Photoformation of Environmentally Persistent Free Radicals During Phototransformation of Poly-Cyclic Aromatic Hydrocarbons (PAHs) on Particles in an Aqueous Solution: The Hydrogenation of PAHs and Effect of Co-Existing Water Matrix Factors.

Environmentally persistent free radicals (EPFRs) generated on particles under irradiation in water have attracted particular attention, and their formation mechanisms are not well understood. This study investigated the photoformation of EPFRs on both actual samples collected from an oil production plant in Panjin, Liaoning, China, and simulated Fe(III)-montmorillonite samples in water. The EPFRs detected on actual samples were not easily generated compared with those in the soil or in the air, based on the concentrations of identified PAHs. EPR signals in the range of 10 to 10 spin/g were detected on the simulated Fe(III)-montmorillonite samples. Their g factors were smaller than 2.0030, which indicated the generation of carbon-centered EPFRs. The primary byproducts were identified by chromatography-mass spectrometry (GC-MS), and a possible EPFR formation pathway during PAH degradation was proposed. Hydrogenation of PAHs during the photoformation of EPFRs was observed and might be due to the catalysis of the simulated particles and the interaction of the intermediates. Meanwhile, the effects of the typical anions (NO and Cl) and the surfactant (TWEEN 80 and sodium dodecyl sulfate) were investigated and indicated that the phototransformation process and adsorption process would affect the formation of EPFRs. Overall, our study provided useful information to understand the photoformation of EPFRs in aqueous environments.