Lecture 1: RDCH 710 Introduction

CHEM 312: Part 1 Lecture 14 Plutonium ChemistryReadingsPu chemistryhttp://radchem.nevada.edu/classes/rdch710/files/plutonium.pdfChallenges of Pu chemistryhttp://radchem.nevada.edu/classes/rdch710/lanl Pu book/LASCIENCE.PDFNuclear properties and isotope productionPu in naturePu solution chemistrySeparation and PurificationAtomic propertiesMetallic stateCompounds

Isotopes from 228A247Important isotopes238Pu237Np(n,g)238Np238Pu from beta decay of 238NpSeparated from unreacted Np by ion exchangeDecay of 242Cm0.57 W/g Power source for space exploration83.5 % 238Pu, chemical form as dioxideEnriched 16O to limit neutron emission6000 n s-1g-10.418 W/g PuO2150 g PuO2 in Ir-0.3 % W container

14-#Pu nuclear properties239Pu2.2E-3 W/gBasis of formation of higher Pu isotopes244-246Pu first from nuclear testHigher isotopes availableLonger half lives suitable for experiments

Most environmental Pu due to anthropogenic sources239,244Pu can be found in nature239Pu from nuclear processes occurring in U oren,g reactionNeutrons fromSF of Uneutron multiplication in 235Ua,n on light elements24.2 fission/g U/hr, need to include neutrons from 235U244PuBased on Xe isotopic ratiosSF of 244Pu1E-18 g 244Pu/g bastnasite mineral

14-#Pu solution chemistryOriginally driven by need to separate and purify PuSpecies data in thermodynamic databaseComplicated solution chemistryFive oxidation states (III to VII)Small energy separations between oxidation statesAll states can be preparedPu(III) and (IV) more stable in acidic solutionsPu(V) in near neutral solutionsDilute Pu solutions favoredPu(VI) and (VII) favored in basic solutionsPu(VII) stable only in highly basic solutions and strong oxidizing conditionsSome evidence of Pu(VIII)

14-#Pu solution spectroscopyA few sharp bands5f-5f transitionsMore intense than 4f of lanthanidesRelativistic effects accentuate spin-orbit couplingTransitions observed spectroscopicallyForbidden transitionsSharp but not very intensePu absorption bands in visible and near IR regionCharacteristic for each oxidation state

14-#Pu solution chemistryRedox chemistryPotentials close to 1 V for 4 common statesKinetics permit coexistence of oxidation statesPu(IV) and Pu(V) tend toward disproportionation 3Pu4++2H2O2Pu3++PuO22++4H+K=0.0089 at 1.0 M I3PuO2++4H+Pu3++2PuO22++2H2OPu concentrationIonic strengthpHKinetics for disproportionation based on time and Pu concentrationMoles seconds (M s)Some redox couples are quasi- or irreversibleBreaking or forming oxo bondsi.e., Pu(V)/Pu(III), Pu(VI)/Pu(III)Equilibrium between redox statesK=Pu(III)Pu(VI)/Pu(IV)Pu(V)K=13.1, corrected for hydrolysis

14-#

Oxidation state distribution diagram showing predominant oxidation state of plutonium in 1 M (H,Na)ClO4 solution as a function of pH and (a) average oxidation statePu(IV), and (b) average oxidation state Pu(V)Kinetics for disproportionation of plutonium in 1 M (H,Na)ClO4 solution at(a) pH 1 and average oxidation state Pu(IV), and (b) pH 3 and average oxidation state Pu(V)Preparation of pure oxidation statesPu(III)Generally below pH 4Dissolve a-Pu metal in 6 M HClReduction of higher oxidation state with Hg or Pt cathode0.75 V vs NHEHydroxylamine or hydrazine as reductantPu(IV)Electrochemical oxidation of Pu(III) at 1.2 VThermodynamically favors Pu(VI), but slow kinetics due to oxo formationPu(V)Electrochemical reduction of Pu(VI) at pH 3 at 0.54 V (vs SCE)Near neutral in 1 micromole/L Pu(V)Pu(VI)Treatment of lower oxidation states with hot HClO4Ozone treatmentPu(VII)Oxidation in alkaline solutionsHexavalent Pu with ozone, anodic oxidation

14-#Pu reductionPu redox by actinidesSimilar to disproportionation Rates can be assessed against redox potentialsPu4+ reduction by different actinides shows different ratesAccompanied by oxidation of An4+ with yl bond formationReduction of Pu(VI) by tetravalent actinides proceeds over pentavalent stateReactions show hydrogen ion dependency Rates are generally dependent upon proton and ligand concentrationHumic acid, oxalic acid, ascorbic acidPoor inorganic complexants can oxidize PuBromate, iodate, dichromateReactions with single electron reductants tend to be rapidReduction by Fe2+Complexation with ligands in solution impacts redoxDifferent rates in carbonate media compared to perchlorateMono or dinitrate formation can effect redoxPu(IV) formation or reaction with pentavalent metal ions proceeds faster in nitrate than perchlorateOxidation of Pu(IV) by Ce(IV) or Np(VI) slower in nitratePu(VI) reduction can be complicated by disproportionationHydroxylamine (NH2OH), nitrous acid, and hydrazine (N2H4)Used in PUREX for Pu redox controlPu(III) oxidized2Pu3++3H++NO3-2Pu4++HNO2+H2ORe-oxidation adds nitrous acid to system which can initiate an autocatalytic reaction

14-#Pu aqueous chemistryAutoradiolysisFormation of radicals and redox agents due to radioactive decayLow reaction if concentrations below 1 MWith nitrate can form other reactive species (HNO2)Formation of Pu(IV).H2O2Rate proportional to Pu concentration and dose ratePu(VI) reduction proceeds over Pu(V)Formation of HNO2 and disproportionation

14-#Pu hydrolysisSize and chargeSmaller ions of same charge higher hydrolysisFor tetravalents Pu>Np>U>Pa>Th

10 mMPu

14-#

Pu(III) 10 mM

Pu(IV) 10 mmol/L

Pu(V) 10 mmol/L

Pu(VI) 10 mmol/L

14-#Pu Hydrolysis/colloid formationIn many systems solubility derived Pu(IV) concentrations vary due to colloid formationColloids are 1- to 1000-nm size particles that remain suspended in solutionx-ray diffraction patterns show Pu(IV) colloids are similar to fcc structure of PuO2 Basis for theory that colloids are tiny crystallites PuO2,May include some water saturated of hydrated surfacePrepared by addition of base or water to acidic solutions

14-#Pu aqueous chemistry: colloidsCharacterization SANSLong, thin rods 4.7 nm x 190 nmLight scatteringSpherical particles1 nm to 370 nmLaser induced breakdown12 nm to 25 nmXAFS studies of Pu(IV) colloidsdemonstrated that average fcc structure is overly simplistic additional chemical forms are present that affect solubilityVariations in measured Pu(IV) concentrations may be related to local structurecolloids displays many discrete PuO distances2.25 Pu-OH to 3.5 amplitude of PuPu is reduced, decrease in number of nearest neighbors four H atoms incorporated into Pu(IV) colloid structure could result in one Pu vacancy. EXAFS reveals that many atoms in colloid structure are distributed in a non-Gaussian way whenseveral different oxygen containing groups are presentO2,, OH-, and OH2

14-#Pu aqueous chemistryComplexing ionsGeneral oxidation state trends for complexation constantsPu(IV)>Pu(VI)Pu(III)>Pu(V)OxoanionsPu complexes based on charge and basicity of ligandClO4-