Formation and stability of environmental persistent free radicals (EPFRs) from the reaction between phenolic...

Environmentally Persistent Free Radicals (EPFRs) are defined as a new type of pollutant, which can cause environmental pollution and do harm to human health. However, few studies have comprehensively explored the formation conditions and mechanism of EPFRs. This study investigated the EPFR formation abilities and mechanisms of two organic phenolic precursors (4-tert-butylcatechol, TBC, and catechol, CT) on model particulate matter (Fe(III)-loaded SiO₂ and kaolin) using electron paramagnetic resonance (EPR) in combination with Fourier-transform infrared attenuated total-reflectance spectroscopy (FTIR-ATR) and Gas Chromatography-Mass Spectrometry (GC-MS) techniques. The results showed that TBC produces more EPFRs than CT, while the matrix kaolin produces a wider linewidth and greater concentration of EPFRs compared to SiO₂. The increasing concentrations of organic phenolic compounds led to a decrease in the species of EPFRs but an increase in their concentration. The variation of EPFR formation from CT and TBC was evaluated through long-term observation. The results revealed that CT could cause a stable increase in EPFR formation during the 456 h, whereas a reverse trend was observed for TBC. The predominant final products of dimers for both precursors indicated that EPFRs might have been intermediate products via polymerization reaction. This study suggests that when predicting the environmental behavior and risks of EPFRs, it is important to consider (1) the chemical properties of the particulate matter and (2) the precursor species and concentration.