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