Adsorption Behavior Of Cs⁺ & Sr²⁺ By Geopolymer And Its Derivate Zeolites

Zeolite is a type of alumino-silicate crystal with clear pore and chamber structure,which has good adsorption performance and excellent prospects in the treatment of nuclear waste water.Natural zeolite is mined directly from zeolite ore,which has obvious disadvantages such as impurity and irregular structure.Geopolymer with"zeolite-like"Si-Al framework is considered as an excellent zeolite precursor.In this study,metakaolin,silicon colloid and hydroxides were used for the preparation of Na-K mixed alkali-stimulated geopolymer Na_xK_1-xGP?x=0.5,0.6,0.7,0.8,0.9,1.0?.A strategy of zeolite synthesis with low cost and great effectiveness was obtained by the study on the hydrothermal process of NaGP in situ conversion into zeolite.Zeolites with various crystalline forms were obtained by adjusting the ratio of Si/Al,as well as species and content of the mixed stimulating ions in the geopolymers.¹³³Cs⁺was used as a substitute of ¹³⁷Cs⁺,and ⁸⁷Sr²⁺was used as a substitute of ⁹⁰Sr²⁺for systematical research on the adsorption effect of zeolite on simulated radioactive ions.The research process and results are as follows:?1?The types and concentration of hydrothermal solvents,temperature and time are important parameters in the hydrothermal process of NaGP conversion into zeolite.In the hydrothermal system,the nucleation of NaGP rarely occurred when Na⁺concentration was low.The reaction required a certain alkaline environment to completely dissolve[AlO₄]^-and[SiO₄]in NaGP to form nucleation sites of zeolites.Also,temperature and time are effective factors for the crystallization of NaGP,because the conversion into crystalline zeolites needed enough energy in the hydrothermal process.The optimum hydrothermal process for NaGP in situ conversion into zeolite is:1 mol/L NaOH solution as reaction medium,160°C and 6 h.Under these conditions,NaGP was transformed into analcime crystals with diameters ranging from 3 to 5 microns?2?The chemical formula of analcime is NaAlSi₂O₆·H₂O,which depends on the Si/Al ratio in NaGP.NaGP with various ratio of Si/Al?Si/Al=1.0,1.25,1.50,1.75,2.0?was prepared by adjusting the contents of silicon colloid.When Si/Al ratio ranged from1.0 to 1.25,NaGP was converted into NaA zeolite.When Si/Al ratio ranged from 1.5 to1.75,analcime phase and a minor amount of analite appeared.When Si/Al ratio was 2.0,NaGP was completely transformed into analcime.After hydrothermal treatment of Na_0.5K_0.5GP,no obvious crystalline phase was formed because of the low Na⁺concentration in the system.With the ratio of Na⁺/K⁺increased,the crystallinity of Na_xK_1-xGP increased,while the grain size of analcime decreased gradually.When the ratio of Na⁺/K⁺was 0.9:0.1,NaP was formed.?3?Batch adsorption experiments,using analcime in situ converted from NaGP,analcime-NaP mixed zeolite in situ converted from Na_0.9K_0.1GP and amorphous NaGP as adsorbents,were conducted with different initial Cs⁺,Sr²⁺concentration and contact time with the adsorbents.Kinetic and isothermal studies on the results explained the mechanism and equilibrium conditions of the adsorption process.For Cs⁺,the two types of zeolite had greater adsorption effect than the amorphous NaGP.For Sr²⁺,the adsorption effect of the analcime-NaP mixed zeolite and the amorphous NaGP was higher than that of analcime.The adsorption rate of zeolite was extremely fast.More than 80%of the ions were adsorbed within 10 minutes of the beginning of adsorption.The system was stabilized after 90 minutes.No new phase was formed in the zeolites after the adsorption of Cs⁺and Sr²⁺.The adsorption mechanism was mainly ion exchange.