Peroxydisulfate activation by a versatile ball-milled nZVI@MoS₂ composite: Performance and potential activation mechanism

The slow regeneration of Fe(II) is a key issue that limits the application of Fe-AOPs. In this study, a series of nZVI@MoS₂ composites were successfully synthesized through ball-milling. Under the optimum MoS₂/nZVI molar ratio (FM3), the k_obs of FM3/PDS system for SMX degradation was shown to be 5.9, 24.8 and 4.3 times higher than that of nZVI/PDS, MoS₂/PDS and physically mixed nZVI/MoS₂/PDS systems, respectively. Through ball-milling treatment, the inner core of nZVI (Fe⁰) was exposed and more S-vacancies were created, thus exposing more reductive Mo (IV) active sites, and facilitating the conversion efficiency of Fe(III)/Fe(II) and reduction of Fe(III) by Fe⁰. Meanwhile, the reductive sulfur species also contributed to the regeneration of Fe(II), evidenced by the elemental analysis. Furthermore, it is found that >82.1 % degradation efficiency of SMX could be achieved during five successive runs, suggesting the outstanding stability of FM3. The dominant active species were identified as SO₄^[rad]− and [rad]OH. The intermediate products were analyzed and four main reaction pathways were proposed. Overall, the catalytic capacity of nZVI@MoS₂ prepared by ball-milling has been significantly improved compared with nZVI, MoS₂ and physically mixed nZVI/MoS₂, and this work provides a novel perspective for efficient PDS activation by iron-based catalysts.