Molecular-level insights into mercury removal mechanism by pyrite

Natural pyrite (FeS₂) has been regarded as a kind of potential sorbents to control mercury emission from coal-fired power plants because of its low cost and high affinity between mercury and FeS₂. Theoretical investigations based on density functional theory (DFT) were carried out to discern mercury adsorption, reaction and desorption mechanisms over pyrite surface. DFT calculation results indicate that Hg⁰ adsorption on FeS₂(100) and FeS₂(110) surfaces is dominated by physisorption and chemisorption mechanisms, respectively. Mercury atom interacts strongly with Fe atom on FeS₂(110) surface through the atomic orbital hybridization and overlap. HgS is chemically adsorbed on FeS₂(100) and FeS₂(110) surfaces. Electron density difference analysis implies that the significant charge accumulation around sulfur atom of adsorbed HgS molecule is closely associated with the strong interaction between gaseous HgS and pyrite surface. The reaction pathway leading to the formation of gaseous HgS is a three-step process: Hg⁰→Hg(ads)→HgS(ads)→HgS. In the second step, the energy barrier of diatomic surface reaction between adsorbed Hg⁰ and S^2- monomer is approximately 17kJ/mol. The third step is an endothermic process which requires an external energy of about 414.60kJ/mol to desorb the formed HgS, and is the rate-determining step.