Membrane Coatings for Catalytic Wet-Air Oxidation, Membrane Distillation, and Oil-Water SeparationsBySeth R. HoggThin polymeric films often impart attractive properties to a substrate without substantially altering the performance of the bulk material. This dissertation demonstrates three potential applications of polymeric films to enhance the functionality of membranes for water production and wastewater remediation, which are becoming increasingly important for combatting water scarcity.Layer-by-layer adsorption of polyelectrolytes and citrate-stabilized nanoparticles creates a catalytically active film for wet-air oxidation of pollutants in water. Incorporation of these catalytic films in membranes allows for improved oxygen availability at the catalyst surface in a three phase reaction. In this work, the use of tubular catalytic membranes with small inner diameters (< 1 mm) reduces diffusion limitations relative to similar systems based on larger ceramic membranes. The reduction in the diffusion distance required for aqueous pollutants to reach the catalyst surface leads to a 5- to 10-fold increase in the single-pass pollutant oxidation compared to previous work with ceramic membranes.Deposition of thin, rough polypropylene (PP) coatings rapidly produces superhydrophobic surfaces. Remarkably, addition of such a PP coating to a poly(vinylidene difluoride) (PVDF) membrane increases the advancing water contact angle from 124ᵒ to 155ᵒ. These superhydrophobic membranes are well suited for membrane distillation (MD), where a nonwetted membrane allows water vapor to transfer from a warm feed solution to a chilled permeate phase. During MD with highly fouling humic acid, membranes coated with porous PP show three-fold higher salt rejections than unmodified PVDF membranes. Moreover, the PP-coated membranes resist pore wetting for at least 20 hours during MD of a solution containing dairy whey. In contrast, bare PVDF membranes show pore wetting within 3.5 h of MD with dairy why solutions.Finally, adsorption of charged polyelectrolyte multilayers (PEMs) on membranes increases their hydrophilicity. This increase in hydrophilicity decreases fouling when filtering aqueous solutions with hydrophobic foulants, such as oil and grease. During cross-flow microfiltration of a 0.2% hexadecane-in-water emulsion, flux declines 80% after two hours for a PEM-modified membrane compared to a >90% flux decrease for an untreated membrane. Fouling resistance should be especially high if the foulant and the polyelectrolyte coating have the same charge. Exploiting electrostatic repulsions, the addition of anionic surfactants to oil emulsions causes a nearly two-fold increase in oil droplet rejection by anionically modified membranes.
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