Circular Valorization of Olive Pomace via Hydrothermal Carbonization and Humification

This study offers a comparative analysis of hydrothermal carbonization (HTC) and hydrothermal humification (HTH) of olive pomace as complementary methods for valorizing agroindustrial residues. While HTC is commonly used to produce hydrochar, HTH has been less studied and is known to promote the formation of artificial humic acids (AHAs). Experiments were conducted at temperatures of 180, 200, and 220 °C with a residence time of 1 hour. In HTH experiments, KOH was added as an alkaline agent to increase the pH. After treatments, the hydrothermal products were separated, and their yields, chemical compositions, and structural features were analyzed. The highest hydrochar yield (72.86%) was achieved in HTC at 180 °C, while the maximum AHA yield (24.44%) was obtained in HTH at 180 °C. Yields decreased significantly at higher temperatures. Analytical techniques, including FTIR, elemental analysis, and TGA, confirmed that AHAs exhibit functional and compositional characteristics similar to natural humic substances. This direct comparison highlights the trade-offs between hydrochar and AHA production, demonstrating that HTC and HTH can serve as complementary circular valorization routes for olive pomace residues, supporting both energy recovery and soil improvement strategies.