Introduction

With the improvement of people’s living standard, the demand for organic compounds has increased, leading to a rapid increase in the number and variety of organic micro-pollutants in water. Since organic micro-pollutants are widely distributed in various kinds of water bodies and difficultly removed or degraded, they seriously threaten the human health and the stability of ecosystem. In particular, the molecular structure of aromatic organic pollutants containing a benzene ring structure is more stable, difficult to decompose, and highly toxic, which can cause serious pollution to the environment. Thus, the efficient water treatment technologies are attracting the increasing attentions.

In recent years, various methods have been reported to decontaminate organic pollutants and remove organic pollutants from domestic sewage or industrial effluents, such as oxidative degradation (Yuan et al. 2021; Kiejza et al. 2021; Ushani et al. 2020) and biological treatments (Ren et al. 2020; Qachach et al. 2021; Kanaujiya et al. 2019). Those methods may still suffer from low efficiency. Appropriate methodologies of removing contaminants rapidly and cleanly are required to be urgently exploited and utilized. Adsorption has attracted special interest in the treatment of organic contaminants due to its accessibility, high efficiency, and environmental friendliness. Various adsorbents, such as bentonite (Wang et al. 2021a; Shirazi et al. 2020; Kong et al. 2019), clays (Awad et al. 2019; Najafi et al. 2021), activated carbons (Jones et al. 2021; Ouyang et al. 2020), cellulose (Wang 2019; Ren et al. 2018; Somsesta et al. 2020), and biochar (Dai et al. 2019; Jang et al. 2018; Patra et al. 2021), have been synthesized to remove contaminants. Among those adsorbents, high porous adsorbents have attracted more attention for the removal of organic pollutants through non-covalent interactions. Compared with other adsorbents, the porous crosslinked polymers performed well in adsorption, due to high chemical stability, thermal stability, and surface area.

β-cyclodextrin (β-CD) is a cyclic oligosaccharide consisting of seven α-D-glucose units connected through α-(1, 4) linkages with a hydrophobic inner cavity and a hydrophilic exterior which can selectively bind various organic and inorganic species in its cavity. A stable host-guest inclusion complex polyhydroxy structure makes it possible for β-CD to form molecular complexes via interaction with other polymer network by virtue of intermolecular force. β-CD exhibited its great potential for the treatment of environmental organic pollutants. Lots of studies have been carried out on the fabrication of crosslinked β-cyclodextrin polymers for the adsorption of organic contaminants. Those crosslinkers include organic acid (citric acid or oxalic acid or terephthalic acid) (Liang and Zou 2019), epichlorohydrin (Crini 2021), and diisocyanate (Dang et al. 2020). However, polymers formed by these crosslinkers and cyclodextrins are not satisfied in terms of porosity and insolubility, which limits their performance for pollutant adsorption. Therefore, improving the insolubility of cyclodextrin and porosity of polymers has become the crucial point. Alsbaiee et al. (2016) prepared a novel copolymer via tetrafluoroterephthalonitrile (TFTPN)-crosslinked β-CD to remove organic pollutants from water rapidly due to its high surface area and permanent porosity. This material can only bind to organic pollutants through hydrogen bonding and the inclusion of cyclodextrins.

As it is known, CD hydroxyl moieties can be functionalized with phenylcarbamoyl moieties to extend the CD cavity. The introduction of different functional groups can lead to diverse pore structures and functionalities to provide abundant adsorption capabilities. In particular, the introduction of benzene ring makes the polymer easier to combine with aromatic organic pollutants through π-π interaction. Although there have appeared various CD polymer materials by far, there is still no publication to explore the potential of phenylcarbamoylated-β-CD (Ph-β-CD) polymers.

This work synthesized a series of Ph-β-CD derivatives with different electronegative groups, which were thereafter crosslinked with TFTPN to fabricate a series of porous multifunctional cyclodextrin polymers (Fig. 1a). Furthermore, TFTPN is used again to crosslink the cyclodextrin polymer twice. This process connects the nanoparticles to each other again, which changes the pore structure of the polymer and enables different electronegative groups to play a synergistic effect (Fig. 1b). This work verified the adsorption performance of the material and the interaction between the material and the adsorbed pollutants through adsorption experiments on typical organic pollutants, such as tetracycline, ibuprofen, dichlorophenol, norfloxacin, bisphenol A, and naphthol.

Fig. 1
figure 1

Synthesis of CD polymers and the network structure

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Materials and methods

Materials

β-CD (98% purity) was purchased from Heowns (Tianjin, China). 4-chlorophenyl isocyanate (98% purity) and 4-trifluoromethyl phenyl isocyanate (98% purity) were purchased from Macklin (Shanghai, China). 4-nitrophenyl isocyanate (98% purity) was purchased from Aladdin (Shanghai, China). Tetrafluoroterephthalonitrile (TFTPN) (98% purity) was purchased from Dibai (Shanghai, China). Whatman inorganic membrane filter (0.2 μm, PTFE) was purchased from Wishes (Shanghai, China). All other reagents were commercially available and of analytical grade.

Preparation of modified β-CD

First, 4-chlorophenyl isocyanate (0.82 g), 4-nitrophenyl isocyanate (0.88 g), and 4-trifluoromethyl phenyl isocyanate (0.99 g) were, respectively, added in three 100 mL dry twin-neck flasks equipped with magnetic stirring bar. Then, β-CD (2.00 g) and anhydrous pyridine (30 mL) were added. The above solution was kept in an oil bath pan at 80 °C for 18 h. After removing the solvent by reduced pressure distillation, the crude product was obtained. The product was washed with water and acetone repeatedly for further purification and then dried under vacuum for 24 h at 80 °C to obtain three modified β-cyclodextrins.

Crosslinking of modified β-CD using TFTPN+

A dry 20 mL three-necked flask was charged with the modified β-CD (0.20 g), TFTPN (0.10 g), and K2CO3 (0.32 g) equipped with a magnetic stir bar. The flask was flushed with N2 gas for 5 min; then, an anhydrous THF/DMF mixture (9:1 v/v, 8 mL) was added under nitrogen atmosphere. The mixture was placed on a hot stirring plate (85 °C) and stirred at 500 rpm for 48 h.

The orange suspension was cooled and then filtered. To remove unreacted K2CO3, 1-mol·L−1 HCl was slowly added until there is no bubble observed. The product was isolated and then immersed in deionized water (2 × 10 mL) for 15 min, THF (2 × 10 mL) for 30 min, and CH2Cl2 (2 × 15 mL) for 15 min. Finally, the solid was dried under liquid nitrogen conditions (77 K) for 10 min and stranded at room temperature for 2–3 days to obtain three cyclodextrin polymers (CDPs) named Cl-CDP, NO2-CDP, and F-CDP, respectively.

Preparation of mixed functionalized β-cyclodextrin

Cl-CDP, NO2-CDP, and F-CDP were mixed with the same mass (0.1g) and added in a dry 20 mL three-necked flask equipped with a magnetic stir bar. TFTPN (0.10 g) and K2CO3 (0.32 g) were also added at the same time. The flask was flushed with N2 gas for 5 min, and then an anhydrous THF/DMF mixture (9:1 v/v, 8 mL) was added under nitrogen atmosphere. The mixture was placed on a hot stirring plate (85 °C) and stirred at 500 rpm for 48 h. The post-treatment steps are the same as described in the section “Crosslinking of modified β-CD using TFTPN+” to obtain a mixed secondary crosslinked β-CD polymer (X-CDP).

Characterization

FTIR analysis of Cl-CDP, NO2-CDP, F-CDP, and X-CDP was performed at 2 cm−1 resolution on a Bio-Rad FTS3000 IR Spectrum Scanner. Pellets were prepared from the powder samples using KBr. Thermogravimetric analysis (TG) of Cl-CDP, NO2-CDP, F-CDP, and X-CDP powders was tested on a ZTY-ZP thermal analyzer. Samples were heated from room temperature to 700°C at a heating rate of 10°C/min in a nitrogen atmosphere. The external surfaces of Cl-CDP, NO2-CDP, F-CDP, and X-CDP powders were viewed using a Hitachi S-8100 scanning electron microscope. The specific surface area is measured by Autosorb-iQ2-MP. Solid-state13C NMR was performed on a Varian Infinityplus 600 NMR spectrometer (600 MHz, 7.0 T; USA). UV spectrum is detected by MAPADA UV-1800 PC (transmittance accuracy = ±0.2%τ). Stock solutions of six pollutants at 0.005mM, 0.01mM, 0.02mM, 0.05mM, and 0.1mM concentrations are prepared in water. The standard curve is measured before each measurement, and the correlation coefficient can reach more than 0.999. The six pollutants can be quantitatively detected at concentrations below 0.1 mM.

Adsorption experiment

Adsorption kinetic studies were performed in 20 mL test tube. All studies were conducted at 25°C on an incubator shaker with a shaken speed of 120 rpm. The polymers (10 mg) and 0.1-mM pollutant stock solution (10 mL) were added to a 20 mL test tube. The mixture was shaken immediately. Of the suspension, 2 mL aliquots were taken at certain intervals via syringe and immediately filtered by a Whatman 0.2-μm inorganic membrane filter. The residual concentration of the pollutant in each sample was determined by UV-vis spectroscopy. Prepare a 0.5-mM pollutant stock solution, and dilute to obtain a series of pollutant solutions with a concentration of 0.4mM, 0.3mM, 0.2mM, 0.1mM, and 0.05mM, which are used for adsorption isotherm experiments. In the regeneration experiment, 10-mg X-CDP after adsorbing organic pollutants was washed with 5 mL methanol and then dried for the next reuse. The cycle was repeated five times. Each experiment was repeated six times. The mean values and standard deviations were presented.

Results and discussion

Characterization of porous CD and derivatives

Figure 2 shows the SEM micrographs of the surface of composites under the magnification. It was observed that the composites exhibit rough surface and porosity. The surfaces of the four adsorbents were dense and porous with a large number of channels, which is conducive to the interaction between the adsorbent and organic pollutants to improve the adsorption efficiency. Figure 2d further reveals that, through the two-step crosslinking reaction, X-CDP obtains a larger pore size and a better pore structure than the other three adsorbents, which makes the material have better adsorption performance.

Fig. 2
figure 2

SEM micrographs of Cl-CDP (a), NO2-CDP (b), F-CDP (c), and X-CDP (d)

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The TGA curves for the pyrolysis of β-CD polymer were shown in Fig. 3a. The mass loss of 3–5% at 150 °C was assigned to the evaporation of the water. At 150–260 °C, the TGA curve kept steady, indicating that the β-CD polymer was thermostable at this temperature range. At temperature above 260 °C, there was a mass loss of 30–40%, ascribed to the decomposition of β-CD monomer and TFTPN. Compared with the other three adsorbents, X-CDP lost the least quality at this stage. The re-crosslinking of TFTPN enhanced the thermal stability of the material. At temperature higher than 370 °C, there was still a mass loss of 15–20%, which was caused by the decomposition of the carbon chain. These indicated that the as-synthesized β-CD polymer was thermostable enough for organic pollutant adsorption.

Fig. 3
figure 3

a TGA curve of Cl-CDP, NO2-CDP, F-CDP, and X-CDP;b the FT-IR spectra of TFTPN, β-CD, NO2-CDP, Cl-CDP, F-CDP, and X-CDP;c the 13C solid-state NMR spectra of X-CDP

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The specific surface area of several different composite materials was measured. SBET of Cl-CDP, NO2-CDP, F-CDP, and X-CDP was 16.863 m2·g−1, 2.125 m2·g−1, 16.532 m2·g−1, and 200.974 m2·g−1, respectively. The obvious increase in the specific surface area of X-CDP indicates that X-CDP is not simply obtained by physical mixing of the first three polymers. During the preparation of X-CDP, the large pores of the polymer were broken, and more small pores appeared. The higher specific surface area makes X-CDP have better adsorption capacity for micro-pollutants.

The FT-IR spectra of TFTPN, β-CD, Cl-CDP, NO2-CDP, F-CDP, and X-CDP are shown in Fig. 3b. For the CDPs, the broad bands and peaks show that the composites contain functional groups of isocyanate and β-CD, which is similar to the previous studies (Li et al. 2018). Particularly, the absorption peak at 3423 cm−1 was ascribed to the –OH groups in β-CD. The absorption at about 1157 cm−1 was from C-O bond stretching in the C-O-H group, and the peaks at 1105 and 1035 cm−1 were ascribed to C-O bond stretching in the C-O-C group of the anhydroglucose ring. The peak at 2924 cm−1 was associated to the stretching vibrations of aliphatic C–H. The peak at 1730 cm−1 is attributed to the vibrational absorption of C=O, which is a characteristic peak of isocyanate modification.

The peaks at 2242 cm−1 and 1626 cm−1, respectively, correspond to the stretching vibration of C≡N and the C-C aromatic extension. Both the TFTPN and the final product composites spectra contained C-F stretching vibration peak at 1477 cm−1. After the cyclodextrins were modified with isocyanate and crosslinked with tetrafluoroterephthalonitrile, the peak intensity was weaker than the reactant material. However, the C-F bond peak can still be clearly observed in the spectrum.

The 13C solid-state NMR spectra of X-CDP are shown in Fig. 3c. The resonance associated with β-CD is exhibited at δ=74 ppm. Resonances at δ=98 and 142 ppm correspond to the newly formed alkoxy groups and aromatic carbons, respectively. Resonances at δ=111 and 124 ppm are related to the two unsubstituted carbons on the benzene ring introduced by isocyanate substitution. This reveals that the cyclodextrin was successfully phenylcarbamoylated and crosslinked by tetrafluoroterephthalonitrile.

The swelling and water retention properties of X-CDP are measured. In many experiments, the polymer can absorb 198% of its own mass of water after soaking for 2 h. Afterwards, the polymer can completely lose water by placing it in an environment of 25°C under normal pressure for 30 min. This shows that X-CDP is a rigid material with good swelling resistance.

The adsorption of organic pollutants

Cl-CDP, NO2-CDP, F-CDP, and X-CDP are used to adsorb the common and commercialized organic pollutants in the field of water purification (Fig. 4). The negative inductive effect of −Cl, −NO2, and −CF3 reduces the electron cloud density on the benzene ring, making it easier for the adsorbent to form hydrogen bonds with pollutants. The pollutants containing benzene ring can be adsorbed by π-π interaction with the benzene ring on the adsorbent. Combined with the inherent inclusion effect of the cyclodextrin cavity, especially non-planar compounds are more easily captured by the cavities of cyclodextrin; the adsorbent exhibits good performance in adsorbing pollutants.

Fig. 4
figure 4

The structures of each tested emerging organic micro-pollutant

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Furthermore, X-CDP has a high adsorption capacity for almost all pollutants due to its porous structure and effect synergy of various functional groups which can form various intermolecular forces and hydrogen bonds. As shown in Fig. 5b, the adsorption effect of X-CDP on a variety of pollutants is stronger than that of the other three polymers. Especially for tetracycline (Fig. 5a), the effect is significantly stronger than that for others, which means that the X-CDP obtained by mixing and crosslinking the three polymers is not only the sum of the effects of the three polymers.

Fig. 5
figure 5

Good performance of compound X-CDP for adsorption of organic micro-pollutants. a Adsorption efficiency of tetracycline by four adsorbents in 30 s. b Percentage removal efficiency of each pollutant obtained by the four adsorbents of NO2-CDP (yellow), Cl-CDP (red), F-CDP (gray), and X-CDP (navy). The error bars represent the standard deviation

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Batch adsorption kinetic modeling

As shown in Fig. 6, when equilibrium is reached, different adsorbents have different adsorption effects for the same compound. The characteristic peaks of ibuprofen, dichlorophenol, naphthol, norfloxacin, bisphenol A, and tetracycline are located at 225 nm, 287 nm, 287 nm, 275 nm, 275 nm, and 355 nm, respectively. Obviously, after adsorption, the characteristic peak intensity decreased with different degrees. Those adsorbents have obvious absorption for the above organic substances. X-CDP has the most obvious adsorption effect on various pollutions. The removal rate of norfloxacin and tetracycline by X-CDP in 30 s is over 95%, and the adsorption equilibrium is basically reached within 2 min. It took 10 min for other adsorbents to reach equilibrium and adsorbed only 46% of its equilibrium value in 30 s. The rapid removal rate of pollutants has never been reported before.

Fig. 6
figure 6

The UV contrast of a ibuprofen, b 2,4-dichlorophenol, c 1-naphthol, d norfloxacin, e bisphenol A, f tetracycline solution before and after adsorption by the four varieties of adsorbents

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The effect of contact time on adsorption of organic contaminants by X-CDP and other adsorbents is shown in Fig. 7a and b. The adsorption of contaminants by X-CDP was high and quick in the initial 2 min due to its porosity. Especially for polycyclic and high molecular weight pollutants, such as tetracycline and norfloxacin, these pollutants are more likely to be quickly included due to the three-dimensional cavity structure of cyclodextrin.

Fig. 7
figure 7

aTime-dependent adsorption of tetracycline (0.1 mmol·L−1) by four adsorbents (1 mg·mL−1), b a time-dependent adsorption of each pollutant (0.1 mmol·L−1) by X-CDP (1 mg·mL−1). The error bars represent the standard deviation

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The pseudo-first-order kinetic model and pseudo-second-order model were used to investigate adsorption kinetics. Using the nonlinear form of pseudo-first-order kinetic model (Eq. (1)) and the pseudo-second-order kinetic model (Eq. (2)), the adsorption process was expressed as:

$$ {q}_{\mathrm{t}}={q}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{k}_1t}\right) $$
(1)
$$ {q}_{\mathrm{t}}=\frac{q_{\mathrm{e}}^2{k}_2t}{1+{k}_2{q}_{\mathrm{e}}t} $$
(2)

where qt (mg·g−1) is the amount of adsorbed organic contaminants at any time t(min);k1 (min−1) is the first-order rate constant; k2 (mg·g−1·min−1) is the second-order rate constant; and qe (mg·g−1) represents the amount of adsorbed organic contaminants at equilibrium. Kinetic constants of the kinetic models are estimated by the experimental data in Tables 1 and 2. The R2 of the pseudo-first-order kinetic model is higher than that of the pseudo-second-order kinetic model, which shows that the adsorption experiment is more in line with the pseudo-second-order kinetic model. The apparent pseudo-second-order rate constant (k2) of the adsorption of tetracycline by X-CDP is 2.02 g·mg−1·min−1, which is higher than that of F-CDP, Cl-CDP, NO2-CDP, and the other studied adsorbents for tetracycline. The k2 values reported in the literature are listed in Table 3. X-CDP’s superior k2 for adsorbing organic contaminants indicates that nearly all of its β-CD binding sites are readily accessible, and the mount of binding sites is higher than many adsorbents.

Table 1 The pseudo-first-order kinetic model data and pseudo-second-order kinetic model data of six pollutants adsorption by X-CDP
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Table 2 The pseudo-first-order kinetic model data and pseudo-second-order kinetic model data of tetracycline adsorption by four adsorbents
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Table 3 The pseudo-first-order kinetic model data and pseudo-second-order kinetic model data of tetracycline adsorption by four adsorbents
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Batch adsorption isotherm modeling

Adsorption isotherm is important for determining the adsorption behavior of an adsorbent. In order to better investigate the adsorption mechanisms, the Langmuir model (Eq. (3)) and the Freundlich model (Eq. (4)) were applied to the experimental data using the following equations:

$$ {q}_{\mathrm{e}}=\frac{q_{\mathrm{max}}{k}_{\mathrm{L}}{c}_{\mathrm{e}}}{1+{k}_{\mathrm{L}}{c}_{\mathrm{e}}} $$
(3)
$$ {q}_{\mathrm{e}}={k}_{\mathrm{F}}{c}_{\mathrm{e}}^{\mathrm{n}} $$
(4)

where qe (mg·g−1) is the adsorption capacity at equilibrium; qmax (mg·g−1) is the maximum adsorption capacity; ce (mg·L−1) is the equilibrium concentration in the solution; and kL (L·mg−1) is a constant related to the affinity between an adsorbent and adsorbate. kF mg·g−1·(mg·L−1)−n is the Freundlich constant, and n(dimensionless) is the Freundlich intensity parameter, which indicates the magnitude of the adsorption driving force or the surface heterogeneity. The data of qe−ce, the Langmuir model data, and the Freundlich model data of the four adsorbents for tetracycline were shown in Fig. 8 and Table 4.

Fig. 8
figure 8

Adsorption isotherms of tetracycline by four adsorbents

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Table 4 The Langmuir model data and the Freundlich model data of tetracycline adsorption by four adsorbents
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According to the equilibrium adsorption value and the concentration of tetracycline after adsorption, the Langmuir model was fitter than the Freundlich model, which means the adsorption process accorded with Langmuir model. The adsorption process is monolayer adsorption, and there is no interaction between adsorbates. Furthermore, the maximum adsorption capacity of X-CDP at equilibrium was found to be 110.56 mg·g−1; the qmax value for X-CDP was competitive with the adsorption capacities of tetracycline of other adsorbents, such as chitosan powder (23.92 mg/g) (Huang et al. 2011), Fe-HAP (45 mg/g) (Li et al. 2017), and BM-biochar (85 mg/g) (Xiang et al. 2020).

The adsorption-desorption cycles of X-CDP

An ideal adsorbent should have high adsorption ability as well as excellent desorption performance, which reduce the operating cost for the adsorbent application. As is shown in Fig. 9, the efficiency (%) was the ratio of the weight of absorbed (or desorbed) tetracycline at any time. Thus, the adsorption capacity of the desorbed X-CDP was examined by using methanol as eluent, and the regenerated X-CDP was reused in tetracycline solution with known concentration. From Fig. 9, the removal rate of tetracycline is still 86.7%, which is only 8.3% lower than the first adsorption after five cycles. This reduction might be ascribed to the loss of binding sites after each desorption procedure. These results demonstrated that the recovery efficiency of X-CDP was relatively high with slightly affected by the five consecutive regeneration cycles and X-CDP showed excellent re-adsorption effect.

Fig. 9
figure 9

The average percentage tetracycline removal efficiency by X-CDP after consecutive regeneration cycles. The error bars represent the standard deviation

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Conclusions

In summary, four new adsorbent materials were prepared by crosslinking a series of modified beta-cyclodextrin with the rigid aromatic compound TFTPN. Both the crosslinking agent TFTPN and the polymerized monomer phenylcarbamoylated-β-cyclodextrin contain a large amount of benzene rings, which makes the polymer easy to combine with aromatic organic pollutants through π-π interaction. The electron-withdrawing effects of chlorine, trifluoromethyl, and nitro further strengthen the interaction between the two. The large number of hydroxyl groups in cyclodextrin and the carbonyl group brought about by isocyanate modification enables the polymer to bond with pollutants through hydrogen bonds. Its three-dimensional cavity is easier to adsorb non-planar compounds. The adsorbent has the superior performance for adsorption of micro-pollutants. Each adsorbent can quickly reach the adsorption equilibrium, especially for X-CDP; it reached almost 95% of its equilibrium in 30 s. In the experiment of adsorption of tetracycline, the rate constant reached 2.02 g·mg−1·min−1, and the maximum adsorption capacity of the mixed adsorbent was more significant, reaching 110.56 mg·g−1 at equilibrium. The characteristics of rapid adsorption, large adsorption capacity, easy separation, and ability to adsorb a variety of pollutants make X-CDP have great potentials to be used in the removal of a broad-spectrum of organic micro-pollutants from water.