Mercury (Hg) is a naturally occurring element found throughout Earth’s environment. It is a silvery-white liquid metal at room temperature, distinguishing it from all other metals. This element can exist in three different states of electrical charge. Its distinctive properties allow it to vaporize into a colorless, odorless gas.
Forms of Mercury in the Environment
Mercury exists primarily in three forms: elemental, inorganic, and organic.
Elemental mercury, also called metallic mercury, is the most common form and appears as a shiny, silver liquid. It readily vaporizes at room temperature, forming an invisible, odorless vapor that can be easily inhaled. While poorly absorbed through ingestion, inhaled elemental mercury vapor can quickly enter the bloodstream and affect the central nervous system.
Inorganic mercury compounds are found as white powders or crystals. These compounds are formed when mercury reacts with other elements like chlorine or sulfur. Inorganic mercury is water-soluble and can cause kidney damage and gastrointestinal issues if ingested, but unlike elemental mercury, it does not easily cross the blood-brain barrier.
Organic mercury, predominantly methylmercury, is the most toxic form due to its ability to accumulate in living tissues. Methylmercury is formed when microorganisms convert inorganic mercury, particularly in aquatic environments. This organic form is highly persistent in tissues and is readily absorbed, posing significant risks to the brain and developing fetuses.
Natural Release and Global Movement
Mercury is naturally released through various geological and biological processes. Volcanic eruptions are a significant natural source, releasing mercury as a gas into the atmosphere. Forest fires also contribute to natural mercury emissions.
The weathering of mercury-containing rocks and geothermal activity also release mercury into the environment. These natural processes contribute to the background levels of mercury found in soils, water, and air. Once released, mercury enters a complex biogeochemical cycle, moving between the atmosphere, land, and oceans.
In the atmosphere, elemental mercury can travel globally before being oxidized and deposited back to Earth’s surface through wet or dry deposition. Once deposited in aquatic and terrestrial ecosystems, microorganisms play a significant role in transforming inorganic mercury into methylmercury. This conversion is an important step in mercury’s environmental journey, as methylmercury is the form that readily enters food webs.
Human Activities and Mercury’s Environmental Presence
Human activities significantly amplify mercury levels in natural environments. Coal combustion, particularly from coal-fired power plants, is a leading anthropogenic source of atmospheric mercury emissions. Mercury is present as an impurity in coal, and when burned, a significant portion is released into the air.
Artisanal and small-scale gold mining is another major contributor, where mercury is intentionally used to extract gold from ores, leading to substantial releases into air, water, and soil. This practice accounts for a large percentage of global anthropogenic mercury emissions. Industrial processes, including cement production, chlorine manufacturing, and the production of other metals like zinc and steel, also release mercury into the environment.
Waste incineration, especially of mercury-containing products and municipal or medical waste, further adds to environmental mercury contamination. These human-induced releases contribute to widespread environmental contamination due to mercury’s long-distance transport through air and water. To address this, the Minamata Convention on Mercury, an international treaty, was adopted to protect human health and the environment by controlling and reducing anthropogenic mercury emissions and releases.
Mercury’s Journey Through Ecosystems
Once mercury enters ecosystems, primarily as methylmercury, it undergoes two ecological processes: bioaccumulation and biomagnification. Bioaccumulation describes the gradual buildup of a substance in an individual organism’s tissues over its lifetime. Organisms absorb methylmercury from their environment, and it accumulates because it is not easily excreted.
Biomagnification refers to the increasing concentration of mercury at successively higher trophic levels within a food chain. As smaller organisms containing mercury are consumed by larger predators, the mercury becomes more concentrated in the tissues of those higher-level consumers. Top predators, such as large fish, birds, and marine mammals, can accumulate significantly higher mercury levels than organisms lower in the food web.
Methylmercury enters aquatic food webs when absorbed by phytoplankton, which are then eaten by zooplankton. Small fish consume zooplankton, and larger predatory fish eat the smaller fish, leading to a progressive increase in mercury concentration up the food chain. Humans who consume contaminated seafood can be exposed to elevated levels of mercury.