Development of Anti-fouling Membranes for Water Treatment

摘要

The goal of my dissertation research was to devise new strategies to combat membrane fouling, which is a major hindrance in water treatment systems. Membrane fouling refers to the blocking of pores and the build-up of material on the membrane surface. There are many types of foulants, including but not limited to bacteria, biopolymers (such as alginate), natural organic matter, oils, proteins, particles, and salts (such as gypsum). Typical ways to combat fouling include expensive pretreatment procedures, physical membrane cleaning, and chemical treatments to the membranes. Chemical treatments tend to decrease the membrane lifetime, especially for nanofiltration and reverse osmosis membranes. One common strategy in the research community is to surface modify membranes to increase their fouling resistance. Chapter 1 reviews the surface modification methods used to reduce membrane fouling.;Chapter 2 presents my work on modifying ultrafiltration membranes with poly(2-((2-hydroxy-3-(methacryloyloxy)propyl)dimethylammonio)acetate) (poly(CBOH), a polymer that switches reversibly between a zwitterionic chemistry and a quaternary amine chemistry. Surface characterization showed successful membrane modification by UV-graft polymerization. Bacteria deposition studies showed that the poly(CBOH) chemistry performed better than other common anti-fouling chemistries. Biofilm studies showed that poly(CBOH) functionalized polyethersulfone membranes accumulated half the biovolume as unmodified membranes. Poly(CBOH) switches from an anti-fouling, zwitterion mode to an anti-microbial, quaternary amine mode by changing the environment pH. Studies were done to characterize the switching pH and time using poly(CBOH) modified silicon wafers. Switching pH was determined to be 1.0, with 15 min being required to switch between the zwitterion and quaternary amine chemistries. Biofilm mortality was elevated on ultrafiltration membranes once the anti-fouling poly(CBOH) zwitterion was switched to the anti-microbial, poly(CB-Ring) quaternary amine, with dead-to-live cell ratio increasing from 0.33 to 1.04.;Chapter 3 describes my work to increase the fouling resistance of nanofiltration membranes by applying both a chemical coating and a nanometer sized pattern to the membrane surfaces. A line and groove nano-pattern was applied by thermal embossing directly onto a commercial polyamide thin-film composite nanofiltration membrane. Poly(ethylene glycol) diglycidyl ether (PEGDE) was reacted onto the patterned membrane surfaces by an epoxide ring opening reaction with unreacted carboxyl groups on the polyamide selective layer. Surface characterization showed successful nano-patterning and chemical modification of the membrane surfaces. Membrane performance (flux and salt rejection) was unaffected by patterning the polyamide membrane surface directly. The fouling results show that combining line and groove nano-patterning with PEGDE chemical modification yields a membrane that was more resistant to fouling than either method alone.;Chapter 4 presents my work to apply a nanometer sized pattern onto numerous commercial nanofiltration and reverse osmosis polyamide membranes. There have been differing views on the ability to pattern polyamide thin-film composite (TFC) membranes directly by nanoimprint lithography. The goal of this study was to understand what factors control patternability, working towards a set of heuristics for use by the membrane community to pattern any polyamide TFC membrane. Initial results showed that each membrane patterned to a different degree. Despite completing a comprehensive set of experiments to investigate the roles played by membrane chemistry, surface properties, mechanical properties, and performance properties on pattern peak heights for thirteen commercial nanofiltration and reverse osmosis membranes, I found no correlation between the variables studied and patternability of individual membranes. I did discover significant differences in patternability between membranes grouped by polyamide class, with those prepared by interfacial polymerization of m-phenylenediame and trimesoyl chloride having the largest pattern peak heights. I further discovered that the humectant (pore filler) used for membrane preservation plays a role on the patternability of the membranes. Upon replacement of the original humectants used by the membrane manufacturers with a 15 wt% glycerol solution, the pattern peak heights approached a similar value for each class of membranes. Tests performed to elucidate the role of the glycerol on patternability were inconclusive. Thus, while the humectant clearly contributes to membrane patternability, the reason why remains unknown.;Overall, my research demonstrates the ability to reduce membrane biofouling by changing surface chemistry and surface features. Results of my work contribute to our understanding of membrane fouling and can be used to develop next-generation water treatment membranes with improved fouling resistance. Such membranes could be expected to lower the operations cost of using membranes to clean water.