Insights into mechanism on organic acids assisted translocation of uranium in Brassica juncea var. foliosa by EXAFS

Rigidity-Flexibility Regulation and Hard-Soft Donor Combination: Dual Strategies in Covalent Organic Frameworks Construction for Actinides/lanthanides Separation

Separating actinides from lanthanides is essential for managing nuclear waste and promoting sustainable nuclear energy development. The recycling of transuranium elements (TRUs: Np, Pu, Am) is also significant for various nuclear technology applications. In this study, a dual strategy is introduced to designing covalent organic frameworks (COFs) that skillfully combines molecular rigidity with flexibility, integrating both hard and soft donor atoms in the synthesis of monomers. This results in a specialized COF that efficiently and selectively captures TRUs from acidic aqueous solutions. By utilizing the topological arrangement of rigid ligands to influence the twisting and stretching of flexible ligands, coordination environment featuring nitrogen and oxygen is created, which enhances the separation of transuranium in various oxidation states over lanthanides. In 0.5 m HNO3 solution, the as-synthesized DAPhen-COF achieves removal rates of 99.1% for Np(V) and 95.8% for Pu(IV). For Am(III), the removal rate reaches 98.6% in 0.01 m HNO3. DAPhen-COF exhibits remarkable selectivity for Np(V), with a separation factor of over 5000 for Np/Gd, outperforming other solid-phase materials. This research provides a comprehensive investigation into the design and synthesis of COFs for actinide capture, marking the first application of COFs in the separation of various TRUs over lanthanides.

Leaching Peculiarity of Uranium-Containing Layered Double Hydroxide Sediment Varied with Environmental Anions

In this research, we focus our attention on the leaching peculiarity of uranium-containing Mg-Al layered double hydroxide (LDH) which is one kind of waste sediment in uranium tailings, generated by the alkalinization of uranyl raffinate. The effect of inorganic (CO32-, SO42-, PO43-) and organic (C2O42-, C6H6O72-, C6H16O24P62-) anions were investigated. Atomic force microscopy result showed that the thickness of CO32--LDH increased to 8.6 nm compared to original LDH whose thickness was 6.7 nm. Compared with the control sample (5.58 μm), the grain size with C6H16O24P62- anion grew to 7.04 μm. A large amount of CO32- can stay in LDH, up to 1.78 mol percent, while the C6H16O24P62- anion was only 0.41 mol percent. X-ray diffraction results showed that the anions could change the crystal structure of LDH, especially the C6H18O24P6 anion, and theoretical calculation also conformed this result. The leaching tests showed that the introduction of anions improved the leaching efficiency of UO22+ from LDH. The introduction of anions destroyed the super buffer property of LDH. Theoretical calculation results indicated that the anions could grab UO22+ and help the UO22+ escape from the LDH. This research gave guidance for long-term disposal of uranium-containing tailings.

Effect of silicon on the distribution and speciation of uranium in sunflower (Helianthus annuus)

Sunflower (Helianthus annuus) can potentially be used for uranium (U) phytoremediation. However, the factors influencing the absorption of U and its subsequent distribution within plant tissues remain unclear, including the effect of silicon (Si) which is known to increase metal tolerance. Here, using hydroponics, the effect of Si on the distribution and speciation of U in sunflower was examined using synchrotron-based X-ray fluorescence and fluorescence-X-ray absorption near-edge spectroscopy. It was found that ∼88 % of U accumulates within the root regardless of treatments. Without the addition of Si, most of the U appeared to bind to epidermis within the roots, whereas in the leaves, U primarily accumulated in the veins. The addition of Si alleviated U phytotoxicity and decreased U concentration in sunflower by an average of 60 %. In the roots, Si enhanced U distribution in cell walls and impeded its entry into cells, likely due to increased callose deposition. In the leaves, Si induced the sequestration of U in trichomes. However, Si did not alter U speciation and U remained in the hexavalent form. These results provide information on U accumulation and distribution within sunflower, and suggest that Si could enhance plant growth under high U stress.

Lactic acid bacteria induce phosphate recrystallization for the in situ remediation of uranium-contaminated topsoil: Principle and application

Uranium (U) contamination often occurs in the topsoil (arable layer), and is a serious threat to crop growth. However, conventional microbial reduction methods are sensitive to oxygen and cannot be used to treat aerobic topsoils. In this study, phosphate-solubilizing microorganisms (PSM) were isolated from U-contaminated topsoil and used for soil remediation. Microbial metabolites and products were analyzed, and the pathways and mechanisms of PSM immobilization were revealed. The results showed that strain PSM8 had the highest phosphate-solubilizing capacity (dissolved P was 208 ± 5 mg/L) and the highest U removal rate (97.3 ± 0.1%). Multi-technical analyses indicated that bacterial surface functional groups adsorbed (UO₂)²⁺ ions on the cell surface, glycolysis produced 3-10 mg/L of lactic acid (pH 4.7-6.0), and lactic acid solubilized Ca₃(PO₄)₂ to form stable chernikovite (a type of uranyl phosphate) on the cell surface. The coupled application of Ca₃(PO₄)₂ and strain PSM8 significantly reduced the bioavailability of soil U (62 ± 11%), converting U from the exchangeable to the residual phase and P from the steady to the available form. In addition, pot experiments showed that soil remediation promoted crop growth and significantly reduced U uptake and toxicity to photosynthetic systems. These findings demonstrate that PSM and Ca₃(PO₄)₂ are good coupled fertilizers for U-contaminated agricultural soil.

Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana

Uranium (U) is a non-essential and toxic element that is taken up by plants from the environment. The assimilation pathway of U is still unknown in plants. In this study, we provide several evidences that U is taken up by the roots of Arabidopsis thaliana through Ca²⁺-permeable cation channels. First, we showed that deprivation of Arabidopsis plants with calcium induces a 1.5-fold increase in the capacity of roots to accumulate U, suggesting that calcium deficiency promotes the radionuclide import pathway. Second, we showed that external calcium inhibits U accumulation in roots, suggesting a common route for the uptake of both cations. Third, we found that gadolinium, nifedipine and verapamil inhibit the absorption of U, suggesting that different types of Ca²⁺-permeable channels serve as a route for U uptake. Last, we showed that U bioaccumulation in Arabidopsis mutants deficient for the Ca²⁺-permeable channels MCA1 and ANN1 is decreased by 40%. This suggests that MCA1 and ANN1 contribute to the absorption of U in different zones and cell layers of the root. Together, our results describe for the first time the involvement of Ca²⁺-permeable cation channels in the cellular uptake of U.

Expression of Cytochrome c3 from Desulfovibrio vulgaris in Plant Leaves Enhances Uranium Uptake and Tolerance of Tobacco

Cytochrome c3 (uranyl reductase) from Desulfovibrio vulgaris can reduce uranium in bacterial cells and in cell-free systems. This gene was introduced in tobacco under control of the RbcS promoter, and the resulting transgenic plants accumulated uranium when grown on a uranyl ion containing medium. The uptaken uranium was detected by EM in chloroplasts. In the presence of uranyl ions in sublethal concentration, the transgenic plants grew phenotypically normal while the control plants' development was impaired. The data on uranium oxidation state in the transgenic plants and the possible uses of uranium hyperaccumulation by plants for environmental cleanup are discussed.

Uranium (U) source, speciation, uptake, toxicity and bioremediation strategies in soil-plant system: A review

Uranium(U), a highly toxic radionuclide, is becoming a great threat to soil health development, as returning nuclear waste containing U into the soil systems is increased. Numerous studies have focused on: i) tracing the source in U contaminated soils; ii) exploring U geochemistry; and iii) assessing U phyto-uptake and its toxicity to plants. Yet, there are few literature reviews that systematically summarized the U in soil-plant system in past decade. Thus, we present its source, geochemical behavior, uptake, toxicity, detoxification, and bioremediation strategies based on available data, especially published from 2018 to 2021. In this review, we examine processes that can lead to the soil U contamination, indicating that mining activities are currently the main sources. We discuss the relationship between U bioavailability in the soil-plant system and soil conditions including redox potential, soil pH, organic matter, and microorganisms. We then review the soil-plant transfer of U, finding that U mainly accumulates in roots with a quite limited translocation. However, plants such as willow, water lily, and sesban are reported to translocate high U levels from roots to aerial parts. Indeed, U does not possess any identified biological role, but provokes numerous deleterious effects such as reducing seed germination, inhibiting plant growth, depressing photosynthesis, interfering with nutrient uptake, as well as oxidative damage and genotoxicity. Yet, plants tolerate U toxicity via various defense strategies including antioxidant enzymes, compartmentalization, and phytochelatin. Moreover, we review two biological remediation strategies for U-contaminated soil: (i) phytoremediation and (ii) microbial remediation. They are quite low-cost and eco-friendly compared with traditional physical or chemical remediation technologies. Finally, we conclude some promising research challenges regarding U biogeochemical behavior in soil-plant systems. This review, thus, further indicates that the combined application of U low accumulators and microbial inoculants may be an effective strategy for the bioremediation of U-contaminated soils.