Tailoring of PVDF Membranes by Blending and Chemical Modification for Enhanced Applications in Wastewater Treatment

Abstract

Membrane technologies are now increasingly being utilized for the treatment of wastewater as well as the production of freshwater via desalination as they offer better operational efficiency and cost reduction. Polyvinylidene fluoride (PVDF) membranes play a key role in water applications due to their favorable properties, including high chemical stability. Fine-tuning the properties of PVDF membranes enables higher flux, lower fouling potential etc. for targetspecific applications. Therefore, this thesis aims at exploring various physical and chemical modifications for improving the performance of PVDF flat-sheet membranes in different processes. Blending modification of PVDF membrane was investigated using various additives such as polyvinylpyrrolidone (PVP), fumed silica (FS) and carbon nanotube (CNT). Chemical modification was explored using KOH-H2SO4. Properties of the membrane were evaluated and characterized using a wide range of tools. Extensive physicochemical (e.g., contact angle, surface free energy, surface tension, and work of adhesion) and morphological (e.g., pore size distribution and roughness) characterizations were done applying goniometric and microscopic methods. Hansen solubility parameters (HSPs) were calculated in order to assess the affinity/compatibility of PVDF-additive-solvent system. Thermodynamic and kinetic aspects of the membrane formation were also studied. Fabricated membranes were tested in membrane bioreactor (MBR), wastewater filtration and removal of methylene blue. In the case of PVDF/PVP blend membranes, enhanced leaching of PVP prior to the filtration operation significantly enlarged membrane pore size and thus reduced the membrane resistance. However, this advantage was dismissed during operation in MBR because PVP leaching also induced surface hydrophobicity that promoted membrane fouling, suggesting the detrimental effect of the post treatment. This slow leaching of PVP was found to be the preferred option. Although PVDF-FS blend membrane showed much higher pure water flux, their fouling propensity was also high due to their increased hydrophobicity. In the case of PVDF-CNT blend membranes, top surface hydrophilicity of dual layer membrane was ensured by blending appropriate quantity of PVP into the polymer dope so that feed water with micropollutants will contact the membrane surface containing CNT. Adsorption results indicated the potential of using blend membranes in the post-treatment of treated wastewater to ensure the complete reuse of treated wastewater and thus to achieve sustainable water management. Modification of the PVDF with KOH/H2SO4 altered the crystallinity of the PVDF from a mixture of both α and β phases to a predominant β phase. Lower work of adhesion of the modified membrane indicated the formation of a more hydrophobic and wetting-resistant membrane surface. Centrifugation of the polymer dope after the modification had a pronounced impact on the properties of resultant membranes.

Date of Award	Dec 2017
Original language	American English