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

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

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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

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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: Hg0 (elemental mercury, pure form) – Oxidized: Hg1+ (mercurous, monovalent) Hg2+ (mercuric, divalent)

  • more stable cation
  • associated with compounds:

1) - inorganic, mercury salts

  • examples: mercuric sulfide (HgS, mineral

cinnabar), mercuric chloride (HgCl2), mercuric oxide (HgO) 2) - organic (carbon-based)

  • example: dimethylmercury (Me2Hg)
  • more toxic than inorganic forms
  • f Hg, bioaccumulates
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Mercury Species: Chemical properties, role in mercury cycle

Form Properties Role in mercury cycle

Hg0

  • not very water soluble
  • Inert, not reactive
  • volatilizes easily

Atmospheric transport Hg2+

  • water soluble
  • reactive

Wet deposition Hgp (Hg0 attached to/adsorbed onto particles)

  • attaches to small

particles Deposition MeHg

  • very water soluble
  • volatile
  • bioaccumulates, toxic

Bioaccumulation

Mercury Species in the Biogeochemical Mercury Cycle

Note importance of:

  • Hg0 in atmospheric

transport

– Total Hg atm ~ 95% Hg0

  • Hg2+, Hgp 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

  • xidation

Hg0 Hg2+

inert

reduction

more reactive

– Methylation-demethylation

methylation

Hg2+ + CH3 CH3Hg+

demethylation

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Mercury Oxidation

  • Oxidation of Hg0 in atmosphere

– Important mechanism involved in deposition of mercury on land and water Hg0 Hg2+

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
  • Long-distance transport

(> thousands of miles before oxidized and redeposited in environment for further cycling

  • Rapidly taken up in rain

water, snow or adsorbed

  • nto small particles
  • Subsequent deposition

in environment via “wet”

  • r “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)

Hg0 Hg2+

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 Hg2+

Hg2+ + CH3 CH3Hg+

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

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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

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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 – Sediments 2.5 x 108 years

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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

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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

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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

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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

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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
  • f 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

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Estimates of global atmospheric releases of mercury by continent from a number of major anthropogenic sources in 1995 (tons/year)

Stationary combustion

  • f fossil

fuels (especially coal) And Incinerator

  • f waste

materials Account for ~ 70% total

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

Long-Range Atmospheric Transport

Global mercury budget complicated due to volatility of Hg0

  • Allows Hg to travel

in multi-step sequence

– Emission to atmosphere – Transportation – Deposition – Re-emission

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Long-Range Atmospheric Transport

  • Recall:

~ 95% of 5000t Hg in atmosphere is Hg0 vapour

  • Mean atmospheric residence time: 1 – 1.5 years

– Conversion (photo-oxidation)

Hg0 Hg2+

Insoluble more reactive water soluble

Mechanism for deposition of Hg0 emission to land, water

Mercury Deposition

  • Gaseous Hg2+

– Travel up to hundreds of km

  • Particulate Hgp

– Hg adsorbed onto other particulate matter – Fall out close to source of emissions

  • Both

– undergo wet/dry deposition to earth’s surface locally – Relatively short residence times in atmosphere

  • Hg2+ : 5-14 days

Atmospheric Circulation

http://www.ec.gc.ca/MERCURY/ EH/EN/eh-t.cfm?SELECT= EH

Net result in Northern hemisphere: concentration

  • f mercury in

Arctic

  • Related to Arctic

mercury depletion phenomenon

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Regional Mercury Deposition in the U.S.

  • Regional mercury

deposition patterns suggested in the US

  • South

– Lower observed mercury deposition

  • North

– Higher values

  • bserved

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

Aquatic Mercury Cycle

wi.water.usgs.gov/pubs/FS-216-95/mercury.pg3.pdf

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Biologically Mediated Mercury Cycle in Aquatic Ecosystems

  • Processes regulating

mercury

– Scavenging – Microbial transformation

  • Methylation-demethylation
  • Reduction

– Bioaccumulation – Photochemical reduction – Settling particles (sedimentation), burial

http://loer.tamug.tamu.edu/Research/Mercury/mercury.htm

Role of Microorganisms in the Mercury Cycle

MeHg HgS Hg0 Hg 2+

Demethylation Methylation Mobilize sink Add to sink Reduction Red = Toxifying effects of microbes Blue =Mercury detoxification by microbes

Adapted from the National Academy of Sciences (1978) and http://www.biosci.ohio-state.edu/~mgonzalez/Micro521/24.html

Summary

  • Mercury naturally occurs in

the environment, but is also generated through human activities

  • Once mercury enters the

ecosystem, it is constantly cycled and recycled

  • A range of biogeochemical

interactions affect mercury in its different physical states and forms

– Speciation is important to assess: environmental fate and impacts of anthropogenic mercury emissions