No abstract

Membrane surface modification with polydopamine (PDA) coatings can reduce fouling in oily water filtration due, at least in part, to enhanced surface hydrophilicity. In this study, polysulfone (PSf) UF membranes were coated with PDA. PDA coating conditions (solution concentration and deposition time) were varied, and the effect of coating conditions on membrane molecular weight cutoff (MWCO) and hydraulic permeability was measured. Membrane MWCO decreased and PDA film thickness increased as initial dopamine coating solution concentration or deposition time increased. The MWCO decrease confirmed that PDA restricted the membrane pores. While the PDA coating thickness on membrane surfaces grew progressively with increasing initial dopamine concentration or coating time, coating inside the membrane pores was limited by the finite membrane pore size. A tradeoff between selectivity and hydraulic permeability of unmodified and PDA-modified membranes was noted. This tradeoff is reminiscent of that observed in other separation membranes. Zydney's hindered solute transport model of flow through porous membranes was used to estimate changes in membrane mean pore size and pore size distribution. Based on the modelling results, membrane mean pore radius increased at low initial dopamine concentrations or short deposition times and decreased at high initial dopamine concentrations or long deposition times with increasing initial dopamine concentration or increasing PDA coating time. The pore size distribution narrowed as the membranes were modified with PDA. The porosity to thickness ratio of PDA-modified membranes remained unchanged or was only slightly higher than that of unmodified membranes.

Surface modification of porous membranes for water filtration has been extensively reported in the literature to improve fouling properties. 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.

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) (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 beads < soybean oil < crude oil < motor oil. A three-stage TMP profile was observed with high fouling media, such as motor and crude oil emulsions. The TMP increased slowly in the early stage, then increased abruptly, and eventually reached a new pseudo-steady state TMP.

Critical and threshold flux concepts were recently developed to distinguish no fouling, slow fouling and rapid fouling regimes. Membrane fouling behavior is expected to vary with respect to the imposed flux relative to the critical and threshold flux values. However, crossflow fouling tests are often performed independent of critical and threshold flux determinations. In this study, constant flux fouling experiments were performed in connection with critical and threshold flux determination. Fouling behavior was examined in the context of critical and threshold flux. A poly(vinylidene fluoride) microfiltration membrane was challenged with various oil-in-water emulsions. The critical and threshold flux values were estimated using the flux-stepping technique. Constant flux crossflow fouling tests were performed at selected fluxes below and above the critical and threshold fluxes. Below the critical flux, mass transfer resistance remained constant at the clean membrane value. Above the critical flux but below the threshold flux, mass transfer resistance approached a steady state resistance, R B , which was determined from the linear regression of flux-stepping experiments. Above the threshold flux, a three-stage transmembrane pressure (TMP) was observed, consisting of:(1) an initial gradual increase, (2) a TMP jump stage, and (3) a pseudo-steady state. The pseudo-steady state TMP corresponded to the estimated critical pressure of the oil layer.