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Effect of surface orientation on dissolution rates and topography of caf2
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
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.

Place, publisher, year, edition, pages
2012. Vol. 86, 392-403 p.
Keyword [en]
CRYSTALLOGRAPHIC PREFERRED ORIENTATION, MINERAL DISSOLUTION, PRECIPITATION CREEP, CAF2(111) SURFACE, ETCH PITS, KINETICS, MORPHOLOGY, GROWTH, FORCE, DISLOCATION
National Category
Geophysics Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-79777DOI: 10.1016/j.gca.2012.02.032ISI: 000303677400025OAI: oai:DiVA.org:su-79777DiVA: diva2:551813
Note

AuthorCount:3;

Available from: 2012-09-12 Created: 2012-09-11 Last updated: 2017-12-07Bibliographically approved
In thesis
1. A surface approach to understanding the dissolution of fluorite type materials: Implications for mineral dissolution kinetic models
Open this publication in new window or tab >>A surface approach to understanding the dissolution of fluorite type materials: Implications for mineral dissolution kinetic models
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
dissolution, topography, fluorite, surface
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-89257 (URN)978-91-7447-701-6 (ISBN)
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

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