Chitosan-based adsorbents for water purification: Mechanisms, performance, and sustainability

Chitosan, a biodegradable polysaccharide derived from chitin, has emerged as a sustainable and high-performance adsorbent for water purification, effectively addressing global challenges posed by heavy metals, dyes, pharmaceuticals, and emerging contaminants. This review comprehensively examines chitosan-based adsorbents, covering their adsorption mechanisms (e.g., electrostatic interactions, chelation, hydrogen bonding, and hydrophobic interactions), performance metrics (typical adsorption capacities of 120–600 mg/g and removal efficiencies of 80–95 % under optimized conditions, influenced by factors such as pH, temperature, pollutant type, and matrix complexity), and environmental benefits. Key advancements in modification strategies—such as cross-linking, grafting, composite formation, and biopolymer blending—improve stability, selectivity, and reusability, with many adsorbents retaining 80–90 % capacity over multiple cycles. Life Cycle Assessments reveal chitosan’s significantly lower carbon footprint (1.5–2.5 kg CO₂-eq/kg) compared to conventional materials like activated carbon (8–12 kg CO₂-eq/kg) and ion-exchange resins (15–20 kg CO₂-eq/kg), owing to its renewable sourcing from seafood waste and inherent biodegradability. The review critically analyzes challenges, including pH sensitivity, ion competition in complex wastewater, and scalability issues, while highlighting pilot studies demonstrating 80–95 % efficacy in real-world applications. Future directions focus on green modification techniques (e.g., solvent-free synthesis, enzymatic grafting), smart adsorbents (pH-responsive, magnetic), and hybrid systems (e.g., membrane filtration, photocatalysis) to enhance industrial viability. By integrating these innovations, chitosan-based adsorbents present a scalable, eco-friendly solution for sustainable wastewater treatment.