Actinide chemistry

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Plutonium is a transuranic radioactive chemical element, an actinide metal. Pu,94-cropped.jpg
Plutonium is a transuranic radioactive chemical element, an actinide metal.

Actinide chemistry (or actinoid chemistry) is one of the main branches of nuclear chemistry that investigates the processes and molecular systems of the actinides. The actinides derive their name from the group 3 element actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. The actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. [1] [2] [3] [4]

Contents

Main branches

Organoactinide chemistry

In contrast to the relatively early flowering of organotransition-metal chemistry (1955 to the present), the corresponding development of actinide organometallic chemistry has taken place largely within the past 15 or so years. During this period, 5f organometallic science has blossomed, and it is now apparent that the actinides have a rich, intricate, and highly informative organometallic chemistry. Intriguing parallels to and sharp differences from the d-block elements have emerged. Actinides can coordinate the organic active groups or bind to carbon by the covalent bonds. [5]

Thermodynamics of actinides

The necessity of obtaining accurate thermodynamic quantities for the actinide elements and their compounds was recognized at the outset of the Manhattan Project, when a dedicated team of scientists and engineers initiated the program to exploit nuclear energy for military purposes. Since the end of World War II, both fundamental and applied objectives have motivated a great deal of further study of actinide thermodynamics. [6]

Nanotechnology and supramolecular chemistry of actinides

The possibility of using unique properties of lanthanides in the nanotechnology is demonstrated. The origination of linear and nonlinear optical properties of lanthanide compounds with phthalocyanines, porphyrins, naphthalocyanines, and their analogs in solutions and condensed state and the prospects of obtaining novel materials on their basis are discussed. Based on the electronic structure and properties of lanthanides and their compounds, namely, optical and magnetic characteristics, electronic and ionic conductivity, and fluctuating valence, molecular engines are classified. High-speed storage engines or memory storage engines; photoconversion molecular engines based on Ln(II) and Ln(III); electrochemical molecular engines involving silicate and phosphate glasses; molecular engines whose operation is based on insulator – semiconductor, semiconductor – metal, and metal – superconductor types of conductivity phase transitions; solid electrolyte molecular engines; and miniaturized molecular engines for medical analysis are distinguished. It is shown that thermodynamically stable nanoparticles of LnxMy composition can be formed by d elements of the second halves of the series, i.e., those arranged after M = Mn, Tc, and Re. [7]

Biological and environmental chemistry of actinides

Generally, ingested insoluble actinide compounds such as high-fired uranium dioxide and mixed oxide (MOX) fuel will pass through the digestive system with little effect since they cannot dissolve and be absorbed by the body. Inhaled actinide compounds, however, will be more damaging as they remain in the lungs and irradiate the lung tissue. Ingested Low-fired oxides and soluble salts such as nitrate can be absorbed into the blood stream. If they are inhaled then it is possible for the solid to dissolve and leave the lungs. Hence the dose to the lungs will be lower for the soluble form.

Radon and radium are not actinides—they are both radioactive daughters from the decay of uranium. Aspects of their biology and environmental behaviour is discussed at radium in the environment.

Monazite, a rare earth and thorium phosphate mineral, is the primary source of the world's thorium. MonaziteUSGOV.jpg
Monazite, a rare earth and thorium phosphate mineral, is the primary source of the world's thorium.

In India, a large amount of thorium ore can be found in the form of monazite in placer deposits of the Western and Eastern coastal dune sands, particularly in the Tamil Nadu coastal areas. The residents of this area are exposed to a naturally occurring radiation dose ten times higher than the worldwide average. [8]

Thorium has been linked to liver cancer. In the past thoria (thorium dioxide) was used as a contrast agent for medical X-ray radiography but its use has been discontinued. It was sold under the name Thorotrast.

Uranium is about as abundant as arsenic or molybdenum. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources). Seawater contains about 3.3 parts per billion of uranium by weight [9] as uranium(VI) forms soluble carbonate complexes. The extraction of uranium from seawater has been considered as a means of obtaining the element. Because of the very low specific activity of uranium the chemical effects of it upon living things can often outweigh the effects of its radioactivity.

Plutonium, like other actinides, readily forms a plutonium dioxide (plutonyl) core (PuO2). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH−, NO−
2, NO−
3, and SO2−
4) to form charged complexes which can be readily mobile with low affinities to soil.

Nuclear reactions

Some early evidence for nuclear fission was the formation of a short-lived radioisotope of barium which was isolated from neutron irradiated uranium (139Ba, with a half-life of 83 minutes and 140Ba, with a half-life of 12.8 days, are major fission products of uranium). At the time, it was thought that this was a new radium isotope, as it was then standard radiochemical practice to use a barium sulfate carrier precipitate to assist in the isolation of radium.

PUREX

The PUREX process is a liquid–liquid extraction ion-exchange method used to reprocess spent nuclear fuel, in order to extract primarily uranium and plutonium, independent of each other, from the other constituents. The current method of choice is to use the PUREX liquid–liquid extraction process which uses a tributyl phosphate/hydrocarbon mixture to extract both uranium and plutonium from nitric acid. This extraction is of the nitrate salts and is classed as being of a solvation mechanism. For example, the extraction of plutonium by an extraction agent (S) in a nitrate medium occurs by the following reaction.

Pu4+
(aq) + 4 NO−
3(aq) + 2 S(organic) → [Pu(NO
3)
4S
2](organic)

A complex bond is formed between the metal cation, the nitrates and the tributyl phosphate, and a model compound of a dioxouranium(VI) complex with two nitrates and two triethyl phosphates has been characterised by X-ray crystallography. [10] After the dissolution step it is normal to remove the fine insoluble solids, because otherwise they will disturb the solvent extraction process by altering the liquid-liquid interface. It is known that the presence of a fine solid can stabilize an emulsion. Emulsions are often referred to as third phases in the solvent extraction community.

An organic solvent composed of 30% tributyl phosphate (TBP) in a hydrocarbon solvent, such as kerosene, is used to extract the uranium as UO2(NO3)2·2TBP complexes, and plutonium as similar complexes, from other fission products, which remain in the aqueous phase. The transuranium elements americium and curium also remain in the aqueous phase. The nature of the organic soluble uranium complex has been the subject of some research. A series of complexes of uranium with nitrate and trialkyl phosphates and phosphine oxides have been characterized. [10]

Plutonium is separated from uranium by treating the kerosene solution with aqueous ferrous sulphamate, which selectively reduces the plutonium to the +3 oxidation state. The plutonium passes into the aqueous phase. The uranium is stripped from the kerosene solution by back-extraction into nitric acid at a concentration of ca. 0.2 mol dm−3. [11]

See also

Related Research Articles

<span class="mw-page-title-main">Actinium</span> Chemical element, symbol Ac and atomic number 89

Actinium is a chemical element; it has symbol Ac and atomic number 89. It was first isolated by Friedrich Oskar Giesel in 1902, who gave it the name emanium; the element got its name by being wrongly identified with a substance André-Louis Debierne found in 1899 and called actinium. Actinium gave the name to the actinide series, a set of 15 elements between actinium and lawrencium in the periodic table. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be isolated.

<span class="mw-page-title-main">Americium</span> Chemical element, symbol Am and atomic number 95

Americium is a synthetic chemical element; it has symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element europium and was thus named after the Americas by analogy.

The actinide or actinoid series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.

<span class="mw-page-title-main">Curium</span> Chemical element, symbol Cm and atomic number 96

Curium is a synthetic chemical element; it has symbol Cm and atomic number 96. This transuranic actinide element was named after eminent scientists Marie and Pierre Curie, both known for their research on radioactivity. Curium was first intentionally made by the team of Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944, using the cyclotron at Berkeley. They bombarded the newly discovered element plutonium with alpha particles. This was then sent to the Metallurgical Laboratory at University of Chicago where a tiny sample of curium was eventually separated and identified. The discovery was kept secret until after the end of World War II. The news was released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains ~20 grams of curium.

<span class="mw-page-title-main">Thorium</span> Chemical element, symbol Th and atomic number 90

Thorium is a chemical element. It has the symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive gray when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

<span class="mw-page-title-main">Monazite</span> Mineral containing rare-earth elements

Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium-dominant member of the group. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm3. There are five different most common species of monazite, depending on the relative amounts of the rare earth elements in the mineral:

<span class="mw-page-title-main">Nuclear reprocessing</span> Chemical operations that separate fissile material from spent fuel to be recycled as new fuel

Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the reprocessed plutonium was recycled back into MOX nuclear fuel for thermal reactors. The reprocessed uranium, also known as the spent fuel material, can in principle also be re-used as fuel, but that is only economical when uranium supply is low and prices are high. Nuclear reprocessing may extend beyond fuel and include the reprocessing of other nuclear reactor material, such as Zircaloy cladding.

A period 7 element is one of the chemical elements in the seventh row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behavior of the elements as their atomic number increases: a new row is begun when chemical behavior begins to repeat, meaning that elements with similar behavior fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells in that order, but there are exceptions, such as uranium.

<span class="mw-page-title-main">Nuclear chemistry</span> Branch of chemistry dealing with radioactivity, transmutation and other nuclear processes

Nuclear chemistry is the sub-field of chemistry dealing with radioactivity, nuclear processes, and transformations in the nuclei of atoms, such as nuclear transmutation and nuclear properties.

Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the chemical formula (CH3CH2CH2CH2O)3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of phosphoric acid with n-butanol.

<span class="mw-page-title-main">PUREX</span> Spent fuel reprocessing process for plutonium and uranium recovery

PUREX is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. PUREX is the de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid–liquid extraction ion-exchange.

<span class="mw-page-title-main">Liquid–liquid extraction</span> Method to separate compounds or metal complexes

Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an organic solvent (non-polar). There is a net transfer of one or more species from one liquid into another liquid phase, generally from aqueous to organic. The transfer is driven by chemical potential, i.e. once the transfer is complete, the overall system of chemical components that make up the solutes and the solvents are in a more stable configuration. The solvent that is enriched in solute(s) is called extract. The feed solution that is depleted in solute(s) is called the raffinate. Liquid-liquid extraction is a basic technique in chemical laboratories, where it is performed using a variety of apparatus, from separatory funnels to countercurrent distribution equipment called as mixer settlers. This type of process is commonly performed after a chemical reaction as part of the work-up, often including an acidic work-up.

The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO2+
2. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing.

<span class="mw-page-title-main">Actinides in the environment</span>

Environmental radioactivity is not limited to actinides; non-actinides such as radon and radium are of note. While all actinides are radioactive, there are a lot of actinides or actinide-relating minerals in the Earth's crust such as uranium and thorium. These minerals are helpful in many ways, such as carbon-dating, most detectors, X-rays, and more.

In nuclear chemistry, the actinide concept proposed that the actinides form a second inner transition series homologous to the lanthanides. Its origins stem from observation of lanthanide-like properties in transuranic elements in contrast to the distinct complex chemistry of previously known actinides. Glenn Theodore Seaborg, one of the researchers who synthesized transuranic elements, proposed the actinide concept in 1944 as an explanation for observed deviations and a hypothesis to guide future experiments. It was accepted shortly thereafter, resulting in the placement of a new actinide series comprising elements 89 (actinium) to 103 (lawrencium) below the lanthanides in Dmitri Mendeleev's periodic table of the elements.

<span class="mw-page-title-main">Organoactinide chemistry</span> Study of chemical compounds containing actinide-carbon bonds

Organoactinide chemistry is the science exploring the properties, structure, and reactivity of organoactinide compounds, which are organometallic compounds containing a carbon to actinide chemical bond.

<span class="mw-page-title-main">BTBP</span> A class of tetradentate ligand compounds

The bis-triazinyl bipyridines (BTBPs) are a class of chemical compounds which are tetradentate ligands similar in shape to quaterpyridine. The BTBPs are made by the reaction of hydrazine and a 1,2-diketone with 6,6'-dicyano-2,2'-bipyridine. The dicyanobipy can be made by reacting 2,2'-bipy with hydrogen peroxide in acetic acid, to form 2,2'-bipyridine-N,N-dioxide. The 2,2'-bipyridine-N,N-dioxide is then converted into the dicyano compound by treatment with potassium cyanide and benzoyl chloride in a mixture of water and THF.

The plutonyl ion is an oxycation of plutonium in the oxidation state +6, with the chemical formula PuO2+
2. It is isostructural with the uranyl ion, compared to which it has a slightly shorter M–O bond. It is easily reduced to plutonium(III). The plutonyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Plutonyl salts are important in nuclear fuel reprocessing.

<span class="mw-page-title-main">Thorium compounds</span> Chemical compounds

Many compounds of thorium are known: this is because thorium and uranium are the most stable and accessible actinides and are the only actinides that can be studied safely and legally in bulk in a normal laboratory. As such, they have the best-known chemistry of the actinides, along with that of plutonium, as the self-heating and radiation from them is not enough to cause radiolysis of chemical bonds as it is for the other actinides. While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.

The advanced reprocessing of spent nuclear fuel is a potential key to achieve a sustainable nuclear fuel cycle and to tackle the heavy burden of nuclear waste management. In particular, the development of such advanced reprocessing systems may save natural resources, reduce waste inventory and enhance the public acceptance of nuclear energy. This strategy relies on the recycling of major actinides and the transmutation of minor actinides in appropriate reactors. In order to fulfill this objective, selective extracting agents need to be designed and developed by investigating their complexation mechanism.

References

  1. ↑ Gray, Theodore (2009). The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog & Leventhal Publishers. p.  240. ISBN   978-1-57912-814-2.
  2. ↑ Actinide element, Encyclopædia Britannica online
  3. ↑ Although "actinoid" (rather than "actinide") means "actinium-like" and therefore should exclude actinium, that element is usually included in the series.
  4. ↑ Connelly, Neil G.; et al. (2005). "Elements". Nomenclature of Inorganic Chemistry. London: Royal Society of Chemistry. p. 52. ISBN   0-85404-438-8.
  5. ↑ Sonnenberger, D. C.; Morss, L. R.; Marks, T. J. (1985). "Organo f-element thermochemistry. Thorium-ligand bond disruption enthalpies in tricyclopentadienylthorium hydrocarbyls". Organometallics. 4 (2): 352–355. doi:10.1021/om00121a028.
  6. ↑ Cordfunke, E (2001). "The enthalpies of formation of lanthanide compounds I. LnCl3(cr), LnBr3(cr) and LnI3(cr)". Thermochimica Acta. 375 (1–2): 17–50. doi:10.1016/S0040-6031(01)00509-3. ISSN   0040-6031.
  7. ↑ Tsivadze, A. Yu.; Ionova, G. V.; Mikhalko, V. K. (2010). "Nanochemistry and supramolecular chemistry of actinides and lanthanides: Problems and prospects". Protection of Metals and Physical Chemistry of Surfaces. 46 (2): 149–169. doi:10.1134/S2070205110020012. ISSN   2070-2051. S2CID   94403630.
  8. ↑ "Compendium Of Policy And Statutory Provisions Relating To Exploitation Of Beach Sand Minerals". Government Of India. Archived from the original on 2008-12-04. Retrieved 2008-12-19.
  9. ↑ "Uranium: the essentials". WebElements. Retrieved 2008-12-19.
  10. 1 2 Burns, J. H. (1983). "Solvent-extraction complexes of the uranyl ion. 2. Crystal and molecular structures of catena-bis(μ-di-n-butyl phosphato-O,O′)dioxouranium(VI) and bis(μ-di-n-butyl phosphato-O,O′)bis[(nitrato)(tri-n-butylphosphine oxide)dioxouranium(VI)]". Inorg. Chem. 22 (8): 1174. doi:10.1021/ic00150a006.
  11. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1261. ISBN   978-0-08-037941-8.
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