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.
Highlights
Graphene oxide (GO) has been considered as an extremely promising material for treatment of radioactive waste and contaminated natural waters due to its high capacity in sorption of radionuclides.[1−3] The processing of radioactive waste aims at converting it into forms convenient for long-term storage or permanent disposal
Defect-rich GO was synthesized in our study using the Hummers oxidation procedure applied to the reduced graphene oxide (rGO) precursor prepared using explosive thermal exfoliation of standard HGO.[54]
In agreement with our earlier fundamental study of radionuclides binding to different types of GO, carboxylic groups located on the edges of small holes are responsible for the sorption of, e.g., uranyl cations
Summary
Graphene oxide (GO) has been considered as an extremely promising material for treatment of radioactive waste and contaminated natural waters due to its high capacity in sorption of radionuclides.[1−3] The processing of radioactive waste aims at converting it into forms convenient for long-term storage or permanent disposal. According to our previous results based on EXAFS and modeling, the main sorption sites in HGO for the binding of radionuclides are carboxylic groups on the inside edges of small ∼1 nm-sized holes.[1] HRTEM images are in agreement with the suggested mechanism, showing a homogeneous distribution of uranium over the entire surface of dGO flakes. Many other defect types, which are inevitably formed in the process of dGO synthesis, do not contribute to the sorption of uranyl and remain unoccupied This interpretation of data is confirmed by the fact that the number of carboxyl groups determined using XPS is about 10 times higher than the molar concentration of sorbed uranium cations. Only about 10% of carboxylic groups are providing sorption sites for uranyl, while the rest are in defects or flake edges that remain not suitable for uranyl sorption
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