CHEM 312: Lecture 15 Americium and Curium
Chemistry Part 1
• Readings: Am and Cm
chemistry chapters
 Link on web page
• Combined due to similar
chemical properties of elements
 Cover Am then Cm
• Nuclear properties
• Production of isotopes
• Separation and purification
• Metallic state
• Compounds
• Solution chemistry
• Coordination chemistry
15-1
Production of Am isotopes
•
•
•
•
•
Am produced in reactors 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
7
5

F0 L6 at 503.2 nm (e=410 L mol cm-1)

Shifts in band position and molar absorbance
indicates changes in water or ligand coordination

9 to 11 inner sphere waters
 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
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
5

I43G5; 513.7 nm; 45 L mol cm-1
5I 3I ; 716.7 nm; 60 L mol cm-1

4
7
Am(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 V

996 nm; 100 L mol cm-1
 Smaller absorbance at 666 nm
Am(VII)

3-4 M NaOH, mM Am(VI) near 0 °C

Gamma2-irradiation 3 M NaOH with N2O
or S2O8 saturated solution
Am(VII)

Broad absorbance at 740 nm
15-4
Am solution chemistry
•
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
5
7

D1 F2 at 836 nm

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
•
•
•
•
•
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)
15-5
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
Redox kinetics

Am(III) oxidation by peroxydisulfate
 Oxidation due to thermal
decomposition products
* SO4.-, HS2O8 Oxidation to Am(VI)
 Acid above 0.3 M limits oxidation
* Decomposition of S2O82 Induction period followed by reduction
 Rates dependent
upon temperature,
[HNO3], [S2O82-], and [Ag+2]
 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
15-6
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
15-7
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 formation of HNO2

In perchlorate numerous species
produced
 Cl2, ClO2, or ClComplexation 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
15-8
Am solution
chemistry
•
•
Hydrolysis

Mono-, di-, and trihydroxide
species

Am(V) appears to have 2 species,
mono- and dihydroxide

Am hydrolysis (from CHESS
database)
 Am3++H2OAmOH2++H+:
log K =-6.402
 Am+3++2H2OAm(OH)2++
2H : log K =-14.11
3++3H OAm(OH) +3
 Am
3
+
H : log K2 =-25.72
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
1mM Am3+;
1 mM Am, 1 mM carbonate
15-9
Am solution chemistry: Organics
• Number of complexes examined

Mainly for Am(III)
• Generally stability of complex
increases with coordination sites
• 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
15-10
Am solvent extraction
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•
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Tributylphosphate (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 HNO3
* Separation factor 50 from Nd

Am separation from lanthanides
 1 M ammonium thiocyanate aqueous phase
Dibutyl butylphosphonate (DBBP)

Phosphonate functional group

Similar to TBP, stronger extractant of Am
Trialkylphophine oxide (TRPO)

Increase in basicity of P=O functional group from TBP to DPPB to
TRPO

Am and Cm extraction from 1-2 M HNO3

30 % TRPO in kerosene
 Am, Cm, tetravalent Np and Pu, hexavalent U extracted
* Actinides stripped with 5.5 M HNO3 (Am fraction)
 TRPO with C6-C8 alkyl group
15-11
Am solvent extraction
•
•
Bis(2-ethylhexyl)phosphoric acid (HDEHP)

Has been used to Am separation

Part of TALSPEAK
 Extracts lanthanides stronger that actinides
 TALSPEAK components
HDEHP
* Bis(2-ethyl-hexyl)phosphoric acid (HDEHP)
* HNO3
* DTPA
* Lactic acid
Carbamoylphosphine oxide (CMPO)

Synthesized by Horwitz
 Based on DHDECMP extractions
* Recognized functional group, simplified ligand synthesis
* Purified by cation exchange

Part of TRUEX
 TRUEX (fission products)
* 0.01 to 7 M HNO3
* 1.4 M TBP
* 0.2 M Diphenyl-N,N-dibutylcarbamoyl phosphine oxide (CMPO)
* 0.5 M Oxalic acid
* 1.5 M Lactic acid
* 0.05 M DTPA
15-12
CMPO
Am solvent extraction
• Tertiary amine salt

Low acid, high nitrate or chloride solution
 (R3NH)2Am(NO3)5
• Quaternary ammonium salts (Aliquat 336)

Low acid, high salt solutions
 Extraction sequence of Cm<Cf<Am<Es

Studies at ANL for process separation of Am
• Amide extractants

(R1,R2)N-C(O)-CR3H-C(O)-N(R1R2)
 Diamide extractant
 Basis of DIAMEX process

N,N’-dimethyl-N,N’-dibutyl-2-tetradecyl-malonamide
(DMDBTDMA)
 DIAMEX with ligand in dodecane with 3-4 M HNO3
* Selective extraction over Nd
15-13
•
•
•
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Am/Ln solvent extraction
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
Distribution ratios of Am(III ) and Ln(III ) in 1.0 M
Cyanex 301‐heptane (16 mol% of Cyanex 301
neutralized before extraction contacts)
15-14
•
•
•
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Ion exchange separation Am
from
Cm
LiCl with ion exchange achieves separation from
lanthanide
Separation of tracer level Am and Cm has been
performed with displacement complexing
chromatography

DTPA and nitrilotriacetic acid in presence of
Cd and Zn as competing cations

displacement complexing chromatography
method is not suitable for large scale
Ion exchange has been used to separate trace levels of
Cm from Am

Am, Cm, and lanthanides sorbed to a cation
exchange resin at pH 2
 Separation of Cm from Am was
performed with 0.01 % ethylenediaminetetramethylphosphonic acid at pH 3.4 in
0.1 M NaNO3
 separation factor of 1.4
Separation of gram scale quantities of Am and Cm by
cation and anion exchange

use of a-hydroxylisobutyrate or
diethylenetriaminepentaacetic acid as an
eluting agent or a variation of eluant
composition by addition of methanol to nitric
acid
 best separations were achieved under
high pressure conditions
* separation factors greater than 400
Distribution
coefficients of actinides and
lanthanides into Dowex 1 8 resin
15-15
from 10 M LiCl
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 (Eichrom)
* 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
15-16
Am separation and purification
• 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)
• 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, volatility separation
15-17
CHEM 312: Lecture 15 Americium and Curium
Chemistry
• Readings: Am and Cm
chemistry chapters
 Link on web page
• Combined due to similar
chemical properties of elements
 Cover Am then Cm
• Nuclear properties
• Production of isotopes
• Separation and purification
• Metallic state
• Compounds
• Solution chemistry
• Coordination chemistry
15-18