Pyrite in recirculating stacking hybrid constructed wetland: Electron transfer for nitrate reduction and phosphorus immobilization

Pyrite is considered as an effective and environmentally friendly substrate in constructed wetlands (CW) for wastewater treatment, but its application in recirculation stacking hybrid constructed wetlands (RSHCW) has been scarcely studied. This study uses varying amounts of pyrite as the substrate in RSHCW, leveraging the recirculation of wastewater to alter microenvironments such as dissolved oxygen (DO) and pH, to explore the potential mechanisms of nitrogen (N) and phosphorus (P) removal in pyrite-based RSHCW. The results show that as the proportion of pyrite increases, the removal rate of total phosphorus (TP) in the effluent also increases (25%→58%), significantly enhancing the deposition of iron-bound phosphorus (Fe-P) on the substrate, thereby turning CW into a P reservoir. Even in the absence of a carbon source, the total nitrogen (TN) removal rate in the CW still increases by 20%, which can be attributed to the enrichment of sulfur autotrophic denitrifying bacteria driving autotrophic denitrification by pyrite. Additionally, the addition of pyrite significantly increases the electron transfer system activity (ETSA) in the CW system by approximately 6.14 times and facilitates a "charging and discharging" function through the sulfur-iron electron cycle. Selective enrichment of microbes in moderated pH environment due to RSHCW recirculation in the pyrite-CW (PCW) enhances the coordination among microbial communities and the interaction among functional genes. This study provides new insights into the mechanisms of N and P removal in CWs under the influence of pyrite.