Efficient removal of recalcitrant naphthenic acids with electro-cocatalytic activation of peroxymonosulfate by Fe(III)-nitrilotriacetic acid complex under neutral initial pH condition.

This work investigated the activation of peroxymonosulfate by electrochemical (EC) system assisted with Fe(III)-nitrilotriacetic acid (NTA) complex for degradation of persistent naphthenic acids (NAs) under neutral initial pH conditions. As NAs are a complicated mixture, 1-adamantanecarboxylic acid (ACA) was selected as the model NA compound for degradation experiment. The addition of NTA is to chelate with Fe(III), gaining stability under neutral pH condition to facilitate the circulation of Fe(II)/Fe(III) by the electrochemical process to activate PMS. The EC/Fe(III)-NTA/PMS system was explored with applicable pH range of 3–9 and an optimized molar ratio 1: 2 for Fe: NTA. Results of quenching and chemical probe experiment together with results of electron paramagnetic resonance (EPR) analysis revealed the main reactive species of the system, including •OH, SO 4 •−, 1O 2 and possibly Fe(IV). With the addition of NTA, the yields of •OH, SO 4 •−, 1O 2 were enhanced. Results of mass spectrometry analysis and DFT calculations indicated the formation of 9 degradation byproducts of ACA via three primary degradation pathways such as hydroxyl substitution, carbonyl substitution, and decarboxylation. Furthermore, the EC/Fe(III)-NTA/PMS system could achieve excellent removal efficiency of ACA with different anions such as Cl-, HCO 3 -, NO 3 - and H 2 PO 4 - in the background. The practical applicability of the system was also verified with the high removal of commercial NAs mixture standard. Overall results have indicated the EC/Fe(III)-NTA/PMS system could be utilized for efficient reclamation of authentic oil and gas industrial wastewater under natural pH conditions. [Display omitted] • The EC/Fe(III)-NTA/PMS system was explored with applicable pH range 3–9. • The redox cycle of Fe(III)-NTA/Fe(II)-NTA was boosted to activate PMS. • Fe(II)-NTA complex maintains high reaction rates to activate PMS at neutral pH. • •OH, SO 4 •−, 1O 2 and Fe(IV) are the main reactive species in the system. • ACA degradation pathways and byproducts were revealed based on mass spectra. [ABSTRACT FROM AUTHOR]