Mercury (Hg)

Introduction

Mercury (chemical symbol Hg) is a silvery white metal that belongs to Group IIB (the zinc group) of the periodic table. It has an atomic number of 80 and an atomic weight of 200.5. It has seven stable isotopes and a density of 13.6 grams per cubic centimeter. Also known as quicksilver, mercury has a melting point of -38.87° Celsius, making it the only metal that is liquid at normal room temperature. It boils at a temperature of 356.9° Celsius and has a constant rate of expansion throughout the entire range of temperature of the liquid. Mercury alloys with most metals and is a good conductor of electricity.

Mercury is a relatively scarce element on Earth, accounting for only 3 parts per billion in crustal rocks. It is found both as free liquid metal and, more commonly, as the sulfide mineral cinnabar (HgS). It is generally found in areas of past volcanic activity.

Mercury compounds are formed from mercury with either a +1 or +2 oxidation state. The most common mercury (I) compound is mercury chloride (Hg₂Cl₂), and the most common mercury (II) compounds are mercury oxide (HgO), mercury bichloride (HgCl₂), and mercury sulfide (HgS). (The Roman numerals refer to the valence state of the mercury.)

Mercury and its compounds have been used in agriculture, industry, and medicine. Because of their highly toxic nature, however, care must be used when applying or using such substances. Efforts have been made to limit or phase out the use of mercury in many applications to protect human and environmental health.

Health Concerns

Mercury is also an extremely toxic element that can be easily released into the environment when mined, processed, or used. Mercury vapors can be inhaled, and mercury compounds can be ingested or absorbed through the skin. Symptoms of mercury poisoning include nausea, irritability, memory problems, headache, muscle weakness, and tremors. It can result in long-term neurological damage and death.

Inorganic mercury compounds can be converted by bacteria into highly toxic organic mercury compounds such as methyl mercury. These organic mercury compounds become concentrated as they move up the food chain to higher-level organisms such as fish, birds, and humans, a process known as bioaccumulation. Because of this the disposal of inorganic mercury waste can become a major environmental hazard.

History of Human Use and Regulation

Mercury has been known since at least the second century BCE. Chinese alchemists used mercury in futile attempts to transform the base metals into gold. Mercury was also used in ancient Egypt. Cinnabar, the red ore mineral of mercury, has long been used by many indigenous peoples as an important pigment. By Roman times the distillation of mercury was known, and a mercury trade between Rome and the rich Spanish cinnabar mines was well established. Beginning with the Renaissance and the scientific revolution in the sixteenth and seventeenth centuries, mercury became important for use in measuring devices such as thermometers and barometers.

The major modern industrial, medicinal, and agricultural uses of mercury were developed in the nineteenth and twentieth centuries. One of the element's main industrial applications was long in the industrial production of chlorine and caustic soda. Mercuric sulfate and mercuric chloride have been used industrially to produce vinyl chloride, vinyl acetate, and acetaldehyde. Pharmacological uses of mercury compounds include mercury bichloride and mercurochrome as skin antiseptics, and mercurous chloride (calomel) as a diuretic. It has also been used in dry cell batteries, paints, dental amalgams, gold mining, scientific measuring instruments, and mercury vapor lamps. Some organic mercury compounds, such as phenylmercury acetate, have been used in agriculture as fungicides to control seed rot, for spraying trees, and for controlling weeds. However, many former uses have been banned or limited in the United States and other countries due to environmental and human health concerns.

The toxicity of mercury compounds has been known since ancient times due to the poisoning of early cinnabar miners. Later, in the early nineteenth century, the mental effects that mercury had on felt makers gave birth to the phrase “mad as a hatter.” Deadly incidents of mass mercury poisoning have continued to gain attention in modern times. In Japan, the release of mercury waste from an industrial plant into the waters of Minamata Bay resulted in the deaths of forty-three people during the 1950s and early 1960s. In 1972, wheat seed treated with methyl mercury fungicide was used by farmers in rural Iraq, leading to at least 460 deaths.

One popular application of mercury is in fluorescent lamps, including compact fluorescent lamps (CFLs) or lightbulbs. CFLs are much more energy efficient than traditional incandescent lamps, which made them increasingly popular in the twenty-first century, but environmentalists often raised concerns about their disposal, especially if broken. The issue of mercury pollution helped drive some manufacturers to embrace LED technology instead of CFLs.

Many countries have enacted strict regulations on mercury to protect public and environmental health. In the US, the Mercury Market Minimization Act of 2008 forbid the sale, distribution, and export of elemental mercury and banned all US exports as of January 1, 2013. The US also signed the United Nations' Minimata Convention on Mercury, developed in 2013 to coordinate international efforts against mercury emissions. Following that treaty, in the European Union, regulations adopted in 2017 and enacted in 2018 banned most products containing mercury and other uses of the material, such as dental treatment. Coal-fired power plants remained a major source of mercury emissions worldwide into the 2020s, however. In the US, and Environmental Protection Agency (EPA) rule released in 2024 included stricter limits on such emissions.

Obtaining Mercury

The primary mercury deposits of the world are found in Spain, China, central Europe, and Algeria. Spain is estimated to have the greatest reserves, almost 60 percent of the world’s total. Mercury is also recovered through the recycling of batteries, dental amalgams, thermostats, fluorescent lamp tubes, and certain industrial sludges and solutions.

Bibliography

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