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Properties of the PuO2 nanoparticles (NPs) formed under acidic conditions (pH 1-4) are explored in this study at the atomic scale. High-resolution transmission electron microscopy (HRTEM) was applied to characterize the crystallinity, morphology and size of the precipitates. It was found that 2 nm crystalline NPs are formed with a crystal structure similar to bulk PuO2. 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 analyzed using the theory based on the Anderson Impurity Model (AIM). The experimental data obtained on NPs show that Pu(IV) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrate the presence of Pu(III) in addition to the Pu(IV), which is associated with redox dissolution of PuO2 NPs under acidic conditions. As shown by UV-Vis the contribution of Pu(III) in the HERFD spectrum for the sample at pH 1 originates from the solution rather than from NPs themselves. We discuss in detail the mechanism that affects the PuO2 NP synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Future investigations of plutonium chemistry will fundamentally benefit from the insights this work provides and further applied to various environmental and technological applications

Earlier studies demonstrated that graphene oxide (GO) with large number of defects is favorable for the sorption of radionuclides. Here we report oxidation treatment which converts high surface area activated reduced graphene oxide (arGO) into a 3D analogue of defect-rich GO (dGO). Oxidation of arGO using ammonium persulfate results in oxidation corresponding to carbon to oxygen ratio C/O=3.3, similar to the oxidation state of graphene oxide while preserving high BET surface area of about 880 m2/g. Analysis of surface oxidized arGO shows high abundance of oxygen functional groups very similar to dGO and hydrophilic properties. The “3D graphene oxide” showed high sorption capacity for U(VI) removal in an extraordinary broad interval of pH. Notably, the surface oxidized carbon material has a rigid 3D structure with micropores accessible for penetration of radionuclide ions. Therefore, the bulk “3D GO” can be used as a sorbent directly without dispersing, the step required for GO to make its surface area accessible for pollutants

Structural characterization of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non‐homogeneous samples with NPs below 3 nm. By combining High Energy X‐ray Scattering (HEXS) and High‐Energy‐Resolution Fluorescence Detected X‐ray Near‐Edge Structure (HERFD XANES), we characterized for the first time both short‐ and medium‐range order of ThO₂ NPs obtained by chemical precipitation. With this methodology, a novel insight into the structure of NPs at different steps of their formation process is achieved. The Pair Distribution Function (PDF) reveals a high concentration of ThO₂ small units similar to Th hexamer clusters mixed with 1 nm ThO₂ NPs in the initial steps of formation. Drying the precipitates at ⁓150 °C promotes recrystallization of the smallest units into more thermodynamically stable ThO₂ NPs. HERFD XANES at Th M₄ edge, a direct probe of the f states, shows variations that we correlate to the break of the local symmetry around Th atoms, which most likely concerns surface atoms. Together, HEXS and HERFD are a powerful methodology to investigate actinide NPs and their formation mechanism.

Fast chemical deposition of uranium(IV) under reducing conditions at pH 8-11 results in the formation of highly crystalline UO2 nanoparticles (NPs) with sizes of 2-3 nm, which is similar to the formation mechanism of PuO2 NPs. UO2 NPs are characterized by various microscopic and spectroscopic techniques including high energy transmission electron microscopy (HRTEM), high energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. Despite U(IV) being the dominant oxidation state of the freshly prepared UO2 NPs, they readily oxidize to U4O9 with time and under the X-ray beam. This oxidation of NPs is accompanied by their growth in size to 6 nm. The high tendency of UO2 NPs towards oxidation differs from PuO2 NPs’ behaviour due to the extremely high stability of Pu(IV) and much lower stability of oxidized Pu(V/VI) as compared to U(V/VI).

Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and con-taminated natural waters. dGO prepared using a modified Hummers oxidation procedure, starting from reduced graphene oxide (rGO) as a precursor, shows significantly higher sorption of U(VI), Am(III) and Eu(III) compared to standard gra-phene oxides (GO). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. There-fore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with large number of defects and holes. De-fects and holes are additionally introduced by Hummers oxidation of rGO thus providing extremely defect-rich material. Analysis of characterization by XPS, TGA, FTIR shows that dGO oxygen functionalization is predominantly related to de-fects, such as flake edges and edge atoms of holes, whereas standard GO exhibits oxygen functional groups mostly on the planar surface. The high abundance of defects in dGO results in a 15-fold increase in sorption capacity of U(VI) compared to standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic groups attached hole edge atoms of GO flakes as revealed by synchrotron-based extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detected X-Ray absorption near edge structure (HERFD-XANES) spectroscopy.