| A large amount of radioactive liquid wastes are produced during the innovation process and application of nuclear technology,and it is a fundamental way to reduce the volume and hazardousness of waste through separating and converting the long-lived radionuclides into stable forms.Nowadays,vitrification has been considered widely as the safest techonology for the long term storage and disposal of radionuclides among all the technologies for highlevel radioactive wastes immobilization.However,owing to the high melting temperature up to more than 1000℃,volatile radionuclides such as 99Tc and 137Cs can not be avoided to form aerosols during the fabrication of borosilicate glass,which could lead to the secondary radioactive pollution.Meanwhile,glass’s liquidlike properties make it neighter a liquid nor a solid,it is a unique nonequilibrium state of matter.And the thermodynamic instability of glasses makes it hard to avoid the occurrence of crystallization when prolonged exposure to high-temperature and humid environments,which contributes to the migration of radionuclides into groundwater during the long term storage process,thus taking an adverse effect on human health and environment.Therefore,it would be necessary and urgent for developing new techniques and materials for the safe treatment and disposal of radionuclides.In recent years,the quantities of radioactive wastes can be reduced by adsorption using porous solids,which is considered as one of the most effective strategies to removal radionuclides.The structural and functional tunability as well as good stability endows metal-organic frameworks many advantages in environmental modification field,which has been studied widely.However,there has been no researches reported on directly vitrificating MOFs that have adsorbed radionuclides for deep geological disposal till now.In this sense,design and synthesis of MOF glasses for immobilizing radionuclides physically and chemically remains highly desirable and is of great significance for the safe treatment and disposal of radioactive waste.In this paper,we report a novel route towards effective adsorption and immobilization of UO22+ for the first time.In particular,we report on the novel use of a crystalline ZIF-62 with ultrahigh glass-forming ability as an efficient adsorbent for UO22+.We investigated the formation of a novel MOF glass alternative waste form by melting the U-containing ZIF-62 at a relative low temperature.Furthermore,with the help of the X-ray photoelectron spectroscopy and EXAFS techniques,the adsorption and immobilization mechanism of UO22+ on ZIF-62 was demonstrated.The detailed results are as follows:Firstly,the crystalline ZIF-62 was successfully synthesized by solvothermal method using the mixed linkers,imidazole(Im)and benzimidazole(bIm),and the unique tetrahedral configuration in the structure was characterized by single crystal X-ray diffraction and powder XRD techniques.Then we studied the chemical stability of ZIF-62 structure in aqueous conditions with various pH values.It was found that under acidic conditions,the imidazole ligands are highly susceptible to proton attack,leading to the depomposition of ZIF-62 structure.In view of the instability of ZIF-62 crystals in strong acidic solutions,we studied the adsorption of UO22-on the materials at mild pH conditions.Static adsorption experiments showed that adsorption equilibrium could be achieved within 720 min for UO22-on ZIF-62 at initial pH value of 5.0,and the equilibrium adsorption capacity calculated could reach 163.02 mg/g at room temperature.The adsorption mechanism was well elucidated by X-ray photoelectron spectroscopy analysis,revealing that the adsorbed UO22-was able to selectively coordinate to Zn-OH sites in the framework to form Zn-O-U bonds.The local coordination environments of uranium in the framework clearly unrevealed the adsorption mechanism of UO22-on ZIF-62,which was further confirmed by EXAFS analysis.In order to investigate the effect of UO22-adsorbed on the melting and glass transition behavior of ZIF-62,TG-DSC measurements were carried out.The results that the enthalpy change occurs during the heating process revealed the changes of melting temperature and glass transition temperature for ZIF-62 with UO22-adsorption.Furthermore,Raman spectroscopy characterization showed that more Zn-N bond broke as a result of UO22+adsorption,which contributing to the lower melting temperature and glass transition temperature of ZIF-62,and the result was consistent with TG-DSC tests.ZIF-62 glass immobilizing uranium was successfully synthesized by melt-quenching at a relative low temperature.Powder XRD suggested that the glass has a maximum solubility of 5 w%for uranium,beyond which UO2 crystals would be identified on the glass metrix.Leaching tests results indicated that not only 42-day normalized leaching rate but 42-day normalized loss of uranium for agZIF-62-U0.05 were significantly less than those for borosilicate glass.In addition,the adsorption-immobilization mechanism of uranium in agZIF-62-U0.05 was unraveled by XPS and EXAFS techniques.The analysis revealed that it was the chemical bonds forming between UO22+and Zn-OH as well as N atoms in the ZIF62 structure that makes it possible to immobilize uranium physically and chemically,and further explained the intrinsic reasons for good leaching resistance of agZIF-62-U0.05 from the microscopic point of view. |
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