Chitin, Chitosan and Its Derivatives: Antimicrobials and/or Mitigators of Water

Abstract

Antimicrobial resistance (AMR) is a major global health problem, exacerbated by the excessive and inappropriate use of antibiotics in human medicine, animal care and agriculture. Therefore, new strategies and compounds are needed to overcome this issue. In this view, it may be appropriate to reconsider existing biomaterials to alleviate antibiotic overuse. Chitin, a naturally abundant amino mucopolysaccharide, is a poly-β-1, 4-N-acetylglucosamine (GlcNAc). It is a white, hard, inelastic, nitrogenous polysaccharide and the major source of surface pollution in coastal areas. Chitosan derives from the partial N-deacetylation of chitin and originates from the shells of crustaceans and the fungi cell walls. It is a nontoxic natural antimicrobial polymer approved by GRAS (Generally Recognized as Safe by the United States Food and Drug Administration). Chitin and chitosan, as non-toxic biopolymers, are useful compounds for wastewater treatment to remove pollutants, such as pharmaceuticals, heavy metals and dyes. The described features make these biopolymers intriguing compounds to be investigated for their application as antibacterials.

1. Introduction

The rise in antimicrobial resistance (AMR) in bacteria that are important human pathogens and the spread of resistance from the closed environment of hospitals to open communities are considered a hazardous threat to human health. One of the main factors determining the emergence and spread of AMR is the imprudent use of antibiotics, which determines a selective pressure towards the affected bacteria [1]. AMR has emerged as one of the most prominent “One Health” strategies, “One Health Joint Plan of Action (2022–2026)”, which leverages transdisciplinary knowledge and experience with multisectoral collaboration supported by transnational cooperation to ameliorate the human, animals and the environment health [2]. AMR is principally related to the nosocomial emergence of a group of pathogens designated with the acronym “ESKAPE” [3,4] by the Hospital and Infectious Diseases Society of America (IDSA), including both Gram-positive and Gram-negative bacteria, namely Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. [5]. The design and synthesis of new antibacterial molecules is one of the research priorities to tackle AMR worldwide, even though molecules with new chemical structures and/or mechanisms of action are increasingly rare. In any case, the use of known antibacterials, such as chitosan and chitin, even by improving their performance, seems to be more convenient. In this context, the importance of chitin and chitosan to mitigate AMT and alleviate antibiotic overuse should be underlined [6]. Chitin and chitosan are abundant biopolymers in the biosphere as essential components of the exoskeletons of many organisms, such as arthropods, crustaceans (shrimp and crabs), insects (beetles, ants, brachiopods, scorpions and cockroaches) and algae (green and brown algae), and as by-products of the global seafood industry [7,8,9]. They are also found in the cell walls of fungi and yeasts. Recently, black soldier fly (Hermetia illucens) culture has been suggested as an economically viable and environmentally friendly source of chitin and chitosan, especially considering the insects’ rapid growth and high biomass yield [10]. Chitin and chitosan could be used in concrete to improve the physicochemical and rheological properties in the construction industry [11]. Chitin and chitosan from North Pacific krill were recently extracted, characterized and suggested as promising compounds for applications in food packaging [12]. Chitosan is produced from chitin deacetylation (Figure 1) and the degree of deacetylation (DD) and acetylation (DA) characterize the chitin/chitosan biopolymers, as well as their degree of polymerization or source.

Chitin and chitosan have a wide spectrum of utilizations, because of their abundance, nonpoisonous effects and good biocompatibility. Several uses have been described in diverse fields, such as bioengineering, agriculture, papermaking, food industry, and textile products. Both biopolymers show limited absorption into the bloodstream, with smaller particles exhibiting better bioavailability [13]. Chitin resists enzymatic degradation in the stomach, but is subjected to microbial fermentation in the large intestine, producing beneficial metabolites. Chitosan is more soluble in the alkaline environment of the small intestine and is more susceptible to enzymatic action. Chitin and chitosan possess anti-inflammatory, antimicrobial and immune-modulating properties and impact cholesterol levels. The anti-inflammatory properties derive from interaction with immune cells, influencing cytokine production and modulating immune responses, which may mitigate conditions characterized by chronic inflammation [14]. These biopolymers can impact cholesterol levels by binding dietary fats and reducing lipid absorption. The antimicrobial properties have been demonstrated against both Gram-positive and Gram-negative bacteria. The most studied are Staphylococcus aureus and Escherichia coli, respectively [15,16,17,18], but some studies also described the antibacterial activity against Staphylococcus epidermidis, Proteus mirabilis, Pseudomonas aeruginosa [19], Salmonella spp., [20] Sarcina sp. [21], methicillin-resistant Staphylococcus aureus (MRSA) [22], Enterococcus faecalis [23], Vibrio cholerae, Shigella dysenteriae, Prevotella melaninogenica and Bacteroides fragilis [24], and fungi [25,26,27]. Moreover, the antimicrobial properties contribute to gut health by controlling harmful pathogens and promoting beneficial microbiota. The employment of chitin and chitosan, as biodegradable and environmentally friendly polymers, is considered also suitable to remove synthetic polymers, which are produced up to 140, 106 tons each year worldwide [28]. Several applications have been reported for chitin and chitosan-based biomaterials for the removal of dyes, pharmaceuticals and metals, also including toxic heavy ones, from water [29,30]. Chitin market currently represents USD ~7900 million, meaning that it has a substantial commercial interest [31]. The application and transformation of what was considered waste has now led to valorization, creating opportunities to capitalize on this co-product in new market segments [32,33]. As an adsorbent, chitosan is preferred to chitin; indeed, although chitin is cheaper, more stable and ready to be recovered than chitosan, its utilization as an adsorbent is restricted due to its relatively low adsorption efficiency [34,35]. The usefulness of chitosan spans through many fields like biomedicine, pharmacy, food industry, and other fields listed in Figure 2 [36].

Several efforts have been made by environmental scientists to identify possible commercial sources of chitin and chitosan in nature as alternative over time, given the disadvantages of the current commercial source, such as seasonality and competition for other uses [37]. This review explores the extensive health benefits and applications of chitin and chitosan, providing a detailed examination of their chemical compositions, dietary sources, and applications, and critically assessing their health-promoting effects in the context of human well-being. Some recent studies (published in the last year) regarding the antibacterial activity of chitosan-based materials and removal activity of dyes, pharmaceuticals and heavy metals from water are reported, in order to highlight the importance and usefulness of chitin and chitosan.

2. Chitin and Chitin Deacetylases

Chitin is one of the most abundant existing biopolymers [38]. It is the second major biopolymer on Earth after cellulose and is mainly produced as a byproduct in shellfish industry, with 1011 tons annually biosynthesized by living organisms as a structural component [39]. Mushrooms possess a significant content of chitin, and many authors reported that the highest content is present in the stalks [40]. Chitin is an essential component of invertebrates but may also be present in vertebrates. There is a report—based on lectin binding, endo-chitinase binding and enzymatic degradation studies—indicating that the epidermal cuticle of a vertebrate, a blenny fish called Paralipophrys trigloides, is chitinous [41]. Throughout the Black Soldier Fly (Hermetia illucens) life cycle, different by-products can be found, mainly in the form of chitin-rich biomass, thus providing a potential stable source of chitin and its derivatives [42]. Several reports also describe that chitin can be obtained from Colorado potato larvae and adults, aquatic invertebrates beetles, resting eggs of Daphnia longispina, Daphnia magna and Daphnia similis [43,44], spider crab shells [45], insects [46] such as Melolontha melolontha [47], Orthoptera species [48], wings of cockroach [49], and grasshopper species [50]. Many organisms possess chitin deacetylase (CDA: E.C. 3.5.1.41), an enzyme that catalyzes the conversion of chitin to chitosan by deacetylation of N-acetylglucosamine groups [51].

2.1. History of Chitin

The name chitin originates from the Greek word “chiton”, which means “mail coat” [37]. The discovery of chitin dates back to the 1980s. Specifically, in 1799 the English researcher A. Hachett reported the existence of a “material particularly resistant to usual chemicals” extracted from the shells of mollusks, crabs, lobsters, prawns and crayfish. Chitin was firstly described in 1811 by Henri Braconnot, a French chemist and professor of natural history who was the director of the Botanical Gardens at the Academy of Sciences in Nancy. During his studies on the composition of edible mushrooms and their nutritional value, by serendipity he extracted a substance from mushrooms that did not dissolve in sulphuric acid and that contained a substantial fraction of nitrogen. He called it “fungine” [52]. In 1823, Antoine Odier found a similar material in the cuticle of insects and plants and named it “chitin”, from the Greek word, meaning “tunic” or “envelope” [53]. In 1843, Jean Louis Lassaigne demonstrated the presence of nitrogen in chitin, while working on the exoskeleton of silkworm butterfly Bombix mori [54]. Crini (2019) [55] published an essay entitled “Historical landmarks in the discovery of chitin” that describes the 220 years of the development of chitin.

2.2. Chemistry of Chitin

Chitin is a highly insoluble material resembling cellulose in its solubility and low chemical reactivity, indeed it may be considered as cellulose with hydroxyl at position C-2 replaced by an acetamido group. Chitin is a polysaccharide consisting of N-acetyl-D-glucosamine monomers (GlcNAc) and non-acetylated D-glucosamine (GlcN) units linked by β(1,4) glycosidic linkages, with a molar GlcNAc fraction >50% [56,57]. Due to this linkage, chitin is extremely robust towards chemical and biological aggression, indeed the β(1,4) chitin bond is similar to the one found in cellulose, contrary to starch that is much more easily digested by enzymes of several (micro)organisms than cellulose and presents an α(1,4) covalent bond between its monomeric units. The N-acetyl moieties of the glucosamine monomeric units of chitin confer extremely poor solubility properties to chitin, making it difficult to process it, thus limiting its potential applications [58]. Chitin is biocompatible, biodegradable and bioabsorbable, and presents antibacterial and wound healing activities along with low immunogenicity [59]. It is also studied in diverse fields, such as food technology, material science, microbiology, agriculture, wastewater treatment, drug delivery systems, tissue engineering and bio nanotechnology [60,61,62].

Chitin has three different allomorphs, α, β and γ, differing in the orientation of the respective polymer chains (Figure 3). The α-chitin is the most abundant and resilient and is formed by anti-parallel aligned polysaccharide chains, whereas the sugar chains are ordered in a parallel manner in β-chitin, thus exhibiting weaker intramolecular interactions. The γ-allomorph of chitin is represented by a mixture of parallel and anti-parallel aligned chains, leading to a polymer with fractions of higher and lower levels of crystallinity [57].

The intrinsic physicochemical properties of chitin might slightly differ depending on the source of extraction [63]. However, its overall functional properties such as biodegradability, non-toxicity and biocompatibility offer this natural polymer a wide range of applications in economically important sectors, including biomedicine, cosmetics and agriculture, among others [64,65,66,67]. In the cell wall of fungi, chitin is often associated with β-glucans linked to proteins to form a stratified polysaccharide matrix. Although in small quantities, the deacetylated form of chitin, i.e., chitosan, is present.

2.3. Chitin Deacetylases

Chitin de-N-acetylases or chitin deacetylases (CDAs), belonging to the fourth family of carbohydrate esterases CE4, are nature’s tools that produce chitosan [68]. They catalyze the deacetylation of N-acetyl-D-glucosamine residues under mild reaction conditions and this results in the production of novel superior-quality chitosan, thus they are used as a biological and environmentally friendly method to produce chitosan [69]. CDAs were first extracted from the fungus Mucor rouxii [70]. The effectiveness of CDAs in removing the acetyl groups depends on the fungi from which they are obtained. The deacetylation patterns are diverse: some CDAs being specific for single positions, others showing multiple attack, processivity or random actions [71]. For example, CDAs from M. rouxii are more effective in catalyzing the N-acetyl glucosamine than CDA from Rhizopus oryzae. The mechanism of action of CDAs has been recently reviewed [72]. CDAs have been proposed as targets for antifungal drugs, deserving attention for drug design.

3. Chitosan

Chitosan is obtained by hydrolysis of the acetyl groups of chitin and, in particular, GlcN units are more than GlcNAc ones (Figure 4).

Chitosan is a versatile biomacromolecule abundantly found in nature. It is marketed in the United States under the category of dietary supplements for weight management. It has been recently defined as an excellent bio-macromolecule due to its properties and the myriads of activities in biomedical applications [73,74]. Chitosan does not melt or dissolve in water, alkaline solutions and general organic solvents, therefore limiting its applications range. Thus, different chitosan derivatives have been obtained in order to expand its use. Thanks to the ability to biodegrade in the body, the lack of toxic reactions and high biocompatibility, chitosan has several beneficial effects for health, such as antioxidant, antimicrobial, antioxidant and anti-inflammatory, and has also been suggested as a potential new-generation drug for its anticancer effects. It is also used in transplantation medicine [75]. Chemical modification of the chitosan matrix by various technologies have been used to improve its solubility and other physicochemical characteristics [76,77]. Chitosan contains three reactive functional groups: an amino group at the C-2, a secondary hydroxyl group at the C-3 and a primary hydroxyl group at the C-6. Chemical modification of chitosan is mainly carried out on amino group at the C-2 and primary hydroxyl group at the C-6. The chemical modification methods of chitosan mainly include esterification, sulfation, acylation, alkylation and metal coordination [78]. In Figure 5, the reactive groups of chitosan are shown. The functional groups allow for a great number of modifications, producing polymers with new properties and behaviors. Complexes of chitosan with metals are widely described for their biological activities [79].

Chitosan and its derivatives are used in medicine, pharmaceuticals, food, cosmetics, agriculture, paper and textile industries, and for industrial sustainability. Their usefulness is widely known in drug delivery, ophthalmology, wound dressing, cell encapsulation, dentistry, bioimaging, tissue engineering. In the food industry, they are used for food packaging, gelling and coating, as food additives and preservatives, as active biopolymeric nanofilms, nutraceuticals, for skin and hair care; in agriculture, chitosan is employed for preventing abiotic stress in flora, and for enhancing water availability in plants, controlled release fertilizers, wastewater and sludge treatment, and extraction of pharmaceuticals, dyes and metals [80]. Chitosan derivatives represent significative compounds in the field of medical materials and biomedical science for their antimicrobial activity [81,82], also in orthopedic and vaginal infections [83,84], antioxidant, antitumor [85], anti-HIV, anti-inflammatory antihypertensive and antidiabetic activity [86] and for the treatment of Alzheimer’s Disease [73,87]. Recently, the use of a drug delivery nanoplatform with chitosan nanobubbles, conjugated with antibodies, has been proposed for the treatment of the glioblastoma multiforme tumor cells, which represent the most aggressive and heterogeneous malignant primary brain tumor [88]. New studies are also aiming to investigate the immunomodulatory properties of chitosan, since it is able to regulate the maturation, activation, cytokine production, and polarization of dendritic cells and macrophages. Several pathways, including the cGAS–STING, STAT-1, and NLRP3 inflammasomes signaling, are involved in chitosan-induced immunomodulation [89]. Moreover, drug delivery systems such as films, fibers, gels, nanoparticles, microparticles, liposomes, and injectable systems, made of chitosan and other polysaccharides [90,91], also used for dental drug delivery [92] and food packaging [93], are widely described. Chitosan derivatives may contain common functional groups, including alkyl and acyl groups, Schiff bases, quaternary ammonia, guanidines, and heterocyclic rings [94,95]. Polysaccharide-based systems containing chitosan have also demonstrated interesting activities for dental dug delivery in the treatment of various diseases, including dental caries, periodontal disease, and endodontic disease [92] and in the agricultural field [96]. Finally, the protonation of the amine groups of chitosan may cause electrostatic attraction of anionic compounds, including metal anions (resulting from metal chelation by chloride, anionic ligands, etc.), or anionic dyes [97].

4. Recent Studies of Chitosan-Based Materials as Antimicrobials

Thanks to the antimicrobial and antioxidant activities, chitosan-based coatings and films are often used for food packaging able to prolonging the shelf-life of foods [93,98,99,100] and in photodynamic therapy, which is considered one of the most promising emerging antibacterial strategies due to its non-invasiveness, low side effects, and lack of drug resistance [101]. It has been demonstrated that chitosan also provides freeze–thaw stability to gluten protein and can maintain the functional characteristics of frozen-thawed gluten protein, with 30 kDa chitosan being the most cryoprotective [102]. Moreover, the antibacterial activity of these compounds is widely exploited in drug delivery [103]. There are some reports in the literature that analyze the antibacterial activity of chitosan and the mode of action between chitosan and bacteria and/or fungi [104,105,106,107]. The antibacterial activity of chitosan is affected by multiple independent factors [108], including electrostatic interactions with components of both the bacterial outer and inner membranes, damage to membrane integrity through interactions with phospholipids and proteins, and interactions with phosphate groups in DNA, which can inhibit mRNA and protein synthesis [106]. Specifically, one of the proposed mechanisms is that chitosan may adhere to the negative charges of bacterial walls, initiating disruption and altering membrane permeability. The following attachment to DNA inhibits its replication, ultimately leading to cell demise. Another mechanism allows chitosan to inhibit microbial proliferation by selectively binding to trace metal elements, acting as a chelating agent. The antibacterial activity of chitosan is pH dependent. In acidic conditions, the electrostatic interaction between the positive charges of chitosan and the negative charges of bacterial surface components is crucial for the antibacterial activity: the higher the charge density, the greater the antibacterial activity. In addition, a greater number of amino groups on the chitosan backbone also increases antibacterial activity. Finally, the size and shape of chitosan particles may affect their mode of action, with larger particles embedding themselves in the cell surface and changing cell permeability [107]. The use of chitosan nanoparticles as antibacterials has been recently reviewed by Akdaşçi et al. [109]. Some recent studies (published in 2025) regarding the antibacterial activity are detailed below.

4.1. Antibacterial Activities of Chitosan-Based Materials for Food Packaging

Sun et al. [110] reported a study on ZnO nanoparticles, loaded on lecithin-montmorillonite and incorporated into chitosan, which demonstrated antibacterial activity against E. coli and S. aureus and higher activities than chitosan in terms of moisture vapor transmission rate and content, with great potential for application in novel active food packaging. Villani et al. [111] demonstrated that the incorporation of silver nanoparticles into thin chitosan films improved the antimicrobial activity against Pseudomonas aeruginosa, the Gram-negative opportunistic pathogen notorious for its role in nosocomial infections and food contamination, demonstrating a synergistic effect between the nanoparticles and the polymeric matrix. A remarkable bactericidal efficacy was observed. A recent study by Hou et al. [112] reported the use of composites, obtained by an inexpensive solvent-casting method fabricated from cellulose and chitosan reinforced with different tannic acid, to increase cherry tomatoes shelf-life of about 15 days in comparison to a polyethylene film, and effective antibacterial activity against S. aureus and E. coli. An interesting study by Liang et al. [113] reported a chitosan-gentamicin derivative synthesized by protection, regioselective bromination, nucleophile substitution reaction, and deprotection. The new derivative showed 2.50- and 2.41-fold higher antibacterial activity than gentamicin against E. coli and S. aureus, respectively. Moreover, an active packaging coating was developed by incorporating glycerol into the chitosan-gentamicin derivative. The coating inhibited microbial growth in preservation experiments with papaya and litchi, and maintained quality and appearance up to 16 and 8 days of storage, respectively. Ma et al. [114] studied several bio-based composite films from dialdehyde nanocellulose, tannin and chitosan. They showed antibacterial properties against both E. coli and S. aureus and demonstrated lower water solubility, swelling ratio, water vapor and oxygen barrier properties than pure chitosan film. The composite films also exhibited high UV blocking and antioxidant capacity and extended postharvest life of cherry tomato more than the pure chitosan film. Zhou et al. [115] suggested the use of a chitosan film blended with Malus ‘Donald Wyman’ crabapple ethanol extract to ameliorate grape preservation, as it enhanced the preservative effect of chitosan film on grapes.

4.2. Antibacterial Activities of Chitosan-Based Materials in Photodynamic Therapy and/or for Wound Infections

Photodynamic therapy has emerged as a potential treatment strategy for bacteria-infected wounds since it does not lead to the development of drug resistance [116]. Zhao et al. (2025) [117] used chitosan-based fluorescent copolymers containing borodipyrromethene in combination with photodynamic therapy, demonstrating significant antibacterial efficacy, even at low concentrations, against E. coli and S. aureus. The synergism of antibacterial effects and photodynamic capabilities lead to eradication of bacteria by loading the copolymers onto transparent band-aid, under white light irradiation. Luo et al. [118] described a study on nano-micelles obtained by conjugating chitosan with the photosensitizer chlorin e6, promoting its solubility and stability, acting as antibacterials, without inducing resistance, and disrupting mature biofilms. The nano-micelles, along with laser treatment, maintained favorable biocompatibility and demonstrated high in vivo antibacterial efficacy, accelerating the healing of skin wounds infected with MRSA. Soleimani et al. [119] reported the synthesis of chitosan-based hydrogels, incorporating bicyclo[2.2.2]octect-7-ene-2,3,5,6-tetracarboxylic acid dianhydride as a cross-linking agent and riboflavin as a photosensitizer, for photodynamic therapy applications. The hydrogels showed remarkable biocompatibility, underscoring their potential as a promising platform for antibacterial and photodynamic therapy applications. Wang et al. [120] reported the synthesis of a chitosan-based hydrogel with bacteria capturing and combined photothermal/photodynamic sterilization functions, prepared by mixing chitosan as matrix, protoporphyrin as photosensitizer and polydopamine as photothermal agent and then chemically cross-linking chitosan with glutaraldehyde. The hydrogel demonstrated excellent swelling capabilities and rheological properties. Under the synergetic illumination of a laser, it induced the death of 99.9999% of E. coli and 99.99999% of S. aureus. The hydrogel also demonstrated anti-inflammatory properties and upregulated Heat Shock Protein 90 expression, thus promoting collagen deposition and facilitating wound healing. Liu et al. [121] reported the study on a series of quaternized chitosan-oxidized pullulan polysaccharide-dopamine-coated polypyrrole containing microalgae hydrogels prepared based on quaternized chitosan, oxidized pullulan polysaccharide, dopamine-coated polypyrrole, and Chlorella vulgaris. These compounds demonstrated interesting antibacterial (E. coli and S. aureus), anti-inflammatory, oxygenation and wound-monitoring capabilities, and were suggested for the treatment of chronic wounds caused by hyperglycaemia in diabetic patients. Cao et al. [122] reported the study on a chitosan sponge prepared by self-assembling chitosan and quaternized chitosan without acid retention and chemical crosslinker introduction using the freeze-drying technique. The sponge showed high expandability, injectability, and shape memory property activated by and water/blood interaction, as well as good antibacterial properties, thus suggesting its use to protect the wound from bacterial invasion in hemostasis, following acute hemorrhage caused by accidents and surgery. Recently, bio-nanocomposites containing chitosan with minerals, such as montmorillonite and halloysite, have been described for their antibacterial activity in drug delivery and in the treatment of topical skin infections and wound healing [123]. The benefits of chitosan-based nanofibers as protecting wounds from bacteria proliferation and wound healing without leaving scars have been recently reported by Goel and Bano [124].

4.3. Antibacterial Activity of Chitosan-Based Materials for Dental Drug Delivery

Chitosan is used in dentistry, thanks to its remineralizing property that hardens tissues of the tooth; therefore, it is used as a desensitizer in toothpastes. Moreover, the use of chitosan produced better surgical healing of post-extraction oral wounds and a reduction in bacterial biofilm when used in dental cements [125]. It has been suggested as a biomaterial for the prevention and treatment of dental caries for the antibacterial effect, and drug delivery [126,127]. Chitosan may inhibit the development of biofilms and the growth of bacteria linked to caries, with the level of the antibacterial activity depending on the MW and DD of chitosan. The combination of chitosan and its derivatives with other antibacterial composite materials or methods can significantly enhance the antibacterial effect. Moreover, chitosan can act as a reservoir for calcium and phosphorus ion deposition, thus improving the remineralization of enamel caries sites. Chitosan can serve as an efficient drug delivery vehicle for remineralizing or antibacterial agents, which helps to increase medication bioavailability, lower dosages, and preserve long-term treatment efficacy [128]. Recently, the use of azithromycin-loaded nanoparticles incorporated in chitosan-based soft hydrogels has been suggested as a novel approach for dental drug delivery. The azithromycin-based soft hydrogels significantly improved solubility, controlled release, and biological activity, being active against efficacy against S. mutans and S. aureus, showing strong potential for dental drug delivery [126].

5. Chitosan for Removing Pharmaceutical Pollutants, Dyes and Heavy Metals from Water Sources

Some new studies (published in 2025) on the use of chitosan to remove pollutants, pharmaceuticals, dyes and metals from water and its usefulness in wastewater treatment are reported and summarized in

Table 1. Contaminants removed from water.

Contaminant		Ref.
Pharmaceuticals	Retinoids (isotretinoin, adapalene)	[129]
	Amoxicillin	[130]
Dyes	Methylene blue	[132]
	Congo red	[135]
Metals	Cu(II)	[132,133,134,140]
	Co(II)	[132]
	Mn(II)	[135]
	Pb(II)	[133,138]
	Ni(II)	[136]
	Cd(II)	[136]
	U(VI)	[137]
	Cr(VI)	[131]
	Cr(III)	[139]
	As(III)	[141]

. Ehsanfar et al. [129] described the adsorption and controlled release of isotretinoin and adapalene retinoid drugs from water by using chitosan-modified manganese ferrite (MnFe₂O₄) nanoparticles. The use of green and high-performance biomass-based adsorbents have been described for the removal of heavy metals. Specifically, Alakayleh [130] described a chitosan-olive leaf biomass composite behaving as an efficient, sustainable and eco-friendly adsorbent for the removal of amoxicillin from water. Alyasi et al. [131] reported the use of a magnetic Ti₃C₂Tx MXene chitosan-lignosulfonate composite synthesized by integrating Fe₃O₄, chitosan-lignosulfonate nanospheres and delaminated Ti₃C₂T_x for the removal of heavy metals. The composite demonstrated high selectivity towards Cr(VI) ions and improved magnetic recovery from the aqueous media. The adsorption mechanisms suggested were electrostatic interactions, complexation, surface intercalation, and reduction in toxic Cr(VI) to Cr(III) on the composite adsorbent. The composite effectively removed also other heavy metals even though with low affinity (Cr(VI) > Ni(II) > Cu(II) ≈ Co(II)). Yu et al. [132] reported the preparation of a catalytic nanocomposite nanofiltration membrane, obtained by using two-dimensional materials MXene and graphene oxide, for the removal of dye (methylene blue) and heavy metals (cobalt and copper ions) without requiring UV irradiation. The incorporated MXene catalytically decomposed the hydrogen peroxide and generated reactive oxygen species, which oxidize methylene blue and reduce Co²⁺ and Cu²⁺. The chitosan/MXene/graphene oxide membrane showed removal efficiency higher than that of a neat membrane, both for the dye and metal ions (>75%). Zhao et al. [133] recently reported some chitosan/lignin hydrogels obtained by a simple one-pot method through the Mannich reaction, as adsorbents for heavy metals. The prepared hydrogels showed higher adsorption selectivity for Pb(II) and Cu(II) than other heavy metals, with maximum adsorption capacities obtained for one hydrogel of 139.86 and 98.71 mg/g for Pb(II) and Cu(II), respectively. Yuan et al. [134] described the synthesis of a ferric oxide–chitosan composite obtained by loading chitosan with Fe₂O₃, in turn obtained from a kind of ferrous sulfate waste liquid by chemical precipitation. The ferric oxide–chitosan composite was used to adsorb the copper(II) ion from polluted water. Ibrahim et al. [135] described the preparation of three cost-effective bio-composites: PUF@chitosan, PUF@chitosan@activated-carbon, and PUF@chitosan@polyaniline, obtained from polyurethane foam waste (PUF), for the removal of Congo red dye and manganese ion (Mn²⁺). The PUF@chitosan@polyaniline composite showed the highest Congo red column total dye removal (about 63%), whereas PUF@chitosan@activated carbon was the most effective for Mn²⁺ adsorption (about 95%). Allahkarami et al. [136] studied a nepheline syenite–chitosan composite (NS–CS) for the adsorption of heavy metals (Ni(II) and Cd(II)) from synthetic and electroplating industrial effluents, and suggested the mechanism by statistical physical modeling. Results suggested that the adsorption process is likely physical in nature. Zhou et al. [137] studied a poly ethanolamine amidoxime modified winter melon and chitosan-derived biochar using chemical crosslinking method for the removal of U(VI). It effectively removed U(VI), with a maximum adsorption capacity of 485.89 mg/g at pH = 5.0 and showed good reusability for U(VI) in aqueous solutions. It was suggested as an economical, efficient and stable composite material by the authors. Zhong et al. [138] reported COFs–OH/chitosan composite aerogels obtained by phenolic hydroxyl anchored covalent organic frameworks (COFs) powders (COFs–OH) integrated into chitosan gels. The obtained aerogels realized the fast solid–liquid separation and also rapidly adsorbed Pb(II) from water compared to the pure chitosan and COFs–OH powders. Kurniawan et al. [139] described the use of a chitosan-coated coconut shell composite for treatment of Cr(III)-contaminated wastewater deriving from tannery industry that generates a large amount of Cr(III)-contaminated wastewater. The utilization of these composites for the removal of Cr(III) instead of the toxic form of chromium (Cr(VI)) was based on practical considerations, since Cr(III) was the prevalent form of chromium in the wastewater obtained. The authors suggested the use of the same methodology for the removal of Cr(VI). Buoghanmi et al. [140] reported freeze-dried polyelectrolyte complexes deriving from chitosan and pectin to enhance ion adsorption from aqueous media for water treatment, targeting the removal of metals (the order of removal efficiency was Cu²⁺ > Fe^2+/3+ > Cd²⁺ > Ni²⁺ > Mn²⁺). Tri et al. [141] proposed the so-called “waste to adsorbent” strategy, in which solid waste from heavy industrial processes is treated to produce nanostructured chitosan containing Fe₃O₄ materials for As(III) pollution remediation.

6. Chitosan Market Development

The worldwide demand for chitin in 2015 was above 60,000 T, whereas its production was around 28,000 T. The worldwide market for chitin derivatives (including chitosans) should reach 63 billion USD by 2024, following a report from Global Industry Analysts Inc. (chitin and chitosan derivatives market report—2015) [142]. The Global Chitosan Market was valued at USD 1.83 billion in 2020 and will grow at a compound annual growth rate (CAGR) of around 14.8% from 2021 to 2027, owing to product usage in a wide range of end-user industries including water treatment, healthcare, food and beverages, cosmetics, and others [143]. The rapid growth of the food and beverage industry and the growing requirement for advanced packaging materials with eco-friendly characteristics will support the use of chitosan-based packaging in this sector. The growing water treatment activities in Asia, coupled with rising R&D investments for the usage of chitosan in biomedical applications offer lucrative opportunities for chitosan market trends [144]. The problem of heavy metal ion pollution from industrial water wastes and discharge in freshwater sources poses a serious threat to human health. Moreover, rising consumer spending in emerging economies has led to strong growth in cosmetics, which are used in the production of personal care products, such as anti-aging creams, hair care products, and oral care products. Chitosan forms an elastic film on the hair, thus providing softness and physical strength to the hair fiber; therefore, it is used in styling gels, hair sprays, shampoos, and hair colorants. Moreover, it is also used to manufacture mouthwashes, toothpaste, and chewing gums due to its anti-plaque and anti-caries properties. In addition, the growing demand for organic ingredients in cosmetic products will increase the chitosan market share during the forecast period. The growing awareness of the negative impact of chemical pesticides and growing demand for environmentally friendly measures for crop disease management are expected to boost the demand for natural components, such as chitosan. In agriculture, chitosan is widely used in fruit, vegetable and seed coatings along with use as a fertilizer to control the release of agrochemicals, thus enhancing plant production. In addition, the antibacterial and antifungal properties possessed by chitosan, are highly effective against seed-borne pathogens [145].

7. Conclusions

Chitin and chitosan, a versatile hydrophilic polysaccharide derived from chitin, have broad biological activities. They are natural nontoxic biopolymers, obtainable from easily available sources, such as crustacean shells, fungi and insects, and insoluble in water as well as in most organic solvents. The latter property is the major limiting factor for their utilization in living systems. Several derivatives of chitin and chitosan have been studied in order to improve their physicochemical properties. Their antimicrobial activity is widely exploited to improve food safety and for wound healing applications, behaving as a multifunctional and promising biomaterial within the domain of tissue engineering. Moreover, their usefulness is underlined by their capability of removing contaminants, such as pharmaceuticals, dyes and heavy metals, in wastewater treatment and in an environmentally friendly manner. Chitosan has shown more than 50% efficiency in removing heavy metal ions from water compared to carbon quantum dots and semiconductors. Recent studies regarding these activities are described in this review, once again underlining the importance of chitin and chitosan. Chitin, chitosan and their derivatives are among the active ingredients largely used in cosmetics industry, including preparations for skin, hair, gums and teeth care. Chitosan is used in dentistry, thanks to its bioactivity and anti-inflammatory effects, and in the food industry, playing the role of a food preservative, acting as a natural antioxidant, and improving the quality of food safety and shelf life. It is also used in agriculture as it triggers the body’s defense against pathogenic microorganisms, showing a wide spectrum of antimicrobial activity against bacteria, fungi and viruses.

8. Perspectives

Chitin is an unused waste from the food industry, as it is insoluble in water and basic solvents. Nevertheless, there are possibilities of secondary management. Research has led to the achievement of a soluble and environmentally friendly form of chitin, which could potentially be applied in many fields, including medicine, cosmetics, food and textile industries, agriculture, etc. On the other hand, chitosan has a number of beneficial properties and wide possibilities for modification. Thus, chitosan can be obtained with the desired functional properties, facilitating, for example, the processing of this polymer and expanding the possibilities of its application, including biomimetic materials. The study of the life cycle of chitosan demonstrated that it is safe for the environment. The growing demand for chitosan-based products indicates an increase in demand for this polymer in many sectors. Therefore, the increasingly widespread and subsequent work of researchers on the improvement of many functional properties could in the future completely replace many artificial polymers used so far. The potential of chitosan will promote crop productivity and food safety. Furthermore, new technologies could incorporate chitosan for production that is expected to increase in the near future. The demand for chitosan is expected to increase in the near future as more technologies incorporate chitosan as a basic ingredient in food packaging and preservation, drug formulation and delivery systems, and in water treatment systems and agriculture. Moreover, chitosan as anticancer and antiviral will be essential in the fight against antimicrobial resistance. Chitosan has been explored in membrane filtration system for water purification. The usefulness of chitosan in the treatment of saline water for desalination has not been explored yet, as well as the application of chitosan in air filtration system for better air quality. However, practical applications of chitosan in drug delivery, formulation, water purification and food packaging are limited due to limited in vivo studies of chitosan. More in vivo studies are expected in the near future. Looking ahead, future studies should continue to investigate the potential of these processable natural biopolymers in cutting-edge technologies, such as biomedical engineering, biosensors, the food and cosmetic industries, and the pharmaceutical industry to enhance drug formulations and develop personalized therapeutic solutions, taking advantage of the non-toxic characteristics of these excellent compounds.