Fabrication of Superhydrophobic and Omniphobic Membrane via Dual Electrospinning and Electrospray Technique for Membrane Distillation Application

採用靜電紡絲與靜電塗噴兩種技術製備超疏水及疏油膜並應用於膜蒸餾

Student thesis: Doctoral Thesis

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Award date12 Aug 2019

Abstract

The availability of safe and potable water is vitally important for human health, whether it is for drinking, domestic usage, food production, or recreational purposes. Water stress has become a serious global issue due to the rapid increase in population, global warming, and climate change. In addressing this vital global question, membrane-based desalination technology has been playing a significant role in the cognizance of pursuing sustainability. Desalination using the seawater reverse osmosis (SWRO) process, for instance, has been adopted by many countries around the world for producing drinking water. However, despite its wide use, SWRO has raised new concerns due to its daily global generation of 142 million cubic metres of hypersaline brine, calling for reconsiderations over the economic aspects of the mounting brine issue generated from SWRO plants. Compared to SWRO, the membrane distillation (MD) process, especially when coupled with industrial exhaust stream or waste heat, is one of the novel technology for the economical production of high quality water as well as for the recovery of other valuable resources. MD technology is not without its limitations, which hinder its broader application. The major limitations of the MD process include membrane fouling and wetting issues and low water flux when treating certain types of feeds. To expand MD’s practical applicability, the development of a sustainable membrane with anti-wetting, anti-fouling properties and capabilities for high-water flux is essential. Thus, this work utilizes electrospinning and electrospray technologies to fabricate superhydrophobic membranes and tests the robustness of fabricated membranes in lab-scale direct contact membrane distillation (DCMD) with various feed water conditions. 

Firstly, the surface of poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) membrane fabricated by electrospinning method was further modified by electrospraying low surface energy (∼23.5 mN m-1) poly (dimethylsiloxane) (PDMS) polymeric microspheres to lower the membrane’s overall surface energy. The hierarchical microspheres were achieved without employing any nano/micro-composites as the tetrahydrofuran (THF) solvent evaporated and separated from condensed water vapor. The resulted lab-made membrane achieved a high water contact angle (CA) of 156.9° and a low contact-angle hysteresis of 11.3°. During the DCMD experiment with a low surface tension saline solution made of sodium dodecyl sulfate (SDS; surface tension 28.9 mN m-1) and sodium chloride (NaCl), the fabricated membrane showed high anti-wetting compared to the commercial polyvinylidene fluoride (C-PVDF) membrane used as reference. The dual-layer membrane whose surface was modified by PDMS showed smaller pore sizes and a higher liquid entry pressure (LEP) compared to a single-layer PVDF-HFP membrane. Although SDS concentration of the feed was increased by 0.1 mM over the 4 h DCMD operation, the proposed dual-layer membrane exhibited stable desalination whereas the PVDF-HFP membrane suffered partial wetting with both 0.1 and 0.2 mM SDS concentrations. Furthermore, high desalination performance was recorded when treating low surface tension water (0.3 mM SDS with 3.5 % NaCl) with the optimized surface-modified PDMS dual-layer membrane. 

Next, we report the enhancement of membrane hydrophobicity (superhydrophobic) and surface roughness with re-entrant structures (convex textures) using green polymers silica aerogel to maintain a stable Cassie-Baxter state and prevent partial/full wetting (Wenzel state) and fouling. The optimized surface modification with aerogel/PDMS, i.e. 30% aerogel with PDMS, over the PVDF-HFP bottom layer (E-M3-A30) enabled high flux and anti-wetting with low saline surface tension feed water (0.5 mM SDS with 3.5% NaCl) compared to the control C-PVDF membrane, the PVDF-HFP membrane, and the PDMS modified-only PVDF-HFP dual-layer membrane. In terms of fouling, the E-M3-A30 membrane showed significant resistance against synthetic algal organic matter (AOM). Moreover, the E-M3-A30 membrane achieved a water CA of 170˚, a high liquid entry pressure (LEP) of 129.5 ± 3.4 kPa, vibrant short water droplet bouncing performance (11.6 ms), high surface roughness (Ra) of 5.04 µm, and low surface energy (4.13 ± 0.02 mN m-1), which fit into the key profile of an ideal MD membrane. 

Further, we report the fabrication of an omniphobic membrane by electrospraying fluorinated zinc oxide (ZnO) nanoparticles (NPs) mixed with PVDF-HFP on the surface of an organosilane-functionalized PVDF membrane. Our results revealed that the functionalized ZnO NPs membrane exhibited a rough hierarchical re-entrant morphology with a low surface energy, thereby achieving high omniphobic characteristics. The addition of 30% ZnO (w/w of PVDF-HFP) was found to be the optimal concentration of ZnO NP incorporation, upon which the membrane gained a high repulsive characteristic. The optimized C-PVDF/ZnO(30)/FAS/PVDF-HFP membrane, referred as the cPFP-30Z membrane, exhibited high contact angle values of 159.0 ± 3.1˚, 129.6 ± 2.2˚, 130.4 ± 4.1˚, and 126.1 ± 1.2˚ for water, SDS saline solution (0.3 mM in 3.5% NaCl), ethanol, and vegetable oil, respectively. The low surface energy and high surface roughness (Ra) of the optimised membrane was assessed as 0.78 ± 0.14 mN m-1 and 1.37 µm, respectively. Additionally, in contrast to the commercial C-PVDF membrane, the cPFP-30Z membrane exhibited superior anti-wetting/anti-fouling characteristics and high salt rejection performance (>99%) when treating saline oil (0.015 v/v) and SDS (0.4 mM) feed solutions. 

Finally, a robust dual-layer omniphobic membrane was fabricated by adopting the electrospinning/electrospray technique followed by organosilane-functionalisation and ZnO NPs grafting to form re-entrant structures. PVDF-HFP was adopted to fabricate the nanofiber bottom layer substrate, which was thereafter crosslinked with FAS. Grafting with fluorinated ZnO NPs resulted in the successful formation of hierarchical micro/nano re-entrant structures with hillock valley-like rough structures. The optimised 25% ZnO membrane, referred as the ePFP-25Z membrane, achieved superhydrophobicity (water contact angle (CA) > 161˚) as well as omniphobic characteristics (e.g., vegetable oil and ethanol CAs were 131.5 ± 1.8˚ and 131 ± 2.9˚, respectively), low surface energy (0.75 ± 0.43 mN m-1), and high surface roughness (3.26 µm). The dual-layer electrospun membrane without any surface modification, referred as the E-PVDF-HFP membrane and used as a reference for comparison, faced serious instant wetting with ethanol and oil. Notably, the dual-layer omniphobic membrane presented stable anti-wetting and stable water flux with low surface tension synthetic brine feed composed of 0.5 mM SDS and 1M NaCl, whereas the E-PVDF-HFP membrane suffered with severe partial wetting and exhibited poor water permeate quality with the 0.1 mM SDS feed. These results evidenced that the lab-made electrosprayed omniphobic membrane possessed high anti-wetting features along with a high water production rate under various feed water conditions. 

As a whole, the research presented in this dissertation presents an attempt to fabricate electrospun membranes with the ability to overcome the major limitations of MD technology such as wetting, fouling, and low water flux. The superhydrophobic membrane fabricated using electrospinning/electrospray technology and low surface energy material and the further modifications on the membranes through the application of aerogel and ZnO with FAS unlocked the strong potential of electrospun MD membranes to withstand low surface tension saline liquids including oily wastewater, thereby opening new windows for adopting the MD process in practical and industrial water and wastewater treatment.

    Research areas

  • Cassie-Baxter state, Nanoparticles, Re-entrant structure, Brinewater, Omniphobic, Superhydrophobic, membrane distillation