Microscopic and Spectroscopic Characterization of Hg(II) Immobilization by Mackinawite (FeS)

This study investigated the solid-phase Hg formed by reacting 0.005 or 0.01 M Hg(II) with 10 g/L mackinawite (FeS) as a function of pH in 0.2 M chloride solutions using X-ray diffraction (XRD), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) analyses. Under all experimental conditions, XRD analysis showed formation of metacinnabar (beta-HgS) as a bulk-phase sorption product, in agreement with the results from high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and selected area electron diffraction (SAED) in TEM analysis. HAADF-STEM and energy dispersive X-ray (EDX) analyses also suggested formation of Hg(II) surface precipitates. EXAFS analysis indicated that metacinnabar was the dominant product under most conditions, with Hg(II) chlorosulfide-like surface precipitates having increased contribution at lower Hg(II) concentration and higher pH. This finding is consistent with the results of desorption experiments using Hg(II)-complexing ligands. Considering the low solubility and high stability of metacinnabar, our results support the potential application of mackinawite for sequestering Hg(II) in anoxic environments.