Hydrothermal humification and fulvification of grass for artificial humic substance production and by-product applications

This study compares hydrothermal carbonization (HTC), humification (HTH), and fulvification (HTF) of grass biomass to show how increasing alkalinity governs carbon conversion pathways and liquid product formation. Under HTC without alkali addition, carbon conversion was dominated by dehydration and condensation reactions, yielding the highest solid yield (54.10 wt%) with high carbon content (60.6 wt%), elevated energy potential (HHV = 25.70 MJ/kg), and the greatest intrinsic thermal stability (T 50 = 435 °C), while artificial humic acids were not formed and carbon transfer to the liquid phase remained limited (TC = 15.85 g/L; TOC = 15.75 g/L), accompanied by minimal carbon leachability. Moderate alkalinity under HTH reduced solid yield to 44.50 wt% and promoted partial depolymerization and humification, increasing liquid-phase carbon concentrations (TC = 40.47 g/L; TOC = 36.96 g/L) and enabling artificial humic acid formation (2.25 wt%), while producing hydrochar with favorable surface chemistry for adsorption, reflected by the highest crystal violet uptake (196.28 mg/g). Further increasing alkalinity under HTF shifted carbon partitioning toward the liquid phase, reducing solid yield to 13.18 wt% while maximizing artificial humic acid formation (8.80 wt%) and liquid-phase carbon concentrations (TC = 62.15 g/L; TOC = 58.52 g/L); HTF liquids were enriched in lactic acid, characterized by sub-100 nm colloids, the most negative zeta potential (−8.72 mV), and enhanced seed germination up to 95% at 50-fold dilution. Overall, alkalinity is identified as a key parameter directing integrated carbon valorization from energy-dense solids to adsorption-active humified materials and bioactive liquid products within a circular biomass framework.