Optimizing catalytic activity of cobalt borates/peroxymonosulfate for degradation of various organic pollutants.

The investigation of transition metal borates and borides as electrocatalysts for hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction has garnered significant research attention due to their unique electrocatalytic activity attributed to the synergistic effects of the borate ligand and redox-active metal center.  Previously, our research group synthesized polycrystalline cobalt borate (Co₂B₂O₅) which demonstrated superior catalytic activity in activating peroxymonosulfate (PMS) for tetracycline (TC) degradation compared to cobalt oxides (CoO).   Herein, we optimized the catalytic activity of Co₂B₂O₅ for the degradation of various classes of pollutants, including azo dyes (methylene blue), aromatic compounds (4-nitrophenol) and sulfamethoxazole (SMX), a class of difficult-to-degrade antibiotics.  The most optimal degradation condition using minimal Co₂B₂O₅ and PMS loading achieved >96% of degradation efficiency in 30 minutes was investigated.  In this system, the pseudo first-order reaction TC degradation increased with increasing catalyst loading and PMS concentration.  For practical application in real wastewater containing natural organic materials (NOM), we observed that hydrogen phosphate and nitrates decreased the Co₂B₂O₅/PMS catalytic activity by 8% and 13%, respectively while other NOMs has an insignificant effect on TC degradation.   Furthermore, we have also confirmed that the main reactive oxygen species in the Co₂B₂O₅/PMS AOP system is surface bound hydroxyl radical.  In conclusion, polycrystalline cobalt borates demonstrate promising catalytic activity in the degradation of various classes of organic pollutants.