Preferential and Enhanced Adsorption of Dyes on Alum Doped

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

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Preferential and Enhanced Adsorption of Dyes on Alum Doped Nanopolyaniline Deola Majhi† and Braja N. Patra*,†,‡ †

School of Chemistry, Sambalpur University, Sambalpur, Odisha, India 768019 Department of Chemistry, Utkal University, Bhubaneswar, Odisha, India 751004

‡

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S Supporting Information *

ABSTRACT: This work demonstrates an easy and green method to synthesize alum doped nanopolyaniline (NDPANI). Nanopolyaniline was synthesized by using a template free interfacial polymerization of aniline using APS as oxidant. An environment friendly and nontoxic substance, potash alum, in contrast to hazardous mineral acid, was used as a dopant to create positive charge in the polymer backbone. This material promises a greener method for selective removal of anionic dyes like Orange-II (O-II) and Mordant Yellow (MY) from wastewater. The synthesized alum doped nanopolyaniline (NDPANI) was characterized by FTIR, Xray fluorescence (XRF), SEM, and DLS. The adsorption was studied by variation of adsorbent dosage, pH, temperature, time, and initial concentration of anionic dyes under different reaction conditions. Adsorption of dye follows pseudo-second-order kinetics. Langmuir isotherm was found to be the best fitting model with the maximum adsorption capacity for O-II being 667 mg/g at 308 K. The thermodynamic parameters such as the Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy changes (ΔS°) were calculated and revealed that the adsorption process is spontaneous and endothermic in nature. The material exhibits enhanced dye adsorption capacity and can be used for removal of O-II effectively.

1. INTRODUCTION The major pollutant present in water is toxic dyes, which cause a threat to human health and are effluents of different chemical, textile, printing, leather, and even biotechnology industries.1 These dyes are resistant to biodegradation and photochemical degradation due to their complex chemical structure. Thus, efficient dye removal methods based on costeffective and eco-friendly techniques are essential.1,2 Different strategies have been developed to address the growing need for the removal or degradation of these dyes from the industrial wastes. The most commonly used methods are coagulation, chemical oxidation, membrane separation, electrochemical processes, photodegradation, microbial degradation, and adsorption.3−7 Among these, adsorption is an efficient, economical, and versatile method for the removal of dyes from waste waters and it produces water free of any harmful aromatic amines and free radicals produced by other techniques such as microbial and photodegradation.8 Actived carbon is most extensively used as an adsorbent for removal of traditional pollutants such as dyes, phenols, pharamaceutical pollutants, organic acids, and heavy metals.9−11 However, activated carbon has some shortcomings such as costly in regeneration and nonselective. To avoid the above limitations, other adsorbents have been reported, for instance, macro algae, natural or synthetic organic and polymeric materials, inorganic solid, and metal−organic frameworks for effective removal of dyes.12−22 The drawbacks associated with these materials are effectiveness and their cost.23 Therefore, it becomes highly © XXXX American Chemical Society

imperative to synthesize new low cost adsorbents for removal of dyes from wastewater efficiently. Polyaniline (PANI) is attracting the attention of researchers, as it is inexpensive, is environmentally stable, has a high electrical conductivity, has a simple synthetic procedure, and has a wide range of applications such as solar cell, lightemitting display devices, rechargeable battery, and optical storage lithography.24 Polyaniline and its composites have been used as adsorbents to remove dyes from water.25−28 Removal of anionic and cationic dyes by using PANI can be achieved only when the backbone of the polymer is positively charged or neutral, respectively.29−32 Doping with an acid, for instance, ptoluenesulfonic acid, camphorsulfonic acid, or simple mineral acids, which have a pH less than 4, is responsible for creation of positive charge in the PANI backbone.33 This doping condition in turn causes widespread damage to the environment and is not suitable for industrial applications. In our previous study, we demonstrated the adsorption of anionic dyes in the presence of potash alum doped PANI, which provides a nontoxic, eco-friendly environment for adsorption.34,35 However, synthesizing the material in nano form could lead to improvements in the adsorption capacity with superior dye removal properties. Therefore, better routes to Received: April 18, 2018 Accepted: August 14, 2018

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DOI: 10.1021/acs.jced.8b00312 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2.3. Adsorption and Desorption Experiment. The adsorption behavior of the dyes onto NDPANI was studied by varying the initial concentration of dye, pH, contact time, amount of adsorbent, and temperature under the aspect of thermodynamics study, adsorption isotherm, and kinetics of adsorption. In a typical experiment, 50 mL portions of different dye solutions having initial concentrations of 100−1000 ppm were added to 60 mg of NDPANI and stirred for 8 h. The concentration of dye at different time intervals was determined by UV−vis spectroscopy at their respective absorbance maxima (354 nm for MY and 483 nm for O-II). The amount of dye adsorbed at equilibrium, qe (mg/g), was calculated by following eq 1

highly efficient, low cost, and environment friendly materials are clearly needed. In this study, we report the synthesis of alum doped nanopolyaniline (NDPANI) by using a cheap environment friendly potash alum and preferential adsorption of anionic dyes from the mixture of dyes. The effects of various operational parameters such as initial dye concentration, adsorbent doses, pH, temperature, contact time on adsorption process and thermodynamic parameters, and adsorption kinetics were investigated. Moreover, we made a comparison of the dye adsorption capacity of different adsorbents with alum doped nanopolyaniline (NDPANI).

2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium persulfate (APS) [(NH4)2S2O8], potash alum, and tetrahydrofuran (THF) were used as received from Merck (India). Aniline was obtained from Merck (India) and distilled under a vacuum before use. Rhodamine-B (RB) (Lobachemie, India), Methylene Blue (MB) (Nice Chemicals, India), Mordant Yellow-10 (MY), and Orange-II (O-II) (Acros Organics) were used without further purification. For the preparation of all of the required solutions, deionized water was used. 2.2. Synthesis and Characterization of Alum Doped Nanopolyaniline. Nanopolyaniline was synthesized by a little modification of the procedure described by Huang et al.36 In a typical procedure, 4 mmol (0.3642 mL) of freshly distilled aniline was added to 10 mL of toluene in a flask and stirred for 30 min with a magnetic strirrer. A solution of 1 mmol (0.228 g) of APS in 10 mL of 1 M HCl was taken in another flask. The solutions of two flasks are mixed and allowed to stir for 24 h for complete polymerization. A bilayer solution mixture was formed in which nanopolyaniline was precipited out and floated on the upper layer. It was filtered out and collected on a Buchner funnel. The precipitate cake was washed exhaustively with deionized water. The precipitate cake was further treated with 0.1 M NaOH solution and stirred for 3 h to obtain the emeraldine base (EB) form of the nanopolyaniline. The nanopolyaniline is then filtered off, washed with distilled water, and dried in a vacuum for at least 24 h. A 200 mg portion of nanopolyaniline base in 15 mL of THF was taken, and to this, 50 mL of 0.1 M aqueous potash alum solution was added and the stirring was continued for 24 h. Then, the NDPANI was filtered and washed thoroughly with distilled water and dried at 50 °C for 24 h under a vacuum. The scheme of preparation of NDPANI is shown in Supporting Information Figure S1. The NDPANI samples were characterized by XRF, scanning electron microscopy (SEM), FTIR, and dynamic light scattering graph (DLS). A PerkinElmer Spectrum-2000 (FTIR) spectrophotometer was used to obtain the IR spectra between 400 and 4000 cm−1. The morphology was determined by using a scanning electron microscope, Zeiss EVO50. A wavelength dispersive X-ray fluorescence spectrophotometer (WD-XRF) (make, Bruker; model, S8 Tiger) was used to determine the Al contents in NDPANI. The pellet was prepared by mixing powder NDPANI and binder wax (ratio 80:20, w/w) and exposed to 40 t of pressure. The pellet was subjected to analysis by vacuum mode XRF. The BET surface areas were measured with an Autosorb-iQ and ASiQWiN device from Quantachrome. Specific surface areas were calculated using the BET (Brunauer−Emmett−Teller) method. The particle size distribution and zeta potential were measured with a Microtrac Zetatrac Particle Size Analyzer.

qe =

V (C0 − Ce) m −1

(1) −1

where Ce (mg L ) and C0 (mg L ) represent the equilibrium and initial concentration of dye solution, respectively; m (g) is the mass of the adsorbent used; and V (L) is the volume of dye solution. For desorption experiments, dye loaded NDPANI was used. At first, dye solution was added to NDPANI and stirred for 8 h. After adsorption of dye, the dye loaded NDPANI samples were removed by centrifugation and washed repeatedly with distilled water and dried under a vacuum. The above sample was treated with basic solutions (pH 12) and stirred for 5 h. The concentration of dye was determined by UV−vis spectroscopy after separation of adsorbent by centrifuge. 0.1 M NaOH and 0.1 M HCl were used to adjust the pH of the system. The UV−vis measurements were carried out with a Hitachi U3210 spectrophotometer in the range 200−800 nm.

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbents. The nanopolyaniline was synthesized by interfacial polymerization of aniline using ammonium persulfate as oxidant. Doping of nanopolyaniline was done by using 0.1 M aqueous potash alum solution. Figure 1 represents the FTIR spectra of nanopolyaniline and NDPANI. When comparing with the NDPANI, the bands at 1583 and 1492 cm−1 (attributed to CC stretching of the quinoid and

Figure 1. FTIR spectra of nanopolyaniline (a) and NDPANI (b). B



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benzoid unit, respectively) present in the EB form of nanopolyaniline are shifted to lower wavenumbers.37 A significant shift of the peaks at 1289 and 1161 cm−1 to 1294 and 1107 cm−1, respectively, indicates the structural changes in nanopolyaniline. The degree of doping or delocalization of electron can be estimated from the intensity of the peak at 1107 cm−1. The dynamic light scattering (DLS) technique was used to determine the particle size of NDPANI and is shown in Figure 2.38 From the study, a bimodal distribution was observed with

Figure 4. Proposed structure of NDPANI.

al. in the case of alkali metal salt doped polyaniline.40 Three sulfate ions, acting as counteranions, are responsible for neutralization of total charge present in the polymer backbone. 3.2. Adsorption Study. At the beginning, various classes of dyes with different skeletons including azo (Mordant Yellow-10, Orange-II), heteropolyaromatic (Methylene Blue), and xanthane (Rhodamine-B) were used for adsorption study (Figure S2). In this study, bulk alum doped polyaniline (BDPANI) was synthesized by following the same procedure which was described in our previous work.34 Variation of dye concentration with respect to time in the presence of 60 mg of each adsorbent, i.e., BDPANI and NDPANI, is shown in Figure 5. It can be noticed that adsorption of cationic dyes (MB, RB) on the surface of both BDPANI and NDPANI was negligible, whereas a considerable amount of anionic dyes (MY and O-II) was adsorbed by using both adsorbents. The results reveals that adsorption is more when NDPANI is used as adsorbent and O-II is used as dye. Moreover, we did not observe adsorption of anionic dye when nanopolyaniline base (emeraldine form without doping with potash alum) was used as adsorbent. This clearly shows that positive charge is generated in the backbone of the polymer after doping which is responsible for electrostatic attraction of anionic dye. Effect of Adsorbent Dose. In order to evaluate the maximum adsorption with the minimum amount of adsorbent, the adsorption of dye in the presence of different amounts of adsorbent was carried out (Figure 6). As shown in Figure 6, the percentage of O-II removal increases with increased adsorbent dose and after 60 mg of adsorbent there is no significant change in the percentage of removal. At higher adsorbent dose to O-II concentration ratios, there is a higher percentage of O-II removal. This can be explained as a certain amount of dye that can be adsorbed by a fixed dose of adsorbent. Thus, the more the volume of effluent containing a fixed concentration of dye, the more dosage of adsorbent is required to purify it. Effect of Initial Concentration of Dye and Contact Time. The initial concentration of anionic dye provides an essential driving force to overcome the mass transfer resistance of the

Figure 2. DLS graph of NDPANI.

the main peak mode consistently around 40 nm diameter and a second peak mode around 110 nm. The average particle size distribution of the NDPANI is between 40 and 110 nm (Figure 2). It is speculated that the nano size of the material can make it a promising candidate for adsorption of dyes. The SEM micrographs in Figure 3 show the sample morphology at two different magnifications. The image in Figure 3b clearly shows spherical particles with a size in the range from 40 to 130 nm. The distribution of both bigger and smaller particles as observed in the SEM image matches well with DLS results. In this agglomerated nanostructure, nanosize pores may also be seen in Figure 3b (marked by an arrow). We propose that, during the process of washing and drying of the sample, spherical NDPANI particles self-aggregate to form porous structures. Adsorption of dye is facilitated by the pores present in the material.39 The Brunauer−Emmett−Teller (BET) measurement indicates that the specific surface area of NDPANI (54.6 m2/g) is larger than that of nanopolyaniline (28 m2/g), which is favorable for more adsorption. The proposed structure of nanopolyaniline is shown in Figure 4 below. The XRF analysis shows that 1.4% of Al atoms are present in the doped sample. The aluminum is doped to the polymer backbone, resulting in a pseudoprotonation similar to the results reported by Chen et

Figure 3. SEM micrographs of NDPANI at lower resolution (a) and at higher resolution (b). C



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Figure 5. Variation of dye concentration in the presence of 60 mg of BDPANI and NDPANI with time.

reached very quickly. At higher initial dye concentrations, the adsorption of dye reaches equilibrium quite slower. In the case of lower dye concentrations, the number of available adsorption sites is more compared to the number of dye molecules; hence, quicker adsorption takes place. At higher concentrations, the number of available adsorption sites becomes lower as compared to the number of dye molecules, and reaches dye removal equilibrium quite slower. Effect of Temperature. The effect of temperature on adsorption was studied and represented in Figure 8. The Figure 6. Percentage of removal of O-II as a function of dosage of adsorbent at 1000 mg/L initial concentration dye.

dye between the aqueous and the solid phase adsorbent. The effect of the initial concentration of dyes in the range from 100 to 1000 mg/L on dye removal using NDPANI is shown in Figure 7.

Figure 8. Variation of the percentage of removal of 1000 mg/L of OII with temperature.

adsorption parameters were studied at four different temperatures (15, 25, 35, and 45 °C) with 1000 mg/L initial dye concentration in contact with 60 mg of NDPANI. It was observed that as the temperature increases the extent of adsorption of dye on the surface of NDPANI increases, which indicates the endothermic nature of the adsorption process. Effect of pH. The influence of pH, ranging from 3 to 9, on the removal of dye was studied by using 50 mL of 1000 mg/L Orange-II solution in contact with 60 mg of adsorbents, and the result is shown in Figure 9. The result indicated that pH variations highly influence the dye adsorption capacity. It can be observed that the highest removal of dyes occurs in an acidic medium (pH 3). For an in-depth understanding of the influence of pH on the adsorption process, the zeta potential at different pH values was measured, which provides information about the surface charge of NDPANI. At low pH, the surface charge of NDPANI is positive. With an increase in pH, the positive charge decreases gradually and passes through zero potential at pH 8.4 (Figure S3). The higher adsorption at lower pH is probably due to the electrostatic force of attraction

Figure 7. Plot of qt vs time of O-II in the presence of 60 mg of NDPANI.

This is clearly ensuing from the graph that with an increase in the initial dye concentration the equilibrium capacity increases. This can be attributed to active sites available on the adsorbent for dye removal. For a specific amount of adsorbent, the amount of dye adsorption onto NDPANI increases upon increasing the initial dye concentration. This can be attributed to an increase in the concentration gradient at higher initial concentrations of dye. Dye adsorption on NDPANI is very intense at low initial dye concentrations; thus, equilibrium is D



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where KF and n represent the Freundlich constants related to the bonding energy and the adsorption intensity, respectively. The adsorption isotherms of Orange-II are studied on the surface of NDPANI at different temperatures and are presented in Figure 10 (equilibrium data, Table S1). The related parameters KL, aL, RL, KF, n, R2, and χ2 are shown in Table 1. The dimensionless constant is called the separation factor (RL also known as the equilibrium parameter) and is defined by eq 446 RL = Figure 9. Removal percentage of anionic dye in the presence of 60 mg of NDPANI.

1 a qe = KL + L KL Ce −1

where C0 (mg L ) is the initial concentration of dye and aL (L mg−1) is calculated from the slope of the Langmuir isotherm related to the energy of adsorption. The value of RL assumes the nature and feasibility of the Langmuir isotherm which can be either favorable (RL < 1) or unfavorable (RL > 1) or linear (RL = 1) or irreversible (R = 0).47 Data analysis based on high correlation coefficients R2 was performed to justify the best fitting isotherm for the adsorption process. As the Langmuir isotherms give very high values of R2, this model is the best fit model for the adsorption. Furthermore, the chi-square test was performed to find the suitability of an isotherm that best fits the experimental data. 2

χ =

∑

(qe,exp − qe,theo)2 qe,theo

(5)

The chi-square statistic test (eq 5) is basically the sum of the squares of the differences between the experimental data and theoretically predicted data from models, where qe,exp (mg/g) and qe,theo (mg/g) are the equilibrium capacity obtained from experimental and model data, respectively. If data from the model and experiment are close to each other, χ2 would be a small value and vice versa. The values of χ2 of each model at different temperatures are shown in Table 1. The lowest χ2 values observed in the Langmuir isotherm suggest that the Langmuir isotherm provides the best fit to the experimental data. 3.4. Adsorption Kinetics. Adsorption kinetics can give information regarding the uptake rate of adsorbents, thereby enabling the mechanism of adsorption to be determined.48 To gain a better insight into the adsorption process, pseudo-firstorder, pseudo-second-order, and intraparticle diffusion models have been used to fit the experimental data, which are expressed by the following equations ln(qe − qt) = ln qe − k1t t

(2) −1

where aL (L mg ) and KL (L g ) represent the Langmuir constants and KL/aL corresponds to the Langmuir monolayer saturated adsorption capacity (Q0). qe (mg g−1) is the amount of dye adsorbed per unit mass of adsorbate at equilibrium, and Ce is the equilibrium concentration of dye. The Freundlich isotherm model assumes multilayer adsorption at heterogeneous surfaces having interaction between adsorbed molecules.44 This heterogeneous isotherm system is expressed as follows qe = KFCe1/ n

(4)

−1

existing between the negatively charged dye molecule and the positively charged adsorbent surface. As the pH of the system increases, the decrease in the positive charge in the backbone of the polymer is responsible for the poor electrostatic attraction leading to lower adsorption. The negative surface charge at pH above 8.4 is responsible for the decrease in the electrostatic force of attraction between the adsorbent and negatively charged dye molecule, which hinders the adsorption. However, physical forces, for instance, van der Waals force, hydrogen bonding, etc., in the adsorption process are responsible for the substantial amount of adsorption in this pH range. A similar observation is reported for polypyrrole and polyaniline nanofiber by Bhaumik et al.41 Therefore, surface charge, structure of dye, and electrostatic attraction could play a vital role in the adsorption of dye molecules on NDPANI at various pH values.42 3.3. Adsorption Isotherm. The adsorption isotherm plays an important role in the prediction of the appropriate model that the adsorption follows and gives information about the maximum capacity of adsorption and the equilibrium relationship between adsorbent and adsorbate.43 Out of the several isotherm models available for analyzing experimental data, the Langmuir and Freundlich isotherm models are employed for describing the equilibrium data of adsorption. The Langmuir model is based on the assumption that a monolayer adsorption of dyes occurs at the surface of homogeneous adsorbent having no interaction between the adsorbate molecules.44 Since all of the adsoption sites are of equivalent energy, it is believed that no multilayer adsorption occurs once the adsorbent site is covered with the dye molecules.45 The linear form of Langmuir eq 2 is given as Ce

1 1 + aLC0

(6)

1 1 qt = k 2qe 2 + qe t

(7)

qt = k it 1/2 + C −1

(8) −1

−1

−1

−1/2

where k1 (min ), k2 (g mg min ), and ki (mg g min ) represent the rate constants of the pseudo-first-order, pseudosecond-order, and intraparticle diffusion for the adsorption process, respectively. qe and qt (mg g−1) are the amount of dye adsorbed per unit mass of NDPANI at equilibrium and at time t, respectively. C (mg g−1) is the constant associated with the thickness of the boundary layer, and t (min) is the time. The kinetic model fitting curves of the pseudo-first-order, pseudo-

(3) E



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Figure 10. Langmuir adsorption isotherms at temperatures of (a) 15 °C, (b) 25 °C, (c) 35 °C, and (d) 45 °C and Freundlich adsorption isotherms at temperatures of (e) 15 °C, (f) 25 °C, (g) 35 °C, and (h) 45 °C of O-II onto NDPANI.

Table 1. Isotherm Constants and Correlation Coefficients of the Langmuir and Freundlich Models at Different Temperatures and at pH 7 Langmuir model

Freundlich model 2

temp (K)

aL (L/mg)

KL (L/g)

RL

R

288 298 308 318

1.4324 0.0340 0.0254 0.0263

27.0270 15.4560 16.9205 17.5439

0.0652−0.0069 0.2272−0.0285 0.2826−0.0379 0.2753−0.0366

0.993 0.996 0.991 0.995

second-order, and intraparticle diffusion models are shown in parts a, b, and c of Figure 11, respectively.

χ

2

26.03 9.61 7.52 5.69

nF

KF (L/mg)

R2

χ2

5.757052 2.876043 1.756235 1.798238

69.6145 62.3448 36.2994 37.9577

0.892 0.991 0.975 0.975

31.94 34.27 35.92 35.25

Kinetic parameters were calculated to evaluate the most suitable model fitting for the data and summarized in Table 2. F



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Figure 11. Plots of pseudo-first-order kinetics (a), pseudo-second-order kinetics (b), and the intraparticle diffusion model (c). Reaction conditions: material mass 60 mg, volume 50 mL, pH 7.0, 35 °C.

Table 2. Kinetic Parameters Calculated at Different Initial Dye Concentrations (C0) for O-II Adsorption onto NDPAN at 35 °C pseudo-first-order kinetics

pseudo-second-order kinetics

intraparticle diffusion

C0 (mg/L)

K1 (min−1)

R2

SSE

K2 (g mg−1 min−1)

R2

SSE

Ki (mg g−1 min−1/2)

R2

SSE

100 500 1000

0.0269 0.0154 0.0267

0.9936 0.9567 0.9202

0.002322 0.038566 0.101036

0.00607768 0.00160952 0.00036981

0.9946 0.9916 0.9924

0.000145 0.000026 0.000017

6.0064 40.1142 60.4163

0.7575 0.9671 0.9172

0.488314 0.588967 0.536952

Figure 12. Plots of ln Kd vs 1/T (a) and ln k vs 1/T (b) for the adsorption of O-II onto NDPANI.

From Table 2, it can be observed that R2 values are near unity for all dye concentrations by choosing the pseudo-secondorder kinetic model. This indicates that adsorption kinetics can be most effectively described by the pseudo-second-order model. Further, the sum of square error (SSE) test was used to estimate the best fit of the model (eq 9) ÄÅ ÉÑ 2Ñ ÅÅ (q ÅÅ t ,exp − qt ,theo) ÑÑÑ ÑÑ SSE = ∑ ÅÅÅ ÑÑ ÅÅ (qt ,exp)2 ÑÑ ÅÅÇ (9) ÑÖ

values for all dye concentrations when compared with the other two models, i.e., the pseudo-first-order and intraparticle diffusion models. This suggests that adsorption of anionic dye on the NDPANI surface follows pseudo-second-order kinetics. Adsorption Thermodynamics. Thermodynamic parameters are an essential component to estimate the practical application of adsorption processes. Thermodynamic parameters, namely, the Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°), were calculated using the following equations

where qt,exp is the experimental value of the adsorption capacity at time t and qt,theo is the corresponding value obtained from the kinetic models. The values are summarized in Table 2. It is found that the pseudo-second-order model has the lowest SSE

ΔG° = − RT ln Kd G

(10) DOI: 10.1021/acs.jced.8b00312 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data ln Kd = −

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ΔH ° ΔS° + RT R

(11)

Kd = qe /Ce

(12) −1

where Kd is the equilibrium constant, qe (mg g ) and Ce (mg L−1) are the amount and concentration of dye adsorbed at equilibrium, T is the temperature in Kelvin, and R is the universal gas constant (8.314 J K−1 mol−1). The slope and intercept obtained from the plots of ln Kd versus 1/T (Figure 12a) are used to calculate the values of ΔH° and ΔS° and mentioned in Table 3. Table 3. Thermodynamic Parameters for the Adsorption of O-II onto NDPANI T (K)

ΔG° (kJ·mol−1)

ΔH° (kJ·mol−1)

ΔS° (J K−1 mol−1)

288 298 308 318

−4.7884 −5.4006 −6.5605 −6.8344

16.3436

73.3984

Figure 13. FTIR spectra of O-II (a), NDPANI (b), and O-II loaded NDPANI (c).

On the basis of the negative values of ΔG° and the positive value of ΔH°, the adsorption process is found to be spontaneous and endothermic in nature49−53 and the positive values of ΔS° reflect the increased randomness during the adsorption.54 The magnitude of activation energy gives information about the adsorption mechanism. Plots of ln k versus 1/T (Figure 12b) determine the activation energy, Ea, by using the Arrhenius equation (eq 13) ln k = ln A − Ea /RT

the −SO3− group of the dye molecule and the positively charged nitrogen moiety of NDPANI. 3.6. Regeneration. In this work, we employed base treatment to investigate the regeneration performance of NDPANI. After the first adsorption run (C0 = 500 mg/L, T = 35 °C, adsorbent dosage = 60 mg/L), the dye loaded adsorbent was treated with base followed by centrifugation, dried in vacuum desiccators overnight, and doped with potash alum to regenerate NDPANI. The above NDPANI was used in the next adsorption run. A relative decrease in adsorption was seen on application of preceding adsorbent in further cycles. With advantages like fast adsorption rate, recyclability, high adsorption capacity, and better selectivity, the NDPANI is considered a promising candidate for the removal of anionic dye (Figure 14).

(13)

where k is the rate constant, T is the temperature in Kelvin, A is the Arrhenius pre-exponential factor, and R is the universal gas constant. The activation energy is calculated from the slope. The activation energy for O-II is found to be 8.921 kJ mol−1. The lower activation energy indicates that the adsorption process is faster in comparison to our previous report.34 3.5. Plausible Mechanism of Adsorption. In order to establish the mechanism of adsorption, FTIR spectra of dye and dye loaded NDPANI were recorded. The bands appearing at 1034 and 1122 cm−1 are attributed to ν(SO) of the  SO3− group present in the O-II dye.55 The bands at 1228 and 1451 cm−1 are due to ν(CN) and ν(NN), respectively. The bands at 1506 and 1616 cm−1 appear due to aromatic rings. The ν(SO) band significantly weakened and shifted to lower wavelength after adsorption of the dye on NDPANI. This observation confirms the electrostatic interaction exists between the −SO3− group present in the dyes and the positively charged nitrogen of the polymer backbone (Figure 13). It should be noted that all peak positions of the NDPANI before dye adsorption are shifted and the intensities of the peaks are diminished when compared with NDPANI after adsorption. There is considerable shift of the vibrational band due to the aromatic CC bond of NDPANI from 1573 to 1568 cm−1 which could be due to localization of aromatic πelectrons of NDPANI owing to interaction with the dye molecule. The peak 1107 cm−1 (CN+) of NDPANI shifted to higher wavenumber (1122 cm−1) after dye adsorption which may be due to electrostatic attraction of

Figure 14. Regeneration and adsorption performance of NDPANI.

3.7. Comparative Study. The maximum adsorption capacity for O-II onto different adsorbents as well as PANI based adsorbents, reported in the literature, is shown in Table 4. The data indicated that, in comparison to other material, the material developed in this work shows better removal of O-II dye and adsorption equilibrium can be achieved within 60 min with the monolayer adsorption capacity calculated from the Langmuir equation being 667 mg/g. This high value may be H



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would like to extend thanks to the UGC for the RGNF fellowship.

Table 4. Maximum Adsorption Capacity for Materials Reported in the Literature for Orange-II

■

max. adsorption capacity, qe (mg/g)

adsorbent 56

functionalized titanosilicates de-oiled soya57 sludge58 activated carbon fibers58 unmodified zeolite59 surfactant-modified zeolites59 fly ash60 poly(N-isopropylacrylamide) microgels61 kapok fiber oriented polyaniline62 (KF-O-PAN) BDPANI34 NDPANI

49.98 10.5 98.196 101.150 0.7 3.15 84 11.505 188.7

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357.14 667

attributed to the low activation energy of the adsorption process and electrostatic attraction between dye and adsorbents.

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CONCLUSION Our work demonstrates synthesis of NDPANI for the first time, supported by the characterization techniques FTIR, XRF, SEM, and BET. Morphology characterization of samples indicates the porous nature of the adsorbent. The adsorption of O-II dye onto NDPANI was dependent upon parameters such as the initial dye concentration, contact time, pH, and temperature. The material was proven to be a low cost adsorbent for selective removal of anionic dyes from water effectively. The highest removal efficiency for O-II dye (∼96%) was achieved by 60 mg of NDPANI at 308 K temperature. This material has enhanced adsorption capacity toward O-II dye when compared to other adsorbents. The adsorption of OII follows Langmuir isotherm, pseudo-second-order kinetics and was observed to be spontaneous and endothermic in nature, as evident by the thermodynamic parameters. Simple base treatment may lead to the regeneration of the adsorbent. The work described here has several merits such as low cost material from cheap precursors and an environment friendly method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00312. Scheme of the preparation of NDPANI, structure of dyes, zeta potential of NDPANI, and equilibrium data for adsorption of O-II onto NDPANI (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 91-8895019001. E-mail: [email protected]. ORCID

Braja N. Patra: 0000-0002-6783-0311 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge DST-FIST Govt. India and UGC-DRS, New Delhi, India, for providing the instrumental facility. D.M. I



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