Progress toward Hydrogels in Removing Heavy Metals from Water: Problems and Solutions—A Review

Hydrogels, as a new type of biomaterial with unique physical and chemical characteristics, have three-dimensional solid networks constructed by hydrophilic polymer chains. The superior swell-ability, biocompatibility, and functionalization have made them widely explored and applied in the removal of heavy metals from water. Due to the potential threats of heavy metals, most wastewater systems control industrial discharges at a multiple (e.g., 100 times higher) of the maximum contaminated levels of heavy metals in drinking water. Nevertheless, how to employ the hydrogel adsorbent fabricated in the lab for expanded applications in real wastewater treatment is still a critical challenge. In this review, we scrutinize and emphasize the latest developments in the synthesis and application of hydrogels in removing and recycling heavy metals from water. In particular, the barriers restraining development and scale-up applications are presented, accompanied by the corresponding solutions raised by our point of view. A detailed review is started from the typical synthesis of hydrogels. Afterward, the innovative and representative strategies with computer-aided design for synthesizing hydrogels with the desired capacities are discussed and evaluated. Challenges in perfecting the hydrogels with outstanding properties in improving the anti-interference capability, accelerating the adsorption rate, broadening the operational pH range, enhancing the selectivity, and minimizing the toxicity are clarified. In addition, the mechanical strength, resource recovery, and reusability of the hydrogels as well as reasonable mass transfer models for adsorber design are used in engineering applications. We also shed light on further improving the features of the expected hydrogels.