Enhanced nitrate removal by ternary electron donors of solid carbon source, pyrite, and iron-scrap in stormwater bioretention systems: Impact of natural and synthetic carbon sources.

Insufficient availability of electron donors is challenging for nitrate removal in bioretention systems when treating carbon-limited stormwater runoff. This study constructed two ternary electron-donor systems incorporating solid carbon source, pyrite, and iron-scrap to enhance mixotrophic denitrification, and investigated the impacts of natural carbon source (woodchip) and synthetic carbon source (polycaprolactone) on denitrification performance and by-product generation. Results showed that both systems exhibited stable and efficient nitrate removal (85.76 ± 11.00 % and 89.67 ± 5.75 %) under 120 days of variable stormwater conditions, attributing to the synergy of multi-electron donors. Compared with the polycaprolactone-packed system, the woodchip-packed system showed substantial reductions in ammonia nitrogen, dissolved organic carbon, and sulfate production, indicating better comprehensive pollutant control. Woodchips avoided excessive corrosion and passivation of iron-scrap by maintaining low dissolved organic carbon conditions and preventing high OH production, and promoted the reduction of iron oxides through redox-active groups. Meanwhile, woodchips prevented the inhibition of autotrophic denitrification and released volatile fatty acids, stimulating pyrite bio-oxidation and sulfate reduction. The generated abundant free soluble iron and polysulfides immobilized each other and further enhanced autotrophic denitrification. The microbial community and PICRUSt2-based predicted functional analysis revealed that the woodchip-packed system enriched the heterotrophic denitrifiers, sulfur-oxidizing, sulfate-reducing, iron-oxidizing, and iron-reducing bacteria, along with the genes related to carbon, nitrogen, sulfur, iron metabolism, and electron transfer. The findings of this study demonstrated that coupling woodchips with pyrite and iron-scrap strengthened substrate cooperation and microbial synergy, offering a sustainable solution for efficient nitrate removal from stormwater runoff.