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 is present at liquid state in room temperature, and is used in thermometers and some electrical switches. Inorganic merc
  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: Hg 0 (elemental mercury), Hg 2+2 (mercurousmercury) and Hg +2 (mercuric mercury). These forms can be organized under threeheadings: metallic mercury, inorganic mercury, and organic mercury. Metallic mercury ispresent at liquid state in room temperature, and is used in thermometers and someelectrical switches. Inorganic mercury compounds occur when mercury combines withelements such as chlorine, sulfur, or oxygen. These mercury compounds are also calledmercury salts. Most inorganic mercury compounds are white powders or crystals, exceptfor mercuric sulfide (also known as cinnabar), which is red and turns black after exposureto light. Organic mercury compounds (or organomercurials) occur when mercurycombines with carbon. In the environment there is a large number of organic mercurycompounds; however, by far the most common organic mercury compound in theenvironment is methyl mercury (or MeHg + ). Methyl mercury is of particular concern because it can build up in certain edible freshwater and saltwater fish and marinemammals 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 1950’s it was established that emissions of mercury to the environment couldhave serious effects on human health. These early studies demonstrated that fish andother wildlife from various ecosystems commonly attain mercury levels of toxicologicalconcern when directly affected by mercury-containing emissions from human-relatedactivities. Mercury in the air may settle into water bodies and affect water quality. Thisairborne 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, orestuaries. 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 underanaerobic 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  ⎯    →    ⎯     ⎯     ⎯    CH  3  Hg (  II  )  X  ( aq ) + H  2 S  ( g )  Where X is a ligand (an ion, a molecule, or a molecular group that binds to anotherchemical 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 aquaticenvironment, 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 itcan quickly adsorb to plankton. Plankton may also be consumed by the next level in thefood chain, or after dying, settle to the bottom of the lake and are incorporated into  bottom sediments. Studies of sediment cores show that younger sediments depositedsince industrialization have mercury concentrations that are about 3-5 times that of historical sediments. Mercury in aquatic environment Most of the mercury entering aquatic environments is Hg +2 . Methyl mercuryaccumulates in fish at levels that may harm the fish and the other animals that eat themvia bioaccumulation and biomagnification. Bioaccumulation is the process by whichorganisms (including humans) can take up contaminants more rapidly than their bodiescan 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 reachtoxic 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 eachlevel of a food chain. This phenomenon occurs because the food source for organismshigher on the food chain is progressively more concentrated in mercury and othercontaminants, thus magnifying bioaccumulation rates at the top of the food chain. Thebioaccumulation 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 fattissues, mercury concentrates in muscle tissues. This implies that mercury cannot befilleted 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, animprint 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 anthropogenicsources. Mercury ore is found in all classes of rocks including limestone, calcareous  shales, sandstones, serpentine, chert, andesite, basalt, and rhyolite. Mercury is released tothe atmosphere mostly by the burning of fossil fuels, which naturally contain mercury.There are three main species of mercury occurring in the atmosphere: elemental mercuryvapor (Hg 0 ), gaseous divalent mercury (Hg 2+2 and Hg +2 ) and particulate mercury (Hg p ).  Elemental mercury (Hg 0 ) Elemental mercury is the primary form of atmospheric mercury, accounting for 90%in the atmosphere. In ambient air, elemental mercury is present globally at concentrationsof the order of 1.5-2.0 ng/m 3 . The fact that the reactivity of elemental mercury in water isvery 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 theconcentration of elemental mercury in the atmosphere has been increasing. It wasestimated that 50-75 percent of this increase srcinates from anthropogenic sources(Expert Panel on Mercury Atmospheric Processes, 1994). Removal of elemental mercuryoccurs by dry or wet deposition after oxidization by ozone (O 3 ) to gaseous divalentmercury, and is removed from the atmosphere by precipitation. The overall organicperoxy compounds or radicals may also occur in the atmosphere. In clouds, however, afast oxidation reaction on the order of hours may occur between elemental mercury andozone, as described by the following reactions: (1)  Hg + hv (254 nm ) →  Hg *(2)  Hg * +   O 2 →  Hg + O 2 *(3) O 2 * +   O 2 → O 3 + O (4) O + O 2 → O 3 (5) O 3 + Hg →  HgO + O 2 (6) O + Hg →  HgO    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) toproduce elemental mercury and an oxygen molecule in an excited state. The speciesresponsible 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 oxygenradical, which also reacts with oxygen to form ozone. Both ozone and oxygen radicalsreact 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. Theoverall reaction is obtained by adding reactions (1) through (5): (7)  Hg + 2 O 2 + hv →  HgO + O 3   Gaseous Divalent Mercury (Hg 2+2 and Hg +2 ) Hg 2+2 and Hg +2 account for about 1-3 percent of total gaseous mercury in theatmosphere (Lindberg and Stratton, 1998). It is believed that the most part of gaseousdivalent mercury consists of Hg 2+2 (mercurous mercury) and Hg +2 (mercuric mercury),  but other divalent mercury species are also possible (like mercury dichloride HgCl 2 ).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 gaseousmercury. Air concentrations of gaseous divalent mercury are likely to be highly related tolocal sources, meteorological conditions and some other pollutants. Particulate Mercury (Hg  p ) Particulate mercury occurs in both gaseous and aqueous phases. It can be formed byphysical adsorption of gaseous divalent mercury to atmospheric particulate matter inaqueous phase as the secondary particulate mercury. It can be emitted directly into theatmosphere from anthropogenic and natural sources. Nevertheless, it is largely consistedof anthropogenic srcin. Background concentration of particulate mercury indicated thatit was a minor constituent – only 0.39 percent of total gaseous mercury. Particulatemercury tends to be dry deposited at significant rates when and where measurableconcentrations of these mercury species exist. Figure 2. Transformation of mercury in air, water and sediments
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