Revisit the selectivity of metal-free biochar activated periodate for the oxidation of emerging contaminants

Selecting appropriate biochar catalyzed advanced oxidation processes (BC-AOPs) for the abatement of emerging organic contaminants remained challenging. In this study, five BC-AOPs with different chemical oxidants were established and employed to degrade organic contaminants. Among others, BC activated periodate (PI) process showed advantages in oxidation rate and selectivity. Specifically, both BC350/PI and BC800/PI showed extremely rapid degradation kinetics towards sulfadiazine (SDZ, kapp = 0.1503 min−1 and 0.0905 min−1) while bisphenol A (BPA) was relatively recalcitrant (kapp = 0.0170 min−1 and 0.0475 min−1). Intriguingly, SDZ was primarily eliminated by singlet oxygen (1O2)/superoxide radicals (O2–) (accounted for 78.4% in BC350/PI/SDZ system), while BPA was mainly oxidized through the electron-transfer mechanism (accounted for 96.8% in BC800/PI/BPA system). Density functional theory (DFT) calculations revealed that BPA was more inclined to provide electrons to metastable complexes compared with SDZ due to the more positive highest occupied molecular orbital. Therefore, the selectivity behavior was further expanded from the perspective of the electron-donating capabilities (i.e., half-wave potential φ1/2) of 8 organic pollutants and a threshold of 0.9 V was clearly identified. This study revealed the difference in oxidation rate towards emerging contaminants of multiple BC-AOPs and uncovered the mysterious selective oxidation behaviors of the BC activated PI process for the first time. In summary, the outcomes of this work provided guidelines for the application of BC-AOPs in treating wastewater containing different organic pollutants.