Surfactant-Triggered Alkyl-like Radical Formation in Anaerobic Thermally Activated Persulfate System for Deep Removal of Highly Chlorinated Contaminants

Achieving the deep removal of highly chlorinated contaminants (HCCs) from soil using traditional oxidation processes presents challenges owing to strong electron-absorbing carbon-halogen bonds resisting attack by electrophilic oxidative free radicals. Surfactants are commonly employed for solubilizing HCCs from contaminated soil; however, this study demonstrates for the first time that surfactants induce the formation of reductive alkyl-like radicals (R^•) in anaerobic thermal/persulfate (PS) systems, thereby achieving the deep removal of HCCs. The results showed that alkyl oxyethylene (OE) units of polyoxyethylene (23) lauryl ether (Brij 35) triggered the hydrogen abstraction reaction with electrophilic SO₄^•-/HO^• in the thermal/PS system for R^• formation, with the α-H of OE units being the main active site. Meanwhile, substances with OE units, including Brij 35 decomposition products and analogues, led to R^• generation, with R^• generation exhibiting a remarkable positive relationship with the number of OE units. With the regulation of the Brij 35 concentration, the process of R^• reduction followed by SO₄^•-/HO^• oxidation was further established, achieving almost complete dichlorodiphenyltrichloroethane (DDT) degradation and 65.83% mineralization, which was 9.46 and 1.40 times higher than that in oxidation-alone and reduction-dominant systems. This system also exhibited broad applicability to various halogenated organic pollutants and was resilient to environmental conditions. This study offers novel perspectives on the formation mechanism of reductive free radicals derived from surfactant/thermal/PS systems, enhancing the sustainable environmental remediation of HCC-contaminated sites.