Abstract
Magnetic chitosan composite microparticles (MCCPs) were successfully prepared using a simple one-step co-precipitation method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometer (VSM). The experimental results showed that the particles possessed a honeycomb-like porous structure and had super-paramagnetic properties, with a saturation magnetization of about 33.3 emu/g. Congo red (CR), an anionic azo dye, was used to investigate the adsorption properties of the MCCPs. The adsorption kinetics data and isotherms produced from these experiments indicated that CR adsorption onto the MCCPs was best fitted with a pseudo-second-order kinetic equation and was well described by the Langmuir model. Thermodynamic parameters such as the changes in Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were also estimated; the results revealed that the adsorption process was spontaneous and endothermic. The regeneration studies demonstrated that the MCCPs can be used as a reusable adsorbent for CR adsorption from aqueous solution. The molecular similarity between chitosan and cellulose suggests that the present results might serve as a model of what might be achieved with a cationic derivative of cellulose.
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Removal of Congo Red from Aqueous Solution Using Magnetic Chitosan Composite Microparticles
Peng Wang, Tingguo Yan, and Lijuan Wang*
Magnetic chitosan composite microparticles (MCCPs) were successfully prepared using a simple one-step co-precipitation method and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometer (VSM). The experimental results showed that the particles possessed a honeycomb-like porous structure and had super-paramagnetic properties, with a saturation magnetization of about 33.3 emu/g. Congo red (CR), an anionic azo dye, was used to investigate the adsorption properties of the MCCPs. The adsorption kinetics data and isotherms produced from these experiments indicated that CR adsorption onto the MCCPs was best fitted with a pseudo-second-order kinetic equation and was well described by the Langmuir model. Thermodynamic parameters such as the changes in Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were also estimated; the results revealed that the adsorption process was spontaneous and endothermic. The regeneration studies demonstrated that the MCCPs can be used as a reusable adsorbent for CR adsorption from aqueous solution. The molecular similarity between chitosan and cellulose suggests that the present results might serve as a model of what might be achieved with a cationic derivative of cellulose.
Keywords: Chitosan; Fe3O4; Magnetic microparticles; Removal; Congo red; Regeneration
Contact information: Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, 26 Hexing Road, Harbin 150040, China;
*Corresponding author: donglinwlj@163.com
INTRODUCTION
The large quantities of dyes discharged from the textile, clothing, and printing industries are major sources of aquatic pollution. Most of these dyes are very colorful, but have high biochemical and chemical oxygen demands, smell unpleasant, and are toxic (Crini and Badot 2008). These toxic effluents can cause considerable damage to human and marine life if they are not treated prior to discharge. Unfortunately, dyes are rather difficult to remove from wastewater because of their synthetic origins and complex aromatic structures (Srinivasan and Viraraghavan 2010). Several physical, chemical, and biological techniques have been used to remove dyes from wastewater. These techniques include coagulation (Shi et al. 2007), chemical treatment (Wang et al. 2007; Mishra and Bajpai 2006), oxidation (Hage and Lienke 2005; Ertugay and Acar 2013), electrochemical methods (Gupta et al. 2007; Dogan and Türkdemir 2005), biological treatment (Barragán et al. 2007; Bromley-Challenor 2000; Sani and Banerjee 1999), adsorption, and ion exchange (Liu et al. 2007; Raghu and Ahmed Basha 2007). Among them, adsorption is considered to be an effective and versatile method for removing dyes from aqueous solutions. Activated carbon is an effective adsorbent, but it is too expensive for practical use. Many researchers have studied the feasibility of low-cost, natural materials for dye removal, such as wood, natural coal, peat, clays, and silica (Srinivasan and Viraraghavan 2010; Gupta et al. 2009; Morais et al. 1999; Venkata Mohan et al. 2002; Sun and Yang 2003). However, these low-cost adsorbents generally have low adsorption capacities. Thus, there is still a need to find economical, easily available, and highly effective alternative adsorbents.
Chitosan is a type of natural polyaminosaccharide obtained from chitin partial deacetylation; it is the second-most abundant polymer in nature after the lignocellulose group (Wan Ngah et al. 2011). It can be extracted from the shells of prawns, crabs, fungi, insects, and other crustaceans (Ngah and Isa 1998). The carbohydrate backbone of chitosan is very similar to that of cellulose; chitosan may be regarded as cellulose with the hydroxyl at position C-2 replaced by an amino group. Thus, chitosan is a copolymer consisting of N-acetyl-2-amino-2-deoxy-D-glucopyranose, where the two types of repeating units are linked by (1→4)-β-glycosidic bonds. It can therefore serve as a model for what might be done with cellulose-based polymers.
Recent studies demonstrate that cationic cellulose derivatives have many useful characteristics, such as hydrophilicity, biodegradability, antibacterial properties, and dyeability. Rodríguez et al. (2003) synthesized cationic cellulose hydrogels and evaluated their utility for controlling the release of an anionic amphiphilic drug, diclofenac sodium. Jia et al. (2011) reported the preparation of nano-fiber mats with antibacterial activity by electrospinning of a blended solution of cationic cellulose derivates and polyvinyl alcohol. Khatri et al. (2013) prepared cationic cellulose nanofibers for enhanced color yields and dye fixation. Sirviö et al. (2011) reported the synthesis of a highly cationic water-soluble cellulose derivative as a novel biopolymeric flocculation agent.
Chitosan, regarded as a kind of cationic cellulose derivative, exhibits a high adsorption capacity for many classes of dyes. However, powdery chitosan has some disadvantages that frustrate its use in practical applications; it is easily dissolved under acidic conditions and has poor mechanical strength (Zhu et al. 2010). Many studies have investigated the use of various materials to form chitosan composites; these materials include montmorillonite, polyurethane, activated clay, bentonite, oil palm ash, and kaolin (Wang and Wang 2007b; Lin et al. 2004; Lee et al. 2009; Nandi et al. 2009). However, the separation and recovery of them are still problematic for researchers.
Magnetic separation technology is a promising method to separate powdery adsorbents from the adsorbed solution effectively and in an environmentally friendly way. In recent years, many studies have reported on removing dyes (Fan et al. 2013; Travlou et al. 2013; Yan et al. 2013; Yan et al. 2012; Yang et al. 2013; Zhu et al. 2012) and metal ions (Kyzas and Deliyanni 2013; Abou El-Reash et al. 2011; Donia et al. 2008; Elwakeel 2010; Li et al. 2011; Yu et al. 2013) using magnetic chitosan composite adsorbents. To prepare magnetic chitosan composites, most of the researchers have applied a two-step method based on the inverse emulsion process (Lee et al. 2005; Fan et al. 2012b; Qu et al. 2010; Li et al. 2008). However, it is not known if more than a few researchers have reported preparing a Fe3O4/chitosan composite using a one-step approach and using it to remove dyes from an aqueous solution.
In this work, magnetic chitosan composite microparticles (MCCPs) were prepared using a facile one-step in situ co-precipitation process, then characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometer (VSM). Congo red (CR), which has a large and complicated structure, was used as a model dye for bath adsorption experiments to investigate the adsorption properties of the MCCPs. The effects of the solution pH and initial dye concentration on CR adsorption were investigated. Adsorption kinetics was evaluated with Lagergren pseudo-first-order and pseudo-second-order models. Equilibrium isotherms and thermodynamic parameters were also determined and will be discussed. These results will be useful for further applications involving dye removal from aqueous solutions.
EXPERIMENTAL
Materials
Chitosan (Deacetylation Degree: 85%; Molecular Weight: 3.0×105 g mol-1) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. CR (Molecular Weight: 696.68;C32H22N6O6S2Na2; Color index No.: 22120; Purity: 85%) was purchased from Tianjin Bodi Chemical Co., Ltd., China. The FeCl3·6H2O, FeSO4·7H2O, and glutaraldehyde compounds used were of analytical grade and supplied by Tianjin Kermel Chemical Reagent Co., Ltd., China. A stock solution of Congo red (1000 mg L-1) was prepared in double-distilled water; desired CR solution concentrations were obtained by successive dilutions of the stock solution.
Preparation of MCCPs
Magnetic chitosan microparticles were prepared by a one-step co-precipitation method (Tran et al. 2010; Liu et al. 2011). First, 1.5 g of chitosan was dissolved overnight in 100 mL of 2% (v/v) acetic acid solution. Afterwards, 0.03 mol FeCl3·6H2O and 0.018 mol FeSO4·7H2O were added into the solution, which was stirred for 30 min. The resulting solution was dripped into a 30 wt.% NaOH solution under vigorous stirring at 60 °C in an N2 atmosphere. After 6 h of reaction, the suspension was magnetically separated and washed several times until its pH became neutral. The black products were then dispersed in a 1.0 wt.% glutaraldehyde solution for crosslinking. After 3 h, the final products were separated from the solution and rinsed with ethanol and deionized water three times. The magnetic chitosan composite microparticles were then dried in a vacuum at 70 °C for 24 h.
Characterization of the MCCPs
The morphologies and microstructures of the samples were observed by scanning electron microscopy (SEM, Quanta 200) with an accelerating voltage of 15 kV. The crystal structures of the microparticles were determined by X-ray diffraction (XRD, Rigaku D/max-2200 diffractometer) using Cu Kα radiation at 40 kV and 30 mA. FT-IR spectra were measured at room temperature on an FT-IR spectrometer (Nicolet 560) to investigate the functional groups of the microparticles. Magnetic hysteresis loops were obtained at 298 K using a vibrating sample magnetometer (VSM).
Batch Adsorption Experiments
The adsorption of CR on the MCCPs was studied by batch experiments using a thermostatic rotary shaker at 120 rpm. Standard solutions of different concentrations were prepared by diluting the 1000 mg L-1 CR stock solution. For a typical adsorption experiment, 30 mg of MCCPs was mixed with 50 mL of a CR solution at a 50 mg L-1 concentration. At given time intervals, the microparticles were removed from the solution using an adscititious magnet. The CR concentration in the solution was determined by a UV-vis spectrophotometer at λmax = 494 nm. The amount of CR adsorbed on the MCCPs, qt (mg g-1), was calculated as follows,
(1)
where C0 and Ct are the initial CR concentration (mg L-1) and instantaneous CR concentration (mg L-1), respectively, V is the volume of the solution (L), and W is the weight of the adsorbent (g).
RESULTS AND DISCUSSION
Characterization of the MCCPs
The morphology and microstructure of the MCCPs were observed using SEM, as shown in Fig. 1. Honeycomb-like porous structures are clearly seen in Fig. 1(a). The shapes of the microparticles are irregular because of aggregation, and their surfaces are relatively rough. Figure 1(b) shows that the pore size was fairly uniform and that the mean pore diameter was about 4 μm. Similar to activated carbon (Tan et al. 2007), the well-developed pores present on the surface led to the large surface area and increased the possibility of dye trapping and adsorption.
Fig. 1. SEM micrographs of the MCCPs
Figure 2 shows the XRD patterns for the plain Fe3O4 microparticles and MCCPs. Six characteristic peaks for Fe3O4 at 2θ values of 30.72°, 35.38°, 43.72°, 53.64°, 57.24°, and 62.86° were marked by their corresponding indices: (220), (311), (400), (422), (511), and (440), respectively; these peaks were observed for both samples. These peaks revealed that the particles were pure Fe3O4 with a spinel structure (JCPDS database file, No.79-0418). These results also suggest that the Fe3O4 in the MCCPs prepared by the one-step co-precipitation process has the same crystal structure as plain Fe3O4.
Fig. 2. XRD patterns for naked Fe3O4 microparticles (a) and MCCPs (b)
Figure 3 shows the FT-IR spectra of plain Fe3O4, chitosan, and the MCCPs. As Fig. 3(a) shows, the peak at 587 cm-1 is related to the Fe-O group (Ma et al. 2007). For the FT-IR spectrum of chitosan in Fig. 3(b), the adsorption band around 3330 cm-1 is assigned to the –OH and –NH2 stretching vibrations. The peaks at 1583 cm-1 and 1374 cm-1 are ascribed to the N-H bending vibration and –C-O stretching of alcoholic groups in chitosan, respectively (Zhang et al. 2010). For the MCCPs in Fig. 3(c), the sharp peak at 583 cm-1 is from Fe-O stretching of Fe3O4. A new peak at 1631 cm-1 is present, which is attributed to an imine bond (N=C), indicating that the chitosan was successfully cross-linked by the glutaraldehyde (Elwakeel 2010). Additionally, the negatively charged surface of iron oxide has an affinity toward chitosan. This means that protonated chitosan could be coated on the particles by electrostatic interaction and chemical reactions, specifically, glutaraldehyde crosslinking (Lin et al. 2010).
Fig. 3. FT-IR spectra of (a) naked Fe3O4, (b) chitosan, and (c) MCCPs
The magnetic properties of the MCCPs were measured using VSM. Figure 4(a) shows the magnetization hysteresis loop for the MCCPs obtained at 298 K. There was no remanence or coercivity observed, indicating that the particles were superparamagnetic (Li et al. 2008). The saturation magnetization of the MCCPs obtained from the hysteresis loop was 33.3 emu/g. Figure 4(b) shows that the MCCPs dispersed well in water before magnetic separation. However, almost all of the MCCPs could be quickly separated from the solution when an external magnetic field was introduced, as shown in Fig. 4(c). These results illustrate that the MCCPs have a sensitive magnetic response, which is important for their use in wastewater treatment.
Fig. 4. (a) Magnetic hysteresis curves of MCCPs, and photographs of MCCPs suspended in water, (b) in the absence, and (c) in the presence of an externally placed magnet
Effect of pH
Solution pH is one of the dominant parameters affecting adsorption processes (Dogan et al. 2009). When the pH of the CR solution was reduced to below 4, its color changed from red to blue. Thus, the effect of the solution pH on CR adsorption capacity for a pH range of 5 to 10 was studied. As Fig. 5 shows, the effect of pH on CR adsorption was obvious in the initial stages of the adsorption process.
Fig. 5. Effect of initial solution pH for the adsorption of CR onto MCCPs (initial CR concentration: 50 mg L-1, 50 mL, adsorbent dose: 30 mg, temperature: 30 °C)
The removal efficiency declined as the pH increased. When the adsorption reached equilibrium, the removal rate did not greatly differ between pH 5, pH 6, and pH 8, while the rate was lower for pH 10. During the initial stages of adsorption, a high concentration of hydrogen in the solution led to protonation of the amine groups in the chitosan chain, causing enhanced electrostatic interactions between the CR anions and chitosan (Kyzas and Lazaridis 2009). When the hydrogen atoms (H+) were nearly consumed at equilibrium, the protonation of amine groups was not effective because of the low concentration of free H+ in the solution. Nevertheless, for highly alkaline conditions, no free amino groups of chitosan could be protonated and the abundant OH− competed with the dye anions (Chatterjee et al. 2005), resulting in decreased CR removal efficiency.
Effect of Initial CR Concentration
Figure 6 shows the adsorption capacities of CR onto MCCPs at various concentrations for different contact times. It is clear that the adsorption increased in the first 120 min, after which the adsorption slowed down, eventually approaching equilibrium at 480 min. The adsorption capacity increased as the initial dye concentration increased from 40 mg L-1 to 70 mg L-1. This behavior may be caused by the increase in the mass concentration gradient pressure as the initial concentration is increased. The initial dye concentration provides the necessary driving force to transfer the CR molecules from the bulk solution to the surface of the MCCPs (Senthil Kumar et al. 2010).
Fig. 6. Effect of initial dye concentration on the adsorption of CR onto MCCPs (adsorbent dose: 30 mg, temperature: 30 °C)
Adsorption Kinetics
The kinetic parameters of adsorption help predict the dominant adsorption mechanism and the adsorption rate of the adsorbate (Chen et al. 2009). Two simplified kinetic models, including Lagergren pseudo-first-order and pseudo-second-order equations, were used to fit the adsorption kinetic data.
The Lagergren pseudo-first-order model is given as (Ho and McKay 1998),
(2)
where qe and qt are the amounts of CR adsorbed (mg g-1) at equilibrium and at time t (min), respectively, and k1 is the Lagergren pseudo-first-order rate constant (min-1) of adsorption. Values of k1 can be calculated from plots of ln(qe − qt) versus t, as shown in Fig. 7(a).
The pseudo-second-order model is expressed as (Ho and McKay 1999),
(3)
where k2 is the pseudo-second-order rate constant (g mg-1 min-1). Values of k2 and qe can be directly obtained from the intercept and slope of the plot of t/qt versus t, as shown in Fig. 7(b).
The kinetic parameters of CR adsorption onto the MCCPs for different CR initial concentrations at 303 K are shown in Table 1.
Table 1. Kinetic Parameters for CR Adsorption onto MCCPs
Fig. 7. Fitting curves of (a) pseudo-first-order model and (b) pseudo-second-order model for CR adsorption onto MCCPs at 303 K
The coefficients of determination (R2) for the pseudo-second-order kinetic model were high (> 0.999), and the adsorption capacities calculated by the model (q2e,cal ) were also close to those determined by experiments (qe,exp). The pseudo-first-order model did not perform as well as the pseudo-second-order model, suggesting that the pseudo-second-order kinetic model was more valid for describing this adsorption process. Thus, chemical adsorption is the rate-limiting step of the adsorption mechanism (Fan et al. 2012a). Additionally, the values of the pseudo-second-order rate constants (k2) increased as the initial CR concentration decreased, indicating that the adsorption reaction is more favorable for low initial CR concentrations.
Most adsorption processes have multiple steps, including surface diffusion and intraparticle diffusion (Zhu et al. 2011b). To determine whether intraparticle diffusion was the rate-determining step, the intraparticle mass transfer diffusion model was applied and expressed as (Wu et al. 2001),
(4)
where c is the intercept (mg g-1) and kid is the intraparticle diffusion rate constant (mg g-1 min-1/2), which can be evaluated from the slope of the linear plot of qt versus t1/2.
Fig. 8. Intraparticle diffusion plots for adsorption of CR onto MCCPs at 303 K
A plot of qt versus t1/2 is shown in Fig. 8. The adsorption process tended to have two phases: the initial curved portion and the second linear portion. The curved portion of the plot was attributed to surface adsorption and external diffusion, indicating a boundary-layer effect, whereas the linear portion was ascribed to intraparticle or pore diffusion (Zhu et al. 2011b). The linear portions of the curves at each concentration did not pass through the origin, suggesting that intraparticle diffusion was not the sole rate-limiting step (Bayramoglu et al. 2009). Additionally, the intercept value decreased with increasing initial CR concentration, indicating a decrease in the thickness of the boundary layer and its effect on adsorption.
Adsorption kinetics of CR onto MCCPs was better described by the pseudo-second-order model. The rate constants of the pseudo-second-order model were adopted to calculate the activation energy of the adsorption process by the following Arrhenius equation (Konicki et al. 2013),
(5)
where k2 (g mg-1 min-1) is the pseudo-second-order rate constant, A (g mg-1 min-1) is the Arrhenius factor, Ea (J mol-1) is the Arrhenius activation energy, R (8.314 J mol-1 K-1) is the gas constant, and T (K) is the absolute temperature. Plotting lnk2 as a function of 1/T (Fig. 9) yielded a straight line, from which Ea and A can be calculated using the slope and intercept, respectively. The magnitude of the activation energy gives an idea of the type of adsorption, which is primarily either physical or chemical. The physisorption process normally has a low activation energy (5 to 40 kJ mol-1), while chemisorption has a higher activation energy (40 to 800 kJ mol-1). The value of Ea for the adsorption CR onto MCCPs was 25.50 kJ mol-1 (R2 = 0.958), indicating that the adsorption had a low potential barrier and corresponded to physisorption. Similar results have been observed for the adsorption of Acid Red 88 onto magnetic ZnFe2O4 spinel ferrite nanoparticles (37.5 kJ mol-1) (Konicki et al. 2013).
Fig. 9. Arrhenius plot for the adsorption of CR onto MCCPs
Adsorption Isotherms
Adsorption isotherm models are widely used to describe the interactions between adsorbates and adsorbents. Two well-known adsorption isotherms models, the Langmuir and Freundlich isotherms, have been used to study adsorption. The Langmuir and Freundlich isotherms can be respectively expressed as follows (Zhu et al. 2010),
(6)
(7)
where Ce is the equilibrium concentration of CR in solution (mg L-1); qe is the adsorbed value of CR at equilibrium concentration (mg g-1); qm is the maximum adsorption capacity; KL is the Langmuir adsorption constant, which is related to the energy of adsorption (L mg-1); and KF and n are the Freundlich constants related to the adsorption capacity and the surface heterogeneity, respectively.
Plotting Ce/qe against Ce gave a straight line with a slope and intercept equal to 1/qm and 1/(KLqm), respectively. The slope and intercept of the linear plots of lnqe against lnce were 1/n and lnKF, respectively. The calculated Langmuir and Freundlich isotherm constants for CR adsorption at 303 K, 313 K, and 323 K are summarized in Table 2. As the R2 values indicate, the Langmuir isotherm better described the adsorption of CR onto the MCCPs versus the Freundlich isotherm. This result shows that CR was adsorbed on the MCCPs by a monolayer adsorption process (Fan et al. 2012a). Notably, the adsorption capacity increased with temperature, showing that the adsorption process was endothermic.
The essential feature of the Langmuir isotherm can be described by a separation factor (RL) using the following equation (Qi and Xu 2004),
(8)
where KL is the Langmuir constant (L mg-1) and C0 is the initial concentration (mg L-1). The range 0 < RL < 1 indicates that adsorption is favored. The values of RL for the MCCPs toward the adsorption of CR at 303 K, 313 K, and 323 K are 0.114, 0.085, and 0.127, respectively; these values indicate that the adsorption of CR onto MCCPs is favorable.
Table 2. Langmuir and Freundlich Isotherm Constants at Different Temperatures
Adsorption Thermodynamics
Thermodynamic parameters such as enthalpy change (ΔH0), entropy change (ΔS0), and Gibbs free energy change (ΔG0) were studied to determine the adsorption spontaneity and understand the effect of temperature on the adsorption. Thermodynamic experiments were conducted at 303, 313, 323, and 333 K, with a 50-mg L-1 initial CR concentration and a 30-mg adsorbent dosage. These thermodynamic parameters can be calculated using the following equations (Smith and Van Ness 1987),
(9)
(10)
(11)
where Kc, the equilibrium constant, is the ratio of the concentration of CR on the adsorbent at equilibrium (cs) to the remaining concentration of the dye in solution at equilibrium (ce); ΔG0(kJ mol-1), ΔH0 (kJ mol-1), and ΔS0 (J mol-1K-1) are the changes in Gibbs free energy, enthalpy, and entropy, respectively; R is the ideal gas constant (8.314 J mol-1K-1); and T is the absolute temperature (K).
Plotting lnKc against 1/T gave a straight line with a slope and intercept equal to ΔH0/R and ΔS0/R, respectively. The values of ΔG0, ΔH0, and ΔS0 are listed in Table 3. The negative ΔG0values imply spontaneous adsorption. The positive value of ΔS0 suggests increased randomness during the adsorption of CR. The positive value of ΔH0 shows that the adsorption process was endothermic; therefore, higher temperatures will facilitate the adsorption of CR onto the MCCPs, which is consistent with the results of the adsorption isotherms.
Table 3. Thermodynamic Parameters for the Adsorption of CR onto MCCPs
Comparison with Other Adsorbents
The maximum adsorption capacity (qmax) of CR onto the MCCPs from the Langmuir isotherm was calculated. This was then compared with those of other biosorbents reported by other researchers; these data are listed in Table 4. The table shows that the qmax value of the MCCPs was generally higher than those of other chitosan-based adsorbents (Chatterjee et al. 2007; Wang and Wang 2007b; Chatterjee et al. 2009), and cellulose-based adsorbents (Annadurai et al. 2002; Panda et al. 2009; Zhu et al. 2011a), except for the chitosan/organo-montomorillonite adsorbent (Wang and Wang 2007a). However, the simplicity of the MCCP preparation process and their magnetic properties make them more suitable for CR adsorption.
Table 4. qmax Values for the Adsorption of CR on Different Adsorbents
Regeneration of MCCPs
The regeneration of spent adsorbent is considered an important economical aspect to minimize the cost of the adsorption process. In this study, 0.5 M NaOH was selected as the eluent for desorption; after desorption, the MCCPs were washed with deionized water and 0.5 M HCl until the residual solution became colorless. Then, the recovered adsorbents were used for the next loading cycle. The regeneration studies were repeated seven times, and the results are shown in Fig. 10. It can be seen that the adsorption capacity decreased slightly with increasing cycles. In the first three cycles, the adsorption efficiency of the MCCPs remained almost constant: after being regenerated five times, the removal rate of CR was still more than 80%. This fact indicates that the MCCPs can be used as a reusable adsorbent for CR adsorption from aqueous solution.
.
Fig. 10. Effect of regeneration cycles on CR adsorption (initial CR concentration: 50 mg L-1, 50 mL, adsorbent dose: 30 mg, temperature: 30 °C)
CONCLUSIONS
- Magnetic chitosan composite microparticles were prepared with a simple one-step co-precipitation method; they had a honeycomb-like porous structure and were superparamagnetic.
- The FT-IR spectra revealed that the chitosan was bound to the magnetic Fe3O4 microparticles successfully without damaging the crystal structure of Fe3O4.
- The removal efficiency increased as the initial concentration increased and as the pH declined in the range from 10 to 5.
- The adsorption process was best described by a pseudo-second-order equation, and equilibrium experiments agreed well with the Langmuir isotherm model.
- Thermodynamic calculations indicated that the adsorption process was spontaneous and endothermic.
- The MCCPs have a great potential to be used as an environmentally friendly and economical adsorbent for removal of CR from aqueous solution.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the Fundamental Research Funds for the Central Universities (DL12DB04).
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Article submitted: August 27, 2013; Peer review completed: September 28, 2013; Revised version received and accepted: September 30, 2013; Published: October 3, 2013.