Progress, adsorption mechanisms, and future perspectives of biopolymer-based chitosan adsorbents for dye removal from wastewater

ABSTRACT

Industrial expansion has increased the discharge of hazardous chemicals into aquatic systems, making water pollution a persistent global challenge. Synthetic dyes are among the most common contaminants in industrial effluents and pose serious environmental and public-health risks because of their strong color, toxicity, and resistance to biodegradation. Conventional treatment methods often show limited efficiency, especially at low dye concentrations or in complex wastewater matrices. Adsorption remains one of the most practical dye removal methods because it is simple, economical, and adaptable to bio-based sorbents. This review critically evaluates chitosan-based adsorbents for dye-contaminated wastewater, with emphasis on material design, adsorption mechanisms, treatment efficiency, regeneration, and practical limitations. The reviewed studies were compared according to modification strategy, dye class, adsorption capacity, operating conditions, reusability, and practical relevance. Findings indicate that pristine chitosan can remove over 85% of anionic dyes under acidic conditions near pH 5, but its weak mechanical stability limits application. Modified chitosan materials generally provide higher adsorption capacity, wider effective pH range (2–10), and better durability and regeneration. Adsorption is mainly controlled by electrostatic attraction, hydrogen bonding, π–π interactions, and chelation. Overall, composites and functionalized chitosan outperform native chitosan, although scalability, selectivity in real wastewater, and cost remain major challenges.