Carbon Nanotube Immobilized Composite Hollow Fiber Membranes for PervaporativeRemoval of Volatile Organics from Water

Ornthida Sae-Khow and Somenath Mitra*Department of Chemistry and EnVironmental Science, New Jersey Institute of Technology,Newark, New Jersey 07102

ReceiVed: July 12, 2010; ReVised Manuscript ReceiVed: August 25, 2010

This paper reports the development of novel carbon nanotube immobilized composite membranes (CNIM)for pervaporative removal of organics from an aqueous matrix. The nanotubes were immobilized into thepores of a composite, where they served as sorption sites that provided additional pathways for enhancedsolute transport, affecting both the partitioning and diffusion through the membrane. Depending upon theprocess conditions, the enhancement in organic removal and mass transfer rates were higher by 108 and95%, respectively. The CNIM demonstrated several advantages including enhanced recovery at lowconcentrations, lower temperatures, and higher flow rates. Overall, these lead to more energy efficient processes.

Introduction

Membrane separation has undergone rapid development inrecent years. Serving as a selective barrier, its function is toseparate two bulk phases and control the mass transfer betweenthem. Some real-world applications of membranes includedesalination, dialysis, ultrafiltration, gas separation, electrodi-alysis, and dehumidification.1 It has also been used in analyticaltechniques for monitoring water pollutants.2-4 The use ofmembranes for the separation of volatile organic compounds(VOCs) from water via pervaporation is a promising technique,which involves both the phenomenon of permeation andevaporation. This technique is a promising alternative toconventional energy intensive processes such as evaporation ordistillation.5-7 Often referred to as a “clean technology”, theseparation in pervaporation is not based on relative volatilitiesas in the case of thermal processes but on the rates of permeationthrough a membrane.

Studies on pervaporation of VOCs have focused on polymericmembranes because of their high selective sorption anddiffusion,8-10 where a vacuum or a sweep gas is used to enhancemass transfer. Solid, microporous, as well as compositemembranes have been used to improve the mass transfer rate,and more recent efforts have included the development ofasymmetric, thin film composite, and mixed matrixes toovercome the trade-off between permeability and selectivity.11-14

These are often compromising factors, and conventional mem-branes appear to be reaching their limits as far as separationcapability is concerned.15 The mixed matrix membranes consist-ing of interpenetrating polymer materials and solid fillers exhibitgreater permeation rates and selectivity in gas,16 liquid,17 as wellas in bioseparations.18 Recently, our group has reported thedevelopment of novel membranes by immobilizing CNTs intoporous hollow fiber membrane pores.19,20 Referred to as thecarbon nanotube immobilized membrane (CNIM), here theCNTs served as a sorbent and provide an additional pathwayfor solute transport. These membranes have been used to extracta variety of organic compounds from water and into an organicsolvent while demonstrating higher enrichment and extractionefficiency.20,21

A pervaporation process is comprised of discrete steps suchas diffusion through the boundary layer on the feed side of themembrane, selective partitioning of molecules into the mem-brane, diffusion across the membrane under concentrationgradient, and vapor removal. Typically, solid membranesprovide high selectivity, porous membranes show higher flux,and composite membranes comprised of a thin (micrometerscale) polymeric film on a porous matrix represent the optimumcompromise where high flux is obtained along with highselectivity.7 The immobilization of CNTs on a compositemembrane could potentially affect both partitioning and diffu-sion rate through the membrane, leading to enhanced perfor-mance. The objective of this study is to study the effect ofcarbon nanotubes on the pervaporation of VOCs from anaqueous matrix.

Experimental Section

A hydrophobic hollow fiber membrane module consisted ofhollow fiber membranes (HFM) held within a 30 cm longstainless steel casement. A stainless steel, Swagelok-type, “T”connector (Component and Controls, Carlstadt, NJ) placed ateach end of the module coupled the casement and HFM strands,and were sealed using a fast drying epoxy resin (ResinTechnology Group, LLC, S Easton, MA). The sealed “T” unitsprevented intermixing of the lumen and permeated contents. Italso served as the inlet/outlet for the sample and the permeatestripping N2 gas. The membrane module used in this studyconsisted of 10 strands of a composite hollow fiber membrane(0.260 mm o.d. × 0.206 mm i.d.).

Carbon nanotube immobilized membranes were produced bymodifying a composite membrane comprising of a microporoushydrophobic polypropylene matrix with a 1 µm thick film of ahomogeneous siloxane (Applied Membrane Technology, Min-netonka, MN). This was done by sonicating hollow fibermembrane with CNT dispersions, which led to the introductionof individual CNTs into the membrane pores. The purified CNTswere first dispersed in a solution of polyvinylidene fluoride(PVDF), as described in our previous publications.20,21 Thestarting hollow fibers were sonicated in this solution for 3 h.The PVDF served as glue that held the CNTs in place. Themembrane was flushed with acetone to remove the extra* Corresponding author. E-mail: mitra@njit.edu. Phone: (973) 596 5611.

J. Phys. Chem. C 2010, 114, 16351–16356 16351

10.1021/jp1064402 2010 American Chemical SocietyPublished on Web 09/08/2010

PVDF-CNT suspensions. The original siloxane treated mem-brane was sonicated in PVDF solution without the CNTs, andthis served as the control.

The CNIM was characterized using scanning electron mi-croscopy (SEM) (a Leo 1530 VP (Carl Zeiss SMT AGCompany, Oberkochen, Germany) field emission scanningelectron microscope). This was done by cutting the membranesinto 0.5 cm long pieces and coating with carbon films.Thermogravimetric analysis (TGA) was used to investigate thedegradation of membrane material during heating. TGA wascarried out using a Perkin-Elmer Pyris 7 TGA system with aheating rate of 10 °C/min under air atmosphere.

Figure 1 is a schematic of the pervaporation system used forVOC removal. The sample was delivered through the membranelumen by a HPLC pump (Hewlett-Packard 1050). A countercurrent nitrogen flow on the permeate side removed the VOCsfrom the feed stream. Dichloromethane, chloroform, benzene,trichloroethylene, and toluene were used as test molecules. Thetemperature of the pervaporation module was controlled usinga heating tape and with an associated temperature controller.Due to the hydrophobic nature of the fiber membrane, only traceamounts of water passed through the membrane to the gas phase.The water sample was collected at the outlet of the pervaporationmodule into a vial. The composition of the feed and collectedsamples was analyzed immediately. This was carried out usinga 100 µm polydimethyl siloxane (PDMS) solid-phase microex-traction (SPME) fiber, and analysis was carried out using aVarian 3800 gas chromatograph coupled to a flame ionizationdetector (Varian Inc. Scientific Instrument, Walnut Creek, CA).The compounds were separated on a 30 m long, and 0.32 mmi.d., DB-5 (J&W Scientific, Folsom, CA) column which had a0.25 µm of 95% dimethypolysiloxane-5% diphenyl stationaryphase.

Results and Discussion

Membrane Characterization. The typical morphology ofthe CNIM is shown in Figure 2. The outer surface of the

composite membrane had a solid, homogeneous siloxane film.An unmodified membrane without CNTs is shown in Figure2A. The polymer served as the binder that immobilized theCNTs in the membrane pores. The CNT immobilized membranesurface is shown in Figure 2B. The organic solvent allowedthe membrane materials, especially the siloxane layer, to swell,and the CNTs were able to penetrate this layer and get embeddedin the pores. The nanotubes were uniformly distributed through-out the membrane matrix, as shown in Figure 2B.

Thermogravimetric analysis of the membranes is shown inFigure 3. The addition of CNTs also increased the thermalstability of the membranes. The degradation of the blankmembrane occurred in the 155-471 °C range, while thedegradation of CNIM occurred between 183 and 613 °C. Onthe basis of the TGA analysis, the CNT content of the membranewas estimated to be 0.1 wt % and even such a small amount ofCNT significantly enhanced its thermal stability.

The Role of CNTs on Pervaporation. Immobilizing theCNTs in the membrane pores altered the analyte-polymerinteractions, which was one of the major physicochemical factorsaffecting the permeability and selectivity of a polymer mem-brane.22 The CNTs acted as sorption-active sites that interactedwith the analytes.23 The CNTs also provided an alternate routefor mass transport via diffusion along its smooth surface.24,25

The analytes may also transport through the inner tubes of theCNTs, allowing the analytes to penetrate the membrane interfacemore easily.26 Figure 4 shows the proposed mechanisms for thetransport of organic analytes during pervaporation across aCNIM. The rate of mass transport through the membrane, Q, iscontrolled by the diffusion of analytes under a concentrationgradient, and can be estimated under steady-state conditions byuse of the following equation:27

Figure 2. SEM images of (A) unmodified composite membrane and (B) membrane immobilizing with CNTs.

Figure 1. Schematic diagram of the pervaporation process.

Figure 3. TGA of unmodified composite membrane and CNIM.

16352 J. Phys. Chem. C, Vol. 114, No. 39, 2010 Sae-Khow and Mitra

where A is the surface area of the membrane, De is the diffusioncoefficient in the membrane material, Kes is the membranematerial/sample distribution constant, b is the thickness of themembrane, B4 is a geometric factor defined by the shape ofthe membrane, and Cw is the VOC concentration in water. Thepresence of CNTs affects several of these parameters; De andB4 are altered by the presence of the smooth and frictionlesssurface of CNTs, while the partition coefficient is affected bythe excellent sorbent characteristics of the CNTs and their highsurface area facilitates higher flux.

The Effect of CNTs on VOC Removal. The efficiency ofthe pervaporation process was determined on the basis of theremoval of the organics from water according to

where Ci and Co were the concentrations of the solution at theinlet and outlet, respectively.

Pervaporation was studied in the temperature range 30-75°C. The higher temperature produced free volumes in polymerchains,28 thus allowing enhanced permeation rate. Figure 5shows removal efficiency and enhancement for toluene as afunction of the operating temperature. The sample flow rate andthe permeate stripping gas were kept constant while thetemperature was varied. An increase in operating temperatureincreased the removal of toluene on both membranes. Theremoval efficiency of toluene on the composite membraneincreased from 38 to 73% when the temperature was increasedfrom 30 to 75 °C. In the same temperature range, tolueneremoval efficiency increased from 59 to 79% in CNIM.Therefore, the enhancing effect of the CNTs was higher at lowertemperatures, which ranged from 55% at 30 °C to 8% at 75°C. This is because at higher temperature the vapor pressurewas very high and the driving force was provided by thevaporization rather than enhanced permeability. For example,60% toluene removal was achieved at 30 °C in CNIM, whilethe unmodified membrane achieved the same level of separationonly at 45 °C. This demonstrates the energy saving advantageof the CNIM, where higher pervaporative flux can be achievedat a lower temperature. The other organic solvents studied hereshowed similar trends, as shown in Table 1. Both the boilingpoint and aqueous solubility of the solvents were importantfactors, but in the case of CNIM, no direct correlations wereobserved. The diffusion coefficient through the membrane andthe partition coefficient are probably more important factors.

The effect of sample flow rate on the removal efficiency ofall studied compounds in plain membrane and CNIM ispresented in Table 2. The operating temperature (30 °C) andthe permeate stripping gas were maintained constant, while thesample flow rate was varied from 0.5 to 4 mL/min. The removalefficiency decreased with the increase in sample flow ratebecause it lowered the residence time. The unmodified mem-brane and CNIM exhibited similar trends, but the latter showedconsiderably higher removal efficiency. Interestingly, the en-hancement in CNIM increased with the increase in sample flowrate. At a sample flow rate of 0.5 mL/min, the enhancementsin organic removal due to the presence of the CNT for allcompounds were between 5 and 26%, and the same rangeincreased to 73-108% at 4 mL/min. This demonstrated thatthe organic flux was much higher in the presence of CNTs, andthe CNIM was more effective in pervaporating the VOCs fromwater. The combination of lower temperature operation andhigher flow rate is an excellent way to enhance productivity.

The removal efficiency for unmodified composite membraneand the CNIM increased at higher inlet concentrations due toan increase in the concentration gradient or driving force acrossthe membrane. The results are shown in Figure 6. Once again,the CNIM was more effective in the removal of VOCs fromwater, as seen from the consistently higher removal efficiencies.The equilibrium solubilities of the different compounds studied

Figure 4. CNT assisted pervaporation process. Triangles represent theanalyte molecules.

Figure 5. Influence of temperature on the removal efficiency of tolueneon two membranes and enhancement by CNIM.

TABLE 1: Removal Efficiency and Enhancement of Unmodified Composite Membrane and CNIM at Different Temperatures

% removal at varied temperature (°C)

unmodified membrane CNIM % enhancement

temperature (°C) temperature (°C) temperature (°C)

compound 30 45 60 75 30 45 60 75 30 45 60 75

dichloromethane 50 64 74 78 68 70 75 79 36 10 2 1chloroform 37 59 68 76 56 63 71 78 49 7 5 3benzene 51 58 66 76 68 65 72 78 33 13 9 2trichloroethylene 53 64 74 77 67 70 76 80 27 10 3 3toluene 38 57 65 73 59 72 75 79 55 27 15 8

Q ) B4ADeKesCw/b (1)

% removal )Ci - Co

Ci× 100 (2)

Pervaporative Removal of Volatile Organics from Water J. Phys. Chem. C, Vol. 114, No. 39, 2010 16353

here were quite different (Table 2). Therefore, dichloromethaneand chloroform which had higher solubility were studied in the0.5-4 mg/mL concentration range, while that for trichloroet-hylene was 0.125-1.00 mg/mL and for benzene and toluene itwas 25-200 mg/mL. Figure 6 (A-D) shows the results fromdichloromethane, chloroform, benzene, and toluene, respectively.The measurements were made at 30 °C and at a flow rate of 2mL/min. The removal efficiency of VOCs using the compositemembrane was in the range 39-73%, which increased to52-85% using CNIM.

The enhancement in organic removal using CNIM was higherat lower concentrations. For example, at 1 mg/mL sample

concentration, the removal efficiency of dichloromethane andchloroform was 48 and 60% higher using CNIM than theunmodified composite membrane, but the corresponding valuesdropped to 18% when the inlet concentration was increased to4 mg/mL. This is shown in Figure 6 (A and B). In the case ofbenzene removal, the enhancement by CNIM decreases from64 to 24%, while in the case of toluene the drop was from 31to 9%. This is shown in Figure 6 (C-D). This is because, athigher concentrations, the relative high concentration gradientsprovide a high flux and the effect of the CNTs is lesspronounced. The enhancement in removal efficiency at lowconcentration by the CNTs opens up the exciting possibility of

Figure 6. Removal efficiency and enhancement by CNIM with the function of concentration of (A) dichloromethane (DCM), (B) chloroform(CHCl3), (C) benzene (Benz), and (D) toluene (Tol).

TABLE 2: Removal Efficiency and Enhancement of Unmodified Composite Membrane and CNIM at Different Flow Rates

% removal at varied flow rate (mL/min)

unmodified membrane CNIM % enhancement

flow rate (mL/min) flow rate (mL/min) flow rate (mL/min)

compound solubility in water (g/L) boiling point (°C) 0.5 1 2 3 4 0.5 1 2 3 4 0.5 1 2 3 4

dichloromethane 13 40 78 72 50 36 28 91 87 68 61 54 17 21 36 69 93chloroform 8 61 69 55 37 30 23 87 82 56 48 45 26 49 51 60 96benzene 0.8 80 80 71 51 34 26 89 85 68 59 54 11 20 33 74 108trichloroethylene 1.28 87 87 77 53 38 30 91 87 67 59 52 5 13 26 55 73toluene 0.47 110 81 60 38 32 27 91 80 59 56 52 12 18 55 75 93

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pervaporative separation to be carried out at much lowerconcentrations, thus extending the overall applicable range ofthis technique.

The Effect of CNTs on Mass Transfer Coefficients. Thepervaporation of one or more components of a solution involvesthe following steps: (1) transport of the diffusing species fromthe bulk to the membrane-liquid interface through the boundarylayer, (2) sorption of the solute at the membrane surface, (3)diffusion of the species through the membrane, (4) desorptionfrom the membrane at the vapor side, and (5) transport throughthe vapor phase boundary layer. The rate of transport of soluteacross the membrane can be expressed in terms of flux. Theflux of compound through the membrane can be written as

where J is the solute flux through the membrane, k is the overallmass transfer coefficient, and CL and CV are the liquid- andvapor-phase concentration, respectively. The reciprocal of k isthe overall resistance to mass transfer, which is the sum of theliquid boundary layer resistance (1/kL), membrane resistance(1/kM), and vapor boundary resistance (1/kV):

The partitioning on the membrane material may be consideredinstantaneous, and step 2 does not involve any transportresistance. Also, steps 4 and 5 would offer negligible masstransfer resistance because the vapor is removed by the inertgas as soon as it is desorbed from the membrane surface.29

Therefore, steps 1 and 3 would contribute to the maximum masstransfer resistance. The liquid boundary layer resistance isdependent on a number of parameters including feed flow rate,viscosity, compound diffusivity, and module geometry. Mem-brane resistance is a function of the membrane thickness,temperature, and permeability of a compound through themembrane.29

The VOC fluxes through the membrane were computedaccording to the following relationships:30

where Jvoc is the VOC flux (kg/m2 ·h), t is the permeation timeperiod (h), A is the membrane area (m2), and wvoc

P is the totalmass of permeate (kg). The presence of stripping gas on thepermeate side is sufficient to make the vapor phase concentration(CV) negligible. Thus, the overall mass transfer coefficient (kvoc)for each component was calculated as follows:31

where Cvoc is the feed concentration of the individual species.The extent to which either the liquid boundary layer or themembrane diffusion controls the overall mass transfer resistancecan be evaluated by measuring the effect of flow rate on kvoc.Figure 7 shows the effect of flow rate on mass transfercoefficient for the pervaporation of toluene and dichloromethane.At low flow rates, the overall mass transfer is controlled by

diffusion through the boundary layer, while high flow ratesincrease turbulence and reduce the boundary layer at themembrane interface. At this point, kvoc is no longer a functionof flow rate. The flattening of the profile was observed for theunmodified membrane but not CNIM. This shows that theoverall mass transfer coefficient on the unmodified membranewas controlled by the mass transport resistance of the membranewhile that on the CNIM was controlled by the liquid boundarylayer. As the flow rate of toluene was increased from 0.5 to 4.0mL/min, the mass transfer coefficient of toluene in the unmodi-fied membrane increased from 7.7 × 10-6 to 2.1 × 10-5 m/s,and the corresponding increase for CNIM was from 8.6 × 10-6

to 3.9 × 10-5 m/s. Interestingly, the overall mass transfercoefficient was less affected by the presence of the CNTs atlow flow rates, and the difference increased with rising flowrate. At a flow rate of 0.5 mL/min, the mass transfer coefficientof the CNIM was 12% higher than the unmodified membrane,but it increased to 86% at 4 mL/min. The results were similarfor dichloromethane, where the mass transfer coefficient inCNIM was 16 and 95% higher than the unmodified membraneat 0.5 and 4.0 mL/min, respectively. In general, the presenceof the CNTs led to enhanced permeability through the mem-brane, and at higher flow rates where the mass transfer wasless controlled by diffusion through the boundary layer, theCNIM showed a significantly higher overall mass transfercoefficient.

Conclusions

The removal of organics from water was dramaticallyimproved using CNIM. For all the organics studied here, it was

J ) k(CL - CV) (3)

1k) 1

kL+ 1

kM+ 1

kV(4)

Jvoc )wvoc

P

tA(5)

kvoc )Jvoc

Cvoc(6)

Figure 7. Effect of flow rate on mass transfer coefficient for thepervaporation of (A) toluene and (B) dichloromethane at 30 °C.

Pervaporative Removal of Volatile Organics from Water J. Phys. Chem. C, Vol. 114, No. 39, 2010 16355

found that the operating temperature, sample flow rate, andsample concentration affected the pervaporation efficiency. Theperformance enhancement by CNIM over the plain membranewas higher at lower temperatures, and higher flow rates whenthe residence times were shorter. This implies that the presenceof CNTs leads to higher permeation as well as a faster rate ofmass transfer. Enhancement was also higher at lower concentra-tions, thus allowing pervaporation to be used with more dilutestreams.

Acknowledgment. This work was supported by NationalInstitute of Environmental Health Sciences and US Departmentof Energy.

References and Notes

(1) Baker, R. W. Membrane Technology and Applications; John Wiley& Sons: England, 2004.

(2) Kou, D.; Mitra, S. Anal. Chem. 2003, 75, 6355–6360.(3) Wang, X.; Kou, D.; Mitra, S. J. Chromatogr., A 2005, 1089, 39–

44.(4) Hylton, K.; Sangwan, M.; Mitra, S. Anal. Chim. Acta 2009, 653,

116–120.(5) Panek, D.; Konieczny, K. Desalination 2009, 241, 197–200.(6) Smitha, B.; Suhanya, D.; Sridhar, S.; Ramakrishna, M. J. Membr.

Sci. 2004, 241, 1–21.(7) Sae-Khow, O.; Mitra, S. J. Chromatogr., A 2010, 1217, 2736–

2746.(8) Chapman, P. D.; Oliveira, T.; Livingston, A. G.; Li, K. J. Membr.

Sci. 2008, 318, 5–37.(9) Gonzalez-Rodrıguez, J.; Perez-Juan, P.; Castro, M. D. L. d. Talanta

2003, 59, 691–696.(10) Ruiz-Jimenez, J.; Castro, M. D. L. d. J. Chromatogr., A 2006, 1110,

245–253.

(11) Uragami, T.; Okazaki, K.; Matsugi, H.; Miyata, T. Macromolecules2002, 35, 9156–9163.

(12) Qiu, J.; Peinemann, K.-V. Desalination 2006, 200, 435–436.(13) Li, C.-L.; Huang, S.-H.; Liaw, D.-J.; Lee, K.-R.; Lai, J.-Y. Sep.

Purif. Technol. 2008, 62, 694–701.(14) Choi, J.-H.; Jegal, J.; Kim, W.-N.; Choi, H.-S. J. Appl. Polym. Sci.

2009, 111, 2186–2193.(15) Freeman, B. D. Macromolecules 1999, 32, 375–380.(16) Jiang, L. Y.; Chung, T.-S.; Rajagopalan, R. Carbon 2007, 45, 166–

172.(17) Bowen, T. C.; Meier, R. G.; Vane, L. M. J. Membr. Sci. 2007,

298, 117–125.(18) Avramescu, M. E.; Bornerman, S. Z.; Wessling, M. J. Chromatogr.,

A 2003, 1006, 171–183.(19) Hylton, K.; Chen, Y.; Mitra, S. J. Chromatogr., A 2008, 1211, 43–

48.(20) Sae-Khow, O.; Mitra, S. J. Mater. Chem. 2009, 19, 3713–3718.(21) Sae-Khow, O.; Mitra, S. Anal. Chem. 2010, 82, 5561–5567.(22) Polotskaya, G. A.; Penkova, A. V.; Toikka, A. M. Desalination

2006, 200, 400–402.(23) Hussain, C. M.; Saridara, C.; Mitra, S. J. Chromatogr., A 2008,

1185, 161–166.(24) Striolo, A. Nano Lett. 2006, 6, 633–639.(25) Thomas, J. A.; McGaughey, A. J. H. Nano Lett. 2008, 8, 2788–

2793.(26) Fujiwara, A.; Ishii, K.; Suematsu, H.; Kataura, H.; Maniwa, Y.;

Suzuki, S.; Achiba, Y. Chem. Phys. Lett. 2001, 336, 205–211.(27) Pawliszyn, J. Anal. Chem. 2003, 75, 2543–2558.(28) Yeom, C. K.; Kim, H. K.; Rhim, J. W. J. Appl. Polym. Sci. 1999,

73, 601–611.(29) Dutta, B. K.; Sikdar, S. K. EnViron. Sci. Technol. 1999, 33, 1709–

1716.(30) Vane, L. M.; Alvarez, F. R.; Mullins, B. EnViron. Sci. Technol.

2001, 35, 391–397.(31) Vane, L. M.; Alvarez, F. R.; Giroux, E. L. J. Membr. Sci. 1999,

153, 233–241.

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