Structure–Function Engineering of Lignin-Based Hydrogels for Adsorptive Removal of Organic Dyes and Heavy Metal Ions: A Category-Oriented Review

Given the widespread contamination of water bodies by diverse pollutants, particularly heavy metal ions and organic dyes, there is an urgent need to develop efficient and sustainable biomass adsorbents. Lignin is rich in active groups such as phenolic hydroxyl and carboxyl groups, making it a natural adsorbent. However, its application is still hindered by limitations, including restricted solubility and low reactivity. Converting lignin into three-dimensional porous hydrogels not only overcomes the inherent structural brittleness of lignin-based materials but also accelerates the diffusion kinetics of pollutants through well-developed pore structures, thereby fully exposing the active adsorption sites. This paper systematically reviews the latest progress in lignin-based hydrogels for water treatment and discusses in depth the underlying logic of “structure construction–micromorphology–adsorption performance.” First, this review summarizes synthesis strategies ranging from molecular-level modification to morphology regulation, including nano-reinforcement, magnetic functionalization, and interpenetrating polymer networks. It then provides a pollutant-specific analysis of the adsorption mechanisms of lignin-based adsorbents. For heavy metal ions, such as Pb2+ and Cr(VI), removal is mainly associated with coordination/complexation, ion exchange, and redox reactions. For typical organic dyes, adsorption is primarily driven by π–π interactions, hydrogen bonding, and electrostatic attraction. The effects of environmental factors, such as pH, are also systematically discussed. Finally, considering current challenges related to mechanical strength, regeneration performance, and practical application, this review outlines future research directions for the development of multifunctional, integrated, and stimuli-responsive lignin-based adsorbents.