Turning peroxymonosulfate activation into singlet oxygen-dominated pathway for ofloxacin degradation by co-doping N and S into durian peel-derived biochar

Biochar is an environmentally friendly material with various potential applications in water treatment. Herein, nitrogen and sulfur co-doped biochars (N,S-BCD-X) were fabricated from durian peel as an agricultural waste by coupling with thiourea via thermal treatment to activate peroxymonosulfate (PMS) for degradation of ofloxacin (OFX) in water. The OFX removal using PMS activated by N,S-BCD-800 was over 85 % after 120 min of reaction, which was 17.78 times higher than that by pure BCD. The OFX degradation was not inhibited over a wide pH range of 3 to 9. Through radical quenching tests, electron paramagnetic resonance, and DFT calculations, 1O2 was revealed as the key reactive species in the N,S-BCD-800/PMS system because of PMS's elongated O-O bond-induced promoted generation of •OH and SO4•- with an insignificant contribution and the generation of 1O2 via various reactions associated with •OH, SO4•-, and PMS. The boosted catalytic activity of N,S-BCD-800 mainly came from synergetic effect of N and S co-doping due to the enhanced electron-donating capability caused by decrease in the work function. Furthermore, the increase in electrostatic potential and the difference in electron density and atomic charge could also account for stronger adsorption and activation of PMS on N,S-BCD-800 to generate more 1O2 besides the N,S-BCD-800/PMS complex as a minor species responsible for the degradation of OFX via electron transfer. Finally, the attainment of high efficiency of N,S-BCD/PMS for OFX degradation in various water matrices as well as after 5 cycles proved that N,S-BCD had high stability and applicability.