Removal and recovery of CI reactive red 195 from effluent by solvent extraction using reverse micelles

Abstract

To reduce environmental pollution by CI reactive red 195, solvent extraction of the dye from water was performed by using reverse micelles prepared from hexadecyltrimethylammonium bromide and amyl alcohol. The effects of dye concentration, surfactant concentration, sodium chloride concentration, and pH on the removal percentage of the dye were investigated. A pseudo-first-order kinetic model and an ion-exchange reaction model were used to fit the experimental data. Ultraviolet-visible absorption spectra and particle size distribution were analyzed to evaluate the state of the dyes in the reverse micelles. In addition, the dyes were recovered by backward extraction and then reused for dyeing cotton fabric. The removal percentage of CI reactive red 195 increased with increasing concentrations of surfactant and sodium chloride. Increasing dye concentration and pH values resulted in less removal of dyes. The dye extraction process could be described by the pseudo-first-order kinetic model and by the ion-exchange reaction equations. The state of the dye in the reverse micelles was similar to that in bulk water. The mean particle diameter of the reverse micelles was 10.8 nm. Dye recovery was improved by adding a counterionic surfactant. The recovered CI reactive red 195 had a good dyeing property for cotton fabric.

Keywords

reactive dye extraction reverse micelles dyeing cotton

Dye-containing wastewater from the textile industry is a principal cause of environmental contamination. Over 15% of dyes in wastewater from dye synthesis and dyeing processes remains untreated.^1,2 Such dyes are largely non-biodegradable and toxic to aquatic plants and animals.³ Water-soluble azo dyes are generally the most difficult to remove from the dyeing effluent because of their high stability and the number of aromatic rings in their structure. To mitigate pollution, methods such as oxidation technology,⁴ adsorption,⁵ flocculation–precipitation,⁶ and membrane technology⁷ have been employed for removing dyes from wastewater. All of these methods have been compared and all have been found to have advantages and disadvantages.⁸

Reverse micelles are nanosized spherical aggregates formed by surfactant molecules in an apolar solvent. They are able to solubilize small amounts of water or aqueous solutions in their interior polar core, thereby offering a unique and versatile water pool.⁹ The tendency of many water-soluble solutes to partition into the aqueous inner core of reverse micelles in an organic phase has spawned much interest, as such systems may be used as continuous extractants for proteins,^10,11 amino acids,^12,13 and enzymes.¹⁴ In the last decade, there have been studies on the removal of methyl orange and methylene blue from wastewater by solvent extraction using reverse micelles (Figure 1).^15,16 In this system, dyes are extracted and separated from water. The recovered dyes and spent solvent may be recovered and recycled through backward extraction.¹⁷ This new technology for dye removal has competitive advantages because of its economics and its minimal environmental impact. Thus, it has captured the attention of investigators.^18,19

Figure 1.

Schematic drawing of extraction using reverse micelles.

In the present work, CI reactive red 195, which is used for dyeing cotton fabrics, was removed and recovered by solvent extraction using reverse micelles. The effects of dye concentration, surfactant concentration, sodium chloride (NaCl) concentration, and solution pH on the amount of dye removed were investigated. A pseudo-first-order kinetic model and a theoretical model based on the ion-exchange reaction between surfactant and dye were used to analyze the dye removal process. Ultraviolet-visible (UV-vis) absorption spectra and particle size distribution were analyzed to determine the state of the dyes in the reverse micelles. The dyes were then recovered by backward extraction in the presence of counterionic surfactants. Finally, the recovered dye was reused for dyeing cotton fabrics. Curves for color strength (in terms of K/S value, where K is the absorption coefficient and S is the scattering coefficient) of the dyed cotton fabrics were also examined.

Experimental details

Materials and reagents

Commercially scoured, bleached, and mercerized cotton woven fabric was used in this study. Hexadecyltrimethylammonium bromide (HTAB) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) were of reagent grade. Amyl alcohol and all other chemicals were of analytical grade. A commercial sample of CI reactive red 195, whose molecular structure is presented in Figure 2, was supplied by Chongqing Qiuhong Chemical Company, China. Double-distilled and deionized water were used throughout the study.

Figure 2.

The chemical structure of CI reactive red 195.

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 counterionic surfactants. A schematic diagram of the removal and recovery process is shown in Figure 3.

Figure 3.

Schematic diagram of the removal and recovery process.

Extraction

HTAB was dissolved in 50 ml of amyl alcohol to a specified HTAB concentration. CI reactive red 195 aqueous solution (100 ml) was added to the organic surfactant solution. The aqueous phase and the solvent phase were mixed thoroughly at 100 rpm for 5 min by using a stirrer. The solvent and aqueous phases were subsequently allowed to separate for 10 min. Samples of the aqueous phase were collected and then analyzed on a UV-2401 spectrophotometer (Shimadzu Co., Japan) to determine the amount of dye removed. Each experiment was duplicated under identical conditions. The removal percentage of dyes was calculated as follows:

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).

Backward extraction

Solvent phase (50 ml) containing the extracted dye was added to 100 ml of fresh aqueous phase containing a counterionic surfactant. The mixture was mixed thoroughly with a stirrer at 100 rpm for 5 min. The phases were then allowed to separate. Finally, the dyes were extracted from the solvent phase to the aqueous phase.

Pseudo-first-order kinetic model

The removal capacity for the dye as a function of time was measured to determine the optimum reaction time for dye extraction. The relationship between the removal percentage and the reaction time (t) at different initial concentrations of CI reactive red 195 was analyzed. A pseudo-first-order kinetic model was employed to fit the experimental data. The rate constant is defined as the slope of the plot for the equation for the first-order reaction:²⁰

\ln (C_{D}^{0} / C_{D}) = kt + const

(2)

where k is the rate constant (1/min) and t is the removal time (min).

Ion-exchange reaction model

The ion-exchange reaction model was employed to fit the experimental data. The surfactant in the reverse micelles is assumed to be chemically active because of the strong electrostatic effect of the surfactant head groups. One molecule of the dye reacts with one molecule of the oppositely charged surfactant to form a complex. The model equation was transformed to a straight line equation:¹⁸

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

(3)

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

(4)

y = mx + c

(5)

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

(6)

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

(7)

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 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, K_C and K_D values were estimated.

UV-vis absorption spectroscopy

UV-vis absorption spectra of CI reactive red 195 in the reverse micelles and in the bulk water 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.

Particle size analysis

The mean particle size and size distribution of the reverse micelles were determined on a DelsaTM Nano C particle size analyzer (Beckman Coulter, USA).

Dyeing method

Dyeing experiments were carried out at 60℃ for 40 min (bath ratio, 1:50; CI reactive red 195 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.

Measurement of color strength

The K/S value of the dyed fabric was 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 K/S values were measured by using barium sulfate as 100% white reference standard. K/S values of the dyed fabrics were established with the aid of the Kubelka–Munk equation (Equation (8)):

K / S = (1 - R) 2 / 2 R

(8)

where R is the fraction of reflected light.

Results and discussion

Effect of dye concentration

The effect of CI reactive red 195 concentration on the removal percentage was investigated. A pseudo-first-order kinetic model was employed to fit the experimental data (results are shown in Figures 4 and 5 and in Table 1).

Figure 4.

Plots for the pseudo-first-order kinetic equation.

Figure 5.

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

Table 1.

Results from linear regression of the plots for the pseudo-first-order kinetic equation

Dye (mg)	Rate equation	k (1/min)	R
25	ln(C₀/C) = 0.5105t + 0.0903	0.5105	0.9961
50	ln(C₀/C) = 0.2724t + 0.1094	0.2724	0.9905
75	ln(C₀/C) = 0.1834t + 0.0722	0.1834	0.9918
100	ln(C₀/C) = 0.1267t + 0.0511	0.1267	0.9914

Figure 6 shows that the removal capacity for CI reactive red 195 increased with reaction time at all initial concentrations and that a higher initial concentration caused a decrease in removal percentage. The decrease is due to the constant number of reverse micelles for encapsulating the dye molecules at a given surfactant concentration. Dye removal was rapid at the beginning, proceeded toward a lower rate, and finally attained equilibrium. Figure 4 and Table 1 show that the dye removal could be described by using a pseudo-first-order kinetic equation. The regression coefficient R was >0.99, and k increased with increasing initial concentration of dye. These results indicate that the initial concentration of dye had a significant impact on the removal percentage and reaction time.

Figure 6.

Effect of dye concentration on dye removal. Conditions: 100 mL water, 30 mg hexadecyltrimethylammonium bromide, 50 mL amyl alcohol.

The ion-exchange reaction model was employed to determine the equilibrium constants (results are shown in Figure 5).

The R value was >0.99. K_C and K_D values for the dye-surfactant ion-exchange reactions, as determined from the slope and intercept of the straight line, were 8.10 and 2.06 mmol/L, respectively. These values are higher than that for similar ion-exchange reactions of proteins reported by Rabie and Vera.²¹ This difference may be due to the greater solubilization of dye compared with that of the protein molecules in the respective reverse micelles, which favors dye removal. Our results are similar to those for methylene blue removal from the aqueous phase in another study.¹⁴

Effect of surfactant concentration

To study the effect of surfactant concentration on dye removal, the removal percentage of CI reactive red 195 at different HTAB concentrations was measured (results are shown in Figure 7).

Figure 7.

Effect of hexadecyltrimethylammonium bromide (HTAB) concentration on dye removal. Conditions: 75 mg dye, 100 mL water, 50 mL amyl alcohol.

Figure 7 shows that the removal percentage increased from 72% to 99% with increasing HTAB concentration. The dyes were captured in the reverse micelles because of coulombic attraction between the anionic group of CI reactive red 195 (–SO₃⁻) and the cationic head group of HTAB (–N⁺R₃). There was greater coulombic attraction force. This might be due to the higher concentration of HTAB. On the other hand, the reverse micelles were formed with surfactant, water, and solvent. Usually, the number of reverse micelles increased with increasing concentration of surfactant in the presence of water and solvent.⁹ Then the maximum amount of solubilization of water was enhanced in reverse micelles. Finally, the removal percentage of dyes also increased. It is worth pointing out that the amount of surfactants used in this study was very small. The recovered solvent containing surfactant can be reused for forward extraction.¹⁷ In addition, the solvent and surfactant can be separated and collected by distillation. Thus, this method was fundamentally eco friendly to nature.⁸

Effect of salt concentration

Cotton fiber in contact with water produces a slightly negative charge due to ionization of hydroxyl groups; similarly, most of the reactive dye suitable for cotton is anionic in solution.²² The repulsive charge between dye and cotton fiber was overcome by adding an electrolyte such as sodium chloride, which screens the surface charge on the fiber.²³ In our study, the effect of NaCl concentration on dye removal was examined (results are shown in Figure 8).

Figure 8.

Effect of NaCl concentration on dye removal. Conditions: 75 mg dye, 100 mL water, 30 mg hexadecyltrimethylammonium bromide, 50 mL amyl alcohol.

Figure 8 illustrates that the removal percentage of dyes increased with increasing concentration of NaCl. This increase is due to the formation of dyes and HTAB complexes at the interface between amyl alcohol and water in the presence of NaCl. Thus, the dye concentration in the water phase decreased with increasing amount of NaCl. The results obtained were similar to those for methyl orange reported by Pandit and Basu.¹⁵

Effect of pH value

Fixation of reactive dye, the final step of reactive dyeing, is generally effected by covalent bonding between reactive dye and cotton fiber. This bonding results in good fastness toward washing and toward other wetting processes.⁹ During fixation, an appropriate alkaline substance is added to the dye bath to increase its pH to initiate the desired dye–fiber reaction. Thus, it is necessary to investigate the effect of pH on dye removal (results are shown in Figure 9).

Figure 9.

Effect of pH on dye removal. Conditions: 75 mg dye, 100 mL water, 30 mg hexadecyltrimethylammonium bromide, 50 mL amyl alcohol.

As shown in Figure 9, the removal percentage of CI reactive red 195 decreased with increasing pH. This decrease is due to the removal of dyes by extraction using reverse micelles via electrostatic interaction between the surfactant and dye. CI reactive red 195 usually hydrolyzes under alkaline conditions, such as those during the dyeing process, resulting in substitution of –OSO₃⁻ by –OH⁻ of some vinyl sulfone groups.²⁴ Thus, the force of electrostatic interaction between HTAB and dyes weakened under alkaline conditions, causing less dye removal:

\binom{D - {SO}_{2} C_{2} H_{4} {OSO}_{3} H OH -}{- - - - \to D - {SO}_{2} C_{2} H_{4} OH}

Characterization of reverse micelles

Absorption spectra of the dye in the reverse micelles

Generally, the state of the dye is affected by the polarity of the medium. There is a dynamic equilibrium between the monomolecular state and aggregate state in the medium when the dye is dissolved in the solvent. To examine the state of CI reactive red 195 in the reverse micelles, UV-vis absorption spectra of the dye in the reverse micelles and bulk water (Figure 10) were obtained.

Figure 10.

Ultraviolet-visible spectra of CI reactive red 195 in bulk water and in reverse micelles: (a) in reverse micelles; (b) in bulk water.

Deviations from Beer’s law with the appearance of H- or J-bands have been rationalized in terms of dye aggregation, that is, the formation of dimers, trimers, and n-mers, and the accompanying spectral changes were first interpreted by Forster with a classical oscillator model. Subsequent treatments explained these spectral manifestations of dye/dye interactions with the theory of energetically delocalized states, that is, excitons, which had been applied by Davydov to spectra of molecular crystals.²⁵ Thus, the environmental changes may cause the appearance of a new maximum at wavelengths.²⁶ Figure 10(a) exhibits a strong absorption peak at 524 nm and a weak absorption peak at 375 nm. Notably, the absorption spectra of the dye in the reverse micelles are very similar to that in bulk water (Figure 10(b)). These results suggest that no significant changes in the existing state of CI reactive red 195 occurred in either media. Thus, it is believe that the existing state of reactive red 195 in bulk water was similar to that in the microenvironment of reverse micelles. The reactive red 195 with four sulfonic groups dissolved in the both systems exists mainly in the monomer state for good solubility.

Particle size analysis

During removal, CI reactive red 195 was encapsulated by the HTAB reverse micelles in the solvent phase. To examine the particle size of HTAB reverse micelles, the solvent phase containing the dye removed from the water phase was collected and analyzed (results are shown in Figure 11).

Figure 11.

Particle size distribution of hexadecyltrimethylammonium bromide (HTAB) reverse micelles. Conditions: 75 mg dye, 100 mL water, 30 mg HTAB, 50 mL amyl alcohol.

Figure 11 shows the particle size distribution of the reverse micelles. The mean particle diameter of the reverse micelles was 10.8 nm, and most of the reverse micelles had particle sizes of 7–20 nm. These results are similar to those for methyl orange in reverse micelles reported by Mangat and Kaur.⁸ The size of the water pool was larger than that of CI reactive red 195 (2.79 nm, as calculated with ChemOffice software); thus, the dye was encapsulated by the reverse micelles.

Recovery of the dye

To recover the dye in the reverse micelles, backward extraction was carried out by adding a counterionic surfactant, AOT. The effect of AOT concentration on dye recovery was investigated (results are shown in Figure 12).

Figure 12.

Effect of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) concentration on dye removal. Conditions: 25 mg dye, 100 mL water, 30 mg hexadecyltrimethylammonium bromide, 50 mL amyl alcohol.

The removal percentage of dyes decreased with increasing surfactant concentration. This increase implies that the dyes could be extracted by the water phase by addition of the counterionic surfactant. Backward extraction of dyes was performed by breaking reverse micelles in the presence of AOT. The electrostatic interaction between the oppositely charged AOT and HTAB surfactant molecules might have led to breaking of the micelles. Thus, the dyes could be recovered and reused by solvent extraction using reverse micelles. This process is very important from the viewpoint of economics and reduction of environmental impact of the dye.

Dyeing property

CI reactive red 195 recovered by backward extraction was used for dyeing cotton fabrics, and K/S curves were constructed (results are shown in Figure 13).

Figure 13.

K/S curves of cotton fabric dyed with both systems.

Figure 13 reveals that the K/S curves of cotton fabric dyed with recovered dyes are similar to that of cotton fabric dyed with commercial dye. There was no difference in the maximum absorption wavelength (530 nm) of cotton fabrics dyed with either system. This implies that the recovered dyes could be reused for dyeing cotton fabrics and that the solvent used for dye removal could be recovered and reused.⁸ Thus, liquid/liquid extraction using reverse micelles was feasible. However, a comprehensive study on removal and reuse of mixed dyes in aqueous solution is needed.

Conclusions

Solvent extraction using reverse micelles prepared with HTAB and amyl alcohol was performed to remove CI reactive red 195 from the aqueous phase. The removal percentage of dyes increased with increasing concentrations of HTAB and salt. The dye concentration and pH values exhibited the reverse trend. The extraction process could be described by pseudo-first-order kinetics and by ion-exchange reaction equations. The state of the dyes in the reverse micelles was similar to that in the aqueous phase. The mean particle diameter of the reverse micelles was 10.8 nm. Dyes recovered by adding counterionic surfactant had a good dyeing property for cotton fabric. This technique would be more practical to apply to a concentrated dye bath effluent, rather than a full plant effluent, taking their relative wastewater flow rates into account and considering the cost of application.

Footnotes

Funding

This research was supported by the Fundamental Research Funds for the Central Universities (XDJK2013B025, XDJK2014C128, XDJK2013A021) and by the Doctoral Fund of the Southwest University (SWU113010).

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