Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide

Abstract

Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate-and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the reactivity of Hg, including the propensity of inorganic Hg to be methylated, are poorly understood. Under abiotic conditions, using a laboratory flow reactor, bisulfide (HS À ) was added at 40 to 250 lM h À1 to 5 g L À1 goethite (a-FeOOH) suspensions to which Hg(II) was adsorbed (30-100 nmol m À2 ) at pH 7.5. Dissolved Hg initially decreased from 10 3 or 10 4 nM (depending on initial conditions) to 10 À1 nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (bHgS (s) ) formed, based on identification using Hg L III -edge extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. The apparent coordination of oxygens surrounding Hg(II), measured with EXAFS spectroscopy, increased during one flow experiment, suggesting desorption of monodentate-bound Hg(II) while bidentate-bound Hg(II) persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 nM or 30 nM), suggesting that byproducts of bisulfide oxidation and Fe(III) reduction, primarily polysulfide and potentially Fe(II), enhanced the dissolution of b-HgS (s) and/or desorption of Hg(II). Rapid accumulation of Fe(II) in the solid phase (up to 40 lmol g À1 ) coincided with faster elevation of dissolved Hg concentrations. Fe(II) served as a proxy for elemental sulfur [S(0)], as S(0) was the dominant bisulfide oxidation product coupled to Fe(III) reduction, based on sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. In one experiment, dissolved Hg concentrations tracked those of all sulfide species [S(-II)]. These results suggest that S(-II) reacted with S(0) to form polysulfide, which then caused the dissolution of b-HgS (s) . A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy, although there was no clear evidence that either surface-bound or mineralized Fe(II) strongly affected Hg speciation. Examination of interrelated processes involving S(-II) and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg reactivity models.

Immobilization of Hg(II) by Coprecipitation in Sulfate-Cement Systems

“…Second-neighbor Hg–Fe distances in both samples (3.46 and 3.49 Å) were slightly longer than the Hg–Fe distance (3.40 Å) reported previously for Hg sorption on goethite in one study. 42 These Hg–Fe distances are significantly longer than the Hg–Fe distances (3.19–3.29 Å) reported in other studies for Hg complexes on goethite surfaces. 25,26,43 These prior studies have attributed the observed differences in Hg–Fe distances to the presence of monodentate (longer distance) and bidentate (shorter distance) inner-sphere sorption complexes on goethite.…”

Fish Mercury and Surface Water Sulfate Relationships in the Everglades Protection Area

“…This is caused by several factors affecting mercury methylation and fish mercury bioaccumulation. Primary factors affecting mercury methylation are spatiotemporal variation in quality and quantity of DOC (Cai et al 1999; Benoit et al 2001; Reddy and Aiken 2001; Drexel et al 2002; Miller et al 2007), bioavailable inorganic mercury (Benoit et al 1999a, b; Kelly et al 2003), redox conditions (Marvin-DiPasquale et al 2003; Hollweg et al 2009), concentration of other dissolved ions (Jay et al 2002; Slowey and Brown 2007), and pH (Kelly et al 2003). Primary factors affecting mercury bioaccumulation are water temperature (Bodaly et al 1993; Ethier et al 2008), fish feeding patterns (Li et al 2009; Riva-Murry et al 2011), growth rates (Simoneau et al 2005), and migration (Eagles-Smith et al 2008; Baker et al 2009).…”

Using X-ray Microscopy and Hg L3 XANES To Study Hg Binding in the Rhizosphere of Spartina Cordgrass

“…This avenue can be explored in future studies of environmental Spartina samples. Precipitation of HgS has also been seen in conditions mimicking the presence of SRB and FeRB (15). The precipitated HgS would most likely be in the form of metacinnabar, the main product of Hg precipitation with sulfides.…”