flocculation and adsorption on activated carbon

flocculation and adsorption on activated carbon

Resources, Conservation and Recycling 54 (2010) 283–290 Contents lists available at ScienceDirect Resources, Conservation and Recycling journal home...

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Resources, Conservation and Recycling 54 (2010) 283–290

Contents lists available at ScienceDirect

Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec

Removal of reactive dyes from aqueous solutions using combined coagulation/flocculation and adsorption on activated carbon Franciele Regina Furlan ∗ , Laís Graziela de Melo da Silva, Ayres Ferreira Morgado, Antônio Augusto Ulson de Souza, Selene Maria Arruda Guelli Ulson de Souza Chemical Engineering Department, Federal University of Santa Catarina, P.O. Box 476, CEP 88040-900, Florianópolis, SC, Brazil1

a r t i c l e

i n f o

Article history: Received 12 September 2008 Received in revised form 20 August 2009 Accepted 1 September 2009 Keywords: Coagulation Adsorption Reactive dyes Aluminum chloride Textile effluent

a b s t r a c t The removal of two reactive dyes (Black 5 and Orange 16) was investigated. The objective of this study was to investigate the removal of reactive dyes through a combined treatment process with coagulation/adsorption on activated carbon. Activated carbon derived from coconut shells was used as the adsorbent and aluminum chloride was used as the coagulant. In order to obtain the best conditions for the removal of the dyes, the influence of the following parameters was verified: coagulant and alkalizer dosage, aqueous solution pH, temperature of the mixture and salt addition (sodium chloride). Spectrophotometry was the analysis technique used to measure the concentration of dye remaining in the fluid phase. The results for the adsorption of the reactive dyes were fitted to the models of the Langmuir, Freundlich and Radke-Prausnitz isotherms and showed good correlation. The removal efficiencies were approximately 90% and 84% for the Black 5 and Orange 16, respectively. In order to evaluate the final effluent obtained after the coagulation and adsorption process, acute toxicity tests were carried out with Artemia salina and Daphnia magna, which verified that the effluent was atoxic. The combined coagulation/adsorption process was shown to be an excellent option for the removal of reactive dyes. © 2009 Published by Elsevier B.V.

1. Introduction The textile industry generates effluents with an extremely heterogeneous composition and a great quantity of toxic and recalcitrant material, which makes its treatment more difficult. The effluents have a strong coloration, a great quantity of suspended solids, a highly fluctuating pH, high temperatures, high concentrations of COD, and considerable quantities of heavy metals (Cr, Ni, Cu), chlorinated organic compounds and surfactants (Araújo and Yokoyama, 2006). The reactive dyes are widely used in the textile industry due to their high level of stability during washing and because of their simple dyeing procedures, these being the main class of dyes used to dye cellulose and cotton (Araújo and Yokoyama, 2006). However, these dyes are highly soluble in water and also have low levels of fixing to fibers, most of their initial concentration being lost to the effluent. Many of the contaminating substances present in a textile effluent cannot be removed by simple physical separation (Dogan

∗ Corresponding author. Tel.: +55 48 3721 9448; fax: +55 48 3721 9687. E-mail addresses: qui [email protected] (F.R. Furlan), lais [email protected] (L.G. de Melo da Silva), [email protected] (A.F. Morgado), [email protected] (A.A.U. de Souza), [email protected] (S.M.A. Guelli Ulson de Souza). 1 http://www.enq.ufsc.br/labs/LABSIN. 0921-3449/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.resconrec.2009.09.001

and Alkan, 2003; Kim et al., 2004; Senthillkumaar et al., 2006). The physico-chemical processes applied to clarify effluents are based on the destabilization of the colloids by coagulation–flocculation, and phase separation through sedimentation or floatation (Solmaz et al., 2006). Many treatment processes for textile effluents have been studied. Biological treatment by activated sludge offers high COD removal efficiency, but is not very effective in the removal of colors. According to Solmaz et al. (2006), chemical oxidation with ozone, or a combination of UV radiation, ozone and H2 O2 , are of great interest, but their costs are very high for effluent treatment. One alternative would be the application of these techniques in combination with conventional treatments (Barreto-Damas et al., 2005; Korbutowicz-Kabsch, 2006; Harrelkas et al., 2009; Narayanan and Ganesan, 2009). The combined methods can be used in a complementary manner, in such a way as to compensate for deficiencies of the processes when applied in isolation (Faria et al., 2005; Choo et al., 2007). Lee et al. (2006), tested the coagulation–flocculation and adsorption processes independently and together and concluded that the combined process was more efficient with a 99.9% removal of the dyes studied, working with coagulation and adsorption pH values of 6 and 4, respectively. In studies carried out by Ahmad and Puasa (2007), the maximum removal found for Black 5 was 90.64% and for Orange 16 was 77.5%, at acid pH, using an initial concentration of

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Table 1 Structure of the dyes and wavelength. Dye

max (nm)

Structure

Reactive Black 5

598

Reactive Orange 16

491

500 mg/L of dye solution with a cationic flocculant and a surfactant. According to the authors, with an increase in the positive charge of this matrix (or region), the coagulant became more effective in the removal of the high negative charges of the reactive dyes, resulting in the neutralization of the solution charges. However, with an increase in the pH, the charge concentration of the coagulant decrease resulting in a low removal of color. Studies carried out by Harrelkas et al. (2009) using the coagulation–flocculation process coupled with membranes or adsorption on activated carbon for the removal of dyes, determined that the most appropriate medium in terms of the efficiency of the coagulation process was at pH 5. Demirbas and Nas (2009), in adsorption studies with Reactive Blue 21 (RB21), achieved the best dye removal in acid medium (pH 2). Similarly, Abdelwahab (2009) obtained the best adsorption results for the removal of reactive orange dye at pH 1. Lee et al. (2006) reported that the optimum value for the dosage of aluminum chloride was 250 mg/L for the dye Black 5 and 350 mg/L for Orange 16. Based on these results we can conclude that there is a need for optimization in the process even when working with the same dyes and coagulants, in order to reduce the effluent treatment costs. According to Gregor et al. (1997), there is a stoichiometric relation between the negative charge of the dye and the quantity of coagulant required for the coagulation. Relevant data using the same combination of processes were obtained by Papic et al. (2004), with a decolorization of 99.9% for the reactive dyes analyzed and removals of over 90% for total organic carbon and chemical oxygen demand. It has become ever more evident that with the use of combined processes the drawbacks of a single individual operation can be overcome and complete treatment can be achieved (Chakraborty et al., 2005; Solmaz et al., 2007). The objective of this study was to evaluate the efficiency of a coagulation–flocculation–sedimentation pretreatment combined with adsorption for the removal of color from textile effluents. Also, the optimum operational conditions for the two processes were determined, as well as the toxicity of the effluent before and after the treatment, using the bioindicators Daphnia magna and Artemia salina. 2. Experimental procedure The coagulation and adsorption processes were optimized in order to determine the effectiveness of the combined processes in

the complete removal of the dyes. The pH was adjusted at each stage of the treatment and the effluent toxicity was determined after the pretreatment stage and after the complete color removal treatment. 2.1. Materials The experiments were carried out using two reactive dyes, Black 5 and Orange 16, with raw effluent (effluent which has not been treated). The wavelength values and the molecular structures are shown in Table 1. A carbon adsorbent derived from coconut shells was used, with a surface area of 754 m2 /g. As a coagulant, aluminum chloride (AlCl3 ·6H2 O—99.5% purity, with a molecular mass of 241.45 g/gmol) was used. Sodium carbonate (Na2 CO3 —99% purity) was used as alkalizer. A stock solution was prepared by mixing pre-established quantities of the dye with distilled water. The stock solution was used to obtain all of the other solutions used in the experiments through careful dilution. The concentration of the dyes was analyzed using UV spectrophotometer (Shimadzu, mini 1240) at the wavelength of maximum absorbance of the synthetic effluent, in the visible range. 2.2. Methods 2.2.1. Coagulation studies For the coagulation/flocculation tests, the raw effluent was used with the aim of optimizing the parameters for the textile effluent treatment. The stock solution (2000 mg/L) was used to obtain all the other solutions for the experiment by dilution. The coagulation tests were carried out in Nova Ética static reactors (Jar Test), composed of 6 prismatic stems, square cross-section reactors (jars), each reactor having a 2 L capacity. All the experiments were conducted with 1 L of solution in each reactor. The coagulant and alkalizer were added simultaneously to the dye solution with rapid mixing at 175 rpm for 1 min 40 s, followed by slow mixing at 15 rpm for 10 and 60 min of sedimentation. At the end of the sedimentation period, samples were removed at 2 cm below the effluent surface for the determination of the residual color. In order to adjust the pH, sodium hydroxide (NaOH) 0.1 M and hydrochloric acid (HCl) 0.1 M were added to the aqueous solutions. The pH measurements were carried out using a Quimis pH meter (model Q-400M2), calibrated using buffer solutions of pH 7.0 and 4.0, as recommended by the Standard Methods for the Examination of Water and Wastewater, AWWA (1989).

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2.2.2. Adsorption studies The experimental adsorption tests were carried out in batches with the pretreated synthetic effluent (effluent after the coagulation/flocculation process) using activated carbon as the adsorbent, where the kinetic and equilibrium data were determined. For the removal of the dye Orange 16, the pretreatment parameters were optimized and the most suitable parameters obtained were employed with the dye Black 5 in the adsorption stage in order to verify the efficiency of the combined process with different dye colors. In order to carry out the experiments, a volume of 20 mL dye solution was mixed with 1 g of adsorbent (particles between 1.0 and 0.6 mm) in a 40 mL test tube. The adsorption process was carried out in a thermostated bath (25 ± 1 ◦ C), under constant shaking at 100 rpm in a Dist 940 Shaker for times pre-determined in the kinetic tests. The same experiment was repeated with variations in some of the parameters such as: pH (2, 3, 4 and 7), quantity of sodium chloride (0%, 1%, 3%, 6% and 10% of the solution mass) and temperature (25, 45 and 70 ◦ C). The results for the adsorption experiments were used to determine the equilibrium isotherms. The concentration of solute adsorbed in the adsorbent phase can be determined carrying out a mass balance of the adsorbate, according to Eq. (1): qe =

(C0 − Ce ) × V W

(1)

where qe is the quantity of solute adsorbed in the solute phase (mg/g), C0 and Ce are the initial and equilibrium concentrations of the adsorbate (mg/L), respectively, V is the volume of the solution (L) and W is the adsorbent mass (g). 2.2.3. Toxicological studies with Artemia salina Acute toxicity tests were carried out using the microcrustacean Artemia salina. Samples of the raw effluent, the effluents after the coagulation/flocculation process and the effluent after the complete treatment process (coagulation followed by adsorption) were analyzed. The experiments with Artemia salina were carried out according to the modified method described by Matthews (1995), where the procedure of outbreak and growth of the microcrustaceans occurred for a period of 60 h of incubation in a synthetic marine saline solution (32 g/L), pH 6.5, with aeration and at a temperature of 30 ◦ C. After the hatching, ten larvae of the microcrustacean were selected and incubated in a multiwell plate. Negative controls were conducted in parallel using a synthetic marine saline solution. The solutions were diluted in a concentration series (100%, 90%, 80%, 70%, 60% and 50%) in order to obtain the LC50 . After 24 h of incubation the number of dead larvae was counted and the LC50 was calculated. For each concentration tested the analysis was carried out in quadruplicate.

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standard solution NaCl 0.01 M in 9 Erlenmeyer flasks, to which appropriate quantities of HCl and NaOH were added to obtain the following pH values: 2.5; 4.0; 5.0; 6.0; 7.0; 8.0; 9.0; 10.0 and 11.0. The mixture was shaken in a DIST shaker for 24 h at ambient temperature. After 24 h, the equilibrium pH was measured with a Quimis pH meter, model 400A. The pHpzc is the pH at which the initial value is the same as the final value. Activated carbons can have an acid, basic or neutral nature depending on the pHpcz determined.

3. Results and discussion 3.1. Process 1: coagulation–flocculation–sedimentation tests For the batch coagulation–flocculation–sedimentation tests, the jar tests were carried out to determine the best pH and dosage of coagulant and alkalizer. 3.1.1. Optimum pH for coagulation After the jar tests, the optimum pH results were obtained, where the best performance in terms of color removal was in the acid range of pH 4–6.5 (the pH ranged studied varied from 4 to 11), the highest removal efficiency being at pH 6, for the two dyes studied, as shown in Fig. 1. The conditions used were: dosage of AlCl3 of 250 mg/L for the two dyes; dosage of Na2 CO3 of 100 mg/L for Black 5 and 110 mg/L for Orange 16; initial dye concentration of 100 mg/L; sedimentation time of 30 min; and temperature of 25 ± 1 ◦ C. Therefore, we can verify that the coagulation process has a better performance in the acid to neutral pH range. The best performance of the aluminum chloride coagulant at pH 6.0 is due to the charge equilibrium at this pH. The percentage removals were 96.8% and 78.2% for the dyes Black 5 and Orange 16, respectively. As can be observed in Fig. 1, the removal of Black 5 was greater than that of Orange 16. This may be due to the different structures and molecular masses of the two dyes. In these structures it can be observed that the black dye contains four sulfonic acid groups whereas the orange dye has only two of these groups. The removal using the coagulation process was more effective for the black dye which settles easier than the orange dye, using aluminum chloride as the coagulant. When the dyes and the coagulant are dissolved in water and mixed the formation of cations and anions occurs, which, having opposite ionic charges, combine to form an insoluble

2.2.4. Toxicological studies with Daphnia magna The tests with the microcrustacean Daphnia magna were standardized according to the ABNT norms (1993), and the results were expressed as dilution factor (DF). The DF represents the first of a series of sample dilutions at which acute toxic effects to the test organism are no longer observed. The exposure time of the test organism to the effluent evaluated was 48 h, after which the number of immobile organisms was counted. The microorganisms are considered immobile when there is no movement for 20 s. The analyses were carried out in duplicate. According to the legislation (article no 017/02 of FATMA), the maximum dilution factor for an effluent of textile origin is two (2) (FATMA, 2002). 2.2.5. Determination of pHpzc of activated carbon The pHpzc, known as the point of zero charge, was obtained through the mixture of 0.1 g of activated carbon with 50 mL of a

Fig. 1. The optimum pH for the removal of the reactive dyes in the coagulation–flocculation–sedimentation process.

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F.R. Furlan et al. / Resources, Conservation and Recycling 54 (2010) 283–290 Table 2 Parameters and isotherm models. Isotherms

Equations

Parameters

Langmuir

qm × KL × Ce qe = 1 + KL × Ce

KL , qm

Freundlich Radke-Prausnitz

qe = KF × qe =

Cen

qm × KL × Ce 1 + KL × Ceb

KF , n KL , qm , b

As determined in the previous tests, the pH was fixed at 6 and the dosage of coagulant at 200 and 250 mg/L for the dyes Black 5 and Orange 16, respectively. Harrelkas et al. (2009) carried out studies with the coagulant Al2 (SO4 )3 and determined that the optimum concentrations were 100 mg/L of coagulant and 4 mg/L of flocculant, for the removal of dyes from textile effluents. The alkalizer concentration applied in each jar was also optimized since this is an important stage in terms of the effluent treatment costs, in the range of 40–250 mg/L. The results for the effects of the different alkalizer dosages are given in Fig. 2b. The addition of sodium carbonate to the solution provided a medium suitable for the formation of the insoluble Al(OH)3 precipitate, resulting in a better efficiency of the formation of the decantation flocs. The maximum color removal values for Black 5 and Orange 16 were obtained with concentrations of 110 and 140 mg/L of alkalizer, respectively. 3.2. Process 2: adsorption tests The adsorption results obtained in this study were fitted to the Langmuir, Freundlich and Radke-Prausnitz isotherm models. The equations and parameters of each isotherm are given in Table 2. Fig. 2. The optimum dosage of coagulant (a) and alkalizer (b) for the removal of reactive dyes in the coagulation–flocculation–sedimentation process.

complex, according to reactions, given by Eqs. (2), (3) and (4). + RSO3 Na ↔ RSO− 3 + Na

(2)

↔ Al3+ + 3Cl−

(3)

(dye)

AlCl3 (coagulant)

Al3+ + RSO− 3 (insoluble complex)

→ RSO3 − Al+

(4)

3.1.2. Optimum dosage of coagulant and alkalizer The new tests were carried out with the pre-determined value of pH 6, initial dye concentration of 100 mg/L, sedimentation time of 30 min and temperature of 25 ± 1 ◦ C, varying the dosage of coagulant from 150 to 750 mg/L. The dye removal as a function of coagulant concentration is shown in Fig. 2a. The removal efficiency for the two dyes increased with the coagulant dose up to a certain dose where it remained constant for the dye Black 5 and began to decrease gradually for the dye Orange 16. The maximum color removal efficiency was for a coagulant dose of 200 mg/L for the dye Black 5. For the dye Orange 16 the maximum removal was achieved using 250 mg/L of coagulant. Higher dosages for the orange dye were required due to its low molecular mass compared to the black dye. Since the coagulant dose alters the quantity of flocs formed and their tendency to settle, the dye removal efficiency decreases with the quantity of coagulant above or below a determined optimum dose. According to studies carried out by Wu et al. (2009), these results indicate a strong association between the coagulant type and dose.

3.2.1. Influence of pH The influence of pH on the adsorption of the dyes on activated carbon was analyzed through the adsorption kinetics at 25 ◦ C, in order to select the most favorable pH for the fixing of the carbon dyes. The adsorption kinetics data are shown in Fig. 3a. With the equilibrium time defined in the kinetic tests, it was possible to construct the adsorption isotherms for the different pH values 2, 3, 4 and 7, as shown in Fig. 3b. The maximum adsorption capacity increases from 16.6 to 19 mg/g with a pH increase from 2 to 3, however, it decreases to 5.8 mg/g with an increase to pH 7. Thus, it was concluded that pH 3 is the optimum value for the color removal process. The pH was shown to be an important parameter in terms of the adsorption capacity, influencing not only the surface charge of the adsorbent but also the level of ionization of the material present in the solution. According to a study reported by Malik (2004), on the adsorption of direct dyes Blue 2B and Green B on activated carbon, one of the reasons for the greater adsorption in acid medium could be that at low pH values there is an increase in the positive ion (H+ ) concentration of the system and the surface of the activated carbon acquires positive charges through the adsorption of these ions. The point of zero charge (pHpzc) of the activated carbon under study was determined, and the active carbon can be considered as neutral since the pHpzc value was found to be 7.0. With the activated carbon surface being neutral it could be positively or negatively charged depending on whether the medium is acid or basic. However, the explanation for the greater adsorption in acid medium is that in this medium the activated carbon surface will be positively charged and the negative molecules of the dyes will be electrostatically attracted to the surface resulting in maximum adsorption. When the pH of the system is increased to alkaline values, the number of negatively charged sites increases, decreasing the num-

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Fig. 3. Adsorption kinetics (a) and Langmuir isotherms (b) for different pH values—reactive dye Black 5.

ber of positive sites. The negatively charged adsorbent sites do not favor the adsorption of anionic dye molecules due to electrostatic repulsion. The low adsorption of the reactive dyes in alkaline medium is also due to the competition of excess OH− ions with anionic molecules of the dyes for the adsorption sites. 3.2.2. Influence of temperature On evaluating the results shown in Fig. 4a, it was verified that the time required to reach equilibrium was approximately 6 h 20 min, for the temperatures studied. Some kinetics studies were carried out for a period of 24 h and it was verified that during this time there were some oscillations in the adsorption and desorption, but after 6 h 20 min there was little variation in the maximum adsorption capacity. The equilibrium time obtained with the kinetic experiments was used to carry out the adsorption tests, and the data obtained in the adsorption experiments were used to construct the isotherms shown in Fig. 4b, for the temperatures of 25, 45 and 70 ◦ C, respectively. The maximum adsorption capacity, qm , increased from 19 to 24.4, with an increase in temperature from 25 to 70 ◦ C, demonstrating that the behavior of this process is endothermic. This endothermic behavior is probably due to a greater interaction between the dye and adsorbent molecules, caused by the action of salt, at high temperatures. It is possible that in the presence of salt there is an increase in the degree of ionization at the adsorbent surface, increasing the number of active sites. Since the increase in temperature generates a greater degree of agitation between the molecules, a greater number of dye molecules can be transferred to these active sites, favoring the adsorption process. Singh et al.

287

Fig. 4. Adsorption kinetics (a) and Langmuir isotherms (b) for the temperatures of 25, 45 and 70 ◦ C—reactive dye Black 5.

(2003) suggested that the increase in temperature can produce a dilation of the internal structure of activated carbon, enabling greater diffusion of the dye molecules in the adsorbent. 3.2.3. Influence of salt addition The influence of sodium chloride on the adsorption kinetics of the dyes is shown in Fig. 5. It was found that the time required for the solutions with salt addition to reach equilibrium was approximately 5 h 40 min; in the case of the solution without sodium chloride the adsorption was slower, with equilibrium being reached in 6 h 20 min, for both dyes. Kinetic tests were also carried out with up to 24 h duration; however, as mentioned in Section 3.2.2, there was little alteration in the maximum adsorption capacity for longer equilibrium times. The isotherms constructed for different NaCl quantities for the dyes Black 5 and Orange 16 are shown in Fig. 6. On analyzing the Langmuir parameters for the dye Black 5, it is verified that the maximum adsorption capacity increases from 19 to 87.7 mg/g with an increase in the sodium chloride content from 0% to 6%. However, this decreases to 43.7 mg/g with a further increase in salt content to 10%. Regarding the Orange 16 dye it was found that the maximum adsorption capacity increased from 27.5 to 74.1 mg/g while the sodium chloride content increased from 0% to 1%. According to Guelli Ulson de Souza et al. (2007), the presence of salt positively influences the adsorption when compared with the experiments in its absence. It can be suggested that the salt cations neutralize the negative charge of the carbon surface enabling the adsorption of more molecules or the cations to act directly on the negative adsorbate ions.

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Fig. 6. Langmuir isotherms of the reactive dyes Black 5 (a) and Orange 16 (b) for the different quantities of sodium chloride. Fig. 5. Adsorption kinetics of the reactive dyes Black 5 (a) and Orange 16 (b) for the different quantities of sodium chloride.

The best operational conditions are shown in Table 3, and the parameters of the isotherms obtained through the experimental tests with the best operational conditions are shown in Table 4. All of the parameters studied fulfilled the favorable adsorption conditions criteria.

Table 4 Parameters of the isotherms for the reactive dyes under the best operational conditions. Isotherms

Parameters

Black 5

Orange 16

Langmuir

qm KL R2

95 0.008 0.9600

99.7 0.006 0.9908

Freundlich

KF n R2

0.776 0.917 0.9869

0.801 0.818 0.9977

Radke-Prusnitz

qm KL b R2

0.702 0.066 0.319 0.9964

0.950 0.359 0.334 0.9985

3.3. Acute toxicity tests The acute effects indicated by the toxicity tests with Artemia salina are evaluated according to the lethal concentration (LC50 ), that is, the concentration of the toxic agent, present in the aquatic environment, which causes 50% of lethality after 24 h of exposure to the effluent. The samples analyzed were the raw effluent (solution without treatment), the effluent after the coagulation/flocculation process and the effluent after the complete treatment process (coagulation followed by adsorption). The lethal concentration, LC50 , found for the dye Black 5, for a solution without treatment, was 83.3 mg/L. The LC50 value was Table 3 Best conditions for the removal of reactive dyes Black 5 and Orange 16. Parameters

Agitation pH Sodium chloride Temperature Particle size

Best condition Black 5

Orange 16

With agitation 3 6% 70 ◦ C 1.0–0.6 mm

With agitation 3 1% 70 ◦ C 1.0–0.6 mm

found through an interpolation of the experimental results given in Table 5 (the values studied varied within the range 50–100 mg/L). For the dye solutions after the coagulation–flocculation pretreatment and after the complete treatment process, no toxicity of the effluent was detected, indicating that the treatments were efficient in the removal of the elements under study. The criterion used for the determination of acute toxicity with Daphnia magna was the DF, which establishes the effluent concentration which can cause the mortality or immobility of up to 10% of organisms exposed for a period of 48 h to different effluent concentrations. The results with Daphnia magna are given in Table 6. It can be observed that for the non-treated effluent the toxicity was above the permitted limit, indicating the need for treatment before it can

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289

Table 5 Acute toxicity tests with Artemia salina after 24 h of incubation. Dye concentration (%)

Mortality (%) in the initial solution

Mortality (%) in the after pretreatment with coagulation

Mortality (%) in the after the complete treatment process

100a 90 80 70 60 50

80 60 45 45 32.5 25

5 0 0 0 0 0

0 0 0 0 0 0

a

The solution concentration of 100% refers to a concentration of 100 mg/L.

Table 6 Acute toxicity tests with Daphnia magna after 24 h of exposure. Sample

DFa

DF (maximum limit, Article 017/02; FATMA [16])

(1) Non-treated dye solution (2) Dye solution after pretreatment with coagulation/flocculation (3) Solution after the complete treatment process (adsorption after coagulation)

256 16

2 2

1

2

a

Dilution factor without effect.

be discharged in accordance with the environmental legislation. The toxicity of the dye solution decreased considerably after the coagulation/flocculation pretreatment process. It can therefore be concluded that the dye under study (reactive Black 5) had a harmful effect on the test organism used. After the complete treatment process no toxicity of the effluent was observed, indicating that the products used during the pretreatment stage (coagulant and alkalizer) do not cause toxicity in the effluent generated at the end of the process. The acute toxicity tests for the textile effluent with the bioindicators Daphnia magna and Artemia salina showed that there was a reduction in the effluent toxicity after the pretreatment with coagulation–flocculation and after the complete treatment process, coagulation followed by adsorption. It is thus concluded that the complete treatment process, besides removing most of the color content, also reduced the toxicity of the effluent generated at the end of the process. 4. Conclusions The effluent coagulation–flocculation tests determined that aluminum chloride requires a certain quantity of alkali in order to maintain the alkalinity required for a good flocculation performance. The optimum dosages for the coagulant were 200 and 250 mg/L for the dyes Black 5 and Orange 16, respectively, and in the case of sodium carbonate they were 110 and 140 mg/L for Black 5 and Orange 16, respectively. The adjustment of the pH was necessary in order to increase the color removal efficiency. For both dyes, the maximum removal efficiency was achieved with a pH of around 6 for the pretreatment with coagulation/flocculation and pH 3 for the adsorption tests. The adsorption capacity of the dyes increased with temperature from 25 to 70 ◦ C, thus showing the endothermic behavior of this adsorption process. The time required for the solutions with the addition of salt to reach equilibrium is shorter, approximately 5 h 40 min and in the case of the solution without sodium chloride the adsorption is slower and equilibrium is reached in 6 h 20 min, for both dyes. The equilibrium tests showed that the dye adsorption capacity increased with the addition of salt to the solution, and that equilibrium times longer than those used would not increase the adsorption capacity.

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