Fe-assisted low-temperature torrefaction of composted pig manure: Regulating intermediate redistribution toward enhanced humification and agronomic performance

Composted pig manure (CPM) contains abundant organic matter; however, its complex matrix tends to undergo competitive carbonization during pyrolysis, limiting humification efficiency. In this study, a two-step process combining Fe-assisted low-temperature pyrolysis with alkaline extraction was developed to convert CPM into potassium humate (HA-K). Under optimized conditions (225 °C, 120 min, 5 wt% KOH), the HA-K yield increased from 49.07% to 78.14% with the addition of 1.5 wt% Fe, indicating a substantial enhancement in conversion efficiency. Fe incorporation promoted oxygen functionalization, aromatic condensation, and molecular reorganization, resulting in more humified structures approaching natural humic substances. Mechanistically, Fe facilitated the redistribution of reactive intermediates during pyrolysis, favoring the formation of nitrogen-containing heterocycles and stable aromatic frameworks. Correspondingly, the obtained humic substances exhibited enhanced bioactivity, significantly promoting wheat growth. At the optimal concentration (100 mg/L), shoot and root lengths increased by 8.15% and 54.78%, respectively, accompanied by increased biomass accumulation and protein content. This work provides new insight into Fe-driven thermochemical humification and offers a scalable route for the high-value utilization of livestock waste.