Preparation And Characterization Of The Silica-based Supramolecular Recognition Materials And Their Adsorption For Cesium And Strontium

The treatment and disposal of high level liquid waste (HLLW) is a crucial part of nuclear fuel cycle, which is beneficial to the sustainable development of nuclear power. Radioactive 137Cs and 90Sr, with the half-lives of 30 and 28 years, are heat generators contained in HLLW. It is reported that they have harm for the vitrified HLLW. The elimination of them can reduce the need of cooling solutions and the time of vitrified waste storage before disposal in geological formation. To minimize the long-term radiological risk and facilitate the management of HLLW, effective partitioning and recovery of Cs(I) and Sr(II) is desired. In the present work, several novel macroporous silica-based supramolecular recognition materials were prepared, characterized, and their adsoption properties for Cs(I) and Sr(II) were investigated.Six calix[4]arene-crown-6 compounds (calix[4]arene-crown-6= BnPCalix[4]C6, BiPCalix[4]C6, BnACalix[4]C6, BnOCalix[4]C6, BiOCalix[4]C6, BnUCalix[4]C6) were synthesized and characterized by elemental analysis, FT-IR, ESI-MS, XRD, and 'H NMR. The synthesized procedures of calix[4]arene-crown-6 compound and its intermediates were improved. To obtain the relationship of their structures and properties, the DSC, FT-IR, and PXRD spectra of calix[4]arene-crown-6 compounds were discussed. The odd-even alternation phenomenon of melting point in calix[4]arene-crown-6 compounds was found. C-H stretching vibration absorption bands exhibited red shift with increasing the alkyl chain size while blue shift occured in the C-O stretching vibration absorption bands. Diffraction angles followed the decreasing trends, indicating the d-spacings of these crystal facets were increased as a function of alkyl chain size. Single crystal structure revealled that the calix[4]arene-crown-6 molecules were packed in terms of "head to head, tail to tail" pattern, indicative of amphiphilic characteristic. The calix[4]arene-crown-6 molecules with even-number alkyl chain were closely piled up in staggered-paralleled way, which could explain why the melting points of even ones were higher than odd ones.Several novel macroporous silica-calix[4]arene-crown supramolecular recognition materials, Calix[4]arene-crown-6/SiO2-P, were prepared through impregantion and immobilization of the corresponding calix[4]arene-crown-6 coumpound into the pores of the macroporous SiO2-P support. Their appearence, chemical composition, and inner structure were characterized by SEM, N2 adsorption-desorption isotherm, TG-DSC, FT-IR,29Si NMR, and PXRD techniques. It was found that the composite mechanism between calix[4]arene-crown-6, polymer, and silica was intermolecular interaction rather than grafting or chemical reaction.The effects of HNO3 concentration, contact time, and temperature on the adsorption of Cs(I), Sr(II), and more than 10 typical elements onto the Calix[4]arene-crown-6/SiO2-P materials were investigated at 25℃. The Calix[4]arene-crown-6 /SiO2-P materials showed excellent adsorption ability and selectivity for Cs(I) over all of the other tested metals. The uptake of Cs(I) increased as a function of HNO3 concentration and decreased at higher acidity. The optium acidity for the Cs(I) adsorption was 2.0 M HNO3 or 3.0 M HNO3. With an increase in alkyl chain size, the distribution coefficient of Cs(I) increased and the adsorption dynamics became slow. Compared to the linear calix[4]arene-crown-6 molecules, it was not benificial for the branched ones to preorganize in the pores of SiO2-P, which resulted in lower adsorption ability and poorer chemical stability for Calix[4]arene-crown-6/SiO2-P materials. In 3.0 M HNO3, the distribution coefficients of Cs(I) onto Calix[4]arene-crown-6/SiO2-P were ranked as follows:BnUCalix[4]C6/SiO2-P> BnOCalix[4]C6/SiO2-P> BnACalix[4]C6/SiO2-P> BiPCalix[4]C6/SiO2-P> BnPCalix[4]C6/SiO2-P> BiOCalix[4]C6/SiO2-P.BiPCalix[4]C6/SiO2-P, a representive of the Calix[4]arene-crown-6/SiO2-P materials, was chosen to understand the adsorption capacity and mechanism. The results showed that the adsorption of Cs(I) onto BiPCalix[4]C6/SiO2-P was monolayer and chemical adsorption, belonging to Langmuir isothermic adsorption model. The adsorption capacity of BiPCalix[4]C6/SiO2-P for Cs(I) in 3.0 M HNO3 was 0.20 mmol/g. The complex composition between BiPCalix[4]C6/SiO2-P and Cs(I) was determined as 1:1 type.Four novel macroporous silica-crown-ether supramolecular recognition materials, Crown-ether/SiO2-P(Crown ether= DCH18C6, DB18C6, DtBuDB18C6, and DtBuCH18C6), were prepared. They were characterized by N2 adsorption-desorption isotherm, TG-DSC, FT-IR, and PXRD techniques. The impacts of the HNO3 concentration in the range of 0.4 M to 5.0 M on the adsorption of Cs(Ⅰ), Sr(Ⅱ), and more than 10 typical elements onto the Crown-ether/SiO2-P materials were investigated. Compared to the other Crown-ether/Si02-P materials, DtBuCH18C6 /SiO2-P exihibited excellent recognition ability for Sr(IIⅡ) and high chemical stability in aqueous solution.A novel synergistic macroporous silica-based supramolecular recognition material, BiPDtBu/SiO2-P, was prepared and characterized by N2 adsorption-desorption isotherm, TG-DSC, FT-IR, and PXRD techniques. The BiPDtBu/SiO2-P material was integrated both of calix[4]arene-crown-6 and crown-ether compound with SiO2-P, showing strong adsorption ability and selectivity for Cs(Ⅰ) and Sr(Ⅲ) over all the other tested metals except Rb(Ⅰ) and Ba(Ⅱ). The optimum acidity in the adsorption of Cs(Ⅰ) and Sr(Ⅱ) was 3.0 M HNO3. On the other hand, the BiPDtBu/SiO2-P material exhibited quick adsorption equilibrium and good chemical stability.Because the HNO3 concentration in genuine HLLW is around 3.0 M, the results demonstrated that in 3.0 M HNO3, the macroporous silica-based supramolecular recognition material has excellent adsorption ability and high selectivity for Cs(Ⅰ) with BnUCalix[4]C6/SiO2-P, for Sr(Ⅱ) with DtBuCH18C6/SiO2-P, and for Cs(Ⅰ)+Sr(Ⅱ) together with BiPDtBu/SiO2-P. It is of great beneficial to effective partitioning of Cs(Ⅰ) and/or Sr(Ⅱ) from HLLW by extraction chromatography.