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Carbon nanotube enhanced membrane distillation for online preconcentrationof trace pharmaceuticals in polar solvents

Ken Gethard and Somenath Mitra*

Received 18th February 2011, Accepted 18th April 2011

DOI: 10.1039/c1an15140a

Carbon nanotube enhanced membrane distillation (MD) is presented as a novel, online analytical

preconcentration method for removing polar solvents thereby concentrating the analytes, making this

technique an alternate to conventional thermal evaporation. In a carbon nanotube immobilized

membrane (CNIM), the CNTs serve as sorbent sites and provide additional pathways for enhanced

solvent vapor transport, thus enhancing preconcentration. Enrichment using CNIM doubled

compared to membranes without CNTs, while the methanol flux and mass transfer coefficients

increased by 61% and 519% respectively. The carbon nanotube enhancedMD process showed excellent

precision (RSD of 3–5%), linearity, and the detection limits were in the range of 0.001 to 0.009 mg L�1

by HPLC analysis.

Introduction

A membrane is a selective barrier across which two separate

phases can come in contact without direct mixing. Because of

this, there has been significant interest in the analytical

membrane separation of a variety of organic and inorganic

analytes.1–8 Numerous applications that rely on solvent extrac-

tion,9 pervaporation,10 and supported liquid extraction have

been developed, including some microconcentration techniques

using microlitres of solvent.11 Real time monitoring systems

based on direct interfacing with mass spectroscopy,12,13 chro-

matography, flow injection analysis14,15 and atomic spectros-

copy16 have also been developed.

A major advantage of membranes is that they allow transfer of

solutes across a semi-permeable barrier that prevents mixing.

This allows a sample stream to be processed on a continuous

basis for online sample treatment.17–21 While this concept has

been extensively used in analytical extractions, sample concen-

tration using membranes is yet to be studied.

Conventional concentration methods such as distillation rely

on thermal evaporation of a solvent from a solution, and it is

conceivable that this can also be accomplished by a membrane,

where the solvent is selectively removed. Membrane distillation

(MD) is a low temperature (40–90 �C) process that has been used

for water desalination and the concentration of aqueous solu-

tions such as fruit juice.22–26 Here, a hydrophobic hollow fiber

prevents the passage of the liquid phase across the membrane

pores. The relatively high temperature leads to high vapor

pressure, and MD relies on the net flux of solvent vapor from the

Department of Chemistry and Environmental Science, New Jersey Instituteof Technology, Newark, NJ, 07102, USA. E-mail: mitra@njit.edu; Tel: +1(973) 5965611

This journal is ª The Royal Society of Chemistry 2011

warm to the cool side of the membrane. MD is similar to per-

vaporation where the driving force is a vapor pressure differen-

tial but simultaneous heat transfer is involved in this approach.

The partial pressure gradient is maintained by having the bulk

feed solution at a temperature higher than the condensed

permeate.27 In an analytical MD application, preconcentration

can be accomplished by the elimination of the solvent rather than

selective permeation of an analyte through a membrane, which

makes it a versatile method.

Typical MD processes provide relatively low yields, and

improved membrane systems are needed. Novel membranes have

been developed recently that use the incorporation of nano-

particles to enhance solute transport.28,29 In recent days, several

research groups including ours have incorporated carbon

nanotubes (CNTs) into membranes, where the nanotubes have

acted as novel transport media including channels for the mass

transport of gases30,31 and higher than expected flux rates have

been reported. This has been attributed to the atomic-scale

smoothness of the CNT walls and also to molecular ordering of

the gaseous molecules inside the nanopores.32–34 Besides this,

CNTs also act as sorbents that enhance the partition coefficient

and the overall permeability of solutes.35,36 Of particular rele-

vance to MD is the fact that CNTs also have high thermal

conductivity, which can reduce the temperature gradient thus

preventing solvent condensation at membrane pores.

Recently we have demonstrated that carbon nanotube

immobilized membranes (CNIMs) are effective in enhancing

performance of a variety of processes including extraction and

pervaporation.37–39 The objective of this work is to develop

analytical concentration techniques by the selective removal of

organic solvents. Of particular interest is the monitoring of drug

molecules in methanol, which is a common solvent used in many

pharmaceutical manufacturing operations.

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Experimental

The membrane material used for all work was Celgard type X-50

hollow fiber membrane, from Membrana-Charlotte Division of

Celgard, LLC, (Charlotte, NC, USA). Membrane modules were

constructed in a shell and tube format using threaded brass pipe

fittings. The ‘‘shell’’ portion of the module was a 1/4 inch ID� 1.5

inch long brass threaded pipe fitting. To each end of this was

attached a ‘‘T’’ fitting, through which the membrane was intro-

duced. The ends were then sealed with epoxy to prevent leakage

into the shell side and the assembled module was insulated with

fiberglass insulation. A total of 36 membrane strands were used

in each module, and the total surface area was 0.84 cm2 based on

membrane internal diameter. Polyflo tubing was attached to the

base leg of one of the T fittings and the other end of the tubing

was attached to a vacuum source. When vacuum was turned on,

room temperature air was drawn in through the other ‘‘T’’ fitting

at a rate of 1 L min�1.

Experiments were conducted using modules with plain

membranes as well as ones containing CNIM. The CNIM was

prepared as follows. Ten milligrams of multiwall nanotubes

(Cheap Tubes, Inc, Brattleboro, VT, USA) were dispersed in

a solution containing 0.1 mg of polyvinylidene fluoride (PVDF)

in 15 mL of acetone via sonication. The PVDF/CNTs dispersion

was forced under vacuum into the pore structure of the poly-

propylene membrane. The CNIM was produced during this step

and the PVDF served as a bonding agent that held the CNTs in

place. The membrane was flushed with acetone to remove excess

CNTs. The membrane was characterized by scanning electron

microscopy using a LEO 1530 VP instrument (Gottingen, GER)

and by thermo gravimetric analysis (TGA) using a Perkin Elmer

(Waltham, MA, USA) Pyris instrument.

A schematic of the membrane module and experimental setup

is shown in Fig. 1.

Test solution was pumped through the module using a Hewlett

Packard HPLC 1050 pump (Palo Alto, CA, USA). The solution

travelled through 1/8 inch Teflon tubing that was coiled and

immersed in a water bath, which was controlled to a set

temperature. The Teflon tubing was connected to the inlet of the

module. As solution traveled up the length of the module,

permeate was discharged through the drain port fittings. The

concentrate at the far end of the module was collected into 2 mL

HPLC vials. All analysis was completed using a Hewlett Packard

1050 HPLC system and a Perkin Elmer (Waltham, MA, USA)

785 UV-Vis analyzer. SRI’s (Torrance, CA, USA) Peak Simple

Version 3.29 was used for HPLC data analysis.

Fig. 1 Schematic diagram of the experimental setup.

2644 | Analyst, 2011, 136, 2643–2648

Four pharmaceutical active ingredients were used in this study:

ibuprofen, dibucaine, acetaminophen and diphenhydramine.

Ibuprofen, acetaminophen and dibucaine were analyzed using

Supelco (Sigma-Aldrich, St Louis, MO, USA) C-18 250 mm �4.6 mm columns. The diphenhydramine was analyzed using

a Zorbax (Agilent Technologies, Santa Clara, CA, USA) SB-CN

250 mm � 4.8 mm column. The analysis for acetaminophen was

completed using a method found in the literature.40 Analyses for

ibuprofen, dibucaine and diphenhydramine were completed

using methods detailed in the USP.41

Results and discussion

Scanning electron microscopy (SEM) images of membranes with

and without CNTs are shown in Fig. 2a and b. In Fig. 2a, the

membrane pores are clearly visible, while Fig. 2b showed the

presence of CNTs throughout the membrane surface and in

the pores structure. The TGA analysis indicated the presence of

CNTs caused thermal degradation to occur at a higher temper-

ature. The increase was 30 �C. Further, it was determined the

CNIM contained approximately 0.5 weight% of CNTs.

In MD the heated solution passes through a membrane lumen

and a portion is transformed to vapor. The mechanisms of vapor

transport in conventional MD include molecular diffusion,

Knudsen diffusion, surface diffusion and viscous flow. Knudsen

Fig. 2 Scanning electron micrographs, 25 000� magnification of (a)

original membrane and (b) CNIM.

This journal is ª The Royal Society of Chemistry 2011

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diffusion is the most commonly accepted mechanism, and

molecular diffusion becomes dominant when pore sizes are

larger. Fig. 3 shows the proposed mechanisms in the presence of

CNTs for the selective transport of solvent vapors across

a CNIM.

Immobilizing the CNTs in the pores alters the solvent–

membrane interactions. CNTs are known to be highly hydro-

phobic.42 Since one of the main issues with MD is loss of hydro-

phobicity43 due to pore wetting, the presence of CNTs decreases

the tendency of a pore to become wet with methanol, so higher

transport of pure vapor could occur. CNTs are also known to

have rapid sorption and desorption capacity.44–46This allowed the

solvent vapors to sorb on the surface and then desorb leading to

faster mass transfer. This allows activated surface diffusion which

allows the quickmovement of solvent vapormolecules (a hopping

like motion) from one site to another resulting in an increased

overall vapor transport rate. Further, it has been reported in the

literature that water transport around and through CNTs

proceeds at a high rate, several orders of magnitude higher than

expected. This is due to the fact that CNTs can provide an alter-

nate route for fast mass transport via diffusion along their smooth

surface,47,48 and also through the inner tubes of the CNTs.49

Temperature of the bulk feed solution is the single most

important factor in MD, affecting the vapor pressure and the

diffusion coefficient. Vapor pressure increases exponentially with

increasing temperature according to Antoine’s equation50 and

the diffusion coefficient follows an Arrhenius type expression.51

Since MD is a thermal process, heat transfer plays an important

role. It is also well established that CNTs have high thermal

conductivity, which reduces the temperature gradient in the

membranes, thus reducing condensation and allowing more

vapor to permeate through the pores. This reduces the temper-

ature gradient in the membranes, allowing a more equal intra-

pore temperature distribution and vapor condensation.

The preconcentration effect was measured as the Enrichment

Factor (EF):

EF ¼�Co

Ci

�(1)

where Co is the outlet analyte concentration and Ci is the inlet

analyte concentration.

Fig. 3 Mechanisms of solvent vapor transport across a carbon nanotube

immobilized membrane via membrane distillation.

This journal is ª The Royal Society of Chemistry 2011

Percent solvent reduction (%SR) was defined as:

%SR ¼ Vi � Vo

Vi

� 100 (2)

where Vi and Vo are the inlet and outlet volumes.

Process optimization was carried out to determine the optimal

operating conditions using ibuprofen as the test solution.

The results for EF at varying temperature with constant flow

(0.75 mL min�1) and feed solution concentration (5 mg L�1) are

shown in Fig. 4a. Experiments were carried out in the range of

40–90 �C.Maximum enrichment was reached at a temperature of

70 �C for both the CNIM and the plain membrane. The CNIM

consistently showed higher EF than the plain membrane. For

example, at 70 �C, EF was 45% with the CNIM. Significantly

higher EF could be accomplished using the CNIM at a lower

temperature, implying that the same preconcentration could be

carried out under cooler conditions.

EF at constant feed solution temperature (70 �C) and feed

solution concentration (5 mg L�1) was studied in the flow rate

range of 0.5 to 1.0 mLmin�1. EF began to reduce beyond 0.75mL

min�1 for both the membranes, which was attributed to lower

residence time. The data are presented in Fig. 4b. Once again, for

all measurements, EF for the CNIM was higher than the plain

membrane. In fact, the best EF for the plain membrane was lower

than theworst values obtained forCNIM(1.0mLmin�1 feedflow).

The effect of inlet concentration in the range of 0.1 to 50 mg

L�1 on EF was studied at the optimal conditions and these results

are shown in Fig. 4c. It was observed that in the range studied,

EF did not change significantly with concentration. Once again,

the CNIM consistently showed higher EF than the original

membrane at all concentrations.

The results for %SR at varying temperature with constant flow

(0.75 mL min�1) and feed solution concentration (5 mg L�1) are

shown in Fig. 5a. Experiments were carried out in the range of 40–

90 �C.Maximum solvent reduction was reached at a temperature

of 70 �C for both the CNIM and the plain membrane. The CNIM

consistently showed higher %SR than the plain membrane.

%SR at constant feed solution temperature (70 �C) and feed

solution concentration (5 mg L�1) was studied in the flow rate

range of 0.5 to 1.0 mL min�1. The data are presented in Fig. 5b.

Once again, for all measurements, %SR for the CNIM was

higher than the plain membrane.

The effect of inlet concentration in the range of 0.1 to 50 mg

L�1 on %SR was studied at the optimal conditions and these

results are shown in Fig. 5c.

The presence of CNTs affects the rate of mass transport. The

water vapor flux, Jw, through the membrane is estimated by:52

Jw ¼ k(CL � CV) (3)

where k is the mass transfer coefficient and CL and CV are the

liquid and vapor-phase concentrations, in mg L�1. The reciprocal

of k is the overall resistance to mass transfer53 and:

1

k¼ 1

kLþ 1

kMþ 1

kV(4)

where 1/kL is the liquid boundary layer resistance, 1/kM is the

membrane resistance and 1/kV is the permeate side boundary

resistance. Membrane resistance is a function of the membrane

Analyst, 2011, 136, 2643–2648 | 2645

Fig. 4 (a) EF as a function of temperature at a flow rate of 0.75 mL

min�1 and 5 mg L�1 ibuprofen feed solution; (b) EF as a function of feed

solution flow rate; at 70 �C and 5 mg L�1 ibuprofen feed solution; (c) EF

as a function of inlet concentration of ibuprofen, 70 �C and 0.75 mL

min�1 feed flow.

Fig. 5 (a) %SR as a function of temperature at a flow rate of 0.75 mL

min�1 and 5 mg L�1 ibuprofen feed solution; (b) %SR as a function of feed

solution flow rate; at 70 �C and 5 mg L�1 ibuprofen feed solution; (c) %SR

as a function of inlet concentration of ibuprofen, 70 �C and 0.75 mL

min�1 feed flow.

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thickness, pore size and temperature among other things. Vapor

phase boundary layer resistance is generally not considered to be

a contributor because the condensed permeate is immediately

removed. The flux through the membrane, J, is calculated as:

2646 | Analyst, 2011, 136, 2643–2648

J ¼ wp

tA(5)

where wp is the total mass of permeate, t is the permeate collec-

tion time and A is the membrane surface area.

The overall mass transfer coefficient is:

k ¼ J

c(6)

This journal is ª The Royal Society of Chemistry 2011

Fig. 6 (a) Mass transfer coefficient (m s�1) as a function of temperature

at a flow rate of 0.75 mL min�1 and 5 mg L�1 ibuprofen feed solution. (b)

Mass transfer coefficient (m s�1) as a function of feed flow rate at 70 �Cand 5 mg L�1 ibuprofen feed solution.

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where k is the mass transfer coefficient and c is the average feed

concentration in mg L�1.

The mass transfer coefficients at different temperatures are

presented in Fig. 6a.

The CNIM had significantly higher mass transfer coefficients

at all temperatures. The effect of temperature on k was signifi-

cantly more pronounced for the plain membrane in the 40 �C to

80 �C range. This was attributed to an increase in the membrane

diffusion coefficient.

Mass transfer was studied in the flow rate range of 0.5 to 1.0

mL min�1. Fig. 6b shows the effect of flow rate on the mass

Table 1 EF and %SR for four pharmaceutical compounds (EF and %SR fr

Analytes

EF

Detection limits/mg mL�1 CNIM Plain %En

Ibuprofen 0.001 48 29 65.5Acetaminophen 0.009 13 6 116.6Diphenhydramine 0.004 29 15 93.3Dibucaine 0.003 37 14 164.2

This journal is ª The Royal Society of Chemistry 2011

transfer coefficients. There was a flattening of the profile with the

plain membrane but not for the CNIM. As the flow rate of bulk

feed was increased from 0.5 to 0.75 mL min�1, k in the unmod-

ified membrane increased from 1.53 � 10�6 to 2.14 � 10�6 m s�1,

then stayed constant. This was attributed to boundary layer

diffusion. The presence of the CNTs led to enhanced perme-

ability through the membrane, and mass transfer was not limited

by diffusion through the boundary layer at high flow rates.

The effect of inlet concentration in the range of 0.1 to 50 mg

L�1 was studied at the optimal conditions. The mass transfer

coefficients were calculated, and these were relatively constant

independent of the concentration, and are not shown here for

brevity. Once again, the CNIM consistently showed higher mass

transfer coefficients than the original membrane.

Adding CNTs to the membrane affected the kM (membrane

resistance) in eqn (4), leading to an increased mass transport.

Membrane resistance was decreased by preventing pore wetting

due to the hydrophobic nature of CNTs,54 activated diffusion55

and the rapid transport rate through CNT channels.56

Analytical performance

Other pharmaceutical compounds, namely, acetaminophen,

diphenhydramine and dibucaine, were studied. All compounds

showed similar trends as a function of temperature, flow rate,

concentration and mass transfer coefficient. EF, SR and mass

transfer coefficient were determined for each compound at 0.1,

0.5, 5, 25 and 50 mg L�1. All experiments andmeasurements at all

concentrations were measured in triplicate. All results were linear

in the concentration range measured. The results are presented in

Table 1, which represent measurements at 70 �C and 0.75 mL

min�1 feed flow rate and 0.5 mg L�1 feed concentration.

As seen, the EF and SR were significantly higher in the case of

CNIM. The EF using the plain membrane varied between 6 and

29, while the CNIM showed 13 to 48. The CNIM led to 65.52 to

164.29% enhancement in EF and up to 61.40% enhancement in

SR.

The MD process was highly reproducible with RSD ranging

between 2 and 4%. The calibration curves showed excellent

linearity in the range of 0.1 to 50 mg L�1 with an r2 of greater

than 0.992 for all the compounds, so this allowed quantification

by the method of external standards. The detection limits using

CNIM were 0.001, 0.009, 0.004, and 0.003 mg mL�1 for

ibuprofen, acetaminophen, diphenhydramine and dibucaine

respectively. Corresponding limits for the plain membrane were

0.002, 0.021, 0.011 and 0.010. Significantly lower detection

limits may be possible by increasing the number of membrane

strands.

om measurements at 0.5 mg L�1)

%SR

hancement by CNIM CNIM Plain %Enhancement by CNIM

2 92 57 61.407 80 55 45.453 92 64 43.759 95 61 55.74

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Conclusions

MD via CNIM is an excellent preconcentration method for polar

organic solvents that can be used online. The approach is an

alternative to conventional distillation and is universal because it

relies on the removal of solvent rather than the selective perme-

ation of the analytes across a membrane. Conventional MD

provided low enrichment, but the introduction of CNTs

dramatically increased the performance. The enrichment factor

increased by up to 164%, flux increased by 61% and mass transfer

at the optimal conditions increased by up to 519%. Further

optimization including the use of other types of membranes

could be explored to improve the performance of MD.

Notes and references

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