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 NiCo2S4 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 NiCo2S4 was successfully synthesized through sulfide ion-exchange with its oxide counterpart (NiCo2O4), showing much more excellent catalytic performance in activating PMS than NiCo2O4 for the degradation of ATZ. The ATZ (46 mu M) degradation efficiency (R-D) in the NiCo2S4/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 NiCo2O4/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 NiCo2S4 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 Co3+/Ni3+ to Co2+/Ni2+. More reactive oxygen species (ROSs), including sulfate radical (SO4 center dot-), hydroxyl radical ((OH)-O-center dot), superoxide radical (O-2(center dot-)) and singlet oxygen (O-1(2)) were produced for attacking ATZ molecules, leading to their degradation into the intermediates with reduced toxicity.