Mercury

by Andy Mu


Mercury is a silver-colored metal and one of the chemical elements (symbol Hg, atomic number 80). Mercury, nicknamed quicksilver, exists as a silvery white liquid at room temperature and will boil at 357.25 C under atmospheric pressure. The thermal expansion of mercury metal is relatively large, and between 0 and 100 C, the thermal expansion approximates that of gases. The specific gravity of metallic mercury is 13.54616 at 20 C. In terms of vapor pressure, it rapidly increases with temperature. For example, in 1 cubic meters of air there are 14 mg of Hg at 20 C and 2.42 g at 100 C, if the air is sa turated with mercury vapor. The thermal conductivity of mercury at 0 C, on the other hand, is only 2.2% that of silver and the electrical conductivity is only 1.58% that of silver. The surface tension of liquid mercury is 484 dynes/cm, or six times greater than that of water in contact with air. Thus, mercury does not wet surfaces with which it is in contact. In addition, since mercury is a noble metal, it is soluble only in oxidizing solutions. In its solid state, mercury is as soft as lead and in all states mercury and its compounds are extremely toxic. With some metals (gold, silver, platinum) mercury forms solutions called amalgams (mercury alloys). Although no one knows exactly when or who discovered mercury, it is k nown that the ancient Chinese, Egyptians, Greeks, Hindus, and Romans were familiar with the metal. Most mercury used by people is found in an ore called cinnabar. Today, it is most commonly found as the sulfide, HgS, frequently as the red cinnabar and l ess often as the black metacinnabar. Mercury is used in many different applications, but because of its tremendous toxicity, there have been severe restrictions on its usage. Since mercury itself is a chemical element, there is not much to say about the chemical composition of this metal. In addition to being diamagnetic (due to the electron configuration 5d10), it occurs naturally as a mixture of the following isotopes in t he proportions shown: 202 (29.8%), 200 (23.1%), 199 (16.9%), 201 (13.2%), 198 (10.0%), 204, (6.8%), and 196 (0.15%). The signs and symptoms of neurological damage in mercury poisoning is predominant. Chronic mercury poisoning (mercurialism) produces fi ve main symptoms: numbness, staggering gait, constriction of visual fields (tunnel vision), garbled speech (dysarthria), and tremor. Autopsies of mercury poisoning victims have shown degenerative lesions in the liver, heart muscle fibers, and tubules of the kidneys. The reason mercury and its compounds are so extremely toxic is their biochemical nature. In high doses, methylmercury (a mercury compound) is cytoclastic, or destructive to cells, while in low doses it decreases the number of cells in some organs by decreased cell proliferation. This is not surprising as it is a well known and established fact that mercury has a high affinity for SH groups on proteins and biochemically is a very effective enzyme inhibitor. However, since there are many mo re SH groups available to bind mercury in the body than there are molecules of mercury, there must be some selectivity of the site of action. As for morphogenic effects of mercury, cell proliferation rate could be decreased by methylmercury binding to de oxyribonucleic acid (DNA) bases or DNA polymerases (or other SH-containing nuclear enzymes, cell membranes, or mitochondrial enzymes (energy metabolism). Mercury may also alter microtubule assembly as a principal mechanism of subcellular injury as microt ubules are a main component in cell proliferation, and are important in intraneuronal transport, in cell secretory exocytosis and in the structure and function of sperm flagella. Important deposits of the mineral ore that mercury is extracted from are found in Spain, Italy, California, Nevada, Oregon, Texas, Mexico, Canada, Brazil, Peru, China, Japan and the Soviet Union. The ore found in Spain has the highest mercury content, w ith an average of 0.5-1.2% Hg, sometimes as much as 10% Hg. A less common ore is the mercury(I) chloride found in Texas. Some of the main uses of mercury are for making thermometers, barometers, diffusion pumps, mercury-vapor lamps and advertising signs , mercury switches and other electrical apparatus, making some pesticides, making mercury cells, caustic soda and chlorine production, making dental preparations, making antifouling paint, making batteries and making catalysts. Mercury thermometers have been phasing out in the past decades as the potential harmful effects of mercury have become common knowledge. However, in many laboratories and such, mercury thermometers are still prevalent. These thermometers are created based on the large thermal ex pansion factor of mercury. Thus, as the temperature increases, the difference in volume of mercury is noticeable and distinguishable. Mercury switches is an innovative device that utilizes the dual nature of mercury. Since mercury is both a liquid meta l, it is able to flow and conduct electricity. Thus, a mercury switch connects two ends of a circuit when the mercury has flowed to both sides. This is particularly useful in cases where a switch needs to be turned on automatically given some condition. One last use that can be expounded upon is the making of mercury cells. The primary dry-cell battery consists of a zinc anode, a cathode of mercuric oxide (HgO) mixed with graphite, and an electrolyte of potassium hydroxide (KOH). With carefully purif ied materials and balanced amounts of ZnO and HgO, the cell has very low self-discharge and makes efficient use of the active materials. Despite the many products and uses listed above, mercury is primarily absent from consumer items and is mostly concentrated in research laboratories and facilities. The work of these research laboratories have been able to come up with some ways of cleaning up mercury spills and reducing the outbreak of chronic mercury poisoning. The main process in "making" mercury is metallurgical extraction, which is basically the separation of mercury metal from ores. The process, in its most basic explanation, uses an airstream in a rotary kiln where the ore is heated. In this process, the sulfide ore is roasted, or reduced to metallic mercury (1) by the addition of iron (2), or of quicklime (3).
HgS + O2 --> Hg + SO2

HgS + Fe --> Hg + FeS

4HgS + 4CaO --> 4Hg + 3CaS + CaSO4
The metal vapor is carried over with the combustion gases and condenses in vertical clay pipes having open bottoms and standing in water. The mercury metal collects under the water in a layer of condensation products called mercurial soot, which also contains mercury salts, soot, and tar, and from which the metal is separated by filtration through a cloth. The soot is then pressed with quicklime, whereby additional liquid mercury is obtained. Final purification of the mercury metal is best done by vacuum distillation, by dropping it into a 1.5-m layer of 5% nitric acid or by pressing it through leather. As one can see, besides the mercury itself, there is very little by-product and what is produced as by-products is not toxic itself. In addition to this extraction method, there have also been processes developed to extract and detoxify the mercury that has been absorbed into the environment or other locations that pose a threat to human or animal life. A new chemical process has been studied to clean up a mercury-polluted soil by using acidic potassium iodide (KI) solution. The acidic KI solution was passed through a column packed with 9.8g polluted soil at flow rate of 25 ml/h. After 15 fraction volumes, the mercury content decreased from 113. 5 to 26.2 mg. The leachate from the column test containing Hgl24 was then treated with granular activated carbon. Yet another process, developed by GE Research & Development targets mercury over other metallic contaminants in a variety of tainted waste streams. It is able to isolate and recover mercury in whatever form it is present. The closed-loop process incorporates three basic steps: reagent-based extraction; reduction and removal of extracted mercury; and regeneration and recycle of the extractio n re agent. During mercury extraction, which is carried out in agitated vessels or open heap-leach piles, the impacted materials are mixed with an aqueous, halide-based extraction reagent. The extracted mercury remains in the aqueous phase, which is sent on to the next step of the process, while the leached soil or sediments are dewatered, rinsed and either backfilled or disposed offsite as hazardous waste. Thanks to its ability to extract and recover mercury from mixed-waste streams, the Gemep process can remediate mercury-laden soils, and waste originating from the manufacture of paints, pesticides, and chlor-alkali batteries. A patented, halide-based reagent is the key to the Gemep extraction process. An oxidant in the reagent readily oxidizes a wide range of mercury compounds, to liberate elemental mercury. This mercury is then free to be complexed with the halide ions in the reagent, to form a water- soluble mercurichalide complex. Once this complex is in the aqueous phase, a number of chemical reduction techniques -- including conventional metals reduction and electrowinning -- may be used to reduce it and recover the target contaminant as elemental mercury. The processes in making other products vary greatly, but they generally combine the me rcury with another chemical to form a compound whose properties suit the products intentions or purpose. One such example is the creation of vermilion, a color of paint created by manufacturers using mercuric sulfide. As stated before, mercury batteries use a compound called mercuric oxide (HgO). The uses of mercury is forever being researched and explored, as is the safe creation, use and disposal of this toxic chemical. This paper has presented the basic background, sources, uses, processes, and chemical compositions of mercury. As technology continues to expand in the field of material science, so will the knowledge and uses of mercury.

[Back to the Toxic Materials Page]