Polycyclic aromatic hydrocarbons degradation mechanisms in methods using activated persulfate: Radical and non-radical pathways

Polycyclic aromatic hydrocarbons (PAHs) have become a worldwide public health concern due to their toxicity, carcinogenicity, and co-existence. Persulfate (PS)-based advanced oxidation processes for PAH degradation have been widely investigated, because of their high-activity, environmental sustainability, and high potential for generating active free radicals. Although various PS activation methods have been reported, a systematical classification is lacking and the possible mechanisms involved remain unclear. Most studies have concentrated on the investigating of radical pathways with activated PS, but the reaction mechanisms in non-radical pathways have not been specifically reviewed. Thus, metal-free (i.e., thermal, microwave, alkaline, ultraviolet, ultrasonic, and carbon materials) and metal-based (i.e., iron, nickel, zinc, copper, and bimetal-based particles) activated PS for PAH degradation have been described in detail. Active free radical species such as the sulfate radicals (SO4−·), hydroxyl radical (·OH), and superoxide free radical (·OOH) play key roles in activated PS oxidation processes. Additionally, carbon-based materials could enhance the oxidation activity of PS, through non-radical pathways, which include singlet oxygen (1O2), direct electron transfer, and surface-bound active species. Importantly, a suitable bimetal-based particle activation method can exhibit obvious synergistic effects on the generation of active free radicals and non-radical species; it can also promote the electron transfer process. The paper provides an overview of the various methods that use activated PS for PAH degradation through radical and non-radical pathways, enabling an understanding of electron transfer between PS and activators. Furthermore, the review provides an essential scientific basis for PAH-contaminated soil remediation and environmental protection.