Effect of Nanopatterning on Concentration Polarization during Nanofiltration in Direct Flow Studies

Nanofiltration and reverse osmosis membranes are the two main type of membranes that are used in salt rejection. During saltwater filtration, there is a higher concentration of salt right above the membrane surface. This is because as the membrane allows water to go through, the salt stays behind and concentrates significantly above the membrane surface. This phenomenon is called concentration polarization. Concentration polarization affects the overall salt rejection of the process because it impacts the ability of the membrane to reject salt. Thus, if new methods can be developed to limit or prevent concentration polarization, it would have a significant impact on the membrane field.
This study investigated applying line-and-groove nanopatterns to a nanofiltration membrane to influence the concentration polarization at the membrane surface and increase the rejection of salts. Dupont NF270 membranes were patterned with two silicon stamps for a total patterned area of 6.4 cm2. Then, pristine and patterned membranes were tested in a direct flow cell with deionized water, 2,000 ppm Na2SO4, and 10,000 ppm Na2SO4 solutions. Each membrane was tested with pure water before being tested with any of the salt solutions. Each solution was allowed to permeate for 30 min before a sample was collected. The 30 min allowed the membrane to compact and reach steady state. From these results, it can be inferred that at higher feed concentrations, the concentration above the membrane is lower on a patterned membrane than on a pristine membrane.