Journal of Membrane Science 325 (2008) 357–361
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Removal of organics from produced water by reverse osmosis using MFI-type zeolite membranes Ning Liu a,∗ , Liangxiong Li a , Brian McPherson a,b , Robert Lee a a b
Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Department of Civil and Environmental Engineering, University of Utah, Salt Lake City, USA
a r t i c l e
i n f o
Article history: Received 14 April 2008 Received in revised form 23 July 2008 Accepted 26 July 2008 Available online 12 August 2008 Keywords: MFI-type zeolite membrane Reverse osmosis Ions rejection Organics rejection Water ﬂux
a b s t r a c t Separation of an organics/water mixture was carried out by reverse osmosis using an ␣-aluminasupported MFI-type zeolite membrane. The organic rejection performance is strongly dependent on the ionic species and dynamic size of dissolved organics. The membrane showed high rejection efﬁciency for electrolytes such as pentanoic acid. An organic rejection of 96.5% with a water ﬂux of 0.33 kg m−2 h−1 was obtained for 100 ppm pentanoic acid solution at an operation pressure of 2.76 MPa. For non-electrolyte organics, separation efﬁciency is governed by the molecular dynamic size; the organics with larger molecular dynamic size show higher separation efﬁciency. The zeolite membrane gives an organic rejection of 99.5% and 17% for 100 ppm toluene and 100 ppm ethanol, respectively, with a water ﬂux of 0.03 kg m−2 h−1 , 0.31 kg m−2 h−1 at an operation pressure of 2.76 MPa. It was observed that organic rejection and water ﬂux were affected by the organic concentration. As pentanoic acid concentration increased from 100 ppm to 500 ppm, both organic rejection and water ﬂux decreased slightly. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Population expansion and intermittent water shortages in various regions in the United States have stimulated a resurgence of interest in desalination for water supply augmentation. Several reverse osmosis (RO) plants for processing brackish water, wastewater, sea water, or groundwater are currently operating [1,2]. Among these types of waters, desalinated produced water from oil and gas industry is an attractive option for providing potable water in arid regions [3–6]. However, the organics contained in produced water always damage the polymeric RO membranes by serious fouling [7,8]. A new type of membrane that can resist the effects of organic foulants has become an urgent in the wastewater puriﬁcation industry. Zeolite membranes may offer an alternative choice for produced water treatment. Zeolites are crystalline aluminosilicate materials with uniform sub-nanometer- or nanometer-scale pores. For example, the MFI-type zeolite has a three-dimensional pore system with straight channels in the b-direction (5.4 Å × 5.6 Å) and sinusoidal channels in the a-direction (5.1 Å × 5.5 Å). Due to the inert property of aluminosilicate crystal, zeolite membranes have superior thermal and chemical stabilities, hence holding great potential for application in difﬁcult separations such as produced water puriﬁ-
∗ Corresponding author. Tel.: +1 505 835 5739; fax: +1 505 835 6031. E-mail address: [email protected]
t.edu (N. Liu). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.056
cation and radioactive wastewater treatment. Recently, molecular dynamic simulation has shown that zeolite membranes are theoretically suitable for ion removal from aqueous solutions by RO processes . The simulation revealed that 100% Na+ rejection could be achieved on a perfect (single crystal), ZK-4 membrane through RO. The separation mechanism of the perfect ZK-4 zeolite membranes is the size exclusion of hydrated ions, which have kinetic sizes signiﬁcantly larger than the aperture of the ZK-4 zeolite. Effects of membrane thickness, pore size, and external electric ﬁelds on membrane performance were also studied by molecular dynamic simulation [10–15]. Research results demonstrated that zeolite membrane could separate Na+ , Cl− , Li+ , and Br− from water, methanolic, and ethanolic electrolyte solutions. Li et al. [16,17] reported using MFI-type zeolite membranes in RO separation. These membranes showed 77% rejection of Na+ and water ﬂux of 0.12 kg m−2 h−1 for 0.1 M NaCl solution at the applied feed pressure of 2.07 MPa. In a complex feed solution containing 0.1 M NaCl + 0.1 M KCl + 0.1 M NH4 Cl + 0.1 M CaCl2 + 0.1 M MgCl2 , rejections of Na+ , K+ , NH4 + , Ca2+ , and Mg2+ were 58.1%, 62.6%, 79.9%, 80.7%, and 88.4%, respectively. Besides different kinds of ions, produced water also contains a variety of dissolved organics. The dissolved organics in produced water are mainly fatty acids, benzene, toluene, ethylbenzene, xylene (BTEX), and other aliphatic organics . These dissolved organics usually lead to severe fouling on the polymeric RO membranes surface, which kills the membranes in RO processes. Separation of dissolved organics from water with zeolite ceramic
N. Liu et al. / Journal of Membrane Science 325 (2008) 357–361
membranes has, to date, seldom been reported. Kumakiri et al.  reported using an A-type zeolite membrane (pore size 0.42 nm) in RO separation of water–ethanol mixtures. The hydrophilic A-type zeolite membrane showed 44% rejection of ethanol and a water ﬂux of 0.058 kg m−2 h−1 under an applied feed pressure of 1.5 MPa. In the research described in this paper, we investigated the inﬂuence of dissolved organics, pentanoic acid, toluene, and ethanol on MFI zeolite membranes’ RO performance. Organic rejection and water ﬂux through the membrane with operation time is discussed.
carbon analyzer (TOC-V CPN, SHIMADZU) were used to determine ion and organic concentrations in the sample, respectively. Reverse osmosis experiments were conducted at room temperature for 0.1 M NaCl and different organic solutions, respectively. Water ﬂux and ion/organics rejection were measured as functions of operation time. Time to onset of visually observable ﬁrst drop of liquid was regarded as the starting point (i.e. t = 0 h) of the RO process. The deﬁnitions of water ﬂux (F) and ion/organics rejection (R) are as follows: F=
2. Experimental MFI zeolite membranes were synthesized by in situ crystallization on the inner surface of tubular ␣-alumina substrates (Pall Corp., NY, USA). The alumina tube was 8 cm in length with inner and outer diameters of 7 mm and 10 mm, respectively. Chemicals used in this study included NaOH pellets (99.99%, Aldrich), fumed SiO2 (99.98%, Aldrich), 1 M tetrapropylammonium hydroxide (TPAOH) solution (Aldrich), toluene (Fisher, 99.8%), and pentanoic acid (EM Science, 98%). The zeolite synthesis solution was obtained by dissolving 0.35 g NaOH pellets and 5 g fumed SiO2 in 25 ml 1 M TPAOH solution . The mixture was stirred in a water bath with temperature controlled at 80 ◦ C. Hydrothermal treatment was conducted in an autoclave at 180 ◦ C for 20 h. During synthesis, the cylindrical autoclave was rotated around its axes at a speed of 3.0 × 10−1 rad/s (3 rpm). After synthesis, the membrane was washed, dried, and ﬁred at 450 ◦ C for 8 h with a heating rate of 0.3 ◦ C/min. The above synthesis process was repeated one more time to improve the membrane density. After synthesis, the morphology of the zeolite crystal and the membrane thickness were investigated by a scanning electron microscope (SEM, JEOL 5800 LV). Fig. 1 shows the schematic diagram of the RO separation experiment. The membrane tube was mounted in the separation cell and sealed by silicone O-rings. The tube had an effective membrane area of ∼11.0 cm2 . The feed solution containing dissolved organic substances was ﬁlled in the feed tank and the feed pressure was maintained by a nitrogen cylinder. The feed ﬂow rate was controlled by a needle valve. The liquid permeate samples were collected at every interval. The sample bottle was connected to the gas phase of a water ﬂask to prevent evaporation of the received liquid. Dualcolumn ion chromatography (IC, DX120, Dionex) and a total organic
Qw , Am t
Cf − Cp Cf
where Qw is the amount of sample collected in the time period t, and Am is the effective membrane area, 1.1 × 10−3 m2 in this study, and Cf and Cp are ion/organics concentrations of the feed and permeate solutions, respectively. 3. Results and discussions 3.1. Membrane synthesis Fig. 2 shows the scanning electron microscopic (SEM) images of the tubular MFI zeolite membrane. The crystals have a characteristic cofﬁn shape with crystal size about 0.4 m in length and 0.2 m in width. Fig. 2 also displays the as-synthesized crystals having random orientations (including a, b, c-oriented) [21,22]. It is known that random-oriented crystals have been obtained when the OH− /Si ratio was higher than 0.64 [22,23]. In this study, OH− /Si ratio calculated from the synthesis solution is 1.05. The thickness of the membrane is about 1.2 m according to the SEM observation. A thicker membrane can be obtained by repeating the synthesis process one more time. However, our study revealed that water ﬂux decreased quickly with increase of membrane thickness. In another word, a thinner membrane contributes higher water ﬂux, but loses the separation efﬁciency. 3.2. Ion rejection Fig. 3 shows the Na+ rejection and water ﬂux of 0.1 M NaCl solution as a function of RO operation time. Reverse osmosis was conducted at a feed pressure of 2.76 MPa. At the beginning, water ﬂux decreased and Na+ rejection increased with the time. After 20 h,
Fig. 1. Schematic diagram of the RO system.
N. Liu et al. / Journal of Membrane Science 325 (2008) 357–361
Fig. 2. SEM images of (a) surface and (b) cross-section zeolite membrane.
both the water ﬂux and ion rejection leveled off at 0.35 kg m−2 h−1 and 99.4%, respectively. The mechanism of ion separation with zeolite membrane involves size exclusion of hydrated ions [16,24] and electrostatic repulsion (Donnan exclusion) at the intercrystalline pore entrance . In aqueous, Na+ (molecular diameter 0.19 nm) exists tightly bound with the water molecule and forms a rather stable ionic cluster [9–11,13,15]. The formed ionic clusters increase the effective size of Na+ ions signiﬁcantly (from 0.8 nm to 1.1 nm) and prevent them from entering the zeolite cavities. Possible ionic clusters formed due to the interaction between the ions and polar solvent molecules (water) were reported by Murad et al. [9,12–14]. In addition, larger clusters, which involved more than one ion, were also found in molecular dynamic simulation results [9,12]. The formed hydration Na+ ions were found to have a high de-solvation energy, which effectively prevented the ions from breaking away from such clusters. On the other hand, Na+ transport through the intercrystalline pore is restricted by the overlapping charged double layers . Rejection of 99.4% Na+ indicates the membrane is almost a perfect membrane (single crystal with only zeolitic pores), which was revealed by the molecular dynamic simulation . Fig. 3 also shows that water ﬂux decline and ion rejection increase at the beginning of the RO process. This may be explained by the following analysis. For the MFI zeolite, which has an aperture size of 0.56 nm, molecular species as large as 0.84 nm of kinetic diameter (e.g. triisopropylbenzene) can still get into the zeolite channels but its mobility in the zeolitic pores is extremely low (<10−14 cm2 /s) . Therefore, it is possible that some hydrated Na+ ions entered the zeolitic pores and bound to the pore surface,
Fig. 3. Na+ rejection and water ﬂux as functions of RO operation time for 0.1 M NaCl.
thus hindering the diffusion of water molecules. On the other hand, the existing membrane intercrystalline pores may allow the subnanometer-sized hydrated ions to pass through in the beginning. However, with time, overlapping double layers may develop in the microporous intercrystalline boundaries due to ion adsorption on the external surface of the zeolite crystals . The overlapping double layers can restrict ion transport while allowing water molecular to enter freely. Thus, ion rejection increases and water ﬂux decreases in the initial stage of operation. 3.3. Ionic organic Fatty acids are the major components of dissolved organics in produced water, contributing over 90% of TOC. In this work, pentanoic acid was used as a simulator to study the membrane performance on organic rejection, particularly ionic organics. Before ﬁlling the pentanoic acid solution in the feed tank, we adjusted the pH value of the solution with 0.1 M NaOH to 7.0, which is consistant with the pH value of the actual produced water. Fig. 4 shows the results of the rejection and water ﬂux with operation time for 100 ppm pentanoic acid. From Fig. 4, we can see that organic rejection for 100 ppm pentanoic acid is about 96.5%. Water ﬂux through the membrane decreased at the beginning, then rose a little and then leveled off after 50 h. Considering the molecular size of pentanoic acid (0.40 nm ) and zeolite pore size (0.56 nm), high organic rejection for 100 ppm pentanoic acid can be explained as following. At pH 7.0, pentanoic acid (AH) dissociates and exists as A−
Fig. 4. Organic rejection and water ﬂux as functions of operation time for 100 ppm pentanoic acid.
N. Liu et al. / Journal of Membrane Science 325 (2008) 357–361
in solution (pKa = 4.82 for pentanoic acid). In the meantime, a negatively charged zeolite surface is expected to develop on the synthesized membrane due to the substitution of Si4+ with Al3+ under hydrothermal synthesis conditions. Thus, a signiﬁcant repulsion exists between the negatively charged MFI zeolite framework and the pentanoic anion A− . Moreover, the length and spatial conﬁguration of the pentanoic molecule also help the high organic rejection. The pentanoic molecule has a zigzag spatial conﬁguration with each C C C bond angle of 109.5◦ . Such a zigzag-conﬁguration molecule is difﬁcult to diffuse in the zeolite pore channel. In addition, the orientation of lineal pentanoic molecule during its contact with the zeolite pore aperture (perpendicular or parallel to pore axis) could also keep it from entering the pore channel. On the other hand, pentanoic anion transport through the membrane intercrystalline pores is also hindered by the charged double layers. Thus, a high organic rejection obtained for 100 ppm pentanoic acid. Fig. 4 also shows the water ﬂux changing with the operation time for 100 ppm pentanoic acid. Water ﬂux decreased from 0.37 kg m−2 h−1 to 0.31 kg m−2 h−1 and then rose a little to 0.33 kg m−2 h−1 and stayed around 0.33 kg m−2 h−1 . The decrease of water ﬂux at the beginning probably contributed to the pentanoic molecular adsorption on the zeolite membrane surface. When dilute solutions are in contact with the solid surfaces, more ions can be adsorbed during the initial stages . van der Waals interaction was regarded as the adsorption force between the zeolite membrane and the adsorbed hydrocarbon molecular [28,29]. The adsorbed pentanoic molecule on the zeolite surface can hinder the diffusion of water molecules and also decrease the net driving force for water transport across the membrane, resulting in water ﬂux decline at the beginning. On the other hand, a negatively charged layer developed on the zeolite membrane surface in the RO process. The developed negatively charged layer, on the contrary, tries to repulse pentanoic anion deposition on the membrane surface. These two factors in the RO process lead to the water ﬂux ﬂuctuant of around 0.33 kg m−2 h−1 . Fig. 5 reveals the results of organic rejection and water ﬂux for different concentrations of pentanoic acid. As the concentration of pentanoic acid increased, both the water ﬂux and organic rejection decreased. Water ﬂux and organic rejection for 50 ppm pentanoic acid were 0.38 kg m−2 h−1 and 99.8%, respectively. For 500 ppm pentanoic acid, they were 0.32 kg m−2 h−1 and 95.7%. The decline in water ﬂux with increase of pentanoic acid concentration can be explained by the reduced driving force in the presence of a high-concentration ionic solution. At the same operating pres-
Table 1 Reverse osmosis results of different organic solutions (operation pressure: 2.76 MPa) Organic solution
Water ﬂuxa (kg m−2 h−1 )
Organic rejectiona (%)
100 ppm pentanoic acid 100 ppm toluene 100 ppm ethanol
0.33 0.03 0.31
96.0 99.5 17.0
a The organic permeation test has been of sufﬁcient length. Data appearing here are the ﬁnal equilibrium data.
sure (P = 2.76 MPa), the permeation driving force decreases with an increase in solute concentration because of the increased osmotic pressure at high ion concentrations . Water ﬂux decreases linearly when the pentanoic acid concentration increases from 0 ppm to 100 ppm. However, it decreases slowly as the pentanoic acid concentration changes from 100 ppm to 500 ppm. To understand this observation, we should remember that the mobility and transport behavior of water and hydrated ions in microchannels is more complicated. It is not only affected by the driving force, but also affected by the strong interactions between pore surface and ionic species [31,32]. Factors affecting water diffusion in zeolite channels include surface charge , hydrophilicity , and pore wall roughness . We believe that ion (A− ) adsorption on zeolite channel surface has a dominant effect on water transport in the microchannel as the pentanoic acid concentration changes from 100 ppm to 500 ppm. 3.4. Organics rejection In order to investigate the separation efﬁciency of the membrane for different organics, 100 ppm of pentanoic acid, toluene, and ethanol solutions were prepared and tested for the RO experiments. Table 1 gives the organic rejection and water ﬂux for these solutions. Water ﬂux for 100 ppm toluene solution is very small; 0.03 kg m−2 h−1 compared with 0.33 kg m−2 h−1 for 100 ppm pentanoic acid, and 0.31 kg m−2 h−1 for 100 ppm ethanol. Unlike pentanoic acid, toluene exists in solution as a neutral molecule with molecular kinetic diameter of 0.6 nm , which is bigger than the zeolite pore size. A toluene molecule cannot easily enter the zeolite pore channel. Furthermore, since a zeolite membrane has a large surface area and hence high surface potential, it prefers to adsorb the toluene molecular onto its surface. Low water ﬂux for 100 ppm toluene solution contributes to the adsorbed toluene molecules, which block water molecules transport in the microchannel. On the other hand, the organic rejection for 100 ppm toluene is 99.5%, due to its large molecular size. Table 1 also shows that the organic rejection is around 17% for 100 ppm ethanol. Ethanol molecular size is smaller compared with zeolite pore size. The molecular kinetic diameter for ethanol is 0.44 nm , so ethanol can easily enter the membrane channel. Molecular dynamic simulation results by Yan et al. revealed that ethanol molecular could easily penetrate a zeolite membrane with pore size 0.50 nm . The observed 17% organic rejection for 100 ppm ethanol probably is due to the different diffusivities between ethanol and water molecules in the membrane channel. 4. Conclusions
Fig. 5. Organic rejection and water ﬂux with different concentrations of pentanoic acid.
MFI silicate zeolite membranes were successfully synthesized on the inner surface of tubular ␣-alumina substrates. The membrane has a thickness of 1.2 m and shows high Na+ rejection (99.4%) for 0.1 M NaCl. Organic separation from aqueous solution by zeolite membranes indicated that the membrane has great potential for dissolved organic separation. The separation efﬁciency is determined by the charge and dynamic size of organic
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species. For 100 ppm pentanoic acid solution, a signiﬁcant repulsion exists between the negatively charged MFI zeolite framework and the pentanoic anion. The membrane was found to have 96.5% organic rejection with a water ﬂux of 0.33 kg m−2 h−1 . For 100 ppm toluene solution, a larger molecule prevented toluene from entering the zeolite pore and resulted in 99.5% organic rejection and 0.03 kg m−2 h−1 water ﬂux. Ethanol is a small, neutral molecule. The synthesized membrane showed 17% organic rejection and 0.31 kg m−2 h−1 water ﬂux for 100 ppm ethanol solutions. Acknowledgements This project was funded by the US Department of Energy, National Energy Technology Laboratory, contract no. DE-FC2604NT15548. The authors would like to thank Ms. Liz Bustamante for editing the manuscript. References  B.E. Smith, Desalting and ground water management in the San Joaquin Valley, California, Desalination 87 (1992) 151. ´  B.J. Marinas, R.E. Selleck, Reverse osmosis treatment of multicomponent electrolyte solution, J. Membr. Sci. 72 (1992) 211.  L. Li, A. Ryan, Tina, M. Nenoff, J. Dong, R. Lee, Puriﬁcation of coal-bed methane produced water by zeolite membranes, Paper SPE 89892, presented at the SPE Annual Technical Conference and Exhibition, Houston, Tx, September, 26–29, 2004.  F.T. Tao, S. Curtice, R.D. Hobbs, J.L. Sides, J.D. Wieser, C.A. Dyke, D. Tuohey, F.F. Pilger, Reverse osmosis process successfully converts oil ﬁeld brine into freshwater, Oil Gas J. 91 (1993) 88.  C. Visvanathan, P. Svenstrup, P. Arlyamethee, Volume reduction of produced water generated from natural gas production process using membrane technology, Water Sci. Technol. 41 (2000) 117.  T. Sirivedhin, J. McCue, L. Dallbauman, Reclaiming produced water for beneﬁcial use: salt removal by electrodialysis, J. Membr. Sci. 243 (2004) 335.  E.M. Gwon, M.J. Yu, H.K. Oh, Y.H. Ylee, Fouling characteristics of NF and RO operated for removal of dissolved matter from groundwater, Water Res. 37 (2003) 2989.  X. Zhu, M. Elimelech, Fouling of reverse osmosis membranes by aluminum oxide colloids, J. Environ. Eng.-ASCE 121 (1995) 884.  J. Lin, S. Murad, A computer simulation study of the separation of aqueous solution using thin zeolite membranes, Mol. Phys. 99 (2001) 1175.  S. Murad, L.C. Nitche, The effect of thickness, pore size and structure of a nanomembrane on the ﬂux and selectivity in reverse osmosis separations: a molecular dynamics study, Chem. Phys. Lett. 397 (2004) 211.  J. Lin, S. Murad, The role of external electric ﬁelds in membrane-based separation processes: a molecular dynamics study, Mol. Phys. 99 (2001) 463.  S. Murad, K. Oder, J. Lin, Molecular simulation of osmosis, reverse osmosis, and electro-osmosis in aqueous and methanolic electrolyte solutions, Mol. Phys. 95 (1998) 401.  S. Murad, W. Jia, M. Krishnamurthy, Ion-exchange of monovalent and bivalent cations with NaA zeolite membranes: a molecular dynamics study, Mol. Phys. 102 (2004) 2103.
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