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 Fe_1.5MBC₆₀₀ 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.