Equilibrium Modeling, Kinetic, and Thermodynamic Studies on

Article pubs.acs.org/IECR

Equilibrium Modeling, Kinetic, and Thermodynamic Studies on Adsorption of Pb(II) by a Hybrid Inorganic−Organic Material: Polyacrylamide Zirconium(IV) Iodate Nafisur Rahman* and Uzma Haseen Department of Chemistry, Aligarh Muslim University, Aligarh - 202002, Uttar Pradesh, India S Supporting Information *

ABSTRACT: The sorption of Pb(II) onto polyacrylamide zirconium(IV) iodate, a hybrid inorganic−organic material, has been studied through the batch equilibrium technique. The experimental equilibrium data have been analyzed using Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich isotherm models. The accuracy of the fit of isotherm models to the experimental equilibrium data was assessed using error analysis.The Temkin model yields the best fit between experimental and simulated data. The kinetic data obtained at different concentrations have been analyzed using pseudo-first-order, pseudosecond-order, and intraparticle diffusion models. The experimental data obey the pseudo-second-order kinetic model. The plot of qt versus t1/2 suggests that intraparticle diffusion is not the sole rate-controlling step; some degree of boundary layer diffusion is also involved. The calculated thermodynamic parameters ΔG°, ΔH°, and ΔS° indicate that the adsorption of Pb(II) onto the sorbent is a spontaneous, endothermic, and physical sorption process. membrane separation.17 The choice of treatment technology depends on the effluent characteristics such as concentration of Pb(II), pH, temperature, biological oxygen demand, and the permissible limit set by the government agencies. The adsorption method is considered quite attractive in terms of high efficiency, ease of handling, and availability of different adsorbents for removal of Pb(II) from aqueous systems.18,19 Various adsorbent materials have been studied for their ability to remove Pb(II) from water samples. Nanocomposite of Chitosan with methyl cellulose and nanochitosan with kaolin clay have been used as adsorbents to remove Pb(II) ions from synthetic wastewater.20 The adsorption of Pb(II) on both adsorbents obeyed the Freundlich isotherm.The decontamination of Pb(II) ions from aqueous media has been investigated using styrene-divinylbenzene copolymer beads as adsorbent.21 The adsorption data obeyed Langmuir, Freundlich and Dubinin−Radushkevich isotherms over the lead concentration range of 1.207 × 10−3 to 2.413 × 10−2 molL−1. Jiang and coworkers22 have studied the sorption of Pb(II) ions on amorphous and crystalline zirconium phosphate particles. The sorption isotherms onto both sorbents were well-correlated by the Langmuir model. The sorption of Pb(II) ions on NKF-6 zeolite was reported to be dominated by ion-exchange or outersphere complexation at low pH values and an inner-sphere complexation at higher pH values.23 In addition, the effect of pH on the adsorption of Pb(II) ions on zeolite was also investigated.24Moreover, some minerals and nanomaterials have also been used as sorbents for removal of Pb(II) from aqueous media. Minerals such as Na-rectorite25 and diatomite26 have been tested for their potential application in the removal of

1. INTRODUCTION The excessive release of heavy metals into the surface and groundwater is a major concern for developing countries. Heavy metal pollution is a widespread environmental problem because of their high toxicity and mobility.1−5 Heavy metals that have been identified in the polluted environment include As(III/V), Cu(II), Cd(II), Pb(II),Cr(III/VI), Ni(II), Hg(II) and Zn(II). It is well-documented that lead is one of the contaminants of industrial wastewater that is generated by several industrial processes and is also a leftover of pesticide and fertilizer industries. Human exposure to lead can occur via food, water, air, soil, and dust.6 Food, including drinking water, is the major source of exposure to lead for the majority of the population. Pb(II) interferes with a number of body functions and primarily affects the central nervous, hematopoetic, hepatic, and renal systems, producing serious disorders.7 Chronic toxicity due to lead, the symptoms of which include delirium, lack of coordination, convulsions, paralysis, coma, and ataxia, is much more common and occurs at blood lead levels of about 40−60 μg dL−1.8 Renal dysfunction occurs at a blood lead level greater than 60 μg dL−1, but damage may also occur at lower levels (10 μg dL−1).9 The World Health Organization10 has recommended 0.01 mg L−1 as a maximum permissible limit for Pb(II) in drinking water. According to the Indian Standard Institution, the tolerance limit of lead in drinking water is 0.05 mg L−1 and in land surface water is 0.10 mg L−1.11 Therefore, water streams containing Pb(II) must be treated before being discharged into the water resources. Strict environmental protection legislation and public environmental concerns have created a growing interest in the development of conventional treatment processes to remove heavy metals from industrial wastewater. Various treatment technologies have been employed in the removal of heavy metals from aqueous systems such as electrochemical treatment,12,13 chemical precipitation,14 reverse osmosis, 15 ion exchange,16 and © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8198

January 22, 2014 April 17, 2014 April 24, 2014 April 24, 2014 dx.doi.org/10.1021/ie500139k | Ind. Eng. Chem. Res. 2014, 53, 8198−8207

Industrial & Engineering Chemistry Research

Article

where qe is the amount of Pb(II) adsorbed (mg g−1); C0 (mg L−1) and Ct (mg L−1) are the concentrations in solution at t = 0 and at time t, respectively; V is the volume of solution (L); and m is the amount of adsorbent (g) added. 2.5. Effect of Solution pH. The effect of pH of the initial solution on the uptake of Pb(II) was studied in the pH range of 3.0 to 7.0. For this, 20 mL of Pb(II) solution (200 mg L−1) was agitated separately with 0.2 g of polyacrylamide zirconium(IV) iodate in a series of 100 mL conical flasks at 303 K for a period of 80 min. The solution was filtered at different time intervals throughout the equilibrium time period. The final pH was also measured, and the residual Pb(II) concentration in the filtrate was determined by titrating against 0.002 mol L−1 EDTA solution. 2.6. Kinetics of Pb(II) Adsorption. The kinetics of Pb(II) adsorption on the adsorbent was studied at four different Pb(II) concentrations, i.e., 50,100,150, and 200 mg L−1, and at a fixed pH value using the same procedure as described in Adsorption Procedure. 2.7. Error Analysis. To determine the validity of isotherm and kinetics models, statistical analysis was carried out. In all regression cases, four different error functions, i.e., the sum of the square of the error (SSE), sum of absolute error (SAE), chisquare, and standard deviation Δq (%), between the experimental data and calculated values were evaluated using the following equations:

Pb(II) ions from water. In addition, few-layered graphene oxide nanosheets,27 oxidized multiwalled carbon nanotubes28 and multiwalled carbon nanotubes−polyacrylamide composites29 have been employed as sorbents to study the sorption of Pb(II) from aqueous solutions. The present study deals with the sorption behavior of Pb(II) ions onto polyacrylamide zirconium(IV) iodate. Batch adsorption experiments were conducted to investigate the adsorption ability of Pb(II) ions from aqueous solution. Equilibrium data were fitted to Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich (D-R) equations to determine the best fitted isotherm. The kinetics and thermodynamics of Pb(II) adsorption onto polyacrylamide zirconium(IV) iodate in aqueous systems were investigated by considering the effects of Pb(II) concentration, temperature, and solution pH. The kinetic adsorption results have been analyzed using pseudofirst-order and pseudo-second-order reactions and an intraparticle diffusion model.

2. EXPERIMENTAL SECTION 2.1. Reagents. All chemicals used were of analytical grade except as noted. Zirconium(IV) oxychloride octahydrate (Otto Chemi Pvt. Ltd., Mumbai, India), acrylamide (Ottto Chemie Pvt. Ltd., Mumbai, India), and potassium iodate (Merk, India) were used for the synthesis of the material. 2.2. Preparation of Polyacrylamide Zirconium(IV) Iodate. Polyacrylamide zirconium(IV) iodate was prepared according to our previous study.2 In detail, 500 mL of 0.10 mol L−1 zirconium(IV) oxychloride octahydrate was gradually added into a flask in a ratio of (1:1) mixture of 0.1 mol L−1 potassium iodate and 0.4 mol L−1 acrylamide with constant stirring using a magnetic stirrer at 70 °C. The pH was maintained at 1 by adding 1 mol L−1 HNO3. The reaction mixture was stirred at 70 °C for 6 h and left for another 24 h at room temperature. The gelatinous precipitate was filtered and washed with distilled water until neutral pH. The material was dried in an oven at 50 °C. The dried material was broken into small granules and treated with 1 mol L−1 HNO3 for 24 h with occasional shaking. After that, the solid particles were collected through filtration, washed with distilled water until neutral pH, and finally dried at 50 °C. 2.3. Preparation of Adsorbate Solution. A stock solution of Pb(II) was prepared (1000 mg L−1) by dissolving the desired quantity of Pb(NO3)2 in distilled water. The test solutions were prepared by diluting the stock solution to the desired Pb(II) concentrations (50, 80, 100, 120, 150, and 200 mg L−1) 2.4. Adsorption Procedure. The adsorption experiments were carried out by the batch equilibrium method. A known amount of polyacrylamide zirconium(IV) iodate was added to 20 mL of an aqueous solution of Pb(II) in a series of 100 mL stoppard conical flasks. The pH of the solution was adjusted with dilute HNO3 or NaOH solution. The mixture was shaken at a constant temperature using a thermostated shaker at a speed of 120 rpm for a given time. The solution was then filtered, and the residual Pb(II) concentration was determined by EDTA titration. The experiments were carried out by varying the concentration of initial Pb(II) solution, contact time, temperature, and pH of initial solution. The Pb(II) uptake by the adsorbent was calculated from the following expression: qe =

(C0 − Ct )V m

n

∑i = 0 (qe,exp − qe,cal)i2

SSE =

n n

SAE =

∑ |qe,exp − qe,cal|i i=1 n

X2 =

⎡ (q

∑ ⎢⎢ i=1

Δq(%) =

⎣

− qe,cal) ⎤ ⎥ ⎥⎦ qe,cal i

e,exp

[(qe,exp − qe,cal)/qe,cal]2 n−1

× 100

where qe,exp and qe,cal are the experimental and calculated adsorption capacity (mg g−1) and n is the number of measurements.

3. RESULTS AND DISCUSSION 3.1. Effect of pH on Sorption. The pH of the medium is considered to be the most important parameter controlling the removal of adsorbate by an adsorbent. The pH of the solution may alter the surface charge on the sorbent and also influences the solution chemistry of metal ions. The effect of the solution pH on the Pb(II) adsorption by polyacrylamide zirconium(IV) iodate was studied in the pH range 3.0−7.0 using 0.2 g of the adsorbent and 20 mL of solution of Pb(II) ion (200 mg L−1) at 303 K. Initial pH of the working solution was adjusted by adding HNO3 or NaOH solution. The pH of the system after reaching adsorption equilibrium was also measured. (Table S7 of Supporting Information contains the initial and final pH values.) Figure 1 shows the adsorption of Pb(II) as a function of final pH. As can be seen from Figure 1, the sorption of Pb(II) increases with increase in pH of the solution, reaches a maximum at pH 6.0, and remains constant up to 6.5. The low removal efficiency at lower pH is due to the presence of higher

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species is Pb2+ and removal of Pb2+ is mainly due to adsorption reaction. The adsorption of Pb2+ is partly attributed to ion exchange between Pb2+ and H+ on the surface. The exchanged H+ ions are released to the solution, which causes the lowering in pH values. At pH > 7.0, precipitation occurs; hence, adsorption of Pb(II) on the sorbent was not studied above pH 7.0. The adsorption process can be shown as

3.2. Effect of Contact Time. The effect of contact time on the adsorption of Pb(II) on polyacrylamide zirconium(IV) iodate was checked by shaking 20 mL of 50, 100, 150 and 200 mg L−1 Pb(II) solution at initial pH 6.6 with 0.2 g of adsorbent. The contact time varied from 5 to 80 min (Figure S1 of Supporting Information). The optimum time for the adsorption process was 50 min. Therefore, 60 min of shaking time was fixed as the contact time for further studies. 3.3. Effect of Initial Concentration of Pb(II). The rate of adsorption is considered to be an important factor which depends upon the initial concentration of adsorbate. The effect of initial Pb(II) ion concentration (i.e., 50, 100, 150, 200 mg L−1) on the uptake of Pb(II) by polyacrylamide zirconium(IV) iodate was studied (Figure S2 of Supporting Information). The adsorption capacity at equilibrium increases with increase in initial Pb(II) ion concentration. This can be explained by considering that the increasing concentration gradient acts as an enhanced driving force which overcomes the resistances to mass transfer of Pb(II) between aqueous phase and solid phase.31

Figure 1. Effect of final pH on uptake of Pb(II) by polyacrylamide zirconium(IV) iodate. Temperature, 303 K; concentration, 200 mg L−1; amount, 0.2 g; volume, 20 mL; time, 60 min.

concentration of H+ in the solution which compete with Pb(II) ions for adsorption sites of the polyacrylamide zirconium(IV) iodate. At low pH, high positive charge density on the surface of sorbent due to the amide group causes low sorption of Pb(II) ions. The hydrolysis reaction of Pb(II) can be written as Pb2 + + OH− ⇌ Pb(OH)+

log k1 = 6.48

Pb(OH)+ + OH− ⇌ Pb(OH)2 0

log k 2 = 11.16

Pb(OH)02 + OH− ⇌ Pb(OH)3−

log k 3 = 14.16

30

Weng has determined the relative distribution of Pb(II) species as a function of pH and suggested that Pb(II) ions are present in the form of Pb2+, Pb(OH)+, Pb(OH)20, and Pb(OH)3− at various pH values. At pH < 6.5, the predominant

Table 1. Adsorption Isotherm Constants for Adsorption of Pb(II) onto Polyacrylamide Zirconium(IV) Iodate parameters isotherm

temp (K)

KL (L mg−1)

qm (mg g−1)

Langmuir

303 313 323

0.0122 0.0287 0.0247

5.98 6.22 7.01

R2 0.961 0.990 0.959 parameters

X2

SSE

SAE

0.32 0.25 0.36

1.38 1.36 1.32

2.044 1.84 1.74

isotherm

temp (K)

KF

n

R2

X2

SSE

SAE

Freundlich

303 313 323

0.255 0.256 0.721

1.91 2.85 2.41

0.963 0.910 0.963

0.01 0.03 8.0 × 10−3

0.81 0.45 0.23

0.66 0.20 0.05

parameters isotherm

temp (K)

AT

bT

R2

Temkin

303 313 323

0.0830 0.1386 0.0458

1599.45 1927.60 1643.40

0.932 0.951 0.997

X2 0.04 0.02 3.6 × 10−3 parameters

SSE

SAE

0.42 0.39 0.06

0.178 0.152 3.6 × 10−3

isotherm

temp (K)

qd

β

R2

X2

SSE

SAE

D-R

303 313 323

3.65 4.68 5.03

−0.014 −0.007 −0.006

0.756 0.941 0.763

0.22 6.8 × 10−3 0.087

0.90 0.18 0.66

0.81 0.03 0.43

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3.4. Adsorption Isotherms. Adsorption isotherms are represented as the amount of adsorbate molecules per unit mass of adsorbent as a function of equilibrium concentration in bulk solution at a constant temperature. The fitting of equilibrium data to different isotherm models is an important step in establishing the most appropriate model for designing the sorption system to remove Pb(II) ions from aqueous media. In this study, adsorption of Pb(II) ions on polyacrylamide zirconium(IV) iodate was carried out and four isotherm equations have been tested: Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich. The Langmuir model is based on the assumption that a fixed number of adsorption sites are available on the surface of the adsorbent; each site can take up a maximum of one molecule, and the energy of adsorption is constant. The Langmuir equation is represented as32 ⎛ 1 ⎞1 1 ⎟⎟ + 1 = ⎜⎜ qe K q qm ⎝ L m ⎠ Ce

ln qe = ln KF +

qe =

Table 2. Comparison of Maximum Adsorption Capacities (qm) for Pb(II) by Different Adsorbents

manganese oxide coated zeolite kaolin activated carbon activated carbon (prepared from maiz cob) olive stone activated carbon from coconut 2-aminothiophenyl-S-acetic acid grafted on polystyrene resin bael tree leaf powder bagase fly ash palm shell activated carbon polyacrylamide zirconium(IV) iodate

RT RT ln A T + ln Ce bT bT

(4)

where T is the absolute temperature (K), R the universal gas constant (8.314 J (mol K)−1), AT the equilibrium binding constant (L g−1), and bT the Temkin constant related to heat of adsorption (J mol−1). The Temkin isotherm plots, i.e., qe versus ln Ce, for adsorption of Pb(II) on polyacrylamide Zr(IV) iodate at 303, 313, and 323 K are presented in Figure 2. The Temkin

(2)

qm (mg g−1)

(3)

The constant KF and 1/n are Freundlich constants denoting the approximate indicator of adsorption capacity and intensity of adsorption in the adsorption process, respectively (Figure S5 of Supporting Information). The values of KF and n are given in Table 1. (Description is given in Supporting Information). The Temkin model was tested for equilibrium description at different temperatures, i.e., 303, 313, and 323 K. This model takes into account the adsorbent−adsorbate interactions, and it is expressed by the equation44

where Ce is the equilibrium concentration of the adsorbate in solution (mg L−1) and qe is the amount adsorbed per unit mass of adsorbent (mg g−1); qm is the amount of adsorbate at complete monolayer coverage (mg g−1) and gives the maximum sorption capacity of sorbent; KL (L mg−1) is the Langmuir isotherm constant, which relates to the energy of adsorption. When 1/qe was plotted against 1/Ce, a straight line with slope 1/(qmKL) and intercept 1/qm was obtained. (Figure S3 of Supporting Information shows the Langmuir plots.) Langmuir parameters and correlation coefficients calculated from the isotherms are given in Table 1. (A description is given in Supporting Information). The comparison of maximum monolayer adsorption capacity (in milligrams per gram) for Pb(II) on various adsorbents is presented in Table 2. The

adsorbent

1 ln Ce n

Figure 2. Temkin adsorption isotherm for Pb(II) adsorption at different temperatures.

ref

1.12 4.50 6.68 3.15 5.80 4.38 6.22

33 34 34 35 36 37 38

4.07 2.5 1.34 5.75

39 40 41 this work

constants bT and AT were calculated from the slope and intercept, respectively, and are summarized in Table 1. The Temkin adsorption potentials, A T , of polyacrylamide zirconium(IV) iodate at 303, 313, and 323 K are 0.030, 0.1386 and 0.0458, respectively, which indicated lower polyacrylamide zirconium(IV) iodate-Pb(II) potential. The Temkin constant bT related to the heat of adsorption was found to be in the range of 1.599−1.927 kJ mol−1. It has been reported45,46 that heat of sorption values lower than 20 kJ mol−1 was characteristic of the physiosorption. The low values of bT (1.599−1.927 kJ mol−1) obtained in this case indicated a weak interaction between the polyacrylamide zirconium(IV) iodate and Pb(II) which supported the physical sorption. As can be seen from Table 1, the values of R2 (Temkin isotherm) varied in the range of 0.932−0.997, which indicated a close fit to the adsorption of Pb(II) at 323 K. Radushkevich47 and Dubinin48 have established a relationship between adsorption and the porous structure of adsorbents. The Dubinin−Radushkevich isotherm49,50 is generally expressed as

adsorption capacity of polyacrylamide zirconium(IV) iodate for Pb(II) is greater than that for many other adsorbents such as manganese oxide coated zeolite, kaolin, activated carbons (maiz cob, palm shell), bagasse fly ash, and bael tree leaf powder. It is concluded that the polyacrylamide zirconium(IV) iodate showed better performance than many other adsorbents in terms of adsorption capacity. The adsorption data were also analyzed by Freundlich isotherm model, which is suitable for a heterogeneous surface.42 The linear form of the Freundlich equation is expressed as43

2⎫ ⎧ ⎡ ⎛ ⎪ 1 ⎞⎤ ⎪ ⎢ ⎥ qe = qDR exp⎨−β RT ln⎜1 + ⎟ ⎬ ⎪ ⎪ Ce ⎠⎥ ⎭ ⎝ ⎩ ⎢

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The linear form of D-R isotherm equation is represented as ln qe = ln qDR

2 ⎡ ⎛ 1 ⎞⎤ − β ⎢RT ln⎜1 + ⎟⎥ ⎢⎣ Ce ⎠⎥⎦ ⎝

(6)

where qe (mg g−1) is the amount of metal ion adsorbed, qDR (mg g−1) the maximum adsorption capacity of the metal ion, β (mol2 kJ−2) a D-R isotherm constant, Ce (mg L−1) the equilibrium concentration of metal ion, R the gas constant (8.314 J (mol K)−1), and T the absolute temperature (K). The D-R isotherm constant β is related to the mean free energy of adsorption E (kilojoules per mole), and this can be computed using the following expression.51 E=

1 2β

(7)

Figure 3. Error bar for different isotherm models.

Figure S6 of Supporting Information shows the D-R isotherm plots. The D-R constants qDR and β were calculated from intercept and slope, respectively, and are given in Table 1. As can be seen from the table, the values of R2 are in the range of 0.756−0.941, which revealed that the experimental data fitted well with the D-R isotherm model at only 313 K. The sorption mechanism can be predicted on the basis of the value of sorption energy. If the magnitude of E is between 8 and 16 kJ mol−1, the adsorption process proceeds by ion exchange, while values of E less than 8 kJ mol−1 indicate that the sorption process is of a physical nature.52 In this study the value of E was found to be 5.97, 8.45, and 9.12 kJ mol−1 at 303, 313, and 323 K, respectively, for adsorption of Pb(II) on polyacrylamide zirconium(IV) iodate. It has been suggested that linearization plots may not be a sufficient basis for rejecting or accepting a model.53,54 In view of this, further analysis is required to judge the suitability of the adsorption isotherm models, namely, Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich, and their fit to the experimental data. The fitness of the data was established using the statistical parameters such as R2, X2, SSE, and SAE. As can be seen from Table 1, the value of R2 for all the models, except the D-R model at 303 and 323 K, is greater than 0.90, which indicates the applicability of all four models in describing the data. Moreover, the higher R2 value and lower values of X2, SSE, and SAE for the Temkin isotherm indicated the applicability of this model is better than that of the Langmuir, Freundlich, and Dubinin−Radushkevich models. The experimental (qe) and simulated (qm) data for various isotherm models are shown in Figure 3. As can be seen from figure, the least error is observed in Temkin isotherm model. 3.5. Adsorption Kinetics. To understand the adsorption mechanism of Pb(II) on polyacrylamide zirconium(IV) iodate, a kinetic investigation was conducted. An adsorption kinetic model, usually applied for estimation of adsorption rate, provides valuable insights into the mechanism of adsorption reaction.55 In this study, pseudo-first-order, pseudo-secondorder, and intraparticle diffusion models have been used for testing the experimental data. The Lagergren’s first-order kinetic equation has been used to describe the solute adsorption on various adsorbents on the basis of adsorbent capacity. Its linear form is expressed as56 ln(qe − qt ) = ln qe − k1t

(min), respectively. k1 (min−1) is the rate constant in the pseudo-first-order adsorption process. The values of the constants qe,cal and k1 were calculated from the intercept and slope of the plot of ln (qe − qt) versus t (Figure 4), respectively,

Figure 4. Lagergren’s pseudo-first-order kinetic plot at different temperatures. concentration of Pb(II), 200 mg L−1; amount, 0.2 g; volume, 20 mL; pH, 6.6.

and their values are reported in Table 3. As shown in the table, the correlation coefficient R2 values for pseudo-first-order rate equation varied in the range of 0.8782−0.9440. The values of equilibrium capacity qe,cal obtained by the pseudo-first-order model were much smaller than the experimental values. Moreover, the values of Δq (%), X2, SSE, and SAE were also much higher, indicating that the adsorption process did not follow the Lagergren model. The adsorption data were then analyzed by a pseudo-secondorder kinetic model.57,58 The linear form of this model is expressed as ⎛1⎞ 1 t = + ⎜⎜ ⎟⎟t 2 qt k 2qe ⎝ qe ⎠

(9)

where k2 (g mg−1 min−1) is the pseudo-second-order rate constant. The initial adsorption rate, ho (mg g−1 min−1) at t → 0 was computed using the expression

(8)

where qe (mg g−1) and qt (mg g−1) are the amount of metal adsorbed per unit mass of adsorbent at equilibrium and time t

ho = k 2qe 2 8202

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SSE

0.81 0.09 0.08 0.90 0.30 0.29

Figure 5. Pseudo-second-order kinetic plot at different temperatures. Concentration of Pb(II), 200 mg L−1; amount, 0.2 g; volume, 20 mL; pH, 6.6.

0.26 0.03 0.02 0.777 0.682 0.518

0.9971 0.9974 0.9990

X2 Δq (%) R2 h (mg g−1 min−1)

A plot of (t/qt) versus t (Figure 5) yielded good straight lines (correlation coefficient, R2 > 0.99) as compared to that of a

pseudo-first-order plot. The values of qe, k2, and ho were evaluated from the slope and intercept of the plot, respectively, and are summarized in Table 3. The calculated qe values were very close to the experimental value. In addition, error analysis was also conducted, which yielded low values for Δq (%), X2, SSE, and SAE. These suggested the applicability of the pseudosecond-order kinetic model. As can be seen from the table, the initial adsorption rate increases while the values of k2 decreases with increasing initial Pb(II) concentration. Similar observations were also reported for the adsorption of Pb(II) on Bael tree leaf powder39 and dendrimer−titania composites.59 To understand the mechanism and rate-controlling steps, it is essential to identify the steps involved in the adsorption process. Generally, if the investigated sorption process involves a metal species and a porous sorbent, then the following steps may be considered: (I) Transport of ingoing ions (adsorbate) across the liquid film around the adsorbent particles (film diffusion). (II) Binding of adsorbate on the active sites distributed on the outer surface of adsorbent particle. (III) Transport of adsorbate within the pores of the adsorbent (particle diffusion). (IV) Adsorption of adsorbate into the active sites distributed within the adsorbent particles. Steps II and IV are considered very fast; hence, they do not have a determining role in governing adsorption rates. The following three possibilities may arise if steps I and III are considered: (a) If external transport is less than internal transport, the film diffusion is the rate controlling step. (b) If the external transport is greater than internal transport, the rate is governed by particle diffusion. (c) External transport may be equal to internal transport. In this case the adsorbate ions are transported to the boundary and later form a liquid film surrounded by the adsorbent particles. Therefore, the external mass transfer and intraparticle diffusion warrant further investigation because the pseudofirst-order and pseudo-second-order kinetic models do not provide any information regarding diffusion of adsorption

k2 (g mg min )

3.60 5.16 5.57 5.9 8.9 10.3 2.4 3.0 3.2 2.9 4.7 5.0 22.04 24.93 31.82 0.8782 0.9031 0.9440

SAE SSE X

Δq (%) R (mg g )

0.0600 0.0256 0.0167

−1 −1 2

2.076 1.894 2.072

e,cal

0.0159 0.0554 0.0691 303 313 323

q k1 (min ) temp (K)

pseudo-first-order

2 −1 −1

Table 3. Kinetic Parameters for Pb(II) Adsorption onto Polyacrylamide Zirconium(IV) Iodate

−1

qe,cal (mg g )

pseudo-second-order

Article

0.23 0.02 0.02

SAE


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did not pass through origin, which suggested the involvement of an intraparticle diffusion process, but it was not the sole ratecontrolling process. The positive value of the intercept is indicative of some degree of boundary layer control. The intraparticle diffusion plot shows multilinearity in the adsorption of Pb(II) onto polyacrylamide zirconium(IV) iodate, which indicated that several steps are operational. The first step can be attributed to boundary layer diffusion of Pb(II). The second stage described the gradual sorption where intraparticle diffusion is the rate-controlling step. In the final stage, the intraparticle diffusion process became slow because of the low concentration of Pb(II) left in the solution. Similar observations were reported for removal of adsorption of Pb(II) by mansonia wood sawdust.63 3.6. Adsorption Thermodynamics. The feasibility and orientation of the physicochemical adsorptive reaction can be evaluated by thermodynamic parameters. The thermodynamic parameters such as change in free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) associated with the adsorption of Pb(II) onto polyacrylamide zirconium(IV) iodate can be evaluated using the following equations:53

process. The kinetic experimental data were then analyzed by intraparticle diffusion model60 to gain insight into the diffusion mechanism. This model is expressed as qt = K idt 1/2 + C

(11)

where C is the intercept, which is proportional to the boundary layer thickness (mg g−1),61 and Kid is an intraparticle diffusion rate constant (mg g−1 min1/2), which can be calculated from the slope of a linear plot of qt versus t1/2. The plots of qt versus t1/2 at different initial Pb(II) concentrations and temperatures are shown in Figures 6 and 7, respectively. (Tables S8 and S9 of

ΔG° = −RT ln Kc

ln Kc = Kc =

ΔH ° ΔS° + RT R

(12) (13)

qe Ce

(14)

where Kc is the equilibrium constant, qe the solid-phase concentration at equilibrium (mg g−1), Ce the equilibrium concentration in solution (mg L−1), R the universal gas constant (8.314 J (mol K)−1), T the absolute temperature (K). The values of (ΔH°) and (ΔS°) were obtained from the slope and intercept of the plot of ln Kc versus 1/T (Figure 8). The

Figure 6. Plots for evaluating intraparticle diffusion rate constants for adsorption of Pb(II) onto polyacrylamide zirconium(IV) iodate at different temperatures.

Figure 8. Von’t Hoff plot for adsorption of Pb(II) ion.

thermodynamic analysis was carried out at 303, 313, and 323 K. The calculated thermodynamic parameters for the adsorption of Pb(II) onto polyacrylamide zirconium(IV) iodate are given in Table 4. The negative values of ΔG° indicated the spontaneous nature of the adsorption. The value of ΔG° became more negative with increasing temperature, which suggested that the adsorption was more favorable at higher temperature. It has been reported58,64 that ΔG° values up to −20 kJ mol−1 are involved in electrostatic interaction between surface sites and metal ions, i.e., physiosorption. In our studies,

Figure 7. Intraparticle diffusion plots for different initial concentration of Pb(II) at 323 K.

Supporting Information contain intraparticle diffusion constant values.) If the regression of qt versus t1/2 is linear and passes through the origin,62 then the intraparticle diffusion is the only rate-determining step. If it is not so, some other mechanisms are also involved. In this study, the intraparticle diffusion plots 8204


Industrial & Engineering Chemistry Research Table 4. Thermodynamic Parameters for Adsorption of Pb(II) onto Polyacrylamide Zirconium(IV) Iodate at Different Temperatures temperature (K)

ΔG○ (kJ mol−1)

ΔH○ (kJ mol−1)

ΔS○ (J (mol K)−1)

303 313 323

−0.151 −1.173 −2.194

30.812

102.191

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REFERENCES

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4. CONCLUSIONS The adsorption equilibrium and kinetics of Pb(II) ions onto polyacrylamide zirconium(IV) iodate have been studied in the present work. The equilibrium data have been analyzed by Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich isotherm models. The error analysis suggested that Temkin isotherm model yields the best fit between experimental and simulated data. Kinetic studies showed that the adsorption adhered to the pseudo-second-order model because the theoretical and experimental sorption capacities were in excellent agreement (R2 ≥ 0.997). The intraparticle diffusion model revealed that pore diffusion was not the only ratecontrolling step and indicated some boundary layer control in the process of Pb(II) adsorption by the sorbent. The change in Gibb’s free energy ΔG ° was found to be negative at all temperatures, indicating the feasibility and spontaneity of the adsorption process. The value of ΔH° (30.812 kJ mol−1) and ΔS° (102.19 J (mol K)−1) indicated the endothermic nature of adsorption and randomness at the solution−solid interface, respectively. ASSOCIATED CONTENT

S Supporting Information *

Figures describing effect of contact time and initial concentration; plots of isotherm models; tables describing effect of initial and final pH and intraparticle diffusion parameters; text describing isotherm models. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors thank the Chairman, Department of Chemistry, Aligarh Muslim University, Aligarh, for providing research facilities. U.H. thanks UGC for providing a Non-Net fellowship to carry out this work. The authors acknowledge the partial support provided by DRS-I (SAP, UGC) program and DST (FIST & PURSE), New Delhi.

ΔG° values were found to be in the range of −0.151 to −2.195 kJ mol−1, which suggested that the physical adsorption was the predominant mechanism in the sorption process. It was observed that adsorption capacity of the sorbent for Pb(II) increased with increasing temperature. In addition, the positive value of ΔH° (30.813 kJ mol−1) indicated that the adsorption phenomenon was endothermic, which was further supported by an increase in adsorption capacity with an increase in temperature. Similar observations were also reported65 for adsorption of Pb(II) on iron oxide nanoparticle immobilized Phanerochaete chrysosporium. Therefore, higher temperature favored the adsorption of Pb(II) onto polyacrylamide zirconium(IV) iodate. The positive values of ΔS° (102.19 J mol−1 K−1), which is a measure of randomness at the solid− liquid interface, indicated the interaction of Pb(II) with the active groups.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: nafi[email protected]. Tel: +91-9412501208. Notes

The authors declare no competing financial interest. 8205



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