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
 Structure and coordination
chemistry
 Analytical Chemistry
7-1
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
7-2
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
http://www.webelements.com/webelements/elements/text/Np/redn.html
7-3
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
7-4
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 tertbutylhydrazine
selective of Np(VI)
reduction over Pu(IV)
7-5
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=45
Np(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->SO42NpO2+ forms cation-cation
complexes
7-6

Fe>In>Sc>Ga>Al
•
•
•
•
•
•
•
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
7-7
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
O
C
H3C
OH
N
H
7-8
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
7-9
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
HDEHP
7-10
Metallic Np
• First synthesis from NpF3 with Ba at 1473 K
• Current methods
 NpF4 with 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
7-11
•
•
•
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
7-12
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
7-13
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
7-14
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
7-15
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
7-16
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
7-17
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
CP
 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-, CH7-18
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
7-19
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
7-20
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
7-21
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
7-22
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
7-23