Phosphoric Acid Addition: Insight into the Mechanism Governing Biochar Structural Evolution

Phosphoric acid (H3PO4) pretreatment is an effective method to improve biochar properties, yet its evolution mechanism remains incompletely elucidated. This study investigated the synergistic pyrolysis of H3PO4 and mottled bamboo at different temperatures in a fixed-bed reactor. Results showed that during impregnation, H3PO4 promoted the partial dissolution of hemicellulose and reduced cellulose polymerization, resulting in a decrease in the activation energy of the fast pyrolysis stage from 96.72 kJ/mol (pristine bamboo biochar, MB) to 75.75 kJ/mol (H3PO4-modified bamboo biochar, MB/H3PO4). With increasing temperature, the pore structure of the modified biochar was enhanced while its graphitization degree decreased, owing to the catalytic effect of H+ and the cross-linking action of the acid. Meanwhile, the addition of H3PO4 facilitated the rearrangement of oxygen-containing heterocycles, and the incorporation of small-molecule benzene rings further improved the aromatization degree of the modified biochar. In conclusion, it functions as a catalyst, reactant, and pore-expanding agent during pyrolysis. This study further broadens the understanding of biochar evolution mechanisms regulated by phosphorus-containing additives, and provides a theoretical basis for optimizing biochar properties and producing phosphorus-rich biochar.