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Pervaporation characteristics of cross-linked poly(dimethylsiloxane) membranes for removal of various volatile organic compounds from water
Journal of Membrane Science 260 (2005) 156–163
Pervaporation characteristics of cross-linked poly(dimethylsiloxane) membranes for removal of various volatile organic compounds from water Tadahiro Ohshima, Yoshihisa Kogami, Takashi Miyata, Tadashi Uragami ∗ Unit of Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan Received 26 October 2004; received in revised form 7 March 2005; accepted 9 March 2005 Available online 28 April 2005
Abstract The permeation and separation characteristics of volatile organic compounds (VOCs), such as chloroform, benzene, and toluene, from water by pervaporation through cross-linked poly(dimethylsiloxane) membranes prepared from poly(dimethylsiloxane) dimethylmethacrylate macromonomer (PDMSDMMA) and divinyl compounds, such as ethylene glycol dimethylmethacrylate (EGDM), divinyl benzene (DVB), divinyl siloxane (DVS), and divinyl perfluoro-n-hexane (DVF) are described. When aqueous solutions containing 0.05 wt.% VOCs were permeated through cross-linked PDMSDMMA membranes, these membranes showed high VOC/water selectivity and permeability. Both VOC/water selectivity and permeability were affected significantly by the divinyl compound. Furthermore cross-linked PDMSDMMA membranes showed the highest chloroform/water selectivity. The VOC/water selectivity was mainly governed by the sorption selectivity rather than the diffusion selectivity. However, the difference in the selectivity between different types of VOCs depended on differences in the diffusivity of permeants. With increasing downstream pressure, the VOC/water selectivity of all cross-linked PDMSDMMA membranes increased, but the permeability decreased. A PDMSDMMA–DVF membrane exhibited a normalized permeation rate of 1.9 × 10−5 kg m/m2 h and a separation factor for chloroform/water of 4850, yielding a separation index of 9110. The pervaporation characteristics of the cross-linked PDMSDMMA membranes are discussed based on their chemical and physical structures as well as the chemical and physical properties of the permeants. © 2005 Elsevier B.V. All rights reserved. Keywords: Poly(dimethylsiloxane); Membrane; Pervaporation; VOCs; Cross-link
1. Introduction Wastewater containing volatile organic compounds (VOCs) generated from various production lines in the chemical and petroleum industries has recently caused serious environmental problems. The removal of VOCs from wastewater is typically carried out by adsorption methods using active carbon and biological treatment. However, these methods are not very energy-efficient. More efficient, continuous methods
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0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.03.027
that require less energy and water for treatment of VOCs from wastewater are required [1–3]. Recently, pervaporation (PV), which is a membranebased separation technique, has been investigated for wastewater treatment. Many different membrane types, such as poly(dimethylsiloxane) (PDMS) [4,5], polyamide [6,7], copolymers [8,9], and ceramic-filled [10,11] membranes have been examined for removal of VOCs from water by PV. Of these membranes, PDMS has high selectivity and permeability for organic chemicals in water, because PDMS is hydrophobic and rubbery. However, the mechanical strength of PDMS is notably weak. In order to improve its mechanical strength, selectivity, and per-
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meability, various PDMS derivative membranes were prepared. In previous work [12,13], we studied the relationship between the structure of multicomponent polymer membranes containing PDMS and their VOC/water selectivity during PV. We demonstrated that the selectivity of multicomponent polymer membranes depended strongly on the morphology of their microphase-separated structures. We demonstrated that in graft copolymer membranes, the microphase-separated structure of poly(methylmethacrylate) (PMMA) and PDMS significantly influenced their permeability and selectivity for removal of dilute VOCs from an aqueous solution. Furthermore, the continuous PDMS phase in the microphase-separated structure played an important role in the selective removal of VOCs. In other studies [14–16], addition of polymer additives, such as fluorine-containing PDMS graft and block copolymers, and tert-butyl calix[4]arene, to PMMA-g-PDMS and PMMA-b-PDMS membranes enhanced the VOC/water selectivity and permeability. Because our results demonstrated that the PDMS component in multicomponent polymer membranes is very important for VOC/water selectivity, we prepared polymer membranes consisting of pure PDMS. However, it was very difficult to prepare mechanically strong PDMS membranes from a PDMS macromonomer. Therefore, cross-linked PDMS membranes were prepared by copolymerization of PDMS dimethylmethacrylate macromonomer (PDMSDMMA) as the PDMS component, with various divinyl compounds as cross-linkers. These divinyl compounds were selected to have a high affinity for benzene to improve the performance of the PDMS membranes. The cross-linked PDMSDMMA membranes showed high benzene/water selectivity. The benzene/water selectivity and permeability of these membranes depended significantly on the divinyl cross-linker type and were enhanced by increasing the divinyl content [17,18]. In this study, the PV performance of cross-linked PDMSDMMA membranes was investigated in more detail for removal of other VOCs such as toluene and chloroform from water. The results are discussed and compared with those previously reported for benzene/water mixtures.
2. Experimental 2.1. Materials Poly(dimethylsiloxane) dimethylmethacrylate macromonomer (PDMSDMMA) (1) as a base PDMS polymer for the membrane matrix was supplied by Toray Dow Corning Silicone Co. Ltd. Ethylene glycol dimethylmethacrylate (EGDM) (2), divinylbenzene (DVB) (3), divinylsiloxane (DVS) (4) and divinylperfluoron-hexane (DVF) (5) were used as cross-linkers. 2,2Azobis(2-methylpropionnitrile) (AIBN) was recrystallized
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from methanol solution and used as the initiator. All other solvents and reagents were of analytical grade from commercial sources and were used without further purification. 2.2. Preparation of cross-linked PDMSDMMA membranes The cross-linked PDMSDMMA membranes (PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMSDMMA– DVS and PDMSDMMA–DVF) were prepared by copolymerization of PDMSDMMA with the divinyl cross-linkers using AIBN as an initiator, in a mold plate between two glass plates at 80 ◦ C for 24 h under nitrogen atmosphere, as reported in previous studies [17–19]. In Table 1, the preparation conditions of the cross-linked PDMSDMMA membranes are summarized. In order to remove any unreacted monomers, the membranes were soaked in n-hexane for 24 h and dried completely under reduced pressure at room temperature. All cross-linked PDMSDMMA membranes were rubbery. 2.3. Pervaporation apparatus and measurement conditions The PV cell and apparatus have been described in a previous paper [20]. The effective membrane area was about Table 1 Preparation conditions of the cross-linked PDMSDMMA membranes Membrane
Cross-linker content (mol%)
PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
70 80 90 90
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13.8 cm2 . The PV experiments were carried out under the following conditions: permeation temperature: 40 ◦ C; downstream pressure: 0.01, 0.05, 0.1, 1, and 4 Torr. The feed VOC solutions, aqueous solutions of dilute chloroform, benzene and toluene, were circulated between the PV cell and the feed tank to maintain an essentially constant concentration of the feed solution during PV. The permeation rate (kg/m2 h) for the aqueous VOC solutions during PV was determined from the weight (kg) of permeate collected in a cold trap, the permeation time (h) and the effective membrane area (m2 ). To provide a comparison of the steady-state permeation rates of membranes with different thickness, the normalized permeation rate (kg m/m2 h), which is the product of the permeation rate and the membrane thickness, was used. The VOC concentration in the feed and permeate was determined by a gas chromatograph (Shimadzu GC-14A) using a flame ionization detector (FID) equipped with a capillary column (Shimadzu Co. Ltd., PorapacQ and P) heated to 150 ◦ C. The separation factor, αsep. VOC/H2 O , was calculated from Eq. (1) αsep. VOC/H2 O =
PVOC /PH2 O FVOC /FH2 O
(1)
where FVOC and FH2 O are the weight fractions of VOC and water in the feed solution and PVOC and PH2 O are those in the permeate, respectively. 2.4. Composition of VOC/water in cross-linked PDMSDMMA membranes The cross-linked PDMSDMMA membranes were dried completely under reduced pressure at room temperature. The dried membranes were immersed in an aqueous solution containing 0.05 wt.% VOC in a sealed vessel at 40 ◦ C until equilibrium was reached. The sorbed solution in the swollen membranes was completely desorbed under reduced pressure and was collected in a cold trap. The compositions of the solutions in the cross-linked PDMSDMMA membranes were then determined by measuring the VOC concentration in the collected solution by gas chromatography (Shimadzu GC-14A). The composition of VOC/water in the cross-linked PDMSDMMA membranes and in the feed solution yielded the sorption selectivity, αsorp. VOC/H2 O , as expressed in Eq. (2), αsorp. VOC/H2 O =
MVOC /MH2 O FVOC /FH2 O
(2)
where FVOC and FH2 O are the weight fractions of VOC and water in the feed solution and MVOC and MH2 O are those in the membrane, respectively. 2.5. Degree of swelling of cross-linked PDMSDMMA membrane The dried, cross-linked PDMSDMMA membranes (0.5–0.6 g) were weighed and immersed in an aqueous so-
lution of 0.05 wt.% VOC (300 ml) in a sealed vessel at 40 ◦ C until equilibrium was reached. The membranes were then taken out of the vessel, wiped for 20 s using ADVANTEC qualitative filter paper no. 2, and weighed. These procedures were several times repeated. The measurements of the degree of swelling were reproducible, and the errors inherent in these measurements ranged within a few percent for the degree of swelling. The degree of swelling (DS) of the cross-linked PDMSDMMA membranes was then determined from Eq. (3), DS =
Ws Wd
(3)
where Ws is the weight of the membrane swollen in an aqueous solution of 0.05 wt.% VOC and Wd the weight of the dried membrane.
3. Results and discussions 3.1. Pervaporation characteristics of cross-linked PDMSDMMA membranes The PV performance of the PDMSDMMA membrane and the PDMSDMMA membranes cross-linked with EGDM, DVB, DVS and DVF for removal of VOCs from water is shown in Fig. 1. As can be seen in Fig. 1, although the permeability for all of the different kinds of VOC were not equally enhanced by cross-linking, none of the cross-linked PDMADMMA membranes had lower permeability than that of the uncross-linked PDMSDMMA sample. Moreover, as shown by the higher VOC permeate concentrations for the cross-linked samples, the VOC/water selectivity was obviously enhanced by cross-linking in all cases. Especially DVF, a fluorine-containing cross-linker, significantly improved the PV performance compared to that of the PDMS homopolymer membrane. In general, when the selectivity of a membrane is increased by modification, the permeability of the membrane decreases due to the well-known permeability/selectivity trade-off relationship [21]. As shown in Fig. 1, however, both the selectivity and permeability of the cross-linked PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMS–DVS, and PDMSDMMA–DVF membranes increased by cross-linking. This result suggests that the affinity of the cross-linked PDMSDMMA membranes for VOCs was enhanced by increasing the hydrophobic interactions between the membranes and the VOCs. Furthermore, swelling by the feed solution was controlled due to introduction of cross-link sites and a resulting increase in hydrophobicity. On the other hand, there was a difference of selectivity and permeability for various VOCs for the same membrane. For all membranes, the best PV performance was achieved for the separation of chloroform from water. For comparison, chloroform was removed more efficiently by about 25% based on the VOC/water selectivity and had about 12–30% higher permeability than toluene.
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Fig. 2. Effect of cross-linker type on the VOC concentration in membranes immersed in aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform ().
Fig. 1. Effects of cross-linker type on the VOC concentration in the permeate, and the normalized permeation rate for aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform () through PDMSDMMA, PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMSDMMA–DVS and PDMSDMMA–DVF membranes.
3.2. Structure of the cross-linked PDMSDMMA membranes The VOC concentration sorbed in the cross-linked PDMSDMMA membranes and the degree of swelling of these membranes immersed in aqueous solutions of 0.05 wt.% VOC are shown in Figs. 2 and 3. The VOC concentration in all membranes was significantly higher than that in the feed solution (0.05 wt.%). This result suggests that the cross-linked PDMSDMMA membranes have high VOC/water sorption selectivity. Similar to the results obtained in PV experiments, the sorbed VOC concentration in the membrane was higher in a PDMSDMMA membrane cross-linked with more hydrophobic cross-linkers. This result can be attributed to a strong affinity between the cross-linked PDMSDMMA membranes and the hydrophobic VOCs. However, the absolute amount of VOC concentration in the membranes depended on the type of VOC. Interestingly, the chloroform concentrations in these membranes were the lowest of all VOCs tested. This result differed significantly from that obtained during PV through cross-linked PDMSDMMA membranes as shown in Fig. 1.
On the other hand, the degree of swelling of cross-linked PDMSDMMA membranes was not greatly influenced by the cross-linker type. This result is mainly due to the fact that the feed was a dilute VOC solution. If the degree of swelling can be ranked based on extremely slight differences of the cross-linked PDMSDMMA membranes it followed the order: toluene > benzene > chloroform. This tendency was similar to the sorbed VOC concentration in the membrane, but differed from that observed under PV conditions. These composite results suggest that the mechanisms of permeation and separation of VOCs from aqueous solutions through PDMSDMMA membranes by PV are mainly governed by the sorption process. However, physical factors, such as the molecular size of the VOCs cannot be neglected and greatly influenced the PV performance of cross-linked PDMSDMMA membranes.
Fig. 3. Effect of the cross-linker type on the degree of swelling of cross-linked PDMSDMMA membranes immersed in aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform ().
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Table 2 Contact angle for water, membrane density, and cross-linking density of the PDMSDMMA membranes Membrane
Contact angle (◦ )
Membrane density (g/cm3 )
Cross-linking density (×10−4 mol/g)
PDMSDMMA PDMSDMMA–EGDMa PDMSDMMA–DVBb PDMSDMMA–DVSc PDMSDMMA–DVFd
109.3 108.4 114.0 114.8 115.3
0.993 1.017 1.002 0.999 1.011
2.633 5.147 4.696 1.034 0.935
a b c d
EGDM content: 70 mol%. DVB content: 90 mol%. DVS content: 90 mol%. DVF content: 90 mol%.
In Table 2, the contact angle for water, the membrane density and the cross-link density of the cross-linked PDMSDMMA membranes are summarized. The contact angle for water of the PDMSDMMA–DVF membrane showed the highest value of all membranes. This result suggests that the PDMSDMMA membrane cross-linked with DVF is the most hydrophobic. In addition, it can be assumed that the order of hydrophobicity of the cross-linkers used in this study are: DVF > DVS > DVB > EGDM. On the other hand, the membrane density and the cross-linking density of PDMSDMMA membranes are not always comparable, i.e., the membrane density followed the order: EGDM > DVF > DVB > DVS, whereas that of the crosslinking density was: EGDM > DVB > DVS > DVF. This result suggests that the degree of cross-linking is significantly different for each cross-linker. EGDM and DVB gave a highly cross-linked structure, whereas DVS and DVF formed a less cross-linked structure. Presumably, the PDMSDMMA–DVS and PDMSDMMA–DVF membranes have a pendant structure containing DVS and DVF. This is supported by the fact that the contact angle for water of these membranes was higher. Consequently, cross-linking of the PDMSDMMA membranes with DVS and DVF favor a chemical factor induced by the cross-linker; however, these cross-linkers provide negative physical factors, such as the cross-linking density. On the basis of the above discussion, the increase in the normalized permeation rates of the PDMSDMMA–DVS and PDMSDMMA–DVF membranes with introduction of DVS and DVF, as shown in Fig. 1, can be rationalized.
3.3. Mechanism of permeation and separation In order to discuss the effect of VOCs on the PV performance of cross-linked PDMSDMMA membranes in more detail, the solubility parameter and molar volume of each component were calculated and the relationship between the VOC/water selectivity was investigated. The solubility parameter of each component calculated by the group contribution method [21–24] and the difference in the solubility parameter between the membrane and the permeants are shown in Table 3. The difference in the solubility parameter, ∆, was calculated from the solubility parameter of VOC (δVOC ) or water (δwater ) and the membrane (δmembrane ) using Eqs. (4)
Table 3 Solubility parameter of the cross-linked PDMSDMMA membranes and molar volume of permeants Material
Solubility parameter (cal/cm)1/2
PDMSDMMA PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
Molar volume (cm3 /mol)
8.06 8.70 8.16 8.06 7.59
H2 O Chloroform Benzene Toluene
– – – – –
23.4 9.3 9.2 8.9
18.0 80.4 88.9 106.2
and (5): ∆ = |δmembrane − δVOC |
(4)
∆ = |δmembrane − δwater |
(5)
A small ∆ means that the membrane has a strong affinity for VOC and water. The solubility parameter, δmembrane , of the cross-linked PDMSDMMA membranes was calculated from the solubility parameters and the volume ratios of PDMSDMMA and the cross-linker, based on the group contribution method. The ∆ values are shown in Table 4. It is obvious that these membranes have a higher affinity for VOCs than water. When more hydrophobic cross-linkers were introduced into the membranes, the affinity of the membranes for the VOCs increased. In addition, it is obvious that cross-linked PDMSDMMA membranes have higher affinity for toluene relative to chloroform and benzene. Analysis of the solubility parameters of the membranes and those of the VOCs agrees with the tendency of VOC concentration sorbed in the cross-linked PDMSDMMA membranes. All cross-linked PDMSDMMA Table 4 Difference of solubility parameter between the membranes and permeants Membrane
PDMSDMMA PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
∆ = |δmembrane − δpermeant | H2 O
Chloroform
Benzene
Toluene
15.34 14.70 15.24 15.34 15.81
1.24 0.60 1.14 1.24 1.71
1.14 0.50 1.04 1.14 1.61
0.84 0.20 0.74 0.84 1.31
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Fig. 4. Tentative model of the permeation and separation for removal of different VOCs through cross-linked PDMSDMMA membranes.
membranes exhibit the highest affinity for toluene. As shown in Table 3, the order of the molecular size in VOCs used in this PV experiment is: toluene > benzene > chloroform. This order is the same as those of the VOC/water selectivity and the normalized permeation rates, as shown in Fig. 1. Hence, it can be concluded that the difference in the VOC/water selectivity of cross-linked PDMSDMMA membranes depends significantly on the difference in the diffusivity of the permeants. The diffusion-based selectivity for VOCs depended strongly on the cross-linked structure. From the above results, the removal mechanism of VOCs from water through cross-linked PDMSDMMA membranes can be explained by the qualitative model, shown in Fig. 4, based on the sorptiondiffusion model [25,26]. 3.4. Effect of downstream pressure on PV performance The effect of the downstream pressure on the VOC/water selectivity and the permeability through the PDMSDMMA– DVF membrane is shown in Fig. 5. The downstream pressure was varied from 0.01, 0.05, 0.1, 1, and 4 Torr during PV. An increase in downstream pressure resulted in a decrease of the driving force during PV experiments. As can be seen in Fig. 5, the VOC/water selectivity of the PDMSDMMA– DVF membrane increased for all permeants with increasing downstream pressure. At different downstream pressures, the VOC/water selectivity was almost constant. This result suggests that a higher downstream pressure results in higher VOC/water selectivity. Huang and coworkers reported that the selectivity for ethyl butanoate (ETB) in the separation of ETB from water through PDMS membranes by PV decreased with increasing downstream pressure [27], supporting the results of this study.
Fig. 5. Effects of the downstream pressure on the VOC concentration in the permeate, and the normalized permeation rate for aqueous solutions of 0.05 wt.% VOCs through the PDMSDMMA–DVF membrane. VOCs: () chloroform, () benzene and (䊉) toluene.
In general, the separation of liquid mixtures through polymer membranes results from a difference in the solubility of the permeating molecules into the polymer membrane, and in the diffusivity of the permeating molecules through the polymer membranes. The diffusivity selectivity depends on the molecular size of the permeant and the driving force. Hence, the selectivity of the permeants with a larger molecular size may be expected to decrease with increasing downstream pressure. However, in this study, the selectivity for VOCs increased with increasing downstream pressure. This result may be attributed to the fact that the PDMSDMMA–DVF membrane has a high affinity for VOCs, as shown in Fig. 4. A high VOC/water selectivity with increasing downstream pressure is due to the more open structure of the dry layer of the membrane surface on the downstream side [28]. On the other hand, the normalized permeation rate decreased with increasing downstream pressure, due to a decrease in the driving force. To investigate in detail the decrease in the normalized permeation rate with downstream pressure, partial normalized permeation rates for water and VOCs were analyzed. Fig. 6 shows the effects of downstream pressure on the normalized partial permeation rates of water and VOCs in
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T. Ohshima et al. / Journal of Membrane Science 260 (2005) 156–163 Table 5 Performance of cross-linked PDMSDMMA membranes for an aqueous solution of 0.05 wt.% chloroform during PV Membrane
αCH3 Cl
NPRa
PSIb
PDMSDMMA PDMSDMMA–EGDMc PDMSDMMA–DVBd PDMSDMMA–DVSe PDMSDMMA–DVFf
2726 2937 3648 3498 4847
1.88 1.86 1.97 2.06 1.88
5125 5462 7187 7196 9112
a b c d e f
Fig. 6. Partial normalized permeation rates for VOC and water through a PDMSDMMA–DVF membrane as a function of the different downstream pressure. VOCs: () chloroform, () benzene and (䊉) toluene.
the PV treatment of aqueous solutions of 0.05 wt.% VOCs through the PDMSDMMA–DVF membrane. In each case, the normalized partial permeation rates for water and VOC decreased with increasing downstream pressure, as shown in Fig. 5. The decrease of the normalized partial permeation rate for water was smaller than that for the VOCs. This result suggests that water molecules, which are smaller than VOCs, do not depend much on the diffusion process. The results shown in Fig. 6 demonstrate that with increasing downstream pressure, the decrease in the normalized permeation rate in Fig. 5 is mainly influenced by a decrease in the partial normalized permeation rate for the VOC. 3.5. Performance of the cross-linked PDMSDMMA membranes In Table 5, the normalized permeation rate, the separation factor, αCHCl3 /water, and the pervaporation separation index (PSI) [21,29] for an aqueous solution of 0.05 wt.% chloroform through the cross-linked PDMSDMMMA membranes during PV are listed. PSI is the product of the normalized permeation rate and the separation factor, and can be used as a measure of PV membrane performance. As can be seen in Table 5, the normalized permeation rates
The normalized permeation rate (×10−5 kg m/m2 h). Pervaporation separation index (αsep. CHCl3 /H2 O × NPR). EGDM content: 70 mol%. DVB content: 90 mol%. DVS content: 90 mol%. DVF content: 90 mol%.
for an aqueous solution of 0.05 wt.% chloroform through cross-linked PDMSDMMA–DVF were almost equal, but the separation factor of the PDMSDMMA–DVF membrane was much higher than that of the other membranes. Consequently, the PDMSDMMA–DVF membranes show a higher pervaporation separation index. On the basis of these results, the introduction of a fluorine-containing cross-linker into the PDMSDMMA is a very effective way to provide improved membrane performance. The reason why a fluorine-containing cross-linker improved the membrane performance may be speculated as given below. As shown in Table 2, the contact angle for water of cross-linked PDMSDMMA membrane was the highest in the cross-linked PDMSDMMA membranes. This fact implies that the surface of cross-linked PDMSDMMA–DVF membrane is more hydrophobic, an absorption of water in the feed into this membrane is significantly suppressed, and consequently, the VOC/water selectivity is improved.
4. Conclusions The separation and permeation characteristics of dilute aqueous solutions of VOCs, such as chloroform, benzene and toluene, through cross-linked PDMSDMMA membranes by PV were investigated with respect to different permeants. Cross-linked PDMSDMMA membranes showed high VOC/water selectivity and permeability. Especially, PDMSDMMA membranes cross-linked with more hydrophobic cross-linkers, such as a DVF, showed high VOC/water selectivity and permeability. The PV characteristics of PDMSDMMA membranes depended significantly on the VOC type, and the selective removal of chloroform from water was greater than that of benzene and toluene. The VOC/water selectivity and permeability were significantly influenced by the cross-linker and permeant type. The PV behavior of cross-linked PDMSDMMA membranes can be rationalized by differences in the chemical and physical structures of the membranes and those of the permeants. The VOC/water selectivity of the cross-linked PDMSDMMA membranes was mainly governed by the sorption selectivity of the permeants.
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However, the difference in the selectivity between different types of VOCs depended on differences in the diffusivity of the permeants. The mechanism of permeation and separation follows the sorption-diffusion model. We discovered that the best performance of PDMS membranes for removal of VOCs is achieved using a fluorine-containing cross-linker.
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[14] T. Uragami, T. Meotoiwa, T. Miyata, Effects of addition of calixarene to microphase-separated membranes for the removal of volatile organic compounds from dilute aqueous solutions, Macromolecules 34 (2001) 6806. [15] T. Miyata, H. Yamada, T. Uragami, Surface modification of microphase-separated membranes by fluorine-containing polymer additive and removal of dilute benzene in water through these membranes, Macromolecules 34 (2001) 8026. [16] T. Uragami, T. Meotoiwa, T. Miyata, Effects of morphology of multicomponent polymer membranes containing calixarene on permselective removal of benzene from a dilute aqueous solution of benzene, Macromolecules 36 (2003) 2041. [17] T. Uragami, T. Ohshima, T. Miyata, Removal of benzene from an aqueous solution of dilute benzene by various cross-linked poly(dimethyl siloxane) membrane during pervaporation, Macromolecules 36 (2003) 9430. [18] T. Ohshima, T. Miyata, T. Uragami, Cross-linked smart poly(dimethylsiloxane) membranes for removal of volatile organic compounds in water, J. Mol. Struct. 739 (2005) 47. [19] K. Inui, H. Okumura, T. Miyata, T. Uragami, Permeation and separation of benzene/cyclohexane mixture through cross-linked poly(alkyl methacrylate) membrane, J. Membr. Sci. 132 (1997) 193. [20] K. Inui, H. Okumura, T. Miyata, T. Uragami, Permeation and separation of organic liquid mixtures through liquid crystalline polymer networks, Angew. Makromol. Chem. 240 (1996) 241. [21] R.Y.M. Huang, Membrane Science and Technology Series 1, Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991. [22] R.F. Fedors, A method for estimating both the solubility parameters and molar volumes of liquids, Polym. Eng. Sci. 14 (1974) 147. [23] J. Brandrup, E.H. Immergut, Polymer Handbook, 3rd ed., Wiley, New York, 1989. [24] C.M. Hansen, The universality of the solubility parameter, Ind. Eng. Chem. Prod. Res. Develop. 8 (1969) 2. [25] R.C. Binning, R.J. Lee, J.F. Jennings, E.C. Martin, Separation of liquid mixtures by permeation: liquid-phase permeation through thin plastic films has commercial potential for separating azeotropes and other organic mixtures, Ind. Eng. Chem. 53 (1961) 47. [26] P. Aptel, J. Cuny, J. Jozefonvicz, G. Morel, J. Neel, Liquid transport through membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. III. Steady-state distribution in membrane during pervaporation, J. Appl. Polym. Sci. 18 (1974) 365. [27] P. Sampranpiboon, R. Jiraratananon, D. Uttapap, X. Feng, R.Y.M. Huang, Separation of aroma compounds from aqueous solutions by pervaporation using polyoctylmethyl siloxane (POMS) and polydimethyl siloxane (PDMS) membranes, J. Membr. Sci. 174 (2000) 55. [28] F.W. Greenlaw, W.D. Prince, R.A. Shelden, E.V. Thompson, Dependence of diffusive permeation rates on upstream and downstream pressures, J. Membr. Sci. 2 (1977) 141. [29] T. Uragami, S. Yamamoto, T. Miyata, Dehydration from alcohols by polymer complex cross-linked chitosan composite membranes during evapomeation, Biomacromolecules 4 (2003) 37.
Pervaporation characteristics of cross-linked poly(dimethylsiloxane) membranes for removal of various volatile organic compounds from water Tadahiro Ohshima, Yoshihisa Kogami, Takashi Miyata, Tadashi Uragami ∗ Unit of Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan Received 26 October 2004; received in revised form 7 March 2005; accepted 9 March 2005 Available online 28 April 2005
Abstract The permeation and separation characteristics of volatile organic compounds (VOCs), such as chloroform, benzene, and toluene, from water by pervaporation through cross-linked poly(dimethylsiloxane) membranes prepared from poly(dimethylsiloxane) dimethylmethacrylate macromonomer (PDMSDMMA) and divinyl compounds, such as ethylene glycol dimethylmethacrylate (EGDM), divinyl benzene (DVB), divinyl siloxane (DVS), and divinyl perfluoro-n-hexane (DVF) are described. When aqueous solutions containing 0.05 wt.% VOCs were permeated through cross-linked PDMSDMMA membranes, these membranes showed high VOC/water selectivity and permeability. Both VOC/water selectivity and permeability were affected significantly by the divinyl compound. Furthermore cross-linked PDMSDMMA membranes showed the highest chloroform/water selectivity. The VOC/water selectivity was mainly governed by the sorption selectivity rather than the diffusion selectivity. However, the difference in the selectivity between different types of VOCs depended on differences in the diffusivity of permeants. With increasing downstream pressure, the VOC/water selectivity of all cross-linked PDMSDMMA membranes increased, but the permeability decreased. A PDMSDMMA–DVF membrane exhibited a normalized permeation rate of 1.9 × 10−5 kg m/m2 h and a separation factor for chloroform/water of 4850, yielding a separation index of 9110. The pervaporation characteristics of the cross-linked PDMSDMMA membranes are discussed based on their chemical and physical structures as well as the chemical and physical properties of the permeants. © 2005 Elsevier B.V. All rights reserved. Keywords: Poly(dimethylsiloxane); Membrane; Pervaporation; VOCs; Cross-link
1. Introduction Wastewater containing volatile organic compounds (VOCs) generated from various production lines in the chemical and petroleum industries has recently caused serious environmental problems. The removal of VOCs from wastewater is typically carried out by adsorption methods using active carbon and biological treatment. However, these methods are not very energy-efficient. More efficient, continuous methods
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that require less energy and water for treatment of VOCs from wastewater are required [1–3]. Recently, pervaporation (PV), which is a membranebased separation technique, has been investigated for wastewater treatment. Many different membrane types, such as poly(dimethylsiloxane) (PDMS) [4,5], polyamide [6,7], copolymers [8,9], and ceramic-filled [10,11] membranes have been examined for removal of VOCs from water by PV. Of these membranes, PDMS has high selectivity and permeability for organic chemicals in water, because PDMS is hydrophobic and rubbery. However, the mechanical strength of PDMS is notably weak. In order to improve its mechanical strength, selectivity, and per-
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meability, various PDMS derivative membranes were prepared. In previous work [12,13], we studied the relationship between the structure of multicomponent polymer membranes containing PDMS and their VOC/water selectivity during PV. We demonstrated that the selectivity of multicomponent polymer membranes depended strongly on the morphology of their microphase-separated structures. We demonstrated that in graft copolymer membranes, the microphase-separated structure of poly(methylmethacrylate) (PMMA) and PDMS significantly influenced their permeability and selectivity for removal of dilute VOCs from an aqueous solution. Furthermore, the continuous PDMS phase in the microphase-separated structure played an important role in the selective removal of VOCs. In other studies [14–16], addition of polymer additives, such as fluorine-containing PDMS graft and block copolymers, and tert-butyl calix[4]arene, to PMMA-g-PDMS and PMMA-b-PDMS membranes enhanced the VOC/water selectivity and permeability. Because our results demonstrated that the PDMS component in multicomponent polymer membranes is very important for VOC/water selectivity, we prepared polymer membranes consisting of pure PDMS. However, it was very difficult to prepare mechanically strong PDMS membranes from a PDMS macromonomer. Therefore, cross-linked PDMS membranes were prepared by copolymerization of PDMS dimethylmethacrylate macromonomer (PDMSDMMA) as the PDMS component, with various divinyl compounds as cross-linkers. These divinyl compounds were selected to have a high affinity for benzene to improve the performance of the PDMS membranes. The cross-linked PDMSDMMA membranes showed high benzene/water selectivity. The benzene/water selectivity and permeability of these membranes depended significantly on the divinyl cross-linker type and were enhanced by increasing the divinyl content [17,18]. In this study, the PV performance of cross-linked PDMSDMMA membranes was investigated in more detail for removal of other VOCs such as toluene and chloroform from water. The results are discussed and compared with those previously reported for benzene/water mixtures.
2. Experimental 2.1. Materials Poly(dimethylsiloxane) dimethylmethacrylate macromonomer (PDMSDMMA) (1) as a base PDMS polymer for the membrane matrix was supplied by Toray Dow Corning Silicone Co. Ltd. Ethylene glycol dimethylmethacrylate (EGDM) (2), divinylbenzene (DVB) (3), divinylsiloxane (DVS) (4) and divinylperfluoron-hexane (DVF) (5) were used as cross-linkers. 2,2Azobis(2-methylpropionnitrile) (AIBN) was recrystallized
157
from methanol solution and used as the initiator. All other solvents and reagents were of analytical grade from commercial sources and were used without further purification. 2.2. Preparation of cross-linked PDMSDMMA membranes The cross-linked PDMSDMMA membranes (PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMSDMMA– DVS and PDMSDMMA–DVF) were prepared by copolymerization of PDMSDMMA with the divinyl cross-linkers using AIBN as an initiator, in a mold plate between two glass plates at 80 ◦ C for 24 h under nitrogen atmosphere, as reported in previous studies [17–19]. In Table 1, the preparation conditions of the cross-linked PDMSDMMA membranes are summarized. In order to remove any unreacted monomers, the membranes were soaked in n-hexane for 24 h and dried completely under reduced pressure at room temperature. All cross-linked PDMSDMMA membranes were rubbery. 2.3. Pervaporation apparatus and measurement conditions The PV cell and apparatus have been described in a previous paper [20]. The effective membrane area was about Table 1 Preparation conditions of the cross-linked PDMSDMMA membranes Membrane
Cross-linker content (mol%)
PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
70 80 90 90
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13.8 cm2 . The PV experiments were carried out under the following conditions: permeation temperature: 40 ◦ C; downstream pressure: 0.01, 0.05, 0.1, 1, and 4 Torr. The feed VOC solutions, aqueous solutions of dilute chloroform, benzene and toluene, were circulated between the PV cell and the feed tank to maintain an essentially constant concentration of the feed solution during PV. The permeation rate (kg/m2 h) for the aqueous VOC solutions during PV was determined from the weight (kg) of permeate collected in a cold trap, the permeation time (h) and the effective membrane area (m2 ). To provide a comparison of the steady-state permeation rates of membranes with different thickness, the normalized permeation rate (kg m/m2 h), which is the product of the permeation rate and the membrane thickness, was used. The VOC concentration in the feed and permeate was determined by a gas chromatograph (Shimadzu GC-14A) using a flame ionization detector (FID) equipped with a capillary column (Shimadzu Co. Ltd., PorapacQ and P) heated to 150 ◦ C. The separation factor, αsep. VOC/H2 O , was calculated from Eq. (1) αsep. VOC/H2 O =
PVOC /PH2 O FVOC /FH2 O
(1)
where FVOC and FH2 O are the weight fractions of VOC and water in the feed solution and PVOC and PH2 O are those in the permeate, respectively. 2.4. Composition of VOC/water in cross-linked PDMSDMMA membranes The cross-linked PDMSDMMA membranes were dried completely under reduced pressure at room temperature. The dried membranes were immersed in an aqueous solution containing 0.05 wt.% VOC in a sealed vessel at 40 ◦ C until equilibrium was reached. The sorbed solution in the swollen membranes was completely desorbed under reduced pressure and was collected in a cold trap. The compositions of the solutions in the cross-linked PDMSDMMA membranes were then determined by measuring the VOC concentration in the collected solution by gas chromatography (Shimadzu GC-14A). The composition of VOC/water in the cross-linked PDMSDMMA membranes and in the feed solution yielded the sorption selectivity, αsorp. VOC/H2 O , as expressed in Eq. (2), αsorp. VOC/H2 O =
MVOC /MH2 O FVOC /FH2 O
(2)
where FVOC and FH2 O are the weight fractions of VOC and water in the feed solution and MVOC and MH2 O are those in the membrane, respectively. 2.5. Degree of swelling of cross-linked PDMSDMMA membrane The dried, cross-linked PDMSDMMA membranes (0.5–0.6 g) were weighed and immersed in an aqueous so-
lution of 0.05 wt.% VOC (300 ml) in a sealed vessel at 40 ◦ C until equilibrium was reached. The membranes were then taken out of the vessel, wiped for 20 s using ADVANTEC qualitative filter paper no. 2, and weighed. These procedures were several times repeated. The measurements of the degree of swelling were reproducible, and the errors inherent in these measurements ranged within a few percent for the degree of swelling. The degree of swelling (DS) of the cross-linked PDMSDMMA membranes was then determined from Eq. (3), DS =
Ws Wd
(3)
where Ws is the weight of the membrane swollen in an aqueous solution of 0.05 wt.% VOC and Wd the weight of the dried membrane.
3. Results and discussions 3.1. Pervaporation characteristics of cross-linked PDMSDMMA membranes The PV performance of the PDMSDMMA membrane and the PDMSDMMA membranes cross-linked with EGDM, DVB, DVS and DVF for removal of VOCs from water is shown in Fig. 1. As can be seen in Fig. 1, although the permeability for all of the different kinds of VOC were not equally enhanced by cross-linking, none of the cross-linked PDMADMMA membranes had lower permeability than that of the uncross-linked PDMSDMMA sample. Moreover, as shown by the higher VOC permeate concentrations for the cross-linked samples, the VOC/water selectivity was obviously enhanced by cross-linking in all cases. Especially DVF, a fluorine-containing cross-linker, significantly improved the PV performance compared to that of the PDMS homopolymer membrane. In general, when the selectivity of a membrane is increased by modification, the permeability of the membrane decreases due to the well-known permeability/selectivity trade-off relationship [21]. As shown in Fig. 1, however, both the selectivity and permeability of the cross-linked PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMS–DVS, and PDMSDMMA–DVF membranes increased by cross-linking. This result suggests that the affinity of the cross-linked PDMSDMMA membranes for VOCs was enhanced by increasing the hydrophobic interactions between the membranes and the VOCs. Furthermore, swelling by the feed solution was controlled due to introduction of cross-link sites and a resulting increase in hydrophobicity. On the other hand, there was a difference of selectivity and permeability for various VOCs for the same membrane. For all membranes, the best PV performance was achieved for the separation of chloroform from water. For comparison, chloroform was removed more efficiently by about 25% based on the VOC/water selectivity and had about 12–30% higher permeability than toluene.
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Fig. 2. Effect of cross-linker type on the VOC concentration in membranes immersed in aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform ().
Fig. 1. Effects of cross-linker type on the VOC concentration in the permeate, and the normalized permeation rate for aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform () through PDMSDMMA, PDMSDMMA–EGDM, PDMSDMMA–DVB, PDMSDMMA–DVS and PDMSDMMA–DVF membranes.
3.2. Structure of the cross-linked PDMSDMMA membranes The VOC concentration sorbed in the cross-linked PDMSDMMA membranes and the degree of swelling of these membranes immersed in aqueous solutions of 0.05 wt.% VOC are shown in Figs. 2 and 3. The VOC concentration in all membranes was significantly higher than that in the feed solution (0.05 wt.%). This result suggests that the cross-linked PDMSDMMA membranes have high VOC/water sorption selectivity. Similar to the results obtained in PV experiments, the sorbed VOC concentration in the membrane was higher in a PDMSDMMA membrane cross-linked with more hydrophobic cross-linkers. This result can be attributed to a strong affinity between the cross-linked PDMSDMMA membranes and the hydrophobic VOCs. However, the absolute amount of VOC concentration in the membranes depended on the type of VOC. Interestingly, the chloroform concentrations in these membranes were the lowest of all VOCs tested. This result differed significantly from that obtained during PV through cross-linked PDMSDMMA membranes as shown in Fig. 1.
On the other hand, the degree of swelling of cross-linked PDMSDMMA membranes was not greatly influenced by the cross-linker type. This result is mainly due to the fact that the feed was a dilute VOC solution. If the degree of swelling can be ranked based on extremely slight differences of the cross-linked PDMSDMMA membranes it followed the order: toluene > benzene > chloroform. This tendency was similar to the sorbed VOC concentration in the membrane, but differed from that observed under PV conditions. These composite results suggest that the mechanisms of permeation and separation of VOCs from aqueous solutions through PDMSDMMA membranes by PV are mainly governed by the sorption process. However, physical factors, such as the molecular size of the VOCs cannot be neglected and greatly influenced the PV performance of cross-linked PDMSDMMA membranes.
Fig. 3. Effect of the cross-linker type on the degree of swelling of cross-linked PDMSDMMA membranes immersed in aqueous solutions of 0.05 wt.% benzene (), toluene (䊉), chloroform ().
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Table 2 Contact angle for water, membrane density, and cross-linking density of the PDMSDMMA membranes Membrane
Contact angle (◦ )
Membrane density (g/cm3 )
Cross-linking density (×10−4 mol/g)
PDMSDMMA PDMSDMMA–EGDMa PDMSDMMA–DVBb PDMSDMMA–DVSc PDMSDMMA–DVFd
109.3 108.4 114.0 114.8 115.3
0.993 1.017 1.002 0.999 1.011
2.633 5.147 4.696 1.034 0.935
a b c d
EGDM content: 70 mol%. DVB content: 90 mol%. DVS content: 90 mol%. DVF content: 90 mol%.
In Table 2, the contact angle for water, the membrane density and the cross-link density of the cross-linked PDMSDMMA membranes are summarized. The contact angle for water of the PDMSDMMA–DVF membrane showed the highest value of all membranes. This result suggests that the PDMSDMMA membrane cross-linked with DVF is the most hydrophobic. In addition, it can be assumed that the order of hydrophobicity of the cross-linkers used in this study are: DVF > DVS > DVB > EGDM. On the other hand, the membrane density and the cross-linking density of PDMSDMMA membranes are not always comparable, i.e., the membrane density followed the order: EGDM > DVF > DVB > DVS, whereas that of the crosslinking density was: EGDM > DVB > DVS > DVF. This result suggests that the degree of cross-linking is significantly different for each cross-linker. EGDM and DVB gave a highly cross-linked structure, whereas DVS and DVF formed a less cross-linked structure. Presumably, the PDMSDMMA–DVS and PDMSDMMA–DVF membranes have a pendant structure containing DVS and DVF. This is supported by the fact that the contact angle for water of these membranes was higher. Consequently, cross-linking of the PDMSDMMA membranes with DVS and DVF favor a chemical factor induced by the cross-linker; however, these cross-linkers provide negative physical factors, such as the cross-linking density. On the basis of the above discussion, the increase in the normalized permeation rates of the PDMSDMMA–DVS and PDMSDMMA–DVF membranes with introduction of DVS and DVF, as shown in Fig. 1, can be rationalized.
3.3. Mechanism of permeation and separation In order to discuss the effect of VOCs on the PV performance of cross-linked PDMSDMMA membranes in more detail, the solubility parameter and molar volume of each component were calculated and the relationship between the VOC/water selectivity was investigated. The solubility parameter of each component calculated by the group contribution method [21–24] and the difference in the solubility parameter between the membrane and the permeants are shown in Table 3. The difference in the solubility parameter, ∆, was calculated from the solubility parameter of VOC (δVOC ) or water (δwater ) and the membrane (δmembrane ) using Eqs. (4)
Table 3 Solubility parameter of the cross-linked PDMSDMMA membranes and molar volume of permeants Material
Solubility parameter (cal/cm)1/2
PDMSDMMA PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
Molar volume (cm3 /mol)
8.06 8.70 8.16 8.06 7.59
H2 O Chloroform Benzene Toluene
– – – – –
23.4 9.3 9.2 8.9
18.0 80.4 88.9 106.2
and (5): ∆ = |δmembrane − δVOC |
(4)
∆ = |δmembrane − δwater |
(5)
A small ∆ means that the membrane has a strong affinity for VOC and water. The solubility parameter, δmembrane , of the cross-linked PDMSDMMA membranes was calculated from the solubility parameters and the volume ratios of PDMSDMMA and the cross-linker, based on the group contribution method. The ∆ values are shown in Table 4. It is obvious that these membranes have a higher affinity for VOCs than water. When more hydrophobic cross-linkers were introduced into the membranes, the affinity of the membranes for the VOCs increased. In addition, it is obvious that cross-linked PDMSDMMA membranes have higher affinity for toluene relative to chloroform and benzene. Analysis of the solubility parameters of the membranes and those of the VOCs agrees with the tendency of VOC concentration sorbed in the cross-linked PDMSDMMA membranes. All cross-linked PDMSDMMA Table 4 Difference of solubility parameter between the membranes and permeants Membrane
PDMSDMMA PDMSDMMA–EGDM PDMSDMMA–DVB PDMSDMMA–DVS PDMSDMMA–DVF
∆ = |δmembrane − δpermeant | H2 O
Chloroform
Benzene
Toluene
15.34 14.70 15.24 15.34 15.81
1.24 0.60 1.14 1.24 1.71
1.14 0.50 1.04 1.14 1.61
0.84 0.20 0.74 0.84 1.31
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161
Fig. 4. Tentative model of the permeation and separation for removal of different VOCs through cross-linked PDMSDMMA membranes.
membranes exhibit the highest affinity for toluene. As shown in Table 3, the order of the molecular size in VOCs used in this PV experiment is: toluene > benzene > chloroform. This order is the same as those of the VOC/water selectivity and the normalized permeation rates, as shown in Fig. 1. Hence, it can be concluded that the difference in the VOC/water selectivity of cross-linked PDMSDMMA membranes depends significantly on the difference in the diffusivity of the permeants. The diffusion-based selectivity for VOCs depended strongly on the cross-linked structure. From the above results, the removal mechanism of VOCs from water through cross-linked PDMSDMMA membranes can be explained by the qualitative model, shown in Fig. 4, based on the sorptiondiffusion model [25,26]. 3.4. Effect of downstream pressure on PV performance The effect of the downstream pressure on the VOC/water selectivity and the permeability through the PDMSDMMA– DVF membrane is shown in Fig. 5. The downstream pressure was varied from 0.01, 0.05, 0.1, 1, and 4 Torr during PV. An increase in downstream pressure resulted in a decrease of the driving force during PV experiments. As can be seen in Fig. 5, the VOC/water selectivity of the PDMSDMMA– DVF membrane increased for all permeants with increasing downstream pressure. At different downstream pressures, the VOC/water selectivity was almost constant. This result suggests that a higher downstream pressure results in higher VOC/water selectivity. Huang and coworkers reported that the selectivity for ethyl butanoate (ETB) in the separation of ETB from water through PDMS membranes by PV decreased with increasing downstream pressure [27], supporting the results of this study.
Fig. 5. Effects of the downstream pressure on the VOC concentration in the permeate, and the normalized permeation rate for aqueous solutions of 0.05 wt.% VOCs through the PDMSDMMA–DVF membrane. VOCs: () chloroform, () benzene and (䊉) toluene.
In general, the separation of liquid mixtures through polymer membranes results from a difference in the solubility of the permeating molecules into the polymer membrane, and in the diffusivity of the permeating molecules through the polymer membranes. The diffusivity selectivity depends on the molecular size of the permeant and the driving force. Hence, the selectivity of the permeants with a larger molecular size may be expected to decrease with increasing downstream pressure. However, in this study, the selectivity for VOCs increased with increasing downstream pressure. This result may be attributed to the fact that the PDMSDMMA–DVF membrane has a high affinity for VOCs, as shown in Fig. 4. A high VOC/water selectivity with increasing downstream pressure is due to the more open structure of the dry layer of the membrane surface on the downstream side [28]. On the other hand, the normalized permeation rate decreased with increasing downstream pressure, due to a decrease in the driving force. To investigate in detail the decrease in the normalized permeation rate with downstream pressure, partial normalized permeation rates for water and VOCs were analyzed. Fig. 6 shows the effects of downstream pressure on the normalized partial permeation rates of water and VOCs in
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T. Ohshima et al. / Journal of Membrane Science 260 (2005) 156–163 Table 5 Performance of cross-linked PDMSDMMA membranes for an aqueous solution of 0.05 wt.% chloroform during PV Membrane
αCH3 Cl
NPRa
PSIb
PDMSDMMA PDMSDMMA–EGDMc PDMSDMMA–DVBd PDMSDMMA–DVSe PDMSDMMA–DVFf
2726 2937 3648 3498 4847
1.88 1.86 1.97 2.06 1.88
5125 5462 7187 7196 9112
a b c d e f
Fig. 6. Partial normalized permeation rates for VOC and water through a PDMSDMMA–DVF membrane as a function of the different downstream pressure. VOCs: () chloroform, () benzene and (䊉) toluene.
the PV treatment of aqueous solutions of 0.05 wt.% VOCs through the PDMSDMMA–DVF membrane. In each case, the normalized partial permeation rates for water and VOC decreased with increasing downstream pressure, as shown in Fig. 5. The decrease of the normalized partial permeation rate for water was smaller than that for the VOCs. This result suggests that water molecules, which are smaller than VOCs, do not depend much on the diffusion process. The results shown in Fig. 6 demonstrate that with increasing downstream pressure, the decrease in the normalized permeation rate in Fig. 5 is mainly influenced by a decrease in the partial normalized permeation rate for the VOC. 3.5. Performance of the cross-linked PDMSDMMA membranes In Table 5, the normalized permeation rate, the separation factor, αCHCl3 /water, and the pervaporation separation index (PSI) [21,29] for an aqueous solution of 0.05 wt.% chloroform through the cross-linked PDMSDMMMA membranes during PV are listed. PSI is the product of the normalized permeation rate and the separation factor, and can be used as a measure of PV membrane performance. As can be seen in Table 5, the normalized permeation rates
The normalized permeation rate (×10−5 kg m/m2 h). Pervaporation separation index (αsep. CHCl3 /H2 O × NPR). EGDM content: 70 mol%. DVB content: 90 mol%. DVS content: 90 mol%. DVF content: 90 mol%.
for an aqueous solution of 0.05 wt.% chloroform through cross-linked PDMSDMMA–DVF were almost equal, but the separation factor of the PDMSDMMA–DVF membrane was much higher than that of the other membranes. Consequently, the PDMSDMMA–DVF membranes show a higher pervaporation separation index. On the basis of these results, the introduction of a fluorine-containing cross-linker into the PDMSDMMA is a very effective way to provide improved membrane performance. The reason why a fluorine-containing cross-linker improved the membrane performance may be speculated as given below. As shown in Table 2, the contact angle for water of cross-linked PDMSDMMA membrane was the highest in the cross-linked PDMSDMMA membranes. This fact implies that the surface of cross-linked PDMSDMMA–DVF membrane is more hydrophobic, an absorption of water in the feed into this membrane is significantly suppressed, and consequently, the VOC/water selectivity is improved.
4. Conclusions The separation and permeation characteristics of dilute aqueous solutions of VOCs, such as chloroform, benzene and toluene, through cross-linked PDMSDMMA membranes by PV were investigated with respect to different permeants. Cross-linked PDMSDMMA membranes showed high VOC/water selectivity and permeability. Especially, PDMSDMMA membranes cross-linked with more hydrophobic cross-linkers, such as a DVF, showed high VOC/water selectivity and permeability. The PV characteristics of PDMSDMMA membranes depended significantly on the VOC type, and the selective removal of chloroform from water was greater than that of benzene and toluene. The VOC/water selectivity and permeability were significantly influenced by the cross-linker and permeant type. The PV behavior of cross-linked PDMSDMMA membranes can be rationalized by differences in the chemical and physical structures of the membranes and those of the permeants. The VOC/water selectivity of the cross-linked PDMSDMMA membranes was mainly governed by the sorption selectivity of the permeants.
T. Ohshima et al. / Journal of Membrane Science 260 (2005) 156–163
However, the difference in the selectivity between different types of VOCs depended on differences in the diffusivity of the permeants. The mechanism of permeation and separation follows the sorption-diffusion model. We discovered that the best performance of PDMS membranes for removal of VOCs is achieved using a fluorine-containing cross-linker.
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