Photoformation of persistent free radicals on a montmorillonite-humic acid complex simulated as particulate organic matter in an aqueous solution

Molecular characterization of diverse quinone analogs for discrimination of aerosol-bound persistent pyrolytic and photolytic radicals

Aerosol-bound organic radicals, including environmentally persistent free radicals (EPFRs), are key components that affect climate, air quality, and human health. While putative structures have been proposed, the molecular characteristics of EPFRs remain unknown. Here, we report a surrogate method to characterize EPFRs in real ambient samples using mass spectrometry. The method identifies chemically relevant oxygenated polycyclic aromatic hydrocarbons (OxPAH) that interconvert with oxygen-centered EPFR (OC-EPFR). We found OxPAH compounds most relevant to OC-EPFRs are structurally rich and diverse quinones, whose diversity is strongly associated with OC-EPFR levels. Both atmospheric oxidation and combustion contributed to OC-EPFR formation. Redundancy analysis and photochemical aging model show pyrolytic sources generated more oxidized OC-EPFRs than photolytic sources. Our study reveals the detailed molecular characteristics of OC-EPFRs and shows that oxidation states can be used to identify the origins of OC-EPFRs, offering a way to track the development and evolution of aerosol particles in the environment.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

Photoformation of environmentally persistent free radicals on particulate organic matter in aqueous solution: Role of anthracene and formation mechanism

Environmentally persistent free radicals (EPFRs) generated under irradiation have been widely detected in soil particles, atmospheric particles and microplastic particles, but the formation of EPFRs in water is not well understood. This study investigated the formation of EPFRs on particulate organic matter (POM) in water contaminated by anthracene (Ant) under irradiation. The photoformation and decay progress of EPFRs was represented with the help of electron paramagnetic resonance (EPR) technique on both actual POM and Fe(III)-montmorillonite simulated samples. EPR signals at the range of 10¹⁶ to 10¹⁷ spin/g were detected and the half-life time of EPFRs stored in water was at around 16.62 h and 60.80 h, much shorter than those in the air. The g factors were all larger than 2.0040, which indicated the generation of oxygen centered EPFRs. The primary intermediates were identified by gas chromatography-mass spectrometer (GC-MS) and a possible EPFR formation pathway during Ant degradation was proposed. The interaction between Ant and POM, and the hydroxylation and carbonylation of the intermediates made contributions to the generation of EPFRs. Meanwhile, the indirect photodegradation of bisphenol A (BPA) has been demonstrated by analyzing the reactive oxygen species (ROS) and photogenerated electrons in the solution with POM containing EPFRs. It is found that hydroxyl radicals (•OH) and singlet oxygen (¹O₂) were induced and might promote the photodegration. Overall, our present study provided useful information to understand the photoformation of EPFRs on POM and their fate in aqueous environments.