Gadolinium: An Introduction
Gadolinium (Gd) is a silvery-white, malleable, and ductile rare-earth metal that belongs to the lanthanide series in the periodic table. It is soft enough to be cut with a knife and exhibits ferromagnetic properties below room temperature. While often referred to as a “rare earth,” gadolinium is relatively abundant in the Earth’s crust, occurring in minerals like monazite and bastnäsite, which are mined in various regions globally, including China, the United States, and Australia.
Chemical Reactivity
Reaction with Air
Gadolinium tarnishes slowly in dry air, forming a protective dull grey oxide layer. In moist air, the reaction proceeds more rapidly, leading to the formation of gadolinium(III) oxide ($\text{Gd}_2\text{O}_3$). This oxidation process is characteristic of many electropositive metals, where the metal loses electrons to oxygen.
Reaction with Water
Gadolinium reacts slowly with cold water and more vigorously with hot water to produce gadolinium hydroxide ($\text{Gd(OH)}_3$) and hydrogen gas ($\text{H}_2$). This reactivity is greater than that of transition metals like iron but less pronounced than that of alkali metals such as sodium. The general reaction can be represented as:
$\text{2Gd(s) + 6H}_2\text{O(l)} \rightarrow \text{2Gd(OH)}_3\text{(aq) + 3H}_2\text{(g)}$
Reaction with Acids
Gadolinium readily dissolves in most dilute acids, such as hydrochloric acid ($\text{HCl}$) or sulfuric acid ($\text{H}_2\text{SO}_4$), to form gadolinium(III) salts and hydrogen gas. For example, with hydrochloric acid:
$\text{2Gd(s) + 6HCl(aq)} \rightarrow \text{2GdCl}_3\text{(aq) + 3H}_2\text{(g)}$
However, with hydrofluoric acid ($\text{HF}$), a protective fluoride layer can form on the surface, which slows down or prevents further reaction.
Safety Profile
Toxicity
Elemental gadolinium exhibits low acute toxicity. However, certain gadolinium compounds, particularly the chelates used as magnetic resonance imaging (MRI) contrast agents, have raised safety concerns. In individuals with severely impaired kidney function, these compounds can accumulate in the body and lead to a rare but serious condition known as Nephrogenic Systemic Fibrosis (NSF). This adverse reaction has led to careful clinical guidelines for the use of gadolinium-based contrast agents in medical facilities worldwide, including hospitals in Europe, Asia, and North America.
Radioactivity
Natural gadolinium is not radioactive. It is composed of a mixture of seven stable isotopes, with $\text{^{158}Gd}$ being the most abundant. While some synthetic isotopes of gadolinium, such as $\text{^{153}Gd}$, are radioactive and are used in specific industrial and medical applications (e.g., bone densitometry), these are not present in naturally occurring gadolinium. A particularly notable property of the stable isotope $\text{^{157}Gd}$ is its extraordinarily high neutron capture cross-section, meaning it is highly effective at absorbing neutrons.
Flammability
Like many metals, gadolinium in a finely divided form, such as powder, is flammable. It can ignite and burn when exposed to heat or flame. In this state, it can also react explosively with strong oxidizing agents. Large, solid pieces of gadolinium are not considered flammable under normal conditions.
Notable Chemical Reaction: Neutron Absorption
One of the most significant chemical-physical interactions involving gadolinium is its role as a neutron absorber. The isotope $\text{^{157}Gd}$ possesses the highest known thermal neutron capture cross-section among all stable nuclides, making it exceptionally efficient at absorbing neutrons. This property is crucial in nuclear technology.
Gadolinium is utilized in nuclear reactors globally, including those in countries with advanced nuclear power programs like France, Japan, and the United States. It is incorporated into control rods or as a “burnable poison” within nuclear fuel assemblies. By absorbing excess neutrons, gadolinium helps regulate the chain reaction, ensuring safe and efficient operation of the reactor core. As the reactor operates, the gadolinium isotopes transmute into other elements through neutron capture, and its neutron-absorbing capacity gradually decreases, a predictable process that is factored into reactor design and fuel management.