Flue gas mercury removal is mandatory for decreasing global mercury background concentration and ecosystem protection, but it severely suffers from the instability of traditional demercury products (e.g., HgCl2, HgO, HgS, and HgSe). Herein, we demonstrate a superstable Hg3Se2Cl2 compound, which offers a promising next-generation flue gas mercury removal strategy. Theoretical calculations revealed a superstable Hg bonding structure in Hg3Se2Cl2, with the highest mercury dissociation energy (4.71 eV) among all known mercury compounds. Experiments demonstrate its unprecedentedly high thermal stability (>400 °C) and strong acid resistance (5% H2SO4). The Hg3Se2Cl2 compound could be produced via the reduction of SeO3 2– to nascent active Se0 by the flue gas component SO2 and the subsequent combination of Se0 with Hg0 and Cl– ions or HgCl2. During a laboratory-simulated experiment, this Hg3Se2Cl2-based strategy achieves >96% removal efficiencies of both Hg0 and HgCl2 enabling nearly zero Hg0 re-emission. As expected, real mercury removal efficiency under Se-rich industrial flue gas conditions is much more efficient than Se-poor counterparts, confirming the feasibility of this Hg3Se2Cl2-based strategy for practical applications. This study sheds light on the importance of stable demercury products in flue gas mercury treatment and also provides a highly efficient and safe flue gas demercury strategy.

Gaseous elemental mercury (Hg 0 ) extraction from industrial flue gases is undergoing intense research due to its unique properties. Selective adsorption that renders Hg 0 to HgO or HgS over metal oxide-or sulfide-based sorbents is a promising method, yet the sorbents are easily poisoned by sulfur dioxide (SO 2 ) and H 2 O vapor. The Se-Cl intermediate derived from SeO 2 and HCl driven by SO 2 has been demonstrated to stabilize Hg 0 . Thus, a surface-induced method was put forward when using γ-Al 2 O 3 supported selenite-chloride (xSeO 3 2− -yCl − , named xSe-yCl) for mercury deposition. Results confirmed that under 3000 ppm SO 2 and 4% H 2 O, Se-2Cl exhibited the highest induced adsorption performance at 160 °C and higher humidity can accelerate the induction process. Driven by SO 2 under the wet interface, the in situ generated active Se 0 has high affinity toward Hg 0 , and the introduction of Cl − enabled the fast-trapping and stabilization of Hg 0 due to its intercalation in the HgSe product. Additionally, the long-time scale-up experiment showed a gradient color change of the Se-2Cl-induced surface, which maintained almost 100% Hg 0 removal efficiency over 180 h with a normalized adsorption capacity of 157.26 mg/g. This surface-induced method has the potential for practical application and offers a guideline for reversing the negative effect of SO 2 on gaseous pollutant removal. KEYWORDS: high concentrations of SO 2 and H 2 O, fast-trapping of Hg 0 , selenite-chloride surface, induced adsorption, stability