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Transport of sorbed species in clays

Applicant Gimmi Thomas
Number 166287
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Other disciplines of Environmental Sciences
Start/End 01.12.2016 - 30.11.2020
Approved amount 240'554.00
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All Disciplines (2)

Discipline
Other disciplines of Environmental Sciences
Geochemistry

Keywords (6)

sorption; ion diffusion; solute transport; coupled modelling; clay; surface diffusion

Lay Summary (German)

Lead
Tone und Tongesteine haben eine sehr geringe Wasserdurchlässigkeit. Sie werden oft gebraucht, um Abfalldeponien abzudichten. Um das Langzeitverhalten solcher Tonbarrieren vorherzusagen, braucht es mathematische Modelle, die den Transport von Schadstoffen simulieren können. Die Schadstoffe sind im Porenwasser des Tons gelöst, können jedoch auch durch chemische Interaktionen (Ionentausch, Adsorption, Komplexbildung) an die Oberfläche der Tone gebunden werden. Dies verringert die Ausbreitung von gelösten Stoffen stark. Traditionelle Transportcodes (Programme zur Simulation des Schadstofftransportes) betrachten sorbierte Ionen als immobil. Jüngste Studien weisen jedoch darauf hin, dass dies nicht für alle Arten von Sorption gilt und der diffusive Transport u.U. massgeblich von sorbierten Kationen bestimmt wird. Manche Ergebnisse von Diffusionsexperimenten können deshalb im Moment nicht korrekt mit bestehenden Transportcodes simuliert werden.
Lay summary

Zielsetzung
Dieses Projekt zielt darauf ab, das Anwendungspotential von Transportcodes durch die Berücksichtigung der Mobilität von sorbierten Ionen in den mathematischen Transportgleichungen zu vergrössern. Es werden verschiedene Ansätze zur Beschreibung dieses Prozesses in den Codes implementiert. Die modifizierten Codes werden mit Ergebnissen aus Diffusionsexperimenten getestet und können schliesslich auch für die Vorhersage des Langzeitverhaltens von Tonbarrieren eingesetzt werden.

Kontext
Dieses Projekt trägt dazu bei, die Zuverlässigkeit von Langzeitvorhersagen für Schadstofftransport durch Tonbarrieren zu erhöhen. Es wird erwartet, dass die Anwendung der neuen Modelle (1) zu einem grösseren Verständnis der Diffusionsprozesse von Ionen in Tonen führt und (2) zu einem robusten Parametersatz für Diffusion von Kationen, welcher konsistent für sehr unterschiedliche experimentelle Bedingungen verwendet werden kann.

 
Direct link to Lay Summary Last update: 28.12.2016

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Abstract

Clays and claystones are often used as sealing materials to isolate waste from the environment. Moreover, shale caprocks play an important role during the accumulation of petroleum and as natural seals in the concept of carbon sequestration. Mathematical models describing the transport of dissolved ions through the water-filled pore space of a clay are required to predict the long-term performance of clay barriers. These models have to be able to represent solute-solid interaction processes such as surface complexation or sorption by cation exchange. The default assumption of available transport codes is that sorbed ions are immobile. Experiments indicate, however, that sorbed cations can contribute significantly to the overall diffusive mass flux in clays. This means that sorbed cations are partly mobile. Moreover, it appears that the diffusive mobility of sorbed species varies for different sorption sites. These features are presently not implemented in any available transport code. Accordingly, cation diffusion data cannot be modelled in a mechanistic way currently.We propose to expand the capabilities of transport codes by taking into account the variable mobility of sorbed species on different sites. This expansion will be done in steps of increasing complexity. Available, well-established sorption models relating the amounts of sorbed species on each sorption site to the pore water composition are required in any case. The first and simplest approach (implicit approach) starts by assuming local equilibrium between the concentrations on all sorption sites and in the pore solution. Then, it is possible to derive a lumped, time- and space-dependent diffusion coefficient which includes the mobilities of all sorbed species. This is an implicit way to include site-specific surface mobilities. A second approach (explicit approach) is to introduce mobile surface species and surface sites explicitly as components in the transport equations. Each component would have its own transport parameters i.e. diffusion coefficient and advective mobility. Thus, the concept of species-specific diffusion coefficients is extended to sorbed species. Species-dependent advective mobility is a newly introduced parameter that can vary between 1 (component moves with the pore fluid) and 0 (component does not move advectively). Solid surfaces remain immobile if their diffusion coefficient and their advective mobility are set to zero, whereas sorbed species can have a certain diffusive and advective mobility. Finally, a third approach (pore-scale approach) will be explored. It considers the arrangement of different sorption sites on different mineral surfaces (or pore environments) as well as cation mobilities derived from molecular dynamics (MD) simulations. Such pore-scale simulations mainly serve to validate assumptions used in the former two approaches. The newly developed models will be applied to sets of recent diffusion data for cations such as Cs, Sr, Co and Zn in clays, which could not be explained satisfactorily so far with existing codes. The need to improve the modelling capabilities in the proposed direction became evident from a preliminary interpretation of these data. The proposed new code features will make it possible to test the hypothesis regarding the variable mobility of surface species, and allow the derivation of consistent sets of cation transport parameters for different experimental conditions, which is not possible at present. The proposed work will thus strongly improve the capabilities of reactive transport simulations, notably for conditions with complex solution chemistry, which is essential in many applications.
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