A critical review of secondary environmentally persistent free radicals: formation, properties, environmental and health impacts

Environmentally persistent free radicals (EPFRs) are a class of emerging organic pollutants widely present in various environmental media. They may pose health risks through the generation of reactive oxygen species (ROS). Although previous studies have focused on primary EPFRs generated directly from thermal reactions, research on secondary EPFRs generated through atmospheric oxidation and metal catalysis remains fragmented and lacks a systematic review. To fill this research gap, this review presents the first comprehensive and systematic overview of secondary EPFRs, summarizing the formation mechanisms of secondary EPFRs, key influencing factors, physicochemical properties, and their environmental and health impacts. Our findings revealed that photooxidation plays a dominant role in secondary EPFR formation. Meanwhile, we found that in addition to aromatic compounds as precursors, non-aromatic compounds such as polyethylene, polyamide, and ceftriaxone sodium can also generate secondary EPFRs under certain environmental conditions, expanding the scope of potential precursors. Moreover, we revealed that secondary EPFRs may exhibit distinct physicochemical properties compared to primary EPFRs, including higher water solubility and greater efficiency in ROS production. These differences may significantly influence their environmental fate, toxicity, and impact on human health. Finally, by synthesizing the key findings in secondary EPFR research, we provide a prospective outlook on the future research directions of secondary EPFRs.