An Analysis Of The Influence Degrees Of Physicochemicaland Biological Factors On The Release Of Uranium From Uranium Tailings

The principal wastes produced in large quantities from uranium mining and milling are the uranium tailings,and they are usually dumped in special engineered pond called tailings repository.Due to the influence of physical weathering,chemical weathering,biological weathering and rainwater leaching,uranium from uranium tailings will be released into the environment,which causing damage to ecosystem and human health.The release of uranium from uranium tailings is firstly dependent upon the speciation of uranium.Water-soluble and exchangeable uranium have the highest activity in the environment and they are defined as the easily mobile species of uranium;bound to carbonates,Fe/Mn oxides and organic matter uranium have the lower activity,which belong to potential migration species of uranium;residual uranium is defined as stable species of uranium which has the lowest activity.Secondly,uranium release from uranium tailings is also strongly influenced by many physical and chemical factors,including liquid-solid ratio,pH,redox conditions,inorganic ions,certain salt ligands,as well as soluble organic matter;at the same time,the catalytic oxidation of microorganisms also affects the release of uranium from uranium tailings.This research taked the uranium tailings in uranium tailings reservoir in southern China as the research object,the influences of liquid-solid ratio,PO₄^3-concentration,leaching time,CO₃^2-and Ca²⁺concentration,acid rain pH,and humic acid concentration on the release of different speciation uranium from uranium tailings in a uranium tailings repository were investigated by using the orthogonal tests,sequential extraction and principal component analysis;at the same time,in combination with static leaching experiments of Acidithiobacillus ferrooxidans?A.f?to investigate the effects of microbe on uranium release ratio and dissolution rate of uranium in the leaching solution and the content of different speciation uranium in the tailings after leaching.The PCA results show that the influence of the seven factors on the release of the easily mobile species of uranium was in the order Ca²⁺>acid rain pH>CO₃^2->humic acid>leaching time>PO₄^3->liquid-solid ratio,the first four factors were the main ones.The influence of the seven factors on the release of the potential migration species of uranium was in the order Ca²⁺>liquid-solid ratio>leaching time>CO₃^2->humic acid>acid rain pH>PO₄^3-,the first four factors were the main ones.The influence of seven factors on the release of the stable species of uranium was in the order PO₄^3->humic acid>CO₃^2->acid rain pH>liquid-solid ratio>Ca²⁺>leaching time,the first four factors were the main ones.The results of bioleaching experiments show that the uranium release ratios in the two groups experiments(inoculanted A.f and without Fe²⁺,inoculant A.f and with Fe²⁺)were 84.17%and 88.45%,respectively,and the dissolution rate constants of uranium were 0.0233 d^-11 and 0.0297d^-1,respectively.The sequential extraction results of leached tailings show that the changes for content of different speciation uranium in the tailings inoculated with A.f were divided into three stages which include a sharp decline period,a slow decline period and a stable period.After the reaction,for the two group expriments(inoculanted A.f and without Fe²⁺,inoculant A.f and with Fe²⁺),exchangeable and bound to carbonates uranium in leaching tailings are almost zero,and bound to Fe/Mn oxides uranium are reduced by 85.52%and 89.31%,respectively;bound to organic matter uranium are reduced by 98.56%and 91.88%,respectively;residual uranium are reduced by 73.59%,and 82.73%,respectively;bioleaching promotes the release of uranium from uranium tailings by changing the speciation of uranium so as to reduce the migration of uranium in tailings.The SEM-EDS analyses show that there were a lot of flocculent material voids and pores on the surface of the tailings inoculated with A.f,and the bacteria adhered to the dissolution pits,and the bacteria had an oxidative and dissolution effect on uranium tailings during the leaching process.These findings provide a reference for further research on the control methods for uranium pollution in uranium tailings.