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Mercury in the Environment What is Mercury (Hg) • Hg is a silvery, liquid metal at room temperature • "heavy metals." • Like water, Hg can evaporate and become airborne. • Because it is an element, mercury does not break down into less toxic substances. • Once mercury escapes to the environment, it circulates in and out of the atmosphere until it ends up in the bottoms of lakes and oceans. Where Does Mercury Come From? • Mercury is a naturally occurring element. • Mercury ore - cinnabar - is mined • History of SJ Mercury enters the environment from: • • • • • Natural sources such as volcanoes and the weathering of rocks; Our intentional uses of mercury; Our unintentional releases of mercury from burning fossil fuels and smelting metals. CFL E-waste Bioaccumulation = an increase in the concentration of a chemical in an organism over time, compared to the chemical's concentration in the environment. • Occurs naturally – And necessary for certain minerals and macromolecules • Problematic when bioaccumulate toxins Bioaccumulation of Hg • Hg enteres food chain via anaerobic bacteria (SRBs) • Why does Tuna have such high [Hg]? Basic Chemistry of Hg Hgo (g) Hg(II) (s) Air deposition volatilization Water reduction Hg(II) oxidation Hgo (aq) Natural concentrations: 5 to 100 pM (1 – 20 ng /L) ng/L = ppt; µg/L = ppb; mg/L = ppm dissolution Hgo (l) Morel et al., 2002 Oxidation-Reduction Reactions reduction Hgo (aq) Hg(II) - Done by bacteria Hgo oxidation (aq) Hg(II) - Limited in freshwater (since not many molecules to bond with) Hgo (g) Hg(II) (s) Air Water reduction Hg(II) oxidation Versions of Hg (II): Hg2+, HgCl2o, o, Hg(OH)2 Hg(SH)2 HgS(SH)-, CH3Hg(SH)o o, Hgo (aq) Natural concentrations: 5 to 100 pM (1 – 20 ng /L) Sulfide and Methyl Mercury SO42HgS(HS)MeHg SRB Hg(HS)2 Hg(Sn)HS- reduction Hg(II) oxidation H2S, HS- (these by-products perpetuate methylation, since they cycle back into the rxn) SRB = Sulfide reducing Bacteria Hgo (aq) Guadalupe River Watershed River system low [methylated] Hg since low [SRB] Bay has highest [methylated Hg] since high [SRB] San Francisco Bay, ‘Stinky Mud’ Salt H2O has 1000x more sulfate than fresh H2O Sulfide Complexes of Hg Hg(SH)2o Hg2+ + HS- HgS(SH)- Hg(Sn)SH- Methyl Mercury (MeHg) Hg(HS)2 HgS(HS)- SRB MeHg More toxic Less toxic MeHg = CH3HgSCH3HgCl CH3HgOH Interaction with Solids Hgo (g) Hg(II) (s) Air deposition volatilization Water reduction Hg(II) oxidation Hgo (aq) Hgo (l) Dissolution/precipitation HgS (mined mercury) Sediment (solid) Interaction with Solids Cylcing of Mercury