Pretreatment-guided design of Fe-doped soybean straw biochar: Linking waste valorization, carbon stability, and functional performance

Abstract

Sustainable valorization of agricultural residues is critical for advancing the circular bioeconomy as well as renewable material development. Soybean straw, an abundant yet underutilized lignocellulosic biomass, offers significant potential as a precursor for engineered carbon materials. This study investigates a pretreatment-guided thermochemical approach for converting soybean straw into biochars and elucidates how the contrast between low- and high-severity pyrolysis conditions, combined with mineral-assisted pathways, governs carbon structure, surface chemistry, and functional performance. Biomass was subjected to thermal pretreatment or ferric chloride (FeCl3) impregnation, followed by pyrolysis at 250°C and 500°C. The resulting biochars were characterized based on their elemental composition, surface functionality, cation exchange capacity, oxidation resistance, and thermal stability to establish processing-structure-property relationships. Biochars produced at 500°C exhibited higher carbon content (61.3–67.2%) and reduced oxygen content (9.3–16.6%), indicating enhanced aromaticity and structural stability. In contrast, ferric chloride pretreatment promoted mineral-carbon interactions and surface charge development, significantly improving cation exchange capacity (CEC) (up to 56 cmol kg⁻¹) without compromising stability. Phytotoxicity assays confirmed high germination rates, indicating environmental compatibility. The results reveal a distinct stability function trade-off

demonstrating that the low- versus high-severity pyrolysis conditions examined here control carbon permanence, while mineral-assisted pretreatment governs surface reactivity. Thus, the rational design of tunable biomass-derived carbon materials is warranted. © 2026 Elsevier B.V.