Synthesis and adsorption properties of chitosan-silica nanocomposite prepared by sol-gel method
© Budnyak et al.; licensee Springer. 2015
Received: 20 October 2014
Accepted: 29 December 2014
Published: 28 February 2015
A hybrid nanocomposite material has been obtained by in situ formation of an inorganic network in the presence of a preformed organic polymer. Chitosan biopolymer and tetraethoxysilane (TEOS), which is the most common silica precursor, were used for the sol-gel reaction. The obtained composite chitosan-silica material has been characterized by physicochemical methods such as differential thermal analyses (DTA); carbon, hydrogen, and nitrogen (CHN) elemental analysis; nitrogen adsorption/desorption isotherms, scanning electron microscopy (SEM); and Fourier transform infrared (FTIR) spectroscopy to determine possible interactions between silica and chitosan macromolecules. Adsorption of microquantities of V(V), Mo(VI), and Cr(VI) oxoanions from the aqueous solutions by the obtained composite has been studied in comparison with the chitosan beads, previously crosslinked with glutaraldehyde. The adsorption capacity and kinetic sorption characteristics of the composite material were estimated.
Nowadays, there is a great interest in the development of low-cost sorbents for water remediation. The use of low-cost sorbents has become an alternative to expensive methods such as membrane filtration or ion exchange [1,2]. Recently, numerous approaches have been studied for the development of cheaper and more effective adsorbents containing polysaccharides [3,4].
Natural polysaccharide chitosan, a derivative of chitin [5-11], is of great interest as an organic component in the composites developed for water treatment because of the high quantity of amino and hydroxyl groups, which is very important for sorption processes [12-19]. Moreover, chitosan possesses such properties as good biocompatibility, high adhesion to the surface, a wide range of pH stability, and expressed chelating properties. It has been proved that chitosan is an effective bioadsorbent towards some toxic ions, dyes, and organic contaminants [20-23]. Silicas are characterized by advanced surface stability in the acidic medium and highly developed surface, acceptable kinetics, thermal stability, resistance to microbial attack, and low cost.
Various methods of preparation of hybrid materials based on inorganic materials and polysaccharides such as chitin and chitosan for different applications have been studied [24-30]. The method of obtaining chitin and chitosan hybrid mesoporous composites applied for dye removal from natural waters and industrial effluents was presented in . The authors of  described the adsorption of chitosan on the nanosilicas (SiO2, TiO2/SiO2, and Al2O3/SiO2). Preparation of the hybrid chitosan-silica sorbent was carried out in , and the obtained nanocomposite was used in high-performance liquid chromatography. The chromatographic results showed that the obtained hybrid chitosan-silica sorbent and silica gel encapsulated by chitosan have similar properties according to the separation efficiency. The authors of [34,35] studied the preparation of zirconia-chitin-based composites and their possible application as a template for the in vitro formation of zirconium dioxide nanophase from ammonium zirconium(IV) carbonate under extreme conditions. For that purpose, novel zirconia-chitin-based composites were prepared using hydrothermal synthesis. It was proved that chitin can be effectively silicified by the two-step method with the use of Stöber silica micro- and nanodispersions under extreme biomimetic conditions . In that case, the chitin-silica composites obtained at 120°C and characterized by the presence of spherical SiO2 particles homogeneously distributed over the chitin fibers.
Sol-gel reaction has been extensively studied for several decades as a method to prepare ceramic precursors and inorganic glasses at relatively low temperatures. The major advantage of the process is that mild conditions, such as relatively low temperature and pressure, are used in this type of ceramics processing. Within the past years, the sol-gel process was widely used to create novel hybrid nanoscaled materials based on organic and inorganic components [37-40]. For obtaining such composite materials, the sol-gel reaction is carried out in the presence of organic molecules that are typically polymeric, contain functional groups, and could be immobilized on the inorganic component. Simplicity and possibility of variation of the quantitative ratio of reagents, as well as the nature of initial materials, provide a wide range of applications of the materials. One of the possible applications for those organic-mineral composites is sorption processes and treatment of water solutions from toxic substances.
Peculiarities of preparation of chitosan-silica hybrid materials by using a sol-gel process in a wide range of silica content were studied , as well as the properties of prepared hybrid materials towards adsorption of dyes  and rare earths . Moreover, recent studies in that field have shown that composites of silica and chitosan [44-49] can be used for extraction and concentration of toxic metals from solutions. However, systematic investigations of adsorption properties of chitosan-silica composites, in particular an effect of nature and the pH of solutions as well as determination of achievable values of adsorption capacity, are necessary.
This work describes the synthesis of the nanocomposite material chitosan-silica for their use as a biosorbent. The hybrid material was obtained by the sol-gel method using tetraethoxysilane (TEOS) as the SiO2 precursor. Adsorption properties of the obtained hybrid material were studied with respect to highly toxic metals, such as vanadium(V), molybdenum(VI), and chromium(VI) oxoanions, which are common contaminants in industrial waste waters. Conditions connected with the optimum pH value of the medium, interaction time, and adsorption capacity were studied.
Chitosan (No 417963, Sigma-Aldrich, St. Louis, MO, USA) with a molecular weight from 190,000 to 370,000 Da, degree of deacetylation not less than 75%, and solubility of 10 mg/ml and 99.9% tetraethoxysilane precursor (Sigma-Aldrich, St. Louis, MO, USA) were used for the synthesis. All chemicals purchased from Sigma Aldrich were reagent grade.
Composite chitosan-silica was obtained by the sol-gel method through hydrolysis of tetraethoxysilane in the chitosan solution. Composite chitosan-silica was synthesized by following technique: 30 ml of ethanol, 1 ml of distilled water, and 0.5 ml of concentrated hydrochloric acid were added to 46.5 ml of tetraethoxysilane. The obtained mixture was stirred using the magnetic stirrer MM-5 for 10 min and slowly added dropwise to the previously prepared chitosan solution (0.5 g of chitosan was dissolved in 100 ml of 2% acetic acid) and stirred for a day. After 7 days, when the sol became mature, the obtained substance was dried at 60°C.
Thus, in the case of the 100% yield of silica generation by the sol-gel reaction from the tetraethoxysilane precursor, the theoretical mass ratio of the obtained organic and mineral components of the composite was as follows: chitosan:silica = 1:25.
Partially crosslinked chitosan beads were obtained applying the following technique: 2.5 g of chitosan was dissolved in 85 ml of 2% acetic acid and the solution was stirred using the magnetic stirrer MM-5 for 2 h and allowed to stand for 2 days. The obtained solution was added dropwise into the concentrated ammonia solution. After that, the obtained chitosan beads were washed with distilled water several times until neutral pH was achieved. The obtained chitosan beads were placed in 12.5 ml of 0.25% solution of glutaraldehyde in water and heated at 50°C for 2 h. Such a quantity of glutaraldehyde is proper for the crosslinking of 5% of accessible amino groups of polymer. The crosslinked chitosan beads were washed with distilled water and dried at 50°C.
Buffer solutions with pH 1.0 prepared from the standard titrimetric substance of HCl acid, pH 2.5 and pH 5.0 from glacial acetic acid, and pH 8.0 were prepared from 17 ml of 1 M acetic acid and 5 ml of 25% ammonia solution, adding distilled water up to 1 l. The pH values of all buffer solutions were controlled by pH-meter.
Fourier transform infrared (FTIR) spectra of the samples of the initial chitosan and reaction products were recorded using an IR spectrometer with Fourier transformation (Thermo Nicolet Nexus FT-IR, Waltham, MA, USA). For this purpose, the samples were ground in an agate mortar and pressed with KBr.
The concentration of chitosan in the composite was determined by the thermogravimetric method on the derivatograph Q-1500 MOM (Mateszalka, Hungary) with the computer data registration in the temperature range of 15°C to 1,000°C. The samples heating rate was 10°/min. The differential thermal analyses (DTA), TG, and DTG curves were recorded simultaneously.
The specific surface area and the average pore diameter of the composite were determined with the ASAP 2405 (Micromeritics Instrument Co., Norcross, GA, USA). The isotherm plots were used to calculate the specific surface area and the average pore diameter of the chitosan-silica composite.
Elemental analysis of the chitosan-silica composite was carried out by using a carbon, hydrogen, and nitrogen (CHN)/O analyzer (Series II CHNS/O Analyzer 2400, PerkinElmer, Waltham, MA, USA). The analysis was carried out at the combustion temperature of 925°C and the reduction temperature of 640°C.
The surface morphology of the chitosan-silica composite was observed by using a scanning electron microscope (SEM, LEO 1430VP, Carl Zeiss, Inc., Oberkochen, Germany).
The investigations of the adsorption properties of the obtained composite with respect to V(V), Mo(VI), and Cr(VI) oxoanions were carried out in static mode with periodic hand stirring. The sample of 0.1 g of synthesized adsorbent contacted with 25 ml of solutions at different concentrations of salts: NH4VO3, (NH4)6Mo7O24 · 4H2O, (NH4)2Cr2O7 which were prepared according to . Photometric studies of equilibrium solutions were performed according to the methods described in  using a SF-46 spectrophotometer (LOMO, St. Petersburg, Russia) with square cuvettes (optical path length l = 1 cm).
Results and discussion
Physicochemical characteristics of the composite
The sol-gel process can be viewed as a two-network forming process, the first step being the hydrolysis of silicon alkoxide and the second consisting in a polycondensation reaction. Most interest in this method is focused on metal-organic alkoxides, especially silica since they can form an oxide network in organic matrices. The sol-gel reactions of alkoxysilane can be described as follows :
There are many different synthetic techniques used in the sol-gel process to generate polymer-silica hybrid materials. One of them is the in situ formation of an inorganic network in the presence of a preformed organic polymer. Those hybrid materials possess strong chemical bonds (covalent or ionic) between the organic and inorganic phases. Also, physical- or weak-phase interactions can be observed between phases, for example, hydrogen bonding or van der Waals attraction.
The FTIR spectrum of the synthesized composite (Figure 2B) has shown a shift of the band 1,528 сm-1 of -NH2 deformation vibrations in comparison with the spectrum of the initial chitosan. An intensive absorbance at 1,100 сm−1 represents the Si-O stretching vibrations.
The thermogravimetric curve of the chitosan-silica composite (Figure 3B) is characterized by the decomposition region from 30°C to 200°C which is similar to that of the initial chitosan and corresponds to water desorption followed by decomposition of the organic part of the composite. At higher temperatures (200°C to 1,000°C), the process of condensation and elimination of hydroxyl groups could also take place. At that temperature range, decomposition of chitosan was also confirmed by the increasing stretching vibration of C-O in the molecule of CO2 at 2,350 cm−1 (Figure 3C). Thus, the comparison of TG-curves of chitosan and chitosan-silica composite shows that in the temperature range from 200°C to 1,000°C weight losses of the chitosan-silica composite reach about 10%, which are most likely caused by destruction of the organic component of the composite and hydroxyl groups.
Comparing the results of TGA and mass ratio of initial components for the synthesis, the quantity of chitosan in the composite is in the concentration range from 38 to 100 mg/g.
Percentage of carbon, hydrogen, and nitrogen and the C/N ratio for the chitosan-silica composite
Influence of pH on adsorption
Degree of adsorption of V(V), Mo(VI), and Cr(VI) oxoanions by composite chitosan-silica as a function of medium acidity
Degree of adsorption (%)
СН 3 СООН
СН 3 СООН
Ammonium acetate buffer
Thus, the synthesized composite showed adsorption activity with respect to the investigated ions in different pH ranges, but the highest degree of adsorption was observed in the acidic medium. The values of medium acidity, at which the maximum adsorption activities of the chitosan-silica composite for each of the studied oxoanions were achieved, correspond to the published data of complexation conditions of these ions with amino groups of chitosan in solutions .
Influence of initial metal ion concentration on adsorption
From the distilled water, the chitosan-silica composite removed НCrO4 − ions in the amount of 35.2 mg/g (0.68 mmol/g). But the composite adsorbed these ions with the highest degree of adsorption (94%) at the initial solution concentration 8 μg/cm3.
The synthesized chitosan-silica composite showed the lowest adsorption activity with respect to vanadium(V) oxoanions at pH 2.5, which were presented as a mixture of ions [V10О28]6−, [НV10О28]5−, and [Н2V10О28]4−; the adsorption capacity in that case was 2.5 mg/g (or 0.05 mmol/g). The chitosan-silica composite quantitatively adsorbed VO3 − ions in the neutral medium in the case of metal concentration up to 20 mg/dm3, but at the higher concentration (until 80 μg/cm3), V(V) oxoanions were extracted at 90%. Adsorption isotherms of all investigated ions belong to the L-type, where the ratio between the concentration of the compound remaining in the solution and the concentration adsorbed on the solid decreases when the solute concentration increases, providing a concave curve suggesting a progressive saturation of the solid . L-isotherms of the Langmuir model are common for the monolayer adsorption, where the adsorbed layer is one-molecule thick.
Adsorption capacity of chitosan-silica composite compared to a partly crosslinked chitosan
Partially crosslinked chitosan beads
[V10О28]6−, [НV10О28]5−, [Н2V10О28]4−
MoO4 2−, [Mo6O21]6−, [Mo7O24]6−
In the acidic medium generated by acetic ions (pH 2.5), partially crosslinked chitosan adsorbs molybdenum with q e 39 mg/g, but under the same conditions, the immobilized chitosan extracts three times more than chitosan beads (q e is equal to 121 mg/g). It is interesting that in the neutral medium 1 g of partially crosslinked chitosan concentrated four times less molybdenum than the 1 g of chitosan that was part of the chitosan-silica composite: 10 and 37 mg/g, respectively. The synthesized composite adsorbs chromium(VI) in the neutral medium with an adsorption capacity of 35 mg/g which is seven times higher than that of a partially crosslinked chitosan of 5 mg/g.
An increase in adsorption capacity of immobilized chitosan compared to partially crosslinked chitosan could be explained by the expanded quantity of accessible adsorption sites of the chitosan-silica composite and high surface area, as well as a more suitable morphology of synthesized composite for adsorption of V(V), Mo(VI), and Cr(VI) oxoanions.
Influence of contact time on adsorption
The chitosan-silica composite was synthesized by the sol-gel method through the hydrolysis of tetraethoxysilane in the chitosan solution. IR spectroscopy confirmed the fact of Si-O-Si polymeric network formation in the presence of chitosan polymer. According to the thermogravimetric analysis and theoretical calculations of the reaction mixture, the obtained composite contains from 3.7% to 9.1% of chitosan.
The comparison of adsorption capacity of biopolymer chitosan (partially crosslinked with glutaraldehyde) and the synthesis via the sol-gel reaction chitosan-silica composite showed that 1 g of immobilized chitosan has better adsorption capacity than the initial chitosan with respect to the studied oxoanions. The obtained organic-inorganic composite contained a small part of chitosan, in comparison with the inorganic part of silica—1 g of composite included from 38 to 100 mg of chitosan. It was found that a small quantity of polymer in the composition of the composite makes it easier to develop increased adsorption properties and kinetic characteristics towards the studied ions. For instance, adsorption capacity of the synthesized composite increased several times in comparison to that polymer with respect to the following: vanadium—9.9 mg/g in the neutral medium, molybdenum—121.0 and 37.4 mg/g at рН 2.5 and in the neutral medium, respectively, and chromium—35.2 mg/g in the neutral medium. It was shown that the synthesized composite extracted the studied metal ions in a day.
The research leading to these results was financed from the Visegrad 4 Eastern Partnership Program of the International Visegrad Fund under the contract for financing Visegrad/V4EaP Scholarship No 51400001 and the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under REA grant agreement No° PIRSES-GA-2013-612484.
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