Degradation of sulfamethoxazole by a heterogeneous Fenton-like system with microscale zero-valent iron: Kinetics, effect factors, and pathways

Five commercial microscale zero-valent iron (mZVI) samples showed different reactivity in sulfamethoxazole (SMX) removal in the presence of H₂O₂, which was independent of respective iron surface area but was consistent with the dissolved iron releasing rate. Of five mZVI samples, mZVI_YF2 possessed the highest reactivity in H₂O₂ and the mZVI_YF2/H₂O₂ system could remove 97.9% SMX at room temperature within 10 min. The degradation curve of SMX by mZVI/H₂O₂ at pH_ini 3.0 could be divided into three phases: a lag phase, a rapid degradation phase, and a final stationary phase. Effects of pH_ini, mZVI loading, SMX concentration, and H₂O₂ concentration on SMX degradation by mZVI/H₂O₂ were analyzed. Removal rates of SMX by mZVI/H₂O₂ dropped sharply upon increasing pH_ini. Increasing dosages of Fe⁰ ranged from 25 to 75 mg/L and H₂O₂ at low concentrations (below 0.5 mM) both enhanced SMX degradation, but a higher concentration of Fe⁰ and H₂O₂ also quickly decreased oxidative efficiency of SMX. Increasing dosages of SMX within the range of 5–100 µM progressively decreased the oxidation rates of SMX by mZVI/H₂O₂. Furthermore, three degradation pathways of SMX by mZVI/H₂O₂ process were proposed based on the combination of theoretical calculations and intermediates identification.