Date of Award

December 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Nidal Abu-Zahra

Committee Members

Benjamin Church, Xiaoli Ma, Krishna Pillai, Anna Benko

Abstract

The accumulation and growth of microorganisms on a membrane surface, known as membrane biofouling, has been a significant issue in the effective use of membrane technology for water and wastewater treatment. To overcome these challenges, this study aimed to modify the surface of a polyethersulfone (PES) membrane through the use of a multi-functionalized thermo-responsive polymer. The primary objectives of chemical treatment on membrane surfaces are to enhance surface hydrophilicity and provide anti-bacterial/biocidal properties.To evaluate the effectiveness of these modifications, the performance of the modified membranes was tested for their ability to resist biofouling through filtration of bovine serum albumin (BSA), and to inhibit adhesion and growth of Escherichia coli bacteria, respectively. The membrane's separation performance was evaluated based on its ability to separate oil-water emulsions and bovine serum albumin (BSA). The experimental results clearly demonstrated that grafting poly(N-isopropyl acrylamide) and immobilizing copper oxide nanoparticles on the membrane surface enhanced its hydrophilicity, which was indicated by a significant decrease in bulk contact angle. This increase in contact angle plays a crucial role in preventing bio-foulants from attaching to the membrane surface. The mechanism by which increasing the hydrophilicity reduces the biofouling is related to the interactions between the membrane surface and the foulants. A more hydrophilic membrane surface can form a thin layer of water on its surface, which can act as a barrier to prevent the adhesion of foulants[1], [2]. A more hydrophilic membrane surface can also reduce the interfacial tension between the membrane and the feed water, which can lower the resistance to water permeation and enhance the self-cleaning effect of the membrane. Additionally, a more hydrophilic membrane surface can reduce the electrostatic attraction between the membrane and the foulants, especially for negatively charged foulants such as bacteria or proteins [3]–[6]. Moreover, the receding contact angle of the thermo-responsive grafted membrane exhibited changes with variations in solution temperature, whereas this value remained constant for both the pristine PES membrane and unmodified membranes, indicating the phase volume transition behavior of PNIPAm in this study [7]. The surface charge of the membrane demonstrates (in sections 5.1.1.4 and 5.1.2.4) that grafting thermo-responsive polymers does not have a significant effect on the surface charge. However, the immobilization of copper oxide nanoparticles enhances the surface charge negativity, which occurs due to the deprotonation of the membrane's surface in the presence of metal oxide nanoparticles [8]. In addition, the synthesized membranes were subjected to surface characterization using scanning electron microscopy (SEM). The results showed that the membrane morphology and pore size were not significantly affected by the polymer grafting and copper oxide functionalization as discussed in “Chapter 5-Results and Discussion”. A water filtration test was conducted using a cross-flow filtration system, and the results indicated an improvement (from 68.8±0.16 L/m²h for bare PES to 80.0±0.19 L/m²h for PES-g-15% PNIPAm) in water flux rate after the modification processes, likely due to the enhanced hydrophilicity of the membrane surface. Since more hydrophilic membrane surface can reduce the interfacial tension between the membrane and the feed water, the resistance to water permeation can be lowered and enhance the water flux rate. Moreover, after cleaning the membrane with cold and warm water cycle, the thermo-responsive grafted membrane exhibited a higher flux recovery ratio and reversible fouling ratio, verifying that fluctuations in temperature did result in phase transition, and that the phase transitioning resulted in foulant detachment. Furthermore, the results revealed that the incorporation of organic and inorganic materials had an adverse effect on the phase transition behavior of PNIPAm, resulting in a decrease in the membrane's cleaning efficiency. This was evidenced by lower flux recovery ratio and reversible fouling amount in the modified PES membrane. The membrane's separation performance was evaluated in terms of its ability to separate oil-water emulsions. All membranes showed good performance in removing oil-water emulsions, but the membrane functionalized with copper oxide nanoparticles exhibited a superior rejection rate compared to the others, with a separation efficiency of >99% due to its increased surface hydrophilicity. Meanwhile, the PNIPAm-grafted PES membranes exhibited only slightly improved oil rejection performance compared to the unmodified PES membrane. Specifically, the bare PES membrane had a rejection rate of 96.9%, while the PES-g-25%PNIPAm membrane showed an even better rejection rate of 98.4%. In general, the oil-in-water emulsion separation capability in membrane filtration is determined by two mechanisms, namely size exclusion and hydrophobic/hydrophilic interactions between the feed solution and the membrane surface. The hydrophilic nature of the membrane surface plays a crucial role in regulating the water/oil attraction. Therefore, a membrane surface with increased hydrophilicity will easily form a hydration layer to impede oil passage, resulting in improved water permeability and higher oil rejection rate. Additionally, the membranes functionalized with copper oxide nanoparticles demonstrated high toxicity towards E. coli and B. cereus bacteria strains, effectively killing >99% of bacteria in the suspension after 90 minutes of contact time [9]. Furthermore, we assessed the concentration of copper ions released into the permeate water through ICP-MS analysis. The findings revealed an exceptionally low concentration of copper ions, measuring less than 14.5 ppb (the ratio of leached Cu ions to total CuO on the membrane=2.40%). The achieved value resulted from five recycling runs, with each run involving 60 minutes of water flow followed by 150 minutes of BSA solution flow. This value represents approximately 2.40% of the total copper that can leach into the solution. The maximum allowable level of copper in drinking water, as set by the United States Environmental Protection Agency (EPA), is 1.3 mg per liter (1.3 ppm). This underscores the remarkable stability of copper nanoparticles on the membrane surface, achieved by employing organic ligands to firmly immobilize the copper oxide nanoparticles through chemical bonds. These findings demonstrate the potential of modified PES membranes for water treatment applications and highlight the effectiveness of polymer grafting as a membrane modification method.

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