Fabrication of multi-walled carbon nanotubes and carbon black co-modified graphite felt cathode for amoxicillin removal by electrochemical advanced oxidation processes under mild pH condition

Hydrogen peroxide (H₂O₂) electrogenerated via two-electron oxygen reduction reaction at cathode plays an important role in electrochemical advanced oxidation processes for organic pollutants removal from wastewater. Herein, multi-walled carbon nanotubes and carbon black co-modified graphite felt electrode (MWCNTs-CB/GF) was prepared as an efficient cathode for H₂O₂ electrogeneration and amoxicillin removal by anodic oxidation with hydrogen peroxide (AO-H₂O₂) and electro-Fenton (EF) under mild pH condition. Besides, the physicochemical and electrochemical properties of MWCNTs-CB/GF were characterized by scanning electron microscopy, N₂ adsorption and desorption experiment, contact angle measurement, X-ray photoelectron spectroscopy, and linear sweep voltammetry. Compared with GF, MWCNTs-CB/GF showed a higher H₂O₂ generation of 309.0 mg L⁻¹ with a current efficiency of 60.9% (after 120 min) and more effective amoxicillin removal efficiencies of 97.5% (after 120 min) and 98.7% (after 30 min) in AO-H₂O₂ and EF (with 0.5 mM Fe²⁺) processes, under the condition of current density 12 mA cm⁻² and initial pH 5.5. Meanwhile, the TOC removal efficiency was 45.2% during EF process after 120 min. Anodic oxidation, H₂O₂ oxidation, and methanol capture indicated that ∙OH generated via electro-activation reaction at MWCNTs-CB/GF and Fenton reaction in solution played the dominant role in amoxicillin removal. Moreover, the TOC removal was associated with ∙OH generated during Fenton reaction in the solution. The major intermediates of AMX degradation by EF process were identified using LC-MS and the possible degradation pathways were proposed containing of β-lactam ring opening, hydroxylation reaction, decarboxylation reaction, methyl groups in the thiazolidine ring oxidation reaction, bond cleavage, and rearrangement processes. All of the above results proved that MWCNTs-CB/GF was an excellent cathode for AMX degradation under mild pH condition.