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