Factors affecting the surface-mediated reduction of plutonium in the presence of pure mineral phases.

다국어 초록 (Multilingual Abstract)

The overall objective of the research is to investigate the mechanistic and kinetic aspects of the surface-mediated reduction of plutonium. The first chapter is a critical review of plutonium sorption to pure mineral phases. The objective is to study if and how the mechanisms by which plutonium interacts with mineral surfaces are connected to mechanisms for the surface-mediated reduction of plutonium. The main conclusion of the review is that advanced spectroscopy, microscopy, and molecular modeling are needed to fully understand not only the surface structure of pure mineral phases, but also the coordination and bonding environment of plutonium surface complexes. The objective of the second chapter is to determine if radiolysis at the mineral surface is 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 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. In the final chapter, changes in aqueous- and solid-phase plutonium oxidation state were monitored as a function of time and plutonium concentration in hematite (&agr;-Fe2O3) suspensions containing initially Pu(V). Batch kinetic experiments were conducted at plutonium concentrations between 10-8 and 10-6 M at pH 5 and 0.3 g/L (9.3 m2/L) hematite. Surface-mediated reduction of Pu(V) was observed under all conditions studied. However, differences in the reaction kinetics demonstrate the rate and mechanism of Pu(V) reduction changes as a function of plutonium concentration. Adsorption of Pu(V) was found to be the rate-limiting step at plutonium concentrations less than approximately 10-7 M Pu(V). Plutonium reduction in systems containing low concentrations of plutonium was attributed to trace amounts of Fe(II) in the hematite structure. Reduction of Pu(V) was found to be the rate-limiting step at concentrations higher than approximately 10-6 M Pu(V) and is attributed to the Nernstian favorability of Pu(IV) surface complexes. The reaction order with respect to plutonium concentration was found to be -1.09 +/- 0.13 and -0.90 +/- 0.29 for the adsorption and reduction rate-limiting steps, respectively. The results of this chapter demonstrate that the rate of Pu(V) adsorption and reduction in the presence of hematite decreases with increasing plutonium concentration and that disproportionation of Pu(V) is not the mechanism for the surface-mediated reduction of plutonium in these systems. This work strongly suggests that experiments carried out under high plutonium concentrations (i.e., > 10-7 M Pu) cannot be directly extrapolated to environmental concentrations of plutonium.