Understanding the electro-cocatalytic peroxymonosulfate-based systems with BDD versus DSA anodes: Radical versus nonradical dominated degradation mechanisms

Herein, peroxymonosulfate (PMS) activation via electro-cocatalytic system with circulated Fe(III)/Fe(II) was investigated with boron-doped diamond (BDD) anode and dimensionally stable anode (DSA) for degradation of sulfamethoxazole (SMX). Radicals of OH and SO4− as well as non-radicals of 1O2 and Fe(IV) were all present in BDD/PMS/Fe(III) and DSA/PMS/Fe(III) systems, according to the results of scavenging experiment and electron paramagnetic resonance (EPR) analysis. Interestingly based on the results of chemical probe experiments, the ratio of OH, SO4−, and 1O2 were 9% (7.19 μmol/L), 76% (58.39 μmol/L), and 15% (11.45 μmol/L) in BDD/PMS/Fe(III) system, and were 28% (5.81 μmol/L), 18% (3.76 μmol/L), and 54% (10.93 μmol/L) in DSA/PMS/Fe(III) system. The yield of Fe(IV) in DSA/PMS/Fe(III) process was relatively higher than that in BDD/PMS/Fe(III) process. These results indicated that BDD/PMS/Fe(III) is a radical oxidation system while DSA/PMS/Fe(III) system is mainly non-radical system. Due to the high electrochemical activity of BDD anode, the one-electron transfer reaction might occur to produce more SO4−. The co-existing ions including Cl−, H2PO4−, and NO3− showed significant effects on the degradation efficiency of BDD/PMS/Fe(III) and nonsignificant impacts on DSA/PMS/Fe(III) process, indicating that DSA/PMS/Fe(III) process could be less affected by complex water matrix. BDD/PMS/Fe(III) is inclined to oxidize electron-rich contaminants, while DSA/PMS/Fe(III) is apt to oxidize electron-deficient contaminants. The degradation routes of SMX and its toxicity evolution were systematically studied with the analysis of byproducts. Overall results compared the oxidation mechanisms and primary oxidation pathways of electro-cocatalytic systems with BDD versus DSA anodes, and revealed the merits of each system for future selection and system optimization of treatment processes regarding targeted contaminants.