Stabilization of Supported Liquid Membranes by Î³-Radiation and Their

Ind. Eng. Chem. Res. 2005, 44, 7659-7667

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Stabilization of Supported Liquid Membranes by γ-Radiation and Their Performance in the Membrane Aromatic Recovery System Muhammad G. Dastgir,† Ludmila G. Peeva,† Andrew G. Livingston,*,† Timothy A. Morley,‡ and Joachim H. G. Steinke‡ Department of Chemical Engineering and Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.

This study reports the stabilization of supported liquid membranes, through cross-linking the liquid membrane phase by using γ-radiation, and the performance of the resulting membranes under Membrane Aromatic Recovery System (MARS) operating conditions. The membranes were prepared by impregnating polypropylene glycol (PPG) into the pores of microporous flat sheet membranes [polyvinylidene fluoride (PVDF) or polypropylene (PP)] and subsequently exposing them to γ-radiation. The membranes prepared exhibited operational stability when investigated under MARS operating conditions for more than 1 month. The PP (0.05 mm) membrane provided the best combination of high phenol mass transfer rates and low water flux for the MARS process among the membranes tested in this study. The phenol mass transfer rate through this membrane (27 × 10-7 m s-1) was 18 times higher than that of silicone rubber tubing (1.5 × 10-7 m s-1). This study also reports the effect of radiation dose on the mass transfer rates and on the membrane support itself. 1. Introduction The Membrane Aromatic Recovery System (MARS) is a new process for the recovery of aromatic acids and bases.1-3 In the MARS process, an aromatic acid or base is extracted from a wastewater stream via a nonporous membrane (permeable for the neutral aromatic compounds but impermeable for the ionic species) to a caustic or acidic stripping solution. The extracted aromatic acid/base is recovered by neutralization of the stripping solution. MARS technology has been successfully applied for the recovery of phenol and aniline on laboratory and pilot plant scale.4 It has also been applied on full plant scale for recovery of p-cresol since Dec 2002 at a Degussa plant in Knottingley, U.K. In previous studies of MARS technology, commercially available silicone tubing was used as a membrane. However, this membrane has relatively low organic mass transfer rates and is not very stable under strong acid/base environment and high temperatures.5 In recent years, supported liquid membranes (SLM) have received considerable attention for the separation of organic compounds6 and recovery of metallic ions7 from wastewater streams, due to characteristics such as low energy consumption, high selectivity, and rapid extraction capacity factors. However, despite all these advantages and extensive research in recent years, the application of supported liquid membranes on a large scale is still limited due to inadequate membrane stability.8 Several techniques have been reported to improve the stability and lifetime of supported liquid membranes. Neplenbroek et al.9 proposed that stability of SLM can be enhanced by gelling the liquid membrane phase in * To whom correspondence should be addressed. Tel: +44 (0)20 7594 5582. Fax: +44 (0)20 7594 5629. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Department of Chemistry.

the pores of the support. After the failure of the gelation technique to stabilize the SLMs, Kemperman et al.10 applied the interfacial polymerization technique. Wijers et al.,11 using the same interfacial polymerization technique, proposed the composite SLM configuration, for transport of copper ions, to improve the lifetime of SLM by using sulfonated poly (ether ether ketone) (SPEEK) as a material for the stabilization layer. Ho et al.12 introduced supported polymeric liquid membranes (SPLM) for both removal and concentration of organics from aqueous streams. They enhanced the stability of liquid membranes by cross-linking PPG inside the pores of a porous support using toluene diisocyanate. However, the polyurethane nature of this cross-linked PPG material is susceptible to hydrolytic degradation by caustic, enzymatic degradation by lipases and biodegradation under composting conditions.13 Yang et al.14 suggested plasma polymerization surface coating for stabilization of SLMs, and Wang et al.15 proposed the formation of semipermeable polyamide skin layers on the surface of supported liquid membranes. The use of nuclear radiation in bringing about chemical reactions is well-known.16 In recent years, considerable research has been reported on the preparation of cross-linked polymer gels,17 for pharmaceutical applications, from their aqueous solutions by γ-ray or electron beam irradiation. Lopina et al.18 reported the formation of carbohydrate-modified hydrogels based on radiationcross-linked star poly(ethylene oxide). Savas et al.19 investigated the poly(ethylene oxide) gels produced by γ-radiation cross-linking of PEO from its aqueous solution. Yoshii et al.20 used electron beam irradiation to cross-link PEO and PEO/PVA for production of respective hydrogels for wound dressing. Recently, radiation cross-linking of sodium carboxymethylcellulose (CMCNa) has been reported by Liu et al.21 In this study, the liquid membrane phase inside the pores of a porous support was stabilized by cross-linking

10.1021/ie050140n CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005

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it with γ-radiation. A PVDF flat sheet membrane was used as a porous support due to its excellent thermal and chemical resistance,22 and PPG was used as a liquid membrane phase because of its high affinity for phenol.12 Phenol was used as a model compound because of its moderate mass transfer rates for the MARS technology.1 The membrane was investigated under real MARS operating conditions and it exhibited a mass transfer rate of 8.0 × 10-7 m s-1 [5 times higher than that of silicone rubber tubing (1.5 × 10-7 m s-1)] and operational stability during the period of study. The mass transfer rates were further improved to 27 × 10-7 and 80 × 10-7 m s-1, which are 18 and 53 times higher than that of silicone rubber tubing by using relatively thinner commercially available polypropylene (PP) supports (from Celgard USA) of thickness 0.05 mm and 0.025 mm, respectively. The effect of radiation dose on the mass transfer rates and on the membrane support itself was investigated using 0.025 mm PP membrane support. 2. Mathematical Analysis 2.1. Mass Transfer Coefficients. Assuming no accumulation of the penetrant (phenol) in the membrane or liquid films, transport of penetrant through a membrane with liquid films on both sides can1 be described as

N ) Kov(Cf - Cs)

(1)

and Kov, in terms of individual resistances,23 can be written as

1 1 1 + 1/Kov ) + kf km ksE

(2)

where the membrane resistance is

stripping solutions as a function of run time, the values of Kov,f (based on feed solution side phenol concentration) and Kov,s (based on stripping solution side phenol concentration) can be calculated from eqs 3 and 4, respectively. The overall mass transfer coefficient (Kov) for any test is calculated as the average of these two values. In the MARS continuous process, phenol was extracted across a membrane from wastewater into a caustic stripping solution using the apparatus shown in Figure 2. This is used for long-term testing of membranes under actual process conditions, including exposure over extended periods to an alkaline solution of sodium phenolate. In this apparatus, a phenol mass balance over the contactor cell can be written as

F(Cf - Cf,o) ) AKov(Cf,c - Cs)

(5)

Phenol (PhOH) is a weak acid that dissociates into PhO- and H+, with a dissociation acid constant (Ka) of 10-10 at 25 °C defined as

Ka )

δτ 1 ) km DKP

[PhO-][H+] [PhOH]

(6)

For a given pH, the concentration of neutral phenol (Cs) can be calculated as

The reaction in the stripping solution is instantaneous and reversible. The enhancement factor (E) for such reactions has been developed by Olander24 and further applied to the MARS process by Ferreira et al.25,26 For extraction of phenol (pKa ) 10) to a typical MARS stripping solution at pH ) 13, used in this work, the enhancement factor is large due to high hydroxide concentration and the stripping solution resistance term in eq 2 can be neglected. 2.2. Determination of Kov. Membranes were initially tested for mass transfer properties using the stirred cell shown in Figure 1. Here the stripping solution was not caustic and so phenol transfer was from one water solution to another. This apparatus was used for testing different membranes over short batch tests to ascertain their mass transfer properties. For batch experiments, a phenol mass balance with equal volume V of feed and stripping solutions yields

A t V

() (VA)t

0.5[ln Cw0 - ln(2Cwt - Cw0)] ) Kov,f 0.5[ln Cw0 - ln(Cw0 - 2Cst)] ) Kov,s

Figure 1. Stirred cell setup for batch experiments.

Cs )

Ctot 1 + Ka/10-pH

(7)

Typically the pKa values found in the literature are for dilute solutions at room temperature. However, the stripping solution is highly concentrated in organic salt (∼2 M), so deviation from dilute solution ideal behavior is expected. Therefore, the neutral phenol concentration was determined experimentally by circulating the stripping solution simultaneously through a membrane tube immersed in an aqueous solution (Figure 2). Since only neutral phenol can diffuse through the PDMS membrane, the concentration of phenol in the aqueous solution at equilibrium should be equal to Cs. The overall mass transfer coefficient Kov can then be calculated from eq 5. 3. Materials and Methods

(3) (4)

By measuring the organic concentrations of feed and

3.1. Chemicals and Membranes. Phenol and polypropylene glycol (PPG) Mn ) 4000 g mol-1 were supplied by Aldrich Chemical Co. Sodium hydroxide (pellets, 99%) and HCl solution (35.4 wt. %) were obtained from Merck. All solutions were prepared using deionized water. Polyvinylidene fluoride (PVDF) and polypropy-

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Figure 2. Experimental setup for MARS process in continuous mode.

lene (PP) microporous flat sheet membranes were kindly supplied by GKSS and Celgard USA, respectively. The PVDF membranes consist of a polypropylene nonwoven backing and a porous PVDF top layer with asymmetric pores, while polypropylene membranes have symmetric pores formed by the stretching method. 3.2. Analytical Methods. Phenol content was determined by spectrometric absorption (Unicam UV1) at 270 nm. The stripping solution samples were neutralized by using 5 wt % HCl and were appropriately diluted before analysis. The coefficient of variation of the UV assay was less than 1% over five independent measurements of each sample at a concentration of 100 mg L-1. Concentration of Na+ was obtained using an ion chromatograph (Dionex, DX-120 ion chromatograph). GC analyses were made by using an Agilent 6850 series GC system with an autosampler and a program of 2 min at 50 °C, followed by an increase in temperature to 190 °C at rate of 20 °C min-1. Further details of analyses can be found elsewhere.1 3.3. Preparation of Membranes. The membranes were prepared by filling the pores of the microporous supports with PPG. The microporous membranes were first rubbed gently with PPG-wetted tissue paper. The PPG penetrated spontaneously into the membrane pores. To ensure complete filling of the pores, these membranes were soaked in the PPG overnight. The excess PPG from the surface was then wiped off. The membranes were then exposed to γ-radiation to crosslink the PPG inside the pores. 3.3.1. Membrane Irradiation. The PPG impregnated PVDF or PP flat sheet membranes were placed around the circumference of a glass beaker that was subsequently positioned inside a steel chamber (i.d. ) 10 cm, h ) 20 cm). The chamber was positioned inside a γ-cell 3000 irradiator with a 137Cs γ-emitting source and irradiated for various doses. 3.4. Batch Extraction Experiments. The stirred cell (Figure 1) was used for testing mass transfer through membranes. Phenol transferred from one compartment to the other, across the membrane. Both compartments are equipped with motor-driven stirrers to ensure adequate mixing. The liquid film resistances

Table 1. MARS Process Operating Conditions feed concentration feed pH average feed flow rate feed circulation flow rate feed circulation pH HCl concentration used stripping solution conductivity stripping circulation rate concentration of NaOH used operation temperature thickness of PVDF support thickness of PP (Celgard 2402) support thickness of PP (Celgard 2400) support membrane area

∼7 g L-1 ∼6.5 ∼1.25 × 10-4 L min-1 ∼0.4 L min-1 ∼3 5 wt % ∼66 mS cm-1 ∼0.4 L min-1 10 wt % 30 °C 0.230 mm 0.05 mm 0.025 mm 0.00385 m2

were found to be negligible when the stirrer speeds were above 500 rpm.5 The experiments were run under this condition. Liquid samples were taken from this apparatus over time for analysis of concentrations in feed and stripping solutions. The mass transfer coefficients were calculated using eqs 3 and 4. The temperature was maintained at 30 °C with a constant temperature water bath. Some of the experiments were repeated with different pieces of membrane but prepared with the same cross-linking dose, to test for variations in experimental results. 3.5. MARS Process Experiments. Figure 2 shows the bench-scale experimental setup of the MARS process in a continuous mode. Operating conditions are summarized in Table 1. The membrane, prepared as in section 3.3.1, was fixed inside the contactor cell to separate the feed and stripping solution circulating inside the two compartments of the cell. Gear pumps were used to circulate the feed and stripping solutions at a flow rate of 0.4 L min-1. The pH of the circulatory feed solution was held at a value of 3 via a feedback loop with a pH probe immersed inside the feed circulation solution and a pump adding 5 wt % HCl solution. The conductivity of the stripping solution tank was maintained constant at 66 mS cm-1 (equivalent to pH ) 13) by another feedback loop with a conductivity probe and a pump, adding 10 wt % NaOH solution, when required. The initial stripping solution charge was prepared by neutralizing 10 wt % NaOH with phenol

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Table 2. Mass Transfer Coefficients through the Membranes Tested in the Stirred Cella mass transfer coefficient × 107 (m s-1) non-cross-linked membrane

1

2

PVDF 9.9 8.9 (0.230 mm) PP 35.7 31.9 (0.05 mm) PP 78.1 75.7 (0.025 mm)

radiation-cross-linked

average SD 9.4

0.7

1

2

average

SD

8.1

8.2

8.15

0.07

33.8

2.6 28.4 22.0

25.2

4.5

76.9

1.6 67.5 62.3

64.9

3.7

Conditions: PPG, 4000 g mol-1; feed, 1 g L-1 phenol; stripping solution, distilled water; temperature, 30 °C; radiation dose, 8.97 kGy.

Figure 3. Mass transfer coefficients with time.

a

until pH ) 13. In this way Cs and pH were kept constant in the stripping solution vessel throughout the experiment. To evaluate Cs experimentally, stripping solution was circulated simultaneously with a peristaltic pump through a PDMS tubular membrane, which was contained in water. To ensure quick equilibrium, the water was circulated with another peristaltic pump to provide good mixing throughout and a comparatively large area (0.013 m2) of PDMS tube was used. Concentration analyses of the water phase at equilibrium provided an experimental estimate of Cs. The overflows from feed and stripping solution circulation tanks were collected. Samples from feed, feed circulation, feed circulation over flow, and stripping solution were taken over time. The experiments were carried out at a temperature of 30° C using a constant temperature water bath. 3.6. Effect of Radiation Dose on the Membrane Performance. There were two objectives of studying the effect of radiation dose on the membrane performance. First, it establishes an appropriate radiation dose for the membranes to be exposed to, and second, it may reveal any adverse affects of long time exposure to γ-radiation, if any. The PPG-impregnated membranes (section 3.3) were exposed to a γ-radiation source for different lengths of time to vary the radiation doses from 0 to 46 kGy. These membranes were then investigated in the stirred cell shown in Figure 1. 4. Results and Discussion 4.1. Batch Extraction Experiments. The mass transfer coefficients through the membranes tested in the stirred cell are shown in Table 2. In the stirred cell, the liquid film resistances were found to be negligible (section 3.4) because of vigorous mixing of feed and stripping solutions. Therefore, overall mass transfer coefficients are the mass transfer coefficient through the membrane alone. This fact is further supported by the evidence that the mass transfer coefficient increases accordingly as the relatively thinner membranes are used. It could be inferred from Table 2 that the mass transfer coefficient increases with decreasing thicknesses of the membranes. 4.2. Investigation of PPG-PVDF RadiationCross-Linked Membrane under MARS Operating Conditions. The PPG-impregnated radiation-crosslinked PVDF (0.230 mm) membrane was investigated under the MARS operating conditions for more than 600 h. This membrane was found to be more stable than a PVDF membrane with non-cross-linked PPG. The overall mass balance and phenol mass balance were close, within 4 and 6% error, respectively.

4.2.1. Mass Transfer Coefficient. The mass transfer coefficients through radiation-cross-linked PPGPVDF membranes with time are shown in Figure 3. The mass transfer coefficients through non-cross-linked PPG-PVDF membranes are also shown for comparison. The mass transfer coefficient (7.9 × 10-7 m s-1) through radiation-cross-linked PPG-PVDF membrane is lower than that with non-cross-linked PPG-PVDF membrane (10.6 × 10-7 m s-1). This may be attributed to reduction in the flexibility of polymer chain segments due to crosslinking of polymer chains leading to reduced diffusivity of phenol through cross-linked PPG. A similar difference of mass transfer coefficients was observed when these membranes were investigated for short batch experiments in the stirred cell (Table 2). The partition coefficient could be expected to be the same for the noncross-linked and cross-linked PPG, because all phenol binding sites (ether oxygen) should remain intact during the cross-linking.12 The mass transfer coefficient through radiation-crosslinked PPG-PVDF membrane is fairly constant over time, indicating that the membrane retained both its uniform selectivity and operational stability during the period of study. However, the mass transfer coefficient through non-cross-linked PPG-PVDF membrane started increasing at ∼300 h. This could be due to the displacement of PPG (caused by osmotic pressure due to ∼2 M sodium phenolate in the stripping solution) from the pores of the support. The overall mass transfer coefficient (7.9 × 10-7 m s-1) through the PPG-PVDF radiation-cross-linked membrane in the MARS process compares well with the overall mass transfer coefficient (Table 2) when this membrane was tested in the stirred cell. In the stirred cell, the liquid film resistances are negligible due to vigorous mixing of the feed and stripping solutions; i.e., the membrane is the main resistance to the phenol transfer. It can be inferred from the good comparison of the overall mass transfer coefficients in the stirred cell and the MARS continuous process that the membrane is the main resistance in both cases. The value of neutral phenol in the stripping solution calculated by eq 7 is 0.2 g L-1. The experimentally measured value was 0.5 g L-1. As already discussed in section 2.2, this discrepancy is probably due to the deviation of the stripping solution from ideal behavior. Similar discrepancies between experimental and theoretical values have been reported by Ferreira et al.27 4.2.2. Water Flux and Dilution of Stripping Solution. Water can diffuse from the feed to the stripping solution through the membrane due to existence of an osmotic pressure difference across the membrane. In general, any water flux is undesirable for the MARS process. The dilution of stripping solution adversely affects the phenol recovery efficiency. In a


Figure 4. Cumulative water transfer with time.

Figure 5. Sodium transfer from stripping to feed solution.

typical MARS process, the steady-state stripping solution contains ∼20 wt % phenol, and a dilution factor of 2 would reduce this concentration to 10 wt %, which is near the solubility limit of phenol in water (∼8% at 30 °C).1 Thus, the recovery of phenol would become difficult or even impossible during the recovery stage of the MARS process. Therefore, dilution factors in the range of 2 are totally unacceptable. Water transfer through the membrane was determined by a mass balance over the stripping solution tank and is reported as the cumulative water transfer through the membrane with time as shown in Figure 4. The water flux through the radiation-cross-linked PPG-PVDF membrane is 5 times less than that of the non-cross-linked PPG-PVDF membrane (0.016 vs 0.080 L m-2 h-1). This difference could be primarily due to the instability of the later membrane and secondarily due to the reduced diffusion of water through cross-linked PPG in the former membrane. The water transferred through radiation-cross-linked PPG-PVDF membrane from the feed to the stripping solution would dilute the striping solution only by a factor of 1.2, which is reasonably low and acceptable for MARS applications. On the other hand, water transferred through the non-cross-linked PPG-PVDF membrane could dilute the stripping solution by a factor of 2.2, which would subsequently reduce the recovery to an unacceptable level at the MARS recovery stage. 4.2.3. Sodium Ion Transfer. In the MARS extraction stage, the driving force is maintained by the pH differential between the feed and stripping solution across the membrane. Therefore, it is important that the membrane effectively rejects the transfer of sodium and hydroxide ions from the stripping to the feed side to avoid a pH increase in the feed solution. The concentration of sodium ion in the feed circulation solution was measured by ion chromatograph and is shown as ppm of Na+ with time in Figure 5. This transfer of sodium ion could either be in the form of sodium phenolate (neutral molecule) or as Na+ (charged species). The sodium ion fluxes were calculated by using the steady part of the curves (experimental points with

standard deviation of less than 10%). The sodium ion transfer through radiation-cross-linked PPG-PVDF membrane (0.8 × 10-4 kg m-2 h-1) is 6 times less than that through the corresponding non-cross-linked membrane (5.0 × 10-4 kg m-2 h-1). The sodium transfer through the non-cross-linked PPG-PVDF membrane remained relatively constant for the period between 160 and 300 h (see Figure 5). After that, a rapid increase of the Na+ concentration on the feed side was observed, which coincides with the increase of the phenol masstransfer coefficient (see Figure 3). Thus, we speculate that these sudden changes of the mass-transfer rates indicate the displacement of PPG from some of the membrane pores. This provides further evidence that radiation cross-linking stabilizes the PPG inside the pores while the non-cross-linked PPG membrane is unstable. The slightly lower mass transfer coefficient through the radiation-cross-linked PPG-PVDF as compared to non-cross-linked PPG-PVDF membrane is a trade off given the improved stability of the former membrane. The mass transfer coefficients (7.9 × 10-7 m s-1) through the radiation-cross-linked PPG-PVDF membrane is 5 times higher than that of through 0.5 mm silicone rubber tubing (1.5 × 10-7 m s-1) measured in a separate experiment using single tube mass exchanger.3 Overall, the markedly increased stability of the crosslinked membrane more than compensates for the slight decrease in mass transfer. 4.3. PPG-PP Radiation-Cross-Linked Membranes under MARS Operating Conditions. As discussed above, radiation-cross-linked PPG-PVDF membrane showed encouraging results in terms of stability and mass transfer coefficients. This membrane also proved suitable for the MARS process regarding water flux and sodium ion transfer. However, mass transfer coefficients could further be improved by decreasing the thickness of the membrane support. Therefore, relatively thinner (0.025 and 0.05 mm) commercially available polypropylene microporous supports (from Celgard USA) were chosen for further investigation of the MARS continuous process. These supports were used to prepare radiation-cross-linked membranes. Due to the thinner membrane support, mass transfer coefficients could be expected to be higher than that of membrane with PVDF support. SEMs of the polypropylene microporous support (0.025 mm) and corresponding PPG-PP radiation-cross-linked membranes on this support are shown in Figure 6, parts a and b, respectively. Although the membranes were carefully wiped to remove the PPG from the membrane surface, a very thin film may still exist that would also be cross-linked. The PPG-filled pores along with this thin film give the appearance of a homogeneous top layer (Figure 6b). Since the phenol transport takes place through the PPG-filled pores only, this film should not affect the performance of the membranes. 4.3.1. Mass Transfer Coefficient for PPG-PP Membranes. Mass transfer coefficients are shown in Figure 7. The PPG-PP (0.025 mm) radiation-crosslinked membrane exhibited Kov of 80 × 10-7 m s-1. The mass transfer coefficient through the same PP (0.025 mm) radiation-cross-linked membrane in the stirred cell was 65 × 107 m s-1. This discrepancy is probably due to the fact that this 25-µm-thick membrane was relatively less selective (higher water flux, higher Na+ transfer, etc.) due to the existence of osmotic pressure

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Figure 8. Cumulative water transfer with time.

Figure 9. Sodium ion transfer from stripping to feed solution. Table 3. Summary of Phenol, Water Flux, and Sodium Ion Flux Data through the Membranes Tested in This Study membrane support

δ (mm)

PPa PPa PVDFa PVDFb PDMSc

0.025 0.050 0.230 0.230 0.500

φwater (L m-2 h-1)

Kov × 107 (m s-1)

φNa+ × 104 (kg m-2 h-1)

0.110 0.040 0.016 0.080

80.0 27.0 7.9 10.6 1.5

3.0 1.6 0.8 5.0

a Radiation-cross-linked flat sheet membranes. b Non-crosslinked flat sheet membranes. c Nonporous tubular membranes.

Figure 6. (a) Polypropylene microporous support; the view from the top shows the porous nature of the PP membrane formed by stretching. (b) PPG-PP radiation-cross-linked membrane shows a uniform nonporous top layer.

Figure 7. Mass transfer coefficients with time.

across the membrane under the MARS operating conditions. The higher water flux (Figure 8) could enhance28,29 the transfer of phenol, which could result in higher mass transfer rates. The mass transfer coefficient through the PPG-PP (0.05 mm) radiation-cross-linked membrane is 27 × 10-7 m s-1 (Figure 7). This value is consistent with a mass transfer coefficient of 25 × 10-7 m s-1 (Table 2) through the same PPG-PP radiationcross-linked membrane when tested in the stirred cell.

4.3.2. Water Flux and Stripping Solution Dilution. The cumulative water fluxes through PPG-PP radiation-cross-linked membranes are shown in Figure 8. The 0.025-mm PP membrane shows higher water flux (0.11 L m-2 h-1) than the 0.05-mm PP. This water flux will dilute the stripping solution concentration by a factor of 2.1. This dilution of stripping solution is unacceptable, so this membrane is not suitable for the MARS process. On the other hand, water flux was reasonably low 0.04 (L m-2 h-1) through the relatively thicker (0.05 mm) PP membrane. The amount of water transfer through this membrane will dilute the stripping solution by a factor of 1.4, which is acceptable for the MARS applications. It seems from Figure 8 that water flux decreases with increasing thicknesses of the membranes. Therefore, 0.05-mm-thick PP is better than 0.025-mm-thick PP (in terms of water flux). 4.3.3. Sodium Ion Transfer. The sodium ion transfer data through the radiation-cross-linked PPG-PP membranes is shown in Figure 9. It can be seen from Figure 9 that Na+ transfer decreases with increasing thickness of the membrane. The sodium ion fluxes through 0.025- and 0.5-mm PP were 3.0 × 10-4 and 1.6 × 10-4 kg m-2 h-1, respectively. Thus, 0.05-mm PP is also better in terms of ions/salt transfer from the MARS point of view. 4.4. Selection of a Suitable Membrane. Table 3 shows a summary of the phenol transfer, water flux, and sodium ion flux data through the radiation-crosslinked membranes. The 0.025-mm-thick PP membrane


exhibited the highest mass transport rates among the radiation-cross-linked membranes tested in this study. However, the water flux and sodium ion transfer through this membrane are both higher, making this membrane unsuitable for the MARS applications. The water flux through both 0.05-mm PP and PVDF membranes is acceptable, while the mass transfer rate of phenol through 0.05-mm PP membrane is higher than that of PVDF membrane. Therefore, the 0.05-mm PP membrane with mass transfer coefficient 3-4 times higher than that of PVDF is the most suitable membrane for the MARS process. 4.4.1. Advantages of Radiation-Cross-Linked 0.05mm PPG-PP Membrane. This membrane exhibited ∼18 times higher mass transfer rates for phenol than that of silicone rubber tubing. This could improve the MARS process extraction stage efficiencies remarkably. The mass transfer coefficient through 0.05-mm PP cross-linked membrane is fairly constant over time, which indicates that the membrane retained its stability, selectivity, and integrity over the period of operation. The combination of high mass transfer rates, low Na+ transfer, and low water fluxes through the membrane makes it suitable for the MARS process. The overall mass transfer coefficient is inversely related to the membrane area required. Therefore, for a given separation with a 0.05-mm PP radiation-crosslinked membrane, the required membrane area will be ∼18 times less than that required with silicone rubber. This will result in cost savings for both the membrane and process plant, assuming that the radiation-crosslinked PPG-PP (0.05 mm) membrane costs less than 18 times as much per square meter as the tubular silicone rubber membrane. In previous MARS studies, the process was run at 50 °C to increase mass transfer rates. However, with PP (0.05 mm) radiation-crosslinked membrane, the process can be run at 30 °C, and this will reduce the thermal energy required and in turn will result in further cost savings. Although we did not find any previous studies using a radiation technique to produce such type of membranes, nuclear radiation has been used for production of grafted polymeric membranes.16 While it seems that the mass production of these membranes for industrial applications might not be easy in this way, the use of gamma or electron beam irradiation is not uncommon on a commercial scale. During the past 15 years, medical devices have been increasingly sterilized by γ (and electron beam) irradiation and this trend is expected to continue.30 Radiation techniques are efficient with regard to power and need only a small space to be set up. Despite the relatively high installation cost and risk associated with radiation, it has proved its superiority and cost-effectiveness in a number of chemical processes over that of other forms of energy, such as heat or electricity.16 4.5. Effect of Radiation Dose on Membrane Performance. After establishing experimentally that the radiation cross-linking improves the stability of the impregnated phase, the next step was to investigate how the membrane supports and liquid phase inside the pores respond to the varying radiation doses in terms of mass transfer rates and stability. For this purpose, 0.025-mm-thick PP membrane, the thinnest support among the membranes tested, was selected, because this membrane will respond more quickly to any adverse/ favorable effects caused by variation in radiation doses.

Figure 10. Mass transfer coefficients with radiation dose (0.025mm-thick PPG-PP membrane).

The mass transfer coefficient data obtained from the stirred cell with the membranes irradiated for different lengths of time (different radiation doses) is shown in Figure 10. The phenol mass transfer coefficients slightly decrease (Figure 10) with increasing radiation dose. This may be due to the fact that the degree of crosslinking is proportional to the radiation dose.16 When the membranes are irradiated for longer time, the radiation dose is increased, which may result in a higher degree of cross-linking. Therefore, reduced diffusion of phenol through less flexible polymer chains may result in decreased mass transfer rates. In the study of diffusion of a series of alkanes through rubber, it was reported that diffusivity decreases linearly with increasing crosslinking density and that the rate of decrease of diffusivity leveled off at higher levels of cross-link density.31 Ho et al.12 reported the very small reduction (from 43 to 38 × 10-7 m s-1) in transport rates of p-nitrophenol through cross-linked PPG (using TDI and glycerine for cross-linking) compared to the non-cross-linked PPG. On the other hand, a higher degree of cross-linking will result in a more stable liquid membrane phase. Hence, the slight decrease in mass transfer rates could be compromised for better stability. It could also be inferred from experimental results (Figure 10) that radiation doses used in this study do not affect the PPG adversely; otherwise, it could have resulted in dramatic changes in mass transfer rates. However, when the radiation dose was increased to 46 kGy, the polypropylene support was destroyed, becoming fragile and tearing apart, and it was not possible to use the resulting membrane. This is probably due to the fact that if the energy of radiation is higher, chain breaking could occur due to the cleavage of C-C bond in the polymer backbone.16 The destruction of the PP support restricts the use of higher radiation doses and higher cross-linking densities. However, radiation doses below 46 kGy can safely be used to irradiate the PP membranes. 5. Conclusion The supported liquid membranes with PPG offer high mass transfer rates for phenol. The γ-radiation crosslinking of PPG inside the pores of the microporous supports improves the operational stability of the membranes. The radiation dose affects both the mass transfer rates and the membrane supports. The phenol mass transfer rates seem to be slightly decreasing with increasing radiation dose. The PP support was destroyed at a radiation dose of 46 kGy. The phenol mass transfer rates and water flux both decrease when the membrane thickness increases. The 0.05-mm PP membrane provides the best combination of mass transfer rates and water flux for the MARS process among the membranes tested in this study. The phenol mass transfer rate through this membrane (27 × 10-7 m s-1) is 18 times higher than that of silicone rubber tubing

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(1.5 × 10-7 m s-1) under MARS operating conditions, which could reduce significantly the membrane area required for a given separation, resulting in attractive economical benefits. The membrane retained both its selectivity and integrity over a period of more than 1 month under MARS operating conditions. This longterm operational stability under MARS process conditions implies that this membrane could be scaled up for application in the MARS process. Acknowledgment This work was funded by the U.K. Engineering and Physical Sciences Research Council (EPSRC), Grant GR/ R57188/01. M.G.D. acknowledges financial support from Ministry of Science and Technology (S&TR Division), Government of Pakistan, Grant 28(58)E-37/(CE)/2001DSA(HRD). The authors also thank GKSS and Celgard USA for kindly supplying membrane samples. Nomenclature A ) area of membrane (m2) Cf ) concentration of phenol in feed solution (kg m-3) Cf,c ) concentration of phenol in feed circulation solution (kg m-3) Cf,o ) concentration of phenol in feed over flow solution (kg m-3) Cs ) concentration of neutral phenol in stripping solution (kg m-3) Cst ) concentration of phenol in stripping solution at time t (kg m-3) Ctot ) total concentration of undissociated and dissociated phenol in stripping solution (kg m-3) Cw0 ) initial concentration of phenol in feed solution (kg m-3) Cwt ) concentration of phenol in feed solution at time t (kg m-3) D ) diffusion coefficient (m2 s-1) E ) enhancement due to chemical reaction F ) flow rate (m3 s-1) Ka ) acid dissociation constant (M) kf ) liquid film mass transfer coefficient on feed side (m s-1) km ) membrane mass transfer coefficient (m s-1) Kov ) overall mass transfer coefficient (m s-1) Kov,f ) overall mass transfer coefficient based on feed side concentration (m s-1) Kov,s ) overall mass transfer coefficient based on stripping side concentration (m s-1) KP ) partition coefficient of phenol in polymer/water ks ) liquid film mass transfer coefficient on stripping side (m s-1) N ) phenol flux (kg m-2 s-1) PDMS ) poly(dimethylsiloxane) PhO- ) phenolate PhOH ) phenol t ) time (s) V ) volume of feed or aqueous solution (m3) Greek Letters ) porosity of the membrane δ ) thickness of the membrane (m) τ ) tortuosity factor for the membrane φNa+ ) sodium ion flux (kg m-2 h-1) φwater ) water flux (L m-2 h-1)

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Received for review February 4, 2005 Revised manuscript received April 12, 2005 Accepted April 14, 2005 IE050140N

Stabilization of Supported Liquid Membranes by Î³-Radiation and Their

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