Transforming spent coffee grounds into sulfonated char

Spent coffee grounds (SCG) are an abundant yet underutilized lignocellulosic waste with a high potential for sustainable valorization in environmental applications. Addressing the limitations of conventional pyrolysis for carbon material production, this study aims to develop an integrated biorefinery approach to convert SCG into a sulfonated carbon adsorbent (S-char) while co-producing value-added byproducts. Unlike conventional pyrolysis, which requires high temperatures and post-synthesis activation, the proposed simultaneous carbonization-sulfonation (SCS) process enables in situ functionalization at significantly lower temperatures (100–180 °C). Through sequential delipidation, acid hydrolysis, and SCS, the process achieves three key outcomes: (1) lipid recovery, (2) fermentable sugar production, and (3) functionalized S-char with superior adsorption properties. The resulting S-char exhibited a methylene blue (MB) adsorption capacity of 103.6 mg/g, fivefold higher than conventional pyrolyzed char, owing to its engineered sulfonate groups and optimized porosity. Characterization confirms enhanced surface acidity (2.59 mmol H+/g) and preserved mesoporous structure, enabling efficient dye removal through combined electrostatic, π-π, and hydrogen bonding interactions. This approach maintains environmental sustainability, with an E-factor (31.4) comparable to traditional pyrolysis despite its performance advantages—the cascade process addressing waste management and resource recovery challenges. By simultaneously producing high-performance adsorbents and biorefinery co-products, this work establishes a blueprint for sustainable biomass conversion with applications in wastewater treatment, biofuel production, and circular bioeconomy strategies.