Actinium extraction

As the parent of actinium in this series it was named protoactinium, shortened in 1949 to protactinium. Because of its low natural abundance its chemistry was obscure until 1960 when A. G. Maddock and co-workers at the UK Atomic Energy Authority worked up about 130g from 60 tons of sludge which had accumulated during the extraction of uranium from UO2 ores. It is from this sample, distributed to numerous laboratories throughout the world, that the bulk of our knowledge of the element s chemistry was gleaned. 

Lin [ 1 ] used coprecipitation with lead sulfate to separate 237-actinium from sea water samples. The 237-actinium was purified by extraction with HDEHP, and determined by alpha spectrometry via Si (Au) surface barrier detection. The method has a sensitivity of 10 3 pCig"1 of ashed sample. 

ISOTOPES There are a total of 35 isotopes of actinium, none of which are stable. All are radioactive, and none exist in the Earth s crust in any large amounts, although a few can be extracted from large quantities of pitchblende and other minerals. All are extremely scarce. Those produced artificially in nuclear reactors, cyclotrons, or linear accelerators have relatively short half-lives, ranging from 69 nanoseconds to 21 years. 

Actinium is a rare element that is found in very small amounts in uranium ore (pitchblende), making it difficult and expensive to extract even a small quantity. It is less expensive and easier to produce small amounts by bombarding the element radium with neutrons in a nuclear reactor. Actinium has few commercial uses. 

Pa, protactinium, was first identified in 1913 in the decay products of U-238 as the Pa-234 isotope (6.7 h) by Kasimir Fajans and Otto H. Gohring. In 1916, two groups, Otto Hahn and Lisa Meitner, and Frederick Soddy and John A. Cranston, found Pa-231 (10 years) as a decay product of U-235. This isotope is the parent of Ac-227 in the U-235 decay series, hence it was named protactinium (before actinium). Isolation from U extraction sludges yielded over 100 g in 1960. 

Presently. 24 isotopes of actinium, with mass numbers ranging from 207 to 2.30, have been identified. All are radioactive. One year after the discovery of polonium and radinm by the Curies, A. Debierne found an unidentified radioactive substance in the residue after treatment of pitchblende. Debierne named the new material actinium after the Greek word for ray. F. Giesel, independently in 1902, also found a radioactive material in the rare-earth extracts of pitchblende. He named... 

In chemical behavior, actinium acts even more basic than lanthanum (the most basic element of the lanthanide series). The mineral salts of actinium are extracted with difficulty from their aqueous solutions by means of an organic solvent. Thus, they generally are extracted as chelates with... 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

Since the chemistry of actinium is confined to the Ac + ion, it can readily be separated from thorium (and the lanthanides, for that matter) by processes like solvent extraction with thenoyltrifluoroacetone (TTFA) and by cation-exchange chromatography. The latter is an excellent means of purification, as the Ac + ion is much more strongly bound by the resin than its decay products. 

In natural U, the radionuclides of the uranium family and the actinium family are present, and sometimes also radionuclides of the thorium family. Therefore, direct determination of U in ores without chemical separation is difficult, especially since the absorption of the radiation depends on the nature of the minerals. Generally, the samples are dissolved and Th is separated, e.g. by coprecipitation or by extraction with thenoyltrifluoroacetone (TTA). Radioactive equilibrium between " Th and the daughter nuclide 234mp jg rather quickly attained, and the high-energy yS" radiation of the latter can easily be measured. A prerequisite of the determination of U by measuring the activity of either " Th or 234mp jg establishment of radioactive equilibrium. This means that the uranium compound must not have been treated chemically for about 8 months. 

Roy, S.C., Extraction studies of trans-actinium elements through microporous membrane, PhD thesis. University of Mumbai, Mumbai, 2007 (submitted). 

Actinium is rarely, if ever, extracted ftom natural sources. 

Recent papers (29) report the use of TOPO for the extraction of actinium from nitrate media. The maximum extraction coefficient from >2.0 M NaN03 at pH 2 with 0.05 M TOPO in cyclohexane was noted to be greater than 10, and the extracted complex was reported as Ac(N03)3 4T0P0 (30). 

From this mineral it was formerly customary to extract the uranium and discard the residue. The chemical study of such a complex mixture is an exceedingly difficult task, but by patient effort M. and Mme. Curie succeeded > in 1898 in separating two new radioactive substances to which the names radium and polonium were applied. The latter is now commonly called radium F. Later Debieme discovered 2 a third radioactive constituent of pitchblende residues and named the new substance actinium. 

Actinium was originally isolated from uranium minerals in which it occurs in traces, but it is now made on a milligram scale by neutron capture in radium (Table 28-2). The actinium +3 ion is separated from the excess of radium and isotopes of Th, Po, Bi and Pb formed simultaneously by ion-exchange elution or by solvent-extraction with thenoyltrifluoroacetone. 

Dehierne started his work with a few hundreds kilograms of uranium ore extracting the active principle from it. After, he had extracted uranium, radium, and polonium he was left with a small amount of a substance whose activity was much higher than the activity of uranium (approximately, by a factor of 100 000). At first, Dehierne assumed that this highly radioactive substance was similar to titanium in its chemical properties. Then he corrected himself and suggested a similarity with thorium. Later, in spring of 1899 he announced the discovery of a new element and called it actinium (from the Greek for radiation). 

Then what did Dehierne discover It was a complex mixture of radioactive substances including actinium. But the weak beta radiation of actinium was quite indistinguishable against the background of the alpha rays emitted by the products of actinium decay. It took several years to extract the real actinium from this mixture of radioactive products. 

For a long time extraction of metallic actinium was just out of question. Indeed, one ton of pitchblende contains only 0.15 mg of actinium while the content of radium is as high as 400 mg. A few milligrams of metallic actinium were obtained only in 1953 after reduction of AcCla with potassium vapour. 

Artificial synthesis of francium is much more difficult and less reliable method than extraction of francium as a product of decay of natural actinium. But natural actinium is rare. What to do A current method is to irradiate the main isotope of radium with a mass number of 226 (its half-life is 1 622 years) with fast neutrons. Radium-226 absorbs a neutron and converts into radium-227 with a half-life of about 40 min. Its decay gives rise to pure actinium-227 whose alpha decay in its turn produces francium-223. 

The product actinium can be separated from the precursor radium by solvent extraction or ion exchange, and gram amounts of actinium have been obtained by this procedure. This is not at all an easy task, considering the highly radioactive substances involved, but is preferable by far to extraction from natural sources. Protactinium can be produced by the nuclear reactions ... 

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