Adsorption of Phenolic Compounds from Aqueous Solutions onto

Evaluation of chitosan-carbon based biocomposite for efficient removal of phenols from aqueous solutions. Usha Soni , Jaya Bajpai , Sunil Kumar Singh ...
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Ind. Eng. Chem. Res. 2010, 49, 9238–9247

Adsorption of Phenolic Compounds from Aqueous Solutions onto Chitosan-Coated Perlite Beads as Biosorbent N. S. Kumar,†,‡ M. Suguna,† M. V. Subbaiah,† A. S. Reddy,† N. P. Kumar,† and A. Krishnaiah*,† Biopolymers and Thermophysical Laboratories, Department of Chemistry, Sri Venkateswara UniVersity, Tirupati - 517 502, A.P., India, and Department of Safety EnVironmental System Engineering, Dongguk UniVersity, Gyeongju 780-714, Republic of Korea

Chitosan-coated perlite (CCP) beads were prepared by dropwise addition of a liquid slurry containing chitosan and perlite to an alkaline bath. The resulting beads were characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and surface area analysis. The chitosan content of the beads is 23% as determined by a pyrolysis method. Adsorption of phenolic compounds (phenol, 2-chlorophenol, and 4- chlorophenol) from aqueous solutions on chitosan-coated perlite beads was studied under batch equilibrium and column flow conditions. The binding capacity of the biosorbent was investigated as a function of initial pH, contact time, initial concentration of adsorbate, and dosage of adsorbent. Adsorption kinetic and isotherm studies, respectively, showed that the adsorption process followed a pseudo-first-order kinetic model and the Langmuir isotherm. The maximum monolayer adsorption capacity of phenol, 2-CP, and 4-CP on to the chitosan-coated perlite beads was found to be 192, 263, and 322 mg g-1, respectively. 1. Introduction Phenols are generally considered to be one of the important organic pollutants discharged into the environment causing unpleasant taste and odor. The major sources of phenol pollution in the aquatic environment are waste waters from the paint, pesticide, coal conversion, polymeric resin, petroleum, and petrochemicals industries. Degradation of these substances produces phenol and its derivatives in the environment. The chlorination of natural waters for disinfection produces chlorinated phenols. Phenols are considered as priority pollutants since they are harmful to organisms at low concentrations. Phenol contents in the drinking water should not exceed 0.002 mg L-1 as per the Indian standard.1 In recent years, interest has been focused on the removal of phenols from aqueous solution. A variety of techniques have been implemented to purify water contaminated by phenols. Ozonolysis, photolysis, and photocatalytic decomposition have been used with limited success.2 Traditionally, biological treatment, activated carbon adsorption, reverse osmosis, ion exchange, and solvent extraction are the most widely used techniques for removing phenols and related organic substances.3-6 Adsorption of phenols onto solid supports such as activated carbons allows for their removal from water without the addition of chemicals. Activated carbon exhibits good adsorption ability for many organic pollutants but is expensive due to its difficult regeneration and high disposal cost.7 In recent years, polymeric adsorbents have been used increasingly as an alternative to activated carbon due to their economic feasibility, adsorption-regeneration properties and mechanical strength. Chitosan is a polysaccharide prepared by the de-Nacetylation of chitin, which makes up shells and shrimps.8 Due to the primary, secondary hydroxyl groups and highly reactive amino groups of chitosan as well as the property of nontoxicity and biodegradability, it has been regarded as a useful material to remove inorganic and organic substances from wastewater.9 However, several investigators have attempted to modify * To whom correspondence should be addressed. Tel.: +919393621986. E-mail address: [email protected]. † Sri Venkateswara University. ‡ Dongguk University.

chitosan to facilitate mass transfer and to expose the active binding sites to enhance the adsorption capacity. Grafting specific functional groups onto a native chitosan backbone allows its sorption properties to be enhanced.10 Many applications are due to the secondary amino groups of chitosan which show polycationic, chelating, and film-forming properties along with high solubility in dilute acids. Chitosan has already been described as a suitable natural polymer for the collection of phenolic compounds, through chelation, due to the presence of an amino and hydroxyl groups on the glucosamine unit.11 In most of the studies chitosan has been used in the form of flakes, powder, or hydrogel beads. Enzymatic removal of various phenol compounds from a synthetic water sample was studied by the use of mushroom tyrosinase and chitosan beads as a function of pH, temperature, tyrosinase dose, and the hydrogen peroxide-to-substrate ratio.12 The adsorption of 4-nonylphenol ethoxylates (NPEs) onto chitosan beads having cyclodextrin was investigated by Aoki et al.13 Adsorption of phenol onto chitosancoated bentonite was studied by Cheng et al.14 Biosorption of phenol and o-chlorophenol from aqueous solutions onto chitosan-calcium alginate blended beads was reported by Siva Kumar et al.15 Removal of chlorophenols from groundwater by chitosan sorption was studied by Zheng et al.16 Adsorption of phenol, p-chlorophenol, and p-nitrophenol onto functional chitosan was studied by Jian-Mei et al.17 Biosorption of phenolic compounds from aqueous solutions onto chitosan-abrus precatorius blended beads was studied by Siva Kumar et al.18 The maximum adsorption capacities of different adsorbents obtained from different sources are included in Table 1 along with the values obtained in the present study. In this study a new composite chitosan biosorbent is prepared by coating chitosan, a glucosamine biopolymer, over perlite, an inorganic porous aluminosilicate and formed into beads. Perlite is a siliceous volcanic glassy rock with an amorphous structure. It is expected that the more active sites of chitosan will be available due to the coating thus enhancing the adsorption capacity. The percent of chitosan coated on perlite was determined by pyrolysis technique. Surface area, pore volume, and pore diameter were obtained on the basis of Brunauer, Emmet, and Teller (BET) measurements. The chito-

10.1021/ie901171b  2010 American Chemical Society Published on Web 08/26/2010

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 -1

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Table 1. Maximum Capacity, Q (mg g ), for Adsorption of Phenolic Compounds by Various Adsorbents adsorbates, Q0 (mg g-1) sorbent CS/CA blended beads functional chitosan CS CS-SA CS-CD EPI-CD CS/Ab blended beads olive stone- based activated aarbon sugar cane bagasse fly ash ratan sawdust based activated carbon granular activated carbon commercial activated carbon activated bentonites M-bentonite Al-bentonite CTAB-bentonite T-bentonite CS850A fly ash activated carbon modified bentonite modified starch perlite chitosan-coated perlite beads

phenol 109 2.22 8.50 34.93 131.50 156 189

2-CP

4-CP

97

204

refs 15 17

2.58 20.49 179.73 74.25 278 436

18 24

23.83 149.25

26 27

165.80 49.72

36 37 38

9.9 8.7 8.4 8.2 205.8 3.85 380.2

192

263

422.1 176.6 5.84 322

39 40 41 42 43 44 present study

san-coated perlite (CCP) beads were characterized before and after adsorption of phenolic compounds by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and surface area analysis. In the present investigation, equilibrium and dynamic column adsorption characteristics of phenolic compounds on chitosan-coated perlite beads were studied. The column flow data were used to generate break through curves. The loaded adsorbent with phenolic compounds was regenerated by solvent elution method using 0.1 M NaOH as eluent. 2. Materials and Methods 2.1. Materials. Perlite is not a trade name but a generic term for naturally occurring siliceous rock. The distinguishing feature, which sets perlite apart from other volcanic glasses, is that, when heated to a suitable point in its softening range, it expands from four to twenty times of its original volume. This expansion process also creates one of perlite’s most distinguishing characteristicssits white colorswhile the crude rock may range from transparent light gray to glassy black. The expanded form of perlite is obtained from Silbrico Corporations, IL, USA, and was used as a substrate for the preparation of beads. Chitosan (molecular weight 1-3 lakhs), 99% pure oxalic acid dihydrate, and NaOH beads (95-100%) were purchased from Fisher Scientific Company. All the working solutions are obtained by diluting the stock solution with double distilled water. Phenol (Ranbaxy, India, A.R. grade), 2-chlorophenol, and 4-chlorophenol (Spectrochem, India, A.R. grade) were used without further purification. Stock solutions were prepared by dissolving 1.0 g of phenol, 2-chlorophenol, and 4-chlorophenol individually in 1 L of double distilled water. These stock solutions were used to prepare 100, 200, 300, and 400 mg L-1 solutions of phenol, 2-chlorophenol, and 4-chlorophenol. Water used for preparation of solutions and cleaning adsorbents was generated in the laboratory by double distilling the deionized water in a quartz distillation unit.

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2.2. Preparation of Chitosan-Coated Perlite Beads. Perlite, which is composed mainly of alumina and silica, is used as a substrate for the preparation of beads.19 Perlite is first mixed with 0.2 M oxalic acid, and the mixture is stirred for 12 h at room temperature (30 °C) and filtered. The filtered perlite was washed with deionized water and dried overnight at 70 °C and sieved through 100-mesh size. The acid-treated perlite was stored in desiccator. About 30 g of medium molecular weight chitosan was slowly added to 1 L of 0.2 M oxalic acid solution under continuous stirring at 40-50 °C to facilitate the formation of a viscous gel. About 60 g of acid-treated perlite powder was mixed with deionized water and slowly added to the diluted gel and stirred for 12 h at 40-50 °C. The highly porous beads were then prepared by dropwise addition of the perlite gel mixture into a 0.7 M NaOH precipitation bath.20 The purpose of adding the acidic perlite-chitosan mixture to NaOH solution was to assist rapid neutralization of oxalic acid, so that the spherical shape could be retained. The beads were separated from the NaOH bath and washed several times with deionized water to a neutral pH. The beads were dried in a freeze drier, oven, and by air. 2.3. Analysis of Phenolic Compounds. The concentration of phenol, 2-chlorophenol, and 4- chlorophenol, in aqueous medium, was determined by measuring absorbance at wavelengths of 270, 274, and 280 nm, respectively, using a UVspectrophotometer (model Shimadzu UV-2450). In order to reduce measurement errors in all the experiments, the UV absorption intensity of each solution sample was measured in triplicates and the average value was used to calculate the equilibrium concentration based on standard calibration curve, whose correlation coefficient square (R2) was 0.999. The experimental error was observed to be within (2%. 2.4. Batch Studies. In order to study the effect of different controlling parameters, such as solution pH, contact time, quantity of adsorbent, and the initial concentration of adsorbate, the experiments were conducted by varying one parameter at a time keeping all other parameters constant. The solution pH was adjusted by adding 0.1 M HCl or 0.1 M NaOH solutions. Batch adsorption experiments were conducted in 125 mL Erlenmeyer flasks with a specified amount (0.1 g) of adsorbent in contact with 100 mL of phenolic solutions of desired concentration at a desired pH. The contents of the flasks were shaken at 175 rpm on a mechanical shaker at room temperature. It was confirmed through the preliminary experiments that 180 min is sufficient to attain equilibrium between adsorbent and adsorbate. The samples were filtered through Whatman no. 5 filter paper (2.5 µm size particle retention) to eliminate any fine particules. The sorption capacity of the biosorbent was determined by material balance of the initial and equilibrium concentrations of the solution. Adsorption on the glassware was found to be negligible and was determined by running blank experiments. The amount of solute adsorbed per unit mass of adsorbent was calculated using the following equation: qe ) (C0 - Ce) ×

V W

(1)

where qe is the adsorption capacity (mg g-1) at equilibrium; C0 and Ce are the initial and equilibrium concentrations of solute (mg L-1), respectively; V is the volume of the aqueous solution (L); and W is the mass (g) of adsorbent. 2.5. Column Adsorption Studies. Dynamic flow adsorption studies were carried out in a column made of Pyrex glass of 1.2 cm internal diameter and 30 cm length. The column is filled with 1 g of chitosan-coated perlite beads by tapping so that the column is filled without gaps. The column was fully jacketed

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to circulate water from a constant temperature water bath, enabling the experiment to be carried out at a constant temperature. The adsorbent was washed thoroughly with water and dried prior to use. The influent solution of known concentration of aqueous solution of phenolic compounds was allowed to pass through the bed at a constant flow rate (1 mL min-1) in a downflow manner. The complete cycle of operation of each column experiment includes three steps: pH precondition of the adsorbate solution, solution flow, and adsorption of solute until column exhaustion occurs. The effluent solution was collected at different time intervals and concentrations of phenolic compounds were determined by measuring absorbance using Shimadzu UV-2450 spectrophotometer. The solutions were diluted appropriately prior to analysis. All experiments were carried out at room temperature. Breakthrough curves were obtained by plotting volume of the solution passed through the column vs ratio of the column outlet concentration to the initial concentration, C0/Ci. 2.6. Desorption Studies. Desorption (recovery) studies were very important since the success of adsorption process depends on the regeneration of adsorbent. After the column was completely exhausted, the remaining aqueous solution in the column was drained off by pumping air through the column. Desorption of phenolic compounds (phenol, 2-CP, and 4-CP) was tried with a number of eluents, and it was found that the desorption occurred by sodium hydroxide easily. Desorption of solutes from the loaded adsorbent were carried out by a solvent elution method using 0.1 M NaOH as an eluent maintained at constant temperature at a fixed flow rate (1 mL min-1). The effluent samples at different time intervals were collected at the bottom of the column for analysis. When the concentration of the outlet solution was zero or close to zero, it was assumed that the column is regenerated. After the regeneration, the adsorbent column was washed with distilled water to remove NaOH from the column before the influent adsorbate solution was reintroduced for the subsequent adsorption-desorption cycles. The adsorption-desorption cycles were performed thrice for each phenolic solution using the same bed to check the sustainability of the bed for repeated use. Regeneration curves were obtained by plotting volume of the solution passed through the column vs concentration of the column outlet solution. 3. Results and Discussion 3.1. Characterization of Chitosan-Coated Perlite (CCP) Beads. 3.1.1. Pyrolysis Studies. The amount of chitosan-coated perlite is obtained by measuring the weight loss of biosorbent from pyrolysis. These experiments are conducted at high temperature (800 °C) to determine the amount of chitosan coated over perlite. Two ceramic crucibles, one containing acid washed pure perlite and the other containing chitosan-coated perlite (CCP) beads, are placed inside a furnace heated to 800 °C. The chitosan burnt out at this temperature, and the chitosan content is determined from weight difference. The results indicated that 23% of chitosan is coated over perlite.21 3.2. Fourier Transform Infrared (FTIR) Studies. FTIR spectra of CCP in virgin form and loaded with phenolic compounds, obtained using a Nicolet-740, Perkin-Elmer model 283B (USA). The FTIR spectra of the chitosan-coated perlite beads before and after adsorption are shown in Figure 1a-d, respectively. The FTIR spectrum of CCP beads before adsorption (Figure 1a) shows a broad absorption peak at 3435 cm-1 corresponding to the overlapping of -OH and -NH peaks. A peak at 2926 cm-1 represents the C-H group. A peak appearing at 1649 cm-1 is due to the bending mode of N-H in primary amines. A significant difference can be seen in the FTIR spectra

of biosorbent before and after adsorption. As we observe in Figure 1b-d, some peaks are shifted and/or broadened indicating that the functional groups present on the biosorbent is involved in interaction with the phenolic compounds. These results confirm the participation of amino, carboxylic, and hydroxyl groups of CCP beads as potential active binding sites for adsorption of phenolic compounds. 3.3. Surface Area Analysis. Surface area, density, pore volume, pore diameter, and porosity of the composite biosorbent were determined with a BET (Brunauer, Emmett and Teller) instrument (model no: Micromeritirics, USA). Surface area was measured by assuming that the adsorbed nitrogen forms a monolayer and possess a molecular cross sectional area of 16.2 Å2 /molecule. The isotherm plots were used to calculate the specific surface area (N2/BET method) and average pore diameter of CCP, while micropore volume was calculated from the volume of nitrogen adsorbed at P/Po ) 1.4. The sorbent material shows an average surface area of 112.25 m2 g-1, pore volume of 0.47 cm3 g-1, porosity of 43.41%, pore diameter of 0.97 nm, and density of 3.13 g cm-3. 3.4. Scanning Electron Microscopic (SEM) Studies. Scanning electron micrographs of CCP beads recorded, using a software controlled digital scanning electron microscopeJEOLJSM 5410 (Eucentric Gonimeter state type) Japan, are given in Figure 2. The SEM micrograph of the pure perlite powder, outer surface, and cross section of CCP beads are shown in Figure 2a-c, respectively. The figure also illustrates the surface texture and porosity of CCP beads with holes and small openings on the surface, thereby increasing the contact area, which facilitates the pore diffusion during adsorption. The porous nature is clearly evident from this micrograph. The inner surface appears to have similar type of texture and morphology as the outer surface. The surface morphology of the pure perlite appears to change significantly following coating with chitosan. 3.5. Effect of pH. The pH of aqueous medium is an important factor that may influence the uptake of the adsorbate. The pH of the solution affects the degree of ionization and speciation of various pollutants which subsequently leads to a change in reaction kinetics and equilibrium characteristics of the adsorption process. In order to optimize the pH for maximum removal efficiency, experiments were conducted in the pH range from 4.0 to 10.0 using 0.1 g of CCP beads with 100 mL of 100 mg L-1 adsorbate solutions at room temperature. In the alkaline range, the pH was varied using aqueous NaOH, whereas in the acidic range, pH was varied using HCl. Results are shown in Figure 3. The adsorption capacities increase with increase in pH up to pH 7 and decreases from there. The interaction forces between phenol or chlorophenols and biomass are rather weak in the acidic solutions. The highest adsorption of phenolic compounds occurred at pH 7.0 for all species. The decrease in the sorption capacity of the biosorbent toward the chlorophenols as the pH increases above 7 can be explained as being the result of increased electrostatic repulsion between the sorbate and sorbent, since both are negatively charged over this pH range. The phenolic compounds considered in this study, viz. phenol, 2-CP, and 4-CP, have pKa values of 9.9, 8.3, and 9.2, respectively. When the pH of a solution goes beyond the pKa, phenols chiefly exist as negative phenolate ions, whereas they exist as neutral molecules below the pKa. Due to the electron rich nature of the oxygen atom in phenolate ions, the hydrogen bonding efficiency decreases. Therefore, phenols effectively adsorbed on to the adsorbent as molecules but not phenolate ions. Thus, it may be concluded that the molecular interactions involved in the adsorption process are through hydrogen bonding

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Figure 1. FTIR spectra of phenolic compounds: (a) before adsorption of chitosan-coated perlite beads; (b) after phenol adsorption; (c) after 2-chlorophenol adsorption; (d) after 4-chlorophenol adsorption.

and van-der Waals forces. A similar behavior for phenols, in general, has been reported by Gupta et al.,22 Zogoroski et al.,23 Termoul et al.,24 Mangrulkara et al.,25 Srivastava et al.,26 and Hameed et al.27 3.6. Effect of Agitation Time. The effect of agitation time on the extent of adsorption of phenolic compounds at different concentrations is shown in Figures 4, 5, and 6 for phenol, 2-CP, and 4-CP, respectively. The extent of adsorption increases with time and attained equilibrium for all the concentrations of phenol, 2-CP, and 4-CP studied (100, 200, 300, and 400 mg L-1) at 180 min. After this equilibrium period, the amount of solute adsorbed did not change significantly with time, indicating that this time is sufficient to attain equilibrium for the maximum removal of phenolic compounds from aqueous solutions by CCP beads. The adsorption capacity for chlorophenols is higher than that of phenol, possibly due to the higher solubility of phenol in water.

3.7. Batch Adsorption Kinetic Modeling. Kinetic models are used to determine the rate of adsorption process. Data on removal of phenol, 2-CP, and 4-CP by CCP beads as a function of time at pH 7.0 at various initial concentrations (100-400 mg L-1) are presented graphically in Figures 7-9. Both pseudofirst-order and pseudo-second-order kinetic models were used to correlate the adsorption data28,29 log(qe - qt) ) log qe -

()

t 1 1 ) + t qt qe k2qe2

k1t 2.303

(2) (3)

Where, qt is the amount adsorbed at time t, and k1 and k2 are the kinetic parameters to be determined. The slope and intercept

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Figure 4. Effect of agitation time on adsorption of phenol on chitosancoated perlite beads at different initial concentrations.

Figure 5. Effect of agitation time on adsorption of 2-chlorophenol on chitosan-coated perlite beads at different initial concentrations.

Figure 2. SEM of (a) pure perlite powder, (b) outer surface of chitosancoated perlite bead, and (c) cross section of chitosan-coated perlite bead. Figure 6. Effect of agitation time on adsorption of 4-chlorophenol on chitosan-coated perlite beads at different initial concentrations.

Figure 3. Effect of pH on the adsorption of ([) phenol, (9) 2-chlorophenol, and (2) 4-chlorophenol onto chitosan-coated perlite beads.

of plot of log(qe - qt) versus t were used to determine the firstorder rate constant k1. The slope and intercept of the plot of t/qt versus t were used to calculate the second-order rate constant

k2. It is more likely to predict behavior over the whole range of adsorption and is in agreement with the chemisorption mechanism being the rate controlling step. The kinetic parameters are included (Table 2). Higher correlation coefficients of the pseudo-second-order model and agreement between experimental and calculated qe values indicate that the adsorption of phenol, 2-CP, and 4-CP on to CCP beads follows second-order kinetics. 3.8. Intraparticle Diffusion Model. The intraparticle diffusion model is used to investigate the diffusion controlled adsorption system. A process is diffusion controlled if its rate is dependent upon the rate at which components diffuse toward each another.30 The intraparticle diffusion equation is expressed in the following form. qt ) Kidt0.5 + C

(4)

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Figure 7. Effect of adsorbent dose (percent removal of phenol and adsorption capacity (mg g-1)) for adsorption of phenol on chitosan-coated perlite beads.

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For example, the first step might be due to the boundary layer diffusion at the initial stage of the adsorption and the intraparticle diffusion which gives the other two linear parts. The intraparticle diffusion starts with a rapid transport of adsorbate molecules in to macropores and wider mesopores and then, penetrating the smaller meso- and micropores at a much slower pace. Therefore, the second portion of linear curve attributes to the gradual adsorption, where intraparticle diffusion is a rate limiting. The third portion refers to the final equilibrium stage signified by a formation of plateau, indicating a weak activity of the intraparticle diffusion due to low adsorbate concentration left in the solution.31 On the other hand, most of the adsorption sites have been occupied after lapse of time thus limited free sites for the adsorbate molecules to attach on. If the intraparticle diffusion is the only rate-controlling step, then the plot passes through the origin; otherwise, the boundary layer diffusion affects the adsorption to some degree.32 4. Effect of Adsorbent Dose

Figure 8. Effect of adsorbent dose (percent removal of 2-chlorophenol and adsorption capacity (mg g-1)) for adsorption of phenol on chitosan-coated perlite beads.

One of the parameters that strongly affect the biosorption capacity is the amount of the biosorbent. The effect of adsorbent dose on the uptake of phenol, 2-CP, and 4-CP on CCP beads was studied and is shown in Figures 7, 8, and 9, respectively. It can be seen from the figures, that percentage removal of phenol, 2-CP, and 4-CP increases with the increase in adsorbent dose while the adsorption capacity at equilibrium, qe (mg g-1), decreases. The latter result can be explained as a consequence of partial aggregation, which occurs at high biomass concentration giving rise a decrease of active sites.26 It is apparent that the percent removal of phenol, 2-CP, and 4-CP increases rapidly with increase in the dose of CCP beads due to the availability of greater amount of active sites of adsorbent. It can also be seen from these figures that the uptake of solute markedly increased up to adsorbent dose of 0.5 g and thereafter no significant increase was observed. Adsorption is maximum with 0.5 g of CCP beads and the maximum percent removal is about 91% for phenol, about 95% for 2-CP, and about 98% for 4-CP. 5. Adsorption Isotherm Models The experimental data on adsorption were analyzed using Langmuir, Freundlich, and Dubinin-Radushkevich (DR) isotherm models. The Langmuir isotherm model assumes uniform energies of adsorption onto the surface with no transmigration of adsorbate in the plane of the surface. The linear form of the Langmuir isotherm is given by the following equation.22 1 1 1 ) + qe Q0 bQ0Ce

Figure 9. Effect of adsorbent dose (percent removal of 4-chlorophenol and adsorption capacity (mg g-1)) for adsorption of phenol on chitosan-coated perlite beads.

where Kid (mg g-1 min-1/2) is the rate constant of intraparticle diffusion. C is the value of the intercept of qt versus t0.5, which gives an idea about the boundary layer thickness, i.e. the larger the intercept, the greater the boundary layer effect. The plots are not linear over the whole time range, indicating that more than one step affecting the adsorption of phenolic compounds.

(5)

where qe is the amount adsorbed (mg g-1), Ce is the equilibrium concentration of the adsorbate (mg L-1), and Q0 and b are the Langmuir constants related to maximum adsorption capacity and energy of adsorption, respectively. The values of the parameters were obtained from the plots of 1/qe versus 1/Ce. Langmuir parameter, b, can be used to predict the affinity between the sorbate and adsorbent using the dimensionless separation factor, RL, and defined by6 RL )

1 1 + bC0

(6)

If the value of RL is equal to zero or one, the adsorption is either linear or irreversible, and if the value is in between zero and one, adsorption is favorable to chemisorption. The value

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Table 2. Adsorption Rate Constants of Phenolic Compounds on Chitosan-Coated Perlite Beads first-order kinetic model (mg L-1)

q(e,exp) (mg g-1)

q(e,cal) (mg g-1)

second-order kinetic model

k1 (min-1)

R2

q(e,cal) (mg g-1)

Weber-Morris

k2 (g mg-1 min-1)

R2

Kid

C

R2

3.06 × 10-4 3.05 × 10-4 1.90 × 10-4 1.59 × 10-4

0.992 0.996 0.993 0.994

3.961 4.604 6.258 7.479

11.49 19.87 17.47 20.06

0.994 0.993 0.994 0.995

3.29 × 10-4 1.77 × 10-4 1.49 × 10-4 1.22 × 10-4

0.998 0.997 0.998 0.982

4.275 6.130 7.974 9.939

17.26 11.35 20.15 35.52

0.984 0.983 0.984 0.992

3.11 × 10-4 1.70 × 10-4 1.21 × 10-4 1.09 × 10-4

0.998 0.996 0.999 0.997

4.780 7.304 9.266 11.37

20.17 20.55 17.98 18.56

0.989 0.990 0.982 0.990

phenol 100 200 300 400

74.6 94.3 117 139.2

53.8 62.5 84.7 100

0.0085 0.0080 0.0085 0.0082

0.993 0.997 0.995 0.997

100 200 300 400

85.3 109.4 144.3 192.1

56.5 81.7 103.8 137.4

0.0085 0.0085 0.0087 0.0089

0.990 0.991 0.993 0.991

100 200 300 400

92.3 134.6 167.9 202.7

63.9 96.4 123.8 153.3

0.0096 0.0087 0.0078 0.0078

0.980 0.995 0.988 0.993

75.1 91.7 119 140

2-chlorophenol 84.03 112.3 149.2 196.1

4-chlorophenol 93.4 136.9 169.4 200

Table 3. Isotherm Parameters of Langmuir, Freundlich, and DR Isotherms for Adsorption of Phenol, 2-Chlorophenol, and 4-Chlorophenol on Chitosan-Coated Perlite Beads Langmuir constants

Freundlich constants

Dubinin-Raduskvich isotherm

adsorbates

Q0

b

R2

KF

n

R2

B

qs

E

R2

phenol 2-chlorophenol 4-chlorophenol

192 263 322

0.004 0.003 0.005

0.997 0.993 0.999

1.068 1.437 1.825

1.097 1.037 1.078

0.987 0.997 0.993

0.0042 0.0037 0.0028

3.394 3.276 3.512

10.20 11.62 13.36

0.8359 0.7895 0.8123

of RL is less than 1 and great than 0, suggesting the favorable uptake of phenolic compounds by chitosan-coated perlite beads.33 The Freundlich isotherm is given as qe ) KFCe

1/n

of the solid from infinity in the solution and can be computed by using the relationship: E)

(7)

where KF ((mg g-1)(L mg-1)1/n) and 1/n are indicators of the adsorption capacity and the adsorption intensity, respectively. The values of KF and 1/n were calculated by plotting ln(qe) against ln(Ce). The values of Langmuir constants Q0 and b and Freundlich parameters KF and n along with the correlation coefficients (R2) are presented in Table 3. Among these two models, the Langmuir isotherm gives a better representation of adsorption of phenol, 2-CP, and 4-CP on chitosan-coated perlite beads compared to the Freundlich model. Another model for the analysis of isotherms of a high degree of rectangularity is the Dubinin-Radushkevich isotherm. The equilibrium data were also correlated with the DR model to determine if adsorption occurred by physical or chemical processes. In the case of liquid phase adsorption, several studies have shown that the adsorption energy can be estimated according to the Dubinin-Radushkevich equation. Assuming that the adsorption in micropores is limited to a monolayer and the DR equation is applicable, the adsorption capacity per unit surface area of the adsorbent at equilibrium, qe, can be written as34 qe ) qs exp(-Bε2)

(8)

where ε can be correlated by

1

√2B

(10)

where R is the gas constant (8.314 J mol-1 K-1) and T is the absolute temperature. A plot of ln(qe) versus ε2 enables the constants qs and E to be determined (Table 3). It is known that Freundlich and Langmuir isotherms could not reveal the adsorption mechanism. The purpose of applying equilibrium data to the DR model is mainly to clarify the adsorption type and evaluate the nature of interaction between sorbate and solid. On the basis of the theory of the DR model, the sorption space in the vicinity of the solid surface is characterized by a series of equipotential surfaces having same sorption potential. The sorption mean free energy is the energy required to transfer 1 mol of the sorbate from infinity in solution to the surface of the solid. The magnitude of sorption mean free energy E is widely used for estimating the type of adsorption.35 The mean adsorption energy (E) was found to be 10.20 kJ mol-1 for phenol, 11.62 kJ mol-1 for 2-CP, and 13.36 kJ mol-1 for 4-CP onto chitosan-coated perlite beads. These values indicate that the adsorption processes is associated with chemical ion-exchange mechanism. The parameters of the three isotherms were computed and listed in Table 3. The Langmuir isotherm fitted quite well with the experimental data (correlation coefficient R2 ) 0.99), compared to the other two isotherm models. 6. Column Adsorption Data

[

ε ) RT ln 1 +

1 Ce

]

(9)

where qs is the ultimate capacity per unit area of the adsorbent and the constant B gives the mean free energy E of adsorption per molecule of the adsorbate when it is transferred to the surface

To be useful in separation and removal processes, adsorbed species should be easily desorbed under mild conditions and adsorbents should be reused many times in order to decrease material costs. The results of dynamic flow experiments were used to obtain the breakthrough curves for adsorption of phenol, 2-CP, and 4-CP from aqueous solutions by plotting

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010

Figure 10. Column breakthrough curves for adsorption of phenol on chitosan-coated perlite beads.

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Figure 13. Regeneration curves of chitosan-coated perlite beads loaded with phenol.

Figure 14. Regeneration curves of chitosan-coated perlite beads loaded with 2-chlorophenol. Figure 11. Column breakthrough curves for adsorption of 2-chlorophenol on chitosan-coated perlite beads.

Figure 12. Column breakthrough curves for adsorption of 4-chlorophenol on chitosan-coated perlite beads.

the volume of effluent vs Ce/Ci. The breakthrough curves are shown in Figures 10-12. Breakthrough capacities, the amount adsorbed until the effluent concentration of the adsorbate is equal to the influent solution concentration, are computed from the breakthrough curves. An examination of the curves indicates that no leakage of solute is observed up

to a volume of about 100 mL of influent sorbate solution in all cases in the first cycle. When the bed is exhausted or the effluent coming out of the column reaches the allowable maximum discharge level, the regeneration of the adsorption bed to recover the adsorbed material and/or to regenerate the adsorbent becomes quite essential. The regeneration could be accomplished by a variety of techniques such as thermal desorption, steam washing, solvent extraction, etc. Each method has inherent advantages and limitations. In this study, several solvents were tried to regenerate the adsorption bed. A 0.1 M NaOH solution is found to be effective in desorbing and recovering adsorbates quantitatively from the adsorption bed. The fixed bed columns of CCP beads saturated with phenol or chlorophenol is regenerated by passing 0.1 M NaOH solution as an eluent at a fixed flow rate of 1 mL min-1. To evaluate the sorbate recovery efficiency, the percent of phenol, 2-CP, and 4-CP recovered is calculated from the breakthrough and recovery curves. The desorption profile is graphically represented in Figures 13-15. From the plots, it is observed that the rate of desorption increases sharply reaching a maximum with 4 mL of 0.1 M NaOH solution and complete regeneration occurred at about 30 mL. The regenerated column is further used for the removal of phenol. The results indicate that the column gets saturated early and adsorption capacity decreases. As a result, the percent desorption also decreases from the first to the third cycle.

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Figure 15. Regeneration curves of chitosan-coated perlite beads loaded with 4-chlorophenol.

7. Conclusions Chitosan is effectively coated on an inert substrate, perlite, and is made in the form of spherical beads. Pyrolysis results indicated that 23% of chitosan was coated on perlite. Scanning electron micrographs showed the beads to be porous in nature. The effect of pH on the extent of adsorption was investigated. The adsorption process was found to follow the second-order kinetic model. The Langmuir isotherm model represents the experimental data adequately compared to Freundlich and DR isotherm models. The equilibrium adsorption data show that chitosan-coated beads adsorb a significant amount of phenolic compounds compared to chitosan or other chemically modified chitosan as reported in the literature. Sodium hydroxide (0.1 M) was found to be effective in regenerating the column loaded with phenolic compounds. The present work elucidates that the CCP beads are potential biosorbents for their application in the removal of phenol and its derivatives. The results indicate that CCP beads show higher adsorption capacity for chlorophenols than phenol. This could be attributed to the difference in solubility and hydrophobicity of phenol, 2-CP, and 4-CP in water. Literature Cited (1) Ahmaruzzaman, Md. Adsorption of phenolic compounds on lowcost adsorbents: A review. AdV. Colloid Interface Sci. 2008, 143, 48. (2) Hu, M. Q.; Xu, Y. M.; Zhao, J. C. Efficient photosensitized degradation of 4-chlorophenol over immobilized aluminium tetrasulfophthalocyanine in the presence of hydrogen peroxide. Langmuir 2004, 20, 6302. (3) Tepe, O.; Dursun, A. Y. Combined effects of external mass transfer and biodegradation rates on removal of phenol by immobilized Ralstonia eutropha in a packed bed reactor. J. Hazard. Mater. 2008, 151, 9. (4) Dabrowski, A.; Podkoscielny, P.; Hubicki, M.; Barczak, M. Adsorption of phenolic compounds by activated carbonsa critical review. Chemosphere 2005, 58, 1049. (5) Patterson, J. F. Industrial Waste water Treatment Technology, 2nd ed.; Butterworths: London, 1985. (6) Dursun, G.; Cecek, H.; Dursun, A. Y. Adsorption of phenol from aqueous solution by using carbonised beet pulp. J. Hazard. Mater. 2005, 125, 175. (7) Maugans, C. B.; Akgerman, A. Catalytic wet oxidation of phenol in a trickle bed reactor over a pt/Tio2 catalyst. Water Res. 2003, 37, 319. (8) Chao, A. C.; Shyu, S. S.; Lin, Y. C.; Mi, F. L. Enzymatic grafting of carboxyl groups on to chitosan-to confer on chitosan the property of a cationic dye adsorbent. Biores. Technol. 2004, 91, 157. (9) Crini, G. Non-Conventional low-cost adsorbents for dye removal: a review. Biores. Technol. 2006, 97, 1061. (10) Juang, R. S.; Ju, C. Y. Kinetics of sorption of Cu (II)- ethylenediaminetetraacetic acid chelated anions on cross-linked, polyaminated chitosan beads. Ind. Eng. Chem. Res. 1998, 32, 386.

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ReceiVed for reView July 23, 2009 ReVised manuscript receiVed July 27, 2010 Accepted August 13, 2010 IE901171B