Ceramic membranes activation via piranha reagent– A facile way for significant enhancement in membrane performance

Abstract

A simple, green, and highly effective method for the performance improvement of titania membranes applied for vacuum membrane distillation (VMD) was developed. The activation step turned the hydrophilic material into superhydrophilic (CA ca. 5°) and superaerophobic (underwater CA 160°) owing to the abundance of hydroxyl moieties. Such pretreatment boosted the functionalization efficiency with silane-based modifiers by 20% generating highly hydrophobic membranes (104.1°–148.2°) suitable for membrane distillation. The in-depth studies showing the meaning of modifier type (number of carbon atoms in the alkyl chain, presence of fluorine, number of reactive groups) on membrane performance and physiochemical membrane features (surface charge, wettability, critical surface tension, spreading pressure) were accomplished. All materials were successfully applied in vacuum membrane distillation for the removal of hazardous VOCs (methyl-tert-butyl ether - MTBE and ethyl acetate -EtAc). A significant improvement in separation (separation factor β increased up to 2.5 times) and transport (process separation index PSI was improved up to 8 times ) was noticed owing to membrane activation, particularly for samples modified with nonfluorinated molecules and FC6Cl3. The best performance was found for FC6Cl3 and C6Cl3. Owing to the formation of semi-dense membranes (pore size of ca. 2 nm) after the functionalization process, it was revealed that the mechanism of separation is a solution-diffusion one. The observed significant differences in the McCabe-Thiele diagram (separation efficiency) additionally confirmed the type of mechanism controlling separation and ensured that grafted nanolayer is taking an active part in the separation process. Combined with good stability (long-lasting test of up to 4 years), the results have proven the technical feasibility of applying piranha pretreated membranes for highly effective VOCs removal by vacuum membrane distillation.