Mercury in the Environment

The Traprock, Vol. 2, December 2003, pp. 33 - 36

Nikki La Bella and Amy Hilliker

Mercury is an element that has many uses but when its presence in the environment exceeds its natural amount, it can be considered harmful. Through years of research and environmental studies, scientists have discovered that mercury has risen to abnormally high levels. This increase, in turn, has caused negative effects on both humans and the environment alike, such as health hazards and the disruption of ecosystems. Fortunately, projects such as METAALICUS and the Clean Air Act are being undertaken to better control the presence of mercury in the environment and its damaging impact.

Mercury has been under strict scrutiny in recent years due to the fact that its presence in the environment is detrimental to wildlife and humans alike. It is widely dispersed throughout the environment, and can therefore affect several factors. Although it has many uses and unique properties, mercury has caused more harm than help to our society over the years. It is a concern that the presence of mercury in the environment has negative effects.

Mercury is a toxic element that can be found in the environment. This neurotoxin accumulates in the atmosphere and, depending on the chemical type, is released into the environment. Most of the mercury released into the environment is inorganic mercury, which can have neurobehavioral effects. However, the form that we are dealing with is methylmercury. Methylmercury occurs when inorganic mercury undergoes methylation (gains a methyl group to its molecular structure) to create its most toxic form. Methylation is a biogeochemical process attributed in part to environmental variables that influence the development of certain methylating microbial populations and the accessibility of mercuric ions. Methylmercury has an easier time forming in aquatic environments when microbial populations are high. Usually these populations are high under low pH and anoxic conditions. Organic matter supports the growth of organisms and decreases the level of oxygen, which also helps to form methylmercury. Methylmercury creation varies during different seasons due to changes in the nutrients and oxygen level, temperature, and water composition of organic matter. Studies have shown that methylmercury production was higher during the warmer seasons than the colder seasons (NOAA, 2003).

There are several ways in which mercury enters the environment. Sources of mercury occur both naturally and anthropogenically, and can be naturally found in many locations such as sediment outcrops and clouds. The burning of fossil fuels can deposit methylmercury particles into clouds, creating acid rain. Acid rain has a low pH compared to natural rainwater, which has a pH of 6. Acid rain low pH is an environment that is conducive to the methylation process, so mercury can thrive there and also easily be transported. Mercury can also be found in the Earth crust, and its geochemical cycle begins with volcanic activity. The magma pushes through sedimentary rocks and combines with mercury vapors to form the sulfide cinnabar (Anonymous, 2003c). Cinnabar is mainly found near the earth surface where volcanic rocks are located. It is a reddish color depending on the purity of the mineral. If there is an underground source, mercury can be found in geothermal hot springs. Certain microorganisms are accustomed to living in high-mercury environments such as geothermal springs. These microorganisms in the hot springs easily transform mercury to methylmercury. In general, hot areas with high metal concentration and sulfide content will have a higher concentration of mercury than other areas. Other naturally occurring mercury sources are soils, undersea vents, mercury-rich geological zones such as volcanoes and hot springs, oceans and freshwater, plants, forest fires, sea-salt spray, and meteoric dust (The Green Lane, 2002).

Since the industrial age, human- made mercury sources have come into existence. One of the major sources of mercury attributed to humans is the burning of fossil fuels, such as coal, which emit large amounts of mercury into the environment

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(Nater and Grigal, 1992). According to Snzopek and Goonan, coal was responsible for 87 percent of mercury emissions in 1996. The combustion of oil and fossil fuels also contributes to mercury emissions. Different kinds of waste, such as medicinal and municipal waste, are also to blame for mercury emissions.

Mercury also gets in the environment through a process known as the Mercury Cycle. This is responsible for the circulation of mercury throughout the environment. The first step of the cycle involves the release of mercury in the gas form from rock, soils, water, volcanoes, and human activity (Childress et. al., 1996). These emissions then move throughout the atmosphere. A portion of the mercury binds itself to mineral particles in rocks or in the soil and just remains in the atmosphere, non-reactive. Some mercury is deposited right away, but the remaining mercury combines with the leftover mercury circulating in the atmosphere. After the mercury circulates through the atmosphere, it is deposited in two forms: dry and wet. Dry deposition occurs through settling and scavenging of mercury, and eliminates the more non-soluble form of mercury in the atmosphere (Anonymous, 2003b). Wet deposition occurs through types of precipitation, such as snow and rain, and it occurs more rapidly than dry deposition (Anonymous, 2003b). These deposition patterns occur in both terrestrial and aquatic settings.

During subsequent sedimentation, mercury is pulled down by gravity and settles either on land or in underwater assemblages. Sediment deposition may be the main source of mercury in most aquatic environments. Water sources have perfect conditions for sedimentation (Childress, et.al., 1996). The process of sedimentation traps mercury particles between layers. When sediment is transported through streams, the mercury trapped within is taken along with it to its next destination. Sedimentary rocks also undergo erosion, exposing layers with trapped mercury. Bits and pieces of rock and mercury can then be carried or washed away to their next destination. The pieces can then be deposited and sedimentation can occur all over again. As aforementioned, methylation occurs, and mercury binds to sulfides. However, sulfates limit methylation, but promote the growth of methylating microbes that transform the mercury compound to methyl mercury. This specific methylation process is known as biomethylation. This process thrives among low pH and oxygen levels with high amounts of organic matter.

Since mercury has a high solubility and low vapor pressure, it can diffuse out of the sediments or be resuspended into the water (Krabbenhoft and Rickert, 2003). From there, the mercury can return to the atmosphere through volatilization, a process that involves the vaporization or evaporation of mercury. Mercury can also begin to accumulate in the food web. These two characteristics regarding high solubility and vaporization allow mercury to be easily transported and enter the food web. The mercury cycle continues on. Figure 1 is a pictorial

representation of the mercury cycle.

Figure 1. The mercury cycle in an aquatic environment. (Anonymous, 2003a)

Unfortunately, the presence of mercury in the environment has become such a huge concern because of the health hazards studies have shown it poses. Exposure to high levels of mercury can lead to severe neurological disorders, such as mercury poisoning. This can deteriorate the nervous system, impair hearing, speech, vision, and gait, cause involuntary muscle movements, corrode skin and mucus membranes, and makes it difficult to chew and swallow (Krabbenhoft and Rickert, 2003). Mercury poisoning can progress and possibly even lead to death. It may also create problems for pregnant mothers, leading to birth defects in their

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children.

One of the main causes for mercury poisoning has been the consumption of exposed fish. Going back to the mercury cycle, many fish types have had an active part in perpetuating the bioaccumulation of mercury in the cycle. The primary carriers of mercury are the swordfish, shark, and ahi tuna. When water of any kind is contaminated, methyl mercury cysteine (the form of mercury scientists have discovered in fish) accumulates in the tissues of certain fish and the mercury content of these tissues increases. Those who consume that fish put themselves at risk, whether they are another fish or a human. However, humans are more at a risk than fish because the bio-magnification of mercury is so great at that level because it has passed through the whole food web to the humans at the top. Biomagnification is the intensification of the mercury in each succeeding trophic level. Cases of mercury poisoning have been prevalent throughout the decades. In the 1940 , there was a massive case of mercury poisoning in the town of Minamarta, Japan. Seventy nine people died and at least 600 people felt severe effects from mercury poisoning (Byrne, 1992). In more recent news, Brazil held a summit at Rio de Janeiro to discuss the harmful effects that they will inevitably have to face due to mercury poisoning. Mercury from mines has escaped into the Amazon River and its tributaries and is getting deposited into the fish in waterside communities (Bryne, 1992).

The disruption of certain ecosystems, such as ponds and lakes, is a consequence of the dangerous forms of mercury. In places where eutrophication already exists, mercury flourishes. The low oxygen and pH levels and the warmer temperatures make it favorable for the methylation process to occur. Thus the unhealthy eutrophication goes hand in hand with increased levels of mercury in aquatic systems. Finally, as an ultimate effect from ecosystem disruption and high levels of mercury, the fish industry is being affected. Dangerous levels of mercury cause health problems as aforesaid and will eventually lead to a lower fish demand, cheaper prices, and a greater supply of mostly contaminated fish.

In response to mercury concerns both in the environment and health matters, some legislation has been enacted. The Environmental Protection Agency has passed the Clean Air Act and under this law, the emissions from municipal and medicinal waste incinerations should decrease. By the year 2005, the EPA has predicted waste containing mercury would be eliminated with regards to waste management (Sznopek and Goonan, 1996). Finally, the Chlorine Institute plans to reduce the amount of mercury in their chlor-alkali industry by fifty percent by the year 2005 (Sznopek and Goonan, 1996).

Other projects have been instituted to study mercury increases and their effects. One such program is METAALICUS, the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States. This program charts the route mercury takes by adding mercury isotopes to the lakes in Ontario (Krabbenhoft, 2003). Another project, called ACME, is being undertaken to examine the association between mercury, dissolved organic carbon, and sulfur (Krabbenhoft, 2003). Both projects aim to put an increasing amount of significance on the effects caused by mercury.

Despite mercury unusual characteristics and ample uses, it has presented more problems than solutions to the environment. Mercury has poisoned organisms leading to fatalities. Ironically, the most negatively affected group has been humans even though they are the main cause of the increases in mercury levels recently. In order to control the high levels of anthropogenically caused mercury, more research projects need to be undertaken. To alleviate the harmful effects mercury has caused, mercury intricate cycles need to be respected and understood.

References

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