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= Cadmium =
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Introduction
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Cadmium is a chemical element; it has symbol Cd and atomic number 48.
This soft, silvery-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. It was used for a long time in the 1900s
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's use is generally decreasing because it is
toxic (it is specifically listed in the European Restriction of
Hazardous Substances Directive
) and nickel-cadmium batteries have been replaced with nickel-metal
hydride and lithium-ion batteries. Because it is a neutron poison,
cadmium is also used as a component of control rods in nuclear fission
reactors. One of its few new uses is in 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.
Physical properties
=====================
Cadmium is a soft, malleable, ductile, silvery-white divalent metal.
It is similar in many respects to zinc but forms complex compounds.
Unlike most other metals, cadmium is resistant to corrosion and is
used as a protective plate on other metals. As a bulk metal, cadmium
is insoluble in water and is not flammable; however, in its powdered
form it may burn and release toxic fumes.
Chemical properties
=====================
Although cadmium usually has an oxidation state of +2, it also exists
in the +1 state. Cadmium and its congeners are not always considered
transition metals, in that they do not have partly filled d or f
electron shells in the elemental or common oxidation states.
Cadmium burns in air to form brown amorphous cadmium oxide (CdO); the
crystalline form of this compound is a dark red which changes color
when heated, similar to zinc oxide. Hydrochloric acid, sulfuric acid,
and nitric acid dissolve cadmium by forming cadmium chloride (),
cadmium sulfate (), and cadmium nitrate () respectively. The oxidation
state +1 can be produced by dissolving cadmium in a mixture of cadmium
chloride and aluminium chloride, forming the cation as cadmium(I)
tetrachloroaluminate, which is similar to the cation in mercury(I)
chloride.
The structures of many cadmium complexes with nucleobases, amino
acids, and vitamins have been determined.
Isotopes
==========
Naturally occurring cadmium is composed of eight isotopes. Two of them
are radioactive, and three are expected to decay but have not
measurably done so under laboratory conditions. The two natural
radioactive isotopes are 113Cd (beta decay, half-life is ) and 116Cd
(two-neutrino double beta decay, half-life is ). The other three are
106Cd, 108Cd (both double electron capture), and 114Cd (double beta
decay); only lower limits on these half-lives have been determined. At
least three isotopes - 110Cd, 111Cd, and 112Cd - are stable. Among the
isotopes that do not occur naturally, the most long-lived are 109Cd
with a half-life of 462.6 days, and 115Cd with a half-life of 53.46
hours. All of the remaining radioactive isotopes have half-lives of
less than 2.5 hours, and the majority have half-lives of less than 5
minutes. Cadmium has 8 known meta states, with the most stable being
113mCd ('t'1⁄2 = 14.1 years), 115mCd ('t'1⁄2 = 44.6 days), and 117mCd
('t'1⁄2 = 3.36 hours).
The known isotopes of cadmium range in atomic mass from (95Cd) to
(132Cd). For isotopes lighter than 112 Da, the primary decay mode is
electron capture and the dominant decay product is element 47
(silver). Heavier isotopes decay mostly through beta emission
producing element 49 (indium).
One isotope of cadmium, 113Cd, absorbs neutrons with high selectivity:
With very high probability, neutrons with energy below the 'cadmium
cut-off' will be absorbed; those higher than the cut-off will be
transmitted. The cadmium cut-off is about 0.5 eV, and neutrons below
that level are deemed slow neutrons, distinct from intermediate and
fast neutrons.
Cadmium is created via the s-process in low- to medium-mass stars with
masses of 0.6 to 10 solar masses, over thousands of years. In that
process, a silver atom captures a neutron and then undergoes beta
decay.
History
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Cadmium (Latin 'cadmia', Greek 'καδμεία' meaning "calamine", a
cadmium-bearing mixture of minerals that was named after the Greek
mythological character Κάδμος, Cadmus, the founder of Thebes) was
discovered in contaminated zinc compounds sold in pharmacies in
Germany in 1817 by Friedrich Stromeyer.
Karl Samuel Leberecht Hermann simultaneously investigated the
discoloration in zinc oxide and found an impurity, first suspected to
be arsenic, because of the yellow precipitate with hydrogen sulfide.
Additionally Stromeyer discovered that one supplier sold zinc
carbonate instead of zinc oxide. Stromeyer found the new element as an
impurity in zinc carbonate (calamine), and, for 100 years, Germany
remained the only important producer of the metal. The metal was named
after the Latin word for calamine, because it was found in this zinc
ore. Stromeyer noted that some impure samples of calamine changed
color when heated but pure calamine did not. He was persistent in
studying these results and eventually isolated cadmium metal by
roasting and reducing the sulfide. The potential for cadmium yellow as
pigment was recognized in the 1840s, but the early scarcity of cadmium
limited this application.
Even though cadmium and its compounds are toxic in certain forms and
concentrations, the British Pharmaceutical Codex from 1907 states that
cadmium iodide was used as a medication to treat "enlarged joints,
scrofulous glands, and chilblains".
In 1907, the International Astronomical Union defined the
international ångström in terms of a red cadmium spectral line (1
wavelength = 6438.46963 Å). This was adopted by the 7th General
Conference on Weights and Measures in 1927. In 1960, the definitions
of both the metre and ångström were changed to use krypton.
After the industrial scale production of cadmium started in the 1930s
and 1940s, the major application of cadmium was the coating of iron
and steel to prevent corrosion; in 1944, 62% and in 1956, 59% of the
cadmium in the United States was used for plating.
In 1956, 24% of the cadmium in the United States was used for a
second application in red, orange and yellow pigments from sulfides
and selenides of cadmium.
The stabilizing effect of cadmium chemicals like the carboxylates
cadmium laurate and cadmium stearate on PVC led to an increased use of
those compounds in the 1970s and 1980s. The demand for cadmium in
pigments, coatings, stabilizers, and alloys declined as a result of
environmental and health regulations in the 1980s and 1990s; in 2006,
only 7% of total cadmium consumption was used for plating, and only
10% was used for pigments.
At the same time, these decreases in consumption were compensated by a
growing demand for cadmium for nickel-cadmium batteries, which
accounted for 81% of the cadmium consumption in the United States in
2006.
Occurrence
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Cadmium makes up about 0.1 ppm of Earth's crust and is the 65th most
abundant element. It is much rarer than zinc, which makes up about 65
ppm.
No significant deposits of cadmium-containing ores are known. The
only cadmium mineral of importance, greenockite (CdS), is nearly
always associated with sphalerite (ZnS). This association is caused by
geochemical similarity between zinc and cadmium, with no geological
process likely to separate them. Thus, cadmium is produced mainly as a
byproduct of mining, smelting, and refining sulfidic ores of zinc,
and, to a lesser degree, lead and copper. Small amounts of cadmium,
about 10% of consumption, are produced from secondary sources, mainly
from dust generated by recycling iron and steel scrap. Production in
the United States began in 1907, but wide use began after World War I.
Metallic cadmium can be found in the Vilyuy River basin in Siberia.
Rocks mined for phosphate fertilizers contain varying amounts of
cadmium, resulting in a cadmium concentration of as much as 300 mg/kg
in the fertilizers and a high cadmium content in agricultural soils.
Coal can contain significant amounts of cadmium, which ends up mostly
in coal fly ash.
Cadmium in soil can be absorbed by crops such as rice and cocoa. In
2002, the Chinese ministry of agriculture measured that 28% of rice it
sampled had excess lead and 10% had excess cadmium above limits
defined by law. 'Consumer Reports' tested 28 brands of dark chocolate
sold in the United States in 2022, and found cadmium in all of them,
with 13 exceeding the California Maximum Allowable Dose level.
Some plants such as willow trees and poplars have been found to clean
both lead and cadmium from soil.
Typical background concentrations of cadmium do not exceed 5 ng/m3 in
the atmosphere; 2 mg/kg in soil; 1 μg/L in freshwater and 50 ng/L in
seawater.
Concentrations of cadmium above 10 μg/L may be stable in water having
low total solute concentrations and 'p' H and can be difficult to
remove by conventional water treatment processes.
Production
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Cadmium is a common impurity in zinc ores, and it is most often
isolated during the production of zinc. Some zinc ores concentrates
from zinc sulfate ores contain up to 1.4% of cadmium.
In the 1970s, the output of cadmium was 6.5 lb per ton of zinc. Zinc
sulfide ores are roasted in the presence of oxygen, converting the
zinc sulfide to the oxide. Zinc metal is produced either by smelting
the oxide with carbon or by electrolysis in sulfuric acid. Cadmium is
isolated from the zinc metal by vacuum distillation if the zinc is
smelted, or cadmium sulfate is precipitated from the electrolysis
solution.
The British Geological Survey reports that in 2001, China was the top
producer of cadmium with almost one-sixth of the world's production,
closely followed by South Korea and Japan.
File:Cadmium - world production trend.svg|History of the world
production of cadmium
File:2022cadmium.png|Cadmium production in 2010
Applications
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Cadmium is a common component of electric batteries, pigments,
coatings, and electroplating.
Batteries
===========
In 2009, 86% of cadmium was used in batteries, predominantly in
rechargeable nickel-cadmium batteries. Nickel-cadmium cells have a
nominal cell potential of 1.2 V. The cell consists of a positive
nickel hydroxide electrode and a negative cadmium electrode plate
separated by an alkaline electrolyte (potassium hydroxide). The
European Union put a limit on cadmium in electronics in 2004 of 0.01%,
with some exceptions, and in 2006 reduced the limit on cadmium content
to 0.002%. Another type of battery based on cadmium is the
silver-cadmium battery.
Electroplating
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Cadmium electroplating, consuming 6% of the global production, is used
in the aircraft industry to reduce corrosion of steel components. This
coating is passivated by chromate salts. A limitation of cadmium
plating is hydrogen embrittlement of high-strength steels from the
electroplating process. Therefore, steel parts heat-treated to tensile
strength above 1300 MPa (200 ksi) should be coated by an alternative
method (such as special low-embrittlement cadmium electroplating
processes or physical vapor deposition).
Titanium embrittlement from cadmium-plated tool residues resulted in
banishment of those tools (and the implementation of routine tool
testing to detect cadmium contamination) in the A-12/SR-71, U-2, and
subsequent aircraft programs that use titanium.
Nuclear technology
====================
Cadmium is used in the control rods of nuclear reactors, acting as a
very effective neutron poison to control neutron flux in nuclear
fission. When cadmium rods are inserted in the core of a nuclear
reactor, cadmium absorbs neutrons, preventing them from creating
additional fission events, thus controlling the amount of reactivity.
The pressurized water reactor designed by Westinghouse Electric
Company uses an alloy consisting of 80% silver, 15% indium, and 5%
cadmium.
Televisions
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QLED TVs have been starting to include cadmium in construction. Some
companies have been looking to reduce the environmental impact of
human exposure and pollution of the material in televisions during
production.
Anticancer drugs
==================
Complexes based on cadmium and other heavy metals have potential for
the treatment of cancer, but their use is often limited due to toxic
side effects.
Compounds
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Cadmium oxide was used in black and white television phosphors and in
the blue and green phosphors of color television cathode ray tubes.
Cadmium sulfide (CdS) is used as a photoconductive surface coating for
photocopier drums.
Various cadmium salts are used in paint pigments, with CdS as a yellow
pigment being the most common. Cadmium selenide is a red pigment,
commonly called 'cadmium red'. To painters who work with the pigment,
cadmium provides the most brilliant and durable yellows, oranges, and
reds - so much so that during production, these colors are
significantly toned down before they are ground with oils and binders
or blended into watercolors, gouaches, acrylics, and other paint and
pigment formulations. Because these pigments are potentially toxic,
for safety users normally use a barrier cream on the hands to prevent
absorption through the skin even though the amount of cadmium absorbed
into the body through the skin is reported to be less than 1%.
In PVC, cadmium was used as heat, light, and weathering stabilizers.
Currently, cadmium stabilizers have been completely replaced with
barium-zinc, calcium-zinc and organo-tin stabilizers. Cadmium is used
in many kinds of solder and bearing alloys, because it has a low
coefficient of friction and fatigue resistance. It is also found in
some of the lowest-melting alloys, such as Wood's metal.
Semiconductors
================
Cadmium is an element in some semiconductor materials. Cadmium
sulfide, cadmium selenide, and cadmium telluride are used in some
photodetectors and solar cells. HgCdTe detectors are sensitive to
mid-infrared light and used in some motion detectors.
Laboratory uses
=================
Helium-cadmium lasers are a common source of blue or ultraviolet laser
light. Lasers at wavelengths of 325, 354 and 442 nm are made using
this gain medium; some models can switch between these wavelengths.
They are notably used in fluorescence microscopy as well as various
laboratory uses requiring laser light at these wavelengths.
Cadmium selenide quantum dots emit bright luminescence under UV
excitation (He-Cd laser, for example). The color of this luminescence
can be green, yellow or red depending on the particle size. Colloidal
solutions of those particles are used for imaging of biological
tissues and solutions with a fluorescence microscope.
In molecular biology, cadmium is used to block voltage-dependent
calcium channels from fluxing calcium ions, as well as in hypoxia
research to stimulate proteasome-dependent degradation of Hif-1α.
Cadmium-selective sensors based on the fluorophore BODIPY have been
developed for imaging and sensing of cadmium in cells.
One powerful method for monitoring cadmium in aqueous environments
involves electrochemistry. By employing a self-assembled monolayer one
can obtain a cadmium selective electrode with a ppt-level sensitivity.
Biological role
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Cadmium has no known function in higher organisms and is considered
toxic. Cadmium is considered an environmental pollutant hazardous to
living organisms.
A cadmium-dependent carbonic anhydrase has been found in some marine
diatoms,
which live in environments with low zinc concentrations.
Exposure to cadmium leads to raised levels in the blood cells for a
number of months.
In vertebrates cadmium is preferentially absorbed in the kidneys but
also in the liver and bones. Up to about 30 mg of cadmium is commonly
inhaled throughout human childhood and adolescence.
Cadmium is eliminated from the body in very small amounts and mainly
through urine resulting in a biological half-life of 20 to 40 years.
Cadmium is under research for its potential toxicity to increase the
risk of cancer, cardiovascular disease, and osteoporosis.
Environmental impact
======================
The biogeochemistry of cadmium and its release to the environment is
under research.
Safety
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Individuals and organizations have been reviewing cadmium's
bioinorganic aspects for its toxicity.
The most dangerous form of occupational exposure to cadmium is
inhalation of fine dust and fumes, or ingestion of highly soluble
cadmium compounds. Inhalation of cadmium fumes can result initially in
metal fume fever, but may progress to chemical pneumonitis, pulmonary
edema,necrosis and death.
Cadmium is also an environmental hazard. Human exposure is primarily
from fossil fuel combustion, phosphate fertilizers, natural sources,
iron and steel production, cement production and related activities,
nonferrous metals production, and municipal solid waste incineration.
Other sources of cadmium include bread, root crops, and vegetables.
There have been a few instances of general population poisoning as the
result of long-term exposure to cadmium in contaminated food and
water. Research into an estrogen mimicry that may induce breast cancer
is ongoing, . In the decades leading up to World War II, mining
operations contaminated the Jinzū River in Japan with cadmium and
traces of other toxic metals. As a consequence, cadmium accumulated in
the rice crops along the riverbanks downstream of the mines. Some
members of the local agricultural communities consumed the
contaminated rice and developed itai-itai disease and renal
abnormalities, including proteinuria and glucosuria. The victims of
this poisoning were almost exclusively post-menopausal women with low
iron and low body stores of other minerals. Similar general population
cadmium exposures in other parts of the world have not resulted in the
same health problems because the populations maintained sufficient
iron and other mineral levels. Thus, although cadmium is a major
factor in the itai-itai disease in Japan, most researchers have
concluded that it was one of several factors.
Cadmium is one of ten substances banned by the European Union's
Restriction of Hazardous Substances (RoHS) directive, which regulates
hazardous substances in electrical and electronic equipment, but
allows for certain exemptions and exclusions from the scope of the
law.
The International Agency for Research on Cancer has classified cadmium
and cadmium compounds as carcinogenic to humans. Although occupational
exposure to cadmium is linked to lung and prostate cancer, there is
still uncertainty about the carcinogenicity of cadmium in low
environmental exposure. Recent data from epidemiological studies
suggest that intake of cadmium through diet is associated with a
higher risk of endometrial, breast, and prostate cancer as well as
with osteoporosis in humans.
A recent study has demonstrated that endometrial tissue is
characterized by higher levels of cadmium in current and former
smoking females.
Cadmium exposure is associated with a large number of illnesses
including kidney disease, early atherosclerosis, hypertension, and
cardiovascular diseases. Although studies show a significant
correlation between cadmium exposure and occurrence of disease in
human populations, a molecular mechanism has not yet been identified.
One hypothesis holds that cadmium is an endocrine disruptor and some
experimental studies have shown that it can interact with different
hormonal signaling pathways. For example, cadmium can bind to the
estrogen receptor alpha,
and affect signal transduction along the estrogen and MAPK signaling
pathways at low doses.
The tobacco plant absorbs and accumulates heavy metals such as cadmium
from the surrounding soil into its leaves. Following tobacco smoke
inhalation, these are readily absorbed into the body of users. Tobacco
smoking is the most important single source of cadmium exposure in the
general population. An estimated 10% of the cadmium content of a
cigarette is inhaled through smoking. Absorption of cadmium through
the lungs is more effective than through the gut. As much as 50% of
the cadmium inhaled in cigarette smoke may be absorbed.
On average, cadmium concentrations in the blood of smokers is 4 to 5
times greater than non-smokers and in the kidney, 2-3 times greater
than in non-smokers. Despite the high cadmium content in cigarette
smoke, there seems to be little exposure to cadmium from passive
smoking.
In a non-smoking population, food accounts for around 90% of cadmium
uptake.
High quantities of cadmium can be found in crustaceans, mollusks,
offal, frog legs, cocoa solids, bitter and semi-bitter chocolate,
seaweed, fungi and algae products. However, grains, vegetables, and
starchy roots and tubers are consumed in much greater quantity in the
U.S., and are the source of the greatest dietary exposure there.
Most plants bio-accumulate metal toxins such as cadmium and when
composted to form organic fertilizers, yield a product that often can
contain high amounts (e.g., over 0.5 mg) of metal toxins for every
kilogram of fertilizer. Fertilizers made from animal dung (e.g., cow
dung) or urban waste can contain similar amounts of cadmium. The
cadmium added to the soil from fertilizers (rock phosphates or organic
fertilizers) become bio-available and toxic only if the soil pH is low
(i.e., acidic soils). In the European Union, an analysis of almost
22,000 topsoil samples with LUCAS survey concluded that 5.5% of
samples have concentrations higher than 1 mg kg−1.
Zinc, copper, calcium, and iron ions, and selenium with vitamin C are
used to treat cadmium intoxication, although it is not easily
reversed.
Regulations
=============
Because of the adverse effects of cadmium on the environment and human
health, the supply and use of cadmium is restricted in Europe under
the REACH Regulation.
The EFSA Panel on Contaminants in the Food Chain specifies that 2.5
μg/kg body weight is a tolerable weekly intake for humans. The Joint
FAO/WHO Expert Committee on Food Additives has declared 7 μg/kg body
weight to be the provisional tolerable weekly intake level. The state
of California requires a food label to carry a warning about potential
exposure to cadmium on products such as cocoa powder. The European
Commission has put in place the EU regulation (2019/1009) on
fertilizing products (EU, 2019), adopted in June 2019 and fully
applicable as of July 2022. It sets a Cd limit value in phosphate
fertilizers to 60 mg kg−1 of .
The U.S. Occupational Safety and Health Administration (OSHA) has set
the permissible exposure limit (PEL) for cadmium at a time-weighted
average (TWA) of 0.005 ppm. The National Institute for Occupational
Safety and Health (NIOSH) has not set a recommended exposure limit
(REL) and has designated cadmium as a known human carcinogen. The IDLH
(immediately dangerous to life and health) level for cadmium is 9
mg/m3.
!Lethal dose !Organism !Route !Time
|LD50: 225 mg/kg |rat |oral |n/a
|LD50: 890 mg/kg |mouse |oral |n/a
|LC50: 25 mg/m3 |rat |airborne |30 min
In addition to mercury, the presence of cadmium in some batteries has
led to the requirement of proper disposal (or recycling) of batteries.
Product recalls
=================
In May 2006, a sale of the seats from Arsenal F.C.'s old stadium,
Highbury in London, England was cancelled when the seats were
discovered to contain trace amounts of cadmium. Reports of high levels
of cadmium use in children's jewelry in 2010 led to a US Consumer
Product Safety Commission investigation. The U.S. CPSC issued specific
recall notices for cadmium content in jewelry sold by Claire's and
Wal-Mart stores.
In June 2010, McDonald's voluntarily recalled more than 12 million
promotional 'Shrek Forever After 3D' Collectible Drinking Glasses
because of the cadmium levels in paint pigments on the glassware. The
glasses were manufactured by Arc International, of Millville, New
Jersey, USA.
See also
======================================================================
* Red List building materials
* Toxic heavy metal
Further reading
======================================================================
*
* Agency for Toxic Substances and Disease Registry (ATSDR) (2012).
Toxicological Profile for Cadmium. U.S. Department of Health and Human
Services, Public Health Service.
https://www.atsdr.cdc.gov/toxprofiles/tp5.pdf
*
External links
======================================================================
* [
http://www.periodicvideos.com/videos/048.htm Cadmium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.atsdr.cdc.gov/csem/csem.asp?csem=6&po=0 ATSDR Case
Studies in Environmental Medicine: Cadmium Toxicity] U.S. Department
of Health and Human Services
* [
https://www.cdc.gov/niosh/topics/Cadmium National Institute for
Occupational Safety and Health - Cadmium Page]
*
[
https://web.archive.org/web/20170424180105/https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@na+@rel+cadmium,+elemental
NLM Hazardous Substances Databank - Cadmium, Elemental]
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=========
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Original Article:
http://en.wikipedia.org/wiki/Cadmium
