Research on performance differences and mechanisms of sulfur-iron composite packing materials prepared from different iron sources in nitrogen and phosphorus removal from wastewater.

To address the structural instability and limited nutrient removal of traditional fillers, this study fabricated four novel composite fillers-incorporating zero-valent iron (Fe0), siderite (FeCO3), pyrite (FeS2), and calcium carbonate-via a melt-encapsulation method. Batch and continuous experiments systematically revealed distinct nitrogen and phosphorus removal mechanisms and microbial architectures among these fillers. While all exhibited denitrification potential, S-FeCO3 demonstrated superior shock resistance, maintaining 78.36-94.71 % nitrogen removal and reducing sulfate accumulation by 30.92 %. Conversely, S-Fe0 caused significant nitrite accumulation (2.83 mg/L). For phosphorus, S-FeCO3 (80.53-84.49 %) significantly outperformed S-FeS2 (70.84-78.57 %) and S-Fe0. Microbial analysis showed a transition from Thiobacillus dominance in S-CaCO3 to Thiobacillus-Ferritrophicum co-dominance in iron-coupled systems. At the molecular level, sulfur-iron coupling up-regulated key denitrification genes (narG, nirS, nirK, nosZ) by accelerating electron transfer and relieving Fur-mediated repression, providing a systematic strategy for filler optimization in simultaneous nutrient removal processes.