Basic Chemical Principals of Mercury

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Presentation transcript:

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

Air Water Hg(II) (s) Hgo (g) Hg(II) Hgo (aq) Hgo (l) 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

Air Water Hg(II) (s) Hgo (g) Hg(II) Hgo (aq) Hgo (l) deposition Air volatilization Water reduction Hg(II) Hgo (aq) Hgo (l) oxidation 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

Air Water Hg(II) (s) Hgo (g) Hg(II) Hgo (aq) Hgo (l) reduction oxidation dissolution

Oxidation-Reduction Reactions Hg(II) Hgo (aq) Microbially mediated (dominant) Photoreduction oxidation Hgo (aq) Hg(II) Limited in freshwater Particle surfaces catalyze O2 oxidation of Hg-halides (HgCl2, for example)

Air Water Hg(II) (s) Hgo (g) Hg(II) Hgo (aq) reduction oxidation Hg2+, HgCl2o, Hg(OH)2o, Hg(SH)2o, HgS(SH)-, CH3Hg(SH)o Natural concentrations: 5 to 100 pM (1 – 20 ng /L)

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

Ion Complexes 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) DG = -RT ln K

Oxic (Aerated) Waters Morel et al. (2002)

Sulfide and Methyl Mercury SO42- HgS(HS)- Hg(HS)2 Hg(Sn)HS- reduction MeHg SRB Hg(II) Hgo (aq) oxidation H2S, HS-

Guadalupe River Watershed

San Francisco Bay, ‘Stinky Mud’

Energy Electron Donor Electron Acceptor Yield (food) (breathing) CH2O  CO2 DGdonor - DGacceptor SO42-  H2S NH4+  NO3- Fe3+  Fe2+ Fe2+  Fe3+ NO3-  NH4+ H2S  SO42- O2  H2O Energy Energy

Sulfide Complexes of Hg Hg(SH)2o HgS(SH)- Hg(Sn)SH- Hg2+ + HS-

Methyl Mercury (MeHg) Hg(HS)2 SRB HgS(HS)- MeHg MeHg: CH3HgS- CH3HgCl CH3HgOH

Methylated Species of Hg

Air Water Interaction with Solids Sediment Hg(II) (s) Hgo (g) Hg(II) deposition volatilization Water reduction Hg(II) Hgo (aq) Hgo (l) oxidation 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 Concentration vs Activity HgS = Hg2+ + S2- log Ksp = -53 Ksp = (Hg2+) (S2-) Concentration vs Activity

Calculating Activity Coefficients Debye-Hueckel Limiting Equation: log g = - 0.511 Z2 I1/2 for I < 0.01 Extended Debye-Hueckel Equation: for I < 0.1 Davies Equation: for I < 0.5 B is a temperature dependent constant (0.33 @ 25 °C) a is an effective ion size parameter