Abstract
AbstractBentonite colloids generated from the backfill barrier in nuclear waste repositories may act as radionuclide carriers, if they are stable and mobile. Repository scenarios with highly saline groundwater inhibit colloid stability as particles tend to aggregate but, in the time frame of repositories, groundwater conditions may evolve, promoting particle disaggregation and stabilization. The disaggregation of FEBEX bentonite colloids by fast dilution to lower ionic strength was analysed in this study. Time-resolved dynamic light-scattering experiments were carried out to evaluate the kinetics of bentonite colloid aggregation and disaggregation processes in Na+ and Na+-Ca2+ mixed electrolytes of low ionic strength. Attachment and detachment efficiencies were determined.Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations. Once bentonite colloids are aggregated, a decrease in ionic strength facilitates disaggregation, but the process is not fully reversible as the initial size of the stable bentonite colloids at low ionic strength is not fully recovered. Particle-size distribution and concentration in suspension were analysed on disaggregated samples by single particle-counting measurements. Small colloids were measured in the disaggregated samples but their population was smaller than in the initial stable sample, especially in the presence of Ca2+.
Highlights
Aggregation and disaggregation kineticsAggregation/disaggregation processes are followed by measuring the mean hydrodynamic diameter of the particles, their size distribution and their concentration in solution
Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations
Bentonite colloids released from the bentonite barrier surface can act as contaminant carriers when radionuclide-bentonite colloid complexes are formed and are stable in suspension (McCarthy & Zachara, 1989)
Summary
Aggregation/disaggregation processes are followed by measuring the mean hydrodynamic diameter of the particles, their size distribution and their concentration in solution. Photon correlation spectroscopy (PCS) was used to determine the mean hydrodynamic diameter of particles in suspension (Novich, 1984) and the highly sensitive single-particle counting (SPC) was used to analyse the size distribution of particles and their concentration (Degueldre et al, 1996b; Rossé & Loizeau, 2003). Kinetic aggregation tests were carried out at a bentonite colloid concentration in suspensions of 2 mg L–1 and the average hydrodynamic diameter was measured by PCS for 60 min. Attachment efficiency is the ratio between the variability of hydrodynamic diameter in aregsaimmeplðedddðthdÞddfthaÞstt,!w0 hwicihthisrecsopnescitdteoretdhehefraestaasgthgerergaatteioonf aggregation of bentonite colloids in 0.1 M Na+-Ca2+ electrolyte. Disaggregation kinetic tests were carried out at a bentonite colloid concentration of 2 mg L–1. To follow the disaggregation kinetics, the average hydrodynamic diameter was measured by PCS for 60 min. Detachment efficiency is determined as the ratio of the variability of hydrodynamic diameter in a sample ðredddgthiÞmt!e0ðdddwth Þiftahst , respect which is to the fast considered to disaggregation be the theoretical disaggregation rate at which to reach an initial stable hydrodynamic diameter
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