Mercury, a naturally occurring heavy metal, exists in various forms that influence its movement and impact within the environment. Its journey through the atmosphere, water, and land dictates where it ultimately ends up and how it affects living organisms. The unique chemical properties of these forms determine its behavior and pathways.
The Forms of Mercury
Mercury exists in three primary chemical forms, each with distinct characteristics that govern its environmental fate. Elemental mercury, or Hg(0), is a liquid at room temperature with high volatility, easily evaporating into the atmosphere as a vapor. It is commonly found in older thermometers, barometers, and certain industrial processes. Once airborne, elemental mercury can circulate globally for up to a year.
Inorganic mercury encompasses mercury salts, found as Hg(I) and Hg(II). These forms arise when elemental mercury reacts with other elements. Inorganic mercury compounds are less volatile than elemental mercury and are frequently present in soil, water, and air, often adsorbed to particles.
Organic mercury, with methylmercury (CH3Hg+) being the most recognized example, is the most toxic form. This compound is primarily generated by microorganisms in aquatic environments. Methylmercury has a strong tendency to accumulate in the tissues of living organisms, posing a significant risk to ecosystems and human health.
Mercury’s Journey Through the Atmosphere
Mercury enters the atmosphere from natural and human sources, traveling globally. Natural emissions stem from volcanic eruptions, forest fires, and the weathering of rocks and soils. Human activities account for a substantial portion of atmospheric mercury, with coal combustion in power plants being the largest contributor, making up about 42% of emissions. Other significant anthropogenic sources include artisanal and small-scale gold mining, waste incineration, and chlor-alkali plants.
Once released, elemental mercury vapor can remain in the atmosphere for several months to a year, allowing long-range transport by wind. While airborne, elemental mercury can undergo oxidation, converting into oxidized, soluble forms (e.g., Hg(II)) and particulate mercury. These oxidized forms then deposit back to Earth through wet deposition (rain and snow) or dry deposition (settling particles). This cycling between land, water, and the atmosphere, often referred to as the “global distillation” or “grasshopper effect,” allows mercury to be transported from warmer to colder regions, leading to its accumulation in remote areas like the Arctic.
Movement in Water and Land
After atmospheric deposition, mercury continues its journey through water and land. It enters water bodies through atmospheric deposition, runoff from surrounding land, and industrial discharges. In water bodies, mercury can exist dissolved or adsorbed to particles, moving with currents.
Over time, mercury settles into sediments, acting as a sink. Sediments can also re-emit mercury if conditions change. On land, deposited mercury binds to soil particles. Soil serves as a significant reservoir, holding approximately 4.7 million tons in the first meter, three times the amount in oceans. This mercury can be remobilized through erosion, leach into groundwater, or re-evaporate back into the atmosphere, continuing its cycle.
Biological Cycling and Food Web Entry
The transformation of inorganic mercury into methylmercury is a key step in its biological cycling and entry into the food web. This process, known as methylation, occurs in oxygen-poor environments like wetlands and sediments. Anaerobic microorganisms, particularly sulfate-reducing bacteria, convert inorganic mercury into methylmercury. Methylmercury is much more readily absorbed and retained by living organisms than inorganic forms.
Once formed, methylmercury is efficiently taken up by aquatic organisms like phytoplankton, which can concentrate it over 100,000 times. This uptake and retention in an organism’s tissues, faster than it can be eliminated, is called bioaccumulation. As organisms consume others, methylmercury concentrations increase up the food chain, a process known as biomagnification. For instance, small fish consume contaminated plankton, and larger predatory fish then eat the smaller fish, leading to higher levels in their tissues. Top predators, including humans consuming fish, face the highest exposure risks due to this escalating concentration.