Fabrication Of A Silicate Composite Based Microfiltration Membrane And Its Application In Polluted Water Treatment

Water pollution is becoming a much severer issue. Traditional drinking water treatment process has been unable to meet the requirement of water quality. Therefore, researchers has paid more attention on some new drinking water treatment processes, such as membrane filtration and advanced oxidation process. However, the membrane filtration technology and the advanced oxidation process are not suitable for large-scale water treatment processes because of their high cost. The cost of membrane and catalyzer in these technologies are critical factors. Thus, developing some novel membranes and catalyzers with high efficiency and low cost is necessary.A new plate silicate composite based microfiltration membrane was fabricated using low-cost cement and quartz under room temperature. Multiple membrane fabrication parameters were studied and an optimum condition was screened out. Moreover, pore formation mechanisms of the membrane was confirmed using scanning electron microscope(SEM), pore size analysis and energy dispersive X-Ray spectroscopy(EDX). After that, the membrane was characterized from the point of membrane structure, membrane composition and application performance. Also, the fabricated membrane was used to catalyze dissolved ozone and degrade p-chloronitrobenzene(p-CNB) in aqueous conditions. In this process, the p-CNB degradation efficiency and mechanisms were studied.By a deep analysis of membrane fabricaiton processes, some membrane fabrication parameters were fixed. For example, 6 MPa of membrane forming pressure, 40.6-50.0 μm of quartz particle size, 14.6 μm of cement particle size, 0.4 of water-to-cement ratio, 12 days of membrane curing under 20℃ and 95% of relative humidity. Based on above conditions, a silicate composite based microfiltration membrane could be produced with 2.3 μm of mean pore size, 34% of porosity, 13 m3?m-2?h-1?bar-1 of water flux and 4 MPa of bending strength. However, it must be pointed out that membrane pore size distribution was imperfect under this conditions. The fabricated membrane just showed a bimodal pore distribution.By decreasing quartz-to-cement ratios(q/c) to 2.0 in the membrane fabrication process, the membrane pore size distribution was successfully optimized from a bimodal distribution to a unimodal distribution. Based on above pore size changement, pore formation mechanisms of the membrane was confirmed. In the membrane, three types of pores(I, II and III) were formed:(1) The formation of type I pores(with 7-8μm of pore size) was mainly attributed to the stacking of quartz particles;(2) The formation of type II pores was mainly attributed to the stacking of cement particles. Type II pores occupied most of pores in the membrane and mostly had a pore size of 1-3 μm;(3) Type III pores were less prevalent than type II pores and mostly had a pore size of below 1 μm. The formation of type III pores was attributed to the division of bigger pores by the thin needle-like ettringite. In the membrane producing process, using 2.0 of q/c not only optimized the pore size distribution, but also increased membrane bending strength to 5-6 MPa. Moreover, the target membrane had a mean pore size of 1 μm. Membrane porosity and water flux could also be accepted.A deep characterization of the optimized membrane was carried out from the point of membrane structure and performance. By characterizing membrane sructure, polymeric calcium silicate hydrate was proved to be the main composition of multilple cement hydration byproducts, which excited in gel state in the target membrane. Other cement hydration byproducts were also observed, such as plate calcium hydroxide, needle-like ettringite and sheet-like ettringite. A detailed pore analysis confirmed that membrane pores were in two size grade: micron scale pores and nano scale pores. The micron scale pores were dominant in the membrane with a mean pore size of 1μm. The nano scale pores had a mean pore size of 13.5 nm. By analysizing membrane performance, the fabricated membrane was found to obtain an acceptable gas and water flux, compared with traditional ceramic membrane. Membrane rejection study confirmed that more than 80% of Chlorella vulgaris and inorganic particles were stopped by the membrane used. And the removal of Chlorella vulgaris showed better. However, membrane fouling from the Chlorella vulgaris were more difficult to clean, compared to the inorganic fouling. A further study of membrane performance presented that the silicate composite based membrane could bear 500℃ of heat treatment and 0.01 mol/L of alkaline corrosion. It had the ability of operation to ensure water security and membrane stability.The fabricated silicate composite based membrane was futher applied to catalyze dissolved ozone and degrade p-chloronitrobenzene(p-CNB) in aqueous conditions. Results indicated that, the hybrid ozone-membrane process successfully increased the degradation efficiency of p-CNB compared with the ozone-alone process under continuous flow. The ozone-membrane process decomposed 1.5 mg/L of dissolved ozone and increased the p-CNB removal by 50% with little adsorption. The results of electron paramagnetic resonance(EPR) and tert-butanol affection experiments confirmed that p-CNB degradation followed the mechanism of hydroxyl radical oxidation during the ozone-membrane process. The alkaline hydration products and metal oxides appeared on the surface of membrane pores possibly promoted the hydroxyl radical generation and enhanced the p-CNB degradation. The hybrid process was observed to maintain the ability of removal p-CNB efficiently in different water sources. Moreover, The toxicity of the degradation byproducts were also less than p-CNB. Therefore, the membrane used was an low-cost but efficacious catalyzer for ozonation process. And the hybrid ozone-cementitous membrane process could be used as a kind of emergency treatment technology to deal with some sudden organic pollutions in water.