Gadolinium

Typical wastewaters

• hospital effluent and municipal sewage (MRI contrast agents) intact Gd(III) polyaminocarboxylate chelates, anionic (linear, gadopentetate type) or neutral (macrocyclic), truly dissolved below 10 kDa; influent and effluent of metropolitan sewage plants enriched, sludge not four full scale metropolitan plants; a large plant's biosolids were not enriched although its effluent was
• water recycling (reverse osmosis concentrate) the rejected chelate concentrated 6 to 7 fold in the brine, 2.6 nmol/kg against 0.39 nmol/kg in the effluent feed proposed as a tracer of the concentrate's fate
• coal mine drainage (acid) Gd³⁺ and the GdSO₄⁺ ion pair dissolved at low pH in sulfate rich drainage; total rare earths averaged 282 µg/L over 141 Appalachian sites, per-element values not read co-precipitates into the iron, aluminium and manganese hydroxide sludge when the drainage is neutralised

1 · Identity

Symbol, number

Gd, 64

Oxidation states in water

+3 only (Gd³⁺); in water almost all anthropogenic gadolinium is Gd(III) held in linear or macrocyclic polyaminocarboxylate chelates

Note

Free Gd³⁺ is toxic and is what the chelate exists to prevent; the chelates are stable enough to survive the water cycle, which is the whole problem.

2 · Occurrence in water

Natural sources

Weathering of monazite and bastnaesite; geogenic Gd in the lower Rhine is 1.5 to 3.6 ng/kg, in seawater 1.53 to 9.65 pmol/L. Middle lanthanide.

Anthropogenic sources

Gadolinium-based contrast agents excreted in urine within hours of an MRI examination, reaching sewers, hospital effluent, sewage treatment plants and receiving water. Bau and Dulski (1996) showed positive Gd anomalies in rivers of densely populated central Europe and North America and none in thinly populated Sweden and Japan, almost entirely in the dissolved fraction below 0.2 µm, and traced them to gadopentetate. Since then: anthropogenic Gd of 7.67 to 25.0 ng/kg on a geogenic 1.5 to 3.6 ng/kg in the lower Rhine (up to 99 percent anthropogenic in some samples, up to 730 kg a year to the North Sea); an order of magnitude rise in San Francisco Bay from 8.27 to 112 pmol/kg over two decades; 85 to 99 percent anthropogenic Gd in Berlin and Dusseldorf tap water produced by bank filtration and 91 percent in Munich tap water from Alpine valley groundwater; a 1.5 to 11.5 fold rise in Berlin tap water between 2009 and 2012. Sewage treatment plant effluents show the anomaly wherever the catchment has medical facilities; the one plant serving 1,200 people with none showed no anomaly.

3 · Speciation

Trivalent Gd³⁺ throughout the natural pH and Eh range. Below about pH 6, and in sulfate rich water such as mine drainage, the free ion and the GdSO₄⁺ ion pair dominate (sulfate complexes above 90 percent at pH 3.1 to 3.4 in the EPA groundwater study). From neutral to alkaline pH the carbonate complexes GdCO₃⁺ and Gd(CO₃)₂⁻ take over, and they bind the heavy lanthanides more strongly than the light ones (at pH 5.1 to 6.1 the carbonate share rose from 5.0 percent for La to 19.2 percent for Lu). Phosphate, carbonate and fluoride solids and sorption to iron and manganese oxides cap the dissolved concentration at ng/L in oxic neutral water, and the light lanthanides ride partly on colloids. Anthropogenic gadolinium is not Gd³⁺ at all but the intact chelate: an anionic (linear, gadopentetate type) or neutral (macrocyclic) complex that is not particle reactive, stays in the truly dissolved pool below 10 kDa, is not taken into mussel shells, and behaves conservatively through estuaries and aquifers. Speciation modelling of effluent affected river water shows that even geogenic REE stay dissolved there because phosphate and carbonate complexes dominate over the free ion.

Solubility

Controlled by the phosphate, carbonate and fluoride solids and by sorption; the RIVM report puts total dissolved lanthanum and cerium in Dutch surface water at about 1 µg/L or less and the free ion fraction at 0.3 fM to 9 pM. No solubility product for a gadolinium solid was read this session.

Hydrolysis

Hydrolysis of Gd³⁺ is minor in natural water; the hydroxide Gd(OH)₃ forms only at high pH, so lime or caustic neutralisation of acid mine water strips the element with the iron and aluminium hydroxides rather than as its own hydroxide (no hydrolysis constant read this session).

Complexation

The whole story: polyaminocarboxylate chelates with conditional stability high enough to survive sewage treatment, bank filtration and years of aquifer transit; carbonate and phosphate complexes for the geogenic fraction. Brunjes and Hofmann (2020) warn that the chelates do degrade, ligand by ligand and with UV end-of-pipe treatment, and that degradation products in raw water with a high recycled wastewater share raise the health question.

Precipitates

For geogenic Gd: GdPO₄ (hydrated), Gd₂(CO₃)₃, Gd(OH)₃ under lime. The chelates form no precipitate in treatment.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Filtration defines the result: 0.2 or 0.45 µm filtrates still carry colloid bound lanthanides, and only ultrafiltration (10 kDa) separates the truly dissolved pool, which matters most for the light lanthanides. Acidify after filtration. Report the shale-normalised pattern so that anthropogenic anomalies (gadolinium, lanthanum, samarium) are visible. Anthropogenic Gd is a difference: total Gd minus geogenic Gd interpolated from the shale-normalised neighbours (Sm, Eu, Tb, Dy); the interpolation method matters when the anomaly is small (Brunjes and Hofmann). Speciation of individual contrast agents needs hydrophilic interaction chromatography coupled to ICP-MS or electrospray MS.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

No water guideline. In patients: gadolinium from contrast agents is retained in the body, including the brain, for months to years (FDA class warning of December 2017); the one known adverse effect is nephrogenic systemic fibrosis in a small subgroup of patients with pre-existing kidney failure; linear agents retain more gadolinium than macrocyclic ones. Free Gd³⁺ is the toxic form; the drinking water question is degradation products in raw water with a high recycled wastewater share (Brunjes and Hofmann), not the ng/L of intact chelate.

Bioaccumulation

Field bioconcentration factors in Rhine estuary amphipods fall from the light to the heavy lanthanides (La 28,840 to Lu 4,786; Gd 13,183); carp muscle takes up little (BCF 0.22 to 1.10 for Ce, La, Nd, Pr, Sm) while internal organs reach 634 to 978. Anthropogenic La and Sm in the Rhine are taken into mussel shells, anthropogenic Gd is not, so speciation decides bioavailability.

Ecotoxicity

RIVM (2000) environmental risk limits, not discharge limits: MPC 7.1 µg/L in fresh surface water and 0.85 µg/L in salt water (negligible concentrations 0.40 and 0.34 µg/L), derived as lowest LC₅₀ divided by 1000 plus a background set at the detection limit (0.33 µg/L). Acute data behind it: Daphnia magna 48 h EC₅₀ 6.8 mg/L, zebrafish 96 h LC₅₀ 19 mg/L; chronic: no chronic test listed. Field bioconcentration factor in amphipods (porewater basis) 13,183. Anthropogenic Gd is not taken into freshwater mussel shells, unlike anthropogenic La and Sm, so the RIVM figures, which are for dissolved gadolinium salts, do not describe the chelate.

Flags

• Schmidt 2019 tap water concentrations are in the full text; only the percentages of the abstract are written.
• The Tepe 2014 figure (1.5 to 11.5 fold rise) is taken from the abstract as summarised in search results, not from the paper.
• Bau and Dulski 1996 is cited from its abstract.
• The FDA communication names contrast agent brands; only the linear and macrocyclic classes are written.
• The Hatje paper names a chelating resin brand; it is written as a chelating resin.
• The RIVM MPC (7.1 µg/L fresh, 0.85 µg/L salt) is for gadolinium salts, LC₅₀ divided by 1000, and does not apply to the chelates.

Gaps

• WHO GDWQ and EU DWD 2020/2184 Annex I were not read this session (eur-lex returned the articles without the annexes), so their absence of a rare earth parameter is not asserted here; only the US EPA table was read.
• No GCC discharge standard was read; no GCC row is written.
• World average river water concentrations (Gaillardet et al. 2003) were not reachable; river figures come from the Rhine and Dutch waters only.
• No source read this session gives municipal wastewater concentrations for this element beyond the gadolinium literature.
• ICP-MS oxide interference corrections between lanthanides are standard practice but no method text describing them was read, so none is written.
• No stability constants or solubility products for this element's carbonate, phosphate or hydroxide were read; the speciation is qualitative, taken from the EPA groundwater study and the RIVM report.
• Hospital effluent and sewage influent Gd concentrations were not read as numbers.
• Removal by nanofiltration, ozonation and activated carbon was not read as numbers; the Brunjes and Hofmann review covers them but only its abstract was read.
• No groundwater Gd concentration was read as a number (the tap water percentages stand in).

Sources

Identity

Name and symbol

Gadolinium, Gd

Atomic number

64 protons

Position

no group (f-block) · period 6 · f-block · lanthanide

CAS number

7440-54-2

Atomic structure

Atomic mass

157.249 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f⁷ 5d¹
[Xe] 6s²⁴f⁷⁵d¹

Electrons per shell

2, 8, 18, 25, 9, 2

Valence electrons

10 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,586 K (1,312.85 °C)

Boiling point

3,546 K (3,272.85 °C)

Density

7.9 g/cm3

Appearance

silvery white

Thermal conductivity

10.6 W/(m·K)

Electrical resistivity

α, poly: 1.310 µΩ·m

Electrical conductivity

763,358.779 S/m

Crystal structure

hexagonal close packed

Molar heat capacity

37.03 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.2 (Pauling Scale)

Ionisation energy

6.15 eV
1st 593.4, 2nd 1,170, 3rd 1,990 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 196, covalent 196, van der Waals 237 pm

Ionic radius

Gd³⁺ 94 pm

Reactivity

A middle lanthanide (4f7 5d1 6s2) that is trivalent in essentially all its chemistry and a strong reducing agent; unusually for a rare earth the bulk metal is fairly stable in dry air, but moist air and hot water attack it.

with water

Reacts slowly with cold water and quickly with hot water to the hydroxide and hydrogen: $\ce{2 Gd + 6 H2O -> 2 Gd(OH)3 + 3 H2}$, so moisture must be kept off the metal.

with oxygen, air

Stable in dry air but tarnishes in moist air to a loosely adhering oxide that flakes off and exposes fresh metal: $\ce{4 Gd + 3 O2 -> 2 Gd2O3}$, and the metal burns on heating.

with acids

Dissolves in dilute acids; with dilute sulfuric acid it gives colourless Gd3+ solutions and hydrogen: $\ce{2 Gd + 3 H2SO4 -> Gd2(SO4)3 + 3 H2}$, the salt.

with halogens

Combines with all four halogens to the trihalides, white except the yellow iodide: $\ce{2 Gd + 3 Cl2 -> 2 GdCl3}$, the most used of them.

Typical compounds

• Gd₂O₃ gadolinium(III) oxide gadolinia, dissolves in acids to the salts
• GdCl₃ gadolinium(III) chloride the commonest halide, white and hygroscopic
• GdF₃ gadolinium(III) fluoride reduced with calcium to make the metal
• Gd₂(SO₄)₃ gadolinium(III) sulfate paramagnetic salt used in low-temperature magnetic cooling
• Gd(NO₃)₃ gadolinium(III) nitrate soluble salt, feedstock for MRI chelates and phosphors

Occurrence, production and use

Crustal abundance

6.2 milligrams per kilogram

Oceanic abundance

7×10-7 milligrams per liter

Occurrence and sources

Gadolinium is found in several other minerals, including monazite and bastnasite, both of which are commercially important. With the development of ion-exchange and solvent extraction techniques, the availability and prices of gadolinium and the other rare-earth metals have greatly improved. The metal can be prepared by the reduction of the anhydrous fluoride with metallic calcium.

• monazite (phosphate) and bastnaesite (fluorocarbonate) the main minerals, with the other lanthanides; gadolinite is the historical mineral

Extraction, production

Ion exchange and solvent extraction from monazite and bastnaesite

no balanced equation printed by the source

Reduction of anhydrous gadolinium fluoride with calcium metal to the metal

$\ce{2 GdF3 + 3 Ca -> 2 Gd + 3 CaF2}$

stoichiometry not printed, so no equation is written

Uses

Gadolinium has the greatest ability to capture thermal neutrons of all known elements and can be used as control rods for nuclear reactors. Unfortunately, the two isotopes best suited for neutron capture, gadolinium-155 and gadolinium-157, are present in gadolinium in small amounts. As a result, gadolinium control rods quickly lose their effectiveness.

Gadolinium can be combined with yttrium to form garnets that have applications in microwave technology. Gadolinium can be alloyed with iron, chromium and other metals to improve their workability and their resistance to high temperatures and oxidation. Gadolinium compounds are used to make phosphors for color televisions.

Gadolinium yttrium garnets are used in microwave applications and gadolinium compounds are used as phosphors in color television sets.

The metal has unusual superconductive properties. As little as 1 percent gadolinium improves the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation.

Gadolinium ethyl sulfate has extremely low noise characteristics and may find use in duplicating the performance of amplifiers, such as the maser.

The metal is ferromagnetic. Gadolinium is unique for its high magnetic movement and for its special Curie temperature (above which ferromagnetism vanishes) lying just at room temperature, meaning it could be used as a magnetic component that can sense hot and cold.

• Medicine: gadolinium chelates as magnetic resonance imaging contrast agents, particularly for cancer diagnosis
• Steel and metallurgy: about 1 percent gadolinium to improve the workability, high-temperature strength and oxidation resistance of iron and chromium alloys
• Electronics and magnets: alloys for magnets, electronic components and data storage disks
• Nuclear: neutron absorber in reactor cores
• Rare earth mining and separation: mined as bastnaesite (a rare-earth fluorocarbonate) at Mountain Pass, California, and as monazite (a phosphate) in heavy-mineral-sand concentrates; separated from the other rare earths by ion exchange and solvent extraction world mine production of rare earths 390,000 t of rare-earth-oxide equivalent in 2024 (USGS estimate), of which China 270,000 t (production quota), the United States 45,000 t, Burma 31,000 t, Australia, Nigeria and Thailand 13,000 t each; reserves over 90 million t, China 44 million t, Brazil 21 million t (usgs-mcs2025, PDF p149, printed p145)

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H261 In contact with water releases flammable gas Substances and mixtures which in contact with water, emit flammable gases

Discovery and name

Discovered by

Jean Charles Galissard de Marignac

Discovered

1880

First isolated

Félix Trombe

Named by

not in sources

Origin of the name

after the mineral gadolinite (itself named after Johan Gadolin)

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235°C, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure.

The metal is relatively stable in dry air, but tarnishes in moist air and forms a loosely adhering oxide film which falls off and exposes more surface to oxidation. The metal reacts slowly with water and is soluble in dilute acid.

Gadolinium has the highest thermal neutron capture cross-section of any known element (49,000 barns).