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MERCURY POLLUTION

Extent of pollution?

Chemistry of Mercury

Production and Uses

Mercury emissions and deposition

Mercury cycling, stores and fluxes

Methylation of mercury

Methylmercury release by flooding

Methylmercury in food chain and bioaccumulation

Indicators of mercury sensitivity

Effects of mercury on wildlife

Human effects of mercury consumption/exposure

Acceptable levels of mercury exposure

Sources of these notes –

Driscoll et al., 2007 – BioScience (required reading)

Aquatic pollution (2000, third edition) by Edward Laws

And some other sources…..

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MERCURY POLLUTION – THE EXTENT OF THE PROBLEM?

Across the nation

Mercury had the highest number of advisories across the

nation (79% of all advisories)!

Get the latest advisories at the EPA web site

2004 Fish consumption advisories

Advisories based on the more than allowable concentration of

Mercury measured in fish (varies with states, for NY = 1ppm,

FDA action limit; Michigan ~ 0.5 ppm)

Although current advisories in the United States have been

issued for 36 different pollutants, most advisories involve five

primary bioaccumulative contaminants:

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Mercury—2,436 advisories active in 2004 (up from 2,362

advisories in 2003)

PCBs—873 advisories active in 2004 (down from 884

advisories in 2003)

Chlordane—79 advisories active in 2004 (down from 89

advisories in 2003)

Dioxins—106 advisories active in 2004 (up from 90

advisories in 2003)

DDT and metabolites—67 advisories active in 2004 (up

from 52 in 2003)

An increase in advisories issued by the states generally reflects

an increase in the number of assessments of contaminants in fish

and wildlife tissues.

In 2004 – mercury advisories represented 53,000 km2 of lakes

and 1230,000 km of streams!

**** In the US, mercury only a problem with respect to fish.

Direct exposure to air and water not an issue.

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Bioaccumulation – net accumulation of contaminants in an

organism from all routes of exposure (water, sediment,

food, air..)

Bioconcentration – accumulation in organism directly

from water

Biomagnification – increase in contaminant concentrations

at higher levels in the food chain

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Mercury – the chemistry

Mercury – heavy silvery white liquid metal – only common

metal which is in liquid form at ordinary temperatures

High vapor pressure and low solubility

Mercury vaporizes readily under ambient conditions

Its saturation vapor pressure of 14 mg/m3 greatly exceeds the

average permissible concentrations for occupational (0.05

mg/m3) or continuous environmental exposure (0.015mg/ m3)

--- which mean air saturated with mercury vapors can be

extremely hazardous to human health!!

Three oxidation states of Inorganic Mercury:

1. Hg 0 – elemental, or metallic mercury

2. Hg2 +2 – mercurous ion, a divalent mercury form --- also

indicated as -- Hg(I)

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3. Hg +2 – mercury II, the mercuric ion, a divalent ion – also

indicated as – Hg(II)

Mercurous and mercuric forms can form numerous inorganic

and organic compounds, but mercurous ion is rarely stable under

ordinary environmental conditions.

Organic Forms:

1. Phenyl Hg (phenylmercuric acetate or PMA)

2. Methoxy Hg (methoxyethyl mercury acetate)

3. Alkyl Hg (methylmercuric acetate)

The compounds most likely to be found under environmental

conditions are:

Mercuric salts HgCl2, Hg(OH) 2 and HgS;

Methylmercury compounds, methylmercuric chloride (CH3

HgCl) and methylmercuric hydroxide (CH3 HgOH);

organomercurics C-Hg covalent bond (i.e., dimethylmercury

and phenylmercury).

Only Hg(II) can be converted to Methyl Mercury

Hg0 and Hg(I) cannot be transformed directly to Methyl

Mercury [they have to be first converted to Hg(II)]

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Three most common forms of Mercury:

1. Elemental Mercury – Hg 0

2. Inorganic Mercury --- Divalent Mercuric Ion

3. Organic Mercury form -- Methyl Mercury

Most of the Atmospheric Mercury - Hg 0

Most of the Mercury in soil, water, sediments – Inorganic

Mercuric compounds

Most of the Mercury in animal tissue – Methylmercury

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PRODUCTION & USES

Hg in most soils and rocks is low ~ 60ppb

Highest conc. in Ore – Cinnabar – HgS – mercuric sulfide

HgS production peak in 1970s ~ 10,000 tons/yr

Currently ~ 3000 t/yr

What is it used for?

Used in Chlor-alkali plants for the production of Cl2 and

NaOH

o Hg used as a cathode in the electrolytic process

o In recent years – more recycling and resuse

o Use of cell membrane technology – avoids Hg

Discharges from chlor-alkali plants were significant point

sources of Hg

Hg Dry Cell Batteries

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Toxic qualities of Hg – antifouling and mildew proofing

paints

o Use in former banned in 1972 under FIFRA (Federal

insecticides, fungicides, and rodenticides act)

Use in electrical apparatus – neon lights, switches, …….

Thermometers, manometers, barometers,…..

Each year up to 100 million dental fillings in the US –

amalgam – in combination with silver and tin and other

metals

o Only 50% of that prepared is actually used – some

wastage

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Hg used as catalyst in various processes – PVC, synthetic

acetate fiber

Used to coat seeds, Hg-based fungicides

o Restricted under FIFRA since 1970 and 1972

o Impacts on seed-eating birds

o 1969 case in New Mexico – farmer fed grain to pigs

and then ate the pigs!

o 1972 Iraq – bread prepared from treated wheat – 450

people died!

Used in mining industry to purify metal through the

amalgamation process – Silver in Mexico, Gold in Brazil

and Peru

Hg separated via electrolysis or Vaporization

I

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MERCURY EMISSIONS AND DEPOSITION

atmospheric emissions and deposition – primary nonpoint

source of Hg

Globally, 6600 metric tons of Hg emitted to the

atmosphere

2/3rd of this – direct or remitted anthropogenic sources

(1/3rd new, 1/3rd old recycled emissions)

Coal powered plants – 50-60% of all anthropogenic – 1450

metric tons.

Anthropogenic Hg emissions in the US = 103 tons.

Hg emissions in US from medical waste and municipal

incinerators have declined

Hg emissions from coal power plants have declined since

CAA

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Hg forms in emissions (1999, in the northeast) –

Hg0 = 57%

Reactive gaseous mercury (RGM) = 33%

Particulate mercury (PM) = 10%

Residence time and transport potential of Hg forms –

Hg0 – 0.5 to 2 years – tens of thousands of kilometers

RGM – 0.5 to 2 days – tens to a few hundred kilometers

PM – 0.5 to 3 days – tens to hundred kilometers

Hg0 can be oxidized to Hg(II) (by Ozone); Similarly Hg(II)

can be reduced by sulfides back to Hg0

Clean Air Mercury Rule (CAMR) – May 2005 USEPA

70% reductions in Hg by 2025 from Coal-fired power

plants.

Current level = 48 tons from coal powered plants

Allows a “cap-and-trade” approach.

Maximum allowable control technology – MACT rule

scrapped by Bush Administration in 2003

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Atmospheric Deposition

Direct absorption of Hg0 by vegetative surfaces – through

stomatal exchange!!

o Mosses and lichens serve as a good indicator of this

type of source!

Wet & Dry deposition

Peak deposition in the 70s and 80s???? – as indicated by

Hg concentrations in lake sediments

Figure 1 from Driscoll et al 2007

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Current US deposition levels = 52 metric tons annually.

Highest deposition in the South east US!

Northeast US deposition – from US (local/regional) sources

– coal power plants, etc.

Southeast US deposition – from Global Mercury pool

(Hg0)

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MERCURY CYCLING AND STORES

Main Stores of Mercury:

1. Atmosphere

2. Soils

3. Water

4. Sediments

Mercury in the Soil

Largest store of mercury (90% or more of the terrestrial

portion) – even if all mercury emissions were stopped today –

the soil store would continue to maintain current pollution

levels for at least 50 years!!!!

Most of the Mercury in the soil – Inorganic forms of Hg(II) –

nearly 97 to 99% - HgCl2, Hg(OH)2, HgS

Inorganic Hg(II) forms complexes/sorption with organic

matter (fulvic and humic acids) and mineral matter ---

resulting in reduced mobility of mercury

Some of the Hg(II) may complex with DOC and thus leach

out - strong correlation of Hg with DOC in uplands draining

to surface waters

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Sorption affected by – type of soil, dissolved organic carbon,

and species like S- and Cl-

Biomass or organic matter burning, forest fires, etc. can lead

to release and oxidation of Hg to the atmosphere!

The other form of Hg in soil -- methylmercury – CH3Hg+–

formed due to the process of methylation of Hg(II)

CH3Hg – typically less than 3% of the total mercury soil pool

– but very potent!

Mercury Uptake by Plant and Animals

Plant uptake of Mercury although possible is very small under

normal environmental conditions – which means that plants as

a mercury source to animals or other consumers can be

neglected

Mercury in foliage – mostly from atmosphere – stomatal

exchange

Mercury in roots – uptake from soil

Mercury not a concern along the terrestrial chain as it is for

the aquatic food chain!

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Mercury in Freshwater Ecosystems

Pathways of entry:

Hg(II) and CH3Hg+ from dry and wet deposition

Hg(II) and CH3Hg+ with runoff – dissolved or bound to

sediment or attached to DOC

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o Surface and near surface flow paths carry greater

amounts of Hg

o Strong correlation with DOC

Hg(II) and CH3Hg+ from groundwater flow

A significant amount of Hg(II) may partition to the water

column, especially if there is a high concentration of suspended

material in the water column.

Most of the mercury in the water column will be bound to

organic matter, either to –

dissolved organic carbon (DOC; consisting of fulvic and

humic acids, carbohydrates, carboxylic acids, amino

acids and hydrocarbons; or to

suspended particulate matter.

25 to 60% of the organic-complexes of mercury in the

water column – may be particulate bound, rest is the

dissolved-DOC phase

Hg0 concentrations in the water column are very low – Hg0 may

be formed due to reduction of Hg(II) which may then be lost via

volatilization.

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Most of the volatilization losses from water and soils – recent

mercury deposition – referred to as “prompt recycling”

Methylmercury in the water column – less than 25% of the total

mercury, typically less than 10%

Studies have shown that total and methylmercury concentrations

have a positive correlation with the DOC of the waters – work

by Driscoll at Syracuse U.

Methylmercury losses may occur to – volatilization, runoff,

biotic uptake, or demethylation to Hg0.

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METHYLATION OF MERCURY

A very important process – with regards to the potential of

mercury as a toxic substance!

Conversion of Inorganic Mercury to MethylMercury

(MeHg)

Mono-methylmercury - CH3Hg+ (CH3HgOH or CH3HgCl) or

di-methylmercury could be formed – (CH3)2Hg

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Di-methylmercury (CH3)2Hg volatilizes from surface water

and is generally not persistent in aquatic environments

Mercury methylation is brought about by sulfate-reducing

bacteria (SRB) and certain molds typically under anaerobic

environments (could also occur under aerobic conditions)

Higher MeHg concentrations during late summer (in

wetlands)??? – warmer temps cause higher rate of microbial

activity

Factors affecting Methylation:

1. Availability of Hg(II)

2. Oxygen concentration

Although methylation may occur under aerobic

conditions – the process is much more accelerated

under anaerobic conditions.

Methylation will be greatest at the sediment-water

interface as opposed to in the water column

3. pH

low pH – favors generation of methylmercury –

essentially releasing Inorganic mercury from complexes

– greater availability of inorganic mercury

*** unexpected consequences of acid deposition!

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4. Redox potential

5. Presence of sulfate and sulfide

In presence of high pH and sulfide – mercury will be

precipitated as mercuric sulfide and will not be available

for methylation, --- If the sulfide is oxidized to sulfate

mercury will be released and be available for methylation.

Sulfate may also stimulate SRB production and hence the

methylation process.

6. Complexing inorganic and organic agents

Greater amounts of humic/fulvic acids mean greater sites

for mercury binding – but in low pH conditions mercury

releases will occur from these sites.

7. Salinity

There appears to be a negative correlation between the

rate of methylmercury formation and salinity in

estuarine sediments. The rate is lower in more saline

environments because the bicarbonate component of

seawater slows methylation of Hg [II] under both

aerobic and anaerobic conditions.

8. Organic carbon

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DOC may actually bind up free mercuric ion and thus

reduce methylation. However in freshwater lakes,

DOC and pH may interact in such a way that less

mercuric ion will be bound to DOC and more will be

available for methylation.

Nutrients and organic matter can stimulate the

bacterial growth rates. Decaying matter can create

anaerobic environments – which may increase the

methylation process.

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METHYLMERCURY RELEASE DUE TO FLOODING CAUSED BY

HYDRO-ELECTRIC PROJECTS IN CANADA

Mercury stored in soils in inorganic form is released as MeHg

following flooding.

Flooded soils provide the anaerobic environments favorable

for release of MeHg

Alternating cycles of flooding may enhance MeHg

methylation

Research on methylmercury release being performed at the

Environmental Lakes Area – Manitoba

http://www.umanitoba.ca/institutes/fisheries/index.html

http://www.umanitoba.ca/institutes/fisheries/fludex.html

Kelly, C. A., J.W.M. Rudd, R.A. Bodaly, N.T. Roulet, V.L.

St.Louis, A. Heyes, T.R. Moore, S. Schiff, R. Aravena, K.J.

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Scott, B. Dyck, R. Harris, B. Warner, and G. Edwards. 1997.

Increases in fluxes of greenhouse gases and methyl mercury

following flooding of an experimental reservoir. Environ. Sci.

Technol. 31:1334-1344.

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MERCURY IN THE FOOD CHAIN AND BIOACCUMULATION (trophic transfer)

Definitions –

Bioaccumulation – the net accumulation of contaminant by an

organism due to uptake by all routes (water, sediment, food, air)

Bioconcentration – net accumulation of contaminants by

organisms by uptake from water

Biomagnification – tendency of contaminant to accumulate at

higher concentrations at higher levels in the food web due to

dietary uptake

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Both Inorganic and Methylmercury are taken up directly from

water and food (or ingested sediment) – but it is the MeHg

that bioaccumulates and biomagnifies and is toxic

Elemental and inorganic forms of mercury are poorly

absorbed in the organisms (a large portion of what is

consumed is excreted)

Very large amounts of inorganic mercury are required for it to

be toxic – because of its low bioassimilation

o A person can swallow upto ½ kg of metallic Hg and

show no adverse effects

o 98% of the Hg is excreted with urine and feces

o Inorganic (solid or liquid) Hg does not penetrate the

blood-brain barrier

o Hg vapor may be more toxic – problems in felt hat

industry! – mad hatter disease!

o Inorganic Hg may get methylated by microbes within

the bodies of some birds and fish

In contrast, Methylmercury is readily transferred across

biological membranes – and is strongly bound to sulfhydryl

groups on proteins of tissue such as muscle – disrupt the cell

membranes and destroy the cells

The problem is that Mercury binds with muscle as

opposed to other toxics that bind with fatty tissues or skin

tissues – this means Mercury cannot be filleted out or cooked

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out of consumable fish -- actually, mercury concentrations

increase after cooking because moisture is lost.

******??? which other toxics are unlike mercury in this

regard?

Trophic transfer – greatest jump in bioaccumulation occurs

from water to phytoplankton (algae) – 105 to 106

****Figure 3 from Driscoll et al. 2007

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MeHg continues to increase up the food chain -- Top aquatic

predators such as freshwater largemouth bass, pike and

walleye and marine fish such as king mackerel, sharks, and

swordfish may contain concentrations of mercury 10,000 to

100,000 times greater than that found in the surrounding

water

***Fish Hg concentrations -

Positively correlated with lake or watershed drainage

area

Negatively correlated with – pH, ANC, nutrient

concentrations, zooplankton density, and human land use

*** Why is phytoplankton or fish Hg negatively correlated

with high nutrient concentrations (nutrient

enrichment)??????

Within fish populations – Hg concentrations or burdens

increase with size and age of populations – slower rates of

elimination, longer exposure, feeding habits at higher trophic

levels

Greater than 90% of the bioaccumulated Mercury is MeHg

that’s why EPA recommends determination of total mercury

as a measure of MeHg (MeHg measurements are much more

expensive that total mercury)

Premier lab that measures mercury –

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http://www.frontiergeosciences.com/

INDICATORS OF MERCURY SENSITIVITY

Fish Hg concentrations related to water parameters (Chen et al.,

2005)–

DOC

ANC

pH

total phosphorus

Driscoll et al 2007 tested these indicators for northeastern lakes

Using the 0.3 ppm MeHg USEPA criterion for yellow perch

******Figure 4 from Driscoll et al. 2007

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Lakes with MeHg levels greater than 0.3 ppm in yellow perch

had significantly

- higher DOC

- lower pH

- lower total phosphorus

than lakes with levels less than 0.3 ppm.

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EFFECTS OF MERCURY ON WILDLIFE SPECIES

Deformities

Carcinogenic effects

Reproductive failure

Impacts on Adirondack Loons:

105 loons found dead or debilitated in the state between 1972

and 1999

Mercury was found in the liver of 83 loons

Mercury interferes with the bird’s ability to raise offsprings

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Mercury will interfere with muscle coordination and vision

making it harder for the loon to feed itself and its young

Mercury Impacts on Livestock

In cattle and sheep, dietary intake of 0.2 mg/kg mercury will

cause uncoordination, unsteady gait, and eventual death.

Mortality in poultry begins with mercury levels of 5.0 ppm.

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HUMAN EFFECTS OF MERCURY CONSUMPTION

Highest MeHg levels are generally found in human kidneys.

MeHg readily crosses the placental and the blood/brain

barriers

Pregnant women may discharge the Hg from their body into

the fetus!

Estimates of half-life in human body 44 to 80 days

Excretion of MeHg occurs via – feces, urine, breastmilk

MeHg – may cause daughter cells to get unequal numbers of

chromosomes – genetic impacts

Mercury vapor – inflammation of the gums, metallic taste,

diarrhea, mental instability, and tremors

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Acute Toxicity

Lethal dose – 10 to 60 mg/kg

High Doses of MeHg may cause –

Impaired central nervous system

Kidney damage and failure

Gastro-intestinal damage

Cardiovascular collapse, shock and death

Chronic Toxicity

Deterioration of the nervous system

Impairment of hearing, speech, vision and gait

Involuntary muscle movement

Corrosion of skin and mucous membranes

Difficulty in chewing and swallowing

Largest case of human mercury poisoning – Minamata Bay,

Japan.

Chisso Co. started using the bay in 1932 for dumping mercury

waste – continued till 1968!

More than 900 dead, 12,615 affected. 26 yr ban on fishing.

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residents who ate the fish displayed numbness, tunnel vision,

slurred speech, spasms. Many suffered violent convulsions -- a

strange dance of death -- before going mad and dying. CONCENTRATIONS IN PPM !

Fish &

Shellfish

Cats Humans

oyster 5.6

control 0.9-3.66 control less

than

3.0

gray mullet 10.6 kidney 12.2-

36.1

kidney 3.1-

144.0

short-

necked

clam

20.0

liver 37-145.5 liver 0.3-

70.5

china fish 24.1 brain 8-18 brain 0.1-

24.8

crab 35.7 hair 21-70 hair 96-705

Minamata's food chains dramatically illustrate the

`concentration of elements'--in this case, of mercury--in

successive trophic levels. Assays of tissue from fish and

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shellfish from the bay, and from cats and humans who died from

the poisoning, show high concentrations of mercury. Kidney and

liver concentrations indicate how the bodies tried--

unsuccessfully--to excrete and detoxify the heavy metal.

_http://www1.umn.edu/ships/ethics/minamata.htm

ACCEPTABLE LEVELS OF MERCURY EXPOSURE Human body burden of – 25-30 mg of MeHg is dangerous.

Based on this number and a loss rate of 1% per day, intake rate

should not exceed = 0.02 mg or 20 µg per day

Other viewpoint – no Hg is best – because any Hg will damage

cells – at the cellular level there is no threshold!

Background level in food = 0.02-0.05 ppm.

Meat and fish have higher levels

Pork & beef = 0.1 ppm

Fish = 0.2 ppm

Tuna = 0.21

Swordfish = 0.95

Sharks = 1.33 ppm

May 1971 – FDA advised Americans to stop eating swordfish

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1969 – FDA set a safety level of 0.5 ppm of Hg

Changed to 1.0 ppm in 1973

And then changed to 1.0 ppm of MeHg in 1984.

Drinking water standard = 2µ L-1

Acute and chronic levels for aquatic organisms – table 12.8

OSHA limit in workplace = 0.1 mg/m3