Removal and separation of mixed ionic dyes by solvent extraction

Abstract

To reduce environmental pollution from dyeing wastewater, reverse micelles were prepared and used for the removal and separation of anionic Color Index (CI) Reactive Yellow 3 and cationic CI Basic Red 14 in mixed dye aqueous solution. The effect of the amount of dye and surfactant on removal rates of dyes was investigated. An ion-exchange reaction model was used to fit the experimental data. Ultraviolet-visible absorption spectra were employed to evaluate the state of dyes in mixed dye aqueous solution during the removal process. In addition, the removed dyes were recovered by back extraction and reused for dyeing fabrics. The results obtained indicated that the removal of dyes was increased with increasing the amount of surfactants, while the dye concentration exhibited the reverse trend. The dye extraction process could be described by the ion-exchange reaction equations. The mixed dyes could be removed and separated for the attractive force between dyes and the counter ionic surfactant. The mixed dyes were separated and recovered, which exhibited a good dyeing property.

Keywords

removal separation mixed dyes extraction

Textile dyeing is a significant consumer of water and producer of contaminated aqueous waste streams.¹ There are more than 100,000 commercially available dyes and more than 7 × 10⁵ metric tons of dyes are produced worldwide annually.^2,3 In a typical dyeing factory, about 280–300 liters of water are consumed on average for every kilogram of cloth processed. Over 15% of the textile dyes are lost in the wastewater stream during the dyeing operation, in general.^4,5 To mitigate pollution, some methods, such as oxidation technology,⁶ adsorption,⁷ flocculation–precipitation,⁸ and membrane technology, have been employed for removing dyes from wastewater.⁹ All of these methods have been compared and all have been found to have advantages and disadvantages.^10–12 Therefore, for the treatment of dye wastewater, it is very important to develop new technology that has a lower cost and produces less pollution.

In recent years, reverse micelles have been used for the removal and separation of dyes from wastewater by solvent extraction, which is of interest from the viewpoint of water-saving, lower cost, environmental friendliness, and work safety.^13,14 More importantly, the dyes from wastewater can be recovered and reused for dyeing fabrics. The solvent could be recovered and reused for solvent extraction by the distillation method.⁸ In our previous study, the anionic Color Index (CI) Reactive Red 195 was removed and recovered by counter ionic reverse micelles (shown in Figure 1), which exhibited a good dyeing property for cotton fabrics.¹⁵ However, in fact, it should be noted that the effluent from the textile industry would contain a mixture of dyes.¹⁶

Figure 1.

Schematic drawing of solvent extraction using reverse micelles.

In this work, mixed dye aqueous solution containing anionic CI Reactive Yellow 3 and cationic CI Basic Red 14 were removed and separated by solvent extraction using reverse micellar systems. The ion-exchange reaction model was used to fit the experimental data. In addition, the conductivity and ultraviolet-visible absorption spectra of the mixed dye aqueous solution were measured and compared during the removal process. Finally, the recovered dyes were reused for dyeing fabrics. The dyeing property was also evaluated.

Experimental details

Materials and reagents

Commercial acrylic fabrics and cotton woven fabric were used in this study. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and hexadecyltrimethylammonium bromide (HTAB) were of reagent grade. They were used as surfactants to form micelles. Amyl alcohol and all other chemicals were of analytical grade. The commercial samples of CI Reactive Yellow 3, CI Basic Red 14, AOT, and HTAB, whose molecular structures are presented in Figure 2, were supplied by Chongqing Qiuhong Chemical Company, China. Double-distilled and deionized water were used throughout the study.

Figure 2.

Chemical structures of ionic dyes and surfactants. AOT: sodium bis(2-ethylhexyl)sulfosuccinate; HTAB: hexadecyltrimethylammonium bromide.

Methods

Experiments were conducted in two steps. The first step is dye removal (extraction), in which dye is removed from water by solvent extraction using reverse micelles. The second step is recovery of dye and solvent (backward extraction), in which the dye is extracted back to water by adding counter ionic surfactants.

Extraction and backward extraction process

The AOT was dissolved in amyl alcohol to a specified concentration mixture. Then it was added to the mixed dye aqueous solution. The aqueous and solvent phases were mixed thoroughly at 100 r/min for 15 min using a stirrer. The solvent and aqueous phases were subsequently allowed to separate for 15 min by gravity. The CI Basic Red 14 was removed gradually from the water phase to the solvent phase. Then, the organic solvent containing the extracted dye was added to the fresh aqueous phase containing HTAB. The mixture was mixed thoroughly with a stirrer at 100 r/min for 15 min and was then allowed to separate. Finally, the dyes were recovered from the solvent phase to the new water phase. The CI Reactive Yellow 3 in aqueous solution was removed and recovered by a similar extraction and backward extraction process. A schematic diagram of the removal and recovery process is shown in Figure 3.

Figure 3.

Schematic diagram of extraction and backward extraction process for the mixed ionic dyes.

Measurement of removal

During the removal process, samples of dyes were collected and then analyzed on a UV-2401 spectrophotometer (Shimadzu Co., Japan) to determine the amount of dye removed.¹⁵ Each experiment was duplicated under identical conditions. The removal percentage of dyes was calculated as in equation (1)

Removal % = \frac{C_{D}^{0} - C_{D}}{C_{D}^{0}} \times 100 %

(1)

where

C_{D}^{0}

and C_D are the dye concentrations in the aqueous phase before and after removal, respectively (g/L).

Ion-exchange reaction model

The ion-exchange reaction model was used to fit the experimental results.¹⁷ The surfactant in the reverse micelles is assumed to be chemically active for the strong electrostatic effect of the surfactant head groups. The reaction between the dye molecule and the oppositely charged surfactant is carried out to form a complex. The model equation was transformed to a straight line equation

y = 1 / [C_{D} (r C_{S}^{0} - C_{D}^{0} + C_{D} + K_{D}^{s / w} C_{D}^{sol})]^{1 / 2}

(2)

x = (C_{D}^{0} - C_{D}) / [C_{D} (r C_{S}^{0} - C_{D}^{0} + C_{D} + K_{D}^{s / w} C_{D}^{sol})]

(3)

y = mx + c

(4)

m = (K_{C} K_{D})^{- 1 / 2}

(5)

c = - (K_{C} K_{D})^{1 / 2}

(6)

where r is the volume ratio of the organic phase to the aqueous phase;

C_{S}^{0}

is the initial surfactant concentration in the solvent phase (mmol/L); K_D^s^/ ^w is the concentration partition coefficient of the dye between the solvent and aqueous phases in the absence of surfactant; and C_D^sol is the dye concentration in the bulk aqueous phase after mixing with the solvent phase in the absence of surfactant (mmol/L). K_C is the equilibrium constant for the ion-exchange reaction between the dye and the surfactant (L/mmol) and K_D is the equilibrium constant for the dissociation reaction in the ion-exchange reaction model (mmol/L). By fitting the experimental data to a straight line and by determining the slope and intercept, the K_C and K_D values were estimated.

Ultraviolet-visible absorption spectroscopy

Ultraviolet-visible absorption spectra of different dye aqueous solutions during the removal process were recorded by using a UV-2401 Shimadzu spectrophotometer (Shimadzu Co., Japan). All spectral measurements were performed in duplicate at 25℃ at an accuracy of ±0.5℃, and the mean values were processed for data analysis.

Measurement of conductivity

During the extraction process, the conductivity values for dye aqueous solution with different concentrations of surfactants were determined using a DDSJ-308 A conductivity meter (Shanghai Jingmi Instrumental Co., China). All the measurements were performed at 25 ± 1℃.

Dyeing method

Cotton fabrics

Dyeing experiments were carried out at 60℃ for 40 min (bath ratio, 1:50; CI Reactive Yellow 3 concentration, 2% owf; NaCl concentration, 40 g/L). Fixation was subsequently conducted by adding 10 g/L Na₂CO₃ aqueous solution at 60℃ for 30 min. After the cotton fabric was dyed, it was immediately soaked at 95℃ for 10 min in a mixture of 2 g/L soap powder and 2 g/L sodium carbonate, washed thoroughly, and then dried.

Acrylic fabrics

Dyeing experiments were carried out at 85℃ for 40 min (bath ratio, 1:100; CI Basic Red 14 concentration, 2% owf; glacial acetic acid concentration, 2.5% owf; leveling agent 1227 concentration, 2% owf). Then the dyeing temperature was raised to 100℃ for 20 min. After this time, the dyebath temperature was reduced to 50℃ and the samples were removed from the dye pots, washed thoroughly, and then dried.

Measurement of color strength

The K/S values of the dyed fabrics were obtained on an SF-600 spectrophotometer (Datacolor International, USA) using illuminant D65 and 10° standard observer. The instrument was standardized with a white tile, and the K/S values were measured by using barium sulfate as the 100% white reference standard. The K/S values of the dyed fabrics were established with the aid of the Kubelka–Munk equation (7)

K / S = (1 - R)^{2} / 2 R

(7)

where R is the fraction of reflected light.

The measurement of color fastness

The color fastness to soaping and rubbing was examined in accordance with Textiles Test Specification for Color Fastness (GB/T3921.3-1997 and GB/T3920-1997).

Results and discussion

Single dye aqueous solution

Effect of surfactant and dye concentration

In this work, the removal of anionic CI Reactive Yellow 3 and cationic CI Basic Red 14 in single dye aqueous solution was carried out. The effect of the amount of dyes and surfactants on dye removal was also investigated and discussed. The ion-exchange reaction model was employed to determine the equilibrium constants. The results obtained are shown in Figure 4 –6 and Table 1.

Figure 4.

Effect of surfactant concentration on dye removal in different dye aqueous solutions. Conditions: 75 mg dye, 100 mL water, 50 mL amyl alcohol.

Table 1.

Equilibrium constant values of dyes in the ion-exchange reaction model

Dye	K_C (L/mmol)	K_D (mmol/L)	R
CI Basic Red 14	5.60	2.27	0.9989
CI Reactive Yellow 3	10.9	4.41	0.9990

It was seen from Figure 4 that the removal of dyes for CI Reactive Yellow 3 and CI Basic Red 14 was increased with increasing concentration of surfactants. This is attributed to the attraction force between dyes and counter ionic surfactants, which caused the removal of dyes from the water phase to reverse micelles in the solvent phase. The amount of reverse micelles was increased by adding surfactants. More dye molecules were attracted to the organic phase and were enwrapped in the reverse micelles. Thus, the dye concentration was decreased gradually in aqueous solution while the dye removal was improved. It is shown in Figure 5 that the removal of dyes was decreased with increased amount of dyes. When the number of reverse micelles formed is constant, the reverse micelles have a fixed capacity for encapsulating dye molecules for a given HTAB concentration.¹⁶ The amount of reverse micelles was not enough to enwrap the increased concentration of dyes. So, a lower removal rate of dyes was obtained.

Figure 5.

Effect of dye concentration on dye removal in different dye aqueous solutions. Conditions: 30 mg surfactant, 100 mL water, 50 mL amyl alcohol.

Figure 6.

Experimental data fitted with the ion-exchange reaction model for dye removal.

It should be noted that the removal rate of CI Reactive Yellow 3 was higher than that of CI Basic Red 14 at the same condition. It is well known that the dyes were captured in the reverse micelles because of coulombic attraction between the dyes and surfactants.¹⁵ It was seen from Figure 2 that CI Reactive Yellow 3 was removed by the coulombic attraction between its anionic group (–SO₃⁻) and the cationic head group of HTAB (–N⁺R₃) at 2:1 ionic ratio, while CI Basic Red 14 was removed by a similar extraction for the attraction forces between cationic CI Basic Red 14 (–N⁺R₃) and anionic AOT (–SO₃⁻) at 1:1 ionic ratio. Thus, it is easier for CI Reactive Yellow 3 to be removed from the water phase to the solvent phase.

It is seen from Figure 6 and Table 1 that the K_C and K_D values of CI Reactive Yellow 3 for the dye–surfactant ion-exchange reactions, as determined from the slope and intercept of the straight line, were 10.9 and 4.41 mmol/L, which were higher than that of CI Basic Red 14 (5.60 and 2.27 mmol/L), respectively. In addition, the correlation coefficient (R) was > 0.99. These values are higher than those for similar ion-exchange reactions of proteins reported by Rabie,¹⁸ which caused greater solubilization of dye. The results are also similar to those for removal process of CI Reactive Red 195 in our previous study.¹⁵

In order to observe the removal process of dyes, the counter ionic surfactant and amyl alcohol were added in the dye aqueous solution. The solvent and aqueous phases were mixed and subsequently allowed to separate. Photographs of dyes during the removal process were obtained and are shown in Figures 7(a)–(f) and 8(a)–(f).

Figure 7.

Photograph of CI Reactive Yellow 3 during the removal process.

It is seen from Figure 7 that the CI Reactive Yellow 3 was removed gradually from the water phase to the organic phase due to the attraction force between cationic HTAB and anionic dyes. The removal process was finished within 30 s. After removal, the water phase became colorless. Figure 8 shows that CI Basic Red 14 was removed from the water phase to the organic phase by adding amyl alcohol and anionic AOT. The removal time was measured to be about 60 s, which is longer than that of CI Reactive Yellow 3.

Figure 8.

Photograph of CI Basic Red 14 during the removal process.

Mixed dye aqueous solution

In this study, the mixed dye aqueous solutions of CI Reactive Yellow 3 and CI Basic Red 14 were removed and separated by adding counter ionic surfactants and amyl alcohol, as shown in Figure 3. The results are shown in Figure 9. In addition, ultraviolet-visible absorption spectra were employed to evaluate the state of dyes in the mixed dye aqueous solution during the removal process. The absorption curves of the mixed dye aqueous solution were measured and compared. Before each measurement, the samples were diluted by one 10th with water. The results are shown in Figure 10.

Figure 9.

Effect of hexadecyltrimethylammonium bromide concentration on dye removal in mixed dye aqueous solution. Conditions: 75 mg dye, 100 mL water, 50 mL amyl alcohol.

Figure 10.

Ultraviolet-visible absorption spectra of dyes. Mixed dye aqueous solution with different sodium bis(2-ethylhexyl)sulfosuccinate (AOT) concentrations. Conditions: 40 mg CI Basic Red 14, 40 mg CI Reactive Yellow 3, 50 mL amyl alcohol, 100 mL water. Concentration of CI Basic Red 14 and CI Reactive Yellow 3 in single dye aqueous solution: 0.20 g/L. Before each measurement, the samples were diluted by one 10th with water.

It is seen from Figure 9 that the removal of dyes in mixed aqueous solution was increased with increasing the amount of surfactants, which was close to that of dyes in the single dye aqueous solution. It was concluded that the interaction effect of dyes in the mixed dye aqueous solution has no effect on the removal of dyes. It is seen in Figure 10 that the absorption curves of CI Reactive Yellow 3 exhibited a strong absorption peak at 390 nm, while the absorption curve of CI Basic Red 14 exhibited a strong absorption peak at 520 nm. The absorption curves of mixed dye aqueous solution exhibited a strong absorption peak at 520 nm with a shoulder at 390 nm, which was characterized by the overlap of the principal peak assigned to CI Basic Red 14 and CI Reactive Yellow 3, respectively. As the amount of AOT increased, the absorption peak at 520 nm decreased significantly while the absorption peak at 390 nm was changed to be the maximum absorption peak. When the content of AOT was 80 mg, the absorption curve of the mixed dye aqueous solution was similar to that of CI Reactive Yellow 3 aqueous solution, which indicated that CI Basic Red 14 was removed from the water phase to the solvent phase by the attraction force between CI Basic Red 14 (–N⁺R₃) and AOT (–SO₃⁻).

During the removal process, the conductivity of mixed dye aqueous solution was measured and compared at different surfactant concentrations. The results are shown in Figure 11.

Figure 11.

Effect of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) on the conductivity of the mixed dye aqueous solution during the removal process. Conditions: 75 mg dye, 100 mL water, 50 mL amyl alcohol.

It is seen from Figure 11 that the conductivity values of the aqueous solution were decreased with increasing the amount of AOT from 1204 to 822 us/cm. This result was attributed to the ionic concentration in the aqueous solution being decreased for the removal of CI Reactive Yellow 3, which caused the lower conductivity. When the mass of AOT was 100 mg, the conductivity of mixed dye aqueous solution was close to that of CI Basic Red 14 aqueous solution (794 us/cm).

A photograph of mixed ionic dye during the removal and recovery process was obtained and is shown in Figure 12. The five samples are marked as (a)–(e) from left to right. They were as follows. (a) Mixed dye aqueous solution. (b) Dye separation. AOT and amyl alcohol were added in the mixed dye aqueous solution. The solvent and aqueous phases were stirred and subsequently allowed to separate. Then CI Basic Red 14 was removed from the aqueous solution to the organic solvent. (c) Recovery of CI Basic Red 14. CI Basic Red 14 was recovered by adding a fresh aqueous phase containing the same ionic surfactant (cationic HTAB). Then CI Basic Red 14 was backward extracted from the solvent phase to the aqueous phase. Finally, CI Basic Red 14 aqueous solution was removed with a pipette. (d) Removal of CI Reactive Yellow 3. CI Reactive Yellow 3 aqueous solution was dissolved in a mixture containing HTAB and amyl alcohol. After stirring and separation, CI Reactive Yellow 3 was removed to the organic solvent phase. (e) Recovery of CI Reactive Yellow 3. After extraction, the organic solvent containing CI Reactive Yellow 3 was added to AOT and a new water phase for backward extraction. CI Reactive Yellow 3 was recovered from the organic phase to the new aqueous phase. Finally, the CI Reactive Yellow 3 aqueous solution was removed with a pipette.

Figure 12.

Photograph of the mixed dye during the separation and recovery process.

The separated and recovered dyes were reused for dyeing fabrics. The K/S curves of fabrics dyed with recovered dyes and commercial dyes were measured and compared. The results obtained are shown in Figure 13.

Figure 13.

K/S curves of fabrics dyed with different dyes: (a) CI Basic Red 14; (b) CI Reactive Yellow 3.

It is seen from Figure 13 that the K/S curves of the acrylic fabrics and cotton fabrics dyed with the recovered dyes are similar to that of acrylic fabrics and cotton fabrics dyed with commercial dyes. There was no difference in the maximum absorption wavelength (acrylic fabrics at 510 nm and cotton fabrics at 420 nm) of the fabrics dyed with different dyeing systems. The wash fastness and rubbing fastness of dyed fabrics are listed in Table 2. The staining on samples dyed with different dyes revealed good fastness with a grade range between 4 and 5. This indicated that the mixed dyes could be separated by solvent extraction using reverse micellar systems, and the recovered dyes can be reused for dyeing fabrics. Thus, the solvent extraction method using reverse micelles for the removal and separation of mixed ionic dyes was feasible.

Table 2.

Fastness properties of fabrics dyed with different dyes

Dye	Soaping fastness		Rubbing fastness
Dye	Staining	Fading	Dry	Wet
CI Basic Red 14
Commercial	5	5	5	4–5
Recovered	5	5	5	4–5
CI Reactive Yellow 3
Commercial	4–5	5	4–5	4–5
Recovered	5	4–5	4–5	4–5

Conclusion

The removal and separation of mixed ionic dyes were performed by the solvent extraction process using reverse micelles. The removal of dyes in the mixed dye aqueous solution was similar to that in the single dye aqueous solution. The mixed dyes in aqueous solution can be separated and recovered by extraction and back extraction. The recovered dyes were reused for dyeing fabrics, which exhibited a good dyeing property.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant Number 21506173), the China Postdoctoral Science Foundation funded project (Grant Number 2015M582503), the Key Laboratory of Science & Technology of Eco-Textile (Donghua University), Ministry of Education, China, and the China Scholarship Fund (Grant Number 201508505167).

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