recent advances in the aqueous chemistry and thermodynamics of actinide · · 2019-04-26chemistry...
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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
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
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
Sorption mechanismsSorption mechanisms
A. Manceau et al. 2002, Reviews in Mineralogy and Geochemistry, 49, p. 344
metal ion polynuclearspecies/colloid
outer-spheresorption
incorporation
inner-sphere sorptionsurface precipitation
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
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
Sorption mechanismsSorption mechanisms
A. Manceau et al. 2002, Reviews in Mineralogy and Geochemistry, 49, p. 344
metal ion polynuclearspecies/colloid
outer-spheresorption
incorporation
inner-sphere sorptionsurface precipitation
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
Knopp et al. '99Rai et al. '99
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
Knopp et al. '99Rai et al. '99
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 !