Preparation And Properties Of Chitosan Modified Anti - Pollution PVDF Membrane

In many membrane materials of research and application, poly(vinylidene fluoride) (PVDF) has received great attention as a membrane material with regard to its outstanding properties such as high mechanical strength chemical resistance and thermal stability. Nevertheless, PVDF has an obvious shortcoming, that is, strong hydrophobicity. The strong hydrophobic nature of PVDF causes the low surface energy and the poor wettability of the PVDF membrane, leading to a low pure water flux. This work aims to improve the hydrophilicity and anti-fouling ability of PVDF membranes through a series of methods from the design of membrane material.In this paper, chitin was directly blended with the PVDF powder to improve the hydrophilicity of PVDF membrane via the immersion-precipitation phase transformation method. It was found that the hydrophilic chitin chains were segregated onto the membrane and pore channel surface, increasing the hydrophilicity of blend membranes. Chitin exhibited a pore-forming ability during the membrane formation process. The membrane mean pore size and porosity increased with the increase of chitin concentration in the casting solution, resulting in the high pure water flux and low Lys retention. Moreover, the flux recovery ratio increased, and the fouling resistance decreased with chitin content increasing, indicating that the anti-fouling ability of the membrane was improved.In order to improve the chitin content on the surface of PVDF/CH blend membranes, HAc that was chose to chitin was added into coagulation bath. During the membrane formation process, better incorporation of chitin with acid aqueous made it easier for chitin to enrich on the blend membranes surfaces and pore channel surface, making the chitin content on the membrane surface increase with the increase of HAc percentages in coagulation bath. Adding HAc to coagulation bath also could slow the membrane-forming speed significantly, leading to the changement of pore structure from finger-like pores to sponge-like pores and the decrease of porosity. Increasing the HAc concentration induced an improved hydrophilicity of membranes. The prepared blend membranes to Escherichia coli and Staphylococcus aureus all had certain antibacterial effects. When increasing HAc concentration, water flux and flux recovery ratio increased, the membrane fouling transformed from’irreversible’to ’reversible’.A two-step method to graft chitosan onto the surface of PVDF membrane was proposed, making chitosan fixed on the membrane surface. The PVDF powder was pretreated with alkali solution, the copolymer (PVDF-g-PAA) having the PVDF chains and polyacrylic acid (PAA) was synthesized via radical grafting copolymerization. The copolymer was cast into a flat membrane via immersed phase inversion method. The PVDF-g-PAA-CTS membrane was prepared using the amide reaction between carboxy group of PVDF-g-PAA membrane and amino group of chitosan. It was found that the graft reaction of chitosan would not change the pore structure of membrane obviously. Moreover, the content of chitosan on the surface of the composite membranes increased with the chitosan concentration increasing, and the hydrophilicity increased with it. In the filtration process, the water flux of the composite membrane increased and the flux recovery (FRR) reached 91.2%, which was significantly higher than that of pure PVDF membrane. It was found from bacterial culture that the antibacterial rate of composite membrane increased with the increase of chitosan grafting ratio, and the highest antibacterial rate reached 89.6%.