Chalcopyrite mitigates N₂O emission in bioretention systems: promoting denitrification and activating nitrous oxide...

Recently, various control measures have been explored for urban stormwater runoff treatment. Bioretention systems are typical green infrastructure for nitrogen removal. However, the potential emission of nitrous oxide (N₂O) makes the systems possible greenhouse gas sources. In this study, we established chalcopyrite-based bioretention (CuFeS₂, CB) columns and compared them with pyrite-based bioretention (FeS₂, PB) and sand-based bioretention (SB) columns. The N₂O flux in the CB group was 18.49 ± 2.05 μg N m^-2·h^-1, which was 26.2% and 40.3% lower than that in the PB and SB groups. Concurrently, the CB group achieved average removal efficiencies of 97.0% for nitrate and 75.7% for total dissolved nitrogen, with only limited leaching of sulfate and total iron. SEM-EDS characterization revealed distinct spherical and rod-shaped features indicative of microbial etching, along with a decrease in the proportions of copper, iron, and sulfur on the surface of the used chalcopyrite. Dissolved copper generated from chalcopyrite dissolution enriched N₂O-reducing bacteria, increasing the abundance of Gemmatimonadota to 8%. The absolute abundance of the nosZ gene in the submerged zone of the CB group was 14-fold and 6-fold higher than that in the SB and PB groups, respectively, suggesting a potential promotion of nitrous oxide reductase synthesis and activity. These results demonstrate that chalcopyrite-driven autotrophic denitrification occurred via sulfur and iron oxidation, and reduced N₂O emission through copper-induced enhancement of nosZ gene abundance. These findings highlight that chalcopyrite-based bioretention can simultaneously achieve effective nitrogen removal and N₂O emission mitigation.