Tailoring biochar porosity via fermentation-pyrolysis integration: Trade-offs between pore creation and blockage

Biochar properties are governed by feedstock composition and pyrolysis conditions, yet pretreatment strategies that are simultaneously effective and sustainable remain scarce. To address this gap, this study systematically investigates the coupled effects of fermentation duration (0-40 d) and pyrolysis temperature (400-700 °C) on the pore structure of corn stover-derived biochar. Pyrolysis temperature governs specific surface area, with non-fermented samples reaching 134.2 m2·g-1 at 700 °C, but extending fermentation to 40 d reduces SBET by 66 % and microporosity by 73 % relative to the non-fermented control at the same pyrolysis temperature. This decline likely results from three superimposed mechanisms: (i) melting of alkali metal eutectics that infiltrate micropores, (ii) deposition of anorthite derived from aluminosilicates, and (iii) physical occlusion by carbonized microbial residues. Conversely, fermentation is associated with a 100-200 °C reduction in the temperature required for carbon structural ordering, as microbial nitrogen evolves from pyridinic/pyrrolic to graphitic N, potentially nucleating short-range ordered domains at 500 °C. The optimal processing window is application-dependent: short fermentation (0-10 d) coupled with 700 °C maximizes specific surface area for adsorption, whereas moderate fermentation (20-30 d) at 500-600 °C allows for lower‑temperature fabrication of biochars with improved short-range structural order. This mechanistic framework provides a basis for rational design of biochar properties via integrated fermentation-pyrolysis processing based on correlative evidence, withdirect confirmation of pore‑blocking deposits via TEM‑EDS and true graphitization via HRTEM is required in future work.