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

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Surface immobilization of β-cyclodextrin on hybrid silica and its

fast-adsorption performance to p-nitrophenol from aqueous

phase

Received 00th January 20xx,

Accepted 00th January 20xx

Hai-Min Shen,^aGong-Yuan Zhu,^aWu-Bin Yu,^aHong-Ke Wu,^aHong-Bing Ji,^bHong-Xin Shi,^aYi-Fan

Zheng*^aand Yuan-Bin She*^a

DOI: 10.1039/x0xx00000x

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Renewable β-cyclodextrin (β-CD) was immobilized onto the surface of hybrid silica using ethylenediamine as linking groups

to construct 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 to p-nitrophenol, and the adsorption kinetics, adsorption isotherms, adsorption

thermodynamics, reusability and adsorption mechanism were investigated systematically, which indicate the adsorption of

p-nitrophenol onto CD@Si is a very fast process, the adsorption equilibrium can be reached in 15 s with acceptable

equilibrium adsorption capacity of 69.6 mg/g in pH = 7.0, 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 in lower temperature. And the formation of

inclusion complex and hydrogen bond interaction are two origins of p-nitrophenol being adsorbed onto CD@Si.

Additionally, CD@Si can be recycled and reused at least five runs with acceptable adsorption capacity, is a very promising

adsorbent in the fast adsorption of p-nitrophenol or its analogues from aqueous phase. Additionally, this work also

provides a strategy to increase the adsorption rate of adsorbent based on β-CD.

nowadays. And it is known that p-nitrophenol has been

categorized as one of the 126 priority pollutants by the U.S.

1. Introduction

Environmental Protection Agency (U.S. EPA) for its persistency,

a variety of organic molecules are

In modern society,

bioaccumulation and high toxicity even at very low

concentration,^8-12and frequent exposure to p-nitrophenol can

cause methemoglobin formation, anemia, liver and kidney

discharged into the natural environment as 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 for the negative effect of the organic

molecules 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,^1-6and it can get into the environment system

during the above processes and pollute the nature water. The

other source of p-nitrophenol in the environment is the

hydrolysis or degradation of organic molecules containing p-

nitrophenol moiety.⁷Therefore, p-nitrophenol has become a

commonly encountered pollutant in water pollution

damage, eye and skin irritation, tumor formation, cancer, and

9, 13, 14

Therefore, fast and effective

systemic poisoning.^8,

removal of p-nitrophenol from industrial wastewater and

nature water is becoming more and more urgent.

Several water treatment technologies have been developed

with the aim to remove p-nitrophenol or its analogues from

industrial wastewater and nature water effectively, such as

adsorption,^{4, 15-18}oxidation,^19-21membrane separation,^{22, 23}and

extraction.²⁴Among 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

4, 13, 17, 28

But as adsorbent, activated

adsorption capacity.^3,

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

adsorbent with satisfying adsorption capacity, high mechanical

strength, high reusability, and ready availability, especially the

adsorbent based on natural products for their ready

^a.College of Chemical Engineering, Zhejiang University of Technology, Hangzhou

310014, China.

^b.School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou

510275, China

* Corresponding author. E-mail address: haimshen@zjut.edu.cn. Fax: (+86)571-

8832-0533. Tel. : (+86)571-8832-0533.

Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See

DOI: 10.1039/x0xx00000x

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availability and excellent biocompatibility, which will not cause emergency. This work also provides a strategy to increase the

additional environmental pollution in their application.

β-Cyclodextrin (β-CD), is water-soluble macrocyclic

adsorption rate of adsorbent based on β-CD.

a

oligomer of D-(+)-glucopyranosyl units linked by 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 possessing

excellent biocompatibility.^29-32Because of its hydrophilic

exterior and hydrophobic cavity, β-CD can form inclusion

complexes with a wide range of organic molecules possessing

suitable shape and size in water spontaneously,^33-36which

brings a possibility to remove the organic molecules from

polluted water. Thus, β-CD has become a unique structural

unit in the design and preparation of adsorbent employed in

the water treatment.^37-44In the water treatment process, β-CD

unit can capture the pollutant molecules through 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

2. Experimental section

2.1. Materials and chemicals

β-Cyclodextrin in 99% purity was purchased from Shanghai Bio

Science

chloride in 99% purity, and ethylenediamine in 99% purity

were Shanghai Aladdin reagents. (3-Chloropropyl)

triethoxysilane (CPTES) in 99% purity, orthosilicate (TEOS) in 99%

purity, tetrabutylammonium fluoride in THF, p-

& Technology Co. Ltd., China. p-Toluenesulfonyl

1

M

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

were 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.

pollution. But at present, in the immobilization of β-CD onto

45-47

water-insoluble supporters such as Fe₃O₄,^40,

SiO₂,^48-50

52

54

graphene oxide,^51,

carbon nanotube,⁵³and chitosan,^42,

linking groups mostly employed are epichlorohydrin,⁴²citric

acid,⁴⁸carbodiimide,⁴⁶3-glycidyloxypropyl trimethoxysilane,⁴⁷

hexamethylene diisocyanate,⁵³glutaraldehyde,⁵⁴and so on.

These linking groups possess so high reactivity that multiple

hydroxyl groups in β-CD molecule may react with linking

groups, which will weaken the intramolecular hydrogen bond

in β-CD molecule, 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 adsorbent 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,^55-57using ethylenediamine as

linking groups to construct adsorbent in 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 measurement, 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 satisfying fast-adsorption performance, and

adsorption equilibrium can be reached in 15 s with the

equilibrium adsorption capacity of 69.6 mg/g, a very fast

adsorption process. The effect of 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

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.^58-60A 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 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 in

vacuum at 80 °C for 8.0 h to afford white solid powder 8.17 g

in 12.67% yield. [α]²_D⁵= +124.15° (c = 0.8044, DMF); m.p. >

1

160 °C (decomp.); H 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 ppm (s, 3H); 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 of β-CD, which produces an important

intermediate in the preparation of 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

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70 °C for 24.0 h, and then cooled to room temperature. The S-Twin transmission electron microscope to investigate the

resultant solution was poured into ethanol (400 mL) slowly elementary composition of Cl@Si and CD@Si. Contact angle

forming white precipitate immediately. The white precipitate measurements were conducted through dropping water

was collected by suction filtration and dried in vacuum at 80 °C droplets (6 µL) on the surface of Cl@Si and CD@Si respectively,

for 8.0 h. Then obtained white power was recrystallized two and after waiting for 30 s to let the water droplets on the

times in water (2×10 mL), dried in vacuum at 80 °C for 8.0 h to surface of Cl@Si and CD@Si become stable, the KRÜSS DSA100

afford white crystal 3.18 g in 54.03% yield. [α]²_D⁵= +149.91° (c = Drop Shape Analysis System was employed to measure the

0.8036, H₂O); m.p. > 245 °C (decomp.); ¹H NMR (400 MHz, contact angel and determine their hydrophobicity

/

D₂O): δ = 5.12-5.10 (m, 7H), 4.03-3.89 (m, 26H), 3.70-3.59 (m, hydrophilicity level. The TGA were carried out under nitrogen

14H), 3.48 (t, J = 9.4 Hz, 1H), 3.11-3.08(m, 1H), 2.87-2.68 ppm atmosphere from room temperature to 800 °C at a speed of

(m, 7H); ¹³C NMR (400 MHz, D₂O): δ = 101.81, 100.47, 83.56, 10 °C/min employing a STA 449 F3 Jupiter® spectrometer to

81.09, 80.86, 73.06-72.96 (m), 72.05-71.99 (m), 71.78, 70.42, monitor the weight loss of Cl@Si and CD@Si, and to determine

60.25, 50.66, 49.29, 42.19, 39.73 ppm; MS (ESI): m/z: 1177.5 the immobilized amount of β-CD on the surface of hybrid silica.

[M+H]⁺. Mono(6-ethylenediamine)-β-CD which can be The solid state NMR (¹³C CP MAS) data of Cl@Si and CD@Si

immobilized onto the surface of hybrid silica (Cl@Si) directly, is was collected using BRUKER AVANCE II 300MHZ spectrometer

the second important intermediate in the preparation of to determine the successful immobilization of β-CD on the

adsorbent CD@Si.

surface of hybrid silica. The morphologies of Cl@Si and CD@Si

were observed by SEM (Hitachi S-4700 (II) instrument). The

2.4. Preparation of hybrid silica (Cl@Si)

Cl@Si was prepared according to the literature procedure with diffractometer with a Cu Kα radiation (λ = 1.5418Å) in the

some modification.^55-57

solution of (3-chloropropyl) range of 5~60º.

XRD patterns were recorded on a X′ Pert PRO X-ray

A

triethoxysilane (CPTES) (2.41g, 10 mmol), orthosilicate (TEOS)

(10.42g, 50 mmol) in anhydrous ethanol (30 mL) was added to

a

solution of distilled water (3.60g, 200 mmol), 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 in vacuum at 80 °C for

24.0 h to afford water-insoluble supporter Cl@Si as 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), 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 in vacuum at 80 °C

for 24.0 h to afford CD@Si as white power 12.30 g.

The preparation procedure of CD@Si is illustrated in Scheme

1.

Scheme 1 The schematic preparation of CD@Si.

2.7. Adsorption experiment

Batch experiments were carried out to investigate the

adsorption performance of adsorbent CD@Si to p-nitrophenol

because of the simplicity and reliability of this method. In

experiments, CD@Si (0.10 g) was added to a 25-mL glass

conical flask placed in a thermostatic water-bath shaking

incubator at 283K, 293K, 303K and 313K respectively, and then

aqueous solution of p-nitrophenol (10.0 mL) in the initial

concentration of 0.10 g/L, 0.50 g/L, 1.00 g/L, 1.50 g/L, 2.00 g/L,

2.50 g/L, 3.00 g/L and 4.00 g/L, and at 283K, 293K, 303K and

313K was added, respectively. After shaking at 150 rpm for 2.0

h, the suspensions was poured into distilled water (90 mL)

quickly to weaken the further adsorption and 10 mL of the

upper liquid was filtered through 0.45 µm filter membrane

quickly to remove the adsorbent residue. The concentration of

2.6. Characterization

FT-IR spectra of Cl@Si and CD@Si were record on a Nicolet

6700 spectrometer using KBr as background over the

frequency range of 4000~400 cm^-1. XPS analysis was carried

out on a Kratos AXIS Ultra DLD spectrometer with 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 the Philips-FEI Tecnai G2 F30

p-nitrophenol in the filtrate was detected on

a UV-vis

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spectrophotometer (Shimadzu, UV-2250) at wavelength of 398 where C_eis the equilibrium concentration of p-nitrophenol in

nm. In order to investigate the effect of the solution pH, the aqueous solution (g/L), q_eis the equilibrium adsorption

KH₂PO₄-K₂HPO₄buffer solution was employed. In the capacity (mg/g), q_mis the maximum adsorption capacity

experiments to determine the effect of temperature, the (mg/g), and K_Lis the equilibrium adsorption constant (L/g).

aqueous solution of p-nitrophenol was held at 283K, 293K,

The Freundlich model can be expressed in the linear form as

303K and 313K respectively, and the adsorption experiments following:

were also performed at above temperature. Each experiment

1

ln =ln

q_e

+ ln

K _F

C_e

was carried out twice, and the experiment data are the

average values. The amount of p-nitrophenol adsorbed onto

CD@Si was calculated according the following equation:

n

(6)

where q_eand C_eare the same as in Langmuir model, K_F(L/g)

and

are the Freundlich constants.^{37, 39}

n

(

− )×V

C₀C_t

m

where q_tis the adsorption capacity (mg/g) at time

The Tempkin model can be expressed in the linear form as

following:

=

q_t

(1)

=

ln

+ ln

t, C₀is the

q_e

k₁k₂k₁C_e

(7)

initial concentration of p-nitrophenol (g/L), C_tis the

where k₁is the Tempkin constant related to the adsorption

heat (L/g), k₂is the Tempkin isotherm constant.^{1, 14, 62}

concentration of p-nitrophenol (g/L) at time

t

(s),

V

is the

volume of the aqueous solution of p-nitrophenol (mL),

mass of adsorbent CD@Si (g) employed.^5,

m

18, 37

Thus the

2.8.3. Adsorption thermodynamics

equilibrium adsorption amount was calculated according the

following equation:

In order to investigate the effect of temperature on the

adsorption process, thermodynamic parameters containing

Gibbs free energy change (ΔG), enthalpy change (ΔH) and

entropy change (ΔS) were determined employing the following

equation:

(

−

)×V

C₀C_e

m

=

q_e

(2)

where q_eis the equilibrium adsorption capacity (mg/g), C₀is

the initial concentration of p-nitrophenol (g/L), C_eis the

ꢀG =−RT lnK_d

(8)

equilibrium concentration of p-nitrophenol (g/L),

V

is the

ꢀG =ꢀH −TꢀS

(9)

volume of the aqueous solution of p-nitrophenol (mL),

mass of adsorbent CD@Si (g) employed.^{5, 18, 37}

m is the

ꢀS ꢀH

lnK_d=

−

R

RT

(10)

2.8. Data analysis

2.8.1. Adsorption kinetics

q_e

K_d=

C_e

(11)

In kinetic study, two kinetic models, pseudo-first-order and

pseudo-second-order models were employed to analysis the

experimental data and predict the adsorption kinetics in this

work. The linear form of equation for the pseudo-first-order

model is described as following:

ln( − )=ln − t

q_eq_tq_e

k₁

where q_eand q_tare the adsorption amount of p-nitrophenol

where K_dis the distribution coefficient, R (8.314 J·mol^-1·K^-1) is

the universal gas constant, and

63-65

ΔH and ΔS can be obtained from the slope and intercept

T

is Kelvin temperature (

K

).^{6, 14,}

of the plots of lnK_dversus 1/T, thus ΔG was obtained

employing equation (9).

(3)

2.9. Reusability of CD@Si

on CD@Si (mg/g) at equilibrium and at time t, respectively,

and k₁is the pseudo-first-order adsorption rate constant (s^-1).^18,

39, 61

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 aqueous solution of p-nitrophenol (30.0 mL) in 2.0

g/L and pH 7.0 was added. After shaking at 150 rpm for 2.0 h,

the suspensions was centrifuged at 4500 rpm for 10.0 min. The

upper liquid was filtered through 0.45 µm filter membrane

quickly to remove the adsorbent residue. The concentration of

The linear form of equation for the pseudo-second-order

model is expressed as following:

t

1

t

=

+

q²

q_t

q_e

k₂

e

(4)

p-nitrophenol in the filtrate was detected on

a UV-vis

where q_eand q_tare the same as in the pseudo-first-order

model, and k₂is the pseudo-second-order adsorption rate

constant (g/(mg·s)).^{18, 39, 61}

spectrophotometer (Shimadzu, UV-2250) at wavelength of 398

nm to determined the amount of p-nitrophenol adsorbed onto

CD@Si.The obtained solid was washed successively with

enthanol (3 × 10 mL) and water (3 × 10 mL) under ultrasonic

and centrifugal condition, and then dried in 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

p-nitrophenol aqueous solution added should be decreased

proportionally.

2.8.2. Adsorption isotherm

In adsorption isotherm study, Langmuir, Freundlich and

Tempkin models were employed. The linear form of equation

for the Langmuir model is described as following:

1

C_e

q_e

C_e

q_m

=

+

q

K _L

m

(5)

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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 ºC to 800 ºC, the weight loss of Cl@Si is about

17 %, which is mainly contributed to the thermal

decomposition of CPTES moieties. Thus, the organic layer on

hybrid silica (Cl@Si) is about 17%. As for CD@Si, the weight

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^-1are

attributed to the O-H stretching vibrations, the peaks at

around 2900 cm^-1belong to the C-H stretching vibrations and

the peaks at around 1650 cm^-1are the N-H deformation

vibrations in secondary amine. Comparing the FT-IR spectrum

of CD@Si with Cl@Si, the O-H stretching vibration in CD@Si

become stronger for the abundant hydroxyl groups existing in

β-CD molecule, and the difference of C-H stretching vibration

at around 2900 cm^-1is not very obvious. When β-CD was

immobilized onto the surface of Cl@Si, the N-H deformation

vibration at around 1650 cm^-1appeared in CD@Si for the

existence of secondary amine groups. And the not obvious

peak at around 1650 cm^-1appeared in the spectrum of Cl@Si

mainly comes from the bending vibrations of a little adsorbed

water, not the N-H deformation vibrations in amine groups.

Thus, it can be concluded from the FT-IR spectra that β-CD

have 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, chlorine element decreased and

nitrogen element was detected for the nucleophilic

substitution reaction between nitrogen atoms and chlorine

atoms. The 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 for the existence of

hydrophobic -CH₂CH₂CH₂Cl, so the contact angle of water on it

can reach up to 143.6º. But the surface of CD@Si is hydrophilic

for the existence of massive -OH, so water droplet (6 µL) on

the surface of CD@Si can seep into it quickly (within 30 s) as

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

a

b

c

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm^ꢀ1)

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

120000

O 1s

100000

80000

O KL1

Cl 2p

60000

40000

20000

0

C 1s

Cl LM1

Si 2p

1400 1200 1000 800

600

400

200

0

Binding Energy (eV)

a

100000

80000

60000

40000

20000

0

O 1s

C 1s

O KL1

Si 2p

Cl 2p

N 1s

1400 1200 1000 800 600 400 200

0

characterize the successful immobilization as shown in Fig.4

.

Binding Energy (eV)

The broad signals from 55.5 to 6.5 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 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.

b

60000

50000

40000

30000

20000

10000

0

O 1s

C 1s

N KL1

O KL1

The amount of β-CD immobilized on the surface of hybrid

silica was evaluated through TGA spectra of Cl@Si and CD@Si

under the 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, dehydration of the hydroxyl groups on the surface, and

thermal decomposition of CPTES moieties. Before 250 ºC, the

N KL2

N 1s

1400 1200 1000 800 600 400 200

0

Binding Energy (eV)

c

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loss before 150 ºC 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 ºC to 800 ºC, the weight loss

of CD@Si is about 25%, which is contributed to 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) with omitting of the mass

contribution of -CH₂CH₂CH₂- moieties. The TGA curves also

provide a further evidence for the successful immobilization of

β-CD on the surface of hybrid silica.

250

200

150

100

50

C

O

Si

Cu

Cl

0

2

4

6

8

）

10

12

（

Energy keV

d

100

95

a

1000

800

600

400

200

0

Si

90

85

O

b

80

75

70

65

C

N

0

100 200 300 400 500 600 700 800

Cu

Temperature (^oC)

0

2

4

6

8

10

12

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

Energy (keV)

e

The morphologies of Cl@Si and CD@Si were analyzed by

SEM as 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 were listed in Fig.7, in

which both of Cl@Si and CD@Si exhibit a broad peak centered

at about 22º which is the characteristic peak of SiO₂.⁶⁶Thus,

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 lost of the β-CD's

characteristic peaks 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's characteristic peaks in XRD measurement can not be

observed.

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

spectra of (d) Cl@Si, (e) CD@Si.

a

b

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

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.

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

(b) CD@Si.

a

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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 the neutral condition (pH 7.0), CD@Si can

reach its optimal adsorption performance.

b

70

60

50

40

30

20

10

0

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

5

6

7

8

9

10

c

pH

b

a

Fig. 8 Effect of solution initial pH on the adsorption of p-

nitrophenol by CD@Si.

Conditions: Initial concentration of p-nitrophenol, 0.50 g/L, ●

■

10

20

30

40

50

60

2 Theta (^o)

▲

▼

2.00 g/L, 3.00 g/L, 4.00 g/L in 0.05 mol/L 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.

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

3.2. Effect of solution initial pH on the adsorption

capacity

3.3. Effect of contact time 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

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,

19.6 to 40.4 mg/g, 27.5 to 53.7 mg/g, and 34.3 to 69.6 mg/g in

the initial concentration of 0.50, 2.00, 3.00, and 4.00 g/L,

respectively. 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 following: in this

adsorption system, the adsorption capacity contains two parts:

inclusion in the hydrophobic cavity of β-CD and hydrogen bond

interaction between β-CD unit on CD@Si and p-nitrophenol,

and in the acidic condition, both β-CD unit on CD@Si and p-

nitrophenol become protonated, so p-nitrophenol can not be

adsorbed onto CD@Si efficiently for the electrostatic repulsion

between them. And when pH 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 interaction between β-CD unit on CD@Si and p-

nitrophenol. In the neutral condition, the protonation of

adsorbent CD@Si and p-nitrophenol disappears, and the

hydrogen bond interaction between them favours the

adsorption of p-nitrophenol onto CD@Si. In the basic condition,

the ionization of p-nitrophenol occurs, which brings about the

electrostatic repulsion between p-nitrophenol molecules,

The objective of our study is to develop an adsorbent with high

adsorption rate and 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 in the initial

concentration of 1.00, 2.00, 3.00, and 4.00 g/L were 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) was poured

into distilled water (90 mL) quickly to weaken the further

adsorption. Then 10 mL of the upper liquid was filtered

through 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,

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 unit, and the

hydrogen bond interaction between p-nitrophenol and β-CD

unit on CD@Si are reversible, high initial concentration favours

p-nitrophenol adsorbed onto the surface of CD@Si.

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 in the adsorption of p-nitrophenol

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or its analogues were listed in Table 1 for comparison. In Table

, it is obvious that the equilibrium adsorption capacity of Table 1 Comparison of some reported adsorbents based on β-CD in

1

CD@Si is not the most satisfying one, but considering the time the adsorption of p-nitrophenol or its analogues.

needed to reach adsorption equilibrium, CD@Si is the most

Equilibrium

Adsorption

(mg/g)

69.6

Equilibrium

promising one, because the adsorption equilibrium can be

achieved within 15 with an acceptable equilibrium

NO

Adsorbent

Solute

Time

s

adsorption capacity, 69.6 mg/g, which is the highest

adsorption capacity per unit time for the above adsorbent

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 nature water. In

that case, to prevent p-nitrophenol from diffusing with the

flow of water quickly is the first issue needed to be taken into

consideration, and the maximum removal of p-nitrophenol in

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 for only one

hydroxyl group in the β-CD molecule was employed in the

preparation of CD@Si. And the distribution of β-CD unit on the

surface of hybrid silica is another important reason for the fast

adsorption because no diffusion resistance exists. When

CD@Si enters into the aqueous solution of p-nitrophenol, p-

nitrophenol molecules transfer from the solution into the

hydrophobic cavity of β-CD unit and onto the surface of β-CD

layer quickly, thus adsorption equilibrium is obtained within

only 15 s with acceptable adsorption capacity.

1

CD@Si

Cl@Si

p-Nitrophenol

15 s

2

3⁶⁷

15 s

5.8

β-CD-

60 min

0.3

Zeolite

β-CD-

4²

p-Nitrophenol

180 min

50 min

33.3

Carbon

nanotube-

Fe₃O₄

5⁶⁸

β-CD-

Hydroquinol

192.7

Chitosan

-Fe₃O₄

6⁶⁹

7⁶⁹

8⁷⁰

β-CD-

2,6-

100 min

58.1

55.2

61.7

Attapulgite

β-CD-

Dichlorophenol

2,6-

Attapulgite

β-CD-

Dichlorophenol

2-Chlorophenol 180 min

Poly(4-

vinylbenzyl

chloride)

β-CD

9⁷¹

Phenol

300 min

29.0

35.7

Polymer

10⁴²β-CD-

4-Chlorophenol 60 min

By comparing the adsorption capacity of CD@Si (69.6mg/g)

with Cl@Si (5.8 mg/g), 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 to p-

nitrophenol is in a negligible level. The poor adsorption

performance of Cl@Si is mainly because of its hydrophobicity,

which can not be dispersed well in aqueous solution at all. The

above experimental phenomenon demonstrates the

importance of β-CD unit in the construction of adsorbent too,

especially the one applied in aqueous solution.

Chitosan

3.4. Adsorption kinetics

The adsorption capacity of CD@Si to p-nitrophenol versus

contact time has been illustrated in Fig.9 in the initial

concentration of 1.00, 2.00, 3.00, and 4.00 g/L 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

acceptable adsorption capacity. In order to further investigate

this adsorption process and the adsorption mechanism, two

kinetic models, pseudo-first-order and pseudo-second-order

models were employed to analysis the adsorption kinetics of

p-nitrophenol onto CD@Si following the Equ. (3) and (4). The

plots of ln(q_e-q_t) versus t ( pseudo-first-order model) and t/q_t

80

70

60

50

40

30

20

10

0

versus

t

(pseudo-second-order model) in the initial

concentrations of 1.00, 2.00, 3.00, and 4.00 g/L are presented

in Fig. 10. It is very clear that the pseudo-second-order model

fits the adsorption kinetic of p-nitrophenol onto CD@Si well

with 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 kinetic in this work. The

pseudo-second-order kinetic model also was confirmed

through the similar values of the experimental equilibrium

adsorption capacity q_e,expand the calculated ones q_e,calas

shown in Table 2. The experimental equilibrium adsorption

capacity q_e,expare 23.7, 40.4, 53.7, and 69.6 mg/g, and

meanwhile the calculated ones q_e,calare 24.7, 42.5, 55.7 and

0

5

10 15 20 25 30 35 40

t (s)

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, ●

■

▲

▼

2.00 g/L, 3.00 g/L, 4.00 g/L in 0.05 mol/L 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.

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71.5 mg/g in the initial concentration of 1.00, 2.00, 3.00, and 3.5. Adsorption isotherms

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4.00 g/L respectively, a very good consistency with tiny error. In order to further understand the adsorption mechanism, the

The statistical errors in the adsorption kinetic study are mainly adsorption isotherms of CD@Si to p-nitrophenol at four

attributed to the errors in the weight of adsorbent CD@Si and different temperature (283K, 293K, 303K, and 313K) were

solute p-nitrophenol, and in the preparation of aqueous measured, which is an important illustration of how the

solution of p-nitrophenol. Another origin of the statistical error adsorbate interacts with the adsorbent in equilibrium state.

is the systematic error in the UV measurement. The pseudo- From the study of adsorption isotherms, the maximum

second-order kinetic also suggests that in the adsorption of p- adsorption capacity of CD@Si to p-nitrophenol also can be

nitrophenol onto CD@Si, the rate-limiting step may involve obtained. Several adsorption isotherm models have been

valence forces through electrons sharing or exchange between developed to fit the adsorption data. In this work, three

p-nitrophenol and CD@Si such as hydrogen bond,^{72, 73}which isotherm models, Langmuir model, Freundlich model and

has been mentioned above. The pseudo-second-order Tempkin model, were employed, which can be expressed

adsorption mechanism is the predominant mechanism in the linearly following Equ. (5), Equ. (6) and Equ. (7). For the

adsorption of p-nitrophenol onto CD@Si.

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 adjacent adsorption site. In Langmuir

adsorption model, the adsorbate molecules usually form a

monomolecular layer on the surface of adsorbent.^{5, 74}For the

Freundlich model, the basic theory assumption is that the

distribution of adsorption sites on the adsorbent surface are

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 adsorbent with heterogeneous surface, Freundlich model

is closer to the reality than 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 as the increase in the

adsorbent coverage.^{1, 62}

5

4

3

2

1

0

ꢀ1

ꢀ2

ꢀ3

ꢀ4

0

5

10 15 20 25 30 35 40

t (s)

a

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

The adsorption isotherm fitting was conducted employed

Equ. (5), Equ. (6) and Equ. (7) for Langmuir model, Freundlich

model and Tempkin model respectively. The plots of C_e/q_e

versus C_e(Langmuir model), lnq_eversus lnC_e(Freundlich

model), and q_eversus lnC_e(Tempkin model) at 283K, 293K,

303K, and 313K are presented in Fig. 11

. The initial

0

5

10

15

20

25

30

35

40

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 4.00g/L.

And the adsorption isotherm parameters were 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 high correlation coefficient (R²>98%) in all the

temperature involved fits the adsorption data better than

t (s)

b

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, ●

▲

▼

2.00 g/L, 3.00 g/L, 4.00 g/L in 0.05 mol/L 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.

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Table 2 The pseudo-first-order and pseudo-second-order kinetic

parameters for the adsorption of p-nitrophenol by CD@Si at 303K.

Pseudo-first-order

Pseudo-second-order

C₀

q_e,exp.

k₁×10^-2

q_{e1, cal}

(mg/g)

8.484

6.886

15.87

15.56

R²

q_{e2, cal}

k₂×10^-2

(g/(mg·s))

3.776

R²

(g/L)

(mg/g)

(1/s)

(mg/g)

1.00

2.00

3.00

4.00

23.7

40.4

53.7

69.6

15.97

13.29

13.01

11.34

0.7058 24.7

0.4845 42.5

0.6742 55.7

0.4888 71.5

0.9992

0.9997

0.9988

0.9987

0.00

6.327

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

3.272

C_e

1.753

a

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3.6. Adsorption thermodynamics

In order to access the thermodynamics parameters, the

adsorption isotherms of p-nitrophenol onto CD@Si was

analyzed at 283K, 293K, and 303K in the initial concentration

of 0.10 g/L, 0.50 g/L, and 4.00 g/L respectively. The plots of

4.5

4.0

3.5

3.0

2.5

2.0

1.5

lnK_dversus 1/T were listed in Fig. 12 and the thermodynamic

parameters obtained from the slope and intercept following

equation (10) were summarized in Table 4. It is obvious that

the ΔG values (-7.474 kJ/mol ~ -11.91 kJ/mol) are negative at

all the temperature and initial concentration studied, which

means that the adsorption of p-nitrophenol onto CD@Si is a

spontaneous process, and does not need any energy from

external environment. We can also observe that the Gibbs free

ꢀ3

ꢀ2

ꢀ1

0

1

2

ln C_e

b

90

80

70

60

50

40

30

20

10

0

energy change

(ΔG) increases as the increase in the

temperature, which means that 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 adsorption of p-

nitrophenol onto CD@Si is more favourable at lower

temperature. And the negative values of entropy change (ΔS

)

ꢀ10

tell us that the randomness decreases when p-nitrophenol was

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 in

lower temperature.

ꢀ3

ꢀ2

ꢀ1

0

1

2

ln C_e

c

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. Iniꢀal 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.

5.0

4.5

4.0

3.5

3.0

Langmuir model and Tempkin model, which indicates that the

distribution of adsorption sites on the surface of CD@Si are

heterogeneous, and the stronger adsorption sites are occupied

firstly. Both the correlation coefficients for Langmuir model

and Tempkin model are lower than Freundlich model. The

greater n value in Freundlich model in all the temperature

involved means 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.

0.0033

0.0034

1/T(K^-1)

0.0035

Fig. 12 Plots of lnK_dversus 1/T

nitrophenol by CD@Si. Conditions: 0.10 g/L, ● 0.50 g/L,

for the adsorption of p-

■

▲

4.00 g/L. Temperature, 283K, 293K, and 303K. 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.

Parameter Value

Model

Parameters

283K

293K

89.1

303K

313K

80.3

3.7. Reusability of CD@Si

q_m(mg/g)

108.5

116.1

Reusability is a very important requirement for adsorbent,

which will reduce the material-cost significantly in practical

application. The reusability of CD@Si in the adsorption of p-

nitrophenol was illustrated in Fig.13. For the loss of adsorbent

in each reusability experiment, the volume of 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 to 31.7 mg/g, an acceptable adsorption capacity, which is

Langmuir

model

K_L(L/g)

R²

0.6144 0.8548

0.7501 0.9330

0.4048 0.7016

0.4999 0.9277

n

1.776

37.53

1.708

36.13

1.553

34.10

1.555

29.13

Freundlich

model

K_F(mg/g)

R²

k₁

k₂

R²

0.9845 0.9912

0.9833 0.9855

16.09

16.95

15.06

17.72

16.00

12.19

13.74

13.78

Tempkin

model

0.7644 0.8913

0.7257 0.8840

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ROESY NMR study. Employing mono(6-ethylenediamine)-β-CD

as the representative of β-CD unit on CD@Si, obvious

correlation peaks between the H atoms located in the phenyl

ring of p-nitrophenol and the H-3, H-5 in the cavity of the

parent β-CD can be observed (Fig. 14), which is a strong

evidence for the formation of inclusion complexes. The

existence of hydrogen bond interaction 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 β-

CD unit on CD@Si shift from around 3388 cm^-1to around 3261

cm^-1, which is attributed to the formation of hydrogen bond

between p-nitrophenol and the β-CD unit on CD@Si. And the

appearance of peaks at 1592 cm^-1and 1496 cm^-1in the FT-IR

spectrum of CD@Si after the adsorption process, is another

powerful evidence to the adsorption of p-nitrophenol onto

CD@Si, which belong to the stretching vibrations of C=C in

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

interaction are two origins of p-nitrophenol being adsorbed

onto CD@Si.

about 76% of the initial adsorption capacity. The decrease in

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 application which can be

reused at least five runs with acceptable adsorption capacity.

Table 4 The thermodynamic parameters for the adsorption of p-

nitrophenol by CD@Si.

C₀

Temperature

(K)

ΔH

ΔS

ΔG

(g/L)

(kJ/mol)

(J/mol/K)

(kJ/mol)

-11.91

-11.77

-11.64

-9.511

-9.437

-9.363

-7.519

-7.496

-7.474

283

0.10

0.50

4.00

293

-15.71

-11.60

-8.155

-13.44

-7.383

-2.249

303

283

293

303

283

293

303

H-2

H-5

H-6

H-3

H-4

45

40

35

30

25

20

15

10

5

6.0

6.5

7.0

7.5

8.0

8.5

HO

N⁺

O

O^ꢀ

0

1

2

3

4

5

Run

Fig. 13 Reusability of CD@Si in the adsorption of p-nitrophenol.

Conditions: Initial concentration of p-nitrophenol, 2.00 g/L in

0.05 mol/L KH₂PO₄-K₂HPO₄buffer solution. Solution volume of

4.1

3.9

3.7

3.5

3.3

3.1

δ

p-nitrophenol, 30.0 mL. Adsorbent CD@Si, 0.30 g. Shaking Fig. 14 Partial ¹H ROESY NMR spectrum of the inclusion

speed, 150 rpm. Temperature, 303 K. Contact time, 2.0 h.

complex between mono(6-ethylenediamine)-β-CD and p-

nitrophenol.

3.8. Adsorption mechanism

Based on the systematical study on adsorption kinetics,

adsorption isotherms, and adsorption thermodynamics above,

the adsorption mechanism of p-nitrophenol onto CD@Si was

speculated. In TGA spectrum of CD@Si (Fig.5), the amount of

β-CD immobilized on the surface of hybrid silica is about 25%

(m/m), 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 to p-nitrophenol is

about 30.6 mg/g. Considering the inclusion equilibrium

between β-CD unit and p-nitrophenol, not all the β-CD unit

could be occupied by p-nitrophenol, thus the maximum

adsorption capacity in this work should be below 30.6 mg/g.

But in fact, the maximum adsorption capacity is higher than

30.6 mg/g, especially in high initial concentration, 3.00 g/L and

4.00 g/L. Thus besides forming inclusion complexes, there is

another interaction form between CD@Si and p-nitrophenol,

which is hydrogen bond as reported in some similar

literature.⁵⁴The formation of inclusion complexes between β-

a

b

c

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm^ꢀ1)

Fig. 15 FT-IR spectra of (a) CD@Si (before adsorption), (b)

CD@Si (after adsorption), and (c) p-nitrophenol.

4. Conclusions

1

CD unit and p-nitrophenol could be illustrated through the H

RSC Adv., 2015, 00, 1-3 | 11

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and Zhejiang Provincial Natural Science Foundation (Grant No.

LQ14B020002).

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Graphical Abstract

Surface immobilization of β-cyclodextrin on hybrid silica

and its fast-adsorption performance to p-nitrophenol from

aqueous phase

Hai-Min Shen^a, Gong-Yuan Zhu^a, Wu-Bin Yu^a, Hong-Ke Wu^a, Hong-Bing

Ji^b, Hong-Xin Shi^a, Yi-Fan Zheng^a∗and Yuan-Bin She^a∗

^aCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou

310014, China

^bSchool of Chemistry and Chemical Engineering, Sun Yatꢀsen University, Guangzhou

510275, China

Fast-adsorption of p-nitrophenol was achieved through surface immobilization of

β-cyclodextrin onto hybrid silica and maintenance of its hydrophobic cavity.

^∗Corresponding author. E-mail address: haimshen@zjut.edu.cn. Fax: (+86)571-8832-0533. Tel. :

(+86)571-8832-0533.

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

DOI: 10.1039/c5ra15592d

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