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Americium and Curium Chemistry

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. Production of Am isotopes.

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Americium and Curium Chemistry

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  1. 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

  2. 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

  3. Am solution chemistry • Oxidation states III-VI in solution • Am(III,V) stable in dilute acid • Am(V, VI) form dioxocations • 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, AmO2dissolution) • 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

  4. 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 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 • Am(VII) • 3-4 M NaOH, mM Am(VI) near 0 °C • Gamma irradiation 3 M NaOH with N2O or S2O82- saturated solution

  5. 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 H2Oand D2O

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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

  11. 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 Cl-

  12. 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

  13. 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(2-pyridylmethyl)ethyleneamine) • 0.1 M NaClO4, complexation constant for Am 2 orders greater than Sm

  14. Am(IV) solution chemistry • Am(IV) can be stabilized by heteropolyanions • P2W17O61anion; 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. 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)

  16. 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 Dfof 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 lanthanides

  17. 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

  18. 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

  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

  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

  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

  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

  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

  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

  25. Solution absorption spectroscopy • Am(III) • 7F05L6 at 503.2 nm (e=410 L mol cm-1) • 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

  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

  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

  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

  29. Solution absorption spectroscopy • Am(VII) • Broad absorbance at 740 nm • Am(III) luminescence • 7F05L6 at 503 nm • Then conversion to other excited state • Emission to 7FJ • 5D17F1 at 685 nm • 5D17F2 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

  30. 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)

  31. 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

  32. 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 247Cmare 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

  33. 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

  34. 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

  35. 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

  36. Absorption and fluorescence process of Cm3+ Optical Spectra Fluorescence Process H G F Emissionless Relaxation A 7/2 Excitation Fluorescence Emission Z 7/2

  37. 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

  38. 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

  39. 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

  40. 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

  41. Cm oxide compounds • Cm2O3 • Thermal decomposition of CmO2 at 600 °C and 10-4torr • 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 • Cm compounds do not conduct

  42. 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

  43. 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

  44. 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?

  45. 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?

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