Performance comparison and mechanism investigation of Co3O4-modified different crystallographic MnO2 (α, β, γ, and δ) as an activator of peroxymonosulfate (PMS) for sulfisoxazole degradation

The hybrids of Co3O4 anchored on different crystallographic MnO2 (α-MnO2@Co3O4, β-MnO2@Co3O4, γ-MnO2@Co3O4, and δ-MnO2@Co3O4) were synthesized and applied in peroxymonosulfate (PMS) activation for the degradation of sulfisoxazole (SIZ). Various characterizations were conducted to explore the physicochemical properties of the hybrids, and their catalytic activities were compared. The SIZ degradation followed pseudo-first-order kinetics, and the rate constants (kobs) were 0.1135, 0.0307, 0.1080, and 0.0285 min−1 for α, γ, β, and δ-MnO2, respectively. The introduction of Co3O4 greatly improved the catalytic activities of the MnO2 hybrids, and more than 94.7% of SIZ was degraded within 20 min. The kobs was in the order of γ-MnO2@Co3O4 (0.3952 min−1) > α-MnO2@Co3O4 (0.1573 min−1) > β-MnO2@Co3O4 (0.1536 min−1) > δ-MnO2@Co3O4 (0.1319 min−1). Obviously, the synergy of Co3O4 and MnO2 weakened the differences of catalytic performance of MnO2 caused by their microstructures among α, β, and δ-MnO2. Due to its large surface area, γ-MnO2@Co3O4 exhibited the most excellent catalytic performance for the SIZ removal. All the hybrids also displayed superior stability and reusability, and the degradation rates of SIZ maintained were above 97% during five consecutive cycles. The reactive oxygen species (ROS) including SO4•−, •OH, and 1O2 were responsible for the SIZ degradation in the system of MnO2@Co3O4/PMS, which was confirmed by electron paramagnetic resonance (EPR) technique and scavenger tests. Twelve intermediates of the SIZ degradation were identified via HPLC-TOF-MS2. Combined with the results of characterizations together with ROS identification, the possible catalytic mechanisms and pathways of the SIZ degradation were proposed. This study may shed some light toward the application of different crystallographic MnO2 hybridized with other transition metals in PMS activation.