Transcript No Slide Title

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