Stability and sorption capacity of montmorillonite colloids : Investigation of size fractional differences and effects of γ-irradiation

Sammanfattning: Bentonite clay is intended to form one of the barriers in most repositories of spent nuclear fuel located in granite. One important function of the bentonite barrier is to retard transport of radionuclides in the event of waste canister failure. Bentonite has a high sorption capacity of cations and its main constituent is montmorillonite. In contact with groundwater of low ionic strength, montmorillonite colloids can be released from bentonite and thereby control transport of radionuclides sorbed onto the colloids.In colloid transport in bedrock fractures, size separation of clay colloids may occur due to physical and chemical interactions with the bedrock fracture surface. This may enhance or retard the overall transport of radionuclides, depending on the sorption capacities and stability of the differently sized clay colloids. The bentonite barrier will be exposed to γ-radiation from the spent nuclear fuel. Irradiation affects surface-related properties of bentonite. If an average sorption capacity value cannot be used for all colloid sizes or if sorption is affected by exposure to γ-irradiation, corrected sorption capacity values would give higher resolution in current reactive transport models.In order to study the size separation process, a protocol was developed and successfully applied to fractionate montmorillonite into different-sized colloid suspensions by means of sequential or direct centrifugation. The stability and sorption capacity were studied using these fractions. Both stability and sorption capacity were found to be similar for all colloid sizes.Bentonite exposed to γ-radiation sorbed less divalent cations with increasing radiation dose. The effect was not large enough to have any impact on diffusion. The presence of bentonite enhanced irradiation-induced corrosion of copper under anaerobic atmosphere.An average sorption capacity value for montmorillonite can be used for all colloid sizes in reactive transport models. The effect of γ-irradiation on sorption capacity is sufficiently large to require consideration in transport modelling.