Preparation Of PVDF/SiO₂Composite Ultrafiltration Membranes And Its Application In The Separation Of Chinese Medicine Volatile Oil

Polyvinylidene fluoride (PVDF) has been widely used as a separation membrane material because it has unique properties such as thermal stability, radiation resistance, abrasion resistance, as well as excellent chemical resistance in acids, alkaline, strong oxidants, and halogens. PVDF membranes have been used extensively in many water treatments such as oily wastewater and potable water. However, its severe membrane fouling and decline of permeability due to its hydrophobic nature, has been a barrier to its application in water treatment. Many studies have been attempted to improve the performance of PVDF membranes using various techniques, including chemical grafting, physical blending, and surface modifications. Among those methods, organic-inorganic composite membranes prepared by blending method and sol-gel method have super performances, because organic polymers have typical advantages of flexibility, whereas ceramics have optical properties, such as surface strength, heat resistance, and chemical stability, the combination of organic polymers and ceramics can improve some separation properties of membranes.In this paper, organic-inorganic PVDF/SiO2composite membranes were prepared by so-gel method and blending method, respectively. PVDF/SiO2composite membranes prepared by so-gel method and blending method were compared in terms of hydrophilicity, permeation performance, rejection, porosity, mean pore size and separating performance of Foeniculum vulgare volatile oil and Caryophyllix flos volatile oil.In the blending experiments, unmodified, hydrophilic and hydrophobic nano-sized SiO2were added into the PVDF casting solution respectively, and PVDF/SiO2hybrid membranes were prepared by phase inversion method. The effects of SiO2concentration and the above-mentioned three types of SiO2on the PVDF membrane performances such as pure water flux, rejection ratio, anti-fouling capability were examined. The experimental results indicated that membrane porosity, mean membrane pore size, pure water flux, rejection ration, anti-fouling capability increased with rising SiO2concentration firstly and then decreased. Besides, when the SiO2concentration was2%, in terms of pure water flux, unmodified nano-sized SiO2hybrid membrane was the best, followed by hydrophobic nano-sized SiO2hybrid membrane and hydrophilic nano-sized SiO2hybrid membrane. Inversely, in terms of anti-fouling capability, hydrophilic nano-sized SiO2hybrid membrane was the optimal, followed by unmodified nano-sized SiO2hybrid membrane and hydrophobic nano-sized SiO2hybrid membrane.In the so-gel experiments, different amount of TEOS were added to PVDF casting solution, the membranes were characterized in terms of FI-IR, XRD, thermal properties, mechanical properties, hydrophilicity, permeation performance, rejection, porosity, and mean pore size. The experimental results indicated that hydrophilicity, permeation, porosity and mean pore size increased with increasing TEOS content; however, high TEOS concentration had negative effect on those properties. In particular, FI-IR and XRD indicated that with increasing SiO2content, the crystalline structure of PVDF changed from β-phase to α-phase when TEOS content surpassed20%. Moreover, Compared PVDF/SiO2composite membranes prepared by so-gel method with blending method, the former had better performances on permeation and flux recovery.PVDF/SiO2composite membranes prepared by so-gel method and blending method were used to separate Foeniculum vulgare volatile oil and Caryophyllix flos volatile oil. By using oil flux and COD removal rate as evaluation indicators, the separation performances of above membranes were compared. The results showed that membranes prepared by blending method had higher COD removal rate than membranes prepared by sol-gel method, but had lower oil flux.