Adsorption of Anionic-Azo Dye from Aqueous Solution by

Article pubs.acs.org/IECR

Adsorption of Anionic-Azo Dye from Aqueous Solution by Lignocellulose-Biomass Jute Fiber: Equilibrium, Kinetics, and Thermodynamics Study Aparna Roy, Sumit Chakraborty, Sarada Prasad Kundu, Basudam Adhikari, and S. B. Majumder* Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal 721302, India S Supporting Information *

ABSTRACT: The present investigation describes the evaluation of feasibility of lignocellulosic-biomass jute fiber (JF) toward adsorptive removal of anionic-azo dye from aqueous solution. Batch studies illustrated that dye uptake was highly dependent on different process variables, pH, initial dye concentration of solution, adsorbent dosage, and temperature. Further, an attempt has been taken to correlate these process variables with dye absorption and was optimized through a full-factorial central composite design (CCD) in response surface methodology (RSM). Maximum adsorption capacity (29.697 mg/g) under optimum conditions of variables (pH 3.91, adsorbent dose 2.04 g/L, adsorbate concentration 244.05 mg/L, and temperature 30 °C), as predicted by RSM, was found to be very close to the experimentally determined value (28.940 mg/g). Exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. Equilibrium adsorption data were highly consistent with Langmuir isotherm yielding R2 = 0.999. Kinetic studies revealed that adsorption followed pseudo second-order model regarding the intraparticle diffusion. Activation parameters for the adsorption process were computed using Arrhenius and Eyring equations. Maximum desorption efficiency of spent adsorbent was achieved using sodium hydroxide solution (0.1 M).

1. INTRODUCTION Recently, a progressive increase in industrialization and urbanization has substantially enhanced the aquatic environmental pollution by the discharge of industrial effluents containing highly toxic organic and inorganic pollutants. Various dye producing and consuming industries such as textile, leather, paper, petroleum, printing, cosmetics, paint, rubber, plastic, food, and pharmaceutical industries generate a huge volume of toxic wastewater contaminated with highly colored synthetic dyes.1 The presence of dyes in aquatic bodies increases chemical oxygen demand, color contents, dissolved and suspended solids, impedes light penetration into water, interferes photosynthesis of aquatic plants, hinders the growth of microbes, creates microtoxicity to fish and other organisms, etc. Above all, the carcinogenic and mutagenic nature of the dyes is detrimental to human beings.2 The removal of toxic dyes from wastewater emerges as a major challenge due to their highly stable and complex aromatic structures, which makes them difficult to degrade by conventional oxidation or biodegradation methods. Several traditional methods, for example, coagulation and flocculation, ozonation, membrane-filtration processes, ionexchange, chemical precipitation, etc., are known for the treatment of dye-containing effluents. However, these techniques scuffle with several disadvantages such as inefficiency at lower concentration, high energy and chemical reagents requirement, generation of toxic sludge or other wastes as byproduct that need careful treatment and disposal, high capital and operational costs, etc.3 Accordingly, adsorption technology by activated carbon gained approval for dye removal from wastewater. Nevertheless, their use is restricted due to the high cost of adsorbent-grade carbon and difficulties associated with regeneration and disposal of the spent carbon.4 These constraints have rendered the search for alternative adsorbents from © 2012 American Chemical Society

biological origin, which would be inexpensive, simple, highly abundant, efficient, and viable for regeneration. Several natural fiber-based materials, for example, sugar cane bagasse,1 kenaf core,2 Luffa cylindrical,3 kapok,4 palm kernel,5 rice husk,6 etc., have been explored for this purpose. Jute, a highly abundant lignocellulosic bast fiber mainly found in India, Bangladesh, China, and Myanmar, has extensively been used for making geotextiles and composites. Jute fiber (JF), in its chemically modified form, has also been investigated for the removal of heavy metal ions like nickel, zinc, and copper from aqueous solution.7 Currently, significant effort is being undertaken to explore the possible utilization of this locally produced lignocellulosic fiber for socio-economic development. Considering all of these facts mentioned above, the present investigation reports a cost-effective removal of anionic-azo dye from aqueous solution using very cheap, renewable, and abundantly available JF. However, to the best of our knowledge, JF has not been explored toward the dye removal so far. A systematic investigation of the effects of initial pH, adsorbent dose, temperature, and initial dye concentration of solution on dye uptake and plausible dye−adsorbent interaction during adsorption in the single component system was explored in this study. Further, the process variables for dye adsorption by JF were optimized using RSM. Received: Revised: Accepted: Published: 12095

June 27, 2012 August 22, 2012 August 26, 2012 August 27, 2012 dx.doi.org/10.1021/ie301708e | Ind. Eng. Chem. Res. 2012, 51, 12095−12106

Industrial & Engineering Chemistry Research

Article

Table 1. Experimental Design for Batch Adsorption Study set no. 1 2 3 4

control parameters

variable parameters

initial CR concentration, 50 mg/L; temperature, 303 K; adsorbent dosage, 10 g/L; agitation time, 3 h; agitation speed, 140 rpm solution pH, ∼6.2; temperature, 303 K; adsorbent dosage, 10 g/L; agitation time, 3 h; agitation speed, 140 rpm initial CR concentration, 50 mg/L; solution pH, ∼6.2; temperature, 303 K ; agitation time, 3 h; agitation speed, 140 rpm initial CR concentration, 50 mg/L; solution pH, ∼6.2; adsorbent dosage, 10 g/L; agitation time, 3 h; agitation speed, 140 rpm

2. MATERIALS AND METHODS 2.1. Congo Red (CR) and Stock Solution. Congo Red, used as a model anionic-azo dye, was purchased from Loba Chemicals, India. A stock solution of CR in deionized water (1000 mg/L) was prepared. The working solutions were prepared by diluting the stock solution. 2.2. Preparation and Characterization of Adsorbent. JF (extracted from Corchorus olitorius), collected from Gloster Jute Mill, India, was cut into ∼5 mm of length. Per gram of chopped JF was washed with 100 mL of deionized water to remove any adhering substances and dust. The fibers were then air-dried at room temperature for 24 h followed by oven drying at 55 °C for 24 h. 2.2.1. FTIR Study. FTIR spectroscopic study (Thermo Nicolet, Nexus 870) of JF before and after CR adsorption was done to determine the plausible involvement of functional groups on the fiber surface in the dye sorption process. 2.2.2. SEM Analysis. The surface topography of fibers before and after CR adsorption was investigated using a scanning electron microscope (TESCAN VegaLSV SEM). 2.2.3. Point of Zero Charge (PZC) Determination. The PZC of JF was determined in batch mode adopting the titration method described elsewhere.8 JF (1 g) was added to KNO3 solutions (0.1 M, 100 mL) of different initial pH’s. After equilibration of the suspensions for 24 h at 303 K, the final pH values of the supernatant were measured. The surface charges (Q, C/m2) of JF at different pH values were calculated using eq 1: Q=

(Ca − C b − [H+] + [OH−]) F M S

qe =

solution pH: 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 initial concentration of CR (mg/L): 10, 50, 100, 150, 200, 250, 300 adsorbent dosage (g/L): 2, 4, 6, 8, 10 temperature (K): 303, 313, 323

(Co − Ce)V m

(2)

where C o and C e are the initial and equilibrium dye concentrations (mg/L), respectively, V is the volume of solution (L), and m is the adsorbent dosage (g). Four sets of experiments were conducted for complete investigation of CR adsorption on JF, and the details are given in Table 1. The results presented here were the mean of triplicate determinations, and the standard deviations were calculated to be within the range of 0.1−1.5. 2.4. Batch Desorption Studies. For the desorption study, the JF was loaded with 50 mg/L of dye solution, washed gently with water to remove any unadsorbed dye, and dried. The spent adsorbent (10 g/L), suspended in eluting solvents (viz., neutral deionized water, 0.1 M HCl, 0.1 M NaOH, and 0.1 M CH3COOH) was agitated (130−140 rpm) at 303 K for 3 h. The desorbed CR concentration was quantified using a UV/ visible spectrophotometer. Desorption efficiency of the adsorbent can be calculated as:2 desorption efficiency (%) amount of dye desorbed from adsorbent = × 100 amount of dye adsorbed on adsorbent

(3)

2.5. Experimental Design and Statistical Analysis for RSM. Four-factor CCD of RSM was applied to evaluate optimum process conditions and the individual and combined effects of the independent test variables on the response (adsorption capacity). RSM is an efficient statistical method that uses quantitative data of appropriate experiments for designing experiments, analyzing the relationships between the response and the independent variables, developing response surface models, and ultimately optimizing the process variables to achieve maximum response.9 A second-degree polynomial equation approximated in this study for evaluating the effect of each independent variable on the response is as follows:10

(1)

where Ca, Cb, [H+], and [OH−] are the concentrations of acid, base, H+, and OH−, respectively, M is the weight of added JF (g/ L), F is Faraday’s constant, and S is the specific surface area of JF. The specific surface area of JF, determined using the Methylene Blue adsorption method, was found to be 16.41 m2/g (Supporting Information). The surface charge versus pH curve was drawn. The intersecting point of the resulting curve corresponds to the pHPZC of JF. 2.3. Batch Adsorption Studies. Batch adsorption studies were performed by suspending JF (2−10 g/L) in CR solutions of different initial concentrations (10−250 mg/L) having different initial pH values (3−12). The suspensions were continuously agitated (140 rpm) at constant temperature (303−323 K), and aliquots were drawn at regular intervals of time until equilibrium was reached. The concentration of CR in the supernatant was estimated by a UV/visible spectrophotometer (Perkin-Elmer, Lambda 750) at 497 nm, and the amount of CR adsorbed by JF from aqueous solution at equilibrium (qe, mg/g) was calculated as follows:5

n

y = βo +

n

n−1 n

∑ βi xi+ ∑ βiixi2+ ∑ ∑ βijxixj i=1

i=1

i=1 i=1

(4)

where y is the response, xi and xj are independent variables, βo is the constant coefficient, βi is the ith linear coefficient, βii is the quadratic coefficient, βij is the ijth interaction coefficient, and n is the number of independent process variables. In this study, four important process parameters, initial pH of solution (A), adsorbent dose (B), initial dye concentration (C), and temperature (D), that predominantly affected the extent of adsorption by JF were identified as independent test variables (Table 2). The experimental data obtained from 30 experiments (Table S1 in the Supporting Information), including eight for factorial design, six for axial points, and six repetitions at central 12096

dx.doi.org/10.1021/ie301708e | Ind. Eng. Chem. Res. 2012, 51, 12095−12106


Article

3.2. Effect of pH. The initial pH of the dye solution plays a significant role in the biosorption process of CR by JF. The dye removal was increased as the initial pH of the solution was increased from 3 to 4, but decreased drastically as the pH was further increased from 4 to 12 (Figure 3b). The adsorption capacity of adsorbent is affected by the change in solution pH due to the protonation and deprotonation of the active functional groups of adsorbent and adsorbate.13 CR, having a pKa value at 4.5−5.5,14 can exist in both anionic form at basic pH and in cationic form at acidic pH (Figure 3c). Lower adsorption at pH 3 is due to the interionic repulsion between the positively charged dye molecule and adsorbent. At pH 4, the nitrogen atoms and sulfonate groups of the dye molecules become protonated,15 whereas at pH > 3.6 the surface charge of adsorbent remains negative because the pHPZC of JF is 3.6. This results in an electrostatic attraction between positively charged dye molecule and negatively charged adsorbent and leads to the maximum removal of CR (86%) by JF at pH 4. On the contrary, as pH increases from 4 to 12, the negative surface charge of JF hinders the adsorption of CR by electrostatic repulsion between deprotonated dye molecules and negatively charged JF; consequently, the dye removal decreases to 50.8%. A considerable amount of adsorption in this range indicates that the electrostatic mechanism is not the only sole mechanism for dye adsorption; the physical forces are also responsible for adsorption.16 3.3. Effect of Adsorbent Dose and Initial Dye Concentration. Figure 4a represents the effect of adsorbent dosage on adsorption capacity of JF for CR. The qe of JF was decreased at higher adsorbent dosage due to significant unsaturation of adsorption sites at constant dye concentration and volume.17 However, the removal of CR increases from 17.89% to 55.74% with an increase in the amount of JF from 2 to 10 g/L. The qe of JF for CR enhances from 2.8 to 9.0, with increasing initial dye concentration from 10 to 250 mg/L (Figure 4b). As the dye concentration in aqueous solution increases, more dye molecules become available for adsorption on the adsorbent surface. It is probably due to the effect of concentration gradient between dye in solution and dye on JF surface at higher dye concentration, producing the main driving force for mass transfer during adsorption process.18 However, the dye removal percent at equilibrium reduces from 85 to 30 as the initial dye concentration increases from 10 to 250 mg/L. 3.4. Effect of Temperature. It can be perceived from Figure 4c that the temperature adversely affects the removal efficacy of JF for CR. The adsorption capacity of JF diminishes from 2.818 to 1.984 mg/g as the temperature rises from 303 to 323 K. This may be due to the fact that at higher temperature the solute molecules show an inclination to escape from the solid phase and re-enter the liquid phase due to increased mobility.3 Thermodynamic parameters were calculated to evaluate the thermodynamic feasibility of the process using the following equations:19

Table 2. Independent Process Variables and Their Levels for Central Composite Design variable level variable pH adsorbent dosage (g/L) initial dye concentration (mg/L) temperature (°C)

symbol

−2 (−α)

−1

0

+1

+2 (+α)

A B C

3 2 50

5 4 100

7 6 150

9 8 200

11 10 250

D

30

35

40

45

50

point, were used to validate the model of reaction process. The experimental results were analyzed using Design-Expert software version 8.0.6.2 (Stat-Ease, U.S.).

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent. 3.1.1. FTIR Study. Figure 1 illustrates the FTIR spectra of JF before and after CR

Figure 1. FTIR spectra of JF (a) before and (b) after dye adsorption.

adsorption. A characteristic broad band in the range of 3200− 3600 cm−1 due to hydrogen-bonded −OH stretching was observed for both the virgin and the CR treated JF. Absorption at 2910 cm−1 represents −CH stretching vibration of methyl and methylene groups in cellulose and hemicellulose. Carbonyl (>CO) stretching for ester groups in hemicellulose of the fiber appeared at1738 cm−1.11 The absorbance maxima ratio of the −OH group (3380 cm−1) to the internal standard peak (2920 cm−1) of methyl and methylene groups of JF was found to decrease. This clearly indicates the participation of the −OH group for the CR adsorption. 3.1.2. SEM Analysis. The SEM micrographs showed that the surface of pristine JF is smooth, but an uneven and irregular surface morphology was observed for spent JF (Figure 2a and b). Furthermore, it was observed that the natural golden color of JF had become red after the adsorption process, which clearly indicates that the surface of the fiber was covered with the dye molecules (Figure 2c and d). 3.1.3. Determination of Point of Zero Charge (PZC). Figure 3a presents the result of the experimental measurement of PZC study. The resulting curve has an intersection at a pH of around 3.6, at which the surface charge is zero. This result indicates that the PZC of JF is approximately 3.6. A similar phenomenon was also observed by Bismarck et al.12

ln K C =

ΔS o ΔH o − R RT

(5)

ΔGo = ΔH o − T ΔS o

(6) −1

where ΔG is the Gibbs free energy change (kJ mol ), ΔH is the in enthalpy change (kJ mol−1), ΔSo is the change in entropy (J mol−1 K−1), KC is the distribution coefficient of adsorption, and T is the adsorption temperature (K). ΔHo and ΔSo were o

12097

o



Article

Figure 2. SEM images of (a) pristine and (b) dye adsorbed JF; photographs of (c) pristine and (d) dye adsorbed JF; and (e) schematic representation of Congo Red−JF cellulose unit interaction.

calculated from the slope and intercept of the linear plot of ln KC versus 1/T. Once these two parameters were obtained, ΔGo was determined. The negative value of ΔHo confirmed the exothermic nature of CR adsorption by JF. The values of ΔGo were negative at all temperatures, demonstrating the thermodynamic feasibility and the spontaneity of the adsorption process. The adsorption was associated with a negative value of ΔSo (Table 3). 3.5. Adsorption Isotherm. The adsorption isotherms, Langmuir and Freundlich, were constructed using experimental equilibrium data obtained from the study of CR adsorption on JF at initial dye concentrations of 10−250 mg/L at pH 6.2. 3.5.1. Langmuir Isotherm. Langmuir theory presumes that adsorption is limited to the formation of monolayer coverage of adsorbate on homogeneous adsorbent surface. The linearized Langmuir equation is given below:20 ⎛ 1 ⎞1 1 1 ⎟⎟ = ⎜⎜ + qe qmax ⎝ bqmax ⎠ Ce

RL =

1 1 + bCo

(8)

A favorable condition for adsorption occurs only when RL values are within the range 0 < RL < 1.21 RL values obtained for different initial dye concentrations were well within the defined range (0.2960−0.0165) (Table 3). This suggests that the adsorption of dye onto JF is a favorable process under the conditions used for the experiments. 3.5.2. Freundlich Isotherm. The Freundlich model is based on the assumption that multilayer adsorption occurs on a heterogeneous adsorption surface containing unequally available sites of different adsorption energies and is given by the relation:22 log qe = log KF +

1 log Ce n

(9)

where KF is the Freundlich constant (mg/g), and n is the heterogeneity factor. The graphical presentation of the Freundlich isotherm model is expressed in Figure 5b. The model parameters and error functions reflected that this isotherm model showed poorer fit to the experimental data as compared to Langmuir isotherm under the studied initial concentration range of dye solution (Table 3). 3.6. Adsorption Kinetics. The kinetic behavior of CR adsorption onto JF was studied with different initial dye concentrations. It can be observed from Figure 4 that the dye adsorption by JF from its aqueous solution increases with increasing contact time, irrespective of adsorbent dose, initial dye concentration, and temperature. At the initial stage of the adsorption process, the rate of dye adsorption was very rapid and reaches equilibrium within 25−35 min of contact. The phenomenon of quick adsorption of pollutants to be removed from wastewater by adsorbent would be attractive and advantageous for practical application purpose. To study

(7)

where qmax is the maximum monolayer capacity of adsorbent (mg/g), and b is the Langmuir constant that represents the energy of adsorption process (L/mg). qmax and b were computed from the slopes and intercepts of the straight line obtained from the plot (Figure 5a). The theoretical monolayer adsorption capacity of JF for CR was calculated to be 8.12 mg/g. The adsorption of CR on JF fit very well to the Langmuir model having a correlation coefficient of 0.999 and a small χ2 value (Table 3). Thus, the Langmuir isotherm model described the experimental data of adsorption of CR by JF precisely. The essential feature of the Langmuir adsorption isotherm can be expressed in terms of a dimensionless constant, separation factor (RL). The following equation defines RL: 12098



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Figure 3. (a) Point of zero net charge of jute fiber; (b) dye adsorption onto jute as a function of pH; and (c) structure of Congo Red at (i) pH > 4.5 and (ii) pH < 4.5.

quite agreeable with that obtained from experimental findings, which confirms the applicability of the pseudo second-order equation to the adsorption process (Table 4). This indicated that for the studied cases and conditions, the adsorption kinetics of CR onto JF can be accurately described by the pseudo secondorder rather than the pseudo first-order kinetic model. 3.7. Intraparticle and Liquid Film Diffusion Model. During adsorption, the migration of dye molecules from bulk solution through liquid film to the exterior surface of adsorbent and the intraparticle diffusion into the interior of adsorbent may play an important role in determining the rate of the adsorption process. To predict the potential rate-controlling step, the adsorption dynamics of CR on JF was investigated utilizing intraparticle and liquid film diffusion models (eqs 12 and 13):17

adsorption kinetics of CR adsorption on JF, pseudo first-order and pseudo second-order kinetic models are used. 3.6.1. Pseudo First-Order Model. The linearized form of the Lagergren pseudo first-order rate equation is given by:22 log(qe − qt ) = log qe −

k1 t 2.303

(10)

where qt is the amount of adsorbed CR at time t (mg/g), and k1 is the rate constant of pseudo first-order adsorption (min−1). The linear plot of log(qe − qt) versus t (Figure 5c) provides a poor R2 value, a large χ2 value, and high disparity between the experimental adsorption capacity and the calculated value (Table 4). This indicated that the first-order model had very limited applicability in adsorption kinetics of CR on JF.17 3.6.2. Pseudo Second-Order Model. The linear form of the pseudo second-order kinetic model is expressed as:22 t 1 1 = + t 2 qt qe k 2qe

(11)

qt = k idt 0.5

(12)

ln(1 − F ) = −k fdt

(13)

where kid (mg g−1 min−0.5) and kfd (min−1) are intraparticle and liquid film diffusion rate constants, respectively, F = qt/qe. The multilinearity of the plots, qt against t0.5 (Figure 6a) for intraparticle diffusion, implied the complexity of the CR adsorption by JF. The short initial linear portion of plots described the external mass transfer, in which the adsorbate migrated through solution to the external surface of adsorbent via

where k2 (g/(mg min)) is the rate constant of second-order adsorption. When the experimental data for the studied range of initial dye concentration were analyzed by the pseudo secondorder model, the straight lines obtained by plotting t/qt versus t (Figure 5d) generated R2 values close to 1 and negligible χ2 values (Table 4). The calculated adsorption capacity (qe,cal) was 12099



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Figure 4. Effect of (a) adsorbent dose, (b) initial dye concentration of solution ((left-facing triangle) 10 mg/L, (■) 50 mg/L, (●) 100 mg/L, (▲) 150 mg/L, (◆) 200 mg/L, (▼) 250 mg/L), and (c) temperature on dye adsorption by JF.

Table 3. Thermodynamic Parameters, Isotherm Parameters, and Separation Factors (R) for Adsorption of CR onto Jute Fiber thermodynamic parameters −1

temperature (K)

ΔG° (kJ mol )

ΔH° (kJ mol−1)

ΔS° (J mol−1 K−1)

303 313 323

−24.53 −24.25 −23.97

−33.14

−28.39

isotherm model parameters Langmuir isotherm temp (K)

qe (mg/g)

qmax (mg/g)

303

7.382

8.116

b (L/mg)

Freundlich isotherm χ2

R2

K (mg/g)

0.2378 0.9998 0.1507 2.993 RL values at different initial dye concentrations

1/n

R2

χ2

0.597

0.9648

16.9851

initial dye concentration (mg/L) temp (K)

10

50

100

150

200

250

303

0.2960

0.0776

0.0404

0.0273

0.0206

0.0165

3.8. Activation Parameters. The activation energy (Ea, kJ mol−1) for adsorption of CR by JF was evaluated using the pseudo second-order rate constant k2 in the Arrhenius equation (eq 14).

macropore diffusion at an early stage of adsorption. On the other hand, the long successive linear portion may be attributed to the gradual adsorption stage with intraparticle diffusion via micropore.17 The plots ln(1 − F) versus t, for liquid film diffusion (Figure 6b), were linear, having R2 and intercepts (Ifd) within the range from 0.9694 to 0.8848 and from −0.813 to −1.411, respectively (Table 4). The deviation of the straight lines from the origin in the plots suggested that the liquid film diffusion model cannot describe the present adsorption system accurately.

ln k 2 = ln A −

Ea RT

(14)

where A is the Arrhenius constant, R is the universal gas constant, and T is the temperature (K). According to the literature, Ea < 40 kJ mol−1 generally signifies the predominance of physisorption, and a higher Ea value designates an activated chemical adsorption 12100



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Figure 5. (a) Langmuir and (b) Freundlich isotherm plots, (c) pseudo first-order, and (d) pseudo second-order kinetic plots for CR adsorption using JF.

Table 4. Kinetic Parameters for Adsorption of CR onto Jute Fiber at Different Initial Dye Concentrations (mg/L) initial dye concentration (mg/L) kinetic model pseudo first-order

pseudo second-order

intraparticle diffusion

liquid film diffusion

model parameter

50

100

150

200

250

qe,exp (mg/L) qe,cal (mg/L) k1 (g mg−1 min−1) R2 χ2 qe,cal (mg/L) k2 (g mg−1 min−1) R2 χ2 kid1 (mg g−1 min−0.5) kid2 (mg g−1 min−0.5) Iid1 Iid2 R2 kfd (min−1) × 10−3 Ifd R2

2.818 1.471 0.0368 0.9584 3.353 2.804 0.1341 0.9998 0.001 0.1432 0.0166 1.9851 2.3906 0.9993 −16.02 −1.411 0.9584

3.507 1.909 0.0467 0.9555 1.337 3.536 0.0763 0.9999 0.002 0.1828 0.0275 2.9193 3.2838 0.9827 −23.01 −1.901 0.9555

5.390 2.217 0.0416 0.9693 11.452 5.441 0.0272 0.9997 0.002 0.4989 0.0534 3.3242 4.6781 0.9995 −18.09 −0.877 0.9694

6.227 2.759 0.0345 0.9196 13.357 6.305 0.0169 0.9984 0.002 0.5304 0.0630 3.9713 5.2548 0.9862 −15.00 −0.813 0.9196

7.382 1.879 0.0352 0.8847 12.653 7.123 0.0156 0.9989 0.001 0.5487 0.0462 4.6540 5.9968 0.9497 −15.26 −1.328 0.8848

process. The value of Ea (31.11 kJ mol−1) obtained from the linear plot of ln k2 versus 1/T indicated that the studied adsorption may be a physisorption process. The enthalpy, ΔH⧧ (measure of the energy barrier that must be overcome by reacting molecules), and entropy of activation, ΔS⧧ (indication of associative or dissociative mechanism of adsorption), were determined using the Eyring equation (eq 15):

ln

⎛ k k2 ΔS ⧧ ⎞ ΔH ⧧ ⎟− = ⎜ln B + T R ⎠ RT ⎝ h

(15)

where kB is the Boltzmann constant (J K−1), and h is Planck’s constant (J s).The free energy of activation (ΔG⧧) was also determined from the equation: ΔG⧧ = ΔH⧧ − TΔS⧧. The values of ΔH⧧ and ΔS⧧ calculated from the slope and intercept of the plot of ln(k2/T) versus 1/T were −28.47 kJ mol−1 and −72.88 J 12101



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Figure 6. Plots of (a) intraparticle and (b) liquid film diffusion model for CR adsorption by JF at different initial dye concentrations ((■) 50 mg/L, (●) 100 mg/L, (▲) 150 mg/L, (◆) 200 mg/L, (▼) 250 mg/L).

mol−1 K−1, respectively. The negative values of both ΔH⧧ and ΔS⧧ implied that the adsorption of CR onto JF is exothermic in nature and an associative mechanism.23 The ΔG⧧ controls the rate of the reaction. As the ΔG⧧ value decreases, the reaction rate increases. The ΔG⧧ values were determined to be −6.39, −5.66, and −4.93 kJ mol−1 at temperatures of 303, 313, and 323 K, respectively. Small and negative values of ΔG⧧ suggested that the adsorption was spontaneous.24 3.9. Response Surface Modeling. 3.9.1. Model Verification and Development of Regression Model Equation. CCD was used to define the relationships between response (adsorption capacity, mg/g) and individual process variables. Among linear, two-factor interaction (2FI), quadratic, and cubic polynomials, the quadratic model was considered to be most suitable for this process by Design Expert software as it exhibited a lower standard deviation along with higher R2 values and insignificant lack of fit (Table 5).25

The influence of the independent process variables on response factor was estimated by the coefficients of eq 16. Both magnitude and sign are vital for the regression coefficients, as the earlier indicates the importance of the variables on the response factor and the later decides its effect direction. The constant, 8.804, was independent of any factor and interaction of factors. A positive sign of the coefficients indicated a synergistic effect, while a negative sign denoted an antagonistic effect on response.26 The linear terms (except C), interaction terms (A × C), (A × D), (B × C), (B × D), and second-order terms A2 and C2 had a negative effect on response. Hence, the response will decrease as these terms increase, whereas the terms (A × B), (C × D), B2, and D2 had a positive influence, which indicated that with an increase of these factors there will be an increase in adsorption capacity. 3.9.2. Analysis of Variance (ANOVA). The adequacy of the developed quadratic model was justified performing ANOVA. The model and model terms are regarded as significant only when the values of Prob > F are less than 0.0500. The terms with higher F-value have greater importance to produce an effect on response. The model P-value and F-value were F)

remarks

model A B C D AB AC AD BC BD CD A2 B2 C2 D2

526.7253 19.5334 128.8800 287.6976 34.63833 6.3307 1.5634 4.0792 0.7517 0.2284 0.1443 0.5134 25.1361 3.2498 11.2786

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1

37.6232 19.5334 128.8800 287.6976 34.6383 6.3308 1.5634 4.0791 0.7517 0.2284 0.1443 0.5134 25.1361 3.2498 11.2786

30.5382 15.8549 104.6099 233.5197 28.1154 5.1386 1.2690 3.3109 0.6102 0.1854 0.1172 0.4167 20.4025 2.6378 9.1546

Adsorption of Anionic-Azo Dye from Aqueous Solution by

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