Characterization
The characterization of the as-synthesized polymers was investigated by thermal gravimetric analyzer, scanning electron microscopy, Fourier transform infrared spectroscopy, and N2 adsorption-desorption method. The detailed introduction about instruments were provided in Supplementary data. Firstly, the structures of β-CD, β-CDP, HE-β-CD, HE-β-CDP, HP-β-CD, HP-β-CDP, Me-β-CD, Me-β-CDP and TFPN were characterized by FT-IR. As presented in Fig. 1, the characteristic absorption peaks of C ≡ N and C-F vibration are obviously observed at 2252 cm− 1 and 1260 cm− 1 in the FT-IR spectrum of TFPN, respectively. The absorption peaks of hydroxyl group at 3386 cm− 1, aliphatic C-H stretch at 2929 cm− 1, strong C-O stretch at 1033 cm− 1 are observed in the FT-IR spectrum of β-CD, HE-β-CD, HP-β-CD, and Me-β-CD. All the above characteristic absorption peaks of C ≡ N vibration, C-F vibration, hydroxyl group vibration, aliphatic C-H stretch, and strong C-O stretch are obviously observed in the FT-IR spectra of β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP, which indicates that the four polymers of β-CD, HE-β-CD, HP-β-CD, and Me-β-CD cross-linked with TFPPN have been successfully prepared.
The surface morphologies of four cyclodextrin polymers were observed by scanning electron microscope. As revealed in Fig. 2, the surface shapes of the four polymers are irregular, and it can be clearly seen that the four cyclodextrin polymers have rough surface morphology and uneven distribution of pores. This structure is conducive to the adsorption of pollutants. The specific surface areas and porosity of β-CDP、HE-β-CDP、HP-β-CDP and Me-β-CDP calculated by BJH method according to N2 adsorption-desorption isotherms (Fig. 3) were 20.09 m2·g− 1, 19.02 m2·g− 1, 21.37 m2·g− 1 and 23.51 m2·g− 1, respectively. The inset figures in Fig. 3 illustrate the pore size of the four CD polymers ranges from 2 nm to 5 nm, indicating β-CDP、HE-β-CDP、HP-β-CDP and Me-β-CDP have the characteristics of mesoporous materials.
The four cyclodextrin polymers were also characterized by thermogravimetry, exploring the stability of materials at different temperatures. Figure 4 shows that the thermal stability of the four cyclodextrin polymers is similar. The mass loss of water adsorbed on β-CDP, HE-β-CDP, HP-β-CDP and Me-β-CDP are 6.72%, 6.66%, 6.73% and 7.31% respectively in the temperature range of 25 oC to 100 oC. From 100 oC to 245 oC, the thermogravimetric analysis curves remain stable, suggesting that the synthetic cyclodextrin polymers are stable in this range of temperature studied. The mass loss of β-CDP, HE-β-CDP, HP-β-CDP and Me-β-CDP was 70.39%, 73.43%, 65.95% and 67.6% respectively in the temperature ranging from 245 oC to 325 oC, due to the decomposition of materials. When the temperature is higher than 325 oC, the carbon chains start to break down, making the decomposition rates slow down. The above results show that the prepared polymers have good thermal stability. Kinetics study
To study the adsorption equilibrium time of the four CD polymers for naphthol and naphthylamine, and the kinetics of the adsorption processes, the adsorption time are set from 0.5 to 180 min. As depicted in Fig. 5, the uptake capacities of the four cyclodextrin polymers for 1-naphthylamine, 2-naphthylamine, 1-naphthol and 2-naphthol increase rapidly at first, then increase slowly until to be constant with the increasing of the adsorption time. The adsorption equilibrium can be achieved within 60 min. In addition, the four CD polymers present fast adsorption kinetics towards naphthol and naphthylamine, attaining 93%~100% of adsorption equilibrium uptake for 1-naphthylamine, 2-naphthylamine and 1-naphthol and 87%~90% of equilibrium uptake for 2-naphthol in 15 min, as presented in Table 1.
The kinetics of the adsorption processes were analyzed by the pseudo first-order (PFO) kinetic model (Eq. S(3)) and pseudo second-order (PSO) kinetic model (Eq. S(4)) (Simonin 2016; Ho 2006). As displayed in Fig. S1 and Table S1, the adsorption process of the four cyclodextrin polymers for four naphthalene derivatives can be depicted well by PSO kinetic model, suggesting chemical adsorption may be involved in the above adsorption processes. The adsorption rate constant of HP-β-CDP for 1-naphthol (Table S1) is larger than that of the other three cyclodextrin polymers, while there is no significant difference in the adsorption rate constant (K2) of the four cyclodextrin polymers for 1-naphthylamine, 2-naphthylamine and 2-naphthol. Additionally, Fig. 5 and Table S1 also show that the equilibrium uptake capacity (Qe) of the four cyclodextrin polymers for the four substances has certain regularity. For 1-naphthylamine and 2-naphthylamine, the order of adsorption effect of the four cyclodextrin polymers is as follows: β-CDP > Me-β-CDP > HP-β-CDP > HE-β-CDP. For 1-naphthol and 2-naphthol, the order of adsorption effect of the four cyclodextrin polymers is as follows: Me-β-CDP > HP-β-CDP > β-CDP > HE-β-CDP.
Table 1
The Qt at 15 min vs. the Qe of the four CD polymers towards the naphthalene derivatives, %
CD polymers
|
1-naphthylamine
|
2-naphthylamine
|
1- naphthol
|
2- naphthol
|
β-CDP
|
94
|
96
|
97
|
89
|
HE-β-CDP
|
94
|
96
|
93
|
87
|
HP-β-CDP
|
97
|
97
|
100
|
90
|
Me-β-CDP
|
96
|
97
|
99
|
90
|
Adsorption Isotherm Studying
Adsorption isotherms play a significant part in the study of adsorption mechanism and practical application. Figure 6 shows the adsorption isotherms of four cyclodextrin polymers towards the four naphthalene derivatives at 25 oC. The uptake capacity increases with the increase of adsorption equilibrium concentration as demonstrated in Fig. 6. The nonlinear Langmuir, Freundlich, Redlich Peterson and sips adsorption isotherm models (Foo and Hameed 2010) (Eqs. (S5)-(S8)) were used to fit the above adsorption isotherms. The fitting results are shown in Table S2. Comparing the square of the correlation coefficient (R2) and chi square coefficient (χ2) of four adsorption isotherm models, it is found that the three-parameter Redlich Peterson or sips adsorption isotherm model can better fit the experimental data. In addition, Fig. 6 also shows that the order of adsorption effect of the four cyclodextrin polymers for 1-naphthylamine and 2-naphthylamine is as follows: β-CDP > Me-β-CDP > HP-β-CDP > HE-β-CDP, while the order of adsorption effect for 1-naphthol and 2-naphthol is as follows: Me-β-CDP > HP-β-CDP > β-CDP > HE-β-CDP. This is consistent with the results of kinetics studying.
Furthermore, the adsorption ratio of the four CD polymers towards the naphthalene derivatives has a significant increase with the decrease of the initial concentration of pollutants as displayed in Fig. 7. When the initial concentration (C0) of the pollutant is 10 mg/L, the adsorption ratio is up to 85.3% for the uptake of 1-naphthylamine by β-CDP, 94.8% for the uptake of 2-naphthylamine by Me-β-CDP, 94.0% for the uptake of 1-naphthol by Me-β-CDP, and 90.9% for the uptake of 2-naphthol by Me-β-CDP. When the C0 of the contaminant is 5 mg/L, the adsorption ratio is increased to 95.6% for the uptake of 1-naphthylamine by β-CDP, 96.6% for the uptake of 2-naphthylamine by Me-β-CDP, 96.3% for the uptake of 1-naphthol by Me-β-CDP, and 95.2% for the uptake of 2-naphthol by Me-β-CDP. This phenomenon suggests that the four CD polymers are applicable to removal of naphthylamine and naphthol with low concentration, especially for micropollutant.
Study of temperature on naphthalene derivatives removal by CD polymers
The impact of temperature on the adsorption of 1-naphthylamine, 2-naphthylamine, 1-naphthol and 2-naphthol by the four CD polymers were studied ranging from 15 oC to 35 oC. Figure 8 shows that high temperature is conducive to the above adsorption process. With the increment of the temperature from 25 oC to 35 oC, the adsorption ratio could be improved to 96.5% from 95.6% for the adsorption of 1-naphthylamine in 5 mg/L by β-CDP, to 98.5% from 96.6% for the adsorption of 2-naphthylamine in 5 mg/L by Me-β-CDP, to 98.0% from 96.3% for the adsorption of 1-naphthol in 5 mg/L by Me-β-CDP, and to 97.6% from 95.2% for the adsorption of 2-naphthol in 5 mg/L by Me-β-CDP. In addition, Fig. 8 also shows that the adsorption of four cyclodextrin polymers for the four substances has certain regularity. For 1-naphthylamine and 2-naphthylamine, the order of adsorption effect of the four cyclodextrin polymers is as follows: β-CDP > Me-β-CDP > HP-β-CDP > HE-β-CDP. For 1-naphthol and 2-naphthol, the adsorption effects of β-CDP and HP-β-CDP are similar. Overall, for 1-naphthol and 2-naphthol, the order of adsorption effect of the four cyclodextrin polymers is as follows: Me-β-CDP > HP-β-CDP > β-CDP > HE-β-CDP when the temperature is higher than 25 oC. This is consistent with the kinetic results.
Reutilization Performance Studying
The adsorbent with excellent reusability will have good practicability. In this study, methanol was selected as the desorption agent to desorb the four cyclodextrin polymers with 1-naphthylamine, 2-naphthylamine, 1-naphthol and 2-naphthol. As depicted in Fig. 9, β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP have a small decrease in the adsorption ratio of four naphthalene derivatives after five adsorption-desorption processes, but not more than 5%. In detail, for 1-naphthylamine, the adsorption ratio of β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP decreased by 3.38%, 2.39%, 3.46% and 2.47% respectively. For 2-naphthylamine, the adsorption ratio of β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP decreased by 2.49%, 3.50%, 3.46% and 2.49%, respectively. For 1-naphthol, the adsorption ratio of β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP decreased by 2.46%, 5.46%, 6.24% and 4.24% respectively. For 2-naphthol, the adsorption ratio of β-CDP, HE-β-CDP, HP-β-CDP, and Me-β-CDP decreased by 4.24%, 3.34%, 2.38% and 3.28%, respectively. The results show that the four cyclodextrin polymers have excellent recycling performance on the adsorption of 1-naphthylamine, 2-naphthylamine, 1-naphthol and 2-naphthol.