High ionic strength brine systems (typically I&gt; 5 M) represent extreme subsurface environments that can have many impacts on the overall speciation and oxidation state distribution of actinide species.  These systems are encountered in many high-level nuclear waste repository scenarios where there may be cyclic drying/heating as a transitional phase of the repository history.  More importantly, it is the expected environment for inundation scenarios in salt repository concepts (e.g., WIPP TRU repository). <br/>An overview discussion of the current status of key effects of high ionic strength on actinide speciation will be given.  Most high ionic-strength studies relate to process chemistry under highly acidic conditions ([acid] &gt; 3 M).  In the geologic repository case, however, high sodium-magnesium-chloride brines can be formed or are encountered under high pH conditions (typically pH 7-13) depending on the prevailing conditions in the repository.   A key impact is the loss of ideality due to ion interactions that require empirical modeling approaches (e.g., Pitzer parameter approach) and modified formation constants for complexants.  High ionic strength also shifts the relatively importance of the oxidation state distribution relative to low ionic strength systems.  This can be demonstrated in shifts in Eh-pH diagrams, the enhanced stability of An(VI) at higher pH, and the stabilization of An(III) under some moderate pH conditions.  High ionic strength also impacts the relative stability of nano-structural species that lead to behavioral differences and long-term metastability.  This complexity has been most studied in the thorium system but applies to other actinide systems as well.  Lastly, high ionic strength chemistry shuts down many microbial effects on actinide speciation.  In this context, many microbial effects (e.g., bioreduction, and biodegradation) generally important for long-term containment strategies in low I systems are mitigated by the inability of many microorganisms to survive or function at I &gt; 4 M.   Collectively, these high ionic-strength effects can have a pronounced effect on the fate and transport of actinide contaminants in the subsurface and need to be addressed in the performance assessment and long-term safety case. <br/>

Effects of high ionic strength on actinide speciation and oxidation state distribution in subsurface environments

253rd National Meeting (2017)

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