Examination of the effect of alpha radiolysis on plutonium(V) sorption to quartz using multiple plutonium isotopes

• We use 238Pu and 242Pu to vary solution radioactivity but keep a constant molarity.
• XANES fitting shows a mixture of Pu(IV) and Pu(V) associated with quartz.
• Radiolysis is not responsible for Pu(V) reduction to Pu(IV) on the quartz surface.
• The sorption rate constant does not change with increasing radioactivity.
• The formation of Pu(IV) surface complexes drives Pu(V) reduction to Pu(IV).

The objective of this research was to determine if radiolysis at the mineral surface was a plausible mechanism for 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 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.

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