Cadmium(Cadmium), Cd, a chemical element of group II of the Mendeleev Periodic Table; atomic number 48, atomic mass 112.40; white, shiny, heavy, soft, malleable metal. The element consists of a mixture of 8 stable isotopes with mass numbers: 106 (1.215%), 108 (0.875%), 110 (12.39%), 111 (12.75%), 112 (24.07%), 113 (12 .26%), 114 (28.86%), 116 (7.58%).

Historical reference. In 1817, the German chemist F. Stromeyer, during the revision of one of the pharmacies, discovered that the zinc carbonate present there contained an admixture of an unknown metal, which precipitated in the form of yellow hydrogen sulfide from an acidic solution. Stromeyer called the metal he discovered cadmium (from the Greek kadmeia - impure zinc oxide, also zinc ore). Independently of him, the German scientists K. Hermann, K. Karsten and W. Meissner discovered cadmium in Silesian zinc ores in 1818.

Distribution of cadmium in nature. Cadmium is a rare and trace element with a clarke of the lithosphere of 1.3·10 -5% by weight. Cadmium is characterized by migration in hot groundwater along with zinc and other chalcophile elements and concentration in hydrothermal deposits. The mineral spelerite ZnS in some places contains up to 0.5-1% Cd, up to a maximum of 5%. Greenockite CdS is less common. Cadmium is concentrated in marine sedimentary rocks - shales (Mansfeld, Germany), in sandstones, in which it is also associated with zinc and other chalcophile elements. Three very rare independent minerals of Cadmium are known in the biosphere - carbonate CdCO 3 (stave), oxide CdO (monteponite) and selenide CdSe.

Physical properties of cadmium. The crystal lattice of Cadmium is hexagonal, a = 2.97311 Å, c = 5.60694 Å (at 25 °C); atomic radius 1.56 Å, ionic radius Cd 2+ 1.03 Å. Density 8.65 g / cm 3 (20 ° C), t pl 320.9 ° C, t kip 767 ° C, thermal expansion coefficient 29.8 10 -6 (at 25 ° C); thermal conductivity (at 0°C) 97.55 W/(m K) or 0.233 cal/(cm sec °C); specific heat capacity (at 25 °C) 225.02 J/(kg K) or 0.055 cal/(g °C); electrical resistivity (at 20 °C) 7.4 10 -8 ohm m (7.4 10 -6 ohm cm); temperature coefficient of electrical resistance 4.3 10 -3 (0-100 ° C). Tensile strength 64 MN / m 2 (6.4 kgf / mm 2), elongation 20%, Brinell hardness 160 MN / m 2 (16 kgf / mm 2).

Chemical properties of cadmium. In accordance with the external electronic configuration of the 4d 10 5s 2 atom, the valency of Cadmium in compounds is 2. Cadmium tarnishes in air, being covered with a thin film of CdO oxide, which protects the metal from further oxidation. When strongly heated in air, Cadmium burns into CdO oxide - a crystalline powder from light brown to dark brown in color, density 8.15 g/cm 3 ; at 700°C CdO sublimates without melting. Cadmium combines directly with halogens; these compounds are colorless; CdCl 2 , CdBr 2 and CdI 2 are very easily soluble in water (about 1 part of anhydrous salt in 1 part of water at 20 ° C), CdF 2 is more difficult to dissolve (1 part in 25 parts of water). With sulfur, cadmium forms lemon-yellow to orange-red CdS sulfide, insoluble in water and dilute acids. Cadmium is readily soluble in nitric acid with the release of nitrogen oxides and the formation of nitrate, which gives hydrate Cd (NOa) 2 4H 2 O. From acids - hydrochloric and dilute sulfuric Cadmium slowly releases hydrogen, when the solutions are evaporated, hydrates of chloride 2CdCl 2 5H 2 O and sulfate 3CdSO crystallize from them 4 8H 2 O. Cadmium salt solutions are acidic due to hydrolysis; caustic alkalis precipitate white hydroxide Cd (OH) 2 from them, insoluble in excess of the reagent; however, under the action of concentrated alkali solutions on Cd (OH) 2, hydrooxocadmates, for example Na 2, were obtained. The Cd 2+ cation easily forms complex ions with ammonia 2+ and with cyan 2- and 4-. Numerous basic, double and complex salts Cadmium. Cadmium compounds are poisonous; especially dangerous is the inhalation of vapors of its oxide.

Getting Cadmium. Cadmium is obtained from by-products of processing zinc, lead-zinc and copper-zinc ores. These products (containing 0.2-7% Cadmium) are treated with dilute sulfuric acid, which dissolves Cadmium and zinc oxides. Cadmium is precipitated from the solution with zinc dust; the spongy residue (a mixture of Cadmium and zinc) is dissolved in dilute sulfuric acid and Cadmium is isolated by electrolysis of this solution. Electrolytic cadmium is melted down under a layer of caustic soda and cast into sticks; metal purity - not less than 99.98%.

The use of cadmium. Metallic cadmium is used in nuclear reactors, for anti-corrosion and decorative coatings, and in batteries. Cadmium serves as the basis of some bearing alloys, is part of low-melting alloys (for example, Wood's alloy). Low-melting alloys are used for soldering glass with metal, in automatic fire extinguishers, for thin and complex castings in plaster molds, and others. Cadmium sulfide (cadmium yellow) - paint for painting. Cadmium sulfate and amalgam are used in the normal Weston cell.

Cadmium in the body. The content of cadmium in plants is 10 -4% (per dry matter); in some animals (sponges, coelenterates, worms, echinoderms and tunicates) - 4-10 -5 - 3-10 -3% of dry matter. Found in all vertebrates. The liver is richest in cadmium. Cadmium affects carbohydrate metabolism, the synthesis of hippuric acid in the liver, and the activity of certain enzymes.

Cadmium - uncommon toxic and unknown
a wide range of silver hazardous metal
Toxic and poisonous stones and minerals

Cadmium(Latin Cadmium, denoted by the symbol Cd) is an element with atomic number 48 and atomic mass 112.411. It is an element of a secondary subgroup of the second group, the fifth period of the periodic system chemical elements DI. Mendeleev. Under normal conditions, a simple substance cadmium is a heavy (density 8.65 g / cm3 - lighter than uranium) soft malleable ductile transition metal silver white colors (does not devour the flesh, like the "Kerbersky stone" of the Zhytomyr region of Ukraine - not uranium oxide pitchblende, brown dangerous stone). On the picture - cadmium sulfide, greenockite(earthy crusts yellow colors).

Natural cadmium consists of eight isotopes, six of which are stable: 106Cd (isotope abundance 1.22%), 108Cd (0.88%), 110Cd (12.39%), 111Cd (12.75%), 112Cd (24, 07%), 114Cd (28.85%). Radioactivity was detected for two other natural isotopes: 113Cd (isotopic abundance 12.22%, β-decay with a half-life of 7.7∙1015 years) and 116Cd (isotopic abundance 7.49%, double β-decay with a half-life of 3.0 ∙1019 years).

Cadmium of the periodic system was partially described by the German professor Friedrich Stromeyer in 1817 (distinguished from zinc). Magdeburg pharmacists, when studying preparations containing zinc oxide ZnO, suspected the presence of arsenic (oxidation catalyst from sulfide) in them. Since zinc oxide is included in many ointments, powders and emulsions used for various skin diseases, the inspectors categorically banned the sale of suspicious drugs.

Naturally, the manufacturer of medicines, defending his personal interests, demanded an examination. Stromeyer acted as an expert. He isolated a brown-brown oxide from ZnO, reduced it with hydrogen and obtained a silver-white metal, which he called "cadmium" (from the Greek kadmeia - zinc oxide, also zinc ore). Regardless of Professor Stromeyer, cadmium was discovered in Silesian zinc ores (satellite) by a group of scientists - K. Hermann, K. Carsten and W. Meisner in 1818.

Cadmium absorbs slow neutrons, for this reason cadmium rods are used in nuclear reactors to control the rate of a chain reaction (ChNPP). Cadmium is used in alkaline batteries and is included as a component in some alloys. So, for example, copper alloys containing about 1% Cd (cadmium bronzes) are used for the manufacture of telegraph, telephone, trolleybus and tram wires, subway cables, since these alloys have greater strength and wear resistance than copper.

Greenockite (yellow dope) on calcite. Yunnan, China. 7x5 cm. Photo: A.A. Evseev.

A number of fusible alloys, such as those used in fire extinguishers, contain cadmium. In addition, cadmium is a part of substandard jewelry alloys (soldering after evaporation of the amalgam component from amalgam alloys bursting from temperature, prohibited in the open sale - amalgams of gold, silver and platinum with toxic mercury).

This metal is used for cadmium plating of steel products, because it carries an oxide film on its surface, which has a protective effect. The fact is that in sea water and in a number of other media, cadmium plating is more effective than galvanizing. Cadmium has a long history of use in homeopathic (basic treatment with herbs and microdoses - the so-called "Dietary Supplements in Food" - dietary supplements and animal feed) medicine. Cadmium compounds have also found wide application - cadmium sulfide is used to make yellow paint and colored glasses, and cadmium fluoroborate is a flux used to solder aluminum and other metals.

Cadmium is found in the body of vertebrates (bones, ligaments, tendons and muscles), it is established that it affects carbon metabolism, the activity of a number of enzymes and the synthesis of hippuric acid in the liver. However, cadmium compounds are poisonous, and the metal itself is a carcinogen. Especially dangerous is the inhalation of vapors of cadmium oxide CdO, fatal cases are not uncommon. The penetration of cadmium into the gastrointestinal tract is also harmful, but no cases of fatal poisoning have been recorded, most likely this is due to the fact that the body seeks to get rid of the toxin (vomiting).

Biological properties

It turns out that cadmium is present in almost all living organisms - in terrestrial cadmium content is approximately equal to 0.5 mg per 1 kg of mass, in marine organisms (sponges, coelenterates, echinoderms, worms Pacific Ocean) - from 0.15 to 3 mg/kg, the content of cadmium in plants is about 10-4% (on dry matter). Despite the presence of cadmium in most living organisms, its specific physiological significance has not been fully established (growth hormone). Scientists managed to find out that this element affects carbohydrate metabolism, the synthesis of hippuric acid in the liver, the activity of a number of enzymes, as well as the metabolism of zinc, copper, iron and calcium in the body (a favorite stone of bodybuilders who increase muscle mass and strengthen their bones in sports - in microdoses).

Greenockite (yellow). Curly volcano, about. Iturup, Kuril Islands, Russia. Photo: A.A. Evseev.
Can be issued for talc, sulfur and other greenoctite-like minerals

There is a suggestion, supported by research, that microscopic amounts of cadmium in food can stimulate body growth in mammals. For this reason, scientists have long ranked cadmium as a conditionally essential trace element, that is, vital, but toxic in certain doses. The body of a healthy person contains a small amount of cadmium. Sung in ancient Greek and Roman epic - Cadmeus(place poison trade in the south-east of Europe ("Shield on the gates of Tsaregrad", Istanbul), in Greece (porticos and amphitheaters) and in the Mediterranean near Turkey - a drug). On slang miners and stone miners cadmium called " snake poison" (jargon).

Cadmium is one of the most toxic heavy metals- in Russia (metrology) it is assigned to the 2nd hazard class - highly hazardous substances - which include antimony, strontium, phenol and other toxic substances (equivalent to ADR dangerous goods N 6 - poison, skull and crossbones in a rhombus). In the Bulletin of the Russian Federation on Environmental Safety and Poison Transportation Technologies "Problems of Chemical Safety" dated April 29, 1999, cadmium appears as "the most dangerous ecotoxicant at the turn of the millennium"!

Like other heavy metals, cadmium is a cumulative poison, that is, it can accumulate in the body - its half-life is from 10 to 35 years. Human body by the age of fifty it is able to accumulate from 30 to 50 mg of cadmium. The main "depots" of cadmium in the human body are the kidneys, which contain from 30 to 60% of the total amount of this metal in the body, and the liver (20-25%). The following are able to accumulate cadmium to a lesser extent: the pancreas, spleen, tubular bones, and other organs and tissues. Small amounts of cadmium are present even in the blood. However, unlike lead or mercury, cadmium does not enter the brain.

For the most part, cadmium in the body is in a bound state - in combination with the protein metallothionein - this is a kind of protective mechanism, the body's reaction to the presence of a heavy metal. In this form, cadmium is less toxic, however, even when bound, it does not become harmless - accumulating over the years, this metal can lead to disruption of the kidneys and an increased likelihood of kidney stones. Much more dangerous is cadmium, which is in ionic form, because it is chemically very close to zinc and is able to replace it in biochemical reactions, acting as a pseudo-activator or, conversely, an inhibitor of zinc-containing proteins and enzymes.

Cadmium binds to the cytoplasmic and nuclear material of cells of a living organism and damages them, changes the activity of many hormones and enzymes, which is explained by its ability to bind sulfhydryl (-SH) groups. In addition, cadmium, due to the proximity of the ionic radii of calcium and cadmium, is able to replace calcium in bone tissue. The same situation is with iron, which cadmium is also able to replace. For this reason, the lack of calcium, zinc and iron in the body can lead to an increase in the absorption of cadmium from the gastrointestinal tract up to 15-20%. It is believed that a harmless daily dose of cadmium for an adult is 1 μg of cadmium per 1 kg of body weight, large amounts of cadmium are extremely hazardous to health.

What are the mechanisms of entry of cadmium and its compounds into the body? Poisoning occurs when drinking water (MPC for drinking water is 0.01 mg/l) contaminated with cadmium-containing waste, as well as when eating vegetables and grains growing on lands located near oil refineries and metallurgical enterprises. The use of mushrooms from such territories is especially dangerous, since, according to some information, they are able to accumulate more than 100 mg of cadmium per kg of their own weight. Smoking is another source of cadmium intake into the body, both of the smoker himself and of the people around him, because the metal is found in tobacco smoke.

The characteristic signs of chronic cadmium poisoning are, as mentioned earlier, kidney damage, muscle pain, bone tissue destruction, and anemia. Acute food poisoning with cadmium occurs when large single doses are taken with food (15-30 mg) or with water (13-15 mg). At the same time, signs of acute gastroenteritis are observed - vomiting, pain and convulsions in the epigastric region, however, cases of fatal poisoning with cadmium compounds that have entered the body with food are unknown to science, but according to WHO estimates, a lethal single dose can be 350-3500 mg.

Much more dangerous is cadmium poisoning by inhalation of its vapors (CdO) or cadmium-containing dust (as a rule, this occurs in industries related to the use of cadmium) - similar to liquid mercury and red cinnabar (by toxicity). Symptoms of such poisoning are pulmonary edema, headache, nausea or vomiting, chills, weakness, and diarrhea (diarrhea). As a result of such poisoning, deaths have been recorded.

The antidote for cadmium poisoning is selenium, which helps to reduce the absorption of cadmium (they work on copiers and printers in modern data centers and refill cartridges for office equipment). However, a balanced intake of selenium is still required, this is due to the fact that its excess in the body leads to a decrease in sulfur content (forms sulfur sulfide - binds it), and this will certainly lead to the fact that cadmium will again be absorbed by the body.

Interesting Facts

It has been established that one cigarette contains from 1 to 2 micrograms of cadmium. It turns out that a person who smokes a pack of cigarettes per day (20 pcs.) Gets about 20 micrograms of cadmium! The danger lies in the fact that the absorption of cadmium through the lungs maximum- from 10 to 20%, thus, in the body of a smoker, from 2 to 4 micrograms of cadmium is absorbed with each pack of cigarettes! The carcinogenic effect of nicotine contained in tobacco smoke is usually associated with the presence of cadmium, and it is not retained even by carbon filters - lung cancer.

An example of chronic cadmium poisoning with numerous fatal outcomes was described in the late 1950s. On the territory of Japan, there have been cases of a disease that the locals called "itai-itai" ("Italian disease"), which can also be translated into the local dialect as "oh, how it hurts!" (poisoning). Symptoms of the disease were severe lumbar pain, which, as it turned out later, was caused by irreversible kidney damage; severe pain in the muscles. The ubiquitous spread of the disease and its severe course were caused by high pollution environment in Japan at that time and the specifics of the Japanese diet (rice and seafood accumulate a large amount of cadmium). It was found that those who fell ill with this disease consumed about 600 micrograms of cadmium daily!

Despite the fact that cadmium is recognized as one of the most toxic substances, it has also found application in medicine! Thus, inserted into the chest of a patient suffering from heart failure, a nickel-cadmium battery provides energy to a mechanical stimulator of the heart. The convenience of such a battery is that the patient does not have to lie down on the operating table to recharge or replace it. For uninterrupted battery life, it is enough to wear a special magnetized jacket once a week for just an hour and a half.

Cadmium is used in homeopathy, experimental medicine, and more recently it has been used to create new anticancer drugs.

Wood's metal alloy, containing 50% bismuth, 12.5% ​​tin, 25% lead, 12.5% ​​cadmium, can be melted in boiling water. The alloy was invented in 1860 by engineer B. Wood ) Several interesting facts are associated with this low-melting alloy: firstly, the first letters of the components of Wood's alloy form the abbreviation "WAX", and secondly, the invention is also attributed to B. Wood's namesake - American physicist Robert Williams Wood, who was born eight years later ( peers fought at VAK).

Not so long ago, cadmium of the periodic system entered the "armament" of the police and forensic experts: with the help of the thinnest layer of cadmium deposited on the surface being examined, it is possible to identify human fingerprints.

Scientists have established interesting fact: cadmium tin in the atmosphere of rural areas has a much greater resistance to corrosion than in the atmosphere of industrial areas. Such a coating fails especially quickly if the content of sulfurous or sulfuric anhydrides is increased in the air.

In 1968, one of the US health officials (Dr. Carroll) discovered a direct relationship between mortality from cardiovascular diseases and the content of cadmium in the atmosphere. He came to such conclusions by analyzing the data of 28 cities. In four of them - New York, Chicago, Philadelphia and Indianapolis - the content of cadmium in the air was higher than in other cities; the proportion of deaths due to heart disease was also higher.

In addition to "standard" measures to limit cadmium emissions into the atmosphere, water and soil (filters and cleaners at enterprises, removal of housing and crop fields from such enterprises), scientists are also developing new - promising ones. So scientists in the bay of the Mississippi River planted water hyacinths, believing that with their help it would be possible to clean the water from elements such as cadmium and mercury.

Story

History knows many "discoveries" that were made during fictitious checks, reviews and revisions. However, such finds are more criminal in nature than scientific. And yet there was such a case when the revision that had begun eventually led to the discovery of a new chemical element. It happened in Germany early XIX century. The district doctor R. Rolov checked the pharmacies of his district, during the audit - in a number of pharmacies near Magdeburg - he discovered zinc oxide, the appearance of which aroused suspicion and suggested that it contained arsenic (pharmacolyte). To confirm the assumptions, Rolov dissolved the seized drug in acid and passed it through a solution of hydrogen sulfide, which led to the precipitation of a yellow precipitate, similar to arsenic sulfide. All suspicious medications- ointments, powders, emulsions, powders.

Such a move outraged the owner of the factory in Schenebek, which produced all the drugs rejected by Rolov. This businessman - Herman, being a chemist by profession, conducted his own examination of the goods. Having tried the entire arsenal of experiments known at that time for the detection of arsenic, he was convinced that his products were pure in this respect, and iron, which confused the auditor, gave the yellow color of zinc oxide.

Having reported the results of his experiments to Rolov and the authorities of the land of Hanover, Herman demanded an independent examination and complete "rehabilitation" of his product. As a result, it was decided to find out the opinion of Professor Stromeyer, who headed the Department of Chemistry at the University of Göttingen, and concurrently held the post of Inspector General of all Hanoverian pharmacies. Naturally, Stromeyer was sent for verification not only zinc oxide, but also other zinc preparations from the Shenebek factory, including zinc carbonate, from which this oxide was obtained.

By calcining zinc carbonate ZnCO3, Friedrich Stromeyer obtained oxide, but not white, as it should have been, but yellowish. As a result of further research, it turned out that the drugs do not contain either arsenic, as Rolov suggested, or iron, as German thought. The reason for the unusual color was a completely different metal - previously unknown and very similar in properties to zinc. The only difference was that its hydroxide, unlike Zn (OH) 2, was not amphoteric, but had pronounced basic properties.

Stromeyer named the new metal cadmium, hinting at the strong similarity of the new element with zinc - the Greek word καδμεια (kadmeia) has long denoted zinc ores (for example, smithsonite ZnCO3) and zinc oxide. In turn, this word comes from the name of the Phoenician Cadmus, who, according to legend, was the first to find a zinc stone and discovered its ability to give copper (when smelted from ore) a golden color. According to ancient Greek myths, there was another Cadmus - a hero who defeated the Dragon and built the Cadmeus fortress on the lands of the enemy defeated by him, around which the great seven-gate city of Thebes subsequently grew. In the Semitic languages, "kadmos" means "eastern" or "serpentine" (Fergana, Kyrgyzstan, Central Asia - there are places where snakes accumulate), which, perhaps, builds the name of the mineral from the places of its extraction or export from any eastern country or province.

In 1818, Friedrich Stromeyer published a detailed description of the metal, the properties of which he had already studied well. In its free form, the new element was a white metal, soft and not very strong, covered with a brownish oxide film on top. Pretty soon, as often happens, Strohmeyer's priority in the discovery of cadmium began to be challenged, but all claims were rejected. Somewhat later, another chemist, Kersten, found a new element in Silesian zinc ore and named it mellin (from the Latin mellinus, "yellow like quince"). The reason for this name was the color of the precipitate formed under the action of hydrogen sulfide.

To Kersten's chagrin, the "mellin" turned out to be Stromeyer's "cadmium". Even later, other names were proposed for the forty-eighth element: in 1821, John proposed naming the new element "claprotium" - in honor of the famous chemist Martin Klaproth - the discoverer of uranium, zirconium and titanium, and Gilbert "junonium" - after the asteroid discovered in 1804 Juno. But no matter how great Klaproth's merits to science, his name was not destined to gain a foothold in the list of chemical elements: cadmium remained cadmium. True, in Russian chemical literature the first half of XIX For centuries, cadmium was often called cadmium.

Being in nature

Cadmium is a typically rare and rather scattered element, the average content of this metal in earth's crust(Clark) is estimated at about 1.3 * 10–5% or 1.6 * 10–5% by weight, it turns out that cadmium in the lithosphere is approximately 130 mg/t. There is so little cadmium in the bowels of our planet that even germanium, which is considered rare, is 25 times more! Approximately the same ratios for cadmium and other rare metals: beryllium, cesium, scandium and indium. Cadmium is close in abundance to antimony (2 * 10–5%) and twice as common as mercury (8 * 10–6%).

Cadmium is characterized by migration in hot groundwater along with zinc (cadmium is found as an isomorphic impurity in many minerals and always in zinc minerals) and other chalcophilic elements, that is, chemical elements prone to the formation of natural sulfides, selenides, tellurides, sulfosalts and sometimes found in the native state. In addition, cadmium is concentrated in hydrothermal deposits. Volcanic rocks are quite rich in cadmium, containing up to 0.2 mg of cadmium per kg; among sedimentary rocks, clay is the richest in the forty-eighth element - up to 0.3 mg / kg (for comparison, limestones contain cadmium 0.035 mg / kg, sandstones - 0.03 mg / kg). The average content of cadmium in the soil is 0.06 mg/kg.

Also, this rare metal is present in water - in dissolved form (sulfate, chloride, cadmium nitrate) and in suspension as part of organo-mineral complexes. IN natural conditions Cadmium enters groundwater as a result of leaching of non-ferrous metal ores, as well as as a result of the decomposition of aquatic plants and organisms capable of accumulating it. Since the beginning of the 20th century, anthropogenic contamination of natural waters with cadmium has become the predominant factor in the entry of cadmium into water and soil. The content of cadmium in water is significantly affected by the pH of the medium (in an alkaline medium, cadmium precipitates in the form of hydroxide), as well as sorption processes. For the same anthropogenic reason, cadmium is also present in the air.

In rural areas, the content of cadmium in the air is 0.1-5.0 ng / m3 (1 ng or 1 nanogram = 10-9 grams), in cities - 2-15 ng / m3, in industrial areas - from 15 to 150 ng /m3. Mainly the entry of cadmium into atmospheric air due to the fact that many coals burned in thermal power plants contain this element. Being deposited from the air, cadmium enters the water and soil. The increase in the content of cadmium in the soil is facilitated by the use of mineral fertilizers, because almost all of them contain minor impurities of this metal. From water and soil, cadmium enters plants and living organisms and further along the food chain can be "supplied" to humans.

Cadmium has its own minerals: howliite, otavite CdCO3, montemponite CdO (contains 87.5% Cd), greenockite CdS (77.8% Cd), xanthochroite CdS(H2O)x (77.2% Cd) cadmoselite CdSe (47% Cd ). However, they do not form their own deposits, but are present as impurities in zinc, copper, lead and polymetallic ores (more than 50), which are the main source of industrial production of cadmium. Moreover, the main role is played by zinc ores, where the concentration of cadmium ranges from 0.01 to 5% (in sphalerite ZnS). In most cases, the content of cadmium in sphalerite does not exceed 0.4 - 0.6%. Cadmium accumulates in galena (0.005 - 0.02%), stannite (0.003 - 0.2%), pyrite (up to 0.02%), chalcopyrite (0.006 - 0.12%), cadmium is extracted from these sulfides.

Cadmium is able to accumulate in plants (most of all in fungi) and living organisms (especially in water), for this reason, cadmium can be found in marine sedimentary rocks - shales (Mansfeld, Germany).

Application

The main consumer of cadmium is the production of chemical current sources: nickel-cadmium and silver-cadmium batteries, lead-cadmium and mercury-cadmium cells in backup batteries, normal Weston cells. Cadmium nickel batteries (AKN) used in industry are one of the most popular among other chemical current sources.

The negative plates of such accumulators are made of iron grids with cadmium sponge as an active agent, and the positive plates are coated with nickel oxide. The electrolyte is a solution of caustic potash (potassium hydroxide). Nickel-cadmium alkaline batteries are more reliable than lead acid batteries. Chemical current sources using cadmium are distinguished by a long service life, stable operation and high electrical characteristics. In addition, recharging these batteries takes less than one hour! However, AKN cannot be recharged without a complete preliminary discharge, and in this they, of course, are inferior to metal hydride batteries.

Another wide field of application of cadmium is the deposition of protective anticorrosive coatings on metals (cadmium plating). Cadmium coating reliably protects iron and steel products from atmospheric corrosion. In the past, cadmium plating was done by immersing the metal in molten cadmium, modern process carried out exclusively by electrolysis. Cadmium plating is applied to the most critical parts of aircraft, ships, as well as parts and mechanisms designed to operate in tropical climates.

It is known that some properties of zinc and cadmium are similar, but the cadmium coating has certain advantages over galvanized coating: firstly, it is more resistant to corrosion, and secondly, it is easier to make it even and smooth. In addition, unlike zinc, cadmium is stable in an alkaline environment. Cadmium sheet is used quite widely, however, there is an area in which the use of cadmium coating is strictly prohibited - this food industry. This is due to the high toxicity of cadmium.

Until a certain point, the spread of cadmium coatings was also limited for another reason - when cadmium is applied electrolytically to a steel part, hydrogen contained in the electrolyte can penetrate into the metal, and, as is known, this element causes hydrogen embrittlement in high-strength steels, leading to unexpected destruction of the metal under load . The problem was solved by Soviet scientists from the Institute physical chemistry Academy of Sciences of the USSR. It turned out that a negligible addition of titanium (one atom of titanium per thousand atoms of cadmium) protects the cadmium-plated steel part from the occurrence of hydrogen embrittlement, since the titanium absorbs all the hydrogen from the steel during the coating process.

About a tenth of the world production of cadmium is spent on the production of alloys. The low melting point is one of the reasons for the widespread use of cadmium in low-melting alloys. Such, for example, is Wood's alloy containing 12.5% ​​cadmium. Such alloys are used as solders, as a material for obtaining thin and complex castings, in automatic fire-fighting systems, for soldering glass with metal. Solders containing cadmium are quite resistant to temperature fluctuations.

Other distinguishing feature cadmium alloys - their high anti-friction properties. Thus, an alloy containing 99% cadmium and 1% nickel is used for the manufacture of bearings operating in automobile, aircraft and marine engines. Since cadmium is not sufficiently resistant to acids, including organic acids contained in lubricants, cadmium-based bearing alloys are coated with indium. Alloying copper with small additions of cadmium (less than 1%) makes it possible to make more wear-resistant wires on electric transport lines. Such insignificant additions of cadmium can significantly increase the strength and hardness of copper, practically without worsening it. electrical properties. Cadmium amalgam (a solution of cadmium in mercury) is used in dental technology for the manufacture of dental fillings.

In the forties of the XX century, cadmium acquired a new role - they began to make control and emergency rods of nuclear reactors from it. The reason why cadmium quickly became a strategic material was that it absorbs thermal neutrons very well. But the first reactors of the beginning of the "atomic age" worked exclusively on thermal neutrons. Later it turned out that fast neutron reactors are more promising both for energy and for obtaining nuclear fuel - 239Pu, and cadmium is powerless against fast neutrons, it does not delay them. In the days of thermal neutron reactors, cadmium lost its dominant role, giving way to boron and its compounds (actually, coal and graphite).

About 20% of cadmium (in the form of compounds) is used for the production of inorganic colorants. Cadmium sulfide CdS is an important mineral dye formerly called cadmium yellow. Already at the beginning of the 20th century, it was known that cadmium yellow could be obtained in six shades, ranging from lemon yellow to orange. The resulting paints are resistant to weak alkalis and acids, and are completely insensitive to hydrogen sulfide.

Paints based on CdS were used in many areas - painting, printing, porcelain painting, they covered passenger cars, protecting them from locomotive smoke. Dyes containing cadmium sulfide were used in the textile and soap industries. However, at present, rather expensive cadmium sulfide is often replaced with cheaper dyes - cadmopone (a mixture of cadmium sulfide and barium sulfate) and zinc-cadmium litopone (composition, like that of cadmopone, plus zinc sulfide).

Another cadmium compound, cadmium selenide CdSe, is used as a red dye. However, cadmium compounds have found their application not only in the production of dyes - cadmium sulfide, for example, is also used for the production of film solar cells, the efficiency of which is about 10-16%. In addition, CdS is a fairly good thermoelectric material, which is used as a component of semiconductor materials and phosphors. Sometimes cadmium is used in cryogenic technology, which is associated with its maximum thermal conductivity (relative to other metals) near absolute zero (vacuum).

Production

The main "suppliers" of cadmium are by-products of processing zinc, copper-zinc and lead-zinc ores. As for the native minerals of cadmium, the only one of interest in obtaining cadmium is greenockite CdS, the so-called "cadmium blende". Greenockite is mined together with faerite during the development of zinc ores. During the recycling process, cadmium accumulates in the by-products of the process, from where it is then recovered.

In the processing of polymetallic ores, as mentioned earlier, cadmium is often a by-product of zinc production. These are either copper-cadmium cakes (metal precipitates obtained as a result of cleaning solutions of zinc sulfate ZnSO4 by the action of zinc dust), which contain from 2 to 12% Cd, or pussieres (volatile fractions formed during the distillation of zinc), containing from 0.7 to 1.1% cadmium.

The richest in the forty-eighth element are concentrates obtained during the rectification purification of zinc, they can contain up to 40% cadmium. From copper-cadmium cakes and other products with a high content of cadmium, it is usually leached with sulfuric acid H2SO4 with simultaneous air aeration. The process is carried out in the presence of an oxidizing agent - manganese ore or recycled manganese sludge from electrolysis baths.

In addition, cadmium is recovered from dust from lead and copper smelters (it can contain 0.5 to 5% and 0.2 to 0.5% cadmium, respectively). In such cases, the dust is usually treated with concentrated H2SO4 sulfuric acid, and then the resulting cadmium sulfate is leached with water. Cadmium sponge is precipitated from the resulting cadmium sulfate solution by the action of zinc dust, after which it is dissolved in sulfuric acid and the solution is purified from impurities by the action of sodium carbonate Na2CO3 or zinc oxide ZnO, it is also possible to use ion exchange methods.

Cadmium metal is isolated by electrolysis on aluminum cathodes or by zinc reduction (displacement of cadmium oxide CdO by zinc from CdSO4 solutions) using centrifugal separator reactors. The refining of cadmium metal usually consists in melting the metal under a layer of alkali (to remove zinc and lead), while using Na2CO3 is possible; treatment of the melt with aluminum (to remove nickel) and ammonium chloride NH4Cl (to remove thallium).

Cadmium of higher purity is obtained by electrolytic refining with intermediate purification of the electrolyte, which is carried out using ion exchange or extraction; rectification of the metal (usually under reduced pressure), zone melting or other crystallization methods. Combining the above purification methods, it is possible to obtain metallic cadmium with the content of the main impurities (zinc, copper and others) of only 10-5% by weight. In addition, the methods of electrotransfer in liquid cadmium, electrorefining in a melt of sodium hydroxide NaOH, and amalgam electrolysis can be used to purify cadmium. When zone melting is combined with electrotransfer, separation of cadmium isotopes can occur along with purification.

The world production of cadmium is largely related to the scale of zinc production and has increased significantly over the past decades - according to 2006 data, about 21 thousand tons of cadmium were produced in the world, while in 1980 this figure was only 15 thousand tons. The growth of cadmium consumption continues even now. The main producers of this metal are Asian countries: China, Japan, Korea, Kazakhstan. They account for 12 thousand tons of total production.

Russia, Canada and Mexico can also be considered major producers of cadmium. The shift of mass production of cadmium towards Asia is due to the fact that in Europe there has been a reduction in the use of cadmium, and in the Asian region, on the contrary, the demand for nickel-cadmium elements is growing, which forces many to transfer production to Asian countries.

Physical properties

Cadmium is a silvery-white metal that shimmers blue when freshly cut, but tarnishes in air due to the formation of a protective oxide film. Cadmium is a rather soft metal - it is harder than tin, but softer than zinc, it is quite possible to cut it with a knife. In combination with softness, cadmium has such qualities that are important for industry, such as ductility and ductility - it is perfectly rolled into sheets and drawn into wire, and can be polished without any problems.

When heated above 80 o C, cadmium loses its elasticity, and so much so that it can easily be crushed into powder. The hardness of cadmium according to Mohs is equal to two, according to Brinell (for an annealed sample) 200-275 MPa. Tensile strength 64 MN/m2 or 6.4 kgf/mm2, relative elongation 50% (at 20 o C), yield strength 9.8 MPa.

Cadmium has a hexagonal close-packed crystal lattice with periods: a = 0.296 nm, c = 0.563 nm, c/a ratio = 1.882, z = 2, crystal lattice energy 116 μJ/kmol. Space group С6/mmm, atomic radius 0.156 nm, ionic radius Cd2+ 0.099 nm, atomic volume 13.01∙10-6 m3/mol.

A rod made of pure cadmium, when bent, emits a weak crack like tin ("tin scream") - this is metal microcrystals rubbing against each other, however, any impurities in the metal destroy this effect. In general, in terms of its physical, chemical and pharmacological properties, cadmium belongs to the group of heavy metals, having the most similarities with zinc and mercury.

The melting point of cadmium (321.1 o C) is quite low and can be compared with the melting points of lead (327.4 o C) or thallium (303.6 o C). However, it differs from the melting points of metals similar in a number of properties - lower than that of zinc (419.5 o C), but higher than that of tin (231.9 o C). The boiling point of cadmium is also low - only 770 o C, which is quite interesting - lead, like most other metals, has a large difference between melting and boiling points.

So lead has a boiling point (1745 o C) 5 times higher than the melting point, and tin, whose boiling point is 2620 o C, is 11 times higher than the melting point! At the same time, zinc, similar to cadmium, has a boiling point of only 960 o C at a melting point of 419.5 o C. The thermal expansion coefficient for cadmium is 29.8 * 10-6 (at a temperature of 25 o C). Below 0.519 K, cadmium becomes a superconductor. The thermal conductivity of cadmium at 0 o C is 97.55 W / (m * K) or 0.233 cal / (cm * sec * o C).

The specific heat capacity of cadmium (at a temperature of 25 o C) is 225.02 j/(kg * K) or 0.055 cal/(g * o C). The temperature coefficient of electrical resistance of cadmium in the temperature range from 0 o C to 100 o C is 4.3 * 10-3, the electrical resistivity of cadmium (at a temperature of 20 o C) is 7.4 * 10-8 ohm * m (7.4 * 10-6 ohm*cm). Cadmium is diamagnetic, its magnetic susceptibility is -0.176.10-9 (at a temperature of 20 o C). The standard electrode potential is -0.403 V. The electronegativity of cadmium is 1.7. The effective cross section of capture of thermal neutrons is 2450-2900-10 ~ 28 m2. Work function of electrons = 4.1 eV.

The density (at room temperature) of cadmium is 8.65 g/cm3, which makes it possible to classify cadmium as a heavy metal. According to the classification of N. Reimers, metals with a density of more than 8 g/cm3 should be considered heavy. Thus, heavy metals include Pb, Cu, Zn, Ni, Cd, Co, Sb, Sn, Bi, Hg. And although cadmium is lighter than lead (density 11.34 g/cm3) or mercury (13.546 g/cm3), it is heavier than tin (7.31 g/cm3).

Chemical properties

IN chemical compounds cadmium always exhibits a valence of 2 (the configuration of the outer electron layer is 5s2) - the fact is that for the atoms of the elements of the side subgroup of the second group (zinc, cadmium, mercury), as well as for the atoms of the elements of the copper subgroup, the d-sublevel of the second outer electronic layer is completely filled . However, for elements of the zinc subgroup, this sublevel is already quite stable, and the removal of electrons from it requires a very large expenditure of energy. Another characteristic feature of the elements of the zinc subgroup, which brings them closer to the elements of the copper subgroup, is their tendency to complex formation.

As already mentioned, cadmium is located in the same group of the periodic system with zinc and mercury, occupying an intermediate position between them, for this reason a number of chemical properties of all these elements are similar. For example, oxides and sulfides of these metals are practically insoluble in water.

In dry air, cadmium is stable, but in humid air, a thin film of CdO oxide slowly forms on the metal surface, protecting the metal from further oxidation. With strong incandescence, cadmium burns out, also turning into cadmium oxide - a crystalline powder from light brown to dark brown in color (the difference in color gamut is partly due to particle size, but to a greater extent is the result of crystal lattice defects), CdO density 8.15 g /cm3; above 900 o C cadmium oxide is volatile, and at 1570 o C it sublimates completely. Vapors of cadmium react with water vapor to release hydrogen.

Acids react with cadmium to form salts of this metal. Nitric acid HNO3 easily dissolves cadmium, while nitric oxide is released and nitrate is formed, which gives the hydrate Cd (NO3) 2 * 4H2O. From other acids - hydrochloric and dilute sulfuric - cadmium slowly displaces hydrogen, this is explained by the fact that in the series of voltages cadmium is further than zinc, but ahead of hydrogen. Unlike zinc, cadmium does not interact with alkali solutions. Cadmium reduces ammonium nitrate NH4NO3 in concentrated solutions to ammonium nitrite NH4NO2.

Above the melting point, cadmium combines directly with halogens, forming colorless compounds - cadmium halides. CdCl2, CdBr2 and CdI2 are very easily soluble in water (53.2% by mass at 20 o C), much more difficult to dissolve cadmium fluoride CdF2 (4.06% by mass at 20 o C), which is completely insoluble in ethanol. It can be obtained by the action of fluorine on a metal or hydrogen fluoride on cadmium carbonate. Cadmium chloride is obtained by reacting cadmium with concentrated hydrochloric acid or metal chlorination at 500 o C.

Cadmium bromide is obtained by metal bromination or by the action of hydrogen bromide on cadmium carbonate. When heated, cadmium reacts with sulfur to form CdS sulfide (lemon yellow to orange red), insoluble in water and dilute acids. When cadmium is fused with phosphorus and arsenic, phosphides and arsenides of compositions Cd3P2 and CdAs2 are formed, respectively, with antimony - cadmium antimonide. Cadmium does not react with hydrogen, nitrogen, carbon, silicon and boron. CdH2 hydride and Cd3N2 nitride, which readily decompose on heating, were obtained indirectly.

Solutions of cadmium salts are acidic due to hydrolysis, caustic alkalis precipitate white hydroxide Cd (OH) 2 from them. Under the action of very concentrated alkali solutions, it is converted into hydroxocadmates, such as Na2. Cadmium hydroxide reacts with ammonia to form soluble complexes:

Cd(OH)2 + 6NH3 * H2O → (OH)2 + 6H2O

In addition, Cd(OH)2 goes into solution under the action of alkali cyanides. Above 170 o With it decomposes to CdO. The interaction of cadmium hydroxide with hydrogen peroxide (peroxide) in an aqueous solution leads to the formation of peroxides (peroxides) of various compositions.

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Where does cadmium come from? Cadmium is always found in ores from which zinc, lead are mined, and sometimes in copper ore. Therefore, it inevitably ends up in the waste products of the production of these metals. But they are not thrown away, but they are trying to recycle, because there are many others necessary to a person elements. The proportion of cadmium is very high - 0.3–0.5% by weight of zinc concentrate, and 95% is selected from there. Actually, cadmium was discovered in the study of zinc compounds. They tell such a story (see "Chemistry and Life", 1970, No. 9). In 1817, a conflict arose in Magdeburg: the district doctor Rolov ordered all preparations with zinc oxide to be withdrawn from sale, suspecting that there was arsenic in it. Pharmacists swore that there was no arsenic in the preparations, except perhaps iron oxide, which gives the ointment a yellowish color. The arbitrator was Professor Friedrich Stromeyer of the University of Göttingen, who was then Chief Pharmaceutical Inspector. He actually succeeded in isolating a yellowish compound from the preparation. However, it had nothing to do with either arsenic or iron, but turned out to be an oxide of a new element. In the autumn of 1817, in conversations with colleagues, Strohmeyer called it cadmium, which is given the following explanation. The legendary Phoenician prince Cadmus, having come to Boeotia in search of his sister Europe, stolen by Zeus, built the fortress of Cadmeus there. Then the ancient Greek Thebes grew up around it. In ancient times, a specific mixture of zinc compounds was found near this city, called "Cadmean earth" or cadmea. Stromeyer used this name.

Rolov also soon became convinced that the suspicious impurity was not arsenic, but a compound of a new metal. But his article sent to “ Journal fur der praktischen Heilkunde”, was delayed and came out in April 1818, when among chemists they already knew about the discovery of Stromeyer.

How did the yellow color of the compound affect interest in cadmium? In the most direct way: shortly after the discovery of Stromeyer, a certain Carsten, senior adviser for metallurgy at the plant in Breslau (now Wroclaw), found in Silesian zinc ore an element that gave a yellow precipitate when hydrogen sulfide was passed through a solution of hydrogen sulfide, and called it "melinium" from the Latin word " mellis", which means honey. It was still the same cadmium, and its sulfide became an excellent yellow pigment, first for artists, and then, when the price dropped, in the paint business. Getting cadmium sulfide in different ways, you can make a beautiful paint of different shades - from lemon to orange. Since it is resistant to acids, alkalis and strong heat, cadmium yellow was also suitable for painting ceramics. In addition, when cadmium sulfide is mixed with ultramarine, an excellent green dye is formed - cadmium green. When burning, cadmium gives a blue color, so it was also used in pyrotechnics. Thus, in the 90s of the XX century, 17% of cadmium was used for the preparation of paints for various purposes.

What is the main application of cadmium? Nickel-cadmium batteries: one of the electrodes in them is made of cadmium or its hydroxide, their production consumes more than 60% of all cadmium mined. These batteries are very durable: they can provide several times more discharge-charge cycles than their closest competitors - lead batteries, however, they cost ten times more. And in terms of the ratio of stored electricity to weight, Ni-Cd is twice as superior to Pb, which makes them promising for electric vehicles. The life of modern nickel-cadmium batteries is more than 30 years. They charge quickly and release energy quickly, and due to their low internal resistance, they can provide high current density without heating. Therefore, they are used wherever high current densities are required - in electric cars, trolleybuses, trams, electric trains, screwdrivers, as well as in radio equipment and household appliances. Until recently, they also supplied power to computers and Cell Phones, but now their place is taken by lithium-ion. Nickel-cadmium batteries are also expected to be used in alternative energy systems, where from time to time it is necessary to pump excess energy somewhere, which then compensates for the lack of production due to bad weather: such batteries can provide reliable storage of up to 6.5 MWh of electricity, which puts them on a par with lead and sodium sulfide.

Among the disadvantages of nickel-cadmium batteries is a large self-discharge and memory effect: if you charge a battery that is not completely discharged, it will accumulate less and less energy each time. It is believed that this effect can be combated if such a battery is very strongly discharged from time to time. But their main drawback is the toxicity of cadmium; because of it, the use of nickel-cadmium batteries, however, as well as cadmium pigments for paints, stabilizers for polymers (10% of metal production), coatings for metals (5%), is constantly decreasing.

What application of cadmium is on the rise? Production of solar panels. Cadmium telluride converts sunlight into electricity quite well, although it is inferior to silicon batteries: the efficiency of modules available on the market is 8–9% and 13–16%, respectively. However, cadmium telluride is deposited as thin films on conductive glass, which requires much less energy and materials than the production of silicon batteries. As a result (" ”, 2012, 16, 5245–5259; doi:10.1016/j.rser.2012.04.034) the energy costs for the production of the battery pay off by generating energy in a year, which is two to three times (as well as carbon dioxide emissions per kilowatt of electricity it produces in Europe) less than that of silicon batteries. In other words, batteries using cadmium compounds are very environmentally friendly. With the growth of efficiency, this difference will increase even more, and there are prospects here, since the record efficiency values ​​for cadmium telluride were 15.6 and 13.8% in 2011 when applying its thin film to glass and flexible polyimide, respectively. Polymer-based batteries weigh hundreds of times less than glass batteries and are easily mounted on curved surfaces, which attract the attention of researchers.

Thin films are not everything. Elements based on quantum dots from chalcogenides - cadmium sulfide, telluride and selenide - are promising representatives of third-generation solar cells, which, according to experts, are finally able to ensure self-sufficiency for this energy source. The dots attract the attention of researchers, because due to the dependence of their properties on size, it is possible to achieve absorption and conversion into electricity of the entire solar spectrum. In addition, in some experiments, chalcogenide quantum dots have shown the ability to obtain several electrons from one photon - the effect of multiple generation of excitons. Obviously, with proper use, it will greatly increase the efficiency of light conversion, and this allows us to count on the convergence of the cost of electricity from the Sun and burning coal.

So far, however, the potential of quantum dots has not been fully disclosed - a record efficiency of 5.42% at the beginning of 2013 was demonstrated by an element based on quantum dots from cadmium sulfide and selenide with manganese additives (“ Renewable and Sustainable Energy Reviews”, 2013, 22, 148–167; doi:10.1016/j.rser.2013.01.030). It is believed that the points themselves are not to blame for this - the optimal material of the electrodes has not yet been selected, which ensures the complete removal of charge carriers from them resulting from the photoreaction. It is possible that cadmium will also be useful in the manufacture of electrodes - experiments with an electrode from cadmium stannate CdSnO 3 for solar batteries show good results (“ Solar Energy Materials & Solar Cells”, 2013, 117, 300–305; doi:10.1016/j.solmat.2013.06.009).

What other nanoparticles are made from cadmium compounds? The most diverse: nanorods, nanotubes and even structures similar to sea urchins. It is possible that some of them will find application in the technologies of the future.

Is there cadmium in tin soldiers? It may well be there, because a small addition of cadmium greatly reduces the melting point of other metals and, accordingly, provides a better filling of the mold with a casting alloy. It is not surprising that it is part of the famous Wood's alloy and its varieties. Such alloys are widely used in metallography (they are poured into thin sections, samples for microscopic examination), in precision casting, they serve as investment rods in the manufacture of hollow figures, as well as fusible fuses. Apparently, it was the English engineer Barnaba Wood who was the first to discover the ability of cadmium to lower the melting point of other metals, because the elements that make up the alloy of his name - seven to eight parts of bismuth, four lead and two each of tin and cadmium - have melting points of 271, respectively, 327, 231 and 742°C. And all melt together at 69°C! This result in 1860 was so unexpected that the editorial board of the magazine " The American Journal of Science and Arts” added the following postscript to Wood’s article: “We had enough time to repeat only a few interesting experiences Dr. Wood, relating to the amazing effect that cadmium has in lowering the melting points of various alloys. Now the ability of cadmium to reduce the melting point of metals is used by adding it to solders - this is 2% of the world's metal production. Moreover, in solders, not only industrial, but also home-made. Here, for example, on the forum of jewelers, craftsmen give the following recommendations: “Add a little cadmium to gold, its melting point will be lower than that of the metal of the product, and it will be possible to solder the required part. Since cadmium is likely to evaporate during soldering, the sample of the product may not change. Only you need to solder under the draft, so as not to get poisoned.

What is the pathway of cadmium into the body?“Cadmium in children's toys is impossible, it is poisonous,” the reader will say. And he will be right, but only in part, since it is unlikely that cadmium from a tin soldier (any figurine made of silvery heavy metal cast in a small workshop) or from a yellow pattern on a salad bowl can somehow enter the human body. He has completely different paths. There are three of them. Firstly, with cigarette smoke: cadmium is perfectly accumulated in tobacco leaves. Secondly, from the air, especially urban air: it contains a lot of road dust resulting from abrasion of tires and brake pads (and cadmium is part of them); the more you breathe this dust, the higher the content of cadmium in the body. Thus, for traffic controllers it is one and a half times more than for road workers from rural areas (“ Chemosphere”, 2013, 90, 7, 2077–2084). Cadmium is also present in the smoke of thermal stations, if they run on coal, and in the smoke from burning firewood, since trees extract it from the soil. The third source is food, especially the roots, leaves and grains of plants: this is where cadmium accumulates. Studies conducted by scientists from Seattle showed that in young women living in places not polluted with cadmium, smoking is the main source of cadmium, it increases the content of this metal by one and a half times. But among food products, tofu turned out to be a significant source of cadmium - one serving of it per week increases the content of cadmium in the body by 22% (“ Science of the Total Environment”, 2011, 409, 9, 1632–1637). A lot of cadmium is found in mollusks and crustaceans that feed on plankton. New Zealand biologists have found that cadmium in sea water (its concentration in it is 0.11 μg / l) most likely ended up there through the fault of man. Cadmium is contained in phosphate fertilizers, from where, by the way, it mainly enters edible plants. Rains wash fertilizers into rivers, then into the sea. Cadmium travels on the surface of the microparticles. Once in salt water, it is released and ends up in phytoplankton, and with it in oysters. As a result, molluscs grown higher in river mouths, where cadmium has not yet been washed away from microparticles, are relatively pure, and those below contain especially a lot of this metal (“ Science of the Total Environment”, 1996, 181, 1, 31–44). The cadmium content of oysters is 13–26 micrograms per gram of dry weight. For comparison: in sunflower seeds, which are also considered an important source of cadmium, - 0.2–2.5 μg per gram of grains, in tobacco leaves - 0.5–1 μg per gram of dry weight. Because plankton isn't just for oysters, cadmium also ends up in fish caught in dirty seas. And the dirtiest is the Baltic Sea, where many rivers flow from industrial areas and areas with intensive agriculture.

How does anthropogenic cadmium get into the environment? In addition to phosphate fertilizers, road dust and fuel combustion, there are two other ways. The first is non-ferrous metallurgy: with all the efforts aimed at cleaning emissions, a certain amount of it inevitably passes through all filters. The second is landfills and recycling sites, for example, when plastic burns there. However, in a landfill, even without heating, cadmium leaches and enters the soil with water. In general, non-ferrous metallurgy produces 5 thousand tons of cadmium emissions per year, waste incineration - 1.5, and the production of phosphate fertilizers and wood burning - 0.2 thousand tons each of the more than seven thousand tons that a person dissipates in the environment approximately since the 30s of the XX century. Nature's own possibilities are more modest: 0.52 thousand tons are produced by volcanoes and 0.2 thousand tons - by plant excretions, a total of 0.83 thousand tons (see "Chemistry and Life", 1979, No. 12). In other words, no more than two-thirds of the cadmium extracted from the earth's interior can be converted into metal (and the world output has been fluctuating between 17-20 thousand tons per year for decades), so the prospects for recycling are very wide. However, there is no incentive, which will be discussed further.

How will new materials containing cadmium behave in a landfill? Differently. Detailed Analysis conducted by Vasily Ftenakos of the Brookhaven National Laboratory (USA), who described in detail the life cycle of a cadmium telluride battery (“ Renewable and Sustainable Energy Reviews”, 2004, 8, 303–334; doi:10.1016/j.rser.2003.12.001). He talks like this. In a solar cell, the cadmium compound is sandwiched between layers of glass or plastic. Therefore, particles containing cadmium can only appear in the environment when the element is destroyed, which happens either in very dusty areas or when it breaks. But even then, as the experiment showed, no rain is able to wash any noticeable amount of cadmium out of the element. The evaporation temperature of CdTe exceeds 1000°C, and CdS, also present in these cells, is 1700°C, so there will be no evaporation during operation.

But what if the element is on the roof of a private house in which there was a fire? In air, cadmium telluride remains stable up to temperatures of 1050°C, which is less heating during a conventional fire. Direct experiments have shown that if the battery is made on a glass substrate, almost all of the cadmium will remain in the molten glass - only 0.6% of its already small amount (after all, this is a thin film) can be released. Some elements, when broken in a landfill, do break down, releasing cadmium, while others, more modern, do not. Legislative regulation can ensure that only harmless elements are thrown away. And it would be better not to throw them away at all, because they contain valuable tellurium.

Unfortunately, Fthenakos does not say anything about polymer-based elements, which are likely to burn out, and no cadmium fusing into glass will occur. But he notes that bans on the use of cadmium can lead to much worse consequences: having lost a sales market, manufacturers of zinc, lead and copper will stop extracting cadmium from waste and they will pollute everything around them much more than landfills (recall a third of cadmium flying into a pipe ). Therefore, the use of cadmium should be expanded with the tightening of measures for the disposal of products.

Separately, there is the issue of devices based on nanodots: when destroyed, these materials will inevitably scatter nanoparticles that can move along the food chain. There is data (“ Journal of Hazardous Materials”, 2011, 192, 15, 192–199; doi:10.1016/j.jhazmat.2011.05.003) that they will by no means remain unchanged: an increase in free cadmium was noted in the liver and kidneys of rats injected with cadmium selenide nanodots into the abdominal cavity. The effect was most pronounced if the nanoparticles were illuminated with ultraviolet light before use (apparently, this will be the case with nanodust under natural conditions). Obviously, the requirements for the disposal of solar cells and other devices based on such nanoparticles should be stricter than when using monolithic products.

Why is cadmium dangerous? The question is much more complicated than it might seem, since cadmium enters the body in microscopic quantities and does not act instantly. Researchers from the University of North Dakota, led by Soisunwan Satarug, write about this in detail (“ ”, 2010, 118, 182–190; doi:10.1289/ehp.0901234). Let's review this review.

It can be considered proven that people living in areas where the soil contains a significant amount of cadmium and food is constantly contaminated with it, there is an increased fragility of the bones. The Japanese called this disease itai-itai: it appeared in the 1940s in Toyama Prefecture, where farmers used water from a zinc mine to irrigate their fields. The content of cadmium in rice was so high that daily intake was 600 micrograms per day, or 4200 micrograms per week, or up to 2 grams per person for a lifetime. It is not difficult to identify a causal relationship here, which cannot be said about the chronic consumption of cadmium in small doses. It all comes down to the percentage risk of getting a particular disease. It is still not completely known what doses of cadmium can be considered harmless. The World Health Organization in 1989 named the maximum allowable intake of cadmium per week as 400–500 micrograms, based on the fact that 2 g in a lifetime is a lot, leads to itai-itai. In 1992, the norm was recalculated, it amounted to 7 micrograms per day per kilogram of weight. It is easy to see that the weekly dose for a person weighing 70 kg is the same - 490 mcg. When calculating, it was assumed that the body absorbs 5% of the cadmium entering it, and 0.005% of the amount of metal that is already in it is excreted in the urine. However, some doctors question this model, pointing out that they have seen cases when the body absorbed even 40% of the cadmium that entered it. Moreover, measurements have shown that consumption as low as 1 microgram per kg per day leads to 2 micrograms of cadmium per gram of creatinine in the urine, and unpleasant effects appear even at much lower levels. (The content in the urine of cadmium and other harmful metals, the concentration of which is low, is usually expressed in micrograms per gram of creatinine - this substance is formed during the work of the muscles and is constantly excreted in the urine. The result presented in such units does not depend on the dilution of the sample. Further, the word " creatinine" will be omitted. It is obvious that measuring cadmium in the urine is much easier than its intake from various sources)

What are these effects? Reading the review, one gets the impression that cadmium causes symptoms of old age. First of all, accumulating in the kidneys, it accelerates the degradation of the renal tubules. According to some data, if 2–4 μg of cadmium is excreted in the urine per day, the probability of kidney degradation is 10%; according to others, when not the daily excretion is measured, but the concentration in the test sample, the cadmium content in the urine of 0.67 μg / g is already dangerous. (If we assume that 1-2 grams of creatinine is excreted in the urine per day, then it turns out that a dangerous daily dose of cadmium excretion is about 1 mcg.) As a result of the degradation of the tubules, the ability of the kidneys to return vitamins, minerals and other useful substances to the body is weakened, for example associated with metallothioneins zinc and copper, calcium, phosphates, glucose, amino acids. A twofold increase in the level of cadmium in the urine increases the content of calcium in it by 2 mg per day. It is not hard to guess that calcium loss increases the risk of osteoporosis. Indeed, in a group of women over 50 with more than 1 µg/g of cadmium in the urine, the risk of osteoporosis is 43% higher than in those who had less than 0.5 µg/g. With a cadmium content between 1 and 2 µg/g, the risk of elevated glucose and the development of type 2 diabetes is 1.48 and 1.24, respectively, compared with those with less than 1 µg/g. A survey of Koreans, a quarter of whom suffered from high blood pressure, showed that the risk of this ailment in people with a high content of cadmium is one and a half times higher than with a low one. The risk of heart attack in women with more than 0.88 mcg/g of cadmium in the urine is 1.8 times higher compared to those with less than 0.43 mcg/g. The probability of death from cancer in men with less than 0.22 and more than 0.48 µg/g of cadmium in the urine differs by 4.3 times. There are suspicions that cadmium reduces fertility in men.

In general, from the data of the work of Dr. Sataruga and colleagues, it follows that it is environmental pollution with cadmium that is to blame for the fact that age-related diseases have become much “younger” over the course of the 20th century.

There are also strange data. Thus, a strong relationship has been observed between the content of cadmium in the urine and the risk of getting high blood pressure in Americans who do not smoke, while such a relationship has not been seen in smokers. Meanwhile, the consumption of cadmium among cigarette lovers is obviously higher, and, in addition, the content of cadmium in the urine of Americans is generally more than three times less than that of the Koreans mentioned above. Smokers with senile retinal degradation had urinary cadmium levels of 1.18 µg/g, almost twice as high as smokers without the disease and healthy non-smokers. However, even those non-smokers who developed the disease had just as little cadmium as healthy people - which means that it is not only about him. Such conflicting data make one ask the question: maybe the increased content of cadmium in the urine reflects not the cause, but the consequence of some systemic processes in the body? After all, cadmium consumption was not measured in most of the studies mentioned in the review, only its output.

How to deal with cadmium in the body? Scientific research there is little on this topic, and the principle is indicated in the same work of researchers from North Dakota. Cadmium is not one of the vital elements, so there are no special mechanisms for its absorption in the body - cadmium uses those that are provided for heavy metals similar to it, which form divalent ions: zinc, iron, manganese and calcium. The lack of any of these elements immediately leads to increased absorption of cadmium. For example, iron deficiency increases cadmium levels in Thai women three to four times. The same was found in a study of Bangladeshi women, but zinc was also in play. It follows from this how important it is to maintain the correct microelement balance in the body.

There are other ideas as well. For example, the Brazilians show that caffeine significantly, more than two times, reduces the content of cadmium both in the blood and in tissues, including genital ones, in experimental rats (“ Reproductive Toxicology”, 2013, 35, 137–143; doi:10.1016/j.reprotox.2012.10.009). According to researchers, caffeine forms complexes with cadmium, preventing its absorption. The conclusion suggests itself: the custom of drinking coffee or tea with a meal, which also contains caffeine, is correct.

Sometimes there is a paradox: food with a high content of cadmium does not affect the body. For example, a 1986 study of oyster drinkers came up with a surprise: at a maximum intake of 72 oysters per week, they ate a whopping 1,750 micrograms of cadmium, but this did not show up in either urine or hair. Where all this cadmium went remains a mystery. There is an assumption that selenium, the content of which in those oysters was high, somehow interfered with the absorption of cadmium, and he apparently came out with other inedible substances through the intestines. However, in 2008, compliance with the general line was restored: among oyster farm workers who ate 18 oysters every week for more than 12 years, the cadmium content in the urine increased 2.5 times compared to the average in the United States - up to 0, 76 mcg/g.

Or maybe it is better to deal with cadmium before it enters the body, for example, make sure that it does not get into the soil and air? It is hardly possible to free phosphate fertilizers from cadmium, it is long and expensive to breed plants with reduced cadmium digestibility, although attempts are being made with respect to tobacco, but it is possible to clean the soil with hyperaccumulator plants - in the case of cadmium, this is black nightshade Solanum nigrum, he is an edible berry of a funnel, a French variety of a shepherd's bag or mustard of a bluish or alpine yarutka ( thlaspi caerulescens) and Chinese stonecrop Sedum alfredii. True, it is not clear what to do with the parts of these plants enriched with cadmium - they are clearly not suitable for compost and ashes obtained in the garden. With the industrial combustion of the so-called solid biofuels - straw, brushwood, etc. - there are opportunities to get rid of harmful metal: it is necessary to separate the high-temperature smoke fractions containing it from the low-temperature ones - then the resulting ash can be safely brought back to the field, restoring its fertility.

But the main thing that should be cleaned is the air. The most radical method was chosen by the American, and now the European Union authorities - an uncompromising fight against tobacco smoking (“ Environmental Health Perspectives”, 2012, 120, 2, 204–209; doi:10.1289/ehp.1104020). The results are clear: the average cadmium content in the urine of Americans has decreased from 0.36 mcg/g in 1988 to 0.26 mcg/g in 2008. Since even for heavy smokers (20 or more packs a year by American standards) it fell from 0.71 to 0.49, and for non-smokers from 0.26 to 0.19, it should be assumed that smoking bans in in public places significantly reduced the effects of passive tobacco smoke consumption. Given the above data on the harmfulness of microdoses of cadmium, such bans seem to be the most easily implemented and very significant contribution to public health. It would also be worthwhile to tighten the requirements for emissions from non-ferrous metallurgy plants, boiler houses and cars, and at the same time make sure that less harmful dust flies from under the wheels “shod” in rubber.

In the autumn of 1817 when checking some pharmacies in the district of Magdeburg in Germany, zinc oxide was found containing some kind of impurity. The district doctor R. Rolov suspected the presence of arsenic in it and banned the sale of the drug. The owner of a factory that produced zinc oxide, K. Hermann did not agree with this decision and began to study the ill-fated product. As a result of his experiments, he concluded that the zinc oxide produced by his factory contained an admixture of some unknown metal. The obtained data were published by K. Hermann in April 1818 in the article "On Silesian zinc oxide and on a probably still unknown metal found in it." At the same time, a favorable conclusion was published by F. Stromeyer, who confirmed the conclusions of Hermann and proposed that the new metal be called cadmium.

F. Stromeyer, who was the general inspector of pharmacies in the province of Hanover, published a detailed article about the new metal in another journal. The article, dated April 26, 1818, was published in an issue with the date 1817 on the cover. Apparently, this circumstance, combined with the fact that Strohmeyer (with the consent of Hermann) gave the name to the discovered metal, led to errors in determining both the dates and the author of the discovery.

physical properties.

Cadmium - silver white shimmering blue metal, which tarnishes in air due to the formation of a protective oxide film. Melting point - 321 ° C, boiling point - 770 ° C. A stick of pure cadmium crunches like tin when bent, but any impurities in the metal destroy this effect. Cadmium is harder than tin, but softer than chink - it can be cut with a knife. When heated above 80°C, cadmium loses its elasticity to such an extent that it can be ground into a powder.

Cadmium forms alloys and compounds with many metals and is highly soluble in mercury.

General chemical characteristics of cadmium.

When heated, oxidation becomes more intense and ignition of the metal is possible. Powdered cadmium easily ignites in air with a bright red flame, forming an oxide.

If powdered cadmium is vigorously mixed with water, hydrogen evolution is observed and the presence of hydrogen peroxide can be detected.

Diluted hydrochloric and sulfuric acid when heated, they gradually react with cadmium, releasing hydrogen. Dry hydrogen chloride interacts with cadmium at a temperature of 440 °C. Dry sulfur dioxide also reacts with the metal, forming cadmium sulfide CdS and partly its sulfate CdSO 4 . Nitric acid, interacting with cadmium under normal conditions, releases ammonia, and when heated, nitrogen oxides.

Cadmium, unlike zinc, insoluble in caustic alkalis, but also soluble in ammonium hydroxide. When cadmium reacts with an ammonium nitrate solution, nitrates are formed.

Aluminum, zinc and iron displace cadmium from solutions of its compounds. He himself precipitates copper and other more electropositive elements from solutions. When heated, cadmium combines directly with phosphorus, sulfur, selenium, tellurium and halogens, but it is not possible to obtain its hydride and nitride by direct interaction with hydrogen and nitrogen.

The most important compounds of cadmium.

cadmium oxideCdO can be obtained by burning the metal in air or oxygen, by roasting its sulfide, or by thermal decomposition of certain compounds. This is a powder of different colors, depending on the temperature at which it was obtained: greenish-yellow (350-370 ° C.), thick dark blue (800 ° C.), brown, black.

cadmium hydroxideCD(Oh) 2 in the form of a white gelatinous precipitate is released from solutions of its salts under the action of alkalis.

Cadmium sulfideCDS- one of the most important compounds of cadmium. Depending on the physicochemical conditions of obtaining, it can be from lemon yellow to red.

Halogenites cadmium is quite easily obtained by direct interaction of the elements, as well as by dissolving cadmium, its oxide or carbonate in the corresponding acids. All forming salts are colorless crystalline substances.

Cadmium carbonatecdcAbout 3 in the form of a white amorphous precipitate precipitates from cadmium solutions when alkali carbonates are added to them.

Raw sources of cadmium. Getting cadmium.

Cadmium is scattered element, i.e. it almost does not form its own minerals, and the deposits of such minerals are not known at all. Cadmium is present in ores of other metals in concentrations of hundredths and thousandths of a percent. Some ores containing 1-1.5% cadmium are considered extremely rich in this metal.

The only cadmium mineral of some interest is its natural sulfide, greenockite, or cadmium blende. When developing deposits of zinc ores, greenockite is mined together with phaerite and ends up in zinc plants. During processing, cadmium is concentrated in some intermediate products of the process, from which it is then extracted.

Thus, the real raw material for the production of cadmium is the cakes of zinc-electrolyte plants, lead and copper smelters.

The first production was organized in Upper Silesia in 1829.

Currently, the world produces over 10,000 tons of cadmium per year.

Application of cadmium.

The main part of the industrial consumption of cadmium falls on cadmium protective coatings protecting metals from corrosion. These coatings have a significant advantage over nickel, zinc or tin coatings. do not exfoliate from parts during deformation.

Cadmium coatings in some cases are superior to all others: 1) in protection against sea ​​water, 2) for parts operating in enclosed spaces with high humidity, 3) to protect electrical contacts.

The second area of ​​​​application of cadmium is alloy production. Cadmium alloys are silvery-white, ductile, well machinable. Cadmium alloys with small additions of nickel, copper and silver are used to manufacture bearings for powerful ship, aircraft and automobile engines.

Copper wire with only 1% cadmium added is twice as strong, while its electrical conductivity decreases slightly.

Copper-cadmium alloy with zirconium addition has even greater strength and is used for high-voltage transmission lines.

Pure cadmium, due to its remarkable property - a high thermal neutron capture cross section, is used for the manufacture of control and emergency rods. nuclear reactors on slow neutrons.

IN jewelry business alloys of gold and cadmium are used. By changing the ratio of components, different color shades are obtained.

Nickel-cadmium accumulators, even completely discharged do not become completely unusable.

Cadmium amalgam is used in dentistry for making fillings.

Biological properties of cadmium.

Cadmium coatings are unacceptable when they must come into contact with food. The metal itself is non-toxic, but extremely poisonous soluble cadmium compounds. Moreover, any way they enter the body and in any state (solution, dust, smoke, fog) is dangerous. In terms of toxicity, cadmium is not inferior to mercury and arsenic. Cadmium compounds have a depressing effect on nervous system, affect the respiratory tract and cause changes in internal organs.

Large concentrations of cadmium can lead to acute poisoning: a minute stay in a room containing 2500 mg / m 3 of its compounds leads to death. In acute poisoning, the symptoms of the lesion do not develop immediately, but after a certain latent period, which can last from 1-2 to 30-40 hours.

Despite the toxicity, it has been proven that cadmium is a trace element vital for the development of living organisms. Its function is still unclear. Feeding plants favorably affects their development.

Cadmium is an element of a side subgroup of the second group, the fifth period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 48. It is designated by the symbol Cd (lat. Cadmium). Soft malleable ductile silver-white transition metal.

History of the discovery of cadmium

The district doctor Rolov was distinguished by a sharp temper. So, in 1817, he ordered that all preparations with zinc oxide produced at Herman's Shenebek factory be withdrawn from sale. By appearance preparations, he suspected that there was arsenic in zinc oxide! (Zinc oxide is still used in skin diseases; ointments, powders, emulsions are made from it.)

To prove his case, the strict auditor dissolved the suspected oxide in acid and passed hydrogen sulfide through this solution: a yellow precipitate fell out. Arsenic sulfides are just yellow!

The owner of the factory began to challenge Rolov's decision. He himself was a chemist and, having personally analyzed product samples, did not find any arsenic in them. He reported the results of the analysis to Rolov, and at the same time to the authorities of the land of Hanover. The authorities, of course, requested samples in order to send them for analysis to one of the reputable chemists. It was decided that the judge in the dispute between Rolov and Herman should be Professor Friedrich Stromeyer, who since 1802 had been the chair of chemistry at the University of Göttingen and the post of general inspector of all Hanoverian pharmacies.

Stromeyer was sent not only zinc oxide, but also other zinc preparations from the Hermann factory, including ZnCO 3 , from which this oxide was obtained. Having calcined zinc carbonate, Strohmeyer obtained oxide, but not white, as it should have been, but yellowish. The owner of the factory explained the coloring with an admixture of iron, but Stromeyer was not satisfied with this explanation. Having bought more zinc preparations, he made a complete analysis of them and without much difficulty isolated the element that caused yellowing. The analysis said that it was not arsenic (as Rolov claimed), but not iron (as Herman claimed).

It was a new, previously unknown metal, chemically very similar to zinc. Only its hydroxide, unlike Zn(OH) 2 , was not amphoteric, but had pronounced basic properties.

In its free form, the new element was a white metal, soft and not very strong, covered with a brownish oxide film on top. Stromeyer called this metal cadmium, clearly alluding to its "zinc" origin: the Greek word καδμεια has long denoted zinc ores and zinc oxide.

In 1818, Stromeyer published detailed information about the new chemical element, and almost immediately its priority began to be encroached upon. The first to speak was the same Rolov, who previously believed that there was arsenic in the preparations from the German factory. Shortly after Stromeyer, another German chemist, Kersten, discovered a new element in Silesian zinc ore and named it mellin (from the Latin mellinus, “yellow like quince”) because of the color of the precipitate formed by the action of hydrogen sulfide. But it was cadmium already discovered by Strohmeyer. Later, two more names were proposed for this element: klaprotium - in honor of the famous chemist Martin Klaproth and junonium - after the asteroid Juno discovered in 1804. But the name given to the element by its discoverer was nevertheless established. True, in Russian chemical literature of the first half of the 19th century. cadmium was often called cadmium.

The average content of cadmium in the earth's crust is 130 mg/t. Cadmium is one of the rare, trace elements: it is found as an isomorphic impurity in many minerals and always in zinc minerals. Only 6 cadmium minerals are known. Very rare cadmium minerals are greenockite CdS (77.8% Cd), howliite (the same), otavite CdCO 3 , montemponite CdO (87.5% Cd), cadmoselite CdSe (47% Cd), xantochroite CdS (H 2 O) x (77.2% Cd). The bulk of cadmium is dispersed in a large number of minerals (more than 50), mainly in zinc, lead, copper, iron, manganese, and mercury sulfides.

Although independent minerals of cadmium are known - greenockite(CdS), otavite(CdCO 3), monteponite(CdO) and selenide(CdSe), they do not form their own deposits, but are present as impurities in zinc, lead, copper and polymetallic ores, which are the main source of industrial production of cadmium. The maximum concentration is noted in zinc minerals and, first of all, in sphalerite (up to 5%). In most cases, the content of cadmium in sphalerite does not exceed 0.4 - 0.6%. In other sulfides, for example, in the frame, the cadmium content is 0.003 - 0.2%, in galena 0.005 - 0.02%, in chalcopyrite 0.006 - 0.12%; cadmium is usually not recovered from these sulfides.
Cadmium is present, by the way, in certain quantities in the air. According to foreign data, the content of cadmium in the air is 0.1-5.0 ng / m 3 in rural areas (1 ng or 1 nanogram = 10 -9 grams), 2 - 15 ng / m 3 - in cities and from 15 to 150 ng / m 3 - in industrial areas. This is due, in particular, to the fact that many coals contain cadmium as an impurity and, when burned at thermal power plants, it enters the atmosphere. At the same time, a significant part of it settles on the soil. Also, the increase in the content of cadmium in the soil contributes to the use of mineral fertilizers, because. almost all of them contain minor impurities of cadmium.
Cadmium is able to accumulate in plants (most of all in fungi) and living organisms (especially in aquatic ones) and further along the food chain can be "supplied" to humans. Lots of cadmium in cigarette smoke.

IN vivo Cadmium enters groundwater as a result of leaching of non-ferrous metal ores, as well as as a result of the decomposition of aquatic plants and organisms capable of accumulating it. In recent decades, the anthropogenic factor of cadmium pollution of natural waters has become prevalent. Cadmium is present in water in dissolved form (sulfate, chloride, cadmium nitrate) and in suspended form as part of organo-mineral complexes. The content of cadmium in water is significantly affected by the pH of the medium (in an alkaline medium, cadmium precipitates in the form of hydroxide), as well as sorption processes.

The only mineral that is of interest in obtaining cadmium is greenockite, the so-called "cadmium blende". It is mined together with faerite in the development of zinc ores. During processing, cadmium is concentrated in the by-products of the process, from where it is then recovered. Currently, over 10³ tons of cadmium are produced per year.

In the processing of polymetallic ores, it, an analogue of zinc, invariably turns out mainly in zinc concentrate. And cadmium is reduced even more easily than zinc, and has a lower boiling point (767 and 906°C, respectively). Therefore, at a temperature of about 800°C, it is not difficult to separate zinc and cadmium.

Silvery white soft metal with a hexagonal lattice. If the cadmium stick is bent, then a slight crack can be heard - these are metal microcrystals rubbing against each other (a tin bar also cracks).

Cadmium is soft, malleable, easily machinable. This also facilitated and accelerated his path to atomic technology. The high selectivity of cadmium, its sensitivity to thermal neutrons, also played into the hands of physicists. And according to the main performance characteristic - the capture cross section of thermal neutrons - cadmium occupies one of the first places among all elements of the periodic system - 2400 barn. (Recall that the capture cross section is the ability to “take in” neutrons, measured in conventional units of barns.)

Natural cadmium consists of eight isotopes (with mass numbers 106, 108, 110, 111, 112, 113, 114 and 116), and the capture cross section is a characteristic in which the isotopes of one element can differ very much. In a natural mixture of cadmium isotopes, the main "neutron-eater" is an isotope with a mass number of 113. Its individual capture cross section is enormous - 25,000 barns!

By attaching a neutron, cadmium-113 turns into the most common (28.86% of the natural mixture) isotope of element No. 48 - cadmium-114. The share of cadmium-113 itself is only 12.26%. Unfortunately, separating eight isotopes of cadmium is much more difficult than separating two isotopes of boron.

The crystal lattice of Cadmium is hexagonal, a = 2.97311 Å, c = 5.60694 Å (at 25 °C); atomic radius 1.56 Å, ionic radius Cd 2+ 1.03 Å. Density 8.65 g / cm 3 (20 ° C), t pl 320.9 ° C, t kip 767 ° C, thermal expansion coefficient 29.8 10 -6 (at 25 ° C); thermal conductivity (at 0°C) 97.55 W/(m K) or 0.233 cal/(cm sec °C); specific heat capacity (at 25 °C) 225.02 J/(kg K) or 0.055 cal/(g °C); electrical resistivity (at 20 °C) 7.4 10 -8 ohm m (7.4 10 -6 ohm cm); temperature coefficient of electrical resistance 4.3 10 -3 (0-100 ° C). Tensile strength 64 MN / m 2 (6.4 kgf / mm 2), elongation 20%, Brinell hardness 160 MN / m 2 (16 kgf / mm 2).

Cadmium is located in the same group of the periodic table with zinc and mercury, occupying an intermediate position between them, so some Chemical properties these elements are similar. Thus, sulfides and oxides of these elements are practically insoluble in water. Cadmium does not interact with carbon, hence it follows that cadmium does not form carbides.

In accordance with the external electronic configuration of the 4d 10 5s 2 atom, the valency of Cadmium in compounds is 2. Cadmium tarnishes in air, being covered with a thin film of CdO oxide, which protects the metal from further oxidation. When strongly heated in air, Cadmium burns into CdO oxide - a crystalline powder from light brown to dark brown in color, density 8.15 g/cm 3 ; at 700°C CdO sublimates without melting. Cadmium combines directly with halogens; these compounds are colorless; CdCl 2 , CdBr 2 and CdI 2 are very easily soluble in water (about 1 part of anhydrous salt in 1 part of water at 20 ° C), CdF 2 is more difficult to dissolve (1 part in 25 parts of water). With sulfur, cadmium forms lemon-yellow to orange-red CdS sulfide, insoluble in water and dilute acids. Cadmium readily dissolves in nitric acid with the release of nitrogen oxides and the formation of nitrate, which gives hydrate Cd (NOa) 2 4H 2 O. From acids - hydrochloric and dilute sulfuric Cadmium slowly releases hydrogen, when the solutions are evaporated, chloride hydrates 2CdCl 2 crystallize from them 5H 2 O and sulfate 3CdSO 4 8H 2 O. Cadmium salt solutions are acidic due to hydrolysis; caustic alkalis precipitate white hydroxide Cd (OH) 2 from them, insoluble in excess of the reagent; however, under the action of concentrated alkali solutions on Cd (OH) 2, hydrooxocadmates, for example Na 2, were obtained. The Cd 2+ cation easily forms complex ions with ammonia 2+ and with cyan 2- and 4-. Numerous basic, double and complex Cadmium salts are known. Cadmium compounds are poisonous; especially dangerous is the inhalation of vapors of its oxide.

Cadmium gained popularity in the 40s of the 20th century. It was at this time that cadmium turned into a strategic material - they began to make control and emergency rods of nuclear reactors from it.

Cadmium at first turned out to be the main "core" material, primarily because it absorbs thermal neutrons well. All reactors of the beginning of the "atomic age" (and the first of them was built by Enrico Fermi in 1942) worked on thermal neutrons. Only many years later it became clear that fast neutron reactors are more promising both for energy and for obtaining nuclear fuel - plutonium-239. And cadmium is powerless against fast neutrons, it does not delay them.

However, the role of cadmium in reactor construction should not be exaggerated, since the physical and chemical properties of this metal (strength, hardness, heat resistance - its melting point is only 321 ° C) leave much to be desired. Cadmium was the first core material. Then boron and its compounds began to play the leading roles. But cadmium is easier to obtain in large quantities.

About a tenth of the world's cadmium production is spent on the production of alloys. Cadmium alloys are mainly used as antifriction materials and solders. A well-known alloy composition of 99% Cd and 1% Ni is used for the manufacture of bearings operating in automobile, aircraft and marine engines at high temperatures. Since cadmium is not sufficiently resistant to acids, including organic acids contained in lubricants, sometimes cadmium-based bearing alloys are coated with indium.

Alloying copper with small additions of cadmium makes it possible to make more wear-resistant wires on electric transport lines. Copper with the addition of cadmium almost does not differ in electrical conductivity from pure copper, but it noticeably surpasses it in strength and hardness.

An alloy of cadmium with gold has a greenish color. An alloy of cadmium with tungsten, rhenium and 0.15% uranium 235 - sky blue was obtained by Spanish scientists in 1998.

Everyone knows galvanized tin, but not everyone knows that not only galvanizing, but also cadmium plating is used to protect iron from corrosion. Cadmium coating is now applied only electrolytically, most often in industrial conditions cyanide baths are used. Previously, iron and other metals were cadmium-plated by immersing products in molten cadmium.

Despite the similar properties of cadmium and zinc, cadmium coating has several advantages: it is more resistant to corrosion, it is easier to make it even and smooth. In addition, cadmium, unlike zinc, is stable in an alkaline environment. Cadmium tin is used quite widely; it is denied access only to the production of food containers, because cadmium is toxic. Cadmium coatings have another interesting feature: in the atmosphere of rural areas, they are much more resistant to corrosion than in the atmosphere of industrial areas. Such a coating fails especially quickly if the content of sulfurous or sulfuric anhydrides is increased in the air.

The most important field of application of cadmium is the production of chemical current sources. Cadmium electrodes are used in batteries and accumulators. The negative plates of nickel-cadmium batteries are made of iron meshes with sponge cadmium as the active agent. Positive plates coated with nickel hydroxide. The electrolyte is a potassium hydroxide solution. On the basis of cadmium and nickel, compact batteries for guided missiles are also made, only in this case, not iron, but nickel grids are installed as the basis.

Nickel-cadmium alkaline batteries are more reliable than lead (acid) batteries. These current sources are distinguished by high electrical characteristics, stable operation, and long service life. They can be charged in just one hour. However, nickel-cadmium batteries cannot be recharged without being fully discharged first (they are inferior to metal hydride batteries in this respect).

About 20% of cadmium is used for the manufacture of cadmium electrodes used in batteries (nickel-cadmium and silver-cadmium), normal Weston cells, in backup batteries (lead-cadmium cell, mercury-cadmium cell, etc.).

About 20% of cadmium is used for the production of inorganic dyes (sulfides and selenides, mixed salts, for example, cadmium sulfide - cadmium citric).

• Cadmium is sometimes used in experimental medicine.

Cadmium is used in homeopathic medicine.

• IN last years cadmium began to be used in the creation of new antitumor nano-medicines. In Russia, in the early 1950s, the first successful experiments were carried out related to the development of antitumor drugs based on cadmium compounds.

• Cadmium sulfide is used for the production of film solar cells with an efficiency of about 10-16%, and also as a very good thermoelectric material.

• It is used as a component of semiconductor materials and phosphors.

• The thermal conductivity of a metal near absolute zero is the highest among all metals, so cadmium is sometimes used for cryogenic technology.

The effect of cadmium on the human body

Cadmium is one of the most toxic heavy metals and therefore it is assigned to the 2nd hazard class by the Russian SanPiN.

Cadmium compounds are poisonous. A particularly dangerous case is the inhalation of vapors of its oxide (CdO). Cadmium is a cumulative poison (capable of accumulating in the body). In drinking water MPC for cadmium is 0.001 mg/dm³

Soluble cadmium compounds, after being absorbed into the blood, affect the central nervous system, liver and kidneys, and disrupt phosphorus-calcium metabolism. Chronic poisoning leads to anemia and bone destruction.

Cadmium is normally present in small amounts in the body of a healthy person. Cadmium easily accumulates in rapidly proliferating cells (for example, in tumor or sex cells). It binds to the cytoplasmic and nuclear material of cells and damages them. It alters the activity of many hormones and enzymes. This is due to its ability to bind sulfhydryl (-SH) groups.

In 1968, an article appeared in a well-known magazine, which was called "Cadmium and the Heart." It said that Dr. Carroll, a US public health officer, had discovered a relationship between atmospheric cadmium levels and the incidence of deaths from cardiovascular disease. If, say, in city A the content of cadmium in the air is higher than in city B, then the cores of city A die earlier than if they lived in city B. Carroll made this conclusion after analyzing data for 28 cities.

According to USEPA, WHO and Health Canada, the total daily intake of cadmium in the human body from all sources is 10-50 mcg. The main and most "stable" source is food - an average of 10 to 30-40 micrograms of cadmium per day. Vegetables, fruits, animal meat, fish usually contain 10-20 micrograms of cadmium per kilogram of body weight. However, there are no rules without exceptions. Cereal crops grown on soil contaminated with cadmium or watered with water containing cadmium may contain an increased amount of cadmium (more than 25 µg/kg).

A significant "increase" of cadmium is obtained by smokers. One cigarette contains 1 microgram (and sometimes more - up to 2 micrograms) of cadmium. So consider - a person who smokes a pack of cigarettes per day exposes his body to additional effects of at least 20 micrograms of cadmium, which, for reference, are not retained even by a carbon filter.
It should also be noted that cadmium is more easily absorbed by the body through the lungs - up to 10-20%. Those. from one pack of cigarettes, 2-4 micrograms of cadmium will be absorbed. When taken through the gastrointestinal tract, the percentage of digestibility is only 4-7% (0.2 - 5 μg of cadmium per day in absolute terms). Thus, a smoker at least 1.5-2 times increases the "load" on his body for cadmium, which is fraught with adverse health consequences.

World cadmium market

About 20 thousand tons of cadmium are produced per year. The volume of its production is largely related to the scale of zinc production.

About 82% of the world's refined cadmium supply comes from nickel-cadmium power supplies, but after restrictions on their production in Europe, one third of cadmium consumption will be affected. As a result of the increase in zinc production in Europe and the reduction in the use of cadmium, there may be “free” cadmium, most often in the form of solid waste, but the production of nickel-cadmium batteries is growing in Asia, there is a transfer of production to Asia and, as a result, the demand for cadmium in Asian region. For the time being, this will allow keeping world consumption of cadmium at the current level. In 2007, cadmium prices, starting from $4.18/kg, rose to $13/kg, but by the end of the year they were $7/kg.

In 2010, the South Korean Young Poong Corp. increased cadmium production by 75% to 1,400 tons per year and plans to launch new facilities soon, a company official said.