Sorption,Desorption And Transport Of U (?) On GMZ Bentonite And A Sand Rock

GMZ bentonite(Gaomiaozi,Mongoliar,China)has been chosen as the buffer/backfill materials for high level radioactive waste disposal repository in China.Sorption and desorption of radionuclides on the buffering/backfill material is an intensive concern of performance and safety assessment of the disposal repository.In addition,it has been stated that colloids generated when bentonite contacts with groundwater act as carriers for radionuclides and are responsible for the unexpected long-distance migration of radionuclides from the source,which will fail the retardation performance of the repository as well as the prediction model.Unfortunately,no light has been shed on the colloid generation or the role of colloid in radionuclides transport in terms of the disposal repository in China.There also is a very low level radioactive waste disposal repository in located in west China,which is designed to dispose the very low level radioactive waste generated in nuclear facility decommissioning and environmental remediation.In order to predict the risk of radionuclides' migration to the environment as well as to illuminate the diffusion and transport of radionuclides to the surroundings,it is necessary to investigate the radionuclides'sorption and transport on/in the geological medium adjacent to the repository.238U is one of the most important nuclides in radioactive waste and has ubiquitous existence.Consequently,in the present study,1)the influence of pH,ionic strength,cations,anions,humic substances on the sorption of U(VI)on GMZ bentonite and sand rock from the very low level radioactive waste repository are checked;2)the influence of pH,ionic strength,HA and addition sequence on the sorption-desorption hysteresis were examined and the sorption-desorption hysteresis was quantitatively calculated.3)the GMZ bentonite colloid was prepared and carefully characterized;The colloidal stability was checked as well;4)the role of the GMZ bentonite colloid in U(VI)transport though quartz sand column is evaluated in order to provide a comprehensive database and theoretical reference for the safety assessment of high level radioactive disposal repository in China.The main conclusions are as follows:1)It was stated that U(?)sorption on GMZ bentonite and the sand rock was greatly influenced by pH and ionic strength due to the varied U(?)species,surface charge of the solid surface and the solid structure.Mechanisms involved in the sorption behaviors included ion exchange,surface complexation,(surface)precipitation.In addition,impact on U(?)species from CO2 solubility varies with ionic strength.Large improvement in U(?)sorption could be observed in the presence of organic ligands like humic acid(HA)and inorganic ligands like PO43-.2)Sorption-desorption hysteresis of U(?)on/from GMZ bentonite was investigated.The sorption of U(?)on GMZ bentonite showed high reversibility at acidic pH,while gradually decreased as pH decreased.High reversibility was found when sorption occurred at low ionic strength.At high ionic strength,sorption turned to be totally irreversible as the desorption hysteresis became near 100%.At both pH 3 and pH 5,introduction of HA led to higher reversibility,because sorption of U(?)on HA was almost totally reversible.Different addition sequence caused different hysteresis,implying different sorption structures were formed due to the different addition sequence.3)GMZ bentonite colloid was prepared and characterized.Effect of pH,ionic strength and PO43-on colloid stability was discussed.The average size of GMZ bentonite colloid was 425±15 nm,and the zeta potential was-31±1 mV,indicating a stable colloidal system.The colloidal stability was sensitive to pH and ionic strength.Low pH and high ionic strength resulted to the rapid aggregation and settlement of colloid particles.However,GMZ bentonite colloids could maintain stable in the presence of 10-6-10-3 M NaH2PO4 for a long time(>1 month),even on unfavorable conditions like low pH and high ionic strength.4)It was proved that colloid and PO43-,both separately and together,could facilitate the transport of U(?)in quartz sand column.Growing enhancement could be observed with increasing colloid concentration(30-500 mg/L).It was believed that the results help understanding the role of PO43-played in the sorption and transport of radionuclides.The present work help understanding the environmental behavior of U(?)on the GMZ bentonite and the sand rock,which was believed to provide theoretical basis for environmental safety assessment of HLW and VVLW disposal repository,as well as to have reference values for further research on transuranic nuclides' sorption,desorption and colloid-facilitated transport on GMZ bentonite and sand rock.