Application of Hydrogels in Adsorptive Removal of Aqueous Pollutants

Heavy metal ions (e.g., Cd2+, Pb2+, Cu2+, Mg2+) and organic pollutants (like dyes) present in industrial wastewaters are one of the major causes of pollution of groundwater sources. These pollutants are toxic to humans, plants and aquatic life and should be removed from wastewater before disposal. Various treatment technologies have been reported to treat pollutants from aqueous media, such as coagulation and flocculation, oxidation, membrane filtration, and adsorption. Most of these methods are associated with some shortcomings and challenges in terms of applicability, efficiency, and cost. Based on economical aspect, flexibility and simplicity of design, and availability of wide range of adsorbents, adsorption is recommended as an effective method for removal of organic/inorganic pollutants from aqueous media. Hydrogels are three-dimensional flexible polymeric networks with extensive applications in the biomedical, pharmaceutical, agriculture, biotechnology and separation processes fields. Due to the unique physical and chemical characteristics of hydrogels, such as hydrophilicity, swelling ability, high adsorption capacity, the presence of various specific functional groups and modifiability, an increasing research interest in the development and application of novel hydrogels in water and wastewater treatment has emerged. Hydrogels have exhibited superior performance in the adsorptive removal of a wide range of aqueous pollutants including heavy metals and toxic dyes. Due to the lack of an overview on applications of hydrogels in adsorption of pollutants, this review investigates the different steps involved in the hydrogel-based treatment systems, the influencing factors and mechanisms of pollutants removal. Major challenges about adsorption kinetics, operational pH range, interference, and hydrogel recovery are discussed. Finally, important considerations like stability, reusability of hydrogels and resource recovery are discussed for economic and sustainability concerns.