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