Sophia Belfer

Effects of surface modification on antifouling and performance properties of reverse osmosis membranes

Abstract The effects of surface modification of commercial reverse osmosis (RO) polyamide membranes were studied in terms of flux/rejection and surface properties. Selected RO membranes, spanning ultra-low pressure to seawater membranes, were modified by in-situ graft polymerization using redox generation of monomer radicals. Modification of the membrane significantly reduced both receding and advancing contact angles of some of the membranes as measured by a modified Wilhelmy plate method. Lower contact angles indicate more hydrophilic membranes and such membranes will potentially show better resistance to fouling by hydrophobic foulants. This was confirmed by adsorption measurements carried out on modified and unmodified membranes with a highly fouling surface water. Modified membranes adsorbed less organic material and were more easily cleaned than unmodified membranes. Specific fluxes were not changed by more than 0–25% and NaCl rejection was unchanged or increased slightly.

Surface properties and reduced biofouling of graft-copolymers that possess oppositely charged groups.

Microbial biofilms and their components present a major obstacle for ensuring the long-term effectiveness of membrane processes. Graft polymerization on membrane surfaces, in general, and grafting with oppositely charged monomers, have been shown to reduce biofouling significantly. In this study, surface forces and macromolecular properties of graft copolymers that possess oppositely charged groups were related to their potent antibiofouling behavior. Graft polymerization was performed using the negatively charged 3-sulphopropyl methacrylate (SPM) and positively charged [2-(methacryloyloxy)ethyl]-trimethylammonium (MOETMA) monomers to yield a copolymer layer on polyvinylidene fluoride (PVDF) surface. Quartz crystal microbalance with dissipation monitoring (QCM-D) technology was used to monitor the reduced adsorption of extracellular polymeric substances (EPS) extracted from a membrane bioreactor (MBR) wastewater treatment facility. Complemented measurements of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy provided evaluation of the antifouling properties of the surface. Increase in water content in grafted layer exposed to 100 mM aqueous NaCl solution was observed by QCM-D. Therefore, the grafted copolymer layer is swelled in the presence of 100 mM NaCl because of reversing of polymer self-association by counterions. Force measurements by atomic force microscopy (AFM) showed an increased repulsion between a carboxylate-modified latex (CML) particle probe and a modified PVDF surface, especially in the presence of 100 mM NaCl. The hydration and swelling of the grafted polymer layer are shown to repel EPS and reduce their adsorption. Delineating the surface properties of antifouling grafted layers may lead to the design of novel antifouling surfaces.