Mercury is a neurotoxic heavy metal and has a bad reputation in the public mind. Customers buying lamps are concerned about its effects on their health. However, for light sources such as fluorescent lamps or mercury lamps, mercury is completely indispensable. End-of-life light sources need to be disposed of properly so that they do not become a burden on the environment. Let's talk more about this interesting element.

The roots of the Czech name are unclear [1], a possible Slavic origin is the transliteration of truji (= poison poison). Polish is mercury rtęć, Slovak mercury and Russian ртуть. It may also come from Arabic utarid (planet Mercury), just as other (al) chemical terms come from Arabic - e.g. alcohol or alkali. It has entered many languages as "living silver" from Latin argentum vivum or as "liquid silver" from Greek hydrargyros. In English quicksilver and German Quecksilber, besides silver, speed and mobility, attributes of Mercury, from which the name mercury is derived in many languages, are also evident. Mercury is higany in Hungarian, živa in Croatian, and 汞 (gông) in Chinese.

Mercury, an element with a proton (atomic) number of 80, is adjacent to gold and thallium in the periodic table of elements and is in the twelfth group below zinc and cadmium. Under normal conditions, it is a silvery liquid metal. Mercury has a melting point of -38.8 °C, a boiling point of 356.7 °C and a density of 13.534 g-cm-3. Its specific electrical resistance of 961 nΩ-m is quite large for a metal, similar to that of stainless steel. The thermal conductivity of 8.3 W-m-1-K-1 is one of the lowest for metals.

Light sources

Due to its fluidity and high vapour tension, mercury is one of the few elements with suitable properties for the construction of discharge lamps with good efficiency and durability. The nature of the radiation of mercury discharge depends greatly on the mercury vapour pressure. At low saturation mercury vapour pressures (around 1 Pa), mercury discharge is a source of intense UV-C radiation with a wavelength of mainly 253,7 nm and also 184,9 nm (see Figure 3) with minimum luminous efficiency, which only has to be transformed into the visible spectrum by a suitable phosphor. This type of discharge is used in fluorescent lamps, compact fluorescent lamps, induction lamps, etc. At increasing pressure (around 100 kPa), the proportion of radiation in the visible region and the efficiency of the conversion of electrical energy into light energy (about 50 lm/W) increases significantly, but the red component is completely absent in this discharge as well, and for general lighting purposes it is also necessary to use suitable phosphors (this type of discharge is used, e.g., in The mercury spectrum is supplemented by the radiation of other elements which suitably complement the line spectrum of mercury in the visible region (metal halide lamps). Only at pressures higher than 0,1 MPa is the discharge spectrum continuous, with efficiencies up to 75 lm/W, which is, however, mainly used for special-purpose lamps. At the same time, mercury is relatively insensitive to electrodes and modern phosphors.

The RoHS Directive2 (2011/65/EU) limits the presence of mercury in products to 0.1% by weight for homogeneous materials and lists exemptions - for most types of fluorescent lamps the content is limited to 2.5 to 5 mg per piece, which corresponds to a sphere with a diameter of 0.7 to 0.9 mm. In the past, T12 fluorescent lamps were produced with more than 100 mg of mercury. This was to compensate for the operational loss of mercury in the phosphor and the difficulty of reliably and reproducibly dosing smaller amounts of mercury into the lamp, as well as the loss of mercury during mass production of the lamps to achieve the required lifetime. Once the mercury has been used up, the fluorescent lamp glows a weak pink-orange discharge in neon and argon. Mercury incorporation technologies, in the form of a suitable amalgam, allow mercury to be safely dispensed in the form of a solid alloy in a specified small amount and to maintain optimum mercury vapour pressure in the fluorescent lamp over a wide range of ambient temperatures. For example, indium, bismuth and zinc amalgams are used. Under normal pressure (during manufacture or when the fluorescent lamp is broken), less mercury vapour is released from the amalgam than from the liquid phase. The disadvantage of amalgam fluorescent lamps is the slower start-up of the source and the more difficult start-up at low temperatures (this problem can, however, be solved by using a starter amalgam suitably placed in the light source).

Mercury in the environment

Like other elements, mercury is part of the natural background of our environment. This background increases for several years when large volcanoes erupt, see Figure 4. Of human activities, the largest contributor is the burning of coal for electricity generation. Coal always contains a certain proportion of mercury, depending on the location. This applies to almost all of the industrialised world. Another contribution is the primitive processing of gold ore, which is leached in mercury. The resulting gold amalgam is then burned and the mercury escapes into the atmosphere. This is currently a problem particularly in the Amazon. In our territory, the greatest mercury pollution is in the vicinity of Spolana Neratovice and Spolchemie Ústí nad Labem, where chlorine and sodium hydroxide are produced by electrolysis of brine, and sodium is taken into the mercury cathode. Both companies are preparing to introduce mercury-free membrane technology in approximately 2016. Also polluted is the Skalka dam near Cheb, into which flows the Reslava river, the tributary of which was the site of Germany's oldest chemical plant, Chemische Fabrik Marktredwitz, a traditional producer of mercury and its compounds, closed in 1985 due to catastrophic pollution. Fish caught in the adjacent waters have greatly increased concentrations of mercury in their muscle tissue. Environmental remediation has been or is underway at all three sites.

Mercury poisoning

Acute poisoning by elemental mercury is very rare in our country. Elemental mercury enters the body mainly through the respiratory system and also through the skin. This usually involves accidents with mercury-containing devices, handling of contaminated objects or activities in buildings with unresolved historical burdens, such as older buildings converted to housing. The first symptoms of acute mercury poisoning are usually neurological: tremors, aggression, disturbances in perception and coordination, and muscle weakness. In severe poisoning, the lungs may fail. Later, kidney function fails. The dependence of symptoms on the concentration of mercury in the blood is highly individual. Treatment consists of chelation of mercury (removal of the metal by an organic agent), e.g. with unithiol (DMPS), and its subsequent elimination from the body.

Elementární rtuť má velkou vazební afinitu k síře, která je využívána při sanacích nehod se rtutí. Vzniklý sulfid rtuťnatý (HgS) je hlavní rtuťnatou rudou (cinabarit). V těle se rtuť váže se na disulfidické můstky v bílkovinách. Napadené bílkoviny nejsou funkční a pomalu jsou vylučovány močí. Při chelataci unithiolem se rtuť rovněž váže se sírou. Rtuť velmi ochotně reaguje se selenem. V těle pak blokuje selenoenzymy, které jsou významnými antioxidanty, podílejí se na imunitní odezvě a regulují působení hormonů štítné žlázy. Podle nařízení vlády, kterým se stanovují podmínky ochrany zdraví při práci (zákon 361/2007 Sb.), je pro rtuť přípustný expoziční limit PEL = 0,02 mg·m–3 a nejvyšší přípustná koncentrace NPK-P = 0,15 mg·m–3. U člověka je běžná koncentrace rtuti 3 až 7 μg·l–1 v moči a 4 až 10 μ g·l–1 v krvi. Akutní toxicita LC50 pro krysu je < 27 mg·m–3 vzduchu po dobu 2 h (dávka, při které uhyne polovina testovací skupiny). U ryb je LC50 přibližně 0,5 mg·l–1 vody po dobu 96 h. Příklad: Kdyby se odpařilo celých 5 mg rtuti z rozbité zářivky a toto množství se rovnoměrně rozptýlilo v místnosti 4 x 3,3 x 2,5 m, způsobí právě dosažení limitu nejvyšší přípustné koncentrace 0,15 mg·m–3. Ve skutečnosti je rozložení nerovnoměrné a do vzduchu se ihned uvolní pouze plynná rtuť, které je u kompaktní zářivky asi 50 μg [3], a další se začne odpařovat z tekuté fáze. Tenze par, a tedy i rychlost odpařování prudce rostou s teplotou. Pokusy ukazují, že už po asi 15 minutách intenzivního větrání klesá koncentrace rtuti ve vzduchu na hodnotu pozadí. V případě nehod se zářivkami je vhodné ihned začít intenzivně větrat a z prostoru se na chvíli vzdálit. Přibližně 80 % vdechnuté páry rtuti zůstává v těle a pouze 20 % odchází s výdechem. Téměř všechna rtuť uvolněná do ovzduší nakonec vlhkostí zoxiduje a déšť ji spláchne do půdy a do řek, kde se jí otráví bakterie, které na rtuťnaté ionty navážou metylovou skupinu –CH3, aby je dokázaly v podobě methylrtuti vyloučit. Methylrtuť je přitom pro vodní živočichy i člověka (vrchol potravního řetězce) velmi jedovatá, neboť prochází hematoencefalickou bariérou do mozku, kde poškozuje neurony.

Biotransformation

The mechanism of mercury biotransformation was discovered in the 1950s in Japan in Minamata Bay, where the Chisso chemical plant was discharging waste containing inorganic mercury. The inhabitants (fish consumers) suffered severe neurological disorders caused by methylmercury poisoning. Many children were born with birth defects and in total more than 10,000 people were affected.

Bioconcentration

Mercury has another dangerous property for living organisms, which is bioconcentration. The bioaccumulation factor (BCF) indicates how many times the concentration of a substance from the environment is reflected in the concentration in the organism. For example, in fish, the BCF for mercury is 156. Predatory fish have much higher mercury levels because they feed on contaminated smaller fish. Sharks, which are at the top of the oceanic food chain, have the highest mercury concentrations and can have tissue mercury concentrations more than a million times higher than in the surrounding water. According to the HPA survey, the average mercury level in the Czech population is low due to relatively low fish consumption. However, in countries where fish forms a major part of the diet, the situation is reversed.

Methylmercury poisoning

Methylmercury poisoning has a latency of weeks to months. Prolonged poisoning with bioaccumulation of small doses may take years to manifest. Mild poisoning is manifested by disruption of the cardiovascular and immune systems. At higher doses, severe nervous system disturbances occur, the first symptom being uncontrollable trembling of the eyelids. Mercury exposure has been speculated to be linked to autism, multiple sclerosis, Parkinson's disease or Alzheimer's dementia [4].

It is therefore very sensible to prevent mercury pollution. The European Union's approach is that mercury is very dangerous and, as ordinary citizens, we should have as little contact with it as possible. In particular, there is a ban on the marketing of thermometers and pressure gauges containing mercury. It is forbidden to export mercury from the EU. Last year, the Czech Republic acceded to the so-called Minamata Convention on mercury, which should be a step towards reducing the environmental burden of this element. The take-back of mercury-containing light sources from the professional sector is very good in the Czech Republic, but 60% of household fluorescent lamps end up in mixed waste containers or in landfills. Some education is still needed in this respect.

Attractions

Mercury is mostly made up of divalent (mercuric) compounds. In mercury (monovalent) compounds, two mercury atoms are bonded together. At low temperatures, they form a tetravalent fluoride (HgF4). It is the only element known to form compounds (helides) with otherwise insoluble helium (HgHex). Liquid mercury forms diatomic molecules (Hg2), but mercury pairs are monatomic. Quantum theory predicts its melting point at +82 °C, and its fluidity at room temperature is an example of relativistic orbital contraction. The alchemists considered mercury to be the primordial metal from which all other metals are created, differing in the proportion of sulfur. Mercury is likely to be part of the philosopher's stone.

In the continuation of this article, we will discuss the historical use of mercury, the methods of measuring its content in air and solids, and the neutron activation analysis method, which is being improved by the Centre for (Nuclear) Research Řež, s. r. o. for the purpose of measuring mercury content in fluorescent lamps.

fuksa@nasli.net

Literature:

[1] MACHEK, V.: Etymological dictionary of the Czech language. Prague, Academia, 1968.

[2] KRABBENHOFT, D. - SCHUSTER, P.: Glacial Ice Cores Reveal A Record of Natural and Anthropogenic Atmospheric Mercury Deposition for the Last 270 Years. USGS Fact Sheet FS-051-02. 2002. Available at: http://toxics.usgs.gov/pubs/FS-051-02/.

[3] KRBAL, M. - BAXANT, P. - ISKANDIROVA, M. - ŠKODA, J. - SUMEC, S.: Light sources and ecology. Light, 2014, vol. 17, no. 2, pp. 48-50. Available at: http://www.odbornecasopisy.cz/flipviewer/Svetlo/2014/02/Svetlo_02_2014_output/web/Svetlo_02_2014_opf_files/Web-Search/page0050.html.

[4] URBAN, P.: Current problems of mercury neurotoxicity. Neurology for practice, 2006, no. 5, pp. 251-253.

Review: Ing. Vladimír Dvořáček

Fig. 1. The astronomical sign of Mercury and the alchemical symbol of Mercury

Fig. 2. Pictograms for mercury according to the CLP Directive (1272/2008/EU) If inhaled, it can cause death. May damage the fetus in the mother's body. Causes organ damage with prolonged or repeated exposure. Highly toxic to aquatic organisms + long-lasting effects

Fig. 3. Emission spectrum of low-pressure mercury discharge

Fig. 4. Mercury concentration in the Fremont Glacier, Wyoming, USA, taken from [2]

Author. Antonín Fuksa, NASLI & Blue Step spol. s r. o.
Published in Světlo 4/2014