The presence of plutonium in the geosphere poses a long-term environmental concern due to its toxicity and the long half-lives of several isotopes.  Therefore, understanding the mechanisms responsible for enhancing or retarding the mobility of plutonium in the environment is important for risk management.  Surface-mediated reduction of Pu(V) to Pu(IV) has been observed on a variety of surfaces.  Although numerous studies have observed the capacity of pure minerals to reduce or oxidize plutonium, the exact mechanism(s) responsible for surface-mediated redox reactions remain unclear.  The objective of this research was to determine if radiolysis at the mineral surface was a plausible mechanism for the surface-mediated reduction of plutonium.  Batch sorption experiments were used to monitor the amount of plutonium sorbed to high-purity quartz as a function of time, pH, and total alpha radioactivity.  Three systems were prepared using both 238Pu and 242Pu in order to increase the total alpha radioactivity of the mineral suspensions while maintaining a constant plutonium concentration.  The fraction of sorbed plutonium increased with increasing time and pH regardless of the total alpha radioactivity of the system.  Increasing the total alpha radioactivity of the solution had a negligible effect on the sorption rate.  This indicated that surface-mediated reduction of Pu(V) in these systems was not due to radiolysis.  Additionally, literature values for the Pu(V) disproportionation rate constant did not describe the experimental results.  Therefore, Pu(V) disproportionation was also not a main driver for the surface-mediated reduction of plutonium.  Batch desorption experiments and X-ray absorption near edge structure spectroscopy were used to show that Pu(IV) was the dominant oxidation state of sorbed plutonium.  Thus, it appears that the observed surface-mediated reduction of Pu(V) in the presence of high-purity quartz was based on the thermodynamic favorability of a Pu(IV) surface complex.

Effect of alpha radiolysis on plutonium(V) sorption to quartz

247th National Meeting (2014)

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