Dyes Removal from Simulated and Industrial Textile Effluents by

In this study, which complements the previously published work of this team on dyes removal, dispersed- and dissolved-air flotation were employed in o...
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Ind. Eng. Chem. Res. 2008, 47, 5594–5601

Dyes Removal from Simulated and Industrial Textile Effluents by Dissolved-Air and Dispersed-Air Flotation Techniques Evangelia K. Dafnopatidou and Nikolaos K. Lazaridis* DiVision of Chemical Technology, School of Chemistry, Aristotle UniVersity, GR-54124, Thessaloniki, Greece

In this study, which complements the previously published work of this team on dyes removal, dispersedand dissolved-air flotation were employed in order to remove dyestuffs from simulated and industrial textile effluents, originating either from the dyeing mill or from the equalization tank of a dye-house. The influence of initial dyes, sodium chloride, and surfactant and polyelectrolyte content was investigated. A first-order kinetic model could adequately describe dispersed-air flotation time profiles. The results show that both flotation techniques are realistic methods for the treatment of effluents originating from the equalization tank by employing 100 mg/L cetyltrimethylammonium bromide (CTAB). Dissolved-air flotation can be used for effluents originating from the dyeing mill with the addition of polyelectrolyte, flocculant, dodecyl hydrogen sulfate sodium salt (SDS), and CTAB. Dye removal was higher than 97%, and the residual dye content could be lower than the limit of 300 American Dyestuff Manufacturer’s Institute units. Furthermore, the possibility for reuse of the decolorized water was evaluated in dyeing experiments. However, the results should be confirmed in full-scale experiments. 1. Introduction The dyeing process is one of the largest contributors to textile effluent, and such colored wastewater has a seriously destructive impact on the environment.1 The textile industry generally discharges large volumes of wastewater, which has high pH, alkalinity, temperature, organic matter, nonbiodegradable matter, toxic substances, detergents, oils, sulfide, and suspended/ dissolved solids. Therefore, textile wastewater has to be discolored before being discharged to the environment.2 Reactive dyes are some of the most commonly used dyes in the textile industry. They are colored compounds, which have high solubility in water and reactive groups able to form covalent bonds between dye and fiber. These dyes require a high salt concentration (up to 1.5 mol/L) to promote exhaustion. However, even with the assistance of salt, about 30% of the dye is still commonly discharged into effluent streams.3 A great variety of methods has been developed to remove color from dye-house effluents, varying in effectiveness, economic cost, and environmental impact (of the treatment process itself).1 The most widely used methods of dye removal from aqueous solutions are divided into three categories: chemical, physical, and biological.4,5 Among them, several studies cover attempts to treat real wastewater to improve the treated water quality so that it can be reused in the industry.2,6–10 An alternative decolorization method is ion flotation, one of the adsorptive bubble separation techniques, which involves the removal of surface inactive ions from aqueous solutions by the introduction of a surfactant and the subsequent passage of the gas bubbles through the solution.11 As a result of the process, a solid known as sublate appears on the gas/aqueous solution surface. Higher concentrations of floated ions and surfactant may lead to precipitation of sublate in the bulk solution.12 Flotation, depending on the method used to generate the bubbles, is classified as follows: dispersed-air flotation (DispAF), dissolved-air flotation (DAF), and electroflotation.13–15 In DispAF, compressed air is forced through pores of sintered-glass * To whom correspondence should be addressed. Tel.: +32310 997807. Fax: +32310 997859. E-mail: [email protected].

disks in order to produce bubbles with diameters usually ranging from 75 to 655 µm. In DAF, air is first pressurized with water into a saturation vessel and then is released through needle valves into the flotation cell at atmospheric pressure. After the pressure is reduced, the air transfers out of solution and forms bubbles that rise to the surface of the liquid. The reported typical diameter range for bubbles generated using DAF is 10-120 µm, with a mean of 40 µm. In electroflotation, water is split into its molecular constituents by applying a current to the solution that is being treated. Bubbles of H2 are formed at the cathode and bubbles of O2 are formed at the anode. This method generates bubble diameters ranging from 22 to 50 µm, depending on the experimental conditions.13 An initial study of this team has already been published, dealing with the removal of reactive dyestuffs from simulated aqueous solutions, by DispAF.8 The originality of this study is the expansion of the research in order to demonstrate the feasibility of DispAF and DAF in the treatment of real industrial effluents, as well as water reuse in the dyeing process. The main objectives were to characterize the experimental parameters affecting the two flotation processes on the removal of three reactive dyestuffs (red, yellow, and navy) from simulated aqueous solutions and, subsequently, to apply the results in real industrial textile effluents, originating either from the dyeing reactor or from the equalization tank of a dye-house, bearing the same dyes as the simulated effluents. 2. Materials and Methods 2.1. Materials. Cetyltrimethylammonium bromide (CTAB) and dodecyl hydrogen sulfate sodium salt (SDS) were used as cationic and anionic surfactants (both Panreac, Pro-analysis), respectively. Sodium chloride (Panreac, Pro-analysis) was used as background electrolyte. The pH of the solution was adjusted, when necessary, by adding NaOH/HNO3. The cationic coagulant Neoflock (PE), a medium molecular weight polyamine, and the high molecular weight cationic flocculant Zetag 87 (Floc) were kindly supplied by a Greek chemical company (Ballis Chemicals) and Allied Colloids, respectively. The anionic reactive dyestuffs, Red 3BS, Navy SG, and Yellow S3R, were kindly

10.1021/ie071235n CCC: $40.75  2008 American Chemical Society Published on Web 06/27/2008

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supplied by CIBA. The molecular structure of the above dyestuffs was not made available by the supplier. All the above reagents were used without any further purification. While this approach could induce differential effects, it is precisely this information that we seek to obtain. Real textile effluents were kindly supplied by a dye-house located in the industrial area of Thessaloniki. Two kinds of industrial effluents were taken, one directly from the dyeing mill, just after the end of the dyeing process, and the other from the equalization tank of the factory. The first effluent contained the three reactive dyes, along with the auxiliary dyeing reagents (crease preventing agent, bleaching agent, lubricant, stabilizer, wetting agent, and sequestering-dispersing agent, etc.)16 had pH ) 10-11.5, salt content of 1 mol/L NaCl, and American Dyestuff Manufacturer’s Institute (ADMI) units around 6000. The second effluent contained the aforementioned dyes, mixed with other dyes and dyeing auxiliaries, resulting from the segregation of the exhausted dye-baths and the intermediate washing processes. This effluent had pH ) 9-10, a salt content of 0.01-0.1 mol/L NaCl, and ADMI units of 1500-4000. 2.2. Dissolved-Air Flotation. The system used was purchased from AZTEC (U.K), furnished with acrylic jars holding a volume of 500 mL solution, each jar having its own mixer. In general, chemical reagents were added into the solution with the following order: NaCl, CTAB, Floc, PE, SDS; each addition was followed by a conditioning time. It must be emphasized that all reagents were not added in all cases. Subsequently, water-air saturated solution was forced through a nozzle arrangement at the bottom of the jar in a single-pulse mode. A 10% recycle was applied by activating the solenoid valves that are connected to the nozzle. When the air bubbles disappeared from the bulk, a small sample was withdrawn. The same procedure was followed for the flotation of real effluents, but without the addition of salt. 2.3. Dispersed-Air Flotation. The bench-scale dispersedair flotation system was constructed from a Perspex column (internal diameter of 4 cm and total height of 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 in order to estimate the removal of dyestuffs. Simulated solutions bearing desired concentrations of dyestuffs and NaCl were conditioned by agitation (300 rpm for 50 min), after the addition of CTAB and the pH adjustment. Subsequently, they were transferred into the flotation column where the flotation experiment was initiated by feeding air continuously at a flow rate of 100 mL/min. All experimental runs were realized at pH 10, except as otherwise stated. 2.4. Dyeing. Dyeing was carried out in a Zeltex Vitacolor Lowboy apparatus (Switzerland) at pH 10, at a constant temperature of 60 °C, for 1.5 h. The reaction vessel, holding a volume of 80 mL, contained NaCl (1 mol/L), Na2CO3 (4 g/L), and cotton specimens (1 g). After the dyeing experiments the cotton specimens were rinsed with water (boiling for 10 min). 2.5. Analyses. Dyestuff concentration in single solutions was estimated spectrophotometrically by monitoring the absorbance of the dyestuff using UV-vis spectrophotometer (model U-2000, Hitachi), at the following λmax ) 525 nm (red). The dyestuff content of mixtures and real effluents was estimated in ADMI units. 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.8,17 The synthetic single solutions were bearing either 5 or 50 mg/L

Figure 1. Plots of turbidity (NTU units) of the red dyestuff versus CTAB concentrations, pH ) 10, and for dye concentrations (a) 5 and (b) 50 mg/L.

dye, while the mixtures contained either 5 or 50 mg/L of each dye. The resulting ADMI content of the low-concentration mixture (about 2000 ADMI) was close to the ADMI content of the effluent originating from the equalization tank, while that of the high-concentration mixture (about 5000 ADMI) was close to the one originating from the dyeing mill. Turbidity measurements of the dye-surfactant solutions were carried out with a WWT turb 550 turbidimeter. A reflectance spectrometer Macbeth CE 3000 was used for the colorimetric measurements on the dyed samples in K/S values, where K ) absorbance coefficient and S ) scattering coefficient.8 Surface tension was measured at 298 K with a Kru¨ss tensiometer using the De Nou¨y ring detachment method. 3. Results and Discussion 3.1. Dye-Surfactant Interactions. The addition of a surfactant of opposite charge to a dye solution, at concentrations below the cmc (critical micelle concentration), may bring about the formation of colloidal dye-surfactant submicellar aggregates (mixed micelles) or insoluble dye-surfactant salts. The actual species formed depends, mainly, on the nature of the dye. Formation of an insoluble salt between ionic dyes and oppositely charged surfactants is most common, but it is not a completely general phenomenon.18,19 In recent publications, it has been shown that it is possible to organize azo-dyes, through simple electrostatic interactions, by complexation with oppositely charged surfactants into supramolecular assemblies of high mesoscopic order in a strict fulfillment of charge stoichiometry.20,21 Since the presence of sublate in the bulk solution determines the ion flotation process, the sublate’s solubility has been evaluated by measuring the turbidity of the dye-surfactant solution (Figure 1).12 In our case, given that the molecular structure of the dyes was unknown, in order to find out the

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required stoichiometry for the complete association of dyesurfactant, a series of mixed solutions was prepared and the nephelometric turbidity units (NTU) of each solution were measured. Figure 1 shows the dependence of turbidity upon CTAB concentration in the presence of 5 mg/L (Figure 1a) and 50 mg/L red dye (Figure 1b), at 0.001 and 1 M ionic strength. For all the experimental conditions there is a monotonous increasing trend with increasing CTAB concentration, succeeded by decay. The qualitative resemblance among the curves is considerable, despite the fact that the peak is located in a different surfactant concentration. Thus, when employing 5 and 50 mg/L dye concentration, the peak is observed at approximately 7 and 50 mg/L surfactant concentration, respectively. Surface tension measurements of CTAB aqueous solutions have shown that the critical micelle concentration was 305 and 15 mg/L, at 10-3 and 1 mol/L NaCl concentration, respectively. Regarding the usual decrease observed in the cmc values after salt addition, it is speculated that it is related to the ability of the salt to “melt” Frank-Evans “icebergs”; micelle formation is an entropy-directed process and is influenced by changes in the water structure surrounding surfactant ions. If structurebreaking ions are present in solution, the water icebergs will “thaw” to a more random state. Destruction of icebergs around the monomeric surfactant ions would result in easier micelle formation at a lower surfactant level.18 According to the above, the solubility behavior of Figure 1 couldbedistinguishedintotwocases:(i)lowionicstrength-absence of micelles, where the increasingly surfactant (S+) dose in the dye (Dn-) solution generates dye-surfactant aggregates (SnD) reaching the complete charge neutralization (peak point), followed by a steep dissolution of the aggregate due to the generation of soluble complexes (Sn+zDz+) in the excess of surfactant, as follows22

Figure 2. Effect of CTAB dose on red dyestuff concentration (DAF), V ) 500 mL, pH ) 10, and dye concentrations (a) 5 and (b) 50 mg/L.

zS+

S+ + Dn- f SnD 98 Sn+zDz+

(1)

(ii) high ionic strength-presence of micelles, where, again, the increasingly surfactant dose generates dye-surfactant aggregates reaching the complete charge neutralization (peak point), because of the dynamic equilibrium with the monomer from which it is formed, followed by a steep dissolution of the aggregate due to the dye solubilization in the surfactant micelles.18 Among the curves of Figure 1a,b, there is a shift to higher NTU values, especially when higher dye and salt content are used. This fact could be attributed to the higher degree of flocculation of hydrophobic aggregates. It is known that a higher concentration of electrolytes suppresses the electrical double layer surrounding the particles and increases their destabilization.23 3.2. Simulated Effluents. 3.2.1. DAF. Parts a and b of Figure 2 depict the influence of CTAB concentration on dyestuff removal, at high and low salt content, for a system loaded with 5 and 50 mg/L red dyestuff, respectively. For low dye content, minimum residual concentration was achieved with 7 mg/L CTAB, while for high dye content, with 50 mg/L CTAB. These results are in strict accordance with the results of Figure 1. The needed CTAB concentration for minimum residual dye after flotation, coincides with the CTAB concentration for maximum turbidity. However, the range of effective flotation in Figure 2b was wider in the case of high ionic strength, spanning from 40 to 80 mg/L CTAB. This fact could be attributed to the higher degree of flocculation of hydrophobic aggregates, which eventu-

Figure 3. Effect of CTAB and SDS dose on the mixture of three dyestuffs content in the presence and absence of polyelectrolyte (Neoflock PE) and flocculant (Zetag 87) (DAF): V ) 500 mL, pH ) 10, dye concentration ) 50 mg/L each, and [NaCl] ) 1 mol/L.

ally improves their floatability. The same trend has also been observed for the flotation of the two other dyes (data not shown). Following the successful removal of dyestuff from single solutions, the flotation of mixtures was examined. Figure 3 depicts the influence of CTAB concentration on dyestuff removal, in the absence and in the presence of auxiliary flotation reagents (PE, flocculant, collector), for a system loaded with 50 mg/L of each of the three dyestuffs. In the absence of auxiliary reagents, by increasing CTAB concentration, an increase in flotation was recorded, presenting a minimum at 150 mg/L CTAB (600 ADMI units). Above 150 mg/L a decrease in the flotation was noted, which is in agreement with the previous findings. The unfloatable residual dye content, at the optimum CTAB concentration, was attributed to the colloidal size of the remaining aggregates, unsuitable for flotation. An intrinsic problem associated to froth flotation is that fine particles, of sizes less than about 5-10 µm, are too small to float.24,25 Aggregation of fine particles is a common method of

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Figure 4. Mechanism of dye-CTAB-polyelectrolyte-SDS interaction.

increasing their size and improving the efficiency of flotation.23 Basically, particles need to be destabilized so that any repulsion between them is reduced, and then collisions must occur, resulting in aggregates or flocs. Long-chain polymers may act by bridging particles together, in order to give strong flocs, which are more capable of withstanding disruptive forces than aggregates formed simply as a result of charge neutralization. For this reason, the usage of polymer reagents was tested, in order to increase the size of the aggregates.23 Among numerous trials, the most promising auxiliary reagents were found to be a combination of cationic polyelectrolyte (PE, Neoflock) and cationic flocculant (Zetag 87) (at 1.5 g/L and 50 mg/L, respectively), after the addition of CTAB. However, the resulting well-flocculated aggregates were unfloatable. This may be due to the disappearance of the hydrophobic character of the aggregates. Most polyelectrolytes used in water treatment are weakly hydrophobic at best and cannot be relied on to impart any hydrophobicity to the flocs they produce. Due to the flexibility of the polymer’s molecule, the hydrophobic moiety would almost certainly be enfolded deep within the molecule when dispersed in water. A way of imparting hydrophobicity to the surface of flocculated aggregates is through adsorption of soluble surfactants. If there are adventitious surfactant ions in solution (i.e., SDS), they will adsorb at the surface of flocculants and will therefore impart hydrophobicity to the aggregates.26 Given that, the extra surfactant sodium dodecyl hydrogen sulfate, with an opposite charge to the cationic polyelectrolytes, was employed in various concentrations (Figure 3). The optimum removal was achieved when 200 mg/L of CTAB and SDS were used and the remaining dyestuff was approximately 300 ADMI. The use of collectors in excess, above 250 mg/L, led to higher residual dyestuff concentrations. It should be clarified that a great number of experiments was performed, with the flotation auxiliaries added in various concentrations and in every possible sequence. In Figure 3, only four of these experiments are shown since all the others were not successful. The only sequence that came up with better results (floatable aggregates) was the one suggested in this work, which led to the generation of the following mechanism. The interaction of dye-surfactants-polyelectrolyte-floc (Figure 4) can be analyzed as follows: (i) dye-surfactant aggregation and charge neutralization, where hydrophobic surfaces are generated, (ii) flocculation by the cationic flocculant through a bridging mechanism, (iii) grafting of the cationic polyelectrolyte onto a solid particle surface, that is, generation of polyelectrolyte brushes, and (iv) formation of a complex between anionic surfactant and polyelectrolyte chains. This tendency for assembly formation is driven by ionic attraction and by the hydrophobic interaction of the alkyl moieties, resulting in supramolecular structure. However, at high ionic strength, the polyelectrolyte layer may be collapsed, losing its lateral continuity, and split into pinched domains.27–29

Figure 5. Red dyestuff concentrations versus flotation time (DispAF): V ) 500 mL, pH ) 10, dye concentration ) 5 mg/L, and in the inset dye concentration ) 50 mg/L.

Polyelectrolyte-surfactant complexes (PSC) have received a lot of attention recently, partly because of the fundamental interest in these systems and partly because of the potential application that these systems may have. While polyelectrolyte chains tethered to solid surfaces, that is, polyelectrolyte brushes, have been the subject of intense research, only a few studies have been devoted to the interaction of these systems with surfactants in aqueous solutions.29 3.2.2. DispAF. The optimum collector concentrations, as found from the turbidity experiments (Figure 1), were confirmed from the DAF experiments (Figure 2) and, thus, were simply applied in DispAF. So, Figure 5 presents the time variation of 5 and 50 mg/L initial red dye concentration, at high and low ionic strength and different CTAB concentration, in a DispAF modulation. For all the employed concentrations there is a monotonous decreasing trend with time, with the steep descent at the beginning of flotation being followed by a plateau. It is evident that the dye removal was satisfactory. The residual concentration was lower than 1 mg/L, and in all cases equilibrium was almost reached within 10 min. An additional reason, which contributes positively in the floatability, is the dramatic decrease of bubble sizes by increasing salt content.30,31 Preliminary measurement of bubble size, with a high-speed camera, showed that the bubble size decreased from 400-750 to 150-270 µm in the presence of low and high ionic strength, respectively. In Figure 6a, the influence of CTAB concentration on dyestuff removal is shown for a system containing 1 M NaCl and 5 mg/L of each dyestuff, in a DispAF modulation. When we increased CTAB concentration from 5 to 15 mg/L, residual dye content was decreased below the discharge limit. After 30 min of flotation, with CTAB higher than 10 mg/L, the residual content was lower than 250 ADMI units. The same behavior was observed for 50 mg/L of each dye in Figure 6b. The four curves were very similar, albeit they were displaced to lower ADMI units, as the CTAB concentration was increased. The discharge

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Figure 8. Effect of CTAB dose on ADMI units of effluents from the dyehouse equalization tank (DAF): V ) 500 mL and pH ) 11.

Figure 6. Plots of ADMI units of mixture dyestuffs versus flotation time (DispAF), with the following dye concentrations: (a) 5 mg/L each and (b) 50 mg/L each.

Figure 7. Effect of CTAB and SDS dose on ADMI units of effluents from the dye-house reactor in the presence and absence of polyelectrolyte (Neoflock PE) and flocculant (Zetag 87) (DAF): V ) 500 mL and pH ) 11.

limit was achieved with 150 mg/L CTAB, and it was improved with 200 mg/L, within 20 min of flotation. This concentration is in agreement with our previous published findings for the flotation of a trichromatic mixture (20-20-20 mg/L) of different reactive dyes, where 70 mg/L CTAB was needed. 3.3. Dyeing Reactor Effluents. 3.3.1. DAF. Figure 7 presents the variation of ADMI units versus CTAB concentration after flotation of textile effluent originating directly from the dyeing reactor. The industrial effluent had pH 11.5 and consisted of the three dyestuffs, as well as of the auxiliary dyeing effluents, with an approximate salt content of 1 mol/L NaCl and initial ADMI units around 6000. The experiments were performed in the absence and in the presence of auxiliary flotation reagents. By employing solely CTAB, it was obvious that an increase in the collector concentration from 0 to 400 mg/L resulted in an increase in the dyestuff removal. However, the residual dyestuff was more than 3000 ADMI units. Eventually, by applying floc, PE, and SDS, the dyestuff removal was high and the residual color was slightly higher than 300 ADMI, in all experiments. Best results, around 200 ADMI units were obtained in all cases, when 3 g/L PE was used.

3.3.2. DispAF. The dispersed-air flotation of effluents from the dyeing reactor was impossible due to the excessive foam production, caused by a crease-preventing agent and a wetting agent, two of the dyeing auxiliaries present in the dyeing bath. These auxiliaries, even though they practically do not effervesce while agitated in the dyeing reactor, did foam excessively when air was added continuously in the effluent (DispAF). The foam transferred the liquid content outside the flotation column, thus making this method inapplicable. It must be stressed that in all the other cases foaming did not exceed 5% of the initial volume. For this reason, we continued to test effluents, solely from the equalization tank. 3.4. Equalization Tank Effluents. 3.4.1. DAF. The effect of CTAB concentration (25, 50, 75, 100, 150, 200, and 300 mg/L) on dyestuff removal, for effluents coming from the equalization tank, is depicted in Figure 8. The general trend is the same as in Figures 2 and 3; namely, by increasing the collector concentration, a minimum in residual dye, less than 200 ADMI units, is appearing at 100 mg/L CTAB. This means that effluents originating from the equalization tank presented almost the same behavior with the single dye solutions, e.g., Figure 2. 3.4.2. DispAF. Figure 9a depicts the influence of different pH values (4, 7, and 9) on dyestuff removal for effluents deriving from the equalization tank. It was clear that the acidic pH decelerated the removal of color from the effluent, which could be explained by the competition of the hydrogen ions with CTAB cations, in order to aggregate with the dyestuff anions. The effect of CTAB concentration (25, 50, 75, and 100 mg/L) on dyestuff removal, for effluents coming from the equalization tank, is shown in Figure 9b. The color was quickly removed within 15 min with a dose of 75 and 100 mg/L CTAB. The final dyestuff concentration was 300 ADMI units with the addition of 75 mg/L CTAB and 145 ADMI units with 100 mg/L CTAB. 3.5. DispAF-Kinetics. The removal rate of sublate or the rate of flotation from the solution is the consequence of (i) the collision between sublate and bubbles and (ii) the attachment and detachment of sublate and bubbles. Most of the flotation models are based on a singlebubble-single-particle system, which does not represent the real flotation process where a swarm of bubbles is necessary. The prediction of particle-bubble interaction in a real system is complicated from the gas hold-up effect, the interaction between neighboring bubbles and the presence of a multilayer of bubbles, all of which tend to straighten the liquid streamlines around a bubble and, thus, to increase the overall probability of collection.32 However, the whole process has been macroscopically assumed of first-order kinetics, which is convenient and simple,

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Figure 9. Plots of ADMI units of the effluent from the equalization tank versus flotation time (DispAF), V ) 500 mL: (a) under different pH values, [CTAB] ) 50 mg/L, and (b) under various CTAB concentrations at pH ) 9.

and has been used satisfactorily for decades.8,33,34 For this reason, a pseudo-first-order kinetic model (eq 2) was used to describe DispAF data in Figures 5, 6, and 9 by employing nonlinear regression analysis. Ct ) Ce + e-k1t(C0 - Ce)

(2)

where C0 is the initial dye concentration; Ct, the dye concentration at time t; Ce, the equilibrium concentration; and k1, the rate constant. The correlation coefficients (R2) are given as insets in the respective figures and indicate an adequate fitting of the kinetic model. According to the application of extended DLVO theory and the derivation of flotation rate equation, the rate constant, k, is defined as

[

]( )

1 1 3 4Re0.72 R1 1 k ) SbP ) SbPCPa(1 - Pd) ) Sb + 4 4 4 2 15 R2

( ){

exp -

E1 Ek

[

1 - exp -

Figure 10. Effect of dissolved-air flotation (DAF) effluent recycling on dyestuff uptake by the textile expressed (a) as equilibrium isotherms in ADMI units and (b) as K/S values: V ) 80 mL, textile amount ) 1 g, [NaCl] ) 1 mol/L, [Na2CO3] ) 4 g/L, T ) 60 °C, pH ) 10, and tdyeing ) 90 min.

2

γlvπR12(1 - cos θ)2 + Ε1 Ek ’

×

]}

(3)

where Sb is the superficial surface area rate of bubbles, P is the probability of collection, Pc is the probability of bubble-particle collision, Pa is the probability of adhesion, Pd is the probability of detachment, Re is the Reynolds number of the bubble, R1 is the particle radius, R2 is the bubble radius, E1 is the energy barrier for bubble-particle interaction, Ek is the kinetic energy of a particle approaching a bubble, Ek′ represents the kinetic energy that tears the particle off the bubble surface, γlν is the surface free energy at the liquid/vapor interface, and θ is the contact angle.35 Equation 3 shows that k is a function of both the hydrodynamic parameters (R1, R2, Ek, Ek′, Re, and Sb) and the surface chemistry parameters (E1, θ, and γlν). The latter indicates the influence of surfactant concentration and pH on the floatation efficiency. 3.6. Water Reuse in the Dyeing Process. Next, the possibility of reuse of treated effluent, by DAF (Figure 10) and

DispAF (Figure 11), from the equalization tank was evaluated in the dyeing process. We performed dyeing experiments, with various initial red dye concentrations, in which the liquid medium was a mixture of clean to treated effluent (100:0, 80: 20, 60:40, 40:60, 20:80 and 0:100 (v/v)). The effluent from the equalization tank was previously treated with 100 mg/L CTAB, the optimum condition for both DispAF and DAF. The evaluation of the dyeing process was performed by estimating the residual dye concentration (liquid phase), as well as by colorimetric measurements on the dyed samples (solid phase). Figures 10a and 11a present the equilibrium concentration of the dye in the solid phase versus the equilibrium concentration in water for DAF and DispAF, respectively. The correlation between equilibrium concentrations of the dye in the liquid and solid phases, after sorption onto textile, was realized by the Freudlich isotherm (eq 4).36 qe ) KFCe

(4)

where qe is the equilibrium concentration in the solid phase ((mg/ L)/(g of sorbent)), Ce is the equilibrium dyestuff concentration in the bulk (mg/L), KF is the Freudlich constant ((mg1-n Ln)/(g of sorbent)), and n is a constant depicting the sorption intensity. Isotherm parameters and correlation coefficients were estimated by nonlinear regression analysis, and they are given as insets in the above figures. The latter show a satisfactory convergence between experimental and predicted data. The colorimetric measurements of the solid phase versus initial dye concentration are shown in Figures 10b and 11b for DAF and DispAF, respectively. Both patterns of evaluation reveal that by increasing the percentage of treated water, the equilibrium parameters and K/S values present a reduction. This

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The results of this and of our previous study show that flotation seems to be an effective process for the removal of coloring matter from textile industry effluents. High-volume effluents, containing low dye concentrations, originating from the equalization tank are easily decolorized, since only the addition of a small concentration of surfactant is needed. Lowvolume effluents, with high dye concentrations, originating from the dyeing mill need the addition of a significant amount of chemicals, as polyelectrolyte, flocculant, and two surfactants. However, this high chemical consumption could be balanced by the great difference in volume treatment. Effluent discoloration could be applied next to the dyeing reactor without causing color pollution to the overall dye-house effluents. However, the findings should be confirmed in full-scale experiments. Acknowledgment The financial support received for this work from the Greek Ministry of Education (Pythagoras I) is gratefully acknowledged. The authors are grateful to Mr. J. Kyridis director/owner of the “Dyeing-Finishing-mills of Thessaloniki” who, very kindly, collaborated with us by providing the dyestuffs and the textile effluents examined in this study, as well as some valuable information on the dyeing procedure. Literature Cited

Figure 11. Effect of dipsersed-air flotation (DispAF) effluent recycling on dyestuff uptake by the textile expressed (a) as equilibrium isotherms in ADMI units and (b) as K/S values: V ) 80 mL, textile amount ) 1 g, [NaCl] ) 1 mol/L, [Na2CO3] ) 4 g/L, T ) 60 °C, pH ) 10, and tdyeing ) 90 min.

means that an interaction between dye and residual surfactant possibly takes place, which deactivates a percentage of available dye. Despite this, dyeing seems to be feasible by employing solely treated water, if a small amount of supplement dye is added in order to produce the same shade as with clean water. 4. Conclusions In this study, dissolved-air and dispersed-air flotation experiments were performed with simulated and industrial colored aqueous solutions originated from the dyeing reactor and the equalization tank of a dye-house, in order to evaluate the removal of dyestuff. The following conclusions can be drawn from the data obtained under the experimental conditions of the study: (1) Simulated effluents: Mixtures of dyestuffs, with an initial ADMI content of approximately 6000 units, could be decolorized by DAF and DispAF. To meet the permitted limit of 300 ADMI units, DAF needs the addition of 1.5 g/L polyelectrolyte, 50 mg/L flocculant, and 150 mg/L CTAB and SDS, each; DispAF needs 200 mg/L CTAB. (2) Dyeing reactor effluents: These types of effluents could be decolorized only by DAF due to exceptional foaming by DispAF. To meet the permitted limit of 300 ADMI units, DAF needs the addition of 3 g/L polyelectrolyte, 50 mg/L flocculant, and 300 mg/L CTAB and SDS each. (3) Equalization tank effluents: These types of effluents could be decolorized by DAF and DispAF. To meet the permitted limit of 300 ADMI, both techniques need the addition of 100 mg/L CTAB. (4) Water reuse: Treated water could be totally reused in textile dyeing, if a small amount of supplement dye is used.

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ReceiVed for reView September 12, 2007 ReVised manuscript receiVed May 9, 2008 Accepted May 16, 2008 IE071235N