Development of Synthetic Nanofiltration Membranes for Water Treatment

118 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Synthetic nanofiltration (NF) membrane technologies are capable of removing colloidal, organic, and multivalent ionic contaminants in natural water resources and are considered an attractive means for providing safe drinking water due to their much lower energy consumption compared to reverse osmosis and conventional distillation technologies. State-of-the-art commercial NF membranes have limited types based on either Loeb-Sourirajan or thin film composite anisotropic membranes. For these reasons, NF membrane development in other preparation methods and active layer chemical compositions to improve membrane productivity and cost-effectiveness has tremendous growth in the global water treatment market. This dissertation describes the exploration of a new NF membrane preparation approach based on small molecule pre-deposition and subsequent in situ stitching. Shape-persistent macrocycles (phenylene-ethynylene pentamer, hexamer, heptamer, benzocyclyne derivatives) and polyaromatic hydrocarbons (HPB, FIBC derivatives) of nanoscale dimension were designed and synthesized via cyclooligomerization reactions, serving as potential molecular building blocks for construction of membrane active layers. Among these molecules, rigid star amphiphiles (RSAs) with a series of hydrophobicity and functional properties were selected to fabricate a new generation of NF membranes. Membranes were prepared by direct percolation of methanol solutions of the RSAs through an asymmetric polyethersulfone support film that had been previously conditioned with methanol and cross-linked polyvinyl alcohol. The resulting RSA membranes (RSAMs) have been shown to exhibit significantly enhanced water permeability while maintaining high rejection of water contaminants compared to commercial NF membranes. Characterization results from confocal microscopy and Rutherford back-scattering spectrometry provided visual and semi-quantitative evidence suggesting the formation of an ultra-thin active layer of RSA molecules atop the modified support after lining its nano-pores with sizes similar to those of the RSAs. Such active layer constitution was found crucial for rejecting organic contaminants and achieving high water flux as a result of the reduced active layer thickness compared to that of commercial NF membranes. In addition, SEM and AFM images revealed that the new membranes achieved a uniform, flat surface which might help alleviate the fouling problems associated with rough membrane surfaces of commercial NF membranes. In situ stitching of RSAs to stabilize active layers by Cu(I)-catalyzed azide-alkyne cycloaddition reaction and UV-induced nitrene C-H bond insertion reaction were conducted, however, preliminary results revealed these attempts were unsuccessful when applied to tiny amounts of crosslinkers under wet conditions in a membrane system. Current research efforts are focusing on varying the in situ chemical stitching methods and conditions to construct robust membrane active layers preserving any inherent selectivity that the RSA building blocks might have. The molecular deposition approach is expected to provide an alternative, versatile channel to the creation of ultra-thin membrane active layers with tunable chemical compositions to remove challenging water contaminants while producing high water fluxes.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD