Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury Immobilization by Mineral Sulfide

Mineral sulfide based sorbents were superior alternatives to traditional activated carbons for elemental mercury (Hg-0) immobilization in industrial flue gas. A systematical study concerning the influence of sulfur trioxide (SO3) on Hg-0 adsorption over a nanosized copper sulfide (Nano-CuS) was for the first time conducted. SO3 was found to significantly inhibit the Hg-0 removal over Nano-CuS partially because SO3 oxidized the reduced sulfur species (sulfide) with high affinity to mercury to its oxidized sulfur species (sulfate). Moreover, a brand new oxidation-reduction mechanism that led to a simultaneous oxidation of sulfide and reduction of mercury on the immobilized mercury sulfide (HgS) was responsible for the inhibitory effect. Even though the released Hg-0 from the reduction of mercury in HgS could be oxidized by SO3 into its sulfate form (HgSO4) and recaptured by the sorbent, the oxidation-reduction mechanism still compromised the Hg-0 capture performance of the Nano-CuS because HgSO4 deposited on the sorbent surface could be easily leached out when environmentally exposed. These new insights into the role of SO3 in Hg-0 capture over Nano-CuS can help to determine possible solutions and facilitate the application of mineral sulfide sorbents as outstanding alternatives to activated carbons for Hg-0 immobilization in industrial flue gas.