Transitioning from hydrothermal carbonization to humification for producing artificial humic substances

Hydrothermal humification (HTH) is an emerging green route for converting biomass into artificial humic substances (A-HS), oxygen-rich macromolecules resembling natural humics and relevant for carbon sequestration and soil improvement. HTH represents the alkaline version of hydrothermal carbonization (HTC), acidic and yielding to insoluble hydrochars. Here, we investigate the pathways behind this transition by treating glucose, cellulose, and lignin at 220 °C for 4 h under controlled concentrations of alkali (KOH). Under HTH, carbohydrates undergo retro-aldol cleavage, yielding low solid yields and lactic acid and aldehydes that condense into phenolic/aromatic A-HS, as shown by solid-state NMR. In contrast, HTC mainly forms furanic hydrochars. Lignin under HTH undergoes near-complete depolymerization/defragmentation (<1.1 % residue), yielding oxygen-rich aromatic A-HS, whereas it remains unreacted under HTC. Overall, the resulting A-HS are aromatic, rich in oxygenated functionalities, and fluorescent, resembling natural HS and highlighting the potential of HTH for producing soil-relevant humics from biomass.