Mechanistic study of cobalt and iron based Prussian blue analogues to activate peroxymonosulfate for efficient diclofenac degradation

Cobalt and iron-based Prussian blue analogues (PBAs) have been assumed as highly efficient peroxymonosulfate (PMS) activators. Herein, four PBAs using iron and/or cobalt as the coordination metals were synthesized by coprecipitation method to investigate the degradation of diclofenac (DCF). Results showed that Co3[Fe(CN)6]2 (CoFe) and Co3[Co(CN)6]2 (CoCo) exhibited much better PMS activating capabilities than Fe3[Co(CN)6]2 (FeCo) and Fe3[Fe(CN)6]2 (FeFe). The optimum performance achieved by CoCo/PMS resulted in a complete degradation of 0.1 mM (30 mg/L) DCF within 2 min under a broad pH range of 4–10. The leaching of Co2+ of both CoFe and CoCo was more significant than that of FeCo, but the leaching of iron species was very low in all FeContaining PBAs. However, both homogeneous and heterogeneous mechanisms were assumed to contribute to DCF degradation, and the high spin state cobalt existing in CoFe and CoCo that was linked to nitrogen was considered as main active site. Moreover, based on principal component analysis the most influential parameter was the initial pH. Electron spin resonance (ESR) and quenching experiments exhibited that SO4•−, 1O2 and O2•− all led to DCF degradation. The high-valent cobalt-oxo species [Co(IV)] was for the first time proposed in both CoFe/PMS and CoCo/PMS processes, also contributing to DCF degradation. Additionally, CoCo exhibited better reusability and DCF mineralization performance than CoFe.