chang2008 chitosan based aerogels with high adsorption performance

The aerogels exhibit high adsorption capability, can remove SDBS from acidic aqueous solutions, and have potential applications in controlling SDBS pollution.. Only a few studies have em

carbon dioxide (CO2) fluid extraction The resulting materials were characterized using scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis, and Fourier transform infrared (FT-IR) spectroscopy Furthermore, the adsorption of the anionic surfactant sodium dodecylbenzene-sulfonate (SDBS) from aqueous solution by the materials was investigated The aerogels exhibit high adsorption capability, can remove SDBS from acidic aqueous solutions, and have potential applications in controlling SDBS pollution

1 Introduction

As low-density solids, aerogels have an open

three-dimen-sional mesoporous structure and high specific surface area, and

thus have potential applications as adsorbents, thermal insulators,

acoustic absorbers, catalysts, and catalyst supports.1Therefore,

much research has been dedicated to aerogels, including their

preparation, properties, and applications Besides inorganic

aerogels such as silicon dioxide,2metal oxides,2,3chalcogenides,4

and carbon aerogels,5the organic or hybrid aerogels have also

been extensively investigated.6–8However, among the organic

aerogels that have been prepared,6most are made from artificial

polymers Only a few studies have employed natural polymers

or their derivatives, mainly alginate7and cellulose-based

aero-gels.8

Chitosan from natural polymer chitin has good

biocompat-ibility, as well as extensive applications in pharmacology,

biomedicine, agriculture, food, and waste treatment.9 It is

reasonable to infer that its porous structure should have a higher

adsorption capability and further applications such as drug

delivery.10However, the polymer backbone is highly polar and

capable of forming strong hydrogen bonds between adjacent

chains, so all attempts to prepare cross-linked aerogels from it

have been unsuccessful due to the severe shrinkage and

deformation of the gel.11 Quignard and co-workers prepared

chitosan microspheres with a small surface area of 110 m2/g

by supercritical CO2drying of gel beads,12and Renzo’s group

fabricated chitosan films with a surface area of 175 m2/g,13but

neither of these cases involves cross-linking One of the

distinguishing features of chitosan is that it can be cross-linked

by reagents such as glutaradehyde, glyoxal, and formaldehyde

to form rigid aqueous gels

As the most extensively used surfactant in detergents and

surface cleaners, SDBS can often be detected in the effluents

from many industries such as textiles, paper and pulp, and food

processing, and so SDBS enters the environment primarily

through wastewater, harming human beings, fish, and

vegeta-tion.14SDBS in wastewater is adsorbed by soil, slowly degrading

and impeding the microbial processes within it.15In the past

few decades, many efforts have been made to develop both

organic and inorganic adsorbents such as carbon,16minerals,17

poly(vinylchloride) latexes,18cellulose, and chitosan19in order

to remove SDBS molecules from wastewater However, most

of these materials exhibit low adsorption capacities, and inorganic adsorbents almost always cannot be reused Synthetic resins such as acrylic ester resins have high adsorption capaci-ties, but their tedious preparation and expense may obstruct their application.20In the present research, as-prepared cross-linked chitosan aerogel adsorbents exhibit a high absorption capability Additionally, the aerogels are biocompatible and innocuous to organisms.10In aqueous solutions, the amine groups of chitosan are easy to ionize, and they adsorb the SDBS molecules by electrostatic attraction However, chitosan dissolves below pH 5.5, severely limiting the use of native chitosan as an adsorbent

to remove SDBS molecules from acidic effluents.21 One objective of this work is to demonstrate the feasibility

of preparing a new class of cross-linked chitosan aerogels which are stable at low pH The other objective is to investigate the uptake of typical anionic surfactants such as sodium dodecyl-benzenesulfonate by the cross-linked chitosan-based aerogels Herein, cross-linked chitosan aerogels were first prepared by the sol-gel route combined with drying by supercritical CO2 The new porous materials were structurally characterized, and their adsorption of SDBS from aqueous solutions was also investigated

2 Experimental Section 2.1 Materials Chitosan (g90% deacetylated) and SDBS

were purchased from China Medicine Co Glutaraldehyde (50%

by weight in water), glyoxal (40% by weight in water), formaldehyde (40% by weight in water), acetic acid (HAc) and hydrochloric acid (HCl) were purchased from Tianjin Chemical Co., and sodium hydroxide (NaOH) was obtained from Shan-dong Chemical Co All reagents were analytical grade SDBS was recrystallized according to the following procedures: (1) SDBS was dissolved in hot methanol and filtered to remove sodium sulfate (Na2SO4), and (2) the solution was mixed with water and evaporated at 70°C until dry sample was obtained.22 Other reagents were used without further purification

2.2 Aerogel Preparation In a typical preparation process,

1.0 g of chitosan was dissolved in 50.0 mL 1.0 vol % acetic acid solution, and 1.0 mL glutaraldehyde (50 wt %) was dissolved in 20.0 mL deionized water The two solutions were mixed under vigorous stirring for 2 min and gradually

trans-* Corresponding author Phone: 88364280 Fax:

+86-0531-88364281 E-mail: cdr@sdu.edu.cn

10.1021/jp8011359 CCC: $40.75 2008 American Chemical Society

Published on Web 06/11/2008

formed into wet gels within 10 min and were aged for 24 h.

After the solvent, unreacted acetic acid and aldehyde in the

wet-gels were exchanged with absolute ethanol at 25°C, and the

aerogel was obtained by drying the alcogels beyond the critical

point of CO2 (304.1 K, 738 MPa) in a supercritical extractor

(HL-0.5L, Huali Co., China)

2.3 Characterization The BET surface area (SBET) and

Barrett-Joyner-Halenda (BJH) pore size distribution (PSD)

were measured by nitrogen (N2) adsorption/desorption at 77 K

using a QuadraSorb SI surface area analyzer after degassing

the samples at 100 °C for 10 h The surface mesoporous morphology of the aerogels was observed using an SEM (Hitachi S-520, JXA-840), and the bulk densities of the monolithic aerogels were obtained from the weight and volume

of the aerogels The Fourier transform infrared (FT-IR) spectra were recorded on a FT-IR spectrometer (Nicolet 5DX) using the KBr pellet method in the range 400-4000 cm-1 The

ζ-potentials of as-prepared aerogels in different pH solutions

were measured using a Zeta potential analyzer (Zeta-plus, Shanghai Zhongchen Instruments Corp., China)

2.4 Adsorption Tests 2.4.1 Effect of pH on the

Adsorp-tion Performance The adsorpAdsorp-tion of SDBS was studied in the

pH range 2-12 The pH value of the initial solution was adjusted with 0.10 M HCl or NaOH solution The pH value was measured using a pH meter (PHS-2, Shanghai Leisheng Instrument Co., China) The as-prepared aerogels (10 mg) were added into the SDBS solution (50 mL, 50 ppm) The solutions were shaken at 25 °C for 24 h until the adsorption reached equilibrium The SDBS concentrations were determined by the UV-vis absorption (UV-3100, Shimadzu) at 224 nm

2.4.2 Adsorption Kinetics Experiments The SDBS solution

(1000 mL, 50 ppm) was mixed with the as-prepared aerogels (0.2 g) in a stoppered 1000 mL Erlenmeyer flask and placed

on a rotary shaker at 200 rpm Several milliliters of reaction solution were sampled with a pipet at various time intervals between 0 and 24 h The sample solution was immediately filtered through a 0.45 µm membrane filter, and the UV

absorbance of the filtrate was measured to determine the concentration of SDBS

2.4.3 Adsorption Isotherm Experiments Adsorption

iso-therms were determined by measuring the depletion of different

Figure 1 N2 adsorption-desorption isotherms (A) and pore size distributions (B) of the chitosan- based aerogels Curves a-c are samples 1 (glutaraldehyde), 6 (glyoxal), and 11 (formaldehyde) as listed in Table 1.

TABLE 1: Properties of Chitosan-Based Aerogels

sample cross-linker

mass ratio of chitosen to water

volume ratio of cross-linker to water

surface area (m 2 /g)

pore diameter (nm)

pore volume (cm 3 /g)

bulk density (g/cm 3 )

Figure 2 (a) Photographs of the wet gel (a1) and aerogel (a2) and

SEM image (a3) of chitosan- glutaraldehyde, (b) chitosan-glyoxal,

and (c) chitosan-formaldehyde.

concentrations of SDBS (50 mL) at 25°C and pH 3 The

as-prepared aerogels (10 mg) were added, and the solutions were

shaken until the adsorption reached equilibrium (at the

equili-bration time determined for each material from the kinetics

experiments)

2.4.4 Elution of SDBS Adsorbed on the Aerogels To

remove the SDBS from the aerogel, 10 mg of the aerogel with

adsorbed SDBS was placed in NaOH (50 mL, 0.1 M) aqueous

solution and shaken for 24 h at 25°C The amount of SDBS

eluted was estimated by measuring the absorbance of the

solution at 224 nm

2.4.5 Reuse of Aerogels After elution of SDBS with 0.1

M NaOH, the aerogel was recovered by dipping in deionized

water for 24 h, then reused to adsorb SDBS as described above

Up to 10 cycles were performed

3 Results and Discussion

Cross-linked chitosan aerogels were successfully prepared with our procedure using three different cross-linkers: glutaral-dehyde, glyoxal, and formaldehyde As listed in Table 1, the aerogels have large specific surface areas that increased as the cross-linker content decreased which may be due to the formation of strong hydrogen bonds between adjacent chains which result in shrinkage The surface area increased as the mass percent of chitosan (CA) in the solution increased The N2 adsorption-desorption isotherms and corresponding BJH (Barret-Joyner-Halenda) pore size distribution (PSD) curves for cross-linked chitosan-based aerogels (Figure 1) show a type IV-like isotherm with an H1-type hysteresis loop, indicating the presence of mesopores in the monolithic aerogels, and that all aerogels have a narrow PSD The Brunauer-Emmett-Teller (BET) analyses indicate that the largest surface areas for the chitosan-glutaraldehyde/-glyoxal and -formaldehyde aerogels are 569, 707, and 845 m2/g, respectively We believe that the

N2adsorption-desorption hysteresis is a permanent type Every aerogel sample was tested for 3 times repeatedly for the N2 adsorption-desorption experiment; the adsorption-desorption hysteresis curves and the PSD curves are same This indicates the mesopores are stable.23The SEM images (Figure 2) also demonstrate the mesoporous structures of these materials, and the pore size is consistent with the BJH PSD within the error However, the aerogels melt under the electron beam of SEM within a short period of time, causing their mesoporous structures to gradually disappear Considering that the aerogels were degassed at 100 °C for 10 h, the high specific surface area indicated that the mesoporous structures were stable at

e100 °C The key factor to obtaining stable aerogels with a high specific surface area is the full replacement of the solvent with absolute ethanol If the solvent is not fully replaced by absolute ethanol, only a broken wet-gel will be produced We also tried to prepare cross-linked chitosan-based aerogels by freeze-drying and atmospheric drying, but only dry gels without porous structures were achieved

IR spectra of chitosan (1) and cross-linked chitosan-based aerogels (2, 3, 4) shown in Figure 3A indicate that the primary amine peak at 1653 cm-1decreased as the chitosan was

cross-linked, while a new peak for CdN amine appeared at 1653-1656

cm-1 The peak at 1602 cm-1disappeared in the aerogels due

to the loss of free amines, indicating a Schiff-base amine functionality.24According to the literature,25the adsorption at

ca 1700 cm-1 appeared in the presence of free cross-linker molecules In the present experiment, the IR spectra for the aerogels obtained with different cross-linkers did not show adsorptions at ca 1700 cm-1, which indicates that there are no

Figure 3 IR spectra (A) and TG curves (B) of chitosan (a), chitosan-glutaraldehyde (b), chitosan-glyoxal (c), and chitosan-formaldehyde (d)

aerogels.

Figure 4 Effects of pH on the adsorption of SDBS (50 ppm) on the

chitosan-based aerogels, with the cross-linkers (a) glutaraldehyde, (b)

glyoxal, and (c) formaldehyde.

Figure 5 The relationship of zeta-potential of the aerogels to the pH

of the solution (25 ° C) in absence of SDBS, with the cross-linkers (a)

glutaraldehyde, (b) glyoxal, and (c) formaldehyde.

free cross-linker molecules in the samples (Figure 3A) Low

concentration of cross-linkers used and full exchange of

unreacted aldehyde with absolute ethanol may cause the result

The TG curve of the chitosan-based aerogels shown in Figure

3B demonstrates that the first mass loss from ca 25 to 220°C

concerns the loss of water, which is adsorbed both on the surface

and in the pores of the aerogels The decomposition of the

chitosan-based aerogel is observed from ca 220 to 450°C, and

the profiles of the TG curves for both non-cross-linked and

cross-linked materials are similar It was also found that carbon

aerogels could not be produced after calcining the

chitosan-based aerogels under a flow of nitrogen because the thermal

degradation of the main chains in chitosan and chitosan-based

aerogels during heating destroyed the network structure.26

The adsorptions of SDBS on chitosan aerogels prepared with

different cross-linkers show similar trends The pH affected the

adsorption considerably (Figure 4) For example, over a pH

range from 2 to 12, the absorption maxima for all aerogels in

50 ppm SDBS solution occur at pH 3 The maximum adsorption

amounts for chitosan-glutaraldehyde/-glyoxal/-formaldehyde

aerogels are 96, 246, and 222 mg/g, respectively With the pH

value increasing from 3 to 10, the adsorption amount grandually

decreased, and reduced to 0 at pH 10

It is known that surfactant adsorption may result from several

interactions at the solid-solution interface, such as the

hydro-phobic interaction, hydrogen bonding, dispersion forces,

elec-trostatic attraction, and ion exchange.16bThe aerogel is positively

charged when the pH value is less than 9 from the determined

zeta potential (Figure 5), while SDBS is an anionic surfactant,

and the adsorption amount in the pH range 3-10 increased with

the positive charge density of the aerogel increasing Thus, it

is considered that the electrostatic adsorbent-adsorbate

interac-tion may be the dominant one in the present adsorpinterac-tion of SDBS

The exception at pH 2 may be due to the protonated form of

the SDBS in the acid solution27and the competition of the Cl

-anions in the solution Figure 6B shows that the required time

for SDBS to reach adsorption equilibrium is ca 600 min for

the aerogels formed in presence of glutaraldehyde or glyoxal

as cross-linker, and ca 300 min for the aerogel formed in the

presence of formaldehyde as cross-linker It can be seen that

the equilibrium time decreases with the pore size increasing,

although all the aerogels exhibit similar zeta potential at pH 3

Thus, it is considered that this long equilibrium time might result from the small pore size in the aerogel into which the SDBS molecules enter slowly

As shown in Figure 6, for each aerogel, the amount of SDBS adsorbed increased with the increase of the equilibrium con-centration of SDBS up to a maximum amount For the chitosan-glutaraldehyde aerogel (sample 1, the concentration

of glutaraldehyde in the sol process is 0.8 wt %), the largest amount adsorbed is 869 mg/g (equilibrium concentration ) 590 ppm) For the chitosan-glyoxal aerogel (sample 6, the con-centration of glyoxal in the sol process is 1.2 wt %), 1735 mg/g SDBS is adsorbed (equilibrium concentration ) 302 ppm) For the chitosan-formaldehyde aerogel (sample 11, the concentra-tion of formaldehyde in the sol process is 1.2 wt %), the largest amount adsorbed is up to 1800 mg/g (equilibrium concentration ) 199 ppm) After reaching a maximum concentration, the amount adsorbed remained constant within the error, as the aerogel samples became saturated In comparison to the adsorp-tion capacities of other adsorbents, these mesoporous aerogels have a much higher SDBS removal capability than all previously tested materials except the acrylic ester resins,20especially at

pH 3 Further analysis showed that the SDBS molecules were adsorbed on the chitosan-glutaraldehyde aerogels rapidly in the first 4 h and slowly toward the end of the run (Figure 6B)

At ca 4 h, ca 68% of the SDBS was adsorbed, and only ca 12% was adsorbed in the next ca 6 h After the adsorption equilibrium time was reached, the aerogels did not adsorb additional SDBS The experiments indicate that the shortest adsorption equilibrium time of the chitosan-formaldehyde aerogels is ca 5 h, and those of the chitosan-glutaraldehyde aerogel and chitosan-glyoxal aerogels are ca 10 h Thus, it is generally concluded that the adsorption capacity and rate increase as the specific surface area and pore diameter of the aerogels increase Table 2 shows adsorption results of different cross-linker aerogels

It is worthwhile here to investigate the elution behavior of SDBS from the aerogels and the possibility of reuse of the aerogels The result shows more than 97% of the SDBS was eluted from the aerogels after being dipped in 0.1 M NaOH solution for 24 h This result also implies that the adsorption and elution of SDBS are mainly based on electrostatic interac-tions between the chitosan aerogels and SDBS When the

Figure 6 Adsorption isotherms (A) and kinetic curves (B) of SDBS molecules on the aerogels, with cross-linkers (a) glutaraldehyde (top axis),

(b) glyoxal (bottom axis), and (c) formaldehyde (bottom axis).

TABLE 2: Adsorption Results of Aerogels with Different Cross-Linkers

sample cross-linker

mass ratio of chitosen to water

volume ratio of cross-linker to water

SDBS adsorption maximun (mg/g)

equilibrium time (min)

a fibrillar solid network It has been demonstrated that different

cross-linkers and their content have an important effect on the

aerogel textures as well as the bulk density The gels exhibited

shrinkage during the supercritical CO2drying process, due to

the strong hydrogen bonds between the adjacent polymer chains

The key factor for obtaining aerogels with a high specific surface

area is complete removal of water in the pores by displacement

with absolute ethanol The aerogels are insoluble in acidic

solution and have good adsorption performance for SDBS in

acidic solution, much higher than that of the previously reported

materials except the acrylic ester resins,20so it appears

techni-cally feasible to use these materials to remove SDBS from acidic

aqueous wastewater and control SDBS pollution

Acknowledgment This work was supported by the Program

for New Century Excellent Talents in University

(NCET-05-0580), People’s Republic of China

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JP8011359