Novel preparation method of ceramic nanofiltration (NF) membrane for tunable molecular weight cut-off (MWCO)

Abstract

Nanofiltration (NF) membranes have been applied in the advanced water treatment process to remove micropollutants causing disinfection by-products, tastes & odors, bacterial antibiotic resistances, etc. Although inorganic NF membranes (specifically, ceramic membranes) possessed superior properties including physical strength, chemical durability, lower fouling potentials than those of polymeric membranes, preparation methods including dip-, spin-, and spray-coating have been failed to produce reproducible ceramic NF membranes with a precisely controlled pore sizes.

In this study, we suggested a simple and novel filtration-coating method as the preparation of ceramic NF membranes, which variously sized nanoparticles penetrated and filled previously formed membrane pores to form nano-sized pores, and examined potential applications for the separation of micropollutants and radioactive chemicals.

To achieve this, the alumina-zirconia (Al-Zr) nanoparticles ranged from 40 nm to 100 nm in diameter were prepared and filled pores of pre-casted ceramic ultrafiltration (UF) membranes via the application of trans-membrane pressure (TMP) and cross-flow velocity (CFV) to prevent the formation of irregular cake layer and crack on UF membrane surfaces. As a result, the pore size of ceramic NF membrane was successfully reduced from 303.2±118.3 nm to 4.3±0.7 nm with selective nanoparticle layer on and inside of UF membrane surface. The prepared ceramic NF membrane showed excellent performance with around 1,000 Da of molecular weight cut-off (MWCO), 58% of CaCl2 rejection, and 92% of Suwannee river-natural organic matter (SR-NOM) rejection.

Next, the method to control the MWCO of ceramic NF membranes was proposed and verified by the filtration of variously sized Al-Zr nanoparticles. The average sizes of the prepared nanoparticles were changed 42.3 nm, 60.8 nm, 69.3 nm, and 101.3 nm by changing pH and the concentration of surfactant during the preparation of Al-Zr nanoparticles. Respectively, the selective ceramic NF membranes, which have around 1,000 Da, 2,000 Da, 2,000 Da, and 5,000 Da of MWCO with narrow distribution of pore size, were successfully prepared after the filtration-coating process.

In the third section, sphere-packing theory and Hagen-Poiseuille equation were applied to theoretically explain the relation between the size of nanoparticles and MWCO. All MWCO results are located in the theoretical range obtained from minimum (0.155 for tetrahedral site) and maximum void ratio (0.414 for hexagonal site) of packed nanoparticles. It implied that the pore size of ceramic NF membranes can be precisely controlled by preparing desired size of nanoparticles. Furthermore, the packed structure of nanoparticles inside of the ceramic membrane surface could be explained by the theory of multi-sized sphere packing, which has minimum porosity of 24.0% in the experimental condition. It implies that dense random packing of the Al-Zr nanoparticles in the ceramic UF membrane pores occurred due to TMP during the filtration-coating process.

Finally, the prepared ceramic NF membrane was successfully applied for the removal of radioactive uranium (U) species in ground water. The removal mechanism of uranium species by the ceramic NF membrane was explained by size exclusion and Donnan effect.In conclusion, the novel filtration coating method can enable the precise pore control and provide tunable MWCO to ceramic NF membranes by preparing various size of nanoparticles, thus be applied for the removal of various micropollutants, and for the separation and concentration of low molecular weight compounds.