Gadolinium

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 (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 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 (68 °F) 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 (5 °C (41 °F)) for the gadolinium alloy Gd₈₅Er₁₅, and the effect is considerably stronger for the alloy Gd₅(Si₂Ge₂), but at a much lower temperature (<85 K (âˆ'188.2 °C; âˆ'306.7 °F)). A significant magnetocaloric effect is observed at higher temperatures, up to 300 K, in the Gd₅(Si_xGe_1-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 (Gd₂O₃), which spalls off, exposing more surface to oxidation.

4 Gd + 3 O₂ â†' 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 â†' 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 â†' 2 [Gd(H₂O)₉]³⁺ + 3 SO2âˆ'
4 + 3 H₂

Gadolinium metal reacts with the halogens (X₂) at temperature about 200 °C:

2 Gd + 3 X₂ â†' 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 the chelating agent DOTA, an octadentate ligand. Salts of [Gd(DOTA)]^âˆ' 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 (t_1/2=110 seconds), ^145mGd (t_1/2=85 seconds) and ^141mGd (t_1/2=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 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, 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.

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 3â€"4. Thorium precipitates as its hydroxide and is removed.
3. The remaining solution is treated with ammonium oxalate to convert rare earths into 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 0.1â€"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 color 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 (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.

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) 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(Si_xGe_1-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(Si_xGe_1-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. 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 100â€"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 (which also improves its solubility) decreases its toxicity with regard to the free ion by at least a factor of 100 (i.e., the lethal dose for the Gd-chelate increases by 100 times). 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 0.03â€"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 (NSF) 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 â€" Complication of Gadolinium MR Contrast (series of images at MedPix website)
• It's Elemental â€" 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

Share on Google Plus

About Unknown

This is a short description in the author block about the author. You edit it by entering text in the "Biographical Info" field in the user admin panel.