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

Extraction of heavy metals from copper tailings by ryegrass (Lolium perenne L.) with the assistance of degradable chelating agents

Heavy metal contamination is an urgent ecological governance problem in mining areas. In order to seek for a green and environmentally friendly reagent with better plant restoration effect to solve the problem of low efficiency in plant restoration in heavy metal pollution soil. In this study, we evaluated the effects of three biodegradable chelating agents, namely citric acid (CA), fulvic acid (FA) and polyaspartic acid (PASP), on the physicochemical properties of copper tailings, growth of ryegrass (Lolium perenne L.) and heavy metal accumulation therein. The results showed that the chelating agent application improved the physicochemical properties of copper tailings, increased the biomass of ryegrass and enriched more Cu and Cd in copper tailings. In the control group, the main existing forms of Cu and Cd were oxidizable state, followed by residual, weak acid soluble and reducible states. After the CA, FA or PASP application, Cu and Cd were converted from the residual and oxidizable states to the reducible and weak acid soluble states, whose bioavailability in copper tailings were thus enhanced. Besides, the chelating agent incorporation improved the Cu and Cd extraction efficiencies of ryegrass from copper tailings, as manifested by increased root and stem contents of Cu and Cd by 30.29-103.42%, 11.43-74.29%, 2.98-110.98% and 11.11-111.11%, respectively, in comparison with the control group. In the presence of multiple heavy metals, CA, FA or PASP showed selectivity regarding the ryegrass extraction of heavy metals from copper tailings. PCA analysis revealed that the CA-4 and PASP-7 treatment had great remediation potentials against Cu and Cd in copper tailings, respectively, as manifested by increases in Cu and Cd contents in ryegrass by 90.98% and 74.29% compared to the CK group.

Biodegradable chelating agents for enhancing phytoremediation: Mechanisms, market feasibility, and future studies

Heavy metals in soil significantly threaten human health, and their remediation is essential. Among the various techniques used, phytoremediation is one of the safest, most innovative, and effective. In recent years, the use of biodegradable chelators to assist plants in improving their remediation efficiency has gained popularity. These biodegradable chelators aid in the transformation of metal ions or metalloids, thereby facilitating their mobilization and uptake by plants. Developed countries are increasingly adopting biodegradable chelators for phytoremediation, with a growing emphasis on green manufacturing and technological innovation in the chelating agent market. Therefore, it is crucial to gain a comprehensive understanding of the mechanisms and market prospects of biodegradable chelators for phytoremediation. This review focuses on elucidating the uptake, translocation, and detoxification mechanisms of chelators in plants. In this study, we focused on the effects of biodegradable chelators on the growth and environmental development of plants treated with phytoremediation agents. Finally, the potential risks associated with biodegradable chelator-assisted phytoremediation are presented in terms of their availability and application prospects in the market. This study provides a valuable reference for future research in this field.

Metabolomic profiles reveals the dose-dependent effects of rice grain yield and nutritional quality upon exposure zero-valent iron nanoparticles

Zero-valent iron nanoparticles (nZVI) were widely used material in environmental remediation, which has attracted increasing concern for their safety. Previous studies have shown that the addition of nZVI could inhibit rice seedling growth. However, the effect of nZVI on the soil-rice system during the entire life cycle was not reported. Furthermore, the effect of nZVI on the quality of rice grain has also not been studied. Therefore, we investigated the effects of rice grain yield and nutritional quality upon exposure nZVI. The results showed that the soil pH value, redox potential and Fe (II) content in the nZVI-treated group were decreased in a dose-dependent manner. Interestingly, 2500 mg/kg nZVI significantly decreased the relative abundance of several functional microbial communities (10.52-73.53 %) associated with carbon and nitrogen cycles in response to plants compared to the control. Meanwhile, the nZVI treatment clearly reduced grain yield (8.71-18.21 %). Furthermore, the content of protein (51.72-57.79 %) and several essential nutrients (Zn, Cu, Mn and Mo) in the nZVI-treated grains was also decreased in a dose-dependent manner. The results of grain metabolomics indicated that nZVI could interfere with the relative expression of lysine and glutathione by regulating the metabolic pathways of antioxidant and protein synthesis in rice.

Non-targeted metabolomics and 16s rDNA reveal the impact of uranium stress on rhizosphere and non-rhizosphere soil of ryegrass

As a radioactive heavy metal element with a long half-life, uranium causes environmental pollution when it enters the surrounding soil. This study analyzed the changes about soil enzyme activity, non-targeted metabolomics, microbial community structure and function microbial community structure and function to assess the differences in the effects of uranium stress on rhizosphere and non-rhizosphere soil. Results showed that uranium stress significantly inhibited the activities of urease and sucrase in rhizosphere and non-rhizosphere, which had less effect on rhizosphere. Compare to the non-rhizosphere soil, the uranium stress induced the production of gibberellin A1, to promoted several metabolic pathways, such as nitrogen and PTS (Phosphotransferase system) metabolic in rhizosphere soil. The species and abundance of Aspergillus, Acidobacter, and Synechococcus in both rhizosphere and non-rhizosphere soil were decreased by uranium stress. However, the microorganisms in rhizosphere soil were less inhibited according to the soil metabolism and microbial network map analysis. Furthermore, the Chujaibacter in rhizosphere soil under uranium stress was found significantly positively correlated with lipid and organic oxygen compounds. Overall, the results indicated that ryegrass roots significantly alleviated the effects of uranium stress on soil microbial activity and population abundances, thus playing a protective role. The study also provided a theoretical basis for in-depth understanding of the biological effects, prevention and control mechanisms of uranium-contaminated soil.

Ryegrass extraction of heavy metals from municipal sewage sludge compost-amended soils assisted with citric acid

Land use is an effective way to reduce carbon emission in the recycling process of municipal sludge compost; meanwhile, heavy metals (HMs) in the sludge can be phytoextracted by ornamental plants. As an eco-friendly soil amendment, citric acid (CA) has been reported to be of great potential aid to phytoremediation, and its effect on ryegrass (Lolium perenne L.) extraction of HMs (Zn, Ni, Pb, Cu, and Cd) from municipal sewage sludge compost-amended (MSSC) soils has been investigated through pot experiments in the study. The growth of ryegrass was significantly promoted under 2 and 4 mmol kg^-1 CA treatments. The concentrations of HMs in MSSC soil after 45-day planting were significantly reduced ([Formula: see text]), and they were further reduced except for Cu while CA treated. The acid-extractable fraction of HMs in the soil was increased significantly as CA treated, and further improvement could be found when CA dose increased, which was due to the decreased soil pH and the complexation of CA with metal ions. The phytoremediation factor (PRF) was proposed to assess the phytoremediation efficiency, which was obtained as a ratio of the product of the biomass and metal concentration of plant shoot between the CA-treated group and the control group. When the CA dose was 6 mmol kg^-1, the average PRF of five heavy metals reached 2.29, and Cd was the highest (3.72), demonstrating that CA had great promotion on phytoremediation of heavy metals. This study made a contribution to the research of phytoremediation in sludge land use by demonstrating ryegrass as an ideal bioaccumulator for heavy metals, especially for Cd.

Chelating Agents in Assisting Phytoremediation of Uranium-Contaminated Soils: A Review

Massive stockpiles of uranium (U) mine tailings have resulted in soil contamination with U. Plants for soil remediation have low extraction efficiency of U. Chelating agents can mobilize U in soils and, hence, enhance phytoextraction of U from the soil. However, the rapid mobilization rate of soil U by chelating agents in a short period than plant uptake rate could increase the risk of groundwater contamination with soluble U leaching down the soil profile. This review summarizes recent progresses in synthesis and application of chelating agents for assisting phytoremediation of U-contaminated soils. In detail, the interactions between chelating agents and U ions are initially elucidated. Subsequently, the mechanisms of phytoextraction and effectiveness of different chelating agents for phytoremediation of U-contaminated soils are given. Moreover, the potential risks associated with chelating agents are discussed. Finally, the synthesis and application of slow-release chelating agents for slowing down metal mobilization in soils are presented. The application of slow-release chelating agents for enhancing phytoextraction of soil U is still scarce. Hence, we propose the preparation of slow-release biodegradable chelating agents, which can control the release speed of chelating agent into the soil in order to match the mobilization rate of soil U with plant uptake rate, while diminishing the risk of residual chelating agent leaching to groundwater.