Effect Of Ferrihydrite Colloid On U(Ⅵ) Transport Through Saturated Sand Columns

Colloids have great influence on the transport of radionuclide in the environment. After being corroded, containers used for geological disposal of high-level radioactive wastes can produce ferrihydrite colloids, which will be carriers for the migration of key radionuclides. Therefore, it is of profound significance to study the effects of ferrihydrite colloids on radionuclide migration. In the present dissertation,the effects of ferrihydrite colloids on the transport of U(VI) migration in saturated quartz sand columns as model systems were studies. The findings may contribute to a better understanding of the transport of ferrihydrite colloids and actinides of +6 oxidation state in saturated porous medium, and may benefit safety assessment of high-level radioactive waste repository.Ferrihydrite colloids were prepared and characterized by X-ray powder diffraction, scanning electron microscopy, zeta potential analysis and particle size analysis. The transport of ferrihydrite colloids and / or U(VI) in single and coexisting system were studied as a function of flow rate, ionic strength and pH value. Effluent pH, zeta potential and particle size of ferrihydrite colloids in effluent were analyzed. The results showed that the greater the flow rate was, the faster the ferrihydrite colloids transported; when pH value or ionic strength increased, ferrihydrite colloid breakthrough curves were back shifted and recovery(C/C0) decreased, which might be related to decreased stability of colloids. In comparison, flow rate and ionic strength had no obvious effects on U(VI) transport, which was possibly due to the formation of inner-layer surface complexes and their dynamics. As pH value increased or U(VI) concentration decreased, U(VI) transport became slow and C/C0 decreased with long tail of elution curve. In the coexistence of U(VI) and ferrihydrite colloids, flow rate had also no obvious effects on U(VI) transport, whereas the increases in pH, ionic strength, U(VI) transport slowed down and decreased C/C0, possibly due to decreased stability of ferrihydrite colloids. Moreover as compared to U(VI) transport without ferrihydrite colloids, U(VI) transport sped up at low U(VI) concentration whereas it was slowed down at high U(VI) concentration. The two site dynamic retention model and the two site non-equilibrium model were used to describe the transport of ferrihydrite colloids and U(VI), respectively.