Lignin-Derived Biochar in Biorefineries: Linking Structure–Property Relationships to Emerging Contaminant Removal and Controlled Release Applications

Lignin is an abundant aromatic biopolymer generated as a major by-product in lignocellulosic biorefineries, and its efficient valorization is essential for improving process sustainability and economic viability. Among current upgrading strategies, the conversion of lignin into lignin-derived biochar (LDB) has emerged as a promising route because of its high carbon yield, scalable production, and tunable physicochemical properties. This review examines the relationships between lignin structure, thermochemical conversion pathways, and the resulting properties of LDB materials within biorefinery systems. The influence of different technical lignins and conversion routes, including pyrolysis and hydrothermal carbonization, is critically discussed together with post-functionalization strategies. Particular attention is devoted to emerging applications in contaminant adsorption and controlled release systems for agrochemicals. The adsorption mechanisms governing pharmaceuticals, pesticides, microplastics, and PFAS removal are analyzed, while the dual role of LDB as both adsorbent and delivery platform is highlighted. Current limitations include lignin heterogeneity, lack of standardized evaluation protocols, and insufficient validation under realistic environmental conditions. Overall, LDB represents a versatile and scalable platform for lignin valorization and sustainable material design within circular bioeconomy frameworks.