Neglected Role of Iron Minerals in Mixotrophic Aerobic Denitrification: Enhanced Electron Transfer and Metabolic Activity in Micropolluted Water Bacterial Community

The limited availability of organic carbon concentrations in surface water resources has consistently challenged in situ nitrogen removal processes. Natural iron minerals have gained attention as inorganic electron donors that enhance denitrification. However, the mechanisms underlying the enhancement of aerobic denitrification by various iron minerals in micropolluted lake water remain unclear. This study investigates the use of magnetite, siderite, and pyrite as electron donors to enhance in situ nitrate removal. The results show that the pyrite aerobic denitrification reactor achieved a higher removal efficiency of nitrate and organic matter, reaching 82.29% and 63.83%, respectively. Mechanistic analysis revealed that pyrite promotes electron transfer system activity, enhancing microbial metabolism and denitrifying enzymatic activity. Additionally, pyrite stimulated sulfur-metabolizing microbes (Ottowia sp.) and the synergistic effects of iron and sulfur minerals further amplified denitrification capacity. Statistical models indicated that iron minerals enhanced the response among the environmental factors, the microbial community, and nitrates within the reaction system. These findings provide a theoretical foundation for leveraging pyrite to stimulate indigenous microorganisms for efficient in situ removal of low-concentration nitrates from micropolluted lake water.