Tc In Beishan Granite Adsorption And Diffusion Behavior Research

The sorption and diffusion of Tc on granite under aerobic and anaerobic conditions were investigated. Influences of pH values,ferrous minerals(reduced iron powder or magnetite),bentonite,concentration of cations and anion (Fe3+, Fe2+ and CO32-) on the distribution coefficient,Kd,were studied. The sorption mechanism of 99Tc on granite was discussed by the desorption method of adding H2O2 into desorption solvent. Based on the diffusion coefficient measured and the solution of the diffusion equation,the distance of Tc diffusion after ten thousand years was predicted. The major results in this work are as follows:(1) The distribution coefficients,Kd,of Tc on granite under aerobic and anaerobic conditions are measured to be 16mL/g (pH=7.96) and 8.8X102mL/g (pH=9.68) respectively. The apparent diffusion coefficients on the granite plate with the thickness of 1.50mm are found to be 4.3X 10-11m2/s and 1.1 X 10-13m2/s respectively under aerobic and anaerobic conditions,these data show that the sorption and retardation of Tc on granite under anaerobic condition is strongly superior than that under aerobic condition.(2) The Kd values of Tc on granite are almost unchanged with the increase of pH value under aerobic condition,while the Kd values increas firstly,then drop under anaerobic condition. Under aerobic condition the changes of Kd values are not evident when the magnetite and bentonite are added into granite;while the Kd values of Tc on the granite mixed with reduced iron powder increase drastically. Under anaerobic condition the Kd values of Tc on the granite mixed with those minerals increase evidently .(3) Under aerobic and anaerobic conditions,the Kd values of TcO/ on granite drop with the increase of Fe3+concentration,- Under aerobic condition,the changes of Kd values are not evident when the concentration of Fe2 and CO32- increase respectively. Under anaerobic condition,the Kd values increase firstly,then drop with the increase of Fe-- concentration. It is believed that TcO4- is reduced on the surface of granite:The Kd values decrease with the increase of CC2 concentration.(4) Tc is difficult to be desorbed when preequlibrated water is used as desorption solvent under aerobic and anaerobic conditions. Under anaerobic condition,The desorption rate of Tc rises with the increase of HaCh concentration It appears that the Tc sorption process is dominated by oxidation-reduction mechanism.(5) The Kd values of Tc on biotite are higher than that on granite and the sorption properties is similar to that of granite;The Kd values of Tc on quartz are very low.(6) Using the diffusion coefficient determined under anaerobic condition,it is predicted that the concentration of Tc 1m away from the source of diffusion would be zero after ten thousand years,which indicates that Tc is retarded by granite effectively under long-term geologic disposal condition.(7) In this work,granite samples are collected from Bei Shan geologic layer of Gansu Province,the preselected site in our country for the high level radioactive waste disposal. This is the first time in China in the studies of Tc sorption and diffusion. The results show some actual referential values for the determination of preselected sites and the safety assessment of disposal repository.