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Determining pseudo-equilibrium of montmorillonite colloids in generation and sedimentation experiments as a function of ionic strength, cationic form, and elevation
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology. (Nuclear Chemistry)
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology. (Nuclear Chemistry)
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.

Place, publisher, year, edition, pages
2009. Vol. 335, no 1, 54-61 p.
Keyword [en]
Ca-montmorillonite, Colloids, Generation, Na-montmorillonite, Sedimentation, Size distribution
National Category
Chemical Sciences
URN: urn:nbn:se:kth:diva-11850DOI: 10.1016/j.jcis.2009.02.048ISI: 000266615400009ScopusID: 2-s2.0-65549100552OAI: diva2:285478
QC 20100908Available from: 2010-01-12 Created: 2010-01-12 Last updated: 2010-09-08Bibliographically approved
In thesis
1. Generation, stability and migration of montmorillonite colloids in aqueous systems
Open this publication in new window or tab >>Generation, stability and migration of montmorillonite colloids in aqueous systems
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.

xii, 60 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2010:3
Bentonite; montmorillonite; colloid stability; colloid migration; geological disposal; nuclear waste; granitic fractures;
National Category
Environmental Sciences Environmental Sciences Physical Chemistry Inorganic Chemistry
urn:nbn:se:kth:diva-11847 (URN)978-91-7415-535-8 (ISBN)
Public defence
2010-01-29, F1, Lindstedtsvägen 22, KTH, Stockholm, 10:00 (English)
Available from: 2010-01-12 Created: 2010-01-08 Last updated: 2010-04-28

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