Plant-microbial Remediation Of Uranium-contaminated Soil And The Mechanism Analysis

With the rapid development of nuclear power industry in China, the demand of uranium fuel increases sharply. While uranium deposits have made important contributions to the development of national defense and nuclear energy, they also bring seriouspollutions in the surrounding environment. Uranium contaminated soil has the characteristics of low radionuclide content, large volume and area.Uranium polluted soils are difficult to recover, since uranium can not be degraded, with both chemical toxicity and radioactivity.In this study, wecarried out a series of experiments to screen uranium tolerant plants, evaluate techniques in phyto-remediation, and understand underlying mechanisms of plants responses to uranium stress.In experiment 1, plant species were exposed to four uranium concentrations and physiological and biochemicalparameters were measured to evaluate uranium tolerance of plants. Four plant species, i.e. Medicagosativa, Lolium Perenne,Sorghumsudanense, Brassica juncea and a hybrid sorghum strainwere subjected to the uranium toleranceevaluation. The results showed that contents of photosynthetic pigments and soluble proteinof plants increasedunder low concentration of uranium, and decreased gradually with the increase of uranium concentration. The contents of MDA in plants induced by uranium stress significantly increased and the enrichment of uranium in these all test plants increased with the increase of uranium concentration.Of all the test plant species and strains, L. Perenne had the most uranium resistance based on its growth and antioxidant system performances.A lot of studies showed that chelating agents could activate the heavy metal ions in the soil, increase the bioavailability of heavy metals, reduce the toxicity of heavy metals to plants, and promote the absorption of heavy metals by plants and the ability to transfer from root to shoot.In experiment 2, different concentrations of citricacid( 1, 5 and 10mmol/kg) were addedto uraniumpolluted soils one week before harvesting, and physiological parameters of plants were measured. The results showed that 5 mmol/kg addition of citricacid significantly decreased the relative electrical conductivity and MDA content, enhanced the content of soluble protein, plant height, photosynthetic pigment content, the antioxidant activity, and the ability to maintain metabolic balance.The enrichment of uranium by L. Perenne increased with the increase of the concentration of citric acid, and highest in 5mmol/kg citric acid treaments. At the same time, similar results were also observed for the subcellular distribution of uranium in Lolium Perenne, and the enrichment conditions were greater than that of the root, and the cell wall and the residue were higher than that of the cell and membrane. And the application of 5mmol/kg citric acid increased the ability of it.Arbuscular mycorrhizal fungi(AMF) inoculation have been found to enhance the resistance of plants to heavy metal stresses. In experiment 3, three AMF species(i.e. Glomus tortuosum 、 G. Claroideum and G. mosseae) were inoculated to roots of L. Perenne to evaluate the role of AMF fungi in uranium absorption, allocation and resistance of host plants. The results showed that AMF inoculation significantly enhanced the uranium resistance of L. Perenne, with increasing in photosynthetic pigments, soluble protein content, activity of antioxidant enzyme system and decreasing in MDA content.The relationship between the heavy metal pollutants and the tolerance and transport mechanism has been a hot and important research area. In this study, the ratio of the uptake of and succinate ehydrogenase(SDH) in the shoots of plants were determined and the activity of and alkaline phosphatase(ALP) was determined. It is found that the soil uranium treatment inhibited the colonization rate of AMF and the activity of the enzyme.In the compartment cultivation, the uranium content in the soil from the uranium contaminated soil was 10.25% and 8.70% in the pot experiment. The contents of G. mosseae in the upper part and the root of the L. Perenne L. were 12.20% and 10.83%, respectively.In summary, our study showed that the inoculation of AM fungi, the adding of citric acid and mycorrhizal bioremediation have increased content of biological enrichment of uranium in Lolium Perenne, and plant-microbial and chemical combined remediation are two good systems to repair uranium mining and metallurgy area of uranium in contaminated soil.