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= Gadolinium =
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Introduction
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Gadolinium is a chemical element; it has symbol Gd and atomic number
64. It is a silvery-white metal when oxidation is removed. Gadolinium
is a malleable and ductile rare-earth element. It reacts with
atmospheric oxygen or moisture slowly to form a black coating.
Gadolinium below its Curie point of 20 C is ferromagnetic, with an
attraction to a magnetic field higher than that of nickel. Above this
temperature it is the most paramagnetic element. It is found in nature
only in an oxidized form. When separated, it usually has impurities of
the other rare earths because of their similar chemical properties.
Gadolinium was discovered in 1880 by Jean Charles de Marignac, who
detected its oxide by using spectroscopy. It is named after the
mineral gadolinite, one of the minerals in which gadolinium is found,
itself named for the Finnish chemist Johan Gadolin. Pure gadolinium
was first isolated by the chemist Félix Trombe in 1935.
Gadolinium possesses unusual metallurgical properties, to the extent
that as little as 1% of gadolinium can significantly improve the
workability and resistance to oxidation at high temperatures of iron,
chromium, and related metals. Gadolinium as a metal or a salt absorbs
neutrons and is, therefore, used sometimes for shielding in neutron
radiography and in nuclear reactors.
Like most of the rare earths, gadolinium forms trivalent ions with
fluorescent properties, and salts of gadolinium(III) are used as
phosphors in various applications.
Gadolinium(III) ions in water-soluble salts are highly toxic to
mammals. However, chelated gadolinium(III) compounds prevent the
gadolinium(III) from being exposed to the organism, and the majority
is excreted by healthy kidneys before it can deposit in tissues.
Because of its paramagnetic properties, solutions of chelated organic
gadolinium complexes are used as intravenously administered
gadolinium-based MRI contrast agents in medical magnetic resonance
imaging.
The main uses of gadolinium, in addition to use as a contrast agent
for MRI scans, are in nuclear reactors, in alloys, as a phosphor in
medical imaging, as a gamma ray emitter, in electronic devices, in
optical devices, and in superconductors.
Physical properties
=====================
Gadolinium is the eighth member of the lanthanide series. In the
periodic table, it appears between the elements europium to its left
and terbium to its right, and above the actinide curium. It is a
silvery-white, malleable, ductile rare-earth element. Its 64 electrons
are arranged in the configuration of [Xe]4f75d16s2, of which the ten
4f, 5d, and 6s electrons are valence.
Like most other metals in the lanthanide series, three electrons are
usually available as valence electrons. The remaining 4f electrons are
too strongly bound: this is because the 4f orbitals penetrate the most
through the inert xenon core of electrons to the nucleus, followed by
5d and 6s, and this increases with higher ionic charge. Gadolinium
crystallizes in the hexagonal close-packed α-form at room temperature.
At temperatures above 1235 C, it forms or transforms into its β-form,
which has a body-centered cubic structure.
The isotope gadolinium-157 has the highest thermal-neutron capture
cross-section among any stable nuclide: about 259,000 barns. Only
xenon-135 has a higher capture cross-section, about 2.0 million barns,
but this isotope is radioactive.
Gadolinium is believed to be ferromagnetic at temperatures below 20 C
and is strongly paramagnetic above this temperature. In fact, at body
temperature, gadolinium exhibits the greatest paramagnetic effect of
any element. There is evidence that gadolinium is a helical
antiferromagnetic, rather than a ferromagnetic, below 20 C. Gadolinium
demonstrates a magnetocaloric effect whereby its temperature increases
when it enters a magnetic field and decreases when it leaves the
magnetic field. A significant magnetocaloric effect is observed at
higher temperatures, up to about 300 kelvins, in the compounds
Gd5(Si1−'x'Ge'x')4.
Individual gadolinium atoms can be isolated by encapsulating them into
fullerene molecules, where they can be visualized with a transmission
electron microscope. Individual Gd atoms and small Gd clusters can be
incorporated into carbon nanotubes.
Chemical properties
=====================
Gadolinium combines with most elements to form Gd(III) derivatives. It
also combines with nitrogen, carbon, sulfur, phosphorus, boron,
selenium, silicon, and arsenic at elevated temperatures, forming
binary compounds.
Unlike the other rare-earth elements, metallic gadolinium is
relatively stable in dry air. However, it tarnishes quickly in moist
air, forming a loosely-adhering gadolinium(III) oxide ():
:,
which spalls off, exposing more surface to oxidation.
Gadolinium is a strong reducing agent, which reduces oxides of several
metals into their elements. Gadolinium is quite electropositive and
reacts slowly with cold water and quite quickly with hot water to form
gadolinium(III) hydroxide ():
:.
Gadolinium metal is attacked readily by dilute sulfuric acid to form
solutions containing the colorless Gd(III) ions, which exist as
complexes:
:.
Chemical compounds
====================
In the great majority of its compounds, like many rare-earth metals,
gadolinium adopts the oxidation state +3. However, gadolinium can be
found on rare occasions in the 0, +1 and +2 oxidation states. All four
trihalides are known. All are white, except for the iodide, which is
yellow. Most commonly encountered of the halides is gadolinium(III)
chloride (). The oxide dissolves in acids to give the salts, such as
gadolinium(III) nitrate.
Gadolinium(III), like most lanthanide ions, forms complexes with high
coordination numbers. This tendency is illustrated by the use of the
chelating agent DOTA, an octadentate ligand. Salts of [Gd(DOTA)]− are
useful in magnetic resonance imaging. A variety of related chelate
complexes have been developed, including gadodiamide.
Reduced gadolinium compounds are known, especially in the solid state.
Gadolinium(II) halides are obtained by heating Gd(III) halides in
presence of metallic Gd in tantalum containers. Gadolinium also forms
the sesquichloride , which can be further reduced to GdCl by annealing
at 800 C. This gadolinium(I) chloride forms platelets with layered
graphite-like structure.
Isotopes
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Naturally occurring gadolinium is composed of six stable isotopes,
154Gd, 155Gd, 156Gd, 157Gd, 158Gd and 160Gd, and one radioisotope,
152Gd, with the isotope 158Gd being the most abundant (24.8% natural
abundance). The predicted double beta decay of 160Gd has never been
observed (an experimental lower limit on its half-life of more than
1.3×1021 years has been measured).
Thirty-three radioisotopes of gadolinium have been observed, with the
most stable being 152Gd (naturally occurring), with a half-life of
about 1.08×1014 years, and 150Gd, with a half-life of 1.79×106 years.
All of the remaining radioactive isotopes have half-lives of less than
75 years. The majority of these have half-lives of less than 25
seconds. Gadolinium isotopes have four metastable isomers, with the
most stable being 143mGd ('t'1/2= 110 seconds), 145mGd ('t'1/2= 85
seconds) and 141mGd ('t'1/2= 24.5 seconds).
The isotopes with atomic masses lower than the most abundant stable
isotope, 158Gd, primarily decay by electron capture to isotopes of
europium. At higher atomic masses, the primary decay mode is beta
decay, and the primary products are isotopes of terbium.
History
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Gadolinium is named after the mineral gadolinite. Gadolinite was first
chemically analyzed by the Finnish chemist Johan Gadolin in 1794. In
1802 German chemist Martin Klaproth gave gadolinite its name. In 1880,
the Swiss chemist Jean Charles Galissard de Marignac observed the
spectroscopic lines from gadolinium in samples of gadolinite (which
actually contains relatively little gadolinium, but enough to show a
spectrum) and in the separate mineral cerite. The latter mineral
proved to contain far more of the element with the new spectral line.
De Marignac eventually separated a mineral oxide from cerite, which he
realized was the oxide of this new element. He designated the element
with the provisional symbol Yα. The French chemist Paul-Émile Lecoq de
Boisbaudran named the element "gadolinium" in 1886. The pure element
was isolated in 1935 by Félix Trombe.
Occurrence
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Gadolinium is a constituent in many minerals, such as monazite and
bastnäsite. The metal is too reactive to exist naturally.
Paradoxically, as noted above, the mineral gadolinite actually
contains only traces of this element. The abundance in the Earth's
crust is about 6.2 mg/kg. The main mining areas are in China, the US,
Brazil, Sri Lanka, India, and Australia with reserves expected to
exceed one million tonnes. World production of pure gadolinium is
about 400 tonnes per year. The only known mineral with essential
gadolinium, lepersonnite-(Gd), is very rare.
Production
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Gadolinium is produced both from monazite and bastnäsite.
# Crushed minerals are extracted with hydrochloric acid or sulfuric
acid, which converts the insoluble oxides into soluble chlorides or
sulfates.
# The acidic filtrates are partially neutralized with caustic soda to
pH 3-4. Thorium precipitates as its hydroxide, and is then removed.
# The remaining solution is treated with ammonium oxalate to convert
rare earths into their insoluble oxalates. The oxalates are converted
to oxides by heating.
# The oxides are dissolved in nitric acid that excludes one of the
main components, cerium, whose oxide is insoluble in HNO3.
# The solution is treated with magnesium nitrate to produce a
crystallized mixture of double salts of gadolinium, samarium and
europium.
# The salts are separated by ion exchange chromatography.
# The rare-earth ions are then selectively washed out by a suitable
complexing agent.
Gadolinium metal is obtained from its oxide or salts by heating it
with calcium at 1450 C in an argon atmosphere. Sponge gadolinium can
be produced by reducing molten GdCl3 with an appropriate metal at
temperatures below 1312 C (the melting point of Gd) at reduced
pressure.
Applications
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Gadolinium has no large-scale applications, but it has a variety of
specialized uses.
Neutron absorber
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Because gadolinium has a high neutron cross-section, it is effective
for use with neutron radiography and in shielding of nuclear reactors.
It is used as a secondary, emergency shut-down measure in some nuclear
reactors, particularly of the CANDU reactor type. Gadolinium is used
in nuclear marine propulsion systems as a burnable poison. The use of
gadolinium in neutron capture therapy to target tumors has been
investigated, and gadolinium-containing compounds have proven
promising.
Alloys
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Gadolinium possesses unusual metallurgic properties, with as little as
1% of gadolinium improving the workability of iron, chromium, and
related alloys, and their resistance to high temperatures and
oxidation.
Magnetic contrast agent
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Gadolinium is paramagnetic at room temperature, with a ferromagnetic
Curie point of 20 C. Paramagnetic ions, such as gadolinium, increase
nuclear spin relaxation rates, making gadolinium useful as a contrast
agent for magnetic resonance imaging (MRI). Solutions of organic
gadolinium complexes and gadolinium compounds are used as intravenous
contrast agents to enhance images in medical and magnetic resonance
angiography (MRA) procedures. Magnevist is the most widespread
example. Nanotubes packed with gadolinium, called "gadonanotubes", are
40 times more effective than the usual gadolinium contrast agent.
Traditional gadolinium-based contrast agents are un-targeted,
generally distributing throughout the body after injection, but will
not readily cross the intact blood-brain barrier. Brain tumors, and
other disorders that degrade the blood-brain barrier, allow these
agents to penetrate into the brain and facilitate their detection by
contrast-enhanced MRI. Similarly, delayed gadolinium-enhanced
magnetic resonance imaging of cartilage uses an ionic compound agent,
originally Magnevist, that is excluded from healthy cartilage based on
electrostatic repulsion but will enter proteoglycan-depleted cartilage
in diseases such as osteoarthritis.
Phosphors
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Gadolinium is used as a phosphor in medical imaging. It is contained
in the phosphor layer of X-ray detectors, suspended in a polymer
matrix. Terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) at the
phosphor layer converts the X-rays released from the source into
light. This material emits green light at 540 nm because of the
presence of Tb3+, which is very useful for enhancing the imaging
quality. The energy conversion of Gd is up to 20%, which means that
one fifth of the X-ray energy striking the phosphor layer can be
converted into visible photons. Gadolinium oxyorthosilicate (Gd2SiO5,
GSO; usually doped by 0.1-1.0% of Ce) is a single crystal that is used
as a scintillator in medical imaging such as positron emission
tomography, and for detecting neutrons.
Gadolinium compounds were also used for making green phosphors for
color TV tubes.
Gamma ray emitter
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Gadolinium-153 is produced in a nuclear reactor from elemental
europium or enriched gadolinium targets. It has a half-life of days
and emits gamma radiation with strong peaks at 41 keV and 102 keV. It
is used in many quality-assurance applications, such as line sources
and calibration phantoms, to ensure that nuclear-medicine imaging
systems operate correctly and produce useful images of radioisotope
distribution inside the patient. It is also used as a gamma-ray source
in X-ray absorption measurements and in bone density gauges for
osteoporosis screening.
Electronic and optical devices
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Gadolinium is used for making gadolinium yttrium garnet (Gd:Y3Al5O12),
which has microwave applications and is used in fabrication of various
optical components and as substrate material for magneto-optical
films.
Electrolyte in fuel cells
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Gadolinium can also serve as an electrolyte in solid oxide fuel cells
(SOFCs). Using gadolinium as a dopant for materials like cerium oxide
(in the form of gadolinium-doped ceria) gives an electrolyte having
both high ionic conductivity and low operating temperatures.
Magnetic refrigeration via magnetocalorics
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Gadolinium is the standard reference material in the study of magnetic
refrigeration near room temperature. Pure Gd itself exhibits a large
magnetocaloric effect near its Curie temperature of 20 C, and this has
sparked interest into producing Gd alloys having a larger effect and
tunable Curie temperature. In Gd5(Si'x'Ge1−'x')4, Si and Ge
compositions can be varied to adjust the Curie temperature.
Gadolinium-based materials, such as Gd5(Si'x'Ge1−'x')4, are currently
the most promising materials, owing to their high Curie temperature
and giant magneto-caloric effect.
Magnetic refrigeration could provide significant efficiency and
environmental advantages over conventional refrigeration methods.
Superconductors
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Gadolinium barium copper oxide (GdBCO) is a superconductor with
applications in superconducting motors or generators such as in wind
turbines. It can be manufactured in the same way as the most widely
researched cuprate high temperature superconductor, yttrium barium
copper oxide (YBCO) and uses an analogous chemical composition
(GdBa2Cu3O7−'δ' ). It was used in 2014 to set a new world record for
the highest trapped magnetic field in a bulk high temperature
superconductor, with a field of 17.6T being trapped within two GdBCO
bulks.
Asthma treatment
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Gadolinium is being investigated as a possible treatment for
preventing lung tissue scarring in asthma. A positive effect has been
observed in mice.
Niche and former applications
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Gadolinium is used for antineutrino detection in the Japanese
Super-Kamiokande detector in order to sense supernova explosions.
Low-energy neutrons that arise from antineutrino absorption by protons
in the detector's ultrapure water are captured by gadolinium nuclei,
which subsequently emit gamma rays that are detected as part of the
antineutrino signature.
Gadolinium gallium garnet (GGG, Gd3Ga5O12) was used for imitation
diamonds and for computer bubble memory.
Safety
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As a free ion, gadolinium is reported often to be highly toxic, but
MRI contrast agents are chelated compounds and are considered safe
enough to be used in most persons. The toxicity of free gadolinium
ions in animals is due to interference with a number of calcium-ion
channel dependent processes. The 50% lethal dose is about 0.34 mmol/kg
(IV, mouse) or 100-200 mg/kg. Toxicity studies in rodents show that
chelation of gadolinium (which also improves its solubility) decreases
its toxicity with regard to the free ion by a factor of 31 (i.e., the
lethal dose for the Gd-chelate increases by 31 times). It is believed
therefore that clinical toxicity of gadolinium-based contrast agents
(GBCAs) in humans will depend on the strength of the chelating agent;
however this research is still not complete. About a dozen different
Gd-chelated agents have been approved as MRI contrast agents around
the world.
Use of gadolinium-based contrast agents results in deposition of
gadolinium in tissues of the brain, bone, skin, and other tissues in
amounts that depend on kidney function, structure of the chelates
(linear or macrocyclic) and the dose administered. In patients with
kidney failure, there is a risk of a rare but serious illness called
nephrogenic systemic fibrosis (NSF) that is caused by the use of
gadolinium-based contrast agents. The disease resembles scleromyxedema
and to some extent scleroderma. It may occur months after a contrast
agent has been injected. Its association with gadolinium and not the
carrier molecule is confirmed by its occurrence with various contrast
materials in which gadolinium is carried by very different carrier
molecules. Because of the risk of NSF, use of these agents is not
recommended for any individual with end-stage kidney failure as they
may require emergent dialysis.
Included in the current guidelines from the Canadian Association of
Radiologists are that dialysis patients should receive gadolinium
agents only where essential and that they should receive dialysis
after the exam. If a contrast-enhanced MRI must be performed on a
dialysis patient, it is recommended that certain high-risk contrast
agents be avoided but not that a lower dose be considered. The
American College of Radiology recommends that contrast-enhanced MRI
examinations be performed as closely before dialysis as possible as a
precautionary measure, although this has not been proven to reduce the
likelihood of developing NSF. The FDA recommends that potential for
gadolinium retention be considered when choosing the type of GBCA used
in patients requiring multiple lifetime doses, pregnant women,
children, and patients with inflammatory conditions.
Anaphylactoid reactions are rare, occurring in approximately
0.03-0.1%.
Long-term environmental impacts of gadolinium contamination due to
human usage are a topic of ongoing research.
Biological use
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Gadolinium has no known native biological role, but its compounds are
used as research tools in biomedicine. Gd3+ compounds are components
of MRI contrast agents. It is used in various ion channel
electrophysiology experiments to block sodium leak channels and
stretch activated ion channels. Gadolinium has recently been used to
measure the distance between two points in a protein via electron
paramagnetic resonance, something that gadolinium is especially
amenable to thanks to EPR sensitivity at w-band (95 GHz) frequencies.
External links
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*
[
https://web.archive.org/web/20070927220031/http://rad.usuhs.edu/medpix/master.php3?mode=slide_sorter&pt_id=10978&quiz=#top
Nephrogenic Systemic Fibrosis - Complication of Gadolinium MR
Contrast] (series of images at MedPix website)
* [
https://education.jlab.org/itselemental/ele064.html It's Elemental
- Gadolinium]
*
[
https://web.archive.org/web/20100323011159/http://www.external.ameslab.gov/news/release/01magneticrefrig.htm
Refrigerator uses gadolinium metal that heats up when exposed to
magnetic field]
*
[
https://web.archive.org/web/20090309174807/https://www.fda.gov/cder/drug/infopage/gcca/qa_200705.htm
FDA advisory on gadolinium-based contrast]
* [
http://www.siaecm.org/gadolinium/index.asp Abdominal MR imaging:
important considerations for evaluation of gadolinium enhancement]
Rafael O.P. de Campos, Vasco Herédia, Ersan Altun, Richard C. Semelka,
Department of Radiology University of North Carolina Hospitals Chapel
Hill
*
[
https://mobile.abc.net.au/news/2019-06-17/inside-super-kamiokande-360-tour/11209104
Inside Japan’s Super Kamiokande 360 degree tour including details on
adding Gadolinium to the pure water to aid in studying neutrinos]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Gadolinium
