(698a) Surface Modification to Enhance the Fouling Resistance of Commercial Polyamide Membranes

The desalination market, using polyamide reverse osmosis membranes, is growing steadily around the world. However, these membranes are limited by their susceptibility to fouling, a phenomenon that decreases water flux and membrane lifetime, thus increasing the cost of the desalination process. Alternative water sources such as produced water, a byproduct of oil and gas production, present even more potential for fouling, but are of interest due to growing water shortages and the resulting need to consider previously unused water sources. Depending on the level of purification achieved, produced water could provide a new water source for agricultural irrigation, livestock watering, process water, or even human consumption. Surface modification of commercially available polyamide membranes is one possible means of improving the fouling resistance of membranes for desalination and produced water purification.

This study employed three commercial membranes made by Dow Water Solutions, the LE, XLE and NF90. All three share the same general chemistry of the FT-30 polyamide membrane, but have different water and salt permeances. Their water fluxes increase in the order LE, XLE, NF90, while their NaCl rejections increase in the order NF90, XLE, LE. The LE membrane was chosen as the control, while the higher flux XLE and NF90 membranes were used for surface modification. Since surface modification lowers membrane water flux, this strategy allowed comparison of control and modified membranes with more similar water fluxes. Surface modification was carried out using poly(ethylene glycol) diglycidyl ether (PEGDE). Aqueous solutions of PEGDE were contacted with the top surface of the membrane to effect grafting of the chains to the membrane surface, and the desalination properties (water flux and NaCl rejection) of the modified membranes were measured. Fouling resistance was studied using charged surfactants (cationic dodecyltrimethylammonium bromide or anionic sodium dodecyl sulfate) and oil in water emulsions using decane and charged surfactant. In addition to measuring flux decline during fouling, the ability of the membranes to regain their pre-fouling flux after undergoing a cleaning protocol was also evaluated.

Techniques employed to verify the presence of PEGDE on the membrane surface included FTIR, XPS and SEM, while a magnetic suspension balance was used to estimate grafting density. Properties thought to influence fouling (i.e., surface roughness, charge and hydrophilicity) were evaluated using AFM, zeta potential analysis and contact angle measurements, to correlate observed fouling behavior with surface properties of the modified and control membranes.