Magnetic Rattle-Type Fe<inf>3</inf>O<inf>4</inf>@CuS Nanoparticles as Recyclable Sorbents for Mercury Capture from Coal Combustion Flue Gas

Abstract

Rattle-type Fe3O4@CuS synthesized using a two-step method was applied for elemental mercury (Hg0) adsorption in coal combustion flue gas for the first time. The Fe3O4 with strong magnetization was an ideal candidate as a core to make the sorbent recyclable, while the stabilized ultrathin CuS shell assured that the Fe3O4@CuS had a higher Brunauer-Emmett-Teller (BET) surface area with more exposed active sites and stronger magnetization. The optimal operating temperature of 75 °C allowed for the injection of the sorbent between the wet desulfurization (WFGD) and wet electrostatic precipitator (WESP), which removed the detrimental influence of nitrogen oxides. Simulated flue gas (SFG) in this section showed a slight inhibitive effect on Hg0 adsorption over the Fe3O4@CuS, mainly due to the presence of water vapor (H2O). The inhibition of H2O was proven to be the result of an active site prevention effect instead of the widely recognized competitive adsorption effect. The adsorption capacity and rate of the Fe3O4@CuS for Hg0 capture reached 80.73 mg g-1 and 13.22 μg (g min)-1, which were the highest values among the magnetic sorbents currently reported for Hg0 removal from coal combustion flue gas. These properties allowed the sorbent to maintain a 100% Hg0 capture efficiency for more than 20 h with only a 50 mg dosage when no regeneration step was applied. Meanwhile, the contacting time between the sorbent and Hg0 was generally less than 5 s in a typical sorbent injection process. Polysulfides dominated the capturing process and primarily contributed to the extremely high adsorption capacity/rate. A multistep reaction mechanism was proposed to explain the Hg0 adsorption over Fe3O4@CuS. At the first stage, polysulfide participated in Hg0 adsorption as the most active component and was consumed rapidly. After that, S-S dimers, sulfides, and even copper-terminated sites functioned as the adsorption centers. The as-formed mercury-copper (Hg-Cu) amalgam was transformed into mercury sulfide (HgS), a process that was dependent on the extent of the saturation of sulfide sites. With these advantages, Fe3O4@CuS is a promising, cost-effective, highly recyclable, and efficient alternative to the traditional activated carbon for capturing Hg0 in coal combustion flue gases.