Boosting Denitrification in Pyrite Bioretention Through Biochar-Mediated Electron Transfer

The pyrite bioretention system has been increasingly used to control dissolved nutrients in stormwater runoff. However, its low electron supply rate cannot adapt to the demand for denitrification under high nitrogen-loading conditions. To address this limitation, we constructed a mixed biochar–pyrite bioretention system (BP) by optimizing the structural composition of the fill media. Under simulated complex rainfall conditions, the nitrogen removal efficiency, by-product generation, and filler physicochemical properties of system were evaluated. Results demonstrated that the BP system significantly enhanced denitrification performance, achieving average NOx−-N and TN removal rates of 63.3% and 67.8%, respectively. This represented improvements of 79.1% and 45.9% over the conventional pyrite bioretention system. Moreover, the composite system exhibited a sustained and effective denitrification even under low C/N ratio conditions. This enhancement is attributed to biochar’s dual role as an electron shuttle and an electron reservoir, which facilitated microbial nitrate reduction. XPS analysis further confirmed that biochar addition effectively reduced the oxidation degree of pyrite, thereby protecting it from rapid oxidative degradation. Microbial analysis revealed that biochar supplementation in the BP system increased microbial diversity in the saturated zone, which contributed to improved ecosystem function and stability, including the promotion for key denitrification processes.