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. The threshold flux increased when PDA was deposited at low initial dopamine concentrations or short coating times. 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. PDA coatings significantly increased surface hydrophilicity, but they did not markedly change the surface roughness or zeta potential.
Membrane properties, including surface hydrophilicity, roughness, and zeta potential, were characterized. Polysulfone ultrafiltration membranes were modified with polydopamine (PDA) at different initial dopamine concentrations and deposition times. membrane comparison strategies to better understand the complex, competing phenomena occurring when membranes are surface modified. This study focused on demonstrating surface modification strategies and. However, surface modification can significantly change the membrane filtration properties, sometimes resulting in more severe fouling than with the original, unmodified membrane. Surface modification of porous membranes for water filtration has been extensively reported in the literature to improve fouling properties. Results from studies exploring the effect of dopamine and related compounds on the fouling properties of UF and MF membranes used in oil/water emulsion filtration will be reported. Polydopamine modification conditions (i.e., dopamine concentration, deposition time, and dopamine solution pH) influence polydopamine film properties, such as thickness, hydrophilicity, roughness, etc. Membrane surface coating using dopamine solution (and related compounds such as hydroquinone), which undergoes polymerization under alkaline, aerobic conditions to form polydopamine, has previously been shown to significantly improve membrane fouling resistance to oil/water emulsions. Surface modification has been widely studied as a mean to reduce membrane fouling. Possible reasons for similarities and differences in membrane behavior for each mode of operation will be discussed.
At fluxes higher than the critical flux, the membrane resistances differed for the two operating modes, in some cases by more than an order of magnitude. At fluxes lower than the critical flux, the membrane resistance during constant flux and constant TMP fouling were similar. Membrane resistance as a function of time and as a function of permeate volume during fouling were compared for constant flux and constant TMP operation. The membrane performance during fouling tests in both constant flux and constant TMP experiments was investigated at fluxes higher and lower than the critical flux. For a given set of experimental conditions, the critical flux was determined using a flux-stepping method in constant flux crossflow filtration. To explore the similarities and differences between operation at constant flux and constant TMP, this presentation shows a comparison of constant-flux and constant-TMP operation of flat-sheet polysulfone ultrafiltration membranes during oil/water emulsion filtration. Many literature studies of fouling of water purification membranes report data from constant TMP experiments, and industrial filtration systems often operate at constant flux. Two methods of operating water filtration membrane systems are: (1) maintaining constant permeate flux by controlling transmembrane pressure (TMP) difference or (2) maintaining constant TMP and obtaining the permeate flux associated with that TMP.
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