Gadolinium is a chemical element with symbol Gd
and atomic number 64. It is a silvery-white,
malleable and ductile rare-earth metal. It
is found in nature only in combined form.
Gadolinium was first detected spectroscopically
in 1880 by de Marignac who separated its oxide
and is credited with its discovery. It is
named for gadolinite, one of the minerals
in which it was found, in turn named for chemist
Johan Gadolin. The metal was isolated by Paul
Emile Lecoq de Boisbaudran in 1886.
Gadolinium metal possesses unusual metallurgic
properties, to the extent that as little as
1% gadolinium can significantly improve the
workability and resistance to high temperature
oxidation of iron, chromium, and related alloys.
Gadolinium as a metal or salt has exceptionally
high absorption of neutrons and therefore
is used for shielding in neutron radiography
and in nuclear reactors. Like most rare earths,
gadolinium forms trivalent ions which have
fluorescent properties. Gadolinium(III) salts
have therefore been used as green phosphors
in various applications.
The gadolinium(III) ion occurring in water-soluble
salts is quite toxic to mammals. However,
chelated gadolinium(III) compounds are far
less toxic because they carry gadolinium(III)
through the kidneys and out of the body before
the free ion can be released into tissue.
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. However, in a small minority of patients
with renal failure, at least four such agents
have been associated with development of the
rare nodular inflammatory disease nephrogenic
systemic fibrosis. This is thought to be due
to the gadolinium ion itself, since gadolinium(III)
carrier molecules associated with the disease
differ.
Characteristics
Physical properties
Gadolinium is a silvery-white malleable and
ductile rare-earth metal. It crystallizes
in hexagonal, close-packed α- form at room
temperature, but, when heated to temperatures
above 1235 °C, it transforms into its β-
form, which has a body-centered cubic structure.
Gadolinium-157 has the highest thermal neutron
capture cross-section among any stable nuclides:
259,000 barns. Only xenon-135 has a higher
cross section, 2 million barns, but that isotope
is unstable.
Gadolinium is ferromagnetic at temperatures
below 20 °C and is strongly paramagnetic
above this temperature. Gadolinium demonstrates
a magnetocaloric effect whereby its temperature
increases when it enters a magnetic field
and decreases when it leaves the magnetic
field. The temperature is lowered to) for
the gadolinium alloy Gd85Er15, and the effect
is considerably stronger for the alloy Gd5(Si2Ge2),
but at a much lower temperature). A significant
magnetocaloric effect is observed at higher
temperatures, up to 300 K, in the Gd5(SixGe1-x)4
compounds.
Individual gadolinium atoms have been isolated
by encapsulating them into fullerene molecules
and visualized with transmission electron
microscope. Individual Gd atoms and small
Gd clusters have also been incorporated into
carbon nanotubes.
Chemical properties
Gadolinium combines with most elements to
form Gd(III) derivatives. nitrogen, carbon,
sulfur, phosphorus, boron, selenium, silicon
and arsenic at elevated temperatures, forming
binary compounds.
Unlike 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.
4 Gd + 3 O2 → 2 Gd2O3
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
hydroxide:
2 Gd + 6 H2O → 2 Gd(OH)3 + 3 H2
Gadolinium metal is attacked readily by dilute
sulfuric acid to form solutions containing
the colorless Gd(III) ions, which exist as
[Gd(H2O)9]3+ complexes:
2 Gd + 3 H2SO4 + 18 H2O → 2 [Gd(H2O)9]3+
+ 3 SO2−
4 + 3 H2
Gadolinium metal reacts with the halogens
at temperature about 200 °C:
2 Gd + 3 X2 → 2 GdX3
Chemical compounds
In the great majority of its compounds, Gd
adopts the oxidation state +3. 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 form sesquichloride Gd2Cl3,
which can be further reduced to GdCl by annealing
at 800 °C. This gadolinium(I) chloride forms
platelets with layered graphite-like structure.
Isotopes
Naturally occurring gadolinium is composed
of 6 stable isotopes, 154Gd, 155Gd, 156Gd,
157Gd, 158Gd and 160Gd, and 1 radioisotope,
152Gd, with 158Gd being the most abundant.
The predicted double beta decay of 160Gd has
never been observed.
Twenty-nine radioisotopes have been characterized,
with the most stable being alpha-decaying
152Gd with a half-life of 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 74.7 years. The
majority of these have half-lives of less
than 24.6 seconds. Gadolinium isotopes have
4 metastable isomers, with the most stable
being 143mGd, 145mGd and 141mGd.
Isotopes with atomic masses lower than the
most abundant stable isotope, 158Gd, primarily
decay via electron capture to Eu isotopes.
At higher atomic masses, the primary decay
mode is beta decay, and the primary products
are Tb isotopes.
History
Gadolinium is named from the mineral gadolinite,
in turn named for Finnish chemist and geologist
Johan Gadolin. In 1880, Swiss chemist Jean
Charles Galissard de Marignac observed spectroscopic
lines due to gadolinium in samples of gadolinite,
and in the separate mineral cerite. The latter
mineral proved to contain far more of the
element with the new spectral line, and Jean
Charles Galissard de Marignac eventually separated
a mineral oxide from cerite which he realized
was the oxide of this new element. He named
the oxide "gadolinia." Because he realized
that "gadolinia" was the oxide of a new element,
he is credited with discovery of gadolinium.
French chemist Paul Émile Lecoq de Boisbaudran
actually carried out the separation of gadolinium
metal from gadolinia, in 1886.
Occurrence
Gadolinium is a constituent in many minerals
such as monazite and bastnäsite, which are
oxides. The metal is too reactive to exist
naturally. Ironically, as noted above, the
mineral gadolinite actually contains only
traces of Gd. The abundance in the earth crust
is about 6.2 mg/kg. The main mining areas
are China, USA, 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.
Production
Gadolinium is produced both from monazite
and bastnäsite.
Crushed minerals are extracted with hydrochloric
or sulfuric acids, 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 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 suitable complexing agent.
Gadolinium metal is obtained from its oxide
or salts by heating with calcium at 1450 °C
under argon atmosphere. Sponge gadolinium
can be produced by reducing molten GdCl3 with
an appropriate metal at temperatures below
1312 °C in a reduced pressure.
Applications
Gadolinium has no large-scale applications
but has a variety of specialized uses.
Gadolinium has the highest neutron cross-section
among any stable nuclides: 61,000 barns for
155Gd and 259,000 barns for 157Gd. 157Gd has
been used to target tumors in neutron therapy.
This element is very 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 type. Gadolinium
is also used in nuclear marine propulsion
systems as a burnable poison.
Gadolinium also possesses unusual metallurgic
properties, with as little as 1% of gadolinium
improving the workability and resistance of
iron, chromium, and related alloys to high
temperatures and oxidation.
Gadolinium is paramagnetic at room temperature,
with a ferromagnetic Curie point of 20 °C.
Paramagnetic ions, such as gadolinium, move
differently within a magnetic field. This
trait makes gadolinium useful for magnetic
resonance imaging. Solutions of organic gadolinium
complexes and gadolinium compounds are used
as intravenous MRI contrast agent to enhance
images in medical magnetic resonance imaging
and magnetic resonance angiography procedures.
Magnevist is the most widespread example.
Nanotubes packed with gadolinium, dubbed "gadonanotubes",
are 40 times more effective than this traditional
gadolinium contrast agent. Once injected,
gadolinium-based contrast agents accumulate
in abnormal tissues of the brain and body.
This accumulation provides a greater contrast
between normal and abnormal tissues, allowing
doctors to better locate uncommon cell growths
and tumors.
Gadolinium as a phosphor is also used in other
imaging. In X-ray systems, gadolinium is contained
in the phosphor layer, suspended in a polymer
matrix at the detector. Terbium-doped gadolinium
oxysulfide at the phosphor layer converts
the X-rays released from the source into light.
This material emits green light at 540 nm
due to 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-rays striking
the phosphor layer can be converted into light
photons. Gadolinium oxyorthosilicate is a
single crystal that is used as a scintillator
in medical imaging such as positron emission
tomography or for detecting neutrons.
Gadolinium compounds are also used for making
green phosphors for colour TV tubes.
Gadolinium-153 is produced in a nuclear reactor
from elemental europium or enriched gadolinium
targets. It has a half-life of 240±10 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 or in bone density gauges for
osteoporosis screening, as well as in the
Lixiscope portable X-ray imaging system.
Gadolinium is used for making gadolinium yttrium
garnet; it has microwave applications and
is used in fabrication of various optical
components and as substrate material for magneto-optical
films.
Gadolinium gallium garnet was used for imitation
diamonds and for computer bubble memory.
Gadolinium can also serve as an electrolyte
in solid oxide fuel cells. Using gadolinium
as a dopant for materials like cerium oxide
creates an electrolyte with both high ionic
conductivity and low operating temperatures
that are optimal for cost-effective production
of fuel cells.
Research is being conducted on magnetic refrigeration
near room temperature, which could provide
significant efficiency and environmental advantages
over conventional refrigeration methods. Gadolinium-based
materials, such as Gd5(SixGe1-x)4, are currently
the most promising materials owing to their
high Curie temperature and giant magnetocaloric
effect. Pure Gd itself exhibits a large magnetocaloric
effect near its Curie temperature of 20 °C,
and this has sparked great interest into producing
Gd alloys with a larger effect and tunable
Curie temperature. In Gd5(SixGe1-x)4, Si and
Ge compositions can be varied to adjust the
Curie temperature. This technology is still
very early in development and significant
material improvements still need to be made
before it is commercially viable.
Biological role
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.
Safety
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. Free
gadolinium ions toxicity in animals is due
to interfering with a number of calcium-ion
channel dependent processes. The 50% lethal
dose is about 100–200 mg/kg. No prolonged
toxicities have been reported following low
dose exposure to gadolinium ions. Toxicity
studies in rodents, however show that chelation
of gadolinium decreases its toxicity with
regard to the free ion by at least a factor
of 100. It is believed therefore that clinical
toxicity of Gd contrast agents 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.
Gadolinium MRI contrast agents have proved
safer than the iodinated contrast agents used
in X-ray radiography or computed tomography.
Anaphylactoid reactions are rare, occurring
in approximately 0.03–0.1%.
Although gadolinium agents have proved useful
for patients with renal impairment, in patients
with severe renal failure requiring dialysis,
there is a risk of a rare but serious illnesses,
called nephrogenic systemic fibrosis or nephrogenic
fibrosing dermopathy, that has been linked
to the use of four gadolinium-containing MRI
contrast agents. The disease resembles scleromyxedema
and to some extent scleroderma. It may occur
months after contrast has been injected. Its
association with gadolinium and not the carrier
molecule is confirmed by its occurrence in
from contrast materials in which gadolinium
is carried by very different carrier molecules.
Current guidelines in the United States are
that dialysis patients should only receive
gadolinium agents where essential and to consider
performing an iodinated contrast enhanced
CT when feasible. If a contrast enhanced MRI
must be performed on a dialysis patient, it
is recommended that certain high-risk contrast
agents be avoided and 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.
References
External links
Nephrogenic Systemic Fibrosis – Complication
of Gadolinium MR Contrast
It's Elemental – Gadolinium
Refrigerator uses gadolinium metal that heats
up when exposed to magnetic field
FDA advisory on gadolinium-based contrast
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
