mercury as an air pollutant

8/8/2019 Mercury as an Air Pollutant

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Characterization of Mercury as an Air Pollutant

ABSTRACT: Mercury is a naturally occurring metal found in air, water and soil.Mercury exists in any of three valence states: Hg0 (elemental mercury), Hg2

+2 (mercurous

mercury) and Hg+2 (mercuric mercury). These forms can be organized under three

headings: metallic mercury, inorganic mercury, and organic mercury. Metallic mercury ispresent at liquid state in room temperature, and is used in thermometers and some

electrical switches. Inorganic mercury compounds occur when mercury combines with

elements such as chlorine, sulfur, or oxygen. These mercury compounds are also called

mercury salts. Most inorganic mercury compounds are white powders or crystals, exceptfor mercuric sulfide (also known as cinnabar), which is red and turns black after exposure

to light. Organic mercury compounds (or organomercurials) occur when mercurycombines with carbon. In the environment there is a large number of organic mercury

compounds; however, by far the most common organic mercury compound in the

environment is methyl mercury (or MeHg+). Methyl mercury is of particular concern

because it can build up in certain edible freshwater and saltwater fish and marine

mammals to levels that are many times greater than levels in the surrounding water.

INTRODUCTION

Mercury has been well known as an environmental pollutant for several decades. Asearly as the 1950s it was established that emissions of mercury to the environment could

have serious effects on human health. These early studies demonstrated that fish and

other wildlife from various ecosystems commonly attain mercury levels of toxicological

concern when directly affected by mercury-containing emissions from human-relatedactivities. Mercury in the air may settle into water bodies and affect water quality. This

airborne mercury can fall to the ground in raindrops, in dust, or simply due to gravity

(known as air deposition). After the mercury falls, it can end up in streams, lakes, or

estuaries. Over time, the mercury mostly precipitates to the red mineral cinnabar, HgS,which is responsible for soil contamination. With the aid sulfate-reducing bacteria (SRB)

or iron-reducing bacteria (IRB), the cinnabar is converted to methyl mercury under

anaerobic and acidic conditions, which are typical of the well-buried muddy sediments of

rivers, lakes, and oceans. SRB and IRB use sulfur rather than oxygen as their cellularenergy-driving system.

HgS(s)SRB /IRB

CH3Hg(II)X(aq ) +H2S(g ) Where X is a ligand (an ion, a molecule, or a molecular group that binds to another

chemical entity to form a larger complex), typically Cl- or OH-, are the most toxic forms.

Upon methylation, the SRB transport the new mercury complex back to the aquatic

environment, where it is taken up by other microorganisms. The conversion of inorganicmercury to methyl mercury is important for two reasons:

(1)Methyl mercury is much more toxic than inorganic mercury.(2)Organisms require a considerably longer period to eliminate methyl mercury.

At this point, the next higher level in the food chain may consume the methyl mercury-

containing bacteria, or the bacteria may release the methyl mercury to the water where it

can quickly adsorb to plankton. Plankton may also be consumed by the next level in the

food chain, or after dying, settle to the bottom of the lake and are incorporated into


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bottom sediments. Studies of sediment cores show that younger sediments deposited

since industrialization have mercury concentrations that are about 3-5 times that ofhistorical sediments.

Mercury in aquatic environment

Most of the mercury entering aquatic environments is Hg+2. Methyl mercury

accumulates in fish at levels that may harm the fish and the other animals that eat themvia bioaccumulation and biomagnification. Bioaccumulation is the process by which

organisms (including humans) can take up contaminants more rapidly than their bodies

can eliminate them, thus the amount of mercury in their body accumulates over time. If

for a period of time an organism does not ingest mercury, its body burden of mercury willdecline. However, if an organism continually ingests mercury, its body burden can reach

toxic levels. The rate of increase or decline in body burden is specific to each organism.

Biomagnification is the incremental increase in concentration of a contaminant at each

level of a food chain. This phenomenon occurs because the food source for organismshigher on the food chain is progressively more concentrated in mercury and other

contaminants, thus magnifying bioaccumulation rates at the top of the food chain. The

bioaccumulation effect is generally compounded the longer an organism lives, so thatlarger predatory fish will likely have the highest mercury levels. In addition, unlikeorganic contaminants (such as dioxin and PCBs) that concentrate in the skin and fat

tissues, mercury concentrates in muscle tissues. This implies that mercury cannot be

filleted or cooked out of consumable fish. Figure 1 illustrates the aquatic mercury cycle.

Figure 1. Mercury cycling pathways in aquatic environments. Reprinted with permission

from Mercury Pollution: Integration and Synthesis. Copyright Lewis Publishers, an

imprint of CRC Press.

Atmospheric Mercury

Mercury is a naturally occurring metal that is omnipresent in the environment.

Mercury is released to environmental media by both natural processes and anthropogenic

sources. Mercury ore is found in all classes of rocks including limestone, calcareous


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shales, sandstones, serpentine, chert, andesite, basalt, and rhyolite. Mercury is released to

the atmosphere mostly by the burning of fossil fuels, which naturally contain mercury.There are three main species of mercury occurring in the atmosphere: elemental mercury

vapor (Hg0), gaseous divalent mercury (Hg2+2 and Hg+2) and particulate mercury (Hgp).

Elemental mercury (Hg0)

Elemental mercury is the primary form of atmospheric mercury, accounting for 90%

in the atmosphere. In ambient air, elemental mercury is present globally at concentrations

of the order of 1.5-2.0 ng/m3. The fact that the reactivity of elemental mercury in water is

very low compared to the other forms of atmospheric mercury, allows it to persist in theatmosphere with a lifetime of 1-2 years, as well as being capable to transport globally,

therefore making it a true global pollutant. Over the past century, it was shown that the

concentration of elemental mercury in the atmosphere has been increasing. It was

estimated that 50-75 percent of this increase originates from anthropogenic sources(Expert Panel on Mercury Atmospheric Processes, 1994). Removal of elemental mercury

occurs by dry or wet deposition after oxidization by ozone (O3) to gaseous divalent

mercury, and is removed from the atmosphere by precipitation. The overall organicperoxy compounds or radicals may also occur in the atmosphere. In clouds, however, a

fast oxidation reaction on the order of hours may occur between elemental mercury and

ozone, as described by the following reactions:

(1) Hg+ hv (254nm)Hg*

(2) Hg*+O2 Hg+O2 *

(3) O2 *+O2 O3 +O

(4) O+O2 O3

(5) O3 +HgHgO+O2

(6) O+HgHgO

In the reactions above, under light of wavelength of 254 nm light, elemental mercuryis excited in reaction (1). The excited mercury reacts with oxygen in reaction (2) to

produce elemental mercury and an oxygen molecule in an excited state. The species

responsible for oxidation of elemental mercury are formed in reactions (3) and (4)through the reaction of excited state oxygen with oxygen to form ozone and oxygen

radical, which also reacts with oxygen to form ozone. Both ozone and oxygen radicals

react with elemental mercury to form mercuric oxide (HgO) as shown in (5) and (6).Most of the mercuric oxide is formed through the thermal reaction with ozone. The

overall reaction is obtained by adding reactions (1) through (5):

(7) Hg+ 2O2 + hv HgO+O3

Gaseous Divalent Mercury (Hg2+2 and Hg+2)

Hg2+2 and Hg+2 account for about 1-3 percent of total gaseous mercury in the

atmosphere (Lindberg and Stratton, 1998). It is believed that the most part of gaseous

divalent mercury consists of Hg2+2 (mercurous mercury) and Hg+2 (mercuric mercury),


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but other divalent mercury species are also possible (like mercury dichloride HgCl2).

Gaseous divalent mercury is likely to be rapidly scavenged via dry and wet depositionprocesses withing approximately 100 to 1000 kilometers as a result of its high water-

solubility and chemical reactivity, thus having much shorter life than elemental gaseous

mercury. Air concentrations of gaseous divalent mercury are likely to be highly related to

local sources, meteorological conditions and some other pollutants.

Particulate Mercury (Hgp)

Particulate mercury occurs in both gaseous and aqueous phases. It can be formed by

physical adsorption of gaseous divalent mercury to atmospheric particulate matter inaqueous phase as the secondary particulate mercury. It can be emitted directly into the

atmosphere from anthropogenic and natural sources. Nevertheless, it is largely consisted

of anthropogenic origin. Background concentration of particulate mercury indicated that

it was a minor constituent only 0.39 percent of total gaseous mercury. Particulatemercury tends to be dry deposited at significant rates when and where measurable

concentrations of these mercury species exist.

Figure 2. Transformation of mercury in air, water and sediments


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Famous outbreaks of mercury poisoning

In the early days of the Industrial Revolution, mercury nitrate, Hg(NO3)2, was used tosoften rabbit fur to make felt hats. Mercury nitrate is a form of inorganic mercury that

isnt absorbed by the body as easily as other species, but it was toxic enough to affect the

brains of many hat makers, that suffered from the toxic effect of mercury on their central

nervous system that eventually led to the death of many hat makers.Over the history numerous cases of mercury contamination were reported. The

extensive toxicity of mercury was first revealed in 1955 in Minamata City in Japan,

where there was a mercury-poisoning outbreak as a result from a continuous poisoning of

the local bay by Chisso Corporations chemical factory. The outbreak was responsible tothe immediate death of 887 people, and to an overall death of 2,265 people (as of march

2001). The factory had been releasing methyl mercury to the Minamata Bay and the

Shiranui Sea between 1932 and 1968. Methyl mercury was then bio-accumulated in the

shellfish and fish in, which were then eaten by the local population, resulting in mercurypoisoning, also known as the Minamata disease, having similar symptoms in both

animals and humans. The neurological syndrome was characterized by a long list of

symptoms including prickling, tingling sensation in the extremities (paresthesia),impaired peripheral vision, hearing, taste and smell, slurred speech, unsteadiness of gaitand limbs, muscle weakness, irritability, memory loss, depression, and sleeping

difficulties. The extensive mercury poisoning affected children and fetuses much more

than it affected adults, leading to severe brain damage in 22 infants whose mothers had

ingested fish contaminated with methyl mercury during pregnancy.Another mercury poisoning outbreak also occurred in Iraq in 1971 and 1972, when

locals ate bread prepared from wheat and other cereals treated with a methyl mercury

fungicide. The outbreak caused to the hospitalization of more than 6,530 patients and 459deaths occurred, probably due to central nervous system damage.In comparison to the

outbreak in Minamata Bay, the breast milk concentrations were reported to be as high as

200 ppb. Breast milk of individuals who lived in nearby areas to Minamata and hadconsumed highly mercury-contaminated fish, had a total mercury concentrations on theorder of 63 ppb. Generally, breast milk with total mercury levels greater than 4 ppb

would exceed the safe level (2 g methyl mercury/day for an average 5-kg infant).

Figure 3. Defects resulting from the

Minamata Bay mercury poisoning can be

observed at fisherman Akinori Moris hand


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CHEMICAL AND PHYSICAL PROPERTIES OF MERCURY

Mercury has been recognized for decades as a persistent and bio-accumulative toxicsubstance in the environment. It is the 6th most toxic in a universe of 6 million substances,

and the 16th most rare element of Earth. The origin of the name comes from the Latin

word hydrargyrum meaning liquid silver. It is a heavy, silver-white metal, liquid at room

temperature (25

0

C), stable in air and water. Mercury is unreactive with alkalis and mostacids. It gives off poisonous elemental mercury vapor that has a chronic cumulative

effect. In the environment, mercury is a very persistent pollutant that is very mobile due

to its low vapor pressure (0.25 Pa at 250C), low melting point (-38.870C), and because of

among all metals, it has the highest solubility in water (56.1*10-6 g/L at 250C). Thephysical properties of mercury allow it to be readily released from soil, water and plant

canopies; therefore it can be transported in the air and deposited back to the Earths

surface, as well as staying in the atmosphere for long time (up to two years). Mercury

also has a relatively poor thermal conductivity, good electric conductivity, and the abilityto expand and contract evenly with temperature changes. Its abilities to easily form alloys

with other metals and the high surface tension make it widely used in the manufacture of

industrial chemicals and electrical applications.Inorganic mercury compounds include mercuric sulfide (HgS), mercurous chloride(Hg2Cl2), mercuric chloride (HgCl2), mercuric nitrate (Hg(NO3)2) and mercuric oxide

HgO. Mercuric chloride is the major part of inorganic mercury compounds (U.S EPA,

1999). Most inorganic mercury compounds are white powders or crystals, unstable when

exposed to heat and light, and readily decompose to elemental mercury. Some mercurysalts such as mercuric chloride are volatile enough to evaporate into the atmosphere at

room temperature and pressure. However, their high water solubility and chemical

reactivity make them deposit from the atmosphere faster than elemental mercury.

Organic mercury compounds (also known as organomercurials) are differentiated from

inorganic mercury since they contain a covalent bond between carbon and mercury

atoms. Organic mercury compounds include methyl mercury, dimethyl-mercury andmethyl-mercury chloride. Organic mercury compounds include a large number of

compounds, however methyl mercury is by far the most toxic and prevalent form of

organic mercury compounds in the environment.The toxicity of inorganic and organic mercury compounds is due to their strong

affinity for sulfur-containing organic compounds, such as enzymes and other proteins.

For this reason these compounds are extremely toxic to biological systems.Mercury is

unusual among metals because it tends to form covalent rather than ionic bonds. Most ofthe mercury encountered in water/soil/sediments/biota (all environmental media except

the atmosphere) is in the form of inorganic mercuric salts and organomercurics. The

compounds most likely to be found under environmental conditions are: the mercuric

salts HgCl2, Hg(OH)2 and HgS; the methyl mercury compounds, methyl mercuricchloride (CH3HgCl) and methyl mercuric hydroxide (CH3HgOH); and, in small fractions,

other organomercurics (for instance dimethylmercury and phenylmercury). Table 1

illustrates the difference in chemical properties of different mercury compounds.


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Table 1. Properties of elemental mercury and selected mercury compounds

Hg0 HgCl2 HgS HgO (CH3)2Hg CH3HgCl

Melting Point (0C) -38.87 277

583

sublimation

500

decomposition - 170

Boiling Point (0C) 356.72 302 - - 96 -

Water Solubility

(g/L) 56.1*10-6 66 2*10-24 5.3*10-2 2.95


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Thermal Properties:

Thermal conductivity / W m-1K-1: 8.34Thermal expansion / m m-1K-1: 60.4 --> constant rate over wide temperature range, thus

is used in thermometers and barometers

Heat of fusion / kJ mol-1: 2.331

Heat of vaporization / kJ mol-1

: 59.11Heat of atomization / kJ mol-1: 64.463

Ionization Energy:

1st ionization energy / kJ mol-1: 1007.07

2nd ionization energy / kJ mol-1: 1809.69

3rd ionization energy / kJ mol-1: 3299.82

Isotopes:

Isotope Relative atomic mass Mass percent Stable with

196Hg 195.965815(4) 0.15(1) 116 neutrons

198Hg 197.966752(3) 9.97(8) 118 neutrons

199Hg 198.968262(3) 16.87(10) 119 neutrons

200Hg 199.968309(3) 23.10(16) 120 neutrons

201Hg 200.970285(3) 13.18(8) 121 neutrons

202Hg 201.970626(3) 29.86(20) 122 neutrons

204Hg 203.973476(3) 6.87(4) 124 neutrons

Radiological Data of203Hg:

Nuclear and Emissions:

Half-life: 47 daysBeta particles: 0.210 MeV maximum energy (100 %) and 0.070 MeV average energy.

Gamma rays: 0.279 MeV (100%).

Beta Maximum Range: 34 cm in air; 0.04 cm in tissue; 0.04 cm in Plexiglas

Dose and Shielding

Dose rate to the skin at 30 cm: 15.2 mrem/hour/mCi (for an unshielded point source)

Gamma Dose rate (deep tissue dose) at 30 cm: 1.63 mrem/hour/mCi (for an unshielded

point source).Dose rate to epidermal basal cells from skin contamination of 1 Ci/cm2: 3296mrem/hour

Shielding: Shield stock vials with lead.Half-Value Layer: 0.2 cm lead The half-value layer is the amount of material required to

reduce the radiation intensity by 50%.

Annual Limit on Intake (ALI): 500 microcuries via ingestion and 800 microcuries via

inhalation. The intake of one ALI will produce a dose of 5 rem.


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Crystallography of Mercury:

Crystal structure: rhombohedral

Unit-cell dimensions / pm: a=299.25, =7044.6'

Space group: R3

m

Classification of Mercury in Hazardous Materials Identification System (HMIS):

Figure 4. Elemental mercury at room Figure 5. Density of mercury is high

temperature looks like liquid water enough to allow an adult to sit on it


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Physical and Chemical Transformations of Mercury in the Atmosphere

Gas phase reactions

The only important gaseous reaction globally identified so far is that between

Hg0 and O3 (marked in green), with an expected lifetime of 1.4 years at an ozone

concentration of 30 ppb.Reaction Rate (cm3*molec1*s1) Reference

Hg0(g) + O3(g)HgO(g,s) 3.0 2 1019 Hall, 1995

Hg0(g) + H2O2(g) HgOH2(g,s) 8.5 1019 Tokos et al., 1998

Hg0(g) + Cl2(g) HgCl2(g,s) 4.8 1018 Calhoun & Prestbo, 2001

Hg0(g) + HOHg+2 8.7 1014 Sommar et al.,2001

Hg0(g) + HCl(g) products 1.0 1019 Hall et al., 1993

Hg0(g) + NO3(g) products 4.0 1015 Sommar et al., 1997

Br(g) + Hg0(g) products 4.0 10

15 Sommar et al., 1997

Br2(g) + Hg0

(g) products 4.0 1015 Sommar et al., 1997

BrO(g) + Hg0

(g) products 4.0 1015 Sommar et al., 1997

Aquatic phase reactions

Aqueous phase reactions of mercury or mercury compounds occur in rainwater, cloud-

water or fog-water in the atmosphere. In aqueous phase, the chemical conversion of Hg +2

to Hg0 takes place. The proposed reaction mechanism involves a formation of an

intermediate HgSO3 that decomposes to produce Hg+2, which in turn is rapidly reduced to

Hg0. The overall rate of the reaction is inversely dependent on the sulfite concentration.

This reaction may influence the concentration of mercury in cloud and rain water byreducing water soluble Hg+2 to volatile Hg0. Hg+2 and the hydroxide ion, OH, form

HgOH+ and Hg(OH)2, and the divalent mercury bound as Hg(OH)2 can be reduced back

to Hg0(aq) by photolysis.

Reaction Rate Reference

Hg0(aq) + O3(aq)Hg+2

(aq) + products 4.7 107 M1s1 Munthe(1991)

HgSO3(aq)Hg0

(aq) + products Texp((31.971T)12595T)Ts1 Van Loon (2000)

Hg(OH)2(aq) + h Hg0(aq) + products 6.0 107 s1(maximum)a Xiao et al.,(1994)

Hg0(aq) + OH(aq)Hg+2

(aq) + products 2.0 109 M1s1 Lin (1997)

Hg+2 + HO2(aq) Hg0(aq) + products 4.7 107 M1s1 Lin (1998)

Hg0(aq) + HOCl(aq) Hg+2


Hg0(aq) + OCl

(aq) Hg+2


Hg0(aq) + H2O2(aq)

HgO(s) + Hg+2 + products 6.0 M1s1 Lin (1998)

Hg+2 + SO2 HgSO3 2.0 1013 M Smith (1976)

HgSO3 + SO-2 Hg(SO)2

-2 4.0 1012 M Smith (1976)

Hg+2 + 2Cl HgCl2 1.0 1014 M2 Smith (1976)

Hg+2 + OH HgOH+ 2.51 1011 M Lin (1999)

HgOH+ + OH Hg(OH)2 6.31 1012 M Smith (1976)

HgOH+ + Cl HgOHCl 3.72 108 M Smith (1976)

Hg(OH)2(s) Hg(OH)2(aq) 3.5 104 M Smith (1976)


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HgCl2(s) HgCl2(aq) 0.27 M Smith (1976)

Dissolved Hg0 in cloud droplets is oxidized by O3, hydroxyl radical (HO) and

chlorine (HOCl/OCl), leading to the formation of Hg+2 in rain or cloud water, which maythen be removed from the atmosphere on shorter time scales due to the increase in water

solubility. After oxidized by Ozone, elemental mercury in cloud droplets mainly formsHgO(aq), which reacts further to divalent mercury, Hg2+2 or Hg+2. Another oxidant of

potential significance is OH(aq), which is also capable of oxidizing elemental mercury incloud droplets to divalent mercury. A modeling study involving known atmospheric

reactions of mercury concluded that the two most important oxidation reactions of

elemental mercury in aqueous phase involve hydroxyl group (OH) in daytime and

chlorine atom (Cl) at nighttime.

Figure 6. Physical and chemical transformation of mercury in the atmosphere (Bullock,

2003)

The complexity of the various atmospheric chemical reactions is not fully understood

yet. It is evident that mercury is emitted and returns to the ground by precipitation, yet the

conditions at which the transformations among the different mercury types are notknown. Further explanation on the environmental fate of mercury is presented later on in

this project.


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SOURCES OF MERCURY IN THE ENVIRONMENT

Mercury is released to the environment by both natural processes and anthropogenicsources. Natural mercury emission is defined as the mobilization or release of

geologically bound mercury by natural processes, with mass transfer of mercury to the

atmosphere (e.g., volcanic activity and weathering of mercury-containing rocks);

Anthropogenic mercury emission is the mobilization or release of the mercury by humanactivities, with mass transfer of mercury to the atmosphere. Currently, the average

mercury level in the atmosphere is about 3 to 6 times higher than the estimated level in

the preindustrial atmosphere. However, a degree of uncertainty exists with respect to

estimates of the relative contributions of natural and anthropogenic sources of mercuryemissions to the environment due to the fact that no previous data was collected in the

past.

Anthropogenic Sources

Approximately 25% of atmospheric mercury is attributed to the continuous releasing

of mercury in different anthropogenic activities. The natural component of the total

atmospheric burden is difficult to estimate, although anthropogenic releases of mercury tothe atmosphere were estimated to cause triple its concentration in air and marine surface

waters since the pre-industrial era. Recent estimates of anthropogenic releases of mercury

to the atmosphere range from 2,000-4,500 metric tons/year, mostly from the mining and

smelting of mercury and other metal sulfide ores. An estimated 10,000 metric tons ofmercury are mined each year, although there is considerable year-to-year variation (WHO

1990).

Figure 7. Industrial power plant is the primary source of anthropogenic mercury

Emission to the atmosphere


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In the past, chlor-alkali plants were thought to be the single largest source category of

anthropogenic mercury emissions to the environment in many industrialized countriesuntil the 1970s. After that, its ranking among the remaining source categories has been

substantially changed due to antipollution measures. Coal combustion, waste

incineration, metal smelting, refining and manufacturing are currently major source

categories in the industrialized world. Until few years ago, mercury was commonly usedin thermometers and barometers due to its high rate of thermal expansion that is fairly

constant over a wide temperature range. Other potential emission sources include copper

and zinc smelting operations, paint applications, waste oil combustion, geothermal

energy plants, crematories, and incineration of agricultural wastes. The incineration ofmedical equipment has also been found to be a significant source of atmospheric

mercury, releasing up to 12.3 mg/m3 of mercury. Medical wastes release approximately

110 mercury mg/kg of uncontrolled emissions from medical waste incinerators,

compared with 25.5 mercury mg/kg general municipal waste. Table 2 shows the differentanthropogenic source types of mercury in the US.

Table 2. Atmospheric mercury emission inventory for the US by anthropogenic source

type (Bullock, 2003)

Anthropogenic mercury emission has been a global issue, forcing the leading countriesto innovate new technologies in order to reduce the environmental effect resulted to

aquatic environments. In Canada, anthropogenic mercury emissions were reduced fromapproximately 32 to 8 metric tons from 1990 to 2000. The largest source of mercury prior

to 1995 was base metal mining industry. From 1995 to 2000, electricity generation andmetal smelting were equally the largest sources of mercury into the atmosphere, each

accounting for 25% of Canadian emissions. In the US, emissions of mercury to the air

from anthropogenic sources have fallen by more than 45% since passage of the 1990Clean Air Act Amendments, which provided new authority to EPA to reduce emissions


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of mercury and other toxic pollutants to the air (U.S. EPA, 2006). Figure 8 shows the

anthropogenic mercury emissions in the US over the past decade.

Figure 8. U.S. anthropogenic mercury emissions over past decades (U.S. EPA, 2006)

Anthropogenic emissions from a number of major sources have been estimated to bedecreasing in North America and Europe due to reduction efforts during the last decade,

while the anthropogenic emissions from some developing countries have been increasing

dramatically over past ten years (Figure 9). In recent years, Asia was the biggest

anthropogenic mercury emission region, accounting for about 52% of the global

anthropogenic emission.

Figure 9. Anthropogenic mercury emissions: distributed by region in 1990 and 2000

(Cain, 2006)


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Natural Sources

Mercury is a naturally occurring element that is present in the Earths crust, with anaverage abundance of 0.5 ppm (g/g). It is considered to have been a component of thelithosphere since the planet was formed approximately 4.5 billion years ago. Low levels

of mercury can be found everywhere in the environment - in rocks, plants, animals, water

and the air. Mercury ore is found in all classes of rocks, including limestone, calcareousshales, sandstone, serpentine, chert, andesite, basalt, and rhyolite. The normal

concentration of mercury in igneous and sedimentary rocks and minerals appears to be

10-50 ppb; however, the mineral cinnabar (mercuric sulfide, HgS) contains 86.2%

mercury.The high temperature in the Earths mantle layer results in high mercury mobility,

allows mercury to continuously diffuse to the Earths surface. Elemental and some forms

of oxidized mercury come to the atmosphere due to their volatility. In the environment,

mercury is emitted naturally in the form of volcanic activity, deposition of mercury,weathering of mercury-containing rocks, degassing of the earths mantle/crustal material,

evasion from soil, water, vegetation surfaces, wild fires, and geothermal sources. All of

which result in higher concentrations of mercury in certain compartments within thenatural environment.Active volcanoes release substantial quantities of volatile materials into the

atmosphere in the form of gases and aerosol. It was estimated that that the time-averaged

volcanic mercury emission is about 700 Mg/yr, or 20-40% of total natural emissions

(Pyle and Mather, 2003). Continuous degassing accounts for only 10% of this flux, while75% of volcanic mercury is released during smaller sporadic eruptions (103 Mg) explosive eruptions overwhelm the total atmospheric

burden several times per century, and account for about 15% of total volcanic mercuryemissions. However, the extent of the volcanic contribution to global mercury emission

remains highly uncertain, therefore current atmospheric mercury modeling studies

ignored the volcanic contribution of mercury to the atmosphere.Emission from soil, water and vegetation surfaces is another important pathway.Several estimates of natural mercury emission have been made, but the estimates differ

significantly. Earth surfaces might act as dynamic exchange interfaces which can be

sources or sinks of atmospheric mercury depending on the ambient mercury

concentration, mercury deposition velocities, micro-meteorological conditions andmercury concentration in the transpiration stream, water or soil.

Roots of trees are also considered as a natural source of mercury emission. Mercury

can be taken up by the roots of trees and later released to the environment when that

wood is burned in a stove or a forest fire. Emission of mercury due to biomass burning iscurrently being studied. In 2001, a scholar names Brunke measured mercury in the plume

of a wild land fire in South Africa and estimated the global contribution to theatmospheric emission to be as high as 1106 kg/yr.Other pathways of evasion of mercury include emission from the bottom of the oceans

and from geothermal or tectonically active areas. To date, considerable uncertainties still

remain in the estimations of the quantitative significance of evasion. Very little is known

about the amounts of mercury evading from soil, water and vegetation.


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Re-Emission Sources

Re-emitted mercury is defined as the mass transfer of mercury to the atmosphere by the

processes drawing on mercury that was deposited to the earths surface after initial

mobilization by either anthropogenic or natural activities. To date, very little is known

about the natural emissions and re-emissions of mercury. Current emissions of mercury

from soil, water and vegetation not only include emission of naturally occurring mercurythat exists in the substrates but also include re-emission of previously deposited mercuryfrom anthropogenic sources. This makes it very difficult to discriminate actual natural

emissions from re-emission. Currently, there is still no effective way to strictly

distinguish between natural emissions and re-emissions, except in cases where there is a

strong natural source signal (e.g., areas geologically enriched in mercury such asvolcanoes). In addition, there are few measurements available and current estimates are to

a large extent extrapolated from a few data points and constrained by global mass balance

estimates.


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MEASUREMENT OF ATMOSPHERIC MERCURY

Attempts to measure atmospheric mercury have been established globally in order toforecast mercury contamination that may alter life quality, as well as to predict movement

of atmospheric mercury to different regions through wind and precipitation. Elemental

mercury is not soluble in water, however gaseous divalent mercury, Hg+2 and Hg2+2, are

very water-soluble, making the measurement of atmospheric mercury more challenging.Currently there are a number of experimental methods used in the measurement of

atmospheric mercury deposition. Some of the methods include the dynamic flux

chambers (DFC), surrogate tanks (SS), micrometeorological methods like the modified

Brown ratio (MBR), the aerodynamic (AER) and the relaxed eddy accumulation (REA).The degree of uncertainty in the measurement varies considerably with the measurement

method used. The use of two systems of the same method reduces the degree of

uncertainty, like in Fritscher et.al (2008a), a research that measured elemental

atmospheric mercury air-surface exchange over grassland, used two micrometeorologicalmethods simultaneously.

The DFC measurement method employs flux chambers to measure the fluxes. DFC

methods have been used in wetlands in Quebec, Ontario, the Florida Everglades, forestareas in Tennessee, in background soils across the US, and in over lakes and forest soilsin southwestern Sweden. DFC can provide immediate results and are relatively simple to

use, however they can alter local conditions such as temperature, humidity and

turbulence.

A surrogate surface is a surface composed of a material such as Teflon or polysulfonethat is placed at a site for a designated amount of time, allowing mercury to deposit onto

it. The surface is then washed and analyzed. Drawbacks to the use of surrogate surfaces

include the need to cover the surfaces during rain showers, therefore it creates gaps in thedata collection and a difficulty in determining the speciation of mercury (gaseous vs.

particulate) and the uncertainties associated with the fact that these surfaces are not real

life and therefore cannot accurately duplicate every aspect of deposition. Surrogatesurfaces and water surface samplers have been used to measure mercury at a hardwoodforest site in Michigan, over wetlands in Florida and over arid lands in south central New

Mexico. Surrogate surfaces have also been used to observe dew over wetland vegetation

in Florida and to measure particulate mercury on rooftops in Tokyo.

Micrometeorological methods have been used to measure fluxes for several differentsurface types. The modified Brown ratio (MBR) method provides relatively accurate

results but is complicated by the fact that it requires measurements of a second gas in

addition to mercury. The MBR method has been used over wetlands in Quebec and

Florida, forests and their background soils within the Walker Branch Watershed, youngpine plantation in Tennessee, boreal forest floor in Sweden, grassland sites in the US,

Austria and Switzerland and pavement surfaces in Indiana. The flux measured whenusing micrometeorological methods is higher than flux measured in enclosure methods

that may encounter condensation building up inside the bag during the nighttime.The REA method is a relatively new method that has been used for flux measurement

of RGM over snow in Alaska, modified soil surfaces in Sweden, hardwood forest in

Connecticut and over a cornfield in Minnesota. The REA method is attractive since it

requires measurements at only one height and for only one gas. However, its accuracycan be decreased by small gradients.


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The AER method has been used to measure total gaseous mercury fluxes over a rice

paddy field in Korea, elemental mercury fluxes over a snow surface in Nunavut, anagricultural field in Ontario and subalpine grassland sites in Austria and Switzerland. The

AER method requires measurements at various heights, thereby increasing the amount of

data analysis required.

An additional measurement, known as the

222

Rn/Hg

0

method, has recently been usedfor the measurement of fluxes of elemental mercury at a grassland site in Seebodenalp,

Switzerland. It was used during periods with a stable nocturnal boundary layer and was

found to be an effective method for the measurement of elemental mercury fluxes in

situations where the atmospheric conditions are non-turbulent, the fluxes are small, or thesurface is highly heterogeneous.

In the measurement of mercury, two kinds of denuders are used: tubular and annular:

1) Tubular denuder consists of 6 mm quartz tubes coated with KCl. During sampling, the

denuders are electrically heated to approximately 450C to avoid water vaporcondensation. The sampling flow rate is of 1 L/min. Analysis is made using thermal

desorption and CVAFS detection (Cold Vapor Atomic Fluorescence Spectrophotometry).

The denuders is heated to 450

o

C and purged with N2 in order to remove any traces ofmercury associated with the coating procedure. Mercury released from the denuder iscollected on a gold trap, which then is analyzed using CVAFS. The detection limit is of 5

pg/m3for a 24 hours integrated sample.

2) Annular denuder consists of a 15 mm outer diameter quartz tube with an inner,enclosed 8 mm tube. Air is pulled through the space between the two tubes. Both the

inner surface of the outer tube and the outer surface of the inner tube are coated with KCl.

The sampling flow rate is of 5-10 L/min. In the analysis step the denuder is heated to

5000C, which converts the adsorbed RGM to elemental mercury vapor, which is pre-concentrated on a gold trap. The gold trap is then analyzed using the normal desorption

and CVAFS detection procedure. The detection limit for a measurement with 2 hours

sampling time is under 2-3 pg/m

3

.

Hg0 measurements

The gaseous elemental mercury vapor exits in the largest portion in the atmosphere

(95-99%), making the measurement techniques crucial to protect the welfare of the publicand the environment. Measurements of elemental mercury are conducted by using CEMS

(continuous emission monitoring systems) as of the beginning of the third millennia. The

measurement of elemental mercury is done by gold trap. Gold traps consist of a wafer

thin ceramic tube, which has been carefully crafted to achieve minimum thermal inertia.The tube has been packed with pure gold and supports a heating coil on its outside. The

mercury gold trap reveals sharp and high peaks and an excellent long-term stability. As

air is used for purging the trap during the heating step, possible contaminations areoxidized and swept off thus preventing from passivation.The primary reason why gold traps work is because mercury sticks to gold at room

temperature. A flue gas used in the measurement of elemental mercury is carrying

gaseous mercury (mostly elemental mercury and mercuric chloride). In order to convert

the other mercury species into an elemental form, there is a use in thermo-catalyticconverter that uses wet chemical reactions that converts mercuric chloride to elemental

mercury vapor. Commonly used reagents to accomplish the conversion to elemental


19/34

mercury were Stannous Chloride, Sodium Tetrahydroborate and Ascorbic Acid. Due to

the presence of reactors and tubings associated with wet chemistry, first generations ofthe CEMS suffered from problems related to tubing pluggage, corrosion and inefficient

conversion of mercuric chloride to mercury.

Once solely elemental mercury is present in the flue gas (typically between 5 0C and

35

0

C), it is then pulled through the trap, the gas continues to flow through the trap but themercury stays behind since it got trapped on the gold. After the mercury is captured, the

gold trap is rapidly heated to approximately 2000C. As a result, the mercury is released as

a gas (thermal desorption). The gaseous mercury is then swept by the flow of purified

mercury free air (composed of a very high content of nitrogen gas) into the optical cell ofthe detector, which measures the mercury in a UV Atomic Absorption Spectrometry

(AAS) operating at 253.7 nm. It is essential that the trap is free of mercury before sampling. This is ensured by acleaning step, which is automatically performed. To keep particles out of the system andto protect the trap from passivation, an easily replaceable filter is installed upstream the

sample inlet. The filter is made of low-interactive material; the filter membrane has a

porosity of 0.45 m and is made of PTFE. It is possible to control sample volume basedon the expected mercury concentration. An illustration of the measurement of elementalmercury is shown in figure 10.

Figure 10 Process flow diagram of the measurement of elemental mercury

Hg+2/ Hg2+2 (HgII) measurements

In spite of the fact that atmospheric mercury mostly consists in an elemental form, the

remainder is mostly attributed to particulate and gaseous HgII species.

Gaseous HgII species, termed Reactive Gaseous Mercury (RGM), are approximately 105more soluble than elemental mercury, mostly composed of HgCl2 and HgO in minute

amounts (pg/m3). This fact strongly influences the extent of removal from the atmosphere


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and subsequent deposition to the biosphere, which may affect marine aquatics once

deposited and methylated into toxic methyl mercury.Measurements of RGM are more complicated than of elemental mercury that uses

gold trap. Due to the minute amounts of mercury, there is a use in an apparatus called

denuder. The denuder is an apparatus that separates gases and aerosols (over a given

diameter) based upon the difference in diffusion velocity between gases and aerosolparticles. Usually, a denuder apparatus contains tubes with selective internal wall coating

that removes the gaseous compounds at the wall (IUPAC Compendium of Chemical

Terminology, 1997). In general, an adsorbent is used to trap reactive polar gases that

diffuse along its surface. In measurement of RGM, the chosen sorbent must solely sorbs

the reactive form of mercury, Hg+2 and Hg2+2 and not the elemental mercury. The most

common sorbent that is used in this case is 2M of potassium chloride solution (KCl) dueto its high tendency to absorb the RGM species from the gas stream to its surface.

However, the use of KCl is subject to high maintenance when used for extended periods

of time.

A third, less used method of RGM measurement (after tubular and annular denuders)

is called Mist Chamber (MC) technique that was developed in 1995 by Lindberg andStratton. In this technique, air is drawn through a Pyrex glass chamber of 100 ml total

volume containing 30 ml diluted HCl solution. Part of the MC solution is dispersed as a

fine aerosol, by a nebulizer (a device for producing a fine spray of liquid) inside thechamber. A hydrophobic filter at the top of the MC separates the droplets from the air

and allows the liquid to drain back into the chamber. SnCl2 and CVAFS detection analyze

HgII species adsorbed in the MC solution after reduction to elemental mercury. The air

sampling flow rate is 10-15 L/min and the detection limit for a 6-hour sample is 1 pg/m 3.

In the measurements of RGM, annular denuders are found to be more suitable for

mercury monitoring efforts than tubular denuders since the improved surface area can

facilitate larger sampling times that allows sufficient amounts of samples collected.

Figure 11 Annular denuder Figure 12 Mist Chamber in ambient air at theFlorida Everglades


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Hgp measurements

The total mercury content of airborne particles (TPM) in ambient air is in the range ofless than 1 pg/m3to some hundreds of pg/m3. The low concentrations level makes

adequate quantification difficult. The traditional procedure is to sample particles on fiber

or membrane filters. The filters are made of glass fiber and quartz. To remove larger

particles to the upstream of the filter, collection time should be longer (around 24 hours)and with a relatively larger ambient air flow rate (10-30 L/min) compared to the

measurement of RGM. Following sampling, the filters are placed into acid digestion (20

mLs of a 10% dilution of concentrated nitric acid (HNO3)with a concentration of 1.6M)

followed by digestion of the filter in a vessel made of Teflon for 20 minutes at 160C and4.7 atmospheres that uses CEM computer that controls microwave unit. The mercury

forms in the nitric acid solution is then oxidized by bromine monochloride (BrCl) and isleft overnight. The mercury content of the digestion solution is determined by standardmethods, using SnCl2 for reduction (of Hg

II to an elemental form) and pre-concentration

via gold amalgamation followed by CVAFS or CVAAS (Cold Vapor Atomic Absorption

Spectrometry) detection. The method allows large air volumes to be sampled in a

relatively short time thereby collecting enough mercury to improve analysis accuracy.The disadvantage of this traditional method of sampling particles on fiber or membrane

filters is the risk of contamination in combination with sample handling. The analysis

procedure requires a fair amount of time and involves treatment with chemicals that arenot totally free of mercury, hence limiting the detection limit of the method.

Another method to measure particulate mercury is the Miniaturised TPM-traps. This

new technique does not require treatment with chemicals, no manual sample transfer or

sample handling and has a low total cost per sample. Hence, the risk of contamination islow. In contrast to the previous technique, the airflow of the sample is limited to only 4-5

L/min. These traps serve as both particulate trap and pyrolyzer that converts any mercury

form to a gaseous elemental form. The trap consists of a small quartz glass fiber filter

supported in a small quarts glass tube. After sampling the device is analyzed by pyrolyzisfollowed by either amalgamation, thermal desorption or CVAFS as the detection step.

The detection limit for a 24-hour measurement is close to 1 pg/m3.


22/34

MEASUREMENT DATA OF ATMOSPHERIC MERCURY

Using the measurement techniques previously described, measurements ofatmospheric mercury are conducted across the world to monitor its levels. Mercury

interacts, transforms and transports in the atmosphere from very short (e.g.12 days in the

spring time in a gaseous divalent form) to very large time and space scales (12 years

lifetime of elemental mercury). Therefore, multi-scale atmospheric models are needed tounderstand mercury cycling in the atmosphere. During the last decade, a number of

numerical regional models and a few global models with varying degrees of complexity

have been developed for the transport and transformation of mercury in the atmosphere.

Nevertheless, the complete understanding of mercury sources is not fully understood. Inorder to improve the quantitative environmental fate component of the risk assessment

for mercury and mercury compounds, more and better mercury emissions data are

needed. Measured mercury data near sources of concern, as well as a better quantitative

understanding of mercury chemistry in the atmosphere are essential for future predictionof atmospheric mercury. Examples of future measurement fields include:

Aqueous oxidation-reduction kinetics in atmospheric water droplets

Physical adsorption and condensation of divalent mercury gas to ambientparticulate matter

Photolytic reduction of particle-bound divalent mercury by sunlight Convincing evidence that gas-phase oxidation of mercury is insignificant

From figure 9 it was concluded that Asia, North America and Europe emitted the

largest quantity of atmospheric mercury at the end of the 20 th century. Figure 13 showsthe 1990 Global Emission Inventory Activity of atmospheric mercury.

Figure 13. Global anthropogenic emission of atmospheric mercury in 1990 (Dastoor and

Larocque, 2003)

The three major source areas are Europe, China and North America but, There are also

significant emissions in South Africa. The total global anthropogenic emission for 1990,according to EPA, was 1881 tons/year which approximately 33% was from Europe, 38%

from China and Japan and 14% from North America. More recent global emissions are


23/34

being compiled and they show that the geographical distribution is changing. However,

the total global emissions of atmospheric mercury are not declining. Figure 14 shows thedistribution of the total gaseous mercury on the surface in 1997 at the North Hemisphere

(NH), the South Hemisphere (SH) and globally.

Figure 14. Monthly mean total gaseous mercury surface average air concentrations in

1997 (Dastoor and Larocque, 2003)

Figure 14 shows an average seasonal cycle in Northern and Southern Hemispheres with

higher concentrations in fall and winter due to the increase in coal combustion to provideelectricity and heat. Conversely, during the spring and summer there are lower

concentrations, as the weather gets warmer.

From table 2 previously presented, the major anthropogenic sources of atmospheric

mercury were medical waste incineration (26%), municipal waste collection (22%),electric utility boilers (22%) and power & heat generation plants (13%). Figures 14-16

illustrate the emission of elemental mercury, RGM and particulate mercury into the

atmosphere across the US in 1990, respectively:


24/34

Figure 13. Elemental mercury emissions (tons per year) from anthropogenic sources at

the US in 1990 (EPA: 1997)

Figure 14. Reactive gaseous mercury emissions (tons per year) from anthropogenic

sources at the US in 1990 (EPA: 1997)


25/34

Figure 15. Particulate mercury emissions (tons per year) from anthropogenic sources at

the US in 1990 (EPA: 1997)

From observation of figures 13, 14 and 15, it is evident that the eastern side of the US

contains more point sources of atmospheric mercury emission. This is due to the presence

of coal-burning power plants that generate electricity that are mostly at the northeastern

side of the US. Most of the emission of reactive gaseous mercury and particulate mercuryare due to medical waste incineration facilities and municipal waste collections.

The EPA model, IEM-2M, was presented to the congress in 1997. The model divided

the US into Western and Eastern halves along the line of 90 degrees west longitude. IEM-

2M is composed of two integrated modules that simulate mercury fate using massbalance equations describing watershed soils and a shallow lake. The results of these

terrestrial and aquatic models were used to predict mercury exposure to hypothetical

humans through inhalation, consumption of drinking water, and ingestion of soil, farmproducts (e.g., beef products and vegetables), and fish. These models were also used to

predict mercury exposure in hypothetical fish-eating birds and mammals through their

consumption of fish. The model compared pre-industrialized atmospheric mercury

emission with industrialized ones. It was estimate that pre-industrial (startingapproximately around the 17th century) atmospheric mercury levels were roughly one

third of current levels. Table 3 presents the inputs to the IEM-2M models on both the

Western and Eastern sides of the 90 degrees west longitude, comparing the emission of

mercury from the pre-industrial and the industrial eras. Once the input to the model hasbeen inserted, an atmospheric model for mercury transportation can be made.


26/34

Table 3. Inputs to IEM-2M model obtained from atmospheric mercury measurements

As expected, once the industrial era started, all measurement parameters across the US

have increased considerably. From the atmospheric modeling analyses of mercurydeposition and on a comparative basis, a facility located in a humid climate has a higher

annual rate of mercury deposition than a facility located in an arid climate. Precipitation

removes various forms of mercury from the atmosphere and deposits mercury to thesurface of the earth. Of the species of mercury that are emitted, divalent mercury is

predicted to generally deposit to local environments near sources. Elemental mercury is

predicted to generally remain in the atmosphere for longer periods until atmospheric

conversion to divalent species occurs.Given the simulated deposition efficiencies for each form of mercury air emission

(elemental mercury 1%, divalent mercury vapor 70%, and particulate mercury 38%)

the relative source contributions to the total anthropogenic mercury deposited to the

continental U.S. are strongly and positively correlated to the mass of emissions inoxidized form. This oxidized mercury occurs in both gaseous (Hg+2) and particulate (Hgp)

forms. While coal combustion is responsible for more than half of all emissions of

mercury in the inventory of U.S. anthropogenic sources, the fraction of coal combustion

emissions in oxidized form is thought to be less than that from waste incineration andcombustion. The true speciation of mercury emissions from the various source types

modeled is still uncertain and is thought to vary, not only among source types, but also

for individual plants as feedstock and operating conditions change. With further research,


27/34

it may be possible to make a confident ranking of relative source contributions to

mercury deposition in the continental U.S. Table 4 shows a comparison of mercuryconcentrations between rural and urban areas.

Table 4. Summary of measures mercury concentrations in rural and urban areas

Table 4 shows that the measured U.S. atmospheric mercury concentrations are generally

very low. The dominant form in the atmosphere is vapor-phase elemental mercury,

although close to emission sources, higher concentrations of the divalent form may bepresent due to its strong ability to dissolve in water. Small fractions of particulate

mercury and methyl mercury may also be measured in ambient air. The formation of

methyl mercury in the atmosphere still remains a great unknown. In rural areas, airborneparticulate mercury is typically 4% or less of the total (particulate + gas phase) mercuryin air

From the modeling analysis and a review of field measurement studies, it is concluded

that mercury deposition appears to be ubiquitous across the continental U.S. and at, or

above, detection limits when measured with current analytic methods. Due to the use ofatmospheric models to detect atmospheric mercury, it was discovered that of the total

amount of elemental mercury vapor that is emitted, about 1 percent (0.9 metric tons/yr)

may be atmospherically transformed into divalent mercury by tropospheric ozone and

adsorbed to particulate soot in the air and subsequently deposited in rainfall and snowfallto the surface of the continental U.S. The vast majority of emitted elemental mercury

does not become part of the global mercury cycle due to the fact that it does not diffusevertically to the free atmosphere. However, nearly all of the elemental mercury vapor

emitted from other sources around the globe also enters the global cycle and can bedeposited slowly to the U.S. Over 30 times as much elemental mercury vapor is deposited

from these other sources than from stationary point sources and area sources within the

U.S.


28/34

ENVIRONMENTAL FATE OF ATMOSPHERIC MERCURY

Mercury is emitted to the environment by both anthropogenic and natural processes.Due to its chemical properties, mercury is thought to move through various

environmental compartments, possibly changing form and species during process.

Mercury, like other elements such as nitrogen, sulfur and carbon, has a cycle that

describes its environmental fate. The mercury cycle has been studies by many scholarsover the years, and the understanding of it continues to undergo refinement. The

movement and distribution of mercury in the environment can be confidentially described

in general terms, since not all its details are agreed due to lack of information. Currently

it is estimated that about half of total anthropogenic mercury emissions eventually enterthe global atmospheric cycle. The remainder is removed through local or regional cycles.

An estimated 5 to 10 percent of primary RGM emissions are deposited within 100 km of

the point of emission and a larger fraction on a regional scale. Elemental mercury that is

emitted may be removed on a local and regional scale to the extent that it is oxidized toRGM. Some elemental mercury may also be taken up directly by plants and trees.

However, most elemental mercury that is not oxidized will undergo long-range transport

due to its insolubility in water. In general, primary RGM emissions will be deposited on alocal and regional scale to the degree that wet deposition processes remove the solubleRGM. The wet and dry deposition phenomena will be discussed in this section. Figure 16

below illustrates the mercury cycle, illustrating the natural and anthropogenic sources of

mercury in the environment.

Figure 16. Global mercury cycle schematic (Bullock, 2000a)


29/34

From figure 16 it can be seen that the atmospheric mercury is continuously

transforming to different forms and deposited at land and aquatic sources. Of the totalamount of global divalent mercury vapor that is emitted, about 70 percent (36.8 metric

tons/year) deposits to the surface through wet or dry processes within the U.S. The

remaining 30 percent is transported outside the U.S. or is vertically diffused to the free

atmosphere to become part of the global cycle. Of the total amount of particulate mercurythat is emitted, about 38 percent (10.0 metric tons/year) deposits to the surface through

wet or dry processes within the U.S. The remaining 62 percent is transported outside the

U.S. or is vertically diffused to the free atmosphere to become part of the global cycle.

All three major forms of mercury can be dry deposited and the rates of deposition varydepending upon surface characteristics and meteorological conditions. The dry deposition

velocity of a gaseous pollutant is governed by three factors: turbulent diffusive transport

in the atmosphere, molecular diffusive transport through the quasi-laminar sublayer

(boundary level resistance) at the ground surface and uptake by the surface or plants.Elemental mercury is relatively inert, therefore it is likely that elemental mercury

deposits with a small dry deposition velocity. Due to high background concentration of

elemental mercury, the total dry deposition of mercury is sensitive to dry depositionvelocity of elemental mercury. Both particulate and gaseous divalent mercury (RGM) arethought to dry deposit at large rates, where measurable concentrations of these mercury

species exist. The deposition velocity of particulate mercury is dependent on atmospheric

conditions and particle size.

Wet deposition is often parameterized through the precipitation rate and the washoutratio, which is the ratio of the mercury concentration in rainwater divided by the mercury

concentration in air. Scavenging of mercury by precipitation varies considerably. It is

generally accepted that elemental mercury is not susceptible to any major process ofdirect wet deposition. Precipitation is the major way the gaseous divalent mercury is

readily expected to be collected. Due to its higher solubility, divalent mercury is thought

to wet deposit at much higher rates than that of elemental mercury vapor. The washoutratio used for divalent mercury vapor is based on an assumed similarity between divalentmercury and nitric acid. Particulate mercury is also subject to wet deposition due to

scavenging by cloud microphysics and precipitation. Roughly half of particulate mercury

may be scavenged by atmospheric water.

Transport and transformation of mercury in the environment

Atmospheric emissions are a major concern with respect to the mercury entering the

environment. Mercury that is released to the atmosphere is transported, transformed, and

deposited back to the earths surface as can be seen from figure 16. The distance ofmercury transport depends upon the chemical form of the emitted mercury, the height of

the emissions, the chemical and physical processes, and the atmospheric conditions (e.g.,wind speeds, precipitation, composition of oxidizing and reducing species). Among the

three forms of atmospheric mercury, an atmospheric transport is likely to be the primermechanism by which elemental mercury is distributed throughout the environment.

However, there is an indirect pathway by which elemental mercury vapor released into

the atmosphere may be removed and deposited to the earth's surface. Chemical reactions

occur in the aqueous phase (cloud droplets) that both oxidize elemental mercury todivalent mercury and reduce the divalent mercury to elemental mercury. The most


30/34

important reactions in this aqueous reduction-oxidation balance are thought to be

oxidation of elemental mercury with ozone, reduction of divalent mercury by sulfite(SO3

-2) ions, or complexation of divalent mercury with soot to form particulate divalent

mercury:

Hg0(g) Hg0

(aq)

Hg0(aq) + O3(aq) Hg+2(aq)

Hg+2(aq) + soot/possible evaporation Hg+2

(p)

Hg+2(aq) + SO3-2

(aq) Hg0(aq)

The RGM produced from oxidation of elemental mercury by ozone can be reducedback to elemental mercury by sulfite ion. However, the oxidation of elemental mercury

by ozone is a much faster reaction than the reduction of RGM by sulfite. The RGM(aq)

produced would then be susceptible to atmospheric removal via wet deposition. The third

reaction, however, may transform most of the RGM(aq) into the particulate form, due tothe much greater amounts of soot than mercury in the atmosphere. The resulting RGM (p)

can then be removed from the atmosphere by wet deposition (if the particle is still

associated with the cloud droplet) or dry deposition (following cloud dropletevaporation).

Airborne mercury can also undergo chemical reactions, which may significantly affect

the lifetime of atmospheric mercury. Particulate mercury is likely to be transported and

deposited at an intermediate distances depending on aerosol diameter or mass.Before mercury is deposited back to the earths surfaces, a series of complex physical and

chemical transformations of mercury can take place in the atmosphere as was illustrated

in figure 6. The transformations include the equilibriums of mercury species among

gaseous, aqueous and solid phases, the aqueous phase chemistry of mercury, and thegaseous phase chemistry of mercury. Often the equilibrium relationship between the

partial pressure in the gas phase and the mole fraction in the liquid phase can be

expressed by a straight-line Henrys law equation at low concentrations:pA=H*xA

Where:pA is the partial in the gas, His the Henrys law constant in atm/mole fraction for

the given system and xAis the mole fraction in the liquid phase.

Henrys law governs the gas-liquid equilibriums of mercury species. The Henrys lawconstant for elemental mercury is quite small in the atmosphere, which lowers its partial

pressure; therefore it can travel for long distances. Although elemental mercury is the

dominant constitute of atmospheric mercury, the cloud-water concentration of elemental

mercury in Henrys equilibrium with gaseous elemental mercury range only from 1.3 to5.3 10-14 M, which is relatively low compared to the total dissolved and absorbed reactivegaseous mercury. In contrast, the Henrys law constants for Hg(OH)2 and HgCl2 are 4 and

6 orders of magnitude bigger than that for elemental mercury. Solid mercury species arenot likely present in atmospheric water given the low atmospheric concentration of

mercury. The quantitative description of gas-solid equilibriums is rather limited due to

uncertain nature of atmospheric particles.

In the aqueous phase, mercury compounds often remain as not disassociatedmolecules, and the reported solubility values reflect this. Most organomercurics are not


31/34

soluble and do not react with weak acids or bases due to the low affinity of the mercury

for oxygen bonded to carbon. Hydroxy methyl mercury, CH3HgOH, however, is highlysoluble due to the strong hydrogen bonding capability of the hydroxide group. The

mercuric salts vary widely in solubility. For example HgCl2 is readily soluble in water,

and HgS is as unreactive as the organomercurics due to the high affinity of mercury for

sulfur.The phenomenon of re-emission of deposited mercury results most significantly from

the evasion of elemental mercury from the oceans. In this process, anthropogenically

emitted mercury is deposited to the oceans as reactive gaseous mercury and then reduced

to volatile elemental mercury and re-emitted. According to one estimate, this processaccounts for approximately 30% (10 Mmol/year) of the total mercury flux to the

atmosphere. The slow release of mercury from terrestrial sinks to freshwater and coastal

waters will likely persist for much longer, though, effectively increasing the lifetime of

anthropogenic mercury further. This may be particularly significant considering thatsurface soils currently contain most of the pollution-derived mercury of the industrial

period. However, recently published studies indicate that mercury in soil may be reduced

and re-volatilized, in that the capacity of soils to isolate airborne mercury must bereconsidered. Thus, re-emissions of anthropogenic mercury will contribute to long-terminfluences on the global biogeochemical cycle for mercury.

Environmental fate of mercury in marine environment

There are a number of pathways by which mercury can enter the freshwaterenvironment: RGM and methyl mercury from atmospheric deposition (wet and dry) can

enter water bodies directly; RGM and methyl mercury can be transported to water bodies

when it is bound to suspended soil or attached to dissolved organic carbon; or RGM andmethyl mercury can leach into the water body from groundwater flow in the upper soil

layers. Once in the freshwater system, mercury can remain in the water column, be lost

from the lake through drainage water, re-volatilize into the atmosphere, settle into thesediment or be taken up by aquatic biota. After entry, the movements of mercury throughany specific water body may be unique. Mercury in the water, in the sediment, and in

other aquatic biota appears to be available to aquatic organisms for uptake. However, not

all mercury compounds entering an aquatic ecosystem are methylated by sulfur reducing

bacteria, and demethylation reactions as well as volatilization of dimethyl mercurydecrease the amount of methyl mercury available in the aquatic environment. There is a

large degree of scientific uncertainty and variability among water bodies concerning the

processes that methylate mercury.

Bacterial methylation rates appear to increase under anaerobic conditions, hightemperatures and low pH a relationship exists between methyl mercury content in fish

and lake pH, with higher methyl mercury content in fish tissue typically found in moreacidic lakes. In addition, anthropogenic acidification of lakes appears to increase

methylation rates, as well increased quantities of the mercuric species, the proper biologiccommunity, adequate suspended soil load and sedimentation rate. Methyl mercury

accumulates in fish through the aquatic food web. Nearly 100% of the mercury found in

fish muscle tissue is methylated. Methyl mercury appears to be primarily passed to

planktivorous and piscivorous fish via their diets. Larger, longer-lived fish species at theupper end of the food web typically have the highest concentrations of methyl mercury in


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a given water source. Most of the total methyl mercury production ends up in biota,

particularly fish. Methyl mercury appears to be efficiently passed through the aquaticfood web to the highest tropic level consumers in the community (for instance the

piscivorous fish). At this point, humans and fish-consuming wildlife may ingest fish

contaminated with methyl mercury. The mechanisms for the behavior of methyl mercury

and the increased biomagnifications are still unknown.This bioaccumulation of methyl mercury in fish muscle tissue occurs in water bodies

that are remote from emission sources and seemingly immaculate, as well as in water

bodies that are less isolated. Methyl mercury appears to pass from the gastrointestinal

tract into the bloodstream more efficiently than the divalent species. In humans,consumption of fish contaminated with methyl mercury is particularly dangerous for

pregnant women, infants and young children. Once consumed into the body, the human

body degrades slowly methyl mercury (with half-life of 45 to 70 days). It builds up in the

brain and is secreted in breast milk. Potential consequences of methyl mercury includedeath, kidney toxicity, cardiovascular toxicity, immunotoxicity and neurotoxicity (the

development of the fetus is the most sensitive). Figure 17 illustrates the mercury cycle in

the environment.

Figure 17. Environmental fate of mercury

By considering the current global mercury budget and estimates of the preindustrialmercury fluxes, it was estimated that total emissions have increased by a factor of

approximately 4.5 since preindustrial times, which has subsequently increased the

atmospheric and oceanic reservoirs by a factor of 3. The difference is attributed to local

deposition near anthropogenic sources. Although the estimated residence time of


33/34

elemental mercury in the atmosphere is about 1 year, the equilibrium between the

atmosphere and ocean waters results in a longer time period needed for overall change totake place for reservoir amounts. Therefore, by substantially increasing the size of the

oceanic mercury pool, anthropogenic sources have introduced long-term perturbations

into the global mercury cycle. Modeled results estimated that if all anthropogenic

emissions were ceased today, it would take about 15 years for mercury pools in theoceans and the atmosphere to return to pre-industrial conditions. However, The Science

Advisory Board concluded that it could take significantly longer.

CONCLUSION

Mercury is an element that is naturally present in the environment. Mercury is

persistent pollutant that is released to the atmosphere mostly by coal-burning power

plants. Atmospheric mercury is characterized into three types: elemental, reactive

gaseous and particulate. Elemental mercury is insoluble in water and can be transportedfor long distances, whereas the reactive gaseous form is highly soluble in water and as a

result lands in nearby areas to the pollution sources via wet deposition.

When enters to aquatic systems, mercury is transformed by sulfur and iron reducingbacteria to an organic form methyl mercury, the most toxic form of mercury. In aquaticsystems, fish eat different foods that were contaminated with methyl mercury, staying in

their body for long periods of time. Human consuming fish containing methyl mercury

experience a biomagnification of methyl mercury in their body, resulting in severe

diseases and eventually - death. Up to date, it is still unknown how mercury istransformed form its inorganic form to the organic one, in both aquatic and atmospheric

matrices.

Currently, mercury is monitored in many areas around the world, trying to predict ormodel future transportation or behavior of the pollutant in the atmosphere. Understanding

the movement of mercury in the atmosphere will help to alert populations with different

health risks that are associated with mercury. Future research in the investigation ofmercury as an atmospheric pollutant can focus on understanding to greater details how toanalyze obtained data within three or four years since measurement dates instead of eight

or nine years as it stands now. In addition, more accurate equipments generating less

uncertainty per given measurement will greatly help in the future understanding and

prediction of atmospheric mercury pollution.


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