Managing long-term passivation in pyrite-based constructed wetlands for low C/N wastewater: roles of Fe-P precipitation, EPS evolution and microbial succession.

Pyrite-based constructed wetlands can enhance autotrophic denitrification in low-C/N wastewater, but progressive mineral coating may impair long-term electron-donor availability at the biofilm-pyrite interface. This study investigated mechanisms associated with the performance of a pyrite-based constructed wetland (Pyrite-CW) during approximately 700 days of operation. During the stable stage (days 100-400), Pyrite-CW achieved a TN removal efficiency of 50.0 ± 3.4% and an average effluent TN concentration of 8.37 mg/L, outperforming coarse sand-CW and quartz sand-CW. After approximately day 550, TN and NO3--N removal declined to 33.4 ± 3.4% and 28.9 ± 2.2%, respectively, indicating sustained performance deterioration. Controlled batch tests showed that additional and in situ-produced Fe-P phases reduced TN removal by approximately 30-50%, while Fe(OH)3 treatments caused approximately 25% inhibition. Surface characterization and reduced electron transport system activity strongly supported interfacial passivation. EPS shifted from a protein/humic-like-substance-rich matrix to a polysaccharide-enriched structure, and microbial succession was associated with a decline in Sulfurimonas and enrichment of Acidobacteriota and Geothrix. These convergent results provide strong evidence that Fe-P-centered surface passivation was a major pyrite-specific mechanism contributing to declining denitrification. Although hydraulic aging may have contributed at the whole-wetland scale, it cannot explain the treatment-specific inhibition independently reproduced in the controlled batch systems. Phosphorus control, pH buffering, staged substrate design, and combined biochemical and hydraulic diagnostics are recommended for maintaining long-term Pyrite-CW performance.