Thermal-Alkaline-Activated Persulfate for Remediation of PAH-Contaminated Soils: Natural Organic Matter Regulation, Degradation Mechanisms, and Toxicity Assessment

Polycyclic aromatic hydrocarbons (PAHs), characterized by their high stability, are typical persistent organic pollutants that pose irreversible risks to human health. Conventional chemical oxidation methods exhibit limitations that hinder effective remediation in practice. In contrast, sulfate-radical-based advanced oxidation processes have emerged as promising alternatives, among which the heat-alkaline activation system for persulfate (PS) demonstrates distinct advantages. In this study, a heat-alkaline-activated PS system was established to investigate the degradation of PAHs in both simulated contaminated soils and coal chemical industrial site soils, as well as the modulatory effects of natural organic matter (NOM). Response surface methodology optimized critical experimental parameters to 12.53 mmol PS dosage, 60.31 °C reaction temperature, and a 1.55 CaO/PS molar ratio. Under these conditions, degradation efficiencies of 98.32% and 82.26% were achieved in simulated and field soils, respectively. Radical test experiments revealed a cooperative mechanism dominated by SO4−• > •OH > O2−• radicals, accompanied by auxiliary involvement of non-radical 1O2. Low concentrations of NOM plausibly facilitate degradation via a hypothesized electron transfer protective effect and boosted radical generation, whereas excessive NOM inhibits degradation through competitive consumption of reactive radicals. Density functional theory calculations identified preferred radical attack sites on the aromatic rings of PAHs and corroborated the degradation pathway involving aromatic ring oxidation, functional group addition, ring cleavage, and mineralization. QSAR-based theoretical toxicity predictions via T.E.S.T. suggested that the ultimate degradation products exhibit lower potential toxicity than parent PAHs. Experiments fill the knowledge gap regarding NOM-mediated regulation in heat-alkaline activated PS systems, and elucidate degradation mechanisms and toxicity evolution.