Thermodynamic, kinetic, and equilibrium studies ... - Wiley Online Library

THERMODYNAMIC, KINETIC, AND EQUILIBRIUM STUDIES ON PHENOL REMOVAL BY USE OF CASHEW NUT SHELL P. Senthil Kumar,1 * K. Ramakrishnan,1 S. Dinesh Kirupha2 and S. Sivanesan2 1. Department of Chemical Engineering, SSN College of Engineering, Chennai 603 110, India 2. Environmental Management Laboratory, AC Tech, Anna University, Chennai 600 025, India

Adsorption of phenol from aqueous solution onto cashew nut shell (CNS) was investigated to assess the possible use of this adsorbent. The influence of various parameters such as contact time, phenol concentration, adsorbent dose, pH, and temperature has been studied. Studies showed that the pH of aqueous solutions affected phenol removal as a result of decrease in removal efficiency with increasing solution pH. The experimental data were analysed by the Langmuir equation. Equilibrium data fitted well with the Langmuir model with maximum monolayer adsorption capacity of 5.405 mg/g. Thermodynamic parameters such as G◦ , H◦ , and S◦ have also been evaluated and it has been found that the sorption process was feasible, spontaneous, and exothermic in nature. The pseudo-first-order and pseudo-second-order kinetic models were selected to follow the adsorption process. Kinetic parameters, rate constants, equilibrium sorption capacities and related correlation coefficients, for each kinetic model were calculated and discussed. It was shown that the adsorption of phenol could be described by the pseudo-second-order equation, suggesting that the adsorption process is presumable a chemisorption. The CNS investigated in this study showed good application potential for the removal of phenol from aqueous solution. L’adsorption du phénol d’une solution aqueuse sur une coque de noix de cajou (CNC) a e´ té analysée pour e´ valuer l’utilisation possible de cet adsorbant. L’influence de divers paramètres comme le temps de contact, la concentration de phénol, la dose d’adsorbant, le pH et la température a e´ té e´ tudiée. Des e´ tudes ont démontré que le pH des solutions aqueuses avait un effet sur le retrait du phénol comme résultat de la diminution de l’efficacité de retrait avec un pH de solution croissant. Les données expérimentales ont e´ té analysées par l’équation de Langmuir. Les données d’équilibre correspondaient bien au modèle de Langmuir avec une capacité d’adsorption monocouche maximale de 5.405 mg/g. Des paramètres thermodynamiques comme G◦ , H◦ et S◦ ont e´ galement e´ té e´ valués et on a découvert que le processus de sorption e´ tait faisable, spontané et de nature exothermique. Les modèles cinétiques de pseudo premier ordre et pseudo second ordre ont e´ té choisis pour suivre le processus d’adsorption. Les paramètres cinétiques, constantes de vitesse, capacités de sorption d’équilibre et cœfficients de corrélation connexes pour chaque modèle cinétique ont e´ té calculés et discutés. On a découvert que l’on peut décrire l’adsorption de phénol par l’équation de pseudo second ordre, suggérant que le processus d’adsorption est sans doute une chimisorption. La CNC analysée dans cette e´ tude démontrait un bon potentiel d’application pour le retrait du phénol d’une solution aqueuse. Keywords: phenol, CNS, isotherms, kinetics, thermodynamics

INTRODUCTION

P

ollution by phenols is an important environmental issue. Phenol is an organic compound widely used in industries such as plastics, lubricants, paints, pharmaceuticals, herbicides, and resins (El-Geundi, 1997). Chemically, phenol is a hazardous compound, if absorbed through the skin, inhaled, or swallowed may lead to serious injuries or fatalities. One of the major threats due to phenol-toxicity lies in its ability to penetrate the skin rapidly. Various common phenolated industrial effluents have been reported to range between 35 and 8000 ppm (Mahajan, 1994). As per the Environmental Protection Agency (EPA), the permissible limit of phenol in drinking water is 0.002 ppm. The phenol removal has been achieved by various methods such as sorption (Juang et al., 2004), biological degradation (Haghighi-

|

284

|

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING

|

Podeh et al., 1995), chemical oxidation (Dieckmann and Gray, 1996), and solvent extraction (Ashutosh and Sharma, 1998). Out of all methods, sorption technique is the most commonly used method due to its simplicity and easy operation.

Additional Supporting Information may be found in the online version of this article. ∗ Author to whom correspondence may be addressed. E-mail addresses: [email protected], [email protected] Can. J. Chem. Eng. 89:284–291, 2011 © 2010 Canadian Society for Chemical Engineering DOI 10.1002/cjce.20396 Published online 5 November 2010 in Wiley Online Library (wileyonlinelibrary.com).

|

VOLUME 89, APRIL 2011

|

Many researchers have carried out studies on different types of wastes, which include community or agricultural waste (viz. sludge, roots/barks/stems/leaves/hulls/husks/pills/seeds of various plants), industrial wastes (viz. fly ash, bottom ash) (Calace et al., 2000; Gaikwad, 2002; Vasanth, 2003; Srihari and Ashutosh, 2004; Lua and Jia, 2007; Nagda et al., 2007; Shawabkeh and Abu-Namesh, 2007; Srihari and Das, 2008; Cherifi et al., 2009; Tor et al., 2009) as an adsorbent in search of a cost effective alternative to activated carbon. Because of the low cost and high availability of these materials, it is not essential to have complicated regeneration process. This low cost adsorption method has been attracting many scientists and engineers. The objective of this research is to develop inexpensive and effective adsorbents from the sources of natural wastes, such as cashew nut shell (CNS), to replace the existing commercial materials. In this present study, CNS from which the cashew nut shell liquid (CNSL) was extracted (Kumar et al., 2009) is used as an adsorbent and it was examined for their sorption properties towards phenol. The influence of experimental parameter such as pH, contact time, temperature, CNS dosage, and initial phenol concentrations were studied. The adsorption process is studied from isotherm, kinetic, and thermodynamic standpoints.

EXPERIMENTAL Adsorbent The raw CNS was collected from Karaikudi, Sivagangai District, Tamil Nadu, India and the treated CNS was collected from the CNSL recovery unit and it was used as an adsorbent. This natural waste was thoroughly rinsed with water to remove dust and soluble material and was allowed to dry at room temperature. The above dried natural waste was crushed into powder and sieved. The particle size used in this study was 200-30 mesh. The proximate and ultimate analysis of CNSs are shown in Table 1.

Adsorbate All the chemicals used were of analytical reagent (AR) grade. A stock solution was obtained by dissolving 1.0 g of phenol (Merck, Mumbai, India) in de-ionised water and diluted to 1000 mL. Desired test solutions of phenol were prepared using appropriate subsequent dilutions of stock solution. The range in

Table 1. Properties of the cashew nut shells Proximate analysis (wt.%) Volatile matter Moisture Ash Fixed carbon Ultimate analysis (wt.%) Carbon Hydrogen Oxygen Nitrogen Sulphur Moisture Ash Ash chemical composition (wt.%) Silica Iron oxide Aluminium oxide Calcium oxide Magnesium oxide Sodium oxide

|


|

65.21 9.83 2.75 22.21 45.21 4.25 37.75 0.21 Nil 9.83 2.75 64.53 3.27 2.19 26.89 2.49 0.63

concentrations of phenol prepared from standard solution varied between 20 and 100 mg/L. Before mixing the adsorbent, the pH of each test solution was adjusted to the required value with 0.1 M NaOH or HCl.

Analysis The concentration of residual phenol in the sorption medium was determined with direct photometric method (Eaton et al., 1995). At the end, after the preparation of samples according to the standard methods, the residual phenol concentrations were measured using spectrophotometer equipment (Spectrophotometer DR-2000 HACH company, Loveland). The absorbance of the coloured complex of phenol with 4-aminoantipyrine was read at 500 nm (Eaton et al., 1995). The pH of solution was measured with a Hanna pH meter using a combined glass electrode (Model HI 9025C, Singapore).

Adsorption Experiment Adsorption experiments were conducted by varying pH, contact time, adsorbent dose, temperature, adsorbate concentration under the aspect of thermodynamic study, adsorption isotherms, and adsorption kinetics. The experiments were carried out in 250 mL Erlenmeyer flasks and the total volume of the reaction mixture was kept at 100 mL. The pH of solution was maintained at a desired value by adding 0.1 M NaOH or HCl. The flasks were shaken for the required time period in a water bath shaker. The kinetic studies was carried out by agitating 250 mL flasks containing 20 g/L of CNS and 100 mL phenol solutions of different concentrations, that is, from 20 to 100 mg/L in water bath shaker. The mixture was agitated at 120 rpm at 30◦ C. The contact time was varied from 0 to 60 min. At predetermined time, the flasks were withdrawn from the shaker and the reaction mixtures were filtered through Whatman filter paper No. 42. For thermodynamic study, the experiments were performed by varying temperatures from 30 to 60◦ C using 20 g/L CNS added to 100 mL of phenol solution in 250 mL flasks. The flasks were shaken at 120 rpm for 60 min at pH 5. The initial phenol concentration used in this study was 50 and 100 mg/L. The isotherm study was performed using various concentrations of phenol solutions, that is, from 20 to 100 mg/L. A 20 g/L CNS with 100 mL phenol solutions of various initial concentrations was shaken at 120 rpm for 60 min at 30◦ C. The initial pH of the solution was adjusted to 5. All the experiments were performed in duplicates. The filtrate samples were analysed by spectrophotometer. The amount of phenol adsorbed onto CNS, qe (mg/g), was calculated by the following mass balance relationship: qe =

(Ci -Ce )V W

(1)

where Ci and Ce are the initial and equilibrium concentrations (mg/L) of phenol solution, respectively; V is the volume (L), and W is the mass (g) of the adsorbent.

RESULTS AND DISCUSSION Characterization of CNS The adsorption capacity of CNS depends upon porosity as well as chemical reactivity of functional groups at the surface. This reactivity creates an imbalance between forces at the surface when compared to those within the body, thus leading to molecular adsorption by the van der Waals force. Knowledge on surface

|


|

285

|

Figure 1. FT-IR spectrum of cashew nut shells.

functional groups would give insight to the adsorption capability of the CNS. The FT-IR spectrums of the CNS are shown in Figure 1. OH and NH stretching between 3100 and 3500 cm−1 , C–H aromatic in between 3000 and 3100 cm−1 , C–H aliphatic in between 2800 and 3000 cm−1 . The spectrum shows a broad band near 3399 cm−1 , which indicates the presence of hydroxyl groups on the CNS surface. The stretching was attributed to the absorbed water on the surface of CNS. The peak at 2925 cm−1 is due to C–H stretching of CH2 groups. The stretching frequencies of the aromatic C C and aromatic C–H groups give rise to peaks at 3011 and 2854 cm−1 , respectively. The bands near 1630 cm−1 indicates fingerprint region of C O, C–O, and O–H groups that exist as functional groups of CNS. The peaks at 1542 and 1515 cm−1 is assigned to a conjugated hydrogen bonded carboxyl group. The peak at 1454 cm−1 (vC–O) indicates the presence of carboxylic groups. The peak at 1374 cm−1 indicates the presence of C–H aliphatic bending. The peaks at 1232 cm−1 indicates the presence of the C–N from amine. The two peaks at 1156 cm−1 (P O) and 1035 cm−1 (P–OH) was characteristic of PO2 stretching. These results agree with the surface chemistries of other agricultural by-products, such as coconut coir pith (Anirudhan et al., 2009) and rubber wood saw dust (Zakaria et al., 2009). The specific surface area and pore structure of the CNS was determined by using surface area and pore size analyser (Quantachrome, Autosorb-I) on nitrogen adsorption at 77 K. The specific surface area was calculated by BET equation (Brunauer et al., 1938). It was found that the BET surface area, pore volume, average pore diameter, and bulk density of the CNS were 395 m2 /g, 0.4732 cm3 /g, 5.89 nm, and 0.415 g/cm3 , respectively. The pH dependent surface charge is very widely used in predicting the adsorption characteristics of CNS. The potentiometric titration method was used to determine the surface charge density 0 as a function of pH and ionic strength and it was calculated using Equation (2): (2)

where F is the Faraday’s constant, CA and CB are the concentrations of the acid (HCl) and base (NaOH) after each addition, [OH− ] and

286

|


[H+ ] are the concentration of free hydroxide and hydronium ion bound to suspension surface and A is the specific surface area of the suspension surface (cm2 /L). The plot of 0 versus pH is given in Figure 2. The point of intersection of 0 with the pH curve gives the pHPZC and was found to be 5.0.

Effect of pH The adsorption of phenol from aqueous solution is dependent on the pH of the solution, which affects the surface charge of the adsorbent, degree of ionisation, and speciation of the adsorbate species. The adsorption of phenol by CNS was studied at various pH values. Figure 3 shows that the percentage of phenol removal decreases with increase in solution pH. This can be attributed to the dependency of phenol ionisation on the pH value. The ionic fraction of phenolate ion ϕions can be calculated from the following equation (Banat et al., 2000): ϕions =

F(CA -CB ) + ([OH- ]-[H+ ]) 0 = A

|

Figure 2. Surface charge density ( 0 ) as a function of pH in aqueous solution of NaCl.

|

1 [1 + 10(pKa -pH) ]

(3)

Obviously, ϕions increases as the pH value is increased. Accordingly, phenol, which is a weak acid (pKa = 10), will be adsorbed to a lesser extent at higher pH values due to the repulsive force prevailing at higher pH value (Banat et al., 2000; Khalid et al.,

|


|

Figure 3. Effect of pH/ionic fraction of phenolated ions on phenol removal onto CNS (phenol concentration = 50 mg/L, CNS dose = 20 g/L, volume of sample = 100 Ml, and equilibrium time = 1 h).

2000). Also, in the higher pH range, phenol forms salts, which readily ionise leaving negative charge on the phenolic group. At the same time the presence of OH− ions on the adsorbent prevents the uptake of phenolate ions (Khalid et al., 2000; Rengaraj et al., 2002). Similar behaviour has been reported for the adsorption of phenol by activated carbon (Halouli and Drawish, 1995) and adsorption of phenol onto bentonite (Banat et al., 2000). The decrease in adsorption capacity with increase in solution pH from 5.0 to 11.0 can be attributed to the electrostatic repulsion existing between CNS surface and the phenolate anions in solution. As the pHZPC for CNS was found to be 5.0, the surface of CNS is positively charged, when pH < pHZPC . Therefore, in the lower pH range the protonated species will be repelled by the CNS. In this case a reduction in the adsorption at a higher pH is possibly due to the increased solubility of phenols and the abundance of OH− ions therefore increases hindrance to diffusion of phenolate anions (Anirudhan et al., 2009).

Effect of CNS Dose The effect of the adsorbent dose was studied at room temperature (30◦ C) by varying the sorbent amounts from 5 to 30 g/L. For all these runs, initial concentration of phenol was fixed as 50 mg/L. Figure 4 shows that the adsorption of phenol increases rapidly with increase in the amount of CNS, due to greater availability of the surface area at higher concentration of the adsorbent. The adsorption of phenol is site specific. Due to hydrophobic nature of the phenol molecules and higher affinity of the solute for the solid adsorbent surface, the molecules of phenol get adsorbed to the surface. However, with the increase in amount of adsorbent, the site for phenol adsorption increases. From the results, it is revealed that within a certain range of initial phenol concentration, the percentage of phenol adsorption on CNS is determined by the sorption capacity of the CNS.

Figure 4. Effect of CNS dose on phenol removal (phenol concentration = 50 mg/L, volume of sample = 100 mL, pH = 5, and equilibrium time = 1 h).

about 60 min of shaking time at different initial concentrations. The increasing contact time increased the phenol adsorption and it remained constant after equilibrium was reached in 30 min for different initial concentrations.

Effect of Initial Phenol Concentration The equilibrium sorption capacities of the sorbents obtained from experimental data at five different initial phenol concentrations were presented in Figure 6. As seen from the results, the percentage of phenol removal decreases with increase in initial phenol concentration while the sorption capacities of CNS showed the opposite trend. The initial phenol concentration provides the necessary driving force to overcome the resistances to the mass transfer of phenol between the aqueous phase and the solid phase. The increase in initial phenol concentration also enhances the interaction between phenol and CNS. Therefore, an increase in initial concentration of phenol enhances the adsorption uptake of phenol. This is due to increase in the driving force of the concentration gradient, as the initial phenol concentration increases. While the percentage of phenol removal was found to be 92.56% for 20 mg/L of initial concentration, this value was 82.33% for that of 100 mg/L.

Effect of Temperature and Thermodynamic Parameters The adsorption of phenol on CNS was investigated as a function of temperature and the maximum removal of phenol was obtained at 30◦ C. Experiments were performed at different temperatures of

Effect of Contact Time Adsorption of phenol was measured at given contact times for five different initial phenol concentrations. Figure 5 shows the effect of contact time on the removal of phenol by CNS. The plot reveals that the percentage of phenol removal is higher at the beginning. This is probably due to the larger surface area of the CNS being available at beginning for the adsorption of phenol. As the surface adsorption sites become exhausted, the uptake rate is controlled by the rate at which the adsorbate is transported from the exterior to the interior sites of the adsorbent particles. Most of the maximum percentage of phenol removal was attained after

|


|

Figure 5. Effect of contact time on phenol removal onto CNS (phenol concentration = 20–100 mg/L, CNS dose = 20 g/L, volume of sample = 100 mL, pH = 5, and equilibrium time = 1 h).

|


|

287

|

Figure 6. Effect of initial phenol concentration on phenol removal onto CNS (phenol concentration = 20–100 mg/L, CNS dose = 20 g/L, volume of sample = 100 mL, pH = 5, and equilibrium time = 1 h).

Figure 8. Thermodynamic study. ◦

log Kc =

Figure 7. Effect of temperature on phenol removal onto CNS (phenol concentration = 50 and 100 mg/L, CNS dose = 20 g/L, volume of sample = 100 mL, pH = 5, temperature = 30–60◦ C, and equilibrium time = 1 h).

30, 40, 50, and 60◦ C for the initial phenol concentrations of 50 and 100 mg/L at constant adsorbent dose of 20 g/L and pH of 5. The adsorption decreased from 89.09% to 77.89% and 82.38% to 69.71% for the initial phenol concentrations of 50 and 100 mg/L, respectively, with the rise in temperature from 30 to 60◦ C (Figure 7). This is mainly due to the decrease in surface activity suggesting that adsorption between phenol and CNS is an exothermic process. Thermodynamic parameters such as free energy (G◦ ), enthalpy (H◦ ), and entropy (S◦ ) change of adsorption can be evaluated from the following equations (4–6): Kc =

CAe Ce

◦

G = −RT ln Kc ◦

◦

G = H −TS

◦

◦

H S − 2.303 R 2.303 RT

(7)

where Kc is the equilibrium constant, Ce the equilibrium concentration in solution (mg/L), and CAe the amount of phenol adsorbed on the adsorbent per litre of solution at equilibrium (mg/L). G◦ , H◦ , and S◦ are changes in Gibbs free energy (kJ/mol), enthalpy (kJ/mol), and entropy (J/mol/K), respectively. R is the gas constant (8.314 J/mol/K), T is the temperature (K). The values of H◦ and S◦ are determined from the slope and the intercept of the plots of log Kc versus 1/T (Figure 8). The G◦ values were calculated using Equation (5). Adsorption of phenol on CNS decreased, when the temperature was increased from 303 to 333 K is shown in Figure 8. The process was thus exothermic in nature. The plots were used to compute the values of thermodynamic parameters (Table 2). The value of enthalpy change (H◦ ) and the entropy change (S◦ ) recorded from this work were presented in Table 2. The negative G◦ value indicates that the process is feasible and spontaneous in the nature of adsorption; negative H◦ value suggests the exothermic nature of adsorption and the S◦ can be used to describe the randomness at the CNS–solution interface during the sorption.

Adsorption Isotherm Adsorption isotherm, which represents the amount of solute adsorbed per unit of adsorbent, as a function of equilibrium concentration in bulk solution at constant temperature, were studied. In order to optimise the design of a sorption system to remove phenol from aqueous solutions, it is important to establish the most appropriate correlation for the equilibrium curve. A linear form of Langmuir isotherm (1915) is:

(4)

1 1 1 1 = + qe qm qm KL Ce

(5)

where qm and KL are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption, respec-

(6)

(8)

Table 2. Thermodynamic parameters for the adsorption of phenol onto CNS Initial phenol conc. (mg/L)

H◦ (kJ/mol)

S◦ (J/mol/K)

G◦ (kJ/mol) 30◦ C

−24.336 −20.334

50 100

|

288

|


−62.822 −54.109

|

−5.290 −3.885

40◦ C

50◦ C

60◦ C

−4.776 −3.519

−3.914 −2.797

−3.486 −2.317

|


|

Figure 9. The linearised Langmuir adsorption isotherm for phenol with CNS at 30◦ C.

tively. The values of qm (5.405 mg/g) and KL (0.138 L/mg) were determined from Figure 9. The Langmuir equation represents the best fit for the experimental data. The comparison of the monolayer sorption capacity, qm , and sorption equilibrium constant, KL for the adsorption of phenol by non-conventional adsorbents is presented in Table 3.

Kinetic Study In order to investigate the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, a suitable kinetic model is needed to analyse the rate data. Any kinetic or mass transfer representation is likely to be global. From a system design viewpoint, a lumped analysis of kinetic data is sufficient for practical operations. The sorption kinetics may be described by a pseudo-first-order equation (Lagregren, 1898). The differential equation is as follows: dqt = kad (qe -qt ) dt

(9)

After integration by applying the initial conditions qt = 0 at t = 0 and qt = qt at t = t, Equation (9) becomes:

log

qe qe -qt

=

kad t 2.303

(10)

Equation (10) can be rearranged to obtain a linear form: log(qe -qt ) = log qe −

kad t 2.303

(11)

where qt and qe are the adsorption capacity at time t (mg/g) and at equilibrium, respectively, and kad (min−1 ) is the rate constant of the pseudo-first-order adsorption, was applied to the present

Figure 10. Pseudo-first-order kinetic fit for adsorption of phenol by CNS at 30◦ C.

study of phenol adsorption. The rate constant kad and correlation coefficients of phenol under different concentrations were calculated from the linear plots of log(qe –qt ) versus t (Figure 10) and listed in Table 4. The correlation coefficients for the pseudofirst-order kinetic model are low. Moreover, a large difference of equilibrium adsorption capacity (qe ) between the experiment and calculation was observed indicating a poor pseudo-first-order fit to the experimental data. The adsorption kinetics may also be described by a pseudosecond-order equation (Ho and McKay, 1999). The differential equation is as follows: dqt = k(qe -qt )2 dt

(12)

Integrating Equation (12) and applying the boundary conditions, gives:

1 qe -qt

=

1 + kt qe

(13)

Equation (13) can be rearranged to obtain a linear form: 1 1 t = + t qt h qe

(14)

where h = kq2e (mg g−1 min−1 ) can be regarded as the initial adsorption rate as t → 0 and k is the rate constant of pseudosecond-order adsorption (g mg−1 min−1 ). The plot t/qt versus t (Figure 11) should give a straight line if pseudo-second-order kinetics is applicable and qe , k, and h can be determined from the slope and intercept of the plot, respectively. At all studied initial phenol concentrations, the straight lines with extremely high correlation coefficients (>0.99) were obtained. In addition,

Table 3. Comparison of monolayer sorption capacity, qm , and sorption equilibrium constant, KL , for the adsorption of phenol by non-conventional adsorbents

|

Adsorbents

qm (mg/g)

KL (L/mg)

Refs.

Petroleum coke treated with KOH Granulated activated carbon Bagasse fly ash Coal Cashew nut shell Rice husk Coke breeze

158.0 49.720 23.832 13.230 5.405 4.508 0.171

0.391 0.109 0.088 0.008 0.138 0.001 0.001

Asyhar et al. (2002) Ozkaya (2006) Srivastava et al. (2006) Ahmaruzzaman and Sharma (2005) Present study Ahmaruzzaman and Sharma (2005) Ahmaruzzaman and Sharma (2005)


|

|


|

289

|

Table 4. Comparison between the adsorption rate constants, qe , estimated and correlation coefficients associated with pseudo-first-order and to the pseudo-second-order rate equations Initial phenol conc. (mg/L)

20 40 60 80 100

Pseudo-first-order rate equation

pseudo-second-order rate equation

kad (min−1 )

qe,cal (mg/g)

R2

k (g mg−1 min−1 )

qe,cal (mg/g)

R2

h (mg g−1 min−1 )

qe,exp (mg/g)

0.170 0.168 0.170 0.170 0.173

1.137 2.594 4.276 5.470 7.998

0.926 0.930 0.934 0.915 0.923

0.282 0.110 0.065 0.044 0.028

0.966 1.980 2.923 3.802 4.785

0.998 0.997 0.997 0.995 0.993

0.263 0.432 0.552 0.635 0.644

0.926 1.806 2.632 3.371 4.119

NOMENCLATURE A CA CB Ci Ce F h k kad KL Figure 11. Pseudo-second-order kinetic fit for adsorption of phenol by CNS at 30◦ C.

the calculated qe values also agree with the experimental data in the case of pseudo-second-order kinetics. These suggest that the adsorption data are well represented by pseudo-second-order kinetics and supports the assumption that the rate-limiting step of phenol onto CNS may be chemisorption. From Table 4, the values of the rate constant k decreased with increasing initial phenol concentration for the CNS. The reason for this behaviour can be attributed to the lower competition for the sorption surface sites at lower concentration. At higher concentrations, the competition for the surface active sites will be high and consequently lower sorption rates are obtained.

CONCLUSION Equilibrium, kinetic, and thermodynamic studies were made for the adsorption of phenol from aqueous solution of different initial phenol concentrations onto CNS at pH 5. The equilibrium data have been analysed using Langmuir isotherm. The characteristic parameters for Langmuir isotherm and related correlation coefficient have been determined. The Langmuir isotherm was demonstrated to provide the best correlation for the sorption of phenol onto CNS. The suitability of the pseudo first- and secondorder equations for the sorption of phenol onto CNS is also discussed. The pseudo-second-order kinetic model agrees very well with the dynamic behaviour of the adsorption of phenol onto CNS for different initial phenol concentrations over the entire range studied. It may be concluded that CNS may be used as a lowcost, natural and abundant source for the removal of phenol from wastewater.

|

290

|


|

qe qm qt R 0 t T V W

specific surface area of the suspension surface (cm2 /L) concentration of the acid (HCl) concentration of the base (NaOH) initial concentration of phenol solution (mg/L) equilibrium concentration of phenol solution (mg/L) Faraday’s constant (96 500 C/mol) the initial adsorption rate (mg g−1 min−1 ) the rate constant of pseudo-second-order adsorption (g mg−1 min−1 ) the rate constant of the pseudo-first-order adsorption process (min−1 ) Langmuir constant represents the energy constant related to the heat of adsorption (L/mg) the amount adsorbed at equilibrium (mg/g) Langmuir constant represents the maximum adsorption capacity for the solid phase loading (mg/g) the amount adsorbed at time t (mg/g) gas constant (8.314 J/mol K) surface charge density (C/cm2 ) time (min) temperature (K) volume (L) mass of the adsorbent (g)

REFERENCES Ahmaruzzaman, M. and D. K. Sharma, “Adsorption of Phenols from Wastewater,” J. Colloid Interface Sci. 287, 14–24 (2005). Anirudhan, T. S., S. S. Sreekumari and C. D. Bringle, “Removal of Phenol from Water and Petroleum Industry Refinery Effluents by Activated Carbon Obtained from Coconut Coir Pith,” Adsorption 15, 439–451 (2009). Ashutosh, D. and D. K. Sharma, “Adsorption of Phenol from Aqueous Solution by Oxidized and Solvent-Extracted Residual Coal,” Energy Source 20, 821–830 (1998). Asyhar, R., H. Wichmann, M. Bahadir and H. K. Cammenga, “Equilibrium Adsorption Studies of Activated Coke Towards Phenol and 4-Nitrophenol,” Fres. Environ. Bull. 11, 270–277 (2002). Banat, F. A., B. Al-Bashir, S. Al-Asheh and O. Hayajneh, “Adsorption of Phenol by Bentonite,” Environ. Poll. 107, 391–398 (2000). Brunauer, S., P. H. Emmett and E. Teller, “Adsorption of Gases in Multimolecular Layers,” J. Am. Chem. Soc. 60, 309–319 (1938). Calace, N., E. Nardi, B. M. Petronio and M. Pietroletti, “Adsorption of Phenols by Paper Mill Sludges,” Environ. Poll. 118, 315–319 (2000).

|


|

Cherifi, H., S. Hanini and F. Bentahar, “Adsorption of Phenol from Wastewater Using Vegetal Cords as a New Adsorbent,” Desalination 244, 177–187 (2009). Dieckmann, M. S. and K. A. Gray, “A Comparison of the Degradation of 4-Nitrophenol via Direct and Sensitized Photocatalysis in TiO2 Slurries,” Water Res. 30, 1169–1183 (1996). Eaton A. D., L. S. Clesceri and A. E. Greenberg, “Standard Methods for the Examination of Water and Wastewater,” 19th ed., APHA-AWWA-WEF, Washington (1995). El-Geundi, M. S., “Adsorbents for Industrial Pollution Control,” Adsorpt. Sci. Technol. 15, 777–787 (1997). Gaikwad, R. W., “Adsorption of Detergent from Aqueous Solution on Fly Ash,” J. IPHE 1, 5–7 (2002). Haghighi-Podeh, M. R., S. K. Bhattacharya and M. Qu, “Effects of Nitrophenols on Acetate Utilizing Methanogenic Systems,” Water Res. 29, 391–399 (1995). Halouli, Kh. A. and N. M. Drawish, “Effects of pH and Inorganic Salts on the Adsorption of Phenol from Aqueous Systems on Activated Decolourising Charcoal,” Sep. Sci. Tech. 30, 3313–3324 (1995). Ho, Y. S. and G. McKay, “Pseudo Second Order Model for Sorption Processes,” Process Biochem. 34, 451–465 (1999). Juang, R. S., L. Su-Hsia and T. Kung-Hsuen, “Sorption of Phenols from Water in Column Systems Using Surfactant-Modified Montmorillonite,” J. Colloid Interface Sci. 269, 46–52 (2004). Khalid, N., S. Ahmad, A. Toheed and J. Ahmad, “Potential of Rice Husks for Antimony Removal,” Appl. Rad. Isotop. 52, 30–38 (2000). Kumar, P. S., N. A. Kumar, R. Sivakumar and C. Kaushik, “Experimentation on Solvent Extraction of Polyphenols from Natural Waste,” J. Mater. Sci. 44, 5894–5899 (2009). Lagregren, S., “About the Theory of So-Called Adsorption of Soluble Substances,” Kungl. Sven. Veten. Akad. Handl. 24, 1–39 (1898). Langmuir, I., “Chemical Reactions at Low Pressures,” J. Am. Chem. Soc. 27, 1139–1143 (1915). Lua, A. C. and Q. Jia, “Adsorption of Phenol by Oil-Palm-Shell Activated Carbons,” Adsorption 13, 129–137 (2007). Mahajan, S. P., “Pollution Control in Processes Industries,” Tata McGraw-Hill, New Delhi (1994), pp. 115–125. Nagda, G. K., A. M. Diwan and V. S. Ghole, “Potential of Tendu Leaf Refuse for Phenol Removal in Aqueous Systems,” Appl. Ecol. Environ. Res. 5, 1–9 (2007). Ozkaya, B., “Adsorption and Desorption of Phenol on Activated Carbon and a Comparison of Isotherm Models,” J. Hazard. Mater. 129, 158–163 (2006). Rengaraj, S., M. Seuny-Hyeon and R. Sivabalan, “Agricultural Solid Waste for the Removal of Organics: Adsorption of Phenol from Water and Wastewater by Palm Seed Coat Activated Carbon,” Waste Manage. 22, 543–548 (2002). Shawabkeh, R. A. and E. S. M. Abu-Namesh, “Absorption of Phenol and Methylene Blue by Activated Carbon from Pecan Shells,” Colloid J. 69, 355–359 (2007). Srihari, V. and D. Ashutosh, “Study on Phenol Sorption of Two Homogenized Industrial-Wastewater Blend Activated Carbon,” J. Ecol. Environ. Cons. 10, 443–445 (2004). Srihari, V. and A. Das, “The Kinetic and Thermodynamic Studies of Phenol-Sorption Onto Three Agro-Based Carbons,” Desalination 225, 220–234 (2008). Srivastava, V. C., M. M. Swamy, I. D. Mall, B. Prasad and I. M. Mishra, “Adsorptive Removal of Phenol by Bagasse Fly Ash and Activated Carbon: Equilibrium, Kinetics and

|


|

Thermodynamics,” Coll. Surf. A: Physicochem. Eng. Asp. 272, 89–104 (2006). Tor, A., Y. Cengeloglu and M. Ersoz, “Increasing the Phenol Adsorption Capacity of Neutralized Red Mud by Application of Acid Activation Procedure,” Desalination 242, 19–28 (2009). Vasanth, K. K., “Adsorption Isotherms for Basic Dye on to Low Cost Adsorbents,” Res. J. Chem. Environ. 7, 72–77 (2003). Zakaria, Z. A., M. Suratman, N. Mohammed and W. A. Ahmad, “Chromium (VI) Removal from Aqueous Solution by Untreated Rubber Wood Sawdust,” Desalination 244, 109–121 (2009).

Manuscript received October 9, 2009; revised manuscript received March 30, 2010; accepted for publication April 6, 2010.

|


|

291

|