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MWNTs/Polyester Thin Film Nanocomposite Membrane: An Approach To Overcome the Trade-Off Effect between Permeability and Selectivity Huiqing Wu, Beibei Tang,* and Peiyi Wu* The Key Laboratory of Molecular Engineering of Polymers (Ministry of Education) and Department of Macromolecular Science and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: August 3, 2010; ReVised Manuscript ReceiVed: August 23, 2010
An improved process to prepare MWNTs/polyester thin film nanocomposite membranes was initiated by interfacial polymerization of trimesoyl chloride (TMC) and triethanolamine (TEOA) solution containing MWNTs. The improved process was facilely done by immersing the support membrane into the organic phase before the conventional process of interfacial polymerization. The TEM images showed that the MWNTs were embedded throughout the polyester thin film layer. The MWNTs/polyester thin film nanocomposite membrane prepared via the improved process exhibited both high permeability and excellent selectivity when compared with the thin film composite membrane without MWNTs and the MWNTs/polyester thin film nanocomposite membrane prepared via the conventional process. The water permeability increased upon an increase of reaction time of TMC-saturated support membrane immersed into aqueous phase (step-1), reaching a maximum of 4.7 L/m2 h at 25 min, while the membrane rejection rate kept increasing. The role of step-1 played a positive role in the performance and the morphology of the thin film composite membrane. It was found that the process of step-1 produced a low degree of cross-linking thin layer with high amount of hydrophilic and negatively charged carboxyl groups. This improved process of interfacial polymerization to prepare MWNTs/polymer thin film nanocomposite membranes provides a new approach to overcome the trade-off effect between permeability and selectivity. 1. Introduction In recent years, nanotechnology has captured the world’s attention and is being considered as a key technology that might have the potential to produce sweeping changes to almost all aspects of human society beyond the scope of conventional technologies.1 The breakthrough in nanotechnology includes the rapid development and extensive application of nanomaterials. Among its versatile applications, the application of nanotechnology in membrane separation is developing gradually and still needs more exploration. Inorganic nanomaterials, such as zeolite, titanium dioxide, carbon graphite, and silica, have been used to improve the performances of polymeric membranes, which usually have an inherent drawback: the trade-off effect between permeability and selectivity.2-8 Carbon nanotubes (CNTs) attracted considerable attention from both academia and industry due to their excellent mechanical, electrical, and thermal properties.9-13 The rapid mass transport behavior of CNTs has also been gaining interest. The high fluid fluxes reported in the recent publications contribute to atomic-scale smoothness of the CNT walls and the high amount of hydrophilic groups on it.13-15 Therefore, advances in nanoporous CNTs/polymer membrane design for improved properties such as chemical selectivity and permeability can be attained by incorporation and alignment of CNTs across a polymer film to form a well-ordered membrane structure. Unfortunately, it is very difficult to align the nanotubes due to the extremely flexible and high aspect ratio of CNTs. Incorporation and dispersion of modified CNTs in the polymer matrix is another facile approach to obtain a high hydrophilic membrane with fast mass transport. Many researchers have been * Corresponding authors. E-mail:
[email protected] (B.T.) and
[email protected] (P.W.).
dedicated to this work, and some have achieved positive results. For instance, Choi et al. prepared an acidified-MWNTs/ polysulfone (PSf) blend membrane through an immersed phase inversion process using N-methyl-2-pyrrolidinone (NMP) as a solvent and water as a coagulant.15,16 Qiu et al. prepared a MWNTs/PSf blend ultrafiltration membrane with MWNTs functionalized by isocyanate and isophthaloyl chloride groups via the reaction between carboxylated carbon nanotubes and 5-isocyanato-isophthaloyl chloride.17 However, mixed matrix membranes based on CNTs dispersed inside a polymer matrix are usually for ultrafiltration separation. Preparation of CNTs/ polymer membrane for nanofiltration is rarely reported. A nanofiltration (NF) membrane is a kind of pressure driven membrane with separation characteristics between reverse osmosis (RO) and ultrafiltration (UF) membranes.18-20 Because NF membranes take the advantages of high permeation flux, high retention of multivalent ion salts, low operation pressure, and low maintenance cost, they have been used in various industrial fields, such as water treatment, pharmaceuticals, biochemical industries, etc.21,22 The current worldwide expansion and diverse applications of NF technology result from the introduction of thin-film composite (TFC) membranes by interfacial polymerization in 1972.23,24 The TFC membrane is characterized by an ultrathin separation selective layer supported on a porous substrate wherein the benefits of two separate polymeric layers could be combined to obtain the desired performance properties for various applications. Recently, a new class of thin film nanocomposite (TFN) membranes was constructed by interfacial polymerization with silver, zeolite, and titania nanoparticles incorporated within the thin film layer.2-4,25 The TFN membranes were prepared by interfacial polymerization through conventional processes, which
10.1021/jp107280m 2010 American Chemical Society Published on Web 09/09/2010
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SCHEME 1: Schematic Description of an Improved Process for Preparing a TFN Membrane by Interfacial Polymerization of TEOA and TMC on the Polysulfone Support Membrane
were done by immersing the support membrane into an aqueous phase followed by an organic phase containing nanoparticles. The obtained TFN membranes had improved performance in some respects, such as water flux, rejection, thermal stability, or mechanical property. However, it is not quite compatible to prepare the MWNTs/polymer thin film nanocomposite membrane through the conventional process, due to the worse dispersion of MWNTs in the nonpolar solvent of the organic phase. In addition, it is still challenging to explore the greater potential of performance improvement. In this research, we initiated an improved process to prepare MWNTs/polyester thin film nanocomposite membranes with MWNTs being embedded throughout the polyester thin film layer. The role of the procedure of the improved process in the membrane structure and formation was explored. 2. Experimental Section 2.1. Materials. The microporous polysulphone support film was supplied by the Development Center of Water Treatment Technology (Hangzhou, China). Triethanolamine (TEOA) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Trimesoyl chloride (TMC) was purchased from Qindao Sanli Chemical Engineering Technology Co., Ltd. (China). n-Hexane was supplied by Shanghai Feida Industry & Trade Co., Ltd. (China). Sodium dodecyl sulfate (SDS) was achieved from Wenmin Biochemistry Science and Technology Co., Ltd. (Shanghai, China). Carboxyl multiwalled carbon nanotubes (MWNTs, purity >95%, COOH content 3.86 wt %, OD < 8 nm, i.d. 2-5 nm, length 10-30 µm) were purchased from Chengdu Organic Chemicals Co., Ltd. (Chinese Academy of Sciences). Na2CO3, NaOH, and Na2SO4 were analytical grade and used without further purification. 2.2. Membrane Preparation. The MWNTs/polyester thin film nanocomposite membranes were prepared through an improved process, which involved initial preparation of an aqueous phase solution: TEOA (6%, w/v), SDS (0.3%, w/v), and MWNTs (0.05%, w/v) were placed in deionized water with the pH adjusted with a mixture of NaOH and Na2CO3, blended in 1:2 proportion. The organic phase solution was composed of TMC (0.6%, w/v) in n-hexane. The aqueous phase solution was sonicated for approximately 1 h for better dispersion of MWNTs. Therefore, the improved process for preparing MWNTs/ polyester thin film nanocomposite membranes by interfacial polymerization of TEOA and TMC on the polysulfone (PSf) support membrane proceeded as follows and is schematically shown in Scheme 1. First, the microporous PSf support membrane was immersed into the organic phase for 30 min. The membrane surface was rolled with a soft rubber roller to eliminate any little bubbles during the soaking procedure. Then, the TMC-saturated support membrane was immersed into the
aqueous phase for a certain time (step-1). The excess solution was drained from the membrane surface and air-dried at room temperature until no liquid remained. Afterward, the membrane was put into the organic phase again for 30 min (step-2). And then the membrane was post-treated in a 60 °C oven for 30 min for further polymerization, leading to the formation of a skin layer. The final membranes were washed in deionized water repeatedly and stored in deionized water. The above-mentioned operation proceeded in a 35 °C water bath. It should be noted that the TFC membrane is meant to represent the thin film composite membrane without MWNTs, while the TFN membrane denotes the thin film MWNTs/ polyester nanocomposite membrane. 2.3. Membrane Characterization. The measurements of pure water flux and protein rejection were performed using a cross-flow membrane module at an operation pressure of 0.6 MPa. The water flux was calculated in eq 1
F ) V/At
(1)
where V is the total volume of permeated pure water, A is the membrane area, and t is the operation time. Deionized water was used for this measurement. The rejection was measured with 5 mmol/L Na2SO4 solution at an operation pressure of 0.6 MPa. The concentrations of the permeation and feed solutions were determined by electrical conductivity HANA-EC215 (Italy). The rejection, R, was calculated in eq 2
R)1-
Cp Cf
(2)
where Cp and Cf are the concentrations of the permeation and feed solutions, respectively. Water was used as the probe liquid for determination of the hydrophilicity at the membrane surface. The static contact angle of water on the surface of a polymer membrane was measured by using OCA15 (Dataphysics Co., Germany) and following the sessile drop method at 25 °C and a relative humidity of 65%. Drops were formed using a 10-µL Hamilton positive displacement syringe. The average value of the contact angle on each polymer membrane was calculated using at least five different locations on each membrane. Reversible Ag/AgCl electrodes, placed on both sides of the membrane, were used to measure the resulting electrical potential difference (∆E) as the pressure difference across the membrane (∆F) changed through a digital electrometer (VC 890D, Shenzhen Victor Hi-tech Co. Ltd.). Then, the streaming
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Figure 1. TEM images of the cross-sections of the TFN membrane.
potential was calculated as SP ) ∆E/∆P. The pressure difference ranged from 0 to 4 × 105 Pa. XPS analysis was performed using a RBD upgraded PHI5000CESCA system (Perkin-Elmer) with Mg KR radiation (hν ) 1253.6 eV). The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 54°. The base pressure of the analyzer chamber was about 5 × 10-8 Pa. Binding energies were calibrated by using the containment carbon (C 1s ) 284.6 eV). The thermal stability of the membranes was analyzed by means of thermogravimetic analysis with a Perkin-Elmer Pyris 1 TGA instrument. The membrane samples were heated at a rate of 20 °C/min under a nitrogen atmosphere. The morphologies of the surface of membranes were observed with a scanning electron microscope (TESCAN 5136MM) after being coated with gold. Quantitative surface roughness analysis of the nanocomposite membranes was measured using AFM imaging and analysis (Nanoscope IV). The surface roughness was reported in terms of the average plane roughness (Ra) and root-mean-square roughness (Rms). Membrane samples were prepared for TEM imaging by peeling away the polyester backing fabric, gently to ensure polysulfone and polyester layers remained together. Small pieces of the fabric free membrane samples were embedded in Epon resin. Approximately 60-80 nm thick sections were cut on an LKB ultramicrotome and placed on Formvar-coated copper grids. Then, the membrane samples were studied with transmission electron microscope, TEM (JEOL-2100 and Hitachi H-600). 3. Results and Discussion 3.1. Characterization of TFC and TFN Membranes. The TEM images of TFN membrane by the improved process are shown in Figure 1. The membrane exhibits nanoscale roughness (peak-and-valley structures) (Figure 1a), which is the typical characteristic of thin film composite membranes prepared by interfacial polymerization.26,27 The irregular morphology precludes quantification of a single film layer thickness, but the thin film is approximately 100-500 nm. In Figure 1b, MWNTs show tubelike appearance and are clearly located within the cross-section of the thin film, but also at the interface. The TEM results suggest the improved process can successfully introduce MWNTs into a membrane and make MWNTs embed throughout the polyester thin film layer of an interfacially polymerized composite nanofiltration membrane. Figure 2 presents the performance of TFC and TFN membranes, respectively, prepared by the conventional process and the improved process of interfacial polymerization at the same preparation parameters (for the preparation of TFN membrane, the aqueous phase contained 0.05% (w/v) MWNTs). It is clear that the presence of MWNT in the composite membrane can
Figure 2. Performance of TFN membranes through different processes of membrane preparation.
Figure 3. SEM images of the surfaces of TFN membranes (0.05% (w/v) MWNTs in the aqueous phase) through different processes of membrane preparation: (a) conventional process and (b) improved process.
effectively enhance the membrane performance including both the water permeability and the salt separation property. Furthermore, compared with the conventional process, the TFC and TFN membranes prepared by the improved process exhibit a significant increase in pure water permeation rate. Very interestingly, the salt rejection is found to be improved as well. Figure 3 shows the SEM images of the surfaces of the TFN membrane through the conventional and improved process of interfacial polymerization. The TFN membrane prepared by the improved process has a surface layer covered by greater fluctuant structures. The results above suggest that the preparation process of interfacial polymerization plays an important role in separation property and morphology of the composite membranes. Also, TFN membrane with high performance can be constructed through the improved process of interfacial polymerization. Then, what induces the improvement in the membrane properties during the improved process of interfacial polymerization? And what is the role of step-1 of the improved process in membrane structure and formation? The chemical composition of ultrathin polyester on the surface of the membrane is analyzed by XPS. To accurately determine the atomic composition of a newly formed layer of membrane surface, the membrane samples
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TABLE 1: XPS Results for TFC Membranes Prepared by the Conventional and Improved Processes of Interfacial Polymerization preparation process
C (%)
O (%)
N (%)
O/N
conventional process improved process
67.33 64.86
28.34 29.91
5.33 5.23
5.32 5.72
examined are TFC membranes that do not contain MWNTs. The elemental compositions are summarized in Table 1. The reaction of TEOA and TMC can lead to two types of polymer chains.28 One is a totally cross-linked polyester structure, which is formed by cross-linking reaction of TEOA and TMC via ester linkage (-CO-O-). The other is a linear hydrophilic structure, which results from hydrolysis of the third acid chloride group of TMC to a carboxylic acid. Thus, the extent of polyester thin film cross-linking is typically estimated by looking at the atomic ratios of O/N. As shown in Table 1, a lower degree of crosslinking (i.e., a higher ratio of O/N) is observed for the TFC membrane prepared by the improved process. The result suggests that the improved process of interfacial polymerization produces TFC membrane with a more linear structure containing pendant COOH. Therefore, it is assumed that the function of step-1 during the improved process of interfacial polymerization is forming a hydrophilic and loose layer on the surface of the membrane. To prove the analysis above, the membrane was prepared by immersing the support membrane into organic phase followed by aqueous phase (i.e., only step-1 while no step-2, without MWNTs). The water flux of this membrane is 446 L/m2 h at 0.6 MPa pressure, which is much less than that of the microporous support membrane (1600 L/m2 h at 0.2 MPa pressure) but higher than that of the TFC membrane prepared through the conventional process. The result implies that the polymerization reaction also takes place during step-1 and a permeable thin film layer with a large amount of pendant COOH is formed. When the aqueous phase contains MWNTs, MWNTs can simultaneously be deposited in the layer. This layer promotes an enhancement in permeability and separation property of the final membrane due to the negative charge on the membrane surface. Figure 4 shows the TG/DTG curves of TFC and TFN membranes. The peak temperature corresponding to the highest decomposition rate for the TFN membrane (209 °C) is 12 °C higher than that for the TFC membrane (197 °C). The result suggests that the addition of MWNTs improves the thermal stability of the membrane. 3.2. Effect of Step-1 Reaction Time on the TFN Membrane Performance and Morphology. To further reveal the role of step-1 of the improved process in the TFN membrane
Figure 4. TG and DTG curves of TFC and TFN membranes.
Figure 5. Effect of reaction time of step-1 on the pure water flux and Na2SO4 rejection of TFN membrane at 0.6 MPa.
Figure 6. Streaming potentials of TFN membrane with different reaction times of step-1.
formation, the influence of reaction time of step-1 ranging from 0 to 40 min on the TFN membrane properties is investigated. A plot of pure water flux and rejection to Na2SO4 versus reaction time of step-1 is shown in Figure 5. As the reaction time of step-1 increases, the pure water flux continuously increases. When the reaction time is 5 min, the flux is 1.9 L/m2 h. The flux increases to a maximum of 4.7 L/m2 h at 25 min, but decreases at longer reaction time, while the rejection to Na2SO4 keeps increasing with the reaction time.
Figure 7. Contact angles of the TFN membranes against water as a function of reaction time of step-1.
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Figure 8. SEM images of the surfaces of TFN membranes with different reaction times of step-1: (a) 5 min, (b) 10 min, (c) 15 min, (d) 20 min, (e) 25 min, (f) 30 min, (g) 35 min, and (h) 40 min.
When a TMC-saturated support membrane is directly immersed into the aqueous phase, two competitive reactions take place. One is the TMC hydrolysis reaction, and the other is the polymerization between TMC and TEOA. Because the concentration of water is always much higher than that of TEOA in the aqueous phase, the amount of TMC that can react with TEOA is limited, or partly hydrolyzed TMC may take part in the polymerization with TEOA. So, a permeable thin film layer with a large number of pendant COOH is formed. As a result, the hydrophilicity of membrane is enhanced. Therefore, both the water flux and salt rejection of membrane increase in the initial stage of polymerization. However, the density of the thin film layer will remain constant if the reaction time exceeds a threshold value due to the intensive hydrolysis of TMC. Then, prolonging the reaction time increases the thickness of the thin film layer rather than the density. That is the reason why the water flux decreases in the later stage of reaction. During the process of step-1, some hydrophilic MWNTs can be incorporated into the newly formed thin film layer, leading to a higher loading of MWNTs on the membrane and contributing to the enhanced water flux and rejection. Figure 6 is a plot of the streaming potential of the TFN membrane versus reaction time of step-1. The streaming potentials of the TFN membranes are all negative and decrease with an increase in the reaction time of step-1. The observed trend indicates that step-1 produces a negatively charged layer. An increase in the reaction time of step-1 results in an increase in the content of negative charges, and thus, the hydrophilicity of membrane increases correspondingly. This can be proved by examining the contact angle of the TFN membrane (Figure 7). As the reaction time of step-1 increases, the contact angle of the membrane decreases, suggesting that the hydrophilicity of the membrane surface increases. Therefore, the membrane hydrophilicity and the pure water flux can be enhanced by the increasing reaction time of step-1. The results of streaming potential and contact angle are consistent with previous analysis. Figure 8 shows SEM images of the surfaces of TFN membranes with various reaction times of step-1. The roughness of the membrane surface can be quantified from the AFM results as presented in Figure 9 and Table 2. Clearly, the surface of
Figure 9. AFM images of the surfaces of the TFN membranes with different reaction times of step-1: (a) 5 min and (b) 35 min.
TABLE 2: Surface Roughness Values of TFN Membranes with Different Reaction Times of Step-1 by AFM reaction time (min)
Ra (nm)
Rms (nm)
5 15 35
16.228 21.956 32.809
20.265 28.514 41.216
the TFN membrane becomes rougher as the reaction time of step-1 increases. 4. Conclusions We initiated an improved process to prepare MWNTs/ polyester thin film nanocomposite (TFN) membranes, which is facilely done by immersing the support membrane into organic
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phase before the conventional process of interfacial polymerization. This improved process results in a TFN membrane with dramatically improved permeability and selectivity properties when compared with the thin film composite membrane without MWNTs and TFN membrane prepared by the conventional interfacial polymerization. Step-1 of the improved process played a positive role in the membrane properties which was realized by producing a permeable thin layer with a large amount of hydrophilic and negatively charged carboxyl groups on the membrane surface. Also, step-1 is favorable for the incorporation of MWNTs in the thin film layer. This improved process of interfacial polymerization can provide a paradigm for the preparation of TFN membrane and an approach to overcome the trade-off effect between permeability and selectivity, on the basis of which some other nanocomposite NF membranes could be prepared and contribute to the NF membrane with high performance. Acknowledgment. Financial support of this research was provided by the National Science of Foundation of China (NSFC) (nos. 20876028, 20934002, 20774022), and the National Basic Research Program of China (2005CB623800, 2009CB930000). References and Notes (1) Islam, N.; Miyazaki, K. TechnoVation 2010, 30, 229–237. (2) Jeong, B. H.; Hoek, E. M. V.; Yan, Y. S.; Subramani, A.; Huang, X. F.; Hurwitz, G.; Ghosh, A. K.; Jawor, A. J. Membr. Sci. 2007, 294, 1–7. (3) Lind, M. L.; Ghosh, A. K.; Jawor, A.; Huang, X. F.; Hou, W.; Yang, Y.; Hoek, E. M. V. Langmuir 2009, 25, 10139–10145. (4) Lee, H. S.; Im, S. J.; Kim, J. H.; Kim, H. J.; Kim, J. P.; Min, B. R. Desalination 2008, 219, 48–56. (5) Uragami, T.; Okazaki, K.; Matsugi, H.; Miyata, T. Macromolecules 2002, 35, 9156–9163. (6) Lu, L. Y.; Sun, H. L.; Peng, F. B.; Jiang, Z. Y. J. Membr. Sci. 2006, 281, 245–252. (7) Freeman, B. D. Macromolecules 1999, 32, 375–380.
Wu et al. (8) Peng, F. B.; Lu, L. Y.; Sun, H. L.; Wang, Y. Q.; Liu, J. Q.; Jiang, Z. Y. Chem. Mater. 2005, 17, 6790–6796. (9) Coleman, J. N.; Khan, U.; Gun’ko, Y. K. AdV. Mater. 2006, 18, 689–706. (10) Manchado, M. A. L.; Valentini, L.; Biagiotti, J.; Kenny, J. M. Carbon 2005, 43, 1499–1505. (11) Spinks, G. M.; Mottaghitalab, V.; Bahrami-Saniani, M.; Whitten, P. G.; Wallace, G. G. AdV. Mater. 2006, 18, 637. (12) Hinds, B. J.; Chopra, N.; Rantell, T.; Andrews, R.; Gavalas, V.; Bachas, L. G. Science 2004, 303, 62–65. (13) Holt, J. K.; Park, H. G.; Wang, Y. M.; Stadermann, M.; Artyukhin, A. B.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Science 2006, 312, 1034– 1037. (14) Holt, J. K.; Noy, A.; Huser, T.; Eaglesham, D.; Bakajin, O. Nano Lett. 2004, 4, 2245–2250. (15) Choi, J. H.; Jegal, J.; Kim, W. N. J. Membr. Sci. 2006, 284, 406– 415. (16) Choi, J. H.; Jegal, J.; Kim, W. N. Macromol. Symp. 2007, 249, 610–617. (17) Qiu, S.; Wu, L. G.; Pan, X. J.; Zhang, L.; Chen, H. L.; Gao, C. J. J. Membr. Sci. 2009, 342, 165–172. (18) Lu, X. F.; Bian, X. K.; Shi, L. Q. J. Membr. Sci. 2002, 210, 3–11. (19) Chen, S. H.; Chang, D. J.; Liou, M. R.; Hsu, C. S.; Lin, S. S. J. Appl. Polym. Sci. 2002, 83, 1112–1118. (20) Chan, W. H.; Tsao, S. C. J. Appl. Polym. Sci. 2003, 87, 1803– 1810. (21) Zhang, W.; He, G. H.; Gao, P.; Chen, G. H. Sep. Purif. Technol. 2003, 30, 27–35. (22) Verissimo, S.; Peinemann, K. V.; Bordado, J. Desalination 2005, 184, 1–11. (23) Zhou, Y.; Yu, S. C.; Liu, M. H.; Gao, C. J. Desalination 2005, 180, 189–196. (24) Ji, J.; Mehta, M. J. Membr. Sci. 2001, 192, 41–54. (25) Lee, S. Y.; Kim, H. J.; Patel, R.; Im, S. J.; Kim, J. H.; Min, B. R. Polym. AdV. Technol. 2007, 18, 562–568. (26) Hoek, E. M. V.; Bhattacharjee, S.; Elimelech, M. Langmuir 2003, 19, 4836–4847. (27) Vrijenhoek, E. M.; Hong, S.; Elimelech, M. J. Membr. Sci. 2001, 188, 115–128. (28) Tang, B. B.; Huo, Z. B.; Wu, P. Y. J. Membr. Sci. 2008, 320, 198–205.
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