Lecture 7: Separation and quantification of radionuclides

Report
Nuclear Forensics Summer School
Radiochemical 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
6-1
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
Polonium
235U
fission yield
6-2
Fundamentals of separations
• Oxidation state
 Elements of different oxidation states easier to
separate
Anionic and cationic speciation
* UO22+,TcO4 Variation of oxidation state
Addition of reductants/oxidants to control
speciation
* Method for separation of Pu from U
Varied stability of oxidation states
6-3
Fundamentals of separation
• Ion size
 Concentration of counter anion
Can form anionic species
* ThCl4 and PuCl5- will behave
differently
Counter anion can effect overall charge
* Varied by acid concentration or
addition of salt
 Ionic size difference basis of lanthanide
separations
6-4
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
6-5
Solvent Extraction
•
•
•
•
•
Two phase system for separation

Sample dissolved in aqueous phase
 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 needed
 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
6-6
Sr separations
• Sr only as divalent cation
 Isotopes
88 (stable), 89 (50.5 d), 90
(28.78 a)
90Sr/90Y (3.19 h for
metastable, 2.76 d) can be
exploited
• Eichrom Sr Resin
 1.0 M 4,4'(5')-di-tbutylcyclohexano 18-crown-6
(crown ether) in 1-octanol
6-7
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
6-8
Technetium separation
[Tc] M in solution
• Exploit redox chemistry of Tc

TcO4- in aqueous phase

Separation from cations in near
neutral pH solution
 Anion exchange methods
 Interference from other
anions
* Nitrate
 Use Tc redox
chemistry
 Remove nitrates
 Precipitate Tc
(tetrabutylamonium)
• Solvent extraction

UREX (i.e., 1 M HNO3, 0.7 M
AHA)
 UO22+ and TcO4- extracted
 Back extraction (pH 2 acid),
separate
2.00 10
-2
1.50 10
-2
[Tc] Dowex
[Tc] Reillex
1.00 10
-2
5.00 10
-3
0.00
0
50
100
150
200
time (min)
250
300
6-9
350
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
6-10
Pd to Sb
• Extraction with HDEPH
HDEHP
• Vary aqueous phase
 Basic (pH 10)
 Citric acid at pH 8
 6 M HNO3
• Elements into different
fractions
6-11
In, Sn, and Sb
• Extraction with HCl and HI
 Control of redox chemistry to enhance
separations
 Varied organics
Isoamyl acetate, benzene
6-12
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
6-13
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
6-14
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
6-15
Polonium extraction
6-16
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
6-17
 DOS < Cyanex 301 < Cyanex 302 < Cyanex 272.
Lanthanides
• Size separations
• Lanthanide and
actinide by elution
with ammonium ahydroxyisobutyrate
from Dowex 50-X4
resin columns
 pH variation
 Determination of
peak position
with pH
6-18
Lanthanides
• Ln separation by
HPLC using Di-(2ethylhexyl)
phosphoric acid
(HDEHP) coated
reverse phase
column
 a-hydroxy
isobutyric acid
for elution
HDEHP separations
6-19
Th Solution chemistry
• Only one oxidation state in solution
• Th(III) is claimed
 Th4+ + HN3  Th3+ +1.5N2 + H+
 IV/III greater than 3.0 V
* Unlikely based on reduction by HN3
 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 Å
 Th-O distance 2.45 Å
 O from H2O
6-20
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
 Least hydrolyzable tetravalent
 Can be examined at higher pH, up to 4
 Tends to form colloids
* Discrepancies in oxide and hydroxide solubility

Range of data
 Different measurement conditions
 Normalize by evaluation at zero ionic strength
6-21
6-22
6-23
6-24
6-25
Solution chemistry
• Complexing media
 Carbonate forms soluble species
 Mixed carbonate hydroxide species can
form
Th(OH)3CO31,5
 Phosphate shown to form soluble species
Controlled by precipitation of
Th2(PO4)2(HPO4).H2O
* logKsp=-66.6
6-26
Complexation
• Inorganic ligands
 Fluoride, chloride, sulfate, nitrate
 Data is lacking for complexing
 Re-evaluation based pm semiemperical approach
* Interligand repulsion
 Decrease from 1,4 to 1,5
 Strong decrease from 1,5 to 1,6
• Organic ligands
 Oxalate, citrate, EDTA, humic substance
 Form strong complexes
 Determined by potentiometry and solvent extraction
 Choice of data (i.e., hydrolysis constants) impacts
6-27
evaluation
Th analytical methods
• Low concentrations
 Without complexing agent
• Indicator dyes
 Arzenazo-III
• ICP-MS
• Radiometric methods
 Alpha spectroscopy
 Liquid scintillation
May require preconcentration
Need to include daughters in evaluation
6-28
Th ore processing
• Main Th bearing mineral is monazite

Phosphate mineral
 strong acid for dissolution results in water soluble salts
 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
 precipitate at pH 1
* A number of different precipitation steps can be used
 Hydroxide
 Phosphate
 Peroxide
 Carbonate (lanthanides from U and Th)
 U from Th by solvent extraction
6-29
6-30
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
6-31
6-32
Solution chemistry
• Pa(V) in mineral acid

Normally present as mixed species

Characterized by solvent extraction or anion exchange

Relative complexing tendencies
 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
 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
6-33
6-34
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
6-35
6-36
Solution chemistry
• Redox behavior
 Reduction in Zn amalgam
 Electrochemistry methods
 Pt-H2 electrode
 Acidic solution
 Polarographic methods
* One wave
 V to IV
 Calculation of divalent redox
• Pa(IV) solution
 Oxidized by air
 Rate decreases in absence of O2 and complexing
ions
6-37
Solution chemistry
•
•
Pa(IV)

Precipitates in acidic solutions
 i.e., HF
Spectroscopy

6d15f1
 Peak at 460 nm
6-38
Pa Analytical methods
• Radiochemical

Alpha and gamma spectroscopy for 231Pa

Beta spectroscopy for 234Pa
 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
6-39
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
 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
 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
6-40
Pa purification
• Difficult to separate from Zr, Ta, and Nb with macro amounts of
Pa
• Precipitation

Addition of KF
 K2PaF7
* Separates Pa from Zr, Nb, Ti, and Ta
 NH4+ double salt
* Pa crystallizes before Zr but after Ti and Ta

Reduction in presence of fluorides
 Zn amalgam in 2 M HF
 PaF4 precipitates
* Redissolve with H2O2 or air current

H2O2 precipitation
 No Nb, Ta, and Ti precipitates

Silicates
 K, Na silicates with alumina
6-41
Pa purification
• Ion exchange
 Anion exchange with HCl
 Adhere to column in 9-10 M HCl
* Fe(III), Ta, Nb, Zr, U(IV/VI) also sorbs
 Elute with mixture of HCl/HF
 HF
 Sorbs to column
 Elute with the addition of acid
* Suppresses dissociation of HF
* Lowers Kd
 Addition of NH4SCN
* Numerous species formed, including mixed
oxide and fluoride thiocyanates
6-42
6-43
Pa purification
• Solvent extraction
 At trace levels (<1E-4 M) extraction effective from
aqueous phase into a range of organics
 Di-isobutylketone
* Pa extracted into organic from 4.5 M H2SO4
and 6 M HCl
* Removal from organic by 9 M H2SO4 and
H2 O2
 Di-isopropylketone
* Used to examine Pa, Nb, Db
 Concentrated HBr
 Pa>Nb>Db
 Dimethyl sulfoxide
6-44
Pa purification
• TTA
 10 M HCl
 PaOCl63 With TBP, Tri-n-octylphosphine oxide (TOPO), or
triphenylphosphine oxide (TPPO)
• Triisooctylamine
 Mixture of HCl and HF
 0.5 M HCl and 0.01 M HF
* Used to examine the column extraction
 Sorbed with 12 M HCl and 0.02 M HF
 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
 Extraction sequence Ta>Nb>Db>Pa
6-45
Pa purification
• Aliquat 336
 Methyltrioctylammonium
chloride
 Extraction from
HF, HCl, and HBr
6-46
Uranyl chemical bonding
•
•
Bonding molecular orbitals

sg2 su2 pg4 pu4
 Order of HOMO is unclear
* pg< pu< sg<< su proposed
 Gap for s based on 6p orbitals interactions

5fd and 5ff LUMO

Bonding orbitals O 2p characteristics

Non bonding, antibonding 5f and 6d

Isoelectronic with UN2
Pentavalent has electron in non-bonding orbital
6-47
6-48
6-49
f orbitals
From LANL Pu chemistry
6-50
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
 Net negative 0.43 on oxygens
 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
6-51
Uranium aqueous solution complexes
• Strong Lewis acid
• Hard electron acceptor

F->>Cl->Br-I
Same trend for O and N group
 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
 Inductive effect
* Electron redistribution of coordinated ligand
* Exploited in synthetic chemistry
• U(III) and U(V)

No data in solution
 Base information on lanthanide or pentavalent actinides
6-52
Np chemistry
• Basic solutions
 Difficulty in
understanding
data
 Chemical
forms of
species
• Determine ratios of
each redox species
from XANES
 Use Nernst
equation to
determine
potentials
6-53
Np solution chemistry
• Disproportionation
 NpO2+ forms Np4+ and NpO22+
 Favored in high acidity and Np concentration
 2NpO2+ +4 H+Np4+ + NpO22+ + 2H2O
 K for reaction increased by addition of complexing
reagents
 K=4E-7 in 1 M HClO4 and 2.4E-2 in H2SO4
* Suggested reaction rate
 -d[NpO2+]/dt=k[NpO2+][H+]2
• Control of redox species
 Important consideration for experiments
 LANL write on methods
6-54
Np solution chemistry
• Oxidation state control
 Redox reagents
 Adjustment from one redox state to another
 Best for reversible couples
* No change in oxo group
* If oxo group change occurs need to know
kinetics
 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
 1,1 dimethylhydrazine and tert-butylhydrazine
selective of Np(VI) reduction over Pu(IV)
6-55
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
 No effect from 1 to 6 M nitric
Np(VII) only in basic media

NpO65 2 long (2.2 Å) and 4 short (1.85 Å)
 Absorbance at 412 nm and 620 nm
* O pi 5f
* Number of vibrational states
 Between 681 cm-1 and 2338 cm-1
Np(VI)

Studies in Cs2UO2Cl4 lattice

Electronic levels identified at following wavenumbers (cm-1)
 6880, 13277, 15426, 17478, and 19358
* 6880 cm-1 belongs to 5f1 configuration
6-56
Np solution chemistry
• Np(IV)
 Absorbance from 300 nm to 1800 nm
permitted assignment at 17 excited state
transitions
 IR identified Np-O vibrational bands
825 cm-1
 Absorbance in nitrate
Variation seen for nitrate due to
coordination sphere
6-57
Np(III)
Np(V)
Np(IV)
Np(VI)
6-58
Np solution chemistry
6-59
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
 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
6-60
Np separation chemistry
• Most methods exploit redox chemistry of Np
• Solvent extraction

2-thenoyltrifluoroacetone
 Reduction to Np(IV)
* Extraction in 0.5 M HNO3
* Back extract in 8 M HNO3
 Oxidation to Np(V), extraction into 1 M HNO3

Pyrazolone derivatives
 Np(IV) extracted from 1 to 4 M HNO3
 Prevents Np(IV) hydrolysis
 No extraction of Np(V) or Np(VI)

Pyrazolone derivatives synergistic extraction with tri-noctylphosphine oxide (TOPO)
 Separate Np(V) from Am, Cm, U(VI), Pu(IV) and lanthanides

1:2 Np:ligand ratio as extracted species
6-61
6-62
Np solvent extraction
• Tributylphosphate

NpO2(NO3)2(TBP)2 and Np(NO3)4(TBP)2 are extracted
species
 Extraction increases with increase concentration of TBP
and nitric acid
* 1-10 M HNO3
 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
 Reduction of Pu and Np by Fe(II) sulfamate
 Np(IV) extracted into organic, then removed with
carbonate, oxalate, or EDTA
6-63
Np solvent extraction
• HDEHP
 In 1 M HNO3 with addition of NaNO2
 U, Pu, Np, Am in most stable oxidation states
 Np(V) is not extracted
 Oxidized to Np(VI) then extracted
 Reduced to Np(V) and back extracted into 0.1
M HNO3
• Tri-n-octylamine
 Used for separation of Np from environmental
samples
 Extracted from 10 M HCl
 Back extracted with 1 M HCl+0.1 M HF
6-64
Chromatography with Chelating Resins
• Resin loaded with
Aliquat 336
 TEVA resin
Np controlled by
redox state
* Reduction with
Fe(II) sulfamate
and ascorbic
acid
Ascorbic acid
6-65
6-66
6-67
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)
 Small energy separations between oxidation states
 All states can be prepared
* Pu(III) and (IV) more stable in acidic solutions
* Pu(V) in near neutral solutions
 Dilute Pu solutions favored
* Pu(VI) and (VII) favored in basic solutions
 Pu(VII) stable only in highly basic
solutions and strong oxidizing conditions
 Some evidence of Pu(VIII)
6-68
6-69
6-70
Pu solution chemistry
• Other spectroscopic methods employed in Pu
analysis
 Photoacoustic spectroscopy
 Thermal lensing
• Vibrational spectroscopy
 Oxo species
Asymmetric stretch 930-970 cm-1
* 962 cm-1 in perchloric acid
Linear arrangement of oxygen
 Raman shifts observed
Sensitive to complexation
* Changes by 40 cm-1
6-71
6-72
6-73
Pu solution chemistry
•
Preparation of pure oxidation states

Pu(III)
 Generally below pH 4
 Dissolve a-Pu metal in 6 M HCl
 Reduction of higher oxidation state with Hg or Pt cathode
* 0.75 V vs NHE
 Hydroxylamine or hydrazine as reductant

Pu(IV)
 Electrochemical oxidation of Pu(III) at 1.2 V
* Thermodynamically favors Pu(VI), but slow kinetics due to oxo
formation

Pu(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 HClO4
 Ozone treatment

Pu(VII)
 Oxidation in alkaline solutions
6-74
* Hexavalent Pu with ozone, anodic oxidation
Pu solution chemistry
•
•
Pu(VI) oxo oxygen exchange with water
18O enriched water exchange

 need to maintain hexavalent oxidation state
* Exchange rate increases with lower oxidation state

Exchange half life = 4.55E4 hr at 23 °C
 Two reaction paths
* Reaction of water with Pu(VI)
* Breaking of P=O bonds by alpha decay
 Faster exchange rate measured with 238Pu
Pu redox by actinides

Similar to diproportionation

Rates can be assessed against redox potentials
 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
6-75
Pu solution chemistry
•
Pu reduction by other metal ions and ligands

Rates are generally dependent upon proton and ligand concentration
 Humic acid, oxalic acid, ascorbic acid

Poor inorganic complexants can oxidize Pu
 Bromate, iodate, dichromate

Reactions with single electron reductants tend to be rapid
 Reduction by Fe2+

Complexation with ligands in solution impacts redox
 Different rates in carbonate media compared to perchlorate
 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)
 Used in PUREX for Pu redox control
 Pu(III) oxidized
* 2Pu3++3H++NO3-2Pu4++HNO2+H2O
* Re-oxidation adds nitrous acid to the system which can initiate an
autocatalytic reaction
6-76
Pu anion exchange
6-77
6-78
6-79
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)
6-80
Pu separations
• Alkaline solutions
 Need strong ligands that can compete with hydroxide
to form different species
 F-, CO32-, H2O2
* High solubility, based on oxidation state
* Stabilize Pu(VII)
• Room temperature ionic liquids
 Quaternary ammonium with anions
 AlCl4-, PF6O
O
N
 Liquid-liquid extraction
S
S
CF
F C
O
O
 Electrochemical disposition
3
3
O
NTf2
N
N
N
NTf2
N
NTf2
N R
6-81
NTf2
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)
 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)
 Pink in mineral acids, yellow in HClO4 when Am is 0.1 M
• Am(IV)

Requires complexation to stabilize
 dissolving Am(OH)4 in NH4F
 Phosphoric or pyrophosphate (P2O74-) solution with anodic
oxidation
 Ag3PO4 and (NH4)4S2O8
 Carbonate solution with electrolytic oxidation
6-82
Am solution chemistry
• Am(V)

Oxidation of Am(III) in near neutral solution
 Ozone, hypochlorate (ClO-), peroxydisulfate
 Reduction of Am(VI) with bromide
• Am(VI)

Oxidation of Am(III) with S2O82- or Ag2+ in dilute nonreducing 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 S2O82- saturated
solution
6-83
Am solution chemistry
• Am(III) has 9 inner sphere waters

Others have calculated 11 and 10 (XAFS)

Based on fluorescence spectroscopy
 Lifetime related to coordination
* nH2O=(x/t)-y
 x=2.56E-7 s, y=1.43
 Measurement of fluorescence lifetime in H2O and
D2O
6-84
Am solution chemistry
• Autoreduction
 Formation of H2O2 and HO2 radicals from
radiation reduces Am to trivalent states
 Difference between 241Am and 243Am
 Rate decreases with increase acid for perchloric
and sulfuric
 Some disagreement role of Am concentration
 Concentration of Am total or oxidation state
 Rates of reduction dependent upon
 Acid, acid concentration,
 mechanism
* Am(VI) to Am(III) can go stepwise
 starting ion
* Am(V) slower than Am(VI)
6-85
Am solution chemistry
•
Disproportionation

Am(IV)
 In nitric and perchloric acid
 Second order with Am(IV)
* 2 Am(IV)Am(III) + Am(V)
* Am(IV) + Am(V)Am(III) + Am(VI)

Am(VI) increases with sulfate

Am(V)
 3-8 M HClO4 and HCl
* 3 Am(V) + 4 H+Am(III)+2Am(VI)+2 H2O
 Solution can impact oxidation state stability
6-86
Am solution chemistry
•
Redox kinetics

Am(III) oxidation by peroxydisulfate
 Oxidation due to thermal decomposition products
* SO4.-, HS2O8 Oxidation to Am(VI)
* 0.1 M to 10 nM Am(III)
 Acid above 0.3 M limits oxidation
* Decomposition of S2O82 Induction period followed by reduction
 Rates dependent upon temperature, [HNO3], [S2O82-], and [Ag+2]
 3/2 S2O82- + Am3++2 H2O3 SO42- +AmO22++4H+
* Evaluation of rate constants can yield 4 due to peroxydisulfate
decomposition
 In carbonate proceeds through Am(V)
* Rate to Am(V) is proportional to oxidant
* Am(V) to Am(VI)
 Proportional to total Am and oxidant
 Inversely proportional to K2CO3
6-87
6-88
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
 Am hydroxide carbonate species
 Based on solid phase analysis
 Am(IV)
 Pentacarbonate studied (log b=39.3)
 Am(V) solubility examined
6-89
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(2pyridylmethyl)ethyleneamine)

0.1 M NaClO4, complexation
constant for Am 2 orders
greater than Sm
6-90
Am solution chemistry
•
•
•
•
•
Fluorides

Inner sphere complexes, complexation constants much higher than other
halides
 1,1 and 1,2 Am:F complexes identified
 Only 1,1 for Cl
Sulfates

1,1 and 1,2 constants known

No evidence of AmHSO42+ species
Thiocyanate (SCN-)

Useful ligand for Ln/Ac separations

1,1 to 1,3 complex forms
 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 species
 From cation exchange, spectroscopic and solvent extraction data
6-91
Am(IV) solution chemistry
• Am(IV) can be stabilized by heteropolyanions

P2W17O61 anion; formation of 1,1 and 1,2 complex
 Examined by absorbance at 789 nm and 560 nm
 Autoradiolytic reduction
* Independent of complex formation
 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
 Am(V) spectroscopic shift from 716-733 nm to 765 nm
6-92
Am solvent extraction
• Lanthanide/actinide separation

Extraction reaction
 Am3++2(HA)2AmA3HA+3 H+
* Release of protons upon complexation requires pH
adjustment to achieve extraction
 Maintain pH greater than 3

Cyanex 301 stable in acid
 HCl, H2SO4, HNO3
* Below 2 M

Irradiation produces acids and phosphorus compounds
 Problematic extractions when dosed 104 to 105 gray

New dithiophosphinic acid less sensitive to acid concentration
 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
6-93
6-94
Ion exchange
• Cation exchange

Am3+ sorbs to cation exchange resin in dilute acid
 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
 Trivalents sorb
* Oxidation of Am to AmO2+ achieves separation

TiSb (titanium antimonate)
 Am3+ sorption in HNO3
 Adjustment of aqueous phase to achieve separation
6-95
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 2
 separation was achieved by adjusting pH and organic complexant
 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 acid
 best separations were achieved under high pressure conditions
 repeating the procedure separation factors greater than 400 were
obtained
6-96
Extraction chromatography
• Mobile liquid phase and stationary liquid phase

Apply results from solvent extraction
 HDEHP, Aliquat 336, CMPO
* Basis for Eichrom resins
* Limited use for solutions with fluoride, oxalate, or
phosphate
 DIPEX resin
* Bis(2-ethylhexylmethanediphosphonic acid on inert support
* Lipophilic molecule
 Extraction of 3+, 4+, and 6+ actinides
* Strongly binds metal ions
 Need to remove organics from support

Variation of support
 Silica for covalent bonding
 Functional organics on coated ferromagnetic particles
* Magnetic separation after sorption
6-97
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?
6-98

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