Actinides Q &A

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1/27/2019

Alena Paulenova

NSE/CH‐516

What Will You Learn About Actinides Today?

This presentation is an introduction to rich and intricate chemistry of actinides. Today we will speak about:

 Natural Actinides  Artificial Actinides • Actinide Theory • Oxidation State and Redox Behavior • Coordination Structure and Bonding • General Trends of Actinide Chemistry

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Natural Actinides

 The actinides (AN) are all radioactive elements.

 U, Th, Pa and Ac are the only four actinides that have been found in environment. They are mothers of their decay chains and a progeny radionuclides, occurring in the nature.

 Other AN are artificial, being produced through various nuclear reactions.

 At the creation of the universe, some amount of 244Pu could have been formed; however, with an 80 million year half‐life, it would have fully decayed during the past 10 billion years.

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Uranium Discovery

 Uranium was the first actinide element discovered.

 The use of uranium in its natural oxide form dates back to at least the year 79 CE (Italy) since when it

was used to add a yellow color to ceramic glazes.

 Starting in the late Middle Ages, pitchblende (impure, mineralized form of UO2) was extracted

from the Habsburg silver mines in Joachimsthal,

Bohemia (now Jáchymov in the Czech Republic).

 For centuries, it was used as a coloring agent in the local glassmaking industry.

Silver mining in Joachimsthal (1548), (now Jáchymov)

Uranium discovery 1789 ‐ M. H. Klaproth identified the presence of a new chemical element in a sample of

pitchblende. He named the new element “uranite” after the recently discovered

planet Uranus; however, it was only uranium oxide

1841 ‐ Eugene Peligot, French chemist, insulated uranium metal. The atomic mass of uranium

was then calculated as 120.

1872 ‐Dmitri Mendeleev corrected it to 240 using his periodicity laws.

1882 ‐ K. Zimmerman experimentally confirmed the value calculated by Mendeleev.

6 Beta Roentgen's hand

1896 ‐ Henri Becquerel made the initial discovery of the ”uranium rays” through experiments with uranium minerals and photographic plates.

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Radio–active?  Marie Sklodowska, a recent graduate in physics in Paris, became interested in “uranium rays”, recently discovered by H. Becquerel.

 Using just the electrometer invented by her husband P. Curie, she discovered that uranium rays caused the air around a sample to conduct electricity.

 1898 ‐ she coined term “Radioactivity”.

 Marie Sklodowska‐Curie also showed that thorium is radioactive, but was late to claim her discovery (Gerhard C. Schmidt , England).

 The same year (1898) she discovered and separated polonium and radium (after processing tons of pitchblende by bare hands).

Radiochemistry was born

1898 ‐ Po and Ra were discovered after processing tons of pitchblende (UO2) by bare hands.

1903 ‐ Nobel Prize in Physics for H. Becquerel, P. Curie and M. Sklodowska‐Curie for discovering radioactivity.

1911 – Nobel Prize in Chemistry for Madam Curie for discovering Ra and Po.

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Thorium

1827 ‐ Thorium oxide was discovered by Fridrich Wöhler in a Norwegian mineral.

1828 ‐ Berzellius characterized this material and discovered and insulated a new element and named it thorium (Th) after Thor, a mythological Norse god of thunder and lightening.

J. J. Berzelius

• Berzelius applied reduction of ThCl4 with potassium ‐ later used also for reduction of uranium by Peligot (1841).

• Radioactivity of Th was discovered only 70 years later, in 1898, independently by Marie Sklodowska‐Curie (France) and Gerhard C. Schmidt (England).

Actinium

‐ from ‘aktinos’, Greek word for “ray”

1899 ‐ the earliest discovery of actinium (Ac) is attributed to A.L. Debierne, Curie’s collaborator on isolation of Ra; he described its behavior, however, never insulated actinium.

1902 ‐ F. Giesel identified and isolated the element Ac in pitchblende.

pitchblende

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Protactinium

Pa, mother of Ac, was discovered as the last from the naturally occurring actinides.

1913 – identified as short‐lived isotope 234mPa (half‐life 1.17 minutes) by K. Fajans and O. H. Goehring during their studies of the 238U decay, and named brevium (latin for ‘brief ’).

1918 ‐ O. Hahn and L. Meitner (Germany) and independently F. Soddy and J. Cranston (UK) discovered 231Pa and renamed it to “protoactinium” (Greek word ‘protos’ =first, meaning the first element).

1949 – IUPAC ⇛ protactinium.

O. Hahn and L. Meitner [wiki ]

Natural Actinides Series

A=4n+2

http://www.gemoc.mq.edu.au/Participants/Research/ADosseto/research.html

• Named after the longest lived member

• Contribute to natural radioactivity

• All decay to lead

• A= atomic mass number of members

• All members are in secular equilibrium

A=4n+3

A=4n

* Secular equilibrium is formed by a so long-lived parent and so short-lived daughter that after a time long about 10 half-lives of daughter their activity are equal to each other.

• Each series contains 2 isotopes of Th

• Two Ac (227/U‐235; 228/Th‐232

• Pa‐231, formed from U‐235

Uranium: U‐ 238 (99.3%), U‐ 235 (0.7%) and trace of U‐234 (by weight).

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Atomic Era begins:

The long form of the Periodic Table published by A. Werner (1905).

Th is the only correctly localized radionuclide.

1911 – Discrete structure of atom (Rutheford’s laboratory)

1913 ‐ Solar model of atom (Niels Bohr)

1928 ‐ Joliot‐Curie observed positron and neutron, but failed interpretation 13

First Artificial Radioactivity • 1932 ‐ Neutron discovery: Chadwick observed “beryllium rays” and

correctly identified them as neutrons:

2α + 4Be ⇾ 1n + 6C

• 1934 – Joliot‐Curie observed the first artificial radioactivity:

27Al + 4He ⇾ 30P + 1n

• 1940 ‐ Neptunium was the first transuranium element produced

synthetically by bombarding uranium with slow neutrons:

92U 238+ 0n

1 ⇾ 92U 239 ⇾ 93Np

239

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.).

Joliot Curie and her mother Maria Sklodowska-Curie [wiki]

• 239Np isotope (half‐life 2.4 days), discovered by Edwin McMillan and Philip H. Abelson in Berkeley, CA and

named for the planet Neptune (the next planet out from Uranus, after which uranium was named).

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Synthesis of Transuranium Elements

1940‐ Edwin McMillan was the first ever to produce a transuranium element. He also started bombarding U239 with deuterons, but had to leave to MIT to work on the radar project.

1940 ‐ Glenn T. Seaborg joined McMillan’s project

1941‐ February – Seaborg, Kennedy, Wahl and McMillan synthesized element 94‐Pu

Since then, Seaborg and his group at the University of California at Berkeley discovered 10 actinide elements and synthesized more than 100 atomic actinide isotopes: Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, including element 106‐ Sg (named in his honor while he was still living).

Heave elements were also synthesized at:

GSI, Darmstadt (Germany)

Joint Institute for Nuclear Research, Dubna (Russia)

Element Year Method

Neptunium 1940 Bombarding2 38U by neutrons

Plutonium 1941 Bombarding2 38U by deuterons

Americium 1944 Bombarding 239Pu by neutrons

Curium 1944 Bombarding 239Pu by α-particles

Berkelium 1949 Bombarding 241Am by α-particles

Californium 1950 Bombarding 242Cm by α-particles

Einsteinium 1952 As product of nuclear explosion

Fermium 1952 As product of nuclear explosion

Mendelevium 1955 Bombarding 253Es by α-particles

Nobelium 1965 B-g 243Am by 15N or 238U with α-particles

Lawrencium 1961–1971 B-g 252Cf by 10B or 11B and of 243Am with 18O

Synthesis of Transuranium Elements)

1951 - Nobel Prize, for Seaborg and McMillan for "their discoveries in the chemistry of the first transuranium elements."

[www. wikipedia.org]

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Reactor Chemistry

Adopted from : E. Holm, J. Rioseco and H. Peterson; J. Radioanal. Nucl.,Chem. Articles, 1992, 156, 183

Formation of transuranic elements in nuclear fuel or nuclear weapons material

↑ ‐ neutron activation ↙ ‐ spontaneous α-decay → ‐ spontaneous β-decay

Decay Properties

Element Major radionuclides Half-life Decay mode Actinium 227Ac 22 y β-/α Thorium 232Th 1.4x1010y α 230Th 7.6x104 y α 228Th 5.8 y α Protactinium 231Pa 3.3 x 104 y α Uranium 238U 4.5x109 y α/SF 235U 7.0x108 y α 236U 2.3x107 y α 234U 2.5x105 y α Neptunium 237Np 2.1x106 y α Plutonium 238Pu 88 y α 239Pu 2.4 x104 y α 240Pu 6500 y α 241Pu 14 y β- Americium 241Am 433 y α Curium 244Cm 18 y α/SF 242Cm 0.45 y α/SF Berkelium 247Bk 1380 y α Californium 251Cf 898 y α Einsteinium 252Es 1.3 y α/β+/EC Fermium 257Fm 0.27 y α/SF Mendelevium 258Md 0.14 y α/SF/β+/EC Nobelium 259No 1 h α/EC/SF Lawrencium 262Lr 3.6 h SF SF=Spontaneous fission EC=Electron capture

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Applications

 Nuclear reactor fuel  Nuclear weapons  Depleted uranium armor and projectiles  Heat sources  Smoke detectors  Lantern mantles  Catalysis

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Chemistry of Actinides

Since the synthesis of Pu, transuranium chemistry was extensively studied; information was needed

to insure that plutonium produced could be successfully extracted from the irradiated uranium:

• Seaborg isolated a weighable sample of plutonium, using lanthanum fluoride as a carrier (1942).

• Isadore Perlman and William J. Knox investigated the peroxide method of separation.

• John E. Willard studied various sorption materials.

• Theodore T. Magel and Daniel K. Koshland, Jr., researched solvent-extraction processes.

• Harrison S. Brown and Orville F. Hill investigated volatility

• Stanley G. Thompson (school friend of Seaborg) found that bismuth phosphate retained over ninety-eight percent plutonium in a precipitate.

• Basic research on plutonium's chemistry continued as did work on radiation and fission products.

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Glenn T. Seaborg

Theory of Actinides

With the development of transuranic chemistry, soon it become clear that they do not fit into Mendeleev Periodic Table of Elements:

• What is the relation of actinides to lanthanides and other chemical elements?

• What is their position in Table of Elements?

Table of Elements

III B

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III B

IV B

Seaborg's actinide theory resulted in a redrawing of

the Periodic Table of the Elements into its current

configuration with the actinide series appearing

below the lanthanide series (next slide).

As a result of this concept, the transactinide and

superactinide series of elements (Z > 103) were also

properly placed within the d‐block elements.

Actinide Theory

Glenn Seaborg - father of the new table of elements [wiki]

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Position in Table of Elements

Transactinides (z>103)

La Lu

Ac Lr

III B

Position in Periodic Table- Fundamental issue

 Numerous experiments confirmed the position of actinides in the periodic table as a series of 5‐f elements, first proposed by G. T. Seaborg.

 While other designs of the periodic table could also be considered (for example, 3D with the f‐block extending behind the main table), the separation of f‐blocks is a fundamental classification issue.

 Chemical behavior and electronic structural evidence established the actinides (Ac‐Lr; atomic numbers Z=89‐103) as an inner transition series, analogous to the lanthanide transition series (La‐Lu; Z=57‐71).

 First member in series ‐ actinium? Traditionally, Ac has been considered a group III element and Th, the first f‐block; however, the chemical behavior confirms that all 15 elements, Ac

through Lr, may be considered together, hence, also Lr should occupy the position as the

first group III element. 28

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Thermodynamic Properties of Compounds

• Oxidation states of An (particularly in the first half of the series) in solutions are far more variable than those of the lanthanides.

• Multiple oxidation states for the An‐ions are guaranteed by a close proximity of the energy levels of the 7s, 6d, and 5f electrons.

• The rich chemistry of lighter actinides, from Pa to Am, is based on their:

• multiple oxidation states,

• hydrolytic behavior of their cations and,

• strong coordination of organic ligands

Chemistry of Ans is the most complex and intricate among all elements in the periodic table.

Actinide Oxidation States

VII+  

VI+    

V+      

IV+         

III+               

II+     

An Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Importance:  >  > 

32→ TPU→ TRU

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III+ Oxidation State

 While trivalent species are typical for the transplutonium actinides (Am and beyond), the lighter actinides are less stable in trivalent oxidation state

 In acidic solutions: • U(III) is oxidized by water • Np(III) is oxidized by dissolved oxygen in water • Pu(III) is stable, but easily oxidized to Pu(IV) by a variety of mild oxidants. • Th and Pa do not even exhibit the trivalent state in solutions. Stable Th(III) has been

reported only in organometallic compounds. 33

IV+ Oxidation State of Actinides:

 The tetravalent species also can be considered transient, since a stable 4+ state is observed only for elements from Th through Pu and for Bk.

 Other actinides are unstable in IV+ oxidation state:

• Am(IV) in aqueous media must be stabilized by very strong complexing agents like carbonate, phosphate or fluoride.

• The Cm(IV) state is confined to a few solid compounds, particularly CmO2 and CmF4, and appears to be present in a stable complex ion that exists in concentrated cesium fluoride solution.

• The Cf(IV) state is limited to the solid compounds CfO2, CfF4, a complex oxide BaCfO3, and in tungstophosphate solutions; the oxidation of Cf(III) to Cf(IV) in strong carbonate solutions is a disputed topic.

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V+ and VI+ Actinide Oxidation States

 The 5+ oxidation state is well established for the elements Pa through Am; and the 6+ state in the elements U through Am.

 If Am can be stabilized (even as a transient) in its various upper oxidation states, unique options are available for potential Am/Ln group separations.

Colors of Actinides

Plutonium: Colors of the different oxidation states of plutonium, in 1 M HClO4. (Pu(V) is in NaClO4 at pH=7, Pu(VII) is in 2.5 M NaOH.)

Uranium Aqueous solutions of U (III, IV, V, VI) salts.

Neptunium: Colors of the different oxidation states of Np, in 1 M HClO4. (Np(V) is in NaClO4 at pH=7, Np(VII) is in 2.5 M NaOH.)

Light actinides (U, Np, Pu) have very rich redox chemistry and consequently, also a very colorful solution chemistry.

These pictures were taken in Los Alamos Laboratory several decades ago. Available at http://gotexassoccer.com/elements/094Pu/Pu.htm

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Beautiful Colors of Plutonium

These pictures were taken in Los Alamos Laboratory several decades ago. Available at http://gotexassoccer.com/elements/094Pu/Pu.htm

Each oxidation state has its own characteristic color (upper figure), which is also influenced by interaction with other species in solution (figure below).

Uncomplexed Pu(IV), slightly brownish in HClO4 solution (non‐complexing medium) changes its color from red through green to almost yellow when it is dissolved in chloric acid (red), nitric acid (green) or neutral solution (colloid).

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Actinides in Solutions

 The solution chemistry of the actinide elements has been investigated in both aqueous and selected organic solutions.

 A variety of oxidation states (2+ to 7+) in aqueous solution possible, but the light actinides in aqueous acidic solutions are in III, IV, V and VI oxidation states.

 The stability of a particular oxidation state is quite variable, and for some actinides (Np, Pu) several oxidation states can coexist in the same solution.

 Pu – the most evident example: there are small differences in the redox potentials of Pu(III), Pu(IV), Pu(V), and Pu(VI) over a range of pH values:

Edelstein, Thermodynamic of Actinides, 2006

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0.2673 -0.553 0.447 0.088 U3+ → U4+ → UO2

+ → UO2 2+ (5)

0.882 0.219 0.604 1.159 2.04*

Np3+ → Np4+ → NpO2 + → NpO2

2+ → NpO3+ (6)

0.983

1.047 1.031 0.936 2.31* Pu3+ → Pu4+ → PuO2

+ → PuO2 2+ → PuO3+ (7)

1.683(3) 2.62(11) 0.84 1.60(9 2.5(2)

Am3+ → Am4+ → AmO2 + → AmO2

2+ → AmO3+ (8)

3.0 (9)Cm3+ → Cm4+

The scheme of standard redox potentials (in volts) for U, Pu, Np, Am and Cm in 1M HCl or 1M HClO4 (*) [Edelstein, 2006]

Redox Potentials of Actinides

Redox Chemistry in Aqueous Solutions: Pu

 The reduction potentials for the four common oxidation states of plutonium (III‐ VI) under acidic conditions are all near 1 V, and as a result, all four oxidation states can coexist in aqueous solutions.

 The equilibrium concentrations of Pu species existing simultaneously will be determined by these equations:

Pu4+ + PuO2 + ↔ Pu3+ +PuO2

2+ (1)

Pu4+ + 2H2O ↔ Pu 3+ +PuO2

+ + 4H+ (2)

 The equilibrium constant for Equation 2 is dependent on [H+]4; hence, the position of this disproportionation equilibrium changes significantly with acidity.

42 [Edelstein, Thermodynamic of Actinides, 2006]

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Redox Chemistry in Aqueous Solutions: Np

The couples of species in which only an electron is transferred between them, e.g., Pu3+/Pu4+

or NpO2 +/NpO2

2+:

 are electrochemically reversible

 redox reactions between them are rapid

Redox reactions that involve forming or rupturing of the An‐O bond, e.g., Np4+/NpO2 + and

Pu4+/PuO2 2+:

 are not electrochemically reversible

 have a slower reaction rate because of the barrier introduced by the subsequent reorganization of the solvent shell and also because some of these are two‐electron reductions

Redox Chemistry in Aqueous Solutions: Pu/Np

 Disproportionation of Np is appreciable only for the Np(V) oxidation state, and the reaction is favored by high concentration of acid:

2NpO2 + + 4H+ ↔ Np4+ + NpO2

2+ + 2H2O (3)

 Both the tetravalent and hexavalent cations, having higher effective cationic charge, are more strongly bound by complexing ligands, thus the disproportionation reaction will be accelerated toward complexation by an addition of complexing agents.

 Obviously, the reproportionation reaction will be promoted by lower acidity of solution:

Np4+ + NpO2 2+ + 2H2O ↔ 2NpO

2+ + 4H+ (4)

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Np, Pu Redox Chemistry in HNO3 Solutions

 In the solutions of nitric acid HNO3, always some portion of the nitrous acid HNO2 is generated (by heat, light, and in this case also by radiation). Like HNO3, it dissociates, releasing hydronium and nitrous ion NO2

‐.

 In the case of Pu, the presence of the nitrous ion NO2 ‐ stabilizes the tetravalent Pu4+ speciation,

and conversely,

 At high concentrations of HNO2, Np(VI) is rapidly reduced to Np(V).

 The oxidation of Np(V) to Np(VI) by nitrate ion is favored by high concentration of HNO3 and low HNO2 concentrations.

 Rate of oxidation of Np(V) is dependent on the [HNO2]/[Np(V)] ratio and the nitrate concentration (Moulin, 1978).

Role of Proton in Redox Processes

 The role of proton in redox processes is paramount.

 For example, increase from 1 M acid to 10 M base causes a change in redox potential of 2 V and makes possible the oxidation of Np(VI) to Np(VII).

 Np(VII) can be prepared only in strongly basic solution; and similarly Pu(VII), though higher concentrations of base are required.

 Generating an unusual oxidation states opens new separation opportunities.

 E.g., typically trivalent Am, if oxidized to hexavalent Am(VI), can be separated from lanthanides by extraction with PUREX solvent (PuO2

2+ or UO2 2+ by TBP).

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Ionic Radius and Oxidation State of Actinides

• With increase of atomic number, the nuclear charge increases and attracts more effectively the 5f electron shell ⇾ ionic radii of the same oxidation states systematically decrease .

• Decrease of ionic radius affects the hydrolysis and complex formation:

• Smaller radius ⇾ larger cationic charge density ⇾ more readily the ion hydrolyses and forms complexes.

Coordination of Actinide Ions in Solutions

 An coordination can be split into two groups: lower oxidation (di‐, tri‐ and tetravalent) and higher oxidation state (penta‐, hexa‐ and heptavalent) ions.

 The coordination number (CN) and geometry of their aqueous complexes is determined by the electronic configuration and steric size and shape of the ligands.

 While ionicity is the predominant characteristic of both lanthanide and actinide bonding, an appreciable covalency, stronger in the actinide bonds, has been confirmed by many spectroscopic studies.

 Covalency is attributed to the 6d orbital interactions with the ligands, which are significantly stronger than the 5f interactions.

→ See structures on the following slides 48

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Coordination and Hydration of Actinides  The iconicity for the same charge increases with decreasing ionic radius:

• Ionic radii range from 0.112 to 0.095 nm for AN(III), from 0.094 to 0.082 nm for AN(IV).

 The actinide ions are relatively large cations with high coordination numbers. Coordination numbers range from 6 to 14.

 The uncertainty in the structures can be explained by the limited number of crystal structures that exist for their aquo complexes, related to the difficulty in crystallizing materials from aqueous solutions.

 For a majority of the actinides, the exact numbers of water molecules that are bound to the metal centers in the hydrated metal ions are still controversial.

 Structures with 9–12 molecules of water have been proposed for tetravalent actinides in aqueous solutions.

 In general, the most accepted values for the number of H2O molecules bound to the metal center are 10 for Th and 9 for U to Pu.

 The An‐OH2 distances in these ions range from 0.25 to 0.24 nm. 49

OH 2

H 2O

H2O

OH 2

3+/4+

OH 2

H 2O

H2O

OH 2

3+/4+

OH 2

H 2O

H2O

OH 2

3+/4+

H2O

OH 2

H 2O

H2O

H 2O

H2O

OH 2

O O

O O O

O O

(1) (3)

(4) (5) (6)

(2)

Structures of Aquo-complexes An(III) and (IV): Typical examples of coordination geometries of octa‐aquo [An(H2O)8]

z+ and nona‐aquo ions [An(H2O) 9] z+ of

An3+ and An4+:

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Penta– and Hexavalent Actinides  Typical aqueous species of penta‐ and hexavalent actinides include the linear dioxo unit, AnO2

+/2+, with two oxygen atoms positioned at 180º (next slide).

 An average M=O distance in dioxocations:  An(VI): 0.175‐0.180 nm  An(V): 0.181‐0.193 nm

 All “secondary” ligands are coordinated in the perpendicular equatorial plane with typical M‐X bond distances of 0.24‐0.26 nm.

X – atom of coordinating molecule of water or a ligand.

Actinyl Coordination Complexes of ANV/VI+:

 The structures of a variety of aqueous‐based coordination complexes have been observed.

 Examples of tetragonal symmetry:

[AnO2Cl4] 2− [NpO4(OH)2]

3−

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Penta– and Hexavalent Actinides (cont.)

 The bonding for these ions has significant covalency with the axial An‐O ligands, while the bonding for the majority of the ligands residing in the equatorial plane is primarily ionic.

 As a result of this dual behavior (covalency and ionicity) of the trans dioxo ions, the linear dioxo unit is unperturbed (with the exception of bond distance changes) in all of the aqueous‐based complexes.

 The coordination numbers of the central actinide cation are defined by the equatorial size of ligands and their electronic properties.

Aqueous-based Coordination Complexes (cont.)

AnH2O

H2O

OH2

2+ O

AnF

(A) (B)

 The penta‐aqua ion (A) and pentafluoro‐complex (B) for the hexavalent actinides are seven‐coordinate structures, prevalent in actinide chemistry.

 They are the highest coordination numbers achievable with all monodentate ligands; however, coordination complexes with eight atoms bound to the actinide are achievable.

 The most well studied aquo‐ion is UO2(H2O)5 2+ (A), uranyl pentahydrate:

pentafluoro‐penta‐aqua‐ 54

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Actinides are Lewis Acids

 Actinides in aqueous solution form aqua ions, of the general formula M(H2O)nm+.

 The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as:

M(H2O)n m+ + H2O ↔ M(H2O)n‐1(OH)

(m‐1)+ + H3O +

 Thus, the aqua ion is behaving as an acid in terms of Brønsted‐Lowry acid‐base theory (Lewis acids).

 This is easily explained by considering the inductive effect of the positively charged metal ion, which weakens the O‐H bond of an attached water molecule, making the liberation of a proton relatively easy.

Hydrolysis and Polymerization of An

An4+ > AnO2 2+ > An3+ > AnO2

• OH‐bridged polynuclear complexes observed for actinide cations

• Tendency toward polymer formation is a function of the charge density of the actinide cation and drops in the order: An4+ > AnO2

2+ > An3+ > AnO2 +

• The slower rate of depolymerization compared with the rate of polymer formation is due to an equilibrium between hydroxo and oxo bridge formation with aging.

• The kinetics of polymerization–depolymerization becomes more complicated for Pu4+.

H O

O H

AnAn

AnVI(OH)4 + An VI(OH) 4 +…+ An

VI(OH) 4 ⇾

⇾ Hydroxide‐bridged polynuclear complexes

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59 Fraction of mononuclear plutonium(IV) hydrolysis products as a function of pH in 1 M NaClO4 solution (Choppin, 2003).

Hydrolysis of actinide cations

Fraction of Am(III) species in water in equilibrium with atmospheric CO2 as a function of pH

In carbonate‐free environments, Am(OH)2+ and Am(OH)2+ are the major species at pH 8.2, while, in carbonate‐rich waters, Am(CO3)

+ and Am(CO3)2 ‐ may also be significant components

(Choppin, Jensen, 2006).

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Actinide ions in their common solution oxidation states (3+ to 6+) are all hard Lewis acids, and their bonds with aqueous ligand are predominantly ionic. They have a great complex ability.

For a given cation, the strength of actinide complexes decreases in orders:

Monovalent inorganic ligands: F‐ > NO3 ‐ > Cl‐ > ClO4

‐

Divalent inorganic ligands: CO3 2‐ > C2O4

2‐ > SO4 2‐

For a given ligand, the strength of actinide complexes increases with the “effective” cationic electrostatic charge of the actinide ions:

AnO2 +< An3+< AnO2

2+ < An4+

Solution Chemistry of Actinides

Strength of Complexes

 The effective cationic charges of both the actinyl(V) and actinyl(VI) ions, larger than their overall, formal charge, suggest that the oxygen atoms of both O=An5/6=O cations retain a partial negative charge and

 the bonds between the actinide cations and the ligands in the equatorial plane are considerably stronger than would be indicated by their formal charge of +1/+2.

 The values of the effective charge, determined experimentally (Choppin and Rao, 1984) have been confirmed by theoretical calculations. They provide theoretical foundations for the observed behavior of actinides (Choppin, Jensen, 2006).

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Effective Cationic Charge of Actinides

Effective Cationic Charge 2.0 2.1 3.0 3.3 4.0

Metal Cation Valence (oxidation state)

2 5 3 6 4

Overall Formal Charge of Cation (“gross” charge)

2 1 3 2 4

Am3+

Important Actinide Species

 U (III, IV, V, VI): environmentally mobile, extractable, ubiquitous

 Np (III, IV, V, VI): environmentally mobile as NpO2 +

 Pu (III, IV, V, VI): environmentally mobile as 4+‐colloid polymer, extractable, co‐ exist in several oxidation states

 Am (III, IV, V, VI)

 Cm (III)

64 64Wymer, R. G., Complexation Reaction, In: CRESP Introduction to Nuclear Chemistry and Fuel Cycle Separations, Vanderbilt U, Nashville, 2008

Most significant aqueous complex species ⇛ review ⇛

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Uranium Aqueous Species

U3+, U4+, UO2 +, UO2

2+ :

 UO2 2+ forms complexes with many anions

(Cl‐, SO4 2‐, NO3

‐, PO4 3‐…)

 UO2 2+ forms diuranates (Na2U2O7) when fused with ammonium and

sodium hydroxides

 UO2 + is unstable and rapidly disproportionates into U3+ and UO2

 UO2 2+ salts in acidic solutions are stable up 300oC

65 65

Stable Aqueous Species of Uranyl: UO2+ is unstable and rapidly disproportionates into U3+ and UO22+

UO2 2+ forms complexes:

• with many anions Cl‐, SO4 2‐, NO3

‐, PO4 3‐…

 very strong anion carbonate [UO2(CO3)3] 4‐

 anion acetate complexes NaZn[UO2(C2H3O2)3] 4‐

 nitrate complex that extracts with tributylphosphate as UO2(NO3)2.2TBP

 diuranates (Na2U2O7) when fused with ammonium and sodium hydroxides

 UO2 2+ precipitates as the peroxide UO4.2H2O

 UO2 2+ salts in acidic solutions are stable up 300oC

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Plutonium Aqueous Species Pu3+, Pu4+, PuO2

+, PuO2 2+:

• Pu3+ more stable than Np3+:

– fluoride and peroxide are insoluble

– precipitated by carbonate and oxalate

• Pu4+ is predominate species:

–relatively easily oxidizes (to PuO2 2+) or reduces (to Pu3+)

–forms complexes with nitrate, peroxide, fluoride and chloride; nitrate and chloride form anionic complexes,

–with oxalate and peroxide precipitate Pu4+: 2 H2C2O4 + Pu(NO3)4 ↔ Pu(C2O4)2 + 4 HNO3

–nitrate complex that extracts with tributylphosphate as Pu(NO3)4.2TBP or can be retained by extraction resin (TRU)

67 67

Plutonium Stable Aqueous Species

Pu3+, Pu4+, PuO2 +, PuO2

2+:

PuO2 + more stable than Np3+

 fluoride and peroxide are insoluble, precipitated by carbonate and oxalate

PuO2 2+:

 forms complexes with carbonate, fluoride chloride, sulfates, etc.;

 similarly to UO2 2+, its nitrate complex extracts with tributylphosphate as

PuO2(NO3)2.2TBP

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Neptunium Stable Aqueous Species

Np3+, Np4+, NpO2 +, NpO2

2+:

 Np3+ behaves analogically to REE: precipitates with OH‐, PO4 3‐, and F‐

 Np4+ behaves like Pu4+: hydrolyzes and forms stable SO4 2‐, F‐, C2O4

2‐ complexes

 NpO2 + is not easily complexed, precipitated or extracted

 NpO2 2+ behaves like UO2

2+ and PuO2 2+, forms similar complexes and is

extracted by organic solvents

Americium Aqueous Species

Am3+, Am4+, Am5+, Am6+

 Am3+ behaves analogically to REE: hydrolyzes, precipitates with OH‐, PO4 3‐, and F‐

 Am3+ forms stable complexes with Cl‐, NO3 ‐ CNS‐, and SiF6

2‐

 Am3+ forms a soluble carbonate complex, while Cm3+ doesn’t, is not easily complexed,

precipitated or extracted

 AmO2 + (Am=5+) forms insoluble KAmO2CO3 (Am

3+ or Ln3+ don’t)

 AmO2 2+ (Am=6+) is a strong oxidizer and very unstable in solutions

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Curium Aqueous Species  Cm(III) is the only oxidation state found normally in solution

 Cm(III) fluoride, oxalate, phosphate, iodate, and hydroxide are insoluble

 Cm(III) is very stable toward oxidation

 Cm(III) chemistry studies are hampered by radiolytic and heating effects

 CmF3 can be precipitated from solution

 Cm(III) forms complexes with α‐hydroxyisobutyrate and CNS that can be separated from Am,

other TRU‐elements, and rare earths using ion exchange

71 71

72 72 UV-vis-NIR Spectra of Np (Runde, 2000)

Speciation of Actinides

Aqueous/ organic phase

 Optical spectroscopy

 Vibrational spectroscopy

 Nuclear Magnetic Resonance

 X‐Ray Absorption Fine Structure (XAFS) Spectroscopy

 Laser‐Induced Fluorescence (LIF) Spectroscopy

 Etc.

Speciation Modeling

 PHREEQC, Hyperquad (Peter Gans et al.)

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Conclusion  It has been more than 65 years since the first reactors and huge chemical separation plants at Hanford were constructed.

 Since the first industrial‐scale separations of Pu began in December 1944, fundamental studies of the chemistry, physics, and nuclear properties of Pu and other actinides have advanced understanding of their properties.

 Actinide studies also have had a significant influence on the development of all other aspects of chemistry, physics, and engineering.

 From the very beginning (M. Curie!), separations methods (precipitation/coprecipitation, ion‐exchange, solvent extraction) have played a crucial role in the development of nuclear chemistry and discovery of transuranic elements.

 Rapid and reliable separation methods are key to solving problems related to use of radionuclides:s pent nuclear fuel reprocessing, environmental monitoring, decontamination and radioecological problems.

References Ahrland, S., Solution Chemistry and Kinetics of ionic reactions, in Chemistry of the Actinide Elements, 2nd Edn, vol. 2 (eds. J. J. Katz, G. T. Seaborg, and L. R. Morss), Chapman&Hall, New York, (1986) p. 1480.

Albrecht‐Schmitt, T. E., Ed. Organometallic and Coordination Chemistry of the Actinides, Struct Bond 127 (2008) 1–85

Antonio, M. R., Soderholm, L., Williams, C. W., Blaudeau, J.‐P., and Bursten, B. E. Radiochim. Acta, 89, (2001) 17

Choppin, G. R., Jensen, M.P., Actinides in Solution: Complexation and Kinetics, In: The Chemistry of the Actinides and Transactinides, Ed. L.R.Morss, N. M. Edelstein and J. Fuger, Springer, 2006

Clark, D.L., Hecker, S. S,. Jarvinen, G. D., Neu, M.P., Plutonium, in: The Chemistry of the Actinides and Transactinides, Ed. L.R. Morss, N. M. Edelstein and J. Fuger, Springer, 2006

Edelstein, Thermodynamic of Actinides, In: The Chemistry of the Actinides and Transactinides, Ed. L.R. Morss, N. M. Edelstein and J. Fuger, Springer, 2006

Hoffman, D., Advances in Plutonium Chemistry, 1967‐2000 (a multiauthored review) book, ANS, 2002

Keogh, D.W., Actinides: Inorganic & Coordination Chemistry. In Encyclopedia of Inorganic Chemistry; King, R. B., Crabtree, R. H., Eds.; John Wiley & Sons: Chichester, England, (2005) Vol. 1, p 2

Lehto, J. , Hou, X., Chemistry and Analysis of Radionuclides, Wiley‐CH, 2011

Moulin, J. P., Oxidation ‐ Reduction Kinetics of Neptunium in Nitric Acid Solution, Comm. a l'Energie At. Rep. CEA‐R‐4912, Fontenay‐aux‐Roses, France, 1978; INIS Atomindex, 451540

Nash, K. L., Sullivan, J.C., Kinetics and Mechanisms of Actinide Redox and Complexation Reactions. In: Advances in Inorganic and Bioinorganic Reaction Mechanisms, (Ed. A. G. Sykes) Vol.5 p. 185 Academic Press, New York (1986)

Nash, K. L., Sullivan, J.C., Kinetics of Complexation and Redox Reactions of Lanthanides in Aqueous Solutions, In: Handbook on the Physics and Chemistry of Rare Earths; Eds: Gschneidner, K. A. Jr.; Eying, L.; Choppin, G. R.; Lander, G. H., Vol. 15, 287; Elsevier Science, Amsterdam (1991)

Paulenova, A. Physical and Chemical Properties of Actinides in Nuclear Fuel Reprocessing, In: Advanced Separation Techniques for Nuclear Fuel Reprocessing and Radioactive Waste Treatment, Ed. K. Nash and G. Lumetta, Woodhead Publishing, 2010

Siddall, T.H., Dukes, E.K., J. Am. Chem. Soc., 81 (1959) 790

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