Recent advances in lignin-based carbon materials for pollutant removal and oil-water separation: a review

The rapid escalation of water pollution from industrial effluents, oil spills, and emerging contaminants has created an urgent need for sustainable, high-performance remediation materials. Although surface water covers nearly 71% of the Earth's surface, only about 2.5% constitutes freshwater, and less than 1.2% is readily accessible for human use. Moreover, over 113 billion m³ of untreated wastewater is discharged annually into natural water bodies, further exacerbating global water scarcity and pollution. Lignin accounts for 15-30% of lignocellulosic biomass and contributes more than 60% of its carbon content. It has emerged as a promising precursor for advanced carbon materials. Globally, over 100 million tonnes of lignin are generated annually by the pulp and paper industry, yet 98% is still underutilised or combusted for low-value energy recovery. This review critically summarises recent advances in the conversion of lignin into porous, heteroatom-doped carbon materials via thermochemical routes, including pyrolysis, hydrothermal carbonisation, and chemical activation. Lignin-derived carbons exhibit exceptionally higher surface areas (1700-2285 m² g^-1), tunable pore structures, and rich surface functionalities, enabling efficient removal of heavy metals, dyes, pharmaceuticals, and oils. Furthermore, the adsorption capacities exceed 300 mg g^-1 for organic pollutants, while lignin-based foams, aerogels, and sponges demonstrate oil absorption efficiencies greater than 20-40 g g^-1 with excellent recyclability. Key adsorption mechanisms, including electrostatic attraction, π-π interactions, surface complexation, and pore filling, are systematically discussed. Furthermore, current challenges related to scalability, regeneration, and process economics are highlighted. Overall, lignin-derived carbon materials offer a low-cost, sustainable, and scalable solution for advanced wastewater treatment and oil-water separation, aligning closely with the principles of the circular bioeconomy and UN Sustainable Development Goals (SDGs 6, 12, and 13).