Advances in the management of radioactive wastes and radionuclide contamination in environmental compartments: a review

Several anthropogenic activities produce radioactive materials into the environment. According to reports, exposure to high concentrations of radioactive elements such as potassium (⁴⁰K), uranium (²³⁸U and ²³⁵U), and thorium (²³²Th) poses serious health concerns. The scarcity of reviews addressing the occurrence/sources, distribution, and remedial solutions of radioactive contamination in the ecosystems has fueled data collection for this bibliometric survey. In rivers and potable water, reports show that several parts of Europe and Asia have recorded radionuclide concentrations much higher than the permissible level of 1 Bq/L. According to various investigations, activity concentrations of gamma-emitting radioactive elements discovered in soils are higher than the global average crustal values, especially around mining activities. Adsorption technique is the most prevalent remedial method for decontaminating radiochemically polluted sites. However, there is a need to investigate integrated approaches/combination techniques. Although complete radionuclide decontamination utilizing the various technologies is feasible, future research should focus on cost-effectiveness, waste minimization, sustainability, and rapid radionuclide decontamination. Radioactive materials can be harnessed as fuel for nuclear power generation to meet worldwide energy demand. However, proper infrastructure must be put in place to prevent catastrophic disasters.

Synthesis of Novel Hierarchical Rod-like Mg-Al bimetallic oxides for enhanced removal of uranium (VI) from wastewater

As one of the most frequently used nuclides for nuclear fuel and toxic heavy metal in polluted solutions, the removal and recovery of U(VI) from wastewater is significant both for nuclear energy and human health. Herein, the novel hierarchical Mg-Al bimetallic oxides (Mg/Al-BOs) were successfully synthesized by a facile hydrothermal-lyophilization-calcination method for enhanced removal of uranium (U(VI)) from wastewater. The as-synthesized Mg/Al-BOs adsorbents were characterized by a variety of techniques including SEM-EDS, XRD, high temperature in-situ XRD, TG-DSC, N₂ adsorption-desorption isotherm and XPS. Batch experiments including the effects of pH, hydration species, interfering ions on U(VI) removal, adsorption kinetics, isotherms and recyclability were systematically studied. Results showed that calcined Mg/Al-BO-24 inherited the hierarchical structure from its hydrotalcite-like precursor and grew the bimetallic oxides of Al₂O₃/MgO into a 3D rod-like and mesoporous network with the large BET surface area (472.4 m²∙g-¹), which presented abundant binding sites on the surface and contributed to preventing the aggregation of Al₂O₃/MgO nanoparticles, allowing the fast uptake of U(VI) for equilibrium within 180 min and the significant increase of maximum adsorption capacity to 411.5 mg∙g-¹. The uptake kinetics and isotherms of U(VI) removal could be well represented by the pseudo-second-order and Langmuir models, respectively. Further, it was demonstrated that U(VI) removal by Mg/Al-BO-24 was less influenced by coexisting cations and the regeneration cycles, indicating the excellent selectivity and reusability for U(VI) by the as-prepared composites. Based on the XPS analysis results, the mechanisms for U(VI) sorption onto the Mg/Al-BO-24 were mainly ascribed to the synergistic surface complexation and electrostatic interaction. These results suggested that Mg/Al-BO-24 prepared by the method reported here was available for developing other multiple metal oxides and would be a promising material for the effective treatment of wastewater with U(VI)-contamination.

Highly-efficient and easy separation of γ-Fe2O3 selectively adsorbs U(Ⅵ) in waters

The migration and transformation of uranyl [U (Ⅵ)] ions in the environment are quite dependent on the geological condition in particular with the site enriched in Fe. In this study, the interfacial interaction of U (Ⅵ) ions with maghemite (γ-Fe₂O₃) particles was studied and the interaction mechanism was explored as well. Batch experiments confirm that γ-Fe₂O₃ can effectively remove U (Ⅵ) from an aqueous solution within a relatively short reaction time (R% > 92.01% within 3 min) and has a considerable capacity for U (Ⅵ) uptake (q_t: 87.35 mg/g). γ-Fe₂O₃ displays an excellent selectivity for U (Ⅵ) elimination. Results on the effects of natural organic matter such as humic acid (HA) indicated that HA could promote the interfacial interaction between γ-Fe₂O₃ and U (Ⅵ) under acidic conditions. Compared with other radionuclides (e.g., Sr(Ⅱ) and Cs(Ⅰ)), U (Ⅵ) was more effectively removed by γ-Fe₂O₃. The U (Ⅵ) removal by γ-Fe₂O₃ is primarily due to electrostatic interactions and precipitation that result in the long-term retardation of uranium. γ-Fe₂O₃ not only can fast and selectively adsorb U (Ⅵ) but also can be magnetically recycled, demonstrating that γ-Fe₂O₃ is a cost-effective and promising material for the clean-up of uranyl ions from radioactive wastewater.

NH3 Plasma Functionalization of UiO-66-NH2 for Highly Enhanced Selective Fluorescence Detection of U(VI) in Water

Radioactive U(VI) in nuclear wastewater is a global environmental pollutant that poses a great threat to human health. Therefore, it is of great significance to develop a U(VI) sensor with desirable sensitivity and selectivity. Inspired by electron-donating group modification for enhancement of binding affinity toward U(VI), we report an amine group functionalization of UiO-66-NH₂, using a low-cost, environmentally friendly, and low-temperature NH₃ plasma technique as a fluorescence switching nanoprobe for highly sensitive and selective detection of U(VI). The resulting amine-functionalized UiO-66-NH₂ (LTP@UiO-66-NH₂) shows dramatically enhanced fluorescence emission and selective sensitivity for U(VI) on the basis of the quenching effect. The quenching efficiency increases from 58 to 80% with the same U(VI) concentration (17.63 μM) after NH₃ plasma functionalization. As a result, the LTP@UiO-66-NH₂ has the best K_sv (1.81 × 10⁵ M^-1, 298 K) and among the lowest LODs (0.08 μM, 19.04 ppb) compared with those reported in the literature. Intraday and interday precision and application in real environment experiments indicate stable and accurate U(VI) detection performance. Fluorescence lifetime and temperature-dependent detection experiments reveal that the quenching mechanism belongs to the static quenching interaction. The highly selective fluorescence detection is attributed to the selective binding of U(VI) by the rich functionalized amine groups of LTP@UiO-66-NH₂. This work provides an efficient fluorescence probe for highly sensitive U(VI) detection in water, and a new strategy of tailored plasma functionalization for developing a practical MOF sensor platform for enhanced fluorescence emission, sensitivity, and selectivity for detecting trace amounts of radioactive species in the environment.

Easily synthesized mesoporous aluminum phosphate for the enhanced adsorption performance of U(VI) from aqueous solution

High-yield selective adsorbents and suitable modification methods are both significant for the efficient treatment of U-contaminated wastewater. In this work, a rich-mesoporous aluminum phosphate adsorbent (APO-10) was synthesized by simply increasing the mass of reactants under a fixed solvent volume. After increasing the mass of reactants ten times, APO-10 has the added defect level, the increased specific surface area, and mesoporous structure, and the increased number and enhanced adsorption ability of adsorption active sites (phosphorus-oxygen groups) on the surface, resulting in an enhanced adsorption performance of U(VI) in various environmental conditions. Its ultrahigh adsorption capacity calculated by the Langmuir model can reach 826.44 mg g^-1 at pH = 5.5 and T = 298 K. Its crystal structure did not change after adsorption and remained at 584.40 mg g^-1 after 6 cycles. Additionally, APO-10 shows an excellent uranium-selectivity over 68% from a mixed aqueous solution and has excellent applicability in the acidic and alkaline environment based on dynamic adsorption and desorption column experiments. This study not only provides a high-yield efficient selective adsorbent (APO-10) with excellent anti-radiation structure stability for the treatment of radioactive contamination but also provides a feasible modification method by simply increasing the mass of reactants.

Metal oxide aerogels: Preparation and application for the uranium removal from aqueous solution

The low density CeO₂, Pr₂O₃ and Nd₂O₃ aerogels were synthesized by a novel solution-freeze-drying-calcination route. The bulk densities of the CeO₂, Pr₂O₃ and Nd₂O₃ aerogels were calculated to be 8.10, 10.67 and 9.80 mg/cm³, respectively. Moreover, the structure of metal oxide aerogels was similar due to the same template materials, which made them possible to be appealing materials for adsorption. The maximum adsorption capacity for U(VI) of the CeO₂, Pr₂O₃ and Nd₂O₃ aerogels reached 481.5, 840.6 and 587.3 mg/g (pH = 7, T = 25 °C), respectively, which were much higher than most of other modified metal oxides. Moreover, at low concentration of U(VI) (5 mg/L), it was completely dislodged by CeO₂ aerogel and the remaining U(VI) was below 0.006 mg/L, which was lower than the effluent standards of United States Environmental Protection Agency and World Health Organization. Besides, after five cycles, the adsorption efficiency of metal oxide aerogels almost remained at a high level. Due to the excellent adsorption performance and high reusability, the three metal oxide aerogels would be promising adsorbents for the removal of U(VI).

Design of 3D alumina-doped magnesium oxide aerogels with a high efficiency removal of uranium(vi) from wastewater

3D alumina-doped magnesium oxide (Al₂O₃/MgO) aerogels with remarkable adsorption properties for U(vi) were prepared via a simple lyophilization–calcination method.