Synthesis of Microporous Covalent Phosphazene-Based Frameworks for Selective Separation of Uranium in Highly Acidic Media Based on Size-Matching Effect

On the basis of high stability of phosphorus-oxygen linkage, we constructed two microporous covalent phosphazene-based frameworks (CPFs), for the first time, by choosing hexachlorocyclotriphosphazene as a core unit and polyhydroxy aromatic compounds (hydroquinone or phloroglucinol) as monomers, named CPF-D and CPF-T, respectively. Characterization studies by using Fourier transform infrared, nuclear magnetic resonance, thermal gravimetric analysis, Co-60 gamma-ray irradiation, and so forth, demonstrated that both of the CPF materials have excellent acid and radiation stability and relatively higher thermal stability. The results of batch adsorption experiments show that CPF-T is significantly more capable of sorbing uranium than CPF-D. In a pure uranium system with higher acidity (pH 1), the uranium sorption amount of CPF-T can reach up to 140 mg g(-1). Distinctively, in mixed-metal solution with 12 coexisting cations, CPF-T shows relatively stable and excellent uranium adsorption capability over a wide range of acidity (pH 4 to 3 M HNO3), and the difference in uranium sorption amounts is less than 30% with the maximum of 0.26 mmol g(-1) at pH 4 and the minimum of 0.20 mmol g-(1) at 3 M HNO3, which is far superior to that of the conventional solid-phase extractant (SPE) materials previously reported. The research results suggested that the sorption model based on the speculated mechanism of size-matching plus hydrogen bond network has played a dominant role in the process of uranium adsorption. The proposed strategy for the one-pot fabrication of an acid-resistant microporous framework materials by bridging the aromatic monomers via P-O bonds provides an alternative approach for the design and synthesis of new SPE materials with size-matching function desired for effective separation of uranium or other valuable metals from highly acidic environments.