Immobilization Of Cs~+ By Na/Cs-based Geopolymer

Pollucite is widely used in the field of ¹³⁷Cs immobilization because of its high inclusion of Cs⁺,good thermal stability and low dissolution rate of Cs⁺.However,the traditional pollucite is prepared at a high temperature?>1100??,which not only has a high cost of synthesis,but also causes volatilization of Cs⁺at high temperature,resulting in secondary pollution.Geopolymer has zeolite-like structure,which has a good immobilization effect on Cs⁺and can be directly converted to pollucite at lower temperature through appropriate follow-up treatment,thus realizing the efficient sealing of Cs⁺and has broad application prospcets in the field of nuclear waste treatment.This paper firstly synthesizes Na/Cs-based geopolymer solidified body by using sodium silicate,cesium silicate and metakaolin as raw materials,and then the pollucite is obtained by hydrothermal and high temperature treatment respectively on the solidified body to achieve high-efficiency encapsulation of Cs⁺.Four variables of hydrothermal temperature,hydrothermal time,sodium-cesium ratio and hydrothermal medium were designed in the hydrothermal section.Two variables of boron oxide content and treatment temperature were designed in the high temperature treatment section.The effects of various variables on the microstructure,phase composition,mechanical properties and Cs⁺immobilization performance of the solidified body were studied.The orthogonal experiments were conducted to investigate the effects of sodium-cesium ratio?x=0,0.2,0.4,0.6,0.8?,hydrothermal temperature?140?,170?,200?,230?,260??and hydrothermal time?6 h,12 h,18 h,24 h,30 h?on the Cs⁺immobilization performance of Na/Cs-based geopolymer solidified body?xNa-?1-x?CsGP?.The results of variance analysis showed that the effect of sodium-cesium ratio was the most significant,and the hydrothermal temperature had a certain effect.The hydrothermal time of more than 6 h had no obvious effect.After hydrothermal treatment of 0.2Na-0.8CsGP at 230°C for 6 h,each gram of solidified body can solidify the Cs⁺with a mass of 0.3097 g.After the hydrothermal treatment at 170°C,the pollucite and the analcime first appeared.The increase of the hydrothermal temperature is conducive to the continuous accumulation of nano-sized particles of geopolymer into approximate spherical pollucite and analcime particles,and the crystallinity of which are further improved.On this basis,the influence of hydrothermal medium was studied.TCLP results show that the original Cs⁺in the hydrothermal medium can promote the formation of pollucite in the hydrothermal product of the solidified body,and increase the total immobilization ratio of Cs⁺in the solidified body?from 86.24%to 94.95%?.The hydrothermal environment with high temperature and high pressure can significantly reduce the mechanical properties of the solidified body.After hydrothermal treatment in deionized water,the compressive strength of 0.2Na-0.8CsGP decreased from 44.1 MPa to 10.5 MPa.The increase of the concentration of alkali in the hydrothermal medium gradually increases the compressive strength of the product to18.9 MPa,which meets the minimum mechanical performance requirements specified in the national standard.When boron oxide is doped into Na/Cs-based geopolymer solidified body?0.3Na-0.7CsGP?,liquid boron oxide at high temperature will accelerate the flow,mass transfer and diffusion of solid particles,promote liquid phase sintering,thus significantly reducing the ceramic transition temperature.When the content of boron oxide is 7.5 wt%,the transition temperature of ceramic is as low as 731.5?,which is 180?lower than that of the comparison group without boron oxide.When excessive boron oxide is added at room temperature,the compressive strength of the solidified body gradually decreases,which may be caused by the decrease in the degree of polymerization of the geopolymer.However,it is still significantly higher than the minimum requirement of 7 MPa as stipulated in the national standard.The formation of pollucite will greatly improve the Cs⁺solidification performance of the solidified body.The TCLP results showed that when the content of boron oxide was7.5 wt%,Cs⁺immobilization rate reached 99.91%after 800°C treatment,which was31.28%higher than that before treatment,and had excellent immobilization performance.After treatment at 600?and 700?,the evaporation of liquid boron oxide resulted in more pore structure and higher apparent porosity in the microstructure of the experimental group with boron oxide added.At the same time,the experimental group with boron oxide has not reached the ceramization transition temperature at this temperature,and Cs⁺mainly exists in amorphous phase.A large amount of dissolution of amorphous phase in acetic acid solution makes it have relatively low Cs⁺immobilization rate.However,after being treated at 800°C or higher temperature treatment,the Cs⁺in the experimental group with boron oxide was mainly present in pollucite,and the apparent porosity of the material decreased,which made the immobilization rate higher than that of the comparison group without boron oxide.The bulk density of the solidified body with a boron oxide content of 7.5 wt%after treatment at 1000°C was 2.88 g/cm³,which reached 98.3%of the theoretical density of pollucite.The long-term leaching experiments of deionized water showed that the Cs⁺leaching rate in each experimental group gradually decreased and stabilized with the leaching time.The Cs⁺cumulative leaching fraction in the hydrothermal group decreased from 7.4×10^-3 cm before hydrothermal to 1.26×10^-3 cm after hydrothermal.The Cs⁺cumulative leaching fraction within 5 days of the experimental group of 7.5 wt%was reduced from 1.86×10^-2 cm before high temperature treatment to 6.57×10^-5 cm after treatment at 800°C.After hydrothermal treatment or higher temperature treatment,the leaching rate and cumulative leaching fraction of Cs⁺in the solidified body will reach a stable value more quickly.For solidified bodies without hydrothermal or high temperature treatment,the leaching index is less than 8,which does not meet the standard for safe landfill disposal of nuclear waste.After hydrothermal treatment at 230?or high temperature treatment at 800°C,the leaching index is greater than 9,reaching the requirement of"controlled utilization".This also means that it is feasible to subject Na/Cs-based geopolymer solidified body to hydrothermal or high temperature treatment to synthesize the pollucite to improve its immobilization performance performance.