Biopolymer-based hydrogels for the remediation of heavy metals and dyes from wastewater

The increasing accumulation of heavy metals in aquatic environments poses a persistent threat to both environmental sustainability and public health. Conventional water treatment technologies often suffer from drawbacks such as high operational cost, limited selectivity, and poor regeneration efficiency. In recent years, biopolymer-based hydrogels have gained attention as efficient, eco-friendly, and tunable adsorbents for the removal of heavy metal ions from aqueous systems, owing to their high-water retention capacity, surface functional versatility, biodegradability, and environmental compatibility. Despite significant research efforts, current findings remain scattered, and the lack of a unified framework for evaluating hydrogel performance presents a challenge for practical implementation. This review provides a comprehensive and critical synthesis of recent progress in the design, functionalization, and application of natural biopolymers, particularly cellulose, starch, and chitosan, in hydrogel fabrication for waterpurification. Special emphasis is placed on chemical modification strategies, including carboxymethylation, graft copolymerization, and sulfonation, that introduce or enhance reactive functional groups responsible for metal ion binding. The mechanisms underlyingmetal ion removal, such as ion exchange, electrostatic interaction, coordination, and chelation, are discussed in detail. The role of crosslinking methods in controlling hydrogel structure, porosity, stability, and recyclability is also addressed. In addition, the integration of nanomaterials and biochar into hydrogel matrices for performance enhancement is highlighted. Finally, the review outlines key challenges and future directions in translating these materials from laboratory-scale studies to practical, scalable, and sustainable water treatment solutions.