| As a kind of the important membrane materials, Poly(vinylidene fluoride)(PVDF) has been widely used in many fields. Due to its strong hydrophobicity, the application of PVDF is limited significantly. In this experiment, the adsorption behavior of the different modified PVDF membrane as the research objects is studied by quartz crystal microbalance with dissipation(QCM-D) and atomic force microscope(AFM) from the microscopic point of view. Based on the depth analysis of the interaction between the membrane surface and the pollutants, and the corresponding adsorption models are established. The experimental results are as follows:(1) PVDF membrane is modified with organic additives, such as polyvinyl pyrrolidone(PVP), polyvinyl alcohol(PVA), polyethylene glycol(PEG), and bovine serum albumin(BSA) acts as representative pollutants. The hydrophilic angle is controlled in order to obtain different degrees of modified membrane. The result shows that, for different hydrophilic angle, the corresponding change of the adsorption capacity and adsorption layer of the same modified membranes are not the same, with the decrease of hydrophilic angle, the value of frequency of PVDF/PVP membrane decrease significantly, the change value of dissipation become larger, in other words, the decrease of adsorption, the structure of adsorption layer become more loose. But the PVDF/PEG membrane is almost not affected by the hydrophilic angle, and the value of the frequency and the dissipation have no change obviously. It is also found that the adsorption behavior of the modified membrane is not the same as that of the same hydrophilic angle, which showed that the adsorption process of BSA on the membrane surface was related to other properties of the surface except the hydrophilic angle. When the hydrophilic angle is 85 degrees, the adsorption behavior of the PVDF/PVP membrane is similar to PVDF/PVA membrane, but the adsorption capacity is much larger than the PVDF/PEG membrane, which indicates a small amount of PEG can reduce BSA absorption. When the value of hydrophilic angle is 75 degrees, PVDF/PVA membrane begins to exhibit different adsorption behavior with PVDF/PVP membranes, and the adsorption amount obviously reduces, the adsorption layer has become more loose and viscoelastic, and the PVDF/PVP membrane still showed larger changes of the frequency value, and smaller dissipation value. When the hydrophilic angle is 65 degrees, the PVDF/PVP and PVDF/PVA membrane both show less frequency value and larger dissipative value, namely, the layer become more viscoelastic.(2) The results also show that the adsorption behavior of the membrane surface was greatly influenced by the chemical properties of the membrane surface. Due to the rich hydroxyl of PVA, the membrane surface is easy to form hydrogen bonds with water molecules in the solution, the existence of hydrogen bonds further makes the membrane surface layer form a thin layer of hydration, the hydration layer directly weakens the force between the membrane and the pollutant in a large extent. At the same time, the adsorption of BSA pollutants accompanied with water molecules accumulates on the surface of membrane, resulting in the smaller amount of pollutants to adsorb on the membrane surface and the looser structure of the adsorbed layer. Different from PVA and PVP, except some hydroxyl group internal PEG molecules, it also contains a lot of ether group, and the ether group with weak polar can form hydrogen bonds with water molecules and reduce the adsorption of BSA to a certain extent. Once BSA is adsorbed onto the membrane surface, due to the interaction between ether and BSA, the conformation of the BSA absorbed on membrane will soon change, and forms compacted layer, at the same time water molecules are very difficult to enter into the layer, the compacted layer reduces the interaction between membrane surface and BSA in the external solution, also further cuts interactions with water molecules of the external solution. In a word, PEG can reduce BSA adsorption on the membrane surface in a great extent, but the dense structure would not be conducive to the water molecules through, and the layer is hard to wash down.(3) Through study the effluent(EfOM) adsorption on the PVDF/PVP/SiO2 membrane, the hydrophilicity of the membrane surface was significantly improved by adding different content of nano-SiO2 particles. QCM-D experimental result shows that EfOM is more easily adsorbed to the surface of the membrane with poor hydrophilicity, and the formation of the pollution layer is more rigid. we also found that EfOM adsorption process has two stages: a) the stage that EfOM rapidly accumulate on the membrane surface, and the frequency(F) rapidly decline, the dissipation(D) sharply rise; b) the adsorption equilibrium reach, the frequency(F) no significantly changes, the dissipation(D) are still on the rise, the conformation of the adsorption layer changes. The result of atomic force(AFM) force curves shows that the decrease of the force between EfOM and the membrane can not only reduce the adsorption amount of EfOM effectively, but also affect the structure of EfOM adsorption layer. Therefore, controlling the force between EfOM and the membrane can greatly improve the anti-fouling of the membrane surface. |
