Recent advances in alginate-based hydrogels for the adsorption–desorption of heavy metal ions from water: A review

Water pollution by heavy metal ions (HMs) is a major issue requiring highly efficient treatment methods such as enhanced adsorption. In particular, alginate-based hydrogel has sparked great interest as an ideal candidate for the advanced adsorbents due to their abundant oxygen-containing functional groups, simplicity of modification, abundant availability, eco-friendliness, and sustainability, etc. We focus on presenting a comprehensive overview of recent advances in HMs adsorption–desorption by alginate-based hydrogel. First, the basic and intrinsic gelling properties of alginate, and corresponding synthesis methods of alginate-based hydrogels are concluded systematically. Preparation of alginate-based hydrogels mainly done by the ionotropic gelation method are discussed here. Then, the adsorption mechanisms of HMs by plain alginate hydrogels are simple sorting. Furthermore, synthesis modifications including surface functionalization, blending and composite modification of alginate-based hydrogels for enhanced HMs adsorption are summarized. Besides, the challenges in pH-dependence, low adsorption rate, and poor selectivity for adsorption of HMs from water by alginate-based hydrogels are also discussed. Additionally, we further emphatically discuss the desorption and reusability of alginate-based hydrogels. Finally, challenges and future prospects of fabrications and applications are presented. We expect that the documentation of relevant information will stimulate scientific efforts to further develop functional alginate-based hydrogels in adsorption application.