ABSTRACT
Herein, neptunium(V) carbonates containing sodium or potassium cations were synthesized via chemical precipitation. Various techniques such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetry combined with differential scanning calorimetry, X-ray diffraction, and X-ray absorption spectroscopy were used to analyze the microstructures and elemental compositions of these samples. The crystal structures of hydrated NaNpO2CO3·3H2O (P1, a = 4.3420(2) Å, b = 4.8962(2) Å, c = 10.0933(11) Å, α = 91.014(7)°, ß = 77.834(11)°, and γ = 90.004(10)°) and KNpO2CO3 (P63/mmc, a = b = 5.0994(2) Å, c = 10.2210(15) Å) were determined for the first time using the Rietveld method. The synthesized carbonates exhibited distinct structural features and decomposition behaviors, as demonstrated through thermogravimetry analysis, which revealed the presence of crystalline hydrate water in sodium neptunium(V) carbonate. Furthermore, calcium-containing neptunium(V) carbonates were synthesized and characterized. Samples with the general composition Ca0.5NpO2CO3 were obtained using the ion exchange method and chemical precipitation from solutions containing competing cations (Ca2+, Na+, K+, and Mg2+). The synthesis conditions notably affected the diffraction patterns of the obtained calcium neptunium(V) carbonates. This investigation enhances our understanding of the structural properties and thermodynamic stability of neptunium(V) carbonates in the presence of diverse cations commonly found under radioactive waste disposal conditions.
ABSTRACT
Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and contaminated 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) than standard graphene oxides (GOs). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. Therefore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with a large number of defects and holes. Defects and holes are additionally introduced by Hummers oxidation of rGO, thus providing an extremely defect-rich material. Analysis of characterization by XPS, TGA, and FTIR shows that dGO oxygen functionalization is predominantly related to defects, 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 that in standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic group 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.
