7-1

RFSS: Lecture 13 Neptunium Chemistry

• From: Chemistry of actinidesà 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

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Neptunium nuclear properties• 22 known Np isotopes

§ 237Np longest livedà Neutron irradiation of U

* Consecutive neutron capture on 235U* 238U(n,2n)237U237Np + b-

* Alpha decay of 241Amà Used at target for 238Pu production by neutron irradiationà Reaction with 23 MeV and 30 MeV electrons to produce 236Puà Critical mass is 73 kgà 2500 kg in environment from fallout

§ 238,239Npà Short half-life, useful radiotracers

* From neutron irradiation of 237Np and 238U§ 235,236Np

à Cyclotron irradiation of 235U* 235U(d,n)236Np* 235U(p,n)235Np

• Np isotopes formed in Earth’s crust§ Dynamic equilibrium established

7-3

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 solutionsà Difficulty in

understanding dataà Chemical forms of

species§ Determine ratios of each

redox species from XANESà Use Nernst equation

to determine potentials

http://www.webelements.com/webelements/elements/text/Np/redn.html

7-4

Np solution chemistry

• Disproportionation§ NpO2

+ forms Np4+ and NpO22+

à Favored in high acidity and Np concentration§ 2NpO2

+ +4 H+Np4+ + NpO22+ + 2H2O

§ K for reaction increased by addition of complexing reagentsà K=4E-7 in 1 M HClO4 and 2.4E-2 in H2SO4

* Suggested reaction rateØ -d[NpO2

+]/dt=k[NpO2+][H+]2

• Control of redox species§ Important consideration for experiments

7-5

Np solution chemistry• Oxidation state control

§ Redox reagentsà Adjustment from one

redox state to another à Best for reversible couples

* No change in oxo group

* If oxo group change occurs need to know kinetics

à 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 reductantà 1,1 dimethylhydrazine

and tert-butylhydrazine selective of Np(VI) reduction over Pu(IV)

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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§ NpO6

5- à 2 long (2.2 Å) and 4

short (1.85 Å) à Absorbance at 412 nm

and 620 nm* O pi 5f* Number of

vibrational statesØ 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->HPO4

2->SO42-

• NpO2+ forms cation-cation

complexes§ Fe>In>Sc>Ga>Al

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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

à 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

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PUREX separations• Np(V) not extracted in PUREX

§ Np(V) slowly disproportionates in high acidà Formation of extractable Np(IV,VI)à Variation of Np behavior based on redox

* Need to understand redox kinetics* Reduction of Np(VI) by a range of compounds

à Back extraction of Np(V) can be used to separate from Pu and U* Controlled Np(VI) reduction in presence of Pu(III)

Ø Hydrazine derivatives Ø N-butyraldehyde Ø Hydroxamic acidsØ Acetohydroxamic acid shows preferential

complexation with tetravalent Np and Pu

H3C

C

N

OH

O

H

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Np solvent extraction

• Tributylphosphate § NpO2(NO3)2(TBP)2 and

Np(NO3)4(TBP)2 are extracted speciesà Extraction increases with

increase concentration of TBP and nitric acid* 1-10 M HNO3

à 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 adjustmentà Reduction of Pu and Np by

Fe(II) sulfamateà Np(IV) extracted into organic,

then removed with carbonate, oxalate, or EDTA

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Np solvent extraction

• HDEHP (Bis(2-ethyl-hexyl)phosphoric acid )§ In 1 M HNO3 with addition of NaNO2

à U, Pu, Np, Am in most stable oxidation statesà Np(V) is not extractedà Oxidized to Np(VI) then extractedà Reduced to Np(V) and back extracted into 0.1 M HNO3

• Tri-n-octylamine§ Used for separation of Np from environmental samples

à Extracted from 10 M HClà Back extracted with 1 M HCl+0.1 M HF

HDEHP

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Metallic Np

• First synthesis from NpF3 with Ba at 1473 K• Current methods

§ NpF4 with excess Ca

§ NpO2 in a molten salt processà Can also use Cs2NpO2Cl4 and Cs3NpO2Cl4

à LiCl/KCl as electrolyte at 723 K§ NpC reduction with Ta followed by volatilization of

Np§ Electrodepostion from aqueous solution

à Amalgamation with Hg from 1 M CH3COOH and 0.3 M CH3COONa at pH 3.5

à Distillation to remove Hg

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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 à ab

* Transition T 553 K* ΔS=10.1 JK-1mol-1

* ΔH=5.607 kJmol-1

à bg* Transition T 856 K* ΔS=6.23 JK-1mol-1

* ΔH=5.272 kJmol-1

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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 GPaà 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

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Np halides

• Fluorides§ NpF3, NpF4, NpF5, and NpF6

§ Prepared from reactions with HF at 773 Kà NpO2+1/2H2+3HFNpF3 + 2H2Oà NpF3+1/4O2+HF NpF4 + 1/2H2Oà NpO2+4HFNpF4 + 2H2Oà 10NpF6+I210NpF5+2IF5

* Other route where Np(VI) is reduced§ NpF6 is volatile

à Melting point at 327.8 K* Higher vapor pressure that U and Pu compound

à Can form Np(V) species upon reaction with NaF* NpF6+3NaFNa3NpF8 + 1/2F2

Ø U will stay as hexavalent compoundØ Range of monovalent species with Np fluorides

à Synthesis similar to U compoundà NpO2F2 intermediate speciesà KrF2 used as fluorinating agent for some synthetic routes

7-15

Np halides

• NpCl4

§ From the reaction of NpO2 with CCl4

à Addition of H2 yields NpCl3

à Similar to U reactions§ Several melting point reported

à Heating for NpOCl2

• NpBr4

§ NpO2 with AlBr3

§ Reaction of elementsà Same for AlI3 for NpI3

• Synthesis reactions similar to U species• Measured data on Np compounds limited

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Np coordination compounds• Interests driven from different Np oxidation states and systematic studies of

actinides• Np3+

§ Very little dataà Instable in aqueous solutions under air

§ Trivalent state stabilized by sodium formaldehyde sulfoxylate (NaHSO2

.CH2O.2H2O)à Formation of oxalate and salicylate species

* 2 Np, 3 ligands* No O2 in synthesis

• Np4+

§ Et4NNp(NCS)8

à Isostructural with U complex§ Range of nitrate compounds

• Np(V)§ Exhibit cation-cation interaction§ Na4(NpO4)2C12O12

à Dissolve neptunium hydroxide in solution with mellitic acidà Adjust to pH 6.5 with baseà Slowly evaporate

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Np coordination compounds

• Np(VI)§ Some simple synthesis

à Oxalic acid to Np(VI) solutions* Reduction of Np over time

à Ammonium carbonate species* Excess (NH4)2CO3 to nitrate solutions of

Np(VI)• Np(VII)

§ Some disagreement on exact speciesà Mixed species with Co, Li, NH3 and OH

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Np Organometallic compounds

• Mainly cyclopentadienyl and cyclooctatetraenyl compounds• Np cyclopentadienyl

§ Reduction of Np4+ complex with Na§ Np(C5H5)3Cl + Na Np(C5H5)3

.3THF + NaClà Difficult to remove THF

* Heating and vacuum§ Np4+

§ NpCl4+4KC5H5Np(C5H5)4+4KClà Dissolves in benzene and THF

* Less sensitive to H2O and O2 than tetravalent Pu and Am compound

à Halide salt of Np compound reported* NpX4 + 3 KC5H5 Np(C5H5)3X +3KX* Can use as starting material and replace X with ligands

Ø Inorganic (other halides); NC4H4-, N2C3H3

-, CH-

CP

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Analytical methods

• Environmental levels§ General levels 1E-15 g/L§ Elevated levels up to 1E-11 g/L

• Radiometric methods§ Alpha

à 2.6E7 Bq/gà Isolation from seawater

* Hydroxide co-precipitation, ion-exchange, LaF3, solvent extraction with HTTA

§ Liquid scintillation§ Activation analysis

à Formation of 238Np* 170 barns, 2.117 day half life for 238Np* 500 more sensitive than alpha spectroscopy

7-20

Analytical methods

• Spectrophotometric methods§ Direct absorbance

à 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

à Laser induced photoacoustic spectroscopy (LIPAS)à Increase factor by over an order of magnitude

§ Indicator dyes§ Fluorescence

à New work in tetrachlorides and solidsà Luminescence at 651 nm and 663 nm from Np in CaF2

at 77 K• X-ray fluorescence• Mass spectroscopy

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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

7-22

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

7-23

Homework question

• What are the forms of solid binary neptunium oxides compounds?

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