Facile synthesis of phosphorus-doped porous biochars for efficient removal of elemental mercury from coal combustion flue gas

Heteroatom doping is an effective method to modify carbonaceous sorbents and improve their chemical reactivity. In this study, P-doped biochars (PBCs) derived from one-step pyrolysis of H3PO4-laden biomass were developed for elemental mercury (Hg0) removal from coal-fired flue gas. Sample characterization showed that there were massive micropores and slit-shaped mesoporous in the PBCs. The specific surface area and pore volume of PBCs was obviously enhanced after P doping. In addition, more organic functional groups were generated on the PBCs surface, particularly the C-P=O and C=O groups. The PBCs presented far higher mercury removal efficiency compared with raw biochars (BCs). The influences of pyrolysis temperature (700 °C-1000 °C), adsorption temperature (25 °C-180 °C), and various flue gas components (NO, SO2, O2, HCl, and H2O) on mercury removal performance were also analyzed. At the optimum temperature (100 °C), the Hg0 adsorption capacity of PBC900 was increased by more than 400 times compared with BC900, which was also higher than that of a commercial brominated activated carbon. The mechanism responsible for Hg0 removal was further revealed. The results suggested that chemisorption dominated the Hg0 removal process, where the C-P=O, C=O, and O-C=O groups could serve as electron acceptors, accelerating the electron migration process for Hg0 oxidization.