Effects Of Humic Acid And Goethite Colloids On U(?) Transport Through Water-saturated Granular Granite Columns

Colloids widely present in the environment and can significantly affect the environmental behavior of radionuclides.The Beishan area?Gansu province,China?is the key candidate site for high-level waste disposal in China.Therefore,it is of great significance to study the effect of colloid on the transport behavior of radionuclide in the granite media of Beishan.In this paper,goethite colloid?Gt?and humic acid?HA?were selected as representative of typical inorganic colloids and organic colloids in the environment,respectively.We studied the stability of single colloids?Gt and HA?and composite colloids?Gt–HA?.The systematical column experiments were performed to study the individual and co-transport behavior of these colloids and U???in saturated granite particle under different physical and chemical conditions?pH,ionic strength,HA concentration,U???concentration,etc.?.The changes of effluent properties?pH,zeta potential and particle size?were also measured.The transport results of U???and colloid were fitted by two-site chemical non-equilibrium model and double dynamic site model respectively to obtain the transport parameters of different systems.Combined with the changes in the characteristics of the effluent and the theoretical calculations by DLVO and CFT,the transport mechanism of different systems was discussed.This paper mainly draws the following conclusions:?1?Individual transport of U???.The individual transport behavior of U???is mainly controlled by adsorption/desorption,U???concentration and U???speciation.The transport of U???is inhibited with the increase of pH?5.0–6.6?,whereas the transport of U???is promoted at pH=7.6.The ionic strength does not affect the transport of U???.The retention of U???was significantly enhanced with decreasing U???concentration.?2?Individual transport of Gt and HA.The transport behavior of Gt mainly depends on its stability.The transport of Gt is inhibited with the increase of pH and ionic strength.During transport,Gt may undergo ligand exchange reaction with hydroxyl groups?OH^–?on the granite surface and size exclusion effects may exist.The transport behavior of HA is mainly determined by the effect of charge exclusion and size exclusion.High pH is beneficial to the HA dissociation and transport,while the effect of ionic strength on the transport of HA is not obvious.?3?The effects of Gt and HA on U???transport.The co-transport of Gt and U???is sensitive to changes in pH,ionic strength,and U???concentration.When pH?5.0–6.6?and ionic strength increase,the stability of Gt–U???decreases and transport is inhibited,whereas when pH=7.6,the surface charge of Gt–U???changes from positive to negative,and the transport is promoted.Gt significantly promoted the transport of low concentrations of U???(2×10^–6,2×10^–7 M)and retarded the transport of high concentrations of U???(2×10^–5 M).The co-transport of HA and U???is less affected by pH and ionic strength.HA promotes U???slightly at high pH or low ionic strength,but the recovery of U???is reduced due to the coordination of HA and U???.?4?The effects of composite colloids on U???transport.The transport behavior of U???in the presence of composite colloids mainly depends on the colloid stability.Increasing the pH or decreasing the ionic strength facilitates the transport of U???.HA of low concentration?<5 mg/L?reduced the stability of the composite colloid,thereby blocking the transport of U???.HA of high concentration?5–20 mg/L?alerts surface charge of composite colloid from positive to negative,enhance colloidal stability,and significantly promotes the U???transport.?5?Model fitting,DLVO calculation and CFT calculation.U???and colloidal transport data in saturated porous media can be well fitted by the two-site chemical non-equilibrium model and the dual-kinetic site model,respectively.The DLVO and CFT calculation results are in good agreement with the experimental phenomena.