Assessing the HClO/Fe^2+Fenton-like reaction for the degradation of metronidazole in a flow cell equipped with a Ti-Ru-Ir oxides anode

Abstract

This work examines the electrochemical mineralization of metronidazole (MTZ) via the photoelectro-Fenton-like (PEF-like) process in a flow plant utilizing a filter-press electrolyzer with a Ti|Ti-Ru-Ir-oxides anode. The process uses the chloride ions contained in a dilute solution to form HClO through direct anodic oxidation on the Ti|Ti-Ru-Ir-oxides anode. Then, HClO reacts with ferrous ion (Fe²⁺) to yield ferryl-oxo species (Fe^IVO²⁺) and hydroxyl radicals (^•OH), which are responsible for the mineralization of MTZ to CO₂. The influence of the current density (<em>j</em>), as well as the initial Cl^-, Fe²⁺, and MTZ concentrations was systematically examined to determine the best experimental conditions for MTZ degradation. A comparison of electro-oxidation aided with active chlorine (EO-HClO), electro-Fenton-like (EF-like), and PEF-like processes for the elimination of MTZ indicates the superiority of the latter one. The use of scavengers indicates that Fe^IVO²⁺ is the main oxidant (58 %), with minor contributions from ^•OH (13 %) and other routes, such as electro-oxidation and chlorination. Optimum PEF-like conditions for the complete degradation of 20 mg L^-1 MTZ involved the use of 0.4 mM Fe²⁺ in a 35 mM NaCl+ 50 mM Na₂SO₄ electrolyte, at <em>j</em> = 16 mA cm^-2 and <em>u</em> = 10 cm s^-1, with an electrolytic energy consumption of 0.9 kWh (g TOC)^-1. Up to 78 % of mineralization was attained, where the residual dissolved organic matter mainly consisted of oxalic acid and N-derivatives.