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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 Hg(II) (s) Hgo (g) Air deposition volatilization Water reduction Hg(II) Hgo (aq) Hgo (l) oxidation dissolution Natural concentrations: 5 to 100 pM (1 – 20 ng /L) ng/L = ppt; µg/L = ppb; mg/L = ppm
Oxidation-Reduction Reactions reduction Hg(II) Hgo (aq) • Done by bacteria oxidation Hgo (aq) Hg(II) • Limited in freshwater (since not many molecules to bond with)
Hg(II) (s) Hgo (g) Air Water reduction Hg(II) Hgo (aq) oxidation Versions of Hg (II): Hg2+, HgCl2o, Hg(OH)2o, Hg(SH)2o, HgS(SH)-, CH3Hg(SH)o Natural concentrations: 5 to 100 pM (1 – 20 ng /L)
Sulfide and Methyl Mercury SO42- HgS(HS)- Hg(HS)2 Hg(Sn)HS- reduction MeHg SRB Hgo (aq) Hg(II) oxidation H2S, HS- (these by-products perpetuate methylation, since they cycle back into the rxn) SRB = Sulfide reducing Bacteria
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 HgS(SH)- Hg(Sn)SH- Hg2+ + HS-
Methyl Mercury (MeHg) SRB Hg(HS)2 HgS(HS)- MeHg More toxic Less toxic MeHg = CH3HgS- CH3HgCl CH3HgOH
Interaction with Solids Hg(II) (s) Hgo (g) Air deposition volatilization Water reduction Hg(II) Hgo (aq) Hgo (l) oxidation Dissolution/precipitation HgS (mined mercury) Sediment (solid)