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Kinetic Transfer of Radionuclides in
Aqueous Suspensions of the Marine
Environment
A. Laissaoui
IAEA Regional Training Course on Sediment Core Dating Techniques
RAF7/008 Project
Rabat, 05 – 09 July 2010
Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco
INTRODUCTION
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Aqueous suspension: is a mixture of insoluble
particles in water
Sorption: partitioning between water and solid
phases
process in which radionuclides become associated
with solid phases
Adsorption: process in which a radionuclide
becomes associated with the surface of a particle
Desorption: the release of radionuclide already
associated with particles
Why is Sorption Important?
- Sorption affects the fate and distribution of radionuclides in the
marine environment
- Radionuclides behave differently when associated with a solid
Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco
INTRODUCTION
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Radionuclides pathways is essential for both radioecology
purposes and their use as tracers of marine processes
The uptake and release by SPM combined with
sedimentation and resuspension of the solids play a
significant role
Reactive transport models were based on the equilibrium
distribution coefficients, Kd Steady state condition
Recent models are based on kinetic rates of
uptake/release of radionuclides between waters and the
solid phases when short timescales are involved
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INTRODUCTION
Sorption of radionuclides by particles in aqueous
suspensions can be regarded as a physico-chemical
process
Active sites
Effect of active sites
=
Electrical neutrality
Exchangeable ions
Active sites
Ref.: J.M. Abril, 1998. JER
Particle
Water
The most common adsorption reaction in aqueous
suspensions is ion exchange
R
S1
+
S2
k1
R
S2 +
S1
k2
Centre National de l’Énergie des Sciences et des Techniques Nucléaires, Rabat - Morocco
The experimental approach
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The experimental approach consists in spiking the studied
suspension to record the time course of the tracer
concentrations in the dissolved phase.
-Use natural samples
-Spiking (analogues)
-Permanent Stirring
-Control of room temperature
-Extraction of aliquots
-Separation of SPM from water
-Conditioning and measurements
The uptake curve
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Application of box models
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Each box in the model represents a phase and within a
particle, sites of different accessibility or energy are
represented by boxes.
da w = - k .a + k .a
1
w
2
p
dt
da p
= + k1 .a w - k 2 .a p
dt
k1
Solids
ap
Water
aw
k2
k1 .a w (0) - k2 .a p (0) -( k + k
a w (t ) =
e
k1 + k2
(
1
2 ).t
)
- 1 + a w (0)
Time evolution of 133Ba conc. in estuarine
aqueous suspension (Laissaoui et al. 1998. J.
Radioanal. Nucl. Chem.)
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Application of box models
Sr-85 conc. in water (Bq/L)
220
220
One-step model
200
200
180
180
160
160
140
140
120
120
0
20
40
60
Time (hours)
80
100
120
Two-step model
0
20
40
60
80
100
120
Time (hours)
Sorption kinetics of 85Sr in aqueous suspension (Benkdad et al.
2008. JER).
The use of one-step model is a subparametrisation of the real
process
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Application of box models
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Two Parallel and Reversible Reaction Model
k11
Water
aw
da w
= -k 11 .a w + k 21 .a s1 - k 12 .a w + k 22 .a s 2
dt
da s1
= k 11 .a w - k 21 .a s1
dt
da s 2
= k 12 .a w - k 22 .a s 2
dt
Site 1
as1
k21
k12
k22
Site 2
as2
[ (
)
(
) ]
aw ( t ) = aw ( t = 0 ). A1 . e P1 .t - 1 + A2 . e P2 .t - 1 + 1
Assumptions:
-Discrimination between reactive sites
-No intercation between the adsorbed ions
-Different species compete for the same sites
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Application of box models
k11 (s-1)
0.0018  0.0003
k21 (s-1)
0.028  0.005
k12 (s-1)
0.0172  0.0015
k22 (s-1)
0.35  0.03
kd (l/kg)
19.9  2.1
120
110
Water
100
kd
16
12
8
Site 1
4
Site 2
Kd (l/kg)
as1; as2 (Bq/kg)
aw (Bq/l)
130
0
0
20
40
60
80
100
120
140
160
Time (hours)
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Application of box models
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1
Fraction in water
0.8
0.6
Fast
slow
0.4
0.2
internediate
0
0.01
0.1
1
10
100
Time (hours)
Time evolution of 133Ba concentrations in water
(Data from Barros and Abril, 2003)
Three-step model involving four compartments
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Application of box models
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Three-step models
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Application of box models
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Equivalence between models
The fitting parameters are usually obtained from the observed
time course of tracer concentrations in the dissolved phase.
The same analytical function aw(t) is the solution of the five
models
Then, the models are mathematically equivalent. once
obtained an uptake curve for water concentration, it is not clear
to decide which model best describes the adsorption process
in a given experiment.
What is the physical significance of the kinetic transfer
coefficients?
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Effect of SPM concentration
Adsorption coefficients are
environmental conditions
sensitive
to
changes
in
0,16
0,14
k1(h-1)
0,12
0,10
0,08
0,06
Variation of Sr adsorption coefficient
with SMC
0,04
0,02
100
200
300
400
500
SMC (ppm)
The rate of uptake increases as SPM concentration
increases
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Competition effect
Activities in site 2 (Bq/kg)
70
C = 0.022 mS
60
50
C = 0.78 mS
40
30
20
C = 21 mS
10
0
1E-3
0,01
0,1
1
10
100
Time (hours)
Time evolution of 85Sr activities in particles provided by
the two-step model for different conductivities of the
solutions (major competitors are Ca2+ and Mg2+).
Adsorption is important when cations content is small
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Conlusion
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Adsorption is important in understanding the behavior of
radionuclides in marine environments
Desorption is still unpredictable
No theoretical aspect could predict the variation of desorption
coefficients to changes in physico-chemical properties
Experimental efforts are being done to understand desorption
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