Preparation and characterization of quaternized poly(phthalazinone ether sulfone ketone) NF membranes

Journal of Membrane Science 241 (2004) 225–233

Preparation and characterization of quaternized poly(phthalazinone ether sulfone ketone) NF membranes Yi Su a , Xigao Jian a,b,∗ , Shouhai Zhang a , Guoqing Wang a a

Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116012, PR China b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, PR China Received 12 December 2003; received in revised form 21 April 2004; accepted 26 April 2004

Abstract A novel nanofiltration membrane was prepared and applied for the rejection of dye and divalent salt and for the separation between dye and NaCl. The membrane was obtained by two steps: At first chloromethylated poly(phthalazinone ether sulfone ketone) (CMPPESK) was prepared into the membrane by a phase inversion method, and then CMPPESK membrane was immersed into an aqueous trimethylamine solution to induct quaternary nitrogen groups into the membrane. The resulting membrane owns positive charges and is called quaternized PPESK (QAPPESK) NF membrane. Compared with CMPPESK membranes, QAPPESK membranes have dramatic increase in the pure water flux and the rejection of dyes and MgCl2 . The factors affecting the membrane properties were investigated, such as evaporation time, coagulation temperature and additives, as well as quaternization conditions including trimethylamine concentration, quaternization time and quaternization temperature. The results show QAPPESK membrane with polyethyleneglycol 400 (PEG400) as additive has perfect properties for the removal of dyes and the separation between dyes and NaCl, and the highest permeation flux is about 193 kg/m2 h at 0.25 MPa, and the membrane with ethyleneglycol monomethylether (EGME) as additive is more suitable for the rejection of MgCl2 . The flux increases significantly with the increase of trimethylamine concentration, quaternization temperature and quaternization time. Furthermore, QAPPESK membranes were pretreated or posttreated by boiling in water or heating at 80 ◦ C in the air. It is found that the two methods are effective for increasing the rejection of MgCl2 . The chemical stability of QAPPESK membranes was also investigated. The results indicate that they have excellent acid resistance and oxide resistance. © 2004 Elsevier B.V. All rights reserved. Keywords: Chloromethylation; Quaternization; Poly(phthalazinone ether sulfone ketone); Nanofiltration; Membrane properties

1. Introduction Currently, there is significant interest in NF separation, which have application as follows: (1) removal of organic compounds (100–1000 Da) from water; (2) separation of organic compounds having different MW; (3) separation between divalent salts and monovalent salts; (4) separation between salts and their corresponding acids. NF separations occupy an area that lies between reverse osmosis (RO) and ultrafiltration (UF) and one that has become an active research area in membrane separation [1]. In the last decade, some researchers have begun to concentrate on the NF membrane application in water softening, purification of dye product and reuse of textile wastewater [2–8]. ∗

Corresponding author. Fax: +86-411-83639223. E-mail addresses: [email protected] (Y. Su), [email protected] (X. Jian). 0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.04.040

Compared with neutral NF membranes, charged NF membranes have usually better permeation and separation properties because they reject a solute not only by the steric effect but also by the Donnan effect. They can be used at low operation pressure and thereby save more energy. Moreover, the ionic character themselves also endows them with good fouling resistance. Although many studies on charged NF membranes have been reported [9–13], the research on the preparation and performance of positively charged NF membrane is still little. Quaternization is an effective modification method to induct positive charges into polymer [14–17]. Ishii et al. synthesized quaternized polysulfone (QAPSF) and prepared it into membranes to deal with 0.25 wt.% treating esoine yellowish solution [18]. The results showed that QAPSF membrane had the rejection of 63–98% and the flux of 0.6–5.3 m3 /m2 day at 3 kg/cm2 . The membrane made from the same polymer was applied for the removal of ions in

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Scheme 1. Chloromethylation/quaternization of PPESK.

tap water by Hao et al. [19]. Under operation pressure of 1.5 MPa, the membrane had 93% salt rejection and 0.8 m3 /m2 day product rate for tap water. It also exhibited stable performance when immersed in pH 1–12 and 50 ppm NaClO and 50 ppm H2 O2 solutions. Xu and Yang prepared a novel positively charged composite membrane with poly(2,6-dimethyl-1,4-phenylene oxide) as a support to separate divalent salt from monovalent salt [20]. The rejections for 1000 ppm MgCl2 and NaCl were 32–73% and 6–36%, respectively, and the pure water flux changed from 32 to 38 kg/m2 h. In this research, our intention is to prepare a novel positively charged NF membrane with high permeation flux and high rejection. And we are interested to use the membrane in the rejection of divalent salt and dyes, and the separation between dye and inorganic salt. Furthermore, it is also expected to apply in the industrial fields, such as water softening, purity of dye product and reuse of textile wastewater. A series of poly(phthalazinone ether sulfone ketone) (PPESK) containing different ratios of sulfone and ketone unites (S/K) were previously synthesized [21,22]. These polymers, whose glass transition temperatures are in the range of 263–305 ◦ C, show excellent comprehensive properties and outstanding thermostability in contrast to polysulfone. In the previous investigation, we reported that dense

and asymmetric membranes made from PPESK had shown good separation and permeation properties for both gas and liquid separation [23,24]. Quaternization is applied to modify PPESK in order to increase the hydrophilicity and ionic character. As shown in Scheme 1, at first, chloromethylated PPESK(CMPPESK) was prepared from previously synthesized polymer and thereby utilize this type of polymer for asymmetric membrane, and then put the membrane into trimethylamine solution to obtain final product – QAPPESK NF membrane. In this paper, QAPPESK NF membranes were prepared and characterized. The effects of membrane-forming conditions and quaternization conditions on membrane properties were studied. The chemical stability of QAPPESK membrane was investigated.

2. Experimental 2.1. Materials and methods CMPPESK with the degree of chloromethylation of 1.60 was prepared from PPESK containing a sulfone/ketone ratio of 1/l. N-methyl-2-pyrrolidine (NMP) as solvent was industrial grade and the other chemicals were analytical

Y. Su et al. / Journal of Membrane Science 241 (2004) 225–233

227

Fig. 1. Experimental membrane test cell.

grade. All chemicals were commercial without further purification. Clayton Yellow (CY, MW 696), Methyl Orange (MO, MW 327), X-6G (MW 670), X-8G (MW 556), Methylene Blue (MB, MW 374), Centian Violet (CV, MW 394), Malachite Green (MG, MW 365), MgCl2 and NaCl were used as challenge solutes for the membrane characterization. The concentrations of kinds of dyes were measured using a Spectrophotometer-751 (Shanghai Instrument, China). The dye solution with different concentration showed different absorbency on its max-absorbed wavelength. Thus, the rejection of dye can be calculated based on the absorbency of feed solution and permeation solution. Salt concentrations were determined by a DDS-11A Electrical Conductivity Instrument (Shanghai Leichi Instrument, China). The membrane feed solution side was stirred magnetically to reduce concentration polarization. A flat-sheet dead-end membrane module (Ecological Environment Center of Chinese Academy of Science) having an effective separation area of 41 cm2 and a feed volume of 550 ml was used in all membrane flux characterization and separation experiments as shown in Fig. 1.

solution for 1 h. At last, QAPPESK NF membrane was obtained with the group of −CH2 N(CH3 )3 Cl. 2.3. 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 rejections of 100 ppm dye and 1000 ppm MgCl2 and NaCl solutions (the concentrations were adopted in all the experiments except for extra label)were measured under a pressure difference of 0.2 MPa at room temperature. In every series of comparison experiments, all the tested membranes have similar thickness about 200 ␮m. The permeation flux, Jw (J) is calculated as Jw (J) = W/At (1), where W is the total weight of the pure water (or solution) permeated during the experiment; A represents the membrane area; t denotes the operation time. Rejection (R) is calculated as R = 1 − Cp /Cf (2), where Cp , and Cf , are the concentrations of the permeate and the feed, respectively. In this paper, the concentrations are replaced by the absorbency of dye solutions or the conductivity of salt solutions.

2.2. Membrane preparation Several sheets of asymmetric membranes were cast from each of the CMPPESK solutions in NMP onto glass at a temperature of 60 ◦ C and kept for some time, usually 5 min in the air. The casting solutions were precipitated by immersion into water bath at <5 ◦ C until the membranes formed and came off from glass, and then the membranes were moved into another water bath at ambient temperature and further kept for 36 h. This extended immersion time was to allow adequate time for the solvent to be replaced by non-solvent water. The resulting membrane called CMPPESK membrane was rinsed after immersion. CMPPESK membrane was further immersed into trimethylamine solution of certain concentration at certain temperature for some time and then put into a 5 wt.% HCl

3. Results and discussions 3.1. The influence of type of additive on NF membrane properties As shown in Table 1, n-butanol (n-BuOH), 1,4-dioxane (DO), polyethyleneglycol 400 (PEG400) and ethyleneglycol monomethylether (EGME) were selected as additives. The membranes were cast from NMP solutions containing 19 wt.%, CMPPESK and 15 wt.% additive and then immersed into 0.2 mol/l trimethylamine solution at 40 ◦ C for 2 h. The pure water flux of QAPPESK membranes rank: Jw(PEG400) > Jw(DO) > Jw(EGME) > Jw(n-BuOH) , and the rejection of MgCl2 ranks: Rn -BuOH > RDO > REGME > RPEG400 .

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Table 1 The effect of type of additive on QAPPESK membrane propertiesa No.

Ml M2 M3 M4

Additives

n-BuOH DO PEG400 EGME

RMgCl2 (%)

Jw (kg/m2 h)

RCY (%)

RMO (%)

Beforeb

Afterc

Beforeb

Afterc

Beforeb

Afterc

Beforeb

Afterc

58 106 240 85

28 75 154 50

0 0 0 0

49 45 20 35

98 98 95 96

99.9 99.9 99.9 99.3

70 64 38 67

100 99.9 100 100

Membrane formulation: polymer (19 wt.%) + additive (15 wt.%); Quaternization conditions: 40 ◦ C, 0.2 mol/l, 2 h. Before quaternization. c After quaternization. a

b

The results suggest that QAPPESK membrane with PEG400 as additive has the highest pure water flux and the lowest rejection for MgCl2 . Furthermore, all the QAPPESK membranes have nearly 100% rejection for CY and MO dyes. Thus, it can be concluded that PEG400 is the optimum additive for the rejection of dyes and the separation between dye and salt. And the casting solution containing n-BuOH or DO was so sticking that a smooth membrane could not form if polymer concentration was promoted further. Thus, although QAPPESK membrane with n-BuOH as additive has the highest rejection of MgCl2 , we still select EGME as additive when dealing with MgCl2 . Table 1 also shows the comparison of membrane properties for before and after quaternization. Compared with CMPPESK membranes, QAPPESK membranes have significant increase in the rejection of MO dye and MgCl2 and obvious decrease in pure water flux. When CMPPESK membrane was quaternized, the membrane swelled and large amounts of chloromethylated groups were substituted by the quaternary nitrogen groups. Due to the steric resistance of the quaternary nitrogen groups, they stuffed the pores of the dense layer. Thus, although the membrane swelled, as a whole, the pores of the dense layer diminished. And at the same time Donnan effect was promoted significantly due to the induction of many quaternary nitrogen groups. All these resulted in the increase of the rejections of MgCl2 and dyes. In addition, CY and MO dyes possess counter charges in contrast to QAPPESK membrane. They were easy to be absorbed onto QAPPESK membrane surface due to electrostatic interaction and thereby form further resistance for themselves. This was the other cause for the increase of rejection of dyes. For the flux, the pore radius of both the dense layer and the sublayer controlled it. When the resistance decrease resulted from the pore swell of the sublayer was not enough to compensate for the resistance increase resulted from the pore diminution of the dense layer, the flux decreased. 3.2. The influence of membrane thickness on QAPPESK membrane properties The same formulation as M3 in Table 1 was selected to investigate the effect of membrane thickness on QAPPESK

Table 2 The effect of membrane thickness on QAPPESK membrane properties No.

Membrane thickness (␮m)

Jw (kg/m2 h)

RMO (%)

M3-1 M3-2 M3-3

193 238 312

154 91 74

100 100 100

membrane properties. The results are listed in Table 2. With the increase of membrane thickness, the permeation resistance increases and thereby the flux decreases. However, the rejection of membranes has no changes. Thus, the membrane thickness about 200 ␮m was selected for further experiments. 3.3. The influence of inorganic salt in the casting membrane solution on QAPPESK membrane properties In this series of experiments, EGME and LiNO3 were selected as additives, and the highest LiNO3 concentration selected was 4 wt.% because the cast solution containing more LiNO3 than 4 wt.% was too sticking to form a smooth membrane. The effect of ratio of EGME to LiNO3 on QAPPESK membrane properties was determined and shown in Table 3. Under the total additive concentration of 15 wt.%, as the ratio of EGME to LiNO3 changes from 14/1 to 11/4, the pure water flux increases from 9.4 to 23.3 kg/m2 h and the rejection of MgCl2 has no obvious change. Thus, the increase of LiNO3 concentration in the casting solution is beneficial for improving the membrane properties. In the phase inversion process, a membrane is made by casting a polymer solution on a support and then bringing the solution to phase separation by means of solvent outTable 3 The effect of ratio of EGME to LiNO3 on QAPPESK membrane propertiesa No.

Ratio of EGME/LiNO3

Jw (kg/m2 h)

RMgCl2 (%)

1 2 3 4

14/l 13/2 12/3 11/4

9.4 13.1 16.8 23.3

73 74 75 76

a Membrane formulation: polymer (24 wt.%) + additives (15 wt.%); quaternization conditions: 40 ◦ C, 0.2 mol/l, 2 h.

Y. Su et al. / Journal of Membrane Science 241 (2004) 225–233

flow and/or non-solvent inflow. Thus, in most cases at least three components are involved: a polymer, a solvent and a non-solvent. Exchange of the latter two leads to a phase transition in the at first homogeneous polymer solution and the membrane structure is formed. The cross-section of this structure is usually asymmetric: a thin and dense skin layer is supported by a porous sublayer. Therefore, the exchange speed plays an important role in the membrane structure. The more quickly the solvent exchanges with the non-solvent, the looser the sublayer structure is and the lower the permeation resistance is. In the experiment, the solvent was composed of NMP, EGME and LiNO3 . Due to the perfect solubility of LiNO3 in water, it promoted the exchange speed between the mixed solvent and non-solvent water and thereby led to the looser structure in the sublayer. Thus, the pure water flux increased. 3.4. The influence of preparation conditions on QAPPESK membrane properties 3.4.1. The influence of evaporation time in the air As noted earlier in the membrane preparation, the membranes are allowed to stay in the air for a given time to evaporation the solvent or additives in order to form a dense skin layer. The longer the evaporation time is, the more the solvent and additives evaporate and the denser the top layer is, which will lead to a lower flux and a higher rejection. To check such as expectation and optimize the preparation parameters, membranes with different evaporation times were prepared and characterized. The results are listed in Table 4. Ml, M2 and M3 stand for membranes made from the same preparation parameters except for the evaporation time of 3, 5 and 7 min, respectively. It is obvious that the flux and rejection tendency agree with expectation. Furthermore, considering the flux and rejection of MgCl2 integrally, evaporation time of 5 min is proper. 3.4.2. The influence of coagulation temperature For this test, under the same preparation conditions, two membranes from the coagulation temperatures of 4 and 34 ◦ C, respectively, were prepared and measured. As shown in Table 5, when the coagulation temperature changes from 4 to 34 ◦ C the pure water flux increases from 13.0 to 19.0 kg/m2 h, whereas the rejection of MgCl2 decreases from 74% to 64%. These prove that the higher coagulation temperature is beneficial for increasing the flux and adverse for the rejection of MgCl2 . Table 4 The effect of evaporation time on QAPPESK membrane propertiesa No.

Evaporation time

Jw (kg/m2 h)

RMgCl2 (%)

1 2 3

3 5 7

12.4 11.6 9.4

65 73 75

Membrane formulation: polymer (24 wt.%) + EGME (15 wt.%) + LiNO3 (2 wt.%); quaternization conditions: 40 ◦ C, 0.2 mol/l, 2 h. a

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Table 5 The effect of coagulation temperature on QAPPESK membrane propertiesa No.

Coagulation temperature (◦ C)

Jw (kg/m2 h)

RMgCl2 (%)

1 2

4 34

13.0 19.0

74 64

a Membrane formulation: polymer (24 wt.%) + EGME (13 wt.%) + LiNO3 (2 wt.%); quaternization conditions: 40 ◦ C, 0.2 mol/l, 2 h.

The results are similar to those reported by Gao and Ye [25]. They gave a further explanatory for that. They thought a high coagulation temperature expedited the exchange between solvent and non-solvent markedly. The quicker exchange helped the sublayer structure looser and the permeation resistance lower. On the other hand, the high coagulation temperature caused the polymer on the surface (in direct contact with the coagulation bath) to aggregate suddenly. The sudden aggregation came to a result that some of neighboring polymer micelles touched each other, whereas the other polymer micelles became more independent. Thus, the distribution of pore radius of the skin layer formed became wider and the rejection decreased. 3.5. The influence of quaternization conditions on QAPPESK membrane properties From the results in Table 1, the pure water flux decreases obviously after quaternization, which is not our desire. To improve the Jw further, three quaternization parameters were investigated and optimized, including trimethylamine concentration, quaternization temperature and quaternization time. All CMPPESK membranes in Table 6 were prepared from the same casting solution and preparation conditions. The results in Table 6 indicate that quaternization conditions play an important role in the permeation flux of QAPPESK membrane. For example, for Ml, M2 and M3, as the trimethylamine concentration increases from 0.2 to 5.0 mol/l, the flux has a threefold increase, from 37 to 104 kg/m2 h, whereas the rejection has only a slight change. The same phenomenon occurs when quaternization temperature is enhanced or quaternization time is prolonged. In the experiment, the highest flux obtained is 104 kg/m2 h, which is a twofold increase when compared with its corresponding CMPPESK membrane. In a word, the higher trimethylamine concentration, the higher quaternization temperature and the longer quaternization time are helpful for improving the flux of QAPPESK membrane with slight changes in rejection, and the optimum quaternization condition is just that of M3. 3.6. The influence of heat pretreatment and posttreatment on QAPPESK membrane properties Heat treatment usually causes the membrane to shrink and the surface dense layer to be even denser. Therefore, after heat treatment, the rejection would increase and the flux

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Table 6 The effect of quaternization conditions on QAPPESK membrane propertiesa No.

Quaternization conditions Temperature

Ml M2 M3 M4 M5 M6

(◦ C)

40 40 40 60 26 40

Time (h)

N(CH3 )3 Con. (mol/l)

2 2 2 2 10 10

0.2 1.0 5.0 1.0 0.2 0.2

Jw (before)b (kg/m2 h)

Jw (after)c (kg/m2 h)

RMgCl2 (%)

66 65 64 69 76 76

37 58 104 94 44 62

48 47 48 41 41 51

Membrane formulation: polymer (20 wt.%) + EGME (15 wt.%). Before quaternization. c After quaternization. a

b

Table 7 The effect of pretreatment and posttreatment on QAPPESK membrane propertiesa No. Ml M2 M3 M4 M5b M6c

Pretreatment for CMPPESK membrane

Posttreatment for QAPPESK membrane

Jw (kg/m2 h)

RMgCl2 (%)

RNaCl (%)

– – Boiling for 10 min 80 ◦ C for 10 min – –

80 ◦ C

82 76 100 61 26 104

72 80 78 78 80 59

– 22 14 20 21 14

for 10 min 80 ◦ C for 15 min – – – –

Membrane formulation: polymer (20 wt.%) + EGME (15 wt.%); quaternization conditions: 40 ◦ C, 5.0 mol/l, 2 h. Feed solution: 500 ppm MgCl2 , 500 ppm NaCl. b N,N,N,N-tert-methyl ethylenediamine as the quaternization agent. c The properties of untreated membrane. a

would decrease. In this experiment, two methods including boiling the membrane in a 100 ◦ C water or heating the membrane at 80 ◦ C in an oven for some time, were used to treat CMPPESK or QAPPESK membranes. For example, for Ml, the CMPPESK membrane was prepared and at first quaternized, and then put in an oven at 80 ◦ C for 10 min. However, for M3, the CMPPESK membrane was prepared and at first boiled in 100 ◦ C water for 10 min, and then quaternized. The M6 was not treated by any method and showed the original properties. All the membranes treated by different methods were measured and the results are listed in Table 7. Compared with M6, the other membranes have higher rejection of MgCl2 and irregular flux changes. The highest rejection, 80%, was obtained when QAPPESK membrane was heated at 80 ◦ C in the oven for 15 min and at the same time the pure water flux was 76 kg/m2 h. In addition, N,N,N,N-tert-methyl ethylenediamine was also used as the quaternization agent to lead to a crosslinking reaction to promote the rejection of MgCl2 . However, it is not satisfactory that the rejection of M5 is similar to M2 and its flux is only 26 kg/m2 h. Table 7 also shows the rejections of all the membranes for NaCl. It is found that although all the membranes have high rejection for MgCl2 , the rejection for NaCl is not still over 22%. Thus, QAPPESK membrane can be expected to apply in both the rejection of MgCl2 and the separation between divalent salt and monovalent salt.

3.7. The application for treating dye solution and dye solution containing NaCl 3.7.1. The application for treating dye solution In this series of experiments, sorts of dye solution of 100 ppm were used to characterize QAPPESK NF membranes. The membranes were prepared with different polymer concentration of 19, 21 and 23 wt.%, respectively and presented different rejection property of dye, as shown in Table 8. For six dyes, QAPPESK membranes have the higher rejection for X-6G, X-8B and MO than for MB, CV and MG. For a charged membrane, a solute is retained through steric effect or Donnan effect or a combination of steric and

Table 8 The dye separation with QAPPESK membranesa Dyes

X-6G (MW 670) X-8B (MW 556) MO (MW 327) MB (MW 374) CV (MW 394) MG (MW 365)

Rdye (%) M1 (23 wt.%)

M2 (21 wt.%)

M3 (19 wt.%)

100 100 100 99 99 93

100 100 100 93 90 88

100 95 100 78 74 39

a Membrane formulation: polymer + PEG400 (15 wt.%); quaternization conditions: 40 ◦ C, 0.2 mol/l, 2 h.

Y. Su et al. / Journal of Membrane Science 241 (2004) 225–233

Donnan effects. If Donnan effect was the main cause for the solute rejection, the membrane would have high rejection for the solute with the same charges as the membrane. However, QAPPESK membrane with positive charges presents an abnormal phenomenon. They have high rejections for the negatively charged dyes (X-6G, X-8B and MO), and low rejections for the positively charged dyes (MB, CV and MG). Thus, it can be concluded that steric effect dominates the dye rejection. For X-6G, X-8B and MO dyes, they are possibly easier to aggregate and thereby have larger effective hydrodynamic radius than MB, CV and MG. As a consequence, the rejections of X-6G, X-8B and MO dyes are higher than those of MB, CV and MG. In addition, higher polymer concentration results in higher polymer net density, which means the pore size is smaller and number of pores is fewer in the membrane formed. Thus, with the increase of polymer concentration, the rejections of all the dyes were promoted except for X-6G and MO whose rejections are always being 100%, and the highest rejections of all the dyes were obtained at the polymer concentration of 23 wt.%. These reveal that although the requirement for membrane is different for different type dye separation, there can be a membrane optimally prepared which can be applied to a variety of dyes. 3.7.2. The application for treating dye solution containing NaCl Wastewaters from the textile industry usually contain certain concentration of inorganic salt. Inorganic salt would affect the removal of dye when using NF as separation method, especially for charged NF membrane because Donnan effect is reduced in the presence of inorganic salt. For the reason, we investigated elementarily QAPPESK membrane performance for treating dye solution containing NaCl. The membrane M3 in Table 8 was used and two series of experiments were conducted with 100 ppm CY or MO dye solution containing different NaCl concentration. As shown in Figs. 2 and 3, the permeation flux increases with increasing pressure for all the solutions. The highest flux value obtained is 193 kg/m2 h at 0.25 MPa when using MO and NaCl mixture as the feed solution. All the rejec-

Fig. 2. QAPPESK membrane properties versus pressure for NaCl and CY mixtures (䉬) 500 ppm NaCl; (䊏) 1000 ppm NaCl; (䉱) 2000 ppm NaCl.

231

Fig. 3. QAPPESK membrane properties versus pressure for NaCl and MO mixtures (䉬) 100 ppm NaCl; (䊏) 200 ppm NaCl; (䉱) 500 ppm NaCl.

tions for dyes are over 99%. For a dye solution containing salt, the anions can be absorbed onto the positively charged membrane surface, shielding the positive groups. Therefore, the Donnan effect is reduced. However, QAPPESK membrane has stable rejection of nearly 100% with the increase of salt concentration in the solution. These prove further the dominative role of steric effect. Further, all the results suggest QAPPESK membrane is hopeful to be used in the textile industry. 3.7.3. The application for the separation between dye and inorganic salt Inorganic salt is usually used in dye preparation process, which will lead to retain some salt in the final dye products and affect the purity of dye products. Thus, the purification of dye is a necessary step for producing quality dye. In this experiment, two solutions composed of 100 ppm CY and 1000 ppm NaCl or 100 ppm MO and 1000 ppm NaCl were dispensed and separated by the membrane M3 in Table 8 to determine if QAPPESK membrane is suitable for the separation between dye and NaCl. The permeation solution was collected every 10 ml of volume and measured. To avoid the error led by the membrane module, the first two experiment data were deserted. As shown in Fig. 4a and b, the abscissa stands for the total volume of permeation solution and the ordinate stands for the dye and NaCl rejection, respectively. When the mixed solution containing CY was concentrated from 300 ml originally to 60 ml, and the one containing MO from 400 to 70 ml, the rejections for CY or MO dye are always being 100% and the rejection for NaCl is so low as to be close to 0% or even show minus. In the dye concentration process, with the increase of dye concentration in the feed solution, the permeation flux of membrane decreased gradually due to the dye concentration polarization. At the same time, for NaCl, the Donnan effect was also reduced when the membrane surface absorbed more dye with negative charges, which led more NaCl to pass through the membrane. All these made the rejection of NaCl decrease gradually and even show minus. The results in Fig. 4a and b suggest QAPPESK membrane has a good

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Fig. 5. The acid resistance of QAPPESK membrane (䉬) pure water flux; (䊏) rejection.

Fig. 4. QAPPESK NF membrane application in the separation between dye and salt: (a) (䉬) CY rejection and (䊏) NaCl rejection; (b) (䉬) MO rejection and (䊏) NaCl rejection.

application prospect for the separation between dye and salt in industry. 3.8. The chemical stability of QAPPESK membrane QAPPESK membranes from different formulations or different preparation parameters were measured in the special conditions to investigate their chemical stability, such as acid resistance and oxide resistance. Two of them were put in 10 wt.% HCl solution and in 500 ppm H2 O2 solution, respectively, and measured every dozens of hours. As shown in Fig. 5, QAPPESK membrane has a perfect stability in 10 wt.% HCl solution. There is an increase in Jw and no obvious change in the rejection of MgCl2 . In Fig. 6, the tendency for Jw of membrane in 500 ppm H2 O2 solution is

Fig. 6. The oxide resistance of QAPPESK membrane (䉬) pure water flux; (䊏) rejection.

similar to that in Fig. 5, whereas its rejection has a slight decrease. Some of QAPPESK membranes in Table 6 were kept in tap water for 2 months at a temperature of about 30 ◦ C, and then measured. Further, they were immersed into a 5 wt.% HCl solution for 42 h to rinse. The results are listed in Table 9. To show more clearly, the number of membranes in Table 9 correspond to the ones in Table 6. After these membranes were kept in tap water for 2 months, they showed the decrease in both the Jw and the rejection of MgCl2 , but after immersed in 5 wt.% HCl solution for 42 h, they restituted to nearly original properties. Due to QAPPESK membrane excellent acid resistance and the good rinsed results in HCl solution, we conclude elementarily that QAPPESK membrane may be reserved for a long time in HCl solution.

Table 9 The rinse of QAPPESK membranesa No.

Ml M4 M6 a

Original properties

After 2 months in tap water

After 42 h in 5 wt.% HCl solution further

Jw (kg/m2 h)

RMgCl2 (%)

Jw (kg/m2 h)

RMgCl2 (%)

Jw (kg/m2 h)

RMgCl2 (%)

37 94 62

48 41 51

26 63 39

40 38 47

41 103 67

44 45 48

Membrane formulation: polymer (20 wt.%) + EGME (15 wt.%). Membranes derived from different quaternization conditions.

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4. Conclusions Positively charged NF membranes have been prepared successfully from quaternized PPESK(S/K, 1:1). The effects of evaporation time, coagulation temperature and additives on QAPPESK membrane performance were evaluated and these parameters were optimized. The results show that evaporation time of 5 min and the low coagulation temperature of 4 ◦ C are proper. LiNO3 as additive is beneficial for increasing the flux with the stable rejection of MgCl2 . All the QAPPESK membranes from different additives have the rejection of 100% for CY and MO dyes. QAPPESK membrane with PEG400 as additive has the highest pure water flux of 154 kg/m2 h and the lowest rejection of 20% for MgCl2 . Thus, PEG400 is the optimal additive for the rejection of dyes or the separation between dye and salt, and EGME is the optimal additives for the rejection of MgCl2 . Quaternization conditions play an important role in QAPPESK membrane permeation flux. If proper quaternization condition was chosen, QAPPESK membrane would have dramatic increase from 64 to 104 kg/m2 h in the permeation flux and from 38 to 100% in the rejection of MO dye in contrast to its corresponding CMPPESK membrane. We also obtain perfect results when applying them in the rejection of dye solution containing NaCl and the separation between dyes and NaCl. In the experiments, the highest flux obtained is 193 kg/m2 h at 0.25 MPa with nearly the 100% rejection of dye, and at the same time the rejection of NaCl is close to 0% or even shows minus. Pretreatment and posttreatment are the effective methods for promoting the rejection of MgCl2 . The pretreated membrane by boiling in water bath for 10 min has Jw of 100 kg/m2 h and the rejection of 78% for MgCl2 at 0.2 MPa. The chemical stability of QAPPESK membrane has also been investigated. The results indicate elementarily that it has excellent acid resistance and oxide resistance, and can be rinsed with HCl solution.

Acknowledgements This work was supported by the National 863 Project of China (2001AA334020-3).

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