Engineering chitosan-based metal oxide and metal nanoparticle composites for organic dye remediation: mechanisms, performance, and future perspectives.

The discharge of synthetic dyes from industrial effluents poses a critical threat to public health and aquatic ecosystems owing to their toxicity, persistence, and resistance to biodegradation. Chitosan (CS)-based hybrid materials have emerged as suitable platforms for dye remediation because of the abundant amino and hydroxyl functional groups of CS, its biodegradability, biocompatibility, adsorption capacity, and ability to stabilize inorganic nanoparticles. This review critically examines recent advances in the catalytic reduction and degradation of organic dyes using chitosan-metal nanoparticle and chitosan-metal oxide composites with emphasis on the relationships between composite structure, surface properties, and removal mechanism. The fundamental mechanisms of catalytic reduction and photocatalytic degradation are systematically discussed which highlight the synergistic contribution of chitosan through pollutant pre-concentration, surface interaction, nanoparticle stabilization, morphological control, and enhanced catalyst recovery. A particular attention is given to chitosan-based hybrid with mono-, binary and ternary systems of metal nanoparticles or metal oxides which can provide improved charge separation, reduced electron-hole recombination, catalytic reduction performance, stability of metal nanoparticles/metal oxides in CS matrix, broader light utilization, and easy recyclability after catalysis or photocatalysis. The review shows the influence of pH, temperature, irradiation conditions, and surface characteristics on treatment efficiency of both CS-metal nanoparticles and CS-metal oxides as well as their recovery, regeneration, recyclability, activity retention, and potential metal leaching. Current challenges and future opportunities involving multifunctional architectures, stimuli-responsive systems, immobilization, real wastewater applications, mechanistic characterization, and standardized performance evaluation are identified. This review provides a practical framework for researchers to design efficient, recyclable, and environmentally sustainable chitosan-based catalytic systems for advanced dye wastewater treatment.