Crystal face-dependent methylmercury adsorption onto mackinawite (FeS) nanocrystals: A DFT-D3 study

Abstract

• Facet-dependent MeHg adsorption onto mackinawite (FeS) was calculated by DFT-D3. • The (0 1 1) surface is more favorable than (0 0 1) and (1 1 1) surfaces for MeHg adsorption. • MeHg adsorbs preferentially onto Fe sites of FeS surfaces relative to the S sites. Reactions occurring at mackinawite (FeS) surfaces play an important role in controlling methylmercury (MeHg) bioavailability and mobility. MeHg adsorption onto FeS is morphology dependent, and the mineral surface structure imparts distinct interfacial chemical properties to individual facets and can control the adsorption capacity at these interfaces. However, the underlying mechanisms for facet-specific adsorption of MeHg remain poorly understood at the molecular scale. A systematic first-principles study of MeHg adsorption onto the (0 0 1), (0 1 1), and (1 1 1) surfaces of FeS was conducted using density functional calculations, and the preferred adsorption sites and adsorption mechanisms were determined. MeHg binds preferentially with Fe sites relative to S sites on the FeS surface, with stronger adsorption onto the FeS (0 1 1) and FeS (1 1 1) surfaces compared to the FeS (0 0 1) surface. MeHg displays a variety of chemisorption geometries on each of the FeS surfaces. The most stable configuration on the FeS (0 0 1) surface is a monodentate S–Hg complex. In contrast, the most stable configuration on the FeS (0 1 1) surface is a monodentate Fe–Hg complex, and a bidentate Fe–Hg–Fe complex was found to be the most stable configuration on the FeS (1 1 1) surface. This work provides an important stepping stone in understanding facet effects and bonding mechanisms of MeHg adsorption on FeS surfaces at the molecular level and provides a framework to guide experimental studies that optimize the use of FeS as a scavenger material for MeHg in groundwater and soil systems.