Plutonium is present in the environment due nuclear weapons production and testing and poses an extensive threat to public health because of its radiological toxicity and long half-lives (e.g., t1/2, Pu-239 = 24,100 years). The primary factor in determining the mobility of plutonium in the subsurface environment is oxidation state. While Pu(IV) is generally assumed to be the least mobile oxidation state and Pu(V) the most mobile oxidation state, certain caveats exist, such as the potential for nanoparticle-facilitated transport. Previous studies report that decreasing the diameter of nanoparticles can impact the chemical behavior of metal due to the increase of chemical reactivity and surface area at smaller sizes. Thus, we hypothesize that nanoparticles may influence the sorption and reduction of Pu due to the presence of more undercoordinated and defective sites at the nanoparticle surface. To test this hypothesis, we conducted batch sorption experiments with hematite (α-FeIII2O3) and magnetite (FeIIFeIII2O4) nanoparticles as a function of nanoparticle diameter under environmental conditions (e.g., constant ionic strength of 0.1 M NaCl and pH 8). Preliminary sorption data shows that Pu sorption to nano-magnetite is faster than Pu sorption to nano-hematite. However, nano-hematite shows more Pu(V) reduction than nano-magnetite. Further research is needed determine if this behavior is due to the presence of Fe(II) in the magnetite structure or the smaller particle size of nano-magnetite (8 nm) as compared to nano-hematite (40 nm). The results of these experiments will lead to an understanding of how changes in particle size affect the rate and extent of Pu sorption and reduction and, ultimately, how these effects impact the chemical behavior of Pu at the mineral-water interface.

Role of mineral particle size on the sorption behavior and redox chemistry of plutonium

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ACS Fall 2022

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