Role of direct current on thermal activated peroxydisulfate to degrade phenanthrene in soil: Conversion of sulfate radical and hydroxyl radical to singlet oxygen, accelerated degradation rate and reduced efficiency

Electrokinetic (EK) delivery followed by thermal activated peroxydisulfate (PS) has turned out to be a potential in situ chemical oxidation technology for soil remediation , but the activation behavior of PS in an electrical coupled thermal environment and the effect of direct current (DC) intervention on PS in heating soil has not been explored. In this paper, a DC coupled thermal activated PS (DC-heat/PS) system was constructed to degrade Phenanthrene (Phe) in soil. The results indicated that DC could force PS to migrate in soil, changing the degradation rate-limiting step in heat/PS system from PS diffusion to PS decomposition, which greatly accelerated the degradation rate. In DC/PS system, 1 O 2 was the only reactive species directly detected at platinum (Pt)-anode, confirming that S 2 O 8 2- could not directly obtain electrons at the Pt-cathode to decompose into SO 4 •- . By comparing DC/PS and DC-heat/PS system, it was found that DC could significantly promote the conversion of SO 4 •- and •OH generated by thermal activation of PS to 1 O 2 , which was attributed to the hydrogen evolution caused by DC that destroys the reaction balance in system. It was also the fundamental reason that DC leaded to the reduction of oxidation capacity of DC-heat/PS system. Finally, the possible degradation pathways of phenanthrene were proposed on the basis of seven detected intermediates.