Alkali pre-soaking facilitates artificial hydrothermal humification of tobacco bio-waste by enhanced oxidative condensation

In this work, a humification pathway of KOH pre-soaking coupled with hydrothermal conversion of tobacco waste by reconfiguring the reaction pathway was mentioned. The hydrochar and liquid products from KOH pre-soaking hydrothermal humification was investigated by XPS, FTIR (2D-COS), XRD, TG, and 3D-EEM analysis. Results revealed that this coupled strategy achieves liquid-phase humic acids (HAs) concentrations of 23.17 g/L within 2 h-equivalent to conventional 24 h alkali-assisted hydrothermal humification. The pre-soaking selectively cleaves lignin-carbohydrate ester/ether bonds and swells cellulose, facilitating oxygen incorporation, with O/C increased 0.30-0.44 compared to hydrochar obtained directly hydrothermal humification 24 h, though the C content of hydrochar derived from KOH pre-soaking hydrothermal humification (1-24 h) increased from 41.0% to 43.20%. The pre-depolymerization shifts the rate-limiting step from slow hydrochar formation to rapid liquid-phase condensation of reactive oxygen-containing intermediates. This KOH pre-soaking shorten the residence time of hydothermal humification by the fact that pre-soaking strengthened the following oxidation and condensation process with high humification degree of HAs production.