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Pure and Applied Chemistry
ISSN: 1060-1325 (Print) 1520-5738 (Online) Journal homepage: https://www.tandfonline.com/loi/lmsa20
Synthesis and Preparation of Novel Magnetite
Nanoparticles Containing Calix[4]arenes With
Different Chelating Group Towards Uranium
Anions
Fatih Ozcan, Mevlüt Bayrakcı & Şeref Ertul
To cite this article: Fatih Ozcan, Mevlüt Bayrakcı & Şeref Ertul (2015) Synthesis and Preparation of Novel Magnetite Nanoparticles Containing Calix[4]arenes With Different Chelating Group Towards Uranium Anions, Journal of Macromolecular Science, Part A, 52:8, 599-608, DOI: 10.1080/10601325.2015.1050631
To link to this article: https://doi.org/10.1080/10601325.2015.1050631
Published online: 24 Jun 2015.
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Synthesis and Preparation of Novel Magnetite Nanoparticles
Containing Calix[4]arenes With Different Chelating Group
Towards Uranium Anions
FATIH OZCAN1,2, MEVL €UT BAYRAKCI3,*, and¸SEREF ERTUL1
1Faculty of Science, Department of Chemistry, Selcuk University, 42075 Konya, Turkey 2Advanced Technology Research and Application Center, 42075 Konya, Turkey
3Faculty of Engineering, Department of Bioengineering, Karamanoglu Mehmetbey University, 70200 Karaman, Turkey
Received February 2015, Revised and Accepted March 2015
In this study, four new magnetite nanoparticle containing different chelating groups as ester, amide and azacrown moieties at the lower rim of calix[4]arenes were prepared by the reaction of formylated calix[4]arenes and amino propyl modified magnetite nanoparticles (APTMS-MN). All of the newly synthesized and prepared compounds were characterized by using different analytical techniques such as nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and thermogravimetric analyses (TGA). Furthermore, uranium binding abilities of newly prepared magnetite nanoparticles were evaluated by using solid-liquid phase extraction process and obtained results showed that magnetite nanoparticles containing picolinamide units exhibited superior and more efficient extraction percentage for uranium ions at lower pH values.
Keywords: Calixarene, magnetite nanoparticle, uranium ion, picolinamide, azacrown
1 Introduction
Uranium is a naturally existing element the significant use of this element has been observed as fuel in nuclear power reactors and nuclear weapons. The nuclear industry requires about tens of thousand tons of uranium every year (1). During its processing, in spite of the protection means implemented in the workplace, the workers may inhale or ingest some uranium by inhalation, ingestion or penetration through wounds or intact skin, which could result in radiation doses to the body. Furthermore, the common peoples may be exposed to low levels of uranium by inhalation; but they are affected by drinking water sup-plies through the mining and milling of uranium ores (2). The WHO recommended level of uranium is 9 mgL¡1 in drinking water (3). The long-term excessive intake of ura-nium through diet can cause serious health effect (4). Therefore, the removal of uranyl ion from waters has great
importance. Various researchers have reported different methods/technologies for the removal of uranyl ion from drinking/waste waters (5, 6). Moreover, various research-ers have reported the advantages of solid phase extraction (SPE) over other preconcentration techniques and in par-ticular over liquid–liquid extraction (7). The different sor-bent materials have been used; but in thisfield magnetite nanoparticles immobilized with the different calixarene derivatives have great importance. Calixarenes are cyclic oligomers which can be conveniently obtained through acid- or base-catalyzed condensation of p-substituted phe-nols and formaldehyde and are regarded as the third gen-eration of supramolecular chemistry because of their binding abilities towards cations, anions, and neutral mol-ecules (8, 9). Calixarenes can be chemically modified by substitution of the phenolic hydrogens with various types of function (10–12). Furthermore, calixarene skeleton can be functionalized on the para position of phenolic units to make the calixarene skeleton either lipophobic or lipo-philic (13).
Magnetic iron oxide nanoparticles have attracted con-siderable interest in thefields of biomedical and biotechno-logical applications, including target drug delivery, MRI contrast enhancement, separation and purification of pro-tein and cell, and proteomics (14–19). For many of these
*Address correspondence to: Mevl€ut Bayrakcı, Faculty of Engineering, Department of Bioengineering, Karamanoglu Mehmetbey University, 70200 Karaman, Turkey. E-mail: mevlutbayrakci@gmail.com
Color versions of one or more figures in this article can be found online at www.tandfonline.com/lmsa.
Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2015) 52, 599–608 Copyright © Taylor & Francis Group, LLC
ISSN: 1060-1325 print / 1520-5738 online DOI: 10.1080/10601325.2015.1050631
applications, surface modification of magnetite nanopar-ticles is a key challenge. However, due to the magnetic dipolar attraction, the naked magnetite nanoparticles tend to aggregate into clusters and have a limited application in field of biological sciences (20). To overcome the above drawbacks, a magnetic core coated with organic or inor-ganic materials can be accomplished by physical/chemical adsorption or functionalization to introduce the required groups for specific applications. The core–shell nanostruc-ture composites may combine the advantages of “core” and “shell” and show enhanced physical and chemical properties. Thus, many of study showing the synthesis of the magnetic core–shell nanocomposites, in which the functional shell including polymers, ligands, metals or oxides is coated on the Fe3O4core have been reported (21)
In the last decade, significant interests have been devel-oped in thefield of nano-sized magnetite particles (gener-ally maghemite, g-Fe2O3, or magnetite, Fe3O4, single
domains of about 5–20 nm in diameter). The Fe3O4have
received more attention for their reliable chemical stabil-ity, biocompatibilstabil-ity, good conductivity and optical prop-erties (22).
Until now, the use of calixarene-based materials as uranophiles with either a pseudo planar pentaco-ordi-nate or hexaco-ordipentaco-ordi-nate structure have been reported very few (23, 24). It is well known that the high selec-tivity is attributed to the rigid skeleton of calixarenes towards the uranyl ion since these derivatives can pro-vide the pre-organized hexa- or penta-coordination geometry required for the binding of UO22C ions (25,
26). Although there are numerous examples of lower rim modified calixarene and their corresponding poly-meric derivatives that act as hosts and complexants for cations, (27) relatively few upper rim modified calixar-enes and their polymeric analogs have been reported (28). Thus, the development of efficient upper rim and lower rim functionalized calixarenes for metal cations as uranium has received considerable attention in recent years. Compared to the number of reports on the bind-ing of metal ions with both lower rim and upper rim modified calixarenes, reports on the uranium binding properties of them are still limited. As mentioned above, both upper rim and lower rim functionalized calixarenes and their grafted derivatives onto magnetite nanoparticle are expected to be an artificial linker mol-ecule system for uranium ions. From this point of view, suitable modified calixarene monomers and their mag-netite nanoparticle analogs are at the center of interest. Therefore, in this study, we aimed to synthesize substi-tuted calix[4]arene receptors 5a, 5b and 7 that was functionalized with formyl units at the upper rim and with amide, ester and azacrown units at the lower rim and prepare their magnetite nanoparticle derivatives 1, 2a, 2b and 3 in order to systematically study the in flu-ence of the molecular structure on uranium recognition by means of solid-liquid phase extraction processes.
2 Experimental
2.1 Apparatus
Melting points were determined on a Gallenkamp appara-tus in a sealed capillary glass tube and were uncorrec-ted.1H-NMR spectra were recorded on a Varian 400 MHz spectrometer. IR spectra were obtained on Perkin–Elmer spectrum 100 FTIR spectrometer (ATR). UV-Vis spectra were obtained on a Shimadzu 160A spectrophotometer. Elemental analyses were performed using a Leco CHNS-932 analyzer. Thermogravimetric analysis (TGA) was car-ried out with a Setaram SETSYS thermal analyzer at tem-perature range of 25–950C at a heating rate of 10C
min¡1 under argon atmosphere with a gas flow rate of 20 mL min¡1.The size and shape of the NPs were deter-mined by transmission electron microscope (TEM, FEI Company-Tecnai G2 Spirit/Biotwin). The pH measure-ments were carried out an Orion 410AC pH meter.
2.2 Materials
TLC analyses were carried out on DC Alufolien Kieselgel 60 F254(Merck). The solvents were dried by storing
over-molecular sieves (Aldrich; 4 A, 8–12 mesh). All reactions, unless otherwise noted, were conducted under a nitrogen atmosphere. All starting materials and reagents used were of standard analytical grade from Merck or Aldrich and used without further purification. CH2Cl2 was distilled
from CaCl2, while MeOH was distilled over Mg and
stored over molecular sieves. All commercial grade sol-vents were distilled, and then stored over molecular sieves. The drying agent employed was anhydrous magnesium sulfate. All aqueous solutions were prepared with deion-ized water that was passed through a Millipore Milli-Q Plus water purification system. Arsenazo III, uranyl ace-tate dihydrate were purchased from Fluka. Standard stock solution of 0.9787 g/mL uranium (VI) was prepared by dissolving the appropriate amounts of uranyl acetate dihy-drate in deionized water. A stock arsenazo III solution (0.01%) was prepared by dissolving reagent. Adjusting the pH values of the working solutions was carried out using 5 M of sodium acetate buffer to determination of UO22C
in aqueous solution.
2.3 Synthesis
Pure Fe3O4 nanoparticles,
3-aminopropyltrimethoxysi-lane-modified Fe3O4(Sch. 1), and calixarene derivatives
such as 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahy-droxycalix[4]arene1, 5,11,17,23-Tetra-tert-butyl-25,27-bis (methoxycarbonylmethoxy)-26,28-dihydroxycalix[4]aren 2, 5,17-bisformylated-11,23-di-tert-butyl-25,27-bis(metho-xycarbonylmethoxy)-26,28-dihydroxycalix[4]aren 3, 5,11, 17,23-Tetra-tert-butyl-25,27-bis(2-aminomethyl-pyridinea-mido)-26,28-dihydroxycalix[4]arene 4a,
5,11,17,23-Tetra-Sch. 1. Preparation of the Fe3O4magnetite nanoparticles and APTMS-MN magnetite nanoparticles.
Sch. 2. The synthetic route for synthesis of calixarene derivatives.
tert-butyl-25,27-bis(3-aminomethyl-pyridineamido)-26,28-dihydroxycalix[4]arene 4b and 5,11,17,23-Tetra-tert-butyl-25,27-bis(amido)crown 6 compounds (Sch. 2) were synthe-sized according to the reported literatures (29–34).
2.4 General Procedure for the Synthesis of New Bisformylated Calixamide Derivatives
Bisformylated calixamide derivatives were synthesized by modified literature procedure as mentioned Scheme 2 (34). Calixarene derivatives (4a, 4b and 6) (0.15 mol) and hexa-methylenetetramine (HMTA) (6.17 mmol) were taken in trifluoroacetic acid (TFA) (50 mL). The reaction mixture was refluxed until the starting materials had disappeared (TLC). On completion, the mixture was quenched with ice cold water and extracted with chloroform. The organic layer was washed with water and dried (Na2SO4). The
sol-vent was evaporated under reduced pressure, and the resi-due was purified as mentioned to yield the desired bisformylated calix[4]arene products 5a, 5b and 7.
5,17-bisformylated-11,23-di-tert-butyl-25,27-bis(2-amino-methyl-pyridineamido)-26,28-dihydroxycalix[4]arene 5a: Yellow powder 80.2% yield-m.p.: 158–162C. IR n
max/
cm¡1: 3340 (OH), 2957 (CH2), 1724 (COH), 1658 (CONH); 1
H-NMR (400 MHz CDCl3): dH0.96 (s, 18H,tBu), 3.42 (d,
4H, ArCH2Ar, JD 12.8 Hz), 3.92 (d, 4H, ArCH2Ar, JD
12.8 Hz), 3.96 (d, 4H, Ar–CH2–NH), 4.70 (s, 4H, OCH2),
6.88 (m, 4H, PyH), 7.10 (m, 4H, ArH), 7.80 (m, 4H, ArH, J D 7.8 Hz), 8.21 (s, 2H, NH), 9.90 (s, 2H, H-CDO ), Elemen-tal Anal. Calc. For C54H56N4O8: C, 72.95; H, 6.35; N,
6.30%.Found: C, 72.86; H, 6.32; N, 6.22%.
5,17-bisformylated-11,23-di-tert-butyl-25,27-bis(2-ami-nomethyl-pyridineamido)-26,28-dihydroxycalix[4]arene 5b: Yellow powder 72.5% yield-m.p.: 165–169C. IR n
max/
cm¡1: 3352 (OH), 2953 (CH2), 1758 (COH), 1678
(CONH). 1H-NMR (400 MHz CDCl3): dH0.97 (s, 18H, tBu), 3.47 (d, 4H, ArCH
2Ar, J D 13.0 Hz), 3.86 (d, 4H,
ArCH2Ar, J D 13.0 Hz),3.90 (d,4H, Ar–CH2–NH), 4.55
(s, 4H, OCH2), 6.85 (m, 4H, PyH), 7.00 (m, 4H, ArH),
7.61 (m, 4H, ArH, JD 7.8 Hz), 8.14 (s,2H, NH), 9.82 (s, 2H, H-CDO), Elemental Anal. Calc. For C54H56N4O8(889.04 g/mol): C, 72.95; H, 6.35; N, 6.30%.
Found: C, 72.87; H, 6.33; N, 6.27%.
5,17-bisformylated-11,23-di-tert-butyl-25,27-bis(amido) crown 7: Yellow powder 58.2 % yield-m.p.: 115–118C. IR nmax/ cm¡1: 3331 (OH), 2957 (CH2), 1728 (COH), 1676
(CONH). 1H-NMR (400 MHz CDCl3): dH0.88 (s, 18H, tBu), 3.46 (m, 4H, -CH
2-), 3.54 (d, 4H, ArCH2Ar, J D
13.1 Hz), 3.64 (m, 4H, -CH2-), 4.25 (d, 4H,-ArCH2Ar, J
D 13.1 Hz), 4.52 (s, 4H, OCH2), 6.71 (s, 4H, ArH), 7.13
(s, 4H, ArH), 8.78 (s, 2H, NH), 9.90 (s, 2H, H-CDO). Ele-mental Anal. Calc. For C46H52N2O9: C, 71.11; H, 6.75;
N, 3.61%. Found: C, 70.98; H, 6.68; N, 3.57%.
2.5 General Procedure for the Synthesis of Calix[4]Arene-Grafted Fe3O4Nanoparticles
1 g Fe3O4–APTMS were dispersed in 60 ml of dry ethanol
by sonication for about 1 h at room temperature. Then, a 0.5 g amount of the bisformylated calix[4]arene derivatives (3, 5a, 5b or 7) was refluxed in the Fe3O4–APTMS solution
with dry MgSO4and catalytic acetic acid for 12 h with
stir-ring. The resultant products were obtained by magnetic sep-aration with permanent magnet and were thoroughly washed with water, ethanol and CH2Cl2(Sch. 3). The brown
precipitates were dried at room temperature under vacuum. For Polymer-1-Magnetic Calix[4]arene: The polymer
resulted with 0.95 g. IRnmax/cm¡1: 3126 (OH), 2957
(CH2) and 1748 (COOC2H5), 1112, 893 and 792 (Si-O).
For Polymer-2a-Magnetic Calix[4]arene: The polymer resulted with 1.05 g ; IRnmax/cm1: 3400 (OH), 2957
(CH2), 1636 (CONH), 1047, 1001 and 879 (Si-O).
For Polymer-2b-Magnetic Calix[4]arene: The polymer resulted with 0.88 g; IRnmax/cm¡1: 3436 (OH), 2961
(CH2),1634 (CONH), 1045, 999 and 879 (Si-O).
For Polymer-3-Magnetic Calix[4]arene: The polymer resulted with 1.03 g; IR nmax/cm¡1: 3404 (OH), 2965
(CH2),1672 (CONH), 1481, 1023, 893 and 792 (Si-O).
2.6 Sorption Procedure
The sorption efficiencies of the magnetite nanoparticles based calixarene with chelating groups such as ester, amide and azacrown units were determined by the following tech-nique (35) An aqueous solution (10 mL) of UO2
(O-COCH3)2¢2H2O (1.15¢10¡5M) and magnetite nanoparticle
based calix[4]arenes 1, 2a, 2b and 3(25 mg) were taken in a stopperedflask that was shaken at 180 rpm and 25C. The sorbents were separated easily by using a magnet before measurements. The residual uranium concentration of aque-ous solute was determined spectrophotometrically by UV-Vis analyses at 652 nm (for pH 4.5–8.5). The effect of pH was studied by adjusting the pH of aqueous solutions using diluted 0.01 M HCl/KOH solutions at 25C.
The percent sorption (S %) was calculated according to Equation 1;
S % D .S0¡ S/=S0£ 100 (1)
Where S0and S are the initial andfinal concentrations of
the uranium ion before and after the sorption, respectively.
3 Results and Discussion
3.1 Synthesis and Characterization
The synthesis of a new calix[4]arene derivative have been given in Scheme 1. In order to access the target
bisformylated calixarene derivatives 3, 5a, 5b and 7 syn-thetically, p-tert-butylcalix[4]arene ester derivative 2, 4a, 4b and 6 was interacted with hexamethylenetetramine (HMTA) in the presence of trifluoroacetic acid (CF3COOH) with for 2 h. The structure of
bisformy-lated calixarene derivatives 3, 5a, 5b and 7 was estab-lished by the analysis of their 1H-NMR spectra, as well as Fourier Transform Infrared FT-IR (ATR) and ele-mental analyses. The formation of bisformylated calix [4]arene(3, 5a, 5b and 7) was confirmed by the appear-ance of the aldehyde bands at about 1799, 1724, 1758 and 1728 cm¡1in the FT-IR spectra, respectively. Fur-thermore, in the 1H-NMR spectrum, it was determined that the pair of doublets at 3.46 and 4.45 ppm (J D 12.9 Hz), 3.42 and 3.92 ppm (J D 12.8 Hz), 3.47 and
3.90 ppm (J D 13.0 Hz), 3.54 and 4.25 ppm (J D 13.31 Hz) for ArCH2Ar protons of bisformylated calix
[4]arene3, 5a, 5b and 7, respectively. The high field doublet around d 3.46, 3.42, 3.47 and 3.54 was assigned to the equatorial protons of methylene groups of bisfor-mylated calix[4]arene 3, 5a, 5b and 7, respectively, whereas the low field signal around d 4.45, 3.92, 3.90 and 4.25 was assigned to the axial protons of calixarene scaffold for compounds 3, 5a, 5b and 7 respectively. These data suggested that bisformylated calix[4]arenes3, 5a, 5b and 7 possessed cone structures. In addition to above signals bisformylated calix[4]arene3, 5a, 5b and 7 also showed one singlet around d 9.77, 9.90, 9.82 and 9.97 corresponding to the aldehyde (H-CDO) protons at the upper rim of calixarene skeleton, respectively.
Sch. 3. The synthetic route for synthesis of Polymer-1, 2a, 2b and 3 magnetite calix[4]arene.
It is well known that in extraction processes, separation is a time consuming task. Therefore, immobilization of calixarene compounds onto surface of the magnetite nano-particles for easy separation of these from solvents by using a magnet and reuse of them are most important point in separation process. In this point of view, in this study four new bisformylated calixarene derivatives 3, 5a, 5b and 7 were immobilized on the surface of the magnetite nanoparticles with amino propyl group (APTMS-MN) that prepared according to the modified procedures (22, 32)and four new magnetic calixarene derivatives (Poly-mer-1, 2a, 2b and 3) were prepared as shown in Scheme 3. The formation of polymers 1,2a, 2b or 3 were confirmed by a combination of FT-IR, TEM, TGA and elemental analysis. In order to receive more direct information on the particle size and morphology, transmission electron microscopy (TEM) micrographs of pure Fe3O4
nanopar-ticles and magnetic calixarene derivatives polymers 1, 2a, 2b and 3 were investigated (Fig. 1). Observing the TEM micrographs, nanoparticles formed dense aggregates due to the lack of any repulsive force between the magnetic nanoparticles. This force is mainly due to the nano-size of the APTMS-modified Fe3O4, which is about 10§ 2 nm.
This may be considered as indirect evidence that the mag-netic core of the APTMS-modified magnetite particles consist of a single magnetic crystallite with a typical diam-eter of 8 §3 nm, and that difference corresponds to the APTMS coating. After bisformyl calix[4]arene immobili-zation, the dispersion of particles was improved greatly, which can easily be explained by the electrostatic repulsion force and steric hindrance between the calix[4]arene units on the surface of Fe3O4nanoparticles.
The thermal properties of APTMS-MN and magnetic calixarene derivatives (Polymers 1, 2a, 2b and 3) were ana-lyzed by the thermogravimetric method (Fig. 2). The TGA curves also confirmed the immobilization of calixar-ene derivatives 3, 5a, 5b and 7 onto the magnetite nanopar-ticles surface. Upon heating, the weight loss of
APTMS-modified magnetite nanoparticles (APTMS-MN) were shown to be about 5% within a broad temperature range of 250 and 650C. The decomposition of Polymer 1 was observed as H2O 5% between 25–150C, 18%
3-amino-propyl groups between 150–450C and between 450– 800C iron oxide. Furthermore, the decomposition of Pol-ymers 2a and 2b were observed as H2O 7% between 25–
125C, 23% 3-aminopropyl groups between 125–450C and between 450–850C iron oxide. The last
decomposi-tion which belongs to Polymer 3 was observed as H2O 4%
between 25–125C, 21% 3-aminopropyl groups between
125–450C and between 450–825C iron oxide.
FT-IR spectroscopy was used to elaborate the structure of Fe3O4, APTMS-MN and polymers 1, 2a, 2b and 3. The IR
peak at 568 cm¡1belongs to the stretching vibration mode of Fe-O bonds in Fe3O4and 3235, 1614 and 1545 (NH2), 1069,
895 and 795 cm¡1(Si–O) bonds in APTMS-Fe3O4(Fig. 3).
Compared to the IR spectrum of APTMS-MN and poly-mers 1, 2a, 2b and 3, the calix[4]arene derivatives (polypoly-mers 1, 2a, 2b and 3) possessed peaks for 2a, 2b and 3at 1636 cm¡1, 1634 cm¡1and 1672 cm¡1which are stretching vibrations of amide carbonyl (N-CDO) respectively and at 1748 cm¡1
which are stretching vibrations of ester carbonyl COOC2H5)
for nanoparticle 1. Furthermore, the bending vibrations assigned to the aromatic CDC bonds of calix[4]arene skeleton for nanoparticles (polymers 1, 2a, 2b and 3) were seen around 2957 cm¡1, 2958 cm¡1, 2961 cm¡1, 2965 cm¡1, respectively. Also, additional peaks around1112, 893 and 792 cm¡1for 1, 1047, 1001 and 879 for 2a, 1045, 999 and 879 for 2b and 1023, 893, 792 for 3 were most probably due to the symmetric and asymmetric stretching vibration of framework and terminal Si-O- groups.
3.2 Solid-Liquid Sorption
In spite of the recent Fukushima accident, nuclear energy is still expected to play an important role for the future
energy mix. Uranium has a key role in generation of nuclear power. The selective isolation of uranium is of par-ticular interest in the context of both energy resources and treatment of nuclear wastes. In aqueous solution, uranium (VI) exists as a linear UO22C, and it forms stable
com-plexes with both organic and inorganic ligands (36) The results showed that calixarene modified magnetite nano-particles are suitable for the separation of uranium ions from aqueous solution. The results of the separation experiments in dependence on pH value are shown in Table 1. The uranium separation was 80.40 %, 71.70%, 81.40% and 84.64% for nanoparticles (polymers 1, 2a, 2b and 3), respectively, based on triplicate analysis at pH 4.5. However, at pH 5.5, uranium extraction percentage was seen over 90% based on triplicate analysis for nanopar-ticles (polymers 1, 2a, 2b and 3). The extractability of ura-nium ions dropped to slightly lower level with an increasing pH values (from 4.5 to 8.5). This indicates that effectively the applicability of the calixarene modified magnetite nanoparticles in the higher pH range is limited. But, from the obtained extraction results, it was observed that uranium could be extracted from aqueous phase as shown in Table 1. Comparing the nanoparticles (polymers 1, 2a, 2b and 3), obtained results show that nanoparticle 2a and 2b with picolinamide units are suitable extractant for uranium. This is not a surprising result because it is expected that calixarene skeleton having chelating groups as picolin amide would geometrically be more suited for
effectively interaction with uranium ion than suitable ester and azacrown derivatives of the calix[4]arene onto magne-tite nanoparticle surface (37–39. Moreover, the sorption results indicated that the complexation of the uranyl cation depends on the structural properties of the receptors, such as stability or rigidity and hydrogen binding ability. There-fore, this improved performance for 2a and 2b compared to 1 and 3, is attributed to more rigid structure of calixar-ene skeleton owing to the support material and the pres-ence of the two nitrogen atoms of the picolyl moieties that may form a good chelating site and help generate a suit-able geometry for the uranium ions. Furthermore, the structural changes in the calixarenes like removing the para substituents affect the molecular interactions (40). Furthermore, the interfering effect of othercations such as Fe3C, Ca2C, NaC and KC and their mixture on uranium retention of polymers 1, 2a, 2b and 3 was examined. The extraction of uranium cations with polymers 1, 2a, 2b and 3 was not affected by the presence of Fe3C, Ca2Cand KC. Conversely, the uranium cations extraction values by poly-mers 1, 2a and 2b showed a slightly negative effect on the binding of uranium in presence of NaC. This behavior could be explained with a better structural complementar-ity between spherical NaC and the ester and amide cavity of the calix[4]arene (10). On the other hand, hydrolysis of UO22Cmay also affect adsorption. Extraction experiments
showed a little decreasing for the adsorption of uranyl ions by 1, 2a, 2b and 3 over the pH range of 7 to 8.5. This
Fig. 2. TG and theirfirst derivatives (dTG) of Polymer 1, 2a and 3.
decrease was paralleled by a reduction in the aqueous con-centration of UO22C in favor of UO2(OH)C and
(UO2)3(OH)5C.Uranium can exist in the 3C, 4C, 5C, and
6C oxidation states, of which the 4C and 6C states are the most common states found in the environment. The mobil-ity of U(VI) was enhanced in oxidizing environments by
the formation of uranyl (UO22C), which hydrolyzes to
form different species as given in Table 2. (41) It is well-known that uranium may form a series of aqua-complexes, such as UO2(OH)C, (UO2)2(OH)22C and (UO2)3(OH)5C
ions through a hydrolysis process (42,43). The ratio of these species depends on the concentration as well as pH of the of uranyl solution. Moreover, the 1.3–4.0 pH
Fig. 3. The FT-IR spectrum of Fe3O4, APTMS- MN, Polymer 1, 2a, 2b and 3.
Table 1. Percentage extraction of uranium ions by extractants1, 2a, 2b and 3 at different pH values
pHa Compound 4.5 5.5 7.0 8.0 8.5 APTMS-MN 42.37 48.82 39.72 32.69 19.84 Polymer-1 80.40 96.21 87.90 85.50 76.00 Polymer-2a 71.70 93.40 77.20 71.40 67.45 Polymer-2b 81.40 94.20 93.00 88.80 86.30 Polymer-3 84.64 92.17 78.35 73.62 61.75 a
Aqueous phase, [cation uranyl] D 1.15¢10¡5 M; solid phase, 25 mg [ligand: Polymer-1, 2a, 2b and 3] at 25C, for 1 h. The percentage extraction is given by [initial aqueous anion] - [final aqueous anion]/[ini-tial aqueous anion]£ 100.
Table 2. Equilibrium constants
Reaction Log Ka
UO22CC H2OD UO2(OH)CC HC ¡5.20
2UO22CC 2H2OD (UO2)2(OH)22CC 2HC ¡5.62
3UO22CC 5H2OD (UO2)3(OH)5CC 5HC ¡15.55
UO22CC 3H2OD UO2(OH)3¡C 3HC ¡21.0
Ion exchange reactions
UO22CC 2X¡D UO2X2 30.9
UO2(OH)CC X¡D UO2(OH)X 8.6
(UO2)3(OH)5CC X¡D (UO2)3(OH)5X ¡1.75
a
interval the predominant species are UO22C,
(UO2)2(OH)22C and (UO2)3(OH)5C. Increasing pH i.e. >
4.0, uranium adsorption decreases because in this range, the soluble complexes of uranyl ions are the predominant species and at pH<4, only a low amount of non-hydro-lyzed UO22C ions are adsorbed. Furthermore, at pH<6,
the uranyl contaminant solution results in the formation of uranium(VI)- hydroxo or uranium(VI)- aquo-complexes and at pH 5.3 the cationic uranium(VI) species, UO22C
and UO2OHC, are predominant in the solution and the
competition of protons, regarding cation exchange, is rela-tively low.
4 Conclusions
In the present study, a set of four calixarene derivatives, incorporating ester, picolin amide and azacrown chelating groups at the lower rims and bisformyl groups at the upper rim were synthesized and immobilized onto modified mag-netite nanoparticles. The spectroscopic data indicated that the new compounds are in the cone conformation. The uranyl ion binding studies was explored using different polymers 1, 2a, 2b, 3 and APTMS-MN. Comparative ura-nyl ion sorption study demonstrated that magnetite nano-particles based bisformyl calix[4]arene with picolinamide units were excellent chelating groups for the removal of uranyl ion as compared to bisformyl calix[4]arene with azacrown and ester units and ungrafted APTMS-MN. The APTMS-MN only provided rigid structural features and to prevent solubility of calix[4]arene ionophores. Calixar-ene based receptors with picolinamide units in extraction processes were found to be very useful in transportation of uranium in laboratory-scale applications. However, large amount of calixarene compound and magnetite nanoparti-cle used in this method restricts its application for indus-trial-scale. This issue needs further studies that can suggest suitable methodologies by which both magnetite nanopar-ticle and calixarene compounds can be recovered and/or reused. But in the light of this present study, the most affin-ity of these magnetite nanoparticles based bisformyl calix [4]arene with picolinamide units looks promising especially for the treatment of nuclear waste. Currently, further stud-ies in this field, especially for a comparison of the some parameters on uranium extraction process such as the number of substituents, the position of the nitrogen on picolinamide and the size of calixarene are in progress by our group.
Funding
The authors wish to thank Selcuk University and Karama-noglu Mehmetbey University for the facilities provided.
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