Modeling and Experimental Evaluation of Ni(II) and ... - ACS Publications

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Modeling and Experimental Evaluation of Ni(II) and Pb(II) Sorption from Aqueous Solutions Using a Polyaniline/CoFeC6N6 Nanocomposite Nima Moazezi,*,† Majid Baghdadi,‡ Michael A. Hickner,§ and Mohammad Ali Moosavian† †

Department of Chemical Engineering, Faculty of Engineering, University of Tehran, Tehran 11365-4563, Iran Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, Tehran, P.O. Box 14178-53111, Iran § Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡

S Supporting Information *

ABSTRACT: Herein, a modified polyaniline nanocomposite containing CoFeC6N6 was prepared and tested for its sorption of Pb2+ and Ni2+ ions from aqueous solutions. The adsorbent was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Brunauer−Emmett− Teller and Fourier transform infrared analyses. The adsorption percentage increased with increasing CoFeC6N6 in the nanocomposite material. A pseudo-second-order model and Langmuir adsorption described the behavior of the material. Additionally, the adsorption capacity was increased by increasing the temperature. The prediction of concentrations was performed using a model based on pore diffusion and the implicit finite difference method. From a mass transfer standpoint, the best data set of De for Pb2+ (1.12 × 10−9 m2 s−1) and Ni2+ (0.98 × 10−13 m2 s−1) was determined.

1. INTRODUCTION Because of growing concerns about environmental pollution caused by heavy metals,1,2 various processes have been suggested for efficient removal of these contaminants from water, including precipitation, ion exchange, ion flotation, chemical and electrochemical techniques, adsorption, membrane filtration, reverse osmosis, and coagulation.3,4 Unlike organic contaminants, heavy metals are nonbiodegradable and persistent in the food chain.5 The adverse effects of heavy metals on living organisms are intensified as a result of bioaccumulation.6 Hence, ecological benefits can be achieved by the removal of heavy metals from water or wastewaters and effective sequestration of these contaminants. All of the mentioned treatment techniques are currently being widely employed for removal of heavy metals from the environment, and each one has its inherent benefits and limitations.7 Among them, adsorption processes have received much attention owing to their simplicity, convenience, high removal efficiency, and the reversible and reusable nature of many adsorbents.8 The reusability of adsorbent materials and their ability to concentrate contaminants for easy sequestration and disposal make these types of adsorption processes more cost-effective.9 Therefore, the selection of an operational adsorbent is of great importance for the widespread application of this technology.10 Functionalization of polymeric materials © XXXX American Chemical Society

can result in thermally stable adsorbents containing active sites with a high affinity toward heavy metals.11 These polymeric adsorbents have high adsorption capacities because of their porous structures, and polymeric sorbents can have desirable mechanical properties.12 Polyaniline has been extensively studied for the removal of heavy metal ions as a result of its desirable attributes such as low cost, ease of synthesis, and environmental stability. It is expected that polyaniline-based sorbents have a strong affinity toward heavy metals due to its high concentration of nitrogencontaining functional groups (imine and amine) in the polymer structure.13 In addition, polyaniline exhibits good resistance against acid or alkaline environments, which makes it more suitable for removal of some metals, such as Cr(VI). On the basis of these characteristics, many researchers have focused on the application of polyaniline for the removal of heavy metal ions from aqueous solutions. Thus far, polyaniline and its composites have been studied for the removal of Pb2+ (polyaniline−cation exchange composite),14 Cd2+ (polyaniline−sawdust composite),15 Hg2+ (polyaniline),16 arsenate Received: October 13, 2017 Accepted: February 7, 2018

A

DOI: 10.1021/acs.jced.7b00897 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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CoCl2·6H2O, KOH, Ni(NO3)2·6H2O, Pb(NO3)2, and nitric acid (65 wt %) were purchased from Merck (Darmstadt, Germany). Transmission electron microscopy conducted at 300 kV (TEM, Philips CM30) was used for the investigation of the size and morphology of the polyaniline/CoFeC6N6 nanocomposite. Morphological studies of the bare polyaniline were carried out using a field emission scanning electron microscope (FE-SEM, Hitachi S4160, Japan). Nanocomposite formation was confirmed using Fourier transform infrared spectroscopy (FTIR, Bruker Vector 22) within the wavenumber range of 400−4000 cm−1 and X-ray diffraction (XRD, STOE, STIDY-MP, Germany) with Cu Kα (λ = 1.54056 Å) radiation over Bragg angles ranging from 0° to 100°. The pore volume and specific surface area were investigated using the Brunauer−Emmett− Teller (BET, Quantachrome, NOVA 2000) method. An Oxford ED2000 X-ray fluorescence (XRF) spectrometer was used for the elemental analysis of polyaniline/CoFeC6N6 nanocomposite (Table S1). An inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer, Optima 7300 DV, USA) was used for the determination of Pb2+ and Ni2+ concentrations. 2.2. Synthesis of Adsorbents. 2.2.1. Preparation of Polyaniline. The polymerization of aniline can be carried out by numerous methods, such as chemical polymerization, photochemical polymerization, and electrochemical polymerization.32 In this research, aniline was chemically polymerized in HCl solution in the presence of (NH4)2S2O8 as an oxidant. In brief, (NH4)2S2O8 (1 g) was dissolved in 100 mL of aqueous solution of HCl (1 mol L−1). Then 1 mL of freshly distilled aniline was added dropwise to the oxidant solution at 25 °C, and the reaction mixture was stirred for 5 h. Afterward, the precipitated polymer was filtered, washed with deionized water, and finally dried at room temperature. 2.2.2. Preparation of Polyaniline/CoFeC6N6 Nanocomposites. KxMy[Fe(CN)6]z (MFeC6N6) was synthesized through a stoichiometric reaction between potassium ferricyanide and a metal chloride salt (MClp).33 First, cobalt(II) chloride hexahydrate was used for CoFeC6N6 synthesis. Second, the polyaniline/CoFeC 6 N 6 nanocomposite was synthesized through in situ polymerization in the presence of CoFeC6N6. CoCl2·6H2O (0.238 g), K3[Fe(CN)6] (0.329 g), and (NH4)2S2O8 (1 g) were dissolved in HCl solution (100 mL, 1 mol L−1), followed by stirring for 30 min at room temperature. Afterward, 1 mL of freshly distilled aniline was added dropwise with constant stirring to the oxidant solution containing CoFeC6N6. A dark green solution was obtained after 5 min. Then the solution was vigorously stirred for 5 h. Finally, the resultant green colored precipitate was filtered and then washed with deionized water for removal of impurities, followed by drying in a convection oven at 25 °C. 2.3. Batch Sorption Experiments. Batch sorption experiments were conducted to obtain the adsorption equilibrium and kinetic data for Pb2+ and Ni2+ uptake. Samples were stirred using a thermostatic mechanical shaker (WNB-14, Mennert), followed by filtration through a membrane filter (0.45 μm, Whatman 42). Metal concentrations were determined using ICP-OES. The calculation of adsorbent dosage can be found in the Supporting Information (section S1 and Figure S1), which shows that the optimum amount of adsorbent is 3.5 g L−1. The isotherm experiments were carried out at three temperatures (25, 40, and 60 °C), 3.5 g L−1 of adsorbent dosage, and different concentrations of Pb2+ and Ni2+, (10−100 mg L−1).

(activated carbon−polyaniline composite),17 As(III),18 and chromium (polyaniline−poly(ethylene glycol) composite).19 Inorganic ion exchange materials, such as hexacyanoferrates,20 titanates,21 phosphates,22 and hydrous metal oxides23 have high selectivity and efficiencies for separating and removing heavy metal from aqueous waste streams. Moreover, potassium cobalt hexacyanoferrates (K0.01CoFeC6N6) have been applied as selective inorganic adsorbents for cesium removal. 24 Among the metal hexacyanoferrates, cobalt hexacyanoferrate (CoFeC6N6) is a favorable candidate for the surface modification of polymeric matrixes. Therefore, preparation of a new organic−inorganic hybrid materials using polyaniline and CoFeC6N6 was proposed to exploit the reactive functionalities of each component for the removal of hazardous heavy metals from aqueous solutions. As a crucial scientific component to gain insights into problems in chemistry, environmental science, and engineering, mathematical modeling has attracted wide attention in water and wastewater treatment, including its use in membrane processes, bioreactors, and adsorption technology.25 Regularly, a momentum balance, an energy balance, and mass balances have been employed to describe the adsorption process. Among these balance equations, mass balances, because of their ability to predict the concentrations in fluid outside particles, in pore fluid, and in a sorbed phase, are particularly attractive.26 Several mathematical models have been reported for the prediction of the adsorption characteristics of such processes based on diffusional mass transport models, such as the film− pore diffusion model, the film−surface diffusion model, intraparticle diffusion model, and external film diffusion model,27 which can lead to solutions requiring nonlinear partial differential equations.28 Despite their potential for a variety of applications, however, they do not have exact analytic solutions. Hence, numerical techniques have become popular for arriving at solutions.29 Some of the numerical methods which have been previously reported for batch adsorption, including the integral formulation, the finite difference,30 and orthogonal collocation,31 have been used for solving the governing equations. This paper reports two major activities: (a) the fabrication of a new polymeric nanocomposite for the sorption of Pb2+ and Ni2+ from aqueous solutions and (b) development of a pore diffusion-based model for the prediction of concentration decay curves. The nanocomposite was composed of polyaniline to which the CoFeC6N6 was added in order to modify the surface of the porous sorbent. The structure and morphology of nanocomposite were investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) analyses. The effects of the experimental parameters influencing the Pb2+ and Ni2+ ion sorption into the polyaniline/CoFeC6N6 material, such as pH, adsorbent dosage, the initial concentration of Pb2+ and Ni2+, contact time, and temperature are discussed. Equilibrium data were investigated using Langmuir and Freundlich isotherms. Among various numerical solution methods, an implicit finite difference method was selected for solving the resultant mathematical model.

2. EXPERIMENTAL PROCEDURES 2.1. Materials and Instrumentation. Analytical grade chemicals were used in all experiments. Aniline (C6H7N), hydrochloric acid (37 wt %), K3[Fe(CN)6], (NH4)2S2O8, B



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The influence of the initial pH of the solutions was studied (section S2 and Figure S2). The experiments were studied at the natural pH, without any changes in initial pH of the solution. The kinetics of the adsorption process was investigated at 25 °C with an adsorbent dosage of 3.5 g L−1 over a period of 1440 min. The effect of CoFeC6N6 on the functional group content of the nanocomposite and subsequent adsorption of Pb2+ and Ni2+ were observed in the range of 0.015−0.1 mol L−1. The adsorption percentage and the amount of adsorbed metal ions were calculated, respectively, using the following equations:15 %removal =

(Ci − Ce) × 100 Ci

q e = (C i − Ce)

V W

C b,0 = g (Ce , V , m)

(6)

(1) (2)

where W is the adsorbent mass (g), V is the sample volume (L), and Ce and Ci (mg L−1) are equilibrium and the initial concentrations of metal ions, respectively.

3. MATHEMATICAL MODELING 3.1. Assumptions. The model was developed with the assumptions similar to that in LeVan et al.26 and Sadeghi Pouya et al.:28 • Temperature is considered to be constant during the adsorption process. • The mass transfer in the pore phase of the adsorbent is described by Fick’s law. • Diffusion in the pores reaches equilibrium rapidly. • Diffusion of the adsorbate takes place from the film surrounding the particle toward the core through the pores with sorption occurring on the active sites. • The adsorbent particles are spherical with uniform sizes. 3.2. Mass Balance. With the above assumptions, we can set up a mass balance to derive partial differential equations. We consider three different phases and conditions, including steady-state, liquid phase, and solid phase. 3.2.1. Equilibrium (Steady Condition). The adsorption reaction between the polyaniline/CoFeC6N6 and metal ions reaches an equilibrium state in aqueous solution. Equation 3 is a popular form of the overall mass balance for batch adsorption calculations at the equilibrium state. mbefore adsorption,0 = mS,eq + mL,eq (3)

Figure 1. Schematic diagram of the polyaniline/CoFeC6N6 single particle used in the modeling.

3.2.2. Adsorbent Phase. According to Figure 1, the governing equation for transport in the polyaniline/CoFeC6N6 grain is expressed by DeεP

∂q ∂q ∂C P = ∂t ∂C P ∂t

(8)

It was considered that q is a function of CP, which itself is a function of t, as a consequence q is also a function of t. So 1 ∂ ⎛ 2 ∂C P ⎞ ∂C P ⎜r ⎟= (1 − εP) ∂q ⎤ r 2 ∂r ⎝ ⎡ ∂r ⎠ ∂t ⎢⎣1 + ρS εP ∂CP ⎥⎦ De

(4)

(9)

and defining

Here, V is the solution volume, m is the mass of polyaniline/ CoFeC6N6, Cb,0 is the concentration of ions in the bulk phase at initial condition, Ce is the concentration of the ions in liquid phase at equilibrium state, and qe is the adsorption capacity of the ions in the solid phase at the equilibrium state, which is a function of Ce obtained using adsorption isotherm models (eq 5). section 3.2.3 discusses the adsorption isotherms.

qe = f (Ce)

(7)

where r is the radial coordinate of the grain, εP is the adsorbent porosity, ρS is the skeletal density, CP is the adsorbate concentration in the pore liquid phase, De is the effective diffusivity, and t is time. The chain rule was used according to Leibniz’s notation as follows:

where mbefore adsorption,0 is the weight of the ions in the bulk phase at the initial condition, mS,eq is the ion mass in the solid phase at equilibrium state, and mL,eq is the ion mass in the liquid phase at the equilibrium state. Therefore, the overall mass balance is, C b,0V = CeV + qem

∂q 1 ∂ ⎛ 2 ∂C P ⎞ ∂C P ⎜r ⎟= εP + ρS (1 − εP) 2 ∂t ∂t r ∂r ⎝ ∂r ⎠

F=

De

(1 − εP) ∂q ⎤ ⎡ ⎣⎢1 + ρS εP ∂CP ⎦⎥

(10)

Now eqs 9 and 10 become ∂C P 1 ∂ ⎛ ∂C ⎞ = F 2 ⎜r 2 P ⎟ ∂t r ∂r ⎝ ∂r ⎠

(5)

(11)

Equations 12 and 13 are subject to the following boundary and initial conditions:

By rearranging, eqs 4 and 5 become C


Journal of Chemical & Engineering Data ∂C P(r = 0, t) = 0; ∂t

C P(r , t = 0) = 0;

C P(r = RP , t ) = C b(t )

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(12)

C P(r = RP , t = 0) = C b(0) = C b,0 (13)

where RP is the average radius of the grain and Cb represents the adsorbate concentration in the bulk phase. 3.2.3. Bulk Liquid Phase. The material balance of the bulk liquid phase for the batch adsorption is shown as V

∂C b ∂C (R , t ) m =−4 3 (4πRP 2εP)De P P dt ∂r πRPρs (1 − εP) 3

Figure 2. Effect of molar ratio of CoFeC6N6 concentration in the nanocomposite on the adsorption removal of metal ions. Conditions: Pb 2+, 100 mg L −1; Ni2+, 100 mg L−1; polyaniline/CoFeC6N6 nanocomposite, 0.1 g; sample volume, 0.02 L; aniline, 1 mL.

(14)

This result is attractive because of the ion exchange properties of CoFeC6N6 and its adsorption capacity for Pb2+ and Ni2+ ions. As a result, the polyaniline/CoFeC6N6 nanocomposite at the molar ratio of 4:1 (CoFeC6N6/aniline) was selected for further experiments. A comparison of the adsorption capacity of polyaniline/CoFeC6N6 nanocomposite with an individual polyaniline and CoFeC6N6 can be found in the Supporting Information (section S3 and Figure S3). The morphology of the products using SEM and TEM was investigated (Figure 3). It is notable that the formation of a porous larger particle was due to the agglomeration of small particles. The SEM image of the bare polyaniline is shown in Figure 3a. As can be seen, the bare polyaniline was composed of rod-shaped particles with an average size of around 175 nm × 75 nm. Furthermore, the TEM image of the polyaniline/ CoFeC6N6 nanocomposite was presented in Figure 3b, showing that the nanocomposite contained polyhedral particles (50 to 70 nm). The SEM image of the polyaniline/CoFeC6N6 nanocomposite can be found in Figure 3c. The morphology of polyaniline/CoFeC6N6 nanocomposite is clearly shown. The FTIR spectra of the samples (Figure 3d) indicated a broad band between 3100 and 3650 cm−1, which is a result of O−H stretching. The bands at 1483 and 1569 cm−1 are related to benzenoid and quinoid rings, respectively. The band around 2850−2950 cm−1 is attributed to the alkyl stretching groups. C−H bending was observed as a sharp band at 788 cm−1. The two characteristic bands appearing at 1105 and 1286 cm−1 are related to C−N vibrations in the benzenoid and quinoid rings.34 An FeII−CN−CoII band was seen at 2090 cm−1,35 which shows that CoFeC6N6 has been successfully incorporated into the nanocomposite structure. The XRD pattern of the bare polyaniline (Figure 3e) showed no sharp peak, indicating an amorphous structure; on the other hand, polyaniline/CoFeC6N6 nanocomposite had a singlecrystal nanosphere. The six reflections of polyaniline/CoFeC6N6 nanocomposite that appeared at around 2θ = 18° (200), 2θ = 24.8° (220), 2θ = 35.5° (400), 2θ = 40° (420), 2θ = 43.9° (422), 2θ = 51° (440) (JCPDS 5-0036, 5-0037), was consistent with those expected for a face-centered cubic lattice.36 The sharp peaks indicated formation of CoFeC6N6 crystals.37 The pore volume of the samples, average pore diameter, and the Brunauer−Emmett−Teller (BET) surface area are presented in Table 1, based on BJH theory and nitrogen adsorption−desorption isotherms for polyaniline/CoFeC6N6 nanocomposite; see Figure 4. The adsorbent pores are commonly classified as mesopores (in the range of 2−50 nm), macropores (greater than 50 nm), and micropores (2 nm or less).38 Therefore, adsorbates are likely diffusing in mesopores polyaniline/CoFeC6N6 nanocomposites.

and defining

B=−

3εPDem RPρs (1 − εP)V

(15)

and by rearranging, this becomes ∂C b ∂C (R , t ) =B P P dt ∂r

(16)

3.3. Numerical Solution. The governing equations have been solved numerically using the fourth order Runge-Kutta algorithm, which is based on an implicit finite difference method. These equations are obtained by the three-point central difference scheme. The boundary conditions with either a backward or forward difference are used together in each boundary node. In the line scheme method, the partial differential eqs (eqs 11 and 16), their boundary conditions, and initial conditions take the form dC P,(j) dt

= F(j) +

dC b,(j) dt

2 ⎛ C P,(j + 1) + C P,(j − 1) − 2C P,(j) ⎜ r(j) ⎝ Δr 2 C P,(j + 1) − C P,(j − 1) ⎞ ⎟ 2Δr ⎠

⎛ C b − C P,(J − 1) ⎞ = B⎜ ⎟ Δr ⎝ ⎠

(17)

(18)

where index j signifies for the jth internal node in the r direction, and J is the number of discretized nodes; see Figure 1. A MATLAB code is written (MATLAB 7.14.0.739, The MathWorks, Inc., R2012a, 2012) to carry out the calculations and to solve the above equations.

4. RESULTS AND DISCUSSION 4.1. Synthesis and Characterization of Polyaniline/ CoFeC6N6 Nanocomposite. The concentration of CoFeC6N6 in the nanocomposite was optimized through a stoichiometric reaction between Co2+ and K3[Fe(CN)6] at 0.00, 0.01, 0.05, and 0.1 mol L−1. The effect of CoFeC6N6 on the functional group content of the nanocomposite and the subsequent adsorption of Pb2+ and Ni2+ were investigated; see Figure 2. As can be seen, the adsorption percentages of Pb2+ and Ni2+ ions reached their maximum values at a molar ratio of 4:1 (CoFeC6N6/aniline) due to the increase of active sites adsorbing metal ions by increasing the content of CoFeC6N6 in the polyaniline/CoFeC6N6 nanocomposite. The adsorption percentages of the polyaniline/CoFeC6N6 nanocomposite for Pb2+ (80%) and Ni2+ (37%) were significantly higher than those of the bare polyaniline for Pb2+ (42%) and Ni2+ (8%). D



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Figure 3. (a) SEM top-view image of the polyaniline; (b) transmission electron micrographs of the polyaniline/CoFeC6N6 nanocomposite; (c) SEM top-view image of the polyaniline/CoFeC6N6 nanocomposite; (d) FTIR spectra of the bare polyaniline and polyaniline/CoFeC6N6 nanocomposite; (e) XRD pattern of the bare polyaniline and polyaniline/CoFeC6N6 nanocomposite.

isotherm type is representative of capillary condensation in mesopores.40 The hysteresis loop for each of the nanocomposites is shown in Figure 4, which correspond to a type H3 hysteresis,41 and it is as a result of different adsorption and desorption mechanisms due to capillary condensation inside mesopores,42 the presence of narrow slit-shaped pores, and some mesoporosity.41 4.2. Adsorption on the Optimized Polyaniline/ CoFeC6N6 Nanocomposite. 4.2.1. Contact Time and Adsorption Kinetics Studies. Kinetic uptake investigations are essential for designing an effective adsorption process.43 The volume of the treatment tank depends on its hydraulic retention time (HRT). By increasing the adsorption rate and the lowering the HRT, a smaller mixing tank can be considered for the adsorption process. Figure 5a represents the adsorption capacity versus contact time for Pb2+ and Ni2+. It was observed that the adsorption was faster at the contact time lower than 240 min. This phenomenon is a result of a large number of surface active sites accessible to the adsorbate. The equilibrium sorption was achieved after 300 min for both Pb2+ and Ni2+. Several models are available for investigating the adsorption kinetics. In this study, pseudo-first-order and pseudo-secondorder models were applied for analyzing the experimental data. The linear forms of models and the obtained results are presented in Table 2. The intercept and the slope of the plot of log (qe − qt) vs t were investigated for the determination of pseudo-first-order model parameters (k1 and qe). However, the rate constant of the pseudo-second-order equation (k2) was obtained using intercept of the plots of t/q versus t. On the other hand, qe was calculated from the slope of the related equation.44 According to the linear regression coefficients, it was found that the pseudo-second-order model well described the effect of

Table 1. Pore Structure Analysis of the Polyaniline and Polyaniline/CoFeC6N6 Nanocomposite sample polyaniline polyaniline/CoFeC6N6 nanocomposite 0.05 M polyaniline/CoFeC6N6 nanocomposite 0.1 M

SBET (m2 g−1)

average pore diameter (nm)

pore volume (cm3 g−1)

106.6 90.30

0.3301 0.2371

10.59 12.39

35.21

0.2175

24.71

Figure 4. Nitrogen adsorption−desorption isotherms for the bare polyaniline, polyaniline/CoFeC6N6 nanocomposite (0.05 mol L−1), and polyaniline/CoFeC6N6 nanocomposite (0.1 mol L−1).

It is clear from the nitrogen adsorption−desorption isotherm of the polyaniline/CoFeC6N6 nanocomposite, in comparison with that of the bare polyaniline, that the inflection position shifted to lower relative pressure. The volume of adsorbed nitrogen was also decreased, indicating a reduction in pore size in the presence of CoFeC6N6.39 The isotherm is of the type IV IUPAC classification group, suggesting that interaction between adsorbate, the adsorbed ions onto the adsorbent, and adsorptive, foreign ions in the liquid, is lower than the interaction between the adsorbent and adsorptive. Thus, the E



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Figure 5. (a) The effect of contact time on adsorption; (b) kinetic models on the adsorption of Pb2+ and Ni2+. Conditions (a,b): Pb2+, 100 mg L−1; Ni2+, 100 mg L−1. (c−f) Fit of the data to the Freundlich and Langmuir isotherm for adsorption of the ions. Conditions (c−f): time, 500 min; temperatures, 296, 313, and 333 K. Conditions (a−f): polyaniline/CoFeC6N6 nanocomposite, 0.07 g; sample volume, 0.02 L.

seen, the pseudo-second-order model represents a good fit to experimental data for both Pb2+ and Ni2+. 4.2.2. Adsorption Isotherms. The relationship between the equilibrium concentration of adsorbate in the solution and adsorbent phase is described using adsorption isotherms; for this purpose, various models have been developed.45 In this research, Freundlich and Langmuir isotherms were applied to investigate the adsorption of Pb2+ and Ni2+ using prepared adsorbent at various temperatures (296, 313, and 333 K). Table 3 shows the linear forms of adsorption isotherms and the obtained results. The Langmuir model considered monolayer adsorption onto identical active sites of adsorbent without any interaction between the adsorbed molecules, and the Freundlich isotherm considered the multilayer adsorption on the heterogeneous surface. The Langmuir constant (KL) is an equilibrium adsorption constant; however, the Freundlich constant (KF) indicates adsorption capacity. The higher is KL, the higher is the affinity of adsorbate for adsorbent sites. The other constant of Freundlich (n) indicates the linearity deviation of the adsorption. The higher n-value shows that the adsorption intensity is relatively strong.39 On the basis of the R2 and Figure 5c−f, the adsorption of Pb2+ and Ni2+ seems to be well described using the Langmuir model. The n-value is greater than unity, suggesting that Pb2+ and Ni2+ were favorably adsorbed on the surface of polyaniline/ CoFeC6N6 nanocomposite. The maximum adsorption capacities (qm), Freundlich constants (KF), and Langmuir constants of Pb2+ and Ni2+ were increased by increasing the temperature, which indicates the adsorption process is endothermic. The obtained maximum capacities of Pb2+ and Ni2+ using the

Table 2. Adsorption Kinetics Parameters Calculated by Pseudo-First-Order and Pseudo-Second-Order Models for Adsorption of Pb2+ and Ni2+ Using Polyaniline/CoFeC6N6 Nanocompositea adsorbate model

parameters

Pb

2+

Ni2+

pseudo-first-order

log(qe − qt ) = log(qe) −

k1t 2.303

R2 k1: min−1 qe(calc): mg g−1

0.9496 0.0047 13.78

0.9841 0.0041 4.40

R2 k2: g mg−1 min−1 qe (calc): mg g−1 h = k2q2e : mg g−1 min−1

0.9950 0.0014 18.59

0.9909 0.00092 5.12

pseudo-second-order

⎛1⎞ t 1 = + ⎜⎜ ⎟⎟t 2 qt k 2qe ⎝ qe ⎠ a

qPb2+ e,exp ,

0.32

0.04

qNi2+ e,exp ,

Notation: 17.98; 4.7. k1, rate constant of pseudo-firstorder adsorption (min−1); k2, second-order rate constant of adsorption (g mg−1 min−1); qe (calcd), equilibrium capacity (mg g−1); h, initial adsorption rate.

time on the adsorption of Pb2+ and Ni2+. On the other hand, the adsorption capacities predicted using the pseudo-secondorder model were much closer to experimental capacities. The pseudo-second-order rate constant of Pb2+ (0.0014 g mg−1 min−1) was higher than that of Ni2+ (0.00092 g mg−1 min−1); therefore, Pb2+ was adsorbed faster than Ni2+. Furthermore, the slope of the plot of % adsorption vs t (Figure 5a) for Pb2+ was higher than that of Ni2+, which confirms faster adsorption of Pb2+ on the adsorbent. Figure 5b shows the experimental and related calculated data using the investigated models. As can be F



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Table 3. Isotherm Parameters Obtained with Linear Regression for Adsorption of Pb2+ and Ni2+ Using Polyaniline/CoFeC6N6 at Different Temperatures (296, 313, and 333 K) Pb2+ isotherm

parameters

Ni2+

298 K

313 K

333 K

298 K

313 K

333 K

0.9989 18.34 0.53

0.9996 20.88 0.62

0.9964 21.55 1.20

0.9993 4.76 0.21

0.9988 9.60 0.35

0.9983 13.58 0.45

0.9511 2.45 5.44

0.9106 1.99 6.39

0.9679 1.96 8.99

0.9448 1.45 1.46

0.9646 1.57 2.82

0.9730 1.30 3.84

Langmuir R2 qm: mg L−1 KL: L mg−1

⎛C ⎞ Ce 1 = + ⎜⎜ e ⎟⎟ qe kLqm ⎝ qm ⎠ Freundlich

ln qe = ln KF +

1 nF

R2 nF KF: (mg g−1)(L mg−1)1/n

ln Ce

Langmuir model were 18.34 and 4.76 mg g−1, respectively. Table 4 shows the adsorption capacities of previously reported adsorbents.

Table 5. Thermodynamic Parameters for Adsorption of Pb2+ and Ni2+ Using Polyaniline/CoFeC6N6 Pb

Table 4. Comparison of Current Adsorbent Capacity with Previously Reported in the Literature

Ni2+

maximum adsorbed amount, qmax [mg g−1] Pb2+

adsorbent waste eggshell expanded perlite rice bran beech sawdust Fe3O4 dithizone-anchored poly (EGDMA-HEMA) microbeads polyaniline/CoFeC6N6 nanocomposite

Ni2+

2.2 2.24 8 4 0.19 7.2 17.56 (296 K) − 20 (333 K)

4.54 (296 K) − 12.8 (333 K)

ref 46 47 48 49 50 51 this study

(20)

Standard enthalpy was obtained from the slope of the Van’t Hoff equation (eq 21) by plotting ln(Ke) vs 1/T. ln(Ke) = −

ΔH 0 ΔS 0 + RT R

T (K)

ΔS° (J mol−1 K−1)

ΔH° (kJ mol−1)

ΔG° (kJ mol−1)

298 313 333 298 313 333

160.3

19.19

138.60

17.80

−47.77 −50.18 −53.38 −41.28 −43.37 −46.13

The mechanism of adsorption of these ions follows four steps:8,43,52 (i) diffusion of Pb2+/Ni2+ from the bulk of solution to the adsorptive sites through external film and entrance into the pores; (ii) dehydration of Pb2+/Ni2+ (an endothermic process) in the proximity of adsorptive sites. According to FTIR spectra of the samples, adsorptive sites can react with Pb2+/Ni2+ to adsorb a large number of Pb2+/Ni2+; (iii) hydration of the counterions of the adsorbent; (iv) binding of adsorbates on the active sites (an exothermic process). According to Table 5, the positive value of ΔH° confirms that the energy of binding of adsorbates is lower than the energy of dehydration. 4.3. Mathematical Modeling. The results showed that the Langmuir model is more accurate than the Freundlich model for equilibrium adsorption data of Pb2+ and Ni2+. The skeletal density and porosity were calculated to be 22 × 105 g m−3 and 0.72, respectively. The effective diffusivity can be calculated using the adsorbent porosity and diffusivity of adsorbate in pure water:53

4.2.3. Thermodynamic Studies. The endothermic or exothermic nature of the adsorption process and its feasibility were evaluated by estimating standard enthalpy (ΔH°, J mol−1) and standard free energy (ΔG°, J mol−1), respectively. Standard free energy was obtained from the following equations:8 q Ke = e Ce (19) ΔG 0 = −RT ln(Ke)

2+

De = εP2Dadsorbate − water

(22)

According to the model results, De was estimated in order to minimize the average absolute radial error. The best data set of De for Pb2+ and Ni2+ was determined to be 1.12 × 10−9 and 0.98 × 10−13 m2 s−1, respectively. The diffusion coefficients for Pb2+ and Ni2+ were reported within the range of 7.70 × 10−11 to 1.39 × 10−10 m2 s−1,54,55 and 1.3 × 10−13 to 2.5 × 10−13 m2 s−1,56,57 respectively. According to eq 17, the estimated values for the De were different from the reported range due to the simplifying assumptions, such as a spherical adsorbent with uniform sizes and Fick’s law simplification. The effective diffusivity of Pb2+ was higher than that of Ni2+, which may be due to the lower atomic weight of Ni2+ in comparison with Pb2+.58 Figure 6a,b presents the fit of the experimental results to modeling data at different initial concentrations. As can be seen, the obtained models for both ions were well converted to the

(21)

where T is the absolute temperature (K), Ke is the thermodynamic equilibrium constant, R is the universal gas constant (8.314 J mol−1 K−1), and ΔS° is standard entropy. The calculated thermodynamic parameters are shown in Table 5. The positive values of ΔH° and ΔS° showed the endothermic nature of the adsorption process and randomness at the solid/solution interface during adsorption. The negative value of ΔG° showed that adsorption process was thermodynamically favorable. The adsorption of Pb2+ on the surface of the proposed adsorbent was more endothermic and more feasible than that of Ni2+. These results are in good agreement with higher Langmuir constants and adsorption capacities at higher temperatures. G



Article

Figure 6. Comparisons of the experimental data (●) and model (□) for adsorption of (a) Ni2+ and (b) Pb2+. Pore concentration profiles at different times for removal of (c) Ni2+ and (d) Pb2+ with an initial concentration of 100 mg L−1 at 25 °C.

■

experimental data, which proves the accuracy of these mathematical equations and MATLAB codes. The pore concentration profiles illustrate the removal of ions as a function of particle radius and contact time with an initial concentration of 100 mg L−1 at 25 °C; see Figure 6c,d. Penetration depth increased with increasing the contact time, which was achieved with increasing the diffusion of ions. The removal of Pb2+ was higher than that of Ni2+ (Figure 6); in addition, from the mass transfer standpoint, the diffusivity of Pb2+ was higher than that of Ni2+. Therefore, the mass transfer rate of Pb2+ from the bulk liquid to the pore of the polyaniline/ CoFeC6N6 nanocomposite was higher than that of Ni2+.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00897. A comparison of the adsorption capacity of the nanocomposites with different composition, the effect of pH, the effect of adsorbent dosage, and elemental analysis of polyaniline/CoFeC6 N 6 nanocomposite (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98 (912) 299-1998. ORCID

5. SUMMARY AND CONCLUSIONS

Nima Moazezi: 0000-0003-0555-7628

In this research, polyaniline/CoFeC6N6 nanocomposites were synthesized as high performance adsorbents for heavy metal ions. Characterization was performed using TEM, BET, XRD, and FTIR techniques. The prepared adsorbent was utilized for adsorption of Pb2+ and Ni2+ ions in batch sorption processes. The adsorption kinetics of both ions using the prepared nanocomposite adsorbent followed a pseudo-second-order model with rate constants of 0.0014 and 0.00092 g mg−1 min−1 for Pb2+ and Ni2+, respectively. The adsorption of Pb2+ and Ni2+ was described well using a Langmuir model. An increase in maximum adsorption capacities (qm) and Freundlich constants (KF) was observed with an increase in temperature. The negative value of ΔG° showed that the adsorption process was thermodynamically favored. The adsorption of Pb2+ on the surface of proposed adsorbent was more endothermic and more feasible than that of Ni2+. A model based on pore diffusion was presented for the prediction of concentration versus contact time. Among various numerical solution methods, an implicit finite difference method was selected to solve the resultant government equations. The obtained models for both ions fitted well to the experimental data, which demonstrates the accuracy of this mathematical study. From a mass transfer standpoint, the obtained diffusivity of Pb2+ was higher than that of Ni2+. Therefore, the mass transfer rate of Pb2+ from the bulk liquid to the pore of the polyaniline/ CoFeC6N6 nanocomposite was higher than that of Ni2+, and these results were confirmed using experimental kinetics data. The experiments showed a calculated De for Pb2+ and Ni2+ to be 1.12 × 10−9 and 0.98 × 10−13 m2 s−1, respectively.

Funding

The authors would like to thank the University of Tehran for providing the financial support for this project (ID: 0010708170). Notes

The authors declare no competing financial interest.

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