Mercury Removal by Magnetic Biochar Derived from Simultaneous Activation and Magnetization of Sawdust

Novel magnetic biochars (MBC) were prepared by one-step pyrolysis of FeCl3-laden biomass and employed for Hg0 removal in simulated combustion flue gas. The sample characterization indicated that highly dispersed Fe3O4 particles could be deposited on the MBC surface. Both enhanced surface area and excellent magnetization property were obtained. With the activation of FeCl3, more oxygen-rich functional groups were formed on the MBC, especially the C═O group. The MBC exhibited far greater Hg0 removal performance compared to the nonmagnetic biochar (NMBC) under N2 + 4% O2 atmosphere in a wide reaction temperature window (120-250 °C). The optimal pyrolysis temperature for the preparation of MBC is 600 °C, and the best FeCl3/biomass impregnation mass ratio is 1.5 g/g. At the optimal temperature (120 °C), the Fe1.5MBC600 was superior in both Hg0 adsorption capacity and adsorption rate to a commercial brominated activated carbon (Br-AC) used for mercury removal in power plants. The mechanism of Hg0 removal was proposed, and there are two types of active adsorption/oxidation sites for Hg0: Fe3O4 and oxygen-rich functional groups. The role of Fe3O4 in Hg0 removal was attributed to the Fe3+(t) coordination and lattice oxygen. The C═O group could act as act as electron acceptors, facilitating the electron transfer for Hg0 oxidation.