Competition among thiols and inorganic sulfides and polysulfides for Hg and MeHg in wetland soils and sediments under suboxic conditions: Illumination of controversies and implications for MeHg net production

[1] Current research focus in mercury biogeochemistry is on the net production and accumulation of methyl mercury (MeHg) in organisms. The activity of iron- and sulfate-reducing bacteria (FeRB and SRB) has been identified as important for MeHg production. There are indications of a passive uptake of neutral Hg-sulfides by SRB, as well as of a facilitated bacterial uptake of Hg complexed by small organic molecules. In order to understand these processes, the chemical speciation of Hg and MeHg, and most important, the competition among organic thiols and inorganic sulfides and polysulfides, needs to be clarified under suboxic conditions (nM to low μM range of total sulfide concentrations) in wetland soils and sediments. In this paper the chemical speciation of Hg and MeHg is modeled at pH 4.0 and 7.0 in a conceptual wetland soil/sediment with typical concentrations of thiols, sulfides, Hg, and MeHg. Effects of precipitated HgS(s), the formation of Hg-polysulfides, and the size of the controversial stability constant for the formation of HOHgSH 0 (aq) are emphasized. The outcome of the modeling is discussed in light of chosen stability constants for Hg complexes with thiols, sulfides, and polysulfides. It is concluded that organic thiols are competitive with inorganic sulfides in the approximate total sulfide concentration range 0-1 μm. It is also concluded that increases in absolute aqueous concentrations of MeHg, or the molar ratio of dissolved MeHg/Hg, are not appropriate as indirect measures of MeHg net production, unless changes and differences in solubility of MeHg and Hg are corrected for.

[1]  R. Pattrick,et al.  Structure of mercury(II)–sulfur complexes by EXAFS spectroscopic measurements , 2003 .

[2]  G. Gill,et al.  Complexation of mercury by dissolved organic matter in surface waters of Galveston Bay, Texas , 2006 .

[3]  J. Casas,et al.  Mercury(II) complexes with sulfhydryl containing chelating agents: Stability constant inconsistencies and their resolution , 1980 .

[4]  G. Schwarzenbach,et al.  Die Komplexchemie des Methylquecksilber‐Kations , 1965 .

[5]  B. Branfireun,et al.  Assessing sulfate and carbon controls on net methylmercury production in peatlands: An in situ mesocosm approach , 2008 .

[6]  C. Lamborg,et al.  Determination of the mercury complexation characteristics of dissolved organic matter in natural waters with "reducible Hg" titrations. , 2003, Environmental science & technology.

[7]  A. E. Torma,et al.  THE STABILITY OF THIOCOMPLEXES AND SOLUBILITY PRODUCTS OF METAL SULPHIDES: I. CADMIUM SULPHIDE , 1964 .

[8]  P. Bloom,et al.  Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups. , 2006, Environmental science & technology.

[9]  G. Aiken,et al.  Dissolution of cinnabar (HgS) in the presence of natural organic matter , 2005 .

[10]  D. Nelson,et al.  Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium , 2006, Applied and Environmental Microbiology.

[11]  Robert P. Mason,et al.  Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades , 2001 .

[12]  C. Lamborg,et al.  Marine biogeochemical cycling of mercury. , 2007, Chemical reviews.

[13]  M. Nilsson,et al.  Mercury cycling in boreal ecosystems: The long‐term effect of acid rain constituents on peatland pore water methylmercury concentrations , 2001 .

[14]  G. Gill,et al.  Determination of mercury complexation in coastal and estuarine waters using competitive ligand exchange method. , 2005, Environmental science & technology.

[15]  Jinzhong Zhang,et al.  Thiols in wetland interstitial waters and their role in mercury and methylmercury speciation , 2004 .

[16]  G. Benoit,et al.  Distribution of the thiols glutathione and 3‐mercaptopropionic acid in Connecticut lakes , 2006 .

[17]  C. Hammerschmidt,et al.  Geochemical controls on the production and distribution of methylmercury in near-shore marine sediments. , 2004, Environmental science & technology.

[18]  R. Mason,et al.  Mercury speciation in the San Francisco Bay estuary , 2003 .

[19]  C. Gilmour,et al.  Influence of dissolved organic matter on the complexation of mercury under sulfidic conditions , 2007, Environmental toxicology and chemistry.

[20]  F. Morel,et al.  Mercury speciation in the presence of polysulfides , 2000 .

[21]  E. Tipping.  Modelling the interactions of Hg(II) and methylmercury with humic substances using WHAM/Model VI , 2007 .

[22]  G. Gill,et al.  Chemical and physical speciation of mercury in Offatts Bayou: A seasonally anoxic bayou in Galveston Bay , 2007 .

[23]  P. Bloom,et al.  Binding constants of divalent mercury (Hg2+) in soil humic acids and soil organic matter. , 2006, Environmental science & technology.

[24]  G. Gill,et al.  Distribution of particulate, colloidal, and dissolved mercury in San Francisco Bay estuary. 1. Total mercury , 2003 .

[25]  G. Aiken,et al.  Binding of mercury(II) to dissolved organic matter: the role of the mercury-to-DOM concentration ratio. , 2002, Environmental science & technology.

[26]  M. Gillman,et al.  Effects of dissolved organic carbon and salinity on bioavailability of mercury , 1997, Applied and environmental microbiology.

[27]  J. Holmes,et al.  Mercury transformations and fluxes in sediments of a riverine wetland , 2007 .

[28]  G. A. Gill,et al.  Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades , 1998 .

[29]  Reed C. Harris,et al.  Increases in Fluxes of Greenhouse Gases and Methyl Mercury following Flooding of an Experimental Reservoir , 1997 .

[30]  D. Engstrom,et al.  Sulfate addition increases methylmercury production in an experimental wetland. , 2006, Environmental science & technology.

[31]  J. Rudd,et al.  Evidence for facilitated uptake of Hg(II) by Vibrio anguillarum and Escherichia coli under anaerobic and aerobic conditions , 2002 .

[32]  F. Black,et al.  Competing ligand exchange-solid phase extraction method for the determination of the complexation of dissolved inorganic mercury (II) in natural waters. , 2007, Analytica chimica acta.

[33]  A. O. Gubeli,et al.  Constantes de stabilité de thiocomplexes et produits de solubilité de sulfures de métaux. II Sulfure de zinc , 1967 .

[34]  Nigel T. Roulet,et al.  In situ sulphate stimulation of mercury methylation in a boreal peatland: Toward a link between acid rain and methylmercury contamination in remote environments , 1999 .

[35]  H. Kőszegi-Szalai,et al.  Equilibrium studies of mercury(II) complexes with penicillamine. , 1999, Talanta.

[36]  G. Helz,et al.  Inorganic Speciation of Mercury in Sulfidic Waters: The Importance of Zero-Valent Sulfur , 1997 .

[37]  R. Bartha,et al.  Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment , 1985, Applied and environmental microbiology.

[38]  Ralph Mitchell,et al.  Sulfate stimulation of mercury methylation in freshwater sediments , 1992 .

[39]  P. Bloom,et al.  Bonding of methyl mercury to reduced sulfur groups in soil and stream organic matter as determined by x-ray absorption spectroscopy and binding affinity studies , 2002 .

[40]  R. D. Evans,et al.  Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability , 2000 .

[41]  George R. Aiken,et al.  A critical review of three methods used for the measurement of mercury (Hg2+)-dissolved organic matter stability constants , 2007 .

[42]  E. Björn,et al.  Do potential methylation rates reflect accumulated methyl mercury in contaminated sediments? , 2008, Environmental science & technology.

[43]  Michael M. Reddy,et al.  Inhibition of Precipitation and Aggregation of Metacinnabar (Mercuric Sulfide) by Dissolved Organic Matter Isolated from the Florida Everglades , 1999 .

[44]  D. Sedlak,et al.  Strong hg(II) complexation in municipal wastewater effluent and surface waters. , 2003, Environmental Science and Technology.

[45]  K. Xia,et al.  Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment , 2003 .

[46]  C. Gilmour,et al.  Estimation of mercury‐sulfide speciation in sediment pore waters using octanol—water partitioning and implications for availability to methylating bacteria , 1999, Environmental toxicology and chemistry.

[47]  E. Björn,et al.  Importance of dissolved neutral mercury sulfides for methyl mercury production in contaminated sediments. , 2007, Environmental science & technology.

[48]  U. Skyllberg,et al.  Bonding of ppb levels of methyl mercury to reduced sulfur groups in soil organic matter. , 2003, Environmental science & technology.

[49]  Bonnie O. Leung,et al.  Mercury(II) penicillamine complex formation in alkaline aqueous solution. , 2007, Dalton transactions.

[50]  Robert M. Smith,et al.  NIST Critically Selected Stability Constants of Metal Complexes Database , 2004 .

[51]  J. Tossell.  Calculation of the Structures, Stabilities, and Properties of Mercury Sulfide Species in Aqueous Solution , 2001 .

[52]  D. Dyrssen,et al.  Titration of sulphides and thiols in natural waters , 1986 .

[53]  M. Frischer,et al.  A quantitative relationship that demonstrates mercury methylation rates in marine sediments are based on the community composition and activity of sulfate-reducing bacteria. , 2001, Environmental science & technology.

[54]  C. Gilmour,et al.  Mercury Methylation by Dissimilatory Iron-Reducing Bacteria , 2006, Applied and Environmental Microbiology.

[55]  J. Starý,et al.  Radiometric determination of stability constants of mercury species complexes with L-cysteine , 1988 .

[56]  C. Gilmour,et al.  The influence of sulfide on solid-phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3). , 2001, Environmental science & technology.

[57]  P. Bloom,et al.  Binding of mercury(II) to reduced sulfur in soil organic matter along upland-peat soil transects , 2000 .

[58]  B. Branfireun,et al.  Spatial characteristics of net methylmercury production hot spots in peatlands. , 2008, Environmental science & technology.

[59]  J. Qian,et al.  Combined XANES and EXAFS study on the bonding of methyl mercury to thiol groups in soil and aquatic organic matter , 2005 .

[60]  L. Newman,et al.  Methylmercury formation in a wetland mesocosm amended with sulfate. , 2004, Environmental science & technology.

[61]  D. Dyrssen,et al.  The sulphur-mercury(II) system in natural waters , 1991 .

[62]  K. Daskalakis,et al.  The solubility of sphalerite (ZnS) in sulfidic solutions at 25°C and 1 atm pressure , 1993 .

[63]  J. Casas,et al.  Structural requirements for Hg(II) antidotes , 1981 .

[64]  G. Schwarzenbach,et al.  Die Löslichkeit von Metallsulfiden I. Schwarzes Quecksilbersulfid , 1963 .

[65]  G. Aiken,et al.  Mercury(II) sorption to two Florida Everglades peats: evidence for strong and weak binding and competition by dissolved organic matter released from the peat. , 2000, Environmental science & technology.