Decrease in Net Mercury Methylation Rates Following Iron Amendment to Anoxic Wetland Sediment Slurries

Abstract

The rate of mercury methylation in anoxic wetland sediments is affected by the concentration of bioavailable complexes between Hg and sulfide. Previous research with pure bacterial cultures has shown that addition of ferrous iron reduces the net rate of mercury methylation by decreasing the concentration of dissolved sulfide. To assess the possibility of using this approach to decrease net mercury methylation in restored and constructed wetlands, laboratory experiments were conducted by adding Hg(II) and Fe(II) to sediments collected from six sites in five estuarine wetlands. Addition of 30 mM (0.07 mmol g(-1) or 3.9 mg g(-1)) Fe(II) decreased net mercury methylation relative to that of unamended controls by a factor of 2.1-6.6. In all cases, the observed decrease in net mercury methylation was accompanied by a decrease in the concentrations of sulfide and filterable mercury in the water overlying the sediments. When iron was added to one of the sediment samples at doses that were small relative to the concentration of sulfide present, net mercury methylation either increased slightly or was unaffected. Comparison of the results to speciation model predictions suggests that dissolved reduced sulfur-containing species play a role in the formation of uncharged, bioavailable Hg complexes. Although further research is needed to determine the long-term effect of iron amendment, these results suggest that iron addition decreases mercury methylation in authentic wetland sediments.

Assessment of the use of sorbent amendments for reduction of mercury methylation in wetland sediments at Acadia National Park, Maine

“…One approach is to apply a chemical amendment that has the potential to reduce the concentration of Hg(II) available for Hg(II)-methylation by reduction to volatile elemental Hg(0) (Vernon and Bonzongo, 2014). Another approach is to add a complexation agent that has the potential to reduce the concentration of sulfides that are reported to facilitate Hg(II)-methylation by the formation of neutral mercury sulfide (HgS) complexes (Mehrotra and Sedlak, 2005). One of the more common approaches to remediation is to add a sorbent to adsorb Hg(II) and MeHg, thus decreasing the concentrations of the MeHg precursor and product (Weisener and others, 2005;Vernon and Bonzongo, 2014.…”

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

“…Warner et al (67) demonstrated that freshwater sediments with iron reduction as the dominant process had methylation potentials similar to those of sediments in which sulfate reduction was the dominant terminal electron accepting process. Recent studies (46) showing stimulation of methylation by low concentrations of iron (0.3 and 3.0 mM) suggest (assuming some oxidation of the added ferrous chloride) that methylation by iron-reducing bacteria is also important in marine sediments. These increased methylation rates may be due to increases in the bioavailability of mercury for methylators in general, via dissolution of iron colloids or via electron acceptor stimulation of methylation by metal-reducing bacteria.…”

Radial oxygen loss has different effects on the accumulation of total mercury and methylmercury in rice

“…The formation of Fe plaque on roots significantly inhibited the uptake of As by rice, under both glasshouse (Chen et al 2005) and paddy field conditions (Garnier et al 2010). It has also been reported that adding ferrous iron to sulphidic wetland sediments decreased Hg bioavailability and net methylation due to a decrease in sulphide activity and a concomitant decrease in the concentration of dissolved mercury (Mehrotra et al 2003;Mehrotra and Sedlak 2005). However, the exact effects of ROL and Fe plaque formation on Hg and MeHg uptake by rice plants have not been reported.…”