Chitosan-functionalized nanoparticles have emerged as effective adsorbents for removing textile dyes from wastewater, combining the advantages of nanoscale materials with the unique properties of chitosan. These hybrid systems, such as chitosan-coated iron oxide (Fe3O4) nanoparticles, exhibit high adsorption capacity, selectivity, and reusability, making them suitable for environmental remediation. The performance of these materials depends on their physicochemical properties, including surface charge, functional group availability, and magnetic responsiveness, which can be tailored for specific dye removal applications.

The adsorption mechanism of chitosan-functionalized nanoparticles is heavily influenced by pH due to the protonation or deprotonation of chitosan’s amino and hydroxyl groups. Chitosan contains primary amine groups that become protonated in acidic conditions (pH < 6.5), forming positively charged NH3+ sites. This enhances electrostatic interactions with anionic dyes, such as reactive or acid dyes, which are prevalent in textile effluents. At higher pH levels (pH > 6.5), chitosan’s amine groups deprotonate, reducing electrostatic attraction but enabling other interactions like hydrogen bonding and chelation. For cationic dyes, such as basic dyes, adsorption is more favorable at neutral to alkaline pH, where the chitosan surface is less positively charged, minimizing repulsion. The pH-dependent behavior allows selective adsorption of different dye classes by adjusting the solution conditions.

Binding kinetics of chitosan-Fe3O4 nanoparticles typically follow a pseudo-second-order model, indicating chemisorption as the rate-limiting step. The initial adsorption rate is fast, with most dye uptake occurring within the first 30 to 60 minutes, followed by a slower equilibrium phase. The high surface area of nanoparticles (often exceeding 50 m²/g) provides abundant active sites, while the porous structure of chitosan facilitates intraparticle diffusion. Studies have shown that increasing chitosan loading on Fe3O4 nanoparticles can enhance adsorption capacity but may reduce diffusion rates if the coating becomes too thick. Optimal chitosan functionalization balances surface accessibility with functional group density.

The adsorption isotherms often fit the Langmuir model, suggesting monolayer coverage of dyes on the nanoparticle surface. Maximum adsorption capacities for common textile dyes, such as methylene blue or Congo red, range from 50 to 300 mg/g, depending on nanoparticle size, chitosan degree of deacetylation, and dye structure. Competitive adsorption occurs in mixed dye systems, where sulfonated dyes with higher charge density are preferentially adsorbed over neutral or weakly charged molecules. Temperature also affects adsorption, with endothermic processes showing increased capacity at higher temperatures due to enhanced diffusion and binding site accessibility.

Chelation plays a critical role in dye adsorption, particularly for metal-complex dyes used in textile printing. Chitosan’s amino and hydroxyl groups coordinate with metal ions (e.g., chromium or copper) in dye molecules, forming stable complexes. This chelation is pH-dependent, with optimal binding occurring near neutral pH for many metal-dye systems. The presence of Fe3O4 further enhances metal ion capture through additional surface interactions, enabling dual mechanisms for dye removal. The magnetic core also simplifies nanoparticle recovery after adsorption, as an external magnetic field can separate the spent adsorbent from treated water without filtration.

Biocompatibility is a key advantage of chitosan-functionalized nanoparticles over conventional adsorbents like activated carbon or synthetic polymers. Chitosan is biodegradable, non-toxic, and derived from renewable sources (e.g., crustacean shells), aligning with green chemistry principles. Fe3O4 is generally considered safe in environmental applications due to its low solubility and minimal iron leaching under neutral conditions. However, nanoparticle stability must be ensured to prevent aggregation or dissolution in extreme pH environments. Crosslinking agents like glutaraldehyde or genipin can improve chitosan stability without significantly compromising adsorption capacity.

Regeneration and reuse of chitosan-Fe3O4 nanoparticles are feasible through desorption at extreme pH or with organic solvents. Acidic or alkaline washes (pH < 3 or pH > 10) disrupt dye-nanoparticle interactions, releasing adsorbed dyes while retaining nanoparticle structure. After regeneration, adsorption capacity typically remains above 80% of the initial value for at least five cycles, making the process economically viable. The magnetic properties of Fe3O4 simplify recovery between cycles, reducing operational costs compared to non-magnetic adsorbents.

Scalability challenges include maintaining nanoparticle dispersion in large-scale systems and optimizing chitosan functionalization for consistent batch production. Spray drying or freeze-drying can produce powdered adsorbent forms suitable for column or batch treatment systems. Pilot-scale studies demonstrate effective dye removal from real textile wastewater, though competing ions (e.g., salts or organic matter) may reduce efficiency compared to synthetic dye solutions. Pre-treatment steps like pH adjustment or filtration can mitigate interference.

Future developments may focus on enhancing selectivity for specific dye classes through chitosan modification (e.g., grafting with carboxyl or sulfonate groups) or combining chitosan-Fe3O4 with other nanomaterials like graphene oxide for synergistic effects. Life cycle assessments are needed to evaluate environmental impacts across full production and application scales. Standardized testing protocols would facilitate comparison between different chitosan-functionalized adsorbents and industrial adoption.

In summary, chitosan-functionalized Fe3O4 nanoparticles offer a sustainable and efficient solution for textile dye removal, leveraging pH-responsive adsorption, magnetic separation, and biocompatibility. Their performance is tunable through material design and process optimization, addressing key challenges in wastewater treatment while minimizing secondary pollution risks. Continued research into scalable synthesis and application methods will further advance their practical implementation in the textile industry.