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The impact of groundwater chemistry on the stability of bentonite colloids
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
2007 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [sv]

I det svenska djupförvaret för kärnbränsle ska kompakterad bentonit användas som barriär mellan kopparkapslar med utbränt kärnbränsle och berget. I kontakt med vattenförande sprickor kan bentonitbarriären under vissa omständigheter avge montmorillonitkolloider. Förutom att barriärens egenskaper urholkas pga förlusten av material kan kolloiderna, om de är stabila, underlätta transporten av sorberade radionuklider ut mot biosfären.

Den här studien fokuserar på att undersöka vilka effekter grundvattenkemin har på montmorillonitkolloiders stabilitet. Grundvattnets sammansättning, pH och jonstyrka, kommer sannolikt att förändras under djupförvarets livslängd, delvis pga inträngandet av glactialt smältvatten. Initialt kommer omgivande berg att värmas upp pga värmealstring från det radioaktiva sönderfallet i det utbrända kärnbränslet. Effekterna av pH, jonstyrka och temperatur på montmorillonitkolloiders stabilitet har analyserats genom att följa hur kolloiderna aggregerar med tiden. Minskningen av partikelkoncentration med tiden mättes med Photon Correlation Spectroscopy (PCS).

Aggregationsexperimenten visar att, vid ett givet pH och en given temperatur, ökar hastighetskonstanten för aggregation med ökande jonstyrka. Kritiska koaguleringskoncentrationen (CCC) för NaCl och CaCl2 för Na-montmorillonit och Ca-montmorillonit beräknas utifrån ett samband mellan hastighetskonstanterna och jonstyrkan.

Hastighetskonstanten för aggregation minskar med ökande pH eftersom ytpotentialen ökar. Effekten blir tydligare vid högre jonstyrkor och högre temperaturer, men kan däremot inte observeras vid låga temperaturer.

Temperatureffekten på bentonitkolloidernas stabilitet är pH-beroende. Vid pH≤4 ökar hastighetskonstanten för kolloidaggregation med ökande temperatur, oavsett jonstyrka.Vid pH≥10 minskar hastighetskonstanten med ökande temperatur. I mellanliggande pH-område minskar hastighetskonstanten för aggregation med ökande temperatur, förutom vid den högsta jonstyrkan, där den ökade. Beräkningar baserade på DLVO-teori matchar de experimentella resultaten.

Abstract [en]

In deep geological repositories in Sweden, encapsulated nuclear waste will be surrounded by compacted bentonite in the host rock. In future contact with water-bearing fractures, this bentonite barrier can release montmorillonite colloids under certain conditions. This process can lead to loss of buffer material. Furthermore, these colloids, if stable, may facilitate the transport of associated radionuclides towards the biosphere. Colloid stability is determined by groundwater chemistry.

This study addresses the effects of groundwater chemistry on the stability of montmorillonite colloids. During the lifetime of the repository, the pH and ionic strength of the groundwater are expected to vary, partly due to intrusion of glacial melt water. Initially, the temperature will be higher in the surrounding host rock due to heat released from radioactive decay in the spent nuclear fuel. The effects of these parameters on the stability of montmorillonite suspensions were evaluated by studying the aggregation kinetics. The change in particle concentration with time was monitored by Photon Correlation Spectroscopy (PCS).

Aggregation kinetics experiments showed that for a given pH and temperature, the rate constant for colloid aggregation increased with increasing ionic strength. The relationship between the rate constant and the ionic strength allowed the NaCl and CaCl2 critical coagulation concentration (CCC) for Na- and Ca-montmorillonite to be determined.

The aggregation rate constant decreased with increasing pH as the surface potential increased. This effect became more pronounced at higher ionic strengths and higher temperatures but could not be observed at low temperature.

The effect of temperature on the stability of the suspensions 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 experimental results were in agreement with DLVO calculations.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , vii, 40 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:61
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4504ISBN: 978-91-7178-760-6 (print)OAI: oai:DiVA.org:kth-4504DiVA: diva2:12589
Presentation
2007-10-05, Sal E2, KTH, Lindstedtsvägen 3, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20101105Available from: 2007-10-03 Created: 2007-10-03 Last updated: 2012-04-02Bibliographically approved
List of papers
1. 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: 2010-09-21Bibliographically approved
2. 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: 2010-10-01Bibliographically approved
3. 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: 2010-09-10Bibliographically approved

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