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Keywords:

  • Adsorption;
  • Diphenyl methane diisocyanate;
  • Ultrasound;
  • Water pollution

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

A crosslinked β-cyclodextrin polymer (β-CD-MDI), synthesized by the modification of β-cyclodextrin (β-CD) with crosslinking reagent diphenyl methane diisocyanate (MDI), was prepared and used as an adsorbent in the treatment of phenol wastewater ultrasonic aided. An infrared spectroscopy test was conducted to characterize β-CD-MDI. The adsorption capacity of phenol in wastewater was determined by assessing the corresponding parameters, such as the amount of adsorbent β-CD-MDI, the initial concentration of phenol, the influence of pH and ultrasonic intensity. The adsorption efficiency of β-CD-MDI is up to 87.3% with an amount of crosslinked polymer of 40 g/L, an initial phenol concentration of 100 mg/L and a wastewater pH of 6. The application of ultrasound can facilitate the purification of wastewater and reduce the adsorption time remarkably.

Abbreviations
CD

cyclodextrin

β-CD

β-cyclodextrin

DMF

N,N-dimethylformamide

MDI

methane diisocyanate

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

The developments of industry production cause a dramatic increase of industrial wastewaters, which are considerably harmful to the ecological environment and human health. Although traditional methods of water decontamination could partly mitigate water pollution, most of treatments were chemical, costly, and complex [1-5]. Physical absorption, which possessed the advantages of non-expensive and environmental friendly was increasingly used [6-9].

Cyclodextrins (CDs), natural cyclic oligosaccharides, arouse great interest for its configuration with a hydrophobic inner cavity and a hydrophilic outer surface [10-12]. In recent years, due to the controllable cavity structure, CDs are widely applied to the field of medicine, food, and chemical industry, and especially served as physical absorbent in wastewater treatment [13-18]. However, pure CDs have some solubility in water, resulting in secondary pollutions. Exploitation of sorbents containing modified CDs for isolating specific materials commenced in the late sixties. In 1979, CDs were applied first in the treatment of wastewater when Tabushi et al. [19] modified CDs on polystyrene to eliminate organic substances from pharmaceutical wastewater. Crini et al. [20] prepared β-cyclodextrin polymer (β-CD) by polymerization using epichlorohydrin as a crosslinking agent to separate aromatic pollutants. Salipira et al. [21] modified β-CD with carbon nanotubes for removal trichloroethylene. Zhao et al. [22] reported that β-CD polymer modified by sulfonate groups to adsorption of methylene blue and basic magenta. Nevertheless, the phenol adsorption of as-prepared product is rarely reported [23, 24]. In this work, we present the crosslinked β-cyclodextrin polymer (β-CD-MDI) to eliminate the pollutants in industrial wastewater, especially, an ultrasonic-assisted [25-28] method could be employed to improve adsorption efficiency and reduce the adsorption time.

2 Materials and methods

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

2.1 Chemicals

β-CD was purchased from Sinopharm and purified by recrystallization, diphenyl methane diisocyanate (MDI) was obtained from Sigma–Aldrich, N,N-dimethylformamide (DMF) was obtained from Sinopharm and purified by vacuum distillation, phenol, and triethylenediamine were obtained from Sinopharm, chloroform was purchased from Shanghai Zhongshi, dibutyltin dilaurate (BTL) was obtained from Shanghai Fourth Reagent Factory.

2.2 Synthesis of crosslinked β-CD

Series of crosslinked β-CD were synthesized according to the combination of the type of β-CD and crosslinker, as described in Fig. 1. A proper amount of β-CD (4.09 g) and 0.04-g complex catalyst of dibutyltin dilaurate/triethylenediamine (1:2) was added in DMF (25 mL) and stirred protected by nitrogen atmosphere. A solution of MDI (7.98 g) was added in DMF (5 mL) after the solution reached an appropriate temperature, and the reaction mixture was heated at 80°C for 10 h under nitrogen atmosphere. Terminator was then introduced to end the polymerization. After that, the samples were taken out to wipe off the impurities with deionized water and then dried in the vacuum. Finally, samples were grinded for further use.

image

Figure 1. Preparation of crosslinked polymers.

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2.3 Adsorption experiments

β-CD-MDI was added to phenol-contained solutions of different concentrations, with mechanical agitation. Meanwhile, ultrasonic treatment was carried out as assistance. All experiments were performed under the following conditions: temperature of 40°C, stirring speed of 150 rpm, phenol solution volume of 100 mL with the desired adsorption time. At specified intervals of time, the residual solution was sampled, then centrifugally separated and filtrated. Spectrophotometer was conducted to measure the absorbance of the sample in the phenol solutions. The adsorption efficiency of the phenol was calculated from the concentration difference of phenol before and after adsorption [23, 29].

3 Results and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

3.1 Infrared spectrum analysis of crosslinked β-CD

IR spectra of β-CD (1), MDI (2), and β-CD-MDI (3) are shown in Fig. 2. In these spectra, β-CD and β-CD-MDI have stretching vibration absorption peaks of –C–H at 2927/cm and cavity peaks of β-CD at 1162/cm [18], which shows that the crosslinked polymer still retains the skeleton structure of the β-CD. In the IR spectra of β-CD-MDI, the characteristic absorption peak of –N–H bond appears at 1655/cm [30], and the characteristic absorption peak of carboxylate –C=O appears at 1722/cm [31]. An obvious characteristic peak of –NCO at 2280/cm for MDI sample is observed, whereas this peak nearly disappears in the β-CD-MDI sample. It can be explained by the fact that –NCO functional group and hydroxyl group react and generate urethane group.

image

Figure 2. IR spectra of β-CD (1), MDI (2), and β-CD-MDI (3).

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3.2 Phenol wastewater treatment of β-CD-MDI

3.2.1 Effect of the amount of β-CD-MDI

Phenol adsorptions of β-CD-MDI were carried out in the different amount of samples (20–50 g/L). Results can be seen in Fig. 3. The adsorption capacity of β-CD-MDI enhances as the amount of β-CD-MDI increases. The adsorption efficiencies of samples in phenol solution at different amounts of β-CD-MDI 20, 30, 40, 50 g/L are 72.2, 78.4, 87.3, and 88.5%, respectively. As can be seen in Fig. 3, the adsorption efficiencies of samples increase sharply in the first 60 min, and have a turning point at 60 min. In brief, the adsorption equilibrium for the whole system is nearly achieved after 60 min of adsorption time. Results showed that increasing the amount of β-CD-MDI helped to raise the adsorption efficiencies, but the optimum value might be 40 g/L.

image

Figure 3. Effect of the amount of β-CD-MDI on the adsorption of phenol in different time at pH 6.0, phenol = 100 mg/L.

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The reason could be that the crosslinking reaction led to the generation of hydrophobic β-CD, which induced the interaction with hydrophobic guests. Therefore, inclusion complexes were formed through this host–guest reaction process. As seen in the Fig. 3, it can be found that the adsorption exhibits an appreciable adsorption capacity.

3.2.2 Effect of pH

In Fig. 4, it can be observed that the adsorption efficiency of the as-prepared product decreases with the enhancement of caustic or acidic solutions. The best adsorption can reach 87.3% at pH 6. The adsorption efficiencies are >80% when the pH is between 5 and 8. Because in acidic conditions, the hydrogen bonds are dominant in the host–guest assembly, which are formed between the primary hydroxyl group of the β-CD and the hydroxyl group of phenol. In terms of the above-mentioned results, the pH of solution may be controlled in the range of 5–7 in the wastewater treatment process.

image

Figure 4. Effect of pH on the adsorption of phenol at β-CD-MDI = 4 g/L, phenol = 100 mg/L, adsorption time = 120 min.

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3.2.3 Effect of initial concentration of phenol

In order to investigate the effect of initial concentration of phenol, the same amount of CD was joined in the different initial concentrations of phenol. As shown in Fig. 5, the adsorption efficiency of β-CD-MDI decreases with the increase in initial concentration of phenol. Less the initial concentration is, better the adsorption of phenol possessed. The adsorption efficiencies of samples in phenol solution at initial concentration of phenol = 100, 150, 200, 250 mg/L are 87.3, 85.1, 83.2, and 79.0%, respectively. The experimental results indicate that due to the predetermined amount of β-CD-MDI, the hollow cavities are not enough to include the growing number of phenol molecules.

image

Figure 5. Effect of initial concentration of phenol on the adsorption of phenol in different times at pH 6.0, β-CD-MDI = 4 g/L.

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3.3 Ultrasound-assisted phenol wastewater treatment with β-CD-MDI

3.3.1 Effect of the amount of β-CD-MDI

To investigate the effect of the amount of β-CD-MDI with ultrasound-assisted; the different amounts of CD were joined in the same concentrations of phenol. The amounts of β-CD-MDI were adjusted in the range of 20–50 g/L. The result presented in Fig. 6 demonstrates that the adsorption capacity of β-CD-MDI increases as the increasing of β-CD-MDI. The adsorption efficiencies of samples in phenol solution at β-CD-MDI = 20, 30, 40, 50 g/L are 77.3, 84.7, 89.5, and 92.3%, respectively. The adsorption efficiency of as-prepared polymer can increase 4 to 6% in the process with ultrasonic-assisted. As can be easily seen in the figure, the adsorption time reduces, and the adsorption equilibrium for the whole system is nearly achieved after 30 min of adsorption time. According to the experimental results, it is speculated that ultrasonic vibration can promote phenol molecule to enter the inner cavities of CD. The growth of the cavitation bubbles may occur by cavitation nucleus, and the collapse of cavitation bubbles must need a lot of energy [32]. The collapse process reduces the phenol molecules collision and competition, therefore promotes the phenol molecule entering into the inner cavities of CD.

image

Figure 6. Effect of the amount of β-CD-MDI under ultrasound. Experimental conditions: pH 6.0, phenol = 100 mg/L, ultrasonic frequency: 20 kHz, ultrasonic intensity: 0.2 W/cm2.

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3.3.2 Effect of pH

To evaluate the effect of pH on the adsorption of phenol in ultrasonic treatment, as shown in Fig. 7, the best adsorption can reach 89.5% at pH 6, and adsorption efficiency can increase by 2.2% comparing with those in Fig. 4. The trend of curve has a similarity with the individual use of as-prepared product. The adsorption efficiencies are >85% when the pH values are in the range of 5–8. Ultrasonic treatment may make the whole solution remain homogenized, and ultrasound provides the energy to reinforce the ability of the phenol molecule entering into the hollow cavities of CD.

image

Figure 7. Effect of pH under ultrasound. Experimental conditions: β-CD-MDI = 4 g/L, phenol = 100 mg/L, adsorption time = 120 min, ultrasonic frequency: 20 kHz, ultrasonic intensity: 0.2 W/cm2.

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3.3.3 Effect of initial concentration of phenol

We investigated the adsorption efficiency of β-CD-MDI compounds obtained via crosslinking reaction with different initial concentrations of phenol by using spectrophotometer analysis (Fig. 8). With the increase of initial concentration of phenol, the adsorption capacity of phenol decreases. Less the initial concentration is, better the adsorption of phenol will be. The adsorption efficiencies of samples in phenol solution at initial concentration of phenol = 100, 150, 200, 250 mg/L are 89.5, 87.0, 83.0, and 81.0%, respectively. It should be noticed that the adsorption efficiency increases slowly after 30 min, that is, the adsorption reaches equilibrium. The high concentration of phenol has a lot of phenol molecules, which lead to strong competition in the case of a same amount of positions of hollow cavities. From these results, ultrasonic vibration can promote phenol molecule to enter the hollow cavities of CD, therefore improving the adsorption efficiency.

image

Figure 8. Effect of initial concentration of phenol under ultrasound. Experimental conditions: pH 6.0, β-CD-MDI = 4 g/L, ultrasonic frequency: 20 kHz, ultrasonic intensity: 0.2 W/cm2.

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3.3.4 Effect of ultrasonic intensity

Different ultrasonic intensities were adjusted in the range of 0.05–0.2 W/cm2. The corresponding values of adsorption efficiency were measured and shown in Fig. 9. Results show that increasing ultrasonic intensity helps to raise the adsorption efficiency of β-CD-MDI. Certainly, the adsorption efficiency could reach the maximum value of 89.5% by regulating ultrasonic intensity to 0.20 W/cm2. The adsorption capacity increases with the increasing of ultrasonic intensity, namely, the positive correlation between the adsorption efficiency and ultrasonic intensity. The result also implies that the high ultrasonic intensity is favorable for the adsorption. This is due to the fact that the mechanism of ultrasound decomposes organics' cavitation effect of ultrasound, and ultrasonic can produce cavitation effect of ultrasound when ultrasonic intensity reaches the threshold of cavitation. The cavitation effect of ultrasound enhances with the increasing of the sound energy density, which promotes the phenol molecule into the hollow cavities of CD and diffusion speed. In an extremely high ultrasonic intensity, cavitation bubbles collapse was incomplete. The cavitation effect of ultrasound may be weakened at a higher intensity of ultrasound. According to the above-discussions, the increase in the rate of adsorption efficiency decreases as the ultrasonic intensity increases.

image

Figure 9. Effect of ultrasonic intensity. Experimental conditions: pH 6.0, β-CD-MDI = 4 g/L, phenol = 100 mg/L, ultrasonic frequency: 20 kHz.

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4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

The β-CD-MDI polymer was synthesized via polyaddition crosslinking reaction of β-CD with MDI. The IR result of the product exhibits that the crosslinked reaction immobilized urethane bond into the β-CD network, while the β-CD-MDI still possesses the original cavity structure of β-CD. The optimum adsorption efficiency of the β-CD-MDI is achieved at 87.3% (in the condition of 40 g/L polymer, the initial concentration of phenol 100 mg/L and pH 6). Interestingly, the ultrasound treatment could give rise to the adsorption enhancement of β-CD-MDI in phenol wastewater, and the highest adsorption can reach 92.3%. The results show that the treatment of ultrasound affords a superior absorption efficiency and lower absorption time.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References

This work was supported by the Cooperative Foundation of Nanjing University of Technology and Shanghai Baosteel Chemical Co., Ltd. (K10SHAM280).

The authors have declared no conflict of interest.

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  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. Acknowledgements
  8. References
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