Freezing-Enhanced Accumulation of ROS Generated by Fulvic Acid: Implications for As(III) Oxidation

Humic substances (HS), as redox-active organic macromolecules, are capable of generating reactive oxygen species (ROS) through redox-active moieties such as quinones and phenolic groups. Despite extensive research having elucidated these redox processes in aqueous systems, there is a paucity of knowledge regarding the electron transfer pathways of these processes under freezing conditions, a common environmental scenario in temperate, polar, and high-altitude regions. Here, we demonstrated a previously unrecognized and environmentally significant pathway: freezing dramatically enhanced fulvic acid (FA)-mediated oxidation of As(III) in ice (from -5 to -18 °C), achieving 68% conversion within 96 h at -18 °C compared to negligible transformation in aqueous phases (4-25 °C). Mechanistic investigations revealed that this cryogenic acceleration was ascribed to a synergistic freeze-concentration effect and a kinetic decoupling of ROS generation from scavenging. In situ fluorescence imaging directly visualized ROS accumulation within ice grain boundaries, while complementary spectroscopic (EPR and 3D-EEM) and chromatographic (GPC) analyses revealed that freezing suppressed FA humification, thereby redirecting ROS from self-consumption toward As(III) oxidation. H₂O₂ and O₂^•- served as the dominant oxidants. These findings redefine our understanding of arsenic redox cycling in cold environments, highlighting cryogenic conditions as a critical period for geochemical and pollutant redox dynamics in the environment containing HS.