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A surface approach to understanding the dissolution of fluorite type materials: Implications for mineral dissolution kinetic models
Stockholm University, Faculty of Science, Department of Geological Sciences. (Geochemistry)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Traditional dissolution models are based in the analyses of bulk solution compositions and ignore the fact that different sites of a surface dissolve at different rates. Consequently, the variation of surface area and surface reactivity during dissolution are not considered for the calculation of the overall dissolution rate, which is expected to remain constant with time. The results presented here show the limitations of this approach suggesting that dissolution rates should be calculated as a function of an overall surface reactivity term that accounts for the reactivity of each of the sites that constitute the surface. In contrast to previous studies, here the focus is put on studying the surface at different dissolution times. Significant changes in surface topography of CaF2 were observed during the initial seconds and up to 3200 hours of dissolution. The observed changes include the increase of surface area and progressive exposure of the most stable planes, with consequent decrease in overall reactivity of the surface. The novelty of a proposed dissolution model for fluorite surfaces, when compared with traditional dissolution models, is that it differentiates the reactivity of each characteristic site on a surface, e.g. plane or step edge, and considers the time dynamics. The time dependency of dissolution rates is a major factor of uncertainty when calculating long term dissolution rates using equations derived from dissolution experiments running for short periods of time and using materials with different surface properties. An additional factor of uncertainty is that the initial dissolution times are the most dynamic periods of dissolution, when significant variations of surface area and reactivity occur. The results are expected to have impact in the field of nuclear waste management and to the larger geological and material science community.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2013. , 26 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 352
Keyword [en]
dissolution, topography, fluorite, surface
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-89257ISBN: 978-91-7447-701-6 (print)OAI: oai:DiVA.org:su-89257DiVA: diva2:616609
Public defence
2013-05-29, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Submitted. Paper 5: Submitted.

Available from: 2013-05-07 Created: 2013-04-17 Last updated: 2013-08-09Bibliographically approved
List of papers
1. Sintering of CaF2 pellets as nuclear fuel analog for surface stability experiments
Open this publication in new window or tab >>Sintering of CaF2 pellets as nuclear fuel analog for surface stability experiments
2011 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 419, no 1-3, 46-51 p.Article in journal (Refereed) Published
Abstract [en]

To enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel, it is necessary to produce analogs that closely resemble nuclear fuel in terms of crystallography and microstructure. One such analog can be obtained by sintering CaF2 powder.

This paper reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240 °C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900 °C and 1000 °C. When CaF2 was sintered above 1100 °C the formation of CaO at the grain boundaries caused the disintegration of the pellet due to hydration occurring after sintering.

First results from a novel set-up of dissolution experiments show that changes in roughness, dissolution rate and etch pit shape of fluorite surfaces are strongly dependent on the crystallographic orientation of the expose surface. Consequently, the differences observed for each orientation will affect the overall dissolution rate and will lead to uncertainties in the estimation of dissolution rates of spent nuclear fuel.

Place, publisher, year, edition, pages
Elsevier, 2011
National Category
Materials Chemistry
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-65942 (URN)10.1016/j.jnucmat.2011.08.031 (DOI)000298936600007 ()
Note
We describe the microstructures of CaF2 that can be obtained by sintering. ► Microstructures that resemble nuclear waste pellets were obtained in the range 900 °C–1000 °C. ► We demonstrate the importance of surface orientation on dissolution behavior and surface properties of a mineral. ► New procedure for studying the role of crystallography and surface chemistry in dissolution using 3-D confocal profilometry.Available from: 2011-12-16 Created: 2011-12-16 Last updated: 2017-12-08Bibliographically approved
2. Effect of surface orientation on dissolution rates and topography of caf2
Open this publication in new window or tab >>Effect of surface orientation on dissolution rates and topography of caf2
2012 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 86, 392-403 p.Article in journal (Refereed) Published
Abstract [en]

This paper reports how during dissolution differences in surface chemistry affect the evolution of topography of CaF2 pellets with a microstructure similar to UO2 spent nuclear fuel. 3D confocal profilometry and atomic force microscopy were used to quantify retreat rates and analyze topography changes on surfaces with different orientations as dissolution proceeds up to 468 h. A NaClO4 (0.05 M) solution with pH 3.6 which was far from equilibrium relative to CaF2 was used. Measured dissolution rates depend directly on the orientation of the exposed planes. The {111} is the most stable plane with a dissolution rate of (1.2 +/- 0.8) x 10(-9) mol m(-2) s(-1), and {112} the least stable plane with a dissolution rate 33 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plane dissolve faster. Dissolution rates were found to be correlated to surface orientation which is characterized by a specific surface chemistry and therefore related to surface energy. It is proposed that every surface is characterized by the relative proportions of the three reference planes {111}, {100} and {110}, and by the high energy sites at their interceptions. Based on the different dissolution rates observed we propose a dissolution model to explain changes of topography during dissolution. Surfaces with slower dissolution rate, and inferred lower surface energy, tend to form while dissolution proceeds leading to an increase of roughness and surface area. This adjustment of the surface suggests that dissolution rates during early stages of dissolution are different from the later stages. The time-dependency of this dynamic system needs to be taken into consideration when predicting long-term dissolution rates.

Keyword
CRYSTALLOGRAPHIC PREFERRED ORIENTATION, MINERAL DISSOLUTION, PRECIPITATION CREEP, CAF2(111) SURFACE, ETCH PITS, KINETICS, MORPHOLOGY, GROWTH, FORCE, DISLOCATION
National Category
Geophysics Geochemistry
Identifiers
urn:nbn:se:su:diva-79777 (URN)10.1016/j.gca.2012.02.032 (DOI)000303677400025 ()
Note

AuthorCount:3;

Available from: 2012-09-12 Created: 2012-09-11 Last updated: 2017-12-07Bibliographically approved
3. Ab Initio Prediction of Surface Stability of Fluorite Materials and Experimental Verification
Open this publication in new window or tab >>Ab Initio Prediction of Surface Stability of Fluorite Materials and Experimental Verification
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 13, 6639-6650 p.Article in journal (Refereed) Published
Abstract [en]

Utilizing first-principle simulations [based on density functional theory (DFT) corrected for on-site Coulomb interactions (DFT+U)], we develop a model to explain the experimental stability in solution of materials having the fluorite structure, such as CaF2 and CeO2. It is shown that the stability of a surface is mainly dependent on its atomic structure and the presence of sites where atoms are deficiently bonded. Using as reference planes the surfaces with low surface formation energies, viz., (111), (100), and (110), our results reveal the relation between the surface energy of any Miller-indexed plane and the surface energy of those reference planes, being dependent on the fluorite surface structure only. Therefore, they follow the same trend for CaF2 and CeO2. Comparison with experimental results shows a correlation between the trends of dry surface energies and surface stabilities during dissolution of both CaF2 and CeO2, even though the chemical processes of dissolution of CeO2 and CaF2 are different. A deviation between ab initio predictions and experiments for some surfaces highlights the sensitivity of the developed model to the treatment of surface dipolar moments.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013
National Category
Nano Technology Physical Chemistry
Identifiers
urn:nbn:se:su:diva-89273 (URN)10.1021/jp312645f (DOI)000317317600017 ()
Available from: 2013-04-18 Created: 2013-04-18 Last updated: 2017-12-06Bibliographically approved
4. Effect of surface structure for the development of topography during dissolution of fluorite surfaces
Open this publication in new window or tab >>Effect of surface structure for the development of topography during dissolution of fluorite surfaces
(English)Article in journal (Other academic) Submitted
National Category
Geology
Identifiers
urn:nbn:se:su:diva-89281 (URN)
Available from: 2013-04-18 Created: 2013-04-18 Last updated: 2013-04-21Bibliographically approved
5. Direct observations of the structures developed on fluorite surfaces with different orientations during dissolution
Open this publication in new window or tab >>Direct observations of the structures developed on fluorite surfaces with different orientations during dissolution
(English)Article in journal (Other academic) Submitted
National Category
Geochemistry
Identifiers
urn:nbn:se:su:diva-89282 (URN)
Available from: 2013-04-21 Created: 2013-04-18 Last updated: 2013-04-21Bibliographically approved

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