Structure-tailored hierarchical porous biochar from banana straw: Pore evolution and formation mechanisms

A well-developed pore structure is essential for the application of biochar in agricultural and environmental fields, and its formation is largely determined by pyrolysis conditions and intrinsic feedstock properties. In this study, banana straw biochar (BSBC) was selected as a representative herbaceous biochar to investigate pore formation mechanisms during pyrolysis. The results show that pore evolution follows a four-stage transition from “biological inheritance” to “thermochemical reconstruction,” in which I pores are morphology-retaining structures inherited from plant tissues and organs; II pores are multilayer structures formed within stomata under high-temperature pyrolysis; III pores are hemispherical surface features locally induced by moderate KOH activation; and IV pores are interconnected networks generated through the synergistic effects of high pyrolysis temperature and intensive KOH activation. Pore density increases progressively from Pore I to Pore IV, while circularity reaches a minimum in Pore II and a maximum in Pore III. Further analysis indicates that endogenous elements in the raw material play a significant regulatory role in pore evolution. Specifically, Si acts as a structural stabilizing factor that partially suppresses the collapse of the carbon framework, whereas K promotes pore formation and development by catalyzing carbon structural rearrangement and etching reactions. With increasing thermochemical severity, the specific surface area of biochar increases markedly from 7.72 m 2 ·g −1 to 962.09 m 2 ·g −1 , indicating a transformation from natural biological structures to high-surface-area porous functional materials, in which micropores dominate the overall surface area contribution. This study provides a structural evolution framework and theoretical basis for the rational design of hierarchically porous biochars derived from agricultural residues. • A morphology-based framework elucidates hierarchical pore development pathways. • Intrinsic properties of banana straw determine biochar pore evolution. • Thermal and chemical perturbations synergistically regulate pore transformation.