Environ. Sci. Technol. 2005, 39, 3110-3115
Cloud Point Extraction of Direct Yellow ELZBIETA TATARA,† K A T A R Z Y N A M A T E R N A , * ,† ACHIM SCHAADT,‡ HANS-JO ¨ RG BART,‡ AND JAN SZYMANOWSKI† Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland, and Kaiserslautern University, Institute of Thermal Process-Engineering, Kaiserslautern, Germany
The cloud point separation of Direct Yellow from micellar solution of various nonionic surfactants, containing a polyoxyethylene chain, was studied. The separation of dyes is an important environmental problem. Moreover, such separation can also be considered as a preliminary step for further studies of biochemical recovery. Some dyes can be considered as affinity ligands, which form complexes with biochemicals. The use of dye enabled the observation of the dynamics of surfactant-rich phase separation by means of a color video. It was found that the separation of phases was incomplete. The aqueous phase contained some amounts of surfactant globules with the dye. The surfactant-rich phase was usually more heterogenic than the aqueous phase. The recovery of Direct Yellow was very effective in the presence of electrolyte (NaCl). The distribution coefficients were high and equal to a few hundreds in the presence of sodium chloride. Under optimum conditions 98-99.9% of the dye could be removed in the one step.
Introduction One of the main problems associated with the treatment of textile wastewater is the removal of dyes. Most (60-70%) of the more than 10000 dyes applied in textile processing industries are azo compounds, i.e., molecules with one or more azo (NdN) bridges linking substituted aromatic structures (1). Azo dyes are also widely employed to color solvents, inks, paints, varnishes, paper, plastic, rubber, foods, drugs, and cosmetics. More than two thousand azo dyes are known and over half of the commercial dyestuffs are azo dyes. The manufacturing of azo dyes and their application produce wastewater contaminated with azo dyes, which are discharged through conventional wastewater treatment plants. Unfortunately, the activated sludge process does not decompose most azo dyes, causing a potentially serious disposal problem. Discharge of azo dyes is undesirable, not only for aesthetic reasons, but also because many azo dyes and their breakdown products are toxic to aquatic life (2) and mutagenic to human (3). Therefore, studying the destruction of azo dyes in wastewater treatment processes is of utmost importance. Consequently, it is necessary to * Corresponding author. Tel./fax:
[email protected]. † Poznan University of Technology. ‡ Kaiserslautern University. 3110
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(+48)616653649. E-mail:
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 9, 2005
explore alternative technologies, which can remove azo dyes from water. Solvent extraction is an effective and economical process, but it also shows some drawbacks. The worst is the use of diluents, which are considered as environmentally harmful. An alternative is the use of aqueous biphasic or bipseudophasic systems. Aqueous biphasic systems can be obtained by an addition of appropriate electrolytes to hydrophilic polymers, especially polyoxyethylene glycols (4, 5) or by an increase of temperature of solutions containing nonionic surfactants (6, 7). Surfactant enhanced ultrafiltration is another process, which enables the separation of micellar pseudophase from aqueous pseudophase by filtration through ultrafilters (8, 9). When solutions containing nonionic surfactants are heated above the cloud point, in a proper period of time, two phases are formed. The first one is a surfactant-rich phase containing a high concentration of surfactant; the second one is the aqueous phase containing a low concentration of surfactant. Any substance that binds to the micellar aggregates in the aqueous feed can be extracted from the original solution and concentrated in the small volume of surfactant-rich phase. Thus, the surfactant concentration must be above the critical micellar concentration of surfactant to ensure the formation of micelles (10). Several trends in cloud point with surfactant molecular structure are commonly known: cloud point increases with the relative polyoxyethylene chain content and decreases with increasing alkyl carbon chain length. It is worth noting that the cloud point of aqueous surfactant solutions can be strongly influenced by the presence of other materials. Cloud point can be decreased by the introduction of polar compounds, anions that are water-structure formers (hard bases, F-, OH-, SO42-, Cl-, and PO43-), and certain cations (NH4+, alkali metal ions except for Li+). Cloud point can be increased by addition of long chain nonpolar material, anions that are water-structure breakers (large, polarizable anions, soft bases, SCN-, I-), and certain cations (polyvalent cations, H+, Li+) (11-14). The cloud point technique is often used as a preconcentration method prior to instrumental analysis, e.g., HPLC. However, the technique can be also used to recover various organic pollutants and metal cations (15-17), the latter after complexation with hydrophilic reagents (18). In analytical applications oxyethylated alkylphenols and oxyethylated alcohols are usually used as surface-active agents. However, such surfactants may cause an additional environmental problem connected with their biodegradability and toxicity. This means that appropriate surfactants must be selected. The problem was broadly discussed in the papers of Sabate et al. (19), who used surfactants for groundwater remediation. Oxyethylated fatty acid methyl esters are new biodegradable surfactants (20), which hydrolyze easily in sewage systems to nontoxic and nonsurface active components. These surfactants show higher biodegradability and lower aquatic toxicity compared to oxyethylated alcohols. Generally, aquatic toxicity of oxyethylated fatty acid methyl esters is one order lower in comparison to oxyethylated alcohols (21, 22). The basic aim of the work was to investigate the cloud point separation of Direct Yellow from micellar solution of various nonionic surfactants containing a polyoxyethylene chain. The study of the dynamics of the surfactant-rich phase separation and the determination of dye removal were additional auxiliary aims. 10.1021/es049381x CCC: $30.25
2005 American Chemical Society Published on Web 03/25/2005
FIGURE 1. Experimental setup.
Materials and Methods Direct Yellow was used as a representative of azo dyes (Zachem, Bydgoszcz, Poland). It has the maximum absorption at 424 nm ( ) 15300 L‚mol-1‚cm-1 for concentration of dye equal to 2.95‚10-5 mol/L). The formula of Direct Yellow (DY) is as follows:
Oxyethylated nonylphenol with the average degree of oxyethylation equal to 8 - Rokafenol N8 (abbreviated further as RokN8), oxyethylated and oxypropylated nonylphenol with the average degrees of oxyethylation and oxypropylation equal to 8 and 7, respectively Rokafenol N8p7 (RokN8p7), (both from Chemical Plant “Rokita”, Poland), oxyethylated dodecanol with the average degree of oxyethylation equal to 8 (OD8), and oxyethylated methyl dodecanoate with the average degree of oxyethylation equal to 9 (OMD9), (both from the Institute of Heavy Organic Synthesis, KedzierzynKozle, Poland) were used as nonionic surfactants. Sodium and potassium chlorides, iodides and thiocyanates were used as electrolytes. Deionized water was used for the preparation of aqueous solutions. The average formulas of the surfactants are as follows:
RokN8:
C9H19C6H4(OCH2CH2)8OH
RokN8p7:
C9H19C6H4(OCH2CH2)8(OCH2CHCH3)7OH
OD8:
C12H25(OCH2CH2)8OH
OMD9:
C11H23CO(OCH2CH2)9OCH3
Cloud Point Determination. The cloud points were determined in a classical way by heating micellar solutions until they became turbid and then slowly decreasing the temperature of the solutions with intensive mixing until the instant of complete disappearance of turbidity. The temperatures at which the solutions became transparent were assumed to be the cloud points. The determination was repeated at least three times for each solution and the error of the average values of cloud point did not exceed 1 °C. Dynamics of Rich-Phase Separation. The dynamics of surfactant-rich phase separation was studied using the experimental setup (Figure 1), elaborated in the University of Kaiserslautern and used to study the coalescence extraction (23). Solutions were placed in a temperature controlled cuvette and heated above the cloud point. The experimental setup (Figure 1) included a temperature controlled cuvette with a stopper, volume equal to 0.7 mL,
and size equal to 48 × 12,5 × 4.5 mm. It also contained a microscope, which was connected with a video camera, TK-C1380, produced by JVC. Pictures were obtained and collected by a computer. The system included a thermostat, which allowed the heating of the solution and the thermoelements, which were used to control the temperature inside solution. Observations were carried out until the concentration of dye was low enough to be detected by camera. Obtained pictures were processed using Image C software, which enables the determination of the grayness in dimensionless units. The grayness was measured in the relative scale between the values of 0 and 256 attributed to the black and the white, respectively. The concentration of Direct Yellow in the aqueous phase after phase separation was determined from the calibration straight line relationship: G ) -0.374‚CDY + 163.8, where G denotes grayness and CDY is the concentration of the Direct Yellow up to 25 mg/L. Thus, the pictures could only be qualitatively interpreted in this region and the concentration of the dye in the separated aqueous phase determined. Separation of Direct Yellow. Separations were carried out into calibrated test-glasses, heated in laboratory oven Memmert 400, Germany, above their cloud points. One part of the samples were heated at temperature equal to 10, 20, or 30 °C higher than CP (overheating ∆CP ) t - CP ) 10, 20, or 30 °C) and held at constant temperature for the proper period of time (5 or 15 h). The second part of the samples was additionally centrifuged, using Centrifuge 5804, Eppendorf, Germany. This type of a procedure is often used in analytical works. In this case, samples were heated at 100 °C for 60 min and rotated for 10 min with the speed of 2500 rpm. After phase separation, the aqueous phase was analyzed for the dye content, using a Specol 1200 spectrophotometer, Jena, Germany. The measurements were made at 424 nm wavelength and the content of the dye was calculated from the straight line calibration curve. The pH of the aqueous samples changed from 6 to 8. Surfactant-Rich Phase Separation. Surfactant concentrations in the aqueous phase were estimated from surface tension measurements using a drop shape method, tensiometer Tracker, I. T. Concept, France. The samples were diluted with an appropriate volume of water and the linear part of the surface tension isotherm was used as a calibration curve. The content of water in the separated surfactant-rich phase was determined by the Karl Fischer titration, by using a Metrohm 702 SM Titrino, Switzerland.
Results and Discussion The cloud point (CP) of aqueous solutions containing the considered surfactants depends only slightly upon surfactant concentration changed from 0.2 to 5%. Direct Yellow increase the CP of surfactants containing the polyoxyethylene chain but this effect is weak compared to the reverse effect of NaCl addition. Electrolytes can change the CP in different ways (Figures 2-4). Salting-out electrolytes such as NaCl and KCl decrease the cloud point, even by 30 °C. Thus, the separation of the surfactant-rich phase can be obtained at lower temperatures, which is very important from practical point of view. The observed effect of salt can be separated into effects of ions. Cations such as Na+ and K+ decrease the CP due to dehydration of the polyoxyethylene chain. Chloride anion causes an increase of water molecules self-association decreasing; in this way, the hydration of polyoxyethylene chain and the solubility of surfactants in water. SCN- and I- anions decrease self-association of water molecules and destroy the structure of water. Thus, they enhance the VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of chlorides on cloud point of 2.5% surfactant aqueous solutions (0 9, OD8; b, RokN8; 4, 2, RokN8p7; ], [, OMD9; full and empty symbols correspond to 1 M NaCl and KCl, respectively), with the absence of dye.
FIGURE 3. Effect of thiocyanates on cloud point of 2.5% surfactant aqueous solutions (0 9, OD8; b, RokN8; 4, 2, RokN8p7; ], [, OMD9; full and empty symbols correspond to NaSCN and KSCN, respectively), with the absence of dye. hydration of the polyoxyethylene chain, causing an increase of the cloud point. As is seen in Figure 3, the effect of SCNanion is stronger than the effects of cations (Na+ and K+) when the concentration of salt is low (below 3.5 and 0.5 M for RokN8 and RokN8p7 in case of NaSCN, below 2.5, 2.0, and 0.5 M for OD8, RokN8, RokN8p7 in case of KSCN). The effect of cations becomes dominant at higher concentrations. The results also indicate a stronger effect of Na+ in comparison to K+, in an agreement with Gibbs free energy of ions hydration, -375 and -307 kJ/mol for Na+ and K+, respectively (24). The dominant effect of Na+ is strongly observed when NaI is used. As a result, although the effect of iodide is similar to SCN-, the CP strongly decreases with an increase of NaI concentration. The opposite effects of cation and anion are almost compensated by KI. The observed effects of sodium and potassium thiocyanates and iodides are interesting from the fundamental point of view. However, the effects of sodium and potassium chlorides are important because the cost of the eventual process depends on the amount of heat needed to obtain the separation of phases. Before heating, the solutions containing Direct Yellow are yellow and homogeneous without any structures notice3112
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FIGURE 4. Effect of iodides on cloud point of 2.5% surfactant aqueous solutions (0 9, OD8; b, RokN8; 4, 2, RokN8p7; ], [, OMD9; full and empty symbols correspond to NaI and KI, respectively), with the absence of dye. able under the microscope. When the solutions are heated in the cuvette (Figure 1), clouding is first observed near the heated surface area, due to the temperature gradient. Further increase in temperature causes the turbidity through the whole volume and then the separation of the surfactant-rich phase, which forms viscous globules (Figure 5a). Due to it, coalescence of the globules is slow and several hours are needed to obtain a “homogeneous” surfactant-rich phase. The separated aqueous phase also contains globules of a nonseparated surfactant-rich phase (Figure 5b). Separated aqueous phase still contains some amounts of dye, probably solubilized in the micelles of the surfactants. The intensity of the phase separation depends on the temperature of separation and the content of electrolyte. The surfactantrich phase is usually more heterogenic than the aqueous phase. The separation of the phases is incomplete. It can be improved by an increase of temperature and the presence of electrolyte. The interfacial region (Figure 5c) is very fragile and can be easily destroyed, even in the case of delicate mixing or vial movement. The considered separation system can be characterized quantitatively by the changes in profile’s grayness with the change of position (from 0 µm at the top of the pictures to 575 µm at the bottom) as is presented in example Figure 6. The lower the concentration of dye, the higher grayness is observed. Thus, the grayness of the aqueous phase and surfactant-rich phase become higher and lower with the time, respectively. A rapid change of the grayness is observed in the interfacial zone. The heterogeneity of both the separated phases and the interfacial region can be qualitatively characterized by the standard deviation of the average values of grayness. The confidence limit can also be considered. Table 1 shows the average grayness of the initial solutions and separated phases in the interfacial region together with the statistical assessment calculated from 300 to 600 points. The results prove the heterogeneity of the separated phase and that the surfactant-rich phase is more heterogenic than the aqueous phase. Figure 7 shows the extraction isotherms (co vs cw) obtained for various experimental conditions. Linear relationships extrapolate near zero for a system free of dye. Thus, distribution coefficients could be calculated as the slopes of the linear relations (Table 2). The results show a very strong effect of NaCl presence on the separation of the dye. An effective separation is only possible in the presence of the electrolyte. NaCl is not
FIGURE 5. Separation of Direct Yellow from OD8 after 20 min (a, the surfactant-rich phase), 55 min (b, the aqueous phase), and 60 min (c, interfacial region) (2.5% OD8, 60 °C, 1 M NaCl, 20 min), 100 fold magnification of photograph.
TABLE 1. Separation of Direct Yellow (250 mg/L) from Solution of RokN8 (2.5%) in 55 °Ca grayness syst em
time, min
average values
standard deviation
confidence levelb
number of data
IS SPh SPh APh IA
0 22 32 34 39
122.64 94.68 80.42 141.77 157.48 77.05
4.51 16.78 13.94 15.00 9.55 6.37
0.77 3.09 3.11 2.36 1.46 1.48
570 570 570 570 166 404
a
APh SPh
Dye’ s concentration [mg/ dm3]
17
IS, inital solution; SPh, surfactant-rich phase; Aph, aqueous phase; IA, interface area. b Calculated for R ) 0.05.
FIGURE 6. Profiles of grayness after 35 min (2.5% RokN8, 55 °C; Direct Yellow, 250 mg/dm3; IS, inital solution; SPh, surfactant-rich phase; Aph, aqueous phase, IA - interface area). transferred to the surfactant-rich phase and remains in the micellar phase after phase separation. In the case of separation 2.5% OMD9 with 1 M NaCl, concentration of electrolyte after separation was equal to (1.00 ( 0.01) M in the aqueous phase and 0 M in the surfactant-rich phase (for overheating ∆CP ) 10, 15, 20, 25 °C). Thus, the salt causes the partial dehydration both of the surfactant and the dye by the breaking of hydrogen bonds with water molecules. The surfactant-rich phase becomes less hydrophilic, and the dye is salted out from the aqueous micellar solution. The effect is reflected in the water content in the surfactant-rich phase. At the cloud point, the surfactant phase contains large amounts of water. When deionized water is used, the surfactant-rich phase overheated by 20 °C (at temperature equal to 20 °C higher than CP, ∆CP ) 20 °C) contains approximately 35, 47, and 56% of water for OMD9, OD8, and RokN8, respectively. An addition of NaCl causes a decrease of water content in the surfactant-rich phase. In the presence of 1 M NaCl, the surfactant-rich phase contains only 12, 29, and 43% of water for OMD9, OD8, and RokN8.
FIGURE 7. Distribution of the dye between surfactant-rich phase (Co) and aqueous phase (Cw) (2.5% RokN8, empty and full symbols correspond to the system with (1 M) and without NaCl): 0, 9 71 °C, b, O 81 °C, 4, 2 91 °C, heated for 15 h; >, < 71 °C, ], [ 81 °C, f, g 91 °C, heated for 5 h; × centrifugation with 1 M NaCl. An increase of temperature also causes a decrease of the water content in the surfactant-rich phase. However, the effect is weaker in comparison with the addition of electrolyte. In our previous work (25), it was presented that the curves concentration of OMD-n in surfactant-rich phase versus the overheating above the cloud point ∆CP, coincide approaching asymptotes at the content of water equal 15%. The effect of the electrolyte content is very strong, especially in the region of low overheating. The asymptotic value of the content of water is near 5% in the presence of NaCl. Temperature affects interactions in both phases, decreasing the hydration of solutes, i.e., dye and surfactant, in the micellar aqueous phase and surfactant-rich phase. Due to it, depending upon experimental conditions, an increase of temperature can cause an increase or a decrease of dye recovery. In the case considered in the work, a decrease of the dye recovery is observed with an increase of temperature from 70 to 90 °C for OD-8 and OMD-9, both in the absence VOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Distribution Coefficients for Systems Containing Direct Yellow and 2.5% of Different Surfactants (Heated for 5 h) distribution coefficient surfactant OD8 OMD9 RokN8
temp [°C]
without NaCl
with 1 M NaCl
94 84 79 69 59 91 81 71
4.2 9.1 0.9 1.7 6.8 4.0 3.7 2.5
311.6 425.5 116.7 139.0 257.6 246.5 220.0 186.6
FIGURE 8. Relationship between dye recovery in percents and distribution coefficient (2.5% 0, OD8; 4, RokN8; and ], OMD9; system with (1 M) and without NaCl, heated for 5 and 15 h, heated for 10 min and separated by centrifuging). and presence of electrolyte. Thus, the results can suggest that temperature increase decreases interactions in the surfactant-rich phase (water-dye, water-surfactant and surfactant-dye) stronger than it does in the aqueous micellar phase. Such effect can be explained by a very strong hydrophilicity of the Direct Yellow. The opposite effect of temperature is observed when RokN8 containing an aromatic ring is used as surfactant. As a result, heating at 100 °C and centrifuging gives better separation of the dye. Linear relationships enabled to determine the distribution coefficients (Table 2), which can be used to design the separation process. The results indicate that the separation of such hydrophilic dye is only reasonable in the presence of electrolyte. The distribution coefficients are relatively low (1-9) in the electrolyte absence, but they increase to a few hundreds (100-900) in the presence of 1 M NaCl. As is seen in Figure 7, the best recovery of the dye is obtained when centrifuging is used for phase separation. The distribution coefficients for the considered system, containing RokN8, are estimated as equal to 1028, 574, and 246 for the separation of phases obtained by centrifuging, prolong heating at 71 and 91 °C during 15 and 5 h. Thus, the method enables up to one hundred enrichment of the dye in the surfactant-rich phase with almost complete recovery of the dye (98-99.9%). Figure 8 indicates that the dye recovery percentage sharply increases when the distribution coefficient increases up to 200 and approaches to the asymptotic value of 98%. The scaleup of the separation process is possible by heating the surfactant solution in a batch tank, followed periodically by continuous centrifuging in a liquid centrifugal separator. 3114
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Acknowledgments The work was supported by Grant KBN 3 T09D 055 26.
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Received for review April 23, 2004. Revised manuscript received February 15, 2005. Accepted February 23, 2005. ES049381X
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