chang2008 chitosan based aerogels with high adsorption performance

J Phys Chem B 2008, 112, 7721–7725 7721 Chitosan-Based Aerogels with High Adsorption Performance Xinhong Chang, Dairong Chen,* and Xiuling Jiao School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100 P R China ReceiVed: February 2, 2008; ReVised Manuscript ReceiVed: April 2, 2008 New natural polymer-based aerogels, cross-linked chitosan aerogels, were prepared by the sol-gel route with glutaradehyde, glyoxal, and formaldehyde as cross-linkers The alcogels were dried by supercritical 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 Introduction As low-density solids, aerogels have an open three-dimensional mesoporous structure and high specific surface area, and thus have potential applications as adsorbents, thermal insulators, acoustic absorbers, catalysts, and catalyst supports.1 Therefore, much research has been dedicated to aerogels, including their preparation, properties, and applications Besides inorganic aerogels such as silicon dioxide,2 metal oxides,2,3 chalcogenides,4 and carbon aerogels,5 the organic or hybrid aerogels have also been extensively investigated.6–8 However, among the organic aerogels that have been prepared,6 most are made from artificial polymers Only a few studies have employed natural polymers or their derivatives, mainly alginate7 and cellulose-based aerogels.8 Chitosan from natural polymer chitin has good biocompatibility, 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.10 However, 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 CO2 drying of gel beads,12 and Renzo’s group fabricated chitosan films with a surface area of 175 m2/g,13 but 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 vegetation.14 SDBS in wastewater is adsorbed by soil, slowly degrading and impeding the microbial processes within it.15 In the past few decades, many efforts have been made to develop both organic and inorganic adsorbents such as carbon,16 minerals,17 * Corresponding author Phone: +86-0531-88364280 Fax: +86-053188364281 E-mail: cdr@sdu.edu.cn poly(vinylchloride) latexes,18 cellulose, and chitosan19 in 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 capacities, but their tedious preparation and expense may obstruct their application.20 In the present research, as-prepared cross-linked chitosan aerogel adsorbents exhibit a high absorption capability Additionally, the aerogels are biocompatible and innocuous to organisms.10 In 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 dodecylbenzenesulfonate 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 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 Shandong 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 and gradually trans- 10.1021/jp8011359 CCC: $40.75  2008 American Chemical Society Published on Web 06/11/2008 7722 J Phys Chem B, Vol 112, No 26, 2008 Chang et al Figure N2 adsorption-desorption isotherms (A) and pore size distributions (B) of the chitosan- based aerogels Curves a-c are samples (glutaraldehyde), (glyoxal), and 11 (formaldehyde) as listed in Table 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 (m2/g) pore diameter (nm) pore volume (cm3/g) bulk density (g/cm3) 10 11 12 13 14 15 glutaraldehyde glutaraldehyde glutaraldehyde glutaraldehyde glutaraldehyde glyoxal glyoxal glyoxal glyoxal glyoxal formaldehyde formaldehyde formaldehyde formaldehyde formaldehyde 0.014 0.014 0.014 0.018 0.021 0.014 0.014 0.014 0.018 0.021 0.014 0.014 0.014 0.018 0.021 1:70 1:35 1:14 1:70 1:70 1:35 1:14 1:7 1:35 1:35 1:35 1:14 1:7 1:35 1:35 504 392 66 569 566 612 574 420 707 686 747 716 525 821 845 5.40 3.62 3.29 4.59 4.37 6.40 4.20 3.89 5.38 4.37 8.50, 11.13 6.87 8.50 10.16 9.28 0.90 0.40 0.06 0.78 0.74 0.99 0.64 0.48 1.11 0.85 2.89 1.48 1.24 3.46 2.65 0.57 0.78 0.92 0.51 0.49 0.53 0.62 0.78 0.47 0.49 0.48 0.52 0.59 0.38 0.43 formed into wet gels within 10 and were aged for 24 h After the solvent, unreacted acetic acid and aldehyde in the wetgels 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 Figure (a) Photographs of the wet gel (a1) and aerogel (a2) and SEM image (a3) of chitosan- glutaraldehyde, (b) chitosan-glyoxal, and (c) chitosan-formaldehyde 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 -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 Adsorption Performance The adsorption 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 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 isotherms were determined by measuring the depletion of different Chitosan-Based Aerogels J Phys Chem B, Vol 112, No 26, 2008 7723 Figure IR spectra (A) and TG curves (B) of chitosan (a), chitosan-glutaraldehyde (b), chitosan-glyoxal (c), and chitosan-formaldehyde (d) aerogels Results and Discussion Figure 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 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 concentrations of SDBS (50 mL) at 25 °C and pH The asprepared aerogels (10 mg) were added, and the solutions were shaken until the adsorption reached equilibrium (at the equilibration 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 Cross-linked chitosan aerogels were successfully prepared with our procedure using three different cross-linkers: glutaraldehyde, 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 N2 adsorption-desorption hysteresis is a permanent type Every aerogel sample was tested for 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.23 The 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-1 decreased as the chitosan was crosslinked, while a new peak for CdN amine appeared at 1653-1656 cm-1 The peak at 1602 cm-1 disappeared in the aerogels due to the loss of free amines, indicating a Schiff-base amine functionality.24 According to the literature,25 the 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 7724 J Phys Chem B, Vol 112, No 26, 2008 Chang et al Figure 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) 11 glutaraldehyde glyoxal formaldehyde 0.014 0.014 0.014 1:70 1:35 1:35 869 1735 1800 600 600 300 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 chitosanbased 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 to 12, the absorption maxima for all aerogels in 50 ppm SDBS solution occur at pH The maximum adsorption amounts for chitosan-glutaraldehyde/-glyoxal/-formaldehyde aerogels are 96, 246, and 222 mg/g, respectively With the pH value increasing from to 10, the adsorption amount grandually decreased, and reduced to at pH 10 It is known that surfactant adsorption may result from several interactions at the solid-solution interface, such as the hydrophobic interaction, hydrogen bonding, dispersion forces, electrostatic attraction, and ion exchange.16b The aerogel is positively charged when the pH value is less than 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 interaction may be the dominant one in the present adsorption of SDBS The exception at pH may be due to the protonated form of the SDBS in the acid solution27 and the competition of the Clanions in the solution Figure 6B shows that the required time for SDBS to reach adsorption equilibrium is ca 600 for the aerogels formed in presence of glutaraldehyde or glyoxal as cross-linker, and ca 300 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 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 concentration 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 concentration 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 concentration 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 adsorption capacities of other adsorbents, these mesoporous aerogels have a much higher SDBS removal capability than all previously tested materials except the acrylic ester resins,20 especially at pH Further analysis showed that the SDBS molecules were adsorbed on the chitosan-glutaraldehyde aerogels rapidly in the first h and slowly toward the end of the run (Figure 6B) At ca h, ca 68% of the SDBS was adsorbed, and only ca 12% was adsorbed in the next ca 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 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 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 interactions between the chitosan aerogels and SDBS When the Chitosan-Based Aerogels aerogels were recycled and reused for adsorption of SDBS, the 90% of the original adsorbed capacity still remained after 10 cycles, showing high stability of the aerogels during the chemical treatment Conclusions In summary, we have synthesized mesoporous chitosan-based aerogels for the first time The sol-gel and subsequent supercritical CO2 drying process generate aerogels with high surface areas and mesopores The pore volume and PSD of the final materials were dependent on the properties and contents of the cross-linkers SEM shows that the as-synthesized aerogels have a fibrillar solid network It has been demonstrated that different cross-linkers 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Carbohydr Polym 2005, 62, 97 (c) Santos, J E.; Dockal, E R.; Cavalheiro, E T G J Therm Anal Calorim 2005, 79, 24 (27) Introduction to Nonionic Surfactants In Surfactant Science Series: Nonionic Surfactants Chemical Analysis; John, C., Eds.; Marcel Dekker Inc.: New York, 1987; Vol 19 JP8011359 ... carbon aerogels could not be produced after calcining the chitosanbased aerogels under a flow of nitrogen because the thermal degradation of the main chains in chitosan and chitosan- based aerogels. .. elution of SDBS are mainly based on electrostatic interactions between the chitosan aerogels and SDBS When the Chitosan- Based Aerogels aerogels were recycled and reused for adsorption of SDBS, the... chitosan (a), chitosan- glutaraldehyde (b), chitosan- glyoxal (c), and chitosan- formaldehyde (d) aerogels Results and Discussion Figure Effects of pH on the adsorption of SDBS (50 ppm) on the chitosan- based