| Pollucite has a wide application in nuclear waste encapsulation because of its advantages of large Cs+ package capacity,chemical resistance,and structural stability,but there is a problem of secondary pollution caused by Cs+ volatilization in high temperature due to the high synthesis temperature of traditional pollucite.Therefore,in this paper,meta-kaolin and strontium silicate are used as the main raw materials to synthesize cerium ion-excited geopolymer(Cs GP),and the formation of pollucite is achieved by controlling the species and relative content of mixing-excited ions and the ratio of silicon to aluminum within 1000 °C,to immobilize Cs+efficiently.systematically studied the influence of parameters such as the type and content of alkali ion and the ratio of silicon to aluminum on the microstructure,thermal evolution mechanism,and influence law for Cs+immobilization performance.The leaching behavior of Cs+ in the storage environment is simulated by long-term leaching test,the immobilization mechanisms of geopolymers and their ceramization products on Cs+ are revealed,for developing new Cs+ stabilization materials with low-temperature preparation and excellent immobilization performance.The main research results are as follows:The substitution of Li,Na,K for 30 at% Cs to synthesize geopolymer(0.3M-0.7Cs GP,M is Li,Na,K)can significantly reduce the formation temperature of the pollucite,and with decreasing of ion radius,the generation temperature of pollucite gradually decreased from 1144.4 °C(Cs GP)to 916.7 °C(0.3Na-0.7Cs GP)and 891.3 °C(0.3Li-0.7Cs GP),which was significantly lower than the high temperature evaporation temperature of Cs+(1100°C).After the heat treatment at800 °C,a large amount of pollucite grains were formed in the 0.3Na-0.7Cs GP,and the amorphous phase was reduced.At the processing temperature of 1000 °C,the amorphous phase in the 0.3M-0.7Cs GP was obviously reduced and the degree of crystallization was significantly higher,Cs+ exists stably in the form of pollucite,which can significantly inhibit the diffusion of Cs+ from the amorphous phase and increase the immobilization content of Cs+: the immobilization of Cs+ after treatment at 1000 °C increase to 97.82% from 58.26%(before high temperature treatment)in the TCLP test,and the mixing-excited groups reached 99.60%.The0.3Na-0.7Cs GP has the best effect with its immobilization of Cs+ reaching 99.93%.The compressive strength of 0.3M-0.7Cs GP after mixed excitation showed a decreasing trend,but it was still significantly higher than the national standard's strength requirement for the stabilize body of nuclear waste.After replacing different molal weight Cs with Na to synthesize geopolymer(Na/Cs = 17:3,14:6,8:12,6:14,2:18),different Na/Cs geopolymer phases are mainly amorphous before the high-temperature treatment,and the presence of a small amount of Na+(10 at%)can promote the nucleation and generation of pollucites,the Cs+ immobilization content in Cs GP increased from 58.26% to82.65 %(compared with before heat treatment);the crystallinity of pollucites in different Na/Cs ratios of geopolymers also increased gradually with increasing temperature after treatment at 1000 °C,The Cs+ immobilization content has increased to more than 98.75% after treatment at 1000 °C.The low Cs content is not conducive to the formation of pollucite,and the samples with Na/Cs of 17:3 and14:6 only generate pollucites after heat treatment at 1000 °C,while the pollucite crystallization phase is present at 800 °C when the cerium content is high.At the same time as the content of Cs is low,the high Na content is beneficial to the vitrification of the geopolymer after high temperature treatment at 1000°C.The presence of glass phase and pollucite is beneficial to the solidification of Cs+ and promotes the leach content of Cs+ reducing to 7 ‰ in the sample of Na/Cs = 14:6.In the geopolymer system of replacing 40 at% Cs by Na with different Si/Al ratios(Si/Al = 1.5,2.0,2.5,3.0),the low Si/Al ratio(1.5)is not conducive to the formation of pollucite,and the high Si/Al(3.0)sample forms a new cerium-containing crystal phase-Cs0.841(Al Si5O12)after treated at 800 °C.The existence of this crystal phase facilitates the immobilization of Cs+: 800 °C.The Cs+immobilization content was increased from 85.03 %(after treatment at 600 °C)to96.19%(after treatment at 800 °C).The long-term leaching test shows that mixed excitation and high temperature treatment can improve the leaching behavior of Cs+,and the Cs+ leaching rate slowly decreases and gradually keeps stable as time increases.The Cs0.841(Al Si5O12)and pollucite phase generated after high temperature treatment can significantly reduce the leach fractions of Cs+,and the leach fractions of Cs+ in the material was significantly reduced after 1000 °C high temperature treatment.In the deionized water,the 5-day cumulative leaching fraction of 0.3Na-0.7Cs GP decreased from3.91 %(before high temperature treatment)to 0.0038 %(after high temperature treatment at 1000 °C).Simulated seawater is more aggressive to geopolymers,The leaching rate of Cs+ is significantly faster than that of deionized water.Compared to the deionized water environment,the Cs+ cumulative leaching fraction in Cs GP rises from 7.19% to 39.13%(in simulated seawater)before high temperature treatment.The leachability indexes L value of matrix that have not been treated with high temperature is less than 8.The leaching mechanism of Cs+ is mainly diffusionand is not suitable for nuclear waste landfill treatment.Then the mechanism transforms to dissolution during thermal treatment,the pollucite in surface is dissolved and Cs+ is leached with weak diffusion.The L values of geopolymers after high temperature treatment at 1000 °C are all higher than 9(Cs GP is 8.93)and can all be used in “controlled utilization”. |
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