Detachment of colloidal particles from bentonites in water

Abstract

Bentonites are currently investigated as geotechnical barrier for sealing radioactive waste e.g. in HLRW repositories (HLRW = high level radioactive waste). Their favourable properties are low hydraulic permeability, cation exchange capacity as well as swelling capacity in contact with aqueous solutions. The pre-requisite for this application is the stability of bentonite under the conditions expected. Accordingly, several studies are available dealing with different scenarios of possible alteration processes (e.g. high pH, high salinity, extensive drying,…). In most of the studies either only a few different bentonites are investigated or a number of bentonites are compared based on only a few parameters. Different bentonites perform rather different in most fields of applications. These differences cannot always be explained by the dominating exchangeable cation only. Therefore, comparative studies considering various different bentonites sometimes allow for the identification of relations between properties and performance. This study was conducted in order to identify the differences of stability (detachment of colloidal particles and/or dissolution) of bentonites in contact with deionized water, representing the simplest aqueous solution. Some of the bentonites release ultrafine colloidal particles upon shaking of suspensions which cannot be centrifuged even by using an ultracentrifuge with 46,000 g. After centrifugation the supernatant containing the colloidal particles was separated. ESEM, XRD, and IR prove that the colloidal particles are mainly montmorillonites. Considering the stochiometrical composition of montmorillonites suggests that approximately 10% of the elemental concentration measured in the supernatant stems from dissolution of octahedral sheet. However, detachment of colloidal particles was found to be the dominating mechanism. As expected, the amount of released colloidal particles strongly depends on the amount of exchangeable Na +. However, in the present study we show that exchangeable Na + and pH show a good correlation. In a separate test the facilitating effect of alkalinity (at a given Na + content) on detachment of colloidal particles was proved. This can be either due to the increase of solubility of alumosilicates at pH > 10 and/or due to the stabilization of the dispersion. In advance of this study it was expected, that bentonites showing a high degree of intergrowth of the fine constituents (e.g. montmorillonite intergrowth with relict volcanic glass) do not release the same amount of colloidal particles at a given exchangeable Na + content and pH compared to bentonites showing a loose microfabric. This was not confirmed in this study. We, conclude that alkaline Na + bentonites generally are less suitable for the application as geotechnical barrier in HLRW repositories since the probability that colloidal particles are released is higher than in the case of pH neutral Ca 2+ bentonites. It is conceivable that such colloidal particles are able to transport strongly adsorbed radionuclides. On the other hand detachment of colloidal particles reduces the thickness of the geotechnical barrier itself.