Chitosan-based nanocomposites have emerged as promising materials for the removal of dyes from textile wastewater due to their high adsorption capacity, biodegradability, and cost-effectiveness. These materials combine the natural polymer chitosan with inorganic nanoparticles such as zinc oxide (ZnO) or clay minerals like montmorillonite, enhancing their mechanical stability and adsorption efficiency. The functional groups in chitosan, including amino and hydroxyl groups, facilitate dye binding through electrostatic interactions, hydrogen bonding, and chelation. Cross-linking methods further improve the stability and reusability of these composites in aqueous environments.

Cross-linking is a critical step in modifying chitosan-based nanocomposites to prevent dissolution in acidic conditions while maintaining adsorption performance. Common cross-linking agents include glutaraldehyde, epichlorohydrin, and genipin. Glutaraldehyde reacts with the amino groups of chitosan, forming Schiff base linkages that enhance structural integrity. However, excessive cross-linking can reduce the number of available adsorption sites, decreasing dye uptake efficiency. Epichlorohydrin introduces ether linkages, improving chemical stability without significantly compromising adsorption capacity. Genipin, a natural cross-linker, offers an eco-friendly alternative, though its higher cost limits large-scale applications. The choice of cross-linking agent influences the nanocomposite’s mechanical strength, swelling behavior, and adsorption kinetics.

Adsorption kinetics play a crucial role in determining the efficiency of chitosan-based nanocomposites in dye removal. The process typically follows pseudo-first-order or pseudo-second-order kinetics, depending on the dominant adsorption mechanism. Pseudo-first-order kinetics suggest physical adsorption, where dye molecules adhere to the surface of the nanocomposite. In contrast, pseudo-second-order kinetics indicate chemisorption, involving stronger interactions such as electrostatic attraction or covalent bonding. The Elovich model describes heterogeneous surface adsorption, common in systems with multiple active sites. Intraparticle diffusion models help identify whether pore diffusion limits the adsorption rate. Studies show that chitosan-ZnO nanocomposites exhibit rapid initial adsorption, achieving equilibrium within 60 to 120 minutes for cationic dyes like methylene blue, with capacities ranging from 150 to 300 mg/g depending on ZnO loading.

The performance of chitosan-based nanocomposites is highly pH-dependent due to the protonation or deprotonation of functional groups. At low pH (below 4), the amino groups of chitosan become protonated, enhancing electrostatic interactions with anionic dyes such as Congo red. Conversely, at higher pH (above 7), the deprotonated amino groups favor the adsorption of cationic dyes through hydrogen bonding and chelation. However, extreme pH conditions can degrade the nanocomposite structure or reduce adsorption efficiency. Optimal pH ranges vary with dye chemistry; for instance, chitosan-montmorillonite composites show maximum adsorption for methyl orange at pH 3, while methylene blue removal peaks at pH 8. Adjusting pH during wastewater treatment can thus optimize dye removal efficiency.

Biodegradability is a key advantage of chitosan-based nanocomposites over synthetic adsorbents like ion-exchange resins. Chitosan degrades naturally through enzymatic action, reducing environmental impact. The incorporation of nanoparticles does not significantly hinder biodegradation, though some studies report slower degradation rates for cross-linked composites. In contrast, ion-exchange resins, while highly effective in dye removal, are non-biodegradable and require energy-intensive regeneration processes. Their disposal poses environmental risks due to the release of toxic by-products. Chitosan-based materials offer a sustainable alternative, particularly in applications where single-use adsorbents are preferred.

Despite their advantages, chitosan-based nanocomposites face limitations in mechanical strength, especially in continuous flow systems. The soft nature of chitosan leads to swelling and structural deformation under prolonged hydraulic pressure, reducing adsorption efficiency over time. Reinforcing agents like montmorillonite or carbon nanotubes can mitigate this issue by improving tensile strength and reducing swelling. However, achieving mechanical stability comparable to synthetic polymers remains challenging. Continuous flow systems require robust materials capable of withstanding repeated regeneration cycles, an area where ion-exchange resins still outperform chitosan-based composites.

Comparative studies highlight the trade-offs between chitosan-based nanocomposites and ion-exchange resins. While resins exhibit higher mechanical stability and consistent performance in flow-through systems, their high cost and environmental drawbacks limit widespread adoption. Chitosan composites, though less durable, provide a low-cost, eco-friendly solution for batch treatment processes. Future research should focus on enhancing mechanical properties through advanced cross-linking techniques or hybrid material designs, enabling their use in continuous systems without compromising biodegradability.

In summary, chitosan-based nanocomposites represent a viable solution for dye removal in textile wastewater, combining high adsorption capacity with environmental sustainability. Cross-linking methods enhance stability, while pH-dependent performance allows for tailored treatment strategies. Their biodegradability offers a clear advantage over synthetic resins, though mechanical limitations must be addressed for broader application in industrial settings. Advances in material design and processing will further improve their competitiveness as sustainable adsorbents.