Biogeochemical controls on Hg transformations at a contaminated site: The role of dissolved organic matter and redox gradients

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1 Biogeochemical controls on Hg transformations at a contaminated site: The role of dissolved organic matter and redox gradients Carrie Miller Oak Ridge National Laboratory Oak Ridge National Laboratory SBR Science Focus Area (SFA)

2 What controls methylmercury concentrations in contaminated systems?" Microbially driven production of methymercury (MeHg) results in human health risks Brooks and Southworth, 2011 Factors controlling MeHg concentrations in contaminated creek systems not understood

3 Mercury contamination in East Fork Poplar Creek (EFPC)" 350,000 kg Hg lost to the environment between at the Y-12 facility in Oak Ridge Elevated concentration of Hg in East Fork Poplar Creek (EFPC) Outfall 200 OF200

4 Biochemical and molecular mechanisms controlling contaminant transformation in the environment" Field & geochemistry Rates and mechanisms Microbiology and genetics Molecular structure Hg e - = Hg(0) catalytic domain Haitzer et al. (2003) (Luther et al. 1999) Transformation in field Speciation & mechanisms Molecular dynamics Field studies: Redox gradient important in understanding porewater MeHg DOM aromaticity related to MeHg concentrations; floodplains a potential source of MeHg Laboratory Studies: Equilibrium complexation does not exist in upper EFPC Complexation of Hg with DOM important in the reduction of Hg (II)

5 Biogeochemical redox controls on Hg methylation" Accumulation and production of MeHg observed in zones of Fe reduction and sulfate reduction Microbially driven redox processes Sulfate and iron reducing bacterial can methylate Hg Lack of understanding of zones of MeHg production in creek systems Desulfobulbus propionicus identified in EFPC, pure culture methylation studies underway (Vishnivetskaya et al. 2011) Hg methylation

6 Hg-DOM complexation" Complex mixture: LMW ligands to macromolecular structures Hg-DOM complexes dominate in freshwater oxic systems Important in Hg cycling Organic ligands important for methylation D. desulfuricans ND132 (Gilmour et al., in press) Cysteine 2MPA GSH Geobacter sulfurreducens PCA (Schaefer et al., in press)

7 Understanding controls on MeHg in East Fork Poplar Creek (EFPC)" Quarterly sampling initiated August 2010 Intense sampling (surface water, porewater and two sites Longitudinal study of surface 8 locations in EFPC

8 Center Redox gradients present in the creek margin porewater! Margin

9 MeHg porewater concentration higher in margin sediments! Center channel: Well mixed with surface water Summer concentrations higher than winter Margin: MeHg higher relative to center channel MeHg increases with depth

10 MeHg concentration increases with DOM aromaticity! DOM characterization Dissolved organic carbon (DOC): measure of DOM quantity Specific UV absorbance (SUVA 254 ): increase SUVA related to increase in DOM aromaticity Total Hg not related to DOC concentration or aromaticity MeHg not related to DOC concentration MeHg increases with increasing DOM aromaticity Highest porewater MeHg in samples containing sulfide Spearman s Rho = P<0.0001

11 Methylation potentials" Intact core incubations Sediment microcosms 201 Hg Me 202 Hg Production Me 201 Hg Methylation potential Loss Me 202 Hg Demethylation potential

12 DOM aromaticity increases downstream" Floodplain Downstream SUVA downstream Terrestrial derived, more aromatic DOM

13 MeHg increases downstream and is correlated with DOM aromaticity" Floodplain Spearman's Rho = p< Downstream MeHg downstream Highest surface water MeHg in floodplain Total Hg not related to DOC concentration or aromaticity MeHg increases with increasing DOM aromaticity

14 Laboratory studies: How is Hg interacting with DOM" Hg-DOM complexes dominate Hg speciation under oxic conditions Is equilibrium established in EFPC?

15 Equilibrium complexation not established in UEFPC" Headwaters: OF200: Hg source (1000 ng/l) Melton Hill Lake Dissolved and reducible Hg (HgR) measured in upper 2.5 km of EFPC MHL OF200 Stannous chloride reducible Hg (HgR) Hg 2+ -ligand + Sn 2+ Hg 0 + Sn 4+ Equilibrium Inorganic and LMW organic ligands: ~100 %HgR EFPC: ~20% HgR Miller et al. 2009"

16 Formation Hg-DOM complex kinetically hindered Melton Hill Lake water spiked with Hg(II) to simulate conditions in EFPC Decrease in HgR also observed when solution of SRNOM spiked with Hg(II) HydrophobicHg HgR Complexation of Hg with DOM in EFPC kinetically hindered Does the strength of the Hg-DOM complex change? Miller et al. 2009"

17 Hg-DOM complex changing over time" Competitive ligand titration w/ glutathione Hg GSH Hg(GSH) 2 Log K = Hg-DOM (Hg-NR) + 2GSH Hg(GSH) 2 (HgR) Hg-DOM (Hg-NR) SRNOM Solutions prepared with Hg equilibrated with SRNOM After 1 hr solution titrated with glutathione ~90% Hg removed from Hg-DOM complex HB EFPC Longer equilibration time; more Hg unreactive toward glutathione Similar results with DOM isolated from EFPC

18 Unreactive complexes present in EFPC" Hg forming unreactive complexes Very strong complexes Hydrophobic complexes not accessible by glutathione EFPC spike Unspiked creek water Collected 2.5 km donwstream of OF200 >50% Hg not reactive to glutathione

19 Importance of Hg-DOM complexation on Hg reactivity" Oxidation-Reduction reaction: Hg (II) Hg(0) Laboratory experiments: Hg(II) spiked into solutions of reduced and oxidized DOM; Hg(0) measured Hg(II) readily reduced by DOM, especially at low DOM concentrations under anoxic conditions Complexation occurs simultaneously, leading to decreased Hg(0) production at higher DOM concentrations Hg(0) interacts with DOM, particularly the reduced DOM - inhibiting DGM Gu B., Y, Bian, C. L. Miller, W. Dong, X. Jiang, and L. Liang Proc. Natl. Acad. Sci. USA. 108, BER Overview Department of Energy Office of Science Biological and Environmental Research

20 Complex nature of DOM makes understanding Hg- DOM interactions a challenge" [Hg(H 2 O) 6 ] 2+, hydration of Hg(II) thiosalicyclic acid in complex with Hg 2+ Hg: silver; O: red; H: white; S: yellow thioglycolate in complex with Hg 2+ Hg(II)-Ligand quantum chemical complexation calculations

21 Summary" Hg complexation in upper EFPC not at equilibrium Hg:DOM ratio and DOM redox state important in production and fate of Hg(0) Catalyzed and photochemical demethylation Speciation [Hg 0, Hg(II ), Hg(I ), MeHg ] Coupled redox reactions and transformation Complexation and stabilization Biological methylation and demethylation Abiotic methylation and demethylation MeHg Adsorption in surface and water catalyzed and reactions Photochemical reactions sediment porewater correlated with DOM aromaticity Water - sediment interface Biological and abiotic methylation and demethylation Adsorption and surface reactions Precipitation and co - precipitation Dissolution and transformation in Complexation creek margin: and stabilization MeHg highest Fate and transport in these regions Redox gradients established Redox /DOM MeHg Hg - DOM photo/redox Hg(0) (n- 2) - HgCl n HgCl(OH ) HgS(HS ) - Reduction MeHg cell Hg(II ) Hg(0) Hg(OH) 2 Oxidation Biogeochemical redox gradients Dissolved Particulate HgS (s ) Hg - particle Hg(0) Hg - NOM HgS(HS ) - Reduction MeHg cell Hg(HS) 2 Hg(II ) Hg(0) Oxidation Biological demethylation HgS (s ) Hg - clay/oxide Water Sediment Modified from Morel et al. 1998

22 Acknowledgements" Aqueous, environmental chemistry and geochemistry S Brooks (ORNL) Chemical speciation B Gu (ORNL) biogeochemistry and humics F He (ORNL) Contaminant remediation D Kocman (ORISE) Mercury behavior in environment L Liang (ORNL) Environmental surface chemistry C Miller (ORNL) Mercury behavior in environment Y Qian (UTK) Mercury and methylmercury analyses H Zhang (TTU) Mercury chemistry and photochemistry W Zheng (ORISE) Isotope photochemistry R Landis/J.Dyer (Dupont) Mercury contaminated sites Computational simulation J Smith (UTK-ORNL) Biophysicist, molecular dynamics and enzyme simulation JM Parks (ORNL) Computational Chemistry H-B Guo (ORISE) Computational Chemistry H Guo /Qin Xu (UTK) Computational Chemistry D Riccardi (UTK) Computational Chemistry Microbiology S Brown (ORNL) Genomics, gene expression D Elias (ORNL) Mutagenesis, physiology C Gilmour (Smithsonian) Mercury methylation R Hurt (ORISE) Molecular microbiology J Moberly (ORISE) Microbiology A Palumbo (ORNL) Community analysis J Schaefer (Princeton) Microbial methylation by IRB J Wall (UMC) Mutagenesis, anaerobic culture Statistics C Brandt (ORNL) Data analysis Biochemistry and biophysics A Johs (ORNL) Molecular mechanisms L Shi (PNNL) Protein biochemistry S Miller (UCSF) Hg molecular biology A Summers (UGA) Hg biochemistry, bacteria resistance S Tomanicek (ORISE) Biochemistry, protein crystallography X-ray spectroscopy Ken Kemner and B Mishra (APS) EXAFS