Closing the Loop: Transforming Black Liquor Waste into Sustainable Porous Carbon Materials

Abstract Black liquor, an industrially generated, low-value by-product of pulp and paper manufacturing, is produced in significant quantities worldwide (around 100 million tons annually). Transforming black liquor into valuable products is crucial but remains challenging. In this work, lignin was recovered from actual industrial black liquor collected from Kerala Paper Products Limited (KPPL), Kerala, India, via acid precipitation using dilute sulfuric acid at a selected pH of 2, achieving an extraction yield of approximately 13–18 %. The lignin derived from black liquor was pyrolysed at 500 o C to 800 o C under an inert atmosphere. Comprehensive characterization of the lignin derived from black liquor and commercial lignin and its carbonized derivatives was performed and compared, which revealed significant temperature dependent evolution in chemical structure, morphology, and porosity. Carbonization resulted in yields of 44–53 % for lignin derived from black liquor. The surface area increased with temperature, reaching a maximum of 385.16 m 2 /g for carbonized lignin derived from black liquor at 700 o C, compared to 278.37 m 2 /g for carbonized commercial lignin at the same temperature. Optimal pore development and mesoporosity were observed at 700 o C, while higher temperatures (800 o C) promoted enhanced graphitic ordering with evidence of further structural ordering and a slight reduction in pore development. The carbonized lignin derived from black liquor exhibited a three-dimensional porous structure with average pore radii of 1.14–2.63 nm and exhibited physicochemical characteristics indicating potential for applications where porous carbon materials are desirable. LC700 and BLC700 were further evaluated as electrode materials for energy storage applications using cyclic voltammetry and galvanostatic charge–discharge measurements, with BLC700 showing enhanced electrochemical charge–storage performance compared with LC700. This study highlights an eco-friendly approach to valorizing industrial black liquor through its conversion into biomass-based porous carbon materials, contributing to sustainable materials development and circular economy principles.