Gadolinium

Gadolinium is a chemical element with the 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 (salt) 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 geologist Johan Gadolin. The metal was isolated by Lecoq de Boisbaudran in 1886.

Gadolinium metal 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 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. Gd (III) salts have therefore been used as green phosphors in various applications.

The Gd(III) ion occurring in water-soluble salts is quite toxic to mammals. However, chelated Gd(III) compounds are far less toxic because they carry Gd(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 gadolinium ion itself, since Gd(III) carrier molecules associated with the disease differ.

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 effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2).

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 (Gd2O3), 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 a [Gd(OH2)9]3+ complexes:
 * 2 Gd + 3 H2SO4 + 18 H2O → 2 [Gd(H2O)9]3+ + 3 SO42-  +  3 H2

Gadolinium metal reacts with the halogens (X2) 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 (GdCl3). 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 DOTA chelating agent, 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 (24.84% natural abundance). The predicted double beta decay of 160Gd has never been observed (the only lower limit on its half-life of more than 1.3×1021 years has been set experimentally ).

Twenty-nine radioisotopes have been characterized, with the most stable being alpha-decaying 152Gd (naturally occurring) 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 (T½=110 seconds), 145mGd (T½=85 seconds) and 141mGd (T½=24.5 seconds).

Isotopes with atomic masses lower than the most abundant stable isotope, 158Gd, primarily decay via electron capture to Eu (europium) isotopes. At higher atomic masses, the primary decay mode is beta decay, and the primary products are Tb (terbium) 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 didymium and gadolinite, and separated from them "gadolinia," gadolinium oxide. 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 carried out the separation of gadolinium metal from the oxide 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, the mineral gadolinite 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.
 * 1) Crushed minerals are extracted with hydrochloric or sulfuric acids, which converts the insoluble oxides into soluble chlorides or sulfates.
 * 2) The acidic filtrates are partially neutralized with caustic soda to pH 3–4. Thorium precipitates as its hydroxide and is removed.
 * 3) The remaining solution is treated with ammonium oxalate to convert rare earths in to their insoluble oxalates. The oxalates are converted to oxides by heating.
 * 4) The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO3.
 * 5) The solution is treated with magnesium nitrate to produce a crystallized mixture of double salts of gadolinium, samarium and europium.
 * 6) The salts are separated by ion exchange chromatography.
 * 7) 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 (melting point of Gd) 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 (MRI). 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 (MRA) 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 (Gd2O2S: Tb) 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 (Gd2SiO5, GSO; usually doped by 0.1–1% of Ce) 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 and compact discs.

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 (Gd:Y3Al5O12); it has microwave applications and is used in fabrication of various optical components and as substrate material for magneto–optical films.

Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) was used for imitation diamonds and for computer bubble memory.

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, as well as to stretch activated ion channels.

Safety
As a free ion, gadolinium is highly toxic, but MRI contrast agents are chelated compounds and are considered safe enough to be used in most persons. The toxicity depends on the strength of the chelating agent. US Food and Drug Administration approved Gd chelated contrast agents include: Omniscan, Multihance, Magnevist, ProHance, Vasovist, Eovist and OptiMARK.

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 approx. 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 that dialysis should be performed as soon as possible after the scan is complete, in order to remove the agent from the body promptly. After several years of controversy during which up to 100 Danish patients have been gadolinium poisoned (and some died) after Omniscan use, it has been admitted by the Norwegian medical company Nycomed that they were aware of the dangers of using gadolinium based agents for their product.