What is Mercury?
Mercury is a naturally occurring element of remarkable qualities, a volatile liquid metal (at normal temperatures) that easily becomes a gas. Mercury (chemical symbol Hg, atomic number 80) conducts electricity, and can be used to directly measure temperature and pressure. Its applications can be found everywhere in human society:
Mercury is one of the group of elements known as heavy metals. Many of these (including lead, cadmium, and selenium) are toxic to living things. Mercury too can kill living things, from bacteria to human beings. In particular, it can be converted into an organic form, methyl mercury, which is especially toxic.
Presence of Mercury in the Environment
Mercury is a naturally occurring mineral that can be found throughout the environment. Mercury forms can be found as the elemental metal or in a wide variety of organic and inorganic compounds.
There is a constant biogeochemical cycle of mercury. This cycle includes:
Natural and Manmade Emissions of Mercury
Mercury is released to environmental media (air, water, soil) by a wide variety of natural processes and human interventions. Worldwide, natural emissions of mercury from physical and biological processes may equal or exceed manmade emissions.
The global anthropogenic emission rate for mercury is estimated to be 650 metric tons (650,000 Kg) annually, while natural emissions could be as much as 1020 metric tons (1,020,000 Kg). While the totals are quite uncertain, natural emissions may comprise about 50 percent of them.
Even if all manmade emissions of mercury were eliminated, a significant natural discharge to the environment --through both biological and physical processes-- would persist.
Mercury in Nature: Chemical and Biology
Significant amounts of mercury are directly released from the earth's crust by the process of degassing. Both natural and manmade emissions are modified by biological processes into forms more directly harmful to human beings. Mercury is somehow less toxic in its volatile form, mercury-zero, than in organic compounds like methyl mercury or inorganic salts (mercury-two).
Common bacteria of the soil and water have adapted to the presence of mercury. They have developed methods to detoxify its organic compounds and salts to the elemental form of mercury zero. Mercury zero, however, is volatile, and thus can spread throughout the environment through secondary biological mechanisms. Once it reaches inland aquatic environments, mercury zero can again accumulate and be transformed into methyl mercury, the toxic form that bioaccumulates in fish, animals, and humans. This toxic transformation can occur from any of three causes:
Mercury in the Atmosphere
At least 99 percent of all mercury in the atmosphere exists in the elemental gaseous form of mercury-zero. Much of the remainder is mercury-two, which is water soluble and the form most often deposited by rainfall.
Most of the mercury in the atmosphere comes from natural degassing from water and rocks. Major manmade discharges to the atmosphere come from:
Less common manmade sources of atmospheric mercury include:
One important local source of atmospheric mercury is the incineration of medical wastes.
Mercury in the Water
Mercury can enter water by many different routes and processes:
Manmade discharges may result from industrial processes, such as:
Trouble in the Waters: Methyl Mercury
In the aquatic environment, mercury can be:
Methyl mercury is the form of mercury most available and most toxic to biota (including zooplankton, insects, fish, and humans). This form of mercury is easily taken up by biota and bioaccumulates in their tissues. Unlike many other fish contaminants, such as PCBs, dioxin, and DDT, mercury does not concentrate in the fat, but in the muscle tissue. Thus, there is no simple way to remove mercury-contaminated portions from fish that is to be eaten.
The local aquatic environment largely determines how much available mercury takes the accessible toxic for of methylmercury. Research suggests that sulfur-using bacteria are a major source. The extent of biomethylation may depend upon such factors such as pH, (i.e. alkalinity), available sulfur sources, and dissolved organic materials.
Mercury in the Soil
Human agricultural activities may release mercury to the soil through direct applications, such as:
Once in the soil, mercury compounds may undergo the same chemical and biological transformations found in aquatic systems. Elemental mercury (mercury zero) will form various compounds with the chloride and hydroxide ions of soils. The exact result will depend upon the pH, salt content, and other characteristics of the soil.
For soil, like water, both inorganic chemistry and the actions of living things will affect the formation and degradation of organic mercuric compounds. For example, elevated levels of chloride ions will reduce methylation of mercury in river sediments, streams and soils. In contrast, increased levels of organic carbon and sulfate ions will increase methylation in sediments.
Mercury in the Food Chain
Benthic (bottom-dwelling) invertebrates may face exposure to mercury released directly from sediments. They may also release mercury bound in the sediment by direct ingestion, having thus entered the food chain, mercury bioaccumulates as bottom-dwellers are consumed by others.
Toxic methyl mercury can inflict increasing levels of harm upon species near the top of the aquatic food chain. These likely victims include: predatory fish such as ocean swordfish; fish-eating birds such as loons, cormorants, pelicans, ospreys, and eagles; and humans.
We can be certain of the harm resulting from methyl mercury contamination of fish, even if the exact degree of risk is yet uncertain. Reasonable preventive actions that will safeguard humans from methyl mercury poisoning should also protect birds that eat freshwater fish. Controlled feeding studies suggest that the methyl mercury threshold dose for adverse effects to wildlife is the same or higher than for humans.
The Worldwide Mercury Cycle
There is a worldwide cycling and recycling of mercury through the environment, called the biogeochemical cycle of mercury. The cycle has six steps, as shown in the adjoining diagram:
Elemental metallic mercury ("mercury zero"), released to the atmosphere in vapor form, can be transported very long distances. Eventually, wet and dry deposition processes return it to land or water surfaces in the form of compounds.
Wet depositions of mercury by precipitation (rain, snow, etc.) is the primary method of mercury removal from the atmosphere (perhaps 66 percent of the total). Mercury can also be removed from the atmosphere by sorbtion of the vapor onto soil or water surfaces.
The particular form of mercury and its compounds strongly influence the movement and partitioning of mercury among surface waters and soils. Ninety seven percent of all the gaseous mercury dissolved in water is the elemental form -- "mercury zero". Volatile forms of mercury, such as the metallic liquid and dimethyl mercury, will evaporate into the atmosphere. Solid forms particulates in the soil or the water column, and once in the water column are transported downward to the sediments.
What makes Mercury Run?
Sorbtion of nonvolatile forms of mercury onto soil and sediment particulates is the central process that determines the distribution of mercury compounds in the environment. This sorbtion process varies according to the organic matter content of the soil or sediment.
Inorganic mecury sorbed onto particulate material is not easily desorbed. This means that freshwater and marine sediments will be important storehouses for inorganic forms of mercury, and that leaching from soils will play a minor role in mercury transport.
Where the soils are rich in humus, surface runoff will be an important route moving mercury from soil to water.
There are processes that will release the sorbed mercury from particulates: