Microwave Synthesis Of Zr/Ti/Ce Pyrophosphates And Their Adsorption Properties For Uranyl Ions

In the disposal of high-level radioactive nuclear wastes,buffering absorption materials in the deep multilayer geological repository were used to adsorb leaked radioactive nuclear wastes to prevent migration to the biosphere.Pyrophosphates show a potential application propect as potent buffering absorption materials due to their strong irradiation resistance,insolubility,negative thermal expansion properties and irreversible adsorption for uranyl ions.However,there are some problems for preparing pyrophosphate such as high reaction temperature,long reaction time,and complicated procedures.Meanwhile,smaller absorption capacity of pyrophosphates for uranyl ion and lack of insight of adsorption mechanism largely restrict their practical application.In this thesis,we are going to design and prepare pyrophosphate buffering materials with high adsorption capacity for uranyl ions,to exploit the simple,quick,energy－saving preparation methods,to investigate the influence regularities of synthesis conditions on structure and adsorption performance of pyrophosphates,and to reveal their adsorption mechanism.The main research contents are as follows:（1）Selection of preparation methods for pyrophosphates.Zirconium pyrophosphate（ZrP₂O₇）was prepared by microwave heating and calcination method,and their structure were studied.The effect of microwave and calcination method on the structure and adsorption performance for uranyl ion of ZrP₂O₇ was also investigated.ZrP₂O₇ prepared by microwave method possesses better dispersion,smaller particle size and more uniform distribution,and the higher adsorption capacity for uranyl ions than those of sample prepared via the calcination route.It was proved that the pyrophosphates prepared by microwave methods were suitable for absorption materials.（2）Preparation and structural research of different pyrophosphates.Three types of pyrophosphates,ZrP₂O₇,TiP₂O₇ and CeP₂O₇,prepared by microwave methods were studied.The effect of microwave preparative conditions,such as temperature,raw materials,reaction time,on product structure was investigated.X-ray diffraction analysis showed that pure ZrP₂O₇ particles with 30-300 nm in size and round-likely roundness in shape,spherical TiP₂O₇ particles with 20-100 nm in size,and bulk CeP₂O₇ particles with 0.1-3 μm in size were obtained.The synthetic process research showed that pure ZrP₂O₇,CeP₂O₇,and TiP₂O₇ could be prepared at microwave power 800 W for 10 min,800 W for 5 min and 400 W for 5 min,respectively.With the increasing of microwave power and heating time,degree of crystallinity of products TiP₂O₇ gradually raised until the power reached to 640 W.（3）Adsorption performance of pyrophosphates.The adsorption performance of ZrP₂O₇、TiP₂O₇ and CeP₂O₇ for uranyl ions were investigated by the adsorption isotherm,thermodynamic calculations and kinetics research.The adsorption processes of pyrophosphate for uranyl ions belong to chemisorption of monolayer coverage and proceed spontaneously.The dependence of the adsorption behavior of pyrophosphates on coexisting ions and fulvic acid was also studied.The results showed that the effect of Mg²⁺,Ca²⁺,Cl^- and fulvic acid is negligent and the existence of HSO₄^-slightly decreases the adsorption ability.It is noteworthy that the adsorption process of pyrophosphates for uranyl ions was irreversible by desorption results analysis.（4）Adsorption mechanism of pyrophosphates.A series of pyrophosphates（Zr1-xTixP₂O₇）with various Ti content were designed to prepare.The difference of adsorption capacity of ZrP₂O₇,Zr_1-xTi_xP₂O₇ and TiP₂O₇ was revealed,and adsorption mechanism was also illuminated.Under the same adsorption conditions,the difference of adsorption sites which could result in the difference of adsorption capacity,could be attributed to the radius difference between Zr and Ti.Nano-size effects had an influence on the adsorption performance of pyrophosphates.The adsorption mechanism of pyrophosphates for uranyl ions was revealed as follows:the uranyl ions are adsorbed on O atom of =M-O sites and =P-O sites to form（=M-OH）2UO₂²⁺ and（=P-O）₂UO₂;the difference of different surface structure and adsorption sites resulted in different adsorption performance.