Abstract
M.Sc. (Nanoscience)
Polychlorinated biphenyls (PCBs) are endocrine disrupting compounds (EDCs) and are harmful to humans and the environment. These PCBs are grouped under chlorinated organic compounds (COCs). The PCBs find their way to the environment through human activity such as industrialization and farming. Such activity produces wastes and runoffs that eventually end up in the water we use for drinking, farming and sanitation. It has then become necessary for researchers to find viable methods to remove these compounds from the environment. This is because current water treatment methods are not effective in the removal of the PCBs from water. The stages in the conventional treatment methods may include sand filtration, advanced oxidative processes and coagulation among others. These methods need to be energetically eco-friendly to drive the PCB dechlorination processes.
Researchers have used a variety of metallic nanoparticles including bimetallic nanoparticles for the removal of COCs from water. However, nanoparticles tend to agglomerate when not supported - leading to a decrease in their activity. Hence it has become necessary to stabilize or immobilize these nanoparticles on suitable support materials, such as, polymer solutions or solid substrates. Solid substrates including metal oxides, carbon and membranes, are currently being explored. Poly(vinylidene difluoride) microfiltration membranes are especially suitable for this function given the high porosity, chemical inertness and other outstanding physical properties.
In this work, the objective was to modify commercially hydrophilized poly(vinylidine)difluoride (PVDF) membranes with poly(ethylene glycol) (PEG). PEG is a bidentate polymer with two –OH groups found on either side of the molecule. The -OH groups allows PEG binding to the PVDF polymer backbone and hence high ability to capture or chelate the metal ions followed by their reduction. Nano-zerovalent metal nanoparticles were formed from these metal ions and chelated into the PEG grafted PVDF membrane to give the composite PVDF-PEG-Fe0. Post addition of the secondary metal was then followed by the introduction of the precomposite to a Pd solution to give the final catalytic membrane (PVDF-PEG-Fe0/Pd0). The use of PEG in this system allows for an even dispersion of the nanoparticles in the composite. The resulting nanocomposite membrane was used for the dechlorination of a polychlorinated biphenyl (PCB 77). Attenuated total reflection- Fourier transform infra red spectroscopy (ATR-FTIR) showed that PEG was successfully grafted onto the PVDF backbone. Optical contact angle measurements (OCA) were taken to determine the change in hydrophilicity of the
membrane upon modification. X-ray diffraction spectroscopy (XRD) proved that the Pd and Fe nanoparticles immobilized on the system were indeed zerovalent.
Scanning electron microscopy (SEM) images and contact angle measurements suggested a less porous membrane and slightly decreased hydrophilicity after modification. On the SEM micrographs the nanoparticles were observed to be quite evenly distributed in the membrane. Transmission electron microscopy (TEM) showed that the nanoparticles were in the range 20-30 nm in diameter, confirming the particle size values as determined by SEM. For the preliminary dechlorination studies done under ambient conditions, two dimensional column gas chromatography- time of flight- mass spectrometry (GCxGC-TOF-MS) results showed a complete dechlorination of PCB 77. A comparative study of the bare PVDF and catalytic membranes showed a slight difference in adsorption of the total PCB 77 concentrations. The catalytic membrane maintained its activity towards the dechlorination of PCB 77 after multiple regeneration cycles.