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.
무기계열의 세라믹막은 강한 내구성을 지니며, 기존의 한외여과막 수준의 표면 공극 크기를 나노여과막 수준으로 개질할 경우 저분자량 물질의 분리에 효과적이다. 본 연구에서는 알루미나-지르코니아 나노물질을 제조하였고, 새로운 여과코팅 방법으로 세라믹막 표면을 나노여과막 수준으로 개질을 하였다. SEM-EDX을 통해 나노입자가 막 표면을 도포한 것을 확인하였으며, 막 단면에는 투과된 나노물질에 의해 약 15 μm의 코팅 층이 형성된 것을 확인 하였다. 제조된 세라믹 나노여과막의 성능 평가 결과, 1,000 Da의 분획분자량, 58%와 92%의 염(CaCl2) 과 Suwannee river 자연유기물 제거율을 각각 나타내었다. 이를 통해 여과코팅 방법을 통해 세라믹 나노여과막이 성공적으로 제조되었음을 확인하였다. 또한, 나노입자의 크기가 약 40 nm 에서 100 nm까지 커짐에 따라 분획분자량 또한 약 1,000 Da 에서 5,000 Da 까지 증가하는 것을 확인하였으며, 나노입자크기와 제조된 세라믹 나노여과막의 분획분자량간의 상관관계를 Sphere-packing theory 와 Hagen-Poiseuille equation 이론으로 설명하였다.최종적으로 제조된 세라믹 나노여과막의 방사선물질 제거 평가로 우라늄(Uranyl nitrate hexahydrate) 2 mg/L 수용액을 사용하였고, ICP-MS 분석결과 실제 지하수 조건과 유사한 pH 7.4 조건에서 최대 90% 이상의 제거율을 확인하였다.결론적으로, 새롭게 개발된 여과코팅 방법을 통해 세라믹 나노여과막을 성공적으로 제조하였고, 나노입자 크기를 조절함으로써 분획분자량 또한 조절할 수 있음을 본 연구에서는 이론적으로 설명하였다. 또한 고분자 나노여과막이 사용되기 어려운 극한 조건에서 세라믹 나노여과막이 적용가능할 것으로 사료된다.