From biomass to humic acid: Is there an accelerated way to go?

Acid hydrothermal treatment of biowaste is favorable for producing hydrochar and corresponding hydrothermal solution containing saccharides, furans and aromatic compounds, while the alkaline hydrothermal condition is beneficial for their humification to form hydrothermal humic acid (HHA). Herein, the humification potential of hydrochar, hydrothermal solution, and their mixture under alkaline hydrothermal conditions was compared to further reveal their contribution to HHA. The result showed the main source of HHA was hydrochar, occupying 70 wt% of total HHA yield from their mixture while hydrothermal solution only contributed 17 wt%. Moreover, the HHA that existed in hydrochar (denoted as HHA acid ) was 32 wt% while the hydrochar-derived HHA obtained after alkaline hydrothermal treatment (denoted as HHA alk ) increased to 65 wt%, close to that from commercial coal or lignite. The hydrochar was further separated into four components, i.e. hydrothermal fulvic acid (HFA), stable HHA acid (S-HHA acid ) non-stable HHA acid ( N -HHA acid ) and remaining substance (RS), aiming to reveal the major pathway of the incremental HHA production under alkaline hydrothermal condition. Reactions of RS + HFA, RS +  N -HHA acid and self-synthesizing of RS all contributed to the incremental HHA alk yield with the major contribution (75C%) from the first combination (RS + HFA). The pathways of alkaline hydrothermal humification of hydrochar were revealed after analyzing the molecular level structure of HHA alk by UPLC-MS. This in-depth analysis fills the knowledge gap in the mechanism of hydrothermal humification of biowaste.