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.