Insights into hydrangea-like NiCo2S4 activating peroxymonosulfate for efficient degradation of atrazine

Developing a highly efficient heterogeneous catalyst for activating peroxymonosulfate (PMS) in the degradation of persistent organic pollutants (POPs) remains a challenge, and transition bimetallic sulfide is considered a promising candidate. Herein, a hydrangea-like NiCo 2 S 4 was designed for activating PMS in the removal of a typical POP (atrazine, ATZ), and the activation mechanism of the catalyst was elucidated. The hydrangea-like NiCo 2 S 4 was successfully synthesized through sulfide ion-exchange with its oxide counterpart (NiCo 2 O 4 ), showing much more excellent catalytic performance in activating PMS than NiCo 2 O 4 for the degradation of ATZ. The ATZ (46 μM) degradation efficiency ( R D ) in the NiCo 2 S 4 /PMS system (40 mg L −1 catalyst and 0.4 mM PMS) achieves nearly 100 % within 10 min with an apparent rate constant ( k ) of 0.583 min −1 , superior to those of NiCo 2 O 4 /PMS system with values of R D  = 26.7 % and k  = 0.033 min −1 . The role of sulfur species was uncovered and the origin of the exceptional catalytic performance of NiCo 2 S 4 was disclosed through various techniques. The results demonstrated that sulfur species not only accelerated electron transfer but also promoted the regeneration of reactive sites by facilitating the conversion of Co 3+ /Ni 3+ to Co 2+ /Ni 2+ . More reactive oxygen species (ROSs), including sulfate radical (SO 4 •− ), hydroxyl radical ( • OH), superoxide radical (O 2 •− ) and singlet oxygen ( 1 O 2 ) were produced for attacking ATZ molecules, leading to their degradation into the intermediates with reduced toxicity.