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]?

Hg(II) Hgo (aq)

Hgo (g)Hg(II) (s)

volatilization

deposition

oxidation

reduction

Water

Air

Natural concentrations: 5 to 100 pM

(1 – 20 ng /L)

Hgo (l)

dissolution

ng/L = ppt; µg/L = ppb; mg/L = ppm

Basic Chemistry of Hg

Morel et al., 2002

Hg(II) Hgo (aq)

oxidation

reduction

- Done by bacteria

Hg(II)Hgo (aq)

- Limited in freshwater (since not many molecules to bond with)

Oxidation-Reduction Reactions

Hg(II)

Water

Air

Natural concentrations: 5 to 100 pM

(1 – 20 ng /L)

Hgo (aq)

oxidation

reduction

Hgo (g)Hg(II) (s)

Hg2+, HgCl2o,

Hg(OH)2o, Hg(SH)2

o,

HgS(SH)-,

CH3Hg(SH)o

Versions of Hg (II):

MeHgMeHg

HgS(HS)-

Hg(HS)2

Hg(Sn)HS-

Hgo (aq)

oxidation

reduction

Hg(II)

SO42-

H2S, HS-

SRB

Sulfide and Methyl Mercury

SRB = Sulfide reducing Bacteria

(these by-products perpetuate methylation, since they cycle back into the rxn)

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

Hg(SH)2o

HgS(SH)-

Hg(Sn)SH-

Hg2+ + HS-

Sulfide Complexes of Hg

Hg(HS)2

HgS(HS)-SRB MeHg

MeHg = CH3HgS-

CH3HgCl

CH3HgOH

Methyl Mercury (MeHg)

Less toxicMore toxic

Hg(II) Hgo (aq)

Hgo (g)Hg(II) (s)

volatilization

deposition

oxidation

reduction

Water

Air

Hgo (l)

Dissolution/precipitation

Sediment(solid)

Interaction with Solids

HgS

(mined mercury)

Interaction with Solids

Cylcing of Mercury