Humification of organic matter and passivation of heavy metals during the hydrothermal carbonization of swine manure

• HTC enhances humification of organic matter in swine manure, with optimal effect at 240°C, increasing humus content from 41.97 to 83.84 mg·g−1 and humification rate from 15.73% to 28.89%. • Heavy metals (Cu, Zn, Pb, Cr) are enriched in hydrochars but significantly passivated, reducing ecological risk from moderate to low. • The passivation of heavy metals is positively correlated with dissolved organic carbon content and negatively correlated with the humic acid to fulvic acid ratio, indicating that humification promotes metal passivation. • HTC is a promising technology for safe land application of swine manure-derived hydrochars, mitigating environmental risks from heavy metals.