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Separation of stable oil–water emulsion by the hydrophilic nano-sized ZrO2 modified Al2O3 microfiltration membrane
Separation and Purification Technology 75 (2010) 243–248
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Separation of stable oil–water emulsion by the hydrophilic nano-sized ZrO2 modified Al2 O3 microfiltration membrane Jian-er Zhou a , Qibing Chang a,b,∗ , Yongqing Wang a , Jinming Wang a , Guangyao Meng b a b
Jingdezhen Ceramic Institute, Jingdezhen 333001, China University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 13 July 2009 Received in revised form 2 August 2010 Accepted 9 August 2010 Keywords: Membrane fouling Nano-sized ZrO2 Modification Ceramic membrane Emulsion separation
a b s t r a c t Membrane fouling weakens seriously the membrane’s separating performance, which can be reduced by the membrane surface modification. Commercial Al2 O3 microfiltration membranes were modified by the nano-sized ZrO2 coating to reduce the membrane fouling by oil droplets and to keep the unique character of the inorganic membrane. The nano-sized ZrO2 coating is prepared by in situ hydrolysis of ZrCl4 . The results show that the nano-coating does not form a separating layer, and just makes the ceramic membrane more hydrophilic. Using the stable 1 g/L 20# engine oil–water emulsion as feed, the steady flux of the modified membrane is obtained in a very short time. The steady flux keeps 88% of the initial flux and the oil rejection is above 97.8%. The hydrophilic nano-sized coating, the backflushing and the appropriate cross-flow velocity contribute to reducing the membrane fouling by oil droplets. The modified membrane has the good performance in the oil–water separation of stable emulsions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Oily wastewater should be treated before being discharged due to its ecological hazard [1]. The traditional methods to treat the unstable oil/water emulsion include gravity settling (API separator), skimming, dissolved air flotation, coalescence and centrifuging, etc. [2,3]. However, the oil emulsion is so stable that it is hardly treated by the traditional methods if the drop sizes of oil droplets are less than 20 m. In the developing methods, membrane technology shows a good perspective in the separation of oil emulsions, as it is of high efficiency and quite economical [4]. However, oil droplets stick easily on the membrane surface resulting in the formation of the cake layer. More seriously, the oil droplets can be squeezed into and through the membrane channels even if the size of oil droplets is far larger than the pores diameter of the membrane under the high applied pressure [1,5]. It means that the membrane fouling is generated. Membrane fouling degrades the membrane filtration performance and ultimately shortens the membrane life. To avoid this, optimizing the operation condition and changing membrane surface character by modification are the dominating methods. Comparably, modification is
∗ Corresponding author at: Jingdezhen Ceramic Institute, Taoyang Road, Jingdezhen 333001, Jiangxi Province, China. Tel.: +86 798 8499162; fax: +86 798 8494973. E-mail address: [email protected] (Q. Chang). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.08.008
more effective than the optimization of the operation condition because the increase of the repulsing force between the fouling and the membrane surface reduces the formation chance of the cake. Ochoa et al. [6] found that hydrophilic poly(vinylidene fluoride) (PVDF) membrane modified with poly(methyl methacrylate) (PMMA) had a low fouling to treat the oil/water emulsion. Ceramic membranes offer some unique advantages in the application of oil–water separation, such as good mechanical resistance, chemical inactivity, non-swelling and thermal stability. More importantly, the oil sticking on ceramic membrane can be removed by thermal treatment to recover the membrane performance. Ceramic membranes are usually hydrophilic due to the surface hydroxyl (–OH) groups [7]. However, the hydrophilic character is weakened due to the high-temperature calcination during the ceramic membrane preparation processes. To increase the membrane surface’s hydrophilic character, ceramic membrane can be modified by polymers [8–10]. However, polymer weakens the unique performance of ceramic membrane. Our group had researched the modification of ceramic membrane with nanometal oxide coating 10 years ago by homogenous precipitation and sol–gel method [11–13]. We found that the nano-metal oxide particles could effectively increase the ceramic membrane’s performance. Li and coworkers [14] and Yan and coworkers [15] found that the nano-sized alumina particles improved the anti-fouling performance of poly(vinylidene fluoride) membrane, and the flux recovery ratio of modified membranes reached 100% after washing. Yang et al. [16] found that the 2 wt% TiO2 –PSF UF composite membranes held the excellent water permeability, the hydrophilicity,
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Fig. 1. Flow chart of the preparation of the modified membrane by nano-zirconia coating.
the mechanical strength and the good anti-fouling ability without changing retentions. Several studies of zirconia membrane have indicated its superior performance in the separation of emulsion due to its special surface character [17–21], such as the high surface density and the strong polar. The oil droplet hardly adheres on the membrane surface. As zirconia is expensive, zirconia separating layer or modifying coating usually are fabricated on alumina support to obtain zirconia membrane. Del Colle et al. [20] impregnated the ceramic membrane with zirconium citrate solution to obtain zirconia nanoparticles distributed throughout the membrane surface. However, the zirconia nanoparticles agglomerate, which maybe weaken the permeate performance of the modified membrane. In the present work, the Al2 O3 microfiltration membrane (MF) was modified with nano-sized ZrO2 coating prepared through in situ hydrolysis of zirconium tetrachloride (ZrCl4 ). The nano-sized zirconia coating and the anti-fouling of the modified membrane were characterized.
2. Experimental 2.1. Preparation of modified membrane The used tubular Al2 O3 microfiltration membranes, commercially available (Nanjing Jiusi High-Tech Co. Ltd.), have a configuration of 7-channels. The other parameters are as follows: the outer diameter is 31 mm, the inter-diameter is 6 mm, the length is 250 mm, the porosity is 40%, and the mean pore diameter is 0.2 m. ZrCl4 (chemical pure grade) and absolute alcohol (reagent pure grade) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. 24.00 g of ZrCl4 was dissolved into 500 mL of the absolute alcohol. The as-prepared solution was used to modify Al2 O3 microfiltration membranes. The Al2 O3 membranes were firstly dried at 110 ◦ C for 12 h in oven, and then were saturated with the as-prepared ZrCl4 alcohol solution for 24 h at room temperature. After washed by absolute alcohol three times, the wet Al2 O3 membranes were dried at 60 ◦ C for 4 h again. Then, the Al2 O3 membranes were placed above the boiling water and bathed by hot water stream for 6 h. The membranes were directly calcined at 600 ◦ C for 2 h at heating rate of 1 ◦ C/min. The flow chart is shown in Fig. 1.
The modified Al2 O3 membrane was cut, and then its crosssection was observed by scanning electron microscopic (SEM) (JSM-6700F, JEOL). The energy dispersive spectrums (EDS) of the cross-section were also measured synchronously. The samples for TEM observation were prepared by mashing (not triturating) the modified membrane in the mortar until the particles were suitable for TEM observation. The particles coated by zirconia coating are observed on a JEOL-2010 electronic microscope. 2.3. Cross-flow filtration of modified membrane The oil–water emulsion consisted of 20# engine oil (1 g/L), Tween 80 (0.5 g/L), Span 80 (0.5 g/L) and distilled water. The oil and the surfactants were added to the distilled water. The emulsion was prepared by mixing at mild speed in the food blender (Fluko Equipment Shanghai Co., Ltd.) for 2 min. The stable oil–water emulsion has an average droplet size of 1.79 m and 90% of the oil droplets between the limit of 0.67 and 7.4 m, as measured by the Malvern Mastersizer Particle Size Analyzer (Nano ZS, Malvern). The similar procedures were reported in Ref. [22]. The water fluxes of the unmodified and modified membranes were tested in the filtration systems. Fig. 2 shows the schematic diagram of cross-flow microfiltration system. The stable emulsion was delivered from the feed tank by means of a centrifugal recirculation pump into the membrane module. The cross-flow velocity and trans-membrane pressure were adjusted by regulating the valves fitted at the inlet and outlet of membrane module. Both concentrate and filtrate should be sent back to the feed tank to maintain constant feed concentration during membrane operation in crossflow filtration mode. The fluxes were monitored by the rotometer recorded at an interval of 60 s. The trans-membrane pressure was 0.16 MPa and the cross-flow velocity was 5 m/s. For the filtration
2.2. Characterization of modified membrane To avoid the disturbance of the membrane pores on the measurement of contact angle, the nano-sized ZrO2 coating was prepared on a dense Al2 O3 discs. The preparation process was the same as that of the membrane. The contact angle was tested by the contact angle meter (SL-200B) by the sessile drop method.
Fig. 2. Schematic diagram of the cross-flow filtration system: 1, feed tank; 2, pump; 3, 3 valve; 4, 4 pressure gauge; 5, rotometer; 6, air compressor; 7, electromagnetic valve; 8, backflushing controlling panel; 9, membrane module.
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Fig. 3. SEM image and EDS spectra of the cross-section of the modified Al2 O3 membrane.
with different cross-flow velocity under backflushing conditions, the cross-flow velocities were 5 m/s, 3 m/s and 7 m/s in turn. The backflushing procedure was performed by using permeate water under high-pressured air in air compressor. The backflushing pressure was 0.6 MPa. The backflushing interval was 10 min, and the specified duration of backflushing was 5 s. The operating temperature was 30 ◦ C. The emulsion has the max adsorption at 220 nm. Therefore, The 220 nm UV was used for UV spectrometer (WFZ800-D3B, Beijing, China) to measure the oil concentration in feed and permeate. 3. Results and discussion 3.1. Characterization of nano-sized ZrO2 coating Fig. 3 shows the SEM image of the cross-section of the modified Al2 O3 membrane. The EDS spectra at points 1 and point 2 are also enclosed. The SEM image shows several Al2 O3 particles in the support. The alumina particles’ surfaces can be divided into two regions: fresh Al2 O3 cross-section area generated in the sample preparation process for SEM observation, and the surface covered with nano-sized ZrO2 coating, which are represented by point 1 and point 2, respectively. It is verified by the EDS spectra that the element composition at point 1 predominantly are Al and O, and at point 2 are Al, Zr and O. As can be seen, the nano-sized ZrO2 coating distributes uniformly on the alumina particle’s surface. The ZrO2 particles do not agglomerate at the sintering necks between the Al2 O3 particles. The coating has less influence on the pore structures. It can be explained that the coating is formed by not the deposition of zirconia grains but the adsorption of ZrCl4 forms. When the alumina membrane was soaked in the ZrCl4 alcohol solution, ZrCl4 chemically adsorbed on the alumina surface with which the solution could contact. The thickness of ZrCl4 depends on the adsorbing equilibrium of ZrCl4 on the alumina particles surface. Therefore, the adsorbed layer is isopachous, does not depend on the arrangement of Al2 O3 particles and follows the particle’ surface. After in situ hydrolysis and calcination, ZrCl4 transforms into the nano-ZrO2 coating without the morphological changes. The transformation can be denoted as: ZrCl4 + 9H2 O = ZrOCl2 ·8H2 O + 2HCl
ZrOCl2 · 8H2 O −→ ZrO2 + 2HCl + 7H2 O
gation of nano-sized zirconia grains, which is obviously different with the Al2 O3 grain. It implies that the numerous amounts of surface hydroxyl (–OH) groups exist in the coating, which makes the membrane more hydrophilic. Synchronously, the aggregation of zirconia grains increases the surface roughness of the membrane, comparing with the naked alumina grains. The apparent contact angle decrease with the surface roughness increases if the intrinsic contact angle of the material is less than 90◦ [23]. Fig. 5 shows the shape of a water droplet on the ZrO2 coating and the dense alumina disc, respectively. As can be seen, the apparent contact angle on the ZrO2 coating is 20◦ , which is far lower than that of a water droplet on dense, clean ZrO2 disc (r = 71.8◦ ) [24,25]. The contact angle is also lower than that of the dense alumina disc (r = 33◦ ), implying the modified membrane becomes more hydrophilic. The result is conflict with the reported by Del Colle et al. [20]. It maybe the nano-zirconia coating has the higher surface hydroxyl (–OH) groups than the sintered commercial zirconia membrane. 3.2. Performance of the modified microfiltration membrane Fig. 6 shows the flux declines of the virgin and the nano-sized ZrO2 modified Al2 O3 membrane. For the virgin Al2 O3 membrane, as can be seen, the flux declines sharply in the first 60 min, from 446 L/m2 h to 159 L/m2 h. The steady flux is only about 30% of the
(1) (2)
Therefore, the coating distributes uniformly on the alumina surface. However, it does not mean that the coating is uniform because it follows the surface morphology of the alumina particles. Fig. 4 shows the TEM image of a modified Al2 O3 particle spitted from the modified membrane. As can be seen, the porous ZrO2 coating with the thickness of about 100 nm is formed by the aggre-
Fig. 4. TEM image of modified Al2 O3 grain spitted from the modified membrane.
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Fig. 5. Shape of the water droplets on the dense Al2 O3 disc (A) and the ZrO2 coating (B).
initial flux, implying that the Al2 O3 membrane has a serious membrane fouling. However, the flux of the modified membrane quickly reaches a constant in 10 min. The flux declines from 506 L/m2 h to 441 L/m2 h. The steady flux keeps 88% of the initial flux. It could be explained that the hydrophilic nano-ZrO2 coating repulses the hydrophobic oil droplets. This result in: (I) the oil droplets are easily washed out by the feed with the appropriate cross-flow velocity and backflushing, even they have adhered on the membrane surface. The nano-coating prevents the oil droplets from adsorbing on the membrane surface. The cake layer could hardly be generated or its thickness hardly increases continuously; (II) the blocking of membrane pore is avoided because the hydrophilic membrane channels have the high capillary repulsing force to prevent the oil droplets from transformation. After modification, the membrane channels become thinner and more hydrophilic than the unmodified one. The repulsing force is high enough to repulse the oil droplet. The balance between the driving force by the permeate flow and the repulsing force by the nano-coating is easily reached. The steady flux is reached in a short time. The permeating flux of the modified membrane is higher than that of the unmodified one because the nano-coating reduces the thickness of the cake layer. As a whole, the nano-sized ZrO2 coating reduces remarkably the membrane fouling. It is well known that the decline of the permeating flux is caused by the concentration polarization, the cake layer or the pore blockage. Generally, the membrane fouling caused by concentration polarization is reversible and cake layer can be destroyed by the backflushing. However, the flux decline caused by pore block-
ing is hardly recovered by the backflushing and is regarded as the irreversible fouling. From this point, it is reasonable to define the maximum flux as the maximum flux at the time when the backflushing is finished in this paper. The maximum flux is obtained because the backflushing destroys the cake layer and recovery some membrane channels. Thus, the max flux reflects the degree of the destroyed cake layer in some degree. The declines of the max flux reflect the degree of the irreversible fouling because the backflushing cannot destroy the pore blockage. The steady flux is defined as the stable flux in the interval of backflushing. The steady flux reflects the degree of the total membrane fouling due to the cake layer and pore blockage. Fig. 7 shows the comparison of the steady fluxes of the modified membrane at different cross-flow velocities. The fluxes were measured using the same ceramic membrane. The cross-flow velocities were 5 m/s, 3 m/s and 7 m/s in turn. Therefore, the initial fluxes are different at each cross-flow velocity. Fig. 7 shows that both the initial and the steady fluxes at 3 m/s and 7 m/s are lower than those at 5 m/s. For the velocity is 3 m/s, the max flux almost keeps a constant, the steady flux declines slightly, and becomes unstable especially after 60 min. According to the definition, the max flux keeps the constant at 3 m/s, implying no oil droplet can be squeezed into the membrane channels due to the existence of the nano-zirconia coating. The function of the nanocoating is also embodied in the steady flux. The dragging force to make the oil droplet move to the membrane surface is lower than the repulse force of the nano-coating if the velocity is low. Thus, the cake layer is hardly formed when the original cake layer is
Fig. 6. Flux declines of the virgin and the nano-sized ZrO2 modified Al2 O3 membrane, cross-flow velocity is 5 m/s, backflushing pressure is 0.6 MPa, backflushing interval is 10 min, backflushing duration is 5 s, and operating temperature is 30 ◦ C.
Fig. 7. Fluxes of the modified membrane at different cross-flow velocity, backflushing pressure is 0.6 MPa, backflushing interval is 10 min, backflushing duration is 5 s, and operating temperature is 30 ◦ C.
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further increases, the cake layer is easily formed. Cake layer contributes to increasing the oil rejection as a “second filter layer” due to its dense structure. The oil concentration in the filtrate is lower again. According to the concentration of the feed, the oil rejections are 97.8%, 98.8% and 99.2%, respectively. It can be concluded that the modified microfiltration membrane can successfully separate the stable oil–water emulsion. 4. Conclusions The commercial Al2 O3 microfiltration membrane is successfully modified with nano-sized ZrO2 coating by in situ hydrolysis of ZrCl4 . The porous zirconia coating with the thickness of about 100 nm is formed on the membrane surface and the membrane channels, which makes the membrane is more hydrophilic. The nano-coating contribute to reducing the membrane fouling of oil droplets. The stable 1 g/L 20# engine oil–water emulsion is successfully separated by the modified Al2 O3 microfiltration membrane. Fig. 8. Oil concentration of filtrates obtained at different cross-flow velocities.
Acknowledgements destroyed by backflushing [26]. The flux decline shows that the flux cannot be steady in the interval of backflushing. Under this condition, concentration polarization is the major factor of the membrane fouling resulting in the lower flux [27]. The permeate flux at 7 m/s was measured following the flux at 3 m/s. The dragging force to make the oil droplet move to the membrane surface increases with the feed velocity. The cake layer is easily formed under this condition. Therefore, the steady flux is reached in a short time after the backflushing. At the same time, the driving force increases with the cross-flow velocity increases for the pressure-driven membrane, resulting in the increase of the flux. The steady flux increases with the filtrate time until the new steady flux is reached, implying the new balance of high dragging force generated by the permeating flux and the repulsing force of the nano-coating. Specially, the max flux increases gradually before 60 min, and then keeps a constant after 60 min. It is explained that the dragging force at the high cross-flow velocity compress the cake layer and make the cake layer denser. For the modified membrane, the hydrophilic nano-coating repulses the cake layer. The compressed cake layer as a body can be easily removed by the backflushing. More the membrane channels can be recovered when a mass of the cake layer are destroyed. Thus, the max flux increases. The compressed cake layer, as a “second filter layer” with the narrower pore size, prevents the oil droplets from permeating. The irreversible fouling can be avoided and the max flux shows a constant. On the whole, the high cross-flow velocity makes the oil droplets partially wet and stick on the membrane surface [28]. The compressed cake layer becomes the major factor resulting in the lower flux. Fig. 8 shows the oil concentration of filtrates obtained at different cross-flow velocities. As can be seen, the oil concentrations in the filtrate decrease when the cross-flow velocity is in the order of 5 m/s, 3 m/s and 7 m/s. As discussed above, no oil droplet can be squeezed into the membrane channels due to the existence of the nano-zirconia coating when the cross-flow velocity is 3 m/s. Only the oil droplets whose sizes are less than or equal to the max membrane pores penetrates and contaminates the filtrate because of low cross-flow velocity. The oil concentration is in the range of 9–13 mg/L. When the cross-flow velocity increases to 5 m/s, the cake layer still does not be formed or the thickness of the cake layer is small because the flux is high. The higher velocity makes the oil droplets can penetrate the membrane channels by transformation even their drop sizes are large than the membrane channels. The oil concentration increases remarkably. If the cross-flow velocity
The authors gratefully acknowledge the financial support provided by Sino-French International Science and Technology Cooperation Program (No. 2009DFB50490) and Education Department of Jiangxi Province (Nos. GJJ09651, GJJ09652, GJJ09653). References [1] B. Chakrabarty, A.K. Ghoshal, M.K. Purkait, Ultrafiltration of stable oil-in-water emulsion by polysulfone membrane, J. Membr. Sci. 325 (2008) 427–437. [2] F.L. Hua, Y.F. Tsang, Y.J. Wang, S.Y. Chan, H. Chua, S.N. Si, Performance study of ceramic microfiltration membrane for oily wastewater treatment, Chem. Eng. J. 128 (2007) 169–175. [3] J.M. Benito, G. Rios, E. Ortea, E. Fernhdez, A. Cambiella, C. Pazos, J. Coca, Design and construction of a modular pilot plant for the treatment of oil-containing waste waters, Desalination 147 (2002) 5–10. [4] P. Srijaroonrat, E. Julien, Y. Aurelle, Unstable secondary oil/water emulsion treatment using ultrafiltration: fouling control by backflushing, J. Membr. Sci. 159 (1999) 11–20. [5] P. Lipp, C.H. Lee, A.G. Fane, C.J.D. Fell, A fundamental study of the ultrafiltration of oil–water emulsions, J. Membr. Sci. 36 (1988) 161–177. [6] N.A. Ochoa, M. Masuelli, J. Marchese, Effect of hydrophilicity on fouling of an emulsified oil wastewater with PVDF/PMMA membranes, J. Membr. Sci. 226 (2003) 203–211. [7] S.R. Krajewski, W. Kujawski, F. Dijoux, C. Picard, A. Larbot, Grafting of ZrO2 powder and ZrO2 membrane by fluoroalkylsilanes, Colloids Surf. A 243 (2004) 43–47. [8] R.S. Faibish, Y. Cohen, Fouling-resistant ceramic-supported polymer membranes for ultrafiltration of oil-in-water microemulsions, J. Membr. Sci. 185 (2001) 129–143. [9] A. Dafinov, R. Garcia-Valls, J. Font, Modification of ceramic membranes by alcohol adsorption, J. Membr. Sci. 196 (2002) 69–77. [10] Y.S. Li, L. Yan, C.B. Xiang, L.J. Hong, Treatment of oily wastewater by organic–inorganic composite tubular ultrafiltration (UF) membranes, Desalination 196 (2006) 76–83. [11] J. Zhou, Y. Wang, G. Ma, X. Gu, X. Zhao, Study on modified film with Zn–Al spine is made on the Al2 O3 microfiltration membrane, China Ceram. Ind. 9 (4) (2002) 1–5. [12] J. Zhou, J. Wu, Y. Wang, J. Liang, X. Zhang, R. Cao, Modification of Al2 O3 microfiltration membrane by nano-crystalline TiO2 coating, J. Inorg. Mater. 21 (3) (2006) 725–730. [13] J. Zhou, J. Wu, Y. Wang, W. Jiang, G. Ma, R. Cao, Modification for Al2 O3 microfiltration membrane with nano-crystalline ZnO coating, J. Chin. Ceram. Soc. 32 (12) (2004) 1464–1469. [14] Y. Shui Li, L. Yan, C.B. Xiang, L.J. Hong, Treatment of oily wastewater by organic–inorganic composite tubular ultrafiltration (UF) membranes, Desalination 196 (2006) 76–83. [15] L. Yan, Y.S. Li, C.B. Xiang, S. Xianda, Effect of nano-sized Al2 O3 -particle addition on PVDF ultrafiltration membrane performance, J. Membr. Sci. 276 (2006) 162–167. [16] Y. Yang, H. Zhang, P. Wang, Q. Zheng, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (2007) 231–238. [17] J.K. Ting, J.C.S. Wu, Cross-flow ultrafiltration of oil/water emulsions using porous ceramic membrane, J. Chin. Inst. Chem. Eng. 30 (3) (1999) 207–214.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Separation of stable oil–water emulsion by the hydrophilic nano-sized ZrO2 modified Al2 O3 microfiltration membrane Jian-er Zhou a , Qibing Chang a,b,∗ , Yongqing Wang a , Jinming Wang a , Guangyao Meng b a b
Jingdezhen Ceramic Institute, Jingdezhen 333001, China University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 13 July 2009 Received in revised form 2 August 2010 Accepted 9 August 2010 Keywords: Membrane fouling Nano-sized ZrO2 Modification Ceramic membrane Emulsion separation
a b s t r a c t Membrane fouling weakens seriously the membrane’s separating performance, which can be reduced by the membrane surface modification. Commercial Al2 O3 microfiltration membranes were modified by the nano-sized ZrO2 coating to reduce the membrane fouling by oil droplets and to keep the unique character of the inorganic membrane. The nano-sized ZrO2 coating is prepared by in situ hydrolysis of ZrCl4 . The results show that the nano-coating does not form a separating layer, and just makes the ceramic membrane more hydrophilic. Using the stable 1 g/L 20# engine oil–water emulsion as feed, the steady flux of the modified membrane is obtained in a very short time. The steady flux keeps 88% of the initial flux and the oil rejection is above 97.8%. The hydrophilic nano-sized coating, the backflushing and the appropriate cross-flow velocity contribute to reducing the membrane fouling by oil droplets. The modified membrane has the good performance in the oil–water separation of stable emulsions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Oily wastewater should be treated before being discharged due to its ecological hazard [1]. The traditional methods to treat the unstable oil/water emulsion include gravity settling (API separator), skimming, dissolved air flotation, coalescence and centrifuging, etc. [2,3]. However, the oil emulsion is so stable that it is hardly treated by the traditional methods if the drop sizes of oil droplets are less than 20 m. In the developing methods, membrane technology shows a good perspective in the separation of oil emulsions, as it is of high efficiency and quite economical [4]. However, oil droplets stick easily on the membrane surface resulting in the formation of the cake layer. More seriously, the oil droplets can be squeezed into and through the membrane channels even if the size of oil droplets is far larger than the pores diameter of the membrane under the high applied pressure [1,5]. It means that the membrane fouling is generated. Membrane fouling degrades the membrane filtration performance and ultimately shortens the membrane life. To avoid this, optimizing the operation condition and changing membrane surface character by modification are the dominating methods. Comparably, modification is
∗ Corresponding author at: Jingdezhen Ceramic Institute, Taoyang Road, Jingdezhen 333001, Jiangxi Province, China. Tel.: +86 798 8499162; fax: +86 798 8494973. E-mail address: [email protected] (Q. Chang). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.08.008
more effective than the optimization of the operation condition because the increase of the repulsing force between the fouling and the membrane surface reduces the formation chance of the cake. Ochoa et al. [6] found that hydrophilic poly(vinylidene fluoride) (PVDF) membrane modified with poly(methyl methacrylate) (PMMA) had a low fouling to treat the oil/water emulsion. Ceramic membranes offer some unique advantages in the application of oil–water separation, such as good mechanical resistance, chemical inactivity, non-swelling and thermal stability. More importantly, the oil sticking on ceramic membrane can be removed by thermal treatment to recover the membrane performance. Ceramic membranes are usually hydrophilic due to the surface hydroxyl (–OH) groups [7]. However, the hydrophilic character is weakened due to the high-temperature calcination during the ceramic membrane preparation processes. To increase the membrane surface’s hydrophilic character, ceramic membrane can be modified by polymers [8–10]. However, polymer weakens the unique performance of ceramic membrane. Our group had researched the modification of ceramic membrane with nanometal oxide coating 10 years ago by homogenous precipitation and sol–gel method [11–13]. We found that the nano-metal oxide particles could effectively increase the ceramic membrane’s performance. Li and coworkers [14] and Yan and coworkers [15] found that the nano-sized alumina particles improved the anti-fouling performance of poly(vinylidene fluoride) membrane, and the flux recovery ratio of modified membranes reached 100% after washing. Yang et al. [16] found that the 2 wt% TiO2 –PSF UF composite membranes held the excellent water permeability, the hydrophilicity,
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Fig. 1. Flow chart of the preparation of the modified membrane by nano-zirconia coating.
the mechanical strength and the good anti-fouling ability without changing retentions. Several studies of zirconia membrane have indicated its superior performance in the separation of emulsion due to its special surface character [17–21], such as the high surface density and the strong polar. The oil droplet hardly adheres on the membrane surface. As zirconia is expensive, zirconia separating layer or modifying coating usually are fabricated on alumina support to obtain zirconia membrane. Del Colle et al. [20] impregnated the ceramic membrane with zirconium citrate solution to obtain zirconia nanoparticles distributed throughout the membrane surface. However, the zirconia nanoparticles agglomerate, which maybe weaken the permeate performance of the modified membrane. In the present work, the Al2 O3 microfiltration membrane (MF) was modified with nano-sized ZrO2 coating prepared through in situ hydrolysis of zirconium tetrachloride (ZrCl4 ). The nano-sized zirconia coating and the anti-fouling of the modified membrane were characterized.
2. Experimental 2.1. Preparation of modified membrane The used tubular Al2 O3 microfiltration membranes, commercially available (Nanjing Jiusi High-Tech Co. Ltd.), have a configuration of 7-channels. The other parameters are as follows: the outer diameter is 31 mm, the inter-diameter is 6 mm, the length is 250 mm, the porosity is 40%, and the mean pore diameter is 0.2 m. ZrCl4 (chemical pure grade) and absolute alcohol (reagent pure grade) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. 24.00 g of ZrCl4 was dissolved into 500 mL of the absolute alcohol. The as-prepared solution was used to modify Al2 O3 microfiltration membranes. The Al2 O3 membranes were firstly dried at 110 ◦ C for 12 h in oven, and then were saturated with the as-prepared ZrCl4 alcohol solution for 24 h at room temperature. After washed by absolute alcohol three times, the wet Al2 O3 membranes were dried at 60 ◦ C for 4 h again. Then, the Al2 O3 membranes were placed above the boiling water and bathed by hot water stream for 6 h. The membranes were directly calcined at 600 ◦ C for 2 h at heating rate of 1 ◦ C/min. The flow chart is shown in Fig. 1.
The modified Al2 O3 membrane was cut, and then its crosssection was observed by scanning electron microscopic (SEM) (JSM-6700F, JEOL). The energy dispersive spectrums (EDS) of the cross-section were also measured synchronously. The samples for TEM observation were prepared by mashing (not triturating) the modified membrane in the mortar until the particles were suitable for TEM observation. The particles coated by zirconia coating are observed on a JEOL-2010 electronic microscope. 2.3. Cross-flow filtration of modified membrane The oil–water emulsion consisted of 20# engine oil (1 g/L), Tween 80 (0.5 g/L), Span 80 (0.5 g/L) and distilled water. The oil and the surfactants were added to the distilled water. The emulsion was prepared by mixing at mild speed in the food blender (Fluko Equipment Shanghai Co., Ltd.) for 2 min. The stable oil–water emulsion has an average droplet size of 1.79 m and 90% of the oil droplets between the limit of 0.67 and 7.4 m, as measured by the Malvern Mastersizer Particle Size Analyzer (Nano ZS, Malvern). The similar procedures were reported in Ref. [22]. The water fluxes of the unmodified and modified membranes were tested in the filtration systems. Fig. 2 shows the schematic diagram of cross-flow microfiltration system. The stable emulsion was delivered from the feed tank by means of a centrifugal recirculation pump into the membrane module. The cross-flow velocity and trans-membrane pressure were adjusted by regulating the valves fitted at the inlet and outlet of membrane module. Both concentrate and filtrate should be sent back to the feed tank to maintain constant feed concentration during membrane operation in crossflow filtration mode. The fluxes were monitored by the rotometer recorded at an interval of 60 s. The trans-membrane pressure was 0.16 MPa and the cross-flow velocity was 5 m/s. For the filtration
2.2. Characterization of modified membrane To avoid the disturbance of the membrane pores on the measurement of contact angle, the nano-sized ZrO2 coating was prepared on a dense Al2 O3 discs. The preparation process was the same as that of the membrane. The contact angle was tested by the contact angle meter (SL-200B) by the sessile drop method.
Fig. 2. Schematic diagram of the cross-flow filtration system: 1, feed tank; 2, pump; 3, 3 valve; 4, 4 pressure gauge; 5, rotometer; 6, air compressor; 7, electromagnetic valve; 8, backflushing controlling panel; 9, membrane module.
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Fig. 3. SEM image and EDS spectra of the cross-section of the modified Al2 O3 membrane.
with different cross-flow velocity under backflushing conditions, the cross-flow velocities were 5 m/s, 3 m/s and 7 m/s in turn. The backflushing procedure was performed by using permeate water under high-pressured air in air compressor. The backflushing pressure was 0.6 MPa. The backflushing interval was 10 min, and the specified duration of backflushing was 5 s. The operating temperature was 30 ◦ C. The emulsion has the max adsorption at 220 nm. Therefore, The 220 nm UV was used for UV spectrometer (WFZ800-D3B, Beijing, China) to measure the oil concentration in feed and permeate. 3. Results and discussion 3.1. Characterization of nano-sized ZrO2 coating Fig. 3 shows the SEM image of the cross-section of the modified Al2 O3 membrane. The EDS spectra at points 1 and point 2 are also enclosed. The SEM image shows several Al2 O3 particles in the support. The alumina particles’ surfaces can be divided into two regions: fresh Al2 O3 cross-section area generated in the sample preparation process for SEM observation, and the surface covered with nano-sized ZrO2 coating, which are represented by point 1 and point 2, respectively. It is verified by the EDS spectra that the element composition at point 1 predominantly are Al and O, and at point 2 are Al, Zr and O. As can be seen, the nano-sized ZrO2 coating distributes uniformly on the alumina particle’s surface. The ZrO2 particles do not agglomerate at the sintering necks between the Al2 O3 particles. The coating has less influence on the pore structures. It can be explained that the coating is formed by not the deposition of zirconia grains but the adsorption of ZrCl4 forms. When the alumina membrane was soaked in the ZrCl4 alcohol solution, ZrCl4 chemically adsorbed on the alumina surface with which the solution could contact. The thickness of ZrCl4 depends on the adsorbing equilibrium of ZrCl4 on the alumina particles surface. Therefore, the adsorbed layer is isopachous, does not depend on the arrangement of Al2 O3 particles and follows the particle’ surface. After in situ hydrolysis and calcination, ZrCl4 transforms into the nano-ZrO2 coating without the morphological changes. The transformation can be denoted as: ZrCl4 + 9H2 O = ZrOCl2 ·8H2 O + 2HCl
ZrOCl2 · 8H2 O −→ ZrO2 + 2HCl + 7H2 O
gation of nano-sized zirconia grains, which is obviously different with the Al2 O3 grain. It implies that the numerous amounts of surface hydroxyl (–OH) groups exist in the coating, which makes the membrane more hydrophilic. Synchronously, the aggregation of zirconia grains increases the surface roughness of the membrane, comparing with the naked alumina grains. The apparent contact angle decrease with the surface roughness increases if the intrinsic contact angle of the material is less than 90◦ [23]. Fig. 5 shows the shape of a water droplet on the ZrO2 coating and the dense alumina disc, respectively. As can be seen, the apparent contact angle on the ZrO2 coating is 20◦ , which is far lower than that of a water droplet on dense, clean ZrO2 disc (r = 71.8◦ ) [24,25]. The contact angle is also lower than that of the dense alumina disc (r = 33◦ ), implying the modified membrane becomes more hydrophilic. The result is conflict with the reported by Del Colle et al. [20]. It maybe the nano-zirconia coating has the higher surface hydroxyl (–OH) groups than the sintered commercial zirconia membrane. 3.2. Performance of the modified microfiltration membrane Fig. 6 shows the flux declines of the virgin and the nano-sized ZrO2 modified Al2 O3 membrane. For the virgin Al2 O3 membrane, as can be seen, the flux declines sharply in the first 60 min, from 446 L/m2 h to 159 L/m2 h. The steady flux is only about 30% of the
(1) (2)
Therefore, the coating distributes uniformly on the alumina surface. However, it does not mean that the coating is uniform because it follows the surface morphology of the alumina particles. Fig. 4 shows the TEM image of a modified Al2 O3 particle spitted from the modified membrane. As can be seen, the porous ZrO2 coating with the thickness of about 100 nm is formed by the aggre-
Fig. 4. TEM image of modified Al2 O3 grain spitted from the modified membrane.
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Fig. 5. Shape of the water droplets on the dense Al2 O3 disc (A) and the ZrO2 coating (B).
initial flux, implying that the Al2 O3 membrane has a serious membrane fouling. However, the flux of the modified membrane quickly reaches a constant in 10 min. The flux declines from 506 L/m2 h to 441 L/m2 h. The steady flux keeps 88% of the initial flux. It could be explained that the hydrophilic nano-ZrO2 coating repulses the hydrophobic oil droplets. This result in: (I) the oil droplets are easily washed out by the feed with the appropriate cross-flow velocity and backflushing, even they have adhered on the membrane surface. The nano-coating prevents the oil droplets from adsorbing on the membrane surface. The cake layer could hardly be generated or its thickness hardly increases continuously; (II) the blocking of membrane pore is avoided because the hydrophilic membrane channels have the high capillary repulsing force to prevent the oil droplets from transformation. After modification, the membrane channels become thinner and more hydrophilic than the unmodified one. The repulsing force is high enough to repulse the oil droplet. The balance between the driving force by the permeate flow and the repulsing force by the nano-coating is easily reached. The steady flux is reached in a short time. The permeating flux of the modified membrane is higher than that of the unmodified one because the nano-coating reduces the thickness of the cake layer. As a whole, the nano-sized ZrO2 coating reduces remarkably the membrane fouling. It is well known that the decline of the permeating flux is caused by the concentration polarization, the cake layer or the pore blockage. Generally, the membrane fouling caused by concentration polarization is reversible and cake layer can be destroyed by the backflushing. However, the flux decline caused by pore block-
ing is hardly recovered by the backflushing and is regarded as the irreversible fouling. From this point, it is reasonable to define the maximum flux as the maximum flux at the time when the backflushing is finished in this paper. The maximum flux is obtained because the backflushing destroys the cake layer and recovery some membrane channels. Thus, the max flux reflects the degree of the destroyed cake layer in some degree. The declines of the max flux reflect the degree of the irreversible fouling because the backflushing cannot destroy the pore blockage. The steady flux is defined as the stable flux in the interval of backflushing. The steady flux reflects the degree of the total membrane fouling due to the cake layer and pore blockage. Fig. 7 shows the comparison of the steady fluxes of the modified membrane at different cross-flow velocities. The fluxes were measured using the same ceramic membrane. The cross-flow velocities were 5 m/s, 3 m/s and 7 m/s in turn. Therefore, the initial fluxes are different at each cross-flow velocity. Fig. 7 shows that both the initial and the steady fluxes at 3 m/s and 7 m/s are lower than those at 5 m/s. For the velocity is 3 m/s, the max flux almost keeps a constant, the steady flux declines slightly, and becomes unstable especially after 60 min. According to the definition, the max flux keeps the constant at 3 m/s, implying no oil droplet can be squeezed into the membrane channels due to the existence of the nano-zirconia coating. The function of the nanocoating is also embodied in the steady flux. The dragging force to make the oil droplet move to the membrane surface is lower than the repulse force of the nano-coating if the velocity is low. Thus, the cake layer is hardly formed when the original cake layer is
Fig. 6. Flux declines of the virgin and the nano-sized ZrO2 modified Al2 O3 membrane, cross-flow velocity is 5 m/s, backflushing pressure is 0.6 MPa, backflushing interval is 10 min, backflushing duration is 5 s, and operating temperature is 30 ◦ C.
Fig. 7. Fluxes of the modified membrane at different cross-flow velocity, backflushing pressure is 0.6 MPa, backflushing interval is 10 min, backflushing duration is 5 s, and operating temperature is 30 ◦ C.
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further increases, the cake layer is easily formed. Cake layer contributes to increasing the oil rejection as a “second filter layer” due to its dense structure. The oil concentration in the filtrate is lower again. According to the concentration of the feed, the oil rejections are 97.8%, 98.8% and 99.2%, respectively. It can be concluded that the modified microfiltration membrane can successfully separate the stable oil–water emulsion. 4. Conclusions The commercial Al2 O3 microfiltration membrane is successfully modified with nano-sized ZrO2 coating by in situ hydrolysis of ZrCl4 . The porous zirconia coating with the thickness of about 100 nm is formed on the membrane surface and the membrane channels, which makes the membrane is more hydrophilic. The nano-coating contribute to reducing the membrane fouling of oil droplets. The stable 1 g/L 20# engine oil–water emulsion is successfully separated by the modified Al2 O3 microfiltration membrane. Fig. 8. Oil concentration of filtrates obtained at different cross-flow velocities.
Acknowledgements destroyed by backflushing [26]. The flux decline shows that the flux cannot be steady in the interval of backflushing. Under this condition, concentration polarization is the major factor of the membrane fouling resulting in the lower flux [27]. The permeate flux at 7 m/s was measured following the flux at 3 m/s. The dragging force to make the oil droplet move to the membrane surface increases with the feed velocity. The cake layer is easily formed under this condition. Therefore, the steady flux is reached in a short time after the backflushing. At the same time, the driving force increases with the cross-flow velocity increases for the pressure-driven membrane, resulting in the increase of the flux. The steady flux increases with the filtrate time until the new steady flux is reached, implying the new balance of high dragging force generated by the permeating flux and the repulsing force of the nano-coating. Specially, the max flux increases gradually before 60 min, and then keeps a constant after 60 min. It is explained that the dragging force at the high cross-flow velocity compress the cake layer and make the cake layer denser. For the modified membrane, the hydrophilic nano-coating repulses the cake layer. The compressed cake layer as a body can be easily removed by the backflushing. More the membrane channels can be recovered when a mass of the cake layer are destroyed. Thus, the max flux increases. The compressed cake layer, as a “second filter layer” with the narrower pore size, prevents the oil droplets from permeating. The irreversible fouling can be avoided and the max flux shows a constant. On the whole, the high cross-flow velocity makes the oil droplets partially wet and stick on the membrane surface [28]. The compressed cake layer becomes the major factor resulting in the lower flux. Fig. 8 shows the oil concentration of filtrates obtained at different cross-flow velocities. As can be seen, the oil concentrations in the filtrate decrease when the cross-flow velocity is in the order of 5 m/s, 3 m/s and 7 m/s. As discussed above, no oil droplet can be squeezed into the membrane channels due to the existence of the nano-zirconia coating when the cross-flow velocity is 3 m/s. Only the oil droplets whose sizes are less than or equal to the max membrane pores penetrates and contaminates the filtrate because of low cross-flow velocity. The oil concentration is in the range of 9–13 mg/L. When the cross-flow velocity increases to 5 m/s, the cake layer still does not be formed or the thickness of the cake layer is small because the flux is high. The higher velocity makes the oil droplets can penetrate the membrane channels by transformation even their drop sizes are large than the membrane channels. The oil concentration increases remarkably. If the cross-flow velocity
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