Oxygen-driven thiol-activated ROS generation by particulate organic matter: Deciphering the role of interface-accelerated organic radical chain reaction.

Particulate organic matter (POM), a major component of natural organic matter (NOM), plays a crucial but poorly understood role in dark reactive oxygen species (ROS) generation. This study revealed that thiols exhibited superior activation capacity for POM-mediated ROS production, with hydroxyl radical (OH) yields 1.4-6.1 times higher than that of dissolved Fe(II). Oxygen acted as a terminal electron acceptor in thiol-activated POM systems, sustaining ROS generation via organic radical chain reactions. Batch experiments demonstrated that thiol donated electrons via -SH groups, with surface-active quinones on POM facilitating electron transfer, as evidenced by C NMR, FTIR, and XPS analyses. Model experiment and DFT calculation confirmed that quinones lower the energy barrier for Cys oxidation, initiating radical chain reactions to produce RS and SQ, leading to significant OH generation in the Cys/BQ system (120.7 µM). Kaolinite particulates in POM further enhanced OH yields by 88 % through surface-mediated radical chain propagation. This ROS-mediated process not only enhanced Cys transformation, but also altered POM surface properties and facilitated As(III) oxidation. These findings provide a deeper understanding of the role of POM in environmental redox chemistry and its implications for contaminant transformation and geochemical cycling, and pave the way for further research into the behavior of free radical intermediates in environmental applications.