Ball-milling medium dictates the reconstruction pathway of hydrochar: Coupling pore opening with functionality retention.

Hydrochar retains abundant oxygen-containing functional groups but little porosity, and thermal upgrading opens pores at the expense of surface chemistry. Ball milling requires no external heating, so its structural outcome is decided by how freshly fragmented particles reorganize. Here, dry and water-mediated ball milling of corn-straw hydrochar were compared to resolve how the milling medium directs this reorganization, with 600℃ biochar as a thermal reference. Without a liquid phase, fragments recontacted and densified: the specific surface area fell from 11.23 to 2.91 m2·g-1, the total pore volume from 0.0818 to 0.0179 cm3·g-1, and the CO proportion from 26.2 % to 19.0 %, so pore closure and carbonyl loss occurred together. In water, dispersion suppressed reagglomeration, increasing the specific surface area, total pore volume, and CO proportion to 34.26 m2·g-1, 0.1903 cm3·g-1, and 33.3 %, respectively. HTC-WBM adsorbed 2.53, 2.41, and 3.15 times as much TC as HTC, HTC-DBM, and BC, respectively. Although BC had the highest specific surface area (54.99 m2·g-1), its total pore volume and BET-equivalent average pore diameter were only 0.0439 cm3·g-1 and 3.19 nm, compared with 0.1903 cm3·g-1 and 22.22 nm for HTC-WBM. Open meso-/macropores facilitated TC transport to retained oxygen-containing sites, where hydrogen bonding and electrostatic interactions contributed to adsorption. Spectroscopic evidence and SHAP analysis were consistent with this coupled pore-accessibility/functionality effect. Medium-controlled milling therefore couples pore opening with functionality retention.