Carbon materials for extraction of uranium from seawater

With the rapid growth of population and industrialization, the energy crisis and environmental pollution as two main difficulties urgently need to be solved nowadays. The development and utilization of nuclear energy is of great significance for solving energy support, national security and environmental protection. As the raw material of nuclear energy, a lot of uranium in seawater provide a guarantee for the sustainable and green development of nuclear power plants. Recently, various new carbon-based materials (e.g., carbon nanofibers, multiwalled carbon nanotube, graphene) have been attracted widely intense interest in extraction of uranium from seawater due to large specific surface area, excellent acid-base resistance, high adsorption performance, environmental friendly and low cost. Thus, the systematic reviews concerning the extraction of uranium from seawater on various carbon-based materials were highly desirable. In this review, the extraction methods of uranium from seawater, including electrochemical, photocatalytic and adsorption methods are briefly introduced. Then the application and mechanism of four generation carbon-based materials on the extraction of uranium from seawater are systematically reviewed in details. Finally, the current challenges and future trends of uranium extraction from seawaters are proposed. This review provides the guideline for designing carbon-based materials with high adsorption capacity and exceptional selectivity for U(VI) extraction from seawater.

Highly efficient U(VI) capture by amidoxime/carbon nitride composites: Evidence of EXAFS and modeling

The recovery of uranium from wastewater and safe treatment of U(VI)-containing wastewater are of great important to ensure the sustainable development of nuclear-related energy. Although abundant studies of U(VI) sorption on various adsorbents have been widely achieved, U(VI) sorption at extreme pH and trace concentration is challenging issues due to limited sorption activity of natural adsorbents. The development of novel materials with highly efficient and excellent selectivity for capturing U(VI) from nuclear-related wastewater and seawater is highly desirable. In this study, amidoxime/carbon nitride (AO/g-C₃N₄) was fabricated and captured U(VI) under a variety of water chemistry. We demonstrated that AO/g-C₃N₄ exhibited the high adsorption capacities (312 mg/g at pH 6.8), fast removal equilibrium (>98% at 10 min) and superior selectivity for U(VI) compared with the other radionuclides (e.g., 19.76 mg/g of Cs(I)). In addition, AO/g-C₃N₄ exhibited the high uranium extraction capacity from natural seawater (9.55 mg/g at saturation time of 5.5 days) compared to vanadium (1.85 mg/g). U(VI) adsorption behavior at different pH can be excellently fitted by the surface complexation modeling with three inner sphere surface complexes (i.e., SOUO₂(CO₃)₂^3-, SO(UO₂)₃(OH)₅⁰ and SOUO₂⁺ species). According to XPS (X-ray Photoelectron Spectroscopy) analysis, the strong complexation of U(VI) with AO groups retained in C₃N₄ nanosheet. The split of U-O_eq2 subshell and the occurrence of U-C shell further demonstrated inner-sphere surface complexation by EXAFS (X-Ray Absorption Fine Structure) spectra analyses. These results revealed that the high potential of AO/g-C₃N₄ materials for selective U(VI) capture from wastewater and seawater.

The photocatalytic reduction of U(VI) into U(IV) by ZIF-8/g-C3N4 composites at visible light

The development of new photocatalyst towards the highly efficient photo-reduction of U(VI) was highly desirable. In this study, ZIF-8/g-C₃N₄ photocatalyst was fabricated to photo-reduce U(VI) from aqueous solutions under different water chemistry. It is demonstrated that ZIF-8/g-C₃N₄ exhibited the fast-photocatalytic rate (completely photoreduction within 30 min), high photocatalytic activity (K_d > 10⁵ mL/g) and superior chemical stability (No significant decrease after fifth cycles). The photoreduction rate of U(VI) significantly decreased with increasing pH, H₂O₂ radicals and photo-generated electrons play an important role in U(VI) photoreduction by quenching experiments and ESR analysis. According to XPS and XANES analysis, adsorbed U(VI) was partly photo-reduced into U(IV) by ZIF-8/g-C₃N₄ photocatalyst. The highly efficient removal of U(VI) on ZIF-8/g-C₃N₄ photocatalyst was attributed to the synergistic effect of ZIF-8 and g-C₃N₄ photocatalyst. The present study may provide a new strategy to apply new photocatalyst for in-situ photoreduction of U(VI) in actual environmental remediation.