Enhanced production of hydrothermal humic acid in a two-step hydrothermal process with acid hydrothermal solution recycling

Acid hydrothermal treatment of biowaste is a beneficial approach for producing hydrochar with high hydrothermal humic acid (HHA) selectivity, along with a by-product hydrothermal solution containing saccharides, furans, and aromatic compounds. Conversely, alkaline hydrothermal conditions are suitable for humification to form HHA. This study explored the recycling of hydrothermal solution as a solvent for hydrochar production, aiming to reduce salt content in wastewater and the need for acid/alkali inputs. The acidity of the recycled hydrothermal solution significantly influenced the quality of the prepared hydrochar in terms of alkaline hydrothermal humification. Insufficient solution acidity resulted in a decrease in hydrochar-derived HHA yield (65.0 wt% to 20.2 wt%). Maintaining sufficient acidity (pH 0) in the recycled hydrothermal solution not only improved the hydrochar yield (29.6 wt% to 33.1 wt%) via polymerization or condensation reactions but also maintained ab high level of hydrochar-derived HHA yield (68.2 wt% to 71.2 wt%). This can be attributed to adequate hydrolysis of biowaste and polymerization of soluble chemicals into the hydrochar. Furthermore, the total yields of hydrochar-derived and solution-derived humic substances were higher compared to values obtained without hydrothermal solution recycling. This study presents an innovative hydrothermal process for HHA production, offering cost savings and environmental benefits.