Advanced water treatment and contaminant adsorption is gaining importance due to its growing concerns related to water scarcity, pollution, and the requirement for water management. One of the most used water treatment technologies is novel chitosan-based materials and their derivatives.

Introduction

Water is an important resource in today’s world, and preserving its purity is crucial for health and survival. Many traditional methods for water purification are insufficient in dealing with modern technologies. Among various biosorbents, chitosan, which is a derivative generated by the N-deacetylation of chitin, is a naturally occurring and flexible biopolymer with immense significance in wastewater treatment (Rhazi et al., 2002).

Background of Chitosan:

Chitosan is formed with a copolymer of glucosamine and N-acetyl glucosamine copolymer produced from chitin. Chitin is mostly found in insects, cell walls of crustaceans, algae, fungi, microbes, and some invertebrate creatures (Pellis et al., 2022). The most common methods used for the extraction of chitosan are alternative fermentation processes, conventional lactic fermentation bioprocess, and enzymatic hydrolysis of crustacean biowaste.

Chemical Structure of Chitosan:

Chitosan is a linear polysaccharide primarily composed of randomly dispersed β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). Chitosan is also known by various names like Poliglusam; Deacetylchitin; Poly-(D)glucosamine; BC; Chitopearl; Chitopharm; Flonac; Kytex. The chemical formula of Chitosan can be represented as C56H103N9O39.

Properties of Chitosan:

Chemical properties:

• Linear Polysaccharide
• Soluble in acidic solutions
• Highly viscous
• Strong chelation with metals like Cu²⁺, Zn²⁺, and Fe³⁺

 Biological properties:

• Biodegradable by enzymes such as lysozyme, chitosanase, and glycosidases
• Antimicrobial
• Hemostatic
• Immune modulator
• Anti-inflammatory
• Non-toxic

Contaminant adsorption Mechanism of Chitosan:

The adsorption process of chitosan can be classified into two types such as physical adsorption (physisorption) and chemical adsorption (chemisorption). Mild attractive forces cause physical adsorption while chemical adsorption requires the formation of strong chemical bonds through electron exchange (Kajjumba et al., 2019).

Physical adsorption (Physisorption) in Chitosan:

1. Van der Waals forces: Adsorption can also happen through weak interactions between adsorption molecules and the surface of chitosan, majorly for smaller organic molecules and gases.
2. Hydrogen bonding: Hydrogen bond is more common than van der Waals force. Hydrogen bonds can also be a part of physical adsorption if the interactions are too weak to be classified as chemical adsorption. Hydrogen bonding occurs only when polar adsorbate molecules interact with amino groups on chitosan and hydroxyl.

Chemical adsorption (Chemisorption) in Chitosan:

1. Ionic relations: In acidic conditions, the amino acids in chitosan can be protonated, which results in the formation of –NH3+ groups. These positively charged particles can form strong ionic relations with the adsorbates that are negatively charged, similar to certain metal ions or anionic dyes.
2. Coordination bonds: Chitosan can form collaboration complexes with the metal ions which are stable complexes produced when the lone pairs of electrons on the amino and hydroxyl groups interact with metal ions.
3. Covalent bonding: To create covalent connections, adsorbates can chemically react with chitosan’s functional groups. For example, glutaraldehyde can create covalent bonds with the amino groups in chitosan.

Contaminants adsorbed by chitosan:

Chitosan acts as the best adsorbent for removing a wide range of contaminants from water. The contaminants that chitosan can remove include:

1. Heavy metals: Heavy metals are the primary pollutants present in industrial wastewater and they can have very toxic effects on plants and animals. Chitosan can be used to remove heavy metals like lead, copper, zinc, cadmium, nickel, and mercury.
2. Dyes: Chitosan can also be used in the adsorption of various dyes used in textile, paper, and other industries. Increasing the dose of chitosan powder in water can remove the amount of dye removal. It includes cationic dyes, anionic dyes, and reactive dyes.
3. Organic Pollutants: Chitosan has a high density of amino and hydroxyl groups, which helps to remove organic pollutants including phenol, polyaromatic hydrocarbons, and surfactants. Chitosan is also used in the filtration of drinking water.

 Modifications of Chitosan:

Chitosan, a chitin-derived biopolymer, can be modified to improve its properties and functionality in a variety of applications. Modifications in chitosan can help to increase the solubility, physical reinforcement of structural components, and improve bioactivity, which makes it suitable for adsorption processes. Chitosan can be modified in following ways:

Chemical Modifications:

Chemical modifications change the chemical structure of chitosan by reacting with different chemicals. Chemical modifications include:

Grafting:

Grafting chitosan with carboxyl and amine groups can modify its properties and applications.

• Carboxyl Group Grafting: Grafting chitosan molecules with a carboxyl group can enhance its water soluble properties. This type of grafting is achieved by reactions with carboxyl-containing molecules or polymers like acrylic acid or poly acrylic acid.
• Amine group grafting: It can be obtained by grafting amine-containing polymers like polyethyleneimine (PEI) with chitosan. It leads to an increase in the adsorption of dyes and heavy metals.

Cross-linking:

• Chemical cross-linking: Using agents like genipin, glutaraldehyde, or epichlorohydrin to cross-link chitosan molecules, can improve its mechanical stability and decrease its solubility.
• Ionic cross-linking: Multivalent ions like tripolyphosphate (TPP) can cross-link chitosan through ionic interactions that are mainly used in drug delivery.

Functionalization:

• Quaternization: It involves the conversion of amino groups into quaternary ammonium groups by reacting with alkyl halides. Quaternization of chitosan increases the antimicrobial property. It is mostly preferred in wound dressing, antimicrobial coatings, and disinfectants.
• Carboxymethylation: In this process, carboxymethyl groups are added to improve the solubility of water and increase the adsorption capacity for dyes and heavy metals. The water soluble property of chitosan increases due to carboxymethylation.

Physical Modifications:

Physical modifications are the changes made in chitosan’s physical properties to improve its quality without changing its chemical structure. Physical modifications include:

Nanoparticles:

Nanoparticles of chitosan are created using methods like ionic gelation, emulsion cross-linking, or solvent evaporation to improve its bioavailability in drug delivery and to increase surface area for adsorption.

Films and Membranes:

• Chitosan films: They are produced by casting or drying chitosan solutions for applications in food packaging, water treatment, and wound dressings.
• Chitosan membranes: Chitosan membranes offer benefits like biocompatibility and antimicrobial properties so they are preferred in the filtration and separation process.
• Hydrogels: Hydrogels of chitosan are formed by cross-linking with chitosan agents like glutaraldehyde or genipin, which are used in drug delivery, tissue engineering, and wound healing.

Chitosan-based nanocomposites:

These are the materials formed by combining chitosan with nanoscale additives such as nanoparticles, nanofibers, and nanofillers under controlled conditions to generate a new material with increased properties (Azmana et al., 2021).

Applications of modified Chitosan in Water treatment and contaminant adsorption:

Modified chitosan is widely used in water treatment applications due to its high adsorption properties, biodegradability, non-toxicity, and ability to be modified to improve overall performance. Various applications of modified chitosan in water treatment are as follows:

• Heavy metal removal: Heavy metals present in the water are one of the most dangerous water pollutants. The high adsorption rate of chitosan due to the presence of amino and hydroxyl groups can help to remove heavy metals. By adding carboxyl, thiol, or phosphate groups, the ability of chitosan to adsorb different metals like lead, cadmium, mercury, and arsenic is increased.

Example: Chitosan can be used in powder and bead form cross-linked with glutaraldehyde to remove lead present in the water.

• Dye removal: Modified chitosan is effective, for eliminating dyes from wastewater by enhancing adsorption capacity and selectivity for different colors like cationic, ionic, and relative dyes. To achieve this chitosan undergoes modifications with groups like carboxyl and quaternary ammonium.

Example: For instance when chitosan is grafted with acrylic acid it shows increased adsorption capacity for dyes through the interaction, between positively charged chitosan and negatively charged dye molecules.

• Microbial Contaminant Removal: Removing Microbial Contaminants; altered chitosan possesses innate qualities making it proficient, in eradicating impurities from water. Additionally it can inhibit the proliferation of bacteria, fungi, and viruses in facilities that treat water.

Example: Variants of chitosan, for combatting pollution in potable water include chitosan nanoparticles and chitosan silver nanocomposites.

• Removal of Organic Pollutants: Chitosan can be used in absorbing organic pollutants, such, as pesticides, drugs, and chemicals that disrupt hormones. Enhancing chitosan adsorption efficiency can be achieved by incorporating hydrophobic groups or connecting it with polymers.

Example: β cyclodextrin-modified cross-linked chitosan beads effectively eliminate pharmaceuticals from water by forming blends, with pollutants.

Case studies of Chitosan in contaminant adsorption:

1. Chitosan Hydrogels for Water Purification Applications

Water treatment has seen an overwhelming demand for chitosan-based hydrogels to include them in its systems, because they are biodegradable, bioavailable, biocompatible, green, and fast absorbers of contaminants and water. This paper covers current advances in the structural characteristics and properties of hydrogels applicable to water decontamination. Some ways such as nanoparticles, graphene and metal-organic frameworks (MOFs), and nanoparticles among others have been explored for their potential usefulness in clearing heavy metals, organic pollutants as well as bacteria from water supplies. It explains how some purification processes are done including adsorption, filtration, and antibacterial activity. Chitosan-based hydrogels possess great potential for combating world water pollution challenges (Chelu et al., 2023).

2. Chitosan–Silica Composites for Adsorption Application in the Treatment of Water and Wastewater from Anionic Dyes

Chitosan-nano silica (ChNS) and chitosan-silica gel (ChSG) are two kinds of biopolymer-silica composites that are prepared as highly efficient adsorbents. These composites exhibit optimum adsorption efficiency at pH 2. Raising the chitosan content presents an increment in the amount of adsorbate about 1.8 times for nanosilica and 1.6 times for silica nanoparticles, accordingly. ChNS showed rapid adsorption kinetics for anionic dyes such as acid red AR88 compared to ChSG. Techniques like X-ray scattering, electron microscopy, and nitrogen sorption were used to study structural and physicochemical properties. The dye adsorption ability was augmented by chitosan acting collectively with the Silica Gel’s dye binding groups and interfacial surface (Blachino et al., 2023).

3. Chitosan, chitosan derivatives, and chitosan-based nanocomposites: eco-friendly materials for advanced applications

While chitosan is renowned for being biocompatible, biodegradable, non-toxic, and easy to modify the issues which make it up are its molecular weight and degree of deacetylation. Chitosan derivatives are now receiving more attention and their applications are expected to grow in the future. Concerning a recent study conducted on chitosan, production processes were discussed as they affect physicochemical properties including solubility, biological activities, and extraction methods. Modification techniques to improve existing chitosan derivatives were investigated as well as the current nanocomposites on the market were researched in this study. Chitosan’s amazing properties make it valuable in healthcare, ecology solutions, food industries, cosmetics, agricultural treatment products, pharmaceuticals manufacturing medical devices, and finally in wastewater treatment (Janati et al.,2024).

Advantages of Chitosan in Water Treatment and contaminant adsorption:

1. Biodegradability: Chitosan is extracted from renewable sources and is biodegradable, which makes it a more environmentally friendly option than artificial alternatives.
2. Adsorption Properties: Chitosan has excellent adsorption properties, which efficiently remove heavy metals, chemical compounds, and colors from water through binding mechanisms.
3. Antimicrobial Properties: Its antimicrobial characteristics restrict the growth of bacteria and fungi, making it useful for disinfection purposes in water treatment plants.
4. Low Cost: Chitosan is inexpensive as compared to some other water treatment methods, especially when it is made up of seafood industry waste.
5. Flocculation and Coagulation: Chitosan can act as a flocculants or coagulants, helping to accumulate and settle suspended solids, thereby clarifying water.
6. Selective Binding: Chitosan can bind with certain contaminants, offering specificity in pollutant removal without affecting desired ions content or compounds in water.

Research gap:

Despite the significant potential chitosan has in water treatment on account of its biocompatibility and ability to adsorb pollutants, it is still evident that further research needs to be done on this. The research gap was identified after several studies that tackled the reuse, long-term stability, and cost-effectiveness of chitosan including its production process. Moreover, the interaction between chitosan and new pollutants like drugs and microplastics needs to be researched further. It is crucial for the practical and long-term utilization of chitosan in water treatment that this research gap is overcome.

Future scope:

Chitosan is widely used in water treatment because of its unique properties. It removes heavy metals and dyes as well as acts as a flocculant and antibacterial agent. The future studies on chitosan need to be concentrated on the improvement of its efficiency and stability via different chemical modifications and composite materials with graphene and carbon nanotubes. Enhancing the reuse and regeneration processes, tailoring for optimum performance under various conditions, deciphering the molecular level interactions, and scaling up manufacturing are all key areas to focus on in terms of economic sustainability in large-scale applications. All these things combined could significantly decrease both the world’s water scarcity and pollution.

Conclusion:

An important polymer that has its origins in chitin is chitosan and this finds a prominent place in advanced water treatment due to the exceptional adsorption as well as antibacterial properties. Chitosan gets rid of impurities quite effectively, heavy metals, and organic pollutants so they are not toxic or polluting, and in addition, it is biodegradable. Chemical modifications and nanocomposites enhance its efficacy but their long-term stability, reusability and cost-effectiveness require further research. More investigations and new applications regarding chitosan could be helpful for sustainable water management systems as they would increase both efficiency and scalability of the above-mentioned polymer.

References:

1. Rhazi, M., Desbrières, J., Tolaimate, A., Rinaudo, M., Vottero, P., Alagui, A., & El Meray, M. (2002). Influence of the nature of the metal ions on the complexation with chitosan: Application to the treatment of liquid waste. European Polymer Journal, 38(8), 1523-1530. https://doi.org/10.1016/S0014-3057(02)00026-5
2. Pellis, A., Guebitz, G. M., & Nyanhongo, G. S. (2022). Chitosan: Sources, processing and modification techniques. Gels, 8(7), 393. https://doi.org/10.3390/gels8070393
3. Kajjumba, G. W., Emik, S., Öngen, A., Özcan, H. K., & Aydın, S. (2019). Modelling of adsorption kinetic processes—Errors, theory and application. In S. Edebali (Ed.), Advanced sorption process applications. IntechOpen. https://doi.org/10.5772/intechopen.80495
4. Azmana, M., Mahmood, S., Hilles, A. R., Rahman, A., Arifin, M. A. B., & Ahmed, S. (2021). A review on chitosan and chitosan-based bionanocomposites: Promising material for combatting global issues and its applications. International Journal of Biological Macromolecules, 185, 832–848. https://doi.org/10.1016/j.ijbiomac.2021.07.023
5. Chelu, M., Musuc, A. M., Popa, M., & Calderon Moreno, J. M. (2023). Chitosan hydrogels for water purification applications. Gels, 9(8), 664. https://doi.org/10.3390/gels9080664
6. Blachnio, M., Zienkiewicz-Strzałka, M., Deryło-Marczewska, A., Nosach, L. V., & Voronin, E. F. (2023). Chitosan–silica composites for adsorption application in the treatment of water and wastewater from anionic dyes. International Journal of Molecular Sciences, 24(14), 11818. https://doi.org/10.3390/ijms241411818
7. Janati, W., Ullah, R., Ercisli, S., & Errachidi, F. (2024). Chitosan, chitosan derivatives, and chitosan-based nanocomposites: Eco-friendly materials for advanced applications (a review). Frontiers in Chemistry, 11, 1327426. https://doi.org/10.3389/fchem.2023.1327426

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