustainable. Furthermore, the second most common biopolymer, chitin, is the source of chitosan, which has several benefits that make it ideal for the construction of membranes. This review article presents an evaluation of current developments in the utilization of chitosan membranes. The applications of interest in this review are nanofiltration, pervaporation and ion exchange. The chitosan based nanofiltration membranes are comprehensively reviewed with respect to various factors (e.g., solvent, pH resistant, etc.). The development of water permselective, organic permselective, and organic-organic separation films, as well as its permeability and segregation properties, are addressed in pervaporation (PV) section.

1. Introduction

In recent decades, membrane technology has gained prominence due to its efficient energy use, ease of operation, flexibility, and scalability [1,2]. Because membrane technology supports continuous operation, eliminating the need for sorbent regeneration or desorption due to temperature or pressure variations, it is extremely energy efficient. Additionally, the membrane apparatus is small, has no moving components and is simple to use, regulate, and scale up [3]. Over the past twenty years, there has been a marked increase in the motivation on research and growth action within the larger discipline of membrane science and technology. As a result, membrane-based technology has been utilized in many different industries, including food processing, pharmaceuticals, gas purification, environmental protection, treatment of water, and healthcare applications [4,5,6,7,8,9]. The most essential element of membrane separation technology is the membrane, which has a direct impact on the efficiency of the procedure [10]. An appropriate membrane should possess various structural and functional characteristics, including a long and stable service life, low fouling rate, excellent selectivity, high permeation rate, and adequate mechanical, chemical, and thermal stability during operation [3]. With growing environmental and public health awareness, alongside increasingly stringent regulations on waste discharge, there is a rising consideration for using biopolymers derived from sustainable sources as substitutes for synthetic polymers. In the natural biocycle, biopolymers are manufactured by plants and other living things and are eventually broken down and reabsorbed by the environment [11]. Natural chitin, also identified as poly (β-(1,4)-N-acetyl-D-glucosamine), is the next largest biopolymer [12]. The primary supply of chitin is found in the crustacean exteriors, such as crabs and shrimp, which are residues of marine plantations [13]. The deacetylated chitin, called chitosan (CS), is made up of linear β-1,4-linked D-glucosamine (GlcN) and N-acetyl glucosamine (GlcNAc). The molecular weight of chitosan obtained from crustaceans typically ranges from 50 kDa to 1000 kDa (kilodaltons), depending on the degree of deacetylation and the extraction method. Chitosan from crustaceans generally has a high degree of deacetylation, about 70% to 95%. Chitosan attained from crustaceans typically comprises impurities similar to proteins, minerals, lipids, and sometimes carotenoids, which can contribute to a yellowish color. Insects, such as silkworms, mealworms, and beetles, are also a feasible source of chitosan. Insects have chitin in their exoskeletons, comparable to crustaceans. The molecular weight of chitosan derived from insects can vary from 50 kDa to 500 kDa, with lower molecular weight typically observed in insect-derived chitosan associated with that from crustaceans. The degree of deacetylation in insect-derived chitosan is characteristically lower, about 50% to 85%, which can affect its solubility and functionality. Proteins, fatty acids, and chitin-derived pigments are general impurities in chitosan obtained from insect. These impurities can alter its texture, functionality, and color. Fungi, such as mushrooms and certain molds (e.g., Aspergillus), also comprise chitin in their cell walls. This offers a source for chitosan manufacture. Chitosan extracted from fungi often has a lesser molecular weight, ranging from 10 kDa to 200 kDa. Polysaccharides from the fungi, proteins, and usually fungal metabolites are common impurities. The level of these impurities can vary depending on the extraction method The chitin and CS nanostructures are depicted in Figure 1 [14].

The biological and physicochemical characteristics of chitosan are often characterized by the degree of deacetylation (%DD). CS is a naturally occurring biopolymer that has several beneficial properties, including biocompatibility, biodegradability, non-toxicity, affordability, and excellent sorption capabilities. Furthermore, the presence of free aminohydroxyl groups imparts additional benefits such as antibacterial activity, the capacity to chelate heavy metal ions, gel-forming characteristics, exceptional protein affinity, solubility in water, and simplicity of alteration and preparation [2]. Many CS variants have been produced so far for various functions. Chitosan is widely used in water treatment applications because it is an inexpensive and efficient adsorbent for removing dyes, heavy metal ions, and other pollutants. Moreover, CS has significant applications in a variety of different industries, including food, cosmetics, agriculture, and others [2]. There are numerous kinds of CS that are manufactured for different purposes, such as beads, fibers, microcapsules, gels, microspheres and films. CS membranes are frequently utilized for addressing chemical, biological, energy, and ecological concerns due to their low energy costs and simple operation, which enable the selective transport of materials [11]. Chitosan membranes are widely used due to their excellent affinity for dyes, metals, and other materials, as well as their hydrophilicity, biocompatibility, ease of modification, and cationic properties. The applications of CS membranes in gas separation, Direct methanol fuel cell (DMFC), pervaporation, water treatment, and other fields have been reported in several studies. Figure 2 lists all the studies that have been published to date.

This review article offers several novel contributions to the discipline of chitosan-based membranes. The review distinctively examines chitosan-based membranes across three advanced separation processes. This broad scope is unprecedented in existing literature, providing an additional complete understanding of chitosan’s applications. Unlike previous studies that focus on single applications or other membrane materials, this review highlights chitosan’s adaptability and multifunctionality. It explains how chitosan can be efficiently used in several membrane processes, showcasing its versatility. Furthermore, it addresses new developments in membrane production methods, surface alterations, and performance-enhancing hybrid composite membranes, offering an original view on developing trends in that sector. This thorough approach delivers readers with a broader understanding of chitosan’s potential in membrane technology, making it a valuable contribution to the existing literature.

2. Chitosan

Chitosan is a cationic polymer that is produced when chitin is alkaline deacetylated [16,17,18]. Chitosan is made commercially by treating shrimp and lobster shells with sodium hydroxide in an alkaline solution, a process known as deacetylation [16,19,20]. Due to its amino (-NH₂) moiety, chitosan is incredibly hydrophilic and can generate strong bonds through hydrogen bonding with water and other polar substances. Additionally, because chitosan is almost completely soluble in basic and neutral aqueous solutions with non-polar organic solvents, this mechanism essentially permits chitosan to dissolve in acidified aqueous solutions [18,21]. In addition to its biodegradability, a quality petroleum-derived polymers lack, this substance is viewed as a biological and environmentally friendly substitute for petroleum-derived plastics [18,19,20]. Chitosan’s biocompatibility makes it suitable for use in healthcare fields as well. Research is being carried out on chitosan’s potential as a compostable bandage and component of dressings for wounds, particularly for the care of burns [17,18,19]. An overview of chitosan uses is presented in Figure 3.

Several analyses have been published previously that use chitosan as a base material (virgin, as the primary ingredient, or combined with additional biopolymers) to produce films, primarily for active wrapping in food applications [23]. Positive outcomes on the food’s qualities and preservation duration were observed when chitosan and natural oil blends were used in smart packaging methods. Research utilizing chitosan for the microencapsulation of aromatic oils and membrane formation for various types of separations have also been established [24,25]. The development of biodegradable films, which are employed in things from active packaging to medicine, was one of the earliest uses of chitosan [17,20]. The present era of research in active packaging has produced chitosan as well as alternative natural polymers (including cellulose, starch and gelatin) and several reports in the membrane making industry. The research is primarily focused on producing packages with biodegradable properties, excellent durability, and derived from renewable resources [16,17,18,19]. Blends of essential oils and chitosan-based films have been used in a number of research, mostly for proactive packaging of food. Water decontamination, particularly heavy metals, has been identified to be accomplished through the application of chitosan-based processes for the microencapsulation of oils and functional components (regulated discharge and administration of medications) in the pharmaceutical and food sectors. A comprehensive analysis examined the promising applications and advantages of chitosan and chitosan-derived substances [26,27]. Among the alternatives mentioned by the authors consist of the utilization of films which might fulfill the role of cation-exchange resins, as an addition to the current resources, chitosan in granulated form, containing micro-sized pellets. Even as a greener substitute for certain techniques and feedstocks, chitosan derived products and blends are currently being investigated in areas of application beyond the conventional ones. On the other hand, chitosan’s primary published research and potential uses are as an active food packaging ingredient or feedstock [17]. Employing chitosan in film synthesis either as a primary component or as a supplementary element to achieve numerous objectives in a manner comparable to film production is another significant research area. Some chitosan films and membranes are an excellent option in these areas of research because of their comparatively simple manufacturing process [28].

3. Chitosan-Based Membranes

Chitosan is one of the most extensively researched biopolymers, alongside starch, cellulose, and gelatin, and is commonly used to manufacture sheets and membranes for various processes, including pervaporation and reverse osmosis [17,19,28]. Chitosan-based membranes are available in various configurations, including blended, multilayer, sub-layer, and composite membranes (Figure 4) [29,30,31,32,33].

Strong acids, such as hydrochloric and sulfuric acids, are rarely used in the synthesis of chitosan-derived membranes; instead, lactic and acetic acids are the most frequently used solubilizing agents, typically at concentrations of about 1% by weight or volume [34,35,36]. Several crosslinking agents have been reported, including glyoxal, tetraethyl orthosilicate, glutaraldehyde, glycerol, epichlorohydrin, and sodium tripolyphosphate (

Table 1. List of chemical moieties used to modify chitosan.

Name	Chemical Structure
Glyoxal
Tetraethyl orthosilicate
Glutaraldehyde
Epichlorohydrin
N,O-Carboxymethyl chitosan
Poly(acrylic acid)
Diethyleneglycol diglycidyl ether
2-formyl benzene sulfonate polysiloxane(SBAPTS)
Benzoylchitosan

). Depending on the intended membrane features or the method of manufacture, a particular crosslinker may need to be employed [37,38]. Chitosan has been used to modify conventional polymer films, enhancing specific characteristics such as increased affinity between molecular groups, reduced fouling, and improved chemical resistance [28]. Gayed et al. investigated the external treatment of a polyamide membrane intended for reverse osmosis using irradiated TiO₂ and chitosan nanoparticles as additives. They found that increasing the TiO₂ concentration to 0.125 weight percent improved the membrane’s permeate flow rate; at lower concentrations of nanoparticles in the chitosan solution, salt rejection increased [39]. Liao et al. used chitosan to develop a cellulose triacetate/chitosan blend reverse osmosis membrane, resulting in enhanced antibacterial properties [40]. The findings indicated that incorporating chitosan improved the composite membrane’s hydrophilicity, water flux, and mechanical properties. At chitosan levels that varied between 0.75 and 1.00 wt%, the film demonstrated an elevated salt removal (R > 90%) without appreciably altering its crystallinity or strain at failure. The results of the dynamic contact antibacterial evaluation showed that the blend inhibited the growth of E. coli and S. aureus. To enhance the interaction between the supporting and active components of a hybrid polyacrylonitrile (PAN) nanofiber support layer and a polyamide active layer for forward osmosis, Shi et al. incorporated a chitosan sub-layer. This improved the overall mechanical properties of the membrane and acted as an effective intermediate layer between the supporting and active layers. In addition, the reverse salt flux decreased as matched to the membrane lacking the sub-layer [41].

Kazemi, Jahanshahi, and Peyravi demonstrated that chromium (VI) (Cr6+) can be purified from wastewater through photocatalysis using optical light [42]. They created a chitosan-alginate composite ultrafiltration (UF) membrane incorporating Fe° nanoparticles infused with WO₃. The efficiency of Cr6+ ion removal ranged from 84.9% at a feed level of 10 mg/L to 71.3% at 30 mg/L. Figoli et al. discussed the use of chitosan-derived materials in the manufacture of various types of membranes, including multilayer, composite, hybrid, chitosan-modified, and crosslinked membranes, for a range of applications such as gas separation (primarily CO₂), nanofiltration, pervaporation, and both reverse and forward osmosis [43].

3.1. Application of Chitosan Derived Membranes in Nanofiltration

Nanofiltration process shares similarities with reverse osmosis. However, they are not identical. In nanofiltration, a semipermeable membrane selectively rejects multivalent ions (Figure 5).

Several researchers have significantly utilized chitosan polymer in the development of diverse nanofiltration membranes, each designed for a wide range of applications. In one of the studies, the authors focused on the durability of newly developed surface crosslinked CS/PAN composite nanofiltration (NF) membranes under varying pH environment and exposure to organic solvents. Chitosan, a naturally hydrophilic biopolymer, also exhibits an outstanding resistance to solvents. To estimate solvent resistance, a series of tests including swelling, immersion, and permeation were conducted by various industrially important organic solvents. The solvents employed in these assessments were ethanol, methyl ethyl ketone, methanol, iso-propanol, hexane, and ethyl acetate. For membranes crosslinked with 0.5% glutaraldehyde, the solvent flux of both ethanol (EtOH) and isopropyl alcohol (IPA) reduces over time. This drop in flux is more evident for IPA compared to EtOH. This implies noteworthy hydrophobic interactions between the relatively hydrophobic alcohols and the membranes, which show the maximum hydrophobicity in this study. This could lead to the easing of the membrane morphology and a related variation in the pore structure of the selective Chitosan layer [44].

Furthermore, chitosan (CS) is combined with polyacrylonitrile (PAN) polymer to make composite membranes. Authors observed how different levels of glutaraldehyde and cross-linking times affected the surface chemical bonding of CS/PAN composite nanofiltration membranes. They studied the way these factors influenced key membrane properties, such as molecular weight cut-off, water permeability, and the efficiency to separate out simple salts and short-chain carbohydrates. As the concentration of glutaraldehyde grew, the water permeation-swelling ratio declined and the removal of carbohydrates and electrolytes improved. The observed phenomenon could be explained by numerous factors working together as the glutaraldehyde concentration rises in chitosan/PAN polymer matrix: increased hydrophobicity, reduced swelling which caused pore contraction and pore tortuosity. The surface cross-linked films had molecular weight thresholds between 550 and 700 Da [45].

Additionally, a chitosan derivative, N,O-carboxymethyl chitosan (NOCC) is utilized by researchers to fabricate active layer of polysulfone (PSF) composite NF membranes. The consequent NOCC/polysulfone (PSF) NF membranes were used for treating the fermentation wastewater from a wine plant [46]. The study explored the effect of input flow, driving force, and process duration on the nanofiltration (NF) membranes. Particularly, it evaluated the membranes’ diffusion flow and their efficiency in removing several components from fermentation wastewater, through total organic carbon (TOC), chemical oxygen demand (CODCr), conductivity, and color. It was observed that when the driving force or input flow increased, so did the flux of permeate and the rejection proficiency. The rejection values for color, CODCr, TOC, and conductivity were 95.5%, 70.7%, 72.6%, and 31.6%, respectively, over 0.40 MPa and room temperature. For the 10 h operation, the fermentation wastewater treatment membrane was found to be stable.

Additionally, chitosan polymer is used in composite NF membranes and is further grafted with mesogenic structure to alter the morphology resulting in good NF performance. By coating the PSF ultrafiltration membrane with a blend of CS and mesogenic compound-modified CS, various unique composite NF membranes were produced. The mesogenic chemical grafted onto CS and its structure were the only factors affecting the rejection rate and flux of the mixed matrix NF membrane (Figure 6).

Modified chitosan mixed matrix NF membrane showed an exceptionally high flow of 2543.3 l m⁻² h⁻¹, and the rejection rate continued to be higher as 66.3% at 0.4 MPa via 1000 mg/L NaCl. The water-soluble nature and relatively large volume of the modified chitosan probably caused it to initially adhere to the pore walls. This adhesion developed in a reduction in flux and an enhancement in rejection values [47].

Moreover, chitosan polymer has been used to generate composite nanofiltration membranes with polyethersulfone (PES) substrate to treat wastewater generated by various industries. A pore former polyvinylpyrrolidone (PVP) were also employed to ensure the required pore dimension and porosity of the composite films for specific application. Studies were conducted on the influence of the casting solution’s formulation on the structure of the composite membrane and water permeability [48]. In comparison to other ratio, the membrane composed of 15 weight percent PES and 2.25 weight percent PVP showed superior water permeability. The flux and rejection of the CS/PES composite film were 5.2 l m⁻² h⁻¹ and 76.15%, respectively. The probable interaction between chitosan and PVP may have resulted in enlarged pore sizes, which consequently led to increased flux in the composite membranes.

Also, chitosan underwent additional modifications, combining a chiral organic compound and a positively charged molecule. This modified chitosan was then utilized as a selective layer in nanofiltration (NF) composite membranes, fabricated to efficiently reject multivalent cations. The composite nanofiltration membrane, with a 70% grafting level of positively charged molecules, showed remarkable performance under particular environments. When subjected to a pressure of 0.4 MPa and exposed to a calcium chloride (CaCl₂) solution with a concentration of 1000 mg/L, the membrane maintained a high flux rate of 687.4 l m⁻² h⁻¹. Simultaneously, it achieved a highest rejection rate of 95.2%, indicating excellent filtration efficiency. The analysis indicated that both chiral organic compound and positively charged molecule modified chitosan, as well as unmodified chitosan, generated an extensive interconnected morphology. As the degree of chiral organic compound grafting on chitosan increased, the material’s porosity also improved. Subsequently, this led to enhanced flux, however resulted in reduced rejection percentage of NaCl salt [49].

Chitosan membranes owing to their abundant amine and hydroxy groups in the structure have been widely applied for adsorbent membranes to remove contaminants from gaseous and aqueous environments. The electron lone pair of nitrogen and oxygen molecules facilitated the adsorptive reactivity of chitosan-based membranes. Figure 7 demonstrates the complex development involving chitosan active sites and usual metal ion cationic and anionic dyes. The complexation and affinity of hydroxyl moities towards metal cations helped the chitosan/cellulose blend membranes for the absorption of Cd(II), Pb(II) and Cu(II) from water [50]. Besides cation absorption, the chitosan composite membranes have also been utilized for anion exclusion from sea ecosystems.

3.2. Use of Chitosan-Based Membranes in Pervaporation

A certain feed mixture constituent may permeate the membrane significantly during pervaporation (PV), as seen in Figure 8, when feed integrates are introduced to a membrane surface and the alternative surface goes through evacuation [51,52]. Variations in the diffusivity, solubility, and differential volatility of permeants within and outside of the membrane can affect the properties of separation and permeation in pervaporation systems. The distinguishing of structural isomers, close-boiling point combinations, and azeotropic mixtures can be accomplished through the PV method. Pervaporation can be useful mainly in three areas such as (1) hydrophilic pervaporation (2) hydrophobic pervaporation (3) separation of organic-organic solvent mixtures. Permeance and selectivity are the ideal ways of reporting the pervaporation data as they both describe the permeation of a specific compound [52].

3.2.1. Water Selective Membranes

Chitosan (CS) is a hydrophilic polymeric material.

Table 2. Separation and permeation and properties of water based alcohol solutions by chitosan membrane in pervaporation. Reproduced/Adapted with permission from [54], Willey, 2017.

Methanol	Feed (wt%)	0	10	30	50	70	90		100
	Rate of Permeation (103 kg/m2h)	18.6	15.0	16.0	9.4	6.8	4.3		2.4
	${\alpha }_{H_{2}O/MeOH}$	-	0.7	1	1	2	2		-
Ethanol	Feed (wt%)	0	10	30	50	70	90	96.5 a	100
	Rate of Permeation (103 kg/m2h)	186.0	150.0	136.0	67.1	34.6	12.3	6.5 a	2.9
	${\alpha }_{H_{2}O/MeOH}$	-	0.7	2	13	50	31	17 a	-
1-Propanol	Feed (wt%)	0	10	30	50	71.8 a	90		100
	Rate of Permeation (103 kg/m2h)	337.4	111.0	141.7	106.2	50.5 a	8.2		1.3

^a Azeotropic composition.

delivers an outline of the separation and permeability properties via the CS membrane in PV in alcohol–water mixtures. In Table 2 separation factor can be calculated by Equation (1) [53]

$${\mathsf{\alpha }}_{\mathrm{s}\mathrm{e}\mathrm{p}(H_2\mathrm{O}/\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H})}=\left({\displaystyle \raisebox{1ex}{$P_{H_{2}O}$}\!\left/ \!\raisebox{-1ex}{$P_{EtOH}$}\right.}\right)\left({\displaystyle \raisebox{1ex}{$F_{H_{2}O}$}\!\left/ \!\raisebox{-1ex}{$F_{EtOH}$}\right.}\right)$$

(1)

wherein water and alcohol weight fractions in the permeate and feed liquid are represented by the variables P_H2O, P_EtOH, F_H2O, and F_EtOH, respectively. As the amount of alcohol in feed solutions increased, the diffusion rate of all water-based solutions went down. Because.

Methanol has considerable volatility and a relatively very tiny molecular dimensions, its separation factor figures were minimal in water-based solutions. The separation ratios for the aqueous 1-propanol mixtures used in pervaporation systems were larger and developed considerably as the feed’s 1-propanol percentage rose. These findings demonstrate that aqueous alcoholic solutions with larger alcohol contents can be concentrated via the CS membrane with greater efficiency than those that have lesser alcohol contents [55]. Chemical modifications were made to the CS membranes to raise their water/ethanol selectivity [56]. Although the CS membrane’s permeation speed rises as the percentage of water does, the split parameter intended for the water/ethanol selectivity maximized at roughly 25% of the total water content. Figure 9 illustrates the properties of ethanol/water combinations’ permeation and segregation via the CS and the glutaraldehyde cross-linked chitosan (GAC) membrane produced by PV [56].

The CS and GAC membranes demonstrated significant water-permeable properties. Furthermore, both membranes lacked evidence of azeotropic elements, particularly when the feed mixture contained 96.5% ethanol by weight. Researchers have discovered an equisorptic chemical structure within the CS membrane. The membrane is not capable of absolute segregation, based on the statement. The CS membrane experiences significant expansion when exposed to the feed solution, likely due to the existence of an equisorptic point.

In the pervaporation (PV) process, the CS membrane selectively absorbs ethanol from feed mixtures containing low concentrations of ethanol. At lower ethanol concentrations in the feed, ethanol’s preferential permeation is primarily due to its volatility. This volatility has a more significant impact on separation during evaporation compared to its effect through diffusion processes.

The GAC membrane, however, was found to be without an equisorptic point. This study suggests that cross-linking the CS membrane with glutaraldehyde significantly decreases membrane swelling. This reduction occurs due to exposure to an aqueous solution containing a small amount of ethanol. In terms of permeability, GAC membranes outperformed CS membranes. With lower ethanol concentrations in the feed, the trend became more significant.

The following interpretation might be made of the GAC membrane’s superior rate of permeation and separation properties compared to the CS membrane. In general, the polymer membrane becomes denser after cross-linking. This process enhances its separation capabilities. However, it also reduces the rate of permeation. However, the GAC membrane enhances both the separation properties and the permeation rate. This study involved cross-linking the CS membrane through the application of glutaraldehyde in an aqueous medium. Consequently, the CS membrane undergoes swelling in an aqueous solution, after which it is subjected to cross-linking using glutaraldehyde. Hence, a dense GAC membrane emerges, possessing substantial hydrophilic properties. The GAC membrane’s dense structure and remarkable hydrophilicity are essential for achieving both enhanced flow rates and superior filtration performance.

Feng et al. fabricated a composite membrane using chitosan (CS) and polysulfone (PSF) [57]. In this work, the chitosan was dissolved in a diluted solution of acetic acid and water, to generate a CS salt. This salt solution was then applied as a coating onto a porous PSF substrate. The membrane’s capacity to selectively separate water from ethylene glycol–water mixtures was evaluated using pervaporation techniques. Research was carried out to investigate how various operational factors, including feed quantity, thermal conditions, and pressure on the output side, influenced the membrane’s separation efficiency.

The system achieved a permeation rate of 300 g/m²h, with the resulting permeate containing over 92 wt% water. These results were obtained under specific conditions: the initial feed contained 10 wt% water, the temperature was maintained at 35 °C, and the downstream pressure was set at 60 Pa. This study demonstrated that membrane pervaporation could be a viable alternative to traditional distillation for certain separation processes. Operating the pervaporation system’s membrane at its maximum allowable temperature was shown to induce a nearly irreversible change in the membrane’s selective permeability characteristics.

Lee et al. synthesized a variety membrane derived from CS and evaluated their effectiveness in pervaporation processes for mixed liquid solutions. This study evaluates the pervaporation (PV) efficiency of four different modified chitosan (CS) membranes in separating water from an ethanol–water solution [58]. CS was produced by first extracting chitin from crab shells, then treating it with a sodium hydroxide solution to remove acetyl groups. The focus of this study was on chitosan (CS) membranes with specific modifications: carboxyethyl, amidoxime, cyanoethyl, and carboxymethyl. Modified CS membranes that retain their carboxy groups demonstrate the highest PV efficiency. Carboxymethyl chitosan (CS) membranes demonstrate peak ethanol permeation and swelling when exposed to a feed solution containing approximately 15 wt% ethanol. This behavior is attributed to the membranes’ coupling effects and plasticizing properties.

In another study, Lee et al. investigated the pervaporation performance of novel phosphorylated chitosan membranes for dehydrating ethanol–water mixtures. Phosphorylated CS films were fabricated through a specific process. This process involved a chemical reaction, which took place on the outermost layer of the CS membrane. Two precursors were used in the reaction: urea and orthophosphoric acid. These substances were combined in a solution of N,N-dimethylformamide. The result of this reaction was the production of phosphorylated CS films. The membrane’s phosphorus concentration, which ranged between 1 and 80 mg/m², was directly related to the reaction time. CS membranes exhibited superior functionality in the pervaporation process after phosphorylation.

The phosphorylated chitosan membrane, containing 56 mg/m² of phosphorus, exhibited the highest pervaporation efficiency. The flow rate was measured at approximately 200 g/m²h at 70 °C. Additionally, the membrane exhibited a selectivity of about 600 when tested using a solution of 90% ethanol by weight, indicating its effectiveness in separating ethanol from water. The newly developed phosphorylated membrane exhibited a quadruple increase in permeate flow rate while preserving its ability to selectively filter water, outperforming previously reported sulfonated and carboxymethylated chitosan membranes [59].

A composite membrane consisting of poly(vinyl alcohol) (PVA) and CS has been fabricated through solvent casting techniques. This membrane was designed to efficiently separate the ethanol–water mixture using pervaporation (PV) technology. The composite membrane exhibited a water selectivity of 450 and a flow rate of 470 g/m²h, at glutaraldehyde cross-linking concentration of 4 × 10⁻⁶ mol/g [60].

Nam et al. used chitosan and poly(acrylic acid) [PAAc], two polymer mixtures which were mixed in varying weight ratios to generate polyelectrolyte complex membranes. Various membranes were used to execute the pervaporation of the water–alcohol and MeOH-MTBE (Methyl t-butyl ether) combinations. When the PAAc concentration in the water–alcohol solution improved, the permeation flux reduced and the water level in the permeate enhanced significantly. Excellent separation ratios were demonstrated by these films up to 4000 and 19,000 in 80 and 95 weight percent feed ethanol levels, respectively [61].

A chitosan composite membrane crosslinked with an ion interface has been developed to facilitate the pervaporation of a mixture of ethylene glycol (EG) and water by Nam et al. Permeation flow of 1130 g/m²h and water levels of permeate more than 99.5 wt% were attained at 80 °C and 80 wt% feed EG content. At each EG content evaluated the current chitosan membrane demonstrated permselectivity towards water. The pervaporation characteristics of chitosan composite films were reduced upon annealing. The operating temperature was directly correlated with the permeability flux of chitosan composite membranes [62].

Lee et al. studied the pervaporation performance of water/alcohol mixture with composite membranes composed of β-chitosan from squid and different crosslinking durations were achieved using sulfuric acid. The β-chitosan composite membrane demonstrated a separation factor (α) of 270 and a flux (J) of 700 g/m²h for the water/ethanol combination. Feed content and feed temperature exhibited a substantial influence on the β-chitosan composite membrane’s efficiency intended for the pervaporation of a water/iso propyl alcohol (IPA) mixture. The permeation flow declined, and the separation factor improved as the amount of feed water lowered. Pervaporation results for β-chitosan at 15 wt% feed water content and 80 °C was α-=150, J=1800 (g/m²h). The estimated activation energies (KJ/mol) for 15 wt% water content have been calculated as follows from the correlation among flux and reciprocal absolute temperature: E_water = 2.4 and E_IPA = 8.0. It has been established that P-chitosan, which is permselective toward water, is an appropriate choice with respect to the pervaporation membrane in combinations of water/alcohol [63].

In a remarkable study, Lee et al. explored the significance of chitosan composite membranes’ degree of deacetylation on ethanol dehydration pervaporation capacity. The chitosan has acetylation levels of 91, 96, and 99%, respectively. A microporous polyethersulfone membrane having pore diameters ranging from 3 to 7 nm was coated using a chitosan solution to produce chitosan composite membranes. Subsequently, sulfuric acid was used to crosslink the chitosan layer of the well-dried membranes, and various parameters were evaluated for water/ethanol binary mixtures pervaporation. Since chitosan deacetylation improves the fraction of free volume of the chitosan polymer through the elimination of comparatively abundant acetyl groups, it additionally boosts the diffusing rates of permeating species. As a result, as chitosan deacetylation grows, the overall permeation flux of chitosan films improves, and the separation component declines [64].

An entirely novel organic polymer blend film termed the CS/silk fibroin blend film was developed. The study focused on the alcohol–water solution’s preferential solubility and pervaporation characteristics. Studies demonstrated that, whenever silk fibroin concentration in composite membrane was slightly above 40 weight percent, the membrane was water-specific and the coefficient of separation of the ethanol–water combination could be enhanced versus pristine CS film. As the silk concentration was 20 weight percent and the cross-linking agent–glutaraldehyde level was 0.5 mol%, the composite membrane performed at its best, signifying that the water in the permeate was greater than 99 weight percent. Since CS and silk fibroin in the composite membrane generate an excellent intermolecular hydrogen link, an approach for enhancement in PV characteristics was described by lowering the free volume and displacing hydrophilic pairs of CS. Furthermore, the impacts of feed content and operational temperature, along with the PV characteristics of the isopropanol–water solution, were examined [65]. Diethyleneglycol diglycidyl ether (DEDGE)-crosslinked CS membranes have been utilized for examining the PV removal of a water-based solution including urine constituents (ammonia, uric acid, or creatinine). None of the membrane penetrates under investigation included uric acid, creatinine, or creatine. Ammonia was eliminated by the DEDGE cross-linked CS membranes, which also demonstrated a strong water permselectivity at roughly 20 weight percent of DEDGE each amino unit in the CS component [66]. Utilizing recycled cellulose and CS membranes, Hirabayashi et al. also explored the PV method for separating a water-based solution including urine components (uric acid, ammonia or creatinine) [67]. When the temperature of the input liquid rose and approached the upstream end of the membrane segment, the scale at which water permeated grew as well. There is no creatinine, uric acid or creatine were observed in the permeate across any of the examined membranes. Membranes are necessary for the selective penetration of water and ammonia. Ammonia disposal via the CS barrier increased from 57% to 59%. Additionally, it has been determined that ammonia may be desorbed from the adsorption agents by heating them at low pressure to regain their capacity for adsorption. With the introduction of the new PV equipment, ammonia was virtually entirely eliminated by the use of PV in conjunction with the adsorption/desorption procedure, resulting in the eventual production of purified distilled water in the ice trapping. To reduce the swelling of q-Chito membranes, hydrophilic quaternized CS (q-Chito) and tetraethoxysilane (TEOS) were used together to generate hydrophilic organic-inorganic composite films utilizing a sol–gel approach. Excellent water permselectivity was demonstrated by q-Chito/TEOS composite membranes when they were permeabilized via a water-based solution containing 96.5 wt% ethanol undergoing pervaporation. Figure 10 illustrates that the membranes’ water permselectivity tended to somewhat decline when the TEOS content increased by more than 45 mol% [68]. N-o-sulfonic acid benzyl chitosan (NSBC) was produced by modifying CS with sodium 2-formylbenzenesulfonate polysiloxane (SBAPTS). With the purpose of PV dehydration of a water–ethanol solution, NSBC–SBAPTS composite films were developed. It was possible to attach -SO3 H units onto the organic and inorganic membrane components. The impact of the membrane morphology upon PV efficiency was investigated by a methodical optimization of the components of the membrane and cross-linking intensity. The most appropriate composite film (CPS-a) demonstrated 5282 selectivity and 0.59 kg/L² h permeation flow for the dehydration of ethanol (90 wt%) @ 30 °C [69].

For effective water/ethanol segregation, Xia et al. employed a thin film composite (TFC) hollow fiber (HF) pervaporation film that included an in situ crosslinked ultrathin chitosan selective barrier developed through several supramolecular interaction-crosslinking techniques. PAN HF membranes were first made during their manufacture through a wet spinning procedure, and to assure that a selective layer would only appear on the membrane’s external surface, epoxy curing was used for securing the membrane’s edges. Subsequently, the hydrolyzed polyacrylonitrile (HPAN) HF substrates were submerged in water-based solutions containing 0.06 M FeCl₃ and 0.042 M phytic acid (PhA) in order to deposit a coating of Fe³⁺-PhA complex on the substrate’s edge. Finally, these membranes were soaked in a chitosan solution resulting in a composite membrane. Having an ultra-high flux of 2.87 kg/m² h and a water permeate level of 99.5 wt%, the TFC-CS-Fe₂PhA₂ membrane, which has a 60 nm selective layer with excellent anti-swelling characteristics, demonstrated exceptional water/ethanol separation efficiency. It also demonstrated useful for a long-time dehydration durability in a 85 wt% ethanol fluid at 50 °C [70].

3.2.2. Organic-Permselective Membranes

The organic permselective membranes facilitates the extraction of non-polar organic compounds from aqueous solutions during the process of pervaporation. Wang et al. examined the PV characteristics of water/ethanol solutions across the polyethylene oxide (PEO)/CS composite film. The findings indicate that ethanol is predominantly permeabilized through the Chitosan and PEO/Chitosan membrane at reduced alcohol concentrations in feed solution. Additionally, the addition of hydrophilic polymers such as PEO to Chitosan can significantly increase the membrane’s selectivity for alcohol. With 8% weight percentage of ethanol in the feed, the PEO/Chitosan composite film exhibited an isolation coefficient of 4.4 and a rate of permeation of 0.9 kg/m² h [71].

3.2.3. Organic–Organic Separation Membranes

The effectiveness of PV isolation of a methanol/methyl-t-butyl ether (MTBE) solution using a CS hybrid film altered through sulfuric acid and four surfactants has been researched by Nam et al. [72]. The effect of temperature, feed amount, intensity of cross-linking, and surfactant kind were addressed. This study indicates that chitosan membranes combined with sodium lauryl sulfate (SLS) at a concentration of 0.30 g/l exhibited optimal efficiency in pervaporation processes. The achieved methanol concentration in the permeate was 98.3 wt%, with a flux of 470 g/m²h.

Authors observed that both the separation factor and flux declined concurrently as the concentration of SLS was reduced. This can be attributed to the ionic properties of the surfactant–chitosan complex membrane, which favor the permeation of polar methanol through the membrane over non-polar MTBE.

Furthermore, it can be postulated that when surfactants form complexes with chitosan, they may lead to an expansion of the free spaces within the polymer chain structure. The linear alkyl chain of the SLS surfactant, when complexed with chitosan, creates significant steric hindrance. This structural arrangement acts as a more effective barrier against the comparatively bulky MTBE molecules than it does against smaller methanol molecules.

With the objective to separate MTBE/methanol solution, the PV properties were studied. The polyion complex composite (PIC) hybrid films were produced by complexing the ionic bonds between sodium alginate (SA) and CS [73]. The protonated amines (-NH₃⁺) of CS and the carboxylate groups (-COO-) of sodium alginate performed an ionic bridging to initiate the PIC reaction. The amount of counter ions present regulated the polyion the complexation process. The fabricated membranes in the present research functioned exceptionally well in PV splitting of MTBE/methanol solutions. PIC film composed of 2.0 weight percent SA and CS solution were observed to selectively permeate methanol from the feed, through a flow of more than 240 g/m² h. Methanol permeability improved yet MTBE permeation reduced when the operational temperature went up from 40 to 55 °C.

The mobility of polymer chains containing varying weight percentage of SA in chitosan matrix hinder the passage of MTBE molecules, which are larger in size compared to methanol molecules. The favorable movements of polymer molecules within membrane structures primarily contribute to enhancing selectivity and inhibiting the diffusion of MTBE.

The effectiveness of CS using poly(N-vinyl-2-pyrrolidone) (PVP) composite films in separating methanol through MTBE was measured. MeOH was found to permeate all investigated films with a preference, and as the PVP concentration grew, so did the partial flux of methanol. PVP, acting as a proton acceptor within membranes, makes hydrogen bonds with the hydroxyl groups of chitosan. Such interaction weakens the chitosan network structure, resulting in reduced chain-to-chain interactions among chitosan molecules. Conversely, it also acts as a proton receptor, allowing the formation of hydrogen bonds with methanol molecules present in the feed. Consequently, PVP works as a transport medium for separating methanol from MTBE through pervaporation. This results in an increase in the partial methanol flux as the concentration of PVP rises [74].

Methanol selective CS hybrid membranes have been developed for the purpose of separating MeOH/MTBE solutions. The PV properties of CS films treated with surfactants significantly improved because of the reduced membrane thickness and potential increased methanol compatibility [75]. DMC/water, methanol/water and Dimethyl carbonate (DMC)/methanol combinations, in addition to the ternary DMC/methanol/water solution, have been studied by Won et al. employing PV and CS membranes [76]. It pertains to the production of DMC, whereby the reaction solutions are typically separated using energy-intensive extracted distillation or pressure swing a distillation process. Solution casting was the method employed to make the CS membranes, and then they were alkali processed. It was shown that the membrane functioned effectively in both the DMC/methanol isolation and the DMC dehydration processes. In this study, crosslinking led to a higher permeation flux. Sorption measurements have verified that the membrane’s swelling degree reduced by about 40% following its crosslinking. This implies that the rise in permeability resulted from an improvement in the diffusion process. In the crosslinked membranes, the amino groups became protonated, leading to electrostatic interactions between the ammonium ions and the sulfate ions. Consequently, the crystalline area of the chitosan chain was altered, resulting in a higher proportion of the amorphous phase. Probably, when crosslinking was low, the expansion of amorphous regions due to chitosan molecule shapes was larger than the effects of reduced free volume and chain mobility in the membrane. Though its permselectivity was decreased, the membrane could still dehydrate methanol. It has been demonstrated that relationships between the permeating constituents profoundly influence the functionality of the membrane when it comes to the extraction of ternary DMC/methanol/water combinations.

A permeable polyetherimide substrate was covered with chitin films in order to perform pervaporative extraction from ethanol/toluene and methanol/toluene solutions. The overall permeation rate was reduced, and the separation measure grew when more acetyl groups were added to the CS framework. The incorporation of acetyl groups into the chitosan matrix enhances the bulkiness and rigidity of the membrane’s chemical structure, ultimately making the acetylated chitosan mechanically brittle. Consequently, the membrane’s chain mobility was significantly restricted by the presence of bulky acetyl groups, while the available free volume for accommodating the permeating component gradually decreased [77].

Benzoyl-chitosans (BzCS) were synthesized with several degrees of benzoylation. These BzCS serve as membrane components. They demonstrate exceptional durability when used to separate mixtures of benzene and cyclohexane (Bz/Chx). BzCS membranes showed superior benzene permselectivity in pervaporation systems. Specifically, it applied to Bz/Chx combinations with 50 weight percent benzene. Benzene holds π electrons in its structure. Thus, it may interact more strongly with molecules that have polar groups. In contrast, the cyclohexane molecule expected to have weaker interactions with polar groups. The significant hydrogen bonding ability of benzene will make interaction with exposed polar groups in the benzoyl chitosan membrane structure. This interaction may slightly affect how selectively the membrane permits benzene to pass through. The benzene permselectivity of BzCS membranes changes based on their benzoylation degrees. These variations are closely related to changes in the membrane properties. This correlation can be attributed to the inherent characteristics of such membranes [22,54,78,79]. Figure 11 illustrates the performance of BzCS membranes when exposed to a 50 weight percent benzene solution in Bz/Chx. The results show an enhancement in permeation rates and a marginal decline in benzene permselectivity with respect to varying degrees of benzoylation.

The use of deep eutectic solvents (DESs), a green solvent as choice to standard solvents facilitates molecular adsorption and transportation across the functional groups present in it. Roberto et al. reported a threefold efficiency on segregation of methyl t-butyl ether (MTBE)-methanol (MeOH) azeotropic combination with chitosan-L-proline:sulfolane (1:2 molar ratio) DES membrane compared to the pristine chitosan membrane [80].

The selectivity of the pristine CS membrane shows a specific behavior. Its separation factor reduces as the temperature increases. The selectivity was significantly enhanced by incorporating the DES on chitosan polymer matrix through in situ cross-linking. This modification resulted in a selectivity value of up to 35 at 25 °C. Mostly, for all membranes better separation factors and reduced permeation rates were achieved at lower temperatures. The results support the hypothesis of free volume theory. According to this theory, the amorphous regions in polymers contain chains that move due to thermal energy, which ultimately increases the free volume within the polymer matrix.

Three different non-ionic DES (proline–glucose, 2-pyrrolidone-5-carboxylic acid–sulfolane, and proline–xylitol) were utilized as additives into chitosan membrane for the pervaporative separation of MeOH MTBE azeotropic solution. These DES were selected based on their constituents of naturally derived compounds, little toxicity, cheapness and compatibility with chitosan matrices. The membrane fabricated using proline–xylitol/chitosan were shown results comparable to the other previous reported membranes. The flux calculated was 90 g/m².h, at 25 °C and 0.05 mbar pressure, when 14.3 wt% MeOH concentration have been used. These results could be attributed to the best compatibility thus resulting in a denser and more selective membrane structure when chitosan is combined with proline–xylitol additive which was confirmed by Hansen’s solubility theory [81].

3.3. Chitosan Based Polymer Electrolyte Fuel Cell Membranes (PEMFCs)

Fuel cell technological advances, with minimal pollutants and excellent efficiency, have drawn a lot of interest recently owing to increasing fears regarding global warming and the diminishing supply of petroleum-based reserves of energy. Based on the ion conduction mechanism, polymer electrolyte fuel cells are of two types: Alkaline PEMFCs, sometimes referred to as anion exchange membrane fuel cells, AEMFCs, use an anion exchange membrane (AEM) for carrying hydroxide ions within an elevated pH atmosphere, while acidic PEMFCs use a proton-conducting membrane with a solid electrolyte with a lower pH area. Figure 12 presents a schematic representation of a PEMFC along with the processes arising in its function. The working principle of a PEMFC is as follows: at the anode, hydrogen fuel (H₂) undergoes oxidation, releasing electrons and generating protons. The electrons move over an external circuit, while the protons move through the proton exchange membrane, which is placed between the anode and cathode. At the cathode, these electrons and protons react with the dissolved oxygen (O₂), resulting in the formation of water and the liberation of heat. Polymer electrolyte membranes are a crucial component of fuel cells. An ideal polymer electrolyte membrane (PEM) should possess high ion conductivity, minimal methyl alcohol boundary, longstanding hydrolytic and chemical endurance in a humidified and heated environment etc.

Chitosan has garnered significant attention in this field due to its distinctive characteristics, which include easy chemical alteration, strong proton conductivity, outstanding alcohol barrier features, superior resistance to chemical and heat, great hydrophilicity, inexpensiveness, and appropriate film-forming abilities. Three atoms of hydrogen that are closely linked to the amino and hydroxyl moieties found in CS monomers provide a tough lattice with elevated crystallinity that immobilize the structure and limit proton conduction. To increase the ion conductivity and selectivity of different membranes, techniques such as cross linkage, doping, multilayering, grafting, combining other kinds of polymers and hybrid compounds containing extra inorganic additives are employed [83,84].

Silica and sulfonated graphene oxide nanosheets are added to chitosan-based films in an attempt to boost the efficiency of a direct methanol fuel cell [85]. A layered structure of sulfonated poly(vinylidene fluoride-co-hexafluoropropylene) (SPVDF-HFP) or sulfonated poly(vinylidene fluoride) (SPVDF) demonstrated an excellent equilibrium between mechanical durability and proton conductivity. This was achieved by alternately dumping graphene oxide (GO) and chitosan (CS) on the membrane surfaces [86]. The ultra-thin membranes, which were produced by layer-by-layer assembling chitosan (CS), carbon nanotube (CNT) doped with phosphoric acid, and alternate polyurethane (PU) polycations ((PU/CNT-CdTe/PU/CS)₁₅₀/60% PA), with an optimal proton conductivity of 68.2 mS cm⁻¹ at 150 °C and an activation energy of 22.9 KJ/mol, also demonstrated the capacity to serve as effective proton recipients in the production of extreme temperatures PEMS [87].

Due to its low ability to retain water, pure chitosan has inadequate self-humidification. Water sorption is often enhanced when amine and hydroxyl functionalities are modified using acid moiety. Swaghatha et al. reported an eco-friendly organic–inorganic hybrid composite PEM from chitosan/geopolymer and heptanedioic acid crosslinker. Alkali-activated aluminosilicates can yield geopolymers (GP), which are inorganic, amorphous polymer substances with three-dimensional porous structures. Its long-range covalent connections, that connect alumina tetrahedra and oxygen-shared silica, may improve the chitosan’s mechanical strength, oxidation resistance, and ionic conductivity. The hydrophilic gaps in GP help films self-humidify by retaining molecules of water [88].

There are many ways such as electrophilic substitution, radiation grafting, doping with alkali metal hydroxides, fabricating composites, etc., to manufacture anion exchange membranes. Membranes are composed of quaternized poly(vinyl alcohol) (QAPVA) and chitosan (2-hydroxypropyl trimethyl ammonium chloride chitosan, HACC), cross-linked using glutaraldehyde. They have a relatively low methanol permeability (from 5.68 × 10⁻⁷ to 4.42 × 10⁻⁶ cm² s⁻¹) at 30 °C and an elevated conductivity (10⁻³ to 10⁻² Scm⁻¹) (Figure 13). The effective application of chitosan within fuel cells is attributed to the presence of glutaraldehyde (GA), which promotes a crosslinking reaction among the -OH molecules on HACC and QAPVA and the -C=O groups on (GA). This deteriorates the internal hydrogen bonding and lowers crystal structure and conductivity [89,90].

Blending quaternary ammonium functionalized polyvinyl alcohol and chitosan exhibited an enhanced conductivity, as well as alkaline stability [91]. At extremely elevated pH levels, copolymerizing a vinyl imidazole derivative-based chitosan membrane demonstrated strong ionic conductivity and durability in structure [92].

A maximum ion conductivity of 32 mS cm⁻¹ at 90 was seen upon adding chitosan to poly (2,6-dimethyl-1,4-phenylene oxide)s (CS-QAPPO) via the Menshutkin synthesis. Additionally, the material exhibited outstanding durability within a 2 M NaOH solution at ambient temperature, even after 2000 h [93]. Within chitosan/poly(bis(2-chloroethyl) ether-1,3-bis[3-(dimethyl-amino) propyl] urea copolymer/poly-(acrylamide-co-diallyl dimethyl-ammonium chloride) (CS/PUB/PAADDA), the semi-interpenetrating (semi-IPN) arrangement can generate a conductivity of 1.6 × 10⁻² Scm⁻¹ at room temperature and 3.12 × 10⁻² Scm⁻¹ at 80 °C, along with an alkaline resistance for up to 320 h [94]. Han et al. attempted to reduce the methanol permeability by fabricating a composite membrane using schiff base functionalized chitosan (CS-PY) as the polymer medium using 1,4-dichlorobutane (DL) crosslinker. Iodomethane was used as quaternization component. The methanol permeability in 3 M methanol at 60 °C was only 0.72% of Nafion^®-117 [95]. By 1967, Pedersen developed the very first crown ether, which is mostly utilized for ionic migration and metal ion the complexation process. It has a distinct hole structure and electronegativity because it contains oxygen atom electrons. After being submerged in 6 M KOH liquids at 70 °C over 480 h, Zheng et al. described an AEM type in alkaline fuel cells using cross-linked polyphosphazene crown ether films that had just 5% conductivity degradation [96].

Based on the relatively small ion conductivity of the additives and the substantial dispersion of ion exchange bonds within the parent polymer structure, numerous studies have shown how an upsurge in the proportion of inorganic filler led to a reduction in ion conductivity. Various studies reported that the functionalization of inorganic filler decreases the methanol permeability and enhance ion conductivity [97].

The composite membrane made of polyvinyl alcohol/quaternized chitosan/flower-like layered hydroxide displayed an extreme hydroxide conductivity of 25.7 mS cm⁻¹ and 92% retention of conductivity next to immersion in 2 M KOH for 100 h as well as good methanol barrier properties [98]. Superior conductivity and minimal methanol permeability were shown via the membranes made by adding molybdenum disulfide (MoS₂), silica-coated carbon nanotubes (CNTs), and layered double hydroxide (LDH) coated carbon nanotubes (LDH@CNTs) into QPVA/CS/MoS₂, QCS/QSiO₂@CNTs and QCS/PVA-1%-LDH@CNTs, respectively [99,100,101]. Being employed 2 M MeOH along with 5 M Potassium hydroxide as the anode fuel at 80 °C, DMFC constructed using QCS/PVA-1%-LDH@CNTs electrolyte yields an open circuit voltage of 0.83 V and a maximum power output of 107 mW cm⁻² [100]. Utilizing benzyl trimethylammonium chloride (BTMAC) as the hydroxide conductor, AEMs based on chitosan (CS), graphene oxide (GO) and magnesium hydroxide (Mg(OH)₂) demonstrated, an ethanol permeability of 6.17 × 10⁻⁷ ± 1.17 × 10⁻⁷ cm²S⁻¹, a peak power density of 72.7 mW cm⁻² at 209 mA cm⁻² and a hydroxide conductivity of 142.5 ± 4.0 mS cm⁻¹ at 40 °C if utilized in an alkaline ethanol fuel cell operating at 80 °C [102].

Mechanical durability and superior ion conductivity were demonstrated by layered double hydroxides (LDHs) incorporated with glycine betaine embedded quaternized chitosan and polyvinyl alcohol blend film. Intercalated organic ions in the LDHs help homogeneous dispersion that promotes weight transmission commencing the matrix to strong LDHs and also act as the OH⁻ conductive channels. When the KOH level increased from 1 M to 7 M, the composite AEMs likewise showed a rise in peak power density from 20.2 to 99.1 mW cm⁻² [103].

Chitosan-based electrolyte membranes offer numerous advantages and disadvantages with respect to regularly employed battery separators in the market and literature. Chitosan-based membranes propose enhanced mechanical properties compared to specific commercial separators. Lignin-chitosan gel polymer electrolytes (GPEs) show semi-ductile behavior with Young’s modulus ranging from 55 to 940 MPa and elongation at break of 14.1% to 43.9%. This compares favorably to brittle glass microfiber separators with ~5.8% elongation at break [104].

These membranes generally exhibit lesser ionic conductivity compared to commercial separators like Nafion. Though, modifications can substantially upgrade their performance. Pristine chitosan membranes exhibit proton conductivity around 10⁻³ S/cm, which is lower than Nafion’s 10⁻¹ S/cm [83]. Tailored chitosan membranes, such as those with nanosilica, can attain conductivity up to 0.231 S/cm, which is comparable to commercial separator performance [105].

Chitosan membranes excel in decreasing methanol crossover, a crucial factor in direct methanol fuel cells. Chitosan membranes exhibit lesser methanol permeability compared to Nafion, making them appropriate for DMFC applications. Nanosilica-altered chitosan membranes can attain methanol permeability as low as 5.43 × 10⁻⁷ cm²/s [105].

Although, chitosan-based membranes have shown several good performances in the field of various applications. However, brittleness, poor mechanical strength and less stability in the long-term operational process are some of the limitations which need to be addressed. Additionally, these membranes show limited permeability and selectivity trade-off, in case nanofiltration applications. Moreover, chitosan despite being considered hydrophilic, a few of these membranes may show inadequate hydrophilicity for specific pervaporation applications. Also, Chitosan membranes may demonstrate elevated gas permeability compared to Nafion, which can lead to improved fuel crossover (especially for methanol in DMFC), lowering fuel efficiency and overall cell performance [106].

4. Conclusions and Future Prospects

This review has demonstrated the versatility of chitosan as a natural material by highlighting its potential in a variety of human activities. Although there are numerous possible precursors for the synthesis of chitosan, solely crustaceans are used to produce chitosan of a high enough quality for commercial use. The global demand for chitosan has the potential to develop significantly in the years to come, so it is necessary to investigate alternatives.

Chitosan, a biopolymer made from chitin, has gained widespread acceptance as a versatile material for membrane technologies because of its distinctive physicochemical characteristics, biological compatibility, and availability. Membranes derived from chitosan have been substantially studied recently for the applications such as ion exchange (IE), pervaporation (PV), and nanofiltration (NF) techniques. Since such membranes are considerably hydrophilic, biodegradable, and have chemically modifiable functional groups, they provide sustainable substitutes for membranes made of synthetic polymers.

Chitosan-based nanofiltration membranes are frequently utilized for organic component separation, desalination, and water purification. With regard to its natural hydrophilicity and chemical adaptability, chitosan can be used to construct membranes that are highly permeable to water and that selectively reject organic and salt ions. Glutaraldehyde, citric acid, and genipin are among the many of the crosslinking strategies that investigators are looking into to improve the mechanical durability and stability of such membranes in harsh surroundings. Selectivity, fouling resistance, and efficiency of chitosan-based membranes have been demonstrated to be significantly enhanced by blending chitosan with additional polymers or by adding nanomaterials.

Chitosan membranes have demonstrated significant potential in pervaporation to facilitate the dehydration of organic solvents, especially when it comes to the extraction of water from alcohols such as ethanol and isopropanol. Chitosan is perfect for water-selective pervaporation because of their hydrophilic characteristics and protonated amine groups, which give it an extreme preference for water atoms. Furthermore, its selectivity and permeability have been further improved by its adaptability via crosslinking or grafting. In order to maximize pervaporation efficacy, particularly for commercial uses such as solvent recovery and biofuel generation, experts are also investigating chitosan composites incorporating nanomaterials like graphene oxide, zeolites, and metal–organic frameworks (MOFs).

Chitosan’s abundant amino and hydroxyl groups provide active sites for ion exchange, making it an excellent candidate for heavy metal removal from wastewater and for water softening applications. Researchers have greatly improved the adsorption capability, durability, and recycling of such membranes via functionalizing chitosan using different ligands or adding nanomaterials (such as silica or modified graphene oxide).

To overcome the limitations correlated with chitosan-based membranes, future research should focus on advanced approaches to improve their mechanical strength, stability, and overall performance. The incorporation of nanomaterials (such as cellulose nanocrystals, TiO₂, carbon nanotubes, etc.), polymer blends or crosslinking agents can notably enhance brittleness and long-term stability while sustaining the intrinsic properties of chitosan. For nanofiltration applications, advanced fabrication techniques such as surface modifications or layer-by-layer assembly can be explored to improve the permeability-selectivity trade-off. Optimization of pore size distribution can be performed to achieve better separation performance.

In pervaporation applications, enhancing the hydrophilicity of chitosan membranes through chemical modifications or blending with hydrophilic polymers could be a potential solution. Researchers should also focus on the surface treatments which can enhance water affinity without cooperating membrane integrity. Furthermore, for fuel cell applications, modifying chitosan membranes with ion-conductive additives or structural variations may assist in reducing fuel crossover and improve whole cell efficiency. Future studies should also explore scalable manufacture processes and eco-friendly production methods to uphold the commercialization of chitosan-based membranes for sustainable industrial applications.

Overall, chitosan-based membranes provide a flexible and environment friendly foundation for a variety of uses in ion exchange, pervaporation, and nanofiltration. With further advancements in surface engineering, hybridization, and material science, such membranes will likely become more crucial in solving industrial and environmental problems. Future research on chitosan-based membranes will concentrate on improving performance, cutting manufacturing expenses, and broadening the range of sectors in which they can be used.