Valorization of corn straw digestate compost via hydrothermal humification for sustainable humic acid production

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Corn straw (CS) digestate, rich in lignocellulose, generates substantial residues after anaerobic digestion. This study explored the hydrothermal humification (HTH) characteristics of CS digestate (CSD) and CSD compost (CSDC) from a continuous-feed biogas project. After HTH at 180 °C for 4 h, the humic-like fluorescence signals and humic acid (HA) yield of CSDC were significantly higher than those of CS and CSD, with a maximum HA yield of 31.66 wt%, 3.1 and 2.6 times greater, respectively. Lignin degradation during HTH (removal rates of 71.78 %–87.58 %) promoted HA formation. The HA from CSDC exhibited a composition and morphology more similar to lignite HA, featuring higher aliphatic carbon and surface oxygen content, closely resembling black soil HA. These properties endowed it with enhanced aromaticity, ion-exchange capacity, and biodegradability. A germination test showed that 0.001 % HA treatment achieved a germination index of 132 %. Thus, CSDC is an excellent feedstock for artificial HA production, supporting the high-value utilization of CS digestate.