Synthesis and Characterization of Novel Nanofiber Based Calixarene

J Incl Phenom Macrocycl Chem (2016) 85:49–58 DOI 10.1007/s10847-016-0604-5

ORIGINAL ARTICLE

Synthesis and characterization of novel nanoﬁber based calixarene and its binding efﬁciency towards chromium and uranium ions

1,2 3 1 Fatih O¨ zcan • Mevlu¨t Bayrakcı • S¸ eref Ertul

Received: 19 January 2016 / Accepted: 11 March 2016 / Published online: 18 March 2016 Ó Springer Science+Business Media Dordrecht 2016

Abstract The objective of this study is to obtain Introduction nanofibrous precursor based calixarene with high ion adsorption capacity by electrospinning of blended solution Nanotechnology has gained interest in recent years from of polyacrylonitrile (PAN) and upper rim functionalized both public and private sector organizations. Therefore, calix[4]arene bearing N-methylglucamine (Calix-NMG). nanotechnology has offered several novel products, which The obtained electrospun fibers were characterized using have superior properties making them valuable for a wide fourier transform infrared (FT-IR-ATR), scanning electron range of applications. One of the major successes of nan- microscope (SEM) equipped with energy-dispersive X-ray otechnology has been nanofibers. Nanofibers can be man- spectrometry (EDX) and thermogravimetric analyses (TGA ufactured by electrospinning which is a technique to obtain and DSC). Analysis indicated that preparation of nanofi- fiber structures that can range in diameter from nanometers bers based Calix-NMG was successfully achieved. The ion to micrometers [1–4]. Electrospun fibers can be made of binding studies exhibited that the nanofiber based Calix- many types of polymers and composite combinations that NMG could be efficiently used for the binding of chromate can be utilized to imitate properties of native extracellular anions and uranium cations. Nanofiber based calix[4]arene matrix or to optimize various chemical, mechanical, elec- with N-methylglucamine chelating groups may be a good trical, architectural, and biological properties. Polymeric candidate as a filter material for treatment of a large nanofibers have enormous specific surface area and high quantity of wastewater owing to their very large surface flexibility. As a result, nanofiber skeletons have large sur- area as well as both the inclusion and donor–acceptor face-to-volume ratio, micropores, and high porosity [5]. complexation capability of all surfaces associated with the Owing to the special properties of nanofibers, they are used calixarene skeleton. in a wide range of applications such as battery separators, aerospace, transistors, drug delivery systems, capacitors, Keywords Calixarene Á Nanofiber Á N-methylglucamine Á energy storage, information technology, filtration, super Uranium Á Chromate absorbents, as scaffolds for tissue engineering, wound dressings, fuel cells and many electronic applications [6– 16], Highly purified air, water, gasses, chemicals, biological agents without contaminants are needed intensively in & Mevlu¨t Bayrakcı food, pharmaceuticals and biotechnology industries. [email protected] Therefore, compared the microfibers, nanofibrous media undoubtedly hold great advantage in filtration and nanofi- 1 Department of Chemistry, Faculty of Science, Selcuk University, 42075 Konya, Turkey ber structures can act as protective materials against environmental and infectious agents in hospitals, offices 2 Advanced Technology Research and Application Center, 42075 Konya, Turkey and homes. From this point of view functionalized nanofibers will be most important topic and used as affinity 3 Department of Bioengineering, Faculty of Engineering, Karamanog˘lu Mehmetbey University, Karaman 70200, membranes for filtering heavy metals and toxic anions that Turkey are difficult to purify by conventional purification methods. 123 50 J Incl Phenom Macrocycl Chem (2016) 85:49–58

Furthermore, it is estimated that the current nanofiber addition, the presence of functionalized calixarene in market worldwide may be $400 million and will be worth polymeric matrix provides the functional properties of more than $1 billion by 2020 [17]. This increase also shows solid supports. Electrospinning of nanofibers with cal- the importance of the investigations on functionalized ixarene units is of particular interest, since in this way, nanofibers. Careful examination of the literature reveals nanofibers with specific function groups can be produced. that considerable work has been reported on the design and The polystyrene structures based N-methylglucamine synthesis of nanofibers with supramolecules as cyclodex- (NMG) have been developed for boron removal from trins and their analytical applications [18, 19]. But very irrigation water [38]. Along with boron removal, (NMG) little information has appeared in the literature concerning containing several macroporous resins have been studied nanofibers derived from calixarene supramolecular and for its affinity towards oxoanions of Ge(IV), V(V), Mo(VI), their possible applications [15, 20]. Calixarenes are often As(V), As(III), W(VI), Se(IV), Se(VI), and Sn(IV) [39], referred to as the third generation of supramolecular uranium sorption studies were not observed. N-methyl- receptors after crown ethers and cyclodextrins [21, 22]. glucamine (NMG) has a high interaction capability with Calixarenes are cyclic oligomers made of several phenolic different types of species owing to the presence chelating units bound with methylene bridges [23–26]. The phenolic tertiary amine and vic diol groups that interacted with OH groups of the calixarene lower rim can be further different ion species. Therefore, there is a growing interest functionalized to give various ionic receptors for anions, on (NMG)-based support materials by surface modification cations, as well as hosts of neutral molecules [27–29]. This methods. Compared the support materials, nanofibers excellent skeleton enables calixarenes to act as ‘‘molecular mainly including higher durability in acidic environment, baskets’’ towards neutral or ionic guests. Many different large surface-to-volume ratio, micropores, high porosity studies about the chromate and dichromate anions and flexibility are excellent support materials especially for À2 À (Cr2O7 /HCr2O7 ) from contaminated water have been separation science. At this context, it is a good tentative reported, but the toxic effect of chromate is not well doc- idea to combine the multifunctional character of calixarene umented despite its importance as an environmental pol- skeleton based NMG and PAN nanofibers. Even though lutant [30]. Chromium is one of the most toxic pollutants surface modifications of fibers by calixarene compounds generated from industrial applications [31]. Chromium were reported [15, 20], to the best of our knowledge, the generally appears both in trivalent Cr(III) and hexavalent synthesis of nanofibers with calix [4] arene containing N- Cr(VI) forms in aqueous media. Especially, trivalent methylglucamine chelating groups and their extraction chromium is considered as an essential micronutrient for studies were not reported previously. With this in mind, we human, plant and animal metabolism and less toxic than set out to synthesize PAN nanofiber based calix [4] arene hexavalent chromium which is extremely toxic and car- containing N-methylglucamine chelating groups at upper cinogenic to living organism [32, 33]. Depending on pH rim of calixarene skeleton and explore its ion binding and the total concentration of the solution, Cr(VI) is existed properties towards chromate anions and uranium cations. in different oxy-anion forms. Cr(VI) is unstable and shows very oxidizing behaviors in the presence of the electron donor in acidic medium. At lower pH values, the dominant Experimental À 2À form of the chromium is HCrO4 and only CrO4 ions exist above pH 7. The synthesis of supports and hosts for General specific anions as dichromate is an important goal. Because Cr(VI) has well known effects on environment and living 1H and 13C NMR spectra were obtained using a Varian organism, it is necessarily to remove Cr(VI) from 400 MHz spectrometer operating at 400 MHz. IR spectra wastewater. Owing to the oxide moieties, the dichromate were recorded on a Perkin-Elmer spectrum 100 FT-IR ions are known as oxyanions. These oxide moieties are spectrometer (ATR). Thermogravimetric analysis (TGA) potential sites for hydrogen bonding to the host molecule. data were obtained with a Setaram SETSYS thermal ana- Pyridine, amino and imino derivatives of calixarene are lyzer. SEM images were received using a Zeiss LS-10 field extremely efficient extractant for oxoanions [34]. Nowa- emission SEM instrument. UV–Visible spectra were days many researchers are interested in surface modifica- recorded on Jenway 6105 and Shimadzu160A UV–Visible tion of all kinds of materials such as nanoparticles [35], recording spectrophotometers. Millipore Milli-Q Plus resin [36] and silica gel [37] with calixarene compounds to water purification system is used for the distilled water. For obtain new composite materials. Therefore, simple nano- the pH measurements, An Orion 410Aþ pH meter was fibers containing calixarenes may result in composites used. All of the reagents used in this study were obtained having a range of useful and interesting properties. In from analytical grade and used without further purification.

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Arsenazo III, uranyl acetate dihydrate were purchased from performed three times. From the blank experiments data it Fluka. Standard stock solution of 0.9787 g/mL ura- was observed that no dichromate extraction occurred in the nium(VI) was prepared by dissolving the appropriate absence of the Calix-NMG/PAN nanoﬁbers. The percent amounts of uranyl acetate dihydrate in deionized water. A extraction (E %) was calculated through the absorbance of stock arsenazo III solution (0.01 %) was prepared by dis- the aqueous phase measured using the following solving reagent. Adjusting the pH values of the working expression:] solutions was carried out using 5 M of sodium acetate Extraction E% ¼ ðÞA0 À A =A0 Â 100 ð1Þ 2þ buffer to determination of UO2 in aqueous solution. Thin layer chromatography (TLC) was performed using silica where A0 and A are the initial and ﬁnal concentrations of gel on glass TLC plates (silica gel H, type 60, Merck). the dichromate ion before and after the extraction, respectively (Eq. 1). Synthesis

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahydroxycalix Results and discussion [4] arene (1), 25,26,27,28-tetrahydroxycalix [4] arene (2) and 5,17-bis[(N-methylglucamine)methyl]-25,26,27,28-te- Synthesis of calix[4]arene bearing N- trahydroxycalix [4] arene (3) compounds were synthesized methylglucamine according to the literature procedures [21–23, 40, 41]. In this study, we aimed to design a new calix[4]arene Electrospinning nanofiber structure decorated with N-methylglucamine chelating units and explore its sorption behaviors towards Fibers were electrospun as reported in literature [15]. The dichromate and uranium ions. For this purpose, the polymer solution were prepared by dissolving calixarene required starting materials, p-tert-butylcalix[4]arene 1 and compounds in DMF which is containing 20 % (w/v) PAN. calix[4]arene 2 were synthesized by following the proce- The concentration of calixarenes was 20 wt% with respect dure available in the literature [43] as shown in Scheme 1. to the PAN concentration in DMF. The polymer solution N-methylglucamine groups were attached to the upper rim was held in a horizontally plastic syringe fitted with a of calix[4]arene skeleton 2 by the substitution reaction so- metallic needle of 0.7 mm inner diameter. A stainless steel called Mannich reaction. In this reaction calix[4]arene 2 electrode was immersed in the solution and connected to a was interacted with a secondary amine N-methylglucamine high voltage power supply. A metal plate coated with and formaldehyde to obtain the cone conformer calix[4]- aluminum foil placed opposite served as a counter elec- arene based N-methylglucamine (Calix-NMG) 3 in the trode. The applied voltages between the needle tip and presence of AcOH in THF. All of the structures were in the collector were set at 20 kV with a tip-to-collector distance cone conformation in solution. of 15 cm. The electrospinning temperature and the relative humidity were 25 °C and 50 %, respectively. The synthesis and characterization of nanofibers based calixarene Adsorption experiment In this report, PAN nanofibers based Calix-NMG were The sorption capacities of the synthesized Calix-NMG/ obtained by electrospinning as shown in Scheme 2. Elec- PAN were determined by the following technique. Aqueous trospinning technique was used for the modification of solution (10 mL) of Na2Cr2O7 or UO2(CH3COO)2Á2H2O PAN nanofibers with calixarene molecules bearing N- -4 with 1.0 9 10 M (for Na2Cr2O7) concentration and methylglucamine units because of the absence of the any -5 1.15 9 10 M (for UO2(CH3COO)2Á2H2O) and 25 mg of reactive groups of the PAN polymer backbone which are the sorbent were pipetted in a stoppered flask that was capable of covalent bound formation with calixarene shaken at 175 rpm and 25 °C for 1 h. The sorbent was skeleton. Two different PAN nanofibers with and without separated before measurements. The residual dichromate calixarene were also electrospun for the comparison study. and uranium concentration of aqueous solute was deter- SEM, FT-IR (ATR) and TGA-DSC spectroscopic tech- mined spectrophotometrically by UV–Vis analyses at niques were used for the characterization of the newly 346 nm for dichromate [23] anions and 652 nm for ura- prepared fiber structures. Morphological changes of fiber nium cations [42]. The effect of pH was studied by structures after modification of PAN nanofibers with and adjusting the pH of aqueous solutions using diluted HCl without calixarene derivative were investigated by scan- and KOH solutions at 25 °C. The experiments were ning electron microscopy (SEM) analysis.

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Scheme 1 Schematic illustration of the synthesis of calix[4]arene derivatives. (i) Formaldehyde, NaOH, diphenyl ether; (ii) AlCl3, phenol, toluene, rt, 3 h; (iii) N- methylglucamine, formaldehyde, CH3COOH, THF

SEM images of the pure PAN and Calix-NMG/PAN observation is consistent with the absence of the free nanofibers were obtained from the homogeneous solution reactive groups on polyacrylonitrile polymer scaffold. In of calixarene derivative and PAN as shown in Fig. 1. From Fig. 2a, typical bands stretching vibrations of CH and CH2 the SEM images, it is clearly observed that bead-free groups, CN and CH/CH2 deformation vibration signals for nanofibers were produced with diameters mostly ranging PAN was observed around 2920–2930, 2242 and from 274 to 290 nm. The surface morphologies of Calix- 1250–1453 cm-1 respectively [19]. In case of FT-IR NMG/PAN nanofibers were obviously different from the spectrum of Calix-NMG/PAN (Fig. 2b), the new and broad unmodified PAN nanofibers. While the surface of the band at 3300–3500 cm-1, indicating phenolic hydroxyl unmodified PAN nanofibers was both smooth and uniform, groups of calixarene skeleton was observed for the Calix- the surfaces of the Calix-NMG/PAN nanofibers seen rough NMG/PAN compared to the spectrum of pure PAN. In feasibly due to presence of the calixarene molecules onto addition, new peak appeared at 1058 cm-1, which can be PAN nanofibers. In brief, the rough and irregular surface of assigned to the amine C–N stretching vibrations [44]. All modified PAN nanofibers showed that the modification of these results provide clear evidence the presence of the of PAN nanofibers with calixarene molecules was occurred calixaren molecules onto PAN nanofibers. successfully. The presence of calixarene derivatives in the TGA and DSC analysis are used to observe the thermal PAN nanofibers was also confirmed by Fourier transform behavior of prepared PAN nanofibers. In Fig. 3, the major infrared spectroscopy (FT-IR) studies. weight loss for the PAN nanofibers was recorded at around The FT-IR (ATR) spectra of nanofiber based Calix- 290–350 °C which is consistent with the main degradation NMG are shown in Fig. 2 and the FT-IR spectrum of pure temperature reported for the PAN nanofibers. The weight PAN nanofiber is also given for comparison. The over- loss from 365 to 440 °C was mainly caused by decompo- lapping of absorption peaks of calixarene bearing N- sition of the carbon–carbon main chains. Compared to pure methylglucamine units and PAN makes the identification PAN nanofibers, it is clearly seen that Calix-NMG/PAN of the individual components in the nanofiber structures nanofibers have demonstrated slightly higher degradation rather complicated. One can observe the widening and the temperature. This situation exhibited that the incorporation overlapping of the bands in the FT-IR spectra of the of the calixarene molecules onto the fiber structure resulted nanofibers. The FT-IR spectra do not indicate any in higher thermal stability. appearance of new signals or disappearance of existing Exothermic and endothermic peaks, exhibiting thermal signals which is assigned to the possible covalent bond stabilizing, cyclization of nitrile group and melting pro- formation between calixarene and polyacrylonitrile. This cess of PAN nanofibers were observed in DSC curves

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Scheme 2 Schematic of the electrospinning setup and synthesis of PAN nanoﬁber based calix[4]arenebearing N-methylglucamine chelating groups

(Fig. 3). An exothermic peak attributable multiple com- Adsorption studies plex chemical reactions, such as dehydrogenation, cyclization and crosslinking reactions of PAN was seen Dichromate anion sorption studies around 291 °C for pure PAN nanofiber [44], while in the presence of calixarene molecules this weak exothermic The extraction efficiency of nanofibers at different pH peak shifted to some extent. The residue of thermal values toward chromate ions are studied and obtained decomposition at 555 °C is around 41 %, probably con- results are given in Table 1. From the previous study, it is sisting of ash. All of these data confirmed the presence of well known that pure PAN nanofibers have no adsorption the calixarene molecules onto the PAN nanofibers. In capacity toward chromate anions [45]. Whereas excellent addition to the characterization experiments mentioned adsorption percentage using calixarene-modified nanofiber above, the sorption behavior of the Calix-NMG/PAN was observed for chromate ions. nanofiber gave valuable information about the modifica- From the chromate extraction experiments especially at tion of the PAN nanofiber upon the addition of calixarene low pH values, it was determined that Calix-NMG/PAN is molecules based N-methylglucamine units. If the cal- more effective solid support and could be used as adsorbent ixarene molecules with chelating group had not been for chromate ions. The maximum percentage of dichromate attached to the nanofiber structure, it would not have ions extracted was 87 % for Calix-NMG/PAN when the pH shown any sorption behavior [20]. (As will be shown of the aqueous solution was 1.5 and they attained minimum later, the pure PAN nanofiber had no oxoanion sorption level of 41 % for Calix-NMG/PAN when the pH of the capability whereas it was significantly adsorbed oxoanions aqueous solution was increased to 4.5. Furthermore, the after modification with calixarene molecules bearing N- presence of the chromium on the surface of the nanofibers methylglucamine groups) after dichromate extraction is supported by the EDX

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Fig. 1 SEM images of PAN nanoﬁbers. a Calix-NMG/PAN (1 lm), b Calix-NMG/PAN (200 nm) c pure PAN (1 lm) and d pure PAN (200 nm)

Fig. 2 The FT-IR (ATR) spectra of prepared nanoﬁbers. a pure PAN, b Calix-NMG/ PAN nanoﬁbers

results. EDX analysis of Calix-NMG/PAN shows that methylglucamine units at para position of calixarene 3.90 % of the chromium was found at surface of the Calix- skeleton and can easily form complexes with chromate NMG/PAN which is having main elements such as carbon, anions by electrostatic interactions and hydrogen bonding. nitrogen and oxygen before extraction experiments. This Furthermore, the presence of the vic diols groups of the N- increase at lower pH value can be explained by the fact that methylglucamine units also support the sorption behavior the Calix-NMG/PAN can be protonated in acidic condi- of the nanoﬁbers structure via hydrogen bonding between tions due to the presence of the tertiary amine groups of N- oxoanion and vic diol groups as well as ion-dipol

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Fig. 3 TG and DSC curves of PAN nanoﬁbers (TG lines and DSC dashed lines)

Table 1 Percentage extraction of dichromate anions by extractants probably due to the special and large surface area, high pure PAN and Calix-NMG/PAN at different pH values porosity, microporesity and high flexibility properties of Nanofibers pH the fiber structures of Calix-NMG/PAN. In aqueous solutions having a lower pH, the dichromate will be primarily 1.5 2.5 3.5 4.5 À in its protonated form HCr2O7 . Compared the dianionic Pure PAN 1.1 ± 0.1 – – – 2 À Cr2O7 and monoanionic HCr2O7 form, monoanion will Calix-NMG/PAN 87 ± 0.3 72 ± 0.3 64 ± 0.2 41 ± 0.1 have a smaller free hydration energy. As a result, -4 À Aqueous phase, (metal dichromate) = 1 9 10 M; solid phase, monoanionic HCr2O7 form is easily transferred from the 25 mg (fiber) at 25 °C, for 1 h aqueous phase to the organic phase owing to this smaller Averages and Standard deviations calculated for data obtained from hydration energy. Another advantage of monoanionic form three independent extraction experiments À HCr2O7 is also the only one sodium ion needs to be co- 2À extracted to maintain charge balance, whereas for Cr2O7 two sodium ions are extracted, with additional loss of interaction between Na? ion and diol groups. According to hydration energy [46]. Furthermore, the interfering effect - À 2À - our knowledge the data obtained in extraction Calix-NMG/ of other anions such as Cl ,NO3 ,SO4 and CH3COO on PAN can be attributed to a number of reasons. Calix dichromate anion extraction for the selectivity properties of nanofibers possess a tertiary amine and vic diols, facili- Calix-NMG/PAN was examined. Obtained results showed tating hydrogen bonding with the dichromate anion. The that the extraction of chromate ions was not affected by the next reason is that PAN nanofibers based calixarene bear- - À 2À presence of some selected anions, such as Cl ,NO3 ,SO4 ing N-methylglucamine units have a more stable and - and CH3COO although they used more concentrated than functionalized groups of the calixarene skeleton, and also the concentration of chromate anions. As a results, fiber upon the nature of the aggregations of the ions around the type of solid supports with protonable groups as pyridine in calixarenes skeleton. On the other hand, the slight increase solvent extraction processes were found to be very useful in chromate binding was observed at 3.5 and 4.5 pH values. in the purification of waters contaminated with the chro- From the our knowledge, this increase at these pH values is mate anions. probably due to formation of an ion-pair interaction between sodium ion (Na2Cr2O7) and N-methylglucamine units of Calix-NMG/PAN as well as more rigid and Uranium sorption studies appropriate structure of calixarene derivatives in the polymeric matrix. Furthermore, it is expected that cal- The increasing usage of nuclear reactors for large-scale ixarene skeleton having chelating groups as N-methylglu- energy production leads to radioactive contamination; camine would geometrically be more suited for effectively hence, research concerning the separation of U(VI) ions interaction with dichromate anions. Compared the pub- from water has become a critical environmental issue in the lished literature results about dichromate extraction by last decade [47, 48]. For the extraction of uranium (U) from solid materials such as magnetite nanoparticle and epoxy seawater and brines, a number of chelating polymeric resin with N-methylglucamine units [40, 41], Calix-NMG/ adsorbents were employed. Micro and macroporous poly- PAN showed excellent extraction results. These results are mers having different chelating groups were studied for the

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Table 2 Percentage extraction Nanofibers pH of uranium ions by pure PAN and Calix-NMG/PAN at 4.5 5.5 7.0 8.0 8.5 different pH values Pure PAN 5.5 ± 0.1 4.9 ± 0.3 4.3 ± 0.2 3.3 ± 0.2 3.2 ± 0.2 Calix-NMG/PAN 92 ± 0.3 85 ± 0.2 74 ± 0.3 67 ± 0.3 60 ± 0.3 Aqueous phase, (uranium acetate) = 1.15 9 10-5 M; solid phase, 25 mg (fiber) at 25 °C, for 1 h Averages and Standard deviations calculated for data obtained from three independent extraction experiments uranium uptake from seawater [49, 50]. The adsorption nature, with the lone pairs of vic diols and tertiary amine percentage of uranium anions onto pure PAN and Calix- groups on the surface of fiber structures playing the main NMG/PAN at different pH values are shown in Table 2. role in interaction with the uranium species. With a pH of The results showed that Calix-NMG/PAN nanofibers are 4.5, the adsorption capacity of uranium by the prepared suitable for the separation of uranium ions from aqueous Calix-NMG/PAN reached its maximum value and the solution. The uranium separation was 90 % for Calix- sorption of uranium ions decreased at pH values greater 2þ NMG/PAN nanofiber, based on triplicate analysis at pH than 6. Approximately 85 % of the U(VI) exists in UO2 4.5. The extractability of uranium ions dropped to slightly chemical forms at 4.5 pH value. In pH greater than 6, lower level with an increasing pH values (from 4.5 to 8.5). 2þ hydrolysis of UO2 may also affect adsorption. Extraction This indicates that effectively the applicability of the experiments showed a little decreasing for the adsorption Calix-NMG/PAN nanofibers in the higher pH range is of uranium ions by Calix-NMG/PAN nanofiber over the pH limited. range of 7–8.5. This decrease was paralleled by a reduction The sorption capability of Calix-NMG/PAN nanofibers 2þ in the aqueous concentration of UO2 in favor of UO2 was compared with that of pure PAN nanofibers. The pure ? 5? (OH) and (UO2)3(OH) .It is well-known that uranium PAN nanofibers exhibited around 5.5 % sorption; on the ? may form a series of aqua-complexes, such as UO2(OH) , other hand, Calix-NMG/PAN nanofibers showed higher ðÞUO ðÞOH 2þ andðÞ UO ðÞOH þ ions through a hydrol- sorption capacity. The presence of the calixarene mole- 2 2 2 2 3 5 ysis process [51]. cules with N-methylglucamine chelating groups on the surface of the fiber structures converted hydrophobic properties of the fiber structure to hydrophilic, and the Conclusion sorption of uranium ions increased due to the chelating ability of the N-methylglucamine units. Comparing the This study demonstrated that PAN nanofibers based cal- pure PAN nanofiber and Calix-NMG/PAN nanofiber, ixarene bearing N-methylglucamine units Calix-NMG/ obtained results show that Calix-NMG/PAN nanofiber with PAN produced by electrospinning are an effective sorbent N-methylglucamine chelating units are suitable extractant for the removal of chromate anions and uranium cations. In for uranium cations. Moreover, the sorption results indi- the fabrication process, PAN nanofibers based calixarene cated that the complexation of the uranium cation depends bearing N-methylglucamine units were first produced with on the structural properties of the receptors, such as sta- the aim to develop functional nanofibers. The Calix-NMG/ bility or rigidity and hydrogen binding ability. Therefore, PAN fiber network provided higher ion binding ability due this improved performance for Calix-NMG/PAN nanofiber to the very high surface area, porosity, flexibility and compared to pure PAN nanofiber, is attributed to more microporesity of PAN nanofibers and surface associated rigid structure of calixarene skeleton owing to the support with calixarene molecules containing N-methylglucamine material and the presence of the two N-methylglucamine chelating groups. Calixarenes are already being used in units that may form a good chelating site and help generate variety areas such as pharmaceuticals, catalyst, filtrations a suitable geometry for the uranium ions. In addition, and controlled drug delivery systems, therefore, having surface of the functionalized fiber Calix-NMG/PAN is nanofiber structures might hopefully open up the possibil- dominated by vic diols, and tertiary amine groups. In acidic ities and extend the use of calixarenes in these fields or in media these groups would normally be expected to undergo other functional systems. Moreover, our findings may protonation. Thus, at lower pH values, the chemical forms contribute to the fabrication of new functional nanofibers of the fiber surface and the uranium ions do not support an from other types of calixarene and/or other supramolecular electrostatic hypothesis of sorption. The fixation of the systems via electrospinning. The good adsorption capaci- 2? UO2 ions might therefore be expected to proceed via ties of the by Calix-NMG/PAN have been shown in labo- formation of surface complexes that possess a coordinative ratory studies, and indicate the potential of these fiber 123 J Incl Phenom Macrocycl Chem (2016) 85:49–58 57 structures towards cationic and anionic ion species. 14. 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