Separation and Puri ﬁ cation Technology

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Introduction
Many chemical substances used in the production and processing of textile industry are highly toxic, for example dyes, which in some cases appear as mutagenic and carcinogenic materials.Dyes are used in the dyeing stage, which aims to give textile substrate its color, in this stage the industry consumes a large part of the water used in their processes, the average water use rate being 40 m 3 /ton of product [1].In addition, conventional cities effluent treatment plants remove only 20-30% of the color associated with synthetic dyes [2], therefore, development of efficient processes for treatment of textile effluent is very important for the environment.
Textile dyes can be divided into different classes; among them are reactive dyes, which differ from other types of dyes by forming a covalent bond with the substrate.These are normally applied in cotton dyeing.However, some disadvantages associated with reactive dyes are the use of high concentrations of electrolytes in dyeing [3], presence of unfixed dyes in the substrate, 30-50% of the dye applied, and resistance to light and oxidizing agents, which makes them difficult to degrade once released into aquatic systems [4].
Several effluent treatment processes containing dyes have been developed, aiming to achieve high levels of efficiency and low cost.Among these processes are; the use of adsorption [5][6][7], coagulationflocculation [8,9], advanced oxidative processes [10][11][12], biological treatment [13,14] and membrane filtration [15,16].An alternative is the use of surfactants.Surfactants are substances widely used in separation processes, including treatment of textile effluents, such as: microemulsions [17,18], micellar-enhanced ultrafiltration [19], cloud point extraction [20,21].In this work, ionic flocculation with anionic surfactants was evaluated as an alternative in dye removal from textile effluent.Ionic flocculation process begins with the mixture of anionic surfactants in the effluent allowing its interaction with the dye, this interaction occurs through micellar solubilization, where the solute, according to its affinity for aqueous medium, is positioned within the micelle, thus decreasing its concentration in the form of free monomer.Subsequent addition of multivalent metals such as calcium ion (Ca 2+ ) promotes formation of surfactants, where sodium is replaced by calcium, and remains in the medium as an insoluble precipitate.When submitted to stirring, this precipitate aggregates to form flocs which maintain the interaction of the surfactant with the dye.In this way, surfactant floc acts as an adsorbent surface, retaining the dye.Thus, a filtration step to separate the floc allows removal of the dye, providing an effluent with less pollutant load.
A biodegradable anionic surfactant, obtained from natural materials (coconut oil and bovine tallow), was used to perform ionic flocculation to treat the synthetic textile effluent containing RB14 dye as a model pollutant.The efficiency of dye removal was evaluated by factors such as: surfactant concentration, temperature, contact time, stirring speed, pH and electrolyte concentration.

Materials
The surfactant used in this work was base soap (average molecular weight = 289 g/mol, c.m.c.= 1.35.10 −3 M) consisting of a mixture of carboxylate surfactants of different lengths of alkyl chains.The dye RB14 (λ max = 590 nm, CAS 12236-85-0) was obtained from Ciba-Geigy.The calcium used in the experiments was obtained from solutions of calcium chloride (purity 99%).In experiments that carried out adjustment of pH, solutions of hydrochloric acid and sodium hydroxide (purity 97.8%) were used.The study of the process in the presence of electrolytes was carried out applying sodium chloride (purity 99%).

Effect of surfactant concentration and temperature
The synthetic effluent samples were prepared so that the dye concentration was 50 ppm, which is close to that found in the textile effluent [22][23][24], in a volume of 100 mL.After this step, the samples were taken to a thermostatic bath (Koehler Instrument Company, Inc, USA) equipped with stirrer so that the surfactant was dissolved under vigorous stirring (> 700 rpm).With bath temperature already controlled between 30 and 50 °C, the calcium solution was added to the effluent so that its concentration was half that of surfactant, which is a concentration that ensures interaction of the entire surfactant with calcium [25].The medium was then homogenized under a stirring speed of 100 rpm for 3 min to promote contact of calcium with the surfactant to form insoluble salt of the surfactant and then under a stirring rate of 50 rpm for 2 min so that small particles aggregate to form big flocs.
Floc separation was performed by filtration using 0.7 μm Millipore membrane.The dye concentration in the filtrate was determined by molecular absorption spectrophotometer (Varian Analytical Instruments, Cary 50 Conc, USA), and the dye removal efficiency calculated by Eq. ( 1).
where E(%) is dye removal efficiency in percentage, C RB,initial and C RB,final are the initial and final RB14 concentrations, respectively.

Evaluation of stirring speed and contact time
In order to verify the effect of the agitation speed and contact time in the process, same procedure described in Section 2.2.1 was adopted, maintaining concentration of surfactant and temperature constant at 650 ppm and 30 °C, respectively.The stirring speed control occurred in two stages: fast and slow.The rapid stirring was evaluated at 100, 200, 300, 400 and 500 rpm while slow stirring was half the value corresponding to rapid stirring.
Evaluation of contact time was made to define the best moment to collect samples for subsequent filtration.The contact time was measured as the slow stirring was stopped.Time between 5 and 150 min were evaluated.

Evaluation of the presence of electrolytes
The effect of the presence of electrolyte is necessary since it is present in textile effluent [26].The experiments to evaluate effect of electrolyte concentration were performed using NaCl, with surfactant concentrations used between 390 and 1300 ppm and NaCl concentrations were 0.0, 0.2 and 0.4 M. Temperature was maintained at 30 °C and the fast and slow stirring rates were 100 and 50 rpm, respectively.Samples were collected after five minutes of rest.

Evaluation of pH
Evaluation of pH was performed following the methodology described in Section 2.2.1, but pH adjustment was performed before the addition of calcium.The efficiency of the process was evaluated in pH range of 7-13 for surfactant concentrations of 1040, 1170 and 1300 ppm.

Effect of surfactant concentration and temperature
In order to determine the best process conditions, effects of the surfactant concentration and temperature were evaluated, maintaining the initial dye concentration constant at 50 ppm.Surfactant concentrations used are in the range of 390-1300 ppm.The working range for surfactant concentration was obtained in preliminary experiments, which demonstrated a negligible removal efficiency below 390 ppm and a approximately constant removal above 1300 ppm, which resulted only in higher amounts of surfactant without significant gains in process efficiency, In relation to the temperature, a maximum of 50 °C was established, as it was found in the range of 30-50 °C, that as the temperature increases the removal decreases, therefore it was unnecessary to study at higher temperatures.Fig. 1 shows the floc of the surfactant with the adsorbed dye and Fig. 2 shows the relationship of the removal efficiency with the surfactant concentration and the temperature.
According to Fig. 2 it can be noted that the removal efficiency depends on the surfactant concentration and the temperature.In relation to surfactant concentration, it is observed that as it increases the removal efficiency also increases, being a consequence of formation of a greater number of surfactant flocs, which provides greater dye removal.The process is also influenced by temperature, showing a decrease in removal efficiency as temperature increases.This is related to increased solubility of the surfactant at higher temperatures, impairing floc formation.The best result obtained from combination of surfactant concentration and temperature was using 1300 ppm of surfactant at 30 °C, reaching 86% of removal.Therefore, this process is shown to be efficient at room temperature.
The data obtained from removal of dye as a function of surfactant concentration and temperature were statistically evaluated to obtain a model that represents the experimental points.This evaluation shows that the quadratic model provides a greater approximation to the experimental data as can be observed in Fig. 3.The equation obtained for quadratic model is given by Eq. ( 2).
where [S] represents surfactant concentration (mg/L) and T is the temperature (°C).
As observed in Fig. 3, experimental results approximate well with those obtained with the quadratic model, indicating that this model is adequate.In addition, the coefficient of determination for the quadratic model was 0.9910.

Assessment of contact time and stirring speed
The evaluation of contact time was performed in order to verify the best time to collect the sample.The results obtained are shown in Fig. 4.
Fig. 4 shows that contact time influences the result of the process.Dye removal was 39.65% and 21.48% for contact times of 5 and 150 min, respectively.This shows that the dye RB14 has an affinity with water, which makes it difficult to fix it on the surface of the surfactant floc, causing a reduction of the removal in long times until reaching equilibrium in 90 min.
Agitation speed evaluation was performed to verify its effect on the efficiency of the process.Fig. 5 shows the relationship between dye removal and stirring speed.
Fig. 5 shows that stirring speed does not interfere with dye removal efficiency, since it remained constant at the rates assessed.Therefore, the value of the agitation speed used in other stages of this work was maintained at 100 rpm, because higher velocities are unnecessary.

Effect of pH
The effect of the pH was evaluated because stability of anionic surfactant is affected by it, because surfactant used in this work is the result of reaction of fatty acid with strong base NaOH, and this type of salt undergoes great influence of pH in its dissociation [27].The pH range evaluated was between 7 and 13.Removal results obtained for different pH values are shown in Fig. 6 at different surfactant concentrations.
Fig. 6 shows that dye removal is affected by pH, this behavior being related to stability of the surfactant.The effluent samples were prepared at a fixed dye concentration, and after the dissolution step of the  surfactant, pH between 10.4 and 10.6 were obtained.Therefore, experiments in pH range of 7-10 were adjusted by the addition of a 0.1 M hydrochloric acid solution, while for pH range above 10.6, a solution of NaOH, 1.0 M.
In cases where the pH is less than 10, when mixing the calcium to form surfactant floc, a lower amount of surfactant is available due to formation of fatty acids, which prevents formation of a larger amount of flocs, thereby reducing dye removal.At high pH (> 10.6) the process removal efficiency remains high, reaching a maximum of 86% at pH 12 and a surfactant concentration of 1300 ppm.This result shows that adjusting pH to a value higher than that found after surfactant dissolution will not cause significant gains of dye removal because without pH correction, dye removal was 86% applying 1300 ppm.
In Fig. 6 it can still be observed that the increase of the surfactant concentration promotes a significant increase in extraction efficiency.

Effect of NaCl concentration
The effect of the NaCl concentration was studied using two different molarities for NaCl (0.2 and 0.4 M).The evaluation of effect of sodium and chloride ions in textile effluent is important, as they are present in the actual effluents [28,29].Fig. 7 shows a sample after treatment with surfactant in the presence of NaCl, results obtained are shown in Fig. 8 at different concentrations of surfactant (390-1300 ppm).
Fig. 8 shows that the presence of electrolytes reduces dye removal, and it is observed that the higher the electrolyte concentration, the lower the removal.At surfactant concentration of 1300 ppm the removal was 86%, 81% and 70% for NaCl concentrations of 0.0 M, 0.2 M and 0.4 M, respectively.This is because, as shown in works of Stellner & Scamehorn [30] and Noïk et al. [31], the presence of electrolytes increases the tolerance to surfactant to calcium to precipitate.In addition, the addition of salts decreases to c.m.c.promoting formation of micelles that can attract calcium [31].

Determination of thermodynamic parameters
From data of dye concentration in aqueous phase and of surfactant used in the samples, a mass balance was performed to obtain the value of retention capacity of surfactant floc.Thermodynamic data presented in Tables 1 and 2 refer to the balance between dye retained in the floc and dye in the aqueous phase.In the calculations to determine thermodynamic parameters it was considered that only surfactant molecules which are in the form of micelles before addition of calcium are able to draw the dye onto the surface of the floc.
To assert this assumption, were evaluated systems with a surfactant concentration below 390 ppm (c.m.c.concentration).It was verified that despite the reaction of the surfactant with calcium forming the floc, no dye removal was observed.This fact proves that ionic flocculation only removes solutes, when dyes solubilized in the micelles are present.With the equilibrium data, calculations of thermodynamic parameters: Gibbs energy (ΔG°), entropy (ΔS°) and enthalpy (ΔH°) were performed by applying Eqs. ( 3) and (4).Eq. ( 4) is obtained by equality of the dye chemical potentials in aqueous phase and in micellar phase [20], where the q e .w/Ce ratio represents dye distribution coefficient between these phases.Table 1 shows enthalpy and entropy values obtained.
where q e (mg/g) is the amount of dye retained in the micelle, C e (mg/L) is the equilibrium dye concentration in the aqueous phase, w is the amount of surfactant used (g/L) and T is the temperature in Kelvin.The enthalpy and entropy values of Eq. ( 4) can be obtained using the angular and linear coefficients in the line log(q e .w/Ce ) vs 1/T, and the Gibbs energy values can then be determined with Eq. ( 3).Table 1 shows that the enthalpy and entropy values remain practically constant while surfactant concentration increases except for the 390 and 1300 ppm points.The fact that the enthalpy is negative indicates that retention of dye on the surfactant is exothermic, so it is expected that the efficiency of the process will decrease with increasing temperature.
The decrease in entropy indicates reduction of randomness during the process, since the dye molecules are less randomly organized after being retained.All these results are in accordance with what the theory of thermodynamic adsorption predicts, both ΔH°and ΔS°, must be negative [32].In Table 2 the positive values of the Gibbs energy indicating a non-spontaneous process agree with the experimental result shown in Fig. 4. The adhesion of dye molecules to the surface of the floc is not spontaneous, since the dye is gradually desorbed the floc when they remain in contact with aqueous medium.Dye desorption occurs after the time in that the dye is solubilized by the micelle, adsorbed therein in aqueous phase, and when the addition of the calcium is carried out causing the dye to be entrained by the surfactant into formed floc.In turn, the system, in condition of flocs, responds in the direction of a new equilibrium between dye on the surface of the floc/ dye in aqueous phase, which results in a lower dye removal efficiency, because at this moment there are no more micelles and affinity of the surfactant by the dye decreases.

Conclusions
In this work, ionic flocculation of anionic surfactants was applied using the calcium ion for the treatment of synthetic textile effluent containing the RB14 dye.It has been shown that ionic flocculation of surfactant promotes phase separation of the flocs containing dyes, although its affinity for it is not so high, a fact that is clearly demonstrated by thermodynamic parameters.It is noteworthy that after forming floc, desorption occurs indicating that a new equilibrium is established, thus corroborating with the fact that surfactant in the micellar form, initial stage, retains more dye than surfactant in its floc form.With dye removal efficiency of 86%, this process is promising and can be applied in the treatment of effluents.

Fig. 1 .
Fig. 1.Dye adsorbed on the surface of the surfactant floc.

Fig. 2 .Fig. 3 .
Fig. 2. Removal of RB14 as a function of the surfactant concentration and temperature.

Fig. 8 .
Fig. 8. Removal of RB14 dye from effluents to different concentrations of surfactant and NaCl.

Table 2
Gibbs energy (kJ/mol) of the process for different temperatures and surfactant concentrations.