What is Mercury? Naturally occurring element (atomic number 80) - PDF document

The Global Biogeochemical Mercury Cycle • What is mercury? • Mercury species in nature – Their properties and place in the cycle • How is mercury converted among its various forms? • The mercury cycle: – Major steps: in general – Budget (fluxes and pools) – Major steps: details What is Mercury? • Naturally occurring element (atomic number 80) • Heavy metal, can be toxic to organisms • Only metal that is liquid at room temperature • Highest volatility of any metal • Exists in several forms – Form strongly influences • Biochemistry and bioavailability of mercury • Movement of mercury through environment Mercury Species in Nature • 3 possible valence states (electrical charge) – No charge: Hg 0 (elemental mercury, pure form) – Oxidized: Hg 1+ (mercurous, monovalent) Hg 2+ (mercuric, divalent) - more stable cation - associated with compounds: 1) - inorganic, mercury salts - examples: mercuric sulfide (HgS, mineral cinnabar), mercuric chloride (HgCl 2 ), mercuric oxide (HgO) 2) - organic (carbon-based) -example: dimethylmercury (Me 2 Hg) - more toxic than inorganic forms of Hg, bioaccumulates •1

Mercury Species: Chemical properties, role in mercury cycle Form Properties Role in mercury cycle Hg 0 -not very water soluble Atmospheric transport -Inert, not reactive -volatilizes easily Hg 2+ -water soluble Wet deposition -reactive Hg p -attaches to small Deposition particles (Hg 0 attached to/adsorbed onto particles) MeHg -very water soluble Bioaccumulation -volatile -bioaccumulates, toxic Mercury Species in the Biogeochemical Mercury Cycle Note importance of: Hg 0 in atmospheric • transport – Total Hg atm ~ 95% Hg 0 • Hg 2+ , Hg p in deposition • HgS (insoluble) as sink • MeHg in bioaccumulation • Inorganic and organic complexes – Represent most of the Hg in water, soil, sediments, plants and animals http://www.ec.gc.ca/MERCURY/ EH/EN/eh-b.cfm?SELECT= EH How is Mercury Converted Among its Various Forms? • 2 main types of reactions convert mercury through its various forms: – Oxidation-reduction oxidation Hg 0 Hg 2+ reduction inert more reactive – Methylation-demethylation methylation Hg 2+ + CH 3 CH 3 Hg + demethylation •2

Mercury Oxidation • Oxidation of Hg 0 in atmosphere – Important mechanism involved in deposition of mercury on land and water Hg 0 Hg 2+ Reactivity Inert More reactive Volatility High Low Solubility in water Low High Atmospheric residence time ~ 1 -1 1.5 years < 2 weeks Relevance to cycle - Emitted to atmosphere - Rapidly taken up in rain water, snow or adsorbed - Long-distance transport onto small particles (> thousands of miles before oxidized and - Subsequent deposition redeposited in environment in environment via “wet” for further cycling or “dry” deposition” Mercury Oxidation in the Arctic • Arctic phenomenon called “Mercury depletion” or “Mercury sunrise” – Occurs at end of dark polar winters when sun rises in spring – Rapid photochemical conversion of Bromine, chlorine ions (reactive chemicals released from sea salt) Hg 0 Hg 2+ Atmospheric deposited on snow/ice mercury levels (“depleted”) pulse of reactive mercury at start of growing season http://www.ec.gc.ca/science/sandemar99/article3_e.html Mercury Methylation • Methylation of Hg 2+ Hg 2+ CH 3 Hg + + CH 3 mercuric species methyl group methylmercury highly toxic bioaccumulative (builds up in living tissues, food chain) – A natural, biological process • Occurs in a variety of bacteria, especially methanogens – use enzymes containing vitamin B-12 •3

Mercury Methylation – Factors influencing formation of methylmercury: • Methylating microbes – Methanogenic (methane producing) and sulfate- dependent bacteria • Anoxic/Anaerobic (oxygen poor) water and sediments – Wetlands and river sediments • Low pH (acidic) environments with high concentrations of organic matter • Temperature The Mercury Cycle The Mercury Cycle: 6 Major steps 1. Release from sources • Emission from natural and anthropogenic sources 2. Transportation and circulation • Movement in gaseous form through atmosphere 3. Deposition • On land and surface waters 4. Conversion into insoluble mercury sulfide (HgS) • Ocean sediments represent final sink 5. Bioconversion into more volatile or soluble forms such as methylmercury 6. Reentry into the atmosphere or Bioaccumulation in foodchains •4

Global Mercury Budget Note: 1) Sources 2) Deposition • Local vs. regional/global 3) Global deposition • Terrestrial vs. marine 4) Atmospheric fluxes 5) Atmospheric reservoir 6) Riverine transport http://www.ec.gc.ca/MERCURY/ EH/EN/eh-mb.cfm?SELECT= EH Global Mercury Budget: some salient points • Anthropogenic emissions ≥ (greater than or exceed; estimates vary) natural releases • ~ ½ of mercury released falls out locally – Other ½ travels, changes in chemical and physical form • Most local deposition � dry particles • Global deposition � rain, snow • Long-range fallout affects terrestrial and marine ecosystems • ~ 1/2 of global deposition lands on terrestrial ecosystems; rest to marine Global Mercury Budget: some salient points • Fluxes between – Atmosphere and land, atmosphere and ocean are much greater than transport from land � ocean via riverine discharge • Mercury storage in atmospheric reservoir has increased by factor of 3 Transport rate of mercury from land � oceans has • increased by factor of 4 • Average residence time of mercury in – Atmosphere 11 days – Soil 1000 years – Oceans 3200 years 2.5 x 10 8 years – Sediments •5

Global Mercury Cycle: “Pre-man” Global Mercury Cycle: Present day The Mercury Cycle: 6 Major steps 1. Release from sources Emission from natural and anthropogenic sources • 2. Transportation and circulation • Movement in gaseous form through atmosphere 3. Deposition • On land and surface waters 4. Conversion into insoluble mercury sulfide (HgS) • Ocean sediments represent final sink 5. Bioconversion into more volatile or soluble forms such as methylmercury 6. Reentry into the atmosphere or Bioaccumulation in foodchains •6

Sources • “Geologic” mercury – Mercury that exists in a stable state in Earth’s crust • Active mercury cycle begins when mercury is released from this stable form through – Natural processes – Human intervention Sources: Emissions estimates • Total global mercury emissions ~ 5000t/yr – Recent scientific studies estimate ~ 50-80% mercury emitted is result of human activities; rest is natural • Estimates very uncertain – Why difficult to distinguish between natural and anthropogenic emissions? • Lack accurate data on emissions past and present • Leap-frogging – Mercury deposition and re-emission Types of Sources • Natural • Anthropogenic – Direct – Re-emitted • We emitted years ago • Land, surface waters re-emitted mercury into atmosphere http://www.epa.gov/mercury/control_emissions/global.htm •7

Natural sources of Mercury • Mercury – Occurs naturally throughout our solar system – On Earth • Geological deposits – cinnabar (HgS mineral) up to 86% Hg – Rocks » Granite 0.2 ppm Hg » Other crustal rocks 0.1 ppm Hg http://www.ec.gc.ca/MERCURY/ SM/EN/sm-ns.cfm?SELECT= SM Natural Sources of Mercury • Natural processes can release mercury from crust to water, soil, atmosphere – Examples: • Volcanic eruptions • Weathering of rocks • Under sea vents • Hot springs • Mercury also concentrates in – Plants – Sediments rich in organic matter – Fossil fuels (examples: coal, oil) http://www.pollutionprobe.org/Reports/mercuryprimer.pdf Natural sources of Mercury • As mercury is incorporated in biosphere, releases attributed to – Vegetation – Forest fires – Water bodies – Sea salt spray – Soils •8

Natural sources of Mercury • Natural emission from continental sources 1000 t/yr • Evasion from oceans – Pre-industrial 600 t/yr – Today 2000 t/yr • Higher due to re- emission of mercury deposited from human activities http://www.ec.gc.ca/MERCURY/ EH/EN/eh-mb.cfm?SELECT= EH Anthropogenic Sources • Types of anthropogenic releases – Incidental release • Result of an activity that does not involve direct/deliberate use of mercury • Examples: burning coal, processing metals (Cu, Zn), generate electricity from energy sources (coal, other fossil fuels), cremation – Direct/deliberate release • Deliberate use of mercury in products and processes • Examples: intentional extraction, electrical switches Presence of Mercury from Deliberate Uses in Canada •9

Canadian Anthropogenic Mercury Emissions by Source (2001) • Coal fired power and heat production – Largest single source of mercury emissions http://pubs.acs.org/hotartcl/est/98/apr/4903hanB.ev.html U.S. Anthropogenic Emission Sources (1994-1995) Coal-fired utility boilers - Largest point source of mercury emissions Electric utilities, Municipal waste combustors, Commercial/industrial boilers, Medical waste incinerators ~ 80 % of total amount http://pubs.acs.org/hotartcl/est/98/apr/mer.html Worldwide distribution of emissions • Asia ~ ½ • US anthropogenic mercury emissions ~ 3% of global total • Emissions from US power sector ~ 1% of total global emissions http://www.epa.gov/mercury/control_emissions/global.htm •10