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作者： Bengani-Lutz, Prity

Tufts University

 

摘要：Membranes with ~1 nanometer size-based selectivity have numerous applications in the biochemical and pharmaceutical industries, as well as in wastewater treatment processes. However, most commercial membranes show poor selectivity in this size range due to wide pore size distributions, and charged surfaces that complicate the separation mechanisms involved. There is a need for new membrane materials that enable these separations while exhibiting high flux and fouling resistance, and are readily fabricated into large-area membranes.  Zwitterionic groups strongly resist biomacromolecular fouling due to their high degree of hydration, which makes them promising materials for membrane applications. Zwitterions are also documented to self-assemble into channel-type clusters 0.6-2 nm in size. We introduce a new class of membranes whose selective layers are made of zwitterionic amphiphilic random copolymers. These membranes derive their permeability, selectivity and fouling resistance from this self-assembled nanostructure. We synthesized random copolymers of hydrophobic monomers (acrylonitrile, methyl methacrylate, trifluoroethyl methacrylate) and zwitterionic monomers (sulfobetaine methacrylate, sulfobetaine vinylpyridines, and phosphorylcholine) by free radical polymerization. These copolymers self-assemble into bicontinuous networks of ~1 nm nanochannels, driven by the strong dipoles of the zwitterionic groups, as documented by transmission electron microscopy (TEM) imaging. Membranes prepared by coating these polymers on porous supports exhibit high flux, size-based  selectivity with a ~1 nm size cut-off, and excellent fouling resistance to common foulants. We have systematically investigated how copolymer composition affects nanostructure (self-assembled domain size) and the membrane performance.

We have also used ionic liquid co-solvents and other additives in the coating solutions to boost and alter the performance of these membranes. Membranes prepared by coating copolymer solutions with sufficient amounts of selected additives on commercial ultrafiltration membrane supports exhibit permeances as high as 50 L/m2 .hr.bar, up to 10 times higher than membranes formed without additives. These membranes also exhibit a narrow pore size distribution, retaining the same size-based selectivity with a ~1 nm size cut-off. Performance of these membranes depends on the amount of additive as well as the membrane manufacturing method (non-solvent, drying time etc.).

We have also applied two of the best performing membranes to real world applications such as treating municipal, oily and textile wastewaters. In singleand mixed-solute fouling tests with common components of effluent organic matter in municipal wastewater feeds (proteins, polysaccharides, natural organic matter, and fatty acids), these membranes showed complete resistance to irreversible fouling. Excellent fouling resistance was also observed with oil emulsions common in industrial wastewater. These membranes also exhibited no irreversible flux decline after filtering real wastewater samples from a textile dying plant in Turkey. High rejections of dyes and colored substances were achieved while salts were allowed to permeate, allowing low pressure operation and potential reuse of effluent. Thus, zwitterionic copolymer membranes are very promising for municipal and industrial wastewater treatment and reuse.