Role of sulfide-modified nanoscale zero-valent iron on carbon nanotubes in nonradical activation of peroxydisulfate.

Sulfamethoxazole (SMX) is highly persistent and difficult to remove, making it urgent to find an efficient method for alleviating the enormous environmental pressure of SMX. In this study, sulfide-modified nanoscale zero-valent iron on carbon nanotubes (S-nZVI@CNTs) was prepared to activate peroxydisulfate (PDS) for the degradation of SMX. The results showed that SMX was completely removed within 40 min (k=0.1058 min) in the S-nZVI@CNTs/PDS system. By analyzing quenching experiments and electron paramagnetic resonance (EPR), singlet oxygen (O) was the main active species of the S-nZVI@CNTs/PDS system. O might be mediated by the abundant carbonyl groups (CO) on carbon nanotubes through spectroscopic analyses. In addition, sulfur doping transitioned the activation pathway to a nonradical pathway. Spectroscopic analyses and electrochemical experiments confirmed that the formation of CNTs-PDS complexes and S-nZVI could promote electron transfer on the catalyst surface. Furthermore, the main degradation intermediates of SMX were identified, and five possible transformation pathways were proposed. The S-nZVI@CNTs/PDS system possessed advantages including high anti-interference (Cl, NO, HA), a strong applicability, recyclability and a low PDS consumption, offering new insight into the degradation of antibiotic wastewater.