Efficient Mineralization of Sulfamethoxazole by a Tandem Dual-System Electro-Fenton Process Using a Gas Diffusion Electrode for H2o2 Generation and an Activated Carbon Fiber Cathode for Fe2+ Regeneration

Both H2O2 generation and Fe3+/Fe2+ cycle play significant roles in electro-Fenton (EF) that produces hydroxyl radicals (•OH) to degrade refractory organic pollutants. Nevertheless, it is typically difficult to achieve simultaneously due to the electron transfer selectivity of cathode material. To address this challenge, herein, a novel tandem dual-system EF process was developed to realize efficient mineralization of sulfamethoxazole (SMX), in which a gas diffusion electrode (GDE) was used mainly for H2O2 accumulation (GDE-EF), while an activated carbon fiber (ACF) cathode was applied primarily for Fe2+ regeneration (ACF-EF). H2O2 accumulation by the GDE cathode was about 22 times higher than that by the ACF cathode. The ACF cathode significantly accelerated the conversion of Fe3+ to Fe2+, i.e., about 17.0 mg L-1 (30.6%) of Fe2+ was maintained at 120 min in ACF-EF, much higher than 3.4 mg L-1 in GDE-EF (6.1%). Up to 87.6% mineralization was achieved after 360 min of GDE+ACF-EF treatment. Effects of Fe2+ concentration, current density, pH, and SMX concentration on pollutant removal were investigated. Evolutions of inorganic ions and short-chain carboxylic acids were determined. Seven aromatic intermediates were identified by UPLC-QTOF-MS, while their ecotoxicity were also assessed. Finally, a reaction sequence for SMX mineralization was proposed.