Elimination of the pharmaceutical sulfadoxine by electro-Fenton: Reaction kinetics, oxidation pathway and stability of the electrode

In this study, the removal of sulfadoxine (SDX) with the electro-Fenton (EF) process is for the first time investigated, in a commercially available filter press electrochemical cell. A Gas Diffusion Electrode (GDE) was used as the cathode and a Dimensionally Stable Anode (DSA) as the anode. The effects of current density, recirculation flow, and pH on H₂O₂ generation were examined. The maximum H₂O₂ concentration was 457 mg/L at 50 mA/cm², 200 mL/min, and pH 7 after 2 h of operation. Under the same conditions, the effect of Fe²⁺ on SDX oxidation and mineralization was studied. With 0.5 mM Fe²⁺, SDX was eliminated within 10 min, with 77.4% total organic carbon (TOC) removal after 2 h. Notably, Fe precipitation was observed at near-neutral pH, indicating that removal via coagulation and adsorption may have contributed to SDX elimination in addition to oxidation. Five transformation products (TPs) were identified via LC/HRMS Orbitrap, with four newly reported. The acute and chronic toxicity of the TPs was assessed in fish, daphnia, and green algae using ECOSAR software. Most TPs showed lower toxicity than SDX, though some exhibited increased toxicity, suggesting longer or more intense treatments may be needed. The stability of the GDE was evaluated over 10 consecutive cycles with 0.1 mM Fe²⁺. SDX removal remained stable, ranging from 72.2% to 85.4%, though H₂O₂ electrogeneration decreased steadily from 481.4 mg/L to 200.6 mg/L by the final cycle. These results suggest that while EF effectively removes SDX, improvements in H₂O₂ generation stability are necessary for long-term use.