2.1. Structural Characterization of the Polymers.
In the present study, β-CD was expected to be chemically modified on CMPS according to the nucleophilic substitution. The FT-IR spectra in Fig. 1(a) revealed that the C-Cl [12, 13] stretching, with the frequency at 1265 cm-1 and 670 cm-1, was significantly weakened after the reaction, and β-CD-PS displayed much lower chlorine content (0.541 mmol/g) than CMPS (4.88 mmol/g). In addition, β-CD-PS had strong vibration at 1157 cm-1 and 1008 cm-1, which can be assigned to the C-O-C [14, 15] and C-OH [16, 17] and stretching. While another vibration ranged at 3650 cm-1-3200 cm-1 presented, which can be assigned to the O-H [18] stretching of β-CD. The chemical composition of the polymer was also characterized by XPS analysis. As shown in Fig. 1 (b), the Cl species was absent after the substitution, while a significant increase of O content (9.04 wt%) was presented, implying enough consumption of the -CH2Cl groups and plentiful addition of the -OH on polymer. The high-resolution C1s revealed that the C-Cl configuration of CMPS at 286.06 eV obviously vanished, while the C-O configuration at 286.74 eV (Fig. 1 (c)) appeared for β-CD-PS. The high-resolution O1s in Fig. 1 (d) also showed the C-O and C-OH configurations at 532.26eV and 532.81 eV for β-CD-PS [4, 19, 20]. This further illustrates that the modification was successful.
Table 1
Pore structural parameters of the polymers
|
CMPS
|
β-CD-PS
|
SBET (m2 /g) a
|
78
|
71
|
Smicro (m2 /g) b
|
-
|
-
|
Vtotal (cm3 /g) c
|
0.68
|
0.53
|
Vmicro (cm3 /g) b
|
-
|
-
|
Pore size (nm)
|
34.8
|
30.2
|
Chlorine (%)
|
17.32
|
1.94
|
Oxygen (%)
|
-
|
9.04
|
Moisture content
|
44.82
|
64.85
|
CA (deg)
|
130
|
45
|
a Calculated using the BET model.
b Calculated using nonlocal density functional theory (DFT) model.
c Calculated at P/P0 = 0.99.
As shown in Figure S1, the contact angle (CA) reduced from 135° (CMPS) to 45° (β-CD-PS) after introducing large amounts of β-CD molecules, and the resulting polymer β-CD-PS had strong hydrophilicity. In Table 1, it is clear that the values of CMPS are 78 m2/g and 0.68 cm3/g, respectively. They were lightly reduced to 71 m2/g and 0.53 cm3/g after the nucleophilic substitution reaction. Chlorine in CMPS is consumed, and β- CD is uploaded as a functional group onto the polymer, which leads to a decrease in the SBET and Vtotal of the polymer. According to the IUPAC classification, the N2 isotherms of β-CD-PS had a type-II profile (Fig. 2), characteristic of the macropores. In addition, the N2 capacity raised sharply at relatively high pressure (P/P0 > 0.9), suggesting that the polymer contained considerable macropores [21, 22]. The nucleophilic substitution reaction brings brought a little change of β-CD-PS, slightly decreasing from 34.8 nm to 30.2 nm of the average pore size, mesopores and macropores ranging from 20–60 nm were the main pores for β-CD-PS and CMPS.
2.2 The adsorption of the acid compound on β-CD-PS
Compared with CMPS, β-CD-PS had better adsorption capacity for BNAP, NAP, PHE, SA, PA, and GA which was exhibited in Fig. 3. The adsorption isotherm of the different temperature was exhibited in Fig. 4. At 298K, when the equilibrium concentration is 150mg/L, the equilibrium capacity (qe) ranked an order of SA (260.09 mg/g) > PA (254.18 mg/g) > BNAP (227.07 mg/g) > GA (204.27 mg/g) > NAP (169.91 mg/g) > PHE (39.01 mg/g). Evidently, its adsorption capacity for PHE was relatively less because the molecular diameter of PHE was tinier than the pore size of β-CD, and the relative distance between the molecule and the cavity wall was broad, which made the hydrophobic force between the PHE and the inner wall of β-CD reduced, resulting in a reduced adsorption capacity. β-CD has more active hydroxyl (-OH), which is effective for hydrogen bonding adsorption of such acidic compounds. Its molecular structure is like naphthalene, and suitable for the aperture of the β-CD.
Table 2
Comparison of adsorption capacities of salicylic acid on β-CD-PS with some other adsorbents in the literature.
Adsorbate
|
Adsorbents
|
T(K)
|
qmax /(mg∙g− 1)
|
References
|
SA
|
GQ-11
|
298
|
295.4
|
[23]
|
Zirconia-carbon composites
|
298
|
109
|
[24]
|
Filtrasorb F400
|
298
|
351
|
[25]
|
Amberlite XAD-16
|
298
|
85
|
[26]
|
Magnetic polar resins
|
298
|
252
|
[27]
|
M-PMD-P-A
|
298
|
251.9
|
[28]
|
PDVB/PMAEM IPN
|
293
|
151.72
|
[29]
|
M-PMD-P-A
|
298
|
251.9
|
[28]
|
β-CD-PS
|
298
|
407.47
|
This study
|
GA
|
HF-02
|
298
|
160.1
|
[30]
|
PDDA/PGV
|
298
|
43.08
|
[31]
|
ZA1
|
298
|
79.10
|
[32]
|
β-CD-PS
|
298
|
218.67
|
This study
|
NAP
|
MGCH
|
298
|
169.00
|
[33]
|
Me-β-CDP
|
298
|
35.92
|
[34]
|
GAC
|
298
|
169.70
|
[35]
|
β-CD-PS
|
298
|
246.39
|
This study
|
BNAP
|
β-CD-PS
|
298
|
246.39
|
This study
|
PA
|
β-CD-PS
|
298
|
302.17
|
This study
|
Langmuir [36] and Freundlich [37] models were used to fitting the adsorption behavior, and the relevant parameters were listed in Table S1. From the R2, the adsorption of β-CD-PS on BNAP, PA, and GA is more consistent with the Langmuir equation, which means that the adsorption of β-CD-PS on the above compounds tends to be monolayer adsorption, while SA, NAP, and PHE are more in line with the Freundlich equation, which means that the adsorption of the above compounds by β-CD-PS is more inclined towards multilayer covering adsorption [38]. Predicting the maximum adsorption capacity (qmax) on β-CD-PS of SA, PA, NAP, BNAP, GA, and PHE (Table S1), the qmax was 407.47 mg/g, 312.42 mg/g, 246.39 mg/g, 320.75 mg/g and 254.40mg/g, respectively, which were higher than other adsorbents in Table 2. To sum up, β-CD-PS exhibited excellent adsorption ability for organic acids. Meanwhile, it can be seen from Fig. 4 that increasing the temperature could promote the adsorption of BNAP, PA and GA but restrain the adsorption of NAP, SA, and PHE. The increase of the temperature of the adsorption system is not conducive to the adsorption of NAP, SA, and PHE by β-CD-PS, which indicates that the adsorption process of NAP, SA and PHE is exothermic [39], whereas the adsorption process of BNAP, PA and GA were endothermic. Figure S2 further analyzed the thermodynamics of β-CD-PS to BNAP, NAP, SA, PA, GA, and PHE. Under different equilibrium concentrations, the Figure S2 showed a good linear relationship. Conforming to Freundlich model indicate that the adsorption process obeyed the Clausius-Clapeyron relation [40], and the enthalpy (ΔH, kJ/mol) of adsorption could be calculated.
The Gibbs equation is derived from the equilibrium adsorption isotherm and the Gibbs free energy (ΔG, kJ/mol) can be calculated by the constant n of the Freundlich model.
The entropy (ΔS, J/(mol·K) can be further calculated according to the Gibbs-Helmholtz equation.
Conforming to Langmuir model indicate that the thermodynamic parameters for the adsorption of BNAP, PA and GA on β-CD-PS were calculated based on the Vant-Hoff equation [41].
The enthalpy (ΔH, kJ/mol) of NAP, PHE and SA were negative (Table S2), indicating that the adsorption process was exothermic, the enthalpy (ΔH, kJ/mol) of BNAP, PA and GA were positive (Table S3), indicating that the adsorption process was endothermic, which was consistent with the Fig. 4. The Gibbs free energy (ΔG, kJ/mol) were also negative, indicating that the adsorption of six organic acids at 298K, 308 K and 318 K were spontaneous.
2.3 Adsorption kinetics
Figure 5 (a) (b) mainly displays the adsorption kinetics curves of BNAP, NAP, SA, PA, GA, and PHE on β-CD-PS at 298K. In addition to the longer time it takes for BNAP to reach equilibrium, the speed at SA, PA, GA, PHE and NAP to reach equilibrium is around 100 minutes. This is because compare to the pore size of β-CD-PS, the diameter of BNAP molecules is large, which hinders their flow in the pores. The kinetic data were commonly fitted by pseudo-first-order (PFO) [42] and pseudo-second-order (PSO) [43] rate models. Table S4 listed the corresponding parameters. From the perspective of R2, the adsorption of these six compounds is more in line with PSO. Figure 5 (c) also shows the same result. Due to the difference in the size of adsorbent molecules and hydrogen bonding ability, the diffusion rates of six adsorbent compounds are different. The diffusion rate (k2) ranked an order of PHE (2.73×10− 3 g/(mg·min)) > SA (5.83×10− 4 g/(mg·min)) > GA (5.73×10− 4 g/(mg·min)) > NAP (2.57×10− 4 g/(mg·min)) > PA (1.92×10− 4 g/(mg·min)) > BNAP (1.06×10− 5 g/(mg·min)), The larger the k2, the faster the adsorption rate. PHE with the smallest diameter has the fastest diffusion rate in macroporous polymer, while BNAP with the largest diameter has the slowest diffusion rate.
Figure 5 (d), (e) and (f) depicted the kinetic adsorption of SA at different initial concentrations. It can be known that when the initial concentration was 1000 mg/L, the removal efficiency could reach 42%. As the initial concentration decreased, the removal efficiency continued to improve. When the initial concentration was 100mg/L, SA could be completely adsorbed. β-CD-PS could achieve adsorption of SA at high concentration and completely remove at low concentration, which had high application value.
2.5 Influence of other factors on Adsorption
Figure 6 (a) exhibited the effect of increasing the amount of β-CD-PS on the removal of adsorbates in the solution. As the amount of polymer in the reaction system increases, the removal rate of adsorbed compounds gradually increases, and the removal rate of BNAP, NAP, SA, PA, and GA could reach 99%. Figure 6 (b) revealed the effect of different concentrations of Na2SO4 on β-CD-PS for the removal of organic acids in the solution. It can be concluded that the amount of coexisting ions has little effect on adsorption, and it slightly increases with the increase of coexisting ions, which may be the result of the “salt out effect”. Figure 6 (c) investigated the effect of different pH on adsorption. The adsorption capacity of NAP and PHE increased with the increase of pH, as the hydroxyl group of PHE became an oxygen anion with six lone pair electrons, the ability of β-CD-PS to form hydrogen bonds has become stronger, and the hydrogen bonds have become stronger, making it less likely to detach from the polymer. As the pH increased, the adsorption capacity of NAP gradually increases, enabling efficient adsorption in alkaline media, which has not been reported in other literature and make β-CD-PS had higher research and application value. The adsorption capacity of SA decreases with the increase of pH. The reason is that the carboxyl and hydroxyl groups of SA become oxygen negative ions, which cannot form intramolecular hydrogen bonds, resulting in a decrease in the molecular size of SA. The mismatch in pore size is not conducive to adsorption. In Fig. 6 (d), using anhydrous ethanol and 0.01 mol/L NaOH as the eluent, the adsorption efficiency of the β-CD-PS can still reach 93% after five adsorption-desorption cycles, indicating good reusability.
2.6 Possible adsorption mechanism
From the above analysis and Fig. 3, it can be seen that compared to CMPS, β-CD-PS has better adsorption effects on BNAP, NAP, SA, PA, GA, and PHE. The reason is that the introduction of macromolecules β-CD provides a hydrophobic inner cavity and more hydrogen bonding donors and receptors. The O and H atoms of carboxyl and hydroxyl groups in acidic compounds can interact with β-CD to form hydrogen bonds, coupled with the hydrophobic effect of the benzene ring structure and hydrophobic cavity, leading to an increase in adsorption capacity.
In order to better understand the adsorption mechanism, FT-IR spectra and XPS analysis were conducted on several compounds after adsorption. From the full spectrum of XPS and C1s, it can be seen that the O content further increases, and a small C = O peak appears in the O1s spectrum whose binding energy is 532.57 eV, the binding energy of C-O and C-OH blue-shifted from 532.80 eV, 532.26 eV to 532.94 eV, 532.61 eV, respectively. This indicates a strong interaction between organic acids and β-CD-PS, further proving the occurrence of adsorption.