Actinium

Properties

Actinium is a soft, silvery-white, radioactive, metallic element. Its estimated shear modulus is similar to that of lead. Owing to its strong radioactivity, actinium glows in the dark with a pale blue light, which originates from the surrounding air ionized by the emitted energetic particles. Actinium has similar chemical properties to lanthanum and other lanthanides, and therefore these elements are difficult to separate when extracting from uranium ores. Solvent extraction and ion chromatography are commonly used for the separation.

The first element of the actinides, actinium, gave the set its name, much as lanthanum had done for the lanthanides. The actinides are much more diverse than the lanthanides and therefore it was not until 1945 that the most significant change to Dmitri Mendeleev's periodic table since the recognition of the lanthanides, the introduction of the actinides, was generally accepted after Glenn T. Seaborg's research on the transuranium elements (although it had been proposed as early as 1892 by British chemist Henry Bassett).

Actinium reacts rapidly with oxygen and moisture in air, forming a white coating of actinium oxide that impedes further oxidation. As with most lanthanides and actinides, actinium exists in the oxidation state +3, and the Ac³⁺ ions are colorless in solutions. The oxidation state +3 originates from the [Rn] 6d¹7s² electronic configuration of actinium, with three valence electrons that are easily donated to give the stable closed-shell structure of the noble gas radon. Although the 5f orbitals are unoccupied in an actinium atom, it can be used as a valence orbital in actinium complexes and hence it is generally considered the first 5f element by authors working on it. Ac³⁺ is the largest of all known tripositive ions and its first coordination sphere contains approximately 10.9 ± 0.5 water molecules.

Chemical compounds

Due to actinium's intense radioactivity, only a limited number of actinium compounds are known. These include: AcF₃, AcCl₃, AcBr₃, AcOF, AcOCl, AcOBr, Ac₂S₃, Ac₂O₃, AcPO₄ and Ac(NO₃)₃. They all contain actinium in the oxidation state +3. In particular, the lattice constants of the analogous lanthanum and actinium compounds differ by only a few percent.

Here a, b and c are lattice constants, No is the space group number, and Z is the number of formula units per unit cell. Density was not measured directly but calculated from the lattice parameters.

Oxides

Actinium oxide (Ac₂O₃) can be obtained by heating the hydroxide at 500 °C (932 °F) or the oxalate at 1,100 °C (2,010 °F), in vacuum. Its crystal lattice is isotypic with the oxides of most trivalent rare-earth metals.

Halides

Actinium trifluoride can be produced either in solution or in solid reaction. The former reaction is carried out at room temperature, by adding hydrofluoric acid to a solution containing actinium ions. In the latter method, actinium metal is treated with hydrogen fluoride vapors at 700 °C (1,292 °F) in an all-platinum setup. Treating actinium trifluoride with ammonium hydroxide at 900–1,000 °C (1,650–1,830 °F) yields oxyfluoride AcOF. Whereas lanthanum oxyfluoride can be easily obtained by burning lanthanum trifluoride in air at 800 °C (1,470 °F) for an hour, similar treatment of actinium trifluoride yields no AcOF and only results in melting of the initial product.

AcF₃ + 2 NH₃ + H₂O → AcOF + 2 NH₄F

Actinium trichloride is obtained by reacting actinium hydroxide or oxalate with carbon tetrachloride vapors at temperatures above 960 °C (1,760 °F). Similarly to the oxyfluoride, actinium oxychloride can be prepared by hydrolyzing actinium trichloride with ammonium hydroxide at 1,000 °C (1,830 °F). However, in contrast to the oxyfluoride, the oxychloride could well be synthesized by igniting a solution of actinium trichloride in hydrochloric acid with ammonia.

Reaction of aluminium bromide and actinium oxide yields actinium tribromide:

Ac₂O₃ + 2 AlBr₃ → 2 AcBr₃ + Al₂O₃

and treating it with ammonium hydroxide at 500 °C (932 °F) results in the oxybromide AcOBr.

Other compounds

Actinium hydride was obtained by reduction of actinium trichloride with potassium at 300 °C (572 °F), and its structure was deduced by analogy with the corresponding LaH₂ hydride. The source of hydrogen in the reaction was uncertain.

Mixing monosodium phosphate (NaH₂PO₄) with a solution of actinium in hydrochloric acid yields white-colored actinium phosphate hemihydrate (AcPO₄·0.5H₂O), and heating actinium oxalate with hydrogen sulfide vapors at 1,400 °C (2,550 °F) for a few minutes results in a black actinium sulfide Ac₂S₃. It may be produced by acting with a mixture of hydrogen sulfide and carbon disulfide on actinium oxide at 1,000 °C (1,830 °F).

Isotopes

Naturally occurring actinium is principally composed of two radioactive isotopes; ²²⁷Ac (from the radioactive family of ²³⁵U) and ²²⁸Ac (a granddaughter of ²³²Th). ²²⁷Ac decays mainly as a beta emitter with a very small energy, but in 1.38% of cases it emits an alpha particle, so it can readily be identified through alpha spectrometry. Thirty-three radioisotopes have been identified, the most stable being ²²⁷Ac with a half-life of 21.772 years, ²²⁵Ac with a half-life of 10.0 days and ²²⁶Ac with a half-life of 29.37 hours. All remaining radioactive isotopes have half-lives that are less than 10 hours, and the majority of them have half-lives shorter than one minute. The shortest-lived known isotope of actinium is ²¹⁷Ac (half-life of 69 nanoseconds) which decays through alpha decay. Actinium also has two known meta states. The most significant isotopes for chemistry are ²²⁵Ac, ²²⁷Ac, and ²²⁸Ac.

Purified ²²⁷Ac comes into equilibrium with its decay products after about a half-year. It decays according to its 21.772-year half-life, emitting mostly beta (98.62%) and some alpha particles (1.38%); the successive decay products are part of the actinium series. Owing to the low available amounts, low energy of its beta particles (maximum 44.8 keV) and low intensity of alpha radiation, ²²⁷Ac is difficult to detect directly by its emission, and it is therefore traced via its decay products. The isotopes of actinium range in atomic weight from 203 Da (²⁰³Ac) to 236 Da (²³⁶Ac).

Occurrence and synthesis

Actinium is found only in traces in uranium ores – one tonne of uranium in ore contains about 0.2 milligrams of ²²⁷Ac – and in thorium ores, which contain about 5 nanograms of ²²⁸Ac per one tonne of thorium. The actinium isotope ²²⁷Ac is a transient member of the uranium-actinium series decay chain, which begins with the parent isotope ²³⁵U (or ²³⁹Pu) and ends with the stable lead isotope ²⁰⁷Pb. The isotope ²²⁸Ac is a transient member of the thorium series decay chain, which begins with the parent isotope ²³²Th and ends with the stable lead isotope ²⁰⁸Pb. Another actinium isotope (²²⁵Ac) is transiently present in the neptunium series decay chain, beginning with ²³⁷Np (or ²³³U) and ending with thallium (²⁰⁵Tl) and near-stable bismuth (²⁰⁹Bi); even though all primordial ²³⁷Np has decayed away, it is continuously produced by neutron knock-out reactions on natural ²³⁸U.

The low natural concentration and the close similarity of physical and chemical properties to those of lanthanum and other lanthanides, which are always abundant in actinium-bearing ores, render separation of actinium from the ore impractical. The most concentrated actinium sample prepared from raw material consisted of 7 micrograms of ²²⁷Ac in less than 0.1 milligrams of La₂O₃, and complete separation was never achieved. Instead, actinium is prepared, in milligram amounts, by the neutron irradiation of ²²⁶Ra in a nuclear reactor.

${\displaystyle {}_{88}{}^{226}\text{Ra}+{}_0{}^1\text{n}⟶{}_{88}{}^{227}\text{Ra}\underset{42.2\text{min}}{\overset{{\beta }^{-}}{\to }}{}_{89}{}^{227}\text{Ac}}$

The reaction yield is about 2% of the radium weight. ²²⁷Ac can further capture neutrons resulting in small amounts of ²²⁸Ac. After the synthesis, actinium is separated from radium and from the products of decay and nuclear fusion, such as thorium, polonium, lead, and bismuth. The extraction can be performed with thenoyltrifluoroacetone-benzene solution from an aqueous solution of the radiation products, and the selectivity to a certain element is achieved by adjusting the pH (to about 6.0 for actinium). An alternative procedure is anion exchange with an appropriate resin in nitric acid, which can result in a separation factor of 1,000,000 for radium and actinium vs. thorium in a two-stage process. Actinium can then be separated from radium, with a ratio of about 100, using a low cross-linking cation exchange resin and nitric acid as eluant.

²²⁵Ac was first produced artificially at the Institute for Transuranium Elements (ITU) in Germany using a cyclotron and at St George Hospital in Sydney using a linac in 2000. This rare isotope has potential applications in radiation therapy and is most efficiently produced by bombarding a radium-226 target with 20–30 MeV deuterium ions. This reaction also yields ²²⁶Ac which however decays with a half-life of 29 hours and thus does not contaminate ²²⁵Ac.

Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor in vacuum at a temperature between 1,100 and 1,300 °C (2,010 and 2,370 °F). Higher temperatures resulted in evaporation of the product, and lower temperatures led to an incomplete transformation. Lithium was chosen among other alkali metals because its fluoride is the most volatile.

Applications

Owing to its scarcity, high price and radioactivity, ²²⁷Ac currently has no significant industrial use, but ²²⁵Ac is currently being studied for use in cancer treatments such as targeted alpha therapies. ²²⁷Ac is highly radioactive and was therefore studied for use as an active element of radioisotope thermoelectric generators, for example, in spacecraft. The oxide of ²²⁷Ac pressed with beryllium is also an efficient neutron source with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs. In all those applications, ²²⁷Ac (a beta source) is merely a progenitor which generates alpha-emitting isotopes upon its decay. Beryllium captures alpha particles and emits neutrons owing to its large cross-section for the (α,n) nuclear reaction:

${\displaystyle {}_4{}^9\text{Be}+{}_2{}^4\text{He}⟶{}_6{}^{12}\text{C}+{}_0{}^1\text{n}+\gamma }$

The ²²⁷AcBe neutron sources can be applied in a neutron probe – a standard device for measuring the quantity of water present in soil, as well as moisture/density for quality control in highway construction. Such probes are also used in well logging applications, in neutron radiography, tomography, and other radiochemical investigations.

²²⁵Ac is applied in medicine to produce ²¹³Bi in a reusable generator or can be used alone as an agent for radiation therapy, in particular targeted alpha therapy (TAT). This isotope has a half-life of 10 days, making it much more suitable for radiation therapy than ²¹³Bi (half-life 46 minutes). Additionally, ²²⁵Ac decays to nontoxic ²⁰⁹Bi rather than toxic lead, which is the final product in the decay chains of several other candidate isotopes, namely ²²⁷Th, ²²⁸Th, and ²³⁰U. Not only ²²⁵Ac itself, but also its daughters, emit alpha particles which kill cancer cells in the body. The major difficulty with the application of ²²⁵Ac was that intravenous injection of simple actinium complexes resulted in their accumulation in the bones and liver for a period of tens of years. As a result, after the cancer cells were quickly killed by alpha particles from ²²⁵Ac, the radiation from the actinium and its daughters might induce new mutations. To solve this problem, ²²⁵Ac was bound to a chelating agent, such as citrate, ethylenediaminetetraacetic acid (EDTA) or diethylene triamine pentaacetic acid (DTPA). This reduced actinium accumulation in the bones, but the excretion from the body remained slow. Much better results were obtained with such chelating agents as HEHA (1,4,7,10,13,16-hexaazacyclohexadecane-N,N′,N″,N‴,N‴′,N‴″-hexaacetic acid) or DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) coupled to trastuzumab, a monoclonal antibody that interferes with the HER2/neu receptor. The latter delivery combination was tested on mice and proved to be effective against leukemia, lymphoma, breast, ovarian, neuroblastoma and prostate cancers.

The medium half-life of ²²⁷Ac (21.77 years) makes it a very convenient radioactive isotope in modeling the slow vertical mixing of oceanic waters. The associated processes cannot be studied with the required accuracy by direct measurements of current velocities (of the order 50 meters per year). However, evaluation of the concentration depth profiles for different isotopes allows estimating the mixing rates. The physics behind this method is as follows: oceanic waters contain homogeneously dispersed ²³⁵U. Its decay product, ²³¹Pa, gradually precipitates to the bottom, so that its concentration first increases with depth and then stays nearly constant. ²³¹Pa decays to ²²⁷Ac; however, the concentration of the latter isotope does not follow the ²³¹Pa depth profile, but instead increases toward the sea bottom. This occurs because of the mixing processes, which raise some additional ²²⁷Ac from the sea bottom. Thus, analysis of both ²³¹Pa and ²²⁷Ac depth profiles allows researchers to model the mixing behavior.

There are theoretical predictions that AcH_x hydrides (in this case with very high pressure) are a candidate for a near room-temperature superconductor as they have T_c significantly higher than H₃S, possibly near 250 K.

Precautions

²²⁷Ac is highly radioactive and experiments with it are carried out in a specially designed laboratory equipped with a tight glove box. When actinium trichloride is administered intravenously to rats, about 33% of actinium is deposited into the bones and 50% into the liver. Its toxicity is comparable to, but slightly lower than, that of americium and plutonium. For trace quantities, fume hoods with good aeration suffice; for gram amounts, hot cells with shielding from the intense gamma radiation emitted by ²²⁷Ac are necessary.