Effect of Ionic Strength on the Stability of Colloids Released from Injection Grout Silica Sol
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Effect of Ionic Strength on the Stability of Colloids Released from Injection Grout Silica Sol Pirkko Hölttä1, Mari Lahtinen1, Martti Hakanen1, Jukka Lehto1 and Piia Juhola2 1 University of Helsinki, Department of Chemistry, P.O. Box 55, 00014 University of Helsinki, Finland. 2 Posiva Oy, Olkiluoto, 27160 Eurajoki, Finland. ABSTRACT In Olkiluoto Finland colloidal silica called silica sol (EKA Chemicals) will be used as a non-cementitious grout for the sealing of fractures of the hydraulic apertures of 0.05 mm or less. The use of colloidal material has to be considered in the long-term safety assessment of a spent nuclear fuel repository. The potential relevance of colloid-mediated radionuclide transport is highly dependent on their stability in different geochemical environments. Objective of this work was to study the effect of ionic strength on stability of silica colloids released from silica gel. Silica gel samples were stored in contact with NaCl and CaCl2 electrolyte solutions and in deionized water. Colloid release and stability were followed for two years by taking the samples after one month and then twice in a year. The release and stability of colloids were followed by measuring particle size, colloidal silica concentrations and zeta potential. The particle size distributions were determined applying the dynamic light scattering (DLS) method and zeta potential based on dynamic electrophoretic mobility. In dilute NaCl (10-7–10-2 M) and CaCl2 (3 10-7– 3 10-3 M) solutions, a mean colloid diameter was less than 100 nm and high negative zeta potential values suggests the existence of stable silica colloids. After two years, the mean particle diameter was increased but it was still less than 500 nm and absolute value of zeta potential was decreased. In 0.1–1 M NaCl and 0.03– 3 M CaCl2 solutions, wide particle size distribution and zeta potential values around zero suggested particle aggregation and instable colloids. In deionized water, particle size remained rather stable and zeta potential remained high negative suggests stable silica colloids. The threshold value of ionic strength was 0.03–0.1 M when salinity had an effect on the stability of colloids. In Olkiluoto, the ionic strength of saline groundwater is order of magnitude higher than the range of effect value obtained in this study. Under the prevailing conditions in Olkiluoto, silica colloids are instable, but the possible influence of glacial melt waters has to be considered. INTRODUCTION In Finland the final disposal of spent nuclear fuel is investigated by Posiva Oy in Olkiluoto utilizing the underground rock characterization facility ONKALO. The project seeks to obtain information about the bedrock at the site planned for the final disposal repository and assess its safety and test final disposal technology in actual deep underground conditions. Cement is typically used for permeation grouting in hard rock. Because high pH can be harmful for the Engineered Barrier System (EBS) and cement-based grouts may have limited penetration, non
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