Tuning the Separation Properties of Thin-Film Nanocomposite Membranes and Aerogels by Polymeric Functionalization of Graphene Oxide Nanosheets and Cellulose Nanocrystals

Abstract

Water contamination from dyes has emerged as a serious environmental issue, owing to their prevalent use in industries such as textiles, food, and pharmaceuticals. Yearly, billions of tons of dye-laden effluent are dumped into water systems, posing serious threats to aquatic ecosystems and public health. Untreated wastewater not only changes the color of water bodies, but it also inhibits the penetration of light, which is essential for photosynthesis in aquatic plants. Microalgae, which are essential organisms in the aquatic food chain, experience growth suppression and morphological abnormalities when exposed to these contaminants. Toxic effects travel up the food chain, affecting fish and resulting in bioaccumulation of hazardous compounds that endanger human health through contaminated water and food. In light of the significant contamination issues, numerous regions, particularly those classified as low- and middle-income, are deficient in the necessary infrastructure and regulatory systems to manage dye wastewater in an effective manner. To mitigate the severe issue of dye pollution in bodies of water, there is a need to research into possible techniques for efficient removal of such contaminants. Among the several techniques, the use of advanced materials used for separation and filtration technologies is an economical, efficient and sustainable method. This thesis studies novel approaches to improving the performance of thin-film nanocomposite (TFN) membranes and aerogels for wastewater treatment applications. This work primarily focuses on the development of graphene oxide (GO) and cellulose nanocrystals (CNCs) based composite filler materials with poly (glutamic acid) and poly (bis[2-(methacryloyloxy)ethyl] phosphate) brushes grown on them through surface initiated atom-transfer radical polymerization (SI-ATRP). These materials enhance thin film composite (TFC) membranes properties such as hydrophilicity, rejection rates, antifouling properties, and extend membranes longevity. All nanomaterials were synthesized and characterized using elemental Carbon, Hydrogen, Nitrogen, and Sulphur (CHNS) analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), 13C nuclear magnetic resonance spectroscopy (13C NMR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Scanning electron microscopy (SEM), FTIR, atomic force microscopy (AFM) were employed to examine the fabricated membranes. In the first study, glutamic acid brush modified GO (GO-GLU) was introduced to the selective polyamide (PA) layer. This layer was produced through interfacial polymerization (IP) of organic trimesoyl chloride (TMC) and aqueous mphenylenediamine (MPD) on a polysulfone (PSF) support. Water permeability and dye (methyl orange (MO) and methylene blue (MB)) rejection were used to evaluate membranes performance. Contact angle measurements proved that the incorporation of these modified GO nanosheets significantly boosted the membranes' hydrophilicity and water permeability. The study revealed that at the optimal GO-GLU loading of 0.10 wt%, GO-GLU TFN membrane attained a water permeance of 14.11 L/m²h with impressive rejection rates of 99.18% for MB and 67.18% for MO. Furthermore, it displayed superior antifouling characteristics against Bovine Serum Albumin (BSA) when compared to the virgin TFC membrane and potentially good operation stability performance after 3 cycles. Secondly, TFN membranes were prepared through IP, incorporating bis[2 (methacryloyloxy)ethyl] phosphate (BMEP)-grafted graphene oxide (GO-PMEP) into the selective PA layer for the removal of dyes/salts in water. The results revealed that at an optimal GO-PMEP loading of 0.025 wt% (TFN-1), the membrane exhibited significantly enhanced wettability, hydrophilicity, and pure water permeability attributable to the increased oxygenated functional groups located on the membrane surface, which promote higher affinity to water molecules. Furthermore, the TFN-1 membrane demonstrated superior performance under low-pressure conditions (400 kPa), achieving a water flux of 9.32 ± 0.89 L/m²h, representing a 1.5-fold increase compared to conventional TFC membranes. Furthermore, the TFN-1 membrane demonstrated outstanding rejection rates, surpassing 80% for MO, achieving 100% for MB, and approximately 86% and 31% for MgSO4 and NaCl, respectively. Moreover, the efficacy of these membranes was further validated using real water samples, confirming their potential for practical application in dye removal under environmental conditions. Thirdly, this research explored the integration of hemp derived cellulose nanocrystals (CNCs) modified with glutamic acid (CNC-GLU) into another set of TFN membranes. The resulting CNC-GLU TFN membranes (0.10 wt%) were more hydrophilic and smoother than the comparable TFC membrane. The CNC-GLU TFN membrane outperformed pristine TFC membranes with a pure water flux of 15.45 ±1.03 L/m2h, high dye rejection (99.83% for MB, 90.63% for MO) and salt removal (MgSO4 = 86%; NaCl = 38%) under low pressure conditions (400 kPa). This was attributed to the increased hydrophilic active layer, steric hindrance, and Donnan exclusion. Furthermore, the TFN membrane effectively removed MB and MO dyes from real water samples and demonstrated good operational stability after three cycles. Moreover, the potential application of glutamic acid modified graphene oxide (GG-PVA) aerogels fabricated through a simplistic sol gel and freeze-drying process utilizing polyvinyl alcohol (PVA) as a cross-linker in dye removal was also investigated. These aerogels exhibited a maximum adsorption capacity (qmax) for MB of 121.95 mg/g defined by the Langmuir Isotherm model. Maximum adsorption of the MB was favored by near neutral to alkaline conditions, reaching 88.4% efficiency and equilibrium reached in the first 30 minutes of the adsorption process. The kinetics of the MB removal followed the pseudo-second order model. Furthermore, thermodynamic studies revealed endothermic (ΔHo = 80.598 kJ mol-1) and spontaneous (ΔGo = -8.219 kJ mol-1) nature of the adsorption process. Overall, this thesis contributes to a deeper understanding of how advanced nanocomposite materials can be utilized to create more effective and sustainable solutions for wastewater treatment, particularly in removing toxic dyes from industrial effluents. The integration of these innovative materials into membrane technology represents a significant step forward in developing efficient strategies for mitigating water contamination and promoting environmental sustainability.