Methylene Blue - American Chemical Society

Aug 10, 2010 - such as methylene blue (MB) from aqueous solution. This observation was ... parameters of initial concentration of MB and contact time...
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J. Phys. Chem. C 2010, 114, 14377–14383

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Adsorption of Cationic Dye (Methylene Blue) from Water Using Polyaniline Nanotubes Base Mohamad M. Ayad* and Ahmed Abu El-Nasr Department of Chemistry, Faculty of Science, UniVersity of Tanta, Tanta, Egypt ReceiVed: April 27, 2010; ReVised Manuscript ReceiVed: July 17, 2010

Polyaniline nanotubes (PANI NTs) base has been utilized as an adsorbent for the removal of cationic dyes such as methylene blue (MB) from aqueous solution. This observation was evidenced from the measurements of quartz crystal microbalance (QCM) and UV-visible spectroscopy. Experiments were conducted by varying parameters of initial concentration of MB and contact time. The percentage of color removal decreased with the increase of initial dye concentration. Adsorption equilibrium of the dye was reached after 120 min of contact time. Equilibrium data were fit to Langmuir, Freundlich, and Tempkin isotherms, and their constants were determined. The linear correlations coefficient showed that the Langmuir isotherm best fits the MB adsorption data on PANI NTs. The pseudo-first-order, pseudo-second-order, and the intraparticle diffusion kinetic models were applied to the experimental data. It was observed that the pseudo-second-order kinetic model described the adsorption process better than any other kinetic model. Results obtained indicate that PANI NTs could be employed as an efficient adsorbent much more than the conventional PANI powder for dye removal from water. 1. Introduction Dyes and pigments have been a subject of much interest in recent years, and removing them from wastewater is very important. These dyes are most widely used in industries such as textiles, paper, plastics, leather, food, and cosmetic to color products. The release of colored wastewater from these industries may present an eco-toxic hazard and introduce the potential danger of bioaccumulation.1,2 Various techniques such as precipitation, membrane filtration, coagulation, electrochemical, ion exchange, chemical oxidation, adsorption,3,4 etc., are used for the removal of dyes from wastewater. Adsorption is a procedure of choice for the removal of dyes from wastewater.5-10 The major advantages of this technique are its low generation of residues and the possibility of recycling and reuse of the adsorbent .11 Several effective, selective, and cheaper adsorbent materials have been developed such as cellulosic orange peel waste,12 banana pith,13 rice husk,14 clay,15 neem leaf powder,16 powdered activated sludge,17 activated carbon,18 gram husk,19 coal bottom ash,20 bagasse fly ash,21 deoiled soya,22 red mud,23 and sawdust.24 Recently, conducting polymers were tested in the adsorption of dye effluent.25-27 Conducting polymers have attracted great interest in the world of research due to their various physical and chemical properties and their numerous possible applications.28 One of these polymers is polyaniline (PANI). Polyaniline is considered to be one of the most promising classes of organic conducting polymers due to their well-behaved electrochemistry, easy protonation reversibility, excellent redox properties, good environmental stability, electrochromism, ease of doping, and ease of preparation. Aniline polymerization in an aqueous acidic medium yields the most conductive form of PANI: the emeraldine salt (ES). The ES may be converted to the corresponding emeraldine base (EB) by treatment with an alkali solution or by rinsing with a large excess of water29 as shown in * Corresponding author. Telephone: 02-040-3404398. Fax: 02-0403350804. E-mail: [email protected].

SCHEME 1: Protonated (Doped) Polyaniline Emeraldine Salt (ES) is Deprotonated (Dedoped) by Treatment with an Alkali to Polyaniline Emeraldine Base (EB)

Scheme 1. The presence of the amino and imine groups may be important for the PANI in adsorption of analyte molecules. Recently, we have explored PANI coating on the electrode of the quartz crystal microbalance (QCM) as chlorinated hydrocarbons and alcohol vapor30-32 sensors. The diffusion and the adsorption kinetics of the adsorbed vapors into the polymer were studied. The use of PANI powder as adsorbent, however, could be limited by the polymer surface area and complicated by diffusion processes. This drawback of using the PANI powder may be overcome by using nanostructured PANI such as nanotubes (NTs).33,34 Recently PANI NTs were used for the reduction of silver nitrate to produce PANI-silver nanoparticle composites.35 The PANI NTs35 would provide a profound improvement to metal reduction efficiency due to the high surface area of the polymer. The high surface area of the polymer NTs would also provide a profound improvement to adsorption efficiency in comparison to the conventional PANI powder. The PANI powder was used recently to absorb some anionic and cationic dyes.25-27 The kinetics of the dye adsorption were characterized. The driving forces of the adsorption of the dyes into the polymer are the electrostatic interaction between the basic sites of the

10.1021/jp103780w  2010 American Chemical Society Published on Web 08/10/2010

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SCHEME 2: Molecular Structure of MB

PANI (amine and imine of nitrogen) and the dye. In the literature, no investigation was carried out to see the effect of the surface area increases of the PANI polymer on the adsorption of the dyes. Therefore, the present paper was devoted to the study of the adsorption of cationic methylene blue (MB) into PANI NTs as a model, using UV-visible absorption spectroscopy and QCM. The isotherm and the kinetics of the adsorption were discussed. The difference between PANI NTs and the PANI powder toward the adsorption of MB was considered. 2. Experimental Section 2.1. Chemicals. Aniline (ADWIC, Egypt) was distilled twice under atmospheric pressure using zinc dust. Ammonium peroxodisulphate (APS) (MP Biomedicals, LLC), acetic acid (ADWIC, Egypt), sulphuric acid (ADWIC, Egypt), methylene blue (MB) (Aldrich, U.S.A.) were used without further purification. 2.2. Preparation of PANI NTs and Conventional PANI Powder. The PANI NTs were prepared in the bulk solution as described by Stejskal et al.34 Typical solutions used for the preparation were 0.2 mol L-1 aniline and 0.25 mol L-1 APS in an aqueous medium of 0.5 mol L-1 acetic acid. In the present investigation, the polymer was prepared as NTs in the bulk and as in situ deposited films. On the following day the precipitate was collected on a filter paper, rinsed with 0.5 mol L-1 acetic acid, and dried. The PANI NTs salt was deprotonated with excess 0.1 mol L-1 ammonia, and the resulting PANI NTs base was again dried in dynamic vacuum. The PANI powder base was prepared in the bulk solution as described earlier for the PANI NTs base, except that acetic acid was changed to aqueous sulphuric acid of 0.1 M concentration. 2.3. Preparation of Methylene Blue (MB) Solution. Scheme 2 shows the molecular structure of MB. It has a molecular formula C16H18N3ClS with molecular weight 319.85. It is a water-soluble nontoxic dye, which is blue in color (λmax 664 nm).27 A stock solution was prepared by dissolving an appropriate quantity of MB in a volume of distilled water. The working solutions were prepared by diluting the stock solution with distilled water to give the appropriate concentration of the working solutions. 2.4. Quartz Crystal Microbalance Measurements. Aniline and APS, dissolved separately in 0.5 mol L-1 acetic acid as mentioned in the previous section, were added to a polypropylene bottle. A hole was made in the cap of the bottle, and a 5-MHz AT-cut quartz resonator, 2.5 cm in diameter, covered this hole and was sealed with silicon rubber. Once the reactants were added, PANI NTs salt film was deposited in situ onto the QCM electrode. The film was converted to the base form by exposing to 0.1 M ammonia. The crystal frequency was measured using a Fluke/Phillips PM 6654 frequency counter. Details of the QCM experimental setup and the experimental conditions are described elsewhere.36 The conventional PANI film was also formed onto the QCM electrode as described previously, except that 0.1 M aqueous sulphuric acid was used instead of acetic acid.

Figure 1. Time-dependence of quartz-oscillation frequency during the in situ deposition of nanotubular PANI film prepared by the oxidation of 0.2 mol L-1 aniline with 0.25 mol L-1 APS in 0.5 mol L-1 acetic acid, followed by deprotonation with 0.1 mol L-1 ammonia.

2.5. Adsorption Experiments. The initial and final concentrations of MB solutions were determined by measuring absorbance at 664 nm by using UV-visible absorption spectroscopy (Labomed, Inc.). MB solutions (100 mL) of different concentrations were mixed with 0.05 g of PANI NTs or PANI powder and were stirred at 700 rpm by an electric stirrer for different times in the dark at 25 °C. The mixtures of the polymer and dye were separated first by syringe and then by a centrifuge (Hettich, EBA 20). 3. Results and Discussion 3.1. Adsorption of MB into PANI Film Coating the QCM Electrode. The ability of PANI NTs film to adsorb MB molecules from aqueous solution was examined by QCM. Initially, the PANI NTs film was grown onto the electrode of QCM. Figure 1 shows the variation of frequency during the PANI NTs film deposition. The polymerization was finished by the formation of ES film, the frequency difference, ∆f, equals to 2374 Hz. The film was subsequently deprotonated by using 0.1 mol L-1 ammonia solution to obtain the corresponding EB film. The deprotonation, associated with the loss of an acid molecule from ES, is connected with the mass decrease, as reflected by the increase in the frequency ∆f by 312 Hz. Consequently, the ∆f corresponding to the weight of the PANI NTs base film in the form of EB is 2062 Hz. The morphology of the PANI NTs film in the form of EB was described earlier in our previous work.35 At the early stages of polymerization, homogeneous and smooth film was initially formed on the substrate, and then NTs of dimeters of 100-300 nm grew at the surface of the film, Figure 2a,b. The cavities in the NTs were seen by using the transmission electron micrographs, Figure 2c. The inner diameter differed substantially among the individual NTs, in the range 20-50 nm, but it was relatively constant within each individual nanotube. Some nanorods with no cavities were also formed. It was proposed34 that aniline oxidation, which starts in acetic acid, results in the coupling of aniline molecules in ortho- and para-positions to produce oligomers. As the acidity increases, the ortho-coupled units become oxidized to phenazine units and serve as template sites for the growth of nanotubes after they become coated with PANI. The terminal phenazine units selfassemble and guide the growth of PANI chains, thus producing a nanotube wall. Hydrogen bonding and ionic interactions between the neighboring PANI chains stabilize the supramo-

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Figure 2. SEM micrographs of PANI NTs base (A) 10 µm and (B) 100 nm; and TEM micrograph of PANI NTs base (C) 80 nm.

hence the PANI NTs provide more efficient adsorbent than PANI powder substrates. 3.2. Adsorption of MB into the PANI NTs Using UV-Visible Absorption Spectroscopy. 3.2.1. Adsorption Studies. The adsorption of MB into the PANI NTs and PANI powder using UV-visible spectroscopy was studied, as shown in Figure 4. Additions of 0.05 g PANI NTs to 3.1 mg L-1 MB solution lead to noticeable decreases in the absorbance of MB with increasing time, Figure 4a. However, the addition of the PANI powder to the MB leads to small change in absorbance at the same conditions, Figure 4b. The amount of dye adsorbed into unit weight of polymer adsorbent, Qe (mg g-1), was calculated from the mass balance equation given by:

Qe ) (Co - Ce)Vm-1 Figure 3. (a) The frequency difference of the QCM electrode coated with PANI NTs film exposed to an aqueous solution of 0.3 mg L-1 MB and (b) PANI film exposed to an aqueous solution of 0.3 mg L-1 MB.

lecular nanotubular structure. However, under highly acidic conditions such as sulphuric acid,28 which are typical for PANI preparation, the aniline is present as an anilinium cation, and its oxidation leads to PANI in which the aniline constitutional units are linked with a strong preference for the para-positions resulting in particulates of different size. The PANI NTs film coating on the electrode of QCM was subsequently exposed to an aqueous solution of 0.3 mg L-1 MB, Figure 3a. The crystal frequency difference, ∆f, increases until it reaches steady-state value. The increasing in ∆f is attributed to a mass increase in the film, corresponding to the adsorption of MB. The frequency difference, ∆f ) 76 Hz, attained after 30 min, represents the mass increase of 3.686% [(76/2062) × 100]. Figure 3b shows the conventional PANI base film coating on the electrode of QCM that was exposed to the same concentration of MB solution used in the previous experiment. The mass increase in the film was calculated and equals 0.39% [(9/2290) × 100]. It can be seen that the MB adsorption into the PANI NTs is much more noticeable than the corresponding adsorption into the PANI film. This justifies the surface area effect in the adsorption of MB dye, and

(1)

where Co is the initial dye concentration in liquid phase (mg L-1), Ce is the liquid phase dye concentration at equilibrium (mg L-1), V is the volume of dye solution used (L), and m is the mass of adsorbent used (g). The amount of dye adsorbed into the polymer at different times is shown in Figure 4c. It can be shown that the amounts adsorbed into the PANI NTs and PANI powder matrices are equal to 4.8 mg g-1 and 0.6 mg g-1, respectively. The MB adsorption capacity of PANI NTs is 8 times higher than that of PANI powder; i.e., the extent of the adsorption of the MB dye into the PANI NTs is highly accessible in comparison to the PANI powder. This is in accord with the conclusion obtained previously from QCM measurements. Chowdhury et al.25 have studied the adsorption of MB in acidic (pH 1.71), alkaline (pH 10.01), and neutral (pH 6.86) media into PANI powder salt, base, and neutral, respectively. The neutral PANI was obtained from the PANI base by washing the latter with distilled water until the pH of the supernatant reached 6.86. The extent of MB adsorption into the PANI matrices was in the order: PANI base > PANI salt > PANI neutral. They explained this behavior on the basis of electrostatic force attraction influencing the cationic MB adsorption into the PANI. The results are considered a net negative surface charge on the PANI base matrix at the pH value (pH 10.01). The interesting result in their finding is the adsorption of MB in aqueous medium. It is very small and

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Figure 4. Time-resolved absorption spectra of the adsorption of MB dye [ 3.1 mg L-1] with 0.05 g PANI NTs base (a), 0.05 g PANI powder base (b) in dark at 25 °C, and (c) concentration profiles of MB dye with time in the presence of PANI NTs base (a) and PANI powder base (b).

log(Qe - Qt) ) log Qe consistent with the present data in the case of the adsorption of MB into the PANI powder base. Recently Patil et al.26 have studied the adsorption of different dyes including MB into PANI powder salt in aqueous medium. There was no significant adsorption of the MB dye. However, one has to consider that stirring of the aqueous MB solution with the PANI salt should change the PANI salt to PANI base.29 In that case we are dealing with the adsorption of MB into PANI base. Again the results of Patil et al. justify the negligible adsorption of the MB into the PANI powder base. This is in contrast to that obtained for the PANI NTs base, which is attributed to the higher surface area of the PANI NTs. The binding sites of the interactions available in the NTs (amine and imine nitrogens) and according to Langmuir adsorption isotherm the majority of the adsorption was due to one site (imine nitrogens) which has a lone pair of electrons that can increase interaction with the cationic MB dye. Consequently, the current study presented PANI NTs base as a good adsorbent for cationic dyes. 3.2.2. Adsorption Kinetics. In order to evaluate the kinetic mechanism which controls the process, the pseudo-first-order,37 pseudo-second-order,38,39 and intraparticle diffusion40 models were tested, and the validity of the models were verified by the linear equation analysis: log(Qe - Qt) vs t, (t/Qt) vs t and Qt vs t1/2, respectively. Good correlation with the kinetic data explains the dye adsorption mechanism in the solid phase.37-40 The first model is the pseudo-first-order rate equation:

k1 t 2.303

(2)

where Qe and Qt (mg/g) refer to the amount of dye adsorbed at equilibrium and time t (min), respectively, and k1 is the rate constant. Figure 5a shows the plot of the pseudo-first-order and the parameters k1, Qe, and the correlation coefficient (R2) values were determined (Table 1). The curve-fitting plots with correlation coefficient (R2 ) 0.9432). The second model is pseudo-second-order reaction and is dependent on the amount of solute adsorbed on the surface of adsorbent and the amount adsorbed at equilibrium. It can be represented in the following form:

t/Qt ) 1/k2Q2e + t/Qe

(3)

where k2 is the rate constant of pseudo-second-order model (g mol-1 min). Figure 5b shows the curve-fitting plot of equation 3 (t/ Qt) vs t), and the parameters k2, Qe, and R2 values were determined (Table 1). The curve-fitting plots with the excellent linearity (R2 ) 0.9976), confirm the applicability of the pseudosecond-order equation. The third model is an intraparticle diffusion model. Weber and Morris41 stated that, if intraparticle diffusion is the ratecontrolling factor, uptake of the adsorbate varies with the square root of time. The root time dependence was expressed by:

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Figure 5. Pseudo-first order(a), pseudo-second-order and (b) and intraparticle diffusion kinetic plot for adsorption of MB on PANI NTs base (c) at 25 °C. Dye concentration is 3.1 mg L-1, and adsorbent dose is 0.05 g.

TABLE 1: Kinetic Parameters for the Adsorption of MB on PANI NTs Base (25) °C, Initial MB Concentration ) 3.1 mg L-1 models

R2

model coefficients -1

pseudo-first order

Qe ) 2.147 mg g K1 ) 0.0260 min-1

pseudo-second order

Qe ) 5.015 mg g-1 R2 ) 0.99767 K2 ) 0.03595 g mg-1 min-1

intraparticle diffusion Ki ) 0.26965 mg g-1 min-1 t1/2 ) 5.546 min C ) 2.2426 mg g-1

Qt ) kit1/2 + C

R ) 0.94322 2

of 2.2426 mg g-1), indicating that intraparticle diffusion is involved in the adsorption process but it is not the only rate-limiting mechanism and that some other mechanisms also play an important role. Surface adsorption and intraparticle diffusion were likely to take place simultaneously. 3.2.3. Adsorption Isotherms. The most common sorption models were used to fit the experimental data: Langmuir,42 Freundlich,42 and Tempkin43 isotherms. The model of Langmuir isotherm is given as:

R2 ) 0.94268

Ce /Qe ) 1/Kl + alCe /Kl

(4)

where ki is an intraparticle diffusion rate parameter. Figure 5c shows a plot of Qt versus t1/2. As shown in Figure 5c the external surface adsorption (stage 1) is the fastest and completed before 10 min, and then the stage of intraparticle diffusion control (stage 2) is attained and continues from 10-90 min. The slope of the first linear portion (stage 2) characterizes the rate parameter corresponding to the intraparticle diffusion, whereas the intercept of this portion is proportional to the boundary layer thickness. The R2 value for this diffusion model is 0.94268, and the other values were determined (Table 1). This indicates that the adsorption of MB onto PANI NTs can be followed by an intraparticle diffusion in about 120 min. However, the lines do not pass through the origin (the plots have an intercept

(5)

where Ce is the equilibrium concentration of the adsorbate (mg L-1), Qe is the amount of adsorbed per unit mass of adsorbent (mg g-1), Kl (L mg-1) is a constant related to the affinity between the adsorbent and the adsorbate, Kl/al is the theoretical monolayer saturation capacity Qo. A linear plot was obtained when Ce/Qe was plotted against Ce as shown in Figure 6a. The correlation coefficient (R2) was calculated and equals 0.98159. The second model is the Freundlich model and is given by:

ln Qe ) ln Kf + 1/n ln Ce

(6)

where n is the Freundlich constant and Kf [(L mg-1)1/n] is the constant correlated to the maximum adsorption capacity. Figure 6b shows the plots of ln Qe versus ln Ce are linear with R2 equals 0.87539.

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Figure 6. Langmuir isotherm model (a), Freundlich isotherm model (b), Tempkin isotherm model (c), and separation factor versus initial MB concentration (d).

The third model is the Tempkin isotherm and is represented by the following equation:

Qe ) B ln Kt + B ln Ce

(7)

Kt is the equilibrium binding constant and corresponds to the maximum binding energy and B is constant related to the heat of adsorption. A linear plot was obtained when Qe was plotted against ln Ce as shown in Figure 6c. The R2 was calculated and equals 0.97268. The results obtained from adsorption isotherms for the dye by the polymer are shown in Table 2. For the three studied systems, the Langmuir isotherm correlated better than Freundlich and Tempkin isotherms. The Langmuir sorption isotherm is the most widely used for the sorption of a pollutant from a liquid solution, assuming that the sorption takes place at specific homogeneous sites within the adsorbent.44 The essential feature of the Langmuir isotherm can be expressed in terms of a dimensionless factor called separation factor (Rl) which is defined by the following equation:45

Rl ) 1/(1 + alCo)

(8)

where Co (mg L-1) ) the initial adsorbate concentration, al (L mg-1) ) the Langmuir constant related to the energy of adsorption. The value of Rl indicates the shape of the isotherm to be either unfavorable (Rl > 1), linear (Rl ) 1), favorable

TABLE 2: Summary of the Longmuir, Freundlich, and Tempkin Isotherm Constants, Separation Factor (Rl) and Linear (R2) Regression Coefficients model

parameters

Langmuir

Qo ) 9.21(mg g-1) Kl ) 1.29(L g-1) al ) 0.146 (L mg-1) Rl ) 0.68 R2 ) 0.98159

Freundlich

Kf ) 4.392 (L g-1) n ) 2.057 R2 ) 0.87539

Tempkin

Kt ) 12.202 (L/mg) B ) 1.866 R2 ) 0.97268

(0 < Rl < 1), or irreversible (Rl ) 0). Figure 6d shows the variation of Rl with initial MB concentrations. The results indicate that Rl values were in the range of 0-1, indicating that the adsorption of MB onto PANI NTs base is favorable. 3.3. Effect of Initial MB Concentration. An experiment was conducted with different initial concentrations of MB in the presence of 0.05 g of PANI NTs for 120 min. When the initial concentration of the dye was 0.95 mg L-1, the dye was completely absorbed in 20 min. At higher concentrations, the dye was not completely absorbed, indicating that there is a saturation limit for the polymer above which it does not remove the dye. The effects of initial concentration and time on the adsorption of MB by PANI NTs are shown in Figure 7.

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Figure 7. Effect of initial concentration (mg L-1) and time on the adsorption of MB by PANI NTs base.

At lower concentrations all MB present in the adsorption medium could interact with the binding sites on the surface of adsorbent, and thus, higher adsorption yields were obtained. At higher concentrations, lower adsorption yields were observed because of the saturation of the adsorption sites. 4. Conclusion In this study, equilibrium and kinetic studies for the adsorption of cationic MB dye from aqueous solutions onto PANI NTs polymer were studied with variations in MB concentration. The uptake of the dye was studied using quartz crystal microbalance and UV-visible spectroscopy. It is seen that the MB adsorption into the NTs is much more noticeable than the corresponding occurrence into the conventional PANI powder. This is due to the increases in the surface area of the substrate, and hence, the PANI NTs provide more efficient adsorbent than PANI powder substrates. The kinetic mechanism was tested by using the pseudo-firstorder, pseudo-second-order, and intraparticle diffusion models. The straight lines in plots of t/Qt versus t showed good agreement of experimental data with the pseudo-second-order kinetic model. The equilibrium data have been analyzed using Langmuir, Freundlich and Tempkin isotherm models. The results showed that experimental data were correlated reasonably well by the Langmuir adsorption isotherm. Surface adsorption and intraparticle diffusion were likely to take place simultaneously. References and Notes (1) Vandevivere, P. C.; Bianchi, R.; Verstraete, W. J. Chem. Technol. Biotechnol. 1998, 72, 289–302. (2) Pak, D.; Chang, W. Water.Sci Technol. 1999, 40, 115–121. (3) Stephenson, R. J.; Sheldon, J. B. Water Res. 1996, 30, 781–792. (4) Chiou, M. S.; Chuang, G. S. Chemosphere 2006, 62, 731–740. (5) Forgacs, E.; Tibor, C.; Gyula, O. EnViron. Int. 2004, 30, 953–971.

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