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Kinetic determination of critical coagulation concentrations for sodium- and calcium-montmorillonite colloids in NaCl and CaCl2 aqueous solutions
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.ORCID iD: 0000-0003-0663-0751
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.

Place, publisher, year, edition, pages
2007. Vol. 315, no 2, 512-519 p.
Keyword [en]
Aggregation, CCC, DLVO theory, Montmorillonite colloids, PCS, Zeta potential
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-7532DOI: 10.1016/j.jcis.2007.07.002ISI: 000250064100013Scopus ID: 2-s2.0-34548860946OAI: oai:DiVA.org:kth-7532DiVA: diva2:12586
Note
QC 20100921Available from: 2007-10-03 Created: 2007-10-03 Last updated: 2017-12-14Bibliographically approved
In thesis
1. The impact of groundwater chemistry on the stability of bentonite colloids
Open this publication in new window or tab >>The impact of groundwater chemistry on the stability of bentonite colloids
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:nbn:se:kth:diva-4504 (URN)978-91-7178-760-6 (ISBN)
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
2. 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.

Publisher
xii, 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:3
Keyword
Bentonite; montmorillonite; colloid stability; colloid migration; geological disposal; nuclear waste; granitic fractures;
National Category
Environmental Sciences Environmental Sciences Physical Chemistry Inorganic Chemistry
Identifiers
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)
Opponent
Supervisors
Available from: 2010-01-12 Created: 2010-01-08 Last updated: 2010-04-28

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