Freeze-thaw priming enhances artificial humification of rice straw through oxidative condensation of hydrochar and liquid-phase.

The effect of freeze-thaw pretreatment on the artificial humification of rice straw via hydrothermal carbonization was evaluated in this work. The physical-chemical characteristics of hydrochar were characterized using XRD, XPS, FTIR, SEM, and the liquid was analyzed by FT-ICR-MS. Results showed that one cycle of freeze-thaw at -20 °C for 24h increased total humic acid yield by 7.2 % compared to untreated straw, with a 6.8 % enhancement under KOH-assisted hydrothermal humification conditions (KOH-to-straw mass ratio of 1:5, 200 °C with reaction time of 2h). The pretreatment reduced crystallinity from 20.53 % to 12.85 %, exposing amorphous cellulose for preferential hydrolysis. FT-ICR-MS reveals that freeze-thaw more than doubles carbohydrate diversity in dissolved organic matter while increasing aliphatic species by 18 %. XPS confirms the emergence of carboxyl groups (4.65 % of surface carbon) and a 7.5 % increase in surface oxygen content. These cryogenic with modifications promoted oxidative condensation, yielding lower-molecular-weight humic substances with enhanced aromaticity.