Preparation of sulfonated poly(phthalazinone ether sulfone ketone) composite nanofiltration membrane

Journal of Membrane Science 246 (2005) 121–126

Preparation of sulfonated poly(phthalazinone ether sulfone ketone) composite nanofiltration membrane Shouhai Zhang a , Xigao Jian a,b,∗ , Ying Dai c a College of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China c National Research Council of Canada, Institute for Chemical Process and Environmental Technology, Ottawa, Ont., Canada K1A 0R6

b

Received 24 December 2003; received in revised form 28 April 2004; accepted 28 April 2004

Abstract Thin film composite (TFC) membranes were prepared from sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) as a top layer coated onto poly(phthalazinone ether sulfone ketone) (PPESK) ultrafiltration (UF) support membranes. The effects of different preparation conditions such as the SPPESK concentration, organic additives, solvent, degree of substitution (DS) of SPPEK and curing treatment temperature and time on the membrane performance were studied. The SPPESK concentration in the coating solution was the dominant factor for the rejection and permeation flux. The TFC membranes prepared from glycerol as an organic additive show better performance then those prepared from other additives. The rejection increased and the flux decreased with increasing curing treatment temperatures. The salt rejections of the TFC nanofiltration (NF) membranes increased in the order MgCl2 < MgSO4 < NaCl < Na2 SO4 . TFC membranes showed high water flux at low pressure. SPPESK composite membranes rejections for a 1000 mg L−1 Na2 SO4 feed solution was 82%, and solution flux was 68 L m−2 h−1 at 0.25 MPa pressure. © 2004 Elsevier B.V. All rights reserved. Keywords: Thin films; Composite membranes; Sulfonated poly(phthalazinone ether sulfone ketone); Nanofiltration

1. Introduction In recent years, a number of composite nanofiltration (NF) membranes have been developed. The term NF is drawn from the observation that size selectivity of the membrane towards non-charged solute approximates 10 Å, i.e. a nanometer cut-off [1]. The performance characteristics of the NF membrane stand at between those of reverse osmosis and ultrafiltration (UF) membrane. The NF process has been used in many applications such as wastewater reclamation, water softening, and separation of organic compounds having different molecular weights. Most NF membranes developed to date are thin film composite (TFC) structures. The composite membrane approach has some key advantages relative to the asymmetric approach. In a TFC membrane, each individual layer can be optimized for its particular function, i.e. the thin barrier layer ∗

Corresponding author. Fax: +86 411 83639223. E-mail address: [email protected] (X. Jian).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.04.013

can be optimized for the desired combination of solvent flux and solute rejection, while the porous support layer can be optimized for maximum strength and compression resistance combined with minimum resistance to permeation flow [2]. Studies on the performance and preparation of composite membranes have been reported by a number of authors. Many materials such as polyamide [3,4], sulfonated polysulfone [5,6], sulfonated polyethersulfone [7], and sulfonated polyphenylene oxide [8–10] were used to prepare the composite NF membrane. PPESKs were previously synthesized [11–13]. These novel polymers with high glass transition temperatures (263–305 ◦ C) show excellent comprehensive properties and outstanding thermal stabilities. Dense films and asymmetric membranes made from PPESKs show good properties for gas and liquid separation [13,14]. Sulfonation is commonly applied to modify polymers in order to increase their hydrophilicity. The sulfonated poly(phthalazinone ether sulfone ketone)s (SPPESK) were prepared from previously synthesized polymers in order to improve their expected

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Scheme 1. Structure of SPPESK with DS 200%.

membrane properties such as better hydrophilicity, and higher water flux. The structure of SPPESK (S:K = 1:1) [15] with degree of substitution (DS) 200% is shown in Scheme 1. UF and NF asymmetric membranes prepared from SPPESK with a low degree of sulfonation showed high water flux and high rejection [15,16]. The performances of SPPESK TFC membranes have been studied by Dai et al. [17]. The SPPESK TFC membranes have negligible changes in rejection, but solution fluxes increased 2.7-fold as operating temperatures were increased from 20 to 100 ◦ C. The process operating temperature was maintained at 130 ◦ C for 1 h and then decreased to 20 ◦ C. The flux and rejection measurements were repeated and found to be almost unchanged from the original values. This indicates that the composite membranes have excellent thermostability. Although the preparation and characterization of SPPESK TFC membranes were described by Dai et al. [17], a systematic comparative study of the preparation conditions for SPPESK TFC membranes was not described. The objective of this work was to investigate systematically the effects of preparation variables of SPPESK TFC membranes, SPPESK concentration, additives and coating time on the membrane performance. For this purpose, SPPESK TFC membranes were prepared by coating SPPESK solution of different polymer concentration onto PPESK UF support membranes. SPPESK with different DS were used for this study. Additives such as polyethyleneglycol, glycerol, N,N-dimethylacetamide and dimethylsulfoxide were used in the preparation of SPPESK coating solutions.

2. Experimental

glycerol, glycol, tetrachloroethane, chloroform, sodium sulfate, sodium chloride, magnesium sulfate, magnesium chloride and fuming sulfuric acid were obtained commercially as reagent grad chemicals and used as received. Salt concentrations were measured by a Conductometer Model DDS-11A (Shanghai Leici Instruments, China). The membrane feed solution side was stirred magnetically to reduce concentration polarization. A flat-sheet dead-end membrane cell (Ecological Environment Center of Chinese Academy of Science) having an effective separation area of 41 cm2 and a feed volume of 500 mL was used in all membrane flux characterization and separation experiments. 2.2. Sulfonation of PPESK The sulfonation procedures of PPESK were similar to procedures reported previously [18,19]. In a typical procedure, 10 wt.% PPESK solution was prepared by dissolving PPESK in chloroform. Fuming sulfuric acid was added dropwise to the above PPESK solution. After reaction, the acid solution was poured into ice water to precipitate SPPESK. The precipitate was washed with water and dried in an oven. SPPESKs with different DS were obtained by the addition of gotten from different quantities of fuming sulfuric acid. The DS of SPPESK was determined by the acid–base titration method. 2.3. PPESK and SPPESK solubility test SPPESK with a DS ranging from 132 to 230% and PPESK and were placed in respective solvents at room or higher temperatures. Solubilities of the polymers were determined by visual observation after 2 h.

2.1. Materials and instrument 2.4. Membrane preparation PPESK (S:K = 1:1) was provided by Dalian New Polymer Co. (PR China). N-methyl-2-pyrrolidone (NMP), chloroform, N,N-dimethylacetamide (DMAc), butanone (BO), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), ethylene glycol monomethyl ether (EGME), polyethyleneglycol (PEG) which molecular weight is 200,

The PPESK UF membrane was prepared by casting a 15% solution of PPESK in a solvent mixture comprising of NMP and 15% EGME on non-woven cloth, then gelling the solution in water within 15 s. The resulting UF membrane was washed with water at room temperature over 24 h. The

S. Zhang et al. / Journal of Membrane Science 246 (2005) 121–126

rejection for PEG10000 is 95% and the water flux is about 300 L m−2 h−1 . TFC membranes were prepared by coating the SPPESK solutions onto the surface of PPESK UF membranes taped to a glass plate or the PPESK UF membrane was soaked in the solution for a certain time. The concentration of SPPESK solutions in EGME was in the range of 0.5–2.0 wt.%. The SPPESK solution was coated onto PPESK UF membrane. The solution on the UF membrane surface was drained by holding membrane vertically, leaving a thin layer of the SPPESK solution. The coated layer was dried at ambient temperature, then cured at 60–140 ◦ C for a certain time. The composite membranes were stored in water until ready to be used. The thickness of SPPESK coating solution on the membrane surface is about 40 ␮m.

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Fig. 1. Effect of SPPESK concentration on TFC membrane performances for a 1000 mg L−1 Na2 SO4 solution.

2.5. Membrane characterization The membranes were characterized in the module after pretreatment with pure water under 0.3 MPa pressure for 30 min. The pure water flux and the rejection of Na2 SO4 , NaCl, MgSO4 and MgCl2 1000 mg L−1 solutions were measured under a pressure difference of 0.25 or 0.3 MPa at room temperature. The permeation flux, F, is calculated according to F = V/At, where V is the total volume of the water or solution permeated during the experiment; A represents the membrane area; and t denotes the operation time. Rejection, R, is calculated according to R = 1 − Cp /Cf , where Cp and Cf are permeate concentration and feed concentration, respectively.

3. Results and discussion 3.1. Solubility of SPPESK and PPESK Prior to the preparation of TFC membranes, the solubility of SPPESK and PPESK were determined in order to select appropriate solvents for the coating solution. SPPESK must be soluble in solvents that do not solubilize the PPESK UF support membranes. The solubilities of SPPESK and PPESK in selected solvents at room temperature are shown in Table 1. As can be seen, PPESK is soluble in selected po-

lar aprotic solvents and in some chlorinated solvents such as tetrachloroethane and chloroform. SPPESK with a DS ranging from 132 to 230% are hydrophilic enough to be soluble in the water, alcohols and EGME. 3.2. Effect of SPPESK coating solution concentration A mixture of EGME and alcohol was selected as the solvent for SPPESK. The TFC membranes were prepared using SPPESK solutions of different concentration in the range of 0.5–2.0%. The substrate membranes were coated with the SPPESK solution, dried at ambient temperature, and then cured at 80 ◦ C for 30 min. The effect of SPPESK concentration in the coating solution on the performance of TFC membranes tested under 0.25 MPa for a 1000 mg L−1 Na2 SO4 is shown in Fig. 1. The Na2 SO4 rejection increased, and water flux decreased with increasing SPPESK concentration. As the solution concentration of the coating polymer increased, the thickness of the selective layer also increased, leading to lower flux and higher rejection. Using the coating procedure, composite membranes prepared from the optimum SPPESK solution concentration of 1.5% have high Na2 SO4 rejection and moderate flux. Therefore, a SPPESK solution concentration of 1.5% was selected, while the effects of other preparation condition on the membrane performance were investigated. 3.3. Effect of organic additives and aprotic solvents

Table 1 The solubility of PPESK and SPPESK Solvent

PPESK

SPPESK

NMP DMAc Water Alcohol EMGE Chloroform

+ + − − − +

+ + + + + −

+, soluble; −, insoluble.

The TFC membranes were prepared from the SPPESK coating solution in the presence of different organic additives or different aprotic solvents. Organic additives such as glycerol, PEG and glycol were used. The substrate membranes were coated with the 1.5% SPPESK solution, dried at ambient temperature, and then cured by heat treatment at 80 ◦ C for 30 min. When the concentration of additives was 25 wt.% and the operating pressure was 0.3 MPa, as can be seen from Table 2, the Na2 SO4 rejection increased in the order PEG < glycol < glycerol; however the water flux de-

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Table 2 Effect of organic additives in SPPESK solution on the TFC membrane performance Organic additives

Rejection (%)

Flux (L m−2 h−1 )

PEG Glycol Glycerol

38 64 94

63 29 10

Table 3 Effect of aprotic solvent in SPPESK solution on the TFC membrane performance Aprotic solvent

Rejection (%)

Flux (L m−2 h−1 )

DMAc DMF DMSO –

63 55 36 45

76 62 167 112

creased. In Table 3, aprotic solvents such as DMAc, DMF and DMSO were used for this study and the concentration of aprotic solvent was 5 wt.% and the operating pressure was 0.25 MPa. PPESK was soluble in DMAc and DMF. The surface strain between the SPPESK coating solution in presence of DMAc or DMF and the surface of the PPESK UF membrane decreased, which was advantageous for preparing TFC membranes with high rejection and high flux. The presence of DMSO in the SPPESK coating solution resulted in higher flux and lower rejection because PPESK was completely insoluble and non-swelling in DMSO. 3.4. Effect of degree of sulfonation of SPPESK TFC membranes were prepared from 1.5% SPPESK solutions which have different DSs in the range of 132–230% coated on substrate membranes. They were initially dried at ambient temperature, then cured by heat treatment at 100 ◦ C for 30 min. The effect of DS on the performance of membranes was studied under 0.25 MPa for a 1000 mg L−1 Na2 SO4 solution. As can be seen from Fig. 2, the flux of TFC membranes increased as the DS of SPPESK increased.

Fig. 3. Effect of soaking time on the TFC membrane performances for a 1000 mg L−1 Na2 SO4 solution.

This is because when DS of SPPESK increases, the membrane hydrophilicity increases, which increased both the water flux and the rejection. Fig. 2 indicated a slight increase in rejection from the DS values of 132–212% and a decrease from DS values of 212–230%. 3.5. Effect of soaking time on TFC membrane performances Fig. 3 illustrates the effect of soaking time on the performance of SPPESK composite membranes under 0.25 MPa and 1000 mg L−1 Na2 SO4 concentration. The TFC membranes were prepared from a 1.5% SPPESK coating solution and dried at 100 ◦ C. With the soaking time being prolonged, the top layer in the TFC membrane thickened, so the rejection of the TFC membrane increased and the flux decreased. 3.6. Effect of membrane curing treatment temperature The TFC membranes were prepared from a 1.5% SPPESK coating solution. PPESK UF membranes were coated with SPPESK solution for 5 min, dried at ambient temperature, then cured at different temperatures ranging from 60 to 120 ◦ C for 1 h. Fig. 4 shows the effect of curing temperature on the performance of composite membranes. The rejection increased with an increase in curing temperature and simultaneously the water flux decreased under 0.25 MPa and for 1000 mg L−1 Na2 SO4 solution. In SPPESK solution, glycerol was used as a cross-linking agent for the selective skin layer. The SO3 H group in SPPESK can react with the OH group in glycerol during curing. Elevated temperature promotes crosslinking of polymers and shrinkage of the surface of the substrate membrane, which results in higher rejection and lower water flux. 3.7. Effect of membrane curing treatment time

Fig. 2. Effect of DS of SPPESK on the TFC membrane performances for a 1000 mg L−1 Na2 SO4 solution.

Fig. 5 illustrates the effect of the membrane curing treatment time on the performance of SPPESK composite mem-

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Table 4 Performances of the SPPESK/PPESK TFC membranes

R (%) F (L m−2 h−1 )

Na2 SO4

NaCl

MgSO4

MgCl2

82 68

25 83

18 65

8 71

order. The flux for NaCl is higher than for other solutes. This sequence shows the predictable Donnan characteristic [2] of salt rejection for a negatively charged nanofiltration membrane.

4. Conclusions Fig. 4. Effect of curing temperature for 1 h on the TFC membrane performances for a 1000 mg L−1 Na2 SO4 solution.

brane. For this study, TFC membranes were prepared from a 1.5% SPPESK coating solution and cured at 100 ◦ C for different curing times ranging from 0 to 60 min. The rejection increased with an increase in curing treatment time and simultaneously the flux decreased under 0.25 MPa for 1000 mg L−1 Na2 SO4 solution. 3.8. Performance of SPPESK TFC membrane TFC membranes were prepared from a 1.5% SPPESK solution in the presence of 25% glycerol and 5% DMAc. PPESK UF membranes were coated with the SPPESK solution for 5 min, dried at ambient temperature, and then cured at 80 ◦ C for 30 min. The performance of SPPESK composite membranes under 0.25 MPa pressure for a 1000 mg L−1 feed solution was shown in Table 4. The salt rejection increased in the order MgCl2 < MgSO4 < NaCl < Na2 SO4 for composite membranes thus prepared. It is obvious that the rejection for mono-valent anion is lower than for divalent anions; however, the rejection for cations is in the reverse

SPPESK composite nanofiltration membranes were prepared by coating a 0.5–2.0% SPPESK solution on the surface of PPESK UF membranes. The composition of the coating solution and the condition of the prepared membranes were studied these include such as SPPESK concentration, organic additives, soaking time, curing time and curing temperature. SPPESK concentration has a great influence on the performance of TFC membranes. Rejections and fluxes for a 1000 mg L−1 Na2 SO4 feed solution were 51–95% and 11–36 L m−2 h−1 , respectively. Elevated curing treatment temperatures promoted production of dense top layer, which resulted in high rejection and low flux. TFC membranes show nanofiltration characteristics. The salt rejection of SPPESK membranes increased in the order MgCl2 < MgSO4 < NaCl < Na2 SO4 . TFC membranes exhibit high rejection and high flux. Rejections and fluxes for a 1000 mg L−1 Na2 SO4 feed solution were 18–95% and 8–167 L m−2 h−1 , respectively, depending on the different conditions of the prepared membrane.

Acknowledgements The authors wish to thank China National Natural Science Fund for financial support.

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Fig. 5. Effect of curing time at 100 ◦ C on the TFC membrane performances for a 1000 mg L−1 Na2 SO4 solution.

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