The mechanism of pyrite passivation in bioretention systems: Electron donor termination and electron transfer...

Pyrite-based bioretention system hold considerable promise for nitrogen removal from stormwater runoff. However, long-term performance attenuation, caused by surface passivation of the filter media, remains a critical bottleneck for practical engineering applications. This study elucidated the formation mechanism of the passivation layer and its inhibition of denitrification through a six-month continuous operation experiment, comparing a pure pyrite system (PB) with a pyrite-wheat straw composite system (WPB). Results revealed that prolonged operation led to a dense passivation layer on pyrite surfaces, predominantly composed of crystalline iron (hydr)oxides and iron phosphate precipitates (FePO₄), a process further aggravated by Ca²⁺/Mg²⁺ co-precipitation. Passivation impaired denitrification through two synergistic mechanisms: a physical barrier that reduced Fe²⁺ release by up to 71.4% (severing the electron donor supply), and an electron-insulating effect that decreased microbial electron transfer system activity (ETSA) by 40-65%. The addition of wheat straw as a slow-release carbon source effectively attenuated passivation by stimulating heterotrophic denitrification and modulating the microbial community (e.g., enriching Fe(III)-reducing bacteria). This work clarifies the microscale mechanism of iron-phosphate-mediated pyrite passivation and provides a critical theoretical basis for optimizing long-term bioretention system operation and developing anti-passivation filter media.