Effects of cationic and anionic surfactants on the adsorption of toluidine blue onto fly ash

Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 15–22

Effects of cationic and anionic surfactants on the adsorption of toluidine blue onto fly ash R. Yes¸im Talman a,∗ , G¨ulten Atun b a

Marmara University, Faculty of Pharmacy, Department of Basic Pharmaceutical Sciences, Haydarpa¸sa Campus Tıbbiye cad. 34668, Usk¨udar-Istanbul, Turkey b Istanbul University, Faculty of Engineering, Department of Chemistry, 34850 Istanbul, Turkey Received 30 September 2005; received in revised form 27 January 2006; accepted 6 February 2006 Available online 23 March 2006

Abstract The adsorption of cationic dye toluidine blue (TB) onto fly ash has been studied from aqueous solution, in the presence of cationic surfactant cetyl pyridinium chloride (CPC) and anionic surfactant sodium dodecyl sulfate (SDS) at three different temperatures. Two-step adsorption isotherms are observed for TB adsorption in aqueous solution. Isotherm shape is also preserved in the presence of CPC. The equilibrium saturation adsorption capacities of fly ash for TB decreased in the presence of CPC as a result of the competitive adsorption for the same sites between the same charged dye and surfactant molecules. Four-region adsorption isotherms are obtained in the presence of SDS. The adsorption of the cationic dye at low SDS concentration is enhanced by adsolubilization phenomenon because of the favorable interaction with the negatively charged adsorbed micelles. Above the critical micelle concentration free micelles are formed, TB is distributed between the free and the adsorbed aggregates, and therefore the global adsolubilization decreases. The order of changing of Freundlich isotherm constant (n) gives information about isotherm shape whether two-step or four-region adsorption isotherm. Adsorption isotherms of SDS and CPC in aqueous solution have also been constructed for interpreting adsorption results of TB in the presence of surfactants. © 2006 Elsevier B.V. All rights reserved. Keywords: Toluidine blue; Fly ash; Cetyl pyridinium chloride; Sodium dodecyl sulfate; Adsorption isotherms

1. Introduction Dyes are widely used in various fields, and their discharge into water causes environmental pollution. Effluents from dyeing and finishing processes in the textile industry are known to contain color and high amount of surfactants. The effluents from the dyestuff manufacturing and some similar industries are also generally highly colored with a large amount of suspended organic solids and hence are the important sources of water pollution. Nowadays, the use of surfactants in many industrial processes has increased considerably. A problem associated with this fact is the presence of surfactants in land water as contaminants; so there is great concern about suitable methods to remove them from wastewater. One of the more extended techniques for this

∗

Corresponding author. Tel.: +90 216 414 29 63; fax: +90 216 345 29 52. E-mail addresses: [email protected], [email protected] (R.Y. Talman). 0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.02.006

purpose is their adsorption onto solids with a large adsorptive capacity [1]. The main effect of adsorption of a surfactant onto a solid surface is to modify the surface properties of that solid. The electrical charge may be neutralized or even reversed, changing the particle from hydrophobic to hydrophilic. The addition of surfactants in controlled concentrations changes the surface properties of the solid particles from hydrophobic to uncharged hydrophilic entities. Surfactants may be used either for transportation or for the immobilization of the toxic chemicals such as heavy metal ions and dyes [2]. Adsorption has been used extensively in industrial process for many purposes of separation and purification. The removal of colored and colorless organic pollutants from industrial wastewater is considered as an important application of adsorption process using suitable adsorbent. The most widely used adsorbent is activated carbon to remove dyes in wastewater. In spite of its widespread use in various cleaning procedures, activated carbon remains expensive; therefore, the development of low-cost alternative adsorbents has been the focus of recent researches. A wide variety of low cost materials such as bagasse fly ash [3–5],

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shale oil ash [6], blast furnace slag [7], peat [8,9], palm-fruit bunch particles [10], bagasse pith, sunflower stalks, hardwood [3], clay minerals [11–14], perlite [15], waste red mud [16], carbonaceous materials [17] and fly ash are being used as low cost alternatives to activated carbon. Fly ash is a finely divided residue resulting from the combustion of ground or powdered coal. These micron-sized earth elements consist primarily of silica, alumina and iron. They consist mainly of glassy-spherical particles as well as residues of hematite and magnetite, char, and some crystalline phases formed during cooling. Several investigators [18–20] explored the use of fly ash as an adsorbent for the treatment of wastewater to remove a variety of organic compounds and color. The aim of the present research is to study the adsorptive capabilities of fly ash as a possible cheap adsorption media for removing toluidine blue in aqueous solutions and in the presence of cationic surfactant cetyl pyridinium chloride and anionic surfactant sodium dodecyl sulfate. 2. Materials and methods The fly ash sample was obtained from Ankara C¸ayırhan Thermal Power Plant, Turkey. Toluidine blue, a cationic thiazine dye was obtained from Carlo Erba:

Cetyl pyridinium chloride and sodium dodecyl sulfate were obtained from Aldrich and Sigma, respectively. Perkin-Elmer 554 UV–vis Spectrophotometer was used for absorbance measurements. Surface tension measurements were made by Sensodyne QC3000 Surface Tensiometer using the maximum bubble pressure method. 2.1. Adsorbent specifications The fly ash (FA) was repeatedly washed with distilled water to remove impurities until the leachate had a constant conduc-

Table 1 Chemical composition of fly ash (%w/w) Component

%

SiO2 CaO Al2 O3 Fe2 O3 K2 O MgO SO3 Na2 O TiO2 P2 O5 MnO NiO SrO Cr2 O3 ZrO2 V2 O 5 As2 O3 ZnO CuO Rb2 O PbO Nb2 O5 LOIa

43.40 14.70 12.80 11.60 5.68 4.91 3.40 1.24 1.05 0.679 0.187 0.095 0.088 0.075 0.062 0.052 0.030 0.026 0.012 0.011 0.010 0.004 0.00

a

Loss on ignition indicates the carbon content.

tivity and then dried at 378 K for 24 h before it was used as an adsorbent. The soluble metal concentrations of Ca2+ , Mn2+ , Cu2+ , Sr2+ and Zn2+ in the supernatant determined by Atomic Absorption Spectrometer (AAS) were found to be 1.88 × 10−3 , 2 × 10−7 , 3.1 × 10−8 , 4.7 × 10−6 and 7.3 × 10−8 mol/L, respectively. These concentrations of heavy metals in the leachate do not exceed regulatory limits [20]. The chemical compositions of washed fly ash sample obtained from X-ray Fluorescence (XRF) were given in Table 1. The main crystalline phases detected from X-ray Diffraction (XRD) pattern in Fig. 1 are quartz and mullite; non-identified amorphous phases are also present. BET surface area of adsorbent was determined as 7.1 m2 /g by analyzing the data obtained from nitrogen adsorption experiments using a Quantachrome Autosorb Automated Gas Sorption System.

Fig. 1. XRD spectrum of the fly ash: Q, quartz; M, mullite.

R.Y. Talman, G. Atun / Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 15–22

2.2. Determination of point of zero charge of FA surface Acid-base titration was applied to determine point of zero charge (pHpzc ) of fly ash sample. 0.025 g amount of fly ash was added to a conical flask containing 25 mL of 0.1 M NaCl supporting electrolyte and 0.1 M HCl. After equilibration for about 24h, suspension was back titrated with 1 M NaOH until the pH exceeded 12.00. Acidic supernatant, separated by centrifuging, was utilized as the blank system and titrated with hydroxide in a manner identical to that of the sample suspension. Surface charge density σ (Coulomb/g) is defined by the net uptake of protons, i.e., σ = F(Q+ − Q− ), where Q+ and Q− are the moles of H+ and OH− ions adsorbed per unit weight (mol/g) and F is Faraday constant. The pHpzc value for fly ash determined from the σ-pH curve (not shown here) is 2.1 [21,22]. pH measurements were made by using Mettler Toledo MP220 pH-meter. The values of pH of the TB, CPC and SDS solutions under experimental condition were measured as 9.14–9.33, 9.93–9.88 and 10.26–10.82 range, respectively. The surface charge of FA under this situation was negative because of pH > pHpzc . 2.3. Determination of critical micelle concentrations (CMC) of surfactants The critical micelle concentrations (CMC) of CPC and SDS in pure water are 9.0 × 10−4 M and 8.2 × 10−3 M, respectively. In the water treated with the fly ash (WTFA), CMC of CPC and SDS were determined by measuring surface tension as a function of surfactant concentration, and the values were 4.5 × 10−4 M and 4.0 × 10−3 M, respectively. Dyes can provide a simple and rapid method for determination of the CMC owing to the large change in their spectrophotometric properties that accompanies micelles formation [23–26]. In this paper, the CMC value of SDS in WTFA in the presence of TB was determined by means of spectrophotometrically. The absorption spectra of 1 × 10−5 M TB in the presence of various SDS concentrations were measured with a Schimadzu Spectrophotometer (model 2100S) at laboratory temperature (298 K ± 1). The concentration of dye was kept constant during an experimental run. The CMC of SDS in the presence of TB in WTFA was found to be 1.6 × 10−4 M. The effect of CPC on the absorption spectra of TB was also studied and no change in absorbance was observed. 2.4. Adsorption studies 2.4.1. Adsorption of TB Previous experiments were conducted to optimize V/m (the ratio of volume of TB solution to that of the mass of FA). The effect of V/m on the distribution ratio (the ratio of amount of TB adsorbed on solid phase to that of to the solution phase) was studied by adding different amounts of FA in 25 mL of TB solution of 1 × 10−5 M. An initial strong dependence of the distribution ratio on V/m was observed up to 0.5 L/g. Further increase of V/m did not change the distribution ratio appreciably, therefore this V/m ratio was chosen for all experiments.

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Stock solution of TB 1 × 10−3 M was prepared in doubledistilled water. Six joint-stoppered conical flasks were used to study the effect of CPC on TB adsorption equilibria. Three of them were used for blank samples (25 mL WTFA, 25 mL 1 × 10−4 M CPC in WTFA and 25 mL 9 × 10−4 M CPC in WTFA). The optimized amount of adsorbent (0.05 g) was added into the other three flasks containing the same solutions as described above. 0.1 mL of stock solution of TB was added into each six flasks. Three flasks containing fly ash were agitated in a thermostatic shaker (Memmert WB 14) for 4 h at 120 rpm until reaching to the equilibrium. Samples were left overnight and then a definite volume of the supernatant was pipetted. UV–vis spectra were recorded in the range of 550–700 nm wavelengths and the equilibrium concentration of TB was determined by absorbance measurement at λmax = 630 nm. Then the supernatant was put back into flask. 0.1 mL of stock solution of TB was added again into each six flasks and this procedure was repeated until the TB spectra became over scale. Amount of molecules adsorbed in mol/g on solid surface (X), was calculated from the depletion of TB from solution, by comparison with the blank solution to which no fly ash was added using the mass balance Eq. (1): (C0 − C)V (1) m where V, m, C0 and C are the volume of solution (L), fly ash mass (g), and the initial and equilibrium concentration (M), respectively. Adsorption experiments were run at 298, 308 and 318 K to study the effect of temperature. The effect of anionic surfactant on TB adsorption was also investigated at similar conditions in SDS solutions. Two surfactant concentrations of 1 × 10−4 M and 8 × 10−4 M were chosen. Since turbidity was observed at concentrations higher than 8 × 10−4 M SDS in WTFA the absorbance could not be followed spectrophotometrically. In the presence of SDS absorbance measurements were performed at λmax = 632 nm. All spectrophotometric measurements were made in a quartz cell with pathway of 1 cm and WTFA was used as reference solution. X=

2.4.2. Adsorption of surfactants Adsorption of CPC and SDS onto FA at three temperatures was studied by measuring surface tension in the concentration range (0.1–1.1) × 10−3 M and (0.1–8.0) × 10−3 M, respectively. Remaining concentrations of surfactants were determined by using surface tension versus concentration curves by dissolving of CPC or SDS in WTFA. Another one method was also used to show that both methods are comparable. A concentration cell for determining dodecyl sulfate was constructed using two ion selective electrodes for dodecyl sulfate anions (ISEDS) as follows [27]: ISEDS/reference DS− solution (C0 ) |agar bridge| test DS− solution (C)/ISEDS Potentiometric measurements were made in a thermostated chamber and the electromotive force (E) was

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read using a digital voltmeter (Jenway 3010). E=

RT C ln F C0

(2)

where R is the gas constant (J/mol K) and T the temperature (K). Adsorption of CPC was also studied spectroscopically by monitoring the absorbance at λmax = 258 nm. Deviations between two methods were ±2% for SDS and ±3% for CPC. 3. Results and discussion 3.1. Adsorption isotherms 3.1.1. TB adsorption in aqueous solution Adsorption isotherms of TB in aqueous solution are included in Fig. 2 (a, a ), (b, b ) and (c, c ) at 298, 308 and 318 K, respectively. All isotherms of TB in aqueous solution typically display two plateau regions. Many descriptions of two-step isotherms are available in the literature [28–33] for a wide variety of ionic surfactant onto oppositely charged solid surface. This suggests that adsorption of positively charged cationic TB on oppositely charged FA surface shows a similar behavior with cationic surfactant. The two-step isotherm is interpreted by dividing it into four regions. The regions suggested are a low surface excess region (I), a first plateau region (II), a solute-solute interaction region (III) and a second plateau (IV) [34]. Depending on several factors in the region (IV) may show a maximum as referenced in the literature [35]. At 298 K, in the region (I) TB monomers are adsorbed via electrostatic interactions on FA surface. The first adsorption plateau begins at an equilibrium value of ∼0.65 × 10−5 mol/g. Since BET surface area of adsorbent is 7.1 m2 /g its surface is not completely covered with TB molecules having a ˚ 2 even a face-tocross section molecular area around 100 A face orientation. The sudden increase in region (III) up to 1.95 × 10−5 mol/g suggests that solution concentration is sufficient to lead to interactions between dye monomers. TB monomers adsorbed in the region (II) are thought to act as nucleation sites for formation of adsorbed aggregates. The difference of ∼1.3 × 10−5 mol/g between adsorbed amounts corresponding to the second and the first plateau suggests that bi-layer structure is formed, similar to hemimicelle. TB spectra supported this assumption which show a band associated with monomeric TB at 630 nm with a shoulder at 590 nm in dilute solution. At higher concentrations corresponding to second plateau the shoulder at 590 nm became a peak, which is attributed to formation of dimers and higher aggregates on external surface [36–43,45]. Adsorption capacity of TB in water medium is ∼1.95 × 10−5 mol/g for the first plateau at 308 K. This result suggests that TB molecules are adsorbed in trimer form because of increasing hydrophobic interactions between the dye molecules. The beginning of the second plateau is observed at ∼2.6 × 10−5 mol/g. An increase of 0.65 × 10−5 mol/g for second plateau may be attributed to formation of a dye layer on

preadsorbed trimers. The scattering in the experimental points may arise from the loosely bounded last layer. For all systems, adsorbed amount of TB decreased again at 318 K. These results suggest that desorption process affects on adsorption mechanism. It can be deduced from Fig. 2c and c , in water medium, the first plateau appears at ∼0.65 × 10−5 mol/g and the latter 1.3 × 10−5 mol/g suggesting that the second layer of dye forms on preadsorbed TB monomers. A slight decrease observed in the forth regions of isotherms for all temperatures may be arise from desorption of some dye monomers at higher concentrations [42]. 3.1.2. Effect of CPC on TB adsorption At the beginning, all isotherms show a concave profile corresponding to S shaped behavior. In CPC solutions the S shape region moves toward the right, indicating a lower coverage of the adsorbate at higher CPC concentration at 298 K (Fig. 2a). This clearly shows the competition between TB+ and cetyl pyridinium cations for the same sites available on fly ash surface [43]. In order to better understand the adsorption behavior of TB in the presence of CPC, adsorption isotherms of CPC are also presented in Fig. 3. At low concentrations, no significant change was observed in amount adsorbed depending on temperature and the second step started at around CMC of 0.45 mM. When temperature increased the second step of isotherm slightly shifted towards left as expected but a plateau region couldn’t be observed under experimental conditions. The flat plateau suggests that CPC is electrostatically adsorbed as monomers below CMC. The abrupt increase in CPC adsorption following the first plateau denotes hemimicelle formation, which defined as a spherical structure. CPC is adsorbed as monomers at the low CPC concentration of 1.0 × 10−4 M and competes with TB for neutralizing surface charge. Thus the shape of two-step isotherm profile is preserved but the values of TB adsorbed decrease. On the other hand, at high CPC concentration of 9.0 × 10−4 M, TB is hardly adsorbed at 298 K because of the electrostatic repulsion forces between TB ions in solution and CPC head groups of adsorbed bilayer facing towards solution. Two-step isotherm shape is preserved at higher temperatures because TB molecules may exchange with CPC adsorbed as monomers. 3.1.3. Effect of SDS on TB adsorption The effect of anionic surfactant SDS on TB adsorption isotherms are shown in Fig. 2a , b and c at 298, 308 and 318 K, respectively. Adsorption isotherm of TB in the presence of 1.0 × 10−4 M SDS in linear scale at 298 K represents similar shape to fourregion adsorption model (Fig. 2a ). SDS isotherm presented in Fig. 4 also shows four-region model at 298 K and surface excess of SDS decreases with increasing temperature. This model for surfactants predicts that hemimicelle formation takes place in the second region. The beginning of the plateau at 4 mM corresponds to CMC of SDS in (WTFA). This is a characteristic behavior of SDS-adsorbed on mineral oxides that can be related

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Fig. 2. Comparison of the isotherm plots for TB in aqueous medium and in the presence of cationic surfactant CPC (a–c) as well as in the presence of anionic surfactant SDS (a –c ) depending on temperature. The lines were drawn by hand to guide the eye.

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R.Y. Talman, G. Atun / Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 15–22 Table 2 Comparison of the maximum amounts of TB adsorbed (Xm ) in different conditions depending on temperature T (K)

298 308 318

Fig. 3. Adsorption isotherms of CPC at different temperatures.

to an overpopulation of unimers in the electric double layer at concentrations near the CMC [44]. Decreasing in maximum amount adsorbed of SDS and shifting of plateau with temperature towards left direction may be arise from lower CMC values at higher temperatures, which is consistent with the reported study on SDS adsorption. Similarly, it can be concluded that from Fig. 2a dye aggregation begins around 1.0 × 10−5 M solution concentration in the presence of 1.0 × 10−4 M SDS. Hydrocarbon tail groups of surfactant monomers may interact with any hydrophobic regions of FA surface at the low SDS concentration. Surfactant monomers first adsorbing to existing on hydrophobic sites on

Xm (mol/g) Water

1 × 10−4 M CPC

9 × 10−4 M CPC

1 × 10−4 M SDS

8 × 10−4 M SDS

1.84 2.66 1.19

1.01 1.08 0.74

0.50 0.73 0.32

2.08 3.29 1.65

0.50 1.20 0.75

the substrate create additional negative charges, which increase TB adsorption electrostatically. A well-defined four-region adsorption isotherm is also observed at 308 K whereas isotherm somewhat disorders at 318 K (Fig. 2b and c ). Since aggregation number of hemimicelles increase at the higher surfactant concentration of 8.0 × 10−4 M SDS [32], TB monomers at low dye concentrations can be incorporated in the surfactant hemimicelles by an adsolubilization process [43,45]. At low dye concentrations, surface excess of TB in the presence of SDS above CMC of 1.6 × 10−4 M is higher than that found for aqueous solution. The amount of TB adsorbed increased with increasing TB concentration and reached a maximum, and then decreased gradually for all temperatures indicating disordered adsorption isotherms. The decrease in the adsorption of TB after passing a maximum with increasing concentration is probably due to a formation of SDS micelles in solution. Dye molecules are distributed between the adsorbed SDS aggregates and the free micelles in the bulk phase. At higher concentrations, TB may be completely desorbed from the solid/water interface as the result of a near complete solubilization in the free micelles [46]. The maximum amounts of TB adsorbed (Xm ) in different conditions are compared depending on temperature and presented in Table 2. The highest Xm values are obtained at 308 K. 3.1.4. Freundlich isotherm When four-region and two-step adsorption isotherms are presented in a log–log scale four straight lines are obtained which correspond to Freundlich isotherm. The linear form of Freundlich isotherm is: log X = log k + n log C

Fig. 4. Adsorption isotherms of SDS at different temperatures.

(2)

where k and n are Freundlich constants. log X versus log C plots give four straight lines for two-step and four-region adsorption models [34,35]. Representative Freundlich isotherms for TB in aqueous solution are depicted in Fig. 5. These two-step isotherms gave straight lines with slopes following the order n1 > n2 < n3 > n4 . On the other hand, n parameters followed the order n1 < n2 > n3 > n4 for four-region model. The values of n parameters of TB adsorption from aqueous solution, in the presence of CPC and SDS at low and high concentrations are presented in Table 3. Although Freundlich isotherm does not into account for a finite capacity constant (k) is considered a relative measure of adsorbent capacity. Therefore, it might be reasonable only for forth region of isotherm

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Table 3 The values of n parameter evaluated from Freundlich isotherms for TB adsorption from different media T (K)

298

Water

308

318

298

1 × 10−4 M CPC

308

318

298

9 × 10−4 M CPC

308

318

298

1 × 10−4 M SDS

308

318

298

8 × 10−4 M SDS

308

318

Regions

Experimental points

n

r

1 2 3 4 1 2 3 4 1 2 3 4

6 4 5 3 10 6 3 4 4 3 3 6

1.35 0.59 2.19 −0.05 1.61 0.19 0.75 −0.01 2.55 0.63 3.80 −0.32

0.96 0.97 0.93 0.87 0.99 0.85 0.87 0.82 0.97 0.78 0.97 0.84

1 2 3 4 1 2 3 4 1 2 3 4

4 2 4 3 5 4 7 9 4 3 3 6

3.79 0.90 3.21 1.14 0.65 0.49 1.27 0.35 2.28 2.27 0.71 0.25

0.99 0.94 0.92 0.97 0.93 0.95 0.98 0.85 0.96 0.92 0.94 0.88

1 2 3 4 1 2 3 4 1 2 3 4

10 – – – 2 7 9 6 3 4 2 4

2.67 – – – 0.62 0.17 1.54 0.11 4.30 0.06 1.40 –0.22

0.88 – – – 1.00 0.97 0.89 0.78 1.00 0.76 1.00 0.80

1 2 3 4 1 2 3 4 1 2 3 4

3 5 5 3 4 13 6 3 – 6 9 5

0.31 1.21 0.83 −0.17 0.78 2.71 0.58 0.03 – 1.34 0.10 −0.20

1.00 0.97 0.93 0.93 1.00 0.82 0.80 0.77 – 0.89 0.83 0.84

1 2 3 4 1 2 3 4 1 2 3 4

2 7 3 – 9 4 6 – 2 10 – –

2.99 −0.09 −2.30 – 1.09 −0.06 −1.40 – 0.98 −0.33 – –

1.00 0.78 0.86 – 0.96 0.89 0.94 – 1.00 0.92 – –

Fig. 5. Representative Freundlich isotherms for TB in aqueous solution.

for comparison purposes. Since constant (n4 ) has almost negative values the values of k for forth region were not given in Table 3. 4. Conclusions The adsorption of TB into a fly ash/water interface has been studied by adding an adsorbing cationic surfactant CPC or anionic surfactant SDS to the solution. It was concluded that the adsorption behavior of a cationic dye depends on both the charge and the bulk concentration of the surfactants. The adsorption of the TB decreased in the presence of CPC as a result of the competition between the cationic dye and the surfactant with the same charge for the same adsorption sites. The isotherms in CPC solutions exhibit two-step model, which decreases depending on increasing CPC concentration. The result indicates that due to repulsive electrostatic interaction of TB does not solubilize in CPC micelles but remains in the water phase. The adsorption isotherm of TB in the presence of SDS could be explained in terms of four-region model. Experiments performed in the presence of SDS at concentration below CMC show that the adsorption mechanism is affected by the adsolubilization process. The cationic TB molecules are incorporated into the adsorbed surfactant aggregates at low SDS concentration whereas the solute adsolubilization decreases above CMC. At high SDS concentration TB molecules are desorbed from the solid/water interface corresponding to a decreasing plateau on the isotherm curves. The findings of this study can be useful in determining optimum conditions for cationic dye removal both from pure water, in the presence of anionic surfactant SDS and cationic surfactant CPC by a waste product FA.

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Acknowledgment This work was supported by the Research Fund of the University of Istanbul, Project T-1030/19022001. References [1] C.M. Gonzalez-Garcia, M.L. Gonzalez-Martin, A.M. Gallardo-Moreno, V. Gomez-Serrano, L. Labajos-Broncano, J.M. Bruque, Separ. Sci. Technol. 37 (2002) 2823. [2] C. Treiner, Surfactant adsorption and adsorption induced by surfactants at solid-water interfaces, in: P. Somasundaran, A. Hubbard (Eds.), Encyclopedia of Surface and Colloid Science, Marcel Dekker Inc., New York, 2002, pp. 5154–5168. [3] V.K. Gupta, D. Mohan, S. Sharma, M. Sharma, Separ. Sci. Technol. 35 (2000) 2097. [4] V.K. Gupta, I. Ali, Water Res. 35 (2001) 33. [5] V.K. Gupta, C.K. Jain, I. Ali, S. Chandra, S. Agarwal, Water Res. 36 (2002) 2483. [6] Z. Al-Qodah, Water Res. 34 (2000) 4295. [7] V.K. Gupta, I. Ali, D. Suhas, Mohan, J. Colloid Interf. Sci. 265 (2003) 257. [8] Y.S. Ho, G. McKay, Chem. Eng. J. 70 (1998) 115. [9] D. Couillard, Water Res. 28 (1994) 1261. [10] M.M. Nassar, Y.H. Magdy, Chem. Eng. J. 66 (1997) 223. [11] F.L. Arbeloa, T.L. Arbeloa, I.L. Arbeloa, J. Colloid Interf. Sci. 187 (1997) 105. [12] U. Herwig, E. Klumpp, H.-D. Narres, M.J. Schwuger, Appl. Clay Sci. 18 (2001) 211. [13] G. Rytwo, S. Nir, M. Crespin, L. Margulie, J. Colloid Interf. Sci. 222 (2000) 12. [14] C. Breen, H. Loughlin, Clay Clay Miner. 29 (1994) 775. [15] O. Demirbas, M. Alkan, M. Dogan, Adsorption 8 (2002) 341. [16] C. Namasivayam, R.T. Yamuna, D.J.S.E. Arasi, Separ. Sci. Technol. 37 (2002) 2421. [17] N. Kannan, M.M. Sundaram, Dyes Pigments 51 (2001) 25. [18] K. Banergee, P.N. Cheremisinoff, S.L. Cheng, Water Res. 31 (1997) 249. [19] K.R. Ramakrishna, T. Viraraghavan, Water Sci. Technol. 36 (1997) 189.

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