15-1 RFSS: Lecture 15 Americium and Curium Chemistry Part 1 Readings: Am and Cm chemistry chapters...
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15-1 RFSS: 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 RFSS: Lecture 15 Americium and Curium Chemistry Part 1 Readings: Am and Cm chemistry chapters §Link on web page Combined due to similar chemical
15-1 RFSS: 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
Slide 3
15-2 Production of Am isotopes Am produced in reactors from
neutron irradiation of Pu 239 Pu to 240 Pu to 241 Pu, then beta
decay of 241 Pu 241,243 Am main isotopes of interest Long
half-lives Produced in kilogram quantity Chemical studies Both
isotopes produced in reactor 241 Am source for low energy gamma and
alpha Alpha energy 5.44 MeV and 5.49 MeV Smoke detectors Neutron
sources ( ,n) on Be Thickness gauging and density 242 Cm production
from thermal neutron capture 243 Am Irradiation of 242 Pu, beta
decay of 243 Pu Critical mass 242 Am in solution 23 g at 5 g/L
Requires isotopic separation
Slide 4
15-3 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 *T 1/2 of oxidation state 5E-6
seconds Am(III) Easy to prepare (metal dissolved in acid, AmO 2
dissolution) Pink in mineral acids, yellow in HClO 4 when Am is 0.1
M 7 F 0 5 L 6 at 503.2 nm ( =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 n H2O =(x/ )-y
x=2.56E-7 s, y=1.43 Measurement of fluorescence lifetime in H 2 O
and D 2 O Am(IV) Requires complexation to stabilize dissolving
Am(OH) 4 in NH 4 F Phosphoric or pyrophosphate (P 2 O 7 4- )
solution with anodic oxidation Ag 3 PO 4 and (NH 4 ) 4 S 2 O 8
Carbonate solution with electrolytic oxidation
Slide 5
15-4 Am solution chemistry Am(V) Oxidation of Am(III) in near
neutral solution Ozone, hypochlorate (ClO - ), peroxydisulfate
Reduction of Am(VI) with bromide 5 I 4 3 G 5 ; 513.7 nm; 45 L mol
cm -1 5 I 4 3 I 7 ; 716.7 nm; 60 L mol cm -1 Am(VI) Oxidation of
Am(III) with S 2 O 8 2- or Ag 2+ 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
Gamma irradiation 3 M NaOH with N 2 O or S 2 O 8 2- saturated
solution Am(VII) Broad absorbance at 740 nm
Slide 6
15-5 Am solution chemistry Am(III) luminescence 7 F 0 5 L 6 at
503 nm Then conversion to other excited state Emission to 7 F J 5 D
1 7 F 1 at 685 nm 5 D 1 7 F 2 at 836 nm Lifetime for aquo ion is 20
ns 155 ns in D 2 O Emission and lifetime changes with speciation Am
triscarbonate lifetime = 34.5 ns, emission at 693 nm Autoreduction
Formation of H 2 O 2 and HO 2 radicals from radiation reduces Am to
trivalent states Difference between 241 Am and 243 Am 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)
Slide 7
15-6 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 HClO 4 and HCl *3 Am(V) + 4 H +
Am(III)+2Am(VI)+2 H 2 O Solution can impact oxidation state
stability Redox kinetics Am(III) oxidation by peroxydisulfate
Oxidation due to thermal decomposition products *SO 4.-, HS 2 O 8 -
Oxidation to Am(VI) Acid above 0.3 M limits oxidation
*Decomposition of S 2 O 8 2- Induction period followed by reduction
Rates dependent upon temperature, [HNO 3 ], [S 2 O 8 2- ], 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 K 2 CO 3
Slide 8
15-7 Am solution chemistry: Redox kinetics Am(VI) reduction H 2
O 2 in perchlorate is 1 st order for peroxide and Am 2 AmO 2 2+ +H
2 O 2 2 AmO 2 + + 2 H + + O 2 NpO 2 + 1 st 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 (Na 2 S 2 O 4 ), sulfite
(SO 3 2- ), or thiourea dioxide ((NH 2 ) 2 CSO 2 ) 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
Slide 9
15-8 Am solution chemistry Radiolysis From alpha decay 1 mg 241
Am release 7E14 eV/s Reduction of higher valent Am related to dose
and electrolyte concentration In nitric acid formation of HNO 2 In
perchlorate numerous species produced Cl 2, ClO 2, or Cl -
Complexation chemistry Primarily for Am(III) F - >H 2 PO 4 -
>SCN - >NO 3 - >Cl - >ClO 4 - 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
Slide 10
15-9 Am solution chemistry Hydrolysis Mono-, di-, and
trihydroxide species Am(V) appears to have 2 species, mono- and
dihydroxide Am hydrolysis (from CHESS database) Am 3+ +H 2 O AmOH
2+ +H + : log K =-6.402 Am 3+ +2H 2 O Am(OH) 2 + + 2H + : log K
=-14.11 Am 3+ +3H 2 O Am(OH) 3 +3 H + : log K =-25.72 Carbonate
Evaluated by spectroscopy Includes mixed species Am hydroxide
carbonate species Based on solid phase analysis Am(IV)
Pentacarbonate studied (log =39.3) Am(V) solubility examined 1mM Am
3+ ; 1 mM Am, 1 mM carbonate
Slide 11
15-10 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(2-
pyridylmethyl)ethyleneamine) 0.1 M NaClO 4, complexation constant
for Am 2 orders greater than Sm
Slide 12
15-11 Am solvent extraction Tributylphosphate (TBP) Am
extracted from neutral or low acid solutions with high nitrate
Am(VI) Oxidation with (NH 4 ) 10 P 2 W 17 O 61 to stabilize Am(VI)
100 % TBP from 1 M HNO 3 *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 HNO 3 30 % TRPO in
kerosene Am, Cm, tetravalent Np and Pu, hexavalent U extracted
*Actinides stripped with 5.5 M HNO 3 (Am fraction) TRPO with C 6 -C
8 alkyl group
Slide 13
15-12 HDEHP 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
*Bis(2-ethyl-hexyl)phosphoric acid (HDEHP) *HNO 3 *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 HNO 3 *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 CMPO
Slide 14
15-13 Am solvent extraction Tertiary amine salt Low acid, high
nitrate or chloride solution (R 3 NH) 2 Am(NO 3 ) 5 Quaternary
ammonium salts (Aliquat 336) Low acid, high salt solutions
Extraction sequence of Cm