Experimental Study On The Immobilization Of Uranium In Uranium Tailings Reservoir By Nano Zeolite,Potassium Dihydrogen Phosphate And Modified Biochar

The development and utilization of nuclear energy resources have exacerbated radioactive pollution problems such as uranium waste rocks and tailings,and posed a serious threat to the ecological environment and the health of residents.In this paper,the surface soil of reddish brown soil（0²0cm）in a uranium tailings pond in the south-central part of Hunan Province is taken as the research object,and the nuclide uranium in the uranium tailings pond is immobilized by using nano-zeolite,potassium dihydrogen phosphate and modified biochar as fixatives,and its influence on the physical and chemical properties of soil is determined.Toxic leaching（TCLP）and stepwise chemical extraction.were used to evaluate the fixation effect.The correlation between soil pH and available phosphorus content and TCLP leached uranium content was analyzed by Pearson correlation matrix method to explore the influencing factors of fixed treatment of uranium-contaminated soil by nanozeolite,potassium dihydrogen phosphate,modified biochar and their combination.Finally,the soil surface micro-morphology and structure were analyzed by X-ray Diffraction（XRD）and Scanning Electron Microscope Energy Spectrum（SEM）,to explore the further immobilization mechanism.The main results are as follows:（1）Except the addition of modified biochars alone,the addition of other fixatives alone and in combination can increase the pH value of soil.Potassium dihydrogen phosphate has a significant effect on the content of available phosphorus in soil.When the addition amount is 6%,the content of available phosphorus in soil increases from 59.49 mg/kg to3745.83 mg/kg.The results of TCLP leaching showed that potassium dihydrogen phosphate had the best stabilizing effect on nuclide uranium in soil when fixative was applied alone,when the dosage was 3%,the leaching amount of uranium reached the lowest level,which decreased by56.92%compared with the control group,followed by 6%Nanozeolite and 6%modified biochar,and the leaching amount of uranium decreased by 55.03%and 56.03%,respectively.Among the fixed agent group application treatments,the best effect was achieved when nano-zeolite,potassium dihydrogen phosphate and modified biochar were applied at a mass ratio of 1:1:1 by 6%,and the uranium leaching amount was reduced by 69.26%compared with the control group.The results of Pearson correlation matrix analysis showed that the content of available phosphorus was not the main factor affecting the activity of uranium,but it would affect the activity of uranium to some extent,and the soil pH value was the key factor controlling the activity of uranium in soil.（2）Stepwise chemical extraction showed that after the soil of uranium tailings pond was repaired with three kinds of fixatives,the morphological distribution of uranium changed obviously,the plant availability and bioavailability decreased,and the effect of fixatives applied alone was in the following order:Nanozeolite>potassium dihydrogen phosphate>modified biochar,the optimal dosage was 6%,3%,6%,44.74%,26.68%,2%,respectively.5.25%of exchangeable states（including water-soluble states）are converted to other more stable forms.In the immobilization experiments of the composite materials,the repair effects were in the following order:Nanozeolite+potassium dihydrogen phosphate+modified biochars>Nanozeolite+potassium dihydrogen phosphate>Nanozeolite+modified biochars.The ratio of exchangeable uranium in soil can be reduced by 49.61%when three fixatives are added at the same time with a mass ratio of 1:1:1.Among the different ratios of nano-zeolite to potassium dihydrogen phosphate and modified biocharcoal,the best fixed ratio was 1:1 and 1:2,which reduced the ratio of exchangeable uranium in soil by 38.15%and 18.66%,respectively.Mixed application of the three fixatives can not only enhance the fixing effect synergistically,but also overcome the defects of the single fixing agent and achieve good repair results.（3）According to the characterization and analysis of soils before and after fixing by XRD and SEM,the mechanism of remediation of uranium tailings soil by three fixing agents is mainly through phosphate dissolution/precipitation,which converts U（VI）into uranium phosphate mineral precipitating Co（UO₂PO₄）₂ and mixed heavy metal mineral Fe（UO₂）₂（PO₄）₂OH,so that uranium can be converted to residue state to effectively fix heavy metals.