Temperature-Dependent ROS Generation by Humic Substance-Iron in Bulk Solutions and Microdroplets

Humic substances (HSs) and/or iron can drive reactive oxygen species (ROS) generation in various environments; however, most reported systems involve light irradiation or rely on the reduced status of HSs and/or iron. Here, we identify an overlooked abiotic, nonphotochemical pathway for ROS generation and pollutant degradation via thermally induced activation of original HSs and Fe³⁺ in both bulk solutions and microdroplets at environmentally relevant temperatures. The HO• generation increased by 5.7-18.1-fold with elevated temperatures (20-70 °C) and rising concentrations of HSs and Fe³⁺. Mechanistic studies involving Fe²⁺ quantification, ROS scavenging assays, and functional group identification revealed that reductive moieties (e.g., HS(Ar-OH)) within HSs simultaneously facilitated H₂O₂ and Fe²⁺ generation. In this process, O₂ and Fe³⁺ functioned as single or dual electron acceptors, with semiquinone radicals (HS(Ar-O•)) acting as key intermediates and HO• and quinone species (HS(Ar═O)) as terminal oxidation products. Remarkably, HS-Fe³⁺ in microdroplets exhibited ROS production up to 2 orders of magnitude higher than in bulk solution due to interfacial enhanced reactivity, resulting in accelerated micropollutant degradation. A microfluidic device was further developed for in situ visualization, confirming the temperature dependence of ROS generation in HS-Fe³⁺ microdroplets. This study highlights a thermally driven oxidative pathway with implications for contaminant decomposition in high-temperature or microdroplet-rich environments.