Utilization of Chitosan/Bamboo Charcoal Composite as Reactive Dye Adsorbent

Transcription

1 174 Chiang Mai J. Sci. 2014; 41(1) Chiang Mai J. Sci. 2014; 41(1) : Contributed Paper Utilization of Chitosan/Bamboo Charcoal Composite as Reactive Dye Adsorbent Walaikorn Nitayaphat* Department of Home Economics, Faculty of Science, Srinakharinwirot University, Bangkok, 10110, Thailand. *Author for correspondence; walaikorn@swu.ac.th Received: 7 November 2012 Accepted: 27 February 2013 ABSTRACT Chitosan/bamboo charcoal composites were prepared by blending chitosan with bamboo charcoal and forming composite beads. The composites were used as reactive dye adsorbents. Adsorption equilibrium experiments were carried out as a function of contact time, bamboo charcoal concentration, ph value, and adsorbent dosage level. The equilibrium time of dye adsorption was found to be 8 h. Composite adsorbent had the highest adsorption efficiency when the weight ratio was 50/50. The maximum dye removal took place at the initial ph value of 4.0. The optimum adsorbent dosage for dye removal was 6.0 g. Under above optimal conditions the maximum dye removal was 98.4%. The adsorption isotherm of chitosan and chitosan/bamboo charcoal composite beads agreed well with the Langmuir model. The maximum adsorption capacity was 3.47 mg/g for chitosan bead and 4.32 mg/g for chitosan/bamboo charcoal composite bead, respectively. SEM micrographs confirm that after adsorption the pores were packed with Reactive Red 152. Keywords: chitosan, bamboo charcoal, composite, reactive dye, adsorbent 1. INTRODUCTION The textile industry is one of the most important and rapidly expanding industrial sectors in developing countries. Among the various processes in the textile industry, the dyeing process uses large volumes of water for the dyeing, fixing and washing processes. Therefore, the wastewater generated form the textile processing industries contains suspended solids, high amounts of dissolved solids, un-reacted dyestuffs and other auxiliary chemicals which are used in the various stages of dyeing and with other processing [1]. The water coloration from the presence of dyes, even in small concentrations, is highly visible and affects the esthetics, water transparency and the gas solubility of water bodies [2]. Among several classes of textile dyestuffs, the reactive dyes represent about 50% [3] of the total market share due to their advantages, such as with a wide variety of color shades, bright colors, excellent color fastness, easy application, and minimal energy consumption [4]. A large fraction, typically about 30%, of applied reactive dye is wasted because of the dye hydrolysis in the alkaline dyebath. Several wastewater treatment technologies have been applied for color removal, including physical, chemical and biological processes. In the past years,

2 Chiang Mai J. Sci. 2014; 41(1) 175 adsorption processes have been shown to be effective along with economical treatment processes, thus many low-cost adsorbents have been investigated, such as clay minerals, rice husk, leaf powder, fly ash, and bacterial biosorbents [5-11]. However, low adsorption capacities of these adsorbents toward dyes limit their applications in practical fields. Chitosan, the deacetylated product of chitin, which are extracted from various animals and plants, are the second most abundant natural biopolymers found on earth, next to cellulose. Due to its biocompatibility, biodegradability, antimicrobial activity, non-toxicity, and exhibits a high adsorption capacity, chitosan has been extensively investigated for several decades for use in wastewater treatment towards many classes of dyes, such as reactive dyes [12-15], acid dyes [16-17], and direct dyes [18]. Chitosan contains high contents of amino functional groups, which might form electrostatic attraction between chitosan and solutes to adsorb the dyes [19]. Nevertheless, the market cost of chitosan is relatively high and its specific gravity should be improved with practical operations. Bamboo charcoal powder contains many pores and gaps in its structure, making it excellent for adsorption. In this study, chitosan/bamboo charcoal composite beads were used to remove reactive dye from an aqueous solution by batch adsorption process and the parameters affecting the adsorption capacity of the chitosan/bamboo charcoal composite beads, including bamboo charcoal concentration variation and ph, were investigated. 2. MATERIALS AND METHODS 2.1 Materials Chitosan powder with an average molecular weight of 150 kda and a deacetylation degree of 90% was purchased from Seafresh Chitosan (Lab) Co., Ltd. (Thailand). Bamboo charcoal powder, with a particle size range of micron and specific surface area about 743 m 2 /g, used as filler. C. I. Reactive Red 152 (Figure 1) was used as a model anionic dye. Glacial acetic acid was purchased from J.T. Baker (Thailand). Figure 1. Structure of C. I. Reactive Red 152.

3 176 Chiang Mai J. Sci. 2014; 41(1) 2.2 Preparation of Chitosan /Bamboo Charcoal Composite Beads Pure chitosan beads were prepared by dissolving 2 g of chitosan powder in 50 ml of 2% (v/v) acetic acid solution and stirring for 3 h at room temperature. This mixture was dropped through a syringe into a precipitation bath containing 1 dm 3 of an alkaline coagulating mixture (H 2 O: EtOH: NaOH = 4:5:1, w/w) gave rise to the chitosan beads. The beads were extensively washed with de-ionized water and preserved in an aqueous environment for future use. Different weights of bamboo charcoal powder (0.2, 0.6, and 1.0 g) were added into 50 ml of 2% (v/v) acetic acid solution together with 1.8, 1.4, and 1.0 g of chitosan, respectively. The mixtures were stirred for 3 h at room temperature. Then, the mixtures with different ratios by weight (chitosan/ bamboo charcoal: 90/10, 70/30, and 50/50) were dropped through a syringe into a precipitation bath containing 1 dm 3 of an alkaline coagulating mixture gave rise to the chitosan/bamboo charcoal composite beads. 2.3 Surface Area and Porosity Analysis The specific surface area and pore size distribution were determined by surface analyzer (Autosorb-1, Quantachrome) using N 2 as adsorbate. The diameter (D) and porosity (ε) of the chitosan and chitosan/ bamboo charcoal composite beads were determined by the amount of water within the pores of the chitosan and chitosan/ bamboo charcoal composite beads [20]. The diameter (D) and porosity (ε) can be calculated using these equations: 1/3 D = [ W 6 D /ρ Mat +(W W -W D )/ρ W ] π ε = (W W -W D )/ρ W x 100% W D /ρ Mat +(W W -W D )/ρ W where W W (g) is the wet weight of the beads before drying; W D (g) is the wet weight of the beads after drying; ρ w is the density of water, 1.0 g/cm 3 ; and ρ Mat is the density of material. 2.4 Adsorption Experiments Adsorption experiments were carried out by using chitosan and chitosan composite samples (with different bamboo charcoal powder content) as adsorbents. Batch adsorption experiments were carried out using a water bath shaker (Model RAPID). For a typical adsorption experiment, 1.0 g adsorbent was dispersed in 50 ml of 50 mg/ L aqueous Reactive Red 152 solution without adjusting the ph value. The dispersion was stirred at a speed of 120 rpm at 30 C. The dye concentrations were determined by measuring the absorbance at the maximum absorption of Reactive Red 152 (520 nm) using spectrophotometer (Model ICS- TEXICON). The effect of ph on adsorption capacities was determined in the ph range from 3 to 9. The ph was adjusted with 0.1 mol/l NaOH or 0.1 mol/l HCl. Different amounts of adsorbent in the range of 1.0 to 6.0 g were used to examine the effect of an adsorbent dosage on adsorption of Reactive Red Adsorption Isotherms Adsorption isotherms were obtained by using 1 g of adsorbent beads and 50 ml of dye solution with different concentrations ( mg/l). These solutions were buffered at an optimum ph (ph 6.0) for adsorption and stirred in a water bath shaker until they reached adsorption equilibrium, i.e., 8 h. The quantity of dye adsorbed was derived from the concentration change.

4 Chiang Mai J. Sci. 2014; 41(1) Scanning Electron Micrograph (SEM) Study Morphological features and surface characteristics of chitosan/bamboo charcoal composite beads were obtained from the scanning electron microscopy (SEM) using a JEOL JSM-5400 microscope. Before observing the SEM, all the samples were fixed on aluminum stubs and coated with gold. 3. RESULTS AND DISCUSSION 3.1 Surface Area and Porosity Analysis The porosity and diameter of chitosan and chitosan/bamboo charcoal composite beads are shown in Table 1. The porosity of chitosan/bamboo charcoal composite beads was significantly higher than of the chitosan beads due to the presence of bamboo charcoal. The diameter of chitosan/bamboo charcoal composite beads was less than that of chitosan beads, indicating bamboo charcoal addition makes chitosan/bamboo charcoal composite beads smaller than chitosan beads. Table 2 shows the specific surface area and the pore size distribution of chitosan and chitosan/bamboo charcoal composite beads. The total porosity is classified into three categories according to the pore diameter (d). The categories are: macropores (d>50 nm), mesopores (2<d <50 nm), and micropores (d<2 nm) [21]. Based on Table 2, chitosan and chitosan/bamboo charcoal composite beads are mesopore. It is clear that with the addition of bamboo charcoal, the surface areas of chitosan/ bamboo charcoal composite beads became larger than that of chitosan beads, resulting that the chitosan/bamboo charcoal composite beads might be able to enhance the reactive dye adsorption. Table 1. Porosity and diameter of chitosan and chitosan/bamboo charcoal composite beads. Adsorbent Wet weight (W W, mg ) Dry weight (W D, mg ) Porosity (ε, %) Diameter (D, mm) CTS 90:10 CTS/BC composite 70:30 CTS/BC composite 50:50 CTS/BC composite Table 2. Specific surface area and pore size distribution of chitosan and chitosan/bamboo charcoal composite beads. Adsorbent CTS 90:10 CTS/BC composite 70:30 CTS/BC composite 50:50 CTS/BC composite Specific surface area (m 2 /g) Average pore diameter (nm) Pore volume (cm 3 /g)

5 178 Chiang Mai J. Sci. 2014; 41(1) 3.2 Adsorption Studies Effect of contact time and bamboo charcoal concentration in beads The effect of contact time and the inclusion of different bamboo charcoal concentration on the removal of Reactive Red 152 are summarized in Figure 2. It can be observed that the chitosan bead has good adsorption for Reactive Red 152. Adsorptions of anionic dyes occur mainly due to the electrostatic interactions between the protonated amine groups on the chitosan (-NH 3+ ) and the SO 3 - groups of the anionic dyes structures [12]. All of the studied chitosan and chitosan/bamboo charcoal composite beads presented similar trends. The Reactive Red 152 was rapidly adsorbed in the first 1 h, and then the adsorption rate decreased gradually from 1 h to 7 h and finally reached equilibrium in 8 h. This observation could be explained as at the very beginning of the adsorption process, abundant active sites are available on the surface of chitosan/bamboo charcoal composite bead, which makes the adsorption process easier. As time went by, the adsorption sites become scarce, thus resulting in a decrease of adsorption efficiency. Thus, in the following experiments the equilibrium time was fixed at 8 h. Figure 2. Effects of contact time and bamboo charcoal concentration in beads on Reactive Red 152 removal (adsorbent dosage = 1.0 g, dye concentration = 50 mg/l, volume = 50 ml, ph = 6.0). The dye removal of the chitosan/ bamboo charcoal composite beads increased with the increasing bamboo charcoal concentrations in the composite beads. The maximum dye removal (84.4%) was observed at the weight ratio of 50/50 of chitosan to bamboo charcoal in chitosan/ bamboo charcoal composite beads. Bamboo charcoal is already reported as good adsorbents for various materials due to its large specific surface area, and porous structure [22]. It is clear that with the addition of bamboo charcoal, the surface areas of chitosan/bamboo charcoal composite beads became larger than that of chitosan beads, resulting that the chitosan/bamboo charcoal composite beads might be able to enhance the dye removal. However, the dye removal of the chitosan/bamboo charcoal composite beads increased with increase in bamboo charcoal concentration up to a certain level (weight ratio of chitosan to bamboo charcoal

6 Chiang Mai J. Sci. 2014; 41(1) 179 = 50/50). However, the higher bamboo charcoal concentrations in the chitosan/ bamboo charcoal composite beads (weight ratio of chitosan to bamboo charcoal less than 50/50) appear to induce the formation of larger aggregates of bamboo charcoal, which obstruct the formation of the chitosan/ bamboo charcoal composite beads Effect of ph The effect of the initial solution ph on Reactive Red 152 adsorption is shown in Figure 3. The maximum dye removal was 87.5% at ph 4 followed by a slight decrease from ph 5 to 9. During adsorption, protonation of amine groups is necessary for its interaction with negatively charged Reactive Red 152 molecules. At lower ph levels more protons will be available to protonate amine groups of chitosan molecules, thereby increasing the electrostatic attraction of dye molecules to active sites and causing the observed increase in dye adsorption [23-24]. On the other hand, in alkaline conditions of ph (>7), the amino groups are not protonated, and the interaction between the dye and the adsorbent occurs by van der Waals forces. The adsorption occurs preferentially for physical interaction, decreasing the dye removal. Figure 3. Effect of initial solution ph on Reactive Red 152 removal (weight ratio of chitosan to bamboo charcoal = 50/50, adsorbent dosage = 1.0 g, dye concentration = 50 mg/l, volume = 50 ml) Effect of adsorbent dosage Adsorbent dosage is an essential parameter which must be carefully adjusted during wastewater treatment. The effect of adsorbent dosage (varying from 1.0 g to 6.0 g) on Reactive Red 152 dye removal is presented in Figure 4. Initially, a rapid increase of adsorption with the increasing adsorbent dosage was attributed to the availability of a larger surface area and more adsorption sites [25]. A further increase of the adsorbent dosage from 2.0 g to 6.0 g didn t increase dye removal too much (only from 93.8% to 98.4% at an equilibrium time of 8 h). It was also noted that the equilibrium time decreased with the increasing adsorbent dosage.

7 180 Chiang Mai J. Sci. 2014; 41(1) Figure 4. Effect of adsorbent dosage on Reactive Red 152 removal (weight ratio of chitosan to bamboo charcoal = 50/50, ph = 4, dye concentration = 50 mg/l, volume = 50 ml). 3.3 Adsorption Isotherms The linear form of the Langmuir isotherm is expressed as: Ce qe = 1 Ce ( ) + ( ) bxm Xm where q e is the amount of dye adsorbed per unit mass of adsorbent (mg/g) and C e is the equilibrium concentration of dye in solution (mg/l). The constant X m is the monolayer adsorption capacity (mg/g) and b is the Langmuir constant. The Langmuir isotherms of chitosan and chitosan/bamboo charcoal composite beads are shown in Figure 5 for chitosan and chitosan/bamboo charcoal composite beads, the absorption capacity (X m ), and the Langmuir constant derived from the linear regression plots are shown in Table 3. The adsorption isotherms of chitosan and chitosan/bamboo charcoal composite beads could be described very well by the Langmuir equation. The chitosan/bamboo charcoal composite beads had a higher adsorption capacity (X m =4.32) than that of chitosan (X m =3.47). Figure 5. The Langmuir adsorption isotherm of chitosan and chitosan/bamboo charcoal composite (weight ratio of chitosan to bamboo charcoal = 50/50) beads for Reactive Red 152.

8 Chiang Mai J. Sci. 2014; 41(1) 181 Table 3. Parameter of the Langmuir isotherm and the relative correlation coefficients for chitosan and chitosan/bamboo charcoal composite (weight ratio of chitosan to bamboo charcoal = 50/50). Langmuir equation Ce/qe = (1/bXm)+(Ce/Xm) Type of adsorbent Xm (mg/g) b R2 CTS CTS/BC composite Scanning Electron Micrograph (SEM) Study The SEM micrograph of the fresh chitosan/bamboo charcoal composite bead and Reactive Red 152 dye adsorbed chitosan/ bamboo charcoal composite bead were presented in Figure 6(a) and (b), respectively. After adsorption the pores were packed with Reactive Red 152. The micrograph showed that the heights of heterogeneous pores within chitosan particle were decreased where adsorption could occur. The dye had densely and homogeneously adhered to the surface of the adsorbent, as a result of physical adsorption onto chitosan due to electrostatic forces and natural entrapment in to the porous bamboo charcoal material. Figure 6. SEM micrograph of chitosan/bamboo charcoal composite (weight ratio of chitosan to bamboo charcoal = 50/50) beads (a) before and (b) after the adsorption of Reactive Red CONCLUSIONS Chitosan/bamboo charcoal composites were made by adding bamboo charcoal into a chitosan solution and forming the composite beads. The adsorption of the reactive dye (Reactive Red 152) from an aqueous solution by using composite beads was investigated. Adsorption equilibrium experiments were carried out as a function of contact time, bamboo charcoal concentration, ph value, and adsorbent dosage level. The equilibrium time of dye adsorption was found to be 8 h. Composite adsorbent had the highest adsorption efficiency when the weight ratio was 50/50. The maximum dye removal took place at the initial ph 4.0. The optimum adsorbent dosage for dye removal was 6.0 g. Under above optimal conditions the maximum dye removal was 98.4%. The adsorption isotherm of chitosan and chitosan/bamboo charcoal composite beads followed the Langmuir isotherm model very well. The chitosan/bamboo charcoal composite beads had a higher adsorption

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