Enhanced desalination using carboxylated carbon nanotube immobilized membranes

Enhanced desalination using carboxylated carbon nanotube immobilized membranes

Separation and Purification Technology 120 (2013) 373–377 Contents lists available at ScienceDirect Separation and Purification Technology journal hom...

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Separation and Purification Technology 120 (2013) 373–377

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Enhanced desalination using carboxylated carbon nanotube immobilized membranes Madhuleena Bhadra, Sagar Roy, Somenath Mitra ⇑ Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ 07102, USA

a r t i c l e

i n f o

Article history: Received 24 April 2013 Received in revised form 29 August 2013 Accepted 9 October 2013 Available online 21 October 2013 Keywords: Desalination Carbon nanotubes Membranes Membrane distillation

a b s t r a c t In carbon nanotube immobilized membrane (CNIM), the nanotubes serve as a sorbent that provides additional pathways for solute transport. In this paper we present that carboxylated nanotubes which are significantly more polar and can increase interactions with the water vapor in CNIM to improve desalination efficiency in membrane distillation (MD). The encapsulation of the nanotubes in PVDF prevented the carboxylated nanotubes from making the overall membrane more hydrophilic and thus retain its performance. Overall, desalination was consistently better with carboxylated nanotubes than with unfunctionalized ones with flux reaching as high as 19.2 kg/m2 h in a sweep gas membrane distillation mode. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction As the shortage of clean water looms in the horizon, there is much interest in developing novel, cost effective desalination technology. Current methodologies include thermal, chemical and reverse osmosis [1–6]. Membrane distillation (MD) has emerged as an alternative to address some issues related to the current technologies [7–9]. Here a hot salt solution such as sea or brackish water is passed through (or across) a hydrophobic membrane which acts as a physical barrier separating the warm solution from a cooler permeate. The permeation is driven by a vapor pressure gradient resulting from the temperature difference and solution composition gradients across the membrane. Typically, MD is carried out at 60–90 °C, which is significantly lower than conventional distillation. Therefore, it has the potential to generate high quality drinking water using only low temperature heat sources such as waste heat from industrial processes and solar energy. The main effort in optimal design involves the maximization of solute rejection and flux, which would make MD commercially viable. A key component in such a process is the membrane itself because it determines both flux and selectivity. Several membrane material based on polypropylene, Polyvinylidene fluoride (PVDF) and Teflon have been used in MD [10,11]. Some recent developments include surfaces made of zeolite [12], clay nanocomposites nanofiber [13], silane grafting [14] and modification by hydrophobic porous alumina [15]. Carbon nanotube based membranes have been used in a variety of separation applications in various formats [16,17], that range ⇑ Corresponding author. Tel.: +1 (973) 5965611. E-mail address: [email protected] (S. Mitra). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.10.020

from forward osmosis [18] to nanofiltration [19,20]. The CNTs have been incorporated in the membranes by making polymer composites [19] and via chemical vapor deposition [21]. Recently we have demonstrated that immobilizing CNTs in different types of prefabricated membranes alter the solute-membrane interactions, which is one of the major physicochemical factors affecting the permeability and selectivity. Referred to as carbon nanotube immobilized membrane (CNIM), here the CNTs serve as a sorbent and provide an additional pathway for solute transport. These membranes have been used in nanofiltration, MD, solvent extraction and pervaporation, and have demonstrated superior performance [19,22,23]. An important consideration that is yet to be fully utilized in membrane separation is that CNTs can be effectively functionalized to alter its chemical properties, which could lead to specific interactions with solutes, or just a change in hydrophilicity. This has been demonstrated in nanofiltration applications by our group [19]. In our previous efforts in MD using CNIM with plain CNTs, we attributed enhanced flux to the sorption of water vapor on the nanotube surface [22]. The objective of this research is to study if functionalization, in particular carboxylation will increase the interaction of nanotubes with the polar water vapor and result in improved desalination efficiency.

2. Experimental 2.1. Materials and methods The membrane modules for MD were constructed in a shell and tube format using 1/4 in. polypropylene tubing. Ten, 16.6 cm long hollow fiber strands were used in the module. Each module


M. Bhadra et al. / Separation and Purification Technology 120 (2013) 373–377

contained approximately 12.50 cm2 of effective membrane contact area (based on internal surface). The ends were then sealed with epoxy to prevent leakage into the shell side. Vacuum was applied to one drain port to draw dry air through the other port, which created a higher pressure differential and provided a sweep air. The synthesis of carboxylated CNTs (MWCNT–COOH) was carried out as follows. Pristine MWCNT was purchased from Cheap Tubes, Inc., Brattleboro, VT, USA. As previously reported by our group [24], CNT carboxylation was carried out in a Microwave Accelerated Reaction system (CEM Mars) fitted with internal temperature and pressure controls. Three hundred milligram of original MWCNTs was added to the reaction chamber together with 25 ml 1:1 conc. H2SO4 and HNO3. The reaction was carried out at 120 °C for 40 min. After cooling, the product was vacuum filtered using a Teflon membrane with pore size (0.45 lm), and the solid was dried in a vacuum oven at 70 °C for 5 h. This led to the formation of carboxylated MWCNTs (MWCNT–COOH) which was characterized by FTIR which confirmed the presence of carboxyl groups. The results are not presented here for brevity. The CNIM with pure CNT (referred to as CNIM) and functionalized CNT (referred to as CNIM-f) were prepared using Celgard type X30-240 (Celgard, LLC, and Charlotte, NC, USA) hollow fiber with pore size (0.04 lm) as the starting material. For the preparation of CNIM and CNIM-f, each of 10 mg of MWCNT and MWCNT–COOH were dispersed in a solution containing 0.1 mg of polyvinylidene fluoride in 15 ml of acetone by sonicating for 3 h. The Polyvinylidene fluoride (PVDF)-nanotube dispersion was forced under controlled vacuum into the bore of the polypropylene hollow fiber membrane. The PVDF served as glue that held the CNTs in place and led to its encapsulation within the membrane, and this may also affect the membrane performance. The membrane was flushed with acetone to remove excess nanotubes. The original polypropylene membrane was sonicated in PVDF solution in acetone without the CNTs, and this served as the control. The morphology of CNIM and CNIM-f were studied using scanning electron microscopy (SEM, Model LEO 1530), and Thermogravimetric analysis (TGA) was performed using a Perkin Elmer Pyris 7 TGA instrument to study the thermal stability of the membrane. Differential scanning calorimetry (DSC) was also carried out using a Universal V4.5A TA instrument to observe the alterations in thermal properties. The schematic of experimental system is shown in Fig. 1. The feed used in these experiments contained 3.4 wt% NaCl solutions (Sigma Aldrich). This was pumped through the module using a Master flex 7519-10 peristaltic pump. The preheated hot feed solution travelled through a heat exchanger which was used to maintain the desired temperature throughout the experiment. Dry air was passed into the shell side and the permeate was collected in a trap. Air flow was maintained at 1 l min1. The ionic strength of the original solution, the permeate and the concentrate were measured using a Jenway Electrode Conductivity Meter 4310. Each experiment was repeated 3 times to check the reproducibility and relative standard deviation was less than 1%.

Fig. 1. Schematic diagram of the experimental system

3. Results and discussions 3.1. Characterization of the prepared membranes Scanning electron micrographs of the original membrane and CNIM-f are shown in Fig. 2a and b. The incorporation of the carboxylated CNTs is clearly evident in Fig. 2b. Additionally, Fig. 2c depicts the intactness of CNT–COOH within the membranes after 90 days of continuous usage. The TGA curve is shown in Fig. 3a. As observed, the thermal degradation of unmodified polypropylene membrane started at around 260 °C. However, in line with previous observations, the presence of CNT–COOH increased the degradation temperature by 40 °C. This implies that the CNT–COOH was highly stable and enhanced the thermal stability of the membrane. This is an important factor for MD, where the elevated temperatures can be used for desalination [22]. This data was also supported by differential scanning calorimetry (DSC) and is presented in Fig. 3b. No specific degradation or alterations was observed in the CNIM-f. 3.2. Desalination using CNIM-f The CNIM and CNIM-f were tested for MD. The water vapor flux, Jw, across the membrane can be expressed as:

Jw ¼

wp tA


where wp is the total mass of permeate, t is the permeate collection time and A is the membrane surface area. Also, Jw can be denoted as:

J w ¼ kðC f  C p Þ


where k is the mass transfer coefficient, Cf and CP is the water vapor concentration in feed and permeate side. Usually Cp is close to zero, since we utilize dry air as sweep gas. So overall mass transfer coefficient was calculated as:

Jw Cf


As can be observed in Fig. 4a, increasing temperature increased flux for all three-membrane types. Flux at 70 °C using the either CNIM or CNIM-f was higher than what was obtained by the unmodified membrane. Maximum flux reached up to 19.2 kg/m2 h for CNIIM-f membrane and 15.6 kg/m2 h using CNIM. The enhanced performance on CNIM as compared to unmodified membrane has already been studied previously in our group [22]. However, what is unique here is that, the permeate flux was the highest for CNIM-f membrane. Desalination as a function of flow rate is shown in Fig. 4b. It can be observed that increasing flow rate increased permeate flux. As observed, compared to unmodified membrane, CNIM and CNIM-f demonstrated higher flux at all flow rates. At elevated flow rate, there was reduced boundary layer and adsorption–desorption processes were faster. Fig. 4c depicts the effect of varying of feed concentration on permeate flux. It is well known that concentration polarization is more important at higher feed concentration. At higher feed concentration, a more significant boundary layer develops next to the membrane interface and this reduces driving force of mass transfer. This leads to the decrease in permeate flux in case of unmodified membrane modules. On the other hand, in case of CNIM, and CNIM-f, the flux remained unchanged. The presence of CNTs increased the surface roughness that prevented the formation of stable boundary layers. As observed from Fig. 4c, for CNIM-F, the flux remained constant with increasing salt concentration, reaching up to 19.2 kg/m2 h.

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Fig. 2. (a) SEM images of the original membrane, (b) CNIM-f, and (c) CNIM-f after 90 days of operation.

Incorporation of various weights of CNT loadings/cm2 of membrane area was also investigated. An optimum value of 0.005 mg per centimeter square loadings of MWCNT was required to enhance the overall percent removal and flux. A further increase of CNT loading (0.008 mg per centimeter square) did not showed any further enhancement. It was estimated that significantly higher MWCNT amount would block the pores of the hydrophobic membrane, thereby reducing flux and removal efficiency. Additionally, as observed from Table 1, the mass transfer coefficients enhancements were found to be significantly higher for CNIM-f as compared to the unmodified membrane. Enhancement for CNIM ranged between 50% and 77%. However, for CNIM-f, enhancement ranged from 95% to 116%. Table 2 indicates the effect of feed flow rate on mass transfer coefficients. As observed, the overall mass transfer coefficient was enhanced by presence of CNIM-f. Interestingly, the enhancement in mass transfer coefficient was higher at a low flow rate. At a flow rate of 10 ml/min, the mass transfer coefficient of the CNIM-f was 145% higher than the unmodified membrane, whereas for CNIM enhancement was just 56% but the corresponding values dropped to 27% and 59% when inlet feed flow rate was 24 ml/min. In general, the presence of the CNT–COOH led to enhanced permeability through the membrane, and the CNIM-f showed a significantly higher overall mass transfer coefficient. An important observation from Fig. 5 is that, whereas an increase in feed concentration decreased k for the unmodified membrane, but remained almost constant and showed negligible decease for CNIM and CNIM-f. At 34,000 mg L1, the mass transfer coefficient was more than double for CNIM-f than the plain membrane, which was significantly higher than what was previously as reported [22]. 3.3. Salt breakthrough and stability of the CNIM and CNIM-f

Fig. 3. Thermo gravimetric analysis of unmodified membrane, CNIM, CNIM-f; (b) Differential Scanning Colorimetry of unmodified membrane, CNIM, CNIM-f.

There was no observable salt breakthrough in any of the experiments, and the permeate showed low conductivity of 1– 2.5 ls/cm at 20 °C, implying that the water had over 99.9% purity.

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unmo dified CNIM -f CNIM

Mass transfer coefficientX10 7(kg/m 2.s.Pa)


0.7 unmodified




0.3 4000




Feed concentration (ppm) Fig. 5. Effect of feed concentration on mass transfer coefficient at a feed flow rate of 20 ml min1, 90 °C.


(c) unmo dified


Fig. 6. Operational period stability study of CNIM and CNIM-f membrane. CNIM

Membrane Enhanced adsorption and fast transport by polarpolar interaction by COOH on CNT surface Fig. 4. (a) Effect of temperature on permeate flux at a feed flow rate of 20 ml min1; (b) effect of flow rate on permeate flux at 90 °C and (c) effect of feed concentration on permeate flux at a feed flow rate of 20 ml min1.

Feed solution

Activated diffusion via adsorption desorption on CNT surface

Table 1 Mass transfer coefficient and enhancement% at various feed temperature at 20 ml/ min. Temp(°C)

70 80 90

Mass transfer coefficient  107 (kg/m2 s Pa)

Enhancement (%)






0.499 0.469 0.349

0.856 0.704 0.618

1.07 0.915 0.753

72 50 77

114 95 116

Sweep Direct air permeation through membrane pores

Enhanced Hydrophobic effect by PVDF

Table 2 Mass transfer coefficient and enhancement% at various feed flow rate at 90 °C. Flow rate (ml/min)

10 20 24

Mass transfer coefficient  107 (kg/ m2 s Pa)

Enhancement (%)






0.285 0.349 0.5

0.444 0.618 0.634

0.697 0.753 0.793

56 77 27

145 116 59

Water molecule

Water Vapor molecule

CNT with PVDF surface cover

Fig. 7. Mechanism of action on CNIM-f.

The stability of the membrane, especially the ability to retain the CNT coating on the surface was tested for long-term operation. A test was carried out for 90 days and there was no observable

decrease in flux over this period of time using either CNIM or CNIM-f. This is shown in Fig. 6. The SEM images of CNIM-f after 90 days of operation also did not show any visible signs of CNT erosion or damage.

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4. Mechanism Due to a combination of factors mentioned above, significantly higher flux was observed for CNIM and CNIM-f as compared to conventional membrane. This was attributed to the fact that the CNTs serves as sorbent sites for vapor transport while rejecting the liquid water [22]. The carboxylated CNTs are polar and they provided higher sorption for the water vapors than unfunctionalized CNTs, thus enhancing flux (Fig. 7). Under normal circumstances one would expect the hydrophilic CNT–COOH to decrease the overall hydrophobicity of the membrane and also interact with the sodium ions. Therefore, one would expect the performance of CNIM-f to be lower than CNIM. However, since PVDF dispersion was used to immobilize the CNT–COOH, the former encapsulated the latter, which prevented water as well as Na+ ions from reaching the nanotubes. On the other hand, the water vapors that permeated through the PVDF surface was able to partition on the CNT-f and effectively permeate through the membrane. 5. Conclusions Carboxylated CNTs were incorporated into CNIM to enhance pure water flux in membrane distillation. With the incorporation of CNTs, the desalination performance was consistently higher than the conventional membrane. The carboxylated CNTs showed higher performance than their unfunctionalized analogs. The permeate flux achieved up to a maximum of 19.2 kg/m2 h and salt reduction higher than 99% in all cases. These results indicate that the incorporation of carboxylated CNTs favorably altered the water-membrane interactions to enhance vapor permeability while preventing liquid penetration into the membrane pores. The membranes were stable over long periods of operation without any salt leakage. Acknowledgement The authors wish to acknowledge the support of Mr. Anthony Mancusi from Membrana Charlotte for their contribution of X30240 membrane material. References [1] A.M. Delgado-Torres, L.G. Rodriguez, Status of solar thermal-driven reverse osmosis desalination, Desalination 216 (2007) 242–251. [2] A.D. Khawaji, I.K. Kutubkhana, J.M. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69. [3] B. Peñate, L.G. Rodríguez, Current trends and future prospects in the design of seawater reverse osmosis desalination technology, Desalination 284 (2012) 1–8.


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