Membrane distillation omniphobic mullite hollow fibre membrane with fluoroalkylsilane-functionalised titania deposition

Abstract

Membrane distillation (MD), which is capable of achieving high solute rejection, has recently attracted significant attention for desalination. However, membrane fouling and wetting are still the major challenges in membrane distillation systems. Hence, it is essential to prepare an omniphobic membrane for anti-fouling and anti-wetting performance to overcome the issue. This work suggested a facile fabrication method of omniphobic mullite hollow fibre membrane via a one-step synthesis of growing hierarchical titania (TiO2) particles on the membrane surface through hydrothermal method and followed by surface fluorination. Mullite hollow fibre membrane (HFM) was prepared as a substrate from ball clay using phase inversion and sintering technique. The composition of raw ball clay consisted of 85.9% kaolinite, 9.5% illite, 3.6% quartz and 1% maghemite. The particles of raw ball clay were irregular in shape, and some particles reach dimensions of over 50 µm size. After pre-treatment processes, the particle size of ball clay powder was remarkably reduced to 4.96 µm. The physico-chemical and permeation properties of a membrane were investigated by varying ball clay loading and sintering temperature. After the sintering process, major composition of the hollow fibre membranes was mullite with minor traces of quartz. When the membrane with ball clay loading of 47.5 wt.% was sintered at 1250 °C, its mechanical strength and permeability were comparable to that of membranes fabricated from pure metal oxides. The membrane had an average porosity and pore size of about 50.5 ± 2.1% and 0.61µm, respectively. Subsequently, hydrothermal treatment was carried out at 150°C to acquire re-entrant structures on the hollow fibre membrane’s surface followed by the fluorination with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (C8). The formation of rod-like (RL) and flower-like (FL) TiO2 structures was observed after 5 and 10 hours of hydrothermal process. After surface texturing and fluorination at 48 hours, the hollow fibre membrane, which was initially hydrophilic in nature, exhibited high liquid repellence towards water and low surface tension liquids such as ethylene glycol and olive oil. The order of the membranes in terms of wetting resistance for low surface tension liquids is as follows: C8-FL/TiO2-HFM > C8-RL/TiO2-HFM> C8-HFM. The value of contact angle for water on C8-FL/TiO2-HFM was around 162o, which is among the highest of previously reported contact angle of ceramic membranes in MD system. Also, the membrane exhibited nearly superomniphobic properties towards olive oil, ~140o. In addition, the formation of air layers was observed on submerged C8-FL/TiO2-HFM and C8- RL/TiO2-HFM, which significantly reduced the deposition of organic substances after 500 minutes of MD with an aqueous NaCl (3.5 wt.%) containing humic acid (10 mg/L). A rise in the permeate salt concentration was observed for C8-HFM but not for C8-FL/TiO2-HFM and C8-RL/TiO2-HFM. Moreover, no significant fouling was observed for C8-FL/TiO2-HFM and the membrane exhibited the most stable flux and the highest salt rejection compared to other membranes. These results suggest that the fabricated membrane with micro/nano-roughness from flower-like structures is potential for a robust MD process as compared to other membranes.