Bio-Sourced and Biodegradable Membranes

Abstract

Biodegradable membranes with innovative antifouling properties are emerging as possible substitutes for conventional membranes. These types of membranes have the potential to be applied in a wide range of applications, from water treatment to food packaging and energy production. Nevertheless, there are several existing challenges and limitations associated with the use of biodegradable membranes in large scale applications, and further studies are required to determine the degradation mechanisms and their scalability. Biodegradable membranes can be produced from either renewable natural resources or synthesized from low-molecular monomers that increase the number of possible structures and, as a result, greatly expand the membrane application possibilities. This study focused on bio-sourced and synthesized biodegradable polymers as green membrane materials. Moreover, the article highlighted the excellent antifouling properties of biodegradable membranes that assist in improving membrane lifetime during filtration processes, preventing chemical/biological disposal due to frequent cleaning processes and ultimately reducing the maintenance cost. The industrial and biomedical applications of biodegradable membranes were also summarized, along with their limitations. Finally, an overview of challenges and future trends regarding the use of biodegradable membranes in various industries was thoroughly analyzed.

1. Introduction

Membrane technology has been extensively used in various applications, including but not limited to waste/wastewater treatment, gas separation, hemodialysis, energy production, drug delivery, and the food industry. Polymeric membranes are mainly employed in all those separation processes [1,2]. The production and disposal processes of polymeric membranes result in unsustainable accumulations of waste and several environmental concerns [3,4,5,6,7]. Even though these types of membranes are more prevalent and dominate the membrane market, they are still subject to waste disposal challenges, chemical, mechanical, and thermal stability concerns, as well as the membrane fouling issue [7]. As an environmentally friendly and sustainable approach to solving these problems, biodegradable membranes have been introduced, especially to reduce the amount of waste disposal. Under favorable conditions in the environment in terms of pH, humidity, and temperature, biodegradable membranes are broken down into non-toxic compounds by enzymes and microorganisms in nature [1]. Biodegradable membranes proved to be potential substitutes for conventional membranes in various applications such as oil/water separation, dye removal, tissue engineering, food packaging, fuel cells, etc. [1,8,9,10,11].

2. Green Membrane Materials

Synthetic polymer materials have been developed less than 100 years ago but because of their low ability to be utilized in nature are considered to have serious ecological impact on the environment. The interest in biodegradable plastics is constantly increasing, and their production is forecasted to reach more than 1.3 million tons in 2024 [12]. The main position on the biopolymers market belongs to the Asia-Pacific region, North America, and Europe [13]. All biodegradable polymeric materials considered in this chapter could be divided into two main classes: bio-sourced and synthetic. Synthetic materials, in turn, fall into two categories: petrochemical and made out of renewable sources, which is illustrated on Figure 1.

2.1. Bio-Sourced Polymers for Membrane Fabrication

Bio-sourced materials have been used by humanity since ancient times as materials for construction (wood), tools (wood, bones), cloth (plants, leather, wool, fur), jewelry (bones, wood, amber), and other. Nowadays we also use bio-sourced crude to create new materials, including membranes. The majority of bio-sourced polymers used for membrane fabrication are carbohydrates/polysaccharides and their derivatives. The origin of bio-sourced materials possesses a high degree of biodegradability that makes them attractive as green materials to be used for reducing pollution of the environment with plastic wastes.

2.1.1. Cellulose and Its Derivatives

Cellulose is one of the first materials used for membrane production. Thus, the first membrane for hemodialysis was produced from cellophane, which is the product of cellulose treatment. Cellulose is one of the most prevalent polysaccharides that occurs in natural sources like trees and plants. Saccharide monomer units in cellulose (see Figure 2A) macromolecules are arranged predominately in a linear manner, which results in its fiber structure and high mechanical properties.

Due to the presence of active hydroxyl groups in the saccharide unit, cellulose is widely used for its modification to obtain new artificial materials with improved properties. Some of the structures that resulted from cellulose modification are presented on Figure 2A.

Thus, the most common cellulose derivative is cellulose triacetate, which is, along with other cellulose derivatives, used as the fabrication material for membranes applied in many industries such as oil-water separation [14,15,16], antifouling improvement [17], virus removal [18], heavy metal adsorption [19,20], desalination [21,22,23], hemodialysis [24], and gas separation [25,26,27].

2.1.2. Chitin and Chitosan

Chitosan is the second (after cellulose) most abundant biopolymer in nature that can be found in insects, shells, mollusks, shrimps, crabs, fungi etc. It also can be derived from chitin (the exoskeleton of many living creatures, insects in particular) by its deacetylation (see Figure 2B).

Though this biopolymer is widely spread in nature, the research of chitosan began only in the 1980s because of the structure’s features and insolubility in water [28].

Examples of chitin and chitosan membranes applications are ion exchange membranes [29,30,31], wastewater treatment [32,33], oil-water separation [34], dye removal [35], etc.

2.1.3. Carrageenan

Carrageenan is a naturally occurred polysaccharide containing sulphate groups. This polymer can be extracted from red algae, particularly from Rhodophyceae [36]. Depending on the degree of sulfanation, there are three classes of carrageenan: kappa, iota, and lambda. The structure of carrageenan is shown on Figure 2C. One carrageenan application example is the super-oleophobic membrane for the removal of dyes and heavy metals [37].

2.1.4. Other Polysaccharides and Biopolymers

Starch is another example of a polysaccharide. It is one of the most abundant biopolymers in nature and can be found in such common products as potato, corn, and rice. Due to its branched structure, resulting in poor mechanical properties, starch can be hardly used as the main material for membrane production [38]. Thus, starch was reported to be used as an additive to produce membranes for wastewater treatment [39,40,41].

Cyclodextrin (CD) can also be used as an additive for the improvement of hydrophilicity and permeability of membranes used for water treatment [42]. The functionalization of hydroxyl groups to amino, sulphonyl, and other groups resulted in the further improvement of membrane selectivity by enhancing porosity and hydrophilicity [43].

Alginate is a naturally occurring anionic water-soluble linear polysaccharide composed of α-L-guluronic acid and β-D-mannuronic acid (see Figure 2D), typically obtained from brown seaweed (Phaeophyceae), including Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera [44]. Alginate-containing membranes were reported to be used for dyes and heavy metal removal [45,46,47,48,49].

Silk fibroin is another natural biopolymer, extracted from insects’ cocoons when treating them with hot alkali solution [50]. This protein is added to membranes to improve their adsorption of heavy metal ions [51,52].

Similar to fibroin, collagen can be extracted from animal connective tissues [53]. Despite being sensitive to pH, temperature, and bacteria, collagen can be used for membrane fabrication, though its application is limited. Due to its protein nature, collagen is perfectly suitable to create fiber-type membranes for oil-water separation [54] and pervaporation [55,56].

Polyhydroxybutyrate (PHB) is bio-derived polyester (see Figure 3) that is produced by some microorganisms such as Cupriavidus necator, Methylobacterium rhodesianum, or Bacillus megaterium during assimilation of glucose or starch [57].

This polymer has a tensile strength close to polypropylene, making it suitable to manufacture fibers. As a result, PHB has been used to prepare electrospun fibers-based membranes with improved antibacterial properties for medical applications, dye removal, and microfiltration [58,59,60,61].

2.2. Synthetic Biodegradable Polymers

2.2.1. Polymers Synthesized from Renewable Sources

Polylactic Acid

Polylactic acid (PLA) is a thermoplastic polyester that is produced by polycondensation of lactic acid or ring-opening polymerization of cyclic diester-lactide (see Figure 4). Lactic acid, in turn, is produced from a renewable bio-source, such as sugarcane or starch, [62] by aerobic fermentation [63]. PLA is one of the most researched and mass-produced biodegradable polymers.

PLA belongs to semicrystalline polymers. PLA-application temperatures are limited by its low glass-transition temperature (T_g = 55–60 °C) and melting temperature (T_m = 170–180 °C) [64]. PLA is suitable to prepare membranes based on electro-spun fibers for oil-water separation [65] and tissue regeneration [66]. PLA biodegradation proceeds quite fast to CO₂ and water [67], which meets all the requirements of all standards for biodegradable plastics [68]. PLA is also suitable for decomposition as a part of compost [69].

Polybutylene Succinate

Polybutylene succinate (PBS) is another (like PLA) example of a biodegradable polyester that is produced by polycondensation of succinic acid and 1,4-butanediol. Succinic acid can be produced through biomass fermentation [70] by further hydrogenation to 1,4-butanediol [71]. The further polycondensation process of succinic acid and 1,4-butanediol proceeds in two steps and requires catalysts to produce a polymeric PBS from its oligomers, obtained in the first step (see Figure 5). The first mass production of this polymer began in 1990s in Japan by the Showa Highpolymer company [72]. PBS are reported to be used for the fabrication of membranes for pervaporation and other applications [73,74,75]. PBS biodegradation in soil was reported to proceed with 65% carbon mineralization (conversion to CO₂) for 425 days [76].

2.2.2. Petrochemical Polymers

Poly-ε-caprolactone

Poly-ε-caprolactone is an aliphatic polyester, produced by ring-opening polymerization of ε-caprolactone (see Figure 6A) [77]. This polymer has a low Tg = −55–60 °C and low melting point of Tm = 60–65 °C and good resistance to oil and water. Examples of using poly-ε-caprolactone for membranes include dye removal [78], wound dressing [79], antifouling [80], treatment blood infections [81], and guided tissue regeneration [82] applications. Furthermore, poly-ε-caprolactone is highly degradable polymer. Thus, Pseudozyma japonica-Y7-09 enzyme, which is present in different microorganisms, results in Poly-ε-caprolactone degradation by 93.3% for 15 days for polymer film and 43.2% for 30 days when the polymer was made as foam plastic [83].

Poly-propylene Fumarate

Polypropylene fumarate (PPF) is an aliphatic polyester, containing a double bond in the main chain. This polymer can be produced by multistep synthesis (see Figure 6B) [84]. Because of the presence of double bonds, this polymer can be cross-linked by various methods which, results in the broad mechanical properties of the resultant structures [85]. Although, the mechanical properties are still low and limit the use of PPF for the membrane and other applications. However, some new approaches have been applied by the introduction of nanofillers into PPF to improve its properties [86,87,88]. The in vitro biodegradation of PPF scaffolds revealed nearly 17% mass loss at 6 weeks [89].

Poly-ethylene Glycol

Polyethylene glycol (PEG) is a synthetic polyether that can be produced by various ways: from ethylene oxide, ethylene glycol, or ethylene glycol oligomers (see Figure 6C).

PEG production has been reported in 1859 by both A. V. Lourenço and Charles Adolphe Wurtz, independently. PEG membranes are predominately used for drug release systems [90,91,92,93,94]. In addition, PEG is well known to possess sensitivity to oxidative degradation [95]; thus, this material is substantial to oxidative biological degradation by different microorganisms [96,97,98].

Poly-vinyl Alcohol

Polyvinyl alcohol (PVA), unlike most of other vinyl polymers, cannot be produced by polymerization of its monomer units because vinyl alcohol is thermodynamically unstable. PVA is predominately produced by hydrolysis of polyvinyl acetate (see Figure 7). Hydrolysis reaction is usually proceeded in the presence of ethanol but can be easily done without it.

PVA has been reported to use for membrane preparation for wound dressing applications [99,100]. In addition, PVA is biodegradable in the presence of different microorganisms under two-step metabolic processes, including oxidation and hydrolysis [101].

Polyurethane and Polyurethane Urea

Polyurethanes (PU) are one of the most produced polymers in the world. Around 25 million tons of this class of polymer was produced in 2019, which is about of 6% of all manufactured polymers [102]. The interest to polyurethanes is determined by the broad chemical variety of monomer units, resulting in a wide range of physical properties–from rigid and strong plastics to flexible elastomers [103]. Linear PU are basically produced during polycondensation reaction between diisocyanates and diols (see Figure 8A, step 2A) with the ability to produce polyurethane urea when a diamine chain extender is used on the second synthesis step (see Figure 8A, step 2B).

Polyurethanes were known for a long time as non-degradable polymers. For the last decades, biodegradable PU were developed by using monomer units with hydrolysable bonds (e.g., ester, amide) groups [104,105]. Examples of oligomeric diols [106,107] used for biodegradable PU synthesis are given on Figure 8B. A broad opportunity to adjust and control PU chemistry and structure makes PU degradable by enzymes, fungi, and bacteria [108].

PU applications include sprayable membranes [109], tissue repair [110], and many other biomedical applications [110].

The biodegradable membranes synthesis routes and main applications are summarized in

Table 1. Green chemistry biodegradable membranes chemistry and applications.

No.	Membrane Material	Synthesis Method	Main Applications
Bio-sourced
1	Cellulose derivatives (acetates, carboxymethyl etc.)	Processing of cellulose containing biomaterials (plants, trees)	Water treatment and separation, gas separation, biomedical applications
2	Chitin and chitosan	Processing of chitin containing biomaterials (insects, shrimps, crabs etc.). Deacetylation results in chitosan	Ion exchange, water treatment and separation, dye removal
3	Carrageenan	Processing of biomaterials (algae)	Dyes and heavy metals removal
4	Starch	Processing of starch containing biomaterials (potato, corn, etc.)	Used as additive and source of other materials
5	Cyclodextrin	Enzymatic conversion of starch	Water treatment
6	Alginate	Processing of biomaterials (algae)	Dyes and heavy metals removal
7	Silk fibroin	Processing of biomaterials (insects cocoons)	Heavy metals removal
8	Collagen	Processing of biomaterials (animal connecting tissues)	Water separation, pervaporation
9	Polyhydroxybutyrate	Glucose and starch conversion by microorganisms	Biomedical application, dye removal, microfiltration
Synthesized from bio-sourced chemicals
1	Polylactic acid	Polycondensation or ring-opening polymerization of lactic acid, produced by aerobic fermentation of glucose or starch.	Water separation, tissue regeneration
2	Polybutylene succinate	Polycondensation of succinic acid and 1,4-butanediol. Succinic acid is produced by biomass fermentation. 1,4-butanediol is produced by succinic acid hydrogenation	Pervaporation
Petrochemical
1	Poly-ε-caprolactone	Ring-opening polymerization of ε-caprolactone	Biomedical applications
2	Polypropylene fumarate	Polycondensation of fumarate with propylene glycol	Biomedical applications
3	Poly-ethylene glycol	Polycondensation of ethylene glycol or ring-opening polymerization of ethylene oxide	Drug release systems
4	Poly-vinyl alcohol	Hydrolysis of polyvinyl acetate	Wound dressing
5	Polyurethane and polyurethane urea	Polycondensation of diisocyanates with diols and diamines	Biomedical application, agriculture

.

3. Biodegradable Antifouling Membrane Properties

Fouling is a serious drawback of polymeric membranes used in membrane-based separation processes. This phenomenon occurs as a result of particle-membrane and particle-particle attachments on the membrane surface or within the pores, which is mostly due to the hydrophobicity of the membranes [111]. This phenomenon can be observed at the membrane surface as well as in the membrane pores as a result of sieving and adsorption. In the sieving mechanism, particles larger than the membrane pores agglomerate and partially/completely block the pores (intermediate/complete blockage). A further accumulation of particles on the membrane surface would lead to the formation of cake layer. In addition, small particles enter the pores and are adsorbed (intermolecular interaction) to the pore wall, causing an internal blockage (standard blockage) [112]. Figure 9 shows the schematic of membrane fouling mechanisms.

Several methods have been employed to prevent fouling formation (feed pre-treatment or membrane modification) or clean the membrane surface (chemical/biological cleaning, electrochemical, advanced oxidation, ultrasonic cleaning, forward/backflushing, pulsing mode, vibration, air sparging, and electrical field). Many of these methods suffer from limitations such as reduced membrane lifetime due to frequent cleaning, higher operating costs and energy consumption due to system downtime and interruption of filtering, and secondary pollutants as a result of the use of chemical/biological agents for surface cleaning [112,113]. The foulant-membrane interaction and fouling behavior are mainly controlled by the chemistry/surface properties of membranes. Therefore, there is a great demand for membranes with antifouling properties to reduce the cleaning costs and prevent membrane damage. Natural antifoulant materials have shown promising results and proved to be an environmentally friendly solution to membrane fouling [114,115,116,117]. Figure 10 shows the antifouling properties of various types of membranes.

3.1. Bio-Based Materials

In recent decades, bio-based materials have been widely used to enhance the antifouling property of the membranes [118,119,120,121,122]. According to Ye et al. [54], the collagen-based nanofibrous membranes incorporated with carbon nano-tubes (CNT) were capable of separating the viscous emulsions (up to 200.0 mPa.s) by more than 99%, whereas the conventional membranes, such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), failed to separate these types of emulsions. The fabricated membranes exhibited a high separation flux of 1582 Lm⁻²h⁻¹ bar⁻¹ and robust reusability after 10 filtration cycles, which prove their antifouling properties. Due to the presence of the mixture of -NH₂ and -COOH functional groups in collagen/CNT nanofibrous membranes, these materials show amphiprotic properties and act as demulsifiers. They separate the emulsions by selectively ionizing, targeting, and capturing oppositely charged molecules. Biomimetic membranes are also among the potential candidates for wastewater treatment and seawater desalination, owing to their excellent antifouling, water permeability, and selectivity characteristics [111,117,123,124,125]. Generally, biological surfaces contain particular functional groups (like zwitterionic groups on lipid head groups or glycocalyx oligosaccharide chains) that inhibit the foulant adhesion. Zwitterionics prevent the protein adhesion to the exterior surface of the cells by causing electrostatic hydration and strong binding to water molecules. Glycocalyx chains also hinder foulant adhesion by electrostatic and entropic forces [117]. Biological cells also contain protein water channels called aquaporins. Aquaporins have excellent water permeability and high solute rejection [126]. Water flux and permeate flux recovery, which show the antifouling property, are improved by incorporating these water channels in membranes. The antifouling properties of cell membranes inspired the fabrication of biomimetic membranes for separation processes [127,128,129,130]. A summary of the antifouling properties of common biodegradable membranes from several studies is provided in the following.

3.2. Biodegradable Membranes for Oil Fouling

Oil fouling is one of the major challenges involved in oil/water separation owing to the low surface energy of oil droplets. In this regard, several bio-inspired materials have been employed to fabricate different filters with self-cleaning and antifouling properties [131]. Chitosan (CS) is among the bio-based polymers, containing hydroxyl and amino groups and showing hydrophilic behavior in air and oleophobic properties in water. In addition to those characteristics, its biocompatibility, non-toxicity, and low cost allow chitosan to be an excellent alternative to treat oily wastewater with a separation efficiency of more than 99% [113,132,133,134,135,136]. Chitosan can also be used as an adsorbent for removing dyes, macromolecules, and heavy metals from a system with a high permeate flux recovery rate [137,138]. Zhang et al. [113] used the chitosan-coated mesh to remove the oil from the hypersaline environments and solution at various pH levels. Their research on fabricating a bio-inspired filter with stable wettability in complex environments was inspired by shrimp shells. Shrimp shells are both hydrophilic and oleophilic in the air (contact angles of about 41° and 24.7° for water and oil, respectively). However, the shells exhibit a superoleophobic property once placed in seawater (oil contact angle > 150°). CS is the deacetylation form of chitin, which is the major component of shrimp shells. Despite chitin, CS can be dissolved in an acid solution and form filtration films. The spin coating technique was used to fabricate the CS film, and the result showed that the water contact angle of the film in the air and the oil contact angle in the water was about 7° and 155°, respectively. Krishnamoorthi et al. [116] also used chitosan (CS) and caffein acid (CAF) as the biodegradable coating agents on cotton fibres to prepare a superwettable filter for oil/water separation. The hydrophilicity (contact angle of almost 0°) and underwater superoleophobicity (oil contact angle ≥ 160°) of those polymers result in high permeate flux (up to 50,050 L h⁻¹ m⁻² bar⁻¹) and separation efficiency (>99.9%) under low applied pressure (0.1 bar). Despite its inability to form a film, chitin can be used as an additive to the polymer solution or as a coating agent to modify the surface properties of membranes. In the research conducted by Goetz et al. [139], chitin nanocrystals were coated on electrospun cellulose acetate (CA) to improve its antifouling properties. The nanofibrous CA has hydrophobic surfaces with a water contact angle of about 132°. However, the results showed that the cellulose acetate/chitin membrane turned superhydrophilic characteristics (water contact angle of almost 0°). The modified membranes were found to be less prone to biofouling, especially toward the solutions which contain bovine serum albumin (BSA) and humic acid (HA), such that no flux decay was observed over a complete filtration cycle. In recent decades, chitosan-based composite membranes have gained increasing attention. The antifouling properties of chitosan have been found to be useful in developing membrane separation processes for water purification and wastewater treatment [138,140,141,142,143,144,145,146].

3.3. Polyvinyl Alcohol (PVA) Membranes

Polyvinyl alcohol (PVA) is another biodegradable, non-toxic polymer with inherent hydrophilicity and antifouling properties. PVA is one of the most favorable polymers that can be used for membrane fabrication and exhibits high mechanical, thermal, and pH stability [143,147,148,149,150]. PVA membranes are useful for wastewater treatment applications due to their oleophobic and highly polar nature, which minimizes foulant adsorption and the adhesion between the membrane surface and water contaminants [11,143,147,151,152]. Hydrophilic membranes appear to be less prone to fouling formation, especially toward organic matters like proteins in water, and it is much easier to clean the attached particles or recover the permeate flux [153]. Such membranes absorb water molecules much more easily, resulting in the formation of a hydration layer that protects the membrane from particle accumulation [114,117]. Amanda et al. [153] evaluated the protein fouling of PVA ultrafiltration membranes using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). They compared their results with fouling formation on polyethersulfone (PES) as one of the most commonly used ultrafiltration membranes. According to their findings, PVA membranes had a higher resistance to protein fouling. PVA membranes can also be used for oil/water separation. According to Wu et al. [154], the flux recovery ratio of ultrafiltration PVA membrane was about 95% after washing the fouled membrane with pure water, which demonstrates the excellent antifouling properties of the membrane toward the oily wastewater. Wang et al. [155] also fabricated a high performance PVA-based composite membrane for oil/water separation. As the support layer, they prepared a PVA nanofibrous scaffold and coated it with PVA hydrogel as the anti-fouling layer. The hydrophilic nanofibrous membrane was found to have a filtration efficiency of more than 99% with no flux decay over the filtration time, which was similar to the commercial UF membrane with Pebax 1074 coating. However, the permeate flux of the prepared membrane was more than twofold that of commercial UF membrane (about 130 LMH for electrospun composite PVA membrane and 60 LMH for commercial UF membrane). Similar to chitosan, PVA has been employed in several studies to fabricate biodegradable fouling-resistant composite membranes [128,156,157,158,159,160,161].

3.4. Cellulose Acetate (CA) Membranes

Cellulose acetate (CA) is among the natural polymers with excellent mechanical stability. In light of their high hydrophilicity, antifouling, and film-forming properties, cellulose and its derivatives have attracted considerable interest as potential membranes for water/wastewater filtration [127,162,163,164,165,166,167,168,169,170,171]. Several membrane filtration applications have been conducted with cellulose-based membranes, including separating bacteria, salt, dye, protein, oil, and heavy metal from the aqueous solutions [2,131,162,172,173,174,175,176]. Lv et al. [172] reported the antifouling property and filtration performance of the fabricated cellulose-based membranes for protein solution filtration. To improve the hydrophilicity and antifouling property of the flat sheet cellulose diacetate membrane (CDA), cellulose nanocrystals (CNCs) were added into the casting solution. The results of their research indicated that the protein adsorption (bovine serum albumin) decreased by about 48%, and the flux recovery ratio (FRR) after three complete filtration cycles improved by 50%. In another study by Kanagaraj et al. [177], hydrophilic CA ultrafiltration membranes modified with biodegradable materials, polyurethane, and lactic acid, were prepared for the removal of humic acid and copper ions. The prepared membrane exhibited enhanced antifouling and hydrophilicity properties. The flux recovery rate was 97.8% and 92% for humic acid and copper ultrafiltration, respectively. Additionally, low irreversible fouling was reported for both contaminants (2.2% for humic acid and 7.8% for copper), demonstrating the superior antifouling property of the prepared biodegradable membrane. Ye et al. [178] compared the adsorption of blood proteins by CA membranes and non-biodegradable polyethersulfone (PES) membranes. The adsorption of albumin protein by PES membrane was about 2.5 times that of CA membrane. Protein adsorption decreased by 50% (<0.5 μg.com⁻²) for the modified CA membrane with a biocompatible phospholipid polymer (poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-co-n-butyl methacrylate (BMA))).

3.5. Polylactic Acid (PLA) Membranes

Recent studies have shown that polylactic acid (PLA) biopolymers can be an excellent alternative to conventional thermoplastic membranes used in separation applications [179,180,181,182]. The polar oxygen linkage in PLA membranes contacts the water molecules over the separation of aqueous solutions and results in a higher permeate flux and decreased foulant attachment [183]. Khalil et al. [180] prepared an ultrafiltration PLA membrane through the phase inversion method and evaluated its antifouling property using the BSA solution. After one filtration cycle, the flux recovery ratio was about 57% for membranes containing 15 wt% PLA polymer, and it reached 93% as the PLA concentration increased to 22 wt%. For membranes containing 22 wt% of PLA, the permeate flux recovery ratio after the second cycle was about 85% (decreased only 8%), indicating high fouling resistance of the fabricated membranes. Although PLA membranes demonstrated good antifouling properties, their hydrophobic characteristic and brittleness at room temperature may hinder their use in various applications. Therefore, a number of techniques, including blending, self-assembly, and surface grafting and coating have been used to enhance the hydrophilicity, mechanical stability, and fouling resistance of those membranes [1,184]. In terms of fouling resistance and hydrophilicity, Ouda et al. [7] proposed hydroxyapatite (Hap) as a filler and polydopamine (PDA) as the surface coating for PLA membranes. It was reported that the water permeability of PLA membrane improved by 120% when only 2 wt% Hap/1-h PDA was employed in the system. After three complete filtration cycles, the flux recovery ratio (FRR) improved from 20% for pristine PLA membranes to 54% for composite membranes. The irreversible fouling of non-organic material removal was also decreased from 79.7% to 70.7%. In fact, all of those promising findings were the result of improvements in membrane contact angle, porosity, and mean pore size. In another research, the bio-based β-cyclodextrin (β-CD) was also employed as the pore foaming to improve water permeability in the PLA membrane due to its hydrophilic exterior [185]. The SEM images of the surface and cross-section of the PLA membrane show that it was composed of a dense skin layer with interconnected internal pores. However, the addition of 5 wt% of β-CD to PLA solution led to the fabrication of membranes with homogenous pores on the surface (mean size of about 1 µm) and some finger-shaped pores in cross-section. As a result of changes in membrane morphology, the pure water flux and the flux recovery ratio improved by about 230% and 50%, respectively.

4. Green Membrane Technology Applications

Biodegradable membranes have been employed in a variety of industrial and biomedical applications. Figure 11 summarized the applications of biodegradable membrane. In Table 1, various applications of green membranes and their advantages/disadvantages are discussed. Following are some of the most prevalent applications of these membranes.

4.1. Wastewater Treatment

Membrane technology has been extensively used for water purification, wastewater treatment, and seawater desalination due to its superior separation performance, ease of operation, and reasonable energy consumption [2]. One of the most common applications of biodegradable membranes is wastewater treatment, including but not limited to oil separation and removing heavy metals, dye, pathogens, and proteins from aqueous solutions. Biodegradable membranes are strong candidates for wastewater treatment owing to their excellent stability, high durability, energy efficiency, high permeability, and selectivity [2]. Depending on the industry that releases wastewater and the pollution type, various membranes can be used in filtration processes. For instance, in oil absorption (oil/water separation), membranes with underwater superoleophilic/superhydrophobic characteristics are more desirable. On the other hand, composite membranes, containing hydrophilic materials such as PVA, CS, CA, etc., have been widely used to remove dyes from industrial wastewater [8,78,186,187,188].

PLA is among the biodegradable materials that can be used to prepare a porous superhydrophobic membrane and replace the non-biodegradable commercial membranes such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polypropylene (PP), etc. [113,116,179,189,190,191,192,193]. Su et al. [179] prepared the superhydrophobic PLA membrane with excellent oil adsorption capacity using non-solvent induced phase separation. The preparation method may affect the surface roughness, porosity, and free energy of the membrane and its hydrophobicity. In their study, the prepared membranes displayed a high water contact angle of about 152°, and they were capable of absorbing oils by 4–5 times their weight.

Colorant effluent is one source of severe water pollution that can cause serious concerns for human and marine lives. The colorant effluent, mainly from the textile industry, may have low molecular concentrations and inert properties, which makes it difficult to remove from wastewater streams [143]. Adsorption techniques using bridgeable materials are innovative methods for removing dyes from industrial effluents. Incorporating nature-based nanoparticles or polymers with high adsorption capacities would also improve the separation efficiency of the filtration membranes and at the same time maintain their biodegradability [194]. In this regard, Tahazadeh et al. [176] employed porous carbon nanoparticles to improve the adsorption capacity of biodegradable CA membrane for Methylene Blue (MB) removal. The prepared membrane showed excellent reusability with 98.2% of MB removal and 76.03 LMH of water flux. Aside from dye removal, biodegradable composite membranes have been employed in several applications for heavy metal, pathogen organic, and inorganic matter removal [7,138,182,195,196,197,198,199]. Table 1 represents various studies that utilized biodegradable membranes in wastewater treatment applications.

4.2. Gas Separation

The separation of pure gases from gas mixtures is another critical application of biodegradable membranes. Gas separation in membrane-based applications mostly relies on the solution-diffusion mechanisms. Thus, the permeability and selectivity of each component in membranes play a crucial role in the separation process [2]. A common gas separation process is removing CO₂ and H₂S from natural gas or to recover N₂, H₂, and O₂ from gas mixtures [2,200]. Some functional groups in the membrane structure could facilitate gas transportation and improve the separation efficiency. For instance, amine groups in biodegradable membranes like chitosan and PVA could enhance the CO₂ separation from a gas mixture [200,201]. In fact, amine groups act as carriers for Co₂ molecules and improve the gas transport through the reversible reactions presented in Equation (1) [201].

CO₂ + NH₂ + H₂O ↔ HCO₃⁻ + NH₃⁺

(1)

Upon exposure to gas mixture, amino groups and water in the film react with carbon dioxide to form bicarbonate ions. As the bicarbonate ions passed through the film, they were converted back into carbon dioxide at the permeate side [201].

Furthermore, the morphology and structure of polymers influence gas selectivity and separation efficiency such that glassy polymers pose the biggest challenge to gas separation processes because of non-selective voids in their structure. Therefore, rubbery polymers are more favorable for light gas separation [200]. Biodegradable rubbery polymers like polyurethanes have been widely used in gas separation applications, owing to their film-forming property, excellent selectivity, and permeability [200,202,203]. Considering the effect of polymer structure, the free volume, packing density, and chain mobility would also affect the separation efficiency [204]. To address this challenge, biodegradable fillers with desirable selectivity would be incorporated into the polymeric membranes (mix matrix membranes) to modify the polymer structure and improve the gas permeability. In order to robust the permeability and selectivity or even mechanical and thermal stability of the membrane, a composition of biodegradable polymers (mix matrix membranes) would also be used in the separation process [200,201,204,205,206]. A number of studies have examined the use of biodegradable membranes in gas separation, as shown in Table 1.

4.3. Pervaporation

As one of the emerging technologies, Pervaporation has experienced rapid development in recent decades. This technology involves the membrane separation and liquid-to-vapor phase transition through the pressure difference between the feed and permeate sides (as the driving force) and works based on the difference between the diffusion/sorption of the components in the mixtures [207,208]. In order to create the pressure difference, a vacuum pump is employed at the permeate side, resulting in the vaporization and transport of the components with a higher affinity. Pervaporation is an easily performed, which is an environmentally friendly and energy-efficient process used to separate an organic compound from a dilute liquid mixture or an organic mixture with a close boiling point and dehydration of azeotropic- aqueous organic mixtures [208,209,210,211]. Biodegradable membranes have been increasingly used in pervaporation systems to improve the sustainability of the process. However, the efficiency and performance of this process are highly dependent upon the membrane’s selectivity and its ability to separate the particular components from liquid mixtures [207,209]. Membranes with polar characteristics and aromatic groups such as CS, PVA, agarose, alginate, and fluorinated polymers are favorable for organic dehydration due to their hydrophilic property and high water selectivity [211]. Conversely, hydrophobic membranes like poly-dimethyl siloxane [211] polyurethane urea [212] are more suitable to remove non-polar components (mostly organic matters) from dilute solutions. PLA, PHA, CS-based, PVA-based, and mixed matrix membranes are the most common biodegradable membranes for boiling azeotrope mixtures like methanol-methyl tert-butyl ether or methanol-toluene [210,213,214,215]. Table 2 summarizes the use of biodegradable membranes in various pervaporation processes.

4.4. Biomedical Applications

Bio-inspired and biodegradable membranes have been gaining increasing attention for their potential applications in biomedical fields, including tissue engineering (dental implants, rebuilding of bone, artificial skin, liver, nerve, etc.), drug delivery, and hemodialysis/hemodiafiltration due to their low toxicity, osteoconductivity, antibacterial properties, physical/mechanical properties, formability, versatility with multiple functions, and sterilization capability [9,216,217,218,219,220,221]. The main purpose of tissue engineering is to recover or replace damaged body tissues and organs [217]. A variety of bio-inspired materials are being investigated for use in tissue engineering. Biodegradability, bioadhesivity, and bioactivity of chitosan make it a highly biocompatible biopolymer suitable for use in a variety of biomedical applications [222]. Chitosan and chitin-based materials are commonly used in tissue engineering in a variety of physical forms since they are capable of accelerating the wound healing by providing the tissue cells with an extracellular microenvironment [9,223,224,225]. Vivcharenko et al. [225] prepared a biodegradable chitosan-based film and used it to stimulate the wound healing process and repair the injured skin. The results of their research demonstrated that the prepared chitosan/agarose film has all the required characteristics to be served as an artificial skin substitute. Biodegradable membranes have also exhibited to be promising candidates for drug delivery systems and improving the treatment efficiency [216,217,226]. Obtaining therapeutic effects in host cells requires control of the dose and localization of drug release by preparing the porous materials with relevant size scales (nanometer for small molecules to a few tens of nanometer for macromolecules’ delivery). In this regard, Bernards and Desai [226] used the template etching method to prepare a nanoporous membrane for drug delivery purposes using the poly(caprolactone) (PCL) biodegradable polymer. For the diffusion experiment, they employed sodium fluorescein (SFC) as the small molecule ($~$1 nm) and fluorescein isothiocyanate-labelled bovine serum albumin (FITC-BSA) as the macromolecule ($~$7 nm). FC and FITC-BSA released at a normal first-order diffusion (exponential release) and zero-order (constant release), respectively. The thin film porous PCL membrane fabricated in this study was found to be useful in flexible therapeutic devices. Hemodialysis is another process in which membranes are used to remove blood toxins. Conventional membranes have been widely used for hemodialysis; however, their hydrophobic nature could negatively affect their biocompatibility and cause blood clotting [178,227]. Cellulose based membranes with high hydrophilicity are still used for HD membranes production, despite of development of other types of polymeric materials for biomedical applications [168,228]. PES is still the most spread polymer for HM membrane fabrication, though its surface modification is required to improve its hemocompatibility [229,230]. Table 1 presents further studies on biomedical applications of biodegradable membranes.

4.5. Food Packaging

In recent decades, a number of studies have been conducted on the development and application of biodegradable membranes/films in food packaging due to their antimicrobial, non-toxicity, antifungal, controlled permeability, good flexibility, lightness, and transparency [10,145,223,231,232,233,234,235,236,237]. Antimicrobial packaging is a promising method to improve food safety by preventing the growth of pathogenic, spoilage microorganisms, and certain bacteria [231,238]. Among the biodegradable materials, chitosan and bacterial cellulose have shown excellent antimicrobial properties and have been extensively used in food packaging [223,232,235,237]. The chitosan membranes also proved to have a low permeability to O₂, N₂, and CO₂ and moderate permeability to water [223]. PLA is the most used biodegradable material for food packaging applications [239]. However, there are some limitations to using PLA films in the food industry, including its mechanical stability, antimicrobial, and barrier performance. In order to overcome those limitations, several techniques have been implemented, and PLA films have been modified with other biodegradable materials such as PHB, tea polyphenols (TP), and limonene [239,240,241,242,243]. PVA is another biodegradable polymer, commonly used with CS to fabricate the composite film for food packages owing to its good film-forming property [145,244,245]. Water and water vapor resistance property is a key factor for membranes/films employed in food packaging applications. Incorporating nanocellulose into polymeric membranes could improve the water barrier feature of biodegradable films by the creation of nanocrystal networks that prevent swelling and water absorption [234]. According to Chen et al. [246], the nanocellulose filled PVA films (1% nanocellulose loading) showed a 9.5% improvement in water vapor barrier property after 166 h of testing. An overview of biodegradable membranes in food packaging is presented in Table 1.

4.6. Fuel Cell

Fuel cells (FC) are a sustainable energy source with high efficiency that converts the chemical energy of the organic matters (fuels) into electricity and can replace fossil fuels [247]. This technology has a low environmental impact and can be employed in various devices to reduce global pollutions [2]. A fuel cell consists of an anodic and cathodic chamber as well as an electrolyte that separates the two chambers and facilitates the transfer of ions. In FC, membranes can be used as electrolytes, known as proton exchange membranes [248,249]. The most common membrane used in FC is Nafion, which has a high production cost and environmental impact [250]. Therefore, using the biodegradable membranes could be a promising alternative to Nafion. González-Pabón et al. [251] prepared biodegradable membranes using PVA, CS, and their compositions, and they compared their performance in a microbial fuel cell (MFC) to Nafion commercial membrane. According to their findings, PVA/CS membranes had a lower O₂ permeability compared to Nafion, resulting in maintaining the anaerobic condition in MFC. They also reported that the PVA/CS composite membrane led to higher power production and lower cost, as well as less environmental impact. Aside from their high cost and environmental impact, Nafion membranes have shown a high fuel crossover (methanol or hydrogen leakage) and low ion conductivity in electrolyte membrane FC. Chitosan and alginate-based bio-membranes are strong candidates to tackle this issue [30,250,252,253]. Smitha et al. [254] prepared a chitosan/sodium alginate composite membrane and used that for the methanol fuel cell. The composite membrane exhibited an excellent mechanical property with low methanol permeability (4.4 × 10⁻⁸ cm²/s for CS/SA and 27.6 × 10⁻⁸ cm²/s for Nafion117) and favorable proton conductivity. In addition, they noted that the membrane was fabricated using a simple and cost-effective technique. In another study [253], they prepared PVA/CA and PVA/CS composite membranes and evaluated their performance in methanol fuel cell. Similar to previous research, the composite membranes demonstrated lower methanol permeabilities in comparison with Nafion117. More researches on using biodegradable membranes in proton exchange membrane fuel cells are presented in Table 2.

Table 2. An overview of the applications of biodegradable membranes.

Polymer	Application	Advantage	Disadvantage	Ref
PLA/PBS/SNPs PLA/PPC/SNPs PLA/PHB/SNPs	Oil/water separation	Favourable thermal stability, high separation efficiency and turbidity removal (>98% and >89%, respectively)	Low metal ion and dissolved solid removal	[1]
CS	Oil/water separation	Excellent underwater superoleophobicity, stable wettability in various pH, >99% oil separation efficiency	Low diesel removal in saline solutions	[113]
CAF/CS/CF	Oil/water separation	Superhydrophilicity and underwater superoleophobicity property, >99% separation efficiency, good reusability, high recyclability		[116]
PLA	Oil/water separation	Superhydrophobic properties, excellent oil absorption capacity, antifouling property, not complicated preparation method		[179]
PLA	Oil/water separation	Superoleophilic and superhydrophobic property, excellent reusability	Low oil adsorption capacity	[189]
PLA/TiO2	Oil/water separation	Superhydrophilicity and underwater superoleophobicity, long-term behavior, excellent separation efficiency (>99%), high permeate flux, antifouling property	Low BSA adsorption capacity	[190]
PLA	Oil/water separation	Robust recyclability, excellent adsorption capacity		[191]
CA/PVP	Oil, dye, metal removal	Excellent removal efficiency for oil, dye and metal ions (>99%), long-term reusability, super hydrophilicity and underwater superoleophobicity		[192]
CA	Oil/water separation	Robust recyclability, antifouling property, high infuse flux, enhanced adsorption capacity	lower infused flux for solutions, containing crude oil	[193]
CA/MOFDPC	Dye removal	Excellent reusability, High MB removal, long term behaviour, high water flux	Lower tensile strength and flexibility compared to CA membrane	[176]
CA/Cotton	Dye removal	MB rejection of 98% over 8 filtration cycle, high durability, low fouling, high mechanical stability	Tested for low volume wastewater (5–10 mL, dead end filtration setup)	[186]
CS/PVA	Dye removal	Strong interaction between CS and PVA, higher tensile strength, stability in DI, acidic and alkaline medium	Lower thermal stability compared to pristine membranes	[8]
CS/GO	Dye removal	At least 95% of cationic MB removal	GO size affected the anionic MB removal such that the maximum removal efficiency was 64% for nanoscale GO	[187]
PLA/CS	Dye removal	Large porous framework though a nanofibrous structure, High removal capacity of 86.43 and 82.37 mg/g for rhodamine B and MB	Adsorption capacity reduction after fourth cycle	[255]
PCL/MXene	Dye removal	High hydrophilicity and water permeability by adding only 4 wt% of MXene, more than 99% or crystal violet removal	Lower CV rejection after adding MXene nanoparticles	[78]
CS/PVA/MMT	Heavy metal removal	High hydrophilicity and water flux, antibiofouling property, 84–88.34% of Chromium removal efficiency	Low removal efficacy after 1 h and at high pH (pH of 9)	[138]
PLA/PBS	Heavy metal removal	Heat resistance improvement, enhanced hydrophilicity and mechanical stability, about 83% of ion removal efficiency (cobalt and nickel), antifouling property		[182]
CS/PVA	Heavy metal removal	Improved chemical stability in acidic medium, enhanced hydrophilicity and anti-swelling, good adsorption recovery	Adsorption capacity decay by increasing the Cd (II) concentration (about 95% at 40 mg/L to 75% at 50 mg/L)	[195]
CS/Cellulose	Heavy metal removal	High removal efficiency of 85% and 94% for Pb and Cd, respectively, Excellent adsorption capacity for Pb, Cd	Low removal efficacy and adsorption capacity for Cr.	[256]
SCS/PVA	Heavy metal removal	Up to 90% of CU ion removal after 3 h, improved areal swelling	Low Ni ion removal	[196]
PLA-HAp/PDA	NOM removal	Antifouling properties, Flux improvement, thermal stability, more resistance to damage in harsh environment	Reversible fouling increased from 3.6% to 10.5 %	[7]
PLA	NOM removal	Enhanced BSA removal up to 92%, improved antifouling property, increased FRR from 57% to 93%	Lower water flux by increasing the PLA concentration	[180]
PCL/PBS	Wastewater treatment	Enhanced hydrophilicity and biodegradability, improved water flux and FRR, and pollution rejection	Lower mechanical properties compared to neat PCL	[197]
PLLA/PDLA/AlCl3	Pathogen removal	Excellent filtration efficiency, high porosity, small pore size, low pressure drops		[198]
PBS/CA/DEX	Dairy wastewater treatment	Enhanced porosity and hydrophilicity, robust permeate flux, antifouling property, >99% turbidity removal	Foulant rejection decreased, decreased mechanical property, low TDS removal	[199]
PU/PVA	Gas separation	Enhanced CO2 solubility, high CO2/N2 and CO2/CH4 selectivity	Low permeability for pure gases such as N2 and O2	[200]
CS/DS-PVA	Gas separation	Robust self-healing efficiency, enhanced CO2/N2 selectivity, amino groups increase the CO2 permeance	Lower CO2 and N2 permeability by increasing the PVA concentration	[201]
PU	Gas separation	Improved mechanical and chemical stability, high CO2 separation	Low CO2/H2 selectivity	[203]
PLA/PBS/MWCNT	Gas separation	Improved hydrophilicity, tensile strength, porosity, and crystallinity, enhanced pure gas permeability (Ar, CO2, H2, and N2), H2/N2 selectivity improvement	Low selectivity and no improvement for CO2/N2, Ar/N2, and CO2/Ar	[204]
PLA/layer silicate	Gas separation	Improved thermal and mechanical stability, enhanced pure gas permeability by increasing silicate concentration	Decreased CO2, N2 and O2 permeability by increasing clay content	[205]
CS/PVA	Gas separation	Improved specific surface area, high CO2 adsorption capacity		[206]
PVA/CNC	Gas separation	Increased CO2 permeability and CO2/N2 separation factor		[257]
CMS	Gas separation	High specific area, pore volume, and CO2 absorption capacity, improved N2 absorption,		[258]
CS/PHB/MWCNT	Pervaporation (1,4-dioxane/water)	Low swelling degree compared to pristine CS, improved mechanical properties, improved water selectivity by increasing the 1,4-dioxane concentration in feed	Not significant water selectivity improvement for mixed matrix membrane compared to pristine CS	[207]
SA/PCL/GO	Pervaporation (Alcohol/water)	Excellent membrane hydrophilicity and dehydration performance for alcohol-water, water flux and separation factor improvement	Increased swelling degree by increasing GO content	[208]
CS/FGS	Pervaporation (isopropanol/water and ethanol-water)	Good isopropanol and ethanol barrier, good water selectivity	Low water permeability	[209]
PLA	Pervaporation (MeOH/MTBE)	Good mechanical properties, good methanol selectivity	Enrichment factor dropped drastically by 75% as the methanol concentration changed from 1 wt% to 10 wt%	[210]
PUU	Pervaporation (phenol/water)	Low swelling degree, good separation factor and flux for phenolic components	Separation factor dropped by 60% as the phenol concentration changed from 0.1% to 0.4%	[212]
PHA/PHBHV	Pervaporation (MeOH/MTBE)	Good mechanical properties, favourable methanol selectivity	Reduced methanol selectivity by adding the additives (PEG, EBO) to the polymer solution	[215]
CS/PVA/NH2-MWCNT	Pervaporation (Isopropyl alcohol/water)	Improved mechanical property and separation factor and reduced swelling degree by adding NH2-MWCNT, excellent water flux and PSI	Lower separation factor compared to similar studies	[259]
PLA	Pervaporation (MeOH/MTBE)	Good mechanical and chemical stability, good methanol selectivity	Low permeate flux compared to similar researches	[260]
PVA/APS/MBA	Pervaporation (Ethanol/water)	Reduced swelling rate, high separation efficiency (95%), water flux, and membrane durability	Reduced selectivity by increasing the feed temperature (from 40 °C to 70 °C)	[261]
PGS APS	Pervaporation (Organic solvents/water)	Higher separation factor and PSI compared to commercial membrane (PDMS), good stability		[262]
PLA/Fe-MOF	Pervaporation (MeOH/MTBE)	Improved methanol selectivity by adding Fe-MO and pressure increase (from 0 to 7.5 mbar)	Lower flexibility and mechanical strength by adding 0.5 wt% of Fe-MO, reduced selectivity by increasing the feed temperature (from 25 °C to 45 °C)	[263]
Agarose	Pervaporation (Organic solvents/water)	High water flux and permselectivity		[264]
CS/Agarose	Artificial skin	Nontoxicity, high exudate absorption capacity, high elastic deformations, extracellular matrix similarity, support reproduction of skin fibroblast		[225]
ALG/GC	Hepatocyte attachment	Increase spheroid formation, higher viability and mechanical property rather than alginate sponge		[265]
PLLA/CS	Periodontitis treatment	Higher hydrophilicity, biocompatibility and bioactivity rather than electrospun PLLA membrane, higher cell reproduction, fibroblast barrier (mitigate the destructive effect of fibroblast on tissue recovery)	Lower degradation rate due to the presence of electrospun PLLA	[266]
CS/PEEK	Culturing the liver cells	Favourable microenvironment for liver cells, higher cell proliferation in 8–11 days, higher level of specific functions for longer time (compared to former substrates like collagen and PSCD)		[267]
PCE	Biomedical implants	Elastomeric behavior, antibacterial activity, biomimetic mechanical property, photoluminescent capacity (favourable for real-time monitoring), high cytocompatibility and hemocompatibility, low inflammatory response		[268]
GEL	Tissue regeneration	High tensile strength, improved water resistance, good biocompatibility	Low elastic and ductile characteristic in dry state	[269]
PU/PGSAP	Nerve tissue	Increasing the Schwann cells’ (SCs) myelin gene expressions, higher neurotrophin secretion, inducing neurite growth and elongation of PC12 cells, reducing the intracellular Ca+2 level		[270]
PCL	Drug delivery	Constrained diffusion for drug release control, no negative effect on cells’ growth, cellular compatibility	Low diffusivity of water-soluble components like FS and FITC-BSA due to the hydrophobic nature of PCL	[226]
CA/Phospholipid	Blood purification	Good water and solute permeability, lower protein adsorption (compared to CA and commercial PES membranes), sharp molecule weight cut-off, permselectivity and antifouling property over a long-term filtration, good hemocompatibility	Lower dye and protein retention (compare to pristine CA membrane)	[178]
PLA (blended with three different copolymers: PA, PD, and PH)	Hemodiafiltration	Desirable protein and urea retention, improved hydrophilicity and permeate flux (compared to PLA), antifouling property, hemocompatibility	Relatively low lysozyme retention (17.4%) for PLA/PA	[184]
PLA/DA/HEP	Hemodialysis	Improved hemocompatibility, low platelet adhesion and hemolysis ratio, reasonable urea and BSA separation	Relatively low lysozyme retention (18%), it was bioincompatible with human blood.	[227]
PLA/ DA/GOC	Hemodialysis	High hydrophilicity and electronegativity, improved hemocompatibility, low platelet adhesion and hemolysis ratio, longer plasma recalcification time, high BSA separation	Lower lysozyme retention compared to pristine PLA, the hemocompatibility was not proved for human serum.	[228]
CS/PVA	Food packaging	Good antimicrobial property, molecular miscibility between PVA and CS, improved crystallinity		[238]
PLA/limonene	Food packaging	Improved water barrier ability, excellent flexibility	Reduced oxygen barrier (still acceptable for food packaging)	[240]
PLA/TP	Food packaging	Improved antioxidant and antimicrobial activity	Reduced tensile strength and breakage elongation	[241]
PLA/PHB/limonene	Food packaging	Robust elongation at break, improved oxygen barrier, increased hydrophobicity		[242]
PLA/ATBC/CS	Food packaging	Improved thermal and mechanical properties, reduced brittleness, increased transparency, good antifungal and antibacterial activity	Low water vapour barrier	[243]
PVA/CS	Food packaging	Good antimicrobial activity, improved thermal property	Reduced film stretchability, reduced water vapour barrier	[244]
PVA/CS/Silica	Food packaging	Improved tensile strength, increased oxygen and moisture barrier	Deferred resolving time	[245]
PVA/CS	Microbial fuel cell	Compared to Nafion: Reduced O2 permeability, lower cost, higher power generation, higher water uptake capacity environmentally friendly	Low proton conductivity	[251]
CSS	Methanol fuel cell	Much lower methanol cross over compared to Nafion 112, improved mechanical strength	Low breaking elongation compared to Nafion112	[30]
PVA/CS/SA	Methanol fuel cell	Low methanol permeability, high ion exchange capacity, excellent thermal and mechanical stability	Low proton conductivity compared to Nafion117	[253]
CS/SA	Methanol fuel cell	Low methanol permeability, excellent mechanical property, low cost	Low proton conductivity compared to Nafion117	[254]
Graphene-PVA/CS	Methanol fuel cell	Improved methanol barrier, conductivity and ion selectivity compared to Nafion117	Lower elongation break by adding graphene	[271]
CCS	Direct borohydride fuel cell	Higher ionic conductivity and power performance compared to Nafion212, cost effective, stable performance for >100 h	Higher borohydride crossover compared to Nafion212	[272]
CS/PEO	Fuel cell	Improved conductivity, reduced swelling, low production cost		[273]
PVA/GO	Ethanol fuel cell	Reduced water uptake and ethanol permeability, and improved proton conductivity by adding GO, higher power density compared to Nafion117	Reduced elongation at break ratio	[274]
CS/PMC	Microbial fuel cell	Lower bioelectricity start-up time compared to Nafion117 and Agar salt bridge, acceptable power density, improved retention time	Antifouling property was not investigated	[275]

Table 2. An overview of the applications of biodegradable membranes.

Polymer	Application	Advantage	Disadvantage	Ref
PLA/PBS/SNPs PLA/PPC/SNPs PLA/PHB/SNPs	Oil/water separation	Favourable thermal stability, high separation efficiency and turbidity removal (>98% and >89%, respectively)	Low metal ion and dissolved solid removal	[1]
CS	Oil/water separation	Excellent underwater superoleophobicity, stable wettability in various pH, >99% oil separation efficiency	Low diesel removal in saline solutions	[113]
CAF/CS/CF	Oil/water separation	Superhydrophilicity and underwater superoleophobicity property, >99% separation efficiency, good reusability, high recyclability		[116]
PLA	Oil/water separation	Superhydrophobic properties, excellent oil absorption capacity, antifouling property, not complicated preparation method		[179]
PLA	Oil/water separation	Superoleophilic and superhydrophobic property, excellent reusability	Low oil adsorption capacity	[189]
PLA/TiO2	Oil/water separation	Superhydrophilicity and underwater superoleophobicity, long-term behavior, excellent separation efficiency (>99%), high permeate flux, antifouling property	Low BSA adsorption capacity	[190]
PLA	Oil/water separation	Robust recyclability, excellent adsorption capacity		[191]
CA/PVP	Oil, dye, metal removal	Excellent removal efficiency for oil, dye and metal ions (>99%), long-term reusability, super hydrophilicity and underwater superoleophobicity		[192]
CA	Oil/water separation	Robust recyclability, antifouling property, high infuse flux, enhanced adsorption capacity	lower infused flux for solutions, containing crude oil	[193]
CA/MOFDPC	Dye removal	Excellent reusability, High MB removal, long term behaviour, high water flux	Lower tensile strength and flexibility compared to CA membrane	[176]
CA/Cotton	Dye removal	MB rejection of 98% over 8 filtration cycle, high durability, low fouling, high mechanical stability	Tested for low volume wastewater (5–10 mL, dead end filtration setup)	[186]
CS/PVA	Dye removal	Strong interaction between CS and PVA, higher tensile strength, stability in DI, acidic and alkaline medium	Lower thermal stability compared to pristine membranes	[8]
CS/GO	Dye removal	At least 95% of cationic MB removal	GO size affected the anionic MB removal such that the maximum removal efficiency was 64% for nanoscale GO	[187]
PLA/CS	Dye removal	Large porous framework though a nanofibrous structure, High removal capacity of 86.43 and 82.37 mg/g for rhodamine B and MB	Adsorption capacity reduction after fourth cycle	[255]
PCL/MXene	Dye removal	High hydrophilicity and water permeability by adding only 4 wt% of MXene, more than 99% or crystal violet removal	Lower CV rejection after adding MXene nanoparticles	[78]
CS/PVA/MMT	Heavy metal removal	High hydrophilicity and water flux, antibiofouling property, 84–88.34% of Chromium removal efficiency	Low removal efficacy after 1 h and at high pH (pH of 9)	[138]
PLA/PBS	Heavy metal removal	Heat resistance improvement, enhanced hydrophilicity and mechanical stability, about 83% of ion removal efficiency (cobalt and nickel), antifouling property		[182]
CS/PVA	Heavy metal removal	Improved chemical stability in acidic medium, enhanced hydrophilicity and anti-swelling, good adsorption recovery	Adsorption capacity decay by increasing the Cd (II) concentration (about 95% at 40 mg/L to 75% at 50 mg/L)	[195]
CS/Cellulose	Heavy metal removal	High removal efficiency of 85% and 94% for Pb and Cd, respectively, Excellent adsorption capacity for Pb, Cd	Low removal efficacy and adsorption capacity for Cr.	[256]
SCS/PVA	Heavy metal removal	Up to 90% of CU ion removal after 3 h, improved areal swelling	Low Ni ion removal	[196]
PLA-HAp/PDA	NOM removal	Antifouling properties, Flux improvement, thermal stability, more resistance to damage in harsh environment	Reversible fouling increased from 3.6% to 10.5 %	[7]
PLA	NOM removal	Enhanced BSA removal up to 92%, improved antifouling property, increased FRR from 57% to 93%	Lower water flux by increasing the PLA concentration	[180]
PCL/PBS	Wastewater treatment	Enhanced hydrophilicity and biodegradability, improved water flux and FRR, and pollution rejection	Lower mechanical properties compared to neat PCL	[197]
PLLA/PDLA/AlCl3	Pathogen removal	Excellent filtration efficiency, high porosity, small pore size, low pressure drops		[198]
PBS/CA/DEX	Dairy wastewater treatment	Enhanced porosity and hydrophilicity, robust permeate flux, antifouling property, >99% turbidity removal	Foulant rejection decreased, decreased mechanical property, low TDS removal	[199]
PU/PVA	Gas separation	Enhanced CO2 solubility, high CO2/N2 and CO2/CH4 selectivity	Low permeability for pure gases such as N2 and O2	[200]
CS/DS-PVA	Gas separation	Robust self-healing efficiency, enhanced CO2/N2 selectivity, amino groups increase the CO2 permeance	Lower CO2 and N2 permeability by increasing the PVA concentration	[201]
PU	Gas separation	Improved mechanical and chemical stability, high CO2 separation	Low CO2/H2 selectivity	[203]
PLA/PBS/MWCNT	Gas separation	Improved hydrophilicity, tensile strength, porosity, and crystallinity, enhanced pure gas permeability (Ar, CO2, H2, and N2), H2/N2 selectivity improvement	Low selectivity and no improvement for CO2/N2, Ar/N2, and CO2/Ar	[204]
PLA/layer silicate	Gas separation	Improved thermal and mechanical stability, enhanced pure gas permeability by increasing silicate concentration	Decreased CO2, N2 and O2 permeability by increasing clay content	[205]
CS/PVA	Gas separation	Improved specific surface area, high CO2 adsorption capacity		[206]
PVA/CNC	Gas separation	Increased CO2 permeability and CO2/N2 separation factor		[257]
CMS	Gas separation	High specific area, pore volume, and CO2 absorption capacity, improved N2 absorption,		[258]
CS/PHB/MWCNT	Pervaporation (1,4-dioxane/water)	Low swelling degree compared to pristine CS, improved mechanical properties, improved water selectivity by increasing the 1,4-dioxane concentration in feed	Not significant water selectivity improvement for mixed matrix membrane compared to pristine CS	[207]
SA/PCL/GO	Pervaporation (Alcohol/water)	Excellent membrane hydrophilicity and dehydration performance for alcohol-water, water flux and separation factor improvement	Increased swelling degree by increasing GO content	[208]
CS/FGS	Pervaporation (isopropanol/water and ethanol-water)	Good isopropanol and ethanol barrier, good water selectivity	Low water permeability	[209]
PLA	Pervaporation (MeOH/MTBE)	Good mechanical properties, good methanol selectivity	Enrichment factor dropped drastically by 75% as the methanol concentration changed from 1 wt% to 10 wt%	[210]
PUU	Pervaporation (phenol/water)	Low swelling degree, good separation factor and flux for phenolic components	Separation factor dropped by 60% as the phenol concentration changed from 0.1% to 0.4%	[212]
PHA/PHBHV	Pervaporation (MeOH/MTBE)	Good mechanical properties, favourable methanol selectivity	Reduced methanol selectivity by adding the additives (PEG, EBO) to the polymer solution	[215]
CS/PVA/NH2-MWCNT	Pervaporation (Isopropyl alcohol/water)	Improved mechanical property and separation factor and reduced swelling degree by adding NH2-MWCNT, excellent water flux and PSI	Lower separation factor compared to similar studies	[259]
PLA	Pervaporation (MeOH/MTBE)	Good mechanical and chemical stability, good methanol selectivity	Low permeate flux compared to similar researches	[260]
PVA/APS/MBA	Pervaporation (Ethanol/water)	Reduced swelling rate, high separation efficiency (95%), water flux, and membrane durability	Reduced selectivity by increasing the feed temperature (from 40 °C to 70 °C)	[261]
PGS APS	Pervaporation (Organic solvents/water)	Higher separation factor and PSI compared to commercial membrane (PDMS), good stability		[262]
PLA/Fe-MOF	Pervaporation (MeOH/MTBE)	Improved methanol selectivity by adding Fe-MO and pressure increase (from 0 to 7.5 mbar)	Lower flexibility and mechanical strength by adding 0.5 wt% of Fe-MO, reduced selectivity by increasing the feed temperature (from 25 °C to 45 °C)	[263]
Agarose	Pervaporation (Organic solvents/water)	High water flux and permselectivity		[264]
CS/Agarose	Artificial skin	Nontoxicity, high exudate absorption capacity, high elastic deformations, extracellular matrix similarity, support reproduction of skin fibroblast		[225]
ALG/GC	Hepatocyte attachment	Increase spheroid formation, higher viability and mechanical property rather than alginate sponge		[265]
PLLA/CS	Periodontitis treatment	Higher hydrophilicity, biocompatibility and bioactivity rather than electrospun PLLA membrane, higher cell reproduction, fibroblast barrier (mitigate the destructive effect of fibroblast on tissue recovery)	Lower degradation rate due to the presence of electrospun PLLA	[266]
CS/PEEK	Culturing the liver cells	Favourable microenvironment for liver cells, higher cell proliferation in 8–11 days, higher level of specific functions for longer time (compared to former substrates like collagen and PSCD)		[267]
PCE	Biomedical implants	Elastomeric behavior, antibacterial activity, biomimetic mechanical property, photoluminescent capacity (favourable for real-time monitoring), high cytocompatibility and hemocompatibility, low inflammatory response		[268]
GEL	Tissue regeneration	High tensile strength, improved water resistance, good biocompatibility	Low elastic and ductile characteristic in dry state	[269]
PU/PGSAP	Nerve tissue	Increasing the Schwann cells’ (SCs) myelin gene expressions, higher neurotrophin secretion, inducing neurite growth and elongation of PC12 cells, reducing the intracellular Ca+2 level		[270]
PCL	Drug delivery	Constrained diffusion for drug release control, no negative effect on cells’ growth, cellular compatibility	Low diffusivity of water-soluble components like FS and FITC-BSA due to the hydrophobic nature of PCL	[226]
CA/Phospholipid	Blood purification	Good water and solute permeability, lower protein adsorption (compared to CA and commercial PES membranes), sharp molecule weight cut-off, permselectivity and antifouling property over a long-term filtration, good hemocompatibility	Lower dye and protein retention (compare to pristine CA membrane)	[178]
PLA (blended with three different copolymers: PA, PD, and PH)	Hemodiafiltration	Desirable protein and urea retention, improved hydrophilicity and permeate flux (compared to PLA), antifouling property, hemocompatibility	Relatively low lysozyme retention (17.4%) for PLA/PA	[184]
PLA/DA/HEP	Hemodialysis	Improved hemocompatibility, low platelet adhesion and hemolysis ratio, reasonable urea and BSA separation	Relatively low lysozyme retention (18%), it was bioincompatible with human blood.	[227]
PLA/ DA/GOC	Hemodialysis	High hydrophilicity and electronegativity, improved hemocompatibility, low platelet adhesion and hemolysis ratio, longer plasma recalcification time, high BSA separation	Lower lysozyme retention compared to pristine PLA, the hemocompatibility was not proved for human serum.	[228]
CS/PVA	Food packaging	Good antimicrobial property, molecular miscibility between PVA and CS, improved crystallinity		[238]
PLA/limonene	Food packaging	Improved water barrier ability, excellent flexibility	Reduced oxygen barrier (still acceptable for food packaging)	[240]
PLA/TP	Food packaging	Improved antioxidant and antimicrobial activity	Reduced tensile strength and breakage elongation	[241]
PLA/PHB/limonene	Food packaging	Robust elongation at break, improved oxygen barrier, increased hydrophobicity		[242]
PLA/ATBC/CS	Food packaging	Improved thermal and mechanical properties, reduced brittleness, increased transparency, good antifungal and antibacterial activity	Low water vapour barrier	[243]
PVA/CS	Food packaging	Good antimicrobial activity, improved thermal property	Reduced film stretchability, reduced water vapour barrier	[244]
PVA/CS/Silica	Food packaging	Improved tensile strength, increased oxygen and moisture barrier	Deferred resolving time	[245]
PVA/CS	Microbial fuel cell	Compared to Nafion: Reduced O2 permeability, lower cost, higher power generation, higher water uptake capacity environmentally friendly	Low proton conductivity	[251]
CSS	Methanol fuel cell	Much lower methanol cross over compared to Nafion 112, improved mechanical strength	Low breaking elongation compared to Nafion112	[30]
PVA/CS/SA	Methanol fuel cell	Low methanol permeability, high ion exchange capacity, excellent thermal and mechanical stability	Low proton conductivity compared to Nafion117	[253]
CS/SA	Methanol fuel cell	Low methanol permeability, excellent mechanical property, low cost	Low proton conductivity compared to Nafion117	[254]
Graphene-PVA/CS	Methanol fuel cell	Improved methanol barrier, conductivity and ion selectivity compared to Nafion117	Lower elongation break by adding graphene	[271]
CCS	Direct borohydride fuel cell	Higher ionic conductivity and power performance compared to Nafion212, cost effective, stable performance for >100 h	Higher borohydride crossover compared to Nafion212	[272]
CS/PEO	Fuel cell	Improved conductivity, reduced swelling, low production cost		[273]
PVA/GO	Ethanol fuel cell	Reduced water uptake and ethanol permeability, and improved proton conductivity by adding GO, higher power density compared to Nafion117	Reduced elongation at break ratio	[274]
CS/PMC	Microbial fuel cell	Lower bioelectricity start-up time compared to Nafion117 and Agar salt bridge, acceptable power density, improved retention time	Antifouling property was not investigated	[275]

5. Green Membranes Challenges and Limitations

Despite their wide range of potential applications, biodegradable membranes pose a number of challenges and limitations. The biocompatibility, mechanical properties, and degradability of the membranes, in terms of degradation behavior and degradation rate, must be thoroughly evaluated to determine their suitability for the desired applications [276]. Degradation rate/time is directly related to the type of the materials, the concentration of enzymes and microorganisms, pH, humidity, oxygen, and light levels in the surrounding environment. In other words, the degradability of different materials might vary depending on their surrounding environment [277]. The long-term biodegradation of some membranes would cause serious environmental problems owing to the increased waste disposal in nature. On the other hand, in biomedical applications where drugs should be slowly released into the environment or membranes should not degrade prior to tissue regeneration, rapid membrane degradation would have negative impacts. Thus, a thorough understanding of the membrane degradation mechanisms is necessary to determine the membrane’s performance in a particular application.

Another challenge is incorporating non-biodegradable or toxic materials into the biodegradable membranes (especially bio-based materials) through the blending, crosslinking, and copolymerization processes to improve their mechanical stability, thermal resistance, or to gain structural diversity and specific properties. The composition of green membranes with non-environment friendly materials results in secondary pollution since the biodegradable part degrades and the non-biodegradable material converts into small molecules that would persist in the environment for a very long time [2,10,231]. In addition, some biodegradable membranes are water-soluble. Therefore, degraded materials dissolve in water (ground/surface water) due to membrane breakdown and negatively affect the water quality and plant growth [2].

The economic performance of bio-based biodegradable membrane production has been evaluated and compared with conventional membrane production in several studies. Although it is essential to develop viable biodegradable alternatives in order to overcome the challenges associated with conventional membranes, bio-based biodegradable polymers have not been accounted for more than 1% of the total plastics production (335 million tons) in 2019. This is mainly due to their uneconomical production processes and expensive raw materials compared to low-cost fossil-fuel-based plastics [278,279,280,281].

6. Future Trends

Biodegradable polymers have been considered sustainable materials and proved to be promising alternatives for conventional membranes. As mentioned earlier, the biodegradability and degradation rate of the green membranes depend on the environment in which they are used. Biodegradable membranes may also cause waste disposal once the required conditions for their degradation are not provided. Thus, it is necessary to investigate the presence of various enzymes and microorganisms in different environments, their operating mechanisms, and required physical conditions to design and prepare a suitable membrane with enhanced biodegradation ability in nature. Over the past decade, society has increasingly moved toward environment-friendly materials that minimize the reliance on single-use polymeric membranes. Therefore, besides their biodegradability, the reusability of the membranes also plays a crucial role in preventing increased waste disposal. It is important to note that thermal and mechanical stability and swelling rates of biodegradable membranes should be thoroughly investigated. In order to improve the performance of the biodegradable membranes and non-toxic and biodegradable additives, nanoparticles and cross-linking agents are required to be incorporated into the membranes. These environmentally responsible materials preserve membrane sustainability and improve its biodegradation rate in nature.

Biodegradable membranes should also compete with conventional membranes in terms of production cost. In some cases, biodegradable membranes (especially biomimetic membranes) are manufactured on a pilot-scale due to their unique raw materials, resulting in high production costs. However, in order to run a sustainable experimental application, tons of those membranes must be produced. Thus, efficient scale-up models are required to fully address the production cost of biodegradable membranes [117]. In addition to existing biodegradable membranes, designing a new sustainable membrane with competitive performance and scalability needs to be explored in future studies.

7. Conclusions

Biodegradable membranes have proven to be an alternative to conventional membranes, especially in terms of permeate flux and antifouling properties. It has been successfully employed in various applications including wastewater treatment, gas separation, pervaporation, tissue engineering, food packaging, and fuel cell. In many cases, biodegradable membranes have shown comparable results to the commercial membranes such as PES, PVDF, and PTFE. The possibility to synthesize biodegradable membranes as well as to modify natural materials opens a wide opportunity to adjust and control the properties of resulted membranes, which significantly enlarges their application. One of the directions to create biodegradable materials is to incorporate biodegradable bonds into the initial chemical structure to retain the physical properties close to the properties of original material. Nevertheless, there are still a number of challenges and limitations associated with biodegradable membranes, including their degradation rate and its composition with non-biodegradable materials, which results in the waste disposal and secondary pollutions. Further studies are needed to determine the degradation mechanisms and behavior of microorganisms and enzymes under different environmental conditions. As biodegradable membranes are typically produced on pilot scales, efficient scale-up models are also needed to scale up the production process.