|N / A||Mg||Al||Si||P||S.||Cl||Ar|
Heavy metals are combined with non-uniform definitions metals whose density or atomic mass exceeds a certain value. In some cases, other properties such as atomic number and toxicity are also included in the definition . Many sources classify a heavy metal as a metal whose density is greater than 5.0 g / cm³ or - in the case of older sources - greater than 4.5 g / cm³.
In nuclear engineering , "heavy metal" is used in two different special meanings:
- as a collective term for all nuclides that can be split by neutrons ,
- in the case of burn-up considerations for the fissile metal content ( uranium , plutonium ) of the fresh nuclear fuel.
In technology (only non-ferrous metals ) and chemistry, the term “heavy metal” includes metals with a density> 5 g / cm³. These include the precious metals , the base metals iron , copper , lead , zinc , tin and nickel, as well as bismuth , cadmium , chromium and uranium . However, an IUPAC study found at least 38 definitions for the designation “heavy metal”, ranging from density , atomic weight or atomic number to chemical properties or toxicity . Thus lists of "heavy metals" differ from one set of guidelines to another; while often also metalloids such. B. arsenic included. The term is often used without specifying the metals to which it refers. For the reasons listed above, the designation of all other metals as light metals is also undefined. In the public eye, all substances designated as “heavy metals” (and their compounds and alloys are often also included) are considered toxic. The following elements have a density over 5 g / cm³:
** Since these elements cannot be synthesized in measurable quantities, many of their properties such as density cannot be measured. However, model calculations suggest ranges of values for these quantities.
The table contains elements with a density from 5 g / cm³. Elements with a known density between 5 and 10 g / cm³ have a yellow background, between 10 and 20 g / cm³ orange and over 20 g / cm³ brown.
Occurrence and origin
Heavy metals occur in the rocks of the earth's crust, where they are firmly bound in ores as oxides , sulfides and carbonates and are also enclosed in silicates or are partly native . Their concentration in the hydrosphere , atmosphere and pedosphere fluctuates over many orders of magnitude. Their concentration in the earth's crust ranges from single-digit parts per billion (ppb) (iridium, gold, platinum) to 5 percent (iron). Through weathering and erosion, these naturally find their way into the soil and groundwater. Some rocks such as picrite , serpentinite , basalts and, above all, ores contain high concentrations of chromium, nickel and cobalt in some cases, which leads to a high natural heavy metal load on the soil in their vicinity. The amount of material cycles and the accumulation in the environment have increased rapidly since industrialization in the 19th century due to growing emissions from various anthropogenic sources. This includes the extraction of heavy metals and their processing, the production of fertilizers, the burning of coal, garbage and sewage sludge, motor vehicle traffic and steel, cement and glass production. The mining of "heavy metal ores" is often associated with high levels of heavy metal pollution in the soil. In some places in the Harz , Siegerland and the Aachen area, for example, azonal vegetation of specific plant communities has formed on the soils polluted by ore mining . There the galmei plants form what are known as " heavy metal turf ".
4.5 billion years ago - when the earth's mantle was still liquid - the heavy metals sank to the center of the earth and formed the earth's core . Regarding the occurrence of heavy metals in the earth's crust , geologists assume that most of them come from asteroids . This assumption is supported by a study with tungsten , which comes from a rock sample from Greenland . The isotope 182 W was found 13 times more often in this rock sample than in rock samples from other locations. Matthias Willbold of the University of Bristol , first author of the study, says: “Most of the precious metals on which our economy and many important industrial processes are based came to our planet by lucky coincidence - when the earth was made up of around 20 trillion tons of asteroid material was hit."
Biological properties and environmental impact
By nature, heavy metals and their compounds only occur in traces in the biosphere. Some of them are vital for plants, animals and humans in small amounts; they are then referred to as essential heavy metals or trace elements . These include chromium , iron , cobalt , copper , manganese , molybdenum , nickel , vanadium , zinc and tin . Many heavy metals, including the essential ones, can be harmful or toxic to the human organism even in slight overconcentration , whereby their toxic effect also depends strongly on the chemical compound of the heavy metal. One example of this is chromium, which is nontoxic in its elementary form, essential as chromium (III) and toxic and carcinogenic as chromium (VI). In general, the dangerousness of the compounds increases with their water and fat solubility. The substances are usually absorbed through the food chain and thus get into the human body . Plants play an important role here, as they can absorb and accumulate heavy metals. In humans, chronic heavy metal poisoning often has a specific effect on certain organs and causes characteristic clinical pictures.
Since 2006 the American Blacksmith Institute has published a list of the ten most polluted places in the world. All heavy metals - mostly emitted by mining or smelting - are represented in various ways every time.
Lead accumulates in the human organism when it is absorbed through food and breath and acts as a chronic poison even in small traces . It accumulates in bones , teeth and in the brain and affects the functionality of the nervous system . Children in particular are at risk, they often show intelligence , learning and concentration disorders . The immune defense is also damaged by lead poisoning, which leads to an increased susceptibility to infection.
The largest source of lead poisoning used to be leaded gasoline, developed in the United States in the 1920s, to which tetraethyl lead was added to increase knock resistance . In the USA, this fuel was gradually withdrawn from use from 1973. In Europe, unleaded gasoline was sold again for the first time in Germany in 1983. After being gradually replaced and abolished, leaded petrol was banned across the EU at the beginning of 2000. Worldwide, however, leaded petrol is still used in Africa and large parts of Asia - with the corresponding health consequences.
Since 1973 no lead pipes have been built into the house as water pipes in Germany. The whole of southern Germany is practically free of lead pipes, as none have been laid there for over a hundred years. The limit value for lead in tap water was 25 µg / L from December 1, 2003 and was reduced to 10 µg / L on December 1, 2013.
Cadmium and its compounds are toxic even in low concentrations. It has been shown to be carcinogenic in animal experiments and is mutagenic and teratogenic. An adult's body contains around 30 mg of cadmium without it being needed for the construction of body substances. It is one of the non-essential elements . Ingestion of soluble cadmium salts can cause vomiting and digestive disorders , liver damage and cramps . Inhalation of cadmium vapors causes irritation of the respiratory tract and headaches. Chronic poisoning manifests itself in the loss of the ability to smell , yellowing of the tooth necks, anemia and vertebral pain, in an advanced stage through bone marrow damage and osteoporosis . Cadmium has increasingly come into disrepute since the appearance of the often fatal Itai-Itai disease in Japan , which is associated with severe skeletal changes. The accumulation of cadmium in the liver and especially in the kidneys is particularly worrying. About twice as high levels of cadmium were found in smokers as in non-smokers. The average exposure to cadmium through smoking is 2 to 4 µg per day. Humans ingest between 10 and 35 µg cadmium with their food every day. According to the WHO , the critical limit is 10 µg per day and kilogram of body mass. The biological half-life in humans is between 10 and 35 years.
Copper is one of the essential trace elements. Special compounds, however, can cause weakness, vomiting and inflammation in the digestive tract if large amounts are swallowed . Acute poisoning due to very high amounts is rare in humans, as vomiting is inevitably triggered. Copper has a catalytic effect in numerous chemical processes, including metabolic processes.
Copper has to be consumed in sufficient quantities every day. The storage capacity in the body is limited. The daily requirement of an adult is around 1 to 2 mg. Numerous foods contain this trace element, including nuts, certain types of fish and meat, and some vegetables. Copper can also get into drinking water through copper-containing water pipes, but only if the drinking water has been in the pipes for a long time. This is only important in terms of quantity for water with a low pH value . In this case, it is recommended to drain any stale water. Fresh water that does not stagnate in pipes is basically not changed in its composition by the materials used in the house installation. The drinking water standards of the WHO and the EU allow a maximum copper content of 2 mg / L. The German Drinking Water Ordinance adopted this value, which was specified to 2.0 mg / L in the 2011 Amendment to the Drinking Water Ordinance.
A copper content of 2 mg / L already gives water a metallic taste, 5 mg / L make it inedible. According to current knowledge, an average drinking water content of 2 mg / L is regarded as harmless to health, this applies to lifelong enjoyment. A very excessive copper intake through water or food can lead to cirrhosis of the liver in infants and toddlers whose copper metabolism is not yet fully developed . One of the reasons for this is that the specific total amount of copper in the body of infants is naturally relatively high at birth. In adolescents and adults, excess copper is excreted similarly to vitamin C.
In 2011, the Federal Environment Agency published the draft of the metallic materials suitable for drinking water hygiene, copper is included for all component types. In water with a low pH value, copper components should be tinned on the inner surface - DIN 50930-6 gives detailed descriptions of the water-side framework conditions. A precise test is necessary for house wells because house well water is often not treated. Apart from this exception, the drinking water is much better than it is reputed to be and it is safe for children to drink plenty of it.
Although copper is one of the essential trace elements for humans, it has a growth-inhibiting or even active antimicrobial effect on many microorganisms. This property is used specifically for contact surfaces in the medical field as a supplementary measure in the fight against antibiotic-resistant microorganisms.
The lethal dose for a human is likely to be in the tens of milligrams. Much more dangerous than the chemical effects, however, is its radioactivity , which can cause cancer . An amount on the order of a few micrograms is probably sufficient for the development of cancer. The widespread misunderstanding about the particular danger posed by plutonium was derived from this estimate . Since the emitted alpha radiation is already shielded by the outermost layers of the cornea, plutonium is only harmful to health when it is incorporated (for example, the inhalation of dust containing plutonium).
Metallic mercury can be absorbed into the body as mercury vapor through the lungs . It irritates the respiratory and digestive tracts , can cause vomiting with abdominal pain, and also damage the kidneys and central nervous system .
Thallium and compounds containing thallium are highly toxic and must be handled with great care.
Heavy metals are used in many areas, but mostly for metal finishing . This gives the selected materials special properties . The following areas of application are prohibited today due to their harmful effects:
- Lead in PVC and drinking water pipes
- According to the RoHS directive, lead in solder is no longer allowed (with a few exceptions, see below)
- Cadmium in cosmetics , crop protection and PVC, and earlier also in accumulators
- Mercury in wood preservatives, impregnating agents, antifouling paints and for water treatment
Heavy metals still used:
- Lead in solder in medical devices, in monitoring and control instruments, in the aerospace and military sectors, and for repairs
- Chromium and nickel for steel , nickel also in accumulators
- Lead for accumulators (= rechargeable batteries ), cable sheaths, pigments , alloys and for radiation protection
- Mercury in small amounts in fluorescent lamps and energy-saving lamps , in thermometers, in apparatus technology and in amalgam tooth fillings
- Cadmium for accumulators (nickel-cadmium and silver-cadmium), as corrosion protection for iron and similar metals ( cadmium coatings produced by electrolytic deposition or physical vapor deposition protect against corrosion even in a thickness of 0.008 mm ), cadmium pigments and cadmium soap as stabilizers for PVC, cadmium (also as alloys) for shielding against thermal neutrons and for control rods in reactors (cadmium-113 has a particularly large cross-section for neutron capture ).
Use in medicine:
- Lanthanum is used as a lanthanum carbonate ( phosphate binder ) to treat hyperphosphataemia in patients with kidney failure .
- Gadolinium is used in complex-bound form as a contrast agent in magnetic resonance imaging , such as gadopentetate dimeglumine . There is a risk of nephrogenic systemic fibrosis (NSF) in patients with renal insufficiency.
- Heavy metals such as mercury used to have a wide range of indications, for example in the treatment of syphilis .
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Few, older sources give a limit value of <4.5 g / cm³
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