Phytoremediation through Bidens pilosa L., a nonhazardous approach for uranium remediation of contaminated water

Role of arbuscular mycorrhizal fungi in lead translocation from Bidens pilosa L. plants to soil

Bidens pilosa frequently forms a symbiotic association with arbuscular mycorrhizal fungi (AMF). This plant species can grow in Pb-polluted soils, accumulating Pb in its tissues. The aims of the study were to determine whether Pb accumulated in the tissues of B. pilosa can be transferred to the soil through AMF and to compare the role of AMF communities that have a history of exposure to the contaminant with those that have never been exposed. The experiment combined plants with and without Pb accumulated in their tissues, and inoculated with AMF collected from the rhizosphere of B. pilosa in soils contaminated and not contaminated with Pb. The results showed that AMF participate in the removal of Pb that had entered the plant and release it into the soil, as evidenced by the presence of Pb in the AMF spores and in the glomalin produced by AMF. We propose that Pb accumulation in AMF spores would be a protection mechanism that interrupts Pb uptake by the plant; however, that mechanism would not be fully exploited in detoxification, whereas the production of Pb-enriched glomalin could be an important detoxification mechanism to eliminate Pb already taken up by plants. AMF with a history of Pb exposure achieved only higher rates of root colonization, while AMF without previous exposure showed higher Pb concentration in the spores and higher glomalin production, and successfully removed Pb from both the roots and aboveground parts of the plant. The use of AMF communities not adapted to Pb may be a more effective option for microbe-mediated phytoremediation methods in which detoxification mechanisms are desirable.

Screening and signifying the uranium remediation level of Alternanthera philoxeroides and Eichhornia crassipes from aquatic medium

Uranium is causing a hazardous impact on the human population throughout the globe. Different methods of remediation have been documented but the approach of phytoremediation has been praised throughout the globe. The bioaccumulation of uranium especially as a hyper-accumulator, has been documented in limited plant species. Therefore the current studies were conducted to elaborate on the overall U accumulation, biochemical and photochemical reactions in Alternanthera philoxeroides and Eichhornia crassipes to different concentrations of Uranium. The results showed that the accumulation of U in A.philoxeroides is higher; followed by E.crassipes; with maximum amounts of roots accumulation. Overall A.philoxeroides and E.crassipes accumulate as much as 948.88 mg/kg and 801.87 mg/kg on a dry weight basis. The biochemical results showed that Superoxide dismutase (SOD) decreased in the leaves and stem of A.philoxeroides; whereas an increase has been seen in E.crassipes in response to all treatments. peroxidase (POD) and Catalase (CAT) showed irregular response to all treatments; where the main increase was observed at T3 (120 μmol/L) and 72 h up to 138 μ/g-FW (POD) and 178 μ/g-FW (CAT) in A.philoxeroides and 1870 μ/g-FW (POD) and 73 μ/g-FW (CAT) in E.crassipes, respectively. The correlation coefficient between the fluorescence ratio Fv/Fm and the concentrations of U-treatment was significantly negative. It is concluded from the results that Uranium halted the biochemical and photochemical reaction but the plants resisted its impact while accumulating a good amount of uranium which is a good prospect for future interventions for the in-situ remediation of uranium-affected sites.

Uranium bioavailability in soil pore water: A long-term investigation in a contaminated soil mesocosm

Uranium in soil can exist in both (IV) and (VI) oxidation states, distributed among different soil fractions (exchangeable, carbonate, oxidizable, reducible, and residual). Following its release from these fractions, uranium enters the soil pore water, becoming bioavailable and potentially posing risks due to its radio and chemical toxicity. Given the significant health and environmental risks associated with uranium, it is crucial to understand its behaviour in contaminated soil pore water and how it changes over time, especially in response to seasonal variations. To address this issue, study was designed to investigate the temporal changes in uranium availability in soil pore water, with a special focus on effect of seasonal variations and biogeochemical reactions that govern the bioavailability of uranium in a contaminated soil mesocosm. This field investigation was carried out for two consecutive years, and revealed that, seasonal variation has a significant effect on the bioavailability of the uranium in the upper soil layers (<30 cm). The biogenic NO3- induced oxidative dissolution of uranium was found to be the predominant reaction causing the dissolution of uranium into soil pore water, followed by ion-exchange in upper layer, whereas at higher depths (30 cm < d < 70 cm) bioavailability is predominantly controlled by ion-exchange reaction. Furthermore, the study shows that at upper layers bioavailability is high during the summer, which is attributed to higher rate of biogenic denitrification and ion exchange reactions. Fast vertical migration of uranium in the soil column is attributed to formation of stable mobile species such as hydroxo‑carbonato ((UO2)xCO3(OH)y-), hydroxo (UO2)x(OH)y and carbonato (UO2CO3) complexes, identified by speciation modelling. For the first time, this study reports the process controlling uranium behaviour in soil pore water and the effect of seasonal variation on it in a contaminated soil. The findings are essential for assessing its potential radiological impact and remediation planning.

Phytotoxicity of radionuclides: A review of sources, impacts and remediation strategies

Various anthropogenic activities and natural sources contribute to the presence of radioactive materials in the environment, posing a serious threat to phytotoxicity. Contamination of soil and water by radioactive isotopes degrades the environmental quality and biodiversity. They persist in soils for a considerable amount of time and disturb the fauna and flora of any affected area. Hence, their removal from the contaminated medium is inevitable to prevent their entry into the food chain and the organisms at higher levels of the food chain. Physicochemical methods for radioactive element remediation are effective; however, they are not eco-friendly, can be expensive and impractical for large-scale remediation. Contrastingly, different bioremediation approaches, such as phytoremediation using appropriate plant species for removing the radionuclides from the polluted sites, and microbe-based remediation, represent promising alternatives for cleanup. In this review, sources of radionuclides in soil as well as their hazardous impacts on plants are discussed. Moreover, various conventional physicochemical approaches used for remediation discussed in detail. Similarly, the effectiveness and superiority of various bioremediation approaches, such as phytoremediation and microbe-based remediation, over traditional approaches have been explained in detail. In the end, future perspectives related to enhancing the efficiency of the phytoremediation process have been elaborated.

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.

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.

Pb tolerance and accumulation capabilities of Bidens pilosa L. growing in polluted soils depend on the history of exposure

Phytoremediation, especially phytoextraction, is a good alternative for remediation of soils contaminated with heavy metals. This method requires selection of species for their tolerance, high accumulation levels in harvestable parts, and high biomass production. Bidens pilosa L. has been reported as tolerant to and potentially hyperaccumulator of several heavy metals, including Pb, but with variable results in terms of effectiveness. The aim of this study was to analyse the intra- and interpopulation variability of B. pilosa in response to Pb in individuals from two populations: one historically exposed to Pb and another with no history of exposure. Bidens pilosa L. presented tolerance to Pb pollution in soil, evidenced in a higher survival rate, a better antioxidant response, and an efficient reduction in cell membrane damage mainly due to history of exposure. The period of exposure (30 years) was not long enough to obtain a B. pilosa population that provides seeds for phytoextraction projects, since the average value of total extraction was relatively low. Collecting seeds from a historically exposed population will provide some suitable individuals with Pb accumulation and translocation capabilities, but not a sufficient amount to conduct a large phytoremediation project. The individual accumulator profile of B. pilosa is not related to the physiological behaviour or to the Pb entry into the vascular bundle in root, but to the incorporation of other heavy metals that are micronutrients.

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.