Influence of the anode material on the degradation of naproxen by Fenton-based electrochemical processes

This study focuses on the role of the anode material for the electrochemical degradation of the top-selling anti-inflammatory drug naproxen (NPX). Aqueous solutions containing 40 mg L−1 NPX sodium in 0.050 M NaClO4 at pH 3.0 were comparatively treated by electrochemical advanced oxidation processes (EAOPs) like electro-oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). The experiments were performed in a 2.5 L flow plant equipped with an annular glass photoreactor coupled to a cell with a Pt, IrO2-based (DSA-O2), RuO2-based (DSA-Cl2) or boron-doped diamond (BDD) anode and an air-diffusion cathode to electrogenerate H2O2. In EF and PEF, 0.50 mM Fe2+ was added as catalyst. At 50 mA cm−2, the oxidation power of EAOPs rose in the order: EO-H2O2 < EF < PEF, regardless of the anode used. The IrO2-based anode led to greater mineralization in EO-H2O2 and EF. In contrast, the BDD anode allowed an almost total mineralization in PEF, being superior to 85% attained with the other three materials. DSA, a significantly cheap anode compared to Pt and BDD, can then be a suitable candidate for treating NPX solutions by EAOPs. For each process, the mineralization current efficiency and specific energy consumption were determined. The NPX concentration decay always followed a pseudo-first-order kinetics and, in PEF, it was enhanced in the sequence: RuO2-based < Pt < BDD < IrO2-based. GC-MS analysis of treated solutions allowed detecting six aromatic products, whereas maleic and oxalic acids were identified by ion-exclusion HPLC. A reaction sequence for the degradation of NPX by EAOPs is finally proposed.