Effects of alkaline hydrothermal extraction on the molecular organization and antioxidant activity of humic substances derived from sugarcane straw hydrochar

Hydrothermal humic acids (HHA) have attracted attention due to their redox-active properties and potential for lignocellulosic waste valorization. However, the effects of alkaline hydrothermal extraction temperature on their molecular organization and antioxidant activity remain poorly understood. This study investigated the influence of extraction temperature (120–200 °C) on the structural characteristics and antioxidant properties of HHA produced from sugarcane straw hydrochar, with humic acid obtained by conventional alkaline extraction at 90 °C (HA90) included for comparison. HHA were characterized by SEM/EDS, elemental analysis, TGA, FTIR, and one- and two-dimensional NMR spectroscopy, together with measurements of total acidity and carboxylic and phenolic hydroxyl groups. Total phenolic content was determined, and antioxidant activity was evaluated by DPPH radical scavenging. Extraction yields remained similar among treatments, whereas increasing temperature promoted changes in chemical composition, thermal stability, and the distribution of oxygen-containing functional groups. 1 H NMR showed an increase in the relative contribution of the δ 1.6–3.0 region, from 13.35% in HA90 to 21.13% in HHA200. HSQC and HMBC analyses indicated the presence of methoxyl groups associated with lignin-derived oxygenated aromatic structures. Increasing extraction temperature enhanced total phenolic content from 85.32 to 146.56 mg GAE g −1 and improved antioxidant activity, with IC50 decreasing from 1.34 to 0.72 g L −1 . Exploratory correlation analysis indicated significant associations between antioxidant activity, phenolic content, phenolic hydroxyl groups, and other chemical constituents. The alkaline hydrothermal extraction modified the molecular organization and antioxidant properties of HHA, supporting the valorization of sugarcane residues into redox-active humic materials.