Efficient degradation of sulfamethoxazole using peracetic acid activated by zero-valent cobalt

A novel heterogeneous peracetic acid (PAA)-based advanced oxidation process (AOP), viz. zero-valent cobalt activated PAA (ZVCo/PAA), was developed to remove sulfamethoxazole (SMX) in this study. About 99% SMX could be removed by ZVCo/PAA under neutral condition in 5 min. According to the characterization results of ZVCo before and after reaction by scanning electron microscopy (SEM), ZVCo was corroded to provide Co2+ which was primarily responsible for the activation of PAA. The X-ray photoelectron spectroscopy (XPS) analysis of fresh and used ZVCo suggested that heterogeneous catalysis also occurred on the surface of ZVCo. Radical scavenging experiments indicated that organic radicals (i.e., CH3C(O)O• and CH3C(O)OO•) were the dominant reactive species for SMX degradation in ZVCo/PAA system. SMX could be degraded well in a wide pH range especially at near-neutral pH. Increasing PAA or ZVCo dosage could enhance SMX removal, while excess ZVCo inhibited its degradation. The addition of SO42−, NO3− or Cl− had a negligible effect on SMX degradation, while an obvious inhibition was observed in the presence of HCO3− or natural organic matter (NOM). ZVCo still presented an excellent activation ability for PAA to degrade SMX after 4 cycles, indicating its good stability and reusability. The probable reaction sites in SMX molecular structure were predicted by density functional theory (DFT) calculation and wave function analysis. Combining the results of DFT calculation and five identified transformation products, four possible degradation pathways of SMX in ZVCo/PAA system were proposed, including nitration, cleavage of S-N bond, coupling reaction and hydroxylation.