Abstract

Membrane-based separation technologies, as versatile platforms, have attracted considerable attention for water recycling and reuse, due to their ease of operation, high separation efficiency, excellent sustainability, relatively low energy consumption, and industrial viability. Among them, thin-film composite (TFC) membranes have been widely employed for the desalination of seawater and separation of small organic compounds. However, the state-of-the-art polyamide TFC membranes still suffer from the severe fouling and limited water permeability, which are mainly ascribed to their intrinsic surface hydrophobicity and uncontrollable interfacial polymerization process. To address the current challenges and limitations in TFC membranes, it is essential to develop new methodologies and design new functional materials for the fabrication of TFC membranes.
In this research, a new strategy based on the bioinspired chemistry has been explored for the fabrication of TFC membranes. In the strategy, polydopamine-modified cellulose nanocrystals (CNCs) with substantial quinonoid active sties have been prepared through oxidative auto-polymerization of dopamine, which enable the subsequent crosslinking with polyethylenimine (PEI). Electrospun nanofiber mats (ENMs) produced from electrospinning has been employed as the supporting layer in order to enhance the membrane permeability. TFC membranes are fabricated with the modified CNCs as the active layer via facile vacuum filtration on ENMs followed by further cross-linking with PEI. The achieved ultrahigh pure water permeability (PWP), superior dye rejection, and remarkable salt permeation demonstrate its great potential for the effective separation of dye/salt mixtures and recovery of these valuable components.
To expand the application of bioinspired chemistry in TFC membranes, a rapid co-deposition of dopamine with zwitterions (Z-DNMA) via covalent polymerization triggered by CuSO4/H2O2 has been further proposed to construct the thin film selective layer. The fabricated TFC membranes with the incorporated zwitterionic structure from Z-DNMA show enhanced surface hydrophilicity and superior fouling-resistant performance towards both typical hydrophobic contaminants (e.g., proteins) and organic molecules (e.g., organic dyes).
In addition to the development of new strategies, a new diamine monomer featured with trimethylamine N-oxide (TMAO) structure, N,N-bis (3-aminopropyl)methylamine N-oxide (DNMAO), has been designed for the fabrication of polyamide TFC membranes via interfacial polymerization. Its charged group (N+-O-) is directly connected without extra atoms and has the typical characteristics of zwitterions. The fabricated TFC membranes show high water permeability, high dye/salt selectivity, and improved antifouling ability. Apart from the polyamide TFC membranes, zwitterionic triethanolamine-based (Z-TEOA) polyester TFC membranes have been also fabricated via interfacial polymerization in this research. Z-TEOA endows a typical sulfonbetaine (SB) -based zwitterionic structure, which is one of the most widely used zwitterions in the fabrication of membranes. The performance of the polyester TFC membranes has been systematically investigated on the purification of dye- and antibiotic-contaminated wastewaters.
The results in this thesis demonstrate the promising application of bioinspired chemistry in the fabrication of TFC membranes for the treatment of organic compounds-contaminated wastewaters. Meanwhile, the new generation of zwitterions based on TMAO-derived structure shows a great potential for the modification of the conventional polyamide TFC membranes to alleviate fouling propensity. Furthermore, this research also highlights the promising application of zwitterionic-modified polyhydroxyl monomers in the fabrication of polyester TFC membranes to enhance fouling-resistant performance.