Reactive Dyestuffs Removal from Aqueous Solutions by Flotation

Feb 24, 2007 - Reactive Dyestuffs Removal from Aqueous Solutions by Flotation, Possibility of .... Journal of Environmental Management 2012 104, 127-1...
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Ind. Eng. Chem. Res. 2007, 46, 2125-2132

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Reactive Dyestuffs Removal from Aqueous Solutions by Flotation, Possibility of Water Reuse, and Dyestuff Degradation Evangelia K. Dafnopatidou, George P. Gallios, Euforia G. Tsatsaroni, and Nikolaos K. Lazaridis* DiVision of Chemical Technology and Industrial Chemistry, School of Chemistry, Aristotle UniVersity, GR-54124 Thessaloniki, Greece

In this study, the removal of three reactive dyestuffs (Remazol Brillantrot 3BS, Gelb 3RS 133%, and Blue RN new) from aqueous solutions was realized by dispersed-air flotation. The influence of pH, background electrolyte, and surfactant concentration was evaluated. Alkaline pH, high collector concentration, and high sodium chloride concentration had a positive effect on dyestuff removal. Cetyltrimethylammonium bromide was found to be effective as the collector. A first-order model could adequately describe flotation kinetics. The results of this study show that flotation seems to be a realistic method for direct treatment of dyestuff baths in the textile industry. The remaining dyestuff concentration could be lowered to less than 1 mg/L for single solutions and to 300 ADMI units for the mixture. Furthermore, the possibility of decolorizing water for reuse in dyeing experiments and the destruction of the dyestuffs by ultrasonic irradiation were evaluated. However, the results should be confirmed in full-scale experiments. 1. Introduction Research on textile effluent decolorization has often been focused on reactive dyestuffs for three reasons. First, reactive dyestuffs represent an increasing market share, ∼20-30% of the total market for dyestuffs. Second, a large fraction, typically ∼30% of the applied dyestuff, is wasted through hydrolysis. Third, conventional wastewater treatment plants, which rely on sorption and aerobic biodegradation, have low removal efficiency for reactive and other anionic dyestuffs.1 Several review papers mention various techniques for the treatment of aqueous streams to eliminate dyestuffs. These can be classified as (i) chemical (oxidative processes), (ii) physical (sorption, coagulation-flocculation), and (iii) biological (living or dead microbial cultures).2-4 During the reactive dyeing of cotton, salts such as sodium chloride are added to the dye bath to aid the exhaustion of dyestuffs onto the fabric, while bases are added to raise the pH from around neutral to pH 10-11 to achieve fixation. Subsequently, the used dye bath solution is discharged with almost all the added bases and salts, as well as with the unfixed dyestuffs.5 Discharging the wastewater costs textile companies for wastewater discharge, apart from letting raw materials down the drain and damaging the environment. Several studies cover attempts to treat wastewater to improve the treated water quality so that it can be reused in the industry.5-12 Despite that, limited studies are focused on the simultaneous water reuse and dye degradation.13-17 In this study, flotation is used as an alternative method for the removal of dyestuffs from aqueous solutions. Flotation is a relatively simple process capable of removing flocs almost totally, as well as lowering turbidity.18 It exploits the natural or induced hydrophobicity of the dyestuff when a stream of gas bubbles is introduced into the solution. The hydrophobic material adheres to bubble surfaces and floats to the top of the liquid phase, forming a froth layer rich in solute.19,20 Reactive dyestuffs form a covalent bond with the fiber in the dyeing process. However, unfixed dyestuff reacts with water * To whom correspondence should be addressed. Tel.: +32310 997807. Fax: +32310 997859. E-mail: [email protected].

to form hydrolyzed or oxy-dyestuff that has lost its bonding capacity and, thus, cannot be reused. Therefore, dyestuff recovery is not an option, and the treatment process must lead to final destruction or disposal of these contaminants.21 Recently, ultrasonics has been found to be a very suitable method for the degradation of organic compounds.22,23 Ultrasound produces high-energy chemical reactions in liquids, primarily by acoustic cavitation, which consists of the formation, growth, and implosive collapse of bubbles. In this work, the removal of three reactive dyestuffs (red, yellow, and blue) from solutions by dispersed-air flotation in a flotation column was investigated. The selection of this technique was dictated by the need to determine an efficient process for wastewater treatment, which is fast and cheap and requires the smallest possible floor space. The particular dyestuffs were chosen because they are commonly used in dyeing mills, to produce various trichromatic shades. To meet the demands of conservation of resources and sustainable development, this study was followed up by dyeing experiments and ultrasonic irradiation in order to investigate the possibility of water reuse and dyestuff degradation, respectively. 2. Materials and Methods 2.1. Materials. The following Remazol reactive dyestuffs were kindly supplied by DyeStar: (i) Brillantrot 3BS (Red, C.I. 239), (ii) Gelb 3RS 133% (Yellow, C.I. 176), and (iii) Br blue RN new (Blue, C.I. 19), which were used without any further purification. The molecular structure of two of the above dyestuffs is presented in Figure 1. Cetyltrimethylammonium bromide (CTAB) and sodium chloride (both Panreac, Proanalysis) were used as surfactant and background electrolyte, respectively. The pH of the solution was adjusted by adding NaOH/HNO3. 2.2. Flotation. Solutions bearing desired concentrations of dyestuffs and NaCl were conditioned by agitation (200 rpm for 20 min) after the addition of CTAB and pH adjustment. Subsequently, they were transferred into the flotation column, where the flotation experiment was initiated by feeding air at a flow rate of 100 mL/min. The dispersed-air flotation system used had a typical arrangement.20 The bench-scale flotation cell

10.1021/ie060993v CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007

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Figure 1. 3-D structure of (a) blue and (b) red dyestuffs generated with HyperChem 7.5. Geometry optimized with the Polak-Ribere algorithm.

was constructed from a Perspex column (internal diameter 4 cm and total height 60 cm). A cylindrical ceramic porous diffuser, with a range of pore diameters from 16 to 40 µm, was used as a gas sparger. During each experiment, small samples were withdrawn from the side sampler at various times. The removal of dyestuffs (Re%) was estimated from the following equation,

Re% )

(C0 - Ct) 100 C0

(1)

where C0 and Ct are the initial and transient dyestuff concentrations, respectively. 2.3. Dyeing. Dyeing was carried out in a Rotadyer apparatus (John Jeffreys Ltd., Rochdale Banbury), at pH 10, at a constant temperature of 60 °C, for 1.5 h. The reaction vessel, holding a volume of 150 mL, contained NaCl (1 mol/L), Na2CO3 (4 g/L), and cotton specimens (0.5 g). 2.4. Ultrasonic Irradiation. Ultrasonic irradiation of the dyestuff solutions was carried out with a VibraCell model VCX750 direct immersion ultrasonic horn (Sonics & Materials, Inc.) operated at a frequency of 20 kHz. Reactions were carried out in a glass sonication cell encased in a water jacket, holding a volume of 50 mL, which was cooled by a recirculating water bath. The temperature in the sonoreactor was 55 ( 2 °C. The horn titanium tip (1.3 cm diameter) was immersed in the center of the solution. 2.5. Analysis. Dyestuff concentration in single solutions was estimated spectrophotometrically by monitoring the absorbance of each dyestuff using a UV-vis spectrophotometer (model

U-2000, Hitachi), at the following λmax: 541 nm (red), 588 nm (blue), and 419 nm (yellow). The dyestuff content of mixtures was estimated in ADMI units (American Dyestuff Manufacterer’s Institute). The method involves measuring the absorbance at a set of 30 wavelengths. According to the U.S. Pollutant Discharge System, the permitted level is 300 ADMI units.4,24,25 A reflectance spectrometer Macbeth CE 3000 was used for the colorimetric measurements on the dyed samples. The values K/S given by the device are directly correlated into dye concentration on the substrate according to the Kubelka-Munk equation26

K (1 - R) ) ) aCt S 2R 2

(2)

where K ) absorbance coefficient, S ) scattering coefficient, R ) reflectance ratio, a ) constant, and Ct ) transient dye concentration. 3. Results and Discussion 3.1. Flotation Experiments. 3.1.1. Ion Flotation Mechanism. Ion flotation involves the removal of surface-inactive ions from aqueous solutions by the addition of surfactants and the subsequent passage of gas through the solutions.27 The result of the flotation procedure is a solid (sublate), which contains the surfactant as a chemical constituent and appears on the surface of the solution. Usually the surfactant (collector) is an ion of opposite charge to the surface-inactive ion (colligend),

Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2127 Table 1. Initial Concentrations of Dye/Surfactant, Estimated φ Ratios, and Residual Noncomplexed Surfactant Concentration, for Each Experimental Run Fig.

NaCl (mol/L)

2 3 a b c 4 a b c 5 a b c

1 10-3 5 × 10-1 1 1 1 1 1 1 1

[CTAB]initial

[dye]initial

(mg/L) (mmol/L) (mg/L) (mmol/L) 50 50 50 50 30 50 70 50 70 150

0.137 0.137 0.137 0.137 0.082 0.137 0.192 0.137 0.192 0.411

50 50 50 50 50 50 50 60 60 60

0.080 0.056 0.056 0.056 0.043 0.043 0.043 0.071 0.071 0.071

φ

n

1.7 2.4 2.4 2.4 1.9 3.2 4.5 1.9 2.7 5.8

5 3 3 3 2 2 2 3.7 3.7 3.7

[CTAB]res (mg/L)

∼19 ∼39 ∼54

and thus, cations and anions are floated with anionic and cationic collectors, respectively.28 Because sublate formed in ion flotation is a chemical compound of the collector and the colligend, the ratio φ (in molar basis) of the two required for complete flotation must at least be the stoichiometric required n.28 Since reactive dyes are anionic (Dn-), in the form of sodium salts, the cationic cetyltrimethylammonium bromide (S+) was chosen as surfactant in order to artificially induce hydrophobicity and promote the formation of sublates (SnD). The optimum φ ratios that have been reported vary widely. For example, complete removal of Y3+ ions has been achieved with φ ) 3, while Pb2+ removal was achieved with φ ) 44.28 If initial surfactant concentration exceeds its critical micelle concentration into the mixing reactor and hydrophilic micelles M(1-β)n are formed, they could disintegrate very rapidly and n the resulting monomers could be consumed for the formation of sublates, up to complete dye complexation.29 Our unpublished work on surface tension of CTAB aqueous solutions has shown that the critical micelle concentrations (CMCs) were 305 (0.837 mmol/L) and 15 mg/L (0.041 mmol/ L) at 10-3 and 1 mol/L NaCl concentration, respectively. The latter value is slightly lower than the published one, 0.049 mmol/ L, for 10-1 mol/L KBr.30 Generally, increasing the background electrolyte (counterion) concentration decreases the extent of the electric double layer and, thus, decreases the repulsions between the headgroups of the surfactant molecules and increases the tendency toward aggregation, thus lowering the critical micelle concentration (CMC) for the “salted surfactant”. The addition of electrolyte to solutions of CTAB changes the shape of their micelles from spheres to cylindrical rods. This change in the shape of the aggregate is due to closer packing of headgroups in the presence of excess counterion, which happens at surfactant concentrations above the critical micelle concentration or when the concentration of added counterion exceeds 0.1 mol/L.30 Table 1 presents the experimental conditions for all the experimental runs along with φ ratios, respective stoichiometric ratios n, and the estimated residual surfactant concentration, which is the difference between the initial and stoichiometric concentration. According to the previous assumptions, there is a residual surfactant excess, greater than the CMC, in three cases, namely, 4b, 4c, and 5c. In general, when transferring the solution into the flotation reactor, the sublate species and nonassociated surfactant molecules are competing for adhesion onto the air bubbles in order to be elevated on the top of the flotation reactor. When there is no more surfactant excess, which could trigger deleterious effects on flotation, the bulk micelles start to disintegrate into

Figure 2. Plots of red dyestuff concentration histories as a function of pH: [dyestuff] ) 50 mg/L, V ) 500 mL.

monomers. All the above could be pictorially given by the following mechanism:

where β is the degree of counterion binding. 3.1.2. Flotation Kinetics. A number of kinetic models have been developed for the flotation process, which can be divided as chemical kinetic analogy and analytical. In the first case, the models are determined by the analogy with a similar rate process, which is usually drawn from chemical kinetics. Indeed, flotation can be considered as a reaction between bubbles and particles, and therefore, it invokes a chemical reactor analogy. In the second case, the models are developed on the basis of bubble-particle interactions and other subprocesses that occur in flotation systems. Because there are many variables involved in flotation, it is quite difficult to develop analytical models. However, when successfully developed, they will be the most valuable because of the insight they give to the physics and chemistry of the process. The first type of kinetic models require empirical determination of the rate constant, which is generally accepted as first order, with respect to the number of particles and bubbles. This is in line with the analytical fundamental models.31,32 Recently, flotation models based on computational fluid dynamics (CFD) have been published, which eventually compare the resulting flotation rates with the corresponding ones from first-order kinetic models.31 Taking into account that analytical kinetic models were beyond the scope of the present research, the rate of dyestuff decay from the solution is written as first-order kinetics,20,33

Ct ) Ce + (C0 - Ce) e-kt

(4)

where Ce is the equilibrium dye concentration in the liquid phase. 3.1.3. Remazol Brillantrot 3BS (Red Dye). Figure 2 presents the influence of pH on the flotation behavior of the red dyestuff in the presence of 1 mol/L electrolyte. Changing the pH from acidic to alkaline had a dramatic effect on flotation kinetics, even though the remaining dyestuff concentration was almost the same. In alkaline pHs, equilibrium was attained in 5 min, while in the acidic pH, it was attained in 60 min. The residual concentrations, after 90 min flotation, were 1.20, 0.93, and 0.44 mg/L for pH 4, 8, and 10, respectively. Although the initial surfactant concentration was greater than the respective CMC value, for 1 mol/L electrolyte, it is clear that there were no

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Figure 3. Plots of yellow dyestuff concentration as a function of flotation time: [dyestuff] ) 50 mg/L, V ) 500 mL.

Figure 4. Plots of blue dyestuff concentration as a function of flotation time: [dyestuff] ) 50 mg/L, V ) 500 mL, pH ) 10.

deleterious effects on flotation. This could be attributed to the fact that the stoichiometric ratio n ) 5 (five reactive groups in the red dye molecule) was higher than the molar ratio φ ) 1.7 and all CTAB was consumed by the dye. In order to shed more light on flotation delay at pH 4, with respect to pH 8 and 10, potentiometric titrations of the red dyestuff were realized at 1 mol/L background electrolyte (NaCl) concentration, and the resulting pKa was 6.23. This means that at pH two units below the pKa, the red dye will be ∼99% protonated (Dn-Hn), while at pH two units above the pKa, the red dye will be ∼99% deprotonated (Dn-). This finding fits well with the experimental results since, at the alkaline environment, the dye anions associate easily with the surfactant, while at the acidic region, there is a competition of the hydrogen ion, making difficult the formation of dye-surfactant aggregates. 3.1.4. Remazol Gelb 3RS 133% (Yellow Dye). Figure 3 shows the effect of background electrolyte concentration on flotation time profiles of the yellow dyestuff. It is clear that, by increasing the electrolyte concentration, the rate of dyestuff removal was increased. The residual concentrations, after 90 min flotation, were 2.50, 0.84, and 0.10 mg/L for [NaCl] ) 10-3, 5 × 10-1, and 1 mol/L, respectively. In this series, initial surfactant concentration was lower than the respective CMC only for the lower ionic strength (10-3 mol/L), while for the other two, the concentration was exceeding it. Given that the molecular structure of the yellow dye was unknown, a mean value for the molecular weight and valence, of the other two dyes, was adopted. Under this assumption, flotation performance is in line with the previous findings, since molar ratio φ ) 2.4 is lower than the stoichiometric ratio n ) 3. It is generally agreed that the presence of salts decreases the efficiency of metal ion flotation, and this arises from (i) competition for collector between the colligend and the added salts and (ii) surfactant inactivation by forming micelles. As expected, ions of a charge opposite to that of the collector have the most effect, but this was most intense in the case of polyvalent ions. Under our experimental conditions, higher NaCl concentration favored dye flotation instead of generating adverse effects. This fact could be attributed to the lack of competition between dye molecules and NaCl for CTAB as well as to the higher degree of flocculation of sublate, which eventually becomes more floatable. It is known that a high concentration of electrolytes suppresses the electrical double layer surrounding the particles and increases their destabilization.34 3.1.5. Remazol Br Blue RN New (Blue Dye). Figure 4 presents the influence of collector concentration on the blue dyestuff removal. It was observed that the three curves are very similar and show the same trends. For all cases, there was a

steep decline at the beginning of sorption (first 15 min) followed by a more gradual decrease. As the amount of collector increased (30, 50, and 70 mg/L), the residual dyestuff concentration decreased 2.50, 1.80, and 1.50 mg/L, respectively. In all experimental runs, the initial collector concentrations, as well as the noncomplexed residual collector concentrations for cases 4b and 4c, were higher than the CMC values. In many cases, a complete cessation of flotation has been observed whenever the surfactant concentrations reached their CMCs. On the other hand, there are also results that show good floatability above the CMC and a cessation at a considerably higher concentration. A cessation of the floatability of quartz has been observed, in solutions of cetylpiridinium chloride, in the concentration range between 10-3 and 10-2 mol/L, even when the CMC was shifted to much lower concentrations by the addition of sodium chloride. The same behavior was also observed for the flotation of aluminum oxide by sodium octadecyl sulfate, which exhibited a cessation in the concentration range between 10-2 and 10-1 mol/L, although the respective CMC was much lower.35 At the CMC, a particle is probably covered by a surfactant double layer.35,36 While in the primary adsorbed layer, the surfactant molecules are oriented in a way that a hydrophobic surface is formed; the second, inverse oriented surfactant layer again renders the particle hydrophilic. Bubble-particle attachment requires the removal of this adsorption layer, so that the primary adsorbed, hydrophobic layer comes into contact with the gas phase of the bubble. This may be possible if the bubble itself is not strongly armored by a surfactant adsorption layer. Dynamic surface tension measurements demonstrated that, at small bubble age with collector concentrations above the CMC, changes in the surface tension occur, dependent on the bubble age. This relates to a still incomplete surfactant adsorption, which may permit bubble attachment at the CMC. 3.1.6. Mixture of Dyestuffs. Following the successful removal of dyestuff from simple dyestuff solutions, the flotation of a mixture of dyes has been examined. The results are presented in Figure 5, where the influence of CTAB concentration (50-150 mg/L) on dyestuff removal is shown for a system containing 1 mol/L NaCl as background electrolyte. In this experimental set, all initial collector concentrations were greater than the CMC value. The color was quickly removed within 20 min with 50 and 70 mg/L CTAB concentration. The final dyestuff content (432 ADMI units) was over the limit of 300 ADMI units for collector concentration 50 mg/L, but well below (176 ADMI) the limit for 70 mg/L CTAB. With more collector (150 mg/L), the shape of the curve is totally different. There

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Figure 5. Plots of ADMI units of mixture dyestuffs as a function of flotation time: [dyestuff] ) 20 mg/L each, V ) 500 mL, pH ) 10.

are two separate sections: one starting from the beginning up to ∼30 min flotation and a second one from 30 to 60 min. It seems that, at higher surfactant concentrations, there is a change in the mechanism of flotation, because the rate of surfactant removal prevails at the first section.37 In case 5c, the estimated noncomplexed residual collector was the greatest among all the experiments, ∼54 mg/L, causing either hydrophilic double layers or micellar rod structures on the surface of aggregates. As the flotation proceeds, parts of the collector and hydrophobic aggregates are removed and the collector concentration decreases. The aggregates reform, as the collector molecules have more space and separate from the SnD aggregates, which then become hydrophobic and float with the air bubbles. So, it seems that, in the first phase, the system works to lower the collector concentration, and in the second phase, the behavior is similar to the curves for 50 and 70 mg/L collector, but shifted right in time. The final result is still good dye removal with final concentrations well below the limit (61 ADMI units), but with much higher flotation time (∼60 min). Also, the increased collector concentration might result in foam fractionation of the surfactant not adsorbed onto aggregate surfaces. In Figures 2-5, the solid lines represent eq 4; the respective kinetic parameters, as well as the correlation coefficients (R2), are given as insets. Since the R2 values are higher than 0.964, it is confirmed that the model effectively represents the experimental data. The fit of the results of Figure 5 for 150 mg/L was produced after splitting the curves into two sections. In order to explore the feasibility of flotation for the removal of coloring matter from aqueous solutions, a comparison with other techniques, regarding the removal of the Remazol Blue RN, was performed (data available on demand). Despite the fact that the fundamentals of the methods are different, i.e., removal or destruction, it is obvious that flotation is a fast and effective procedure (e.g., 96% removal within 20 min). 3.2. Water Reuse in the Dyeing Process. Next, the possibility of reusing decolorized water in the dyeing process was examined by performing experiments with clean water but in the presence of CTAB. The latter could be present as residual from the flotation procedure and could cause problems in dye fixation on the textiles. The dyeing results are given in a form of isotherms (Figure 6a) that correlate the equilibrium ADMI units in the aqueous solutions with the respective ADMI units in the textile phase. It is clear that the increase of the CTAB concentration does not have any significant effect on sorption

Figure 6. Effect of CTAB concentration on dyestuff uptake by the textile expressed (a) as equilibrium isotherms in ADMI units and (b) as K/S values; V ) 30 mL, textile ) 3 g, [NaCl] ) 1 mol/L, [Na2CO3] ) 4 g/L, T ) 60 °C, pH ) 10, tdyeing ) 90 min.

loading. The experimental data were fitted to the Langmuir isotherm, which is given below,

qe ) qmKLCe/(1 + KLCe)

(5)

where qe is the equilibrium equilibrium sorption capacity ((ADMI units)/(g of sorbent)), qm is the maximum sorption capacity ((ADMI units)/(g of sorbent)), and KL is the Langmuir constant (L/mg). The fitted equilibrium parameters are shown in the inset table of the graph in Figure 6a. Figure 6b presents the colorimetric measurements on the dyed samples in K/S values against mixture dyestuff concentration, under the same conditions as in Figure 6a. The experimental data verify the previous findings. Dye fixation decreased very slightly in the presence of 50 mg/L CTAB. Next, the possibility of reuse of treated water (by flotation) was evaluated in the dyeing process by performing experiments in which the liquid medium was a mixture of clean and treated water (0, 1, 5, and 10% v/v in treated water). Again, the data are presented either as isotherms (Figure 7a), with the Langmuir parameters given as an inset table, or as colorimetric measurements on the dyed samples (Figure 7b). It is evident that as much as 10% recycling of the treated water does not significantly affect the dyeing process, which is a promising result. 3.3. Dyestuff Degradation by Ultrasonic Irradiation. 3.3.1. Sonolysis Kinetics. During sonolysis, the collapse of bubbles produces intense local heating and high pressures for a very short period of a few microseconds. Thus, the cavitation energy serves as a mean of concentrating the diffuse energy of sound into microreactors. Heat from cavity implosion decomposes water into extremely reactive hydrogen atoms and hydroxyl

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Figure 7. Effect of flotation effluent recycling on dyestuff uptake by the textile expressed (a) as equilibrium isotherms in ADMI units and (b) as K/S values; V ) 30 mL, textile ) 3 g, [NaCl] ) 1 mol/L, [Na2CO3] ) 4 g/L, T ) 60 °C, pH ) 10, tdyeing ) 90 min.

Figure 8. Plots of mixture dyestuffs content as a function of sonication time, under various CTAB concentrations: (a) in the absence of electrolyte and (b) in the presence of electrolyte, [NaCl] ) 1 mol/L, T ) 55 °C, pH ) 10, and amplitude ) 70%.

radicals, which recombine to form hydrogen peroxide and molecular hydrogen. In this molecular environment, organic compounds are either oxidized or reduced, as follows38

8b) of background electrolyte. As shown in Figure 8a, the increase in the surfactant concentration caused a decrease in the residual ADMI units. By employing 50 mg/L CTAB, the remaining ADMI units were reduced to almost 4 within 150 min. A similar effect was also observed in the presence of sodium chloride (Figure 8b), with the exception of the higher concentration of 100 mg/L CTAB. This pattern could be attributed to the fact that, with increasing surfactant concentrations, a greater part of the dyestuff becomes hydrophobic and is transferred to the bubble/water interface. If the surfactant concentration reaches higher levels (100 mg/L), then competing effects could come into play between micelles and dyestuffssurfactant coordination molecules, since part of the sonication power is consumed by the presence of micelles in the bulk of the solution. Despite this, within 150 min of sonication in the presence of 50 mg/L CTAB, the units of remaining ADMI were reduced to 276, which is below the limit. 3.4. Potential Application of Combined Flotation with Ultrasonic Irradiation. Taking into account these findings, it seems possible that a combination of flotation and ultrasonic irradiation could be an alternative technique for the complete treatment of colored exhausted baths in dyeing mills. This combination, presented in Figure 9, could lead to the recycling of the majority of the used water. Additionally, the decolorized water after sonolysis could be reused or expressed as effluent according to the environmental regulations. Even though the lab-scale results have confirmed the proposed scheme with artificial mixtures, pilot-scale experiments with real effluents should be performed before considering industrial application.

H2O + ))) f •OH + •H

(6)

2•OH f H2O2

(7)

dyestuff + •OH f products

(8)

where ))) denotes the ultrasounds. Although various sonochemical reactions have already been reported, the kinetics for the reactions are still unclear. In most of the reports discussing the reaction kinetics, the obtained data were roughly analyzed to obey a pseudo-first-order kinetics. In practice, however, one can imagine that the reaction conditions will change during sonication because products formed from sonolysis can affect the cavitation phenomenon or scavenge the formed OH radicals.39 In our study, eq 4 was also employed to fit sonolysis kinetics. Generally, the efficiency of ultrasounds to destroy organic substances may be related to the availability of hydroxyl radicals escaping from the cavitation.40 In this study, it could be presumed that the reaction occurs mainly at the bubble/liquid interface, since the frequency of sonolysis (20 kHz) belongs to the range of low frequencies (20-100 kHz), providing only small quantities of hydroxyl radicals to be ejected into the solution. 3.3.2. Degradation of Mixtures of Dyestuffs. Figure 8 displays the changes in dimensionless ADMI color units against time, at various initial CTAB concentrations (0, 25, 50, and 100 mg/L), in the absence (Figure 8a) and the presence (Figure

4. Conclusions In this study, experiments were performed with colored aqueous solutions in order to test the removal and/or degradation

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Figure 9. Proposed flotation-sonochemical process for the treatment of dyeing bath effluents containing reactive dyes.

of dyestuff and the possibility to recycle the used water. The experiments were conducted under conditions similar to those used in industrial practice (e.g., pH ) 10, NaCl concentration ) 1 mol/L, trichromatic dyestuff content). The following conclusions can be drawn from the data obtained under the experimental conditions of the study. Flotation: (1) Higher NaCl concentration plays a positive role in flotation kinetics and in the residual dyestuff concentration. (2) Higher CTAB concentration increases flotation removal, up to a limit. (3) Flotation rate is accelerated in alkaline pH conditions. (4) The residual dye content from trichromatic mixtures is below the limit of 300 ADMI units, after 20 min of flotation with 70 mg/L CTAB. Dyeing: (1) Textile dyeing could be carried out effectively in the presence of small concentrations of CTAB, while 10% of treated water from the flotation unit could be recycled without problems. Sonication: (1) Degradation of trichromatic dye mixture by ultrasonic irradiation is improved in the presence of 50 mg/L CTAB yielding effluents with 300 ADMI units. Higher CTAB concentrations have an adverse effect. Acknowledgment The financial support for this work was provided by the Greek Ministry of Education (Pythagoras I). The authors gratefully acknowledge (i) the constructive comments of the reviewers and their suggestions for improving the paper, (ii) Prof. K. A. Matis for his valuable suggestions, and (iii) Ms. K. Karatisoglou and Mr. G. Kyzas for their help with the experimental work. Nomenclature a ) constant C0 ) initial dyestuff concentration (mg/L) Ce ) equilibrium dyestuff concentration in bulk (mg/L) Ct ) dyestuff concentration at time t (mg/L) K ) absorbance coefficient KL ) Langmuir constant (L/mg) k ) rate constant (min-1) qe ) equilibrium sorption capacity ((ADMI units)/(g of sorbent)) qm ) maximum sorption capacity ((ADMI units)/(g of sorbent)) Re% ) removal % R ) reflectance ratio R2 ) correlation coefficients

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ReceiVed for reView July 28, 2006 ReVised manuscript receiVed December 8, 2006 Accepted January 9, 2007 IE060993V