Application of Fe-ethylene diamine tetraacetic acid complex to enhance the electrocatalytic activation of peroxymonosulfate under neutral pH condition for removal of refractory naphthenic acids

Abstract

In this work, ethylene diamine tetraacetic acid (EDTA) was applied as a chelating agent to stabilize Fe(III)/Fe(II) cycle to effectively activate peroxymonosulfate (PMS) in an electrochemical (EC) co-catalytic system for elimination of naphthenic acids (NAs) under neutral pH condition. 1-Adamantanecarboxylic acid (ACA) was applied as the model compound for NAs that are persistent contaminants in petroleum industrial wastewater. In the PMS/EC/Fe(III)-EDTA system, complete removal of ACA has been achieved in 20-min reaction under neutral pH condition. Degradation mechanisms were investigated via electron paramagnetic resonance (EPR) analysis, probe experiments and quenching experiments to reveal the reactive species •OH, SO4•− and 1O2 for ACA removal, the production of which have been significantly facilitated by the introduction of EDTA complex. A new pathway has been found for the production of 1O2 via several intermediates including hydrogen peroxide and superoxide radicals. Density functional theory (DFT) calculations and mass spectrometry analysis were applied to predict ACA degradation pathways and byproducts. Decarboxylation, hydroxyl substitution and carbonyl substitution byproducts of ACA were detected. The PMS/EC/Fe(III)-EDTA system suffered nonimportant influence at the presence of humic acid, H2PO4-, HCO3–, NO3– and Cl- for ACA removal. Overall results demonstrate the effectiveness and reliability of the PMS/EC/Fe(III)-EDTA system for efficient removal of NAs in petroleum wastewater in natural environment under neutral pH condition.