A Comprehensive Guide to Chitosan-Based Adsorbents for Water Remediation: From Isolation to Chemical Functionalization and Nanocomposite Engineering

Chitosan, a natural polysaccharide obtained from crustaceans, insects, and fungi, has been extensively investigated for the development of adsorbents for wastewater treatment. This biopolymer is considered one of the most promising materials for environmental remediation owing to its abundance, renewability, biodegradability, and unique chemical structure, which enables a wide range of chemical modifications. This review provides a comprehensive overview of the state of the art in chitosan functionalization for environmental applications. Chemical modification, including N-acylation, esterification, carboxylation, grafting, amination, sulfonation, phosphorylation, quaternization, Schiff-base formation, and crosslinking, is critically discussed. In addition, physical crosslinking and freeze–thaw processes aimed at improving adsorbent stability are reviewed. Plasma treatment as a strategy to increase the surface area of the adsorbents is also examined. The role of chitosan as a supporting matrix for the preparation of nanocomposites containing inorganic oxides and carbon-based nanomaterials is discussed. Particular attention is given to the influence of structural parameters of chitosan, including crystallinity, molecular weight, and biological source, on adsorption performance. The adsorption capacities and regeneration potential of modified chitosan-based adsorbents were analyzed. Overall, this review highlights recent advances in chitosan functionalization. In particular, it emphasized the need for life cycle assessment studies from cradle to grave to better evaluate the sustainability of the prepared materials and guide development.