Removal of direct Yellow 27 dye using animal fat and vegetable oil-based surfactant

The textile industry uses large amounts of water in its processes. Such water ends up contaminated by a number of substances, including dyes, creating an environmental challenge. The present study aims at removing dye by applying the precipitation process using a sodium soap (anionic surfactant), derived from animal fat and vegetable oil, promoting dye decontamination using biodegradable materials. Sodium soap, in the presence of calcium ions, forms another insoluble surfactant. This new surfactant, under stir-ring, aggregates and forms hydrophobic ﬂocs that are capable of adsorbing organic materials dissolved in aqueous solutions, including dyes. Dye removal was assessed considering the inﬂuence of initial surfactant concentration, temperature, pH, presence of electrolyte, equilibrium time, and stirring rate. Dye removal efﬁciency reached 97.6%.


Introduction
The textile industry is one of the largest polluters worldwide due, primarily, to the high water consumption and the elevated volume of colored wastewater production.Dye bath wastewaters can interfere in the penetration of solar radiation in aquatic systems, thereby disturbing biological processes [1,2].In the dyeing process, it is usual to add chemicals such as dyes with trace amounts of heavy metals (such as chromium, cadmium, and zinc), acids, alkali, nitrate and sulfate salts, surfactants, and formaldehyde to improve dye fixation [3,4].Therefore, adequate water treatment is important before discharging this wastewater into the environment.
Synthetic dyes bring considerable industrial interest, since they are relatively stable compounds, with widespread applications in the dyeing processes of textile companies.However, these dyes are difficult to remove in wastewater treatment plants, whether by physical, chemical, biological, or a combination of these processes [4].Dyes have complex structures.Direct dyes are anionic watersoluble dyes and, when in the presence of electrolytes in aqueous solutions, exhibit a considerable affinity for cellulose fibers [5].Nowadays, several technologies are being used to produce better fade-resistant fabrics, and increase the variety of shades available [6].
Surfactants are amphiphilic molecules composed of at least two parts, one of them polar or hydrophilic (head group) and the other one nonpolar or hydrophobic (tail).Surfactant monomers act as typical electrolyte when they are present in diluted solutions, accumulating at interfaces and reducing the interface tension.When the surfactant concentration is increased, colloidal organized aggregates called micelles are formed by self-association of a large number of monomers.The critical micelle concentration (c.m.c.) is the concentration at which micellization starts [24].
The carboxylate soaps can interact with calcium ions (Ca 2+ ) forming salts of low water solubility and precipitating as scummy deposits [25] that can aggregate to form flocs when submitted to stirring.This interaction can be described by a solubility product relationship between the free or unassociated species involved [26].The flocs end up exhibiting significant decontamination potential for organic compounds, because they exhibit the hydrophobic character of the non-polar tail of the surfactant.Depending on their weight, they will decant or remain suspended in the medium, being separated by filtration.The use of surfactants to treat dye-bearing wastewaters is the object of study of our research group [27,28].The treatment proposed in this research is based on the use of carboxylate surfactants, which have the advantage of being low-cost, biodegradable, and low-toxicity products.Another advantage is the use of low surfactant concentrations.Experiments were conducted to remove the Direct Yellow 27 dye by flocculation, assessing the influence of surfactant concentration, stirring rate, equilibrium time, electrolytes concentration, pH, and temperature on removal efficiency.

Materials
The surfactant used was synthesized in the laboratory from animal fat and coconut oil with a mass percentage of 95% and 5%, respectively.Table 1 presents the mean composition of fatty acids present in animal fat and coconut oil.The surfactant obtained was a mixture of surfactants, commercially known as base soap (SB), including sodium dodecanoate and sodium hexadecanoate.The mean molecular mass of the obtained surfactant was 289 g/mol and its critical micelle concentration (c.m.c.) was 0.0063 M (1820 ppm) [29].Sodium dodecanoate and sodium hexadecanoate were also used as surfactants, both synthesized in the laboratory.Direct Yellow 27 (DY27 -Ciba-Geigy; molar mass = 662.62g/mol; max = 393 nm; empirical formula: C 25 H 20 N 4 Na 2 O 9 S 3 ; CAS number: 10190-68-8) was used as dye to prepare the synthetic wastewater.Fig. 1 presents the chemical structure of DY27.Calcium chloride (CRQ) was used to obtain the calcium solution to perform flocculation.In the experiments that assessed the effect of pH, this was adjusted using hydrochloric acid (Vetec) and sodium hydroxide (NEON) solutions.NaCl (Anidrol) was used in the experiments to evaluate the presence of electrolytes in the dye removal process.All reagents were of analytical grade.Aqueous solutions were prepared using distilled water.

Dye extraction experiments
Initially, stock solutions of dye and calcium were prepared with 1000 and 2000 ppm, respectively.In all experiments the concentration of DY27 was maintained at 100 ppm, which corresponds to a higher concentration than that found in textile wastewaters containing direct dyes [30].In 100-mL test tubes, the dye solution was added and, soon after, SB was weighed and dissolved according to the required concentration (220, 260, 290, 330, 360, 390, 520, and 650 ppm -all lower than the c.m.c.value).The calcium solution was added in a concentration corresponding to half the SB concentration.The tests were conducted in a thermostatic bath (Koehler Instrument Company, Inc., USA) combined with a stirring system (EUROSTAR Power control-visc KIKA ® -WERKE) (Fig. 2), at constant temperature (30, 40, 50, 60, and 70 • C), 100 rpm stirring rate during 3 min, then 50 rpm for 2 min [31,32].After stirring, the samples were allowed to rest for 5 min and, then, filtered using a 0.7 m glass fiber membrane (AP40, Millipore).The filtrate was analyzed in a molecular absorption spectrophotometer (Varian Analytical Instruments, Cary 50 Conc USA) and in an atomic absorption spectrophotometer (Varian, AA240) to determine final dye and residual calcium concentrations, respectively.Dye removal efficiency was calculated by using Eq. ( 1): where, C DY27 initial is the initial concentration of dye and C DY27 dilute is the concentration of DY27 in the dilute phase, after floc separation.

Effect of pH on dye removal efficiency
In experiments to evaluate the effect of pH, the solution containing SB was restricted to only three concentrations (390, 520, and 650 ppm), at 30 • C. The process was assessed at pH 7-13.The pH adjustment stage, using a Digimed DM-22 potentiometer, was carried out after the addition of the SB to the dye solution and before addition of the calcium solution.At pH 7 and 8, the solutions of SB become cloudy, which hinders spectrophotometric analysis of molecular absorption, requiring the use of high-performance liquid chromatography (HPLC).The samples analyzed by HPLC were previously treated with sodium carbonate (0.01 M) for calcium precipitation.
The experiments to analyze dye removal efficiency as a function of pH also included assessing dye removal capacity with the addition of calcium in the absence of SB.The same pH range was studied (7-13).

Influence of electrolytes
The textile wastewater presents a high concentration of electrolytes [8,33], mainly sodium chloride or sodium sulfate, that are employed in the dyeing specially with direct dyes [5].To assess the influence of electrolytes, sodium chloride was used at concentrations of 0.2 M and 0.4 M.

Effect of equilibrium time and stirring rate
In order to determine the equilibrium time, samples of dye solution (100 ppm) were deposited into 100-mL test tubes.The SB concentration was fixed at 650 ppm.The experiments were carried out in a thermostatic bath (30 • C-100 rpm stirring rate during 3 min and after 50 rpm for 2 min).Contact times assessed were 5, 15, 30, 45, 60, 90, and 120 min.
To evaluate the effect of stirring rate, samples were prepared under the same conditions to determine equilibrium time.The samples were stirred during 3 min at a rapid stirring rate (100, 200, 300, 400, and 500 rpm), then during 2 min in a slow stirring rate, always being half that of the rapid one.

Effect of SB concentration and temperature on dye removal efficiency
Fig. 3 shows the dye removal efficiency for different SB concentrations and temperatures.The working range for the SB concentration remained between 220 and 650 ppm.When the SB concentration is below the aforementioned range, dye removal is negligible and at concentrations above that range removal is practically constant.The temperature range was set between 30 and 70 • C, in order to determine its effect on equilibrium.
Fig. 3 shows that dye removal efficiency increases as SB concentrations rises.At lower SB concentration (ca.300 ppm), removal efficiency reaches 70% at 30 • C. In this region, flocculation is low due to the low amount of SB, which is essential for floc formation.Nevertheless, when SB concentrations are higher, a large amount of flocs are formed, allowing more dye removal.At SB concentrations above 500 ppm dye removal reaches 90%, remaining practically constant thereafter.Another characteristic of the process is that, for a same SB concentration, a decline in dye removal efficiency is observed by increasing the temperature.This occurs because at high temperatures, the sodium hexadecanoate and sodium dode-  canoate, that are present in the surfactant used in this research, reduce its ability for dye removal, as shown in Figs. 4 and 5.
Fig. 5 shows that the removal efficiency of sodium dodecanoate is greater than that observed for SB when the same surfactant concentration is used.As explained, SB is a blend of surfactants with different alkyl chain lengths, resulting in an average dye removal efficiency that is based on the contribution of each one of the surfactants.
Eq. ( 2) can be used to evaluate the process from a thermodynamic point of view [34]: where m is the adsorbent concentration (g/L), q e is the adsorption capacity on equilibrium (mg/g), T is the temperature (K), H • is the enthalpy of adsorption, and S • is the entropy of adsorption.If a surfactant concentration (m) equal to 0.33 g/L is used in Eq. ( 2), the obtained result for H • is −29 kJ/mol, showing an exothermic interaction between surfactant and dye.An increase in the temperature may cause the solubilization of the formed flocs.A fact observed during the experiments is that at 30 • C the flocs decant while above 40 • C remains in the medium as a supernatant, which may be related to a reduction in its density.Another important parameter in the flocculation process is the residual calcium levels, responsible for floc formation.The amount of calcium present in the floc is practically the same at all temperatures assessed, showing a constant Ca 2+ /SB ratio, i.e., 0.337 ± 0.007 mg/mg.For systems with calcium initial concentration up to 325 ppm, after extraction experiments, a residual concentration around 40 ppm Ca 2+ .Considering surfactant solution with an initial concentration up to 1300 ppm, its residual concentration is very low (≤13 ppm), with 98% total organic carbon (TOC) removal being observed after floc formation (calcium-surfactant interaction).Fig. 6 shows that dye removal efficiency is practically constant during the studied time, with a slight tendency to increase ( ∼ =2%).This increase in efficiency is too low to justify an increase in contact time, considering the operational conditions in a wastewater treatment unit, because in 5 min dye removal efficiency reaches 92%.It is expected that this instantaneous equilibrium occurs due to intermolecular interaction between the dye molecule and the nonpolar tail of the surfactant, before calcium addition.After adding calcium to the system, it reacts with the polar head group of the surfactant forming the floc, thereby dragging the dye along with it.Fig. 7 shows the effect of the stirring rate on dye removal efficiency, demonstrating that it does not interfere in process efficiency.It is observed experimentally that the floc size decreases when stirring rate increases, as shows Fig. 8.This phenomenon, in a wastewater treatment unit, hinders the filtration stage for floc removal.

Effect of pH
The pH was assessed between 7 and 13 since visual observations show that the medium becomes murky below pH 7 (Fig. 9), indicating the formation of an oil-in-water emulsion.This occurs because the surfactant returns to its respective fatty acids as a result of the lower pH, precluding the formation of flocs due to the absence of carboxyl anion, rendering the process unfeasible.
During the experiments it was found that, without adjusting the pH, the samples exhibit a pH between 9.1 and 9.3, which corresponds to the pH resulting from the reactions between fatty acids with strong bases, pK a of approximately 9. Figs. 10 and 11 show the effect of pH on dye removal efficiency in the presence and absence of SB, respectively.Fig. 10 shows that a pH between 9 and 12 produces the best results.For pH 11, with 650 ppm SB and 325 ppm calcium (SB concentration being always two times calcium concentration, as explained in experimental section), dye removal reaches 97.6%, an increase of 7% in relation to the experiment under the same conditions without adjusting pH (pH ∼ = 9.2).At pH 13, removal rates decrease due to the effect of the interaction between calcium and hydroxyl, which ends up competing with the surfactant anion, thereby inhibiting floc formation.
To determine whether the presence of calcium is sufficient to cause dye removal, samples were prepared without surfactant addition and dye removal efficiency was assessed.Fig. 11 shows that little dye was removed ( ∼ =10%).The presence of calcium in the environment is not sufficient to remove the dye.As explained, the dye removal efficiency is directly dependent of the interaction between dye molecule and the non-polar tail of the surfactant.

Effect of electrolyte
Fig. 12 illustrates dye removal efficiency as a function of SB and sodium chloride concentrations.One can observe that the presence of electrolyte lowers dye removal efficiency.These results demonstrate that for surfactant concentration of 650 ppm with 0.0 M, 0.2 M, and 0.4 M NaCl, the maximum dye removals are 90.6%,68.5%, and 60.2%, respectively.The addition of electrolytes reduces floc formation, since tolerance to precipitation (minimum calcium concentration required to cause precipitation) increases when electrolyte concentration rises [26,35].This is due to the reduced concentration of anionic surfactant monomers in the environment when NaCl is added [35].In addition, the surfactant precipitation occurs at the evaluated NaCl concentrations and the produced precipitate is not able to attract and remove the dye molecules, as occurs with the surfactant flocs.

Conclusion
In the present study, precipitation of SB was used to remove Direct Yellow 27 dye (DY27).The results show that there is a SB concentration limit for promoting dye removal.Equilibrium time and stirring speed has no significant influence on the process.The addition of electrolytes to the environment reduces precipitation, making the process less efficient.The suitable pH range for precipitation is between 10 and 11, whereas low pH values make the process unfeasible.These results demonstrate that the process proposed here is a new alternative for removing dye from textile wastewaters.

Fig. 2 .
Fig. 2. Photograph of the experimental setup used to perform dye removal experiments.

Figs. 6
Figs. 6 and 7 present dye removal efficiency as a function of equilibrium time and stirring rate, respectively.Fig.6shows that dye removal efficiency is practically constant during the studied time, with a slight tendency to increase ( ∼ =2%).This increase in efficiency is too low to justify an increase in contact time, considering the operational conditions in a wastewater treatment unit, because in 5 min dye removal efficiency reaches 92%.

Table 1
Mean composition (wt.%) in fatty acids in the animal fat and coconut oil.