Pyrolysis temperature-regulated evolution of micro-morphology and surface chemical structure of biochar derived from papermaking black liquor lignin

Biochars possess distinct, application-specific functions owing to their different physicochemical properties. To achieve the valorization of black liquor lignin (LG) from the pulp and paper industry, LG-derived biochars were prepared at pyrolysis temperatures of 100–1000 °C. The resulting biochars were comprehensively characterized in terms of micro-morphology, carbon structure/speciation, surface functional groups, thermal stability, pore structure, and elemental composition, and the evolution of physicochemical properties regulated by pyrolysis temperature was elucidated. The results show that at relatively low pyrolysis temperatures (≤200 °C), LG does not decompose, and the physicochemical properties of the obtained biochar are essentially consistent with those of LG. At 300–500 °C, extensive decomposition of LG occurs, and the resulting biochars exhibit abundant pore-channel structures predominantly assigned to macropores; the carbon structure is dominated by amorphous carbon; oxygen-containing and aliphatic functional groups weaken or disappear; and the thermal stability is poor. With further increases in pyrolysis temperature (≥600 ℃), the pore structure of LG-derived biochar becomes more developed, the carbon structure gradually graphitizes, aromatic functional groups increase, and thermal stability is enhanced. Overall, this study provides a fundamental basis for the targeted design of black-liquor-derived carbons. Specifically, the macropore-abundant biochars produced at moderate temperatures (300–500 ℃) present highly favorable structural prerequisites for mass-transfer-demanding applications (e.g., bulky-molecule adsorption or catalyst supports), whereas the highly graphitized carbons generated at elevated temperatures (≥ 800 ℃) offer promising potential for electrochemical energy storage and conductive composites.