Transcript PowerPoint

Americium and Curium Chemistry
• From: Chemistry of actinides
 Nuclear properties
 Production of Am isotopes
 Am separation and purification
 Metallic state
 Compounds
 Solution chemistry
 Coordination chemistry
 Analytical Chemistry
15-1
Production of Am isotopes
•
•
•
•
•
Am first produced from neutron irradiation of Pu
239Pu to 240Pu to 241Pu, then beta decay of

241Pu
241,243Am main isotopes of interest

Long half-lives

Produced in kilogram quantity

Chemical studies

Both isotopes produced in reactor
241Am

source for low energy gamma and alpha
 Alpha energy 5.44 MeV and 5.49 MeV

Smoke detectors

Neutron sources
 (a,n) on Be

Thickness gauging and density
242Cm production from thermal neutron

capture
243Am

Irradiation of 242Pu, beta decay of 243Pu
Critical mass
242Am in solution

 23 g at 5 g/L
 Requires isotopic separation
15-2
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
15-3
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
15-4
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
D2 O
15-5
Am solution chemistry
•
Thermodynamic data available (NEA data)

Systematic differences at Am
 Thermodynamic changes with atomic number
 Deviation at Am due to positive entropy of vaporization
15-6
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)
15-7
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
15-8
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
15-9
Am solution chemistry: Redox kinetics
• Am(VI) reduction

H2O2 in perchlorate is 1st order for peroxide and Am
 2 AmO22++H2O22 AmO2+ + 2 H++ O2

NpO2+
 1st order with Am(VI) and Np(V)
* k=2.45E4 L / mol s

Oxalic acid reduces to equal molar Am(III) and Am(V)
• Am(V) reduction

Reduced to Am(III) in NaOH solutions
 Slow reduction with dithionite (Na2S2O4), sulfite (SO32-),
or thiourea dioxide ((NH2)2CSO2)

Np(IV) and Np(V)
 In both acidic and carbonate conditions
* For Np(IV) reaction products either Np(V) or Np(VI)
 Depends upon initial relative concentration of
Am and Np
 U(IV) examined in carbonate
15-10
Am solution chemistry
• Radiolysis
 From alpha decay
1 mg 241Am release 7E14 eV/s
 Reduction of higher valent Am related to
dose and electrolyte concentration
 In nitric acid need to include role of HNO2
 In perchlorate numerous species produced
Cl2, ClO2, or Cl15-11
Am solution chemistry
• Complexation chemistry
 Primarily for Am(III)
 F->H2PO4->SCN->NO3->Cl->ClO4 Hard acid reactions
 Electrostatic interactions
* Inner sphere and outer sphere
 Outer sphere for weaker ligands
 Stabilities similar to trivalent lanthanides
 Some enhanced stability due to participation of
5f electron in bonding
15-12
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
15-13
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
15-14
Am separation and purification
• Pyrochemical process

Am from Pu
 O2 in molten salt, PuO2 forms and precipitates
 Partitioning of Am between liquid Bi or Al and molten
salts
* Kd of 2 for Al system
 Separation of Am from PuF4 in salt by addition of OF2
* Formation of PuF6
• Precipitation method

Formation of insoluble Am species
 AmF3, K8Am2(SO4)7 , Am2(C2O4)3, K3AmO2(CO3)2
* Am(V) carbonate useful for separation from Cm
* Am from lanthanides by oxalate precipitation
 Slow hydrolysis of dimethyloxalate
 Oxalate precipitate enriched in Am
 50 % lanthanide rejection, 4 % Am

Oxidation of Am(VI) by K2S2O8 and precipitation of Cm(III)
15-15
Am solvent extraction
• Am from lanthanides

HDEHP extract lanthanides better than actinides
 Harder metal-ligand interaction
 Basis of TALSPEAK
 Preferential removal of actinides by contact with DTPA
solution
* Reverse-TALSPEAK
* Also useful with DIDPA
 Selective actinide extraction with DTPA and 0.4 M NaNO3
* Ce/Am Df of 72

Recent efforts based on soft donor molecules
 Sulfur and nitrogen containing ligands
 Tripyridyltriazene (TPTZ) (C5H4N: pyridyl, (R-N:, azene)
and dinonylnapthalene sulfonic acid (HDNNS) in CCl4
and dilute nitric acid
* Preferential extraction of Am from trivalent
15-16
lanthanides
Am solvent extraction
• Am from lanthanides
 Initial work effected direction of further research
Focus on nitrogen and sulfur containing
ligands
* Thione (Phosphine SO), pyridenes,
thiophosphonic acid
 Research does not follow CHON principles
 Efforts with Cyanex 301 achieved
lanthanide/actinide separation in pH 3 solution
Bis (2,4,4-trimethylpentyl)dithiophosphinic
acid
15-17
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
15-18
15-19
Ion exchange
• Cation exchange

Am3+ sorbs to cation exchange resin in dilute acid
 Elution with a-hydroxylisobutyrate 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
15-20
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
15-21
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
15-22
Am metal and alloys
•
•
Preparation of Am metal

Reduction of AmF3 with Ba or Li

Reduction of AmO2 with La

Bomb reduction of AmF3 with Ca

Decomposition of Pt5Am
 1550 °C at 10-6 torr

La or Th reduction of AmO2 with distillation of Am
Metal properties

Ductile, non-magnetic

Double hexagonal closed packed (dhcp) and fcc

Evidence of three phase between room temperature and melting point at 1170
°C
 Alpha phase up to 658 °C
 Beta phase from 793 °C to 1004 °C
 Gamma above 1050 °C

Some debate in literature
 Evidence of dhcp to fcc at 771 °C

Interests in metal properties due to 5f electron behavior
 Delocalization under pressure
 Different crystal structures
* Conversion of dhcp to fcc
 Discrepancies between different experiments and theory
15-23
Am metal, alloys, and compounds
•
•
•
Alloys investigated with 23 different
elements
Phase diagrams available for Np, Pu,
and U alloys
Am compounds

Oxides and hydroxides
 AmO, Am2O3, AmO2
* Non-stoichiometric
phases between Am2O3
and AmO2
 AmO lattice parameters
varied in experiments
* 4.95 Å and 5.045 Å
* Difficulty in stabilizing
divalent Am
 Am2O3
* Prepared in H2 at 600
°C
* Oxidizes in air
* Phase transitions with
temperature
 bcc to
monoclinic
between 460 °C
and 650 °C
 Monoclinic to
hexagonal
between 800 °C
and 900 °C
15-24
Am compounds
•
Am oxides and hydroxides

AmO2
 Heating Am hydroxides, carbonates, oxalates, or nitrates in air
or O2 from 600 °C to 800 °C
 fcc lattice
* Expands due to radiation damage

Higher oxidation states can be stabilized
 Cs2AmO4 and Ba3AmO6

Am hydroxide
 Isostructural with Nd hydroxides
 Cystalline Am(OH)3 can be formed, but becomes amorphous due
to radiation damage
* Complete degradation in 5 months for 241Am hydroxide
 Am(OH)3+3H+,Am3++3H2O
* logK=15.2 for crystalline
* Log K=17.0 for amorphous

Am hydrolysis (from CHESS database)
 Am3++H2OAmOH2++H+: log K =-6.402
 Am3++2H2OAm(OH)2++2H+: log K =-14.11
 Am3++3H2OAm(OH)3+3H+: log K =-25.72
15-25
Solution absorption spectroscopy
•
Am(III)
7F 5L at 503.2 nm (e=410 L mol cm-1)

0
6

Shifts in band position and molar absorbance indicates changes in water or
ligand coordination

Solution spectroscopy compared to Am doped in crystals

Absorbance measured in acids and carbonate
15-26
Solution absorption spectroscopy
• Am(IV)
 In acidic media, broad absorption bands
 13 M HF, 12 M KF, 12 M H3PO4
 Resembles solid AmF4 spectrum
15-27
Solution absorption spectroscopy
• Am(V)
 5I43G5; 513.7 nm; 45 L mol cm-1
 5I43I7; 716.7 nm; 60 L mol cm-1
 Collected in acid, NaCl, and carbonate
15-28
Solution absorption spectroscopy
•
Am(VI)

996 nm; 100 L mol cm-1

Smaller absorbance at 666 nm
 Comparable to position in Am(V)
 Based on comparison with uranyl, permits analysis based on uranyl core with addition of electrons
15-29
Solution absorption spectroscopy
• Am(VII)

Broad absorbance at 740 nm
• Am(III) luminescence
7F 5L at 503 nm

0
6
 Then conversion to other excited state

Emission to 7FJ
5D 7F at 685 nm

1
1
5D 7F at 836 nm

1
2

Lifetime for aquo ion is 20 ns
 155 ns in D2O

Emission and lifetime changes with speciation
 Am triscarbonate lifetime = 34.5 ns, emission at 693 nm
15-30
15-31
Am spectroscopy
• Vibrational
 AmO2+
 Antisymmetric vibration in solids at 802 cm-1
 Raman of Am(III) phosphate
 Symmetric stretch of PO43- at 973 cm-1
 PO3- groups at 1195 cm-1
• X-ray absorption
 Absorption edge at 18504 eV
 4 eV difference between Am(IV) and Am(III)
15-32
Cm nuclear properties
• Isotopes from mass 237 to 251
 Three isotopes available in quantity for chemical
studies
 242Cm, t1/2=163 d
* 122 W/g
* Grams of the oxide glows
* Low flux of 241Am target decrease fission of
242Am, increase yield of 242Cm
 244Cm, t1/2=18.1 a
* 2.8 W/g
 248Cm, t1/2= 3.48E5 a
* 8.39% SF yield
* Limits quantities to 10-20 mg
* Target for production of transactinide
elements
15-33
Cm Production
• From successive neutron capture of higher Pu isotopes
242Pu+n243Pu (b-, 4.95 h)243Am+n244Am (b-, 10.1 h)244Cm


Favors production of 244,246,248Cm
 Isotopes above 244Cm to 247Cm are not isotopically pure
 Pure 248Cm available from alpha decay of 252Cf
• Large campaign to product Cm from kilos of Pu
• 244Cm separation

Dissolve target in HNO3 and remove Pu by solvent extraction

Am/Cm chlorides extracted with tertiary amines from 11 M LiCl
in weak acid
 Back extracted into 7 M HCl

Am oxidation and precipitation of Am(V) carbonate
• Other methods for Cm purification included NaOH, HDEHP, and
EDTA

Discussed for Am
15-34
Cm aqueous chemistry
• Trivalent Cm
• 242Cm at 1g/L will boil
• 9 coordinating H2O from fluorescence
 Decreases above 5 M HCl
 7 waters at 11 M HCl
 In HNO3 steady decrease from 0 to 13 M
 5 waters at 13 M
 Stronger complexation with NO3• Inorganic complexes similar to data for Am
 Many constants determined by TRLFS
• Hydrolysis constants (Cm3++H2OCmOH2++H+)
 K11=1.2E-6
 Evaluated under different ionic strength
15-35
15-36
Cm atomic and spectroscopic data
•
•
•
5f7 has enhanced stability

Half filled orbital
 Large oxidation potential for
IIIIV
 Cm(IV) is metastable
Cm(III) absorbance

Weak absorption in near-violet
region

Solution absorbance shifted 20-30 Å
compared to solid
 Reduction of intensity in solid
due to high symmetry
* f-f transitions are symmetry
forbidden

Spin-orbit coupling acts to reduce
transition energies when compared
to lanthanides
Cm(IV) absorbance

Prepared from dissolution of CmF4
 CmF3 under strong fluorination
conditions
15-37
Atomic and spectroscopic data
• Cm fluorescence
 Fluoresce from 595-613 nm
Attributed to 6D7/28S7/2 transition
Energy dependent upon coordination
environment
* Speciation
* Hydration
* complexation constants
15-38
Absorption and fluorescence process of Cm3+
Optical Spectra
Fluorescence Process
30
W av en um b er ( 10
3
cm
-1
)
H
G
F
Emissionless
Relaxation
20
A
7/2
Excitation
10
Fluorescence
Emission
15-39
0
Z
7/2
15-40
15-41
Cm separation and purification
• Solvent extraction

Fundamentally the same as Am

Organic phosphates
 Function of ligand structure
* Mixed with 6 to 8 carbon chain better than TBP

HDEHP
 From HNO3 and LiCl
* Use of membrane can result in Am/Cm separation

CMPO
 Oxidation state based removal with different stripping
agent

Extraction of Cm from carbonate and hydroxide solutions,
need to keep metal ions in solution
 Organics with quaternary ammonium bases, primary
amines, alkylpyrocatechols, b-diketones, phenols
15-42
Cm separations
• Ion exchange (similar to Am conditions)

Anion exchange with HCl, LiCl, and HNO3
 Includes aqueous/alcohol mixtures
 Formation of CmCl4- at 14 M LiCl
* From fluorescence spectroscopy

TEVA resins
 Same range of organic phases
• Precipitation

Separation from higher valent Am
 10 g/L solution in base
 Precipitation of K5AmO2(CO3)3 at 85 °C
 Precipitation of Cm with hydroxide, oxalate, or fluoride
15-43
Cm metallic state
• Melting point 1345 °C

Higher than lighter actinides Np-Am

Similar to Gd (1312 °C)
• Two states

Double hexagonal close-packed (dhcp)
 Neutron diffraction down to 5 K
 No structure change

fcc at higher temperature
• XRD studies on 248Cm
• Magnetic susceptibility studies

Antiferrimagnetic transition near 65 K
 200 K for fcc phase
• Metal susceptible to corrosion due to self heating

Formation of oxide on surface
15-44
Cm metallic state
•
•
Preparation of Cm metal

CmF3 reduction with Ba or Li
 Dry, O2 free, and above 1600 K

Reduction of CmO2 with Mg-Zn alloy in MgF2/MgCl2
Alloys

Cm-Pu phase diagram studied

Noble metal compounds
 CmO2 and H2 heated to 1500 K in Pt, Ir, or Rh
* Pt5Cm, Pt2Cm, Ir2Cm, Pd3Cm, Rh3Cm
15-45
Cm oxide compounds
• Cm2O3

Thermal decomposition of CmO2 at 600 °C and 10-4 torr

Mn2O3 type cubic lattice
 Transforms to hexagonal structure due to radiation
damage
 Monoclinic at 800 °C
• CmO2

Heating in air, thermal treatment of Cm loaded resin, heating
Cm2O3 at 600 °C under O2, heating of Cm oxalate

Shown to form in O2 as low as 400 °C
 Evidence of CmO1.95 at lower temperature

fcc structure

Magnetic data indicates paramagnetic moment attributed to
Cm(III)
 Need to re-evaluate electronic ground state in oxides
• Oxides

Similar to oxides of Pu, Pr, and Tb
 Basis of phase diagram

BaCmO3 and Cm2CuO4
 Based on high T superconductors
15-46
 Cm compounds do not conduct
Cm compounds
• Cm(OH)3

From aqueous solution, crystallized by aging in water

Same structure as La(OH)3; hexagonal
• Cm2(C2O4)3.10H2O

From aqueous solution

Stepwise dehydration when heated under He
 Anhydrous at 280 °C
 Converts to carbonate above 360 °C
* TGA analysis showed release of water (starting at 145 °C)
 Converts to Cm2O3 above 500 °C’
• Cm(NO3)3

Evaporation of Cm in nitric acid

From TGA, decomposition same under O2 and He
 Dehydration up 180 °C, melting at 400 °C

Final product CmO2
 Oxidation of Cm during decomposition
15-47
Review
• Production and purification of Am and Cm isotopes
 Suitable reactions
 Basis of separations from other actinides
• Formation of Am and Cm metallic state and properties
 Number of phases, melting points
• Compounds
 Range of compounds, limitations on data
• Solution chemistry
 Oxidation states
• Coordination chemistry
 Organic chemistry reactions
15-48
Questions
• Which Cm isotopes are available for chemical
studies?
• Describe the fluorescence process for Cm
 What is a good excitation wavelength?
• What methods can be use to separate Cm from
Am?
• How many states does Cm metal have? What
is its melting point?
• What are the binary oxides of Cm? Which will
form upon heating in normal atmosphere?
15-49
Questions
• What is the longest lived isotope of Am?
• Which Am isotope has the highest neutron induced
fission cross section?
• What are 3 ligands used in the separation of Am?
 What are the solution conditions?
• What column methods are useful for separating Am
from the lanthanides?
• Which compounds can be made by elemental reactions
with Am?
• What Am coordination compounds have been
produced?
• What is the absorbance spectra of Am for the different
oxidation states?
• How can Am be detected?
15-50
Pop Quiz
• How can high valent oxidation states of Am be
made?
• Why does Cm have fewer accessible oxidation
states than Am?
15-51