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
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Transcript 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
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
13-1
Neptunium nuclear properties
•
•
22 known Np isotopes
237Np longest lived
Neutron irradiation of U
* Consecutive neutron capture on 235U
* 238U(n,2n)237U237Np + 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
13-2
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
13-3
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
13-4
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)
13-5
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
13-6
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
13-7
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
13-8
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
13-9
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
13-10
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
13-11
•
•
•
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
ab
* Transition T 553 K
* ΔS=10.1 JK-1mol-1
* ΔH=5.607 kJmol-1
bg
* Transition T 856 K
* ΔS=6.23 JK-1mol-1
* ΔH=5.272 kJmol-1
13-12
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
13-13
Np halides
•
Fluorides
NpF3, NpF4, NpF5, and NpF6
Prepared from reactions with HF at 773 K
NpO2+1/2H2+3HFNpF3 + 2H2O
NpF3+1/4O2+HF NpF4 + 1/2H2O
NpO2+4HFNpF4 + 2H2O
10NpF6+I210NpF5+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+3NaFNa3NpF8 + 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
13-14
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
13-15
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
13-16
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
13-17
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+4KC5H5Np(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-, CH13-18
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
13-19
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
13-20
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
13-21
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
13-22
Question
• Respond to PDF Quiz 13
• Post comments on the blog
http://rfssunlv.blogspot.com/
13-23