6-1

Nuclear Forensics Summer SchoolRadiochemical separations and quantification• Aqueous chemical behavior of key radionuclides

§ Oxidation state variation§ Solution phase speciation

• General separations§ Ion exchange/column chromatography § Solvent extraction§ Precipitation/carrier

• Quantification§ Radiochemical methods§ Spectroscopic§ BOMARC example (at a later date)

• Provide basis for linking chemical behavior with separations• Provide range of techniques suitable for quantification of

radionuclides

6-2

Radionuclides of interest• Can differentiate fissile material and neutron energetics from fission

products§ A near 90 (Sr, Zr), 100 (Tc) and 105 (Pd)§ Mass 110-125 (Pd, Ag, Cd, In, Sn, Sb)§ Lanthanides (140 < A < 150)§ Actinides

• Polonium235U fission yield

6-3

Fundamentals of separations

• Oxidation state§ Elements of different oxidation states easier to

separateà Anionic and cationic speciation

* UO22+,TcO4

-

§ Variation of oxidation stateà Addition of reductants/oxidants to control

speciation* Method for separation of Pu from U

à Varied stability of oxidation states

6-4

Fundamentals of separation

• Ion size§ Concentration of counter anion

à Can form anionic species

* ThCl4 and PuCl5

- will behave differently

à Counter anion can effect overall charge* Varied by acid concentration or

addition of salt§ Ionic size difference basis of lanthanide

separations

6-5

Chromatography Separations

• Sample dissolution• Adjustment of solution matrix

§ Based on column chemistry and other elements in solution

• Retention of target radionuclide on column§ Removal of other elements

• Solution adjustment§ Acid concentration, counter ion

variation§ Addition of redox agent

• Elute target radionuclide• Can include addition of isotopic tracer

to determine yield• Chemical behavior measured by

distribution

6-6

Solvent Extraction

• Two phase system for separation§ Sample dissolved in aqueous phase

à Normally acidic phase• Aqueous phase contacted with organic

containing ligand§ Formation of neutral metal-ligand

species drives solubility in organic phase

• Organic phase contains target radionuclide§ May have other metal ions, further

separation neededà Variation of redox state, contact

with different aqueous phase• Back extraction of target radionuclide into

aqueous phase• Distribution between organic and aqueous

phase measured to evaluate chemical behavior

6-7

Sr separations

• Sr only as divalent cation§ Isotopes

à 88 (stable), 89 (50.5 d), 90 (28.78 a)

à 90Sr/90Y (3.19 h for metastable, 2.76 d) can be exploited

• Eichrom Sr Resin § 1.0 M 4,4'(5')-di-t-

butylcyclohexano 18-crown-6 (crown ether) in 1-octanol

6-8

Sr separation• 8 M nitric acid, k' is

approximately 90§ falls to less than 1

at 0.05 M nitric acid

• Tetravalent actinide sorption can be limited by addition of oxalic acid

• 90Sr determined by beta counting

6-9

Technetium separation• Exploit redox chemistry of Tc

§ TcO4- in aqueous phase

§ Separation from cations in near neutral pH solutionà Anion exchange methodsà Interference from other anions

* NitrateØ Use Tc redox chemistryØ Remove nitratesØ Precipitate Tc

(tetrabutylamonium)• Solvent extraction

§ UREX (i.e., 1 M HNO3, 0.7 M AHA)

à UO22+ and TcO4

- extracted

à Back extraction (pH 2 acid), separate

0.00

5.00 10-3

1.00 10-2

1.50 10-2

2.00 10-2

0 50 100 150 200 250 300 350

[Tc] Dowex

[Tc] Reillex

[Tc]

M in

so

lutio

n

time (min)

6-10

Mass 110-125 (Pd, Ag, Cd, In, Sn, Sb)

• Noble metals to group 15§ Divalent Pd and Cd§ Monovalent Ag§ Trivalent In§ Sn di- and tetravalent§ Sb stable as trivalent, pentavalent

• Separation by changing conditions to target specific elements

6-11

Pd to Sb

• Extraction with HDEPH

• Vary aqueous phase§ Basic (pH 10)§ Citric acid at pH 8

§ 6 M HNO3

• Elements into different fractions

HDEHP

6-12

In, Sn, and Sb

• Extraction with HCl and HI§ Control of redox chemistry to enhance separations§ Varied organics

à Isoamyl acetate, benzene

6-13

In, Sn, and Sb

• The extraction behavior of In, Sn and Sb in HI and HCl examined§ Extraction of Sb(V) from Sn(IV) in 7 M HCl

solution with isoamylacetate. § Selective removal of Sn(IV) or In (III) from Sb(V)

by extraction into benzene or isopropylether from HI

6-14

Polonium

• Essentially tracer chemistry due to short half-life of isotopes§ 206Po 8.8 d EC to 206Bi; α to 202Pb § 207Po 5.80 h EC to 207Bi; α to 203Pb § 208Po 2.898 y EC to 208Bi; α to 204Pb § 209Po 102 y EC to 209Bi; α to 205Pb § 210Po 138.38 d α to 206Pb

• Range of separations from environmental samples§ Sediment§ seawater

6-15

Polonium extraction

• From aqueous α-hydroxyisobutyric acid• Varied organic phase

§ dioctyl sulphide, Cyanex 272, Cyanex 301 or Cyanex 302 in toluene

• 2 mL each phase

6-16

Polonium extraction

6-17

Polonium extraction

• Extraction of Po from 1M α-HIBA increases § Cyanex 272 < DOS < Cyanex 302 < Cyanex 301

• Extraction of Po with 1M extractants without α-HIBA aqueous phase§ DOS < Cyanex 301 < Cyanex 302 < Cyanex 272.

6-18

Lanthanides

• Size separations• Lanthanide and

actinide by elution with ammonium a-hydroxyisobutyrate from Dowex 50-X4 resin columns§ pH variation § Determination of

peak position with pH

6-19

Lanthanides

• Ln separation by HPLC using Di-(2-ethylhexyl) phosphoric acid (HDEHP) coated reverse phase column § a-hydroxy

isobutyric acid for elution

HDEHP separations

6-20

Th Solution chemistry

• Only one oxidation state in solution• Th(III) is claimed

§ Th4+ + HN3 Th3+ +1.5N2 + H+

à IV/III greater than 3.0 V* Unlikely based on reduction by HN3

à Claimed by spectroscopy* 460 nm, 392 nm, 190 nm, below 185 nm * Th(IV) azido chloride species

• Structure of Th4+

§ Around 11 coordination§ Ionic radius 1.178 ŧ Th-O distance 2.45 Å

à O from H2O

6-21

Solution chemistry

• Thermodynamic data§ Eº= 1.828 V (Th4+/Th)§ ΔfHº= -769 kJ/mol§ ΔfGº= -705.5 kJ/mol§ Sº= -422.6 J/Kmol

• Hydrolysis§ Largest tetravalent actinide ion

à Least hydrolyzable tetravalentà Can be examined at higher pH, up to 4à Tends to form colloids

* Discrepancies in oxide and hydroxide solubility§ Range of data

à Different measurement conditionsà Normalize by evaluation at zero ionic strength

6-22

6-23

6-24

6-25

6-26

Solution chemistry

• Complexing media§ Carbonate forms soluble species§ Mixed carbonate hydroxide species can

formà Th(OH)3CO3

-

à 1,5 § Phosphate shown to form soluble species

à Controlled by precipitation of Th2(PO4)2(HPO4).H2O* logKsp=-66.6

6-27

Complexation

• Inorganic ligands§ Fluoride, chloride, sulfate, nitrate§ Data is lacking for complexing

à Re-evaluation based pm semiemperical approach* Interligand repulsion

Ø Decrease from 1,4 to 1,5Ø Strong decrease from 1,5 to 1,6

• Organic ligands§ Oxalate, citrate, EDTA, humic substance

à Form strong complexes§ Determined by potentiometry and solvent extraction

à Choice of data (i.e., hydrolysis constants) impacts evaluation

6-28

Th analytical methods

• Low concentrations§ Without complexing agent

• Indicator dyes§ Arzenazo-III

• ICP-MS• Radiometric methods

§ Alpha spectroscopy§ Liquid scintillation

à May require preconcentrationà Need to include daughters in evaluation

6-29

Th ore processing• Main Th bearing mineral is monazite

§ Phosphate mineral à strong acid for dissolution results in water soluble saltsà Strong base converts phosphates to hydroxides

* Dissolve hydroxides in acid• Th goes with lanthanides

§ Separate by precipitation§ Lower Th solubility based on difference in oxidation state

à precipitate at pH 1* A number of different precipitation steps can be used

Ø HydroxideØ PhosphateØ PeroxideØ Carbonate (lanthanides from U and Th)Ø U from Th by solvent extraction

6-30

6-31

Pa Solution chemistry• Both tetravalent and pentavalent states in solution

§ No conclusive results on the formation of Pa(III)§ Solution states tend to hydrolyze

• Hydrolysis of Pa(V)§ Usually examined in perchlorate media§ 1st hydrolyzed species is PaOOH2+

§ PaO(OH)2+ dominates around pH 3

§ Neutral Pa(OH)5 form at higher pH§ Pa polymers form at higher concentrations

• Constants obtained from TTA extractions§ Evaluated at various TTA and proton

concentrations and varied ionic strength § Fit with specific ion interaction theory

• Absorption due to Pa=O

6-32

6-33

Solution chemistry• Pa(V) in mineral acid

§ Normally present as mixed species§ Characterized by solvent extraction or anion exchange§ Relative complexing tendencies

à F->OH->SO42->Cl->Br->I->NO3

-≥ClO4-

• Nitric acid§ Pa(V) stabilized in [HNO3]M>1§ Transition to anionic at 4 M HNO3

• HCl§ Precipitation starts when Pa is above 1E-3 M§ Pa(V) stable between 1 and 3 M

à PaOOHCl+ above 3 M HCl• HF

§ High solubility of Pa(V) with increasing HF concentration§ Up to 200 g/L in 20 M HF§ Range of species form, including anionic

6-34

6-35

Solution chemistry• Sulfuric acid

§ Pa(V) hydroxide soluble in H2SO4

§ At low acid (less than 1 M) formation of hydrated oxides or colloids

§ At high acid formation of H3PaO(SO4)3

6-36

6-37

Solution chemistry

• Redox behavior§ Reduction in Zn amalgam § Electrochemistry methods

à Pt-H2 electrodeà Acidic solutionà Polarographic methods

* One waveØ V to IV

§ Calculation of divalent redox• Pa(IV) solution

§ Oxidized by air§ Rate decreases in absence of O2 and complexing

ions

6-38

Solution chemistry• Pa(IV)

§ Precipitates in acidic solutionsà i.e., HF

• Spectroscopy§ 6d15f1

à Peak at 460 nm

6-39

Pa Analytical methods

• Radiochemical§ Alpha and gamma spectroscopy for 231Pa§ Beta spectroscopy for 234Pa

à Overlap with 234Th• Activation analysis

§ 231Pa(n,g)232Pa, 211 barns• Spectral methods

§ 263 lines from 264 nm to 437 nm§ Microgram levels

• Electrochemical methods§ Potentiometric oxidation of Pa(V)

• Absorbance§ Requires high concentrations§ Arsenazo-III

• Gravimetric methods§ Hydroxide from precipitation with ammonium hydroxide

6-40

Pa Preparation and purification

• Pa is primarily pentavalent• Pa has been separated in weighable amounts during U

purification § Diethylether separation of U§ Precipitation as carbonate

à Use of Ta as carrier• Sulfate precipitation of Ra at pH 2

§ Inclusion of H2O2 removes U and 80 % of Pa§ Isolated and redissolved in nitric acid

à Pa remains in siliceous sludge• Ability to separate Pa from Th and lanthanides by

fluoride precipitation§ Pa forms anionic species that remain in solution§ Addition of Al3+ forms precipitate that carriers Pa

6-41

Pa purification

• Difficult to separate from Zr, Ta, and Nb with macro amounts of Pa

• Precipitation§ Addition of KF

à K2PaF7

* Separates Pa from Zr, Nb, Ti, and Taà NH4

+ double salt* Pa crystallizes before Zr but after Ti and Ta

§ Reduction in presence of fluoridesà Zn amalgam in 2 M HFà PaF4 precipitates

* Redissolve with H2O2 or air current§ H2O2 precipitation

à No Nb, Ta, and Ti precipitates§ Silicates

à K, Na silicates with alumina

6-42

Pa purification

• Ion exchange§ Anion exchange with HCl

à Adhere to column in 9-10 M HCl* Fe(III), Ta, Nb, Zr, U(IV/VI) also sorbs

à Elute with mixture of HCl/HF§ HF

à Sorbs to columnà Elute with the addition of acid

* Suppresses dissociation of HF* Lowers Kd

à Addition of NH4SCN* Numerous species formed, including mixed

oxide and fluoride thiocyanates

6-43

6-44

Pa purification

• Solvent extraction§ At trace levels (<1E-4 M) extraction effective

from aqueous phase into a range of organicsà Di-isobutylketone

* Pa extracted into organic from 4.5 M H2SO4 and 6 M HCl

* Removal from organic by 9 M H2SO4 and H2O2

à Di-isopropylketone* Used to examine Pa, Nb, Db

Ø Concentrated HBrØ Pa>Nb>Db

à Dimethyl sulfoxide

6-45

Pa purification

• TTA§ 10 M HCl

à PaOCl63-

§ With TBP, Tri-n-octylphosphine oxide (TOPO), or triphenylphosphine oxide (TPPO)

• Triisooctylamine§ Mixture of HCl and HF

à 0.5 M HCl and 0.01 M HF* Used to examine the column extraction

Ø Sorbed with 12 M HCl and 0.02 M HFØ Elute with 10 M HCl and 0.025 M HF, 4

M HCl and 0.02 M HF, and 0.5 M HCl and 0.01 M HF

Ø Extraction sequence Ta>Nb>Db>Pa

6-46

Pa purification

• Aliquat 336§ Methyl-

trioctylammonium chloride

§ Extraction from HF, HCl, and HBr

6-47

Uranyl chemical bonding• Bonding molecular orbitals

§ sg2 su

2 pg4 pu

4

à Order of HOMO is unclear* pg< pu< sg<< su

proposedØ Gap for s based on 6p orbitals interactions

§ 5fd and 5f f LUMO§ Bonding orbitals O 2p characteristics§ Non bonding, antibonding 5f and 6d§ Isoelectronic with UN2

• Pentavalent has electron in non-bonding orbital

6-48

6-49

6-50

f orbitals

From LANL Pu chemistry

6-51

Uranyl chemical bonding• Linear yl oxygens from 5f characteristic

§ 6d promotes cis geometry• yl oxygens force formal charge on U below 6

§ Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride systems

à Net negative 0.43 on oxygensà Lewis bases

* Can vary with ligand in equatorial plane* Responsible for cation-cation interaction* O=U=O- - -M* Pentavalent U yl oxygens more basic

• Small changes in U=O bond distance with variation in equatorial ligand

• Small changes in IR and Raman frequencies§ Lower frequency for pentavalent U§ Weaker bond

6-52

Uranium aqueous solution complexes• Strong Lewis acid• Hard electron acceptor

§ F->>Cl->Br-I-

§ Same trend for O and N groupà based on electrostatic force as dominant factor

• Hydrolysis behavior§ U(IV)>U(VI)>>>U(III)>U(V)

• Uranium coordination with ligand can change protonation behavior § HOCH2COO- pKa=17, 3.6 upon complexation of UO2

à Inductive effect* Electron redistribution of coordinated ligand* Exploited in synthetic chemistry

• U(III) and U(V)§ No data in solution

à Base information on lanthanide or pentavalent actinides

6-53

Np chemistry• Basic solutions

§ Difficulty in understanding dataà Chemical

forms of species

• Determine ratios of each redox species from XANES§ Use Nernst

equation to determine potentials

6-54

Np solution chemistry

• Disproportionation§ NpO2

+ forms Np4+ and NpO22+

à Favored in high acidity and Np concentration§ 2NpO2

+ +4 H+Np4+ + NpO22+ + 2H2O

§ K for reaction increased by addition of complexing reagentsà K=4E-7 in 1 M HClO4 and 2.4E-2 in H2SO4

* Suggested reaction rateØ -d[NpO2

+]/dt=k[NpO2+][H+]2

• Control of redox species§ Important consideration for experiments§ LANL write on methods

6-55

Np solution chemistry

• Oxidation state control§ Redox reagents

à Adjustment from one redox state to another à Best for reversible couples

* No change in oxo group* If oxo group change occurs need to know

kineticsà Effort in PUREX process for controlled

separation of Np focused on organics* HAN and derivates for Np(VI) reduction* Rate 1st order for Np in excess reductant

à 1,1 dimethylhydrazine and tert-butylhydrazine selective of Np(VI) reduction over Pu(IV)

6-56

Np solution chemistry• Applied to Np(III) to Np(VII) and coordination complexes

§ Applied to Np(V) spin-orbit coupling for 5f2

• Absorption in HNO3

§ Np(IV): 715 nm§ Np(V): weak band at 617 nm§ Np(VI): below 400 nm

à No effect from 1 to 6 M nitric• Np(VII) only in basic media

§ NpO65-

à 2 long (2.2 Å) and 4 short (1.85 Å) à Absorbance at 412 nm and 620 nm

* O pi 5f* Number of vibrational states

Ø Between 681 cm-1 and 2338 cm-1

• Np(VI)§ Studies in Cs2UO2Cl4 lattice§ Electronic levels identified at following wavenumbers (cm-1)

à 6880, 13277, 15426, 17478, and 19358* 6880 cm-1 belongs to 5f1 configuration

6-57

Np solution chemistry

• Np(IV)§ Absorbance from 300 nm to 1800 nm

permitted assignment at 17 excited state transitions

§ IR identified Np-O vibrational bandsà 825 cm-1

§ Absorbance in nitrateà Variation seen for nitrate due to

coordination sphere

6-58

Np(III)Np(IV)

Np(V) Np(VI)

6-59

Np solution chemistry

6-60

Np solution chemistry

• Np hydrolysis§ Np(IV)>Np(VI)>Np(III)>Np(V)§ For actinides trends with ionic radius

• Np(III) § below pH 4§ Stable in acidic solution, oxidizes in air§ Potentiometric analysis for determining K§ No Ksp data

• Np(IV) § hydrolyzes above pH 1

à Tetrahydroxide main solution species in equilibrium with solid based on pH independence of solution species concentration

• Np(V) § not hydrolyzed below pH 7

• Np(VI) § below pH 3-4

• Np(VII)§ No data available

6-61

Np separation chemistry• Most methods exploit redox chemistry of Np• Solvent extraction

§ 2-thenoyltrifluoroacetoneà Reduction to Np(IV)

* Extraction in 0.5 M HNO3

* Back extract in 8 M HNO3

Ø Oxidation to Np(V), extraction into 1 M HNO3

§ Pyrazolone derivativesà Np(IV) extracted from 1 to 4 M HNO3

à Prevents Np(IV) hydrolysisà No extraction of Np(V) or Np(VI)

§ Pyrazolone derivatives synergistic extraction with tri-n-octylphosphine oxide (TOPO)à Separate Np(V) from Am, Cm, U(VI), Pu(IV) and lanthanides

§ 1:2 Np:ligand ratio as extracted species

6-62

6-63

Np solvent extraction

• Tributylphosphate § NpO2(NO3)2(TBP)2 and Np(NO3)4(TBP)2 are extracted

speciesà Extraction increases with increase concentration of TBP

and nitric acid* 1-10 M HNO3

à Separation from other actinides achieved by controlling Np oxidation state

• CMPO (Diphenyl-N,N-dibutylcarbamoyl phosphine oxide)§ Usually used with TBP§ Nitric acid solutions§ Separation achieved with oxidation state adjustment

à Reduction of Pu and Np by Fe(II) sulfamateà Np(IV) extracted into organic, then removed with

carbonate, oxalate, or EDTA

6-64

Np solvent extraction

• HDEHP§ In 1 M HNO3 with addition of NaNO2

à U, Pu, Np, Am in most stable oxidation statesà Np(V) is not extractedà Oxidized to Np(VI) then extractedà Reduced to Np(V) and back extracted into 0.1

M HNO3

• Tri-n-octylamine§ Used for separation of Np from environmental

samplesà Extracted from 10 M HClà Back extracted with 1 M HCl+0.1 M HF

6-65

Chromatography with Chelating Resins

• Resin loaded with Aliquat 336§ TEVA resin

à Np controlled by redox state* Reduction with

Fe(II) sulfamate and ascorbic acid

Ascorbic acid

6-66

6-67

                                               

6-68

Pu solution chemistry• Originally driven by the need to separate and purify Pu• Species data in thermodynamic database• Complicated solution chemistry

§ Five oxidation states (III to VII)à Small energy separations between oxidation statesà All states can be prepared

* Pu(III) and (IV) more stable in acidic solutions* Pu(V) in near neutral solutions

Ø Dilute Pu solutions favored* Pu(VI) and (VII) favored in basic solutions

Ø Pu(VII) stable only in highly basic solutions and strong oxidizing conditions

§ Some evidence of Pu(VIII)

6-69

6-70

6-71

Pu solution chemistry• Other spectroscopic methods employed in Pu

analysis§ Photoacoustic spectroscopy§ Thermal lensing

• Vibrational spectroscopy§ Oxo species

à Asymmetric stretch 930-970 cm-1

* 962 cm-1 in perchloric acidà Linear arrangement of oxygen

§ Raman shifts observedà Sensitive to complexation

* Changes by 40 cm-1

6-72

6-73

6-74

Pu solution chemistry• Preparation of pure oxidation states

§ Pu(III)à Generally below pH 4à Dissolve a-Pu metal in 6 M HClà Reduction of higher oxidation state with Hg or Pt cathode

* 0.75 V vs NHEà Hydroxylamine or hydrazine as reductant

§ Pu(IV)à Electrochemical oxidation of Pu(III) at 1.2 V

* Thermodynamically favors Pu(VI), but slow kinetics due to oxo formation

§ Pu(V)à Electrochemical reduction of Pu(VI) at pH 3 at 0.54 V (vs SCE)

* Near neutral in 1 micromole/L Pu(V)§ Pu(VI)

à Treatment of lower oxidation states with hot HClO4

à Ozone treatment§ Pu(VII)

à Oxidation in alkaline solutions* Hexavalent Pu with ozone, anodic oxidation

6-75

Pu solution chemistry

• Pu(VI) oxo oxygen exchange with water§ 18O enriched water exchange

à need to maintain hexavalent oxidation state* Exchange rate increases with lower oxidation state

§ Exchange half life = 4.55E4 hr at 23 °Cà Two reaction paths

* Reaction of water with Pu(VI)* Breaking of P=O bonds by alpha decay

Ø Faster exchange rate measured with 238Pu• Pu redox by actinides

§ Similar to diproportionation§ Rates can be assessed against redox potentials

à Pu4+ reduction by different actinides shows different rates* Accompanied by oxidation of An4+ with yl bond formation

§ Reduction of Pu(VI) by tetravalent actinides proceeds over pentavalent state

§ Reactions show hydrogen ion dependency

6-76

Pu solution chemistry

• Pu reduction by other metal ions and ligands§ Rates are generally dependent upon proton and ligand concentration

à Humic acid, oxalic acid, ascorbic acid§ Poor inorganic complexants can oxidize Pu

à Bromate, iodate, dichromate§ Reactions with single electron reductants tend to be rapid

à Reduction by Fe2+

§ Complexation with ligands in solution impacts redoxà Different rates in carbonate media compared to perchlorateà Mono or dinitrate formation can effect redox

* Pu(IV) formation or reaction with pentavalent metal ions proceeds faster in nitrate than perchlorate

* Oxidation of Pu(IV) by Ce(IV) or Np(VI) slower in nitrate§ Pu(VI) reduction can be complicated by disproportionation§ Hydroxylamine (NH2OH), nitrous acid, and hydrazine (N2H4)

à Used in PUREX for Pu redox controlà Pu(III) oxidized

* 2Pu3++3H++NO3-2Pu4++HNO2+H2O

* Re-oxidation adds nitrous acid to the system which can initiate an autocatalytic reaction

6-77

Pu anion exchange

6-78

6-79

6-80

Pu cation exchange

• General cation exchange trends for Pu§ HN03, H2S04, and HC104 show stronger influence than HC1§ Strong increase in distribution coefficient in HClO4 at high

acidities exhibited for Pu(III) and Pu(VI)

6-81

Pu separations• Alkaline solutions

§ Need strong ligands that can compete with hydroxide to form different speciesà F-, CO3

2-, H2O2

* High solubility, based on oxidation state* Stabilize Pu(VII)

• Room temperature ionic liquids§ Quaternary ammonium with anions

à AlCl4-, PF6

-

§ Liquid-liquid extraction§ Electrochemical disposition

N R

NTf2

N NNTf2 N NTf2N

O

NTf2

NS S

O

O O

O

CF3F3C

6-82

Am solution chemistry• Oxidation states III-VI in solution

§ Am(III,V) stable in dilute acid§ Am(V, VI) form dioxo cations

• Am(II)§ Unstable, unlike some lanthanides (Yb, Eu, Sm)

à Formed from pulse radiolysis* Absorbance at 313 nm* T1/2 of oxidation state 5E-6 seconds

• Am(III)§ Easy to prepare (metal dissolved in acid, AmO2 dissolution)

à Pink in mineral acids, yellow in HClO4 when Am is 0.1 M• Am(IV)

§ Requires complexation to stabilizeà dissolving Am(OH)4 in NH4Fà Phosphoric or pyrophosphate (P2O7

4-) solution with anodic oxidation

à Ag3PO4 and (NH4)4S2O8

à Carbonate solution with electrolytic oxidation

6-83

Am solution chemistry

• Am(V)§ Oxidation of Am(III) in near neutral solution

à Ozone, hypochlorate (ClO-), peroxydisulfateà Reduction of Am(VI) with bromide

• Am(VI)§ Oxidation of Am(III) with S2O8

2- 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 V

• Am(VII)§ 3-4 M NaOH, mM Am(VI) near 0 °C§ Gamma irradiation 3 M NaOH with N2O or S2O8

2- saturated solution

6-84

Am solution chemistry• Am(III) has 9 inner sphere waters

§ Others have calculated 11 and 10 (XAFS)§ Based on fluorescence spectroscopy

à Lifetime related to coordination* nH2O=(x/t)-y

Ø x=2.56E-7 s, y=1.43Ø Measurement of fluorescence lifetime in H2O and

D2O

6-85

Am solution chemistry• Autoreduction

§ Formation of H2O2 and HO2 radicals from radiation reduces Am to trivalent statesà Difference between 241Am and 243Am

§ Rate decreases with increase acid for perchloric and sulfuric

§ Some disagreement role of Am concentrationà Concentration of Am total or oxidation state

§ Rates of reduction dependent uponà Acid, acid concentration, à mechanism

* Am(VI) to Am(III) can go stepwise à starting ion

* Am(V) slower than Am(VI)

6-86

Am solution chemistry• Disproportionation

§ Am(IV) à In nitric and perchloric acidà Second order with Am(IV)

* 2 Am(IV)Am(III) + Am(V)* Am(IV) + Am(V)Am(III) + Am(VI)

Ø Am(VI) increases with sulfate§ Am(V)

à 3-8 M HClO4 and HCl* 3 Am(V) + 4 H+Am(III)+2Am(VI)+2 H2O

à Solution can impact oxidation state stability

6-87

Am solution chemistry

• Redox kinetics§ Am(III) oxidation by peroxydisulfate

à Oxidation due to thermal decomposition products* SO4

.-, HS2O8-

à Oxidation to Am(VI)* 0.1 M to 10 nM Am(III)

à Acid above 0.3 M limits oxidation* Decomposition of S2O8

2-

à Induction period followed by reductionà Rates dependent upon temperature, [HNO3], [S2O8

2-], and [Ag+2]à 3/2 S2O8

2- + Am3++2 H2O3 SO42- +AmO2

2++4H+

* Evaluation of rate constants can yield 4 due to peroxydisulfate decomposition

à In carbonate proceeds through Am(V)* Rate to Am(V) is proportional to oxidant* Am(V) to Am(VI)

Ø Proportional to total Am and oxidantØ Inversely proportional to K2CO3

6-88

6-89

Am solution chemistry

• Hydrolysis§ Mono-, di-, and trihydroxide species§ Am(V) appears to have 2 species, mono- and

dihydroxide• Carbonate

§ Evaluated by spectroscopy§ Includes mixed species

à Am hydroxide carbonate speciesà Based on solid phase analysis

§ Am(IV)à Pentacarbonate studied (log b=39.3)

§ Am(V) solubility examined

6-90

Am solution chemistry: Organics• Number of complexes examined

§ Mainly for Am(III)• Stability of complex decreases with

increasing number of carbon atoms• With aminopolycarboxylic acids,

complexation constant increases with ligand coordination

• Natural organic acid§ Number of measurements

conducted§ Measured by spectroscopy

and ion exchange• TPEN (N,N,N’,N’-tetrakis(2-

pyridylmethyl)ethyleneamine)§ 0.1 M NaClO4, complexation

constant for Am 2 orders greater than Sm

6-91

Am solution chemistry

• Fluorides§ Inner sphere complexes, complexation constants much higher than other

halidesà 1,1 and 1,2 Am:F complexes identifiedà Only 1,1 for Cl

• Sulfates§ 1,1 and 1,2 constants known§ No evidence of AmHSO4

2+ species• Thiocyanate (SCN-)

§ Useful ligand for Ln/Ac separations§ 1,1 to 1,3 complex forms

à Examined by solvent extraction and spectroscopy• Nitrate

§ 1,1 and 1,2 for interpreting solvent extraction data§ Constant for 1,1 species

• Phosphate§ Interpretation of data complicated due to degree of phosphate protonation§ AmHPO4

+

§ Complexation with H2PO4; 1,1 to 1,4 speciesà From cation exchange, spectroscopic and solvent extraction data

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Am(IV) solution chemistry

• Am(IV) can be stabilized by heteropolyanions§ P2W17O61

anion; formation of 1,1 and 1,2 complexà Examined by absorbance at 789 nm and 560 nmà Autoradiolytic reduction

* Independent of complex formationà Displacement by addition of Th(IV)

* Disproportionation of Am(IV) to Am(III) and Am(VI)

§ EXAFS used with AmP5W30O11012-

• Cation-cation interaction§ Am(V)-U(VI) interaction in perchlorate

à Am(V) spectroscopic shift from 716-733 nm to 765 nm

6-93

Am solvent extraction• Lanthanide/actinide separation

§ Extraction reactionà Am3++2(HA)2AmA3HA+3 H+

* Release of protons upon complexation requires pH adjustment to achieve extractionØ Maintain pH greater than 3

§ Cyanex 301 stable in acidà HCl, H2SO4, HNO3

* Below 2 M§ Irradiation produces acids and phosphorus compounds

à Problematic extractions when dosed 104 to 105 gray§ New dithiophosphinic acid less sensitive to acid concentration

à R2PSSH; R=C6H5, ClC6H4, FC6H4, CH3C6H4 * Only synergistic extractions with, TBP, TOPO, or

tributylphosphine oxide* Aqueous phase 0.1-1 M HNO3

* Increased radiation resistance

6-94

6-95

Ion exchange• Cation exchange

§ Am3+ sorbs to cation exchange resin in dilute acidà Elution with a-hydroxyisobutyrate and

aminopolycarboxylic acids• Anion exchange

§ Sorption to resin from thiocyanate, chloride, and to a limited degree nitrate solutions

• Inorganic exchangers§ Zirconium phosphate

à Trivalents sorb* Oxidation of Am to AmO2

+ achieves separation§ TiSb (titanium antimonate)

à Am3+ sorption in HNO3

à Adjustment of aqueous phase to achieve separation

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Ion exchange separation Am from Cm• Separation of tracer level Am and Cm has been performed with displacement

complexing chromatography § separations were examined with DTPA and nitrilotriacetic acid in the

presence of Cd and Zn as competing cations§ use of Cd and nitrilotriacetic acid separated trace levels of Am from Cm§ displacement complexing chromatography method is too cumbersome to use

on a large scale• Ion exchange has been used to separate trace levels of Cm from Am

§ Am, Cm, and lanthanides were sorbed to a cation exchange resin at pH 2à separation was achieved by adjusting pH and organic complexantà Separation of Cm from Am was performed with 0.01 %

ethylenediamine-tetramethylphosphonic acid at pH 3.4 in 0.1 M NaNO3 with a separation factor of 1.4

• Separation of gram scale quantities of Am and Cm has been achieved by cation and anion exchange § methods rely upon use of a-hydroxylisobutyrate or

diethylenetriaminepentaacetic acid as an eluting agent or a variation of the eluant composition by the addition of methanol to nitric acidà best separations were achieved under high pressure conditionsà repeating the procedure separation factors greater than 400 were

obtained

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Extraction chromatography• Mobile liquid phase and stationary liquid phase

§ Apply results from solvent extractionà HDEHP, Aliquat 336, CMPO

* Basis for Eichrom resins* Limited use for solutions with fluoride, oxalate, or

phosphateà DIPEX resin

* Bis(2-ethylhexylmethanediphosphonic acid on inert support* Lipophilic molecule

Ø Extraction of 3+, 4+, and 6+ actinides* Strongly binds metal ions

Ø Need to remove organics from support§ Variation of support

à Silica for covalent bondingà Functional organics on coated ferromagnetic particles

* Magnetic separation after sorption

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Questions

1. What are some key fission products for nuclear forensics? Why?

2. Describe a method for the separation of Sr

3. What methods are suitable for the separation of Pd and In? How would these be quantified? When would it necessary to investigate these isotopes?

4. What is the fundamental chemistry that control lanthanide separation?

5. Describe two methods for the separation of U from Pu. Under which conditions would it be preferable to separate Pu from U for forensics applications?