Hierarchically structural PAN/UiO-66-(COOH)2 nanofibrous membranes for effective recovery of Terbium(III) and Europium(III) ions and their photoluminescence performances
Graphical abstract
Introduction
In recent years, rare earth elements (REEs) have been widely used in many high-tech industries such as magnetics, catalysts, superconductors, laser technology, phosphors and electronics and so on, mainly due to their unique magnetic, catalytic, optical, electrical and chemical properties [1], [2]. In 2015, the global consumption for REEs was as high as about 120,000 metric tons, and the annual growth rate of global REEs demand was predicted to increase by 5% until 2020 [3]. Obviously, such ever-increasing REEs demand would create a great pressure as well as an enormous challenge to the global supply chain of REEs. Nevertheless, the recycling amount of REEs was reported to be only 1% in 2012, which was extremely low compared with their consumption [4]. Furthermore, in the process of REEs exploitation and postproduction, a large number of rare earth pollutants were produced and were directly discharged into the environment without any treatment, which could bring about serious water pollution and further threaten human health [5]. Consequently, the effective recovery of REEs has been becoming an urgent issue.
So far, substantial technologies have been constructed to enrich and recover rare earth ions, such as ion-exchange [6], solvent extraction [7], [8], adsorption [9], [10] and chemical precipitation [11], etc. Among these separation methods, adsorption has been considered as the most efficient, economical and eco-friendly method for the recovery of Ln3+ ions, ascribing to its simple operation, high separation efficiency, low cost and non-toxicity. To date, various adsorbents used for recovery of rare earth ions including activated carbon [12], ion exchange resin [6], organic-inorganic composite materials [4] and biomaterials [13] have been extensively studied. However, these materials often suffer from the limitations of low adsorption capacity and poor recyclability. Therefore, it is necessary to synthesize novel, efficient and reliable materials for rare earth adsorption.
Metal-organic frameworks (MOFs) are a novel class of crystalline microporous materials, which are constructed through the strong coordination bonding of metal nodes and organic ligands [14]. Nowadays, MOFs have received considerable attention owing to their high porosity, large specific surface area, tunable pore sizes and high density of active sites. They have been widely applied in a wide range of fields including gas storage and separation, catalysis, magnetism, energy storage, chemical sensors and adsorbents [15], [16]. In particular, MOFs as adsorbents were extensively investigated and exhibited good adsorption performance. For example, Jung et al. synthetized a high efficient mesoporous zeolitic imidazolate framework-8 for the removal of p-arsanilic acid with a high adsorption capacity of 791.1 mg/g [17]. Moreover, Li et al. fabricated a novel carboxyl-functionalized MIL-101 and the MOF showed adsorption capacity for uranyl ions as high as 314 mg/g [18]. In spite of having high adsorption quantity, the applications of MOFs nanoparticles were still heavily restricted because of their poor separation ability from the aqueous phase, which would result in their poor reusability. To this end, many researchers combined the MOFs nanoparticles with the substrate materials. For instance, Zhu et al. reported a core-double-shell structured magnetic polydopamine@zeolitic idazolate frameworks-8 (MP@ZIF-8) hybrid microsphere for the adsorptive removal of Cr(VI), where the MP@ZIF-8 adsorbent could be easily recycled [19]. Zhuo et al. incorporated MIL-101(Cr) into sodium alginate (SA) and chitosan (CS) to prepare the MIL-101(Cr)/SA and MIL-101(Cr)/CS composite beads for the adsorption of pharmaceuticals and personal care products [20]. In addition, Efome et al. produced novel nanofibrous MOFs membranes by doping MOF 808 and F300 into electrospun polyacrylonitrile nanofibers for the removal of heavy metal ions from aqueous solution [21]. Actually, the electrospun nanofibrous scaffolds were often considered to be ideal carriers for nanoparticles immobilization due to the high porosity, large specific surface area and good stability [22], [23]. And also, colloid electrospinning technique was a simple and effective method to prepare inorganic/organic composite nanofibers with multilevel structure and functionality by electrospinning the functional inorganic colloidal nanoparticles and polymer matrices [24]. For example, Wanjale et al. fabricated PS/TiO2 nanofibrous membranes by introducing TiO2 nanoparticles into polystyrene spinning solution, and the composite membranes exhibited excellent adsorption capacity for Cu2+ ions [25]. Liu et al. developed a novel polystyrene/metal-organic framework-199 (PS/MOF-199) nanofiber adsorbent by electrospinning the MOF-199 functional nanoparticles and PS for extraction of aldehydes in human urine [26].
Rare earth elements are highly oxyphilic and can easily coordinate with oxygenic ligands [27]. UiO-66-(COOH)2 [28], a promising MOF, not only owns superior chemical stability but also contains twenty-four free carboxyl functional groups, which would endow the UiO-66-(COOH)2 nanoparticles with powerful affinity to the rare earth ions. Meanwhile, PAN is an ideal spinning carrier material owing to its hydrophilicity, low cost and excellent chemical stability. Accordingly, in our current work, necklace-like PAN/UiO-66-(COOH)2 nanofibrous membranes were constructed by encapsulating the UiO-66-(COOH)2 functional nanoparticles into PAN nanofibers by means of colloid electrospinning technique acting as an efficient affinity agent for the recovery of Ln3+ ions (as shown in Scheme 1). What’s more, benefiting from the carboxyl functional groups of PAN/UiO-66-(COOH)2 NFMs serving as good sensitizers for Tb3+ and Eu3+ ions, the flexible PAN/UiO-66-(COOH)2 NFMs with adsorbed Ln3+ ions presented various fascinating characteristics such as outstanding photoluminescence performances and tunable emission colors from green to red, which made the luminescent membranes promising for applications in luminescent patterning and optoelectronics. In summary, the resultant hierarchically structural PAN/UiO-66-(COOH)2 NFMs were bifunctional, they not only could act as adsorbents for effective recovery of Ln3+ ions, but the Ln3+-loaded nanofibrous membranes could be used as luminescent materials, indicating that this work would be of great significance to fabricate the self-supported and flexible MOF/polymer composite membranes with multiple functions.
Section snippets
Chemicals
Polyacrylonitrile (PAN, Mw = 120 000 g/mol) was purchased from Shanghai Jinshan Petroleum Chemical Co., Ltd. 1,2,4,5-benzenetetracarboxylic acid (H4btec, >98%) was supplied by Shanghai Tokyo Chemical Industry Co., Ltd. Zirconium tetrachloride (ZrCl4, 98%) was purchased from Beijing J&K Scientific Ltd. Acetic acid, anhydrous methanol, acetone, N,N-dimethylformamide (DMF) were provided by Shanghai Lingfeng Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH), KCl, MgCl2, CaCl2, CuSO4·5H2O,
Characterization of UiO-66-(COOH)2 nanoparticles
UiO-66-(COOH)2, this porous material was constructed of Zr6-octahedra [Zr6O4(OH)4] but bounded to 1,2,4,5-benzenetetracarboxylic acid (H4btec) ligands to shape cubic three-dimensional microporous structure (Fig. 1a), exhibiting the high chemical and thermal stability of UiO-66(Zr) [29], [30]. Note that the UiO-66-(COOH)2 contained two free carboxylic acid functional groups per ligand owing to only two carboxylate arms of the H4btec ligands acting as linkers (Fig. 1a). The representative TEM and
Conclusions
In this article, we demonstrated the novel MOFs functionalized nanofibrous membranes with hierarchical structure via employing a colloid-electrospinning technique. The resultant PAN/UiO-66-(COOH)2-60 NFMs took full advantages of UiO-66-(COOH)2 (strong chelating agent) and electrospun nanofibers (high specific surface area, good flexibility, safe to be used and easy to be separated), exhibiting high adsorption capacities and good reusability for Ln3+ ions. The adsorption process followed the
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central Universities and Graduate Student Innovation Fund of Donghua University (CUSF-DH-D-2019001), National Science Foundation of China (51273042), Program of Shanghai Science and Technology Innovation International Exchange and Cooperation (15230724700) and Program for Innovative Research Team in University of Ministry of Education of China (IRT_16R13).
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