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Dye and Solvent Recovery in Solvent Extraction Using Reverse Micelles for the Removal of Ionic Dyes P. Pandit and S. Basu* Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India
Among the several methods, dye removal from water by solvent extraction using reverse micelles is promising in terms of the simplicity and efficiency of the process. The recovery of solvent and reuse of dye after its removal is related to the economic viability of the process. In the present study, backward extraction or recovery of dye into an aqueous phase from the solvent phase by using a counterionic surfactant after forward extraction is experimentally studied using a simple mixer and settler arrangement. The backward extraction of dye also resulted in the recovery of solvent. The effects of different parameters, such as the surfactant concentration, pH, and KCl concentration on the recovery of methyl orange and methylene blue dyes, are investigated. The percentage of both cationic and anionic dyes recovered increases with an increase in the counterionic surfactant concentration or with a decrease in the KCl concentration. The percentage of recovery of anionic methyl orange increases with a decrease in pH, whereas that for cationic methylene blue increases with an increase in pH. Selective removal of dye from a cationic and anionic dye mixture was experimentally studied by treating the dye mixture with reverse micelles of anionic and cationic surfactants in a stepwise manner. The results obtained are explained based on electrostatic interaction between the surfactant and dye. Introduction Almost every industry, but mainly the textile industry, uses dyes to color its products. Effluent water from carpet manufacturing, dyeing, leather, distilleries, textile, pulp, and paper industries contains various types of dyes that should be removed before discharging the effluent to the environment to avoid health hazards and destruction of the ecosystem. Effluent containing dye is responsible for water-borne diseases exhibiting symptoms such as hemorrhage, nausea, dermatitis, ulceration of the skin and mucous membranes, kidney damage, and a loss of bone marrow leading to anemia.1,2 Most dyestuffs are designed to be resistant to environmental conditions such as light, pH, and microbial attack.3 Hence, their presence in wastewater is unwarranted, and it is desirable to remove dyes from effluents before their discharge to the environment. This is important to regions where water resources might be scarce or sensitive to the maintenance of the ecosystem. There are four major technologies available to remove dyes from water, i.e., oxidation, adsorption, flocculation-precipitation, and membrane technology. Among the oxidation methods, UV/ozone or UV/H2O2 is one of the best technologies for the total removal of dye from wastewater.4,5 These methods are only effective for low concentrations of organic matter present in water. The separation of dyes based on adsorption on peat, wood,6 silica,7 bagasse pith,8 activated carbon and slag,2 and bagasse fly ash9 has been proposed and the adsorption kinetics studied in detail. These adsorption methods are capable of removing the dyes from concentrated wastewater. However, regeneration of most of the adsorbents is difficult except for activated carbon. The adsorption treatment using activated carbon as the adsorbent is quite expensive.5,6 On the other hand, in the floccula* To whom correspondence should be addressed. Tel.: +91 11 26591035. Fax: +91 11 26581120. E-mail: sbasu@ chemical.iitd.ernet.in.
tion-precipitation process, dye forms a complex10 with the flocculant, and thus the reuse of dye is not possible. The removal of color from wastewater can also be carried out by a flotation technique.11-13 However, the flotation technique is not as efficient as the present method described below. Pandit and Basu14-16 investigated the application of reverse micelles in removing ionic dyes from water by solvent extraction using reverse micelles. The purpose of the present study is to show that the removed dyes and used solvent can be recovered through backward extraction and they may be reused. This is important from the viewpoint of economics of the dye removal process by reverse micelles. Further selective removal of dye from a dye mixture is studied. It should be noted that the effluent from the textile industry would contain a mixture of dyes. Experimental Section Material. The surfactants used to prepare reverse micelles were sodium dodecylbenzenesulfonate (SDBS) and hexadecyltrimethylammonium bromide (HTAB). SDBS (Beijing Chemical Works) is an anionic surfactant, whereas HTAB (Lancaster) is a cationic surfactant. The physical properties of the surfactants are given in detail by Pandit and Basu.15 The anionic dyes, methyl orange (MO) and eosin yellow, were obtained from E. Merck. The cationic dye, methylene blue (MB), was obtained from Qualigens. The physical properties of the dyes are given by Pandit and Basu.15 The solvent (analytical grade) used for the removal of the dyes from water was amyl alcohol. Freshly prepared distilled water was used in all of the experiments. Analytical-grade HCl or NaOH was added for the variation of pH. Analytical-grade KCl (E. Merck) was used to study the effect of salt. Experimental Setup. A schematic view of the experimental setup is shown in Figure 1. A simple
10.1021/ie0402160 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/02/2004
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Figure 2. Schematic diagram for the treatment of a mixture of dyes by the reverse micelles method.
Figure 1. Schematic diagram of the experimental setup for the removal of dye from the aqueous phase, backward extraction of dye, and solvent recovery.
stirrer connected to a 1/12 hp Remi motor with an adjustable rpm was used for the mixing of solvent and aqueous phases containing dye in a beaker. A threebladed axial-type impeller was used. The beaker was connected to a graduated cylinder for the collection of solvent/water dispersion and the separation of solvent and aqueous phases by gravity. An output from the top of the separation vessel is connected to similar mixer and gravity settler, as described above for the backward extraction of dye and recovery of solvent. An UV spectrophotometer (Shimadzu, UV 1201) was used to measure the removal of dye from an aqueous phase. Method. The experiments were conducted in two steps. The first step is the dye removal step, where dye was removed from water by solvent extraction using reverse micelles. The second step is the dye and solvent recovery step, where dye is backward extracted to water by adding counterionic surfactants. (a) Dye Removal. The removal of organic dyes by liquid-liquid extraction using reverse micelles was conducted in a simple stirred vessel as shown in Figure 1. A given volume of the aqueous solution with known anionic and cationic dye concentrations was added to a known volume of the solvent containing a known quantity of the cationic or anionic surfactant. The surfactant is chosen such that it forms reverse micelles in the solvent phase and its charge is opposite to that of dye. The aqueous and solvent phases were mixed thoroughly using a stirrer at a fixed rpm for 5 min. The two-phase dispersion was transferred to a graduated cylinder to separate the solvent and aqueous phases by gravity. This resulted in the formation of two clear liquid phases: the solvent phase containing the dye encapsulated in the reverse micelles and the clear aqueous phase. The samples were collected from the aqueous phase and analyzed in an UV spectrophotometer (Shimadzu, UV 1201) to determine the amount of dye separated. The details of the experimental methods were given by Pandit and Basu.15,16 (b) Recovery of Dye and Solvent. The experiments on backward transfer were conducted using an experimental setup similar to that of the forward transfer (Figure 1). A known volume of the solvent phase containing the extracted dye was added to a given volume of the fresh aqueous phase containing a counterionic surfactant. The mixture was dispersed thor-
oughly using a stirrer. The phases were separated by gravity and resulted in the formation of two clear phases: an aqueous phase containing dye and a clear solvent phase. The aqueous phase was analyzed using the UV spectrophotometer to quantify the dye that was backward transferred. The recovered solvent containing surfactant was tested for forward extraction (i.e., removal of dye from water). (c) Removal of a Dye Mixture. The separation of a mixture of anionic and cationic dyes was conducted in a stepwise manner. A known volume of the aqueous phase containing the mixture of dyes was treated with a known volume of the solvent phase containing cationic surfactants. The aqueous phase was then treated with a known volume of the solvent phase containing anionic surfactants. The schematic diagram for the process is shown in Figure 2. Because a mixture of dyes is difficult to analyze using an UV spectrophotometer, chemical oxygen demand (COD) was determined for the whole mixture of dyes in each step. The open reflux method was used for the COD analysis.16 Results and Discussion Dye and Solvent Recovery. (a) Effect of the Surfactant Concentration. Dye and solvent recovery is related to the economics of the dye removal process by solvent extraction using reverse micelles. Dye and solvent recovery experiments were carried out for two different dyes (e.g., cationic MB and anionic MO) separately. At first, MB or MO dye is totally forward transferred into amyl alcohol from the aqueous phase by the usual procedure using reverse micelles of SDBS or HTAB. After the forward transfer, the counterionic surfactant to remove MB, HTAB, or that for MO, SDBS, was used to backward transfer dye into the aqueous phase. The backward transfer of the dye into the aqueous phase also results in the recovery of the solvent. The solvent may be reused without further addition of the surfactant for forward extraction. However, a makeup quantity of surfactant may be required because of various losses. The dye backward extracted into the aqueous phase would contain the counterionic surfactants. The effect of the presence of a counterionic surfactant on the reuse of dye is not known and needs to be investigated. It is well-known that surfactants are used in the textile industry to enhance dye solubility through solubilization. In Figure 3, the percentage of recovery of both cationic MB and anionic MO in the aqueous phase from amyl alcohol using two respective counterionic surfactants, HTAB and SDBS, is shown. In both cases, for two different dye concentrations, the percentage of recovery of dye in the aqueous phase increases with an increase in the counterionic surfactant
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Figure 3. Effect of the counterionic surfactant concentration on the recovery of MO or MB from 50 mL of amyl alcohol into 100 mL of the aqueous phase. Initially, MB or MO in 100 mL of the aqueous phase was removed using reverse micelles of 40 mg of SDBS or 30 mg of HTAB in 50 mL of amyl alcohol.
Figure 4. Effect of pH on the recovery of MO or MB from the solvent phase using 25 mg of SDBS or 30 mg of HTAB. Initially, MO or MB in 100 mL of the aqueous phase was removed using reverse micelles of 30 mg of HTAB or 40 mg of SDBS in 50 mL of amyl alcohol.
concentration. This substantiates the fact that the backward transfer of dye is achieved by breaking of reverse micelles in the presence of counterionic surfactants. The electrostatic interaction between the oppositely charged SDBS and HTAB surfactant molecules may lead to breaking of reverse micelles and backward transfer of dye to the aqueous phase. The analyses of experimental data for backward extraction may be carried out based on the same model as that for forward extraction.15 (b) Effect of pH. The results pertaining to the effect of pH on the percentage of recovery of the dye from amyl alcohol into a fresh aqueous phase are shown in Figure 4. Figure 4 shows that the percentage of recovery of MO decreases with an increase in pH, whereas that for MB increases with an increase in pH. The pH effect on backward transfer of MB and MO by breaking reverse micelles is just the opposite to that obtained for forward transfer using reverse micelles.14 The backward transfer takes place through the breaking of reverse micelles and subsequent transfer of dye into the aqueous phase,
Figure 5. Effect of KCl on the recovery of MO or MB from 50 mL of amyl alcohol to 100 mL of the aqueous phase using 45 mg of SDBS or 30 mg of HTAB. Initially, MO or MB in 100 mL of the aqueous phase was removed using reverse micelles of 30 mg of HTAB or 40 mg of SDBS in 50 mL of amyl alcohol.
Figure 6. Effect of the dye concentration on COD removal for a mixture of MB and eosin yellow (1:1) dyes using 35 mg of SDBS and 25 mg of HTAB separately in 50 mL of amyl alcohol in two steps.
whereas the forward transfer takes place through the forming of reverse micelles and subsequent solubilization of dyes. Note that forward and backward transfer mechanisms are opposite in nature and the same explanation as that given by Pandit and Basu14 (see the Supporting Information) may be applied to explain the above results. (c) Effect of Salt. The decrease in MB and MO recovery with an increase in the KCl concentration is shown in Figure 5. The decrease in MB and MO recovery is due to the screening of the charge of counterionic surfactants in the presence of KCl. Therefore, the extent of dye and solvent recovery can be controlled by proper choices of pH and KCl concentration. Treatment of a Mixture of Dyes. In reality, effluents from textile and dyeing industries would contain a mixture of dyes in the aqueous phase. The mixture of dyes is separated in a stepwise manner in two steps. At first, the anionic dyes are separated by treating the aqueous phase with the reverse micelles of cationic surfactants, and then the cationic dyes are separated
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surfactants to remove cationic dye and then with reverse micelles of cationic surfactants to remove anionic dye in a stepwise manner. Supporting Information Available: Explanation of the effect of pH on dye removal. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited
Figure 7. Effect of the surfactant concentration on COD removal for a mixture of 20 mg of MB and 20 mg of eosin yellow dyes in 100 mL of the aqueous phase, using SDBS and HTAB surfactants in two steps. The surfactants are added in the following proportions:
by treating the effluent of the first step with the reverse micelles of anionic surfactants (Figure 2). Figure 6 shows that the percentage of COD removal from the aqueous phase decreases with an increase of the dye mixture (eosin yellow and MB) concentration for given surfactant, HTAB and SDBS, concentrations. Further, it is seen in Figure 7 that the percentage of COD removal from the aqueous phase increases with an increase in the HTAB and SDBS concentrations for a given dye concentration. These results are similar to that obtained for a single dye experiment. Conclusions The recovery of solvent and dye is possible in a dye removal process using reverse micelles through back extraction using a counterionic surfactant. The backward transfer is governed by the concentration of the counterionic surfactant, pH, and KCl concentration in water. The percentage of backward transfer of both cationic and anionic dyes increases with an increase in the counterionic surfactant concentration. The percentage of backward transfer of anionic MO increases with a decrease in pH, whereas the percentage of backward transfer of cationic MB increases with an increase in pH. The percentage of backward transfer of both cationic MB and anionic MO decreases with an increase in the KCl concentration in the aqueous phase. The mixture of cationic and anionic dyes was selectively separated by treating it first with reverse micelles of anionic
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Received for review August 1, 2004 Revised manuscript received September 15, 2004 Accepted September 15, 2004 IE0402160
