Removal of Anionic Dyes from Water by Potash Alum Doped

Article pubs.acs.org/JPCB

Removal of Anionic Dyes from Water by Potash Alum Doped Polyaniline: Investigation of Kinetics and Thermodynamic Parameters of Adsorption Braja N. Patra* and Deola Majhi School of Chemistry, Sambalpur University, Sambalpur, Odisha, India 768019 ABSTRACT: Polyaniline was synthesized by the oxidative polymerization method by using ammonium persulfate as an oxidant. The positive charge in the backbone of the polymer was generated by using Potash alum as a dopant. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR), X-ray fluorescence (XRF), and X-ray diffraction (XRD) techniques were used for characterization of doped polyaniline. The doped polyaniline can be used for selective adsorption of various dyes (selectively sulfonated dyes) from aqueous solution. Adsorption studies regarding the effect of contact time, initial dye concentration, pH, doses of adsorbent, and temperature on adsorption kinetics were investigated. The influence of other anions like Cl−, NO3−, and SO42− on the adsorption density of dyes onto doped polyaniline was also explored. Langmuir isotherm and pseudo-second-order kinetics were found to be the most appropriate models to describe the removal of anionic dyes from water through adsorption. Thermodynamic parameters such as free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) changes were also evaluated. The interaction of dyes with doped polyaniline was also investigated by FTIR and UV spectroscopy.

1. INTRODUCTION In recent years, water pollution has been attracting attention of researchers, as it causes severe damage to the environment. Synthetic dyes, the potential pollutant present in the industrial effluents from leather, textile, paper, and food, are almost all soluble in water which is difficult to remove.1 Therefore, efficient separation and remediation technologies are to be explored for these dyes. Various treatment techniques and processes like chemical oxidation, coagulation, membrane separation, electrochemical processes, and adsorption techniques have been employed to remove the dyes from wastewater.2,3 Out of these techniques, adsorption has been reported as an easy, promising, and cost-effective process for dye removal from wastewater. The most extensively used adsorbent is activated carbon which is suitable for removal of traditional pollutants such as dyes, phenols, organic acids, and heavy metals.4 However, there are some drawbacks related with activated carbon, which is costly in regeneration and nonselective. To circumvent these drawbacks related with activated carbon, some other materials have been reported, for instance, natural organic matter, natural and synthetic polymers, and inorganic solid which are effective for dye removal.5−9 In the past two decades, polyaniline (PANI), the conducting polymer, has attracted great interest of researchers because of the physical and chemical properties such as environmental stability, high electrical conductivity, and easy synthesis as well as its numerous applications in plastic batteries, display devices, and optical storage lithography.10 PANI can be used as an adsorbent for the removal of anionic dyes or cationic dyes when the backbone of the polymer is positively charged or neutral, respectively.11−14 The driving forces of this adsorption phenomenon are electrostatic attraction of the positively © XXXX American Chemical Society

charged polymeric backbone with anionic dyes or the basic site of the PANI with cationic dyes.11 The positive charge is created in the PANI backbone by doping with an acid, for instance, camphorsulfonic acid, p-toluenesulfonic acid, or simple mineral acid which have a pH less than 4.15 This doping condition renders a harsh and corrosive environment which is not suitable for industrial applications. Thus, there is a tremendous need to develop the methods based on environment friendly and nontoxic substances. Anticipating a nontoxic environment for doping, we explored the use of Potash alum, a well-known double salt as a dopant, in order to create a positive charge in the PANI backbone. In this Article, we report a new method of preparation of doped polyaniline by using cheap Potash alum and the preferable adsorption of anionic dyes as compared to cationic dyes on the doped PANI. The kinetics of adsorption, thermodynamic parameters, and effect of different parameters such as initial dye concentration, temperature, pH, and contact time on the adsorption process was studies in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Potash alum (Merck), ammonium persulfate (APS) [(NH4)2S2O8] (Merck), tetrahydrofuran (THF) (Merck India), and N-methyl-2-pyrrolidone (NMP) (Spectrochem India) were used as received. Aniline (Merck India) was distilled under a vacuum before use. Methylene Blue (MB) (Nice Chemicals, India), Rhodamine-B (RB) (Lobachemie, India), Mordant Yellow-10 (MY) and Orange II (O-II) (Acros Received: January 19, 2015 Revised: May 22, 2015

A

DOI: 10.1021/acs.jpcb.5b00535 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. FTIR spectra of pure PANI and Potash alum doped PANI.

The desorption experiment was conducted with dye loaded doped PANI, in which doped PANI was treated with dye for around 9−10 h. Before subjecting to the desorption experiment, the dye loaded PANI was separated from the solution by centrifuge, washed with distilled water several times, and finally dried under a vacuum. Then, the dye loaded samples were mixed with 50 mL of basic solutions of different pH’s and stirred for 5 h. The polyaniline was separated by centrifuge, and the concentrations of the dyes were determined by UV−vis spectroscopy at their respective absorbance maxima. A stock solution of 0.1 M NaOH and 0.1 M HCl was prepared and further diluted 10−20 times (as required) before adding them in the system to adjust the pH of solution. A Hitachi U3210 spectrophotometer was used for recording UV−vis spectra in the range 200−800 nm.

Organic), sodium chloride (Merck India), sodium nitrate (Merck India), and sodium sulfate (Fisher Scientific, India) were used as received. 2.2. Synthesis and Characterization of Doped Polyaniline. APS was used as an oxidant for synthesis of polyaniline (emeraldine base), described in detail elsewhere.16 The emeraldine base was doped with Potash alum. In a typical procedure, 200 mg of polyaniline base was taken in a flask. To this, 15 mL of THF was added and then it was stirred for 1 h. A 35 mL portion of 0.1 M solution of Potash alum in water was added to the flask and stirred for 24 h. Then, the doped polyaniline was filtered and washed with 300 mL of water. The powder was dried in the vacuum at 50 °C for 24 h. The doped polyaniline samples were characterized by X-ray diffraction (XRD), FTIR, scanning electron microscopy (SEM), and X-ray fluorescence (XRF) spectroscopy. FTIR spectra between 400 and 4000 cm−1 were recorded by using a PerkinElmer Spectrum-2000 (FTIR) spectrophotometer. A Zeiss EVO50 scanning electron microscope was used for SEM measurements. The X-ray diffraction patterns were recorded on a Rigaku Miniflex II X-ray diffractometer using Cu Kα as the source. Al and S contents in PANI powders were determined by using a wavelength dispersive X-ray fluorescence spectrophotometer (WD-XRF) (make, Bruker; model, S8 Tiger). The powder PANI sample was thoroughly mixed with binder wax (ratio 80:20, w/w) and pressed at 40 ton pressure to make a pellet. The pressed pellet was then analyzed using XRF in vacuum mode. Adsorption and Desorption Experiment. The adsorption kinetics of the dyes onto doped polyaniline were carried out by varying different parameters such as the initial dye concentration, pH, contact time, temperature, and amount of adsorbent in the context of thermodynamics study, adsorption isotherm, and adsorption kinetics. In a typical experiment, 50 mL of different dye solutions of different initial concentrations (100−500 ppm) was stirred for 8 h in the presence of 80 mg of doped PANI. The concentrations of the dyes at different times were determined by UV−vis spectroscopy at their respective absorbance maxima (483 nm for O-II and 354 nm for MY).

3. RESULTS AND DISCUSSION 3.1. Characterization of Doped Polyaniline. The PANI was synthesized by oxidative polymerization of aniline. The synthesized polyaniline was doped with 0.1 M aqueous Potash alum solution. The FTIR spectrum of the doped polyaniline and pure polyaniline is shown in Figure 1. When comparing with the doped PANI, the band at 1585 cm−1 (CC stretching of the quinoid) present in PANI base is shifted by 12 cm−1 to lower wavenumbers while the band at 1502 cm−1 (CC stretching of the benzenoid unit) remains almost unchanged.17−20 It demonstrates that the electronic structure of the quinoid units is mainly affected by doping whereas benzenoid units remain almost unaffected. A structural change in polyaniline is manifested by a substantial shift of the peaks at 1323 cm−1 (CN stretching) and 1165 cm−1 (electronic-like absorption of NQN, where Q denotes the quinoid ring) to the lower frequency at 1303 and 1143 cm−1, respectively. As the system does not possess any protons, the doping must occur due to pseudoprotonation of the nitrogen in the imine unit by the aluminum ion. The intensity of the peak at 1143 cm −1 is associated with the degree of doping or delocalization of electron. The microstructure analysis of the materials was carried out by SEM, and the scanning electron micrograph is shown in B


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The imine nitrogen present in polyaniline can act as a Lewis base, and the cations (Al3+) present in the medium serve the role of Lewis acid, resulting in a pseudoprotonation of the polymer backbone. The result is similar to the doping of polyaniline by alkali metal salt.20 The total charge is neutralized by three sulfate ions, which act counteranions. 3.2. Adsorption and Desorption Study. Various classes of dyes with different skeletons including heteropolyaromatic (Methylene Blue), xanthane (Rhodamine-B), and azo (Mordant Yellow-10, Orange-II) were used for adsorption study. Figure 5 shows the variation of the concentration of dyes with

Figure 2. The obtained micrograph details the morphology of the material. It is observed that the porous or sponge structure

Figure 2. SEM micrograph of Potash alum doped PANI.

was resulting from the aggregation of small granules. The porous nature is responsible for the adsorption of dye. Figure 3 represents the X-ray diffraction patterns of the polyaniline base and doped polyaniline. The X-ray diffraction Figure 5. Concentration profile of various dyes in the presence of 80 mg of doped PANI with 100 ppm initial concentration at 25 °C.

time in the presence of 80 mg of doped polyaniline. It was noticed that adsorption of cationic dyes (MB, RB) was not significant, whereas in the case of anionic dyes MY and O-II the adsorption is significant (Scheme 1). This clearly suggests that the polymer backbone is positively charged and there is chemical interaction between the negatively charged sulfonated group of dye with the positively charged PANI. Furthermore, the adsorption of anionic dyes is not possible, when polyaniline base (emeraldine form without doping with Potash alum) is used as an adsorbent. Comparing the adsorption behavior of both of the sulfonated azo dyes, one containing a naphthyl ring (O-II) and another containing a benzene ring (MY), it was observed that the adsorption is almost the same at the initial stage. However, after 20 min, the adsorption of O-II is more compared to MY. This may be attributed to the easy approach of O-II dye toward PANI due to its planar structure attained by intramolecular hydrogen bonding between the OH group and NN group. The adsorption becomes almost saturated after 100 min. Also, pure γ-Al2O3 as an adsorbent does not show any adsorption of the above anionic dyes. However, nanosized mesoporous alumina is reported to be a potential adsorbent for removal of Congo red dyes from the aqueous medium.22 Effect of Initial Dye Concentration and Contact Time. The main driving force to overcome the mass transfer resistance of the dye from the aqueous to the solid phase is the initial dye concentration. The effect of initial dye concentrations in the range from 100 to 500 ppm on dye removal using doped polyaniline is shown in Figure 6. The higher the initial dye concentration, the more the driving force for the adsorption, and this indicates the strong interaction between the adsorbent and dye molecules. The amount of dye adsorbed at time t (qt) was obtained by using eq 1

Figure 3. XRD of PANI and alum doped PANI.

pattern of Potash alum doped polyaniline is similar to that of polyaniline base, and the peak positions are a little bit shifted to higher 2θ values in the case of doped polyaniline. A shoulder at around 2θ = 15.5 present in the case of polyaniline base is missing in the case of Potash alum doped polyaniline. The broad structures in the diffraction patterns is attributed to the diffraction by an amorphous polymer, as reported in the literature.21 On the basis of the above results, the proposed structure of doped PANI can be represented as in Figure 4. The proposed structure is further supported by the XRF analysis. The Al and S contents of the doped polyaniline determined by XRF were found to be Al (7.4%) and S (13.5%). The elemental analyses are consistent with the proposed structure of doped polyaniline.

qt =

Figure 4. Proposed structure of Potash alum doped PANI. C

V (C0 − Ct ) m

(1) DOI: 10.1021/acs.jpcb.5b00535 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B Scheme 1. Preferential Adsorption of Anionic Dyes (O-II and MY) over Cationic Dyes (RB and MB)

Figure 6. Plot of qt vs time of O-II (a) and MY (b) in the presence of doped PANI.

where V is the volume of dye solution (L), C0 and Ct are the initial and final dye concentrations at time t (mg L−1), respectively, and m is the mass of the adsorbent used (g). As expected, the value of qt increases with increase of time and reaches a plateau. This indicates that a dynamic equilibrium exists between the amount of dye adsorbed and desorbed from adsorbent. With an increase of the initial dye concentration, the time necessary to reach the asymptotic value of qt decreases in the case of both MY and O-II. Thus, qt becomes constant for different dyes after a certain time and after this time no more dyes are adsorbed even if the contact time is greater. Effect of the Adsorbent. To observe the effect of adsorbent on dye removal, 100 ppm of dye at 25 °C is put in contact with various amounts of adsorbent (20−100 mg). It is observed that the removal of both dyes increases by increasing the adsorbent doses. The percentage of removal of dyes (R) was calculated by using eq 2 R (%) =

100(C0 − Ct ) C0

Figure 7. Effect of dosage of adsorbent on percentage of removal of dyes.

after attending critical doses. At lower doses of adsorbent, removal of MY is more as compared to O-II. Mahanta et al. reported complete adsorption of initial concentration of 100 ppm sulfonated dye by using 100 mg of the doped PANI in 120 min.12,13 Compared with those previous studies, a relatively lower amount of adsorbent (80 mg of doped PANI) can adsorb the initial concentration of 100 ppm of dye in 100 min. Effect of Temperature. The percentage of removal of both dyes was investigated by varying the temperature, and the results are presented in Figure 8. These experiments were carried out at four different temperatures (15, 25, 35, and 45 °C) for the initial concentration of 100 ppm and by using 80 mg of doped PANI. It is observed that the adsorption of both dyes on doped polyaniline increases with an increase of

(2)

where C0 and Ct are the initial and final dye concentrations at time t (mg·L−1), respectively. At least 80 mg of adsorbent is required for almost complete removal (>99%) of the dyes from water. Initially, a sharp increase in adsorption was noticed up to 60 mg (critical dose) of adsorbent after which the rate of dye removal increases slowly (Figure 7). This can be attributed to the availability of more adsorption sites initially and the splitting effect of the flux between adsorbate and adsorbent D


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removal of dyes in both cases occurs at acidic medium (pH 3). Since at lower pH the polymer backbone is positively charged, a strong electrostatic attraction between the positively charged polyaniline and anionic dye molecule occurs expedite to maximum adsorption. As the pH of the system increases, the decrease in positive charge in the backbone is leading to the lower adsorption. The adsorption of MY is more as compared to O-II at higher pH, that may be due to the presence of the ionized COO− group, in addition to the SO3− group. Kinetics of Adsorption. The adsorption kinetics is an important characteristic which gives the information regarding the adsorption efficiency, and a possible rate controlling step. The kinetics data were analyzed by pseudo-first-order, pseudosecond-order, and intraparticle diffusion models by using eqs 3−5.

Figure 8. Effect of temperature on percentage of removal of dyes.

temperature. The positive effect of temperature on adsorption of both dyes indicates that the adsorption process is endothermic in nature. Effect of pH. pH plays a key role in adsorption of dyes. The effects of pH on adsorption of dyes are studied for both of the dyes, and the plot of percentage of removal versus pH is shown in Figure 9. It is apparent from the figure that the highest

ln(qe − qt ) = ln qe − k1t

(3)

t 1 1 = + t 2 qt qe k 2qe

(4)

qt = kit 1/2 + C

(5)

k1, k2, and ki are the pseudo-first-order rate constant (min−1), pseudo-second-order rate constant (g mg−1 min−1), and intraparticle diffusion rate constant (mg g−1 min−1/2) for the adsorption process, respectively. qe and qt are the adsorption capacity of dye molecules onto doped PANI at equilibrium (mg g−1) and at time t, respectively. C (mg g−1) is the constant, and t is the time (min). Figure 10a, b, and c and Figure 11a, b, and c show the fitted curves of the pseudo-first-order kinetics model, pseudo-second-order kinetics model, and Weber’s intraparticle diffusion model kinetics for MY and O-II, respectively. Table 1 (MY) and Table 2 (O-II) contain all the parameters mentioned above. To determine the most appropriate model fitting for the data, the correlation coefficient R2 was calculated. Calculation results indicate that the adsorption kinetics can be explained by

Figure 9. Effect of pH on percentage of removal of dyes.

Figure 10. Pseudo-first-order kinetics (a), pseudo-second-order kinetics (b), and intraparticle diffusion model (c) of MY at pH 7.0 and 25 °C; volume = 50 mL; material mass = 80 mg. E


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Figure 11. Pseudo-first-order kinetics (a), pseudo-second-order kinetics (b), and intraparticle diffusion model (c) of O-II at pH 7.0 and 25 °C; volume = 50 mL; material mass = 80 mg.

Table 1. Parameters of Kinetic Models for MY Adsorption onto Alum Doped Polyaniline at Different Initial Concentrations of Dye (C0) (pH 7.0; temp = 25 °C; volume = 50 mL; material mass = 80 mg) pseudo-first-order kinetics

pseudo-second-order kinetics

intraparticle diffusion

C0 (mg L−1)

qe,exp (mg g−1)

k1 (min−1)

qe,calc (mg g−1)

R2

k2 (g mg−1 min−1)

qe,calc (mg g−1)

R2

ki (mg g−1 min−1/2)

C (mg g−1)

R2

100 200 300 400 500

63 106 133 139 142

0.027866 0.016121 0.009212 0.008521 0.008121

57.8 77.4 85.1 87.0 87.5

0.9793 0.9750 0.9264 0.8662 0.8687

0.001014 0.000350 0.000280 0.000332 0.000544

65 110 139 143 145

0.9991 0.9989 0.9995 0.9997 0.9994

5.70500 6.67220 7.23260 7.82790 8.51320

6.4501 21.938 34.233 37.887 54.327

0.9752 0.9749 0.9214 0.8419 0.9128

Table 2. Parameters of Kinetic Models for O-II Adsorption onto Alum Doped Polyaniline at Different Initial Concentrations of Dye (C0) (pH 7.0; temp = 25 °C; volume = 50 mL; material mass = 80 mg) pseudo-first-order kinetics C0 (mg L−1)

qe,exp (mg g−1)

100 200 300 400 500

63 88 91 96 99

k1 (min−1)

qe,calc (mg g−1)

0.033163 0.007830 0.008290 0.008981 0.008521

55.1 61.1 63.9 72.4 75.2

pseudo-second-order kinetics R2

k2

(g mg−1 min−1)

qe,calc (mg g−1)

0.9968 0.9621 0.9554 0.8468 0.9298

0.001398 0.000323 0.000449 0.000460 0.000501

65 91 93 97 101

the pseudo-second-order model with a high correlation coefficient (R2 = 0.9993 and 0.9997 for O-II and MY, respectively). Further, the calculated qe values are also very close to the experimental values in the case of the pseudosecond-order model. This suggests that adsorption of both of the dyes on a doped polyaniline surface follows the pseudosecond-order kinetics. Adsorption Isotherm. The adsorption isotherm is an important parameter for optimization of the adsorption system, as the equilibrium relationship between the adsorbent and the adsorbate can be best explained by the adsorption isotherm. Two important isotherm Langmuir and Freundlich adsorption models were used to fit the experimental data shown in eq 6 and 7, respectively.23

intraparticle diffusion R2

ki (mg g−1 min−1/2)

C (mg g−1)

R2

0.9993 0.9968 0.9977 0.9977 0.9984

6.1242 6.3839 4.3375 2.7197 2.8799

7.1668 9.6447 33.058 45.775 49.636

0.9642 0.9325 0.8973 0.8882 0.8847

Ce a 1 = + L Ce qe KL KL

(6)

qe = KFCe1/ n

(7)

where KL (L g−1) and aL (L mg−1) are the Langmuir constants and KL/aL corresponds to the Langmuir monolayer adsorption capacity (Q0). qe is the quantity of dye per mass of adsorbent at equilibrium (mg g−1), Ce is the concentration at equilibrium, KF is the Freundlich constant being indicative of adsorption capacity, and n is the Freundlich constant (index of adsorption intensity or surface heterogeneity). F


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Figure 12. Langmuir (a) and Freundlich (b) adsorption isotherm of MY and O-II onto doped PANI.

Table 3. Parameters of the Langmuir and Freundlich Models at pH 7 and 25 °C Langmuir model MY O-II

Freundlich model 2

aL (L/mg)

KL (L/g)

RL

R

0.029767 0.130435

4.31 12.42

0.0629−0.2514 0.0151−0.0712

0.9998 1

n

KF (L/mg)

R2

4.382 24.510

38.2825 74.3875

0.9399 0.9398

Figure 13. Plot of amount of dye adsorbed at equilibrium (qe) vs concentration at equilibrium (Ce) for adsorption of Mordant Yellow-10 (a) and Orange-II (b) on alum doped PANI at pH 7 and 35 °C in the absence and presence of different salts of 1 mM concentration.

and SO42− on the adsorption of both dyes on PANI at a concentration of 1 mM at pH 7 and 35 °C were studied and are shown in Figure 13. The experimental data fitted well with the Langmuir model. Table 4 contains the value of Q0, as determined by using eq 6. It is noticed from Figure 13 and Table 4 that the Q0 value for both dyes follows the order: without salt < Cl− < NO3− < SO42−.

The characteristic feature of the Langmuir isotherm can be expressed in terms of a dimensionless constant called the separation factor (RL also known as the equilibrium parameter) which can be defined by eq 824 RL =

1 1 + aLC0

(8) −1

where C0 is the initial concentration (mg L ) and aL is the Langmuir constant related to the energy of adsorption (L mg−1) calculated from the slope of the Langmuir isotherm. The value of RL indicates the adsorption nature which can be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (R = 0).25 The value of RL for MY (0.0629−0.2514) and for O-II (0.0151−0.0712) shows that the adsorption is favorable in both cases. The Langmuir and Freundlich isotherms fitting the experiment data for both dyes are shown in parts a and b of Figure 12, respectively, and the corresponding parameters aL, KL, RL, KF, n, and R2 are given in Table 3. It can be seen that the R2 values for the Langmuir model are closer to 1, indicating that the Langmuir adsorption model explained the adsorption process of both dyes better than the Freundlich model. Since the value of 1/n is less than 1, it indicates that adsorption of both dyes on the polyaniline surface is favorable. Effect of Other Ions on Adsorption. The industrial effluent may contain different ions in addition to the anionic dyes, which may compete with the anionic dyes toward adsorption on PANI. The effects of the presence of ions like Cl−, NO3−,

Table 4. Value of Maximum Adsorption Capacity (Q0) in mg/g as Determined by Using a Langmuir Plot for the Adsorption of MY and O-II on Doped PANI in the Presence of Different Salts of 1 mM Concentration Q0 (mg/g) MY O-II

without salt

NaCl

NaNO3

Na2SO4

312.50 357.14

384.62 400.00

416.67 416.67

500.00 500.00

The increase in absorption density in the case of SO42− may be attributed to the highest polarizability among the anions (Cl− = 3.76 Å3, NO3− = 4.09 Å3, SO42− = 6.33 Å3) which is an important factor for adsorption of dyes at the PANI and water interface.26 Thermodynamics Parameters. The values of enthalpy change (ΔH0) and entropy change (ΔS0) were calculated from the Van’t Hoff equation G


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The Journal of Physical Chemistry B Table 5. Thermodynamic Parameters for the Adsorption of Dyes onto Doped Polyaniline Mordant Yellow (MY)

Orange-II (O-II)

T (K)

ΔG0 (kJ mol−1)

ΔH0 (kJ mol−1)

ΔS0 (J K−1 mol−1)

ΔG0 (kJ mol−1)

ΔH0 (kJ mol−1)

ΔS0 (J K−1 mol−1)

288 298 308 318

−1.018 −1.971 −2.509 −2.766

15.730

58.738

−1.018 −2.118 −2.856 −4.025

27.621

99.044

ln Kd = −

ΔH 0 ΔS 0 + RT R

physisorption processes are usually in the range 5−40 kJ mol−1, while the chemisorption processes are characterized by higher activation energies (40−800 kJ mol−1).33 The activation energy (Ea) is calculated from the slope of plots of ln k versus 1/T (Figure 15) using the Arrhenius equation (eq 12).

(9)

Kd = qe /Ce

(10)

where Kd is the equilibrium constant, qe is the amount of dye adsorbed at equilibrium (mg g−1), Ce is the concentration at equilibrium (mg L−1), R is the ideal gas constant (8.314 J K−1 mol−1), and T is the temperature in Kelvin. Values of ΔH0 and ΔS0 (Table 5) were calculated from the slope and intercept of Van’t Hoff plots of ln Kd versus 1/T (Figure 14).

ln k = ln A −

Ea RT

(12)

Figure 15. Plot of ln k vs 1/T for the adsorption of dyes onto doped PANI. Figure 14. Plot of ln Kd vs 1/T for the adsorption of dyes onto doped PANI.

where A is the Arrhenius pre-exponential factor, T is the temperature in Kelvin, and R is the universal gas constant. The activation energy for MY and O-II is found to be 30.803 and 23.117 kJ mol−1, respectively, indicating that the adsorption process is governed by physical adsorption in both cases. Desorption of Dye. In order to study the desorption of dyes, the doped polyaniline surface (adsorbed at pH 7 for 24 h) was exposed to solution in the range pH 9−12 and the results are presented in Figure 16. It was noticed that pH has a positive effect on the percentage of desorption. The desorption efficiency of both dyes was very less at pH 9. However, the percentage of desorption at higher pH (>11) increases to 92 and 94% for MY and O-II, respectively. This is mainly due to loss of electrostatic attraction of the positively charged polymer backbone surface and negatively charged dye molecules.

The values of Gibb’s free energy (ΔG0) at different temperatures were calculated according to eq 11. ΔG 0 = −RT ln Kd

(11)

The negative value of ΔG indicates the spontaneous nature of the adsorption process. The ΔG0 value of O-II is more negative compared to MY, indicating that adsorption is more spontaneous in the case of O-II, which is also supported by the fact that the equilibrium time is faster for O-II than for MY. The values of ΔG0 lie in the range from −1.018 to −4.025 kJ mol−1 , which suggests that the adsorption process is physisorption.27−31 Generally, ΔG0 values in the range from −20 to 0 kJ mol−1 are typically those of physisorption, while, in the case of chemisorption, the ΔG0 value ranges from −400 to −80 kJ mol−1.28,29 For both dyes, the ΔG0 value increases with increasing temperature, which suggests that the adsorption is enhanced at high temperature. The ΔH0 values as calculated from eq 9 were found to be 15.730 kJ mol−1 for MY and 27.6 kJ mol−1 for O-II, indicating that the adsorption is an endothermic process in both cases. The adsorption process is governed by a physical process, as the ΔH0 is within the range 1−93 kJ mol−1,31 while the positive value of ΔS0 reveals the increased randomness at the dyes and doped PANI interface during the adsorption process.32 Activation energy determination also gives information about the type of adsorption. The activation energies for 0

Figure 16. Desorption efficiency of MY and O-II from the polyaniline surface at different pH’s. H


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Figure 17. FTIR spectra of (a) MY, doped PANI, and MY loaded PANI and (b) O-II, doped PANI, and O-II loaded PANI.

Scheme 2. Electrostatic Attraction of MY (a) and O-II (b) with Doped PANI

Figure 18. UV−vis spectra of (a) alum doped PANI, (b) O-II, (c) O-II loaded PANI, (d) MY, and (e) MY loaded PANI.

bands at 1034 and 1122 cm−1 correspond to ν (SO) of  SO3−.34 The bands appearing at 1257 and 1451 cm−1 are assigned to ν (CN) and ν (NN), respectively. The bands at 1507 and 1619 cm−1 appear due to aromatic rings. The  SO3− group present in both dyes could lead to electrostatic interaction (Scheme 2) with the positively charged nitrogen of the polymer backbone, which is speculated by observing the decrease in intensity as well as shifting of the ν (SO) bands

Plausible Mechanism of Adsorption. The mechanism of adsorption was studied by recording the FTIR spectrum of both dyes and dye loaded PANI. FTIR spectra of MY (Figure 17a) show bands at 1036 and 1121 cm−1 corresponding to ν (SO) of the SO3 group. The bands appearing at 1296 and 1444 cm−1 are assigned to ν (CN) and ν (NN), respectively. The band at 1663 cm−1 is assigned to ν (CO) of the COOH group. The bands at 1485 and 1614 cm−1 appear due to aromatic rings. In the case of O-II (Figure 17b), the I


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toward lower wavelength after adsorption of both of the dyes on PANI (Figure 17). UV−vis Spectroscopy. The presence of dyes on the alum doped polyaniline surface was confirmed by recording the UV− vis spectrum of dyes and the doped PANI before and after adsorption of dyes, as shown in Figure 18 using NMP as solvent. Alum doped polyaniline shows two absorption peaks at 323 nm due to the π−π* transition and 629 nm due to the exciton-like transition from the benzenoid rings to the quinoid rings, respectively.35,36 O-II dye has its absorption maxima at 483 nm. This peak is shifted to 494 nm in the case of the dye loaded samples, which can be attributed to the interaction of alum doped PANI with O-II. Similarly, the absorption maximum in the case of Mordant Yellow-10 (395 nm in NMP) is shifted to 356 nm, indicating the interaction of the dye with alum doped PANI.

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CONCLUSION We have investigated the adsorption behavior of two anionic dyes on doped polyaniline, synthesized by doping of polyaniline with alum. The synthesized alum doped polyaniline is characterized by SEM, XRD, XRF, and FTIR. Adsorption of the anionic dyes MY and O-II was significantly high as compared to cationic dyes RB and MB. This may be due to the presence of the SO3− group in these anionic dyes, which is responsible for the electrostatic attraction by the positively charged surface of alum doped polyaniline as confirmed by the UV−vis spectrum. The adsorption kinetics of MY and O-II was found to follow a pseudo-second-order model. The temperature has a positive effect on the percentage of dye removal, suggesting that adsorption of the MY and O-II onto polyaniline was an endothermic process. Isotherm analysis reveals that the Langmuir model was the best fitting isotherm model for these two dyes. Adsorption of the dyes to the polyaniline surface was mainly governed by a physisorption process. Moreover, at pH > 11.0, the highest desorption of dyes from the polyaniline surface occurs. This is an economical, ecofriendly, as well as efficient method for removal of anionic dyes from water.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 91-98618131331. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. B. K. Mishra and Dr. P. K. Behera for helpful discussion. Deola Majhi acknowledges UGC India for the RGNF fellowship. We acknowledge DST and UGC, New Delhi, India, for providing instrumental facility under the FIST and DRS programs, respectively.

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REFERENCES

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