1 / 23

RFSS: Lecture 13 Neptunium Chemistry

RFSS: Lecture 13 Neptunium Chemistry. From: Chemistry of actinides http://radchem.nevada.edu/classes/rdch710/lectures%20and%20chapters.html Nuclear properties and isotope production Aqueous phase chemistry Separation and Purification Metallic state Compounds

faxon
Download Presentation

RFSS: Lecture 13 Neptunium Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. RFSS: Lecture 13 Neptunium Chemistry • From: Chemistry of actinides • http://radchem.nevada.edu/classes/rdch710/lectures%20and%20chapters.html • Nuclear properties and isotope production • Aqueous phase chemistry • Separation and Purification • Metallic state • Compounds • Structure and coordination chemistry • Analytical Chemistry

  2. Neptunium nuclear properties • 22 known Np isotopes • 237Np longest lived • Neutron irradiation of U • Consecutive neutron capture on 235U • 238U(n,2n)237U237Np + b- • Alpha decay of 241Am • Used at target for 238Pu production by neutron irradiation • Reaction with 23 MeV and 30 MeV electrons to produce 236Pu • Critical mass is 73 kg • 2500 kg in environment from fallout • 238,239Np • Short half-life, useful radiotracers • From neutron irradiation of 237Np and 238U • 235,236Np • Cyclotron irradiation of 235U • 235U(d,n)236Np • 235U(p,n)235Np • Np isotopes formed in Earth’s crust • Dynamic equilibrium established

  3. http://www.webelements.com/webelements/elements/text/Np/redn.htmlhttp://www.webelements.com/webelements/elements/text/Np/redn.html Np solution chemistry and oxidation states • Np exists from 3+ to 7+ • Stable oxidation state favored by acidity, ligands, Np concentration • 5+ and 6+ forms dioxocations • Redox potentials • Basic solutions • Difficulty in understanding data • Chemical forms of species • Determine ratios of each redox species from XANES • Use Nernst equation to determine potentials

  4. Np solution chemistry • Disproportionation • NpO2+ forms Np4+ and NpO22+ • Favored in high acidity and Np concentration • 2NpO2+ +4 H+Np4+ + NpO22+ + 2H2O • K for reaction increased by addition of complexing reagents • K=4E-7 in 1 M HClO4 and 2.4E-2 in H2SO4 • Suggested reaction rate • -d[NpO2+]/dt=k[NpO2+][H+]2 • Control of redox species • Important consideration for experiments

  5. Np solution chemistry • Oxidation state control • Redox reagents • Adjustment from one redox state to another • Best for reversible couples • No change in oxo group • If oxo group change occurs need to know kinetics • Effort in PUREX process for controlled separation of Np focused on organics • HAN and derivates for Np(VI) reduction • Rate 1st order for Np in excess reductant • 1,1 dimethylhydrazine and tert-butylhydrazine selective of Np(VI) reduction over Pu(IV)

  6. Np solution chemistry • Applied to Np(III) to Np(VII) and coordination complexes • Np(V) spin-orbit coupling for 5f2 • Absorption in 2 M HClO4 • Np(III): 786 nm, e=45Np(IV): 960 nm, e=160 • Np(V): 980 nm, e=395 • Np(VI): 1223 nm, e=45 • Np(VII) only in basic media • NpO65- • 2 long (2.2 Å) and 4 short (1.85 Å) • Absorbance at 412 nm and 620 nm • O pi 5f • Number of vibrational states • Between 681 cm-1 and 2338 cm-1 • Range of complexation constants available • Oxidation state trends same as hydrolysis • Stability trends for inorganic • F->H2PO4->SCN->NO3->Cl->ClO4- • CO32->HPO42->SO42- • NpO2+ forms cation-cation complexes • Fe>In>Sc>Ga>Al

  7. Np solution chemistry • Np hydrolysis • Np(IV)>Np(VI)>Np(III)>Np(V) • For actinides trends with ionic radius • Np(III) • below pH 4 • Stable in acidic solution, oxidizes in air • Potentiometric analysis for determining K • No Ksp data • Np(IV) • hydrolyzes above pH 1 • Tetrahydroxide main solution species in equilibrium with solid based on pH independence of solution species concentration • Np(V) • not hydrolyzed below pH 7 • Np(VI) • below pH 3-4 • Np(VII) • No data available • Most separation methods exploit redox chemistry of Np

  8. PUREX separations • Np(V) not extracted in PUREX • Np(V) slowly disproportionates in high acid • Formation of extractable Np(IV,VI) • Variation of Np behavior based on redox • Need to understand redox kinetics • Reduction of Np(VI) by a range of compounds • Back extraction of Np(V) can be used to separate from Pu and U • Controlled Np(VI) reduction in presence of Pu(III) • Hydrazine derivatives • N-butyraldehyde • Hydroxamic acids • Acetohydroxamic acid shows preferential complexation with tetravalent Np and Pu

  9. Np solvent extraction • Tributylphosphate • NpO2(NO3)2(TBP)2 and Np(NO3)4(TBP)2 are extracted species • Extraction increases with increase concentration of TBP and nitric acid • 1-10 M HNO3 • Separation from other actinides achieved by controlling Np oxidation state • CMPO (Diphenyl-N,N-dibutylcarbamoyl phosphine oxide) • Usually used with TBP • Nitric acid solutions • Separation achieved with oxidation state adjustment • Reduction of Pu and Np by Fe(II) sulfamate • Np(IV) extracted into organic, then removed with carbonate, oxalate, or EDTA

  10. HDEHP Np solvent extraction • HDEHP (Bis(2-ethyl-hexyl)phosphoric acid ) • In 1 M HNO3 with addition of NaNO2 • U, Pu, Np, Am in most stable oxidation states • Np(V) is not extracted • Oxidized to Np(VI) then extracted • Reduced to Np(V) and back extracted into 0.1 M HNO3 • Tri-n-octylamine • Used for separation of Np from environmental samples • Extracted from 10 M HCl • Back extracted with 1 M HCl+0.1 M HF

  11. Metallic Np • First synthesis from NpF3 with Ba at 1473 K • Current methods • NpF4with excess Ca • NpO2 in a molten salt process • Can also use Cs2NpO2Cl4 and Cs3NpO2Cl4 • LiCl/KCl as electrolyte at 723 K • NpC reduction with Ta followed by volatilization of Np • Electrodepostion from aqueous solution • Amalgamation with Hg from 1 M CH3COOH and 0.3 M CH3COONa at pH 3.5 • Distillation to remove Hg

  12. Metallic Np data • Melting point 912 K • Boiling point estimated at 4447 K • Density 19.38 g/mL • Three metallic forms • Enthalpies and entropies of transitions • ab • Transition T 553 K • ΔS=10.1 JK-1mol-1 • ΔH=5.607 kJmol-1 • bg • Transition T 856 K • ΔS=6.23 JK-1mol-1 • ΔH=5.272 kJmol-1

  13. Neptunium oxides • Two known anhydrous oxides • Np2O5 and NpO2 • NpO2 • From thermal decomposition of a range of Np compounds • Isostructural with other actinides • Fluorite lattice parameter • Stable over a range of temperatures • Phase change from fcc to orthorhombic at 33 GPa • Stable to 2.84 MPa and 673 K • Np2O5 • From thermal decomposition of NpO3.H2O or NpO2OH(am) • Np2O5 decomposes to NpO2 from 693 K to 970 K

  14. Np halides • Fluorides • NpF3, NpF4, NpF5, and NpF6 • Prepared from reactions with HF at 773 K • NpO2+1/2H2+3HFNpF3 + 2H2O • NpF3+1/4O2+HF NpF4 + 1/2H2O • NpO2+4HFNpF4 + 2H2O • 10NpF6+I210NpF5+2IF5 • Other route where Np(VI) is reduced • NpF6 is volatile • Melting point at 327.8 K • Higher vapor pressure that U and Pu compound • Can form Np(V) species upon reaction with NaF • NpF6+3NaFNa3NpF8 + 1/2F2 • U will stay as hexavalent compound • Range of monovalent species with Np fluorides • Synthesis similar to U compound • NpO2F2 intermediate species • KrF2 used as fluorinating agent for some synthetic routes

  15. Np halides • NpCl4 • From the reaction of NpO2 with CCl4 • Addition of H2 yields NpCl3 • Similar to U reactions • Several melting point reported • Heating for NpOCl2 • NpBr4 • NpO2 with AlBr3 • Reaction of elements • Same for AlI3 for NpI3 • Synthesis reactions similar to U species • Measured data on Np compounds limited

  16. Np coordination compounds • Interests driven from different Np oxidation states and systematic studies of actinides • Np3+ • Very little data • Instable in aqueous solutions under air • Trivalent state stabilized by sodium formaldehyde sulfoxylate (NaHSO2.CH2O.2H2O) • Formation of oxalate and salicylate species • 2 Np, 3 ligands • No O2 in synthesis • Np4+ • Et4NNp(NCS)8 • Isostructural with U complex • Range of nitrate compounds • Np(V) • Exhibit cation-cation interaction • Na4(NpO4)2C12O12 • Dissolve neptunium hydroxide in solution with mellitic acid • Adjust to pH 6.5 with base • Slowly evaporate

  17. Np coordination compounds • Np(VI) • Some simple synthesis • Oxalic acid to Np(VI) solutions • Reduction of Np over time • Ammonium carbonate species • Excess (NH4)2CO3 to nitrate solutions of Np(VI) • Np(VII) • Some disagreement on exact species • Mixed species with Co, Li, NH3 and OH

  18. CP Np Organometallic compounds • Mainly cyclopentadienyl and cyclooctatetraenyl compounds • Np cyclopentadienyl • Reduction of Np4+ complex with Na • Np(C5H5)3Cl + Na  Np(C5H5)3.3THF + NaCl • Difficult to remove THF • Heating and vacuum • Np4+ • NpCl4+4KC5H5Np(C5H5)4+4KCl • Dissolves in benzene and THF • Less sensitive to H2O and O2 than tetravalent Pu and Am compound • Halide salt of Np compound reported • NpX4 + 3 KC5H5Np(C5H5)3X+3KX • Can use as starting material and replace X with ligands • Inorganic (other halides); NC4H4-, N2C3H3-, CH-

  19. Analytical methods • Environmental levels • General levels 1E-15 g/L • Elevated levels up to 1E-11 g/L • Radiometric methods • Alpha • 2.6E7 Bq/g • Isolation from seawater • Hydroxide co-precipitation, ion-exchange, LaF3, solvent extraction with HTTA • Liquid scintillation • Activation analysis • Formation of 238Np • 170 barns, 2.117 day half life for 238Np • 500 more sensitive than alpha spectroscopy

  20. Analytical methods • Spectrophotometric methods • Direct absorbance • Detection limit in M (1 cm cell, 0.02 absorbance) • Np(III) 5E-4, Np(IV) 1E-4, Np(V) 5E-5, Np(VI) 5E-4 • Laser induced photoacoustic spectroscopy (LIPAS) • Increase factor by over an order of magnitude • Indicator dyes • Fluorescence • New work in tetrachlorides and solids • Luminescence at 651 nm and 663 nm from Np in CaF2 at 77 K • X-ray fluorescence • Mass spectroscopy

  21. Analytical methods: 237Np Moessbauer spectroscopy • 68 ns excited state lifetime • Isomer shift suitable for analysis of chemical bonds • Can record radiation spectrum from absorber • 60 keV from 241Am • Shift correlated with oxidation state and number of 5f electrons present

  22. Review • Oxidation states of Np in solution • Role of different oxidation states in separations • Np separations • Distribution with ligands in solvent extraction • Synthesis of Np metal • Np oxides and fluorides • Coordination and organometallic compounds • Analytical methods

  23. Homework question • What are the forms of solid binary neptunium oxides compounds? • Provide comments on blog • Bring to next class or submit by e-mail

More Related