This research aims to improve our understanding of how novel anti-fouling coatings impact the properties and performance of reverse osmosis and nanofiltration membranes. The coating used for this study is a highly water-permeable polyether-polyamide block copolymer (PEBAX 1657, or ‘PEBAX’). The effects of the PEBAX layer and coating procedure on membrane performance (i.e. water flux and salt rejection) were assessed through water permeation and salt rejection studies. The anti-fouling properties of coated membranes were evaluated both in the lab with a model fouling solution, and in the field with a concentrated brackish groundwater feed. Membrane surface properties were characterized using scanning electron microscopy, atomic force microscopy, and infrared spectroscopy. In addition, gas permeation testing provided new insight on how the separating layers of water treatment membranes are affected by typical RO operating conditions and surface modification procedures.
The first part of this research tested the hypothesis that PEBAX coatings can reduce the rate of membrane flux decline and increase cumulative permeate volume. A 1 wt. % PEBAX coating layer on polyamide RO membranes (Hydranautics seawater type SWC4 and low-pressure type ESPA1) produced smoother membrane surfaces with enhanced fouling resistance against a model oil/surfactant/water emulsion feed. Chemical surface analyses confirmed that PEBAX remained on the membrane surface for the duration of a 106-day fouling study. The slower rate of flux decline led to a higher cumulative permeate production relative to uncoated SWC4 after 30 days of operation. However, the improvement in fouling resistance was not sufficient to compensate for the reduction in intrinsic membrane flux for coated ESPA1 membranes. This drop in water flux was greater than series resistance model predictions, and prompted further study to identify the cause(s) for this discrepancy.
For the second part of this research, a method using gas permeation testing was developed to evaluate how membrane modification procedures change selective layer morphology. Knudsen diffusion-based gas transport properties indicated the existence of nanoscale ‘defects’ in the separating layers of several commercial RO membranes. The contribution of Knudsen diffusion to gas transport was diminished—but not eliminated—by concurrent compression and swelling of the dense polyamide matrix, which are conditions inherent to reverse osmosis treatment. These defects were eliminated when the membrane surface was coated with PEBAX, as indicated by a 25-fold decrease in gas permeance and at least a two-fold increase in most selectivity values. These defects may act as high permeability sites for water permeation, and their elimination may contribute to the observed reduction in water flux after coating.
Reductions in gas permeance and water flux were also observed for membranes after the coating process was repeated without PEBAX in the coating solution. The third part of this research explored how coating process conditions (i.e., solvent exposure and oven-drying) can impact membrane performance. The observed trends in water and salt fluxes were attributed to changes in polyamide matrix permeability and enhancement of transport through defects. Matrix permeability can be altered by increased or decreased water-membrane hydrogen bonding. Ethanol can swell the polymer and break interchain hydrogen bonds, thereby making them available to water; the resultant increase in water flux is not readily reversible, based on a 300-hour flux test. Conversely, solvent evaporation can promote interchain hydrogen bonding, which decreases water permeability. Reduced matrix transport caused by membrane drying can be partially reversed by subsequent ethanol-soaking; however, some irreversible flux loss will remain. Enhanced gas transport through defects was observed after the treatment sequence of ethanol-soaking followed by drying, which suggests that defects became wider or that more defects were exposed. These defects can permit higher concentrations of salt through the membrane, which would account for the observed decreases in salt rejection.
Finally, the last part of this research evaluated the effects of PEBAX coatings on the performance of a smooth nanofiltration membrane (GE Osmonics DL). Coated and uncoated membrane samples had similar pure water fluxes, demonstrating that PEBAX coatings can be applied to certain membranes without causing a significant change in intrinsic water flux. A preliminary field study of PEBAX-coated DL membranes treating brackish groundwater demonstrated the potential for coatings to improve fouling resistance under scaling conditions.
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