In this work, activated carbon (AC) coated by chitosan was synthesized and characterized by Fourier transform infrared spectrophotometer and scanning electron microscope (SEM) techniques. The removal of aniline from aqueous solutions by AC coated by chitosan was investigated. The factors affecting the adsorption of aniline onto AC coated by chitosan, including the ratio of AC to chitosan, adsorbent dosage, pH value of solution, initial aniline concentration, and contact time were evaluated. These results showed that the optimum operating conditions were: the ratio of AC to chitosan = 0.5, adsorbent dosage = 0.2 g, and the adsorption of aniline from aqueous solutions had better removal in the concentration range of 20–50 mg/L. This adsorbent allowed high removal toward aniline in a wide range of pH. The equilibrium time was 100 minutes. The Freundlich model exhibited better correlation of the equilibrium adsorption data. The pseudo-second-order kinetic equation could better describe the kinetic behavior of aniline adsorption.
Aniline is frequently used as a raw material by the chemical industry, i.e. in the manufacture of dyes, rubbers, pharmaceutical preparation, plastic and paint. It is also a common byproduct of the paper and textile industries. It is known to be a toxic pollutant and its presence in wastewater, even in very low concentrations, has been shown to be harmful to aquatic life (An et al. 2009). Traditionally, aniline-containing wastewater was treated using photodecomposition (Chu et al. 2007), electrolysis (Han et al. 2006), adsorption (Al-Johani & Salam 2011), oxidation (Sapurina & Stejskal 2012) and biodegradation (Wang et al. 2011). Among these methods, adsorption has been shown to be the most promising option for the removal of organic contaminants from wastewater in the case of low concentrations, due to the low cost and high efficiency (Hu et al. 2013; Yakout et al. 2013). Activated carbon (AC) has been considered as a popular adsorbent due to its high adsorption capacity, high adsorption rate and good resistance to abrasion. However, the lack of dispersion of AC powder brings into question its further application (Xu et al. 2015). To pave the way for the application of AC in the treatment of wastewater, many efforts have been made to maintain the high adsorption capacity of AC and reduce the cost of AC. The coating of AC with biological materials, such as chitosan and cellulose, is a new method for its modification. By this modification, much lower quantities of AC will be needed in the adsorption process and the removal process becomes cost-effective. Chitosan is a biopolymer with a linear polysaccharide based on glucosamine units, which may be obtained by the deacetylation of chitin (a natural polymer). Meanwhile, chitosan is an environmentally friendly material due to these advantages, including its abundance, non-toxicity, biocompatibility and biodegradability. It has been applied in many fields including food processing, agriculture, medicine, textiles, and wastewater treatment, etc. At present, chitosan and its derivatives, such as cross-linked chitosan, chitosan beads and chitosan composite, have been studied as adsorbents for the removal of organic pollutants and dyes from aqueous solutions (Gupta et al. 2012; Liu et al. 2012; Xie et al. 2012).
In this study, AC coated by chitosan was synthesized and characterized, and the adsorption capacity of AC coated by chitosan for aniline was investigated. The effects of the ratio of AC to chitosan, adsorbent dosage, the initial pH value of the aniline solution and contact time on the adsorption of aniline onto AC coated by chitosan were determined. Further, the experimental adsorption data were analyzed by different kinetic and isotherm models (see Supporting information, available in the online version of this paper).
MATERIALS AND METHODS
Chitosan powder (90% deacetylated) was acquired from the Sinopharm Group Chemical Reagent Limited Company (China). The AC powder was used without further purification. All other reagents used, including acetic acid, sulfuric acid, hydrochloric acid, and sodium hydroxide, were of analytical grade. A stock aniline solution of 1,000 mg/L was prepared by dissolving 1 g of aniline (Tianjin Bodi Chemical Co., Ltd, purity = 99.5%) in 1 L of deionized water. The pH values of aniline solutions were adjusted by adding 0.1 mol/L hydrochloric acid or 0.1 mol/L sodium hydroxide solutions.
Preparation of activated carbon coated by chitosan
AC coated by chitosan was prepared according to the literature (Hydari et al. 2012) with some modifications. Five grams of AC was poured into 2 mol/L of sulfuric acid for 4 hours. The AC was washed with deionized water after filtration and dried in an oven at 60 °C. The chitosan powder was dissolved into 2% (v/v) acetic acid, thereby obtaining a 2 wt% solution. Acid treated AC was added slowly into this solution, the ratio of AC to chitosan being 0.5. After the mixed solution was stirred for 40 minutes, it was coated in culture vessels and then dried at 60 °C to form membranes. Subsequently, these membranes were soaked in 0.1 mol/L sodium hydroxide solution to separate them from the culture dishes. The membranes were washed with distilled water to neutral pH and dried at 60 °C. Finally, these dry membranes were ground and sieved to obtain 100-mesh size particles, which were then applied in the adsorption studies.
The aniline concentration was determined by a double beam UV-vis spectrophotometer (Unicam UV-2, China) at 545 nm by naphthyl ethylendiamine azo photometry. Other instruments included a circulating water pump (Zhengzhou Dufu Instrument Co., Ltd, China, SHB-3), a thermostated shaker (Jintan Kanghua Electronic Instrument Co., Ltd, China, SHA-B), a cantilever constant speed mixer (Jintan Kanghua Electronic Instrument Co., Ltd, China, JJ-1B) and a pH meter (Shanghai LiDa Co., Ltd, China). The surface morphology of AC and AC coated by chitosan was determined by a field emission scanning electronic microscope (FE-SEM) (Hitachi S4800). The Fourier transform infrared spectroscopy (FTIR) spectra of AC, chitosan and AC coated by chitosan were determined by the FTIR. Spectra were collected with a spectrometer using potassium bromide pellets. In each case, 1 mg of dried sample and 100 mg of potassium bromide were homogenized using a pestle and mortar, and pressed into a transparent tablet. The pellets were analyzed with a FTIR spectrometer (Shimadzu 4100) in the transmittance (%) mode with a scan resolution of 4 cm–1 in the range 4000–400 cm–1.
To better understand the mechanism of aniline adsorption onto the adsorbent, desorption experiments were conducted. After performing the equilibrium study with different initial aniline concentrations ranging from 20 to 250 mg/L, aniline-adsorbed particles were collected by filtration and then dried at room temperature. Fifty mL of ethanol and acetone solutions were used for the regeneration of AC coated by chitosan. These solutions were stirred for 6 hours at 200 rpm and 20 °C. The particles were removed and rinsed with distilled water and then reused for the adsorption of aniline solutions.
RESULTS AND DISCUSSION
Characteristics of AC coated by chitosan
Effect of the ratio of activated carbon to chitosan
Effect of adsorbent dosage
Effect of initial aniline concentration
Effect of pH value of aniline solutions
Effect of particle size
Effect of contact time
From Table 1 it is observed that the pseudo-second-order kinetic equation showed high correlation coefficients (R2 > 0.999) for all three concentrations of aniline. In addition, the calculated qe from the pseudo-second-order kinetic model was closely similar to the experimental qe for aniline adsorption by AC coated by chitosan. Therefore, the pseudo-second-order kinetic model may be applied to predict the kinetic behavior of aniline adsorption. Besides, the values of the pseudo-second-order rate constant (k2) were calculated as 0.138 mg/(g·min), 0.268 mg/(g·min) and 0.408 mg/(g·min) at the three selected concentrations: 20 mg/L, 50 mg/L and 100 mg/L, respectively. As observed, the k2 value for aniline adsorption by AC coated by chitosan increased with increasing aniline concentrations. An increase in k2 value indicated an increase in adsorption rate, and thus the equilibrium time was shortened. This result was in accordance with the phenomena mentioned previously in this section.
|Parameters (mg/L)||qe (exp)||k2||qe (cal)||R2|
|Parameters (mg/L)||qe (exp)||k2||qe (cal)||R2|
The adsorption isotherm models of Langmuir (Gao et al. 2009) and Freundich (Barka et al. 2010) are usually used to describe the relationship between the adsorbent and adsorbate. The Langmuir isotherm model assumes a saturated molecular layer (monolayer) on the adsorbent surface, while the Freundlich isotherm model assumes a heterogeneous surface and a multilayer adsorption with an energetic nonuniform distribution.
Desorption and regeneration
FTIR analysis confirmed that AC was successfully coated by chitosan. The combination of AC and chitosan enhanced the adsorption of aniline as compared with chitosan alone. A reduced quantity of AC will be needed in the adsorption process using the composite of AC with chitosan. The adsorption of aniline onto AC coated by chitosan as a function of adsorbent dosage, initial aniline concentration, pH value of aniline solution, and contact time was investigated by using batch experiments. The removal toward aniline decreased with increasing aniline concentration, while it increased with increasing adsorbent dosage. This adsorbent allowed high removal toward aniline across a wide range of pH values. The adsorption of aniline followed the pseudo-second-order equation. The adsorption isotherm of aniline can be well described by the Freundlich equation.
This work was supported by the National Natural Science Foundation of China (Grant No. 51003086).