A fundamental study of organofunctionalised PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams

Journal of Membrane Science 190 (2001) 147–157

A fundamental study of organofunctionalised PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams P. Wu a , R.W. Field a,∗ , R. England a , B.J. Brisdon b a

Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK b Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK Received 22 September 2000; accepted 15 March 2001

Abstract Modified polydimethylsiloxane (PDMS) composite membranes in which a known mol% of the SiMe groups were replaced by a short methylene spacer group terminated by one of the four side-arm functional groups, acetate, –CO2 Me; ethylether, OEt; dimethylamino, –NMe2 ; and pyridyl, –py; were fabricated and tested in short term trials for the pervaporative recovery of cresols from aqueous solution. The influences of functional group type, functional group loading, pH, temperature and purity of the feed solution were investigated. Three commercially available hydrophobic membranes were also examined for comparison. It was found that significant performance enhancements, compared with an unfunctionalised PDMS membrane, were realised for all functionalised PDMS membranes, with dimethylamino and ethylether functionalities proving the most effective. A functional group loading level close to 20% was found to be optimal for this application, The influence of feed pH in the range of 5–8.5 was small on total flux but significant on selectivity for amine loaded membranes. By contrast, the effect of feed temperature was significant on total flux but negligible on selectivity. Low levels of phenolic impurities in a p-cresol feed solution were found to have negligible effect on the separation. Compared with commercially available zeolite filled PDMS and PEBA membranes, the functionalised PDMS membranes showed a better overall performance. The PV performance of all new membranes followed the trend: o-cresol > p-cresol > phenol. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Organofunctionalised PDMS membranes; Pervaporation; Phenolic derivatives

1. Introduction The optimum recovery of dilute organics from aqueous solutions (process fluids or effluents) is of great importance for both economic and environmental reasons. Pervaporation (PV) provides an important option for the recovery and recycling of many volatile organics from aqueous media, and research ∗ Corresponding author. Tel.: +44-1225-826-117; fax: +44-1225-826-894. E-mail address: [email protected] (R.W. Field).

on the development and use of organophilic membranes for such purposes has increased greatly in recent years [1,2]. Much work has been carried out at Bath on the production of modified PDMS membranes containing a wide range of organofunctional side chains [3]. These have been screened previously for the PV separation of four separate organic components, phenol, chloroform, pyridine and methylisobutylketone [3]. In each case significant performance enhancements were realised, over those achieved with an unfunctionalised PDMS membrane.

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 4 0 8 - 2

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Nomenclature List of symbols A effective mass transfer area of membrane (m2 ) J flux (kg/m2 s) P permeability (m2 /s) T temperature (K) t time elapsed (s) X weight fraction of component in feed (wt.%) Y weight fraction of component in permeate (wt.%) Greek letters α separation factor defined as (Yo /Yw ) (Xo /Xw ) ρ density (kg/m3 ) σ thickness of membranes (m) Subscript f m p pc ph oc t w o

feed membrane selective layer permeate p-cresol phenol o-cresol total water organic

separation of phenol, were chosen for this investigation. The influence of functional group type, loading levels, pH, temperature and feed solution purity were determined. Three commercially available organophilic membranes were also examined for comparative purposes.

2. Experimental Two different methodologies for synthesising a wide range of organofunctional polysiloxane membranes have been developed at Bath [4,5]. Both methods enable the reproducible incorporation of a range of organofunctional side-chains into a lightly cross-linked, PDMS polymer from which composite membranes are produced by pressing the fluid polymer on a Celgard 2400/2500 support for 1 h at room temperature for pure PDMS membrane and 2 h at 40◦ C for functionalised PDMS membranes in order to complete the cross-linking process. All resulting membranes are finally cured at 80◦ C for 1 week before use. The membranes produced in this study, each containing a single functional groups at a level of between 10 and 25 mol%, are noted below. The PV experiments were carried out in a stirred cell with a supported magnetic follower (Fig. 1).

Abbreviations PV pervaporation process PDMS polydimethylsiloxanes AMI dimethylamino loaded PDMS membrane AEE ethylether loaded PDMS membrane PY pyridyl loaded PDMS membrane AAC acetate loaded PDMS membrane As part of our continuing efforts in this area, the separation of cresols/water binary systems using modified PDMS membranes has been investigated. In view of the similarities in the chemical and physical properties of cresols and phenol, PDMS modified by the four functional groups, acetate, ethylether, dimethylamino and pyridyl, which were effective for the PV

Fig. 1. Schematic drawing of permeation apparatus.

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Details of the test rig set-up, membrane synthesis and test procedures, and feed and permeate analytical procedures have been reported previously [3]. All pervaporation studies were carried out under the following conditions: mass transfer area: 22.9 cm2 ; top layer thickness: 50–l50 ␮m; support layer: 25 ␮m thick, 38%/45% porosity (Celgard 2400/2500); feed temperature: 70◦ C (standard); 50–90◦ C (temperature influence); feed concentration: 5% phenol/H2 O; 2% cresols/H2 O; feed pH: 7.5 (standard), 5–8.5 (pH influence); stirrer speed: 1000 rev/min; permeate pressure: 0.6–2.0 mbar. Functional group ethylether: dimethylamino: pyridyl: acetate:

types and loadings were: 10, 15, 20 and 25%; 10, 15, 20 and 25%; 10, 15, 20 and 25%; 10 and 20%.

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where Pi is permeability of component (m2 /s), δ m the effective thickness of selective layer (m) (δ m = (δ t − δ s ) + δ s /ϕ), where δ t is total composite membrane thickness (m), δ s the thickness of Celgard support layer (m), ϕ the porosity of Celgard support layer, ρ o the density of organics (kg/m3 ), γ i the activity coefficient of component, xi f the mole fraction of component in feed, yi p the mole fraction of component in permeate, pt the total pressure at permeate side (Pa), Pi ,sat the saturated partial pressure of component (Pa), i the subscript, o for organics and w for water. Measured fluxes were normalised to the standard condition of an effective membrane thickness of 50 ␮m. The results of the first one or two experiments for each new membrane were disregarded as the membrane underwent conditioning. Subsequent experiments were repeated until consecutive sets of results were similar for each set of membranes (±10%). The results reported here are an average of three measurements.

3. Results and discussion

The flux is calculated using the following equation J =

Wp × 24 × 3600 At

(1)

where J is total flux (kg/(m2 day)), Wp the weight of permeate (kg), A the effective mass transfer area of membrane (m2 ), t the time elapsed (s). The separation factor of the membrane is defined as α=

Yop (1 − Xof ) Xof (1 − Yop )

(2)

where α is separation factor, Xof the weight fraction of organics in feed, Yop the weight fraction of organics in permeate. For the test system used in this study, earlier experiments had shown that the resistance of the feed side liquid boundary layer to mass transfer was negligible if the stirrer speed was set above 250 rpm [6]. The permeability of the top layer was then assumed to be constant, and it was determined according to the equation
 Pi pt Ji = γi xif − yip (3) δm /ρo Pi,sat

The p-cresol/water separations were effected using 14 different functionalised PDMS membranes (four different types of functional group and each with between two and four levels of functional group loading). Table 1 summarises the results obtained including component flux and selectivity. For all the membranes examined, the total fluxes and selectivities ranged from 10 to 38 kg m−2 per day and 28 to 68, respectively. 3.1. Effect of functional group type It can be seen from Table 1 that the water flux through pure PDMS was ca. 70% higher than the p-cresol flux, resulting in a separation factor of 28.1. This factor was significantly changed by the incorporation of functional groups into the PDMS matrix. Both ethylether and acetate functionalities were found to be effective for enhancing the component flux rather than improving selectivity. Both water and organic fluxes increased with the flux of the former still significantly greater than that of the latter. The large increase in component flux might reflect intramolecular interactions between the ether/acetate functionality and the polar-OH groups of p-cresol

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Table 1 p-Cresol separation (T = 70◦ C, X of = 2%, p t ≤ 2 mbar, δ m = 50 ␮m, n = 3) Functional group

Chemical structure

Loading (mol%)

Pure PDMS

Total flux (kg m−2 per day)

Water flux (kg m−2 per day)

Organic flux (kg m−2 per day)

Separation factor

10.0

6.3

3.7

28.1

Ethylether

–CH2 OC2 H5

10 15 20 25

19.1 22.9 33.4 29.4

12.0 14.0 19.0 17.2

7.1 8.9 14.6 12.2

28.2 31.7 37.4 34.7

Dimethylamino

–CH2 N(CH3 )2

10 15 20 25

13.3 20.7 24.1 33.6

5.5 8.6 10.5 18.8

7.8 12.0 13.0 14.8

68.4 68.4 63.7 38.4

Pyridyl

–C5 H4 N

10 15 20 25

11.3 16.3 15.2 20.8

48.9 7.5 6.6 10.7

6.4 8.8 8.6 10.1

67.0 54.6 63.9 45.7

Acetate

–CH2 CO2 CH3

10 20

28.9 37.7

16.6 24.4

12.2 13.3

36.1 26.7

and water molecules, respectively. The rather small improvement in the selectivity suggests that these two interactions are competitive. No significant gain in flux was achieved by introducing pyridyl groups into the membrane, although the selectivity was improved significantly. The relatively low fluxes are expected in view of the likelihood of aromatic ␲–␲ interactions between close pyridyl neighbours, which impose geometrical constraints as noted in other system [7], so leading to reduced membrane free volume and diffusion coefficients. The fact that membranes with 20 and 25% pyridyl loadings were much more brittle than analogues with the same loading of other functional groups lends support to this explanation. Finally, the dimethylamino functionalised PDMS membranes displayed both enhanced fluxes and selectivities. The p-cresol fluxes were greater than those of water, resulting in a notable improvement in selectivity. The significant increase in p-cresol flux (by a factor of ca. 1.4 for a 20% amine functionalised membrane) can be attributed to a facilitation effect caused by interactions between the basic amino group and the weakly acidic OH groups of p-cresol. Similarly, the slight enhancement of water flux is probably attributable to weak hydrogen bonding between dimethylamino groups and the polar water molecules.

3.2. Effect of functional group loadings As noted in Table 1 the p-cresol flux increased in general with increasing functional loadings in the range of 10–25% for all the membranes tested, but the trend for selectivity was varied. For the dimethylamino and pyridyl modified membranes the selectivity started to fall significantly when the functional group loading was over 20%. For the ethylether modified membrane selectivity also peaked at a functional loading level of about 20% whereas for the acetate functionalised PDMS membranes selectivity peaked at a loading of around 10%. Thus, for each functional group there existed an optimal functional loading level that would help to produce both high flux and high selectivity for a specific application. In practice, two factors are important in selecting the most appropriate functional loading level for a given separation. One concerns the balance between high flux and high selectivity, as a compromise is often inevitable. Membranes displaying higher selectivity would reduce the specific energy requirement for organic recovery since less water would have to be evaporated and condensed. Membranes with higher organic fluxes would reduce the membrane area requirement and investment cost. The saturated concentration of organic is an important factor when

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conducting an economic analysis of a PV process. In the case concerned, the saturated concentration of p-cresol/water solution is only about 2.3% at room temperature [9]. Even using an unfunctionalised PDMS membrane, the permeate condensate would consist of a two-phase liquid mixture and the aqueous layer could easily be recycled. Accordingly, it seems appropriate to aim for a membrane with a high organic flux rather than a high selectivity for this particular application. The other factor concerns the influence of the functional loading on the fabrication process and characteristics of the membrane itself. It was found that highly loaded membranes (>20%) were the more difficult it fabricate and produce in defect free form. Accordingly, lower functional loading is always preferred provided that the performance of the modified membrane is sufficient for its designated purpose. 3.3. Influence of the pH of the feed solution The influence of the pH of the feed solution in the range of 5.3–8.5 was investigated for a selection of seven membranes for both p-cresol and phenol separations. Sodium hydroxide was used to adjust the acidity of the feed solution. Fig. 2a and b and Fig. 3a and b show variations of the total flux and separation factor (selectivity) with the pH of the feed solution for p-cresol and phenol, respectively. As expected, the pH of the feed solution had no appreciable influence on the flux or selectivity of either unfunctionalised PDMS or 20% ethylether loaded PDMS membranes. For membranes containing 20% dimethylamino and pyridyl functionalities, the results indicated that pH changes have a negligible effect on total flux, but affect p-cresol selectivity significantly. For the 20% dimethylamino loaded PDMS membrane an increase in the pH of the feed solution from 5.3 to 7.6 caused the selectivity to increase by a factor of ca. 2.3, from 28 to 63. For the 20% pyridyl loaded PDMS membrane, selectivity changed significantly only when the pH of the feed was >7. By contrast, variations in the pH of the feed solution showed a noticeable effect on total flux but only a modest influence on selectivity for the phenol/water system. For the 20% dimethylamino loaded PDMS membranes, the total flux was reduced by about 23% and selectivity increased by about 20% as the pH of the

Fig. 2. (a) Influence of feed pH on total fluxes (p-cresol separation), (b) influence of feed pH on selectivity (p-cresol separation).

feed solution increased from 5.2 to 8.5. This change may reflect competition for the basic N-centres of the amino/pyridyl groups in the membrane between protons in the feed solution and the acidic –OH groups of the phenolic compounds, At higher pHs interactions between p-cresol/phenol and the amino/pyridyl group would predominate, resulting in an increase in the adsorption of p-cresol/phenol. The above results imply that by carefully adjusting the pH of the feed solution, the selectivity of a functionalised PDMS membrane containing a basic centre can be enhanced. 3.4. Effect of the temperature of the feed solution The effect of the temperature of the feed solution in the range of 50–90◦ C was examined for pure PDMS, 20% ethylether, and 20% dimethylamino loaded PDMS membranes for the separation of p-cresol/

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Fig. 3. (a) Influence of feed pH on total fluxes (phenol separation), (b) influence of feed pH on selectivity (phenol separation).

water. Variations in the total flux and selectivity as a function of temperature are shown in Fig. 4. An increase in feed temperature from 50 to 90◦ C resulted in an approximately threefold increase in total flux for all three organofunctional membranes, but had little effect on separation factor. The variation of the component flux with temperature was found to follow the Arrhenius relationship, i.e.
 −Eai Ji = Ji∗ exp RT The apparent activation energy of permeation, Eai determined from the slopes of the ln(Ji ) versus 1/T plots, are tabulated in Table 2, and fall within the range of −Eai values for other pervaporation systems [8]. For all three membranes examined the Eao values for the organic component and for water are almost equal. The implication is that both the organic and water fluxes

Fig. 4. (a) Effect of feed temperature on separation factor, (b) effect of feed temperature on selectivity.

increase with temperature at a similar rate, resulting in little change to the selectivity value. 3.5. Influence of impure feed solutions Industrial cresylic wastewater contains about 2% p-cresol and 0.23% o- or m-cresols, phenol and traces of thiophenols. The effects of such feeds on PV separations using unfunctionalised as well as three functionalised PDMS membranes were next assessed. Results showing total flux, component permeability Table 2 Apparent activation energies (kJ/mol) for permeation Membrance

−Eao

−Eaw

Pure PDMS 20% dimethylamino loaded PDMS 20% ethylether loaded PDMS

32.0 33.6 34.7

34.3 34.0 34.2

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Table 3 Comparison of PV performance for industrial cresylic waste water and pure p-cresol feed solutions (T = 70◦ C, X of = 2%. p t ≤ 2 mbar, δ m = 50 ␮m, n = 3, pH = 5.3–5.6) Membrane (mol%)

Pure PDMS AEE (20%) AMI (20%) PY (20%) a b

Total flux (kg m−2 per day)

Ppc × 10−11 (m2 s−1 )

Pw × 10−11 (m2 s−1 )

Separation factor

1a

2b

1

2

1

2

1

2

9.4 33.7 26.1 15.8

8.2 38.6 24.7 17.3

0.33 1.40 0.90 0.76

0.31 1.60 0.87 0.82

0.37 1.13 0.99 0.50

0.30 1.29 0.95 0.51

28.1 37.4 29.0 45.5

31.5 38.4 28.1 48.0

Separation with pure p-cresol/water model solution. Separation with industrial cresylic waste water.

and selectivities are summarised in Table 3. In calculating the permeability values for p-cresol, the activity coefficient was taken to be 190 at 70◦ C. This value was obtained from the solubility method [14,15]. It was found that the maximum difference in flux and selectivity is about 15 and 10%, respectively, which is within the limits of experiment errors.

and selectivities for cresols increased significantly, resulting in an even bigger difference compared with the flux and selectivity values found for phenol. This may reflect either an increase in hydrogen bonding strength caused by the electron releasing methyl group on cresol, or a greater solubility of cresol in polysiloxane membranes containing these functional groups. The dissociation constants in water (1.3 × 10−10 for phenol, 5.4 × 10−4 for p-cresol, 4.8 × 10−11 for o-cresol at 25◦ C) [9,10] provide support for this explanation. It is interesting to note that for all unfunctionalised/ functionalised PDMS membranes tested, the following sequence describes the performance enhancement achieved for each separation: o-cresol > p-cresol > phenol.

3.6. Comparison of phenol, p-cresol and o-cresol separations Table 4 permits comparison between the separations of aqueous phenol, p-cresol and o-cresol carried out under standard test conditions with unfunctionalised and six functionalied PDMS membranes. The experimental phenol fluxes obtained at 5% feed were converted using Eq. (3), to those expected at 2% (γ ph = 20.9 at 5%, and 23.5 at 2%, respectively, UNIFAC method, [6]). It can be seen that for all the membranes tested the total flux and selectivity of phenol were lower than those of o- or p-cresol. On incorporating functional groups into the PDMS membrane, the permeate fluxes

3.7. Comparison with commercially available membranes Whilst organofunctionalised PDMS membranes showed significantly enhanced performances over pure PDMS, it was of great interest to see how they perform in comparison with commercially available

Table 4 Comparison of phenolic separation (T = 70◦ C, X f = 2%, p t ≤ 2 mbar, δ m = 50 ␮m, n = 3) Membrance Total flux (kg m−2 per day) (mol%) Jtph Jtpc Jtoc

Water flux (kg m−2 per day)

Organic flux (kg m−2 per day)

Selectivity

Jwph

Jwpc

Jwoc

Joph

Jopc

Jooc

α ph

α pc

α oc

Pure PDMS 8.9 AEE (10%) 9.4 AEE (20%) 16.3 AMI (10%) 8.2 AM1 (20%) 12.4 PY (10%) 9.6 PY (20%) 10.9

7.2 6.8 11.4 5.8 8.6 7.0 7.8

6.3 12.0 19.1 5.5 10.6 4.9 6.6

8.1 13.9 19.8 10.7 12.5 8.8 10.2

1.7 2.6 4.9 2.4 3.8 2.6 3.1

3.7 7.1 14.6 7.8 13.5 6.4 8.6

7.1 13.2 20.6 15.0 17.9 11.7 12.6

11.6 19.0 21.0 20.8 23.7 17.9 19.4

28.1 28.2 37.4 68.4 63.7 67.0 63.9

43.2 46.7 51.3 69.6 70.5 65.6 61.2

10.0 19.1 33.7 13.3 24.1 11.3 15.2

15.2 27.1 40.4 25.7 30.4 20.5 22.8

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Fig. 5. Total flux and selectivity ratio to unfunctionalised PDMS — phenol separation.

organophilic membranes. At present there are two major types of organophilic membranes commercially suitable for evaluation for the separation of phenolic compounds. Type one is based on PDMS and is represented by pure PDMS (PERVAP® 1060, Sulzer, Germany), or organophilic zeolite filled PDMS. The latter is designed to increase the sorption of organic compound into the membrane surface (PERVAP® 1070, Sulzer, Germany) and is coated on polyacrylonitrile (PAN) support materials [11]. Type two is represented

by polyetherblockamide (PEBA, GKSS, Germany), which is a homogenous polymer supported on estal flees. This polymeric material is composed of a polyamide and polyetherdiol [12,13]. A comparative study was carried out in which these two types of commercially available organophilic membranes were evaluated for the separation of phenol, p-cresol and o-cresol from water. The results of total flux and selectivity measurements are presented in Figs. 5–7.

Fig. 6. Total flux and selectivity ratio to unfunctionalisd PDMS — p-cresol separation.

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Fig. 7. Total flux and selectivity ratio to unfunctionalisd PDMS — o-cresol separation.

It can be seen from Fig. 5 that the commercially available pure PDMS (PERVAP 1060) and our unfunctionalised PDMS are equivalent in terms of the total flux, but the former has a significantly lower selectivity. This is likely to result from the different

methods used for membrane preparation, different molecular weights of PDMS, and different membrane cross-linking procedures. The organophilic zeolite filled PDMS membrane (PERVAP 1070) was not significantly better in

Fig. 8. PEBA membrane after contacted with p/o-cresol solution (about 1.5 h), the dense surface layer is destroyed and the estal fleece becomes visible.

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terms of selectivity for phenol compared with pure PDMS (PERVAP 1060). However, the three types of organofunctionalised membranes reported above show much better performance enhancement for both flux and selectivity for the separation of phenol. As expected, the PEBA membrane shows excellent total flux and selectivity of phenol, which is matched only by the 20% ethylether functionalised PDMS membrane. Similarly, the data in Fig. 6 show that the commercially available pure PDMS membrane (PERVAP® 1060) had a significant lower p-cresol selectivity compared with our unfunctionalised PDMS membrane, whereas the difference in the total flux was small. Again, this may be attributed to the different methods and materials used for the membrane preparation. The zeolite filled PDMS membrane (PERVAP 1070) showed some improvement in selectivity for p-cresol but at the expense of total flux. Obviously, the three types of organofunctionalised membranes tested are far better in terms of both flux and selectivity for the separation of p-cresol. The commercially available PEBA membrane, which showed excellent performance in phenol separation, was unsuitable for p-cresol/water separation because the membrane was destroyed by the chemically aggressive p-cresol (about 1.5 h contacting with p-cresol/water solution). See Fig. 8. As expected, the results of the o-cresol separation (Fig. 7) are similar to those achieved in the p-cresol separation. The selectivity of the pure PDMS membrane (PERVAP® l060) is only about 40% of that of our unfunctionalised PDMS membrane, whereas the difference in the total flux is relatively small. The zeolite filled PDMS membrane (PERVAP 1070) improved the o-cresol selectivity to some extent but at the expense of the total flux. The PEBA membrane was again destroyed by o-cresol solutions.

•

•

•

•

•

•

especially for dimethylamino and ethylether functionalised membranes. There exists an optimal functional loading level for a functional group to give both high flux and high selectivity for a specific application. For the separation of phenol and o- or p-cresol from aqueous solution, 20% dimethylamino and 20% ethylether loaded PDMS membranes showed the best combination of high flux and high selectivity. The pH of the feed solution (within a range of 5–8.5) showed significant influence on the selectivity of the p-cresol/water separation only if the loaded functional group was basic. This influence was only modest for the phenol/water separation. The influence of the temperature of the feed solution over the range of 50–90◦ C was significant on flux but small for the selectivity in the p-cresol/water separation. For all the membranes tested, the following sequence describes the performance enhancement achieved by each separation: o-cresol > p-cresol > phenol. A commercially available organophilic zeolite filled PDMS displayed no significant performance enhancement over our pure PDMS membrane for the separation of phenolic residues. A commercially available PEBA membrane showed excellent performance equivalent to that achieved with 20% ethylether loaded PDMS membrane for the separation of phenol/water solution. However, it was found to be unsuitable for the separation of cresols/water solutions.

Acknowledgements The authors are grateful to the Engineering and Physical Science Research Council (EPSRC) of UK for their financial support of this project (what about the commercial support??)

4. Conclusions References The following conclusions can be drawn from this study: • Significant performance enhancements for the pcresol/water separation were achieved in short-term tests for the functionalised PDMS membranes,

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