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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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