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)-O-circle and SO4 circle- as well as non-radicals of O-1(2) 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)-O-circle, SO4 circle-, and O-1(2) were 9% (7.19 mu mol/L), 76% (58.39 mu mol/L), and 15% (11.45 mu mol/L) in BDD/PMS/ Fe(III) system, and were 28% (5.81 mu mol/L), 18% (3.76 mu mol/L), and 54% (10.93 mu mol/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 circle-. 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.

Automated summary

In this study, the activation of peroxymonosulfate (PMS) via electro-cocatalytic system with circulated Fe(III)/Fe(II) was investigated using boron-doped diamond (BDD) anode and dimensionally stable anode (DSA) for the degradation of sulfamethoxazole (SMX). The results showed the presence of radicals and non-radicals in both BDD/PMS/Fe(III) and DSA/PMS/Fe(III) systems. The degradation efficiency of BDD/PMS/Fe(III) was affected by co-existing ions, while DSA/PMS/Fe(III) process was less affected. The study also compared the oxidation mechanisms and primary oxidation pathways of the BDD and DSA systems.