Enhanced omniphobicity of mullite hollow fiber membrane with organosilane-functionalized TiO2 micro-flowers and nanorods layer deposition for desalination using direct contact membrane distillation

In this study, a novel omniphobic mullite hollow fiber membrane (HFM) was successfully fabricated to reduce the fouling and wetting propensity of direct contact membrane distillation (DCMD). The surface of the mullite hollow fiber membrane was functionalized with flower-like (FL) and rod-like (RL) TiO2 rough layer. Then, surface fluorination was carried out using 1H,1H,2H,2H-perfluorodecyltriethoxysilane (97%) (C8). The modified mullite hollow fiber membrane, which was initially hydrophilic in nature, exhibited high liquid repellency towards water and low surface tension liquids such as ethylene glycol (47.3 mN/m) and olive oil (32 mN/m). 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 omniphobic membrane that was fabricated with TiO2 micro-flowers (C8-FL/TiO2-HFM) was the only membrane that exhibited superomniphobic properties towards ethylene glycol (~150°) and was nearly superomniphobic towards olive oil (~140°). In addition, the formation of air layers was observed on submerged C8-FL/TiO2-HFM and C8-RL/TiO2- HFM, which was proven to significantly reduce organic fouling even after 500 min of DCMD with an aqueous NaCl (3.5%) feed solution containing humic acid (10 mg/L). At the macroscopic level, no significant fouling was observed for the C8-FL/TiO2-HFM. This could be attributed to the hierarchical structure induced by TiO2 micro-flowers, which played a critical role in achieving excellent anti-fouling properties. This has also translated into the high flux stability of the C8-FL/TiO2-HFM. Rise in permeate conductivity was observed for the C8-HFM but not for C8-FL/TiO2-HFM and C8-RL/TiO2- HFM. These results suggest that the fabricated omniphobic mullite hollow fiber membrane with flower-like structure is promising for a robust DCMD process, even for the desalination of real seawater that contains organic contaminants.