Mercury in the Environment

1
Mercury in the Environment
2
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.

3
Where Does Mercury Come From?

• Mercury is a naturally occurring element.
• Mercury ore - cinnabar - is mined
• History of SJ

4
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

5
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

6
Bioaccumulation of Hg

• Hg enteres food chain via anaerobic bacteria
  (SRBs)
• Why does Tuna have such high Hg?

7
(No Transcript)
8
Basic Chemistry of Hg
Hgo (g)
Hg(II) (s)
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
9
Morel et al., 2002
10
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)

11
Hgo (g)
Hg(II) (s)
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)
12
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
13
Guadalupe River Watershed
River system low methylated Hg since low SRB
Bay has highest methylated Hg since high SRB
14
San Francisco Bay, Stinky Mud
Salt H2O has 1000x more sulfate than fresh H2O
15
Sulfide Complexes of Hg
Hg(SH)2o HgS(SH)- Hg(Sn)SH-
Hg2 HS-
16
Methyl Mercury (MeHg)
SRB
Hg(HS)2 HgS(HS)-
MeHg
More toxic
Less toxic
MeHg CH3HgS- CH3HgCl CH3HgOH
17
Interaction with Solids
Hgo (g)
Hg(II) (s)
Air
deposition
volatilization
Water
reduction
Hg(II)
Hgo (aq)
Hgo (l)
oxidation
Dissolution/precipitation
HgS (mined mercury)
Sediment (solid)
18
Interaction with Solids
19
Cylcing of Mercury