Date of Award

8-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Earth Science

Advisor

Ladner, David A

Committee Member

Lee , Cindy

Committee Member

Karanfil , Tanju

Abstract

Reverse osmosis (RO) desalination is increasingly used to produce potable water throughout the world. Despite their promising abilities, membrane filtration processes are limited by fouling. Fouling is a broad term for organics, inorganics, colloids and organisms that interact physically, chemically, or biologically with the membrane surface, resulting in reduced flux and shortened membrane lifespan. There is potential to create a snakeskin-like barrier between the membrane and foulants by electrostatically binding a coating material to the membrane that can be released by pH manipulation, removing foulants in the process. The feasibility of using functionalized nanoparticles as removable adsorptive coatings on RO membranes was evaluated in this study. Several inorganic-polymer composite nanoparticles (NPs) were examined in this study, including titanium dioxide coupled with polydiallyldimethylammonium chloride (polyDADMAC) to impart a positive charge [TiO,2(+)], titanium dioxide incorporated with polyacrylate to impart a negative charge [TiO2(-)] and silver incorporated with polyacrylate, again to incorporate a negative charge [Ag(-)]. PolyDADMAC was used as a positively charged binding layer atop the negatively charged membrane surface to adsorb negatively charged NPs. A series of concentration experiments was performed for each NP to find the lowest effective concentration for self-assembled coatings. The optimal concentration was then used in a series of kinetic experiments to determine the time required for coatings to assemble. Coating removal experiments were performed over a range of high and low pH values, with attention to indications of chemical alterations to the virgin membrane. Titrations were run on pairs of membrane samples in an electrokinetic analyzer to measure the surface zeta potential over a range of pH values. While virgin SW30HR had an isoelectric point ~4, the polyDADMAC-coated membrane was completely positive over the pH range tested (3 to 9). When low pH cleaning was used, polyDADMAC was partially removed from the membrane, indicated by an isoelectric point of ~4.5. NP coatings also resulted in changes to the membrane isoelectric point, which was more closely recovered after low pH cleaning. A secondary method for confirming coating and removal was not found, which is an excellent dilemma: coatings are thin enough to be invisible to scanning electron microscopy (SEM) and infrared spectroscopy analysis, but are definitely present as indicated by significant zeta potential changes induced by extremely small quantities of material. It is expected that X-ray photoelectron spectroscopy (XPS) would effectively detect and quantify coatings. Bench-scale RO experiments were run to test coating efficacy in an applied system. Flux was monitored for deionized water, sodium alginate fouling and acid cleaning. A second deionized water run was performed to determine flux recovery after fouling and cleaning. Membranes were coated similarly to those for electrokinetic studies, but on a larger coupon. RO experiments resulted in 69% recovery of the DI water flux after cleaning a fouled membrane with no coating, compared to a flux of 83% recovery when a NP layer was used.

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