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Removal of Cd2+ from synthetic wastewater using micellar-enhanced ultrafiltration with hollow fiber membrane
Colloids and Surfaces A: Physicochem. Eng. Aspects 294 (2007) 140–146
Removal of Cd2+ from synthetic wastewater using micellar-enhanced ultrafiltration with hollow fiber membrane Xu Ke a , Zeng Guang-ming a,∗ , Huang Jin-hui a , Wu Jiao-yi a , Fang Yao-yao a , Huang Guohe a,c , Li Jianbing a,b , Xi Beidou b , Liu Hongliang a,b b
a College of Environmental Science and Engineering, Hunan University, Changsha, Hunan 410082, China Environmental Engineering Program, University of Northern British Columbia, Prince George, Canada V2N 4Z9 c Chinese Research Academy of Environmental Science, Beijing 100012, PR China
Received 26 January 2006; received in revised form 26 July 2006; accepted 2 August 2006 Available online 18 August 2006
Abstract Micellar-enhanced ultrafiltration (MEUF) was used to remove Cd2+ from synthetic wastewater by applying polysulfone hollow fiber ultrafiltration membrane and with SDS as surfactant. The effects of some important parameters were investigated, including operating time, the concentration of SDS, transmembrane pressure, solution pH, the concentration of feed electrolyte and the mixture of SDS and Brij. The results show that the rejection of Cd2+ can reach 99%. But MEUF is not feasible when the synthetic wastewater is intensively acidic. The presence of electrolyte can decrease the efficiency of MEUF, while the mixture of SDS and Brij can increase it. © 2006 Elsevier B.V. All rights reserved. Keywords: Micellar-enhanced ultrafiltration; Surfactants; Cadmium ions; Brij
1. Introduction Nowadays, Cadmium is used widely in numerous industries. At the same time, Cadmium pollution is a serious problem. After polluting soil and water, Cadmium can enter human bodies by cumulating through the food chain and result in some serious and chronic illnesses. Consequently, most countries prescribe that the wastewater containing cadmium must be treated before discharged. At present, the popular techniques for treating the wastewater containing cadmium are chemical precipitation, adsorption, bleaching powder oxidation, ferrite process, ions exchange, biotechnology and so on. However, these techniques are of their own deficiencies, such as secondary pollution of deposition, inconvenient operation, high cost, difficulty of recycling cadmium and so on. MEUF is a new technology developed for treating the wastewater containing cadmium. In this technology, surfactants are added to the wastewater to promote the removal of cad-
∗
Corresponding author. Tel.: +86 731 8828754; fax: +86 731 8823701. E-mail address: [email protected] (G.-m. Zeng).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.08.017
mium ions. The surfactant molecules will aggregate and form the spherical micelles when the surfactant concentration in the wastewater is higher than its critical micellar concentration (cmc). The anionic micelles, which are negatively charged, can bind to cadmium ions, which are positively charged. Then the wastewater can be ultrafiltered through an ultrafiltration membrane whose pore size is smaller than the micelle size to reject the micelles. At the same time, cadmium ions adsorbed onto the micelles are rejected. The permeation quality is good to be reused or discharged directly. The concentrations of the surfactant and the cadmium ion are high in the retention and the volume of the retention is much smaller than that of the wastewater, so the retention is easier to be further treated than the wastewater, such as recycling the surfactant and cadmium. MEUF can be used to remove single metal ion or several kinds of metal ions simultaneously, whose removal efficiency is high [1–4]. This method is of some advantages such as simple operation, high removal efficiency, metal ion recycle. For the poor development and the high cost of ultrafiltration membrane, this method is just studied in labs. These studies were mostly carried out in batch stirred cells. Fluxes of ultrafiltration membranes used in most studies are very low and the volume of aqueous stream ultrafiltered is only hundreds of milliliters. In these experiments, the
K. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 294 (2007) 140–146
removal efficiency of metal ions and impacts of some parameters on this method were studied [1–7]. With the development of the membrane technique in recent years, the resistance on foul and the service life of membrane increase greatly, and the cost of membrane decreases continually. Therefore, the method becomes more practical and it is of greater potential for application. In this study, Micellar-enhanced ultrafiltration (MEUF) was used to remove Cd2+ from synthetic wastewater with the polysulfone hollow fiber ultrafiltration membrane and SDS as surfactant which is cheap and used widely. The hollow fiber UF device is operated in continuous and cross-flow mode which has much higher flux and much more effective membrane area than conventional batch cell system. The effects of some important parameters were investigated, including operating time, the concentration of SDS, transmembrane pressure, solution pH, the concentration of feed electrolyte and the mixture of SDS and Brij added into the solution. These results will be helpful to realize the practical application of this method. 2. Experimental 2.1. Materials All agents were analysis pure grade (>99%) and used as received. Cadmium ions were from cadmium nitrate, Cd(NO3 )2 ·4H2 O, which was purchased from Shanghai Tingxin chemical factory in China. The anionic surfactant sodium dodecyl sulfate (SDS), whose structure is C12 H25 OSO3 Na, was purchased from Tinajin Miou chemical factory. The nonionic surfactant Polyoxyethyleneglycol dodecyl ether (Brij), whose structure is CH3 (CH2 )10 CH2 O(C2 H4 O)23 H, was purchased from Koch Light Laboratories in England. The deionized water used in all experiments was produced by the deionized water apparatus type 90007-03 purchased from Labconco company in America. 2.2. Surfactants characters The CMCs of SDS and Brij used in the computations were 8 mM [8] and 0.3 mM [9], respectively. In aqueous solution without electrolytes, the reported micelles aggregation numbers for SDS and Brij are 64 [10] and 40 [9], respectively. That gives the micelles equivalent molecular weights of ≈18 and 38 kDa.
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Table 1 UEOS503 UF membrane characteristics Sizing specification (Φ × L/mm) Rejecting molecular weight (Da) Area of membrane (m2 ) pH Membrane material Inner/diameter of fiber (mm) Transmembrane pressure (MPa)
50 × 386 6K 1.5 2–13 PS 0.24/0.4 ≤0.15
In this study, the permeation flux of the UF membrane was defined as Ji =
Qi p × A
Ji is the permeation flux (m3 /m2 s Pa); Qi is the feed flux (m3 /s); p is the transmembrane pressure (Pa); A is the area of membrane (m2 ). The rejection was defined as R=
Cf − Cp × 100% Cf
Cf is the concentration of Cd2+ (mg/L) in the feed solution; Cp is the concentration of Cd2+ in the percolate (mg/L). 2.3.2. Procedure Cd(NO3 )2 ·4H2 O was added into the deionized water to produce the synthetic wastewater with Cd2+ concentration of 100 mg/L. The SDS with concentration pre-determined was added into the synthetic wastewater. Then the aqueous solution was ultrafiltered by the UF membrane. The schematic of the experimental set-up is shown in Fig. 1. 2.4. Analysis methods The concentration of SDS was measured by the methylene blue spectrophotometric method with Shimadzu UV2550(P/N206-55501-93) spectrophotometer from Japan. The concentration of cadmium ions was analyzed by atomic absorption spectrometry with the instrument type of Agilent 3510. The CMCs of the mixed SDS/Brij system were measured by using a conductivity instrument (model DDS-11A, made in Shang Hai, China).
2.3. Apparatus and procedure 2.3.1. Continuous and cross-flow hollow fiber UF device The experimental membrane module used in this study was purchased from Tianjin Motianmo project technology limited company with type of UEOS503. The transmembrane pressure was controlled by peristaltic pump. The UF membrane was polysulfone hollow fiber ultrafiltration membrane. During the ultrafiltration, the flow of the process solution was tangential to the membrane surface in order to minimize concentration polarization. The important parameters of the UF membrane were shown in Table 1.
Fig. 1. Schematic of micellar-enhanced ultrafiltration process. (1) Feed solution; (2) thermostat; (3) peristaltic pump; (4) membrane module; (5) permeate; (6) retentate; (7) pressure control valve; (8) manometer; (9, 10) rotameter.
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Fig. 2. Effect of operating time on permeate flux.
Micelle sizes of SDS were measured by using a laser light scattering instrument (model Zetasizer 3000HS, Malvern, England). 3. Results and discussions 3.1. Effect of operating time on permeation flux and rejection efficiency At the beginning of the experiment, the deionized water was ultrafiltered at the transmembrane pressure of 0.07 MPa and the permeate flux was 6.09 × 10−8 m3 /m2 s Pa. Then the effect of operating time on permeate flux was investigated at the fixed SDS and Cd2+ concentration of 8.0 mM (cmc) and 100 mg/L, respectively, and pH 7.0. As shown in Fig. 2, the initial permeation flux is 3.57 × 10−8 m3 /m2 s Pa which is the half of the permeation flux of deionized water. The reason is that the micelles and the cadmium ions in the feed block the membrane pores. During the operating, the permeation flux decreases. In the first 10 min, the permeation flux decreases quickly, then slowly. After 20 min, the permeation flux remains invariant with operating time. This behavior is attributed to the concentration polarization, namely SDS micelles deposits quickly on the membrane surface in short time. When the micelle concentration on the membrane surface reaches an adequately high value, the gel layer will form. During the process of concentration polarization, SDS micelles block the membrane pores and cause a resistance against the flow, so the permeation flux decreases quickly in the first 10 min in this study. After the gel layer forms, the membrane is polluted and the deposited micelles at the membrane surface do not increase, so the permeation flux reaches a plateau, namely the state of the membrane is steady. Accordingly, the sampling time is 30 min to guarantee the authenticity and the reliability of experimental results in the whole study.
Fig. 3. Effect of surfactant concentration on the Cd2+ rejection.
transmembrane pressure of 0.07 MPa. The results are shown in Figs. 3 and 4. Fig. 3 shows that the Cd2+ rejection increases with the SDS concentration increasing. Theoretically, there are no micelles formed at the SDS concentrations below the cmc, as a result there is no Cd2+ rejection. However, as shown in Fig. 3, the Cd2+ rejection was observed when the SDS concentrations are below the cmc. This unanticipated rejection can be explained by a concentration polarization mechanism. During the process of concentration polarization, the SDS molecules are rejected by the membrane. These rejected SDS molecules deposits on the membrane surface gradually and form the gel layer near the membrane surface where the SDS concentration reaches to the cmc and the micelles form to bind Cd2+ . In Fig. 3, the Cd2+ rejection increases sharply with the increase of the SDS concentration, which is below the cmc. However, when the feed SDS concentration is higher than the cmc, with the increase of the SDS concentration, the Cd2+ rejection reaches an asymptotic value beyond which a further increase in the SDS concentration cannot make the Cd2+ rejection increase. This phenomenon can be explained by the reason that the shape and aggregation number of micelles changes with the increase of the feed SDS concentration, and the efficient binding sites do not increase
3.2. Effect of SDS concentration on permeation flux and rejection efficiency The effect of feed SDS concentration on Cd2+ rejection was investigated at the Cd2+ concentration of 100 mg/L and the
Fig. 4. The change of SDS micelle size with SDS concentration.
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corresponding to the increase of the feed SDS concentration [11]. This explanation can be supported by some literatures. It was reported [12] that the presence of multivalent counterions (Ca2+ , Al3+ ) in solutions of anionic surfactant (sodium dodecyl dioxyethylene sulfate: SDP2S) strongly enhances the formation of rod-like micelles. This observation can be qualitatively explained by the fact that one multivalent counterion, Ca2+ or Al3+ , can bind together two or more anionic surfactant headgroups at the micelle surface, thus decreasing the optimal area per headgroup [12]. This induces a transition from spherical to cylindrical micelles in accordance with the theory by Israelachvili et al. [13,14]. Furthermore, the micelle sizes for different SDS concentration were measured at the fixed Cd2+ concentration of 100 mg/L and depicted in Fig. 4. Here the unit of SDS concentration is CMC. As shown in Fig. 4, the micelle size for the concentration of CMC is 5.07 nm. This value is in good agreement with the values reported in the literature [15–17]. Moreover, the micelle size increases with the increase of SDS concentration. This phenomena supports the above explanation, too. According to the above discussion, it is not necessary to increase the feed SDS concentration excessively considering the economic feasibility. As shown in Fig. 5, the permeation flux decreases with the increase of the feed SDS concentration. When the feed SDS concentration is below the cmc, all the SDS molecules exist as free monomers, whose size is much smaller than the pore diameter. Under these conditions, monomers can easily pass the membrane. So the permeation flux is high. With the increase of the feed SDS concentration, though still below cmc, the concentration polarization has an important effect on the permeation flux. The SDS free monomers deposits on the membrane surface so that the SDS concentration in the region near the membrane is higher than the SDS concentration in the feed. This effect can cause a resistance against the flow. When the feed SDS concentration reaches and exceeds the cmc, lots of micelles form in the solution to block the membrane pores. As a result, the corresponding permeation fluxes are very low and similar.
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Fig. 6. Effect of transmembrane pressure on the permeate flux.
In the practical application of MEUF, the high rejection efficiency and the high permeation flux are necessary, so an appropriate SDS concentration must be chosen. According to what is mentioned in the above, 8.0 mM (cmc) is the appropriate value. 3.3. Effect of transmembrane pressure on permeation flux and Cd2+ rejection efficiency The effect of the transmembrane pressure on the permeation flux and Cd2+ rejection efficiency were investigated with the Cd2+ concentration of 100 mg/L and the SDS concentration of 8.0 mM. Fig. 6 shows the variation of the permeation flux with the transmembrane pressure. As shown in Fig. 6, the permeation flux increases linearly with the increase of the transmembrane pressure below 0.08 MPa. If beyond 0.08 MPa, as the transmembrane pressure increases, the flux reaches an asymptotic value. This behavior is usually explained by the reason that high transmembrane pressure can make more micelles deposit on the membrane surface to enhance the concentration polarization and the gel layer is compressed. As a result, when the transmembrane pressure is beyond the critical concentration which is 0.08 MPa in this experiment, the permeate flux does not increase with the increase of the transmembrane pressure. Fig. 7 shows that the transmembrane pressure has no significant effect on the Cd2+ rejection. The reason is that the ultrafiltration membrane cannot reject free ions at any transmembrane pressure. 3.4. Effect of pH on Cd2+ rejection efficiency
Fig. 5. Effect of surfactant concentration on the permeation flux.
Fig. 8 shows the effect of pH on the Cd2+ rejection efficiency at the SDS concentration of 8.0 mM, Cd2+ concentration of 100 mg/L and the transmembrane pressure of 0.07 MPa. As shown in Fig. 8, the Cd2+ rejection efficiency increases sharply with the increase of pH (<9). Then the increasing trend slows down at a high pH (>9). Then the Cd2+ rejection efficiency reaches 99% at pH of 11. This is due to the competition of H+ trapped on the surface of the micelles with Cd2+ . H+ has the same charge with Cd2+ so that H+ can be bound to SDS micelles and
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Fig. 7. Effect of transmembrane pressure on the Cd2+ rejection.
Fig. 9. Effect of electrolyte concentration on the Cd2+ rejection.
occupies the binding sites. At low pH, there are lots of H+ in the solution, so the corresponding rejection is comparatively low. With the pH of the solution increasing, the amount of H+ in the solution decreases dramatically. As a result, the corresponding rejection increases sharply. At high pH (>9), there are few H+ in the solution, so the corresponding rejection does not increase and reaches a plateau.
of electrolyte cannot increase the Cd2+ rejection efficiency. This behavior may be attributed to the following two factors: (1) the cations of electrolyte, namely Na+ in this experiment, can be bound to the micelles and occupies the binding sites. Accordingly, the Cd2+ rejection efficiency decreases with the increase of the NaCl concentration; (2) the anions of electrolyte, namely Cl− in this experiment, can form the complexes with metal ions, namely Cd2+ in this experiment [19]. In this experiment, the two negative effects of the presence of NaCl may exceed the positive effect of the decrease of cmc due to the presence of NaCl.
3.5. Effect of electrolyte on Cd2+ rejection efficiency The presence of electrolyte can decrease the cmc of ionic surfactants because the electrolyte can weaken the repulsive forces between the head groups, which are normally fighting against the aggregation of surfactant monomers. Therefore, micelles can form comparatively easier in the presence of electrolyte and increase the Cd2+ rejection efficiency [18]. Fig. 9 shows the effect of electrolyte, which is NaCl in this research, on Cd2+ rejection efficiency at the SDS concentration of 8.0 mM, Cd2+ concentration of 100 mg/L and the transmembrane pressure of 0.07 MPa. As shown in Fig. 9, the Cd2+ rejection efficiency decreases with the increase of the NaCl concentration (from 10 to 100 mM) and reaches an unvarying low value. The result does not comply with what is pointed out in the literatures, namely the presence
Fig. 8. Effect of pH on the Cd2+ rejection.
3.6. Cd2+ rejection efficiency by SDS–Brij mixed micelles In MEUF applications, it is highly desirable to minimize the fraction of surfactant in monomeric form in order to reduce the permeate surfactant concentration. This can be achieved by using anionic–nonionic surfactant mixtures rather than single surfactant systems because the cmc of anionic–nonionic mixed surfactant is less than the cmc of single ionic surfactant [19,20]. In this study, nonionic surfactant Brij is chosen to mix with SDS. In the single SDS solution, the electrostatic repulsive forces between the hydrophilic head groups of monomeric SDS molecules can hinder micelles formation. In the mixed SDS/Brij system, Brij molecules can insert SDS micelles and promote micelles formation. Brij has non ionic and amphiphilic molecule structure with ethylene oxide groups. Aether oxide atoms in these groups can combine with trace protons generated from water ionization. Then, ethylene oxide groups become weak electropositive and can reduce the electrostatic repulsive forces between the hydrophilic head groups of SDS. As a result, the mixed SDS/Brij system has lower CMC and micelles are easier to form [21,22]. In this study, the CMCs of the SDS/Brij systems at different α values were measured by conductivity measurement as shown in Fig. 10. Here α is defined as molar ratio of Brij versus SDS. As shown in Fig. 10, the CMC of the SDS/Brij system decreases with the increase of α. When α increases from 0.1 to 0.3, the CMC of the SDS/Brij system decreases sharply. When α is above 0.3, the CMC of the SDS/Brij system decreases tardily to the CMC of Brij. This shows that: Brij molecules inserting
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This may be explained as following. When ionic surfactant is mixed with nonionic surfactant, the positive effect is that more micelles can form compared with single ionic surfactant. The negative effect is that the charge amount on the micelle surface may decrease because electropositive Brij molecules insert SDS micelles and the corresponding metal ions rejection decreases. The two effects are present simultaneously and restrict each other. In this experiment, the negative effect may be comparatively strong when adding Brij so that the improvement of Cd2+ rejection efficiency is not significant. It is recommended to find another nonionic surfactant, which can improve the Cd2+ rejection efficiency significantly. 4. Summary and conclusions Fig. 10. The relation between α and CMC of Brij/SDS.
SDS micelles can promote micelles formation effectively when the molar ration of Brij versus SDS is small because SDS is majority and its CMC is much higher than CMC of Brij; when the molar ratio is becoming higher and Brij is becoming majority in the mixed system, the effect of Brij on decreasing CMC is becoming weak gradually. Fig. 11 shows the Cd2+ rejection efficiency with SDS–Brij mixed surfactant and single SDS. The molar ratio values of SDS/Brij are 5.3 mM/0.59 mM, 1.8 mM/0.77 mM, 0.48 mM/0.48 mM, 0.13 mM/0.30 mM and 0.032 mM/0.29 mM, respectively. As shown in Fig. 11, the Cd2+ rejection efficiency is unexpected low when the SDS concentration is low in the mixed SDS–Brij surfactant. Though the mixed micelles form in the solution, the number of mixed micelles is not enough to bind Cd2+ effectively. Accordingly, the Cd2+ rejection efficiency by mixed surfactant is similar with one of the single SDS. When the SDS concentration increases, the addition of a little Brij can make more mixed micelles form compared with the single SDS. The number of mixed micelles is enough to bind Cd2+ effectively, so the Cd2+ rejection efficiency by mixed surfactant is higher than that by the single SDS. However, as observed, the improvement of the rejection efficiency is not significant.
Micellar-enhanced ultrafiltration was used to remove Cd2+ from synthetic wastewater using polysulfone hollow fiber ultrafiltration membrane and SDS as surfactant. The effect of some important parameters were investigated, including transmembrane time, the SDS concentration, transmembrane pressure, solution pH, electrolyte and the mixture of SDS and Brij. The results show that the rejection of Cd2+ can reach 99% when the transmembrane is 0.07 MPa and the concentrations of Cd2+ and SDS are 100 mg/L and 8 mM, respectively. The SDS–Brij mixed surfactant can improve the Cd2+ rejection efficiency and decrease the SDS dosage in MEUF. Accordingly, MEUF is feasible to treat the waste water containing Cd2+ . The efficiency of MEUF decreases when the wastewater is intensively acidic. The presence of electrolyte can decrease the efficiency of MEUF. Furthermore, the presence of electrolyte can diminish the Cd2+ rejection efficiency by SDS. With the development of the membrane technique in recent years, the resistance on foul and the service life of membrane increase greatly, and the cost of membrane decreases continually. In the future, MEUF will be used widely to treat the waste water containing Cd2+ . Acknowledgements The study was financially supported by the National 863 High Technologies Research Foundation of China (no. 2004AA649370), the National Basic Research Program (973 Program) (no. 2005CB724203), the Natural Foundation for Distinguished Young Scholars (no. 50425927, no. 50225926), the Doctoral Foundation of Ministry of Education of China, the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, P.R.C. (TRAPOYT) in 2000. The National Natural Science Foundation of China (no. 50608028). References
Fig. 11. Effect of Brij–SDS mixed micelles on rejection of Cd2+ .
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Removal of Cd2+ from synthetic wastewater using micellar-enhanced ultrafiltration with hollow fiber membrane Xu Ke a , Zeng Guang-ming a,∗ , Huang Jin-hui a , Wu Jiao-yi a , Fang Yao-yao a , Huang Guohe a,c , Li Jianbing a,b , Xi Beidou b , Liu Hongliang a,b b
a College of Environmental Science and Engineering, Hunan University, Changsha, Hunan 410082, China Environmental Engineering Program, University of Northern British Columbia, Prince George, Canada V2N 4Z9 c Chinese Research Academy of Environmental Science, Beijing 100012, PR China
Received 26 January 2006; received in revised form 26 July 2006; accepted 2 August 2006 Available online 18 August 2006
Abstract Micellar-enhanced ultrafiltration (MEUF) was used to remove Cd2+ from synthetic wastewater by applying polysulfone hollow fiber ultrafiltration membrane and with SDS as surfactant. The effects of some important parameters were investigated, including operating time, the concentration of SDS, transmembrane pressure, solution pH, the concentration of feed electrolyte and the mixture of SDS and Brij. The results show that the rejection of Cd2+ can reach 99%. But MEUF is not feasible when the synthetic wastewater is intensively acidic. The presence of electrolyte can decrease the efficiency of MEUF, while the mixture of SDS and Brij can increase it. © 2006 Elsevier B.V. All rights reserved. Keywords: Micellar-enhanced ultrafiltration; Surfactants; Cadmium ions; Brij
1. Introduction Nowadays, Cadmium is used widely in numerous industries. At the same time, Cadmium pollution is a serious problem. After polluting soil and water, Cadmium can enter human bodies by cumulating through the food chain and result in some serious and chronic illnesses. Consequently, most countries prescribe that the wastewater containing cadmium must be treated before discharged. At present, the popular techniques for treating the wastewater containing cadmium are chemical precipitation, adsorption, bleaching powder oxidation, ferrite process, ions exchange, biotechnology and so on. However, these techniques are of their own deficiencies, such as secondary pollution of deposition, inconvenient operation, high cost, difficulty of recycling cadmium and so on. MEUF is a new technology developed for treating the wastewater containing cadmium. In this technology, surfactants are added to the wastewater to promote the removal of cad-
∗
Corresponding author. Tel.: +86 731 8828754; fax: +86 731 8823701. E-mail address: [email protected] (G.-m. Zeng).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.08.017
mium ions. The surfactant molecules will aggregate and form the spherical micelles when the surfactant concentration in the wastewater is higher than its critical micellar concentration (cmc). The anionic micelles, which are negatively charged, can bind to cadmium ions, which are positively charged. Then the wastewater can be ultrafiltered through an ultrafiltration membrane whose pore size is smaller than the micelle size to reject the micelles. At the same time, cadmium ions adsorbed onto the micelles are rejected. The permeation quality is good to be reused or discharged directly. The concentrations of the surfactant and the cadmium ion are high in the retention and the volume of the retention is much smaller than that of the wastewater, so the retention is easier to be further treated than the wastewater, such as recycling the surfactant and cadmium. MEUF can be used to remove single metal ion or several kinds of metal ions simultaneously, whose removal efficiency is high [1–4]. This method is of some advantages such as simple operation, high removal efficiency, metal ion recycle. For the poor development and the high cost of ultrafiltration membrane, this method is just studied in labs. These studies were mostly carried out in batch stirred cells. Fluxes of ultrafiltration membranes used in most studies are very low and the volume of aqueous stream ultrafiltered is only hundreds of milliliters. In these experiments, the
K. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 294 (2007) 140–146
removal efficiency of metal ions and impacts of some parameters on this method were studied [1–7]. With the development of the membrane technique in recent years, the resistance on foul and the service life of membrane increase greatly, and the cost of membrane decreases continually. Therefore, the method becomes more practical and it is of greater potential for application. In this study, Micellar-enhanced ultrafiltration (MEUF) was used to remove Cd2+ from synthetic wastewater with the polysulfone hollow fiber ultrafiltration membrane and SDS as surfactant which is cheap and used widely. The hollow fiber UF device is operated in continuous and cross-flow mode which has much higher flux and much more effective membrane area than conventional batch cell system. The effects of some important parameters were investigated, including operating time, the concentration of SDS, transmembrane pressure, solution pH, the concentration of feed electrolyte and the mixture of SDS and Brij added into the solution. These results will be helpful to realize the practical application of this method. 2. Experimental 2.1. Materials All agents were analysis pure grade (>99%) and used as received. Cadmium ions were from cadmium nitrate, Cd(NO3 )2 ·4H2 O, which was purchased from Shanghai Tingxin chemical factory in China. The anionic surfactant sodium dodecyl sulfate (SDS), whose structure is C12 H25 OSO3 Na, was purchased from Tinajin Miou chemical factory. The nonionic surfactant Polyoxyethyleneglycol dodecyl ether (Brij), whose structure is CH3 (CH2 )10 CH2 O(C2 H4 O)23 H, was purchased from Koch Light Laboratories in England. The deionized water used in all experiments was produced by the deionized water apparatus type 90007-03 purchased from Labconco company in America. 2.2. Surfactants characters The CMCs of SDS and Brij used in the computations were 8 mM [8] and 0.3 mM [9], respectively. In aqueous solution without electrolytes, the reported micelles aggregation numbers for SDS and Brij are 64 [10] and 40 [9], respectively. That gives the micelles equivalent molecular weights of ≈18 and 38 kDa.
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Table 1 UEOS503 UF membrane characteristics Sizing specification (Φ × L/mm) Rejecting molecular weight (Da) Area of membrane (m2 ) pH Membrane material Inner/diameter of fiber (mm) Transmembrane pressure (MPa)
50 × 386 6K 1.5 2–13 PS 0.24/0.4 ≤0.15
In this study, the permeation flux of the UF membrane was defined as Ji =
Qi p × A
Ji is the permeation flux (m3 /m2 s Pa); Qi is the feed flux (m3 /s); p is the transmembrane pressure (Pa); A is the area of membrane (m2 ). The rejection was defined as R=
Cf − Cp × 100% Cf
Cf is the concentration of Cd2+ (mg/L) in the feed solution; Cp is the concentration of Cd2+ in the percolate (mg/L). 2.3.2. Procedure Cd(NO3 )2 ·4H2 O was added into the deionized water to produce the synthetic wastewater with Cd2+ concentration of 100 mg/L. The SDS with concentration pre-determined was added into the synthetic wastewater. Then the aqueous solution was ultrafiltered by the UF membrane. The schematic of the experimental set-up is shown in Fig. 1. 2.4. Analysis methods The concentration of SDS was measured by the methylene blue spectrophotometric method with Shimadzu UV2550(P/N206-55501-93) spectrophotometer from Japan. The concentration of cadmium ions was analyzed by atomic absorption spectrometry with the instrument type of Agilent 3510. The CMCs of the mixed SDS/Brij system were measured by using a conductivity instrument (model DDS-11A, made in Shang Hai, China).
2.3. Apparatus and procedure 2.3.1. Continuous and cross-flow hollow fiber UF device The experimental membrane module used in this study was purchased from Tianjin Motianmo project technology limited company with type of UEOS503. The transmembrane pressure was controlled by peristaltic pump. The UF membrane was polysulfone hollow fiber ultrafiltration membrane. During the ultrafiltration, the flow of the process solution was tangential to the membrane surface in order to minimize concentration polarization. The important parameters of the UF membrane were shown in Table 1.
Fig. 1. Schematic of micellar-enhanced ultrafiltration process. (1) Feed solution; (2) thermostat; (3) peristaltic pump; (4) membrane module; (5) permeate; (6) retentate; (7) pressure control valve; (8) manometer; (9, 10) rotameter.
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Fig. 2. Effect of operating time on permeate flux.
Micelle sizes of SDS were measured by using a laser light scattering instrument (model Zetasizer 3000HS, Malvern, England). 3. Results and discussions 3.1. Effect of operating time on permeation flux and rejection efficiency At the beginning of the experiment, the deionized water was ultrafiltered at the transmembrane pressure of 0.07 MPa and the permeate flux was 6.09 × 10−8 m3 /m2 s Pa. Then the effect of operating time on permeate flux was investigated at the fixed SDS and Cd2+ concentration of 8.0 mM (cmc) and 100 mg/L, respectively, and pH 7.0. As shown in Fig. 2, the initial permeation flux is 3.57 × 10−8 m3 /m2 s Pa which is the half of the permeation flux of deionized water. The reason is that the micelles and the cadmium ions in the feed block the membrane pores. During the operating, the permeation flux decreases. In the first 10 min, the permeation flux decreases quickly, then slowly. After 20 min, the permeation flux remains invariant with operating time. This behavior is attributed to the concentration polarization, namely SDS micelles deposits quickly on the membrane surface in short time. When the micelle concentration on the membrane surface reaches an adequately high value, the gel layer will form. During the process of concentration polarization, SDS micelles block the membrane pores and cause a resistance against the flow, so the permeation flux decreases quickly in the first 10 min in this study. After the gel layer forms, the membrane is polluted and the deposited micelles at the membrane surface do not increase, so the permeation flux reaches a plateau, namely the state of the membrane is steady. Accordingly, the sampling time is 30 min to guarantee the authenticity and the reliability of experimental results in the whole study.
Fig. 3. Effect of surfactant concentration on the Cd2+ rejection.
transmembrane pressure of 0.07 MPa. The results are shown in Figs. 3 and 4. Fig. 3 shows that the Cd2+ rejection increases with the SDS concentration increasing. Theoretically, there are no micelles formed at the SDS concentrations below the cmc, as a result there is no Cd2+ rejection. However, as shown in Fig. 3, the Cd2+ rejection was observed when the SDS concentrations are below the cmc. This unanticipated rejection can be explained by a concentration polarization mechanism. During the process of concentration polarization, the SDS molecules are rejected by the membrane. These rejected SDS molecules deposits on the membrane surface gradually and form the gel layer near the membrane surface where the SDS concentration reaches to the cmc and the micelles form to bind Cd2+ . In Fig. 3, the Cd2+ rejection increases sharply with the increase of the SDS concentration, which is below the cmc. However, when the feed SDS concentration is higher than the cmc, with the increase of the SDS concentration, the Cd2+ rejection reaches an asymptotic value beyond which a further increase in the SDS concentration cannot make the Cd2+ rejection increase. This phenomenon can be explained by the reason that the shape and aggregation number of micelles changes with the increase of the feed SDS concentration, and the efficient binding sites do not increase
3.2. Effect of SDS concentration on permeation flux and rejection efficiency The effect of feed SDS concentration on Cd2+ rejection was investigated at the Cd2+ concentration of 100 mg/L and the
Fig. 4. The change of SDS micelle size with SDS concentration.
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corresponding to the increase of the feed SDS concentration [11]. This explanation can be supported by some literatures. It was reported [12] that the presence of multivalent counterions (Ca2+ , Al3+ ) in solutions of anionic surfactant (sodium dodecyl dioxyethylene sulfate: SDP2S) strongly enhances the formation of rod-like micelles. This observation can be qualitatively explained by the fact that one multivalent counterion, Ca2+ or Al3+ , can bind together two or more anionic surfactant headgroups at the micelle surface, thus decreasing the optimal area per headgroup [12]. This induces a transition from spherical to cylindrical micelles in accordance with the theory by Israelachvili et al. [13,14]. Furthermore, the micelle sizes for different SDS concentration were measured at the fixed Cd2+ concentration of 100 mg/L and depicted in Fig. 4. Here the unit of SDS concentration is CMC. As shown in Fig. 4, the micelle size for the concentration of CMC is 5.07 nm. This value is in good agreement with the values reported in the literature [15–17]. Moreover, the micelle size increases with the increase of SDS concentration. This phenomena supports the above explanation, too. According to the above discussion, it is not necessary to increase the feed SDS concentration excessively considering the economic feasibility. As shown in Fig. 5, the permeation flux decreases with the increase of the feed SDS concentration. When the feed SDS concentration is below the cmc, all the SDS molecules exist as free monomers, whose size is much smaller than the pore diameter. Under these conditions, monomers can easily pass the membrane. So the permeation flux is high. With the increase of the feed SDS concentration, though still below cmc, the concentration polarization has an important effect on the permeation flux. The SDS free monomers deposits on the membrane surface so that the SDS concentration in the region near the membrane is higher than the SDS concentration in the feed. This effect can cause a resistance against the flow. When the feed SDS concentration reaches and exceeds the cmc, lots of micelles form in the solution to block the membrane pores. As a result, the corresponding permeation fluxes are very low and similar.
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Fig. 6. Effect of transmembrane pressure on the permeate flux.
In the practical application of MEUF, the high rejection efficiency and the high permeation flux are necessary, so an appropriate SDS concentration must be chosen. According to what is mentioned in the above, 8.0 mM (cmc) is the appropriate value. 3.3. Effect of transmembrane pressure on permeation flux and Cd2+ rejection efficiency The effect of the transmembrane pressure on the permeation flux and Cd2+ rejection efficiency were investigated with the Cd2+ concentration of 100 mg/L and the SDS concentration of 8.0 mM. Fig. 6 shows the variation of the permeation flux with the transmembrane pressure. As shown in Fig. 6, the permeation flux increases linearly with the increase of the transmembrane pressure below 0.08 MPa. If beyond 0.08 MPa, as the transmembrane pressure increases, the flux reaches an asymptotic value. This behavior is usually explained by the reason that high transmembrane pressure can make more micelles deposit on the membrane surface to enhance the concentration polarization and the gel layer is compressed. As a result, when the transmembrane pressure is beyond the critical concentration which is 0.08 MPa in this experiment, the permeate flux does not increase with the increase of the transmembrane pressure. Fig. 7 shows that the transmembrane pressure has no significant effect on the Cd2+ rejection. The reason is that the ultrafiltration membrane cannot reject free ions at any transmembrane pressure. 3.4. Effect of pH on Cd2+ rejection efficiency
Fig. 5. Effect of surfactant concentration on the permeation flux.
Fig. 8 shows the effect of pH on the Cd2+ rejection efficiency at the SDS concentration of 8.0 mM, Cd2+ concentration of 100 mg/L and the transmembrane pressure of 0.07 MPa. As shown in Fig. 8, the Cd2+ rejection efficiency increases sharply with the increase of pH (<9). Then the increasing trend slows down at a high pH (>9). Then the Cd2+ rejection efficiency reaches 99% at pH of 11. This is due to the competition of H+ trapped on the surface of the micelles with Cd2+ . H+ has the same charge with Cd2+ so that H+ can be bound to SDS micelles and
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Fig. 7. Effect of transmembrane pressure on the Cd2+ rejection.
Fig. 9. Effect of electrolyte concentration on the Cd2+ rejection.
occupies the binding sites. At low pH, there are lots of H+ in the solution, so the corresponding rejection is comparatively low. With the pH of the solution increasing, the amount of H+ in the solution decreases dramatically. As a result, the corresponding rejection increases sharply. At high pH (>9), there are few H+ in the solution, so the corresponding rejection does not increase and reaches a plateau.
of electrolyte cannot increase the Cd2+ rejection efficiency. This behavior may be attributed to the following two factors: (1) the cations of electrolyte, namely Na+ in this experiment, can be bound to the micelles and occupies the binding sites. Accordingly, the Cd2+ rejection efficiency decreases with the increase of the NaCl concentration; (2) the anions of electrolyte, namely Cl− in this experiment, can form the complexes with metal ions, namely Cd2+ in this experiment [19]. In this experiment, the two negative effects of the presence of NaCl may exceed the positive effect of the decrease of cmc due to the presence of NaCl.
3.5. Effect of electrolyte on Cd2+ rejection efficiency The presence of electrolyte can decrease the cmc of ionic surfactants because the electrolyte can weaken the repulsive forces between the head groups, which are normally fighting against the aggregation of surfactant monomers. Therefore, micelles can form comparatively easier in the presence of electrolyte and increase the Cd2+ rejection efficiency [18]. Fig. 9 shows the effect of electrolyte, which is NaCl in this research, on Cd2+ rejection efficiency at the SDS concentration of 8.0 mM, Cd2+ concentration of 100 mg/L and the transmembrane pressure of 0.07 MPa. As shown in Fig. 9, the Cd2+ rejection efficiency decreases with the increase of the NaCl concentration (from 10 to 100 mM) and reaches an unvarying low value. The result does not comply with what is pointed out in the literatures, namely the presence
Fig. 8. Effect of pH on the Cd2+ rejection.
3.6. Cd2+ rejection efficiency by SDS–Brij mixed micelles In MEUF applications, it is highly desirable to minimize the fraction of surfactant in monomeric form in order to reduce the permeate surfactant concentration. This can be achieved by using anionic–nonionic surfactant mixtures rather than single surfactant systems because the cmc of anionic–nonionic mixed surfactant is less than the cmc of single ionic surfactant [19,20]. In this study, nonionic surfactant Brij is chosen to mix with SDS. In the single SDS solution, the electrostatic repulsive forces between the hydrophilic head groups of monomeric SDS molecules can hinder micelles formation. In the mixed SDS/Brij system, Brij molecules can insert SDS micelles and promote micelles formation. Brij has non ionic and amphiphilic molecule structure with ethylene oxide groups. Aether oxide atoms in these groups can combine with trace protons generated from water ionization. Then, ethylene oxide groups become weak electropositive and can reduce the electrostatic repulsive forces between the hydrophilic head groups of SDS. As a result, the mixed SDS/Brij system has lower CMC and micelles are easier to form [21,22]. In this study, the CMCs of the SDS/Brij systems at different α values were measured by conductivity measurement as shown in Fig. 10. Here α is defined as molar ratio of Brij versus SDS. As shown in Fig. 10, the CMC of the SDS/Brij system decreases with the increase of α. When α increases from 0.1 to 0.3, the CMC of the SDS/Brij system decreases sharply. When α is above 0.3, the CMC of the SDS/Brij system decreases tardily to the CMC of Brij. This shows that: Brij molecules inserting
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This may be explained as following. When ionic surfactant is mixed with nonionic surfactant, the positive effect is that more micelles can form compared with single ionic surfactant. The negative effect is that the charge amount on the micelle surface may decrease because electropositive Brij molecules insert SDS micelles and the corresponding metal ions rejection decreases. The two effects are present simultaneously and restrict each other. In this experiment, the negative effect may be comparatively strong when adding Brij so that the improvement of Cd2+ rejection efficiency is not significant. It is recommended to find another nonionic surfactant, which can improve the Cd2+ rejection efficiency significantly. 4. Summary and conclusions Fig. 10. The relation between α and CMC of Brij/SDS.
SDS micelles can promote micelles formation effectively when the molar ration of Brij versus SDS is small because SDS is majority and its CMC is much higher than CMC of Brij; when the molar ratio is becoming higher and Brij is becoming majority in the mixed system, the effect of Brij on decreasing CMC is becoming weak gradually. Fig. 11 shows the Cd2+ rejection efficiency with SDS–Brij mixed surfactant and single SDS. The molar ratio values of SDS/Brij are 5.3 mM/0.59 mM, 1.8 mM/0.77 mM, 0.48 mM/0.48 mM, 0.13 mM/0.30 mM and 0.032 mM/0.29 mM, respectively. As shown in Fig. 11, the Cd2+ rejection efficiency is unexpected low when the SDS concentration is low in the mixed SDS–Brij surfactant. Though the mixed micelles form in the solution, the number of mixed micelles is not enough to bind Cd2+ effectively. Accordingly, the Cd2+ rejection efficiency by mixed surfactant is similar with one of the single SDS. When the SDS concentration increases, the addition of a little Brij can make more mixed micelles form compared with the single SDS. The number of mixed micelles is enough to bind Cd2+ effectively, so the Cd2+ rejection efficiency by mixed surfactant is higher than that by the single SDS. However, as observed, the improvement of the rejection efficiency is not significant.
Micellar-enhanced ultrafiltration was used to remove Cd2+ from synthetic wastewater using polysulfone hollow fiber ultrafiltration membrane and SDS as surfactant. The effect of some important parameters were investigated, including transmembrane time, the SDS concentration, transmembrane pressure, solution pH, electrolyte and the mixture of SDS and Brij. The results show that the rejection of Cd2+ can reach 99% when the transmembrane is 0.07 MPa and the concentrations of Cd2+ and SDS are 100 mg/L and 8 mM, respectively. The SDS–Brij mixed surfactant can improve the Cd2+ rejection efficiency and decrease the SDS dosage in MEUF. Accordingly, MEUF is feasible to treat the waste water containing Cd2+ . The efficiency of MEUF decreases when the wastewater is intensively acidic. The presence of electrolyte can decrease the efficiency of MEUF. Furthermore, the presence of electrolyte can diminish the Cd2+ rejection efficiency by SDS. With the development of the membrane technique in recent years, the resistance on foul and the service life of membrane increase greatly, and the cost of membrane decreases continually. In the future, MEUF will be used widely to treat the waste water containing Cd2+ . Acknowledgements The study was financially supported by the National 863 High Technologies Research Foundation of China (no. 2004AA649370), the National Basic Research Program (973 Program) (no. 2005CB724203), the Natural Foundation for Distinguished Young Scholars (no. 50425927, no. 50225926), the Doctoral Foundation of Ministry of Education of China, the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, P.R.C. (TRAPOYT) in 2000. The National Natural Science Foundation of China (no. 50608028). References
Fig. 11. Effect of Brij–SDS mixed micelles on rejection of Cd2+ .
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