Nitrogen-Centered Persistent Free Radicals from Roxarsone Phototransformation on Soil Minerals: Magnetic Coupling Mechanism and PFR-Driven Ecological Risks

Abstract Roxarsone (ROX) degradation in soil environments is a recognized source of arsenic contamination, but the concurrent evolution of persistent free radicals (PFRs) presents an overlooked hazard. Here, we report the generation of unique nitrogen-centered PFRs during ROX phototransformation on soil mineral surfaces. Unlike typical environmental PFRs, these species exhibit a distinct triple-peak EPR signature, which is proposed to arise from the magnetic coupling between •NO and •NHR radicals following C–As bond cleavage. Mineral composition strongly governs radical evolution, with MnO2 enhancing and Fe2O3 suppressing PFR stability. These nitrogen-centered radicals act as efficient electron shuttles, sustaining the production of reactive oxygen species (•OH, •O2–, H2O2). Multitrophic toxicity tests (bacteria-plant-earthworm) combined with rigorous arsenic-controlled conditions (As(V)≤150 ppb, As(III)≤20 ppb) reveal that the observed growth inhibition, oxidative damage, and histopathological effects are predominantly driven by PFR-mediated processes rather than by released inorganic arsenic. These findings uncover a previously unrecognized pathway for nitrogen-centered radical formation and highlight the necessity of incorporating radical-driven toxicity into environmental risk assessments of organoarsenic contaminants.