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Stability and sorption
capacity of
montmorillonite colloids
Investigation of size fractional
differences and effects of γ-irradiation
KNAPP KARIN NORRFORS
KTH Royal Institute of Technology
School of Chemical Science and Engineering
Department of Chemistry
Applied Physical Chemistry
SE-100 44 Stockholm, Sweden
Copyright © Knapp Karin Norrfors, 2015. All rights reserved. No parts of
this thesis may be reproduced without permission from the author.
Paper V © 2011 Elsevier B.V.
TRITA-CHE Report 2015:8
ISSN 1654-1081
ISBN 978-91-7595-444-8
Akademisk avhandling som med tillstånd av KTH Kungliga Tekniska
Högskolan i Stockholm framlägges till offentlig granskning för avläggande
av teknologie doktorsexamen fredagen den 6 mars kl 10:00 i sal F3, KTH,
Lindstedtsvägen 26, Stockholm. Avhandlingen försvaras på engelska.
Fakultetsopponent: Dr. Daniel Kaplan, Savannah River National
Laboratory, Aiken, SC, USA
ii
To my family,
The difference between stupidity and genius
is that genius has its limits.
Albert Einstein
iii
iv
Abstract
Bentonite clay is intended to form one of the barriers in most repositories of
spent nuclear fuel located in granite. One important function of the
bentonite barrier is to retard transport of radionuclides in the event of waste
canister failure. Bentonite has a high sorption capacity of cations and its
main constituent is montmorillonite. In contact with groundwater of low
ionic strength, montmorillonite colloids can be released from bentonite and
thereby control transport of radionuclides sorbed onto the colloids.
In colloid transport in bedrock fractures, size separation of clay colloids may
occur due to physical and chemical interactions with the bedrock fracture
surface. This may enhance or retard the overall transport of radionuclides,
depending on the sorption capacities and stability of the differently sized
clay colloids. The bentonite barrier will be exposed to γ-radiation from the
spent nuclear fuel. Irradiation affects surface-related properties of
bentonite. If an average sorption capacity value cannot be used for all colloid
sizes or if sorption is affected by exposure to γ-irradiation, corrected
sorption capacity values would give higher resolution in current reactive
transport models.
In order to study the size separation process, a protocol was developed and
successfully applied to fractionate montmorillonite into different-sized
colloid suspensions by means of sequential or direct centrifugation. The
stability and sorption capacity were studied using these fractions. Both
stability and sorption capacity were found to be similar for all colloid sizes.
Bentonite exposed to γ-radiation sorbed less divalent cations with
increasing radiation dose. The effect was not large enough to have any
impact on diffusion. The presence of bentonite enhanced irradiationinduced corrosion of copper under anaerobic atmosphere.
An average sorption capacity value for montmorillonite can be used for all
colloid sizes in reactive transport models. The effect of γ-irradiation on
sorption capacity is sufficiently large to require consideration in transport
modelling.
Keywords: montmorillonite, colloids, size effect, γ-irradiation, sorption,
colloid stability, radionuclides
v
Sammanfattning
Bentonite är planerad som en av barriärerna i de flesta slutförvar av använt
kärnbränsle. Bentonite har en hög sorptionskapacitet för katjoner. Den
huvudsakliga
beståndsdelen
av
bentonit
är
montmorillonit.
Montmorillonitkolloider kommer att frigöras från bentonitbufferten i
kontakt med grundvatten av låg jonstyrka och på så vis styra transporten av
sorberade radionuklider.
Under den kolloidala transporten i bergsprickorna kan en separation med
avseende på storlek uppstå genom interaktioner mellan kolloiderna och
bergytan. Detta kan få till följd att den genomsnittliga transporten av
radionuklider bromsas eller tilltar beroende på sorptionskapaciteten och
stabiliteten av de olika kolloidstorlekarna. Bentonitbarriären kommer även
att utsättas för γ-bestrålning från det använda kärnbränslet, vilket påverkar
dess ytrelaterade egenskaper. Om inte ett medeltal för sorptionskapaciteten
är giltigt för alla kolloidstorlekar eller om sorptionen påverkas av γbestrålning, behövs nya sorptionskapaciteter bestämmas och impliceras för
noggrannare transportmodeller.
En metod för att separera montmorillonitkolloider med avseende på storlek
via direkt och stegvis centrifugering har utvecklats. Stabiliteten och
sorptionskapaciteten för dessa fraktioner har studerats. Både stabilitet och
sorptionskapacitet visade sig vara lika för alla kolloidstorlekar.
Bestrålad bentonit sorberar mindre andel divalenta katjoner med ökad dos
bestrålning. Effekten är dock inte stor nog för att slå igenom i
diffusionsexperimenten. Förekomst av bentonit ökar även den
strålningsinducerade korrosionen av koppar under anaeroba förhållanden.
Ett medelvärde för sorptionskapaciteten kan användas för alla
kolloidstorlekar i transportmodeller. Effekten av γ-bestrålning är dock stor
nog för att implementeras i modellerna.
Nyckelord: montmorillonit, kolloider,
sorption, kollodial stabilitet, radionuklider
vi
storlekseffekt,
γ-bestrålning,
List of publications
I. Smectite and latex colloid agglomeration at high ionic strength
– Is there any effect of particle size?
Knapp Karin Norrfors, Muriel Bouby, Johannes Luetzenkirchen, Mats
Jonsson and Susanna Wold
Submitted to Colloids and Surfaces A: Physicochemical and Engineering
Aspects
II. Montmorillonite colloids. I: Characterization and stability of
suspensions with different size fractions
Knapp Karin Norrfors, Muriel Bouby, Stephanie Heck, Nicolas Finck, Rémi
Marsac, Thorsten Schäfer, Horst Geckeis and Susanna Wold
Submitted to Applied Clay Science
III. Montmorillonite colloids. II: Dependency of Colloidal size on
Sorption of Radionuclides
Knapp Karin Norrfors, Muriel Bouby, Rémi Marsac, Stephanie Heck,
Thorsten Schäfer, Horst Geckeis and Susanna Wold
Manuscript to be submitted
IV. Montmorillonite colloids. III: Influence of Colloidal Size on
the Sorption Reversibility of Radionuclides
Knapp Karin Norrfors, Muriel Bouby, Rémi Marsac, Stephanie Heck,
Thorsten Schäfer, Horst Geckeis and Susanna Wold
Manuscript in preparation
V. Effect of γ-radiation on radionuclide retention in compacted
bentonite
Michael Holmboe, Knapp Karin Norrfors, Mats Jonsson and Susanna Wold
Radiation Physics and Chemistry, 80 (2011) 1371-1377
VI. Radiation induced corrosion of copper in bentonite-water
systems under anaerobic conditions
Knapp Karin Norrfors, Amanda Kessler, Åsa Björkbacka, Susanna Wold and
Mats Jonsson
Manuscript in preparation
vii
Contributions to the papers
The author’s contributions to the appended papers were as follows:
I.
Principal author. Took part in planning the experiments and
performed all the experimental work. Major part in writing.
II.
Principal author. Planned the experimental work together with
Muriel Bouby and performed all experimental work, except the
ICP-OES/MS, IC, XRD and AsFlFFF-analysis. Major part in
writing.
III.
Principal author. Planned the experimental work together with
Muriel Bouby and performed all experimental work, except the
ICP-OES/MS-analysis. Major part in writing.
IV.
Principal author. Planned the experimental work together with
Muriel Bouby and performed all experimental work, except the
ICP-OES/MS-analysis. Major part in writing.
V.
Took part in planning the experiments and performed the batch
sorption experiments. Minor contribution to writing.
VI.
Principal author. Planned and performed all experimental work.
Minor contribution to writing.
viii
Table of contents
Abstract....................................................................................................... v
Sammanfattning ...................................................................................... vi
List of publications ................................................................................ vii
Contributions to the papers ............................................................... viii
Table of contents ..................................................................................... ix
1. Introduction ........................................................................................... 1
1.1 Background – repository of spent nuclear fuel in Sweden ........ 1
1.2 The bentonite barrier ....................................................................... 2
1.3 Worst case scenario ........................................................................ 3
1.4 Transport of radionuclides by bentonite colloids ......................... 4
1.4.1 Stability of clay colloids ............................................................ 5
1.4.2 Size exclusion of colloids ........................................................ 7
1.4.3 Effect of γ-irradiation ................................................................ 8
1.4.4 Sorption of radionuclides onto clay colloids ......................... 9
1.4.5 Sorption reversibility of radionuclides .................................. 12
1.5 Objectives of this thesis ................................................................ 14
2. Materials ............................................................................................... 15
2.1 Unpurified MX80 ............................................................................ 15
2.2 Homo-ionic montmorillonite .......................................................... 15
2.3 Organic matter ................................................................................ 16
2.3.1 Humic substances .................................................................. 16
2.3.2 Fulvic acid ................................................................................ 16
2.4 Nano-sized polystyrene (latex) particles .................................... 17
2.5 Synthetic groundwater .................................................................. 17
2.6 Copper cubes ................................................................................. 18
3. Methods ................................................................................................ 19
3.1 Size fractionation of MX80............................................................ 19
3.1.1 Fractionation method ............................................................. 19
3.1.2 Characterisation of size fractions ......................................... 21
ix
3.2 Stability studies............................................................................... 22
3.3 Sorption of radionuclides onto size-fractionated MX80 ............ 24
3.3.1 Radionuclide (RN) cocktail .................................................... 24
3.3.2 Preparation of sorption samples ........................................... 24
3.3.3 Determination of sorption ...................................................... 24
3.4 Sorption reversibility studies on radionuclides........................... 25
3.4.1 Change in chemical composition.......................................... 26
3.4.2 Addition of competing ligand ................................................. 27
3.5 γ-irradiation experiments ............................................................... 27
3.5.1 Batch sorption experiments ................................................... 27
3.5.2 Radionuclide retardation in compacted bentonite ............. 28
3.5.3 Copper cube corrosion in the presence of bentonite ........ 28
3.5.4 Effect of total dose on the bentonite sorption capacity of
Cu2+ ..................................................................................................... 29
4. Results and Discussion.................................................................... 31
4.1 Size fractionation of MX80 ............................................................ 31
4.1.1 Size distribution ....................................................................... 31
4.1.2 Characterisation of the colloids............................................. 35
4.1.3 Estimations of edge site densities ........................................ 37
4.2 Size effect on colloid stability ....................................................... 39
4.2.1 Latex particles ......................................................................... 39
4.2.2 Montmorillonite colloids.......................................................... 43
4.3 Sorption properties of RNs and analogues onto montmorillonite
colloids .................................................................................................... 45
4.3.1 Effect of clay colloid size........................................................ 45
4.3.2 Effect of γ-irradiation .............................................................. 48
4.4 Sorption reversibility ...................................................................... 51
4.4.1 Introducing organic matter as competing ligand ................ 51
4.4.2 Decrease in pH........................................................................ 53
4.4.3 Increase in ionic strength ....................................................... 54
4.4.4 Addition of crushed bedrock material (CBM) ...................... 54
4.5 γ-irradiation-induced copper corrosion in the presence of
bentonite................................................................................................. 56
4.6 General discussion ........................................................................ 57
Conclusions ............................................................................................. 59
x
Further work ............................................................................................ 61
List of abbreviations and symbols .................................................... 63
Acknowledgements ............................................................................... 65
References ............................................................................................... 67
xi
xii
1. Introduction
1. Introduction
1.1 Background – repository of spent nuclear fuel in Sweden
Nuclear power has been one of the major energy sources for production of
electricity in Sweden during the last four decades. Every year, each nuclear
reactor in Sweden produces 25-30 tons of highly radioactive spent nuclear
fuel, so hundreds of tons of spent fuel that have to be disposed of safely [1].
Actinides are present in the spent nuclear fuel, making it highly radioactive
and toxic. Therefore, it has to be isolated from the biosphere for a long time,
approximately 100 000 years, before the activity has decayed to the same
levels as natural uranium minerals in the bedrock.
A concept for storing spent nuclear fuel has been proposed and developed
by the Swedish Nuclear Fuel and Waste management company (SKB). This
multi-barrier concept is named KBS-3 [2], and the design includes three
barriers preventing the release of radionuclides (RNs) to the environment
(Figure 1). The spent nuclear fuel is planned to be stored encapsulated in
copper canisters at 500 m depth in the crystalline granite bedrock. Each
canister will be surrounded by bentonite clay to keep it in place.
Figure 1: The KBS-3 concept designed for storage of spent nuclear fuel in Sweden [2].
1
1. Introduction
The outer layer of copper on the canister will provide corrosion resistance,
while the cast iron insert will provide mechanical support. The outermost
barrier, the granite bedrock, will provide an anoxic atmosphere during longterm storage. A bedrock with the lowest fracture frequency possible will be
chosen. In the event of canister failure, most RNs will adsorb to the bedrock
due to its negatively charged surfaces and its ability to reduce redoxsensitive RNs. Transport of RNs with the groundwater will thereby be
delayed. The last barrier, the compacted bentonite clay, will form an
approximately 35 cm thick layer around the copper canister. This clay
barrier is the main subject of this thesis.
1.2 The bentonite barrier
Bentonite has been chosen as a barrier in the KBS-3 repository for spent
nuclear fuel due to its mechanical and chemical properties. The clay swells
in contact with water. Therefore, it can keep the copper canister in place
following any movements in the bedrock and at the same time seal small
fractures present in the adjacent bedrock. In addition to its mechanical
properties, bentonite will prevent corrosive elements from the
surroundings, such as sulphide, thiosulphate and polythionates [3], from
making contact with the copper canister. In the event of canister failure,
bentonite has a high capacity for surface complexation and cation exchange,
allowing potential leaching RNs to sorb onto the bentonite. This will lead to
retention of any RNs migrating towards the biosphere.
The main component of bentonite is montmorillonite, an aluminium (Al)rich smectite mineral. Montmorillonite layers of approximately 1 nm are
stacked onto each other to form one particle, one stack. These layers are
negatively charged at pH > 6. As water is absorbed between the layers, the
clay swells [4]. Due to the negatively charged montmorillonite surfaces,
bentonite has a high sorption capacity for cations. In cases of canister
failure, most RNs will be present as multivalent cations and thereby have a
high affinity for montmorillonite. Therefore, retention of cationic RNs will
be significant in compacted bentonite [5].
2
1. Introduction
Bentonite releases stable colloids (i.e. particles of 1 nm to 1 µm diameter in
at least one dimension in suspension [6]) when in contact with groundwater
of low ionic strength, i.e. < 10-2 M [7-9]. During the intended lifetime of
spent nuclear fuel repositories (~100 000 years), ice age cycles are predicted
at northern latitudes [2]. At the end of an ice age, glacial meltwater of low
ionic strength may press away old, saline groundwater equilibrated with the
bedrock over a long time. In addition, with the pressure release from
melting glaciers, bedrock fractures will be formed. Therefore, the bentonite
barrier may come into contact with water of low ionic strength. From
inventories of existing glaciers, glacial melt water is known to have a pH of
8-9 and low ionic strength, i.e. 5·10-4 M or less [10]. The released
montmorillonite colloids can be transported in bedrock fractures with the
groundwater flow. In the event of large mass loss, the buffer functionality
will be endangered. In the case of canister failure, mobile montmorillonite
colloids will control transport of RNs [5].
1.3 Worst case scenario
To perform safety assessments for the storage of spent nuclear fuel, the
probability of possible events and their consequences have to be studied. A
complete safety assessment must be based on rather extreme extrapolations
in time. In this thesis, the following worst case scenario was considered (see
Figure 2).
• The copper canister is broken, the fuel pellets start to dissolve and release
RNs.
• Released RNs are sorbed onto montmorillonite colloids.
• Groundwater of low ionic strength has pressed away the old, saline
groundwater, opened up fractures in the bedrock and reached repository
depth with high flow.
3
1. Introduction
Figure 2: Schematic diagram of the worst case scenario studied in this thesis.
In addition to this worst case scenario, γ-irradiation from the spent nuclear
fuel will penetrate through the copper canister, exposing the bentonite to γirradiation. This exposure to γ-irradiation could change the transport
properties and the sorption capacity of the clay colloids.
1.4 Transport of radionuclides by bentonite colloids
Radionuclide transport through bedrock fractures is a highly complex
process with many parameters that have to be considered. The transport is
dependent on the interactions between the RNs and the clay colloids and
also on the interactions between the clay colloids themselves and the
bedrock surface. Furthermore, chemical conditions (e.g. water chemistry
and chemical composition of clay colloids and bedrock) and physical
properties (e.g. structure of bedrock fracture and amount of fracture filling
material) have to be considered. The near-field concentration of clay
4
1. Introduction
colloids will affect the stability of the colloids and thereby the transport of
sorbed RNs.
1.4.1 Stability of clay colloids
One of the most important aspects in transport of nano-particles is colloid
stability. Unstable colloid suspensions will sediment and the particles will be
excluded from solution and further transport. However, stability is an
equilibrium with both physical and chemical parts, which means that new
equilibria will be established in new groundwater conditions, such as water
flow or chemical composition. Sedimented colloids can be resuspended back
to the water phase if the surrounding conditions change.
Stability can be expressed both in kinetic and thermodynamic terms. From a
practical point of view, the stability of a colloid suspension is seen as the
life-time of the suspension. The DLVO-theory predicts the stability of
particles in a suspension in terms of energetics, where the electrostatic
).
repulsion (V ) is compared to the van der Waals attraction (V
V
=V
+V
(Equation 1)
Van der Waals forces are always attractive between two particles of the same
material, expressed by the positive Hamaker constant (A) [11, 12]. Assuming
two identical spherical particles in the suspension, it can be seen that
attraction between the particles increases with increasing particle size and
decreasing separation distance (Equation 2) [13-15].
V
=−
+
+ ln
(Equation 2)
where r is the radius of the spherical particles and s is the centre-to-centre
separation.
The stabilising force in suspension is mainly the electrical double layer
repulsion, which can be seen as the activation energy for the agglomeration
process. This repulsion is highly dependent on the ionic strength of the
suspension and the charge of the particle surface. For a symmetric
electrolyte it can be expressed as a function of the surface-to-surface
5
1. Introduction
separation, h, the number concentration of electrolyte, n , the Boltzmann
constant, k , and the absolute temperature, T [11, 16, 17]:
V =
n k Tγ ∙ e
(Equation 3)
where γ is calculated from the valence of the electrolyte, z, the charge of the
electron, e, and the diffuse layer potential, ψ :
γ = tanh
(Equation 4)
The measured ζ-potential can be used to approximate ψ due to the
correlation between the charge at the slipping plane and the surface charge.
The inverse of Debye-Hückel screening length, κ, is given by:
κ=
/
(Equation 5)
where I is the ionic strength, ϵ the permittivity of vacuum and ϵ the
permittivity of the material.
At high ionic strength, where the repulsion between the particles decreases
towards zero, the amount of agglomerates formed is proportional to the
probability of two particles colliding with each other. Therefore, the
agglomeration kinetics is dependent on the particle concentration in
suspension.
There are many methods used for determining colloid stability. Dynamic
light scattering (DLS) can monitor the in situ development of mean particle
size in the systems. However, this technique only monitors the increase in
mean agglomerate size with time. Besides, agglomerates are most often not
spherical and their shape may depend on the kinetics of agglomeration.
Therefore, the treatment of data from DLS-measurements is not straightforward [18-21].
To summarise, particle stability in suspension is dependent on particle
characteristics, such as surface charge and particle composition (giving the
Hamaker constant), and on the background electrolyte and initial particle
concentration. The colloid stability of montmorillonite will determine the
6
1. Introduction
concentration of migrating bentonite colloids in bedrock fractures.
Montmorillonite colloids are stable at low ionic strength [7-9], and thus the
colloid-facilitated transport of RNs will be more likely under these
conditions. Holmboe et al. [22] have found that bentonite colloids are more
stable when γ-irradiated, with the potential to be transported further in the
bedrock. Finally, the relationship between particle stability and particle size
has not been clarified and is thus investigated in this thesis.
1.4.2 Size exclusion of colloids
Montmorillonite colloids may be separated by size during transport due to
interactions between the bedrock and the colloids. In a ‘clean’ fracture with
a low amount of fracture filling material and high water velocity, laminar
water flow is expected. Here, transport of smaller colloids will be enhanced
due to their lower mass and surface area (Figure 3). In contrast, in bedrock
fractures containing a high amount of fracture filling material, the colloids
may be separated by size exclusion effects, where larger colloids are
transported faster than smaller colloids in a process comparable to size
exclusion chromatography. This phenomenon occurs due to chemical and
electrostatic interactions between the clay colloids and the fracture filling
material or bedrock. The smaller colloids are transported more slowly since
they are able to interact with surfaces where large colloids are restricted due
to their size.
Figure 3: Schematic diagram of a plausible clay colloid size separation effect occuring in a
bedrock fracture containing a low (left) or high (right) amount of fracture filling material.
7
1. Introduction
The overall bulk properties of the total clay distribution may differ if its
properties are connected to the exposed surface area and specific sizes of
colloids are transported further. Intuitively, one would expect larger
amounts of RNs to sorb onto smaller montmorillonite colloids per unit mass
because of the excess of lateral surface area, where the sorption active edge
sites are expected to be present [23]. In addition, in laminar water flow,
smaller colloids will be transported further in the bedrock fractures than
large clay colloids (Figure 3). In this case, by only considering bulk
properties in reactive transport models, the RN transport would be
underestimated.
Colloid size effects on the stability or sorption capacity of bentonite colloids
have not previously been reported in these types of systems. If a colloid size
effect exist, these parameters would have to be included in reactive
transport models, where size separation of montmorillonite colloids is not
considered at the present time [24].
1.4.3 Effect of γ-irradiation
The bentonite barrier will be exposed to ionising radiation, mainly γirradiation from 137Cs, under anoxic conditions. Initially, the bentonite
barrier will have low water content and no great effect of γ-irradiation on
the properties of bentonite has been found in such conditions [25-27].
However, the bentonite will be saturated with groundwater over time.
Therefore, γ-irradiation effects on bentonite suspensions with high water
content were studied in this work. At a high water content, several γirradiation effects on clay colloids have been reported, e.g. the stability of
montmorillonite colloids increases upon γ-irradiation [22]. Furthermore,
the structural Fe2+/Fe3+-ratio in montmorillonite colloids increases after
exposure to γ-irradiation [28]. This indicates that the negative surface
charge on the montmorillonite colloids increases accordingly. Sorption is
clearly related to surface charge, which raises the question of whether
exposure to γ-irradiation can also affect the sorption of RNs onto
montmorillonite. Since sorption of RNs onto montmorillonite is described
both by electrostatic interactions such as cation exchange (macroscopic
properties) and surface complexation (microscopic properties), sorption of
different RNs may be affected differently upon γ-irradiation. If the effect on
8
1. Introduction
sorption capacity is sufficiently large, this may also be reflected in the
retention properties of RNs in clays.
1.4.4 Sorption of radionuclides onto clay colloids
Sorption of inorganic cations onto clays has been studied for decades [29,
30]. However, the mechanisms for interactions between RNs and the clay
surface are not fully understood. The sorption mechanisms vary with the
valency and thermodynamic properties of the RN, so no general sorption
model can be used for all RNs. The most important elements to be studied
in the storage of spent high and low level nuclear fuel, and their analogues,
are [31]:
• Actinides: plutonium (Pu), thorium (Th), americium (Am), protactinium
(Pa), neptunium (Np), curium (Cm), actinium (Ac) and uranium (U)
• Other elements: lead (Pb), radium (Ra), technetium (Tc), caesium (Cs),
zirconium (Zr), nickel (Ni), tin (Sn), selenium (Se), palladium (Pd) and
niobium (Nb)
Among these elements, there is great diversity in speciation, ionic radius
and thermodynamic properties.
Sorption capacity is most often given as an equilibrium constant (KD value)
where the amount of RNs sorbed onto the clay surface is normalised to the
amount of clay (mass). These values are useful when comparing sorption of
different RNs, or one element in different oxidation states, onto one type of
clay with a given size distribution. The drawback with KD values is that they
assume a sorption site density per unit mass for the material studied and
not a site density normalised to its exposed surface. Therefore, the question
is whether sorption models for RNs can be developed and applied to
different sizes of clay colloids. In order to do this, sorption properties, such
as specific sorption sites or cation exchange properties, have to be
characterised for each type of clay and its colloid size [23, 29].
Several sorption models have been developed where sorption of RNs onto
clay surfaces is explained by interactions at specific edge sites. Most often,
no physical explanation of how these sites are constructed is given [32, 33].
The only information given is a general edge site density per unit area for a
9
1. Introduction
given clay mineral, which is calculated from the theoretical formula of the
clay [34]. These models have recently been further developed using metal
oxides, such as Al-OH and Si-OH, as the specific sites on the clay colloids
[35]. Interactions between the surface and RNs are explained as:
≡ MeOH + RN
→≡ MeO − RN
(
)
+H
where ≡ Me corresponds to a metal on the clay surface. Even though Al and
silicon (Si) oxides are the most dominant species in the interactions between
RNs and clays [36-38], many other possible sites of metal oxides at the clay
surface may explain the complexity of the sorption mechanism. Up to 27
different metal oxides and oxides formed from two different metals are
suggested for determining the interactions between sorption of protons onto
the clay structure. This development is performed using a non-electrostatic
model based on the MUSIC approach [38, 39]. The question is whether all
these sites are active in sorption of RNs and whether association constants
can be stated for all these sorption mechanisms. As mentioned above, a first
modelling approach, using Al and Si oxides as sorption sites, has been
performed by Gaskova et al. [35] for sorption of U(VI) and Eu(III) onto
bentonite. Even if these results correspond well to the experimental results,
this is most probably not the complete description of the sorption
mechanisms. If possible, all sites which participate in proton exchange [38]
should be included in sorption models of heavier elements, such as RNs.
In addition to the specific sorption sites, cation exchange of the clay
particles has to be considered in the sorption modelling of RNs. Knowledge
about the amount of interlayers and basal planes for the clay stacks is
needed since ion exchange occurs faster at basal planes than on interlayers.
There, the ions have to diffuse into the interlayers before participating in ion
exchange [40, 41].
In all models available for estimation of sorption of RNs onto clay particles,
a cation exchange capacity and an edge site density for the clays are given as
general properties normalised to the surface area of the clays, irrespective of
clay colloid size [23, 32, 35, 41]. In most studies, an average particle size of
the clay is assumed after short-term sedimentation to remove particles
larger than 2 µm. From the estimated area, the edge sorption capacity can
10
1. Introduction
be calculated from structural formula of the clay particles [30]. This
assumes a symmetrical amount of edge site density to mean particle size.
There has been no previous discussion about whether the smaller particles
are miniatures of the larger ones and whether edge sites are distributed in
the same way on the surface.
The number of available sorption sites would be expected to increase with
decreasing particle size since both the surface area and the number of
broken structures in the crystalline clay structure are increased (Figure 4).
In this thesis, changes in sorption with changes in clay colloid size were
studied. If present, a dependency of sorption on colloid size should be
implemented in reactive transport models. If there is no effect of colloid size
on the sorption, an approach is needed to estimate sorption from the
composition and conformation of the clay particles.
In addition to sorption capacity studies at chemical equilibrium, the kinetics
of the sorption process has to be studied over a long time. Sorption of RNs
onto montmorillonite present in bedrock fractures is a dynamic process that
aims to reach the thermodynamically most favourable equilibrium.
Figure 4: Schematic diagram of how the sorption of RNs may differ with size of the clay
colloids. The question is whether the sorption remains unchanged with a change in clay particle
size.
If metal oxides in the clay structure correspond to the specific sorption sites
used in sorption models [38, 42], this information can be used when
11
1. Introduction
comparing the sorption properties of γ-irradiated and unirradiated
bentonite colloids. The specific sorption sites may be affected upon exposure
to ionising irradiation, since the surface characteristics of clay colloids are
known to change [22, 28, 43]. Moreover, organic degradation products are
affected by ionising radiation [44] and are known to be present in bentonite
(approximately 0.2% by mass) [45]. If sorption of RNs onto bentonite
colloids is affected by the amount of organic matter present at the clay
surface, sorption is affected by γ-irradiation.
Transport of RNs is highly dependent on the sorption capacity of RNs onto
clay colloid surfaces. When the sorption mechanisms are fully understood,
risk assessment models with finer resolution can be constructed.
1.4.5 Sorption reversibility of radionuclides
During transport of RNs in bedrock fractures, sorption of RNs is driven to
be in equilibrium with the surrounding chemical conditions. The sorption
equilibrium will be affected if the chemical conditions change over time or
distance or if competing ligands are added to the system (Figure 5). During
the estimated lifetime of a repository for spent nuclear fuel, i.e. at least 100
000 years, the chemical conditions may vary significantly.
Figure 5: Schematic diagram of different sorption reversibility cases.
12
1. Introduction
Two factors with a large impact on RN sorption are pH and ionic strength
[30, 32, 46]. Both the surface charge of the particles and the RN valency are
dependent on pH. The chemical composition of the background electrolyte
is also affected by a change in pH if, for example, carbonate is present in
solution. An increase in ionic strength affects sorption since clay colloids are
unstable at higher ionic strength [47, 48]. Ions in the background electrolyte
may compete in sorption onto the clay by cation exchange. Furthermore, the
charge of the clay particles will be screened at higher ionic strength.
Organic matter present in groundwater (up to 5 mg/L [49]) is known to
form strong and highly soluble complexes with RNs under these conditions
[50-53]. Small organic molecules are also known to interact with clay
particle surfaces by electrostatic interactions [19, 54]. Therefore, it is highly
important to study the sorption behaviour of RNs onto clays in the presence
of organic matter as a competing ligand. Moreover, RNs are known to sorb
onto bedrock surfaces and fracture filling material, surrounding the
bentonite clay [55-57]. This increases the complexity of the sorption system.
By adding samples of fracture filling material or crushed bedrock material to
sorption samples of RNs and bentonite colloids, these ternary systems can
be studied.
13
1. Introduction
1.5 Objectives of this thesis
The aim of this thesis was to determine whether smaller- and larger-sized
montmorillonite colloids have the same stability and sorption properties. In
addition, the effect of γ-irradiation on sorption capacities of
montmorillonite colloids was studied. The main objectives investigated in
the thesis were:
• Do smaller and larger montmorillonite colloids have the same
composition and surface-related physical properties?
• Can montmorillonite colloids be separated by size in suspension?
If so,
• Is there an effect of colloid size on colloid stability?
• Can the sorption capacity of RNs onto montmorillonite colloids be
normalised to exposed lateral surfaces, related to the size of the colloids?
• Is the sorption reversibility of RNs affected by the exposed area of clay
colloids in suspension?
In addition,
• How are the sorption properties of metal cations onto clay colloids
affected by γ-irradiation?
14
2. Materials
2. Materials
2.1 Unpurified MX80
One of the bentonite clays used as a reference material for the storage of
spent nuclear fuel in Sweden is unpurified MX80 bentonite [58]. Wyoming
Volclay bentonite MX-80 from American Colloid Co., consisting of
approximately 82 % montmorillonite, was used as the test material in this
thesis. The structural formula of this montmorillonite is [45]:
Na
.
(Al
.
Fe
.
Mg
.
)(Si
.
Al
.
)O (OH)
The molar weight of the structural unit is MW = 372.6 g/mol [45] and its
cation exchange capacity (CEC) is 0.75 meq/g (measured according to [59]).
In addition to montmorillonite, accessory minerals (e.g. cristobalite, quartz,
gypsum) are present in the clay [45, 60], in separate phases to
montmorillonite. The montmorillonite was purified during colloid size
separation, with the accessory minerals only present in the larger colloid
size fractions [60]. Furthermore, a small amount (approximately 0.2% by
mass) of organic matter is present in unpurified MX80, which may affect its
properties [45, 61].
Due to the high heterogeneity of this material in mineralogical composition
and colloid size, additional purified clays and latex standards were used in
the experiments to better understand the reactive mechanisms of MX80
bentonite.
2.2 Homo-ionic montmorillonite
In order to estimate how the impurities in MX80 bentonite affect clay
properties on exposure to γ-irradiation, the sorption capacity of purified
clays was compared. Apart from Mx80 bentonite, two Na-montmorillonites
and one Ca-montmorillonite were used in the study [61]. Purified Namontmorillonite originating from Wyoming montmorillonite was supplied
from the Clay Mineral Society. This clay is free from both free iron (Fe)
(citrate-borate treatments) and organic matter (H2O2 treatment), and is
15
2. Materials
hereafter denoted SWyNaw. In addition, Na- and Ca-montmorillonite
originating from MX80 (WyNa and WyCa) where the clays are cationexchanged by NaCl or CaCl2, respectively, were included in the analyses. A
detailed description of preparation of these clays is provided elsewhere [45].
2.3 Organic matter
2.3.1 Humic substances
The humic substances (HS) used in the γ-irradiation experiments comprised
a commercial humic standard (1S102) obtained from the International
Humic Substances Society. The HS was first dissolved in NaOH at pH 14
and later neutralised to pH ~7 by addition of H2SO4 under stirring. The
ionic strength of the solution in this thesis is given after adjustment of pH.
2.3.2 Fulvic acid
The organic matter used in all experiments performed on the size-separated
montmorillonite was fulvic acid (FA-573) originating from natural water
(Gohy573) at the Gorleben site, Germany [62]. The element composition of
the FA, given by the manufacturer, is presented in Table 1. Fulvic acid most
often has smaller colloids than HS in suspension and has more functional
groups, resulting in a higher total change. Therefore, FA is more soluble in
acid solution [63].
Table 1: Element composition of fulvic acid (FA).
Element
C
H
O
N
S
Proton exchange
capacity
Amount
54.1 ± 0.1 %
4.23 ± 0.08 %
38.94 ± 0.04 %
1.38 ± 0.02 %
1.32 ± 0.01 %
6.82 ± 0.04 meq/g
16
2. Materials
2.4 Nano-sized polystyrene (latex) particles
In the stability studies, well-defined, spherical and homogeneous
carboxylated polystyrene (latex) nanoparticle standards (Magsphere Inc.,
Pasadena, USA) were used to gain a better fundamental understanding of
agglomeration processes. These latex particles represent a large size range of
nano-particles, from 24 nm up to 495 nm. Their dimensions correlate well
with independent measurements by Scanning Electron Microscopy (SEM;
FEI QUANTA 650 FEG). The sizes and their corresponding charge densities
are presented in Table 2.
Table 2: Characteristics of the standard carboxylated latex particles used in the stability studies
and their corresponding charge density. Particle diameter and surface charge density as given
by the manufacturer.
Sample
LC24
LC40
LC60
LC81
LC97
LC217
LC420
LC495
Particle
diameter [nm]
24 ± 8
40 ± 13
60 ± 20
81 ± 6
97 ± 1.5
217 ± 24
420 ± 29
495 ± 50
SEM measurements
[nm]
30 ± 5
47 ± 6
64 ± 10
88 ± 15
104 ± 19
214 ± 28
419 ± 9
511 ± 79
Surface charge
density [mC/m2]
72.8
94.8
121
45.1
34.1
182
220
209
2.5 Synthetic groundwater
To simulate carbonated groundwater with low ionic strength, representing
glacial meltwater, a synthetic groundwater (SGW) was prepared. This SGW
was used as the background electrolyte in all experiments that included sizeseparated montmorillonite colloids. The SGW was chosen over a natural
groundwater to avoid the influence of natural colloids present in the
groundwater and to provide a homogeneous background throughout all
experiments. The final composition of the SGW is given in Table 3. The
groundwater had a total ionic strength of 1.57 mM and pH 8.4 ± 0.1.
17
2. Materials
Table 3: Chemical composition of the synthetic groundwater (SGW) of Grimsel type used as
background electrolyte in all experiments performed.
Element
Na+
Ca2+
FClSO42Si
HCO3-
Concentration [mM]
1.2
0.05
0.1
0.074
0.04
0.5·10-3
1.4
2.6 Copper cubes
The copper cubes used in the experiments had dimensions 1 × 1 × 1 cm3
and consisted of 99.992% Cu. They were cut from a SKB copper canister
wall and polished on all sides, using SiC abrasive papers of 1200 grit in
99.5% ethanol, prior to addition to the bentonite slurry to avoid effects from
impurities on the copper surfaces. After polishing, the Cu cubes were ultrasonicated during 5 min and later dried under N2 in a glovebox where they
were stored until preparation of γ-irradiation samples (section 3.5.3).
18
3. Methods
3. Methods
3.1 Size fractionation of MX80
3.1.1 Fractionation method
Size fractionation of MX80 bentonite in SGW was performed prior to
stability, sorption and sorption reversibility experiments. All
montmorillonite colloids were separated in size by a sedimentation step,
followed by sequential or direct centrifugation according to a specially
designed protocol for the system. In the sedimentation step, 10 g/L MX80
was diluted in SGW, stirred for one day and left to sediment for three days.
The colloid suspension collected after sedimentation is hereafter referred to
as S0 (Figure 6). The collected suspension S0 was the starting material for
all centrifugation steps.
S0
S0
R0
Figure 6: Colloid suspensions of MX80 bentonite before and after sedimentation for three days
in synthetic ground water (SGW). The suspension S0 and the residual R0 are marked in the
right-hand beaker.
A sequential centrifugation procedure was performed on the S0 suspension
using the centrifugation times and speeds presented in Table 4 and Figure 7.
The ith suspension, Si, was centrifuged according the conditions given in
19
3. Methods
Table 4 and, after centrifugation, resulted in the suspension S(i+1) and the
corresponding residual R(i+1) (Figure 7).
Table 4: Conditions used for fractionation of clay suspensions, and colloid sizes (equivalent
sphere diameters) expected in the residuals (Ri) and the supernatants (Si).
Suspension
Conditions of separation
(C: centrifugation,
UC: ultracentrifugation)
Particle size expected
in the ith residual clay
fraction (Ri) in nm
Mean colloid size
expected in the ith
supernatant (Si) in nm
S0
3 days sedimentation
1000 ≤ R0
0 ≤ S0 ≤ 1000
S1
C: 30 min (S0) at 313 x g
450 ≤ R1 ≤ 1000
0 ≤ S1 ≤ 450
S2
C: 1 h (S1) at 700 x g
200 ≤ R2 ≤ 450
0 ≤ S2 ≤ 200
S3
C: 4 h (S2) at 1.200 x g
70 ≤ R3 ≤ 200
0 ≤ S3 ≤ 70
S3.5
UC: 30 min (S3) at 26.000 x g
50 ≤ R3.5 ≤ 70
0 ≤ S3.5 ≤ 50
S3.5UC
UC: 30 min (S0) at 26.000 x ga
50 ≤ R3.5UC
0 ≤ S3.5UC ≤ 50
S3.5UC, FA
UC: 30 min (S0) at 26.000 x ga
50 ≤ R3.5UC, FA
0 ≤ S3.5UC, FA ≤ 50
a
: One-step ultra-centrifugation from a colloid suspension, S0, obtained after stirring and
sedimentation of an MX80 suspension at 10 g/L in the presence or absence of 11.8 mg/L fulvic
acid (FA) .
Figure 7: Separation procedure used to obtain suspension Si during the sequential
centrifugation separation.
For comparison to sequential centrifugation, the colloids in S0 were
separated by one direct centrifugation with the same centrifugation speed
and time as for the smallest-sized colloids in the sequential centrifugation
20
3. Methods
process (Table 4). This direct centrifugation was performed both in the
absence and presence of FA.
3.1.2 Characterisation of size fractions
All size-fractionated montmorillonite colloids were characterised to study
whether the composition of the colloids remained the same throughout all
separation steps. The element concentrations and size distributions were
determined.
3.1.2.1 Ion and colloid concentrations
The element concentrations in the suspensions presented in Table 4 were
determined by inductively-coupled plasma-optical emission spectrometry
(ICP-OES; Optima 2000 DV, PerkinElmer), inductively-coupled plasmamass spectrometer (ICP-MS; X-Series2, Thermo Scientific, Germany) and
ion chromatography (IC; ICS-3000). The concentrations were then
compared to the structural formula of the montmorillonite colloids to obtain
the clay colloid concentration. Background concentrations for all elements
were taken from fast centrifuged samples where no colloids were detected
[60].
3.1.2.2 Mineral characterisation and content of organic matter
The mineral composition was determined by X-ray diffraction (XRD) where
all suspensions and residuals were dried on low background Si wafers. The
composition of the raw material MX80 was also monitored as a reference.
The powder diffractograms were recorded with a D8 Advance (Bruker)
diffractometer (Cu Kα radiation) equipped with an energy dispersive
detector (Sol-X). The phases were identified with the DIFFRAC.EVA version
2.0 software (Bruker) by comparison with the JCPDS 2 database.
The amount of FA as organic matter in the suspensions was measured using
a total organic carbon analyser (TOC-5000, Shimadzu) before and after
centrifugation. These results indicated that one-third of the FA was
associated to the montmorillonite colloids after size separation.
3.1.2.3 Size distribution measurements
To determine the efficiency of the size fractionation, size distribution
measurements were performed using two methods: i) Photon correlation
21
3. Methods
spectroscopy (PCS; homodyne single beam ZetaPlus System equipped with a
50mW solid-state laser emitting at 632 nm, Brookhaven Inc, USA, using the
Non-Negatively constrained Least Squares (NNLS) algorithm [64]) and ii)
an asymmetric flow field-flow fractionation system (AsFlFFF; HRFFF
10.000 AF4, Postnova Analytics, Landsberg, Germany) coupled to a UV-Vis.
detector (LambdaMax LC Modell 481, Waters, Milford, USA) and an ICPMS unit (X–Series2, Thermo Scientific, Germany). These techniques
complement each other since PCS gives the mean intensity-weighted and
volume weighted colloid size of the measured sample, whereas the
AsFlFFF/UV-Vis./ICP-MS system gives the element concentration as a
function of particle size. A detailed description of the system and method
used for AsFlFFF can be found in the Supporting Information (SI) to [60].
For the conditions used in this thesis, the AsFlFFF/UV-Vis./ICP-MS system
does not detect the largest sized clay colloids, which were detected with PCS.
3.2 Stability studies
Mono-dispersed nano-sized latex particles with well-defined particle sizes
(section 2.4) were used as reference material in the stability studies (Table
5). Agglomeration in the suspensions was taken as the increase in mean
hydrodynamic diameter for the particles as a function of time, monitored by
PCS (same instrument as described in section 3.1.2.3). The conditions
studied for the different particles at pH 7 are summarised in Table 5.
Table 5: Electrolytes and their concentrations expressed in ionic strength used in the stability
studies.
Electrolyte
NaCl
CaCl2
NaCl
CaCl2
MgCl2
Particles
Latex particles
Latex particles
Montmorillonite colloids
Montmorillonite colloids
Montmorillonite colloids
Ionic strength [M]
0.5 and 2
0.5 and 2
0.01, 0.1 and 1
0.03, 0.3 and 3
0.03, 0.3 and 3
The agglomeration rate for the systems was determined by linear
approximations of the increase in hydrodynamic diameter, using the first
10-25 data points for each set of data. This agglomeration rate was then
22
3. Methods
used to estimate the rate constant, k, of the system using the expression [1821, 65]:
k=
∙[ ]
1−
∙
∙
(
)
(Equation 6)
where r is the hydrodynamic radius of a singlet, r the radius of a doublet,
I the intensity of a singlet and I the intensity of a doublet, d
the
hydrodynamic diameter obtained from PCS, d the initial diameter of the
particles, t the time and [X] the initial concentration of particles present in
the samples, valid for low particle concentrations.
Agglomerate rate constants could not be determined for montmorillonite
since data on the initial concentration of particles were needed but were
unavailable due to the polydispersivity. The stability of the bentonite
suspensions was characterised by determining the stability ratio [18, 19, 66,
67] and the master curve [68-72] of the suspensions. The stability ratio, W,
was defined as the ratio between the fast agglomeration rate (suffix f) and
the measured agglomeration rate in the present sample:
W=
→
→
/[ ]
()
(Equation 7)
/[ ]
The master curve of a system was constructed by plotting the increase in
hydrodynamic diameter, normalised to the initial particle size (d /d )
against the time of the measurement (t), normalised to the characteristic
time of the Brownian aggregation process (t ). From the slope of this plot,
the fractal dimension (d ), i.e. the packing density of the agglomerates
formed, was obtained. It gives general information on the mechanisms of
the agglomeration process. Master curves were constructed for the latex
particles too, for comparison.
23
3. Methods
3.3 Sorption of radionuclides onto size-fractionated MX80
3.3.1 Radionuclide (RN) cocktail
The radionuclide cocktail used in the sorption studies was prepared prior to
addition of RNs to the colloid suspensions. It was prepared in an argon
glove box and kept acidified until addition to the pre-size-fractionated clay
colloids. The oxidation states and final concentration of RNs in the sorption
samples are presented in Table 6.
Table 6: Radionuclides, their oxidation state and final concentration in the sorption samples.
Radionuclide
Th(IV)
99Tc(VII)
233U(VI)
237Np(V)
242Pu(IV)
232
Concentration [M]
10-8
5·10-9
10-8
10-8
2·10-9
3.3.2 Preparation of sorption samples
All suspensions in Table 4 were used in the sorption study performed on size
fractionated montmorillonite colloids. This study was performed in a
glovebox under argon atmosphere (<1 ppm O2) at pH 8.9 ± 0.3 and Eh(SHE)
+232 ± 50 mV. Each sorption sample was prepared in duplicate with 20
mg/L montmorillonite colloids and a total sample volume of 50 mL. To
determine the time dependence of the RN sorption, samples were taken
after 72 h (3 days), 260 h (2 weeks), 580 h (1 month) and 3800 h (6
months). The amount of RNs was measured by ICP-MS before and after
ultracentrifugation and compared against the initial amount of RNs present
in the samples to determine the sorption.
3.3.3 Determination of sorption
The RN concentrations were measured with ICP-MS, where the distribution
of RNs is given as particulate (%Part), colloid associated (%Colloid) or free
(%Free) RNs in the sample (Figure 8). Particulates correspond to the
sedimented clay colloids and eigen-colloids formed by RNs. The amount of
particulates was determined from element concentrations in the sorption
24
3. Methods
samples at a given sorption time without any shaking, compared with the
initial concentration of the elements. The change in concentration before
and after ultracentrifugation at a given sorption time corresponded to the
montmorillonite-associated RNs, since ultracentrifugation under the
conditions applied is known to effectively remove clay colloids of nano-size
from solution [60, 73]. Lastly, the remaining concentration of RNs in the
sorption samples was taken to be free RNs in the samples. Note that no
distinction between free RNs and FA-RN-complexes could be made in the
presence of FA in the samples.
Figure 8: Schematic diagram of the radionuclide (RN) distribution in the batch sorption
samples, as detected by ICP-MS.
3.4 Sorption reversibility studies on radionuclides
To study the sorption reversibility processes for size-separated
montmorillonite colloids, the sorption samples described in section 3.3.2
were used. After a given sorption time, the chemical conditions in the
samples were changed or a competing ligand was added. Four different
sorption reversibility studies were performed for the sorption samples
where the sorption reversibility samples were prepared from the same
sorption sample at each sorption time as shown in Figure 9. From each
sorption sample, five sorption reversibility samples were prepared. The
25
3. Methods
sorption reversibility was studied after one week and one year. The
distribution of RNs in the sorption reversibility samples was detected as
described for the sorption samples (see section 3.3.3).
Figure 9: Schematic diagram of preparation of the sorption reversibility samples from the
sorption samples (described in section 3.3.2) at each given sorption time.
3.4.1 Change in chemical composition
3.4.1.1 Decrease in pH
A 0.1 mL aliquot of o.1 M HCL was added to 10 mL sorption sample to lower
the pH. After addition of acid, the pH was decreased from 8.9 to 5.6, and
then buffered back to pH 7.5 after one week. Therefore, an additional 0.15
mL 0.1 M HCl was added to the samples before storing them during one
year. After one year, the pH was decreased to 2.7.
3.4.1.2 Increase in ionic strength
By addition of 5 mL CaCl2 to 5 mL sorption sample, 0.5 M ionic strength
CaCl2 was obtained. The montmorillonite colloids are known to be unstable
at this ionic strength. These samples were prepared in duplicate.
26
3. Methods
3.4.2 Addition of competing ligand
3.4.2.1 FA as organic matter
Addition of FA was studied, since FA is known to form strong complexes
with RNs. A final concentration of 5.1 mg/L FA was obtained in the sorption
reversibility samples. The distribution of RNs in the samples was analysed
after one week and one year.
3.4.2.2 Crushed Bedrock Material (CBM)
Grandiorite from Äspö was used as crushed bedrock material (CBM). The
grandiorite had a grain size of 0.5-1 mm after crushing and was added to the
samples in a 1:4 solid to liquid ratio. The specific surface area of the CBM
grains, measured by BET (Quantachrome Autosorb Automated Gas
Sorption), was approximately 0.1 m2/g. Information on preparation of the
size-fractionated CBM and its composition is presented in [74].
3.5 γ-irradiation experiments
3.5.1 Batch sorption experiments
Two analogues to RNs, relevant for the storage of spent nuclear fuel, Cs+ and
Co2+, were used to study the effect of γ-irradiation on sorption onto
bentonite and montmorillonite. All clays presented in sections 2.1 and 2.2
were used in the study. The solid to liquid ratio for the clays was 1:200, with
trace amounts of 137Cs+ and 60Co2+ and 1 µM inactive carrier isotopes. The
experimental set-up used for the batch sorption samples is summarised in
Table 7. All irradiated clays were irradiated prior to addition of the
analogues in N2-atmosphere or air. For comparison, samples with added
humic substances (HS) prior to γ-irradiation were prepared to detect
whether presence of organic matter during irradiation affects the sorption.
The irradiated samples were irradiated with a γ-dose of approximately 60
kGy using a Gammacell 1000 Elite 137Cs-source. The pH in the samples was
controlled to pH 9 by a borate buffer. After five days of sorption, the RNs
were separated from the clay by centrifugation for 30 min at 6.000 rpm and
analysed with γ-spectroscopy.
27
3. Methods
Table 7: Experimental set-up used for batch sorption experiments studying the effect of γirradiation.
Clay
MX80
Na/Camontmorillonite
Na/Camontmorillonite
Atm.
N2
N2
dry
dry
Air
dry
γ-irradiated
wet
wet + HS
wet
wet + HS
wet
wet + HS
Unirradiated
wet
wet + HS
wet
wet + HS
wet
wet + HS
3.5.2 Radionuclide retardation in compacted bentonite
As a complement to the batch sorption samples described in section 3.5.1,
diffusion cells with the clays MX80 and SWyNaw at a dry density of 1.6 and
1.5 g/cm3, respectively, were prepared. The diffusivity of Co2+ and Cs+ was
studied for both γ-irradiated and unirradiated samples, as shown in Table 7.
During the experimental time (80 days), the outlet solution was collected on
a weekly basis and the γ-activity was analysed with γ-spectroscopy. After the
experiments were terminated, the clay plugs were sliced into thin sections
and their respective γ-activity was analysed.
3.5.3 Copper cube corrosion in the presence of bentonite
The Wyoming Volclay bentonite MX-80 used in this study was unpurified
prior to sample preparation. The bentonite was first dried at 80°C for 12
hours before being suspended at 4 wt% in suprapure MilliQ-water. The clay
slurry was degassed by bubbling N2 for 12 hours and thereafter transferred
to a glovebox.
Six irradiation samples were prepared in a glovebox by adding one copper
cube to 10 mL bentonite slurry. Two additional samples were prepared and
stored in the glovebox as unirradiated references. The irradiated samples
were irradiated for 7 days using a Gammacell 1000 Elite 137Cs-source. The
dose rates and the total doses for all samples are given in Table 8.
28
3. Methods
Table 8: Dose rates and total doses used for studying γ-irradiation induced copper corrosion in
bentonite clay.
Sample
1
2
3
4
5
6
Dose rate (Gy/s)
0.1
0.135
0.134
0.114
0.155
0.168
Total dose (kGy)
60
81
81
69
93
101
The amount of dissolved Cu from the copper cubes entering the suspension
was analysed by ICP-OES (Thermo Scientific iCAP 6000 series ICP
spectrometer) using a multi-element standard (Merck). The Cu
concentration in the suspensions was measured both before and after
centrifugation (30 min at 24.000 x g, Thermo Scientific Heraeus Megafuge
16, Microliter 30x2 mL-sealed rotor). The ICP-OES samples including
bentonite colloids were acidified by suprapure HNO3 (Merck) prior to
analysis.
3.5.4 Effect of total dose on the bentonite sorption capacity of Cu2+
The samples used to study the effect of total dose of γ-irradiation on Cu2+
adsorption onto MX80 were prepared similarly to those in the copper
corrosion experiments. A suspension of 4 wt% MX80 bentonite was
degassed by N2 for 12 hours prior to transfer into a glovebox. Nine samples
of 10 mL bentonite slurry were sealed under anoxic atmosphere and seven
of these were irradiated by the dose rates and total doses presented in Table
9. After irradiation, 3 mM CuCl2 were added to all samples and they were
allowed to equilibrate for 24 hours. Thereafter, the Cu concentration was
measured by ICP-OES before and after centrifugation (30 min at 24.000 x g,
Thermo Scientific Heraeus Megafuge 16, Microliter 30x2 mL-sealed rotor).
The ICP-OES samples including bentonite colloids were acidified prior to
analysis.
29
3. Methods
Table 9: Dose rates and total doses used for studying the sorption capacity of Cu
bentonite.
Sample
1
2
3
4
5
6
7
Dose rate (Gy/s)
0.155
0.297
0.168
0.100
0.135
0.249
0.134
Irradiation
(hours)
262
186
245
138
208
217
208
30
time
Total dose (kGy)
146
199
148
50
101
195
100
2+
onto MX80
4. Results and Discussion
4. Results and Discussion
In the following, the results from all experiments are discussed in terms of
whether the surface characteristics of montmorillonite colloids differ
between the smaller and larger colloids in the size distribution and whether
colloid properties are modified by exposure to γ-irradiation. If there is a
difference in surface properties, this may affect the overall macroscopic
properties of the bentonite barrier.
4.1 Size fractionation of MX80
4.1.1 Size distribution
The colloid size distribution of the isolated, size-fractionated clay
suspensions was monitored by both PCS and AsFlFFF, as described in
section 3.1.2.3. These measurements complement each other, since they
separate the colloid sizes by different physical properties. In addition, the
PCS-measurements are based on scattered laser intensity for the whole
samples, whereas the largest colloids were not eluted in the AsFlFFFmeasurements. As the intensity is highly dependent on particle size, i.e.
large particles result in higher scattered intensities, the mean hydrodynamic
diameters from the PCS-measurements will be larger than those sizes given
by AsFlFFF-measurements, which are dependent on the recovery of the
measurements as mentioned above.
The size distributions from the AsFlFFF-measurements were obtained from
the elution time, calibrated to latex standards of varying sizes. For the
conditions used in this work [60], the largest particles in the suspensions
were moving too slowly in the channel to be detected. The loss of signal for
the larger colloids in the AsFlFFF-measurements was reflected in the
recovery for the measurements. This means that care should be taken when
determining the mean size in colloid clay suspensions by these techniques.
4.1.1.1 Average particle sizes monitored by PCS
The intensity-weighted and volume-weighted mean sizes of the clay
suspensions obtained from PCS-measurements are presented in Table 10.
Since the mean colloid size decreased with increasing number of
31
4. Results and Discussion
centrifugation steps and force, this indicates that colloid size separation
succeeded, even though the mean sizes were larger than expected (Table 4).
In addition, the average count rates obtained from the measurements
decreased with decreasing particle size when all suspensions were
monitored at 10 mg/L clay colloids. Since larger particles result in higher
scattered light than smaller particles, the higher count rates for the
suspensions S0→S2 indicated a higher amount of large particles in these
fractions. The sizes obtained from direct (S3.5UC) and sequential (S3.5)
centrifuged suspensions under the same final centrifugation conditions were
similar. Therefore, the preparatory centrifugation steps did not affect the
final colloid size distribution in the suspensions.
Table 10: Size distributions for the size-fractionated clay colloid suspensions obtained from
PCS-measurements and the respective count rate measured in 10 mg/L colloid suspensions.
PCSI - intensity weighted mean size; PCSV - volume weighted mean size
Mean diameter (nm)
PCSI
PCSV
Average count
rate (kcps)
S0
1452 ± 632
962 ± 225
118 ± 7
S1
513 ± 60
610 ± 57
87 ± 2
S2
404 ± 95
337 ± 29
53 ± 1
S3
S3.5
S3.5UC
S3.5UC, FA
248 ± 28
181 ± 28
180 ± 39
185 ± 47
186 ± 64
172 ± 49
167 ± 50
143 ± 43
37 ± 1
20 ± 1
25 ± 1
29 ± 1
Sample
4.1.1.2 AsFlFFF
Aluminium is one of the main elements in montmorillonite, so the Al signal
was used as a fingerprint for the clay colloids in the AsFlFFF-measurements.
In the Al fractograms, broad size distributions were present in all clay
fractions ranging from 10 to 275 nm. However, the mean colloid size clearly
decrease with increasing fractionation steps, Figure 10. A more defined
separation of the different colloidal sizes was not obtained due to the
conditions used during the measurements [60], supporting information.
The recovery from the AsFlFFF-measurements is also an indication of the
size distribution, since the measurement time was too short for elution of
larger particles and therefore resulted in lower recovery. As shown in the
32
4. Results and Discussion
legend to Figure 10, the recovery of smaller-sized colloids (i.e. S3→S3.5UC)
was higher than that of larger-sized colloids. Consequently, the total size
distributions of the smaller-sized particles are presented in Figure 10,
whereas the larger-sized clay colloids were not apparent in the fractograms
of the largest-sized clay suspensions.
S0 (35 ± 4 %)
3
Al concentration (ng/s)
S1 (62 ± 5 %)
2.5
S2 (77 ± 3 %)
S3 (88 ± 5 %)
2
S3.5 (>90 %)
S3.5UC (> 95 %)
1.5
S3.5UCFA (87 ± 10 %)
1
0.5
0
0
25
50
75
100
125
150
175
200
225
250
275
Hydrodynamic diameter (nm)
Figure 10: Fractograms of monitored size distributions for the size-separated clay suspensions,
as measured by AsFlFFF. Recovery of the sample is indicated in brackets.
4.1.1.3 Comparison of the two techniques
The size distribution of particle suspensions monitored by different
techniques may differ if the particles are non-spherical and the techniques
are based on particle size separation by different physical properties. Most
often, the equivalent spherical diameter (ESD) is given from particle size
measurements. The ESD is defined either as the diameter of a sphere with
the same volume as the monitored particle [75] or as “the diameter of a
spherical particle which will give identical geometric, optical, electrical or
aerodynamic behaviour to that of the particle (non-spherical) being
examined” [76]. Clay colloids are known to be non-spherical [4], while their
size distribution has to be further investigated.
33
4. Results and Discussion
Particle sizes obtained from the volume-weighted PCS-measurements were
frictional translator diffusion data and are denoted dT hereafter [75]. In
contrast, the particle diameter given by AsFlFFF-measurements is the
Stokes diameter, dS [77]. By assuming a geometry of the clay particles and
comparing the PCS- and AsFlFFF-results, estimates of ‘real’ colloid sizes
were obtained from the measured ESD values.
Here, clay particles were assumed to be disc-shaped, for which previous
works have described a mathematical relationship between the ESD for a
translating particle (dT) and its corresponding ESD Stokes diameter (dS)
[75]:
δ
δ
=
=
ρ
ρ∙
ρ
ρ
ρ
(Equation 8)
(Equation 9)
where ρ is the axial (or aspect) ratio (ρ = δ/t
), with δ as the diameter of
the disc-shaped particle and t
the thickness of the disc.
From Equation 8 and 9, the axial ratio (ρ) can be obtained by the dT/dS-ratio
[60]. If ρ, dT and dS are known, the diameter of the discs (δ) can be
calculated, as can their thickness (t
). By assuming a given thickness of
one basal layer in the colloid stack (t ), the amount of clay sheets can be
calculated. In this thesis, t
was assumed to be 1.3 nm, as given by XRDmeasurements (section 4.1.2) and the literature [78]. The ESD values from
the PCS- and AsFlFFF-measurements are presented in Table 10 and Figure
10. The calculated sizes of the disc-shaped clay colloids in all suspensions
are presented in Table 11. For details, see [60].
34
4. Results and Discussion
Table 11: Mean diameters (δ) and thickness (t
) of disc-shaped clay colloids and number of
sheets in a clay stack calculated from PCS and AsFlFFF mean equivalent sphere diameter
(ESD) and mathematical Equations 8 and 9, according to [75]. The number of layers was
= 1.3 nm as the thickness of one clay sheet.
calculated by taking t
S0
66.3
Disc diameter
δ (nm)
1496
S1
34.6
940
27
21
S2
10.3
500
48
37
S3
3.7
250
68
52
S3.5
14.4
258
18
14
S3.5UC
10.0
246
25
19
S3.5UC,FA
3.0
187
63
48
Sample
Axial
ratio (ρ)
Disc thickness
(nm)
23
Number of
sheets
17
4.1.2 Characterisation of the colloids
The element composition of the clay suspensions was measured with ICPOES, IC and XRD, as described in section 3.1.2. If the composition of the
montmorillonite colloids was found to remain similar throughout the colloid
size distribution, this could be a first indication that smaller clay colloids are
miniatures of the larger ones.
The concentration of the main elements in the montmorillonite colloids (i.e.
Al, Mg, Si and Fe) decreased proportionally with increasing number
separation steps. Therefore, the element composition of all colloids,
independent of size, was assumed to be similar. The colloid concentration
calculated from these element concentrations decreased from 1100 mg/L for
S0 down to 90 mg/L clay colloids for S3.5UC. Interestingly, the
montmorillonite suspension which was fractionated in the presence of FA
had a higher clay colloid concentration (125 mg/L). The increase in clay
colloid stability is explained by minor sorption of highly negatively charged
FA, increasing the total surface charge of the colloids and thereby their
stability [19, 51, 79, 80].
35
4. Results and Discussion
There was a direct release of cations and anions from the unpurified MX80
bentonite to the background electrolyte in all suspensions. The largest
release was found for Na+ and SO42-, where the concentrations increased by
a factor of 3 and 10, respectively, compared with the SGW. These elements
were not associated to the clay colloids, since their concentrations remained
unchanged throughout all suspensions. There was an increase in Ca
concentration in the suspensions, which was associated with the amount of
clay colloids in the suspension. Calcium is not generally one of the structural
elements in montmorillonite. However, calcite is present in unpurified
bentonite clay [45, 81, 82]. Therefore, Ca may be dissolved and, by ion
exchange with Na at the basal plane, associated to the clay colloids. The ion
exchange between Ca and Na also explains the large release of Na to the
background electrolyte. To summarise, all size-separated montmorillonite
suspensions had similar element composition and no changes in the
composition were found between the colloid fractions (Table 12).
Table 12: Element composition of the size-fractionated montmorillonite suspensions. The
concentration of clay colloids was calculated from the Al-, Fe-, Si- and Mg-concentrations and
compared to the structural formula. The amount of recovered colloids in the suspensions was
defined as the amount of colloids in suspension relative to the initial suspension, S0.
Suspension
S0
S1
S2
S3
S3.5
S3.5UC
S3.5UC, FA
Suspension
S0
S1
S2
S3
S3.5
S3.5UC
S3.5UC, FA
SGW
[Al]
mg/L
[Mg]
mg/L
[Si]
mg/L
[Fe]
mg/L
[Coll.]
mg/L
133 ± 20
88 ± 8
65 ± 4
33 ± 3
11.3 ± 0.6
10.6 ± 0.6
14.7 ± 0.3
[Ca]
mg/L
8.6 ± 0.5
5.7 ± 0.1
4.0 ± 0.2
2.5
1.1
1.0
1.6
1.6
20 ± 2
13.6 ± 0.4
9.9 ± 0.6
5.5 ± 0.2
1.9 ± 0.1
1.8 ± 0.1
2.5 ± 0.1
[Na]
mg/L
87 ± 1
82 ± 2
76 ± 2
77
73
74
75
28
382 ± 110
252 ± 70
178 ± 46
99 ± 25
40 ± 18
40 ± 18
51 ± 18
[F]
mg/L
2.9 ± 0.5
2.9 ± 0.5
2.9 ± 0.5
3.0 ± 0.4
2.9
2.7
3.0
2.5
32 ± 2
21.4 ± 0.6
15 ± 2
8±1
2.7 ± 0.1
2.8 ± 0.3
3.5 ± 0.1
[SO4]
mg/L
37 ± 3
37 ± 3
37 ± 4
37 ± 4
36
36
36
3.2
1127 ± 170
746 ± 68
551 ± 34
280 ± 26
96 ± 6
90 ± 6
125 ± 3
Recovered
colloids (%)
100 ± 0
67 ± 4
48 ± 5
25 ± 2
9.4 ± 0.1
9.0 ± 0.2
12.1 ± 0.1
36
4. Results and Discussion
XRD was used to monitor the mineral phase composition of the clay
colloids. MX80, prior to any dilution into suspensions, was used as a
reference for the bentonite. Minerals, such as cristobalite, halite and quartz,
were present in the raw material and the largest-sized colloid suspensions
(S0, S1 and S2), as well as their corresponding residuals. In all clay
suspensions, intense reflections corresponding to montmorillonite were
observed, confirming presence of montmorillonite (Figure 11). The XRD
results confirmed that the structural composition of all montmorillonite
colloids was similar, independent of their size.
Figure 11: X-ray diffractograms obtained for (left) all residuals and (right) the respective
supernatants following size fractionation of MX80 bentonite.
4.1.3 Estimations of edge site densities
By approximating the clay colloids to discs with a given diameter and
thickness, the amount of edge sites for each mean particle size could be
estimated. Calculations of number of edge sites performed in this thesis are
based on previous work by White and Zelazny [34] and Tournassat et al.
37
4. Results and Discussion
[23]. Here, the number of aluminol (Al-OH) and silanol (Si-OH) groups was
given by the perimeter to area ratio of the particles, together with the
number of montmorillonite sheets stacked in one colloid and the structural
formula of bentonite. From the particle sizes presented in Table 11, the
number of sheets in one colloid was assumed not to affect the access to
lateral surfaces (where the edge sites were assumed to be present). By
assuming a clay density of 2.7 g/cm3, the number of edge sites was
calculated by equations given by Tournassat et al. [23] and is presented in
Table 13.
Table 13: Number of accessible edge sites calculated by assuming disc-shaped clay colloids
and the particle sizes presented in Table 11. The concentration of aluminol (nAl) and silanol
(nSi) groups was determined by taking into account the total number of layers in the colloids,
considering one clay layer. Number of edge sites (nTot) sum of aluminol + silanol groups.
Sample
S0
S1
S2
S3
S3.5
S3.5UC
S3.5UC,FA
nAl
mmol/kg
4.9
7.8
15
29
28
30
39
nSi
mmol/kg
6.3
10
19
38
36
38
50
nTot
mmol/kg
11
18
34
67
64
68
89
A six-fold difference in number of edge sites was found between the smallest
(S3.5UC) and largest (S0) sized clay fractions (Table 13). However, it should
be borne in mind that the recovery from the AsFlFFF-measurements was
low for the larger-sized clay colloids (i.e. S0 and S1), so the edge site
densities were underestimated for these clay fractions. In addition, the
approximation of the montmorillonite colloids to discs probably cause
underestimation of the exposed surface area of the colloids and thereby the
number of edge sites. These amounts of edge sites were used in interpreting
the results of the sorption and sorption reversibility studies (sections 4.3
and 4.4).
38
4. Results and Discussion
4.2 Size effect on colloid stability
4.2.1 Latex particles
To study a possible particle size effect on the agglomeration process,
homogeneous, spherical and well-characterised polystyrene (latex) particles
in the size range 20-500 nm were first used. Since the latex particles are
monodispersed, the initial particle concentration in the agglomeration
studies was known. The initial concentration is of great importance when
studying the agglomeration kinetics in a system. In addition, latex particles
with carboxylic functional groups have a surface charge close to the charge
of montmorillonite colloids at low ionic strength and pH < 7.
As mentioned in section 1.4.1, the DLVO-theory can be used for predictions
of the total interaction energy between two spherical particles. At high ionic
strength, the electrostatic repulsion between the two particles decreases
towards zero due to screening of the electrical double layer from high
amount of ions in solution. Therefore, the electrostatic repulsion is
negligible at high ionic strength and the total interaction between the
particles is dominated by van der Waals-attraction forces. This results in
agglomerates being formed upon collisions between two particles. However,
diffusion-controlled agglomeration is dependent on the kinetic energy of the
particles, since this controls the collision frequency in the suspension. By
comparing the total interaction energy and the kinetic energy for two
spherical particles as a function of particle size, it was found that the kinetic
energy dominated over the interaction energy, except at very short
(approximately 10% of the particle size) separation distances (Figure 12).
Hence, no difference in agglomeration rate due to initial particle size was
expected.
39
4. Results and Discussion
0
100
200
300
400
Interaction energy /kT
0
-5
24nm Vtot
40nm Vtot
-10
60mn Vtot
81nm Vtot
-15
97nm Vtot
217nm Vtot
-20
420nm Vtot
495nm Vtot
-25
Separation (Å)
Kinetic energy
Figure 12: Total interaction energy (Vtot) calculated between two particles of equal size as a
fuction of separation distance and particle size from DLVO-theory, and compared to the kinetic
energy k T of the particles. The calculations were made at high ionic strength where no
electrostatic repulsion between the particles is expected.
Agglomeration rate constants were calculated from the increase in average
hydrodynamic diameter as a function of time (described in section 3.2 and
Equation 6). From Figure 12, the agglomeration rate constant was expected
to be independent of initial particle size. By comparing the agglomeration
rate constants to the initial particle concentration, it was found that the size
dependence was an artefact and instead the agglomeration rate constants
were dependent on the initial particle concentration (Figure 13). The rate
constants obtained for the suspensions with low particle concentration (i.e.
< 1016 particles/m3) were higher than the theoretical Smoluchowski rate
constant, k = 1.22 ∙ 10 m3/s (Figure 13). These higher rate constants are
probably related to the fact that PCS is more sensitive to larger
agglomerates. Here, the agglomeration rates were taken as agglomeration of
multiples and not aggregation of monomers, since the agglomeration was
very fast.
40
4. Results and Discussion
1E-15
Agglomeration rate constant (m3/s)
24 nm
40 nm
60 nm
1E-16
81 nm
97 nm
217 nm
1E-17
420 nm
495 nm
1E-18
1E-19
1E+14
1E+15
1E+16
1E+17
1E+18
1E+19
Particle concentration (particles/m3)
Figure 13: Agglomeration rate constants in diffusion controlled agglomeration as a function of
initial particle concentration for different sizes of latex particles.
Another way of studying the agglomeration process is to determine the
fractal dimension, d , i.e. the porosity of the agglomerates formed, from a
master curve, as described in section 3.2. In a previous study, Wu et al. [72]
reported dependence of initial particle size on the d -value, which would
imply that the structures of the agglomerates differ with particle size. On
comparing our results with d -values from the literature, summarised by Wu
et al. [72], it was found that they were in line with previous findings (Figure
14a). At d > 2, the agglomeration was assumed to be in the reaction-limited
controlled agglomeration (RLCA) range and therefore negligible. However,
when the d -values were instead plotted as a function of initial particle size,
no correlation to particle size was found. Instead, the d -values increased
with increasing initial particle concentration (Figure 14b). To conclude, no
differences in the agglomeration behaviour between different sizes of
particles were found with regard to agglomeration rate constant or in cluster
density (d -value), as expected from theory. However, there was a large
41
4. Results and Discussion
dependence on initial particle concentration. This indicates that the
agglomeration mechanism was the same for all particle sizes.
3.0
a)
2.8
literature data
this study (averaged over all results)
2.6
2.4
df
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0
100
200
300
400
500
600
Particle diameter (nm)
3.0
2.8
b)
2.6
2.4
df
2.2
2.0
1.8
1.6
1.4
1.2
1.0
1E+13
1E+15
1E+17
1E+19
Initial particle concentration (particles/m3)
1E+21
Figure 14: Fractal dimension (d ) -values obtained from master curves for experimental data
(open squares) and literature data from Wu et al. [72] (filled diamonds) as a function of a) initial
particle size and b) initial particle concentration.
42
4. Results and Discussion
4.2.2 Montmorillonite colloids
From the agglomeration study of latex particles, no differences in stability
properties of montmorillonite colloids by size were expected, since the
chemical composition of the clay colloids was the same for all particle sizes
(section 4.1.2). As the particle concentration in the montmorillonite
suspensions was unknown, no agglomeration rate constants could be
determined.
When constructing a master curve for montmorillonite colloids, the clay
colloids disservice the universality of agglomeration, as found for illite
colloids [83]. From the slope of the master curve the z/d -ratio was 0.28,
indicating that z ≠ 1 and no d -values could be determined. Therefore, no
conclusions regarding particle size effects on cluster density could be drawn
for montmorillonite colloids. Hence, the assumption of non-sphericity of the
clay colloids in section 4.1.1 is supported by the finding that z ≠ 1, since this
indicates that clay colloid agglomerates have different conformations, such
as card-house structures.
However, relative stability, W, was determined by comparing agglomeration
at an ionic strength to the fastest agglomeration rate obtained in that
electrolyte (Equation 7). Due to the polydispersivity of the clay suspensions,
the agglomeration data were scattered. There were no differences in stability
ratios between different sizes of clay suspensions (Figure 15). Calcium ions
in the suspensions decreased the stability ratio more than Na ions at the
same concentrations. This is explained by the fact that divalent ions affect
colloids more efficiently than monovalent ions, from the Schulze-Hardy-rule
[84]. The similarities in stability ratios for all clay suspensions are in line
with the theory in section 4.2.1 where it was concluded that the initial
particle size does not affect agglomeration behaviour. No effect of colloid
size on the stability properties of the particles has been found in previous
studies [66, 85], but here this was explained by limitations in the
instruments.
43
4. Results and Discussion
a)
20
25
S1
S2
20
S3
15
W
S3.5
S3.5,UC
10
S3.5,UC-FA
W
25
S1
b)
S2
S3
S3.5
15
S3.5,UC
10
S3.5,UC-FA
5
5
0
0
0.01
0.1
1
Ionic strength [M]
0.03
3
0.3
Ionic strength [M]
3
Figure 15: Relative stability ratio, W, for all size-fractionated clay suspensions as a function of
ionic strength in a) NaCl and b) CaCl2. The agglomeration was not measured for suspension
S3.5 at 0.01 M NaCl.
The long-term stability of the clay colloids was monitored in all clay
suspensions by determining the concentration of suspended colloids after
two months without shaking. Larger particles, present in the suspensions S0
and S1 (Table 10) sedimented over time [60], whereas the smaller-sized
colloid fractions remained stable.
To summarise, no differences in agglomeration behaviour induced by
addition of electrolyte were observed for the size-separated clay colloids.
Even though universal agglomeration was not obtained for the clay colloids
[86], this is in line with the agglomeration behaviour of latex particles of
varying sizes. The lack of size effect is consistent with the composition of the
clay colloids remaining the same in all suspensions (Table 12 and Figure 11)
and with the overall physical properties remaining the same for all colloid
sizes. As expected, larger clay particles continued to sediment with time,
since complete sedimentation was not achieved before the first
fractionation.
In the case of transport of clay colloids in bedrock fractures, similar stability
properties of all particle sizes would allow the particles to be transported
under the same chemical conditions present in the groundwater.
Consequently, the particles could be separated by size, as described in
section 1.4.2. If the stability differed between particle sizes, the size
44
4. Results and Discussion
separation would be suppressed by the more stable particles being
transported further, even though their conformation is not the most
favourable in the prevailing water flow. Sedimentation of larger clay colloids
would be present, but these could be re-dispersed if the conditions changed,
e.g. higher water velocity, during transport.
4.3 Sorption properties of RNs and analogues onto montmorillonite
colloids
4.3.1 Effect of clay colloid size
4.3.1.1 RNs in the higher oxidation states, Np(V), U(VI) and Tc(VII)
The three RNs Np, U and Tc were mainly present as NpO2CO3-, UO2(CO3)34and TcO4- in the sorption studies performed [87-93]. The negatively
changed complexes and the high affinity to carbonate for Np and U predict
low sorption of these RNs onto montmorillonite. By comparing the
concentrations of the RNs in suspension before and after ultracentrifugation, it was found that their concentrations remained constant,
indicating no sorption of the RNs onto the montmorillonite colloids. The
absence of sorption of the RNs is expected from previous studies, due to the
low mass to liquid ratio of clay colloids. The low concentration of
montmorillonite predicts sorption of 1% for U [89] and even less for Np
since KD values < 102 dm3/kg have been reported [56, 94, 95]. This low RN
uptake was not detectible within the analytical uncertainty in this thesis.
Tc(VII) is known not to sorb onto minerals [87, 93]. In addition, no
reduction to Tc(IV) was found since TcO2(s) was not stable in the SGW [96].
The absence of sorption of Np, U and Tc onto montmorillonite was found
for all suspensions investigated (Table 10).
For the suspension which was isolated in the presence of FA during
centrifugation (S3.5UC,FA), FA is negatively charged at pH 9 and therefore no
sorption of FA onto montmorillonite was expected [51, 80, 97]. Since FA will
remain soluble in the suspensions, no effect on non-sorbing RNs was
expected or monitored.
45
4. Results and Discussion
4.3.1.2 Tetravalent RNs Th(IV) and Pu(IV)
The tetravalent RNs studied, i.e. Th and Pu, sorbed to a high extent onto all
size fractions of montmorillonite. As expected from the previous long term
stability measurements of the suspensions (Table 10), the larger sized clay
colloids sedimented over time. The sorbed Th and Pu remained sorbed onto
montmorillonite. This was seen by comparing the amount of sedimented
concentrations of Al to the Th or Pu concentrations.
From the large decrease in concentration before and after ultracentrifugation, approximately 98% of the initial Th concentration was
associated to montmorillonite. Thorium was probably sorbed as ternary Thcarbonate-montmorillonite complexes (Figure 16). This corresponds to a log
KD value of 6.4 (dm3/kg). No differences in sorption capacity between the
different sized montmorillonite suspensions were observed. The amount of
sorbed Th remained constant over the total sorption time investigated.
Plutonium sorbed initially (after three days of sorption) to 87%, which
increased to 98% sorbed Pu by the end of the sorption experiments (Figure
16). This corresponded to an increase in log KD value (dm3/kg) from 5.5
±0.5 up to 6.3 ±0.5, which was not significant. The higher amount of sorbed
Pu was similar to the amount of sorbed Th, which indicates that several
tetravalent RNs have similar sorption affinities, even if the dynamics in the
sorption process differ.
As mentioned in section 4.3.1.1, FA present in the colloid suspension
S3.5UC,FA will not be sorbed onto the montmorillonite surfaces, and will
instead act as a competing ligand. In the presence of FA, the sorption of Pu
and Th onto montmorillonite was significantly less. The associated Th was
15% less and Pu 10% less, compared with the sorption samples in the
absence of FA. No increase in sorbed Pu was observed for S3.5UC,FA. The log
KD values (dm3/kg) obtained for Th and Pu were 5.3 ±0.5 and 5.1 ±0.5,
respectively. From the concentration of FA and montmorillonite added in
the samples, these KD values are similar to those obtained in previous
studies [51, 93, 97, 98].
46
4. Results and Discussion
S0 Th-232
S0 Pu-242
7
6
5
4
3
2
1
0
120
100
80
60
40
20
0
3 days
7
6
5
4
3
2
1
0
120
100
80
60
40
20
0
2 weeks 1 month 6 months
3 days
S3.5 Th-232
7
6
5
4
3
2
1
0
120
80
60
40
20
0
2 week
80
60
40
20
0
1 month 6 months
3 days
100
80
60
40
20
0
7
6
5
4
3
2
1
0
100
80
60
40
20
0
1 month 6 months
3 days
%Colloid
242
1 month 6 months
120
7
6
5
4
3
2
1
0
%Part
2 week
S3.5uc,FA Pu-242
120
2 week
7
6
5
4
3
2
1
0
120
100
S3.5uc,FA Th-232
3 days
1 month 6 months
S3.5 Pu-242
100
3 days
2 week
%Free
232
2 week
1 month 6 months
Kd-values
Figure 16: Distribution of Pu(IV) and Th(IV) in the presence (20 mg/L) of different-sized
montmorillonite colloids in synthetic ground water (SGW) in the presence and absence of FA.
The x-axis represents the sorption time. The left y-axis represents the percentage of particulate
(%Part, blue), colloid bound (%Colloid, red) and free (%Free, green) radionuclides in solution.
3
The right y-axis is the corresponding log KD value (dm /kg) (purple line).
47
4. Results and Discussion
No differences in sorption capacity for Th and Pu onto montmorillonite
colloids of varying sizes were detected. An estimated six-fold difference in
the sorption was found in the sorption samples (Table 13). By using the
estimated amount of sites for the different size fractions, normalisation of
the sorption to number of sites instead of mass was possible. Given a sixfold difference for the smallest- and largest-sized colloid fractions, a
difference of log 6 = 0.8 in the log KD values was expected. This small
difference in sorption capacity could not be detected within the
experimental uncertainty.
4.3.2 Effect of γ-irradiation
γ-irradiation influence the properties of bentonite and montmorillonite
colloids, by affecting colloid stability and Fe2+/Fe3+-ratio [22, 28]. If the
structure in montmorillonite colloids is changed upon radiation, this may
affect the sorption properties of the clays. Sorption of Co2+ and Cs+ onto γirradiated and unirradiated clays (as references) is presented as massnormalised KD values in Figure 17. There was a large difference in sorption
capacity between the clays, with the Na-montmorillonites sorbing
significantly less than WyCa and MX80. Therefore the irradiation effect was
compared for each clay separately.
48
4. Results and Discussion
Sorption capacity KD (dm3/kg)
16000
a) Co2+
14000
12000
SWyNaw
10000
SWyNaw + Ca
8000
WyNa
6000
WyCa
4000
MX80
MX80 + Ca
2000
0
Dry, Dry, Wet, Wet,
N2, - O2, - N2, - O2, -
Wet, Wet, No,
No,
N2,
O2, N2, - N2,
Hum Hum
Hum
Sorption capacity KD (dm3/kg)
700
b) Cs+
600
500
SWyNaw
400
SWyNaw + Ca
WyNa
300
WyCa
200
MX80
MX80 + Ca
100
0
Dry,
N2, -
Dry,
O2, -
Wet,
N2, -
Wet,
O2, -
Wet, Wet, No,
No,
N2,
O2, N2, - N2,
Hum Hum
Hum
2+
+
Figure 17: Sorption capacity (KD values) for a) Co and b) Cs onto γ-irradiated or unirradiated
clays. The γ-irradiated clays were irradiated under wet or dry conditions, as indicated. The
unirradiated clays are denoted by No. The atmosphere in the samples was N2 or air (denoted
O2). Hum indicates presence of humic substances (HS). ‘+ Ca’ indicates addition of calcium to
the samples.
49
4. Results and Discussion
There was a significant effect of γ-irradiation on the sorption capacity of
Co2+ for all clays, with less sorbing onto the clays after γ-irradiation, except
in the washed Na-montmorillonite (SWyNaw). This trend was more
pronounced for clays with a high affinity to C02+. No significant effect of γirradiation on sorption of Cs+ was found. It is known that Co2+ is mostly
sorbed onto montmorillonite by surface complexation onto specific surface
sites, while Cs+ interacts with the clay mainly by cation exchange. Therefore,
the effect on C02+ sorption may indicate that γ-irradiation affects the
specific sites, whereas the overall CEC of the clay colloids remains
unchanged. The SWyNaw clay was pretreated with H2O2, which is one of the
main products in radiolysis of water. This may be one reason why the
sorption capacity for this clay was not affected by γ-irradiation. It is known
from the literature that Si is dissolved from the montmorillonite structure at
the edges of the sheets during exposure to γ-irradiation [26, 99]. Assuming
that Al-OH and Si-OH are the main sorption sites for higher valence cations
[36, 38], this is supported by the present results. Interestingly, the stability
of montmorillonite colloids, and thereby their charge, was previously found
to increase upon γ-irradiation. A higher colloid charge could be expected to
lead to higher sorption of cations, but the opposite was found in this study.
The change in sorption capacity between irradiated and reference clays did
not affect the diffusivity experiments [61]. This means that the change in
sorption has to be larger to affect the overall diffusion of RNs through
bentonite clay.
Addition of Ca2+, to compare the Na- and Ca-montmorillonites, decreased
sorption of Co2+ onto MX80, as expected due to competition between the
two divalent cations. However, the sorption increased for Co2+ onto
SWyNaw, which cannot be explained.
A second study of sorption capacity of divalent cations onto γ-irradiated
bentonite was performed using Cu2+. Here, the effect of γ-irradiation on the
sorption was studied as a function of total dose (Figure 18). As for sorption
of Co2+, sorption of Cu2+ decreased upon irradiation, although the effect was
not very dramatic [100]. The decrease in sorption capacity was most
pronounced up to a dose of 100 kGy. This result confirms that a change in
sorption capacity upon γ-irradiation occurs for bentonite and has to be
considered when including sorption capacity in reactive transport models.
50
4. Results and Discussion
700
600
KD (dm3/kg)
500
400
300
200
100
0
0
50
100
150
200
250
Total dose (kGy)
2+
Figure 18: Sorption capacity of Cu as a function of total dose of γ-radiation for bentonite prior
to sorption studies.
4.4 Sorption reversibility
4.4.1 Introducing organic matter as competing ligand
4.4.1.1 Effect for size-fractionated clay colloids
Fulvic acid acted as a competing ligand to RN sorption onto
montmorillonite, decreasing the sorption of both Pu and Th, since the
majority of FA was not sorbed onto montmorillonite under the conditions
used in this thesis [51, 80, 97]. When FA was added to the size-fractionated
sorption samples, the amount of reversible Th was approximately 20%
(Figure 19), reported previously [50, 51]. No effect of particle size was found
[101]. For Pu, the amount of FA-complexed Pu was dependent on the
sorption time prior to addition of FA. The reversibility was greater at shorter
sorption time (Figure 19). One explanation may be that Pu sorbs to less
assessable sorption sites over time, decreasing the reversibility with longer
sorption times. The interactions between FA and RNs have previously been
categorised as reversible, pseudo-irreversible and irreversible depending on
the equilibrium time needed [52, 53]. The reversible cation interactions
occur rapidly, whereas the site-specific interactions, which occur after a
51
4. Results and Discussion
longer contact time between the RNs and FA, are classified as irreversible.
Interactions of Th and FA have faster kinetics in the first 10 days, followed
by a slower sorption process over several months [53]. This may be the case
for Pu as well.
Comparing the CEC values for RNs to montmorillonite colloids and FA,
assuming a CEC of 0.75 meq/g for montmorillonite [45] and 6.82 meq/g for
FA [62] gave a two-fold higher CEC per unit volume for FA than for
montmorillonite (0.035 meq/L compared with 0.015 meq/L). This would
result in most RNs which interact via cation exchange being favoured to
bind to the FA. Previous studies of sorption competition between other
minerals and organic matter have found that the CEC has to be three-fold
larger than that of the mineral to affect Th significantly [50], which may
explain the large association of Th to the montmorillonite colloids in this
thesis [101].
Thorium
120
Plutonium
120
100
100
80
80
60
60
40
40
20
20
0
3 days
2 week
1 month
Sorption time
0
6 months
3 days
120
120
100
100
80
80
60
60
40
40
20
20
2 week
1 month
Sorption time
6 months
0
0
3 days
2 week
1 month 6 months
Sorption time prior to addition of FA
232
242
3 days
2 week
1 month 6 months
Sorption time prior to addition of FA
Figure 19: Amount of Th (left) and Pu (right) in their respective phase where the y-axis
represents the percentage of particulate (%Part, blue), colloid bound (%Colloid, red) and free
(%Free, green) radionuclides in solution in the sorption samples (upper figures [102]) and after
one year in contact with 5.1 mg/L fulvic acid (FA) (lower figures [101]). The five bars for each
UC
sorption time are (left to right): S0, S1, S2, S3 and S3.5 .
52
4. Results and Discussion
4.4.1.2 Presence of HS in γ-irradiated montmorillonite suspensions
On adding organic matter as a competing ligand to the sorbing Co2+ and Cs+
in the samples before γ-irradiation, the interactions between the cations, HS
and clay may be different from those in unirradiated samples due to
decomposition of HS by γ-irradiation [103, 104]. The experimental results
obtained in this thesis showed that presence of HS in the samples decreased
the sorption capacity of Co2+ more than in the γ-irradiated clays (Figure 17).
However, sorption of Cs+ was not significantly affected by the presence of
HS, which can be explained by its lower sorption capacity. An effect of γirradiation on the Co-HS complexes was not found. This indicated that even
if HS decomposes under γ-irradiation, its lower molecular units are still
charged and interact with cations in a similar way to the original HS. This
suggests that the main functional groups in HS, which are active in cation
interactions, remain unaffected by γ-irradiation.
4.4.2 Decrease in pH
All samples were buffered back from pH 5.6 to 7.5 after one week due to the
presence of carbonate in the SGW, which has good buffering capacity [105].
Uranium-carbonate complexes were less soluble at lower pH, whereas
approximately 80% U was sorbed onto the montmorillonite after one week
at pH 7.5. No differences in sorption capacity for the different-sized
montmorillonite colloids were found, as in the sorption samples. No
sorption of U onto montmorillonite occurred for the suspension S3.5UC,FA,
since FA strongly interacts with uranyl under the chemical conditions
provided [93].
When additional acid was added to lower the pH further, the final pH
obtained was pH 2.7. In these samples, U was once again free in solution,
since montmorillonite surfaces are positively charged under these
conditions [80]. Thorium, which was associated to montmorillonite to 98%
prior to the decrease in pH, was dissolved to 80% at pH 2.7. The amount of
free Th is in line with the literature where thorium is predicted to be present
as Th(OH)3+, which will not sorb to the positively charged montmorillonite
[106, 107]. Reversibility of Th sorption was found for all clay suspensions,
independent of colloid size. In contrast to Th, all Pu was in particulate form
at pH 2.7 after one year. Montmorillonite colloids were unstable at pH 2.7,
53
4. Results and Discussion
whereas all sorbed Pu remained sorbed onto the sedimented clay. In
addition, the amount of free Pu prior to the decrease in pH was found
among the particles. This can be explained by slow reduction of Pu(V),
present initially, to Pu(IV). Another explanation may be that a small amount
of Pu eigen-colloids may be formed under the conditions applied [108, 109].
4.4.3 Increase in ionic strength
After addition of 0.5 M ionic strength CaCl2, all clay colloids became
unstable and sedimented in the samples, as expected from the stability
studies of montmorillonite reported in section 4.2.2. Both Th and Pu
remained sorbed onto the sedimented montmorillonite. The amount of RNs
associated to FA in fraction S3.5UC,FA was also in the particulate phase in the
samples. This can be explained by FA being unstable at this ionic strength
too [79, 110]. No effect of colloid size was found in these suspensions.
4.4.4 Addition of crushed bedrock material (CBM)
On addition of crushed bedrock material to the sorption samples, Np and Tc
interacted with the solid phase and 41% of Np and 32% of Tc was in the
particulate phase after one week’s equilibrium time (Figure 20). After one
year, 97% of Np and all Tc were in the particulate phase (Figure 20). These
processes are probably a result of reduction of Np(V) to Np(IV) and Tc(VII)
to Tc(IV), where the eigen-colloids NpO2(s) and TcO2(s) are formed. The
tetravalent RNs may also sorb onto grandiorite. One explanation for the
reduction is the Fe present in grandiorite, which is known to reduce
technetium [55, 111]. Interestingly, no, or very little, Tc was reduced in the
presence of fracture filling material (FFM) in previous studies [57], whereas
fast reduction has been found in the presence of grandiorite [56]. The
differences in reactivity between grandiorite and FFM have been attributed
to the fact that grandiorite contains more iron [56]. No influence of colloid
size of the montmorillonite was found.
In the suspension S3.5UC,FA, approximately 11% of Tc was complexed to the
FA, instead of interacting with the CBM. Complexation between FA and Tc
has been found in previous studies in the absence of grandiorite [112-114],
and is therefore a probable explanation why a small amount of Tc remained
54
4. Results and Discussion
free in the solutions. This indicates that FA is a strong competing ligand to
CBM.
Technetium
120
100
100
80
80
60
60
40
40
20
20
0
3 days
120
2 week
1 month
Sorption time
6 months
0
3 days
120
1 week CBM
100
100
80
80
60
60
40
40
20
20
0
0
3 days
2 week
1 month
6 months
Sorption time prior to addition of CBM
120
Neptunium
120
3 days
2 week
1 month
6 months
Sorption time prior to addition of CBM
100
100
80
80
60
60
40
40
20
20
0
0
3 days
2 week
1 month
6 months
Sorption time prior to addition of CBM
99
237
6 months
1 week CBM
120
1 year CBM
2 week
1 month
Sorption time
1 year CBM
3 days
2 week
1 month
6 months
Sorption time prior to addition of CBM
Figure 20: Amount of Tc (left) and Np (right) in their respective phase where the y-axis
represents the percentage of particulates (%Part, blue), colloid bound (%Colloid, red) and free
(%Free, green) radionuclides in solution in the sorption samples (top figures [102]) and after
one week or one year in contact with crushed bedrock material (CBM) [101]. The six bars at
UC
UC,FA
.
each sorption time are (left to right): S0, S1, S2, S3, S3.5 and S3.5
55
4. Results and Discussion
From the literature, it is known that uranium sorbs onto grandiorite, with a
log KD value of 1.8 (dm3/kg) for fractions < 1160 µm and one week of
sorption time [56]. This KD value corresponds to the amount of sorbed 233U
after one week of contact time with CBM in this thesis, where 44% of 233U
sorbed. After one year, most 233U was released as free ions in the
suspensions. One explanation for this may be that natural 238U was released
from the CBM at a higher concentration than that of the added 233U. The
excess 238U may have led to cation exchange with the sorbed 233U, resulting
in 233U being released in suspension. The large amount of 238U release was
because uranium interacts with carbonate to form U-carbonate complexes.
Both Th and Pu remained sorbed onto the montmorillonite colloids, which
partly sedimented in the presence of CBM. In addition, the remaining,
previously free, part of Pu sorbed onto the CBM, resulting in all Pu being in
the particulate phase. The additional sorption of Pu may occur due to
reduction of a small amount of Pu(V) initially present to Pu(IV) by the Fe
known to be present in high amounts in crushed grandiorite [56].
4.5 γ-irradiation-induced copper corrosion in the presence of bentonite
When copper cubes were irradiated in the presence of bentonite, the free Cu
concentration in the aqueous phase increased with dose (Figure 21a) as
previously shown for copper cubes irradiated in pure anaerobic water [115].
However, the concentrations were considerably lower than in presence of
bentonite, owing to the high sorption of Cu onto bentonite as shown in
Figure 18. To determine the amount of sorbed Cu, the bentonite slurry was
acidified in order to desorb any Cu it had immobilised. The total
concentration of Cu released from the cube is presented in Figure 21b. The
same trend of increasing Cu concentration with dose was found in both
cases (Figure 21a and b). The total concentration of Cu was approximately
five-fold higher in the presence of bentonite than earlier found in water
[100]. Hence, it would appear that the effect of adsorption of Cu onto
bentonite is a significant driving force for the corrosion.
56
4. Results and Discussion
600
a)
Concentration Cu (µM)
Concentration Cu (µM)
0.40
0.30
0.20
0.10
0.00
0
50
100
150
Total dose (kGy)
b)
500
400
300
200
100
0
0
50
100
150
Total dose (kGy)
Figure 21: Release of Cu from irradiation of copper cubes under anaerobic conditions in the
presence of 4 wt% bentonite. a) Cu in aqueous phase b) Total concentration of Cu in samples.
4.6 General discussion
There were no significant differences in element composition, stability or
sorption capacity between the montmorillonite colloid sizes studied. The
similarities found in stability criteria indicate that smaller colloids are
miniatures of larger colloids. If the element composition or surface charge
varied by size, this would influence the Hamaker constant and the
electrostatic interactions. Thereby, the stability would also be affected.
Given the similar chemical composition of all colloids, similar sorption site
density per area can be expected. The edge sites are assumed to be present
at the lateral surfaces [23]. The number of edge sites for a colloid suspension
would then be given by the amount of lateral surface. By assuming discshaped colloids, the lateral surface was estimated here from the mean
colloid size. The lateral surface was approximately six-fold higher for the
smallest size fraction in this thesis compared with the largest. Within the
experimental uncertainty, this is not a sufficiently large difference to be
detected for highly sorbing radionuclides.
57
4. Results and Discussion
The remaining question is whether there is a small difference in sorption
capacity between smaller and larger colloids which cannot be detected, or
whether the distribution of sorption sites changes slightly with colloid size.
Bentonite is a natural clay and assuming all colloids to be disc-shaped is a
rough approximation. The surface roughness and the inner surfaces in the
porous colloids may influence sorption so that even smaller differences
between larger and smaller colloids would be expected. Even though
montmorillonite colloids can be separated by size initially, a change in size
distribution over time is expected. Larger colloids will sediment and there
may be a slow agglomeration process present in the suspensions for the
smaller-sized colloids. A more homogeneous distribution of colloid sizes
would also give an even smaller difference in sorption capacity.
More studies of the effects of γ-irradiation on clay surface properties have to
be performed to confirm whether the experimental sorption capacities
reported here are valid. A significant decrease in sorption capacity for
bentonite after exposure to γ-irradiation would call for more refined reactive
transport models.
58
Conclusions
Conclusions
From the studies conducted within the framework of this thesis, the
following conclusions can be drawn:
Dependence on particle size
-
A protocol was developed and successfully applied to fractionate
montmorillonite colloids into different-sized suspensions by
centrifugation. This protocol can be applied to other polydisperse
systems where the initial colloid size distribution cannot be
controlled. All montmorillonite colloid sizes have the same element
composition.
-
No effect of particle size could be detected in agglomeration
behaviour of either size-separated montmorillonite colloids or
homogeneous and well-defined polystyrene (latex) particles.
-
Thorium and plutonium sorbed onto all montmorillonite colloids,
independent of colloid size, in synthetic groundwater.
-
No dependency on colloid size was found in the sorption
reversibility studies.
Effect of γ-irradiation
-
Irradiated bentonite sorbed significantly less surface-complexed,
divalent cations than non-irradiated bentonite. Cation exchange was
not affected by irradiation.
-
Presence of bentonite enhanced corrosion of copper cubes upon γirradiation under anaerobic conditions.
59
Conclusions
General conclusions
-
Average sorption capacities can be used in reactive transport
models, as they are valid for the whole size distribution of
montmorillonite colloids. No dependency on exposed lateral
surfaces of the colloids has to be taken into consideration. This
facilitates model use.
-
The influence of γ-irradiation should be considered in reactive
transport models since divalent cations sorb less to irradiated
bentonite.
60
Further work
Further work
To the best of my knowledge, this thesis presents some of the first studies of
how the colloid size of bentonite affects transport of radionuclides in the
near-field of a planned repository for spent nuclear fuel. Based on the
results, the experimental design of sorption studies onto size-fractionated
clay colloids can be further developed. Firstly, suspensions including
smaller sized montmorillonite colloids with a concentration relevant for
sorption studies of radionuclides are desirable. In addition, narrower-range
suspensions with respect to colloid size, where smaller colloids are not
present in all suspensions, would probably give a clearer trend in sorption
capacity, if present. In order to see whether the colloids continue to differ in
size, the equilibrium colloid size of montmorillonite in suspension after a
longer time (over years) would be interesting to study.
These studies of the effects of colloid size on sorption should be
complemented with sorption studies of trivalent RNs, which were not
included in this thesis. Trivalent RNs are interesting since they can be
released from spent nuclear fuel and have different chemistry than
tetravalent cations. Furthermore, use of higher concentrations of RNs in
experiments would be desirable if radiation protection allows, since the
measurement uncertainty would be lower. Field-flow fractionation
measurements in the sorption studies would give the RN distribution with
colloid size in the suspensions. RNs which sorb to a lower extent onto clay
are interesting for use, since these RNs would give less experimental
uncertainty. These modifications would also be interesting for sorption
reversibility samples. For the sorption reversibility samples, more data
points over time would be needed to monitor the reversibility kinetics.
Moreover, classical desorption studies of different initial sizes of
montmorillonite colloids would give valuable information for reactive
transport models.
Several complementary techniques, such as static light scattering, can be
used to determine the agglomeration rate constants for different-sized
colloids. In suspensions with narrower size-fractionated montmorillonite
colloids, the initial particle concentration would be better estimated and
agglomeration rate constants for these colloids could also be estimated.
61
Further work
The effect of γ-irradiation of clays in the presence of metallic copper needs
more study, since no studies on γ-irradiation-induced copper corrosion or γirradiation effects on bentonite have previously been reported. In addition,
the effect of γ-irradiation on humic substances is currently unknown and
may affect the equilibrium between sorption onto clays and organic matter.
Many of the main objectives in this thesis are interesting for other similar
systems, such as transport of metal ions or organic pollutants in
contaminated soil. There the metal concentrations are often higher, the
groundwater conditions different and the time scale shorter. In such cases,
dependence of sorption capacity related to exposed surface area of the soil
particles would be an interesting topic for study.
62
List of abbreviations and symbols
List of abbreviations and symbols
Symbol
%Colloid
%Free
%Part
[X]
A
AsFlFFF
BET
CBM
CEC
δ
d
d
d
dT
dS
DLCA
DLS
ϵ
ϵ
e
ESD
f
FA
FFM
h
HS
I
I
I
IC
ICP-MS
ICP-OES
κ
k
k
k
n
n
Explanation
Amount of RNs associated to clay colloids in sample
Amount of RNs free in suspension
Amount of RNs associated to particulates in sample
Initial concentration of particles X present in sample
Hamaker constant
Asymmetric flow field-flow fractionation
Brunauer-Emmett-Teller surface
Crushed bedrock material
Cation exchange capacity
Diameter of a disc-shaped particle
Hydrodynamic diameter obtained from DLS
Fractal dimension
Initial diameter of a particle
ESD for a translating particle
ESD Stokes diameter
Diffusion limited controlled agglomeration
Dynamic light scattering
Permittivity of the material
Permittivity of vacuum
Charge of the electron
Equivalent spherical diameter
Suffix for fast agglomeration rate
Fulvic acids
Fracture filling material
Surface-to-surface separation between two particles
Humic substances
Ionic strength
Intensity of a singlet
Intensity of a doublet
Ion chromatography
Inductively coupled plasma-mass spectrometry
Inductively coupled plasma-optical emission spectroscopy
Inverse of Debye-Hückel screening length
Agglomeration rate constant
Boltzmann constant
Smoluchowski’s rate constant of agglomeration
Number concentration of electrolyte
Number of aluminol groups
63
n
n
NNLS
ρ
PCS
PCS
PCS
r
r
r
RLCA
RN
s
SEM
SGW
SI
t
T
t
t
t
TOC
UC
UV-Vis.
V
V
V
W
XRD
ψ
z
Number of silanol groups
Total number of aluminol and silanol groups
Non-negatively constrained least squares
Axial (or aspect) ratio
Photon correlation spectroscopy
Intensity-weighted mean size from PCS-measurement
Volume-weighted mean size from PCS-measurement
Radius of a spherical particle
Hydrodynamic radius of a singlet
Hydrodynamic radius of a doublet
Reaction-limited cluster aggregation
Radionuclide
Centre-to-centre separation between two particles
Scanning electron microscopy
Synthetic groundwater
Supporting information
Time
Absolute temperature,
Thickness of a disc-shaped particle
Thickness of one basal layer in the colloid stack
Characteristic time of the Brownian aggregation process
Total organic carbon
Ultra-centrifugation
Ultraviolet–visible spectroscopy
Electrostatic repulsion
van der Waals attraction
Interaction energy
Stability ratio
X-ray diffraction
Diffuse layer potential
Valence of the electrolyte
64
Acknowledgements
Acknowledgements
First, I would like to express my deepest gratitude to my supervisors Assoc.
Prof. Susanna Wold and Prof. Mats Jonsson for accepting me as a
PhD student in your group and giving me the opportunity to perform this
work. Your support along the way gave me the possibility to grow as a
researcher.
I would also like to gratefully thank Dr. Muriel Bouby for giving me the
opportunity to work with you at KIT-INE, Karlsruhe, Germany, during a
total period of 8 months. I cannot express my gratitude enough for your
supervision in the lab, our interesting discussions, your encouragement and
your friendship.
I wish to thank my co-authors for interesting discussions and fruitful
collaborations.
I would also like to extend my appreciation to my additional colleagues and
friends at INE: Johannes, Nicolas, Stephanie, Thorsten, Horst,
Florian, Tom, Daniel, Patric, Aurélie, Cornelia and especially Rémi.
Thank you for sharing your expertise, for your warm welcome, nonscientific discussion during lunch time and making Karlsruhe into my
second home.
To all my present and former colleagues at Applied Physical Chemistry at
KTH, namely: Michael, Amanda, Åsa, Inna, Kristina, Yang, Alex,
Mats, Johan, Gabor, Björn, Joakim, Camilla, Istvan, Anders,
Martin, Veronica, Sandra, Helena, Sara, Claudio and Marianne.
Thanks for making me looking forward to my working days and always
being available to help out in the lab. Our board game evenings, after-works,
trips etc. will remain as good memories for a very long time.
Prof. Per Claesson is acknowledged for reviewing the thesis and making
valuable comments. Thanks to Joakim, Sara and Björn for giving
feedback to my thesis.
Lena Skowron, Kenneth Andersson, Susanne De Meyere, Ilona
Mozsi, Vera Jovanovic and Dr. Ulla Jacobsson have been very helpful
65
Acknowledgements
with all administrative work during my time at Applied Physical Chemistry,
KTH.
The Swedish Nuclear Fuel and Waste Management Co. (SKB) is gratefully
acknowledged for financial support to this project. Moreover, part of the
work was funded by the project CP-BELBaR Fission 2010-1.1.1. Additional
financial support for conference scholarships and travel grants from
Ångpanneföreningens Forskningsstiftelse, T&E network activity in
TALISMAN and Klasons stiftelse is gratefully acknowledged. Last, this work
was supported by the European FP7 TALISMAN project, under contract
with the European Commission. My last stay in Karlsruhe would not have
been possible without this support.
Without all my friends from KTH, Kårspexet, orchestras etc., this work
would never have been finished. You are my never-ending source of joy.
To my family, for all the patience and support they have given during this
time. To my parents, for always being by my side and believing in me. To
Anders, who teaches me every day to never give up.
66
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