Surface modification of water purification membranes to improve fouling resistance in oily water filtration
One of the biggest challenges in using water purification membranes is fouling. Surface modification using hydrophilic materials can reduce hydrophobic interactions between membrane surface and hydrophobic foulants, thereby alleviating fouling. In this Ph.D. research, polydopamine (PDA), a highly hydrophilic and universal coating agent, was used to surface-modified reverse osmosis (RO) and ultrafiltration (UF) membranes. PDA modification conditions (e.g., dopamine coating solution concentration, coating time, and pH of coating solution) control PDA deposition and can directly influence the modified membrane properties. Thus, the influence of PDA modification conditions on membrane physical, permeation, selective, and fouling properties were investigated systematically. A fundamental understanding relating the physical and permeation properties and the fouling characteristics of PDA-modified membranes was established. The RO membranes were modified with PDA at various modification conditions. Permeate fluxes during pure water and oil/water emulsion filtrations were studied. The PDA modification increased the permeate fluxes during oil/water emulsion filtration (thus, improved membrane fouling resistance) relative to unmodified membranes regardless of the initial dopamine concentration or deposition time used. However, these changes were only observed for the membranes coated under alkaline conditions, suggesting that the PDA did not deposit well under acidic condition. For UF membranes, molecular weight cutoff (MWCO) and pure water permeance decreased with increasing initial dopamine concentration or deposition time. A permeability and selectivity tradeoff was also observed. Membrane mean pore size and pore size distribution (modeled using log-normal pore size distribution) were investigated via modelling using a hindered solute transport model, Hagen-Poiseuille equation, and a stagnant film model. The PDA modification increased UF membrane surface hydrophilicity regardless of the coating conditions used, but it did not clearly change surface roughness or zeta potential (i.e., surface charge). Membrane fouling propensity was characterized using threshold flux. Compared to unmodified membranes, the threshold flux increased at minimal PDA coatings, but decreased at excessive PDA coatings. These threshold flux changes were likely governed by a tradeoff between surface hydrophilicity increase and pure water permeance decrease. Excessive PDA coatings resulted in decreased pure water permeance and possibly, pore blockage and pore size reduction, leading to higher local permeate flux causing severe fouling and decreased threshold flux.