Adsorption and molecular transformation mechanisms of mercury sulfide on mackinawite surfaces: A DFT-D3 study

• Facet-dependent HgS adsorption and molecular transformation onto mackinawite (FeS) were calculated by DFT-D3. • The adsorption capacity of HgS on the FeS surface follows the order of FeS (011) > FeS (111) > FeS (001), and the Fe sites are more reactive than the S sites. • The S-defect FeS (001) and FeS (111) surfaces have a higher capacity than the Fe-defect. • The dissociation of HgS molecules can be more stably adsorbed on the FeS (011) surface rather than releasing Hg. Mackinawite (FeS), a common metallic sulfide mineral, plays a crucial role in regulating the bioavailability and mobility of mercury sulfide (HgS) in the environment. However, molecular-level insights into HgS interactions with FeS surfaces are currently limited. This study used density functional theory (DFT) to investigate HgS adsorption and transformation on FeS (001), FeS (011), and FeS (111) surfaces, including their defect surfaces. Bonding characteristics were analyzed using electron density difference, Bader charge, projected density of states (PDOS), and crystal orbital bonding index (COBI). HgS adsorption capacity on FeS surfaces is determined by surface reactivity in the order FeS (011) > FeS (111) > FeS (001). Additionally, S-defective FeS (001) and FeS (111) surfaces demonstrate enhanced HgS adsorption compared to Fe-defective surfaces. A potential risk of Hg release from HgS exists on FeS (001) and FeS (111) surfaces compared to FeS (011) surfaces. The dissociation of HgS molecules can be more stably adsorbed on the FeS (011) surface rather than releasing Hg. This study enriches the understanding of HgS adsorption and transformation on metal sulfides, shedding light on the microscopic cycling of HgS in soil systems.