Advanced membrane for water desalination and ion separation applications

Abstract

Population growth, contamination of fresh water, and climate change are increasing pressure on water supplies, accelerating the need for technological solutions that will improve access to clean water for drinking and sanitation. Membrane technology especially reverse osmosis (RO) and nanofiltration (NF) processes as sustainable routes for water desalination and purification are valuable from an environmental and economic standpoint. At present, RO and NF are the most energy-efficient technologies that provide us with safe and affordable drinking water, but they still need to be improved in terms of cost, affordability, and energy consumption. To achieve these improvements, advances in membrane materials are needed. The most commonly used semi-permeable membrane in RO and NF are polyamide (PA) thin-film composite (TFC) membranes which are fabricated on porous polymeric supports by in-situ polycondensation of two reactive monomers, namely polyamine and polyacyl chloride, at the interface of two mutually immiscible solvents. The main objective of my thesis was to use functional nanomaterials and nanotechnology tools to develop high-performance polyamide thin-film composite (TFC) membranes for water purification and desalination. PA-TFC membranes are flexible and the chemistry and performance of both top-layer and sublayer can be individually manipulated to maximize the overall membrane performance. In the first phase of my doctoral thesis, for the first time, an approach of using an atomic layer deposited (ALD) monomer-affinitive titanium dioxide (TiO2) nanofilm to modify the sublayer of TFC was proposed to form a thin, smooth, and highly cross-linked PA selective top layer. The functional TiO2 nanofilm increases the affinity between modified sublayer and amine monomer provide a more efficient and subtle tuning of the adsorption and diffusion of amine monomer during the interfacial polymerization process. The obtained TFC membrane with optimal ALD TiO2 coverage improved RO performance by obtaining a high permeance of 1.8 L m-2 h-1 bar-1 and high salt rejection rate of 96% in a dead-end process. This work reveals that coating functional nanomaterials by ALD is a practical manipulation technique for the controllable fabrication of promising TFC membranes and the optimization of sublayer materials. In the second phase of this thesis, we offered a facile, green, and cost-efficient approach for coating a stable layer of plant-derived polyphenol tannic acid (TA) on the surface of MXene (Ti3C2Tx) nanosheets. Then, high-performance reverse osmosis polyamide thin film nanocomposite (RO-PA-TFN) membranes were fabricated by incorporation of the modified MXene (Ti3C2Tx-TA) nanosheets in the polyamide selective layer through interfacial polymerization (IP). The strongly negative charge and hydrophilic multifunctional properties of tannic acid not only boosted the chemical compatibility between Ti3C2Tx MXene nanosheets and polyamide matrix to overcome the formation of nonselective voids, but also generated a tight network with selective interfacial pathways for efficient monovalent salt rejection and water permeation. In comparison to the neat thin film composite (TFC) membrane, the optimum TFN (Ti3C2Tx-TA) membrane with a loading of 0.008%wt nanofiller yielded a 1.4-fold enhancement in the water permeability while maintaining at a high NaCl rejection rate of 96% in a dead-end process and enhanced anti-fouling tendency. To the best of our knowledge, this is the first research on tannic acid-modified Ti3C2Tx MXene nanosheets and their utilization in the IP-based TFN membrane. This research offers a facile way for the development of modified MXene nanosheets to be successfully integrated into the polyamide selective layer to improve the performance and fouling resistance of thin film nanocomposite membranes. In the last phase of this thesis, for the first time, a novel IP template, graphene oxide nanoribbons (GONR) was proposed to act perfectly in response to two needs including minimizing the funnel effect and mediating the IP reaction toward desired PA properties. The coated GONR template not only efficiently served the gutter layer role, but also properly regulated the adsorption and transport of amine monomers at the interface of GONR through manipulating electrostatic interaction, capillary rise, and nanoconfinment of IP template by different loadings of GONR. The optimized loading of GONR at 0.02 g.m-2 resulted in a desired hybrid GONR/PA TFC NF membrane with nano-striped crumple structure beyond the PA context, an ultrathin PA nanofilm with a thickness of 15 nm, and a narrow pore size distribution and high crosslinking degree of 80% that simultaneously improve the permeability and selectivity, and successfully passed the upper bound trade-off with permeance of 21.3 L.m-2.h-1.bar-1 and great rejection of 98% for Na2SO4 under 5 bar of pressure. This research provides a new understanding on taking the advantage of a template method thorough an optimized GONR ultrathin network to make a desired selective TFC membrane for more affordable and efficient nanofiltration and ion separation processes.