Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons

? Regulation of redox conditions promotes the generation of free radicals on HM. ? HM-PFRs can be fractionated into active and inactive types depending on stability. ? The newly produced PFRs readily release electrons to oxygen and generate ROS. ? PFR-induced ROS mediate the transformation of organic contaminants adsorbed on HM. The role of humic substance-associated persistent free radicals (PFRs) in the fate of organic contaminants under various redox conditions remains unknown. This study examined the characterization of original metal-free peat humin (HM), and HM treated with varying concentrations of H₂O₂ and L-ascorbic acid (VC) (assigned as H₂O₂-HM and VC-HM). The concentration of PFRs in HM increased with the addition of VC/H₂O₂ at concentrations less than 0.08 M. The evolution of PFRs in HM under different environmental conditions (e.g., oxic/anoxic and humidity) was investigated. Two types of PFRs were detected in HM: a relatively stable radical existed in the original sample, and the other type, which was generated by redox treatments, was relatively unstable. The spin densities of VC/H₂O₂-HM readily returned to the original value under relatively high humidity and oxic conditions. During this process, the HM-associated “unstable” free radicals released an electron to O₂, inducing the formation of reactive oxygen species (ROS, i.e., ^?OH and ^?O₂^?). The generated ROS promoted the degradation of polycyclic aromatic hydrocarbons based on the radical quenching measurements. The transformation rates followed the order naphthalene>phenanthrene>anthracene>benzoa]pyrene. Our results provide valuable insight into the HM-induced transformation of organic contaminants under natural conditions.