Enhanced phytoremediation of uranium contaminated soil by artificially constructed plant community plots

Effects of biochar pyrolysis temperature on uranium immobilization in soil remediation: Revealed by 16S rDNA and metabolomic analyses

Uranium-stressed soil caused by nuclear industry development and energy acquisition have attracted extensive attentions for a long time. This study investigated the effects of biochar application with different pyrolysis temperatures (300 ℃, 500 ℃ and 700 ℃) on remediation of uranium-stressed soil. The results showed that higher pyrolysis temperature (700 ℃) was benefit for ryegrass growing and caused a lower uranium accumulation in plants. At the same time, uranium immobilization was more effective at higher biochar pyrolysis temperature. Careful investigations indicated that activities of soil urease and sucrase were promoted, and bacterial diversity was strengthened in C700 group, which may contribute to uranium immobilization. The biochar application could activate metabolic of lipids and amino acids, organic acids and derivatives, and organic oxygen compounds. Nicotinate and nicotinamide metabolism, and Benzoxazinoid biosynthesis were unique metabolic pathways in the C700 group, which could enhance the uranium tolerance from different perspectives. Based on these results, we recommend to use biochar with 700 °C pyrolysis temperature when processing remediation of uranium-stressed soil. This study will facilitate the implementation of biochar screening and provide theoretical helps for remediation of uranium-stressed soil.

Phytoremediation of uranium-contaminated soil by perennial ryegrass (Lolium perenne L.) enhanced with citric acid application

Perennial ryegrass (Lolium perenne L.) was planted in uranium-contaminated soil mixtures supplemented with different amounts of citric acid to investigate the defense strategies of perennial ryegrass against U and the enhanced mechanism of citric acid on the remediation efficiency in the laboratory. The uranium content in the plant tissues showed that the roots were the predominant tissue for uranium accumulation. In both root and shoot cells, the majority of U was located in the cell wall fraction. Furthermore, antioxidant enzymes were also stimulated when exposed to U stress. These results suggested that perennial ryegrass had evolved defense strategies, such as U sequestration in root tissue, compartmentalization in the cell wall, and antioxidant enzyme systems, to minimize uranium stress. For an enhanced mechanism, the optimal concentration of citric acid was 5 mmol/kg, and the removal efficiency of U in the shoots and roots increased by 47.37% and 30.10%, respectively. The treatment with 5 mmol/kg citric acid had the highest contents of photosynthetic pigment and soluble protein, the highest activity of antioxidant enzymes, and the lowest content of MDA (malondialdehyde) and relative electrical conductivity. Moreover, the TEM (transmission electron microscope) results revealed that after 5 mmol/kg citric acid was added, the cell structure of plant branches partially returned to normal, the number of mitochondria increased, chloroplast surfaces seemed normal, and the cell wall became visible. The damage to the cell ultrastructure of perennial ryegrass was significantly alleviated by treatment with 5 mmol/kg citric acid. All the results above indicated that perennial ryegrass could accumulate uranium with elevated uranium tolerance and enrichment ability with 5 mmol/kg citric acid.

Radioactive waste treatment technology: a review

Radioactive waste is generated from activities that utilize nuclear materials such as nuclear medicine or power plants. Depending on their half-life, they emit radiation continuously, ranging from seconds to millions of years. Exposure to ionizing radiation can cause serious harm to humans and the environment. Therefore, special attention is paid to the management of radioactive waste in order to deal with its large quantity and dangerous levels. Current treatment technologies are still being developed to improve efficiency in reducing the hazard level and waste volume, to minimize the impact on living organisms. Thus, the aim of this study was to provide an overview of the global radioactive waste treatment technologies that have been released in 2019–2021.

Adsorption and enrichment of U in a cellulase-producing Trichoderma sp. and its physiological response mechanism

The cellulase produced by Trichoderma sp. was characterized by investigating the adsorption and enrichment of U and the physiological response to U exposure. The effects of U exposure (0 and 400 μM) on the growth, morphological characteristics, cellulase production, U adsorption, and U enrichment capacity of the Trichoderma strain were assessed. The effects of U exposure on the basic metabolism of this fungus were also analyzed by non-targeted metabolomics. Exposure to U (400 μM) for 24 h resulted in OD₆₀₀ turbidity of 0.278, and activities of carboxymethyl cellulase (CMC), filter paper enzyme (FPA), and β-glucosidase of 12834 U·mL^-1, 9285 U·mL^-1, and 12574 U·mL^-1, respectively. The measurement of the background α and β radioactivity showed an α activity concentration of 3.35 × 10⁶ Bq·kg^-1 in the fungus, a β activity concentration of 6.28 × 10⁵ Bq·kg^-1, and a U enrichment rate of 70.4 ± 4.5%. GC-MS metabolomics analysis identified a total of 319 metabolites (34 up-regulated and 30 down-regulated), which mainly caused the metabolic imbalance of organic acids and derivatives. The alanine, aspartate, and glutamate metabolic pathways were the most significantly enriched. Trichoderma sp. therefore has a strong ability to tolerate/accumulate U and continues to produce cellulase under U (400 μM) exposure. However, U interferes with the basic metabolism of this fungus.

A novel method for determining the adequate dose of a chelating agent for phytoremediation of radionulides contaminated soils by M. cordata

A chelating agent in an adequate dose used to enhance phytoremediation of radionuclide-contaminated soil should not inhibit the growth of the plant. If this constraint condition is satisfied, the total bioaccumulation amount (TBA) of radionuclide by the plant can be maximized. This is a constrained optimization problem to determine the adequate dose of the chelating agent for phytoremediation of radionuclide-contaminated soil. In this research, an adequate dose of a chelating agent for phytoremediation of radionuclide-contaminated soil was determined by a novel approach using pot experiments. The proposed approach was applied to specify the adequate doses of citric acid (CA) and S,S-ethylenediamine disuccinic acid (EDDS) for phytoremediation of uranium contaminated soil by M. Cordata. By using this method, the adequate doses of CA and EDDS for phytoremediation of ²³⁸U, ²³²Th and ²²⁶Ra contaminated soils by M. cordata were measures as 10.0 and 5.0 mmol kg^-1, respectively. The results showed that the approach could be used to establish the adequate dose of a chelating agent for phytoremediation of radionuclide or other toxic heavy metal contaminated soil by a plant.

Exogenous plant growth regulator alleviate the adverse effects of U and Cd stress in sunflower (Helianthus annuus L.) and improve the efficacy of U and Cd remediation

Plant growth regulators (PGRs) are widely used in agricultural activities and have the potential to improve plant growth and plant tolerance against metal stress. PGR-assisted phytoextraction is now an effective and inexpensive method for enhancing the plant removal of toxic metals from soil. In this study, we conducted experiments to determine the effects of PGR treatments on soil uranium (U) and cadmium (Cd) removal by sunflowers as well as their stress response to U and Cd contamination. We found that the plant growth was inhibited by combined U and Cd stress in sunflowers compared with that of the control; however, the application of exogenous PGR had reduced the combined U and Cd stress by stimulating photosynthesis, decreasing the levels of active oxygen and lipid peroxidation, and enhancing the activity of the antioxidant defence systems. Exogenous PGR also increased the uptake of U and Cd by sunflowers and therefore, improved their U and Cd remediation efficiency. Moreover, indoleacetic acid (IAA) was the most effective PGR at removing U and Cd in the soil; the U and Cd removal efficiency was 484.21% and 238.85% higher in the 500 mg L^-1 IAA application compared with that of the control without PGR application, respectively. Furthermore, none of the PGR treatments significantly influenced the available U and Cd contents in soil. Our results, therefore, may provide some detailed understanding on the technologies for the sustainable remediation of U and Cd contaminated soil by the combination of PGR treatments and phytoextraction.