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Generation, stability and migration of montmorillonite colloids in aqueous systems
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology. (Nuclear Chemistry)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In Sweden the encapsulated nuclear waste will be surrounded by compacted bentonite in the granitic host rock. In contact with water-bearing fractures the bentonite barrier may release montmorillonite colloids that may be further transported in groundwater. If large amounts of material are eroded from the barrier, the buffer functionality can be compromised. Furthermore, in the scenario of a leaking canister, strongly sorbing radionuclides, can be transported by montmorillonite colloids towards the biosphere. This thesis addresses the effects of groundwater chemistry on the generation, stability, sorption and transport of montmorillonite colloids in water bearing rock fractures.

To be able to predict quantities of montmorillonite colloids released from the bentonite barrier in contact with groundwater of varying salinity, generation and sedimentation test were performed. The aim is first to gain understanding on the processes involved in colloid generation from the bentonite barrier. Secondly it is to test if concentration gradients of montmorillonite colloids outside the barrier determined by simple sedimentation experiments are comparable to generation tests. Identical final concentrations and colloid size distributions were achieved in both types of tests.

Colloid stability is strongly correlated to the groundwater chemistry. The impact of pH, ionic strength and temperature was studied. Aggregation kinetics experiments revealed that for colloid aggregation rate increased with increasing ionic strength. The aggregation rate decreased with increasing pH. The temperature effect on montmorillonite colloid stability is pH-dependent. At pH≤4, the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH≥10, the aggregation rate constant decreased with increasing temperature. In the intermediate pH interval, the aggregation rate constant decreased with increasing temperature except at the highest ionic strength, where it increased. The relationship between the rate constant and the ionic strength allowed the critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined.

In order to distinguish the contribution of physical filtration and sorption to colloid retention in transport, the different retention mechanisms were quantified. Sorption on different representative minerals in granite fractures was measured for latex colloids (50, 100, 200 nm) and montmorillonite colloids as a function of ionic strength and pH. Despite of the negative charge in mineral surfaces and colloids, sorption was detected. The sorption is correlated to the mineral point of zero charge and the zeta potential of the colloids, and increases with increasing ionic strength and decreasing pH. In transport experiments with latex colloids in columns packed with fracture filling material, the retention by sorption could clearly be seen. In particular at low flow rates, when the contact time for colloids with the mineral surfaces were the longest, sorption contributed to retention of the transport significantly. The retention of latex colloids appeared to be irreversible in contrary to the reversible montmorillonite colloid retention.

Generation, stability and sorption of the montmorillonite colloids are controlled by electrostatic forces; hence, the results were in qualitative agreement with DLVO.

Place, publisher, year, edition, pages
2010. , xii, 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:3
Keyword [en]
Bentonite; montmorillonite; colloid stability; colloid migration; geological disposal; nuclear waste; granitic fractures;
National Category
Environmental Sciences Environmental Sciences Physical Chemistry Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-11847ISBN: 978-91-7415-535-8 (print)OAI: oai:DiVA.org:kth-11847DiVA: diva2:284953
Public defence
2010-01-29, F1, Lindstedtsvägen 22, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2010-01-12 Created: 2010-01-08 Last updated: 2010-04-28
List of papers
1. Determining pseudo-equilibrium of montmorillonite colloids in generation and sedimentation experiments as a function of ionic strength, cationic form, and elevation
Open this publication in new window or tab >>Determining pseudo-equilibrium of montmorillonite colloids in generation and sedimentation experiments as a function of ionic strength, cationic form, and elevation
2009 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 335, no 1, 54-61 p.Article in journal (Refereed) Published
Abstract [en]

Colloid generation and sedimentation experiments were carried out on Na- and Ca-montmorillonite in order to verify whether pseudo-equilibrium concentrations are reached at the same level in both types of experiments. The size and concentration of colloidal Na- and Ca-montmorillonite particles were monitored as a function of time and distance from the colloid bed in different ionic strength solutions. A stable pseudo-equilibrium concentration was reached after time in generation and sedimentation experiments. The colloid concentration decreased sharply at distances near to the colloid source. Na-montmorillonite concentration at pseudo-equilibrium (roughly quantified at distances ≥7 cm from the colloid source) was 5.2 ± 0.5, 0.5 ± 0.1, and 0.2 ± 0.1 mg L-1 in 0.001, 0.01, and 0.1 M NaCl solution, respectively, while the Ca-montmorillonite concentration was 0.4 ± 0.2 mg L-1 in 0.001 M NaCl.

Keyword
Ca-montmorillonite, Colloids, Generation, Na-montmorillonite, Sedimentation, Size distribution
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-11850 (URN)10.1016/j.jcis.2009.02.048 (DOI)000266615400009 ()2-s2.0-65549100552 (Scopus ID)
Note
QC 20100908Available from: 2010-01-12 Created: 2010-01-12 Last updated: 2017-12-12Bibliographically approved
2. Kinetic determination of critical coagulation concentrations for sodium- and calcium-montmorillonite colloids in NaCl and CaCl2 aqueous solutions
Open this publication in new window or tab >>Kinetic determination of critical coagulation concentrations for sodium- and calcium-montmorillonite colloids in NaCl and CaCl2 aqueous solutions
2007 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 315, no 2, 512-519 p.Article in journal (Refereed) Published
Abstract [en]

The stability of the sodium and calcium forms of montmorillonite was studied at different NaCl and CaCl2 concentrations. The aggregation kinetics was determined from the decrease in particle concentration with time at different electrolyte concentrations. The DLVO theory defines the critical coagulation concentration (CCC) value as the electrolyte concentration that balances the attractive and repulsive potential energies between the particles, making aggregation diffusion-controlled. Therefore CCC values were obtained by extrapolation of the aggregation rate constants measured as a function of ionic strength to conditions where the rate constant value is determined by diffusion only. When the electrolyte was CaCl2, the CCC value was found to be approximately two orders of magnitude lower than the CCC values obtained using NaCl as electrolyte.

Keyword
Aggregation, CCC, DLVO theory, Montmorillonite colloids, PCS, Zeta potential
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7532 (URN)10.1016/j.jcis.2007.07.002 (DOI)000250064100013 ()2-s2.0-34548860946 (Scopus ID)
Note
QC 20100921Available from: 2007-10-03 Created: 2007-10-03 Last updated: 2017-12-14Bibliographically approved
3. Temperature effect on the stability of bentonite colloids in water
Open this publication in new window or tab >>Temperature effect on the stability of bentonite colloids in water
2006 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 298, no 2, 694-705 p.Article in journal (Refereed) Published
Abstract [en]

The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10-3 M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The ζ-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles.

Keyword
Bentonite colloids, DLVO theory, PCS, Sedimentation, Zeta potential
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7533 (URN)10.1016/j.jcis.2006.01.018 (DOI)000237909100026 ()2-s2.0-33646374149 (Scopus ID)
Note
QC 20101001Available from: 2007-10-03 Created: 2007-10-03 Last updated: 2017-12-14Bibliographically approved
4. Effects of temperature on the stability of colloidal montmorillonite particles at different pH and ionic strength
Open this publication in new window or tab >>Effects of temperature on the stability of colloidal montmorillonite particles at different pH and ionic strength
2009 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 43, no 1, 21-26 p.Article in journal (Refereed) Published
Abstract [en]

The effect of temperature at different pH and ionic strengths on the aggregation kinetics of colloidal montmorillonite particles in aqueous dispersions was investigated. For a given temperature and pH, the rate constant for aggregation increased with increasing ionic strength. At pH:54 the rate constant for colloid aggregation increased with increasing temperature, regardless of ionic strength. At pH 10 the aggregation rate constant decreased with increasing temperature as a general trend. In the intermediate pH interval, the aggregation rate constant apparently decreased with increasing temperature except at the highest ionic strength, where it increased with increasing temperature. The aggregation rate constant decreased at alkaline pH compared with the acidic pH range. This effect became more pronounced at higher ionic strengths and higher temperatures but could not be observed at 4 degrees C. These observations are in qualitative agreement with DLVO calculations taking temperature, pH and ionic strength into account.

Keyword
Montmorillonite dispersions, PCS, Surface potential, PZC, DLVO theory, Aggregation
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7534 (URN)10.1016/j.clay.2008.07.011 (DOI)000262658000004 ()2-s2.0-57649117108 (Scopus ID)
Note
QC 20100910. Original titel "Effect of pH and temperature on the stabiligy of bentonite colloid".Available from: 2007-10-03 Created: 2007-10-03 Last updated: 2017-12-14Bibliographically approved

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