Using zwitterionic amino acid l-DOPA to modify the surface of thin film composite polyamide reverse osmosis membranes to increase their fouling resistance

This study focuses on the incorporation of redox functional amino acid 3-(3,4-dihydroxyphenyl)-l-alanine (l-DOPA) onto commercial reverse osmosis (RO) membranes (SW 30 XLE) to create a zwitterionic surface that resists membrane fouling. The top layer of the membranes surfaces was modified by the deposition of l-DOPA from its alkaline solution. Streaming potential and contact angle measurements were conducted to characterise the membrane surface. Zeta potential data showed little or no change in the surface charge of 24-h-coated membrane relative to the original membrane. The contact angle measurements indicated that the hydrophilicity of the coated membranes was significantly improved. UV-visible spectroscopy confirmed the presence of poly DOPA film on the coated membranes. Whilst the salt rejection remained unchanged, a systematic increase in water flux was observed for the samples coated up to 12. h. Static BSA adhesion experiments revealed that l-DOPA coating has reduced the amount of BSA adsorbed to the surface. To investigate the dynamic fouling resistance of the membranes, a series of cross-flow filtration tests were carried out using bovine serum albumin (BSA) and alginic acid sodium salt solution as the feed. The modified membranes exhibited less flux decline than the original untreated membrane. Further, by water-only cleaning 98% water flux recovery ratio was achieved for the 24-h-modified membrane. The coated membranes also showed an improvement in their fouling resistance when testing with the solution containing dodecyltrimethyl ammonium bromide (DTAB), a positively charged surfactant model foulant.