Lecture 1: RDCH 710 Introduction

CHEM 312: Lecture 15 Americium and Curium Chemistry Part 1Readings: Am and Cm chemistry chaptersLink on web pageCombined due to similar chemical properties of elementsCover Am then CmNuclear propertiesProduction of isotopesSeparation and purification Metallic stateCompounds Solution chemistryCoordination chemistry

15-#

Production of Am isotopesAm produced in reactors from neutron irradiation of Pu239Pu to 240Pu to 241Pu, then beta decay of 241Pu241,243Am main isotopes of interestLong half-livesProduced in kilogram quantityChemical studiesBoth isotopes produced in reactor241Am source for low energy gamma and alphaAlpha energy 5.44 MeV and 5.49 MeVSmoke detectors Neutron sources(a,n) on BeThickness gauging and density242Cm production from thermal neutron capture243AmIrradiation of 242Pu, beta decay of 243PuCritical mass242Am in solution23 g at 5 g/LRequires isotopic separation

15-#

Am solution chemistryOxidation states III-VI in solutionAm(III,V) stable in dilute acidAm(V, VI) form dioxo cationsAm(II)Unstable, unlike some lanthanides (Yb, Eu, Sm)Formed from pulse radiolysisAbsorbance at 313 nmT1/2 of oxidation state 5E-6 secondsAm(III)Easy to prepare (metal dissolved in acid, AmO2 dissolution)Pink in mineral acids, yellow in HClO4 when Am is 0.1 M7F05L6 at 503.2 nm (e=410 L mol cm-1)Shifts in band position and molar absorbance indicates changes in water or ligand coordination9 to 11 inner sphere watersBased on fluorescence spectroscopyLifetime related to coordinationnH2O=(x/t)-yx=2.56E-7 s, y=1.43Measurement of fluorescence lifetime in H2O and D2OAm(IV)Requires complexation to stabilizedissolving Am(OH)4 in NH4FPhosphoric or pyrophosphate (P2O74-) solution with anodic oxidationAg3PO4 and (NH4)4S2O8Carbonate solution with electrolytic oxidation

15-#Am solution chemistryAm(V)Oxidation of Am(III) in near neutral solutionOzone, hypochlorate (ClO-), peroxydisulfateReduction of Am(VI) with bromide5I43G5; 513.7 nm; 45 L mol cm-15I43I7; 716.7 nm; 60 L mol cm-1Am(VI)Oxidation of Am(III) with S2O82- or Ag2+ in dilute non-reducing acid (i.e., sulfuric)Ce(IV) oxidizes IV to VI, but not III to VI completely 2 M carbonate and ozone or oxidation at 1.3 V996 nm; 100 L mol cm-1Smaller absorbance at 666 nmAm(VII)3-4 M NaOH, mM Am(VI) near 0 CGamma irradiation 3 M NaOH with N2O or S2O82- saturated solutionAm(VII)Broad absorbance at 740 nm

15-#Am solution chemistryAm(III) luminescence7F05L6 at 503 nmThen conversion to other excited stateEmission to 7FJ 5D17F1 at 685 nm5D17F2 at 836 nmLifetime for aquo ion is 20 ns155 ns in D2OEmission and lifetime changes with speciationAm triscarbonate lifetime = 34.5 ns, emission at 693 nmAutoreduction Formation of H2O2 and HO2 radicals from radiation reduces Am to trivalent statesDifference between 241Am and 243AmRate decreases with increase acid for perchloric and sulfuricSome disagreement role of Am concentrationConcentration of Am total or oxidation stateRates of reduction dependent uponAcid, acid concentration, mechanism Am(VI) to Am(III) can go stepwise starting ion Am(V) slower than Am(VI)

15-#Am solution chemistryDisproportionationAm(IV) In nitric and perchloric acidSecond order with Am(IV)2 Am(IV)Am(III) + Am(V)Am(IV) + Am(V)Am(III) + Am(VI)Am(VI) increases with sulfateAm(V)3-8 M HClO4 and HCl3 Am(V) + 4 H+Am(III)+2Am(VI)+2 H2OSolution can impact oxidation state stabilityRedox kineticsAm(III) oxidation by peroxydisulfateOxidation due to thermal decomposition productsSO4.-, HS2O8-Oxidation to Am(VI)Acid above 0.3 M limits oxidationDecomposition of S2O82-Induction period followed by reductionRates dependent upon temperature, [HNO3], [S2O82-], and [Ag+2]In carbonate proceeds through Am(V)Rate to Am(V) is proportional to oxidantAm(V) to Am(VI)Proportional to total Am and oxidantInversely proportional to K2CO3

15-#Am solution chemistry: Redox kineticsAm(VI) reductionH2O2 in perchlorate is 1st order for peroxide and Am2 AmO22++H2O22 AmO2+ + 2 H++ O2NpO2+1st order with Am(VI) and Np(V)k=2.45E4 L / mol sOxalic acid reduces to equal molar Am(III) and Am(V)Am(V) reductionReduced to Am(III) in NaOH solutionsSlow reduction with dithionite (Na2S2O4), sulfite (SO32-), or thiourea dioxide ((NH2)2CSO2) Np(IV) and Np(V)In both acidic and carbonate conditionsFor Np(IV) reaction products either Np(V) or Np(VI)Depends upon initial relative concentration of Am and NpU(IV) examined in carbonate

15-#Am solution chemistryRadiolysisFrom alpha decay1 mg 241Am release 7E14 eV/sReduction of higher valent Am related to dose and electrolyte concentration In nitric acid formation of HNO2In perchlorate numerous species producedCl2, ClO2, or Cl-Complexation chemistryPrimarily for Am(III)F->H2PO4->SCN->NO3->Cl->ClO4-Hard acid reactionsElectrostatic interactionsInner sphere and outer sphereOuter sphere for weaker ligandsStabilities similar to trivalent lanthanidesSome enhanced stability due to participation of 5f electron in bonding

15-#Am solution chemistryHydrolysisMono-, di-, and trihydroxide speciesAm(V) appears to have 2 species, mono- and dihydroxideAm hydrolysis (from CHESS database)Am3++H2OAmOH2++H+: log K =-6.402Am3++2H2OAm(OH)2++2H+: log K =-14.11Am3++3H2OAm(OH)3+3H+: log K =-25.72CarbonateEvaluated by spectroscopyIncludes mixed speciesAm hydroxide carbonate speciesBased on solid phase analysisAm(IV)Pentacarbonate studied (log b=39.3)Am(V) solubility examined

1mM Am3+; 1 mM Am, 1 mM carbonate

15-#

Am solution chemistry: OrganicsNumber of complexes examinedMainly for Am(III)Generally stability of complex increases with coordination sitesWith aminopolycarboxylic acids, complexation constant increases with ligand coordinationNatural organic acidNumber of measurements conductedMeasured by spectroscopy and ion exchangeTPEN (N,N,N,N-tetrakis(2-pyridylmethyl)ethyleneamine)0.1 M NaClO4, complexation constant for Am 2 orders greater than Sm

15-#Am solvent extractionTributylphosphate (TBP)Am extracted from neutral or low acid solutions with high nitrate Am(VI)Oxidation with (NH4)10P2W17O61 to stabilize Am(VI)100 % TBP from 1 M HNO3Separation factor 50 from NdAm separation from lanthanides1 M ammonium thiocyanate aqueous phaseDibutyl butylphosphonate (DBBP)Phosphonate functional groupSimilar to TBP, stronger extractant of AmTrialkylphophine oxide (TRPO)Increase in basicity of P=O functional group from TBP to DPPB to TRPOAm and Cm extraction from 1-2 M HNO330 % TRPO in kerosene Am, Cm, tetravalent Np and Pu, hexavalent U extractedActinides stripped with 5.5 M HNO3 (Am fraction)TRPO with C6-C8 alkyl group

15-#

HDEHPAm solvent extractionBis(2-ethylhexyl)phosphoric acid (HDEHP)Has been used to Am separationPart of TALSPEAKExtracts lanthanides stronger that actinidesTALSPEAK componentsBis(2-ethyl-hexyl)phosphoric acid (HDEHP)HNO3DTPALactic acidCarbamoylphosphine oxide (CMPO)Synthesized by HorwitzBased on DHDECMP extractionsRecognized functional group, simplified ligand synthesisPurified by cation exchangePart of TRUEXTRUEX (fission products)0.01 to 7 M HNO31.4 M TBP 0.2 M Diphenyl-N,N-dibutylcarbamoyl phosphine oxide (CMPO)0.5 M Oxalic acid1.5 M Lactic acid 0.05 M DTPA

CMPO

15-#Am solvent extractionTertiary amine saltLow acid, high nitrate or chloride solution(R3NH)2Am(NO3)5Quaternary ammonium salts (Aliquat 336)Low acid, high salt solutionsExtraction sequence of Cm