One-Step CO2-Assisted Pyrolysis of Spent Coffee Grounds: A Simpler Route to Highly Porous Biochar Compared with Two-Step Pyrolysis–CO2 Activation

Spent coffee grounds (SCG) are an abundant, carbon-rich residue that can be valorized through thermochemical conversion into biochar. Conventional CO2 activation is typically performed in a two-step process, which is time- and energy-consuming. This study aims to evaluate whether a one-step CO2-assisted pyrolysis can produce biochar with comparable or enhanced structural and textural properties while simplifying the process. We compare a two-step pyrolysis process followed by CO2 activation with a one-step CO2-assisted route for producing biochar from SCG. CO2 treatment markedly increases surface area (from 9.8 m2∙g−1 to 550.6–671.0 m2∙g−1) and pore volume. FTIR and Boehm titration indicate depletion of oxygenated surface groups, while N2 adsorption–desorption analyses and SEM reveal a more uniform micro/mesoporous texture for the one-step sample. Although fixed carbon decreases due to gasification, the one-step route delivers superior textural properties in a single thermal stage, reducing energy demand. These results highlight one-step CO2-assisted pyrolysis as an efficient, scalable option for producing high-porosity biochar from coffee waste.