Design of advanced fouling-resistant and self-cleaning membranes for treatment of oily and impaired waters.

摘要

The overall goal of my PhD research was to design and develop advanced anti-fouling and self-cleaning membranes for treating impaired waters. Initial work focused on the development of membranes to treat produced water, which is oily wastewater that is co-produced during oil and gas exploration. Economical, environmentally sustainable treatment of the large volumes of produced water is a grand challenge for oil and gas companies. While membranes offer many advantages over more conventional treatment methods, membrane-based treatment processes for oily waters often fail due to membrane fouling. Therefore, the primary objective of my doctoral research was to design membranes that limit foulant accumulation and provide an easy, chemical-free way to remove any attached foulants during the filtration of oily and other impaired waters. My strategy was to modify the surface of ultrafiltration (UF) membranes with polymer nanolayer coatings using methods that enabled nano-scale control over the chemical and environmentally responsive conformational properties of grafted polymer layers.;A three step surface-modification procedure was designed and implemented to modify commercial regenerated cellulose UF membranes by grafting block copolymer nanolayers from the membrane surfaces by surface-initiated atom transfer radical polymerization. Membranes were modified by grafting poly(N-isopropylacrylamide) (PNIPAAm)-block-poly(oligoethylene glycol methacrylate) (PPEGMA) nanolayers. The lower block (PNIPAAm) was grafted to make the membrane surfaces temperature responsive while the upper block (PPEGMA) was grafted to suppress attachment of foulants. The physiochemical and performance properties of the modified membranes were characterized using a number of different analytical methods. Polymer grafting led to a roughly 40% decrease in the water flux, however, modified membranes showed slower flux decline than unmodified membranes, and, hence, the modified membranes allowed a 13.8% higher cumulative volume of water to be processed over a 40 h cross-flow filtration run. Flux recovery was better for the modified membranes after a cold water rinse. The flux recovered fully to initial values for the modified membranes; while only ∼81% of the initial flux was recovered for the unmodified membrane. Total organic carbon removal efficiencies were higher than 94% for all the membranes studied and increased slightly with increasing degree of modification; however, all the membranes exhibited poor salt rejection.;After successful demonstration of the modification strategy for preparing fouling-resistant, easily cleanable UF membranes for produced water treatment, I shifted my focus towards a better understanding of the role of polymer nanolayer structure on performance. I used initiator grafting density and average molecular weight of both the PNIPAAm and PPEGMA blocks as independent variables to optimize the performance of the surface-modified membranes. Higher initiator densities and longer polymerization times yielded membranes with stable flux, while lower densities and shorter polymerization times slowed the rate of flux decline but did not eliminate it. The trade-off for the stable flux was lower instantaneous flux. This trade-off was deemed acceptable since the cumulative volume of impaired water that could be treated prior to cleaning was higher for the modified membranes. My results showed that, beyond the chemistry of the coating, its structural properties, especially polymer grafting density and block nanolayer thicknesses, play an important role in determining its effectiveness for fouling control. My membrane surface modification protocol allows one to tailor these structural properties independently, in ways not achievable by standard coating methods, to produce membranes with an optimized combination of high enough instantaneous permeate flux and low enough rate of flux decline.;Having demonstrated that my newly designed, advanced fouling-resistant and self-cleaning membranes could be used for treatment of oily produced water, the possibility of using these membranes for treatment of highly impaired wastewaters generated in rendering facilities was investigated. I evaluated the separation performance of my advanced membranes using impaired waters provided by Carolina By-Products/Valley Proteins Inc., and compared performance metrics to those of commercial wastewater treatment UF membranes. Membrane surfaces were characterized by spectroscopy and electron microscopy pre- and post-filtration to determine the extent of fouling. Low molecular weight cutoff membranes showed stable permeate fluxes for long periods of time without the need for intermittent cleaning, characteristic of systems with low degrees of internal fouling. For 100 kDa molecular weight cutoff membranes, flux decline was more severe. While polymer-modified membranes processed ∼26% more permeate than unmodified membranes in this case, flux recovery after a membrane cleaning step was low and similar for unmodified and modified membranes, characteristic of high degrees of internal fouling. All membranes showed minimal changes in the permeate pH and total dissolved solids, but turbidity was reduced nearly 100% and chemical oxygen demand was reduced by over 70%.;Taken together, results from my doctoral research indicate that well-designed PNIPAAm-b-PPEGMA-modified ultrafiltration membranes can be used to separate organics from large volumes of impaired waters at high flux.