An efficient cloud point extraction method for the ... - NOPR - niscair

Indian Journal of Chemistry Vol. 54A, May 2015, pp. 627-632

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An efficient cloud point extraction method for the separation of congo red using Triton X-100 in the presence additives Nilesh Dharaiya*, Arpan Parmar & Pratap Bahadur Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, India Email: [email protected]/ [email protected] (PB) Received 3 December 2014; revised and accepted 29 April 2015 A new approach to improve the cloud point extraction of congo red (a hazardous anionic azo dye) from aqueous solution using Triton X-100 surfactant is reported. The interaction of congo red with Triton X-100 during micellar solubilization has also been investigated by visible absorption spectral studies. The optimum conditions for surfactant concentration, additives (salts, glycols and their ethers) concentration, temperature and pH have been determined to achieve higher extraction. Among the various glycol ethers studied, ethylene glycol monohexyl ether shows the most efficient extraction of the dye even at ambient temperature, with almost complete removal of the dye from aqueous solution. Keywords: Cloud point extraction, Surfactants, Dyes, Azo dyes, Congo red, Triton X-100, Glycol ethers

Dyes are considered to be major water pollutants released into aquatic bodies mainly from dye manufacturing and textile finishing, food coloring, cosmetics, leather, paper and carpet industries1-3. Synthetic dyes have complex aromatic molecular structure that offers thermal stability and optical characteristcs3,4. The presence of dyes in water manifests a colour even at a very low concentration (< 1 ppm). Several dyes possess toxic, mutagenic and carcinogenic properties which are significantly hazardous to aquatic biota and human life1-3. For these reasons, the removal of dyes from process or waste effluent becomes environmentally important. Various techniques, viz., adsorption3,5, membrane-wet oxidation6, reverse osmosis7, nanofiltration8, micellar enhanced 9 ultrafiltration , liquid-liquid extraction10, solid phase extraction11 and cloud point extraction (CPE)12-17 have been used for the removal, separation and extraction or preconcentration of dyes from waste water. Congo red (CR) a benzidine-based anionic diazo dye is difficult to biodegrade due to its structural stability. It has the tendency to metabolize into benzidine, a human carcinogen18,19. CR has also been

recognized as mutagenic and carcinogenic18-20. CR containing effluents are generated from textiles, printing and dyeing, paper, rubber, plastics industries, etc. Adsorption methods have been widely used for removal of CR by using different absorbents, viz., activated carbon20, 21, red mud22, 23 and waste materials18, 24. Purkait et al.12 have examined cloud point extraction (CPE) of CR using TX-100. The present study was carried out to enhance the efficiency for the CPE method for removal of CR. The CPE is an environment-friendly method which has received attention because the surfactants used are biodegradable and less toxic than the organic solvents25,26. Moreover, this green technique does not require any sophisticated instrument or high energy, and offers higher extraction efficiency at low cost. CPE has been widely used for separation and preconcentration of trace analytes from different matrices26-29. CPE is based on the micellar solution behavior of the polyoxyethylene based non-ionic surfactants, where the micellar solution of non-ionic surfactant undergoes phase separation on heating above a cloud point temperature. One of the two phases is a surfactant-rich phase (coacervate phase) while the other is aqueous phase containing smaller amount of surfactant. The micellar solubilization properties of surfactants allow the extraction of hydrophobic, amphiphilic and even ionic solutes into a small volume of coacervate phase after increasing the temperature above CP26-29. The CPE method was initially described by Watanabe and co-workers for the preconcentration of Zn(II)30. Another application of CPE focuses on the isolation and purification of species of biological interest, mainly proteins31. CPE has also been used for the extraction of organic compounds like phenols32, 33, polychlorinated compounds27, 34 and polyaromatic hydrocarbons27, 34. In the recent past, CPE has also been successfully employed for the extraction/ removal of dyes12-17. As CPE is temperature dependent process, its thermodynamic study was investigated for different dyes35. The aim of the present work is to improve the CPE method for removal of CR by using TX-100 in the presence of various additives and under different conditions. The effects of surfactant concentrations,

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temperature, pH and salts on the extraction of CR were investigated. Since the glycol ethers are mostly biodegradable, eco-friendly and exhibit low toxicity36-38, their effects as additives were determined in the current study. A previous study on the change in clouding and micellar behaviour of TX-100 in the presence of various glycol ethers as additives shows that their effect on the micellar behaviour depends on the solubilization sites in the micelles39. As seen in earlier work, the addition of ethylene glycol monohexyl ether (EGMHE) significantly decreases the CP of TX-100, and allows the CPE procedure at a lower temperature39. Experimental A UV-vis spectrophotometer (Thermo Scientific™ Evolution 300) was used for recording absorption spectra and absorbance measurements (with 1 cm glass cells). Metrohm digital pH-meter (model 632) with a combined glass electrode was used for pH adjustments. A thermostat bath (Remi RSB-12) maintained at the desired temperature (±0.1°C) was used for the cloud point extraction method. Triton X-100 (p-tert-octylphenoxy polyethylene (9.5) ether) was purchased from Sigma Aldrich (USA) and used as received. Congo red (CR) dye was a gift sample by Atul Pvt. Ltd., India. Ethylene glycol (EG), ethylene glycol monomethyl ether (EGMME), ethylene glycol monobutyl ether (EGMBE), ethylene glycol monohexyl ether (EGMHE), propylene glycol (PG), Propylene glycol monomethyl ether (PGMME) and Propylene glycol monobutyl ether (PGMBE) were supplied by Dow Chemical Co. and Sigma Aldrich (USA) and used as received. NaCl, Na2SO4 and Na3PO4 were procured from Merck (India). Millipore water was used for preparing solutions for cloud point extraction (CPE). In a typical CPE experiment, 10 mL sample solutions were prepared in measuring cylinders by dissolving accurately weighed amounts of TX-100, CR and additives. The concentration of CR was kept 550 ppm in all measurements of CPE. The pH of the solutions was adjusted by using standard NaOH or HCl solutions. The measuring cylinders were kept in a thermostated water bath (Remi RSB-12) for 20 min. at the required temperature. After complete phase separation, the measuring cylinders were removed from the temperature bath and cooled for 10 min. Then, the concentration of CR in aqueous phase was determined from calibration plots of CR. The

calibration curve of CR was recorded at its maximum wavelength 498 nm on the UV-vis spectrophotometer and showed linear regression coefficients higher than 0.995. The absorption spectra of CR in TX-100 solution were also recorded on the spectrophotometer. The efficiency (in percentage) of cloud point extraction was determined by using the following expression.

Results and discussion In the present study, TX-100 surfactant was used to extract the congo red dye. TX-100 (CP = 65 °C)39,40 is highly surface active, and the surface tension-log concentration plot shows its critical micelle concentration41, 42 (CMC) ~0.2 mM at 25 °C and area/molocule41 = 64 Å2. The previous dynamic light scattering results show its apparent hydrodynamic micelle radius to be ~5.5 nm, which increases gradually with temperature in the micelle growth seen at the temperature close to CP 39, 40. The earlier SANS results illustrate that micelles are ellipsoidal at 30 °C (Semimajor axis = 6.35 nm and semiminor axis = 1.98 nm)40. For successful CPE, it is desirable to use the minimum amount of surfactant for maximum extraction of dye. Therefore, the effect of the TX-100 at different concentrations was examined in order to ensure maximum extraction efficiency of the target analyte CR (Fig.S1). Purkait et al.12 have examined the effect of CR dye concentration on the extraction efficiency at a fixed concentration of TX-100. To show the improvement in the extraction of CR (using additives) as compared to this previous study a higher concentration of CR (550 ppm) was used. At fixed temperature and feed CR concentration, the extraction efficiency of CR increases sharply when the TX-100 concentration increases from 25–100 mM. Beyond 100 mM, increase in extraction efficiency becomes gradual for CR. Almost complete extraction of CR was achieved at 200 mM concentration of TX-100. This concentration (200 mM) is optimum to achieve extraction of CR up to 99%. The concentration of surfactant in the coacervate phase resides nearly constant at invariable temperature26. Increased concentration of TX-100 usually increases the volume of coacervate phase to maintain material balance12. Consequently, the extraction of CR into the coacervate phase enhances with TX-100 concentration.

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The extraction of a dye depends on hydrophobic interaction between dye and surfactant micelles. The spectral change of the dye in surfactant solution reveals the dye-surfactant interaction43, 44. In the current study, the interaction between CR and TX-100 has been investigated from the visible absorption spectra. The spectra of CR (50 mM) as a function of TX-100 concentration (below and above its CMC) have been recorded (Fig. 1). The CMC of TX-100 is ~0.2 mM as reported in the literature41, 42. No detactable changes in the spectra of the CR were observed below the CMC of TX-100. However, the shift in visible absorption band of CR has been observed with increase in TX-100 concentration above the CMC. A considerable red shift (from 498 to 515 nm) in the λmax of CR was observed in the presence of 1 mM TX-100. The red shift in the λmax of spectra indicates the incorporation of dye into the micelles43, 44. Usually, a shift in λmax demonstrates the strength of interaction between dye and surfactant. Here, the considerable shift in λmax signifies the strong association of CR with the TX-100 micelles. This interaction shows the suitability of TX-100 surfactant for the cloud point extraction of CR. The temperature effect on the extraction efficiency of CR in the presence of 50 mM TX-100 was investigated (Fig. S2). The extraction of CR increased with increasing the temperature of the system. The CP of 50 mM TX-100 was determined to be 66 °C. Addition of the CR (550 ppm) in TX-100 solution leads to increase in the CP up to 68 °C. While, the extraction efficiency is very low at ~70 °C, (i. e., just above the CP temperature), the extraction efficiency is found to be high at temperatures higher (≥ 75 °C) than CP of the system. The aqueous solubility of nonionic surfactant mainly depends on the H-bond interaction between its polyoxyethylene chain (shell) and water. Water becomes a poorer solvent for the TX-100 at higher temperature because the extent of H-bonds decreases as the temperature increases. While as the temperature increases above the cloud point, the surfactant concentration is significantly enhanced in the coacervate phase, it decreases in the diluted phase45. In addition, the polarity of CR molecules reduces with increase in temperature12, thus enhancing the extraction of CR in the coacervate phase was improved. Hence, temperature can be used as a supportive parameter to enhance the extraction efficiency of dyes.

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Fig. 1 − Absorption spectra of CR (50 ppm) in presence of TX-100 surfactant.

The effect of pH on the extraction of CR (550 ppm) using 50 mM TX-100 solution at 70 °C was studied (Fig. S3). The results indicate that the extraction of CR increases from alkaline to acidic condition of solution pH. In acidic pH, CR gets protonated (less than pKa = 3.5 value)46 and is in the tautomeric state. Therefore, the dye molecules tend to aggregates at higher acidic pH47, 48. As a result, CR becomes insoluble in aqueous solution at lower pH49. Consequently, CR was completely extracted in surfactant-rich phase at pH ~2. CR is a pH sensitive dye, and gives a blue colour below pH ~3.0 and red colour above pH~5.0. In the present study, the main attention was to remove CR even if it changes its colour in response to the pH. Generally, in the case of pH-sensitive dyes, it is necessary to adopt an appropriate process where change in colour of the dye with pH does not affect the analysis of dye extraction. Therefore, in the current study, the colour of aliquot was maintained as similar to original CR solution by neutralizing the pH of the solution. Purkait et al.50 noticed higher extraction of chrysoidine dye at basic pH using TX-100 and TX-114 surfactants. Chrysoidine remains as uncharged and more hydrophobic at higher pH, thus its extraction is more feasible in coacervate phase. In the CPE process, it is desirable that the CP of surfactant solution is low. If the surfactant has high CP, it is likely that the CP is decreased by additives including inorganic salts and organic compounds51-58 during CPE. Such additives can modify the

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Fig. 2 − Effect of salts on the extraction efficiency of CR (550 ppm) in the presence of 50 mM TX-100 at 70 °C.

Fig. 3 − Effect of glycols and their ethers on extraction efficiency of CR (550 ppm) in the presence of 50 mM TX-100 at 85 °C.

surfactant-solvent interaction. Some additives behave as water-structure breakers and increase the CP, while several others are water-structure makers and decrease the CP51-58. TX-100 (50 mM) has high cloud point (66 °C) and therefore we have used some salts, glycols and glycol ethers which can alter the CP and thus the efficiency of CPE. Usually, anions of inorganic salts influence the clouding behaviour of nonionic surfactants more as compared to cations53,57,58. Purkait et al.50 have determined the effect of divalent cation Ca2+ and monovalent Na+ using CaCl2 and NaCl respectively on the CPE of the chrysoidine dye. They observed no significant effect of CaCl2 on the extraction efficiency of chrysoidine as compared to NaCl. Anion effect usually follows Hofmeister series, and the effect of anions of salts on CP of polyoxyethylene based non-ionic surfactants and protein denaturation has been extensively studied54,57,59, while cations do not shown substantial effect. Hence, in the present work we have focused on the effects of anions on the CPE of CR. Figure 2 shows the influence of different anions on the extraction efficiencies of CR. Such effects of NaCl, Na2SO4 and Na3PO4 on the extraction of CR have been investigated. It is observed that the extraction efficiency of CR improves with increasing salt concentration. Addition of these salts exhibits salting-out effect and causes a reduction in CP of nonionic surfactants, since they reduce the solvent property of water for surfactants and induce dehydration of micelles54. Hence, the aggregation number and hydrophobic environment of micelle are enhanced with salt concentration55. As a result, the

extraction of CR was improved as a function of salt concentration. The salting-out effect is more prominent for trivalent salt Na3PO4 as compared to divalent Na2SO4, and the divalent salt shows more competence over the monovalent NaCl54,57. Accordingly, the ability of salts to enhance the extraction of CR followed the sequence: Na3PO4 > Na2SO4 > NaCl. In the present study, we have also investigated the effect of varying concentration of different glycols and glycol ethers on the CPE of CR, in the presence of 50 mM TX-100 (Fig. 3). The short chain glycols and their ethers increase the CP of nonionic surfactants39, 56 and therefore CPE was carried out at higher temperature 85 °C. The study shows that ethylene glycol (EG) does not alter noticeably the extraction efficiency of CR, while the propylene glycol (PG) decreases the extraction efficiency with increase in its concentration. Further, ethylene glycol momomethyl ether (EGMME) and propylene glycol momomethyl ether (PGMME) also reduce the extraction efficiency of CR. EG has been identified as a water structure breaker and increases the extent of H-bonds between water and the polyoxyethylene shell of the nonionic micelle42, 56. This causes a reduction of the dielectric constant of water as well as increase of steric repulsion between solvated micelles. This type of system (glycol+water) is a better solvent for both parts (polar and nonpolar) of surfactant than water. However, PG, EGMME and PGMME are more efficient for increasing the solubility of surfactant in water and oppose micellization as compared to EG42, 56. Thus, a higher amount of EG is

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Fig. 4 − (a) Effect of EGMBE and PGMBE on the extraction efficiency of CR (550 ppm) in the presence of 50 mM TX-100 at 70 °C; (b) Effect of EGMHE on the extraction efficiency of CR (550 ppm) in the presence of 25 mM TX-100 at 70 °C; (c) Effect of EGMHE on the extraction efficiency of CR (550 ppm) in the presence of 50 mM TX-100 at different temperatures.

required to increase the CP of non-ionic surfactant than PG, PGMME and EGMME. Accordingly, EG cannot alter the extraction efficiency of CR in the studied concentration. Short chain (2 carbon chain) glycols and glycol ethers do not get incorporated into the TX-100 micelles to a large extent and behave as cosolvent in TX-100 solution39. Here PG, PGMME and EGMME also behave as cosolvent and decrease the dye solubilization in TX-100 micelle. As a result, the extraction of CR decreases with increasing concentration of the cosolvents. Figure 4(a) illustrates the improved extraction of CR in the presence of ethylene glycol monobutyl ether (EGMBE) and propylene glycol monobutyl ether (PGMBE) at 70 °C. These ethers are miscible with water to some extent and not only behave as cosolvents but also insert between the monomer of the TX-100 micelles39. This enhances the dehydration and hydrophobicity of micelles and reduces the CP of TX-100. It was observed that, the PGMBE offers better extraction of CR than EGMBE. The extraction efficiency of CR in the presence of ethylene glycol monohexyl ether (EGMHE) is shown in Fig. 4(b). Here, a lower concentration of TX-100 (25 mM) was used; still the extraction was superior with very small amount of EGMHE as compared to EGMBE and PGMBE. The CP of TX-100 is significantly reduced with only a small concentration of EGMHE39, and CPE could be achieved at ambient temperature. The maximum extraction of CR at varying concentration of EGMHE and the required lower temperature is illustrated in Fig. 4(c). Results show

that only 2% EGMHE was required to achieve almost complete extraction (~98 %) of CR at 31 °C. It has been reported that hexyl alcohol and its glycol ethers are preferentially solubilized deep in core39. Thus, EGMHE penetrates deep into the micelle and leading to noticeable dehydration. This results in the formation of elongated micelles with increased hydrophobic environment39. Consequently, the dye solubilization is increased in these micelles, resulting in higher extraction of CR. EGMHE provides a wide range of temperature for extraction of CR. The proposed CPE procedure is extremely simple and no special equipment is required for sample treatment. Spectral changes of interaction between CR and TX-100 show the suitability of TX-100 for CPE of CR. At a fixed temperature and feed CR concentration, the extraction efficiency of CR increases with increasing TX-100 concentration. As the temperature increases above the CP, the extraction efficiency of CR increases. Higher extraction is observed for CR at acidic pH. Addition of salts shows their dissimilar effects to improve the extraction of CR in the order: NaCl < Na2SO4 < Na3PO4. Short chain alkyl ethers of EG and PG are not effective. However, with increase in the chain length of glycol ethers, extraction efficiency of CR is enhanced significantly. While butyl ethers of EG and PG are helpful in the cloud point extraction, hexyl ether of EG offers almost complete removal of CR at ambient temperature. This is an alternative way to use the non-ionic surfactants, which have higher CP. Results demonstrate the potential and flexibility of CPE method for removal of dyes from aqueous system.


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Supplementary data Supplementary data associated with this article, i.e., Figs S1–S3, are available in the electronic form at http://www.niscair.res.in/jinfo/ijca/IJCA_54A(05)627 -632_SupplData.pdf. Acknowledgement ND gratefully acknowledges University Grants Commission, New Delhi, India, for providing financial assistance in the form of RGNF fellowship (No. F.16-1860(SC)/2010(SA-III)). References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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