Radical-enhanced humification of straw-impregnated biogas slurry toward artificial humic acid fertilizer

The residual biogas slurry (BS) generated after straw impregnation contains dissolved lignocellulosic fragments and other humification precursors but remains insufficiently utilized. Here, a mild radical-enhanced humification strategy was developed to convert post-impregnation BS into a humic acid (HA)-like product at room temperature. An alkaline persulfate (PS) activation system using citrate (CA)-chelated Fe²⁺ was established to promote the oxidative transformation and subsequent condensation of soluble organic precursors. Under the selected conditions (1.0 g KOH, 3.0 g PS, 0.75 g Fe²⁺, and 1.0 g CA per 100 mL BS), the HA yield reached approximately 13.3 g/L. Electron paramagnetic resonance and quenching experiments confirmed that sulfate and hydroxyl radicals were the dominant reactive species. Spectroscopic analyses demonstrated that radical oxidation preferentially transformed labile proteinaceous, polysaccharide-derived, and aliphatic fractions into reactive oxygenated intermediates, which subsequently underwent aromatic coupling, condensation, and polymerization to form humic-like macromolecules. In a bok choy pot experiment, the HA product produced greater plant length, root development, and biomass than untreated BS and mineral HA. Overall, this study provides a rapid, room-temperature approach for upgrading residual BS into a value-added HA-like product and supports the integrated utilization of straw and anaerobic-digestion effluents.