Hydrothermal Humification of Wetland Biomass: Coproduction of Artificial Humic Acid and Endogenous Potassium-Driven Fulvic Acid

The degradation of soil organic matter and nutrient depletion pose challenges for global agricultural sustainability. In this work, a hydrothermal humification (HTH) process was developed to simultaneously address these challenges by converting K-rich Canna indica biomass waste into artificial humic acids (HA) and K-enriched fulvic acids (FA-K) without external potassium sources. The results revealed that alkali concentration was the dominant factor, with 1 mol·L–1 NaOH yielding the highest HA yield (17.91% at 180 °C) by promoting lignocellulose depolymerization and polycondensation. Increasing alkalinity (≥1 mol·L–1 NaOH) promoted the migration of endogenous K, releasing over 81.54%–91.36% into the liquid phase. The released K+ preferentially associated with liquid-phase fulvic acid (LFA), forming −COO-K coordination complexes and reaching concentrations of 50.74–53.62 mg·g–1 under 1 mol·L–1 NaOH conditions. Spectroscopic analyses identified carboxylate groups (−COO–) as the primary coordination sites responsible for K retention within the oxidized aliphatic frameworks of LFA. A dual-pathway mechanism was proposed in which the lignin-derived aromatics polymerized into HA, while lipid/carbohydrate hydrolysates assembled into carboxyl-rich LFA acting as efficient potassium carriers. This study establishes a sustainable waste-to-resource approach for the concurrent production of humic substances and organic K fertilizers, offering a feasible pathway toward resource circularity and enhanced agricultural resilience.