Could sulfidation enhance the long-term performance of nano-zero valent iron in the peroxymonosulfate activation to degrade 2-chlorobiphenyl?

Sulfidation can enhance the hydrophobicity of nano-zero valent iron (nZVI) and improve its long-term degradation performance in reduction technology. However, whether sulfidation can enhance its long-term performance in sulfate radical-based advanced oxidation processes hasn't been systematically studied. Herein sulfide-modified nZVI (S-nZVI) was prepared by different sulfidation methods and S/Fe ratios. The behavior of S-nZVI on the peroxymonosulfatec (PMS) activation to degrade 2-chlorobiphenyl for continuous 5 rounds was investigated. The results showed that sulfidation couldn't always promote the long-term degradation performance. S-nZVI prepared by one-step sulfidation method with high S/Fe ratio (S-nZVIonestep-7%, S-nZVIonestep-14%) exhibited inferior degradation performance than unmodified nZVI (52.2%). This was because that the electron donor Fe0 was consumed rapidly and the crystalline lepidocrocite accumulated on the surface, thus inhibited PMS activation. In contrast, S-nZVI prepared by post-sulfidation method with high S/Fe ratio (S-nZVIpost-7%, S-nZVIpost-14%) exhibited more Fe0 residual, less FeOx accumulation, and more catalytic Fe2+ regeneration. Consequently, S-nZVIpost exhibited superior degradation capacity (69.3%). Moreover, the radical quenching experiments revealed that the primary free radicals involved in the degradation were transformed from SO4•- to •OH with prolongation of the degradation. Additionally, Fe (IV) contributed to the degradation through non-radical mechanism, especially in the S-nZVIpost-7%/PMS system.