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Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–5
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis Farui Chong, Congwei Wang, Jibin Miao∗, Ru Xia, Ming Cao, Peng Chen, Bin Yang, Weibin Zhou, Jiasheng Qian∗ School of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-Friendly Polymer Materials, Anhui University, Hefei 230601, China
a r t i c l e
i n f o
Article history: Received 6 April 2017 Revised 19 June 2017 Accepted 22 June 2017 Available online xxx Keywords: Chlorosulfonated polyethylene (CSM) Alkali recovery Diffusion dialysis (DD) Interpenetrating network structure (IPNs)
a b s t r a c t Cation-exchange membranes for diffusion dialysis (DD) were prepared by selecting chlorosulfonated polyethylene (CSM) and methacrylic acid (MAA) as framework and functional monomer, respectively. An interpenetrating network structure (IPN) was formed by free radical polymerization and consequent crosslinking reaction. The membranes possessed ion exchange capacities (IECs) of 1.98– 3.12 mmol/g, water uptake (WR ) of 12.7–34.8%, swelling degree of 72.7–170% as well as favorable thermal stability and alkali resistance (almost zero mass loss in hot alkali solution). The membranes were applied for DD process to recover alkali from NaOH/Na2 WO4 system at 25 °C and 65 °C. Results indicated that coefficients for OH− (UOH ) were in the range of 0.0015–0.018 m/h and separation factors (S) were in the range of 9–33 at 25 and 65 °C, respectively. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Diffusion dialysis (DD) process has been commercially applied for the recovery of either acids or bases from industrial wastewaters [1–6]. Compared with classical separation methods, DD process exhibited some positive features, which included free operation, non-secondary-pollution, lower energy consumption and cost. Otherwise, commercial DD process for alkali recovery was less reported because of lack of suitable membranes with high OH− flux, high ion selectivity as well as favorable alkali resistance. A large amount of alkaline waste water was produced during the refining of tungsten, molybdenum and rhenium, which was detrimental to environment and hard to treatment. To solve these problems, various efforts have been made by researchers to obtain satisfactory membranes. Organic–inorganic hybrid membrane was one of the candidates. Polymers, such as polyvinyl alcohol (PVA) [7,8] or sulfonated poly (2,6-dimethyl-1,4-phenyleneoxide) (SPPO) [9,10] were mingled with suspension of inorganic nano particles or solution of multi-silicon copolymers to prepare hybrid membranes. Ion flux and selectivity of hybrid membranes
∗
Corresponding authors. E-mail addresses: [email protected] (J. Miao), [email protected] (J. Qian).
improved obviously, while alkali resistance was also out of expectation. The fundamental reason for this was as follows: (1) polymer materials often contain high hydrophilic groups or exchangeable groups (such as –OH, –NH2 , –SO3 − , –COO− ), which could not resist the attack of alkali effectively; (2) most of the polymers swelled seriously under high-temperature alkaline solution and ion selectivity suddenly decreased. Liu et al. [11] obtained cationic ion exchange membranes via the method of building semi-interpenetrating network structure (s-IPN) structure in polyvinylidene fluoride (PVDF) matrix. The membranes indicated high alkali resistance (mass loss was less than 5%), which exhibited higher stability than that of membranes based SPPO and PVA [12–16]. Thus, building an IPN or s-IPN structure could be an effective choice to enhance polymer resistance against alkali. As a kind of special synthetic rubber material, chlorosulfonated polyethylene (CSM) possessed excellent abrasive resistance, corrosion resistance, alkali resistance thermal and mechanical stability as well as ability of mmebrane formation [17–19]. However, CSM was seldom reported to prepare ion exchange membranes due to its saturated structure and strong hydrophobicity. Therefore, methacrylic acid (MAA) was selected as functional monomer to endow the CSM matrix with ion exchange ability and enhanced hydrophilicity in this paper. IPN structure was constructed via free radical polymerization to further promote stability of membranes. Property and microscopic morphology of as-prepared membranes
http://dx.doi.org/10.1016/j.jtice.2017.06.043 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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[m5G;July 8, 2017;14:50]
F. Chong et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–5 Table 1 WR and ion exchange capacity of as-prepared membranes (dry).
were detected and the membranes were introduced to recover alkali from NaOH/Na2 WO4 mixture via DD process. 2. Experimental 2.1. Materials Chlorosulfonated polyethylene (CSM) was kindly supplied by Anhui Zhongding Technology Co., Ltd., methylbenzene, NaOH, HCl and Na2 WO4 with analytical pure were purchased from Sinopharm Chemical Regent Co., Ltd.(Shanghai, China). Methacrylic acid (MAA), diethylenetriamine (DETA) and dipentamethylene thiuram hexasulfide (DPTT) (A.R. grade) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. A mixed solution of NaOH (1.0 mol/L)/Na2 WO4 (0.10 mol/L) was prepared for DD process and deionized (DI) water was utilized throughout. 2.2. Membrane preparation and characterization 2.2.1. Membrane preparation 2 g CSM and 1 g MAA were dissolved completely in 40 mL methylbenzene to form a uniform solution. After that, 5 mg AIBN was added and the solution was stirred at 80 °C for 5 h under nitrogen. Then a series of crosslinking agents (0.5%, 1%, 1.5%, 2%, 2.5%, quality ratio of DETA to MAA) DETA and 0.1 g DPTT were added to the uniform solution. After that, the solution was casted onto a glass plate and dried at room temperature. The obtained membranes were hot-pressed under 185 °C and 10 Mpa. The asprepared membranes were signed as 0.5%, 1%, 1.5%, 2% and 2.5%, respectively by dosage of DETA. The crosslinking reaction process was depicted in Scheme 1. 2.2.2. Characterizations and DD testing The mechanical properties were measured by an electronic computer-control universal testing machine (CMT6104, MTS Industrial System (China) CO., LTD.) at 25 °C with dumbbell and three duplicate samples were tested for each membrane. Thermo gravimetric analysis (TGA) was tested by a Shimadzu TGA-50H analyzer under air flow from 25 °C to 800 °C. The morphology of membranes was observed by a scanning electron microscopy (S-4800, Hitachi Limited, Japan). Membrane samples were fractured in liquid nitrogen and coated with gold before tests. TEM results were observed
Content of DETA (%)
WR (%)
IEC (mmol/g)
0.5 1 1.5 2 2.5
34.78 ± 1.39 22.92 ± 2.50 20.96 ± 1.34 15.88 ± 1.07 12.71 ± 0.33
3.12 ± 0.07 2.71 ± 0.03 2.42 ± 0.04 2.19 ± 0.07 1.98 ± 0.05
by a JEM-2100 HRTEM after slicing into ultrathin section via a Leica EM FC7 UC7 cryosection system. WR , IECs and alkali resistance of as-prepared membranes were tested as reported in our previous works and two duplicate samples were tested simultaneously [13,16]. The DD tests were carried out to investigate potential application of the composite membranes for alkali recovery. NaOH/Na2 WO4 mixture was selected as the feed solution on the basis of our previous work [13,15–16]. The effective area of membrane was 6 cm2 and the DD process was allowed for 1 h. The two compartments were maintained in thermostatic condition and stirred at identical rate to minimize concentration polarization. The OH− concentration in both sides was calculated from titration with HCl, while WO4 2 − concentration was determined with a thiocyanate spectrophotometric method [13,16]. The calculation of dialysis coefficients (U) and separation factors (S) were as follows:
U=
M At C
(1)
where M was the amount of component transported in moles, A was the effective area in square meters, t was the time in hours, and C was the logarithmic mean concentration difference between the two chambers as in Eq. (2):
C =
C 0f − (Ctf − Cdt ) ln[C 0f /(Ctf − Cdt )]
(2)
where C 0f and Ctf were the feed concentrations at time 0 and t, respectively and Cdt was the dialysate concentration at time t. The separation factor (S) was given as the ratio of dialysis coefficients (U) of OH− to that of WO4 2− . 3. Results and discussion 3.1. Water uptake (WR ) and ion exchange capacities (IECs) Results of WR (in the range of 12.71%−34.78%) are shown in Table 1, from which the decreasing trend is observed with an increasing content of DETA. The reasons were as follows: (1) water uptake of membranes was mainly from –COOH, which decreased as the dosage of DETA increased. Thus, WR of membranes declined. (2) The enhanced dosage of DETA promoted density of membranes and made it difficult to adsorb water molecule. Therefore, WR of membranes decreased as the loading content of DETA increased. IECs of as prepared membranes were in the range of 1.98– 3.12 mmol/g as shown in Table 1, which were higher than those reported previously [15, 16]. The results confirmed the crosslinking reaction (as shown in Scheme 1); therefore, IECs decreased with an increasing content of DETA. 3.2. Swelling degree and mass loss
Scheme 1. Crosslinking reaction of composite membranes.
As observed in Fig. 1, swelling degree results of as-prepared membranes were in the range of 72.7–170.1% and this was lower than that found in our previous reports [13,15,16,20]. The explanation was as given: the content of –COOH decreased as dosage
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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F. Chong et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–5
3
Fig. 3. TGA and DTG curves of as-prepared membranes. Fig. 1. Swelling degree and mass loss of as-prepared membranes.
of DETA increased and this prevented attack of OH− on the membranes; an increased dosage of DETA enhanced density of membranes and the hydrophilic –COOH groups were encapsulated by the formed IPNs, which made it difficult for OH− to permeate through the membrane matrix. Thus, the as-prepared membranes exhibited well swelling resistance in hot alkaline solution. Mass loss of as-prepared membranes can also be seen in Fig. 1. The retained mass of all the membranes was higher than 99.8%, which indicated the excellent alkali resistance of as-prepared membranes. Considering the saturated structure of CSM, mass loss of as-prepared membranes was mainly from degradation of PMAA chains and amide bonds. From these results, we concluded that the formation of IPN structure prevented the attack of OH− to PMAA and amide bonds. To confirm this conclusion, slices of as-prepared membranes were prepared and dyed by osmic acid. The slices were observed by TEM and the pictures are shown in Fig. 2. The bright zones in the TEM pictures were the aggregation of –COOH and the dark field was CSM matrix [21,22]. PMAA encapsulated in the CSM phase and dispersed uniformly can be seen from the pictures. The size of PMAA aggregation was around dozens of nanometers and decreased with an increasing dosage of DETA, which indicated the enhanced compatibility between PMAA and CSM matrix. The results indicated that CSM matrix is entangled with PMAA chains closely and this would prevent the degradation
Table 2 Initial decomposition temperature (IDT) of as-prepared membranes. Content of DETA (%)
0.5
1
1.5
2
2.5
IDT (°C)
221
222
225
229
224
of functional groups (i. e. –COOH) in contact with an alkaline solution. This was the main reason for the excellent alkali resistance of as-prepared membranes. 3.3. Thermal stability Thermal testing results of as-prepared membranes are shown in Fig. 3 and Table 2. It could be seen from the results that the initial decomposition temperatures (IDT) of membranes were in the range of 202–229 °C. Considering the fact that the samples were hotpressed at the temperature of 185 °C, the mass loss below 185 °C in Fig. 3 could be ignored. As observed in Fig. 3, boundary between the two mass loss peaks (decomposition of amide bond around 300 °C and breakage of carbon–hydrogen bond between 400 °C and 500 °C) turned unapparent, which was due to the formation of IPNs. To confirm the peaks, DTG curves of as-prepared membranes are shown in the inset of Fig. 3, from which the rate of mass loss can be detected clearly. The decomposition temperature of PMAA was very close to that of CSM and this indicated well compatibility between
Fig. 2. TEM pictures of as-prepared membranes.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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F. Chong et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–5 Table 3 Tensile strength (TS) and elongation at break (Eb ) of as-prepared membranes. Content of DETA (%)
0.5
1
1.5
2
2.5
TS (MPa) Eb (%)
24.43 ± 1.55 479.9 ± 38.56
23.84 ± 1.26 554.4 ± 36.55
29.04 ± 1.05 499.9 ± 37.22
28.92 ± 1.23 474.4 ± 29.22
21.53 ± 2.26 411.6 ± 33.25
Fig. 4. Cross-section SEM of as-prepared membranes.
the two phases, which was supported by the TEM and SEM results. The maintained mass of samples after TGA test was less than 10%, in which the sample with 1.5% content of DETA exhibited the highest value. These results indicated that suitable dosage of DETA was necessary to prepare thermally stable membranes. 3.4. Mechanical properties Mechanical properties of the as-prepared membranes (including tensile strength (TS) and elongation at break (Eb )) are shown in Table 3, from which TS and Eb of membranes were observed in the range of 21.5–29.0 MPa and 411.7−554.4%, respectively. The mechanical properties of the as-prepared membranes are more favorable than those of PVA [23] and SPPO [16], which is a consequence of the inherent flexibility of CSM matrix. These results also indicated that the incorporation of MAA had less effect on the mechanical stability of composite membranes. The membranes possessed optimized mechanical properties while loading content of DETA was 1.5%.
Fig. 5. Dialysis coefficients of OH− (UOH ) of as-prepared membranes at 25 °C and 65 °C.
3.5. Membrane morphologies Cross section morphologies of the as-prepared membranes are shown in Fig. 4. All the samples exhibited uniform cross-section structure, which confirmed good compatibility between CSM matrix and PMAA. With the increasing content of DETA, there was not obvious phase separation or structure cavity as observed in the SEM pictures. This was corresponding to the TEM results well, which confirmed the successful construction of IPN structure [24]. The uniform structure of as-prepared membranes provided possibility for application in cationic DD process. 3.6. Results of DD process The results of DD process with the as-prepared membranes at 25 °C and 65 °C are shown in Figs. 5 and 6. Similar with previous report [16], dialysis coefficient of OH− (UOH ) at 65 °C was higher than that of 25 °C as given in Fig. 5, which was due to the enhanced mobility of ions at higher temperature. The values of UOH were in the range of 0.0015–0.018 m/h, which are comparable
Fig. 6. Separation factors (S) of the membranes at different temperatures.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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with PVA and SPPO membranes. The membranes exhibited a similar decreasing trend with an increasing dosage of DETA at 25 °C and 65 °C. The explanation was as follows: DETA was crosslinked with –COOH groups, so that the content of exchangeable groups decreased as the dosage of DETA increased. The decreased trend of UOH turned slowly while the dosage of DETA was higher than 2%, which was in accordance with the uniform structure of as-prepared membranes. Separation factors of as-prepared membranes at 25 °C and 65 °C are shown in Fig. 6, from which the opposite trend to UOH was observed. Similarly to the previous report [4], the values of UOH and S of as-prepared membranes were limited by the “tradeoff effect” between flux and selectivity. Compared with previous report [13, 15, 16], CSM/PMAA membranes possessed competitive selectivity at higher temperature. The results were due to two aspects: (1) IPNs between PMAA and CSM matrix encapsulated the exchangeable groups (–COO− ) and thus improved stability of composite membrane in basic form; (2) formation of IPN structure enhanced density of membranes and restricted the interfacial relaxation between CSM and PMAA. 4. Conclusions CSM/PMAA composite cationic exchange membranes with IPN structure were prepared by free radical polymerization method. Different amount of diethylenetriamine (DETA) was used to form IPN structure and control density of composite membranes. The as-prepared membranes possessed water uptake of 12.7–34.8%, IEC of 1.98–3.12 mmol/g, as well as favorable thermal and mechanical stability. Even more, all the membranes exhibited excellent resistance to 2.0 mol/L NaOH at 65 °C for 60 h: swelling degree was in the range of 72.7–170% while mass loss was less than 0.2%. Natural property of CSM and formation of IPN structure were concluded to avoid erosion of hot alkaline solution effectively. The as-prepared membranes were applied to recover NaOH from NaOH/Na2 WO4 mixture and the results were found to be acceptable: dialysis coefficients of OH− (UOH ) were in the range of 0.0015–0.018 m/h and the separation factors were in the range of 9.0 ∼ 33. The results indicated that the as-prepared CSM/PMAA composite membranes would be potentially applied in alkaline DD process in commercial scale. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 216060 01, 512730 01), the Anhui Provincial Natural Science Foundation (no. 1708085QE117) and the Doctoral Scientific Research Startup Foundation of Anhui University (no. J01003213). Particularly, the financial supports from Collaborative Innovation Center for Petrochemical New Material and Institute of High Performance Rubber Materials & Products of Anhui Province were appreciated.
5
References [1] Jeong J, Kim MS, Kim BS, Kim SK, Kim WB, Lee JC. Recovery of H2 SO4 from waste acid solution by a diffusion dialysis method. J Hazard Mater 2005;124:230–5. [2] Lan S, Wen X, Zhu Z, Shao F, Zhu C. Recycling of spent nitric acid solution from electrodialysis by diffusion dialysis. Desalination 2011;278:227–30. [3] Luo J, Wu C, Wu Y, Xu T. Diffusion dialysis of hydrochloride acid at different temperatures using PPO–SiO2 hybrid anion exchange membranes. J Membr Sci 2010;347:240–9. [4] Luo J, Wu C, Xu T, Wu Y. Diffusion dialysis-concept, principle and applications. J Membr Sci 2011;366:1–16. [5] Wu Y, Luo J, Zhao L, Zhang G, Wu C, Xu T. QPPO/PVA anion exchange hybrid membranes from double crosslinking agents for acid recovery. J Membr Sci 2013;428:95–103. [6] Xu J, Lu S, Fu D. Recovery of hydrochloric acid from the waste acid solution by diffusion dialysis. J Hazard Mater 2009;165:832–7. [7] Hao J, Gong M, Wu Y, Wu C, Luo J, Xu T. Alkali recovery using PVA/SiO2 cation exchange membranes with different -COOH contents. J Hazard Mater 2013;244–245:348–56. [8] Hao J, Wu Y, Ran J, Wu B, Xu T. A simple and green preparation of PVA-based cation exchange hybrid membranes for alkali recovery. J Membr Sci 2013;433:10–16. [9] Wu Y, Hao J, Wu C, Mao F, Xu T. Cation exchange PVA/SPPO/SiO2 membranes with double organic phases for alkali recovery. J Membr Sci 2012;423:383–91. [10] Xiao X, Wu C, Cui P, Luo J, Wu Y, Xu T. Cation exchange hybrid membranes from SPPO and multi-alkoxy silicon copolymer: preparation, properties and diffusion dialysis performances for sodium hydroxide recovery. J Membr Sci 2011;379:112–20. [11] Liu R, Wu L, Pan J, Jiang C, Xu T. Diffusion dialysis membranes with semiinterpenetrating network for alkali recovery. J Membr Sci 2014;451:18–23. [12] Afsar NU, Miao J, Mondal AN, Yang Z, Yu D, Wu B, et al. Development of PVA/MIDA based hybrid cation exchange membranes for alkali recovery via diffusion dialysis. Sep Purif Technol 2016;164:63–9. [13] Miao J, Li X, Yang Z, Jiang C, Qian J, Xu T. Hybrid membranes from sulphonated poly (2,6-dimethyl-1,4-phenylene oxide) and sulphonated nano silica for alkali recovery. J Membr Sci 2016;498:201–7. [14] Hao J, Wu Y, Xu T. Cation exchange hybrid membranes prepared from PVA and multisilicon copolymer for application in alkali recovery. J Membr Sci 2013;425–426:156–62. [15] Li X, Miao J, Xia R, Yang B, Chen P, Cao M, et al. Preparation and properties of sulfonated poly (2,6-dimethyl-1,4-phenyleneoxide)/mesoporous silica hybrid membranes for alkali recovery. Micropor Mesopor Mat 2016;236:48–53. [16] Miao J, Yao L, Yang Z, Pan J, Qian J, Xu T. Sulfonated poly(2,6-dimethyl-1,4-phenyleneoxide)/nano silica hybrid membranes for alkali recovery via diffusion dialysis. Sep Purif Technol 2015;141:307–13. [17] Markovic G, Marinovic-Cincovic M, Jovanovic V, Samarzija-Jovanovic S, Budinski-Simendic J. NR/CSM/biogenic silica rubber blend composites. Composites Part B 2013;55:368–73. [18] Markovic G, Veljkovic O, Marinovic-Cincovic M, Jovanovic V, Samarzija-Jovanovic S, Budinski-Simendic J. Composites based on waste rubber powder and rubber blends: BR/CSM. Composites Part B 2013;45:178–84. [19] Tabaei TA, Bagheri R, Hesami M. Comparison of cure characteristics and mechanical properties of nano and micro silica-filled CSM elastomers. J Appl Polym Sci 2015;132:511–15. [20] L Y, Li X, Miao J, Qian J, Xia R, Yang B. PVA/nano-TiO2 hybrid membranes: preparation and application for alkali recovery (in Chinese),. Membr Sci Technol 2016;36:81–7. [21] Y L, Liu F, Wang J. The synthesis and characterization of chlorosulfonated polyethylene/polybutyl methacrylate/epoxy resins three-component IPNs. Chem Res Chin Univ 1992;13:1141–66. [22] He S, Lin Y, Wei Z, Zhang L, Lin J, Nazarenko S. Solvent-free fabrication of proton-conducting membranes based on commercial elastomers. Polym Adv Technol 2015;26:300–7. [23] Wu Y, Luo J, Yao L, Wu C, Mao F, Xu T. PVA/SiO2 anion exchange hybrid membranes from multisilicon copolymers with two types of molecular weights. J Membr Sci 2012;399–400:16–27. [24] Zhang X, Wu D, Chu C. Synthesis, characterization and controlled drug release of thermosensitive IPN–PNIPAAm hydrogels. Biomaterials 2004;25:3793–805.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
JID: JTICE
[m5G;July 8, 2017;14:50]
Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–5
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis Farui Chong, Congwei Wang, Jibin Miao∗, Ru Xia, Ming Cao, Peng Chen, Bin Yang, Weibin Zhou, Jiasheng Qian∗ School of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-Friendly Polymer Materials, Anhui University, Hefei 230601, China
a r t i c l e
i n f o
Article history: Received 6 April 2017 Revised 19 June 2017 Accepted 22 June 2017 Available online xxx Keywords: Chlorosulfonated polyethylene (CSM) Alkali recovery Diffusion dialysis (DD) Interpenetrating network structure (IPNs)
a b s t r a c t Cation-exchange membranes for diffusion dialysis (DD) were prepared by selecting chlorosulfonated polyethylene (CSM) and methacrylic acid (MAA) as framework and functional monomer, respectively. An interpenetrating network structure (IPN) was formed by free radical polymerization and consequent crosslinking reaction. The membranes possessed ion exchange capacities (IECs) of 1.98– 3.12 mmol/g, water uptake (WR ) of 12.7–34.8%, swelling degree of 72.7–170% as well as favorable thermal stability and alkali resistance (almost zero mass loss in hot alkali solution). The membranes were applied for DD process to recover alkali from NaOH/Na2 WO4 system at 25 °C and 65 °C. Results indicated that coefficients for OH− (UOH ) were in the range of 0.0015–0.018 m/h and separation factors (S) were in the range of 9–33 at 25 and 65 °C, respectively. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Diffusion dialysis (DD) process has been commercially applied for the recovery of either acids or bases from industrial wastewaters [1–6]. Compared with classical separation methods, DD process exhibited some positive features, which included free operation, non-secondary-pollution, lower energy consumption and cost. Otherwise, commercial DD process for alkali recovery was less reported because of lack of suitable membranes with high OH− flux, high ion selectivity as well as favorable alkali resistance. A large amount of alkaline waste water was produced during the refining of tungsten, molybdenum and rhenium, which was detrimental to environment and hard to treatment. To solve these problems, various efforts have been made by researchers to obtain satisfactory membranes. Organic–inorganic hybrid membrane was one of the candidates. Polymers, such as polyvinyl alcohol (PVA) [7,8] or sulfonated poly (2,6-dimethyl-1,4-phenyleneoxide) (SPPO) [9,10] were mingled with suspension of inorganic nano particles or solution of multi-silicon copolymers to prepare hybrid membranes. Ion flux and selectivity of hybrid membranes
∗
Corresponding authors. E-mail addresses: [email protected] (J. Miao), [email protected] (J. Qian).
improved obviously, while alkali resistance was also out of expectation. The fundamental reason for this was as follows: (1) polymer materials often contain high hydrophilic groups or exchangeable groups (such as –OH, –NH2 , –SO3 − , –COO− ), which could not resist the attack of alkali effectively; (2) most of the polymers swelled seriously under high-temperature alkaline solution and ion selectivity suddenly decreased. Liu et al. [11] obtained cationic ion exchange membranes via the method of building semi-interpenetrating network structure (s-IPN) structure in polyvinylidene fluoride (PVDF) matrix. The membranes indicated high alkali resistance (mass loss was less than 5%), which exhibited higher stability than that of membranes based SPPO and PVA [12–16]. Thus, building an IPN or s-IPN structure could be an effective choice to enhance polymer resistance against alkali. As a kind of special synthetic rubber material, chlorosulfonated polyethylene (CSM) possessed excellent abrasive resistance, corrosion resistance, alkali resistance thermal and mechanical stability as well as ability of mmebrane formation [17–19]. However, CSM was seldom reported to prepare ion exchange membranes due to its saturated structure and strong hydrophobicity. Therefore, methacrylic acid (MAA) was selected as functional monomer to endow the CSM matrix with ion exchange ability and enhanced hydrophilicity in this paper. IPN structure was constructed via free radical polymerization to further promote stability of membranes. Property and microscopic morphology of as-prepared membranes
http://dx.doi.org/10.1016/j.jtice.2017.06.043 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
ARTICLE IN PRESS
JID: JTICE 2
[m5G;July 8, 2017;14:50]
F. Chong et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–5 Table 1 WR and ion exchange capacity of as-prepared membranes (dry).
were detected and the membranes were introduced to recover alkali from NaOH/Na2 WO4 mixture via DD process. 2. Experimental 2.1. Materials Chlorosulfonated polyethylene (CSM) was kindly supplied by Anhui Zhongding Technology Co., Ltd., methylbenzene, NaOH, HCl and Na2 WO4 with analytical pure were purchased from Sinopharm Chemical Regent Co., Ltd.(Shanghai, China). Methacrylic acid (MAA), diethylenetriamine (DETA) and dipentamethylene thiuram hexasulfide (DPTT) (A.R. grade) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. A mixed solution of NaOH (1.0 mol/L)/Na2 WO4 (0.10 mol/L) was prepared for DD process and deionized (DI) water was utilized throughout. 2.2. Membrane preparation and characterization 2.2.1. Membrane preparation 2 g CSM and 1 g MAA were dissolved completely in 40 mL methylbenzene to form a uniform solution. After that, 5 mg AIBN was added and the solution was stirred at 80 °C for 5 h under nitrogen. Then a series of crosslinking agents (0.5%, 1%, 1.5%, 2%, 2.5%, quality ratio of DETA to MAA) DETA and 0.1 g DPTT were added to the uniform solution. After that, the solution was casted onto a glass plate and dried at room temperature. The obtained membranes were hot-pressed under 185 °C and 10 Mpa. The asprepared membranes were signed as 0.5%, 1%, 1.5%, 2% and 2.5%, respectively by dosage of DETA. The crosslinking reaction process was depicted in Scheme 1. 2.2.2. Characterizations and DD testing The mechanical properties were measured by an electronic computer-control universal testing machine (CMT6104, MTS Industrial System (China) CO., LTD.) at 25 °C with dumbbell and three duplicate samples were tested for each membrane. Thermo gravimetric analysis (TGA) was tested by a Shimadzu TGA-50H analyzer under air flow from 25 °C to 800 °C. The morphology of membranes was observed by a scanning electron microscopy (S-4800, Hitachi Limited, Japan). Membrane samples were fractured in liquid nitrogen and coated with gold before tests. TEM results were observed
Content of DETA (%)
WR (%)
IEC (mmol/g)
0.5 1 1.5 2 2.5
34.78 ± 1.39 22.92 ± 2.50 20.96 ± 1.34 15.88 ± 1.07 12.71 ± 0.33
3.12 ± 0.07 2.71 ± 0.03 2.42 ± 0.04 2.19 ± 0.07 1.98 ± 0.05
by a JEM-2100 HRTEM after slicing into ultrathin section via a Leica EM FC7 UC7 cryosection system. WR , IECs and alkali resistance of as-prepared membranes were tested as reported in our previous works and two duplicate samples were tested simultaneously [13,16]. The DD tests were carried out to investigate potential application of the composite membranes for alkali recovery. NaOH/Na2 WO4 mixture was selected as the feed solution on the basis of our previous work [13,15–16]. The effective area of membrane was 6 cm2 and the DD process was allowed for 1 h. The two compartments were maintained in thermostatic condition and stirred at identical rate to minimize concentration polarization. The OH− concentration in both sides was calculated from titration with HCl, while WO4 2 − concentration was determined with a thiocyanate spectrophotometric method [13,16]. The calculation of dialysis coefficients (U) and separation factors (S) were as follows:
U=
M At C
(1)
where M was the amount of component transported in moles, A was the effective area in square meters, t was the time in hours, and C was the logarithmic mean concentration difference between the two chambers as in Eq. (2):
C =
C 0f − (Ctf − Cdt ) ln[C 0f /(Ctf − Cdt )]
(2)
where C 0f and Ctf were the feed concentrations at time 0 and t, respectively and Cdt was the dialysate concentration at time t. The separation factor (S) was given as the ratio of dialysis coefficients (U) of OH− to that of WO4 2− . 3. Results and discussion 3.1. Water uptake (WR ) and ion exchange capacities (IECs) Results of WR (in the range of 12.71%−34.78%) are shown in Table 1, from which the decreasing trend is observed with an increasing content of DETA. The reasons were as follows: (1) water uptake of membranes was mainly from –COOH, which decreased as the dosage of DETA increased. Thus, WR of membranes declined. (2) The enhanced dosage of DETA promoted density of membranes and made it difficult to adsorb water molecule. Therefore, WR of membranes decreased as the loading content of DETA increased. IECs of as prepared membranes were in the range of 1.98– 3.12 mmol/g as shown in Table 1, which were higher than those reported previously [15, 16]. The results confirmed the crosslinking reaction (as shown in Scheme 1); therefore, IECs decreased with an increasing content of DETA. 3.2. Swelling degree and mass loss
Scheme 1. Crosslinking reaction of composite membranes.
As observed in Fig. 1, swelling degree results of as-prepared membranes were in the range of 72.7–170.1% and this was lower than that found in our previous reports [13,15,16,20]. The explanation was as given: the content of –COOH decreased as dosage
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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Fig. 3. TGA and DTG curves of as-prepared membranes. Fig. 1. Swelling degree and mass loss of as-prepared membranes.
of DETA increased and this prevented attack of OH− on the membranes; an increased dosage of DETA enhanced density of membranes and the hydrophilic –COOH groups were encapsulated by the formed IPNs, which made it difficult for OH− to permeate through the membrane matrix. Thus, the as-prepared membranes exhibited well swelling resistance in hot alkaline solution. Mass loss of as-prepared membranes can also be seen in Fig. 1. The retained mass of all the membranes was higher than 99.8%, which indicated the excellent alkali resistance of as-prepared membranes. Considering the saturated structure of CSM, mass loss of as-prepared membranes was mainly from degradation of PMAA chains and amide bonds. From these results, we concluded that the formation of IPN structure prevented the attack of OH− to PMAA and amide bonds. To confirm this conclusion, slices of as-prepared membranes were prepared and dyed by osmic acid. The slices were observed by TEM and the pictures are shown in Fig. 2. The bright zones in the TEM pictures were the aggregation of –COOH and the dark field was CSM matrix [21,22]. PMAA encapsulated in the CSM phase and dispersed uniformly can be seen from the pictures. The size of PMAA aggregation was around dozens of nanometers and decreased with an increasing dosage of DETA, which indicated the enhanced compatibility between PMAA and CSM matrix. The results indicated that CSM matrix is entangled with PMAA chains closely and this would prevent the degradation
Table 2 Initial decomposition temperature (IDT) of as-prepared membranes. Content of DETA (%)
0.5
1
1.5
2
2.5
IDT (°C)
221
222
225
229
224
of functional groups (i. e. –COOH) in contact with an alkaline solution. This was the main reason for the excellent alkali resistance of as-prepared membranes. 3.3. Thermal stability Thermal testing results of as-prepared membranes are shown in Fig. 3 and Table 2. It could be seen from the results that the initial decomposition temperatures (IDT) of membranes were in the range of 202–229 °C. Considering the fact that the samples were hotpressed at the temperature of 185 °C, the mass loss below 185 °C in Fig. 3 could be ignored. As observed in Fig. 3, boundary between the two mass loss peaks (decomposition of amide bond around 300 °C and breakage of carbon–hydrogen bond between 400 °C and 500 °C) turned unapparent, which was due to the formation of IPNs. To confirm the peaks, DTG curves of as-prepared membranes are shown in the inset of Fig. 3, from which the rate of mass loss can be detected clearly. The decomposition temperature of PMAA was very close to that of CSM and this indicated well compatibility between
Fig. 2. TEM pictures of as-prepared membranes.
Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043
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F. Chong et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–5 Table 3 Tensile strength (TS) and elongation at break (Eb ) of as-prepared membranes. Content of DETA (%)
0.5
1
1.5
2
2.5
TS (MPa) Eb (%)
24.43 ± 1.55 479.9 ± 38.56
23.84 ± 1.26 554.4 ± 36.55
29.04 ± 1.05 499.9 ± 37.22
28.92 ± 1.23 474.4 ± 29.22
21.53 ± 2.26 411.6 ± 33.25
Fig. 4. Cross-section SEM of as-prepared membranes.
the two phases, which was supported by the TEM and SEM results. The maintained mass of samples after TGA test was less than 10%, in which the sample with 1.5% content of DETA exhibited the highest value. These results indicated that suitable dosage of DETA was necessary to prepare thermally stable membranes. 3.4. Mechanical properties Mechanical properties of the as-prepared membranes (including tensile strength (TS) and elongation at break (Eb )) are shown in Table 3, from which TS and Eb of membranes were observed in the range of 21.5–29.0 MPa and 411.7−554.4%, respectively. The mechanical properties of the as-prepared membranes are more favorable than those of PVA [23] and SPPO [16], which is a consequence of the inherent flexibility of CSM matrix. These results also indicated that the incorporation of MAA had less effect on the mechanical stability of composite membranes. The membranes possessed optimized mechanical properties while loading content of DETA was 1.5%.
Fig. 5. Dialysis coefficients of OH− (UOH ) of as-prepared membranes at 25 °C and 65 °C.
3.5. Membrane morphologies Cross section morphologies of the as-prepared membranes are shown in Fig. 4. All the samples exhibited uniform cross-section structure, which confirmed good compatibility between CSM matrix and PMAA. With the increasing content of DETA, there was not obvious phase separation or structure cavity as observed in the SEM pictures. This was corresponding to the TEM results well, which confirmed the successful construction of IPN structure [24]. The uniform structure of as-prepared membranes provided possibility for application in cationic DD process. 3.6. Results of DD process The results of DD process with the as-prepared membranes at 25 °C and 65 °C are shown in Figs. 5 and 6. Similar with previous report [16], dialysis coefficient of OH− (UOH ) at 65 °C was higher than that of 25 °C as given in Fig. 5, which was due to the enhanced mobility of ions at higher temperature. The values of UOH were in the range of 0.0015–0.018 m/h, which are comparable
Fig. 6. Separation factors (S) of the membranes at different temperatures.
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with PVA and SPPO membranes. The membranes exhibited a similar decreasing trend with an increasing dosage of DETA at 25 °C and 65 °C. The explanation was as follows: DETA was crosslinked with –COOH groups, so that the content of exchangeable groups decreased as the dosage of DETA increased. The decreased trend of UOH turned slowly while the dosage of DETA was higher than 2%, which was in accordance with the uniform structure of as-prepared membranes. Separation factors of as-prepared membranes at 25 °C and 65 °C are shown in Fig. 6, from which the opposite trend to UOH was observed. Similarly to the previous report [4], the values of UOH and S of as-prepared membranes were limited by the “tradeoff effect” between flux and selectivity. Compared with previous report [13, 15, 16], CSM/PMAA membranes possessed competitive selectivity at higher temperature. The results were due to two aspects: (1) IPNs between PMAA and CSM matrix encapsulated the exchangeable groups (–COO− ) and thus improved stability of composite membrane in basic form; (2) formation of IPN structure enhanced density of membranes and restricted the interfacial relaxation between CSM and PMAA. 4. Conclusions CSM/PMAA composite cationic exchange membranes with IPN structure were prepared by free radical polymerization method. Different amount of diethylenetriamine (DETA) was used to form IPN structure and control density of composite membranes. The as-prepared membranes possessed water uptake of 12.7–34.8%, IEC of 1.98–3.12 mmol/g, as well as favorable thermal and mechanical stability. Even more, all the membranes exhibited excellent resistance to 2.0 mol/L NaOH at 65 °C for 60 h: swelling degree was in the range of 72.7–170% while mass loss was less than 0.2%. Natural property of CSM and formation of IPN structure were concluded to avoid erosion of hot alkaline solution effectively. The as-prepared membranes were applied to recover NaOH from NaOH/Na2 WO4 mixture and the results were found to be acceptable: dialysis coefficients of OH− (UOH ) were in the range of 0.0015–0.018 m/h and the separation factors were in the range of 9.0 ∼ 33. The results indicated that the as-prepared CSM/PMAA composite membranes would be potentially applied in alkaline DD process in commercial scale. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 216060 01, 512730 01), the Anhui Provincial Natural Science Foundation (no. 1708085QE117) and the Doctoral Scientific Research Startup Foundation of Anhui University (no. J01003213). Particularly, the financial supports from Collaborative Innovation Center for Petrochemical New Material and Institute of High Performance Rubber Materials & Products of Anhui Province were appreciated.
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Please cite this article as: F. Chong et al., Preparation and properties of cation-exchange membranes based on commercial chlorosulfonated polyethylene (CSM) for diffusion dialysis, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.043