Actinium

Typical wastewaters

• uranium ore refining raffinate (pitchblende digestion residues, St. Louis airport site and Weldon Spring raffinate pits) Ac-227 with Pa-231 and Th-230 in the raffinate, the filtered solids and slurry left after uranium extraction, stored as AM-7 cake and in raffinate pits source term for ore in equilibrium: Pa-231 and Ac-227 each at 0.02 of the uranium activity, 0.01 of total alpha; for raffinate work the non-uranium alpha is apportioned 94.6 percent Th-230 and 5.4 percent Ac-227; a residue slurry rather than a discharged liquid, and no activity per litre was read

1 · Identity

Symbol, number

Ac, 89

Oxidation states in water

+3 only (Ac³⁺), a heavier lanthanum; only radioactive isotopes

Note

Ac-227 (half-life 21.8 years) is the actinium of natural water, produced by decay of Pa-231 (half-life 32,760 years), itself from U-235; Pa-231 adsorbs to particles and settles, then its Ac-227 daughter is released to the overlying water because actinium is the more soluble of the two.

2 · Occurrence in water

Natural sources

Ac-227 in the ocean: at secular equilibrium with dissolved Pa-231 in mid water, in excess near the seafloor from sediment release (an excess reaching 1.0 dpm/m₃ in the North Atlantic section), and from hydrothermal vents (up to 1.44 dpm/m₃ in a neutrally buoyant plume). Activities are low: below 0.4 dpm/m₃ along the North Atlantic GA₀₁ section, 0.02 to 0.4 dpm/m₃ for most samples, 0.6 and 1.1 dpm/m₃ near the seafloor; 0.05 to 2.68 dpm/m₃ in other basins and 0.1 to 4.8 dpm/m₃ in the compilations cited. In mass terms: below the detection limit of about 0.5 ag/kg in South China Sea surface water, 3.4 ag/kg at 2760 m, 4.2 to 10.9 ag/kg in the Weddell Gyre; 4.1 to 6.1 ag/kg in the Vienne river, France. Actinium also follows uranium into ore, tailings and phosphate rock (book entry).

Anthropogenic sources

None read; Ac-225 and Ac-227 medical and research uses are in the book entry, without a water pathway.

3 · Speciation

Ac³⁺ behaves as a soluble trivalent cation, more soluble than its parent Pa-231, which is particle reactive; that difference is the source of the excess Ac-227 in bottom water. No speciation model for actinium in natural water was read.

Solubility

Ac(OH)₃ is precipitated by ammonia and is less soluble than radium hydroxide (book entry); at ag/kg no solid controls it in nature.

Hydrolysis

Ac³⁺ hydrolyses to Ac(OH)₃ (book entry); no constant read.

Complexation

not read

Precipitates

none at natural levels; co-precipitated with manganese oxide in the analytical method

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

Not relevant or not given for this element.

6 · Analytics

Sampling pitfalls

Volume: tens of litres at least, hundreds for counting methods. Separate Ac-227 supported by dissolved Pa-231 from the excess released by sediments; near-bottom and hydrothermal samples carry the excess.

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

Radiological only; about 150 times as active as radium (book entry). No chemical toxicity data were read.

Bioaccumulation

not read

Ecotoxicity

not read

Flags

• Seawater activities are from one North Atlantic section (2014) and a few Pacific and Southern Ocean stations; the river figure is two quality control duplicates.
• Geibert 2002 and Nozaki 1984 are cited by title only, through the two papers read.

Gaps

• No actinium measurement in groundwater, mine water or uranium mill effluent was read.
• WHO GDWQ radiological screening and EU DWD Annex I Part D were not read this session.
• No removal data for actinium in water treatment were read.

Sources

Identity

Name and symbol

Actinium, Ac

Atomic number

89 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-34-8

Atomic structure

Atomic mass

227 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d¹
[Rn] 7s²⁶d¹

Electrons per shell

2, 8, 18, 32, 18, 9, 2

Valence electrons

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

Physical properties

State at room temperature

Solid

Melting point

1,324 K (1,050.85 °C)

Boiling point

3,471 K (3,197.85 °C)

Density

10.07 g/cm3

Appearance

silvery-white, glowing with an eerie blue light; sometimes with a golden cast

Thermal conductivity

12 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

face-centered cubic

Molar heat capacity

27.2 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.1 (Pauling Scale)

Ionisation energy

5.17 eV
1st 499, 2nd 1,170 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 215, covalent 215, van der Waals 260 pm

Ionic radius

not in sources

Reactivity

The first actinide, chemically a heavier lanthanum: +3 in every known compound, colourless Ac3+ in solution, and lattice constants within a few percent of the lanthanum analogues; its intense radioactivity (it is about 150 times as active as radium) limits the number of compounds that have been characterised.

with water

Reacts rapidly with moisture, the air-exposed metal growing a white oxide coating; in solution Ac3+ hydrolyses to the hydroxide Ac(OH)3, which ammonia precipitates and which is less soluble than radium hydroxide.

with oxygen, air

Reacts rapidly with oxygen in air to a white coating of the sesquioxide: $\ce{4 Ac + 3 O2 -> 2 Ac2O3}$, which protects the metal from further oxidation.

with acids

Dissolves in hydrochloric and nitric acid to Ac3+ solutions, from which the chloride AcCl3, the nitrate Ac(NO3)3 and the insoluble phosphate are obtained; nitric acid solutions serve the ion exchange separation from radium.

with halogens

No direct reaction of the metal with a halogen is reported; the trihalides AcF3, AcCl3 and AcBr3 and the oxyhalides AcOF, AcOCl and AcOBr are known, made from the oxide or hydroxide with the hydrogen halide.

Typical compounds

• Ac₂O₃ actinium(III) oxide white protective coating on the metal
• AcF₃ actinium(III) fluoride reduced with lithium vapour to make the metal
• AcCl₃ actinium(III) chloride from hydrochloric acid solutions
• Ac(NO₃)₃ actinium(III) nitrate soluble salt of the nitric acid separations
• AcPO₄ actinium(III) phosphate white insoluble hemihydrate from phosphate solutions
• Ac₂S₃ actinium(III) sulfide black, from the oxalate and hydrogen sulfide at 1400 C

Occurrence, production and use

Crustal abundance

5.5×10-10 milligrams per kilogram

Oceanic abundance

Not Applicable

Occurrence and sources

• actinium-227 in uranium ores (pitchblende) about 150 mg per tonne of pitchblende, from the decay of uranium-235

Extraction, production

Neutron bombardment of radium-226

no equation printed

Uses

Actinium has no significant commercial applications, although it is used in the production of neutrons.

Safety, toxicity

not in sources

Discovery and name

Discovered by

Friedrich Oskar Giesel

Discovered

1902

First isolated

Joseph G. Stites, Murrell L. Salutsky, Bob D. Stone

Named by

André-Louis Debierne

Origin of the name

from the Ancient Greek ακτίς, "beam" or "ray"

Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha particles. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees C. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons.

In April of 2012, Los Alamos National Laboratory announced a new medical isotope project that shows promise for rapidly producing major quantities of a new cancer-treatment agent, actinium 225 (Ac-225). Both a press release and a video are available.