Synthesis of β-cyclodextrin and starch based polymers for sorption of azo dyes from aqueous solutions

Bioresource Technology 99 (2008) 526–531

Synthesis of -cyclodextrin and starch based polymers for sorption of azo dyes from aqueous solutions Elif Yilmaz Ozmen, Mehmet Sezgin, Aydan Yilmaz, Mustafa Yilmaz

¤

Selçuk University, Department of Chemistry, 42031 Konya, Turkey Received 5 September 2006; received in revised form 3 January 2007; accepted 8 January 2007 Available online 12 March 2007

Abstract Three -cyclodextrin (polymers 1–3) and a starch-based (polymer 4) polymers were synthesized using hexamethylene diisocyanate (HMDI) as a cross-linking agent in dry dimethylformamide and used as a sorbent for the removal of some selected azo dyes from aqueous solutions. The cross-linked polymers were characterized by Fourier transform infrared spectroscopy, thermogravimetric and diVerential scanning calorimetric analysis. Results of sorption showed that cyclodextrin and starch based polymers can be eVectively used as a sorbent for the removal of anionic azo dyes. The InXuence of the amide groups and the chemical structure of azo dyes are also studied. Results of sorption experiments showed that these adsorbent exhibited high sorption capacities toward Direct Violent 51 (80% for polymer 1, 69% for polymer 2, 70% for polymer 3 and 78% for polymer 4). The sorption capacity of dyes on the polymers was dependent on the presence of sulfonate groups of the anionic dyes. In order to explain the results an adsorption mechanism mainly physical adsorption and interactions such as hydrogen bonding, ion-exchange due to the nature of the polymer network and the formation of an inclusion complex due to the -CD molecules through host–guest interaction is proposed. © 2007 Elsevier Ltd. All rights reserved. Keywords: -cyclodextrin (CD); Starch; Azo dye; Solid phase extraction

1. Introduction Dyes have been the subject of much interest in recent years because of increasingly stringent restriction on the organic content of industrial eZuents. Many industries (plastics, paper, textile and cosmetics) use dyes in order to color their products. Over 100,000 commercially available dyes exist and more than 7 £ 105 tones per year are produced annually (Pearce et al., 2003; McMullan et al., 2001). Due to their good solubility, synthetic dyes are common water pollutants and they may frequently be found in trace quantities in industrial wastewater. Many of these dyes are also toxic and even carcinogenic and this pose a serious hazard to aquatic living organism (O-Neill et al., 1999; Vandevivere et al., 1998). Methods of dye wastewater treatment have been reviewed (Crini, 2006; Pokhrel and Virar*

Corresponding author. E-mail address: [email protected] (M. Yilmaz).

0960-8524/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.01.023

aghavan, 2004; Robinson et al., 2001; Fu and Viraraghavan, 2001; Slokar and Majcen Le Marechal, 1998; Banat et al., 1996; Cooper, 1993). During the past three decades, several physical, chemical and biological methods, such as Xocculation, membrane-Wltration, ozonation, coagulation, precipitation, adsorption, fungal decolarization, etc. are used for the removal of dyes from an eZuent (Jain et al., 2003; Ho, 2003; Derbyshire et al., 2001; Dabrowski, 2001). Among these numerous techniques, adsorption process is a procedure of choice for the removal of organic compounds from wastewater. With these methods, a number of non-conventional, low cost adsorbents have been tried for dye removal. These include wood (Asfour et al., 1985), Fuller’s earth and Wred clay (McKay et al., 1985), Xy ash (Khare et al., 1987), biogas waste slurry (Namasivayam and Yamuna, 1992a,b, 1993, 1994, 1995), waste orange peel (Namasivayam et al., 1996), banana pith (Namasivayam and Kanchana, 1992; Namasivayam et al., 1993), peat (McKay, 1982), chitin (McKay et al., 1983),

E.Y. Ozmen et al. / Bioresource Technology 99 (2008) 526–531

chitosan (Juang et al., 1997) silica (McKay, 1984), starch and starch derivatives (Crini and Morcellet, 2002; Delval et al., 2002; Janus et al., 1999; Crini, 2003). In recent years much attention has been paid to chemical separation techniques and the design and synthesis of new extraction reagents for ions and molecules. This attention results in part from environmental concerns, eVorts to save energy and recycling at the industrial level. In this respect, the supramolecular chemistry has provided a much better solution to the 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 delineate a suitable binding site. This was achieved with the development of macrocyclic molecules such as synthetic crown ethers, cryptands, spherands (Asfari et al., 2001), calixarenes (Gutsche, 1998; Yilmaz et al., 2006) and natural cyclodextrins (Bhaskar et al., 2004; Del Valle, 2004). Cyclodextrins (CDs) are glucose-based molecules and produced from the enzymatic degradation of starch by bacteria. They are cyclic oligosaccharides consisting of 6 (), 7 (), 8 () glucopyranose units, which are joined together by (1–4) linkage forming a torus-shaped ring structure. These compounds when cross-linked with suitable monomers form an insoluble resin which exhibits speciWc adsorption based on inclusion complex formation (Crini, 2003; Lee et al., 1999; Bhaskar et al., 2004; Yu et al., 2003; GaVar et al., 2004). The most characteristic feature of CDs is the ability to form inclusion compounds with various organic molecules through host–guest interactions: the interior cavity of the molecule provides a relatively hydrophobic environment into which an apolar pollutant can be trapped. Several review articles were devoted to the detailed description of the industrial applications of CDs (Szejtli, 1998; Del Valle, 2004; Crini and Morcellet, 2002; Singh et al., 2002; Hedges, 1998). -cyclodextrin based materials were used as sorbent in the waste water by Crini and Peindy (2006). They found -cyclodextrin based materials exhibited high adsorption capacities toward phenolic molecules and dyes. In particular the -cyclodextrin based polymers are very diVerent in removing acid, direct, disperse and reactive dyes from solutions. Recently, they prepared cyclodextrin based polymers containing carboxyl groups from reticulation of -CD using epichlorohydrin in the presence of carboxymethyl cellulose for the sorption of C.I. Basic Blue 9 (Crini and Peindy, 2006). The aim of this work is to prepare  CD and starch based sorbents containing urethane groups and to investigate the eYciency of these materials as a solid phase extraction materials for the selected anionic dyes. The inXuence of several parameters (contact time, concentration, NaCl amount and pH) on the adsorption capacity of the synthesized polymers is evaluated and discussed. 2. Methods The azo dyes direct violent 51 (DV-51), methyl orange (MO), and tropaeolin 000 (TP) were obtained from Sigma

527

(St. Lois, MO, USA). Hexamethylenediisocyanate (HMDI) and starch were procured from Aldrich, and -cyclodextrin, N,N⬘-dimethylformamide (DMF) from Merck. IR spectra were recorded on a Perkin Elmer 1605 FTIR spectrometer as KBr pellets. UV–vis. spectra were obtained on a Shimadzu 160A UV–visible recording spectrophotometer. Sodium determinations were made on a Jenway PFP7 Xame photometer. Thermogravimetric analysis (TGA) was carried out with Perkin Elmer Pyris 1 thermogravimetric analyzer. The sample weight was 13–15 mg. Analysis was performed from room temperature to 800 °C at a heating rate of 20 °C/ min in nitrogen atmosphere with a gas Xow rate of 40 ml/ min DSC was performed from 0 to 300 °C at a heating rate of 10 °C/min under a nitrogen atmosphere with a gas Xow rate of 40 ml/min. The analysis was carried out in sealed aluminum pans and the amount of the sample was 8–10 mg. All aqueous solutions were prepared with deionized water that had been passed a Millipore milli-Q Plus water puriWcation system. Commercial grade solvents such as chloroform, methanol, toluene, ethyl acetate and hexane were distilled and stored over 4 Å molecular sieves. 2.1. Preparation of polymers The cross-linked polymers have been prepared in one step by -CD or starch using only hexamethylene diisocyanate (HMDI). The synthetic procedure has already been described in detail (Yilmaz et al., 2007). A typical reticulation reaction was carried as follows (polymers 1–3): -CD or starch (2.0 g) was dissolved in 10 ml of dry DMF in 100 ml round bottom Xask at room temperature. Then various amounts (1.76 mmol for polymer 2, 6.55 mmol for polymer 1, 17.6 mmol for polymer 3 and 6.55 mmol for polymer 4) of hexamethylenediisocyanate (HMDI) were added dropwise. Then the solution was stirred at 70 °C for 4 h. The precipitated polymer (when the ratio of CD to HMDI was taken 1:1, there was not seen any precipitation at the end of the reaction period and in this case the polymer was precipitated by the addition of ethanol) was Wltered and washed with water and ethanol. The product was dried under vacuum for 24 h. 2.2. Sorption method The sorption capacity of the synthesized polymers was determined by the following technique: 25 mg of the sorbent was mixed with 10 ml of aqueous solution of the azo dyes (concentration 2 £ 10¡5 mol l¡1) in a stopped Xask that was shaken 150 rpm in a horizontal shaker water bath at room temperature. The experiments were performed in triplicate. The sorbent was removed by centrifugation before measurements. The residual concentration organic solute was determined by spectrophotometer and absorbance values were recorded at max for each solution: at 484 nm for tropaeolin 000 (TP), 549 nm for direct violent 51 (DV-51), 463 nm for methyl orange (MO), (see Fig. 1 for the structure formulae). The adsorption capacity was then

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SO 3Na

OMe

N N

H3 C CH3

N N

SO 3Na

HO

H3 C

NH

Direct Violet 51

SO 3 Na

N

N

N

CH3 CH3

Methyl Orange

OH N

N

SO3Na

Tropaeolin 000 (Orange II ) Fig. 1. Chemical structure azo dyes.

calculated and expressed in percentage uptake (%) which represents the ratio between the amount of adsorbed dye and the starting amount of dye (McKay et al., 1982; Crini, 2003; El-Geundi, 1991). 3. Results and discussion The polymers have been prepared in one step by reticulation of -CD or starch using HMDI as cross-linking agent to obtain insoluble CD polymers. -Cyclodextrin– polyurethane resins were synthesized by reacting of -CD with diVerent amount of HMDI (1:1, 1:3.7, 1:10 molar ratios) in dry DMF. The polymerization reaction was dependent on the molar ratio of the cyclodextrin and the HMDI (Table 1). For example, there was not observed any precipitation in the reaction period while the polymer 2 was obtained by the addition of ethanol when the molar ratio was less than 1:1. The cross-linked product was precipitated in a short time when the molar ratio was 1:3.7 (polymer 1) and 1:10 (polymer 3). Table 1 Experimental conditions of the synthesis and characteristics of the polymers used during this study Polymer

-CD (g) HMDI (mol) CD/HMDIa Nb (%) HMDId

Polymer 1 Polymer 2 Polymer 3 Polymer 4

2.0 2.0 2.0 2.0c

a b c d

6.55 1.76 17.60 6.55

1:3.7 1:1 1:10 1:3.7

5.12 3.80 7.07 8.07

1.83 1.36 2.54 2.86

In molar ratio. Nitrogen content from elemental analysis. Starch is used. HMDI content in mmol/1 g polymer from elemental analysis.

The cross-linked polymers were found to be insoluble in water as well as other solvents. The consumption of isocyanate was followed by measuring the absorbance of the – NBCBO antisymmetric stretching band at 2275 cm¡1 (corresponding of isocyanate group). By completion of reaction, urethane was present as seen from the 1715 cm¡1 NACBO band. The thermal stability of the -CD and starch polymers was evaluated by using TGA and compared with their precursors. It is seen that the -CD polymers undergo two step thermal degradation. The Wrst step can be mainly attributed to decomposition of -CD, while the second one is mainly to the polymer. In the literature (Li et al., 2004) CD starts to decompose at temperatures ranging from 290 to 300 °C. The onset of degradation of CD polymers was found to occur around 325 °C for polymer 3, around 350 °C for polymer 1 which is quite high compared to conventional polyurethanes which generally start to decompose around 200– 220 °C. The weight loss is maximum 44–45%, during the Wrst stage of thermal decomposition which is due to the cleavage of the -CD units and volatilization of the resultant products of decomposition. The starting temperature of weight loss for starch based polymer (polymer 4) is lower than that of -CD polymers. It was observed that polymer 4 shows that there is no weight loss until 319 °C. At this temperature the weight losses of polymer 4 is around 45%. The diVerential scanning calorimeter studies showed that no transition was observed up to 250 °C. The absence of the glass transition was due to the highly cross-linked nature of polymer. It should be noted that -CD polymers were previously studied and also present no thermal transition before decomposition (Li et al., 2004). The endothermic peak due to the decomposition of the material around 272 °C was the only transition found. The diVerence in the temperatures of decomposition for TGA and DSC was due to the diVerent heating rates adopted for the two techniques. The enhanced thermal stability of all polymers are attributed to the cross-linked nature, it is well-know that cross-linking leads to enhancement of the thermal stability. 3.1. Adsorption behavior of resins toward azo dyes Polymeric sorbents have several advantages over silicabased materials for the extraction of water samples, which leads to higher recovery and avoids the loss of analytes during extraction (Bhaskar et al., 2004). Since removal of azo dyes from aqueous media is environmentally important, their extraction or sorption from aqueous samples was investigated. Fig. 2 shows the sorption capacity of three azo dyes on four polymers, polymers 1–3 (-CD based polymers) and polymer 4 (a starch based polymer). All polymers exhibited higher sorption capacities. However, these polymers exhibit approximately same sorption capacity toward all the azo dyes except DV-51 which contains more sulfonates and azo groups than the others. Hydroxyl and amide groups in the polymer form intermolecular hydrogen bonds between sulfonate groups of DV-51 and the polymer. These results

E.Y. Ozmen et al. / Bioresource Technology 99 (2008) 526–531

100

100

polymer 1 polymer 2 polymer 3 polymer 4

polymer 3 polymer 2 polymer 1

Sorption capacity (%)

80

80

Sorption capacity (%)

529

60

polymer 4 60

40

20

40

0 5

20

7

8.5

9.5

11

pH Fig. 3. pH eVect on per cent sorption (polymers; contact time 1 h , NaCl : 0.4 mol l¡1 , DV-51 concentration 2 £ 10¡5 mol l¡1).

0 DV-51

100

MO

Fig. 2. Comparison between sorption capacity (expressed in percentage uptake) of three azo dyes on polymers. (Polymer dose 25 mg, contact time 1 h, concentration 2 £ 10¡5 mol l¡1, NaCl: 0.4 mol l¡1, pH 11).

were in agreement with the literature data published (Crini, 2003) by using -CD based polymers with epichlorohydrin. It was assumed that the formation of inclusion complex due to the -CD molecules and the presence of other interactions (hydrogen bonding and there are probably very strong hydrophobic interactions between azo dyes and polymer). Comparing the values obtained for starch-based polymer (polymer 4) with the other polymers (polymers 1– 3), however, nearly same sorption capacity was observed. In the literature (Crini, 2003; Delval et al., 2003), when starch was used for the sorption of an anionic azo dye (Direct Red 81), lower sorption capacities was observed. This clearly indicates that the incorporation of amide groups into the polymer 4 network plays a major role. To evaluate the inXuence of the pH on the sorption of the dye, tests were achieved at diVerent pH (5, 7, 8.5, 9.5 and 11) in water with DV-51 (Fig. 3). The values of the adsorption capacity for DV-51 increased with increasing of the pH. It is well-known that the inclusion complex with -CD and aromatic derivatives depend on pH (Crini, 2003). It was observed that the percentage of colour removed was 32.5% for polymer 1 when the DV-51 solution pH was 5.0 and it attained maximum of 80% for same polymer when dye solution pH increased to 11. In this experimental condition, it is well known that deprotonation reaction of hydroxyl groups is very diYcult even at pH 11. –CH2–OH , CH2–O¡ + H+ pKa of the reaction is 18 (Smith and March, 2001). Thus, electrostatic repulsion does not occur between the dye and sorbent. This demonstrates that the three dyes which have sulfonate groups in their structure are able to interact with the amide and hydroxyl protons of the polymers.

Sorption capacity (%)

TP

80 60 40 TP 20

MO DV 51

0 15

30

60 90 120 Contact time (min)

150

180

Fig. 4. EVect of contact time on per cent sorption of dyes by a -CD-based polymer (polymer 1, NaCl: 0.4 mol l¡1, dye concentration 2 £ 10¡5 mol l¡1 pH 11).

As expected, Fig. 4 shows on per cent adsorption capacity of polymer versus the contact time (stirring time) toward the azo dyes used. The kinetics of sorption was fast and the maximum capacity was obtained after 1 h except MO. Fig. 5 shows the inXuence of the amount of NaCl on per cent adsorption of CD polymers toward DV-51. The molar concentration of NaCl was varied between 0.08 and 0.7 mol l¡1. Addition of sodium chloride to aqueous solutions produced an increase of the performance of the polymers. Probably, NaCl minimizes electrical charge on the surface of the polymer and increases the sorption. Similar results have been published by (Shao et al., 1996; Crini, 2003). Fig. 6 shows on per cent sorption versus polymer 1 dose using contact time at 1 h. The molar concentration of dyes (TP and MO) was kept constant while the quantity of polymer 1 was varied between 10 and 50 mg. It was observed that the adsorption capacity of TP–MO was increased with increasing of polymer dose.

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for guest molecules to gain access to modiWed -CD cavity and to have bigger inclusion constants. However the CD polymers were found to be eVective adsorbent toward azo dyes. The possible eVects are hydrogen bonding and other physical surfaces adsorption in the polymeric network. It is important for the long term stability and reproducibility of the sorption properties of the polymers. The polymers were easily regenerated using ethanol as washing solvent by soxhlet extraction. It was observed that the sorption capacity was not changed after this treatment. This showed the chemical stability of the polymers and reproducibility of the values.

100

Sorption capacity (%)

polymer 1 80

60

40

20

0 0

0.09

0.17

0.34

0.68

NaCl concentration (mol/l) Fig. 5. EVect of NaCl concentration on per cent sorption (contact time 1 h; concentration 2 £ 10¡5 mol l¡1, pH 11).

80 TP MO

Sorption capacity (%)

60

40

20

4. Conclusion This study was carried out to synthesize water insoluble three -CD and starch based polymers with HMDI. These materials were characterized FTIR, TGA and DSC analysis. They have been evaluated for the extraction of azo dyes from water. The results indicate that these materials are high-capacity sorbents for the azo dyes. However, the sorption was dependent on the presence of amide and sulfonate groups. Results of sorption experiments showed that -CD and starch based polymers exhibited high sorption capacities toward Direct Violent 51 (80% for polymer 1, 69% for polymer 2, 70% for polymer 3 and 78% for polymer 4) which contains more than one sulfonate group. According to these results, it assumed that in the adsorption mechanism, hydrogen bonding, hydrophobic interactions (pollutant–polymer and pollutant–pollutant interactions), complexation and acid–base interactions between the sorbent and the pollutant, physical adsorption due to the polymer network and chemical interactions of solute dyes via ion exchange are all involved. Acknowledgement

0 10

25

50

Polymer dose (mg) Fig. 6. InXuence of the polymer dose on per cent sorption by polymer 1 (contact time 1 h, NaCl: 0.4 mol l¡1 dye concentration 2 £ 10¡5 pH 11).

Using these preliminary results, we can conclude that the major role of anionic azo dyes sorption is hydrogen bonding due to the amide and hydroxyl protons. Besides, formation of an inclusion complex due to the -CD molecules through host–guest interaction is also important. According to the literature (Guo et al., 2004), it has been concluded that the modiWed -CD (methylated) exhibited stronger binding ability than the parent -CD implying that the capacity of the modiWed -CD provided a better protective microenvironment. Strong inclusive ability can be understood from the fact that the substitution by alkyl groups leads to the enlargement of the bigger opening of the -CD cavity and the contraction of smaller opening, and destroys the strong hydrogen bond network, which makes it easier

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