The law of indestructibility of matter is the greatest contribution of the eighteenth-century French chemist Antoine Lavoisier. But he is also known for being the man who proved the existence of oxygen through a famous mercury experiment jointly carried out with English chemist Joseph Priestley.
From its elemental form of silvery-white liquid, mercury changes into solid at minus 38.83°C. It evaporates at temperatures as low as 25°C, and starts boiling at 356.73°C. As proved by Lavoisier’s law, it can change forms in chemical reactions, but never be destroyed. In 1911, the Dutch physicist Heike Kamerlingh Onnes from Leiden University managed to cool mercury to the temperature of liquid helium, 4°Kelvin, and discovered that its resistance disappeared. This established mercury as the first superconducting substance, and Kamerlingh Onnes would be awarded the Nobel Prize in physics for this research two years later.
Most of the mercury found in the atmosphere is elemental mercury vapour (Hg0). It is also found in two oxidized states (combined with oxygen): mercurous (Hg2 2+) and mercuric (Hg2+), with properties changing according to its state.
By natural processes, mercuric compounds get removed from the atmosphere faster than elemental mercury. They get bound to airborne particles and are removed by precipitation or in some cases even without it. Elemental mercury vapour, on the other hand, has a strong tendency to remain in the atmosphere for long and is not vulnerable to precipitation or getting deposited in the earth’s crust.
Most of the mercury found in water, soil and sediments are in the form of inorganic mercury salts; in living organisms, plants and animals, it is in organic forms like methylmercury, a mercury–carbon compound. Methylmercury, a compound of organic methyl group (three hydrogen and one carbon atoms) bonded to an atom of mercury, is formed from inorganic mercury by microbial action in water bodies – lakes, rivers, wetlands and the ocean.
Mercury moves in the air in many ways. Primarily, it gets transported from atmosphere to water and land as wet deposits. In aquatic ecosystems, it can dissolve or exist in particulate form, besides undergoing chemical changes, depending on the other particles it comes into contact with. Bottom sediments in water bodies serve as reservoirs for mercury where it can bind with contaminants. There, it can get retained for a very long time, before slowly releasing into the water and the atmosphere.
Once any living being, plant or animal, is exposed to mercury, it enters the organism and gets accumulated in the body tissues. This can occur by inhaling gaseous mercury, or by ingesting food or water contaminated by it. In aquatic environments, mercury accumulates in the sediments or binds with the sediments, where it is consumed by small algae known as phytoplankton, which in turn is eaten by larger organisms known as zooplankton. Zooplanktons are consumed by small fish which are in turn eaten by larger fish. From there, mercury moves to fish-eating birds, reptiles and human beings. In scientific parlance, this process becomes known as bioaccumulation.
Once mercury enters the body cells of living beings, it sets off a dangerous chemical process termed methylation. This happens in the water or the sediments too, wherein mercury transforms from its inorganic form to methylated organic form by combining with carbon molecules, assisted by microorganisms like bacteria. Through a series of chemical reactions, mercury can be converted by bacteria in the sediments to methylmercury, a form that is incredibly toxic to humans and wildlife. The body cells of fish absorb the methylmercury from the water as it passes over their gills and upon feeding on other organisms. So its concentrations increase in the bigger fish. Consequently, the larger predator fish usually have higher concentrations of methylmercury from eating contaminated prey.
The production and accumulation of methylmercury in the freshwater ecosystems is an efficient process for accumulating mercury which can then be ingested by piscivores – fish-eating birds, animals and people. Only after being methylated can mercury start moving up from organism to organism through the food chain. The emissions from coal-based power plants are one of the sources of methylated mercury, as proven by a plethora of research on the impacts of mercury emissions from coal-based power plants over the last half century. Simple studies of the mercury levels in the hair and blood of people living near coal-based power plants to researches on its manifestations as illnesses and long-term health effects have all proven beyond doubt the seriousness of the issue.
Of course, this is in addition to living organisms directly inhaling mercury vapour. With global energy consumption going up due to economic growth, coal remains the mainstay in meeting energy needs. Because it is burned in unsafe ways, however, coal will continue to emit dangerous gases like mercury at shockingly high levels for the foreseeable future as the coal corporations are backed by the political leadership in their respective countries. The 2018 Global Mercury Assessment by UN Environment reported that thirty-eight per cent of all mercury emissions came from the coal sector. It sources the largest quantity of mercury, and virtually all the mercury used in its processes is eventually released into the environment.
Besides methylmercury, inorganic mercury too is absorbed by living organisms and bioaccumulates, albeit in a lower quantity and at a slower pace. The removal of methylmercury does take place inside the organisms but is a gradual process that can take years because of its long ‘half-life’, the time taken for the disintegration of radioactive materials to half its original quantity. All isotopes of mercury are either radioactive or observationally active, meaning they can be predicted to be radioactive. Recently, pharmacokinetics studies have proved that methylmercury has a half-life of fifty days in the human body, meaning it takes fifty days for a single gram of mercury in the body cells to denude to half a gram.
This excerpt from ‘Heavy Metal: How a Global Corporation Poisoned Kodaikanal’ by Ameer Shahul has been published with permission from Pan Macmillan India.