Unravelling the formation mechanism and performance of nitrogen, sulfur codoped biochar as peroxymonosulfate activator for gatifloxacin removal

Multi-heteroatom doping is a promising approach to increase the affinity of biochar for catalysis. Herein, a series of N, S-codoped biochar (BSN) were synthesized at different temperatures using a one-pot calcination protocol. Investigation on the physiochemical characteristics of these BSNs revealed that g-C3N4 was first formed from the precursors at lower temperature, engulfing the biochar. At higher synthesis temperature, the g-C3N4 decomposed and coalesce with the biochar to form BSN. The performance of BSN as peroxymonosulfate (PMS) activator for gatifloxacin (GAT) removal was evaluated. The results indicated that BSN prepared at 800 °C (BSN-800) exhibited the greatest performance due to its relatively high specific surface area and synergism between heteroatoms. A kinetic model based on the second-order PMS consumption and first-order GAT removal was developed to describe GAT removal and PMS consumption simultaneously at various operating conditions including BSN-800 loading, PMS dosage and pH. The proposed kinetic model has better fit compared to the conventional pseudo first-order kinetics. The major PMS activation mechanism was identified using chemical scavenger and electrochemical studies indicating that the nonradical pathway involving 1O2 generation and electron mediator mechanisms are dominant with graphitic N and thiophenic S acting as the active sites. Despite its restricted reusability, BSN-800 can be used effectively to remove GAT in various water matrixes including river water, secondary water and tap water. The GAT degradation intermediates were identified, and the degradation pathway was also proposed. Overall, this study provides a better understanding on the development of multi-heteroatom-doped biochar as promising catalyst for antibiotics removal.