Theoretical research on reaction of solid sulfur allotropes with elemental mercury

Abstract

Element sulfur has proven to be active material for hazardous elemental mercury (Hg0) removal. However, the underlying mechanism of elemental sulfur reacting with Hg0 was still unclear. Generally, solid elemental sulfur exists mainly composed of crown-shaped S8 molecules and traces of S6, S7, and S9 molecules. This research employed quantum chemical calculations and electronic wavefunction analysis to investigate the reaction between branched and cyclic Sn isomers (n = 6–9) with Hg0 at the molecular level. Thermodynamic data and kinetics were also computed. It was found that Hg0 react with branched Sn isomers would proceed spontaneously with heat release by analyzing reaction Gibbs free energies and enthalpies. Reaction rate constants of branched Sn molecules are several orders of magnitude higher than those of cyclic Sn molecules. Moreover, the electronic wavefunction analysis was conducted. The Mayer bond order along the reaction pathway could characterize Hg-S and S-S bonds breaking and formation quantitively. Localized molecular orbitals isosurface maps reflect Hg-S bonds show typical σ bond characteristics in the products. Localized orbital locator color-filled maps reveal electrons begin to concentrate in the region between Hg and S atoms as reactions proceeding forward. In summary, this research would help develop sulfur contained adsorbents with superior Hg0 removal performance in the future.