COMPUTATIONAL INVESTIGATION OF THE KINETICS OF AQUEOUS PLUTONYL (PUO22+) REDOX REACTIONS WITH HYDROXYL RADICAL

The solubility of actinides, like plutonium, is largely controlled by their oxidation state. The tetravalent forms of these elements preferentially form insoluble oxide phases (e.g., PuO₂), but penta and hexavalent species are soluble, and thus, mobile, as actinyl ions (e.g., PuO₂²⁺_(aq)). The reactions that these species undergo in the presence of reductants and oxidants are important to determine the extent to which they may be dispersed in a given environment. This work assesses the degree to which different subreactions control the rate of the overall reaction between plutonyl and an oxidant, hydroxyl radical (•OH), which is a radiolysis product of water. In this approach, the redox reaction is broken down into three steps: (1) the diffusion of reactants toward each other in solution to form an outer-sphere complex, (2) the transition from outer- to inner-sphere complex, and (3) the rate of electron transfer. At this time it is prohibitively difficult to separate these subreactions experimentally, so we chose to address the encounter frequency using collision theory, the transition from an outer- to inner-sphere complex using a quantum-mechanical approach to analyze the energy as a function of distance between the two species, and the electron transfer using Marcus theory.

This contribution focuses on the first two subreactions using the reaction of PuO₂²⁺ and •OH as an example. For the first subreaction, we define the distance threshold (R) of outer-sphere complex formation by the energy between the species being on the order of kT lower than their interaction energy at an infinite distance (i.e., where there is no interaction). The initial rate of outer-sphere PuO₂²⁺ – •OH (both at 1 µM) complex formation assuming the rate law r = πR²vN_A[PuO₂²⁺][•OH] (where v is the geometric mean of the diffusion velocities) is found to be 5.74 × 10^‑3 mol L^-1 s^-1. At this rate, the t_1/2 of uncomplexed reactants is 87 µs. From fitting ab initio energy data, the rate of the second step, inner-sphere complex formation, is found to be ~10⁹ s^-1 for these species. These results indicate that the first two steps proceed quickly and are likely not rate-limiting for this reaction. This work shows this methodology can provide insight into rate-limiting subprocesses and allow us to explore the redox behavior of Pu and other metals in greater detail.