Americium

Typical wastewaters

• nuclear fuel reprocessing and radioactive waste streams Am³⁺ in acid, AmCO₃⁺ and Am(CO₃)₂⁻ at neutral to alkaline pH, humate bound in organic rich water; americium-241 grows in from plutonium-241 measured values are in the Sellafield, La Hague, Hanford and Rocky Flats entries below
• drinking water treatment residuals (floc, resin, reverse osmosis concentrate) Am(III) sorbed and co-precipitated on iron or aluminium hydroxide floc, on cation resin and in the RO concentrate coagulation, ion exchange and reverse osmosis all concentrate it into a transuranic waste
• spent fuel reprocessing liquid effluent to sea (Sellafield, Irish Sea) Am-241 in treated effluent discharged by pipeline, part of the total alpha 0.01 TBq in each of 2020 to 2022 (0.02 TBq in 2018 and 2019) against a permit limit of 0.14 TBq per year; the marine critical group dose, 72 µSv in 2022, is dominated by plutonium alpha and Am-241 from historic discharges
• spent fuel reprocessing liquid effluent to sea (La Hague, English Channel; OSPAR reporting) Am-241 within the reported total alpha; OSPAR states total alpha from nuclear sites is mainly Pu-239, Pu-240 and Am-241 (OSPAR 2010) 2023, TBq per year: La Hague Am-241 0.00343 with total alpha 0.0322; Sellafield, reported under decommissioning in 2023, Am-241 0.00909 with total alpha 0.0648
• legacy site groundwater (Hanford 200 West, well 299-W₁₅-8 beside the 216-Z-9 trench) Am-241 in unfiltered groundwater beneath a trench that received acidic, organic rich plutonium processing waste 0.14 pCi/L on 7 May 1990 and 5.9 pCi/L on 13 November 1991, with Pu-239/240 8.3 and 1.9 pCi/L; americium detected to 36.7 m below the trench in sediment
• storm runoff and pond discharge from contaminated soil (Rocky Flats 903 Pad) Am-241 with Pu-239/240, 40 to 90 percent in particles above 0.45 µm and 10 to 60 percent colloidal spring and summer samples 1998 to 2000; colloid bound actinides remobilised by humic and fulvic acids during erosion

1 · Identity

Symbol, number

Am, 95

Oxidation states in water

+3 in every environmental system, as Am³⁺ and its carbonate, hydroxo, sulfate and fluoride complexes; +4, +5 and +6 are strong oxidants stable only where nothing oxidisable is present (EPA Kd volume III). Americium-241 (half life 432.2 years, alpha with a 60 keV gamma) and americium-243 (7,370 years) are the isotopes in waste.

Note

The element entry covers the smoke detector source and the halides. This chapter is about Am³⁺ as a lanthanide look alike in groundwater.

2 · Occurrence in water

Natural sources

None; the traces in uranium minerals (element entry) are below any water relevance.

Anthropogenic sources

Decay of plutonium-241 in nuclear fallout and in waste streams from fuel reprocessing; americium-241 and -243 in radioactive wastes (EPA Kd volume III). Smoke detector sources are sealed and are not a water discharge. No measured concentration in any water was read.

3 · Speciation

In oxidising river water the free ion Am³⁺ dominates from moderately to highly acid pH; at near neutral to alkaline pH the carbonate complexes AmCO₃⁺ and Am(CO₃)₂⁻ take over, Am(CO₃)₃³⁻ growing with dissolved carbonate, and at highly alkaline pH Am(OH)₃ (aq) may exceed the carbonates (EPA Kd volume III, Figure 5.1 at 10⁻12 mol/L). Dissolved americium is capped by Am(OH)₃, AmOHCO₃ and Am₂(CO₃)₃, with AmOHCO₃ the likely control as pH and carbonate rise. Am(III) sorbs readily to minerals, crushed rock and soil, peaking between pH 5 and 6 on single minerals, with soil Kd from 4 mL/g (the screening minimum, pH 7.8) to 225,000 mL/g and typical values of 1,000 to above 100,000 mL/g, though some high values reflect precipitation rather than sorption; it is more mobile at low to moderate pH, where mineral surfaces are positively charged, and in high ionic strength water, and it forms strong complexes with humic substances (EPA Kd volume III).

Solubility

Am(OH)₃, AmOHCO₃ and Am₂(CO₃)₃ control dissolved Am(III) at near neutral and alkaline pH; AmF₃ is poorly soluble (element entry). No numeric solubility was read.

Hydrolysis

AmOH²⁺, Am(OH)₂⁺ and Am(OH)₃ (aq), the last only at high pH.

Complexation

Carbonate strongest at natural pH, then phosphate, sulfate, fluoride, chloride, nitrate weak; humate strong.

Precipitates

AmOHCO₃, Am(OH)₃, Am₂(CO₃)₃, AmF₃; americium carried in iron hydroxide floc.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Am³⁺ sorbs to bottle walls and particles at neutral pH; acidify at collection and filter first if the dissolved fraction is wanted. Humic rich samples carry americium as colloids that pass a 0.45 µm filter.

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

Toxic through its radioactivity (element entry); ingestion dose coefficient 2.0 x 10⁻7 Sv/Bq for americium-241 (WHO Table 9.2).

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The speciation distribution is EPA's calculation for a mean river water at 10⁻12 mol/L; it is illustrative.
• The AmOHCO₃ equation is written from the solid named as the solubility control, not from printed constants.
• Kd values span five orders of magnitude and EPA warns that the highest reflect precipitation; site specific values are essential.
• No measured americium concentration in any water was read.
• The Euratom values were read from the retained UK copy on legislation.gov.uk; its americium detection limit was not returned by the page read.

Gaps

• No stability constants or solubility products were read as numbers; the Silva 1995 NEA review is cited by EPA but was not opened.
• Alpha spectrometry method numbers and detection limits for americium were not read.
• No aquatic toxicity value was read.
• The CWW BAT conclusions have no radioactivity parameter; no discharge row is written.
• No surface complexation stoichiometry was read for americium on iron or aluminium hydroxide floc, so the coagulation row carries no equation; WHO Table 9.4 gives percentages only.

Sources

Identity

Name and symbol

Americium, Am

Atomic number

95 protons

Position

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

CAS number

7440-35-9

Atomic structure

Atomic mass

243 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁷
[Rn] 7s²⁵f⁷

Electrons per shell

2, 8, 18, 32, 25, 8, 2

Valence electrons

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

Physical properties

State at room temperature

Solid

Melting point

1,449 K (1,175.85 °C)

Boiling point

2,284 K (2,010.85 °C)

Density

13.69 g/cm3

Appearance

silvery white

Thermal conductivity

10 W/(m·K)

Electrical resistivity

0.69 µΩ·m

Electrical conductivity

1.45 MS/m

Crystal structure

double hexagonal close packed

Molar heat capacity

62.7 J/(mol·K)

Chemical properties

Oxidation states

+6, +5, +4, +3

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

5.993 eV
1st 578 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 180, covalent 180, van der Waals 244 pm

Ionic radius

Am²⁺ 126 (8-coordinate); Am³⁺ 98; Am⁴⁺ 85 pm

Reactivity

A soft, silvery, malleable actinide that behaves much like a trivalent lanthanide: +3 is by far the most stable state, but +2, +4, +5, +6 and +7 have all been observed, the widest range of any actinide, and Am(IV) and above are oxidants as strong as permanganate.

with water

The sources do not describe the bulk metal with water; the divalent halides AmCl2, AmBr2 and AmI2 are oxidized by water with release of hydrogen and return to Am(III), and the pentavalent ion AmO2+ disproportionates in acid: $\ce{3 AmO2^+ + 4 H^+ -> 2 AmO2^2+ + Am^3+ + 2 H2O}$.

with oxygen, air

Reacts readily with oxygen but tarnishes only slowly in dry air at room temperature; the oxides are AmO (trace amounts only), red-brown Am2O3 and black fluorite-type AmO2, the working form of solid americium.

with acids

Dissolves in aqueous acids to the yellow-red Am3+ ion; Am3+ in weakly acidic solution precipitates AmF3 with fluoride, $\ce{Am^3+ + 3 F^- -> AmF3}$, and persulfate in dilute nitric acid oxidizes it to Am(V) and Am(VI).

with halogens

Trihalides AmF3, AmCl3, AmBr3 and AmI3 are the stable halides; fluorine oxidizes the trifluoride to the tetrafluoride: $\ce{2 AmF3 + F2 -> 2 AmF4}$; the metal with a mercury(II) halide at 400 to 500 C gives the black dihalides AmX2.

Typical compounds

• AmO₂ americium dioxide black, the form used in smoke-detector sources
• Am₂O₃ americium(III) oxide red-brown sesquioxide, melting point 2205 C
• AmF₃ americium(III) fluoride poorly soluble, reduced with barium to the metal
• AmF₄ americium(IV) fluoride stable tetravalent solid, from AmF3 and fluorine
• AmCl₃ americium(III) chloride reddish, hexahydrate crystallizes from HCl solution

Occurrence, production and use

Crustal abundance

Not Applicable

Oceanic abundance

Not Applicable

Occurrence and sources

• trace americium in uranium minerals trace amounts only; the main source is neutron bombardment of plutonium in reactors

Extraction, production

Neutron bombardment of plutonium in a nuclear reactor

a few grams produced each year; no equation printed

Uses

Americium can be produced in kilogram quantities and has a few practical uses. It is used in smoke detectors and can be used as a portable source of gamma rays. Americium-241, with a half-life of 432.2 years, is used in these products because it is easier to produce relatively pure samples of this isotope.

There are many commercial applications for americium isotopes. Americium-241 has been used as a portable source of both gamma rays and alpha particles for a number of medical and industrial uses. The 60-keV gamma ray emissions from 241Am in such sources can be used for indirect analysis of materials in radiography and X-ray fluorescence spectroscopy, as well as for quality control in fixed nuclear density gauges and nuclear densometers. For example, americium has been employed to gauge glass thickness to help create flat glass. Americium-241 is also suitable for calibration of gamma-ray spectrometers in the low-energy range, since its spectrum consists of nearly a single gamma peak. Americium-241 is also used as the ionization source in commercial smoke detectors. Several unusual applications, such as a nuclear battery or fuel for space ships with nuclear propulsion, have been proposed for the isotope 242mAm, but they are as yet hindered by the scarcity and high price of this isomer.

Safety, toxicity

not in sources

Discovery and name

Discovered by

Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, Albert Ghiorso

Discovered

1944

First isolated

not in sources

Named by

not in sources

Origin of the name

after the Americas

Americium does not occur naturally in the Earth’s crust. In 1944, it was first synthesized by Glenn T. Seaborg and his team at the University of California Laboratory in Berkeley via multiple neutron capture reaction on 239Pu to produce 241Am : 239Pu (n, γ) 240Pu, 240Pu (n, γ) 241Pu, and 241Pu→ 241Am+β -.

The initial americium samples weighed a few micrograms; they were barely visible and were identified by their radioactivity. The first substantial amounts of metallic americium were not prepared until 1951 via reduction of americium(III) fluoride with barium metal in high vacuum at 1100 °C, producing up to 200 milligrams. The luster of freshly prepared americium metal is white and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and tarnishes slowly in dry air at room temperature. In solution, oxidation states III, IV, V, and VI are known and there is an unsubstantiated claim of the existence of Am(VII). Am(IV) is unstable in acidic media but in strongly basic carbonate solutions Am(IV) is stable. In fact, in carbonate solutions, americium has been shown to be the second element after plutonium to have in coexistence all four oxidation states simultaneously. There are numerous compounds of americium. Its oxides have the most practical applications.