Actinide Colloids in Natural Aquifer Systems
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Actinide Colloids in Natural Aquifer Systems J.I. Kim Introduction Immobilization and geological isolation are the general goals of nuclear waste disposal. The appropriate isolation of immobilized long-lived radionuclides, mostly actinide elements, from interaction with water for a period of many thousands of years in any given geological formation is a scientific and technological challenge. This challenge can be met by considering a multibarrier concept that appraises engineered, geoengineered, and geological barriers designed to confine the migration of long-lived actinides.1 Once interaction with water occurs, one may expect many complex geochemical reactions of immobilized actinides;2'3 actinides will therefore be distributed into two kinds of chemical states, mobile and immobile species. Reflecting results in laboratory systems, the two species are expected to be chemical partners in equilibrium reactions. In multicomponent geochemical systems, however, the chemistry of the mobile and immobile species of actinides in trace concentration is, in general, masked by geochemical reactions of natural aquatic constituents, which are found in much higher concentrations. The mobile species are either complexed anions2"3 or colloidal species,4 whereas the immobile species are new solid phases5 of very low solubility, which are thermodynamicalry better stabilized than in the original host phases. In all groundwater, aquatic colloids are ubiquitous.6'7 The chemical composition, structure, and particle size of aquatic colloids vary widely, depending on the geochemical nature of given aquifer systems,47 and their particle concentration varies likewise, ranging widely from 108 to 1017 particles per liter of water.3'4 Aquatic colloids are chemically surface active and they therefore readily absorb actinide ions of either hydrolyzed or nonhydrolyzed species through surface complexation and/or ion exchange processes.4"8 The formation
MRS BULLETIN/DECEMBER 1994
of such colloids, called generation of actinide "pseudocolloids,"9 may increase the amount of actinides in groundwater far beyond their thermodynamic solubilities.2'3 Such a geochemical process facilitates the formation of mobile actinide species and may thus enhance their migration in a given aquifer system.10 For these reasons, recently much attention has been directed to the chemistry of aquatic colloids in connection with the migration study of actinides1"4 and of other environmental contaminants7 in the geosphere. The role of colloids is critically important for assessing the near-1114 and far-field8"1" migration behavior of actinides. Failure to account for aquatic colloid transport of actinides may lead to serious underestimates of the actinide migration and hence to unreliable long-term safety assessments for the geological disposal of nuclear wastes, especially high-level wastes (which contain high levels of actinides). This article reviews the present state of the aquatic colloid chemistry, the generation processes of actinide pseudocolloids and their geochem
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