Novel urchin-like Co5Mn-LDH hierarchical nanoarrays: Formation mechanism and its performance in PMS activation and norfloxacin degradation

In this study, Co5Mn-layered double hydroxide (Co5Mn-LDH) was synthesized by hydrothermal process. The SEM image of Co5Mn-LDH shows the urchin-like sphere with the approximate diameter of 7.0–8.0 μm assembled by the numerous rods with a width of about 30–40 nm. Investigation on the formation mechanism of Co5Mn-LDH indicated that NH4F dosage and reaction time played an essential role in modulating the morphology and structure of Co5Mn-LDH. Co5Mn-LDH, as a heterogeneous Fenton-like catalyst, could efficiently activate peroxymonosulfate (PMS) and degrade norfloxacin (NOR) in the pH range of 3.0–9.0. The degradation efficiency of NOR was 98.7% at its natural pH, and the COD removal rate reached 83.3% within 8 min. On the one hand, the abundant Co2+ and Mn3+ in Co5Mn-LDH can simultaneously react with PMS to generate various reactive oxygen species (ROS), while the resulted Co3+ and Mn4+ could convert to original Co2+ and Mn3+, behaving similarly to the Harber-Weiss cycle. On the other hand, the regeneration of Co2+ could also rely on the oxidation and reduction between Co3+/Co2+ and Mn4+/Mn3+. Co(OH)6 octahedron structure and rich surface hydroxyl groups within LDH are favorable to the transformation between Co2+/Mn3+ and Co3+/Mn4+. Such mechanism provides new insights into the nature of reactions in heterogeneous Fenton-like system.