1 Introduction

In recent decades with the rapid development of human society as well as science and technology, no doubt the world is reaching to new sky scraping horizon; but at the same time the cost in which the world is paying or will pay in the near future in the form of extensive global contamination is certainly very high (Gupta 2009). Azo dyes pollution is one of the biggest problems in the world that troubles physically and economically (Vimonses et al. 2009; Olukanni et al. 2006). Azo dyes are synthetic organic colorants commonly prepared by coupling of a diazonium compound with a phenol or an aromatic amine. They are widely used as colorants in a variety of consumer goods such as, leather, textiles, and foodstuff. Some of these dyes can be reduced in vivo through cleavage of the azo groups (N=N), forming mutagenic and carcinogenic aromatic amines (Golka et al. 2004; Chung 2000). With the increased awareness of the potential risk for consumers, the European Parliament recently accepted the 19th amendment of the Council Directive 76/769/EEC and issued the European Directive 2002/61/EC (European Commission 2002). This directive particularly restricts the marketing and use of azo dyes, which may form any of the 22 listed harmful aromatic amines in textile and leather articles after reductive cleavage, and may come into direct and prolonged contact with human skin or oral cavity. Legally permissible limits of the banned amined on textiles is 30 mg/lg or 30 ppm.

Convectional wastewater treatment relying on aerobic biodegradation has low removal efficiency for anionic soluble dyes. Due to the low biodegradation of dyes, a conventional biological treatment process is not very effective in treating dyes in wastewater. Such wastewater is usually treated with either physical or chemical processes. However, these processes are very expensive and cannot effectively be used to treat the wide range of dyes in wastewater (Garg et al. 2003). The adsorption process is one of the effective methods for dye removal from effluents. The process of adsorption has an advantage over other methods due to its sludge free clean operation and complete removal of dyes, even from dilute solutions.

The increasing number of recent publications on adsorption of toxic compounds shows that there is widespread interest in the synthesis of adsorbent resins able to totally eliminate organic pollutants. Various chemical and physical processes are currently in use. Solid phase extraction (SPE) using sorbents is one of the most efficient and well-established procedures in the field of separation science, which finds application in various fields like environmental, food, clinical, pharmaceutical, and industrial chemistry. SPE is usually performed using a small column or cartridge containing an appropriate sorbent. The sorbents may be of mineral or organic origin. Among these, modified silicas (C8 and C18), ion exchangers (Organ et al. 2002), graphitized carbon black (Vial et al. 2001), various polymeric sorbents polystyrene-divinyl benzene (Jung et al. 2001), immunosorbents (Delaunay-Bertoncini et al. 2001), molecularly imprinted polymers (Andersson and Nicholls 2004), conductive polymers (Bagheri and Saraji 2003), porous polymers (Gawdzik and Matynia 1996), polysaccharides such as chitin (Synowiecki and Al-Khateeb 2003), starch (Yuryev et al. 2002) and chitosan (Babel and Kurniawan 2003), etc. are reported. In this respect, the supramolecular chemistry has provided a much beter solution to search for molecular structures that can serve as building blocks for the production of sophisticated molecules by anchoring functional groups oriented in such a way that they provide a suitable binding site. This was achieved with the development of macrocyclic molecules such as synthetic crown ethers, cryptands, spherands (Feber, 1978), natural cyclodextrins (Chern and Huang 1998), and calixarenes (Memon et al. 2005; Memon et al. 2006; Tabakci et al. 2003; Yilmaz et al. 2006).

Calixarenes have generated considerable interest as useful building blocks for the synthesis of hosts for cations, anions, and neutral molecules. During the last two decades, they have attracted much attention as receptors in supramolecular chemistry. The increasing interest in these compounds is stimulated by the simple large-scale synthesis of calixarenes, and the different ways in which they can be selectively functionalized at the narrow (phenolic groups) or at the wide rim (aromatic nuclei; Vicens and Bohmer 1991; Roundhill 1995; Gutsche 1998). Their rigid conformation enables calixarenes to act as host molecules because of their preformed hydrophobic cavities. Due to this ability to form host–guest type complexes with a variety of organic or inorganic compounds, the calixarenes have received increasing attention (Feber 1978; Gutsche 1998; Delval et al. 2002; Janus et al. 1999).

In recent years, superparamagnetic nanoparticles of iron oxides have shown great potential applications in many biological fields. The application for biomolecules immobilization is mainly based on the solid-phase magnetic feature that is able to achieve a rapidly easy separation and recovery from the reaction medium in an external magnetic field. Previously, we have synthesized various derivatives of calix[4]arene and immobilized upon magnetic nanoparticles and investigated to their extraction abilities towards Cr(VI) ion (Ozcan et al. 2009; Sayin et al. 2010a, b). Also, the different derivatives of calix[n]arene (n = 4,6 and 8) was prepared and their sorption efficiencies were investigated for selected azo dye and aromatic amines by our group (Erdemir et al. 2009; Ozmen et al. 2007; Akceylan et al. 2009). In the present study, we realized the synthesis of p-sulfonated calix[4,6]arenes and their immobilization onto 3-(glycidoxy)propyl functionalized magnetic nanoparticles (EP-MN; see Schemes 1 and 2). The new water souble calix[4,6]aren appended magnetic nanoparticles were used as sorbent in removing of selected carcinogenic aromatic amines from aqueous solutions.

Scheme 1
scheme 1

The preparation of 3-(glycidoxy)-propyl functionalized magnetic nanoparticles (EP-MN)

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Scheme 2
scheme 2

The synthetic route for synthesis of p-C[4]-MN and p-C[6]-MN as sorbent

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2 Experimental

2.1 Material and Method

Analytical TLC was performed on precoated silica gel plates (SiO2, Merck PF254), while silica gel 60 (Merck, particle size 0.040–0.063 mm, 230–240 mesh) was used for preparative column chromatography. Generally, solvents were dried by storage over molecular sieves (Aldrich; 4 Å, 8–12 mesh). All chemicals were purchased from Merck and Fluka and used without further purification. All aqueous solutions were prepared with deionized water that had been passed through a Millipore Milli-Q Plus water purification system. Melting points were determined on a Gallenkamp apparatus in a sealed capillary and are uncorrected. 1H NMR spectra were recorded with a Varian 400 MHz spectrometer in CDCl3. FT-IR spectra were recorded on a Perkin Elmer Spectrum 100. UV–vis spectra were obtained with a Shimadzu 160A UV–vis recording spectrophotometer. High-performance liquid chromatography (HPLC) Agilent 1200 Series were carried out using a 1200 model quaternary pump, a G1315B model Diode Array and Multiple Wavelength UV–vis detector, a 1200 model Standard and preparative autosampler, a G1316A model thermostated column compartment, a 1200 model vacuum degasser, and an Agilent Chemstation B.02.01-SR2 Tatch data processor. The amines were separated on a Ace 5 C18 column (25 cm, 4.6 mm). The mobile-phase consisted of acetonitrile (eluent A) and water (eluent B), flow rate: 1 mL/min, at 25°C, injection volume 20 μL, gradient elution: 0 min 20 % A and 80 % B; 25 min 80 % A and 20 % B. Detection was performed at 280 nm. Thermal gravimetric analysis (TGA) was carried out with Seteram thermogravimetric analyzer. The sample weight was 15–17 mg. Analysis was performed from room temperature to 900°C at heating rate of 10°C/ min in argon atmosphere with a gas flow rate of 20 mL/min.

2.2 Synthesis

The synthesis of p-tert-butylcalix[4]arene (1), calix[4]arene (2), p-tert-butylcalix[6]arene (4), and calix[6]arene (5) was conducted using the procedures described by Gutsche (Gutsche and Iqbal 1990a, b; Gutsche and Lin 1986). Then, p-sulfonated calix[4,6]arene derivatives 3, 6 were synthesized using H2SO4 referring to the literature (Shinkai et al. 1986). The structures of these compounds were characterized by FT-IR and 1H NMR analyses.

2.2.1 The Preparation of 3-(Glycidoxy) Propyl Functionalized Magnetic Nanoparticles (EP-MN)

A microemulsion was prepared by dissolving 1.75 g of sodium dodecylbenzenesulfonate in 15 mL of xylene by sonication. Metal salts were dissolved in deionized (DI) water to enhance the hydrolysis reaction of TEOS (tetraethoxysilane). The salt solution was composed of FeCl2·4H2O, Fe(NO3)3·9H2O, and DI water. This salt solution was added to the microemulsion under vigorous stirring, and the solution was kept at room temperature for about 12 h to allow for its stabilization. Then, the reverse-micelle solution was slowly heated to 90°C under continuously flowing nitrogen gas for an hour and 1 mL of hydrazine (34 wt% aqueous solution) was injected into the solution. These particles were aged at 90°C for 3 h, and then cooled down to 40°C within an hour. The temperature was maintained at 40°C to prevent the complete evaporation of the water in the hydrophilic phase containing the magnetite nanoparticles. After the temperature of the magnetite solution reached 40°C and became stabilized, 2 mL of TEOS and 2 mL of 3-(glycidoxy)-propyltrimethoxysilane was injected into the mixture of the as-synthesized magnetite and xylene under vigorous stirring. The silica coated magnetite nanoparticles were segregated from the xylene phase and dispersed well in water. After magnetic separation, the attained 3-(glycidoxy)-propyl functionalized magnetic nanoparticles (EP-MN) were washed three times with ethanol and deionized water turn in and dried at 80°C under vacuum.

2.2.2 Immobilization of p-Sulfonated Calix[4,6]arene on EP-MN

Compound 3 or 6 (0.3 g) and NaOH (0.11 g) were stirred in deionezied water (3 mL) for 30 min at room temperature. Then, 0.9 g of 3-(glycidoxy) propyl functionalized magnetic nanoparticles (EP-MN) and 12 mL of DMSO was added to the this solution. The mixture was stirred for about 48 h at 50°C. After magnetic separation, the resulting compounds (7 and 8) were washed three times with ethanol and water in turn and dried at 80°C under vacuum. The compounds 7 and 8 were named as the p-C[4]-MN and p-C[6]MN, respectively.

2.3 Batch Sorption Studies of Aromatic Amines

An aqueous solution (10 mL) containing aromatic amine (see Fig. 1 for the structure formula) was pipetted into a vial at a concentration of 1 × 10−3 M, and a few drops of 0.01 M KOH/HCl solution in order to obtain the desired pH at equilibrium and maintain the ionic strength and 25 mg of the sorbent were added. The mixture was stirred at 25°C on a horizontal shaker at 170 rpm until equilibrium for 1 h. After the sorbent was removed by centrifugation the residual concentration of the organic moiety was determined by means of HPLC. The sorption capacity was then calculated and expressed in percentage uptake.

Fig. 1
figure 1

The chemical structures of aromatic amines used in experiments

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3 Results and Discussion

3.1 Synthesis and Characterization of p-C[4]-MN and p-C[6]-MN Sorbents

In order to evaluate the aromatic amines removal efficiency of p-C[4]-MN and p-C[6]-MN sorbents, firstly, p-sulfonated calix[n]arenes (n = 4,6) were synthesized according to known procedure (Shinkai et al. 1986). On the other hand, magnetic Fe3O4 nanoparticles were prepared by the chemical co-precipitation of Fe(III) and Fe(II) ions and the nanoparticles were modified directly by 3-(glycidoxy)-propyltrimethoxy silane (GPTMS) to introduce reactive glycidoxy groups onto the particles’ surface (Scheme 1). Then, the synthesized water-soluble p-sulfonated calix[n]arenes (n = 4,6) were immobilized onto 3-(glycidoxy)-propyltrimethoxy functionalized magnetic nanoparticles (EP-MN) in DMSO/H2O to give p-C[4]-MN and p-C[6]-MN sorbents (Scheme 2). It is well-known that in extraction processes, separation is a time consuming task. In view of this, we prepared the p-sulfonated calix[n]arene (n = 4,6) derivatives immobilized magnetic nanoparticles for easy separation of these from solvents by using a magnet (Fig. 2). The synthesized p-C[4]-MN and p-C[6]-MN sorbents have been characterized by FT-IR spectroscopy, TGA, and scanning electron microscopy (SEM) analyses techniques.

Fig. 2
figure 2

Schematic illustration of magnetic separation of azo dyes from aqueous solution

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Thermal properties of GPTMS-modified Fe3O4 (EP-MN) and p-sulfonated calix[4,6]arene immobilized magnetic nanoparticles (p-C[4]-MN and p-C[6]-MN) were analyzed by the thermogravimetric method. The indication of the coating formation on the magnetic nanoparticles’ surface can be obtained from TGA measurement. Figure 3 shows TGA curves of GPTMS-MN, p-C[4]-MN, and p-C[6]-MN magnetic nanoparticles. The total losses temperature range of 40–900°C are 9.5 %, 30.1 %, and 34.2 % for GPTMS-MN, p-C[4]-MN, and p-C[6]-MN, respectively. Thermogravimetric results showed a direct relationship of the loss of mass to the amount of the calixarene molecules anchored on the nanoparticle surfaces. Also, from the Fourier transform infrared spectroscopy (FTIR) results, it was observed that compound 3 and 6 were immobilized onto EP-MN. Exemplary, the FT-IR spectra of compound 3 and p-C[4]-MN was depicted in Fig. 4 The peaks at 1459, 1,409 cm−1 for p-C[4]-MN (1457, 1,410 cm−1 for p-C[6]-MN) are attributed to the bending vibration of the aromatic C=C bonds of the calixarene derivatives. Additional peaks centered at 1085, 913 cm−1 for p-C[4]-MN (1082, 916 cm−1 for p-C[6]-MN) were most probably due to the symmetric and asymmetric stretching vibration of framework and terminal Si–O groups.

Fig. 3
figure 3

TGA curves of EP-MN, p-C[4]-MN, and p-C[6]-MN magnetic nanoparticles

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Fig. 4
figure 4

The FT-IR spectra of compound 3 and p-C[4]-MN

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SEM is a kind of widely used surface analysis technique. It can examine the surface morphology of the solids rapidly without damages to the surface. To confirm the immobilization of the calixarenes on the magnetic nanoparticles surface, we chose the SEM technique to characterize the magnetic nanoparticles surface and displayed it in Fig. 5. A SEM image of the GPTM-MN (Fig. 5a) was compared with images obtained for the p-C[4]-MN and p-C[6]-MN (Fig. 5b, c). The photographs were taken at 5,000 magnification. The SEM micrograph of EP-MN (Fig. 5a) shows a very loose morphology, while after the immobilization of p-sulfonated calix[n]arenes onto the EP-MN (Fig. 5b, c); it shows a regular morphology covered by foreign materials, i.e., p-sulfonated calixarenes. These changes occurred onto the surface of EP-MN confirms the immobilization.

Fig. 5
figure 5

SEM micrographs of EP-MN (a), p-C[4]-MN (b), and p-C[6]-MN (c) magnetic nanoparticles

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3.2 Sorption Studies of Selected Aromatic Amines

Azo dyes are readily decolorized by splitting the azo bond(s) in an anaerobic environment. Azo dye reduction leads to the formation of aromatic amines. Aromatic amines are generally not degraded and accumulate under anaerobic conditions (Neill et al. 2000) with the exception of a few compounds characterized by the presence of hydroxyl and/or carboxyl groups (Razo-Flores et al. 1997). In order to evaluate the aromatic amines removal efficiency of p-C[4]-MN and p-C[6]-MN magnetic nanoparticles, solid–liquid sorption experiments were carried out at different pHs (3.0, 5.0, 7.0, and 8.5). The aromatic amines removal was analyzed by means of HPLC using acetonitrile-water as mobile phase at 280 nm (Fig. 6). The results of the sorption studies are summarized in Table 1 and also depicted in Fig. 7. We reported that the parent calixarenes (1, 2, 4) showed less sorption capacities for all three aromatic amines in our previous studies (Erdemir et al. 2009; Akceylan et al. 2009). After the parent calixarenes were converted to p-sulfonated calix[4,6]arene derivatives and immobilized onto magnetic nanoparticles, the obtained p-C[4]-MN and p-C[6]-MN magnetic nanoparticles showed high sorption ability towards all aromatic amines. The results obtained at pH 5.0, 7.0, and 8.5 are close to each other but different from those obtained at pH 3.0, which show significantly stronger interactions. Furthermore, it was observed that the percentage of benzidine removal was 61–72 % for p-C[4]-MN and 67–76 % for p-C[6]-MN when the pH of the benzidine solution was 5.0–8.5. However, benzidine removal attained a maximum to 78 % for p-C[4]-MN and 94 % for p-C[6]-MN when the amine solution pH decreased to 3.0. On the other hand, the HPLC results indicated that p-C[6]-MN is a more effective sorbent towards α-naphthalamine.

Fig. 6
figure 6

Representative HPLC chromatograms of standard aromatic amines solution (a) and aromatic amines which were treated with the p-C[6]-MN (b), in double distilled water samples. 1 benzidine, 2 p-chloroaniline, 3 α-napthylamine. Conditions—mobile phase, acetonitrile (A) and water (B); flow rate, 1 mL/min, at 25°C, injection volume 20 μL; gradient elution—0 min, 20 % (A) and 80 % (B); 25 min, 80 % (A) and 20 % (B). Detection at 280 nm, pH 8.5

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Table 1 Percent sorption of aromatic amines by sorbents (%)a
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Fig. 7
figure 7

pH effect on the sorption of aromatic amines by p-C[4]-MN (a) and p-C[6]MN (b)

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The higher level of aromatic amine removal by p-C[4]-MN and p-C[6]-MN magnetic nanoparticles suggests that a Coulomb interaction and intermolecular hydrogen bonds exist between the sulfonate groups of calixarene and the amino groups of aromatic amine (Fig. 8). It is seen from Table 1 that p-C[6]-MN forms a stable complex than p-C[4]-MN with a guest molecule by entrapping it into the cavity. Previously, we have reported (Gungor et al. 2008) the synthesis and inclusion abilities of calix[4] and calix[8]arene derivatives for selected azo dyes. The results showed that calix[4]arene and its derivatives have no influence on the extraction of azo dyes. Comparing to calix[8]arene derivatives, the molecular size of calix[4]arene derivatives is smaller and has higher steric hindrance. For this reason, it will be very difficult for azo dye molecules to enter into the cavity of calix[4]arene, resulting in no complex formation. Hence, the cyclic structure, the cavity size, and the functional groups of the calix[n]arene derivatives were found to be the decisive factors for the sorption of aromatic amines.

Fig. 8
figure 8

Proposed interaction model between p-C[6]-MN and α-naphthalamine

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4 Conclusion

In summary, the preparation of two new water-soluble p-sulfonated calix[n]arenes (n = 4,6) immobilized magnetic nanoparticles was successfully achieved. They were utilized to extract selected toxic and carcinogenic aromatic amines from aqueous solution. It was observed that the sorption capacities of the prepared magnetic nanoparticles containing calixarene (p-C[4]-MN) and (p-C[6]-MN) were higher than that of parent calix[n]arenes. Also, it was observed that the highest sorption level for the selected aromatic amines was obtained at pH 3.0. It was concluded that the sorption of aromatic amines by p-C[4]-MN and p-C[6]-MN sorbents indicate that amino and sulfonic acid groups are responsible for the formation of hydrogen bonds and electrostatic interactions.