Mercury

James Richard Fromm


Mercury is one of the metals known to the Greeks and Romans, as is reflected in its elemental symbol Hg (hydrargyrum). It was a curiosity to them because it is the only metal which is liquid at room temperature. It may also have been used in gold mining. The production then was, and still is, from the ore cinnabar (HgS), either by roasting:

HgS(s) + O2(g) SO2(g) + Hg

or by oxidation with lime (CaO):

4HgS(s) + 4CaO(s) 4Hg + 3CaS(s) + CaSO4(s).

The present world production of mercury is about 9000 tonnes/year. The majority of the mercury used through about 1970 was employed in the chloralkali industry, although uses in this area are now declining due to antipollution measures. There are many other uses for mercury, especially in scientific and electrical apparatus, metallurgical plant processes, antifungicides for seed, and antifungicides for paint.

In the chloralkali industry, the major process is electrolysis of aqueous NaCl solution to produce NaOH and chlorine. The aqueous chloride ion is oxidized at a carbon anode in the reaction

2Cl- Cl2(g) + 2e-

while water is reduced at a mercury or carbon cathode:

2e- + 2Na+ + 2H2O 2Na+OH- + H2(g)

Mercury cells are not the only types of cells possible for this process, but they were in wide use since they have been more economic. The NaOH product, the spent NaCl solutions and other plant effluents carry traces of mercury out with them into the discharge tubes and from there into lakes and rivers. Loss in North America was, in 1970, about 600 tonnes/year in this manner, although for both economic and pollution reasons great efforts are being made to reduce this.

Toxic Effects of Mercury

Mercury toxicity depends very heavily upon the form the mercury is in. Mercury (I) chloride, calomel or Hg2Cl2, has been used in medicine as a purgative for a long time; the fillings of teeth are mercury amalgams with gold or silver. Only 2%, or less, of the inorganic mercury breathed or swallowed is absorbed into the bloodstream, and inorganic mercury constitutes only a minor health hazard. It has long been known to be a hazard, however. In the early days of the Industrial Revolution, mercury (II) nitrate, Hg(NO3)2, was used to soften fur in the making of felt hats. The phrase "mad as a hatter" -- and the Mad Hatter of Lewis Carroll's Alice in Wonderland -- both are from the same source, which is the toxic effect of mercury on the central nervous system, producing mental effects and "hatter's shakes". The workers in the felt hat industry, and also those in mercury smelters, were the only ones constantly exposed to high enough mercury levels to exhibit obvious symptoms.

The different forms of organic mercury, such as methylmercury, CH3Hg, and other organomercury compounds such as the mold preventative phenylmercury acetate used in fertilizers,

C6H5-Hg-OCOCH3, are much more dangerous. Over 90% of the intake of methylmercury, for example, is absorbed into the bloodstream.

Organic mercury compounds are manufactured artificially, but can also arise naturally. Some bacteria, especially those found in the slime muds of rivers, have the ability to slowly convert inorganic mercury compounds into organic mercury. As a consequence, pollution by inorganic mercury can be slowly converted into pollution by the much more dangerous organic forms of mercury.

The organic mercury compounds are concentrated in the food chain; over 90% of the methylmercury in fish is held so tightly to the fish protein that it cannot be removed by freezing, boiling, or frying. The organic mercury enters the small fish by direct absorption through the gills, or by their eating of phyto-plankton containing absorbed mercury. Small fish are eaten by game fish or birds, and the organic mercury compounds are further concentrated -- to the point where the contamination level in pike flesh may be 3000 times as great as in the surrounding water.

The background, or natural, levels of mercury in freshwater lakes and rivers is about 0.03 parts per billion (1 ppb = 0.001 ppm); this mercury appears to arise from the weathering of rocks. The mercury level in the oceans is variable depending upon location but is in the range of 0.03 - 5.0 ppb. Concentration of mercury in general oceanic fish flesh is greater than 0.15 ppm and has been so since at least the 1930s. Levels of mercury in polluted water can be much higher. For example, a mercury-cell chloralkali plant sending effluent to a river had 560 ppm mercury in the silt deposits near the outlet, and 50 ppm in the silt deposits 4 miles downstream. For mercury in fish flesh, the "safe level", U.S.A.-Canada, is 0.5 ppm; Sweden recommends 0.2 - 1.0 ppm and then not more than once a week.

There have been several recorded instances of mercury poisoning, of which the most well-known occurred at Minimata Bay, Japan, in 1952. The mercury source was a plant effluent; as a result, 397 are known to have been affected, of which 68 died; of those affected, 22 were unborn children. The methylmercury was absorbed by eating fish and shellfish from contaminated oceanic water. Levels of mercury in fish flesh in Minimata Bay in 1952 were 5 - 10 ppm. The 1965 instance in Niigata, Japan was a similar case to Minimata Bay; 330 persons are known to have been affected, of which 13 died.

Cases of mercury poisoning occurred in Iraq in 1961, in Pakistan in 1963, and in Guatemala in 1966. In each case over 30 persons were poisoned by flour made from seed grains treated with organomercurial preservatives. In Iraq in 1972, there was a similar case, with several hundred deaths reported. In New Mexico in 1961, a farmer and his family ate a hog fed on garbage and fungicide-treated grain. Three children were seriously affected, with sight damage, hearing damage, or coma. In 1970 Ontario, Canada, banned all fishing in the St. Clair River, Lake St. Clair, and the Detroit River. No clear data are available on numbers affected with no known dead. Levels of mercury in fish flesh from Lake St. Clair, in 1935, were 0.07 - 0.01 ppm; but in 1970, some were 7.0 ppm, average 0.5 ppm. Reported (and questionable) levels in fish flesh from Lake Wabigoon, in 1970, were 24 ppm.


Copyright 1997 James R. Fromm