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Study on a novel composite membrane for treatment of sewage containing oil
Desalination 299 (2012) 63–69
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Study on a novel composite membrane for treatment of sewage containing oil Yuqing Zhang a,⁎, Yan Xu a, Simeng Zhang b, Yuyuan Zhang d, Zhiping Xu c a
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130,China c ARC Centre of Excellence for Functional Nanomaterials, AIBN and School of Engineering, The University of Queensland, Brisbane, 4072, Australia d Department of Fundamental Subject, Tianjin Institute of Urban Construction, Tianjin, 300384, China b
a r t i c l e
i n f o
Article history: Received 5 January 2012 Received in revised form 9 May 2012 Accepted 11 May 2012 Available online 12 June 2012 Keywords: Composite membrane Zirconia Defect Hydrophilic property Sewage containing oil
a b s t r a c t When treated with sewage, polysulfone (PSF) membranes have a tendency to be contaminated due to their hydrophobicity, which limits large scale applications. To enhance the hydrophilic and anti-fouling property of PSF membrane, a novel organic–inorganic composite membrane was prepared by adding sulfated Y-doped nonstoichiometric zirconia (zirconia with defects, ZD) to PSF through a sol–gel process. The composite membranes were exposed to sewage containing oil, and the effects of the added amount of ZD, oil concentration, operating pressure, temperature and time on properties of composite membrane were researched. The optimum parameters are: 15 wt.% added ZD; 80 mg/L oil concentration; 0.15 MPa operating pressure; operating temperature between 22 and 30 °C and up to 5 h of operating time. At 80 mg/L sewage containing oil, the oil retention is 99.16% and the oil concentration in the permeate is 0.67 mg/L, which meets the requirement for discharge (less than 10 mg/L). After backflushing and chemical cleaning, the composite membranes could still keep a high permeation flux. It can be concluded that the hydrophilic and anti-fouling property of composite membrane can be significantly enhanced and hence the novel composite membrane is desirable in the treatment of sewage containing oil. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Sewage containing oil produced from industry not only pollutes environment but also wastes crude oil and water resource, so sewage needs to be treated before it is released to the environment. Many conventional methods including gravity setting, dewatering and incineration cannot efficiently treat emulsified and soluble oil in sewage. Membrane technology offers an efficient solution to the problem mentioned above. This is because it has a lot of advantages such as easy operation, low cost and capability of reducing contaminants. However membranes are easy to be contaminated by oil, which forms an oil-layer on membrane surface [1]. Hence, the enhancement of the hydrophilic and anti-fouling property of polysulfone membranes has become the focus problem of many researchers. Nowadays lots of methods to enhance the hydrophilic property of polymer membranes are reported. Among these methods, the approach of doping inorganic oxide particles to polymer to prepare organic–inorganic composite membranes is studied intensively, because the operation process and preparation technology of the composite membranes are simple, besides, the composite membranes have some advantages of both organic membranes and inorganic membranes. By adding ZrO2 particles to polysulfone (PSF), Genn et al. [2]
⁎ Corresponding author. Tel.: + 86 22 2789 0470; fax: + 86 22 2740 3389. E-mail address: [email protected] (Y. Zhang). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.05.020
prepared a novel porous ZrO2/PSF composite membrane to increase the permeability and reduce the resistance of composite membrane. Bottino et al. [3] improved membrane's thermodynamic property, mechanical strength, tenacity and so on by adding nanosilica to poly(vinyl alcohol) membrane. By adding nanosilica to poly-4-methyl-2-pentine membrane, Merkel et al. [4] prepared a sort of composite membrane which had high permeability and selectivity. Nunes et al. [5] prepared composite membrane with SO2 nano-dispersed in polyether imides (PEI) and the resistance of the membrane to compactability has been increased to some extent. After compression, a much higher loss of butane and pentane permeability was observed for the PEI membrane than the one without the inorganic component. Yang et al. [6] added TiO2 to PSF membrane to prepare a sort of composite membrane which had excellent water permeability, hydrophilic property, mechanical strength and good anti-fouling ability. Zhang and Ding [7] doped Al2O3 to PSF membrane to enhance the hydrophilic property and antifouling ability of the membrane. Though the capability of polymer membranes can be enhanced by adding inorganic oxide particles, further enhancement is limited because there are few Lewis acid sites and hydroxide radicals on the surface of stoichiometric monocomponent inorganic oxide particles [8]. Nonstoichiometric inorganic oxide nanoparticles have many point defects inside and lots of exposed hydroxide radicals on the surface, so these nanoparticles show stronger activity in the course of chemical bonding than stoichiometric monocomponent inorganic oxide particles [9]; especially when nonstoichiometric inorganic
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oxide nanoparticles are filled in polymer membranes, the capability of membranes is evidently improved. Zhang and Shan [10] prepared a novel composite membrane with Ce-doped nonstoichiometric nanosilica dispersed in polysulfone and the capability of membrane enhanced further. Sulfated metal oxides, first reported in 1976, have strong solid superacid sites with Hammett acidity greater than 100% pure H2SO4. It is well know that the interaction of sulfated oxides with metals leads to the formation of material with greater acidity and surface areas than one of non-sulfated oxides [11]. When sulfated nonstoichiometric inorganic oxide nanoparticles were filled in polymer membranes, the performance of the membranes is evidently improved. Among the sulfated metal oxides, sulfated zirconia is a typical oxide with 10,000 times Hammett acidity of 100% pure H2SO4. Therefore, in our previous work, Zhang and Cui [12] prepared sulfated Y-doped nonstoichiometric zirconia first, and then added it to polysulfone to prepare a novel organic–inorganic composite membrane by a sol–gel process. The results show that the hydrophilic property and anti-fouling capability of polysulfone membrane are enhanced. However, they did not further investigate and observe the application effect of this novel composite membrane for treating with sewage containing oil. Therefore, in this paper we further study the effect of some parameters such as the added amount of zirconia particles in polysulfone membranes, oil concentration in sewage, operating pressure, operating temperature and operating time on the flux of membranes and determine the optimum parameters. Finally the recycle of these composite membranes is explored. It can be concluded that the composite membrane is desirable to apply to treat with sewage containing oil. 2. Experimental 2.1. Materials and reagents Y2O3, ZrOCl2·8H2O, SiO2 and nonionic surfactant — fatty alcohol polyoxyethylene ether (SA-20) were obtained from Tianjin Jiangtian Chemical Technology Limited Company. N-butyl alcohol and NH3·H2O were supplied by Tianjin Jinyu Fine Chemical Factory. PSF was purchased from Dalian Polysulfone Co., Ltd. and its MW and polydispersity were 84400 Da and 1.37, respectively. Polyethylene glycol with average MW 400 Da (PEG400) was supplied by Tianjin Jinyu Fine Chemical Factory. N-N-Dimethylacetamide (DMAC) was obtained from Tianjin Damao Service of Chemical Instruments. The last two reagents were analytical grade and used as received. 2.2. Preparation of ZD Preparation process of ZD is similar to literature [12,13], the detailed procedures are as follows:Y2O3, ZrOCl2·8H2O (molar ratio is 3:100) and surfactant SA-20 (3 wt.%) were dissolved in a hydrochloride solution of 1 mol/L under stirring while heating at 40–50 °C. Aqueous ammonia of 2 mol/L was then added into the mixed solution until the pH value was adjusted to about 10.0. After aging for 12 h at 25 °C, the ZrO2–Y2O3 gel particles were washed with deionized water to remove chloride ions. After removing chloride ions, ZrO2–Y2O3 gel particles were washed by n-butyl alcohol for several times and vacuum dried at 110 °C to form ZrO2–Y2O3 powder particles. Then the ZrO2–Y2O3 powder particles were dipped in H2SO4 solution while stirring for 30 min, filtered by the Buchner funnel, dried at 110 °C and finally sintered at 650 °C for 2 h. Thus ZD particles were obtained.
dissolved sufficiently with stirring. And then polyethylene glycol 400 (PEG400) with a mass ratio of 10% to PSF was added as porogen to promote the yield of pores in the gelation process. After that, ZD with a mass ratio of 15% to PSF were added into the mixture. They were mixed with vigorous stirring with the use of intermittent ultrasound until a homogenous solution was obtained. After being kept still for 24–48 h, the solution was poured onto a dense glass plate and cast to form thin film (thickness ca circa 300 μm),and the films were then immersed into a water bath at 20 °C after a 10 s exposure period in air (20 °C and 60% relatively humidity). Membranes were leached under running water for at least 2 d prior to being soaked in 30 wt.% glycerin aqueous solution. Finally membranes were stored in de-ionized water containing 1 wt.% formaldehyde to avoid bacteria growth in ambient condition. The ratio between polysulfone and ZD, solvent and polyethylene glycol 400, the evaporating time, the coagulation bath temperature and intermittent ultrasound were fixed as the unchangeable parameters because these parameters were researched in our previous work. Therefore, the added amount of ZD was considered as the chief research parameter in this article. PSF membranes were prepared by using the same procedure mentioned above, except for inorganic oxide particles doped into casting solutions. 2.4. Characterization of ZD particles and composite membranes Sample ZD particles were investigated by the Nicolet NEXUS Fourier transform infrared (FTIR) spectra (America Thermo electron Corporation), by the KBr method. Membrane surfaces and cross-section were observed under a Nanosem 430 scanning electron microscope (SEM, America FEI Corporation) with the voltage of 10 kV. An even specimen of fabricated membrane was tested with an Instron5543 tensile testing machine (America). The condition was applied stroke speed, 15 mm/ min; measuring range, 0–4 MPa; test temperature, 21 °C. 2.5. Effect of membrane material Composite membranes (the added amount of ZD is 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.% respectively) were used to treat sewage containing oil. Oil concentration of feed is 120 mg/L, and the ultrafiltration was carried out with an operating pressure of 0.20 MPa. The fluxes of different composite membranes were noted to determine the optimum quantity of ZD that should be added. Fig. 1 shows the schematic diagram of the experimental device used in this experiment. Feed solution was transported to the membrane module by a diaphragm pump. Flow was regulated using a valve V1 while the pressure on the membrane was adjusted by another valve V2. The feed stream split into two streams: the one of two streams with non-passing components was returned to the feed tank; the other one with passing components (permeation) passed through the membrane and its volume was measured by a measuring cylinder.
2.3. Preparation of composite membranes with different casting solutions [14] Composite membranes were prepared by immersion precipitation (i.e. phase inversion) method. At first, DMAC in a 500 ml flask was heated to 40–50 °C in a water bath. Dried PSF was then added and
Fig. 1. Schematic diagram of the experimental device.
Y. Zhang et al. / Desalination 299 (2012) 63–69
The permeation was returned to the feed tank in order to keep a constant feed concentration. The permeation volume was recorded per 1 h. Permeation flux is calculated by the following equation: J¼
V S·t
ð1Þ
where J is the permeation flux (L/(m 2 h)), V the permeation volume (L), S the effective membrane area (m 2) and t the operating time. 2.6. Effect of oil concentration in the feed Sewage containing oil (oil concentration is 80 mg/L, 150 mg/L, 225 mg/L respectively) was treated with composite membrane (the added amount of ZD is 15 wt.%). The ultrafiltration was carried out with an operating pressure of 0.20 MPa. The fluxes were recorded and the best oil concentration was obtained by analysis and comparison. 2.7. Effect of operating pressure Composite membrane (the added amount of ZD is 15 wt.%) was used to treat sewage containing oil (80 mg/L). The permeation fluxes (at the steady state) were measured at different operating pressures to research the effect of operating pressure. 2.8. Effect of operating temperature Composite membrane (the added amount of ZD is 15 wt.%) was used to treat sewage containing oil, the ultrafiltration was carried out with an operating pressure of 0.15 MPa and oil concentration of 80 mg/L. In addition, recording the fluxes of secondary distilled water treated with the same composite membrane was done to make a comparison with sewage containing oil.
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3. Results and discussion 3.1. FTIR analysis of sample particles It can be observed from Fig. 2 that stretching vibration of OH group for ZD particles at 3400 cm − 1 is stronger than that for ZrO2 particles while distorting vibration of OH group for ZD particles at 1627 cm − 1 is stronger than that for ZrO2 particles. A stretching vibration peak of Zr\O\Zr bond of ZrO2 particles is at 500 cm − 1 (a); in contrast, stretching vibration peaks of Zr\O\Zr bond of ZD particles are at 469 cm − 1 (b), which move toward the lower wave number region. These results show that Zr\O\Y bond could be formed and the Y 3+ cations were doped into the ZrO2 frame structure [17]. The weak absorption band at 2928 cm − 1 is the characteristic peak of ZrO2 and it can also be observed in the FTIR spectrum (b) [18]. The peak of S_O covalent band at 1390 cm − 1 is a characteristic peak of H2SO4 and the bands at 1070 cm − 1 are assigned to a chemical bond formed by chelate reactions between sulfate and metal ions on the surface of the ZrO2. The result could suggest that Y 3+ cations have entered into the ZrO2 frame structure and partly replaced Zr 4+ cations inside the ZrO2.
3.2. SEM of ZD/PSF membrane and pure PSF membrane Fig. 3(a), (b), and (c) shows the SEM images of the cross section of the 15 wt.% ZD/PSF membrane and pure PSF membrane. It is observed from Fig. 3(a) and (c) that dispersibility of ZD particles in the PSF membrane is relatively uniform. In addition, Fig. 3(b) indicates that ZD particles were doped into the PSF membrane structure and have good compatibility with PSF membrane, which could improve the tensile strength and anti-compaction capability of the hybrid membrane.
2.9. Test of oil retention rate
3.3. The tensile strength of membranes
Permeation flux test of membranes for sewage containing oil was carried out under an operating pressure of 0.15 MPa for 9 h (oil concentration of the sewage is 80 mg/L). The oil concentration in the permeation was determined by UV spectrophotometer [15,16] and the retention rate R was calculated by the following equation:
The tensile strength of the three membranes is shown in Table 1. The tensile strength of membranes is improved from 1.925 MPa to 3.315 MPa and tensile strength of the ZD/PSF membrane is higher than SiO2/PSF membranes (2.101 MPa). We can deduce from this result that dispersion of ZD particles and their compatibility with polysulfone matrix are better than that those of SiO2 in the resulting hybrid membrane. Results show that doping ZD particles into PSF membrane effectively improves mechanical performance and anti-compaction property of the ZD/PSF hybrid membrane.
R ¼
ðC1 −C2 Þ 100% C1
ð2Þ
where R is the retention rate (%), C1 the oil concentration in the feed solution (80 mg/L) and C2 the oil concentration in the permeation (mg/L). 2.10. Cleaning and operation studies of membrane After working for a period, the permeation flux of membranes tended to decline owning to fouling and block. In order to resume the permeate flux as much as possible, it is necessary to clean membrane to remove pollutant. In this article, two strategies are adopted, the detailed processes are as follows: Strategy 1: clean with 1%–3% citric acid solution, 1%–2% NaOH solution and light mineral oil respectively, each cleaning time is 30 min, and then rinse with secondary distilled water to neutral. Strategy 2: clean with light mineral oil to remove oil on the surface of membrane, then clean with 1%–3% citric acid solution to remove mechanic impurities and fouling layer, finally clean with NaOH solution to remove absorption layer caused by citric acid solution.
Fig. 2. FTIR spectra of ZrO2 particles, ZD particles sintered at 650 °C.
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Fig. 3. SEM of the 15 wt.% ZD/PSF membrane, pure PSF membrane and 30 wt.% ZD/PSF membrane: (a) cross section of the 15 wt.% ZD/PSF membrane, (b) enlarged cross section of the 15 wt.% ZD/PSF membrane, (c) cross section of pure PSF membrane, (d) upper surface of the 30 wt.% ZD/PSF membrane.
3.4. Effect of different membrane material on flux of membrane Fig. 4 shows the effect of ZD content in the casting solution and the operating time on permeation flux of membrane. PSF membrane is easy to be contaminated because PSF membrane has hydrophobic character, making the flux of membrane decline when used. In contrast, hydrophilic property of composite membrane doped by ZD is stronger than PSF membrane. A thin hydrophilic layer on the composite membrane is formed in the process of oil–water separation, which can evidently enhance antifouling properties of the membrane. Therefore the permeation fluxes of composite membrane doped by ZD (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%) are more than that of PSF membrane under the same operating condition. For composite membranes, permeation flux of composite membranes with 15 wt.% ZD permeation flux is bigger than that of other composite membranes and its steady flux reaches 110 L/m 2 h in the process of oil–water separation. This is because when the added amount of ZD is more than 15 wt.%, the dispersion of ZD in polysulfone membrane becomes more difficult, causing agglomeration of particles (as shown in Fig. 3(d), the added amount of ZD in the membrane is 30 wt.%). Thereby, the hydrophilic property of composite membrane declines. The composite membrane (the added amount of ZD is 15 wt.%) shows the better performance due to existence of many hydrophilic
groups inside the membrane. Therefore, composite membrane with 15 wt.% ZD can be applied in the following experiments of oil–water separation. 3.5. Effect of oil content in the feed on flux of membrane Fig. 5 indicates that as oil concentration in water increases, permeation fluxes decrease; and the steady fluxes are approximate. This is
Table 1 Tensile strength of pure PSF, 15 wt.% SiO2/PSF and 15 wt.% ZD/PSF membranes. Type of membranes
Tensile strength (MPa)
Pure PSF membrane 15 wt.% SiO2/PSF membrane 15 wt.% ZD/PSF membrane
1.925 2.101 3.315
Fig. 4. Effect of ZD content in the casting solution and the operating time on permeation flux (oil concentration of feed is 120 mg/L).
Y. Zhang et al. / Desalination 299 (2012) 63–69
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of operating pressure has a positive effect on the increase of permeation flux. If operating pressure is much lower than critical pressure, it is far more difficult to compensate for the resistance loss along the system flow channel, which recedes efficiency of the entire membrane system. In contrast, when operating pressure exceeds critical pressure, pores are blocked by oil droplets for the compressible property of oil droplets, which induces the declination of permeation flux. Therefore, the optimum operating pressure is 0.15 MPa. 3.7. Effect of operating temperature on flux of membrane
Fig. 5. Effect of operating time and oil content on the permeation flux of the 15 wt.% ZD/PSF membrane.
because that as oil concentration in water increases, the oil layer on the 15 wt.% ZD/PSF membrane surface becomes thicker, meanwhile the flow resistance increases. Oil droplets retained by the membrane build up a boundary layer on the membrane surface which makes oil droplets accumulate and reduces the permeation flux. As operating time increases, the droplet size increases, gradually a gel coating layer forms; at this time, actually the role of filtration is the thin gel layer, so the steady fluxes are approximate even though oil concentrations are different. 3.6. Effect of operating pressure on flux of membrane The plot in Fig. 6 shows the permeation fluxes (at the steady state) of membrane (the added amount of ZD is 15 wt.%) at different operation pressures. Operating pressure is a direct factor to influence permeation flux of membranes for a pressure-actuated ultrafiltration process (as shown in Fig. 6). When operating pressure is low, permeation flux (at the steady state) increases with operating pressure increases, but it declines a little when operating pressure is more than 0.15 MPa. Before reaching critical pressure (0.15 MPa), the increase
Fig. 6. Variation of the permeation flux with the applied pressure on the 15 wt.% ZD/ PSF membrane.
To investigate the effect of operating temperature on membrane separation process, experiment was carried out with composite membrane (the amount of ZD is 15 wt.%), treated secondary distilled water and sewage containing oil under an operating pressure of 0.15 MPa. It can be observed from Fig. 7 that when operating temperature increases, the flux of secondary distilled water increases while the flux of sewage containing oil changes a little. The viscosity of water declines when heating water, which causes permeation flux increase [19]. In contrast, when operating temperature increases, interaction between oil droplets and surface of membrane increases, causing more adsorption pollution of membrane. Therefore, suitable operating temperature is between 22 and 30 °C. 3.8. Effect of operating time on flux of membrane From Figs. 4 and 5, it can be observed that the flux declines rapidly at initial procedure and reaches almost a steady state after operating the separation process for about 5 h. One of the possible reasons for flux declination is concentration polarization due to the increase of retentate concentration. With carrying on ultrafiltration process, retentate concentration on the membrane surface is higher than the feed concentration, and it causes concentration polarization which will form a gel layer on the membrane surface. The other possible reason for flux declination is pore blocking due to the existence of membrane pores impervious for large oil droplets. And pore blocking formed by oil droplets could be the major factor for initial flux declination. 3.9. Rejection of oil The effect of membrane separation can be learned by the rejection of oil. Table 2 shows oil concentration and rejection of oil in
Fig. 7. Effect of temperature and feed composition on fluxes of 15 wt.% ZD/PSF membrane.
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Table 2 Analysis results of water samples.
Table 4 Relation of cleaning solutions with flux recovery ratio.
Membrane
Oil content (mg/L)
Rejection of oil (%)
Detergent
Flux recovery ratio (%)
Cleaning time (min)
Raw water Membrane 1 Membrane 2 Membrane 3 Membrane 4
80.00 1.04 0.86 0.67 0.68
0 98.70 98.93 99.16 99.16
1%–3% citric acid solution 1%–2% NaOH solution Light mineral oil
30 49 42
30 30 30
(5 wt.% ZD) (10 wt.% ZD) (15 wt.% ZD) (20 wt.% ZD)
permeation, the oil concentration was measured when permeation flux test of membranes for sewage containing oil was carried out under an operating pressure of 0.15 MPa for 9 h. As indicated in Table 2, the percent rejection of oil for membranes changes with the added amount of ZD at the optimum operating pressure (0.15 MPa), and membrane 3 (the added amount of ZD is 15 wt.%) has higher rejection of oil (99.16%), so membrane 3 shows better performance. From Fig. 4, it can also be observed that the 15 wt.% composite membrane has better performance attribute to its stronger hydrophilic property and better anti-fouling ability. It can be seen in Table 3 that after doping SiO2 (the added amount is 15 wt.%) to polysulfone, rejection of oil increases a lot (from 93.75% to 96.65%), however, when doping ZD particles (the added amount is 15 wt.%) to polysulfone, rejection of oil increases a lot again (from 96.65% to 99.16%). These data indicate that the use of ZD has real advantages over composite membrane using SiO2. This result can be considered as an evidence for the hydrophilicity of ZD. The oil concentration of 0.67 mg/L in permeation has met the maximum allowable discharge limit (b10 mg/L). In conclusion, this novel composite membrane has a potential application in dealing with sewage containing oil. 3.10. Membrane regeneration As can be seen in Table 4, strategy 1 is inefficient, this is mainly because components of sewage containing oil are complicated. When cleaned with light mineral oil, it can remove oil on membrane surface but cannot remove mechanic impurities; when cleaned with 1%–2% NaOH solution, flocculent precipitant forms and attaches on membrane surface because sewage containing oil has lots of calcium, magnesium and so on; when cleaned with 1%–3% citric acid solution, it cannot efficiently remove mechanic impurities and fouling layer attached on membrane surface due to the protection of oil fouling layer. Compared with strategy 1, strategy 2 is more efficient. It can be seen in Table 5 that when the composite membrane (the added amount of ZD is 15 wt.%) is reused six times, the permeation flux of the composite membrane can still return to about 86% of initial permeation flux. It can be concluded that this novel composite membrane has long life and can be recycled. 4. Conclusions Composite membrane with strong hydrophilic property can be prepared by adding ZD to polysulfone. The optimum parameters for oil– water separation are: 15 wt.% added ZD; 80 mg/L oil concentration; 0.15 MPa operating pressure; operating temperature between 22 and 30 °C and up to 5 h of operating time. Test of oil retention rate indicates that the performance of composite membrane is better than pure PSF Table 3 The rejection of oil of PSF membrane, 15 wt.% SiO2/PSF membrane, 15 wt.% ZD/PSF membrane. Membrane
Oil content (mg/L)
Rejection of oil (%)
Raw water PSF membrane Membrane 5 (15 wt.% SiO2) Membrane 3 (15 wt.% ZD)
80.00 5.00 2.68 0.67
0 93.75 96.65 99.16
Table 5 Recovery ratio of membranes after being cleaned. The adding of ZD particles (wt.%)
Cleaning times 1
2
3
4
5
6
0 5 10 15 20
83.3% 98.3% 98.7% 99.1% 98.8%
82.5% 97.5% 97.4% 97.5% 96.4%
81.2% 94.3% 94.0% 95.6% 95.1%
80.6% 91.0% 91.2% 92.3% 92.1%
79.2% 87.5% 88.0% 88.7% 87.9%
78.0% 85.9% 86.0% 86.2% 86.0%
membrane. This novel composite membrane with strong hydrophilic property and anti-fouling capability has practical application in treating sewage containing oil. Acknowledgment This project is supported by the project supported by the National Natural Science Foundation of China (No. 21076143), by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. 2009‐1341), by RAEng Research Exchanges with China and India award supported by the UK Royal Academy of Engineering (No. 2012‐5502), by the project funded by the Key Laboratory of Inorganic film Materials, Chinese Academy of Sciences (Nos. KLICM-2010-06, KLICM-2011-07), by innovation fund of Tianjin University and Hainan University (No. 1107026), by the Program of Introducing Talents of Discipline to Universities (No. B06006), by the Basic Research of Tianjin Municipal Science and Technology Commission (No. 07JCYBJC00700), and by Petro China Innovation Fund (No. 060511-6-3). References [1] K.K. Malgorzata, M.N. Katarzyna, W. Tjomasz, Analysis of membrane fouling in the treatment of water solutions containing humic acids and mineral salts, Desalination 126 (1999) 179–189. [2] I. Genn, S. Kuypers, R. Leysen, Effect of the addition of ZrO2 to polysulfone based UF membranes, J. Membr. Sci. 113 (1996) 343–350. [3] A. Bottino, G. Capannelli, A. Comite, Preparation and characterization of novel porous PVDF–ZrO2 composite membranes, Desalination 146 (2002) 35–40. [4] T.C. Merkel, B.D. Freeman, R.J. Spontak, et al., Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519–552. [5] S.P. Nunes, K.V. Peinemann, K. Ohlrogge, et al., Membranes of poly (ether imide) and nano dispersed silica, J. Membr. Sci. 157 (1999) 219–226. [6] Y.N. Yang, H.X. Zhang, P. Wang, et al., The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (2007) 231–238. [7] Y.Q. Zhang, J. Ding, Effect of the addition of alumina on performance of polysulfone based membrane, Chin. J. Chem. Eng. 28 (2000) 42–44. [8] V.P. Zlomanov, A.J. Zavrazhnov, A.V. Davydov, Nonstoichiometry and P–T–x diagrams of binary systems, Intermetallics 11 (2003) 1287–1291. [9] G.G. Libowitz, J.B. Lightstone, Characterization of point defects in nonstoichiometric compounds from thermodynamic considerations, J. Phys. Chem. Solids 28 (1967) 1145–1154. [10] Y.Q. Zhang, L.B. Shan, Preparation and Characterization of Novel Ce-doped nonstoichiometric nanosilica/polysulfone composite membranes, Sep. Purif. Technol. 63 (2008) 207–212. [11] R. Silva Rodrigo, J.M. Hernandez Enriquez, A. Castillo Mares, J.A. Melo Banda, R. Garcia Alamilla, M. Picquart, T. Lopez Goerne, Effect of CeO2 on the textural and acid properties of ZrO2–SO42−, Catal. Today 107–108 (2005) 838–884. [12] Y.Q. Zhang, P. Cui, Development of a sulfated Y-doped nonstoichiometric zirconia/polysulfone composite membrane for treatment of wastewater containing oil, Sep. Purif. Technol. 70 (2009) 153–159.
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Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Study on a novel composite membrane for treatment of sewage containing oil Yuqing Zhang a,⁎, Yan Xu a, Simeng Zhang b, Yuyuan Zhang d, Zhiping Xu c a
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130,China c ARC Centre of Excellence for Functional Nanomaterials, AIBN and School of Engineering, The University of Queensland, Brisbane, 4072, Australia d Department of Fundamental Subject, Tianjin Institute of Urban Construction, Tianjin, 300384, China b
a r t i c l e
i n f o
Article history: Received 5 January 2012 Received in revised form 9 May 2012 Accepted 11 May 2012 Available online 12 June 2012 Keywords: Composite membrane Zirconia Defect Hydrophilic property Sewage containing oil
a b s t r a c t When treated with sewage, polysulfone (PSF) membranes have a tendency to be contaminated due to their hydrophobicity, which limits large scale applications. To enhance the hydrophilic and anti-fouling property of PSF membrane, a novel organic–inorganic composite membrane was prepared by adding sulfated Y-doped nonstoichiometric zirconia (zirconia with defects, ZD) to PSF through a sol–gel process. The composite membranes were exposed to sewage containing oil, and the effects of the added amount of ZD, oil concentration, operating pressure, temperature and time on properties of composite membrane were researched. The optimum parameters are: 15 wt.% added ZD; 80 mg/L oil concentration; 0.15 MPa operating pressure; operating temperature between 22 and 30 °C and up to 5 h of operating time. At 80 mg/L sewage containing oil, the oil retention is 99.16% and the oil concentration in the permeate is 0.67 mg/L, which meets the requirement for discharge (less than 10 mg/L). After backflushing and chemical cleaning, the composite membranes could still keep a high permeation flux. It can be concluded that the hydrophilic and anti-fouling property of composite membrane can be significantly enhanced and hence the novel composite membrane is desirable in the treatment of sewage containing oil. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Sewage containing oil produced from industry not only pollutes environment but also wastes crude oil and water resource, so sewage needs to be treated before it is released to the environment. Many conventional methods including gravity setting, dewatering and incineration cannot efficiently treat emulsified and soluble oil in sewage. Membrane technology offers an efficient solution to the problem mentioned above. This is because it has a lot of advantages such as easy operation, low cost and capability of reducing contaminants. However membranes are easy to be contaminated by oil, which forms an oil-layer on membrane surface [1]. Hence, the enhancement of the hydrophilic and anti-fouling property of polysulfone membranes has become the focus problem of many researchers. Nowadays lots of methods to enhance the hydrophilic property of polymer membranes are reported. Among these methods, the approach of doping inorganic oxide particles to polymer to prepare organic–inorganic composite membranes is studied intensively, because the operation process and preparation technology of the composite membranes are simple, besides, the composite membranes have some advantages of both organic membranes and inorganic membranes. By adding ZrO2 particles to polysulfone (PSF), Genn et al. [2]
⁎ Corresponding author. Tel.: + 86 22 2789 0470; fax: + 86 22 2740 3389. E-mail address: [email protected] (Y. Zhang). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.05.020
prepared a novel porous ZrO2/PSF composite membrane to increase the permeability and reduce the resistance of composite membrane. Bottino et al. [3] improved membrane's thermodynamic property, mechanical strength, tenacity and so on by adding nanosilica to poly(vinyl alcohol) membrane. By adding nanosilica to poly-4-methyl-2-pentine membrane, Merkel et al. [4] prepared a sort of composite membrane which had high permeability and selectivity. Nunes et al. [5] prepared composite membrane with SO2 nano-dispersed in polyether imides (PEI) and the resistance of the membrane to compactability has been increased to some extent. After compression, a much higher loss of butane and pentane permeability was observed for the PEI membrane than the one without the inorganic component. Yang et al. [6] added TiO2 to PSF membrane to prepare a sort of composite membrane which had excellent water permeability, hydrophilic property, mechanical strength and good anti-fouling ability. Zhang and Ding [7] doped Al2O3 to PSF membrane to enhance the hydrophilic property and antifouling ability of the membrane. Though the capability of polymer membranes can be enhanced by adding inorganic oxide particles, further enhancement is limited because there are few Lewis acid sites and hydroxide radicals on the surface of stoichiometric monocomponent inorganic oxide particles [8]. Nonstoichiometric inorganic oxide nanoparticles have many point defects inside and lots of exposed hydroxide radicals on the surface, so these nanoparticles show stronger activity in the course of chemical bonding than stoichiometric monocomponent inorganic oxide particles [9]; especially when nonstoichiometric inorganic
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oxide nanoparticles are filled in polymer membranes, the capability of membranes is evidently improved. Zhang and Shan [10] prepared a novel composite membrane with Ce-doped nonstoichiometric nanosilica dispersed in polysulfone and the capability of membrane enhanced further. Sulfated metal oxides, first reported in 1976, have strong solid superacid sites with Hammett acidity greater than 100% pure H2SO4. It is well know that the interaction of sulfated oxides with metals leads to the formation of material with greater acidity and surface areas than one of non-sulfated oxides [11]. When sulfated nonstoichiometric inorganic oxide nanoparticles were filled in polymer membranes, the performance of the membranes is evidently improved. Among the sulfated metal oxides, sulfated zirconia is a typical oxide with 10,000 times Hammett acidity of 100% pure H2SO4. Therefore, in our previous work, Zhang and Cui [12] prepared sulfated Y-doped nonstoichiometric zirconia first, and then added it to polysulfone to prepare a novel organic–inorganic composite membrane by a sol–gel process. The results show that the hydrophilic property and anti-fouling capability of polysulfone membrane are enhanced. However, they did not further investigate and observe the application effect of this novel composite membrane for treating with sewage containing oil. Therefore, in this paper we further study the effect of some parameters such as the added amount of zirconia particles in polysulfone membranes, oil concentration in sewage, operating pressure, operating temperature and operating time on the flux of membranes and determine the optimum parameters. Finally the recycle of these composite membranes is explored. It can be concluded that the composite membrane is desirable to apply to treat with sewage containing oil. 2. Experimental 2.1. Materials and reagents Y2O3, ZrOCl2·8H2O, SiO2 and nonionic surfactant — fatty alcohol polyoxyethylene ether (SA-20) were obtained from Tianjin Jiangtian Chemical Technology Limited Company. N-butyl alcohol and NH3·H2O were supplied by Tianjin Jinyu Fine Chemical Factory. PSF was purchased from Dalian Polysulfone Co., Ltd. and its MW and polydispersity were 84400 Da and 1.37, respectively. Polyethylene glycol with average MW 400 Da (PEG400) was supplied by Tianjin Jinyu Fine Chemical Factory. N-N-Dimethylacetamide (DMAC) was obtained from Tianjin Damao Service of Chemical Instruments. The last two reagents were analytical grade and used as received. 2.2. Preparation of ZD Preparation process of ZD is similar to literature [12,13], the detailed procedures are as follows:Y2O3, ZrOCl2·8H2O (molar ratio is 3:100) and surfactant SA-20 (3 wt.%) were dissolved in a hydrochloride solution of 1 mol/L under stirring while heating at 40–50 °C. Aqueous ammonia of 2 mol/L was then added into the mixed solution until the pH value was adjusted to about 10.0. After aging for 12 h at 25 °C, the ZrO2–Y2O3 gel particles were washed with deionized water to remove chloride ions. After removing chloride ions, ZrO2–Y2O3 gel particles were washed by n-butyl alcohol for several times and vacuum dried at 110 °C to form ZrO2–Y2O3 powder particles. Then the ZrO2–Y2O3 powder particles were dipped in H2SO4 solution while stirring for 30 min, filtered by the Buchner funnel, dried at 110 °C and finally sintered at 650 °C for 2 h. Thus ZD particles were obtained.
dissolved sufficiently with stirring. And then polyethylene glycol 400 (PEG400) with a mass ratio of 10% to PSF was added as porogen to promote the yield of pores in the gelation process. After that, ZD with a mass ratio of 15% to PSF were added into the mixture. They were mixed with vigorous stirring with the use of intermittent ultrasound until a homogenous solution was obtained. After being kept still for 24–48 h, the solution was poured onto a dense glass plate and cast to form thin film (thickness ca circa 300 μm),and the films were then immersed into a water bath at 20 °C after a 10 s exposure period in air (20 °C and 60% relatively humidity). Membranes were leached under running water for at least 2 d prior to being soaked in 30 wt.% glycerin aqueous solution. Finally membranes were stored in de-ionized water containing 1 wt.% formaldehyde to avoid bacteria growth in ambient condition. The ratio between polysulfone and ZD, solvent and polyethylene glycol 400, the evaporating time, the coagulation bath temperature and intermittent ultrasound were fixed as the unchangeable parameters because these parameters were researched in our previous work. Therefore, the added amount of ZD was considered as the chief research parameter in this article. PSF membranes were prepared by using the same procedure mentioned above, except for inorganic oxide particles doped into casting solutions. 2.4. Characterization of ZD particles and composite membranes Sample ZD particles were investigated by the Nicolet NEXUS Fourier transform infrared (FTIR) spectra (America Thermo electron Corporation), by the KBr method. Membrane surfaces and cross-section were observed under a Nanosem 430 scanning electron microscope (SEM, America FEI Corporation) with the voltage of 10 kV. An even specimen of fabricated membrane was tested with an Instron5543 tensile testing machine (America). The condition was applied stroke speed, 15 mm/ min; measuring range, 0–4 MPa; test temperature, 21 °C. 2.5. Effect of membrane material Composite membranes (the added amount of ZD is 0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.% respectively) were used to treat sewage containing oil. Oil concentration of feed is 120 mg/L, and the ultrafiltration was carried out with an operating pressure of 0.20 MPa. The fluxes of different composite membranes were noted to determine the optimum quantity of ZD that should be added. Fig. 1 shows the schematic diagram of the experimental device used in this experiment. Feed solution was transported to the membrane module by a diaphragm pump. Flow was regulated using a valve V1 while the pressure on the membrane was adjusted by another valve V2. The feed stream split into two streams: the one of two streams with non-passing components was returned to the feed tank; the other one with passing components (permeation) passed through the membrane and its volume was measured by a measuring cylinder.
2.3. Preparation of composite membranes with different casting solutions [14] Composite membranes were prepared by immersion precipitation (i.e. phase inversion) method. At first, DMAC in a 500 ml flask was heated to 40–50 °C in a water bath. Dried PSF was then added and
Fig. 1. Schematic diagram of the experimental device.
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The permeation was returned to the feed tank in order to keep a constant feed concentration. The permeation volume was recorded per 1 h. Permeation flux is calculated by the following equation: J¼
V S·t
ð1Þ
where J is the permeation flux (L/(m 2 h)), V the permeation volume (L), S the effective membrane area (m 2) and t the operating time. 2.6. Effect of oil concentration in the feed Sewage containing oil (oil concentration is 80 mg/L, 150 mg/L, 225 mg/L respectively) was treated with composite membrane (the added amount of ZD is 15 wt.%). The ultrafiltration was carried out with an operating pressure of 0.20 MPa. The fluxes were recorded and the best oil concentration was obtained by analysis and comparison. 2.7. Effect of operating pressure Composite membrane (the added amount of ZD is 15 wt.%) was used to treat sewage containing oil (80 mg/L). The permeation fluxes (at the steady state) were measured at different operating pressures to research the effect of operating pressure. 2.8. Effect of operating temperature Composite membrane (the added amount of ZD is 15 wt.%) was used to treat sewage containing oil, the ultrafiltration was carried out with an operating pressure of 0.15 MPa and oil concentration of 80 mg/L. In addition, recording the fluxes of secondary distilled water treated with the same composite membrane was done to make a comparison with sewage containing oil.
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3. Results and discussion 3.1. FTIR analysis of sample particles It can be observed from Fig. 2 that stretching vibration of OH group for ZD particles at 3400 cm − 1 is stronger than that for ZrO2 particles while distorting vibration of OH group for ZD particles at 1627 cm − 1 is stronger than that for ZrO2 particles. A stretching vibration peak of Zr\O\Zr bond of ZrO2 particles is at 500 cm − 1 (a); in contrast, stretching vibration peaks of Zr\O\Zr bond of ZD particles are at 469 cm − 1 (b), which move toward the lower wave number region. These results show that Zr\O\Y bond could be formed and the Y 3+ cations were doped into the ZrO2 frame structure [17]. The weak absorption band at 2928 cm − 1 is the characteristic peak of ZrO2 and it can also be observed in the FTIR spectrum (b) [18]. The peak of S_O covalent band at 1390 cm − 1 is a characteristic peak of H2SO4 and the bands at 1070 cm − 1 are assigned to a chemical bond formed by chelate reactions between sulfate and metal ions on the surface of the ZrO2. The result could suggest that Y 3+ cations have entered into the ZrO2 frame structure and partly replaced Zr 4+ cations inside the ZrO2.
3.2. SEM of ZD/PSF membrane and pure PSF membrane Fig. 3(a), (b), and (c) shows the SEM images of the cross section of the 15 wt.% ZD/PSF membrane and pure PSF membrane. It is observed from Fig. 3(a) and (c) that dispersibility of ZD particles in the PSF membrane is relatively uniform. In addition, Fig. 3(b) indicates that ZD particles were doped into the PSF membrane structure and have good compatibility with PSF membrane, which could improve the tensile strength and anti-compaction capability of the hybrid membrane.
2.9. Test of oil retention rate
3.3. The tensile strength of membranes
Permeation flux test of membranes for sewage containing oil was carried out under an operating pressure of 0.15 MPa for 9 h (oil concentration of the sewage is 80 mg/L). The oil concentration in the permeation was determined by UV spectrophotometer [15,16] and the retention rate R was calculated by the following equation:
The tensile strength of the three membranes is shown in Table 1. The tensile strength of membranes is improved from 1.925 MPa to 3.315 MPa and tensile strength of the ZD/PSF membrane is higher than SiO2/PSF membranes (2.101 MPa). We can deduce from this result that dispersion of ZD particles and their compatibility with polysulfone matrix are better than that those of SiO2 in the resulting hybrid membrane. Results show that doping ZD particles into PSF membrane effectively improves mechanical performance and anti-compaction property of the ZD/PSF hybrid membrane.
R ¼
ðC1 −C2 Þ 100% C1
ð2Þ
where R is the retention rate (%), C1 the oil concentration in the feed solution (80 mg/L) and C2 the oil concentration in the permeation (mg/L). 2.10. Cleaning and operation studies of membrane After working for a period, the permeation flux of membranes tended to decline owning to fouling and block. In order to resume the permeate flux as much as possible, it is necessary to clean membrane to remove pollutant. In this article, two strategies are adopted, the detailed processes are as follows: Strategy 1: clean with 1%–3% citric acid solution, 1%–2% NaOH solution and light mineral oil respectively, each cleaning time is 30 min, and then rinse with secondary distilled water to neutral. Strategy 2: clean with light mineral oil to remove oil on the surface of membrane, then clean with 1%–3% citric acid solution to remove mechanic impurities and fouling layer, finally clean with NaOH solution to remove absorption layer caused by citric acid solution.
Fig. 2. FTIR spectra of ZrO2 particles, ZD particles sintered at 650 °C.
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Fig. 3. SEM of the 15 wt.% ZD/PSF membrane, pure PSF membrane and 30 wt.% ZD/PSF membrane: (a) cross section of the 15 wt.% ZD/PSF membrane, (b) enlarged cross section of the 15 wt.% ZD/PSF membrane, (c) cross section of pure PSF membrane, (d) upper surface of the 30 wt.% ZD/PSF membrane.
3.4. Effect of different membrane material on flux of membrane Fig. 4 shows the effect of ZD content in the casting solution and the operating time on permeation flux of membrane. PSF membrane is easy to be contaminated because PSF membrane has hydrophobic character, making the flux of membrane decline when used. In contrast, hydrophilic property of composite membrane doped by ZD is stronger than PSF membrane. A thin hydrophilic layer on the composite membrane is formed in the process of oil–water separation, which can evidently enhance antifouling properties of the membrane. Therefore the permeation fluxes of composite membrane doped by ZD (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%) are more than that of PSF membrane under the same operating condition. For composite membranes, permeation flux of composite membranes with 15 wt.% ZD permeation flux is bigger than that of other composite membranes and its steady flux reaches 110 L/m 2 h in the process of oil–water separation. This is because when the added amount of ZD is more than 15 wt.%, the dispersion of ZD in polysulfone membrane becomes more difficult, causing agglomeration of particles (as shown in Fig. 3(d), the added amount of ZD in the membrane is 30 wt.%). Thereby, the hydrophilic property of composite membrane declines. The composite membrane (the added amount of ZD is 15 wt.%) shows the better performance due to existence of many hydrophilic
groups inside the membrane. Therefore, composite membrane with 15 wt.% ZD can be applied in the following experiments of oil–water separation. 3.5. Effect of oil content in the feed on flux of membrane Fig. 5 indicates that as oil concentration in water increases, permeation fluxes decrease; and the steady fluxes are approximate. This is
Table 1 Tensile strength of pure PSF, 15 wt.% SiO2/PSF and 15 wt.% ZD/PSF membranes. Type of membranes
Tensile strength (MPa)
Pure PSF membrane 15 wt.% SiO2/PSF membrane 15 wt.% ZD/PSF membrane
1.925 2.101 3.315
Fig. 4. Effect of ZD content in the casting solution and the operating time on permeation flux (oil concentration of feed is 120 mg/L).
Y. Zhang et al. / Desalination 299 (2012) 63–69
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of operating pressure has a positive effect on the increase of permeation flux. If operating pressure is much lower than critical pressure, it is far more difficult to compensate for the resistance loss along the system flow channel, which recedes efficiency of the entire membrane system. In contrast, when operating pressure exceeds critical pressure, pores are blocked by oil droplets for the compressible property of oil droplets, which induces the declination of permeation flux. Therefore, the optimum operating pressure is 0.15 MPa. 3.7. Effect of operating temperature on flux of membrane
Fig. 5. Effect of operating time and oil content on the permeation flux of the 15 wt.% ZD/PSF membrane.
because that as oil concentration in water increases, the oil layer on the 15 wt.% ZD/PSF membrane surface becomes thicker, meanwhile the flow resistance increases. Oil droplets retained by the membrane build up a boundary layer on the membrane surface which makes oil droplets accumulate and reduces the permeation flux. As operating time increases, the droplet size increases, gradually a gel coating layer forms; at this time, actually the role of filtration is the thin gel layer, so the steady fluxes are approximate even though oil concentrations are different. 3.6. Effect of operating pressure on flux of membrane The plot in Fig. 6 shows the permeation fluxes (at the steady state) of membrane (the added amount of ZD is 15 wt.%) at different operation pressures. Operating pressure is a direct factor to influence permeation flux of membranes for a pressure-actuated ultrafiltration process (as shown in Fig. 6). When operating pressure is low, permeation flux (at the steady state) increases with operating pressure increases, but it declines a little when operating pressure is more than 0.15 MPa. Before reaching critical pressure (0.15 MPa), the increase
Fig. 6. Variation of the permeation flux with the applied pressure on the 15 wt.% ZD/ PSF membrane.
To investigate the effect of operating temperature on membrane separation process, experiment was carried out with composite membrane (the amount of ZD is 15 wt.%), treated secondary distilled water and sewage containing oil under an operating pressure of 0.15 MPa. It can be observed from Fig. 7 that when operating temperature increases, the flux of secondary distilled water increases while the flux of sewage containing oil changes a little. The viscosity of water declines when heating water, which causes permeation flux increase [19]. In contrast, when operating temperature increases, interaction between oil droplets and surface of membrane increases, causing more adsorption pollution of membrane. Therefore, suitable operating temperature is between 22 and 30 °C. 3.8. Effect of operating time on flux of membrane From Figs. 4 and 5, it can be observed that the flux declines rapidly at initial procedure and reaches almost a steady state after operating the separation process for about 5 h. One of the possible reasons for flux declination is concentration polarization due to the increase of retentate concentration. With carrying on ultrafiltration process, retentate concentration on the membrane surface is higher than the feed concentration, and it causes concentration polarization which will form a gel layer on the membrane surface. The other possible reason for flux declination is pore blocking due to the existence of membrane pores impervious for large oil droplets. And pore blocking formed by oil droplets could be the major factor for initial flux declination. 3.9. Rejection of oil The effect of membrane separation can be learned by the rejection of oil. Table 2 shows oil concentration and rejection of oil in
Fig. 7. Effect of temperature and feed composition on fluxes of 15 wt.% ZD/PSF membrane.
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Table 2 Analysis results of water samples.
Table 4 Relation of cleaning solutions with flux recovery ratio.
Membrane
Oil content (mg/L)
Rejection of oil (%)
Detergent
Flux recovery ratio (%)
Cleaning time (min)
Raw water Membrane 1 Membrane 2 Membrane 3 Membrane 4
80.00 1.04 0.86 0.67 0.68
0 98.70 98.93 99.16 99.16
1%–3% citric acid solution 1%–2% NaOH solution Light mineral oil
30 49 42
30 30 30
(5 wt.% ZD) (10 wt.% ZD) (15 wt.% ZD) (20 wt.% ZD)
permeation, the oil concentration was measured when permeation flux test of membranes for sewage containing oil was carried out under an operating pressure of 0.15 MPa for 9 h. As indicated in Table 2, the percent rejection of oil for membranes changes with the added amount of ZD at the optimum operating pressure (0.15 MPa), and membrane 3 (the added amount of ZD is 15 wt.%) has higher rejection of oil (99.16%), so membrane 3 shows better performance. From Fig. 4, it can also be observed that the 15 wt.% composite membrane has better performance attribute to its stronger hydrophilic property and better anti-fouling ability. It can be seen in Table 3 that after doping SiO2 (the added amount is 15 wt.%) to polysulfone, rejection of oil increases a lot (from 93.75% to 96.65%), however, when doping ZD particles (the added amount is 15 wt.%) to polysulfone, rejection of oil increases a lot again (from 96.65% to 99.16%). These data indicate that the use of ZD has real advantages over composite membrane using SiO2. This result can be considered as an evidence for the hydrophilicity of ZD. The oil concentration of 0.67 mg/L in permeation has met the maximum allowable discharge limit (b10 mg/L). In conclusion, this novel composite membrane has a potential application in dealing with sewage containing oil. 3.10. Membrane regeneration As can be seen in Table 4, strategy 1 is inefficient, this is mainly because components of sewage containing oil are complicated. When cleaned with light mineral oil, it can remove oil on membrane surface but cannot remove mechanic impurities; when cleaned with 1%–2% NaOH solution, flocculent precipitant forms and attaches on membrane surface because sewage containing oil has lots of calcium, magnesium and so on; when cleaned with 1%–3% citric acid solution, it cannot efficiently remove mechanic impurities and fouling layer attached on membrane surface due to the protection of oil fouling layer. Compared with strategy 1, strategy 2 is more efficient. It can be seen in Table 5 that when the composite membrane (the added amount of ZD is 15 wt.%) is reused six times, the permeation flux of the composite membrane can still return to about 86% of initial permeation flux. It can be concluded that this novel composite membrane has long life and can be recycled. 4. Conclusions Composite membrane with strong hydrophilic property can be prepared by adding ZD to polysulfone. The optimum parameters for oil– water separation are: 15 wt.% added ZD; 80 mg/L oil concentration; 0.15 MPa operating pressure; operating temperature between 22 and 30 °C and up to 5 h of operating time. Test of oil retention rate indicates that the performance of composite membrane is better than pure PSF Table 3 The rejection of oil of PSF membrane, 15 wt.% SiO2/PSF membrane, 15 wt.% ZD/PSF membrane. Membrane
Oil content (mg/L)
Rejection of oil (%)
Raw water PSF membrane Membrane 5 (15 wt.% SiO2) Membrane 3 (15 wt.% ZD)
80.00 5.00 2.68 0.67
0 93.75 96.65 99.16
Table 5 Recovery ratio of membranes after being cleaned. The adding of ZD particles (wt.%)
Cleaning times 1
2
3
4
5
6
0 5 10 15 20
83.3% 98.3% 98.7% 99.1% 98.8%
82.5% 97.5% 97.4% 97.5% 96.4%
81.2% 94.3% 94.0% 95.6% 95.1%
80.6% 91.0% 91.2% 92.3% 92.1%
79.2% 87.5% 88.0% 88.7% 87.9%
78.0% 85.9% 86.0% 86.2% 86.0%
membrane. This novel composite membrane with strong hydrophilic property and anti-fouling capability has practical application in treating sewage containing oil. Acknowledgment This project is supported by the project supported by the National Natural Science Foundation of China (No. 21076143), by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. 2009‐1341), by RAEng Research Exchanges with China and India award supported by the UK Royal Academy of Engineering (No. 2012‐5502), by the project funded by the Key Laboratory of Inorganic film Materials, Chinese Academy of Sciences (Nos. KLICM-2010-06, KLICM-2011-07), by innovation fund of Tianjin University and Hainan University (No. 1107026), by the Program of Introducing Talents of Discipline to Universities (No. B06006), by the Basic Research of Tianjin Municipal Science and Technology Commission (No. 07JCYBJC00700), and by Petro China Innovation Fund (No. 060511-6-3). References [1] K.K. Malgorzata, M.N. Katarzyna, W. Tjomasz, Analysis of membrane fouling in the treatment of water solutions containing humic acids and mineral salts, Desalination 126 (1999) 179–189. [2] I. Genn, S. Kuypers, R. Leysen, Effect of the addition of ZrO2 to polysulfone based UF membranes, J. Membr. Sci. 113 (1996) 343–350. [3] A. Bottino, G. Capannelli, A. Comite, Preparation and characterization of novel porous PVDF–ZrO2 composite membranes, Desalination 146 (2002) 35–40. [4] T.C. Merkel, B.D. Freeman, R.J. Spontak, et al., Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519–552. [5] S.P. Nunes, K.V. Peinemann, K. Ohlrogge, et al., Membranes of poly (ether imide) and nano dispersed silica, J. Membr. Sci. 157 (1999) 219–226. [6] Y.N. Yang, H.X. Zhang, P. Wang, et al., The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, J. Membr. Sci. 288 (2007) 231–238. [7] Y.Q. Zhang, J. Ding, Effect of the addition of alumina on performance of polysulfone based membrane, Chin. J. Chem. Eng. 28 (2000) 42–44. [8] V.P. Zlomanov, A.J. Zavrazhnov, A.V. Davydov, Nonstoichiometry and P–T–x diagrams of binary systems, Intermetallics 11 (2003) 1287–1291. [9] G.G. Libowitz, J.B. Lightstone, Characterization of point defects in nonstoichiometric compounds from thermodynamic considerations, J. Phys. Chem. Solids 28 (1967) 1145–1154. [10] Y.Q. Zhang, L.B. Shan, Preparation and Characterization of Novel Ce-doped nonstoichiometric nanosilica/polysulfone composite membranes, Sep. Purif. Technol. 63 (2008) 207–212. [11] R. Silva Rodrigo, J.M. Hernandez Enriquez, A. Castillo Mares, J.A. Melo Banda, R. Garcia Alamilla, M. Picquart, T. Lopez Goerne, Effect of CeO2 on the textural and acid properties of ZrO2–SO42−, Catal. Today 107–108 (2005) 838–884. [12] Y.Q. Zhang, P. Cui, Development of a sulfated Y-doped nonstoichiometric zirconia/polysulfone composite membrane for treatment of wastewater containing oil, Sep. Purif. Technol. 70 (2009) 153–159.
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