As anyone who’s ever broken a thermometer can attest, mercury is a fascinating substance. It also has an interesting history.
Named after the fleet-footed Roman messenger of the gods, mercury has been used for more than 2,000 years. Mercury ore cinnabar has even been found smeared on Neolithic skulls. Its first recorded mention is a reference by Aristotle in the fourth century B.C., a time when the silvery-white, heavy metal was used in religious ceremonies. Spanish miners used mercury to process gold ore for ancient Rome; when their mercury supply ran out, gold production dropped and set in motion the decline of the Roman Empire.
Today mercury poses a serious threat, this time environmental. Human beings around the world cause mercury to enter the atmosphere primarily when they burn coal or incinerate waste. Studies from Sweden and Florida suggest that mercury also evaporates from landfills, but only 0.0001 per cent. Groundwater contamination from old dumps is of concern but combustion is the major path via which mercury pollutes the globe.
Mercury rarely exists in a free state in nature; it’s recovered from red mercuric sulphide in geologically recent volcanic rocks. The world inventory of mined mercury is estimated at 600,000 tonnes, stored mostly in states of the former Soviet Union. (Mercury has not been mined in Canada since 1975.)
Scientists estimate that man-made mercury releases are two- to four-fold greater than those of nature. The typical mercury content of lakes has increased up to seven-fold since industrialization. Acid rain dissolves lake-rock and releases mercury to water. (Granite contains about 0.2 ppm mercury.) With a melting point of minus 38.87C, metallic or elemental mercury (the form used in thermometers) readily vaporizes and can be transported long distances. The vaporization rate of mercury doubles with every 10C temperature increase and its residence time in the atmosphere is up to three years. Not surprisingly, mercury condenses and accumulates in cold climates such as the Arctic or mountain regions. Though far away from industrial activity, these areas have become repositories of the world’s mercury emissions.
Mercuric chloride, a simple salt, is the predominant form in many surface waters. Almost all the mercury found in animal tissues is methyl mercury, a water-soluble toxic organic mercury compound. Persistent and non-biodegradable, mercury biomagnifies up the food chain. In the Arctic as few as six steps stand between ingestion by microscopic organisms and consumption by human beings. Pregnant women can pass mercury from contaminated fish to their unborn children. Toxic effects include central nervous system and kidney damage, mental retardation, blindness and cerebral palsy.
Mercury’s many commercial applications include electrical apparatus (such as fluorescent lighting), electrolytic preparation of chlorine and caustic soda, measurement/control instruments and dental amalgams. Mercury was once used to make certain latex paints mildew-resistant but this practice was banned in 1991. Mercury from discarded alkaline batteries constituted a terrible problem until industry eliminated it from formulations in 1990. Mercuric oxide batteries (the “button” type commonly used in cameras, hearing aids and watches) have been replaced by silver-oxide and zinc-air cells that contain only trace amounts of mercury (up to 25 milligrams per cell).
With mercury reduced in batteries, the U.S. EPA predicted that mercury in American municipal solid waste would decline from a high of 709 tonnes in 1989 to 173 tonnes by 2000. The next large opportunity to eliminate mercury from municipal waste is fluorescent light tubes. Again, U.S. data quantifies the problem — more than half a billion light tubes are discarded south of the border each year. Manufacturers have reduced the mercury in these lamps but a certain level of the unique metal is essential to maintain desirable properties. Assuming increased sales and decreased mercury content, U.S. EPA estimates that 11.6 tonnes of mercury in light tubes will be discarded in 2000.
Landfilling accounts for 82 per cent of fluorescent tube disposal, incineration 16 per cent and recycling just 2 per cent. There’s comfort in reports that waste incinerators that inject activated carbon into the flue gas prior to the particulate matter control system have achieved mercury removal efficiency rates above 95 per cent. U.S. Clean Air Act requirements are believed to have reduced mercury emissions from municipal waste incinerators overall by about 90 per cent from the 1990 baseline (i.e., from 55 tonnes per year to 4.4 tonnes).
Government reports indicate that mercury emissions from fluorescent light tubes disposed in landfills and incinerators are minimal. However, this observation is misleading since most tubes are broken before they arrive at the disposal site and their mercury escapes. Clearly, mandatory recycling of spent fluorescent light tubes is needed in a system that is separate from curbside collection. This is an example of a material problem for which extended producer responsibility is the solution. The tubes should be placed on deposit, collected in a return-to-retail program and carefully transported to recycling facilities that can capture the mercury vapours and prevent them from contributing further to a daunting global problem.