Abstract
The aim of this study is to synthesize PuO2 nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO2 NPs formed under acidic conditions (pH 1–4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO2 crystal structure. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1–4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO2 NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO2 NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M4 HERFD data on PuF3 and PuF4 compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.
Highlights
PaperEnvironmental Science: NanoPlutonium in the environment can be present in various species, depending on the conditions
We have recently studied the formation of PuO2 NPs at pH 8 and >10 and we have used the total Pu concentration of 6 × 10−5 M, which exceeds its solubility in line with the available thermodynamic prediction.[18,28,44]
We have found by high energy resolution fluorescence detection (HERFD) and high-resolution transmission electron microscopy (HRTEM) that NPs obtained from pH 1, 2 and 4 are very similar to PuO2 NPs obtained from alkaline pH: identical crystal structures and Pu(IV) as the dominating oxidation state in all NPs
Summary
Plutonium in the environment can be present in various species, depending on the conditions. In the solution it may exist in the form of an ion, either hydrated or with other ligands; it can be present as a product of polymerization reaction in the form of mono-/polynuclear species. Despite the fact that PuO2 solubility is extremely low, Pu has been shown to migrate with mineral[3,7] or organic colloids,[8] colloidfacilitated transport might play an important role in the transporting of plutonium.[9]. The formation of PuO2+x NPs during the reactions at solid–liquid interfaces ranging from natural minerals[10–14] to microorganisms[15,16] has been reported. Extremely small and stable Pu NPs can still be present in the solution and affect solubility and redox processes
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