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Synthesis of high-performance thin film composite (TFC) membranes by controlling the preparation conditions: Technical notes
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Synthesis of high-performance thin film composite (TFC) membranes by controlling the preparation conditions: Technical notes Mohammed Kadhoma,b, Baolin Denga,c,
⁎
a
Department of Chemical Engineering, University of Missouri, Columbia, MO 65211, USA Al-Dour Technical Institute, Northern Technical University, Al-Dour, Saladin, Iraq c Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Reverse osmosis (RO) Thin film composite (TFC) membranes Desalination Interfacial polymerization (IP) Phase inversion Membranes preparation
Reverse osmosis (RO) is one of the most globally competitive technologies for desalination and water purification, in which thin film composite (TFC) membrane is the common form of membranes used. In this paper, we examined changes in membrane performance by studying the effects of 1) m-phenylenediamine (MPD) and trimesoyl chloride (TMC) exposure times; 2) different phase inversion methods to prepare polysulfone (PSU) support sheets; 3) the support sheet thickness, and 4) the fabricated membrane’s drying temperature. Operational conditions of 300 psi, 25 °C, 2000 ppm NaCl, and sample run time of at least 8 h were applied. Optimal performance results were achieved when the interfacial polymerization time was short, and a vertical immersion during PSU layer preparation was applied. Increasing PSU sheet thickness increased the salt rejection but decreased the water flux, while rising the curing temperature led to an enhancement in salt rejection and water flux at short reaction times. Based on this study, a TFC membrane of NaCl rejection 98.8 ± 0.5% and water flux 76.1 ± 2.7 L/m2 h was fabricated using 25 s MPD contacting time, 5 s TMC reaction time, and 110 °C drying temperature. This result is considered high among reported outcomes in the literature for simply prepared TFC membranes.
1. Introduction Water shortage is a serious challenge arsing from and limiting human growth and development. Different forms of technologies were developed to treat and use water from different resources [1]. Since the first recorded membrane was invented in the nineteenth century by Traube [2], membrane technology has been increasingly used in different separation systems, such as gas separation and water treatment [3]. Reverse Osmosis (RO) is a relatively mature method of desalination that produces potable water from a saline solution [4]. Large-scale RO plants are of high interest due to the increase in demand for pure water, to illustrate, the number of large-scale plants in operation has doubled within the past decade [5]. This trend also indicates the reliability of this technology and its competitiveness in terms of energy and scaling up in operation. The membrane is the key part of the RO process, its development took place in the late 1950’s when Loeb and Sourirajan invented the first applicable membrane made of cellulose acetate [6]. This membrane dominated research and practical applications till the early 1980s, when John Caddote invented the polyamide (PA) thin film composite membrane by the interfacial polymerization (IP) of MPD in ⁎
aqueous solution and TMC in organic solvent [7]. The TFC membrane has become the state-of-the-art for RO processes, including efforts to improve its performance by incorporating additives. Filling nanoparticles (NPs), such as silica [8,9], clay [10], zeolite [11], and metalorganic frameworks [12,13], inside the membrane results in the thin film nanocomposite (TFN) membranes with enhanced performance. Also, following the preparation by post treatments [14] was another way to improve the membrane product. The TFC membrane is normally few hundred nanometer thick that attached to a polymeric supporting sheet, such as PSU, which is manufactured using the phase inversion phenomenon [15,16]. Desalination through membranes has been explained by different theories including the solution-diffusion theory, in which water and salts molecules diffuse inside the PA layer and transport from one side to another [17]. Here, high membrane hydrophilicity is an important property, since it helps prevent bio-fouling and attracts water molecules [18]. Of course a broader perspective is needed to study the development and optimization of membrane performance. Among many parameters affecting membrane water desalination, Wilf et al. studied the effect of salt concentrations in feed water on membrane salt rejection [19]. Results showed that higher amounts of
Corresponding author at: Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211, USA. E-mail address: [email protected] (B. Deng).
https://doi.org/10.1016/j.jwpe.2017.12.011 Received 23 November 2017; Received in revised form 24 December 2017; Accepted 24 December 2017 2214-7144/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Kadhom, M., Journal of Water Process Engineering (2017), https://doi.org/10.1016/j.jwpe.2017.12.011
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characteristics. PSU sheets were prepared for the SEM analysis by drying them at room temperature and storing at 4 °C until the examination. The sheets were spread on a sticky testing pan and coated with 10 nm platinum layer using an EMS 150TES sputter coater for 1 min at 20 milliAmps, before the specimens were mounted onto an FEI Quanta 600F Environmental SEM for the imaging at different resolutions.
salt passed through the membrane at low and high feed salinity, when compared with moderate salt concentration. The highly negatively charged membranes imparted strong Donnan potential for moderate salinity feed; however, this potential became weak by using high NaCl concentration feed. Ghosh et al. [20] investigated and reported the effects of 1) dissolving TMC in four organic solvents, 2) adding TEACSA salt to MPD solution, 3) the temperature of the TMC solution when the PA layer was synthesized, and 4) the curing temperature on the membranes’ efficiency. A more recent study from our group [21] focused on the role of the PSU support layer showing that by increasing PSU concentration in the casting solution, the water flux decreased. Ho’s group has also examined the effects of controlling the membrane synthesis conditions, including the TMC exposure time, curing time, TEA-CSA salt concentration, TMC and MPD concentrations, and different post treatment approaches [22,23]. These studies showed that many factors and conditions may affect membrane efficiency. Nevertheless, MPD exposure time was not studied as much as the reaction time, even though it influenced the membrane specifications. Also, many groups are using commercial support layers, so the effects of support layer thickness and phase inversion procedures were not commonly reported. In this work, the effects of MPD aqueous solution and TMC-organic solution exposure time were studied. Also, three phase inversion approaches for PSU sheet preparation were applied and the best was selected by the scanning electron microscopy (SEM) examination. The influence on membrane performance by varying the support sheet thickness and increasing the curing temperature during the membrane manufacturing were reported as well. This work produced very high-performance membranes by only controlling the synthesis conditions, the best among them had a water flux of 76.1 ± 2.7 L/m2hr and 98.8 ± 0.5% salt rejection as tested by 2000 ppb NaCl and at 300 psi transmembrane pressure.
2.3. TFC membrane preparation TFC membranes were prepared by the interfacial polymerization reaction of MPD and TMC on a PSU sheet. The PSU layer was spread gently on a plate of glass and the excess water from storage was removed by a squeegee roller. MPD aqueous solution, which consisted of 2% MPD, 1% CSA/TEA salt, and 0.01% CaCl2, was first poured onto the PSU sheet. MPD solution-PSU layer contact time varied in order to evaluate its effect on the membrane performance. A TMC solution, made by dissolving 0.15% TMC in isooctane, was poured onto PSU sheet that contained MPD active sites on its surface and the reaction time was changed to study its effect. The extra TMC solution was removed by a casting knife. The selection of isooctane as organic solvent was based on the recommendations from our previous work [8]. The prepared TFC membrane was dried at 80 °C for 6 min in all preparations, except for the part studying the temperature effect where the temperature was raised to 110 °C. The final membrane product was soaked in DI water and stored in a refrigerator for at least 20 h at 4 °C before the performance assessments were applied. 2.4. Performance assessments Each prepared membrane in this study was placed in a cross-flow testing system as shown in our previous work [8] to measure the water flux and salt rejection. The tests were conducted using 2000 ppm NaCl feed solution and 300 psi cross-membrane pressure at room temperature. The filtration cell had a surface area of 9.6 cm2 (Millipore Corp, stainless steel XX4504700). The water flux was calculated based on the accumulated volume of permeated water as a function of time, whereas the salt rejection was determined by measuring the total dissolved solids (TDS) of the in and out water, using a conductivity meter (HACH Company). The feed tank temperature was maintained at 25 °C by an external water bath. The water flux and salt rejection were calculated by Eqs. (1) and (2), respectively.
2. Materials and methods 2.1. Materials Polysulfone pellets (PSU, MW = 35000) and N,N-dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich and used for PSU sheet preparation. The membrane synthesis materials, m-phenylenediamine (MPD, ≥99%) and trimesoyl chloride (TMC, ≥98.5%), were purchased from Fisher Scientific and Sigma-Aldrich, respectively. 2, 2, 4-trimethylpentane (isooctane, 99%) and calcium chloride (CaCl2) were obtained from Fisher Scientific. Triethylamine (TEA, ≥99%), (1s)(+)-10-camphorsulfonic acid (CSA, 99%), and sodium chloride (NaCl) were purchased from Sigma Aldrich. A Millipore DI water (Synergy185, 18.2 MΩ-cm) system was employed to prepare MPD aqueous solution and for cleaning purposes.
J=
V A. t
(1)
Where J is the water flux (L/m hr), V the product volume (L), A the membrane area (m2), and t the accumulation time (h). 2
Cp ⎞ × 100 R= ⎛1 − Cf ⎠ ⎝
2.2. Preparation and characterization of PSU support layer sheets
(2)
Where R is the salt rejection ratio, Cp the permeate conductivity, and Cf the feed conductivity. It is worth noting that a test under each condition was conducted at least three times, and the average result was reported with the standard deviation shown as error bars.
PSU supporting sheets were prepared by the phase inversion process. Briefly, a solution was made by dissolving polysulfone pellets in DMF solvent in portions of 15: 85 wt.%, respectively. The mixture was heated to 60 °C and stirred for 6 h until a clear solution formed, which was left over night for degassing. The process invovled in taking an aliquot from the solution by a pipette to spread on a glass plate, then a casting knife (MTI Corp, EQ-Se-KTQ-150) was used to cast the solution to the desired thickness, 130 μm in this work except for the section of studying thickness effect. Afterwards, the glass plate with the solution on its top was immersed in water, resulting in immediate formation of the support sheet. Finally, the sheets were collected, rinsed, and stored in DI water for at least 24 h at 4 °C prior to use. Three submerging ways of the glass plate into water were examined, free drop, horizontal, and vertical, during the preparation of PSU support sheets. The SEM images for the membranes were captured to study the impact of the submerging ways on the membrane morphological
3. Results and discussion 3.1. Effect of MPD and TMC exposure times TFC membranes were fabricated by the IP of MPD and TMC, in which the exposure time for both chemicals was found to largely impact the membrane performance. In this section, the study was conducted using 130 μm horizontally immersed PSU sheets and curing temperature of 80 °C. MPD exposure time may affect the density of the active sites on PSU sheet surface, while TMC exposure time is the IP reaction 2
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Fig. 1. The effect of MPD exposure time on membranes performance. Samples were prepared on 130 μm thick horizontally immersed PSU supprt sheets and cured at 80 °C, (a) TMC Exposure Time (5 s). (b) TMC Exposure Time (10 s). (c) TMC Exposure Time (15 s). (d) TMC Exposure Time (20 s). (e) TMC Exposure Time (25 s).
are shown in the following:
time, in which the membrane is generated at the separation interface. Here, since TMC has a low solubility in water, the IP occurred in the organic phase, so it is required to maintain MPD/TMC ratio high to drive MPD diffusion into TMC-isooctane solution [20,24]. However, if the ratio is too high, a reduction in carboxylic acid groups may occur, which decrease the hydrophilicity and surface charge, resulting in the water flux and salt rejection decreases hence [25]. The detailed results
3.1.1. MPD exposing time Pouring MPD aqueous solution on the PSU layer is the first step in TFC membrane synthesis. Four MPD-PSU contact times (10, 20, 25, and 30 s) were investigated to evaluate the influence of changing MPD contact time on the membrane performance at a constant TMC contact 3
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Fig. 2. The effect of TMC exposure time on membranes performance. Samples were prepared on 130 μm thick horizontally immersed PSU support sheets and cured at 80 °C, (a) MPD Exposure Time (10 s). (b) MPD Exposure Time (20 s). (c) MPD Exposure Time (25 s). (d) MPD Exposure Time (30 s). Fig. 3. Phase inversion methods.
increasing the exposure time up to 25 s, but decreased at longer times. The results at times shorter than 25 s are exceptional, which is consistent with the general concept that the increase in thin film thickness leads to decrease the water flux, as reported in the literature [20]. This could be attributed to two reasons. First, when MPD exposure time was increased, more MPD molecules would react with the surface. Some MPD molecules might have one or both amine groups oxidized, which increased the hydrophilicity. The second is related to the cross linking
time (Fig. 1). As shown in Fig. 1a, when the TMC exposure time was 5 s, the salt rejection and water flux didn’t change significantly by changing MPD solution exposure time. By increasing TMC exposure time, it can be noted that MPD contact time started to affect the results. From Fig. 1b–e, it can be concluded that by increasing MPD exposing time, the salt rejection increased. This can be explained [19] by considering that as MPD is one of the IP reactants, increasing its contact time increased the reaction active sites. The water flux also increased by 4
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Fig. 4. The effect of phase inversion method on PSU sheet morphology and membranes performance. (a) and (c) are for sheets horizontally immersed, (b) and (d) for sheets vertically immersed, (e) the influence on salt rejection, and (f) the influence on water flux. Samples were prepared using 25 s MPD contact time, 130 μm PSU sheets, and 80 °C drying temperature.
film composite membrane [27]. Fig. 2a–d demonstrate TMC contact time effects on both salt rejection and water flux at a constant MPD exposure time. From all figures, it is clear that a reaction time of 5 s gave the best results at all MPD contact times. Generally, the water flux decreased by increasing reaction time, while the salt rejection increased. This might be because of the increase in membrane thickness and cross linking. It can be noted that the salt rejection was almost maintained the same at all reaction times, although longer time gave slightly higher results, which can be attributed to the increase of the top dense layer thickness [28]. Here, the flux dropped by increasing the reaction time because of the dense layer growth. Another explanation could be the physiochemical properties of the reaction. MPD-TMC reaction is rapid, so the formed polyamide layer decreased MPD transfer toward the TMC-isooctane solution. For this reason, no large difference was observed in salt rejection by increasing the reaction time. Conversely, at short reaction times, some TMC molecules did not react,
of the membrane. Increasing MPD-PSU contact could produce a wellformed membrane since membrane generation depends on many factors, including the diffusivity of MPD to TMC-organic solution. Higher MPD contact time generally decreased the total performance, which could be attributed to the lack of free carboxylic groups on the membrane surface [26]. MPD contact time of 25 s gave the best results as deduced from Fig. 1. This time could be critical to have well distributed and absorbed MPD molecules over the support layer, which gave the best cross linking when reacted with the TMC later. 3.1.2. TMC exposing time TMC is the second reactant in the IP reaction to produce the thin film composite membrane. After MPD covered the support sheet surface as explained above, TMC solution was poured onto it for different reaction times (5, 10, 15, 20, and 25 s) at room temperature. TMC’s acyl chloride group reacted with MPD’s amine group and generated the thin 5
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Fig. 5. The effect of support layer thickness on membranes performance. Samples were prepared using 25 s MPD contact time, 15 s TMC reaction time, horizontally immersed PSU sheets, and 80 °C drying temperature.
submerging leaded to generate well-organized and distributed pores. However, horizontal dipping produced different-sized pores, including large ones, around 100 nm. This influenced TMC’s water flux and salt rejection, as will be explained later. Thereby, we like to state that the vertical submerging is easier to apply and control and produced higher performance TFC. Fig. 4e and f shows salt rejection and water flux comparisons, respectively, for sheets made by the horizontal and vertical methods. Fig. 4e shows the salt rejection comparison where the immersion method affected it slightly, while Fig. 4f clearly shows the effect of phase inversion methods on membrane water permeability. The comparison was between membranes made by 25 s MPD contact time and different TMC exposure times, 5, 10, and 15 s on 130 μm thick support sheets, and drying temperature of 80 °C. By immersing the plate horizontally, the salt rejection was slightly higher than the membranes on vertically submerged supports, but the water flux was lower. The vertically submerged sheets were smooth and well-shaped, which either allowed the membrane to form properly or the sheet porosity was higher than the horizontally immersed sheets; as a result, higher water flux was achieved. Another explanation attributes this behavior to the pores size; here, for the horizontally immersed sheets, the pores were larger, which allowed MPD solution to diffuse deeper into the support layer and blocked the pores ends, a thicker polyamide layer is generated hence. As a result, salt rejection increased and water flux decreased when compared with the vertical immersed sheets, in which, pores were smaller [29].
which allowed the acyl chloride group to hydrolyze and form the COOH group. The carboxylic group is hydrophilic, which could increase the membrane hydrophilicity and the water flux afterwards [27]. 3.2. PSU sheets casting methods Different phase inversion methods were tested for PSU support layer synthesis. As mentioned before, the support layer solution was made by dissolving polysulfone in DMF. Fig. 3 shows three methods of immersing the glass plate that helds 130 μm thick casting solution on its top into a water bath. The method illustrated in Fig. 3a was the free drop method, where the glass plate was dropped into the water one time. This method is not recommended because of the holes that generated inside the sheet, in addition to low membrane performance results (for the sheet that has no holes). This is explained by the water covering the plate roughly and producing these holes. The membrane performance results made on these layers were not reported. Fig. 3b shows the second method, in which the glass plate was immersed horizontally in the water bath. The water this time covered the plate more gently and uniformly. This type of submerging is commonly used by researchers. The membranes in this paper were synthesized on support layers made by this form of immersion. The last method is the vertical method, as shown in Fig. 3c, which seemed to work better than others. This method produced very flat, uniform and homogeneous sheets. The morphology, by the naked eye, was very good and no holes existed in the sheets. Fig. 4 illustrates SEM images and performance comparison between the horizontal and vertical immersing methods, where a and c are for the horizontal submerging, b and d for the vertical one, and e and f display the results of salt rejection and water flux, respectively. By comparing a with b, it is clear that the vertical submerging produced high uniformly sheets with no wide pores or change in morphology thickness. Sheets attained from the horizontal submerging showed less uniformity comparing with the vertical method; also, different size pores were observed on top of the sheets. This difference in morphology may be attributed to the way that water covered the casting solution, in which, in the vertical way water surrounded PSU sheets neatly. Horizontal immersion has different challenges. First, the way of submerging the sheet would differ depending on experience of the operator. Second, the submerging angle affected how gently the water covers the casted solution. Third, water level in the bath might also influence how the water becomes in contact. Finally, dipping speed, casting speed, room temperature and humidity, etc. may also affect the output. Fig. 4c and d shows the surface pores, it appeared that vertical
3.3. Effect of PSU layer thickness 15% PSU in DMF solution was casted in four different thicknesses by a casting knife, 50, 80, 130, and 150 μm. The preparation conditions of the membranes were 25 s MPD contact time, 15 s TMC reaction time, horizontal immersing for the support layers, and 80 °C curing temperature. Increasing the support layer thickness resulted in an increased salt rejection but decreased water flux. This could be because of the hydrophobic nature of the PSU. Whenever the sheet thickness increased, the water and salt faced higher resistance. Fig. 5 shows support layer thickness effects on the membrane performance. 3.4. Effect of drying temperature The last aim in this work was to study the effect of increasing the drying temperature. After the membranes were prepared, they were 6
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raising the temperature on salt rejection and water flux, respectively. By increasing the curing temperature, both salt rejection and water flux increased. The samples were made by exposure to MPD for 25 s and TMC for 5, 10, and 15 s on 130 μm thick horizontally immersed support sheets. From Fig. 6a, it is clear that by increasing the curing temperature, NaCl rejection increased. This is due to the fast evaporation of the remaining organic solution, which might give better cross linking for the polyamide layer. In terms of flux, the best results were obtained when the reaction time was 5 s. When the reaction time was longer, the water flux dropped as shown in Fig. 6b. Again, this could be because when the reaction time was short, some of the TMC reacted and the rest left, which hydrolyzed and increased the hydrophilicity. Also, the difference in water flux between the two temperatures dropped as the reaction time increased. This could be attributed to the deformation of the top dense layer of the membrane, as it increased by increasing the reaction time. 3.5. Why this work is important? TFC membranes dominate desalination research and markets. Since they have been discovered, the same manufacturing basics are used, with some modifications. In this work, by manipulating some physical conditions during preparation, a big improvement in performance was achieved, without the incorporation of other additives, complex preparation methods, and/or post treatment. Thereby, at the optimum conditions of synthesis, the produced membranes were highly practical. Table 1 shows our best result compared with other control samples (before they had any additives added or post treatment). By applying this paper’s recommendations, previous and future works could obtain improved results using the same procedures and additives already used. 4. Conclusions In this work, the synthesis conditions of reverse osmosis membranes were investigated. MPD and TMC exposure times were studied and the best results were found to be at 25 s and 5 s for MPD and TMC contacting times, respectively. The best phase inversion method was the vertical immersion method. Support layer thickness affected membrane performance as well. By exploring the trade-off relationship between salt rejection and water flux, it was found that a thickness of 80–130 μm could be the best choice. Finally, increasing curing temperature positively affected the performance of membranes produced at short reaction times, but the effect was diminished by increasing the reaction times. The above report describes a technical guide for researchers and workers who can gain higher yield from membranes that they manufacture. Due to the lack in the literature on the impact of the support layer on TFC membrane performance, this work was designed to narrow partially that gap. However, a wide area is open to study other influencing factors relevant to support layer formation and TFC membrane fabrication.
Fig. 6. The effect of increasing drying temperature on (a) salt rejection and (b) water flux; samples were prepared using 25 s MPD contact time, 15 s TMC reaction time, and 130 μm horizontally immersed PSU sheets.
Table 1 A comparison between this work and other works results. No.
Water permeability (L/m2 h bar)
NaCl rejection%
Reference
1 2 3 4 5 6 7 8 9 10
3.684 ± 0.131 2.553 0.756 0.522 1.378 1.885 0.928 2.364 2.422 ∼ 0.725
98.8 ± 0.5 98.8 93.4 90.4 97.9 95.7 ∼95 ∼98 98.5 ∼92
This Work [22] [30] [31] [14] [32] [33] [11] [34] [35]
Acknowledgment We gratefully acknowledge the Higher Committee for Education Development in Iraq (HCED-Iraq) for providing a scholarship to Mohammed Kadhom and the Missouri Water Resources Research Center for partially supporting this project. References [1] B. Hua, H. Xiong, M. Kadhom, L. Wang, G. Zhu, J. Yang, G. Cunningham, B. Deng, Physico-chemical processes, Water Environ. Res. 89 (10) (2017) 974–1028. [2] E.A. Mason, From pig bladders and cracked jars to polysulfones: an historical perspective on membrane transport, J. Membr. Sci. 3 (1991) 125–145. [3] Q. Xu, H. Xu, J. Chen, Y. Lv, C. Dong, T.S. Sreeprasadc, Graphene and graphene oxide: advanced membranes for gas sepatation and water purification, Inorg. Chem.
heated to 80 °C to evaporate any remaining isooctane. In this section, the drying temperature was raised to 110 °C and the results were compared to the original samples. Fig. 6a and b shows the effect of 7
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[21] C. Ding, J. Yin, B. Deng, Effects of polysulfone (PSf) support layer on the performance of thin-film composite (TFC) membranes, J. Chem. Proc. Eng. 1 (102) (2014) 1–8. [22] H. Dong, L. Zhao, L. Zhang, H. Chen, C. Gao, W. Winston Ho, High-flux reverse osmosis membranes incorporated with NaY zeolite nanoparticles for brackish water desalination, J. Membr. Sci. 476 (2015) 373–383. [23] L. Zhao, P.C.-Y. Chang, C. Yen, W.S. Winston Ho, High-flux and fouling-resistant membranes for brackish water desalination, J. Membr. Sci. 426 (2013) 1–10. [24] V. Freger, Kinetics of film formation by interfacial polycondensation, Langmuir 21 (2005) 1884–1894. [25] B. Tang, C. Zou, P. Wu, Study on a novel polyester composite nanofiltration membrane by interfacial polymerization. II. The role of lithium bromide in the performance and formation of composite membrane, J. Membr. Sci. 365 (2010) 276–285. [26] J. Roh, A. Greenberg, V. Khare, Synthesis and characterization of interfacially polymerized polyamide thin films, Desalination 191 (2006) 279–290. [27] S.-T. Kao, S.-H. Huang, D.-J. Liaw, W.-C. Chao, C.-C. Hu, C.-L. Li, D.-M. Wang, K.R. Lee, J.-Y. Lai, Interfacially polymerized thin-film composite polyamide membrane: positron annihilation spectroscopic study, characterization and pervaporation performance, Polym. J. 42 (2010) 242–248. [28] H. Meng, B. Gong, T. Geng, C. Li, Thinning of reverse osmosis membranes by ionic liquids, Appl. Surf. Sci. 292 (2014) 638–644. [29] A. Ghosh, E. Hoek, Impacts of support membrane structure and chemistry on polyamide?polysulfone interfacial composite membranes, J. Membr. Sci. 336 (2009) 140–148. [30] B.-H. Jeong, E.M. Hoek, Y. Yan, A. Subramani, X. Huang, G. Hurwitz, A.K. Ghosh, A. Jawor, Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis membranes, J. Membr. Sci. 294 (2007) 1–7. [31] G.L. Jadav, P.S. Singh, Synthesis of novel silica-polyamide nanocomposite membrane with enhanced properties, J. Membr. Sci. 328 (2009) 257–267. [32] J. Yin, G. Zhu, B. Deng, Graphene oxide (GO) enhanced polyamide (PA) thin-film nanocomposite (TFN) membrane for water purification, Desalination 379 (2016) 93–101. [33] H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256. [34] H.D. Lee, H.W. Kim, Y.H. Cho, H.B. Park, Experimental evidence of rapid water transport through carbon nanotubes embedded in polymeric desalination membranes, Small 10 (13) (2014) 2653–2660. [35] A. Peyki, A. Rahimpour, M. Jahanshahi, Preparation and characterization of thin film composite reverse osmosis membranes incorporated with hydrophilic SiO2 nanoparticles, Desalination 368 (2015) 152–158.
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Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Synthesis of high-performance thin film composite (TFC) membranes by controlling the preparation conditions: Technical notes Mohammed Kadhoma,b, Baolin Denga,c,
⁎
a
Department of Chemical Engineering, University of Missouri, Columbia, MO 65211, USA Al-Dour Technical Institute, Northern Technical University, Al-Dour, Saladin, Iraq c Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Reverse osmosis (RO) Thin film composite (TFC) membranes Desalination Interfacial polymerization (IP) Phase inversion Membranes preparation
Reverse osmosis (RO) is one of the most globally competitive technologies for desalination and water purification, in which thin film composite (TFC) membrane is the common form of membranes used. In this paper, we examined changes in membrane performance by studying the effects of 1) m-phenylenediamine (MPD) and trimesoyl chloride (TMC) exposure times; 2) different phase inversion methods to prepare polysulfone (PSU) support sheets; 3) the support sheet thickness, and 4) the fabricated membrane’s drying temperature. Operational conditions of 300 psi, 25 °C, 2000 ppm NaCl, and sample run time of at least 8 h were applied. Optimal performance results were achieved when the interfacial polymerization time was short, and a vertical immersion during PSU layer preparation was applied. Increasing PSU sheet thickness increased the salt rejection but decreased the water flux, while rising the curing temperature led to an enhancement in salt rejection and water flux at short reaction times. Based on this study, a TFC membrane of NaCl rejection 98.8 ± 0.5% and water flux 76.1 ± 2.7 L/m2 h was fabricated using 25 s MPD contacting time, 5 s TMC reaction time, and 110 °C drying temperature. This result is considered high among reported outcomes in the literature for simply prepared TFC membranes.
1. Introduction Water shortage is a serious challenge arsing from and limiting human growth and development. Different forms of technologies were developed to treat and use water from different resources [1]. Since the first recorded membrane was invented in the nineteenth century by Traube [2], membrane technology has been increasingly used in different separation systems, such as gas separation and water treatment [3]. Reverse Osmosis (RO) is a relatively mature method of desalination that produces potable water from a saline solution [4]. Large-scale RO plants are of high interest due to the increase in demand for pure water, to illustrate, the number of large-scale plants in operation has doubled within the past decade [5]. This trend also indicates the reliability of this technology and its competitiveness in terms of energy and scaling up in operation. The membrane is the key part of the RO process, its development took place in the late 1950’s when Loeb and Sourirajan invented the first applicable membrane made of cellulose acetate [6]. This membrane dominated research and practical applications till the early 1980s, when John Caddote invented the polyamide (PA) thin film composite membrane by the interfacial polymerization (IP) of MPD in ⁎
aqueous solution and TMC in organic solvent [7]. The TFC membrane has become the state-of-the-art for RO processes, including efforts to improve its performance by incorporating additives. Filling nanoparticles (NPs), such as silica [8,9], clay [10], zeolite [11], and metalorganic frameworks [12,13], inside the membrane results in the thin film nanocomposite (TFN) membranes with enhanced performance. Also, following the preparation by post treatments [14] was another way to improve the membrane product. The TFC membrane is normally few hundred nanometer thick that attached to a polymeric supporting sheet, such as PSU, which is manufactured using the phase inversion phenomenon [15,16]. Desalination through membranes has been explained by different theories including the solution-diffusion theory, in which water and salts molecules diffuse inside the PA layer and transport from one side to another [17]. Here, high membrane hydrophilicity is an important property, since it helps prevent bio-fouling and attracts water molecules [18]. Of course a broader perspective is needed to study the development and optimization of membrane performance. Among many parameters affecting membrane water desalination, Wilf et al. studied the effect of salt concentrations in feed water on membrane salt rejection [19]. Results showed that higher amounts of
Corresponding author at: Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211, USA. E-mail address: [email protected] (B. Deng).
https://doi.org/10.1016/j.jwpe.2017.12.011 Received 23 November 2017; Received in revised form 24 December 2017; Accepted 24 December 2017 2214-7144/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Kadhom, M., Journal of Water Process Engineering (2017), https://doi.org/10.1016/j.jwpe.2017.12.011
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characteristics. PSU sheets were prepared for the SEM analysis by drying them at room temperature and storing at 4 °C until the examination. The sheets were spread on a sticky testing pan and coated with 10 nm platinum layer using an EMS 150TES sputter coater for 1 min at 20 milliAmps, before the specimens were mounted onto an FEI Quanta 600F Environmental SEM for the imaging at different resolutions.
salt passed through the membrane at low and high feed salinity, when compared with moderate salt concentration. The highly negatively charged membranes imparted strong Donnan potential for moderate salinity feed; however, this potential became weak by using high NaCl concentration feed. Ghosh et al. [20] investigated and reported the effects of 1) dissolving TMC in four organic solvents, 2) adding TEACSA salt to MPD solution, 3) the temperature of the TMC solution when the PA layer was synthesized, and 4) the curing temperature on the membranes’ efficiency. A more recent study from our group [21] focused on the role of the PSU support layer showing that by increasing PSU concentration in the casting solution, the water flux decreased. Ho’s group has also examined the effects of controlling the membrane synthesis conditions, including the TMC exposure time, curing time, TEA-CSA salt concentration, TMC and MPD concentrations, and different post treatment approaches [22,23]. These studies showed that many factors and conditions may affect membrane efficiency. Nevertheless, MPD exposure time was not studied as much as the reaction time, even though it influenced the membrane specifications. Also, many groups are using commercial support layers, so the effects of support layer thickness and phase inversion procedures were not commonly reported. In this work, the effects of MPD aqueous solution and TMC-organic solution exposure time were studied. Also, three phase inversion approaches for PSU sheet preparation were applied and the best was selected by the scanning electron microscopy (SEM) examination. The influence on membrane performance by varying the support sheet thickness and increasing the curing temperature during the membrane manufacturing were reported as well. This work produced very high-performance membranes by only controlling the synthesis conditions, the best among them had a water flux of 76.1 ± 2.7 L/m2hr and 98.8 ± 0.5% salt rejection as tested by 2000 ppb NaCl and at 300 psi transmembrane pressure.
2.3. TFC membrane preparation TFC membranes were prepared by the interfacial polymerization reaction of MPD and TMC on a PSU sheet. The PSU layer was spread gently on a plate of glass and the excess water from storage was removed by a squeegee roller. MPD aqueous solution, which consisted of 2% MPD, 1% CSA/TEA salt, and 0.01% CaCl2, was first poured onto the PSU sheet. MPD solution-PSU layer contact time varied in order to evaluate its effect on the membrane performance. A TMC solution, made by dissolving 0.15% TMC in isooctane, was poured onto PSU sheet that contained MPD active sites on its surface and the reaction time was changed to study its effect. The extra TMC solution was removed by a casting knife. The selection of isooctane as organic solvent was based on the recommendations from our previous work [8]. The prepared TFC membrane was dried at 80 °C for 6 min in all preparations, except for the part studying the temperature effect where the temperature was raised to 110 °C. The final membrane product was soaked in DI water and stored in a refrigerator for at least 20 h at 4 °C before the performance assessments were applied. 2.4. Performance assessments Each prepared membrane in this study was placed in a cross-flow testing system as shown in our previous work [8] to measure the water flux and salt rejection. The tests were conducted using 2000 ppm NaCl feed solution and 300 psi cross-membrane pressure at room temperature. The filtration cell had a surface area of 9.6 cm2 (Millipore Corp, stainless steel XX4504700). The water flux was calculated based on the accumulated volume of permeated water as a function of time, whereas the salt rejection was determined by measuring the total dissolved solids (TDS) of the in and out water, using a conductivity meter (HACH Company). The feed tank temperature was maintained at 25 °C by an external water bath. The water flux and salt rejection were calculated by Eqs. (1) and (2), respectively.
2. Materials and methods 2.1. Materials Polysulfone pellets (PSU, MW = 35000) and N,N-dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich and used for PSU sheet preparation. The membrane synthesis materials, m-phenylenediamine (MPD, ≥99%) and trimesoyl chloride (TMC, ≥98.5%), were purchased from Fisher Scientific and Sigma-Aldrich, respectively. 2, 2, 4-trimethylpentane (isooctane, 99%) and calcium chloride (CaCl2) were obtained from Fisher Scientific. Triethylamine (TEA, ≥99%), (1s)(+)-10-camphorsulfonic acid (CSA, 99%), and sodium chloride (NaCl) were purchased from Sigma Aldrich. A Millipore DI water (Synergy185, 18.2 MΩ-cm) system was employed to prepare MPD aqueous solution and for cleaning purposes.
J=
V A. t
(1)
Where J is the water flux (L/m hr), V the product volume (L), A the membrane area (m2), and t the accumulation time (h). 2
Cp ⎞ × 100 R= ⎛1 − Cf ⎠ ⎝
2.2. Preparation and characterization of PSU support layer sheets
(2)
Where R is the salt rejection ratio, Cp the permeate conductivity, and Cf the feed conductivity. It is worth noting that a test under each condition was conducted at least three times, and the average result was reported with the standard deviation shown as error bars.
PSU supporting sheets were prepared by the phase inversion process. Briefly, a solution was made by dissolving polysulfone pellets in DMF solvent in portions of 15: 85 wt.%, respectively. The mixture was heated to 60 °C and stirred for 6 h until a clear solution formed, which was left over night for degassing. The process invovled in taking an aliquot from the solution by a pipette to spread on a glass plate, then a casting knife (MTI Corp, EQ-Se-KTQ-150) was used to cast the solution to the desired thickness, 130 μm in this work except for the section of studying thickness effect. Afterwards, the glass plate with the solution on its top was immersed in water, resulting in immediate formation of the support sheet. Finally, the sheets were collected, rinsed, and stored in DI water for at least 24 h at 4 °C prior to use. Three submerging ways of the glass plate into water were examined, free drop, horizontal, and vertical, during the preparation of PSU support sheets. The SEM images for the membranes were captured to study the impact of the submerging ways on the membrane morphological
3. Results and discussion 3.1. Effect of MPD and TMC exposure times TFC membranes were fabricated by the IP of MPD and TMC, in which the exposure time for both chemicals was found to largely impact the membrane performance. In this section, the study was conducted using 130 μm horizontally immersed PSU sheets and curing temperature of 80 °C. MPD exposure time may affect the density of the active sites on PSU sheet surface, while TMC exposure time is the IP reaction 2
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Fig. 1. The effect of MPD exposure time on membranes performance. Samples were prepared on 130 μm thick horizontally immersed PSU supprt sheets and cured at 80 °C, (a) TMC Exposure Time (5 s). (b) TMC Exposure Time (10 s). (c) TMC Exposure Time (15 s). (d) TMC Exposure Time (20 s). (e) TMC Exposure Time (25 s).
are shown in the following:
time, in which the membrane is generated at the separation interface. Here, since TMC has a low solubility in water, the IP occurred in the organic phase, so it is required to maintain MPD/TMC ratio high to drive MPD diffusion into TMC-isooctane solution [20,24]. However, if the ratio is too high, a reduction in carboxylic acid groups may occur, which decrease the hydrophilicity and surface charge, resulting in the water flux and salt rejection decreases hence [25]. The detailed results
3.1.1. MPD exposing time Pouring MPD aqueous solution on the PSU layer is the first step in TFC membrane synthesis. Four MPD-PSU contact times (10, 20, 25, and 30 s) were investigated to evaluate the influence of changing MPD contact time on the membrane performance at a constant TMC contact 3
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Fig. 2. The effect of TMC exposure time on membranes performance. Samples were prepared on 130 μm thick horizontally immersed PSU support sheets and cured at 80 °C, (a) MPD Exposure Time (10 s). (b) MPD Exposure Time (20 s). (c) MPD Exposure Time (25 s). (d) MPD Exposure Time (30 s). Fig. 3. Phase inversion methods.
increasing the exposure time up to 25 s, but decreased at longer times. The results at times shorter than 25 s are exceptional, which is consistent with the general concept that the increase in thin film thickness leads to decrease the water flux, as reported in the literature [20]. This could be attributed to two reasons. First, when MPD exposure time was increased, more MPD molecules would react with the surface. Some MPD molecules might have one or both amine groups oxidized, which increased the hydrophilicity. The second is related to the cross linking
time (Fig. 1). As shown in Fig. 1a, when the TMC exposure time was 5 s, the salt rejection and water flux didn’t change significantly by changing MPD solution exposure time. By increasing TMC exposure time, it can be noted that MPD contact time started to affect the results. From Fig. 1b–e, it can be concluded that by increasing MPD exposing time, the salt rejection increased. This can be explained [19] by considering that as MPD is one of the IP reactants, increasing its contact time increased the reaction active sites. The water flux also increased by 4
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Fig. 4. The effect of phase inversion method on PSU sheet morphology and membranes performance. (a) and (c) are for sheets horizontally immersed, (b) and (d) for sheets vertically immersed, (e) the influence on salt rejection, and (f) the influence on water flux. Samples were prepared using 25 s MPD contact time, 130 μm PSU sheets, and 80 °C drying temperature.
film composite membrane [27]. Fig. 2a–d demonstrate TMC contact time effects on both salt rejection and water flux at a constant MPD exposure time. From all figures, it is clear that a reaction time of 5 s gave the best results at all MPD contact times. Generally, the water flux decreased by increasing reaction time, while the salt rejection increased. This might be because of the increase in membrane thickness and cross linking. It can be noted that the salt rejection was almost maintained the same at all reaction times, although longer time gave slightly higher results, which can be attributed to the increase of the top dense layer thickness [28]. Here, the flux dropped by increasing the reaction time because of the dense layer growth. Another explanation could be the physiochemical properties of the reaction. MPD-TMC reaction is rapid, so the formed polyamide layer decreased MPD transfer toward the TMC-isooctane solution. For this reason, no large difference was observed in salt rejection by increasing the reaction time. Conversely, at short reaction times, some TMC molecules did not react,
of the membrane. Increasing MPD-PSU contact could produce a wellformed membrane since membrane generation depends on many factors, including the diffusivity of MPD to TMC-organic solution. Higher MPD contact time generally decreased the total performance, which could be attributed to the lack of free carboxylic groups on the membrane surface [26]. MPD contact time of 25 s gave the best results as deduced from Fig. 1. This time could be critical to have well distributed and absorbed MPD molecules over the support layer, which gave the best cross linking when reacted with the TMC later. 3.1.2. TMC exposing time TMC is the second reactant in the IP reaction to produce the thin film composite membrane. After MPD covered the support sheet surface as explained above, TMC solution was poured onto it for different reaction times (5, 10, 15, 20, and 25 s) at room temperature. TMC’s acyl chloride group reacted with MPD’s amine group and generated the thin 5
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Fig. 5. The effect of support layer thickness on membranes performance. Samples were prepared using 25 s MPD contact time, 15 s TMC reaction time, horizontally immersed PSU sheets, and 80 °C drying temperature.
submerging leaded to generate well-organized and distributed pores. However, horizontal dipping produced different-sized pores, including large ones, around 100 nm. This influenced TMC’s water flux and salt rejection, as will be explained later. Thereby, we like to state that the vertical submerging is easier to apply and control and produced higher performance TFC. Fig. 4e and f shows salt rejection and water flux comparisons, respectively, for sheets made by the horizontal and vertical methods. Fig. 4e shows the salt rejection comparison where the immersion method affected it slightly, while Fig. 4f clearly shows the effect of phase inversion methods on membrane water permeability. The comparison was between membranes made by 25 s MPD contact time and different TMC exposure times, 5, 10, and 15 s on 130 μm thick support sheets, and drying temperature of 80 °C. By immersing the plate horizontally, the salt rejection was slightly higher than the membranes on vertically submerged supports, but the water flux was lower. The vertically submerged sheets were smooth and well-shaped, which either allowed the membrane to form properly or the sheet porosity was higher than the horizontally immersed sheets; as a result, higher water flux was achieved. Another explanation attributes this behavior to the pores size; here, for the horizontally immersed sheets, the pores were larger, which allowed MPD solution to diffuse deeper into the support layer and blocked the pores ends, a thicker polyamide layer is generated hence. As a result, salt rejection increased and water flux decreased when compared with the vertical immersed sheets, in which, pores were smaller [29].
which allowed the acyl chloride group to hydrolyze and form the COOH group. The carboxylic group is hydrophilic, which could increase the membrane hydrophilicity and the water flux afterwards [27]. 3.2. PSU sheets casting methods Different phase inversion methods were tested for PSU support layer synthesis. As mentioned before, the support layer solution was made by dissolving polysulfone in DMF. Fig. 3 shows three methods of immersing the glass plate that helds 130 μm thick casting solution on its top into a water bath. The method illustrated in Fig. 3a was the free drop method, where the glass plate was dropped into the water one time. This method is not recommended because of the holes that generated inside the sheet, in addition to low membrane performance results (for the sheet that has no holes). This is explained by the water covering the plate roughly and producing these holes. The membrane performance results made on these layers were not reported. Fig. 3b shows the second method, in which the glass plate was immersed horizontally in the water bath. The water this time covered the plate more gently and uniformly. This type of submerging is commonly used by researchers. The membranes in this paper were synthesized on support layers made by this form of immersion. The last method is the vertical method, as shown in Fig. 3c, which seemed to work better than others. This method produced very flat, uniform and homogeneous sheets. The morphology, by the naked eye, was very good and no holes existed in the sheets. Fig. 4 illustrates SEM images and performance comparison between the horizontal and vertical immersing methods, where a and c are for the horizontal submerging, b and d for the vertical one, and e and f display the results of salt rejection and water flux, respectively. By comparing a with b, it is clear that the vertical submerging produced high uniformly sheets with no wide pores or change in morphology thickness. Sheets attained from the horizontal submerging showed less uniformity comparing with the vertical method; also, different size pores were observed on top of the sheets. This difference in morphology may be attributed to the way that water covered the casting solution, in which, in the vertical way water surrounded PSU sheets neatly. Horizontal immersion has different challenges. First, the way of submerging the sheet would differ depending on experience of the operator. Second, the submerging angle affected how gently the water covers the casted solution. Third, water level in the bath might also influence how the water becomes in contact. Finally, dipping speed, casting speed, room temperature and humidity, etc. may also affect the output. Fig. 4c and d shows the surface pores, it appeared that vertical
3.3. Effect of PSU layer thickness 15% PSU in DMF solution was casted in four different thicknesses by a casting knife, 50, 80, 130, and 150 μm. The preparation conditions of the membranes were 25 s MPD contact time, 15 s TMC reaction time, horizontal immersing for the support layers, and 80 °C curing temperature. Increasing the support layer thickness resulted in an increased salt rejection but decreased water flux. This could be because of the hydrophobic nature of the PSU. Whenever the sheet thickness increased, the water and salt faced higher resistance. Fig. 5 shows support layer thickness effects on the membrane performance. 3.4. Effect of drying temperature The last aim in this work was to study the effect of increasing the drying temperature. After the membranes were prepared, they were 6
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raising the temperature on salt rejection and water flux, respectively. By increasing the curing temperature, both salt rejection and water flux increased. The samples were made by exposure to MPD for 25 s and TMC for 5, 10, and 15 s on 130 μm thick horizontally immersed support sheets. From Fig. 6a, it is clear that by increasing the curing temperature, NaCl rejection increased. This is due to the fast evaporation of the remaining organic solution, which might give better cross linking for the polyamide layer. In terms of flux, the best results were obtained when the reaction time was 5 s. When the reaction time was longer, the water flux dropped as shown in Fig. 6b. Again, this could be because when the reaction time was short, some of the TMC reacted and the rest left, which hydrolyzed and increased the hydrophilicity. Also, the difference in water flux between the two temperatures dropped as the reaction time increased. This could be attributed to the deformation of the top dense layer of the membrane, as it increased by increasing the reaction time. 3.5. Why this work is important? TFC membranes dominate desalination research and markets. Since they have been discovered, the same manufacturing basics are used, with some modifications. In this work, by manipulating some physical conditions during preparation, a big improvement in performance was achieved, without the incorporation of other additives, complex preparation methods, and/or post treatment. Thereby, at the optimum conditions of synthesis, the produced membranes were highly practical. Table 1 shows our best result compared with other control samples (before they had any additives added or post treatment). By applying this paper’s recommendations, previous and future works could obtain improved results using the same procedures and additives already used. 4. Conclusions In this work, the synthesis conditions of reverse osmosis membranes were investigated. MPD and TMC exposure times were studied and the best results were found to be at 25 s and 5 s for MPD and TMC contacting times, respectively. The best phase inversion method was the vertical immersion method. Support layer thickness affected membrane performance as well. By exploring the trade-off relationship between salt rejection and water flux, it was found that a thickness of 80–130 μm could be the best choice. Finally, increasing curing temperature positively affected the performance of membranes produced at short reaction times, but the effect was diminished by increasing the reaction times. The above report describes a technical guide for researchers and workers who can gain higher yield from membranes that they manufacture. Due to the lack in the literature on the impact of the support layer on TFC membrane performance, this work was designed to narrow partially that gap. However, a wide area is open to study other influencing factors relevant to support layer formation and TFC membrane fabrication.
Fig. 6. The effect of increasing drying temperature on (a) salt rejection and (b) water flux; samples were prepared using 25 s MPD contact time, 15 s TMC reaction time, and 130 μm horizontally immersed PSU sheets.
Table 1 A comparison between this work and other works results. No.
Water permeability (L/m2 h bar)
NaCl rejection%
Reference
1 2 3 4 5 6 7 8 9 10
3.684 ± 0.131 2.553 0.756 0.522 1.378 1.885 0.928 2.364 2.422 ∼ 0.725
98.8 ± 0.5 98.8 93.4 90.4 97.9 95.7 ∼95 ∼98 98.5 ∼92
This Work [22] [30] [31] [14] [32] [33] [11] [34] [35]
Acknowledgment We gratefully acknowledge the Higher Committee for Education Development in Iraq (HCED-Iraq) for providing a scholarship to Mohammed Kadhom and the Missouri Water Resources Research Center for partially supporting this project. References [1] B. Hua, H. Xiong, M. Kadhom, L. Wang, G. Zhu, J. Yang, G. Cunningham, B. Deng, Physico-chemical processes, Water Environ. Res. 89 (10) (2017) 974–1028. [2] E.A. Mason, From pig bladders and cracked jars to polysulfones: an historical perspective on membrane transport, J. Membr. Sci. 3 (1991) 125–145. [3] Q. Xu, H. Xu, J. Chen, Y. Lv, C. Dong, T.S. Sreeprasadc, Graphene and graphene oxide: advanced membranes for gas sepatation and water purification, Inorg. Chem.
heated to 80 °C to evaporate any remaining isooctane. In this section, the drying temperature was raised to 110 °C and the results were compared to the original samples. Fig. 6a and b shows the effect of 7
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