Lecture 3: Actinium Chemistry

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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

• 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
• 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

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Ac electron binding energies

• 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
• Two waves in HClO4, pH 1.9-3.1
• Ac22e-Hg??Ac(Hg)
• Ac33e-Hg??Ac(Hg)
• Other experiments failed to find divalent state

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Metals

• Preparation of metals difficult
• Tendency to form oxides and hydroxides in water
• Formation in electrolytic reduction
• 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
• Ac2O3 3H2S Ac2S3 3H2O

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Properties of Ac metal

• 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
• 227Ac sg762 barns
• 225Ac from 229Th generator
• Start with 233U from neutron activation of 232Th

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Ac purification

• Liquid-liquid extraction
• Extration with TTA
• pH control of extraction
• Ac extracts well above pH 6
• Hydrolysis in this range
• Synergist extraction with 0.1 TTA in 0.1 M TBP
• Above pH 4

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Ion exchange

• Cation exchange
• Separation of 227Ac from 227Th and 223Ra
• Strong cation exchange
• 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
• Ra(NO3)2 precipitated
• Recycle for Ac production
• Th and Ac remain in solution
• 5 M HNO3, anion exchange
• Th strongly absorbed, Ac only slightly
• Oxalate precipitation of Ac
• 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
• pK1h9.4
• Described by electrostatic model of hydrolysis
• Linearity of log K1h versus 1/ionic radius
• For trivalent metal ions, actually related to
  charge density
• Hydrolyzes less than trivalent lanthanides or
  actinides
• More basic than lanthanides

• http//www.ingentaconnect.com/content/klu/jrnc/200
  4/00000261/00000001/05379859jsessionid4dkgcbb4sv
  85c.alice

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Ac salts

• Fluorides
• AcF3
• Density 7.880 g/mL
• Formation of AcF3
• 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
• Ac oxalate solubility 24 mg/L

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Ac salts

• Chloride
• Melting point 1051C (sublimes)
• Density 4.810 g/mL
• Formation reaction
• 4Ac(OH)3 3CCl4? 4AcCl3 3CO2 6H2O
• Bromide
• Melting point 1051C
• Boiling point 3198C
• Density 5.850 g/mL
• Formation reaction
• Ac2O3 2AlBr3 2AcBr3 Al2O3
• For Iodide
• Ac2O3 2Al 3I2 2AcI3 Al2O3

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Solubility

• Generally co-precipitation with insoluble salts
  from with an cation
• Fluorides, hydroxides of metal ion
• Ac precipitated by Pb sulfate
• Solubility of oxalates evaluated
• Effects of radiolysis
• La solubility half of Ac solubility
• Ksp around 5E-27
• Large decrease in pH due to radiolysis from 227Th
• Purified Ac did not show large pH decrease

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Complexation

• Resembles lanthanum in complexation
• Generally lower for Ac
• Determined in tracer experiments
• Solvent extraction
• Compared to ionic radius to evaluated some
  constants
• HDEPH

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Radiocolloid

• Separation of 227Ac from 227Th and 223Ra
• Sorption of Ac onto filter increases with pH and
  time
• Above pH 5 filter separation of Ac by centrifuge
• Analytical chemistry of Ac
• Neutron activation for 227Ac
• 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
• 225Ac suitable isotope
• Decay series produces alpha and beta
• No hard gammas
• Ac bound by marcocyclic compounds
• HEHA incorporates Ac
• 1,4,7,10,13,16-hexaazacyclohexadecane-N,N',N',N',N
  ',N'-hexaacetic acid
• http//lsd.ornl.gov/highlights/LSDHighlights0302a.
  pdfsearch22actinium20HEHA22
• Geotracer
• Compare 231Pa with 227Ac
• Ac higher than Pa in deep seawater
• Ac as a tracer for deep seawater circulation