Mechanistic insights into nZVI-enhanced denitrification resilience in biological aerated filters under chlorfenapyr stress: Roles of EPS stability and electron-transfer pathways.

To address the decline in nitrogen removal performance of biological aerated filters (BAFs) caused by the inhibition of key denitrification processes by the arylpyrrole insecticide chlorfenapyr, this study established a nanoscale zero-valent iron (nZVI)-enhanced BAF system and systematically investigated the enhancement effects and regulatory mechanisms of nZVI. The results demonstrated that nZVI markedly improved shock load resistance and recovery resilience. The NH4+-N removal efficiency remained above 90% under different chlorfenapyr concentrations, while the total nitrogen (TN) removal efficiency reached up to 88.82%. Extracellular polymeric substance (EPS) analysis indicated that nZVI effectively mitigated the adverse effects of stress on the surface properties of the biofilm, maintaining the protein-to-polysaccharide ratio at approximately 2.0 and thereby preserving biofilm structural stability. High-throughput sequencing revealed that nZVI sustained microbial community diversity and evenness. The relative abundance of the dominant phylum Pseudomonadota consistently remained above 54%, and the targeted enrichment of functional genera such as Paracoccus and members of Rhizobiaceae was promoted. Integrated analysis of carbon and nitrogen metabolic pathways had revealed that nZVI, acting as an electron donor, had promoted the TCA cycle while significantly upregulating the expression of iron transport genes (afuABC) and key denitrification genes (napA, nirB, and nosZ). From the perspectives of electron transfer and microbial community ecology, this study elucidates the intrinsic mechanisms by which nZVI enhances the stress resistance of biological systems, providing a theoretical basis for the engineering treatment of refractory pesticide wastewater.