Highly efficient electro-cocatalytic Fenton-like reactions for the degradation of recalcitrant naphthenic acids: Exploring reaction mechanisms and environmental implications

In this study, an electrochemical/peroxymonosulfate/Fe(III) (EC/PMS/Fe(III)) system was established for efficient degradation of four naphthenic acids (NAs) compounds including cyclohexanecarboxylic (CHA), heptanoic (HPA), benzoic (BA) and 2-methylhexanoic acids (2-MHA). The introduction of electric field enhances the redox cycle of Fe(III)/Fe(II) to activate PMS for degredation of NAs. Complete degradation of NAs compounds (5 mg/L) has been achieved within 30 min in the EC/PMS/Fe(III) system. The results of electron paramagnetic resonance (EPR) analysis, scavenging experiment, and chemical probe experiments indicated hydroxyl radical ( • OH), sulfate radical (SO 4 •− ) and singlet oxygen ( 1 O 2 ) are primary reactive oxygen species (ROS) in the EC/PMS/Fe(III) system. NAs compounds with different structures exhibit inconsistent degradation efficiency in current system ( k CHA  >  k BA  >  k HPA  >  k 2-MHA ), depending on the selectivity of the different ROS and stability of respective intermediate organic radicals. Besides, multiple degradation pathways of CHA, mainly including hydroxylation and carbonylation , were proposed by combining the results of mass spectrometry analysis with the calculated Fukui index based on density functional theory (DFT) calculation. In addition, the EC/PMS/Fe(III) system exhibited robust performance with the addition of co-existing ions. This work provides a novel strategy for efficient degradation of NAs for future remediation of petroleum wastewater.