Research On The Adsorption And Immobilization Mechanism Of Cesium By MgO-SiO₂-H₂O System

As a clean energy source,nuclear energy has brought great economic and social benefits to human,but the radioactive nuclear waste generated in the development and utilization process has caused serious environmental pollution,so the optimal disposal of these waste has become an urgent problem globally.Cesium（Cs）is one of the most common and harmful radionuclides in low and intermediate radioactive solid wastes.However,the curing effect on Cs by conventional cementitious is not obvious.Therefore,it is necessary to select new cementitious materials for curing Cs.Considering the characteristics of the MgO-SiO₂-H₂O（M-S-H）system,such as low porosity,low quantity of heat and high specific surface area,in this paper,the Cs is solidified using M-S-H system.Through phase analysis,micro analysis and durability analysis,the influence of Cs on the hydration reaction and stability of M-S-H system was investigated.The adsorption of hydration product powder of M-S-H system on Cs with different initial concentration,initial pH value and water cement ratio of the gelling system was analyzed.The adsorption of Cs was compared with that of ordinary Portland cement（OPC）.The effect of M-S-H system on the anti-leaching performance of Cs with different leaching conditions,water-cement ratio and molding method of solidified body was investigated.Meanwhile,by analyzing the phase,micromorphology and energy spectrum of the solidified body with increased Cs content,the existence state and immobilization mechanism of Cs were analyzed.The specific results are showed as follows:（1）The addition of Cs can reduce the compressive strength of the sample,but the sample still maintain good mechanical properties.The solidified body exhibits a compressive strength with 1.6%Cs at 90 d can reach 28.33 MPa,which is far higher than the compressive strength of the sample specified in the national standard GB14569.1-2011.The addition of Cs only slightly influences the reaction of the M-S-H system and only slows down the conversion of Mg（OH）₂ into the M-S-H gel to a certain extent.No new substance is found.The impact resistance,soak resistance and freeze-thaw resistance of the M-S-H system solidified body meet the requirements of the national standard GB14569.1-2011.It is proved that the solidified body is stable.（2）The absorptive capacity of Cs by hydrate powder increases with the increase of initial concentration and when the pH=7,the adsorption reaches the highest point.The adsorption rate and equilibrium adsorption capacity are higher when the water cement ratio of the cementitious system is larger.Compared with OPC,the M-S-H system has a smaller adsorption rate of Cs,but has a larger equilibrium adsorption capacity,so M-S-H system has a stronger adsorption capacity.The adsorption process follows the quasi-second-order dynamic model.（3）The leaching rate of Cs at 42 d under different leaching solution and temperature combination conditions is much lower than the requirement of GB14569.1-2011.The cumulative leach fraction is about 4%³0%of the standard.The leachate is simulated sea water,or high leaching temperature is unfavorable to the leaching effect.The solidified body with water cement ratio of 0.7 has the best fixation effect on Cs.In terms of the 42 d leaching rate,the press-formed solidified body whose solidification effect is unsatisfactory is higher than the cast-formed solidified body.The 42 d leaching rate of the M-S-H system solidified body is is lower than that of OPC solidified body and other cement solidified bodies,and the solidification effect is better.The immobilization of Cs by M-S-H system is mainly the physical encapsulation and adsorption of hydration products.The research shows that the M-S-H system has excellent adsorption performance and anti-leaching performance for Cs,and has the potential to be used as the base material for the encapsulation of nuclear waste.This system is of great significance to the sustainable development of the nuclear energy industry and building materials industry as well as the construction of ecological civilization.