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

Actinium Isotopes

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

Ac electron binding energies Atomic radius 195 pm Ionic radius 126 pm 6-coordinate

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 Ac2++2e-+HgAc(Hg) Ac3++3e-+HgAc(Hg) Other experiments failed to find divalent state

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

Properties of Ac metal Density 10.07 g/mL Melting point 1050 °C Boiling Point 3300 °C Crystal structure fcc

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 sg=762 barns 225Ac from 229Th generator Start with 233U from neutron activation of 232Th

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 Keto Enol Hydrate

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

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

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 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/2004/00000261/00000001/05379859;jsessionid=4dkgcbb4sv85c.alice

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

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

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

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

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

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.pdf#search=%22actinium%20HEHA%22 Geotracer Compare 231Pa with 227Ac Ac higher than Pa in deep seawater Ac as a tracer for deep seawater circulation