Advanced mineralogical study of the clay mineral fraction in Boom Clay

Frederickx Lander

Promoter

Elsen Jan, (KULeuven), Jan.Elsen@ees.kuleuven.be

SCK•CEN Mentor

De Craen Mieke
mieke.de.craen@sckcen.be
+32 14 33 32 52

SCK•CEN Co-mentor

Honty Miroslav
miroslav.honty@sckcen.be
+32 14 33 32 23

Expert group

R&D Disposal

PhD started

2015-11-16

Short project description

In Belgium, the Boom Clay is studied as one of the potential host rocks for the geological disposal of radioactive waste. The clay minerals within Boom Clay contribute significantly to the retardation of the release of radionuclides from the potential repository to the geosphere and biosphere due to their sorption capacity. The major processes responsible for this behaviour are cation exchange in the interlayer of (mostly) swelling clays like smectites, and surface complexation on the broken edges of clay platelets (Baeyens and Bradbury, 1997; Bradbury and Baeyens, 1997; 2000). Due to these processes, clay minerals also dictate to a high extent the pore water chemistry of argillaceous rocks (Bradbury and Baeyens, 1998).

Besides a high sorption capacity, clay minerals may equally possess substantial reduction-oxidation potential, depending on their nature and Fe content (Neumann et al., 2011). Several naturally-occurring mechanisms have already been identified which are able to alter the Fe(II)/Fe(III) content of clay minerals (Schaefer et al., 2011), thereby changing both their redox behaviour as well as their layer charge and as such their total adsorptive capacity (Gorski et al., 2013). Novel electrochemical methods were developed to probe the total electron-accepting and electron-donating capacity of clay minerals, based on kinetic probes (Neumann et al., 2008; Gorski et al., 2011).

The detailed study of the clay mineral characteristics is therefore fundamental in our understanding of the Boom Clay composition and geochemical processes, and forms the base for further experimental work and modelling.

Objective

The objectives of this PhD are to study the clay mineral characteristics of various Boom Clay samples, both qualitatively and quantitatively, and to couple this information to geochemical parameters and processes. This coupling would allow to gain a better insight into the retention properties of Boom Clay towards radionuclide dispersion starting from baseline mineralogical information.

1. The PhD student will use Boom Clay samples from various locations, to scope the natural mineralogical diversity. Also, the effect of perturbations will be studied. Numerous processes might influence the characteristics of undisturbed Boom Clay during the installation, operation and post-closure phases of a radioactive waste repository. These processes include oxidation (due to air atmosphere) and reduction (due to H2 gas generated by anaerobic corrosion of stainless steel), and therefore might have an effect on the Fe(II)/Fe(III) in clay minerals (Didier et al., 2012).

2. The selected samples will be studied by qualitative and quantitative mineralogical analysis, mainly through X-ray diffraction (XRD). The main goal is to use the methodology recently developed at KULeuven (Zeelmaekers, 2011) which is based on the use of internal standards, clay fraction analyzed in the context of bulk rocks, diffraction patterns modelled by advanced evolutionary software and, the quantitative results complemented and verified by independent controllers (e.g. chemistry).

3. Geochemically relevant parameters of the clay samples which can be temptatively linked to the mineralogical composition will be analysed. Thereactive specific surface area will be studied through the N2 adsorption technique available at SCK•CEN. In order to study the effect of Fe(II)/Fe(III) changes on the adsorptive capacity, the layer charge of smectites and their distribution will be by e.g. methylene blue, alkylammonium and Rhodamine 6G method. Another important parameter directly related to the surface and charge of clay minerals, in particular smectite, is the cation exchange capacity (CEC). The CEC of a clay/shale is the sum of the cations available for exchange held on the total specific surface area of the rock. Several techniques have been developed to measure CEC with index cations, such as Ag-thiourea, Co-hexamine and Cu-trien. The overarching aspect of the sorption phenomena on clay surfaces is that surface area, layer charge and cation exchange capacity are interrelated. Lastly, the Fe(II)/Fe(III) content of the clay mineral fraction will be investigated through Mössbauer spectroscopy.

4. The studied geochemical parameters will be coupled to the quantitative and qualitative mineralogical data in order to study possible links.

The novelty of this research is the integrated approach, i.e. the combination of different existing analytical techniques and their application on clay minerals from Boom Clay, and especially the coupling of the mineralogical investigation to geochemical processes. These geochemical processes are expected to influence several specific parameters and characteristics which are of pivotal importance for safety assurance. Our results would allow to scope the effectiveness of Boom Clay as a geological barrier based only on mineralogical data, and to predict the change in effectiveity as a result of both physical and chemlical disturbances.