Degradation of atrazine by Ni-doped sulfidated microscale zero-valent iron: Mechanistic insights for enhanced reactivity and selectivity

Sulfidation of microscale zero-valent iron particles (S/mZVI) has been considered as a general means to enhance its reactivity and selectivity. However, further improving of S/mZVI over water while still maintaining high reactivity with target contaminants is still a great challenge. In this study, Ni and Na2S were chosen as the catalyst and sulfidation regent, respectively, to synthesize three different kinds of nickel-sulfidated composite mZVI particles (NSC-mZVI), namely, post-sulfidation (Ni/mZVI-S), pre-sulfidation (S/mZVI-Ni), and co-sulfidation (Ni/S-mZVI), by adjusting the sequence of nickel doping and sulfidation. The reactivity and selec-tivity performance of NSC-mZVI toward atrazine (ATZ) in groundwater were compared, and the underlying mechanisms for the enhancement effect of ATZ and selectivity were also elucidated. Compared with S/mZVI, Ni/S-mZVI not only further increased the ATZ removal rate, but also reduced the hydrogen accumulation, which simultaneously improved the reactivity and selectivity of target contaminants. The enhanced removal rate of ATZ by Ni/S-mZVI could be ascribed to the generated atom hydrogen and the direct reduction of Fe-0 core mediated by the electron transfer of FeS2 and Fe3O4. Meanwhile, the reduced hydrogen evolution reaction (HER) of Ni/S-mZVI can be explained by that Fe oxides conducting minor electrons mediated HER. The pH value of Ni/S-mZVI eventually reached 5.1 with high release of Fe2+. Overall, bimetallic modification of S/mZVI is a prom-ising strategy to improve its reactivity, selectivity, and longevity for groundwater remediation.