Recent Advances in the Aqueous Chemistry and Thermodynamics of Actinide · · 2019-04-26Chemistry and Thermodynamics of Actinide Thomas Fanghänel. Merck Actinides-14 elements following

Physikalisch-Chemisches Institut

Forschungszentrum Karlsruhein der Helmholtz-Gemeinschaft

Institut für Nukleare Entsorgung

Recent Advances in the Aqueous Chemistry and Thermodynamics

of Actinide

Thomas Fanghänel

Merck

Actinides

-14 elements following the element Actinium-5f-Elektrons-formal Analogy to the Lanthanides-Transuranium Elements artificial elements

Periodensystemder Elemente

Radiotoxicity Radiotoxicity of 1 ton of 1 ton spent fuelspent fuelEnrichmentEnrichment: 4,2% U: 4,2% U--235; 235; Burn Burn up: 50 up: 50 GWd/tGWd/t;;

< 70 years FP

> 70 years Pu, MA

>1000 years Pu

Main contribution:

101 102 103 104 105 106102

103

104

105

106

107

108

109

Total Plutonium Minor Actinides FP

Rad

ioto

xici

ty (S

v/tH

M)

Years after discharge

Oxidation states of Actinides in aqueous solution including electron configurations (f-electrons)

Ac Th Pa U Np Pu Am Cm

f0 f1

f2

f3 f4

f5

f6 f7

+7

+3

+4

+5

+6

-Mobility of Actinides is controlled by:

Solubility Colloids

SurfaceInteractions

Aqueous Speciation

Sorption mechanismsSorption mechanisms

metal ion polynuclearspecies/colloid

outer-spheresorption

incorporation

inner-sphere sorptionsurface precipitation

A. Manceau et al. 2002, Reviews in Mineralogy and Geochemistry, 49, p. 344

Interaction of An(III) with mineral surfaces

Cm(III) TRLFS

Fluorescence process of Cm(III)

Nonradiative relaxation

A = 6D7/2

Emission λ = 593.8 nmLifetime 1.3 ms

Z = 8S7/2

Excitation λ = 396.6 nm

1) Fluorescence emission spectrum

- “inner-sphere“ ,“outer-sphere“ complex formation - identification and quantification of different species

2) Fluorescence lifetime- quenching process of excited state by

OH-vibrations→ hydration state (number of quencher)→ information on sorption mechanism

Cm

H H

H H

H H

H

H

H HH H

580 590 600 610 620

Rel

. Flu

ores

cenc

e In

tens

ity

Wavelength / nm

Cm3+

(aq)

593.

8 nm Cm

H H

H H

580 590 600 610 620

Rel

. Flu

ores

cenc

eIn

tens

ity

Wavelength / nm

Cm3+

(aq)

593.

8 nm

H

HH

+-

-

Time Resolved Laser Fluorescence Spectroscopy (TRLFS)

Fluorescence life time /µsln(fl

uor e

scen

c ein

ten s

ity)

/ co u

n ts

τ Cm(aq)

τ Cm(cplx)

τ Cm(aq) + Cm(cpx)

Cm(III) interaction with γ-Al2O3

Cm(III) interaction with clay material

Cm(III) interaction with calcite

Cm(III) interaction with CSH phases

Cm(III) interaction with sapphire single crystals

Examples






Examples

Eu(III) sorption onto smectite and kaolinite

1 2 3 4 5 6 7 8 9 10 11 12

0

20

40

60

80

100

Kaolinite without permanent charge

Smectite with permanent charge

3.3 x 10-6 mol/L Eu(III)0.025 mol/L NaClO4

Sorb

ed E

u(III

) / %

pH

Emission spectra of 3 x 10-7 mol/L Cm(III) in kaolinite and smectite suspension at various pH

580 590 600 610 620

kaolinite

593.8 nm

0.025 mol/L NaClO4

3x10-7 mol/L Cm(III)

pH 1.89 pH 3.68 pH 5.00 pH 5.40 pH 5.37 pH 5.54 pH 5.79 pH 6.18 pH 6.65 pH 7.22 pH 7.68 pH 8.22

Wavelength / nm580 590 600 610 620

smectite

593.8 nm

0.025 mol/L NaClO4

3x10-7 mol/L Cm(III)

pH 1.89 pH 3.75 pH 4.60 pH 5.24 pH 5.40 pH 5.51 pH 5.68 pH 5.85 pH 6.20 pH 6.54 pH 6.90 pH 7.15 pH 7.43 pH 8.23

Wavelength / nm

Spectra of Cm3+, Cm/clay sorption species 1 and 2

580 585 590 595 600 605 610 615 620

603.3 nm

598.2 nm593.8 nm

Cm species 2

Cm species 1Cm3+

3x10-7 mol/L Cm(III)0.025 mol/L NaClO4

Nor

mal

ized

fluo

resc

ence

em

issi

on

Wavelength / nm

Species distribution of Cm(III) as a function of pH

3 4 5 6 7 8 9

0

20

40

60

80

100

3.0x10-7 mol/L Cm(III)0.25 g/L clay0.025 mol/L NaClO4

Cm3+ kaolinite Complex 1 kaolinite Complex 2 kaolinite Cm3+ smectite Complex 1 smectite Complex 2 smectite

Cm

(III)-

spec

ies

/ %

pH

Species distribution of Cm(III) as a function of pH

3 4 5 6 7 8 9

0

20

40

60

80

100

Cm3+ kaolinite Complex 1 kaolinite Complex 2 kaolinite Cm3+ smectite Complex 1 smectite Complex 2 smectite

Cm

(III)-

spec

ies

/ %

pH

1 2 3 4 5 6 7 8 9 10 11 12

0

20

40

60

80

100

3.3x10-6 mol/L Eu(III)6.7x10-4 mol/L smectite9.7x10-4 mol/L kaolinite0.025 mol/L NaClO4

pH

Fluorescence emission spectra of Cm(III) in aqueous suspensions

580 585 590 595 600 605 610 615 620

Cm3+

7x10-7 mol/L Cm(III) SiO2; pH 9.19

3x10-7 mol/L Cm(III)smectite; pH 8.23kaolinite; pH 8.22γ-Al2O3; pH 8.65

Nor

mal

ized

fluo

resc

ence

em

issi

on

Wavelength / nm

Bi-exponential emission decay of Cm(III) in aqueous smectite suspension at pH 5.85

0 100 200 300 400 500

e21

e22

e23

e24

e25

e26

e27

Lifetime: 70 +/- 3 µs and 110 +/- 7 µs9 and 5 water molecules

pH 5.853x10-7 mol/L Cm(III)0.25 g/L smectite

0.025 mol/L NaClO4

ln

Flu

ores

cenc

e em

issi

on

Time / µsn (H2O) = 0.65 K0bs(Cm) – 0.88

At pH < 5 Cm(III) is sorbed as an outer-sphere complex onto smectite

TOT

INT

TOT

Starting at pH ≥ 5 a ≡Al-O-Cm2+(H2O)5sorption species is formed

TOT

The second Cm(III) sorption species : ≡Al-O-Cm+(OH)(H2O)4 or ≡(Al-O)2-Cm+(H2O)5

TOT

TOT





incorporation







Examples

Fluorescence emission spectra of Cm(III) in calcite suspension at different contact times

585 590 595 600 605 610 615 620 625 630

24 h48 h70 h235 h290 h310 h336 h6 month

contact time8.9 x 10-8 mol/L Cm(III)0.1 mol/L NaClO41 g/L CaCO3

Cm

(III)

norm

aliz

ed fl

uore

scen

ce e

mis

sion

Wavelength / nmno pH variation; CO3

2- bufferstrong influence of contact time

s

Cm/calcitsorption species 1

Cm/calcitsorption species 2

Fluorescence emission spectra of Cm3+ aquo ion,Cm/calcite sorption species 1 and 2

585 590 595 600 605 610 615 620 625 630

593.8 nm

Cm/calcitesorption species 2Cm/calcite

sorption species 1

Cm3+(aq)

618.0 nm607.5 nm

Cm

(III)

norm

aliz

ed fl

uore

scen

ce e

mis

sion

Wavelength / nm

Fluorescence emission spectra of Cm(CO3)45- and Cm(III) sorption species 1

580 590 600 610 620 630

Cm(CO3)45-

in solution

Cm/calcitesorption species 1

607.5 nm

Cm

(III)

norm

aliz

ed fl

uore

scen

ce e

mis

sion

Wavelength / nm

Emission spectra of Cm(III) in aqueous calcitesuspension before and after centrifugation

580 590 600 610 620 630

8.9 x 10-8 mol/L Cm(III)0.1 mol/L NaClO41 g/L CaCO3contact time 460 h

suspension solution after centrifugation

25 min at 14000 rpm

Cm

(III)

fluor

esce

nce

emis

sion

Wavelength / nm

Time dependency of the emission decay of Cm(III)in aqueous calcite suspension

0 500 1000 1500 2000 2500 300021

22

23

24

25

26

Lifetime: 314 +/- 6 µs and 1302 +/- 75 µs

8.9 x 10-8 mol/L Cm(III)0.1 mol/L NaClO41 g/L CaCO3contact time 460 h

Cm(III) sorptionspecies 11 H2O molecule

Cm(III) sorption species 2

0 H2O molecules

ln C

m(II

I) flu

ores

cenc

e em

issi

on

time / µs

Cm(III)/calcite sorption species 1 and 2

COin solution

32-

calcite

mineral surface

O2-

Ca2+

C4+

Cm3+

H O2

H O2

Cm(III) sorption species 1 :

- replacement of a calcium ion in thefirst surface layerof the calcite lattice;

- coordinated by eightoxygen atoms fromcarbonate groups andadditionally one water molecule

Cm(III) sorption species 2 :

- lose of its complete hydration sphere

- incorporation into the calcitebulk structure





incorporation


PlutoniumRedox Chemistry

PuV(aq)

log K°IVs/V =- 19.8 ± 0.9 (NEA-TDB)

PuO2(s,hyd)

PuIV(aq)

Known solid-liquid and redox equilibria at 25°C

PuVI(aq)PuIII(aq)

O2(aq)

O2(g)

log K°III-IV = - 17.7 ± 0.1 (NEA-TDB)

log*K°s,0(PuO2) = - 2.3 ± 0.5 (NEA-TDB)

log K°IVs/III = 15.5 ± 0.7Rai et al.’02, Fujiwara et al.’02

log K°V-VI = - 15.8 ± 0.1 (NEA-TDB)

I ≤ 0.1 M

Solubility of PuO2xH2O(am)

Pu(IV) concentration ascertained by spectroscopy or solvent extraction

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

1 2 3 4 5 6 7 8 9 10 11 12 13

log

[Pu]

- log [H+]

Neck, Kim 2001

present work

Rai '84

Kasha '49

Knopp et al. '99Rai et al. '99

Rai et al. '80

Pu(IV)

Solubility of PuO2xH2O(am) under airI ≤ 0.1 M

Pu(IV) concentration ascertained by spectroscopy or solvent extractionSamples in closed vials under air ≠ equilibrated with pO2(air) = 0.2 barEquilibrium O2 concentration in solution: [O2]aq = 2.5.10- 4 M Additional O2 in gas volume over the solution: [O2]gas ∝ (Vgas/Vsoln)

⇒ Maximum concentration of oxidised Pu(V) and Pu(VI) is limited to the available amount of oxygen !

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

1 2 3 4 5 6 7 8 9 10 11 12 13

log

[Pu]

- log [H+]

Neck, Kim 2001

exposed to air

present work

Rai '84

Kasha '49


Rai et al. '80 Total Puslope -1

Pu(V)

Pu(VI) + Pu(V)

Pu(IV)

Solubility of PuO2xH2O(am) under airSolubility of PuO2xH2O(am) under air / Ar + traces O2

Samples in closed vials in Ar box (present work: ca. 10 ppm O2)≠pH < 4: log [Pu(V)] = - 5.0 (0.5 % Pu(VI) in Pu(IV) stock solution) pH > 4: Total Pu comparable to samples exposed to air

I ≤ 0.1 M

⇒ Maximum concentration of oxidised Pu(V) and Pu(VI) is limited to the available amount of oxygen !

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

1 2 3 4 5 6 7 8 9 10 11 12 13

log

[Pu]

- log [H+]

Neck, Kim 2001

exposed to air

present work

Rai '84

Kasha '49


Rai et al. '80 Pu(VI) + Pu(V)

Pu(IV) Ar + traces O2

Total Puslope -1

Pu(V)

Air: Pu(IV) + O2 → PuO2+ + PuO2

2+ C Rai ‘84 (NEA-TDB)

PuO2+ ⇔ PuO2

2+ + e- A log K°V/VI = - 16.16 ± 0.45 (- 15.82 ± 0.09)

PuO2(am) ⇔ PuO2+ + e- A + B log K°IV(s)/V = - 19.45 ± 0.23 (- 19.78 ± 0.86)

log K° = log (PuO2+) - pe C - 12.8 ± 0.8 ⇒ different solid phase !

0

2

4

6

8

10

12

14

16

18

1 2 3 4 5 6 7 8 9 10 11 12 13

pe

A

B

Rai et al. (air) I = 0.005 - 0.1 M0.4 M NaCl0.4 M NaClO4

p.w. (Ar + traces O2)0.1 M NaCl

pH = - log ([H +] γH)

6 days34 days55 & 77 days

log pO2(air) = - 0.7

slope = -1log pO2(g) = - 8

log pO2(g) = - 33

slope = -1C

Solubility of PuO2(s,hyd) Redox potentials (pe = 16.9 Eh)

Rai et al. 2001: Redox potentials at pH > 4 cannot be explained: pe values reliable ? Present study: Reproducible and independent of pO2(g): pe under air = pe under Ar + traces O2

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

1 2 3 4 5 6 7 8 9 10 11 12 13

log

[Pu]

- log [H+]

Pu(IV)

A

C

Pu(VI) + Pu(V)

Pu(V)B

Rai et al. '80 - 2001I = 0.005 - 0.1 M0.4 M NaCl0.4 M NaClO4

present work 0.1 M NaCl

Ar + traces O2

⇒ PuO2(s) + x/2 O2 → PuO2+x(s)

0

2

4

6

8

10

12

14

16

18

1 2 3 4 5 6 7 8 9 10 11 12 13

pe

A

B

Rai et al. (air) I = 0.005 - 0.1 M0.4 M NaCl0.4 M NaClO4

p.w. (Ar + traces O2)0.1 M NaCl

pH = - log ([H +] γH)

6 days34 days55 & 77 days

log pO2(air) = - 0.7

slope = -1log pO2(g) = - 8

log pO2(g) = - 33

slope = -1C

Solubility of PuO2(s,hyd) Redox potentials (pe = 16.9 Eh)

Air: Pu(IV) + O2 → PuO2+ + PuO2

2+ C O2 consumed, but not by [Pu]aq < 10-5 M at pH > 4

PuO2(am) ⇔ PuO2+ + e- A + B log K°IV(s)/V = - 19.45 ± 0.23 Solubility control

C - 12.5 ± 1.2 ⇒ Not PuO2(am) !

pe under air = pe under Ar + traces O2 << pe(pO2(air)

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

1 2 3 4 5 6 7 8 9 10 11 12 13

log

[Pu]

- log [H+]

Pu(IV)

A

C

Pu(VI) + Pu(V)

Pu(V)B

Rai et al. '80 - 2001I = 0.005 - 0.1 M0.4 M NaCl0.4 M NaClO4

present work 0.1 M NaCl

Ar + traces O2

Solubility control by PuO2+x(s)

Haschke et al.: Water catalized oxidation of PuO2(s)

PuO2(s) + x H2O(ads) → PuO2+x(s) + 2 x H(ads)x/2 O2 → x O(ads)

2 x H(ads) + x O(ads) → x H2O(ads)––––––––––––––––––––––––––––––––––––Σ PuO2(s) + x/2 O2 → PuO2+x(s)

EXAFS studies: Solid solution of Pu(IV) and Pu(V) oxide/oxyhydroxide

PuO2+x(s,hyd) = (PuIV)1-2x(PuV)2xO2+x-n(OH) 2n.y H2O(s) or (PuO2)1-2x(PuO2.5)2x(s,hyd)

Stable solid phase: x → 0.5 ⇒ PuO2.5(s) = 1/2 Pu2O5(s) ?Maximum observed: x = 0.27 ⇒ PuO2.27(s) ≈ 1/4 Pu4O9(s) )

Solubility product of PuO2.5(s,hyd) in (PuO2)1-2x(PuO2.5)2x(s,hyd)

PuO2.5(s) + 0.5 H2O ⇔ PuO2+ + OH- Ksp = [PuO2

+] [OH-]

PuO2.5(s) + H+ ⇔ PuO2+ + 0.5 H2O *Ks,0 = [PuO2

+] / [H+]

Solubility products log K°sp (I = 0, 25°C) of analogoushydroxides / oxides of Np(IV), Np(V) and Pu(IV), Pu(V)

NpO2OH(am) PuO2OH(am)- 8.7 ± 0.2 NEA-TDB - 9.0 ± 0.5 NEA-TDB

NpO2.5(cr) PuO2.5(s) in PuO2+x(s, hyd)- 12.2 ± 0.8 NEA-TDB - 14.0 ± 0.8 p.w.

NpO2.5(s, hyd) (pure PuO2.5(s, hyd))- 11.4 ± 0.4 Efurd et al. ‘98 (- 13.0 ± 1.5) estimated from stabilisation- 10.1 ± 0.4 Pan, Campbell ‘98 in mixed valent oxides, e.g.

Fe(II)-Fe(III) and U(IV)-U(VI)

NpO2(am, hyd) PuO2(am, hyd)- 56.7 ± 0.5 NEA-TDB - 58.3 ± 0.5 NEA-TDB

NpO2(cr) PuO2(cr)- 63.7 ± 1.8 Rai et al. ‘87 - 64.0 ± 0.5 NEA-TDB

Normalized molar standard Gibbs energies of formation ∆fG°m(AnO2+x) of crystalline An(IV-V-VI) oxides and Actinyl(VI) oxyhydroxides

PuO2(cr,dry) + x/2 O2(g) –//–> PuO2+x(cr) NpO2(cr,dry) + x/2 O2(g) –//–> NpO2+x(cr)

No oxidation with O2(g) ⇔ ∆rG°m > 0

Filled points from NEA-TDBOpen squares estimated by analogy (p.w.)

-1180

-1160

-1140

-1120

-1100

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

0 0.2 0.4 0.6 0.8 1

∆ fG

o m(A

nO2+

x) (k

J/m

ol)

x (AnO2+x)

UO2(cr)

1/3 U3O8(cr)

An(IV) An(VI)

Solid solution effect

Schoepite

UO3(cr)

Metaschoepite

1/4 U4O9(cr)

1/3 U3O7(cr)

α

γβ

An(V)

NpO2(cr)

PuO2(cr)Neptunyl(VI)oxyhydroxide

Plutonyl(VI)oxyhydroxide

1/2 Np2O5(cr)

{NpO3(cr)}

{PuO3(cr)}

{1/2 Pu2O5(cr)}

Effect of crystallinity & structure

UO2(cr,dry) + x/2 O2(g) → UO2+x(cr)∆rG°m < 0

Normalized molar standard Gibbs energies of formation ∆fG°m(AnO2+x) of crystalline An(IV-V-VI) oxides and Actinyl(VI) oxyhydroxides

Haschke, Allen 2002:PuO2(cr) + x H2O → PuO2+x(cr) + x H2(g)

Reaction is not possible: ∆rG°m > 200 kJ/mol !

Filled points from NEA-TDBOpen squares estimated by analogy (p.w.)

-1180

-1160

-1140

-1120

-1100

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

0 0.2 0.4 0.6 0.8 1

An(IV) An(VI)An(V)

NpO2(cr)

PuO2(cr)Neptunyl(VI)oxyhydroxide

Plutonyl(VI)oxyhydroxide

1/2 Np2O5(cr)

{NpO3(cr)}

{PuO3(cr)}

{1/2 Pu2O5(cr)}

Haschke, Allen 2002PuO2+x(cr)

∆ fG

o m(A

nO2+

x) (k

J/m

ol)

x (AnO2+x)

Normalized molar standard Gibbs energies of formation ∆fG°m(AnO2+x) of hydrated Np(IV-V) and Pu(IV-V) oxides / oxyhydroxides

AnO2(cr,dry) + x/2 O2(g) –//–> AnO2+x(cr) ∆rG°m > 0

AnO2(s,hyd) + x/2 O2(g) ––> AnO2+x(s,hyd) Np: ∆rG°m = - 26.6 ± 13.0 kJ/mol for x = 0.5Data derived from solubility studies Pu: ∆rG°m = - x (11.4 ± 9.0) kJ/mol for x < 0.1

x = 0.25 → 0.5 ?

-1060

-1040

-1020

-1000

-980

-960

0 0.1 0.2 0.3 0.4 0.5

∆ fG

o m(N

pO2+

x) (k

J/m

ol)

x (NpO2+x)

NpO2(cr)NpO2.5(cr)

Np(IV) Np(V)

± 5.7 kJ/mol (1 log10-unit)

NpO2(am,hyd)

[98EFU/RUN]

[98PAN/CAM]Effect of crystallinity, particle size, hydration

∆rGom > 0

∆rGom < 0

O2(g)

O2(g)

NpO2.5(s,hyd)

Solid solution effect

-1040

-1020

-1000

-980

-960

-940

0 0.1 0.2 0.3 0.4 0.5∆ f

Go m

(PuO

2+x)

(kJ/

mol

)x (PuO2+x)

PuO2(cr)

Pu(IV) Pu(V)

Solid solution effect ?

± 5.7 kJ/mol (1 log10-unit)

PuO2(s,hyd)

{PuO2.5(cr)}

∆rGom > 0

∆rGom > 0 ?

∆rGom < 0 ?

O2(g)

PuO2.5(s,hyd) ?

Effect of crystallinityparticle size, hydration

PuO2+x(s,hyd)

PuV(aq)

log K°IVs/V =- 19.8 ± 0.9 (NEA-TDB)

PuO2+x(s,hyd)PuO2(s,hyd) PuO2.5(s,hyd)

log*K°s,0(PuO2.5) = 0.0 ± 0.8

PuIV(aq)

Solubility and pe controlling equilibria at pH > 3 in the presence of oxygen

PuVI(aq)PuIII(aq)

B + CA + B

AO2(aq)

O2(g)

O2

log K°IVcoll/V = - 12.5 ± 1.4

C

C

PuIV(coll)

log K°coll =- 8.3 ± 1.0

Conclusions:

Plutonium chemistry

= Equilibrium chemistry !

Recent Advances in the Aqueous Chemistry and Thermodynamics of Actinide · · 2019-04-26Chemistry and Thermodynamics of Actinide Thomas Fanghänel. Merck Actinides-14 elements following

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