Formation and stability of bentonite colloids

Document available in extended abstract form only. Crystalline rock in Olkiluoto is being considered as a host medium for the repository of highly radioactive spent nuclear fuel in Finland. The spent uranium fuel will be placed in the final disposal tunnels in copper iron canisters, which will be surrounded with bentonite clay. Finally, the tunnels will be closed using a mixture of bentonite and crushed rock. Colloids released from degraded bentonite buffer and bentonite-crushed rock back-fill may effect on the migration of radionuclides and colloid-facilitated transport may be significant to the long-term performance of a spent nuclear fuel repository. The potential relevance of colloids for radionuclide transport is highly dependent on the stability of colloids in different chemical environments and their interaction with radionuclides. The objective of this work was to determine the formation and stability of colloids released from MX-80 type bentonite clay, which mainly consists of montmorillonite. In the batch dispersion experiments, powdered MX-80 type bentonite was added to diluted OLSO reference groundwater, sodium chloride and calcium chloride electrolyte solutions whose ionic strengths were adjusted between 0.001-0.1 M. The colloidal particle fraction was separated by filtration and the pH, particle size distribution, zeta potential, colloidal particle concentration and morphology were determined. Colloidal particle size distribution was determined applying the dynamic light scattering method (Malvern Zetasizer Nano ZS) and zeta potential applying the dynamic electrophoretic mobility. Colloid concentration was determined using a standard series made from MX-80 bentonite and applying the DLS measurement count rate which is roughly proportional to the concentration of particles in the suspension. The particle size as a function of ionic strength is shown in Figure 1. The particle size increased strongly with increasing ionic strength at low ionic strengths. The largest particles sediment during one year and the particle size was decreased. The particle sizes grew as the ionic strength of the solution got higher. In both electrolyte solutions, the particle size was approximately 400 nm at ionic strength 0.001 M. In sodium chloride, the particle size exceeds 1000 nm at ionic strength 0.025 M. In calcium chloride, the particle size exceeds 1000 nm at ionic strength 0.015 M and after that point particles are much larger than in NaCl. The average concentration of bentonite colloids in different solutions of different ionic strengths are shown in Table I. The colloid concentration was very low when the ionic strength was higher than 0.01 M. The colloid concentration exceeded the standard series area when the ionic strength was lower than 0.001 M. Zeta potential for bentonite colloids in diluted OLSO reference groundwater is presented as a function of ionic strength in Figure 2. The zeta potential was approximately -40 mV at 0.005 M ionic strength and increased to -10 mV at ionic strength 0.07 M. In monovalent sodium chloride solution colloids were stable (zeta potential under -30 mV) when the ionic strength was 0.04 M or lower. In divalent calcium chloride solution colloids were only stable in the ionic strengths of 0.01 M or lower. The decrease in zeta potential values in NaCl curve was due to the use of unpurified bentonite and uptake of sodium from the solution. The stability of bentonite colloids in diluted OLSO and electrolyte solutions did not change substantially during one year. The stability of colloids strongly depended on the ionic strength of the solution and the valence of the cation. Colloids are smaller and more stable in monovalent (Na+) than in divalent (Ca2+) dominated solutions. Low ionic strength (<0.04 M in NaCl, <0.01 M in CaCl2 or <0.015 M in OLSO) favors the formation of stable colloids. At current conditions in Olkiluoto, the ionic strength of the groundwater is around 0.5 M and the colloids released from the bentonite barrier are aggregated and unstable do not have an influence on the migration of radionuclides. However, this knowledge and understanding of bentonite erosion in colloidal form can be utilized in the estimation performance of the bentonite barrier. The possibility of a future glacial period and subsequent post-glacial phase when the infiltration of fresh, glacial melt water dilutes groundwater, implies that dilute groundwater conditions cannot be excluded and the influence of bentonite and other colloids has to take into consideration. (authors)