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

Morel et al., 2002

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)

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)

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

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 Natural concentrations: 5 to 100 pM (1 – 20 ng /L)

Ion Coordination: Hg(II)

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)

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

Solubility Products HgS = Hg2+ + S2- log Ksp = -53

Guadalupe River Watershed

San Francisco Bay, ‘Stinky Mud’

Energy Electron Donor Electron Acceptor Yield (food) (breathing) CH2O  CO2 DGdonor - DGdonor 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

Solubility Products 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