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Basic Chemical Principals of Mercury
Solid-Water Interface
Mineralogical transformation
biomineralization
dissolution
precipitation
Mn+
Oxidation
Reduction
Mn+x
release
Bacteria
deposition
Organic
Matter
Mineral
adsorption
Organic ligand
desorption
Soil Profile
complexation
Aqueous Metal Ion
degradation
Metal-Organic Complex
Surface complex
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)
Hgo (g)
Hg(II) (s)
Air
deposition
volatilization
Water
reduction
Hg(II)
oxidation
Hgo (aq)
Hgo (l)  Hgo (aq) K = 10-7.3
dissolution
(mol/L)
Hgo (g)  Hg (aq) K = 2.56 x 10-3
atm)
(mol / L ·
Hgo (l)
Morel et al., 2002
Hgo (g)
Hg(II) (s)
Air
Water
reduction
Hg(II)
oxidation
Hgo (aq)
dissolution
Hgo (l)
Oxidation-Reduction Reactions
reduction
Hgo (aq)
Hg(II)
- Microbially mediated (dominant)
- Photoreduction
Hgo
oxidation
(aq)
Hg(II)
- Limited in freshwater
- Particle surfaces catalyze O2 oxidation
of Hg-halides (HgCl2, for example)
Hgo (g)
Hg(II) (s)
Air
Water
reduction
Hg(II)
oxidation
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)
Ion Coordination: Hg(II)
H2 O
H2 O
Fe
H2 O
Fe 2 +
H2 O
H2 O
H2 O
Hg2+ · nH2O
H2 O
H2 O
H2 O
H2 O
Solutions
Speciation: Differences in molecular
configuration
Hg2+, HgCl2o, Hg(OH)2o,
Hg(SH)2o, HgS(SH)-,
CH3Hg(SH)o
Ion Complexes
Cation + Anion (termed ligand) => Complex
Hg2+ + Cl-  HgCl-
Keq = 107.2
(association reaction)
Hg(SH)2o  Hg2+ + 2 HS(dissociation reaction)
Kdiss = (Hg2+) (HS-)2 / (Hg(SH)2o)
DG = -RT ln K
log K = -36.6
Oxic (Aerated) Waters
Morel et al. (2002)
Sulfide and Methyl Mercury
SO42HgS(HS)MeHg
SRB
Hg(HS)2
Hg(Sn)HS-
H2S, HS-
reduction
Hg(II)
oxidation
Hgo (aq)
Guadalupe River Watershed
San Francisco Bay, ‘Stinky Mud’
Electron Donor
(food)
Electron Acceptor
(breathing)
CH2O  CO2
DGdonor - DGacceptor
SO42-  H2S
NH4+  NO3Fe3+  Fe2+
Fe2+  Fe3+
NO3-  NH4+
H2S  SO42O2  H2O
Energy
Energy
Yield
Energy
Sulfide Complexes of Hg
Hg(SH)2o
Hg2+ + HS-
HgS(SH)-
Hg(Sn)SH-
Methyl Mercury (MeHg)
Hg(HS)2
HgS(HS)-
SRB
MeHg: CH3HgSCH3HgCl
CH3HgOH
MeHg
Methylated Species of Hg
Interaction with Solids
Hgo (g)
Hg(II) (s)
Air
deposition
volatilization
Water
reduction
Hg(II)
oxidation
Hgo (aq)
Hgo (l)
Dissolution/precipitation
HgS
Sediment
Mineral Solubility
HgS = Hg2+ + S2-
log Ksp = -53
What about other Hg(II) species?
Role of Sulfide
with
So
Interaction with Solids
Ion Retention
adsorption
desorption
Aqueous Metal Ion
Adsorption: Chemical versus Electrostatic
(strong)
(weak)
-
-
Hg2+
Cylcing of Mercury
Mineral Solubility
HgS = Hg2+ + S2Ksp = (Hg2+) (S2-)
Concentration vs Activity
log Ksp = -53
Calculating Activity Coefficients
Davies Equation:
log  = – 0.511 Z2 {
I1/2 – 0.3 I}
1 + I 1/2
for I < 0.5
Debye-Hueckel Limiting Equation:
log  = - 0.511 Z2 I1/2
for I < 0.01
Extended Debye-Hueckel Equation:

0.511Z 2 I1/2
log  = – {
1/2 }
1 + Ba I
B is a temperature dependent constant (0.33 @ 25 °C)
a is an effective ion size parameter
for I < 0.1