Catalytic mechanism of nitrogen-doped biochar under different pyrolysis temperatures: The crucial roles of nitrogen incorporation and carbon configuration.

To scrutinize the crucial role of carbon configuration and nitrogen speciation in peroxymonsulfate (PMS) activation, nitrogen-doped biochars (NBCs) were prepared at different pyrolysis temperatures (700, 800 and 900 °C) and named NBC700, NBC800 and NBC900, respectively. Nitrogen doping introduced many nitrogen-containing groups into NBCs and the carbon configuration and nitrogen speciation of NBCs were regularly changed by the pyrolysis temperature. Compared to the phenol (PN) removal in the pristine biochar (BC)/PMS system that mainly depended on adsorption, NBCs showed excellent PMS activation activity for efficient PN degradation and the PMS activation activity was highly dependent on the carbon configuration and nitrogen speciation of NBCs. Furthermore, the PMS activation pathways of NBCs were unveiled to convert O to electron transfer with increasing pyrolysis temperature, which was ascribed to the variation of active sites on NBCs caused by the regular changes in carbon configuration and nitrogen speciation. Pyridinic N and oxygen groups (CO, CO and O-C=O) were proposed as potential active sites on NBC700 and NBC800 for O generation via PMS activation. Differently, the highly sp-hybridized carbon skeleton and graphitic N of NBC900 played an important role in the electron transfer pathway by acting as a carbon bridge to accelerate electron transfer from PN to PMS. This study provides new insight into the effects of carbon configuration and nitrogen speciation on PMS activation mechanism of NBCs and identifies opportunities for the subsequent catalyst design in a specific degradation pathway.