Formation of stable imine intermediates in the coexistence of sulfamethoxazole and humic acid by electrochemical oxidation

The electrochemical degradation performance of sulfamethoxazole (SMX) was studied in the presence of humic acid (HA) by using a Ti/Ti 4 O 7 /β-PbO 2 anode. The electrochemical degradation efficiency of SMX decreased from 93.4% to 45.8% in 50 min after the addition of 25 mg L −1 HA. The pseudo-first-order kinetic rate constant decreased by 71.4%, and the E EO value increased from 63.8 Wh L −1 to 90.9 Wh L −1 . HA and its degradation intermediates could compete for free radicals, especially for ·OH, with SMX. The analytical results obtained using UPLC-ESI-Q-TOF-MS showed that 18 degradation intermediates were identified in the coexistence of SMX and HA. Four imine intermediates were formed through the reactions between the aniline moieties of SMX and quinone groups in the HA structure through covalent bonds. Furthermore, the relative abundances of the intermediates demonstrated that the imine intermediates were complex and stable during electrochemical degradation. • The influence factors were investigated in the electrochemical degradation of sulfamethoxazole. • The presence of humic acid decreased the electrochemical degradation efficiency of sulfamethoxazole. • Stable imine intermediates were identified in the presence of humic acid. • There was accumulation of degradation intermediates of SMX after the addition of humic acid. • Possible electrochemical oxidation mechanisms were proposed in the coexistence of sulfamethoxazole and humic acid.