Background Various pollutants present in industrial wastewater, such as heavy metal ions, radionuclides and organic compounds, are high-toxic and refractory, posing a severe threat to ecological environmental safety and human health. Adsorption method is one of the best technologies for the purification of wastewater due to its advantages of simple operation and controllable cost. Lignin shows innate advantages as an adsorbent for the treatment of industrial wastewater, in terms of unique aromatic skeleton, three-dimensional network structure, and diverse sources.

Analysis/Progress In this paper, the sources, structural characteristics, and reactivity of lignin were elaborated to facilitate the preparation of lignin-based adsorbents. Meanwhile, the core five modification methods, which convert lignin into functional adsorbents, were also analyzed. In detail, activation and carbonization focus on optimizing pore structure and increasing specific surface area, aiming to enhance adsorption capacity. Modification via lignin nanoparticle preparation centers on improving dispersibility and surface activity through nanonization, with an emphasis on accelerating adsorption rate. Composite modification with magnetic materials is based on synergistic adsorption and magnetic separation, focusing on enhancing practical application convenience. Grafting modification targets precise introduction of functional groups, emphasizing improved adsorption selectivity. In practical applications, either a single modification method or a combined modification strategy (such as activation-grafting combination or magnetic-carbonization combination) can be selected according to the type of target pollutants, cost budget, and application scenario. For heavy metal ions, as one of the key concerns in wastewater, which exhibit distinct charge states in aqueous solution, targeted chemical and physical modification of lignin is effective to enhance the adsorption performance. For instance, grafting amino groups onto lignin can achieve the effective chelation of anionic metals (Cr₂O₇²⁻ and AsO₄³⁻), while immobilizing carboxyl groups onto the lignin surface enables the adsorption of cationic metals (Zn²⁺ and Pb²⁺) via forming stable carboxylate complexes. For the organic pollutants, introducing hydrophobic groups (such as alkyl groups and benzene rings) and porous structure onto the lignin surface can realize efficient adsorption of organic contaminants via multiple interaction mechanisms. In particular, the composite modification of lignin with advanced materials, such as reduced graphene oxide and carbon nanotubes, can construct a synergistic system of "porous structure-strong π-π interaction", thereby significantly improving the adsorption capacity of lignin-based materials for refractory organic pollutants. For other categories of contaminants, metal ion doping and amination are recognized as effective approaches for remarkably enhancing the adsorption performance of lignin-based materials. The radionuclides, such as uranium (U(Ⅵ)), cesium (Cs⁺), and strontium (Sr²⁺), are characterized by long half-lives, high toxicity, and high mobility. They can be effectively disposed by lignin-based adsorbents due to the strong functional modifiability and good selectivity, which have become a potential alternative to traditional expensive ion-exchange resins.

Conclusion/Prospect As a novel, low-carbon, green, and low-cost adsorbent material, lignin-based adsorbents show broad application prospects in wastewater treatment. Through techniques such as activation modification, nano crystallization modification, magnetic composite modification, and graft copolymerization modification, the structure and surface properties of lignin can be effectively regulated, thereby significantly improving its adsorption performance and enabling the efficient adsorptive removal of various pollutants from wastewater, including heavy metal ions, organic pollutants, nitrogen and phosphorus compounds, radionuclides, and others. Although current lignin-based adsorbents still face challenges such as unstable raw material quality, difficulties in large-scale preparation, and unsatisfactory regeneration performance, with the continuous optimization and innovation of modification technologies, the precise regulation of adsorbent structures, and the development of green and efficient preparation and regeneration technologies, lignin-based adsorbents are expected to achieve large-scale industrial application. This will provide important support for solving water resource pollution problems and promoting the high-value utilization of renewable resources.