The Formation, Stabilization Mechanism, and Environmental Impacts of Persistent Free Radicals in Soil Humic Substances

Persistent free radicals (PFRs) in humic substances (HSs) are relatively stable free radicals with a longer lifespan compared to transient free radicals. These PFRs are abundant in the ecological environment, such as in soil and water. The formation of PFRs in HSs occurs primarily through the humification of biological residues, electron transfer between quinone and phenolic groups in HSs, and interactions among microorganisms, HSs, and mineral particles. Mineral particles contribute significantly to the stability of these radicals. Conditions such as prolonged exposure to light and redox changes further influence their formation and stability. PFRs in HSs have been applied in the treatment and remediation of environmental pollution due to their ability to adsorb, degrade, and undergo redox reactions with soil pollutants. The formation mechanisms of PFRs (including semiquinone free radicals) in soil HSs are reviewed, and the factors affecting their stability are explored. Additionally, the interaction between PFRs and soil pollutants, and their effects on pollutant migration and transformation, and soil microbial communities, are discussed. Lastly, perspectives for future research aimed at improving innovation in understanding the impact of PFRs on pollutant migration and transformation, soil microbial communities, and the environmental behavior of PFRs are provided.