N-doped biochar mediated peroxydisulfate activation for selective degradation of bisphenol A: The key role of potential difference-driven electron transfer mechanism

Abstract

N-doped biochar has emerged as a promising material for peroxydisulfate (PDS) activation to degrade organic pollutants; however, its reaction mechanism remains elusive. In this study, we synthesized N-doped piggery biogas residue biochar (NBRBC) and investigated an NBRBC-mediated PDS activation system for contaminants degradation. The experimental results show that the NBRBC/PDS system exhibits superior activity for selective degradation of bisphenol A (BPA) via an electron transfer pathway (ETP) but does not work for the degradation of bisphenol S (BPS) and hexafluorobisphenol A (BPAF). Density functional theory (DFT) calculations indicate that the removal efficiencies of contaminants are highly related to the potential difference (EPD) between the highest occupied molecular orbitals (HOMOs) of contaminants and the lowest unoccupied molecular orbital (LUMO) of the NBRBC/PDS system (R2 = 0.9568). Contaminants with low EPD (e.g., BPA) were more easily degraded via ETP, while the oxidation of high EPD contaminants (e.g., BPS, BPAF) was inert to ETP. In addition, N-doping process endowed NBRBC with superior PDS activation activity by lowering the EHOMO−ELUMO gap of the NBRBC/PDS system, thereby accelerating electron transfer. Graphitic carbon, pyridinic N and graphitic N are regarded as the predominant active sites. Our NBRBC/PDS system also possessed resistance to the interference of water matrix and efficient mineralization capacity during BPA degradation. This work further clarified the in-depth mechanism of PDS activation over N-doped biochar catalysts, unraveled the mysterious selective degradation behavior of the NBRBC/PDS system toward BPA, and provides a novel perspective for developing a “fit-for-purpose” strategy for organic wastewater purification.