Molecular-Scale Insights into the Surface Structural Transformation and Light-Driven Production of Reactive Oxygen Species of Goethite Induced by Microplastic-Derived Dissolved Organic Matter

Microplastic-derived dissolved organic matter (MP-DOM) is emerging as a component of environmental dissolved organic matter (DOM), yet the molecular-scale interactions governing its behavior with iron minerals and their implications for photochemical reactivity remain poorly understood. This study investigates the molecular-scale interactions of five representative DOM types with goethite, focusing on reactive oxygen species (ROS) generation under simulated sunlight (UVA, 5.7 mW/cm²). Among the ROS species, hydrogen peroxide (H₂O₂) exhibited the most pronounced variation in yield across the different DOM types. DOM analysis revealed that natural DOM (humic acid [HA] and fulvic acid [FA]) contains hydroxyl/phenolic groups and sulfur-nitrogen heteroatoms, whereas MP-DOM (polystyrene [PS-DOM], polybutylene adipate terephthalate [PBAT-DOM], and polyethylene [PE-DOM]) is rich in aromatic and aliphatic structures. PS-DOM and PBAT-DOM induced significant lattice distortion and Fe(III) reduction, promoting oxygen vacancy formation, while PE-DOM exhibited minimal reactivity due to its hydrophobic structure. Optical and electrochemical characterizations showed that DOM lowered the conduction band position and narrowed the band gap of goethite, enhancing light absorption and charge separation. PS-DOM and HA induced the highest photocurrents and H₂O₂ yields, with PS-DOM enhancing H₂O₂ production via oxygen vacancy formation. Multivariate analysis identified condensed aromatics and sulfur-nitrogen groups as key regulators of ROS generation by promoting electron transfer and defect formation. This work demonstrates that DOM molecular features directly modulate the photoreactivity of goethite by controlling the efficiency of charge separation, defect density, and ultimately the yield of H₂O₂.