Mechanistic insights into nitrite-type denitrifying phosphorus removal driven by iron-sulfur cycle-mediated electron...

Nitrite-type denitrifying phosphorus removal (NiDPR) process can achieve simultaneous nitrogen and phosphorus removal from low carbon/nitrogen municipal wastewater with less carbon and energy input. However, conventional constructed wetlands (CCW) struggle to implement NiDPR process due to the difficulty in maintaining sustainable accumulation of nitrite and effective electron transfer. This study proposes incorporating pyrite in CCW (ICW) to address these problems for achieving efficiently simultaneous nitrogen and phosphorus removal, and unveiled corresponding mechanisms. Results demonstrated that ICW achieved higher nitrogen (90.94 ± 6.40 %) and phosphorus (85.08 ± 9.37 %) removal efficiency compared to CCW. Specific activity batch tests revealed that sulfur intermediates suppressed nitrite-oxidizing process to achieve partial nitrification, which facilitated NiDPR process in ICW. Microbial analysis further proved that ICW accumulated nitrite by promoting ammonia-oxidizing bacteria and suppressing nitrite-oxidizing bacteria, thereby facilitating a 2.26-fold enrichment of denitrifying polyphosphate-accumulating organisms (DPAOs, e.g., Dechloromonas). Furthermore, pyrite addition formed active iron-sulfur cycle which served as an efficient electron shuttle and thereby enhanced the electron transfer efficiency by 2.01-fold. The polysulfide oxidation process provided energy for DPAOs growth which enhanced the abundances of polyphosphate synthesis genes (e.g., ppk) and nitrite reduction functional genes (e.g., nirS) in ICW, confirming the occurrence of NiDPR process. Partial least squares path modeling further revealed that electron transfer was dominant factor for simultaneous nitrogen and phosphorus removal, confirming that pyrite enhanced performance primarily by accelerating iron-sulfur cycle-mediated electron transfer. This study provides insights into the underlying mechanism of simultaneous nitrogen and phosphorus removal in ICW.