Kapok Fiber Oriented Polyaniline for Removal of Sulfonated Dyes

Jul 6, 2012 - The specific surface area of KF-O-PAN was determined by the Brunauer–Emmett–Teller method (BET, ASAP2020, Micromeritics Inc.) with ...
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Kapok Fiber Oriented Polyaniline for Removal of Sulfonated Dyes Yian Zheng,†,‡ Yi Liu,†,‡ and Aiqin Wang*,† †

Center of Eco-materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In the present work, kapok fiber oriented polyaniline (KF-O-PAN) was prepared via a facile polymerization of aniline on the surface of KF and optimized using response surface methodology based on a central composite design. Adsorption characteristics of KF-O-PAN were examined using three sulfonated dyes, that is, Congo Red (CR), Orange II (OG-II), and Orange G (OG-G), as the model adsorbates. The effects of contact time, pH, initial dye concentration, and poly(vinyl alcohol) sizing on the amount of dye adsorbed were investigated using a batch experiment. The amount of dye adsorbed in different water bodies including distilled water, tap water, well water, and lake water was compared to study further the influence of ionic strength. The results indicate that the adsorption equilibrium can be achieved within 4 h, and the monolayer adsorption capacity calculated from the Langmuir equation is 40.82, 188.7, and 192.3 mg/g for CR, OG-II, and OG-G, respectively. KF-O-PAN shows a higher amount of dye adsorbed despite the influences of ionic strength when pH ≤ 8 except for CR. The higher desorption efficiency using NaOH solution and multiple adsorption−desorption cycles suggests the potential of KF-O-PAN for efficient removal of sulfonated dyes from aqueous solution. enticing prospects on the basis of new findings such as conductive thin films, nanorods, chiral materials, nanofibers, helical nanofibers, hollow fibers, hollow nanotubes, metallic polymers, and organic/inorganic hybrids.11,12 In general, PAN has three idealized oxidation states, leucoemeraldine, emeraldine, and pernigraniline, corresponding to reduced, doped, and oxidized states, respectively.13 Emeraldine base (EB) can be reversibly doped preferentially on the imine nitrogen atoms with a protonic acid to yield the conductive emeraldine salt (ES). PAN carries large amounts of amine and imine functional groups and can increase the retention of polar analytes due to the polymeric skeleton (reversed-phase mechanism and π−π interaction), and, accordingly, PAN and its composites have been expected to extract acidic, basic, and neutral compounds and remove them from the aqueous solution.14−25 Furthermore, PAN can be adsorbed and grown on the surface of halloysite nanotubes,14 diatomite,26 attapulgite,27 and silica28 to obtain a series of PAN-coated materials with unique morphologies. On the basis of the above information, KF oriented PAN (KF-O-PAN) is anticipated to be designed and used as an excellent adsorbent to remove sulfonated dyes from aqueous solution. KF has a hollow structure with large lumen, which is guidable for the growth of PAN particles. To optimize the preparation parameters, RSM (response surface methodology) is employed on the basis of a central composite design (CCD). RSM is a statistical-based technique whereby reaction parameters are varied simultaneously in a suitable manner to generate data for the development of empirical models. It is a

1. INTRODUCTION Kapok fiber (KF) is obtained from the seedpods of kapok tree (Ceiba pentandra) and is lustrous, yellowish-brown in color, light, inelastic, and brittle in sense. KF is a type of natural cellulosic fiber with the unique structure of void content as high as 80−90%.1 Conventionally, KF is used as stuffing for bedding, upholstery, life preservers, and other water-safety equipment because of its excellent buoyancy and for insulation against sound and heat because of its air-filled large lumen.2 KF contains waxy cutin on the fiber surface, which provides it with hydrophobic-oleophilic characteristics, and, accordingly, KF is gaining increasing attention as an alternate for oil absorbency in batch and continuous systems.2−5 When KF is treated with alkali, it can also be used as a composite reinforcement material.6 By gamma ray, the combustible compounds from KF can be removed, and functional group as a methoxyl group from lignin polymer can be cleaved, by which KF is given flameresistance characteristics.7 Because of its waxy surface, KF is, however, rarely used as an adsorbent to remove the pollutants from aqueous solution, whereas KF can sorb an extractant used in metal solvent extraction and serve as the support for the impregnated metal sorbent.8,9 Besides, because of its low-cost, abundantly available, and super fine natural fiber, KF can be used as a biotemplate to prepare a series of microtubes originated from organic or inorganic materials.10 KF has a hollow structure with large lumen, and then it is anticipated to guide the growth orientation of a polymerizable monomer by which a series of KF oriented polymers will be designed and used as the adsorbents to remove different types of pollutants from aqueous solution. Polyaniline (PAN) is an important representative of the class of intrinsically conducting polymers and has attracted much attention due to its simple and facile synthesis, good environment stability, and controllability. PAN materials offer © 2012 American Chemical Society

Received: Revised: Accepted: Published: 10079

January 27, 2012 July 4, 2012 July 6, 2012 July 6, 2012 dx.doi.org/10.1021/ie300246m | Ind. Eng. Chem. Res. 2012, 51, 10079−10087

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Figure 1. The molecular structures and ball-stick models of CR, OG-II, and OG-G.

Figure 2. Schematic diagram of preparation process for KF-O-PAN.

2.2. Preparation of KF-O-PAN. KF-O-PAN was prepared by the chemical oxidation method as follows: 2 g of aniline monomer was dissolved in 66 mL of 1.0 mol/L HCl. The mixture was cooled in an ice bath, while an appropriate amount of KF was dispersed and stirred for 30 min. A designed amount of APS in 16 mL of 1.0 mol/L HCl (precooled) was added dropwise to the solution containing KF and PAN. After being stirred for 1 h in an ice bath, the mixture was left for 16 h at room temperature. The product was washed with distilled water until the supernatant became colorless and then washed with industrial alcohol and dried at 50 °C in an oven. The preparation procedure of PAN was similar to that of KF-OPAN except for the addition of KF. Figure 2 showed the schematic diagram of the preparation process for KF-O-PAN. 2.3. Central Composite Design. CCD was employed for the optimization of two variables, the amounts of KF and APS. In the experimental design model, amounts of KF (0.4−1.0 g) and APS (1.62−7.20 g) were taken as input variables, and the amounts of dye adsorbed for CR, OG-II, and OG-G were taken as the responses. The variables obtained from the CCD model were studied, and their low and high levels were coded as −1 and +1, respectively (Table S1, Supporting Information). Response surface was generated using Design Expert Software Version 7.1.3 (STAT-EASE Inc., Minneapolis, MN). 2.4. Batch Adsorption Experiments. The batch adsorption experiments were performed by contacting 25 mg of adsorbent with 25 mL of dye solution in a series of 50 mL conical flasks in a temperature-controlled orbital shaker with constant shaking at 120 rpm. After the adsorption, the KF-OPAN was collected by centrifugation, and the concentration of dye was determined by UV−vis spectrophotometry at 500, 495, and 485 nm for CR, OG-II, and OG-G, respectively. Considering the pH-dependence of absorbency, 0.1 mol/L NaOH or HCl was used to adjust the pH values of dye

faster and more economical analytical approach than the traditional one, which involves variation of one factor at a time while keeping all other factors constant. In our previous studies, RSM has been successfully applied for optimizing the preparation parameters for a hydrogel with excellent adsorption properties for ammonium ion and heavy metals.29,30 The present study is an attempt to obtain KF-O-PAN with unique morphology and optimize the preparation process of KF-OPAN using CCD-based RSM, with the amounts of dye adsorbed of resulting adsorbent for three sulfonated dyes, that is, Congo Red (CR), Orange II (OG-II), and Orange G (OG-G), as the references. The parameters that need to be optimized are amounts of KF and ammonium persulfate (APS). When the preparation parameters were chosen, KF-O-PAN with desired properties was obtained and used as the adsorbent to remove CR, OG-II, and OG-G using batch adsorption experiments. The effects of contact time, pH, initial dye concentration, PVA sizing, and ionic strength were investigated, and the desorption efficiency was also evaluated.

2. MATERIALS AND METHODS 2.1. Materials. Aniline (AN, Guangdong Xilong Chemical Co., Ltd., China) was purified by vacuum distillation before polymerization. Kapok fiber (KF) was provided by Shanghai Panda Industry Co., Ltd., China. Ammonium persulfate (APS), Orange II (OG-II), and Orange G (OG-G) were received from Shanghai Sinopharm Chemical Reagent Co., Ltd., China, and used without further purification. Congo Red (CR) was purchased from Alfa Aesar and used as received. The molecular structures and ball-stick models of these three sulfonated dyes were shown in Figure 1. All other chemicals used were of analytical reagent grade, and all solutions were prepared with distilled water. 10080

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3. RESULTS AND DISCUSSION 3.1. FTIR Analysis. Figure 3 shows the characteristic absorption bands of KF, KF-O-PAN, and PAN. For the FTIR

solutions when drawing the standard curves or determining the amount of dye adsorbed. The amount of dye adsorbed (qe, mg/g), the removal ratio (RR, %), and the desorption ratio (DR, %) were calculated according to the following formulas: qe = (C0 − Ce)V /m

(1)

RR = (C0 − Ce)/C0

(2)

DR = qd /qe

(3)

where qe is the amount adsorbed onto adsorbent (mg/g), qd is the amount desorbed from the adsorbent (mg/g), C0 is the initial dye concentration (mg/L), Ce is the equilibrium dye concentration (mg/L), m is the mass of adsorbent used (mg), V is the volume of dye solution used (mL), RR is the removal ratio (%), and DR is the desorption ratio (%). The dye adsorption on KF-O-PAN was first conducted at various time intervals (0−300 min) with an initial dye concentration of 200 mg/L, to determine the adsorption equilibrium time at the time a defined distribution of the adsorbate between solid and liquid phase was reached. Afterward, pH-dependence of dye adsorption was investigated by adjusting pH values from 3.0 to 12.0 for CR, and 2.0 to 12.0 for OG-II and OG-G by addition of 0.1 and 1.0 mol/L NaOH or HCl solutions. The adsorption isotherm study was carried out with different initial dye concentrations ranging from 50 to 250 mg/L for CR and 100 to 400 mg/L for OG-II and OG-G, keeping the contact time of 5 h. The effect of PVA sizing on the amount of dye adsorbed was performed varying PVA concentration from 0 to 1.0% with the change interval of 0.2% in three dye solutions. To evaluate the effects of ionic strength on dye removal, the adsorption behaviors were also investigated in other water bodies including tap water, well water, and lake water. The analytical study was carried out in triplicates, and the relative standard deviation was less than 5%. 2.5. Desorption and Reusability. The desorption of dyes loaded on KF-O-PAN was performed using distilled water, 0.1 mol/L HCl, 0.1 mol/L NaOH, and industrial alcohol as the desorbing agents. Typically, 25 mg of adsorbent was first contacted with 200 mg/L dye solution at a temperaturecontrolled orbital shaker with constant shaking of 120 rpm, contact time of 5 h, and temperature of 30 °C. After the adsorption was complete, the adsorbent was collected and washed with distilled water and then mixed with 25 mL of different eluting agents for 2 h at 120 rpm. At the end of the experiment, KF-O-PAN was collected from the conical flask, and the concentrations of dyes were determined using a UV− vis spectrophotometer. Afterward, KF-O-PAN was washed with distilled water several times and employed for another adsorption by adjusting the dye solution to pH 3.0. 2.6. Characterization. FTIR spectra were recorded with a Thermo Nicolet NEXUS TM spectrophotometer using KBr pellets in the range of 400−4000 cm−1. For morphological characterization of sample surfaces, a field emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL) was used after coating the samples with gold film. The specific surface area of KF-O-PAN was determined by the Brunauer−Emmett− Teller method (BET, ASAP2020, Micromeritics Inc.) with nitrogen absorption.

Figure 3. FTIR spectra of (a) KF, (b) KF-O-PAN, and (c) PAN.

spectrum of PAN, the absorption bands at 1558 and 1477 cm−1 are assigned to CN and CC stretching vibration of a quinonoid ring and a benzenoid ring, respectively. The absorption bands at 1296 and 1241 cm−1 are assigned to C− N stretching vibration of secondary amine of PAN backbone and bipolaron structure related to the C−N stretching vibration, respectively.31 The absorption band at 1106 cm−1 is attributable to the C−H in-plane bending of 1,4-ring.32 The FTIR spectrum of KF-O-PAN is almost the same as that of PAN, except several characteristic absorption bands have arisen from KF. The important absorption bands at 1741 and 1244 cm−1 are associated with a carbonyl group (CO) in the ester bond, while the absorption band at 1056 cm−1 is within the region of carbohydrate or polysaccharide.4 These absorption bands from KF appear in the FTIR spectrum of KF-O-PAN or overlap with absorption bands from PAN, indicating that KF is combined successfully with PAN. 3.2. SEM Analysis. SEM micrographs of KF, KF-O-PAN, and PAN are shown in Figure 4. It is clearly observed that the outer surface of KF is smooth and silky, while the air-filled lumen is broad but with a thin wall. The as-prepared PAN consisted of plenty of highly aggregated rough surface particles. After in situ oriented growth, PAN has been successfully fabricated on the fibrous surface of KF, rendering that KF has been covered by PAN particles as a newly coated layer. 3.3. RSM Optimization. With the amounts dye adsorbed for CR, OG-II, and OG-G as the responses, the threedimensional response surfaces are displayed in Figure 5. It is observed that the amounts of KF and APS have some influences on the adsorption properties, although the effects are not significant from analysis of variance (Tables S2−S4, Supporting Information). KF is very light, floating on top of the aqueous solution. Also, KF shows no obvious adsorption for the three sulfonated dyes. Therefore, it is not surprising that with increasing amount of KF, KF-O-PAN exhibits a decreasing amount of dye adsorbed for all three sulfonated dyes. Furthermore, the formation of PAN chains is initiated by APS. It is clear that a lower amount of APS cannot produce enough cross-link points to construct PAN chains, by which partial PAN will be dissolved and washed out from KF surface.33 Consequently, the amount of dye adsorbed for these dyes will be reduced due to the availability of fewer polymers. With further increase in amount of APS, the observed decrease in the amount of dye adsorbed may be attributed to the 10081

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Figure 4. SEM micrographs of (a) KF, (b) KF-O-PAN, and (c) PAN.

Figure 5. Response surface graphs of amount of dye adsorbed for (a, Q1) CR, (b, Q2) OG-II, and (c, Q3) OG-G.

formation of soluble quinine-hydroquinone structures, or the overoxidation of the polymer backbone by APS.34 By numerical optimization using CCD-based RSM (Table S5, Supporting Information), the appropriate preparation conditions for KF-OPAN were obtained as follows: 0.40 g of KF and 3.53−4.72 g of APS for CR, 0.40 g of KF and 4.06−4.29 g of APS for OG-II, and 0.40 g of KF and 1.62−1.73 g of APS for OG-G, respectively. To obtain an adsorbent with a high amount adsorbed for all three sulfonated dyes, a compromise was applied using the “point prediction” function of CCD-based RSM. That is, we select some desired operating parameters and discover the higher predicted values. When the amounts of KF and APS were fixed at 0.40 and 4.0 g, KF-O-PAN was prepared with a BET surface area of 21.80 m2/g. Its predicted values were 30.2, 154.7, and 138.6 mg/g for CR, OG-II, and OG-G, while the experimental values were determined to be 31.3, 148.8, and 138.7 mg/g, with the relative deviation of 3.64%, 3.81%, and 0.07%, respectively. 3.4. Adsorption Kinetics. Contact time is an important parameter because this factor can reflect the adsorption kinetics of an adsorbent for a given initial adsorbate concentration. At natural pH, the effects of contact time on dye adsorption were investigated, as shown in Figure 6. It is observed that for OG-II

Figure 6. Effect of contact time on the amount of dye adsorbed. Adsorption experiments: C0, 200 mg/L; adsorbent amount, 1 g/L; 30 °C/120 rpm; natural pH.

and OG-G, the adsorption rates are rapid within 120 min (stage I) and get slower ranging from 120 to 240 min (stage II) (Table S6, Supporting Information). When the contact time is 10082

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considerably increases with increasing dye concentration, as a result of increased driving force of concentration gradient. To analyze the adsorption data and illustrate the adsorption mechanism, two most common isotherm equations, Langmuir and Freundlich models, were applied for fitting the adsorption experimental data. Langmuir model predicts the existence of monolayer homogeneous coverage of an adsorbate at the outer surface of an adsorbent, while Freundlich model endorses the heterogeneity of the surface of an adsorbent, assuming that the adsorption occurs at sites with different adsorption energy.

above 4 h (240 min), the amount of dye adsorbed seems to be a constant, meaning that the entire adsorption system reaches its equilibrium. Yet for CR, after a gradual increase in 90 min, the adsorption is observed to be in equilibrium. To understand better the adsorption process, four typical kinetic equations were used to fit the experimental data, expressed as: first‐order equation:q = a(1 − e−bt )

(4)

second‐order equation:q = abt /(1 + bt )

(5)

power function equation:q = at b

(6)

Elovich equation:q = a + b ln t

(7)

Langmuir equation:

Freundlich equation:log(qe) =

where q is the amount adsorbed (mg/g) at time t (min). The other parameters are different kinetics constants, which can be determined by nonlinear regression of the experimental data. As shown in Table 1, the first-order equation is ruled out because of its poor correlation coefficients (R2), while the other equations seem to be appropriate for describing the experimental data.

first-order equation CR OG-II OG-G

a (mg/g)

b (min−1)

28.86 0.084 0.90 137.3 0.022 0.89 136.4 0.015 0.93 power function equation

a (mg/g)

b (min−1)

R2

Langmuir equation

30.95 0.128 0.94 154.8 0.032 0.94 161.5 0.018 0.95 Elovich equation

adsorbate

a (mg/g·min)

b

R2

a (mg/g)

b (mg/g)

R2

CR OG-II OG-G

13.92 28.66 18.92

0.148 0.292 0.357

0.95 0.99 0.99

10.28 −12.94 −32.46

3.80 27.6 29.5

0.95 0.98 0.96

1 log(Ce) + log K n

(9)

Table 2. Estimated Isotherm Parameters for the Adsorption of Three Sulfonated Dyes, and the Adsorbent Dosage Required with Different Dye Concentrations

second-order equation R2

(8)

where qe is the equilibrium amount adsorbed onto the adsorbent (mg/g), Ce is the equilibrium dye concentration (mg/L), qm is the monolayer adsorption capacity (mg/g), b is the Langmuir adsorption constant (L/mg), and K (L/g) and n (dimensionless) are Freundlich isotherm constants. These parameters can be determined by linear regression of the experimental data. As summarized in Table 2, the Langmuir

Table 1. Estimated Kinetic Model Parameters for the Adsorption of Three Sulfonated Dyes adsorbate

Ce C 1 = + e qe qmb qm

adsorbate CR OG-II OG-G

3.5. Adsorption Isotherms. Figure 7 shows the amounts adsorbed for three sulfonated dyes as a function of initial dye concentration. The plot of dye adsorption against dye concentration indicates that the amount of dye adsorbed

qm (mg/g)

b (L/mg)

40.82 0.024 188.7 0.064 192.3 0.033 calculated m/V values (g/L)

Freundlich equation R2

K (L/g)

0.98 0.98 0.99

n

R2

8.16 3.74 0.96 112 12.8 0.60 49.2 4.33 0.84 calculated m/V values (g/L)

adsorbate

95% removal

99% removal

adsorbate

95% removal

99% removal

OG-IIa OG-IIb OG-IIc

2.58 3.09 4.09

9.26 9.79 10.8

OG-Ga OG-Gb OG-Gc

3.99 4.49 5.47

16.7 17.2 18.2

a c

Initial concentration is 200 mg/L. bInitial concentration is 300 mg/L. Initial concentration is 500 mg/L.

model can well describe the experimental data, and the calculated monolayer adsorption capacity is found to be 40.82 mg/g for CR, 188.7 mg/g for OG-II, and 192.3 mg/g for OGG, respectively. Here, it should be mentioned that at a lower initial dye concentration, KF-O-PAN can remove completely the dyes of OG-II and OG-G from aqueous solution (inset in Figure 7). As we have defined, the amount of dye adsorbed of the asprepared KF-O-PAN can be obtained from eq 1. The adsorbent/solution (in other terms, adsorbent dosage) then can be calculated from eq 10. Combining eqs 8 and eq 10, we can obtain eq 11. m/V = Figure 7. The variation of the equilibrium amount adsorbed with initial dye concentration. Inset shows the corresponding removal ratio. Adsorption experiments: t, 5 h; adsorbent amount, 1 g/L; 30 °C/120 rpm; natural pH.

m/V = 10083

C0 − Ce qe

(10)

C0 − Ce (C0 − Ce)(1 + bCe) = qe qmbCe

(11)

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In general, higher m/V is required to treat the solutions having higher adsorbate concentrations. 35 To have an evaluation about the required KF-O-PAN amount for higher dye concentrations, calculated KF-O-PAN dosages to achieve 95% and 99% removal ratios by using the Langmuir isotherm parameters are also listed in Table 2. Here, it should be mentioned that KF-O-PAN shows a higher amount of dye adsorbed for OG-II and OG-G, and then the required data are evaluated for the dyes of OG-II and OG-G. Obviously, increasing KF-O-PAN dosage is required for the effective removal of OG-II and OG-G with higher initial concentrations such as 200, 300, and 500 mg/L. 3.6. pH-Dependence. To investigate the pH-dependence, the pH values were adjusted from 2.0 to 12.0 for OG-II and OG-G, while for CR, the pH was monitored from 3.0 to 12.0 to avoid precipitation (Figure 8). It is observed that for OG-II and

adsorbed is observed for all three sulfonated dyes. This is ascribed to the transition of ES to EB via in situ dedoping effect (Figure S1, Supporting Information). The transition from the doped ES to the undoped EB leads to the depletion of active adsorption sites in the polymer skeleton, and consequently the interaction of KF-O-PAN with the sulfonated dyes is retarded. 3.7. PVA Sizing. Sizing and desizing are two essential operations in textile processing of either natural fiber or manmade fiber. The desizing wastewater from natural fiber processing mainly contains starch, glucose, and yeast, while the major constituent in man-made fiber desizing wastewater is poly(vinyl alcohol) (PVA). In this section, we investigate the effect of PVA sizing on the amount of dye adsorbed, as shown in Figure 9. It is observed that the amount of dye adsorbed

Figure 9. Effect of PVA sizing on the amount of dye adsorbed. Adsorption experiments: C0, 200 mg/L; t, 5 h; adsorbent amount, 1 g/ L; 30 °C/120 rpm; natural pH.

Figure 8. The changes in amount of dye adsorbed as a function of equilibrium pH values. Adsorption experiments: C0, 200 mg/L; t, 5 h, adsorbent amount, 1 g/L; 30 °C/120 rpm.

shows a gradual decrease with increasing PVA content. That is, the presence of PVA sizing is not beneficial for the adsorption of three sulfonated dyes. This might be attributed to the higher viscosity of PVA solution, which makes the adsorbent not easily dispersed, and, subsequently, the dye molecules are not easily accessible to the adsorption sites on the surface of KF-O-PAN. Nevertheless, the amounts of dye adsorbed of KF-O-PAN for OG-II and OG-G are still considerable even when PVA content reaches 1.0 wt %. At that time, the adsorption value is measured to be 111.7 mg/g for OG-II and 87.0 mg/g for OG-G. 3.8. Ionic Strength. To study the effect of ionic strength on the amount of dye adsorbed, the efficacy of the as-prepared KFO-PAN in different water bodies, including distilled water, tap water, well water, and lake water, was assessed and compared, as shown in Figure 10. Distilled water and tap water were collected from Lanzhou, while well water and lake water were received from Xuyi county and Taihu Lake in Jiangsu, China. The pH and conductivity (μS/cm, 25 °C) were measured to be 6.22 and 18.0 for distilled water, 7.97 and 476 for tap water, 7.70 and 661 for well water, and 8.09 and 588 for lake water. When the adsorption equilibrium was achieved, the final pH values were measured (Table S7, Supporting Information). It is clearly observed that the final solution pH for the distilled water is ∼3 for all three dyes, while the differences in the pH before and after the adsorption are not so significant for tap water, well water, and lake water. As shown in Figure 8, with increasing equilibrium pH from 2.0 to 12.0, the amounts of dye adsorbed

OG-G, the highest amount of dye adsorbed is obtained at pH = 2.0. Yet for CR, the higher amount of dye adsorbed is obtained in pH between 5.0 and 8.0. It is well-known that the degree of ionization of a dye molecule depends on the pH of the aqueous medium. The pKa value for the deprotonation of −SO3H group of OG-II is ∼1.36 Thus, OG-II molecules are negatively charged in almost all of the pH range studied. A similar feature is found for OG-G. Additionally, the dependence of this adsorption process upon the pH can be correlated to the redox character of PAN. In strong acid medium, the PAN can exist as protonated quinonoid diimine structures (ES). This form of PAN is rich with the positively charged polaron/bipolaron sites that undergo interaction with the anionic moiety (D-SO3−) of the dye molecule generated in solution upon dissolution (Figure S1, Supporting Information). The ES form gradually transforms into the EB form with increasing the pH via in situ dedoping effect.34 This pH crossover point for the transformation from ES to EB occurs at pH 6.5 for conventional HCl-doped PAN.37 On the basis of the above discussions, it can be concluded that for OG-II and OG-G, the lower are the solution pH values, the stronger is the adsorption of these dyes on KF-O-PAN surface as a result of electrostatic forces. Yet for CR, the pKa value is reported to be ∼4.0.38 A higher amount of dye adsorbed then is observed in the pH range of 5−8. With further increasing pH from 8.0 to 12.0, a sudden decrease in the amount of dye 10084

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adverse effects on the amounts of dye adsorbed for OG-II and OG-G, while for CR, the presence of “matrix effects” seems to be more favorable. Nevertheless, all of the information indicates that the as-prepared KF-O-PAN shows high affinity to these sulfonated dyes despite the presence of high ionic strength. 3.9. Desorption and Reusability. Desorption studies help to determine the adsorption mechanism and to evaluate the feasibility of regenerating the spent adsorbent. In this section, desorption experiments were conducted using distilled water, 0.1 mol/L HCl, 0.1 mol/L NaOH, and industrial alcohol as the desorbing agents, as shown in Figure 11a. Our previous studies have shown that an acid medium favors the dye adsorption; thus no appreciable desorption is observed using 0.1 mol/L HCl solution. The present data show that these dyes can be simply and efficiently eluted using 0.1 mol/L NaOH solution. Also, preliminary experiments suggest that the desorption of three dyes from KF-O-PAN surface is rapid, for the desorption ratio shows no significant differences at the four desorption times studied (0.5, 1, 2, and 3 h). The addition of a diluted alkaline solution can promote the transition of polyaniline from ES to EB by a dedoping process, and, accordingly, the sulfonated dyes previously adsorbed can be desorbed and the adsorbent is anticipated to be regenerated and reusable for further adsorption. Regeneration ability is one of the important parameters characterizing an adsorbent. In this study, we define the regeneration ratio as the ratio of readsorbed dye amount to the initial adsorbed amount to evaluate the reusability of KF-OPAN for three sulfonated dyes (Figure 11b). The results indicate that due to the dedoping effect during the desorption process, the amount of dye adsorbed shows a sudden decrease for three sulfonated dyes in the second cycle, beyond which the amount of dye adsorbed shows a consecutive decrease as a result of the predominant occupancy of more adsorption sites by accumulative dye molecules. Nevertheless, the adsorption− desorption tests reveal that KF-O-PAN can be used for multiple cycles, demonstrating its potential for removing sulfonated dyes from aqueous solution.

Figure 10. Amounts of dye adsorbed in different water bodies. (1) Distilled water, (2) tap water, (3) well water, (4) lake water.

show a decreasing tendency for OG-II and OG-G, especially for pH ≥ 8.0. When the influences of ionic strength are not considered, the amounts of dye adsorbed, according to the pH effects for OG-II and OG-G, can be reasonably expected to be as follows: distilled water > lake water > tap water > well water. Although the amount of dye adsorbed in well water seems to be the lowest, the amount of dye adsorbed in tap water is higher than that in lake water, suggesting that the ionic strength (as reflected in conductivity) can cause some effects on the adsorption process. Basically, the amount of dye adsorbed is inversely proportional to the ionic strength for the adsorption of three sulfonated dyes except for CR in distilled water. In appearance, the presence of external ions can enhance the adsorption for CR. On the basis of the literature, CR tends to aggregate in aqueous and organic solutions, leading to dimer formation and sometimes even higher order aggregates due to hydrophobic interactions between aromatic rings of dye molecules. This aggregation phenomenon is more noticed for high CR concentrations, at high salinity and/or low pH.39 The formed aggregates can separate and precipitate onto the solid surface of an adsorbent. As a result, the higher amount of dye adsorbed for CR in real water bodies cannot be ascribed to the amount of dye adsorbed of KF-O-PAN but to dye behavior at high salinity. It can then be concluded that, apart from pH effects, the solutions with high ionic strength show some

4. CONCLUSIONS Given CCD-based RSM, we have developed an adsorbent material using KF as the supporting material and PAN as the structural composition with functional adsorption sites for three

Figure 11. (a) Desorption efficiency from KF-O-PAN using different eluting agents. (1) Distilled water, (2) 0.1 mol/L HCl, (3) 0.1 mol/L NaOH, (4) industrial alcohol. (b) The reusable ability of KF-O-PAN for three sulfonated dyes. 10085

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sulfonated dyes: CR, OG-II, and OG-G. The results indicate that KF can guide the growth orientation of PAN, and the resulting adsorbent exhibits a unique KF-aligned morphology. The adsorption equilibrium can be achieved within 4 h, with the monolayer adsorption capacity of 40.82, 188.7, and 192.3 for CR, OG-II, and OG-G, respectively. Solutions with high ionic strength show some adverse effects on the amounts of dye adsorbed for OG-II and OG-G, while for CR, the presence of “matrix effects” seems to be more favorable. This adsorbent can be applied in a wide pH range and can be efficiently regenerated, suggesting its potential for efficient removal of sulfonated dyes from aqueous solution. Furthermore, the findings in this Article demonstrate that KF has an oriented role for designing and developing a series of novel materials, and the unique structure and properties will expand its application fields.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental range and levels of independent variables (Table S1), analysis of variance for the response of the amount of dye adsorbed for CR, OG-II, and OG-G (Tables S2−S4), numerical optimization using CCD-based RSM (Table S5), expressions of the amount of dye adsorbed as a function of contact time (Table S6), pH values before and after the adsorption for three dyes (Table S7), and schematic transition of EB to ES and the interaction between ES and sulfonated dyes, where D denotes the backbone of dye molecule (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-931-4968118. Fax: +86-931-8277088. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 21107116). REFERENCES

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