Zinc

Zinc is a chemical element with symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. In some respects zinc is chemically similar to magnesium: both elements exhibit only one normal oxidation state (+2), and the Zn2+ and Mg2+ ions are of similar size. Zinc is the 24th most abundant element in Earth's crust and has five stable isotopes. The most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. The largest workable lodes are in Australia, Asia, and the United States. Zinc is refined by froth flotation of the ore, roasting, and final extraction using electricity (electrowinning).

Brass, an alloy of copper and zinc in various proportions, was used as early as the third millennium BC in the Aegean, Iraq, the United Arab Emirates, Kalmykia, Turkmenistan and Georgia, and the second millennium BC in West India, Uzbekistan, Iran, Syria, Iraq, and Israel/Palestine.[3][4][5] Zinc metal was not produced on a large scale until the 12th century in India, though it was known to the ancient Romans and Greeks.[6] The mines of Rajasthan have given definite evidence of zinc production going back to the 6th century BC.[7] To date, the oldest evidence of pure zinc comes from Zawar, in Rajasthan, as early as the 9th century AD when a distillation process was employed to make pure zinc.[8] Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow".

The element was probably named by the alchemist Paracelsus after the German word Zinke (prong, tooth). German chemist Andreas Sigismund Marggraf is credited with discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. Corrosion-resistant zinc plating of iron (hot-dip galvanizing) is the major application for zinc. Other applications are in electrical batteries, small non-structural castings, and alloys such as brass. A variety of zinc compounds are commonly used, such as zinc carbonate and zinc gluconate (as dietary supplements), zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory.

Zinc is an essential mineral, including to prenatal and postnatal development.[9] Zinc deficiency affects about two billion people in the developing world and is associated with many diseases.[10] In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea.[9] Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans.[11]

Consumption of excess zinc may cause ataxia, lethargy, and copper deficiency.

Zinc,  30Zn
Zinc fragment sublimed and 1cm3 cube
Zinc
Appearancesilver-gray
Standard atomic weight Ar, std(Zn)65.38(2)[1]
Zinc in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Zn

Cd
copperzincgallium
Atomic number (Z)30
Groupgroup 12
Periodperiod 4
Blockd-block
Element category  post-transition metal, alternatively considered a transition metal
Electron configuration[Ar] 3d10 4s2
Electrons per shell
2, 8, 18, 2
Physical properties
Phase at STPsolid
Melting point692.68 K ​(419.53 °C, ​787.15 °F)
Boiling point1180 K ​(907 °C, ​1665 °F)
Density (near r.t.)7.14 g/cm3
when liquid (at m.p.)6.57 g/cm3
Heat of fusion7.32 kJ/mol
Heat of vaporization115 kJ/mol
Molar heat capacity25.470 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 610 670 750 852 990 1179
Atomic properties
Oxidation states−2, 0, +1, +2 (an amphoteric oxide)
ElectronegativityPauling scale: 1.65
Ionization energies
  • 1st: 906.4 kJ/mol
  • 2nd: 1733.3 kJ/mol
  • 3rd: 3833 kJ/mol
  • (more)
Atomic radiusempirical: 134 pm
Covalent radius122±4 pm
Van der Waals radius139 pm
Color lines in a spectral range
Spectral lines of zinc
Other properties
Natural occurrenceprimordial
Crystal structurehexagonal close-packed (hcp)
Hexagonal close packed crystal structure for zinc
Speed of sound thin rod3850 m/s (at r.t.) (rolled)
Thermal expansion30.2 µm/(m·K) (at 25 °C)
Thermal conductivity116 W/(m·K)
Electrical resistivity59.0 nΩ·m (at 20 °C)
Magnetic orderingdiamagnetic
Magnetic susceptibility−11.4·10−6 cm3/mol (298 K)[2]
Young's modulus108 GPa
Shear modulus43 GPa
Bulk modulus70 GPa
Poisson ratio0.25
Mohs hardness2.5
Brinell hardness327–412 MPa
CAS Number7440-66-6
History
DiscoveryIndian metallurgists (before 1000 BCE)
First isolationAndreas Sigismund Marggraf (1746)
Recognized as a unique metal byRasaratna Samuccaya (800)
Main isotopes of zinc
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
64Zn 49.2% stable
65Zn syn 244 d ε 65Cu
γ
66Zn 27.7% stable
67Zn 4.0% stable
68Zn 18.5% stable
69Zn syn 56 min β 69Ga
69mZn syn 13.8 h β 69Ga
70Zn 0.6% stable
71Zn syn 2.4 min β 71Ga
71mZn syn 4 d β 71Ga
72Zn syn 46.5 h β 72Ga

Characteristics

Physical properties

Zinc is a bluish-white, lustrous, diamagnetic metal,[12] though most common commercial grades of the metal have a dull finish.[13] It is somewhat less dense than iron and has a hexagonal crystal structure, with a distorted form of hexagonal close packing, in which each atom has six nearest neighbors (at 265.9 pm) in its own plane and six others at a greater distance of 290.6 pm.[14] The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C.[12][13] Above 210 °C, the metal becomes brittle again and can be pulverized by beating.[15] Zinc is a fair conductor of electricity.[12] For a metal, zinc has relatively low melting (419.5 °C) and boiling points (907 °C).[16] The melting point is the lowest of all the d-block metals aside from mercury and cadmium; for this, among other reasons, zinc, cadmium, and mercury are often not considered to be transition metals like the rest of the d-block metals are.[16]

Many alloys contain zinc, including brass. Other metals long known to form binary alloys with zinc are aluminium, antimony, bismuth, gold, iron, lead, mercury, silver, tin, magnesium, cobalt, nickel, tellurium, and sodium.[17] Although neither zinc nor zirconium are ferromagnetic, their alloy ZrZn
2
exhibits ferromagnetism below 35 K.[12]

A bar of zinc generates a characteristic sound when bent, similar to tin cry.

Occurrence

Zinc makes up about 75 ppm (0.0075%) of Earth's crust, making it the 24th most abundant element. Soil contains zinc in 5–770 ppm with an average 64 ppm. Seawater has only 30 ppb and the atmosphere, 0.1–4 µg/m3.[18] The element is normally found in association with other base metals such as copper and lead in ores.[19] Zinc is a chalcophile, meaning the element is more likely to be found in minerals together with sulfur and other heavy chalcogens, rather than with the light chalcogen oxygen or with non-chalcogen electronegative elements such as the halogens. Sulfides formed as the crust solidified under the reducing conditions of the early Earth's atmosphere.[20] Sphalerite, which is a form of zinc sulfide, is the most heavily mined zinc-containing ore because its concentrate contains 60–62% zinc.[19]

Other source minerals for zinc include smithsonite (zinc carbonate), hemimorphite (zinc silicate), wurtzite (another zinc sulfide), and sometimes hydrozincite (basic zinc carbonate).[21] With the exception of wurtzite, all these other minerals were formed by weathering of the primordial zinc sulfides.[20]

Identified world zinc resources total about 1.9–2.8 billion tonnes.[22][23] Large deposits are in Australia, Canada and the United States, with the largest reserves in Iran.[20][24][25] The most recent estimate of reserve base for zinc (meets specified minimum physical criteria related to current mining and production practices) was made in 2009 and calculated to be roughly 480 Mt.[26] Zinc reserves, on the other hand, are geologically identified ore bodies whose suitability for recovery is economically based (location, grade, quality, and quantity) at the time of determination. Since exploration and mine development is an ongoing process, the amount of zinc reserves is not a fixed number and sustainability of zinc ore supplies cannot be judged by simply extrapolating the combined mine life of today's zinc mines. This concept is well supported by data from the United States Geological Survey (USGS), which illustrates that although refined zinc production increased 80% between 1990 and 2010, the reserve lifetime for zinc has remained unchanged. About 346 million tonnes have been extracted throughout history to 2002, and scholars have estimated that about 109–305 million tonnes are in use.[27][28][29]

Isotopes

Five stable isotopes of zinc occur in nature, with 64Zn being the most abundant isotope (49.17% natural abundance).[30][31] The other isotopes found in nature are 66
Zn
(27.73%), 67
Zn
(4.04%), 68
Zn
(18.45%), and 70
Zn
(0.61%).[31] The most abundant isotope 64Zn and the rare 70Zn are theoretically unstable on energetic grounds, though their predicted half-lives exceed 4.3×1018 years[32] and 1.3×1016 years,[31] meaning that their radioactivity could be ignored for practical purposes.

Several dozen radioisotopes have been characterized. 65
Zn
, which has a half-life of 243.66 days, is the least active radioisotope, followed by 72
Zn
with a half-life of 46.5 hours.[30] Zinc has 10 nuclear isomers. 69mZn has the longest half-life, 13.76 h.[30] The superscript m indicates a metastable isotope. The nucleus of a metastable isotope is in an excited state and will return to the ground state by emitting a photon in the form of a gamma ray. 61
Zn
has three excited metastable states and 73
Zn
has two.[33] The isotopes 65
Zn
, 71
Zn
, 77
Zn
and 78
Zn
each have only one excited metastable state.[30]

The most common decay mode of a radioisotope of zinc with a mass number lower than 66 is electron capture. The decay product resulting from electron capture is an isotope of copper.[30]

n
30
Zn
+
e
n
29
Cu

The most common decay mode of a radioisotope of zinc with mass number higher than 66 is beta decay), which produces an isotope of gallium.[30]

n
30
Zn
n
31
Ga
+
e
+
ν
e

Compounds and chemistry

Reactivity

Zinc has an electron configuration of [Ar]3d104s2 and is a member of the group 12 of the periodic table. It is a moderately reactive metal and strong reducing agent.[34] The surface of the pure metal tarnishes quickly, eventually forming a protective passivating layer of the basic zinc carbonate, Zn
5
(OH)
6
(CO3)
2
, by reaction with atmospheric carbon dioxide.[35] This layer helps prevent further reaction with air and water.

Zinc burns in air with a bright bluish-green flame, giving off fumes of zinc oxide.[36] Zinc reacts readily with acids, alkalis and other non-metals.[37] Extremely pure zinc reacts only slowly at room temperature with acids.[36] Strong acids, such as hydrochloric or sulfuric acid, can remove the passivating layer and subsequent reaction with water releases hydrogen gas.[36]

The chemistry of zinc is dominated by the +2 oxidation state. When compounds in this oxidation state are formed, the outer shell s electrons are lost, yielding a bare zinc ion with the electronic configuration [Ar]3d10.[38] In aqueous solution an octahedral complex, [Zn(H
2
O)6]2+
is the predominant species.[39] The volatilization of zinc in combination with zinc chloride at temperatures above 285 °C indicates the formation of Zn
2
Cl
2
, a zinc compound with a +1 oxidation state.[36] No compounds of zinc in oxidation states other than +1 or +2 are known.[40] Calculations indicate that a zinc compound with the oxidation state of +4 is unlikely to exist.[41]

Zinc chemistry is similar to the chemistry of the late first-row transition metals, nickel and copper, though it has a filled d-shell and compounds are diamagnetic and mostly colorless.[42] The ionic radii of zinc and magnesium happen to be nearly identical. Because of this some of the equivalent salts have the same crystal structure,[43] and in other circumstances where ionic radius is a determining factor, the chemistry of zinc has much in common with that of magnesium.[36] In other respects, there is little similarity with the late first-row transition metals. Zinc tends to form bonds with a greater degree of covalency and much more stable complexes with N- and S- donors.[42] Complexes of zinc are mostly 4- or 6- coordinate although 5-coordinate complexes are known.[36]

Zinc(I) compounds

Zinc(I) compounds are rare and need bulky ligands to stabilize the low oxidation state. Most zinc(I) compounds contain formally the [Zn2]2+ core, which is analogous to the [Hg2]2+ dimeric cation present in mercury(I) compounds. The diamagnetic nature of the ion confirms its dimeric structure. The first zinc(I) compound containing the Zn–Zn bond, 5-C5Me5)2Zn2, is also the first dimetallocene. The [Zn2]2+ ion rapidly disproportionates into zinc metal and zinc(II), and has been obtained only a yellow glass only by cooling a solution of metallic zinc in molten ZnCl2.[44]

Zinc(II) compounds

Zinc acetate
Zinc acetate
Zinc chloride
Zinc chloride

Binary compounds of zinc are known for most of the metalloids and all the nonmetals except the noble gases. The oxide ZnO is a white powder that is nearly insoluble in neutral aqueous solutions, but is amphoteric, dissolving in both strong basic and acidic solutions.[36] The other chalcogenides (ZnS, ZnSe, and ZnTe) have varied applications in electronics and optics.[45] Pnictogenides (Zn
3
N
2
, Zn
3
P
2
, Zn
3
As
2
and Zn
3
Sb
2
),[46][47] the peroxide (ZnO
2
), the hydride (ZnH
2
), and the carbide (ZnC
2
) are also known.[48] Of the four halides, ZnF
2
has the most ionic character, while the others (ZnCl
2
, ZnBr
2
, and ZnI
2
) have relatively low melting points and are considered to have more covalent character.[49]

In weak basic solutions containing Zn2+
ions, the hydroxide Zn(OH)
2
forms as a white precipitate. In stronger alkaline solutions, this hydroxide is dissolved to form zincates ([Zn(OH)4]2−
).[36] The nitrate Zn(NO3)
2
, chlorate Zn(ClO3)
2
, sulfate ZnSO
4
, phosphate Zn
3
(PO4)
2
, molybdate ZnMoO
4
, cyanide Zn(CN)
2
, arsenite Zn(AsO2)
2
, arsenate Zn(AsO4)
2
·8H
2
O
and the chromate ZnCrO
4
(one of the few colored zinc compounds) are a few examples of other common inorganic compounds of zinc.[50][51] One of the simplest examples of an organic compound of zinc is the acetate (Zn(O
2
CCH3)
2
).

Organozinc compounds are those that contain zinc–carbon covalent bonds. Diethylzinc ((C
2
H5)
2
Zn
) is a reagent in synthetic chemistry. It was first reported in 1848 from the reaction of zinc and ethyl iodide, and was the first compound known to contain a metal–carbon sigma bond.[52]

Test for zinc

Cobalticyanide paper (Rinnmann's test for Zn) can be used as a chemical indicator for zinc. 4 g of K3Co(CN)6 and 1 g of KClO3 is dissolved on 100 ml of water. Paper is dipped in the solution and dried at 100 °C. One drop of the sample is dropped onto the dry paper and heated. A green disc indicates the presence of zinc.[53]

History

Ancient use

Hemmoorer Eimer
Late Roman brass bucket – the Hemmoorer Eimer from Warstade, Germany, second to third century AD

Various isolated examples of the use of impure zinc in ancient times have been discovered. Zinc ores were used to make the zinc–copper alloy brass thousands of years prior to the discovery of zinc as a separate element. Judean brass from the 14th to 10th centuries BC contains 23% zinc.[4]

Knowledge of how to produce brass spread to Ancient Greece by the 7th century BC, but few varieties were made.[5] Ornaments made of alloys containing 80–90% zinc, with lead, iron, antimony, and other metals making up the remainder, have been found that are 2,500 years old.[19] A possibly prehistoric statuette containing 87.5% zinc was found in a Dacian archaeological site.[54]

The oldest known pills were made of the zinc carbonates hydrozincite and smithsonite. The pills were used for sore eyes and were found aboard the Roman ship Relitto del Pozzino, wrecked in 140 BC.[55][56]

The manufacture of brass was known to the Romans by about 30 BC.[57] They made brass by heating powdered calamine (zinc silicate or carbonate), charcoal and copper together in a crucible.[57] The resulting calamine brass was then either cast or hammered into shape for use in weaponry.[58] Some coins struck by Romans in the Christian era are made of what is probably calamine brass.[59]

Strabo writing in the 1st century BC (but quoting a now lost work of the 4th century BC historian Theopompus) mentions "drops of false silver" which when mixed with copper make brass. This may refer to small quantities of zinc that is a by-product of smelting sulfide ores.[60] Zinc in such remnants in smelting ovens was usually discarded as it was thought to be worthless.[61]

The Berne zinc tablet is a votive plaque dating to Roman Gaul made of an alloy that is mostly zinc.[62]

The Charaka Samhita, thought to have been written between 300 and 500 AD,[63] mentions a metal which, when oxidized, produces pushpanjan, thought to be zinc oxide.[64] Zinc mines at Zawar, near Udaipur in India, have been active since the Mauryan period (c.  322 and 187 BCE). The smelting of metallic zinc here, however, appears to have begun around the 12th century AD.[65][66] One estimate is that this location produced an estimated million tonnes of metallic zinc and zinc oxide from the 12th to 16th centuries.[21] Another estimate gives a total production of 60,000 tonnes of metallic zinc over this period.[65] The Rasaratna Samuccaya, written in approximately the 13th century AD, mentions two types of zinc-containing ores: one used for metal extraction and another used for medicinal purposes.[66]

Early studies and naming

Zinc was distinctly recognized as a metal under the designation of Yasada or Jasada in the medical Lexicon ascribed to the Hindu king Madanapala (of Taka dynasty) and written about the year 1374.[67] Smelting and extraction of impure zinc by reducing calamine with wool and other organic substances was accomplished in the 13th century in India.[12][68] The Chinese did not learn of the technique until the 17th century.[68]

Zinc-alchemy symbols
Various alchemical symbols for the element zinc

Alchemists burned zinc metal in air and collected the resulting zinc oxide on a condenser. Some alchemists called this zinc oxide lana philosophica, Latin for "philosopher's wool", because it collected in wooly tufts, whereas others thought it looked like white snow and named it nix album.[69]

The name of the metal was probably first documented by Paracelsus, a Swiss-born German alchemist, who referred to the metal as "zincum" or "zinken" in his book Liber Mineralium II, in the 16th century.[68][70] The word is probably derived from the German zinke, and supposedly meant "tooth-like, pointed or jagged" (metallic zinc crystals have a needle-like appearance).[71] Zink could also imply "tin-like" because of its relation to German zinn meaning tin.[72] Yet another possibility is that the word is derived from the Persian word سنگ seng meaning stone.[73] The metal was also called Indian tin, tutanego, calamine, and spinter.[19]

German metallurgist Andreas Libavius received a quantity of what he called "calay" of Malabar from a cargo ship captured from the Portuguese in 1596.[74] Libavius described the properties of the sample, which may have been zinc. Zinc was regularly imported to Europe from the Orient in the 17th and early 18th centuries,[68] but was at times very expensive.[note 1]

Andreas Sigismund Marggraf-flip
Andreas Sigismund Marggraf is given credit for first isolating pure zinc

Isolation

Metallic zinc was isolated in India by 1300 AD,[75][76][77] much earlier than in the West. Before it was isolated in Europe, it was imported from India in about 1600 CE.[78] Postlewayt's Universal Dictionary, a contemporary source giving technological information in Europe, did not mention zinc before 1751 but the element was studied before then.[66][79]

Flemish metallurgist and alchemist P. M. de Respour reported that he had extracted metallic zinc from zinc oxide in 1668.[21] By the start of the 18th century, Étienne François Geoffroy described how zinc oxide condenses as yellow crystals on bars of iron placed above zinc ore that is being smelted.[21] In Britain, John Lane is said to have carried out experiments to smelt zinc, probably at Landore, prior to his bankruptcy in 1726.[80]

In 1738 in Great Britain, William Champion patented a process to extract zinc from calamine in a vertical retort style smelter.[81] His technique resembled that used at Zawar zinc mines in Rajasthan, but no evidence suggests he visited the Orient.[78] Champion's process was used through 1851.[68]

German chemist Andreas Marggraf normally gets credit for discovering pure metallic zinc, even though Swedish chemist Anton von Swab had distilled zinc from calamine four years previously.[68] In his 1746 experiment, Marggraf heated a mixture of calamine and charcoal in a closed vessel without copper to obtain a metal.[61] This procedure became commercially practical by 1752.[82]

Later work

Luigi Galvani, oil-painting
Galvanization was named after Luigi Galvani.

William Champion's brother, John, patented a process in 1758 for calcining zinc sulfide into an oxide usable in the retort process.[19] Prior to this, only calamine could be used to produce zinc. In 1798, Johann Christian Ruberg improved on the smelting process by building the first horizontal retort smelter.[83] Jean-Jacques Daniel Dony built a different kind of horizontal zinc smelter in Belgium that processed even more zinc.[68] Italian doctor Luigi Galvani discovered in 1780 that connecting the spinal cord of a freshly dissected frog to an iron rail attached by a brass hook caused the frog's leg to twitch.[84] He incorrectly thought he had discovered an ability of nerves and muscles to create electricity and called the effect "animal electricity".[85] The galvanic cell and the process of galvanization were both named for Luigi Galvani, and his discoveries paved the way for electrical batteries, galvanization, and cathodic protection.[85]

Galvani's friend, Alessandro Volta, continued researching the effect and invented the Voltaic pile in 1800.[84] The basic unit of Volta's pile was a simplified galvanic cell, made of plates of copper and zinc separated by an electrolyte and connected by a conductor externally. The units were stacked in series to make the Voltaic cell, which produced electricity by directing electrons from the zinc to the copper and allowing the zinc to corrode.[84]

The non-magnetic character of zinc and its lack of color in solution delayed discovery of its importance to biochemistry and nutrition.[86] This changed in 1940 when carbonic anhydrase, an enzyme that scrubs carbon dioxide from blood, was shown to have zinc in its active site.[86] The digestive enzyme carboxypeptidase became the second known zinc-containing enzyme in 1955.[86]

Production

Mining and processing

Top zinc output countries 2014[22]
Rank Country Tonnes
1 China 5,000,000
2 Australia 1,500,000
3 Peru 1,300,000
4 India 820,000
5 United States 700,000
6 Mexico 700,000
World Zinc Production 2006
Percentage of zinc output in 2006 by countries[87]
Zinc world production.svg&lang=en
World production trend

Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper with an annual production of about 13 million tonnes.[22] The world's largest zinc producer is Nyrstar, a merger of the Australian OZ Minerals and the Belgian Umicore.[88] About 70% of the world's zinc originates from mining, while the remaining 30% comes from recycling secondary zinc.[89] Commercially pure zinc is known as Special High Grade, often abbreviated SHG, and is 99.995% pure.[90]

Worldwide, 95% of new zinc is mined from sulfidic ore deposits, in which sphalerite (ZnS) is nearly always mixed with the sulfides of copper, lead and iron.[91] Zinc mines are scattered throughout the world, with the main areas being China, Australia, and Peru. China produced 38% of the global zinc output in 2014.[22]

Zinc metal is produced using extractive metallurgy.[92] The ore is finely ground, then put through froth flotation to separate minerals from gangue (on the property of hydrophobicity), to get a zinc sulfide ore concentrate[92] consisting of about 50% zinc, 32% sulfur, 13% iron, and 5% SiO
2
.[92]

Roasting converts the zinc sulfide concentrate to zinc oxide:[91]

2 ZnS + 3 O
2
→ 2 ZnO + 2 SO
2

The sulfur dioxide is used for the production of sulfuric acid, which is necessary for the leaching process. If deposits of zinc carbonate, zinc silicate, or zinc spinel (like the Skorpion Deposit in Namibia) are used for zinc production, the roasting can be omitted.[93]

For further processing two basic methods are used: pyrometallurgy or electrowinning. Pyrometallurgy reduces zinc oxide with carbon or carbon monoxide at 950 °C (1,740 °F) into the metal, which is distilled as zinc vapor to separate it from other metals, which are not volatile at those temperatures.[94] The zinc vapor is collected in a condenser.[91] The equations below describe this process:[91]

2 ZnO + C → 2 Zn + CO
2
ZnO + CO → Zn + CO
2

In electrowinning, zinc is leached from the ore concentrate by sulfuric acid:[95]

ZnO + H
2
SO
4
ZnSO
4
+ H
2
O

Finally, the zinc is reduced by electrolysis.[91]

2 ZnSO
4
+ 2 H
2
O
→ 2 Zn + 2 H
2
SO
4
+ O
2

The sulfuric acid is regenerated and recycled to the leaching step.

When galvanised feedstock is fed to an electric arc furnace, the zinc is recovered from the dust by a number of processes, predominately the Waelz process (90% as of 2014).[96]

Environmental impact

Refinement of sulfidic zinc ores produces large volumes of sulfur dioxide and cadmium vapor. Smelter slag and other residues contain significant quantities of metals. About 1.1 million tonnes of metallic zinc and 130 thousand tonnes of lead were mined and smelted in the Belgian towns of La Calamine and Plombières between 1806 and 1882.[97] The dumps of the past mining operations leach zinc and cadmium, and the sediments of the Geul River contain non-trivial amounts of metals.[97] About two thousand years ago, emissions of zinc from mining and smelting totaled 10 thousand tonnes a year. After increasing 10-fold from 1850, zinc emissions peaked at 3.4 million tonnes per year in the 1980s and declined to 2.7 million tonnes in the 1990s, although a 2005 study of the Arctic troposphere found that the concentrations there did not reflect the decline. Anthropogenic and natural emissions occur at a ratio of 20 to 1.[98]

Zinc in rivers flowing through industrial and mining areas can be as high as 20 ppm.[99] Effective sewage treatment greatly reduces this; treatment along the Rhine, for example, has decreased zinc levels to 50 ppb.[99] Concentrations of zinc as low as 2 ppm adversely affects the amount of oxygen that fish can carry in their blood.[100]

Historically responsible for high metal levels in the Derwent River, the zinc works at Lutana is the largest exporter in Tasmania, generating 2.5% of the state's GDP, and producing more than 250 000 tonnes of zinc per year.
Historically responsible for high metal levels in the Derwent River,[101] the zinc works at Lutana is the largest exporter in Tasmania, generating 2.5% of the state's GDP, and producing more than 250 000 tonnes of zinc per year.[102]

Soils contaminated with zinc from mining, refining, or fertilizing with zinc-bearing sludge can contain several grams of zinc per kilogram of dry soil. Levels of zinc in excess of 500 ppm in soil interfere with the ability of plants to absorb other essential metals, such as iron and manganese. Zinc levels of 2000 ppm to 180,000 ppm (18%) have been recorded in some soil samples.[99]

Applications

Major applications of zinc include (numbers are given for the US)[103]

  1. Galvanizing (55%)
  2. Brass and bronze (16%)
  3. Other alloys (21%)
  4. Miscellaneous (8%)

Anti-corrosion and batteries

Feuerverzinkte Oberfläche
Hot-dip handrail galvanized crystalline surface

Zinc is most commonly used as an anti-corrosion agent,[104] and galvanization (coating of iron or steel) is the most familiar form. In 2009 in the United States, 55% or 893,000 tons of the zinc metal was used for galvanization.[103]

Zinc is more reactive than iron or steel and thus will attract almost all local oxidation until it completely corrodes away.[105] A protective surface layer of oxide and carbonate (Zn
5
(OH)
6
(CO
3
)
2
)
forms as the zinc corrodes.[106] This protection lasts even after the zinc layer is scratched but degrades through time as the zinc corrodes away.[106] The zinc is applied electrochemically or as molten zinc by hot-dip galvanizing or spraying. Galvanization is used on chain-link fencing, guard rails, suspension bridges, lightposts, metal roofs, heat exchangers, and car bodies.[18]

The relative reactivity of zinc and its ability to attract oxidation to itself makes it an efficient sacrificial anode in cathodic protection (CP). For example, cathodic protection of a buried pipeline can be achieved by connecting anodes made from zinc to the pipe.[106] Zinc acts as the anode (negative terminus) by slowly corroding away as it passes electric current to the steel pipeline.[106][note 2] Zinc is also used to cathodically protect metals that are exposed to sea water.[107] A zinc disc attached to a ship's iron rudder will slowly corrode while the rudder stays intact.[105] Similarly, a zinc plug attached to a propeller or the metal protective guard for the keel of the ship provides temporary protection.

With a standard electrode potential (SEP) of −0.76 volts, zinc is used as an anode material for batteries. (More reactive lithium (SEP −3.04 V) is used for anodes in lithium batteries ). Powdered zinc is used in this way in alkaline batteries and the case (which also serves as the anode) of zinc–carbon batteries is formed from sheet zinc.[108][109] Zinc is used as the anode or fuel of the zinc-air battery/fuel cell.[110][111][112] The zinc-cerium redox flow battery also relies on a zinc-based negative half-cell.[113]

Alloys

A widely used zinc alloy is brass, in which copper is alloyed with anywhere from 3% to 45% zinc, depending upon the type of brass.[106] Brass is generally more ductile and stronger than copper, and has superior corrosion resistance.[106] These properties make it useful in communication equipment, hardware, musical instruments, and water valves.[106]

Microstructure of rolled and annealed brass; magnification 400X
Cast brass microstructure at magnification 400x

Other widely used zinc alloys include nickel silver, typewriter metal, soft and aluminium solder, and commercial bronze.[12] Zinc is also used in contemporary pipe organs as a substitute for the traditional lead/tin alloy in pipes.[114] Alloys of 85–88% zinc, 4–10% copper, and 2–8% aluminium find limited use in certain types of machine bearings. Zinc is the primary metal in American one cent coins (pennies) since 1982.[115] The zinc core is coated with a thin layer of copper to give the appearance of a copper coin. In 1994, 33,200 tonnes (36,600 short tons) of zinc were used to produce 13.6 billion pennies in the United States.[116]

Alloys of zinc with small amounts of copper, aluminium, and magnesium are useful in die casting as well as spin casting, especially in the automotive, electrical, and hardware industries.[12] These alloys are marketed under the name Zamak.[117] An example of this is zinc aluminium. The low melting point together with the low viscosity of the alloy makes possible the production of small and intricate shapes. The low working temperature leads to rapid cooling of the cast products and fast production for assembly.[12][118] Another alloy, marketed under the brand name Prestal, contains 78% zinc and 22% aluminium, and is reported to be nearly as strong as steel but as malleable as plastic.[12][119] This superplasticity of the alloy allows it to be molded using die casts made of ceramics and cement.[12]

Similar alloys with the addition of a small amount of lead can be cold-rolled into sheets. An alloy of 96% zinc and 4% aluminium is used to make stamping dies for low production run applications for which ferrous metal dies would be too expensive.[120] For building facades, roofing, and other applications for sheet metal formed by deep drawing, roll forming, or bending, zinc alloys with titanium and copper are used.[121] Unalloyed zinc is too brittle for these manufacturing processes.[121]

As a dense, inexpensive, easily worked material, zinc is used as a lead replacement. In the wake of lead concerns, zinc appears in weights for various applications ranging from fishing[122] to tire balances and flywheels.[123]

Cadmium zinc telluride (CZT) is a semiconductive alloy that can be divided into an array of small sensing devices.[124] These devices are similar to an integrated circuit and can detect the energy of incoming gamma ray photons.[124] When behind an absorbing mask, the CZT sensor array can determine the direction of the rays.[124]

Other industrial uses

Zinc oxide
Zinc oxide is used as a white pigment in paints.

Roughly one quarter of all zinc output in the United States in 2009 was consumed in zinc compounds;[103] a variety of which are used industrially. Zinc oxide is widely used as a white pigment in paints and as a catalyst in the manufacture of rubber to disburse heat. Zinc oxide is used to protect rubber polymers and plastics from ultraviolet radiation (UV).[18] The semiconductor properties of zinc oxide make it useful in varistors and photocopying products.[125] The zinc zinc-oxide cycle is a two step thermochemical process based on zinc and zinc oxide for hydrogen production.[126]

Zinc chloride is often added to lumber as a fire retardant[127] and sometimes as a wood preservative.[128] It is used in the manufacture of other chemicals.[127] Zinc methyl (Zn(CH3)
2
) is used in a number of organic syntheses.[129] Zinc sulfide (ZnS) is used in luminescent pigments such as on the hands of clocks, X-ray and television screens, and luminous paints.[130] Crystals of ZnS are used in lasers that operate in the mid-infrared part of the spectrum.[131] Zinc sulfate is a chemical in dyes and pigments.[127] Zinc pyrithione is used in antifouling paints.[132]

Zinc powder is sometimes used as a propellant in model rockets.[133] When a compressed mixture of 70% zinc and 30% sulfur powder is ignited there is a violent chemical reaction.[133] This produces zinc sulfide, together with large amounts of hot gas, heat, and light.[133]

Zinc sheet metal is used to make zinc bars.[134]

64
Zn
, the most abundant isotope of zinc, is very susceptible to neutron activation, being transmuted into the highly radioactive 65
Zn
, which has a half-life of 244 days and produces intense gamma radiation. Because of this, zinc oxide used in nuclear reactors as an anti-corrosion agent is depleted of 64
Zn
before use, this is called depleted zinc oxide. For the same reason, zinc has been proposed as a salting material for nuclear weapons (cobalt is another, better-known salting material).[135] A jacket of isotopically enriched 64
Zn
would be irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, forming a large amount of 65
Zn
significantly increasing the radioactivity of the weapon's fallout.[135] Such a weapon is not known to have ever been built, tested, or used.[135]

65
Zn
is used as a tracer to study how alloys that contain zinc wear out, or the path and the role of zinc in organisms.[136]

Zinc dithiocarbamate complexes are used as agricultural fungicides; these include Zineb, Metiram, Propineb and Ziram.[137] Zinc naphthenate is used as wood preservative.[138] Zinc in the form of ZDDP, is used as an anti-wear additive for metal parts in engine oil.[139]

Organic chemistry

DiphenylzincCarbonylAddition
Addition of diphenylzinc to an aldehyde

Organozinc chemistry is the science of compounds that contain carbon-zinc bonds, describing the physical properties, synthesis, and chemical reactions.Many organozinc compounds are important.[140][141][142][143] Among important applications are

  • The Frankland-Duppa Reaction in which an oxalate ester (ROCOCOOR) reacts with an alkyl halide R'X, zinc and hydrochloric acid to form the α-hydroxycarboxylic esters RR'COHCOOR[144]
  • The Reformatskii reaction in which α-halo-esters and aldehydes are converted to β-hydroxy-esters
  • The Simmons–Smith reaction in which the carbenoid (iodomethyl)zinc iodide reacts with alkene(or alkyne) and converts them to cyclopropane
  • The Addition reaction of organozinc compounds to form carbonyl compounds
  • The Barbier reaction (1899), which is the zinc equivalent of the magnesium Grignard reaction and is the better of the two. In presence of water, formation of the organomagnesium halide will fail, whereas the Barbier reaction can take place in water.
  • On the downside, organozincs are much less nucleophilic than Grignards, and they are expensive and difficult to handle. Commercially available diorganozinc compounds are dimethylzinc, diethylzinc and diphenylzinc. In one study,[145][146] the active organozinc compound is obtained from much cheaper organobromine precursors
  • The Negishi coupling is also an important reaction for the formation of new carbon-carbon bonds between unsaturated carbon atoms in alkenes, arenes and alkynes. The catalysts are nickel and palladium. A key step in the catalytic cycle is a transmetalation in which a zinc halide exchanges its organic substituent for another halogen with the palladium (nickel) metal center.
  • The Fukuyama coupling is another coupling reaction, but it uses a thioester as reactant and produces a ketone.

Zinc has found many applications as catalyst in organic synthesis including asymmetric synthesis, being cheap and easily available alternative to precious metal complexes. The results (yield and ee) obtained with chiral zinc catalysts are comparable to those achieved with palladium, ruthenium, iridium and others, and zinc becomes metal catalyst of choice.[147]

Dietary supplement

Zinc 50 mg
GNC zinc 50 mg tablets. The amount exceeds what is deemed the safe upper limit in the United States (40 mg) and European Union (25 mg)
Zinc gluconate structure
Zinc gluconate is one compound used for the delivery of zinc as a dietary supplement.

In most single-tablet, over-the-counter, daily vitamin and mineral supplements, zinc is included in such forms as zinc oxide, zinc acetate, or zinc gluconate.[148] Zinc is generally considered to be an antioxidant. However, it is redox inert and thus can serve such a function only indirectly.[149] Generally, zinc supplement is recommended where there is high risk of zinc deficiency (such as low and middle income countries) as a preventive measure.[150]

Zinc deficiency has been associated with major depressive disorder (MDD), and zinc supplements may be an effective treatment.[151]

Zinc serves as a simple, inexpensive, and critical tool for treating diarrheal episodes among children in the developing world. Zinc becomes depleted in the body during diarrhea, but recent studies suggest that replenishing zinc with a 10- to 14-day course of treatment can reduce the duration and severity of diarrheal episodes and may also prevent future episodes for as long as three months.[152]

A Cochrane review stated that people taking zinc supplement may be less likely to progress to age-related macular degeneration.[153]

Zinc supplement is an effective treatment for acrodermatitis enteropathica, a genetic disorder affecting zinc absorption that was previously fatal to affected infants.[57]

Gastroenteritis is strongly attenuated by ingestion of zinc, possibly by direct antimicrobial action of the ions in the gastrointestinal tract, or by the absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc), or both.[154][155]

In 2011, researchers reported that adding large amounts of zinc to a urine sample masked detection of drugs. The researchers did not test whether orally consuming a zinc dietary supplement could have the same effect.[156]

Zinc is a negative allosteric modulator of the GABAA receptor.[157]

Common cold

Zinc supplements (frequently zinc acetate or zinc gluconate lozenges) are a group of dietary supplements that are commonly used for the treatment of the common cold.[158] The use of zinc supplements at doses in excess of 75 mg/day within 24 hours of the onset of symptoms has been shown to reduce the duration of cold symptoms by about 1 day.[158][159] Due to a lack of data, there is insufficient evidence to determine whether the preventative use of zinc supplements reduces the likelihood of contracting a cold.[159] Adverse effects with zinc supplements by mouth include bad taste and nausea.[158][159] The intranasal use of zinc-containing nasal sprays has been associated with the loss of the sense of smell;[158] consequently, in June 2009, the United States Food and Drug Administration (USFDA) warned consumers to stop using intranasal zinc products.[158]

The human rhinovirus – the most common viral pathogen in humans – is the predominant cause of the common cold.[160] The hypothesized mechanism of action by which zinc reduces the severity and/or duration of cold symptoms is the suppression of nasal inflammation and the direct inhibition of rhinoviral receptor binding and rhinoviral replication in the nasal mucosa.[158]

Topical use

Topical preparations of zinc include those used on the skin, often in the form of zinc oxide. Zinc preparations can protect against sunburn in the summer and windburn in the winter.[57] Applied thinly to a baby's diaper area (perineum) with each diaper change, it can protect against diaper rash.[57]

Chelated zinc is used in toothpastes and mouthwashes to prevent bad breath.[161]

Zinc pyrithione is widely included in shampoos to prevent dandruff.[162]

Biological role

Zinc is an essential trace element for humans[163][164][165] and other animals,[166] for plants[98] and for microorganisms.[167] Zinc is required for the function of over 300 enzymes and 1000 transcription factors,[165] and is stored and transferred in metallothioneins.[168][169] It is the second most abundant trace metal in humans after iron and it is the only metal which appears in all enzyme classes.[98][165]

In proteins, zinc ions are often coordinated to the amino acid side chains of aspartic acid, glutamic acid, cysteine and histidine. The theoretical and computational description of this zinc binding in proteins (as well as that of other transition metals) is difficult.[170]

Roughly 2–4 grams of zinc[171] are distributed throughout the human body. Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye.[172] Semen is particularly rich in zinc, a key factor in prostate gland function and reproductive organ growth.[173]

In humans, the biological roles of zinc are ubiquitous.[9][164] It interacts with "a wide range of organic ligands",[9] and has roles in the metabolism of RNA and DNA, signal transduction, and gene expression. It also regulates apoptosis. A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds more that transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins.[98]

In the brain, zinc is stored in specific synaptic vesicles by glutamatergic neurons and can modulate neuronal excitability.[164][165][174] It plays a key role in synaptic plasticity and so in learning.[164][175] Zinc homeostasis also plays a critical role in the functional regulation of the central nervous system.[164][174][165] Dysregulation of zinc homeostasis in the central nervous system that results in excessive synaptic zinc concentrations is believed to induce neurotoxicity through mitochondrial oxidative stress (e.g., by disrupting certain enzymes involved in the electron transport chain, including complex I, complex III, and α-ketoglutarate dehydrogenase), the dysregulation of calcium homeostasis, glutamatergic neuronal excitotoxicity, and interference with intraneuronal signal transduction.[164][176] L- and D-histidine facilitate brain zinc uptake.[177] SLC30A3 is the primary zinc transporter involved in cerebral zinc homeostasis.[164]

Enzymes

Carbonic anhydrase
Ribbon diagram of human carbonic anhydrase II, with zinc atom visible in the center
Zinc finger rendered
Zinc fingers help read DNA sequences.

Zinc is an efficient Lewis acid, making it a useful catalytic agent in hydroxylation and other enzymatic reactions.[178] The metal also has a flexible coordination geometry, which allows proteins using it to rapidly shift conformations to perform biological reactions.[179] Two examples of zinc-containing enzymes are carbonic anhydrase and carboxypeptidase, which are vital to the processes of carbon dioxide (CO
2
) regulation and digestion of proteins, respectively.[180]

In vertebrate blood, carbonic anhydrase converts CO
2
into bicarbonate and the same enzyme transforms the bicarbonate back into CO
2
for exhalation through the lungs.[181] Without this enzyme, this conversion would occur about one million times slower[182] at the normal blood pH of 7 or would require a pH of 10 or more.[183] The non-related β-carbonic anhydrase is required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and alcoholic fermentation.[184]

Carboxypeptidase cleaves peptide linkages during digestion of proteins. A coordinate covalent bond is formed between the terminal peptide and a C=O group attached to zinc, which gives the carbon a positive charge. This helps to create a hydrophobic pocket on the enzyme near the zinc, which attracts the non-polar part of the protein being digested.[180]

Signalling

Zinc has been recognized as a messenger, able to activate signalling pathways. Many of these pathways provide the driving force in aberrant cancer growth. They can be targeted through ZIP transporters.[185]

Other proteins

Zinc serves a purely structural role in zinc fingers, twists and clusters.[186] Zinc fingers form parts of some transcription factors, which are proteins that recognize DNA base sequences during the replication and transcription of DNA. Each of the nine or ten Zn2+
ions in a zinc finger helps maintain the finger's structure by coordinately binding to four amino acids in the transcription factor.[182] The transcription factor wraps around the DNA helix and uses its fingers to accurately bind to the DNA sequence.

In blood plasma, zinc is bound to and transported by albumin (60%, low-affinity) and transferrin (10%).[171] Because transferrin also transports iron, excessive iron reduces zinc absorption, and vice versa. A similar antagonism exists with copper.[187] The concentration of zinc in blood plasma stays relatively constant regardless of zinc intake.[178] Cells in the salivary gland, prostate, immune system, and intestine use zinc signaling to communicate with other cells.[188]

Zinc may be held in metallothionein reserves within microorganisms or in the intestines or liver of animals.[189] Metallothionein in intestinal cells is capable of adjusting absorption of zinc by 15–40%.[190] However, inadequate or excessive zinc intake can be harmful; excess zinc particularly impairs copper absorption because metallothionein absorbs both metals.[191]

The human dopamine transporter contains a high affinity extracellular zinc binding site which, upon zinc binding, inhibits dopamine reuptake and amplifies amphetamine-induced dopamine efflux in vitro.[192][193][194] The human serotonin transporter and norepinephrine transporter do not contain zinc binding sites.[194] Some EF-hand calcium binding proteins such as S100 or NCS-1 are also able to bind zinc ions.[195]

Dietary recommendations

The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for zinc in 2001. The current EARs for zinc for women and men ages 14 and up is 6.8 and 9.4 mg/day, respectively. The RDAs are 8 and 11 mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 11 mg/day. RDA for lactation is 12 mg/day. For infants up to 12 months the RDA is 3 mg/day. For children ages 1–13 years the RDA increases with age from 3 to 8 mg/day. As for safety, the IOM sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of zinc the adult UL is 40 mg/day (lower for children). Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs).[178]

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in United States. For people ages 18 and older the PRI calculations are complex, as the EFSA has set higher and higher values as the phytate content of the diet increases. For women, PRIs increase from 7.5 to 12.7 mg/day as phytate intake increases from 300 to 1200 mg/day; for men the range is 9.4 to 16.3 mg/day. These PRIs are higher than the U.S. RDAs.[196] The EFSA reviewed the same safety question and set its UL at 25 mg/day, which is much lower than the U.S. value.[197]

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For zinc labeling purposes 100% of the Daily Value was 15 mg, but on May 27, 2016 it was revised to 11 mg.[198] A table of the old and new adult Daily Values is provided at Reference Daily Intake. Food and supplement companies have until January 1, 2020 to comply with the change.[199]

Dietary intake

Foodstuff-containing-Zinc
Foods and spices containing zinc

Animal products such as meat, fish, shellfish, fowl, eggs, and dairy contain zinc. The concentration of zinc in plants varies with the level in the soil. With adequate zinc in the soil, the food plants that contain the most zinc are wheat (germ and bran) and various seeds, including sesame, poppy, alfalfa, celery, and mustard.[200] Zinc is also found in beans, nuts, almonds, whole grains, pumpkin seeds, sunflower seeds, and blackcurrant.[201] Plant phytates are particularly found in pulses and cereals and interfere with zinc absorption.

Other sources include fortified food and dietary supplements in various forms. A 1998 review concluded that zinc oxide, one of the most common supplements in the United States, and zinc carbonate are nearly insoluble and poorly absorbed in the body.[202] This review cited studies that found lower plasma zinc concentrations in the subjects who consumed zinc oxide and zinc carbonate than in those who took zinc acetate and sulfate salts.[202] For fortification, however, a 2003 review recommended cereals (containing zinc oxide) as a cheap, stable source that is as easily absorbed as the more expensive forms.[203] A 2005 study found that various compounds of zinc, including oxide and sulfate, did not show statistically significant differences in absorption when added as fortificants to maize tortillas.[204]

Deficiency

Nearly two billion people in the developing world are deficient in zinc. Groups at risk include children in developing countries and elderly with chronic illnesses.[10] In children, it causes an increase in infection and diarrhea and contributes to the death of about 800,000 children worldwide per year.[9] The World Health Organization advocates zinc supplementation for severe malnutrition and diarrhea.[205] Zinc supplements help prevent disease and reduce mortality, especially among children with low birth weight or stunted growth.[205] However, zinc supplements should not be administered alone, because many in the developing world have several deficiencies, and zinc interacts with other micronutrients.[206] While zinc deficiency is usually due to insufficient dietary intake, it can be associated with malabsorption, acrodermatitis enteropathica, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses.[10]

In the United States, a federal survey of food consumption determined that for women and men over the age of 19, average consumption was 9.7 and 14.2 mg/day, respectively. For women, 17% consumed less than the EAR, for men 11%. The percentages below EAR increased with age.[207] The most recent published update of the survey (NHANES 2013–2014) reported lower averages – 9.3 and 13.2 mg/day – again with intake decreasing with age.[208]

Symptoms of mild zinc deficiency are diverse.[178] Clinical outcomes include depressed growth, diarrhea, impotence and delayed sexual maturation, alopecia, eye and skin lesions, impaired appetite, altered cognition, impaired immune functions, defects in carbohydrate utilization, and reproductive teratogenesis.[178] Zinc deficiency depresses immunity,[209] but excessive zinc does also.[171]

Despite some concerns,[210] western vegetarians and vegans do not suffer any more from overt zinc deficiency than meat-eaters.[211] Major plant sources of zinc include cooked dried beans, sea vegetables, fortified cereals, soy foods, nuts, peas, and seeds.[210] However, phytates in many whole-grains and fibers may interfere with zinc absorption and marginal zinc intake has poorly understood effects. The zinc chelator phytate, found in seeds and cereal bran, can contribute to zinc malabsorption.[10] Some evidence suggests that more than the US RDA (8 mg/day for adult women; 11 mg/day for adult men) may be needed in those whose diet is high in phytates, such as some vegetarians.[210] The European Food Safety Authority (EFSA) guidelines attempt to compensate for this by recommending higher zinc intake when dietary phytate intake is greater.[196] These considerations must be balanced against the paucity of adequate zinc biomarkers, and the most widely used indicator, plasma zinc, has poor sensitivity and specificity.[212]

Soil remediation

Species of Calluna, Erica and Vaccinium can grow in zinc metalliferous soils, because translocation of toxic ions is prevented by the action of ericoid mycorrhizal fungi.[213]

Agriculture

Zinc deficiency appears to be the most common micronutrient deficiency in crop plants; it is particularly common in high-pH soils.[214] Zinc-deficient soil is cultivated in the cropland of about half of Turkey and India, a third of China, and most of Western Australia. Substantial responses to zinc fertilization have been reported in these areas.[98] Plants that grow in soils that are zinc-deficient are more susceptible to disease. Zinc is added to the soil primarily through the weathering of rocks, but humans have added zinc through fossil fuel combustion, mine waste, phosphate fertilizers, pesticide (zinc phosphide), limestone, manure, sewage sludge, and particles from galvanized surfaces. Excess zinc is toxic to plants, although zinc toxicity is far less widespread.[98]

Precautions

Toxicity

Although zinc is an essential requirement for good health, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption.[191] The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish.[215] The Free Ion Activity Model is well-established in the literature, and shows that just micromolar amounts of the free ion kills some organisms. A recent example showed 6 micromolar killing 93% of all Daphnia in water.[216]

The free zinc ion is a powerful Lewis acid up to the point of being corrosive. Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. Swallowing a post-1982 American one cent piece (97.5% zinc) can cause damage to the stomach lining through the high solubility of the zinc ion in the acidic stomach.[217]

Evidence shows that people taking 100–300 mg of zinc daily may suffer induced copper deficiency. A 2007 trial observed that elderly men taking 80 mg daily were hospitalized for urinary complications more often than those taking a placebo.[218] Levels of 100–300 mg may interfere with the utilization of copper and iron or adversely affect cholesterol.[191] Zinc in excess of 500 ppm in soil interferes with the plant absorption of other essential metals, such as iron and manganese.[99] A condition called the zinc shakes or "zinc chills" can be induced by inhalation of zinc fumes while brazing or welding galvanized materials.[130] Zinc is a common ingredient of denture cream which may contain between 17 and 38 mg of zinc per gram. Disability and even deaths from excessive use of these products have been claimed.[219]

The U.S. Food and Drug Administration (FDA) states that zinc damages nerve receptors in the nose, causing anosmia. Reports of anosmia were also observed in the 1930s when zinc preparations were used in a failed attempt to prevent polio infections.[220] On June 16, 2009, the FDA ordered removal of zinc-based intranasal cold products from store shelves. The FDA said the loss of smell can be life-threatening because people with impaired smell cannot detect leaking gas or smoke, and cannot tell if food has spoiled before they eat it.[221]

Recent research suggests that the topical antimicrobial zinc pyrithione is a potent heat shock response inducer that may impair genomic integrity with induction of PARP-dependent energy crisis in cultured human keratinocytes and melanocytes.[222]

Poisoning

In 1982, the US Mint began minting pennies coated in copper but containing primarily zinc. Zinc pennies pose a risk of zinc toxicosis, which can be fatal. One reported case of chronic ingestion of 425 pennies (over 1 kg of zinc) resulted in death due to gastrointestinal bacterial and fungal sepsis. Another patient who ingested 12 grams of zinc showed only lethargy and ataxia (gross lack of coordination of muscle movements).[223] Several other cases have been reported of humans suffering zinc intoxication by the ingestion of zinc coins.[224][225]

Pennies and other small coins are sometimes ingested by dogs, requiring veterinary removal of the foreign objects. The zinc content of some coins can cause zinc toxicity, commonly fatal in dogs through severe hemolytic anemia and liver or kidney damage; vomiting and diarrhea are possible symptoms.[226] Zinc is highly toxic in parrots and poisoning can often be fatal.[227] The consumption of fruit juices stored in galvanized cans has resulted in mass parrot poisonings with zinc.[57]

See also

Notes

  1. ^ An East India Company ship carrying a cargo of nearly pure zinc metal from the Orient sank off the coast Sweden in 1745.(Emsley 2001, p. 502)
  2. ^ Electric current will naturally flow between zinc and steel but in some circumstances inert anodes are used with an external DC source.

References

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  2. ^ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
  3. ^ Thornton, C. P. (2007). "Of brass and bronze in prehistoric Southwest Asia" (PDF). Papers and Lectures Online. Archetype Publications. ISBN 1-904982-19-0. Archived (PDF) from the original on September 24, 2015.
  4. ^ a b Greenwood 1997, p. 1201
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  10. ^ a b c d Prasad, AS (2003). "Zinc deficiency : Has been known of for 40 years but ignored by global health organisations". British Medical Journal. 326 (7386): 409–410. doi:10.1136/bmj.326.7386.409. PMC 1125304. PMID 12595353.
  11. ^ Maret, Wolfgang (2013). "Chapter 14 Zinc and the Zinc Proteome". In Banci, Lucia. Metallomics and the Cell. Metal Ions in Life Sciences. 12. Springer. doi:10.1007/978-94-007-5561-10_14. ISBN 978-94-007-5561-1.
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Bibliography

External links

Adrenocorticotropic hormone

Adrenocorticotropic hormone (ACTH, also adrenocorticotropin, corticotropin) is a polypeptide tropic hormone produced by and secreted by the anterior pituitary gland. It is also used as a medication and diagnostic agent.

ACTH is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological stress (along with its precursor corticotropin-releasing hormone from the hypothalamus). Its principal effects are increased production and release of cortisol by the cortex of the adrenal gland. ACTH is also related to the circadian rhythm in many organisms.Deficiency of ACTH is a sign of secondary adrenal insufficiency (suppressed production of ACTH due to an impairment of the pituitary gland or hypothalamus, cf. hypopituitarism) or tertiary adrenal insufficiency (disease of the hypothalamus, with a decrease in the release of corticotropin releasing hormone (CRH)). Conversely, chronically elevated ACTH levels occur in primary adrenal insufficiency (e.g. Addison's disease) when adrenal gland production of cortisol is chronically deficient. In Cushing's disease a pituitary tumor is the cause of elevated ACTH (from the anterior pituitary) and an excess of cortisol (hypercortisolism) – this constellation of signs and symptoms is known as Cushing's syndrome.

Brass

Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical and electrical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure. In contrast, bronze is an alloy of copper and tin.Both bronze and brass may include small proportions of a range of other elements including arsenic, lead, phosphorus, aluminium, manganese, and silicon. The distinction is largely historical. Modern practice in museums and archaeology increasingly avoids both terms for historical objects in favour of the all-embracing "copper alloy".Brass is used for decoration for its bright gold-like appearance; for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves; for plumbing and electrical applications; and extensively in brass musical instruments such as horns and bells where a combination of high workability (historically with hand tools) and durability is desired. It is also used in zippers. Brass is often used in situations in which it is important that sparks not be struck, such as in fittings and tools used near flammable or explosive materials.

Bronze

Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (such as aluminium, manganese, nickel or zinc) and sometimes non-metals or metalloids such as arsenic, phosphorus or silicon. These additions produce a range of alloys that may be harder than copper alone, or have other useful properties, such as stiffness, ductility, or machinability.

The archeological period in which bronze was the hardest metal in widespread use is known as the Bronze Age. The beginning of the Bronze Age in India and western Eurasia is conventionally dated to the mid-4th millennium BC, and to the early 2nd millennium BC in China; everywhere it gradually spread across regions. The Bronze Age was followed by the Iron Age starting from about 1300 BC and reaching most of Eurasia by about 500 BC, although bronze continued to be much more widely used than it is in modern times.

Because historical pieces were often made of brasses (copper and zinc) and bronzes with different compositions, modern museum and scholarly descriptions of older objects increasingly use the more inclusive term "copper alloy" instead.

Cadmium

Cadmium is a chemical element with symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of its compounds, and like mercury, it has a lower melting point than the transition metals in groups 3 through 11. Cadmium and its congeners in group 12 are often not considered transition metals, in that they do not have partly filled d or f electron shells in the elemental or common oxidation states. The average concentration of cadmium in Earth's crust is between 0.1 and 0.5 parts per million (ppm). It was discovered in 1817 simultaneously by Stromeyer and Hermann, both in Germany, as an impurity in zinc carbonate.

Cadmium occurs as a minor component in most zinc ores and is a byproduct of zinc production. Cadmium was used for a long time as a corrosion-resistant plating on steel, and cadmium compounds are used as red, orange and yellow pigments, to color glass, and to stabilize plastic. Cadmium use is generally decreasing because it is toxic (it is specifically listed in the European Restriction of Hazardous Substances) and nickel-cadmium batteries have been replaced with nickel-metal hydride and lithium-ion batteries. One of its few new uses is cadmium telluride solar panels.

Although cadmium has no known biological function in higher organisms, a cadmium-dependent carbonic anhydrase has been found in marine diatoms.

Chromosome 19

Chromosome 19 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 19 spans more than 58.6 million base pairs, the building material of DNA.

EGR1

EGR-1 (Early growth response protein 1) also known as Zif268 (zinc finger protein 225) or NGFI-A (nerve growth factor-induced protein A) is a protein that in humans is encoded by the EGR1 gene.

EGR-1 is a mammalian transcription factor. It was also named Krox-24, TIS8, and ZENK. It was originally discovered in mice.

Galvanization

Galvanization or galvanizing is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten zinc.

Group 12 element

Group 12, by modern IUPAC numbering, is a group of chemical elements in the periodic table. It includes zinc (Zn), cadmium (Cd) and mercury (Hg). The further inclusion of copernicium (Cn) in group 12 is supported by recent experiments on individual copernicium atoms. Formerly this group was named IIB (pronounced as "group two B", as the "II" is a Roman numeral) by CAS and old IUPAC system.The three group 12 elements that occur naturally are zinc, cadmium and mercury. They are all widely used in electric and electronic applications, as well as in various alloys. The first two members of the group share similar properties as they are solid metals under standard conditions. Mercury is the only metal that is a liquid at room temperature. While zinc is very important in the biochemistry of living organisms, cadmium and mercury are both highly toxic. As copernicium does not occur in nature, it has to be synthesized in the laboratory.

Insulin

Insulin (from Latin insula, island) is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of carbohydrates, especially glucose from the blood into liver, fat and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both. Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism, especially of reserve body fat.

Beta cells are sensitive to glucose concentrations, also known as blood sugar levels. When the glucose level is high, the beta cells secrete insulin into the blood; when glucose levels are low, secretion of insulin is inhibited. Their neighboring alpha cells, by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon, through stimulating the liver to release glucose by glycogenolysis and gluconeogenesis, has the opposite effect of insulin. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis.If beta cells are destroyed by an autoimmune reaction, insulin can no longer be synthesized or be secreted into the blood. This results in type 1 diabetes mellitus, which is characterized by abnormally high blood glucose concentrations, and generalized body wasting. In type 2 diabetes mellitus the destruction of beta cells is less pronounced than in type 1 diabetes, and is not due to an autoimmune process. Instead there is an accumulation of amyloid in the pancreatic islets, which likely disrupts their anatomy and physiology. The pathogenesis of type 2 diabetes is not well understood but patients exhibit a reduced population of islet beta-cells, reduced secretory function of islet beta-cells that survive, and peripheral tissue insulin resistance. Type 2 diabetes is characterized by high rates of glucagon secretion into the blood which are unaffected by, and unresponsive to the concentration of glucose in the blood. Insulin is still secreted into the blood in response to the blood glucose. As a result, the insulin levels, even when the blood sugar level is normal, are much higher than they are in healthy persons.

The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a dimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies.The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.

Isotopes of zinc

Naturally occurring zinc (30Zn) is composed of the 5 stable isotopes 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn with 64Zn being the most abundant (48.6% natural abundance). Twenty-five radioisotopes have been characterised with the most abundant and stable being 65Zn with a half-life of 244.26 days, and 72Zn with a half-life of 46.5 hours. All of the remaining radioactive isotopes have half-lives that are less than 14 hours and the majority of these have half-lives that are less than 1 second. This element also has 10 meta states.

Zinc has been proposed as a "salting" material for nuclear weapons. A jacket of isotopically enriched 64Zn, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 65Zn with a half-life of 244 days and produce approximately 1.115 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used.

Penny (United States coin)

The United States one-cent coin, often called the penny, is a unit of currency equaling one one-hundredth of a United States dollar. The cent's symbol is ¢. Its obverse has featured the profile of President Abraham Lincoln since 1909, the centennial of his birth. From 1959 (the sesquicentennial of Lincoln's birth) to 2008, the reverse featured the Lincoln Memorial. Four different reverse designs in 2009 honored Lincoln's 200th birthday and a new, "permanent" reverse – the Union Shield – was introduced in 2010. The coin is 0.75 inches (19.05 mm) in diameter and 0.0598 inches (1.52 mm) in thickness. Its weight has varied, depending upon the composition of metals used in its production (see further below).

The U.S. Mint's official name for the coin is "cent" and the U.S. Treasury's official name is "one cent piece". The colloquial term penny derives from the British coin of the same name, the pre-decimal version of which had a similar place in the British system. In American English, pennies is the plural form. (The plural form pence—standard in British English—is not used in reference to American coins.) In the early 2010s the price of metal used to make pennies rose to a noticeable cost to the mint which peaked at a $0.02 for $0.01 ratio. This pushed the mint to look for alternative metals again for the coin, and also brought the penny debate into more focus. There are currently no firm plans to eliminate the penny as arguments for and against the coin continue to be debated.

Sphalerite

Sphalerite ((Zn, Fe)S) is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides along with calcite, dolomite, and fluorite. Miners have also been known to refer to sphalerite as zinc blende, black-jack and ruby jack.

Zinc chloride

Zinc chloride is the name of chemical compounds with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from sources of moisture, including the water vapor present in ambient air. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis. No mineral with this chemical composition is known aside from the very rare mineral simonkolleite, Zn5(OH)8Cl2·H2O.

Zinc deficiency

Zinc deficiency is defined either as insufficient zinc to meet the needs of the body, or as a serum zinc level below the normal range. However, since a decrease in the serum concentration is only detectable after long-term or severe depletion, serum zinc is not a reliable biomarker for zinc status. Common symptoms include increased rates of diarrhea. Zinc deficiency affects the skin and gastrointestinal tract; brain and central nervous system, immune, skeletal, and reproductive systems.

Zinc deficiency in humans is caused by reduced dietary intake, inadequate absorption, increased loss, or increased body system utilization. The most common cause is reduced dietary intake. In the U.S., the Recommended Dietary Allowance (RDA) is 8 mg/day for women and 11 mg/day for men.The highest concentration of dietary zinc is found in oysters, meat, beans, and nuts. Increasing the amount of zinc in the soil and thus in crops and animals is an effective preventive measure. Zinc deficiency may affect up to 2 billion people worldwide.

Zinc finger

A zinc finger is a small protein structural motif that is characterized by the coordination of one or more zinc ions (Zn2+) in order to stabilize the fold. Originally coined to describe the finger-like appearance of a hypothesized structure from Xenopus laevis transcription factor IIIA, the zinc finger name has now come to encompass a wide variety of differing protein structures. Xenopus laevis TFIIIA was originally demonstrated to contain zinc and require the metal for function in 1983, the first such reported zinc requirement for a gene regulatory protein. It often apears as a metal-binding domain in multi-domain proteins.Proteins that contain zinc fingers (zinc finger proteins) are classified into several different structural families. Unlike many other clearly defined supersecondary structures such as Greek keys or β hairpins, there are a number of types of zinc fingers, each with a unique three-dimensional architecture. A particular zinc finger protein's class is determined by this three-dimensional structure, but it can also be recognized based on the primary structure of the protein or the identity of the ligands coordinating the zinc ion. In spite of the large variety of these proteins, however, the vast majority typically function as interaction modules that bind DNA, RNA, proteins, or other small, useful molecules, and variations in structure serve primarily to alter the binding specificity of a particular protein.

Since their original discovery and the elucidation of their structure, these interaction modules have proven ubiquitous in the biological world and may be found in 3% of the genes of the human genome. In addition, zinc fingers have become extremely useful in various therapeutic and research capacities. Engineering zinc fingers to have an affinity for a specific sequence is an area of active research, and zinc finger nucleases and zinc finger transcription factors are two of the most important applications of this to be realized to date.

Zinc finger and BTB domain-containing protein 16

Zinc finger and BTB domain-containing protein 16 is a protein that in humans is encoded by the ZBTB16 gene.

Zinc oxide

Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, plastics, ceramics, glass, cement, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, and first-aid tapes. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically.ZnO is a wide-bandgap semiconductor of the II-VI semiconductor group. The native doping of the semiconductor due to oxygen vacancies or zinc interstitials is n-type. This semiconductor has several favorable properties, including good transparency, high electron mobility, wide bandgap, and strong room-temperature luminescence. Those properties are valuable in emerging applications for: transparent electrodes in liquid crystal displays, energy-saving or heat-protecting windows, and electronics as thin-film transistors and light-emitting diodes.

Zinc sulfate

Zinc sulfate is an inorganic compound and dietary supplement. As a supplement it is used to treat zinc deficiency and to prevent the condition in those at high risk. Side effects of excess supplementation may include abdominal pain, vomiting, headache, and tiredness.It has the formula ZnSO4 as well as any of three hydrates. It was historically known as "white vitriol". All of the various forms are colourless solids. The heptahydrate form is commonly encountered.

Zinc sulfide

Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various impurities, the pure material is white, and it is widely used as a pigment. In its dense synthetic form, zinc sulfide can be transparent, and it is used as a window for visible optics and infrared optics.

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