Efficacy of piezoelectric electrospun nanofiber membrane for water treatment
Graphical abstract
Introduction
Membrane technologies for drinking water production and wastewater treatment have attracted much attention due to various advantages such as their relatively low operational costs, high efficiency, and low footprint [1]. However, membrane technologies have suffered from the issue of fouling. As fouling is a phenomenon involving the adsorption and accumulation of contaminants, it decreases the working life of the membrane, decreases the membrane performance efficiency, and increases operating costs [2]. In order to solve this problem, many groups have researched various methods to counteract fouling, such as pre-treatment (coagulation [3], adsorption [4]), modification of the membrane (functionalization [5], blending [6], [7]), cleaning [8], UV irradiation [9] and control of the operating conditions [10], [11]. In addition to these methods, a technique involving fluid instability has also been reported [12], [13], [14], [15], [16], [17]. This technique reduces the attachment of contaminants to the membrane through turbulence flow, which is generated by sonication [12], [13], and through generating a piezoelectric response [14], [15], [16], [17] on the membrane surface. The former method generates turbulence flow via an external ultrasonic wave [12], [18]. However, the latter method produces instable fluid due to the internal contrast between piezoelectric materials [14].
Piezoelectric membranes have previously been introduced, with one type consisting of a flat sheet commercial polyvinylidene fluoride (PVDF) membrane [14], [15], [16] and the other a lead zirconate titanate (PZT) ceramic membrane [17], which both have piezoelectric properties. When the piezoelectric response was enacted during the filtration process, the membranes exhibited a lower flux decline than when no piezoelectric response was present. The membranes’ piezoelectric properties were activated by poling—a technique involving polarizing the PVDF and PZT molecules in the same direction by applying a high voltage, after the membranes had been fabricated by a conventional method. The electrospinning technique can fabricate a piezoelectric membrane [19]. It can be used to simultaneously prepare the membrane and polarize the membrane, because a high voltage is applied during the membrane fabrication process [19]. In addition to this advantage, electrospun nanofiber membranes have been reported to have a variety of advantages such as a high surface area and high porosity [20], [21]. Despite of its huge potentials of ENM as water treatment membrane, critical drawbacks have been remained for field application of ENM due to its mechanical, geometrical properties. One of the drawbacks of the electrospun nanofibers for use in liquid filtration applications is that membrane cleaning by reverse flow was very difficult. When high-pressure reverse fluid is introduced to the ENM’s bottom, the ENM was delaminated and possibly unusable. Other drawback of ENM was weakness for membrane fouling. Generally, when roughness of membrane surface was high, possibility of membrane fouling is increased. The ENM has very rough surface morphology due to its nonwoven structure, so real field applicability of ENM is decreased. Maeng report that significant short-term fouling occurred when MBR was used with the electrospun nanofibrous membrane [22]. To overcome this limitation, we suggest the piezoelectric electrospun nanofiber membrane to minimize the foulant attachment on the membrane surface in this research. To the best of the authors’ knowledge, a piezoelectric electrospun nanofiber membrane (pENM) for water-treatment applications has not yet been reported.
In this study, a PVDF nanofiber membrane was prepared for a piezoelectric microfiltration (MF) membrane application. The synthetic conditions (solvent ratio, tip-to-collector distance (TCD), and heat treatment time) were controlled to fabricate an optimized pENM. The performance measurements for the pENM were evaluated such as its water permeability, particulate rejection, and antifouling properties in the presence of organic matter.
Section snippets
Materials
PVDF (average molecular weight (Mw) ∼275,000 by GPC), N-methyl-2-pyrrolidone (NMP, anhydrous, 99.5%), acetone (ACS reagent, >99.5%), kaolin, and alginic acid sodium salt were purchased from Sigma-Aldrich. Ethyl alcohol (99.5%) was obtained from SAMCHUN Chemicals in Korea. De-ionized water was purified through a water-purification system (Millipore, 18.2 MΩ cm, USA). All the reagents were used without further purification.
Electrospinning
The 22 wt% PVDF solutions were prepared at various solvent ratios
Solvent ratio effect on phase of nanofiber membrane
The morphology of the PVDF nanofibers is shown in Fig. 2. As shown Fig. 2(f, g), uniform nanofibers were not formed when NMP was both 8 and 9 in the acetone/NMP ratio. This result was caused by low solvent volatility. NMP has very low solvent volatility.
When the polymer solution was stretched from the needle to the collector, NMP was not completely volatilized [23]. Thus, nanofibers with beads were formed. For the nanofibers to have a diameter of 724 and 682 nm, a solvent ratio of 7/3 and 6/4
Conclusions
Membrane technology has been recognized as one of the most important technologies in water treatment. However, this technology is challenged with the fouling phenomenon which causes higher operating pressure, flux decline, frequent replacement of membranes and eventually higher operating costs. Various methods have been used also to reduce the deposition of contaminant materials on the surface of the membranes and to maintain the flux.
Among the various method to reduce the membrane fouling
Acknowledgements
This research was supported by the R&D Program of the Society of the National Research Foundation (NRF) (Grant Nos.: NRF-2015R1A2A2A10027866 and 2015K000377).
This Research has been performed as a project No KK1602-B11 (Development of key membranes for high efficiency seawater desalination) and supported by the Korea Research Institute of Chemical Technology (KRICT).
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Equally contribution: Jiyeol Bae, Inchan Baek.