Co-pyrolysis of agricultural biomass for potentially functional biochar: combined influence of both feedstocks and structural characterization

This study evaluates how the co-pyrolysis of two agricultural residues, corn stalk and rice husk, influences the physicochemical characteristics of the resulting biochar. Biochars were produced at 400 °C from each feedstock and from a 1:1 (w/w) mixture to assess interaction-driven behavior compared with the individual materials. The mixture biochar exhibited a broader particle-size distribution (SPAN = 3.36 ± 0.14) than the single-feedstock biochars, while maintaining a comparable BET surface area (13.17 ± 0.42 m 2 g⁻ 1 ) relative to rice husk biochar (13.90 ± 0.35 m 2 g⁻ 1 ) and slightly higher than corn stalk biochar (11.96 ± 0.28 m 2 g⁻ 1 ). These results indicate that surface development was largely preserved despite particle coarsening, suggesting interaction effects between feedstocks rather than a purely additive mixing behavior. Zeta potential measurements showed negative surface charge for all samples (− 25.7 ± 1.3 to − 33.7 ± 1.2 mV), reflecting electrostatic surface characteristics associated with oxygen-containing functional groups and mineral phases, without being interpreted as direct evidence of adsorption performance. Mineral composition analysis revealed that the blended biochar integrated silica-rich and nutrient-associated inorganic phases, with Si, K, and Ca as major constituents. Spectroscopic and diffraction analyses further indicated a predominantly amorphous carbon matrix with retained mineral phases, while microscopy confirmed heterogeneous morphology consistent with the combined contribution of both biomass sources. Overall, co-pyrolysis at 400 °C produced a biochar with integrated structural and chemical characteristics derived from both residues. These physicochemical properties suggest potential relevance for environmental applications such as soil amendment or contaminant management; however, application-based experiments (e.g., soil incubation, sorption, or column tests) are required to verify nutrient retention and adsorption performance under realistic conditions.