FeSO4-Zeolite Coupled with Low Aeration Enhances Humic Acid Stability by Regulating Synthesis and Iron-Mediated Passivation in Food Waste Composting

Humic acid (HA) stability determines the food waste (FW) compost quality and facilitates organic waste recycling via aerobic composting. This study investigated the synergistic effects of ferrous sulfate (FeSO4)-zeolite and low aeration to enhance the HA stability during FW composting. Composting was carried out for 50 days across four treatments: control (CK), FeSO4-zeolite (FZ), low aeration (L), and combined FZL treatments. Results showed that FZL achieved the highest HA content (33.1 mg/g) and condensed HAC3 proportion (18.3%), raising the HA polymerization degree by 84.9% relative to CK. Low aeration prolonged Fe2+ persistence, stimulating stable iron oxide formation to serve as a nucleation site for HA polymerization and complexation. FZL optimized microbial networks and enriched humification-related microbes. Variance partitioning analysis quantified iron-mediated passivation (45.5%) and its synergy with synthesis (21.8%) as the dominant drivers. This study clarifies the underlying mechanism and offers a practical strategy to produce a high-stability HA compost.