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Advanced multi-nozzle electrospun functionalized titanium dioxide/polyvinylidene fluoride-co-hexafluoropropylene (TiO2/PVDF -HFP) composite membranes for direct contact membrane distillation

Eui-Jong Lee1, Alicia Kyoungjin An1*, Pejman HADI2, Sangho Lee3, Yun Chul Woo4, and Ho Kyong Shon4

1 School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue Kowloon, Hong Kong, China

2 New York State Center for Clean Water Technology, Stony Brook University, NY 11794, USA

3 School of Civil and Environmental Engineering, Kookmin University, Seoul, Korea

4 Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney (UTS), P.O. Box 123, 15 Broadway, NSW 2007, Australia

* Corresponding author. Tel: +(852)-3442-9626, Fax: +(852)-3442-0688, E-mail: alicia.kjan@cityu.edu.hk

Abstract

The unique capabilities of electrospinning technology are being increasingly utilized in the fabrication of hydrophobic membranes to improve the membrane distillation (MD) process in recent years. In this study, hydrophobic titanium dioxide (TiO2) nanoparticles functionalized by fluorosilane were incorporated into electrospun membranes using single, coaxial, and dual nozzles to develop novel membrane architectures for improved physico-chemical properties for MD. By incorporating fluorosilane coated TiO2 into the PVDF-HFP solution during the membrane synthesis and using an advanced multi-nozzle to form various hierarchical membrane structures tuned the size and structure of the nanofibers and made them vastly superior for the application in MD. The single and coaxial nozzle membranes showed contact angles close to 150○ and the dual-nozzle membrane assembled bead-on-string fibers achieved superhydrophobicity (i.e., contact angle of 153.4○). To test the functionalized titanium dioxide/polyvinylidene fluoride-co-hexafluoropropylene (TiO2/ PVDF-HFP) composite membranes for MD performance, the membranes were subjected to long-term direct contact MD for about two days to monitor their water vapor flux and selectivity. Compared to commercial PVDF membranes, all electrospun F-TiO2/ PVDF-HFP membrane achieved higher water vapor flux of 40 Lm-2h-1 (60°C feed and 20°C permeate) with a brine (7.0 wt% NaCl) as the feed solution and also exhibited anti-wetting property while maintaining high water flux compared to the membrane without TiO2 incorporation.

Keywords: Electrospun membrane; Coaxial & dual nozzle; Hydrophobicity; Membrane distillation; Desalination

1. Introduction

Membrane distillation (MD) is based on a thermal gradient applied across a hydrophobic membrane to create a vapor pressure that results in the transportation of vapor molecule through the membrane network [1]. This filtration technique minimizes the contact between the membrane and high salt feed solutions to enable high-quality water production with low-grade energy, as compared to the reverse osmosis (RO) and conventional thermal processes, which makes it an attractive method for seawater desalination and RO brine treatement [2,3]. However, because pure water is obtained via vapor condensation of the water molecules individually transported through the membrane barrier, the effectiveness of the MD process heavily depends on membrane characteristics, such as membrane materials, pore size, porosity, and hydrophobicity. Since commercial membranes fabricated by phase inversion or melting processes are inherently limited to serve as suitable membranes for MD [4], numerous researches have been conducted during the last decade to fabricate appropriate membranes for MD with several researchers reviewing the progresses and limitations so far for full-scale application of MD in the past year [1,2,5–8].

Electrospinning has recently been highlighted as a promising technique for fabricating nanofibrous membranes for MD [9]. In the electrospinning process, various factors including dope solution type, electrospinner operational parameters, and post-treatment conditions can be controlled to tune membrane performance [10]. Nevertheless, as a relatively new approach for membrane fabrication for MD, research has so far been limited to lab-scale MD operation and short operation time, and most fabrications have been conducted using conventional single-nozzle apparatus with polyvinylidene fluoride (PVDF) as the polymer [8]. The utilization of coaxial nozzles for the preparation of core-shell nanofibers, in which the central fiber is surrounded by a concentric annular tube of another polymer, can further expand the application of electrospinning by enabling the transformation of low dielectric constant polymers (such as Teflon) into nanofibers with unique fiber surfaces [11,12]. In addition, the application of dual (or multiple) nozzles can endow benefits from the properties of different types of materials [5] to confer flexibility on the manipulation of the membrane framework and/or surface.

Meanwhile, the incorporation of nanomaterials into the membrane has shown promise in enhancing membrane performance; for instance, carbon nanotube (CNT) for enhancing the mechanical properties of the membrane [13,14] or membrane permeability due to its unique structure [15,16], SiO2 particles for creating excellent dynamic adsorption capacity [17], and TiO2 particles for improving the antifouling property of the membrane [18]. Occasionally, the SiO2 and TiO2 nanoparticles have been used simultaneously for enhanced controllability and synergy effects on membrane properties [19]. Above all, nanoparticles have been used to increase the hydrophobicity via surface deformation in various industries [20,21]. Among inorganic nanoparticles, TiO2 is reported to have potential to improve both the membrane electrospinability due to its electrical conductivity [22] and functionality due to its photocatalytic activity, in addition to its stability and non-toxicity [23]. Hence, it is possible to hypothesize that integrating the electrospinning technique and nanomaterial incorporation will produce tailored MD membranes through which more vapor molecules can pass at a higher rate high hydrophobicity to prevent it from wetting. However, features such as high surface energy, nano-size and anisotropic shape are inherently difficult to be evenly dispersed [24] and thus results in a defective structure (abnormal agglomeration) that may not be in favor of hydrophobic MD process.

In general, fillers are incorporated into composite membranes either through physical blending or the sol-gel method. Between the two, although limited in available fillers, the sol-gel method is beneficial for preventing the agglomeration of fillers. Recent studies by Razmjou et al. [25] and Meng et al. [26] showed that PVDF membranes coated with functionalized TiO2 (based on the sol-gel method) had porous, multi-level structures and superhydrophobic surfaces, which led to improved salt rejection and a substantial reduction in pore wetting. In the case of TiO2-incorporated electrospun membranes [27], while the particles were well-dispersed, the morphology of the fiber surface was not improved significantly. This is because particles were synthesized during fiber formation, resulting in the integration of the particles into the fibers. In another previous study on electrospun membranes incorporating fillers based on the sol-gel method [28], a hierarchical morphology of the fiber surface was observed after calcination at high temperature. Ever-increasing interest in applying nanomaterials in many different fields [29,30] has led to continued efforts to develop dispersion and functionalization techniques, however very limited application of functionalized TiO2 / organic polymer membrane was reported in MD.

In this study, we report a one-step versatile electrospinning technique for incorporating functionalized TiO2 (referred hereafter as F-TiO2) nanoparticles into organic polymer using different types of nozzles (single, coaxial, and dual) that control the microstructure of membrane for MD. TiO2 functionalization using hydroxylated fluorosilane was introduced to guarantee proper dispersion, appropriate interfacial adhesion between the nanomaterials and polymer matrix, and increase the hydrophobicity. This is the first attempt to fabricate F-TiO2 nanocomposite membranes by coaxial and dual nozzle electrospinning techniques to gain high water vapor flux and stable salt rejection performances during long-term direct contact MD (DCMD) operation. Concentrated F-TiO2 nanoparticles were directly added to i) a single-nozzle electrospinning dope solution, ii) shells by coaxial electrospinning to see the effects of F-TiO2 particles on stable nanofiber formation, and iii) dual nozzles, where one nozzle was employed for polymer dope solutions with a high concentration of F-TiO2 particles, while the other one was used to fabricate pure nanofibers by electrospinning. Because adding large amounts of F-TiO2 particles increases the hydrophobicity but can lead to brittle nanofibers with beads, different nozzles were employed to fabricate durable membranes. The properties of all membranes produced by different nozzle types were investigated, and their performances were evaluated in a long-term DCMD operation.

2. Materials and methods

2.1 Materials

Poly(vinylidene fluoride-co-hexafluoropropylene) (referred herein as PVDF-HFP, MW = 455,000 g/mol), lithium chloride (LiCl), N, Ndimethylformamide (DMF), and acetone were purchased from Sigma-Aldrich and were used to make the dope solution for electrospinning. Titanium dioxide (TiO2) powder (particle size: 21 nm) and 1H, 1H, 2H, 2Hperfluorooctyltriethoxysilane (FTES) used for the functionalization of electrospun membranes were also acquired from the same supplier.

2.2 Preparation of functionalized TiO2 nanopar