Study On Adsorption And Enrichment Mechanism Of U(?)by Aspergillus Niger

As a strain of industrial high-yield enzymes,Aspergillus niger(A.niger)is widely studied in the fermentation industry.Due to its strong feature of the external environment stress tolerance,it has certain potential advantages in bioremediation application of raaionuclides-contaminated and associated heavy metal-contaminated environment.In this study,the model strain A.niger was performed as the study object,and a representative pollution around the tailings(radioactive hexavalent uranium U(?))was selected as contaminated element,that was conducted to investigate the tolerance to radioactive stress.Through the study on the different levels of influence of physiological and biochemical indexes of A.niger under different initial U(?)concentrations stress culture,and provied a comprehensively analysis of the cell damage,stress tolerance and potential detoxification mechanism.In addition,an internal anti-stress response mechanism in response to U(?)stress was also investigated.By comparing the adsorption and enrichment process of U(?)by the active and inactive A.niger microspheres,in order to further explore the potential adsorption characteristics and mechanism of the combination of microorganisms and radionuclides.Furthermore,the modification process we proposed in this study could provieae useful insights into the fundamental application between biomass materials in the presence of adsorbents,with implications for the development of new bioremediation methods in radioactive contaminated wastewater purification processes.The results indicate:(1)Apparently,exposure to U(?)led to increases in U(?)bioaccumulation,but the mycelium growth and the germination of spores began to show significant inhibition at 100?125 mg/L tested U(?)concentrations,which demonstrated that A.niger does have some resistance to the nuclides U(?).From the aspects of sorption influencing factors including initial U(?)concentration,medium temperature,pH and dosage indicate that the optimal sorption condition of A.niger was 30?,pH 5,and 0.04 g and the adsorption amount reached at 39.38 mg/g.The long-term exposure to U(?)stress study indicated that A.niger mycelium could reduce the toxic effects of surface enrichment and internal transfer of U(?)through growth and metabolism regulation and synergistic intracellular detoxification resistance mechanisms.In the extra addition of oxalic acid experiment found that the oxalic acid could complex heavy metal ions to forni oxalate complexes to reduce the toxicity of heavy metals.By comparing the internal and external U(?)enrichment of A.niger,and the results was apparently related to the exposure with and without oxalic acid.The samples with oxalic acid addition were found to have higher U(?)adsorption and enrichment ability than those without oxalic acid at all times.It indicated that the oxalic acid addition might enhance and accelerate U(?)uptake and promote the chelated oxalic acid as U-oxalate complex,and thus reduce the activity and metastasis of U(?)in the medium,and achieve the effect of reducing the toxicity stress of U(?).Further study was conducted via investigation of intracellular enzyme activity of A.niger in the growth condition of liquid-state fermentation stress culture.It was found that under the seven-day period U(VI)stress exposure,the activity of carboxymethylcellulose(CMCA)and filter paper(FPA)decreased by 18.86%and 28.32%compared with the control group at the highest initial concentration(125 mg/L),respectively.However,the stress response caused the lignin peroxidase(LiP)and manganese peroxidase(MnP)activities were considerably lower than those in the absence of U(?),The activity of LiP and MnP decreased by 25.57%and 42.06%compared with the control group,which indicated the U(?)-induced enzyme inhibition in A.niger and the internal damage of the cell.(2)To better understand the effects of intracellular oxidative damage and the internal antioxidant mechanism of A.niger under long-term exposure to U(?)stress.The toxicity of the nuclide and the detoxification mechanisms of A.niger were studied.The exposure studies showed that due to the accumulation of U(?)in the internal cell,the stress reaction caused a significant increase in oxygen radical species(O2·-,·HO and H2O2),and finally the oxidative damage of the plasma membrane was further reflected by the higher degree concentration of MDA in the cells.In response to toxic levels of radionuclide,the study on the response of superoxide dismutase(SOD)and catalase(CAT)in the antioxidant system revealed that as the initial concentration of U(?)increased,the activity of the two antioxidant enzymes was also increased significantly compared with those found in controls.Meanwhile,after 3d of growth,the decreasing of damage phenomenon(activity of SOD reached maximum)indicated that A.niger was gradually overcame the damage to cell metabolism by U(?)-exposed and began to show the resistance to U(?)through its own antioxidant mechanism.This result was also consistent with the increase of intracellular GSH-Px content,the appearance of GSH-Px could gradually alleviate the oxidative damage of the nuclides U(?)to cells.The main reason for the stress response of A.niger exposed to U(?)stress is that the excessive bioaccumulation of U(?)affects the growth and metabolism of the cells,induces the production of oxygen free radicals and causes toxicity.By analyzing the correlation between R(SOD/CAT)and intracellular H2O2 content,it was found that the stress response inside A.niger triggered the increase of SOD activity,meanwhile,the free radical scavenging process caused excessive CAT enzyme consumption,which broke the balance of CAT enzyme supply in the early growth stage,which lead to the accumulation of H2O2 in the cells and caused oxidative damage of the plasma membrane.(3)Herein,the adsorption and enrichment experiments of U(?)by two different forms of A.niger microspheres biosorbents were compared.The results shown that due to its particular form,the inactive A.niger microspheres was not affected by adsorption factors such as temperature,pH and high initial concentration of U(?),and exhibited better efficient removal performance than that of active microspheres.Under the optimal adsorption conditions(30 0C,pH=5),the adsorption capacity of the inactive microspheres was 83.40 mg/g,which was much higher than the active microspheres of 50.65 mg/g.Batch experimental results showed that U(?)sorption onto A.niger microspheres were independent of ionic strength,indicating the formation of outer-sphere surface complexes,and the adsorption process could be well fitted by the Langmuir isotherm model and the pseudo-second kinetic.Furthermore,combined with the thermodynamic study,the U(?)adsorption characteristics of the two microspheres could be summarized into three processes:the free U(?)ions in the solution was rapidly coveraged the single layer on the surface of the adsorbents through complexation with a large amount of functional adsorption sites(luohehydroxyl,amino and carboxyl);then the U(?)ions diffused through the energy-consuming biomass transfer process to the inside of the tightly wound hyphae ball until all adsorption sites were fully occupied.In addition,the analysis of EDX,FT-IR and XPS in combination with SEM analytical methods revealed that the interaction adsorption mechanism of U(?)onto A.niger was mainly ascribed to the hydroxyl,amino and carboxyl functional groups via electrostatic interaction and surface complexation.