chitosan composite: An eco-friendly polymer for enhanced removal of dyes from aqueous solution

Synthetic Metals 162 (2012) 974–980

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Polyaniline/chitosan composite: An eco-friendly polymer for enhanced removal of dyes from aqueous solution V. Janaki a,1 , Byung-Taek Oh b,1 , K. Shanthi c , Kui-Jae Lee b , A.K. Ramasamy a,∗∗ , Seralathan Kamala-Kannan b,∗ a

Department of Chemistry, Periyar University, Salem 636011, Tamil Nadu, India Division of Biotechnology, Advanced Institute of Environment and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570752, South Korea c Department of Environmental Science, PSG College of Arts and Science, Coimbatore 641014, Tamil Nadu, India b

a r t i c l e

i n f o

Article history: Received 12 March 2012 Received in revised form 6 April 2012 Accepted 13 April 2012 Keywords: Chitosan Composite Dyes Polyaniline Polymer

a b s t r a c t Batch sorption system using eco-friendly polymer/bio-polymer composite (polyaniline/chitosan) as adsorbent was investigated to remove Congo Red, Coomassie Brilliant Blue, Remazol Brilliant Blue R, and Methylene Blue from aqueous solution. Scanning electron micrograph showed that the surface of the composite was rough with pleats, providing a good possibility for dye adsorption. X-ray diffractogram of the composite showed the main reflections of both chitosan and polyaniline (2 = 9.8, 19.8, 20.0, and 25.3). Experiments were carried out as a function of contact time, initial dye concentration (100 mg L−1 ), pH (3), and adsorbent dose (0.1 g L−1 ). The maximum percentage removal was found to be 95.4% for Congo Red, 98.2% for Coomassie Brilliant Blue, and 99.8% for Remazol Brilliant Blue R. Langmuir model showed satisfactory fit to the equilibrium adsorption data of polyaniline/chitosan composite. The kinetics of the adsorption followed pseudo second-order rate expression, which demonstrates that intraparticle diffusion plays a significant role in the adsorption mechanism. Fourier transform infrared spectroscopy and desorption studies confirmed the involvement of amino and hydroxyl groups in dye adsorption. The results indicate that the polymer matrix could be used as an adsorbent for the removal of sulfonated dyes from aqueous solution. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Many industries such as textile, rubber, leather, paper, cosmetics, plastics, food, and pulp use synthetic organic dyes and water to color their products. Discharging even a small amount of dyes into water resources can affect the biotic communities of ecosystem [1]. Therefore, removal of dyes from the wastewater has been an important environmental concern to minimize the water and soil pollution. Several conventional processes such as coagulation, flocculation, biodegradation, adsorption, ion-exchange, and advanced oxidation are most commonly used for the removal of dyes from wastewaters. Among these methods, adsorption has been found to be one of the most prominent and economic methods for the treatment of dyes from wastewater [2].

∗ Corresponding author. Tel.: +82 63 850 0842; fax: +82 63 850 0834. ∗∗ Corresponding author. Tel.: +91 427 2345271. E-mail addresses: [email protected] (A.K. Ramasamy), [email protected], [email protected] (S. Kamala-Kannan). 1 Both authors contributed equally to this work. 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.04.015

Numerous studies proposed a variety of adsorbent materials for the removal of dyes from wastewater [3–7]. Among these, a natural biopolymer, chitosan, has been recently investigated as adsorbent for the removal of dyes [8–10]. The hydrophilic nature, biocompatibility, biodegradability, antibacterial properties, nontoxicity, low-cost, fast adsorption kinetics, and renewable nature have increased the application of chitosan in adsorption process [11]. Mahmoodi et al. [12] reported that the chitosan had effectively adsorbed anionic dyes in both single and binary system. The amino and hydroxyl groups present in the chitosan can serve as coordination and reaction site for the adsorption of several groups of pollutants [13]. However, limited conductivity, low surface area, pH sensitivity, and low porosity have reduced the applications of chitosan in industrial scale [14]. To overcome these disadvantages, chitosan-based composites have been prepared and proven to have resistance to acidic environment with better adsorption [15]. Polyaniline is a well-known semi-flexible, low-cost, and environmentally stable rod-like conducting polymer [16]. The unique electrical and electrochemical properties of polyaniline have led to use this polymer in various applications such as light-emitting diodes, plastic batteries, solid-state sensors, harmonic generators, energy storage devices, anti-static and anti-corrosion coating

V. Janaki et al. / Synthetic Metals 162 (2012) 974–980

materials [17–19]. It can be easily synthesized from bronsted acidic aqueous solutions. Recently, some investigators directed their research toward application of polyaniline and its composites for environmental remediation. The principle is based on the chelating properties attributed to the electron-donating groups (amine and secondary amino groups) on the polyaniline polymers [20]. Mahanta et al. [21] used polyaniline emeraldine salt for the removal of sulfonated dyes. Ai et al. [22] reported that the polyaniline microspheres possess potential efficiency to remove methyl orange from aqueous solutions. Karthikeyan et al. [23] studied the adsorption of fluoride by polyaniline/chitosan composite. However, there is no report about polyaniline/chitosan composite for the removal of dyes from aqueous solution. Hence, the objective of the present work was to develop simple, cost-effective, and environmentally benign method for the removal of organic dyes from aqueous solution using polyaniline/chitosan composite. The experimental variables affecting optimal adsorption were investigated. Various isotherm models and adsorption kinetics also investigated in detail.

2. Materials and methods 2.1. Materials Chitosan (100%) was procured from YB Bio, Kyoungsangbukdo, Korea. Aniline was purchased from Sigma–Aldrich Co. (St. Louis, MO) and distilled before use. The four dyes Congo Red (CR), Coomassie Brilliant Blue (CBB), Remazol Brilliant Blue R (RBBR), and Methylene Blue (MB) were also purchased from Sigma–Aldrich Co. The absorbance maxima (max ) of the four dyes were observed at 497 nm, 555 nm, 591 nm, and 664 nm, respectively. Nanopure purified water (conductivity = 18 ␮/m, TOC < 3 ppb, Barnstead, Waltham, MA, USA) was used for the preparation of all reagents. All other chemicals used in the experiment were of analytical grade. 2.2. Preparation of polyaniline/chitosan composite Polyaniline/chitosan (Pn/Ch) composite was prepared according to Karthikeyan et al. [23]. Briefly, 1 g of chitosan was dissolved in aqueous acetic acid (20%), and the solution was stirred for 24 h at room temperature. To this about 0.01 M of aniline dissolved in 1 M HCl was added and stirred for 15 min to form a homogenous solution. Ammonium peroxydisulfate (4.4 g) solution in 1 M HCl was dropped into the above solution with constant stirring at 5 ◦ C. After complete addition of the oxidizing agent, the reaction mixture was kept under constant stirring for 6 h. The greenish-black precipitate obtained was washed with water followed by methanol until the filtrate become colorless. The final composite was dried at 60 ◦ C for 24 h and used as adsorbent for the removal of dyes from aqueous solution. 2.3. Characterization of Pn/Ch composite Morphological features and surface characteristics of Pn/Ch composite were obtained from scanning electron microscopy (SEM) using a JEOL JSM-6400 microscope (Tokyo, Japan). X-ray diffractograms (XRD) were obtained using a Cu K␣ incident beam ( = 0.1546 nm), monochromated by a nickel filtering wave at a tube voltage of 40 kV and tube current of 30 mA. The scanning was done in the region of 2 from 5◦ to 50◦ at 0.04◦ min−1 with a time constant of 2 s. The Fourier transform infrared spectroscopy (FTIR) spectra of the Pn/Ch composite before and after adsorption of dyes were obtained on a Perkin-Elmer FTIR spectrophotometer (CA, USA) in the diffuse reflectance mode at a resolution of 4 cm−1 in KBr pellets.

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2.4. Batch experiments for dye removal Batch experiments were performed according to Mahanta et al. [24]. Briefly, 0.1 g of the Pn/Ch composite was agitated with 100 mL of dye solutions individually at 26 ◦ C in a rotary incubated shaker. The shaking speed was maintained at 180 rpm throughout the study. Samples were collected at the predetermined time intervals, and the dye solution was separated from the adsorbent by centrifugation at 6000 rpm for 5 min. The dye concentration was analyzed by monitoring the absorbance of the dyes using UV–Vis spectrophotometer (UV-1800 Shimadzu, Japan). Experimental variables considered were (i) the effect of pH on the adsorption capacities, (ii) dosage of Pn/Ch composite, (iii) and initial dye concentration (100–500 mg L−1 ). 2.5. Desorption study The dye-loaded Pn/Ch composite, which were exposed to 100 mg L−1 of dyes (CR, CBB, RBBR, and MB) at 26 ◦ C, was separated from the solution by centrifugation (6000 rpm for 10 min) and washed several times with water. Later, Pn/Ch composite was dried completely and used for desorption experiments. The dye-loaded composites (0.1 g) were then brought into contact with 0.1 M NaOH, 0.1 M HCl, ethanol (95%), and acetone (99%), separately. The mixture was agitated on a rotary shaker (180 rpm) at 26 ◦ C for 1 h. After desorption, the supernatant was centrifuged, with the remaining procedure being the same as for the sorption experiments. 2.6. Data analysis The percentage of removal of dyes was calculated using the following formula: Removal (%) =

Ci − Ce × 100 Ci

(1)

where Ci is the initial dye concentration and Ce is the equilibrium dye concentration in mg L−1 . The maximum amount of dye adsorption at equilibrium was determined using the following equation: qe = (Ci − Ce )

V M

(2)

where Ci and Ce are the initial and equilibrium dye concentration (mg L−1 ), V the solution volume, and M the mass of Pn/Ch composite used. The resulted equilibrium data were modeled using the Langmuir isotherm which can be expressed by the following equations [25,26]: Langmuir :

Ce = qe

C   1  e qm

+

(3)

KL qm

where qm is the maximum adsorption capacity (mg g−1 ), qe the equilibrium adsorption capacity (mg g−1 ), Ce the equilibrium adsorbate concentration in solution (mg L−1 ), and KL is the Langmuir constant (L mg−1 ). Adsorption kinetics was performed using both pseudo first- and second-order kinetics using the following equation [27,28]: Pseudo first order :

log(qe − qt ) = log(qe ) −

Pseudo second order :

t = qt



 k  1

   1 1 k2 q2e

+

qe

2.303 t

t

(4)

(5)

where qe is the amount of dye adsorbed at equilibrium (mg g−1 ), qt the amount of dye adsorbed at time t (mg g−1 ), k1 (min−1 ) is the pseudo first-order rate constant of the equation calculated from the slope of the plot log (qe − qt ) vs. t, k2 (g mg−1 min−1 ) is the pseudosecond-order rate constant. A plot of t/qt vs. t yields a straight line

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V. Janaki et al. / Synthetic Metals 162 (2012) 974–980 25

30

3378

20

Pn/Ch CR CBBR RBBR

15

3333

10

3384

5 0 4000

3800

3600

3400

3200

3000

1640

% Transmittance

1290

799

20 1565

1108

1240 1487

1640

774 1569

1290 1236

1489

1107

1640 1241

10 1371

799 1295

1504

1165 1111

1579 1640

1293 1241

1485

0 1800

1600

1400

1200

1033 798

1105

1000

800

600

400

Wavenumber cm-1 Fig. 1. FTIR spectra of Pn/Ch composite before and after treatment with dyes.

3. Results and discussion 3.1. Characterization of Pn/Ch composite Typical SEM micrograph showed irregular shape and distribution of Pn/Ch composite. The size of the particles was varied from 150 to 350 nm and mostly present in aggregates. The surface of the composite was rough with pleats, providing a good possibility for dyes to be trapped and adsorbed. The XRD pattern of chitosan exhibited characteristics reflections at 2 = 9.80 and 19.86, which is consistent with previous studies reporting the reflection pattern of chitosan [29]. Two broad diffraction peaks appeared at 2 = 20.0 and 25.3 are the characteristics peaks of the polyaniline, corresponding to the periodicity parallel and perpendicular to polymer chains [30]. The main reflections (2 = 9.8, 19.8, 20.0, and 25.3) of both chitosan and polyaniline were observed in Pn/Ch composite. In addition, a series of new peaks were appeared at 6.4◦ , 11.3◦ , and 18.3◦ . The FTIR spectra of the Pn/Ch composite are shown in Fig. 1. The broad peak at 3400–3500 cm−1 is due to N H and O H stretching vibration. The peaks at 1293 cm−1 and 798 cm−1 can be assigned to C N stretching of the secondary aromatic amine and aromatic C H out-of-plane bending vibrations, respectively [31,32]. A peak at 1640 cm−1 and 1565 cm−1 is attributed to N H bending, while the band at 1485 cm−1 is attributed to the C C aromatic ring stretching of the benzenoid unit [23]. The intense peaks

at 1241 cm−1 and 1105 cm−1 are the characteristic vibrations of C O stretching. 3.2. Removal of dyes Different classes of dyes such as diazo (CR), triaryl methane (CBB), vinyl sulfonated (RBBR), and heteropolyaromatic (MB) were selected as model dyes for this study. Among these dyes, CR, CBB, and RBBR are sulfonated anionic dyes while MB is a cationic dye. Fig. 2 shows the sorption of different dyes (100 mg L−1 ) in the presence of 0.1 g L−1 of Pn/Ch composite. From the figure, it was revealed that there was significant removal for sulfonated anionic dyes (95.4% of CR, 98.2% of CBB, and 99.8% of RBBR) when compared with nonsulfonated cationic dye (10.6% of MB). The significant removal of sulfonated dyes could be due to the chemical interactions between the positively charged Pn/Ch composite and

120 100

Conc (mg/L)

with a slope of 1/qe . The value of k2 is determined from the intercept of the plot. All the model parameters were evaluated by nonlinear regression using the Sigma Plot (version 8.0, SPSS, USA) software. Experiments were repeated for two times and mean values were considered. Blank experiments were carried out without adsorbents to ensure that the sorption of dye on the walls of flasks was negligible.

80 60

CR CBB RBBR MB

40 20 0

0

20

40

60

80

Time (min) Fig. 2. Removal of CR, CBB, RBBR, and MB in the presence of Pn/Ch composite.

V. Janaki et al. / Synthetic Metals 162 (2012) 974–980

100

120

0.1 g/L 0.2 g/L 0.3 g/L 0.4 g/L 0.5 g/L

60

40

20

0

20

40

60

80

100

120

Time (min)

3.3. Effect of adsorbent dosage Adsorbent dosage is an important factor which must be carefully optimized during wastewater treatment. The effect of adsorbent dosage (0.1–0.5 g L−1 ) on CR dye was studied, and the results are presented in Fig. 3. Initially, a rapid removal of dyes with the increasing adsorbent dosage was attributed to the availability of reactive sites [33]. A further increase in the Pn/Ch composite dosage from 0.2 g L−1 to 0.5 g L−1 did not show much increase in the removal rate and adsorption equilibrium. Similar phenomenon was observed for CBB and RBBR. Thus, further experiments were carried out using 0.1 g L−1 of the adsorbent, as it exhibits appreciable removal capacity for the optimization of adsorption parameters. Similar results have been reported for the adsorption of methyl orange on chitosan/kaolin/␥-Fe2 O3 composite [29]. 3.4. Effect of contact time and initial dye concentration The adsorption of CR, CBB, and RBBR dyes on Pn/Ch composite at different initial concentrations (100–500 mg L−1 ) was analyzed as a function of contact time to depict the equilibrium time. Fig. 4 shows the time course of adsorption equilibrium of CR onto Pn/Ch composite. The rapid removal was observed during the first 10 min 500 100 mg/L 200 mg/L 300 mg/L 400 mg/L 500 mg/L

Conc (mg/L)

400

300

200

100

20

40

60

60

40

20

2

4

6

8

10

12

Fig. 5. Effect of pH on CR, RBBR, and CBB removal.

the negatively charged dyes [24]. The adsorption equilibrium of sulfonated anionic dyes by the Pn/Ch composites was further examined under different experimental variables.

0

80

pH

Fig. 3. Effect of adsorbent dosage on adsorption of CR on to Pn/Ch composite.

0

CR RBBR CBB

100

Removal efficiency (%)

Dye removal (%)

80

0

977

80

100

Time (min) Fig. 4. Effect of contact time and initial concentration on adsorption of CR on to Pn/Ch composite.

and gradually decreased with laps of time until equilibrium. The increased activity at initial stage could be due to the availability of more adsorption sites on Pn/Ch composite surface, and gradual occupancy of these sites reduced the reaction rate and the adsorption becomes less efficient. At this point, the amount of dye being adsorbed onto the composite was in a state of dynamic equilibrium with the amount of dye desorbed from the composite. The time required to attain this state of equilibrium was termed as equilibrium time, and the amount of dye adsorbed at the equilibrium time reflected the maximum adsorption capacity of the adsorbent under these particular conditions. It is evident from Fig. 4 that the contact time needed to attain the equilibrium condition for CR was about 60 min. Similar pattern of results were also observed for CBB and RBBR. The time required to reach the equilibrium is in accordance with the results obtained by Mahanta et al. [24] investigating sulfonated dyes adsorption by doped polyaniline. The removal rate of the CR was varied according to the initial concentration of dye (Fig. 4). At low concentration (100 mg L−1 ), 97.5% of CR, 98.2% of CBB, and 99.9% of RBBR were adsorbed by the Pn/Ch composite. The enhanced removal at low concentration could be due to the faster movement of dye into the activated sites of composite. However, in higher concentration (500 mg L−1 ) the removal rate was decreased (57.6% for CR, 59% for CBB, and 59% for RBBR) because the dye molecules needed to diffuse to the adsorbent sites by intraparticle diffusion. In addition, steric repulsion between the solute molecules could slow down the adsorption process and thereby decreases the removal rate. A similar trend was observed for the adsorption of methyl violet by agricultural waste [34].

3.5. Effect of pH Several studies reported the significant role of pH in adsorption reaction. Crini et al. [35] reported that the pH of the solution influences the surface charge and functional groups of the adsorbent. In addition, pH influences the degree of ionization of the materials present in the solution and solution chemistry. Hence, 0.1 g of the Pn/Ch composite was mixed with 100 mL of dye solutions (100 mg L−1 ) at different pH values (3–12). The pH was measured after the addition of the composite. From Fig. 5, it was observed that in acidic pH composite adsorbs dye molecules, while in basic pH the dye adsorption is negligible. The maximum adsorption of dyes (98.5% of CR, 99.2% of CBB, and 99.8% of RBBR) was observed at pH 3 and minimum adsorption (30% of CR, 38% of CBB, and 42% of RBBR) was observed at pH 12. The enhanced adsorption in acidic pH expressed as:

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0.8

Table 1 Langmuir isotherm parameters for adsorption of CR, CBB and RBBR onto Pn/Ch composite.

0.7

Adsorbate

Ce/qe (g/L)

0.6 0.5

CR CBB RBBR

0.4

Langmuir model qm (mg g−1 )

bL (L mg−1 )

R2

KL (L mg−1 )

322.58 357.14 303.03

0.0536 0.0475 0.0962

0.9936 0.9692 0.9927

0.1572 0.1737 0.0941

0.3 0.2 0.1 0.0

0

50

100

150

200

250

Ce (mg/L) Fig. 6. Langmuir isotherms for the sorption of CR on Pn/Ch composite.

Dye molecules in aqueous solution dissociated as: Dye-SO3 Na → Dye-SO3 − + Na+ .

(6)

In acidic pH, the amino group of the Pn/Ch composite protonated as: R-NH2 + H3 O+ → R-NH3 + + H2 O.

(7)

Finally the adsorption process proceeds through the electrostatic attraction between the two counter ions. (Positively charged nitrogen atom and the dye anion.) R-NH3 + + Dye-SO3 − → R-NH3 + –O3 − S-Dye.

(8)

As mentioned in Eq. (8), the degree of protonation for amine groups will be higher in acidic condition, eventually leading to enhanced chemisorption between acidic dyes and Pn/Ch composite. In addition, the electrostatic interaction between the dye anion and positively charged sites of polyaniline backbone could be possible in acidic pH [23]. However, in basic solution the electrostatic interaction between the dye anion and polymer was decreased due to the deprotonation of amino groups and dedoping of composite. Moreover, the presence of excess OH− may compete with the dye anions, which results in lower adsorption of dye on the polymer [9]. Similar trend was observed on the adsorption mechanism of acid dye by polyurethane/chitosan composite [14]. The results clearly bring out the importance of chitosan as well as the doping level of polyaniline in its dye adsorption characteristics. All further experiments were conducted at pH 3, at which maximum dye adsorption has been observed for Pn/Ch composite.

according to the isotherm Eq. (3) and the important parameters are reported in Table 1. The Langmuir model helps to estimate the maximum adsorption capacity (qm ) when these could not be experimentally determined. The qm values observed for CR, CBB and RBBR were 322.58, 303.03, and 357.14, respectively. The adsorption capacity values for all the three dyes were higher to those in the previous published works. Chitosan-zinc oxide nanoparticle exhibited qm 52.63 mg g−1 on Acid Black 26 and 34.48 mg g−1 for Direct Blue 78 [36]. Chitosan intercalated montmorillonite exhibits the qm value of 48.8 mg g−1 for Basic Blue 9, 49.2 mg g−1 for Basic Blue 66 and 45.9 mg g−1 for Basic Yellow 1 [37]. Adsorption of Methyl Orange onto polyaniline exhibits inferior qm value (154.56 mg g−1 ) [22] as compared to the Pn/Ch composite. The correlation coefficient constant R2 values of all the dyes are approximately close to unity and which indicate that the adsorption reaction follows Langmuir isotherm model. 3.7. Adsorption kinetics The kinetic study of the adsorption processes often used to describe the efficiency of adsorption and feasibility of scale-up operation. Kinetics of adsorption equilibrium is also needed to evaluate the effectiveness of the adsorbate [38]. Pseudo first-order and second-order kinetic models were used to gain a better understanding of the adsorption processes. First, the kinetic data were fitted to the pseudo first-order kinetic model. Adsorption rate constants and their regression correlation coefficients were calculated from the linear plot and are summarized in Table 2. The validity of the model is checked by the regression coefficient (R2 ), and the equilibrium sorption capacities appeared that the pseudo first-order model was not fit well with the experimental data. Hence, the kinetic data were further modeled with the pseudo second-order kinetic equation. The applicability of the pseudo second-order kinetic model for CR dye was examined by the linear plots of (t/qt ) vs. t (Fig. 7). Similar phenomenon was also observed for CBB and RBBR and their important parameters are listed in Table 2. The adsorption correlation coefficient R2 was approximately close to unity, which fits 0.8 100 mg/L 200 mg/L 300 mg/L 400 mg/L 500 mg/L

3.6. Equilibrium isotherms 0.6

t/qt (min g/mg)

Adsorption equilibrium isotherm is one of the most important parameter required to understand the behavior of adsorption process. The shape of an isotherm gives the idea about the affinity of dye molecules and possible mechanism of adsorption. In the present study, the experimental data were fitted to the Langmuir models. Langmuir isotherm has found successful applications in many other real sorption processes of monolayer adsorption. It depends on the assumption that intramolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of adsorbate on adsorbent. The isotherm equation further represents that adsorption takes place at the specific homogenous sites within the adsorbent. A plot of Ce vs. (Ce /qe ) resulted in a linear graphical relation indicating the application of the above model for CR (Fig. 6). Similar graphical relations were also observed for CBB and RBBR. The adsorption data were analyzed

0.4

0.2

0.0

0

20

40

60

t (min) Fig. 7. Pseudo second-order kinetics plots for CR on Pn/Ch composite.

80

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Table 2 Kinetic parameters for adsorption of CR, CBB and RBBR onto Pn/Ch composite. Adsorbate

qe,exp (mg g−1 )

Dye concentration (mg L−1 )

Pseudo-first order k1 (min−1 )

Pseudo-second order

R2

qe,cal (mg g−1 )

k2 (mg g−1 min−1 )

R2

qe,cal (mg g−1 )

3

CR

100 200 300 400 500

95.77 175.15 232.96 271.21 287.71

4.6 × 10 4.3 × 102 4.6 × 102 4.2 × 102 6.0 × 102

0.9621 0.9612 0.9468 0.8213 0.8463

87.63 150.10 182.05 257.25 260.18

1.07 × 10 6.5 × 104 6.88 × 104 9.89 × 104 1.96 × 103

0.9602 0.9652 0.9797 0.9897 0.9981

104.16 188.67 232.96 271.21 287.71

CBB

100 200 300 400 500

98.23 176.11 236.43 273.64 294.31

4.1 × 102 4.2 × 102 4.6 × 102 5.2 × 102 6.2 × 102

0.9691 0.9657 0.9483 0.9039 0.8317

86.29 157.14 186.21 254.78 263.59

8.15 × 104 5.53 × 104 6.48 × 104 1.09 × 103 2.35 × 103

0.9381 0.9551 0.9788 0.9932 0.9986

108.69 192.31 250.11 277.77 294.11

RBBR

100 200 300 400 500

99.88 178.47 237.87 277.54 296.35

4.1 × 102 4.3 × 102 4.4 × 102 4.9 × 102 5.8 × 102

0.9684 0.9627 0.9585 0.9253 0.8639

92.46 154.77 199.81 234.36 256.96

8.4 × 104 6.1 × 104 5.1 × 104 8.1 × 104 1.55 × 103

0.9426 0.9625 0.9692 0.9887 0.9972

109.89 192.31 256.41 285.71 303.03

2

the experimental data better than the pseudo first-order for the entire adsorption process. In other words, the adsorption of anionic dyes CR, CBB, and RBBR favorably follows pseudo second-order model, and the rate of the reaction appeared to be controlled by the chemical process. The results are consistent with previous studies reported that the adsorption of Direct Blue 78 and Acid Black 26 on chitosan-zinc oxide nanoparticle followed pseudo-second order kinetics [36]. 3.8. Desorption study After confirming the fact that Pn/Ch polymer is capable of adsorbing the dye molecules onto its surface, it becomes necessary to know the process by which the dye molecules remain adhered to the surface of the polymer. The adherence of dye molecules onto the surface of the Pn/Ch composite was purely chemical interactions, involving chemical binding of the substances. The chemical bonding can be elicited by subjecting the adhered material to desorption and regeneration process [39]. In this study, desorption experiments were conducted using NaOH, HCl, ethanol, and acetone to ascertain the nature of binding of dye molecules onto the Pn/Ch composite surface. Desorption rate of dyes at different chemical treatments is shown in Fig. 8. Treatment with NaOH (0.01 M) shows the considerable recovery of the dyes (84% of CR, 77% of CBB, and 90% of RBBR). Under basic condition, the negatively charged 100

CR RBBR CBB

Dye (%)

80

60

40

20

0

HCl

NaOH

Ethanol

Acetone

Solvents Fig. 8. Desorption rate of CR, RBBR, and CBB from Pn/Ch composite.

sites increase onto the polymeric surface, and it acts as a driving force for the elution of dye anions by electrostatic repulsion and is given by the following equation: R-NH3 + –O3 − S-Dye + OH− → RNH2 + Dye-SO3 − + H2 O

(9)

However, negligible recovery of dyes was observed after treatment with HCl, acetone, and ethanol. The results are consistent with previous study on recovery of reactive dyes from Pn/EPS composite [40]. In the series of five sorption and desorption cycle, the loss in the sorption capacity was less than 7%. Hence, Pn/Ch composite can be easily regenerated and reused for the practical purposes.

3.9. FTIR studies FTIR is a useful tool for obtaining information on the nature of possible interaction between adsorbent and adsorbate. The FTIR spectra of Pn/Ch composite before and after dye adsorption are shown in Fig. 1. The significant shift in the wave number 3421 cm−1 (3333 cm−1 for CR, 3384 cm−1 for CBB, and 3378 cm−1 RBBR) further confirmed the interaction between the dyes and amino and hydroxyl groups of the polymer. However, a minor shift in wave number was observed for the peak at 1565 cm−1 , which correspond to the N H bending vibrations. In addition to amino group, a negligible shift was observed in C O stretching (1105 cm−1 ). Similar trend was observed on the adsorption of malachite green onto chitosan-coated bentonite beads [41].

4. Conclusion The polymer composite showed considerable potential for the removal of CR (95.4%), CBB (98.2%), and RBBR (99.8%) within 60 min from a 100 mg L−1 solution. The adsorption kinetics obeys the pseudo second-order model, and the isotherm follows the Langmuir monolayer model. The interaction between the dyes and polymer was confirmed by FTIR spectroscopy. Amino and hydroxyl groups may be the main functional groups involved in the adsorption of dyes. A strong bond between the dyes and composite was indicated by the negligible recovery of dyes in acetone, HCl, and ethanol treatment. The composite has well flocculation ability in aqueous solution, easy to synthesize, and relative high adsorption capacity. Therefore, the Pn/Ch composite can be effectively used as an adsorbent for the removal of acidic dyes from wastewaters.

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