Natural Organic Matter Association Modulates Pyrite Surface Electron Transfer Kinetics and Path for Reactive Oxygen Species Generation

Surface electron transfer from iron minerals to molecular oxygen (O₂) has been identified as a pivotal natural source of reactive oxygen species (ROS). In water and soils, iron minerals are often associated with natural organic matter (NOM), which can substantially influence surface redox reactions. Yet inconsistencies remain in understanding the impacts of NOM association on ROS production from iron mineral surfaces due to limited insight into surface interfacial electron transfer processes. Here, we show that NOM association significantly modulates both the kinetics and the path of electron transfer on pyrite surfaces. Steric hindrance at O₂ adsorption sites decreased the electron transfer rate by 22.7-38.6%, from 4.4 × 10^-4 to 2.7-3.4 × 10^-4 s^-1. Concomitantly, O₂ reduction shifted from a four-electron path producing H₂O to a two-electron path favoring H₂O₂ formation, increasing H₂O₂ selectivity from 29.6% on pristine pyrite to 49.2-67.1% on NOM-associated pyrite. These changes collectively enhanced ROS production, with H₂O₂ and •OH yields increasing by 1.5-1.9-fold and 1.8-2.3-fold, respectively. Elevated ROS levels increased organic pollutant degradation rates by 1.6-2.7-fold. These findings highlight the multifaceted role of the NOM in shaping surface electron transfer kinetics and ROS generation at iron mineral interfaces.