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

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