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Lecture 3: Actinium Chemistry

• Lecture notes from Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium

• Chemistry of actinides§ Metals§ Soluble and insoluble salts§ Complex ions and chelate compounds§ Organic compounds§ Separations

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Actinium Isotopes

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Actinium Isotopes and fundamental data• Ac is trivalent

§ Very similar chemistry to lanthanides and Y§ Trivalent chemistry§ electropositive

• 227Ac§ From 235U decay§ Beta emitter

• 225Ac has been proposed for nuclear medicine§ Ac from 229Th parent

à Alpha emitting therapeutic agent§ Use of 225Ac to produce 213Bi

• Chemistry of Ac basis of use as radiopharmaceutical § Use of Ac in delivery to tumor based on fundamental chemical interactions§ Separation of Bi daughter from Ac parent

• Atomic radius§ 195 pm

• Ionic radius§ 126 pm§ 6-coordinate

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Redox

• Trivalent is predominant state• Divalent state postulated based on similarities

to Ac behavior with divalent Eu and Sm§ Reinfored with polarogram data

à Two waves in HClO4, pH 1.9-3.1* Ac2++2e-+HgAc(Hg)* Ac3++3e-+HgAc(Hg)

§ Other experiments failed to find divalent state

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Ac Metal• Preparation of metals difficult

§ Tendency to form oxides and hydroxides in water

§ Formation in electrolytic reductionà Molten salt systems

§ Reduction of fluoride salts by metallic Ca

§ Need to melt both CaF2 and resulting metal

• Metal oxidizes in air

§ 4Ac + 3O2 → 2Ac2O3

§ Useful starting material for synthesis

à Ac2O3 + 3H2S Ac2S3 + 3H2O

• Density§ 10.07 g/mL

• Melting point§ 1050 °C

• Boiling Point§ 3300 °C

• Crystal structure§ fcc

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Ac preparation and purification

• Separation from U ores§ Ores also contain a fair amount of lanthanides,

require separation• Nuclear reactions and generators

§ Irradiation of 226Ra§ 226Ra(n,g)227Ra, beta decay to 227Ac

à 227Ac sg=762 barns§ 225Ac from 229Th generator

à Start with 233U from neutron activation of 232Th

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Ac purification: solvent extraction• Extration with TTA• pH control of extraction

§ Ac extracts well above pH 6à Hydrolysis in this

range§ Synergist extraction

with 0.1 TTA in 0.1 M TBPà Above pH 4

Keto Enol Hydrate

aa

S

O O

CF3S

O OH

CF3S

OOH

CF3

HO

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Ion exchange • Cation exchange

§ Separation of 227Ac from 227Th and 223Ra

§ Strong cation exchangeà DOWEX 50

• Organic stationary phases§ Trioctylamine§ Bis(2-ethyl-hexyl)phosphoric

acid (HDEHP)§ TBP§ TTA

• Inorganic

§ MnO2

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Preparation of gram quantities of Ac

• Irradiation of multigram quantities of 226Ra§ Forms both 227Ac and 228Th§ 222Rn daughter from 226Ra

• Irradiation of RaCO3

§ Dissolved in dilute HNO3

à Ra(NO3)2 precipitated* Recycle for further Ac production

à Th and Ac remain in solution§ 5 M HNO3, anion exchange

à Th strongly absorbed, Ac only slightly§ Oxalate precipitation of Ac

à Calcination after precipitationà Used to form Ac2O3

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Ac salts

• Salts are soluble in most acids§ Some salt are insoluble and used in separations based on

precipitation• Most data from one study

§ Each compound prepared from less than 10 µg§ Purified by TTA

• Hydroxides§ pK1h=9.4§ Described by electrostatic model of hydrolysis § Linearity of log K1h versus 1/ionic radius

à For trivalent metal ions, actually related to charge density

à Hydrolyzes less than trivalent lanthanides or actinides* More basic than lanthanides

Ac hydrolysis paper• http://www.ingentaconnect.com/content/klu/jrnc/2004/00000261/00000001/05379859;jsessionid=4dkgcbb4sv85c.alice

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Ac salts• Fluorides

§ AcF3

§ Density 7.880 g/mL§ Formation of AcF3

à Ac(OH)3 + 3HF AcF3 + 3H2O • Oxalate

§ Oxalates used in precipitation of metal ions§ Oxalate salt of actinium used to form oxide§ Ac precipitated as an oxalate by the addition of an

oxalic acid § Oxalates are destroyed by boiling concentrated

HNO3 or HClO4 § In 0.1 M HNO3-0.5 M oxalic acid

à Ac oxalate solubility 24 mg/L

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Ac salts• Chloride

§ Melting point: 1051°C (sublimes) § Density: 4.810 g/mL§ Formation reaction

à 4Ac(OH)3 + 3CCl4 4AcCl3 + 3CO2 + 6H2O • Bromide

§ Melting point: 1051°C § Boiling point: 3198°C § Density: 5.850 g/mL§ Formation reaction

à Ac2O3 + 2AlBr3 2AcBr3 + Al2O3 § For Iodide

à Ac2O3 + 2Al + 3I2 2AcI3 + Al2O3

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Solubility and complexation• Generally co-precipitation with

insoluble salts from cation§ Fluorides, hydroxides of metal

ion§ Ac precipitated by Pb sulfate

• Solubility of oxalates evaluated§ Effects of radiolysis

à La solubility half of Ac solubility

à Ksp around 5E-27* Large decrease in pH

due to radiolysis from 227Th

* Purified Ac did not show large pH decrease

• Resembles lanthanum in complexation§ Generally lower for Ac

• Determined in tracer experiments§ Solvent extraction

• Compared to ionic radius to some evaluated constants§ HDEHP

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Radiocolloid

• Separation of 227Ac from 227Th and 223Ra§ Formation of colliods

à Ac goes into solution, Th remains in solid phase§ Sorption of Ac onto filter increases with pH and time

à Above pH 5 filter separation of Ac by centrifuge

• Analytical chemistry of Ac§ Radiation detection§ Neutron activation for 227Ac

à 1E-17 g level§ Activity used to determine to 1E-20 g

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Actinium uses

• Heat sources§ 227Ac multiCi amount§ 5 alpha particles

• Neutron sources§ a,n source using 227Ac

• Nuclear medicine§ Medical use based on ligand

à Bone treatments (polyphosphonate )§ 225Ac suitable isotope

à Decay series produces alpha and beta* No hard gammas

§ Ac bound by marcocyclic compounds§ HEHA incorporates Ac

à 1,4,7,10,13,16-hexaazacyclohexadecane-N,N′,N′,N′,N′,N′-hexaacetic acid

• Geotracer§ Compare 231Pa with 227Ac

à Ac higher than Pa in deep seawaterà Ac as a tracer for deep seawater circulation

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Review• Actinide isotopes

§ Lifetimes, production• Actinium data

§ X-ray, radii, density• Redox

§ Oxidation states• Preparation and purification

§ Solvent extraction and ion exchange• Compounds and properties• Uses of actinium

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Questions

• What is the longest lived Ac isotope?• What are two different ways to obtain actinium

§ Provide the isotopes• What Ac oxidation state can be found in

solution?• How is Ac separated from U• Which Ac species are insoluble?• What are some uses of Ac?

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Pop Quiz

• Describe a method for separating Ac from the lanthanides.