Tourmaline-mediated electrochemical system for sulfamethoxazole degradation: performance, mechanism, and toxicity evaluation

Antibiotics are emerging contaminants that have become a global concern because of their refractory and toxic characteristics. The treatment of antibiotic contaminants using advanced electrochemical oxidation processes is currently gaining interest. In this study, we constructed a natural iron-mediated electrochemical system for the removal of antibiotics using natural tourmaline (TM) as the heterogeneous iron catalyst, platinum (Pt) as the anode, and nickel foam (NiF) as the cathode. Sulfamethoxazole (SMX) was selected as the target antibiotic. The surface properties and components of TM were characterized using energy-dispersive X-ray spectroscopy, N2 adsorption–desorption isotherms, X-ray diffraction, and X-ray photoelectron spectroscopy. Compared with the Pt-NiF electrochemical system, the Pt-NiF-TM electrochemical system showed a 23 times enhanced reaction rate constant of SMX degradation under the optimal conditions of current density 4.17 mA/cm2, 200–400 mesh, TM dosage 3 g/L, and initial pH 3, in which SMX was almost completely degraded (removal rate = 99.92 %) after 60 min. The ·OH and ·O2– radicals generated in the Pt-NiF-TM electrochemical system were the active species for SMX degradation. Thirteen SMX degradation intermediates were identified and possible SMX degradation pathways were proposed. Density functional theory calculations demonstrated that C3 and N11 are the sites with higher electrophilic reactivity in SMX. Furthermore, toxicity calculations and analysis of SMX and its degradation products showed that after treatment with the Pt-NiF-TM electrochemical system, the toxicity of SMX to the microalgae Oocystis was reduced. Overall, this study provides a novel and environmentally friendly electrochemical system mediated by TM for the highly efficient degradation of antibiotic contaminants.