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Highlights  Highest hydrogen peroxide concentration and MTZ mineralization obtained for 0.07 mA.cm-2  The evolution • OH concentration was not in accordance with that of MTZ mineralization  Dissolved oxygen can become a limiting reagent for • OH quantification with DMSO  DMSO can be oxidized and reduced at the electrodes surface distorting • OH quantification  DMSO efficient as • OH probe until 0. 07 mA.cm-2 in this study, not efficient for electrooxidation

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In order to mineralize Metronidazole (MTZ), a process coupling an electro-Fenton pretreatment and a biological degradation was implemented. A mono-compartment batch reactor containing a carbon-felt cathode and a platinum anode was employed to carry out the electro-Fenton pretreatment of MTZ. A total degradation of MTZ (100 mg L) was observed at 0.07 mA.cm after only 20 min of electrolysis. Yet, after 1 and 2 h of electrolysis, the mineralization level remained low (16.2% and 32% respectively), guaranteeing a significant residual organic content for further biological treatment. LCMS/MS was used to determine the intermediates by-products and hence to propose a plausible degradation pathway. An increase from 0 to 0.44 and 0.6 for 1 and 2 h of electrolysis was observed for the BOD/COD ratio. Thus, from 1 h of electro-Fenton pretreatment, the electrolysis by-products were considered biodegradable. A biological treatment of the electrolysis by-products after 1 and 2 h was then realized. The mineralization yields reached very close values, about 84% for 1 and 2 h of electrolysis after 504 h of biological treatment, namely close to 89% for the overall process, showing the pertinence of the proposed coupled process.

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In this study, the monitoring of reactive oxygen species and the regeneration of the ferrous ions catalyst were performed during electro-Fenton (EF) process to highlight the influence of operating parameters. The removal of metronidazole (MTZ) was implemented in an electrochemical mono-compartment batch reactor under various ranges of current densities, initial MTZ and ferrous ions concentrations, and pH values. It was found that under 0.07 mA cm, 0.1 mM of ferrous ions and pH = 3, the efficiency of 100 mg L MTZ degradation and mineralization were 100% within 20 min and 40% within 135 min of electrolysis, respectively. The highest hydrogen peroxide and hydroxyl radical concentrations, 1.4 mM and 2.28 mM respectively, were obtained at 60 min electrolysis at 0.07 mA cm. Improvement of the biodegradability was reached from 60 min of electrolysis with a BOD/COD ratio above 0.4, which was reinforced by a respirometric study, that supports the feasibility of coupling electro-Fenton and biological treatment for the metronidazole removal.

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