Bubela H. Functionalization of polyvinylidene fluoride membranes. – Qualifying scientific work on the rights of the manuscript.
Thesis for scientific degree of Doctor of Philosophy in specialty 102 – Chemistry (10 – Natural sciences). National University of “Kyiv-Mohyla Academy”, Ministry of Education and Science of Ukraine, Kyiv, 2023.
The thesis is devoted to the development of methods of surface modification of polyvinylidene fluoride (PVDF) membranes in order to provide them the desired characteristics, namely: enhanced antifouling and photocatalytic properties; as well as the study of surface, morphological and transport parameters. In addition, the possible usage of modified PVDF membranes was investigated for water purification from excess of iron (Fe2+) from water, fractionation of proteins and decomposition of dyes (rhodamine B and rhodamine 6G).
To provide magnetic properties to commercial PVDF membranes, a simple modification method with cheap and non-aggressive reagents (nanoparticles of magnetite (Fe3O4) and polyethyleneimine as a polymer linker between the surface of the membrane and Fe3O4) was proposed. Owing to the ability of Fe3O4 nanoparticles to move in the presence of a magnetic field, and therefore to additional turbulence in the membrane diffusion layer, the modified polyvinylidene fluoride membranes were characterized by better transport characteristics (45 L·m−2·h−1·bar−1 for the commercial PVDF membrane and 72 L·m−2·h−1·bar−1 for a modified PVDF membrane) and enhanced antifouling properties. In addition, magnetically active commercial PVDF membranes were applied in the process of separation and fractionation of proteins (rejection of lysozyme was only 5.3% at pH 12.0).
Furthermore, the developed method was applied to the surface modification of self-formed PVDF membranes with additional bulk modification (Fe3O4 was added to the polymer matrix of the membrane). The structure and morphology of the investigated membranes were characterized using various methods (IR spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force spectroscopy, etc.), which confirmed that magnetite nanoparticles were successfully integrated into the membrane matrix and onto its surface. It was established that the surface modification led to the hydrophilization of the membrane surface (the contact angle for the PVDF membrane is 106.1º ± 3, while for the modified PVDF membrane is 74.8º ± 2). The studied PVDF membranes were used for water purification from iron (Fe2+) excess using the ultrafiltration process with polyacrylic acid as a compound capable of binding iron (Fe2+) into a polyelectrolyte complex. The application of modified PVDF membranes allows the removal of the Fe2+ up to a concentration of 0.08 mg·l-1, which meets the standards of the World Health Organization (0.3 mg·l-1) and the European Union Regulation (0.2 mg·l -1).
Polyvinylidene fluoride membranes with photocatalytic properties were developed owing to the surface modification method with graphite carbon nitride (g-C3N4), as well as its derivatives doped with iron. Photocatalysts were obtained via thermal pyrolysis of melamine, and in the case of catalysts doped with iron: thermal pyrolysis of melamine with the appropriate metal salt (ferric (III) chloride hexahydrate). To confirm the synthesis of g-C3N4 and its derivatives, transmission electron spectroscopy, IR spectroscopy, and X-ray structural analysis were used. Subsequent modification of the surface of a commercial PVDF membrane with a cut-off of 150 kDa was carried out after activation of the membrane surface with a carbonate buffer, which resulted in dehydrohalogenation and the formation of double bonds on the membrane surface. In addition, physical modification of the polyvinylidene fluoride membrane surface with polydopamine was performed for effective adsorption of the photocatalyst.
Adsorption of Rhodamine B and Rhodamine 6G on modified membranes and their subsequent decomposition using photocatalytic PVDF membranes under visible light irradiation were investigated. The retention coefficients of Rhodamine B and Rhodamine 6G were 96 and 94%, respectively. For dye concentrations from 5 to 50 mg·l-1 decomposition of dyes was about 80% and 85% for Rhodamine B and Rhodamine 6G, respectively. PVDF-g-C3N4 membranes are characterized by high stability and reusability in the process of ultrafiltration of dyes in cross-flow mode. The loss of membrane activity after 5 cycles of ultrafiltration (total duration 50 h) was approximately 15%.
Key words: membranes, surface modification of polymer membranes; polyvinylidene fluoride membranes, magnetite nanoparticles, magnetoactive membranes, graphitic carbon nitrides, photocatalysis, ultrafiltration, concentration polarization, protein fractionation, water purification from dyes, enhanced ultrafiltration, water purification, purification of aqueous solutions.
