Empowered electron transfer behavior mediated by iron/sulfur cycling drives synergistic transformation of nitrogen and PFOA in constructed wetlands: Overlooked abiotic and biotic mechanisms.

In this study, enhanced dynamic ammonium, nitrate (NO3--N), and total nitrogen (TN) removal reaching 85.87-99.35%, 61.31-99.45%, and 74.61-98.48% were observed in biochar-pyrite (BCP) amended constructed wetland. Among them NO3--N and TN were significantly higher than both control and individual biochar (BC) group. Perfluorooctanoic acid (PFOA) removal efficiency was 74.05-92.58% by BCP amendment, with higher defluorination rate and lower short-chain products accumulation. Increased activity of nitrate/nitrite reductase caused improved denitrification process in BCP system, which could result from enrichment of autotrophic denitrifying bacteria (like Pseudomonas, Thiobacillus, and Gallionella) and promotion of most genes involved in nitrogen/iron/sulfur metabolism. Facilitated redox cycles of iron/sulfur increased electron transfer system activity by 14.03-261.60% and 9.06-93.53% compared to control and single BC system, with higher activity of enzymes related to electron transport chain and encoding genes. Meanwhile, 1.72-3.79 folds enrichment of electroactive bacteria in BCP layer increased extracellular electron transfer (EET) efficiency. In mechanism, both reactive oxygen species mediated degradation and accelerated EET might enhance PFOA mineralization. This work offered a promising technique for synergic removal of nitrogen and PFOA in secondary effluent of WWTPs, which supplied novel insights on abiotic/biotic PFOA degradation mechanisms mediated by iron/sulfur cycling.