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Production of drinking water from saline water by air-gap membrane distillation using polyvinylidene fluoride nanofiber membrane

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Abstract

Polyvinylidene fluoride nanofiber membrane could be used in membrane distillation (MD) to produce drinking water (NaCl concentration <280 ppm) from a saline water of NaCl concentration 6 wt.% by air-gap membrane distillation. This was the first attempt to use electro-spun nanofiber membrane in MD. This new approach may eventually enable the MD process to compete with conventional seawater desalination processes such as distillation and reverse osmosis.

Introduction

As shortage in water supply is becoming a serious concern worldwide, much attention is being paid to the production of drinking water by seawater desalination, particularly in arid regions such as in the Middle East and Singapore. Currently, seawater desalination is conducted by distillation or by reverse osmosis (RO), but new alternative technologies are always sought for and membrane distillation (MD) is one of those alternatives. Furthermore, most of MD research has been conducted by using commercially available microfiltration membranes of hydrophobic nature manufactured by the conventional thermal or dry/wet phase inversion technique. This paper explores the viability of using polyvinylidene fluoride (PVDF) electro-spun nanofiber membrane in air-gap membrane distillation to produce potable water. Up to 22% of saline water was treated with a salt rejection of 98.7–99.9%. The fluxes were as high as or higher than those of commercial PVDF membranes. The initial performance was maintained after 25 days of continuous usage.

MD is a thermally driven process in which a microporous membrane acts as a physical support separating a warm solution from a cooler chamber, which contains either a liquid or a gas. As the process is non-isothermal, vapour molecules migrate through the membrane pores from the high to the low vapour pressure side; that is, from the warmer to the cooler component. MD has been known as an effective desalination process for a long time. More specifically, the first MD patent was obtained by Bodell in 1963 [1] and the first MD publication was made by Findley in 1968 [2]. Another attention on MD was made by Haute and Hendeyckx [3] and Hendeyckx [4], in Europe in the late 1960s, but the technology has not been further developed. By early 1980s the research on membrane distillation became very active and different techniques and configurations of the MD process have been developed; direct contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) and vacuum membrane distillation (VMD) [5], [6], [7], [8], [9], [10], [11], [12]. Interest in MD is still increasing significantly within the academic community.

However, MD has not taken off as a commercially viable process until now, because of the relatively low flux and gradual deterioration of the membrane performance with a prolonged operational period. Hence, development of MD membranes through an entirely novel approach is necessary. Currently, MD is performed by using hydrophobic microfiltration membranes available commercially, such as those made of polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE, Teflon), either in capillary or flat-sheet forms. These membranes were prepared for microfiltration purposes. On the other hand, only a few authors have considered the possibility of novel membranes and membrane module designs for MD [13], [14].

Conversely, electro-spinning is gaining momentum as a viable technique to produce highly porous non-woven nanofibrous membrane. Electro-spinning is a simple and versatile method that creates fibers in the submicron to nano-range through an electrically charged jet of polymer solution/melt. A wide range of polymers and blends can be used to yield nanofibers [15]. Commonly used membrane polymers such as cellulose acetate (CA), polysulfone (PSU) and polyvinylidene fluoride (PVDF) have been successfully electro-spun to form non-woven nanofiber membranes for water filtration. For example, investigations have revealed electro-spun nanofibrous membranes (ENMs) possess high flux rates and low transmembrane pressure [16] and hence making them potentially attractive filters in separation technology. These attractive characteristics are attributable to its (1) high porosity, (2) interconnected open pore structure and (3) tailorable membrane thickness.

In this research, we are demonstrating applicability of electro-spun nanofiber membranes (ENMs) for membrane distillation (MD). It is shown that water of less than 280 ppm can be produced from NaCl solutions up to 6% concentration by air-gap membrane distillation (AGMD). It should be noted that the salinity of seawater is approximately 3.5%.

Section snippets

Experimental

Electro-spun PVDF membranes were prepared using a typical electro-spinning setup (Fig. 1). A 18 wt.% solution of polyvinylidene fluoride (PVDF Kynar761, Elf-Chem USA) in dimethylformamide (DMF) was used as the polymer dope. The polymer solution was electro-spun at a rate of 2 mL/h. Six milliliters of the polymer solution was consumed to make around 0.15 mm thickness of the nanofiber membrane. A high voltage (18 kV) was applied between the needle tip of the spinneret and the collecting metal plate

Results

Fig. 3 shows the SEM image of the surface of the ENF. This resembles the images observed by Nasir et al. [13] for nanofibers prepared from PVDF polymer at different concentrations from 18 to 26 wt.% using N,N-dimethylformamide as solvent. From Fig. 3 the nanofibers possess diameters of about 500 nm, while Nasir et al. [13] reported the diameters of 80–700 nm, depending on the electro-spinning condition.

Fig. 4(A) and (B) show the 2D AFM images at the scan areas 11.5 μm × 11.5 μm (lower magnification)

Conclusions

The PVDF nanofiber membrane can be used in air-gap membrane distillation (AGMD) to produce potable water. The membrane flux is comparable to those obtained by commercial microfiltration membranes (5–28 kg/(m2 h) at temperature differences ranging from 25 to 83 °C). In addition, the membrane was intact and unplugged after many days of operation. Our goal is to optimize the flux by changing the electro-spinning conditions. Currently, there is no theoretical limit for the obtainable flux.

Acknowledgements

The authors wish to acknowledge the financial support of Natural Sciences and Engineering Council of Canada, Spanish Ministry of Science and Education (Project FIS2006-05323), the Middle East Desalination Research Centre and the National University of Singapore.

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