Superoleophobic and superhydrophilic nanocomposite coated hollow fiber ceramic membrane for treatment of oily wastewater

Abstract

Robust and efficient separation of emulsified oil and water mixtures is practically required as it still remains a global challenge. Existing conventional methods of oil-water separation using ceramic membranes still suffer some setbacks from membrane brittleness and fouling due to their innate inflexible ionic and covalent bonding and microfiltration pore size. Simultaneous superoleophobic and superhydrophilic surfaces are desired for various applications where emulsified oil separation is required. This study focused on improving the mechanical strength of the bare membrane (BM), addressing the prevailing fouling of membranes, and interactive effects of material properties and operating variables of the system. In the first stage, BM was successfully fabricated using phase inversion and sintering technique from kaolin-based ceramic material at different bore fluid flow rates (12.0-15.0 mL/min), kaolin content loadings (38-42 wt.%), and sintering temperatures (1300-1400 oC) and was examined for physicochemical properties. BM fabricated at 42 wt.% kaolin loading, 15 mL/min bore fluid flow rate and 1400 oC sintering temperature showed the best mechanical strength of 126.00 MPa, an average pore size of 3.50 µm, and pure water flux of 108.57 L/m2.h. The result suggested an improvement in the mechanical strength and that BM was within range of microfiltration application. Due to its pore structures, it was prone to membrane fouling for oil-water separation. In the second stage, BM was modified through a simple dip-coating process with poly(diallyl dimethylammonium chloride)-alumina-perfluorooctanoic acid (PAP) and poly(diallyl dimethylammonium chloride)-perfluorooctanoic acid (PP) as interfacial materials. The interfacial materials were prepared using sol-gel method. Effects of coating layer cycles on BM were examined for surface wettability. Physicochemical properties of the PAP-BM and PP-BM including the surface morphologies and topologies were also studied. Highest average roughness, Ra of 1.042 was obtained for PAP-BM while BM with Ra of 0.665 showed the lowest. Oil contact angle (OCA) of PAP-BM was found at 155o while water contact angle (WCA) displayed time dependence mode as values decreased from 25o to 5o after 10 min of penetration. The third stage involved the performance evaluation using statistical software Design-Expert 7.0.0 based on response surface methodology. Three independent parameters (feed concentration, pH, and pressure) were examined for interactive effects on the two dependent parameters (oil rejection and water flux). Prior to that, the results of the antifouling test showed the total fouling resistance of ~66% and ~73%, reversible fouling resistance of ~52% and ~26% and irreversible fouling resistance of ~14% and ~48% for PAP-BM and BM, respectively. The ANOVA results revealed that the optimum conditions of the emulsified oily wastewater separation were obtained to be 600 ppm, 3.0 bar, and 9.0 for feed concentration, pressure and pH, respectively. Under these conditions, 174.35 L/m2.h of water flux and 98.63% of oil rejection were achieved experimentally. Oil rejection and water flux were found to be primarily affected by the feed concentration, pressure and pH. Hence, PAP-BM can tolerate and display intermittent superoleophobic and superhydrophilic surfaces, which was further evaluated based on its performance in a cross-flow filtration system.