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Adsorption and recovery of methylene blue from aqueous solution through ultrafiltration technique
Separation and Purification Technology 68 (2009) 244–249
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Adsorption and recovery of methylene blue from aqueous solution through ultrafiltration technique Lili Zheng, Yanlei Su ∗ , Lijun Wang, Zhongyi Jiang Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 2 May 2009 Accepted 12 May 2009 Keywords: Methylene blue Polyethersulfone membrane Adsorption Recovery
a b s t r a c t Adsorption and recovery of methylene blue (MB) from aqueous solution through ultrafiltration technique with polyethersulfone (PES) membrane was developed. Cationic dye MB can be adsorbed by PES membrane from aqueous solution. The amount of adsorbed MB on PES membrane was higher at the condition of basic solution and lower ionic strength. The electrostatic interaction may be the important driving force of MB adsorption. During ultrafiltration, the rejection ratio of MB can reach 100% in the initial operation stage due to convection mass transport through membrane pores and adsorption of MB on the surface and pores of PES membrane. The final recovery of MB in the permeate solution was easily achieved by decreasing the solution pH to desorb MB. The adsorption and recovery of dyes from aqueous solution through ultrafiltration technique have potential application in wastewater treatment. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Textile, paper, carpet, leather, and printing industries generate large volumes of wastewater polluted with various dyes. Due to the toxic nature of most dyes to aquatic life, colored wastewater cannot be discharged without adequate treatment. Many methods are available for the removal of dyes from industrial effluents, the most important of which are biodegradation, flocculation–coagulation, chemical oxidation, adsorption, and membrane separation [1–8]. Since the low biodegradability of dyes due to the complex aromatic molecular structure, conventional biological wastewater treatment processes are not very efficient for the treatment of dyeing waste. The flocculation treatment produces a large amount of sludge, which causes disposal problem, thus increasing the operation cost. After chemical oxidation of dyes, some toxic byproducts are still hazardous to the environment. Adsorption is a comparatively cheap process and effective in the removal of dyes, but adsorbent with large specific surface area, high adsorption capacity, and special surface reactivity must be carefully seek, and the subsequent treatment for the desorption of dyes and the regeneration of adsorbent also must be considered. Ultrafiltration is a pressure-driven membrane process for separating dissolved molecules in solution on the basis of molecular size. The molecules larger than the membrane pore size are retained in the feed solution. Ultrafiltration operates at a lower pressure with
∗ Corresponding author. Fax: +86 22 27890882. E-mail address: [email protected] (Y. Su). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.05.010
a higher flux, lower energy cost, and easy use, but it has a lower dye rejection ratio, since the pore size in the skin layer of ultrafiltration membrane is usually larger that the size of dye molecules. Micellar enhanced ultrafiltration is one of the feasible methods to remove organic dyes from wastewater [9–11]. In this process, surfactant with higher concentration than the critical micelle concentration is added to a polluted aqueous solution. The surfactant molecules form micelles, which can solubilize the organic dyes. The micelles containing the dissolved solutes are rejected during ultrafiltration process by the membrane and the permeated stream passing through the membrane is nearly free from dyes. However, little attention has been given to solve the difficulty for the separation and recovery of dyes from the added surfactant. A combined technique based on ultrafiltration and adsorption is alternative process for dye wastewater treatment. Banat and Al-Bastaki reported that the combination of activated carbon with ultrafiltration simultaneously exploits the high adsorption capability of activated carbon and the particle removal ability of ultrafiltration, the combined process achieved better rejection of dyes than that by ultrafiltration process alone [12]. However, the subsequent treatments, such as desorption of dyes from activated carbon and regeneration of adsorbent, have not reported in their work. The recovery of adsorbed dye from activated carbon usually needs organic solvent and regeneration of activated carbon usually needs thermal treatment. It is significant to develop a simple and effective method for the adsorption and recovery of dyes through ultrafiltration technique. In the present study, a facile method for the adsorption and recovery of dyes from aqueous solution through ultrafiltration process was developed. Polyethersulfone (PES) membrane has
L. Zheng et al. / Separation and Purification Technology 68 (2009) 244–249
negatively charge surface under specified condition, which can combine with the positively charged dye molecules because of the electrostatic attractions. PES membrane was operated in filtration mode and methylene blue (MB) was selected as a model compound to evaluate the efficiency of the adsorption and recovery of dye from aqueous solution through ultrafiltration technique. The effects of solution pH, ionic strength, and operation pressure on the adsorption and rejection of MB were investigated. The recovery of adsorbed MB from PES membrane was easily achieved through decreasing pH value of feed solution.
2. Experimental
2.5. Ultrafiltration experiments A dead-end stirred cell (Model 8200, Millipore Co., USA. The volume capacity is 200 mL and the effective area of membrane is 28.7 cm2 ) filtration system connected with a N2 gas cylinder and solution reservoir was designed to evaluate MB rejection and recovery from aqueous solution through ultrafiltration technique. The operation pressure in the system was maintained by nitrogen gas. PES membrane was first compacted with deionized water at 0.15 MPa for about 30 min. The cell was then emptied and refilled with 0.8 mg/L MB solution immediately; the pressure was lowered to a given operating pressure (most operating press is 0.04 MPa except in the experiment of different flux through adjusting press). The flux (J) is calculated by Eq. (1):
2.1. Materials PES 6020P (BASF Co., Germany) was dried at 110 ◦ C for 12 h prior to use. Methylene blue, N,N-dimethyl formamide (DMF), and poly(ethylene glycol) (PEG2000, the molecular weight is 2000) was purchased from Kewei Chemical Reagent Co. (Tianjin, China). Other reagents were all of analytical grade and used without further purification. Water used in all experiments was the deionized water.
Jw1 =
PES membrane was fabricated through a simple aqueous-based immersion precipitation method. 5.0 g PES (membrane material) and 4.0 g PEG2000 (pore-forming agent) were added into 18.6 g DMF to prepare casting solution. The solution was stirred for 4 h at temperature of 60 ◦ C to ensure a complete dissolution of the polymers. After the bubbles were released completely, the solution was cast on glass plates using a stainless steel knife, and then the glass plates were immersed in a coagulation bath of deionized water. The self-made membrane with a wet thickness of about 240 m was peeled off and subsequently rinsed with water to remove the residual solvent and pore-forming agent. PES membrane was kept in deionized water prior to use.
2.3. Membrane characterization The cross-sectional morphology of PES membrane was observed by scanning electron microscopy (SEM) using a Philips XL30E scanning microscope. The membrane was frozen in liquid nitrogen, broken, and sputtered with gold prior to SEM observation. The surface charge of PES membrane was measured via tangential flow streaming potential measurement. The streaming potentials of membranes were measured using a 0.001 mol/L KCl solution in the pH range from 4 to 9 at a room temperature of 25 ◦ C. The zeta potential () was calculated using the Helmholtz–Smoluchowski equation.
V At
(1)
where V (L) is the volume of permeated water, A (m2 ) is the membrane area, and t (h) is ultrafiltration operation time. The filtration efficiency in removal of MB from the feed solution was evaluated through the dye rejection ratio. The rejection ratio of MB is calculated by Eq. (2): R=
2.2. Membrane preparation
245
1−
Cp Cf
× 100%
(2)
where Cp and Cf (mg/mL) are the MB concentrations of permeate and feed solutions, respectively. In the MB recovery experiments, ultrafiltration was first operated with MB solution at pH value of 9.0, then the operation was stopped and the cell was emptied. The cell was refilled with water free of MB at pH value of 4.0. The permeate solution passing through PES membrane was collected for MB recovery. 3. Results and discussion 3.1. Adsorption of MB on PES membrane Since PES has excellent temperature and pH stabilities, and mechanical strength, PES is one of widely used membrane materials. PES membranes have been applied in wastewater treatment, reverse osmosis pretreatment, separation and purification of proteins in food industry and biotechnology [13,14]. MB is a thiazine cationic dye, which has wide applications in coloring paper, dyeing cottons and wools. MB is also used as an oxidation–reduction indicator and in the trace analysis of anionic surfactant. The molecular structures of PES and MB were shown in Fig. 1. The cross-sectional morphology of self-made PES membrane was shown in Fig. 2. PES
2.4. Adsorption experiments PES membrane was cut into a round shape with external surface of 28.7 cm2 (the weight of dried membrane is 0.16 g) and immersed into glass vial containing 30 mL of 4.0 mg/L MB solution at a given pH value and ionic strength. The pH value of MB solution was adjusted with HCl or NaOH solutions, while the ionic strength was adjusted with NaCl. The concentration of MB in the solution was measured with a UV–vis spectrophotometer (Hitach UV-2800, Japan) at the maximum absorbance at 660 nm after a certain immersion time, the amount of adsorbed MB on PES membrane was then calculated.
Fig. 1. The molecular structure of poly(polyethersulfone) and methylene blue.
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Fig. 2. SEM image of cross-sectional morphology of self-made PES membrane.
membrane exhibited a typical asymmetric structure with a top dense layer, a porous sublayer, and fully developed macropores at the bottom. For evaluation of the surface charge of PES membrane, the zeta potential was measured by the tangential flow mode. As seen in Fig. 3, the surface of PES membrane has a negative charge over the studied pH range from 4.0 to 9.0. The absolute zeta potential of PES membrane was increased at higher pH value. Because PES membrane does not contain anionic groups, the dissociation of surface functional groups would not be the source of negative charge on PES surface [14]. Zimmermann et al. has revealed that the origin of interfacial charge at polymer films was caused by preferential adsorption of electrolyte ions in aqueous environments [15]. Negative zeta potential of polymer membrane was caused presumably by the adsorption of anions, especially preferential adsorption of hydroxide ions. The adsorbed hydroxide ions on PES membrane create the negatively charged surface for PES membrane [14]. The absolute zeta potential of PES membrane is decreased towards acidic pH values due to protonation of the adsorbed hydroxide ions on PES membrane. 3.1.1. Effect of pH value PES membrane can adsorb readily MB molecules from aqueous solution. It was found that white PES membrane was changed into blue color within several minutes when PES membrane was immersed into MB solution. The amount of adsorbed MB on PES
Fig. 3. Zeta potential of PES membrane and PES membrane after MB adsorption was plotted as a function of pH value.
membrane at different solution pH values was given in Fig. 4, which was increased rapidly in the initial immersion stage and then increased slowly in the sequential immersion stage. PES membrane has negatively charged surface, the positively charged MB molecules can be adsorbed easily on PES membrane because of the existence of favorable electrostatic attractions. There was an obvious influence of pH value on the MB adsorption on PES membrane. The amount of adsorbed MB was higher at pH value of 9.0 than that at pH of 6.0 and 4.0 at the same immersion time. The surface charge on PES membrane is lower at acidic pH value according to zeta potential measurement, so that the electrostatic attractions between PES membrane and MB molecules are weakened. The available adsorption site (adsorbed hydroxide ions on PES membrane) is decreased at acidic pH value due to protonation, the lower amount of adsorbed MB was observed at pH value of 4.0. The zeta potential of PES membrane after MB adsorption was also measured and given in Fig. 3. The adsorption of MB dramatically decreased the absolute zeta potential of PES membrane. The adsorbed MB molecules neutralized the negative charge on PES membrane, only slight negative zeta potential on PES membrane after MB adsorption. The results of MB adsorption and zeta potential measurement suggested that the electrostatic interactions play a very important role in MB adsorption on PES membrane. 3.1.2. Effect of ionic strength The electrostatic interactions can be screened by the addition of electrolyte, the amount of adsorbed MB on PES membrane should be decreased with an increased of salt concentration. The effect of NaCl concentration on the amount of adsorbed MB on PES membrane was given in Fig. 5. The amount of adsorbed MB was increased rapidly in the initial immersion stage, and then the amount of adsorbed MB was increased slowly in the sequential immersion stage. The amount of adsorbed MB was decreased dramatically with an increase of NaCl concentration in aqueous solution at the same immersion time. The influence of NaCl concentration on the MB adsorption on PES membrane clearly indicated that electrostatic interactions might be the important driving force of MB adsorption. 3.1.3. Formation of surface complex It was also noted in Figs. 4 and 5, there were still part of MB molecules adsorbed on PES membrane at solution pH value of 4.0 and at higher ionic strength of 0.10 mol/L NaCl solution. MB is cationic dye and possesses positive charge in a wide pH range. The electrostatic interactions certainly play a very important role
Fig. 4. The effect of pH value on the amount of adsorbed methylene blue on PES membrane.
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Fig. 5. The effect of NaCl concentration on the amount of adsorbed methylene blue on PES membrane at pH value of 6.0.
in MB adsorption on PES membrane. However, taking into account of many functional groups in MB molecule and on the surface of PES membrane, other interactions, such as hydrogen bonds, ionic bonds, even hydrophobic interactions, may also take part in the MB adsorption on PES membrane. Deng and Bai have proved that both electrostatic interactions and surface complexation mechanisms played important roles in humic acid adsorption on the aminated polyacrylonitrile fibers [16]. A probable mechanism for the adsorption of MB molecules on PES membrane was put forward. The first adsorption of hydroxide ions creates negatively charged PES surface and provide available adsorption sites, the favorable electrostatic force promotes positively charged MB molecules to approach and adsorb on PES membrane surface, at last, the surface complex is formed on PES membrane surface through ionic bonds, hydrogen bonds, and hydrophobic interactions. There are more MB molecules adsorbed on PES membrane at pH value of 9.0, however, most of adsorbed MB molecules can be desorbed at pH value of 4.0. This means that the surface complex, came from MB adsorption on the sites of adsorbed hydroxide ions, is not very steady and can be destroyed by decreasing pH value. The controlled adsorption and desorption of MB molecules on PES membrane can be easily achieved through adjustment of solution pH. This idea was used in the adsorption and recovery of MB from aqueous solution through ultrafiltration technique.
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Fig. 6. The rejection ratios of MB at different pH value were plotted as a function of ultrafiltration operation time. The operation press is 0.04 MPa and the flow was 50 L/(m2 h).
than the pore size of the self-made PES membrane, it was expected that the phenomenon of no rejection of MB phenomenon should take place. However, this assumption was not consistent with the experimental results. During the ultrafiltration experiments, the permeate solutions were collected at certain time intervals to determine MB concentrations. Figs. 6–8 show that MB rejection ratios under different conditions were plotted as a function of operation time. The rejection ratio of MB is almost 100% in the initial operation stage, which indicated that there were few MB molecules that can transport through PES membrane. It is known that convective mass transport is rapid than diffusion. In the common adsorption, the process can be limited by diffusion into the pores of adsorbent, but in the membrane process, convection mass transport through membrane pores may overcome this problem [18]. The convection mass transport through membrane pores allows very rapid adsorption and separation process [19–21]. The probable reason for the complete rejection of MB in the initial ultrafiltration stage is the convection mass transport through the pores of PES membrane and the easy adsorption of MB molecules on the surface and pores of PES membrane. This phenomenon was similar to that the removal of taste and odor model compound by tight ultrafiltration membranes [20].
3.2. Rejection of MB during ultrafiltration processes Recently, pressure-driven membrane processes, such as reverse osmosis, nanofiltration, and ultrafiltration, have been considered for the treatment of dye wastewater from the industrial effluents [7–12]. The main criteria that describe the performance characteristics of membrane processes are the flux and rejection. Despite the fact that dyes are complete rejected, the flux of reverse osmosis is very low and the process is higher energy cost. Nanofiltration does not reach the retention behavior as reverse osmosis does, but its flux was found acceptable for water reuse. However, the major disadvantage of nanofiltration is the flux decline due to adsorption of organic compounds on the membrane surface [8]. Ultrafiltration operates at a lower pressure with a higher flux and lower energy cost. The pore size of the self-made PES membrane is in the range from 5.3 to 20.0 nm based on the measurement by the liquid porosimetry technique [13]. The molecular diameter of MB is 0.8 nm [17]. Since the molecular size of MB is much less
Fig. 7. The rejection ratios of MB at different NaCl concentration with pH value of 9.0 were plotted as a function of ultrafiltration operation time. The operation press is 0.04 MPa and the flux was 50 L/(m2 h).
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hydroxide ions, the chance for the forming of surface complex is correspondingly decreased. Most MB molecules can transport through the pores in PES membrane at higher ionic strength.
Fig. 8. The rejection ratios of MB at different flux with pH value of 9.0 were plotted as a function of ultrafiltration operation time.
3.2.1. Effect of pH value The pH values of MB solution were controlled at 9.0, 6.0, and 4.0 by the addition of dilute chloride acid or sodium hydroxide solutions; the added dye concentration of feed solution was 0.8 mg/L. The flux remained constant at 50 L/(m2 h) at different pH value in the whole operation. The influence of pH value on the rejection ratio of MB as a function of time was given in Fig. 6. At pH value of 6.0, we observed that the rejection ratio of MB was 100% in the initial operation stage from 0 to 10.7 min, then the retention ratio of MB was decreased gradually with an elapse of operation time, at last the retention ratio of MB was retained at about 23.6% after operation time of 44.1 min. The time range for the complete rejection of MB (rejection ratio is 100%) was decreased to 0 min at solution pH value of 4.0, and remarkably extended from 0 to 51.6 min at solution pH value of 9.0. The solution pH value has remarkable influence on the rejection of MB during ultrafiltration operation. The time range for the complete rejection at solution pH value of 9.0 was longer than that at solution pH of 6.0 and 4.0. There is a highly negatively charged surface and more available adsorption sites at pH value of 9.0. The strong electrostatic attractions promote the positively charged MB molecules to approach and adsorb on PES membrane surface, more adsorbed hydroxide ions on PES membrane can provide more available binding sites for MB adsorption, so that MB was completely rejected in the initial operation. The available adsorption sites for MB adsorption was gradually consumed and the electrostatic attractions were gradually weakened along with ultrafiltration process, so that the rejection ratio of MB was decreased in the sequential operation. 3.2.2. Effect of ionic strength The effect of ionic strength on the rejection of MB was also studied and the results were given in Fig. 7. The flux remained constant with time at different NaCl concentration. MB was completely rejected in the initial operation stage at different NaCl concentrations at solution pH value of 9.0. The time range for the complete rejection of MB (rejection ratio is 100%) was from 0 to 51.6 min at NaCl concentration of 0 mol/L, and from 0 to 7.0 min at NaCl concentration of 0.01 mol/L. There was no time range for the complete rejection of MB at NaCl concentration of 0.10 mol/L. The electrostatic attractions between negatively charged PES membrane and positively charged MB molecules are screened at higher ionic strength, so that the driving force for MB adsorption is decreased. On the other hand, an increase in the ionic strength increases the counter ion concentration around the adsorbed
3.2.3. Effect of press Ultrafiltration is a press driven membrane process; the flux of self-made PES membrane was increased with an increased press. The averaged fluxes were 50, 80 and 120 L/(m2 h) at presses of 0.04, 0.06 and 0.10 MPa, The rejection ratios of MB at different flows with pH value of 9.0 plotted as a function of time were given in Fig. 8. MB was completely rejected in the initial operation stage at different presses. The time range for the complete rejection of MB (rejection ratio is 100%) was from 0 to 51.6 min at flux of 50 L/(m2 h), from 0 to 37.0 min at flux of 80 L/(m2 h), and from 0 to 10.3 min at flux of 120 L/(m2 h). The higher flux would bring more MB molecules to PES membrane surface, the available adsorption sites for MB adsorption was quickly consumed, so that the time range for complete rejection of MB was decreased with an increase of operation press. 3.3. Adsorption and recovery of MB from aqueous solution MB molecules can be adsorbed and form surface complex on PES membrane. The surface complex can be destroyed and the adsorbed MB can be easily desorbed from PES membrane by the decrease of solution pH. A technique based on adsorption and ultrafiltration was developed for the recovery of MB from aqueous solution. The experimental set-up was first run at pH value of 9.0, most MB molecules are completely rejected and adsorbed on PES membrane, then the adsorbed MB molecules was desorbed through adjustment solution pH to 4.0, the permeate solution was collected for MB recovery. The results of adsorption and recovery of MB through ultrafiltration technique with PES membrane from aqueous solution were given in Fig. 9. After fourth run, higher rejection of MB (MB rejection ratio is above 98.0%) was all remained in the adsorption operation stage. The process of desorption of MB from PES membrane at pH value of 4.0 and recovery of MB from permeate solution was quick. MB concentration in the permeate solution of the recovery stage was about 30 times than that of feed solution of the adsorption stage. If the hollow fiber membrane module,
Fig. 9. Adsorption and recovery of MB from aqueous solution through ultrafiltration technique, MB concentrations in permeate solution were plotted as a function of ultrafiltration operation time. In the adsorption stage, ultrafiltration was operated with 0.8 mg/L MB feed solution at pH value of 9.0. In the recovery stage, the cell was emptied and refilled with water free of MB at pH value of 4.0. The operation press is 0.04 MPa and the flux was 50 L/(m2 h).
L. Zheng et al. / Separation and Purification Technology 68 (2009) 244–249
which has a high packing density, is used in the adsorption and ultrafiltrtation technique, a greater treatment capacity and a higher MB concentration in recovered solution should be obtained. The adsorption and recovery of dyes from aqueous solution through ultrafiltration technique have potential application in wastewater treatment. 4. Conclusions The electrostatic attractions between the positively charged dye and negatively charged PES membrane surface play an important role in the MB adsorption. The adsorption of hydroxide ions creates negatively charged PES surface and provides available adsorption sites, the favorable electrostatic force promotes positively charged MB molecules to approach to PES membrane, adsorb and form surface complex on PES membrane surface. The adsorption and recovery of MB from aqueous solution through ultrafiltration technique is facile through adjustment of solution pH value. The probable reason for the complete rejection of MB in the initial ultrafiltration stage is the convection mass transport through the pores of PES membrane and the easy adsorption of MB molecules on the surface and pores of PES membrane. Acknowledgments This research was funded by Tianjin Natural Science Foundation (No. 07JCYBJC00900), the Program of Introducing Talents of Discipline to Universities (No. B06006), and Doctoral Fund of Ministry of Education of China for New Teachers (No. 20070056041). References [1] E.N.E. Qada, S.J. Allen, G.M. Walker, Adsorption of basic dyes onto activated carbon using microcolumns, Ind. Eng. Chem. Res. 45 (2006) 6044–6049. [2] Q.H. Hu, S.Z. Qiao, F. Haghseresht, M.A. Wilson, G.Q. Lu, Adsorption study for removal of basic dye using bentonite, Ind. Eng. Chem. Res. 45 (2006) 733–738. [3] M. Srinivasan, T. White, Degradation of methylene blue by three-dimensionally ordered macroporous titania, Environ. Sci. Technol. 41 (2007) 4405–4409.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Adsorption and recovery of methylene blue from aqueous solution through ultrafiltration technique Lili Zheng, Yanlei Su ∗ , Lijun Wang, Zhongyi Jiang Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 2 May 2009 Accepted 12 May 2009 Keywords: Methylene blue Polyethersulfone membrane Adsorption Recovery
a b s t r a c t Adsorption and recovery of methylene blue (MB) from aqueous solution through ultrafiltration technique with polyethersulfone (PES) membrane was developed. Cationic dye MB can be adsorbed by PES membrane from aqueous solution. The amount of adsorbed MB on PES membrane was higher at the condition of basic solution and lower ionic strength. The electrostatic interaction may be the important driving force of MB adsorption. During ultrafiltration, the rejection ratio of MB can reach 100% in the initial operation stage due to convection mass transport through membrane pores and adsorption of MB on the surface and pores of PES membrane. The final recovery of MB in the permeate solution was easily achieved by decreasing the solution pH to desorb MB. The adsorption and recovery of dyes from aqueous solution through ultrafiltration technique have potential application in wastewater treatment. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Textile, paper, carpet, leather, and printing industries generate large volumes of wastewater polluted with various dyes. Due to the toxic nature of most dyes to aquatic life, colored wastewater cannot be discharged without adequate treatment. Many methods are available for the removal of dyes from industrial effluents, the most important of which are biodegradation, flocculation–coagulation, chemical oxidation, adsorption, and membrane separation [1–8]. Since the low biodegradability of dyes due to the complex aromatic molecular structure, conventional biological wastewater treatment processes are not very efficient for the treatment of dyeing waste. The flocculation treatment produces a large amount of sludge, which causes disposal problem, thus increasing the operation cost. After chemical oxidation of dyes, some toxic byproducts are still hazardous to the environment. Adsorption is a comparatively cheap process and effective in the removal of dyes, but adsorbent with large specific surface area, high adsorption capacity, and special surface reactivity must be carefully seek, and the subsequent treatment for the desorption of dyes and the regeneration of adsorbent also must be considered. Ultrafiltration is a pressure-driven membrane process for separating dissolved molecules in solution on the basis of molecular size. The molecules larger than the membrane pore size are retained in the feed solution. Ultrafiltration operates at a lower pressure with
∗ Corresponding author. Fax: +86 22 27890882. E-mail address: [email protected] (Y. Su). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.05.010
a higher flux, lower energy cost, and easy use, but it has a lower dye rejection ratio, since the pore size in the skin layer of ultrafiltration membrane is usually larger that the size of dye molecules. Micellar enhanced ultrafiltration is one of the feasible methods to remove organic dyes from wastewater [9–11]. In this process, surfactant with higher concentration than the critical micelle concentration is added to a polluted aqueous solution. The surfactant molecules form micelles, which can solubilize the organic dyes. The micelles containing the dissolved solutes are rejected during ultrafiltration process by the membrane and the permeated stream passing through the membrane is nearly free from dyes. However, little attention has been given to solve the difficulty for the separation and recovery of dyes from the added surfactant. A combined technique based on ultrafiltration and adsorption is alternative process for dye wastewater treatment. Banat and Al-Bastaki reported that the combination of activated carbon with ultrafiltration simultaneously exploits the high adsorption capability of activated carbon and the particle removal ability of ultrafiltration, the combined process achieved better rejection of dyes than that by ultrafiltration process alone [12]. However, the subsequent treatments, such as desorption of dyes from activated carbon and regeneration of adsorbent, have not reported in their work. The recovery of adsorbed dye from activated carbon usually needs organic solvent and regeneration of activated carbon usually needs thermal treatment. It is significant to develop a simple and effective method for the adsorption and recovery of dyes through ultrafiltration technique. In the present study, a facile method for the adsorption and recovery of dyes from aqueous solution through ultrafiltration process was developed. Polyethersulfone (PES) membrane has
L. Zheng et al. / Separation and Purification Technology 68 (2009) 244–249
negatively charge surface under specified condition, which can combine with the positively charged dye molecules because of the electrostatic attractions. PES membrane was operated in filtration mode and methylene blue (MB) was selected as a model compound to evaluate the efficiency of the adsorption and recovery of dye from aqueous solution through ultrafiltration technique. The effects of solution pH, ionic strength, and operation pressure on the adsorption and rejection of MB were investigated. The recovery of adsorbed MB from PES membrane was easily achieved through decreasing pH value of feed solution.
2. Experimental
2.5. Ultrafiltration experiments A dead-end stirred cell (Model 8200, Millipore Co., USA. The volume capacity is 200 mL and the effective area of membrane is 28.7 cm2 ) filtration system connected with a N2 gas cylinder and solution reservoir was designed to evaluate MB rejection and recovery from aqueous solution through ultrafiltration technique. The operation pressure in the system was maintained by nitrogen gas. PES membrane was first compacted with deionized water at 0.15 MPa for about 30 min. The cell was then emptied and refilled with 0.8 mg/L MB solution immediately; the pressure was lowered to a given operating pressure (most operating press is 0.04 MPa except in the experiment of different flux through adjusting press). The flux (J) is calculated by Eq. (1):
2.1. Materials PES 6020P (BASF Co., Germany) was dried at 110 ◦ C for 12 h prior to use. Methylene blue, N,N-dimethyl formamide (DMF), and poly(ethylene glycol) (PEG2000, the molecular weight is 2000) was purchased from Kewei Chemical Reagent Co. (Tianjin, China). Other reagents were all of analytical grade and used without further purification. Water used in all experiments was the deionized water.
Jw1 =
PES membrane was fabricated through a simple aqueous-based immersion precipitation method. 5.0 g PES (membrane material) and 4.0 g PEG2000 (pore-forming agent) were added into 18.6 g DMF to prepare casting solution. The solution was stirred for 4 h at temperature of 60 ◦ C to ensure a complete dissolution of the polymers. After the bubbles were released completely, the solution was cast on glass plates using a stainless steel knife, and then the glass plates were immersed in a coagulation bath of deionized water. The self-made membrane with a wet thickness of about 240 m was peeled off and subsequently rinsed with water to remove the residual solvent and pore-forming agent. PES membrane was kept in deionized water prior to use.
2.3. Membrane characterization The cross-sectional morphology of PES membrane was observed by scanning electron microscopy (SEM) using a Philips XL30E scanning microscope. The membrane was frozen in liquid nitrogen, broken, and sputtered with gold prior to SEM observation. The surface charge of PES membrane was measured via tangential flow streaming potential measurement. The streaming potentials of membranes were measured using a 0.001 mol/L KCl solution in the pH range from 4 to 9 at a room temperature of 25 ◦ C. The zeta potential () was calculated using the Helmholtz–Smoluchowski equation.
V At
(1)
where V (L) is the volume of permeated water, A (m2 ) is the membrane area, and t (h) is ultrafiltration operation time. The filtration efficiency in removal of MB from the feed solution was evaluated through the dye rejection ratio. The rejection ratio of MB is calculated by Eq. (2): R=
2.2. Membrane preparation
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1−
Cp Cf
× 100%
(2)
where Cp and Cf (mg/mL) are the MB concentrations of permeate and feed solutions, respectively. In the MB recovery experiments, ultrafiltration was first operated with MB solution at pH value of 9.0, then the operation was stopped and the cell was emptied. The cell was refilled with water free of MB at pH value of 4.0. The permeate solution passing through PES membrane was collected for MB recovery. 3. Results and discussion 3.1. Adsorption of MB on PES membrane Since PES has excellent temperature and pH stabilities, and mechanical strength, PES is one of widely used membrane materials. PES membranes have been applied in wastewater treatment, reverse osmosis pretreatment, separation and purification of proteins in food industry and biotechnology [13,14]. MB is a thiazine cationic dye, which has wide applications in coloring paper, dyeing cottons and wools. MB is also used as an oxidation–reduction indicator and in the trace analysis of anionic surfactant. The molecular structures of PES and MB were shown in Fig. 1. The cross-sectional morphology of self-made PES membrane was shown in Fig. 2. PES
2.4. Adsorption experiments PES membrane was cut into a round shape with external surface of 28.7 cm2 (the weight of dried membrane is 0.16 g) and immersed into glass vial containing 30 mL of 4.0 mg/L MB solution at a given pH value and ionic strength. The pH value of MB solution was adjusted with HCl or NaOH solutions, while the ionic strength was adjusted with NaCl. The concentration of MB in the solution was measured with a UV–vis spectrophotometer (Hitach UV-2800, Japan) at the maximum absorbance at 660 nm after a certain immersion time, the amount of adsorbed MB on PES membrane was then calculated.
Fig. 1. The molecular structure of poly(polyethersulfone) and methylene blue.
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Fig. 2. SEM image of cross-sectional morphology of self-made PES membrane.
membrane exhibited a typical asymmetric structure with a top dense layer, a porous sublayer, and fully developed macropores at the bottom. For evaluation of the surface charge of PES membrane, the zeta potential was measured by the tangential flow mode. As seen in Fig. 3, the surface of PES membrane has a negative charge over the studied pH range from 4.0 to 9.0. The absolute zeta potential of PES membrane was increased at higher pH value. Because PES membrane does not contain anionic groups, the dissociation of surface functional groups would not be the source of negative charge on PES surface [14]. Zimmermann et al. has revealed that the origin of interfacial charge at polymer films was caused by preferential adsorption of electrolyte ions in aqueous environments [15]. Negative zeta potential of polymer membrane was caused presumably by the adsorption of anions, especially preferential adsorption of hydroxide ions. The adsorbed hydroxide ions on PES membrane create the negatively charged surface for PES membrane [14]. The absolute zeta potential of PES membrane is decreased towards acidic pH values due to protonation of the adsorbed hydroxide ions on PES membrane. 3.1.1. Effect of pH value PES membrane can adsorb readily MB molecules from aqueous solution. It was found that white PES membrane was changed into blue color within several minutes when PES membrane was immersed into MB solution. The amount of adsorbed MB on PES
Fig. 3. Zeta potential of PES membrane and PES membrane after MB adsorption was plotted as a function of pH value.
membrane at different solution pH values was given in Fig. 4, which was increased rapidly in the initial immersion stage and then increased slowly in the sequential immersion stage. PES membrane has negatively charged surface, the positively charged MB molecules can be adsorbed easily on PES membrane because of the existence of favorable electrostatic attractions. There was an obvious influence of pH value on the MB adsorption on PES membrane. The amount of adsorbed MB was higher at pH value of 9.0 than that at pH of 6.0 and 4.0 at the same immersion time. The surface charge on PES membrane is lower at acidic pH value according to zeta potential measurement, so that the electrostatic attractions between PES membrane and MB molecules are weakened. The available adsorption site (adsorbed hydroxide ions on PES membrane) is decreased at acidic pH value due to protonation, the lower amount of adsorbed MB was observed at pH value of 4.0. The zeta potential of PES membrane after MB adsorption was also measured and given in Fig. 3. The adsorption of MB dramatically decreased the absolute zeta potential of PES membrane. The adsorbed MB molecules neutralized the negative charge on PES membrane, only slight negative zeta potential on PES membrane after MB adsorption. The results of MB adsorption and zeta potential measurement suggested that the electrostatic interactions play a very important role in MB adsorption on PES membrane. 3.1.2. Effect of ionic strength The electrostatic interactions can be screened by the addition of electrolyte, the amount of adsorbed MB on PES membrane should be decreased with an increased of salt concentration. The effect of NaCl concentration on the amount of adsorbed MB on PES membrane was given in Fig. 5. The amount of adsorbed MB was increased rapidly in the initial immersion stage, and then the amount of adsorbed MB was increased slowly in the sequential immersion stage. The amount of adsorbed MB was decreased dramatically with an increase of NaCl concentration in aqueous solution at the same immersion time. The influence of NaCl concentration on the MB adsorption on PES membrane clearly indicated that electrostatic interactions might be the important driving force of MB adsorption. 3.1.3. Formation of surface complex It was also noted in Figs. 4 and 5, there were still part of MB molecules adsorbed on PES membrane at solution pH value of 4.0 and at higher ionic strength of 0.10 mol/L NaCl solution. MB is cationic dye and possesses positive charge in a wide pH range. The electrostatic interactions certainly play a very important role
Fig. 4. The effect of pH value on the amount of adsorbed methylene blue on PES membrane.
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Fig. 5. The effect of NaCl concentration on the amount of adsorbed methylene blue on PES membrane at pH value of 6.0.
in MB adsorption on PES membrane. However, taking into account of many functional groups in MB molecule and on the surface of PES membrane, other interactions, such as hydrogen bonds, ionic bonds, even hydrophobic interactions, may also take part in the MB adsorption on PES membrane. Deng and Bai have proved that both electrostatic interactions and surface complexation mechanisms played important roles in humic acid adsorption on the aminated polyacrylonitrile fibers [16]. A probable mechanism for the adsorption of MB molecules on PES membrane was put forward. The first adsorption of hydroxide ions creates negatively charged PES surface and provide available adsorption sites, the favorable electrostatic force promotes positively charged MB molecules to approach and adsorb on PES membrane surface, at last, the surface complex is formed on PES membrane surface through ionic bonds, hydrogen bonds, and hydrophobic interactions. There are more MB molecules adsorbed on PES membrane at pH value of 9.0, however, most of adsorbed MB molecules can be desorbed at pH value of 4.0. This means that the surface complex, came from MB adsorption on the sites of adsorbed hydroxide ions, is not very steady and can be destroyed by decreasing pH value. The controlled adsorption and desorption of MB molecules on PES membrane can be easily achieved through adjustment of solution pH. This idea was used in the adsorption and recovery of MB from aqueous solution through ultrafiltration technique.
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Fig. 6. The rejection ratios of MB at different pH value were plotted as a function of ultrafiltration operation time. The operation press is 0.04 MPa and the flow was 50 L/(m2 h).
than the pore size of the self-made PES membrane, it was expected that the phenomenon of no rejection of MB phenomenon should take place. However, this assumption was not consistent with the experimental results. During the ultrafiltration experiments, the permeate solutions were collected at certain time intervals to determine MB concentrations. Figs. 6–8 show that MB rejection ratios under different conditions were plotted as a function of operation time. The rejection ratio of MB is almost 100% in the initial operation stage, which indicated that there were few MB molecules that can transport through PES membrane. It is known that convective mass transport is rapid than diffusion. In the common adsorption, the process can be limited by diffusion into the pores of adsorbent, but in the membrane process, convection mass transport through membrane pores may overcome this problem [18]. The convection mass transport through membrane pores allows very rapid adsorption and separation process [19–21]. The probable reason for the complete rejection of MB in the initial ultrafiltration stage is the convection mass transport through the pores of PES membrane and the easy adsorption of MB molecules on the surface and pores of PES membrane. This phenomenon was similar to that the removal of taste and odor model compound by tight ultrafiltration membranes [20].
3.2. Rejection of MB during ultrafiltration processes Recently, pressure-driven membrane processes, such as reverse osmosis, nanofiltration, and ultrafiltration, have been considered for the treatment of dye wastewater from the industrial effluents [7–12]. The main criteria that describe the performance characteristics of membrane processes are the flux and rejection. Despite the fact that dyes are complete rejected, the flux of reverse osmosis is very low and the process is higher energy cost. Nanofiltration does not reach the retention behavior as reverse osmosis does, but its flux was found acceptable for water reuse. However, the major disadvantage of nanofiltration is the flux decline due to adsorption of organic compounds on the membrane surface [8]. Ultrafiltration operates at a lower pressure with a higher flux and lower energy cost. The pore size of the self-made PES membrane is in the range from 5.3 to 20.0 nm based on the measurement by the liquid porosimetry technique [13]. The molecular diameter of MB is 0.8 nm [17]. Since the molecular size of MB is much less
Fig. 7. The rejection ratios of MB at different NaCl concentration with pH value of 9.0 were plotted as a function of ultrafiltration operation time. The operation press is 0.04 MPa and the flux was 50 L/(m2 h).
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hydroxide ions, the chance for the forming of surface complex is correspondingly decreased. Most MB molecules can transport through the pores in PES membrane at higher ionic strength.
Fig. 8. The rejection ratios of MB at different flux with pH value of 9.0 were plotted as a function of ultrafiltration operation time.
3.2.1. Effect of pH value The pH values of MB solution were controlled at 9.0, 6.0, and 4.0 by the addition of dilute chloride acid or sodium hydroxide solutions; the added dye concentration of feed solution was 0.8 mg/L. The flux remained constant at 50 L/(m2 h) at different pH value in the whole operation. The influence of pH value on the rejection ratio of MB as a function of time was given in Fig. 6. At pH value of 6.0, we observed that the rejection ratio of MB was 100% in the initial operation stage from 0 to 10.7 min, then the retention ratio of MB was decreased gradually with an elapse of operation time, at last the retention ratio of MB was retained at about 23.6% after operation time of 44.1 min. The time range for the complete rejection of MB (rejection ratio is 100%) was decreased to 0 min at solution pH value of 4.0, and remarkably extended from 0 to 51.6 min at solution pH value of 9.0. The solution pH value has remarkable influence on the rejection of MB during ultrafiltration operation. The time range for the complete rejection at solution pH value of 9.0 was longer than that at solution pH of 6.0 and 4.0. There is a highly negatively charged surface and more available adsorption sites at pH value of 9.0. The strong electrostatic attractions promote the positively charged MB molecules to approach and adsorb on PES membrane surface, more adsorbed hydroxide ions on PES membrane can provide more available binding sites for MB adsorption, so that MB was completely rejected in the initial operation. The available adsorption sites for MB adsorption was gradually consumed and the electrostatic attractions were gradually weakened along with ultrafiltration process, so that the rejection ratio of MB was decreased in the sequential operation. 3.2.2. Effect of ionic strength The effect of ionic strength on the rejection of MB was also studied and the results were given in Fig. 7. The flux remained constant with time at different NaCl concentration. MB was completely rejected in the initial operation stage at different NaCl concentrations at solution pH value of 9.0. The time range for the complete rejection of MB (rejection ratio is 100%) was from 0 to 51.6 min at NaCl concentration of 0 mol/L, and from 0 to 7.0 min at NaCl concentration of 0.01 mol/L. There was no time range for the complete rejection of MB at NaCl concentration of 0.10 mol/L. The electrostatic attractions between negatively charged PES membrane and positively charged MB molecules are screened at higher ionic strength, so that the driving force for MB adsorption is decreased. On the other hand, an increase in the ionic strength increases the counter ion concentration around the adsorbed
3.2.3. Effect of press Ultrafiltration is a press driven membrane process; the flux of self-made PES membrane was increased with an increased press. The averaged fluxes were 50, 80 and 120 L/(m2 h) at presses of 0.04, 0.06 and 0.10 MPa, The rejection ratios of MB at different flows with pH value of 9.0 plotted as a function of time were given in Fig. 8. MB was completely rejected in the initial operation stage at different presses. The time range for the complete rejection of MB (rejection ratio is 100%) was from 0 to 51.6 min at flux of 50 L/(m2 h), from 0 to 37.0 min at flux of 80 L/(m2 h), and from 0 to 10.3 min at flux of 120 L/(m2 h). The higher flux would bring more MB molecules to PES membrane surface, the available adsorption sites for MB adsorption was quickly consumed, so that the time range for complete rejection of MB was decreased with an increase of operation press. 3.3. Adsorption and recovery of MB from aqueous solution MB molecules can be adsorbed and form surface complex on PES membrane. The surface complex can be destroyed and the adsorbed MB can be easily desorbed from PES membrane by the decrease of solution pH. A technique based on adsorption and ultrafiltration was developed for the recovery of MB from aqueous solution. The experimental set-up was first run at pH value of 9.0, most MB molecules are completely rejected and adsorbed on PES membrane, then the adsorbed MB molecules was desorbed through adjustment solution pH to 4.0, the permeate solution was collected for MB recovery. The results of adsorption and recovery of MB through ultrafiltration technique with PES membrane from aqueous solution were given in Fig. 9. After fourth run, higher rejection of MB (MB rejection ratio is above 98.0%) was all remained in the adsorption operation stage. The process of desorption of MB from PES membrane at pH value of 4.0 and recovery of MB from permeate solution was quick. MB concentration in the permeate solution of the recovery stage was about 30 times than that of feed solution of the adsorption stage. If the hollow fiber membrane module,
Fig. 9. Adsorption and recovery of MB from aqueous solution through ultrafiltration technique, MB concentrations in permeate solution were plotted as a function of ultrafiltration operation time. In the adsorption stage, ultrafiltration was operated with 0.8 mg/L MB feed solution at pH value of 9.0. In the recovery stage, the cell was emptied and refilled with water free of MB at pH value of 4.0. The operation press is 0.04 MPa and the flux was 50 L/(m2 h).
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which has a high packing density, is used in the adsorption and ultrafiltrtation technique, a greater treatment capacity and a higher MB concentration in recovered solution should be obtained. The adsorption and recovery of dyes from aqueous solution through ultrafiltration technique have potential application in wastewater treatment. 4. Conclusions The electrostatic attractions between the positively charged dye and negatively charged PES membrane surface play an important role in the MB adsorption. The adsorption of hydroxide ions creates negatively charged PES surface and provides available adsorption sites, the favorable electrostatic force promotes positively charged MB molecules to approach to PES membrane, adsorb and form surface complex on PES membrane surface. The adsorption and recovery of MB from aqueous solution through ultrafiltration technique is facile through adjustment of solution pH value. The probable reason for the complete rejection of MB in the initial ultrafiltration stage is the convection mass transport through the pores of PES membrane and the easy adsorption of MB molecules on the surface and pores of PES membrane. Acknowledgments This research was funded by Tianjin Natural Science Foundation (No. 07JCYBJC00900), the Program of Introducing Talents of Discipline to Universities (No. B06006), and Doctoral Fund of Ministry of Education of China for New Teachers (No. 20070056041). References [1] E.N.E. Qada, S.J. Allen, G.M. Walker, Adsorption of basic dyes onto activated carbon using microcolumns, Ind. Eng. Chem. Res. 45 (2006) 6044–6049. [2] Q.H. Hu, S.Z. Qiao, F. Haghseresht, M.A. Wilson, G.Q. Lu, Adsorption study for removal of basic dye using bentonite, Ind. Eng. Chem. Res. 45 (2006) 733–738. [3] M. Srinivasan, T. White, Degradation of methylene blue by three-dimensionally ordered macroporous titania, Environ. Sci. Technol. 41 (2007) 4405–4409.
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