Surface immobilization of β-cyclodextrin on hybrid silica and its fast adsorption performance of p-nitrophenol from the aqueous phase

Abstract

Renewable β-cyclodextrin (β-CD) was immobilized onto the surface of hybrid silica using ethylenediamine as linking groups to construct an adsorbent in water treatment (CD@Si), and the obtained CD@Si was characterized through FT-IR, XPS, EDX, contact angle measurement, TGA, solid-state ¹³C NMR, SEM, and XRD analyses. The effect of initial pH, contact time on the adsorption performance of CD@Si for p-nitrophenol, and the adsorption kinetics, adsorption isotherms, adsorption thermodynamics, reusability and adsorption mechanism were investigated systematically, which indicate that the adsorption of p-nitrophenol onto CD@Si is a very fast process. The adsorption equilibrium can be reached in 15 s with an acceptable equilibrium adsorption capacity of 69.6 mg g⁻¹ at pH 7.0, which is much faster than many reported adsorbents based on β-CD. The adsorption of p-nitrophenol onto CD@Si follows the pseudo-second-order model, obeys the Freundlich model, and is a feasible, spontaneous, and exothermic process which is more favorable at lower temperatures. And the formation of an inclusion complex and a hydrogen bond interaction are two origins of p-nitrophenol being adsorbed onto CD@Si. Additionally, CD@Si can be recycled and reused for at least five runs with an acceptable adsorption capacity, and is a very promising adsorbent for the fast adsorption of p-nitrophenol or its analogues from the aqueous phase. Additionally, this work also provides a strategy to increase the adsorption rate of adsorbents based on β-CD.

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1. Introduction

In modern society, a variety of organic molecules are discharged into the natural environment from the rapid development of industrialization and urbanization. Most of these organic molecules do not exist in the environment naturally and can cause many serious environmental problems such as water pollution, and the organic molecules have a negative effect on human health and ecological systems. p-Nitrophenol is an important chemical raw material and intermediate in the production of pharmaceuticals, pesticides, dyes, artificial resins, explosives, and other valuable fine chemicals,^1–6 and it can get into the environment system during the above processes and pollute natural water. The other source of p-nitrophenol in the environment is the hydrolysis or degradation of organic molecules containing a p-nitrophenol moiety.⁷ Therefore, p-nitrophenol has become a commonly encountered pollutant in water pollution nowadays. And it is known that p-nitrophenol has been categorized as one of the 126 priority pollutants by the U.S. Environmental Protection Agency (U.S. EPA) for its persistency, bioaccumulation and high toxicity even at very low concentrations,^8–12 and frequent exposure to p-nitrophenol can cause methemoglobin formation, anemia, liver and kidney damage, eye and skin irritation, tumor formation, cancer, and systemic poisoning.^8,9,13,14 Therefore, fast and effective removal of p-nitrophenol from industrial wastewater and natural water is becoming more and more urgent.

Several water treatment technologies have been developed with the aim of removing p-nitrophenol or its analogues from industrial wastewater and natural water effectively, such as adsorption,^4,15–18 oxidation,^19–21 membrane separation,^22,23 and extraction.²⁴ Among these technologies, adsorption is recognized as the most applicable one for its simple design, easy operation, and high efficiency.^18,25–27 And activated carbon is the most popular adsorbent employed in the adsorption process for its large specific surface area and high adsorption capacity.^3,4,13,17,28 But as an adsorbent, activated carbon also has its inherent limitations such as poor mechanical strength and difficult regeneration.^3,26,28 So, there is an increasing interest in the research and development of adsorbents with a satisfying adsorption capacity, high mechanical strength, high reusability, and ready availability, especially for adsorbents based on natural products for their ready availability and excellent biocompatibility, which will not cause additional environmental pollution in their application.

β-Cyclodextrin (β-CD) is a water-soluble macrocyclic oligomer of D-(+)-glucopyranosyl units linked by a α-1,4-glycosidic bond, which possesses a hydrophilic exterior and a hydrophobic cavity. And β-CD can be readily available from the enzymatic degradation of starch, which makes it possess excellent biocompatibility.^29–32 Because of its hydrophilic exterior and hydrophobic cavity, β-CD can form inclusion complexes with a wide range of organic molecules possessing a suitable shape and size in water spontaneously,^33–36 which brings the possibility of removing organic molecules from polluted water. Thus, β-CD has become a unique structural unit in the design and preparation of adsorbents employed in water treatment.^37–44 In the water treatment process, β-CD units can capture pollutant molecules through the formation of inclusion complexes effectively, and then the removal of the pollutant is realized through separation of the adsorbent from aqueous solution without any additional environmental pollution. But at present, in the immobilization of β-CD onto water-insoluble supports such as Fe₃O₄,^40,45–47 SiO₂,^48–50 graphene oxide,^51,52 carbon nanotubes,⁵³ and chitosan,^42,54 the linking groups mostly employed are epichlorohydrin,⁴² citric acid,⁴⁸ carbodiimide,⁴⁶ 3-glycidyloxypropyl trimethoxysilane,⁴⁷ hexamethylene diisocyanate,⁵³ glutaraldehyde,⁵⁴ and so on. These linking groups possess so high a reactivity that multiple hydroxyl groups in β-CD molecules may react with linking groups, which will weaken the intramolecular hydrogen bond in β-CD molecules, disfavour the maintenance of the hydrophobic cavity of β-CD and affect the adsorption performance, especially the adsorption rate, which is a very important property of adsorbents employed in fast adsorption.

Thus, in this work, β-CD was immobilized onto the surface of hybrid silica (Cl@Si), which was prepared through sol–gel hydrolysis–condensation of (3-chloropropyl)triethoxysilane (CPTES) and tetraethyl orthosilicate (TEOS) in the presence of tetrabutylammonium fluoride,^55–57 using ethylenediamine as linking groups to construct an adsorbent for water treatment (CD@Si), in which only one hydroxyl group in the β-CD molecule was employed in order to maintain the hydrophobic cavity of β-CD. And the adsorbent was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), contact angle measurements, thermo-gravimetric analysis (TGA), solid-state ¹³C NMR, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Then applied to the adsorption of p-nitrophenol from aqueous solution, CD@Si exhibits a satisfying fast adsorption performance, and the adsorption equilibrium can be reached in 15 s with an equilibrium adsorption capacity of 69.6 mg g⁻¹, which is a very fast adsorption process. The effects of the solution pH and contact time on the adsorption performance, adsorption kinetics, adsorption isotherms, adsorption thermodynamics, reusability and adsorption mechanism were also investigated systematically. It is demonstrated that CD@Si is a very suitable and promising adsorbent for fast adsorption in some cases of emergency. This work also provides a strategy to increase the adsorption rate of adsorbents based on β-CD.

2. Experimental section

2.1. Materials and chemicals

β-Cyclodextrin in 99% purity was purchased from Shanghai Bio Science & Technology Co. Ltd., China. p-Toluenesulfonyl chloride in 99% purity, and ethylenediamine in 99% purity were Shanghai Aladdin reagents. (3-Chloropropyl)triethoxysilane (CPTES) in 99% purity, tetraethyl orthosilicate (TEOS) in 99% purity, 1 M tetrabutylammonium fluoride in THF, p-nitrophenol in 99% purity, potassium dihydrogen phosphate (KH₂PO₄) in 99% purity and dipotassium hydrogen phosphate (K₂HPO₄) in 99% purity were purchased from Shanghai Energy Chemical Co. Ltd., China. Potassium iodide (KI) in 99% purity was purchased from Xilong Chemical Co. Ltd., China. Distilled water was used in all the experiments. All the other common reagents were analytical grade. All of the reagents were used as received without further purification unless otherwise noted.

2.2. Synthesis of mono[6-O-(p-toluenesulfonyl)]-β-CD

Mono[6-O-(p-toluenesulfonyl)]-β-CD was synthesized according to our procedure reported previously.^58–60 A solution of sodium hydroxide (6.00 g, 150 mmol) in water (20 mL) was added dropwise to a solution of β-CD (56.75 g, 50 mmol) in water (500 mL) with magnetic stirring at 10–15 °C over about 15 min. The solution became homogeneous, and then a solution of p-toluenesulfonyl chloride (11.44 g, 60 mmol) in acetonitrile (30 mL) was added dropwise at 10–15 °C over about 45 min forming a white precipitate immediately. The resultant solution was kept stirring for 3.0 h and rose to room temperature. The precipitate formed was obtained by suction filtration and then suspended in water (300 mL) with magnetic stirring at room temperature for 3.0 h. The precipitate collected by suction filtration was washed successively with acetone (100 mL) and water (160 mL), and then dried under vacuum at 80 °C for 8.0 h to afford a white solid powder (8.17 g) in a 12.67% yield. [α]²⁵_D = +124.15° (c = 0.8044, DMF); m.p. > 160 °C (decomp.); ¹H NMR (400 MHz, DMSO-d₆): δ = 7.76 (d, J = 8.3 Hz, 2H), 7.44 (d, J = 8.2 Hz, 2H), 5.83–5.63 (m, 14H), 4.85–4.76 (m, 7H), 4.50–4.44 (m, 5H), 4.37–4.32 (m, 2H), 4.21–4.17 (m, 1H), 3.70–3.43 (m, 26H), 3.40–3.19 (m, 14H, overlaps with H₂O), 2.43 (s, 3H) ppm; ¹³C NMR (400 MHz, DMSO-d₆): δ = 144.67, 132.53, 129.75, 127.44, 102.09–101.14 (m), 81.51–80.63 (m), 72.88–71.69 (m), 69.56, 68.76, 59.74–59.04 (m), 21.06 ppm; MS (ESI): m/z: 1311.3 [M + Na]⁺, 1289.0 [M + H]⁺. The modification of β-CD with p-toluenesulfonyl chloride to construct mono[6-O-(p-toluenesulfonyl)]-β-CD is the first step to activate the hydroxyl group of β-CD, which produces an important intermediate in the preparation of the adsorbent CD@Si.

2.3. Synthesis of mono(6-ethylenediamine)-β-CD

A solution of mono[6-O-(p-toluenesulfonyl)]-β-CD (6.45 g, 5.0 mmol) in ethylenediamine (22.54 g, 375 mmol) was stirred at 70 °C for 24.0 h, and then cooled to room temperature. The resultant solution was slowly poured into ethanol (400 mL) forming a white precipitate immediately. The white precipitate was collected by suction filtration and dried under vacuum at 80 °C for 8.0 h. The obtained white power was recrystallized two times in water (2 × 10 mL), and dried under vacuum at 80 °C for 8.0 h to afford a white crystal (3.18 g) in a 54.03% yield. [α]²⁵_D = +149.91° (c = 0.8036, H₂O); m.p. > 245 °C (decomp.); ¹H NMR (400 MHz, D₂O): δ = 5.12–5.10 (m, 7H), 4.03–3.89 (m, 26H), 3.70–3.59 (m, 14H), 3.48 (t, J = 9.4 Hz, 1H), 3.11–3.08 (m, 1H), 2.87–2.68 (m, 7H) ppm; ¹³C NMR (400 MHz, D₂O): δ = 101.81, 100.47, 83.56, 81.09, 80.86, 73.06–72.96 (m), 72.05–71.99 (m), 71.78, 70.42, 60.25, 50.66, 49.29, 42.19, 39.73 ppm; MS (ESI): m/z: 1177.5 [M + H]⁺. Mono(6-ethylenediamine)-β-CD, which can be immobilized onto the surface of hybrid silica (Cl@Si) directly, is the second important intermediate in the preparation of the adsorbent CD@Si.

2.4. Preparation of hybrid silica (Cl@Si)

Cl@Si was prepared according to the literature procedure with some modification.^55–57 A solution of (3-chloropropyl)triethoxysilane (CPTES) (2.41 g, 10 mmol), and tetraethyl orthosilicate (TEOS) (10.42 g, 50 mmol) in anhydrous ethanol (30 mL) was added to a solution of distilled water (3.60 g, 200 mmol), and 1 M tetrabutylammonium fluoride in THF (3.0 mL) in anhydrous ethanol (20 mL). The resultant mixture was shaken vigorously for 5 s to obtain a homogenous solution and was then kept under static conditions. After 30 min, gelification was observed, and the material was aged at room temperature for 6 days. The obtained gel was pulverized, separated by filtration and washed successively with ethanol (3 × 50 mL) and acetone (2 × 50 mL). The resultant solid was dried under vacuum at 80 °C for 24.0 h to afford a water-insoluble support Cl@Si as a white power (4.46 g).

2.5. Preparation of β-CD immobilized on Cl@Si (CD@Si)

To a solution of mono(6-ethylenediamine)-β-CD (18.00 g) and KI (0.30 g) in dry N,N-dimethylformamide (360 mL), Si@Cl (12.00 g) was added, and the reaction mixture was heated to 110 °C and kept stirring for 72.0 h under nitrogen atmosphere. After cooling to room temperature, the suspension was filtered and the obtained solid was washed successively with N,N-dimethylformamide (100 mL), ethanol (3 × 200 mL) and water (2 × 200 mL). The resultant solid was dried under vacuum at 80 °C for 24.0 h to afford CD@Si as a white power (12.30 g).

The preparation procedure of CD@Si is illustrated in Scheme 1.

Scheme 1 The schematic preparation of CD@Si.

2.6. Characterization

FT-IR spectra of Cl@Si and CD@Si were record on a Nicolet 6700 spectrometer using KBr as the background over the frequency range of 4000–400 cm⁻¹. XPS analysis was carried out on a Kratos AXIS Ultra DLD spectrometer with an Al mono Kα X-ray source (1486.71 eV photons) to investigate the elementary distribution on the surface of Cl@Si and CD@Si. EDX analysis was carried out on a Philips-FEI Tecnai G2 F30 S-Twin transmission electron microscope to investigate the elementary composition of Cl@Si and CD@Si. Contact angle measurements were conducted through dropping water droplets (6 μL) on the surface of Cl@Si and CD@Si respectively, and after waiting for 30 s to let the water droplets on the surface of Cl@Si and CD@Si become stable, the KRÜSS DSA100 Drop Shape Analysis System was employed to measure the contact angle and determine their hydrophobicity/hydrophilicity level. TGA was carried out under nitrogen atmosphere from room temperature to 800 °C at a speed of 10 °C min⁻¹ employing a STA 449 F3 Jupiter® spectrometer to monitor the weight loss of Cl@Si and CD@Si, and to determine the immobilized amount of β-CD on the surface of hybrid silica. The solid state NMR (¹³C CP MAS) data of Cl@Si and CD@Si were collected using a BRUKER AVANCE II 300 MHz spectrometer to determine the successful immobilization of β-CD on the surface of hybrid silica. The morphologies of Cl@Si and CD@Si were observed by SEM (Hitachi S-4700 (II) instrument). The XRD patterns were recorded on a X’ Pert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the range of 5–60°.

2.7. Adsorption experiment

Batch experiments were carried out to investigate the adsorption performance of the adsorbent CD@Si to p-nitrophenol because of the simplicity and reliability of this method. In the experiments, CD@Si (0.10 g) was added to a 25 mL glass conical flask placed in a thermostatic water bath shaking incubator at 283 K, 293 K, 303 K and 313 K respectively, and then an aqueous solution of p-nitrophenol (10.0 mL) at the initial concentration of 0.10 g L⁻¹, 0.50 g L⁻¹, 1.00 g L⁻¹, 1.50 g L⁻¹, 2.00 g L⁻¹, 2.50 g L⁻¹, 3.00 g L⁻¹ and 4.00 g L⁻¹, and at 283 K, 293 K, 303 K and 313 K was added, respectively. After shaking at 150 rpm for 2.0 h, the suspensions were poured into distilled water (90 mL) quickly to weaken the further adsorption and 10 mL of the upper liquid was filtered through a 0.45 μm filter membrane quickly to remove the adsorbent residue. The concentration of p-nitrophenol in the filtrate was detected on a UV-vis spectrophotometer (Shimadzu, UV-2250) at a wavelength of 398 nm. In order to investigate the effect of the solution pH, a KH₂PO₄–K₂HPO₄ buffer solution was employed. In the experiments to determine the effect of temperature, the aqueous solution of p-nitrophenol was held at 283 K, 293 K, 303 K and 313 K respectively, and the adsorption experiments were also performed at above temperature. Each experiment was carried out twice, and the experiment data are the average values. The amount of p-nitrophenol adsorbed onto CD@Si was calculated according to the following equation:where q_t is the adsorption capacity (mg g⁻¹) at time t, C₀ is the initial concentration of p-nitrophenol (g L⁻¹), C_t is the concentration of p-nitrophenol (g L⁻¹) at time t (s), V is the volume of the aqueous solution of p-nitrophenol (mL), and m is the mass of adsorbent CD@Si (g) employed.^5,18,37 Thus the equilibrium adsorption amount was calculated according to the following equation:where q_e is the equilibrium adsorption capacity (mg g⁻¹), C₀ is the initial concentration of p-nitrophenol (g L⁻¹), C_e is the equilibrium concentration of p-nitrophenol (g L⁻¹), V is the volume of the aqueous solution of p-nitrophenol (mL), and m is the mass of adsorbent CD@Si (g) employed.^5,18,37

2.8. Data analysis

2.8.1. Adsorption kinetics. In the kinetic study, two kinetic models, a pseudo-first-order and pseudo-second-order model, were employed to analyse the experimental data and predict the adsorption kinetics in this work. The linear form of the equation for the pseudo-first-order model is described as the following:

ln(q_e − q_t) = ln q_e − k₁t (3)

where q_e and q_t are the adsorption amount of p-nitrophenol on CD@Si (mg g⁻¹) at equilibrium and at time t, respectively, and k₁ is the pseudo-first-order adsorption rate constant (s⁻¹).^18,39,61

The linear form of the equation for the pseudo-second-order model is expressed as the following:

where q_e and q_t are the same as in the pseudo-first-order model, and k₂ is the pseudo-second-order adsorption rate constant (g mg⁻¹ s⁻¹).^18,39,61

2.8.2. Adsorption isotherm. In the adsorption isotherm study, the Langmuir, Freundlich and Tempkin models were employed. The linear form of the equation for the Langmuir model is described as the following:where C_e is the equilibrium concentration of p-nitrophenol in the aqueous solution (g L⁻¹), q_e is the equilibrium adsorption capacity (mg g⁻¹), q_m is the maximum adsorption capacity (mg g⁻¹), and K_L is the equilibrium adsorption constant (L g⁻¹).

The Freundlich model can be expressed in the linear form as the following:

where q_e and C_e are the same as in the Langmuir model, and K_F (L g⁻¹) and n are the Freundlich constants.^37,39

The Tempkin model can be expressed in the linear form as the following:

q_e = k₁ ln k₂ + k₁ ln C_e (7)

where k₁ is the Tempkin constant related to the adsorption heat (L g⁻¹), and k₂ is the Tempkin isotherm constant.^1,14,62

2.8.3. Adsorption thermodynamics. In order to investigate the effect of temperature on the adsorption process, thermodynamic parameters containing the Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were determined employing the following equations:

ΔG = −RT ln K_d (8)

ΔG = ΔH − TΔS (9)

where K_d is the distribution coefficient, R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant, and T is the Kelvin temperature (K).^6,14,63–65 ΔH and ΔS can be obtained from the slope and intercept of the plots of ln K_d versus 1/T, thus ΔG was obtained by employing eqn (9).

2.9. Reusability of CD@Si

CD@Si (0.30 g) was added to a 100 mL glass conical flask placed in a thermostatic water bath shaking incubator at 303 K, and then an aqueous solution of p-nitrophenol (30.0 mL) in 2.0 g L⁻¹ and at pH 7.0 was added. After shaking at 150 rpm for 2.0 h, the suspension was centrifuged at 4500 rpm for 10.0 min. The upper liquid was filtered through a 0.45 μm filter membrane quickly to remove the adsorbent residue. The concentration of p-nitrophenol in the filtrate was detected on a UV-vis spectrophotometer (Shimadzu, UV-2250) at a wavelength of 398 nm to determine the amount of p-nitrophenol adsorbed onto CD@Si. The obtained solid was washed successively with ethanol (3 × 10 mL) and water (3 × 10 mL) under ultrasonic and centrifugal conditions, and then dried under vacuum at 80 °C for 8.0 h. Then the regenerated CD@Si was subjected to the second run under the same adsorption conditions, and for the loss of adsorbent in the reusability experiment, the volume of the p-nitrophenol aqueous solution added should be decreased proportionally.

3. Results and discussion

3.1. Characterization

The FT-IR spectra of Cl@Si, CD@Si, and β-CD are presented in Fig. 1. The strong and broad peaks at around 3380 cm⁻¹ are attributed to the O–H stretching vibrations, the peaks at around 2900 cm⁻¹ belong to the C–H stretching vibrations and the peaks at around 1650 cm⁻¹ are the N–H deformation vibrations in the secondary amine. Comparing the FT-IR spectrum of CD@Si with Cl@Si, the O–H stretching vibration in CD@Si becomes stronger for the abundant hydroxyl groups existing in the β-CD molecule, and the difference of the C–H stretching vibration at around 2900 cm⁻¹ is not very obvious. When β-CD was immobilized onto the surface of Cl@Si, the N–H deformation vibration at around 1650 cm⁻¹ appeared in CD@Si for the existence of the secondary amine groups. And the not obvious peak at around 1650 cm⁻¹ appearing in the spectrum of Cl@Si mainly comes from the bending vibrations of a little adsorbed water, not the N–H deformation vibrations in the amine groups. Thus, it can be concluded from the FT-IR spectra that β-CD has been immobilized on the surface of hybrid silica successfully. Another strong evidence for the successful immobilization of β-CD on the surface of hybrid silica is the XPS and EDX spectra as shown in Fig. 2. Compared with Cl@Si, in the XPS spectrum of CD@Si, the chlorine element decreased and the nitrogen element was detected for the nucleophilic substitution reaction between nitrogen atoms and chlorine atoms. A similar elementary change was also observed in the EDX spectra of Cl@Si and CD@Si. Both the XPS analysis and the DEX analysis are very direct evidences for the successful immobilization of β-CD on the surface of hybrid silica. The successful immobilization of β-CD on the surface of hybrid silica was also illustrated by the contact angle measurement in Fig. 3. The surface of Cl@Si is hydrophobic due to the existence of hydrophobic –CH₂CH₂CH₂Cl groups, so the contact angle of water on it can reach up to 143.6°. But the surface of CD@Si is hydrophilic due to the existence of lots of –OH groups, so a water droplet (6 μL) on the surface of CD@Si can seep into it quickly (within 30 s) as is shown in Fig. 3(b). So the contact angle measurement is also a very visual evidence for the successful immobilization of β-CD on the surface of hybrid silica. Solid-state CP MAS ¹³C NMR analysis of Cl@Si and CD@Si was also conducted to characterize the successful immobilization as shown in Fig. 4. The broad signals from 55.5 to 6.5 ppm in Fig. 4(a) belong to the carbon atoms in CPTES on the surface of hybrid silica (Cl@Si). The broad signals from 160.9 to 5.8 ppm in Fig. 4(b) are attributed to the carbon atoms in CPTES and β-CD. Thus, from the systematic characterization above, it is confirmed that β-CD has been immobilized onto the surface of hybrid silica successfully.

Fig. 1 FT-IR spectra of (a) Cl@Si, (b) CD@Si, and (c) β-CD.

Fig. 2 XPS spectra of (a) Cl@Si, (b) CD@Si, and (c) β-CD, and EDX spectra of (d) Cl@Si and (e) CD@Si.

Fig. 3 Contact angles of water on (a) Cl@Si, and (b) CD@Si.

Fig. 4 Spectra of solid-state CP MAS ¹³C NMR of (a) Cl@Si, and (b) CD@Si.

The amount of β-CD immobilized on the surface of hybrid silica was evaluated through the TGA spectra of Cl@Si and CD@Si under an atmosphere of nitrogen. As shown in Fig. 5, the total weight loss of Cl@Si over the full temperature range is about 19% for the evaporation of adsorbed water on the surface of Cl@Si, the dehydration of the hydroxyl groups on the surface, and the thermal decomposition of CPTES moieties. Before 250 °C, the weight loss is about 2% because the surface of Cl@Si is hydrophobic and no water can be adsorbed on its surface. In the range of 250 °C to 800 °C, the weight loss of Cl@Si is about 17%, which is mainly contributed to by the thermal decomposition of CPTES moieties. Thus, the organic layer on hybrid silica (Cl@Si) is about 17%. As for CD@Si, the weight loss before 150 °C is about 3% for the evaporation of adsorbed water on the surface of CD@Si and the included water in the cavity of β-CD. In the range of 150 °C to 800 °C, the weight loss of CD@Si is about 25%, which is contributed to by the thermal decomposition of β-CD moieties and –CH₂CH₂CH₂– moieties. Thus, the amount of β-CD immobilized on the surface of hybrid silica is about 25% (m m⁻¹) with omission of the mass contribution of –CH₂CH₂CH₂– moieties. The TGA curves also provide further evidence for the successful immobilization of β-CD on the surface of hybrid silica.

Fig. 5 TGA curves of (a) Cl@Si, and (b) CD@Si.

The morphologies of Cl@Si and CD@Si were analyzed by SEM as is shown in Fig. 6. It is obvious that their morphologies are uniform in both shape and size, and the immobilization of β-CD on the surface of hybrid silica did not affect the morphology of hybrid silica. And the morphological differences between Cl@Si and CD@Si are not very significant. The XRD patterns of Cl@Si, CD@Si and β-CD are shown in Fig. 7, in which both Cl@Si and CD@Si exhibit a broad peak centered at about 22° which is the characteristic peak of SiO₂.⁶⁶ Thus, the hybrid silica we prepared was confirmed as SiO₂, and in the immobilization process of β-CD, the original structure of hybrid silica had not been changed. The loss of the characteristic peaks of β-CD when β-CD is immobilized onto Cl@Si is mainly attributed to the monolayer distribution of β-CD on the surface of Cl@Si. And in the monomolecular layer of β-CD, there are not enough molecules to form β-CD crystals, so the β-CD characteristic peaks in the XRD measurement can not be observed.

Fig. 6 SEM images of (a) Cl@Si, and (b) CD@Si.

Fig. 7 XRD patterns of (a) Cl@Si, (b) CD@Si, and (c) β-CD.

From the systematic characterization above, it is confirmed that β-CD has been immobilized onto the surface of hybrid silica successfully, and the immobilization process did not affect the original structure of hybrid silica.

3.2. Effect of the solution initial pH on the adsorption capacity

The adsorption of p-nitrophenol from aqueous solution by CD@Si is influenced significantly by the pH value of the solution, because pH not only affects the protonation of the adsorbent CD@Si, but also plays a crucial role in the ionization of p-nitrophenol.^18,67 Thus, the effect of solution pH on the adsorption performance of CD@Si to p-nitrophenol was investigated systematically in the pH range of 5.0 to 10.0. As shown in Fig. 8, with the increase of solution pH from 5.0 to 7.0, the adsorption capacity increased from 8.9 to 15.2 mg g⁻¹, 19.6 to 40.4 mg g⁻¹, 27.5 to 53.7 mg g⁻¹, and 34.3 to 69.6 mg g⁻¹ for the initial concentration of 0.50, 2.00, 3.00, and 4.00 g L⁻¹, respectively. A further increase in the solution pH leads to a slight decrease in the adsorption capacity. The above profile of adsorption capacity to pH is explained as the following: in this adsorption system, the adsorption capacity contains two parts: inclusion in the hydrophobic cavity of β-CD and hydrogen bond interactions between β-CD units on CD@Si and p-nitrophenol, and in acidic conditions, both the β-CD units on CD@Si and p-nitrophenol become protonated, so p-nitrophenol can not be adsorbed onto CD@Si efficiently due to the electrostatic repulsion between them. And when the pH is increased, both the protonation of CD@Si and p-nitrophenol became weak, so the adsorption capacity increased, which could be attributed to the hydrogen bond interactions between the β-CD units on CD@Si and p-nitrophenol. In neutral conditions, the protonation of the adsorbent CD@Si and p-nitrophenol disappears, and the hydrogen bond interactions between them favour the adsorption of p-nitrophenol onto CD@Si. In basic conditions, ionization of p-nitrophenol occurs, which brings about electrostatic repulsion between the p-nitrophenol molecules, and weakens the adsorption of p-nitrophenol on the surface of CD@Si as multi-layer molecules. Thus the adsorption capacity reaches its maximum in pH 7.0. Therefore, it is concluded that the adsorption of p-nitrophenol by CD@Si is a pH-dependent process, and in neutral conditions (pH 7.0), CD@Si can reach its optimal adsorption performance.

Fig. 8 Effect of the solution initial pH on the adsorption of p-nitrophenol by CD@Si. Conditions: initial concentration of p-nitrophenol: 0.50 g L⁻¹, 2.00 g L⁻¹, 3.00 g L⁻¹, and 4.00 g L⁻¹ in 0.05 mol L⁻¹ KH₂PO₄–K₂HPO₄ buffer solution. Solution volume of p-nitrophenol: 10.0 mL. Adsorbent CD@Si: 0.10 g. Shaking speed: 150 rpm. Temperature: 303 K. Contact time: 2.0 h.

3.3. Effect of contact time on the adsorption capacity

The objective of our study is to develop an adsorbent with a high adsorption rate and a satisfying adsorption capacity for p-nitrophenol and its analogues based on β-CD, so the adsorption equilibrium time and the equilibrium adsorption capacity are two major parameters investigated in this work. The profiles of adsorption capacity to contact time at the initial concentration of 1.00, 2.00, 3.00, and 4.00 g L⁻¹ are presented in Fig. 9, and what should be mentioned here is that in our work, in order to eliminate the effect of adsorption during the separation process of CD@Si from the adsorption system, after shaking for a certain time, the suspensions (10 mL) were poured into distilled water (90 mL) quickly to weaken further adsorption. Then 10 mL of the upper liquid was filtered through a 0.45 μm filter membrane quickly to remove the adsorbent residue. As shown in Fig. 9, the adsorption equilibrium in the adsorption of p-nitrophenol by CD@Si can be obtained within only 15 s for various initial concentrations, which is a very fast adsorption process, and the initial concentration of p-nitrophenol does not have any significant effect on the adsorption rate. It is also observed from Fig. 9 that, with the increase in the initial concentration, the equilibrium adsorption capacity increased, because both the inclusion process between p-nitrophenol and β-CD units, and the hydrogen bond interactions between p-nitrophenol and the β-CD units on CD@Si, are reversible, and a high initial concentration favours p-nitrophenol being adsorbed onto the surface of CD@Si.

Fig. 9 Effect of contact time on the adsorption of p-nitrophenol by CD@Si. Conditions: initial concentration of p-nitrophenol: 1.00 g L⁻¹, 2.00 g L⁻¹, 3.00 g L⁻¹, and 4.00 g L⁻¹ in 0.05 mol L⁻¹ KH₂PO₄–K₂HPO₄ buffer solution. Initial pH of solution: 7.0. Solution volume of p-nitrophenol: 10.0 mL. Adsorbent CD@Si: 0.10 g. Shaking speed: 150 rpm. Temperature: 303 K.

In order to further verify the fast adsorption performance of CD@Si to p-nitrophenol, the performance of some reported adsorbents based on β-CD for the adsorption of p-nitrophenol or its analogues are listed in Table 1 for comparison. In Table 1, it is obvious that the equilibrium adsorption capacity of CD@Si is not the most satisfying one, but considering the time needed to reach adsorption equilibrium, CD@Si is the most promising one, because the adsorption equilibrium can be achieved within 15 s with an acceptable equilibrium adsorption capacity, 69.6 mg g⁻¹, which is the highest adsorption capacity per unit time for the adsorbents listed in Table 1. Thus, it is very suitable to apply CD@Si to some cases of emergency needing fast adsorption such as the leakage of p-nitrophenol in a large amount to natural water. In that case, preventing p-nitrophenol from diffusing with the flow of water quickly is the first issue that needs to be taken into consideration, and the maximum removal of p-nitrophenol in a unit time is the most efficient strategy. Therefore, the prepared adsorbent CD@Si in this work is a very attractive choice in the fast adsorption of p-nitrophenol. The fast adsorption performance is mainly attributed to the maintenance of the hydrophobic cavity of β-CD because only one hydroxyl group in the β-CD molecule was employed in the preparation of CD@Si. And the distribution of β-CD units on the surface of hybrid silica is another important reason for the fast adsorption because no diffusion resistance exists. When CD@Si enters the aqueous solution of p-nitrophenol, p-nitrophenol molecules transfer from the solution into the hydrophobic cavity of β-CD units and onto the surface of the β-CD layer quickly, thus the adsorption equilibrium is obtained within only 15 s with an acceptable adsorption capacity.

Table 1 Comparison of some reported adsorbents based on β-CD for the adsorption of p-nitrophenol or its analogues

No.	Adsorbent	Solute	Equilibrium time	Equilibrium adsorption (mg g^−1)
1	CD@Si	p-Nitrophenol	15 s	69.6
2	Cl@Si	p-Nitrophenol	15 s	5.8
3 (ref. 67)	β-CD–zeolite	p-Nitrophenol	60 min	0.3
4 (ref. 2)	β-CD–carbon nanotube–Fe3O4	p-Nitrophenol	180 min	33.3
5 (ref. 68)	β-CD–chitosan–Fe3O4	Hydroquinol	50 min	192.7
6 (ref. 69)	β-CD–attapulgite	2,6-Dichlorophenol	100 min	58.1
7 (ref. 69)	β-CD–attapulgite	2,6-Dichlorophenol	100 min	55.2
8 (ref. 70)	β-CD–poly(4-vinylbenzyl chloride)	2-Chlorophenol	180 min	61.7
9 (ref. 71)	β-CD polymer	Phenol	300 min	29.0
10 (ref. 42)	β-CD–chitosan	4-Chlorophenol	60 min	35.7

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By comparing the adsorption capacity of CD@Si (69.6 mg g⁻¹) with Cl@Si (5.8 mg g⁻¹), we also can obtain that, in the adsorbent CD@Si, the adsorption function mainly comes from the β-CD unit. The adsorption capacity of Cl@Si for p-nitrophenol is at a negligible level. The poor adsorption performance of Cl@Si is mainly because of its hydrophobicity, so it can not be dispersed well in aqueous solution at all. The above experimental phenomenon demonstrates the importance of β-CD units in the construction of adsorbents too, especially the ones applied in aqueous solution.

3.4. Adsorption kinetics

The adsorption capacity of CD@Si to p-nitrophenol versus contact time is illustrated in Fig. 9 at the initial concentrations of 1.00, 2.00, 3.00, and 4.00 g L⁻¹ at 303 K. It is obvious that the adsorption is a very fast process and the adsorption equilibrium can be obtained within only 15 s with an acceptable adsorption capacity. In order to further investigate this adsorption process and the adsorption mechanism, two kinetic models, the pseudo-first-order and pseudo-second-order model, were employed to analyse the adsorption kinetics of p-nitrophenol onto CD@Si following eqn (3) and (4). The plots of ln(q_e − q_t) versus t (pseudo-first-order model) and t/q_t versus t (pseudo-second-order model) at the initial concentrations of 1.00, 2.00, 3.00, and 4.00 g L⁻¹ are presented in Fig. 10. It is very clear that the pseudo-second-order model fits the adsorption kinetics of p-nitrophenol onto CD@Si well with a high correlation coefficient (R² >99%). As for the pseudo-first-order model, a liner relationship between ln(q_e − q_t) and t can not be obtained, meaning that the pseudo-first-order model can not describe the adsorption kinetics in this work. The pseudo-second-order kinetic model also was confirmed through the similar values of the experimental equilibrium adsorption capacity, q_e,exp, and the calculated ones, q_e,cal, as shown in Table 2. The experimental equilibrium adsorption capacities, q_e,exp, are 23.7, 40.4, 53.7, and 69.6 mg g⁻¹, and meanwhile the calculated ones, q_e,cal, are 24.7, 42.5, 55.7 and 71.5 mg g⁻¹ at the initial concentrations of 1.00, 2.00, 3.00, and 4.00 g L⁻¹ respectively, which shows a very good consistency with little error. The statistical errors in the adsorption kinetic study are mainly attributed to the errors in the weight of the adsorbent CD@Si and solute p-nitrophenol, and in the preparation of the aqueous solution of p-nitrophenol. Another origin of the statistical error is the systematic error in the UV measurement. The pseudo-second-order kinetics also suggests that in the adsorption of p-nitrophenol onto CD@Si, the rate-limiting step may involve valence forces through electrons sharing or exchange between p-nitrophenol and CD@Si such as hydrogen bonding,^72,73 which has been mentioned above. The pseudo-second-order adsorption mechanism is the predominant mechanism in the adsorption of p-nitrophenol onto CD@Si.

Fig. 10 Pseudo-first-order model (a) and pseudo-second-order model (b) fits for the adsorption of p-nitrophenol by CD@Si. Conditions: initial concentration of p-nitrophenol: 1.00 g L⁻¹, 2.00 g L⁻¹, 3.00 g L⁻¹, and 4.00 g L⁻¹ in 0.05 mol L⁻¹ KH₂PO₄–K₂HPO₄ buffer solution. Initial pH of solution: 7.0. Solution volume of p-nitrophenol: 10.0 mL. Adsorbent CD@Si: 0.10 g. Shaking speed: 150 rpm. Temperature: 303 K.

Table 2 The pseudo-first-order and pseudo-second-order kinetic parameters for the adsorption of p-nitrophenol by CD@Si at 303 K

C0 (g L^−1)	qe,exp (mg g^−1)	Pseudo-first-order			Pseudo-second-order
		qe1,cal (mg g^−1)	k1 × 10^−2 (s^−1)	R^2	qe2,cal (mg g^−1)	k2 × 10^−2 (g mg^−1 s^−1)	R^2
1.00	23.7	8.484	15.97	0.7058	24.7	3.776	0.9992
2.00	40.4	6.886	13.29	0.4845	42.5	6.327	0.9997
3.00	53.7	15.87	13.01	0.6742	55.7	3.272	0.9988
4.00	69.6	15.56	11.34	0.4888	71.5	1.753	0.9987

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3.5. Adsorption isotherms

In order to further understand the adsorption mechanism, the adsorption isotherms of CD@Si to p-nitrophenol at four different temperatures (283 K, 293 K, 303 K, and 313 K) were measured, which is an important illustration of how the adsorbate interacts with the adsorbent in the equilibrium state. From the study of adsorption isotherms, the maximum adsorption capacity of CD@Si to p-nitrophenol also can be obtained. Several adsorption isotherm models have been developed to fit the adsorption data. In this work, three isotherm models, the Langmuir model, Freundlich model and Tempkin model, were employed, which can be expressed linearly following eqn (5)–(7). For the Langmuir model, the basic theory assumption is that all the adsorption sites on the adsorbent surface are homogeneous, having equal affinity to the adsorbate molecules, and the occupation of one adsorption site do not have any effect on the performance of the adjacent adsorption site. In the Langmuir adsorption model, the adsorbate molecules usually form a monomolecular layer on the surface of the adsorbent.^5,74 For the Freundlich model, the basic theory assumption is that the distribution of adsorption sites on the adsorbent surface is heterogeneous, the stronger adsorption sites are occupied firstly, and with the increase in the coverage of adsorbent surface, the affinity of adsorbent to adsorbate decreases.^5,74 For adsorbents with a heterogeneous surface, the Freundlich model is closer to reality than the Langmuir model. For the Tempkin model, the basic theory assumption is that in the adsorption process, the distribution of adsorption energy is uniform, and the adsorption energy decreases linearly with the increase in the adsorbent coverage.^1,62

The adsorption isotherm fitting was conducted employing eqn (5)–(7) for the Langmuir model, Freundlich model and Tempkin model respectively. The plots of C_e/q_e versus C_e (Langmuir model), ln q_e versus ln C_e (Freundlich model), and q_e versus ln C_e (Tempkin model) at 283 K, 293 K, 303 K, and 313 K are presented in Fig. 11. The initial concentrations of p-nitrophenol in the adsorption isotherm study are 0.10, 0.50, 1.00, 1.50, 2.00, 2.50, 3.00, and 4.00 g L⁻¹. And the adsorption isotherm parameters are summarized in Table 3 which were obtained from the slope and intercept of the linear plots. It is very obvious that the Freundlich model with a high correlation coefficient (R² >98%) at all the temperatures involved fits the adsorption data better than the Langmuir model and the Tempkin model, which indicates that the distribution of adsorption sites on the surface of CD@Si is heterogeneous, and the stronger adsorption sites are occupied first. Both the correlation coefficients for the Langmuir model and the Tempkin model are lower than that of the Freundlich model. The greater n value in the Freundlich model at all the temperatures involved means that the adsorption process is favourable in a wide temperature range. Thus, in this work, the adsorption of p-nitrophenol onto CD@Si obeys the Freundlich model.

Fig. 11 Adsorption isotherm fits for the adsorption of p-nitrophenol by CD@Si: (a) Langmuir model, (b) Freundlich model, and (c) Tempkin model. Conditions: 283 K, 293 K, 303 K, 313 K. Initial pH of solution: 7.0. Solution volume of p-nitrophenol: 10.0 mL. Adsorbent CD@Si: 0.10 g. Shaking speed: 150 rpm. Contact time: 2.0 h.

Table 3 The adsorption isotherm parameters for the adsorption of p-nitrophenol by CD@Si

Model	Parameters	Parameter value
		283 K	293 K	303 K	313 K
Langmuir model	qm (mg g^−1)	108.5	89.1	116.1	80.3
	KL (L g^−1)	0.6144	0.8548	0.4048	0.7016
	R^2	0.7501	0.9330	0.4999	0.9277
Freundlich model	n	1.776	1.708	1.553	1.555
	KF (mg g^−1)	37.53	36.13	34.10	29.13
	R^2	0.9845	0.9912	0.9833	0.9855
Tempkin model	k1	16.09	15.06	16.00	13.74
	k2	16.95	17.72	12.19	13.78
	R^2	0.7644	0.8913	0.7257	0.8840

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3.6. Adsorption thermodynamics

In order to access the thermodynamics parameters, the adsorption isotherms of p-nitrophenol onto CD@Si were analyzed at 283 K, 293 K, and 303 K at the initial concentrations of 0.10 g L⁻¹, 0.50 g L⁻¹, and 4.00 g L⁻¹ respectively. The plots of ln K_d versus 1/T are listed in Fig. 12 and the thermodynamic parameters obtained from the slope and intercept following eqn (10) are summarized in Table 4. It is obvious that the ΔG values (−7.474 kJ mol⁻¹ ∼ −11.91 kJ mol⁻¹) are negative at all the temperatures and initial concentrations studied, which means that the adsorption of p-nitrophenol onto CD@Si is a spontaneous process, and does not need any energy from the external environment. We can also observe that the Gibbs free energy change (ΔG) increases with the increase in the temperature, which means that a lower temperature favours the adsorption of p-nitrophenol onto CD@Si. The negative values of enthalpy change (ΔH) indicate that the adsorption of p-nitrophenol onto CD@Si is an exothermic process, which is consistent with the phenomenon that the adsorption of p-nitrophenol onto CD@Si is more favourable at lower temperatures. And the negative values of entropy change (ΔS) tell us that the randomness decreases when p-nitrophenol is adsorbed onto CD@Si. Thus, it is concluded from the adsorption thermodynamics study that the adsorption of p-nitrophenol onto CD@Si is a feasible, spontaneous and exothermic process, and this process is more favourable at lower temperatures.

Fig. 12 Plots of ln K_d versus 1/T for the adsorption of p-nitrophenol by CD@Si. Conditions: 0.10 g L⁻¹, 0.50 g L⁻¹, and 4.00 g L⁻¹. Temperature: 283 K, 293 K, and 303 K. Initial pH of solution: 7.0. Solution volume of p-nitrophenol: 10.0 mL. Adsorbent CD@Si: 0.10 g. Shaking speed: 150 rpm. Contact time: 2.0 h.

Table 4 The thermodynamic parameters for the adsorption of p-nitrophenol by CD@Si

C0 (g L^−1)	Temperature (K)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)	ΔG (kJ mol^−1)
0.10	283	−15.71	−13.44	−11.91
	293			−11.77
	303			−11.64
0.50	283	−11.60	−7.383	−9.511
	293			−9.437
	303			−9.363
4.00	283	−8.155	−2.249	−7.519
	293			−7.496
	303			−7.474

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3.7. Reusability of CD@Si

Reusability is a very important requirement for adsorbents, which will reduce the material cost significantly in practical applications. The reusability of CD@Si in the adsorption of p-nitrophenol is illustrated in Fig. 13. For the loss of adsorbent in each reusability experiment, the volume of the p-nitrophenol aqueous solution added should be decreased proportionally in each run. From Fig. 13, it could be observed that as the reusability experiments were conducted in sequence for five runs, the equilibrium adsorption capacity decreased from 41.5 mg g⁻¹ to 31.7 mg g⁻¹, which is an acceptable adsorption capacity, and is about 76% of the initial adsorption capacity. The decrease in the equilibrium adsorption capacity is mainly attributed to the incomplete desorption of p-nitrophenol from CD@Si. On the whole, considering the fast adsorption property, CD@Si is a promising adsorbent in practical applications and can be reused for at least five runs with an acceptable adsorption capacity.

Fig. 13 Reusability of CD@Si in the adsorption of p-nitrophenol. Conditions: initial concentration of p-nitrophenol: 2.00 g L⁻¹ in 0.05 mol L⁻¹ KH₂PO₄–K₂HPO₄ buffer solution. Solution volume of p-nitrophenol: 30.0 mL. Adsorbent CD@Si: 0.30 g. Shaking speed: 150 rpm. Temperature: 303 K. Contact time: 2.0 h.

3.8. Adsorption mechanism

Based on the systematic study on adsorption kinetics, adsorption isotherms, and adsorption thermodynamics above, the adsorption mechanism of p-nitrophenol onto CD@Si was speculated. In the TGA spectrum of CD@Si (Fig. 5), the amount of β-CD immobilized on the surface of hybrid silica is about 25% (m m⁻¹), so even if all the β-CD units form inclusion complexes with p-nitrophenol in the molar ratio of 1 : 1 as reported,^75–78 the maximum adsorption capacity of CD@Si for p-nitrophenol is about 30.6 mg g⁻¹. Considering the inclusion equilibrium between β-CD units and p-nitrophenol, not all of the β-CD units could be occupied by p-nitrophenol, thus the maximum adsorption capacity in this work should be below 30.6 mg g⁻¹. But in fact, the maximum adsorption capacity is higher than 30.6 mg g⁻¹, especially for a high initial concentration of 3.00 g L⁻¹ and 4.00 g L⁻¹. Thus besides forming inclusion complexes, there is another interaction form between CD@Si and p-nitrophenol, which is hydrogen bonding, as reported in some similar literature.⁵⁴ The formation of inclusion complexes between β-CD units and p-nitrophenol could be illustrated through the ¹H ROESY NMR study. Employing mono(6-ethylenediamine)-β-CD as the representative of β-CD units on CD@Si, obvious correlation peaks between the H atoms located in the phenyl ring of p-nitrophenol and H-3, and H-5 in the cavity of the parent β-CD can be observed (Fig. 14), which is strong evidence for the formation of inclusion complexes. The existence of hydrogen bond interactions is indicated through comparing the FT-IR spectra of CD@Si before and after the adsorption process as shown in Fig. 15. When p-nitrophenol was adsorbed onto CD@Si, the O–H stretching vibrations of the β-CD units on CD@Si shift from around 3388 cm⁻¹ to around 3261 cm⁻¹, which is attributed to the formation of hydrogen bonds between p-nitrophenol and the β-CD units on CD@Si. And the appearance of peaks at 1592 cm⁻¹ and 1496 cm⁻¹ in the FT-IR spectrum of CD@Si after the adsorption process is further powerful evidence for the adsorption of p-nitrophenol onto CD@Si, which belong to the stretching vibrations of C=C in the phenyl ring. Thus, based on the analysis above, the adsorption mechanism of p-nitrophenol onto CD@Si can be summarized as shown in Fig. 16. Inclusion complexes and hydrogen bond interactions are two origins of p-nitrophenol being adsorbed onto CD@Si.

Fig. 14 Partial ¹H ROESY NMR spectrum of the inclusion complex between mono(6-ethylenediamine)-β-CD and p-nitrophenol.

Fig. 15 FT-IR spectra of (a) CD@Si (before adsorption), (b) CD@Si (after adsorption), and (c) p-nitrophenol.

Fig. 16 Speculated adsorption mechanism of p-nitrophenol onto CD@Si.

4. Conclusions

In summary, reproducible β-CD has been immobilized onto the surface of hybrid silica successfully in this work, and the successful immobilization was confirmed through FT-IR, XPS, EDX, contact angle measurements, TGA, solid-state ¹³C NMR, SEM, and XRD analyses. Applied in the adsorption of p-nitrophenol from the aqueous phase, CD@Si exhibits satisfying fast adsorption performance. The adsorption equilibrium can be reached in 15 s with an acceptable equilibrium adsorption capacity of 69.6 mg g⁻¹ in neutral conditions (pH = 7.0), which is much faster than many reported adsorbents based on β-CD for the adsorption of p-nitrophenol or its analogues, which is mainly attributed to the maintenance of the hydrophobic cavity of β-CD and the distribution of β-CD units on the surface of hybrid silica. The fast adsorption performance demonstrates that CD@Si is a very suitable and promising adsorbent for fast adsorption in cases of emergency, when a large amount of p-nitrophenol is leaked to natural water. And the adsorption of p-nitrophenol onto CD@Si follows the pseudo-second-order adsorption model, obeys the Freundlich model, and is a feasible, spontaneous, and exothermic process, which is more favorable at lower temperatures. Based on the adsorption mechanism study, it is concluded that an inclusion complex and hydrogen bond interactions are two origins of p-nitrophenol being adsorbed onto CD@Si. At last, from the perspective of economy, CD@Si can be recycled and reused for at least five runs with an acceptable adsorption capacity. In general, CD@Si is a very attractive candidate adsorbent in the fast adsorption of p-nitrophenol or its analogues from the aqueous phase, especially in cases of emergency, and possesses promising application potential in wastewater treatment without any additional environmental pollution in its application, which is a genuine environmental management strategy. And this work also provides a strategy to increase the adsorption rate of adsorbents based on β-CD.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21306176, 21276006, 21476270), Scientific Research Launching Foundation of Zhejiang University of Technology (Grant No. G2817101103) and Zhejiang Provincial Natural Science Foundation (Grant No. LQ14B020002).

References

1. R. Arasteh, M. Masoumi, A. M. Rashidi, L. Moradi, V. Samimi and S. T. Mostafavi, Appl. Surf. Sci., 2010, 256, 4447.
2. W. Liu, X. Y. Jiang and X. Q. Chen, Appl. Surf. Sci., 2014, 320, 764.
3. A. E. Ofomaja and E. I. Unuabonah, Carbohydr. Polym., 2011, 83, 1192.
4. L. M. Cotoruelo, M. D. Marques, F. J. Diaz, J. Rodriguez-Mirasol, J. J. Rodriguez and T. Cordero, Chem. Eng. J., 2012, 184, 176.
5. B. Sarkar, Y. F. Xi, M. Megharaj, G. S. R. Krishnamurti and R. Naidu, J. Colloid Interface Sci., 2010, 350, 295.
6. S. Han, F. Zhao, J. Sun, B. Wang, R. Y. Wei and S. Q. Yan, J. Magn. Magn. Mater., 2013, 341, 133.
7. T. O. Isichei and F. E. Okieimen, Environ. Pollut., 2014, 3, 99.
8. Y. Y. Sun, J. B. Zhou, W. Q. Cai, R. S. Zhao and J. P. Yuan, Appl. Surf. Sci., 2015, 345, 897.
9. B. Sarkar, M. Megharaj, Y. F. Xi and R. Naidu, Chem. Eng. J., 2012, 185, 35.
10. B. J. Liu, F. Yang, Y. X. Zou and Y. Peng, J. Chem. Eng. Data, 2014, 59, 1476.
11. O. E. A. A. Adam and A. H. Al-Dujaili, J. Chem., 2013 DOI:10.1155/2013/694029.
12. Y. Park, G. A. Ayoko, R. Kurdi, E. Horvath, J. Kristof and R. L. Frost, J. Colloid Interface Sci., 2013, 406, 196.
13. F. Ahmad, W. M. A. W. Daud, M. A. Ahmad and R. Radzi, Chem. Eng. J., 2011, 178, 461.
14. G. H. Xue, M. L. Gao, Z. Gu, Z. X. Luo and Z. C. Hu, Chem. Eng. J., 2013, 218, 223.
15. J. Rivera-Utrilla, M. Sanchez-Polo, V. Gomez-Serrano, P. M. Alvarez, M. C. M. Alvim-Ferraz and J. M. Dias, J. Hazard. Mater., 2011, 187, 1.
16. M. H. Entezari and T. R. Bastami, J. Hazard. Mater., 2006, 137, 959.
17. T. R. Bastami and M. H. Entezari, Chem. Eng. J., 2012, 210, 510.
18. B. Zhang, F. Li, T. Wu, D. J. Sun and Y. J. Li, Colloids Surf., A, 2015, 464, 78.
19. O. Gimeno, M. Carbajo, F. J. Beltran and F. J. Rivas, J. Hazard. Mater., 2005, 119, 99.
20. M. Ksibi, A. Zemzemi and R. Boukchina, J. Photochem. Photobiol., A, 2003, 159, 61.
21. P. Canizares, J. Lobato, R. Paz, M. A. Rodrigo and C. Saez, Water Res., 2005, 39, 2687.
22. S. F. Shen, S. E. Kentish and G. W. Stevens, Sep. Purif. Technol., 2012, 95, 80.
23. P. Praveen and K. C. Loh, J. Membr. Sci., 2013, 437, 1.
24. S. W. Peretti, C. J. Tompkins, J. L. Goodall and A. S. Michaels, J. Membr. Sci., 2002, 195, 193.
25. Y. X. Yao, H. B. Li, J. Y. Liu, X. L. Tan, J. G. Yu and Z. G. Peng, J. Nanomater., 2014 DOI:10.1155/2014/571745.
26. M. Erdem, E. Yuksel, T. Tay, Y. Cimen and H. Turk, J. Colloid Interface Sci., 2009, 333, 40.
27. B. Koubaissy, G. Joly, I. Batonneau-Gener and P. Magnoux, Ind. Eng. Chem. Res., 2011, 50, 5705.
28. J. H. Huang, C. Yan and K. L. Huang, J. Colloid Interface Sci., 2009, 332, 60.
29. J. Szejtli, Chem. Rev., 1998, 98, 1743.
30. R. Villalonga, R. Cao and A. Fragoso, Chem. Rev., 2007, 107, 3088.
31. F. Sallas and R. Darcy, Eur. J. Org. Chem., 2008, 957.
32. M. A. Abdel-Naby, H. A. El-Refai and A. F. Abdel-Fattah, J. Appl. Microbiol., 2011, 111, 1129.
33. L. X. Zhang, H. Zhang, F. Gao, H. Y. Peng, Y. H. Ruan, Y. Z. Xu and W. G. Weng, RSC Adv., 2015, 5, 12007.
34. Y. J. Yao, X. W. Liu, T. Liu, J. Zhou, J. Zhu, G. Sun and D. N. He, RSC Adv., 2015, 5, 6305.
35. J. Q. Zhang, D. Wu, K. M. Jiang, D. Zhang, X. Zheng, C. P. Wan, H. Y. Zhu, X. G. Xie, Y. Jin and J. Lin, Carbohydr. Res., 2015, 406, 55.
36. N. Vilanova and C. Solans, Food Chem., 2015, 175, 529.
37. D. X. Wang, L. L. Liu, X. Y. Jiang, J. G. Yu, X. H. Chen and X. Q. Chen, Appl. Surf. Sci., 2015, 329, 197.
38. H. Q. Wu, J. H. Kong, X. Y. Yao, C. Y. Zhao, Y. L. Dong and X. H. Lu, Chem. Eng. J., 2015, 270, 101.
39. D. X. Wang, L. L. Liu, X. Y. Jiang, J. G. Yu and X. Q. Chen, Colloids Surf., A, 2015, 466, 166.
40. M. L. Wang, P. Liu, Y. Wang, D. M. Zhou, C. Ma, D. J. Zhang and J. H. Zhan, J. Colloid Interface Sci., 2015, 447, 1.
41. Y. B. Zhou, X. C. Gu, R. Z. Zhang and J. Lu, Ind. Eng. Chem. Res., 2014, 53, 887.
42. L. C. Zhou, X. G. Meng, J. W. Fu, Y. C. Yang, P. Yang and C. Mi, Appl. Surf. Sci., 2014, 292, 735.
43. L. D. Wilson, D. Y. Pratt and J. A. Kozinski, J. Colloid Interface Sci., 2013, 393, 271.
44. A. Celebioglu, S. Demirci and T. Uyar, Appl. Surf. Sci., 2014, 305, 581.
45. N. J. Wang, L. L. Zhou, J. Guo, Q. Q. Ye, J. M. Lin and J. Y. Yuan, Appl. Surf. Sci., 2014, 305, 267.
46. A. Z. M. Badruddoza, G. S. S. Hazel, K. Hidajat and M. S. Uddin, Colloids Surf., A, 2010, 367, 85.
47. Y. S. Ji, X. Y. Liu, M. Guan, C. D. Zha, H. Y. Huang, H. X. Zhang and C. M. Wang, J. Sep. Sci., 2009, 32, 2139.
48. L. B. de Carvalho, T. G. Carvalho, Z. M. Magriotis, T. D. Ramalho and L. D. A. Pinto, J. Inclusion Phenom. Macrocyclic Chem., 2014, 78, 77.
49. N. V. Roik and L. A. Belyakova, J. Colloid Interface Sci., 2011, 362, 172.
50. M. Chen, W. H. Ding, J. Wang and G. W. Diao, Ind. Eng. Chem. Res., 2013, 52, 2403.
51. X. D. Liu, L. Yan, W. Y. Yin, L. J. Zhou, G. Tian, J. X. Shi, Z. Y. Yang, D. B. Xiao, Z. J. Gu and Y. L. Zhao, J. Mater. Chem. A, 2014, 2, 12296.
52. X. J. Hu, Y. G. Liu, H. Wang, G. M. Zeng, X. Hu, Y. M. Guo, T. T. Li, A. W. Chen, L. H. Jiang and F. Y. Guo, Chem. Eng. Res. Des., 2015, 93, 675.
53. L. P. Lukhele, R. W. M. Krause, B. B. Mamba and M. N. B. Momba, Water SA, 2010, 36, 433.
54. K. G. Chai and H. B. Ji, Chem. Eng. J., 2012, 203, 309.
55. A. G. Stamatis, D. Giasafaki, K. C. Christoforidis, Y. Deligiannakis and M. Louloudi, J. Mol. Catal. A: Chem., 2010, 319, 58.
56. G. Borja, A. Monge-Marcet, R. Pleixats, T. Parella, X. Cattoen and M. W. C. Man, Eur. J. Org. Chem., 2012, 3625.
57. W. S. Guo, A. Monge-Marcet, X. Cattoen, A. Shafir and R. Pleixats, React. Funct. Polym., 2013, 73, 192.
58. H. M. Shen and H. B. Ji, Tetrahedron Lett., 2012, 53, 3541.
59. H. M. Shen and H. B. Ji, Carbohydr. Res., 2012, 354, 49.
60. H. M. Shen and H. B. Ji, Tetrahedron, 2013, 69, 8360.
61. J. Li, C. L. Chen, Y. Zhao, J. Hu, D. D. Shao and X. K. Wang, Chem. Eng. J., 2013, 229, 296.
62. M. T. Sikder, Y. Mihara, M. S. Islam, T. Saito, S. Tanaka and M. Kurasaki, Chem. Eng. J., 2014, 236, 378.
63. J. Chen, R. J. Qu, Y. Zhang, C. M. Sun, C. H. Wang, C. N. Ji, P. Yin, H. Chen and Y. Z. Niu, Chem. Eng. J., 2012, 209, 235.
64. Q. Hu, D. W. Gao, H. Y. Pan, L. L. Hao and P. Wang, RSC Adv., 2014, 4, 40071.
65. L. L. Fan, C. N. Luo, M. Sun, H. M. Qiu and X. J. Li, Colloids Surf., B, 2013, 103, 601.
66. C. Sarmah, D. Sahu and P. Das, Catal. Today, 2012, 198, 197.
67. X. H. Li, B. W. Zhao, K. Zhu and X. K. Hao, Chin. J. Chem. Eng., 2011, 19, 938.
68. L. L. Fan, M. Li, Z. Lv, M. Sun, C. N. Luo, F. G. Lu and H. M. Qiu, Colloids Surf., B, 2012, 95, 42.
69. J. M. Pan, X. H. Zou, X. Wang, W. Guan, C. X. Li, Y. S. Yan and X. Y. Wu, Chem. Eng. J., 2011, 166, 40.
70. W. C. E. Schofield, C. D. Bain and J. P. S. Badyal, Chem. Mater., 2012, 24, 1645.
71. H. Huang, Y. F. Fan, J. W. Wang, H. Q. Gao and S. Y. Tao, Macromol. Res., 2013, 21, 726.
72. A. M. Badruddoza, Z. B. Shawon, T. W. J. Daniel, K. Hidajat and M. S. Uddin, Carbohydr. Polym., 2013, 91, 322.
73. L. L. Li, L. L. Fan, H. M. Duan, X. J. Wang and C. N. Luo, RSC Adv., 2014, 4, 37114.
74. S. H. Lin and R. S. Juang, J. Environ. Manage., 2009, 90, 1336.
75. C. Yuan, Z. F. Lu and Z. Y. Jin, Food Chem., 2014, 152, 140.
76. F. Silva, A. Figueiras, E. Gallardo, C. Nerin and F. C. Domingues, Food Chem., 2014, 145, 115.
77. D. W. Wang, C. B. Ouyang, Q. Liu, H. L. Yuan and X. H. Liu, Carbohydr. Polym., 2013, 93, 753.
78. C. Manivannan, R. V. Solomon, P. Venuvanalingam and R. Renganathan, Spectrochim. Acta, Part A, 2013, 103, 18.

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