Removal of imidacloprid by cobalt oxide-activated persulfate anchored on nitrogen-rich biochar: synergistic effect of nitrogen doping and carbon defects

Nitrogen-doped cotton straws biochar-loaded cobalt tetraoxide (Co 3 O 4 /N-BC) catalysts were prepared by a two-step calcination method, and the calcination temperature of nitrogen-doped biochar (N-BC) was systematically optimized with the loading temperature of Co 3 O 4 nanoparticles, which was then used to activate peroxynitrite (PMS) for the removal of imidacloprid (IMI). Under a catalyst dosage of 0.3 g/L, PMS concentration of 5 mM, and pH of 7, 98.2 % of a 30 mg/L solution of IMI was degraded in 20 min, 40 min later, the removal rate reached 100 %. The leaching rate of cobalt ions after removal is only 0.74 mg/L, which meets the national emission standard (GB 24567-2010). Nitrogen doping was mainly in the form of pyridine nitrogen, pyrrole nitrogen and graphite nitrogen, where pyridine nitrogen was the main reactive nitrogen species that enhanced the activation of PMS. Quenching experiments and electron paramagnetic resonance spectroscopy (EPR) showed that the free radical pathway represented by SO 4 − and OH was the main pathway for IMI removal with a contribution of 93.05 %. The intermediates formed during the removal of IMI were analyzed by LC- MS, and three possible degradation pathways of IMI were proposed based on nine possible intermediates. Most of the degradation intermediates were eventually converted to low or non-toxic substances as analyzed by ECOSAR software. This study introduces a sustainable approach for reusing waste cotton straw and efficiently treating pesticide production wastewater.