Strontium migration in a crystalline medium: effects of the presence of bentonite colloids

• Published in: Journal of contaminant hydrology 2011-03, Vol.122 (1), p.76-85
• Main Authors: Albarran, Nairoby, Missana, Tiziana, García-Gutiérrez, Miguel, Alonso, Ursula, Mingarro, Manuel
• Format: Article
• Language: English
• Subjects: Adsorption; Bentonite; Bentonite - chemistry; Colloids; Colloids - chemistry; Earth sciences; Earth, ocean, space; Exact sciences and technology; Flow rate; Fracture mechanics; Fracture surfaces; Geologic Sediments - chemistry; Granite; Hydrogeology; Hydrology. Hydrogeology; Radioactive waste; Radionuclide migration; Silicon Dioxide - chemistry; Sorption; Strontium; Strontium - chemistry; Transport; Water Movements; Water Pollutants, Radioactive - chemistry
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/j.jconhyd.2010.11.005

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10 − 6 m/s–224 m/yr. Experiments with bentonite colloids alone showed that – at the lowest water flow rate used in our experiments (7.1·10 − 6 m/s) – approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R f ~ 6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded ( R f = 1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20. ► Colloid retention on rocks may occur under unfavorable electrostatic conditions. ► Water velocity determines the mobile colloidal fraction in a granite fracture. ► Transport of Sr is affected by the presence of bentonite colloids. ► Sr desorption from colloids imits the importance of colloid-acilitated transport.