First-Principles Study of Hg0 Immobilization on a Defective Pyrite Surface

The formation of defects over a crystal surface could significantly improve the elemental mercury (Hg0) removal performance of pyrite (FeS2). Herein, the role of surface defects in Hg0 immobilization over FeS2(100) was investigated by adopting quantum chemistry. The contribution of surface defects to mercury immobilization over pyrite was mainly manifested in facilitating Hg0 adsorption. Compared with a perfect pyrite surface, a defective pyrite surface exhibited a higher affinity to Hg0, whose adsorption energy was up to −53.03 kJ/mol. In addition, surface defects could induce the dissociation of HCl, a common component in coal combustion flue gas, which provided active chlorine (Cl*) for subsequent Hg0 oxidation. The Langmuir–Hinshelwood mechanism is responsible for Hg0 oxidation over the defective FeS2(100) surface. First, Hg0 and HCl were simultaneously adsorbed over the defective surface, and the presence of defects could promote Hg0 adsorption and HCl dissociation. Subsequently, adsorbed Hg0 would combine with the adjacent active Cl* site, whose energy barrier was 92.60 kJ/mol. Eventually, HgCl could spontaneously react with another Cl* site to form a HgCl2 molecule. Thus, the formation of a HgCl intermediate was the rate-determining step for Hg0 oxidation over a defective pyrite surface. This work reveals the role of defects in Hg0 immobilization over a pyrite surface, which provides a scientific basis for the design and modification of natural mineral sulfide sorbents.