Co-transport Of Cerium (Ⅳ) Hydroxide Colloid With HA, FA Or Various Cations

The safe disposal of high-level radioactive waste（HLW）generated from nuclear power plant is one of the key issues to ensuring China’s sustainable development of nuclear energy.China will adopt deep geological disposal for HLW.The study on the radionuclide migration is the core foundation in evaluating the HLW disposal safety.Although it has been recognized that An（Ⅳ）may form colloids from hydrolysis and consequently transport,there is still a lack of systematic research on the transport behavior of An（Ⅳ）hydroxide colloid as well as its"carrier"effects on other radioactive nuclides.Ⅰn this context,this work selects Ce（Ⅳ）as an analog of An（Ⅳ）,and studies the co-transport behavior of Ce（Ⅳ）hydroxide colloid with colloidal humic acid（HA）and fulvic acid（FA）,and Cs（Ⅰ）,Co（Ⅱ）,and Eu（Ⅲ）in water-saturated quartz sand medium.The obtained results are helpful to better understand the transport behavior of An（Ⅳ）and other coexisting radionuclides,and provide experimental data for the safety assessment of China’s HLW the disposal.The contents and conclusions of this study are as follows:（1）Ce（Ⅳ）hydroxide colloid was prepared and characterized,and its stability and transport behavior in water-saturated quartz sand column were studied.Ⅰt was found that Ce（Ⅳ）hydroxide colloid is stable under acidic conditions.The increase of concentration and flow rate promotes the transport of Ce（Ⅳ）hydroxide colloid,while the increase of ionic strength or p H inhibits the colloid transport.The transport behavior of Ce（Ⅳ）hydroxide and oxide colloids are different.The transport of Ce（Ⅳ）hydroxide colloid can be quantitatively described by a two kinetic site retention model.（2）The stability and transport behavior of Ce（Ⅳ）hydroxide and oxide colloids in the presence of HA or FA was studied.Ⅰt was found that Ce（Ⅳ）hydroxide colloids exhibit different stability under different p H,HA or FA concentrations,while the stability of Ce（Ⅳ）oxide colloids is only affected by the concentration of FA or HA.When p H=5.3,the transport of Ce（Ⅳ）hydroxide-HA composite colloid was first inhibited and then promoted with the increase of HA concentration,while it only played a promoted effect on the transport of Ce（Ⅳ）oxide colloids.When p H=5.3,the increase of FA or FA concentration showed an inhibitory effect on the transport of Ce（Ⅳ）hydroxide colloids,while it showed a facilitative effect on the transport of Ce（Ⅳ）oxide colloids.When HA concentration is 25 mg/L,the transport of Ce（Ⅳ）hydroxide-HA composite colloid is no longer sensitive to the changes of p H and ionic strength.（3）The adsorption of Cs（Ⅰ）,Co（Ⅱ）,and Eu（Ⅲ）on Ce（Ⅳ）hydroxide colloids and their co-transport were studied.Ⅰt was found that the adsorption of Cs（Ⅰ）and Co（Ⅱ）on Ce（Ⅳ）hydroxide colloids is weak,while Eu（Ⅲ）adsorption on colloids is strong.The effect of Ce（Ⅳ）hydroxide colloid on the transport of Cs（Ⅰ）and Co（Ⅱ）is dependent to ion concentration.The transport of Cs（Ⅰ）and Co（Ⅱ）at high concentrations(1×10^-4mol/L)was not affected by the presence of colloids,while colloid exhibited promoting and inhibiting effect on the transport of Co（Ⅱ）and Cs（Ⅰ）at low concentration(1×10^-6mol/L),respectively.Ce（Ⅳ）hydroxide colloid has an inhibiting effect on the Eu（Ⅲ）transport.The effect of colloids on cation transport depends on not only the interaction between colloids and cations,but also the interaction between cations and solid media.