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.Chemical Engineerin

Effect of polydopamine deposition conditions on polysulfone ultrafiltration membrane properties and threshold flux during oil/water emulsion filtration

Surface modification of porous membranes for water filtration has been extensively reported in the literature to improve fouling resistance. However, surface modification can significantly change the membrane filtration properties, sometimes resulting in more severe fouling than with the original, unmodified membrane. This study focused on demonstrating surface modification strategies and membrane comparison strategies to better understand the complex, competing phenomena occurring when membranes are surface modified. Polysulfone ultrafiltration membranes were modified with polydopamine (PDA) at different initial dopamine concentrations and deposition times. Membrane properties, including surface hydrophilicity, roughness, and zeta potential, were characterized. PDA coatings significantly increased surface hydrophilicity, but they did not markedly change the surface roughness or zeta potential. The threshold flux during oil/water emulsion filtration was determined and used as a fouling parameter for membranes modified with PDA at various modification conditions. The threshold flux increased when PDA was deposited at low initial dopamine concentrations or short coating times. However, PDA deposition at high initial dopamine concentrations or long coating times decreased the threshold flux, suggesting that a tradeoff exists between increased hydrophilicity and reduced pore size due to surface modification. An increase in membrane surface hydrophilicity was observed at all PDA deposition conditions, which tends to reduce foulant adhesion and increase threshold flux. However, extensive PDA coating significantly decreased membrane pure water permeance, suggesting that some membrane pores may have been narrowed or blocked, increasing local permeate flux through the remaining pores in the PDA-modified membranes. This higher local flux would exacerbate fouling and decrease threshold flux. Comparing unmodified and PDA-modified membranes having similar pure water permeance values, the PDA-modified membranes had higher threshold fluxes than the unmodified membranes

Fouling propensity of a poly(vinylidene fluoride) microfiltration membrane to several model oil/water emulsions

© 2016 Elsevier B.V. Laboratory membrane fouling studies are often performed with a single foulant. However, studies comparing the behavior of different foulants using a single membrane are rarely reported. In this study, a poly(vinylidene fluoride) (PVDF) microfiltration membrane was challenged with a series of aqueous-based model fouling media, including a suspension of latex beads, as well as soybean, motor and crude oil emulsions, in constant permeate flux fouling experiments. The critical and threshold fluxes were determined for each membrane-foulant pair. Constant permeate flux crossflow fouling experiments were performed at both low and high fluxes. A direct comparison of the fouling propensity of the PVDF membrane to the four fouling media was made. The fouling propensity was evaluated based on threshold flux values and the extent of transmembrane pressure (TMP) increase during constant permeate flux fouling experiments. In this study, the zeta potential of various fouling media correlated with their fouling propensities. The higher the zeta potential, the lower the fouling propensity. The fouling propensity followed the order of: latex bead