Constant flux crossflow filtration evaluation of surface-modified fouling-resistant membranes

Abstract

The surfaces of polysulfone ultrafiltration membranes were modified with polydopamine and polydopamine-g-poly(ethylene glycol) hydrophilic coatings. Unmodified and modified membranes were challenged with a soybean oil emulsion feed at six different permeate fluxes in constant flux crossflow filtration fouling studies. The threshold flux was determined for each membrane. Above the threshold flux, modified membranes generally exhibited lower transmembrane pressures than unmodified membranes. However, below the threshold flux, modified membranes had higher transmembrane pressures than unmodified membranes, likely due to a decrease in permeance resulting from the surface modification. To account for this difference in permeance, polydopamine-g-poly(ethylene glycol) modified membranes were compared to membranes with a thicker polydopamine coating and to an unmodified membrane with a smaller pore size than that used for the surface modified membranes; in this way, all three membranes had similar pure water permeances. In this case, the unmodified membrane exhibited a much higher transmembrane pressure during fouling than the modified membranes, so when membranes of the same permeance are compared, the surface modifications improved fouling resistance. Therefore, a potential strategy to achieve fouling resistance in a membrane of a desired flux and rejection is to modify the surface of a more permeable (and perhaps lower rejection) membrane, thereby making the resulting modified membrane fouling-resistant but leaving it with the desired flux and rejection characteristics.